Synthesis of 2D zeolitic imidazolate frameworks based on Co(II) and Pd(II): Effect of Pd(II) addition on the CO2 cycloaddition with epichlorohydrin

Abstract

Carbon dioxide (CO2) cycloaddition with epoxide can yield value-added cyclic carbonates with 100 % atom efficiency. Moreover, zeolitic imidazolate frameworks (ZIFs) have emerged as promising catalysts for this reaction as their basic and acidic sites can effectively activate CO2 and epoxide molecules. Here, novel mixed-metal ZIFs containing Pd and Co ions (PdCo-ZIFs) that are linked by benzimidazole ligands were synthesized, after which their structures were comprehensively characterized. Despite the inherent two-dimensional nanosheet structure of Co-ZIF, PdCo-ZIFs exhibited a large surface area, with a hierarchical porous structure and profoundly increased active site concentrations (10–20 times higher than those of Co-ZIFs), exhibiting significantly increased catalytic activity toward CO2 cycloaddition. Additionally, the relationship between the catalytic activity and structural characteristics of ZIFs correlated, and the representative Pd2Co8-ZIF catalyst achieved a maximum product yield of 97.3 %, with a high turnover number (5754) and turnover frequency (479 h−1) for cyclic carbonates, attributed to the well-developed porous structure and defect sites that could not be observed in single-metal ZIF materials.