Post-Synthesis of HZSM-5 by Aluminum Fluoride in an Acidic Fluoride Solution for Enhanced Reforming of Lignite Pyrolytic Volatiles into Light Aromatics

Dealumination by chemical etching with acids has been one of the most common post-treatment methods for secondary porosity engineering of zeolite but inevitably suffers from adverse impacts on its skeleton structure and acidic properties. Herein, we present a novel method to treat HZSM-5 zeolite in a buffer solution of HF/NH 4 F and employ AlF 3 as an external Al source for one-step dealumination and alumination. The as-prepared hierarchical zeolites remain with high crystallinity and acidity, while secondary mesopores are formed. A suitable loading of AlF 3 (1.0 wt %) together with a diluted buffer solution (0.1 M) is the most optimal condition, under which the mesoporous volume of HZSM-5 increases from 23 to 41% from the total volume. Under the synergetic effects of H + and F –, AlF 3 undergoes complexation, hydrolysis, and dehydration reactions and is introduced into the frameworks. The catalytic performances for reforming lignite pyrolytic volatiles into light aromatics at 600 °C showed that the AlF 3 -modified zeolites exhibited higher activity than the parent toward suppressing the coke deposition and yielding the desired products.