Catalytic Investigation of CO<sub>2</sub> Chemical Fixation and the Knoevenagel Condensation Reaction for a Tm<sup>III</sup>–Organic Framework
Tao Zhang, Zhengguo Zhang, Hongtai Chen, Xiutang Zhang, Qiaoling Li
Abstract
The delicate combination of wave-like [Tm2(CO2)6(OH2)2]n chains and [Tm(CO2)4(NO3)(OH2)] units with the aid of 2,6-bis(2,4-dicarboxylphenyl)-4-(4-carboxylphenyl)pyridine (H5BDCP) generates one highly robust dual-channel material of {[(CH3)2NH2]2[Tm3(BDCP)2(NO3)(OH2)3]·4DMF·2H2O}n (NUC-28) with excellent physicochemical properties including solvent-free dual channels, a bigger specific surface area, high porosity, water tolerance, and thermal stability. As far as we know, NUC-28 is one scarcely reported nitrate-functionalized microporous metal–organic framework (MOF) with N═O groups as Lewis base sites protruding on the inner surface of the channels. Thanks to the coexistence of Lewis acid–base sites including rich hexa-/hepta-coordinated Tm3+ ions, N═O and C═O groups from μ1-η1:η1 NO3– and CO2–, and Npyridine atoms, NUC-28 displays high catalytic activity in the cycloaddition reaction of epoxides and CO2 into related cyclic carbonates under mild, solvent-free reaction conditions. In addition, Knoevenagel condensation reactions with aldehydes and malononitrile as substrates could be greatly accelerated in the presence of NUC-28. Hence, these catalytic results confirm that the introduction of Lewis base sites of NO3– anions on open metal sites in MOFs could effectively enhance its catalytic efficiency, which should be attributed to the recognized synergistic effect of Lewis base–acid active sites.