Chapter One - Solid-state NMR studies on pentacoordinated Al species in alumina and related materials: Nature of acid sites and anchoring active metal species

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Alumina and related materials are popular catalysts and supports in various key industrial chemical processes. Unsaturated pentacoordinated Al (AlV) species has been proposed either for Lewis acid sites (LASs) on alumina or for Brønsted acid sites (BASs) formed on alumina-based mixed oxides, as well as key sites for anchoring active metal species. However, the lack of surface AlV species and involved in strong quadrupolar interaction hampers the detailed characterization of AlV species by conventional techniques, such as infrared spectroscopy and 1D SSNMR techniques. This review introduces a comprehensive utilization of various advance solid-state NMR techniques to clarify the nature of AlV species in the local structure and their functions in the formation of acid sites and anchoring active metal species, including heteronuclear/homonuclear correlation and dynamic nuclear polarization surface-enhanced NMR spectroscopy, ultra-high-field NMR techniques. Through NMR studies on the surface hydroxyl groups and the Al coordination transfer, the Lewis acidic AlV species are found to be generated from AlVI involved in the double- and triple-bridged hydroxyl groups in spatial proximity upon dehydration for γ-Al2O3. By combining these NMR techniques with probe molecules, new BASs with versatile acid strength and structures are evidenced in AlV-enriched alumina mixed oxides, which opens a potential way to promote the BAS acidity of alumina-based mixed oxides for tailoring high-performance catalysts. Moreover, AlV as key sites to stabilize various active metal species are identified on alumina and related materials and the mechanism has been uncovered through comprehensive NMR investigations. These studies demonstrates that solid-state NMR provide a state-of-the-art strategy to investigate the coordination, distribution and interaction of Al in the local structure and their contribution in the formation of surface acid sites, which can be extended to study other materials.