Light Olefin Diffusion during the MTO Process on H-SAPO-34: A Complex Interplay of Molecular Factors
Light Olefin Diffusion during the MTO Process on H-SAPO-34: A Complex Interplay of Molecular Factors
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DOI：
10.1021/jacs.9b10249
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发表时间：
2020-04-01
影响因子：
15
通讯作者：
Van Speybroeck, Veronique
中科院分区：
文献类型：
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作者：
Cnudde, Pieter;Demuynck, Ruben;Van Speybroeck, Veronique
H-SAPO-34甲醇制烯烃过程的特点是对轻烯烃具有高的形状选择性。催化剂是一种超分子体系，由纳米尺寸的无机笼组成，由Bronsted酸位点装饰，其中有机化合物（主要是甲基化苯）被捕获。这些烃类是催化甲醇转化所必需的，但也可能堵塞孔隙。因此，乙烯和丙烯的扩散在决定最终产物选择性方面起着至关重要的作用。基于力场或密度泛函理论的增强采样分子动力学模拟用于确定分子因素如何影响轻烯烃通过H-SAPO-34的8环窗口的扩散。我们的模拟表明，通过8环的扩散通常是一个受阻过程，对应于扩散分子在相邻笼间的跳跃事件。在笼中装载不同的甲醇、烯烃和芳香物质可以大大减缓或促进扩散过程。8环中Bronsted酸位点的存在促进了扩散过程，这是由于形成了有利的pi-配合物的主客体相互作用。芳烃池的种类严重阻碍了分子筛的扩散，其在沸石晶体中的空间分布对产物的选择性有显著影响。在此，我们通过在操作条件下的增强分子动力学模拟，揭示了分子因素如何影响轻烯烃在受限烃池物种、高烯烃负荷和酸位点存在的复杂环境中的扩散。
The methanol-to-olefins process over H-SAPO-34 is characterized by its high shape selectivity toward light olefins. The catalyst is a supramolecular system consisting of nanometer-sized inorganic cages, decorated by Bronsted acid sites, in which organic compounds, mostly methylated benzene species, are trapped. These hydrocarbon pool species are essential to catalyze the methanol conversion but may also clog the pores. As such, diffusion of ethene and propene plays an essential role in determining the ultimate product selectivity. Enhanced sampling molecular dynamics simulations based on either force fields or density functional theory are used to determine how molecular factors influence the diffusion of light olefins through the 8-ring windows of H-SAPO-34. Our simulations show that diffusion through the 8-ring in general is a hindered process, corresponding to a hopping event of the diffusing molecule between neighboring cages. The loading of different methanol, alkene, and aromatic species in the cages may substantially slow down or facilitate the diffusion process. The presence of Bronsted acid sites in the 8-ring enhances the diffusion process due to the formation of a favorable pi-complex host-guest interaction. Aromatic hydrocarbon pool species severely hinder the diffusion and their spatial distribution in the zeolite crystal may have a significant effect on the product selectivity. Herein, we unveil how molecular factors influence the diffusion of light olefins in a complex environment with confined hydrocarbon pool species, high olefin loadings, and the presence of acid sites by means of enhanced molecular dynamics simulations under operating conditions.