Tailoring Zeolite Nanosheets via Protozeolite-Directed Growth for Enhanced Methanol-to-Propylene Conversion

Single-crystalline MFI-type zeolite nanosheets with shortened b-axis dimension exhibit unique catalytic selectivity and exceptional separation performance. However, their conventional synthesis heavily relies on expensive organic templates and environmentally harmful additives, while also facing challenges in precisely controlling aspect ratios. Herein, we report an anisotropy-tailored, cost-effective protozeolite-directed strategy for preparing single-crystalline MFI-type zeolite nanosheets under ultralow template usage (Tetrapropylammonium hydroxide/SiO₂ molar ratio of 0.05). The amorphous protozeolite (PZ), featuring abundant silanol groups, tunable morphology, and microporous-mesoporous structures, demonstrates exceptional structure-directing efficiency and growth-kinetic-regulating capability. Compared to the seed-free system that yields no solid product at the same template/SiO₂ of 0.05, protozeolite-existed system enables solid yields exceeding 98% for the large-scale synthesis of S-1 and ZSM-5 nanosheets in a 5 L autoclave. Precise control over aspect ratio (6.0-52.0) was achieved by synergistically adjusting H₂O/SiO₂ molar ratios (15-120) and optimizing protozeolite morphology (spherical and platelet-like) and loading amount (5-13 wt %). Significantly, owing to the high aspect ratio ([(L_a + L_c)/L_b] = 11.6) and shortened b-axis dimension (95 nm) that favor alkene-based over aromatic-based cycles in the methanol-to-propylene (MTP) reaction, the resulting ZSM-5 nanosheets delivered a remarkably prolonged catalytic lifetime (76 h) and an ultrahigh propylene/ethylene ratio up to 10.0 (5.5-fold enhancement over conventional ZSM-5 catalysts) in the MTP reaction at WHSV = 8 h^-1. This work not only provides a practical and environmentally benign pathway toward industrial-scale production of zeolite nanosheets but also offers new insights into the crystallization of zeolites with tailored morphology and enhanced performance.