Effects of the location of the Al species in hierarchical ZSM-11 on its catalytic performances in the methanol-to-propylene process

Abstract

During the methanol-to-propylene (MTP) process, the catalytic performance of zeolite catalysts is closely related to its pore structure and acid property. Exploring the structure–function relationship is of great significance for both academic research and industrial applications. In the present work, a series of hierarchical ZSM-11 zeolites with different SiO₂/Al₂O₃ ratios were synthesized by changing the chemical composition of gel precursors. XRD, SEM, TEM, N₂ adsorption–desorption, NH₃-TPD, ICP-OES, NMR spectroscopy, FT-IR spectroscopy, pyridine-IR spectroscopy, in situ UV/vis spectroscopy and TG-DTG analysis were used to explore the structural and textural properties of the as-synthesized zeolite samples. The results show that the change in the SiO₂/Al₂O₃ ratio in the gel precursors has some effects on the morphology, crystalline sizes and porous structural properties of the samples. Furthermore, it plays an important role in tailoring the distribution of the framework Al species and thus in adjusting the acid sites of the as-prepared ZSM-11 catalysts. The distribution of framework Al species in the intersecting cavity significantly decreases with the increased SiO₂/Al₂O₃ ratio in the gel precursors, which inhibits the reaction paths based on the aromatic cycle, resulting in a reduction of aromatic products and carbon deposition precursors while enhancing the selectivity of the targeted product propylene. Typically, as compared with the low-silicon ZSM-11-60, the relatively high-silica ZSM-11-160 exhibits superior catalytic performance to achieve a high selectivity of propylene in the final products (39.4% vs. 17.6%) and a longer catalytic life (106 h vs. 42 h) because of a low coking deposition rate (0.52 mg g⁻¹ h⁻¹ vs. 2.21 mg g⁻¹ h⁻¹) during the MTP process.