Ligand effects in the rhodium-catalyzed carbonylation of methanol

Abstract

The carbonylation of methanol to give acetic acid is one of the most important homogeneously catalyzed industrial processes. The original [Rh(CO)2I2]- catalyst, developed at the Monsanto laboratories and studied in detail by Forster and co-workers, is largely used for the industrial production of acetic acid and anhydride. The conditions used (30-60 bar pressure and 150-200 °C) have spurred the search for new catalysts which work under milder conditions. However, attempts to increase the activity of the catalyst [Rh(CO)2I2] - by introducing electron-donating ligands are generally hampered by the instability of the complexes formed under the harsh reaction conditions. As iridium complexes are normally more stable than the corresponding rhodium complexes, efforts have been made to find suitable iridium catalysts for the carbonylation of methanol. This resulted in the development of the Cativa process, based on [Ir(CO)2I2]- in combination with Ru(CO)4I2, which is presently the most efficient process for the industrial manufacture of acetic acid. On the other hand recent advances in the design of suitable ligands, mainly based on phosphorus-containing systems, allow the synthesis of highly active and stable rhodium complexes, so that a new impetus for the rhodium-catalyzed carbonylation of methanol is to be expected. In this review, attention is focused on the use of phosphine ligands in order to improve the catalytic activity of the rhodium catalysts. This review also includes our recent research results and implications in developing new multifunctional ligands for the rhodium-catalyzed carbonylation of methanol. © 2003 Elsevier Science B.V. All rights reserved.