催化剂,多孔材料的制备过程.doc

Processing of Catalysts, Porous Materials and Aerogels Processing of High Performance Catalysts INTRODUCTION Because of the purity of metal alkoxides and well understanding of the gelation mechanisms, sol–gel processing using metal alkoxides has been widely used for the preparation of supported metal or metal oxide catalysts. Although most of the work concerning sol–gel-prepared catalysts have been focused on the catalysts supported on silica and/or alumina, but, recently, bimetallic and mixed oxide catalysts prepared through the sol–gel processing have also been extensively studied. Accordingly, catalysts reported in this chapter will be classified into four groups as follows; silica supported catalysts, alumina supported catalysts, bimetallic catalysts and mixed oxide catalysts. SILICA SUPPORTED METAL OR METAL OXIDE CATALYSTS BY SOL–GEL TECHNIQUES Tetraethoxysilane (TEOS) has been generally used as a precursor of SiO2 powder in sol–gel techniques, since gelation mechanisms of TEOS were well revealed by Dislich (1971), Teichner et al. (1976), Mazdiyasni et al. (1969) and Sakka (1982). The gelation mechanisms of TEOS mixed with ethanol, water and a small amount of acid, as a catalyst for hydrolysis of TEOS, consist of two steps; the first step is the hydrolysis of TEOS and the second step is a condensation process, as follows: (22-1) (22-2) The structural frame of –Si–O–Si– generated in reaction (22-2) expands three-dimensionally to be transparent wet gels. The wet gels thus obtained are dried and calcined at temperatures higher than 500°C to be sol–gel SiO2 powder. BET surface area, pore volume and pore size of SiO2 powder are important factors to evaluate catalyst carriers, and can be modified to some extent by an addition of either acids or bases in the TEOS solution during gelation (Sakka, 1988). One can use the sol–gel SiO2 powder for a catalyst preparation by conventional impregnation method using an aqueous solution of metal salts such as metal nitrates and