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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 Gangavarapu Ranga Rao ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00920 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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ACS Applied Nano Materials

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 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 FTIR, Raman, XRD, SEM, TGA, DSC, NMR, EPR, XPS and UV-Vis spectroscopic techniques. These ionic liquids show flake like 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 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 which exhibit enormous structural diversity and properties.2 They readily form POM-based hybrid compounds with positively charged organic *

Corresponding author Tel.: +91 44 2257 4226; Fax: +91 44 2257 4202

E-mail: [email protected] (G. Ranga Rao)

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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 which 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 100oC.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 can be used as electroactive materials.17,22,23 Interaction of POMs with sulfogroup 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 semi-solid materials show intrinsic properties such as ionic conductivity, drug activity and solvent ability. Therefore, negatively charged metal-substituted lacunary POMs can pave 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 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 anti-corrosion 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 which are non-volatile. 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 2 ACS Paragon Plus Environment

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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. It is to be noted that the unsubstituted phosphotungstate (H3PW12O40) reacting with tetraoctylammonium (TOA) cations yields solid hybrid material.44 Further, we have employed the POM-based ionic liquids for the removal of hazardous metal ions, Pb2+and Cd2+, present in the aqueous phase. EXPERIMENTAL Materials and characterization. We used tetraoctyl ammonium bromide and sodium tungstate (Avra, India), disodium hydrogen phosphate and analytical grade toluene (Thermo-Fisher Scientific, India) and metal salts (Alfa Aesar) as received from the suppliers. 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 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 GBXDIGIDROP 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 Perkin-Elmer TA make differential scanning calorimeter (DSC) Q200. NMR spectra were recorded on a Bruker 500-MHz NMR spectrometer by dissolving ionic liquids in DMSO-d6 solvent. The UV-Visible-DR spectra of the ionic liquid materials were recorded on JASCO V660 spectrometer using BaSO4 as a reference. The electron paramagnetic resonance (EPR) spectra of all the ionic liquids was recorded at X-band frequencies (∼9 GHz) using JES FA-200 3 ACS Paragon Plus Environment


spectrometer. A custom-built laboratory version ambient pressure photoelectron spectrometer (Lab-APPES) was used for XPS measurements. The concentrations of Cd2+ and Pb2+ metal ions left in the aqueous phase after treating with ionic liquids were determined by Schimadzu 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 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 tetraoctylammonium bromide (TOA-Br) dissolved in 20ml toluene under continuous stirring for 10 minutes. 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 coloured 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 oC. These POM-based ionic liquids are denoted as TOA-PWMn, TOA-PWFe, 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 towards the Keggin anion due to the electrostatic interactions, as shown in scheme 1(c).


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Scheme 1. General scheme of tetrahedral TOA-Br (a) reacting with in situ generated metal (Mn2+, Co2+, Ni2+, Cu2+ and Zn2+) substituted Keggin anion (b) giving rise to POM-based ionic liquid TOA-PWMn, TOA-PWCo, TOA-PWNi, TOA-PWCu and TOA-PWZn (c). The TOAPWFe ionic liquid possesses only four TOA units.

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 POM-based ionic liquids are 80 °C for 1 h. These are optimized conditions arrived at by performing 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. 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 oC. The viscosity of ionic liquid decreases at higher temperature allowing the mixing of ionic liquid 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%.



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RESULTS AND DISCUSSION The substitution of metal ion in Keggin structure raises the overall charge on the anion from

[ PW12 O 40 ]

3−

to [ PW11MO39 ] which interacts with more number of tetraoctyl ammonium cations, 5−

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, DCM, 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 82o for TOA-PWMn, 86o for TOA-PWFe, 85o for TOA-PWCo, 87o for TOA-PWNi, 86o for TOAPWCu and 88o for TOA-PWZn. These gel-like ionic liquids do not flow at room temperature but show liquid-like behaviour when heated to ~80oC and reverse to gels when cooled to room temperature. This phenomenon of thermoreversibility is shown in Figure 1(b). 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 self-healing property of these ionic liquids is the result of van der Waals interactions among the alkyl chains of ionic liquid moieties.


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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-PWZnwhen heated to 80 °C and then cooled to RT and (c) POMbased ionic liquids(i) TOA-PWCo, and (ii) TOA-PWCu showing self-healing property at room temperature.

Physical characterization. The FTIR spectra of six POM-based ionic liquids in Figure 2 show peaks characteristic of first row transition metal substituted Keggin compounds in the region 700 - 1200 cm-1. The IR spectrum of pristine Keggin ion in phosphotungstic acid shows four characteristic stretching frequencies in Figure 2(g). 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 7 ACS Paragon Plus Environment


signature peaks, the ionic liquids also show a weak peak around 726 cm-1 which is attributed to the substituted transition metal-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 TOABr, the four octyl chains are tetrahedrally bonded to the quaternary ammonium ion (scheme 1(a)). 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 TOABr (Figure 2(h)). Other peaks in the region of 28003000 cm-1are 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.


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Figure 2. FTIR spectra of POM-based ionic liquids (a) TOA-PWMn, (b) TOA-PWFe, (c) TOAPWCo, (d) TOA-PWNi, (e) TOA-PWCu, (f) TOA-PWZn, (g) PWA and (h) TOA-Br. 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 S2(a) 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=O ter ) in Figure S2(a) is shifted to 974 cm-1 in the ionic liquids due to the interaction between terminal oxygens and TOA cations (Figure S2 b to g). The vibrational features in the regions 1030-1500 cm-1 and 2700-3050 cm-1 exactly match with the vibrational features of pure TOA-Br in Figure S2(h). This essentially indicates that the alkyl stretches of TOA moieties are unaffected in the ionic liquids. We have analysed the molecular packing of POM anions and TOA cations in the POM-based ionic liquids from the powder XRD patterns shown in panel-I in Figure 3. The diffraction patterns of all the ionic liquids show intense low angle reflections around 2θ value of 5o and the d-spacing varying between 17.7 Å and 18.4 Å. This indicates the lamellar structure for POMbased ionic liquids with alternate packing of Keggin anions and tetraoctylammonium cations, as 9 ACS Paragon Plus Environment


shown in panel-II in Figure 3.50 The perpendicular height of octyl chain is 5.77 Å and the diameter of Keggin anion is about 10.4 Å. Therefore, the total thickness of each layer would be around 21.94 Å, which is larger than the d-values measured by XRD. The strong hydrophobic and van der Waal’s interactions among the octyl chains can lead to the 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 14.7o, 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.

Figure 3. (Panel I) Powder XRD pattern of POM-based ionic liquids (a) TOA-PWMn, (b) TOAPWFe, (c) TOA-PWCo, (d) TOA-PWNi, (e) TOA-PWCu, (f) TOA-PWZn; (Panel II) schematic representation of layered arrangement of transition metal substituted lacunary Keggin anions and TOA cations. 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 up to 70 oC 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. On cooling, tiny needle-like crystals are formed which can be seen in the optical microscope images in Figure 4. The needle-like 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 needle-like structures are elusive for single crystal diffraction

analysis.


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Figure 4. (a) A vial containing TOA-PWNi ionic liquid with needle-like mesophase texture visible to naked eye, (b) to (g) optical microscope images of needle-like texture of TOA-PWMn, TOA-PWFe, TOA-PWCo, TOA-PWNi, TOA-PWCu and TOA-PWZn, respectively. Furthermore, the microscopic morphology of ionic liquids is analysed by scanning electron microscope at low vacuum. The needle-like features noticed in the ionic liquids in optical microscope images were not strong enough to withstand the mechanical strain which was applied during smearing the sample for SEM analysis. However, these ionic liquids at room temperature show regular flake like structures, as shown in Figure 5. This morphology is stable only at room temperature and disappears at about 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.50 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.



Figure 5. Scanning electron microscope 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 The thermal analyses of POM-based ionic liquids and TOA-Br are shown in panel-I, Figure 6. There is about 5-8% loss of water below 150 oC from the ionic liquid samples. Further the TOABr shows single step complete decomposition in the range of 160-220 oC (g), while all the POMbased ionic liquids show two-step decomposition pattern, Figure 6, panel-I(a) to (f). The decomposition of TOA in ionic liquids starts above 200 oC 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 oC and 330-420 oC, which corresponds to 48% weight loss due to the decomposition of five TOA cations, except for TOA-PWFe where 40% weight is lost by the decomposition of four TOA cations (Figure 6(I) 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 which is lost around 125 oC. Further, the DSC curves of ionic liquids are shown in panel-II, Figure 6. All the POM-based ionic liquids except TOA-PWFe show a sharp peak corresponding to the melting point at 87oC (Figure 6, panel-II, a, and c-f). The small endothermic peak between 40-60 oC shows possible glass transition phase in these ionic liquids. 12 ACS Paragon Plus Environment

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Unlike other ionic liquids, the melting temperature of TOA-PWFe is found to be 108 oC (Figure 6, panel-II, b). The TOA-PWFe 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 formulae of the POM-based ionic liquids can be given as:

( C8 H17 )4 N   PW11M ( H 2O ) O39  , where M= Mn2+, Co2+, Ni2+, Cu2+, Zn2+ and 5 ( C8 H17 ) 4 N  PW11Fe ( H 2O ) O39  for TOA-PWFe. 4

Figure 6. (I) Thermogravimetric and (II) DSC analysis of POM-based ionic liquids, (a) TOAPWMn, (b) TOA-PWFe, (c) TOA-PWCo, (d) TOA-PWNi, (e) TOA-PWCu, (f) TOA-PWZn and (g) TOA-Br The POM-based ionic liquids are further characterised by 1H 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); TOA-PWNi: 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); TOA-PWZn: δ =0.85 (t, 12 H), δ = 1.27 (m, 40 H), δ = 1.56 (bs, 13 ACS Paragon Plus Environment


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8 H), δ = 3.15 (t, 8 H). Here the chemical shift values (δ) are in ppm units. Full spectra are shown in Figure S3. The transition metal substituted Keggin anions are generated in situ and their formation is confirmed by

31

P NMR. The chemical shift for saturated Keggin ( PW12 O 40 )

−15.1 ppm,6 while monolacunary Keggin

( PW11O39 )

7−

3−

is

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 nm and 310 nm are due to the O 2 − → W 6+ charge transfer; attributed to edge-sharing and cornersharing oxygens present in W-O-W bridges in the Keggin units, respectively.6 The intense peak for unsubstituted 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 to 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 colour TOA-PWNi and blue colour TOA-PWCu ionic liquids containing substituted Ni2+ and Cu2+ ions, respectively, are found to absorb in the lower energy region 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 yellowish orange and brownish red coloured TOA-PWMn and TOA-PWCo 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 TOA-PWFe is pale yellow in colour and Zn2+ substituted ionic liquid TOA-PWZn is colourless. They both do not show any prominent diffuse reflectance feature in the visible region. The diffuse reflectance features in the region 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. 14 ACS Paragon Plus Environment

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In order to get further insight into the structural details of these POM-based ionic liquids, we have measured the EPR spectra at X-band frequencies (∼9 GHz) by inserting a small capillary tube containing a droplet of highly viscous ionic liquid into the X-band 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 ½, 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, g factors deviate farther from these values when the crystal field is not strictly axially symmetric (E ≠0). An interesting change takes place at -196 oC, when the features at g=2.00 almost disappears and the g=4.2 features become well resolved with a characteristic Mn2+ hyperfine splitting of 86 G. The spectra are shown in Figure 7(I) 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 non-axial crystal field. The EPR spectra at X-band of TOA-PWFe ionic liquid is shown in Figure 7(II). 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

 g xx + g yy parameters, g = 2.35 and g ⊥ = 2.06 obtained from  2 

 63  . The Cu hyperfine coupling is 

A = 116 G and A⊥ is unresolved.60 This is characteristic of a d x2 − y 2 ground state for Cu2+ with

octahedral coordination around Cu2+. Octahedral Cu2+ complexes are characterized by g > g ⊥ being respectively given by 61,62 g =

8λ 2λ and g ⊥ = ∆ ∆ 15 ACS Paragon Plus Environment


where λ is the spin orbit coupling constant of Cu2+ which is ∼828 cm-1 and ∆ 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 upper most d x2 − y 2 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 liquid-like 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 non-degenerate leading to considerable admixture of the d-orbitals brought about by higher order spin orbit coupling. The ground state is certainly, d x2 − y 2 . Cooling the TOA-PWCu ionic liquid in the X-band spectrometer down to -153 oC did not show any noticeable change in the EPR spectra. At -196 oC, however, the spectra of the TOA-PWCu ionic liquid show the presence of two magnetically distinct sites, populations being in the ratio approximately 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 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 to the room temperature species. It is difficult to decipher the structure of the additional species that arises at low temperature. This temperature dependent behaviour is reproducible. Perhaps, upon cooling the soft ionic crystal freezes into rigid crystalline phase, the presence of two poly-types is possible for the crystals at -196 oC. The EPR spectra of TOAPWCu ionic liquid are shown in Figure 7(III) and the parameters are collected in Table S1.


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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 oC). The magnetic properties of these ionic liquids were carried out 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 analysed by XPS. The TOA-PWCu 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 8(a) shows the characteristic

W 4f 7/2 , W 4f 5/2 , W 4d 5/2 and W 4d 3/2 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 17 ACS Paragon Plus Environment


Page 18 of 29

octahedra of the Keggin unit in the ionic liquid also shows weak 2p signals in Figure 8(b). 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 8(a).

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

Application of POM-based ionic liquids for removal of metal ions from aqueous solutions. POM-based ionic liquids are hydrophobic and contain highly negative 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 oC 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 equation,63 % M eta l ion remova l efficiency of ionic liqu id =

ci − c f ci

× 10 0

where ci and c f 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 ions. Under constant stirring at 80℃ the ionic liquid comes in contact with water phase and facilitates the metals to enter the ionic liquid phase. TOA being 18 ACS Paragon Plus Environment

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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 g/L to 25 g/L by using the six ionic liquids is analysed separately and % efficiency of ionic liquid to remove metal ions is presented in Table 1.

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

POM-based ionic liquid TOA-PWMn TOA-PWFe TOA-PWCo TOA-PWNi TOA-PWCu TOA-PWZn

1 g/L 99.7 99.6 99.8 99.8 99.6 99.8

1 2 3 4 5 6

TOA-PWMn TOA-PWFe TOA-PWCo TOA-PWNi TOA-PWCu TOA-PWZn

99.9 99.8 99.6 99.9 99.6 99.6

% removal of Cd2+ ions 5 g/L 7 g/L 10 g/L 14 g/L 20 g/L 99.6 99.6 99.5 99.4 88.4 99.8 99.4 99.4 88.6 79.1 99.5 99.5 99.2 97.4 89.2 99.7 99.7 99.7 99.1 85.4 99.6 99.5 99.4 99.6 87.7 99.5 99.8 99.6 99.3 88.9 2+ % removal of Pb ions 99.6 99.8 99.7 95.4 85.4 99.5 99.2 98.4 91.6 71.5 99.7 99.5 99.1 97.7 88.2 99.7 99.8 99.9 98.1 85.8 99.1 99.6 99.7 99.5 85.7 99.8 99.5 99.7 99.7 88.8

25 g/L 83.2 71.4 82.7 82.4 81.6 82.4 81.2 68.4 82.9 83.4 79.1 83.3

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 14g/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 Figure 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 counter anion (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. 19 ACS Paragon Plus Environment


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 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, thermoreversible and have melting points less than 100 °C. Needle-like 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 negative 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.

ACKNOWLEDGEMENT The DST-FIST facilities in IIT Madras have been very helpful to carry out this work. Shakeela 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. Ruchi Jain, NCL-Pune, for XPS measurements.

ASSOCIATED CONTENT Supporting Information Supporting Information associated with this article is available.

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First row transition metal substituted Keggin based ionic liquids are synthesized and their thermoreversibility, hydrophobicity and high negative charge on anions helped in almost complete removal of Cd2+ and Pb2+ metal ions from the aqueous solutions. 255x101mm (150 x 150 DPI)

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