Solvent extraction of palladium(II) with newly synthesized asymmetric branched alkyl sulfoxides from hydrochloric acid

Abstract

A group of new asymmetric branched alkyl sulfoxides was synthesized and applied in palladium(II) extraction for the first time. According to the experimental data, the concentration of hydrochloric acid in aqueous phase greatly influenced Pd(II) extraction, which could be interpreted by different mechanisms (ligand substitution mechanism at low acidity and ion association mechanism at high acidity). In regard to structure effect, the steric hindrance of branched alkyl showed remarkable influence on the extraction performance of sulfoxides. On the basis of thermodynamic analysis, it was demonstrated that higher temperature exerted positive influence on the Pd(II) extraction reaction. As for the separation of Pd(II) and Pt(IV), low concentration of hydrochloric acid was considered to be appropriate for highly effective separation. Furthermore, ammonia solution was proven to be an efficient stripping agent for palladium recovery from organic phase.

---

1. Introduction

Palladium together with other platinum-group metals exists in ores in the Earth's crust at very low levels. Palladium and its alloys are widely applied in chemical industries and instrumentations due to their corrosion resistant properties and easy alloying. It has also been proven to be an efficient catalyst in various organic reactions such as hydrogenation, fluorination, C–N cross-coupling reactions, and carbonylation.^1–4

Solvent extraction has shown superior performance in the separation of palladium from aqueous solutions,^5–8 because a lot of soluble palladium complexes could be formed in organic solvents.⁹ A variety of organic compounds that bear donor atoms, such as sulfur, phosphorus and nitrogen, have been used as an extractant for palladium extraction.^10–13 Sulfoxides, which act as soft base ligands containing both S and O atoms, have been proven to be good ligands;¹⁴ in particular, they show excellent property in coordinating with soft Lewis acid metals such as palladium.¹⁵ In previous studies, many sulfoxides have been synthesized and used for palladium extraction. Nevertheless, most of them are symmetrical sulfoxides.^16–18 Compared to symmetrical sulfoxides, asymmetric sulfoxides show superiority in structure design through the introduction of different sorts of substituents group at the same time, which can contribute significantly to the improvement of extraction performance.¹⁹ However, there are only few studies regarding the use of asymmetric sulfoxides for palladium extraction, and the effect of structure for the improvement of extraction performance is still not investigated systematically.

In this study, to obtain superior sulfoxide structure for better extraction performance, a group of asymmetrically branched alkyl sulfoxides were synthesized for the first time (the synthetic route is shown in Fig. 1) and applied to Pd(II) separation. The mechanism of Pd(II) extraction by these sulfoxides from a hydrochloric acid medium and the influence of temperature on the extraction reaction were investigated. As for the structure effect, the influence of the steric hindrance of branched alkyl on sulfoxides extraction capacity was studied systematically, which would be helpful for designing more efficient asymmetric sulfoxides structure. Furthermore, the separation performance of the synthetic sulfoxides for Pd(II) and Pt(IV) from hydrochloric acid medium was also discussed. In the end, palladium loaded in the organic phase can be efficiently stripped by ammonia solution.

Fig. 1 The synthetic route for asymmetric alkyl sulfoxides.

2. Experimental

2.1 Reagents and materials

Various alkyl bromides were purchased from Aladdin Chemistry Co., Ltd (Shanghai, China). Pd(II) stock solution (2.0 × 10⁻⁴ mol L⁻¹) was prepared by dissolving metal chlorides in HCl solution and PdCl₂ was obtained from Sinopharm Chemical Reagent Beijing Co., Ltd (Beijing, China). Toluene was used as a diluent for the sulfoxides. Distilled water was used to prepare aqueous solutions in all the experiments. The abovementioned reactants were of analytical grade and were used without further purification.

2.2 Analytical techniques

Pd(II) concentration in the aqueous phase was determined using an atomic absorption spectrophotometer (3150, Precision & Scientific Instrument Shanghai Co., Ltd, Shanghai, China). IR spectra were obtained on an IR spectrophotometer VERTEX 70 FT-IR (Bruker Optics). ¹H NMR spectra were obtained on a Bruker Avance 300 (300 MHz) spectrometer with CDCl₃ as solvent and tetramethylsilane (TMS) as an internal standard. HRMS spectra were recorded on a Q-TOF6510 spectrograph (Agilent).

2.3 General extraction procedure

Equal volumes of aqueous and organic phases were added to a glass tube and then equilibrated mechanically in an orbital shaker for 40 min (equilibrium was reached in 20 minutes). Subsequently, the organic phase and aqueous phase were separated quickly by centrifugation at 2000 rpm for 5 min. Metal concentration in the aqueous phase was determined by an atomic absorption spectrophotometer at 247.6 nm. Metal concentration in the organic phase was calculated by mass balances. Unless stated specifically, all the experiments were carried out at 298 ± 1 K.

2.4 Synthesis procedure

Alkanethiols, thioethers and sulfoxides were synthesized by previously reported methods with slight modifications.^20–24

Synthesis of alkanethiols. The corresponding alkyl bromide (0.20 mol) and thiourea (0.21 mol) were added to ethanol (120 mL) and the reaction mixture was refluxed and stirred for 7 h. NaOH solution (2.5 mol L⁻¹, 120 mL) was then added into the mixture and refluxed for another 2 h. Subsequently, the reaction solution appeared as two layers, the aqueous layer was separated and acidified by a diluted HCl solution and then extracted twice by petroleum ether. The combined organic phase was washed with water and dried over sodium sulfate. Petroleum ether was removed by distilling under vacuum. Colorless liquids were obtained in all the cases with high purity and good yield (61–82%).

Synthesis of thioethers. NaOH (0.10 mol) was dissolved in absolute ethanol (100 mL) with heating and stirring. After the dissolution was complete and the temperature was dropped to about 25 °C, the corresponding thiol (0.11 mol) was added dropwise for ten minutes, followed by the addition of alkyl bromide (0.10 mol) at the same temperature. Precipitation of sodium chloride appeared almost immediately, and the mixture was then stirred and refluxed for 3 h. Subsequently, the cooled mixture was poured into water (900 mL), and the oil layer was separated by extracting twice with petroleum ether. Combined organic phase was washed with water and dried over sodium sulfate. After removing petroleum ether by distilling under vacuum, colorless liquids were obtained in all the cases with high purity and good yield (81–94%).

Synthesis of sulfoxides. Thioethers (0.05 mol) and 30% aqueous H₂O₂ (5.1 mL) were mixed and stirred at 60 °C overnight. After cooling, the reaction mixture was then extracted twice with petroleum ether; the combined organic phase was washed with water and dried over sodium sulfate. Petroleum ether was then removed by distilling under vacuum. The residue was purified by column chromatography on silica gel with petroleum ether to ethyl acetate as the eluent, and the solvent was removed by rotary evaporation. After drying under high vacuum for 2 h, colorless liquids were obtained with good yield (78–83%).

Synthetic sulfoxides were characterized as follows.

Isooctyl n-amyl sulfoxide. Yield, 81%; colorless liquid; ¹H NMR: δ 0.89–0.95 (m, 9H), δ 1.30–1.71 (m, 12H), δ 1.73–1.83 (p, 2H), δ 1.89–1.93 (m, 1H), δ 2.43–2.51 (m, 1H), δ 2.57–2.73 (m, 3H); IR (KBr): 2959, 2927, 2865, 1738, 1459, 1378, 1033, 827, 730 cm⁻¹; HRMS: calcd for [M + H]⁺ C₁₃H₂₈OS: 233.1934; found: 233.1943.

Isooctyl 1-methylbutyl sulfoxide. Yield, 80%; colorless liquid; ¹H NMR: δ 0.88–0.99 (m, 9H), δ 1.21–1.61 (m, 14H), δ 1.77–1.81 (m, 1H), δ 1.89–1.98 (m, 1H), δ 2.39–2.47 (m, 1H), δ 2.52–2.64 (m, 2H); IR (KBr): 2959, 2929, 2871, 1464, 1403, 1383, 1035, 827, 727 cm⁻¹; HRMS: calcd for [M + H]⁺ C₁₃H₂₈OS: 233.1934; found: 233.1933.

Isooctyl 2-methylbutyl sulfoxide. Yield, 78%; colorless liquid; ¹H NMR: δ 0.89–0.96 (m, 9H), δ 1.06–1.10 (t, 3H), δ 1.27–1.65 (m, 10H), δ 1.91–1.94 (m, 2H), δ 2.27–2.35 (dd, 1H), δ 2.46–2.76 (m, 3H); IR (KBr): 2961, 2929, 2874, 1737, 1461, 1406, 1379, 1034, 774, 729, 695 cm⁻¹; HRMS: calcd for [M + H]⁺ C₁₃H₂₈OS: 233.1934; found: 233.1934.

Isooctyl 3-methylbutyl sulfoxide. Yield, 83%; colorless liquid; ¹H NMR: δ 0.89–0.98 (m, 12H), δ 1.26–1.75 (m, 11H), δ 1.89–1.94 (m, 1H), δ 2.44–2.52 (m, 1H), δ 2.63–2.73 (m, 3H); IR (KBr): 2958, 2928, 2872, 1464, 1408, 1382, 1034, 880, 727, 640 cm⁻¹; HRMS: calcd for [M + H]⁺ C₁₃H₂₈OS: 233.1934; found: 233.1910.

3. Results and discussion

3.1 Influence of extraction time

The effect of contact time on Pd(II) extraction was studied first. Other extraction parameters were fixed as follows: organic phase of 0.1 mol L⁻¹ isooctyl n-amyl sulfoxide (OASO), aqueous phase of 2.0 × 10⁻⁴ mol L⁻¹ Pd(II) in 0.1 mol L⁻¹ HCl solution, and R_w:o = 1. The contact time of the two phases ranged from 5 to 30 min. The extraction equilibrium can be reached in 20 min, then the extraction was independent of the contact time. Therefore, 25 min of contact time was sufficient for the following extraction experiments.

3.2 Influence of hydrochloric acid concentration

The influence of HCl concentration in aqueous phase on the Pd(II) extraction was investigated. As shown in Fig. 2, the concentration of the HCl solution drastically influences the Pd(II) extraction yield, and Pd(II) can be efficiently extracted by OASO under considerably low and considerably high HCl concentration. When the HCl concentration ranged from 0.1 mol L⁻¹ to 7.0 mol L⁻¹, the extraction yield decreased rapidly from 0.1 mol L⁻¹ to 2.0 mol L⁻¹ and then increased gradually from 2.0 mol L⁻¹ to 7.0 mol L⁻¹. The quantitative extraction of palladium was found at 0.1 mol L⁻¹ HCl. This phenomenon can be explained by different extraction mechanisms, which would be discussed in subsequent sections.

Fig. 2 Effect of HCl concentration on the percentage extraction of palladium. [Pd(II)] = 2.0 × 10⁻⁴ mol L⁻¹; organic phase: 0.1 mol L⁻¹ OASO in toluene.

3.3 Extraction mechanism analysis

As different variation tendencies were observed at different concentrations of HCl (Fig. 2), the extraction mechanism was investigated at low and high HCl concentrations. The extraction mechanism of Pd(II) with the synthetic sulfoxides under low HCl concentration ([HCl] < 2.0 mol L⁻¹) was investigated first. The influence of sulfoxide concentration on the extraction behavior of Pd(II) was studied to determine the ratio of sulfoxide and metal ion in the extracted species by plotting lg D (distribution ratio) versus lg[sulfoxide] at constant pH.^25,26 As shown in Fig. 3, when the HCl concentration of aqueous phase was fixed at 0.1 mol L⁻¹, plots of lg D versus lg[sulfoxide] yielded a series of straight lines with slopes of 1.95, 1.91, 1.94 and 1.96 (all of them are very close to 2), which indicated that two sulfoxide molecules were included in the Pd(II)-sulfoxide complex.

Fig. 3 Plots of lg D versus lg[sulfoxide] at 0.1 mol L⁻¹ HCl. [Pd(II)] = 2.0 × 10⁻⁴ mol L⁻¹.

Furthermore, we studied the dependence of Pd(II) extraction on [H⁺]. By fixing [Cl⁻] at 1.0 mol L⁻¹ by adding NaCl, the effect of H⁺ was studied with [H⁺] ranging from 0.1 to 1.0 mol L⁻¹. As shown in Fig. 4, variation of [H⁺] did not influence the distribution ratio with different sulfoxides, which demonstrated that H⁺ did not participate in the extraction reaction at low HCl concentration.

Fig. 4 Plots of lg D versus lg[H⁺] while [HCl] < 2.0 mol L⁻¹. [Pd(II)] = 2.0 × 10⁻⁴ mol L⁻¹; organic phase: 0.1 mol L⁻¹ extractant in toluene.

Because the extraction percentage of Pd(II) decreased drastically as the HCl concentration increased from 0.1 mol L⁻¹ to 2.0 mol L⁻¹, while the extraction was independent of H⁺, the increase in [Cl⁻] was supposed to have a negative effect on the extraction of Pd(II). In the following experiments, to investigate the influence of [Cl⁻], [H⁺] was fixed at 0.1 mol L⁻¹ and [Cl⁻] was varied from 0.1 to 1.0 mol L⁻¹ by adding NaCl. As shown in Fig. 5, in different sulfoxide extractant systems, plots of lg D versus lg[Cl⁻] yielded a series of straight lines with slopes of −1.94, −1.95, −1.91 and −1.92, which were close to −2. These experimental results indicated that two Cl⁻ ions were released in the extraction reaction when [HCl] < 2.0 mol L⁻¹.

Fig. 5 Plots of lg D versus lg[Cl⁻] while [HCl] < 2.0 mol L⁻¹. [Pd(II)] = 2.0 × 10⁻⁴ mol L⁻¹; organic phase: 0.1 mol L⁻¹ extractant in toluene.

To assess the coordination of Pd and sulfoxides, ¹³C NMR spectroscopy was used for the analysis of OASO and Pd-OASO complexes. Extracted Pd(II)-OASO adduct was prepared by the following procedure: 0.5 mol L⁻¹ OASO in toluene was shaken with a Pd(II) aqueous solution (1.0 g L⁻¹ in 0.1 mol L⁻¹ HCl) many times to obtain a saturated extraction organic phase. After removing toluene by distillation, a yellowish complex was obtained and dried under vacuum. As shown in Fig. 6, compared to the peaks that appeared in ¹³C NMR spectrum of free OASO, the peaks of Pd(II)-OASO shifted slightly to lower field and the shape of peaks clearly changed, which indicated the coordination between Pd and OASO.

Fig. 6 ¹³C NMR spectra of Pd(II)-OASO complex and free OASO.

IR spectroscopy was also employed for the analysis of Pd(II)-OASO complex to obtain more information regarding the coordination between the sulfoxides and Pd(II). As shown in Fig. 7, the characteristic S=O stretch peak of free OASO appeared as a strong absorption at 1033 cm⁻¹, and two new absorption peaks appeared at 931 cm⁻¹ and 1140 cm⁻¹ in the infrared spectrum of Pd(II)-OASO. In comparison with the characteristic S=O stretch peak, the new absorption peak appearing at 931 cm⁻¹ (shifted to a lower wavenumber by about 102 cm⁻¹) was attributed to the coordination between Pd(II) and O atom. Another new absorption peak appeared at 1140 cm⁻¹ (shifted to higher wavenumber about 107 cm⁻¹), indicating that S atom was also coordinated with Pd(II).^27,28 The peak appearing at 1033 cm⁻¹ in the infrared spectrum of Pd(II)-OASO may be attributed to the residual free OASO. Based on the discussion above, it can be inferred that these sulfoxides were not only coordinated with Pd(II) through the O or S atom, but both O and S atoms also had a coordination interaction with palladium.

Fig. 7 Infrared spectra of Pd(II)-OASO complex and free OASO.

It is known that palladium is a soft base that prefers to coordinate with S atom rather than O atom. However, the IR spectra (Fig. 7) showed that S and O atoms have similar coordination activity in these sulfoxides, which may be explained by the structure effect of these sulfoxides. S atom suffers greater steric hindrance than O atom, which weakens its coordination activity. In contrast, the steric hindrance on O atom is considerably smaller due to its protruded spatial position. Therefore, S and O atoms in these sulfoxides exhibited similar activity in coordination.

According to the studies mentioned above, we inferred that Pd(II) was extracted by the synthetic asymmetric branched alkyl sulfoxides through the ligand substitution mechanism in low HCl concentrations, and both O and S atoms participated in the coordination interaction. The extraction reaction may be depicted as follows:

The variation tendency in the extraction yield of Pd(II) at low HCl concentrations (Fig. 2) can be explained by this extraction mechanism. H⁺ was independent of the extraction reaction, while two Cl⁻ ions were released during the extraction procedure; therefore, the extraction yield decreased quickly as the HCl concentration increased.

The extraction mechanism at high HCl concentrations ([HCl] > 2.0 mol L⁻¹) was then investigated. The influence of sulfoxide concentration on the extraction behavior of Pd(II) is shown in Fig. 8; plots of lg D versus lg[sulfoxide] yield a series of straight lines with slopes of 2.92, 2.97, 3.03 and 2.94 (all of them are very close to 3), which indicate that three sulfoxide molecules were involved in Pd(II) extraction reaction. In addition, the study of the dependence of Pd(II) extraction on the [H⁺] showed that hydrogen ion had positive impact on the extraction.

Fig. 8 Plots of lg D versus lg[sulfoxide] at 7.0 mol L⁻¹ HCl. [Pd(II)] = 2.0 × 10⁻⁴ mol L⁻¹.

We also tried to interpret the variation tendency at high HCl concentration through infrared spectra analysis. The complex extracted at high acidity was obtained with Pd(II) in 6.0 mol L⁻¹

HCl aqueous solution, and its infrared spectrum is shown in Fig. 9b. Compared with the infrared spectrum of Pd(II)-OASO complex (Fig. 7), the absorption peaks that appear at 931 cm⁻¹ and 1140 cm⁻¹ were considerably weakened and a new strong absorption peak appeared at 1047 cm⁻¹. In view of the high acidity of aqueous phase and the strength of hydrogen bond,²⁹ the shift of 14 cm⁻¹ (from 1033 cm⁻¹ to 1047 cm⁻¹) may be caused by the hydrogen bond between H₃O⁺ and coordinating sulfoxide group. In addition, the peak shifted by 14 cm⁻¹ disappeared after the extracted complex at high acidity came into contact with ammonia solution (Fig. 9c), which further indicates that H₃O⁺ participated in the extraction reaction. Thus, we inferred that at high HCl concentration, these sulfoxides were not supposed to be coordinated with Pd(II) directly but associated with H₃O⁺, and then PdCl₄²⁻ with an opposite charge was attracted to the organic phase to carry out extraction. These experimental results are similar to the mechanism of Pd(II) extraction by dialkyl sulfoxides at a high HCl concentration, which was reported previously.¹⁸ On the basis of the above discussion, the extraction reaction may be depicted as follows:

L₂·H₃O⁺ + L + PdCl₄²⁻ = (L₂·H₃O⁺)PdCl₃L⁻ + Cl⁻

Fig. 9 Infrared spectra of: (a) free OASO; (b) the extracted complex at high acidity; and (c) extracted complex after contact with ammonia solution.

3.4 Effect of sulfoxides structure on the extraction

On the basis of the experimental data shown in Fig. (3–5), the extraction capacity size comparison among these sulfoxides could be obtained as follows: isooctyl 1-methylbutyl sulfoxide < isooctyl 2-methylbutyl sulfoxide < isooctyl n-amyl sulfoxide < isooctyl 3-methylbutyl sulfoxide. The experimental results shown in Fig. 10 demonstrate that the order of sulfoxides extraction capacity size remained constant at various concentrations of HCl.

Fig. 10 Effect of HCl concentration on the distribution ratio of palladium. [Pd(II)] = 2.0 × 10⁻⁴ mol L⁻¹; organic phase: 0.1 mol L⁻¹ sulfoxide in toluene.

To explain this phenomenon, the IR spectra position of S=O stretch was analysed to measure the electron density difference on coordinating sulfoxide group. However, as listed in Table 1, the position of S=O stretch peak showed minor differences (appearing from 1033 cm⁻¹ to 1035 cm⁻¹), which indicates that the electron density of S=O was similar among these sulfoxides, and the electron donating ability of substituent alkyl group was not the main reason to cause extraction with uneven capacities. In addition, the S=O group suffered greater steric hindrance as the position of branched methyl shifted from γ-position (entry 2) to α-position (entry 4), which may be the main reason why the extraction capacity was significantly reduced.¹⁶ Therefore, we inferred that, in these sulfoxides, the steric hindrance instead of electron donating ability of the branched alkyl exerted remarkable influence on the extraction capacity. Moreover, in comparison between entry 1 and entry 2, a slight growth in extraction capacity was observed. This may be attributed to the fact that the alkyl groups exhibit almost equal steric hindrance on S=O, but the electron donating ability of substituent alkyl groups grew slightly, which can be seen by the IR spectra position of S=O stretch of entries 1–2 (appeared from 1033 cm⁻¹ to 1034 cm⁻¹), as shown in Table 1.

Table 1 Synthetic sulfoxides for the extraction of Pd(II)

Entry	sulfoxide	S=O stretch peak (cm^−1)
1		1033
2		1034
3		1034
4		1035

---

3.5 Thermodynamics of extraction

Equilibrium constants and thermodynamic characteristics were determined to investigate the effect of temperature on the extraction with 0.1 mol L⁻¹ HCl. According to the extraction reaction shown in eqn (1), the equilibrium constant K_e can be written as follows:

The distribution ratio (D) of Pd(II) is

Substituting eqn (3) into (2):

With the experimental data, the lg K_e values were calculated by taking logarithm of eqn (4):

lg K_e = lg D + 2 lg[Cl⁻] − 2 lg[L] (5)

The ΔG values were calculated by ΔG⁰ = − RT ln K_e. The ΔH values were obtained via the slope of the plot of the natural logarithm of K_e versus the inverse temperature (Fig. 11), and the ΔS values were calculated by ΔG⁰ = ΔH⁰ − TΔS⁰. All the determined equilibrium constants and thermodynamic characteristics are summarized in Table 2.

Fig. 11 The natural logarithm of K_e versus the inverse temperature.

Table 2 The equilibrium constants and thermodynamic parameters of Pd(II) extraction at 298 K

Entry	Sulfoxide	Ke	ΔH^0/(kJ mol^−1)	ΔG^0/(kJ mol^−1)	ΔS^0/(J K^−1 mol^−1)
1	Isooctyl n-amyl	36.67	85.54	−8.92	316.97
2	Isooctyl 3-methylbutyl	42.78	82.75	−9.32	308.91
3	Isooctyl 2-methylbutyl	13.12	64.19	−6.37	236.81
4	Isooctyl 1-methylbutyl	7.54	57.87	−5.82	213.76

---

According to the determined ΔH values, shown in Table 2, the reaction of Pd(II) extraction by sulfoxides is endothermic. The variation tendency in the determined ΔG values (entries 1–4) indicate that the extraction capacity of sulfoxides could be drastically affected by the steric hindrance of substituent alkyl groups, which is consistent with the discussion on sulfoxides structure effect described earlier.

3.6 Separation of Pd(II) and Pt(IV)

The separation performance of Pd(II) and Pt(IV) from hydrochloric acid solutions were investigated using OASO diluted in toluene. The experiments were conducted with the following parameters: organic phase of 0.2 mol L⁻¹ OASO, aqueous phase of 2.0 × 10⁻³ mol L⁻¹ Pd(II) and Pt(IV) in HCl solution, R_w:o = 1, and 30 min for the contact of two phases.

As shown in Fig. 12, as the concentration of HCl increased from 0.1 mol L⁻¹ to 7.0 mol L⁻¹, the percentage extraction of Pt(IV) increased gradually and reached 90.9% at 7.0 mol L⁻¹ HCl, and the percentage extraction of Pd(II) extraction remained at a relatively high level (varied from 89.6% to 98.9%). The “concave valley” extraction curve of Pd(II) at 2.0 mol L⁻¹ HCl can be explained by the Pd(II) extraction mechanism discussed above. According to the experimental results, Pd(II) and Pt(IV) can be effectively separated at 0.1 mol L⁻¹ HCl with a separation coefficient of 279.3.

Fig. 12 Effect of HCl concentration on the separation performances of Pd(II) and Pt(IV). [Pd(II)] = [Pt(IV)] = 2.0 × 10⁻³ mol L⁻¹; organic phase: 0.2 mol L⁻¹ OASO in toluene.

3.7 Stripping property of palladium

The stripping of Pd(II) from loaded sulfoxides was investigated using ammonia solution. The experiments were carried out at the following fixed parameters: 0.195 mmol L⁻¹ Pd(II) loaded in the organic phase; contact time of the two phases, 30 min; R_w:o = 1. As shown in Fig. 13, 2.5 mol L⁻¹ ammonia solution was adequate for the quantitative stripping of palladium loaded in OASO. Palladium loaded in different sulfoxides was then stripped with 2.5 mol L⁻¹ ammonia solution. The data shown in Table 3 indicate that palladium loaded in this group of sulfoxides can be quantitatively stripped by 2.5 mol L⁻¹ ammonia solution. Therefore, 2.5 mol L⁻¹ ammonia solution can be used as an effective stripping agent.

Fig. 13 Effect of ammonia solution concentration on the stripping of Pd(II) from OASO. 0.195 mmol L⁻¹ Pd(II) in organic phase; R_w:o = 1.

Table 3 The stripping properties of Pd(II) loaded in the sulfoxides with 2.5 mol L⁻¹ ammonia solution

Entry	Sulfoxide	Percentage stripping of Pd(II)
1	Isooctyl n-amyl	99.4
2	Isooctyl 3-methylbutyl	99.4
3	Isooctyl 2-methylbutyl	99.3
4	Isooctyl 1-methylbutyl	99.5

---

4. Conclusions

A series of novel asymmetric branched alkyl sulfoxides were synthesized and employed in Pd(II) extraction. The study on the influence of HCl concentration in an aqueous phase on Pd(II) extraction showed that HCl can drastically affect the extraction, which can be described as follows: the extraction yield of Pd(II) decreased when [HCl] < 2.0 mol L⁻¹, and then it increased when [HCl] > 2.0 mol L⁻¹. This phenomenon can be explained by different extraction mechanisms. According to the experimental results obtained, Pd(II) was extracted by these sulfoxides via ligand substitution mechanism at a low HCl concentration and ion association mechanism at a high HCl concentration. The structure effect analysis demonstrated that the extraction capacity of sulfoxides clearly decreased with the increasing steric hindrance of branched alkyl groups. Based on the thermodynamic analysis, thermodynamic parameters of the Pd(II) extraction reaction were determined, which indicate that higher temperature showed a positive effect on Pd(II) extraction. The discussion on the separation performance of OASO for Pd(II) and Pt(IV) from an HCl medium showed that Pd(II) and Pt(IV) can be effectively separated at low concentrations of HCl. In addition, Pd(II) loaded in organic phase could be stripped efficiently using 2.5 mol L⁻¹ ammonia solution.

Acknowledgements

This work was supported by the Natural Science Foundation of China (Grants 21276142 and 21476129).

Notes and references

1. H. G. Lee, P. J. Milner and S. L. Buchwald, Org. Lett., 2013, 15, 5602–5605.
2. C. X. Cai, N. R. Rivera, J. Balsells, R. R. Sidler, J. C. McWilliams, C. S. Shultz and Y. K. Sun, Org. Lett., 2006, 8, 5161–5164.
3. N. Hiyoshi, O. Sato, A. Yamaguchi and M. Shirai, Chem. Commun., 2011, 47, 11546–11548.
4. B. P. Fors and S. L. Buchwald, J. Am. Chem. Soc., 2010, 132, 15914–15917.
5. T. N. Lokhande, M. A. Anuse and M. B. Chavan, Talanta, 1998, 46, 163–169.
6. J. Ougiyanagi, Y. Meguro, Z. Yoshida, H. Imura and K. Ohashi, Talanta, 2003, 59, 1189–1198.
7. K. Sasaki, K. Takao, T. Suzuki, T. Mori, T. Araia and Y. Ikeda, Dalton Trans., 2014, 43, 5648–5651.
8. J. Traeger, J. König, A. Städtke and H. J. Holdt, Hydrometallurgy, 2012, 127–128, 30–38.
9. R. D. Oleschuk and A. Chow, Talanta, 1998, 45, 1235–1245.
10. C. Fontàs, E. Anticó, F. Vocanson, R. Lamartine and P. Seta, Sep. Purif. Technol., 2007, 54, 322–328.
11. E. A. Mowafy and H. F. Aly, J. Hazard. Mater., 2007, 149, 465–470.
12. M. R. Rosocka, M. Wisniewski, A. B. Resterna, A. Cieszynska and A. M. Sastre, Sep. Purif. Technol., 2007, 53, 337–341.
13. A. Cieszynska and M. Wisniewski, Sep. Purif. Technol., 2011, 80, 385–389.
14. J. S. Preston and A. C. du Preez, Solvent Extr. Ion Exch., 1996, 14, 755–772.
15. L. Pan, X. Bao and G. Gu, J. Min. Metall., Sect. B, 2013, 49(1), 57–63.
16. J. S. Preston and A. C. du Preez, J. Chem. Technol. Biotechnol., 1997, 69, 86–92.
17. L. Pan and Z. D. Zhang, Miner. Eng., 2009, 22, 1271–1276.
18. J. S. Preston and A. C. du Preez, Solvent Extr. Ion Exch., 2002, 20, 359–374.
19. Y. W. Li, G. B. Gu, H. Y. Liu, H. H. Y. Sung, I. D. Williams and C. K. Chang, Molecules, 2005, 10, 912–921.
20. F. Shi, M. K. Tse, H. M. Kaiser and M. Beller, Adv. Synth. Catal., 2007, 349, 2425–2430.
21. Z. F. Li, Z. R. Wu and F. Y. Luo, J. Agric. Food Chem., 2005, 53, 3872–3876.
22. P. Manivel, K. Prabakaran, V. Krishnakumar, F. N. Khan and T. Maiyalagan, Ind. Eng. Chem. Res., 2014, 53, 7866–7870.
23. L. J. Huo, Y. Zhou and Y. F. Li, Macromol. Rapid Commun., 2009, 30, 925–931.
24. F. L. Liu, Z. H. Fu, Y. C. Liu, C. L. Lu, Y. Y. Wu, F. Xie, Z. P. Ye, X. P. Zhou and D. L. Yin, Ind. Eng. Chem. Res., 2010, 49, 2533–2536.
25. M. B. Gholivand and N. Nozari, Talanta, 2000, 52, 1055–1060.
26. T. N. Lokhande, M. A. Anuse and M. B. Chavan, Talanta, 1998, 46, 163–169.
27. W. Kitching, C. J. Moore and D. Doddrell, Inorg. Chem., 1970, 9, 541–549.
28. J. H. Price, A. N. Williamson, R. F. Schramm and B. B. Wayland, Inorg. Chem., 1972, 11, 1280–1284.
29. L. Bondesson, K. V. Mikkelsen, Y. Luo, P. Garberg and H. Agren, Spectrochim. Acta, Part A, 2007, 66, 213–224.

---