Engineered ionic liquids supported on activated carbon as a sustainable and selective catalyst for viable fixation of CO2 into valuable chemicals

Abstract

The use of ionic liquids (ILs) based materials in catalysis, especially for CO2 conversion has been attracted great scientific attention due to green catalytic approach and eco-friendliness of the material. In this effort, especially engineered NH2-functionalized imidazolium based ILs with different anions were prepared to develop new covalent ILs functionalized activated carbon (AC) catalysts. All different AC-ILs namely AC-IL-Br, AC-IL-BF4, AC-IL-NTf2 and AC-IL-HSO4 was successfully synthesized by metathesis reaction, and characterized using various analytical and spectroscopic techniques. With the maximum immobilization of the NH2-functionalized ILs on the AC support, the immobilized AC-ILs showed superior structural advantages such as high thermal stability and appreciable surface area. The newly developed AC-ILs as bi-functional catalysts were applied in the fixation of CO2 into epoxide and it displayed high catalytic activity during the catalytic screening. Various reaction variables were surveyed wherein especially, 10.0 wt% of AC-IL-NTf2 catalyst demonstrated the best catalytic activity with 100 % conversion of styrene oxide and 83 % selectivity towards styrene carbonate at 120 °C, 20 bar CO2 pressure in 13 h under solvent-free conditions. The enhanced performance by AC-IL-NTf2 catalyst was achieved as a result of least co-ordination of the NTf2− anion with the NH2-functionalized imidazolium cation which helped in the opening and stabilization of epoxide ring and increased the reaction rate. The best performing AC-IL-NTf2 catalyst was found to be compatible with a broad substrate scope and could effectively produce the respective cyclic carbonates in good to quantitative yields with different substituted epoxides. Most significantly, AC-IL-NTf2 catalyst could be easily separated from the reaction mixture and recycled it for number of cycles. It maintained its catalytic activity without any evident leaching and structural damage up to six recycles. Additionally, plausible reaction mechanism also has been proposed with AC-IL-NTf2 catalyst with the help of catalytic activity results and characterization support.