Hindered Diffusion in Ordered Mesoporous Silicas: Insights from Pore-Scale Simulations in Physical Reconstructions of SBA-15 and KIT-6 Silica
Hindered Diffusion in Ordered Mesoporous Silicas: Insights from Pore-Scale Simulations in Physical Reconstructions of SBA-15 and KIT-6 Silica
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DOI：
10.1021/acs.jpcc.8b03630
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发表时间：
2018-06-14
影响因子：
3.7
通讯作者：
Tallarek, Ulrich
中科院分区：
文献类型：
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作者：
Reich, Stefan-Johannes;Svidrytski, Artur;Tallarek, Ulrich
SBA-15 和 KIT-6 二氧化硅作为用于多相催化、选择性吸附和控制药物释放的有序介孔载体的性能依赖于它们对有限尺寸溶质或颗粒的扩散传输的特性。我们通过重建模拟方法研究了这个问题，应用于平均孔径为 9.4 nm 的纯介孔 SBA-15 和 KIT-6 二氧化硅样品。通过电子断层扫描进行的物理重建证实了两个样品的高度互连的三维中孔网络，但也揭示了原生孔隙系统中所谓的均匀圆柱体中的收缩（两个样品）和死角（仅限 SBA-15）。重建中点状和有限尺寸示踪剂扩散的孔隙尺度模拟表明，少量的瓶颈足以观察有效扩散系数、可及孔隙率和孔隙互连性随着示踪剂尺寸的增加而急剧下降。当示踪剂尺寸接近平均孔径的三分之一时，扩散传输变得不可能。低于此限制，KIT-6 的性能优于 SBA-15，但两个样品都明显劣于随机介孔二氧化硅。我们的结果表明，SBA-15 和 KIT-6 二氧化硅中的扩散传输可以通过将壁内孔隙向平均孔径扩大来改善，从而绕过原生孔隙系统的瓶颈。
The performance of SBA-15 and KIT-6 silicas as ordered mesoporous supports for heterogeneous catalysis, selective adsorption, and controlled drug release relies on their properties for diffusive transport of finite-size solutes or particles. We investigate this issue through a reconstruction-simulation approach, applied to purely mesoporous SBA-15 and KIT-6 silica samples with a mean pore size of 9.4 nm. Physical reconstruction by electron tomography confirms a highly interconnected, three-dimensional mesopore network for both samples but also reveals constrictions (both samples) and dead ends (SBA-15 only) in the supposedly uniform cylinders of the primary pore system. Pore-scale simulations of the diffusion of pointlike and finite-size tracers in the reconstructions show that a small number of bottlenecks suffices for observing a dramatic decline in the effective diffusion coefficient, accessible porosity, and pore interconnectivity with increasing tracer size. When the tracer size approaches one-third of the mean pore size, diffusive transport becomes impossible. Below this limit, KIT-6 performs better than SBA-15 but both samples are decidedly inferior to random mesoporous silicas. Our results suggest that diffusive transport in SBA-15 and KIT-6 silicas could be improved by widening intrawall pores toward the mean pore size to provide bypasses around bottlenecks in the primary pore system.