Theoretical study on the adsorption and diffusion behavior of methyl oleate catalytic cracking in hierarchical ZSM-5 zeolite

doi:10.11949/0438-1157.20231015

Abstract

Abstract:

The hierarchical ZSM-5 zeolite has abundant pore structure, and its unique micro-mesopic pore structure can significantly promote the occurrence of macromolecular reactions such as methyl oleate (OAME), while the promotion mechanism and structure-activity relationship at the molecular level are still unclear. In this work, we studied the adsorption and diffusion behavior of the reactants and five product molecules (C₃H₆, C₄H₈ and BTX) in the catalytic cracking reaction of OAME via Monte Carlo simulation (MC) combined with molecular dynamics (MD). By analyzing the adsorption isotherms, adsorption density maps and diffusion coefficients of the reacting species, we found that rising temperature was detrimental to the adsorption of OAME, thus inhibiting the reaction, which is consistent with the experimental results. At the same time, the two adsorption sites in the zeolite play significantly different roles. The straight channel provides the aromatization active center in the catalytic creaking reaction, while the role of the hierarchical channel is reflected in promoting the mass transfer and diffusion of the main product molecules of BTX. Such synergistic catalysis effect of hierarchical ZSM-5 zeolite channel increases the BTX yield compared to microporous. In addition, it was found that the main reason why the hierarchical pore ZSM-5 molecular sieve can improve the aromatic yield is that it can accelerate the rapid diffusion of BTX into the mesopores. The above simulation results not only help to determine the experimental conditions of the fatty acid ester catalytic cracking reaction from the microscopic point of view, but also could enrich our understanding of the microcosmic mechanism of hierarchical zeolites for prolific aromatics.

Key words: hierarchical zeolite, diffusion, adsorption, molecular simulation

摘要：

多级孔ZSM-5分子筛具有丰富的孔道结构，其独特的微-介复合孔道结构可显著促进油酸甲酯（OAME）等大分子反应的发生，但其在分子层面上的促进机制及构-效关系仍不明晰。本工作采用Monte Carlo模拟（MC）结合分子动力学（MD）方法，研究了OAME催化裂解反应中反应物和5种产物分子（丙烯、丁烯、苯、甲苯和二甲苯）在多级孔分子筛上的吸附和扩散行为。通过分析吸附等温线、吸附密度图和反应物种的扩散系数，发现升温不利于OAME的吸附，从而抑制了反应的发生，这与实验结果一致。同时，直孔道和多级孔道这两种吸附位点在催化裂解反应中起到的作用截然不同：直孔道提供芳构化反应活性中心，而多级孔道则主要促进BTX主产物分子的传质扩散。这种多孔道的协同催化作用使得多级孔分子筛相较于微孔分子筛展现出优异的催化性能。此外，发现多级孔ZSM-5分子筛能提高芳烃收率的主要原因是其能够加速BTX向介孔的快速扩散。以上模拟结果不仅有助于从微观角度优化脂肪酸酯催化裂解反应的实验条件，同时也丰富了对多级孔分子筛能多产芳烃微观机制的理解。

关键词: 多级孔分子筛, 扩散, 吸附, 分子模拟

CLC Number:

TE 624.4

Hansong QIN, Guoliang LI, Hao YAN, Xiang FENG, Yibin LIU, Xiaobo CHEN, Chaohe YANG. Theoretical study on the adsorption and diffusion behavior of methyl oleate catalytic cracking in hierarchical ZSM-5 zeolite[J]. CIESC Journal, 2024, 75(5): 1870-1881.

秦晗淞, 李国梁, 闫昊, 冯翔, 刘熠斌, 陈小博, 杨朝合. 多级孔ZSM-5分子筛中油酸甲酯催化裂解吸附和扩散行为模拟研究[J]. 化工学报, 2024, 75(5): 1870-1881.

Figures/Tables 17

Fig.1 The pathway of catalytic creaking of methyl oleate to BTX[5]

Table 1 Physical properties of hierarchical ZSM-5 zeolite model

参数	实验值	模拟值
孔径/nm	1.6，3.0	1.5，3.0
硅铝比	12.5	12.5

Fig.2 Hierarchical ZSM-5 zeolite model and molecular structure formula of reactive species

Table 2 Diffusion coefficients of reactive species in microporous ZSM-5 zeolite

反应物种	D_s×10¹⁰/(m²/s)	温度/℃	文献
丙烯	100	280	[31]
丁烯	109	400	[17]
丁烯	0.0001	2~27	[35]
苯	0.74±0.20	427	[18]
苯	0.0001~0.01	50~100	[36-38]
甲苯	0.000065~0.000247	100~200	[34]
甲苯	0.097	400	[32]
二甲苯	0.0000035	210	[14]
	1.96	400	[33]
	0.00004~0.000068	100~200	[34]

Fig.3 Simulated and experimental adsorption isotherms of CO2 in hierarchical ZSM-5 zeolite (0.5NaOHZ5)[39]

Fig.4 Adsorption isotherm of OAME in hierarchical ZSM-5 zeolite at different temperatures

Fig.5 Adsorption density of OAME in hierarchical ZSM-5 zeolite at different temperatures at 101 kPa

Fig.6 Potential energy distribution between OAME and hierarchical ZSM-5 zeolite at 400—600℃

Fig.7 Potential energy distribution between OAME and hierarchical ZSM-5 zeolite under low and high pressures at 500℃

Fig.8 Adsorption configuration of OAME in hierarchical ZSM-5 zeolite under low and high pressures

Fig.9 One-component adsorption isotherm of propylene, butene, benzene, toluene and xylene in hierarchical ZSM-5 zeolite

Fig.10 Competitive adsorption isotherm and potential energy distribution of product molecules in hierarchical ZSM-5 zeolite

Fig.11 Adsorption density diagram of product molecules (C3H6, C4H8 and BTX) in hierarchical ZSM-5 zeolite at 500 kPa

Fig.12 MSD-t profile of product molecules in hierarchical ZSM-5 zeolite at 500℃

Table 3 Ds and Arrhenius parameters of product molecules in hierarchical ZSM-5 zeolite at different temperatures

产物	D_s×10⁸/（m²/s）			D₀×10⁷/(m²/s)	E_D/ (kJ/mol)	R²/%
产物	400℃	500℃	600℃	D₀×10⁷/(m²/s)	E_D/ (kJ/mol)	R²/%
丙烯	1.56	2.67	3.89	8.48	1.17	99.95
丁烯	0.77	2.05	3.07	33.50	1.77	99.73
苯	0.81	1.24	1.65	1.84	0.91	99.85
甲苯	0.62	1.18	1.48	2.97	1.12	98.54
二甲苯	0.76	1.23	1.50	1.56	0.88	97.64

Fig.13 MSD-t profile and graph of Ds and temperature of products in hierarchical ZSM-5 zeolite at different temperatures

Fig.14 MSD-t profile of C3H6 and C4H8 molecular in hierarchical ZSM-5 zeolite containing different amounts of toluene

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