Direct transformation of syngas to lower olefins synthesis over hybrid Zn–Al2O3/SAPO-34 catalysts

Abstract

Syngas is a key platform chemical for the utilization of non-petroleum carbon resources. Therefore, great attention has been paid to syngas conversion to value added chemicals, such as lower olefins (C₂–C⁼₄), liquefied petroleum gas (LPG), gasoline, and aromatics. Among these value added chemicals, the lower olefins are promising chemicals, and the direct synthesis of lower olefins from syngas was investigated over a series of hybrid catalysts composed of Zn–M catalysts (M = Al₂O₃, ZrO₂, Cr₂O₃, and CeO₂) and SAPO-34 zeolite. The Zn–M catalysts and different Si/Al ratios of SAPO-34 zeolites were synthesized by co-precipitation and hydrothermal methods, respectively. The aim of the present study was to investigate the influence of various supports on the catalytic performance of this reaction. The catalysts were well characterized by X-ray diffraction (XRD), SEM, ICP-AES, BET surface area, NH₃-TPD, CO₂-TPD, FT-IR, TEM-EDS, and H₂-TPR techniques. NH₃-TPD results showed that with the increase in the Si/Al ratio in the SAPO-34 zeolite, the catalysts exhibited higher acidity due to the higher amount of silica content. The H₂-TPR and XRD results suggested that the Zn/Al₂O₃ catalyst had strong metal–support interactions that were favorable for the higher activity. Various factors that affect the catalytic activity, including the reaction pressure, catalysts weight ratio, SiO₂/Al₂O₃ ratio, and the integration manner of both active components, were investigated. These results clearly demonstrated that the Si/Al ratio in the SAPO-34 catalyst had a predominant effect on lower olefins selectivity. The Zn/Al₂O₃ with SAPO-34 (0.043) catalyst showed excellent lower olefins selectivity (80%). In terms of the integration manner of the hybrid catalysts, the highest activity was obtained when the powders of both Zn/Al₂O₃ and SAPO-34 catalysts were ground in a mortar for 10 min and pelletized; while the sequential bed and granule mixing of Zn/Al₂O₃ and SAPO-34 catalyst showed less lower olefins selectivity, indicating that the appropriate proximity of the active components favors the lower olefins selectivity.