Effect of alkali treatment on ZnZrO_x/SAPO-34 bifunctional catalyst for catalytic synthesis of light olefins from syngas

A bifunctional catalyst was prepared by physical mixing of ZnZrO_x metal oxide and SAPO-34 zeolite for the one-step conversion of synthesis gas to light olefins (STO) reaction. The effects of triethylamine, tetramethylammonium hydroxide and tetraethylammonium hydroxide solutions and different concentrations of triethylamine solution on the texture, structure and acidity of SAPO-34 zeolite were investigated. XRD, SEM, N₂ adsorption and desorption, NH₃-TPD were used to characterize the SAPO-34 zeolite and the STO reaction performance of the catalyst after alkali treatment was investigated. The results show that all three kinds of organic base with 0.06 mol/L post-treatment can etch some hierarchical channels on the surface of SAPO-34 zeolite, thus accelerating the diffusion of intermediate transition species formed on the surface of metal oxides into the channels of SAPO-34 zeolite in STO reaction, improving the CO conversion rate in STO reaction. At the same time, all three kinds of alkali treatments can reduce the acid amount and acid strength of SAPO-34 zeolite, thereby improving the selectivity for light olefins in the STO reaction. The treatment of SAPO-34 zeolite with 0.02−0.10 mol/L triethylamine resulted in the formation of hierarchical pores etched on the surface of SAPO-34 zeolite, which improved the conversion rate of CO in the STO reaction. Moreover, the acid strength and acidity of SAPO-34 zeolite treated with 0.02 and 0.06 mol/L triethylamine solutions decreased, inhibiting the formation of methane and the hydrogenation of light olefins. Therefore, as the concentration of alkali treatment gradually increased from 0, 0.02 to 0.06 mol/L, the selectivity for light olefins gradually increases. Under the reaction conditions of 400 ℃, 3.0 MPa and GHSV=3600 mL/(g·h), the CO conversion rate increased from 24.0% to 26.4%, and the selectivity of light olefins increased from 78.2% to 84.7% on the bifunctional catalyst composed of 0.06 mol/L triethylamine-treated SAPO-34 compared to untreated SAPO-34 zeolite, and the modified bifunctional catalyst had good catalytic stability.