The rhodium-catalyzed methanol carbonylation to acetic acid at low water concentrations: the effect of iodide and acetate on catalyst activity and stability

Abstract

Addition of iodide salts to rhodium catalyst solutions for methanol carbonylation at low water concentrations (<2 M) promotes the rate of carbonylation and stabilizes the rhodium catalyst. Some iodide salts also react with methyl acetate to form acetate salts which also act as promoters. We suggest that promotion by acetate and iodide ions can be explained by their coordination with Rh(I), generating catalyst species which are more active towards oxidative addition of methyl iodide. Rate dependences on water and methyl acetate concentrations are explained by their role in increasing the steady-state concentration of Rh(I) via the water-gas shift process. Catalyst stabilization is rationalized by formation of soluble rhodium complexes that prevent precipitation of RhI 3. By choosing the proper iodide salt and methyl acetate concentrations, the same rate and catalyst stability observed at high water concentrations can be obtained at much lower water concentrations with improved CO selectivity to acetic acid. Operation with lower water concentrations can provide significant savings in the commercial acetic acid process.