Johannes Due-Hansen

Selective gas absorption by ionic liquids

Emission of acidic gases such as NOX and SOx and COx, which are produced by combustion of fossil fuels during, e.g. energy production in power plants, is a major concern in relation to atmospheric pollution and climate changes by the so-called green-house effect. Accordingly, these gases have to be effectively removed from flue gases. Presently this is mainly achieved by relatively energy intensive and resource demanding technologies via selective catalytic reduction (SCR) of NOX with ammonia, by gypsum formation after SO2 wetscrubbing while organic amines are being used as absorbents in CO2 scrubbers leading to concern about, e.g. intensive energy requirements for desorption, corrosion of steel pipes and pumps, CO2 absorption capacity and thermal decomposition of the amine. In this work, we demonstrate how ionic liquids can be tuned by design to perform as selective, high-capacity absorbents of environmentally problematic flue gases like, e.g. SO2 , NO and CO2 . Reversible absorption performance has been tested for several different ILs at different temperatures and flue gas compositions. The structures of ILs are well-ordered even in the liquid state with regular cavities which can host selected solute species depending on the IL ion composition. ILs can be tuned to absorb selected gas molecules making them promising materials for selective, reversible absorption of gaseous pollutants in, e.g. power plant flue gases and other exhaust or industrial offgases.[1,2] In the present work ionic liquids composed of imidazolium or guanidinium based cations like EMIM, BMIM, OMIM and TMGH and anions like acetate, sulphate, triflate and halogenides have been selected for absorption of SO2 and for the first time as far as we know also for NO . Task-specific ionic liquids have been developed[3] which are able to make a chemical bond between CO2 and amine-functionalized ILs at ambient conditions. It appears, however, that this type of bonding is too strong to make reversible absorption/desorption economic in technical scale. Therefore, we have set out to synthesize new ionic liquids containing alternative nitrogen-based functionalities which absorb CO2 readily but less strongly than corresponding amine-based ILs. It is anticipated that the new ionic liquids could improve both chemical and physical absorption of CO2 by proper choice of substituents while maintaining an acceptable enthalpy of absorption and desorption. Reversible absorption performance has been tested for several different ILs at different temperatures and flue gas compositions. Furthermore, different porous, high surface area carriers have been applied as supports for the ionic liquids to obtain Supported Ionic LiquidPhase (SILP) absorber materials.[4] The use of solid SILP absorbers with selected ILs were found to significantly improve the absorption capacity and sorption dynamics at low flue gas concentration, thus making the applicability of ILs viable in technical, continuous flow processes for flue gas cleaning. The results show that CO2 , NO and SO2 can be reversible and selective absorbed using different ILs and that Supported Ionic Liquid-Phase (SILP) absorbers are promising materials for industrial flue gas cleaning. Absorption / desorption dynamics can be tuned by temperatures, pressures and gas concentrations.