Study on the coke deposition characteristics of hierarchical ZSM-5 zeolites with synergistic regulation of pore structure and temperature in benzene catalysis
Carbon deposition caused by mass transfer limitations is a key challenge for traditional microporous ZSM-5 zeolites in coal tar catalytic cracking. To address this, benzene was used as a model compound. Parent ZSM-5 (NL-ZSM-5) was modified with tetraethylammonium hydroxide (TEAOH) to prepare hierarchical ZSM-5 zeolites with different mesopore sizes. Characterization (XRD, FT-IR, BET, TEM) confirmed successful mesopore introduction via selective desilication while retaining the MFI structure. At TEAOH concentration of 0.4 mol/L (ZSM-5-C), total pore volume increased from 0.24 to 0.43 cm3/g, and Brønsted acid amount increased from 0.28 to 0.67 mmol/g, with improved acid site accessibility. Catalytic experiments and carbon deposition analysis showed that hierarchical pore structure inhibits coking via a synergistic effect of diffusion enhancement and adsorption-site regulation. The coke amount of ZSM-5-C was 4.0%, only one-third of that of NL-ZSM-5 (11.9%). Molecular dynamics simulations confirmed that the diffusion coefficient of benzene in a 3.0 nm mesopore model is an order of magnitude higher than in a 2.0 nm model. Adsorption capacity decreases with increasing mesopore size, shortening residence time. Increasing temperature enhances diffusion but exponentially intensifies surface condensation reactions (Arrhenius effect), which dominates coke formation; hierarchical pores mitigate this negative effect. This research provides a theoretical basis for designing high-efficiency, coke-resistant catalysts for coal tar conversion.