Revisiting Edge-Sites of 𝜸-Al2O3 Using Needle- Shaped Nanocrystals and Recoupling-Time Encoded {27Al}-1H D-HMQC NMR Spectroscopy

04 February 2022, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


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.


NMR Spectroscopy
Heterogeneous Catalysis

Supplementary materials

Supplementary Information
Experimental details on materials preparation and solid-state NMR measurements, additional TEM data, additional NMR spectra, numerical values for line shape analysis


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