Solvent Extraction Research and Development, Japan, Vol. 30, No 2, 149 – 157 (2023)

Effect of Hydrophobicity of Ionic Liquids on the Leaching Selectivity of Platinum from a
Spent Automotive Catalyst
Takafumi HANADA, Sayako TAKAOKA, Mayu KAMISONO,
Adroit T. N. FAJAR and Masahiro GOTO*
Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka,
Nishi-ku, Fukuoka, Japan* Corresponding author: m-goto@mail.cstm.kyushu-u.ac.jp(Received January 25, 2023; Accepted February 24, 2023)
Recycling platinum group metals from secondary resources such as spent automotive exhaust catalysts is
promising for the circular economy. However, the selective separation of Pt over massive amounts ofimpurity metals such as Mg and Al is particularly challenging. In this study, non-aqueous direct leaching of
platinum from a spent automotive catalyst (SAC) using hydrophobic ionic liquids, namely,
trihexyl(tetradecyl)phosphonium chloride (P66614Cl) and trioctyl(dodecyl)phosphonium chloride (P88812Cl)
with the aid of pre-loading hydrochloric acid/hydrogen peroxide was proposed. The more hydrophobic
P88812Cl exhibited more efficient and selective leaching of Pt over Mg and Al. The recovery of Pt from themetal-loaded P88812Cl, and the reusability of the IL for SAC leaching were also demonstrated.

1. Introduction
Platinum group metals (PGMs) are of great interest owing to their high stability and catalytic activity.
Especially platinum (Pt), palladium (Pd), and rhodium (Rh) play a vital role as the main catalytic
component of automotive exhaust catalysts in catalytic converters [1]. Due to their limited reserves anduneven distribution of PGM resources, increasing the end-of-life recycling rate in various regions isessential for the stable supply of PGMs [2]. Even if electric vehicles become more popular andgasoline-powered vehicles become less common; PGM recycling will be increasingly requested owing totheir demand in power source sectors such as fuel cells and electrolyzers [3].
Commercial SAC recycling has been operated by pyrometallurgy as the pretreatment for thesolubilization of inert PGMs [4]. However, the massive energy consumption of pyrometallurgy (typicallyoperated at temperatures higher than 1300 K) has been a great concern from an economic standpoint. The
hydrometallurgical recycling of SACs has attracted wide attention owing to its lower energy consumption
and process scalability [5]. The PGMs in SACs could be leached out along with other metals comprising
the catalyst support by using highly acidic aqueous solutions such as aqua regia or inorganic acids withoxidants [6]. Subsequently, the dissolved PGMs could be separated from other impurity metals by solvent
extraction. However, the consumption of inorganic acids for the leaching of automotive catalysts causes
environmentally impactful acidic wastewater containing heavy metals. Moreover, solvent extraction for
metal separation uses volatile organic solvents.

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For more environmentally friendly recycling of SACs, the leaching and separation of PGMs without
inorganic acids and organic solvents should occur. Solvometallurgy or Ionometallurgy has attracted
attention in this context. These new concepts of metallurgy employ non-aqueous or less-aqueous media
containing organic solvents, ionic liquids (ILs), and deep eutectic solvents as the alternative to conventional
inorganic acids and organic solvents for lower emission processes [7,8]. Notably, water-immisciblehydrophobic solvents for leaching provide an advantage that the leached metals can be easily recovered bycontacting with the aqueous stripping solution. Moreover, the solvent can be repeatedly used for metal
processing. Wellens and colleagues reported the use of an acid-saturated hydrophobic ionic liquid for theleaching of metal oxides and also investigated the stripping behavior of the leached metals [9]. Li andcolleagues reported that a trichloride hydrophobic ionic liquid could be reused after the leaching of aneodymium permanent magnet and stripping of the metals [10]. Non-aqueous direct leaching has beenutilized for the leaching of SACs. Serpe and colleagues reported the leaching of Pd from a SAC using anorganic ligand containing sulfur and iodine in organic solvents [11,12]. Nguyen and co-workers usedacetonitrile with some metal salts as oxidants for the selective leaching of palladium and platinum from a
SAC [13]. Li and co-workers reported the use of trihalide ILs for the selective leaching of palladium from a
SAC. Lanaridi and co-workers used deep eutectic solvents for the leaching of a SAC [14]. The leaching of
metals into organic solvents allows for advanced metal separation that is difficult to achieve in aqueous
systems [15]. The non-aqueous leaching of PGMs from SACs presents advantages such as suppressing theleaching of Al, Mg, and other impurity metals from the catalyst support and improving the leaching
selectivity. ILs could be utilized for the extraction and even non-aqueous direct leaching owing to their highaffinity to PGMs [16]. The non-volatile feature of ILs realized a more environmentally friendly recycling
process than using organic solvents. However, due to the lack of a systematic study of the structure of ILs
on the leaching performance of metals from primary or secondary metal resources, the molecular design
rule of the ILs has not yet been revealed.
In this study, we focused on revealing the molecular structure of the hydrophobic ILs on the leaching
behavior of platinum from a SAC. Two phosphonium-based ILs trihexyl- (tetradecyl)phosphonium chloride(P66614Cl) and trioctyl(dodecyl)phosphonium chloride (P88812Cl) were employed as leaching media for the
SAC. These ILs have already been reported as efficient extractants for PGMs. The acid/oxidant-saturating
treatment was applied by contacting with hydrochloric acid (HCl)/hydrogen peroxide (H2O2) solution forthese ILs to enhance the leaching. Finally, the separation and recovery of Pt from the metal-loaded IL werestudied, and the reusability of the IL as the leaching medium was demonstrated.
2. Experimental
2.1 ReagentsThe ILs, P66614Cl and P88812Cl were purchased from Sigma-Aldrich and Io-li-tec, respectively, with apurity of 95% each, and were used without further purification. The chemical structure of the ILs used inthis study is shown in Figure 1. According to the previous literature, P88812Cl has a higher hydrophobicity
owing to its larger carbon number, as evidenced by the minimum leakage of the phosphonium cation into
the water phase [17]. Hydrochloric acid (10 mol/L) was purchased from Kishida Chemical. Hydrogen
peroxide, ammonium chloride, and ICP standard solution of Pt, Fe, Zn, La, Al, and Mg were purchased- 150 -

from Fujifilm-Wako Chemical. Milli-Q water was used to prepare all the aqueous solutions.

Figure 1. Chemical structures of (a) P66614Cl and (b) P88812Cl.
2.2 Direct leaching of a spent automotive catalyst by ILs
The ILs used in this study did not leach any metals from the SAC when used untreated. Therefore, aknown concentration of HCl/H2O2 solution was contacted with the IL by vortex mixing at room
temperature for 1 h to load acid/oxidant into ILs. The acid-loaded IL was then added to the SAC and heated
up to 343 K with stirring at 400 rpm for 24 h. The resulting leachate was centrifuged at 18,000g to obtainthe supernatant. As a comparison, conventional aqueous leaching using HCl/H2O2 was also conducted byfollowing a similar protocol according to the previous literature [18]. The metal concentration in the IL was
directly measured by an inductively coupled plasma optical emission spectrometer (Optima 8300,
Perkin-Elmer) equipping a 1 mm diameter injector and a torch for organic solvent with a Perche-cooler
chamber. The sample solution was prepared as matrix-matched in 20% aqueous components in ethanol. The
calibration score of this organic system was R2>0.999, showing good linearity. The leaching efficiency(%L) was evaluated by the following equation:
CM,leaching  V
%L 
 100
mM / w(1)

where CM,leaching, V, mM, and w represent the metal concentration of a metal M in the IL phase, volume of IL,
mass ratio of M, and the weight of the SAC, respectively.
2.3 Stripping of metals from P88812Cl after SAC leaching
The stripping of the metals from the IL after leaching was performed by contacting various aqueous
solutions with the IL at an aqueous-to-organic (A/O) ratio of 5/1 at room temperature by vortex mixing.
The stripping percentage (%S) was evaluated by the following equation:%S 

CM,stripping
CM,leaching

 100

(2)

where CM,stripping represents the metal concentration in the IL phase after stripping.
3. Results and Discussion
3.1 Direct leaching of a spent automotive catalyst by ILs

The metal contents in the SAC used in this study are shown in Table 1. Due to the shallow content of
Pd, it was not easy to quantify the metal content in the IL after leaching. Therefore, the rich metals in the
SAC were featured for further leaching experiments. The leaching behavior of metals from the SAC by anaqueous solution of 5 mol/L HCl with 5% H2O2 and the ILs loaded with 5 mol/L HCl with 5% H2O2 is- 151 -

shown in Figure 2. When an aqueous HCl/H2O2 solution was used for leaching, almost all the metals were
efficiently dissolved, which means the leaching selectivity of Pt was very low. On the contrary, the SAC
leaching with HCl/H2O2-loaded ILs could selectively dissolve Pt. The leaching of Mg and Al that were inlarge quantities in the SAC was significantly suppressed by IL-based leaching. The leaching mechanism forPt was presumed to be different in the aqueous and the IL-based leaching. In the aqueous system, Pt wasoxidized by Cl2 which was produced from the reaction between HCl and H2O2 [18]. A comparable reaction
may take place in the IL-based system, where HCl and H2O2 are dissolved at the pre-loading. Additionally,
the Cl2 gas generated in the aqueous HCl/H2O2 solution during the pre-loading could dissolve in the IL,
forming trichloride anions which exhibit reactivity towards the oxidation of precious metals [19].
Comparing P66614Cl and P88812Cl, the latter performed better leaching performance for Pt with less leachingof Mg and Al. The suppression in the leaching of Mg and Al is very beneficial for the purification
complexity of Pt after leaching. The higher leaching efficiency of Pt of P88812Cl could be explained by theunintentional consumption of HCl/H2O2 for Al leaching: a higher %L of Al, presented in much larger
quantities than Pt, was observed for P66614Cl. P88812Cl has more carbon and thus has a higherhydrophobicity. Therefore, the hydrophobicity of ILs might be a key to the selective leaching of Pt from the
SAC.
Table 1. The metal contents in the SAC were determined by the leaching with aqua regia at asolid-to-liquid ratio of 12.5 g/L, 353 K for 24 h. The bold elements were featured for further leachingexperiments.
Pt

Pd

Fe

Zn

Cu

Ce

La

Zr

Mg

Al

Ba

Pr

4.1

1.4

21.4

4.3

0.3

13.2

26.4

1.2

43.8

166.7

32.7

13.5

100

%L

80
60
40
20
0
Pt
HCl+H2O2
HCl+H2O2Fe

Zn
P66614Cl
P66614Cl

La

Mg

Al

P88812ClP88812Cl

Figure 2. Leaching of various metals from the SAC into various leaching solvents. The error bars on
P66614Cl and P88812Cl are standard deviations.
The affinity of the metal ions with the ILs is essential to the leaching selectivity of metals into ILs.
The liquid-liquid extraction behavior of metals with the ILs that could be an indicator of metal-IL affinity isshown in Figure 3. It was obvious that P66614Cl and P88812Cl have a similar extraction performance: Pt, Fe,and Zn were efficiently extracted in the wide range of HCl concentrations. La and Al were very slightly- 152 -

extracted in P66614Cl, and were not extracted in P88812Cl. Mg was not extracted in either IL. These ILs havea higher affinity towards Pt, Fe, and Zn, which likely form anionic chloro-complexes in the chloride media,
while they have a low affinity for La, Mg, and Al which are bare cations. A similar extraction selectivity ofILs to these metals from SAC leaching solutions was reported in the literature [20-23]. Therefore, it was
indicated that the leaching selectivity for ILs observed in Figure 1 followed the affinity of the leachedmetals for ILs.

(a)

(b)
100

100
[HCl]
0.1 M

60

1M
2M

40

[HCl]

80
%E

%E

80

0.1 M

60

1M
2M

40

5M

20

5M

20

0

0
Pt

Fe

Zn

La

Mg

Al

Pt

Fe

Zn

La

Mg

Al

Figure 3. Liquid-liquid extraction of Pt(IV), Fe(III), Zn(II), La(III). Mg(II), and Al(III) from various
HCl concentrations by (a) P66614Cl and (b) P88812Cl.
To further investigate the effect of the hydrophobicity of IL on the leaching selectivity, the watercontent of the ILs after contact with the HCl/H2O2 solution was measured. It is assumed that the loading of
HCl and H2O2 into the IL through pre-loading is accompanied by the water molecules. Hence, the watercontent of the ILs can serve as an indicator of the loading amount of HCl and H2O2. As shown in Figure 4,
the water content in P66614Cl tended to increase with the increase of HCl concentration in the aqueous phase,
while that in P88812Cl was lesser, and was constant even with varied HCl concentrations.

Water content [g/kg]

120
100

[P66614][Cl]P66614Cl

80[P88812][Cl]P88812Cl

60
40
20
0
1 mol/L
1

2 mol/L2

5 mol/L
5

Figure 4. The water content of ILs after loading HCl/5%-H2O2 solution with different HCl concentrations
measured by Karl-Fisher titration. Each titration was triplicated.

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The effect of the HCl concentration in the loading HCl/H2O2 solution on the leaching efficiency ofthe metals from the SAC using ILs is shown in Figure 5. Interestingly, the %L of Pt and Al into P66614Cl
was almost independent of the loading HCl concentration, while the leaching of Fe and La gradually
increased with increasing the HCl concentration. This could be attributed to the increase of H2O loading in
the IL, leading to the undesired leaching of impurity metals such as Fe and La. For P88812Cl, Pt leaching
was significantly improved when the loading HCl concentration was as high as 5 mol/L, while the leachingof Mg and Al was kept low. This suggests that the primary active species for Pt leaching in P88812Cl was not
HCl/H2O2 but trichloride anions without unavoidable H2O loading at a high concentration of pre-loading
HCl.
(b)

100

100

80

80

[HCl]

60

60

1M

%L

%L

(a)

40
20
0

2M
40

5M

20

+5% H2O2

0
Pt

Fe

Zn

La

Mg

Al

Pt

Fe

Zn

La

Mg

Al

Figure 5. Effect of loading HCl concentration on the leaching efficiency of metals from the SAC using (a)P66614Cl and (b) P88812Cl as leaching solvents. The error bars are standard deviations.
3.2 Recovery of Pt from the metal-loaded P88812Cl

After the SAC leaching with P88812Cl, the IL phase contained Pt with some impurity metals.
Therefore, the scrubbing and stripping of metals from the IL phase were studied to recover Pt. According toour preliminary investigation, it was suggested that the use of Na2SO3 with the addition of NH3 could be
used for the removal of impurity metals such as Fe, La, Mg, and Al, while HNO3 for the stripping of Pt.
The effect of contact time on the stripping of various metals from P88812Cl after SAC leaching using
Na2SO3 is shown in Figure 6a. The removal of La, Mg, and Al was easily achieved at any contact time,
while Fe removal required a contact time longer than 2 h. This is because of the stripping mechanism of Fe
by Na2SO3: Fe(III) was reduced to Fe(II) by Na2SO3 to deform the extractable anionic chloro-complex.
However, the re-extraction of Zn(II) occurred with time probably due to the increase of Cl- concentration in
the stripping solution. The effect of the A/O ratio on the stripping of metals using 5 M HNO3 is shown in
Figure 6b. The stripping efficiency of Pt was improved by increasing the A/O ratio from 1:1 to 3:1.
Therefore, the scrubbing of impurity metals and the stripping of Pt could be done by using Na2SO3 andHNO3 at the optimized conditions.

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100

%S

80
60

(b)

0.5h
1h
2h
3h

100

A/O=1:1
A/O=3:1

80
%S

(a)

60

40

40

20

20
0

0
Pt

Fe

Zn

La

Mg

Pt

Al

Fe

Zn

La

Mg

Al

Figure 6. Stripping behavior of various metals from P88812Cl after the SAC leaching, (a) effect of contact
time of 0.5 mol/L Na2SO3 (with 0.5 mol/L NH3) on the removal of impurity metals, A/O=1:1, vortex atroom temperature, and (b) effect of A/O ratio of 5 mol/L HNO3 on Pt stripping, vortex for 1 h at room
temperature.
3.3 Stability and reusability of IL

After the scrubbing and stripping of metals from the metal-loaded P88812Cl, the IL could be further
utilized for the SAC leaching. To ensure the chemical stability of the IL, 31P-NMR of the IL before andafter the SAC leaching was measured. As shown in Figure 7a, there was almost no change in the spectra
before and after the leaching, indicating the IL has high chemical stability even under a strongly oxidative
condition. After the 1st leaching, scrubbing, and stripping, the IL was used for the SAC leaching again andthe leaching performance was investigated, as shown in Figure 7b. The leaching behavior of metals fromthe SAC using the reused P88812Cl was almost the same trend as the 1st leaching. The high stability andreusability of the IL were demonstrated.

Figure 7. The stability and reusability studies of IL, (a) 31P-NMR spectra of the P88812Cl before and after the
SAC leaching, and (b) comparison of the 1st and 2nd SAC leaching performance using P88812Cl.
4. Conclusion

In this study, two hydrophobic ILs with different alkyl chain lengths, namely, P66614Cl and P88812Cl,
were used for non-aqueous direct leaching of PGMs from a SAC. Interestingly, more hydrophobic IL
P88812Cl performed more efficient and selective leaching of Pt compared to P66614Cl. This might occur since- 155 -

the P66614Cl underwent an undesired consumption of loaded HCl/H2O2 to the leaching of the impurity
metals such as Al. The scrubbing and stripping from the metal-loaded P88812Cl allowed the separation andrecovery of Pt from other impurity metals, and the IL could be reused for multiple leaching cycles. The
results of this study indicate that ILs of higher hydrophobicity, which contains less water and highacid/oxidant during the loading of the aqueous acid/oxidant solution, is advantageous for the selective
leaching of Pt. Further PGM selective leaching from SACs will be possible by discovering a method thatallows loading acid/oxidant into ILs while preventing water contamination.
Acknowledgment

This study was supported by the Environment Research and Technology Development Fund (Grant
No. 3-2004 and 3-2302) from the Ministry of the Environment of Japan.
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