A series of hierarchical HZSM-5 zeolites were prepared by post-synthesis modification of conventional bulk crystals of HZSM-5 zeolite with NaOH solution at different concentrations. These micro-mesoporous composite molecular sieves were characterized by powder Xray diffraction (XRD), Transmission electron microscopy (TEM), N 2 adsorption, Fourier transform infrared (FT-IR) spectroscopy techniques and pyridine FT-IR to investigate the changes in crystallinity, acidity, morphology and textural property of HZSM-5 zeolite 10 before and after alkaline treatment. The catalytic performances of these hierarchical HZSM-5 zeolites were evaluated by the aromatization of glycerol with methanol as the solvent, which was a promising route to convert renewable glycerol and methanol into high valuable aromatics. Substantial mesoporosity with sizes centered at around 4 nm could be generated for HZSM-5 zeolites after treated by mild NaOH solution (≤0.4 M), coupled with better retained microporosity, resulting in great improvements in catalytic life time and selectivity to BTX aromatics during the reaction of glycerol to aromatics. Though the larger mesopore surface areas were 15 achieved when treated NaOH concentrations were higher than 0.5 M, the HZSM-5 structure was partly damaged, leading to the reduction of catalytic life time and selectivity to BTX aromatics. The HZSM-5 treated with 0.3 M NaOH solution was found as the optimum catalyst for the transformation of glycerol/methanol to aromatics, producing nearly twofold increase in BTX aromatics (carbon yields of 25.18%) and threefold improvement in catalyst life time (12.5 h) than the parent microporous HZSM-5 (13.9% carbon yields of BTX aromatics and 4h). These improved catalytic performances are mainly attributed to the optimized bimodal micro-mesoporous HZSM-5 20 zeolite during the alkali treatment, which retained sufficient micropores that have the capacity of aromatization while created more mesopores that could shorten the average diffusion path lengths, increase the accessibility to the acid sites and facilitate the transportation of large molecular (e.g. glycerol and carbon precursors) during the aromatization of glycerol.
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