Structure and Dynamics of Hydrous Surface Species on Alumina−Boria Catalysts and Their Precursors from 1H, 2H, 11B, and 27Al MAS NMR Spectroscopy

The hydrous surface of a pseudoboehmite precursor impregnated with 1.3 wt % boron, its dehydration upon gentle heat treatment, and the corresponding alumina boria catalyst obtained by calcination at 750 °C have been studied by 1 H and 2 H MAS NMR as well as double-resonance 1 H{ 11 B}, 1 H{ 27 Al} TRAPDOR, and 11 B{ 1 H}, 27 Al{ 1 H} CP/MAS NMR experiments. Resonances from bulk and surface AlOH groups, surface BOH sites, and water molecules adsorbed on the surface-aluminate and -borate species have been identified and characterized. The dynamics of the water molecules and their removal from the surface by heat treatment have been studied by the double-resonance experiments. Optimizations of the 1 H − 11 B/ 27 Al cross-polarization conditions for the individual 11 B and 27 Al sites demonstrate that the BO 4 as well as the BO 3 sites on the alumina surface exhibit hydroxyl groups in their near vicinity. The same holds for the AlO 4, AlO 5, and AlO 6 sites on the γ-Al 2 O 3 surface of the alumina−boria catalyst.