CATALYTIC PROPERTIES OF LANTHANIDE METALS ON SILICA

Silica (SiO2) powders reacted readily with Eu and Yb which were dissolved in liquid ammonia, resulting in the formation of active lanthanide/SiO2 catalysts (Eu/SiO2 and Yb/SiO2). The reactions of dissolved lanthanide metals in liquid ammonia with SiO2 were characterized by FT-IR studies and various catalytic reactions. The catalytic behavior varied markedly with varied levels of lanthanide loading. At low loadings (< ca. 15 wt%), the dissolved lanthanide selectively reacted with the hydroxyl groups on SiO2 to form = SiO-MNH2 species (M = Eu or Yb). The thermal treatment above 573 K led to the decomposition of this species to the inactive nitride MN. The catalyst with low loadings exhibited very high activity for the disproportionation of cyclohexadiene at room temperature. At high loadings (> ca. 15 wt%), after the first portion of added lanthanide was consumed for the reaction with surface hydroxyl groups, subsequent lanthanide seems to deposit on SiO2 in an active metallic form. The catalytic properties were very similar to those reported for lanthanide metal particles prepared by the metal vapor technique. The catalyst with high loadings discriminated between internal and terminal C-C triple bonds for the hydrogenation of alkyne; thus, 2-butyne was readily converted to cis-2-butene, whereas 1-butyne, propyne, and acetylene were not hydrogenated at all.