Influence of surfactant chain length on basic catalytic properties of Si-MCM-41

Some silica/surfactant mesoporous materials have been synthesized in water:ethanol mixtures by combining tetraethoxysilane with a CnTMA+Br− solution. The effects of the water:ethanol ratio and water:NH3 ratio on the formation of the mesoporous solid and particle size and morphology of the materials are reported. These as-synthesized Si-MCM-41 materials were used for base catalyzed Knoevenagel condensation under mild temperature conditions (30 °C), being useful for general heterogeneous catalysis, particularly for fine chemicals synthesis. The high activity of this hybrid organic–inorganic solid is not due to leached species: the active sites of the Si-MCM-41 catalysts are regarded to be siloxy anions located in the pore-mouth. As-synthesized Si-MCM-41 containing the encapsulated quaternary ammonium ions is not only reusable in several recycling experiments, but it also provides superior activity compared to corresponding cesium impregnated Si-MCM-41, as tested by others authors in similar reaction conditions. Knoevenagel condensation catalyzed by Si-MCM-41 materials depends basically on the amount of surfactant cations which are present in the pore-mouth and on the number of pore-mouths which are presented in Si-MCM-41 external surface. In this study, we report the properties of [CnTMA]Si-MCM-41 materials (10 ⩽ n ⩽ 16) which, due to their specific structure characteristics and composition, should be particularly suitable for base catalyzed reactions. The material with higher catalytic activity is the one with higher ordering degree, prepared with longer carbon chain surfactants and with lower cosolvent concentration in the gel synthesis.