Preparation of a novel high-strength polyzwitterionic liquid hydrogel and application in catalysis

Abstract

A new polymeric hydrogel P(PVIS–AA) based on zwitterionic liquids (1-propyl-3-vinyl imidazole sulfonate) (PVIS) and acrylic acid (AA) was prepared by free-radical polymerization. The resulting three-dimensional (3D) networks can serve as nano-reactors for the production of small size and highly stable palladium nanoparticles by in situ reduction of PdCl₆²⁻ with sodium borohydride (NaBH₄) as a reducing agent. To get enough mechanical strength, ferric (Fe³⁺) was used to form the second crosslinking. The as-prepared hybrid hydrogel (P(PVIS–AA)/Pd@Fe³⁺) was successfully confirmed by means of Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), field emission scanning electron microscopy (FESEM), an electrical universal material testing machine, transmission electron microscopy (TEM) and thermogravimetric analysis (TGA), and then tested to catalyze the reduction of 4-nitrophenol. The catalytic results indicate that within 6 min, 4-nitrophenol with an initial concentration of 5 mM can be reduced completely, in addition, the mechanically strengthened P(PVIS–AA)/Pd@Fe³⁺ hydrogel shows excellent reusability for five successive cycles, with no appreciable decrease in the catalytic effects.

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Introduction

Polymer hydrogels are materials containing large amounts of water within three-dimensional polymer networks. They are used as scaffolds for model extracellular matrices for biological studies,¹ such as actuators for optics and fluidics,² vehicles for drug delivery,³ and tissue engineering,⁴ besides, the 3D adjustable networks of hydrogels can potentially serve in the immobilization of different kinds of metal nanoparticles.

Noble metal nanoparticles have promising applications in the photonic, electronic, catalytic, and sensor fields^5–8 because of their specific properties that differ markedly from those of the bulk metals. In principle, noble metal nanoparticles must be stabilized to ensure the dispersion and stability for application, of many protective systems,^9–11 there are many studies about polymer hydrogel acts as support for the application of catalytic reaction. Anatoly Zinchenko​¹² proposed a hydrogel with three-dimensional molecular architectures which composed of DNA, it was used as foundation of Au nanoparticles and applied in catalytic hydrogenation of nitrophenol, the results demonstrated that it took quite long time to catalyze the reaction. Zheye​ Zhang¹³ had made graphene oxide hydrogels by using glucono-δ-lactone, the hybrid material plays an important role in heterogeneous catalytic reaction, but the synthesis process was complex. Natalya Dolya¹⁴ reported “One-Pot” in situ formation of noble metal nanoparticles within hydrogels, the quantity of the noble metal nanoparticles were relatively low. Moreover, from a materials point of view, most applications is suffered from serious disadvantages of conventional synthetic polymer hydrogels, most typically chemically crosslinked polymer hydrogels, in their mechanical properties (fragility).

In this work, we present preparation of a novel high-strength polyzwitterionic liquids hydrogel and applied in catalysis. Due to their excellent chemical properties, preferable thermal stability and hydration capacity, zwitterionic liquid in which anionic and cationic groups are linearly arrayed on the pendant side chain of the molecular backbone, appear especially promising for various potential applications.^15,16 Significant progress on the development of zwitterionic materials has been made for many applications such as biosensors, biomaterials, drug delivery systems, and marine coatings.^17–19 On the support of polyzwitterionic liquids hydrogel, Pd nanoparticles were immobilized by in situ reduction of PdCl₆²⁻ with NaBH₄ as reducing agent, then the mechanical strength of the hydrogel was enhanced through ionically crosslinking by Fe³⁺. The results demonstrated the formation of ultra-small, non-aggregated, and kinetically stable Pd nanoparticles the small amount of Fe ions were reduced to Fe nanoparticles of 2–5 nm size inside the polyzwitterionic hydrogel, being bimetallic nanoparticles catalysts, which promote the catalytic reaction as well. Therefore, the hybrid system of hydrogel/Pd@Fe³⁺ has been successfully exploited as a potential reusable catalyst support for the reduction of aromatic nitro to amino group at room temperature. The tough hydrogel expand the scope of application in catalytic reaction, which can perform the reduction of 4-nitrophenol under a magnetic stirring for a short time.

Experimental section

Materials

1-Vinylimidazole (≥99%; Aldrich), 1,3-propyl sultone (99%; Aldrich), acrylic acid (AA), NaBH₄ (98%), potassium persulfate (KPS) and ferric chloride (FeCl₃·6H₂O, 99%) were purchased from Tianjin Kaixin Chemical Reagent (China), N,N′-methylenebisacrylamide (Bis, Aldrich, 98.0%) were used as received. Ammonium hexachloropalladate(IV) (NH₄)₂PdCl₆ (99%) was purchased from Kun Ming Boren precious metals co. LTD. All other chemicals were of analytic grade and used as received.

Synthesis of the zwitterionic liquid monomer 1-propyl-3-vinyl imidazole sulfonate (PVIS)

N-Vinylimidazole (4.7 g, 50 mmol) was dissolved in 20 mL methylene chloride, then added dropwise to the solution of 1,3-propanesulfonate (6.71 g, 55 mmol) containing 20 mL of methylene chloride. The reaction proceeded at 45 °C under magnetic stirring for 4 h, finally yielded white solid. The products were washed with methylene chloride several times in order to remove impurities and unreacted substance totally, and then filtered, vacuum drying. Finally white solid zwitterionic liquids (7.4 g, 65%) were got. ¹H-NMR (400 MHz, D₂O, signals referenced to δ 4.8 ppm): 2.33–2.30 (m, 2H, –CH₂–CH₂–CH₂–SO₃⁻), 2.93–2.90 (m, 2H, –CH₂–CH₂–SO₃⁻), 4.40–4.37 (m, 2H, –CH₂–SO₃⁻), 5.41–5.39 (d, 1H, =CHH), 5.80–5.76 (d, 1H, =CHH), 7.14–7.08 (m, 1H, –CH=CH₂), 7.60, 7.76, 9.06 (s, 3H, imidazole) (Fig. S1†) (Scheme 1).

Scheme 1 The synthetic route of zwitterionic liquid: 1-propyl-3-vinyl imidazole sulfonate (PVIS).

Synthesis of the hydrogel of P(PVIS–AA)

The monomers of 1-propyl-3-vinyl imidazole sulfonate (PVIS) (1.946 g, 9 mmol) and acrylic acid (AA) (0.648 g, 9 mmol) were dissolved in 10 mL deionized water, with the molar fraction ratio of 1 : 1, then added N,N′-methylenebisacrylamide (BIS) (0.028 g, 0.018 mmol), with the molar fraction ratio of 1 : 100 (to total monomers), K₂S₂O₈ (0.049 g, 0.018 mmol) in 1 mL of deionized water was injected into the reaction mixture quickly after it was degassed by nitrogen for 15 min. The reaction was allowed to proceed at 65 °C for 4 h. The obtained P(PVIS–AA) hydrogel was opaque, it was immersed in deionized water for at least 6 days by replacing solution every other day to extract any soluble species.

Immobilization of Pd nanoparticles within the P(PVIS–AA) hydrogel

The hydrogel of P(PVIS–AA) (2.0 g) was placed into a beaker, subsequently, (NH₄)₂PdCl₆ solution (50 mL, 0.24 mM) was added to the beaker. After 1 day, freshly prepared NaBH₄ solution (5 mL, 0.2 M) was added rapidly to the hydrogel matrix to induce Pd(II) reduction to form hydrogel/Pd system matrix.

Enhance mechanical strength of the P(PVIS–AA)/Pd hydrogel through Fe³⁺ crosslinking

P(PVIS–AA)/Pd matrix was incubated with 30 mL of FeCl₃ (0.02 M) solution. Subsequently, hybrid gel/Pd matrix kept shrinking and continued for 30 min, finally the volume of hybrid gels/Pd matrix became constant, then filtered and washed with deionized water.

Catalytic reduction of 4-nitrophenol

The catalytic reduction reaction of 4-nitrophenol was tested by UV-vis. The procedure was as follows: 4-nitrophenol (0.15 mL, 5 mM) was mixed with a freshly prepared aqueous solution of NaBH₄ (2.4 mL, 0.2 M). A given amount of the as-prepared P(PVIS–AA)/Pd@Fe³⁺ system matrix was added under constant vigorous stirring. UV-vis absorption spectra monitored the change in the reaction mixture under the same condition. The rate constant of the reaction was measured by the gradual change of the intensity of the peak at 400 nm in the same time interval. To study the reusability of the hydrogel, the P(PVIS–AA)/Pd@Fe³⁺ hydrogel was separated simply, followed by washing with deionized water for subsequent recycle of catalysis under the same reaction conditions.

Characterization

A scanning electron microscope (SEM) (Quanta 200, Philips-FEI) was used to analyze the morphology of the samples. Before the test, all samples were sprayed with gold for 30 min. Transmission Electron Microscopy (TEM) (HitachiModel H-600) was used to observe the morphologies of the products at 100 kV. Hybrid hydrogel was homogenized by sonication, a small amount of resulted solution was placed onto a copper grid and allowed the sample to dry for 30 min. Hydrogen nuclear magnetic resonance spectra was obtained on a Varian Inc., 400 MHz UNITYINOVA spectrometer at room temperature with the resonance frequency of 79.5 MHz, a magic-angle spinning at 5 kHz, 908 pulse length of 6.5 μs and a repetition delay of 60 s. Thermogravimetric analysis (TGA) was performed using TG instrument Q 500 from room temperature to 800 °C at a heating rate of 10 °C min⁻¹ under N₂ gas. Tensile strength measurements were performed using an electrical universal material testing machine (EZ-Test, SHIMADZU), test speed 5 mm min⁻¹. FTIR spectra were recorded on an IFS 66v/S FTIR spectrometer (Bruker, Germany) using the KBr disk method. The UV-vis spectra were recorded on a UV-vis spectrophotometer (TU-1901 Spectrophotometer, Persee, China) in the 200–550 nm.

Results and discussion

Synthesis of the hydrogel of P(PVIS–AA)/Pd@Fe³⁺

Fig. 1 is the synthetic procedure of the hydrogel of P(PVIS–AA)/Pd@Fe³⁺, which was synthesized by simple free-radical polymerization, using PVIS and AA as the comonomer, deionized water as the solvent, BIS as the cross-linking agent and K₂S₂O₈ as the initiator, porous hydrogel was synthesized in one pot. Then the hydrogel matrix were put into (NH₄)₂PdCl₆ solution for the attraction between cationic and PdCl₆²⁻, during the procedure, the color of the hydrogel matrix changed gradually from white to yellow and the hydrogel contracted as time rising, this was attributed to the attraction of [PdC1₆]²⁻ ion to the imidazole ring and the original charge balance was breaked. Then freshly prepared solution of NaBH₄ was added quickly into the prepared metal-ion-loaded P(PVIS–AA) hydrogel matrix at room temperature, with the addition of NaBH₄, the color of the hydrogel matrix changed to brown and black gradually, finally, the hybrid gels swelled to the original state, which was due to the reduction of Pd(II) to Pd(0) and recovery of the charge balance. Finally, mechanical strengthened hydrogels was prepared by immersing in FeCl₃·6H₂O solution through ferric crosslinking.

Fig. 1 The preparation procedure of the hydrogel of P(PVIS–AA)/Pd@Fe³⁺.

FTIR spectroscopy

The FTIR spectra of the AA (a), PVIS (b), P(PVIS–AA) (c), P(PVIS–AA)/Pd (d) and P(PVIS–AA)/Pd@Fe³⁺ (e) are shown in Fig. 2. In the spectrum of AA, the peak of 3264 cm⁻¹ was assigned to the stretching vibrations of O–H, the absorption characteristic peak at 1715 cm⁻¹ was responsible for C=O bending in COOH and the peak at 1627 cm⁻¹ was assigned to C=C characteristic absorption.²⁰ In the spectrum of PVIS, the absorption bands at 3447 cm⁻¹ was due to the stretching vibrations of the O–H, 1647, 1455 cm⁻¹ were ascribed to the C=C stretching vibrations on the imidazole chain bands, and 1190 cm⁻¹, along with band at 1041 cm⁻¹ were corresponding to S=O. Compared with the above spectrum, the absorption peaks of P(PVIS–AA) revealed some changes. The absorption band at 3447, 3140 cm⁻¹, which corresponded to stretching vibrations of O–H and C–H groups, showed a major shift, and the peaks at 1723 cm⁻¹ was attributed to the C=O stretching vibration of P(PVIS–AA), and the characteristic peak at 1447 cm⁻¹ of stretching vibration showed a slight shift. They were due to the chemical interaction of PVIS with the copolymer. Furthermore, an additional absorption peak at 1156, 1018 cm⁻¹ was observed and the peak at 1627 cm⁻¹ of C=C has disappeared,²¹ the results providing evidence of functional groups of PVIS grafting to PAA during the polymerization. The characteristic absorption bands of the Pd entrapped hydrogel (d) are quite similar to those of P(PVIS–AA) (c). Compared to the P(PVIS–AA)/Pd (d), the peaks of P(PVIS–AA)/Pd@Fe³⁺ (e) show a shift for –COO⁻ and –SO₃⁻ stretching vibration from 1557 and 1424 cm⁻¹ to 1565 and 1407 cm⁻¹, 1153 and 1029 cm⁻¹ to 1204 and 1046 cm⁻¹ respectively, which resulted from the attraction among –COO⁻, –SO₃⁻ and Fe³⁺ ions²² and implied the successful ionically crosslinking.

Fig. 2 FTIR spectra of AA (a), PVIS (b), P(PVIS–AA) (c), P(PVIS–AA)/Pd (d) and P(PVIS–AA)/Pd@Fe³⁺ (e).

The morphology of P(PVIS–AA) hydrogel

To get the morphology of P(PVIS–AA) hydrogel, the SEM images were measured and shown in Fig. 3, different morphologies can be found at different magnifications. From Fig. 3a–d, it can be seen that the P(PVIS–AA) hydrogel shows well-defined interconnected 3D porous networks, from the high magnified SEM images (Fig. 3c and d), it is found that the pore size of the networks are in the different micrometers, which suggests the hydrogel can absorb a large amount of water and provide space for the reaction. The thermal stability figure of the hydrogel of P(PVIS–AA) and P(PVIS–AA)/Pd see Fig. S2.†

Fig. 3 SEM images of P(PVIS–AA) (a–d) (at different magnifications).

TEM and EDX spectrum analysis

As showed in Fig. 4, the fabrication of Pd nanoparticles within the hybrid hydrogel matrix was found. It can be seen that Pd nanoparticles dispersed homogeneously inside hydrogel matrix, diameters has been found to be 2–5 nm approximately. The building of ultra-small Pd nanoparticles inside hydrogel matrix provided potential application in catalysis due to the large specific surface area. The sizes distribution of Pd nanoparticles is uniformly within zwitterionic ionic liquid-based hydrogel, which attributed to the well-proportioned distribution of the charge.

Fig. 4 TEM image of Pd nanoparticles containing in P(PVIS–AA) hydrogel matrix (left), EDX spectra of P(PVIS–AA)/Pd (right).

For analyses by EDX, the intensities of the Pd peaks for hydrogel matrix nanoparticles appeared at the different energy level. The elementals of C, N, S, O were attributed to the zwitterionic liquid-based hydrogel. Furthermore, there was an elemental of copper showed due to the copper film. These results collectively confirmed the successful synthesis of P(PVIS–AA)/Pd hydrogel.

XRD characterization

The hydrogel of P(PVIS–AA) and P(PVIS–AA)/Pd@Fe³⁺ was further characterized by XRD. From the XRD patterns, the wide peak at around 20–30° could be assigned to the polymers of the hybrid gel. In addition, the XRD pattern showed the diffraction peaks corresponding to the (111), (200), (220) planes of noble metal Pd nanoparticles (Fig. 5), which demonstrated that the Pd nanoparticles which attribute to the face-centered cubic pattern of palladium (Pd) metal⁵ were successfully immobilized.

Fig. 5 XRD patterns of P(PVIS–AA) hydrogel (black line) and P(PVIS–AA)/Pd@Fe³⁺ hydrogel (red line).

Mechanical properties of P(PVIS–AA)/Pd@Fe³⁺ hydrogel

The as-prepared P(PVIS–AA) hydrogel can be used as scaffold which supporting noble metal Pd nanoparticles and acting as an excellent carrier for catalyst. However, the hydrogel was soft and brittle, it is fragile for successive cycles of reactions, which restrict the scope of application in catalysis. The poor mechanical performance of chemically cross-linked hydrogels originates from their very low resistance to crack propagation due to the lack of an efficient energy dissipation mechanism in the gel network.^23,24 To obtain a hydrogel with a high degree of toughness, one has to increase the overall energy dissipation along the gel sample by introducing dissipative mechanisms at the molecular level.²⁵ In recent years, a number of techniques for toughening of gels have been proposed including the double network gels,²⁶ topological gels,²⁷ nanocomposite hydrogels,²⁸ cryogels,²⁹ betaine microgel,³⁰ and supramolecular polymer network hydrogels.³¹ Compared with these techniques, judicious selection of the cross-linking ion provides a means for optimizing the overall mechanical properties of the material.³²

In this work, the mechanical strength of the P(PVIS–AA) hydrogel was enhanced through ion crosslinking of Fe³⁺. Fig. 6 showed effects of different ion solutions on mechanical properties of hydrogels. From the curves obtained (Fig. 6a–c), the stress and stretch at rupture were recorded, respectively, 21.58 MPa and 49% for the P(PVIS–AA)/Pd@Fe³⁺ hydrogel matrix (Fig. 6c), 12.57 MPa and 69.76% for the P(PVIS–AA)/Pd@Ca²⁺ hydrogel matrix (Fig. 6b), and 0.65 MPa and 59.08% for the P(PVIS–AA)/Pd@H₂O matrix (Fig. 6a). From the results, the properties at rupture of the hybrid gel/Pd@Fe³⁺ matrix far exceeded those of either, which demonstrated the trivalent ionic interaction can improve mechanical property more than mono-/divalent interaction. The sample of the hybrid gel/Pd@Fe³⁺ matrix (Fig. 6c) showed a slight internal damage under compressions, but which recovered rapidly and exhibited much better stress through the structure of trivalent iron ions crosslinking. The formulation of the hybrid P(PVIS–AA)/Pd@Fe³⁺ hydrogel for mechanically tough character was due to the zwitterionic structure of containing both cation and anion, the cation can be attracted by metal ions and the anion could be ion crosslinked by Fe³⁺, furthermore, the –COO⁻ of the hydrogel have the ability to complex with Fe³⁺ leading to more stable, pseudocovalent bonds.

Fig. 6 Mechanical tests: stress–stretch curves of the P(PVIS–AA) hydrogel swelling equilibrium in deionized water (a) CaCl₂ (b) FeCl₃ (c) solution (left), high-strength hydrogel crosslinking network through Fe³⁺ (right).

Catalytic study for reduction of 4-nitrophenol

As well as all known, nitrophenols posses toxicity and possible accumulation in the environment, represent a pollutant of industrial wastewaters, the catalytic reaction was carried out to reduce 4-nitrophenol by NaBH₄ is a model reaction, this reaction was rigorously investigated in the field of environmental chemistry,^33–35 in the experiment, 0.448 g of P(PVIS–AA)/Pd@Fe³⁺ hydrogel catalyst has been added into the mixture of 4-nitrophenol (5 mM), 32 mL of deionized water and NaBH₄ solution (0.2 M). The reaction behavior was monitor by UV-vis spectrometer at room temperature. At first, in the solution containing NaBH₄ solution without Pd catalyst, the main intensity of the peak located at 400 nm, which is the signal of 4-nitrophenol, the peak lowered as the P(PVIS–AA)/Pd@Fe³⁺ hydrogel catalyst were added under vigorous stirring gradually, the absorption at 400 nm decreased and the absorption at 298 nm increased, indicating the reduction of 4-nitrophenol and the formation of 4-aminophenol, the reduction of 4-nitrophenol by NaBH₄ to 4-aminophenol could be finished in 6 min at room temperature (Fig. 7), even the content of Pd inside it is as low as 1.6 wt% (ICP data). The results clearly suggested that large specific surface area of noble metal Pd nanoparticles (2–5 nm) inside the hybrid hydrogel. At the same time, strengthened mechanical character make the reaction could be done under vigorous stirring, and the ionically crosslinking concentrated the Pd nanoparticles as well. Following the catalytic reduction of 4-nitrophenol by NaBH₄, Fe(III) was reduced to Fe(II) and Fe(0) species partly (XPS, Fig. S3†), the catalyst of the reaction therefore include Fe nanoparticles in addition to Pd nanoparticles, which also accelerated the reaction.

Fig. 7 Successive UV-vis absorption spectra of the reduction of 4-nitrophenol by NaBH₄ in the presence of P(PVIS–AA)/Pd@Fe³⁺.

The rate constant was calculated using the rate law: −ln A = kt, where A, k, t, are absorbance of 4-nitrophenol, apparent rate constant, and time, respectively. From the slope of linear part in Fig. 8, k was found to be 1.0909 min⁻¹.

Fig. 8 The straight line of time dependence of nitrophenol absorbance at λ = 400 nm built in logarithmic coordinates.

The reusability of the P(PVIS–AA)/Pd@Fe³⁺ hydrogel

The reusability of the P(PVIS–AA)/Pd@Fe³⁺ hydrogel catalysts were tested in 5 consecutives cycles (Fig. 9). Namely, after each measurement, the P(PVIS–AA)/Pd@Fe³⁺ hydrogel catalysts recycled by simple filtration, followed by washing with deionized water for the next cycles of catalysis. The reusable conversion of the hybrid gel/Pd catalysts in the same reaction time (6 min) and conditions were investigated by repeated monitoring for 5 cycles. The catalyst exhibited an excellent reusability as the conversion just slightly decreased, specifically, it can be easily separated from the reaction medium and washed with deionized water before the next run. The results indicated that the presence of P(PVIS–AA)/Pd@Fe³⁺ hydrogel support sufficiently stabilized the catalytic nanoparticles by preventing their aggregation and insured high activity and stability.

Fig. 9 Reusability of the hybrid gel/Pd@Fe³⁺ matrix catalyst.

To compare the catalytic performance of P(PVIS–AA)/Pd@Fe³⁺, the same amount of P(PVIS–AA)/Pd was carried out in the catalytic reduction of 4-nitrophenol by NaBH₄ under the same conditions see (Fig. S4a–c†). The reduction reaction time, constant rate and the fifth conversion was 15 min, 0.168 min⁻¹, 60.5%, respectively; however, the P(PVIS–AA)/Pd@Fe³⁺ was 6 min, 1.0909 min⁻¹, 91.5% respectively. The reusable conversion data of P(PVIS–AA)/Pd decreased greatly and weight reduced seriously. The results showed that the P(PVIS–AA)/Pd@Fe³⁺ system exhibited excellent catalytic activity, which attributed to the less attrition of Pd nanoparticles result from high mechanical strength, moreover, the small amount of Fe ions were reduced to Fe nanoparticles, which promote the catalytic reaction as well.

A complete comparison of the catalytic performances of hydrogel–metal based catalyst for the catalytic reduction reaction of 4-nitrophenol by NaBH₄ was listed through literature survey in Table 1, from the results, our work showed some advantages.

Table 1 Literature survey of catalytic performances of the hydrogel–metal composite

Catalyst	Catalytic time (min)	Reuse (times)	Rate constant (min^−1)	Particle size (nm)	Reference
Hydrogel–Co	28	5	0.12	50–150	36
Hydrogel–Au	44	3	6.6 × 10^−2	30	37
Hydrogel–Cu	30	5	0.085	10–100	38
Hydrogel–Ru	9.3	3	0.96	5	39
Hydrogel–Ni	60	5	0.0614	50–100	40
Hydrogel–Pt	20	1	0.0276	2.3 ± 0.4	41
Hydrogel–Fe	30	1	—	50–250	42
Hydrogel–Pd@Fe^3+	6	5	1.0909	2–5	Our work
Hydrogel–Pd	15	5	0.168	2–5	Our work

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Conclusions

In summary, the hydrogel P(PVIS–AA)/Pd@Fe³⁺ that combines dual-crosslinking of covalent crosslinking and ionically crosslinking were prepared and applied as a support for homogeneously distributed and small size of Pd nanoparticles. The obtained P(PVIS–AA)/Pd@Fe³⁺ hydrogel system could work efficiently as a catalyst for the reduction reaction of 4-nitrophenol in the presence of NaBH₄.

Meanwhile, it was proved that the P(PVIS–AA)/Pd@Fe³⁺ hydrogel with strengthened mechanical character displayed good reusability as well as excellent catalytic properties. There are several important advantages of this hydrogel system, the porous hydrogel with cross-linked network which can provide space for the nucleation and growth of palladium nanoparticles; owing to this porous structure and hydrophilic group, a large amount of water and 4-nitrophenol can be absorbed, the system can act as a reactor to enhance their catalytic activity; dual-crosslink guarantee the mechanical strength of the hydrogel, which makes catalyst separate and reuse easily.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC) (No. 21164010), the Natural Science Foundation of Gansu Province (No. 1107RJZA149), the Foundation for Distinguished Young Scholars of Gansu Province (No. 145RJDA326).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22699f

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