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

Abstract

The surface hydroxyl groups of γ-alumina dehydroxylated at 500 °C were studied by a combination of one- and two-dimensional homo- and heteronuclear (1)H and (27)Al NMR spectroscopy at high magnetic field. In particular, by harnessing (1)H-(27) Al 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 (1)H magic-angle spinning (MAS) NMR spectrum was demonstrated thanks to (1)H-(27) Al RESPDOR (resonance-echo saturation-pulse double-resonance). In a further step the hydroxyl groups were assigned to their aluminium neighbours thanks to a {(1)H}-(27) Al dipolar heteronuclear multiple quantum correlation (D-HMQC), which was used to establish a first coordination map. Then, in combination with (1)H-(1) H 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.

Flash Catalytic Pyrolysis of Polyethylene over (Alumino)silicate Materials

“…Presence of Al−OH linkages was then monitored with the editing of 2D 1 H/ 27 Al correlation maps. As previously reported, the correlation signals observed here are signature of Al−OH linkages owing to the very short recoupling time used in the pulse sequence . The 2D map recorded on the Si/Al sample indicates that the three Al species are involved in Al−OH site but comparison between the 1D and the 2D Map 27Al projection (Figure , and horizontal axis of Figure a) suggests that Al sites are less involved in the Al/H interaction than the two higher coordination states.…”

Probing Interactions of γ‐Alumina with Water via Multinuclear Solid‐State NMR Spectroscopy

“…Three peaks at 2.0, 0.9 and −0.1 ppm arising from different hydroxyl species on the surface of γ‐alumina nanorods can be observed in 1 H MAS NMR spectrum of the dehydrated sample (Figure a). Previous reports show that the chemical shifts are dependent on the number of Al ion connected to the hydroxyl groups . The signals (2.0 and 0.9 ppm) with higher frequencies can be assigned to two types of doubly bridging hydroxyl groups (μ 2 ‐OH), while the lower frequency resonance (−0.1 ppm) should arise from an isolated hydroxyl group (μ 1 ‐OH).…”

“…Therefore, most of NMR studies focus on 1 H, which uniquely originate from the surface as hydroxyl groups and / or adsorbed water. Consequently, surface‐selection can be achieved by correlating other nuclei with surface 1 H species . Surface‐selective 17 O labelling is also able to assist surface‐selection for 17 O NMR studies of oxides, however, few results have been reported on alumina .…”

Natural Abundance 17O DNP NMR Provides Precise O−H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts

“…[11] In silica-alumina, all four types can be present, along with an additional bridging type (Al(OH)Si), which is typically credited for the Brønsted acidity of the material. [11] In silica-alumina, all four types can be present, along with an additional bridging type (Al(OH)Si), which is typically credited for the Brønsted acidity of the material.…”