Post-synthetic modification of Zr-MOFs using guanidine for cyclic carbonate formation catalysis

A post-synthetic modification of Zr-MOF using ionic nitrogen-based functional groups is proposed for the catalytic cycloaddition reaction of CO2 with epoxides. Herein, guanidinium salt derivatives are used as ionic liquid (IL) catalysts while MOFs are used as a superior absorbent for CO2, allowing the generation of heterogeneous catalysts for CO2 fixation reactions under mild conditions. The UiO66-NH2 and UiO67-NH2 were functionalized with carbodiimides N,N '-dicyclohexylcarbodiimide (DCC) and N,N '-diisopropylcarbodiimide (DIC) to produce cationic guanidinium-containing Zr-MOFs with free bromide ions UiO66-DCCBr, UiO66-DICBr, and UiO67DICBr. The materials were characterized comprehensively by multiple techniques such as PXRD, FTIR, TGA, SEM, EDS, 1H NMR, 13C NMR, elemental analysis, as well as N2 and CO2 adsorption equilibrium measurements. UiO66-DCCBr showed a high TOF for the formation of cyclic carbonates using several small epoxides under mild conditions (0.5 MPa CO2, 90 degrees C, without co-catalyst and solvent). In addition, UiO67- DICBr owing to the larger aperture size of MOF displayed better catalytic performance for the conversion of larger epoxides than UiO66DICBr and UiO66-DCCBr with high yield and selectivity. This result underlines the important role of MOF's aperture size for the diffusion of substrates in CO2 fixation reactions.