Surface properties of supported molybdenum catalysts

Abstract

Heterogeneous catalysts consisting of highly dispersed supported molybdenum have been investigated. Hexacarbonylmolybdenum(0) catalysts supported on TiO 2, Al 2O 3, ZrO 2, MgO, and SiO 2 were characterized by temperature-programmed decomposition (TPDE), XPS, and catalytic activity for CO shift reaction, metathesis, and hydrogenation of alkenes. Mo catalyst supported on TiO 2 showed higher catalytic activity than that of the Mo catalysts on other supports. The TPDE spectrum of Mo(CO) 6 on TiO 2, which was pretreated at 773 K, showed two-stage decomposition peaks due to the elimination of CO ligands with very little evolution of H 2, which resulted in a lowvalent catalyst. After the first stage of TPDE, Mo(CO) 3 species were formed on TiO 2. Those species gave higher catalytic activity for CO shift reaction at 403 K, but not for hydrogenation and metathesis. Metathesis of propene at 323 K and hydrogenation of 1,3-butadiene at 273 K showed higher activity only after the second stage. Those catalytic activities are one or two orders of magnitude higher than those of corresponding oxide catalysts. When the catalyst was pretreated at 600–773 K, slight oxidation of Mo occurred due to the presence of hydroxyls of TiO 2; this suppresses the catalytic activities for CO shift reaction and hydrogenation. However, the activity of metathesis increased with increasing evacuation temperatures. The results suggest that metathesis requires a higher oxidation state of Mo than do CO shift reaction and hydrogenation.