Green and efficient epoxide fixation and CO2 separation using halogen-decorated Zr-based UiO-67 MOFs

Abstract

This study reports the synthesis and characterization of novel halogen-functionalized UiO-67 MOFs for CO₂ capture and conversion. Three mixed-linker Zr-UiO-67 MOFs, [Zr₆O₄(OH)₄(L₁L₂)₁₂], were synthesized, incorporating 2,2′-bipyridine-5,5′-dicarboxylic acid (L₁) and halogen-substituted 4,4′-biphenyl dicarboxylic acids (L₂) with chlorine (MOF-1), bromine (MOF-2), or iodine (MOF-3). Comprehensive characterization confirmed successful MOF synthesis and halogen incorporation. Gas adsorption studies showed selective CO₂ adsorption over N₂ and CH₄. Critically, these MOFs exhibited exceptional catalytic activity for solvent-free CO₂ conversion to cyclic carbonates at ambient temperature and pressure. The Lewis acidic Zr centers facilitated heterogeneous catalysis of CO₂ fixation with epoxides, yielding cyclic carbonates with TBAB as a cocatalyst. High conversion (91–99 %) of epichlorohydrin to 4-(chloromethyl)-1,3-dioxolan-2-one was achieved within 12 h at room temperature and 1 bar CO₂ pressure. A structure-activity relationship was established, revealing a direct correlation between halogen electronegativity and catalytic performance. CO₂ uptake and epoxide conversion rates increased across the halogen series from chlorine to iodine, attributed to the synergistic effect of the Lewis acidic Zr centers and the increasing polarizability of the halogen substituents. Moreover, the MOFs demonstrated robust recyclability, retaining substantial catalytic activity for at least five cycles. These findings underscore the potential of halogen-functionalized UiO-67 MOFs as promising candidates for sustainable CO₂ capture and utilization strategies.