Efficient-selective-catalytic aromatization of methanol over Zn-HZSM-5/SAPO-34: Action mechanism of Zn

To improve the performance and stability of catalytic methanol aromatization, the Zn-modified HZSM-5/SAPO-34(Zn-ZS) composite molecular sieve was synthesized by hydrothermal synthesis, and the modified composite molecular sieve was characterized using XRD, SEM, N2 isothermal adsorption-desorption, NH3-TPD, and XPS, and the catalysts were evaluated in catalytic methanol aromatization reactions. The results showed that the introduction of in-situ Zn greatly changes the morphology of the molecular sieve, presenting a polycrystalline stacked bloom-like structure, which brings abundant micro-pore channels to the molecular sieve, but at the same time, the introduction of metallic Zn also will cause more meso-pore channels losses. The introduction of secondary external Zn brought a large number of weak strong acidic sites, and the amount of molecular sieve acid was nearly doubled; weak acidic sites were strengthened, and the ratio of weak to strong acid amount became smaller. The BTX selectivity of the Zn-doped molecular sieve was significantly enhanced and the anti-coking performance was improved. The composite molecular sieve doped with 2% Zn achieves a 41% selectivity for the BTX within the 47 h lifetime, showing good reaction stability.