Abstract
The study of the influence of synthesis operating variables (nature and concentration of complexing silane, palladium percentage, temperatures of gelling, aging and vacuum drying of xerogels, molar ratio between the complexing silane and palladium, molar concentration of ammonia solution, and use of tetramethylammonium hydroxide as base instead of NH3) allows improving metal dispersion in Pd/SiO2 cogelled xerogel catalysts. The use of 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) or 3-(2-aminoethylamino)propyltriethoxysilane (EDAES) to complex palladium in an ethanolic solution containing tetraethoxysilane (TEOS) and an ammonia solution of 0.54mol/l allows obtaining a Pd/SiO2 xerogel catalyst with a mean metal particle diameter of 2.4nm located inside silica particles. Indeed, complexes Pd(EDA(E)S)xn+ induce a nucleation mechanism because of their higher reactivity compared to the network-reagent (TEOS). Although metal particles are located inside the silica particles, their complete accessibility, via the micropore network, has been shown. 1,2-dichloroethane hydrodechlorination over Pd/SiO2 catalysts mainly produces ethane and the specific hydrodechlorination rate per gram of palladium increases proportionally with palladium dispersion. Hydrodechlorination over Pd/SiO2 cogelled xerogel catalysts is a structure insensitive reaction with regard to the ensemble size concept.
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