Indirectly Detected DNP-Enhanced 17O NMR Spectroscopy: Observation of Non-Protonated Near-Surface Oxygen at Naturally Abundant Silica and Silica-Alumina

Recent studies have shown that dynamic nuclear polarization (DNP) can be used to detect O-17 solid-state NMR spectra of naturally abundant samples within a reasonable experimental time. Observations using indirect DNP, which relies on H-1 mediation in transferring electron hyperpolarization to O-17, are currently limited mostly to hydroxyls. Direct DNP schemes can hyperpolarize non-protonated oxygen near the radicals; however, they generally offer much lower signal enhancements. In this study, we demonstrate the detection of signals from non-protonated O-17 in materials containing silicon. The sensitivity boost that made the experiment possible originates from three sources: indirect DNP excitation of Si-29 via protons, indirect detection of O-17 through Si-29 nuclei using two-dimensional Si-29{O-17} D-HMQC, and Carr-Purcell-Meiboom-Gill refocusing of Si-29 magnetization during acquisition. This Si-29-detected scheme enabled, for the first time, 2D O-17-Si-29 heteronuclear correlation spectroscopy in mesoporous silica and silica-alumina surfaces at natural abundance. In contrast to the silanols showing motion-averaged O-17 signals, the framework oxygens exhibit unperturbed powder patterns as unambiguous fingerprints of surface sites. Along with hydroxyl oxygens, detection of these moieties will help in gaining more atomistic-scale insights into surface chemistry.

Automated summary

Recent studies have shown the potential of using dynamic nuclear polarization (DNP) to detect O-17 solid-state NMR spectra of naturally abundant samples within a reasonable experimental time. This study successfully demonstrated the detection of signals from non-protonated O-17 in materials containing silicon, utilizing a Si-29-detected scheme to achieve 2D O-17-Si-29 heteronuclear correlation spectroscopy in mesoporous silica and silica-alumina surfaces at natural abundance. The sensitivity boost in this experiment came from indirect DNP excitation of Si-29 via protons, indirect detection of O-17 through Si-29 nuclei using two-dimensional Si-29{O-17} D-HMQC, and Carr-Purcell-Meiboom-Gill refocusing of Si-29 magnetization during acquisition.