The metal-organic framework UiO-66 with missing-linker defects: A highly active catalyst for carbon dioxide cycloaddition

Metal-organic frameworks (MOFs) have been confirmed to be a promising material for carbon dioxide capture, while the simultaneous capture and conversion of carbon dioxide over the MOF-based catalysts is furthermore being a hot topic. In this work, we demonstrate that the zirconium-based MOF UiO-66 (UiO for University of Oslo) with missing-linker defects is a highly active catalyst for carbon dioxide cycloaddition with an epoxide, which is a reaction with 100% atom economy and broad industrial uses. A facile, rapid, energy-saving and environmentally friendly argon plasma treatment was employed to create the missing-linker defects in UiO-66 without using hazardous chemicals at atmospheric pressure and temperature below 398 K, whereas the bulk structure of UiO-66 remained stable. The energetic argon plasma species decomposed part of the linkers within UiO-66 structure, leaving unsaturated metal sites. The defect concentration in UiO-66 was tuned by changing the plasma bombardment time. After an argon plasma treatment for 30 min, the number of linker deficiencies per Zr-6 cluster can reach up to 2.3. The product yield of the UiO-66 with abundant missing-linker defects increased of ca. 43% over the defect-free UiO-66. The crucial role of the missing-linker defects of UiO-66 in the enhancement of the catalytic activity was confirmed. The present study will be helpful for the future preparation of defective MOFs with potential applications not only for carbon dioxide conversion but also for energy storage and conversion.