A mechanistic study into the effect of acetic acid on methanol synthesis

The effect of the presence of higher oxygenates on the synthesis of methanol over an unmodified commercial Cu/ZnO/Al2O3 catalyst operating under realistic industrial conditions (523 K, 50 barg) was investigated using acetic acid as a suitable probe molecule. On addition of 1 mol% acetic acid the methanol yield decreased by 36%. The acetic acid was totally converted to a range of products namely, ethanol, methyl acetate, carbon monoxide and carbon dioxide. Use of d4-acetic acid revealed no kinetic isotope effect in the formation of any product. However a range of CX3CH2OH species were formed including CD3CH2OH, CDH2CH2OH and CH3CH2OH revealing H/D exchange in the methyl fragment. Using [1-13C]-acetic acid and [2-13C]-acetic acid it could be shown that the production of CO/CO2 from the adsorbed acetate was through decarboxylation. The residual adsorbed methyl fragment decomposed to give carbon, which was re-oxidised with adsorbed oxygen.