Field of the invention

The present invention relates to a process to produce alkanoic acid esters in a carbonylation process and to a process to produce adipic acid dialkyl ester.

Background of the invention

Alkanoic acid esters are important intermediates in the production of industrially important compounds. For example, adipic acid (1 ,6-hexanedioic acid) is a precursor for the production of polyamides such as polyamide 6,6.

A known process for the production of alkanoic acid esters involves the alkoxycarbonylation of alkenes using a group 8-10 metal in the presence of an alkanol. For example, WO2001/068583 describes the Pd catalyzed carbonylation of ethylenically unsaturated compounds such as 2-butene, 1 -octene, and methyl-3-pentenoate in the presence of methanol. The ligands to the Pd in WO2001/068583 are bidentate biphosphine ligands.

Such alkoxycarbonylation reactions generally require the presence of an acid promoter. The most commonly used acid promoters are Bronsted acids such as methanesulphonic acid, which is the acid promoter used in WO2001/068583.

The inventors have realized that the use of methanesulphonic acid as acid promoter in a group 8-10 metal catalyzed alkoxycarbonylation reaction using a ligand having a pKa of greater than 3 may result in loss of activation of the group 8-10 metal, for example to formation of Pd black when Pd is used. Another problem of the use of methanesulphonic acid is that it is highly corrosive.

It is an aim of the invention to provide a process to produce alkanoic acid esters in a carbonylation process which provides good stability of the group 8-10 metal and/or provides good activity and/or which uses a promoter which is less corrosive. Detailed description of the invention

In a first aspect the invention provides an alkoxycarbonylation process for the preparation of an alkanoic acid ester comprising reacting:

(a) an optionally substituted alkene or a mixture thereof;

(b) a source of a group 8-10 metal;

(c) a phosphorous ligand having a pKa of greater than 3;

(d) a Lewis acid having a water exchange rate constant of substantially 10 ⁷ s ^"1 and a hydrolysis constant of between 4 and 10 M or a mixture thereof;

(e) carbon monoxide; and

(f) an alkanol,

under conditions wherein an alkanoic acid ester is produced.

The inventors have surprisingly found that Lewis acids having a water exchange rate constant of substantially 10 ⁷ s ^"1 and a hydrolysis constant of between 4 and 10 M can be efficiently used as acid promoter in alkoxycarbonylation reactions comprising a group 8-10 metal and a ligand having a pKa of greater than 3, and may provide good TON and/or selectivity and may provide good stability of the group 8-10 metal. The water exchange rate constant (unit: s ^"1 ) and hydrolysis constant (unit: M, or molT ¹ ) can be found in, or can determined as described by Kobayashi et al., J. Am. Chem. Soc. 1998, vol. 120, p. 8287. Chan (J. Mol. Catal. (1989), vol. 53, pp 417-423) describes the Pd- catalyzed methoxycarbonylation of 1 ,4-dimethoxy-2-butene in the presence of methanol with a halide as a ligand to the Pd. The reaction is promoted in the presence of Al, Cu, or B Lewis acids. Chan discloses that the use of non-basic triphenylphosphine as a ligand stabilizes the Pd metal but inhibits the catalytic reaction. Chan is silent on the use of triflate ligands.

Williams et al (Angewandte Chemie (2008), vol. 47, pp 560-563) describe a process for the alkoxycarbonylation of pentene and styrene comprising a catalyst of Pd and triphenylphoshine ligands and using Al trifluoromethanesulfonate (Al-triflate) as acid promoter.

Yang and Yuan (Catal. Lett. (2009), vol. 131 , pp 643-648) describe the Pd- catalyzed methoxycarbonylation of styrene with non-basic PPh3 as a ligand. This process comprises Al, V, Mo, Fe, Cu, or Zn as Lewis acid promoter with p-TsOH and CH ₃ SO ₃ H as ligands to the Lewis acids. Yang and Yuan are silent on the problem of Pd metal stability and on the use of metal triflates. Ferreira et al. (Angewandte Chemie (2007), vol. 46, pp 2273 - 2275) describe the Pd - PPh3 catalyzed methoxycarbonylation of ethylene. They describe that the use of salicylic boric acid ester (BSA) resulted in stabilization of the Pd metal. BSA is not commercially available. However, Ferreira et al. are silent on the use of metal triflates.

The term "Lewis acid" refers to an atomic or molecular species that has an empty atomic or molecular orbital of low energy (LU MO) that can accommodate a pair of electrons; examples are ScCI ₂ and Y(OTf) ₃ . Throughout this specification the Lewis acids are usually denoted using their atomic code, i.e. Fe, Cu; it is understood that these include the ionic species (Fe ²⁺ ).

The pKa of the phosphorous ligand in the process of the invention may be determined according to methods known in the art, and such methods are described for instance by H. R. Hudson, in Patai's Chemistry of functional groups, The Chemistry of Organophosphorus Compounds, Ed. F. R. Hartley, J. Wiley & Sons Ltd, New York, 1990, p 473-487 and references therein. In an embodiment, the phosphorous ligand in the process of the invention has a pKa of greater than 3 on the scale that is listed in W. A. Henderson, Jr., and C. A. Streuli, Journal of the American Chemical Society, 1960, 82, 5791 .

In an embodiment, the Lewis acid or mixture thereof comprises a lanthanide and/or a metal selected from group 3-10.

Particularly, the Lewis acid may be selected from La, Hf, Zn, Ca, Sc, Fe, Y, Yb, and Gd, even more preferably from Sc, Fe, Y, Yb, and Gd.

In an embodiment, the phosphorous ligand in (c) in the process of the invention comprises a phosphorous bidentate ligand, preferably a bidentate phosphine ligand of formula I;

R1 R2P - R3 - R - R4 - PR5R6 (I)

wherein P represents a phosphorous atom; R1 , R2, R5 and R6 can independently represent the same or different optionally substituted organic groups containing a tertiary carbon atom through which the group is linked to the phosphorus atom; R3 and R4 independently represent optionally substituted lower alkylene groups and R represents an optionally substituted aromatic group, preferably wherein R1 , R2, R5, and R6 are tert- butyl, R3 and R4 are methylene, and R is ortho-phenylene.

The process of the invention may also include a mixture of at least one Lewis acids and at least one Bransted acid. This may advantageously improve even more the selectivity and/or TOF whilst reducing or even avoiding the inactivation of the catalytic metal.

The alkene may be a substituted alkene. Such alkenes may carry one or more carboxylic acid, amine, or alcohol groups An example of a suitable (substituted) alkene or mixture thereof comprises at least one isomeric methyl pentenoate, thereby forming an alkanoic acid ester of formula II,

XOOC-(CH2)4-COOY(ll),

wherein X represents alkyl and wherein Y represents H or alkyl.

In one embodiment the alkanoic acid ester of formula II comprises an adipate monoester. In another embodiment the alkanoic acid ester of formula II comprises an adipate dialkylester, more preferably an adipate dimethylester.

In another preferred embodiment the at least one isomeric methyl pentenoate comprises a mixture comprising c/s- and/or irans-methyl 2-pentenoate and c/s- and/or trans- methyl 3-pentenoate and/or methyl-4-pentenoate. Said mixture may for example comprise methyl 2-pentenoate and methyl 3-pentenoate, methyl 2-pentenoate and methyl 4-pentenoate, or methyl 2-pentenoate, methyl 3-pentenoate, and methyl 4- pentenoate.

The composition comprising at least one isomeric methyl pentenoate may comprise other components, such as free pentenoic acids (2-pentenoic acid, 3-pentenoic acid, and/or 4-pentenoic acid). The total amount of said pentenoic acids is preferably less than 10 % wt. The composition comprising at least one isomeric methyl pentenoate may also comprise valerolactone.

The composition comprising at least one isomeric methyl pentenoate may also comprise water, preferably between 0.1 and 3% wt. A small amount of water may be advantageous as it may accelerate the conversion rate (TOF, h ^"1 ). Preferably the amount of water in the composition comprising at least one isomeric methyl pentenoate is between 0.13 and 3% wt, more preferably between 0.19 and 3%, between 0.19 and 2.55% wt, even more preferably between 0.24 and 2.55% wt, even more preferably between 0.51 and 2.55 % wt.

Carbon monoxide partial pressures in the range of 1 -65 bar are preferred. In the process according to the present invention, the carbon monoxide can be used in its pure form or diluted with an inert gas such as nitrogen, carbon dioxide or noble gases such as argon. Small amounts of hydrogen can also be present. In general, the presence of more than 5% hydrogen is undesirable, since this can cause hydroformylation or even hydrogenation of the pentenoate esters.

The group 8-10 metal in (b) preferably comprises Pd. Suitable sources of Pd include its salts, such as for example the salts of palladium and halide acids, nitric acid, sulphuric acid or sulphonic acids; palladium complexes, e. g. with carbon monoxide, dienes, such as dibenzylideneacetone (dba) or acetylacetonate, palladium nanoparticles or palladium combined with a solid carrier material such as carbon, silica or 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, proprionic acid, butanoic acid or 2-ethyl-hexanoic acid, or salts of substituted carboxylic acids such as trichloroacetic acid and trifluoroacetic acid. A very suitable source is palladium (II) acetate. In a preferred embodiment the source of Pd is selected from the group consisting of palladium halide, palladium carboxylate and Pd2(dba)3.

Suitable reaction temperatures are in the range of 20-160°C, more preferably in the range of 50-120°C.

The pressure in the process of the invention is preferably between 5 and 100 bar, more preferably between 10 and 50 bar.

In a preferred embodiment the Lewis acid or mixture thereof comprises trifluoromethanesulfonate and/or halide. Preferably the amount of trifluoromethanesulfonate or halide in (d) is between 0.5 and 20 equivalent (mol/mol) relative to the amount of the group 8-10 metal in (b). Trifluoromethanesulfonate Lewis acid complexes (also referred to as triflate) are commercially available and may be added to the process as-is.

A preferred alkanol comprises methanol.

In another aspect the invention provides a process according to the invention wherein the alkene comprises at least one isomeric methyl pentenoate and wherein the alkanol comprises methanol, to form adipic acid dimethylester, said process further comprising converting said adipic acid dimethylester in a hydrolysis reaction, to form adipic acid.

In a further aspect the invention provides a process to produce adipic acid dialkyl ester, said process comprising:

a. converting valerolactone into a pentenoic acid alkyl ester by treatment with an alkanol, in the presence of an acidic or basic catalyst in the gas phase or in the liquid phase; and b. converting the pentenoic acid alkyl ester produced in step (a) to adipic acid dialkyl ester in a process according to the invention.

The conversion of valerolactone to a pentenoic acid alkyl ester can be done either in the liquid phase or in the gas phase. Such processes are described in WO 2005058793, WO 2004007421 , and US4740613.

In an embodiment the valerolacton is prepared by converting levulinic acid to valerolactone in a hydrogenation reaction. Such processes are described in L. E. Manzer, Appl. Catal. A, 2004, 272, 249-256; J. P. Lange, J. Z. Vestering and R. J. Haan, Chem. Commun., 2007, 3488-3490; R. A. Bourne, J.G. Stevens, J.Ke and M. Poliakoff, Chem. Commun., 2007, 4632-4634; H. S. Broadbent, G. C. Campbell, W. J. Bartley and J. H. Johnson, J. Org. Chem., 1959, 24, 1847-1854; R. V. Christian, H. D. Brown and R. M. Hixon, J. Am. Chem. Soc.,1947, 69, 1961-1963. ;L. P. Kyrides and J. K. Craver, US Patent, 2368366, 1945; H. A. Schuette and R. W. Thomas, J. Am. Chem. Soc, 1930, 52, 3010-3012.

In a further embodiment the levulinic acid is prepared by converting a C6 carbohydrate to levulinic acid in a hydrolysis reaction. Such processes are for example described in L. J. Carlson, US Patent, 3065263, 1962; B. Girisuta, L. P. B. M. Janssen and H. J. Heeres, Chem. Eng. Res.Des., 2006, 84, 339-349; B. F. M. Kuster and H. S. Vanderbaan, Carbohydr. Res., 1977, 54,165-176; S. W. Fitzpatrick, WO8910362, 1989, to Biofine Incorporated; S. W. Fitzpatrick, WO9640609 1996, to Biofine Incorporated. Examples of C6 carbohydrates are glucose, fructose, mannose en galactose. Preferred raw material for the C6 carbohydrates is lignocellulosic material containing carbohydrate based polymers composed partly or entirely from C6 sugars such as lignocellulose, cellulose, starch and hemicellulose. The C6 carbohydrate may comprise other components, such as plant waste, sewage etc.

The adipic acid dialkyl ester may be converted to adipic acid in a hydrolysis reaction. The process to produce adipic acid advantageously allows the use of renewable sources such as plant waste, waste from paper production, sewage waste etceteras instead of using fossil sources.

In another embodiment adipate is converted to ammonium adipate by treatment with ammonia.

In another embodiment ammonium adipate is converted to adiponitril in a dehydration reaction. In another embodiment adiponitril is converted to hexamethylenediamine in a reduction reaction. The conversion of adipate to ammonium adipate, from ammonium adipate to adiponitril and from adiponitril to hexamethylene diamine is known to persons skilled in the art and is for example described by Fernelius et al. (Journal of Chemical Education, 1979, vol. 56, p. 654-656).

In a further aspect the invention provides the use of a Lewis acid having a water exchange rate constant of substantially 10 ⁷ s ^"1 and a hydrolysis constant of between 4 and 10 M in an alkoxycarbonylation process for the preparation of an optionally substituted alkanoic acid ester.

The following examples are for illustrative purposes only and are not to be construed as limiting the invention.

EXAMPLES Example 1

Water exchange rate constants and hydrolysis constants of several Lewis acids according to Kobayashi et al., J. Am. Chem. Soc. 1998, vol. 120, p. 8287.

Example 2

Pd(OAc) ₂ (1 mg, 4.4 μηιοΙ) and 1 ,2-Bis(di-tert-butylphosphinomethyl)benzene (8.9, 22 μηηοΙ) were dissolved in 3.5 mL of methanol. The Lewis acid of choice was added to the catalyst solution (44 μηηοΙ). Then, methylpentenoates (mixture of isomers, 2000 eq., 8.8 mmol, 1 g) were added and the glas inserts were placed in an Endeavor parallel setup. The reactor was purged 5 times with N ₂ and successively pressurized to Pco = 20 bar and heated to T = 100°C. The reaction was allowed to proceed for 4 h. The results are shown in the Table 1 below (nd, not determined).

The triflate-containing Lewis acids were obtained from Sigma-Aldrich and used as received.

Table 1. Acid-promoted methoxycarbonylation of methylpentenoates

Example 3

Pd(OAc) ₂ (1 mg, 4.4 μηηοΙ) and 1 ,2-Bis(di-tert-butylphosphinomethyl)benzene (8.9, 22 μηηοΙ) were dissolved in 3.5 mL of methanol. The Lewis acid of choice was added to the catalyst solution (44 μηηοΙ). Then, methylpentenoates (mixture of isomers, 2000 eq., 8.8 mmol, 1 g) were added and the glas inserts were placed in an Endeavor parallel setup. The reactor was purged 5 times with N ₂ and successively pressurized to Pco = 20 bar and heated to T = 100°C. The reaction was allowed to proceed for 1 h, in order to be able to compare the catalyst activity for the various Lewis acid promotors. The results are shown in the Table 2 below. The results for methanesulfonic acid (MSA) and triflic acid are added as comparative examples.

Table 2. Acid-promoted methoxycarbonylation of methyl pentenoates

Example 4

Pd(OAc) ₂ (1 mg, 4.4 μηηοΙ) and the bidentate phosphine ligand of choice (22 μηηοΙ) were dissolved in 3.5 mL of methanol. DTBPMB, 1 ,2-Bis(di-tert- butylphosphinomethyl)benzene. The Lewis acid of choice was added to the catalyst solution (44 μηηοΙ). Then, methylpentenoates (mixture of isomers, 2000 eq., 8.8 mmol, 1 g) were added and the glas inserts were placed in an Endeavor parallel setup. The reactor was purged 5 times with N ₂ and successively pressurized to P _C o = 20 bar and heated to T = 100°C. The reaction was allowed to proceed for 4 h. The performance of the Gd(OTf) ₃ systems are compared with the performance of MSA systems and the results are shown in the Table 3 below.

Table 3. Acid-promoted methoxycarbonylation of methyl pentenoates

Example 5

Pd(OAc) ₂ (1 mg, 4.4 μηηοΙ) and1 ,2-Bis(di-tert-butylphosphinomethyl)benzene (8.9, 22 μηηοΙ) were dissolved in 3.5 mL of methanol. The Lewis acid of choice was added to the catalyst solution (44 μηηοΙ). Then, methylpentenoates (mixture of isomers, 2000 eq., 8.8 mmol, 1 g) were added and the glas inserts were placed in an Endeavor parallel setup. The reactor was purged 5 times with N ₂ and successively pressurized to Pco = 20 bar and heated to T = 100°C. The reaction was allowed to proceed for 4 h. The results are summarized in Table 4.

Table 4. Acid-promoted methoxycarbonylation of methylpentenoates

Example Acid promotor Yield (%) Selectivity (%)

29 Y(acac)3 0 -

30 Y(OAc)3 0 -

31 Gd(N03)3 1.6 100

32 GdCI3 27 99

33 Yb(N03)3 2.1 100

34 Yb(OAc)3 0 -

35 YbCI3 30 98

36 Yb(acac)3 0 - Example 6

Pd(OAc) ₂ (1 mg, 4.4 μηιοΙ) and1 ,2-Bis(di-tert-butylphosphinomethyl)benzene (8.9, 22 μηιοΙ) were dissolved in 3.5 mL of methanol. The acid of choice was added to the catalyst solution in the desired amount: 5, 2, 1 , 0.5 equivalents with respect to Pd. Then, methylpentenoates (mixture of isomers, 2000 eq., 8.8 mmol, 1 g) were added and the glas inserts were placed in an Endeavor parallel setup. The reactor was purged 5 times with N ₂ and successively pressurized to P _C o = 20 bar and heated to T = 100°C. The reaction was allowed to proceed for 4 h. The results are summarized in Table 5.

Table 5. Acid-promoted methoxycarbonylation of methylpentenoates

Example 7

Pd(OAc) ₂ (1 mg, 4.4 μηηοΙ) and1 ,2-Bis(di-tert-butylphosphinomethyl)benzene (8.9, 22 μηηοΙ) were dissolved in 3.5 mL of methanol. The acid of choice was added to the catalyst solution (44 μηηοΙ). Then, the substrate of choice (2000 eq.) was added and the glas inserts were placed in an Endeavor parallel setup. The reactor was purged 5 times with N ₂ and successively pressurized to P _C o = 20 bar and heated to T = 100°C. The reaction was allowed to proceed for 4 h. The results are summarized in Table 6.

Table 6. Acid-promoted methoxycarbonylation of methyl pentenoates

Example Acid promotor substrate Conversion (%) Pd-black

45 AI(OTf) ₃ Styrene 99 +

46 Gd(OTf) ₃ Styrene 100 -

47 AI(OTf) ₃ 1-Octene 70 +

48 Gd(OTf) ₃ 1-Octene 37 -