Carbonylation process

#This invention pertains to a process for the preparation of carboxylic acids and/or derivatives thereof. More specifically, this invention pertains to a process wherein a mixture of a dialkyl carbonate and halide compound is contacted with carbon monoxide in the presence of a metal selected from Group VIII of the Periodic Table. The carbon dioxide co-product of the process may be reacted with a suitable substrate to produce useful chemicals such as urea or cyclic carbonates.

#It is known that carboxylic acids and carboxylic acid derivatives may be produced by the carbonylation of a mixture of (1) a carbonylation feedstock compound such as alcohols, esters, ethers or mixtures thereof and (2) a halide compound such as hydrogen iodide and/or an alkyl iodide. The products of these known processes are carboxylic acids, esters and/or anhydrides, depending on the specific feedstock and processes conditions employed. Carboxylic esters and anhydrides are the primary products when water is absent, whereas carboxylic acids and esters are the main products when water is present. The most efficient catalysts for these reactions are metals from Group VIII of the Periodic Table. Thus carbonylation of mixtures of methanol and a halide compound produces acetic acid, and methyl acetate and water also may be produced at low methanol conversion. Carbonylation of mixtures of methyl acetate and a halide compound produces acetic anhydride in the absence of water.

#The use of dimethyl ether (DME—also referred to as methyl ether) as the feedstock compound in carbonylation processes produces methyl acetate and acetic anhydride at sufficiently high rates of conversion in the absence of water. DME is an attractive feedstock material for acetic anhydride synthesis because the recycle of acetic acid equivalents through the carbonylation reactor is greatly reduced. Although DME is an attractive feedstock material for the synthesis of acetic acid and derivatives in some ways, it has limitations due to its physical properties. For example, the boiling point of DME is −24° C. at atmospheric pressure. Therefore, DME either must be stored in special pressure vessels or in refrigerated vessels. Either of these options is expensive. An accidental release of dimethyl ether into the environment may result in a fire or an explosion. An alternative carbonylation feedstock compound is needed which avoids high recycle of acetyl equivalents and the troublesome physical properties of DME.

#The alkyl groups of the dialkyl carbonate feedstock material may contain 1 to about 20 carbon atoms. The alkyl groups may be substituted with groups inert under the process reaction conditions (non-reactive groups) such as aryl groups, e.g., phenyl. The two alkyl substituents of the dialkyl carbonate feedstock may be the same or different. It is preferable that the two carbonate alkyl groups are the same. Dimethyl carbonate is the most preferred carbonate. The dialkyl carbonate feedstock may be carbonylated with one equivalent of carbon monoxide to produce an alkyl carboxylate ester of the carboxylic acid derived from the from the alkyl group of the dialkyl carbonate and carbon dioxide (reaction 4 above). The dialkyl carbonate feedstock may be carbonylated with two equivalents of carbon monoxide to produce an anhydride of the carboxylic acid derivable from the alkyl group or groups of the dialkyl carbonate. Thus, dimethyl carbonate may be carbonylated with one carbon monoxide molecule to produce methyl acetate and carbon dioxide (reaction 4 above) or with two carbon monoxide molecules to produce acetic anhydride (reaction 7) and carbon dioxide: Acetic anhydride synthesis: (CH3O)2CO+2CO=CH3C(O)O(O)CCH3+CO2  (7) When dimethyl carbonate is carbonylated in the presence of water and/or methanol, acetic acid also may be produced. The process may be operated to produce mixtures of products. Thus, dimethyl carbonate may be contacted with carbon monoxide in the presence of methanol or water to produce mixtures comprising methyl acetate, acetic anhydride and acetic acid.

#The carbonylation process of the present invention may be operated as a vapor or liquid phase process. In the liquid phase process, the catalyst components are dissolved in a solution typically comprising the feedstock material or materials, products and halide compound and may contain additional liquid components. The liquid phase process may be operated either in a batch mode or a continuous mode with the continuous mode being preferred. In vapor phase operation, the catalyst may be a supported catalyst comprising a metal carbonylation catalyst deposited on an inert support material. Alternatively, the catalyst in vapor phase operation may be a metal carbonylation catalyst compound dissolved in a relatively non-volatile (non-volatile under the carbonylation conditions of pressure and temperature) liquid. Vapor phase operation typically is carried out in a continuous mode of operation wherein the vapor feedstock materials contact the catalyst mixture and the products are removed from the carbonylation zone as a vapor. Carbon is an example of a catalyst support material for vapor phase operation. Molten ammonium, phosphonium and sulfonium salts, typically halides, are examples of non-volatile liquids useful in vapor phase operation. The molten ammonium, phosphonium and sulfonium salts also may be used in the liquid phase operation as solvent components. A carboxylic acid also may be used as a component of the liquid or vapor phase processes, although it can be incorporated in the product if a dialkyl carbonate other than dimethyl carbonate is used. When acetic acid is used as a component in the carbonylation of dimethyl ether, methyl acetate and methanol also may be produced, but this reaction does not represent a net production of acetyl since no carbonylation has occurred. It is preferable to perform the liquid phase in a purged mode so the carbon dioxide produced can leave the reactor.

#The carbonylation process of the present invention may be performed at elevated temperatures ranging from about 100 to 300° C. Liquid phase carbonylation is performed at a lower temperature than the vapor phase reaction. Typical temperatures for liquid phase operation range from about 100 to 220° C. with temperatures of about 150 to 200° C. being preferred. Higher temperatures are preferred for the vapor phase operation wherein the catalyst is a catalytic carbonylation metal deposited on an inert support. For example, the process may be carried out in the vapor phase at temperature of about 150 to 300° C. with temperatures of about 170 to 250° C. being preferred. The upper temperature for vapor phase process using a relatively non-volatile liquid such as a molten ammonium, phosphonium or sulfonium salt is about 250° C., preferably about 230° C.