Melting-salt crystallization assisted controllable synthesis of core-shell zeolite catalyst for effective coupling of microwave hotspot and catalytic active site

Abstract

The applications of zeolites to microwave(MW)-assisted heterogeneous reactions are limited by their MW-transparent properties, which call for essential development of core–shell catalysts with MW-absorbing cores to utilize MW hotspots efficiently. However, current preparation methods fail to address the dilemmas of high heat transfer resistance and low heat generation efficiency caused by excessively thick zeolite shells and insufficient core filling. This study proposes an innovative ship-in-bottle strategy based on melting-salt crystallization for achieving controlled preparation of core–shell zeolite catalysts with ultra-thin shells and large filling rate of MW-absorbing cores. Based on the exploration of optimal conditions, ultra-thin hollow mesoporous zeolite shells around 30 nm thickness are successfully prepared, effectively reducing the heat transfer resistance from cores to catalytic sites. Subsequently, precise adjustment of the shell mesopore size enabled quantitative filling of MW-absorbing cores and therefore successful preparation of β-MnO2@zeolite catalysts with significantly enhanced heat generation capabilities. Compared to conventional surface-loaded catalysts, the new core–shell catalysts improve toluene conversion by 44 % and paraxylene selectivity by 66 % in MW-assisted alkylation reactions. Fine numerical simulations of the catalyst heating process demonstrate that the excellent catalytic performance of core–shell catalysts is due to the effective coupling of MW hotspots and catalytic sites.