F-assisted synthesis of a hierarchical ZSM-5 zeolite for methanol to propylene reaction: a b-oriented thinner dimensional morphology

Abstract

Under neutral fluoride medium at 373 K, a facile strategy has been developed to synthesise a hierarchical ZSM-5 zeolite (M-ZSM-5) using a solid silica source in a dense system. The resulting material shows a hexagonal lamellar shape with b-oriented thinner dimension and bi-modal porosity containing MFI micropores and intracrystal mesopores. The effect of synthesis factors, such as NH₄F/SiO₂, H₂O/SiO₂, crystallization temperature and time, on the zeolite morphology and size is studied and its primary crystallization process is proposed. Through varying the synthesis parameters, the crystal size could be tuned with the aspect ratio at a range of 6.2–12. Typically, M-ZSM-5 with a thinner thickness (100 nm) and a high aspect ratio (AR = 12) shows a highly effective catalytic performance in methanol to propylene (MTP) reaction. Compared to the bulk C-ZSM-5 sample obtained from a hydroxyl system, M-ZSM-5 shows higher selectivity for propylene (45.1% vs. 38.1%) and butylene (27.2% vs. 21.3%), especially prolonged catalytic lifetime (224 h vs. 98 h). This enhanced performance could be contributed to the optimized acidity and superior diffusivity of M-ZSM-5 with lamellar morphology and bimodal porosity, which are crucial for suppressing secondary reactions and inhibiting coke deposition.