Guanidyl-implanted UiO-66 as an efficient catalyst for the enhanced conversion of carbon dioxide into cyclic carbonates

Abstract

The development of heterogeneous catalysts for promoting epoxide cycloaddition with carbon dioxide is highly desirable for recycling CO₂ and achieving the goal of carbon neutrality. Herein, we designed and synthesized Zr-based metal organic frameworks (MOFs) by implanting functional guanidyl into the framework via mixing different molar ratios of 4-guanidinobenzoic acid (Gua) with 1,4-benzenedicarboxylic acid (BDC). Consequently, a small sized Zr-MOF (∼350 nm) can be prepared by implanting Gua with 20% molar ligands, denoted as UiO-66-Gua_0.2(s). Compared to large sized and different guanidyl Zr-MOFs, UiO-66-Gua_0.2(s) exhibited an optimal activity on catalyzing epoxide cycloaddition with CO₂ in the presence of the Bu₄NBr cocatalyst. A yield of 97% for the product of chloropropene carbonate was achieved at 90 °C under 1 atm CO₂. The great performance of UiO-66-Gua_0.2(s) might be attributed to the synergistic effect of guanidyl groups as hydrogen-bond donors and Zr centers acting as Lewis-acidic sites. In addition, the heterogeneous catalyst of UiO-66-Gua_0.2(s) exhibited a great versatility towards converting other epoxides and a satisfactory recyclability for five consecutive runs. Moreover, a plausible reaction mechanism has been proposed for UiO-66-Gua_0.2(s) in promoting CO₂ epoxide cycloaddition reactions.