cs9b05103_si_002.pdf (1.47 MB)
Environment, Stability and Acidity of External Surface Sites of Silicalite‑1 and ZSM‑5 Micro and Nano Slabs, Sheets, and Crystals
Version 2 2021-01-07, 19:03
Version 1 2020-02-21, 20:44
journal contribution
posted on 2021-01-07, 19:03 authored by Laureline Treps, Axel Gomez, Theodorus de Bruin, Céline ChizalletZeolites
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, nanosheets, 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-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 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
AlIII 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 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 the mild Brønsted acid sites Al–(H2O) instead.