The central role of oxo clusters in zirconium- and hafnium-based esterification catalysis.

Oxo clusters are a unique link between oxide nanocrystals and MOFs, representing the limit of downscaling each of the respective crystals. Here, we show the superior catalytic activity of Zr12O8(OH)8(OOCR)24 clusters, compared to zirconium MOF UiO-66 and ZrO2 nanocrystals. We focus on esterification reactions given their general importance in consumer products and the challenge of converting large substrates. Oxo clusters have a higher surface-to-volume ratio than nanocrystals, rendering them more active. For large substrates, e.g., oleic acid, MOF UiO-66 has negligible catalytic activity while clusters provide almost quantitative conversion, a fact we ascribe to limited diffusion of large substrates through the MOF pores. Cluster do not suffer from limited mass transfer and we also obtain high conversion in solvent-free reactions with sterically hindered alcohols (hexanol, 2-ethylhexanol, benzyl alcohol and neopentyl alcohol). The cluster catalyst can be recovered and shows identical activity. The structural integrity of the cluster is confirmed using X-ray total scattering and Pair Distribution Function analysis. Even more, when homogeneous zirconium (or hafnium) alkoxides are used as catalyst, the same oxo cluster is retrieved, showing that oxo clusters are the active catalytic species, even in previously assumed homogeneously catalyzed reactions.