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

Abstract

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.

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“…The mesopore size of the SBA‐15 sample increased from 4.1 to 7.5 nm with a concomitant decrease in micropore volume from 0.17 to 0.05 cm 3 g −1 upon increasing the aging temperature from 50 to 120 °C. This reduction in the micropore volume is consistent with previously reported observations for SBA‐15 . The walls between the mesopores of SBA‐15 precursors consist of an interpenetrating network of the P123 template's ethylene oxide chains and silica.…”

“…The surface area then increased to 861 m 2 g −1 at 100 °C and decreased to 702 m 2 g −1 with a further increase in aging temperature to 120 °C. Contrasting trends for BET surface areas were observed in the literature for SBA‐15 with increase in aging temperature . Interestingly, in all these reports, materials aged at a temperature of ≈100 °C always exhibited the highest surface area as in the present investigation.…”

“…The small angle X‐ray diffraction patterns and N 2 adsorption isotherms (Figures S2 and S3) confirmed the successful synthesis of the two‐dimensional hexagonal lattice ( P 6 mm ) of SBA‐15 with diffraction peaks corresponding to (100), (110), and (200) reflections . A gradual shift of the (100) reflection towards lower angle was observed with aging temperature, implying an increase in d‐spacing.…”

“…(200) reflections. [44] Ag raduals hift of the (100) reflection towards lower angle was observed with aging temperature, implying an increase in d-spacing. The structure was retainedu pon functionalization with propyl sulfonic acid groups.…”

“…This reduction in the micropore volume is consistent with previously reported observations for SBA-15. [44] The walls between the mesopores of SBA-15p recursors consist of an interpenetrating network of the P123 template's ethylene oxide chains and silica. Upon calcination, the template is removed, and microporesa re formed in the mesopore walls.…”