Catalytic conversion of lignite pyrolysis volatiles to light aromatics over ZSM-5: SiO2/Al2O3 ratio effects and mechanism insights

In-situ reforming of lignite pyrolysis volatiles over ZSM-5 zeolites with different SiO2/Al2O3 ratios was carried out to obtain light aromatics including BTEXN (benzene, toluene, ethylbenzene, xylene, and naphthalene). In comparison to the commercial zeolites, the zeolites used in this work were synthesized by a one-step method with controlled Al addition. The high Al-containing ZSM-5 (SiO2/Al2O3 = 50) was observed to be more effective in producing BTEXN (20.8 mg/g), likely by removing oxygen atom of volatiles on acidic sites. Moreover, it was still activated even after several regeneration cycles. As indicated by the GC-MS results, the aliphatic hydroxyl and carboxyl groups were decomposed over ZSM-5. High SiO2/Al2O3 ratio was favorable for polyaromatics elimination. However, the as-synthesized ZSM-5 with SiO2/Al2O3 ratio of 25 was less effective in cleaving aliphatic C-C bonds. Besides, the structural properties of ZSM-5 were significantly affected by the SiO2/Al2O3 ratio. High intergrowth and twinning structures were observed with increasing SiO2/Al2O3 ratio. Regardless of the crystal morphology of zeolite, the SiO2/Al2O3 ratio had significant influence on aromatics types, providing insights into ZSM-5 catalyst for the selective control of BTEXN owing to acidity changes.