(31r) Solvent-Free Cycloaddition of Carbon Dioxide to Epoxide Using Zn/Zr-Based Metal Organic Frameworks (MOF)

The high surge in global energy demand, which has been satisfied largely by the consumption of fossil fuel, it has resulted in severe ecological disturbances, highlighting the urgent need to manage carbon dioxide (CO₂) emissions effectively. Converting the CO₂ to valuable products is one sure mean to optimally managing this CO₂ emission; cyclic carbonate are among the values produced and their approach provides the double advantages as mean of reducing the greenhouse gas while providing the industrially significant compound like propylene carbonate, Styrene carbonate, etc. Among the numerous catalytic systems, porous metal-organic frameworks (MOFs) stand out due to their modular architecture, which facilitates the incorporation of both CO₂-philic Lewis basic (LB) and Lewis acidic (LA) sites. This way the MOFs combine distinctive functionalities showing high efficiency in the cycloaddition of carbon dioxide to cyclic carbonates. Such feasible and sustainable strategy in reducing greenhouse gas level.

In this study, we developed a series of bimetallic Zn-Zr based metal-organic frameworks (MOFs) using a green solvothermal technique, adjusting the Zn and Zr metal molar ratios to identify the optimal composition for maximizing conversion efficiency in cycloaddition reactions. The synthesized Zn-Zr MOFs were extensively characterized to assess their structures and morphologies, using FTIR, XRD, SEM, and nitrogen adsorption-desorption techniques. XRD further confirms that the MOF synthesized in this work shows a well-defined crystalline structure. The shifts of peaks towards higher angles are due to the successful incorporation of Zn, since the ionic radius of Zn is smaller compared to Zr. FTIR spectra reveal various types of functional groups, and slight peak shifts further established the incorporation of Zn. SEM images provided information about MOF morphologies. The irregular, smooth sheet-like structure of the Zn-Zr bimetallic MOF is formed at a metal-to-linker ratio of 1.1:1. This MOF, labeled Zn₀.₇₅-Zr₀.₂₅-HAC, demonstrates excellent catalytic activity for CO₂ cycloaddition with epoxides under mild conditions (1 bar, 50°C, 4 hours) using TBAB as a co-catalyst in a solvent-free environment, achieving yields over 75%. Optimizing the reaction conditions (20 bar, 120°C, 6 hours) resulted in a highly impressive yield of 99% for the conversion of propylene oxide to propylene carbonate, showcasing the catalyst’s strong efficiency. In the same manner, high productivities (>90%) were achieved for styrene oxide (SO) and epichlorohydrin (ECH). The MOF showed remarkable stability and reusability, even after five cycles, which is very important as it indicates that the MOF is the material that demonstrates good recyclability and is stable, and can be reused over extended periods. This work highlights the potential of MOFs as efficient catalysts for CO₂ conversion, contributing to sustainable chemical processes.