Theoretical and experimental studies on co2 cycloaddition to propylene oxide catalyzed by solid supported deep eutectic solvents

Abstract

The increase in the concentration of atmospheric CO2 contributes to the current enhanced greenhouse effect. Due to its abundance, CO2 has gathered significant attention as an ideal carbon building block for organic synthesis in recent times. Among the various routes for chemical CO2 fixation, the cycloaddition of epoxides with CO2 stands out as a promising pathway. This particular route of CO2 conver sion produces cyclic carbonates, that play a vital role in organic synthesis, materials science, and energy storage applications. Oneof the important carbonates produced from utilizing CO2 is propylene carbonate (PC), that serves as a polar solvent. PC is widely employed in the production of environmentally friendly polymers. The high boiling point and aprotic nature of PC makes it a key component in high-permittivity electrolytes used in lithium ion batteries. Overall, the multifaceted applications and environmentally friendly nature of PC confirms its significance in the chemical industries. PC is prepared from the cycloadditon of CO2 to propylene oxide in the presence of catalysts. The list of traditional catalysts include alkali metal salts, calcined hydrotalcites, Schiff base, transition-metal complexes and ion-exchange resins. In recent times, various ionic liquids and deep eutectic solvents are also being used. This thesis presents an investigation on the CO2 cycloaddition to propylene oxide using zeolite-supported deep eutectic solvents (DESs). The research commences with a theoretical investigation employing density func tional theory (DFT) calculations. These simulations aim to reveal the quantum chemistry insights behind the formation of DESs and their applications in chemical reactions. Using computational modeling, the interactions between the DESs and CO2 as well as probable interaction site of DESs for CO2 are discussed. In order to correlate the DFT study with the experimental study, the interaction energy of the DESs obtained from the theoretical study was compared with the catalytic activity of the DESs, supported on zeolites. Prior to the experimental runs, the formation of the DESs were confirmed by Fourier-transform infrared spec troscopy. The zeolite-supported DESs were characterized using thermogravimetric analysis, X-ray diffraction, BET surface area and scanning electron microscope. An increase in the reaction time resulted in a higher conversion of propylene oxide. As the reaction of CO2 with propylene oxide is essentially an irreversible reaction, complete conversion is expected at large reaction times. Among the zeolite-supported DES, the highest catalytic activity was obtained using the DES that possessed the lowest interaction energy according to the DFT study. Usually, the lower the interaction energy between the HBA and HBD of the DES, the more is its ability to interact with other molecules. A similar trend was observed in this study as well, which shows a good agreement between the theoretical study and the experimental study. Further, the effect of zeolites in this reaction was explored by comparing among Zeolite-Y, β-Zeolite and ZSM5 as supports for the DES. The existence of more Lewis acid sites in zeolites enhanced the reaction rate of CO2 cycloaddition to propylene oxide. In summary, the cycloaddition of CO2 to propylene oxide using zeolite-supported DESs shows a promising route for sustainable chemical transformations. This thesis contributes to the fundamental understanding of CO2 cycloaddition to propylene oxide catalyzed by zeolite-supported DESs, offering a synergistic combination of theoretical insights and experimental validation. The findings presented here hold significant implications for the development of sustainable processes for CO2 utiliza tion and the production of value-added chemicals.