Conversion of lignite-derived volatiles into aromatics over Zn@MCM-41 and ZSM-5 tandem catalysts with a high stability

Abstract

Direct conversion of lignite into light aromatics via catalytic fast pyrolysis is a great challenge because of severe catalyst deactivation and low aromatics yield. The tandem catalysis (1Zn@MCM-41 and ZSM-5) system was employed to promote the conversion of thermally-derived volatiles of lignite into aromatics at 600 °C. The mesoporous Zn-incorporated MCM-41 (xZn@MCM-41) with different doping amount of Zn was prepared with a one-pot hydrothermal procedure. The structure and the dispersion of Zn species of 1Zn@MCM-41 were systematically studied with XRD, SEM, BET, NH3-TPD and so on. The presence of 1Zn@MCM-41 significantly expands the lifetime of the microporous ZSM-5. Meanwhile, the yield of aromatics, especially benzene, is also increased to some extent. The content of the long-chain hydrocarbons and the phenolic compounds are decreased significantly after being cracked by the mesoporous 1Zn@MCM-41. More small molecules are able to contact with the active sites located in the micropores of ZSM-5 with the presence of Zn@MCM-41. In addition, the ZSM-5 in the tandem catalysts system presents an excellent stability. Therefore, this meso-microporous tandem catalysis system promotes the preferential cracking of macromolecular compounds, the cracked small molecules generated from which then contact with ZSM-5 to produce aromatics. This work provides a new perspective for the gradient catalyzing lignite-derived volatiles over a tandem catalysis system.