Predicting diffusion barriers and diffusivities of C6–C12 methylbenzenes in MFI zeolites
Predicting diffusion barriers and diffusivities of C6–C12 methylbenzenes in MFI zeolites
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DOI：
10.1016/j.micromeso.2022.111705
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发表时间：
2022-01
影响因子：
5.2
通讯作者：
Mykela DeLuca;D. Hibbitts
中科院分区：
文献类型：
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作者：
Mykela DeLuca;D. Hibbitts
质量传递在沸石催化反应和催化剂失活中起重要作用，然而质量传递的实验测量，特别是超慢扩散过程（例如，体积大的芳族化合物），由于时间尺度的限制而受到限制。在这里，我们使用密度泛函理论来克服这些限制，并计算苯和所有12个C7-C12甲苯通过silicalite-1（MFI框架）的直通道和正弦通道的扩散势垒。直线和正弦扩散障碍是很好地预测的临界直径描述的分子的最小宽度，其中苯，甲苯，和对二甲苯（所有6.6欧姆）扩散通过这两个通道与障碍200 kJ mol− 1低于五甲基苯（8.2欧姆）。MFI骨架扭曲以适应具有较大临界直径的物质，并且这种扭曲与活化屏障相关，其中与较大物质（例如五甲基苯）相比，较小分子（例如苯）使骨架扭曲的程度较小。通过MFI的直通道扩散总是比通过正弦通道更容易，平均为39 kJ mol− 1，因为正弦通道的弯曲度比直通道扩散迫使更大的骨架畸变。我们表明，DFT计算的直通道扩散激活势垒与频率响应实验报告的一致，并可用于计算分子的自扩散系数，适当的熵修正。检查所有芳族化合物提供了对分子大小、通道曲折度和熵在晶内扩散期间的作用的了解，以提供可以合理地通过两个通道扩散的物质的参考点（例如，苯、甲苯、二甲苯、杜烯），仅通过直通道（例如，1，2，3-三甲基苯），或者简单地被“卡”在交叉点内（例如，五甲基苯）。
Mass transport plays an important role in zeolite catalyzed reactions and catalyst deactivation, yet experimental measurement of mass transport, particularly ultra-slow diffusion processes (e.g., of bulky aromatics), is limited because of time scale restraints. Here, we use density functional theory to overcome these limitations and calculate diffusion barriers of benzene and all twelve C7–C12methylbenzenes through the straight and sinusoidal channels of silicalite-1 (MFI framework). Straight and sinusoidal diffusion barriers are well-predicted by a critical diameter describing the minimum width of the molecule, where benzene, toluene, andpara-xylene (all 6.6 Å) diffuse through both channels with barriers 200 kJ mol−1lower than those of pentamethylbenzene (8.2 Å). The MFI framework distorts to accommodate species with larger critical diameters and this distortion correlates to activation barriers where smaller molecules, such as benzene, distort the framework to smaller extents compared to larger species, such as pentamethylbenzene. Diffusing through the straight channel of MFI is always more facile than via the sinusoidal channel, by an average of 39 kJ mol−1because the tortuosity of the sinusoidal channels forces larger framework distortions than straight channel diffusion. We show that DFT-calculated straight channel diffusion activation barriers agree well with those reported by frequency response experiments and can be used to calculate self-diffusivities of molecules, with appropriate entropy corrections. Examining all aromatics provides insights to the role of molecule size, channel tortuosity, and entropy during intracrystalline diffusion to provide a reference point for the species that can reasonably diffuse through both channels (e.g., benzene, toluene, xylenes, durene), through straight channels only (e.g., 1,2,3-trimethylbenzene), or simply are ‘stuck’ within intersections (e.g., pentamethylbenzene) in MFI.