Unveiling the stability of mixed Zn/Co-ZIFs as catalysts for CO2 fixation into cyclic carbonates

Abstract

Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs) which exhibit a zeolitic structure. These materials have been extensively used as catalysts in CO2 cycloaddition with epoxides to form cyclic carbonates. However, their stability under reaction conditions is rarely investigated. In this work, four ZIFs were synthesized: monometallic ZIF-67 (with Co), ZIF-8 (with Zn), bimetallic Zn/Co-ZIF-67 (with three-dimensional structure), and Zn/Co-ZIF-L (with two-dimensional structure). Even though the solids presented different crystalline structures, textural properties, and CO2 adsorption capacities, the bimetallic samples showed similar catalytic behavior with high yield (60-65%) and selectivity (92-94%) to propylene carbonate (PC). In comparison, the monometallic samples exhibited a somewhat higher yield (~ 79%) and selectivity (~ 97%) to PC. However, the characterization of the used catalysts showed a substantial loss in crystallinity and porosity for the monometallic ZIF-67 and ZIF-8. At the same time, the Zn/Co-ZIF-67 exhibited the highest stability in maintaining its original structure and porosity. Conversely, the Zn/Co-ZIF-L recrystallized into the sodalite three-dimensional structure after being used in two catalytic reactions. All used catalysts showed the same Zn local environment after the reaction, while the final Co species formed depended on the catalyst’s initial structure and chemical composition.