Environment, Stability and Acidity of External Surface Sites of Silicalite-1 and ZSM-5 Micro and Nano Slabs, Sheets, and Crystals

Zeolites are nanoporous aluminosilicate crystals of prominent fundamental and industrial importance. Among these, ZSM-5 is the most investigated solid that can be obtained in various forms, with some of these (hierarchical forms, nanoslabs, nano-sheets, and nanocrystals) exhibiting a very high external surface to volume ratio. Whereas most knowledge obtained so far at the atomic level concerns the internal nanopores, we here propose a density functional theory (DFT) study to establish the relative stability of relevant surface orientations for silicalite and ZSM-S crystals ((100), (010), and (101)) at different hydration levels to identify the equilibrium morphology of the particles and the major sites present on their surfaces. Several kinds of surface sites have been identified. Bridging Al-OH-Si groups are present at the pore mouth with stability similar to or higher than those in bulk sites. Yet, these groups are not stable at the outermost surface, where the following groups prevail: Si-OH, Al-OH groups, and most importantly water adsorbed on aluminum, Al-(H2O)(OH)(n). Water desorption reactions occur at temperatures that strongly depend on the local topology of the surface site and on the surface orientation: when a siloxane bridge is present below the surface Al atom, water desorption is promoted by the formation of an additional Al-O bond with the oxygen of the siloxane bridge. However, if such a bridge is not present below the surface aluminum atom, desorption leads to a less stable surface Al-III atom. The desorption temperature is influenced by this feature, as well as by the stabilization of the water molecule by hydrogen bonds, depending on the silanol content of the surface. This has in turn direct consequences on the Bronsted and Lewis acid properties, as probed by pyridine. Strong Lewis acid sites can easily be formed on the (010) orientation (relevant for nanosheets), whereas they are unlikely to occur at the (101) surface (tips of coffin-shaped particles), which promotes the mild Bronsted acid sites Al-(H2O) instead.