Insight into the atomic-level structure of γ-alumina using a multinuclear NMR crystallographic approach

Abstract

The combination of multinuclear NMR spectroscopy with ¹⁷O isotopic enrichment and DFT calculations provided detailed insight into both the bulk and surface structure of γ-Al₂O₃. Comparison of experimental ¹⁷O NMR spectra to computational predictions confirmed that bulk γ-Al₂O₃ contains Al cations primarily in “spinel-like” sites, with roughly equal numbers of alternating Al^VI and Al^IV vacancies in disordered “chains”. The work showed that overlap of signals from O^IV and O^III species complicates detailed spectral analysis and highlighted potential problems with previous work where structural conclusions are based on an unambiguous assignment (and quantification) of these signals. There was no evidence for the presence of H, or for any significant levels of O vacancies, in the bulk structure of γ-Al₂O₃. Computational predictions from structural models for different surfaces showed a wide variety of protonated and non-protonated O species occur. Assignment of signals for two types of protonated O species was achieved using variable temperature CP and TRAPDOR experiments, with the sharper and broader resonances attributed to more accessible surface sites that interact more strongly with water and less accessible aluminols, respectively. DFT-predicted ¹H NMR parameters confirmed the ¹H shift increases with denticity but is also dependent on the coordination number of the next nearest neighbour Al species. Spectral assignments were also supported by ¹H–²⁷Al RESPDOR experiments, which identified spectral components resulting from μ₁, μ₂ and μ₃ aluminols. Combining these with ¹H–²⁷Al D-HMQC experiments showed that (i) μ₁ aluminols are more likely to be bound to Al^IV, (ii) μ₂ aluminols are coordinated to all three types of Al, but with a higher proportion bound to similar types of Al and (iii) μ₃ aluminols are most likely bound to higher coordinated Al species. ¹H DQ MAS spectroscopy confirmed no aluminols exist exclusively in isolation but showed that the closest proximities are between bridging aluminols coordinated to Al^IV and/or Al^V species.