Carbonylation process using palladium phosphine catalyst

The invention relates to a process for the carbonylation of an olefinically or acetylenically unsaturated hydrocarbon compound by reaction with carbon monoxide and a hydroxy compound in the presence of a catalyst system comprising a source of cationic palladium, a source of phosphine and a protonic acid, which reaction is carried out in the presence of a free radical inhibitor.

FIELD OF INVENTION 
This invention relates to a process for the carbonylation of an 
olefinically or acetylenically unsaturated hydrocarbon compound by 
reaction with carbon monoxide and a hydroxy compound in the presence of a 
catalyst system comprising a source of cationic palladium, a source of 
phosphine and a protonic acid. 
BACKGROUND OF THE INVENTION 
A carbonylation process of olefinically or acetylenically unsaturated 
hydrocarbon compound with carbon monoxide and hydroxy compound provides a 
versatile tool for the economic production of various chemicals starting 
from readily available unsaturated hydrocarbon feedstock. The 
carbonylation reaction may be represented by the equation: 
##EQU1## 
where A represent the unsaturated hydrocarbon compound and BOH represents 
the hydroxy compound, such as water (B.dbd.H), alcohols (B.dbd.R), and 
carboxylic acids (B.dbd.RCO). Generally, olefinic precursors provide 
saturated products, whereas acetylenic precursors provide olefinic 
products; multiple carbonylations such as producing saturated products 
from acetylenic precursors not being excluded. Depending on the nature of 
the hydroxy compound BOH, various functionalized products, including 
carboxylic acids, esters, and anhydrides can be obtained. 
It has been found that catalytic systems containing cationic palladium, a 
phosphine ligand, and a protonic acid are well suited for carbonylation 
reactions. These catalyst systems allow the carbonylation reaction to 
proceed at high rate under mild conditions in respect of temperature and 
carbon monoxide pressure. By dedicated choice of the type of phosphine and 
the nature of the protonic acid, extremely high selectivities to specific 
desired product could be obtained. For further details of specific 
carbonylation reactions thus catalyzed reference is made to EP-106379-B, 
U.S. Pat. No. 4,739,109, U.S. Pat. No. 4,739,110, U.S. Pat. No. 4,940,787, 
U.S. Pat. No. 4,786,443, U.S. Pat. No. 5,099,062, U.S. Pat. No. 5,158,921, 
U.S. Pat. No. 5,028,576, U.S. Pat. No. 5,103,043, U.S. Pat. No. 5,158,921, 
U.S. Pat. No. 5,124,300, U.S. Pat. No. 5,166,411, U.S. Pat. No. 5,166,116, 
U.S. Pat. No. 5,177,253, U.S. Pat. No. 5,179,225. The low temperatures of 
these carbonylation reactions are particularly advantageous in that the 
usual problem of polymerization as side reaction in preparations and/or 
purifications involving vinylic precursors or products does not occur to 
an appreciable extent. 
Generally, these catalyst systems should comprise a rather high ratio of 
moles of phosphine to gram atoms of palladium for securing high 
conversions at low palladium concentration. The high phosphine content 
disadvantageously attributes to the waste streams and the economics of the 
above processes, in particularly when using substituted phosphines for 
specific selective processes. Therefore, it would be desirable to reduce 
the phosphine/palladium ratio without affecting the performance of these 
catalyst systems. 
The use of inhibitors in other carbonylation reactions and/or preparation 
processes for methyl methacrylate (MMA) has been reported. However, in 
such cases the inhibitor was added for its well-known function of 
inhibiting polymerization reactions of vinylic compounds, since the 
processes were carried out at temperatures of about 100.degree. C. or much 
higher. For example, U.S. Pat. No. 4,447,640 discloses the preparation of 
MMA by carbonylation of 1,2-dihaloalkanes in the presence of a supported 
palladium catalyst and an inhibitor at temperatures in the range of 
150.degree.-300.degree. C. The specification mentions that homogeneous 
catalysts comprising a group VIII metal salt in conjunction with a 
triorganic phosphine can also be used, without providing further detail. 
The alleged effects of inhibitor addition as elucidated in column 4, lines 
7-37, are prevention of polymerization of the product and increase of the 
active life of the catalyst by prevention of fouling of the catalyst 
through deposit of carbon thereon. The latter problem would clearly seem 
to be confined to heterogeneous catalysts. U.S. Pat. No. 4,480,116 
discloses the preparation of MMA by acid hydrolysis of acetone cyanohydrin 
in the presence of 50-3000 ppm of specific inhibitors, particularly during 
the work up procedures. Again, the alleged effect is prevention of 
polymerization of MMA product, whereas this publication is silent on any 
effect on the life of the catalyst. In Example 22 of EP-A-386833, the 
carbonylation of 3-butynol is carried out in the presence of hydroquinone 
for preventing polymerization of the methylenolactone product formed. U.S. 
Pat. No. 4,416,823 discloses the dimeric hydroesterification of 
1,3-alkadienes in the presence of a palladium/phosphine/thiol stabilized 
complex catalyst at preferred temperatures in the range of 
80.degree.-120.degree. C. Preferably, the reaction is conducted in the 
presence of a vinyl polymerization inhibitor to avoid an increased 
incremental loss of 1,3-butadiene to polymeric byproducts. None of these 
publications give any hint to the reduction of a phosphine/palladium ratio 
in general carbonylation reactions, let alone the type of carbonylation 
reaction of the present invention. 
It is therefore an object of the present invention to provide a process of 
carbonylating olefinically or acetylenically unsaturated compounds with 
carbon monoxide and a hydroxy compound in the presence of a catalyst 
system comprising a source of cationic palladium, a source of phosphine 
and a protonic acid with reduced phosphine/palladium ratio. 
SUMMARY OF THE INVENTION 
According to the invention, a process for the carbonylation of an 
olefinically or acetylenically unsaturated hydrocarbon compound is 
provided, comprising reacting the olefinically or acetylenically 
unsaturated hydrocarbon compound with carbon monoxide and a hydroxy 
compound in the presence of a carbonylation catalyst system comprising a 
source of cationic palladium, a source of phosphine and a protonic acid 
and a free radical inhibitor. 
The present inventive process where the reaction is carried out in the 
presence of the free radical inhibitor, the catalytic system used in the 
present process is effective at a lower initial phosphine proportion.

DETAILED DESCRIPTION OF THE INVENTION 
It is now believed that some phosphine was inactivated during the 
previously described carbonylation reaction and that the high ratio of 
phosphine to palladium in the catalyst system was required for maintaining 
a sufficient supply of available phosphine during the entire course of the 
carbonylation reaction. The invention proposes the use of a free radical 
inhibitor in a process for the carbonylation of an olefinically or 
acetylenically unsaturated hydrocarbon compound where the olefinically or 
acetylenically unsaturated hydrocarbon compounds are reacted with carbon 
monoxide and a hydroxy compound in the presence of a catalyst system 
comprising a source of cationic palladium, a source of phosphine and a 
protonic acid for reducing the rate of consumption of the phosphine. 
It was surprisingly found, that in the present carbonylation reaction the 
free radical inhibitor provides a beneficial effect different from its 
usual effect of inhibiting vinylic polymerization. As a consequence, 
catalytic systems having a lower phosphine/palladium ratio than used 
heretobefore, can be used without negative influence to the catalyst life, 
and therefore the degree of conversion, in the carbonylation process 
conducted under the same conditions. Accordingly, the invention provides a 
reduction of costs of phosphine ligands and a reduction of the disposal of 
the phosphine content of the waste stream of the known process. 
The concentration of the free radical inhibitor may vary within wide limits 
depending on factors such as the duration of the carbonylation, the 
concentration of trace oxygen, the catalyst concentration and the 
temperature. For economically attractive carbonylation reactions, the 
concentration of the free radical inhibitor is preferably in the range of 
about 0.0005-1% by weight, more preferably in the range of about 
0.001-0.1% by weight, based on the total of reaction components. It is an 
additional advantage of the present invention, that these inhibitor 
concentrations also will very effectively inhibit any polymerization 
reaction. At normal process temperatures of about 50.degree. C., this side 
effect is though advantageous in theory, but of secondary significance in 
practice. 
Free radical inhibitors, sometimes referred to as polymerization 
inhibitors, suitable for use in the present process are known free radical 
inhibitors used in conventional polymerization technology, and any such 
free radical inhibitor can be used in the present process. Representative 
suitable free radical inhibitors include aromatic hydroxyl compounds, 
aromatic keto compounds, benzo- and naphthoquinones, phenazines, 
phenoxazines and phenothiazines. Preferred free radical inhibitors are 
selected from the group of substituted phenols, including such phenols 
substituted with further hydroxy groups, for instance hydroquinone. The 
phenols may carry any further inert substituent, in particular alkyl 
groups such as methyl and tert.butyl, and include hydroxylated condensed 
aromatic ring systems, such as naphthol. 
Preferable free radical inhibitors include, for example, monohydric 
phenols, such as 4-methyl-2,6-di-tert.butylphenol ("butylated 
methylphenol"), 2,4-dimethyl-6-tert.butylphenol, beta-naphthol, 
p-methoxyphenol ("methylhydroquinone"); dihydric phenols, such as 
hydroquinones, naphthohydroquinones, catechols, for instance 
p-tert.butylcatechol, and trihydric phenols, such as pyrogallol. 
Due to the presence of the free radical inhibitor, the catalytic system 
used in the present process is effective at lower initial phosphine 
proportion. Accordingly, catalyst systems are advantageous, which 
comprise, at the start of the reaction, a ratio of moles of phosphine to 
gram atoms of palladium in the range of from about 2.5 to about 50, 
preferably of from about 5 to about 30. Higher phosphine proportions do 
not disturb the reaction, but attenuate the economic benefits achieved by 
the invention. 
The proportion of the protonic acid in the catalyst system related to the 
initial proportion of phosphine, is suitably in the range of from about 
0.5 to about 10. It has been found that the use of an acid/phosphine ratio 
around one is beneficial to the rate of phosphine consumption, and 
accordingly it is preferred that the ratio of moles of protonic acid to 
moles of phosphine is in the range of from about 0.7 to about 1.5. 
The olefinically or acetylenically unsaturated compounds to be 
carbonylated, the suitable sources of cationic palladium, phosphine and 
protonic acid, the reaction conditions and further experimental details 
are extensively described in the United States patents mentioned 
hereinbefore, which are incorporated herein by way of reference. 
In summary, olefinically unsaturated hydrocarbons include alkenes, in 
particular 1-alkenes, having generally 2-20 carbon atoms, which may be 
straight or branched and may comprise a plurality of double bonds, for 
example ethene, propene, 1-butene, 2-butene, the isomeric pentenes, 
hexenes, octenes, and 1,5-cyclooctadiene. Acetylenically unsaturated 
compounds include alkynes, in particular 1-alkynes, which may be straight 
or branched unsubstituted and may comprise a plurality of triple bonds or 
further double bonds, for example ethyne, propyne, and 1-butyne. Suitable 
hydroxy compounds include water, alcohols, and carboxylic acids, which may 
be aliphatic, cycloaliphatic or aromatic, preferably contain not more than 
20 carbon atoms, and may have more than one hydroxy function. Examples of 
suitable alcohols include methanol, ethanol, propanol, isobutanol, 
tert.butanol, stearyl alcohol, phenol, ethylene glycol and glycerol. 
Examples of suitable carboxylic acids include acetic acid and propionic 
acid. 
Suitable palladium sources include palladium compounds such as salts, for 
example palladium acetate, and complexes, for example 
tetrakis-triphenylphosphinepalladium and bistriphenylphosphinepalladium 
acetate, but also metallic palladium which is solubilized by the acid 
component of the catalyst system. Suitable phosphines generally include 
triorganic phosphines, of which the organic substituents independently of 
each other may be aliphatic, cycloaliphatic, aromatic or heterocyclic and 
contain 1-10 carbon atoms, for example triphenylphosphine, 
ethyldiphenylphosphine, dicyclohexylphenylphosphine, 
2-pyridyldiphenylphosphine, bis(6-methyl-2pyridyl)phenylphosphine, 
tri-p-chlorophenylphosphine and tri-pmethoxyphenylphosphine. Preferred 
phosphines comprise at least one optionally substituted 2-pyridyl group. 
Suitable protonic acids preferably have a non-coordinating or weakly 
coordinating anion. Generally, such acids are strong acids having a pKa 
below about 4.5, more particularly below about 2 (measured at 18.degree. 
C. in aqueous solution), and include sulfuric acid, sulfonic acids, 
phosphonic acid and certain carboxylic acids. In the present context, the 
protonic acid may be generated by interaction of a Lewis acid, such as 
BF.sub.3, with a proton donor, such as HF, or may be generated in situ. It 
may also be an acidic ion exchange resin. 
The process is conveniently effected in the liquid phase. A separate 
solvent is not essential. Solvents for optional use in the process include 
aromatic hydrocarbons, esters, ethers and sulfones. At the preferred 
concentrations, the free radical inhibitor will readily dissolve in the 
liquid reaction medium. The present process is conveniently carried out at 
a temperature in the range of from about 10.degree. to about 130.degree. 
C. Preferred temperatures are in the range of from about 30.degree. to 
about 90.degree. C. Convenient pressures are in the range of from about 1 
to about 100 bar. The molar ratio between the olefinically or 
acetylenically unsaturated hydrocarbon compound and the hydroxy compound 
is not critical, and may vary within a range of about 0.01:1 to 100:1. The 
quantity of the catalyst system is not critical, and the quantity of 
palladium may conveniently be in the range of about 10.sup.-7 to about 
10.sup.-1 of gram atom palladium per mole of unsaturated compound. 
The carbon monoxide required for the process according to the invention may 
be used in a practically pure form or diluted with an inert gas, for 
example nitrogen. Also hydrogen may be present, if it is substantially 
inert in the particular carbonylation reaction. 
The catalyst systems used in the present process may be prepared by any 
convenient method. Thus they may be prepared by combining a separate 
palladium compound, the phosphine and the protonic acid. Alternatively, 
they may be prepared by combining a palladium compound and an acid 
addition salt of the phosphine. Alternatively, they may be prepared from a 
palladium compound which is a complex of palladium with the phosphine, and 
the protonic acid. The free radical inhibitor may be introduced into the 
reaction by any convenient method. It may, for example, be admixed with 
the catalyst system, or it may be incorporated into one of the precursor 
feeds. 
By way of example, the invention will be demonstrated by reference to the 
carbonylation of an acetylenically unsaturated compound, more particularly 
the preparation of an alkyl methacrylate by reaction of propyne with 
carbon monoxide and an alkanol. Such a process is described in more detail 
in U.S. Pat. No. 4,940,787, and uses a catalyst system that can be formed 
from a palladium compound, a protonic acid and an organic phosphine of the 
general formula PR.sub.1 R.sub.2 R.sub.3, wherein one, two or each of 
R.sub.1, R.sub.2 and R.sub.3 represent a heterocyclic 5 or 6 atom ring 
comprising at least nitrogen as hetero atom, which ring is optionally 
substituted and/or may form part of a larger condensed ring structure that 
is optionally substituted, and any remaining group R.sub.1, R.sub.2 or 
R.sub.3 represents an optionally substituted hydrocarbyl group. 
At the beginning of the reaction, the liquid carrier mainly comprises 
methanol besides liquified propyne if the reaction is conducted at 
increased pressure. In the course of the reaction methanol is replaced by 
methyl methacrylate product further acting as the liquid carrier or 
solvent for the reaction mixture. In a continuously conducted process, 
part of the reaction feed consist of a recycled methyl 
methacrylate/methanol azeotrope stream, and accordingly in each stage of 
the reaction methyl methacrylate is present. 
EXAMPLE 
Representative stability tests were carried out by preparing solutions of 
the indicated molar amounts of the catalyst components in mixtures of 
methyl methacrylate (MMA) and methanol (MeOH) at the indicated weight 
ratios or in methanol only. Palladium acetate was used as source of 
palladium, diphenyl 2-pyridyl phosphine as the ligand and methane sulfonic 
acid as the acid components of the catalyst system. In tests Nos. 1-3, 5, 
6 and 8, furthermore the indicated amount of the indicated type of free 
radical inhibitor was added. Tests Nos, 4 and 7 are for reference 
purposes. The solutions were stored for 24 hours under a gaseous medium 
and at a temperature as indicated. Thereupon, the solutions were analyzed 
on their content of phosphinoxide relative to the total of phosphorus 
compounds, phosphine and phosphinoxide. The initial oxide content of the 
phosphine sample used in theses experiments was 2.4% mol. 
The results of the tests are mentioned below. It is seen that the presence 
of a free radical inhibitor increases the life time of the catalyst 
system. The effect is particularly pronounced, if the reaction solvent 
comprises MMA even under a nitrogen atmosphere. 
______________________________________ 
Test 
(No.) 1 2 3 4 5 6 7 8 
______________________________________ 
MMA 46.8 48.6 49.3 50.2 47.4 48.8 -- -- 
(% wt) 
MeOH 53.2 51.4 50.7 49.8 52.6 51.2 100 100 
(% wt) 
Medium N.sub.2 
N.sub.2 
N.sub.2 
N.sub.2 
N.sub.2 
N.sub.2 
air air 
Pd 0.23 0.12 0.11 0.11 0.12 0.11 0.21 0.23 
(mmol/ 
kg) 
Ligand 2.41 1.20 1.16 1.16 1.23 1.11 2.84 2.92 
(mmol/ 
kg) 
Acid 5.06 2.38 2.40 2.28 2.44 2.24 1.31 1.22 
(mmol/ 
kg) 
H/L 2.1 2.0 2.1 2.0 2.0 2.0 0.46 0.42 
(mol/ 
mol) 
L/Pd 10 10 10 10 10 10 14 13 
(mol/ 
mol) 
Inhibitor 
type*.sup.) 
HQ HQ MEHQ -- HQ BMP -- MEHQ 
(mmol/ 4.94 4.68 9.02 -- 0.97 2.25 -- 3.01 
kg) 
Temp. 45 45 45 45 45 45 20 20 
(.degree.C.) 
Oxide**.sup.) 
t = 20 13.1 10.4 14.5 90.0 21.7 15.3 16.5 6.8 
(% mol) 
______________________________________ 
*.sup.) HQ = hydroquinone; MEHQ = methylhydroquinone; BMP = butylated 
methylphenol. 
**.sup.) molar proportion of phosphinoxide relative to total P compounds 
present; initial oxide content of ligand: 2.4% mol.