High‐Field NMR, Reactivity, and DFT Modeling Reveal the γ‐Al2O3 Surface Hydroxyl Network

Abstract Aluminas are strategic materials used in many major industrial processes, either as catalyst supports or as catalysts in their own right. The transition alumina γ‐Al2O3 is a privileged support, whose reactivity can be tuned by thermal activation. This study provides a qualitative and quantitative assessment of the hydroxyl groups present on the surface of γ‐Al2O3 at three different dehydroxylation temperatures. The principal [AlOH] configurations are identified and described in unprecedented detail at the molecular level. The structures were established by combining information from high‐field 1H and 27Al solid‐state NMR, IR spectroscopy and DFT calculations, as well as selective reactivity studies. Finally, the relationship between the hydroxyl structures and the molecular‐level structures of the active sites in catalytic alkane metathesis is discussed.


General conditions
The -alumina (E7102 from Rhône-Poulenc) was first calcined at 500 °C under a N2/O2 flow overnight, then hydrated and heated at 100 °C. In a further step, the alumina was treated at the desired temperature (300, 500 or 700 °C) under high vacuum (10 -5 Torr) for 12 hours. After this treatment, it was stored and handled in an argon-filled glove box. The specific surface area measured by BET (N2) was 200 m 2 .g -1 . The XRD spectrum ( Figure S1) confirms the structural assignment as -Al2O3, in comparison with the diffraction pattern, and matches that of the reference 00-029-0063 (-Al2O3) from ICDD (International Center for Diffraction Data).
Al2O3-700-CO2 : CO2 adsorption on Al2O3-700 was carried out at room temperature in a 500 ml Schlenk-type glass reactor. CO2 (250 torr) was introduced in the reactor containing about 100 mg of Al2O3-700 through a high vacuum line. Excess of CO2 was removed under high vacuum at room temperature after 100 minutes of reaction. All the samples were stored under argon in a glovebox at ambient temperature.
Al2O3-700-Cl: chlorination of Al2O3-700 was performed following a modified method described by Kytökivi. [2] A 1 g sample of Al2O3-700 previously calcined was treated at 700 °C under high vacuum (10 -5   pulse of 2.75 μs on either side of the 27 Al π pulse. A recycling delay of 2 s was used. The SR4 1 2 dipolar recoupling scheme [3] was applied for 500 μs. The effect of dipolar recoupling time was probed, showing that optimal results were obtained with values between 500 μs and 1 ms. We chose 500 µs as a compromise between signal intensity and better selectivity towards first coordination sphere contribution. Longer recoupling time led to increased bulk signal, which is detrimental to spectral interpretation. Two-dimensional (2D) 1 H− 1 H double-quantum singlequantum magic-angle spinning (DQSQ MAS) experiments were performed at 20 kHz spinning speed using the 12 2 5 symmetry-based recoupling scheme, [4] applied for 190 μs at a radio frequency field strength of 40 kHz. A total of 16 or 32 transients were summed for each of the 100 t1 increments, with a recycle delay of up to 100 s (for Al2O3-300, Al2O3-500 and Al2O3-700).     S9 Figure S7. Evolution of the hydroxyl elongation region of the infrared spectrum of Al2O3-300 (a), Al2O3-500 (b) and Al2O3-700 (c), along with the corresponding assignments for sites A, C and F. Table S1. Evolution of 1 H NMR chemical shift for the different sites of Al2O3-300, Al2O3-500 and Al2O3-700 (from Figure S4).

Computational details
DFT-based simulations were performed with the CP2K/Quickstep package, using a hybrid Gaussian and plane wave method. [5] A double quality DZVP Gaussian basis set was employed for all atoms. The Goedecker-Teter-Hutter [6] pseudopotentials together with a 400 Ry plane wave cutoff were used to expand the densities obtained with the Perdew-Burke-Ernzerhof (PBE) [7] exchange-correlation density functional. Molecular graphics were produced by the CHEMCRAFT graphical package. [8] NMR chemical shifts and the electric field gradient was evaluated by the GIPAW module within the quantum expresso package. [9,10] The nuclear electric quadrupole moment for oxygen and hydrogen atoms are taken from Stone. [11] For 27 Al we use a Q value of -8.0178 x 10 -30 m 2 . [12] Surfaces model. The -alumina bulk model used in this study is taken from theoretical investigations of Digne et al. [13] The structure ( Figure S9, left) contains a fcc sublattice of oxide ions that generates octahedral and tetrahedral interstices which accommodate aluminum ions.  Figure S9, right) while bulk tetrahedral Al centers become pseudo-trigonally planar on the surface (AlIII in Figure   S9, right). Therefore, the surface O ions are found to have either 3-O and 2-O geometries.
The 3-O species are bound to AlIII and AlIV surface ions and to the octahedral AlVI bulk ion, while the 2-O species are bound to the AlIV and to the octahedral AlVI bulk ions.
From experimental data the hydroxyl density of alumina annealed at 700°C is about 1.1 OH/nm 2 . Then, simulations involve the absorption of either one or two water molecules on the modelled surface. In fact, with one adsorbed water molecule we obtain a 0.7 OH/nm 2 value of the hydroxyl density and with two water molecules we obtain a 1.5 OH/nm 2 value of the hydroxyl density.