Construction of a series of pH stable Ca-based MOFs, their CO₂ adsorption and catalytic activity

In this study, three different solvent systems have been employed to investigate the effect of reaction parameters on the synthesis of four alkaline earth metal-based MOFs namely [Ca(0.5 1,4-phenyl diacetic acid)₂(H₂O)DMF]_∞ (Ca-MOF-1), [Ca(1,4-naphthalene dicarboxylate)DMF]_∞ (Ca-MOF-2), [Ca₂(0.5 1,2,4,5-benzene tetracarboxylate)₂(H₂O)₃DMF]_∞ (Ca-MOF-3) and [Ca₂(2,6-naphthalene dicarboxylate)₂(H₂O)6]_∞ (Ca-MOF-4). The crystal structures of these four MOFs have been resolved through single crystal X-ray analysis and the bulk phase purity of these MOFs was assessed using PXRD and FT-IR analysis. To check the stability of these MOFs, thermogravimetric analysis (TGA) was carried out. To analyze the robustness of these MOFs, the PXRD of the samples was also collected at different pH levels. These MOFs were further explored as Lewis acid catalysts for the alcoholysis of epoxides and the activity of these catalysts depend on the open metal sites present in the MOFs. The catalytic activity follows the order: Ca-MOF-2 > Ca-MOF-4 > Ca-MOF-1 > Ca-MOF-3. The activity was also checked with various epoxide substrates using Ca-MOF-2. Density functional theory (DFT) calculations also support this trend with the help of the thermodynamic feasibility of epoxide binding, considering model MOF structures. The weak interaction between the epoxide oxygen and the metal centre of the most stable MOF structure has also been clarified by computational studies.