Morphology and topology assessment in hierarchical zeolite materials: adsorption hysteresis, scanning behavior, and domain theory

Using a prototypical family of hierarchical zeolites, we show how adsorption-based characterization can be extended to provide morphological and topological assessment beyond state-of-the-art tools. The well-controlled materials under study consist of submicron-sized zeolite crystals (silicalite-1) that exhibit large nanoporous cavities in addition to their intrinsic microporosity. Such zeolites nanoboxes can be prepared with a single large cavity, with several independent cavities or with several interconnected cavities depending on synthesis conditions. It is shown that analysis of the adsorption/desorption branches using the Derjaguin model allows determining the cavity size distributions in these materials but also the fraction of pores directly connected to the external surface for each cavity size. Moreover, using the independent domain theory, we illustrate how scanning the capillary hysteresis provides a means to determine whether pores behave independently from each other or are connected to each other. All our findings are found to be consistent with additional electron microscopy data including electron tomography data. The multiscale porosity of hierarchical zeolite materials is analyzed through advanced adsorption-based characterization in conjunction with additional techniques including electron tomography.