BASIC METAL-OXIDES AS COCATALYSTS IN THE CONVERSION OF SYNTHESIS GAS TO METHANOL ON SUPPORTED PALLADIUM CATALYSTS

The catalytic behavior of Pd catalysts supported on ultrapure silica a nd promoted with basic metal oxides was investigated in the hydrogenat ion of CO and CO2 to methanol at high pressure. In contrast to previou s claims, Pd on very pure silica produced hardly any methanol from CO and H-2 or CO2 and H-2, while doping with basic additives led to a hig h activity. Molecular adsorption of CO or CO2 and the availability of activated hydrogen are not sufficient to form methanol; basic metal ox ides are needed to give Pd a high methanol activity. The promoting eff ect on the silica-supported Pd catalysts and the absence of a promotin g effect for alumina-supported Pd catalysts suggest that the basic oxi de additives must be close to or in contact with the Pd particles to b e effective in methanol synthesis. The rate enhancements for methanol formation of the metal oxides followed a volcano curve when plotted as a function of the metal ion electronegativity, with a maximum for met al oxides with a moderate basic nature. Calcium and lanthanum were bes t for CO hydrogenation, and barium and lanthanum were best for CO2 hyd rogenation. In contrast to Cu/ZnO, the methanol activity of Ca/Pd/SiO2 increased with increasing CO content in a CO-CO2-H-2 mixture, suggest ing that CO rather than CO2 is the main carbon source for methanol. (C ) 1998 Academic Press.