A 2-STEP PREPARATION OF SILICA-SUPPORTED CALCIUM-MOLYBDENUM CATALYSTS

In order to investigate the role of Ca content on the nature of the ac tive phase of silica supported molybdenum based catalysts, we have pre pared by a two-step procedure, a series of catalysts containing the sa me amount of molybdenum (1, 2, and 10 Mo wt%) but with variable Ca/Mo atomic ratios, ranging from 0 to 4. We have shown by XRD, Raman spectr oscopy, and methanol oxidation reaction, that molybdenum species react with previously deposited Ca, leading to the formation, for CalMo ato mic ratios lower than one, of two kinds of calcium molybdate species o n the silica support: (0) crystallized CaMoO4 particles originating fr om calcium phase large clusters; (b) highly dispersed CaMoOx species f ormed from highly dispersed Can ions. These latter species have been f ound catalytically very active in methanol oxidation for the selective formation of methyl formate, whereas supported CaMoO4 particles behav e as bulk CaMoO4 (low catalytic activity and carbon oxide formation), as observed for CalMo atomic ratios higher than one, where only crysta llized CaMoO4 particles are formed. (C) 1998 Academic Press.