Hybrid Acid Catalysts Prepared via Grafting Trimethylsilyl Groups onto Aluminosilicates Synthesized by Non-Hydrolytic Sol-Gel

Hybrid materials based on metallosilicates are extensively studied for their enhanced catalytic performance. Introducing organic groups into metal silicate catalysts and thus supposedly change the surface hydrophilicity and hydrophobicity has been shown to have an appreciable effect on the catalyst performance in various reactions, including epoxide ring opening reactions and alcohol dehydration. However, the sole organic groups introduction does not unambiguously guarantee hydrophilicity control and might lead to porosity drop, acidity modulation, structural changes, etc. Therefore, a thorough and complex characterization is necessary to provide a complete view of the interaction between the catalyst surface, reactants, and products. Herein, we have prepared aluminosilicate catalyst with well-dispersed Al atoms in silica-based matrices via the non-hydrolytic sol-gel (NHSG) method. This material was post-synthetically modified with trimethylsilyl groups; their number on the catalyst surface was controlled via a temperature vacuum pretreatment. In such a way, we have obtained aluminosilicate materials with similar porosity, structure, and acid site strength and quality. Notably, the water sorption measurements showed that trimethylsilylated aluminosilicates adsorb 2.5−3 times less water (p/p0 = 0.3) than the parent aluminosilicate catalyst. The turn over frequency in epoxide ring opening and ethanol dehydration scaled up with the number of trimethylsilyl groups grafted on the catalyst surface. Particularly, the heavily trimethylsilylated sample achieved three to five times higher turnover frequency in styrene oxide aminolysis with aniline than the parent aluminosilicate material. To the best of our knowledge, it exhibited the most active Al sites for epoxide aminolysis in the present literature.