Proceedings of MATSUS Spring 2025 Conference (MATSUSSpring25)
Publication date: 16th December 2024
Metal oxo clusters are explored as an atomically defined building unit of nanocrystals where the metal atoms are connected by oxo bridges.[1] Here, we aim to investigate the catalytic activity of zirconium oxo clusters and compare them with nanocrystals through an esterification reaction. Several zirconium alkoxides, oxides and MOFs have been explored as esterification catalysts.[2] Here, we show that oxo clusters can be considered as an uprising catalyst which at times can also outperform nanocrystals and MOFs that have the same active site. The higher surface-to-volume ratio of oxo clusters compared to nanocrystals and their homogeneous nature make them an ideal candidate for further catalytic explorations. Looking into the turnover number (TON) per surface metal site reveals that the clusters are also intrinsically more catalytically active.
Focusing on cluster catalysts for large substrates, we obtain high conversion in solvent-free reactions, for sterically hindered alcohols (hexanol, 2-ethylhexanol, benzyl alcohol and neopentyl alcohol). We further expanded our research to hafnium oxo clusters, showing that they behave similarly to zirconium oxo clusters in catalytic reactivity (hexyl oleate ester formation).
The homogeneous cluster catalyst can be recovered and shows identical activity. The structural integrity of the cluster after catalysis is confirmed using Pair Distribution Function analysis (PDF), Nuclear Magnetic Resonance (NMR) spectroscopy and IR spectroscopy.[3] We have dived deep into zirconium-based esterification catalysis and found that when zirconium alkoxides are used as catalysts, the same oxo cluster is retrieved at the end of the reaction. These results thus shed light on the active catalytic species in homogeneous, zirconium-catalyzed esterification.[4]
The authors thank the SNF (51NF-40-205608, 194172 and 218106) for funding. T.N.P.V. thanks KU Leuven and Research Foundation Flanders (FWO) for funding. C.S. thanks the Research Foundation-Flanders (FWO) for the fellowship grant (68090/11C9320N). They thank Jonas Zurflüh for helping us with the analysis. This research used resources of beamline 28-ID-2 of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. They would like to thank Dr. Sanjit K. Ghose and Dr. Hui Zhong for their assistance. Beamtime was allocated for proposal number 313157. The authors also acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out using beamline P21.1 at DESY and they would like to thank Dr. Ann-Christin Dippel, Dr. Jiatu Liu, and Dr. Fernando Igoa for PDF acquisition (proposal I-20231114 EC). The authors also thank Prof. Dr. Konrad Tiefenbacher, Prof. Dr. Francisco De Azambuja, and Dr. Evert Dhaene for all the fruitful discussions.
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