Reduction of MoO₃ to Porous Molybdenum Oxides and Its Catalytic Properties for Alkane Isomerization

Abstract

The catalytic activity of partially reduced Pt/MoO₃ for alkane isomerization was investigated. The surface area of Pt/MoO₃ was markedly enlarged by H₂ reduction to a maximum after reduction at 673 K for 12 h. Enlargement of the surface area was caused by formation of pores with diameters of 0.6-3 nm. The catalytic activity of partially reduced Pt/MoO₃ for heptane isomerization increased with reduction temperature, and reached a maximum at 723 K. The catalytic activity for 2-propanol dehydration was very similar to that of heptane isomerization. The isomerization and dehydration activities of partially reduced Pt/Na-MoO₃ rapidly decreased with increasing content of Na. In contrast, the hydrogenation of cyclohexene was promoted on catalysts containing Na. The isomerization and dehydration activities were related to the number of acid sites, determined by NH₃-TPD. Therefore, the isomerization activity of partially reduced Pt/MoO₃ depends on the activity as an acid catalyst. H₂ reduction at 673 K enlarged the surface areas of H_1.55MoO₃ and Pt/MoO₃, but not the surface area of MoO₃. H_1.55MoO₃ and Pt/MoO₃ reduced at 673 K had comparable activity for pentane isomerization, and were much more active than MoO₃ reduced at 673 K, even after considering the differences in surface areas. Molybdenum oxyhydride, MoO_xH_y, was formed after the decomposition of hydrogen molybdenum bronze in the reduction of H_1.55MoO₃ and Pt/MoO₃. On the other hand, MoO₃ was reduced to MoO₂ without the formation of hydrogen bronze. These results show that surface area and isomerization activity were improved by the formation of MoO_xH_y from H_xMoO₃.