Hindered Diffusion in Ordered Mesoporous Silicas: Insights from Pore-Scale Simulations in Physical Reconstructions of SBA-15 and KIT-6 Silica

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.