ACTIVE SPECIES OF MOLYBDENUM FOR ALCOHOL SYNTHESIS FROM CO-H2

Molybdenum supported on SiO2 catalyzed CO hydrogenation to produce higher alcohols as well as hydrocarbons. The alcohol formation was found to require two kinds of Mo species, metallic Mo and MoO2, after reduction with flowing H2. Marked increase in alcohol yield with time on stream suggested that active species for alcohol synthesis were formed during CO-H2 reaction. Because the carburization of Mo resulted in insignificant change in the number and nature of the sites for alcohol formation, Mo carbides were excluded from the active sites for alcohol synthesis. Treatment of Mo catalysts with flowing atmospheric CO or CO-H2 was remarkably effective for the formation of sites producing alcohols. The Mo 3d XPS spectra of the CO-H2 conditioned catalyst showed a shoulder on the low binding energy side of the Mo 3d5/2 peak. These findings suggest the formation of CO-reduction-induced defects on MoO2, MoO2-x, during the CO hydrogenation reaction, resulting in the increase in the alcohol synthesis rate. On the other hand, the hydrocarbon synthesis appeared to be solely based on metallic Mo. A dual-site mechanism for the alcohol formation over SiO2-supported Mo has been proposed; CO dissociates on metallic Mo to form surface carbide, followed by hydrogenation to carbene and/or methyl species. Addition of methylene unit to alkyl and the following hydrogenation and/or dehydrogenation of alkyl to give hydrocarbons are also catalyzed by metallic Mo, whereas CO insertion leading to alcohols occurs on MoO2-x species. The mechanism can account for the difference in selectivities to branched products between hydrocarbons and alcohols.