Adsorbate-Induced Reversible Changes in Zeolite ZEO-5 Attributed to its Triple-Four-Silicate-Rings

Extra-large pore zeolites exhibit structural features distinct from those of classical zeolites, with potential consequences in their use as adsorbents and catalysts. The lowest-framework-density known zeolite, ZEO-5, was synthesized via interchain expansion, forming unprecedented triple four-ring (t4r) units, creating a fully connected framework with 20-membered-ring pores. Here, we report that ZEO-5 exhibits unique water adsorption behavior. Initially hydrophobic, it undergoes a sharp increase in water uptake within a narrow range of relative pressure, transitioning into a hydrophilic status, with a pronounced desorption hysteresis. Characterization by powder X-ray diffraction, porosimetry, in situ infrared spectroscopy, and solid-state nuclear magnetic resonance reveals structural degradation via Si-O-Si bond cleavage within the highly strained t4r unit. Remarkably, upon re-calcination, the original structure of ZEO-5, including its t4r units, is fully restored, establishing a reversible adsorption-induced order-disorder structural transformation. Similar behavior occurs with other polar molecules, including ammonia and alcohols, underscoring the broader implications of this ZEO-5 feature for adsorptive separations and for pore functionalization. For example, ZEO-5 at 423K, exhibits high ammonia working capacity between 11 and 1.1 bar adsorption and desorption pressures, respectively, surpassing the corresponding performance of commercial aluminosilicate zeolites. Structure models, consistent with experimental observations, are proposed to explain this phenomenon.