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Article

Highly Selective Recovery of Pt(IV) from HCl Solutions by Precipitation Using 1,4-Bis(aminomethyl)cyclohexane as a Precipitating Agent

by
Kazuya Matsumoto
1,*,
Ryu Sakamoto
2,
Yoshiya Sakuta
1,
Ryota Aoki
1,
Hiroshi Katagiri
3 and
Mitsutoshi Jikei
1
1
Graduate School of Engineering Science, Akita University, 1-1 Tegatagakuen-Machi, Akita-shi 010-8502, Akita, Japan
2
Faculty of Engineering Science, Akita University, 1-1 Tegatagakuen-Machi, Akita-shi 010-8502, Akita, Japan
3
Graduate School of Organic Materials Science, Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510, Yamagata, Japan
*
Author to whom correspondence should be addressed.
Metals 2025, 15(7), 778; https://doi.org/10.3390/met15070778
Submission received: 9 June 2025 / Revised: 3 July 2025 / Accepted: 7 July 2025 / Published: 9 July 2025
(This article belongs to the Special Issue Hydrometallurgical Processes for the Recovery of Critical Metals)
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Versions Notes

Abstract

To ensure the sustainable use of limited resources, it is essential to establish selective and efficient recycling technologies for platinum group metals (PGMs). This study focused on the selective precipitation-based separation of Pt(IV) from hydrochloric acid (HCl) solutions in the presence of various metal ions, using trans-1,4-bis(aminomethyl)cyclohexane (BACT) as a precipitating agent. By using BACT, we succeeded in the selective separation of Pt(IV) by precipitation from HCl solutions containing Pd(II) and Rh(III). Notably, selective and efficient recovery of Pt(IV) was accomplished across various HCl concentrations, with a small amount of BACT and within a short shaking time. To evaluate the practical applicability of the method, Pt(IV) was recovered and purified from the HCl leachate of spent automotive exhaust gas purification catalysts using BACT. As a result, a high Pt recovery of 95.6% and a high purity of 99.3% were achieved. Although Pt(IV) was recovered as a precipitate containing BACT, it was found that Pt black could be readily obtained by dissolving the precipitate in HCl solution followed by reduction with sodium borohydride. Detailed structural analysis of the Pt(IV)-containing precipitate revealed that it is an ionic crystal composed of [PtCl6]2− and protonated BACT. The selective formation of this ionic crystal in HCl solution, along with its stability under such conditions, is the key to the selective recovery of Pt(IV) using BACT.

1. Introduction

Platinum group metals (PGMs) refer to six precious metals: rhodium (Rh), platinum (Pt), palladium (Pd), iridium (Ir), ruthenium (Ru), and osmium (Os). These metals possess unique physical and chemical properties, such as corrosion resistance, heat resistance, electrical and thermal conductivity, magnetism, chemical stability, and catalytic activity, that are highly valued in industrial applications [1,2]. Due to these characteristics, PGMs are considered irreplaceable materials in cutting-edge technologies. However, PGMs are geologically scarce, with major deposits concentrated in South Africa and Russia [3]. Their average concentration in the upper continental crust is less than 1 part per billion (ppb), making them extremely rare and consequently very expensive.
Due to their limited supply and high cost, as mentioned earlier, it is necessary to recover PGMs not only from primary resources but also through recycling from secondary sources [4]. In particular, the recycling of PGMs such as Pt, Pd, and Rh from automotive exhaust gas purification catalysts—one of their primary applications—is of critical importance [5,6,7,8]. One commonly used method for separating and recovering PGMs is the hydrometallurgical process [9,10]. This involves dissolving the target metals using acids such as hydrochloric acid (HCl) or nitric acid, followed by separation and recovery from the resulting solution. Hydrometallurgical methods can be categorized into several techniques depending on the recovery process, including solvent extraction [11], precipitation [12], and ion exchange resin methods [13]. Among these, solvent extraction is widely used.
Solvent extraction is a separation technique that utilizes the distribution of substances between two immiscible phases, typically an aqueous phase (water solution) and a hydrophobic organic phase. The organic phase usually consists of an organic compound (extractant) with high affinity for specific metals, diluted in an organic solvent (diluent). It has been reported that Pd(II) and Pt(IV) can be selectively separated and recovered from metal-containing HCl solutions using this method [14,15,16,17,18,19]. However, regardless of the extraction mechanism, it is known to be difficult to selectively extract Pt(IV) over Pd(II) [20,21,22]. Therefore, achieving highly selective recovery of Pt(IV) using solvent extraction in the presence of Pd(II) remains a challenge. Additionally, solvent extraction poses safety concerns due to the use of organic solvents and environmental burdens, along with the need for improved selectivity to recover only the target metal.
Precipitation recovery, on the other hand, does not use organic solvents and involves a simple operation of filtering the metal-containing precipitate. However, when the concentration of the target metal is low, sufficient aggregation may not occur, making efficient precipitation recovery difficult. Co-precipitation of other metals is also likely, making highly selective recovery challenging. For example, although dimethylglyoxime can precipitate Pd(II), it is also known to precipitate Ni(II) [23,24,25]. Pd(II) can be precipitated and recovered as PdI2 using sodium iodide; however, copper is co-precipitated during this process [26]. Furthermore, ammonium chloride, a widely used precipitant for Pt, also precipitates metals such as Pd, Ir, and Ru, making it less than ideal as a selective precipitant for Pt [27,28,29,30]. Recently, cucurbit[6]uril has been used to achieve separation of Pt(IV) from Pd(II) and Rh(III) by selectively forming unique crystals with chloro-complex anions of Pt(IV). However, in addition to the relatively low recovery rate of Pt(IV) (approximately 80%), the high cost of cucurbit[6]uril remains a significant drawback [31]. We have reported that using primary aliphatic and aromatic amine compounds as precipitants allows for the selective separation of Pt(IV) or Rh(III) by precipitation from HCl solutions [32,33,34]. The key to selective metal precipitation recovery lies in the selective formation of ionic crystals composed of the protonated amine compounds and the metal-chloride complex anions of the target metals. However, 2-ethylhexylamine (2EHA), a Pt precipitant, requires a large amount of amine (2EDA/Pt ≥ 50 mol/mol) to effectively precipitate Pt(IV) from HCl solutions containing 1 mM Pt(IV) [32] and shows insufficient separation from Pd(II) (see Supplementary Materials). Furthermore, although m-phenylenediamine (MPDA), as we recently reported, enables highly selective recovery of Pt(IV), its application is limited by the requirement for HCl concentrations of 8 M or higher for separation from Rh(III) as well as a shaking time of at least one hour for efficient Pt(IV) precipitation [33]. Additionally, since Rh(III) can also be precipitated by this reagent, there is concern about co-precipitation of Rh(III) depending on the metal concentration even at high HCl concentrations.
In this study, we demonstrate the highly selective separation of Pt(IV) by precipitation from HCl solutions containing various metals using 1,4-bis(aminomethyl)cyclohexane, an aliphatic primary diamine compound with a cyclohexane ring, as a precipitant. By adding the trans-isomer of 1,4-bis(aminomethyl)cyclohexane (BACT) to HCl solutions (1–8 M HCl) containing Pt(IV), Pd(II), and Rh(III), selective precipitation of Pt(IV) was successfully achieved. This selective precipitation of Pt(IV) by BACT was accomplished with a small amount of BACT and a very short shaking time. Furthermore, selective precipitation of only Pt(IV) was also achieved from HCl leachates of automotive exhaust gas purification catalysts, which have complex compositions containing various metals. Detailed structural analyses, including single-crystal X-ray diffraction and X-ray photoelectron spectroscopy (XPS), revealed that the recovered precipitate is an ionic crystal composed of [PtCl6]2− and protonated BACT in a 1:1 molar ratio. It was clarified that this ionic crystal selectively forms and remains stable in HCl solution, enabling the selective recovery of Pt(IV).

2. Materials and Methods

2.1. Materials

BACT and a mixture of the trans and cis isomers of 1,4-bis(aminomethyl)cyclohexane (BAC, trans:cis = 67:33) were generously provided by Mitsubishi Gas Chemical Co., Ltd. (Tokyo, Japan) and used as received. HCl solutions of Pd(II) and Pt(IV) were obtained from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan), and a Rh(III) standard solution in HCl was obtained from Kanto Chemical Co., Inc. (Tokyo, Japan). These metal-containing solutions were used without further purification. The catalyst leaching solution was prepared by treating powdered spent automotive exhaust gas purification catalysts with concentrated HCl solution containing 1% hydrogen peroxide at 80 °C for 18 h, following previously reported procedures [33,35].

2.2. Metal Precipitation Experiments from Metal-Containing HCl Solutions

To 1 mL of HCl solutions containing Pd(II), Pt(IV), and Rh(III) (each at a concentration of 5 mM), 1,4-bis(aminomethyl)cyclohexane was added, and the mixtures were shaken vigorously at room temperature. Following centrifugation at 7200× g for 3 min, the concentrations of the metals remaining in the supernatant were measured using a microwave plasma atomic emission spectrometer (MP-AES). The metal precipitation percentages were calculated by comparing the metal concentrations in the supernatants and the initial solutions. During the metal precipitation experiments, the HCl concentration, the amount of amine added, and the shaking time were systematically varied.

2.3. Purification of Pt(IV)-Containing Precipitate

BACT (BACT/Pt = 15 mol/mol) was added to 8 M HCl solutions (2 mL) containing Pd(II), Pt(IV), and Rh(III) (each at a concentration of 5 mM), and the mixtures were vigorously shaken at room temperature for 10 min. The resulting precipitate (initial precipitate) was collected by filtration. This precipitate was then dissolved in 2 mL of 5 M HCl by heating at 80 °C. After cooling the solution to room temperature, BACT (BACT/Pt = 15 mol/mol) was added again, and the mixture was shaken for 10 min at room temperature. The resulting purified precipitate was also collected by filtration. The Pt purity of both the initial and purified precipitates was evaluated by dissolving each in 1 M HCl solution and analyzing the resulting solutions using MP-AES. The Pt recovery yield was calculated based on the metal concentrations in the corresponding filtrates, also determined by MP-AES.

2.4. Metal Precipitation from HCl Leachate of Spent Catalysts

BACT (BACT/Pt = 15 mol/mol) was added to a catalyst leaching solution (2 mL), and the mixture was shaken vigorously for 10 min at room temperature. The precipitate was collected by filtration. Then, the resulting precipitate was dissolved in 4 mL of 5 M HCl solution at 80 °C. After the solution was cooled to room temperature, BACT (BACT/Pt = 15 mol/mol) was added, and the mixture was shaken at room temperature for 10 min. The precipitate was collected by filtration. The Pt purity of the precipitates obtained in the first and second steps was evaluated by dissolving the precipitates in 1 M HCl solution and analyzing the solutions using MP-AES. The Pt recovery yield was calculated based on the metal concentrations in the corresponding filtrates and the initial solutions, also determined by MP-AES.

2.5. Reduction of Pt(IV)-Containing Precipitate

The Pt(IV)-containing precipitate prepared using BACT was dissolved in 1 M HCl solution to obtain a solution with a Pt(IV) concentration of approximately 1000 mg/L. To 10 mL of this solution, sodium borohydride (10-fold molar excess relative to Pt) was added, and the mixture was stirred at room temperature for 1 h. The resulting precipitate was collected by filtration and washed with water to obtain Pt black. The Pt recovery yield was calculated based on the Pt concentration in the filtrate, as determined by MP-AES.

2.6. Single Crystal Preparation

A solution of Pt(IV) (5 mM) in 3 mL of 8 M HCl was treated with BACT at a molar ratio of BACT to Pt(IV) of 2:1. The mixture was then allowed to stand undisturbed for a period of two weeks. Single crystals were obtained as orange-colored crystals.

2.7. Measurements

Quantitative analysis of metal concentrations was performed using MP-AES (Agilent 4210, Agilent Technologies, Santa Clara, CA, USA). 1H NMR spectra were recorded using a JEOL JNM-ECZL 500 NMR spectrometer (Jeol Co., Tokyo, Japan). Powder X-ray diffraction (PXRD) measurements were performed using an Ultima IV instrument (Rigaku, Tokyo, Japan). X-Ray diffraction data for single crystals were collected using a Rigaku XtaLAB Synergy-S diffractometer (Rigaku, Tokyo, Japan) with Mo-Kα radiation (λ = 0.71075 Å) at 296 K. Data collection, cell refinement, and data reduction were carried out using CrysAlisPro software (version 1.171.43.115a) [36]. The structure was solved by direct methods using the program SHELXT [37] and refined by full-matrix least square methods on F2 using SHELXL [38]. All materials for publication were prepared by Yadokari-XG 2009 software [39,40]. All non-hydrogen atoms were refined anisotropically. The H atoms attached to N atoms were located using differential Fourier analysis and refined with Uiso(H) values of 1.5 Ueq(N). The positions of other H atoms were calculated geometrically and refined as riding, with Uiso (H) values of 1.2 Ueq(C). The crystallographic data were deposited at the Cambridge Crystallographic Data Center under the CCDC number 2456029. XPS analysis was conducted using an AXIS-ULTRA instrument (Kratos Analytical Ltd., Manchester, UK). Elemental composition was quantified based on the relative sensitivity factors provided by the instrument’s control software (N 1s: 0.477, Cl 2s: 0.493, Pt 4d: 4.637).

3. Results

3.1. Precipitation Experiments Using BACT

Precipitation experiments were carried out using HCl solutions containing 5 mM concentrations of Pt(IV), Pd(II), and Rh(III), respectively. Although 1,4-bis(aminomethyl)cyclohexane exists as both trans and cis isomers, BACT was used as the precipitant (Figure 1). The metal precipitation percentages were determined by centrifuging the solutions after the experiments, analyzing the metal concentrations in the supernatants using a MP-AES, and comparing them with the initial metal concentrations in the solutions. Metal precipitation experiments were performed by changing the HCl concentration of the metal-containing solutions. The precipitation percentages of Pt(IV) were over 90%, in the range of 1–8 M HCl (Figure 2). On the other hand, within the range of HCl concentrations used in this experiment, the precipitation percentages of Pd(II) and Rh(III) were each below 5%. This clearly demonstrates that by using BACT as a precipitant, Pt(IV) can be selectively precipitated regardless of the HCl concentration. MPDA, which enables highly selective precipitation and recovery of Pt(IV), exhibits selectivity only under HCl concentrations of 8 M or higher, as Rh(III) co-precipitates under conditions below 7 M [33]. In contrast, the use of BACT as a precipitant is noteworthy for its ability to achieve high Pt(IV) selectivity across a wide range of HCl concentrations.
We investigated the relationship between the amount of BACT added as a precipitant and the metal precipitation percentages. As shown in Figure 3a, the Pt(IV) precipitation percentage increased with the amount of BACT added, reaching over 94% when more than 5 molar equivalents of BACT were added relative to Pt(IV). In contrast, the precipitation percentages of Pd(II) and Rh(III) remained below 5% regardless of the amount of BACT added, indicating that high selectivity for Pt(IV) was maintained. Notably, even with just 2 molar equivalents of BACT, the Pt(IV) precipitation percentage reached 85%, demonstrating that both high selectivity and efficient recovery of Pt(IV) can be achieved. In subsequent experiments, the amount of BACT added was set to 15 molar equivalents relative to Pt(IV), which was determined to be the optimal condition for maximizing the precipitation percentage of Pt(IV). The previously reported Pt precipitant, 2EHA, enabled selective recovery of Pt(IV) when the concentrations of Pt(IV), Pd(II), and Rh(III) were each 1 mM [32]. However, under the same conditions as in this study, where each metal was present at a concentration of 5 mM, metal precipitation experiments showed that when the amount of 2EHA added was low (2EDA/Pt ≤ 15 mol/mol), Pd(II) co-precipitated with Pt(IV), and when the amount was high (2EDA/Pt ≥ 30 mol/mol), both Pd(II) and Rh(III) co-precipitated with Pt(IV) (Figure S1 in Supplementary Materials). As a result, high Pt(IV) selectivity could not be achieved. In contrast, BACT demonstrated high selectivity for Pt(IV) even under high metal concentration conditions, where selective separation and recovery are generally difficult. This indicates that BACT is an excellent precipitant for Pt(IV).
Figure 3b shows the effect of shaking time on metal precipitation using BACT. More than 96% of Pt(IV) precipitated within just one minute of shaking. In contrast, even with longer shaking times, no significant co-precipitation of Pd(II) or Rh(III) was observed. These results indicate that BACT enables both rapid precipitation of Pt(IV) and high selectivity. While 2EHA, a Pt precipitant, rapidly precipitates Pt(IV) [32], MPDA requires more than one hour of shaking to precipitate over 80% of Pt(IV) [33]. Based on the results presented thus far, BACT enables selective precipitation and recovery of Pt(IV) under a wide range of HCl concentrations, with a small amount of precipitant and within a short shaking time. These findings demonstrate that BACT is a superior Pt-selective precipitant compared to 2EHA and MPDA, both of which are Pt precipitants possessing primary amino groups.

3.2. Purification of Pt(IV)-Contaning Precipitate

BACT was employed as a precipitant to selectively isolate Pt(IV) from HCl solutions containing a mixture of Pt(IV), Pd(II), and Rh(III). Although the initial precipitate contained trace amounts of Pd(II) and Rh(III), further purification was required to obtain high-purity Pt(IV). Solubility tests revealed that the Pt(IV)-containing precipitate was virtually insoluble in water and HCl solutions at room temperature. However, it dissolved effectively in 1–8 M HCl solutions upon heating at around 80 °C (Pt concentration: at least 10 mM). Notably, no precipitation was observed in the resulting solutions after cooling to room temperature. Based on these findings, the precipitate obtained from a metal-containing 8 M HCl solution (Pd(II), Pt(IV), and Rh(III): 5 mM each) was redissolved in 5 M HCl solution under heating. After cooling the solution to room temperature, BACT was added, and the solution was agitated to induce reprecipitation. Quantitative analysis of the metal content in both the initial and reprecipitated solids indicated that Pd(II) and Rh(III) were effectively removed, while the Pt(IV) content remained nearly unchanged (Table 1). As a result, the purity of Pt(IV) in the precipitate increased from 96.2% to 99.9%. These results demonstrate that redissolution of the initial precipitate in a HCl solution followed by reprecipitation with BACT is an effective method for enhancing the purity of Pt(IV).

3.3. Selective Precipitation of Pt(IV) from HCl Leachate of Spent Catalysts

To evaluate the practical applicability of Pt(IV) recovery using BACT, metal precipitation experiments were conducted using HCl leachate derived from spent automotive exhaust gas purification catalysts. The leachate was prepared by heating and stirring pulverized spent catalysts in concentrated HCl solution with the addition of hydrogen peroxide, as described in previous studies [33,35]. The resulting leachate contained Pt(IV), Pd(II), and Rh(III), along with various other metals, resulting in a complex composition. The metal species and their concentrations in the leachate are summarized in Table 2. Selective precipitation of Pt(IV) from the leachate was carried out as follows: BACT was added to the leachate at a molar ratio of 15:1 relative to Pt(IV), and the mixture was shaken for 10 min. The precipitate obtained was filtered and subsequently dissolved in 5 M HCl solution under heating. After cooling the solution to room temperature, BACT was again added at a 15-fold molar excess relative to Pt(IV), followed by shaking for 10 min. The reprecipitated solid was then recovered by filtration.
The Pt recovery percentage was calculated based on the metal content in the filtrate, while the purity was determined from the metal content in the precipitate dissolved in HCl solution (Table 2). Ce and Mg, which were present in high concentrations in the original leachate, were almost completely removed during the initial precipitation step. The first precipitate contained small amounts of Pd(II) and Rh(III), resulting in a Pt purity of 87.0%. However, after the purification process involving redissolution and reprecipitation with BACT, the resulting precipitate contained almost no other metals, and the Pt purity increased to 99.3%. Furthermore, the Pt recovery percentage remained high, at 95.6%, relative to the original leachate, demonstrating that both high purity and high recovery of Pt(IV) can be achieved through this method.

3.4. Reduction of Pt(IV)-Containing Precipitate

In this study, Pt(IV) was recovered as a precipitate containing BACT. While direct utilization of this precipitate may be considered, Pt is generally recovered as Pt black—Pt in the zero oxidation state—during metal recycling processes. As demonstrated in previous sections, the Pt(IV)-containing precipitate obtained in this study readily dissolves in heated HCl solutions. To convert the precipitate into Pt black, sodium borohydride as a reducing agent was added to the 1 M HCl solution in which the precipitate was dissolved. The BACT originally present in the precipitate remained dissolved in the HCl solution, allowing for the selective and quantitative precipitation (99.0%) of Pt black. The PXRD pattern of the obtained solid, shown in Figure 4a, matched that of Pt(0) [41]. Furthermore, the color and morphology of the product were consistent with those of Pt black, confirming the successful recovery of zero-valent Pt black (Figure 4b).

3.5. Effect of Isomers of Precipitating Agent on Metal Precipitation

1,4-Bis(aminomethyl)cyclohexane, an aliphatic primary diamine compound, exists in both trans and cis isomeric forms (Figure 1). As demonstrated in this study, BACT exhibits excellent performance as a selective precipitating agent for Pt(IV). To investigate the potential of cis-1,4-bis(aminomethyl)cyclohexane (BACC) as a Pt(IV) precipitant, a mixture of the trans and cis isomers (BAC, trans:cis = 33:67) was used in Pt(IV) recovery experiments. Metal-containing HCl solutions (Pd(II), Pt(IV), and Rh(III): 5 mM each) were used in precipitation studies, with variations in either the HCl concentration or the amount of BAC added. The results indicated that the Pt(IV) precipitation efficiency using BAC was slightly lower than that observed with BACT (Figure 5a). Furthermore, the amount of BAC required to achieve quantitative precipitation of Pt(IV) was greater than that required when using BACT (Figure 5b). However, no significant difference in Pt(IV) selectivity was observed between BAC and BACT. These findings suggest that BACT is more efficient and suitable as a selective precipitating agent for Pt(IV).
1H NMR measurements were performed on the Pt(IV)-containing precipitate (Pt-BAC) obtained by adding BAC to a Pt(IV)-containing HCl solution, as well as on the hydrochloride salt of BAC (Figure 6). Based on the 1H NMR spectra of the hydrochloride salts of BACT and BACC (Figure S2 in Supplementary Materials), the signals at 2.63 ppm and 2.73 ppm in the BAC hydrochloride were assigned to the methylene groups of the trans and cis isomers, respectively. Comparison of the integral ratios of the methylene signals between the BAC hydrochloride and Pt-BAC revealed that the proportion of BACC in Pt-BAC (BACC:BACT = 67:33) was lower than that in the BAC hydrochloride (BACC:BACT = 32:68). This result implies that BACT is preferentially consumed during Pt(IV) precipitation, possibly due to its faster precipitation kinetics with Pt(IV) or the higher stability of the Pt-BACT precipitate in HCl solution. Additionally, it was confirmed that BACC gradually converts to BACT over time, indicating that BACT is the more thermodynamically stable isomer. Taken together, these results demonstrate that BACT is superior to BACC as a precipitating agent for Pt(IV).

3.6. Characterization of Pt(IV)-Containing Precipitate and Mechanism of Selective Precipitation

Single crystals of Pt-BACT were obtained by adding BACT to a HCl solution containing Pt(IV) and allowing the mixture to stand for an extended period (Figure 7a). X-ray crystallographic analysis of the resulting single crystals revealed that the compound is an ionic crystal composed of [PtCl6]2− and BACT-2H+ in a 1:1 ratio, as shown in Figure 7b. Subsequently, a powder form of Pt-BACT was prepared by adding BACT to a Pt(IV)-containing HCl solution followed by brief shaking. PXRD analysis of this sample showed a diffraction pattern that closely matched the simulated pattern calculated from the single-crystal structure, indicating that the powder form shares the same crystal structure as the single crystal (Figure 8). XPS analysis of the powdered Pt-BACT revealed elemental ratios of Pt:N:Cl = 1:2:8, further supporting the ionic crystal structure derived from single-crystal X-ray analysis (Figure 9).
Since BACT can quantitatively precipitate Pt(IV) from HCl solution, the resulting Pt-BACT ionic crystal is stable in such an environment. At room temperature, the ionic crystal does not readily redissolve into the HCl solution, indicating high stability. In contrast, no ionic crystal formation was observed between BACT and Pd(II) or Rh(III) under comparable conditions. This is likely due to the equilibrium between complex formation and dissociation being shifted toward dissociation in HCl solution, preventing precipitation. In previous studies involving primary amine compounds for the recovery of PGMs, metal selectivity was attributed to the selective formation of stable ionic crystals with the target metal, while other metals either failed to form such crystals or formed unstable ones that readily dissolved [32,33,34]. Similarly, in this study, the high selectivity of BACT for Pt(IV) is attributed to its ability to selectively form a stable ionic crystal, Pt-BACT, under acidic conditions. Notably, Ir(IV) forms chloro-complex anions in HCl solutions that are structurally similar to those of Pt(IV) [21], and therefore, it is expected that Ir(IV) can also be precipitated and recovered using BACT. In contrast, Ir(III) forms chloro-complex anions similar to those of Rh(III) in HCl solutions [21], suggesting that in a mixed solution of Pt(IV) and Ir(III), only Pt(IV) would be selectively precipitated by BACT. Further investigations into the potential of BACT as a selective precipitating agent for PGMs are currently underway.

4. Conclusions

Selective separation of Pt(IV) from metal-containing HCl solutions (Pt(IV), Pd(II), and Rh(III)) was successfully accomplished through precipitation with BACT. This selective precipitation method was applicable across a broad range of HCl concentrations (1–8 M HCl) and was achieved with a small amount of BACT—only 2 to 15 molar equivalents relative to Pt(IV)—and a short shaking time of approximately one minute. The resulting Pt(IV)-containing precipitate (Pt purity: 96.2%) could be efficiently purified through a reprecipitation process involving dissolution in HCl solution followed by re-addition of BACT (Pt purity: 99.9%). Application of this method to HCl leachates of automotive exhaust gas purification catalysts containing various metal species enabled the high-yield (95.6%) recovery of high-purity Pt(IV) (99.3%). Furthermore, the Pt(IV)-containing precipitate could be easily converted to Pt black via reduction in HCl solution. Detailed structural analysis revealed that the precipitate is an ionic crystal composed of [PtCl6]2− and protonated BACT. The selective formation and stability of this ionic crystal in HCl solution were identified as key factors enabling the selective precipitation and recovery of Pt(IV). These findings demonstrate that this technique is a highly practical and effective method for the selective recovery and purification of Pt(IV).

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/met15070778/s1, Figure S1: Metal precipitation results using 2EHA; Figure S2: 1H NMR spectra of hydrochloride salts of BACT and BACC; Table S1: Crystal data and structure refinement parameters for Pt-BACT; File “Pt-BACT.cif”: Crystallographic information file of Pt-BACT.

Author Contributions

Conceptualization, K.M. and M.J.; methodology, K.M. and M.J.; formal analysis, K.M., R.S., Y.S., R.A. and H.K.; investigation, K.M., R.S., Y.S., R.A. and H.K.; data curation, K.M.; writing—original draft preparation, K.M.; writing—review and editing, K.M., H.K. and M.J.; visualization, K.M.; supervision, K.M.; project administration, K.M.; funding acquisition, K.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant-in-Aid for Scientific Research (C) 20K05578).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Metals 15 00778 g001
Figure 1. Chemical structures of trans- and cis-1,4-bis(aminomethyl)cyclohexane.
Figure 1. Chemical structures of trans- and cis-1,4-bis(aminomethyl)cyclohexane.
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Metals 15 00778 g002
Figure 2. Influence of HCl concentrations on metal precipitation from metal-containing HCl solutions (Pd(II), Pt(IV), and Rh(III): 5 mM each) using BACT (BACT/Pt = 15 mol/mol, 30 min of shaking).
Figure 2. Influence of HCl concentrations on metal precipitation from metal-containing HCl solutions (Pd(II), Pt(IV), and Rh(III): 5 mM each) using BACT (BACT/Pt = 15 mol/mol, 30 min of shaking).
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Metals 15 00778 g003
Figure 3. The effect of (a) loading amount of BACT (30 min of shaking) and (b) shaking time (BACT/Pt = 15 mol/mol) on the metal precipitation from metal-containing 8 M HCl solutions (Pd(II), Pt(IV), and Rh(III): 5 mM each).
Figure 3. The effect of (a) loading amount of BACT (30 min of shaking) and (b) shaking time (BACT/Pt = 15 mol/mol) on the metal precipitation from metal-containing 8 M HCl solutions (Pd(II), Pt(IV), and Rh(III): 5 mM each).
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Figure 4. (a) PXRD pattern of the Pt black obtained in this study. (b) Photos of the Pt(IV)-containing precipitate and the Pt black obtained in this study.
Figure 4. (a) PXRD pattern of the Pt black obtained in this study. (b) Photos of the Pt(IV)-containing precipitate and the Pt black obtained in this study.
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Metals 15 00778 g005
Figure 5. The influence of (a) HCl concentration (BAC/Pt = 15 mol/mol, 30 min of shaking) and (b) loading amount of BAC (8 M HCl, 30 min of shaking) on the metal precipitation from metal-containing HCl solutions (Pd(II), Pt(IV), and Rh(III): 5 mM each).
Figure 5. The influence of (a) HCl concentration (BAC/Pt = 15 mol/mol, 30 min of shaking) and (b) loading amount of BAC (8 M HCl, 30 min of shaking) on the metal precipitation from metal-containing HCl solutions (Pd(II), Pt(IV), and Rh(III): 5 mM each).
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Figure 6. 1H NMR spectra of hydrochloride salt of BAC and BAC in Pt-BAC measured in DMSO-d6.
Figure 6. 1H NMR spectra of hydrochloride salt of BAC and BAC in Pt-BAC measured in DMSO-d6.
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Metals 15 00778 g007
Figure 7. (a) The photo of single crystals of Pt-BACT. (b) The crystal structure of Pt-BACT. Color legend: light-gray sphere, H; deep-gray sphere, C; blue sphere, N; green sphere, Cl; yellow sphere, Pt.
Figure 7. (a) The photo of single crystals of Pt-BACT. (b) The crystal structure of Pt-BACT. Color legend: light-gray sphere, H; deep-gray sphere, C; blue sphere, N; green sphere, Cl; yellow sphere, Pt.
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Figure 8. PXRD patterns of Pt-BACT, showing calculated (blue) and measured (red) traces.
Figure 8. PXRD patterns of Pt-BACT, showing calculated (blue) and measured (red) traces.
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Metals 15 00778 g009
Figure 9. The XPS spectrum of Pt-BACT.
Figure 9. The XPS spectrum of Pt-BACT.
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Table 1. Metal contents in the Pt(IV)-containing precipitate before and after purification.
Table 1. Metal contents in the Pt(IV)-containing precipitate before and after purification.
MetalsInitial PrecipitateReprecipitated Solid
Concentration [mg/L]Concentration [mg/L]
Pt970931
Pd50.6
Rh330.7
Table 2. Metal recovery from catalyst leachate via precipitation using BACT.
Table 2. Metal recovery from catalyst leachate via precipitation using BACT.
MetalsCatalyst LeachateInitial PrecipitatePurified Precipitate
ConcentrationConcentrationRecoveryConcentrationRecovery
[mg/L][mg/L][%][mg/L][%]
Pt16801666.699.21606.195.6
Pd979146.515.05.40.6
Rh25636.714.3<0.1<0.1
Ce70,9970.1<0.1<0.1<0.1
Al15636.50.44.70.3
Fe235144.91.9<0.1<0.1
Zr3280.2<0.10.2<0.1
La12370.7<0.10.2<0.1
Mg36,62513.4<0.1<0.1<0.1
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MDPI and ACS Style

Matsumoto, K.; Sakamoto, R.; Sakuta, Y.; Aoki, R.; Katagiri, H.; Jikei, M. Highly Selective Recovery of Pt(IV) from HCl Solutions by Precipitation Using 1,4-Bis(aminomethyl)cyclohexane as a Precipitating Agent. Metals 2025, 15, 778. https://doi.org/10.3390/met15070778

AMA Style

Matsumoto K, Sakamoto R, Sakuta Y, Aoki R, Katagiri H, Jikei M. Highly Selective Recovery of Pt(IV) from HCl Solutions by Precipitation Using 1,4-Bis(aminomethyl)cyclohexane as a Precipitating Agent. Metals. 2025; 15(7):778. https://doi.org/10.3390/met15070778

Chicago/Turabian Style

Matsumoto, Kazuya, Ryu Sakamoto, Yoshiya Sakuta, Ryota Aoki, Hiroshi Katagiri, and Mitsutoshi Jikei. 2025. "Highly Selective Recovery of Pt(IV) from HCl Solutions by Precipitation Using 1,4-Bis(aminomethyl)cyclohexane as a Precipitating Agent" Metals 15, no. 7: 778. https://doi.org/10.3390/met15070778

APA Style

Matsumoto, K., Sakamoto, R., Sakuta, Y., Aoki, R., Katagiri, H., & Jikei, M. (2025). Highly Selective Recovery of Pt(IV) from HCl Solutions by Precipitation Using 1,4-Bis(aminomethyl)cyclohexane as a Precipitating Agent. Metals, 15(7), 778. https://doi.org/10.3390/met15070778

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