Revisiting Edge Sites of γ-Al <sub>2</sub> O <sub>3</sub> Using Needle-Shaped Nanocrystals and Recoupling-Time-Encoded { <sup>27</sup> Al}- <sup>1</sup> H D-HMQC NMR Spectroscopy
Laura A. Vӧlker, Jordan Meyet, Zachariah J. Berkson, Lukas Rochlitz, Jeroen A. van Bokhoven, Christophe Copéret
Abstract
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 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-Al2O3 materials. This work combines the development of well-defined, needle-shaped γ-Al2O3 nanocrystals 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.