PD SIO2-COGELLED AEROGEL CATALYSTS AND IMPREGNATED AEROGEL AND XEROGEL CATALYSTS - SYNTHESIS AND CHARACTERIZATION/

Pd/SiO2 aerogel catalysts have been synthesized by cogelation of tetra ethoxysilane with complex Pd2+[NH2-CH2-CH2-NH-(CH2)(3)-Si(OCH3)(3)](2) containing a hydrolysable silyl group. Pd catalysts have also been pr epared by impregnation of silica aerogels and xerogels with solutions of this complex or Pd2+(NH3)(4) followed by drying under hypercritical conditions or under vacuum. It appears that, in cogelled samples, the Pd2+ complex acts as a nucleation agent in the formation of silica pa rticles. The resulting catalysts are then composed of completely acces sible palladium crystallites with a diameter of about 2.3 nm located i nside silica particles with a monodisperse microporous distribution ce ntered on a hydraulic radius of 0.43 nm. The continuous meso- and macr opore distribution is located in voids between particles and between a ggregates constituted of those particles. In impregnated samples, it a ppears that Pd particles are located outside SiO2 particles. The relat ion between the texture of cogelled and impregnated catalysts and thei r behavior toward sintering has been studied in detail. In cogelled sa mples, it has been shown that Pd crystallites cannot migrate because t hey are trapped in the pores of SiO2 particles. These catalysts are si nterproof during hypercritical drying, When Pd crystallites are not tr apped inside SiO2 particles, hypercritical drying leads to an extensiv e sintering of metal particles. During calcination of impregnated cata lysts dried under vacuum, sintering occurs to a various extent dependi ng on the support texture. The Pd crystallites size decreases when the pore size distribution is shifted toward the small sizes. (C) 1997 Ac ademic Press.