HK1154233B - Method of producing neopentyl glycol - Google Patents

Description

Process for preparing neopentyl glycol

The invention relates to a method for producing neopentyl glycol by hydrogenating hydroxypivalaldehyde in the liquid phase over a nickel-containing catalyst in the presence of more than 15 wt.%, based on the starting mixture, of water.

Polyols or polyhydric alcohols are of considerable economic importance as condensation components for forming polyesters or polyurethanes, synthetic resin coatings, lubricants and plasticizers. In this connection, useful polyols are in particular those obtained by the mixed aldol addition of formaldehyde to isobutyraldehyde or n-butyraldehyde. The aldol addition of formaldehyde to the appropriate butyraldehyde first forms an aldehyde-type intermediate, which must then be reduced to the polyol. An industrially important example of such a polyol obtainable by this process is neopentyl glycol [ NPG, 2, 2-dimethylpropane-1, 3-diol ] obtained by the mixed aldol condensation of formaldehyde with isobutyraldehyde. The aldol addition reaction is carried out in equimolar amounts in the presence of a basic catalyst, such as an alkali metal hydroxide or an aliphatic amine, and first provides an isolatable Hydroxypivalaldehyde (HPA) intermediate. This intermediate can then be converted to neopentyl glycol with excess formaldehyde to form 1 equivalent of formate salt according to the Cannizzaro reaction. In this configuration of the reduction step, formate is thus obtained as a by-product. However, the gas-phase and liquid-phase catalytic hydrogenation of hydroxypivalaldehyde over metal catalysts is also practiced industrially. According to EP 0278106 a1, it has been found that suitable hydrogenation catalysts are nickel catalysts which may comprise other active metals, such as chromium or copper, and additionally comprise an activator. The crude aldol condensation mixture is subsequently catalytically hydrogenated without prior separation into its components or removal of separate components. Since formaldehyde is generally used in the form of an aqueous solution, for example a 37% by weight solution, water is present in the aldol condensation mixture to be hydrogenated. The crude hydrogenation product obtained can then be worked up by distillation according to the teaching of EP 0278106A 1.

Another process for the hydrogenation of hydroxypivalaldehyde to neopentyl glycol in the liquid phase in the presence of a nickel catalyst is known from WO 99/035112 a 1. Mention is made in particular of the detrimental effect of too high an amount of water on the stability of the nickel catalyst during the hydrogenation. Catalyst destruction by the presence of water and a decrease in selectivity at the expense of neopentyl glycol are reported. WO 99/035112 a1 therefore proposes limiting the amount of water to less than 15% by weight when hydrogenating hydroxypivalaldehyde to neopentyl glycol. The hydrogenation temperature of 100 ℃ should also not be exceeded in the known process, since the use of higher hydrogenation temperatures in the presence of nickel catalysts increases the formation of by-products, such as neopentyl glycol monoisobutyrate or neopentyl glycol monohydroxypivalate.

WO 98/17614 a1 also contemplates the hydrogenation of hydroxypivalaldehyde to neopentyl glycol by a liquid phase process in the presence of a nickel catalyst. In this known process, isobutyraldehyde is first reacted with aqueous formaldehyde in the presence of a tertiary alkylamine to produce a crude mixture comprising hydroxypivalaldehyde, which is subsequently extracted with an aliphatic alcohol. The low-boiling components are distilled off from the organic phase and the high-boiling components comprising hydroxypivalaldehyde are hydrogenated. For work-up, the hydrogenation product is extracted with water, which transfers the neopentyl glycol into the aqueous phase. The neopentyl glycol is subsequently separated from the aqueous phase by distillation. An extraction and distillation step connected upstream of the hydrogenation stage reduces the amount of water present in the hydrogenation stage. In the known processes, the hydrogenation stage should be carried out at a temperature in the range from 120 ℃ to 180 ℃.

According to US 6,268,539B 1, the aldol condensation product obtained from the reaction of isobutyraldehyde with aqueous formaldehyde under the catalysis of triethylamine is first distilled. The resulting aqueous distillation residue is subsequently hydrogenated in the presence of Raney nickel comprising molybdenum as promoter at from 70 to 120 ℃. The known liquid phase processes are characterized by the use of specific self-aspirating stirrers which ensure a strong mixing between the liquid and the gaseous phase. Due to this specific reactor configuration, only low hydrogenation pressures of 0.55 to 12.4MPa are required.

The reaction scheme known from EP 0395681B 1 also allows the liquid phase hydrogenation of hydroxypivalaldehyde in the presence of raney nickel using a specific reactor design, wherein hydrogen is passed vigorously through the liquid reaction mixture. This stripping action removes traces of tertiary amines and their compounds used as aldol condensation catalysts, which promote the decomposition of hydroxypivalaldehyde in the hydrogenation stage. According to the teaching of EP 0395681B 1, the use of high pressure is not required. The crude mixture used in the hydrogenation stage contains 10 to 35% by weight of water.

For the liquid-phase hydrogenation of hydroxypivalaldehyde to neopentyl glycol in the presence of a nickel catalyst, special reactor designs are required or only low water content crude hydroxypivalaldehyde is allowed for the hydrogenation in order to convert hydroxypivalaldehyde to neopentyl glycol with high conversion and high selectivity. In some cases, the crude hydroxypivalaldehyde must first be subjected to additional extraction and distillation to reduce the water content in the product to be hydrogenated.

However, it is desirable to hydrogenate the reaction product from the alkylamine-catalyzed aldol addition of isobutyraldehyde to aqueous formaldehyde in the liquid phase in the presence of common commercially available nickel catalysts without a purification step.

It was therefore an object of the present invention to develop a process which is simple to carry out technically and which makes it possible to obtain neopentyl glycol by alkylamine-catalyzed aldol addition with an economically acceptable apparatus.

The invention therefore relates to a continuous process for preparing neopentyl glycol by the addition of isobutyraldehyde and formaldehyde in the presence of tertiary alkylamines as catalysts to give hydroxypivalaldehyde and subsequent hydrogenation, characterized in that the hydrogenation is carried out in a tubular reactor without internals and without stirring devices and in a homogeneous liquid phase which contains from 15 to 27% by weight, based on the organic components in the starting mixture, of aliphatic alcohols as organic solvents or diluents and more than 15 to 25% by weight, based on the total amount used, of water in the presence of a nickel catalyst at a temperature of from 110 to 180 ℃ and a pressure of from 6 to 18 MPa.

It has been found that, surprisingly, hydroxypivalaldehyde can be selectively hydrogenated to neopentyl glycol and the high boilers formed in the reaction of isobutyraldehyde with formaldehyde to give neopentyl glycol can be very selectively cleaved when the water content is in the range from more than 15 to 25% by weight, preferably from 18 to 22% by weight, of the total amount used and a hydrogenation temperature of from 110 to 180 ℃, preferably from 110 to 140 ℃, is established. The high boilers are oxygen-containing compounds, such as esters or cyclic acetals, to which equivalent amounts of neopentyl glycol are chemically bonded. The proportion of mono-and di-isobutyrate of neopentyl glycol and of the disproportionation product neopentyl glycol monohydroxypivalate formed by the Tishchenko reaction of hydroxypivalaldehyde is particularly high in the high boilers. The adjustment of the hydrogenation step in respect of the water content in the starting material and the precise selection of the hydrogenation temperature according to the invention enable an efficient cleavage of the high boilers already present in the starting material to neopentyl glycol and suppress its formation during the hydrogenation reaction, compared with operating modes in which starting materials having a water content of less than 15% by weight are used or in which hydrogenation temperatures of less than 110 ℃ are used.

When the water content in the starting material is too low, the advantageous effect on the reduction of the high boiler content is no longer observed, and when the hydrogenation temperature is too low, only hydroxypivalaldehyde is incompletely hydrogenated. At too high a water content, valuable reactor volume is unnecessarily occupied and not fully utilized. At too high hydrogenation temperatures, decomposition of the tertiary alkylamines, which act as aldol condensation catalysts, again occurs, which leads to difficult removal and thus undesired conversion products.

The aldol addition of isobutyraldehyde and an aqueous formaldehyde solution is carried out in the presence of a tertiary alkylamine as an aldol addition catalyst, for example, in the presence of trimethylamine, triethylamine, tri-n-propylamine, triisopropylamine, methyldiethylamine, methyldiisopropylamine or tributylamine. Particularly suitable catalysts have been found to be triethylamine and tri-n-propylamine.

The aldehyde may be reacted in a molar ratio, but one of the two reactants may also be used in excess. Formaldehyde is used in the form of an aqueous solution; the aldehyde content is generally from 20 to 50% by weight. The reaction is carried out at 20 to 100 ℃; it is suitable to operate at 80 to 95 ℃. Typically, the reaction is carried out at standard pressure, but elevated pressures may also be used. The tertiary alkyl amine used as the aldol addition catalyst is present in the reaction mixture in an amount of 1 to 20, preferably 2 to 12 mol% of isobutyraldehyde.

In addition to water from the aqueous formaldehyde solution and a small proportion of methanol likewise present in the aqueous formaldehyde solution, optionally isobutanol is also added to the reaction mixture as diluent. The addition of isobutanol is not absolutely necessary; however, if isobutanol is added, its content in the reaction mixture is 10 to 20% by weight of the entire reaction mixture. No further solvents and diluents are required.

In practice, the addition reaction is carried out in stirred tanks or in reaction tubes equipped with random packing in order to better mix the reactants. The reaction proceeds exothermically; which can be accelerated by heating.

The crude mixture obtained after the aldol addition is catalytically hydrogenated without prior separation into its constituents or removal of individual components. It is important for the hydrogenation of the hydroxypivalaldehyde-containing reaction mixture according to the invention that the specified water content, the specified hydrogenation temperature and the specified reaction pressure are met. When the amount of water introduced by the use of aqueous formaldehyde solution is not sufficient to ensure the desired water content, water should be added to the crude product prior to use in the hydrogenation reactor.

The hydrogenation is likewise carried out in the presence of an aliphatic alcohol which is miscible with the crude aldol condensation product. Suitable aliphatic alcohols have been found to be straight-chain or branched alcohols having from 1 to 5 carbon atoms, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol or mixtures thereof. The use of isobutanol is particularly suitable since residual amounts of isobutyraldehyde are hydrogenated to isobutanol. If isobutanol has been used as the diluent in the aldol addition stage, then there is already solvent in the hydrogenation stage. There is also a small amount of methanol introduced via the aqueous formaldehyde solution. The proportion of aliphatic alcohols as organic solvent or diluent is from 15 to 27% by weight, preferably from 15 to 18% by weight, of the organic components in the starting mixture. The addition of a diluent or solvent ensures sufficient solubility of hydroxypivalaldehyde in the liquid phase during the hydrogenation stage, also prevents precipitation of hydroxypivalaldehyde and ensures homogeneity of the liquid phase.

The entire starting mixture used for the hydrogenation is homogeneous and therefore contains more than 15 and up to 25% by weight of water, and, to make up to 100% by weight, of an organic component which in turn contains from 15 to 27% by weight of aliphatic alcohols.

The resulting crude hydroxypivalaldehyde-containing mixture was hydrogenated without further work-up and purification steps.

The hydrogenation of the crude hydroxypivalaldehyde is carried out in the liquid phase in the presence of a nickel catalyst at a temperature of 110 to 180 ℃, preferably 110 to 140 ℃. The reaction pressure is from 6 to 18MPa, preferably from 8 to 15 MPa. At lower reaction pressures, satisfactory hydrogenation of hydroxypivalaldehyde is no longer observed.

The nickel as catalytically active metal is generally applied to the support in an amount of from about 5 to 70% by weight, preferably from about 10 to about 65% by weight, in particular from about 20 to 60% by weight, based in each case on the total weight of the catalyst. Suitable catalyst supports are all customary support materials, for example alumina, hydrated alumina in various forms, silica, polysilicic acid (silica gel), including kieselguhr, silica xerogels, magnesium oxide, zinc oxide, zirconium oxide and activated carbon. In addition to the main nickel and support material components, the catalysts may also contain minor amounts of additives, for example to improve their hydrogenation activity and/or their service life and/or their selectivity. Such additives are known; examples include sodium oxide, potassium oxide, magnesium oxide, calcium oxide, barium oxide, zinc oxide, aluminum oxide, zirconium oxide, and chromium oxide. They are generally added to the catalyst in a total proportion of from 0.1 to 50 parts by weight, based on 100 parts by weight of nickel.

However, Raney nickel can also be used in the form of unsupported catalysts.

The hydrogenation is carried out continuously in the liquid phase, for example by trickle flow mode or liquid phase mode over a fixed bed catalyst.

In continuous mode, it has been found to be suitable for the catalyst hourly space velocity V/Vh, expressed as the volume of material passing per unit volume and time of the catalyst, to be in the range from 0.3 to 2.0h^-1Preferably 0.8 to 1.2h^-1。

Higher space velocities over nickel catalysts should be avoided because the hydroxypivalaldehyde starting compound is then no longer completely hydrogenated and increased by-product formation is observed.

The hydrogenation is carried out continuously in the liquid phase over a fixed bed catalyst in a tubular reactor. A tubular reactor is also understood to mean a bundle of a plurality of tubes connected in close parallel. The hydrogenation of hydroxypivalaldehyde is carried out in a tubular reactor without internals and without stirring.

The hydrogenation is preferably carried out with pure hydrogen. However, it is also possible to use mixtures which comprise free hydrogen and additionally comprise constituents which are inert under the hydrogenation conditions.

Pure neopentyl glycol is obtained from the hydrogenation reaction mixture by conventional distillation methods. The removed solvent or diluent can be recycled back to the aldol addition stage and/or hydrogenation stage.

In the hydrogenation process of the present invention, hydroxypivalaldehyde is converted into neopentyl glycol with high conversion and high selectivity. Of note is the low proportion of high boilers after hydrogenation.

The hydrogenation process of the present invention hydrogenates hydroxypivalaldehyde starting compounds very selectively to neopentyl glycol with high conversion and efficiently cleaves high boilers formed in the preceding aldol addition stage, permanently suppressing their formation in the hydrogenation stage. The cleavage of the tertiary alkylamines into volatile nitrogen-containing compounds, which lead to unwanted impurities and can only be removed with difficulty in the subsequent work-up by distillation and are destructive in the further processing of the neopentyl glycol, is also suppressed.

The process of the present invention is described in detail below with reference to some examples, but it is not limited to the embodiments.

Test set-up

The liquid phase hydrogenation was carried out in liquid phase mode over a commercial supported nickel catalyst in a tubular reactor. The catalyst volume was 1.8 l. The crude aldol addition product containing hydroxypivalaldehyde and hydrogen are continuously supplied at the bottom of the tubular reactor. The hydrogenated material is removed via the top of the tube reactor, passed into a high-pressure separator and discharged from the high-pressure separator by means of liquid level control into an ambient pressure reservoir. The hydrogenation temperature, hydrogen pressure and catalyst hourly space velocity were adjusted according to the conditions in the table below. The crude hydroxypivalaldehyde addition product containing hydroxypivalaldehyde used in the hydrogenation test had the following typical composition.

Organic components (data in percent, determined by gas chromatography):

low boiling point substance	0.1
		Isobutanol	2.0
Methanol	0.9
		Middle distillate fraction	7.5
Isobutanol	21.1
		HPA	62.3
NPG	2.2
		TE	2.9
Final fraction of	1.0

Water (W)

18.5% by weight of the total starting mixture

HPA ═ hydroxypivalaldehyde

NPG ═ neopentyl glycol

TE-Tishchenko ester/NPG diisobutyrate ester

In the analytical data given below for the feed stream, the critical content and water content of the aliphatic alcohol acting as diluent are reported. In the analysis of the hydrogenation output, the residual contents of HPA and ester compounds and the NPG content were indicated.

As a comparison of the experimental data shows, as the water content in the starting mixture increases, the proportion of the desired NPG in the hydrogenation output also increases. When a water content below the critical limit of 15% by weight is established (comparative experiment 5) as in experiment 3, the NPG content in the hydrogenation output decreases. This development is also shown in runs 6, 7 and 8 (comparative), in which the NPG content in the hydrogenation output decreases as the water content in the starting material decreases. The pressure chosen in comparative example 9 is no longer sufficient to achieve a satisfactory conversion of HPA.

Claims (18)

1. Continuous process for the preparation of neopentyl glycol by addition of isobutyraldehyde and formaldehyde in the presence of tertiary alkylamines as catalysts to give hydroxypivalaldehyde and subsequent hydrogenation, characterized in that the hydrogenation is carried out in a tubular reactor without internals and without stirring devices and in a homogeneous liquid phase containing from 15 to 27% by weight, based on the organic components in the starting mixture, of aliphatic alcohols as organic solvents or diluents and from more than 15 to 25% by weight, based on the total amount used, of water in the presence of a nickel catalyst at a temperature of from 110 to 180 ℃ and a pressure of from 6 to 18 MPa.

2. The process according to claim 1, characterized in that the hydrogenation is carried out at a temperature of 110 to 140 ℃ and a pressure of 8 to 15 MPa.

3. A process according to claim 1 or 2, characterized in that the homogeneous liquid phase contains from 18 to 22% by weight of water, based on the total amount used.

4. The process according to claim 1 or 2, wherein the tertiary alkylamine used is triethylamine or tri-n-propylamine.

5. A process according to claim 3, wherein the tertiary alkylamine is triethylamine or tri-n-propylamine.

6. A process according to claim 1 or 2, characterised in that the aliphatic alcohol used comprises a straight-chain or branched alcohol having 1 to 5 carbon atoms.

7. A process as claimed in claim 3, characterized in that the aliphatic alcohols used comprise straight-chain or branched alcohols having from 1 to 5 carbon atoms.

8. A process as claimed in claim 4, characterized in that the aliphatic alcohols used comprise straight-chain or branched alcohols having from 1 to 5 carbon atoms.

9. The process according to claim 6, wherein methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol or mixtures thereof are used.

10. The method of claim 1 or 2, wherein the nickel catalyst comprises a support material.

11. The method of claim 3, wherein the nickel catalyst comprises a support material.

12. The method of claim 4, wherein the nickel catalyst comprises a support material.

13. The method of claim 6, wherein the nickel catalyst comprises a support material.

14. The process according to claim 1 or 2, characterized in that the nickel catalyst comprises sodium oxide, potassium oxide, magnesium oxide, calcium oxide, barium oxide, zinc oxide, aluminum oxide, zirconium oxide, chromium oxide or mixtures thereof as an additive.

15. The process according to claim 3, characterized in that the nickel catalyst comprises as additive an oxide of sodium, potassium, magnesium, calcium, barium, zinc, aluminium, zirconium, chromium or mixtures thereof.

16. The process according to claim 4, characterized in that the nickel catalyst comprises sodium oxide, potassium oxide, magnesium oxide, calcium oxide, barium oxide, zinc oxide, aluminum oxide, zirconium oxide, chromium oxide or mixtures thereof as an additive.

17. The process according to claim 6, characterized in that the nickel catalyst comprises sodium oxide, potassium oxide, magnesium oxide, calcium oxide, barium oxide, zinc oxide, aluminum oxide, zirconium oxide, chromium oxide or mixtures thereof as an additive.

18. The process according to claim 10, characterized in that the nickel catalyst comprises as additive an oxide of sodium, potassium, magnesium, calcium, barium, zinc, aluminium, zirconium, chromium or mixtures thereof.