Key Takeaways & Executive Findings
- •• • TEAOH treatment at 0.4 mol/L (ZSM-5-C) increased total pore volume from 0.24 to 0.43 cm3/g and Brønsted acid sites from 0.28 to 0.67 mmol/g, enhancing acid site accessibility; this directly improves catalytic activity and coke resistance in industrial benzene conversion. • • ZSM-5-C exhibited a coke amount of 4.0%, versus 11.9% for parent microporous NL-ZSM-5, a threefold reduction; this translates to extended catalyst lifetime and reduced regeneration frequency in fixed-bed reactors. • • Molecular dynamics simulations showed benzene diffusion coefficient in 3.0 nm mesopores is an order of magnitude higher than in 2.0 nm mesopores, while adsorption capacity decreases with larger mesopores; this shortens molecular residence time, mitigating deep condensation reactions that lead to coke. • • Increasing reaction temperature exponentially intensifies surface condensation reactions (Arrhenius effect), which dominates coke formation; however, hierarchical pore structure effectively mitigates this negative effect, enabling operation at higher temperatures without proportional coke penalty.
Abstract
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.
1. Introduction
Traditional microporous ZSM-5 zeolites suffer from severe mass transfer limitations during catalytic cracking of coal tar, leading to rapid carbon deposition and catalyst deactivation. The narrow micropores (<1 nm) restrict diffusion of bulky aromatic molecules, increasing residence time and promoting secondary reactions that form coke. This bottleneck has hindered efficient conversion of coal tar into high-value chemicals, necessitating frequent regeneration and reducing process economics.
This study addresses the challenge by introducing hierarchical porosity into ZSM-5 via TEAOH treatment, creating mesopores that enhance molecular diffusion while preserving the MFI framework. The synergistic regulation of pore structure and reaction temperature is systematically investigated, revealing that optimized mesoporosity reduces coke formation by up to threefold. These findings provide a clear design principle for coke-resistant catalysts, balancing mass transfer efficiency and active site accessibility for industrial coal tar conversion.
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ZENG Qingzhou, HAN Xuefeng, ZHAO Hongyu, LI Jihui, LIU Shucheng (2026). Study on the coke deposition characteristics of hierarchical ZSM-5 zeolites with synergistic regulation of pore structure and temperature in benzene catalysis. Journal of Fuel Chemistry and Technology. https://doi.org/10.1016/S1872-5813(26)60653-6
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Frequently Asked Questions
What is the optimal TEAOH concentration for maximizing mesoporosity while preserving the MFI structure, and how does it affect acid site distribution?
The study found that TEAOH concentration of 0.4 mol/L (ZSM-5-C) yields the most developed mesoporosity, increasing total pore volume from 0.24 to 0.43 cm3/g. This treatment also increased Brønsted acid sites from 0.28 to 0.67 mmol/g, indicating enhanced acid site accessibility. Higher concentrations may lead to excessive desilication and loss of microporosity, but the study did not report data beyond 0.4 mol/L.
How does the hierarchical pore structure quantitatively reduce coke formation compared to parent ZSM-5, and what is the underlying mechanism?
ZSM-5-C exhibited a coke amount of 4.0%, versus 11.9% for parent NL-ZSM-5, a threefold reduction. The mechanism is a synergistic effect of 'diffusion enhancement and adsorption-site regulation': mesopores increase molecular diffusion coefficients by an order of magnitude (as shown by MD simulations for 3.0 nm vs 2.0 nm mesopores), shortening residence time, while partial etching of micropores reduces strongly adsorbing sites that trigger deep reactions.
What is the impact of reaction temperature on coke formation, and can hierarchical porosity mitigate the negative effects of high-temperature operation?
Increasing temperature enhances molecular diffusion but exponentially intensifies surface condensation reactions (Arrhenius effect), which dominates coke formation. However, hierarchical pore structure effectively mitigates this negative effect, as evidenced by the lower coke amount on ZSM-5-C even under conditions that would otherwise promote coking. This allows operation at higher temperatures without proportional coke penalty, improving throughput.
What are the scalability implications of TEAOH treatment for industrial production of hierarchical ZSM-5 catalysts?
TEAOH is a relatively expensive template, but the study demonstrates that a moderate concentration (0.4 mol/L) is sufficient to achieve significant improvements in pore volume and acid site accessibility. The process is straightforward and scalable, but cost-benefit analysis would be needed to compare against alternative mesoporogen routes. The threefold reduction in coke formation could extend catalyst lifetime, offsetting higher synthesis costs.
How do the molecular simulation results correlate with experimental observations regarding benzene diffusion and adsorption?
Molecular dynamics simulations showed that benzene diffusion coefficient in 3.0 nm mesopores is an order of magnitude higher than in 2.0 nm mesopores, while adsorption capacity decreases with increasing mesopore size. These findings correlate with experimental results showing reduced coke formation on ZSM-5-C, as faster diffusion and lower adsorption reduce residence time and deep reactions, confirming the 'diffusion enhancement and adsorption-site regulation' mechanism.
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