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, 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−(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 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 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−(H 2 O) instead.

The nature and spectroscopic expression of external surface sites of zeolites, in particular ZSM-5, is a long-debated question. Herein, we use three cutting-edge techniques: Fouriertransform infrared spectroscopy (FTIR) with Fourier selfdeconvolution (FSD), high magnetic field proton NMR spectroscopy under fast magic-angle spinning (MAS) and periodic boundary Density Functional Theory (DFT) calculations to study external surface models and analyze the effect of crystallite size. This provides an unequaled description of the various kinds of hydroxyl groups and of their proximities. The hydrogen-bond donor, acceptor or isolated nature of the hydroxyls results in distinct signals both in FTIR and NMR spectra, but the peak assignment is not the same from one technique to the other when the chemical nature of the hydroxyl changes. Bridging Si−(OH)− Al groups and Al−(H 2 O) lead to overlapping signals in one-dimensional 1 H MAS NMR, whereas their contributions are strongly different in FTIR spectra. However, quantification and proximity assessment could only be obtained by 1 H MAS NMR. With DFT, we confirm previous assignments for silanols and Si−(OH)−Al bridging OH groups. Other signals (between 3750 and 3600 cm −1 , and between 1 and 4 ppm) are not only assigned to extra-framework species (which we confirm with dedicated models), but also enclose the signature of sites exposed at the external surface of ZSM-5. In particular, Al−(H 2 O) species (∼3665 cm −1 ; 3.8, 2.6 ppm) and silanol−Al (∼3740, 3720, 3665 cm −1 ; 2.6, 2.2 ppm) contribute to several features depending on their environment. μ 1 -Al−OH are also present at the external surface in low amount, with a 3780 cm −1 signal in IR, and weak signals in the 0−2 ppm interval in 1 H MAS NMR.

In EPOC the adsorption energies of the reactants and products are modified by applying an electrochemical potential to the catalyst. DFT computations unveil that the C–C and O–O bond dissociation are accelerated under positive and negative potential, respectively.