Enhancing the Stability of Methanol-to-Olefins Reaction Catalyzed by SAPO-34 Zeolite in the Presence of CO2 and Oxygen-Vacancy-Rich ZnCeZrOx

In the methanol-to-olefins (MTO) reaction, coke deposition is a key factor that reduces the catalyst stability. Consequently, this work proposes a strategy: conducting the MTO reaction catalyzed by ZnCeZrOx/SAPO-34 in the presence of CO2 to suppress coke deposition and thus improve the catalyst lifetime. Under a CO2 atmosphere, compared to pristine SAPO-34, the composite catalyst of SAPO-34 and ZnCeZrOx with abundant oxygen vacancies has a 3.4 times longer lifetime and a selectivity to light olefins (C2=-C4=) of 87.1%. The results of various characterization techniques and experiments show that the oxygen vacancies of ZnCeZrOx adsorb and activate CO2, providing active oxygen species for the MTO reaction. The active oxygen species selectively oxidize formaldehyde, thereby blocking the reaction of formaldehyde and olefins to polycyclic aromatic hydrocarbons through the Prins reaction. At the same time, the active oxygen species reduce the amounts of polymethylbenzene and polymethylnaphthalene (coke deposit precursors) in situ during the reaction, thereby significantly improving catalyst stability. The above process simultaneously suppresses the aromatic-based cycle, thereby effectively adjusting the ratio of the aromatic-based cycle to the olefin-based cycle and achieving high propylene and butene selectivity. This work provides an effective strategy for improving the catalyst lifetime and controlling product distribution.