In-situ synthesis of micro/mesoporous HZSM-5 zeolite for catalytic pyrolysis of lignin to produce monocyclic aromatics

Catalytic pyrolysis of lignin is a promising way for lignin upgrading. To date, commercial zeolites have been demonstrated to be active in lignin catalytic pyrolysis for monocyclic aromatics production, but usually exhibit low selectivity and fast deactivation during reaction. These are mainly attributed to the inadequate of mesoporous structure and improper acidic property of commercial zeolites. Herein, HZSM-5 zeolites with different micro/mesoporous morphology were synthesized via the green and energy-efficient in-situ synthesis method. The porous structure of the zeolites was regulated by adjusting the synthesis conditions, i.e., the concentration of tetrapropylammonium hydroxide (TPAOH, used as the microporous templating agent), the synthesis temperature, and synthesis duration. With the HZSM-5 sample synthesized under the optimal condition (S8HZ), the relative content of monocyclic aromatics achieved during lignin catalytic pyrolysis reached 39.3 % at 500 degrees C, which was much higher than that attained over the commercial HZSM-5 (17.3 %). In addition, the acidic property and pore structure of the S8HZ catalyst was further modulated by cobalt impregnation. The introduction of cobalt, rendered a good balance between the structural property and acidity of the catalyst, which eventually facilitated the generation of monocyclic aromatics by both improving mass transfer and reducing coke formation. As it turned out, 1 wt% of cobalt introduction to the S8HZ boosted the generation of monocyclic aromatics to 46.3 % at 550 degrees C. Moreover, the catalyst retained good stability in catalytic lignin pyrolysis after 10 reaction cycles. Overall, this work demonstrates the feasibility of using in-situ synthesized HZSM-5 catalyst with rich micro/mesoporous structure as well as Co modification for catalytic lignin upgrading to monocyclic aromatics, which offers new insights for lignin upgrading via catalytic pyrolysis.