Imidazolium Chloride Ionic Liquid Mixtures as Separating Agents: Fuel Processing and Azeotrope Breaking

Relevant chemical separations for the petrochemical and chemical industries include the removal of aromatic hydrocarbons from aliphatics, the desulfurization and denitrification of fuels, and the separation of azeotropic mixtures containing alkanols. In an attempt to contribute to the development of novel technologies, the potentialities of imidazolium chloride ionic liquid (IL) mixtures as separation agents were investigated. Selectivities, capacities, and solvent performance indices were calculated through the activity coefficients at infinite dilution of organic solutes and water in the imidazolium chloride IL: [C8mim]Cl, [C12mim]Cl, and the equimolar mixture of [C4mim]Cl and [C12mim]Cl. Results show that the imidazolium chloride IL might be appropriately tailored for specific purposes, in which an increase in the proportion of cations containing larger alkyl chains tends to increase the overall affinity with organic solutes. The IL designer solvent concept was explored by comparing the IL equimolar mixture results with the intermediary [C8mim]Cl. The COSMO-RS thermodynamic model was also applied, showing it to be a promising tool for a fast qualitative screening of potential separation agents for specific separation processes.

Section S1 -Experimental details Table S1. Chemical structure of the cation, name, molar mass, melting temperature and purity (mass fraction) of the studied imidazolium-chloride ionic liquids acquired from Iolitec.

Procedure for column packing and chromatographic experiments
An all-glass column ( During the experiments, a precision gas flowmeter (Agilent, model 5067-0223) was used to measure the exit flow rate (relative uncertainty of 6%), the outlet temperature (±0. where 3 is the number of moles of the ionic liquid packed in the column, is the ideal gas constant, T is the column absolute temperature (controlled by the GC oven), is the net retention volume of the solute, 1 * is the saturated vapor pressure of the solute at the column temperature, 11 is the second virial coefficient of the pure solute, 1 * is the molar volume of the pure solute, 0 is the outlet column pressure, 2 3 is the pressure correction term, 12 is the second virial coefficient of the solute in the carrier gas, and 1 ∞ is the partial molar volume of the solute at infinite dilution.
All the thermophysical properties of the pure solute required in Eq. (S1), namely the vapor pressure, the molar volume, and the second virial coefficient, were retrieved from the DIPPR 801 database. 5 The mixed second virial coefficient of the solute and the carrier gas ( 12 ) were estimated by the approach proposed by Tsnonopoulos, 6 and discussed in detail by Poling et al. 7 The net retention volume ( ), representing the total volume of solute that passes through the column, is obtained by the following relationship: in which 0 is the outlet column volumetric flow rate, and and are the retention times of the solute and the non-retained substance introduced into the column along with the solute (frequently air), respectively. Since the flow rate is registered after the carrier gas passes though the detector, the following correction is required to retrieve the flow rate at the column conditions: where , and are the volumetric flow, the pressure, and the temperature measured by the flowmeter after the carrier gas passes through the detector, respectively. The pressure correction factor, 2 3 , required in Eq. (S1) and Eq. (S2) to compensate the pressure drop in the column, is expressed as: 3,8 where is the inlet pressure of the column.
Whenever the 13 ∞ are available at different temperatures, the data can be used to derive some excess partial molar properties, namely the enthalpy ( ̅ ), entropy ( ̅ ) and, Gibbs energy ( ̅ ) by the following equations: in which the subscripts and indicate constant pressure and constant composition conditions, respectively.

Gas-liquid partition coefficients
The gas-liquid partition coefficient of solute partitioning between the stationary phase (IL) and the gas phase (carrier gas) can also be calculated form the GC retention times using the following equation: in which c is the molar concentration of the solute, 3 is the density of the IL and 3 is the mass of the IL packed into the column.

Separation factors
The selectivities, ∞ , and capacities, ∞ , are useful parameters to assess the suitability of the ionic liquid for a specific separation problem, and are directly related with the 13 ∞ by: where the subscripts i and j represent the target solutes, being j the solute with the lower activity coefficient value for a given separation, and 3 refers to the ionic liquid. For the selection of an appropriate separation agent, high selectivities and high capacities are desirable, though these values often present an inverse relationship for several specific separation problems. 9 To best assess the suitability of the solvent by a single quantity, the solvent performance index is included: [10][11][12] Q ∞ = ∞ ∞ (S11) While ∞ , ∞ , and Q ∞ are very useful parameters to evaluate potential separation agents for liquid-liquid extraction processes, the relative volatility ( ) is often desirable for distillation process. It is directly obtained from vapor-liquid equilibrium data: 10,13,14 where y and x are the molar compositions of the vapor and liquid phases, respectively, γ is the activity coefficient, 0 is the pure compound vapor pressure, and the subscripts i and j stand for the targets components to be fractionated, being j the less volatile compound.

S9
Eq. (S12) might also be applied when a solvent/entrainer is added in the target mixture if the compositions in the vapor and liquid phases, or the solute's activity coefficient and vapor pressures, are known. Additionally, a relative volatility at infinite dilution can be also be defined as: 15 where 3 stands for the entrainer/solvent, and the subscripts i and j represent the solutes at infinite dilution.

Statistical analysis
The deviations between the experimental and predicted separation factors (e.g., ∞ , ∞ , Q ∞ , ) were assessed by the average relative deviation (ARD), calculated as follows: where SF stands for the analyzed separation factor, the superscripts "exp" and "calc" mean the experimental and calculated, respectively, is the total number of data points and covers all the separation set under study.

Section S3 -Results and discussion
Activity coefficients at infinite dilution

Thiophene and water
Water Thiophene S13  As can be seen in Eq. (S8), the calculation of depends on the availability of the IL density data, which were experimentally determined before for [C8mim]Cl. [17][18][19][20][21][22][23][24][25]   In this context, a literature review on the available density data for the imidazolium chloride based IL was performed (  Table S4 .

Gas-liquid partition coefficients
To check the reliability of the predictions, the density of [C8mim]Cl was also estimated using the available data for [Cnmim]Cl IL, with n ranging from 2 to 6, achieving an average relative deviation (from the average experimental data found in literature) of 0.21%. Moreover, this methodology has already been successfully used by us to predict the densities of [C12mim][BF4], 27 with an average relative deviation of 0.21% of the experimental data available in literature. To the best of our knowledge, no density data is available for comparison for [C12mim]Cl or for the equimolar mixture.
Since ̅ can be directly obtained from the slope of ln (  To further explore this topic, the partial molar thermodynamic functions were represented as function of ln ( 13 ∞ ) where four distinct regions can be distinguished - Figure S3. Figure S3. Partial molar excess properties as a function of ln ( 13 ∞ ) for the studied imidazolium chloride IL at 373.15 K. The line represents ̅ , the circles correspond to ̅ , and the triangles stand for ̅ .
As can be seen from Figure S3, particularly for alcohols and water, the partial molar      TZVP  TZVPD-FINE  TZVP  TZVPD-FINE  TZVP  TZVPD-FINE  TZVP  TZVPD-FINE  TZVP  TZVPD-FINE  TZVP  TZVPD-FINE  TZVP  TZVPD    Cl. e Predictions were truncated at / = 0.05 due to evidence of the saturation point in the liquid phase. 47