Metal–organic framework-assisted synthesis of Zr-modified SAPO-34 zeolites with hierarchical porous structure for the catalytic transformation of methanol to olefins

Abstract

To introduce a hierarchical porous structure into SAPO-34 zeolites and adjust their acid properties and crystalline size, a series of Zr-modified SAPO-34 zeolites (Zr_x-SAPO-34 (x = 0.25, 0.50 and 0.75)) with hierarchical porous structure were synthesized by hydrothermal crystallization using palygorskite as a silicon and partial aluminum source, morpholine and tetraethylammonium hydroxide as organic structure directing agents and UiO-66 as a co-template agent. The obtained catalysts were characterized via XRD, SEM, TEM, EDX, N₂-adsorption–desorption, NH₃-TPD, FTIR, Py-IR, XRF, ²⁹Si MAS NMR and TG analyses. The catalytic performance of Zr_x-SAPO-34 in the methanol-to-olefin (MTO) transformation was investigated in a fixed bed reactor. The introduction of a hierarchical porous structure effectively enhanced the diffusion and mass transfer of the reactants/products, prolonging the catalytic lifetime. Additionally, the optimization of the acidity dramatically improved the selectivity for light olefins. Under the reaction conditions wherein the reaction temperature was 400 °C, the feed of WHSV was 2.0 h⁻¹, the mass of the catalyst was 0.5 g and at atmospheric pressure, the Zr_0.50-SAPO-34 zeolites exhibited optimum catalytic performance, where the methanol conversion was 100%, the light olefin selectivity was 98.66% and the catalytic lifetime was more than 1500 min.