The Central Role of Oxo Clusters in Zirconium-Based Esterification Catalysis

Oxo clusters are a unique link between oxide nanocrystals and Metal-Organic Frameworks (MOFs), representing the limit of downscaling each of the respective crystals. Herein, the superior catalytic activity of $$ clusters, compared to zirconium MOF UiO-66 and $$ nanocrystals is shown. Focus is 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, for example, 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. Clusters do not suffer from limited mass transfer and we also obtain high conversion in solvent-free reactions with sterically hindered alcohols (hexanol, 2-ethyl hexanol, benzyl alcohol, and neopentyl alcohol). The cluster catalyst can be recovered and shows identical activity when reused. The structural integrity of the cluster is confirmed using X-ray total scattering and pair distribution function analysis. Moreover, when homogeneous zirconium alkoxides are used as catalysts, the same oxo cluster is retrieved, showing that oxo clusters are the active catalytic species, even in previously assumed homogeneously catalyzed reactions.