Dual function of additives in controlling Al siting and nanocrystal assembly of MEL zeolites for enhanced methanol-to-olefins performance

Abstract

The catalytic performance of zeolites is governed by crystal morphology, acidity, and the spatial distribution of framework Al atoms. However, achieving precise control over these physicochemical properties during zeolite synthesis remains challenging. In this study, we investigated the synthesis of MEL zeolites, focusing on the effect of water content, OSDA (Organic Structure-Directing Agent) concentration, and the incorporation of urea and tetramethylguanidine (TMG) additives. Optimizing these parameters accelerates crystallization kinetics and reduces crystal size by decreasing water content while increasing OSDA concentration. The dual role of urea and TMG additives was revealed to promote the formation of flower-like MEL aggregates composed of nanosized crystals and increases the proportion of framework Al atoms located in the straight channels by 42% with respect to the intersections compared to the parent MEL zeolite free of additives. These structural modifications improve active site accessibility and facilitate mass transport within the zeolite crystals, and improve catalytic performance in the methanol-to-olefins (MTO) reaction. As a result, the MEL zeolite with TMG and urea additives exhibits a 2–4-fold extension in catalyst lifetime, with a 35% increase in the propylene-to-ethylene ratio compared to its counterpart synthesized without additives.