Highly Efficient Hydrogenation of α,β-Unsaturated Aldehydes to Unsaturated Alcohols over Defective MOF-808 with Constructed Frustrated Lewis Pairs
Linhao Zhong, Xiaoqing Liao, Haishuai Cui, He’an Luo, Yang Lv, Pingle Liu
Abstract
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 ZrCl 4 and H 3 BTC 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–OH 2 . The coordinated water molecules are removed by the dehydration of MOF-808-PA at 180 °C, and the underlying coordinatively unsaturated Zr 4+ (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.