Revisiting Edge Sites of γ-Al₂O₃Using Needle-Shaped Nanocrystals and Recoupling-Time-Encoded {²⁷Al}-¹H D-HMQC NMR Spectroscopy

Despite being widely used in numerous catalytic applications, our understanding of reactive surface sites of high-surface-area γ-Al₂O₃remains limited to date. Recent contributions have pointed toward the potential role of highly reactive edge sites contained in the high-field signal (-0.5 to 0 ppm) of the ¹H NMR spectrum of γ-Al₂O₃materials. This work combines the development of well-defined, needle-shaped γ-Al₂O₃nanocrystals having a high relative fraction of edge sites with the use of state-of-the-art solid-state NMR to significantly deepen our understanding of this specific signal. We are able to resolve two hydroxyl sites with distinct isotropic chemical shifts of -0.2 and -0.4 ppm and different positions within the dipole-dipole network from ¹H-¹H single-quantum double-quantum NMR. Moreover, the use of recoupling-time-encoded arbitrary-indirect-dwell dipolar heteronuclear multiple quantum coherence allows us to partially revise previous assignments for surface-aluminum sites in the proximity of these hydroxyl sites. Although previous work has ascribed the high-field signal to be correlated with a single four-coordinate Al-site with a substantial quadrupolar broadening of >10 MHz, we can identify the presence of two four-coordinate Al-sites with similar isotropic chemical shifts but different quadrupolar coupling constants of approximately 7 and >10 MHz, respectively. Recoupling-time-encoded data are thus able to differentiate sites that would otherwise only be achievable with access to multiple fields or usage of highly advanced NMR techniques.