Hydrogenation of α,β-unsaturated aldehydes over defective UiO-66 with Frustrated Lewis Pairs: Modulation of densities of defect sites via tailoring ligand-vacancies

In this work, defect-rich UiO-66-FA catalysts were synthesized using a modulator-induced defect formation strategy via tailoring ligand-vacancies and applied in the transfer hydrogenation of α,β-unsaturated aldehydes using cyclohexanol as the hydrogen source. The study reveals that the constructed surface defect in UiO-66 promotes the emergence of Zr-OH sites (Lewis base) and the neighboring unsaturated Zr4+ sites (Lewis acid) on the surface of UiO-66-FA, thereby generating solid Frustrated Lewis Pairs (FLPs) sites. Furthermore, the manipulation of the densities of defect sites can establish Zr4+/Zr-OH FLPs sites with improved activity and selectivity to unsaturated alcohols. The findings of this study indicate that the Zr4+/Zr-OH sites possess the ability to activate the CO of aldehydes and -OH of cyclohexanol, resulting in a smaller activation energy barrier and exceptional catalytic performance. Additionally, it is noteworthy that the defective UiO-66 also demonstrates outstanding hydrogenation activity in the direct hydrogenation of α,β-unsaturated aldehydes, with a mere 6.8 Kal/mol activation energy required for the dissociation of H-H bonds. The possible reaction mechanism was proposed based on in situ DRIFT analysis. This work is valuable for the design and preparation of unsaturated aldehydes/ketones hydrogenation catalysts and provides a new idea for the design of solid FLPs catalysts.