Nanoporous Zeolite UTL Decorated with Sn to Catalyze the Baeyer-Villiger Reaction

Stannosilicates with hierarchical pores are greatly desirable for the liquid-phase conversion of ketones into esters or lactones, in particular, requiring open porous spaces. The nanoporous zeolite UTL with a framework Sn(IV) species was constructed successfully using a fluorine-assisted hydrothermal synthesis strategy. The obtained [Sn, Ge]-UTL in calcined form experienced severe structural collapse during an ultrasonic process under a pure water medium due to the easily hydrolytic Ge-O linkages in double four-ring units. The ultrasonic treatment also caused the irregular arrangement of nanosized Si-rich cfi (mono- and/or multi-) layers, inducing the stacking faults and generating abundant intracrystal mesopores after calcination. In addition, the neutral environment had little adverse influence on the content of the tetrahedrally coordinated Sn ions, irrespective of the extensive removal of Ge species. The resultant Sn-Meso-UTL catalyst favored the diffusion of bulky substrates and oxidants, improving the accessibility of Sn active sites and performing efficient catalytic activity in the Baeyer-Villiger oxidation of ketones with an inorganic hydrogen peroxide (H2O2) or organic tert-butyl hydroperoxide oxidant. Furthermore, high selectivity (95.4%) of the target epsilon-caprolactone product was obtained through optimizing the H2O2/cyclohexanone molar ratio (0.2) and reaction time (40 min), which was excellent in comparison with the representative Sn-beta-F catalyst.