Sn-containing zeolite catalysts exhibit the unique feature of selectively activating carbonyl groups in organic compounds, which demonstrate relatively high activity and selectivity in the Baeyer-Villiger (B V) oxidation of aldehydes or ketones using H 2 O 2 as the oxidant. However, the influence of diffusion property and trace water on the ε-caprolactone selectivity as targeted product in the cyclohexanone's B V oxidation system with the green oxidant of aqueous H 2 O 2 remains unclear. To address this, three kinds of Sn-Beta zeolites are prepared by interzeolite transformation, structural reconstruction, and post-synthesis method to establish structure-property relationships. The physicochemical property and Sn coordinated state are investigated in detail. The existence of trace water in the reaction system negatively affects the conversion and product selectivity by competitive adsorption of water with reaction substrates and facilitated hydrolysis of ε-caprolactone, respectively. DFT calculation implies that kinetics dominates the conversion of cyclohexanone → ε-caprolactone → 6-hydroxycaproic acid. The structure-property relationship indicates that the cyclohexanone conversion and ε-caprolactone selectivity are closely related to the adsorption amounts of substrates and the diffusion performance of products, respectively, over Sn-Beta zeolite catalysts. Finally, the reusability and deactivation mechanism are analyzed. This study aims to provide guidance for designing efficient stannosilicate zeolite catalysts for B V oxidation through precisely modulating the zeolite's diffusion property.
Sn-containing Beta (Sn-Beta) zeolite is extensively applied in industrially-relevant reactions as a highly active and robust Lewis acid heterogeneous catalyst. Nonetheless, environmentally-unfriendly fluoride is generally required to the directly hydrothermal synthesis of Sn-Beta. In this work, an innovative strategy of interzeolite transformation with steam-assisted conversion is provided to directly synthesize Sn-Beta zeolite in the absence of fluoride. Silica source of FAU and steam-assisted conversion were two necessary factors for the successful synthesis of Sn-Beta zeolite. It was found that highly crystallized Sn-Beta zeolite with isolated framework Sn ions was hydrothermally synthesized via degradation of siliceous FAU zeolite and recrystallization of degraded silicate species. As far as we know, the crystal of Sn-Beta in this work was minimum in size among all present directly hydrothermal synthesized ones. More importantly, the Sn contents in Sn-Beta could achieve to 2.39 wt % (Si/Sn = 81), breaking through the Sn content restriction of conventional fluoride-mediated method. In addition, the hydrophobicity of this resultant Sn-Beta zeolite was slightly inferior to Sn-Beta-F obtained via traditional fluoride-assisted route. Whereas the smaller crystal size of this Sn-Beta gave rise to relatively good diffusion performance for organics, which made it reveal high activities in the Baeyer-Villiger oxidant of ketones in particular employing bulky tert -butyl hydroperoxide as the oxidant. The current strategy provides an alternative and environmentally-friendly method for the hydrothermal synthesis of Sn-Beta without the assistance of fluoride and alkali metal ions, which can guide further development in metallosilicates crystal engineering in terms of greenization.
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