Heteronuclear NMR Spectroscopy as a Surface-Selective Technique: A Unique Look at the Hydroxyl Groups of γ-Alumina

The surface hydroxyl groups of -alumina dehydroxylated at 500 degrees C were studied by a combination of one- and two-dimensional homo- and heteronuclear H-1 and Al-27 NMR spectroscopy at high magnetic field. In particular, by harnessing H-1-Al-27 dipolar interactions, a high selectivity was achieved in unveiling the topology of the alumina surface. The terminal versus bridging character of the hydroxyl groups observed in the H-1 magic-angle spinning (MAS) NMR spectrum was demonstrated thanks to H-1-Al-27 RESPDOR (resonance-echo saturation-pulse double-resonance). In a further step the hydroxyl groups were assigned to their aluminium neighbours thanks to a {H-1}-Al-27 dipolar heteronuclear multiple quantum correlation (D-HMQC), which was used to establish a first coordination map. Then, in combination with H-1-H-1 double quantum (DQ) MAS, these elements helped to reveal intimate structural features of the surface hydroxyls. Finally, the nature of a peculiar reactive hydroxyl group was demonstrated following this methodology in the case of CO2 reactivity with alumina.