WO2006125801A1

WO2006125801A1 - Process for the preparation of adipic acid from n-pentenoic acid

Info

Publication number: WO2006125801A1
Application number: PCT/EP2006/062590
Authority: WO
Inventors: Eit Drent; Rene Ernst; Willem Wabe Jager; Cornelia Alida Krom; Timothy Michael Nisbet; Johannes Adrianus Maria Van Broekhoven
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2005-05-27
Filing date: 2006-05-24
Publication date: 2006-11-30

Abstract

A process for the preparation of adipic acid from n-pentenoic acid, comprising the steps of (a) contacting the n-pentenoic acid with carbon monoxide and water in the presence of a catalyst system including a source of palladium and a bidentate diphosphine ligand, to obtain a reaction mixture comprising adipic acid, n-pentenoic acid, and catalyst system; and (b) subjecting the reaction mixture to conditions under which adipic acid precipitates from the reaction mixture to obtain precipitated adipic acid and remaining liquid reaction mixture comprising at least part of the n-pentenoic acid in admixture with the catalyst system; and (c) separating the precipitated adipic acid from the remaining liquid reaction mixture comprising at least part of the n-pentenoic acid in admixture with the catalyst system to obtain separated off adipic acid; and (d) washing the separated off adipic acid with n-pentenoic acid and/or water.

Description

PROCESS FOR THE PREPARATION OF ADIPIC ACID FROM N-PENTENOIC ACID

Field of the invention

The present invention provides a process for the preparation of adipic acid from n-pentenoic acid, and its separation and purification. Background of the invention

Adipic acid, i.e. 1,6-hexane dioic acid is an important raw material for the manufacturing of nylon, plasticizers, lubricants components and polyester polyols for polyurethane systems, as well as gelling aid, acidifier, leavening or buffering agent in food applications. In particular in these applications, adipic acid has to be of a high degree of purity. Conventional oxidation processes produce adipic acid containing nitrogen-containing impurities and heavy metal residuals, and hence require extensive purification before the adipic acid obtained can be employed in most of its applications. A process for the preparation of adipic acid that avoids the presence of nitrogen-containing impurities is the carbonylation of n-pentenoic acid (i.e. 2-, 3- and/or 4-pentenoic acid) under catalysis by a transition metal complex in the presence of carbon monoxide and water. Such a process is for instance described in US-A-β, 008, 408. Although no nitrogen- containing by-products are formed in this process, wherein a catalyst based on a source of iridium and hydrogen iodide is employed, only a limited selectivity for adipic acid is achieved, while a significant amount of carbonylation by-products are produced. The resulting product mixture requires a complex multistage crystallization treatment in order to remove the majority of by-products and catalyst components. Moreover, in order to achieve suitable purity for use in its applications, the thus obtained adipic acid requires extensive further purification, for instance by multiple re-crystallisation from water and/or aliphatic carboxylic acids as described in US-A-β, 008 , 408 or US-A-6, 222, 069. Hence, the described process is unattractive for production of adipic acid on a commercial scale.

Accordingly, there remained the need to provide for a simpler and more selective method for the preparation of adipic acid from n-pentenoic acid. Furthermore, it was desirable to provide for a process that can be continuously operated, and to provide for a process that permits recycling of the catalyst to the carbonylation reaction from the products. Summary of the invention

This has been achieved by the following process: The subject invention provides for a process for the preparation of adipic acid from n-pentenoic acid, comprising the steps of

(a) contacting the n-pentenoic acid with carbon monoxide and water in the presence of a catalyst system including a source of palladium and a bidentate diphosphine ligand, to obtain a reaction mixture comprising adipic acid, n-pentenoic acid, and a catalyst system; and

(b) subjecting the reaction mixture to conditions under which adipic acid precipitates from the reaction mixture to obtain precipitated adipic acid and remaining liquid reaction mixture comprising at least part of the n-pentenoic acid in admixture with the catalyst system; and (c) separating the precipitated adipic acid from the remaining liquid reaction mixture comprising at least part of the n-pentenoic acid in admixture with the catalyst system to obtain separated off adipic acid; and (d) washing the separated off adipic acid with n-pentenoic acid and/or water.

It has been found that adipic acid obtained in the carbonylation of pentenoic acids in the presence of a catalyst system based on a source of palladium and a diphosphine ligand can be isolated and purified from byproducts to the desired level, and that the removed active catalyst can be recycled in a continuous operation.

In the present process, n-pentenoic acid is subjected to carbonylation step in the presence of a carbonylation catalyst system. Principally all isomers of n-pentenoic acid, such as cis/trans 2-pentenoic acid, cis/trans 3-pentenoic acid, 4-pentenoic acid and mixtures thereof can be employed. Accordingly, the term "n-pentenoic acid" within the context of this application describes all linear isomers of pentenoic acid, i.e. cis/trans 2-pentenoic acid, cis/trans 3-pentenoic acid and/or 4-pentenoic acid, and mixtures thereof. It was found that the palladium catalyst employed for the subject process catalyses the isomerisation of these pentenoic acid isomers. Therefore, the amount of the different isomers may vary during the reaction, and all isomers are suitable for use in the process. Preferred sources of n-pentenoic acid are mixtures comprising mainly cis and/or trans 3-pentenoic acid, as obtainable by a process for the carbonylation of 1, 3-butadiene, such as for instance described in WO 2004/103948. Preferably, the catalyst system also comprises a source of an anion. A suitable source of an anion is for instance an acid, preferably a carboxylic acid. Again more preferably, the source of anions is an acid having a pK_a above 2.0 (measured in aqueous solution at 18 ⁰C), and yet more preferably an acid having a pK_a above 3.0, and yet more preferably a pK_a of above 3.6. Examples of preferred acids include carboxylic acids, such as acetic acid, propionic acid, butyric acid, pentanoic acid, pentenoic acid and nonanoic acid, the latter three being highly preferred as their low polarity and high pK_a was found to increase the reactivity of the catalyst system. The most preferred source of anions is the n-pentenoic acid itself. However, adipic acid or any other acid present in the reaction mixture can also serve as source of an anion.

N-pentenoic acid was found to be a good solvent for the reaction of step (a) . Accordingly, the n-pentenoic acid is preferably present in step (a) in an amount suitable to act as a solvent, i.e. to maintain the reaction mixture as a solution.

Although the addition of a strong acid as promoter may be beneficial, preferably no additional promoters such as a strong acid are employed in step (a) in order to avoid contamination and to keep the number of materials employed at a minimum. Suitable sources of palladium for step (a) include palladium metal and complexes and compounds thereof such as palladium salts, for example the salts of palladium and nitric acid, sulphuric acid or sulphonic acids; palladium complexes, e.g. with carbon monoxide or acetyl acetonate, or palladium combined with a solid material such as an ion exchanger. Preferably, a salt of palladium and a carboxylic acid is used, suitably a carboxylic acid with up to 12 carbon atoms, such as salts of acetic acid, propionic acid and butanoic acid, or salts of substituted carboxylic acids such as trichloroacetic acid and trifluoroacetic acid. A very suitable source is palladium (II) acetate. In step (a), any bidentate diphosphine that forms an active carbonylation catalyst with palladium may be used in the subject process. Preferably, a bidentate diphosphine ligand of formula R¹R²P-R-PR³R⁴ is employed, in which ligand R represents a divalent organic bridging group, and R¹, R²,

R³ and R⁴ each represent an organic group that is connected to the phosphorus atom through a tertiary carbon atom. Even more preferably, R represents an aromatic bidentate bridging group that is substituted by one or more alkylene groups, and wherein the phosphino groups R¹R²P- and -PR³R⁴ are bound to the aromatic group or to the alkylene group due to the observed high activity, selectivity and/or stability of these ligands. Examples of suitable bidentate ligands include bis (di t- butyl phosphino) -o-xylene and 1, 3-bis- (di-t-butyl phosphino) propane . Bis (di-t-butyl phosphino) -o-xylene has been described in J. Chem. Soc, Chem. Com., 1976, 365, and it is made for instance by treating o-BrCf^Cg^C^Br with HP(t-Bu)2 and subsequent reaction with a base. Bis

(di-t-butyl phosphino) propane has been described in J. Chem. Soc, Dalton Trans., 1991, 863, and it is for instance made by reacting 1, 3-dibromopropane with LiP(t-Bu)2 in tetrahydrofuran as solvent. Examples of suitable catalyst systems are also those disclosed in EP-A-1282629, EP-A-1163202, WO-A-2004/103948 and/or

WO-A-2004/103942. Most preferably R¹, R², R³ and R⁴ are chosen in such way, that the phosphino group PR1R2 differs from the phosphino group PR^R^ .

The ratio of moles of a bidentate diphosphine per mole atom of palladium is not critical. Preferably it ranges from 100 to 0.001, more preferably from 10 to 0.01, yet more preferably from 5 to 0.5, and most preferably in the range of 2 to 1. Applicant found that slightly higher than stoichiometric amounts are beneficial for the stability of the catalyst. The quantity in which the complete catalyst system is used preferably amounts in the range of 10^~8 to 10^~l, preferably in the range of 10^~^ to 10^~2 mole atom of palladium per mole of n-pentenoic acid, more preferably in the range of 10^~^ to 10^~2 mole atom of palladium per mole of n-pentenoic acid.

The carbonylation reaction according to the present invention in step (a) may be carried out at moderate temperatures and pressures. Suitable reaction temperatures are in the range of 0-250 ⁰C, more preferably in the range of 50-200 ⁰C, yet more preferably in the range of from 80-150 ⁰C.

The reaction pressure is usually at least atmospheric pressure. Suitable pressures are in the range of 0.1 to 25 MPa (1 to 250 bar), preferably in the range of 0.5 to 15 MPa (5 to 150 bar) , again more preferably in the range of 1 to 9,5 MPa (10 to 95 bar). Carbon monoxide partial pressures in the range of 0.1 to 9 MPa (1 to 90 bar) are preferred, the upper range of 5 to 9 MPa being more preferred. Pressures above 8 MPa might require special equipment, although the reaction would be faster since the reactivity was found to increase with increased carbon monoxide pressure. Carbon monoxide can be used in its pure form or diluted with an inert gas such as nitrogen, carbon dioxide, noble gases such as argon, and even hydrogen.

The water concentration in the reaction medium of step (a) is preferably maintained within a range of from to 3 to 50%, more preferably from 4 to 30%, yet more preferably from 5 to 25%, and most preferably from 5 to 10 % (w/w) , based on the amount of the total liquid reaction medium.

Step (a) is suitably performed in a single reactor or a cascade of reactors suitable for gas-liquid reactions. Examples of such reactors include constant flow stirred tank reactors or a series of two or more of these reactors and bubble column type reactors, as for instance described in the "Bubble Column Reactors" by Wolf-Dieter Deckwer, Wiley, 1992. A bubble column reactor is a mass transfer and reaction device in which in one or more gases are brought into contact and react with the liquid phase itself or with components dissolved or suspended therein. Preferably, a reactor with forced circulation is employed, for instance those generally termed "ejector reactors", or if the reaction medium is recycled to the reactor, "ejector loop reactors" as described for instance in US-A-5, 159, 092 and JP-A-Il, 269, 110.

Preferably, step (a) is conducted in such way, that the amount of adipic acid present in the reaction mixture remains below the critical value for precipitation under the reaction conditions. This temperature depends on the concentration of the adipic acid in the reaction mixture and hence on the degree of the conversion in step (a) . The exact conversion and hence critical concentration of adipic acid in the reaction mixture can be easily determined at a certain temperature and pressure of step (a) by a skilled person. Most preferably, step (a) is conducted in such way, that a degree of conversion is achieved that permits the adipic acid to remain dissolved in the reactor at the reaction conditions of step (a) , while upon cooling of the reaction mixture to a desired temperature, precipitation takes place. The amount of n-pentenoic acid converted to adipic acid lies preferably in the range of 20 to 30 %w/w, especially when the solvent used is n-pentenoic acid. The concentration of mg palladium (Pd) per kg adipic acid in the reaction mixture of step (a) preferably lies in the range of

0.1-10,000 mg/kg (ppmw) , more preferably in the range of 1-1,000 ppmw. Yet most preferred in the range of 2-1,000 ppmw.

The adipic acid prepared in step (a) was found to precipitate and/or crystallise in step (b) from the reaction mixture obtained in step (a) . The conditions of step (b) upon which the precipitation takes place generally are those at which the critical solubility product of the adipic acid present in the reaction mixture is surpassed, upon which precipitation begins. This may conveniently be achieved by lowering the temperature and/or pressure, and/or by continuing the reaction of step (a) to a higher degree of conversion, or alternatively, removal of n-pentenoic acid from the reaction mixture, for instance by distillation or membrane separation treatment. In the subject specification, the term precipitation is employed in order to describe the crystallisation of the adipic acid from the reaction mixture. In step (b) , the precipitation or crystallisation of adipic acid is preferably achieved by lowering the temperature of the reaction mixture to the desired value. Upon cooling, the adipic acid spontaneously starts to precipitate from the reaction mixture, unless it is present in a very low concentration only. In such a case, the concentration of adipic acid in the solution may be increased by removal of the n-pentenoic acid as set out above .

Preferably, in step (b) the reaction mixture is cooled to a temperature in the range of from 10 ⁰C to 50 ⁰C to allow fast precipitation of the adipic acid. Suitably, the reaction mixture is also depressurized in step (b) , even more preferably to ambient pressure. The gaseous phase including the carbon monoxide may suitably be recycled to step (a) . The concentration of mg Pd per kg precipitated adipic acid of step (b) preferably lies in the range of 1-10,000 ppmw of Pd, more preferably in the range of 2-1,000 ppmw. Yet most preferred in the range of 5-500 ppmw.

In a preferred embodiment of the subject process, steps (a) to (c) are conducted as a continuous process. In this embodiment, step (a) is preferably conducted in such way that a part of the reaction mixture is withdrawn from the reaction mixture of step (a), and then subjected to step (b) . The adipic acid is then separated off in step (c) , while the remainder comprising the catalyst system and n-pentenoic acid is recycled to step (a) . N-pentenoic acid and/or water is then preferably added to the reaction mixture of step (a) in order to replace converted n-pentenoic acid and converted water. In this way, the concentration of adipic acid can be maintained at a constant level in step (a) , thus avoiding precipitation in the reactor, and maintaining the reaction conditions, in particular temperature and carbon monoxide pressure in the reactor. The additional n-pentenoic acid added preferably is obtained from a process for the carbonylation of a conjugated diene, as set out above.

The separation in step (c) may be performed by any suitable known method. Preferably, the separation in step (c) is performed by subjecting the mixture obtained in step (b) to filtration or centrifugation. This separation yields a solid phase consisting mainly of adipic acid, and a liquid supernatant or filtrate, i.e. a liquid solution or mixture comprising the catalyst system in admixture with n-pentenoic acid and possible carbonylation by-products. The concentration of mg Pd per kg separated off adipic acid of step (c) preferably lies in the range of 1-10,000 ppmw, more preferably in the range of 2-1000 ppmw. Yet most preferred in the range of 5-500 ppmw. Suitably, the process according to the invention comprises an additional step (e) , wherein the supernatant or filtrate obtained in step (c) is cooled to a temperature in the range of from -10 °C to below 10 ⁰C, with the proviso that the temperature is chosen such that it remains above the freezing point of the overall solution, to precipitate carbonylation by-products such as 2-methylglutaric acid (α-methyl glutaric acid) and 2-ethylsuccinic acid (α-ethyl succinic acid) , and wherein these precipitated by-products are subsequently separated from a second supernatant or filtrate. Other by-products include esters of n-pentenoic acid, which appear to be reversible under the reaction conditions in step (a) , notably in the presence of the catalyst. Preferably, the supernatant or filtrate obtained in either step (c) or (d) is recycled to step (a) , more preferably after removal of undesired side-products. Water-soluble products are preferably removed from the supernatant or filtrate by bringing the filtrate in contact with water, and by separating an organic phase from the water phase, and returning the organic phase to step (a) .

In step (d) , the adipic acid obtained in step (c) is subjected to a washing treatment. "Washing" within the context of this specification has the meaning of exposing the precipitated crystals or particles of adipic acid to a washing liquid and subsequently removal of that liquid, and optionally drying of the thus obtained washed crystals or precipitate under a gas stream or under vacuum.

It was found that by-products of the carbonylation of n-pentenoic acid in step (a) , for instance branched diacids such as 2-methyl glutaric acid (also known as α-methyl glutaric acid) and 2-ethylsuccinic acid (α-ethyl succinic acid) were at least in part removed from the adipic acid in step (d) . Also, the catalyst system proved particularly soluble in n-pentenoic acid, and therefore could be removed as well.

As washing liquids, n-pentenoic acid, water, and mixtures thereof are suitably employed. The washing step may comprise one or more washing successive treatments. If more than one washing treatment is applied, than the washing liquids may be varied. For instance, by washing first with water, and subsequently with n-pentenoic acid or a mixture of water and n-pentenoic acid, or vice versa. Preferably, the washing step comprises several successive washing treatments. More preferably, the adipic acid obtained in step (c) is first washed one or more times with n-pentenoic acid, and then one or more times with water. Washing results in mixtures of used washing liquids comprising n-pentenoic acid and/or water, the catalyst system, adipic acid and carbonylation byproducts. Suitably, the n-pentenoic acid is isolated from the used washing liquids and recycled to step (a) , after optional removal of by-products or impurities. Catalyst dissolved in the n-pentenoic acid used as washing liquid, or isolated from the used washing liquid is preferably maintained in the n-pentenoic acid, and hence also recycled to step (a) .

If water is employed in step (d) , n-pentenoic acid is removed from the adipic acid together with the water. Since by-products and catalyst components were found to be highly soluble in the n-pentenoic acid, it was found that they were effectively removed as well. The n-pentenoic acid partially dissolves in water, in particular if warm water is employed.

The used washing liquids in step (c) are suitably collected, and then optionally subjected to treatments to remove impurities and by-products. The collected and optionally purified washing liquids are then preferably recycled to process step (a) or (d) as suitable. If water is employed as washing liquid, it will most likely contain n-pentenoic acid, since this is removed from the precipitated adipic acid together with the water. Applicants have found that a spontaneous phase separation to take place in the used washing liquids if water was employed as washing liquid, resulting in a water phase that contains mainly water-soluble by-products and a small amount of n-pentenoic and adipic acid, and an organic phase that comprises mainly n-pentenoic acid in admixture with the catalyst system. It was also found that the components of the catalyst system were almost exclusively detected in the organic phase. The obtained phases can then be easily separated.

The organic phase comprising the n-pentenoic acid is preferably recycled to step (a) , optionally after removal of by-products. The removal of by-products from the n-pentenoic acid obtained in step (d) may be effected by any suitable method preferably a method that does not affect the catalyst activity. Suitable methods include washing of the n-pentenoic acid with water, membrane separations or crystallization or precipitation of the by-products. Preferably, the organic phase comprising the n-pentenoic acid is cooled to a temperature in the range of from -10 ⁰C to 10 ⁰C, with the proviso that the temperature is chosen such that it remains above the freezing point of the overall solution. By-products that precipitate are separated from the remaining liquid n- pentenoic acid, which then may be recycled to step (a) . Alternatively, the n-pentenoic acid separated from the adipic acid may preferably be washed with water to remove impurities that are more soluble in the water phase.

The water phase obtained in the phase separation is preferably recycled to step (d) , since it becomes saturated with adipic acid over time. Optionally, carbonylation by-products or other undesired impurities dissolved in the water may be removed before the recycling.

In a particularly preferred embodiment of the present process, the precipitated adipic acid is first subjected to one or more washing treatments with n-pentenoic acid, and subsequently to one or more washing treatments with water .

Preferably, n-pentenoic acid used for the washing of the adipic acid in step (d) is recycled to step (a) . In a particularly preferred embodiment of the invention, step (d) of the subject process comprises a first washing step, wherein the separated-off adipic acid obtained according to step (c) is washed with n-pentenoic acid, and a subsequent washing step, wherein the adipic acid is washed with water. The n-pentenoic acid comprised in the water wash is then preferably separated of from the water phase by phase separation or any other suitable means of separation, and preferably recycled to step (a) , while the remaining water phase may be recycled to step (d) .The concentration of mg Pd per kg adipic acid obtained after the washing treatments with n-pentenoic acid and/or water in step (d) lies preferably in the range of 0.0001-2 mg/kg (ppmw) , more preferably in the range of 0.0001-1 ppmw. Yet most preferred in the range of 0.001-1 ppmw.

The present process may optionally be carried out in the presence of a solvent, however preferably n-pentenoic acid serves both as the source of anions and as the reaction solvent. The adipic acid obtained according to the subject process is preferably subjected to one or more re-crystallization treatments if a higher purity is required. The subject invention also relates to a process for preparing adipic acid comprising from 0,0001 to

1 ppmw of Pd, and to the adipic acid obtainable by this process. The invention will be illustrated by the following non-limiting examples. Example 1 : Purification of adipic acid - washing with water

A reaction mixture containing 13 weight % of adipic acid obtained from the carbonylation of n-pentenoic acid obtained from the carbonylation of 1, 3-butadiene in the presence of palladium acetate and 1, 2-bis (di-tert . -butyl- phosphinomethyl) benzene (also described herein as Bis (di-t-butyl phosphino) -o-xylene) according to WO-A-2004/103948 was allowed to cool to 20 ⁰C, and the pressure was released. During cooling the adipic acid precipitated and was subsequently filtered off, leaving a liquid filtrate I comprising mainly n-pentenoic acid. The adipic acid still contained a considerable amount of n-pentenoic acid and catalyst. The adipic acid residue was then washed twice with 100 g of distilled water having a temperature of approx. 90 ⁰C, yielding an adipic acid residue, and filtrates II and III. Filtrate II was cooled to room temperature, whereby a phase separation into a water phase and a phase essentially made up of n-pentenoic acid took place. Elemental analysis on palladium and phosphorus content were performed on all the filtrates and the finally recovered adipic acid. The results for the palladium content are listed in Table 1.

Table 1 : Palladium contents of the filtrates and adipic acid

The separated-off water phase separated from filtrate II contained no measurable amounts of Pd and

Phosphorus, whereas the n-pentenoic acid phase contained both Pd and Phosphorus .

Use of water having ambient temperature gave similar results to those obtained in Example 1. Example 2 : Washing with n-pentenoic acid

A carbonylation reaction mixture (weighing 430 g) comprising 22 weight% adipic acid and 18 mol% 2-methyl glutaric acid dissolved in n-pentenoic acid, and further comprising 108 mg Pd was allowed to cool down to 20 ⁰C, upon which cooling adipic acid started to precipitate. The obtained mixture was filtered, and the adipic acid residue was washed three times with each 240 g of n-pentenoic acid not containing any catalyst. The resulting filtrates were subjected to elemental analysis to analyse phosphorus and palladium content (see Table 2) :

Table 2: Elemental analyses (in mg/kg) :

The obtained adipic acid showed a Pd concentration similar to the concentration obtained in Example 1. The results indicate that the subject process permits to remove the complete catalyst system and to obtain highly purified adipic acid without the need for complex multistage crystallisation. The thus obtained adipic acid can be employed for most of its applications, and can be easily further purified if higher purity levels are desired.

Claims

C L A I M S

1. A process for the preparation of adipic acid from n-pentenoic acid, comprising the steps of

(a) contacting the n-pentenoic acid with carbon monoxide and water in the presence of a catalyst system including a source of palladium and a bidentate diphosphine ligand, to obtain a reaction mixture comprising adipic acid, n-pentenoic acid, and catalyst system; and

(b) subjecting the reaction mixture to conditions under which adipic acid precipitates from the reaction mixture to obtain precipitated adipic acid and remaining liquid reaction mixture comprising at least part of the n-pentenoic acid in admixture with the catalyst system; and

(c) separating the precipitated adipic acid from the remaining liquid reaction mixture comprising at least part of the n-pentenoic acid in admixture with the catalyst system to obtain separated off adipic acid; and

(d) washing the separated off adipic acid with n-pentenoic acid and/or water.

2. A process according to claim 1, wherein steps (a) to (d) are performed in a continuous manner.

3. A process according to claim 1 or 2, wherein in step (a) , a liquid stream comprising part of the adipic acid and n-pentenoic acid is withdrawn from the reaction mixture of step (a) .

4. A process according to any one of claims 1 to 3, wherein in step (b) the reaction mixture is cooled to a temperature in the range of from 10 ⁰C to 50 ⁰C to allow precipitation of the adipic acid.

5. A process according to any one of claims 1 to 4, wherein in step (c) the reaction mixture obtained in step (b) is filtrated and/or centrifuged.

6. A process according to any one of claims 1 to 5, wherein n-pentenoic acid obtained in step (c) and/or (d) is recycled to step (a) .

7. A process according to any one of claims 1 to 6, wherein step (d) comprises washing the adipic acid with water to obtain an aqueous mixture comprising n-pentenoic acid, and wherein the n-pentenoic acid is separated from aqueous mixture and recycled to step (a) .

8. A process according to any one of claims 1 to 7, wherein step (d) comprises a first washing step, wherein the separated-off adipic acid obtained in step (c) is washed with n-pentenoic acid, and a subsequent washing step, wherein the adipic acid is washed with water.

9. A process according to any one of claims 1 to 8, wherein the water used in step (d) is recycled to step (d) .

10. A process according to anyone of claims 1 to 9, wherein as the bidentate diphosphine ligand of formula R¹R²P-R-PR³R⁴ is employed, in which ligand R represents a divalent organic bridging group, and R¹, R², R³ and R⁴ each represent an organic group that is connected to the phosphorus atom through a tertiary carbon atom.

11. A process according to any one of claims 1 to 10, wherein the n-pentenoic acid is directly obtained from the carbonylation of 1, 3-butadiene .

12. A process according to any one of claims 1 to 11, wherein the adipic acid obtained is subjected to one or more re-crystallization treatments.

13. A process according to any one of claims 1 to 12, wherein the content of palladium in the adipic acid obtained in step (d) is in the range of from 0,0001 to 1 ppmw .