Exploring the Effect of Different Secondary Building Units as Lewis Acid Sites in MOF Materials for the CO2 Cycloaddition Reaction

In order to explore the catalytic effect of different Lewis acid sites (LASs) in the CO₂ 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: [Cu₃(tatab)₂(H₂O)₃]·8DMF·9H₂O (1), [Cu₃(tatab)₂(H₂O)₃]·7.5H₂O (2), [Zn₄O(tatab)₂]·3H₂O·17DMF (3), [In₃O(tatab)₂(H₂O)₃](NO₃)·15DMA (4), [Zr₆O₄(OH)₇(tatab)(Htatab)₃(H₂O)₃]·xGuest (5), and [Zr₆O₄(OH)₄(tatab)₄(H₂O)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 CO₂ 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 Zn₄O display better catalytic performances than In₃O and Zr₆ cluster. The experiments investigate the catalytic effects of LAS types and prove that it is feasible to improve CO₂ fixation property by introducing multi-active sites into MOFs.