Modulating the Microenvironment of Silanols in Pure-Silicon Zeolites for Boosting Vapor-phase Beckmann Rearrangement of Cyclohexanone Oxime

Vapor-phase Beckmann rearrangement of cyclohexanone oxime(CHO)to & epsilon;-caprolactam (CPL) is still difficult to commercialize atthe industrial scale due to its relatively low catalytic activityand poor lifetime. Herein, we synthesized a series of pure-siliconzeolites (including MFI, MEL, and -SVR) with three-dimensional10-member-ring topolgies, diverse silanol status, and hierarchicalporosity to investigate the synergistic effects of inner diffusivityand reactivity. S-1 zeolite of MFI-type topology with plentiful silanolnests exhibits a more preferable catalytic performance in terms ofCHO conversion (99.7%) and CPL selectivity (89.7%), much higher thanthose of MEL- and -SVR-type zeolites mainly due to their diversesilanol distribution. With the construction of hierarchical porosity,S-1-P shows improved CPL selectivity of 94.1% owing to the enhanceddiffusivity to shorten the retention time of the reactant and productmolecules. The reaction mechanism and network have been further revealedby density functional theory (DFT) calculations and experimental designs,which indicate that silanol nests are major active sites due to theirsuitable interaction with CHO rather than terminal silanols. Particularly,the microenvironments of silanols can be modulated by alcohol solvents,ascribed to their different charge transfer and steric hindrance.Consequently, S-1-P shows superior CPL selectivity of 97.3% in ethonalsolvents, which have higher adsorb energy of -0.627 eV withsilanol nests than other alcohols. The present study not only providesa fundamental guide for the design of zeolite catalysts but also providesa reference for modulating the microenvironment of active sites accordingto the catalytic mechanism.

Automated summary

In this study, pure-silicon zeolites were synthesized to investigate their catalytic performance in the vapor-phase Beckmann rearrangement of cyclohexanone oxime. Among the various zeolites tested, S-1 zeolite with plentiful silanol nests exhibited superior catalytic performance, attributed to its diverse silanol distribution.