Polyol induced extraction (PIE) of water from organic solvents

Publication Year:
2014
Usage 929
Downloads 800
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Repository URL:
http://scholarship.shu.edu/dissertations/1989
Author(s):
Deshpande, Mithilesh
Tags:
Polyols; Organic solvents; Water; Mass separating agents; Acetonitrile; Chemistry; Other Chemistry
thesis / dissertation description
Acetonitrile (ACN) is used in huge quantities for Reversed Phase High Performance Liquid Chromatography (RP-HPLC) experiments. For most of the part, these results in waste that is only contaminated with large amounts of water, buffer and trace organic impurities. It would seem worthwhile to purify the ACN from water using a mass separating agent (MSA). In a quest for an MSA, our investigation reveals polyols as a new class of MSAs. It is observed that when a polyol such as glycerol, sorbitol, erythritol, xylitol, Isomalt® or maltitol is added to a mixture of ACN-water, the ACN separates out of the mixture to form a new immiscible upper phase. The study reveals that glycerol, initiates a phase separation of 1:1 (v/v) ratio of an ACN-water mixture at 20 oC with a glycerol concentration of 17.5 wt/wt %. At 4 oC, only 5 wt/wt % is required to give separation. The results demonstrate that glycerol can improve purity of ACN up to 96 % at lower temperatures thus capable of breaking the ACN-water azeotrope. We also show that polyols as separating agents are equivalent or better than MSAs such as sugars and salts. Further, liquid polyols such as glycerol are far easier to remove, and are better suited to commercial processes.Equilibrium constants for phase separation (KPS) have been measured by 1H NMR and GC-TCD techniques. For the glycerol/ACN/water and sorbitol/ACN/ water systems KPS is < 1 indicating that the free energy for the reaction is positive. For example, glycerol/ACN/water gives the following KPS values at temperatures indicated in parentheses 0.38 (20 oC), 0.48 (10 oC), 0.60 (0 oC), 0.69 (-10 oC) and 0.81 (-20 oC). Thus, KPS increases as temperature decreases. At -10 oC, glycerol (KPS= 0.69) is a more effective MSA than sorbitol (KPS= 0.57). The enthalpy of the phase separation process was investigated using isothermal calorimetry. At 20 oC, ∆Ho is exothermic (-2.06 kJ/mol) for glycerol induced phase separation in ACN-water system and -7.07 kJ/mol at -10 oC. Considering the glycerol/ACN/water system at -10 oC, DGo is 0.83 kJ and DHo is -7.07 kJ/mol, this indicates that enthalpy is the dominant thermodynamic parameter for phase separation. A reasonable interpretation is that the polyol as a MSA forms such strong hydrogen bonds with water that this polyol displaces acetonitrile from its interaction with water to an extent that it forms a separate phase.Our study also demonstrates that solvents in all of the major categories, non-polar (carbon tetrachloride, gasoline, kerosene, diesel, petroleum distillates, cyclohexane, hexanes), polar aprotic (acetonitrile, 2-butanone, acetone, cyclopentanone, dichloromethane, tetrahydrofuran, 2-methyl tetrahydrofuran, 1,2 dimethoxy ethane, methyl benzoate, propyl carbonate, ethyl acetate, chloroform, diethyl ether, mesitylene, p-xylene, benzene, toluene, t-butyl methyl ether, dimethylpthalate, dibutylpthalate) and polar protic (t-butanol, 1-butanol, isopentyl alcohol, cyclohexanol, 1-octanol) can be purified by the PIE process. Some solvents are not separated such as methanol/H2O, ethanol/H2O, dimethylsulfoxide/H2O. In these cases, it is likely that the solvent-water interaction is stronger than the polyol-water interaction. Broader theory indicates that if we know the enthalpy of the solvent-water interaction and if it is less negative than the polyol-water interaction, then phase separation will occur.These results extended the scope of applications in purification of organic solvents where presence of water as an impurity is of concern.