A novel zirconium-based metal-organic framework covalently modified by methyl pyridinium bromide for mild and co-catalyst free conversion of CO 2 to cyclic carbonates.

Building metal-organic frameworks (MOFs) covalently modified by onium halides is a promising approach to develop efficient MOF-based heterogeneous catalysts for the cycloaddition of CO ₂ to epoxides (CCE) into cyclic carbonates. Herein, we report a novel zirconium-based MOF covalently modified by methyl pyridinium bromide, Zr ₆ O ₄ (OH) ₄ (MPTDC) _2.2 (N-CH ₃ -MPTDC) _3.8 Br _3.8 ((Br-)CH3-Pyridinium-MOF-1), where MPTDC denotes 3-methyl-4-pyridin-4-yl-thieno[2,3- b ] thiophene-2,5-dicarboxylate. The structure and composition of this complex were fully characterized with PXRD, NMR, XPS, TEM and so on. CO ₂ adsorption experiments show that (Br-)CH3-Pyridinium-MOF-1 has a higher affinity for CO ₂ than its electrically neutral precursor, which should be attributed to the fact that charging frameworks containing pyridinium salt have stronger polarization to CO ₂ . (Br-)CH3-Pyridinium-MOF-1 integrated reactive Lewis acid sites and Br ^- nucleophilic anions and exhibited efficient catalytic activity for CCE under ambient pressure in the absence of co-catalysts and solvents. Furthermore, (Br-)CH3-Pyridinium-MOF-1 was recycled after five successive cycles without substantial loss in catalytic activity. The corresponding reaction mechanism also was speculated.