Facile Green Synthesis of New Copper-Based Metal–Organic Frameworks: Experimental and Theoretical Study of the CO₂ Fixation Reaction

Two new entangled Cu­(II)-based metal–organic frameworks (MOFs) have been synthesized, namely [Cu­(BDC)­(BPDB)_0.5]_n (PNU-25) and [Cu­(NH₂-BDC)­(BPDB)_0.5]_n (PNU-25-NH₂), using a H₂O–MeOH solvent mixture. Both the PNU-25 and PNU-25-NH₂ MOF materials were characterized by various analytical techniques, and their potential for catalyzing CO₂ fixation into cyclic carbonates at an atmospheric pressure, at a low reaction temperature, and in the neat conditions was demonstrated. The amine-functionalized PNU-25-NH₂ exhibited a significantly high conversion of epichloro­hydrin (ECH) at 1 bar of CO₂ pressure, at 55 °C, and with a moderate catalyst amount (1 mol%), with over 99% selectivity toward the corresponding cyclic carbonate of ECH. The superior catalytic activity of PNU-25-NH₂ may be attributed to its high amount of acidic–basic sites and large BET surface area in comparison with the PNU-25. The PNU-25-NH₂ catalyst could be reused up to four cycles without compromising its structural integrity and the ECH conversion. The reaction mechanism of the CO₂ and ECH cycloaddition reaction mediated by PNU-25-NH₂ was investigated in detail, based on the experimental inferences and periodic calculations of density functional theory (DFT). The energy barrier of the rate-determining step of the PNU-25-NH₂/TBAB-catalyzed reaction was significantly lower than the barriers of the rate-determining steps of non-catalyzed and TBAB-catalyzed reactions.