Study on preparation and catalytic performance of Zn-Al oxides for tandem reaction of syngas conversion into light olefins

A series of Zn-Al oxides with different Zn/Al atomic ratios were prepared by the microwave-assisted evaporation-induced self-assembly (M-EISA) method, using industrial pseudo-boehmite as aluminum source. The prepared Zn-Al oxides were physically mixed with SAPO-18 zeolite and applied in tandem reaction for direct conversion of syngas to light olefins (\rmC_2^= -\rmC_4^= ). X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ adsorption-desorption, CO and H₂ temperature-programmed desorption (CO-TPD, H₂-TPD) and X-ray photoelectron spectroscopy (XPS) were selected for characterization. As the increase of Zn/Al ratio, the specific surface area and pore volume of Zn-Al oxides gradually decreased, while the average pore diameter firstly increased and then decreased. Compared with the ZnAl-IP prepared by the impregnation (IP) method, the ZnAl₂O_x with the Zn/Al ratio of 1∶2 had a high dispersion of Zn and formed the ZnAl₂O₄ spinel structure that produced more oxygen vacancies. The catalytic results showed that the activity of Zn-Al samples prepared by M-EISA method firstly increased and then decreased as the Zn/Al ratio rose, while the \rmC_2^= -\rmC_4^= selectivity gradually decreased. ZnAl₂O_x sample exhibited the highest CO conversion of 34.8% and almost no obvious deactivation after 50 h reaction, furthermore, its catalytic performance was much better than that of ZnAl-IP sample.