One Robust Microporous TmIII–Organic Framework for Highly Catalytic Activity on Chemical CO2 Fixation and Knoevenagel Condensation

Abstract

In terms of documented references, multifunctional MOFs with high catalytic performance could be constructed from the combination of metal cations and polycarboxyl-pyridine ligands, which could efficiently endow crystallized porous frameworks with the coexisting Lewis acid-base properties. Thus, by employing a ligand-directed synthetic strategy, the exquisite combination of wave-like inorganic chains of [Tm(CO₂)₃(OH₂)]_n and mononuclear units of [Tm(CO₂)₄(OH₂)₂] with the aid of the specially designed ligand of 2,6-bis(2,4-dicarboxylphenyl)-4-(4-carboxylphenyl)pyridine (H₅BDCP) generates one highly robust microporous framework of {(Me₂NH₂)[Tm₃(BDCP)₂)(H₂O)₃]·4DMF·H₂O}_n (simplified as NUC-25), which contains near-rectangular nanochannels and large solvent-residing voids. Furthermore, the activated state of NUC-25 with the removal of associated water molecules is a rarely reported bifunctional heterogeneous catalyst due to the coexistence of Lewis acid-base sites including 6-coordinated Tm³⁺ ions, uncoordinated carboxyl oxygen atoms, and N_pyridine atoms. Just as expected, NUC-25 exhibits greatly high catalytic activity for the cycloaddition reaction of epoxides with CO₂ into alkyl cyclic carbonates under bland solvent-free conditions, which should be ascribed to the polarity of nitrogen-containing pyridine heterocycles as Lewis base sites on the inner surface of nano-caged voids except for recognized Lewis acid sites of rare earth cations. Moreover, the excellent pore-size-dependent catalytic property for Knoevenagel condensation reactions confirms that NUC-25 can be viewed as a recyclable bifunctional heterogeneous catalyst. Therefore, these results strongly demonstrate that microporous MOFs assembled from pre-designed polycarboxyl-heterocyclic ligands display better catalytic performance not only for chemical CO₂ fixation but also for Knoevenagel condensation reactions.