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

Abstract

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.

Unravelling mass transport in hierarchically porous catalysts

“…enhanced interactions with surface sulfonic acid groups or hydroxyls, but rather reects topological constraints of the pore system. This nding agrees with pore-scale diffusion simulations, 67,68 which reveal that the effective tortuosity of mesoporous SBA-15 and KIT-6 silicas depends on the size of the probe molecule. Note that this contrasts with simulations 67,68 and PFG-NMR studies of amorphous mesoporous silicas, for which effective tortuosity is independent of molecular size.…”

Surface modification of pH-responsive poly(2-(tert-butylamino)ethyl methacrylate) brushes grafted on mesoporous silica nanoparticles

“…There has been an increase in research on mesoporous silica nanoparticles (MSNs) during the last decades [1][2][3][4][5].…”

Diffusion in Nanoporous Materials: Novel Insights by Combining MAS and PFG NMR

“…That diffusion in the narrow-pore silica gel is found to be slowed down in comparison with diffusion in the large-pore sample in general, may be easily referred to an associated increase in tortuosity [55][56][57][58]. This effect is well known to increase with the size of the diffusing particle [59][60][61] which explains why the reduction in diffusivity in the narrow-pore silica gel is even more pronounced with acetone than with the n-alkanes (since, in contrast to acetone, only a minor part of the n-alkanes contributes to the complex formation).…”