Robust Cu-URJC-8 with mixed ligands for mild CO2 cycloaddition reaction

Abstract

The synthesis of the new MOF Cu-URJC-8, containing two different organic ligands, 2-aminoterephtalic acid and 4,4-bipyridyl, is reported in this work. Cu-URJC-8 shows a pillared-layer structure consisting of [Cu2(-CO2)4(-N)2] paddlewheel secondary building units, with the 4,4’-bypiridyl linkers acting as pillars in the two-fold interpenetrated networks, which provides a robust structure to the material. Cu-URJC-8 presents Lewis acid and basic sites, constituted by Cu2+ and -NH2 groups, respectively, and it is chemically stable in different commonly used organic solvents. This new material shows a CO2 uptake of 1.59 and 1.07 mmol/g at 25 and 45 °C, respectively, and a Qst value of 27 kJ/mol, higher than those observed for other reported MOFs with higher textural properties, possibly due to the presence of amino groups in the MOF structure. Its catalytic activity in the cycloaddition reaction of epichlorohydrin and CO2 was evaluated under various conditions and the best performance (90 conversion and 99% selectivity) was achieved when using 1% mol of catalyst, 4% mol of co-catalyst, 12 bar of carbon dioxide pressure, and room temperature. When different epoxides with bulkier radical groups are used as substrates, the epoxide conversion decreases, evidencing steric hindrances for diffusion inside cavities of Cu-URJC-8. Finally, Cu-URJC-8 has a high stability, keeping its structural integrity and catalytic activity after successive CO2 cycloaddition reaction cycles. These results show that Cu-URJC-8 is a promising catalyst for CO2 revalorization.