Enthalpy and entropy barriers explain the effects of topology on the kinetics of zeolite-catalyzed reactions.
Enthalpy and entropy barriers explain the effects of topology on the kinetics of zeolite-catalyzed reactions.
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DOI：
10.1002/chem.201301272
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发表时间：
2013-08
期刊：
影响因子：
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通讯作者：
Jeroen Van der Mynsbrugge;J. de Ridder;K. Hemelsoet;M. Waroquier;V. Van Speybroeck
中科院分区：
文献类型：
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作者：
Jeroen Van der Mynsbrugge;J. de Ridder;K. Hemelsoet;M. Waroquier;V. Van Speybroeck
研究了乙烯、丙烯和反式-2-丁烯在H-ZSM-58（DDR）、H-ZSM-22（TON）和H-ZSM-5（MFI）分子筛上的甲基化反应，以阐明拓扑结构对分子筛催化反应动力学的特殊影响。发现与H-ZSM-5相比，H-ZSM-58和H-ZSM-22显示出总体较低的甲基化速率，并且甲基化速率随着烯烃尺寸的增加也呈现不同的趋势。这些变化可以合理化的基础上分解的自由能势垒到熵和熵的贡献，这表明，较低的甲基化率在H-ZSM-58和H-ZSM-22上有几乎相反的原因。在H-ZSM-58上，较低的甲基化速率是由较高的焓垒引起的，这是由于反应中间体在大的笼状孔中的稳定性不足。另一方面，在H-ZSM-22上，甲基化速率大多遭受较高的熵垒，因为在窄通道结构内发生过度的熵损失。这些结果表明，关键的基本步骤的动力学取决于沸石孔内的反应中间体的适当稳定性和由此产生的熵损失之间的平衡。这些对其内部工作原理的基本见解对于最终选择或设计更好的沸石催化剂是必不可少的。
The methylation of ethene, propene, and trans-2-butene on zeolites H-ZSM-58 (DDR), H-ZSM-22 (TON), and H-ZSM-5 (MFI) is studied to elucidate the particular influence of topology on the kinetics of zeolite-catalyzed reactions. H-ZSM-58 and H-ZSM-22 are found to display overall lower methylation rates compared to H-ZSM-5 and also different trends in methylation rates with increasing alkene size. These variations may be rationalized based on a decomposition of the free-energy barriers into enthalpic and entropic contributions, which reveals that the lower methylation rates on H-ZSM-58 and H-ZSM-22 have virtually opposite reasons. On H-ZSM-58, the lower methylation rates are caused by higher enthalpy barriers, owing to inefficient stabilization of the reaction intermediates in the large cage-like pores. On the other hand, on H-ZSM-22, the methylation rates mostly suffer from higher entropy barriers, because excessive entropy losses are incurred inside the narrow-channel structure. These results show that the kinetics of crucial elementary steps hinge on the balance between proper stabilization of the reaction intermediates inside the zeolite pores and the resulting entropy losses. These fundamental insights into their inner workings are indispensable for ultimately selecting or designing better zeolite catalysts.