Process for the production of esters

Carboxylic acid esters are produced by reacting an olefinic hydrocarbon with an ester of formic acid in the presence of both carbon monoxide and oxygen at a temperature of up to 200.degree. and a pressure of up to 300 bar in the presence of water, typically in an amount of from 0.01 to 5% w/w, a source of protons, which is preferably a mineral acid or a tectometallosilicate in the hydrogen form, and as catalyst (a) a source of palladium, and (b) a source of copper.

The present invention relates to a process for the production of carboxylic 
acid esters by the reaction of a formic acid ester with an olefinic 
hydrocarbon. 
The production of carboxylic acid esters by reacting an ester of formic 
acid with an olefin is known from, for example, EP-A-92350 and 
EP-A-106656. 
EP-A-92350 discloses a homogeneous process for converting olefins to 
carboxylic acid esters by reacting an olefin with a formate ester with a 
soluble iridium catalyst and an iodide promoter at temperatures of from 
150.degree. to 300.degree. C. in a carboxylic acid solvent. The process of 
EP-A-92350 does not require carbon monoxide or water and when oxygen is 
present it should only be maintained at low levels. 
EP-A-106656 discloses a process for the production of a carboxylic acid 
ester by reacting at elevated temperature an ester of formic acid with an 
olefin in the presence, as catalyst, of a Group VIII noble metal, 
optionally in the presence of carbon monoxide. In this process the 
preferred Group VIII metal is iridium and it is preferred to employ a 
halide promoter. It is also preferred to employ a strong acid as a 
co-promoter. The process is preferably effected in the substantial absence 
of oxygen and the presence of carbon monoxide is not essential. 
We have now found that by using a palladium/copper catalyst and a source of 
aqueous acid in the presence of both carbon monoxide and oxygen, the use 
of halid promoters, with their associated problems of corrosion and 
separation, can be avoided and that the process can be operated under 
substantially milder conditions than the prior art processes. 
Accordingly, the present invention provides a process for the production of 
a carboxylic acid ester which process comprises reacting an olefinic 
hydrocarbon with an ester of formic acid in the presence of both carbon 
monoxide and oxygen at a temperature of up to 200.degree. C. and a 
pressure of up to 300 bar in the presence of water, a source of protons 
and, as catalyst (a) a source of palladium and (b) a source of copper. 
The process of the present invention is preferably carried out by reacting 
the olefinic hydrocarbon with the formic acid ester in the liquid phase 
under the conditions described above with the catalyst dissolved or 
suspended therein. 
As regards the olefinic hydrocarbon feedstock this is suitably one or more 
linear or cyclic olefins having one or more carbon-carbon double bonds. 
Preferred olefins include C.sub.1 -C.sub.20 aliphatic olefins, C.sub.4 
-C.sub.20 cyclic olefins and aromatic olefins. Most preferred olefins are 
C.sub.1 -C.sub.12 aliphatic mono- and diolefins, C.sub.6 -C.sub.10 cyclic 
olefins and styrene. 
The formic acid ester is suitably an alkyl ester of formic acid and is 
preferably a C.sub.1 -C.sub.12 alkyl ester. Preferred examples include 
methyl formate, ethyl formate, propyl formate, n-butyl formate, and the 
like. 
The carboxylic acid ester produced by the process of the invention is one 
in which (i) the carboxylic acid group has one carbon atom more that the 
starting olefin and (ii) the ester group corresponds to that derived from 
the formic acid ester. Thus, if the olefin is ethylene and the formic acid 
ester used is methyl formate the carboxylic acid ester formed is methyl 
propioniate. Using higher mono-olefins, the possibility exists for the 
production of two or more carboxylic acid ester isomers. It is a feature 
of the present invention that under such circumstances the reaction is 
highly specific to the branched-chain ester. The reaction is completely 
regiospecific, or highly regioselective, when the formate is employed as 
both reactant and solvent. In the case of diolefins, such as 
1,7-octadiene, both mono- and di-esters are formed in contrast to the 
hydroesterification of olefins in alcohol under similar conditions, where 
only the diester is formed. 
As regards the catalyst, this comprises a source of palladium and a source 
of copper. The sources of palladium and copper can be in any convenient 
form e.g. the finely divided metal, simple inorganic salts, as well as 
inorganic or organometallic complexes. For palladium, preferred sources 
are the simple inorganic salts such as the chloride or bromide and the 
nitrate. The copper source is also preferably a copper halide e.g. copper 
(II) chloride, copper(II)bromide, copper(I) chloride or copper (I) 
bromide. 
The sources of palladium and copper are suitably present in amounts such 
that the molar ratio of olefin to palladium or copper is greater than 5:1. 
The presence of water is essential to the operation of the process of the 
invention. Only trace amounts of water are necessary, typically of the 
order of from 0.01 to 5% w/w. For optimum operation of the process the 
presence of large amounts of water should preferably be avoided, otherwise 
side-reactions may occur. 
The source of protons may suitably suitably by a mineral acid or an organic 
acid. Suitably the source of protons and water may be combined in the form 
of an aqueous acid, preferably an aqueous mineral acid, e.g. aqueous 
hydrochloric acid, aqueous sulphuric acid or aqueous hydrobromic acid, or 
the like. Alternatively, a solid source of protons, for example a hydrogen 
ion-exchanged tectometallosilicate or a hydrogen ion-exchanged layered 
clay may be employed. Suitable tectometallosilicates include the 
aluminosilicate zeolites, for example ZSM-5. 
The process of the present invention is carried out in the presence of a 
gas mixture comprising carbon monoxide and oxygen. The carbon 
monoxide/oxygen mixture can be used to provide an overpressure for the 
process if the process is carried out at superatmospheric pressure. The 
carbon monoxide/oxygen gas mixture is suitably one having a molar ratio of 
carbon monoxide to oxygen in the range 5:1 to 1:5. 
Although the process may be carried out at room temperature, elevated 
temperatures up to 200.degree. C. can be used in order to accelerate the 
reaction. Preferably the reaction is carried out at a temperature in the 
range from 25.degree. to 150.degree. C. The reaction may also be carried 
out at atmospheric pressure or at a superatmospheric pressure of up to 300 
bars. When a superatmospheric pressure is used it can be generated by the 
carbon monoxide and oxygen or by the further addition of an inert gas such 
as nitrogen, helium, argon or carbon dioxide. 
As mentioned above, the process described hereinbefore can be carried out 
in the liquid phase using the reactants as the reaction medium. However, a 
solvent may optionally be used in order to dilute the reactants, to assist 
in solubilising the catalyst and to increase the reaction rate. A 
preferred solvent is dioxan, though other solvents such as dimethyl 
sulphoxide and glycol ethers may be used. 
The process can be carried out either batchwise or continuously. 
The invention will now be further illustrated by the following examples.

EXAMPLE 1 
A mixture of palladium chloride [27 mg, 0.15 mmol] and CuCl.sub.2 [41 mg, 
0.30 mmol] in dioxane (10 ml) was stirred under carbon monoxide for 5 
minutes. Then n-butyl formate (4 ml), 1-decene [0.75 g, 5.35 mmol] and 
aqueous hydrochloric acid (0.1 ml) were added and CO/O.sub.2 (1:1) was 
bubbled through the solution at room temperature and atmospheric pressure 
for a period of 24 hours. Analysis of the product gave n-butyl 
2-methyldecanoate and n-butyl undecanoate with respectively 87% and 13% 
selectivity with a 64% conversion of the formic acid ester. 
EXAMPLE 2 
Example 1 was repeated using the following reaction mixture: 
PdCl.sub.2 : 54 mg, 0.30 mmol 
CuCl.sub.2 : 202 mg, 1.5 mmol 
HCL: 0.2 ml 
Dioxan: 10 ml 
HCOOC.sub.4 H.sub.9 : 4 ml 
1-decene: 0.75 g, 5.35 mmol 
After two days reaction 98% of the formic acid ester had been converted 
with a 71.9% selectivity to n-butyl 2-methyldecanoate and a 15.7% 
selectivity to n-butyl undercanoate. 
COMISON TEST A 
The procedure of Example 1 was repeated except that the reaction was 
carried out in a closed system initially charged with a CO/O.sub.2 
mixture, i.e. CO/O.sub.2 was not bubbled through the reaction mixture. The 
reaction ceased after limited conversion. 
This test demonstrates the importance of carrying out the reaction in the 
presence of both carbon monoxide and oxygen. 
COMISON TEST B 
The procedure of Example 1 was repeated except that oxygen was excluded 
from the reaction mixture. Only traces of ester were detected. 
The test was repeated using excess CuCl.sub.2. Again only traces of the 
ester were detected. 
This test demonstrates that the presence of oxygen is essential for the 
performance of the invention. 
COMISON TEST C 
The procedure of Example 1 was repeated except that copper was omitted as a 
component of the catalyst system. No reaction occurred. 
This test demonstrates that under the mild reaction conditions of Example 1 
copper is an essential component of the catalyst system. 
EXAMPLE 3 
The procedure of Example 1 was repeated except that instead of copper 
chloride there was used copper acetate. 
71% esters in a 6:1 ratio of branched:linear isomers were obtained. 
EXAMPLE 4 
The procedure of Example 1 was repeated except that instead of copper 
chloride there was used copper triflate. 
After 28 hours 20% esters were obtained. 
EXAMPLE 5 
The procedure of Example 1 was repeated except that labelled carbon 
monoxide (.sup.13 CO) was employed. Ester was obtained from the olefin 
with greater than 99% of the label located at the carbonyl carbon. This 
means that the ester carbonyl group arises from CO and not from the 
formate reactant. 
EXAMPLE 6 
The procedure of Example 1 was repeated except that 1,7-octadiene was used 
in place of 1-decene. Both mono- and di-esters were formed in a 2:1 ratio.