Enhanced catalytic performance of NH4+-exchanged Ti-MWW zeotypes for the liquid-phase epoxidation of alkenes with H2O2

Abstract

Ti-MWW zeotypes, characterized by their unique dual pore architecture consisting of intralayer 10-membered ring (MR) pore channels and additional interlayer 10-MR pore channels that are interconnected to intracrystalline 12-MR supercages, exhibit a remarkably catalytic superiority in the selective oxidation of small and/or linear alkenes with hydrogen peroxide (H₂O₂), surpassing other titanosilicate zeotypes in performance. However, a further improvement on their catalytic performance is still needed for the industrialization of alkene epoxidation over Ti-MWW. In the present work, the introduction of NH₄⁺ species into Ti-MWW was conducted via a facile but effective treatment of the as-synthesized zeotypes with NH₄Cl, named N-Ti-MWW. The comprehensive characterization of N-Ti-MWW, derived from NH₄⁺-exchanged Ti-MWW, coupled with Density Functional Theory (DFT) calculations, conclusively demonstrates that the integration of NH₄⁺ into Ti-MWW promotes the generation of -Ti-OOH hydroperoxo intermediates, resulting in a higher catalytic activity in alkene epoxidation in comparison to the parent one. Moreover, the solvolysis of epoxides is significantly suppressed on N-Ti-MWW, enabling the catalyst to display a superior selectivity towards the targeted epoxide, thereby improving its overall catalytic performance. Additionally, N-Ti-MWW is reusable, after the regeneration of the deactivated catalyst simply treated with NH₄Cl. As a result, it is anticipated that N-Ti-MWW would be applicable in the industrial epoxidation of alkenes with H₂O₂.