Molybdenum Catalysts for Synthesis of Mixed Alcohols from Synthesis Gas

Highly active Mo catalysts for synthesis of mixed alcohols from synthesis gas were prepared using SÍO2 as a carrier. KC1 promoted the selectivity to alcohols but reduced CO conversions, in particular, to hydrocarbons. High specific activity of 420 g (kg-catalyst)⁻¹-h⁻¹ was attained at 5.0 MPa, 573 K, and W/F=1.4 g-catalysfh•mol⁻¹. The alcohol formation was found to require two kinds of Mo species, metallic Mo and M0O2, after reduction with flowing H2. The presence of K prevented the complete reduction of Mo to metal, resulting in increased production of alcohols. Marked increase in alcohol yield with time on stream indicated that active species for alcohol synthesis were formed during the CO hydrogenation. From the study of the effect of pretreatment of Mo catalysts with CO, CO-H2, and butane-H2, and Mo 3d XPS, it has been concluded that the formation of CO-reduction induced defects on M0O2, M0O2-X, during the CO hydrogenation reaction, is the cause of the increase in the alcohol synthesis rate. On the other hand, the hydrocarbon synthesis appeared to be solely based on metallic Mo. From the study of addition of probe molecules to CO-H2, it has been clarified that the higher alcohol formation from CO-H2 proceeded by way of the same intermediate as the alkene hydrocarbonylation. A dual-site mechanism for the alcohol formation over Si02-supported Mo has been proposed: CO dissociates on metallic Mo to form surface carbide, followed by hydrogenation of carbene and/or methyl species. While addition of methylene unit to surface alkyl species and following hydrogenolysis and/or dehydrogenation of alkyls to give hydrocarbons also occur on metallic Mo, CO insertion leading to alcohols is catalyzed by M0O2-X species. It has been found that K is effective in retarding hydrogenolysis of alkyls, improving the selectivity to alcohols. The mechanism can account for the difference in selectivities to branched products between hydrocarbons and alcohols.