Poly(ionic liquid) Janus nanosheets towards dye degradation

Abstract

Polymeric ionic liquid (PIL) functionalized Janus nanosheets are synthesized by polymerization of ionic liquid monomers onto the ATRP agent modified side of the silica Janus nanosheets. PW₁₂O₄₀³⁻ anion is introduced onto the PIL side through anion exchange. The PW₁₂O₄₀³⁻ based PIL Janus nanosheets at the emulsion interface exhibit excellent catalytic degradation of water soluble dyes for example methyl orange (MO).

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Water pollution by organic dyes is becoming a concern.^1,2 Among the dyes, water soluble azo based dyes are more difficult to treat by conventional methods. Some methods have been proposed including chemical oxidation,³ adsorption⁴ and biological catalytic degradation⁵ to remove the azo dyes. Chemical oxidation is more welcomed due to its high degradation efficiency. Selection of oxidants is key. Recently, highly redox active Keggin structured polyoxometalates (POMs) for example H₃PW₁₂O₄₀ have been reported.⁶ POMs are required to support onto some matrices including silica particles and graphene.^7,8 The dyes are usually degraded into hydrophobic intermediate products.⁹ How to transfer the intermediate products becomes a key restricted step.

We have recently reported the synthesis of silica based Janus nanosheets and their functionalized derivatives including ionic liquid ones.^10–13 We conjecture that POMs based silica Janus nanosheets will facilitate transferring the intermediate products during the oxidative degradation.

Herein, we report the synthesis of PW₁₂O₄₀³⁻ based PIL Janus nanosheets and emulsion interfacial catalytic degradation of the example water soluble dye MO (Scheme 1). PIL functionalized Janus nanosheets are synthesized through a polymerization of an ionic liquid monomer (ViEtIm⁺Br⁻) onto the ATRP agent terminated side of the Janus silica nanosheets. After a simple anion exchanging Br⁻ with PW₁₂O₄₀³⁻, the PW₁₂O₄₀³⁻ based PIL Janus nanosheets are derived. The Janus composite nanosheets are amphiphilic and can serve as a solid emulsifier to stabilize a toluene-in-water emulsion. The example dye MO is preferentially absorbed within the PIL layer and degraded. The intermediate products are hydrophobic, which can transfer into the internal toluene oil phase. The transfer process can greatly facilitate further absorption and degradation. Higher degradation efficiency is expected due to the cooperative interplay between the synchronous absorption, degradation and phase transfer.

Scheme 1 PW₁₂O₄₀³⁻ based PIL Janus nanosheets serve as a solid surfactant. A toluene-in-water emulsion forms. The PW₁₂O₄₀³⁻ side can synchronously absorb methyl orange (MO) from water phase and catalytic degrade MO. The catalytic intermediate products are hydrophobic, which can be transferred to oil phase. The cooperative procedures can further facilitate the catalytic degradation.

The phenyl/amine Janus silica nanosheets are obtained by self-organized sol–gel process at a Janus emulsion interface (Fig. S1a†).¹⁰ They are thick ∼58 nm (Fig. S1b†). An ATRP agent of 2-bromoisobutyryl bromide is selectively grafted onto the amine-group terminated side via amidation between (–COBr) and amine (–NH₂) groups. Both sides of the nanosheets remain smooth after the ATRP agent grafting (Fig. 1a). The nanosheets maintain thick ∼58 nm. An ionic liquid monomer of 1-vinyl-3-ethylimidazolium bromide (ViEtIm⁺Br⁻) is polymerized from the ATRP agent grafted side of the nanosheets. Poly(ViEtIm⁺Br⁻)/silica composite Janus nanosheets are achieved (Fig. 1b). The PIL grafted side becomes slightly coarsening. The PIL Janus nanosheets become thicker ∼83 nm (Fig. 1c). Thermogravimetric analysis (TGA) indicates that the PIL content is 64 wt% (Fig. S2†). In order to prove the distinct compartmentalization of PIL on one side, negatively charged citrate-capped Fe₃O₄ nanoparticles are used to selectively label the positively charged PIL (Fig. 1d). The PIL side becomes more coarsening, while the other side maintains smooth. This indicates that the other phenyl-group terminated side is not affected. The Br⁻ based PIL Janus nanosheets are well dispersible both in oil and water (Fig. S3†). This reveals that the nanosheets are amphiphilic.

Fig. 1 (a) SEM image of the ATRP agent terminated Janus silica nanosheets; (b and c) SEM and AFM images of the Br⁻ based PIL Janus nanosheets; (d) SEM image of the PIL Janus nanosheets after the PIL side is selectively labeled with the trisodium citrate capped Fe₃O₄ nanoparticles.

It is well known that anion type of PILs is broadly tunable by simple anion exchanging. PW₁₂O₄₀³⁻ is introduced within the PIL layer through the anion exchange. The morphology is preserved (Fig. 2c). 2.61% of Br is measured from the Br⁻ based PIL Janus nanosheets (Fig. 2a). After the anion exchange, the signal of Br almost disappear from the PW₁₂O₄₀³⁻ based PIL Janus nanosheets (Fig. 2b). 0.58% of P and 2.05% of W are measured. This indicates that Br⁻ has been completely exchanged with PW₁₂O₄₀³⁻. The entire exchange has also been confirmed by XPS results (Fig. S4†) and FT-IR spectra (Fig. S5†). Presence of the imidazole cation is confirmed by the bands at 1700–1750 cm⁻¹. The bands at 800–1100 cm⁻¹ are assigned to PW₁₂O₄₀³⁻. The PW₁₂O₄₀³⁻ based PIL Janus nanosheets are well dispersible both in oil and water, indicating that the Janus performance is preserved (Fig. S6†). A toluene-in-water emulsion forms in the presence of the PW₁₂O₄₀³⁻ based PIL Janus nanosheets (Fig. 2d). In order to observe orientation of the PIL Janus nanosheets at the oil/water interface, melt paraffin (T_m: 52–54 °C) is employed to form a paraffin-in-water emulsion (Fig. 2e). After the emulsion is cooled down to room temperature, the PIL Janus nanosheets are fixed at the emulsion interface. The coarsening PIL sides of all the nanosheets direct towards the aqueous continuous phase (Fig. 2f).

Fig. 2 (a) SEM image of the PW₁₂O₄₀³⁻ based PIL Janus nanosheets; (b) EDX spectra of the Br⁻ based PIL Janus nanosheets (1) and the PW₁₂O₄₀³⁻ based PIL Janus nanosheets (2); (c) immiscible toluene and water (left), and a toluene-in-water emulsion stabilized with the PW₁₂O₄₀³⁻ based PIL Janus nanosheets, methyl orange is added to water as a chromogenic agent; (d) SEM image of the paraffin droplets stabilized with the PW₁₂O₄₀³⁻ based PIL Janus nanosheets; (e) magnified SEM image of the paraffin droplet surface.

PW₁₂O₄₀³⁻ anion is highly effective heterogeneous catalyst to decompose organic dyes.^14,15 In our concern, water soluble MO is selected as a model organic dye. The intermediate products during degradation are oil soluble.⁹ When the PW₁₂O₄₀³⁻ based PIL Janus nanosheets are used as a solid emulsifier, toluene is emulsified in the water phase containing MO. Both the top emulsion and the bottom water phases are highly orange colorful (left Fig. 3a). A given amount of H₂O₂ for example 10 μL is added to the emulsion to start the degradation at room temperature. After 3 h, the bottom water phase becomes colorless (right Fig. 3a), and the top emulsion phase becomes yellow. The separated Janus nanosheets are yellow (Fig. S7†). This implies that some amount of MO has been absorbed by the Janus nanosheets. After washing with DMF, the yellow compound is confirmed to be MO by UV-vis spectroscopy. The absorption capacity is 12.7 wt% (Table 1). In the UV-vis spectra, the peak at 463 nm is characteristic of MO (curve a, Fig. 3b). After 3 h, MO removal degree is high 99.2 wt% (curve b, Fig. 3b). In comparison, the removal becomes lower (Fig. 3c) when the PW₁₂O₄₀³⁻ based PIL Janus nanosheets are present in water phase (curve c, Fig. 3b). 88.4 wt% removal is achieved. MO absorption capacity is 8.1 wt% (Table 1). Since MO can be easily extracted by ionic liquids, it is understandable that MO is preferentially adsorbed within the PW₁₂O₄₀³⁻ based PIL side of the Janus nanosheets. H₂O₂ is crucial for the oxidative degradation. H₂O₂ can induce PW₁₂O₄₀³⁻ to form more active peroxides such as [(PO₄){WO(O₂)₂}₂{WO(O₂)₂(H₂O)}]³⁻ and [(PO₄){WO(O₂)₂}₄]³⁻.¹⁴ In comparison, in the absence of H₂O₂ (Fig. S8†), 69.2 wt% removal is achieved (curve d, Fig. 3b). MO absorption capacity is 42.8 wt% (Table 1). At low feeding levels below 5 μL, both the degradation degree and rate increase with the amount of H₂O₂. At high feeding levels, the degradation efficiency is less influenced by further increasing H₂O₂ feeding for example from 10 μL to 15 μL. This is understandable that 10 μL of H₂O₂ is sufficient to activate all the PW₁₂O₄₀³⁻ groups. Therefore, the feeding amount of H₂O₂ is fixed at 10 μL in the current work. After a prolonged reaction time for example one week, the top yellow emulsion phase becomes colorless (Fig. S9†). The absorbed MO diffuses slowly and can be degraded eventually. In the emulsion interfacial catalysis, the hydrophobic intermediate products after degradation can be transferred in the oil phase, which can greatly enhance the degradation efficiency. The intermediate products as shown in the scheme have been separated from toluene phase, which are confirmed by new peaks in the UV-vis spectrum (Fig. 3d). Ionic liquid functionalized Janus nanosheets reported in our previous research¹³ also could degrade the MO in similar emulsion system. The degradation efficiency is only 52.0%, which is far lower than the PIL Janus nanosheets due to the amount of PW₁₂O₄₀³⁻ ions decreases substantially.

Fig. 3 (a) A toluene-in-water emulsion stabilized with the PW₁₂O₄₀³⁻ based PIL Janus nanosheets (left); the emulsion after degradation in the presence of 10 μL H₂O₂ after 3 h (right); (b) UV-vis spectra of 50 ppm MO aqueous solution (curve a), MO concentration in the aqueous phase after degradation by PW₁₂O₄₀³⁻ based PIL Janus nanosheets in the oil-in-water emulsion (curve b), MO concentration after degradation by PW₁₂O₄₀³⁻ based PIL Janus nanosheets in water phase (curve c), MO concentration after degradation by PW₁₂O₄₀³⁻ based PIL Janus nanosheets in water phase but in the absence of H₂O₂ (curve d) (c) MO aqueous solution (left) and after degradation (right) by PW₁₂O₄₀³⁻ based PIL Janus nanosheets in water phase in the presence of 10 μL H₂O₂; (d) UV-vis spectrum of the hydrophobic intermediate products extracted in toluene phase after the degradation.

Table 1 Degradation and absorption of MO by the two methods of emulsion system and water phase. T = 25 °C, V_MO = 2 mL, m_catalyst = 5 mg, 10 μL of H₂O₂

Method	MO absorption (wt%)	Total MO removal (wt%)
Emulsion system	12.7	99.2
Water phase	8.1	88.4
Water phase (without H2O2)	42.8	69.2

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Besides degradation degree, degradation kinetics is another concern. Degradation is very fast at early stage in the emulsion method. After 3 h, degradation comes to nearly completion of 99.2%. In contrast, in water phase the degradation occurs slower and proceeds gradually even after 6 h. The PIL based Janus nanosheets can be easily regenerated by centrifugation and washing with DMF. After washing to remove the absorbed MO dye, 100% activity is preserved after 6 recycling (Fig. 4b).

Fig. 4 (a) Degradation kinetics of MO by emulsion system (a) and water phase (b); (b) catalyst recycling of the PIL Janus nanosheets in the emulsion method.

In summary, PW₁₂O₄₀³⁻ based PIL Janus nanosheets are synthesized by ATRP of IL monomer onto the hydrophilic side of the silica Janus nanosheets following by an anion exchange. At an oil/water emulsion interface, the Janus nanosheets demonstrate higher degradation of the example water soluble dye MO. They can be completely regenerated. The PIL Janus nanosheets should be promising in heterogeneous decomposition of water soluble dyes.

Acknowledgements

This work was supported NSFC (51233007, 51173191, and 51273087).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and supplemental figures. See DOI: 10.1039/c5ra02330k

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