Metal phosphonates as heterogeneous catalysts for highly efficient chemical fixation of CO2 under mild conditions

Abstract

In this study, two novel metal phosphonate networks based on zoledronic acid ((1-hydroxy-2-(1H-imidazol-1-yl)ethane-1,1-diyl)bis(phosphonic acid), abbreviated as EIBPA), with chemical formulas [Co₃(HEIBPA)₂(H₃PO₄)₂(H₂O)₂] and [Cd₃(HEIBPA)₂(H₂O)₄]·2H₂O, namely, compounds 1 and 2, were successfully prepared under hydrothermal conditions. The obtained samples were fully characterized using PXRD, FT-IR, TGA and elemental analysis. The cobalt-phosphonate network has been introduced as an efficient catalyst to transform CO₂ gas to cyclic carbonates with almost complete conversion and 100% selectivity under 1 atm pressure and free of solvent. The active centers of the catalyst are inferred to be Lewis acid sites and Brønsted acid sites in the structure of the compound. In addition, the catalyst can be recycled at least eight times without a significant decrease of the activity in the CO₂ cycloaddition reaction.