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Article Cite This: ACS Appl. Nano Mater. 2018, 1, 4642−4651

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Thermoreversible, Hydrophobic Ionic Liquids of Keggin-type Polyanions and Their Application for the Removal of Metal Ions from Water K. Shakeela and G. Ranga Rao* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India

ACS Appl. Nano Mater. 2018.1:4642-4651. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 10/12/18. For personal use only.

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ABSTRACT: A series of Keggin-based ionic liquids are synthesized by reacting in situ generated first-row transition-metal ion (Mn2+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+) substituted monolacunary Keggin with tetraoctylammonium (TOA) cations. These ionic liquids contain highly charged bulky anions and are found to be hydrophobic, thermoreversible, and self-healing with melting temperatures less than 100 °C. The ionic liquids are characterized by Fourier transform infrared, Raman, X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, NMR, electron paramagnetic resonance, X-ray photoelectron spectroscopy, and UV−vis spectroscopic techniques. These ionic liquids show flakelike microscopic morphology and possess lamellar structure packed with Keggin and TOA cation layers alternately at room temperature. They contain highly charged anionic oxide clusters of size ∼1 nm and are excellent hydrophobic solvents for the removal of Cd2+ and Pb2+ metal ions. KEYWORDS: polyoxometalates, monolacunary Keggin, ionic liquids, hydrophobicity, thermoreversible

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°C.20 Metal-substituted lacunary POMs carry relatively higher negative charge, which facilitates the formation of ionic liquids by interacting with large number of bulky cations.21 These POM-based ionic liquids show good electrochemical stability and high conductivity, and they can be used as electroactive materials.17,22,23 Interaction of POMs with sulfo-group grafted ammonium, phosphonium, and imidazolium cations lead to the formation of thermoreversible and conductive gels.24−28 The crystalline POM clusters form soft materials when they interact with poly(urethane amide) dendrimers.29 Such gelled or semisolid materials show intrinsic properties such as ionic conductivity, drug activity, and solvent ability. Therefore, negatively charged metal-substituted lacunary POMs can pave the way for the development of POM-based ionic liquids by softening the crystalline POM clusters. The POM-based ionic liquids often show specific properties related to POMs as well as ionic liquids. The POM-based ionic

INTRODUCTION Hybrid materials are a special class of materials synthesized by blending an organic moiety with an inorganic moiety on the molecular scale. The chemical interactions involved in the formation of these materials include covalent, coordinative, ionic, hydrogen bonding, and van der Waals-type interactions with energies increasing in that order.1 Polyoxometalates (POMs) are the nanosize transition-metal oxide clusters that exhibit enormous structural diversity and properties.2 They readily form POM-based hybrid compounds with positively charged organic moieties.3,4 For instance, Keggin-based heteropolyanions form stable crystalline hybrid salts by interacting with [Bmim]+ cation of imidazolium ionic liquid.5−8 Polyoxometalate hybrids are active compounds in catalysis,9−11 magnetism,12 photochemistry,13−16 and electrochemistry.17−19 There have been attempts to synthesize POM-based ionic liquids that fall in the category of POM-based inorganic− organic hybrid materials. The high electron density present on the POM cluster allows it to interact with bulky cations to form POM-based ionic liquids with melting points below 100 © 2018 American Chemical Society

Received: May 31, 2018 Accepted: August 3, 2018 Published: August 3, 2018 4642

DOI: 10.1021/acsanm.8b00920 ACS Appl. Nano Mater. 2018, 1, 4642−4651

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ACS Applied Nano Materials

all the ionic liquids were recorded at X-band frequencies (∼9 GHz) using JES FA-200 spectrometer. A custom-built laboratory version ambient pressure photoelectron spectrometer (Lab-APPES) was used for X-ray photoelectron spectroscopy (XPS) measurements. The concentrations of Cd2+ and Pb2+ metal ions left in the aqueous phase after treating with ionic liquids were determined by Shimadzu model AA-7000 atomic absorption spectrophotometer. Milli-Q Millipore water was used to prepare the aqueous solutions. Synthesis of Ionic Liquids Using in Situ Generated Metal Substituted Monolacunary Keggin Anions and TOA Cations. The metal-substituted tungstophosphates were synthesized following the general protocol given by Tourné et al.45 Typically, 9.1 mmol of disodium hydrogen phosphate (Na2HPO4), 100 mmol of sodium tungstate (Na2WO4·2H2O), and 12 mmol of metal nitrate (Mn2+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+) were dissolved in 200 mL of distilled water, and pH was adjusted to 4.8 using 1.0 M HNO3. This procedure leads to in situ generation of transition-metal-substituted lacunary Keggins.46,47 This Keggin ion solution was further mixed with 45 mmol of TOA-Br dissolved in 20 mL of toluene under continuous stirring for 10 min. The reaction mixture was then left undisturbed, until the two phases were separated. The originally colorless toluene phase now attained the color due to the transfer of transition-metal-substituted Keggin ions into the organic phase by interaction with tetraoctylammonium cations. The colored organic phase was collected, and toluene was removed using rotavapor. The remaining highly viscous liquid is the desired POM-based ionic liquid (Scheme 1), which was dried in the vacuum oven overnight at 80 °C.

liquids of phosphotungstate and sulfo-group containing ammonium, imidazolium, and pyridinium cations are used as reaction-induced self-separation catalysts.30−34 Some of these ionic liquids also show self-repair and anticorrosion properties.21 The POM-based ionic liquids are employed as catalysts in deep desulfurization of fuels and biodiesel production.35,36 In a recent report, the effect of cationic alkyl chain length on dye removal has been studied using vanadium-substituted POM-based ionic liquids.37 The tetra-alkyl ammonium silicotungstate ionic liquid immobilized on silica serves as an efficient catalyst for the removal of multiple contaminants from water.38 Solvent extraction has been used as a standard method to remove hazardous metal impurities from aquatic systems.39 This process can be made more efficient by using ionic liquids that are nonvolatile. Ionic liquids can be used in biphasic liquid/liquid metal extractions either by dissolving suitable chelating agents40,41 or by designing the task-specific ionic liquids without chelating agents.42,43 The use of task-specific ionic liquids arrests the loss of ligands to aqueous phase. In such cases, POM-based ionic liquids can be the best choice, as they are highly hydrophobic and possess negatively charged POM clusters to attract metal cations. The hydrophobic nature of the POM-based ionic liquid allows biphasic removal of metal ions from aqueous solutions and easy separation. Here we report a facile method of synthesizing hydrophobic tetraoctylammonium-based transition-metal-substituted Keggin anion containing ionic liquids, named here as POM-based ionic liquids, from simple inorganic salts. Note that the unsubstituted phosphotungstate (H3PW12O40) reacting with tetraoctylammonium (TOA) cations yields solid hybrid material.44 Further, we employed the POM-based ionic liquids for the removal of hazardous metal ions Pb2+and Cd2+ present in the aqueous phase.

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Scheme 1. General Scheme of Tetrahedral TOA-Bra

EXPERIMENTAL SECTION

Materials and Characterization. We used tetraoctyl ammonium bromide (TOA-Br) and sodium tungstate (Avra), disodium hydrogen phosphate and analytical grade toluene (Thermo-Fisher Scientific), and metal salts (Alfa Aesar) as received from the suppliers. Fourier transform infrared (FTIR) spectra of the TOA-Br and ionic liquids were recorded on JASCO FT-IR-4100 spectrometer. Raman spectra of the samples were recorded on Horiba LABRAM HR 800 UV spectrometer using He−Ne laser of wavelength 633 nm. The X-ray diffraction (XRD) pattern of all the ionic liquids were recorded with Rigaku smartlab X-ray diffractometer, at a scan rate of 0.05° per second, employing Cu Kα (λ = 1.5414 Å) radiation generated at 40 kV and 100 mA.The contact angle of water on POM-based ionic liquids coated on glass slide was measured by using the sessile drop method with GBX-DIGIDROP Modular Contact Angle Technology, where a water drop of 10 μL was placed on ionic liquid using a syringe. The optical microscopic images were recorded on Inverted Microscope, Leica DMI3000 B Resource. The microscopic morphologies of the ionic liquids were obtained using an FEI Quanta 200F scanning electron microscope (SEM) at low vacuum. The ionic liquids were smeared on a conducting carbon tape before being mounted on the microscope sample holder for analysis. Thermogravimetric analyses (TGA) of the samples were performed on TA make SDT Q 500 instrument under nitrogen flow at a linear heating rate of 10 °C per minute, from room temperature to 900 °C.The phase transitions of ionic liquids were studied on a PerkinElmer TA make differential scanning calorimeter (DSC) Q200. NMR spectra were recorded on a Bruker 500-MHz NMR spectrometer by dissolving ionic liquids in deuterated dimethyl sulfoxide (DMSO-d6) solvent. The UV−visible diffuse reflectance (DR) spectra of the ionic liquid materials were recorded on JASCO V-660 spectrometer using BaSO4 as a reference. The electron paramagnetic resonance (EPR) spectra of

a

(a) Reacting with in situ generated metal (Mn2+, Co2+, Ni2+, Cu2+, and Zn2+) substituted Keggin anion and (b) giving rise to POM-based ionic liquid TOA-PWMn, TOA-PWCo, TOA-PWNi, TOA-PWCu, and TOA-PWZn. (c) The TOA-PWFe ionic liquid possesses only four TOA units. These POM-based ionic liquids are denoted as TOA-PWMn, TOAPWFe, TOA-PWCo, TOA-PWNi, TOA-PWCu, and TOA-PWZn, respectively, representing the transition metal substituted in the monolacunary Keggin ion. The tetrahedral structure of TOA cation undergoes spatial reorientation on interaction with monolacunary Keggin anion. This forms the head−tail rearrangement with the cationic heads pointing toward the Keggin anion due to the electrostatic interactions, as shown in Scheme 1c. Removal of Cd2+ and Pb2+ Ions from Aqueous Solutions. POM-based ionic liquids were used for the removal of heavy metal ions Cd2+and Pb2+ from aqueous solutions. The experimental conditions for the removal of metal ions from water using POMbased ionic liquids are 80 °C for 1 h. These are optimized conditions arrived at by performing a number of experiments at different time periods ranging from 15 min to 1 h. The POM-based ionic liquids are found to be not efficient in removing the metal ions at temperatures below 80 °C because of their high viscosity. Metal acetate salts were dissolved in acetate buffer solution of pH 4.8 to prepare different concentrations of Cd2+ and Pb2+ ions. An aliquot of 1.0 mL of each metal ion solution is added to 100 mg of the POM-based ionic liquid taken in a vial and stirred for 1 h at 80 °C. The viscosity of ionic liquid decreases at higher temperature allowing the mixing of ionic liquid 4643


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Figure 1. (a) Important characteristics of the six POM-based ionic liquids. (b) Thermoreversibility of (i) TOA-PWMn, (ii) TOA-PWFe, (iii) TOA-PWCo, (iv) TOA-PWNi, (v) TOA-PWCu, and (vi) TOA-PWZn when heated to 80 °C and then cooled to room temperature. (c) POMbased ionic liquids (i) TOA-PWCo and (ii) TOA-PWCu showing self-healing property at room temperature. and solution to extract metal ion from the aqueous phase to ionic liquid phase. The biphasic solution was allowed to cool, and the aqueous phase was collected by simply decanting from the vial. The concentration of metal ions left in the aqueous phase was analyzed by atomic absorption spectroscopy (AAS). The concentration of metals ions was determined in triplicate to ensure the repeatability of the tests, and the mean values are reported here. The repeatability of the assay, as measured by the relative standard deviation, was less than 3%.

Waals interactions among the alkyl chains of ionic liquid moieties. Physical Characterization. The FTIR spectra of six POM-based ionic liquids in Figure 2 show peaks characteristic

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RESULTS AND DISCUSSION The substitution of metal ion in Keggin structure raises the overall charge on the anion from [PW 12 O 40 ] 3− to [PW11MO39]5−, which interacts with a greater number of tetraoctyl ammonium cations, resulting in ionic liquid formation. The six POM-based ionic liquids reported here are greasy and gel-like soft viscous materials containing high negative charge oxide nanoclusters [PW11MO39]5− (Figure 1a). They are insoluble in water but soluble mostly in polar protic/aprotic (methanol, ethanol, dichloromethane (DCM), tetrahydrofuran (THF), etc.) and nonpolar protic/aprotic (benzene, chloroform, hexane, toluene, etc.) solvents. The hydrophobicity of POM-based ionic liquids is investigated by sessile water drop method (Figure S1), and the contact angles of water with ionic liquid film are found to be 82° for TOAPWMn, 86° for TOA-PWFe, 85° for TOA-PWCo, 87° for TOA-PWNi, 86° for TOA-PWCu, and 88° for TOA-PWZn. These gel-like ionic liquids do not flow at room temperature but show liquidlike behavior when heated to ∼80 °C and reverse to gels when cooled to room temperature. This phenomenon of thermoreversibility is shown in Figure 1b. These ionic liquids further show self-healing mechanism upon mechanical damage at room temperature (Figure 1c), which is an important property for tribology applications. The selfhealing property of these ionic liquids is the result of van der

Figure 2. FTIR spectra of POM-based ionic liquids (a) TOA-PWMn, (b) TOA-PWFe, (c) TOA-PWCo, (d) TOA-PWNi, (e) TOA-PWCu, (f) TOA-PWZn, (g) PWA, and (h) TOA-Br.

of first-row transition-metal-substituted Keggin compounds in the region of 700−1200 cm−1. The IR spectrum of pristine Keggin ion in phosphotungstic acid shows four characteristic stretching frequencies in Figure 2g. The peaks at 807, 892, 980, and 1080 cm−1 are due to ν(W−Oe−W), ν(W−Oc−W), ν(WOter), and ν(P−O) stretches.6 In addition to the Keggin signature peaks, the ionic liquids also show a weak peak around 726 cm−1 that is attributed to the substituted transition metal− 4644


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Figure 3. (I) Powder XRD pattern of POM-based ionic liquids (a) TOA-PWMn, (b) TOA-PWFe, (c) TOA-PWCo, (d) TOA-PWNi, (e) TOAPWCu, (f) TOA-PWZn; (II) schematic representation of layered arrangement of transition-metal-substituted lacunary Keggin anions and TOA cations.

Figure 4. (a) A vial containing TOA-PWNi ionic liquid with needlelike mesophase texture visible to naked eye, (b−g) optical microscope images of needlelike texture of TOA-PWMn, TOA-PWFe, TOA-PWCo, TOA-PWNi, TOA-PWCu, and TOA-PWZn, respectively.

oxygen stretching frequency. The intense peak at 1080 cm−1, assigned to the ν(P−O) vibration of central PO4 tetrahedron in the unsubstituted Keggin ion, splits into two peaks in the metal-substituted Keggin due to the decreased symmetry of PO4 tetrahedron.47 However, the extent of splitting of this peak depends on the interaction strength of substituted metal with oxygen bonded to phosphorus. Therefore, depending on the extent of splitting in ν(P−O) peak, the interaction strength of metal with oxygen follows the order Ni2+ > Mn2+ > Fe3+ > Co2+ > Zn2+ > Cu2+. The exceptionally low interaction strength of metal with oxygen in case of copper-substituted ionic liquid is caused by the Jahn−Teller distortion in Cu2+ surrounded by octahedral bridged oxygens.47 Further, the cationic fingerprint region at 700−1200 cm−1 is buried in the Keggin anion fingerprint region. In TOA-Br, the four octyl chains are tetrahedrally bonded to the quaternary ammonium ion (Scheme 1a). The 1300−1500 cm−1 range is the bending vibrational region of −CH2 groups. The most prominent peak at 1462 cm−1 corresponds to the C−H bending vibration, and a small split peak at 1380 cm−1 is from the symmetric −CH2 bending vibration in TOA-Br (Figure 2h). Other peaks in the region of 2800−3000 cm−1 are essentially due to the stretching vibrations of alkyl groups, which remain unaffected. Both electrostatic interactions between the ions and the van der Waals interactions among the octyl chains are sufficient to overcome the induced angular strain and play important role in stabilizing the POM-based ionic liquids.

The POM-based ionic liquids are further analyzed by Raman measurements shown in Figure S2. The set of peaks between 50 and 280 cm−1 in Figure S2a correspond to the bridged W− O−W stretching vibrations of the Keggin ion. These features are also present in all the six ionic liquid samples confirming the presence of Keggin ions in the ionic liquid samples (Figure S2).48,49 The intense peak at 1009 cm−1 of ν(WOter) in Figure S2a is shifted to 974 cm−1 in the ionic liquids due to the interaction between terminal oxygens and TOA cations (Figure S2b−g). The vibrational features in the regions 1030−1500 and 2700−3050 cm−1 exactly match with the vibrational features of pure TOA-Br in Figure S2h. This essentially indicates that the alkyl stretches of TOA moieties are unaffected in the ionic liquids. We analyzed the molecular packing of POM anions and TOA cations in the POM-based ionic liquids from the powder XRD patterns shown in Figure 3I. The diffraction patterns of all the ionic liquids show intense low-angle reflections around 2θ value of 5° and the d-spacing varying between 17.7 and 18.4 Å. This indicates the lamellar structure for POM-based ionic liquids with alternate packing of Keggin anions and tetraoctylammonium cations, as shown in Figure 3II.50 The perpendicular height of octyl chain is 5.77 Å, and the diameter of Keggin anion is ∼10.4 Å. Therefore, the total thickness of each layer would be ∼21.94 Å, which is larger than the d-values measured by XRD. The strong hydrophobic and van der Waals interactions among the octyl chains can lead to the 4645


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Figure 5. SEM images of POM-based ionic liquids, (a) TOA-PWMn, (b) TOA-PWFe, (c) TOA-PWCo, (d) TOA-PWNi, (e) TOA-PWCu, and (f) TOA-PWZn

Figure 6. (I) TGA and (II) DSC analysis of POM-based ionic liquids, (a) TOA-PWMn, (b) TOA-PWFe, (c) TOA-PWCo, (d) TOA-PWNi, (e) TOA-PWCu, (f) TOA-PWZn, and (g) TOA-Br.

interdigitation of alkyl chains of two successive layers and decrease the d-spacing in the XRD of POM-based ionic liquids.44 The second intense peak in XRD is found at 14.7°, whose d-value matches with the perpendicular height of octyl chain. These ionic liquids have a very compact structure with intertwined octyl chains held by van der Waals forces and not expected to be porous in nature. These POM-based ionic liquids are highly viscous at room temperature, which makes it difficult to spread these ionic liquids as thin films. The viscosity of these compounds is decreased by gently warming to 70 °C and allowing them to spread as thin films on a glass slide. The viscosity change of these ionic liquids with temperature needs to be studied further. When cooled, tiny needlelike crystals are formed, which can be seen in the optical microscope images in Figure 4. The needlelike mesophase formation is also seen when these ionic liquids are heated and cooled to room temperature, as demonstrated in the case of TOA-PWNi ionic liquid in Figure 4a. However, these needlelike structures are elusive for singlecrystal diffraction analysis. Furthermore, the microscopic morphology of ionic liquids is analyzed by SEM at low vacuum. The needlelike features noticed in the ionic liquids in optical microscope images were

not strong enough to withstand the mechanical strain that was applied during smearing the sample for SEM analysis. However, these ionic liquids at room temperature show regular flakelike structures, as shown in Figure 5. This morphology is stable only at room temperature and disappears at ∼80 °C forming clear viscous liquids. Thus, the layered structure at room temperature breaks down to form ion pairs at high temperatures with reduced viscosity.51 The other possibility is the existence of nanophase segregation of the lamellar phase as nanodomains in the ionic liquids at room temperature. However, the exact nature of the binary phases is elusive in this study. The thermal analyses of POM-based ionic liquids and TOABr are shown in Figure 6I. There is ∼5−8% loss of water below 150 °C from the ionic liquid samples. Further the TOA-Br shows single-step complete decomposition in the range of 160−220 °C (g), while all the POM-based ionic liquids show two-step decomposition pattern, Figure 6I(a−f). The decomposition of TOA in ionic liquids starts above 200 °C, and this indicates increased thermal stability of TOA due to strong interaction with Keggin anions. In case of ionic liquids, the TOA cations are lost in two steps at 200−270 and 330−420 °C, which corresponds to 48% weight loss due to the 4646


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ACS Applied Nano Materials decomposition of five TOA cations, except for TOA-PWFe, where 40% weight is lost by the decomposition of four TOA cations (Figure 6I(b)). This corroborates the presence of five TOA cations per molecule and confirms −5 charge on all metal-substituted Keggin anions, except Fe3+-substituted Keggin anion, on which the charge is −4. The ionic liquids also contain some amount of water that is lost at ∼125 °C. Further, the DSC curves of ionic liquids are shown in Figure 6II. All the POM-based ionic liquids except TOA-PWFe show a sharp peak corresponding to the melting point at 87 °C (Figure 6II(a, c−f). The small endothermic peak between 40 and 60 °C shows possible glass transition phase in these ionic liquids. Unlike other ionic liquids, the melting temperature of TOA-PWFe is found to be 108 °C (Figure 6IIb). The TOAPWFe has only four TOA cations and hence less steric effect, which explains the increase in melting temperature compared to the other ionic liquids, where five cations interact with the metal-substituted Keggins. The molecular formulas of the POM-based ionic liquids can be given as [(C8H17)4N]5[PW11M(H2O)O39], where M = Mn2+, Co2+, Ni2+, Cu2+, and Zn2+, and [(C8H17)4N]4[PW11Fe(H2O)O39] for TOA-PWFe. The POM-based ionic liquids are further characterized by 1 H and 31P NMR. All the ionic liquids dissolved in DMSO-d6 solvent show similar 1H NMR spectra with no impurities present (Figure S3). The 1H NMR spectra show the peaks corresponding to TOA cation. TOA-PWMn: δ = 0.86 (t, 12 H), δ = 1.27 (m, 40 H), δ = 1.59 (bs, 8 H), δ = 3.21 (t, 8 H); TOA-PWFe: δ = 0.85 (t, 12 H), δ = 1.26 (m, 40 H), δ = 1.55 (bs, 8 H), δ = 3.14 (t, 8 H); TOA-PWCo: δ = 0.85 (t, 12 H), δ = 1.28 (m, 40 H), δ = 1.57 (bs, 8 H), δ = 3.17 (t, 8 H); TOAPWNi: δ = 0.86 (t, 12 H), δ = 1.27 (m, 40 H), δ = 1.57 (bs, 8 H), δ = 3.16 (t, 8 H); TOA-PWCu: δ = 0.86 (t, 12 H), δ = 1.27 (m, 40 H), δ = 1.56 (bs, 8 H), δ = 3.21 (t, 8 H); TOAPWZn: δ = 0.85 (t, 12 H), δ = 1.27 (m, 40 H), δ = 1.56 (bs, 8 H), δ = 3.15 (t, 8 H). Here the chemical shift values (δ) are in parts per million units. Full spectra are shown in Figure S3. The transition-metal-substituted Keggin anions are generated in situ, and their formation is confirmed by 31P NMR. The chemical shift for saturated Keggin (PW12O40)3− is −15.1 ppm,6 while monolacunary Keggin (PW11O39)7− has an upfield shift of −10 ppm.52 However, all the POM-based ionic liquids, with first-row transition metal substituted in the lacunary site of Keggin unit, show a single 31P NMR peak at −13.1 ppm, which is a characteristic of metal-substituted monolacunary Keggin species, shown in Figure S4.53 The optical absorption properties of POM-based ionic liquids are studied by UV−visible absorption spectroscopy, and the spectra are presented in Figure S5. Polyoxometalates generally show characteristic oxygen-to-metal charge transfer bands in the UV region. In Figure S5, the weak absorption band at 220 nm is due to O → P transition. The two intense bands at 260 and 310 nm are due to the O2− → W6+ charge transfer, attributed to edge-sharing and corner-sharing oxygens present inW−O−W bridges in the Keggin units, respectively.6 The intense peak for unsubstituted phosphotungstic acid (PWA) at 310 nm has become very broad and red-shifted to 353 nm in all ionic liquids samples except for TOAPWZn ionic liquid, for which the peak shift appears at 336 nm. The reason for this shift is the strong electrostatic interaction between metal-substituted Keggin anions and TOA cations. The diffuse reflectance features observed between 400 and 800 nm visible region are due to the d−d transitions related to the first-row transition-metal ions substituted in the octahedral site of the

monolacunary Keggin ions.54,55 The presence of transition metal in the Keggin unit exhibited significant enhancement in the absorbance. The green color TOA-PWNi and blue color TOA-PWCu ionic liquids containing substituted Ni2+ and Cu2+ ions, respectively, are found to absorb in the lower energy region of 600−800 nm extending to near IR. These broad features are due to the spin-allowed d−d transitions with configurations of d8 (Ni2+) and d9 (Cu2+) ions. The yellowishorange and brownish-red colored TOA-PWMn and TOAPWCo ionic liquids indicate the substitution of Mn2+ and Co2+ ions and show broad diffuse reflectance features in the region of 370−550 nm. However, Fe3+-substituted ionic liquid TOAPWFe is pale yellow in color, and Zn2+-substituted ionic liquid TOA-PWZn is colorless. They both do not show any prominent diffuse reflectance feature in the visible region. The diffuse reflectance features in the region of 200−350 nm are similar to the parent Keggin ion suggesting that the substitution of first-row transition-metal ions in one of octahedral site does not seem to alter the intrinsic electronic properties of Keggin units. To get further insight into the structural details of these POM-based ionic liquids, we measured the EPR spectra at Xband frequencies (∼9 GHz) by inserting a small capillary tube containing a droplet of highly viscous ionic liquid into the Xband quartz tube of the spectrometer. The TOA-PWMn, TOA-PWFe, and TOA-PWCu ionic liquids are only found to be EPR active. The EPR spectrum of TOA-PWMn ionic liquid at room temperature is a poorly resolved hyperfine sextet feature at g = 2.00, a strong hyperfine sextet at g = 4.33, and a weak unresolved feature at g = 8.66.56−58 Mn2+ has spin S = 5/ 2, I = 5/2; these are characteristic of high-spin Mn2+ in a very strong crystal field in which not all the zero-field levels are populated equally, but only the S = ±1/2 zero-field energy levels of spin sextet are populated appreciably. With total spin S = 5/2 and a “fictitious” spin of 1/2, we can expect, correct to the first order, two spectral features at g|| = 2.00 and g⊥ = 6.00 and several forbidden transitions. However, theoretically, gfactors deviate farther from these values when the crystal field is not strictly axially symmetric (E ≠ 0). An interesting change takes place at −196 °C, when the features at g = 2.00 almost disappear, and the g = 4.2 features become well-resolved with a characteristic Mn2+ hyperfine splitting of 86 G. The spectra are shown in Figure 7I, and the EPR parameters are included in Table S1. The TOA-PWFe ionic liquid gives a strong EPR line at g = 4.33 as expected for Fe3+ in a strong rhombic crystal field with D/E = 3, which is supposed to give spectra with gxx = gyy = gzz = 4.33.59 Fe3+ occupies exclusively the octahedral sites and is subjected to a strong nonaxial crystal field. The EPR spectra at X-band of TOA-PWFe ionic liquid is shown in Figure 7II. At room temperatures the TOA-PWCu ionic liquid gave a typical axially symmetric powder pattern with g∥ > g⊥, both of which are larger than free-spin value of 2.0023. The nearly axially symmetric EPR spectrum of the ionic liquid, with gxx and gyy not well-resolved, has g and A parameters, g∥ = 2.35 and

(

g⊥ = 2.06 obtained from

gxx + gyy 2

). The

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Cu hyperfine coupling

is A∥ = 116 G, and A⊥ is unresolved.60 This is characteristic of a dx2−y2ground state for Cu2+ with octahedral coordination around Cu2+. Octahedral Cu2+ complexes are characterized by g∥ > g⊥ being respectively given by61,62 g = 4647

8λ 2λ and g⊥ = Δ Δ DOI: 10.1021/acsanm.8b00920 ACS Appl. Nano Mater. 2018, 1, 4642−4651

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than what would be expected under ideal octahedral coordination. The EPR parameters of the two low-temperature species are quite different, and one of them resembles in magnetic parameters the room temperature species. It is difficult to decipher the structure of the additional species that arises at low temperature. This temperature-dependent behavior is reproducible. Perhaps, when the soft ionic crystal freezes into rigid crystalline phase, the presence of two poly types is possible for the crystals at −196 °C. The EPR spectra of TOA-PWCu ionic liquid are shown in Figure 7III, and the parameters are collected in Table S1. The magnetic properties of these ionic liquids were examined using vibrating sample magnetometer, and the results are presented in Figure S6. Furthermore, the chemical states of the elements present in the TOA-PWCu ionic liquid are analyzed by XPS. The TOAPWCu ionic liquid is coated as a thin film on copper foil to record the survey spectrum shown in Figure 8. The XP spectrum in Figure 8a shows the characteristicW 4f7/2, W 4f5/2, W 4d5/2, and W 4d3/2 peaks at 34.8, 36.8, 246.8, and 258.9 eV, respectively, due to the presence of W6+ in the TOA-PWCu ionic liquid. The Cu2+ ion present in one of the 12 octahedra of the Keggin unit in the ionic liquid also shows weak 2p signals in Figure 8b. The intense C 1s peak arising from the alkyl chains and O 1s peak arising from the Keggin units are also evident in the XP spectrum in Figure 8a. Application of POM-Based Ionic Liquids for Removal of Metal Ions from Aqueous Solutions. POM-based ionic liquids are hydrophobic and contain highly negatively charged Keggin anions. The six ionic liquids are employed for the removal of Cd2+ and Pb2+ ions from the aqueous solutions. The experiments are conducted at 80 °C under constant stirring allowing the ionic liquid to mix with known concentration of aqueous solutions containing metal ions. The metal ion removal efficiency of ionic liquid from the aqueous solutions is evaluated by using the following equation63

Figure 7. EPR spectra of (I) TOA-PWMn, (II) TOA-PWFe, and (III) TOA-PWCu ionic liquids at (a) room temperature and (b) liquid nitrogen temperature (−196 °C).

where λ is the spin−orbit coupling constant of Cu2+, which is ∼828 cm−1, and Δ is 10 Dq, the crystal field splitting between the eg and t2g levels of the d-orbitals of Cu2+ with the unpaired electron occupying the uppermost dx2−y2 orbital. These results indicate that Cu2+ in fact occupies a substitutional tungsten site. Although the TOA-PWCu ionic liquid is a low-melting solid, the spectrum at room temperature does not indicate a liquidlike behavior with highly restricted motion for the molecules. In other words, the time scale of molecular tumbling is not able to average the anisotropy in g and A tensors. However, it is quite clear that the symmetry around Cu2+ in the ionic liquid is a highly distorted octahedron with the dxz and dyz being nondegenerate leading to considerable admixture of the d-orbitals brought about by higher-order spin−orbit coupling. The ground state is certainly dx2−y2. Cooling the TOA-PWCu ionic liquid in the X-band spectrometer to −153 °C did not show any noticeable change in the EPR spectra. At −196 °C, however, the spectra of the TOA-PWCu ionic liquid show the presence of two magnetically distinct sites, populations being in the ratio of ∼2:1. The EPR parameters of both Cu2+ species correspond to basic octahedral coordination but with different distortions and geometries. Both species have g∥ > g⊥and A∥ somewhat less

%metal ion removal efficiency of ionic liquid c − cf = i × 100 ci

where ci and cf refer to the initial and final concentrations of the metal ions in aqueous phases. The separation of two phases is easy due to the hydrophobic nature of the ionic liquids. The negative charge present on the Keggin ion in the ionic liquid is the driving force for the removal of positively charged metal

Figure 8. XPS of TOA-PWCu ionic liquid on copper foil, (a) survey spectrum of ionic liquid showing W, C, and O, and (b) Cu 2p spectrum. 4648


Article

ACS Applied Nano Materials ions. Under constant stirring at 80 °C the ionic liquid comes in contact with water phase and facilitates the metals to enter the ionic liquid phase. TOA being hydrophobic keeps the ionic liquid phase separate from aqueous phase and does not allow the extracted metal ions to go back in to the water phase. The removal of individual metal ions Cd2+ and Pb2+ at different concentrations ranging from 1 to 25 g/L by using the six ionic liquids is analyzed separately, and percent efficiency of ionic liquid to remove metal ions is presented in Table 1.

Keggin ions are treated with TOA cations. Transition-metal substitution in Keggin units is confirmed by 31P NMR, UV−vis DRS, EPR, XPS, and FTIR measurements. These materials are greasy, hydrophobic, and thermoreversible and have melting points less than 100 °C. Needlelike mesophases are formed when these ionic liquids are gently heated and cooled to room temperature. The microscopic structural analysis of these ionic liquids by XRD and SEM reveal the layered arrangement of TOA-Keggin units. The presence of W6+ ion is confirmed by XPS. The properties such as viscosity and physical behavior of these ionic liquids need to be studied further at different temperatures. The highly negatively charged anions and hydrophobic nature of these ionic liquids are extremely useful to remove Cd2+ and Pb2+ metal ions from the aqueous phase. We have used 100 mg of ionic liquid to remove more than 99% of Cd2+ and Pb2+ from the aqueous metal ion solutions of concentration up to 14 g/L. Further work is required to establish the recovery and reusability of these ionic liquids.

Table 1. Efficiency of POM-Based Ionic Liquids for the Removal of Cd2+ and Pb2+ Ions from Aqueous Solutions S. No. 1 2 3 4 5 6

1 2 3 4 5 6

POMbased ionic liquid TOAPWMn TOAPWFe TOAPWCo TOAPWNi TOAPWCu TOAPWZn TOAPWMn TOAPWFe TOAPWCo TOAPWNi TOAPWCu TOAPWZn

14 g/L

20 g/L

25 g/L

99.7

% removal of Cd2+ ions 99.6 99.6 99.5 99.4

88.4

83.2

99.6

99.8

79.1

71.4

1 g/L

5 g/L

7 g/L

99.4

10 g/L

99.4

88.6

■

ASSOCIATED CONTENT

S Supporting Information *

99.8

99.5

99.5

99.2

97.4

89.2

82.7

99.8

99.7

99.7

99.7

99.1

85.4

82.4

99.6

99.6

99.5

99.4

99.6

87.7

81.6

99.8

99.5

99.8

99.6

99.3

88.9

82.4

99.9

% removal of Pb2+ ions 99.6 99.8 99.7 95.4

85.4

81.2

99.8

99.5

99.2

98.4

91.6

71.5

68.4

99.6

99.7

99.5

99.1

97.7

88.2

82.9

99.9

99.7

99.8

99.9

98.1

85.8

83.4

G. Ranga Rao: 0000-0002-2930-3756

99.6

99.1

99.6

99.7

99.5

85.7

79.1

Notes

99.6

99.8

99.5

99.7

99.7

88.8

83.3

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00920.

■

Contact angle measurement, Raman spectra, 1H and 31P NMR spectra, UV−vis and FTIR spectra, EPR and VSM measurements, EPR parameters (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +91 44 2257 4226. Fax: +91 44 2257 4202. E-mail: [email protected]. ORCID

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The DST-FIST facilities in IIT Madras have been very helpful to carry out this work. K.S. would like to thank CSIR, New Delhi, for awarding JRF and SRF fellowships. We thank Prof. S. Subramanian, IITM, for EPR spectral analyses. We also thank Dr. C. S. Gopinath and Ms. R. Jain, NCL-Pune, for XPS measurements.

It is interesting to note that all the ionic liquid samples are very efficient in removing both the metal ions completely from the aqueous solutions up to a concentration of 14 g/L. To analyze the mechanism of metal removal process by ionic liquids, FT-IR spectra of ionic liquids before and after extraction are compared in Figures S7 and S8. The spectra of ionic liquids after metal exclusion process match exactly with the fresh ionic liquids. Additionally, two peaks at 1260 and 1740 cm−1 are noticed, which are possibly due to ν(C−O) and ν(CO) vibrations related to the acetate ions of the metal acetates. This shows that the metal ion would have entered the ionic liquid phase along with its counteranion (acetate). Thus, ion-pair extraction process is possibly taking place in the ionic liquid medium.43 This is a simple and highly efficient procedure to remove heavy metal ions using ionic liquids. The efficiency of various ionic liquids for the removal of metal ions is given in Table S2.

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REFERENCES

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CONCLUSIONS First-row transition-metal ions are substituted in the in situ generated monolacunary Keggin anions in the aqueous medium. Ionic liquids of type TOA-PW-(Mn2+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+) are produced when the metal-substituted 4649


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