Application of Modified COSMO-SAC in LLE
One of the applications where the hydrogen bonding is put to test can be aqueous-based systems. On a similar line, lignocellulosic biomass has received a great attention as a renewable energy resource as they can improve energy security and reduce carbon emissions (Naik, Goud, Rout, & Dalai, 2010; Sims, Mabee, Saddler, & Taylor, 2010). The last decade saw considerable research in the fast pyrolysis process for the production of liquid fuel and chemicals from biomass. Fast pyrolysis of biomass produces 60-75 wt% of liquid bio-oil, 15-25 wt% of solid char and 10-20 wt% of noncondensable gases depending on the feedstock used (Bridgwater, 2003, 2012). Several chemicals have been identified in bio-oil of which the most abundant and of interest are glycolaldehyde (0.9-13 wt%), acetic acid (0.5-12 wt%), formic acid (0.3-9.1 wt%), acetol (0.7-7.4 wt%), furfural alcohol (0.1-5.2 wt%) and furfural (0.1-1.1 wt%) (Bharti & Banerjee, 2015). Due to the high concentration of the value-added chemical compounds, production of chemicals from bio-oil has received considerable interest. To extract these chemicals from bio-oil, LLE is a popular technique and a significant portion of diluents were found in the raffinate phase or the aqueous-rich phase of LLE. This necessitates the use of novel solvents which can serve two purposes, namely negligible concentration in the aqueous phase and higher selectivity and distribution for bio-oil-derived chemicals. Acetic acid and furfural are chosen as model compounds to extract from the aqueous phase of bio oil and the phase behavior was predicted by the modified COSMO- SAC approach. Based on preliminary LLE experiments, hydrophobic ionic liquids are used for the extraction of acetic acid and furfural from aqueous solution. Hydrophobic characteristic of ILs mainly depend on the nature of the anion. Thus commercial hydrophobic imidazolium-based ionic liquid, 1-butyl- 3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([BMIM][Tf2N]), was investigated for the extraction at T = 298.15 K and atmospheric pressure.
A conformal study was carried out to have a close look into the effect of hydrogen bonding in the systems. Furfural (C4H3OCHO, FUR) has two planar rotational conformers: cis-OO and trans-OO (Rivelino, Coutinho, & Canuto, 2002; Rogojerov, Keresztury, & Jordanov, 2005). These two conform- ers are formed by the rotation of the carbonyl (-CHO) group around the C-C single bond. In cis-OO, carbonyl O and furfural ring O atom remain on the same side of the C-C single bond, whereas in trans-OO, they remain opposite to each other with respect to the C-C bond. Figure 5.3a and b shows the optimized equilibrium geometries of two conformers with important geometrical parameters and total energies. The two conformers can be best described by the dihedral angle O1-C4-C5-O2. The calculated dihedral angles for conformers are cis-OO (0°) and trans-OO (180°). Based on the optimized geometries, the energy order of the conformers is predicted to be trans-OO (-342.5186444 Hartree) < cis-OO (-342.5179797 Hartree). This indicates that the trans-OO conformer is more stable than cis-OO by 1.75 kJ/mol. Thus the trans-OO conformer structure is then used for COSMO file generation. In a similar manner, acetic acid monomer (CH3COOH, AA) has two conformers: trans-AA and cis-AA. In trans-AA, hydroxyl H points along the carbonyl O, whereas in cis-AA, it points opposite to carbonyl O (Ma^oas, Khriachtchev, Fausto, & Rasanen, 2004; Senent, 2001). The equilibrium geometries of two conformers along with important geometrical parameters and total energies are shown in Figure 5.3c and d. The trans-AA has been found to be more stable than the cis-AA by 25.3 kJ/mol due to formation of the hydrogen bond between carbonyl O and hydroxyl H with the H-bond length (HbL) of 2.279 A. Here the trans-AA conformer structure is then used for COSMO file generation. In the case of ionic liquid, the COSMO file is generated separately for the cation and anion. Based on the COSMO file, the sigma profiles for the molecules are calculated by the modified COSMO-SAC approach as
FIGURE 5.3
Optimized equilibrium geometry of (a) cis-OO furfural and (b) trans-OO furfural. (Continued)
FIGURE 5.3 (Continued)
Optimized equilibrium geometry of (c) trans-AA (total energy = -228.5682807 Hartree) and (d) cis-AA (total energy = -228.5586313 Hartree).
explained in Section 5.3. For this study, we only apply the modification from Equations 5.8 through 5.26.
Thus the adoption of a continuous probability distribution function of charge density for the acceptor and donor segments increases the possibility of forming a hydrogen bond (Hb). All the compounds considered in this case study, namely, [BMIM][Tf2N], water, furfural and acetic acid, not only have neutral segments for the nonhydrogen bonding c-profile but also have both hydrogen bonding acceptor segments (from the O, N, F atoms) and hydrogen bonding donor segments (from H atoms connected to the O, N, F atoms);
both contribute to the hydrogen bonding o-profile. It should be noted that the parameters aeff and cHb were not reoptimized in this work. Thus the hydrogen bonding portion of the sigma profile was obtained from the combination of the electronegative atom and hydrogen atom only. The hydrogen bonding, nonhydrogen bonding and the total sigma profile for all the species namely BMIM, Tf2N, furfural and acetic acid are depicted in Figures 5.4 through 5.6.
As depicted, the COSMO-SAC model reproduces the correct raffinate phase composition for both the systems. It also predicts a negligible presence of IL in the raffinate phase which is the same as of the experimental trend. The most important aspect of this study is the correct prediction of the slopes of the experimental and predicted tie lines. It should be noted that there was no modification of the COSMO-SAC parameters, where the model
FIGURE 5.4
Hydrogen bonding sigma profile for all components.
FIGURE 5.5
Nonhydrogen bonding sigma profile for all components.
Phase Equilibria in Ionic Liquid Facilitated Liquid-Liquid Extractions
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FIGURE 5.6
Total sigma profile (Hb + non-Hb) for all components.
was reimplemented from Wang et al. (2007). This further verifies the NMR peak assignments and the experimental procedure. The goodness of fit is measured by the root-mean-square deviation (RMSD), which provides the RMSD values of 2.9% (acetic acid) and 2.2% (furfural). These RMSD values indicate a good degree of consistency of the experimental LLE data for the studied systems at 298.15 K (Figures 5.7 and 5.8).
FIGURE 5.7
Experimental and COSMO-SAC predicted tie lines for the ternary system: [BMIM][TF2N] acetic acid-water at T = 298.15 K and p = 1 atm.
FIGURE 5.8
Experimental and COSMO-SAC predicted tie lines for the ternary system: [BMIM][TF2N]- furfural-water at T = 298.15 K and p = 1 atm.
- [1] Section 5.4 reprinted (adapted) from A. Bharti, T. Banerjee, Enhancement of bio-oil-derivedchemicals in aqueous phase using ionic liquids: experimental and COSMO-SAC predictionsusing a modified hydrogen bonding expression. Fluid Phase Equilibria. 400, 27-37, 2015. Copyright2015, with permission from Elsevier.
