Unraveling the role of internal–external metal substitution in Zn3[Co(CN6)]2 for the styrene oxide–CO2 cycloaddition reaction

Abstract

We investigated the influence of the structural and textural properties along with the chemical environment of pure Zn₃[Co(CN)₆]₂ in comparison with the modified phases on the catalytic performance in the cycloaddition reaction between styrene oxide and CO₂. We relate these to the proposed reaction pathways and mechanisms. The natural cubic phase (ZnCoCn) was dehydrated to obtain the rhombohedral phase (ZnCoRn), while the stabilized cubic phase (ZnCoCs) was synthesized by substituting external zinc atoms with cadmium atoms. The rhombohedral stabilized phase (ZnCoRs) was achieved by the internal cobalt change with iron. All the materials were extensively characterized using X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), and N₂ adsorption. The catalytic behavior of the four phases was tested. The crystalline structure of each phase was obtained, and by XPS, it was demonstrated that the chemical environments of all elements conforming to the rhombohedral stabilized phase are different from those of all other materials owing to the exchange of internal metals. The bulk textural properties were similar; only the ZnCoRs presented more micropore area but did not exceed the total surface area of the other materials. The product distribution and yield at reaction times of 2 h and 6 h were closer to those of the cubic phases. The natural rhombohedral phase exhibits the best performance. The tetrabutylammonium bromide (TBAB) and rhombohedral stabilized phase work together to yield a bigger copolymer quantity at the expense of the styrene carbonate (StCO₃) production. From the proposed mechanism, the TBAB cation (TBA⁺) has a “protection” function that drives the closing of the StCO₃ ring; however, the charge distribution anisotropy in the four nitrogen atoms generated by Co replacement in ZnCoRs could hold TBA⁺ as the reaction time progressed, causing an unavailability that triggered the copolymerization propagation step.