Solvent Extraction Research and Development, Japan, Vol. 21, No 2, (2014)

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1 Solvent Extraction Research and Development, Japan, Vol. 21, No 2, (2014) One Step Effective Separation of Platinum and Palladium in an Acidic Chloride Solution by Using Undiluted Ionic Liquids Jian YANG, 1 Fukiko KUBOTA, 1 Yuzo BABA, 1 Noriho KAMIYA 1,2 and Masahiro GOTO 1,2 * 1 Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Fukuoka , Japan 2 Center for Future Chemistry, Kyushu University, 744 Motooka, Fukuoka , Japan (Received January 29, 2014; Accepted March 3, 2014) Imidazolium-based ionic liquids (ILs) with Tf 2 N - as anions were studied as extractants without dilution for the extractive separation of platinum and palladium in an acidic chloride solution. The effect of the alkyl chain length on the extraction performance was evaluated and, based on the performance, the IL, 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide ([Omim][Tf 2 N]), was selected to be studied in detail. The extraction of Pt(IV) underwent little change over the HCl concentration studied, while that of Pd(II) decreased with increasing HCl concentration. The separation of Pt(IV) and Pd(II) was highly effective, and the highest separation factor between Pt(IV) and Pd(II) reached 312 at the optimum conditions in a binary component system composed of Pt(IV) and Pd(II). The effect of the initial Pt(IV) concentration on loading capacity and extraction efficiency of Pt(IV) was also investigated. 1. Introduction Platinum group metals (PGMs), such as platinum(iv), palladium(ii), rhodium(iii) and ruthenium are nowadays widely used in industry. Though the demand for these metals is continuously increasing, their natural resources are limited. On the other hand, rapid technological change has resulted in the accumulation of a large amount of secondary resources, in which PGMs can be contained. One of the main secondary resources for PGMs is vehicle catalysts. Others can be catalysts in organic processes, metal alloys, waste electrical and electronic equipment, etc. In the last decades, attempts have been made to recover PGMs from such secondary resources[1]. Consequently, the separation and purification of PGMs are becoming more and more important. By far, the separation and recovery of Pt(IV) and Pd(II) have mainly been conducted by a hydrometallurgical method, solvent extraction, which is considered to be advantageous to other traditional methods due to its higher selectivity [2]. Nevertheless, conventional extractants, such as Alamine 300 and 336, can only separate Pt(IV) and Pd(II) from Rh(III) and some base metal ions, but are not effective for the extractive separation of Pt(IV) from Pd(II) [3, 4], wherein the separation of Pt(IV) and Pd(II) is achieved in the following stripping process, which makes the separation process complicated and caused more secondary pollution. Moreover, solvent extraction, during which large amounts of organic solvents are used to dilute the extractants, is recently being reconsidered because of the secondary pollution that might be caused by the

2 evaporation and a leakage of organic solvents. Based on such environmental concerns, an attempt has been made to impregnate extractants into a solid support [5] or use ion exchange resins, so that the use of volatile organic compounds (VOCs) can be avoided. More recently, ionic liquids (ILs), which are composed of only ions, have been considered to be promising alternatives to the classical organic solvents as separation media. ILs have been attracting research interest for their potential use in industrial and analytical fields due to their unique properties, such as low melting point, high thermal stability, negligible vapor pressure and their wide electrochemical window. The most attractive advantage of using ILs as separation media is their negligible vapor pressure. Also, the wide electrochemical window of ILs makes it possible to recover the metal ions in the ILs phase via direct electrodeposition [6]. In the last ten years or so, the feasibility of ILs as alternative media for metal extraction has been evaluated and IL-based systems have been used in supported liquid membranes (SLMs) for the first time [7, 8]. In addition to their use as separation media, ILs have also been used as extractants. Being dissolved in diluents such as chloroform or toluene, ILs Aliquat 336 and tri(hexyl)tetradecylphosponium chloride (Cyphos IL 101) have been studied as extractants for the extractive separation of metal ions[6, 9, 10]. Recently, ILs with hydrophobic anions have also been studied as extractants for Au(III), and it was shown that the extraction mechanism was one of anion exchange between the anionic Au(III) chloro-complex and the anion contained in the ILs[11, 12]. However, there are few reports on the separation of precious metal ions by using ILs, especially on the separation of platinum group metal ions [13], despite it is of great importance. In our previous study, we showed that, when using undiluted ionic liquids as the extracting phase, the anion component species contained in the ILs play a significant role in their extraction performance towards precious metal ions, especially in their extraction selectivity [14]. Compared to PF 6 anions, ILs contained Tf 2 N anions showed better extraction selectivity for Au(III) towards Pt(IV) and Pd(II). Though the ILs with PF 6 show better Pt(IV) extraction ability than that of ILs with Tf 2 N, they may not be practical for the extraction of metal ions from water because PF 6 slowly reacts with water [15]. In the present study, the imidazolium-based ILs with Tf 2 N anions have been evaluated as an extracting phase without dilution for the extractive separation of Pt(IV) and Pd(II). The effect of alkyl chain length on the extraction and separation of Pt(IV) and Pd(II), effect of hydrochloric acid concentration and loading capacity of the ionic liquids for the extraction of Pt(IV) were evaluated. We thus developed an one step effective separation process for Pt(IV) and Pd(II) by using the undiluted ionic liquids. 2. Experimental 2.1 Reagents Ionic liquids, 1-butyl-3-methylimidazolium bis(trifluoro-methylsulfonyl)imide ([Bmim][Tf 2 N], 1-hexyl-3-methylimidazolium bis(trifluoro-methylsulfonyl)imide ([Hmim][Tf 2 N] and 1-methyl-3-octylimidazolium bis(trifluoro-methylsulfonyl)imide ([Omim][Tf 2 N], whose structures are shown in Figure 1, were purchased from Sigma-Aldrich, and used as received. Standard aqueous solutions (1000 ppm) of Pt(IV) and Pd(II) were purchased from Wako Pure chemicals. Metal chlorides of Pt(IV) and

3 Pd(II) (H 2 PtCl 6 and PdCl 2 ) were purchased from Sigma-Aldrich. All other reagents used were of analytical reagent grade. Figure 1. Molecular structures and abbreviations of ILs used in this study 2.2. Extraction experiments Aqueous phases containing single or two metal ions (Pt(III) and Pd(III)) were prepared by dissolving given amounts of their corresponding metal chloride salts in a hydrochloric acid solution. Batch experiments were conducted by mixing the aqueous phase containing the metal ions with an IL phase in centrifugation tubes. The mixture was vigorously shaken by a vortex mixer (2000 rpm) at room temperature (298 K) for 1 min, and then left standing overnight to reach extraction equilibrium and phase separation. The metal ion concentrations in the aqueous phase before and after extraction were measured by an ICP-atomic emission spectrometer (Perkin Elmer Co., Optima 5300, MA USA). The extraction efficiency (E) was calculated from the following equation: E(%) 100 (C 0 C eq ) (1) C 0 where C 0 and C eq denote the metal concentrations in the aqueous phase before and after extraction, respectively. The distribution coefficient (D), defined as the ratio of the metal ion concentration in the ionic liquid phase to that in the aqueous phase, was generally represented by the following equation: D [M ] IL [M ] aq (2) In this study, the metal ion concentration in the ionic liquid phase was calculated according to the mass balance by comparing the metal ion concentrations in the aqueous phase before and after extraction. Consequently, the distribution ratio was determined by the following equation: D V aq V IL (C 0 C eq) C eq (3) where V aq and V IL are the volumes of the aqueous phase and the IL phase respectively. The efficiency of separation of Pt(IV) from Pd(II) is described by the separation factor : Pt Pd D Pt D Pd (4)

4 3. Results and discussion 3.1 Effect of alkyl chain length attached to the ILs on the extraction ability In our previous study, we showed that ILs [Bmim][Tf 2 N] and [Bmim][PF 6 ] have an extractive selectivity for Pt(IV) over Pd(II). However, neither of them is sufficiently effective for the separation of Pt(IV) from Pd(II). The effect of alkyl chain length on the extraction performance has been discussed in several cases for rare earth metals and actinides, wherein the effect was not very significant. However, few reports have been made on the effect of the alkyl chain length on the extraction and separation of platinum metals, via adjusting the alkyl group length. Therefore in this study, the effect of alkyl chain length has been studied, and the feasibility of separating Pt from Pd via adjusting the alkyl chain length is discussed. Figure 2 shows the results when using ILs with different alkyl chain lengths. On increasing the alkyl chain length, the extraction efficiency of Pt(IV) was significantly increased, while that of Pd(II) increased slightly, indicating the separation possibility of Pt(IV) from Pd(II). The extraction of Pt(IV) and Pd(II) was considered to take place via an anion exchange mechanism, wherein the dissolved species of metal ions play an important role. Under the present experimental conditions, Pt(IV) and Pd(II) mainly exist as chloro-complexes PtCl 6 and PdCl 4, respectively [16, 17]. Figure 2. Effect of alkyl chain length on the extraction percentage of Pt(IV) and Pd(II) in a single component extraction system. Aqueous phase: [HCl] = 1 M, [M] = 1 mm; Extracting phase: [Bmim][Tf 2 N], [Hmim][Tf 2 N] and [Omim][Tf 2 N], respectively. V aq / V IL = 2. PtCl 6 favors ion pair formation with the cation of IL due to its lower charge density whereas PdCl 4, with a bigger hydration shell, has a lesser interdependency with the counterion, thus resulting in a lower preference for the extraction via ion pair formation. As the alkyl chain length contributes to the ion pair stability, the cation with a longer alkyl chain length resulted in a better extraction efficiency for Pt(IV) extraction

5 3.2. Extractive separation of Pt(IV) and Pd(II) The extractive separation performance of Pt(IV) and Pd(II) by using [Omim][Tf 2 N] was evaluated over a range of HCl concentrations. Figure 3a shows the extraction performance of the IL for the two different metal ions in the single component system, while Figure 3b shows the competitive extraction of the two metals in the binary component system E (%) Pt Pd E (%) Pt Pd HCl (M) HCl (M) Figure 3. Extraction efficiency of Pt(IV) and Pd(II) as a function of HCl concentration. a) single component system. Aqueous phase: [Pt] = [Pd] = 1 mm; Extracting phase: [Omim][Tf 2 N]. V aq / V IL = 2; b) binary component system. Aqueous phase: [Pt] = [Pd] = 1 mm; Extracting phase: [Omim][Tf 2 N]. V aq / V IL = 2 As shown in Figure 3a, in the single component system, the extraction efficiency change in behavior of Pt(IV) and Pd(II) in response to the change of HCl concentration was different. A dramatic increase in the extraction efficiency of Pt(IV) was observed on increasing the HCl concentration from 0.01 M to 0.1 M while thereafter no further increase in the extraction efficiency was observed. As for Pd(II), extraction efficiency was steadily decreased. The extraction was expected to take place via an anion exchange reaction, that is, chloro-complex anions were transferred to the IL phase, while Tf 2 N anions were released to the aqueous phase. The anion exchange equations for the Pt(IV) extraction are shown as follows: PtCl 6 aq + 2Omim + Tf 2 N - IL = PtCl 6 (Omim + ) 2IL + 2Tf 2 N - aq (5) - HPtCl 6 aq + Omim + Tf 2 N - IL = HPtCl - 6 Omim + IL + Tf 2 N - aq (6) At HCl concentrations higher than 0.01 M, it is likely that Pt(IV) was extracted as the monovalent anion HPtCl - 6 as shown in equation (6). This would be responsible for the different extraction efficiencies of Pt(IV) at 0.01 M HCl and higher HCl concentration because PtCl 6 is dominant in this HCl concentration range. It must be pointed out that the pk a of H 2 PtCl 6 is not recorded in the literature. Further investigation on the extraction mechanism is required. In the binary component system, as shown in Figure 3b, the overall extraction efficiency change for Pt(IV) and Pd(II) was similar to that in the single component system, with a competitive extraction

6 phenomenon being observed. The extraction interference of Pd(II) on Pt(IV) extraction was rather limited, except for that at an HCl concentration 0.01 M, where Pd(II) chloro-complexes were monovalent anions. The values of the distribution coefficients and separation factors were calculated according to equations (3) and (4), and the results are shown in Table 1. Table 1. Distribution coefficient and separation factor for Pt(IV) and Pd(II) in binary component system HCl concentration (M) D Pt D Pd Effect of the initial Pt(IV) concentration on the Pt(IV) loading capacity and extraction efficiency In conventional solvent extraction, where extractants are dissolved in diluents, larger concentrations of extractants are needed, in order to treat high concentration metal ion solutions. In this case, the solubility of extractants in the diluents can be a problem. On the other hand, since ILs are liquid at room temperature, they can be used without dilution as extractants, which is a great advantage when employing ILs. As far as we are concerned, the only report on using ILs for the extraction of PGMs did not offer the feasibility of separating Pt(IV) and Pd(II) via one step extraction [13]. Also, the metal ion concentration studied in that report was rather low. 7 6 [Pt] IL / mm [Pt] ini / mm Figure 4. Loading capacity and extraction efficiency of Pt(IV) as a function of the initial concentration of Pt(IV) in the aqueous phase. Aqueous phase: [HCl] = 1 M; Extraction phase: [Omim][Tf 2 N]. V aq / V IL = 2 The initial concentration of Pt(IV) in the aqueous phase was varied from 0.5 to 5 mm (100 to 1000 ppm) to evaluate the extraction and the enrichment of Pt(IV) by [Omim][Tf 2 N]. Figure 4 shows the extraction performance of Pt(IV) from a feed aqueous solution (1MHCl). On increasing the initial Pt(IV) concentration,the loading capacity increased continuously without

7 saturation at the high Pt(IV) concentrations examined. More than 80 % Pt(IV) was extracted when the Pt initial concentration was below 2mM (400 ppm), suggesting a capability of treating acidic solutions with high metal ion concentrations. Direct electrodeposition within the ILs is considered to be a feasible method for the recovery of the extracted Pt(IV), and will be investigated in a subsequent study. 4. Conclusions In this study, the extractive separation of Pt(IV) and Pd(II) by undiluted hydrophobic ionic liquids was investigated. In the [Omim][Tf 2 N] extraction system, Pt(IV) was efficiently extracted, while Pd(II) was poorly extracted. The extraction performance of Pt(IV) could be strongly enhanced by increasing the alkyl group length attached to the cation. This suggested that the extraction ability of ILs for the PGMs is controllable, and separation among the PGMs becomes possible by employing ILs designed to have appropriate properties. Acknowledgements This work was supported by a Grant-in-Aid for Science Research (No ) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. References 1) S. Syed, Hydrometallurgy, 115, (2012). 2) B.R. Reddy, B. Raju, J.Y. Lee, H.K. Park, J. Hazard Mater, 180, (2010). 3) P.P. Sun, M.S. Lee, Hydrometallurgy, 109, (2011). 4) B. Swain, J. Jeong, S.K. Kim, J.C. Lee, Hydrometallurgy, 104, 1-7 (2010). 5) R. Navarro, I. Saucedo, C. Gonzalez, E. Guibal, Chem. Eng. J., 185, (2012). 6) P. Giridhar, K.A. Venkatesan, T.G. Srinivasan, P.R.V. Rao, Hydrometallurgy, 81, (2006). 7) K. Nakashima, F. Kubota, T. Maruyama, M. Goto, Ind. Eng. Chem. Res., 44, (2005). 8) F. Kubota, Y. Shimobori, Y. Koyanagi, K. Shimojo, N. Kamiya, M. Goto, Anal. Sci., 26, (2010). 9) A. Cieszynska, M. Wisniewski, Sep. Purif. Technol., 73, (2010). 10) A. Cieszynska, M. Wisniewski, Sep. Purif. Technol., 80, (2011). 11) T. Kakoi, M. Yoshiyama, H. Ariyoshi, M. Komai, M. Goto, Solvent Extr. Res. Dev. Jpn., 19, (2012). 12) N. Papaiconomou, G. Vite, N. Goujon, J.-M. Lévêque, I. Billard, Green Chem., 14, 2050 (2012). 13) S. Katsuta, Y. Yoshimoto, M. Okai, Y. Takeda, K. Bessho, Ind. Eng. Chem. Res., 50, (2011). 14) J. Yang, F. Kubota, Y. Baba, M. Komai, M. Goto, Solvent Extr. Res. Dev. Jpn., 21, (2014). 15) R.P. Swatloski, J.D. Holbrey, R.D. Rogers, Green Chem., 5, (2003). 16) J.J. Cruywagen, R.J. Kriek, J. Coord. Chem., 60, (2007). 17) H. Yoshizawa, K. Shiomori, S. Yamada, Y. Baba, Y. Kawano, K. Kondo, K. Ijichi, Y. Hatae., Solvent Extr. Res. Dev. Jpn., 4, (1997)