PROCESS FOR THE PREPARATION OF ESTERS OF CARBOXYLIC ACIDS

Abstract: Process for the preparation of esters of carboxylic acids by contacting an olefinically unsaturated compound with CO and an alcohol, in the presence of (a) a ruthenium compound and (b) an iodide salt, LiBr and/or an onium bromide, a molar ratio of CO to olefinically unsaturated compound ≦ 2.0 being applied when "b" is an iodide salt; a catalytic system prepared by combining (a) a ruthenium compound and (b) LiBr and/or an onium bromide.

Description: The invention relates to a process for the preparation of esters of carboxylic acids. The invention also relates to a catalytic system which may be used in this process. U.S. patent specification 3,168,553 discloses a process in which olefins are carbonylated in the presence of a complex comprising a trialkylphosphine together with cobalt, ruthenium, rhodium or iridium. However, this process requires the use of high pressures, and its selectivity towards the desired product is often unsatisfactory. For instance, the carbonylation of ethylene in the presence of ethanol and Co₂(CO)₈ as the catalyst leads to the formation not only of ethyl propionate, but also of large quantities of by-products, such as diethyl ketone and acetaldehyde. J. Molecular Catalysis 40 (1987) 243-254 concerns the synthesis of ketones and esters from olefins, carbon monoxide and alcohols by using ruthenium-halide catalysts. In the carbonylation of ethylene with methanol, use of ionic halides such as sodium iodide gives methyl propionate selectively, using pressures of carbon monoxide and ethylene of 45 and 20 kg/cm², respectively. It has now surprisingly been found that amending the process described in the above-mentioned article by selecting a molar ratio of carbon monoxide to olefinically unsaturated compound below a certain value results in a higher reaction rate and even in a higher selectivity to esters of carboxylic acids. The invention therefore provides a process for the preparation of esters of carboxylic acids which process comprises contacting an olefinically unsaturated compound with carbon monoxide and an alcohol in the presence of a catalytic system which may be prepared by combining the following components:- component (a) - a ruthenium compound, and component (b) - an ionic halide, characterized in that the ionic component (a) - a ruthenium compound, and halide is selected from an iodide salt, lithium bromide and an onium bromide, and that a molar ratio of carbon monoxide to olefinically unsaturated compound of not higher than 2.0 is applied. The selectivity to a certain compound, expressed in a percentage, is defined herein as 100 x c:d in which "c" is the amount of starting olefinically unsaturated compound that has been converted into that certain compound and "d" is the total amount of starting olefinically unsaturated compound that has been converted. Amending the process of the present invention by replacing component (b) with, for example, lithium chloride, sodium bromide, calcium bromide, cobalt(II) bromide, zinc bromide, tin(IV) bromide or an onium chloride results in decreased reaction rates and/or decreased selectivities to esters of carboxylic acids. According to a preferred embodiment of the present invention a molar ratio of carbon monoxide to olefinically unsaturated compound in the range of from 0.2 to 2.0 is used. The onium compounds as meant herein are derived from the elements of Groups 5a and 6a of the Periodic Table of the Elements, i.e. from nitrogen, phosphorus, arsenic, antimony, sulphur, selenium and tellurium, and preferably from nitrogen and phosphorus. Those derived from nitrogen, phosphorus, arsenic and antimony are salts containing quadricovalent such atoms bound to four hydrocarbon groups and those derived from sulphur, selenium and tellurium are salts containing tricovalent such atoms bound to three hydrocarbon groups. The iodide salt of component (b) is preferably an onium iodide or an iodide of an alkali metal, i.e. of lithium, sodium, potassium, rubidium or cesium; lithium is preferred, this alkali metal usually allowing the highest reaction rates and highest selectivities to esters of carboxylic acids. The catalytic system contains two different components, i.e. component (a) and component (b) are not both ruthenium triiodide. Among the iodides and bromides the latter are preferred because they are generally less corrosive. Very good results have been obtained with iodides and bromides derived from quaternary phosphonium hydroxides, particularly those derived from quaternary alkylphosphonium hydroxides having less than 20 and, more particularly, less than 10 carbon atoms per alkyl group. If desired, the process according to the present invention may be carried out in the presence of a crown ether which solubilizes the cation of the iodide salt or the lithium ion. An example of a crown ether is 1,4,7,10,13,16-hexaoxacyclooctadecane, also known under the name "18-crown-6". This compound may be prepared as described in "Synthesis" 1976, pages 515 and 516. Another feature of the present invention is that the catalytic system is very stable and, therefore, can be used for a long time: no plating out of metallic ruthenium has been observed. Replacement of component (a) with a palladium compound would result in plating out of metallic palladium. Examples of suitable ruthenium compounds are ruthenium oxides and ruthenium salts. Very good results have been obtained with compounds of trivalent ruthenium, particularly ruthenium tri(acetylacetonate). Other examples of suitable ruthenium compounds are ruthenium tribromide, ruthenium trichloride and trirutheniumdodecacarbonyl. Component (a) may be a mixture of ruthenium compounds. Amending the present process by using ruthenium tri(acetylacetonate) and deleting component (b) results in hardly any reaction. Amending the present process by using ruthenium trichloride and deleting component (b) results in the formation of diethyl ketone. The quantity of component (a) used may vary within wide ranges and is generally in the range between 10⁻⁶ and 10⁻¹ and preferably between 10⁻⁵ and 10⁻² gram-atom ruthenium per mol starting olefinically unsaturated compound. The quantity of component (b) may also vary within wide ranges; component (b) is preferably present in a quantity in the range of from 0.1 to 100 and particularly from 0.5 to 20 equivalents per gram-atom of ruthenium. The process according to the present invention may be carried out at a temperature and a pressure which are not critical and may vary within wide ranges. The process is preferably carried out at a temperature in the range of from 100 °C to 250 °C, for example 125 to 225 °C and a pressure in the range of from 5 to 200 bar, for example 25 to 100 bar, but temperatures above 250 °C or below 100°C or pressures above 200 bar or below 5 bar may also be used, if desired. The olefinically unsaturated compound may be an unsubstituted or a substituted alkene or cycloalkene preferably having 2-30, and in particular 2-20, carbon atoms and preferably 1-3 double bonds per molecule. The alkene or cycloalkene may be substituted, for instance, with one or more halogen atoms or cyano, ester, alkoxy, hydroxy, carboxy or aryl groups. If the substituents are not inert under the reaction conditions, the carboxylation reaction may be accompanied with other reactions. For instance, the carboxylation of allyl alcohol is accompanied with esterification of the hydroxy group. Examples of suitable olefinic compounds are ethene, propene, 1-butene, 2-butene, isobutene, the isomeric pentenes, hexenes, octenes and dodecenes, 1,5-cyclooctadiene, cyclododecene, 1,5,9-cyclododecatriene, allyl alcohol, methyl acrylate, ethyl acrylate, methyl methacrylate, acrylonitrile, acrylamide, N,N-dimethylacrylamide, vinyl chloride, allyl chloride, acrolein, oleic acid, methyl allyl ether and styrene. Preference is given to alkenes having 1 to 5 carbon atoms per molecule, in particular to ethylene and propylene. The use of ethylene is most preferred, propionates being obtained with a very high selectivity. The alcohols used in the process according to the invention may be aliphatic, cycloaliphatic or aromatic and may be substituted with one or more substituents, such as mentioned hereinbefore in connection with the olefinically unsaturated compounds to be used as starting material. The alcohol may therefore also be a phenol. The alcohols preferably contain not more than 20 carbon atoms per molecule. Examples of suitable alcohols are methanol, ethanol, propanol, isobutanol, tert.butyl alcohol, stearyl alcohol, benzyl alcohol, cyclohexanol, allyl alcohol, chlorocapryl alcohol, ethylene glycol, 1,2-propanediol, 1,4-butanediol, glycerol, polyethylene glycol, 1,6-hexanediol, phenol and cresol. Special preference is given to alkanols having in the range of from 1 to 10 carbon atoms per molecule. If the alcohol has more than one hydroxy group, different products may be formed, depending on the molar ratios existing between the reagents. For instance, depending on the quantity of olefinically unsaturated compound used, either a mono-ester or a di-ester may be produced from glycol. The process according to the present invention is particularly suitable for the preparation of methyl propionate, starting from ethylene, carbon monoxide and methanol, methyl propionate being an important solvent. In the process according to the invention the carbon monoxide may be used pure or diluted with an inert gas, such as nitrogen, noble gases or carbon dioxide. Generally the presence of more than 10% by volume of hydrogen is undesirable. Generally preference is given to the use of carbon monoxide or a carbon monoxide containing gas which contains less than 5% by volume of hydrogen. The molar ratio of the olefinically unsaturated compound to alcohol is not critical. The molar ratio between hydroxy groups and olefinic double bonds may lie for instance between 0.1:1 and 10:1. When using a mono-olefin and a monohydric alcohol preference is usually given to the use of an excess of the hydroxy compound mentioned. However, when using a polyhydric alcohol to prepare a polyester, it will generally be necessary to use an excess of olefinically unsaturated compound. The process according to the invention may be carried out batchwise, continuously or semi-continuously. Generally there is no need for the use of a solvent since usually there will be an excess of one of the reactants - for instance the alcohol - which may serve as a solvent as well. If required, however, a solvent may be used, for instance diisopropyl sulphone, tetrahydrothiophene 1,1-dioxide (also referred to as sulfolane), acetone, chloroform, methyl isobutyl ketone, diglyme (dimethyl ether of diethylene glycol) or diisopropyl ether. The primary reaction product of the carbonylation reaction may also be used as a solvent. The reaction mixtures obtained may be subjected to suitable catalyst and product separating means comprising one or more such steps, for example, as stratification, solvent extraction, distillation, fractionation or adsorption. The catalytic system as well as unconverted starting compounds or solvent, if any, may be recycled, in part or entirely, to the reaction zone. The following Examples further illustrate the invention. The experiments were carried out in a 300 ml magnetically stirred stainless steel autoclave. The reaction mixtures obtained were analyzed by means of gas-liquid chromatography. Examples 1-7 and Comparative Experiments A-I The autoclave was charged with methanol (50 ml, 1.23 mol), ruthenium tri(acetylacetonate) (0.2 mmol) and compound (b), flushed with carbon monoxide, sealed, charged with carbon monoxide, charged with ethylene and heated to a temperature of 175 °C. The Table hereinafter states which compound (b) was used, the amount in which this compound was used and the pressures of carbon monoxide and ethylene to which the autoclave was charged at ambient temperature. Sixteen experiments were carried out in this manner. After a reaction time of 5 h the autoclave was allowed to adopt ambient temperature and then opened for analysis of the reaction mixture. The Table hereinafter presents the reaction rates and the selectivity to methyl propionate. A comparison between the Examples 1, 3 and 5, where a molar ratio carbon monoxide to ethylene of 1.0:1.0 was used with the Examples 2, 4 and 6, respectively, where this molar ratio was 2.0 shows that a lower molar ratio results in a higher reaction rate and a higher selectivity to methyl propionate. Comparative Experiments A and B show that using lithium chloride and sodium bromide results in a low reaction rate. Comparative Experiments C, D and E show that using calcium bromide, cobalt bromide and zinc bromide results in a selectivity to methyl propionate which is lower than 90%. Comparative Experiments G, H and I show that lithium phosphate, sodium phosphate and nickel phosphate do not promote the carbonylation. Example 7 shows that the presence of tetrabutylammonium iodide allows a high reaction rate and high selectivity to methyl propionate. Example 8 An experiment was carried out as Example 1 with the difference that ruthenium tri(acetylacetonate) (0.2 mmol) was replaced with trirutheniumdodecacarbonyl (0.07 mmol). The reaction rate was 180 mol methyl propionate per gram-atom ruthenium per h, with a selectivity to methyl propionate of 97.5%. Comparative Experiment J An experiment was carried out as Example 1 using the following ingredients: water 10 ml ruthenium tri(acetylacetonate) 0.2 mmol diglyme 40 ml lithium bromide 2 mmol. The starting pressures of carbon monoxide and ethylene were 20 bar each and the reaction mixture was kept for 5 h at a temperature of 175 °C. The reaction rate was 100 mol ethylene per gram-atom ruthenium per h, with selectivities to propionic acid and diethyl ketone of 47% and 53%, respectively. This experiment shows that propionic acid is not formed with a high selectivity. Comparative Experiment K An experiment was carried out as Example 1 using the following ingredients: water 10 ml ruthenium tri(acetylacetonate) 0.2 mmol diglyme 40 ml sodium iodide 2 mmol. The starting pressures of carbon monoxide and ethylene were 25 bar each and the reaction mixture was kept for 5 h at a temperature of 175 °C. The reaction rate was 100 mol ethylene per gram-atom ruthenium per h, with selectivities to propionic aldehyde and diethyl ketone of 75% and 18%, respectively, with only traces of propionic acid. Example 9 An experiment was carried out as Example 1 using the following ingredients: methanol 50 ml tetrabutylphosphonium bromide 2 mmol ruthenium tri-(acetylacetonate) 0.2 mmol propylene 30 ml. The autoclave was flushed with carbon monoxide and charged with carbon monoxide until a pressure of 30 bar was obtained. The reaction rate was 40 mol propylene per gram-atom ruthenium per h, with selectivities to n-butyl butyrate and isobutyl butyrate of 45% and 55%, respectively.

Claims: A process for the preparation of esters of carboxylic acids which process comprises contacting an olefinically unsaturated compound with carbon monoxide and an alcohol in the presence of a catalytic system which may be prepared by combining the following components: component (a) - a ruthenium compound, and component (b) - an ionic halide, characterized in that the ionic halide is selected from an iodide salt, lithium bromide and an onium bromide, and that a molar ratio of carbon monoxide to olefinically unsaturated compound of not higher than 2.0 is applied. A process as claimed in claim 1 in which component (a) comprises ruthenium tri(acetylacetonate). A process as claimed in any one of the preceding claims in which the iodide salt is an alkali metal iodide or an onium iodide. A process as claimed in any one of the preceding claims in which component (b) comprises a phosphonium iodide or phosphonium bromide. A process as claimed in any one of the preceding claims in which the alcohol is an alkanol having not more than five carbon atoms per molecule. A process as claimed in any one of the preceding claims in which a molar ratio of carbon monoxide to olefinically unsaturated compound if, the range of from 0.2 to 2.0 is used. A process as claimed in any one of the preceding claims in which component (b) is present in a quantity in the range of from 0.1 to 100 equivalents per gram-atom of ruthenium. A process as claimed in any one of tie preceding claims which is carried out at a temperature in the range of from 100 °C to 250 °C and a pressure in the range of from 5 to 200 bar. A process as claimed in any one of the preceding claims in which the olefinically unsaturated compound is an alkene having in the range of from 2 to 30 carbon atoms per molecule.

Inventors: DRENT, EIT
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1988
Main classification: C07C67
IPC codes: B01J23, C07B61, C07C67, B01J31, C07C69