Here, a Y(III)-based metal–organic
framework, JLU-MOF112 {[Y3(μ3-O)2(μ3-OH)(H2O)2(BTCTBA)2]·2[(CH3)2NH2]·5DMF·C6H5Cl·4H2O}, has been successfully synthesized
under solvothermal conditions. JLU-MOF112 was constructed
with amide-functionalized tricarboxylate ligands and Y(III)-based
infinite chains, where the Y3 repeating units are arranged
in a trans order. The overall framework could be
viewed as a novel (3,5)-connected net with two types of channels along
the [100] and [010] directions. JLU-MOF112 possesses
a large BET surface area (1553 m2 g–1), a permanent pore volume (0.67 cm3 g–1), and outstanding thermal and chemical stability, which give JLU-MOF112 potential for the purification of natural gas,
especially the equimolar separation of C3H8/CH4 with a high selectivity of 176. In addition, benefiting from
the amide functional groups as Brønsted basic sites and the exposure
of open metal sites as Lewis acid sites after activation, JLU-MOF112 can serve as a high-efficiency heterogeneous catalyst for Knoevenagel
condensation by the reactions of malononitrile with benzaldehyde (yield
of 98%, turnover number of 392, and turnover frequency of 3.27 min–1) and diverse aldehyde compounds. A rational mechanism
was put forward that the Knoevenagel condensation was catalyzed by
the synergistic effect of the Lewis acid sites and Brønsted basic
sites, engendering the polarization of the carbonyl groups and the
deprotonation of the methylene groups for nucleophilic attack.
In order to explore the catalytic effect of different Lewis acid sites (LASs) in the CO 2 cycloaddition reaction, different secondary building units and N-rich organic ligand 4,4′,4″-s-triazine-1,3,5-triyltri-p-aminobenzoate were assembled to construct six reported MOF materials: 4), [Zr 6 O 4 (OH) 7 (tatab)(Htatab) 3 (H 2 O) 3 ]•xGuest (5), and [Zr 6 O 4 (OH) 4 (tatab) 4 (H 2 O) 3 ]•xGuest (6) (DMF = N,N-dimethylformamide, and DMA = N,N-dimethylacetamide). Large pore sizes of compound 2 enhance the concentration of substrates, and the multi-active sites inside its framework synergistically promote the process of the CO 2 cycloaddition reaction. Such advantages endow compound 2 with the best catalytic performance among the six compounds and surpass many of the reported MOF-based catalysts. Meanwhile, the comparison of the catalytic efficiency indicated that Cu-paddlewheel and Zn 4 O display better catalytic performances than In 3 O and Zr 6 cluster. The experiments investigate the catalytic effects of LAS types and prove that it is feasible to improve CO 2 fixation property by introducing multi-active sites into MOFs.
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