Mechanism of ethanol formation from synthesis gas over CuO/ZnO/Al 2O 3

Abstract

The formation of ethanol from CO and H 2 over a CuO ZnO methanol catalyst was studied in a fixed-bed microreactor by measuring the isotopic distribution of the carbon in the product ethanol when 13C methanol was added to the feed. The purpose of this work was to determine whether experimental support could be obtained for any of the various mechanisms proposed in the literature for the synthesis of ethanol over methanol catalysts. The methanol was added at six partial pressures so that the ratio of methanol to CO ranged from 1:34 to 1:250. For each feed ratio the isotopic composition of the reaction products was determined at various contact times from 0.5 to 10 s, and the composition at zero contact time was determined by extrapolation. All four isotopic species possible for ethanol, 12CH 3 12CH 2OH, 12CH 3 13CH 2OH, 13CH 3 12CH 2OH, and 13CH 3 13CH 2OH, were observed; there was no scrambling of the carbon atoms of ethanol. Contrary to what is expected for the formation of ethanol either by condensation or by homologation of methanol, the fraction of carbon-13 in the ethanol at zero contact time was a function of the partial pressure of 13C methanol in the feed. Also, at low contact times a large fraction of the product ethanol was doubly labeled which is incompatible with the formation of ethanol by carbonylation of formaldehyde, methoxide, or other C 1 species. The isotopic distribution of the ethanolic carbon was consistent with a mechanism that involves a C 1 species that is an intermediate in the formation of both ethanol and methanol.