External Surface of Zeolite Imidazolate Frameworks Viewed Ab Initio: Multifunctionality at the Organic-Inorganic Interface

The external surface sites of zinc based zeolite Imidazolate frameworks (ZIFs) is investigated by cluster density functional theory calculations, which reveal a great variety of sites. Their stability as a function of temperature and pressure is resolved, thanks to a thermodynamic approach. Low-coordinated zinc cations Zn-II and Zn-III, which can play the role of Lewis acid sites, are stabilized over a wide temperature range. Bronsted acid sites (NH groups), basic sites (N- extremities), as well as OH groups and hydrogenocarbonates (monodentate, mainly, but also bidentate) are also stable. Their calculated vibrational feature is consistent with high-frequency vibrations observed in experimental spectra of ZIF-8 materials reported in the literature. This coexistence of groups of various expected reactivity opens perspectives to multifunctional catalysis at the external surface of ZIF-8.