Method for stabilizing rhodium compound

Abstract

A concentrated catalyst solution used in a carbonylation reaction in which methyl acetate or dimethyl ether is chemically bonded with carbon monoxide in the presence of a catalyst system comprising a rhodium compound and an alkali metal iodide to produce acetic anhydride is subjected to a carbonylation treatment with carbon monoxide, or a mixture of carbon monoxide and hydrogen, before it is applied to a process for separating the tar contained in the catalyst solution, thereby increasing the iodide ion content of the catalyst solution and enabling the rhodium compound to be stabilized. The balances between the rhodium concentration, the alkali metal concentration and the iodine concentration of the carbonylation reaction system are not disturbed, even when the catalyst solution as reprocessed is returned to the carbonylation reaction system. Further, the increase in the iodide ion concentration of the concentrated catalyst solution enables the rhodium compound contained in the concentrated catalyst solution to be stabilized, and prevents the rhodium compound from settling from the catalyst solution, even during heat treatment conducted for the purpose of removing components having a low-boiling point contained in the catalyst solution before the process for separating and removing tar from the concentrated catalyst solution, so that the loss of the very expensive rhodium compound can be minimized.