Long Walks in Hierarchical Porous Materials due to Combined Surface and Configurational Diffusion

Hierarchical materials with porous structures at different length scales (i.e., micropore and mesopore) are an emerging class of materials. However, the lack of fundamental understanding of mass transport properties significantly limits rational development of these materials for applications in catalysis and separation. In this study, we evaluated the mass transport of two probe molecules, cyclohexane and 1-methylnaphthalene, in two different types of hierarchical porous materials, SBA-15 mesoporous silica and three dimensionally ordered mesoporous imprinted (3DOm-i) silicalite-1 zeolite, for comparison with nonmicroporous MCM-41 mesoporous silica. It was observed that the apparent diffusion lengths determined for hierarchical porous materials (i.e., SBA-15 and 3DOm-i silicalite-1) were significantly longer than predicted by the physical structure (i.e., radius) of the adsorbent particle, indicating that diffusion of molecules in hierarchical porous materials is much longer than expected. The unusually long path length is likely due to diffusion on the external surface, followed by re-entering of diffusing molecules from the external surface into the micropores; the large external surface area of hierarchical porous materials enhances the extent of this phenomenon. The observations reported in the study highlight the importance of surface diffusion in hierarchical porous materials. Enhanced mass transport in hierarchical porous materials can be overpredicted without considering the extent of sorbate-sorbent interaction and the actual diffusion length.