Water-zeolite interactions have long been a central focus in zeolite science, yet the site-specific behavior of water at inequivalent oxygen atoms remains poorly understood. Herein, 17O-labeled T-O-T bonds are employed as site-specific probes, combined with 17O NMR spectroscopy and DFT calculations to uncover framework Al-induced differences in water interactions at inequivalent oxygen atoms in Silicalite-1 (S-1) and HZSM-5 zeolites. Using 17O MQMAS NMR, three inequivalent Si-O-Si species, segregated in distinct regions with different environments, are identified. Si-OI-Si species are located at channel intersections pointing to 10-membered ring (10-MR) channels (10-MR-OI), whereas Si-OII-Si and Si-OIII-Si species are oriented toward either 10-MR channels (10-MR-OII/III) or small cavities formed by 5- and 6-membered rings (5/6-MR-OII/III). Their interactions with water are strongly modulated by framework Al and temperature. In S-1, reversible hydrolysis occurs at 10-MR-OI and 10-MR-OII/III sites at 473 K, whereas site selectivity vanishes at 773 K. In contrast, in HZSM-5, framework Al induces preferential hydrolysis at 5/6-MR-OII/III sites, conferring pronounced selectivity for 5/6-MR-OII/III sites at room temperature and enabling uniform hydrolysis across all oxygen sites at 473 K. DFT calculations reveal that water enrichment at Brønsted acid sites limits access to 10-MR-OI and 10-MR-OII/III species, while reversible breaking and re-forming of Si-O-Al bonds facilitates water entry into small cavities, reducing the energy barrier for hydrolysis of 5/6-MR-OII/III species. Moreover, coke deposition weakens water-framework oxygen interactions, partially protecting the framework against hydrolysis. This 17O-based probing strategy offers an efficient approach to elucidate reversible hydrolysis at inequivalent oxygen sites, contributing to the rational design of hydrothermally stable zeolites.
He et al. (Fri,) studied this question.