Enhancement of Aromatic Products from Catalytic Fast Pyrolysis of Lignite over Hierarchical HZSM-5 by Piperidine-Assisted Desilication

Abstract

HZSM-5 was post-treated by piperidine-assisted desilication (PAD) and metallic Co and/or Zr modification to introduce the meso-/microporous system and metal active sites to enhance the activity for catalytic fast pyrolysis (CFP) of lignite for aromatic products. CFP was conducted over parent and hierarchical HZSM-5 in a drop tube reactor at 600 °C and a gas residence time of 1.5 s. The results showed that assisted desilication with piperidine (PI) concentration of 0.3 mol/L (AT0.2–PI0.3), retained the morphology of HZSM-5, and avoided severe alkaline corrosion. This was due to the shield of the zeolite crystals from extensive dissolving of NaOH by organic amines. It not only decreased the deactivation rate of the catalyst, but also enhanced the mass transfer in the catalyst. The selectivity of light aromatics (LAs) such as benzene, toluene, ethylbenzene, xylene, and naphthalene remarkably increased to 24.9% over AT0.2–PI0.3 in comparison to the HZSM-5. In addition, introducing bimetallic Zr–Co facilitated the hydrogen transfer of pyrolysis fragments at the metal sites and sped up the cracking reaction and deoxygenation step of the cascade reactions. 5Zr/Co–AT0.2–PI0.3 with Zr loading of 5% exhibited an excellent activity for upgrading of pyrolysis vapors, and its LA selectivity further increased to 30.5%. Meanwhile, the organic oxygen species and macromolecular compound (C14+ and C18+) contents were decreased gradually. This work provides a potential approach for directional production of LAs from lignite.