Methanol carbonylation catalyzed by a polymer-bound rhodium (I) complex

Abstract

The carbonylation reaction of methanol in the presence of methyl iodide promoter was catalyzed by a matrix-bound Rh(I) complex formed by ligand exchange of RhCl(CO) (PPh 3) 2 with a styrene-divinylbenzene copolymer containing bound -CH 2PPh 2 groups. Infrared spectra of a functioning, 7-μm-thick membrane catalyst were measured simultaneously with reaction rates in a steady-state flow reactor at 1 atm and 90–125 °C. Kinetic experiments were also performed with macroporous catalyst beads suspended in a stirred batch reactor operating at conditions of potential industrial interest, 20 atm and 90 °C. Reaction rates of the order of 10 −5 moles/(sec mole of Rh) were observed at 100 °C. The carbonylation product at 1 atm was methyl acetate, and the only side product was dimethyl ether. The reaction was approximately first order in methyl iodide, zero order in each of the reactants, and second order in the Rh(I) complex. The stretching frequency of the Rh(I)-bound carbonyl (1978 cm −1) indicated that the predominant form of the catalyst was the bound analog of RhCl(CO)(PPh 3) 2. This complex was converted over tens of hours of operation to a coordinatively saturated and inactive Rh(III) complex, probably an iodine adduct. The results point to a mechanism involving slow oxidative addition of methyl iodide to a Rh(I) complex followed by methyl migration and subsequent steps in the coordination sphere of Rh. The suggested transition state for methyl iodide addition involves two Rh(I) centers in the flexible polymer network, one acting as a nucleophile attacking the carbon and the other playing a solvent-like role stabilizing the transition state.