Unexpected Long Walks in Hierarchical Porous Materials with Combined Surface and Configurational Diffusion

Hierarchical zeolites and zeolite nanocrystals with micropore lengths on the order of nanometers have been synthesized with the aim of reducing mass transfer limitation. However, due to the large external surface to volume ratios, the mass transport in these materials can be hindered by a secondary rate limitation step imposed on the external surface of the zeolites. In this talk, I will present our recent study on the mass transport in hierarchical zeolites and zeolite nanoparticles. We show that surface barrier is related to the presence of pore re-entry caused by strong sorbate/sorbent interaction at the zeolite surface. In order to understand the nature of the surface barrier, a set of silica nanoparticle (SNP)/Silicalite-1 composites with different external surface to micropore surface ratios was synthesized. It was found that the strong sorbate/sorbent interaction at the external surface of Silicalite-1 nanoparticles can cause diffusing molecules to re-enter into micropores and repeat the micropore diffusion process. This pore re-entry step can lead to an unusually long micropore diffusion length. We also demonstrate that this repeated micropore diffusion process can be effectively reduced by mixing the zeolite nanoparticles with secondary, nonporous nanoparticles. It was concluded that the surface barrier in hierarchical zeolites and zeolite nanoparticles is likely due to a combination of pore re-entry of adsorbates and pore blockage.