The characterization and catalytic activity of molybdenum-Y zeolites

Abstract

The characterization of several molybdenum zeolites was facilitated using X-ray photoelectron spectroscopy. Samples prepared from the reaction of Mo(CO)(,6) with ultrastable zeolites exhibited superficially high concentrations of molybdenum which were attributed to a higher density of A10(,4) tetrahedra (exchange sites) near the surface. Because of this gradient in the concentration of aluminum, the number and relative acid strengths of protons in the first few atomic layers may be significantly different from those in the bulk phase. In contrast, a homogeneous distribution of molybdenum was apparent in samples prepared from Mo(CO)(,6) and true hydrogen-Y zeolites. The ion exchange of a dimeric form of molybdenum in aqueous solution was accomplished using Mo(,2)(en)(,4)Cl(,4) and a NaY zeolite. However, these cationic molybdenum species were unstable at higher activation temperatures, as molybdenum migrated to the surface. Molybdenum zeolites prepared from the reaction of Mo(CO)(,6) or MoOCl(,3) with ultrastable zeolites exhibited high initial activities and good selectivities for the epoxidation of propylene when tert-butyl hydroperoxide was used as the oxidant. The catalytic activity was due to heterogeneous forms of molybdenum for short reaction times, since only small amounts of molybdenum were leached from the zeolite by the liquid phase. Moreover, solutions containing soluble molybdenum from the catalysts exhibited low activity for epoxidation. Gradual poisoning of the catalyst was a result of the formation of polymeric species, which were produced via the reaction of propylene oxide with protons of the zeolite. First-order dependencies on the concentrations of propylene, hydroperoxide, and catalyst were observed. The activation energy was similar to that reported using homogeneous catalysts which suggested that the mechanism of expoxidation over molybdenum zeolites may involve a molybdenum-hydroperoxide complex.