Highly Efficient Hydrogenation of α,β-Unsaturated Aldehydes to Unsaturated Alcohols over Defective MOF-808 with Constructed Frustrated Lewis Pairs

Solid frustrated Lewis pair (FLP) catalysts have received much attention. In this work, MOF-808 with rich defects was constructed using a facile monocarboxylic acid modulator-induced defect strategy and applied in the transfer hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohol using cyclohexanol as the hydrogen source. MOF-808 was prepared from different zirconium precursors, and it was found that MOF-808-ZT obtained from ZrCl4 and H3BTC forms abundant surface hydroxyl groups (Zr–OH, base sites). Moreover, MOF-808-PA with missing linker defects was constructed by introducing monocarboxylic acid to preoccupy the coordination sites of MOF-808-ZT, leading to abundant Zr–OH2. The coordinated water molecules are removed by the dehydration of MOF-808-PA at 180 °C, and the underlying coordinatively unsaturated Zr4+ (Zr-CUS) is exposed and acts as a Lewis acid site. Characterizations and DFT calculations show that the Zr-CUS/Zr–OH FLPs sites can efficiently activate the C═O of aldehydes and −OH of cyclohexanol and reduce their activation energy barrier, thus exhibiting good catalytic performance. Moreover, these FLP sites can easily easily dissociate the H–H bond with a lower activation energy of 0.15 eV, thereby achieving a better catalytic reactivity in the direct hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols. A possible reaction mechanism was proposed based on in situ DRIFT, vacuum FTIR, and DFT calculations.