Pore-engineering-dependence and hydrophobicity-dependence on stannosilicate zeolite catalyst for efficient Baeyer-Villiger oxidation

As robust heterogeneous catalysts, stannosilicate zeolites (in particular Sn-Beta) exhibit excellent catalytic performance in aqueous Baeyer-Villiger oxidation due to their unique activation ability of carbonyl group. For a specific Sn-containing zeolite, pore-engineering and hydrophobicity are generally two coupled factors to impact catalytic behavior. How to distinguish the function of each factor in this selective oxidation is still a pending question. Bearing this in mind, five stannosilicate zeolites (Sn-MFI, Sn-MWW, Sn-Beta, Sn-MCM-41, and Sn-Y) were prepared to establish the relationship between the structure (Lewis acidity, pore-engineering, and hydrophobicity) and catalytic performance. Particularly, hierarchical Sn-Y zeolite was post-synthesized by a simple solid-state ion-exchange method, where Sn4+ ions were incorporated into zeolite framework via reacting with silanol nests derived from dealumination process. In comparison with other stannosilicate zeolites, the obtained Sn-Y possessed excellent catalytic activity in Baeyer-Villiger oxidation irrespective of using aqueous hydrogen peroxide or bulky tert-butyl hydroperoxide as the oxidant. Structure-performance relationship revealed that Lewis acid site was the catalytically active center and the catalytic activity was depended on zeolite pore-engineering and hydrophobicity in aqueous B-V oxidation system. The relatively high catalytic activity of Sn-Y zeolite was ascribed to its excellent hydrophobicity and opened channel system. The former can be further enhanced by silanization treatment. Finally, the recyclability and deactivation mechanism were given insight. This work is aimed to provide some guidance for elaborately designing efficient stannosilicate zeolite catalyst for Baeyer-Villiger oxidation by precisely modulating pore-engineering and hydrophobicity.