Transport under confinement: Hindrance factors for diffusion in core-shell and fully porous particles with different mesopore space morphologies
Transport under confinement: Hindrance factors for diffusion in core-shell and fully porous particles with different mesopore space morphologies
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DOI：
10.1016/j.micromeso.2019.02.036
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发表时间：
2019-07
影响因子：
5.2
通讯作者：
S. Reich;Artur Svidrytski;A. Höltzel;Wunjun Wang;C. Kübel;D. Hlushkou;U. Tallarek
中科院分区：
文献类型：
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作者：
S. Reich;Artur Svidrytski;A. Höltzel;Wunjun Wang;C. Kübel;D. Hlushkou;U. Tallarek
我们通过使用电子断层扫描获得的介孔空间形态的物理重建作为被动有限尺寸示踪剂直接扩散模拟的几何模型，量化了介孔二氧化硅颗粒中阻碍输运的限制效应。研究了经典溶胶-凝胶工艺制备的平均介孔尺寸为dmeso= 16.0和23.9 nm的全孔颗粒，以及由固体颗粒围绕不透水的固体岩心层层组装而成的固体核-多孔壳颗粒（dmeso= 9.4和16.8 nm）。由于壳层厚度和芯层大小是可独立调节的，因此芯-壳颗粒可以将固定床反应器或色谱柱中的颗粒内扩散距离与颗粒的外表面积和床层的水力渗透性分离开来，这在完全多孔的颗粒中是不可能的。在四次重建中记录的有效扩散率、可达孔隙度和孔隙网络连通性作为示踪剂与平均介孔尺寸之比λ的函数，表明核-壳颗粒的壳形态不利，与完全多孔颗粒的设计优势相反。重建结果表明，核壳颗粒含有更多的狭窄、收缩和封闭的孔隙。这些结构特征反映了压实和烧结填料，很可能是在壳固结过程中形成的。所提出的阻碍扩散和可达孔隙度表达式可用于优化敏感应用中的介孔空间形态，例如限制催化和控制药物释放。
We quantify confinement effects on hindered transport in mesoporous silica particles through using physical reconstructions of their mesopore space morphology obtained by electron tomography as geometric models in direct diffusion simulations for passive, finite-size tracers. Studied are fully porous particles with mean mesopore sizes ofdmeso= 16.0 and 23.9 nm, prepared by classical sol–gel processing, and solid core–porous shell particles (dmeso= 9.4 and 16.8 nm) originating from a layer-by-layer assembly of sol particles around a solid, impermeable core followed by thermal consolidation of the porous shell. Because shell thickness and core size are independently adjustable, core–shell particles allow to decouple the intraparticle diffusion distance in a fixed-bed reactor or chromatographic column from the external surface area of the particles and the hydraulic permeability of the bed, impossible with fully porous particles. Effective diffusivity, accessible porosity, and pore network connectivity recorded in the four reconstructions as a function of λ, the ratio of tracer to mean mesopore size, demonstrate an unfavorable shell morphology for the core–shell particles that opposes their design advantage over fully porous particles. The reconstructions reveal that core–shell particles contain an increased number of narrow and constricted as well as closed pores. These structural features reflect compacted and sintered packings and are most likely formed during shell consolidation. The presented expressions for hindered diffusion and accessible porosity can be used to optimize mesopore space morphologies in sensitive applications, e.g., catalysis under confinement and controlled drug release.