Low-Cost and Seed-Directed Synthesis of ZSM-5 Nanosheets for Enhanced Monocyclic Aromatics Yields and Coke Resistance in Catalytic Fast Pyrolysis of Biomass

Abstract

Developing novel, high-efficiency and low-cost catalysts poses a core challenge to the production of monocyclic aromatics (MAs) via catalytic fast pyrolysis (CFP) of biomass. Conventional synthesis of short-b-axis ZSM-5 nanosheets suffers from excessive feedstock expenses and severe environmental burdens. Herein, we develop a fluoride-free and low-cost route for the controllable synthesis of short-b-axis ZSM-5 nanosheets using low-cost TPABr as the main structure-directing agent and low-cost colloidal silica as the main silica source, with the co-assistance of Silicalite-1 nanoseeds and urea. By systematic optimization of synthesis parameters, uniform ZSM-5 nanosheets with tunable b-axis lengths ranging from 39 to 160 nm were successfully fabricated. Time-resolved characterizations of crystallization evolution confirm that this synthesis system proceeds through coupled classical and nonclassical crystallization pathways, encompassing epitaxial growth and aggregation of seed crystals, oriented attachment and crystal propagation, as well as monomer addition. CFP tests using cotton stalks demonstrate that stronger catalyst acidity facilitates the generation of BTEX (benzene, toluene, ethylbenzene, and xylene). For samples with identical Si/Al ratios, the b/a axis ratio, serving as a morphological descriptor, influences the yields of MAs and BTEX. Lower b/a ratios favor the formation of MAs and BTEX. The maximum yields of MAs (26.05%) and BTEX (18.94%) are achieved at a b/a value of 0.258. This work delivers a sustainable and economical protocol for catalyst fabrication toward high-value biomass valorization and provides mechanistic guidance for morphology-controllable synthesis of short-b-axis ZSM-5 in fluoride-free systems.