Silica-Grafted Sn^IV Catalysts in Hydrogen-Transfer Reactions

Sn^IV-substituted zeolites show remarkable activity for important Lewis acid catalyzed reactions. In such complex catalytic systems it is, however, difficult to untangle the influence of all parameters contributing to the overall performance, and hence to rationally improve the catalyst. In this work, we studied silica-grafted Sn^IV sites to reduce this complexity by eliminating the potential influence of pore confinement. The surface-anchored Sn^IV sites were modified by calcination and silylation to increase Lewis acidity and hydrophobicity. Various techniques were used to characterize the materials, including the adsorption of cyclohexanone by using FTIR spectroscopy. We relate our spectroscopic results with catalytic tests and compare our model catalysts with benchmark Snβ zeolite. This suggested that besides active-site structure, also hydrophobicity and confinement effects influence the activity. Our work demonstrates the utility of model systems to separate the different contributions to catalytic activity. Not every Sn^IV is alike: The catalytic activity of silica-supported site-isolated Sn^IV sites in the Meerwein-Ponndorf-Verley reduction of cyclohexanone with 2-butanol is found to strongly depend on site structure and hydrophobicity. Differences in activity compared to benchmark Snβ zeolite further indicate the influence of confinement effects in zeolite pores.