# Patent

## Title
Process for the manufacture of saturated fatty acid esters

## Abstract

Disclosed is a process for the manufacture of saturated fatty acid esters by subjecting unsaturated fatty acid esters to hydrogenation in the presence of a catalyst suitable for catalysing the saturation of double bonds in the esters, which is characterized in that said catalyst represents a homogenous complex of a Group VIII metal and a sulfonated phosphite as the polar ionic ligand.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is the National Phase entry of PCT/EP2008/004800, filed Jun. 14, 2008, which claims priority to European patent application number EP07012337.7 filed Jun. 23, 2007, which are incorporated herein by reference in their entireties.
FIELD OF THE INVENTION
The present invention is related to the area of oleochemicals and refers to a process for the manufacture of saturated esters by using a special type of homogenous hydrogenation catalyst.
BACKGROUND OF THE INVENTION
Nowadays, renewable resources are considered to be the new challenge in the development of Sustainable/Green chemistry. Interest in the application of biomass has increased considerably during the last decade because biomass-based resources are renewable and CO2 neutral. In addition, the projected long-term limitations on the exploitation of fossil feedstock, the recent increases in crude oil prices and environmental concerns regarding the local air pollution, the global warming problems caused by CO2, the biodegradability and biocompatibility of many petrochemical based products have also played a role in this respect. Today the world production of renewable biomass is about 200·109 t/a of which 130·106 t/a are fats and oils and only 7% of the total biomass production capacity are used for food, feed and non-food applications. These figures compared to the world capacity of extracted fossil fuels which is only 7·109 t/a show the huge potential of renewable biomass for energy, chemicals and material production. According to the Directive 2003/30/EC of the European Parliament and of the Council by 31 Dec. 2010 biofuels shall be 5.75% of the transportation fuels and according to the US roadmap for biomass technologies—2020 vision goals, biofuels will meet 10% of the fuels, and biomass-based chemicals 18% of the chemicals in the US market.
Vegetable oils and their derivatives are important feedstock for the industry with a broad spectrum of applications such as in foodstuff chemistry, pharmacy, cosmetics, plastics, detergents, biolubricants and in the energy field with the production of biodiesel mainly by transesterification reactions with methanol or ethanol to obtain fatty acid methyl (FAME) or ethyl esters (FAEE).
Catalytic hydrogenation of renewable vegetable oils and their derivatives constitutes a major unit operation in the chemical industry. In such kind of hydrogenation processes of C═C units in unsaturated fatty acids of vegetable oils are commonly used heterogeneous catalytic systems based on nickel, palladium, copper, copper-chromite, platinum etc. However, for edible oil hydrogenation heterogeneous catalysts based on nickel has been the choice of industry. Traditional Ni-based commercial heterogeneous edible oil hydrogenation catalysts produced high amounts in trans-fats (up to 40%). In recent years the negative health effects of trans-fats received increasing attention and decisions have been made in Europe to limit and in USA to declare the trans-isomers contained in fatty foodstuffs which caused a demand for hardstocks with lower trans-isomers content. Therefore, there is increasing interest in the development of new industrial hydrogenation processes producing low amounts of trans-fats. One development may involve the use of homogeneous transition metal complexes as edible oil hydrogenation catalysts. Several attempts have been made to develop homogenous systems which would allow conducting hydrogenation also to saturated compounds under milder conditions in order to improve colour quality of the products and to make the process more economic. Unfortunately, homogenous catalysts based on transition metals modified with conventional phosphites as known from the state of the art suffer from many disadvantages: for example they lack stability against water, they are difficult to remove from the hydrogenation products and/or they simply do not show a sufficient activity.
It has therefore the object of the present invention to overcome the disadvantages known from the state of the art and to provide a cost-effective process for hydrogenation of fatty acid esters which involves a homogenous catalyst system based on transition metals modified with sulfonated phosphites which is stable against hydrolysis, resistant to oxidative destruction, easy to remove from the reaction mixture and exhibiting a high activity.


DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention refers to a process for the manufacture of saturated fatty acid esters by subjecting unsaturated fatty acid esters to hydrogenation in the presence of a catalyst suitable for catalysing the saturation of double bonds in the esters, which is characterised in that said catalyst represents a homogenous complex of a Group VIII metal and a sulfonated phosphite as the polar ligand. The process could be also possible to use for the manufacture of partially hydrogenated fatty acid esters which would be valuable products for applications such as “bio-diesel” of improved oxidative stability and cetane number at a low pour point with low NOx emissions, “biolubricants” of higher oxidative stability and a low pour point and “edible oils” of improved oxidative stability and low contents of trans-fats.
Surprisingly it has been observed that homogenous complexes of Group VIII metals comprising polar ligands of the sulfonated phosphite type are more stable in the presence of water compared to conventional phosphites, can be easily separated from the hydrogenation products for example by membrane processes or by phase separation conducting the reaction in an ionic-liquid/organic two-phase system and exhibit an activity which is by far superior compared to other homogenous systems known from the state of the art.
Fatty Acid Esters
Fatty acid esters in the content of this invention shall mean either glycerides or alkyl esters. As far as glycerides are concerned they may represent synthetic or natural mono-, di- or triglyceride or its mixture. Although the nature of the nature, more particular vegetable oils is not critical as long as they include unsaturated fatty acid moieties, the preferred oils are sunflower oil, rape seed oil, linseed oil, olive oil, palm kernel oil, line oil and their mixtures.
Alkyl esters which may also represent suitable starting materials preferably follow general formula (I)

R1CO—OR2  (I)

in which R1CO represents either an unsaturated acyl radical having 16 to 22, preferably 18 carbon atoms or a mixture of saturated and unsaturated acyl radicals having 6 to 22 carbon atoms and a iodine number of at least 5, and R2 stands for a linear or branched alkyl radical having 1 to 6 carbon atoms. Typical examples are methyl or ethyl esters of oleic acid, elaidinic acid, linoleic acid, conjugated linoleic acid, linolenic acid and erucic acid. Also suitable are esters obtained from vegetable oils as described above. The preferred starting material, however, is either a methyl ester of sunflower or palm kernel oil.

In general, said glycerides or esters exhibit a iodine number in the range of 5 to 120, preferably 10 to 100 and more preferably 25 to 95.
Hydrogenation Process
The following Figure shows a simplified pathway of the hydrogenation of C18 compounds in fatty acid methyl esters catalyzed by transition metal trisulfonated triphenylphosphite (TSTPP) complexes in methanol. Methyl α-linolenate (α-MLN), with other C18:3 isomers, are converted to C18:2 isomer mixture and further to C18:1 isomer mixture and finally to the final product methyl stearate (MS). The term “isomer mixture” refers to all regio-isomers obtained from hydrogenation and/or isomerization (along the carbon chain) reactions and all other isomers formed via cis/trans isomerization reactions.




Hydrogenation Catalysts
The homogenous complexes to be used for catalysing the hydrogenation reaction consist of a group VIII metal and a polar ionic ligand system. Preferably said Group VIII metals are selected from the group consisting of rhodium, ruthenium, palladium, cobalt, platinum, nickel, iridium, iron and their mixtures.
The ligands usually follow general formula (II),




In which R1, R2 and R3 independently represent a linear or branched C1-C20 alkyl or cycloalkyl, a alkylaryl, arylalkyl or aryl radical, preferably a phenyl, benzyl, n-toluoyl, pyridyl, biphenyl, naphthyl or binaphthyl group, R4 stands for an alkyl group having 1 to 12 carbon atoms, and n, m and p represent 0 or integers of from 1 to 3 with the condition than the sum of (n+m+p) is different from zero. Typical examples for suitable sulfonic acid ammonium salts are derived from trialkylphosphites, dialkylarylphosphites, alkyldiarylphosphites or triaryl-phosphites like trimethylphosphite, triethylphosphite, butyldiethylphosphite, tri-n-propylphosphite, tri-n-butylphosphite, tri-2-ethylhexylphosphite, tri-n-octylphosphite, tri-n-dodecylphosphite, dimethylphenylphosphite, Diethylphenylphosphite, methyldiphenyl-phosphite, ethyldiphenylphosphite, triphenylphosphite and trinaphthylphosphite.
The preferred ligands are trisulfonated triphenylphospite (TSTPP) according to general formula (III)




in which X represents a R5N+R6R7R8 cation, R5 represents either hydrogen or an alkyl group having 1 to 12, preferably 4 to 8 carbon atoms and R6, R7, R8 represent an alkyl group having 1 to 12, preferably 4 to 8 carbon atoms. The most preferred catalysts represent complexes of Ruthenium or Rhodium with TSTPP. Basically, the molar ratio between the Group VIII metal and the hydrophilic ligand is about 1:1 to about 1:6 and preferably about 1:2 and about 1:4. Overall the content of the Group VIII metal in the reaction mixture is about 10 to about 50 ppm, and preferably about 20 to about 40 ppm. Usually the catalyst amount is adjusted to a molar ratio of C═C/Group VIII metal of about 500 to about 25,000, and preferably 1.000 to 20.000.

Furthermore, suitable polar sulfonated phosphites are given in the following formulas:
Formula (IV):




In which Z represents an alkyl or aromatic group. The alkyl group stands for a linear or branched C1-C30 alkyl or alkyloxyalkyl, alkyl-S-alkyl, or cycloalkyl radical. Aromatic group stands for an aryl, biaryl, naphthyl, binaphthyl, arylalkyl, arylalkylaryl, aryloxyaryl, aryl-S-alkyl, aryl-S-aryl, aryl-N(R5)aryl or aryl-N(R5)-alkyl moiety in which R5 stands for an alkyl or aromatic radical. R4 represents an alkyl or aromatic group. R of the ammonium moiety stands for an alkyl group having 1 to 12 carbon atoms. n1 and n2 represent 0 or integers of from 1 to 6 with the condition than the sum of (n1+n2) is different from zero.
Also preferred sulfonated phosphite ligands according to general formula (IV) are:










Formula (V):




In which Ar1 and Ar2 may be the same or different and represent an arylalkyl, alkylaryl, aryl, biaryl, naphthyl or binaphthyl group. n1 and n2 represent 0 or integers of from 1 to 3 with the condition than the sum of (n1+n2) is different from zero. n3, n4 and n5 may be the same or different and represent 0 or 1. X represents a —CR8R9—, —O—, —S—, —NR10—, —SiR11R12— and —CO-group. R8 and R9 stand for an H, C1-C12-alkyl, phenyl, toluoyl and anisol group. R10 and R12 stands for a H or methyl group. R7 stands for an C1-C12-alkyl, alkylaryl, arylalkyl, cyclo-hexyl, 1-methylcyclohexyl- and a benzyl group. The preferred sulfonated phosphite ligands according to general formula (V) are:




Formula (VI):




In which Z represents an alkyl or aromatic group. The alkyl group stands for a linear or branched C1-C30 alkyl or alkyloxyalkyl, alkyl-S-alkyl, or cycloalkyl radical. Aromatic group stands for an aryl, biaryl, naphthyl, binaphthyl, arylalkyl, arylalkylaryl, aryloxyaryl, aryloxyalkyl, aryl-S-alkyl, aryl-S-aryl, aryl-N(R5)aryl or aryl-N(R5)-alkyl moiety in which R5 stands for an alkyl or aromatic radical. n1, n2 and n3 represent 0 or integers of from 1 to 3 with the condition than the sum of (n1+n2+n3) is different from zero. The preferred sulfonated phosphite ligands according to general formula (VI) are:







The catalysts described above are known from the prior art and disclosed in EP 0435071 B1 and EP 0435084 B1 according to which a sulfonic acid was reacted with the amine and subjected to transesterification with the phosphite compound. According to JP 2000 281689 A1 (Mitsubishi) a sulfonic acid metal salt is reacted with an ammoniumchloride of the amine and then transesterified with a phosphite. Also references are made to J. Prakt. Chem./Chem. Ztg. 335, 75ff (1993) which describes the use of said systems for the monophasic hydroformylation of α-olefins in organic solvents and to Chem. Commun. (2001) 1360 which describes the use of said systems for the biphasic hydroformylation of α-olefines in ionic liquids based on imidazolium cations where the catalyst was easily separated by simple phase separation from organic products and recycled without loss of its high selectivity toward linear aldehydes. As far as the synthesis of TSTPP and the complexes is concerned the teaching of the aforementioned documents is fully incorporated by reference.
Hydrogenation Process
In order to avoid ambiguities about the meaning of the phrase “hydrogenation” it is pointed out that this phrase refers to the saturation of the double bonds only and is equivalent to the phrase “hardening”.
In a preferred embodiment of the process according to the present invention hydrogenation is conducted in the presence of a solvent. Suitable solvents are selected from lower alcohols or dialkyl ethers, like for example methanol, ethanol, butanol and diethyl ether.
Usually, the mixture comprising the ester or glyceride, the catalyst and optionally the organic solvent is transferred into a stirred autoclave purged with argon and after a number of pressurising-depressurising cycles with hydrogen in order to remove all traces of oxygen. Then the reactor is heated up to about 50 to about 120, preferably about 80 to about 100° C. and kept there for a reaction time of about 5 to about 240, preferably about 10 to about 60 minutes. During this time the pressure rises up to about 5 to about 100 bar. Once the reaction has been completed the mixtures is cooled to room temperature and depressurised. The catalyst can be removed and recovered on a quantitative basis by means of a membrane. In the case that the hydrogenation reaction is performed in an ionic-liquid/organic two-phase system the catalyst is removed and recovered on a quantitative basis by a simple phase separation.
INDUSTRIAL APPLICATION
The homogenous complexes to be used according to the process of the present invention show a high activity in catalysing the hydrogenation of unsaturated materials, which is not limited to fatty acid esters, but could be also of value for producing paraffins from olefins and the like. Another object of the present invention is therefore directed to the use of complexes of a Group VIII metal and a sulfonated phosphite as the polar ionic ligand as homogenous catalysts for the hydrogenation of double bonds in general and in particular for the hydrogenation of unsaturated fatty acid esters in organic monophasic or ionic-liquids/organic two-phase systems.
EXAMPLES
Examples 1 to 10
Hydrogenation of C12-C18 Palm Kernel Methyl Ester
The following examples describe the hydrogenation of palm kernel methyl ester (Edenor® PK 12-18 ME, Cognis GmbH) in the presence of homogenous trisulfonated triphenylphospite (TSTPP) Rhodium- or Ruthenium complexes as catalysts. The following reaction conditions were applied:

    
    
        Hydrogenation time: 60 min,
        1.32 mg (0.005 mmol) RhCl3.3H2O except in entries 1-3 when added 1.31 mg (0.005 mmol) RuCl3.3H2O, 28.01 mg (0.02 mmol) TSTPP (P/Rh molar ratio=4), 20 ml methanol, [Rh]=28 ppm except in entries 1 and 2 when was [Ru]=24 ppm and in entry 3 when was [Ru]=26 ppm.
        Stirring rate=850 rpm.
    
    


Additional details of the hydrogenation and the results of the trials are compiled in Table 1.







TABLE 1







Hydrogenation of C12-C18 palm kernel methyl estera
















C═C/Rh










molar
T
PH2
C18:2 total
C18:1 total
cis-C18:1 total
trans-C18:1 total
C18:0


Ex.
ratio
 ° C.
bar
mol %
mol %
mol %
mol %
mol %


















Edenora
—
—
—
9.9a
83.0a
83.0a
—
7.1


 1
500
100
80
2.3
61.4
40.4
21.0
36.3


 2
500
100
100
2.5
62.1
42.7
19.4
35.4


 3
300
100
80
2.0
52.2
39.7
12.5
45.8


 4
300
100
80
0.0
24.0
1.8
22.2
76.0


 5
300
110
80
0.5
35.9
4.7
31.2
63.6


 6
300
90
80
0.4
17.9
1.3
16.6
81.7


 7
300
80
80
0.5
3.7
0.0
3.7
95.8


 8
300
70
80
0.5
13.5
0.1
13.4
86.0


 9
300
80
100
0.6
15.7
0.0
15.7
83.7


10
300
80
50
0.5
15.4
0.2
15.2
84.1





aEdenor ME PK 12-18 F of Cognis except the C18 compounds (14.1 mol %) further contains: 0.8 mol % methyl decanoate (C10:0), 57.1 mol % methyl dodecanoate (C12:0), 18.7 mol % methyl tetradecanoate (C14:0) and 9.3 mol % methyl palmitate (MP, C16:0) which were ignored in the experiments. The C18 total content of 14.1 mol % with the proportion of 1.0 mol % methyl stearate (C18:0), 11.7 mol % methyl oleate (C18:1) and 1.4 mol % methyl linoleate (C18:2) was calculated to 100 mol % to give 7.1 mol % of C18:0, 83.0 mol % of C18:1 and 9.9 mol % of C18:2.






Examples 11-32
Hydrogenation of Sunflower Methyl Ester
The following examples describe the hydrogenation of sunflower methyl ester in the presence of homogenous trisulfonated triphenylphospite (TSTPP) Rhodium-, Ruthenium-, Iridium- or Palladium complexes as catalysts. The following reaction conditions were applied:

    
    
        Reaction time: 60 min; except entries 30 and 31, t=20 min; entry 32, t=30 min;
        1.32 mg (0.005 mmol) RhCl3.3H2O; except entries 23-25, 1.76 mg (0.005 mmol) IrCl3.3H2O; except entries 26, 27 and 30, 1.31 mg (0.005 mmol) RuCl3.3H2O; except entry 28, 0.89 mg (0.005 mmol) PdCl2 and entry 29, 1.12 mg (0.005 mmol) Pd(OAc)2.
        Stirring rate: 850 rpm.
    
    


Additional details of the hydrogenation and the results of the trials are compiled in Table 2.







TABLE 2







Hydrogenation of sunflower methyl esterd





















C═C/M
L/M






cis-
trans-




Catalyst
molar
molar
T
PH2
Sol-
C18:3 total
C18:2 total
C18:1 total
C18:1 total
C18:1 total
C18:0


Ex.
Precursor
ratio
ratio
° C.
bar
vent
mol %
mol %
mol %
mol %
mol %
mol %






















Esterd
—
—
—
—
—
—
1.2e
63.3f
32.0g
32.0g
—
3.5


11
RhCl3•3H2O/
500
3
110
50
MeOH
0.6
0.0
22.0
2.0
20.0
77.4



TSTPP













12
RhCl3•3H2O/
500
4
110
50
MeOH
0.6
0.0
19.4
1.8
17.6
80.0



TSTPP













13
RhCl3•3H2O/
500
5
110
50
MeOH
0.0
10.6
65.6
26.8
38.8
23.8



TSTPP













14
RhCl3•3H2O/
500
4
110
80
MeOH
0.8
7.7
58.0
14.5
43.5
33.5



TSTPP













15
RhCl3•3H2O/
500
4
110
30
MeOH
0.1
4.8
57.7
12.9
44.8
37.4



TSTPP













16
RhCl3•3H2O/
500
4
100
50
MeOH
1.0
0.0
12.0
0.5
11.5
87.0



TSTPP













17
RhCl3•3H2O/
500
4
90
50
MeOH
1.1
13.2
58.6
24.4
34.2
27.1



TSTPP













18
RhCl3•3H2O/
500
4
100
50
—
1.5
58.8
33.3
31.8
1.5
6.4



TSTPP













19
RhCl3•3H2O/
500
4
100
50
Ether
0.0
62.3
33.0
33.0
0.0
4.7



TSTPP













20
RhCl3•3H2O/
500
4
100
50
MeOAc
1.6
59.6
33.3
32.6
0.7
5.5



TSTPP













21
RhCl3•3H2O/
500
4
100
50
MeOH
1.0
4.4
51.4
16.3
35.1
43.2



PPh3













22
RhCl3•3H2O/
500
4
100
50
MeOH
2.4
8.8
60.8
31.5
29.3
28.0



P(OPh)3













23h
IrCl3•3H2O/
500
4
100
30
MeOH
6.4
9.0
46.5
15.6
30.9
21.8



TSTPP













24i
IrCl3•3H2O/
500
4
100
50
MeOH
2.6
3.6
42.0
7.2
34.8
43.3



TSTPP













25j
IrCl3•3H2O/
500
4
100
80
MeOH
1.9
1.8
38.0
4.9
33.1
50.1



STPP













26
RuCl3•3H2O/
500
4
100
50
MeOH
0.0
3.0
44.4
22.8
21.6
52.6



TSTPP













27
RuCl3•3H2O/
500
4
100
80
MeOH
0.0
0.6
10.2
0.0
10.2
89.2



TSTPP













28
PdCl2/TSTPP
500
4
100
50
MeOH
1.1
17.3
70.9
53.2
17.7
10.7


29
Pd(OAc)2/
500
4
100
50
MeOH
0.7
25.4
66.5
48.5
18.0
7.4



TSTPP













30
RuCl3•3H2O/
500
4
100
80
MeOH
2.4
20.9
49.0
41.4
7.6
27.7



TSTPP













31
RhCl3•3H2O/
500
4
100
50
MeOH
1.2
16.3
60.3
31.9
28.7
21.9



TSTPP













32
RhCl3•3H2O/
500
4
100
50
MeOH
0.0
8.5
61.6
20.6
41.0
29.9



TSTPP





dSunflower Fatty Acid ME of Cognis except the C18 compounds with a total content of 94.1 mol % further contains 5.3 mol % methyl palmitate (MP, C16:0) and 0.6 mol % of methyl docosanate (MD, C22:0) which were ignored in the experiments. The C18 total content of 94.1 mol % with the proportion of 1.1 mol % methyl linolenate (C18:3), 59.6 mol % methyl linoleate (C18:2), 30.1 mol % methyl oleate (C18:1) and 3.3 mol % methyl stearate (C18:0) was calculated to 100 mol % to give 1.2 mol % of C18:3, 63.3 mol % C18:2, 32.0 mol % C18:1 and 3.5 mol % C18:0.


eMethyl α-linolenate (α-MLN), C18:3 (9c, 12c, 15c).


fMethyl linoleate (ML), C18:2 (9c, 12c).


gMethyl oleate (MO), C18:1 (9c).


h16.3 mol % from three unidentified products


i8.5 mol % from three unidentified products


j8.2 mol % from three unidentified products

## Claims

1. A process for the manufacture of saturated fatty acid esters comprising subjecting unsaturated fatty acid esters to hydrogenation in the presence of a catalyst suitable for catalysing the saturation of double bonds in the esters, wherein said catalyst is selected from rhodium and/or ruthenium, with at least one polar ionic ligand that is trisulfonated triphenyl phosphite (TSTPP).

2. The process of claim 1, wherein said ester is selected from the group consisting of glycerides and alkyl esters.

3. The process of claim 2, wherein said glyceride is selected from the group consisting of synthetic or natural monoglycerides, diglycerides, triglycerides and combinations thereof.

4. The process of claim 2, wherein said glyceride is selected from the group consisting of sunflower oil, rape seed oil, olive oil, palm kernel oil, linseed oil and combinations thereof.

5. The process of claim 2, wherein said ester is represented by formula (I)


R1CO—OR2  (I)

in which R1CO represents either an unsaturated acyl moiety having 16 to 22 carbon atoms, or a mixture of saturated and unsaturated acyl moieties having 6 to 22 carbon atoms, and having an iodine number of at least 5; and R2 stands for a linear or branched alkyl group having 1 to 6 carbon atoms.

6. The process of claim 5, wherein said ester comprises the methyl ester of sunflower and/or palm kernel oil.

7. The process of claim 2, wherein said glycerides or esters exhibit an iodine value in the range of 5 to 127.

8. The process of claim 1, wherein the molar ratio between the rhodium and/or ruthenium and the trisulfonated triphenyl phosphite (TSTPP) is about 1:1 to about 1:100.

9. The process of claim 1, wherein the content of the rhodium and/or ruthenium in the reaction mixture is about 10 to about 1000 ppm.

10. The process of claim 1, wherein the molar ratio of C═C/rhodium and/or ruthenium is about 200 to about 50,000.

11. The process of claim 1, wherein hydrogenation is conducted in the presence of a solvent.

12. The process of claim 11, wherein said solvent comprises a lower alcohol or a dialkyl ether.

13. The process of claim 1, wherein the hydrogenation is conducted at a temperature within the range of about 50 to about 120° C.

14. The process of claim 1, wherein the hydrogenation is conducted under a pressure in the range of about 5 to about 100 bar.

15. The process of claim 1, wherein the hydrogenation is conducted over a reaction time of about 5 to about 240 minutes.

16. A method for the hydrogenation of double bonds comprising adding at least one complex of rhodium and/or ruthenium and trisulfonated triphenyl phosphite (TSTPP) as a polar ionic ligand, as a homogenous catalyst, to a hydrogenation reaction mixture, and submitting the resulting mixture to hydrogenation conditions.

17. A method for the hydrogenation of unsaturated fatty acid esters comprising adding at least one complex of rhodium and/or ruthenium and TSTPP to a hydrogenation reaction mixture, and submitting the resulting mixture to hydrogenations conditions.