Chemically Tunable Ionic Liquids with Aprotic Heterocyclic Anion (AHA) for CO₂ Capture

Abstract

Ionic liquids (ILs) with aprotic heterocyclic anions, or AHAs, can bind CO2 with reaction enthalpies that are suitable for gas separations and without suffering large viscosity increases. In the present work, we have synthesized ILs bearing an alkyl-phosphonium cation with indazolide, imidazolide, pyrrolide, pyrazolide and triazolide-based anions that span a wide range of predicted reaction enthalpies with CO2. Each AHA-based IL was characterized by NMR spectroscopy and their physical properties (viscosity, glass transition, and thermal decomposition temperature) determined. In addition, the influence of substituent groups on the reaction enthalpy was investigated by measuring the CO2 solubility in each IL at pressures between 0 and 1 bar at 22 °C using a volumetric method. The isotherm-derived enthalpies range between -37 and -54 kJ mol(-1) of CO2, and these values are in good agreement with computed enthalpies of gas-phase IL-CO2 reaction products from molecular electronic structure calculations. The AHA ILs show no substantial increase in viscosity when fully saturated with CO2 at 1 bar. Phase splitting and compositional analysis of one of the IL/H2O and IL/H2O/CO2 systems conclude that protonation of the 2-cyanopyrrolide anion is improbable, and this result was confirmed by the equimolar CO2 absorption in the presence of water. Taking advantage of the tunable binding energy and absence of viscosity increase after the reaction with CO2, AHA ILs are promising candidates for efficient and environmental-friendly absorbents in postcombustion CO2 capture.

Computer‐Aided Design of Ionic Liquids as CO2 Absorbents

“…Note that for the frequently applied D r H values [2,3,14,15] this agreement is significantly rougher,asthese values lack the solvation and entropy contributions. Figure 1s hows the distinction between physical and chemical absorption is possible based on the knowledge of very simple reaction energies and calculated O-C-O angles.…”

“…Foraset of anions,for which also D r G exp data are available with consistently the same cation (trihexyltetradecylphosphonium;[ P 6,6,6,14 ] + ), the calculated and experimental [3] data are listed in À is non-beneficial because of the steric hindrance which is revealed by the additional structures of the two carboxylate complexes and their rotation profile shown in Figure 2. Note that for the frequently applied D r H values [2,3,14,15] this agreement is significantly rougher,asthese values lack the solvation and entropy contributions.…”

“…A predictive estimate with ac lear distinction between physical and chemical absorption can be simply obtained according to geometries optimized in the presence of as olvation model instead of optimizing it only in gas phase as has been done to date.The resulting Gibbsfree energies compare very well with experimental values and the energies were correlated with experimental capacities.P romising anions,f or ionic liquids with reversible CO 2 absorption properties can be defined by areaction Gibbs free energy of absorption in the range of À30 to 16 kJ mol À1 .Ionic liquids (ILs) are promising solvents for CO 2 absorption from various waste gases,a nd this property has been attracting significant scientific and technological attention in the last decade.T oachieve maximum efficiencyregarding the energy needs of both the transport and the regeneration of the absorbent, the absorption enthalpy must be in ar elatively narrow range, [1][2][3] but it can, of course,d eviate from this desired value depending on the aims and the circumstances. Owing to the great variety of ILs (the estimated number of potential IL candidates is ca.…”

Study on Gas Permeation and CO2 Separation through Ionic Liquid-Based Membranes with Siloxane-Functionalized Cations

“…Taking into account the attractive properties of ILs for CO 2 separation, different engineered membrane approaches have been considered, [6][7][8] while gas solubility studies support the importance of the design of task-specific IL-based materials. [9][10][11][12][13] Within the several membrane configurations explored so far, 6 the simplest approach is the use of supported ionic liquid membranes composite membranes have also been intensively studied as a way to prepare membranes that can combine the adequate gas permeability and selectivity of SILMs with the robustness of polymers. [6][7][8] Actually, PILs are functional materials that merge the macromolecular architecture of polymers with the chemistry of ILs, [14][15][16] maintaining in the polymer several of the unique IL features, especially the tunability of their properties.…”

CO₂ Absorption Property of Ionic Liquid and CO₂ Permselectivity for Ionic Liquid Membrane

“…Some kinds of IL chemical absorbents have been reported so far 34) 41) , and we focused on the amino acid ILs (AAILs) with tetrabutylphosphonium 42) . One of the advantages of AAILs is high CO2 solubility; one mole of AAILs absorb an equimolar amount of CO2 through a 1 : 1 stoichiometric reaction.…”