Adsorption of C2-C8 n-Alkanes in Zeolites
Adsorption of C2-C8 n-Alkanes in Zeolites
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DOI：
10.1021/jp106536m
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发表时间：
2011-02-03
影响因子：
3.7
通讯作者：
Marin, Guy B.
中科院分区：
文献类型：
--
作者：
De Moor, Bart A.;Reyniers, Marie-Francoise;Marin, Guy B.
本文采用QM-Pot（MP2//B3 LYP）和统计热力学计算相结合的方法，在假定吸附质为移动的的条件下，研究了正构烷烃在工业上常用的分子筛H-FAU、H-MOR、H-ZSM-5中的吸附。在H-ZSM-5分子筛中，研究了直孔道（SC+I）和曲折孔道（ZC+I）中烃链交叉点的吸附。此外，还用量热法和重量法同时测定了301 ~ 400 K范围内所有C3 ~ C6正构烷烃在H-ZSM-5上的微分吸附热和吸附等温线。计算出的吸附能与温度无关，实际上与吸附能相同。吸附强度按H-FAU < H-BEA < H-MOR < H-ZSM-5（SC+I）< H-ZSM-5（ZC+I）的顺序增加，并与碳数呈线性变化。与实验值相比，计算的吸附强度在FAU中被高估了约2 kJ mol（-1）/CH 2，在H-ZSM-5中被高估了约4 kJ mol（-1）/CH 2，这表明QM-Pot（MP2//B3 LYP）计算高估了货车der Waals稳定相互作用，并提出了一个修正项。吸附熵损失与温度无关，并且根据沸石的孔径大小，以H-FAU <H-MOR <H-ZSM-5（SC+I）< H-ZSM-5（ZC+I）的顺序增加。计算的吸附熵与现有的实验结果在所有沸石同意很好。QM-Pot（MP2//B3 LYP）计算的吸附平衡系数（使用校正的吸附平衡系数）与实验测定值相当吻合。相对周转频率与相对吸附平衡系数的比较表明，平衡系数的变化与碳数或与沸石只能部分解释所观察到的反应性差异，在单分子裂化的正构烷烃。与实验观察一致，我们的研究结果表明，在给定的沸石中的单分子裂化的正构烷烃的反应性的差异主要源于在本征单分子裂化速率系数的差异。
Adsorption of n-alkanes has been studied in the industrially relevant zeolites H-FAU, H-BEA, H-MOR, and H-ZSM-5 combining QM-Pot(MP2//B3LYP) with statistical thermodynamics calculations and assuming a mobile adsorbate. In H-ZSM-5, adsorption at the intersection site with the hydrocarbon chain extending in the straight channel (SC+I) as well as in the zigzag channel (ZC+I) has been studied. In addition, differential heats of adsorption and adsorption isotherms at temperatures from 301 to 400 K of all C3-C6 n-alkane in H-ZSM-5 have been measured simultaneously via calorimetry and gravimetry. Calculated adsorption enthalpies are independent of temperature and are virtually identical to the adsorption energies. The adsorption strength increases in the order H-FAU < H-BEA < H-MOR < H-ZSM-5 (SC+I) < H-ZSM-5 (ZC+I) and varies linearly with the carbon number. As compared to experimental values, the calculated adsorption strength is overestimated by some 2 kJ mol(-1)/CH2 in FAU up to some 4 kJ mol(-1)/CH2 in H-ZSM-5 suggesting that the QM-Pot(MP2//B3LYP) calculations overestimate van der Waals stabilizing interactions and a correction term has been proposed. Adsorption entropy losses are independent of temperature and increase in the order H-FAU < H-BEA < H-MOR < H-ZSM-5 (SC+I) < H-ZSM-5 (ZC+I), according to the pore size of the zeolites. The calculated adsorption entropies agree nicely with available experimental results in all zeolites. QM-Pot(MP2//B3LYP) calculated adsorption equilibrium coefficients (using the corrected adsorption enthalpies) correspond relatively well to experimentally determined values. Comparison of relative turnover frequencies with relative adsorption equilibrium coefficients indicates that the variation of the equilibrium coefficient with the carbon number or with the zeolite can only partly explain the observed reactivity differences in monomolecular cracking of n-alkanes. In agreement with experimental observations, our results indicate that the difference in reactivity of the n-alkanes for monomolecular cracking in a given zeolite mainly originates from a difference in intrinsic monomolecular cracking rate coefficients.