Primostrato Solid-State NMR Enhanced by Dynamic Nuclear Polarization: Pentacoordinated Al3+ Ions Are Only Located at the Surface of Hydrated γ-Alumina

Aluminas (Al2O3) are ubiquitous functional materials. In particular, the gamma-alumina form is extensively used in research and industry as a catalyst and catalyst support. Nevertheles, a fill structural description, which would aid in comprehension of its properties, is lacking and under large debate. Solid-state NMR has been used previously to study gamma-alumina but is limited for certain applications, such as surface studies, due to intrinsic low sensitivity. Here, we detail the implementation of low temperature (similar to 100 K) magic angle spinning combined with dynamic nuclear polarization (MAS-DNP) to significantly enhance the sensitivity of solid-state NMR experiments and gain strucutral insights into this important material. Notabley we analyze hydrophilic and hydrophobic sample preparation protocols and their implications on the sample and resulting NMR parameters. We show that the choice of preparation does not perturb the spectrum, but it does have a large effect on NMR coherence lifetimes, as does the corresponding required (hyper) polarizing agent. We use this preliminary study to optimize the absolute sensitivity of the following experiments. We then show that there are not detectable hydroxyl groups in this bulk of the material and that DNP-enhanced H-1 -> (27)A1 cross polarization experiments are selective to only the first surface layer, enabling a very specific study. this primostrato NMR is integrated with multiple-quantum magic angle spinning (MQMAS) and it is demonstrated interestingly, that pentacoordinated Al3+ ions are only observed in this first surface layer. To highlight that there is no evidence of subsurface pentacoordinated Al3+ a new bulk filtered experiment is described that can eliminate surface signals.