Preparation of a zwitterionic polymer based on L-cysteine for recovery application of precious metals

Abstract

L-Cysteine-grafted polystyrene (Poly(Cys-g-Sty)), was synthesized from a cysteine-styrene monomer (Cys-Sty) in aqueous solution and used as a facile and selective high-recovery material for palladium(II), platinum(IV), and gold(III) ions from aqueous media. The high solubility of the polymer in aqueous metal ion solutions allows for homogeneous and effective adsorption on the polymer in under 1 min. The maximum recovery amounts were found to be 0.442 g g⁻¹ (4.153 mmol g⁻¹), 0.701 g g⁻¹ (3.593 mmol g⁻¹), and as 1.345 g g⁻¹ (6.829 mmol g⁻¹) for palladium(II), platinum(IV), and gold(III), respectively. In the case of platinum and gold, the maximum recovery amounts were greater than for other recovery materials reported in the literature. The obtained Poly(Cys-g-Sty) showed a higher affinity for these three precious metals, even in the presence of other metal ions. The desorption application was performed using various reagents. The solution of 0.5 M NaOH and 1 M KCN provides the maximum desorption efficiency of precious metal ions from the polymer. As one of the applications, the palladium and polymer complex (Poly(Cys-g-Sty)–Pd) was also used as a heterogeneous catalyst for the Suzuki–Miyaura coupling reaction with a high efficiency.

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Introduction

The formation of metal complexes of amino acids is the key step in a wide range of complex and important processes in biochemistry and organometallic chemistry, including catalytic activity in oxidation reactions,^1,2 protection for modification of an amino acid,⁵ modulation of metal–organic frameworks,⁷ and metalloprotein catalysis in bio-systems.⁹ The amino acids enable the formation of a stable five-membered chelate with various metal ions through the amine and carboxylate groups (N,O-chelation). The additional binding sites in several amino acids such as the imidazole ring on histidine (His) and the phenol ring on tyrosine (Tyr), provide a path to a variety of structures.¹¹ In terms of the side chain coordination playing versatile role in the formation of the metal center and its catalytic action in proteins, L-cysteine and S-methylcysteine manifest a rich coordination chemistry because of their high affinity for a variety of transition metal ions.^12–16 This unique affinity to transition metals has led many studies to investigate cysteine metal complexes with palladium,^16–18 platinum^17–19 and gold,¹⁹ which are three noteworthy elements in the transition metal family.

Rare metals, including Pd^II, Pt^IV and Au^III, have been classified as limited resources, existing only in small amounts on the Earth, and acceptably not only in the forms of currency materials but also as useful metals in a variety of well-known advanced applications, such as electronic devices, catalysts, and biomedical instruments. Therefore, the efficient separation and refining of these precious metals from nature, wastewater, and discarded high-tech products is becoming crucial in terms of ensuring their supply, reusability, and environmental sustainability.

Conventional methods for the removal of these metals from water and wastewater, which consist of precipitation, solvent extraction, reduction, oxidation, precipitation, membrane filtration, and ion exchange, are inefficient in terms of their operational complexity, energy consumption, degree of purification, yield, and labour costs.^3,10,20 To replace these traditional approaches, attempts have been made in recent years to develop highly effective and economical adsorption systems that proceed in homogeneous conditions. In this regard, biopolymers, such as chitosan and lignin, have recently received a great deal of attention as renewable resources and environmental friendly materials.^3,8,20 However, metal adsorption by water-soluble polymers bearing thiourea groups²⁰ or a melamine–cyanuric acid combination¹⁰ provide a greater recovery efficiency and higher adsorption selectivity for rare metals, such as Pd^II, Pt^IV, and Au^III, than other polymers reported in the literature.

In addition to recovery systems, the reusability of metals for catalytic reactions is another aspect of the suitability of rare metals. Polymer-supported catalysis in organic chemistry has been used in a number of classes of organic transformations, including cycloaddition, reduction, oxidation, addition, and transitional metal-catalysed carbon–carbon bond formation reactions.²¹ Reusable polymer-supported palladium catalysis in particular for the Suzuki–Miyaura coupling reaction, was also reported^22–25 and represents an efficiency and high selectivity toward biaryl products in different reaction conditions.

Herein, we demonstrate a new and effective metal recovery system for rare metals, such as Pd^II, Pt^IV and Au^III, based on the interaction between cysteine and metals. This system working homogeneously in aqueous media provides the complexation with metals in exceptional rate of adsorption less than one minute. The reaction to convert L-cysteine to the polymerizable amino acid by the nucleophilic attack of the thiol group to 4-vinylbenzyl chloride forms a styrene–cysteine zwitterionic monomer. Green polymerization conditions can be used to avoid the use of organic solvents; free radical polymerization proceeds in water under exceptionally mild reaction conditions, and the conversion reaches 99%. After that, the resulting L-cysteine-grafted polystyrene (Poly(Cys-g-Sty)) was employed for the recovery of Pd^II, Pt^IV and Au^III while considering the fact that sulfur-containing compounds behaving like a soft base and have a high selectivity toward precious metals, which are typically soft acids. This system works in aqueous solutions, and its high affinity for rare metals provides the highest recovery amount for Pt^IV and Au^III in the literature. Additionally, Poly(Cys-g-Sty) and the palladium complex were used to demonstrate their catalytic effect on the Suzuki–Miyaura coupling reaction.

Results and discussion

Synthesis and characterization of cysteine-grafted styrene zwitterionic polymer

The combination of amino acids and available functional groups for radical polymerization was introduced in several methods. Generally, in the functionalization of amino acids, the carboxylic acid and amine sides were used to obtain new amino acid derivatives for similar purposes. Extraordinarily, the thiol side chain in cysteine often participates in enzymatic and substitution reactions as a more nucleophilic group than its amine side. Therefore, we chose thioether formation from the reaction of L-cysteine and 4-vinylbenzyl chloride to prepare the styrene monomer bearing an unprotected amino acid that has zwitterionic properties (Scheme 1). Initially, the total solubility of L-cysteine in the ethanol and water mixture was achieved by the addition of a molar equivalent amount of triethylamine. S-Vinylbenzyl-L-cysteine (Cys-Sty) begins to precipitate after the complete termination of the reaction after stirring for 1 h at room temperature because of the neutralization of reaction medium in which a nearly equimolar amount of released HCl reacts with triethylamine, forming ammonium chloride. After filtration and washing with diethyl ether, the desired compound was obtained as a white powder in high yield (98%). The identity of the monomer was confirmed by proton NMR analyses (Fig. 1), which were performed in a trifluoroacetic acid-d (TFA-d, 20%) and CDCl₃ (80%) mixture so that the non-zwitterion form of the monomer had a solubility resistance in the organic solvent (it was soluble only in acidic or basic conditions).

Scheme 1 The synthetic procedure for cysteine-styrene (Cys-Sty) monomer.

Fig. 1 ¹H-NMR spectrum of Cys-Sty monomer in the mixture of TFA-d/CDCl₃.

We directly examined the polymerization of the Cys-Sty monomer in aqueous solutions by taking advantage of the amino acid side that can be soluble in gently acidic conditions. To prepare the appropriate reaction conditions, the monomer first diffused into water to form a suspension solution. Then, the addition of acetic acid could increase the solubility. As shown in Scheme 2, the polymerization of Cys-Sty in the presence of 2,2′-azobis(2-methylpropionamidine)dihydrochloride (AAPH) as a radical initiator reaches up to 99% conversion after heating at 60 °C for 24 h and follows the purification with a dialysis technique because the separation of the unreacted monomer and oligomeric impurities is virtually impossible by other purification methods. The obtained Poly(Cys-g-Sty) also has non-zwitterionic properties, and its solubility in organic solvents and neutral water is limited. Therefore, to overcome this problem and perform size-exclusion chromatography (SEC), the amine and acid moiety of the repeating units were protected by Boc and tert-butyl groups, respectively. The availability for protection indicates that the general amino acid protection methods are convenient for this polymer, and it can be converted to its soluble form and derivatives. The chemical structure of Poly(Cys-g-Sty) was investigated by ¹H-NMR spectroscopy performed in TFA-d. As shown in Fig. 2, the aromatic protons of styrene correlate with the normal polystyrenes reported in the literature, and the integration of cysteine protons according to styrene protons indicates an exact structural confirmation. To investigate the molecular weight characteristics, SEC was performed with Poly(Cys-g-Sty)-protected instead of Poly(Cys-g-Sty) because of the solubility problem (Fig. 3). The number averages of the molecular weight (M_n) and polydispersity index (M_w/M_n) are estimated to be 19 700 and 1.5, respectively.

Scheme 2 The synthesis of Poly(Cys-g-Sty) via free radical polymerization in aqueous media, and protection of amino and carboxylic acid for Poly(Cys-g-Sty)-protected.

Fig. 2 ¹H-NMR spectrum of Poly(Cys-g-Sty) in TFA-d.

Fig. 3 SEC trace and the molecular weight characteristics of Poly(Cys-g-Sty)-protected.

Adsorption of precious metal ions onto Poly(Cys-g-Sty)

The zwitterionic feature of Poly(Cys-g-Sty) provides the opportunity to have positive and negative charges on the polymer side chains, leading to solubility both in acidic and basic conditions. In addition, the amino acid side enables complex formation with various metal ions through the amine and carboxylate moieties (N,O-chelation) or other functional sides on amino acid such as sulphur atom in cysteine derivatives. In light of this information, we initially investigated the metal complex formation of several water-soluble metal salts including copper, manganese, iron, nickel, cobalt, palladium, platinum and gold. Extraordinarily, palladium, platinum, and gold formed instantly higher amounts of precipitation in less than one minute, indicating the special affinity of the polymer for these three precious metals (Fig. S1†). As seen in Fig. S2,† considering the proper solubility conditions for the metals and the polymer, the workable pH scale range for complex preparation was decided around pH = 1–5. Experimentally, 1.0 M HCl aqueous solutions of Poly(Cys-g-Sty) were added to 1.0 M HCl aqueous solutions of Pd^II, Pt^IV, Au^III, and these mixtures were stirred at 25 °C for 24 h (Fig. 4). Then, the precipitation of the complexes occurred. The precipitate was separated by filtration, and the concentration of the metal ions in the filtrates was measured by atomic absorption spectrometry (AAS).

Fig. 4 The photographs of after metal recoveries. Conditions: 1 M HCl aqueous solution of metals: 25 mL ([Pd^II] = 0.494 g L⁻¹, [Pt^IV] = 0.870 g L⁻¹, [Au^III] = 0.692 g L⁻¹) that of Poly(Cys-g-Sty): 10 mg in 25 mL HCl aqueous solutions, temperature: 25 °C.

The effect of the concentration of metals on the recovery amounts was examined to determine the maximum recovery amount for the polymer. In this study, the metal concentrations increased up to nine-times to the molar equivalent of the polymer to evaluate the adsorption behaviour of the polymer in the presence of excess amounts of metal ions. As seen in Fig. 5a, as the metal adsorption constitute as the metal concentrations increase beyond the molar equivalent of the polymer. This extraordinary result can be explained by a weak interaction between the thioether and phenyl group with the metals and the amino acid side.^14,26–28 For a better understanding of the metal adsorption mechanism, benzyl-L-cysteine (Bn-Cys) was synthesized by the same experimental pathway for the Cys-Sty monomer for use as a model compound (Scheme S1†). The complex between Bn-Cys and the metal ion was prepared with a three-fold molar excess of Pd^II, Pt^IV, and Au^III metal ion solutions. After 24 h of stirring at 25 °C, no precipitation was observed, and all of the complexes were available for ¹H-NMR analysis in D₂O. In Fig. S3,† the peak changes in the –CH₂– group of cysteine and phenyl ring protons can be accepted as clear evidence for the interaction of metals with thiol and phenyl groups. In addition, Fig. S4† gives extra information about complexation mechanism considering the possible metal–polymer complex structures. Poly(Cys-g-Sty) is insoluble in neutral water, however, acidification of amine side of amino acid forms ammonium salt and makes the polymer soluble. After metal complex formation, only the solubility provider group is blocked and the polymer becomes totally insoluble even in harsh acidic and basic condition which is examined in the desorption step. For instance, the formation of complex 2 requires the polymer to be soluble in acidic conditions. Complex 3 and 4 cannot be considered because the metal adsorption ratios reach the molar equivalent of the polymer (beyond the molar equivalent for Au). Normally, the adsorption ratio does not reach the molar equivalent to the instrument as crosslinked complexation occurs. Considering these reasons and literature²⁹ about similar platinum cysteine complexation mechanism supported by X-ray analysis, the main complex structure is accepted as a complex 1.

Fig. 5 (a) Adsorption capacity plots with Poly(Cys-g-Sty) as a function of initial concentration of metal ions. Conditions for 1 : 1 entry: HCl aqueous solution of metals: 25 mL ([Pd^II] = 0.494 g L⁻¹, [Pt^IV] = 0.870 g L⁻¹, [Au^III] = 0.692 g L⁻¹) that of Poly(Cys-g-Sty): 10 mg in 25 mL HCl aqueous solutions, temperature: 25 °C; reaction time: 24 h (for other entries, only metal ratios were changed increasingly). (b) Changes in adsorption of metals by Poly(Cys-g-Sty) as a function of mixing time. Condition: HCl aqueous solution of metals: 25 mL ([Pd^II] = 2.47 g L⁻¹, [Pt^IV] = 4.35 g L⁻¹, [Au^III] = 3.46 g L⁻¹) that of Poly(Cys-g-Sty): 10 mg in 25 mL HCl aqueous solutions, temperature: 25 °C.

The results presented in Table 1, including a detailed explanation of Fig. 5a, suggest that the maximum adsorption capacity and adsorption ratios were obtained for Pt^IV, Pd^II, and Au^III. Taking into the consideration the sensitivity of the AAS instrument, the average results of all entries were also demonstrated. The adsorption kinetics of Pd^II, Pt^IV and Au^III on Poly(Cys-g-Sty) as a function of the mixing time are shown in Fig. 5b. The maximum recovery amount was attained in less than 1 min for Pd^II and Pt^IV, and a similar result was also seen after 18 h of mixing. However, a strange pattern was observed for Au^III, which was previously reported for a similar gold adsorption application.⁸ This distinctive pattern in the gold adsorption process can be explained only by the change in the oxidation state of some of the Au^III once adsorbed, leading to the formation of new ionic species, most likely Au^I, then the formation of elemental gold as time progresses. Additionally, the elemental gold formation occurs only after interaction with the polymer because the precipitation was not observed for the gold solution prepared at the same concentration and kept under the same conditions. Table 2 compares the maximum recovery amounts used for the Pt^IV, Pd^II, and Au^III adsorption previously reported in the literature. Poly(Cys-g-Sty) has a considerably higher recovery amount for Pd^II than several other materials. The essential differences appear for Pt^IV and Au^III, which shows the highest recovery amount compared any other materials. The selective affinity feature of Poly(Cys-g-Sty) was investigated by using equal amounts of metal ions (Pd^II, Pt^IV, Au^III, Co^II, Ni^II, Zn^II, and Mn^II), each at the same concentration with the polymer. Fig. 6a shows that the polymer has a slightly higher affinity for Pt^IV than Pd^II and Au^III. This can be explained by the hard and soft acids and bases (HSAB) principle. The amino acid side chains of the polymer in acidic conditions behave as a soft base and can preferentially coordinate to a soft acid, Pt^IV, which is a softer acid than Pd^II and Au^III. In addition, the same principle explains the results that were illustrated in Fig. 6b–d, indicating that the polymer has a specific affinity to these three metals in the presence of other metals. In addition to the quantitative adsorption, desorption of the metal adsorbed by the polymer is essential to allow the polymer to be reused and achieve practical use. Initially, the desorption efficiency of adsorbed metals onto Poly(Cys-g-Sty) was studied by different well-known desorption agents. As can be seen in Fig. S5† (which describes all of the steps of the desorption process), even harsh concentrations of HNO₃ and NaOH were not able to dissolve the polymer–metal complex and separate the metals from the polymer. The use of a thiourea–HCl solution provided total solubility for the polymer–metal complex; however, desorption of the metals was not archived at a high level. The highest desorption percentage was obtained when a 0.5 M NaOH and 1 M KCN solution was used, leading to the recovery of almost 99% of the metal (Table S1†). Therefore, a reusability study was performed using the 0.5 M NaOH and 1 M KCN solution. Table S2† shows the adsorption and desorption efficiency (%) for every cycle. After first cycle, the polymer adsorption ability for Pd^II and Au^III significantly decreased, and the polymer was not available for reuse using Pt^IV.

Table 1 The maximum adsorption capacity and adsorption ratios of Poly(Cys-g-Sty) in different metal concentrations

[MPoly]0 : [MMetal]0^a		1 : 1	1 : 3	1 : 5	1 : 7	1 : 9	Average^b
a Conditions for 1 : 1 entry: HCl aqueous solution of metals: 25 mL ([Pd^II] = 0.494 g L^−1, [Pt^IV] = 0.870 g L^−1, [Au^III] = 0.692 g L^−1) that of Poly(Cys-g-Sty): 10 mg in 25 mL HCl aqueous solutions, temperature: 25 °C; reaction time: 24 h (For other entries, only metal ratios were changed increasingly).b Average values was calculated using entry from 1 : 1 to 1 : 9.
Qmax (gmetal gPolymer^−1)	Pd^II	0.388	0.388	0.447	0.463	0.522	0.442
	Pt^IV	0.640	0.650	0.670	0.694	0.884	0.701
	Au^III	0.827	1.042	1.36	1.795	1.712	1.345
Adsorption ratio (MolPoly : MolMetal)	Pd^II	1 : 0.87	1 : 0.87	1 : 1	1 : 1.04	1 : 1.17	1 : 0.99
	Pt^IV	1 : 0.78	1 : 0.78	1 : 0.81	1 : 0.85	1 : 1.19	1 : 0.88
	Au^III	1 : 1	1 : 1.26	1 : 1.65	1 : 2.17	1 : 2.07	1 : 1.63

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Table 2 Comparison of the maximum adsorption capacities for the adsorption of Pt^IV, Pd^II and Au^III onto various adsorbents

Adsorbent	Qmax (gmetal gPolymer^−1)			Ref.
	Pd^II	Pt^IV	Au^III
Lysine modified crosslinked chitosan resin	0.101	0.129	0.703	3
Poly(vinylbenzylchloride–acrylonitryle–divinylbenzene) modified with tris(2-aminoethyl)amine	0.280	0.245	0.190	4
Amberlite IRC 718	0.058	0.066	0.135	6
EN-lignin	0.022	0.104	0.606	8
MC system	0.595			10
Poly(Cys-g-Sty)	0.442	0.701	1.345	This work

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Fig. 6 Competitive adsorption of metal ions. Conditions: (a) HCl aqueous solution of metals: 25 mL ([Pd^II] = 0.494 g L⁻¹, [Pt^IV] = 0.870 g L⁻¹, [Au^III] = 0.692 g L⁻¹); that of Poly(Cys-g-Sty): 10 mg in 25 mL HCl aqueous solutions (b), (c), (d) HCl aqueous solution of metal: 25 mL ([Pd^II] = 0.494 g L⁻¹ only in (b), [Pt^IV] = 0.870 g L⁻¹ only in (c), [Au^III] = 0.692 g L⁻¹ only in (d), [Co^II] = 0.472 g L⁻¹, [Ni^II] = 0.400 g L⁻¹, [Zn^II] = 0.483 g L⁻¹, [Mn^II] = 0.332 g L⁻¹); that of Poly(Cys-g-Sty): 10 mg in 25 mL HCl aqueous solutions, temperature: 25 °C.

Heterogeneous catalyst for Suzuki–Miyaura coupling reactions

The strong interaction between these precious metals leads to a low desorption efficiency and insolubility in organic solvents, and harsh acidic and basic conditions. Thus, Poly(Cys-g-Sty) and the palladium complex (Poly(Cys-g-Sty)–Pd) could be used as a heterogeneous catalyst system. In light of this idea, we focused on palladium-catalyzed carbon–carbon bond formation via Suzuki–Miyaura coupling reactions of aryl halides, which is a well-known and versatile route to construct biaryl units. Experimentally, the reaction with halobenzene, benzeneboronic acid (0.6 mmol), base (K₂CO₃, 1 mmol), and 5 mL of solvent (H₂O and tetrahydrofuran, 3 : 1 v/v) were introduced into a Schlenk flask. To reach the maximum catalytic surface and efficiency during the reaction, a catalyst (Poly(Cys-g-Sty)) was used in a 40% molar ratio to halobenzene. The reaction was allowed to proceed at the reaction temperature for the appropriate time, which led to nearly total conversion. After the reaction, the insoluble catalyst was easily filtered, washed, and reused until the reaction yield decreased from 99% to 95%. Then, the turnover number and frequency, which represent the catalytic capacity of Poly(Cys-g-Sty), were calculated (Table 3).

Table 3 Suzuki coupling reaction of different aryl halides with benzeneboronic acid catalysed by Poly(Cys-g-Sty) and palladium complex

R	X	Yield (%)	Catalyst (mol%)	Selectivity toward biaryl product (%)	TON^a	TOF^b
a TON = turnover number (mol of product per mol of catalyst).b TOF = turnover frequency (TON per hour).
H	I	>99	40	>99	52.5	105
H	Br	>99	40	>99	45	90
H	Cl	>99	40	>99	40	80

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Experimental

Materials

4-(Chloromethyl) styrene, sodium tetrachloroplatinate(II)hydrate, benzeneboronic acid, iodobenzene, chlorobenzene, bromobenzene, triethylamine (Et₃N), acetonitrile (ACN), methanol (MeOH), di-tert-butyl carbonate (Boc₂O), 4-dimethylaminopyridine (DMAP), and N,N′-dicyclohexylcarbodiimide (DCC) were purchased from TCI (Tokyo Chemical Industry Co, Ltd). L-Cysteine, 2,2′-azobis(2-methylpropionamidine)dihydrochloride (AAPH), sodium hydroxide, ethanol (EtOH), and acetic acid were purchased from Wako Pure Chemical Industries, Ltd. Hydrogen hexachloroplatinate(IV) hexahydrate, copper(II) sulfate pentahydrate, copper(II)chloride dihydrate, zinc sulfate heptahydrate, iron(III)chloride hexahydrate, and manganese(II)chloride tetrahydrate were purchased from Kanto Chemical. Gold(III)chloride-4-hydrate (Kishida Chemical Co., Ltd) and dialysis membrane (Spectra/Por MWCO 500 D, Spectrum Laboratories, Inc.) were commercially available and used as received.

Instruments

The NMR spectra (400 MHz for ¹H) were recorded with a JEOL ECS-400 spectrometer, and the chemical shifts were obtained in ppm units using tetramethylsilane (TMS) as an internal standard. Size exclusion chromatography (SEC) was performed at 40 °C on a TOSOH HLC-8220 system equipped with three consecutive polystyrene gel columns (TSK-gels (bead size, exclusion limited molecular weight); super-AW4000 (6 μm, >4 × 10⁵), super-AW3000 (4 μm, >6 × 10⁴) and super-AW2500 (4 μm, >2 × 10³)) and refractive index and ultraviolet detectors. This system was operated using dimethylformamide (DMF) containing 10 mM LiBr as the eluent at a flow rate of 0.5 mL min⁻¹. Polystyrene standards were used for the calibration. High-resolution mass spectrometry was conducted using a JEOL JMS-700 mass spectrometer. Flame atomic absorption spectrometry was performed using a Hitachi polarized Zeeman atomic absorption spectrometer (AAS) Z-2310.

Synthesis of S-vinylbenzyl-L-cysteine (Cys-Sty)

4-(Chloromethyl) styrene was passed through an Al₂O₃ column immediately before the reaction. Et₃N (6.3 mL, 41.3 mmol) was slowly added to a stirred heterogeneous solution of L-cysteine (5.0 g, 41.3 mmol) in 250 mL of EtOH/H₂O (3 : 2, v/v). After the complete dissolution of L-cysteine, the purified 4-(chloromethyl)styrene (6.93 g, 45.43 mmol) in 10 mL of EtOH was added dropwise to the solution and stirred at room temperature. After stirring for 2 h, white precipitates were filtered, washed with 1 L of diethyl ether, and dried under vacuum to give 9.6 g of the target compound, Cys-Sty, at a ∼98% overall yield. ¹H-NMR (CDCl₃–TFA-d (8 : 2), 400 MHz): δ 3.08 (ddd, J = 24.0, 15.2, 6.5 Hz, 2H), 3.77 (s, 2H), 4.13 (dd, J = 8.7, 4.0 Hz, 1H), 5.28 (d, J = 10.9 Hz, 1H), 5.75 (d, J = 17.6 Hz, 1H), 6.67 (dd, J = 17.6, 10.9 Hz, 1H), 7.28–7.21 (d, 2H), 7.39 (d, J = 8.1 Hz, 2H). HRMS [MS EI⁺]: m/z [MF-Linear] calc. for C₁₂H₁₅NO₂S: 237.0823, found 237.0824.

Synthesis of the poly(cysteine-graft-styrene) zwitterionic polymer (Poly(Cys-g-Sty))

The monomer, Cys-Sty, (1.2 g, 5.0 mmol), acetic acid (10 mL), and distillated water (20 mL) were placed in a Schlenk flask and stirred at 60 °C for 30 min. The obtained clear mixture was cooled to room temperature and bubbled with nitrogen for 15 min. Then, AAPH (135 mg, 0.5 mmol) was added to the mixture, and a freeze–pump–thaw cycle was performed. The reaction mixture was stirred for 24 h at 60 °C. Then, the resulting polymer was purified by dialysis (500 Dalton membrane) in water (10 L) and freeze-dried for 2 days to yield 1.1 g of the polymer (92%), (conv. of monomer: >99%). ¹H-NMR (TFA-d, 400 MHz): δ 0.9–2.70 (br, 3H), 2.77–3.53 (br, 2H), 3.57–4.15 (br, 2H), 4.16–4.77 (br, 1H), 6.32–7.30 (br, 4H).

To perform the SEC analysis, Poly(Cys-g-Sty) was converted to a soluble derivative by protection of the amino and carboxyl groups according to the following procedure.

Poly(Cys-g-Sty) (500 mg) was dissolved in distillated water (10 mL) and MeOH (20 mL) by the addition of Et₃N (0.7 mL), and the slightly heterogeneous solution was stirred at room temperature until a clear solution was obtained. Then, Boc₂O (1.2 g) in 10 mL ACN was added dropwise for 15 min and stirred at room temperature for 24 h. The reaction mixture was concentrated, precipitated in diethyl ether and dried under vacuum to obtain N-Boc-Poly(Cys-g-Sty) (703 mg) and directly used in the following step.

N-Boc-Poly(Cys-g-Sty) (703 mg), DMAP (50 mg), and t-butanol (t-BuOH, 0.6 mL) were dissolved in dried DMF (5 mL) under a N₂ atmosphere. Then, DCC (620 mg) was added dropwise to the solution. The reaction mixture was stirred at room temperature for 24 h. Then, the resulting polymer was purified by dialysis (1000 Dalton membrane) in water (3 L) and freeze-dried for two days to yield 560 g of Poly(Cys-g-Sty)-protected (66%). ¹H-NMR (CDCl₃, 400 MHz): δ 1.35–1.49 (br, 9H), 1.48–1.62 (br, 9H), 0.9–2.15 (br, overlapped, 3H), 2.40–3.08 (br, 2H), 3.16–3.88 (br, 2H), 4.17–4.74 (br, 1H), 5.18–5.87 (br, 1H), 6.04–7.66 (br, 4H).

Metal adsorption (typical procedure)

Poly(Cys-g-Sty) (10 mg) was dissolved in 25 mL of 1 M HCl aqueous solution. The solution was added to a 1 M HCl aqueous solution of the metal concentrations (25 mL of [Pd^II] = 0.494 g L⁻¹, [Pt^IV] = 0.870 g L⁻¹, [Au^III] = 0.692 g L⁻¹), and the resulting mixture was stirred at 25 °C for 24 h. The formed precipitate was separated by filtration (pore size of filter: 0.45 μm), and washed with distillate water (2 mL). Then, the filtrate volume was adjusted to 50 mL again, and a thousand-fold dilution was performed to reach an appropriate concentration. The metal concentration in the solution was determined by AAS. The recovery amount of the polymer for all entries was calculated based on the following equation:

Recovery amount (g_metal g_polymer⁻¹) = M of metal × recovery amount (mmol)/weights of polymer used (g).

Metal desorption from Poly(Cys-g-Sty)–metal (typical procedure)

After the adsorption process, insoluble precipitates of Poly(Cys-g-Sty)–metal were dissolved in 25 mL of a solution of the desorption agent (a mixture of 0.5 M NaOH and 1.5 M KCN). After stirring for 24 h at 25 °C, the pH of the solution was adjusted to pH = 7 using 1.5 M HCl aqueous solution to precipitate Poly(Cys-g-Sty). Then, the volumes were adjusted to 100 mL. A ten-fold dilution was performed again to reach an appropriate concentration. The metal concentration in the solution was determined by AAS. The desorption efficiency of the polymer for all entries was calculated based on the following equation:

Desorption efficiency (%) = desorption amount (mmol)/recovery amount (mmol) × 100.

Conclusion

In summary, we have reported a new and effective precious metal recovery system for Pd^II, Pt^IV and Au^III using zwitterionic cysteine-grafted poly(styrene)s, which allows for the adsorption and desorption of metals in aqueous solutions in under 1 min. The recovery efficiencies for Pd^II, Pt^IV, and Au^III were 99%, 88%, and 116% respectively, and present extraordinary results compared to other rare metals recovery systems. The system was also capable of selective recovery of Pd^II, Pt^IV, and Au^III in the presences of metal solutions. By taking advantage of the strong interaction between the polymer and metals, even in harsh basic and acidic conditions, the Poly(Cys-g-Sty) and palladium complex was used as a heterogonous catalyst for the Suzuki–Miyaura coupling reaction. The strategy of using a polymer–metal ion complex is expected to be used for designing the functional polymers and could lead to the effective, practical use of rare metal.

Acknowledgements

This work was financially supported by JSR Corporation. The high-resolution mass spectrometry (HRMS) experiment was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices.” We thank Professor Nobuto Oka (Kindai University) for assistance with the AAS measurement.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23359g

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