Palladium catalysts deposited on silica materials: Comparison of catalysts based on mesoporous and amorphous supports in Heck reaction

Palladium catalysts supported on different silica-based materials were investigated in the Heck reaction of n-butyl acrylate with bromobenzene at 160 degrees C. The main objective was to understand the role of mesopores in the catalysts supported on SBA-15 functionalised with (CH2)(3)NH(CH2)(2)NEt2 donor groups at the surface in comparison with the analogous silica gel-based catalysts. The results obtained evidence a crucial influence of the ratio of the anchoring groups to palladium (N/Pd): a higher excess of coordinating groups in the catalysts results in a higher catalytic activity, probably because such supports aid the deposition of the metal onto the support and, simultaneously, prevent aggregation of the 'active' metal species at high reaction temperatures. This is very important as the reactions with deposited palladium catalysts typically take place in solution with the metal first leaching out from the support and then returning back to the solid matrix. In contrast, the nature of the solid support (mesoporous vs. amorphous) seems to play a less important role. The differences in the activity of catalysts deposited on SBA-15 or silica gel can be accounted for a better availability of the active metal species. (C) 2010 Elsevier B.V. All rights reserved.