Carbon Dioxide Capture With Activation And Chemical Transformation By Task-specific Ionic Liquids

Carbon dioxide from combustion of fossil fuels (coal, petroleum and natural gas) is regarded as the most significant greenhouse gas; hence, CO2chemistry (capture and conversion) has attracted significant attention from the scientific community thanks to global warming. CO2capture from flue gases in industrial combustion processes is now being considered as a serious option to reduce or mitigate the so-called green-house effect. On the other hand, as an abundant, nontoxic, non-flammable, easily available, and renewable carbon resource, CO2is very attractive as an environmentally friendly feedstock for making commodity chemicals, fuels, and materials. However, few industrial processes utilize CO2as a raw material, because CO2is the most oxidized state of carbon, namely CO2could be a thermodynamically stable molecule. In this context, the biggest obstacle for establishing industrial processes for CO2conversion would be due to its low energy level. In short, its inherent thermodynamic stability and kinetic inertness hinder the development of efficient catalysts that achieve CO2activation and subsequent functionalization. Accordingly, only if we understand the underlying principles of CO2activation, can the goal of using CO2as an environmentally friendly and economically feasible source of carbon be achieved. ILs are salts composed of distinct cations and anions that are capable of facilely tuning, and whereby can be designed for task-specific applications through smart choice of the respective cation and/or anion, which have distinctive properties such as high thermal stability, negligible vapor pressure, high loading capacity, easy recyclability and diversiform structure/property modulation.Herein, we have proposed the "CO2capture and utilization concept (CCU)" as a part of CO2chemistry. The essence of our strategy is to use CO2captured by Lewis basic ionic liquids or polyethylene glycol/super base binary system, also considered as the activated form of CO2, as a feed-stock, which renders the reaction system suitable for accomplishing chemical transformation of CO2under low pressure (ideally at1atm), and simultaneously getting rid of the desorption step in the CCU process. Indeed, activation of CO2through carbamate/alkyl carbonate formation with amines has been reported and detected by in situ FT-IR under CO2pressure.(1) DABCO-derived(1,4-diazobicyclo[2.2.2]octane) Lewis basic ionic liquids, which possess tertiary nitrogen in the cations and have the potential to form carbamate species with CO2, being assumed to be an activated form of CO2, were developed for efficient synthesis of cyclic carbonates, dimethyl carbonate and oxazolidinones.(2) polyethylene glycol (PEG)-functionalized basic ILs, especially BrTBDPEG150TBDBr were proven to be a highly efficient and recyclable catalyst for the synthesis of cyclic carbonates from cycloaddition reactions of CO2with epoxides under mild conditions (1atm CO2pressure), and also the subsequent transesterification of cyclic carbonate with methanol to form dimethyl carbonate (DMC). This is presumably due to the activation of epoxide assisted by hydrogen bonding and activation of CO2by the ether linkage in the PEG backbone or through the formation of carbamate species with the secondary amino group in the IL cation on the basis of in situ FT-IR study under CO2pressure.(3) Protic ILs, e.g. HPyI and HDBUCl, proved to be highly efficient and recyclable catalysts for the selective synthesis of cyclic carbonates and oxazolidinones under a CO2atmosphere at room temperature, presumably due to epoxides/aziridines activation assisted by hydrogen bonding on the basis of1H NMR and in situ FT IR under CO2pressure study.(4) A highly efficient binary system consisting of polyethylene glycol and an amidine or guanidine superbase was developed for CO2absorption, leading to the activation of CO2molecules, and thus direct conversion of the captured CO2to value-added chemicals or fuels was successfully performed to avoid desorption.(5) DABCO-derived Lewis basic ILs, or binary system consisting of polyethylene glycol (PEG, proton donor)/PEG-functionalized base with suitable basicity was developed for efficient gas desulfurization (GDS). The absorbed SO2, being considered as the activated form of SO2, can be directly transformed into value-added chemicals(e.g. cyclic sulfites) under mild conditions, thus eliminating the energy penalty for SO2desorption and simultaneously realizing recycle of the absorbents.