WO2018065475A1

WO2018065475A1 - Method for the production of methylsuccinic acid and the anhydride thereof from citric acid

Info

Publication number: WO2018065475A1
Application number: PCT/EP2017/075239
Authority: WO
Inventors: Jasper VERDUYCKT; Dirk De Vos
Original assignee: Katholieke Universiteit Leuven
Priority date: 2016-10-05
Filing date: 2017-10-04
Publication date: 2018-04-12
Also published as: GB201616915D0

Abstract

A process for the preparation of methylsuccinic acid in any form, including its salts, its mono- and diester derivatives and the anhydride thereof, which comprises reacting citric acid or a derivative thereof in decarboxylation conditions, said process comprising (i) reacting citric acid or mono- and diester derivatives thereof in a non- aqueous solvent, specifically excluding alcohols, on a metallic catalyst at a temperature between 50 to 400°C and under a partial hydrogen pressure from 0.1 to 50 bar or (ii) reacting citric acid or any salt thereof or mono-, di- and triester derivatives thereof on a metallic catalyst in solvents comprising at least 5% water, at a temperature of from 50 to 400°C under a hydrogen partial pressure from 0.1 to 400 bar

Description

METHOD FOR THE PRODUCTION OF METHYLSUCCINIC ACID AND THE ANHYDRIDE THEREOF FROM CITRIC ACID

FIELD OF THE INVENTION

The present invention relates to a method for producing methylsuccinic acid, said method involving the use of a metallic catalyst under decarboxylation conditions.

BACKGROUND OF THE INVENTION

The present invention relates to a novel route for the production of methylsuccinic acid in any form, including salts, mono- and diester derivatives and the anhydride thereof, by decarboxylation and catalytic hydrogenation of citric acid or any salt thereof or mono-, di- or triester derivatives thereof either in non-aqueous solvents or in solvents comprising at least 5% water.

Methylsuccinic acid constitutes a bifunctional carboxylic acid : such compounds are ubiquitously used as building blocks for the synthesis of e.g. polyamides and polyesters. The introduction of a methyl group in these compounds results in branched bifunctional carboxylic acids, like is the case for methylsuccinic acid. By introducing branched building blocks the properties of the resulting polymers can be tuned. According to Chae et a/. (Journal of Polymer Science Part B: Polymer Physics, Vol. 42, No. 9, 1759-1766, 2004) the addition of methylsuccinic acid to poly(butylenesuccinic acid) improves the tunability of the balance between biodegradability and physical properties, resulting e.g. in a slower crystallization rate and a lower melting temperature (see also: Park et a/., Polymer International, Vol. 51, 239-244, 2002). Novel biodegradable polyesters based on methylsuccinic acid are already claimed in several patents: CN1861660, US8546472 and DE102011080722, which reports the use of methylsuccinic acid in polyesters and polyamides for binders in powder varnishes with good processability. Methylsuccinic anhydride can be used for the same purposes; it is however much more reactive than methylsuccinic acid. In addition, WO2012119861 claims diester derivatives of methylsuccinic acid as solvent in cosmetics.

Methylsuccinic acid is a C5 compound, it is not readily available through the use of fossil feedstock. At the moment, this bio-based building block can only be produced via the hydrogenation of itaconic acid. The conversion of itaconic acid to methylsuccinic acid has been described in patent and open literature (US 2773897, CN 1609089, CN 102617326, Huang et a/., RSC Advances, vol. 5, pp. 97256-97263, 2015). These hydrogenation processes apply typical hydrogenation catalysts, such as Raney Ni, Pd/C, Ru/C etc. at 25-150°C with 1-140 bar H₂. The availability of itaconic acid itself is however very limited (Okabe et al._r Applied microbiology and biotechnology, vol. 84, pp. 597-606, 2009). Methylsuccinic anhydride can be produced via the dehydration of methylsuccinic acid. A method for the catalytic dehydration of succinic acid to succinic anhydride is claimed in CN 105037302 and comprises reacting succinic acid at 150-230°C in the presence of an alkaline earth metal hydroxide or an alkaline earth metal sulphate as catalyst. Cyclic anhydrides, including methylsuccinic anhydride, can also be prepared at a mild temperature of 40°C by reacting the dicarboxylic acid with a dialkyl dicarbonate, such as B0C2O, in the presence of a Lewis acid catalyst, like MgC (Robert et al._r ACS Catalysis, vol. 4, No. 10, pp. 3586-3589, 2014). Other methods involve heating the dicarboxylic acid in acetic anhydride or involve the aid of acid chloride, thionyl chloride, phosphorus pentoxide or other stoichiometric acylating or dehydrating agents (Robert et a/., ACS Catalysis, vol. 4, No. 10, pp. 3586-3589, 2014).

The present invention relates to the direct formation of methylsuccinic acid in any form, including salts, mono- and diester derivatives and the anhydride thereof, from citric acid. As opposed to itaconic acid, citric acid is a much more widely available renewable resource (Berovic et a/., Biotechnology Annual Review, vol. 13, pp. 303- 343, 2007). Therefore, the present invention renders methylsuccinic acid accessible on a global, industrial scale.

The bio-based and widely available citric acid has already been used as platform compound for the synthesis of chemicals. The production of methylsuccinic acid directly from citric acid, however, has never been mentioned. Carlsson et a/. (Industrial & Engineering Chemistry Research, vol. 33, pp. 1989-1996, 1994) reported the spontaneous decarboxylation of citric acid in water at 230-320°C to a mixture of itaconic acid, mesaconic acid, citraconic acid and degradation products. Le Notre et a/. (ChemSusChem, vol. 7, pp. 2712-2720, 2014) reported the double decarboxylation of citric acid to methacrylic acid, which was enhanced by Pt and Pd catalysts in water at 250°C under an inert atmosphere. Methacrylic acid yields up to 41% were noted. US 5391771 claims the hydrogenation of citric acid in non-aqueous solvents at 50-400°C and 1-400 bar. Several hydrogenation products were mentioned using catalysts based on Cu, Co, Ni and Pd : propane-l,2,3-tricarboxylic acid, tetrahydrofurfurylacetic acid and the CI- to C20-alkyl or C7- to C12-aralkyl esters thereof, propane-l,2,3-trimethanol, 3-methyltetrahydrofuran, 3-(2'-hydroxyethyl)tetrahydrofuran, 4-hydroxymethyltetrahydro-pyran, 2-methyl-y- butyrolactone and 3-methyl-y-butyrolactone. Typical products at 200 bar and 200- 250°C include 3-(2'-hydroxyethyl)tetrahydrofuran and 3-methyltetrahydrofuran. EP0277562 describes the hydrogenation of citric acid to 3-methyltetrahydrofuran and 3- and 4-methylbutyrolactone in water using a 1% palladium-4% rhenium catalyst at a temperature of 250°C and a pressure of 69 bar.

SUMMARY OF THE INVENTION

The present invention can be summarized in following statements.

1. A process for the preparation of methylsuccinic acid or salts, mono- and diester derivatives thereof, which comprises reacting citric acid or a derivative thereof in decarboxylation conditions, said process comprising (i) reacting citric acid or mono- and diester derivatives thereof in a non-aqueous solvent, specifically excluding alcohols, on a metallic catalyst at a temperature between 50 to 400°C and under a partial hydrogen pressure from 0.1 to 50 bar or (ii) reacting citric acid or any salt thereof or mono-, di- and triester derivatives thereof on a metallic catalyst in solvents comprising at least 5% water, at a temperature of from 50 to 400°C under a hydrogen partial pressure from 0.1 to 400 bar.

2. The process as described in statement 1 which comprises reacting citric acid or a derivative thereof in decarboxylation conditions, said process comprising reacting citric acid or mono- and diester derivatives thereof in a non-aqueous solvent, specifically excluding alcohols, on a metallic catalyst at a temperature between 50 to 400°C and under a partial hydrogen pressure from 0.1 to 50 bar.

3. The process as described in statement 1 which comprises reacting citric acid or a derivative thereof in decarboxylation conditions, said process comprising reacting citric acid or any salt thereof or mono-, di- and triester derivatives thereof on a metallic catalyst in solvents comprising at least 5% water, at a temperature of from 50 to 400°C under a hydrogen partial pressure from 0.1 to 400 bar.

4. The process as described in statement 2 where the process further comprises the formation of methylsuccinic anhydride by dehydrating the obtained methylsuccinic acid.

5. The process as described in statement 4 wherein the yield of methylsuccinic anhydride is at least 20%.

6. The process as described in statement 5 where a temperature of from 250°C to 400°C is used.

7. The process as described in any one of statements 5 to 6 which comprises the use of a hydrogen partial pressure from 0.1 to 50 bar. 8. The process as described in any one of statements 5 to 7 which comprises the use of a catalyst at a concentration from 0.01 to 20 mol% of active compound relative to the substrate.

9. The process as described in any one of statements 5 to 8, wherein the production of methylsuccinic anhydride is split into two steps, which might be conducted in two vessels.

10. The process as described in any one of statements 5 to 9, wherein a dehydration catalyst and/or a water-removing compound is added.

11. The process as described in any one of statements 1 to 10 wherein the yield of propane-l,2,3-tricarboxylic acid is less than 10%.

12. The process as described in any one of statements 1 to 3 wherein the yield of methylsuccinic acid is at least 30%.

13. The process as described in any one of statements 1 to 12 wherein metallic catalysts are employed whose catalytically active material contains one or more elements from group IB, VIIB or VIIIB of the Periodic Table of the Elements.

14. The process as described in any one of statements 1 to 13, wherein metallic catalysts are employed whose catalytically active material contains one or more elements from group IB, VIIB or VIIIB of the Periodic Table of the Elements and one or more elements from main groups IV and V of the Periodic Table of the Elements and/or one or more elements from groups IIB to VIB of the Periodic Table of the Elements and/or one or more elements from main groups I and II of the Periodic Table of the Elements, specifically excluding the use of rhenium.

15. The process as described in any one of statements 1 to 14, wherein metallic catalysts are employed which contain palladium, rhodium, ruthenium, platinum, nickel, silver, copper and/or cobalt.

16. The process as described in any one of statements 1 to 15, wherein metallic catalysts are employed which contain copper and/or one or more elements from group VIIIB and molybdenum, tungsten, manganese and/or zinc.

17. The process as described in any one of statements 1 to 16, wherein the reaction is fed by a raw, non-purified stream of citric acid.

18. The process as described in any one of statements 1 or 3 or any one of statements 11 to 17, wherein the solvent is an aqueous medium at pH from 0 to 7, including the presence of added acids and bases.

19. The process as described in any one of statements 1 to 17, wherein the organic solvent is selected from the group consisting of ethers, organic acids, esters, organic carbonates, sulfolane, ketones, nitriles, aromatic solvents, amides, dimethyl sulfoxide, alcohols and mixtures thereof, as well as mixtures with water.

20. The process as described in any one of statements 1 to 17, wherein the organic solvent is ethylene glycol dimethyl ether, ethylene glycol diethyl ether, 1,4- dimethoxybutane, 1,4-diethoxybutane, diglyme, tetrahydrofuran, 2- methyltetrahydrofuran, dioxane, sulfolane, ethylene carbonate, dimethyl carbonate or diethyl carbonate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will become more fully understood from the detailed description and the accompanying drawing which are given by way of illustration only, and thus are not limitative of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 gives a schematic representation of the reactions to produce methylsuccinic acid and methylsuccinic anhydride. The sequence of reactions to produce methylsuccinic acid directly from citric acid is: i) dehydration of citric acid to aconitic acid; ii) decarboxylation of aconitic acid to form itaconic acid and its isomers, which occurs spontaneously, but which could also be aided by metallic catalysts and iii) hydrogenation of the C=C double bond. To produce methylsuccinic anhydride the fourth step is the dehydration/cyclisation of methylsuccinic acid.

Important to selectively produce methylsuccinic acid is the delicate balance between the reaction rates of i) the initial dehydration of citric acid to aconitic acid; ii) the decarboxylation of aconitic acid to form itaconic acid and its isomers (mesaconic and citraconic acid); iii) the hydrogenation of the C=C double bond; iv) the further decarboxylation of itaconic acid and its isomers and v) the hydrogenation of the carboxylic acid groups. Surprisingly, it was found that hydrogenating citric acid under decarboxylation conditions gives rise to the production of methylsuccinic acid.

The invention provides a process for the preparation of methylsuccinic acid in any form, including its salts, its mono- and diester derivatives and the anhydride thereof. The process of the present invention comprises reacting citric acid or a derivative thereof under decarboxylation conditions, more specifically said process comprises (i) reacting citric acid or mono- and diester derivatives thereof in a non-aqueous solvent, specifically excluding alcohols, on a metallic catalyst at a temperature between 50 to 400°C and under a partial hydrogen pressure from 0.1 to 50 bar or (ii) reacting citric acid or any salt thereof or mono-, di- and triester derivatives thereof on a metallic catalyst in solvents comprising at least 5% water, at a temperature of from 50 to 400°C under a hydrogen partial pressure from 0.1 to 400 bar.

In a first embodiment of the present invention, the process according to the invention is carried out as follows : citric acid or mono- and diester derivatives thereof are reacted in a non-aqueous solvent, specifically excluding alcohols, on a metallic catalyst at a temperature between 50 to 400°C and under a partial hydrogen pressure from 0.1 to 50 bar.

Under these conditions, citric acid can be added as such, but also as a hydrated form, such as citric acid monohydrate, or as a salt, such as trisodium citrate, tripotassium citrate, trimagnesium dicitrate and tricalcium dicitrate, as well as a combination of both, such as trisodium citrate dihydrate, tripotassium citrate monohydrate, trimagnesium dicitrate nonahydrate and tricalcium dicitrate tetrahydrate. Mono- or diesters of citric acid can also be used as starting compound . Examples of such mono- and diesters of citric acid are: monomethyl citrate, monoethyl citrate, mono-n-propyl citrate, monoisopropyl citrate, mono-n-butyl citrate, monoisobutyl citrate, mono-sec- butyl citrate, mono-tert-butyl citrate, dimethyl citrate, diethyl citrate, di-n-propyl citrate, diisopropyl citrate, di-n-butyl citrate, diisobutyl citrate, di-sec-butyl citrate and di-tert-butyl citrate. Mixtures of the above mentioned compounds can be used as well. All these starting materials can be introduced into the process according to the invention in solid, liquid or gaseous form . The use of citric acid or an hydrated form thereof is preferred . For the production of methylsuccinic anhydride, the use of citric acid in the anhydrous form would be preferred .

Suitable hydrogenation catalysts are all conventional catalysts, as described e.g . in Nishimura, Handbook of heterogeneous catalytic hydrogenation for organic synthesis, Chapter 1 : Hydrogenation catalysts, John Wiley & Sons, New York, 2001. Preferred hydrogenation catalysts are those whose catalytically active material contains one or more metals from group IB, VIIB or VIIIB of the Periodic Table of the Elements, such as copper, silver, gold, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, preferably silver, copper, iron, nickel, cobalt, palladium, platinum, rhodium and ruthenium, particularly preferably palladium, rhodium, ruthenium, silver, copper, nickel and cobalt; and if desired, one or more metals from groups IIB to VIB of the Periodic Table of the Elements, such as zinc, scandium, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten, preferably zinc, chromium, molybdenum, lanthanum, zirconium and tungsten, particularly preferably zinc, chromium, molybdenum and tungsten, and if desired, elements from main groups I and II of the Periodic Table of the Elements, such as lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium and barium, preferably lithium, sodium, potassium, magnesium and calcium, and if desired, elements from main groups IV and V of the Periodic Table of the Elements, such as lead and bismuth. These catalysts might be in the form of their oxides at the start of the hydrogenation and may additionally contain acids, such as phosphoric acid, boric acid, sulfuric acid, hydrofluoric acid and heteropoly acids, preferably phosphoric acid and heteropoly acids, particularly preferably phosphoric acid. The hydrogenation catalysts can be employed as homogeneous or preferably as heterogeneous catalysts. If heterogeneous catalysts are used, they can be employed either as supported catalysts or in compact form. The type of support material is generally not crucial; conventional support materials, such as silicon dioxide, aluminium oxides, titanium dioxide, zirconium dioxide, calcium carbonate, barium sulphate, activated charcoal, silicates or zeolites, can be used. If necessary, binders or shaping auxiliaries can also be employed to prepare the catalysts. In particular, noble-metal catalysts are usually employed on supports, such as charcoal (e.g. activated charcoal), aluminium oxide or zirconium dioxide, e.g. palladium on charcoal, palladium on aluminium oxide, rhodium on charcoal, platinum on charcoal, ruthenium on charcoal and ruthenium on zirconium dioxide. Preferred catalysts are Palladium on charcoal, Pd/BaS0₄, Pd/ZrC>2

Non-aqueous solvents are those solvents to which water is not or has not been added. The non-aqueous solvents may be moist, thus they do not require drying, i.e. the removal of residual water. However, for the production of methylsuccinic anhydride the removal of residual water leaving a maximum of 0.1% water would be beneficial. Examples of suitable non-aqueous solvents are ethers, such as dialkyl ethers, preferably dialkyl ethers containing CI- to C20-alkyl groups, particularly preferably dialkyl ethers containing CI- to C8-alkyl groups, e.g. diethyl ether, methyl tert-butyl ether, di-n-butyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether and diglyme, and cyclic ethers, such as 5- to 8-membered ring ethers, e.g. furan, tetrahydrofuran, 2-methyltetrahydrofuran, pyran, dihydropyran and dioxane, preferably ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diglyme, tetrahydrofuran, 2-methyltetrahydrofuran and dioxane, particularly preferably ethylene glycol diethyl ether, diglyme, 2-methyltetrahydrofuran and tetrahydrofuran; organic acids, such as formic acid and acetic acid; esters, such as ethyl acetate; organic carbonates, such as ethylene carbonate, dimethyl carbonate, diethyl carbonate and diphenyl carbonate; and sulfolane. Also ketones, such as acetone and 2-butanone; nitriles, such as acetonitrile; aromatic solvents, such as benzene, toluene, m-, o- and p-xylene; amides, such as formamide, 2-pyrrolidone, dimethylformamide, N-butyl-2-pyrrolidone, N-propyl-2-pyrrolidone, N-ethyl-2- pyrrolidone and N-methyl-2-pyrrolidone; and dimethyl sulfoxide can be used. It is also possible for citric acid or the other starting compounds or even for the reaction products to function as solvent. The temperature for the reaction is between 50°C and 400°C, preferably between 100°C and 350°C, or more preferably between 150°C and 300°C. The reaction is carried out under a partial hydrogen pressure from 0.1 to 50 bar, preferably from 0.1 to 30 bar, or more preferably from 0.1 to 20 bar.

The process of the present invention can be carried out in the gas or liquid phase, and either batchwise or, preferably, continuously. If a heterogeneous catalyst is used, it can be employed as a suspension or fixed-bed catalyst. Reactors which can be employed are stirred or tubular reactors. A tubular reactor with a fixed catalyst can be operated with upflow or downflow through the catalyst.

When working in non-aqueous solvents the carboxylic acid groups of citric acid or the carboxylic acid esters, if they would be used, can be hydrogenated to the corresponding alcohols, leading to the ethereal products mentioned in patent US 5391771, such as ethyl tetrahydrofurfurylacetate and 3-(2'- hydroxyethyl)tetrahydrofuran. According to the examples in US 5391771, this hydrogenation of carboxylic acid esters starts from 50 bar H 2 onwards, using also temperatures from 175 to 225°C. Therefore, to avoid these products, a H₂ pressure that is not too high (< 50 bar), is mostly preferred for non-aqueous solvents. According to the examples in EP0277562, hydrogenation of the carboxylic acid groups to the corresponding alcohols can also occur in water at 250°C and a pressure of 69 bar with Re: Pd (4: 1) as the catalyst, leading to 3-methyltetrahydrofuran and 3- and 4-methylbutyrolactone. Therefore, the use of rhenium should be avoided under these conditions. The method according to the first embodiment of the present invention can be used to produce methylsuccinic anhydride by dehydration and cyclisation of the obtained methylsuccinic acid. To this end, the dehydration/cyclisation of methylsuccinic acid must be promoted. To promote the dehydration a higher temperature is needed, preferably higher than 200°C, more preferably higher than 250°C if no dehydration catalysts and/or compounds that remove water are added. When methylsuccinic anhydride would be produced in one step from citric acid, further decarboxylation of itaconic acid and its isomers should be avoided. Therefore hydrogenation of the C=C double bond has to be accelerated, by e.g. the use of a highly active hydrogenation catalyst such as the above described catalysts, a higher H 2 pressure with a maximum of 50 bar and/or a larger amount of catalyst with a maximum of 20 mol%. The yield of methylsuccinic anhydride can be increased by adding dehydration catalyst such as alkaline earth metal hydroxide or alkaline earth metal sulphate and/or a compound that removes water from the medium such as calcium sulfate, zeolite 4A, acid chloride, thionyl chloride, phosphorus pentoxide or other stoichiometric acylating compounds. Alternatively, the production of methylsuccinic anhydride can be performed in two steps, wherein first methylsuccinic acid is produced under milder conditions to avoid the further decarboxylation of itaconic acid and its isomers and secondly methylsuccinic acid is dehydrated to methylsuccinic anhydride by increasing the temperature. The addition of a dehydration catalyst and/or a compound that removes water from the medium, can also improve the yield of methylsuccinic anhydride in this case. When this process would be executed in two steps, it is also possible to split the process into two vessels; in the first methylsuccinic acid is then produced under milder conditions and in the second methylsuccinic acid is converted to methylsuccinic anhydride. In this second vessel the reaction conditions can then be optimized for the production of the anhydride; also a dehydration catalyst such as alkaline earth metal hydroxide or alkaline earth metal sulphate and/or a compound that removes water from the medium such as calcium sulfate, zeolite 4A, acid chloride, thionyl chloride, phosphorus pentoxide or other stoichiometric acylating compounds, can be added. In a second embodiment of the present invention, the process according to the invention is carried out as follows: citric acid or any salt thereof or mono-, di- or triester derivatives of citric acid are reacted on a metallic catalyst in water or in a solvent comprising at least 5% water, at a temperature of from 50 to 400°C under a hydrogen partial pressure from 0.1 to 400 bar.

Under these conditions, citric acid can be added as such, but also as a hydrated form, such as citric acid monohydrate, or as a salt, such as trisodium citrate, tripotassium citrate, trimagnesium dicitrate and tricalcium dicitrate, as well as a combination of both, such as trisodium citrate dihydrate, tripotassium citrate monohydrate, trimagnesium dicitrate nonahydrate and tricalcium dicitrate tetrahydrate. Also raw citric acid solutions derived from its industrial production, such as the fermentation medium or fruit juice can be used to feed the reaction. Mono-, di- or triesters of citric acid can also be used as starting compound. Examples of such mono-, di- and triesters of citric acid are: monomethyl citrate, monoethyl citrate, mono-n-propyl citrate, monoisopropyl citrate, mono-n-butyl citrate, monoisobutyl citrate, mono-sec- butyl citrate, mono-tert-butyl citrate, dimethyl citrate, diethyl citrate, di-n-propyl citrate, diisopropyl citrate, di-n-butyl citrate, diisobutyl citrate, di-sec-butyl citrate, di-tert-butyl citrate, trimethyl citrate, triethyl citrate, tri-n-propyl citrate, triisopropyl citrate, tri-n-butyl citrate, triisobutyl citrate, tri-sec-butyl citrate and tri-tert- butyl citrate. Mixtures of the above mentioned compounds can be used as well. All these starting materials can be introduced into the process according to the invention in solid, liquid or gaseous form. The use of citric acid or an hydrated form thereof is preferred.

Suitable hydrogenation catalysts are all conventional catalysts, as described e.g . in Nishimura, Handbook of heterogeneous catalytic hydrogenation for organic synthesis, Chapter 1 : Hydrogenation catalysts, John Wiley & Sons, New York, 2001. Preferred hydrogenation catalysts are those whose catalytically active material contains one or more metals from group IB, VIIB or VIIIB of the Periodic Table of the Elements, such as copper, silver, gold, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, preferably silver, copper, iron, nickel, cobalt, palladium, platinum, rhodium and ruthenium, particularly preferably palladium, rhodium, ruthenium, silver, copper, nickel and cobalt, specifically excluding rhenium; and if desired, one or more metals from groups IIB to VIB of the Periodic Table of the Elements, such as zinc, scandium, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten, preferably zinc, chromium, molybdenum, lanthanum, zirconium and tungsten, particularly preferably zinc, chromium, molybdenum and tungsten, and if desired, elements from main groups I and II of the Periodic Table of the Elements, such as lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium and barium, preferably lithium, sodium, potassium, magnesium and calcium, and if desired, elements from main groups IV and V of the Periodic Table of the Elements, such as lead and bismuth. These catalysts might be in the form of their oxides at the start of the hydrogenation and may additionally contain acids, such as phosphoric acid, boric acid, sulfuric acid, hydrofluoric acid and heteropoly acids, preferably phosphoric acid and heteropoly acids, particularly preferably phosphoric acid. The hydrogenation catalysts can be employed as homogeneous or preferably as heterogeneous catalysts. If heterogeneous catalysts are used, they can be employed either as supported catalysts or in compact form. The type of support material is generally not crucial; conventional support materials, such as silicon dioxide, aluminium oxides, titanium dioxide, zirconium dioxide, calcium carbonate, barium sulphate, activated charcoal, silicates or zeolites, can be used. If necessary, binders or shaping auxiliaries can also be employed to prepare the catalysts. In particular, noble-metal catalysts are usually employed on supports, such as charcoal (e.g. activated charcoal), aluminium oxide or zirconium dioxide, e.g. palladium on charcoal, palladium on aluminium oxide, rhodium on charcoal, platinum on charcoal, ruthenium on charcoal and ruthenium on zirconium dioxide. Preferred catalysts are Palladium on charcoal, Pd/BaS0₄, Pd/Zr0₂ and Ni/Si02-AI₂0₃.

Alcohols, such as methanol, ethanol, n-propanol, isopropanol, 1-butanol, 2-butanol and tert-butyl alcohol, can only be used when water is added to an alcoholic solvent. Ester groups can be hydrolysed to carboxylic acid groups, enabling the decarboxylation. Water should be added to the alcohol solvent such that the concentration of water is at least 5%, preferably at least 10%, more preferably at least 20%, for instance at least 30%, at least 40% or at least 50% with a maximum of 90%, preferably 85%, more preferably 80%.

It is also possible for citric acid or the other starting compounds or even for the reaction products to function as solvent. If water is used as a solvent, this is used at pH from 0 to 14, including the addition of acids, such as hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid, and bases, such as sodium hydroxide, potassium hydroxide, magnesium hydroxide and calcium hydroxide. Preferably an acidic environment is used at a pH between 0 to 7, preferably at a pH between 0 to 5. Citric acid in its acid form without adding the abovementioned acids or bases is preferably used.

When working in solvents comprising at least 5% water, citric acid can also undergo fragmentation reactions that lead a.o. to the formation of acetone, acetic acid and pyruvic acid. This fragmentation is promoted by a more alkaline environment, as well as by a higher temperature. For the selective production of methylsuccinic acid in water an acidic environment (pH< 5) and a temperature that is not too high (T<250°C) are thus preferred. In either embodiment, the use of the process of the present invention leads to formation of methylsuccinic acid with a yield of at least 30%, preferably at least 40%, more preferably at least 50%. Using the process described in both embodiments leads to formation of propane-, 1, 2, 3-tricarboxylic acid with a yield of maximum 10%, preferably maximum 9%, more preferably maximum 8%.

In the first embodiment, the optional use of a process promoting dehydration and cyclisation of methylsuccinic acid leads to formation of methylsuccinic anhydride with a yield of at least 20%, preferably at least 35%, more preferably at least 50%. Using this process in which methylsuccinic anhydride is formed leads to formation of propane-1, 2, 3-tricarboxylic acid with a yield of maximum 10%, preferably maximum 9%, more preferably maximum 8%.

Depending on the solvent and catalyst used, the reaction conditions can be tuned to provide the right balance between the different reaction rates and to selectively produce methylsuccinic acid in any form, including salts, mono- and diester derivatives and the anhydride thereof.

In general if the hydrogenation of the generated C=C double bond (after dehydration of citric acid or the like) exceeds the rate of decarboxylation, the yield of methylsuccinic acid will decrease because of the formation of propane-1, 2, 3- tricarboxylic acid. This is e.g. the case when lower temperatures are used (T< 175°C) : decarboxylation will slow down more rapidly than hydrogenation; or when higher H₂ pressures are used; or when a larger amount of catalyst is used; or when highly active hydrogenation catalysts are used. The activity of the catalysts is e.g. related to the dispersion of the active metal. These parameters are all connected to each other, leading to several possible combinations of reaction conditions, catalysts and solvents that avoid the formation of propane-1, 2, 3-tricarboxylic acid.

On the other hand, if the hydrogenation of the generated C=C double bond (after dehydration of citric acid or the like and a first decarboxylation) becomes too slow, the yield of methylsuccinic acid will decrease because of further decarboxylation of itaconic acid and its isomers, which leads to the formation of isobutyric acid, butyric acid and even propane. This is e.g. the case when higher temperatures are used (T>250°C) : further decarboxylation will increase more rapidly than hydrogenation; or when lower H 2 pressures are used; or when a lower amount of catalyst is used; or when less active hydrogenation catalysts are used. These parameters are all connected to each other, leading to several possible combinations of reaction conditions, catalysts and solvents that minimize the further decarboxylation to isobutyric acid, butyric acid and propane. EXAMPLES

The identification of the reaction products below was done via H-, ¹³C- and ^,¹^- HSQC-NMR, as well as with GC-MS where possible. The yields were determined via ^-NMR. In the tables below, yields on molar basis are given as Y (%). All catalysts used contain 5 wt% of the active metal, unless stated otherwise.

EXAMPLES 1 TO 9

42 mg of citric acid monohydrate was dissolved in 2 mL of water; 0.8 eq. of NaOH was added in some of the reactions. 4 mol% of active metal (catalyst in powder form) was added and the reactor was flushed 6 times with N₂. Then the reactor was loaded with 4 bar H₂ and heated to 225°C for a period of 6 h or 40 min. The conversion of citric acid was >99% in all cases. The highest yields in these conditions (67-85%) were obtained with Pd and Rh catalysts.

Table 1

Example Catalyst t NaOH Ymethylsuccinic YpTA ⁵ YlBA/BA ^b Yfragmentation ° No. (h) acid (%) (%) (%) (%)

1 Pt/C 6 x 31 < 1 23 9

2 Ru/C 0.67 - 48 8 14 5

3 Rh/C 0.67 - 69 5 12 6

4 Pd/C 6 x 67 2 12 7

5 Pd/Zr0₂ 6 x 81 < 1 7 6

6 Pd/MgAI₂0₄ 6 x 71 7 5 12

7 Pd/BaS0₄ 6 x 81 < 1 3 10

8 Pd/Al₂0₃ 6 x 71 < 1 14 9

9 Pd₃Pb/Zr0₂ 0.67 - 85 < 1 1 4 ^a Propane-l,2,3-tricarboxylic acid. ^b Isobutyric acid and butyric acid. ^c Acetone and acetic acid. EXAMPLE 10

42 mg of citric acid monohydrate was dissolved in 2 mL of water and the reactor was flushed 6 times with N₂. No catalyst was added, making this example the blank reaction. Then the reactor was loaded with 4 bar H₂ and heated to 225°C for a period of 6 h. The conversion of citric acid was >99% and the observed products were methacrylic acid (22%), acetone (4%), acetic acid (33%), pyruvic acid (2%), citraconic acid (7%), mesaconic acid (11%) and itaconic acid (9%). EXAMPLES 11-14

42 mg of citric acid monohydrate was dissolved in 2 mL of water, NaOH was added, 4 mol% of Pd (catalyst in powder form) was added and the reactor was flushed 6 times with N₂. Then the reactor was loaded with H₂ and heated to 225°C for a period of 6 h. The conversion of citric acid was >99% in all cases. Examples 12-14 show that a minimal H₂ pressure is needed to attain yields of methylsuccinic acid of 74% or more.

Table 2

Example l P_H2 NaOH Ymethylsuccinic YpTA ⁵ YlBA/BA Yfragmentation °

No. (bar) (eq .) acid (%) (%) * (%) (%)

11 Pd/C 0 ^d 0.8 13 < 1 21 30

12 Pd/BaS0₄ 4 1 75 < 1 5 8

13 Pd/BaS0₄ 8 1 75 < 1 6 9

14 Pd/BaS0₄ 12 1 74 4 3 15 ^a Propane- l,2,3-tricarboxylic acid . ^b Isobutyric acid and butyric acid . ^c Acetone and acetic acid . ^d No externally added H₂; there is H₂ production in situ though. Also methacrylic acid ( 13%) and 2-hydroxyisobutyric acid ( 12%) are formed under these conditions.

EXAMPLES 15-21

42 mg of citric acid monohydrate was dissolved in 2 mL of water. H₃P0₄ or NaOH were added . 4 mol% of Pd/BaS0₄ (catalyst in powder form) was added and the reactor was flushed 6 times with N₂. Then the reactor was loaded with 4 bar H₂ and heated to 225°C for a period of 6 h or 40 min . The conversion of citric acid was >99% in all cases. The results show that high yields of methylsuccinic acid, up to 86%, are obtained in the absence of added base. Addition of base significantly decreases yields.

Table 3

Example t (h) Additive Ymethylsuccinic YpTA ⁵ YlBA/BA ^b Yfragmentation No. acid (%) (%) (%) ^C (%)

15 0.67 1 eq . H₃P0₄ 81 < 1 3 6

16 0.67 - 86 < 1 3 6

17 6 - 84 < 1 3 7

18 6 1 eq . NaOH 75 < 1 5 8

19 6 1.5 eq . NaOH 64 < 1 4 32

20 6 2 eq . NaOH 54 < 1 2 46

21 6 2.5 eq . NaOH 25 < 1 2 72 ^a Propane-l,2,3-tricarboxylic acid . ^b Isobutyric acid and butyric acid. ^c Acetone, acetic acid, pyruvic acid and lactic acid . EXAMPLES 22-25

42 mg of citric acid monohydrate was dissolved in 2 mL of water; 4 mol% of Pd/BaS0₄ (catalyst in powder form) was added and the reactor was flushed 6 times with N₂. Then the reactor was loaded with 4 bar H₂ and heated for a period of 6 h or 40 min. Depending on the reaction times, any reaction temperature between 175 and 225°C allows to reach yields superior to 80%. The best time-temperature combination under these conditions is 40 minutes reaction at 225°C.

Table 4

Example T t (h) Xcitric acid Ymethylsuccinic YpTA ⁵ YlBA/BA Yfrag mentation

No. (°C) (%) acid (%) (%) * (%) ^C (%)

22 175 0.67 12 8 4 < 1 < 1

23 175 6 >99 82 7 1 6

24 200 6 >99 83 7 1 5

16 225 0.67 >99 86 < 1 3 6

17 225 6 >99 84 < 1 3 7

25 250 6 >99 58 < 1 10 9 ^a Propane-l,2,3-tricarboxylic acid. ^b Isobutyric acid and butyric acid. ^c Acetone and acetic acid.

EXAMPLES 26-28

42 mg of citric acid monohydrate was dissolved in 2 mL of water, 0.5 mol% of Pd (catalyst in powder form) was added and the reactor was flushed 6 times with N₂. Then the reactor was loaded with H₂ and heated to 225°C for a period of 40 min. The conversion of citric acid was >99% in all cases. Example 27 shows that a proper combination of H₂ pressure, time and temperature also allows to reach high yields with a minimal amount of the commercial Pd/C catalyst.

Table 5

Example Catalyst P_H2 Ymethylsuccinic YpTA ⁵ YlBA/BA ^b Yfrag mentation

No. (bar) acid (%) (%) (%) ^C (%)

26 Pd/C 4 82 4 5 6

27 Pd/C 8 84 5 2 7

28 Pd/BaS0₄ ^d 20 36 < 1 3 32 ^a Propane-l,2,3-tricarboxylic acid. ^b Isobutyric acid and butyric acid. ^c Acetone, acetic acid and pyruvic acid. ^d Also methacrylic acid (8%), citraconic acid (5%), mesaconic acid (6%) and itaconic acid (6%) are formed under these conditions. EXAMPLES 29-33

42 mg of citric acid monohydrate was dissolved in 2 mL of solvent; 4 mol% of Pd/BaS0₄ (catalyst in powder form) was added and the reactor was flushed 6 times with N₂. Then the reactor was loaded with 4 bar H₂ and heated to 200°C for a period of 6 h . The conversion of citric acid was >99% in all cases. Especially the ether solvent tetrahydrofuran allows to reach hig h yields of methylsuccinic acid .

Table 6

Example No. Solvent Ymethylsuccinic acid (%)

29 Acetic acid 67

30 Methanol 7 ^a

31 Ethanol 19 ^b

32 Ethanol : Water 74

(20 : 80)

33 Tetrahydrofuran 82

a In the form of the dimethyl ester, the main product is the trimethyl ester of citric acid . ^b In the form of the diethyl ester, the main product is the triethyl ester of citric acid .

EXAMPLE 34

420 mg of citric acid monohydrate was dissolved in 20 mL of diglyme, 0.5 mol% of Pd/C (catalyst in powder form) was added and the reactor was flushed 3 times with N₂ and 3 times with H₂. Then the reactor was loaded with 20 bar H₂ and heated to 275°C for a period of 50 min . This reaction resulted in the production of methylsuccinic anhydride (28%), methylsuccinic acid (49%) and isobutyric acid (9%).

EXAMPLE 35

420 mg of citric acid monohydrate was dissolved in 20 mL of water, 0.5 mol% of Pd/C (catalyst in powder form) was added and the reactor was flushed 3 times with N₂ and 3 times with H₂. Then the reactor was loaded with 69 bar H₂ and heated to 250°C for a period of 50 min . This reaction resulted in the production of mainly methylsuccinic acid (75%) and propane-l,2,3-tricarboxylic acid (9%). Using Pd/C as the catalyst, methylsuccinic acid remains the primary product for the reaction in water at 250°C and a pressure of 69 bar H₂. EXAMPLE 36

42 mg of citric acid monohydrate was dissolved in 2 mL of water, 12 mol% of Ni/Si0₂- AI₂C>3 (catalyst in powder form, 65 wt% Ni) was added and the reactor was flushed 6 times with N₂. Then the reactor was loaded with 20 bar H₂ and heated to 175°C for a period of 6 h. This reaction resulted in the production of methylsuccinic acid (83%). Also non-noble metal catalysts, like Ni/Si02-AI₂03, allow to reach high yields of methylsuccinic acid.

Claims

A process for the preparation of methylsuccinic acid or salts, mono- and diester derivatives thereof by reacting citric acid or a derivative thereof in decarboxylation conditions, said process comprises the steps of :

(i) reacting citric acid or mono- and diester derivatives thereof in a nonaqueous solvent, specifically excluding alcohols, on a metallic catalyst at a temperature between 50 to 400°C and under a partial hydrogen pressure from 0.1 to 50 bar or

(ii) reacting citric acid or any salt thereof or mono-, di- and triester derivatives thereof on a metallic catalyst in solvents comprising at least 5% water, at a temperature of from 50 to 400°C under a hydrogen partial pressure from 0.1 to 400 bar.

The process according to claim 1 comprising the step of reacting citric acid or mono- and diester derivatives thereof in a non-aqueous solvent, specifically excluding alcohols, on a metallic catalyst at a temperature between 50 to 400°C and under a partial hydrogen pressure from 0.1 to 50 bar.

The process according to claim 1, comprising the step of reacting citric acid or any salt thereof or mono-, di- and triester derivatives thereof on a metallic catalyst in solvents comprising at least 5% water, at a temperature of from 50 to 400°C under a hydrogen partial pressure from 0.1 to 400 bar.

The process according to claim 2, where the process further comprises the formation of methylsuccinic anhydride by dehydrating the obtained methylsuccinic acid.

The process according to claim 4, wherein the yield of methylsuccinic anhydride is at least 20%.

6. The process according to claim 5, where a temperature between 250°C and 400°C is used. 7. The process according to any one of claims 5 to 6, which comprises the use of a hydrogen partial pressure from 0.1 to 50 bar.

8. The process according to any one of claims 5 to 7, which comprises the use of a catalyst at a concentration from 0.01 to 20 mol% of active compound relative to the substrate.

9. The process according to any one of claims 5 to 8, wherein the production of methylsuccinic anhydride is split into two steps, optionally conducted in two vessels. 10. The process according to any one of claims 5 to 9, wherein a dehydration catalyst and/or a water-removing compound is added.

The process according to any one of claims 1 to 10, wherein the yield of propane-l,2,3-tricarboxylic acid is less than 10%.

The process according to any one of claims 1 to 3, wherein the yield of methylsuccinic acid is at least 30%.

The process according to any one of claims 1 to 12, wherein metallic catalysts are employed whose catalytically active material contains one or more elements from group IB, VIIB or VIIIB of the Periodic Table of the Elements.

The process according to any one of claims 1 to 13, wherein metallic catalysts are employed whose catalytically active material contains one or more elements from group IB, VIIB or VIIIB of the Periodic Table of the Elements and one or more elements from main groups IV and V of the Periodic Table of the Elements and/or one or more elements from groups IIB to VIB of the Periodic Table of the Elements and/or one or more elements from main groups I and II of the Periodic Table of the Elements, specifically excluding the use of rhenium.

The process according to any one of claims 1 to 14, wherein metallic catalysts are employed which contain palladium, rhodium, ruthenium, platinum, nickel, silver, copper and/or cobalt. The process according to any one of claims 1 to 15, wherein metallic catalysts are employed which contain copper and/or one or more elements from group VIIIB and molybdenum, tungsten, manganese and/or zinc. 17. The process according to any one of claims 1 to 16, wherein the reaction is fed by a raw, non-purified stream of citric acid.

The process according to any one of claims 1 or 3 or any one of claims 11 to 17, wherein the solvent is an aqueous medium at pH from 0 to 7, including the presence of added acids and bases.

19. The process according to any one of claims 1 to 17, wherein the organic solvent is selected from the group consisting of ethers, organic acids, esters, organic carbonates, sulfolane, ketones, nitriles, aromatic solvents, amides, dimethyl sulfoxide, alcohols and mixtures thereof, as well as mixtures with water.

20. The process according to any one of claims 1 to 17, wherein the organic solvent is selected from the group consisting of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, 1,4-dimethoxybutane, 1,4- diethoxybutane, diglyme, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, sulfolane, ethylene carbonate, dimethyl carbonate or diethyl carbonate.