Selective Reversible Hydrolysis at Inequivalent Oxygen Sites Driven by Framework Al in MFI Zeolites Revealed by ¹⁷O...

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-O_I-Si species are located at channel intersections pointing to 10-membered ring (10-MR) channels (10-MR-O_I), whereas Si-O_II-Si and Si-O_III-Si species are oriented toward either 10-MR channels (10-MR-O_II/III) or small cavities formed by 5- and 6-membered rings (5/6-MR-O_II/III). Their interactions with water are strongly modulated by framework Al and temperature. In S-1, reversible hydrolysis occurs at 10-MR-O_I and 10-MR-O_II/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-O_II/III sites, conferring pronounced selectivity for 5/6-MR-O_II/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-O_I and 10-MR-O_II/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-O_II/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.