Properties and reactivity of lanthanide (Eu and Yb) catalysts dispersed on SiO2

Abstract

Lanthanide metals (Ln : Eu and Yb) dissolved in liquid ammonia react with SiO₂ to form active Ln/SiO₂ catalysts. The behaviour of lanthanides present on SiO₂ varies markedly with the level of lanthanide loading. For low loadings, lanthanide catalysts immobilized on SiO₂, formed by the preferential reaction of lanthanides with surface hydroxyl groups on SiO₂ and which Fourier-transform infrared studies have shown have the structure Si—O—Ln—NH₂, exhibited selective hydrogenation properties, discriminating between conjugated and non-conjugated dienes. This was further confirmed by using polyvinyl alcohol (PVA) with uniform hydroxyl groups as a model compound. The lanthanide-bound PVA exhibited similar catalytic properties and the turnover frequencies of Ln/PVA were very close to those of Ln/SiO₂. For high loadings of lanthanide, Ln/SiO₂ additionally showed selective hydrogenation of alkynes; only internal CC triple bonds were reduced in marked contrast to terminal CC bonds. The active catalysts were those in which the lanthanide used, in excess compared with the surface hydroxyl groups on SiO₂, was highly dispersed on the support in an active metallic form, leading to the selective hydrogenation of alkynes.