UiO-66 metal-organic frameworks as aldol condensation catalyst: Impact of defects, solvent, functionality on the catalytic activity and selectivity

Abstract

Defected and amine-loaded zirconium-based metal–organic frameworks (MOF) UiO-66(Zr) have been characterized and chemically assessed in a condensation reaction of acetone and 4-nitrobenzaldehyde. The present work focuses primarily on the production of the aldol adduct as the main product while minimizing the formation of the enone adduct as a side product. Four analogues with varying numbers of missing linkers in the hexanuclear secondary building units, from one to four out of a total of 12, were synthesized. Thorough characterization showed that an increased number of defects results in an enhanced acidity, a higher internal surface area, and an increased accessibility of coordinatively unsaturated Zr-sites. The catalytic performance assessment revealed that defects, to a certain extent, positively affect the selectivity towards the aldol adduct, i,e. the catalyst, which is missing 3 of the 12 linkers, exhibits the highest activity in a solvent-free environment with a turnover frequency (TOF) amounting to 8.03 10-3 s−1. This TOF value is approximately double of the catalyst with 1.4 defects, which exhibited a TOF of only 4.41 10-3 s−1. Excluding more than three linkers more likely leads to 'missing cluster defects' and causes a decrease in the reaction rate towards the aldol adduct. A reduction in overall catalytic activity was observed upon functionalization with amine groups (UiO-66-NH2). Functionalization with L-proline (UiO-66Lpr) resulted in a shift from an acid to a basic reaction mechanism which was reflected by an improved selectivity towards the aldol adduct while maintaining a reasonable overall activity level. The best performances were obtained working solvent-free or with hexane as a solvent, while polar/semi-polar solvents hindered the catalyst performance. An 18-hour stability test demonstrated the robustness of defected and functionalized UiO-66.