Complexation of Phenols and Thiophenol by Phosphine Oxides and Phosphates. Extraction, Isothermal Titration Calorimetry, and ab Initio Calculations

Abstract

To develop a new solvent-impregnated resin system for the removal of phenols from water the complex formation of triisobutylphosphine sulfide (TIBPS), tributylphosphate (TBP), and tri-n-octylphosphine oxide (TOPO) with a series of phenols (phenol, thiophenol, 3-chlorophenol, 3,5-dichlorophenol, 4-cyanophenol, and pentachlorophenol) was studied. The investigation of complex formation between the extractants and the phenols in the solvent toluene was carried out using liquid-liquid extraction, isothermal titration calorimetry (ITC), and quantum chemical modeling (B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) and MP2/6-311++G(2d,2p)//B3LYP/6-311G(d,p)). The equilibrium constant (binding affinity, Kchem), enthalpy of complex formation (DeltaH), and stoichiometry (N) were directly measured with ITC, and the entropy of complexation (DeltaS) was derived from these results. A first screening of K chem toward phenol revealed a very high binding affinity for TOPO, and very low binding affinities for the other extractants. Modeling results showed that although 1:1 complexes were formed, the TIBPS and TBP do not form strong hydrogen bonds. Therefore, in the remainder of the research only TOPO was considered. Kchem of TOPO for the phenols in toluene increased from 1,000 to 10,000 M(-1) in the order phenol < pentachlorophenol < 3-chlorophenol < 4-cyanophenol approximately 3,5-dichlorophenol (in line with their pKa values, except for pentachlorophenol) in the absence of water, while the stoichiometric ratio remained 1:1. In water-saturated toluene, the binding affinities are lower due to co-complexation of water with the active site of the extractant. The increase in binding affinity for TOPO in the phenol series was confirmed by a detailed ab initio study, in which Delta H was calculated to range from -10.7 kcal/mol for phenol to -13.4 kcal/mol for 4-cyanophenol. Pentachlorophenol was found to behave quite differently, showing a DeltaH value of -10.5 kcal/mol. In addition, these calculations confirm the formation of 1:1 H-bonded complexes.

Recovery and Separation of Metal Ions from Aqueous Solutions by Solvent‐Impregnated Resins

“…Although the applications of SIRs have been extended to extraction and removal of organics, such as aromatics, aliphatic compounds [59], methyl tert-butyl ether (MTBE) [60], phenols [61], and aldehydes [62], from aqueous solutions, the recovery and separation of metal ion mixtures in hydrometallurgy, analytical and environmental chemistry, and radiochemistry are still intensive research topics for SIRs. In these studies, a series of metal-specific liquid extractants were used in preparation of SIRs to separate and/or concentrate metal ions from aqueous solutions.…”

Complexation of Phenol and Thiophenol by Amine N‐Oxides: Isothermal Titration Calorimetry and ab Initio Calculations

“…[22] In the calorimetric titration of phenol with DMDAO in watersaturated toluene, the binding enthalpy (DH 0 = À7. [7] [amine N-oxide] is typically 10-20 mm) will preferably be present near the most polar positions in the molecules, that is, near the N + ÀO À moiety in amine N-oxides and near the OH and SH groups in phenol and thiophenol, respectively. Multiple water molecules [45] will be replaced on phenol binding, and this water, which is freed from every amine N-oxide and phenol molecule, will form larger water clusters in the apolar solvent and on average become more structured, even if it remains weakly attached to the amine N-oxide-phenol complex as a hydrogen-bonded large cluster (Figure 3 C).…”

“…Only recently have investigations on other applications been performed, for example, the recovery of apolar organics. [6,7] Bench-scale or pilot-scale recovery of polar organics like organic acids, [8,9] amino acids, [10] and flavonoids has also been realized. [11] To enhance extraction, complex-forming extractants can be added to the organic phase.…”

“…It was shown that complex-forming compounds containing a phosphoryl moiety are suitable extractants. [7,12] However, Torgov et al [13][14][15] indicated that amine N-oxides would likely be even better for phenol recovery, which prompted this study with amine N-oxides as potential candidates that can form hydrogen-bonded complexes. For example, an investigation [15] of the removal of low concentrations of phenol (10-100 mg L À1 ) from sodium sulfate solutions (0.25-0.4 m) with tri-…”

“…Although ITC measurements are usually carried out in aqueous solutions, [26,27] its use is also unproblematic with cosolvents like methanol or DMSO, [28] or with organic solvents like chloroform, [29,30] toluene, [7,29,31,32] and others. [33] Benzene, toluene, and mesitylene were used as apolar model solvents.…”