Ionic liquids (ILs) have gained considerable attention in recent years as CO-reactive solvents that could be used to improve the economic efficiency of industrial-scale CO separations. Researchers have demonstrated that IL physical and chemical properties can be optimized for a given application through chemical functionalization of both cations and anions. The tunability of ILs presents both a great potential and a significant challenge due to the complex chemistries and the many ways in which ILs can be made to react with CO. However, computer simulations have demonstrated great potential in understanding the behavior of ILs from the underlying molecular interactions. In the present review, we examine how computer simulations have aided in the design of ILs that chemically bind CO. We present the historical development of CO-reactive ILs while highlighting the insights provided by molecular modeling which aided in understanding IL behavior to further experimental findings. We also provide a brief discussion of simulations focused on ionic liquids that physically dissolve CO. We conclude with a discussion of areas where simulations can yet be used to advance the current understanding of these complex systems and an outlook on the use of computer simulations in the design of optimal ILs for CO separations.

Molecular dynamics simulations were used to compare water solubilities and the effects of water on the structure and dynamics of ionic liquids (ILs) composed of phosphonium cations paired with azolide and phenolate anions. The addition of water decreases ordering of the ions compared to the dry ILs with the exception of anion-anion ordering in the phenolate IL. The result is that the dynamics of the azolide ionic liquids increase significantly upon addition of water, whereas the phenolate IL dynamics show little change. The relative water solubilities were compared through calculation of Henry's law constants. Water is much more soluble in the phenolate IL due to strong hydrogen bonding interactions between water and the phenolate oxygen atom. Anions can therefore be selected to control IL-water hydrogen bonding for optimal performance in applications such as CO separation.

A combination of ab initio calculations and classical molecular dynamics simulations was used to calculate the free energy of reacting an aprotic heterocyclic anion ionic liquid with CO 2 . The overall reaction was broken into a series of steps using a thermodynamic cycle to calculate the free energy of the gas phase reaction and the free energy contributions of solvation environment effects, which make comparable contributions to the total free energy of reaction. CO 2 absorption isotherms that agree reasonably well with experimental data were calculated using a derived expression for the free energy of reaction as a function of temperature, pressure, and the extent of reaction.

A combination of X-ray scattering experiments and molecular dynamics simulations were conducted to investigate the structure of ionic liquids (ILs) which chemically bind CO. The structure functions were measured and computed for four different ILs consisting of two different phosphonium cations, triethyloctylphosphonium ([P]) and trihexyltetradecylphosphonium ([P]), paired with two different aprotic heterocyclic anions which chemically react with CO, 2-cyanopyrrolide, and 1,2,4-triazolide. Simulations were able to reproduce the experimental structure functions, and by deconstructing the simulated structure functions, further information on the liquid structure was obtained. All structure functions of the ILs studied had three primary features which have been seen before in other ILs: a prepeak near 0.3-0.4 Å corresponding to polar/nonpolar domain alternation, a charge alternation peak near 0.8 Å, and a peak near 1.5 Å due to interactions of adjacent molecules. The liquid structure functions were only mildly sensitive to the specific anion and whether or not they were reacted with CO. Upon reacting with CO, small changes were observed in the structure functions of the [P] ILs, whereas virtually no change was observed upon reacting with CO in the corresponding [P] ILs. When the [P] cation was replaced with the [P] cation, there was a significant increase in the intensities of the prepeak and adjacency interaction peak. While many of the liquid structure functions are similar, the actual liquid structures differ as demonstrated by computed spatial distribution functions.