Catalytic Upgrading of Lignite Pyrolysis Volatiles over AlF3-Modified HZSM-5 to Light Aromatics: Synergistic Effects of One-Step Dealumination and Realumination

Aluminum fluoride (AlF 3 )-modified HZSM-5 obtained by simple mechanical mixing and calcination was used to enhance the conversion of the lignite pyrolysis volatiles to light aromatics. The results revealed that the appropriate amount of AlF 3 modification can simultaneously dealuminize and aluminize to enhance the catalytic performance, in which dealumination considerably enlarged the pores while realumination generated more midstrength acid sites to compensate for the loss of acid sites caused by dealumination. HZ-2 (HZSM-5 mixed with 2 wt % AlF 3 ) obtained the maximum light aromatics yield of 31.0 mg/g as a result of its abundant mesopores and suitable midstrength acid sites, especially the yield of benzene increased by 6.3 mg/g in comparison with that of the original HZSM-5. However, mixing with only a small amount of AlF 3 generated the most serious carbon depositions because of the acidity enhancement by the critical role of alumination. Moreover, excessive AlF 3 etched more framework Al that dramatically reduced the midstrength acid sites, inhibiting the formation of light aromatics, although the enlargement of pore size facilitated mass transfer. This one-step modification combining the expansion of the pore size and the optimization of acid sites provides a promising strategy for upgrading the lignite pyrolysis volatiles.