Kinetic and spectroscopic studies of the carbonylation of methanol with an iodide-promoted iridium catalyst

Abstract

A mechanistic interpretation is proposed for the carbonylation of methanol in the presence of iridium halides as the catalyst precursor and methyl iodide as promoter. The scheme is based on a combination of kinetic observations, in situ spectral studies of reactions, and studies of the chemistry of the observed intermediates. It is shown that there are two principal catalytic cycles for the methanol carbonylation, one involving neutral iridium carbonyl iodide complexes and the other involving anionic iridium species. In addition, a competitive water–gas shift reaction is observed under many conditions. The catalytic cycle which predominates is influenced by iodide-ion level and hydrogen iodide concentration. These two factors are influenced by variables such as water, methanol, and methyl iodide concentrations as well as salt additives and temperature. The kinetic dependencies for the reaction are dramatically dependent on the catalytic cycle which predominates.