Method for stabilizing rhodium compound

Abstract

A concentrated catalyst solution provided in the 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, thus enabling the rhodium compound to be stabilized. When the concentration of the iodide ion in the concentrated catalyst solution is increased by this method, there is no possibility of the balances between the rhodium concentration and, the alkali metal concentration and the iodine concentration of the carbonylation reaction system being 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, which prevents the rhodium compound from settling from the catalyst solution even during the heat treatment conducted for the purpose of removing components each 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.