Studies on catalytically active surface compounds: IV. Preparation of MoSiO2 catalysts from MoCl5 studied by ESR and UV-vis spectroscopy

The preparation steps of silica-supported molybdenum catalysts have been followed by ESR and uv-vis spectroscopy. It is shown that the reaction of MoCl5 with silanol groups of the support leads to dimeric (the main part) and monomeric surface Mo species. On adsorbing H2O a surface “solution layer” is formed allowing a fast tumbling of Mo5+ ions. Simultaneously, the concentration of isolated Mo5+ ions is increased; however, it does not exceed about 10% of the total Mo concentration. The final hydrolyzing process which includes drying at 378 K in air produces partly oxidized catalysts containing polymolybdate complexes. Although the interaction between the molybdenum species and the silica surface has been described as being relatively weak, no indication has been found that adsorption of water vapor causes any structural changes within the polymeric Mo complexes or its bonding to the surface. Adsorption of wet HCl causes ligand exchange which is accompanied by destruction of the polymeric molybdenum complexes.