Selective Reversible Hydrolysis at Inequivalent Oxygen Sites Driven by Framework Al in MFI Zeolites Revealed by 17 O NMR Spectroscopy and DFT Calculations

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, 17 O-labeled T–O–T bonds are employed as site-specific probes, combined with 17 O 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 17 O 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 17 O-based probing strategy offers an efficient approach to elucidate reversible hydrolysis at inequivalent oxygen sites, contributing to the rational design of hydrothermally stable zeolites.