Controllable synthesis of a Pd/PdO nanocomposite as a catalyst for hydrogenation of nitroarenes to anilines in water

Abstract

A heterogeneous catalytic assembly of the different Pd/PdO ratio nanoparticles supported on oxide carbon nanotubes (OCNTs) can be controllably synthesized by a one-pot gas–liquid interfacial plasma (GLIP) method via adjusting the glow produced parameter. The Pd/PdO nanoparticles have uniform particle size distribution. It is found that the catalysts with different Pd/PdO ratios could affect catalytic activity during the reduction of 4-nitrophenol (4-NP) in water by NaBH₄. The best turn over frequency (TOF) value is up to 3000 h⁻¹, which is much higher than the case only using Pd nanoparticles as catalysts. Moreover, the catalysts allowed hydrogenation of nitroarenes in water by atmospheric pressure H₂, and it displayed remarkable activity toward the hydrogenation of nitroarenes using low Pd loading in good yields. The catalyst can be simply and efficiently used for ten consecutive runs without significant decrease in activity.

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Introduction

Functionalized anilines are highly valuable chemical intermediates widely used in the production of pharmaceuticals, polymers, pesticides, explosives, fibers, dyes and cosmetics.¹ Current industrial production of aniline involves the catalytic hydrogenation of nitrobenzene performed at 200–300 °C and 1–3 MPa with copper, palladium or palladium–platinum supported on carbon or inorganic oxides as the catalysts.² Obviously, the high temperature and pressure; toxic, flammable, environmentally hazardous organic solvents; unavoidable formation of harmful azo- and azoxy-derivatives are the main disadvantages. For their importance, extensive effort has been dedicated to exploring the effective greener reduction systems, including hydrogenation promoted by various heterogeneous catalysts, such as Pt,³ Pd,^4–8 Rh₃Ni₁,⁹ Rh,¹⁰ Cu,¹¹ Au,¹² Ru,¹³ Ni¹⁴, Ni–NiO;¹⁵ catalytic reduction with various reducing reagents, for example, H₂,^{3–6,8–10,16} hydrazine,^17–21 silanes,^22,23 sodium borohydride,^24–29 and others;^11,30,31 reduction process in various solvent, including water (H₂O), ethanol (EtOH), tetrahydrofuran (THF), ethyl acetate (EtOAc). Among them, catalytic hydrogenation is of particular interest owing to its environmental friendliness, atomic efficiency, and compatibility with industrial process. On one hand, catalysts (e.g., Pd/C) of higher selectivity require high reaction temperature and high H₂ pressure because of their lower intrinsic activity, leading to high energy input and difficulties in H₂ handling. On the other hand, chemical reactions using water as solvent is expected to receive more attention in organic synthesis from both a practical and an economic standpoint due to its harmfulless, inexpensive, safety and segregative.³² Therefore, the development of more effective and handle-convenient catalysts, less toxic solvent, low temperature and pressure, high selective reaction system for this molecular transformation reaction is still highly desirable.

In the past several years, our group has focused on the development of green catalysts and environmentally friendly procedures to various organic molecular transformations reactions by a series of carbon materials supported Pd and Au nanoparticles.^33–38 These catalysts can be successfully applied in catalyst reactions in water due to the functionalized carbon materials, showing good hydrophobic/hydrophilic property. Moreover, the catalysts are fabricated by a gas–liquid interfacial plasma method (GLIP), which shows many merits for synthesis of metal nanoparticles, such as high process rate, preparation of nanomaterials in large scale, avoiding the use of toxic stabilizers and reducing agents, ambient reaction temperature, no need to stir during the nanoparticle formation process, and produced the glow along with the emission of argon.^39–41 The schematic illustration of the GLIP method for synthesis of metal nanoparticles is shown in Fig. 1. Most recently, we have revealed that Pd(NO₃)₂·2H₂O can be decomposed to PdO as well as reduced to Pd under glow condition.^36,37 Therefore, we successful fabricate the Pd/PdO nanoparticles which show better catalytic activity than Pd nanoparticles due to the boundary effect of Pd and PdO component. In continuation of our previous work,³⁶ it is the first time for us to report a controllable synthesis of different ratios Pd/PdO nanoparticles supported on the oxidized multi-walled carbon nanotube by the GLIP method via adjusting the glow produced parameter, and the catalytic performance of different ratios Pd/PdO nanoparticle are examined by the hydrogenation of nitroaromatics to aromatic amines.

Fig. 1 Schematic illustration for synthesis of metal nanoparticles supported on OCNTs by the GLIP method.

Experimental

General information

All chemicals and solvents were purchased from commercial suppliers. Transmission electron microscopy (TEM, Tecnai G2, F20) combined with an energy dispersive X-ray spectroscopy (EDS) at an acceleration voltage of 200 kV were used to measure the size, morphology, size distribution and element content of Pd catalysts. X-ray diffraction (XRD, Bruker D8 Advance Germany) was applied to characterize the crystal structure of the hybrid materials, and the data were collected on a Shimadzu XD-3A diffractometer using Cu Kα radiation. The X-ray photoelectron spectroscopy (XPS, Thermo Fisher K-Alpha American with an Al Kα X-ray source) was used to measure the elemental composition of samples. The Pd loading in the OCNTs samples were determined by inductively coupled plasma optical emission spectrometer (ICP-OES). UV-vis absorption spectra of the samples were recorded at room temperature on a TU-1900 apparatus. ¹H NMR and ¹³C NMR spectra were recorded on JNM-LA300FT-NMR for checking the final product from the hydrogenation of nitroaromatics reaction.

Fabrication of the OCNTs

The carbon nanotubes used in our experiments were purchased from Beijing Cnano Technology Limited (purity: >95%, average length: 10 μm, average diameter: 11 nm). OCNTs were prepared by slight oxidation of 1 g purified CNTs with cooled piranha solutions (4 : 1, vol/vol 96% H₂SO₄/30% H₂O₂) 100 mL at 25 °C for 100 min. The resulting mixture was diluted with ice water. After that, the obtained suspension was centrifuged at 4000 rpm for 10 min and washed with distilled water for several times until the pH of the mixture reached 7. Finally, the precipitate obtained was dried in a vacuum oven for 12 h before use.

Fabrication of the Pd/PdO nanoparticles decorated OCNTs

The Pd/PdO nanoparticles decorated OCNTs catalyst was prepared by a GLIP method with Pd(NO₃)₂·2H₂O, OCNTs at room temperature for 10 min. The glow plasma was generated between the top flat stainless steel (SUS) and the bottom ionic liquid electrode. Ar gas was introduced and used as the plasma-reaction gas. The power source was supplied from CLASSMAN HIGH VOLTAGE INC. with the model as FJ1P120. The V_DC = 260–270 V was applied to a stainless steel electrode in gas phase for the generation of an Ar plasma, where the discharge current I was fixed to 3–44 mA and the Ar gas was introduced up to a pressure of 140–500 Pa. 14 mg palladium salt dissolved in 1 mL ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF₄), 48 mg OCNTs were added to the stainless steel reactor, then the palladium solution was added to the reactor incubation 15 min. For the formation of Pd nanoparticles, electrons were irradiated toward the ionic liquid for 10 min, and then the mixture was sonicated in ethanol to remove the excess impurities and extracted from the ionic liquid by a centrifuge process. Pd/PdO nanoparticles decorated OCNTs were prepared with the Pd/PdO ratio decreased, and the respective materials were called Pd-n, (n = 1, 2, 3, 4), the detail parameter and the Pd/PdO ratio were summarized in Table 1.

Table 1 The synthetic parameters and characterization for Pd/PdO nanoparticles

Sample	Pd-1	Pd-2	Pd-3	Pd-4
a Average size obtained from the size distribution histogram.b The sum of Pd and PdO loading calculated by XPS.c The sum of Pd and PdO loading calculated by ICP-OES.
Pressure	140	140	190	500
Electricity (mA)	0.3	0.5	1.4	4.4
Voltage (V)	268	265	270	260
Pd-n size^a (nm)	4.2	3.5	3.9	3.8
Pd loading^b (wt%)	12.0	10.0	11.6	11.0
Pd loading^c (wt%)	11.2	10.6	10.8	11.0
Pd/PdO^b	61/39	45/55	33/67	23/77

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The typical procedure for catalytic reduction of 4-NP

In a typical catalysis reaction, 2.1 mg Pd-1 catalyst was used, corresponding to a percentage of Pd of 2 mol% with respect to 4-NP, and 14 mg 4-NP (0.1 mmol) were mixed together in 1 mL milliQ-water, then stirred on 600–800 rpm for 1 min in order to mix thoroughly in a micro-reaction vial. Subsequently, 378 mg NaBH₄ (10 mmol) in 3 mL milliQ-water was added into the above solution by vigorous magnetic stirring. After that, 12 μL of the reaction mixture was collected and diluted to 3 mL with milliQ-water for UV-vis measurement every 30 s. UV-vis absorption spectra were recorded to monitor the change in the reaction mixture over the range from 200 to 500 nm at room temperature. The catalytic reaction by using the other three Pd-n catalysts for the conversion of 4-NP to 4-AP were carried out under the same conditions.

The typical procedure for the hydrogenation of 4-nitrotoluene in water

All hydrogenation reactions were carried out under standard conditions (60 °C, 1 atm of H₂). A micro-reaction vial, charged with the Pd-1 (4 mg), corresponding to a percentage of Pd of 0.75 mol% with respect to 4-nitrotoluene was used, and 68.6 mg 4-nitrotoluene in 2 mL water with a magnetic stir was connected to a H₂ balloon. After consumption of 4-nitrotoluene, which was monitored by Thin Layer Chromatography (TLC), the reaction mixture was extract with ethyl acetate, then the catalyst was recovered by filtering off the solution, later on, the catalyst was washed with ethanol for several times for the use in the next run. The HCl ethyl acetate solution was added to the filtrate in order to induce the precipitation of the amine hydrochloride salt.

Results and discussion

Initially, Pd/PdO nanoparticles with different ratios were prepared by the GLIP method. It is necessary to emphasize that samples with the different Pd/PdO ratios can be synthesized through change the parameter of the power source, as described in Table 1. The morphology of Pd-n has been examined by TEM, as shown in Fig. 2. These results demonstrate that all the synthesized Pd-n catalysts exhibit uniform morphologies. The average particle diameters are described by distribution histograms, and the Pd loading contents of Pd-n are calculated by XPS and ICP-OES, which are summarized in Table 1. It indicates that the high yield for Pd loading on the surface of the OCNTs has been obtained, and the average particle sizes are 4.2 nm, 3.5 nm, 3.9 nm and 3.8 nm, respectively.

Fig. 2 TEM images of Pd-1 (a), Pd-2 (b), Pd-3 (c) and Pd-4 (d).

In addition, the SAED of Pd-n is indicated in Fig. 3a. The lattice spacing measured from the diffraction rings are 0.34, 0.23, 0.21, 0.20, 0.17, 0.14, 0.13 and 0.12 nm, corresponding to reflections C(002), Pd(111), C(100), Pd(200), C(004), Pd(220), C(110) and Pd(311), respectively. Fig. 3b shows the XRD patterns of Pd-n, which shows the diffraction peaks at 25.9°, 34.0°, 40.4°, 43.4°, 46.5°, 68.3° and 81.8°, corresponding to the C(002), PdO(101), Pd(111), C(100), Pd(200), Pd(220) and Pd(311). These evidences have confirmed that the Pd/PdO possess a face-centered crystalline structure.

Fig. 3 (a) A SAED image of Pd-3; (b) XRD spectra of Pd-1 (black curve), Pd-2 (red curve), Pd-3 (blue curve) and Pd-4 (purple curve).

In order to certify the Pd element state and the Pd/PdO ratio of the Pd-n, XPS, a powerful technique was used to confirm the component of the Pd/PdO hybrid materials. Wide XPS scan indicate that the Pd-n catalysts are composed of Pd, C, and O, as shown in Fig. 4a. The peaks of the chemical components C 1s and O 1s are at 284.17 and 533.17 eV, respectively. A small peak corresponding to Pd 3p is observed at 562.08 eV. The Pd catalysts show two peaks for Pd 3d_5/2 and Pd 3d_3/2 which are split into two types of Pd electronic states (Pd⁰ and PdO) centred around 335.97, 337.87, 341.17 and 343.37 eV, as shown in Fig. 4b–e. Moreover, the Pd/PdO ratio can be calculated by peak-differentiation-imitating, and the results are summarized in Table 1. These results suggest that the Pd catalyst contains Pd and PdO active species, and the ratio of the Pd/PdO can be controlled through adjusting the plasma parameters. These results demonstrate that the ratio of the PdO is increased with the higher power output, and the PdO ratio can be controlled between 40% and 80% approximately. The synergistic action of the Pd/PdO ratio may play an important role in highly active catalytic species. However, there are few reports about how the Pd/PdO ratio affects the catalytic activity.^42,43 Therefore, in our case the hydrogenation of nitroarenes reaction was used to evaluate the catalyst efficiency in order to reveal the affection of Pd/PdO ratio for the reduction reaction.

Fig. 4 (a) XPS spectra of Pd-1 (black curve), Pd-2 (green curve), Pd-3 (red curve) and Pd-4 (blue curve); high-resolution XPS Pd 3d image of Pd-1 (b), Pd-2 (c), Pd-3 (d) and Pd-4 (e).

Initially, we employed the reduction of 4-nitrophenol as the model reaction to evaluate the Pd-n catalytic performance in micro-reaction vial at room temperature in aqueous solution with NaBH₄ as reduction agent. The evolution of UV-vis spectra with reaction time for the reduction of 4-NP to 4-AP by using different catalysts was monitored, as shown in Fig. 4. With the time increasing, the absorption band at 400 nm was decreased, and the intensity of the new absorption band at 300 nm was increased, being ascribed to the formation of 4-AP. Two isosbestic points are observed at 277 and 317 nm. The turnover frequency (TOF) is a significant parameter for evaluating the catalytic activity in heterogeneous catalytic reaction. The TOF values of various catalysts are 857, 1000, 3000 and 1500 h⁻¹ for Pd-1, Pd-2, Pd-3 and Pd-4 catalysts, respectively. In comparison with other Pd heterogeneous, the Pd-n catalyst demonstrates obviously superior catalytic activity,^44–49 and the Pd-3 catalyst shows the best catalytic activity. However, the reusability of the Pd-n was not performed well, probably due to the PdO active species was reduced by the strong reductant (NaBH₄). Above speculation was proved by which PdO peak was disappeared in the XPS spectra of the recycle Pd-n (Fig. 5).

Fig. 5 (a) Successive absorption spectra of the conversion from 4-NP to 4-AP with Pd-n catalysts: Pd-1 (a), Pd-2 (b), Pd-3 (c) and Pd-4 (d).

Therefore, we try to use H₂ as reductant for further estimating their high catalytic activity, we first use Pd-n as catalyst for selective hydrogenation by using p-nitrotoluene as the model substrate (Table 2). We employed the reactions with a very low catalyst loading (0.75 mol%) under mild reaction conditions (atmospheric H₂ pressure and 60 °C in water). The Pd-1 and Pd-2 catalysts show good activity for 4-nitrotoluene hydrogenation reaction under current conditions (entries 1 and 2). To our great delight, Pd-3 gave the best results (entry 3). Interestingly, a slightly decrease in activity was observed by using Pd-4 as catalyst (entry 4). We have also checked the case only using Pd/OCNTs, PdO/OCNTs and a mixture of Pd/OCNTs and PdO/OCNTs catalysts, and the results show much lower activity compared with Pd-n catalysts (entry 5, 13 and 14). These results demonstrate that the PdO component in the catalyst can enhance the activity, and the PdO ratio is an important factor affecting the reaction. The reaction temperature also shows some influence on the yield of the product, and the reaction carried out at 60 °C shows the best results (entries 3, 6 and 7). We have further identified several solvents, including THF, EtOAc, EtOH, and the reactions are carried out under the benchmark conditions. To our surprise, the EtOAc and EtOH are compatible with this catalytic system (entries 8 and 9), but they still show little lower yield compared with H₂O. The THF did not work well with this catalyst (entry 10). The blank test without Pd/PdO nanoparticles was also carried out, and we found that the reaction did not proceed at all, which means that the Pd/PdO active species is necessary for this reduction reaction. In sharp contrast to the case of Pd/PdO, the reaction catalyzed by Pd/C proceeds rather slowly and stops at a quite low conversion. This result is rationalized by the lack of a suitable hydrophobic environment provided by the OCNTs.⁵⁰ Therefore, the PdO enhance the Pd nanoparticles activity and OCNTs provide the hydrophilic–hydrophobic environmental, resulting in the high activity of Pd-3 for catalytic hydrogenation of nitroarenes in water.

Table 2 Hydrogenation of 4-nitrotoluene under different conditions^a

Entry	Cat.	Solvent	Temperature (°C)	Yield^b (%)
a Reaction conditions: 4-nitrotolune (0.5 mmol), cat. Pd (0.75 mol%), and in 1 mL solvent.b Isolated yield.c Cat. Pd-5 is PdO/OCNTs (0.75 mol%).d Cat. Pd-6 is Pd/OCNTs (0.25 mol%) and PdO/OCNTs (0.5 mol%).
1	Pd-1	H2O	60	81
2	Pd-2	H2O	60	81
3	Pd-3	H2O	60	94
4	Pd-4	H2O	60	88
5	Pd/OCNTs	H2O	60	67
6	Pd-3	H2O	40	85
7	Pd-3	H2O	80	68
8	Pd-3	EtOAc	60	91
9	Pd-3	EtOH	60	92
10	Pd-3	THF	60	52
11	OCNTs	H2O	60	0
12	Pd/C	H2O	60	37
13	Pd-5^c	H2O	60	68
14	Pd-6^d	H2O	60	60

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With the optimized reaction conditions in hand, we have further extended the Pd-3 catalyst to various nitroarenes to examine the generality of the hydrogenation reactions (Table 3). Satisfyingly, in all cases, the hydrogenation of nitrobenzene proceeded smoothly and completely to give aniline in excellent isolated yields with high selectivity. The electronic features of the nitroarenes did not affect the reaction yield or selectivity. Various 4-substituted nitroarenes, bearing either electron-donating or electron-withdrawing groups, such as –COOMe, –COOH, –COCH₃, –CN, –OMe and –OH, provided the corresponding products in good to excellent yields (entries 1–4 and 7, 8). As for the 4-chloronitrobenzene (1e), the hydrogenation reaction was accompanied by dehalogenation reaction, giving the 4-chloroaniline 69% yield (entry 5). Nitrobenzene (1f) also proceeded smoothly to give aniline in excellent yield (entry 6). Importantly, other position substituted nitroarenes group, including 2-COOH (1i), 3-OH (1j), were compatible with the catalysis system (entries 9 and 10). For compound with two nitro groups, 1,3-dinitrobenzene (1k), both nitro groups were reduced to amines (entry 11). Moreover, heterocycle nitroarene 8-nitroquinoline (1l) was converted to 8-aminoquinoline in quantitative yields (entry 12). These results indicate that the Pd-3 shows high activity in nitroarene hydrogenation.

Table 3 Pd-3 catalyzed hydrogenation of nitroarenes in water^a

Entry	Ar-NO2 (1)	Ar-NH2 (2)	Time (h)	Yield^b (%)
a Reaction conditions: nitroarenes (0.5 mmol), Pd-3 (0.75 mol%), and atmospheric H2 in 1 mL H2O at 60 °C.b Isolated yield.
1	4-MeOOCC6H4NO2 (1a)	4-MeCOOC6H4NH2 (2a)	4	87
2	4-COOHC6H4NO2 (1b)	4-COOHC6H4NH2 (2b)	18	79
3	4-CH3OCC6H4NO2 (1c)	4-CH3OCC6H4NH2 (2c)	12	91
4	4-CNC6H4NO2 (1d)	4-CNC6H4NH2 (2d)	18	89
5	4-ClC6H4NO2 (1e)	4-ClC6H4NH2 (2e)	3	69/87
6	C6H5NO2 (1f)	C6H5NH2 (2f)	4	98
7	4-CH3OC6H4NO2 (1g)	4-CH3OC6H4NO2 (2g)	4	97
8	4-OHC6H4NO2 (1h)	4-OHC6H4NH2 (2h)	24	95
9	2-COOHC6H4NO2 (1i)	2-COOHC6H4NH2 (2i)	18	90
10	3-OHC6H4NO2 (1j)	3-OHC6H4NH2 (2j)	24	99
11	3-NO2C6H4NO2 (1k)	3-NH2C6H4NH2 (2k)	15	98
12	8-Nitroquinoline (1l)	8-Aminoquinoline (2l)	4	99

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Reusability is one of the most important features for a heterogeneous catalyst, which is superior to a homogenous one. First, to confirm that the reaction is indeed catalyzed by solid Pd-3 rather than by homogenous palladium species, we have carried out the leaching experiments. After the catalytic hydrogenation of p-nitrotoluene was carried out for 1 h under standard conditions, the Pd-3 was removed from the vessel by centrifugation with p-toluidine produced in 56% yield. No further reaction took place after removing the catalyst in 3 h, then the Pd-3 was put back into the mixture. As a result, the hydrogenation reaction is restarted, and the product aniline was obtained in 95% in 3 h. In addition, the ICP-OES was not detected the Pd nanoparticles in the reaction mixture. The potential recyclability of the catalyst Pd-3 was explored in the model hydrogenation of 4-nitrotoluene. The reaction was carried out under normal atmospheric pressure, and in water at 60 °C for 3 h. The completion of the reaction was monitored by TLC analysis. After completion of the reaction, the catalyst was filtered from the reaction and washed by EtOH for next run. This process was repeated for 10 cycles, giving all cycles in high yield without prolong the reaction time (Fig. 6a). After the reactions for 10 cycles, the Pd-3 was again examined by TEM and XPS. The particle size is a little bit increased to 4.1 nm, as shown in TEM image (Fig. 6b). The XPS spectra of recycled Pd-3 show no difference compare with the fresh Pd-3. These results illustrate that the Pd/PdO nanoparticle are stabilized in the current reduction systems.

Fig. 6 (a) The yields for 10 cycles of the Pd-3 catalyzed 4-nitrotoluene hydrogenation reaction; (b) a TEM image for recovery of Pd-3 catalyst after 10 cycles hydrogenation of 4-nitrotoluene.

Based on the above results, the reason for the high activity of Pd-3 catalyzed hydrogenation of nitroarenes is schematically shown in Scheme 1. Initially, the Pd-3 was dispersed in H₂O, and the H₂ and nitro group was absorbed on the surface of the Pd-3. Next, an H–H bond cleavage occurs in a rate-determining step to give the [Pd]–H species.⁵¹ Such species are responsible for the rapid reduction of nitroarenes into the corresponding anilines. It is proposed that the high catalytic activity of Pd-3 for hydrogenation of nitroarenes is due to the synergistic effect between Pd and PdO crystallographic plane.

Scheme 1 A possible reaction pathway for the high activity of Pd-3.

Conclusions

In conclusion, we have developed an environmentally-friendly approach to controllable synthesis of different ratios Pd/PdO nanoparticles functionalized OCNTs for the first time by the GLIP method through adjusting plasma parameter with Pd(NO₃)₂·2H₂O as a precursor. The Pd/PdO nanoparticles as a new kind catalyst is applied in reduction of 4-nitrophenol (4-NP) in water by NaBH₄. The turn over frequency (TOF) value is up to 3000 h⁻¹, showing much higher catalytic activity than Pd nanoparticles. The catalysts with different Pd/PdO ratios significantly affect the catalytic activity. Moreover, the catalyst demonstrates high catalytic activities for hydrogenation of nitroarenes in water with atmosphere pressure H₂. The as-prepared catalysts display remarkable activity toward various nitroarenes by using low Pd loading in good yields. In addition, the catalyst can be simply and efficiently used for ten consecutive runs without significant decrease in activity. Further work is in progress to extend such kind of catalyst for other applications.

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 21202203, 21576289 and 21322609), Science Foundation Research Funds Provided to New Recruitments of China University of Petroleum, Beijing (No. YJRC-2013-31), Science Foundation of China University of Petroleum, Beijing (No. 2462015YQ0306, 2462014QZDX01), Thousand Talents Program and National High-tech R&D Program of China (863 Program, No. 2015AA034603).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06900b

‡ These authors contributed equally.

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