Shape-Selective Diffusion of Olefins in 8-Ring Solid Acid Microporous Zeolites
Shape-Selective Diffusion of Olefins in 8-Ring Solid Acid Microporous Zeolites
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DOI：
10.1021/acs.jpcc.5b06010
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发表时间：
2015-10-15
影响因子：
3.7
通讯作者：
Van Speybroeck, Veronique
中科院分区：
文献类型：
--
作者：
Ghysels, An;Moors, Samuel L. C.;Van Speybroeck, Veronique
使用分子动力学模拟技术和新开发的柔性力场研究了烯烃通过 8 环固体酸微孔沸石的扩散。在甲醇制烯烃 (MTO) 工艺和观察到的产品分布的背景下，了解扩散路径对于获得对工艺条件的分子水平控制至关重要。八环沸石材料适合用于 MTO 工艺，因为它们可以选择性地向低碳烯烃分配产品。为了研究组成、酸度和柔韧性如何影响乙烯和丙烯的扩散路径，研究了一系列具有或不具有随机分布的酸性位点的同结构铝硅酸盐（沸石）和硅铝磷酸盐（AlPO 和 SAPO）。在温度、成分、酸度和拓扑（AEI、CHA、AFX）方面观察到乙烯扩散的明显变化。一般来说，乙烯的扩散是一个活化过程，可以确定各个环的自由能垒。我们观察到环相关的扩散行为，不能仅用环的组成和拓扑来描述。受蛋白质和小分子隐式溶剂化模型的启发，必须引入一个新的描述符，即可及窗口面积（AWA）。 AWA 可以在整个分子动力学轨迹中确定，并且与分子水平上的环交叉数量以及从一个笼到另一笼的环交叉的自由能垒密切相关。观察到的总体扩散率由各个环的分子特征决定，AWA 是一个合适的描述符。温度引起的框架动力学和扩散率的变化可以通过在整个模拟过程中遵循新的描述符来捕获。
The diffusion of olefins through 8-ring solid acid microporous zeolites is investigated using molecular dynamics simulations techniques and using a newly developed flexible force field. Within the context of the methanol-to-olefin (MTO) process and the observed product distribution, knowledge of the diffusion paths is essential to obtain molecular level control over the process conditions. Eight-ring zeotype materials are favorably used for the MTO process as they give a selective product distribution toward low-carbon olefins. To investigate how composition, acidity, and flexibility influence the diffusion paths of ethene and propene, a series of isostructural aluminosilicates (zeolites) and silicoaluminophosphates (AlPOs and SAPOs) are investigated with and without randomly distributed acidic sites. Distinct variations in diffusion of ethene are observed in terms of temperature, composition, acidity, and topology (AEI, CHA, AFX). In general, diffusion of ethene is an activated process for which free energy barriers for individual rings may be determined. We observe ring-dependent diffusion behavior which cannot be described solely in terms of the composition and topology of the rings. A new descriptor had to be introduced, namely, the accessible window area (AWA), inspired by implicit solvation models of proteins and small molecules. The AWA may be determined throughout the molecular dynamics trajectories and correlates well with the number of ring crossings at the molecular level and the free energy barriers for ring crossings from one cage to the other. The overall observed diffusivity is determined by molecular characteristics of individual rings for which AWA is a proper descriptor. Temperature-induced changes in framework dynamics and diffusivity may be captured by following the new descriptor throughout the simulations.