Combined physical and chemical absorption of carbon dioxide in a mixture of ionic liquids

• Carbon dioxide can be absorbed in mixtures of two ionic liquids: [C2mim][EtSO4] and [C2mim][OAc].
• A combination of physical and chemical absorption mechanisms is observed.
• The CO2 absorption capacity of the mixture of ionic liquids decreases with increasing temperature.
• [C2mim][EtSO4] in the mixture prevents solidification of the product resulting from reaction of [C2mim][OAc] and CO2.
• Density and viscosity studies of the mixture of ionic liquids also lead to synergies, in particular at low temperatures.

Ionic liquids have attracted great interest recently as the basis of a potential alternative technology for the capture of carbon dioxide. Beyond the inherent tunability of properties of individual ionic liquids, a further strategy in optimising the ionic liquid sorbent for this application is the use of mixtures of ‘pure’ ionic liquids. Some ionic liquids absorb CO2 physically, whereas others do so chemically. Both mechanisms of absorption present advantages and disadvantages for a CO2 capture process operating in a continuous regime. In this work, a mixture of 1-ethyl-3-methylimidazolium acetate (an ionic liquid that reacts chemically with CO2) and 1-ethyl-3-methylimidazolium ethylsulfate (an ionic liquid that absorbs CO2 only through a physical mechanism) was investigated for the absorption of CO2 as a function of temperature and at pressures up to 17 bar. The absorption/desorption studies were complemented by the characterisation of thermal and physical properties of the mixture of ionic liquids, which provide extra information on the interactions at a molecular level, and are also critical for the assessment of its suitability for a proposed process and for the subsequent process design.

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