Modulation of Al Sites in MWW Zeolites with Enhanced Catalytic Performance by Dual Organic Structure-Directing Agents.

Abstract

MWW zeolites exhibit a distinctive combination of both 10-ring and 12-ring features, making them highly versatile in catalytic applications. When dealing with bulk molecules, the acid sites located on the external surface often serve as the primary active sites, and thus, the distribution of Al sites significantly impacts the active acidity and catalytic performance. In this study, a precise modulation of Al distribution within MWW zeolites was investigated using commercially available N,N,N-trimethyl-1-adamantylammonium hydroxide (TMAdaOH) and cyclohexylamine as organic structure-directing agents (OSDAs). The effects of both OSDAs, along with the synthesis temperature, duration and Na⁺, on the formation of the MWW framework were systematically examined. Advanced solid-state NMR characterization addressed the correlation between Al and organic species. The presence of TMAdaOH showed significant influence on the distributions of cyclohexylamine and T₂ Al sites as well as the benzene transport rate, resulting in MWW zeolites with enriched external surface accessible active acid sites. Owing to these properties, the MWW zeolites exhibited superior catalytic performance compared to conventional MCM-22 zeolites in the cracking of TiPB and alkylation of benzene with cyclohexene. This study highlights the successful synthesis of MWW zeolites using small amounts of low-toxicity OSDAs, offering a scalable and economical approach for the production of zeolites with enhanced catalytic properties for industrial applications.