Fabrication of Twin-Free Nanoslab ZSM-5 Zeolite with <i>b</i>-Axis Orientation for Super MTP Catalyst

Abstract

ZSM-5 zeolite with well-matched acidity and diffusivity is of significance for the improvement of acid catalytic performance. This study reports a novel synthesis for the twin-free ZSM-5 single crystal with nanoscaled b-axis orientation and its super performance in methanol to propylene (MTP) reaction. The synergistic effect of ZSM-5 nanosheet seed and n-butylamine template, the target ZSM-5 zeolite was achieved at a short crystallization time of 3.0 h. The as-synthesized ZSM-5 zeolite exposes open and high proportion of straight channels and features an inverse Al zoned distribution, which largely improves the intrinsic molecular diffusion and inhibits coke formation at the external surface. Finally, the optimal nanoslab ZSM-5 zeolite performs high MTP performance, i.e., the selectivity to propylene up to 50%, light olefins more than 80%, along with a very long catalytic life of 297 h at WHSV of 6 h–1. The role of the nanosheet seed in the crystallization process was studied by examining the structure evolution of the sol species by SAXS, XRD, SEM, TEM, Raman, FT-IR, etc. Results show that addition of ZSM-5 nanosheet seed can avoid secondary nucleation on the surface of a crystallizing zeolite and proves a quasi-solid-phase crystallization mechanism. Thus, the crystallization process belongs to a classical synthesis route, because the growth structure units are the smaller sized particle according to SAXS analysis.