Basic metal oxides as co-catalysts in the conversion of synthesis gas to methanol on supported palladium catalysts

The catalytic behavior of Pd catalysts supported on ultrapure silica and promoted with basic metal oxides was investigated in the hydrogenation of CO and CO2 to methanol at high pressure. In contrast to previous 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 high activity. Molecular adsorption of CO or CO2 and the availability of activated hydrogen are not sufficient to form methanol; basic metal oxides are needed to give Pd a high methanol activity. The promoting effect on the silica-supported Pd catalysts and the absence of a promoting effect for alumina-supported Pd catalysts suggest that the basic oxide additives must be close to or in contact with the Pd particles to be 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 metal oxides with a moderate basic nature. Calcium and lanthanum were best for CO hydrogenation, and barium and lanthanum were best for CO2 hydrogenation. In contrast to Cu/ZnO, the methanol activity of Ca/Pd/SiO2 increased with increasing CO content in a CO-CO2-H-2 mixture, suggesting that CO rather than CO2 is the main carbon source for methanol. (C) 1998 Academic Press.