Noninvasive Nanoscopy Uncovers the Impact of the Hierarchical Porous Structure on the Catalytic Activity of Single Dealuminated Mordenite Crystals

Abstract

Spatial restrictions around catalytic sites, provided by molecular-sized micropores, are beneficial to reaction selectivity but also inherently limit diffusion. The molecular transport can be enhanced by introducing meso- and macropores. However, the impact of this extraframework porosity on the local nanoscale reactivity is relatively unexplored. Herein we show that the area of enhanced reactivity in hierarchical zeolite, examined with super-resolution fluorescence microscopy, is spatially restricted to narrow zones around meso- and macropores, as observed with focused ion-beam-assisted scanning electron microscopy. This comparison indicates that reagent molecules efficiently reach catalytic active sites only in the micropores surrounding extraframework porosity and that extensive macroporosity does not warrant optimal reactivity distribution throughout a hierarchical porous zeolite. Super resolution for a super framework: The nanoscale reactivity pattern in dealuminated mordenite crystals, recorded with super-resolution fluorescence microscopy, closely matches the extraframework porous network imaged with focused ion-beam-assisted scanning electron microscopy. Reagent molecules can only efficiently reach catalytic sites located in micropores in the zone of only a few hundred nanometers around the meso- and macropores.