Tuning hydrocarbon pool intermediates by the acidity of SAPO-34 catalysts for improving methanol-to-olefins reaction

Methanol-to-olefins (MTO) has received great attention, in which abundant renewable resources of biomass and biogas can be utilized as promising alternatives to crude oil in the production of light olefins. SAPO-34 is one of the most promising catalysts in MTO reaction, providing excellent selectivity toward ethylene and propylene. In this work, highly crystalline SAPO-34 catalysts with different SiO₂/Al₂O₃ ratios have been applied in MTO reaction to elucidate the effects of particle size and acidity on the "hydrocarbon pool" intermediate distribution, which remarkably influence their catalytic performance in the reaction. A smaller particle size of SAPO-34 catalyst (e.g., 120-360 mesh) can improve the catalyst lifetime, but nearly no effect on the product distribution. A suitable density of Brønsted acid sites (BAS) was found to effectively prolong the catalyst lifetime and enhance the total selectivity toward light olefins (ethylene and propylene). GC-MS and NMR analysis demonstrates that the suitable BAS density on SAPO-34 can suppress hydrogen transfer reaction for the formation of paraffins and the formation of polycyclic aromatics. The suitable BAS density was found to promote the formation of active "hydrocarbon pool" species, such as polymethylbenzenes, at a very low aromatic/CH₃ ratio compared to those having a much higher BAS density (>0.2), which can significantly boost the selectivities to olefins as desired products and improve the catalyst lifetime at 100% conversion of methanol. Therefore, this work provides a potential way for the development of suitable catalysts for MTO reaction with high olefin selectivity and improved lifetime.