Cyanide- and carboxylate-functionalized cubic mesoporous silicas SBA-1: Synthesis, characterization and reactivity of organic functional groups

Highly ordered cubic (Pm3¯n) mesoporous silicas SBA-1 functionalized with a high density of cyanide and carboxylate groups were synthesized by one-pot co-condensation of tetramethoxysilane (TMOS) and 2-cyanoethyltriethoxysilane (CNTES) using cetyltriethylammonium bromide (CTEABr) as the structure-directing agent under strongly acidic conditions. A high degree of structural ordering is still retained up to 25 mol% CNTES loading (based on silicon) in the synthesis mixture. The simultaneous existence of chemically attached cyanide and carboxylate ester moieties was evidenced by solid-state 13C CPMAS (cross polarization magic angle spinning) NMR. The relative concentrations between cyanide and carboxylate ester functional groups in the resulting mesoporous silicas are strongly dependent on the synthesis parameters such as acid type, acid concentration and reaction time. By controlling these synthesis parameters, the contents of the cyanide functional groups incorporated can be tuned, and their transformation into carboxylate ester groups can be followed. With increasing HCl concentration and reaction time, the cyanide functional groups incorporated convert progressively into carboxylate ester groups after solvent extraction with an HCl/ethanol mixture. While using H2SO4 as an acid medium, on the other hand, the cyanide functional groups can be totally transformed into carboxylate ester groups. The generation of carboxylic acid functionalities can be achieved either by solvent extraction with only ethanol (without HCl) or post-dealkylation treatment of carboxylate ester groups in concentrated HCl. The acid type and concentration and the silicon source also have a profound influence on the particle morphology of mesoporous silicas.