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

Abstract : 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, some of these (hierarchical forms, nanoslabs, nanosheets and nanocrystals) exhibiting a very high external surface over 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-5 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 similar or higher stability with respect to 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-(H 2 O)(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 bridge is not present below the surface aluminum atom, desorption leads to a less stable surface Al III atom. The desorp-tion 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 Brønsted 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 mild Brønsted acid sites Al-(H 2 O) instead.
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Laureline Treps, Axel Gomez, Theodorus de Bruin, Céline Chizallet. Environment, Stability and Acidity of External Surface Sites of Silicalite-1 and ZSM-5 Micro- and Nano-Slabs, -Sheets and -Crystals. ACS Catalysis, American Chemical Society, 2020, 10 (5), pp.3297-3312. ⟨10.1021/acscatal.9b05103⟩. ⟨hal-02552741⟩



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