WO2008070365A2 - Oxidation reaction for producing aromatic carboxylic acids - Google Patents

Abstract

A method of minimising or preventing catalyst loss and/or the precipitation of metal oxide in a catalytic synthetic oxidation reaction in an aqueous solvent comprising water under supercritical or near-supercritical conditions wherein during the oxidation reaction an oxidant is present and a metal salt is present as a catalyst, said method comprising the step of adding an acidic component comprising one or more acid(s) into the reaction such that contact of at least part of said metal salt with said oxidant is in the presence of said acidic component, particularly wherein the oxidation reaction is a process for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst, within a reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions.

Description

OXIDATION REACTION FOR PRODUCING AROMATIC CARBOXYLIC ACIDS

Field of The Invention

This invention relates to the field of metal salt-catalysed synthetic oxidation processes in supercritical water, particularly the oxidation of alkyl-suhstituted aromatic hydrocarbons to the corresponding aromatic carboxylic acid. The invention is particularly concerned with catalyst stability in these systems, and particularly the maintenance of catalyst activity and/or efficiency, and the avoidance of fouling of the reactor.

Background of The Invention

The dielectric constant of water decreases dramatically from a room temperature value of around 80C²/Nm² to a value of 5C²/Nm² as it approaches its critical point (374⁰C and 220.9bara), allowing it to solubilise organic molecules. As a consequence, water then behaves like an organic solvent to the extent that hydrocarbons, e.g. toluene, are completely miscible with the water under supercritical conditions or near supercritical conditions. Terephthalic acid, for instance, is virtually insoluble in water below about 200⁰C. Dioxygen is also highly soluble in sub- and super-critical water.

Holliday R.L. et al (J. Supercritical Fluids 12, 1998, 255-260) describe a batch process carried out in sealed autoclaves for the synthesis of, inter alia, aromatic carboxylic acids from alkyl aromatics in a reaction medium of sub-critical water using molecular oxygen as the oxidant. The use of supercritical water as a medium for the production of aromatic carboxylic acids in a continuous flow reactor nevertheless presented significant problems.

WO-02/06201-A discloses a process for the production of an aromatic carboxylic acid comprising contacting in the presence of a catalyst, within a continuous flow reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions close to the supercritical point such that said one or more precursors, oxidant and aqueous solvent constitute a substantially single homogeneous phase in the reaction zone. In the process described in WO-02/06201-Λ the contact of at least part of the precursor with the oxidant is contemporaneous with the contact of the catalyst with at least part of the oxidant.

The continuous process of WO-02/06201-A is carried out with the reactants and the solvent forming a substantially single homogeneous fluid phase in which the components in question are mixed at a molecular level. The concentration of dioxygen in water increases markedly as the supercritical point is approached and exceeded, which assists in improving the reaction kinetics. The reaction kinetics are further enhanced by the high temperatures prevailing when the water solvent is under supercritical or near supercritical conditions. The combination of high temperature, high concentration and homogeneity mean that the reaction to convert the precursor(s) to aromatic carboxylic acid can take place extremely rapidly compared with the residence times employed in the production of aromatic carboxylic acids such as terephthalic acid by conventional techniques using a crystallising three phase oxidation reactor. Under these conditions, the intermediate aldehyde (e.g. 4-carboxybenzaldehyde (4-CBA) in the case of terephthalic acid) is readily oxidised to the desired aromatic carboxylic acid which is soluble in the supercritical or near supercritical fluid thereby allowing a significant reduction in contamination of the recovered aromatic carboxylic acid product with the aldehyde intermediate. The process conditions of WO-02/06201-A substantially reduce or avoid autocatalytic destructive reaction between the precursor and the oxidant and consumption of the catalyst. The continuous process involves short residence times and exhibits high yield and good selectivity of product formation.

The preferred catalyst in the supercritical oxidation for the production of aromatic carboxylic acids comprises manganese salts (particularly MnBr₂), but it has been observed that manganese salts are oxidised irreversibly to manganese oxide(s) (including MnO₂, Mn₂O^ and MnO(OH)₂) during the strong oxidative conditions of the reaction. The manganese oxide(s) forms an insoluble precipitate which adheres to internal walls following the initial contact between the catalyst and the oxidant (typically molecular oxygen), resulting in the progressive fouling of the reactor and/or blockages in the pressure let-down equipment. Metal salts are known to oxidise in aqueous supercritical oxidation processes, and it is this feature which is exploited in the production of nano-particles, as described by, for instance, Viswanathan et al. (J. Supercritical Fluids, 2003, 27(2), ρpl87- 193). It is possible under certain conditions to solubilise metal oxides in isolation by reducing them to a more soluble lower oxidation state, and US-4645650 teaches that such a reduction may be effected at atmospheric pressure and relatively low temperature using coal or other reducing carbonaceous material in an amount of at least 50% by mass of the metal oxide and in the presence of mineral acids. However, it is also known that some metal oxides are stable in supercritical water oxidation conditions, which has been exploited by their use as catalysts in extremely oxidative conditions to enhance the total oxidation of certain organic materials, as described for instance by Kranjnc et al. (Applied Catalysis, B: Environmental (1997), 13(2), 93-103).

In the supercritical oxidation reaction for the production of aromatic carboxylic acids, the precipitation of manganese oxide(s) reduces or prevents the opportunity to recycle catalyst for effective operation of the process, and this loss of catalyst is economically undesirable. In addition, the precipitation reduces or prevents flow in a tubular reactor, and the channels in the apparatus need to be cleaned or unblocked in order to continue operation of the reactor, which is uneconomic and inefficient. Whilst the mixing configuration described in WO-02/06201-A minimises catalyst oxidation compared to other configurations, it is desired to make further improvements.

Summary of The Invention

It is an object of this invention to reduce or avoid one or more of the above- mentioned problems. In particular, it is an object of this invention to reduce the amount of precipitation of metal oxides and/or reduce loss of catalyst during a supercritical (or near- supercritical) water synthetic oxidation process in which a metal salt is used as the catalyst. Thus, it is an object of this invention to improve catalyst stability in these systems, particularly to maintain catalyst activity and/or efficiency, and to avoid fouling of the reactor. It is a further object to provide an improved process, particularly a continuous process, for the production of an aromatic carboxylic acid via catalytic oxidation of a precursor in supercritical water, wherein the amount of precipitation of metal oxides is reduced and/or loss of catalyst is reduced, particularly such a process having good selectivity for, and high yield of, the aromatic carboxylic acid.

According to a first aspect of the present invention there is provided the use, in a catalytic synthetic oxidation reaction in an aqueous solvent comprising water under supercritical or near-supercritical conditions wherein a metal salt is present as a catalyst during the oxidation reaction, of an acidic component comprising one or more acid(s) for the purpose of minimising or preventing catalyst loss and/or precipitation of metal oxide during said reaction, wherein said reaction comprises an oxidant and said acidic component is added into the reaction such that contact of at least part of said metal salt with said oxidant is in the presence of said acidic component.

According to a second aspect of the invention, there is provided a method of minimising or preventing catalyst loss and/or the precipitation of metal oxide in a catalytic synthetic oxidation reaction in an aqueous solvent comprising water under supercritical or near-supercritical conditions wherein during the oxidation reaction an oxidant is present and a metal salt is present as a catalyst, said method comprising the step of adding an acidic component comprising one or more acid(s) into the reaction such that contact of at least part of said metal salt with said oxidant is in the presence of said acidic component.

The metal salt referred to herein is a catalyst for the oxidation process which oxidises an oxidisable organic substrate or precursor to one or more (typically one) target reaction product(s). The metal salt typically comprises a transition metal.

According to a third aspect of the present invention there is provided the use of an acidic component comprising one or more acid(s) for the purpose of minimising or preventing catalyst loss and/or precipitation of metal oxide in a process for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst comprising a metal salt (preferably a transition metal salt), within a reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, typically such that said one or more precursors, oxidant and aqueous solvent constitute a single homogeneous phase in the reaction zone, wherein said acidic component is added into the reaction such that contact of at least part of said catalyst with said oxidant is in the presence of said acidic component.

According to a fourth aspect of the invention, there is provided a method of minimising or preventing catalyst loss and/or the precipitation of metal oxide in a process for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst comprising a metal salt (preferably a transition metal salt), within a reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursors) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, typically such that said one or more precursors, oxidant and aqueous solvent constitute a single homogeneous phase in the reaction zone, said method comprising the step of adding an acidic component comprising one or more acid(s) into the oxidation reaction such that contact of at least part of said catalyst with said oxidant is in the presence of said acidic component.

According to a fifth aspect of the present invention there is provided an oxidation process for the production of one or more target organic compound(s) from one or more oxidisable organic precursor(s) thereof, said process comprising contacting in the presence of a catalyst comprising a metal salt (preferably a transition metal salt), within a reactor, one or more of said prccursor(s), such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, typically such that said one or more precursors, oxidant and aqueous solvent constitute a single homogeneous phase in the reaction zone, wherein

(i) an acidic component comprising one or more acid(s) is added to the reaction mixture; and (ii) contact of at least part of said catalyst with said oxidant is in the presence of said acidic component.

According to a sixth aspect of the present invention there is provided an oxidation process for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst comprising a metal salt (preferably a transition metal salt), within a reactor, one or more precursors) of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, typically such that said one or more precursors, oxidant and aqueous solvent constitute a single homogeneous phase in the reaction zone, wherein

(i) an acidic component comprising one or more acid(s) is added to the reaction mixture; and

(ii) contact of at least part of said catalyst with said oxidant is in the presence of said acidic component.

Preferably, the addition of the acidic component comprising one or more acid(s) should be effected such that the acid(s) is/are present in the preferred single homogeneous phase referred to herein and at any location where the metal salt is in contact with the oxidant of the oxidation process. Thus, contact of at least part, and preferably substantially all, of said metal salt with the oxidant is effected in the presence of the acidic component, and preferably subsequent to the contact of said metal salt with said acidic component (i.e. metal salt and acidic component are mixed prior to contact with the oxidant such that said the acidic component is already present at the point of first contact between catalyst and oxidant). The acidic component is present in the preferred single homogeneous phase in the reaction zone, as described herein.

Brief Description of The Drawings

Figure I is a schematic flowsheet illustrating the basic arrangement described for Embodiment I above. Dashed lines represent alternative pathways for the acid component.

Figures 2A and 2B are schematic flowsheets illustrating the basic arrangement described for Embodiment II above. In Figure 2B, the oxidant is introduced in a progressive manner along the reaction zone at multiple injection points. Dashed lines represent alternative pathways for the acid component. Figure 3 is a schematic flowsheet illustrating an arrangement (such as Embodiment III above) where contact of the precursor and oxidant is non-contemporaneous with contact of catalyst and oxidant.

Figure 4 is a schematic flowsheet illustrating in more detail an arrangement wherein the precursor is added to a premixcd stream of oxygen and water (i.e. an arrangement according to the process illustrated in Figure 1);

Figures 5A, 5B, 5C, 5D and 6 illustrate various premixer configurations that may be employed to effect mixing of at least one of the reactants with the aqueous solvent;

Figure 7 is a schematic view illustrating multiple stage injection of oxidant;

Figures 8 and 9 are schematic flowsheets illustrating mother liquor recycle and heat removal from a reactor for use in oxidising a terephthalic acid precursor in supercritical or near supercritical water, substantially pure oxygen being used as the oxidant in the embodiment of Figure 8 and air being the oxidant in the embodiment of Figure 9.

Figure 10 is a detailed illustration of the apparatus used for the laboratory-scale experiments.

Figure 11 is a graphical illustration of the two distinct effects of oxidation promotion and catalyst recovery which the present inventors have observed operating in this system.

Detailed Description_of Ηie Invention

The use of an acid in an oxidation process in aqueous solvent comprising water under supercritical or near-supercritical conditions according to the present invention minimises or avoids the problems associated with the oxidative side-reaction of the metal salt to metal oxide in this oxidation process. While the inventors do not wish to bound by theory, it is believed that the presence of the acid minimises or prevents the oxidation of the metal salt to metal oxide under these supercritical or near-supercritical conditions. Thus, in accordance with the invention, the precipitation of the metal oxide from the metal salt is avoided because the formation of the metal oxide and/or the destruction of said metal salt is minimised or prevented, and therefore the stability of the metal salt is improved, thereby retaining catalyst activity and efficiency, reducing reactor fouling, and improving process economy and efficiency. It is surprising that the acid is capable of achieving such a result, because the metal salt is functional in the supercritical oxidation process and reaction pathway(s) associated therewith and these pathway(s) are in competition not only with reaction pathway(s) associated with the undesirable oxidative conversion of the metal salt to metal oxide, but also with reaction pathway(s) associated with the interaction of the acid with the metal salt. Thus, it is unexpected that the presence of acid minimises the undesirable production of metal oxide and solves the problem addressed by the present invention.

When compared with the continuous process of WO-02/06201 -A, the process according to the present invention provides an unexpected improvement in reducing the oxidative side-reaction of the metal salt to metal oxide in this oxidation process, and a greater proportion of the catalyst is retained in, and/or recoverable from, the system. While the arrangement described in WO-02/06201-A itself represented an unexpected improvement in this regard over other potential arrangements, loss of catalyst still occurs. The present invention shows that the addition of acid can improve catalyst stability and/or recovery. This effect is entirely distinct from any observed increase in yield and/or selectivity of the target molecule as a result of the optional addition of oxidation promoters.

As used herein, the term "acid" means any proton-donating species that has a pKa of less than that of water at ambient temperatures and pressures. In accordance with the Bronsted theory of acidity (see, for instance, Michael B. Smith, Jerry March; March's Advanced Organic Chemistry, 5th Edition, Chapter 8), the acidity of a proton-donor is measured on the pK_a scale that runs from approximately -12 for veiy strong acids, through 15.74 for water and to approximately 50 for veiy weak acids at ambient temperatures and pressures. The acid(s) chosen for the acidic component will depend on the metal of the catalyst system, and in particular on the solubility of the metal salt formed by the acid anion and the metal cation. Thus, sulphuric and carbonic acids are least preferred since they tend to precipitate sulphates and carbonates respectively. The acid chosen will also depend on the stability of the acid at supercritical or near-supercritical conditions, and nitric acid is therefore suitably avoided. Preferably, the acid is substantially oxidation- stable at supercritical or near-supercritical conditions. Preferably, the acid is selected from mineral acids (preferably HX) and preferably from hydrogen halides (typically HCl and HBr), and in a preferred embodiment the acidic component comprise hydrogen bromide (HBr). In an alternative embodiment, the acidic component does not comprise HBr.

The anion of the acid may be the same as, or different from, the anion of the metal salt.

Preferably, said acidic component is present in the reaction zone in an amount such that the molar ratio of [H⁺]: [M], wherein [H⁴] is the molar amount of acid derived from said acidic component and wherein M is the metal of said metal salt present during the oxidation reaction, is at least 0.05: 1 , preferably at least 0.1: 1 , preferably at least 0.2: 1, preferably at least 0.3: 1, preferably at least 0.5: 1, preferably at least 0.75: 1 , preferably at least 1.0: 1 , preferably at least 1.5: 1, preferably at least 2.0: 1, preferably at least 2.05: 1, preferably at least 2.1 : 1 , preferably at least 2.2: 1 , preferably at least 2.3: 1, and typically no more than about 12.0: 1, more typically no more than about 7.0: 1, more typically no more than about 6.0: 1, more typically no more than about 5.0: 1, more typically no more than 4.5: 1, more typically no more than about 4.0: 1, more typically no more than about 3.5:1, and more typically no more than about 3.0: 1. In a preferred embodiment, the [H ] to [M] ratio is in the range of from 0.05: 1 to 5.0: 1, preferably 0.1 : 1 to 3.0: 1 , and preferably 0.2: 1 to 3.0: 1.

Hydrogen halide may be formed in situ by the addition of a proton source and a halide source to the reaction mixture, Thus, where the acidic component added to the reaction does not comprise halide, then in one embodiment, a source of halide ions may be added to the reaction mixture in addition to the acidic component. Preferably, however, the acidic component is also a source of halide. While the addition of HBr improves catalyst recovery, it also causes corrosion in the system, and so a balance must be reached between these two requirements. The amount of mineral acid (preferably hydrogen halide) added is preferably such that the molar ratio ([X]: [M]) of the amount of total anion X (preferably halide) to the metal ion (M) of the catalyst is at least 1.0: 1, preferably at least 1.5: 1, preferably at least

2.0: 1, preferably at least 2.05: 1, preferably at least 2.1 : 1 , preferably at least 2.2: 1, preferably at least 2.3:1, and typically no more than about 12.0: 1 , more typically no more than about 7.0: 1, more typically no more than about 6.0: 1, more typically no more than about 5.0: 1 , more typically no more than 4.5: 1, more typically no more than about 4.0: 1, more typically no more than about 3.5: 1, and more typically no more than about 3.0: 1. In a preferred embodiment, the [X]: [M] ratio is in the range of from 1.5: 1 to 5.0: 1, preferably 2.0: 1 to 3.5: 1, and preferably 2.2: 1 to 3.0: 1.

The inventors of the present invention have observed two distinct effects operating in the addition of hydrogen halide to metal catalyst in super-critical water oxidation reactions. The first effect is an "oxidation promoter" effect, in which the addition of hydrogen halide has the effect of improving yield and selectivity of the target compound(s), but this effect becomes saturated after a relatively small amount of added hydrogen halide, i.e. the effect reaches a plateau, and yields and selectivities improve no further with increasing amounts of hydrogen halide. The oxidation promoter effect relies on the presence of halide [X^"] rather than acid, i.e. [H ]. The second effect is a "catalyst recovery" effect, and it is this effect to which the present invention is directed. The inventors have found that the addition of hydrogen halide has the effect of improving the recovery of catalyst by reducing the amount of precipitation of metal oxide and this effect increases in parallel to the oxidation promoter effect up to the saturation point of the oxidation promoter effect. However, once the oxidation promoter effect has been saturated, the addition of further hydrogen halide continues to improve the recovery of catalyst, until this effect also becomes saturated, although after a relatively larger amount of hydrogen halide. The catalyst recovery effect is not observed unless acid, i.e. [H⁺], is added into the reaction. In one embodiment therefore, the amount of hydrogen halide added should be sufficient to achieve at least 60%, preferably at least 70%, preferably at least 80%, preferably at least 90% of the catalyst recovery effect at saturation, and in one embodiment the amount of hydrogen halide added should be sufficient to achieve saturation of the catalyst recovery effect. In a preferred embodiment, the metal catalyst comprises manganese, and the acidic component is preferably hydrogen halide (HX) and preferably HBr.

In Embodiment A, which is preferred, the metal catalyst comprises manganese bromide MnBr₂, and the amount of hydrogen halide added is preferably such that the molar ratio [X]: [Mn] of the amount of total halide (X) to manganese (Mn) is greater than 2.0: 1 and typically no more than about 12.0: 1. Preferably, [X]: [Mn] is at least 2.05: 1, preferably at least 2.1 : 1, preferably at least 2.2: 1, preferably at least 2.3: 1 , and typically no more than about 12.0: 1, more typically no more than about 7.0: 1, more typically no more than about 6.0:1, more typically no more than about 5.0: 1, more typically no more than 4.5: 1, more typically no more than about 4.0: 1, more typically no more than about 3.5: 1, and more typically no more than about 3.0: 1. The present inventors have found that the oxidation promoter effect is already saturated at a Br:Mn molar ratio of 2:1 (i.e. the ratio which pertains using MnBr₂ as the catalyst in the absence of HBr) and that the catalyst recovery effect is not observed until acid is also added. In Embodiment A, preferably [H]: [Mn] is at least 0.05: 1, preferably at least 0.1: 1, preferably at least 0.2: 1 , preferably at least 0.3: 1, and typically no more than about 10.0: 1, more typically no more than about 5.0: 1, more typically no more than about 4.0:1, more typically no more than about 3.0: 1, more typically no more than 2.5: 1, more typically no more than about 2.0: 1, more typically no more than about 1.5: 1 , and more typically no more than about 1.0: 1. Preferably, [H]: [M] is in the range of from 0.05: 1 to 3.0: 1 , preferably 0.1 : 1 to 1.0: 1 , and preferably 0.2: 1 to 1.0: 1.

In an alternative embodiment, Embodiment B, the metal catalyst comprises manganese acetate Mn(OAc)₂ which, in conventional systems (i.e. non-SCW systems), is used in combination with HBr to form the catalyst MnBr₂ in situ. In this embodiment, the generic and preferred molar ratios [X]: [Mn] of the amount of total halide (X) to manganese (Mn) are as described above for the general metal M, i.e. at least 1.0: 1 and typically no more than about 12.0: 1. The present inventors have found that the oxidation promoter effect becomes saturated at a Br:Mn molar ratio around 1.5: 1, and that the catalyst recoveiy effect does not become saturated until a BnMn molar ratio of greater than 2: 1 is reached, as is evident from Figure 1 1. In Embodiment B, preferably [H]: [Mn] is at least 0.5: 1, preferably at least 0.75: 1, preferably at least 1.0: 1, preferably at least 1.5:1, preferably at least 2.05: 1, preferably at least 2.1 :1, preferably at least 2.2: 1, preferably at least 2.3: 1, and typically no more than about 12.0:1, more typically no more than about 7,0: 1 , more typically no more than about 6.0: 1, more typically no more than about 5.0: 1, more typically no more than 4.5: 1 , more typically no more than about 4.0: 1 , more typically no more than about 3.5: 1, and more typically no more than about 3.0: 1. Preferably, [H]: [M] is in the range of from 2.05: 1 to 5.0: 1, preferably 2.1 : 1 to 3.0: 1, and preferably 2.2: 1 to 3.0: 1.

The invention is described herein primarily with reference to the preferred embodiment of the catalytic oxidation process for the production of an aromatic carboxylic acid. It will be appreciated, however, that the principles described herein are applicable to other synthetic oxidation reactions of organic compound(s) in aqueous solvent comprising water under supercritical or near-supercritical conditions. Other such processes include other metal-salt-catalysed synthetic oxidations; synthetic oxidation of alkanes such as butane and ethane; and synthetic oxidation of other organic materials such as alcohols, ketones, alkenes, alkynes and ethers, etc in which metal catalysis is required.

By the term "synthetic oxidation reaction" we mean the production of one or more target compound(s) from one or more oxidisable precursor(s) thereof by partial oxidation of said precursor(s). By the term "partial oxidation" we mean an oxidation reaction which consists of a degree of oxidation (or uptake of oxygen) less than that required for total oxidation of said precursor(s) to carbon oxides; such reactions are associated with controlled oxidant/precursor stoichiometry, selective reaction for the synthesis of a small number of compounds in high yield, and retention of chemical structure in the aromatic group of the precursor. By the term "total oxidation" we mean oxidation of a compound to carbon oxides (typically carbon dioxide), i.e. destructive oxidation.

The pressure and temperature of the process are selected to secure supercritical or near-supercritical conditions. In one embodiment, the term "near supercritical conditions" means that the solvent is at a temperature which is not less than 100⁰C below, preferably not less than 5O⁰C below, preferably not less than 35⁰C below, more preferably not less than 2O⁰C below the critical temperature of water at 220.9 bara. Thus, operating temperatures are typically in the range of 300 to 48O⁰C, more preferably 330 to 450 ⁰C, typically from a lower limit of about 350 to 370°C to an upper limit of about 370 to about 42O⁰C. Operating pressures are typically in the range from about 40 to 350 bara, preferably 60 to 300 bara, more preferably 220 to 280 bara, and in one embodiment 250 to 270 bara. In an alternative embodiment, the operating pressures are in the range of 230 to 250 bara.

In a preferred embodiment, the term "near supercritical conditions" means that the reactants and the solvent constitute a single homogeneous phase. In practice, this can be achieved under conditions below the critical temperature for water. By the term "single homogeneous phase" as used herein, we mean that at least 80%, typically at least 90%, typically at least 95%, more typically at least 98%, and most typically effectively all, by weight, of each of the precursor, oxidant, aqueous solvent, acidic component, catalyst and reaction product(s) are in the same single homogeneous phase in the reaction zone.

By the term "aromatic carboxylic acid" as used herein, we mean an aromatic compound in which a carboxylic acid group (-CQ₂H) is attached directly to an aromatic group (Ar). The aromatic carboxylic acid may contain one or more carboxylic acid groups attached directly to an aromatic group, and the present invention is particularly directed to aromatic carboxylic acids which contain at least 2, and particularly only 2, carboxylic acid groups (CO₂H) attached directly to an aromatic group. The aromatic group (Ar) may comprise a single aromatic ring or may comprise two or more aromatic rings, for instance two or more fused aromatic rings, the or each ring typically having 5, 6, 7 or 8 ring atoms, more typically 6 ring atoms. Typically, the aromatic group is mono-cyclic. The aromatic group may be a carbocyclic aromatic group or it may comprise one or more heterocyclic aromatic rings (for instance those containing 1, 2 or 3 heteroatoms (typically only 1 heteroatom) selected from N, O and S, typically N). In one embodiment, the aromatic group is phenyl. In an alternative embodiment, the aromatic group is pyridyl. Typical aromatic carboxylic acids which may be synthesised using the present invention include terephthalic acid, isophthalic acid, phthalic acid, trimellitic acid, naphthalene dicarboxylic acid and nicotinic acid.

By the term "precursor of an aromatic carboxylic acid" as used herein we mean an aromatic compound which is oxidisable to the target aromatic carboxylic acid with an oxidant under supercritical conditions or near-supercritical conditions. The precursor is selected from aromatic compounds having at least one substituent which is attached to the aromatic group (Ar; as defined hereinabove) and which is oxidisable to a carboxylic acid moiety. Suitable substituents are typically selected from alkyl, alcohol, alkoxyalkyl and aldehyde groups, particularly from alkyl, alcohol and alkoxyalkyl groups, and preferably from alkyl groups. An alkyl group is particularly selected from Ci_-4 alkyl groups, preferably methyl. An alcohol group is particularly selected from Ci_-4 alcohol groups. An alkoxyalkyl group is particularly selected from (Ci_-4 alkoxy)Ci_₄ alkyl groups. An aldehyde group is particularly selected from Ci_-4 aldehyde groups. Where two or more substituent groups are present, these may be the same or different, and in a preferred embodiment are the same. For instance, a precursor of tcrcphthalic acid may be selected from para-xylene, 4-tolualdehyde and 4-toluic acid, para-xylene being preferred. A precursor of nicotinic acid is, for instance, 3-methylpyridine.

For the avoidance of doubt, reference herein to the term "transition metal" is to the conventional definition of a metal which can accept or donate electrons into its d- or f- orbitals and exhibit a plurality of oxidation states, and includes the lanthanide and actinide series of transition metals.

A reactor suitable for the performance of the present invention is preferably a continuous flow reactor. By "continuous flow reactor" as used herein we mean a reactor in which reactants are introduced and mixed and products withdrawn simultaneously in a continuous manner, as opposed to a batch-type reactor. For example, the reactor may be a tubular flow reactor (with either turbulent or laminar flow) or a continuous stirred-tank reactor (CSTR) although the various aspects of the invention defined herein are not limited to these particular types of continuous flow reactor. The invention defined herein is described primarily in terms of a continuous flow reactor because this is the most likely commercial and industrial embodiment, but the invention may also be conducted using a batch-type reactor.

By carrying out the process in a continuous flow reactor, the residence time for the reaction can be made compatible with the attainment of conversion of the precursor(s) to the desired aromatic carboxylic acid without significant production of degradation products. The residence time of the reaction medium within the reaction zone is generally no more than 10 minutes, preferably no more than 8 minutes, preferably no more than 6 minutes, preferably no more than 5 minutes, preferably no more than 3 minutes, preferably no more than 2 minutes, and preferably no more than 1 minute.

The residence time may be controlled so that the precursor is converted to the aromatic carboxylic acid with high efficiency such that the aromatic carboxylic acid precipitated from the reaction medium following completion of the reaction contains no more than about 5000 ppm, preferably no more than about 3000 ppm, more preferably no more than about 1500 ppm, more preferably no more than about 1000 ppm and most preferably no more than about 500 ppm aldehyde produced as an intermediate in the course of the reaction (e.g. 4-CBA in the case of terephthalic acid production). Typically, there will be at least some aldehyde present after the reaction, and usually at least 5ppm.

The oxidant in the process of the invention is preferably molecular oxygen, e.g. air or oxygen enriched air, but preferably comprises gas containing oxygen as the major constituent thereof, more preferably pure oxygen, or oxygen dissolved in liquid. The use of air is not favoured, although not excluded from the scope of the invention, since large compression costs would arise and off-gas handling equipment would need to cope with large amounts of off-gas owing to the high nitrogen content of air. Pure oxygen or oxygen enriched gas on the other hand permits use of a smaller compressor and smaller off-gas treatment equipment. The use of dioxygen as the oxidant in the process of the invention is particularly advantageous since it is highly soluble in water under supercritical or near supercritical conditions. Thus, at a certain point, the oxygen/water system will become a single homogeneous phase.

Instead of molecular oxygen, the oxidant may comprise atomic oxygen derived from a compound, e.g. a liquid phase compound at room temperature, containing one or more oxygen atoms per molecule. One such compound for example is hydrogen peroxide, which acts as a source of oxygen by reaction or decomposition.

The oxidation reaction, particularly for the production of aromatic carboxylic acids, according to the present invention is carried out in the presence of a homogeneous oxidation catalyst soluble in the reaction medium comprising solvent and precursor(s). The catalyst is typically present in the single homogeneous phase in the reaction zone, as described herein. The catalyst typically comprises one or more heavy metal compounds, e.g. cobalt and/or manganese compounds, and preferably comprises manganese compounds. For instance, the catalyst may take any of the forms that have been used in the liquid phase oxidation of aromatic carboxylic acid precursors such as terephthalic acid precursor(s) in aliphatic carboxylic acid solvent, e.g. bromides, bromoalkanoates, alkanoates (usually C] - C₄ alkanoates such as acetates) or benzoates (or other aromatic acid salts) of cobalt and/or manganese. Compounds of other heavy metals such as vanadium, chromium, iron, zirconium, hafnium molybdenum, a lanthanide such as cerium, and/or nickel may be used instead of cobalt and/or manganese. Advantageously, the catalyst system will include manganese bromide (MnBr₂). The catalyst can be added pre- prepared or it can be formed within the system by adding reagents which subsequently combine to form the catalyst. For instance, in the case of an MnBr₂ catalyst, it is possible either to introduce MnBr₂ itself into the system, or to introduce reagents such as manganese acetate and HBr into the system, which combine to form MnBr₂ under the reaction conditions.

The reactor system suitable for performing the process of the present invention may be generally configured as described below.

The oxidation reaction is initiated by heating and pressurising the reactants followed by bringing the heated and pressurised reactants together in a reaction zone. This may be effected in a number of ways with one or both of the reactants being admixed with the aqueous solvent prior to or after attainment of supercritical or near supercritical conditions, such admixture being effected in such a way as to maintain the reactants isolated from one another until brought together in the reaction zone.

In the continuous process described herein, contact of at least part of said catalyst with said oxidant is effected in the presence of said acidic component. Preferably, contact of substantially all of said catalyst with said oxidant is in the presence of the acidic component. In the continuous process for the production of carboxylic acids described herein, the reactor system is preferably configured such that the contact between the oxidant and at least part, and preferably substantially all, of the precursor is made at the same point in the reactor system as, and contemporaneous with, the contact between the catalyst and at least part, and preferably substantially all, of the oxidant, and such arrangements are shown in Figures 1 , 2 A and 2B. However, other configurations for the production of carboxylic acids, in which the contact of at least part of said precursor with said oxidant is not contemporaneous with contact of said catalyst with at least part of said oxidant, are not excluded, and such arrangements are shown in Figure 3.

In Embodiment I, the oxidant is mixed with the aqueous solvent after the latter has been heated and pressurised to secure the supercritical or near supercritical state, with suitable pressurisation and, if desired, heating, of the oxidant prior to mixing with the aqueous solvent. The precursor is subjected to pressurisation and, if desired, heating. The catalyst-comprising component is subjected to pressurisation and, if desired, heating. The acid component may be subjected to pressurisation and, if desired, heating. The separate streams comprising precursor, catalyst, acid component and the oxidant/solvent mixture may then be contacted simultaneously. In one arrangement, the acid is pre-mixed with the catalyst. A schematic flow diagram representing Embodiment I is presented in Figure 1.

In Embodiment II of the invention, the precursor is mixed with the aqueous solvent after the latter has been heated and pressurised to secure the supercritical or near supercritical state, with suitable pressurisation and, if desired, heating, of the precursor prior to mixing with the aqueous solvent. In one arrangement, a homogenous catalyst component after pressurisation and, if desired, heating, is contacted with the aqueous solvent simultaneously with the contacting of the precursor with the aqueous solvent. The acid component after pressurisation and, if desired heating, may be contacted simultaneously with the contact of catalyst, precursor and aqueous solvent or may be pre- mixed with the catalyst. Alternatively, the acid component after pressurisation and, if desired heating may be fed directly into the reaction vessel (see Figures 2A and 2B). The oxidant after pressurisation and, if desired, heating, is mixed with aqueous solvent after the latter has been heated and pressurised to secure the supercritical or near supercritical state, and the oxidant/aqueous solvent mixture is then contacted with the mixture comprising the precursor, catalyst, acid component and aqueous solvent.

Tn Embodiment III, the oxidant is mixed with aqueous solvent after the latter has been heated and pressurised to secure the supercritical or near supercritical state, with suitable pressurisation and, if desired, heating, of the oxidant prior to mixing with the aqueous solvent. The catalyst may be mixed with oxidant with contemporaneous mixing of the acid component. Alternatively, the acid component may be pre-mixed with the catalyst prior to contact with the oxidant. The catalyst and/or acid component is subjected to pressurisation and, if desired, heating. The precursor is subjected to pressurisation and, if desired, heating, and then contacted with the mixture comprising the oxidant, catalyst and the acid component in the reaction zone. A schematic flow diagram representing

Embodiment III is presented in Figure 3.

Contact of the various streams may be effected by way of separate feeds to a device in which the feeds are united to form the preferred single homogeneous fluid phase thus allowing the oxidant and precursor to react. The device in which the feeds are united may for instance have a Y, T, X or other configuration allowing separate feeds to be united in a single flow passage forming the continuous flow reactor, or in some circumstances multiple flow passages forming two or more continuous flow reactors. The flow passage or passages in which the feeds are united may comprise a section of tubular configuration with or without internal dynamic or static mixing elements.

In a preferred embodiment, in-line or static mixers are advantageously used to ensure rapid mixing and homogeneity, for example to promote dissolution of oxidant into the aqueous solvent and the formation of a single phase.

The oxidant feed and the precursor feed may be brought together at a single location or the contact may be effected in two or more stages so that at least part of one feed or of both feeds are introduced in a progressive manner, e.g. via multiple injection points, relative to the direction of flow through the reactor. For instance, one feed may be passed along a continuous flow passage into which the other feed is introduced at multiple points spaced apart lengthwise of the continuous flow passage so that the reaction is carried out progressively. The feed passed along the continuous flow passage may include the aqueous solvent as may the feed introduced at multiple positions.

Similarly, the addition of catalyst may be effected in a progressive manner, e.g. via multiple injection points, relative to the direction of flow through the reactor.

In one arrangement, the oxidant is introduced to the reaction at two or more locations. Such locations are conveniently so positioned relative to the bulk flow of solvent and reactants through the oxidation zone that oxidant is introduced to the reaction at an initial location and at least one further location downstream of said initial location.

There may be more than one reaction zone in series or in parallel. For instance, where multiple reaction zones in parallel are used, the reactants and solvent may form separate flow streams for passage through the reaction zones and, if desired, the product streams from such multiple reaction zones may be united to form a single product stream. Where more than one reaction zone is used, the conditions, such as temperature, may be the same or different in each reactor. The or each reactor may be operated adiabatically or isothermally. Isothermal or a controlled temperature rise may be maintained by heat exchange to define a predetermined temperature profile as the reaction proceeds through the reactor.

The heat of reaction may be removed from the reaction by heat exchange with a heat-accepting fluid, according to conventional techniques known to those skilled in the art, and described for instance in WO-02/06201-A the disclosure of which techniques is incorporated herein by reference. Conveniently the heat-accepting fluid comprises water.

After traversing the continuous flow reactor and upon completion of the oxidation process, the reaction mixture comprises a solution of aromatic carboxylic acid, which needs to be recovered from the reaction medium. Substantially the entire amount of aromatic carboxylic acid produced in the reaction is in solution at this stage. In the process of the invention, typically at least 80% wt, more usually at least 90% wt, preferably at least 95% wt, more preferably at least 98% wt and most preferably substantially all of the aromatic carboxylic acid produced in the reaction is maintained in solution during the reaction and does not begin to precipitate until the solution leaves the oxidation reaction zone and undergoes cooling. The solution may also contain catalyst, and relatively small quantities of by-products such as intermediates (e.g. p-toluic acid and 4-CBA in the case of terephthalic acid), decarboxylation products such as benzoic acid and degradation products such as trimellitic acid and any excess reactants. The desired product, aromatic carboxylic acid such as terephthalic acid, may be recovered by causing or allowing the aromatic carboxylic acid to crystallise from the solution in one or more stages followed by a solids- liquid separation in one or more stages.

The product stream is subjected to a solids-liquid separation to recover the aromatic carboxylic acid and the mother liquor (which may but need not necessarily contain dissolved catalyst components) is recycled to the oxidation reaction zone. Preferably prior to re-introduction into the oxidation reaction zone, the mother liquor is heated by heat exchange with the product stream thereby cooling the latter.

One or both reactants may be admixed with the mother liquor recycle stream or separate mother liquor recycle streams prior to re-introduction of the mother liquor into the reaction zone and the mother liquor recycle stream (or at least that fraction or those fractions thereof to be combined with the reactant or reactants) may be heated and pressurised to secure supercritical/near supercritical conditions before being admixed with the reactant or respective reactant.

The invention will now be described further by way of example only with reference to the accompanying drawings.

Referring to Figure 1, dioxygen, after pressurisation, is mixed with water after the water has been heated and the mixture pressurised and optionally further heated in preheater 1 to achieve the supercritical state. The precursor and catalyst are added, after pressurisation, to the CVwater stream at the beginning of or immediately before the reactor 2 and the mixture passed through the reactor. The acid component is added, after pressurisation, to the Q₂/water stream contemporaneously with addition of the catalyst, or alternatively is added to the catalyst stream prior to contact with the oxidant stream. Upon exiting the reactor, the stream is cooled and depressurised at the back -pressure regulator 3. The products are carried out in a stream of cooled water.

Referring to Figures 2A and 2B, the precursor and catalyst after pressurisation are added to water after the water has been pressurised and optionally heated. The acid component may be added to the catalyst before, contemporaneously or after mixing with the water. The mixture is optionally further heated in preheater IA to achieve the supercritical state. Alternatively, the acid component after pressurisation and optionally heating may be mixed with the catalyst/precursor stream at the point when the latter stream is mixed with the oxidant. The dioxygen gas, after pressurisation is mixed with water at a supercritical state and optionally further heated in preheater 1. In Figure 2A, the streams are mixed at the beginning of or immediately before the reactor 2 and the mixture passed through the reactor. In Figure 2B, the O₂/water stream is added to the reactor in a progressive manner at multiple injection points. Upon exiting the reactor, the stream is cooled and depressurised at the back pressure regulator 3. The products are earned out in a stream of cooled water.

Figure 3 corresponds to Figure 1 wherein the catalyst and oxidant are mixed prior to contact of either stream with the precursor. The dioxygen gas after pressurisation is mixed with water at a supercritical state and optionally further heated in preheater 1. The acid component is mixed with the catalyst prior to or contemporaneously with contact of the catalyst with the oxidant.

Referring to Figure 4, feedstock components comprising water, precursor (e.g. paraxylene in the process for the production of terephthalic acid) and dioxygen gas are pressurised to operating pressure and continuously supplied from respective sources 10, 12 and 14 through a preheater 16 where the components are heated to a temperature of 300 to 48O⁰C, more preferably 330 to 450 ⁰C, typically from about a lower limit of about 350 to 37O⁰C to an upper limit of about 370 to about 42O⁰C, the pressure and temperature being selected in order to secure supercritical or near supercritical conditions. Part of the heat used to preheat the feedstock components may be derived from the exotherm produced in the course of the subsequent reaction between the precursor and the oxidant. Heat from other sources may be, for example, in the form of high pressure steam and/or heating may be effected by direct fired heating of the water stream. The heat of reaction may be recovered in any suitable manner, e.g. by means of heat exchange between the fluid following reaction and a suitable heat-accepting fluid such as water. For instance, the heat- accepting fluid may be arranged to flow in heat exchange relation, counter-currently and/or co-currently, with the reactants and solvent passing through the reaction zone. The passage or passages along which the heat-accepting fluid flows in traversing the reaction zone may be external to the reaction zone and/or may extend internally through the reaction zone. Such internally extending flow passage(s) may for instance extend generally parallel with and/or transversely of the general direction of flow of the reactant/solvent through the reaction zone. For example, the heat-accepting fluid may traverse the reaction zone by passage through one or more coiled tubes located within the interior of the reactor. The enthalpy of reaction can be used to recover power via a suitable power recovery system such as a turbine; for instance the heat-accepting fluid, e.g. water, can be used to raise high pressure saturated steam at for example temperature and pressure of the order of 300⁰C/ 100 bara which, in turn, can be superheated by external heat and fed to a high efficiency condensing steam turbine to recover power. In this way, the reactor can be maintained at an optimum temperature and effective energy efficiency can be achieved. In an alternative approach, the reactor may be operated under adiabatic conditions and a suitably high rate of water flow through the reaction zone may be employed in order to constrain the temperature rise across the reactor in operation. If desired, a combination of both approaches may be used, i.e. recovery of the enthalpy of reaction via a heat-accepting fluid coupled with a suitable water flow rate through the reaction zone.

Following heating of the feedstock components, oxygen is mixed with water which, as a result of preheating and pressurisation, will be under supercritical or near supercritical conditions and hence capable of solubilising the feedstocks. In the embodiment illustrated in Figure 4, oxygen and water are mixed in premixer 18A. The precursor is also mixed with water in premixer 18B. Of course, the precursor could also be separately premixed with water prior to entry into the preheater 16.

The premixer (or premixers where premixing of each reactant and water is undertaken) may take various forms such as Y, L or T piece, double T configurations or a static mixer, as illustrated in Figures 5A, 5B, 5C, 5D and 6 respectively. In Figures 5A to 5D and 6, reference A depicts the preheated water supply to the premixer, B depicts the reactant (precursor or oxygen) and P depicts the resulting mixed stream. In the double T configuration of Figure 5D, two mixed streams are produced Pl and P2. These may either be passed through separate continuous flow reactors or be combined into a single stream and then passed through a single continuous flow reactor. An X piece configuration may also be used, as known to those skilled in the art. It will also be appreciated that any suitable mixing apparatus may be used in the present invention. It will further be appreciated that the mixing apparatus referred to above are for use in a continuous process apparatus. In a batch system, there is of course no continuous flow and therefore no specific flow-related mixing requirements. In a continuous vessel reactor, reactants can also be fed into the vessel independently.

It will be appreciated that instead of premixing one or each reactant with water prior to introduction into the reaction zone, the reactants and water may be introduced separately into the reaction zone and mixed within the reaction zone with the aid of some form of mixing arrangement (e.g. a static mixer) whereby substantially all mixing of the components occurs within the reaction zone.

The homogeneous catalyst is added as a solution from source 19 to the premixed oxygen/water stream at the same time as the precursor is added to the premixed oxygen/water stream either immediately prior to entering the reactor or at the beginning of the reactor (i.e. as shown in Figure 1). In a preferred embodiment, the acid is included in the catalyst source 19, as shown in Figure 4.

Following preheating and premixing, the feedstock components are combined in a reaction zone 20 to form a single homogeneous fluid phase in which the reactants are brought together. The reaction zone 20 may consist of a simple mixer arrangement in the form of a tubular flow reactor, e.g. a pipe of a length which, in conjunction with the flow rate of the combined reactants, provides a suitable reaction time so as to secure conversion of, for example, paraxylene to terephthalic acid with high conversion efficiency and low 4- CBA content. The reactants may be combined in a progressive manner by injecting one reactant into a stream containing the other reactant at multiple points along the length of the reactor. One way of implementing a multiple injection arrangement is shown in the continuous flow reactor of Figure 7 in which the reactor is constituted by a pipe or vessel P. In an embodiment wherein a premixed oxygen/water stream is added to a premixed precursor/water stream, a premixed precursor/supercritical or near supercritical water stream W is supplied to the upstream end of pipe or vessel P. Water stream W would also contain the homogeneous catalyst plus acid. The stream passes through the reactor pipe or vessel P and at a series of locations spaced at intervals along the length of the pipe or vessel P, preheated and compressed oxygen dissolved in supercritical or near supercritical water is supplied via multiple injection passages A to E to produce a product stream S comprising carboxylic acid product (e.g. terephthalic acid) in supercritical or near supercritical aqueous solution. In this manner, the oxygen necessary to effect complete oxidation of, for example, paraxylene to terephthalic acid is injected progressively with the aim of controlling oxidation and minimising side reactions and possible burning of paraxylene, terephthalic acid or terephthalic acid intermediates.

Referring now back to Figure 4, following the reaction to the desired degree, the supercritical or near supercritical fluid is passed through a heat exchanger 22 through which heat exchange fluid is circulated via closed loop 24 so that heat can be recovered for use in the preheater 16. One scheme (not shown) for post-reaction cooling of the carboxylic acid product solution involves the use of heat exchanger networks to cool the stream to subcritical temperatures, e.g. of the order of 300⁰C to retain the carboxylic acid product in solution and thereby reducing fouling of heat exchange surfaces, followed by use of a train of flashing crystallisers (similar to those employed in conventional terephthalic acid purification by hydrogenation) to cool and precipitate the carboxylic acid product.

The cooled solution is then supplied to a product recovery section 26 in which the carboxylic acid is precipitated from the solution. Any suitable method of product recovery known to those skilled in the art may be used. The product recovery section 26 may comprise one or more stages of cooling or evaporative crystallisation to crystallise the carboxylic acid product to form a slurry of crystals in aqueous mother liquor. Where the product recoveiy section 26 comprises one or more flashing crystallisers, the resulting flash streams from the crystallisers may be used to preheat the inlet water and precursor streams to the reactor, either indirectly via conventional heat exchangers or via direct injection of the flash into the water and/or precursor feeds to the reactor. The slurry obtained following crystallisation may be subjected to a solids-liquid separation process using for example filtration devices operating under super-atmospheric, atmospheric or sub-atmospheric conditions, with or without washing facilities, such as described in prior published International Patent Applications Nos. WO-A-93/24440 and WO-A-94/17982 (the disclosures of which are incorporated herein by this reference). Thus, the solids-liquid separation may be carried out using any device suitable for this purpose and arranged to operate under elevated pressure conditions or at atmospheric pressure depending on the pressure following the final crystallisation stage. The solids-liquid separation can be carried out using an integrated solids separation and water washing apparatus such as a belt filter unit, a rotary cylindrical filter unit, or a drum filter unit (e.g. a BHS-Fest filter drum formed with a plurality of slurry receiving cells in which the mother liquor is displaced from filter cake by water under hydraulic pressure supplied to the cells). After filtering the slurry, the recovered carboxylic acid product may be used directly for the production of polyester, for instance, for packaging, such as bottles, or fibres. Similarly it can be dried. If not already at atmospheric pressure, the filter cake of carboxylic acid product may be transferred to a low pressure zone (e.g. atmospheric pressure) for drying via a suitable pressure let-down device such as a lock hopper arrangement, a rotary valve, a ram-type pump, a screw feed device or a progressive feed device such as a progressive cavity pump of the type used to pump cold pastes of high solids contents.

The temperature of separation and the level of washing required will be dependent on the levels of impurities generated in the reaction, the means of recovering the product and the required product specification. Although in general, it will be desirable to produce carboxylic acid product, such as terephthalic acid, which is sufficiently pure to render further purification unnecessary (e.g. by oxidation and/or hydrogenation of an aqueous solution of terephthalic acid to convert 4-CBA to terephthalic acid or to para-toluic acid, as the case may be), we do not exclude the possibility of carrying out such purification subsequent to the supercritical or near supercritical water oxidation reaction. Following recovery of the aromatic carboxylic acid product, at least part of the aqueous mother liquor may be recycled for re-use in the oxidation reaction, e.g. by admixture with fresh water and/or the reactants. However, if the recycled mother liquor contains catalyst components, it is preferably not added to the O₂/waler stream prior to addition of precursor. The amount recycled will usually be a major fraction of the recovered mother liquor, with a purge being taken in order to reduce standing concentrations of by-products in the process. The purge stream may be treated to recover its catalyst content where applicable and its organic content.

Referring now to Figure 8, in this embodiment liquid oxygen (line 30), liquid precursor (e.g. paraxylene in the case of the process for the production of terephthalic acid) (line 32) and water (line 34) are supplied to a mixing unit 36. The oxygen and precursor supplies are pressurised by pumps 38, 38A and heated to elevated temperature, for example by high pressure steam, in heat exchangers 40, 4OA. The mixing unit 36 is configured, as shown in Figure 4, to mix the reactants with the water supply to produce two streams 42, 44, one stream comprising a water/precursor mixture and the other stream comprising oxygen dissolved in water, which are fed to a continuous flow reactor 46 in the form of a pipe in which the streams are mixed, e.g. by an unshown static mixing arrangement within the pipe, to initiate the reaction. The homogeneous catalyst and acidic component as a solution in water may be added either into the precursor /water stream 42 immediately prior to entering the reactor, or on combination of streams 42 and 44 at the beginning of or immediately before the reactor, using rapid mixing, for example by the use of a static mixer or similar device.

The supply of fresh make-up water to the system may be effected at various points.

One of the most convenient points is upstream of the main pressurisation pump 68, for instance via line 116 which is described in more detail below in relation to Figure 9. Water may also be fed after pressurisation in pump 38C and heating in heat exchanger 4OC via line 35A into line 74, or prior to the exchangers (50,70). Alternatively, water may be fed, after pressurisation in pump 38B and heating in heat exchanger 4OB independently into the preheater 36 via line 35. Following reaction under supercritical or near supercritical conditions, the product stream 48 in the form of a solution of reaction product(s) (plus small amounts of unreacted reactants, intermediates etc) is cooled by passage through heat exchangers 50 and 52 and may be optionally flashed down to a lower pressure and temperature in flash vessel 54. The means of effecting such a step at this point or in the product recovery section 62 may involve known devices, singly or in multiples, but should be configured to avoid deposition of solids, by means such as localised heating, as known to those skilled in the art. Thus, as the stream from reactor 46 is passed through heat exchangers 50 and 52, the temperature of the stream is monitored and controlled so that the product does not precipitate; precipitation should not occur until flash vessel 54. A substantial amount of steam and some gaseous components such as nitrogen, oxygen, carbon oxides are supplied via line 56 to an energy recovery system 58 while the terephthalic acid solution is supplied via line 60 to a product recovery section 62.

In the product recovery section, the solution of carboxylic acid product is processed through a multi-stage crystallisation train in which pressure and temperature are progressively lowered to crystallise the carboxylic acid product in ciystal form. The product of the crystallisation process is a slurry of carboxylic acid crystals in an aqueous mother liquor. After the final crystallisation stage, the slurry may be at any desired pressure, e.g. atmospheric pressure or above. The slurry is then subjected to a solids-liquid separation of any suitable form to separate the crystals from the mother liquor.

In Figure 8, the carboxylic acid crystals recovered are supplied via line 64 to a drier (not shown) or to the direct production of polyester. Where the solids-liquid separation is carried out under elevated pressure conditions, the crystals are conveniently let down to atmospheric pressure using a suitable device (e.g. as disclosed in International Patent Application No. WO-A-95/19355 or US Patent No. 5470473) before being transferred to diying equipment. The mother liquor from the solids-liquid separation is recovered via line 66, repressurised by pump 68 and recycled to the mixer unit 36 via heat exchanger 70, line 72, heat exchanger 50, line 74, start-up/trim heater 76 and line 34. Thus, under steady state operating conditions, the recycled mother liquor may contribute to the source of water for supply to the reactor 46 as well as a vehicle for the recycle of catalyst to the process. The mixture unit 36 is configured such that, where the recycled mother liquor may contain catalyst, i.e. homogeneous catalyst, the recycled mother liquor is preferably mixed with the precursor stream rather than the oxidant stream since the addition of catalyst to oxidant is preferably contemporaneous with the addition of precursor to oxidant. Thus, where the recycled mother liquor contains catalyst, the mixture unit is configured such that the oxidant stream 30 may be mixed with fresh water from line 35. Similarly, additional catalyst or said acidic component, as required, may be added to the mother liquor in line 34, or directly to the reaction zone 46.

Because water is generated in the course of the reaction, a water purge is taken from the system. This may be effected in several ways; for instance, the purge may be taken via line 78 or from a suitable flash condensate (for example as will be described below in connection with the energy recovery system). The latter may be more advantageous as it will be somewhat less contaminated with organics than a purge from the mother liquor recovered via line 66. The purge however recovered may be passed to effluent treatment, e.g. aerobic and/or anaerobic processing.

In the heat exchanger 70, the temperature of the mother liquor is increased by about 30 to 100⁰C, through heat transfer from steam flashed from one or more of the crystallisation stages, e.g. the first stage highest pressure and temperature crystalliser vessel. The flash (line 79) used for this puipose may, following passage through the heat exchanger 70, be returned as condensate to the product recovery section for use as wash water in washing the carboxylic acid product filter cake produced by solids-liquid separation. In the heat exchanger 50, the temperature of the mother liquor is increased still further, for instance by about 100 to 200⁰C, as a result of heat transfer from the high temperature product stream 48 from the reactor 46. In this manner, the product stream is subjected to cooling while significantly increasing the temperature of the mother liquor recycle stream. The trim/start-up heater 76 serves to boost the temperature of the mother liquor recycle stream, if necessary, to secure supercritical or near supercritical conditions. Under steady state operation of the process such boost may be optional since the mother liquor may be rendered supercritical or near supercritical following passage through the heat exchanger 50. The heater 76 may not therefore be necessary under steady state conditions and may be deployed purely for start-up operation, initially using pressurised water from a source other than mother liquor. In this embodiment, the water solvent is rendered supercritical or near supercritical prior to mixing with one or both reactants. However, it will be understood that raising of the temperature to secure the desired supercritical or near supercritical conditions may be effected prior to, during and/or following the mixing stage.

In the embodiment of Figure 8, the heat of reaction generated in the course of reacting the precursor with oxygen is removed at least in part by heat exchange with a heat-accepting fluid, preferably water, which is passed through the interior of the reactor 46 by means of a coiled tube 80 or a series of generally parallel tubes (as in a tube in shell heat exchanger design) or the like. The water employed is pressurised and heated to a temperature sufficiently high that, at the external surface of the conduit or conduits 80 conducting the water through the reactor, localised cooling which could otherwise cause precipitation of components, such as terephthalic acid, in the reaction medium is avoided. The water for this purpose is derived from the energy recoveiy system 58. Thus, in Figure 8, water at elevated pressure and temperature is supplied via line 82 to heat exchanger 52 where it is used to cool the product stream further following passage through the heat exchanger 50. The water then passes via line 83 through the conduit(s) 80 with consequent raising of high pressure, high temperature steam which is fed to the energy recovery system 58 via line 84.

The energy recovery system 58 is also supplied with steam flashed from one or more stages of the crystallisation train. This is depicted by line 88. This steam may for example be used to preheat the water supplied via line 82 to the heat transfer conduit(s) 80. Condensate resulting from processing of the steam feeds supplied to the energy recoveiy system 58 may be passed via line 90 to the product recoveiy section for use for example in washing the terephthalic acid filter cake produced in the solids-liquid separation. A water purge 92 may be taken from line 90 if desired, with the advantage that a purge taken at this point will be less contaminated than a purge taken from the mother liquor via line 78.

In Figure 8, the reactant(s) are shown as being introduced into the recycled mother liquor after the mother liquor has been heated by heat exchange with the product stream in heat exchanger 50. In a modification, a reactant may be admixed with the mother liquor recycle stream upstream of the heat exchange with the product stream. Where both reactants are so admixed with the mother liquor recycle stream, the latter is split into separate streams with which the reactants are respectively admixed so that the reactants are maintained isolated from each other until brought together for reaction. It will also be understood that the embodiment of Figure 8 may be modified in the manner indicated in Figure 7 by introducing one or even both of the reactants via multiple injection points along the flow path of the reaction medium so that the one or both reactants are introduced to the reaction progressively.

In the energy recovery system 58, various heat recoveiy processes may be carried out in order to render the process energy efficient. For instance, the high pressure steam raised following passage of water through the conduit(s) 80 may be superheated in a furnace supplied with combustible fuel and the superheated steam may then be passed through one or more steam condensing turbine stages to recover power. Part of the high pressure steam may be diverted for use in preheating the reactants (heat exchangers 40, 4OA and 40B) or for preheating stream 82 where this is necessary to effect a system of high thcπnal efficiency. The condensed water recovered from the turbine stages and from the heat exchangers 40, 4OA and 4OB may then be passed through a train of heating stages in order to preheat the water for recirculation to the reactor 46 via heat exchanger 52 thus forming a closed loop with make-up water being added as needed. The heating stages typically comprise a cascade of heat exchangers by means of which the rccirculating water flow returning to the reactor 46 is progressively raised in temperature. In some heating stages, the heat-donating fluid may be constituted by the flash steam derived at different pressures and temperatures from different stages of the crystallisation train. In other heating stages, the heat-donating fluid may be combustion gases rising in the furnace stack associated with the furnace used to superheat the high pressure steam supplied via line 84.

The embodiment of Figure 8 employs substantially pure oxygen as the oxidant. Figure 9 illustrates a similar embodiment but using a supply of compressed air (which may be oxygen enriched) as the oxidant. The embodiment of Figure 9 is generally similar to that of Figure 8 and those parts which function in generally the same way are depicted by the same reference numerals in both Figures and will not be described further below unless the context requires otherwise. As shown, the air supply 100 is supplied via an air compressor 102. As a result of using air, a substantial amount of nitrogen is introduced into the process and must therefore be appropriately handled. In this case, the product stream following passage through the heat exchangers 50 and 52 is flashed down in flash vessel 103 to a lower temperature to condense water to a greater extent than in the embodiment of Figure 8 thereby reducing the water content of the overheads. As described in relation to Figure 8, temperature of the product stream through the heat exchangers 50 and 52 is controlled such that precipitation of product occurs only in flash vessel 103. The overheads stream is supplied via line 104, heat exchanger 106 and fuel-fired heater 108 to a gas turbine 110. The overheads stream is passed through heat exchanger 106 in order to transfer heat to the mother liquor recycle stream while knocking out further water which can be passed to the product recovery section 62 via line 112 for use, for example, as wash water. For reasons of energy efficiency, it is desirable to heat the gaseous overheads stream to a high temperature before introduction into the turbine 110, hence the reason for heating the overheads stream by means of heater 108. There may be more than one gas turbine stage, in which case the overheads stream will be heated to an elevated temperature upstream of each such turbine stage. Line 114 depicts the overheads stream exiting the turbine 110 at low pressure and temperature. Where the oxidation process leads to the generation of species such as carbon monoxide etc. which are undesirable, for example for corrosion and/or environmental reasons, provision may be made for treating the overheads stream to reduce/eliminate such components before or after passage through the turbine 1 10 and/or discharge. Such treatment may comprise subjecting the overheads stream to catalytic combustion and/or scrubbing with a suitable reagent, e.g. an alkaline scrubbing liquor. The turbine 110 may be mechanically coupled with the air compressor so that the latter is driven by the turbine.

In the embodiment of Figure 9, water exits the system via the overheads stream. At least part of this water may be recovered if desired and recirculated for use for example as wash water in the product recovery section 62. Alternatively or additionally, make-up water may be supplied via line 116 to the product recovery section to compensate for the water lost in handling the large volumes of nitrogen as a result of compressed air usage. Such make-up water may be preheated and used as wash water, preheating being effected for example by diverting part of the flash streams (collectively depicted by reference numeral 88) via line 1 16 to heat exchanger 120 and returning the water condensed from the flash stream to the product recovery section 62 as wash water. Although the invention has been described mainly with reference to para-xylene as a precursor for terephthalic acid, it will be appreciated that other precursors may be employed instead or in addition to para-xylene for the production of the corresponding carboxylic acid, and such precursors include ortho-xylene, meta-xylene, 4-tolualdehyde, 4- toluic acid and 3-methylpyridine. As noted above, the invention is also applicable to the production of other aromatic carboxylic acids such as isophthalic acid, phthalic acid, trimellitic acid and naphthalene dicarboxylic acid from the corresponding alkyl aromatic compounds (preferably the methyl compounds) or other precursors. The invention is further illustrated below by the following non-limiting Examples.

EXAMPLES

Experimental work was carried out on a laboratory scale by the continuous oxidation of ortho-xylene by O₂ in supercritical water at about 38O⁰C and 230 to 240 bara with a combined catalyst and acid solution (as detailed below and typically with an MnBr₂ or Mn(OAc)₂ catalyst and HBr). The exotherm was minimised by using relatively dilute solutions (<0.5% organic w/w). The basic configuration of the system is as set out in

Figure 1. A more detailed illustration of the system used in these laboratory scale experiments is shown in Figure 10 (which illustrates the use Of MnBr₂ as catalyst and HBr as acid).

O₂ originates from heating an H₂O₂/H₂O mixture in excess of 400⁰C in the preheater 152. The H₂O₂ decomposes to liberate O₂. The O₂/H₂O fluid then passes through the cross-piece 154, where it is contacted with the ortho-xylene and solution of acid and catalyst, fed in from their own pumps. The reaction mixture is passed through the reactor

156.

Other components labelled in Figure 10 are as follows: cooling coil 158; 0.5μm filter 159; back-pressure regulator 160; valves 162 A to C; pressure-release valves 163 A and B; non-return valves 164 A to C; pressure transducers 165 A to D; thermocouple T

(the aluminium heater blocks of preheater 152 and reactor 156 also contain thermocouples, not shown). The pumps were Gilson 302, 305, 306 and 303; the back pressure regulator was obtained from Tescom.

Maximum corrosion occurs in the region of the crosspiece 154 where O₂, feedstock and the catalyst solution meet, particularly at the incoming unheated catalyst feed pipe where a high temperature gradient coincides with bromide ions. Hastelloy was used for the final section of the catalyst feed-pipe and for the reactor, and 316 stainless steel for the other components.

Before each run, the apparatus is hydrostatically pressure-tested when cold, and is then heated with a flow of pure water (5-10mL/min). Once the operating temperature has been reached, H₂O₂/H₂O is fed and the pumps for ortho-xylene and acid/catalyst are started, typically in that order. The residence time for each run remains constant and is typically up to about 1 minute, but in most cases about 10-20 seconds. The yields of manganese were calculated from the flow-out of Mn divided by the flow-in of Mn (where the concentration of manganese is measured by atomic absoprtion).

It should be noted that in these Examples, the concentrations of catalyst and acid are given as concentrations in the catalyst feed stream. The actual concentrations in the reactor correspond to about one-third of these values because the catalyst feed represents about a third of the (mass) flow, with the other two-thirds mostly coming from the other streams.

Example 1

To investigate the effects of HBr in a system using manganese acetate as the catalyst, an experiment was conducted using the following experimental conditions: Temperature = approx. 380 ⁰C Pressure = approx. 230 bara Flow rate of acid/catalyst and o-xylene = 4. 084 mL/min. Flow rate of o-xylene = 0.061 mL/min

Flow rate oxidant (H₂O₂ in H₂O) = 8.1 mL/min. (providing an amount of [O₂] in aqueous H₂O₂ = 0.276 mol.L^"1 (1.5 molar equivalents of the stoichiometry required for complete oxidation of the organic precursor to the aromatic acid, the molar ratio for which in the case of o-xylene is 3CV organic)).

The Br/Mn mass ratio (w/w) was increased from 0 to 4.17 (corresponding to a molar ratio [Br]: [Mn] of 0 to 2.865: 1) by adjusting the Mn concentration between 1200 and 1700 ppm, whilst increasing the HBr concentration from 0 to 6000ppm Br. The results are presented in Table 1 and Figure 11. The yield of o-phthalic acid was calculated by

HPLC.

A further run was conducted using a solution of NaBr as co-catalyst with

Mn(AcO)₂, but the reactor blocked very quickly due to the precipitation of the catalyst and the insolubility of NaBr in ScH₂O, demonstrating that acidity in addition to bromide content is required to avoid the precipitation of the catalyst.

The results demonstrate generally that not only selectivity towards the target dicarboxylic acid, but also Mn recoverability, improves with the addition of HBr. The addition of HBr produces a clear improvement in that the yield of phthalic acid shows a dramatic improvement from about 8 to 65%, and the selectivity increases from about 21 to 88%. However, the yields of the target molecule remain unaffected by increasing the Br/Mn molar ratio beyond a certain point. The inventors have also observed that Mn recovery increases dramatically from 14 to 93% upon the addition of HBr, which clearly indicates that the stabilisation of the catalyst is favoured by increasing the concentration of HBr. The data in Table 1 are presented in Figure 11 and demonstrate two distinct mechanisms which can be correlated to the Br/Mn ratio. The graph of Figure 11 neatly illustrates how increasing amounts of HBr provide the known oxidation promoter effect (i.e. increasing product yield and selectivity) up to a Br/Mn molar ratio of approximately 1.5 at which point the effect is saturated. However, the level of HBr required for the (full) manganese recovery effect is above and beyond that needed for the oxidation promoter effect and saturates at a Br/Mn molar ratio of between about 2.0 and 3.0. PT0085

90 O t~

O Table 1. Oxidation of ø-xylene with different catalytic mixtures OfMn(AcO)₂ and HBr (Example 1) O

H U

a : Selectivities for compounds 1 to 6 calculated (as a percentage) as the molar concentration of that product relative to the total molar

*O f> concentration of products 1 to 6.

O r-- o b : calculated by atomic absorption

00

O O

O

Example 2

Another series of experiments was performed to determine the effect of other acids and the effect of [Br] alone. The results are presented in Table 2 below. Unless otherwise stated, the experiments use the same concentrations Of H⁺ (i.e. 6.25x10^" M) contained in HBr solutions at 5000 ppm Br. A concentration of Mn of 1700ppm was used. The use of NaBr rather than HBr gave no catalyst recovery effect, indicating that protons are a necessary presence in the reaction. The addition of HCl improved Mn recoveiy for both MnBr₂ and Mn(OAc)₂, although HBr is clearly preferred over HCl for its combined catalyst recoveiy effect and its effect on yield/selectivity of the target compound. The data from Run 4, in which the BnMn ratio (w/w) is 5.85: 1 (corresponding to a molar ratio [Br]: [Mn] of 4.02), indicate that increasing concentrations of HBr tend to result in lower yields and selectivities of the target carboxylic acid, and such concentrations also increase corrosion within the apparatus.

Example 3

Using a procedure similar to that of the previous examples, 1700 ppm Mn (as MnBr₂) was used in the absence of HBr according to the prior art, and in the presence of HBr or HCl. The results are shown in Table 3 and Scheme 1 below. In Run 1, the o-xylene stream was initially supplied such that the oxidant/catalyst contact is contemporaneous with the oxidant/precursor contact, as required by WO-02/06201-A, and 75% catalyst recovery was observed, the remaining 25% being lost in the form of manganese oxide precipitate (primarily MnO₂, but also including some Mn₂Cb and MnO(OH)₂, as determined by X-ray diffraction). When the precursor stream is switched off, catalyst recovery drops dramatically, indicating that MnBr₂ catalyst is significantly more stable in the presence of precursor. The effect of acid is illustrated by Runs 2 and 3, in which initial mixing was contemporaneous as in Run 1, and in which catalyst recovery increases significantly in both the presence and absence of precursor. Runs 2 and 3 also demonstrate that, because of the increased stability of the catalyst in the presence of acid but in the absence of precursor, the contemporaneous mixing regime of WO-02/06201-A is desirable but not required. In Runs 4 and 5, the initial contact between catalyst and oxidant is in the absence of precursor, i.e. in contrast to the arrangement of WO-02/06201-A, and the o-xylene precursor is introduced after that initial contact. Run 5 shows again that HBr improves stability and recovery of the MnBr₂ catalyst, and although the contemporaneous arrangement of WO-02/06201-A is desirable, the effect of the present invention is exhibited in the absence of this contemporaneous mixing.

Table 3

MnBr₂ + HBr (0.0125M) 1.No organic stream 25

2. Organic stream switched on 92

Scheme 1

Run 1 Run 4

Runs 2 & 3 (% given for HBr) Run 5

Example 4

Using a procedure similar to Example 1, the oxidation of 3-picoline to nicotinic acid was conducted. The reaction products were separated using HPLC. The results are shown in Table 4 below. The reaction conditions were: Temperature T = approx 360⁰C; Pressure P = approx 220 bara;

Total flow rate = 24.368 mL/min; Flow rate of organic = 0.124 mL/min, [O₂]/org = 1.5 equivalent; Flow rate of catalyst (MnBr₂ with or without HBr) - 8.168 mL/min.

Table 4

3-PyA = nicotinic acid (3-pyridinecarboxylic acid); Py - pyridine;

3-PyAL ^~ 3-pyridinecarboxaldehyde; 3-MPy = 3-picoline (3-methylpyridine)

The results show that the stabilisation of the catalyst MnBr₂ with acid (HBr) produces a significant improvement in catalyst recovery.

Claims

1. The use, in a catalytic synthetic oxidation reaction in an aqueous solvent comprising water under supercritical or near-supercritical conditions wherein a metal salt is present as a catalyst during the oxidation reaction, of an acidic component comprising one or more acid(s) for the purpose of minimising or preventing catalyst loss and/or precipitation of metal oxide during said reaction, wherein said reaction comprises an oxidant and said acidic component is added into the reaction such that contact of at least part of said metal salt with said oxidant is in the presence of said acidic component.

2. A method of minimising or preventing catalyst loss and/or the precipitation of metal oxide in a catalytic synthetic oxidation reaction in an aqueous solvent comprising water under supercritical or near-supercritical conditions wherein during the oxidation reaction an oxidant is present and a metal salt is present as a catalyst, said method comprising the step of adding an acidic component comprising one or more acid(s) into the reaction such that contact of at least part of said metal salt with said oxidant is in the presence of said acidic component.

3. A use or method according to claim 1 or 2 wherein said oxidation reaction is a process for the production of one or more target organic compound(s) from one or more oxidisable organic precursor(s) thereof.

4. An oxidation process for the production of one or more target organic compound(s) from one or more oxidisable organic precursor(s) thereof, said process comprising contacting in the presence of a catalyst comprising a metal salt (preferably a transition metal salt), within a reactor, one or more of said precursor(s), such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, wherein

(i) an acidic component comprising one or more acid(s) is added to the reaction mixture; and (ii) contact of at least part of said catalyst with said oxidant is in the presence of said acidic component.

5. A use or method according to claim 1 or 2 wherein said oxidation reaction is a process for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst comprising a metal salt (preferably a transition metal salt), within a reactor, one or more precursors of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, wherein said acidic component is added into the reaction such that contact of at least part of said catalyst with said oxidant is in the presence of said acidic component.

6. An oxidation process according to claim 4 for the production of an aromatic carboxylic acid, said process comprising contacting in the presence of a catalyst comprising a metal salt, within a reactor, one or more precursor(s) of the aromatic carboxylic acid with an oxidant, such contact being effected with said precursor(s) and the oxidant in an aqueous solvent comprising water under supercritical conditions or near supercritical conditions, wherein

(i) an acidic component comprising one or more acid(s) is added to the reaction mixture; and (ii) contact of at least part of said catalyst with said oxidant is in the presence of said acidic component.

7. A use, method or process according to claims 4, 5 or 6 wherein said one or more precursors, oxidant and aqueous solvent constitute a single homogeneous phase in the reaction zone.

8. A use, method or process according to any preceding claim wherein the metal salt comprises a transition metal.

9. A use, method or process according to claim 8 wherein the metal catalyst comprises manganese.

10. A use, method or process according to claim 8 wherein the metal catalyst comprises manganese bromide MnBr₂.

1 1. A use, method or process according to any preceding claim wherein the addition of said acidic component is effected such that the acidic component is present in any location where the metal salt is in contact with the oxidant of the oxidation process.

12. A use, method or process according to any preceding claim wherein said acidic component is present in the reaction zone in an amount such that the molar ratio of [H⁺] to [M], wherein [H^f] is the molar amount of acid derived from said acidic component and M is the metal of said metal salt present during the oxidation reaction, is at least 0.05:1.

13. A use, method or process according to any preceding claim wherein said acidic component is present in the reaction zone in an amount such that the molar ratio of [H⁺] to [M], wherein [H^"1] is the molar amount of acid derived from said acidic component and M is the metal of said metal salt present during the oxidation reaction, is no more than 12.0: 1.

14. A use, method or process according to any preceding claim wherein said acidic component is present in the reaction zone in an amount such that the molar ratio of [H⁺] to [M], wherein [H⁺] is the molar amount of acid derived from said acidic component and M is the metal of said metal salt present during the oxidation reaction, is in the range of from 0.2: 1 to 3.0: 1.

15. A use, method or process according to any preceding claim wherein said acidic component comprises a mineral acid.

16. A use, method or process according to claim 15 wherein said acidic component comprises a mineral acid selected from HX wherein X is halide.

17. A use, method or process according to claim 15 wherein said acidic component comprises HBr.

18. A use, method or process according to claim 15 wherein said acidic component comprises HCl.

19. A use, method or process according to claim 16, 17 or 18 wherein the amount of hydrogen halide added is such that the molar ratio [X]: [M] of total halide (X) to the metal ion (M) of the catalyst is greater than 2.0: 1.

20. A use, method or process according to claim 16, 17, 18 or 19 wherein the amount of hydrogen halide added is such that the molar ratio [X]: [M] of total halide (X) to the metal ion (M) of the catalyst is no more than 12.0: 1.

21. A use, method or process according to any preceding claim wherein said contact of at least part of said precursor with said oxidant is contemporaneous with contact of said catalyst with at least part of said oxidant.

22. A use, method or process according to any of claims 5 to 21 wherein at least 98% wt of the aromatic carboxylic acid produced is maintained in solution during the reaction.

23. A use, method or process according to any of claims 5 to 22 wherein the aromatic carboxylic acid following reaction is precipitated from the reaction medium and contains no more than 5000 ppm by weight of aldehyde produced as an intermediate in the course of the reaction.

24. A use, method or process according to any of claims 5 to 23 wherein following the reaction the aromatic carboxylic acid-containing solution is processed to precipitate the aromatic carboxylic acid and the precipitate is separated from the mother liquor.

25. A use, method or process according to any preceding claim wherein said oxidation reaction is performed in a continuous flow reactor.

26. A use, method or process according to claim 25 wherein the residence time of the reaction medium within the reaction zone is no more than 10 minutes.

27. A use, method or process according to any of claims 5 to 26 wherein said aromatic carboxylic acid is selected from terephthalic acid, isophthalic acid, phthalic acid, trimellitic acid, naphthalene dicarboxylic acid and nicotinic acid.

28. A use, method or process according to claim 27 wherein said aromatic carboxylic acid is terephthalic acid.

29. A use, method or process according to any of claims 5 to 28 wherein said precursor is selected from the group consisting of aromatic compounds having at least one substituent selected from alkyl, alcohol, alkoxyalkyl and aldehyde groups.

30. A use, method or process according to any of claims 5 to 28 wherein said precursor is selected from the group consisting of aromatic compounds having at least one substituent selected from alkyl, alcohol and alkoxyalkyl groups.

31. A use, method or process according to any of claims 5 to 28 wherein said precursor is selected from the group consisting of aromatic compounds having at least one substituent selected from alkyl groups.

32. A use, method or process according to any of claims 5 to 28 wherein said precursor is selected from the group consisting of aromatic compounds having at least one substituent selected from Ci_-4 alkyl groups.

33. A use, method or process according to claim 28 wherein said precursor is para- xylene.

34. An aromatic carboxylic acid when produced by the process described in any one of claims 6 to 33.