Synthesis of lower olefins from syngas over Zn/Al2O3–SAPO-34 hybrid catalysts: role of doped Zr and influence of the Zn/Al2O3 ratio
Hybrid catalysts composed of different loadings of Zr-promoted Zn/Al2O3 with SAPO-34 zeolite were investigated for the direct synthesis of lower olefins from syngas in a fixed-bed reactor. Zn/Al2O3 catalysts and Zr-promoted Zn/Al2O3 catalysts with different weight ratios were prepared by a co-precipitation method, respectively. Nano-sized SAPO-34 zeolite was synthesized by a hydrothermal method. The crystallinity, morphology, textural properties and acidic properties of the catalysts were characterized well by XRD, ICP-OES, TEM, H2-TPR, N2 adsorption–desorption isotherm analysis, XPS, and NH3-TPD, respectively. The H2-TPR results suggested that the presence of Zr on the surface of the support favors the reducibility of zinc oxide with a shift to a lower reduction temperature. The Zr promoter improved the Zn dispersion and enhanced the surface area of Zn/Al2O3 catalysts. Various factors that influenced the catalytic activity, including the reaction temperature, the different weight ratios of the methanol synthesis catalyst to SAPO-34 zeolite, the gas flow rate and stability, were investigated in detail. The hybrid catalysts composed of Zr·Zn/Al2O3 and SAPO-34 showed excellent catalytic performance in terms of activity, selectivity and stability for the conversion of syngas into lower olefins. When Zr-free Zn/Al2O3 was used for the hybrid catalyst instead of Zr·Zn/Al2O3, the conversion and stability were low. The results show that a synergistic effect existed between the Zr·Zn/Al2O3 component and SAPO-34 zeolite and it was directly related to the resulting catalytic performance. This study will contribute to the development of efficient catalysts for the production of lower olefins from syngas.