Aprotic heterocyclic anion ionic liquids (AHAs) are a promising new class of CO 2 absorbents, with a capacity of one mole of CO 2 chemically absorbed per mole of AHA. By tailoring the substituents on the anion, the AHA properties, in particular the enthalpy of absorption, can be tuned over a wide range. Furthermore, the entropy of absorption can be tuned by tailoring substituents on the cation. This then presents a materials design challengeWhat are the optimal AHA properties? This challenge is addressed by incorporating AHAs into a simple process model of CO 2 capture from postcombustion flue gas, and formulating the question as a type of simultaneous materials and process design problem. New absorption isotherm data is presented, over a larger pressure range than studied previously, for a few AHAs, and is used to suggest a simple thermodynamic model for CO 2 uptake, to be used in connection with the process model. The possibility of ionic liquids (ILs) that exhibit a 2:1 CO 2 uptake (two moles CO 2 per mole IL) with a cooperative binding mechanism is also considered, with absorption isotherm data for one such compound presented, together with a corresponding isotherm model. The process model is an equilibrium-based material and energy balance model, which is used to determine flow rates, heat duties, and process conditions that minimize a simple energy usage objective function. The sensitivity of this optimum with respect to various material properties and process parameters is studied, for flue gas from both pulverized coal and natural gas combined cycle power plants. The results provide materials property targets for the identification of new AHA molecules for CO 2 capture, leading to significant reductions in heat requirements relative to conventional amine technology.

We show that the addition of tetraglyme (TG), which is a low viscosity liquid with relatively low vapor pressure, is effective in reducing the viscosity of ionic liquids (ILs). In particular, we measure the viscosities of mixtures of 18 ionic liquids with tetraglyme at temperatures between 278.15 and 323.15 K, with a focus on mixtures that are primarily ionic liquid. Thirteen of the ionic liquids contain aprotic heterocyclic anions (AHA ILs) paired with tetra-alkylphosphonium and imidazolium cations, which we have developed for cofluid vapor compression refrigeration and postcombustion CO 2 capture applications. Three bis(trifluoromethylsulfonyl)amide ([Tf 2 N] − ) ionic liquids are included for comparison, as well as trihexyltetradecylphosphonium acetate and trihexyltetradecylphosphonium dicyanamide ([P 66614 ][acetate] and [P 66614 ][DCA]). In addition, we present the densities of trihexyltetradecylphosphonium 1,2,3-triazolide ([P 66614 ][3-Triz]) + tetraglyme mixtures at temperatures between 283.15 and 353.15 K. Finally, we show that the solubility of CO 2 in mixtures of [P 66614 ][3-Triz] + 30 mol % tetraglyme and trihexyltetradecylphosphonium 1,2,4-triazolide ([P 66614 ][4-Triz]) + 30 mol % tetraglyme at 313.15, 333.5, and 353.6 K and pressures to 34 bar can be represented reasonably well by a mole fraction weighted sum of the solubilities (on a mole ratio basis) in the two pure components.

Capture me! The first report of an N-heterocyclic carbene (NHC) as a solid-state carbon capture reagent is presented. Experimental and theoretical measurements demonstrate the ability of the NHC to react rapidly and stoichiometrically with CO2 at low partial pressures.