Mechanism and selectivity control of methyl acetate and methyl formate formation from methanol alone with [Ru(SnCl3)5(PPh3)]3- as catalyst

Abstract

Mechanistic studies have been made of the unique one-pot conversion of methanol into acetic acid (and/or methyl acetate due to rapid esterification) with [Ru(SnCl ₃ ) ₅ (PPh ₃ )] ^3- as catalyst. Addition of Cl ^- ion promoted the formation of methyl formate relative to methyl acetate, but scarcely changed their total rate of formation. Further, the activation energies for their formation were virtually identical (≈76 kJ mol ^-1 ). No ¹³ C was incorporated into methyl acetate even when the reaction was performed under a ¹³ CO atmosphere, indicating that the conversion of methanol into methyl acetate does not involve carbonylation. The effect of the addition of Cl ^- ion on the reaction of formaldehyde was similar to that when using methanol as substrate, but quite different activation energies were obtained for the formation of methyl acetate (27.9 kJ mol ^-1 ) and methyl formate (40.6 kJ mol ^-1 ). The results are in accord with rate-determining dehydrogenation of methanol to formaldehyde (as a common intermediate), which is then converted competitively into acetic acid and methyl formate with different activation energies. The effect of added free Cl ^- ion for both substrates suggests the presence of a pre-equilibrium dissociation of Cl ^- from the co-ordinated SnCl ₃ ^- , which is required for the formation of acetic acid but not for methyl formate. A possible reaction scheme is presented, in which reductive elimination of methyl formate is competitive with that of acetic acid.