WO2025262099A1

WO2025262099A1 - A method for preparation of isoprenol and downstream products thereof from isoamyl alcohol

Info

Publication number: WO2025262099A1
Application number: PCT/EP2025/067019
Authority: WO
Inventors: Chiara BOSCAGLI; Alexander Czaja; Christian Gruenanger; Martin Ernst; Alois Kindler; Michael Ludwig Lejkowski; Piyush Ingale
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2024-06-21
Filing date: 2025-06-18
Publication date: 2025-12-26
Anticipated expiration: 2026-12-21

Abstract

The present invention relates to a method for the preparation of 3-methyl-3-butene-1-ol (isoprenol), comprising the steps of preparing isobutylene (also: isobutene, 2-methylpropene) by contacting 3-methylbutan-1-ol (isoamyl alcohol) to a catalyst comprising at least one catalytically active metal and reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol. Furthermore, the present invention refers to processes for the preparation of 3-methyl-2-butene-1-ol (prenol), of 3-methyl-3-butenal (isoprenal), of 3-methyl-2-butenal (prenal), and of 3,7-dimethy l-octa-2, 6-dienal (citral) and further downstream products comprising the use of isoprenol of the present invention and the resulting products and product streams.

Description

A METHOD FOR PREPARATION OF ISOPRENOL AND DOWNSTREAM PRODUCTS THEREOF FROM ISOAMYL ALCOHOL

FIELD OF THE INVENTION

The present invention relates to a method for the preparation of 3-methyl-3-butene-1-ol (isoprenol), comprising the steps of preparing isobutylene (also: isobutene, 2-methylpropene) by contacting 3-methylbutan-1 -ol (isoamyl alcohol) to a catalyst comprising at least one catalytically active metal and reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol. Furthermore, the present invention refers to processes for the preparation of 3-methyl-2-butene-1-ol (prenol), of 3-methyl-3-butenal (isoprenal), of 3-methyl-2-butenal (prenal), and of 3, 7-dimethy l-octa-2, 6-dienal (citral) and further downstream products comprising the use of isoprenol of the present invention and the resulting products and product streams.

BACKGROUND OF THE INVENTION

Isoprenol is of interest for various uses such as, e.g., intermediate for the synthesis of scents, vitamins and carotenoids. For instance, isoprenol may be used as intermediate for the synthesis of scents (also: aroma compounds) such as 3, 7-dimethyl-octa-2, 6-dienal (citral). In this context, WO 2008/037693 discloses a method for producing citral using isoprenol as intermediate. Such method for preparing citral may involve the following steps: a) 3-methyl-3-butene-1-ol (isoprenol) is produced from isobutylene and formaldehyde; b) 3-methyl-2-butenal (prenal) and 3-methyl-3-butenal (isoprenal) are produced from 3 methyl-3-butene-1-ol (isoprenol) by oxidative dehydrogenation by means of an oxygen-containing gas on a silver support catalyst; c) additional 3-methyl-2-butenal (prenal) is produced from a mixture containing 3 methyl-3-butenal (isoprenal) by isomerization; d) 3-methyl-2-butene-1-ol (prenol) is produced from 3-methyl-3-butene-1-ol (isoprenol) by isomerization; e) the unsaturated acetal 3-methyl-2-butenal-diprenylacetal is produced from 3-methyl-2-butene-1-ol (prenol) and 3-methyl-2-butenal (prenal) using an acidic catalyst; and f) 3,7-dimethyl-octa-2,6-diene-al (citral) is obtained from 3-methyl-2-butenal-diprenylacetal by cleavage and subsequently rearranging.

The preparation of isoprenol from formaldehyde and isobutylene has been widely described in the literature. Isobutylene in turn has been typically produced from non-renewable sources, e.g., via dimerization of ethylene derived from catalytic or steam cracking of fossil feedstocks. Light olefins such as isobutylene are building blocks of interest in modern chemical industries.

In recent years, it is of increasing interest to reduce carbon footprint when preparing such building blocks, intermediates and final products. Therefore, it has been considered to prepare building blocks from renewable sources such as biomass, plant-derived material and fusel oil. Fusel oils may be formed as a by-product of alcoholic fermentation and consist of a mixture of several alcohols comprised mainly of amyl alcohols along with lesser amounts of propanol, n-butanol, and iso-butanol depending upon the purification process employed. Fusel oils may be produced by yeast in anaerobiosis from nitrogenous materials. For sustainable development of chemical industry, it is important to develop sustainable and efficient processes for converting renewable biomass derivatives into value-added chemicals like monomers, solvents, intermediate chemicals, and other fine chemicals. This may include catalytic conversion of a large feedstock of renewable oxygenated biomass derivatives into valuable bulk monomers such as olefins.

In recent years, both as a result of market conditions as well as in response to a variety of governmental initiatives and mandates, biomass transformation to produce building blocks, intermediates and products has attracted significant effort and investment. Biomass is considered as a CO2 neutral energy carrier and is one of the most abundant and renewable of natural resources. Renewably-sourced organic molecules that are degraded (e.g., biodegraded or burned) to CO2 do not contribute to global warming as there is no net increase of carbon emitted to the atmosphere.

Therefore, it may be of interest to prepare isoprenol and downstream products thereof from renewable sources. This has been comparably challenging. Alcohol to olefin conversion processes such as alcohol dehydration are well known. However, in particular with branched alcohols, these conversions often proceed with less than optimum selectivity to the desired olefin. US 2021/0040012 A1 describes a process for the preparation of olefin by alcohol dehydration, such as isoamyl alcohol comprised in fusel oil. In the method described therein, the dehydration of isoamyl alcohol yields a mixture of C5 olefins, with the examples specifically indicating 2-methylbut-2-ene and trans-2-pen- tene as major components.

In the case of butylene from renewable sources, the butylene produced must meet critical purity specifications for downstream applications. This requirement is not easily achieved when the starting materials comprise a mixture of compounds and/or the steps involved in the conversion proceed with less than desired selectivity.

There is, therefore, a need for continued development of new and sustainable catalytic processes for conversion of renewable oxygenated biomass derivatives such as alcohols and/or aldehydes into valuable bulk monomers such as olefins.

Another route considered is reverse-hydroformylation or retro-hydroformylation. Hydroformylation reaction or "Oxoprocess” involves addition of carbon monoxide and hydrogen (H2) to an unsaturated carbon-carbon double bond of an unsaturated hydrocarbon to prepare an aldehyde compound. On the contrary, reverse-hydroformylation or retro- hydroformylation or dehydroformylation reaction involves conversion of an aldehyde into a corresponding olefin by eliminating syngas (carbon monoxide and dihydrogen).

Typically, catalytic processes for conversion of an alcohol into an olefin involves dehydration reactions (|3 - elimination reaction). However, alcohol and olefin have the same number of carbon atoms. Processes for conversion of aldehydes into olefins are known in the art. Kusumoto S., etal (Angew. Chem. Int. Ed. 2015, 54, 8458-8461) discloses retro-hydroformylation reaction of an aldehyde into an alkene and synthesis gas (a mixture of carbon monoxide and dihydrogen) in the presence of a cyclopentadienyl iridium catalyst. However, the known processes disclose preparing an olefin from an aldehyde by retro-hydroformylation. Therefore, these processes require extra steps for preparing an olefin from an alcohol, thereby making them cumbersome and expensive.

There is still an unmet need for providing an efficient method for the preparation of isoprenol and downstream products thereof based on one or more renewably resourced educts.

SUMMARY OF THE INVENTION

Surprisingly, it has been found that isoamyl alcohol can be used as educt in an efficient process for preparing isoprenol and downstream products thereof.

An aspect of the present invention relates to a method for the preparation of 3-methyl-3-butene-1-ol (isoprenol), comprising the steps of: a-i) preparing isobutylene by contacting isoamyl alcohol to a catalyst comprising at least one catalytically active metal; and a-ii) reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol.

It has been found that the educt isobutylene is preparable via easy-to-separate intermediates and/or with high selectivity towards the desired compounds. It has been found that the method for the preparation of isoprenol is surprisingly efficient, leads to considerable yields and suitably high selectivity.

The present invention may be associated with at least one of the following advantages:

• The present invention may provide a method for preparing isoprenol and downstream compounds including the conversion of isoamyl alcohol to isobutylene.

• The aforementioned method is useful for preparing isoprenol from renewable resources such as biomass and may reduce carbon footprint.

• The present invention may provide a catalytic dehydroformylation step generating synthesis gas (gaseous CO + H2) as a valuable by-product, which may be usable in other applications and may further reduce carbon footprint.

• The present invention provides a catalytic method involving up to 100% conversion of isoamyl alcohol to isobutylene and further to isoprenol and downstream products thereof.

A further aspect of the present invention relates to a method for the preparation of 3-methyl-2-butene-1-ol (prenol), comprising the steps of: a) providing isoprenol according to the present invention such as according to any one of claims 1 to 8; and b) isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen. A still further aspect of the present invention relates to a method for the preparation of 3-methyl-2-butenal (prenal) and/or 3-methyl-3-butenal (isoprenal), comprising the steps of: a) providing isoprenol according to the present invention such as according to any one of claims 1 to 8; b) optionally isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; and c) providing prenal by at least one of c-i) and c-ii): c-i) subjecting isoprenol obtained in step a) to oxidative dehydrogenation so as to obtain prenal and/or isoprenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; and/or c-ii) oxidizing prenol obtained in step b) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase.

A still further aspect of the present invention relates to a method for the preparation of 3,7-dimethy l-octa-2, 6-dienal (citral), comprising the steps of: a) providing isoprenol according to the present invention such as according to any one of claims 1 to 8; b) isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; c) providing prenal by at least one of c-i) and c-ii): c-i) subjecting isoprenol obtained in step a) to oxidative dehydrogenation so as to obtain prenal and/or isoprenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; c-ii) oxidizing prenol obtained in step b) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase; d) condensing prenol obtained in step b) with prenal obtained in step c) to obtain diprenyl acetal of prenal; and e) subjecting diprenyl acetal of prenal obtained in step d) to cleaving conditions to obtain citral via prenyl (3-me- thyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene.

Moreover, a further aspect of the present invention relates to isoprenol or isoprenol-containing product stream obtainable from a method of the present invention such as obtainable according to any one of claims 1 to 8.

A still further aspect of the present invention relates to prenol or prenol-containing product stream obtainable from a method of the present invention such as obtainable (or obtained) according to any one of claims 9. Furthermore, a still further aspect of the present invention relates to prenal, isoprenal or a mixture thereof or a prenal- and/or isoprenal-containing product stream obtainable from a method of the present invention such as obtainable (or obtained) according to any one of claims 10, 12 and 13.

A still further aspect of the present invention relates to citral or citral-containing product stream obtainable from a method of the present invention such as obtainable (or obtained) according to any one of claims 11 to 13.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, graphically illustrates conversion of isoamyl alcohol vs temperature using 0.25 ml of 1.0 wt.-% Pt 10.5 wt.-% Sn I ZrO2 as catalyst, as represented in Table 1 .

FIG. 2, graphically illustrates conversion of isoamyl alcohol vs temperature using 1 .0 ml of 1 .0 wt.-% Pt 10.5 wt.-% Sn I ZrO2 as catalyst, as represented in Table 2.

FIG. 3, graphically illustrates conversion of isoamyl alcohol vs temperature using 1 .0 ml of 1 .0 wt.-% Pt 10.5 wt.-% Sn I ZrO2 as catalyst at a constant GHSV of 2500 h^-1, as represented in Table 3.

FIG. 4, graphically illustrates conversion of isoamyl alcohol vs temperature using 0.25 ml of 1.0 wt.-% Pt 10.5 wt.-% Sn I ZrO2 as catalyst at a constant GHSV of 9800 h^-1, as represented in Table 4.

FIG. 5, graphically illustrates conversion of isoamyl alcohol vs % of isoamyl alcohol using 1 .0 ml of 1 .0 wt.-% Pt 10.5 wt.-% Sn I ZrO2 as catalyst, at various GHSVs, at Temp = 300°C, and Pressure = 1 bar, as represented in Table 5. FIG. 6, graphically illustrates conversion of isoamyl alcohol vs temperature using 0.25 ml of Nickel (60 wt.-% metal content, Total Pore Volume 0.45 cm³/g) as catalyst, as represented in Table 6.

FIG. 7, graphically illustrates conversion of isoamyl alcohol vs temperature using 1.0 ml of Nickel (60 wt.-% metal content, Total Pore Volume 0.45 cm³/g) as catalyst, as represented in Table 7.

FIG. 8, graphically illustrates conversion of isoamyl alcohol vs temperature using 1.0 ml of Nickel (56 wt.-% metal content, Total Pore Volume 0.3 cm³/g) as catalyst, as represented in Table 8.

FIG. 9, graphically illustrates conversion of isoamyl alcohol vs temperature using 1.0 ml Nickel-Copper as catalyst, as represented in Table 9.

FIG. 10, graphically illustrates conversion of isoamyl alcohol vs temperature using 0.25 ml of Nickel-Copper as catalyst, as represented in Table 10.

FIG. 11, graphically illustrates conversion of isoamyl alcohol vs temperature using 1.0 ml of Nickel-Copper as catalyst, as represented in Table 11 .

FIG. 12, graphically illustrates conversion of isoamyl alcohol vs temperature using 1.0 ml Nickel-Copper as catalyst, as represented in Table 12.

FIG. 13, graphically illustrates conversion of isoamyl alcohol vs temperature using 0.5 ml of 1.0 wt.-% Pt 10.5 wt.-% Sn I ZrO2+ Cu-Zn as catalyst, as represented in Table 13.

FIG. 14, graphically illustrates conversion of isoamyl alcohol vs temperature using 0.5 ml of 1.0 wt.-% Pt 10.5 wt.-% Sn I ZrO2 + Cu-Zn as catalyst, as represented in Table 14.

FIG. 15, graphically illustrates conversion of isoamyl alcohol vs temperature using 1.0 ml of Ru as catalyst, as represented in Table 15. FIG. 16, graphically illustrates conversion of isoamyl alcohol vs temperature using 1 .0 ml of 0.3 % Pd as catalyst, as represented in Table 16.

FIG. 17, graphically illustrates conversion of isoamyl alcohol vs temperature using 1 .0 ml of 0.5 % Pd as catalyst, as represented in Table 17.

FIG. 18, graphically illustrates conversion of isoamyl alcohol vs temperature using 1.0 ml of (Pd) as catalyst, as represented in Table 18.

DETAILED DESCRIPTION

Before the method of the present invention is described, it should be understood that this invention is not limited to particular exemplified method described, since such details of the method may, of course, vary. It should also be understood that the terminology used herein is not intended to be limiting. The invention is defined by the claims.

If hereinafter a group is defined to comprise at least a certain number of embodiments, if not defined otherwise, this is meant to also encompass a group which preferably consists of these embodiments only. Furthermore, the terms 'first', 'second', 'third' or 'a', 'b', 'c', etc. and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the present invention described herein are capable of operation in other sequences than described or illustrated herein. In case the terms 'first', 'second', 'third' or '(A)', '(B)' and '(C)' or '(a)', '(b)', '(c)', '(d)', 'i', 'ii' etc. relate to steps of a method or use or assay there is no time or time interval coherence between the steps, that is, the steps may be carried out simultaneously or there may be time intervals of seconds, minutes, hours, days, weeks, months or even years between such steps, unless otherwise indicated in the application as set forth herein above or below.

Furthermore, the ranges defined throughout the specification include the end values as well i.e. , a range of 1 to 10 implies that both 1 and 10 are included in the range. For the avoidance of doubt, applicant shall be entitled to any equivalents according to applicable law.

In the following passages, different aspects of the present invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature in particular such indicated as being preferred or advantageous, may be combined with any other feature or features, in particular those indicated as being preferred or advantageous.

Reference throughout this specification to 'one embodiment' or 'an embodiment' means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases 'in one embodiment' or 'in an embodiment' in various places throughout this specification are not necessarily all referring to the same embodiment but may refer to so. Two or more embodiments may be combined with one another.

Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some, but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the present invention, and form different embodiments, as would be understood by those in the art. For example, in the appended claims, any of the claimed embodiments can be used in any combination.

Throughout the present invention, the terms “wt.-%”, “wt%”, “wt-%”, "weight percent”, and "% by weight” are used synonymously. If not otherwise stated, percentage may be understood as wt.-% (i.e., percent by weight).

Preparation of Isoprenol

Preparation of Isobutylene (Step a-i)

Step a-i) of preparing isobutylene by contacting isoamyl alcohol to a catalyst comprising at least one catalytically active metal may optionally also be understood as preparing isobutylene by subjecting isoamyl alcohol to retro-hydroformation. Thus, in other words step a-i) may comprise (in other words may be) subjecting isoamyl alcohol to retro-hydroformation. The present invention may also refer to a method for the preparation of isoprenol, comprising the steps of: a-i) preparing isobutylene by subjecting isoamyl alcohol to retro-hydroformation; and a-ii) reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol.

Step a-i) may be conducted by any means as subsumable under the wording as claimed. Step a-i) may be understood as a one-step catalytic step for preparing isobutylene by from isoamyl alcohol.

In an embodiment, step a-i) comprises feeding an alcohol feedstock (essentially) consisting of isoamyl alcohol or a feedstock (also: feed stream) comprising isoamyl alcohol at a concentration of 2 to 100 wt.-%, 10 to 100 wt.-%, 25 to 99.9 wt.-%, or 60. to 99 wt.-%, or 75 wt.-% to 95 wt.-%, related to the total mass of the alcohol feedstock, into a reaction vessel.

In an embodiment, the isoamyl alcohol is partly or completely renewably-sourced.

As used herein, the term "renewably-sourced” may be understood in the broadest sense as being obtained from a renewable source, in other words one or more renewable raw materials. Thus, as used herein, the terms "renewably- sourced” and "derived from renewable raw materials”, "renewably-based” and "renewable” may be understood interchangeably. In an embodiment, the verification that a feedstock was renewably-sourced is possible according to ASTM D6866 via ¹⁴C for example.

A small amount of the carbon atoms of the carbon dioxide in the atmosphere is the radioactive isotope ¹⁴C. This ¹⁴C carbon dioxide is typically created when atmospheric nitrogen is struck by a cosmic ray generated neutron, causing the nitrogen to lose a proton and form carbon of atomic mass 14 (¹⁴C), which is then immediately oxidized to carbon dioxide. A small but measurable fraction of atmospheric carbon is present in the faun of ¹⁴CC>2. Atmospheric carbon dioxide is typically processed by green plants to make organic molecules during the process known as photosynthesis. Virtually all forms of life on Earth depend on this green plant production of organic molecule to produce the chemical energy that facilitates growth and reproduction. Therefore, the 14C that forms in the atmosphere eventually becomes part of all life forms and their biological products, enriching biomass and organisms which feed on biomass with ¹⁴C. In contrast, carbon from fossil fuels does not have the signature ¹⁴C:¹²C ratio of renewable organic molecules derived from atmospheric carbon dioxide.

Renewably-sourced may denote that the carbon content of a biofuel precursor and subsequent products is from a "new carbon” source as measured by ASTM test method D 6866-05, "Determining the Biobased Content of Natural Range Materials Using Radiocarbon and Isotope Ratio Mass Spectrometry Analysis”, incorporated herein by reference in its entirety. A feedstock shall be regarded as "renewably-sourced” for the purposes of the present invention when the carbon-14 (¹⁴C) presence therein corresponds substantially (to within not more than 6%) to the ASTM D6866 content of ¹⁴C in atmospheric CO2. The ¹⁴C content of a material may be determined by determining the decays of ¹⁴C in this material by liquid scintillation. Such raw materials shall preferably be regarded as renewably- sourced, when they have a ¹⁴C content displaying a radioactive decay of not less than 1 .5 dpm/gC (decays per minute per gram of carbon), preferably 2 dpm/gC, more preferably 2.5 dpm/gC and yet more preferably 5 dpm/gC. The test method ASTM D6866(-05) may measure the ¹⁴C/¹²C isotope ratio in a sample and compares it to the ¹⁴C/¹²C isotope ratio in a standard 100% biobased material to give percent biobased content of the sample.

In an embodiment, the feedstock containing isoamyl alcohol that is preferably obtained from fusel oil contains the C5- Cs primary alcohol has a pMC greater than 90 when measured by a method as described in the ASTM norm D6866 (the current version is D6866-22), which defines the concept of "percent Modern Carbon” or pMC. Preferably the pMC is greater than 91 , preferably greater than 93, preferably greater than 95, preferably greater than 96, preferably greater than 97, more preferably greater than 98, even more preferably greater than 99, more preferably about 100.

"Carbon of atmospheric origin” as used herein refers to carbon atoms from carbon dioxide molecules that have recently (e.g., in the last few decades) been free in the earth's atmosphere. Such carbon atoms are identifiable by the ratio of particular radioisotopes as described herein. "Green carbon”, "atmospheric carbon”, "environmentally friendly carbon”, "life-cycle carbon”, "non-fossil fuel-based carbon”, "non-petroleum-based carbon”, "carbon of atmospheric origin”, and "biobased carbon” may be understood synonymously herein. The isoamyl alcohol used in the present invention preferably has a pMC value of greater than 90, preferably greater than 95, preferably greater than 98, more preferably greater than 99, more preferably about 100, inclusive of all values and subranges there-between.

Assessment of the renewably-sourced carbon content and thus material is described in detail above and in ASTM- D6866.

As used herein, the term "partly renewably-sourced” may be understood in the broadest sense as containing at least 5 wt.-%, at least 10 wt.-%, at least 25 wt.-%, at least 50 wt.-%, at least 75 wt.-%, at least 80 wt.-%, at least 90 wt.-%, or at least 95 wt.-% renewably-sourced isoamyl alcohol, related to the total mass of the isoamyl alcohol.

As used herein, the term "completely renewably-sourced” may be understood in the broadest sense as containing at least 99 wt.-% or at least 99.9 wt.-%, renewably-sourced isoamyl alcohol, related to the total mass of the isoamyl alcohol.

In an embodiment, the isoamyl alcohol is partly or completely biobased.

As used herein, "biobased” in the context of isoamyl alcohol may be understood as isoamyl alcohol in which the carbon comes from recently (on a human time scale) fixated CO2 present in the atmosphere using sunlight energy (photosynthesis). On land, this CO² is captured or fixated by plant life (e.g., agricultural crops or forestry materials). In the oceans, the CO² is captured or fixated by photosynthesizing bacteria or phytoplankton. For example, a biobased material has a ¹⁴C/¹²C isotope ratio greater than 0. Contrarily, a fossil-based material, has a ¹⁴C/¹²C isotope ratio of about 0.

As used herein, the term "partly biobased” may be understood in the broadest sense as containing at least 5 wt.-%, at least 10 wt.-%, at least 25 wt.-%, at least 50 wt.-%, at least 75 wt.-%, at least 80 wt.-%, at least 90 wt.-%, or at least 95 wt.-% biobased isoamyl alcohol, related to the total mass of the isoamyl alcohol.

As used herein, the term "completely biobased” may be understood in the broadest sense as containing at least 99 wt.-% or at least 99.9 wt.-%, biobased isoamyl alcohol, related to the total mass of the isoamyl alcohol.

A renewably-sourced and optionally biobased isoamyl alcohol may be obtained from any source. For instance, it may be obtained from biomass using thermochemical methods (e.g., Fischer-Tropsch catalysts), biocatalysts (e.g., fermentation), or other processes.

In an embodiment, the isoamyl alcohol feedstock may consist of or may comprise fusel oil. In an embodiment, the renewably-sourced isoamyl alcohol of step a-i) is obtained from fusel oil. As used herein, the term "fusel oil” may be understood in the broadest sense as a product that may be formed as a by-product of alcoholic fermentation. Fusel oil is well known in the art and typically comprises a mixture of light alcohols, fatty esters, terpenes and furfural. The alcohols comprised in fusel oil are mainly propanol, (iso)butanol, (iso)amyl alcohols, and hexanol and optionally heavier linear alcohols such as Cz or Cs alcohols. Fusel oils, occasionally referred to as "amyl oils” or “fusels”, have compositions which vary depending on their origin (potato, beet, wheat, barley, etc. musts). Fusel oils may form colorless or yellowish liquids, which have a characteristic odor. They may have a density of about 0.83 g/m, which may however vary due to their content ranges. Their boiling point is far from constant, since they are complex mixtures of substances with a very variable boiling point. Boiling commences at about 80° C and rises to 130 to 134°C. Fusel oils insoluble in water and are usually washed with water and separated out by settling of the phases in order to reduce the amount of ethanol they contain by about 4% to 5% by volume. It should be noted that fusel alcohols are natural alcohols directly produced via biotechnology in distilleries, without any intermediate chemical step.

Fusel oil may be obtained by several processes well known from the skilled person, e.g., by direct removal in the distillation column and cooling. The removed fraction can be purified e.g., by extraction and decantation. A liquid/liquid extraction by addition of water followed by a decantation leads to the formation of two phases. The upper phase comprises mainly amyl and butyl alcohols, slightly soluble in water. The various fractions of fusel oil may also be separated by using adsorbents, which are regenerated thereafter. Among the tested adsorbents, granulated vegetal activated charcoal is preferred since it is able to adsorb eight times its weight of fusel oil.

In an embodiment, the fusel oil contains a mixture of linear or branched C5 alcohols, C4 alcohols or C3 alcohols.

In a preferred embodiment, C5 branched alcohol present in the initial composition is a mixture of isoamyl alcohol and amyl alcohol, i.e., 3-methylbutan-1 -ol (isoamyl alcohol) and 2-methylbutan-1-ol (amyl alcohol).

In a preferred embodiment, the initial composition comprises at least 30 wt.-%, preferably at least 40 wt.-%, more preferably at least 50 wt.-%, more preferably at least 60 wt.-%, even more preferably at least 70 wt.-% C5 branched alcohols, based on the total weight of the composition.

C4 alcohols may also be present in the initial composition, for example, butan-1-ol and 2-methyl propan- 1-ol. The initial composition may comprise one of these C4 alcohols or both.

C3 alcohols may also be present in the initial composition, for example, n-propanol. The initial composition may comprise 0.01 to 20 wt.-% of C3 alcohol.

Fusel oil may further contain hexanol and optionally heavier linear alcohols such as C7 or Cs alcohols.

Fusel oil may typically comprise 5 to 20% of water, 60 to 95% of alcohols mainly consisting of linear or branched alkanols containing from 2 to 5 carbon atoms, and impurities including furfurals, ethers and/or fatty acids. In one embodiment, the composition of fusel oil is as follows: ethanol: 5 to 40%,

1 -propanol: 1 to 8%,

2-propanol: 0 to 1 %,

2-methyl propanol: 5 to 15%,

1 -butanol: 0 to 1 %,

2-methyl- 1 -butanol: 10 to 30%,

3-methyl-1 -butanol (isoamyl alcohol): 25 to 70%, the combination of alkanols representing 100%.

In an embodiment, step a-i) comprises feeding an alcohol feedstock comprising isoamyl alcohol at a concentration of 10 to 100 wt.-%, 25 to 99.9 with-%, or 60 to 99 wt.-%, related to the total mass of the alcohol feedstock, into a reaction vessel, and preparing isobutylene by contacting isoamyl alcohol to a catalyst comprising at least one catalytically active metal, and optionally isolating the isobutylene.

In an embodiment, the remainder of the alcohol feedstock that is not isoamyl alcohol comprising at least one C3 C5 alcohol other than isoamyl alcohol. Fusel oil may be commercially obtained.

Catalysts for step a-i)

According to the present invention, any catalyst, preferably any catalytically active metal, that is suitable for catalyzing the conversion of isoamyl alcohol to isobutylene may be used in step a-i) may be used. As experimentally shown, surprisingly a considerable variety of metal catalysts may be used in this step.

In an embodiment, the present claimed invention is directed to a method for preparing an aroma chemical, in particular isoprenol, prenol, prenal, isoprenal or citral and compounds derived therefrom, by contacting isoamyl alcohol with a catalyst comprising at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), Nickel (Ni), Chromium (Cr), Molybdenum (Mo), Ruthenium (Ru), or a combination of two or more thereof.

In an embodiment, step a-i) comprises contacting the isoamyl alcohol with at least one catalyst comprising at least one catalytically active metal selected from the group consisting of Platinum (Pt), Palladium (Pd), Nickel (Ni), Chromium (Cr), Molybdenum (Mo), Ruthenium (Ru), a combination of two or more thereof, and an alloy comprising two or more thereof and optionally one or more further metals.

Step a-i) may comprise contacting the catalytically active metal with the isoamyl alcohol under any conditions suitable for this purpose. In an embodiment, step a-i) is conducted at a partial pressure in the range of 0.02 to 4.0 bar. In a preferred embodiment, step a-i) is conducted at a temperature in the range of 200 to 450°C. In a preferred embodiment, step a-i) is conducted at a partial pressure in the range of 0.02 to 4.0 bar, and at a temperature in the range of 200 to 450°C.

In a preferred embodiment, the step of contacting is carried out at a temperature in the range from 250°C to 450°C, in the range from 300°C to 450°C, or in the range from 300°C to 400°C, in the range from 300°C to 350°C, or in the range from 200°C to 250°C.

In an embodiment, step a-i) comprises contacting the isoamyl alcohol with at least one catalyst comprising at least one catalytically active metal selected from the group consisting of Platinum (Pt), Palladium (Pd), Nickel (Ni), Chromium (Cr), Molybdenum (Mo), Ruthenium (Ru), a combination of two or more thereof, and an alloy comprising two or more thereof and optionally one or more further metals at a partial pressure in the range of 0.02 to 4.0 bar, and at a temperature in the range of 200 to 450°C.

The catalyst may comprise the catalytically active metal at any content suitable for conducting step a-i). In an embodiment, the at least one catalyst comprises the at least one catalytically active metal in an amount of 0.005 to 10.0 wt.- %, based on the total weight of the catalyst. The remaining mass parts of the catalyst may be any components, such as e.g., a carrier.

In an embodiment, the at least one catalytically active metal is present in an amount in the range from 0.005 wt.-% to 5.0 wt.-%, with respect to total weight of the catalyst. In an embodiment, the at least one catalytically active metal is present in an amount in the range from 0.2 wt.-% to 2.0 wt.-%, with respect to total weight of the catalyst. In an embodiment, the at least one catalytically active metal is present in an amount in the range from 0.5 wt.-% to 1 .5 wt.-%, with respect to total weight of the catalyst. In an embodiment, the at least one catalytically active metal is present in an amount of 1 .0 wt.-%, with respect to total weight of the catalyst.

In an embodiment, the catalyst further comprises at least one promoter. In an embodiment, the at least one promoter is present in an amount in the range from 0.1 wt.-% to 3.0 wt.-%, with respect to total weight of the catalyst. In an embodiment, the at least one promoter is present in an amount in the range from 0.2 wt.-% to 2.0 wt.-%, with respect to total weight of the catalyst. In an embodiment, the at least one promoter is present in an amount in the range from 0.3 wt.-% to 1 .5 wt.-%, with respect to total weight of the catalyst. In an embodiment, the at least one promoter is present in an amount of 0.5 wt.-%, with respect to total weight of the catalyst.

In an embodiment, the at least one catalyst comprises at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), Ruthenium (Ru), or combinations thereof, and the step of contacting is carried out at a temperature in the range from 250°C to 450°C, preferably 300 to 450°C. In an embodiment, the at least one catalyst comprises at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), or combinations thereof, at least one promoter, and at least one carrier, and the step of contacting is carried out at a temperature in the range from 250°C to 450°C, preferably 300 to 450°C.

In an embodiment, the at least one catalytically active metal comprises Nickel (Ni). In an embodiment, the catalyst has a Nickel content in the range from 10 wt.-% to 70 wt.-%, preferably from 50.0 wt.-% to 65.0 wt.-%. In an embodiment, the catalyst having catalytically active metal comprising Nickel (Ni) has a total pore volume in the range from 0.25 cm³/g to 0.5 cm³/g.

The remainder of the catalyst may comprise or may be an oxidic bonding material, for example aluminium oxide, or zirconium dioxide. Preferably, the Ni-containing catalysts are bulk catalysts or precipitation-type catalysts, as opposed to supported catalysts. In an embodiment, the at least one catalytically active metal is Nickel (Ni).

In an embodiment, the at least one catalytically active metal is selected from the group consisting of Platinum (Pt), Palladium (Pd) and Ruthenium (Ru).

In an embodiment, the at least one catalytically active metal is or comprises Nickel (Ni). In an embodiment, the at least one catalytically active metal comprises Nickel (Ni), and the step of contacting is carried out at a temperature in the range from 200°C to 400°C, preferably 200 to 300°C, more preferably 200 to 250°C. When the at least one catalytically active metal is Nickel-Copper (Ni-Cu), a temperature in the range of from 250 to 350°C is particularly useful.

In an embodiment, the at least one catalytically active metal is or comprises Nickel-Copper (Ni-Cu). It is known that pure Nickel catalysts may tend to deposit carbon when exposed to an atmosphere containing CO or hydrocarbons (as starting materials or end products). On the other hand .copper, irrespective of whether it is active or inactive as a catalyst for that reaction, does often not show tendency to deposit carbon. Therefore, Nickel-copper catalysts are used for catalyzing reactions in which carbon monoxide and/or hydrocarbons are present in the reaction gas phase (as starting materials or end products) or are intermediately formed, in order to inhibit carbon deposition on the catalyst. Similarly, copper and copper alloys may be used in combination with other catalysts used for catalyzing reactions in which carbon monoxide and/or hydrocarbons are present in the reaction gas phase or are intermediately formed, in order to inhibit carbon deposition on the catalyst. In addition, presence of copper can strengthen the Nickel-based catalysts against attrition during catalytic reactions at high temperature and pressure conditions.

In an embodiment, step a-i) comprises contacting the isoamyl alcohol under one or more of the following conditions:

(a) the at least one catalytically active metal is selected from the group consisting of Platinum (Pt), Palladium (Pd) and Ruthenium (Ru), and the step of contacting is carried out at a temperature in the range from 250°C to 450°C, preferably 300 to 450°C;

(b) the at least one catalytically active metal is Nickel (Ni), and the step of contacting is carried out at a temperature in the range from 200°C to 400°C, preferably 200 to 300°C, in particular 200 to 250°C; or (c) the at least one catalytically active metal is Nickel-Copper (Ni-Cu), and the step of contacting is carried out at a temperature in the range from 250 to 350°C.

In step a-i), isoamyl alcohol (optionally included in a mixture of components such as, e.g., fusel oil or as (essentially) pure compound) may be added under any conditions. In a preferred embodiment, contacting in step a-i) involves contacting a feed stream comprising the isoamyl alcohol in a gas phase.

The at least one catalyst may further comprise at least one promoter. Promoter may help inhibit isomerization and cracking of hydrocarbon feed during the high temperature catalytic processes involving precious metals such as Platinum and Palladium. Promoter has also been found to stabilize the catalyst and extend its life. The catalyst can be regenerated by conventional oxidation of the catalyst.

The amount of the at least one promoter present in the catalyst may vary depending upon the at least one catalytically active metal and the hydrocarbon feed employed.

In an embodiment, the at least one catalyst comprises (or consists of): a. at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), Nickel (Ni), Chromium (Cr), Molybdenum (Mo), Ruthenium (Ru), or combinations thereof; and b. at least one promoter selected from Tin (Sn), Silver (Ag), Iridium (Ir), Rhenium (Re), or combinations thereof, wherein the at least one promoter is present in an amount in the range from 0.005 wt.-% to 5.0 wt.-%, with respect to total weight of the catalyst.

In an embodiment, the at least one catalyst comprises (or consists of):: a. at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), Nickel (Ni), or combinations thereof; and b. at least one promoter selected from Tin (Sn), Silver (Ag), Iridium (Ir), Rhenium (Re), or combinations thereof, wherein the at least one promoter is present in an amount in the range from 0.005 wt.-% to 5.0 wt.-%, with respect to total weight of the catalyst.

In an embodiment, the at least one catalyst comprises (or consists of): a. at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), Ruthenium (Ru), or combinations thereof; and b. at least one promoter selected from Tin (Sn), Silver (Ag), Iridium (Ir), Rhenium (Re), or combinations thereof, wherein the at least one promoter is present in an amount in the range from 0.005 wt.-% to 5.0 wt.-%, with respect to total weight of the catalyst.

In an embodiment, the at least one catalyst comprises (or consists of): a. at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), Nickel (Ni), or combinations thereof; and b. at least one promoter selected from Tin (Sn), Rhenium (Re), or combinations thereof, wherein the at least one promoter is present in an amount in the range from 0.005 wt.-% to 5.0 wt.-%, with respect to total weight of the catalyst.

The at least one catalyst may further comprise at least one carrier, typically solid carrier. In a preferred embodiment, the at least one carrier support is a non-acidic refractory material selected from the group consisting of calcium oxide, titanium oxide, beryllia, magnesia, thoria, zirconia, alumina, silica, silicon carbide, quartz or combinations thereof.

In an embodiment, the at least one carrier support is non-acidic refractory material selected from calcium oxide, titanium oxide, magnesia, zirconia, alumina, silica, or combinations thereof. In an embodiment, the at least one carrier support is non-acidic refractory material selected from magnesia, zirconia, titanium oxide, alumina, silica, or combinations thereof. In an embodiment, the at least one carrier support is non-acidic refractory material zirconia. In an embodiment, the at least one carrier is present in an amount in the range from 95.0 wt.-% to 99.5 wt.-%, with respect to total weight of the catalyst. In an embodiment, the at least one carrier is present in an amount in the range from 97.0 wt.-% to 99.0 wt.-%, with respect to total weight of the catalyst. In an embodiment, the at least one carrier is present in an amount of 98.5 wt.-%, with respect to total weight of the catalyst.

In an embodiment, the at least one catalyst comprises (or consists of): a. at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), Nickel (Ni), Chromium (Cr), Molybdenum (Mo), Ruthenium (Ru), or combinations thereof; and b. at least one carrier on which the catalytically active metal is supported, wherein the carrier is selected from a non-acidic material selected from calcium oxide, titanium oxide, beryllia, magnesia, thoria, zirconia, alumina, silicon carbide, quartz or combinations thereof, and wherein the at least one carrier is present in an amount in the range from 90.0 wt.-% to 99.995 wt.-%, with respect to total weight of the catalyst.

In an embodiment, the at least one catalyst comprises (or consists of): a. at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), Nickel (Ni), or combinations thereof; and b. at least one carrier on which the catalytically active metal is supported, wherein the carrier is selected from a non-acidic material selected from calcium oxide, titanium oxide, beryllia, magnesia, thoria, zirconia, alumina, silicon carbide, quartz or combinations thereof, and wherein the at least one carrier is present in an amount in the range from 90.0 wt.-% to 99.995 wt.-%, with respect to total weight of the catalyst.

In an embodiment, the at least one catalyst comprises (or consists of): a. at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), Nickel (Ni), or combinations thereof; and b. at least one carrier on which the catalytically active metal is supported, wherein the carrier is selected from a non-acidic material selected from titanium oxide, zirconia alumina, or combinations thereof, and wherein the at least one carrier is present in an amount in the range from 90.0 wt.-% to 99.995 wt.-%, with respect to total weight of the catalyst.

In an embodiment, the at least one catalyst comprises (or consists of): a. at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), Nickel (Ni), Chromium (Cr), Molybdenum (Mo), Ruthenium (Ru), or combinations thereof; b. at least one promoter selected from Tin (Sn), Silver (Ag), Iridium (Ir), Rhenium (Re), or combinations thereof; and c. at least one carrier on which the catalytically active metal is supported, wherein the carrier is selected from a non-acidic material selected from calcium oxide, titanium oxide, beryllia, magnesia, thoria, zirconia, alumina, silicon carbide, quartz or combinations thereof.

In an embodiment, the at least one catalyst comprises (or consists of): a. at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), Nickel (Ni), or combinations thereof; b. at least one promoter selected from Tin (Sn), Silver (Ag), Iridium (Ir), Rhenium (Re), or combinations thereof; and c. at least one carrier on which the catalytically active metal is supported, wherein the carrier is selected from a non-acidic material selected from calcium oxide, titanium oxide, beryllia, magnesia, thoria, zirconia, alumina, silicon carbide, quartz or combinations thereof.

In an embodiment, the at least one catalyst comprises (or consists of): a. at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), Nickel (Ni), or combinations thereof; b. at least one promoter selected from Tin (Sn), Rhenium (Re), or combinations thereof; and c. at least one carrier on which the catalytically active metal is supported, wherein the carrier is selected from a non-acidic material selected from titanium oxide, zirconia, alumina, or combinations thereof.

In an embodiment, the at least one catalyst comprises (or consists of): a. at least one catalytically active metal; b. at least one promoter; and c. at least one carrier on which the catalytically active metal is supported, wherein the at least one carrier is selected from a non-acidic material selected from calcium oxide, titanium oxide, beryllia, magnesia, thoria, zirconia, alumina, silicon carbide, quart or combinations thereof, and wherein the at least one carrier is present in an amount in the range from 90.0 wt.-% to 99.995 wt.-%, with respect to total weight of the catalyst. In an embodiment, the at least one catalyst comprises (or consists of): a. at least one catalytically active metal selected from Platinum (Pt), Palladium (Pd), Ruthenium (Ru), or combinations thereof; b. at least one promoter selected from Tin (Sn), Silver (Ag), Iridium (Ir), Rhenium (Re), or combinations thereof; and c. at least one carrier on which the catalytically active metal is supported, wherein the carrier is selected from a non-acidic material selected from calcium oxide, titanium oxide, beryllia, magnesia, thoria, zirconia, alumina, silicon carbide, quartz or combinations thereof.

In an embodiment, the at least one catalyst comprises at least one catalytically active metal Platinum (Pt), at least one promoter Tin (Sn), and at least one carrier Zirconia (ZrC>2).

In an embodiment, the at least one catalyst further comprises other optional elements that are known to influence the acidity of the catalyst surface and/or to stabilize the catalytically active metals against sintering. For example, elements of main groups I and II, i.e., Li, Na, K, Rb, Cs on the one hand and Mg, Ca, Sr and Ba on the other hand, elements of main group III, i.e., gallium, indium and thallium, and elements of transition group III i.e. Y and La and also rare earth elements. When tetragonal ZrC>2 is employed as a carrier, it can be stabilized by doping with Lanthanum (La) or Yttrium (Y). Zinc has also been found to be effective.

The BET surface of the at least one catalyst may be any one that is suitable for the purpose of the present invention. In an embodiment, the at least one catalyst has a BET surface area in the range from 1 to 500 square meters per gram (m²/g), as measured by the Brunauer-Emmett-Teller method. In an embodiment, the at least one catalyst has a BET surface area in the range from 5 to 300 square meters per gram (m²/g), as measured by the Brunauer-Emmett- Teller method. In an embodiment, the at least one catalyst has a BET surface area in the range from 10 to 200 square meters per gram (m²/g), as measured by the Brunauer-Emmett-Teller method. In an embodiment, the at least one catalyst has a BET a surface area in the range from 20 to 100 square meters per gram (m²/g), as measured by the Brunauer-Emmett-Teller method.

In the present invention, the amount of the at least one catalytically active metal that can be effectively disposed upon a suitable carrier varies by any means, depending usually upon the surface area of the carrier.

The at least one catalyst may have any porosity. In an embodiment, the at least one catalyst has a pore volume in the range from 0.1 to 1.0 ml/g, as determined by Mercury (Hg) Porosimetry. In an embodiment, the at least one catalyst has a pore volume in the range from 0.15 to 0.6 ml/g, as determined by Mercury (Hg) Porosimetry. In an embodiment, the at least one catalyst has a pore volume in the range from 0.2 to 0.4 ml/g, as determined by Mercury (Hg) Porosimetry. In an embodiment, the at least one catalyst has a mean pore diameter, as determined by Mercury (Hg) Porosimetry, in the range from 0.008 to 0.06 microns (pi). In an embodiment, the at least one catalyst has a mean pore diameter, as determined by Mercury (Hg) Porosimetry, in the range from 0.01 to 0.04 microns (pi). Preferably, the molar ratio of Nickel to copper is greater than 1 , preferably greater than 1.2, more preferably 1.8 to

8.5. In an embodiment, the at least one catalytically active metal Nickel (Ni) has Nickel content in the range from 50.0 wt.-% to 65.0 wt.-%; and a total pore volume in the range from 0.25 cm³/g to 0.5 cm³/g.

The at least one catalyst may be prepared by any means. Non-limiting examples for the preparation of the at least one catalyst comprising at least one catalytically active metal Platinum (Pt), at least one promoter Tin (Sn), and at least one carrier Zirconia (ZrO2) of the present invention, as disclosed in US patent No. 6,989,346 B2 is specifically incorporated by reference herein. It may be prepared by precipitating. In general, for instance, precipitation methods may be used for the preparation of the Ni-containing catalysts. Thus, they can be obtained, for example, by coprecipitation of the Nickel and, optionally, copper components, from an aqueous salt solution containing these components, by means of a base, in the presence of a suspension of a sparingly soluble, oxidic bonding material, and subsequent washing, drying and calcination of the resulting precipitate. For precipitation, a base, in particular an aqueous alkali metal base, for example sodium carbonate, sodium hydroxide, potassium carbonate or potassium hydroxide, is added to an aqueous salt solution containing catalyst components at elevated temperatures and with stirring, until the precipitation is complete. The type of salts used is in general not critical and salts having high aqueous solubility are generally preferred.

The at least one catalyst may optionally have any particle size. In an embodiment, the at least one catalyst has a particle size in the range from 50 pm to 500 pm. In an embodiment, the at least one catalyst has a particle size in the range from 100 pm to 350 pm.

The at least one catalyst may have any catalyst bed volume. In an embodiment, the at least one catalyst has a bed volume in the range from 0.1 ml to 1.5 ml. In an embodiment, the at least one catalyst has a bed volume in the range from 0.2 ml to 1.1 ml. To keep the catalyst beds on a constant level, the catalyst material may be optionally diluted with inert particles in such way that all reactors had a constant catalyst bed volume.

The step of contacting isoamyl alcohol with the carrier may be conducted by means suitable to provide isobutylene.

In an embodiment, the step of contacting involves contacting a feed stream comprising the at isoamyl alcohol in a gas phase. In an embodiment, the feed stream is fed at a gas hourly space velocity (GHSV) in the range from 100 h^-1 to 15000 h^-1 or 300 h^-1 to 15000 h^-1 In an embodiment, the feed stream is fed at a gas hourly space velocity (GHSV) in the range from 300 h^-1 to 10000 h^-1. In an embodiment, the feed stream is fed at a gas hourly space velocity (GHSV) in the range from 300 h^-1 to 3000 h^-1.

In an embodiment, the feed stream further comprises at least one inert gas, in particular one or more inert gases selected from the group consisting of N2, CO2, CH4 or Ar. Inert gases such as nitrogen (N2), Carbon-di-oxide (CO2), Argon (Ar), or methane (CH4) can be used as diluent to adjust partial pressure of a hydrocarbon feed. Hydrogen (H2) gas when used as a diluent can act as an inhibitor, thereby inhibiting the coke formation on the catalyst and help improve the catalyst performance.

In an embodiment, the method is carried out in an essentially non-oxidative atmosphere, which means that the feed stream is essentially free of gaseous oxidants such as air, oxygen, ozone, nitrous oxide and nitric oxide. In an embodiment, the feed stream is essentially free of molecular oxygen, for example, the feed stream contains less than 5 vol.-%, more preferably less than 1 vol.-% of molecular oxygen.

In an embodiment, in step a-i), the isobutylene is obtained in the form of a product stream. In an embodiment, in step a-i), the product stream comprises isobutylene, carbon monoxide and hydrogen. In an embodiment, the step a-i) of contacting further involves a step of separating carbon monoxide and hydrogen from the product stream to obtain the isobutylene.

Reacting a Formaldehyde Source and Isobutylene to Obtain Isoprenol (Step a-ii)

According to an aspect of the present invention, the isobutylene obtained in step a-i) is reacted in step a-ii) with at least one formaldehyde source to obtain 3-methylbut-3-en-1 -ol (isoprenol). The step a-ii) of reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol may be conducted by any means.

The at least one formaldehyde source and isobutylene are typically reacted in a reactor, in general at elevated temperature and pressure.

As used herein, "formaldehyde source” refers to any source containing formaldehyde or capable of cleaving off formaldehyde. Formaldehyde sources include aqueous formaldehyde solutions and oligomers or polymers of formaldehyde, like paraformaldehyde.

In an embodiment, the method of the present invention, in particular step a-ii, comprises introducing the at least one formaldehyde source and isobutylene into a reactor and reacting the formaldehyde source and isobutylene under supercritical conditions.

In an embodiment, the isoprenol is obtained by introducing, preferably by mixing and injecting, at least one formaldehyde source and isobutylene into a reactor, preferably through at least one nozzle, and reacting the at least one formaldehyde source and isobutylene under supercritical conditions. In order to achieve supercritical conditions, formaldehyde and isobutylene are preferably reacted at a temperature of at least 220°C, for example in the range of 220 to 290°C, and an absolute pressure of at least 200 bara. The reaction of isobutylene and formaldehyde may be carried out without a catalyst as well as in the presence of at least one catalyst. The reaction of isobutylene and formaldehyde source may also be carried out in the presence of one or more auxiliary chemicals such as ammonia and/or hexamethylenetetramine (urotropin). Conducting this reaction in the presence of such auxiliary chemicals, especially ammonia and/or urotropin, has been described, e.g., in DE 1279014 B. The at least one formaldehyde source and isobutylene are preferably introduced into the reactor in a manner which allows for mixing of the reactants so as to obtain an intimate mixture. Introduction methods include injecting, splashing, stirring in and I or spraying into the reactor. Preferably, the at least one formaldehyde source and isobutylene are injected or sprayed into the reactor through at least one nozzle.

Formaldehyde may be provided as a liquid, for example as a solution of paraformaldehyde in methanol. Preferably, the at least one formaldehyde source comprises or is an aqueous formaldehyde solution.

While initial rapid and intense mixing of reactants is desirable, it may be advantageous to continue and complete the reaction under conditions of limited back-mixing. Thus, the reaction mixture may be passed into a post-reaction chamber disposed after the reactor or in a lower portion of the reactor. In the post-reaction chamber, back-mixing is limited.

In one embodiment, the reactor comprises an upper portion and a lower portion. Introduction of the reactants, in particular by injecting and mixing of the reactants, occurs in a mixing chamber of the reactor disposed in the upper portion, and a fluid comprising formaldehyde and/or isobutylene and/or isoprenol is passed from the mixing chamber into a post-reaction chamber disposed in the lower portion.

In one embodiment, reacting at least one formaldehyde source and isobutylene comprises introducing, preferably mixing and injecting, the at least one formaldehyde source and isobutylene into an internal loop reactor through at least one nozzle into first conduit(s), the internal loop reactor comprising: a vertically disposed cylindrical vessel comprising a sidewall; at least one draft tube having a tube inlet end and a tube outlet end , arranged vertically within the vessel, the draft tube(s) being arranged concentrically to the nozzle(s) , and having an inner surface and an outer surface, wherein the draft tube(s) provide(s) the first conduit(s) within the draft tube(s), and a second conduit outside of the draft tube(s) and within the sidewall, the first conduit(s) being in fluid communication with the second conduit; reactor fluid outlet means; wherein the inner surface of the draft tube(s) convexly curves so that the first conduit(s) exhibit(s) an annular constriction of the cross-section between the tube inlet end and the tube outlet end; wherein the constriction is located closer to the tube inlet end; wherein the convex curvature of the inner surface of the draft tube(s) extends over at least 70%, preferably at least 80%, most preferably at least 90% of the length of the draft tube; and wherein the outer surface of the draft tube(s) convexly curves so that the draft tube(s) exhibit(s) a circumferential protuberance between the tube inlet end and the tube outlet end, which circumferential protuberance is preferably located closer to the tube outlet end; wherein the convex curvature of the outer surface of the draft tube(s) extends over at least 70%, preferably at least 80%, most preferably at least 90% of the length of the draft tube; and wherein the edges of the draft tube(s) are rounded so that the at least one formaldehyde source and isobutylene introduced through the nozzles travel generally downward in the first conduit(s) to obtain a reacted fluid, the reacted fluid is then diverted in the opposite direction so as to travel through the second conduit and is subsequently back- mixed with the introduced fluid. In a preferred embodiment, the nozzles are two-component nozzles. It is especially preferable that a two-component nozzle is designed so as to provide an annular jet of isobutylene around a central jet of the at least one formaldehyde source, and that the velocities upon introduction, for example the injection velocities or spraying velocities, of these two jets are different. In this embodiment, the jet of isobutylene has a large shear surface towards both the central jet of the at least one formaldehyde source and the reaction mixture in the reactor, allowing for favourable fast mixing of the reactants.

In a preferred embodiment, the loop reactor comprises deflector means arranged between the nozzle and the draft tube, the deflector means being suitable for deflecting fluid travelling in the second conduit in the opposite direction.

The deflector means suitably comprise a surface which is concave relative to the end of the draft tube which defines the tube inlet end. In a preferred embodiment, the deflector means have a partial toroidal surface. It is especially preferred that the deflector means are provided in the shape of the upper portion of a ring torus bisected in a plane parallel to the toroidal direction. This shape allows for an especially efficient deflection of the fluid travelling in the second conduit. The deflector means may allow for a stabilization of the introduced, for example injected or sprayed fluid stream. This is especially relevant when the flow rate of the fluid travelling in the second conduit is not uniform across the cross section of the reactor, which may lead to an eccentricity of the introduced fluid stream. Such an eccentricity may cause a decrease in circulation ratio if left unattended.

When the first conduit is downcomer conduit and the second conduit is a riser conduit, it is preferred that the shape of the deflector means constitutes the upper portion of a ring torus bisected in a plane parallel to the toroidal direction, wherein the ring torus is bisected at least 50% of its height, such as at least 55% or 65% of its height. Thus, the upper portion of the ring torus is the same size or smaller than the lower portion of the ring torus. In another preferred embodiment, the shape of the deflector means constitutes the upper portion of a ring torus bisected in a plane parallel to the toroidal direction wherein the ring torus is bisected at the side at most 85% of its height, for example 80% of its height. In these ranges, the entry of the deflector means is angled especially suitable for fluid deflection.

Further details regarding aforementioned embodiments concerning loop reactor may be found in WO 2023/104863, which herewith is incorporated by reference in its entirety.

High temperatures are required to obtain a high isoprenol yield in the reaction of formaldehyde with isobutylene. Effective removal of the heat is critical for the product quality and process safety. The heat removed from the isoprenol is used for raising the temperature of isobutylene before it enters the reactor. The stream of the hot isoprenol contains sensible heat from the chemical reaction. The sensible heat is potentially reclaimable energy that can be reused.

Advantageously, reacting at least one formaldehyde source and isobutylene preferably comprises heat-exchanging a stream of hot isoprenol withdrawn from the reactor with a isobutylene stream directed to the reactor; wherein heat- exchanging is performed in one or more shell-and-tube heat exchangers; each of the heat exchangers comprising a plurality of tubes and a shell-side heat exchange passage; wherein the hot isoprenol is directed through the tubes of the heat exchangers; and the isobutylene is guided through the shell-side passage, and in case of more than one heat exchangers at least two of the heat exchangers are connected in series with regard to both the shell-side flow and the tube-side flow.

In a group of the preferred embodiments, the heat-exchanging is performed in one shell-and-tube heat exchanger.

In another group of the preferred embodiments, the heat-exchanging is performed in at least one or more shell-and- tube heat exchangers, wherein the hot isoprenol is directed through the tubes of the heat exchangers; and the isobutylene is guided through the shell-side passage, and in case of at least two of the heat exchangers these are connected in series with regard to both the shell-side flow and the tube-side flow.

Such configurations allow for prolonging operation intervals between maintenance disruptions in such a method. The term "maintenance disruptions” is intended to mean a shutdown of the process that becomes necessary at recurring intervals in order to clear the tubes of the heat exchanger that have been clogged by fouling. An indicator of a necessity of a maintenance disruption is typically when isobutylene leaving the last heat exchanger is insufficiently preheated and that even a subsequent heater is hardly able to put in additional external heat into the isobutylene to bring isobutylene to the required temperature before it enters the reactor. The pre-heating of the isobutylene stream may be maintained for a longer time at levels high enough so that the desired temperature of the isobutylene can easily be reached before the isobutylene enters the reactor.

One particular area prone to fouling in conventional shell-and-tube heat exchangers is the tube area near the tube sheet near the inlet where the tube-side fluid leaves the individual tubes. Excessive fouling in this area can cause clogging of individual tubes and fluid stagnation along the entire length of these tubes. The fluid stagnation generally leads to reduced heat-transfer performance.

As a further consequence of the decreased heat transfer performance caused by fouling, the energy required in a heater to adjust the temperature of the pre-heated isobutylene stream to the desired reaction temperature increases. Consequently, more additional external heat becomes necessary which is detrimental in terms of energy demand and process economy, and often has a negative impact on the carbon dioxide footprint of the product.

By using two or more heat exchangers, the impact of fouling in individual tubes on the overall heat exchange capacity is reduced in comparison to arrangements where only a single heat exchanger is used. As a consequence, the heat transfer rates are maintained at a desired level for longer periods, hence prolonging operation intervals between maintenance disruptions, and the pre-heating of the isobutylene stream requires less additional external heat compared to a plant with a single heat exchanger in an advanced state of fouling. Further details regarding aforementioned embodiments concerning heat exchangers and energy savings and reducing maintenance intervals may be found in WO 2023/198714 A1, which herewith is incorporated by reference in its entirety.

Preparation of Prenol

The present invention also refers to the preparation of 3-methyl-2-buten-1 -ol (prenol).

A further aspect of the invention relates to a method for the preparation of 3-methyl-2-butene-1-ol (prenol), comprising the steps of: a) providing isoprenol according to the present invention; and b) isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol (e.g., steps a-i) and a-ii) above mutatis mutandis apply to the preparation of prenol. In an embodiment, isoprenol is prepared according to any one of claims 1 to 8.

As used herein and hereinafter, the term "reactant stream” refers to a stream comprising a reactant or reactants consumed in the course of a chemical reaction. In this sense, the reactant stream may further comprise solvent(s), catalyses), additive(s) and/or any other substance involved in the chemical reaction.

Generally, the isomerization of isoprenol to 3-methyl-2-buten-1 -ol (prenol) may be carried out over a supported noble metal, preferably in the presence of hydrogen.

A preferred catalyst is a fixed bed catalyst containing Palladium and selenium or tellurium or a mixture of selenium and tellurium supported on silicon dioxide.

The catalyst contains 0.1 to 2.0wt.-% of Palladium and 0.01 to 0.2wt.-% of selenium, tellurium or a mixture of selenium and tellurium, based on the total weight of the catalyst.

The BET surface area is, for example, in the range of 100 to 150 m²/g, in particular in the range of 110 to 130 m²/g. The BET surface area is determined by N2 adsorption according to DIN 66131.

The pore volume in the pore diameter range of from 3 nm to 300 pm is preferably 0.8 to 0.9 cm³/g, in particular 0.8 to 0.85 cm³/g. Thereby, 80 to 95%, preferably 85 to 93% of this pore volume is in the pore diameter range of 10 to 100 nm. The pore volume is determined by Hg Porosimetry. Preferably, the catalyst contains 0.2 to 0.8wt.-%, in particular 0.4 to 0.6wt.-% of Palladium. Preferably, the catalyst contains 0.02 to 0.08, in particular 0.04 to 0.06 wt.-% selenium, tellurium or a mixture of selenium and tellurium, preferably selenium. In addition to the active components mentioned, other metals may be present on the catalyst in small amounts. Preferably, only Palladium, selenium and/or tellurium, in particular only Palladium and selenium, are present on the silica support.

The described isomerization of isoprenol to prenol on a fixed-bed catalyst is also described in EP-A 841 090, to which express reference is made.

The isomerization is carried out at a temperature in the range of 50 to 150°C, preferably in the range of 60 to 130°C, more preferably in the range of 70 to 120°C to produce a reaction mixture of prenol and isoprenol. The isoprenol can be recycled. Further details are provided in WO 2008/037693.

Generally, a regeneration cycle is performed periodically, to remove accumulated coke from the catalyst. The regeneration cycle can be initiated when the pressure drop increased above a threshold value, or at arbitrary time intervals, for example once a week. A regeneration cycle consists of sending diluted air or air for a defined period of time, for example 6 to 24 h, over the reactor while increasing the salt bath temperature, for example 400 to 450°C, to allow coke combustion.

The unreacted isoprenol from the isoprenol isomerization process may be used, i.e., recycled for the isoprenol isomerization.

Reducing the Content of Aldehydes in the Reactant Stream

Surprisingly, it has been found that the presence of aldehydes, especially formaldehyde and/or prenal in the reactant stream, is detrimental to the activity and selectivity of the process and may accelerate catalyst deactivation and/or poisoning in the isomerization of isoprenol to prenol.

As used herein and hereinafter, the term "concentration of aldehydes in the reactant stream” refers to the total concentration of aldehydes existing in the reactant stream. Aldehydes include those intrinsic to the isoprenol preparation process and those formed by oxidation and isomerization. Hence, the aldehydes usually include formaldehyde and prenal. Therefore, if formaldehyde and prenal are the only aldehydes existing in the reactant stream, the concentration of aldehydes in the reactant stream is the sum of the respective concentrations of formaldehyde and prenal.

It has been found that deterioration of catalyst properties is related to the presence of aldehydes, especially formaldehyde and/or prenal in the reactant stream. Formaldehyde is generally considered to be the most critical of these aldehydes. Catalyst-fouling reactions of condensation and polymerization are believed to be the principal reactions involved in carbon or coke formation on the catalyst. It is thought that this carbon formation involves thermal condensation of aldehydes, for example formaldehyde and/or prenal, or of these aldehydes with the olefinic hydrocarbons isoprenol. In the presence of the catalyst the primary condensation products tend to undergo dehydrogenation and polymerization type reactions and to settle on the catalyst and undergo further dehydrogenation and decomposition until carbonaceous deposits are formed.

One of the poisoning mechanisms of the catalyst is supposed to involve a catalytic or non-catalytic dehydrogenation of aldehydes, especially formaldehyde and/or prenal to carbon monoxide, which is chemisorbed on the catalyst and blocks the active centers.

A further cause of catalyst deactivation, which may occur in combination with the previously mentioned cause of catalyst poisoning, is the formation of paraformaldehyde or trioxane which may deposit, in the form of solids, on the catalyst and shield the catalytically active surfaces from the isoprenol being processed. This leads to progressive deactivation of the catalyst.

In one embodiment, the concentration of aldehydes in the reactant stream is, therefore, maintained at a certain level or less, i.e., less than 0.5wt.-%, preferably less than 0.4wt.-%, in particular less than 0.3wt.-%, or less than 0.25wt.- %, based on the total weight of the reactant stream, wherein the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream. As used herein and hereinafter, the term "ppm” refers to parts-per-million (ppm, 10^-6).

In more preferred embodiments, the concentration of aldehydes is maintained at less than 0.2wt.-%, based on the total weight of the reactant stream, wherein the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream.

Aldehydes, preferably formaldehyde and/or prenal may be removed from the streams comprising isoprenol by a conventional separating method such as distillation, selective adsorption and or selective reaction, in particular by the purification process involving the pressure-swing distillation as described herein.

Alternatively, it is feasible to mix the unreacted isoprenol stream with an amount of a sufficiently purified fresh feed stream so as to give in the combined stream a desired weight ratio of formaldehyde to isoprenol.

As used herein and hereinafter, the term "unreacted isoprenol stream” refers to a stream which is derived from an isoprenol isomerization process and comprises unreacted isoprenol of the isoprenol isomerization process. In this sense, the unreacted isoprenol stream may further comprise solvent(s), catalyst(s), additive(s) and/or any other substance involved in the isoprenol isomerization process. As used herein and hereinafter, the term "crude isoprenol stream” refers to a product stream of an isoprenol production process from which unreacted isobutylene has been removed. Removal of aldehydes, such as formaldehyde and/or prenal, is accomplished in a purification unit following the isoprenol synthesis. A preferred method of recovering aldehydes from a crude isoprenol stream to which an unreacted isoprenol stream is admixed, is described in more detail below.

Preferably, the aldehydes existing in the reactant stream comprise formaldehyde. Also preferably, the aldehydes existing in the reactant stream comprise prenal besides formaldehyde.

More preferably, the aldehydes existing in the reactant stream consist of prenal and formaldehyde. In certain instances, the aldehydes existing in the reactant stream consist of formaldehyde.

Preferably, the concentration of aldehydes in the reactant stream is less than 0.5wt.-%, or 0.4wt.-%, or 0.3wt.-%, more preferably less than 0.25wt.-%, or 0.2wt.-%, even more preferably less than 0.15wt.-%, yet even more preferably less than 0.1wt.-%, equal to or less than 0.08wt.-% or less than 0.05wt.-%, based on the total weight of the reactant stream, but at least 10 ppm with respect to the total weight of the reactant stream. In another embodiment, the concentration of aldehydes is less than 0.025wt.-%, more less than 0.02wt.-%, based on the total weight of the reactant stream. In one embodiment, the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream. The skilled person will appreciate that any of the upper limits of aldehyde concentration can be combined with any of the lower limits of aldehyde concentration, wherein in certain embodiments the aldehyde is either formaldehyde, prenal, or formaldehyde and prenal.

Also preferably, the concentration of formaldehyde in the reactant stream is less than 0.5wt.-%, or 0.4wt.-%, or 0.3wt.-%, more preferably less than 0.25wt.-%, or 0.2wt.-%, even more preferably less than 0.15wt.-%, yet even more preferably less than 0.1wt.-%, equal to or less than 0.08wt.-% or less than 0.05wt.-%, based on the total weight of the reactant stream, but at least 10 ppm with respect to the total weight of the reactant stream. In another embodiment, the concentration of formaldehyde is less than 0.025wt.-%, more less than 0.02wt.-%, based on the total weight of the reactant stream, wherein the concentration of formaldehyde in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream.

If the aldehydes existing in the reactant stream comprise or consist of formaldehyde, the concentration of formaldehyde in the reactant stream is preferably less than 0.5wt.-%, or less than 0.4wt.-%, or less than 0.3wt.-%, more preferably less than 0.25wt.-%, or 0.2wt.-%, even more preferably less than 0.15wt.-%, yet even more preferably less than 0.1wt.-%, or less than 0.05wt.-%, most preferably less than 0.025wt.-%, or less than 0.02wt.-%, based on the total weight of the reactant stream, wherein the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream. Preferably, the concentration of prenal in the reactant stream is less than 0.3wt.-%, more preferably less than 0.2wt.- %, even more preferably less than 0.15wt.-%, in particular less than 0.1wt.-%, based on the total weight of the reactant stream, but not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream.

Therefore, in preferred embodiments, the aldehydes in the reactant stream consist of or comprises formaldehyde, and the concentration of formaldehyde is less than 0.5wt.-%, or less than 0.4wt.-%, or less than 0.3wt.-%, more preferably less than 0.25wt.-%, or 0.2wt.-%, even more preferably less than 0.15wt.-%, yet even more preferably less than 0.1wt.-%, equal to or less than 0.08wt.-%, or less than 0.05wt.-%, based on the total weight of the reactant stream, but at least 10 ppm with respect to the total weight of the reactant stream. In another embodiment, the concentration of formaldehyde is less than 0.025wt.-%, more preferably less than 0.02wt.-%, based on the total weight of the reactant stream, but not less than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream. In one embodiment, the concentration of formaldehyde is equal to or less than 0.08 wt.-%, based on the total weight of the reactant stream, but optionally at least 10 ppm with respect to the total weight of the reactant stream.

In an embodiment, the aldehydes existing in the reactant stream consist of prenal and formaldehyde, and therefore the concentration of aldehydes in the reactant stream corresponds to the sum of the concentrations of prenal and formaldehyde, wherein the concentration of aldehydes in the reactant stream, i.e., the sum of the concentrations of prenal and formaldehyde is less than 0.5wt.-%, preferably less than 0.4wt.-%, in particular less than 0.3wt.-% or less than 0.2wt.-%, based on the total weight of the reactant stream, wherein the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream.

In a group of the preferred embodiments, the weight ratio of aldehydes, preferably prenal and/or formaldehyde, to isoprenol in the reactant stream is adjusted at a certain level or less, i.e., less than 0.04, preferably less than 0.03, in particular less than 0.02, or less than 0.01 . In still more preferred embodiments, the weight ratio of aldehydes, preferably formaldehyde and/or prenal to isoprenol is adjusted at less than 0.002, or less than 0.001. In one embodiment, the ratio is lower than 0.0009.

The terms "maintaining in the reactant stream” and "adjusting in the reactant stream” or "maintained in the reactant stream” or "adjusted in the reactant stream” with respect to the aldehyde levels in the reactant stream are used interchangeably herein.

Reducing the weight ratio of aldehydes, preferably formaldehyde and/or prenal to isoprenol in the reactant stream beyond a certain point, however, reaches a point of rapidly diminishing return. Removal of aldehydes, especially of formaldehyde and/or prenal involves additional equipment and operating costs. An economic balance must be taken between the improvement due to reducing the ratio and the cost of maintaining such a ratio. Hence, the weight ratio of aldehydes, preferably formaldehyde and/or prenal to isoprenol is preferably not lower than 0.0005 or, in some instances, not lower than 0.0007.

Since the double-bond isomerization of isomerization of isoprenol to prenol is an equilibrium reaction, a complete conversion of substances is not achieved in a single pass. Instead, a portion of isoprenol always remains, which unreacted isoprenol is suitably separated from the desired prenol. The unreacted isoprenol may be recycled to the isomerization reaction or may be directed to other isoprenol-consuming reactions.

Generally, the reactant stream will comprise or consist of a fresh isoprenol stream. The term "fresh isoprenol stream” refers to a stream of isoprenol directly obtained from the purification unit following the isoprenol synthesis, i.e., from a purification unit wherein a crude isoprenol stream from the reaction of isobutylene and formaldehyde is purified. The reactant stream may further comprise recycled, unreacted isoprenol, and/or isoprenol from other sources.

Preferably, the reactant stream comprises or consists of a fresh isoprenol stream. Also preferably, the reactant stream comprises or consists of a mixture of unreacted isoprenol stream and a fresh isoprenol stream.

In yet another embodiment, the reactant stream consists of a mixture of the unreacted isoprenol stream, and isoprenol from other sources. Other sources of isoprenol are processes other than the reaction of isobutylene and formaldehyde, in which isoprenol is obtained as a by-product or target product, or isoprenol from commercial sources.

The presence of aldehydes, especially formaldehyde and/or prenal in the reactant stream reduces both catalyst activity and selectivity and causes increase in pressure drop and reactor clogging. Besides aldehydes, especially formaldehyde and/or prenal, other impurities which may be present in the reactant stream can cause a decrease in catalyst activity and selectivity. Preferably, the equipment or operations used for maintaining in the reactant stream a certain concentration of aldehydes, preferably formaldehyde and/or prenal, or a certain weight ratio of aldehydes, preferably formaldehyde or prenal to isoprenol is also effective to remove a major portion of these impurities. In preferred embodiments, the concentration in the reactant stream of at least one of the following impurities is kept below the limit indicated, in particular of all of the following impurities:

Compliance with these limits may be of interest when the reactant stream accommodates isoprenol streams from other sources. Reducing the concentration of aldehydes, preferably formaldehyde and/or prenal in the reactant stream will inherently reduce the weight ratio of aldehydes, preferably formaldehyde and/or prenal to isoprenol in the reactant stream. Therefore, the following applies for reducing the concentration of aldehydes, preferably formaldehyde and/or prenal in the reactant stream as well as reducing the weight ratio of aldehydes, preferably formaldehyde and/or prenal to isoprenol in the reactant stream.

The presence of formaldehyde in the reactant stream is due to two main sources. Formaldehyde may be contained in the isoprenol stream sent to the reactor that is as an impurity originating from the isoprenol manufacture step. In industrial practice, isoprenol is synthesized from isobutylene and formaldehyde. All the formaldehyde that cannot be separated in the purification step following the isoprenol synthesis ends up in the reactant stream.

In addition, formaldehyde is also generated in situ. Part of the isoprenol splits back to isobutylene and formaldehyde.

Since most continuous industrial processes operate at single-pass conversion levels of 50 to 60% and with recycling of the unconverted isoprenol, formaldehyde may be present in the recycling stream of unconverted isoprenol, if no steps to purify the stream containing unreacted isoprenol are taken. The recycle stream of unconverted isoprenol has now been found to typically constitute the biggest source of formaldehyde contamination in the reactant stream. The process is generally carried out at partial conversions, for example at conversions of 30 to 70%, preferably 50 to 60%. An unreacted isoprenol stream is separated from the product stream. The unreacted isoprenol stream is recycled, that is, combined with a fresh feed stream comprising isoprenol (a crude isoprenol stream) to provide the reactant stream. The unreacted isoprenol stream comprises isoprenol as a main constituent, but may also comprise prenal, isoprenal, isoamyl alcohol, isovaleraldehyde, isovaleric acid, prenol, formaldehyde. It can also contain traces of other C3 and C2 aldehydes and acids.

Prenal may be contained in the isoprenol stream sent to the reactor that is as an impurity originating from the isoprenol manufacture step. The isoprenol stream may further contain traces of ammonia, and/or Cs-oxygenates other than prenal besides formaldehyde and/or prenal. All the prenal and/or other impurities that cannot be separated in the purification step following the isoprenol synthesis ends up in the reactant stream.

Since the double bond isomerization of isoprenol is an equilibrium reaction, conversion is necessarily incomplete. For economic operation of the process, the unconverted isoprenol has to be removed and recycled. Recycling of isoprenol may therefore inadvertently (re)introduce formaldehyde into the isomerization step if no steps to purify the stream containing unreacted isoprenol are taken.

Reducing the concentration of aldehydes, preferably formaldehyde and/or prenal in the reactant stream or reducing the weight ratio of aldehydes, preferably formaldehyde and/or prenal to isoprenol in the reactant stream can be accomplished in several different ways. In a preferred embodiment, the process includes separating an unreacted isoprenol stream from the prenol-contain- ing product stream, optionally removing at least some aldehydes, preferably some formaldehyde and/or prenal from the unreacted isoprenol stream, followed by combining the unreacted isoprenol stream with a fresh isoprenol stream to form the reactant stream.

In another preferred embodiment, the process includes separating an unreacted isoprenol stream from the prenol- containing product stream, combining the unreacted isoprenol stream with a crude isoprenol stream containing isoprenol, water and aldehydes, and removing aldehydes, preferably water and aldehydes from the combined stream to form the reactant stream.

As mentioned above, the crude isoprenol stream is generally the product stream of an isoprenol production process from which unreacted isobutylene has been removed. This means that formaldehyde removal is accomplished in the purification unit following the isoprenol synthesis. A preferred method of recovering formaldehyde from a crude isoprenol stream to which an unreacted isoprenol stream is admixed, is described in more detail below.

Aldehydes, preferably formaldehyde and/or prenal may be removed from isoprenol streams by a conventional separating method such as distillation, selective adsorption and/or selective reaction.

Removal of aldehydes, preferably formaldehyde and/or prenal by distillation can involve the use of a single distillation column or a train of distillation columns. The towers and columns used may be conventional distillation columns. Suitable types of distillation columns include packed columns, such as columns with random packing or structured packing, plate columns (i.e., tray columns), and mixed columns comprising both packings and trays.

Suitable plate columns may comprise internals over which the liquid phase flows. Suitable internals include sieve trays, bubble cap trays, valve trays, tunnel trays and Thormann® trays, in particular bubble cap trays, valve trays tunnel trays and Thormann® trays. Random packed columns may be filled with a variety of shaped bodies. Heat and mass transfer are improved by enlarging the surface area by means of shaped bodies, which usually have a size in the range of 25 to 80 mm. Suitable shaped bodies include Raschig rings (hollow cylinders), Lessing rings, Pall rings, Hiflow rings and Intalox saddles. The packing materials may be provided in the column in a regular or irregular manner (as bulk material, i.e., loosely filled). Suitable materials include glass, ceramics, metal and plastics.

Structured packings are an advancement of regular packings and have a regularly shaped structure. This allows for the reduction of gas flow pressure loss. Suitable types of structured packings include fabric and metal sheet packings.

Removal of aldehydes, preferably formaldehyde and/or prenal by selective adsorption involves contacting the stream with an adsorbent that exhibits selectivity for low molecular weight aldehydes, especially formaldehyde and/or prenal. Useful adsorbent materials should deliver high selectivity and high adsorption capacity. An additional and critically important requirement is that the adsorbent material should not catalyze or participate in chemical reactions that might lower the recovery of the (iso)prenal and/or render the adsorbent inactive. Adsorbents include ion exchange resins, mesoporous solids, activated carbons, and zeolites.

Removal of aldehydes, preferably formaldehyde and/or prenal by selective reaction involves exposing the stream to reaction conditions under which aldehydes, preferably formaldehyde and/or prenal are (is) selectively reacted to products that are less prone to catalyst deactivation and clogging or to products that can be separated from the stream more easily than aldehydes, preferably formaldehyde and/or prenal.

Preferably, removal of aldehydes, preferably formaldehyde and/or prenal from a stream comprising isoprenol is conducted by distillation, selective adsorption and/or selective reaction, in particular by purification process involving the pressure-swing distillation.

The above described applies for the reducing the concentration of aldehydes other than formaldehyde or prenal and/or of other impurities in the reactant stream as well as reducing the weight ratio of aldehydes other than formaldehyde or prenal to isoprenol.

In case of formaldehyde, difficulties arise from the fact that monomeric formaldehyde (as well as polymeric formaldehyde) forms both hydrates with water and hemiformals with alcohols such as isoprenol, which is the reactant of the isoprenol isomerization and may still remain in the product stream as unreacted reactant. The hydrates and hemiformals of varying formaldehyde polymerization degree have intermingling boiling points. The stability of and the equilibrium between hydrates and hemiformals is temperature-dependent. Formals formed in an upper region of a distillation tower may decompose in the hotter bottom of the tower, which adds additional complexity to the separation task.

In an embodiment, the unreacted isoprenol stream is combined with a crude isoprenol stream containing isoprenol, water and aldehydes, preferably formaldehyde and/or prenal; and removing aldehydes, such as formaldehyde and/or prenal, preferably water and aldehydes, in particular water and formaldehyde and/or prenal, from the combined stream comprises:

(I) directing the combined stream to a first low-boiler separation tower operated at a pressure of 1 .5 bara or lower, to obtain a first bottoms stream containing isoprenol and aldehydes, preferably prenal and I or formaldehyde, and a first distillate stream containing water and low-boilers;

(II) directing the first bottoms stream to a second low-boiler separation tower operated at a pressure of 2 bara or higher, to obtain a second distillate stream containing aqueous aldehydes, preferably prenal and I or formaldehyde, and a second bottoms stream containing isoprenol; and

(ill) directing the second bottoms stream to a finishing tower to obtain a bottoms stream containing high-boilers, and the reactant stream as a distillate stream. In order to permit a first distillation at a temperature below the isoprenol-aldehyde dissociation temperature of the respective aldehyde(s) present, for example for formaldehyde the isoprenol-formaldehyde dissociation temperature and a second distillation at a temperature above the isoprenol-aldehyde dissociation temperature, like the isoprenol- formaldehyde dissociation temperature, the invention envisages two low-boiler separation towers operated at different pressures. Hence, at the relatively low pressure prevailing in the first low-boiler separation tower, a first distillate containing water and low-boilers essentially free of aldehydes, preferably formaldehyde and/or prenal is obtained. At the relatively high pressure prevailing in the second low-boiler separation tower, a virtually all aldehydes, preferably all formaldehyde and/or prenal is separated from the isoprenol. The process of the invention thus allows for obtaining isoprenol essentially free of aldehydes, preferably formaldehyde and/or prenal.

The term "essentially free of aldehydes, preferably formaldehyde and/or prenal” is understood to indicate the absence of significant amounts of aldehydes, preferably formaldehyde and/or prenal in the obtained isoprenol. Thus, the obtained isoprenol preferably comprises less than 0.2 wt.-%, in particular less than 0.15 wt.-%, or less than 0.1 wt.-%, based on the total weight of the obtained isoprenol, of aldehydes, preferably formaldehyde and/or prenal.

Preferably, the crude isoprenol stream is a liquid stream. The liquid stream can be a single-phase liquid stream or a two-phase liquid stream.

The crude isoprenol is directed to a first low-boiler separation tower operated at a pressure of 1 .5 bara or lower. Any higher pressure of the crude isoprenol stream is preferably released before the same is directed to the first low-boiler separation tower. The crude isoprenol stream is preferably fed to the first low-boiler separation tower as a side stream, defining a rectifying section above the location of the feed and a stripping section below the location of the feed.

In the first low-boiler separation tower, a first bottoms stream containing isoprenol and aldehydes, preferably formaldehyde and/or prenal, and a first distillate stream containing water and low-boilers are obtained. The term "low-boilers" is understood to refer to organic compounds (other than aldehydes, especially formaldehyde and/or prenal) having a boiling point lower than that of isoprenol, hence a boiling point of lower than about 130°C, at atmospheric pressure. The most common low-boilers are methanol and/or isoprenyl formate formed as by-products during the process.

In a preferred embodiment, the first low-boiler separation tower is operated at a pressure of 1 .2 bara or lower, preferably 0.5 bara or lower. The bottoms temperature of the first low-boiler separation tower is preferably in the range of 80 to 135°C, more preferably 90 to 115°C, most preferably 95 to 105°C. The temperature at the top of the first low- boiler separation tower is preferably in the range of 45 to 105°C, more preferably 55 to 80°C. In a particularly preferred embodiment, the first low-boiler separation tower is operated at a pressure in the range of 0.2 to 0.5 bara, a bottoms temperature in the range of 90 to 115°C and a temperature at the top in the range of 55 to 80°C.

The first low-boiler separation tower preferably has from 15 to 65 theoretical plates, more preferably from 25 to 40 theoretical plates. In particular, the stripping section of the first low-boiler separation tower preferably has 10 to 25 theoretical plates. The rectifying section of the first low-boiler separation tower preferably has 5 to 40 theoretical plates.

The first bottoms stream preferably comprises 75 to 95 wt.-% of isoprenol, more preferably 80 to 90 wt.-%, based on the total weight of the first bottom stream.

The first distillate is typically withdrawn at the top of the first low-boiler separation tower in gaseous form and condensed to obtain a liquid two-phase stream. The two-phase stream is preferably allowed to phase-separate in a separating vessel to obtain an aqueous phase and an organic phase. The aqueous phase is preferably passed to a wastewater stripping column described below. The organic phase is preferably partially returned to the top of the first low-boiler separation tower as a reflux stream. Another part of the organic phase is preferably discarded from the process to avoid the accumulation of water-insoluble low-boilers in the first low-boiler separation tower.

In a preferred embodiment, at least part of the first distillate stream is directed to a wastewater stripping column to separate low-boilers and entrained isoprenol from water. Preferably, the part of the first distillate stream directed to the wastewater stripping column is an aqueous phase obtained by condensation and phase separation of the first distillate stream, as discussed above.

In the wastewater stripping column, low-boilers are obtained as the low-boiler distillate stream, and wastewater is obtained as a bottoms stream. Both the low-boiler distillate stream and the wastewater bottoms stream are removed from the process, and each stream may be directed to further processing.

Moreover, isoprenol is preferably obtained as a side stream in the wastewater stripping column. The isoprenol side stream is typically a two-phase stream and preferably comprises 15 to 40 wt.-% of isoprenol, more preferably 25 to 35 wt.-%, based on the total weight of the isoprenol side stream. The isoprenol side stream is preferably recycled to the first low-boiler separation tower.

The low-boiler distillate stream preferably comprises 75 to 95 wt.-% of low-boilers, more preferably 80 to 85 wt.-%, based on the total weight of the low-boiler distillate stream. The wastewater bottoms stream preferably comprises less than 1 .2 wt.-% of organic matter, more preferably less than 0.6 wt.-%, based on the total weight of the wastewater bottoms stream. The wastewater bottoms stream typically comprises aldehydes, preferably formaldehyde and/or prenal in a concentration of 0.05 to 1 .5 wt.-% of aldehydes, preferably formaldehyde and/or prenal, such as 0.3 to 0.9 wt.-%, based on the total weight of the wastewater bottoms stream. The wastewater stripping column is preferably operated at a pressure of 1 .5 bara or lower, preferably 1.1 bara or lower. The bottoms temperature of the wastewater stripping column is preferably in the range of 95 to 110°C, more preferably 97 to 103°C. The temperature at the top of the wastewater stripping column is preferably in the range of 65 to 100°C, more preferably 75 to 85°C.

In a particularly preferred embodiment, the wastewater stripping column is operated at a pressure in the range of 0.95 to 1.1 bara, a bottoms temperature in the range of 97 to 103°C and a temperature at the top in the range of 75 to 85°C.

The wastewater stripping column preferably has from 6 to 30 theoretical plates, more preferably from 10 to 20 theoretical plates.

The first bottoms stream obtained in the first low-boiler separation tower is directed to a second low-boiler separation tower operated at a pressure of 2 bara or higher. The first bottoms stream is preferably fed to the second low-boiler separation tower as a side stream, defining a rectifying section above the location of the feed and a stripping section below the location of the feed.

In the second low-boiler separation tower, a second distillate stream containing or consisting essentially of aqueous aldehydes, preferably formaldehyde and/or prenal, and a second bottoms stream containing isoprenol are obtained. The second bottom stream further comprises high-boilers. The term "high-boilers" is understood to refer to organic compounds having a boiling point higher than that of isoprenol, i.e., higher than about 130°C, at atmospheric pressure.

In a preferred embodiment, the second low-boiler separation tower is operated at a pressure of 2.5 bara or higher, preferably 2.8 bara or higher, most preferably 2.9 bara or higher. The bottoms temperature of the second low-boiler separation tower is preferably in the range of 160 to 200°C, more preferably 170 to 185°C, most preferably 175 to 180°C. The temperature at the top of the second low-boiler separation tower is preferably in the range of 115 to 160°C, more preferably 125 to 145°C.

In a particularly preferred embodiment, the second low-boiler separation tower is operated at a pressure in the range of 2.9 to 3.5 bara, a bottoms temperature in the range of 175 to 180°C and a temperature at the top in the range of 130 to 140°C.

The second low-boiler separation tower preferably has from 20 to 60, more preferably from 35 to 60 theoretical plates. In particular, the stripping section of the first low-boiler separation tower preferably has 25 to 45 theoretical plates. The rectifying section of the first low-boiler separation tower preferably has 7 to 20 theoretical plates. At the top of the second low-boiler separation tower, an off-gas (also: offgas) is typically obtained. The off-gas primarily comprises nitrogen and may comprise traces of isoprenol, formic acid, water, aldehydes, preferably formaldehyde, prenal and/or decomposition gases.

The second bottoms stream preferably comprises 82 to 96 wt.-% of isoprenol, more preferably 87 to 91 wt.-%. The relatively high pressure of the second low-boiler separation tower allows for a high degree of separation of aldehydes, preferably formaldehyde and/or prenal, and isoprenol. Thus, the second bottoms stream preferably comprises at most 0.5 wt.-%, more preferably at most 0.1 wt.-%, even more preferably at most 0.008 wt.-% of aldehydes, preferably formaldehyde and/or prenal, based on the total weight of the second bottoms stream.

The second distillate stream is an aqueous stream, which preferably comprises 25 to 60 wt.-%, more preferably 40 to 50 wt.-%, in particular 45 to 50 wt.-%, based on the total weight of the second distillate stream, of aldehydes, preferably formaldehyde and/or prenal. The second distillate stream preferably comprises at most 15 wt.-% of isoprenol, more preferably at most 5 wt.-%, based on the total weight of the second distillate stream, of isoprenol.

Owing to the broad condensation curve of the vapor emerging at the top of the second low-boiler separation tower, it is advantageous to use a condenser with liquid recycling. The direct condensation in a quench with liquid circulation is particularly advantageous. Hence, in a preferred embodiment of the process, a quench section is provided downstream, in vapor flow direction, of the rectifying section of the second low-boiler separation tower. The term "vapor flow direction" relates to the direction of the flow of gaseous components in the separation tower, i.e., upwards, towards the top of the tower. The quench section is preferably provided within the second low-boiler separation tower above the rectifying section.

The direct condensation in a quench also mitigates fouling caused by various condensation and polymerization mechanisms of aldehydes, for example formaldehyde that may occur at spots of high local aldehyde concentrations, like local formaldehyde concentrations. To avoid the risk of fouling in the second low-boiler separation tower and downstream processes, in particular in the off-gas of the second low-boiler separation tower, the concentration of aldehydes, preferably formaldehyde and/or prenal in the second distillate is preferably no higher than 60 wt.-%, more preferably no higher than 55 wt.-% and in particular no higher than 50 wt.-%, based on the total weight of the second distillate stream.

At the lower end of the quench section, an aqueous liquid is collected. When the quench section is provided within the second low-boiler separation tower, the aqueous liquid may be collected, e.g., at a collecting tray above the rectifying section and beneath of the quench section.

The aqueous liquid is partially circulated into the quench section through a circulation line and partially withdrawn as the second distillate. Suitably, the part of the aqueous liquid circulated into the quench section is circulated into the top of the quench section. Circulation of the aqueous liquid is typically achieved by use of a pump. The circulation of a part of the aqueous liquid into the quench section allows for cooling of vapors rising through the quench section, and absorption of aldehydes, preferably formaldehyde and/or prenal from the vapors into the aqueous liquid. Thus, aldehydes, preferably formaldehyde and/or prenal is quenched from the vapors rising through the quench section.

Further, the aqueous liquid is partially returned to the rectifying section of the second low-boiler separation tower as a reflux stream. This may be accomplished by a reflux line, or aqueous liquid may be partially returned to the rectifying section as overflow from a collecting tray beneath the quench section.

The mass flow ratio of the reflux stream to the second distillate is preferably in the range of 2:1 to 10:1, more preferably in the range of 3:1 to 7:1. In a preferred embodiment, the aqueous liquid is cooled before being circulated into the quench section. Preferably, the part of the aqueous liquid withdrawn as the second distillate is a partial stream of the cooled aqueous liquid.

The temperature of the aqueous liquid collected at the lower end of the quench section is preferably in the range of 80 to 140°C, more preferably 125 to 135°C. The temperature of the cooled aqueous liquid circulated into the quench section is preferably 10 to 80°C below the temperature of the aqueous liquid collected at the lower end of the quench section. This allows for an energetically favorable process.

The hot aqueous liquid withdrawn at the lower end of the quench section lends itself to heat-integration. In a suitable embodiment, it is heat-exchanged with the stream of crude isoprenol flowing into the first low-boiler separation tower before being circulated into the quench section.

In one embodiment, a scrubbing section is provided downstream, in vapor flow direction, of the quench section and water is introduced at the top of the scrubbing section. Preferably, the scrubbing section is provided within the second low-boiler separation tower above the quench section. The scrubbing section allows for maintaining the aldehydes, preferably formaldehyde and/or prenal concentration in the second distillate below the critical concentrations described above and thus to avoid depositions for example paraformaldehyde deposition in, e.g., offgas lines.

The mass flow ratio of the water introduced at the top of the scrubbing section to the first bottoms stream obtained in the first low-boiler separation tower is typically in the range of 0.01 :1 to 0.06:1 more preferably in the range of 0.015:1 to 0.03:1.

The second bottoms stream is directed to a finishing tower, in which pure isoprenol is obtained as a distillate stream. High-boilers are withdrawn via a bottoms stream. As the second bottoms stream comprises essentially no aldehydes, preferably no formaldehyde and/or prenal, the separation task of the finishing tower is significantly less complex than in cases where formaldehyde separation is less efficient in the low-boiler separation section. The term "essentially no aldehydes, preferably no formaldehyde and/or prenal” is understood to indicate the absence of significant amounts of aldehydes, preferably formaldehyde and/or prenal in the obtained isoprenol. Thus, the obtained isoprenol preferably comprises less than 0.05 wt.-%, preferably less than 0.01 wt.-%, based on the total weight of the second bottoms stream, of aldehydes, preferably formaldehyde and/or prenal.

The pure isoprenol distillate stream preferably at least 97.0 wt.-% of isoprenol, more preferably 98.0 wt.-%, such as 98.1 to 99.5 wt.-%, based on the total weight of the pure isoprenol distillate stream. The high-boiler bottoms stream preferably comprises 90 to 99.9 wt.-% of high-boilers, more preferably 99 to 99.8 wt.-%, based on the total weight of the high-boiler bottoms stream. Preferably, the high-boiler bottoms stream comprises less than 0.2 wt.-% of aldehydes, preferably formaldehyde and/or prenal, such as less than 0.05 wt.-%, based on the total weight of the high- boiler bottoms stream, of aldehydes, preferably formaldehyde, and/or prenal.

In a preferred embodiment, the finishing tower is operated at a pressure of 0.5 bara or lower, preferably 0.25 bara or lower. The bottoms temperature of the first low-boiler separation tower is preferably in the range of 130 to 190°C, more preferably 150 to 170°C. The temperature at the top of the finishing tower is preferably in the range of 60 to 90°C, more preferably 65 to 85°C.

In a particularly preferred embodiment, the finishing tower is operated at a pressure in the range of 0.05 to 0.2 bara, a bottoms temperature in the range of 150 to 170°C and a temperature at the top in the range of 65 to 85°C.

The finishing tower preferably has from 6 to 40 theoretical plates, more preferably from 10 to 20 theoretical plates.

Unreacted isoprenol from the isomerization of isoprenol to prenol may be used, i.e., directed as feed to an oxidative dehydrogenation step of isoprenol to obtain a stream comprising prenal and/or isoprenal, as described in the following.

The isoprenol obtained in step a-ii) may or may not be purified. Purification may be conducted by any means. In an embodiment, isoprenol obtained in step a-ii) is purified by subjecting a stream of crude isoprenol containing isoprenol, water and formaldehyde, or an isoprenol containing fraction thereof, to distillation in a low-boiler separation tower operated at a pressure of 2 bara or higher, preferably 2.5 bara or higher, to obtain a distillate stream containing aqueous formaldehyde and a bottoms stream containing isoprenol essentially free of formaldehyde.

Providing Prenal and/or Isoprenal

The present invention also refers to the preparation of 3-methyl-2-butenal (prenal) and/or 3-methyl-3-butenal (isoprenal). A further aspect of the invention relates to a method for the preparation of 3-methyl-2-butenal (prenal) and/or 3-me- thyl-3-butenal (isoprenal), comprising the steps of: a) providing isoprenol according to the present invention; b) optionally isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; and c) providing prenal by at least one of c-i) and c-ii): c-i) subjecting isoprenol obtained in step a) to oxidative dehydrogenation so as to obtain prenal and/or isoprenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; and/or c-ii) oxidizing prenol obtained in step b) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol (e.g., steps a-i) and a-ii) and optionally prenol (step b)) above mutatis mutandis apply to the preparation of prenal and/or isoprenal.

In an embodiment, isoprenol is prepared according to any one of claims 1 to 8. In an embodiment, prenol, if prepared, may be prepared as laid out above.

The isoprenol obtained as described above may be converted to prenal, involving oxidative dehydrogenation and optionally isomerization in any order. Thus, it is possible to first isomerize isoprenol to prenol, and subsequently oxidize prenol to prenal; or, to first oxidatively dehydrogenate isoprenol to isoprenal, and optionally isomerize at least part of the isoprenal to prenal.

Oxidizing Prenol to Prenal

The prenol obtained as described above may be oxidized so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase.

Suitable oxidants include hydrogen peroxide and oxygen, in particular oxygen.

The oxidation is preferably carried out in the presence of a liquid phase and with oxygen as the oxidant. The liquid phase preferably comprises at least 25 wt.-% of water, more preferably at least 50 wt.-% of water or at least 70 wt.-% of water, based on the total weight of the liquid phase, determined at a temperature of 20 °C and a pressure of 1 bar. It has been found that these conditions allow for a simple and efficient process for preparing prenal from prenol.

The oxidation is typically carried out in the presence of at least one oxidation catalyst selected from the group consisting of Platinum, Palladium and gold. Preferably, the at least one oxidation catalyst comprises Platinum. In a preferred embodiment, the at least one oxidation catalyst is a supported catalyst.

The oxidation is suitably carried out at a temperature of 20 to 100 °C, preferably, 25 to 80 °C, in particular 30 to 70 °C, in particular 35 to 50 °C. In another embodiment the oxidation is carried out at a temperature of 20 to 70 °C. The oxidation is suitably carried out under a partial pressure of oxygen between 0.2 and 8 bar.

Further details of the oxidation reaction may be found in WO 2023/222895 A1 , which herewith is incorporated by reference in its entirety.

Oxidative Deyhdrogenation of Isoprenol

Oxidative dehydrogenation of isoprenol typically comprises bringing a reactant stream, in particular a gaseous reactant stream, comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in particular at least one silver-containing heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen. The at least one heterogeneous catalyst may consist of an inert support having a smooth surface having an active layer of silver. Alternatively, massive (full-metal) silver bodies may be used.

In an embodiment, the non-reacted isoprenol from the isomerization of isoprenol to prenol, which corresponds to the step b), is used as feed to the dehydrogenation step.

In an embodiment, in step c-i), the reactant stream is gaseous and at least one heterogeneous oxidative dehydrogenation catalyst is a silver-containing heterogeneous oxidative dehydrogenation catalyst.

In an embodiment, in step c-i), is characterized by maintaining in the reactant stream a weight ratio of aldehydes to isoprenol of less than 0.04, and optionally, step b) is characterized by maintaining in the reactant stream a concentration of aldehydes of less than 0.5wt.-%, preferably less than 0.4wt.-%, in particular less than 0.3wt.-%, or less than 0.25wt.-%, based on the total weight of the reactant stream, and, optionally, the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, based on the total weight of the reactant stream.

Hence, in an embodiment, the process includes separating an unreacted isoprenol stream from a prenol-containing product stream obtained in step b) and directing the unreacted isoprenol stream at least partially to step c-i). In an embodiment, oxidative dehydrogenation is carried out by passing the isoprenol through a plurality of reaction tubes of a shell-and-tube heat exchange reactor comprising: a shell-side heat exchange passage for circulating a heat transfer medium and a reaction passage comprising the plurality of reaction tubes; an inlet for introducing the reactant stream to the reaction passage; and an outlet from the reaction passage for recovering an effluent stream from the reaction tubes; wherein the reaction tubes comprise: a reactant pre-heating zone adjacent to the inlet, and a reaction zone downstream of the reactant pre-heating zone, the reaction zone having a catalytically active wire matrix insert having silver at least on a part of its surface.

The term "reactant pre-heating zone” denotes a section of the reaction tube, i.e., a section inside the reaction tube, where essentially no catalytic oxidative dehydrogenation reaction occurs and where the gaseous stream through the reaction tubes is heat-exchanged via the tube wall with the circulating heat transfer medium. The pre-heating zone upstream of the reaction zone involves net heat flow into the reaction tube and ensures that the reactant stream is sufficiently heated up to a temperature close to or at the reaction temperature when it reaches the reaction zone.

Upon contact with the catalytic surface, the oxidative dehydrogenation reaction immediately starts. Otherwise, in the event when a "cold” reactant stream reaches the catalytic surface such that the reaction onset temperature of the reaction is not reached, coke formation may occur. Less coke formation advantageously leads to a prolonged reactor operation without the necessity of burning off the coke from the catalytic surface.

Preferably, the reactant pre-heating zone is adapted to allow for laminar flow of the reactant inside the reactant preheating zone. This means, the reactant pre-heating zone is devoid of any obstacles to the reactant flow that triggers a I aminar-to-turbulent flow transition. Hence, the reactant pre-heating zone preferably has an essentially free cross section, i.e., the pre-heating zone is empty.

In the case of an "essentially free cross section”, the reactant pre-heating zone may be empty. Alternatively, the reactant pre-heating zone may accommodate fixtures made of a material having zero or limited catalytic activity, which fixtures have a negligible cross-section in a plane perpendicular to the longitudinal axis of the reaction tube. Said fixtures may be attached to the catalytically active wire matrix which is present in the reaction zone and allow to easily place said wire-matrix insert into or remove the same from the reaction zone. For example, the negligible mounting may be a stainless-steel wire or rod.

This setup allows for heating up only the portion of the entire reactant stream that travels near the hot reaction tube wall. Consequently, the portion of the reactant stream flowing in the center of the reaction tube is not heated to the reaction temperature and blind reactions of the unstable starting materials are thus reduced or even avoided. A "blind reaction” is an unselective oxidative reaction that occurs in the absence of the catalyst. Once the reactant stream reaches the reaction zone, the oxidative dehydrogenation reaction is initiated. Due to the exothermic nature of this reaction, energy is released and the remainder of the reactant stream is rapidly heated to the reaction onset temperature, and the reaction proceeds. This fast heat up of the predominant part of the reaction mixture reduces unwanted side-reactions and thus leads to an increased selectivity.

Alternatively, the reactant pre-heating zone may have a wire matrix insert having zero or limited catalytic activity. The wire matrix insert may reduce or eliminate temperature gradients without creating any obstruction to flow that would promote turbulent flow characteristics. A wire matrix insert is considered as having zero catalytic activity (or in other words, as being "inert”) if it does not catalyze the gas-phase partial oxidation reaction in question to a significant degree, and the chemical composition of a stream passing the wire matrix insert does not change significantly. Similarly, a matrix insert is considered as having limited catalytic activity if its catalytic activity is less than the activity of a reaction zone. In an embodiment, the wire matrix insert having zero or limited catalytic activity is made of an inert material, preferably stainless steel.

Herein, the term "reaction zone” denotes a region of the reaction tube where the catalytic gas-phase partial oxidation reaction occurs. The reaction zone comprises a catalytically active wire matrix insert having at least on a part of its surface a catalytically active precious metal. Due to the more open structure of the wire matrix contained in the reaction zone as compared to a packing of individual elements, a larger proportion of the reaction heat is discharged to the reaction tube wall by radiation and does not have to be dissipated by the reactant stream. Due the unique flow characteristic of the reactant stream through the reaction tube with the wire matrix insert in place, heat transfer via the tube wall is improved. Formation of prominent hotspots can be avoided. This in turn, avoids deposition of organic constituents of the reactant stream on the surface of the active catalyst material with concomitant pressure drop. Overall, less regular maintenance in the form of regeneration and/or replacement of the catalyst is required. The number of annual operating hours can be increased and the existing production capacities can be fully utilized, reducing operation cost and increasing profit.

In contrast to individually present catalyst bodies, the wire matrix inserts can be formed contiguously, or in one piece. Hence, placing the wire matrix inserts in the catalyst containment region of the reaction tubes, and removal therefrom is much facilitated.

The "reaction zone” may be comprised of a single contiguous reaction zone. Alternatively, the reaction zone may comprise an alternating series of regions having catalytically active wire matrix inserts and regions having an essentially free cross section or having wire matrix inserts having zero or limited catalytic activity.

A "wire matrix insert” is understood to be a self-supporting skeletal-like structure made of coiled, bent or crimped metal wire which is adapted to be inserted into a reaction tube of a shell-and-tube reactor. The wire matrix insert has a more voluminous structure than a longitudinal wire. A fixture such as a stainless steel wire or rod may be attached to the wire matrix insert which allows for easily placing the wire-matrix insert into or removing the same from the reaction zone.

In an embodiment, the catalytically active wire matrix inserts comprise an elongated core having a plurality of wire loops extending from the elongated core, wherein the wire loops are longitudinally arranged and helically shifted, that is, neighboring wire loops have an angular offset. The loops may be formed by helically bending the wire over the length of the wire matrix insert. In view of the ease of manufacture, the elongated core preferably comprises at least two longitudinal core wire members, which are twisted around each other to form core wire windings, and the wire loops are accommodated in the core wire windings.

The wire loops may be formed from one wire, or more than one intertwined wire, preferably 4 intertwined wires.

The wire matrix insert comprised in the reaction zone has silver at least on a part of its surface a catalytically active precious metal. The wire constituting the wire loops may be a massive silver wire, or a wire coated with silver. The core wire may be made of brass alloys, or high-grade steels. The coating layer of silver superimposed on the surface of the core has a thickness of, e.g., 10 m. In general, however, a massive silver wire has better service life and is preferred. If the wire loops are formed from more than one intertwined wire, at least one of the intertwined wires is made of a massive silver wire, or a wire coated with silver while the other intertwined wires can be made of an inert material.

A silver wire which is of the same composition throughout its cross section and comprises at least 92.5 wt.-% Ag can suitably be used. The silver wire is helically bent to form wire loops, and combined with at least two longitudinal core wire members, which are twisted around each other to form core wire windings, and the wire loops are accommodated in the core wire windings. The longitudinal core wire members can also be silver wire or inert metal wire.

In a preferred embodiment, the catalytically active wire matrix inserts comprise an elongated core having a plurality of wire loops extending from the elongated core, wherein the wire loops are longitudinally arranged and helically shifted, and the wire loops comprise a massive silver wire.

Further details of oxidative dehydrogenation carried out by passing the isoprenol through a plurality of reaction tubes of a shell-and-tube heat exchange reactor as described above may be found in WO 2023/241952 A1, which herewith is incorporated by reference in its entirety.

When isoprenol is subjected to oxidative dehydrogenation, it may be favorable to maintain in the reactant stream a weight ratio of aldehydes, preferably prenal and/or formaldehyde to isoprenol of less than 0.04, preferably less than 0.03, in particular less than 0.02, or less than 0.01. In another embodiment, the weight ratio of aldehydes, preferably prenal and/or formaldehyde to isoprenol is maintained at less than 0.002, or less than 0.001 and optionally at least 100 ppm. The weight ratio of aldehydes, preferably prenal and/or formaldehyde to isoprenol in the reactant stream may be maintained at a certain level or less. Reducing the weight ratio of aldehydes, preferably prenal and/or formaldehyde to isoprenol in the reactant stream beyond a certain point, however, reaches a point of rapidly diminishing return. Aldehydes, preferably prenal and/or formaldehyde removal involves additional equipment and operating costs. An economic balance must be taken between the improvement due to reducing the ratio and the cost of maintaining such a ratio. Hence, the weight ratio of aldehydes, preferably prenal and/or formaldehyde to isoprenol is preferably not lower than 0.0005. In an alternative embodiment the weight ratio is not lower than 0.005.

It has been found that reactor clogging and pressure drop increase are significantly affected by the presence of aldehydes, preferably prenal and/or formaldehyde in the reactant stream. Catalyst-fouling reactions of condensation and polymerization are believed to be the principal reactions involved in carbon or coke formation on the catalyst. It is thought that this carbon formation involves thermal condensation of aldehydes, preferably prenal and/or formaldehyde or of aldehydes, preferably prenal and/or formaldehyde with the olefinic hydrocarbons isoprenol and (iso)prenal. In the presence of the catalyst, the primary condensation products tend to undergo dehydrogenation and polymerization type reactions and to settle on the catalyst and undergo further dehydrogenation and decomposition until carbonaceous deposits are formed.

The process of the invention may satisfy the following condition 1), and preferably the following condition 2), or the process meets at least one of the following conditions 1) and 2):

1) Step c-i) is characterized by maintaining in the reactant stream a weight ratio of aldehydes to isoprenol of less than 0.04.

2) Step b) is characterized by maintaining in the reactant stream a concentration of aldehydes of less than 0.5wt.-%, preferably less than 0.4wt.-%, in particular less than 0.3wt.-%, or less than 0.25wt.-%, based on the total weight of the reactant stream, and, optionally, the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, based on the total weight of the reactant stream.

Reducing the weight ratio of aldehydes, preferably prenal and/or formaldehyde to isoprenol in the reactant stream can be accomplished in several different ways. In an embodiment, aldehydes, preferably prenal and/or formaldehyde, are removed from the unreacted isoprenol stream prior to combining the unreacted isoprenol stream with the crude isoprenol stream.

In an embodiment, the unreacted isoprenol stream is combined with the crude isoprenol stream and aldehydes, preferably prenal and/or formaldehyde is removed from the combined stream.

Aldehydes, preferably prenal and/or formaldehyde, may be optionally removed from isoprenol streams by a conventional separating method such as distillation, selective adsorption and or selective reaction, in particular by the purification process involving the pressure-swing distillation as described above. Preparation of diprenyl acetal of prenal

The present invention also refers to the preparation of diprenyl acetal of prenal.

A further aspect of the invention relates to a method of diprenyl acetal of prenal, comprising the steps of: a) providing isoprenol according to any one of claims 1 to 8; b) isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; c) providing prenal by at least one of c-i) and c-ii): c-i) subjecting isoprenol obtained in step a) to oxidative dehydrogenation so as to obtain prenal and/or iso- prenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; c-ii) oxidizing prenol obtained in step b) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase; and d) condensing prenol obtained in step b) with prenal obtained in step c) to obtain diprenyl acetal of prenal.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol (e.g., steps a-i) and a-ii) and optionally prenol (step b)) and optionally prenal/isoprenal (step c)) above mutatis mutandis apply to the preparation of diprenyl acetal of prenal.

In an embodiment, isoprenol is prepared according to any one of claims 1 to 8. In an embodiment, prenol, is prepared as laid out above. In an embodiment, prenal and/or isoprenal may be prepared as laid out above. In an embodiment, isoprenol, prenol, prenal and isoprenal are prepared as laid out above.

In an embodiment, step d) comprises continuously condensing prenol with prenal in the presence of at least one condensation catalyst in a reaction column while continuously withdrawing an acetal fraction comprising diprenyl acetal of prenal from the reaction column.

Preparation of Citral

The present invention also refers to the preparation of 3,7-dimethy l-octa-2,6-dienal (citral),

A further aspect of the invention relates to a method of 3,7-dimethy l-octa-2,6-dienal (citral), comprising the steps of: a) providing isoprenol according to any one of claims 1 to 8; b) isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; c) providing prenal by at least one of c-i) and c-ii): c-i) subjecting isoprenol obtained in step a) to oxidative dehydrogenation so as to obtain prenal and/or iso- prenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; c-ii) oxidizing prenol obtained in step b) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase; d) condensing prenol obtained in step b) with prenal obtained in step c) to obtain diprenyl acetal of prenal; and e) subjecting diprenyl acetal of prenal obtained in step d) to cleaving conditions to obtain citral via prenyl (3-me- thyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol (e.g., steps a-i) and a-ii) and optionally prenol (step b)) and optionally prenal/isoprenal (step c)) above mutatis mutandis apply to the preparation of citral.

In an embodiment, 3,7-dimethyl-octa-2,6-dienal (citral) can be prepared by a process comprising the steps of: condensing the prenal with prenol in the presence of at least one catalyst in a reaction column while withdrawing an acetal fraction comprising the diprenyl acetal of prenal from the reaction column; subjecting the acetal fraction in a cleaving column to cleaving conditions in the presence of at least one catalyst while withdrawing from the cleaving column a cleaving fraction containing at least one of prenyl (3-methy l-butadi- enyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene, and optionally containing citral; and reacting the cleaving fraction in a plug-flow type reactor to obtain citral.

The overall reaction sequence is illustrated by the reaction scheme below.

citral 2,4,4-trimethyl-3- prenyl (3-methyl- formyl- 1 ,5-hexadiene butadienyl) ether

The unsaturated acetal 3-methy l-2-butenal-dipreny I acetal (herein referred to as "diprenyl acetal of prenal” or "diprenyl acetal”) is formed from prenol and prenal using at least one catalyst. For this purpose, prenal may be reacted together with prenol in the presence of catalytic amounts of at least one acid and with separation of the water formed during the reaction in a reaction column.

It has been found that when the conversion rate of diprenyl acetal of prenal is driven to full conversion, the concentration of by-products increases sharply. Accordingly, it is preferred that the conversion rate of diprenyl acetal of prenal is maintained at above 90% and below 100%. Preferably, the conversion rate of diprenyl acetal of prenal in step b) is maintained equal to or below 99.5%, preferably equal to or below 99%, such as equal to or below 98%, or equal to or below 97.5%, or equal to or below 97%. Preferably, the conversion rate of diprenyl acetal of prenal is maintained above 91%, such as above 92%, or above or 93%, or above 94%, or above 95%. In suitable embodiments, the conversion rate of diprenyl acetal of prenal in is above 94% and equal to or below 99%, such as above 95% and equal to or below 98%. Lower conversion rates will render the process economically unprofitable or will otherwise necessitate recovery and recycling of unreacted diprenyl acetal. Complete conversion is however undesirable as it results in a drop of yield of citral building blocks and increasing by-products-formation. The conversion rate is governed by various parameters including cleaving temperature, nature and concentration of the catalyst(s) and residence time in the cleaving column.

The resulting 3-methyl-2-butenal diprenyl acetal (diprenyl acetal) is cleaved in the presence of at least one catalyst in a cleaving column with elimination of 3-methyl-2-buten-1 -ol (prenol) to give prenyl (3-methylbutadienyl) ether. Claisen rearrangement of the obtained prenyl (3-methylbutadienyl) ether yields 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene which subsequently undergoes Cope rearrangement yielding 3,7-dimethyl-2,6-octadienal (citral). In an embodiment, step d) comprises continuously condensing prenol with prenal in the presence of at least one condensation catalyst in a reaction column while continuously withdrawing an acetal fraction comprising diprenyl acetal of prenal from the reaction column.

Cleaving is carried out in the presence of at least one catalyst, preferably an acid catalyst. The catalyst can be a single catalytic species or a combination of two or more different catalytic species. Suitable acid catalysts are selected from non-volatile protic acids such as sulfuric acid, p-toluenesulfonic acid and phosphoric acid. In an embodiment, the catalyst comprises phosphoric acid. In a preferred embodiment, the concentration of the phosphoric acid in the bottoms of the cleaving column is maintained above 100 ppm and below 1500 ppm, preferably above 200 ppm and below 1000 ppm. Higher concentrations of (acid) catalyst may result in reduced yields of citral building blocks.

Condensation of prenol with prenal is carried out in the presence of at least one catalyst, preferably an acid. The catalyst can be a single catalytic species or a combination of two or more different catalytic species. In an embodiment, the catalyst in is nitric acid. Preferably, the concentration of the nitric acid is below 500 ppm, more preferably in the range of from 100 to 300 ppm, relative to the total amount of the starting materials prenol and prenal. Lower amounts of (acid) catalyst may result in a low conversion in the reaction column. Higher amounts of (acid) catalyst may disad- vantageously result in increased formation of by-products and in decreased selectivity.

Preferably, the acetal fraction is continuously subjected to cleaving conditions in a cleaving column. "Cleaving conditions” denotes reaction conditions selected such that the diprenyl acetal contained in the acetal fraction is cleaved to prenyl (3-methylbutadienyl) ether which may subsequently rearrange to 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene and citral.

The acetal fraction comprises diprenyl acetal as a main constituent. The acetal fraction does not necessarily need to consist of pure diprenyl acetal, but may also comprise prenol, prenal and citral building blocks.

Cleaving is carried out in the presence of at least one catalyst, preferably at least one acid catalyst. Suitable acid catalysts are selected from non-volatile protic acids such as sulfuric acid, p-toluenesulfonic acid and phosphoric acid.

Suitably, the continuous cleaving in the cleaving column may be carried out in the lower part or the sump of the distillation column acting as cleaving column. Preferably, the acetal fraction and/or the catalyst(s) are introduced into the lower part of the distillation column, into the sump of the distillation column or into the evaporator of the distillation column.

In an embodiment, step e) comprises continuously subjecting the acetal fraction in a cleaving column to cleaving conditions in the presence of at least one cleaving catalyst while continuously withdrawing from the cleaving column a cleaving fraction containing at least one of prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5-hexa- diene, and optionally containing citral; and reacting the cleaving fraction in a plug-flow type reactor to obtain citral. In an embodiment, the method of the present invention comprises recycling prenol obtained in step e) to step d); wherein the concentration of 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene of the prenol recycled from step e) into step d) is controlled such that the concentration of 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene in step d) is below 1 wt.-%, relative to the total weight of prenol and prenal; and wherein the concentration of citral of the prenol recycled from step e) into step d) is controlled such that the concentration of citral in step d) is below 1 wt.-%, relative to the total weight of prenol and prenal.

If desired, a high-boiling inert compound can be introduced into the sump of the cleaving column in order to ensure a minimum filling level of the sump and the evaporator. Suitable high-boiling inert compounds are selected from liquid compounds which are inert under the reaction conditions and have a higher boiling point than citral and diprenyl acetal. For example, the high-boiling inert compounds may be selected from hydrocarbons such as tetradecane, pentadecane, hexadecane, octadecane, eicosane; or ethers such as diethylene glycol dibutyl ether; white oils; kerosene oils; or mixtures thereof.

Suitably, the distillation conditions are selected such that the diprenyl acetal is predominantly retained in the lower part or the sump of the distillation column. During the cleaving reaction, a cleaving fraction is continuously withdrawn from the cleaving column, the cleaving fraction containing at least one of prenyl (3-methy l-butadieny I) ether and 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene, and optionally containing citral. For the ease of reference, prenyl (3-methyl- butadienyl) ether, 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene and citral are collectively referred to as "citral building blocks”. This is because the former are intermediates on the reaction route to citral and can be converted into citral in the subsequent passage through the plug-flow type reactor.

Additionally, the prenol formed during the cleaving reaction may be continuously removed from the reaction mixture, generally at the top of the cleaving column.

The cleaving fraction together with the formed prenol may be withdrawn at the top of the distillation column.

Alternatively and preferably, it is also possible to withdraw the cleaving fraction in liquid or vaporous form at a side draw of the distillation column.

The cleaving fraction may be reacted in a plug-flow type reactor to obtain citral. To this end, the cleaving fraction is guided through the plug-flow type reactor at a suitable temperature for carrying out the rearrangement reaction(s) yielding citral. By employing a combination of a highly back-mixed cleaving column and a plug-flow reactor, it is possible to increase the selectivity and the yield of the cleaving reaction. All of the catalyst(s) required for the cleaving reaction is/are preferably introduced into the cleaving column and preferably, no catalyst is introduced into the plugflow reactor. In an embodiment, prenol eliminated in the cleaving reaction is recycled to the condensation reaction. This allows for improved yields to be achieved in the process of the invention.

In particular, the inventive process may comprise recycling prenol obtained in step e) to step d); wherein the concentration of 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene of the prenol recycled from step e) into step d) is controlled such that the concentration of 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene in step d) is below 1 wt.-%, relative to the total weight of prenol and prenal; and wherein the concentration of citral of the prenol recycled from step e) into step d) is controlled such that the concentration of citral in step d) is below 1 wt.-%, relative to the total weight of prenol and prenal.

In an embodiment, the method includes separating an unreacted isoprenol stream from the prenol-containing product stream, combining the unreacted isoprenol stream with a crude isoprenol stream containing isoprenol, water and aldehydes, and removing water, preferably water and aldehydes, from the combined stream to form the reactant stream, preferably wherein removing aldehydes from the combined stream comprises:

(i) directing the combined stream to a first low-boiler separation tower operated at a pressure of 1 .5 bara or lower, to obtain a first bottoms stream containing isoprenol and aldehydes, and a first distillate stream containing water and low-boilers;

(ii) directing the first bottoms stream to a second low-boiler separation tower operated at a pressure of 2 bara or higher, to obtain a second distillate stream containing aqueous aldehydes, and a second bottoms stream containing isoprenol; and

(iii) directing the second bottoms stream to a finishing tower to obtain a bottoms stream containing high-boilers, and the reactant stream as a distillate stream.

In an embodiment, the process for preparing citral is a continuous process. In an embodiment, steps for preparing citral are conducted continuously, in particular are conducted as consecutive continuous steps. This may for instance include steps for preparing prenal (which may be steps a) and c)) and/or prenol (which may be steps a) and b)) as laid out above, condensing prenol with prenal to obtain diprenyl acetal of prenal (which may be step d)), rearrangement of citral (which may be step e)), and optional one or more purification steps.

Isoprenol

A further aspect of the present invention relates to isoprenol obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol (e.g., steps a-i) and a-ii) above mutatis mutandis apply to the preparation of isoprenol. It will be understood that the isoprenol has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) isoprenol. In an embodiment, isoprenol may be such obtainable (or obtained) from a method according to any one of claims 1 to 8.

I soprenol-containing product stream

A further aspect of the present invention relates to isoprenol-containing product stream obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol- containing product stream (e.g., steps a-i) and a-ii) above mutatis mutandis apply to the preparation of isoprenol-containing product stream. It will be understood that the isoprenol-containing product stream has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) isoprenol in the isoprenol-containing product stream any may optionally include traces of educts or derivatives thereof that indicate the production path. In an embodiment, isoprenol-containing product stream may be such obtainable (or obtained) from a method according to any one of claims 1 to 8.

Prenol

A further aspect of the present invention relates to prenol obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol (e.g., steps a-i) and a-ii) and prenol (step b) above mutatis mutandis apply to the preparation of prenol. It will be understood that the prenol has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) prenol. In an embodiment, prenol may be such obtainable (or obtained) from a method according to claim 9.

Prenol-containing product stream

A further aspect of the present invention relates to prenol-containing product stream obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing prenol- containing product stream (e.g., steps a-i) and a-ii) and prenol (step b) above mutatis mutandis apply to the preparation of prenol-containing product stream. It will be understood that the prenol-containing product stream has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) prenol in the prenol-containing product stream any may optionally include traces of educts or derivatives thereof that indicate the production path. In an embodiment, prenol-containing product stream may be such obtainable (or obtained) from a method according to claim 9.

Prenal and/or isoprenal

A further aspect of the present invention relates to prenal and/or isoprenal obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenal and/or isoprenal (e.g., steps a-i) and a-ii) and prenal and/or isoprenal above mutatis mutandis apply to the preparation of prenal and/or isoprenal. It will be understood that the prenal and/or isoprenal has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) prenal and/or isoprenal. In an embodiment, prenal and/or isoprenal may be such obtainable (or obtained) from a method according to any one of claims 10, 12 and 13.

Prenal- and/or isoprenal-containing product stream

A further aspect of the present invention relates to prenal and/or isoprenal-containing product stream obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing prenal- and/or isoprenal-containing product stream (e.g., steps a-i) and a-ii) and prenal and/or isoprenal above mutatis mutandis apply to the preparation of prenal- and/or isoprenal-containing product stream. It will be understood that the prenal- and/or isoprenal-containing product stream has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) prenal and/or isoprenal in the prenal- and/or isoprenal-containing product stream any may optionally include traces of educts or derivatives thereof that indicate the production path. In an embodiment, prenal- and/or isoprenal-containing product stream may be such obtainable (or obtained) from a method according to any one of claims 10, 12 and 13.

Diprenyl acetal of prenal

A further aspect of the present invention relates to diprenyl acetal of prenal obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isodiprenyl acetal of prenal (e.g., steps a-i) and a-ii) and the further steps above mutatis mutandis apply to the preparation of diprenyl acetal of prenal. It will be understood that the diprenyl acetal of prenal has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) diprenyl acetal of prenal. Diprenyl acetal of prenal-containing product stream

A further aspect of the present invention relates to diprenyl acetal of prenal-containing product stream obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing diprenyl acetal of prenal-containing product stream (e.g., steps a-i) and a-ii) and the further steps above mutatis mutandis apply to the preparation of diprenyl acetal of prenal-containing product stream. It will be understood that the diprenyl acetal of prenal-containing product stream has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) diprenyl acetal of prenal in the diprenyl acetal of prenal-containing product stream any may optionally include traces of educts or derivatives thereof that indicate the production path.

Citral

A further aspect of the present invention relates to citral obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol (e.g., steps a-i) and a-ii) and the further steps above mutatis mutandis apply to the preparation of citral. It will be understood that the citral has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) citral. In an embodiment, citral may be such obtainable (or obtained) from a method according to any one of claims 11 to 13.

Citral occurs as (2Z)- and (2E)-isomers: the (2Z))-lsomer, neral, as depicted in formula (Vb-1) and the (2E)-lsomer, geranial, as depicted in formula (Va-1).

The term citral obtainable (or obtained) according to the method of the present invention may be any mixture of the two isomers, preferably a mixture having a mass ratio of neral : geranial of between 40 . 60 to 60 : 40, in particular between 45 . 55 to 55 : 45, between 48 . 52 to 52 : 48, between 49 . 51 to 51 : 49, or (approximately) 50 : 50.

As known in the art, neral and geranial may be optionally separated from one another. For instance, neral and geranial may be separated by distillation. This allows adjusting the mass ratio of neral : geranial to a desired degree.

Citral obtainable (or obtained) according to the invention may comprise geranial of the formula (Va-1) and/or of neral of the formula (Vb-1).

A further aspect of the present invention relates to geranial of the formula (Va-1), neral of the formula (Vb-1 ) or a mixture thereof obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol (e.g., steps a-i) and a-ii) and the further steps directed to citral above mutatis mutandis apply to the preparation and product characteristics of geranial of the formula (Va-1), neral of the formula (Vb-1) or a mixture thereof. It will be understood that the geranial of the formula (Va-1), neral of the formula (Vb-1) and a mixture thereof have certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) geranial of the formula (Va-1), neral of the formula (Vb-1) and a mixture thereof.

Citral-containing product stream

A further aspect of the present invention relates to citral-containing product stream obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing citral-containing product stream (e.g., steps a-i) and a-ii) and the further steps above mutatis mutandis apply to the preparation of citral-containing product stream. It will be understood that the citral-containing product stream has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) citral in the citral-containing product stream any may optionally include traces of educts or derivatives thereof that indicate the production path. In an embodiment, citral-containing product stream may be such obtainable (or obtained) from a method according to any one of claims 11 to 13.

Further Conversion to Menthol or Linalool

The obtainable (or obtained) citral is a useful intermediate for, e.g., menthol or linalool.

Menthol (p-menthal-3-ol) is a naturally occurring active ingredient that is widely used in pharmaceuticals, cosmetics and the food industry. Menthol has a cooling effect when it comes into contact with mucous membranes, especially the oral mucosa. In natural sources, for example peppermint oil, menthol occurs in the form of four diastereomeric enantiomer pairs. The following formula depicts the main component, (-)-menthol or L-menthol, which has desired taste and other sensory properties.

IUPAC name: 1 R, 2S, 5R 2-isopropyl-5-methylcyclohexanol

Menthol may be prepared from citral via a process comprising the steps of

- catalytic hydrogenation of citral to obtain citronellal;

- cyclization of citronellal to obtain isopulegol in the presence of at least one acidic catalyst; and

- catalytic hydrogenation of isopulegol to obtain menthol.

The overall reaction sequence is illustrated by the reaction scheme below.

The hydrogenation of citral to obtain citronellal may be achieved by hydrogenation in the presence of a rhodiumphosphine catalyst.

The cyclization of citronellal to isopulegol may be achieved by cyclization in the presence of at least one Lewis-acidic aluminum-containing catalyst, such as a bis(diarylphenoxy)aluminum compound, which may be used in the presence of an auxiliary, such as a carboxylic anhydride. The isopulegol may be recovered from the catalyst-containing reaction product by distillative separation to give an isopulegol-enriched top product and an isopulegol-depleted bottom product. From the bottom product, the at least one catalyst may be regenerated. The isopulegol obtainable in this way by the cyclization of citronellal can be further purified by suitable separating and/or purification methods, in particular by crystallization, and be at least largely freed from undesired impurities or by-products.

A further aspect of the invention is directed to a process for preparation of menthol comprising the steps of:

- catalytic hydrogenation of citral obtainable (or obtained) according to the invention, preferably according to any of the claims 11 to 13, to obtain citronellal; - cyclization of ci tronel lai prepared in this way to obtain isopulegol in the presence of at least one acidic catalyst; and

- catalytic hydrogenation of isopulegol prepared in this way to obtain menthol.

A further aspect of the present invention is directed to a process for the preparation of optically active menthol using citral obtained by the process according to the invention.

A further aspect of the invention is directed to a process for the preparation of optically active menthol, preferably L- menthol, comprising the steps of o) optionally separating the citral obtainable (or obtained) according to the invention, preferably according to any of the claims 11 to 13, into geranial of the formula (Va-1) and neral of the formula (Vb-1 ))

I) preparation of optically active citronel lai by asymmetric hydrogenation of citral obtainable (or obtained) according to the invention, preferably according to any of the claims 11 to 13, geranial of the formula (Va-1) or of neral of the formula (Vb-1) by the process according to the invention,

II) cyclization of the optically active citronellal prepared in this way to give optically active isopulegol in the presence of a suitable acid, preferably a Lewis acid, and ill) hydrogenation of the optically active isopulegol prepared in this way to give optically active menthol.

A further aspect of the present invention relates to menthol, which may be optionally optically active menthol, preferably L-menthol, obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol (e.g., steps a-i) and a-ii), citral and further steps and obtainable (or obtained) products above mutatis mutandis apply to the preparation and product characteristics of menthol. It will be understood that the obtainable (or obtained) menthol has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) menthol.

The hydrogenation of isopulegol may be achieved by hydrogenation in the presence of at least one heterogeneous Nickel-containing catalyst, preferably at least one heterogeneous Nickel- and copper-containing catalyst.

Further details regarding the reaction sequence from citral to menthol may be found in US 2013/46118 A1 , which is incorporated by reference herein.

Isopulegol (5-methyl-2-(1-methylethenyl)-cyclohexanol) has three asymmetric carbon atoms and therefore four stereoisomers, each occurring as a pair of enantiomers. (1 R,3R,4S)-(-)isopulegol is also known as L-isopulegol. A further aspect of the invention is directed to a process for the preparation of isopulegol, preferably optically active isopulegol, preferably L-isopulegol, comprising the steps of o) optionally separating the citral obtainable (or obtained) according to the invention, preferably according to any of the claims 11 to 13, into geranial of the formula (Va-1) and neral of the formula (Vb-1 )),

II) cyclization of the optically active citronellal prepared in this way to give optically active isopulegol in the presence of a suitable acid, preferably a Lewis acid.

A further aspect of the present invention relates to isopulegol, which may be optionally optically active isopulegol, preferably L-isopulegol, obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol (e.g., steps a-i) and a-ii), citral and further steps and obtainable (or obtained) products above mutatis mutandis apply to the preparation and product characteristics of isopulegol. It will be understood that the obtainable (or obtained) isopulegol has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) isopulegol.

In one aspect, the invention thus relates to an improved process for the preparation of menthol by producing citral using the above processes and then producing menthol from the citral. Menthol may be prepared as described herein or by other methods known in the art.

Linalool may be prepared from citral via a process comprising catalytic hydrogenation of citral to obtain nerol and/or geraniol, and isomerization thereof.

The hydrogenation of citral to obtain nerol and/or geraniol may be achieved by hydrogenation in the presence of at least one supported Ruthenium, rhodium, osmium, iridium or Platinum catalyst, preferably at least one Ruthenium catalyst supported on carbon black.

The isomerization of nerol and/or geraniol to obtain linalool may be achieved by isomerization in the presence of at least one tungsten catalyst, in particular a dioxotungsten (VI) complex. Further details regarding the isomerization of nerol and/or geraniol may be found in US 7,126,033 B2.

In one aspect, the invention thus relates to an improved process for the preparation of linalool by producing citral using the above processes and then producing linalool from the citral. Linalool may be prepared as described herein or by other methods known in the art. A further aspect of the present invention relates to linalool obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol (e.g., steps a-i) and a-ii), citral and further steps and obtainable (or obtained) products above mutatis mutandis apply to the preparation and product characteristics of linalool. It will be understood that the obtainable (or obtained) linalool has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) linalool. Further Conversion to Vitamin A

The obtained citral is also a useful intermediate for the synthesis of vitamin A.

Vitamin A may be prepared from citral via the reaction sequence illustrated by the reaction scheme below. Citral (I) can be converted into pseudoionone (II) in reaction step A. Said pseudoionone can be reacted in synthetic step B to obtain p-ionone (III), which is further transformed into p-vinylionol of formula (IV). Phosphorylation of p-vi- nylionol of formula (IV) can yield the 015-salt of formula (V), which upon reacting it with the C5-acetate of formula (VI) can yield vitamin A acetate of formula (VII).

Reaction step A can be realized in the presence of a base selected form metal hydroxides, in particular alkali metal hydroxides and earth alkali metal hydroxides. Said base acts as a catalyst and can be added in one or several portions as, e.g., disclosed in EP 0 062 291 A1 and WO 2004/041764 A1.

Cyclisation of pseudoionone (II) into p-ionone (III) in step B is realized in the presence of an acid, preferably in the presence of a mineral acid. A method of realizing step B is disclosed in EP 0 133 668 A2 and in US 3,840,601.

The vinylionol (IV) can be obtained by reacting the compound of formula (III) with a Grignard reagent.

The Ci5-salt of formula (V) can be obtained from vinylionol (IV) in the presence of a phosphine. A suitable method of obtaining compound (V) is disclosed in WO 2005/058811 A1.

Vitamin A acetate (VII) can finally be obtained by subjecting the Ci5-salt of formula (V) to Wittig conditions in the presence of the acetate of formula (VI). Details of such a Wittig reaction are disclosed in WO 2005/058811 A1 .

A further aspect of the invention is directed to a process for the preparation of vitamin A acetate comprising the steps of

- converting Citral (VI I) obtainable (or obtained) according to the invention, preferably according to any of the claims 11 to 13, into pseudoionone (VIII),

- reacting pseudoionone (VIII) to obtain p-ionone (IX),

- transforming p-ionone (IX) into p-vinylionol of formula (X),

- phosphorylation of p-vinylionol of formula (X) to yield the Ci5-salt of formula (XI), and

- reacting the Ci5-salt of formula (XI) with the Cs-acetate of formula (XII) to yield vitamin A acetate of formula (XIII).

A further aspect of the present invention relates to vitamin A obtainable (or obtained) from a method of the present invention.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol (e.g., steps a-i) and a-ii), citral and further steps and obtainable (or obtained) products above mutatis mutandis apply to the preparation and product characteristics of vitamin A. It will be understood that the obtainable (or obtained) vitamin A has certain characteristics such as a specific composition of carbon isotopes in different regions of the obtainable (or obtained) vitamin A. In a further aspect of the present invention, the isobutene prepared by the method according to this invention is used to produce diisobutene.

It will be understood that the definitions and preferred embodiments as laid out in the context of preparing isoprenol (e.g., steps a-i) and a-ii) above mutatis mutandis apply to the preparation of diisobutene.

The present invention may include a method for the preparation of diisobutene, comprising the steps of: a-i) preparing isobutylene by contacting isoamyl alcohol to a catalyst comprising at least one catalytically active metal; and a'-ii) dimerization of isobutene as obtained in step a-i) to diisobutene (preferably 2,4,4-trimethyl-l-pentene and 2,4,4-trimethyl-2-pentene as the main components); and a'-iii) optionally purifying isobutene as obtained in step a'-ii).

Diisobutene (2,4,4-trimethyl-l-pentene and 2,4,4-trimethyl-2-pentene as the main components) is an important industrial chemical and an important intermediate product in the production of other major industrial compounds. Processes for preparing diisobutene are known and described inter alia in Baerns et. al. Technische Chemie, 1st edition, Wiley-VCH, Weinheim 2006. One well-known way is the acid-catalyzed dimerization of isobutene.

Disobutene may be used for any purpose known in the art such as, e.g., for preparing one or more types of (co)poly- mers.

The object of providing an alternative, improved method for the production of diisobutene preferably from sources of renewable raw materials is achieved by a method of producing diisobutene comprising the steps a) preparation of isobutene by the method according to this invention; b) dimerization of isobutene into diisobutene; c) purifying the diisobutene.

In the following, "diisobutene” means diisobutene produced from isobutene prepared by the method according to this invention.

Another embodiment of the invention are polymers comprising diisobutene in its polymerized form. Preferred are copolymers comprising, in polymerized form, at least one unsaturated carboxylic acid as defined in formula CoC below and diisobutene.

The variables in general formula (CoC) are defined as follows:

R¹, R² and R³ are independently selected from H, linear or branched C1-C12 alkyl, linear or branched C2-C12 alkenyl, wherein alkyl and/or alkenyl may be substituted with -NH2, -OH, or -COCH; -COCH; and -COOR⁵, wherein R⁵ is selected from linear or branched C1-C12 alkyl and linear or branched C2-C12 alkenyl.

R⁴ is selected from a single bond, -(CH2)_n- with n being in the range of 0 to 4, -COO-(CH2)k- with k being in the range of 1 to 6, -C(O)-NH- and -C(O)-NR⁶-, wherein R⁶ is selected from linear or branched C1-C22 alkyl, linear or branched C2-C22 alkenyl, and C6-C22 aryl.

Non-limiting examples of suitable unsaturated carboxylic acids include acrylic acid, methacrylic acid, 2-ethylacrylic acid, 2-phenylacrylic acid, malonic acid, cratonic acid, maleic acid (or maleic anhydride), fumaric acid, itaconic acid, aconitic acid, mesaconic acid, citraconic acid, sorbic acid, cinnamic acid, methylene malonic acid, unsaturated C4-C10 dicarboxylic acids, and mixtures thereof.

One preferred copolymer consists of or comprises maleic acid and diisobutene in their polymerized forms. Another preferred copolymer consists of or comprises maleic anhydride and diisobutene in their polymerized forms.

Another preferred copolymer is the sodium salt of a copolymer consisting of or comprising, in their polymerized form, maleic acid or maleic anhydride and diisobutene, such copolymers having a K-value in the range of about 20 to about 80, preferably in the range of about 0 to about 50, more preferably in the range of about 35 to about 45, wherein the K-value is determined with about 1 % dry substance in distilled water.

In one embodiment, the copolymer according to this invention consists of or comprises maleic acid or maleic anhydride and diisobutene in a weight ratio of about 1 : 1 and has a K-value of about 35.

In another embodiment, the copolymer consisting of or comprising maleic acid or maleic anhydride and diisobutene is modified by esterification of one carboxyl group of the polymerized maleic acid or maleic anhydride with oligo or polyalkoxylene compounds that may bear an alkyl end-capping, such end-capping preferably selected from with C4 to G , Ce to G , C12 to C14 alkyl, whereas the other carboxyl-group of the polymerized maleic acid or maleic anhydride may be neutralized such that the copolymer may contain partially or - preferably - fully neutralized carboxyl groups.

EXAMPLES The present invention is illustrated in detail by non-restrictive working examples which follow. More particularly, the test methods specified hereinafter are part of the general disclosure of the application and are not restricted to the specific working examples.

MATERIALS AND METHOD

Isoamyl alcohol pure (boiling point 131 °C).

Fusel alcohol (70% isoamyl alcohol + others).

Density of Pt/ZrO2 catalyst = 1.18 g/cm³.

The reactions were carried out using a highly flexible reactor (HIFIexT: R0027) equipped with a number of individual reactors, each with a heater capable of achieving temperatures up to 750°C (850°C), at a pressure of up to 100 bar. Each reactor had capability of holding a catalyst up to 2.0 ml.

The powdered catalysts were sieved (or screened) prior to the catalytic testing experiments. For the catalytic testing experiments a split fraction in the range of 100 to 350 pm was used.

Examples were conducted at three different reactor filling levels (or filling volumes), which were 0.25, 0.5 and 1 .0 ml. The details are listed in the respective Tables. This means that the amount of catalyst which was used for an individual experiment was in the range of 0.295 to 1.18 g in case of Pt-metal on zirconium oxide (which has a density of 1.18 g/cm³). To keep the catalyst beds on a constant level, the catalyst material was diluted with inert particles in such way that all reactors had a constant filling volume.

The components of product feed/stream were detected and quantified using a Gas Chromatography system. The Gas Chromatography system was calibrated using GC standard/s for prenol, prenal, and isoprenal. The Gas Chromatography system was calibrated for products such as: CO, H2, Ci-Ce alkanes and alkenes, isoamyl alcohol, 3- methyl butanal, isobutylene, isoprenol, and isoprenol.

The following examples are included to further illustrate the invention.

A series of catalysts were used for the consecutive dehydrogenation I retro-dehydroformylation reaction in a single fixed bed reactor. The catalysts used for these reactions contained 0.005 wt.-% to 10.0 wt.-% of catalytically active metal (based on total weight of the catalyst). Optionally, the catalyst further contained sufficient amount of a promoter in an amount from 0.005 wt.-% to 5.0 wt.-%, with respect to total weight of the catalyst. The catalysts were charged to a conventional vapor phase reactor having temperature control, inlet/s for introduction of feed including I nert/car- rier/hydrogen gas and outlets for withdrawal of products. The reactions were carried out by varying temperatures from 200 to 450°C, and weight ratios of alcohol in the feed from 2.0 wt.-% to 88.0 wt.-% (based on total weight of the feed). The yield of the olefin for each catalyst was determined at different conversion levels.

The data is summarized in the Figures and showed that the catalyst containing a 1 .0 wt.-% Pt 1 0.5 wt.-% Sn I ZrO2 was unique.

Isoamyl alcohol to isobutylene:

Table 1 : Conversion of isoamyl alcohol using 0.25 ml of 1.0 wt.-% Pt / 0.5 wt.-% Sn / ZrO2 as catalyst

From Table 1, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 9970 h^-1 over 0.25 ml of 1.0 wt.-% Pt 10.5 wt.-% Sn I ZrO₂ catalyst, 4.0% to 81.0% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 200 to 325°C. It is also evident that highest conversion (81 .0%) was achieved at a temperature of 325°C. The results of Table 1 are illustrated graphically in Figure 1 .

Table 2: Conversion of isoamyl alcohol using 1.0 ml of 1.0 wt.-% Pt / 0.5 wt.-% Sn / ZrO2 as catalyst From Table 2, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 2500 h^-1 over 1.0 ml of 1 .0 wt.-% Pt 1 0.5 wt.-% Sn I ZrC>2 catalyst, 3.8% to 98.4% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 200 to 325°C. It is also evident that highest conversion (98.4%) was achieved at a temperature of 325°C. The results of Table 2 are presented graphically in Figure 2.

Table 3: Conversion of isoamyl alcohol using 1.0 ml of 1.0 wt.-% Pt / 0.5 wt.-% Sn / ZrO2 as catalyst at a constant GHSV of 2500 h-¹.

From Table 3, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 2500 h^-1 over 1.0 ml of 1 .0 wt.-% Pt 1 0.5 wt.-% Sn I ZrO2 catalyst, 70.6% to 100.0% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 275 to 350°C. It is also evident that highest conversion (100.0%) was achieved at a temperature of 350°C. The results of Table 3 are presented graphically in Figure 3.

From Experiments 10 to 27, it was observed that the yield of the isobutylene was in the range of 1 mol% to 99 mol%. It is evident from Tables 1-3 that the conversion of Isoamyl alcohol increased with an increase in the amount of the catalyst from 0.25 ml to 1 .0 ml, and with a decrease in GHSV of 9.9 vol% isoamyl alcohol from 9970 h^-1 to 2500 h^-1.

It is evident from Tables 1-3 that the conversion of Isoamyl alcohol in the presence of 1.0 ml of 1.0 wt.-% Pt / 0.5 wt.- % Sn I ZrO2 as catalyst, increased with the temperature in the range of 250°C to 350°C. Thus, conversion of Isoamyl alcohol in the presence of the 1 .0 wt.-% Pt 1 0.5 wt.-% Sn I ZrO2 as catalyst was achieved in the range of 250°C to 450°C.

Table 4: Conversion of Isoamyl alcohol using 0.25 ml of 1.0 wt.-% Pt 1 0.5 wt.-% Sn I ZrO2 as catalyst at a constant GHSV of 9800 h-¹.

From Table 4, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 9980 h^-1 over 0.25 ml of 1 .0 wt.-% Pt 10.5 wt.-% Sn I ZrO₂ catalyst, 38.5% to 97.4% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 275 to 350°C. It is also evident that highest conversion (97.4%) was achieved at a temperature of 350°C. The results of Table 4 are presented graphically in Figure 4.

Table 5: Conversion of Isoamyl alcohol using 1.0 ml of 1.0 wt.-% Pt / 0.5 wt.-% Sn / ZrO2 as catalyst at various GHSV's, at Temp = 300, and Pressure = 1 bar From Table 5, it is evident that on passing a gas feed of 2.0 vol% to 80.1 vol% isoamyl alcohol at a GHSV in the range of 320 h^-1 to 2500 h^-1, over 0.25 ml of 1 .0 wt.-% Pt 10.5 wt.-% Sn I ZrO₂ catalyst, at a pressure of 1 .0 bar, and at a temperature of 300°C, 47.9% to 100.0% conversion of isoamyl alcohol was achieved. The results of Table 5 are presented graphically in Figure 5.

It is evident from Tables 3-5 that the conversion of Isoamyl alcohol decreased with the increasing GHSV. This could be because of the reduction in contact of the isoamyl alcohol in the feed with the catalyst due to increased velocity. Thus, the conversion was optimized by adjusting the amount of isoamyl alcohol in the feed with the change in GHSV in order to increase the contact between isoamyl alcohol in the feed and the catalyst. The conversion of Isoamyl alcohol is highest when the GHSV of the isoamyl alcohol in the feed is in the range of 320 h^-1 to 2500 h^-1 and the amount of isoamyl alcohol in the feed is in the range of 2.0 vol% to 80.1 vol%.

Table 6: Conversion of Isoamyl alcohol using 0.25 ml of Nickel (60 wt.-% metal content, Total Pore Volume 0.45 cm³/g) as catalyst at various temperatures and Pressure = 1 bar

From Table 6, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 9920 h^-1 over 0.25 ml of Nickel (60 wt.-% metal content, Total Pore Volume 0.45 cm³/g), 43.8% to 93.1% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 200 to 250°C. It is also evident that highest conversion (93.1%) was achieved at a temperature of 250°C. The results of Table 6 are presented graphically in Figure 6.

It is evident from Table 6 and Figure 6, that 0.25 ml of Nickel (60 wt.-% metal content, Total Pore Volume 0.45 cm³/g) as catalyst is actively involved in 43% to 94% conversion of isoamyl alcohol.

Table 7: Conversion of Isoamyl alcohol using 1.0 ml of Nickel (60 wt.-% metal content, Total Pore Volume 0.45 cm³/g) as catalyst at various temperatures and Pressure = 1 bar

From Table 7, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 2500 h^-1 over 1.0 ml of Nickel (60 wt.-% metal content, Total Pore Volume 0.45 cm³/g), 61.8% to 92.4% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature above 200°C. It is also evident that highest conversion (92.4%) was achieved at a temperature of 250°C. The results of Table 7 are presented graphically in Figure 7. From

Experiments 51 to 58, it was observed that the yield of the isobutylene was in the range of 1 mol% to 99 mol%.

It is evident from Table 7 and Figure 7, that 1.0 ml of Nickel (60 wt.-% metal content, Total Pore Volume 0.45 cm³/g) as catalyst is actively involved in 61 % to 93% conversion of isoamyl alcohol.

Table 8: Conversion of Isoamyl alcohol using 1.0 ml of Nickel (56 wt.-% metal content, Total Pore Volume 0.30 cm³/g) as catalyst at various temperatures and Pressure = 1 bar

From Table 8, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 2500 h^-1 over 1.0 ml of Nickel (56 wt.-% metal content, Total Pore Volume 0.3 cm³/g), 54.4% to 98.4% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 200 to 250°C. It is also evident that highest conversion (98.4%) was achieved at a temperature of 250°C. The results of Table 8 are presented graphically in Figure 8. From Experiments 59 to 66, it was observed that the yield of the isobutylene was in the range of 1 mol% to 99 mol%. It is evident from Table 8 and Figure 8, that although 1.0 ml of Ni (56 wt.-% metal content, Total Pore Volume 0.3 cm³/g) as catalyst is actively involved in 54% to 98.5% conversion of isoamyl alcohol.

Table 9: Conversion of Isoamyl alcohol using 1 .0 ml Nickel-Copper as catalyst at various temperatures and Pressure = 1 bar

From Table 9, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 2500 h^-1 over 1.0 ml of Nickel-Copper as catalyst, 39.3% to 100.0% conversion of isoamyl alcohol was achieved at a pressure of 1.0 bar, and at a temperature in the range of 275 to 350°C. It is also evident that highest conversion (100.0%) was achieved at a temperature above 300°C. The results of Table 9 are presented graphically in Figure 9. From Experiments 67 to 70, it was observed that the yield of the isobutylene was in the range of 1 mol% to 99 mol%.

It is evident from Table 9 and Figure 9, that Nickel-Copper as catalyst is actively involved in 39% to 100% conversion of isoamyl alcohol.

Table 10: Conversion of Isoamyl alcohol using 0.25 ml of Nickel-Copper as catalyst at various temperatures and

Pressure = 1 bar

From Table 10, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 10010 h^-1 over 0.25 ml of Nickel-Copper as catalyst, 41 .4% to 85.9% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 200 to 250°C. It is also evident that highest conversion (85.9%) was achieved at temperature 250°C. The results of Table 10 are presented graphically in Figure 10.

It is evident from Table 10 and Figure 10 that 0.25 ml of Nickel-Copper as catalyst is actively involved in 41 % to 86% conversion of isoamyl alcohol. Table 11 : Isoamyl alcohol to isobutylene using 1 .0 ml of Nickel-Copper as catalyst at various temperatures and Pressure = 1 bar

From Table 11 , it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 2500 h^-1 over 1.0 ml of Nickel-Copper as catalyst, 50.5% to 97.5% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 200 to 250°C. It is also evident that highest conversion (97.5%) was achieved at temperature 250°C. The results of Table 11 are presented graphically in Figure 11 . From Experiments 75 to 83, it was observed that the yield of the isobutylene was in the range of 1 mol% to 99 mol%.

It is evident from Table 11 and Figure 11 , that 1.0 ml of Nickel-Copper as catalyst is actively involved in 50% to 98% conversion of isoamyl alcohol. Table 12: Isoamyl alcohol to isobutylene using 1.0 ml Nickel-Copper as catalyst at various temperatures and Pressure = 1 bar

From Table 12, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 2500 h^-1 over 1.0 ml of Nickel-Copper as catalyst, 22.3% to 98.9% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 275 to 350°C. It is also evident that highest conversion (98.9%) was achieved at temperature 300°C. The results of Table 12 are presented graphically in Figure 11. From Experiments 84 to 87, it was observed that the yield of the isobutylene was in the range of 1 mol% to 99 mol%.

It is evident from Table 12 and Figure 12 that Nickel-Copper as catalyst is actively involved in 22% to 99% conversion of isoamyl alcohol.

Table 13: Isoamyl alcohol to isobutylene using 0.5ml of [1.0 wt.-% Pt 1 0.5 wt.-% Sn I ZrO2 + Cu-Zn] as catalyst at various temperatures and Pressure = 1 bar

From Table 13, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 5200 h^-1 over 0.5ml of [1 .0 wt.-% Pt 1 0.5 wt.-% Sn I ZrO2 + Cu-Zn] as catalyst, 35.3% to 95.4% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 200 to 325°C. It is also evident that highest conversion (95.4%) was achieved at temperature 325°C. The results of Table 13 are presented graphically in Figure 13.

It is evident from Table 13 and Figure 13, that 0.5 ml of [1.0 wt.-% Pt / 0.5 wt.-% Sn / ZrO2 + Cu-Zn] as catalyst is actively involved in 35% to 96% conversion of isoamyl alcohol. Table 14: Isoamyl alcohol to isobutylene using 0.5ml [1.0 wt.-% Pt / 0.5 wt.-% Sn / ZrC>2 + Cu-Zn] as catalyst at various temperatures and Pressure = 1 bar

From Table 14, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 5890 h^-1 over 0.5ml of [1 .0 wt.-% Pt 10.5 wt.-% Sn I ZrC>2 + Cu-Zn] as catalyst, 53.0% to 98.7% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 275 to 350°C. It is also evident that highest conversion (98.7%) was achieved at temperature 350°C. The results of Table 14 are presented graphically in Figure 14.

It is evident from Table 14 and Figure 14 that 1.0 wt.-% Pt / 0.5 wt.-% Sn / ZrO2 + Cu-Zn as catalyst is actively involved in 53% to 99% conversion of isoamyl alcohol.

Table 15: Isoamyl alcohol to isobutylene using 1.0 ml Ru as catalyst at various temperatures and Pressure = 1 bar From Table 15, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 2500 h^-1 over 1.0 ml of Ru as catalyst, 16.1% to 73.8% conversion of isoamyl alcohol was achieved at a pressure of 1.0 bar, and at a temperature in the range of 275 to 350°C. It is also evident that highest conversion (93.8%) was achieved at temperature 350°C. The results of Table 15 are presented graphically in Figure 15. It is evident from Table 15 and Figure 15, that 1.0 ml Ru as catalyst is actively involved in 16% to 74% conversion of isoamyl alcohol.

Table 16: Isoamyl alcohol to isobutylene using (0.3 wt.-% Pd) as catalyst at various temperatures and Pressure = 1 bar From Table 16, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 2490 h^-1 over 1.0 ml of 0.3 % Pd as catalyst, 11 .3% to 80.0% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 275 to 350°C. It is also evident that highest conversion (80.0%) was achieved at temperature 300°C. The results of Table 16 are presented graphically in Figure 16.

It is evident from Table 16 and Figure 16 that (0.3 % Pd) as catalyst is actively involved in 11% to 81% conversion of isoamyl alcohol.

Table 17: Isoamyl alcohol to isobutylene using 1.0 ml of (0.5 wt.-% Pd) as catalyst at various temperatures and Pressure = 1 bar From Table 17, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 2500 h^-1 over 1.0 ml of 0.5 % Pd as catalyst, 45.1 % to 97.1 % conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 275 to 350°C. It is also evident that highest conversion (97.1%) was achieved at temperature 300°C. The results of Table 17 are presented graphically in Figurel 7.

It is evident from Table 17 and Figure 17, that (0.5 % Pd) as catalyst is actively involved in 45% to 97% conversion of isoamyl alcohol.

Table 18: Isoamyl alcohol to isobutylene using 1 ml of (Pd) as catalyst at various temperatures and Pressure = 1 bar

From Table 18, it is evident that on passing a gas feed of 9.9 vol% isoamyl alcohol at a GHSV of 2500 h^-1 over 1.0 ml of (Pd) as catalyst, 0.9% to 71.6% conversion of isoamyl alcohol was achieved at a pressure of 1 .0 bar, and at a temperature in the range of 275 to 350°C. The results of Table 18 are presented graphically in Figure 18. From Experiments 118 to 121, it was observed that the yield of the isobutylene was in the range of 1 mol% to 99 mol%.

It is evident from Table 18 and Figure 18 that (Pd) as catalyst is actively involved in 0.9% to 71% conversion of isoamyl alcohol.

Preparation of Prenol, Prenal, Isoprenal and Citral

The preparation of prenol, prenal, isoprenal and citral in conducted as described in the art such as WO 2008/037693.

Claims

1 . A method for the preparation of 3-methyl-3-butene-1-ol (isoprenol), comprising the steps of: a-i) preparing isobutylene by contacting isoamyl alcohol to a catalyst comprising at least one catalytically active metal; and a-ii) reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol.

2. The method according to claims 1, wherein step a-i) comprises subjecting isoamyl alcohol to retro-hydroformation.

3. The method according to claims 1 or 2, wherein the isoamyl alcohol is partly or completely renewably- sourced.

4. The method according to any one of claims 1 to 3, wherein step a-i) comprises contacting the isoamyl alcohol with at least one catalyst comprising at least one catalytically active metal selected from the group consisting of Platinum (Pt), Palladium (Pd), Nickel (Ni), Chromium (Cr), Molybdenum (Mo), Ruthenium (Ru), a combination of two or more thereof, and an alloy comprising two or more thereof and optionally one or more further metals, preferably at a partial pressure in the range of 0.02 to 4.0 bar, and at a temperature in the range of 200 to 450°C.

5. The method according to any one of claims 1 to 4, wherein the at least one catalyst comprises the at least one catalytically active metal in an amount of 0.005 to 10.0 wt.-%, based on the total weight of the catalyst.

6. The method according to any one of claims 1 to 5, wherein:

(b) the at least one catalytically active metal is Nickel (Ni), and the step of contacting is carried out at a temperature in the range from 200°C to 400°C, preferably 200 to 300°C, in particular 200 to 250°C; or

(c) the at least one catalytically active metal is Nickel-Copper (Ni-Cu), and the step of contacting is carried out at a temperature in the range from 250 to 350°C.

7. The method according to any one of claims 1 to 6, wherein contacting in step a-i) involves contacting a feed stream comprising the isoamyl alcohol in a gas phase.

8. The method according to any one of claims 1 to7, comprising introducing the at least one formaldehyde source and isobutylene into a reactor and reacting the formaldehyde source and isobutylene under supercritical conditions.

9. The method according to any one of claims 1 to 8, wherein isoprenol obtained in step a-ii) is purified by subjecting a stream of crude isoprenol containing isoprenol, water and formaldehyde, or an isoprenol containing fraction thereof, to distillation in a low-boiler separation tower operated at a pressure of 2 bara or higher, preferably 2.5 bara or higher, to obtain a distillate stream containing aqueous formaldehyde and a bottoms stream containing isoprenol essentially free of formaldehyde.

10. A method for the preparation of 3-methyl-2-butene-1-ol (prenol), comprising the steps of: a) providing isoprenol according to any one of claims 1 to 9; and b) isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen.

11. A method for the preparation of 3-methyl-2-butenal (prenal) and/or 3-methyl-3-butenal (isoprenal), comprising the steps of: a) providing isoprenol according to any one of claims 1 to 9; b) optionally isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; and c) providing prenal by at least one of c-i) and c-ii): c-i) subjecting isoprenol obtained in step a) to oxidative dehydrogenation so as to obtain prenal and/or isoprenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; and/or c-ii) oxidizing prenol obtained in step b) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase.

12. A method for the preparation of 3,7-dimethyl-octa-2,6-dienal (citral), comprising the steps of: a) providing isoprenol according to any one of claims 1 to 9; b) isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; c) providing prenal by at least one of c-i) and c-ii): c-i) subjecting isoprenol obtained in step a) to oxidative dehydrogenation so as to obtain prenal and/or isoprenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; c-ii) oxidizing prenol obtained in step b) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase; d) condensing prenol obtained in step b) with prenal obtained in step c) to obtain diprenyl acetal of prenal; and e) subjecting diprenyl acetal of prenal obtained in step d) to cleaving conditions to obtain citral via prenyl (3-me- thyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene.

13. The method according to any one of claims 11 or 12, wherein in step c-i) the reactant stream is gaseous and at least one heterogeneous oxidative dehydrogenation catalyst is a silver-containing heterogeneous oxidative dehydrogenation catalyst.

14. The method according to any one of claims 12 to 13, wherein step d) comprises continuously condensing prenol with prenal in the presence of at least one condensation catalyst in a reaction column while continuously withdrawing an acetal fraction comprising diprenyl acetal of prenal from the reaction column.

15. Isoprenol or isoprenol-containing product stream obtainable or obtained according to any one of claims 1 to 9.

16. Citral or citral-containing product stream obtainable or obtained according to any one of claims 12 to 14.