Caveat on the Actual Robustness of Heteronuclear NMR Methods for Probing the Surface of γ-Alumina and Related Catalysts
2019
Szeto, Kai C. | Merle, Nicolas | Trébosc, Julien | Taoufik, Mostafa | Gauvin, Régis M. | Pourpoint, Frédérique | Delevoye, Laurent
Transitional aluminas are a very important class of materials in catalysis. The detailed understanding of their surface chemistry has attracted much attention with input from spectroscopic and theoretical studies. In this context, NMR is a key technique, which, thanks to recent methodological developments, provides insights into local structures and relative arrangements. In the present paper, we first assess two ¹H–²⁷Al heteronuclear correlation NMR methods for studying the surface of γ-Al₂O₃ dehydroxylated at 500 °C. The commonly used cross-polarization (CP) transfer method is compared to the dipolar heteronuclear multiple quantum coherence (D-HMQC) experiment, revealing that the latter is significantly more robust and efficient with less signal distortion that hampers the recording of meaningful signals. Furthermore, a new MQ-D-HMQC pulse sequence is proposed to confirm the presence of sites, with a large quadrupolar coupling constant at the surface of alumina. The robustness of D-HMQC is further probed by two additional alumina-based catalysts, namely, aluminum hydrides supported on γ-Al₂O₃, which are active in olefin hydrogenation and polymerization, and ZrCp₂Me₂ grafted onto γ-Al₂O₃, a single-site olefin polymerization catalyst. In both cases, this provides information on the configuration of surface aluminum species in the vicinity of catalytically active sites and on the residual hydroxyl networks. The present study combines advanced NMR methods and complex catalytic materials for which a critical, yet fruitful, spectroscopic approach is required for deeper understanding.
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