Revisiting Edge-Sites of 𝜸-Al2O3 Using 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-Al2O3 remains limited to date. Recent contributions have pointed towards the potential role of highly reactive edge-sites contained in the high-field signal of the 1H-NMR spectrum of 𝛾 -Al2O3 materials. This work combines the development of needle-shaped 𝛾-Al2O3 nanocrystals having a high relative fraction of edge sites with the use of state of art solid-state NMR – 1H-1H Single-Quantum (SQ) Double-Quantum (DQ) and Arbitrary- Indirect-Dwell (AID) dipolar Heteronuclear Multiple Quantum Coherence (D-HMQC) – to significantly deepen our understanding of this specific signal. We identify two distinct hydroxyl sites which possess altered isotropic chemical shifts, different positions within the dipole-dipole network and distinct proximities to different aluminum surface sites. Moreover, the use of recoupling-time encoded D-HMQC data allows us to partially revise previous assignments of D- HMQC data of 𝛾-Al2O3 materials. While previous work has ascribed the high-field signal to be correlated to a single four-coordinate Al-site with substantial quadrupolar broadening we can identify the presence of two four-coordinate Al-sites with similar isotropic chemical shifts but different quadrupolar coupling constants. Recoupling-time-encoded data are thus able differentiate sites that would otherwise only be achievable with access to multiple fields or usage of highly advanced NMR techniques.