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Synthesis and characterization of hybrid molecular material prepared by ionic liquid and silicotungstic acid
Materials Chemistry and Physics 112 (2008) 853–857
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis and characterization of hybrid molecular material prepared by ionic liquid and silicotungstic acid T. Rajkumar, G. Ranga Rao ∗ Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e
i n f o
Article history: Received 5 December 2007 Received in revised form 4 June 2008 Accepted 21 June 2008 Keywords: Ionic liquid Silicotungstic acid Keggin unit Near IR Hybrid molecular material
a b s t r a c t A white colour IL-SiWA hybrid molecular material has been synthesized using 1-butyl 3-methyl imidazolium bromide ionic liquid and silicotungstic acid. The material is found to dissolve in polar organic solvent such as DMSO, but not in water. It is characterized by CHN analysis, FTIR, XRD, UV–vis DRS, TGA and SEM. The FTIR spectra indicate the formation of a hybrid compound showing fingerprint vibrational bands of both Keggin ions of silicotungstic acid and imidazolium ions. There is a strong evidence of interaction between the Keggin anions and imidazolium cations from the UV–vis DR spectra and the splitting of imidazolium C–H bands in the IR spectrum of the IL-SiWA. The near IR shows loss of crystallization water from heteropoly acid when it reacts with ionic liquid forming the hybrid molecular material. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Synthesis of hybrid materials using polyoxometalates has been a subject of immense interest in solid-state materials chemistry [1–10]. The new polyoxometalate composite materials offer rich structural chemistry and physicochemical properties of interest in catalysis [7], photochromism [3,8,9], proton conductors [10,11]. The pairing of polyoxometalate (POM) anions with organic cations and ionic liquids (ILs) continue to yield interesting materials, including a recent report on the synthesis of ambient temperature POMbased ionic liquids [12]. The POM-based materials may retain Keggin anion structure and show hybrid properties of both polyoxometalates and the organic part. Inorganic polyoxometalates are solid acid materials having Brønsted acidity and contain Keggin anions such as [XM12 O40 ]ı−8 (X = P5+ or Si4+ and M = W6+ , Mo6+ ), which are interconnected by hydrogen-bonded water molecules (maximum 29). These large metal oxide framework anions (size ∼1 nm) can react with room temperature ionic liquids forming new organic–inorganic hybrid composite materials [12,13]. The purpose of synthesizing the POM-based hybrid molecular salt is to explore the potential applications in POM-based catalysis and catalytic materials chemistry [14–16]. It is also important to understand the nature of interactions between the Keggin anions and organic cations and their stability in the molecular material [17].
∗ Corresponding author. Tel.: +91 44 2257 4226; fax: +91 44 2257 4202. E-mail address: [email protected] (G. Ranga Rao). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.06.046
Here we report for the fist time the synthesis and characterization of a molecular compound based on 1-butyl 3-methyl imidazolium (BmIm) cation and silicotungstic Keggin anion (SiWA). Near IR, IR and UV–vis DR spectroscopy methods show that the molecular compound contains Keggin anions paired with BmIm cations. The split in the C–H stretching region (3050–3200 cm−1 ) and the redshift of the 332 nm transition to 305 nm in the UV–vis DR spectra show strong electrostatic interactions between Keggin anions and BmIm cations. 2. Experimental 2.1. Synthesis of ionic liquid 1-Butyl 3-methyl imidazolium bromide ionic liquid was synthesized using materials as received (Fluka) and following procedure reported [18]. The IL was prepared by taking 1:1 mole ratio of 1-methyl imidazole and 1-bromobutane. In a typical preparation, 33.29 g of liquid 1-bromobutane was added to 20 g of liquid 1methyl imidazole in a round bottom flask fitted with reflux condenser under stirring conditions at 80 ◦ C for 24 h. The top phase of the two phases formed contains unreacted starting material which can be removed by washing twice with ethyl acetate. This also ensures the removal of the unreacted material in the bottom phase containing IL product. The yellowish IL product of 1-butyl 3-methyl imidazolium bromide (BmImBr) liquid was further heated to 50 ◦ C to eliminate any dissolved ethyl acetate solvent and its formation was confirmed by FTIR, 1 H and 13 C NMR [18,19]. 2.2. Synthesis of 1-butyl 3-methyl imidazolium silicotungstic hybrid For the synthesis of 1-butyl 3-methyl imidazolium silicotungstic hybrid molecular material, 1-butyl 3-methyl imidazolium bromide and silicotungstic acid (H4 SiW12 O40 ·nH2 O) were taken in 4:1 mole ratio in order to prepare one mole of 1-butyl 3-methyl imidazolium silicotungstate. Ionic liquid was added drop wise to the solution containing silicotungstic acid (SiWA) under constant stirring at room
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temperature. The white precipitate obtained was washed with distilled water and dried at 80 ◦ C for overnight. It was characterized by CHN analysis which shows that one mole of Keggin anions of [SiW12 O40 ]4− consume four moles of 1-butyl 3-methyl imidazolium cations to form the molecular material. The formula estimated from the C–H–N elemental analysis of the sample is C32.13 H55.85 N8.08 SiW12 O40 which gives an empirical structure of [BmIm]4 [SiW12 O40 ]. This is consistent with 4:1 mole ratio of BmIm cation to [SiW12 O40 ]4− used. The white colour hybrid material has surface area of 0.7 m2 g−1 and has no porous structure. The hybrid material can be dissolved readily in dimethyl sulphoxide (DMSO) and re-crystallized. However, it scarcely dissolves in other standard organic solvents and water. 2.3. Characterization The samples were analysed by X-ray diffraction employing Shimadzu XD-D1 diffractometer using Cu K␣ radiation ( = 1.5418 Å). The IR spectra of samples as KBr pellets were recorded using PerkinElmer infrared spectrometer with a resolution of 4 cm−1 and in the range of 400–4000 cm−1 . Thermogravimetric analysis (TGA) of IL-SiWA hybrid molecular material was conducted in pure N2 gas (30 ml min−1 ) at a heating rate of 20 ◦ C per min on a PerkinElmer TGA-7 instrument. The UV–vis NIR spectra were recorded in diffuse reflectance mode on Jasco V-570 UV-vis NIR spectrophotometer equipped with an integrating sphere in the spectral range of 200–2500 nm. The spectra are presented as F(R), Kubelka–Munk function versus incident photon wavelength. Scanning electron microscopy (SEM) images were taken using a FEI Quanta 200 microscope operating at 30 kV. The sample powders were deposited on a carbon tape before mounting on a sample holder. Fig. 1. FTIR spectra of (a) 1-butyl 3-methyl imidazolium bromide (BmImBr) ionic liquid, (b) H4 SiW12 O40 ·nH2 O and (c) [MmIm]4 [SiW12 O40 ] hybrid molecular material.
3. Results and discussion 3.1. FTIR analysis The FTIR spectra are useful to study the skeletal modes appearing between 700 and 1100 cm−1 of Keggin anions present in the IL-SiWA hybrid molecular material. The parent [SiW12 O40 ]4− Keggin structure shows characteristic bands due to W–O–W vibrations of edge- and corner-sharing WO6 octahedra linked to the central SiO4 tetrahedra [1,20]. Accordingly, the stretching modes of edge sharing (W–Oe –W) and corner sharing (W–Oc –W) units appear between 790 and 890 cm−1 , whereas the Si–O stretching mode appears at 926 and 1014 cm−1 (Fig. 1). The IR spectrum of BmImBr ionic liquid is also presented in Fig. 1, which is consistent with the IR spectrum reported by Wu et al. [19]. It shows the characteristic IR modes at 590–690, 1080–1600 and 2800–3200 cm−1 (alkyl and imidazole ring C–H stretch). In addition to these characteristic bands, both the parent compounds (BmImBr and SiWA) are hydrophilic and show the presence of large amount of water at ∼3450 cm−1 (Table 1) [14,15,19]. However, the water content is reduced significantly in the IL-SiWA hybrid molecular compound, which is discussed in the near IR study in Section 3.5. The IL-SiWA hybrid molecular material is hydrophobic and insoluble in water. The main characteristic vibrations of various modes of organic and inorganic moieties in the IL-SiWA molecular salt are given in Table 1.
The basic structure and geometry of Keggin anions entrapped in the BmIm cations are preserved in the hybrid molecular material. However, a closer examination of IR spectra in the imidazolium ring C–H stretch (3000–3250 cm−1 ) and imidazolium ring stretch (1500–1620 cm−1 ) in Fig. 2 reveals that there is a strong electrostatic interaction between BmIm cation and large Keggin anion [21–23]. Due to this interaction, the two C–H stretching peaks of the imidazolium ring in BmImBr are split into five vibrational peaks in the [MmIm]4 [SiW12 O40 ] molecular salt Fig. 2(A). The imidazolium ring vibration is also split with intensity variations Fig. 2(B). The origin of such C–H split is clearly attributed to specific interaction of BmIm+ ion with bulky Keggin anion. This type of interaction is demonstrated by Jerman and co-workers in the case of MPIm+ I− x compounds [23]. The SEM picture of the [BmIm]4 [SiW12 O40 ] hybrid molecular material is given in Fig. 3 which shows powdery morphology with small clusters of adhered IL-SiWA particles. 3.2. Powder XRD In this work, the bulky anion [SiW12 O40 ]4− , is substituted in place of Br− in the imidazolium-based ionic liquid template. This has lead to the formation of organic–inorganic hybrid solid material. Fig. 4 shows the powder X-ray diffraction (PXRD) pattern of pure silicotungstic acid and IL-SiWA hybrid molecular material.
Table 1 IR bands of BmImBr, H4 SiW12 O40 ·nH2 O and [BmIm]4 [SiW12 O40 ] hybrid molecular material Wavenumber (cm−1 )
Vibration
BmImBr
SiWA
[BmIm]4 [SiW12 O40 ]
3450 3147, 3090 2959, 2936, 2873 1630 1573, 1562 1168 – – – – 840 752 620
3450 – – 1630 – – 1018, 926 980 878 781 – – –
– 3086, 3104, 3112, 3141, 3157 2958, 2933, 2871 – 1562, 1570, 1573 1164 1014, 926 976 883 795 – – 621
Water in the material Imidazole ring (C–H) Aliphatic (C–H) O–H bending Imidazole (ring stretching) Imidazole H–C–C & H–C–N bending (Si–O) (W O) (W–Oc –W) (W–Oe –W) In-plane imidazole ring bending Out-of-plane C–H bending of imidazole ring Imidazole C2 –N1 –C5 bending
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Fig. 2. Imidazole ring C–H stretch (panel A) and imidazole ring stretch (panel B) regions of (a) 1-butyl 3-methyl imidazolium bromide (BmImBr) ionic liquid and (b) [MmIm]4 [SiW12 O40 ] hybrid molecular material.
Using Scherrer’s formula, the average size of the IL-SiWA cluster is estimated to be 18.5 nm. The silicotungstic acid undergoes structural transformation where the four acidic protons and H2 O molecules are replaced by 1-butyl 3-methyl imidazolium cations. The PXRD pattern of IL-SiWA hybrid material reveals different structure compared to pure SiWA salt. The PXRD pattern suggests the existence of host–guest interactions between silicotungstic acid and 1-butyl 3-methyl imidazolium cations in the absence of sandwiched water molecules, which are normally present in the bulk polyoxometalates.
weight loss at 400–580 ◦ C and a minor loss at 580–700 ◦ C. Thermal analysis of polyoxometalates generally shows water loss below 300 ◦ C and the Keggin anion decomposition in the temperature range of 350–600 ◦ C [24,25]. Pure silicotungstic acid decomposes to the constituent oxides around 500 ◦ C which falls in the major decomposition region of IL-SiWA at 400–580 ◦ C. This region is
3.3. TG analysis The thermogravimetric behaviour of IL-SiWA hybrid molecular material is shown in Fig. 5. The TGA curve shows a major
Fig. 3. SEM image of [MmIm]4 [SiW12 O40 ] hybrid molecular material.
Fig. 4. Powder XRD pattern of (a) H4 SiW12 O40 ·nH2 O, and (b) [MmIm]4 [SiW12 O40 ] hybrid molecular material.
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Fig. 5. TGA of [MmIm]4 [SiW12 O40 ] hybrid molecular material.
therefore attributed to the total decomposition of both organic part and the inorganic Keggin anions in the hybrid molecular material. There is no water loss seen below 300 ◦ C indicating that the bonding between BmIm and [SiW12 O40 ]4− eliminates water and four acidic protons associated with bulk silicotungstic acid. The minor loss around 640 ◦ C is attributed to the formation of defective WO3 [25].
Fig. 6. UV–vis DR spectra of (a) BmImBr ionic liquid, (b) H4 SiW12 O40 ·nH2 O and (c) [MmIm]4 [SiW12 O40 ] hybrid molecular material.
3.5. Near IR analysis Near IR spectroscopy can be used to detect water content in polyoxometalates and the hybrid molecular material by studying
3.4. UV–vis DRS analysis UV–vis diffuse reflectance method is often employed to study solids such as dispersed metal oxides, polyoxometalates, layered clays and hybrid materials by measuring d–d, f–d and oxygen–metal ion charge transfer transitions [8,14,15,26,27]. The polyoxometalates in their non-reduced form are generally characterized by ligand to metal charge transfer (LMCT) bands which appear in the UV region between 200 and 400 nm [8,26,27]. When hybrid materials based on polyoxometalates are irradiated with UV light, electrons are excited from the low-energy electronic states (O 2p orbitals) to the high-energy states (metal d orbitals). The energy of the electronic transitions in polyoxometalates depends on the size of the cluster and counter-cation [26]. The UV–vis DR spectra of IL-SiWA hybrid material recorded using integrating sphere are shown in Fig. 6. The electronic spectrum of silicotungstic acid shows three absorption bands at 217, 259 and 330 nm due to LMCT transitions in the Keggin units and there is no electronic transition related to BmImBr ionic liquid in this region. The diffuse reflectance spectrum of the IL-SiWA salt shows the same three transitions in spectrum (c) in Fig. 6. However the position of the bands at 217 and 259 nm remains unchanged while the lowest energy absorption band at 330 nm has become sharp and blue shifted to lower wavelength at 304 nm. This indicates again that there is an interaction between Keggin ions and BmIm cations. The blue shift is attributed to large size of the BmIm cations substituted in place of Br− ions. Due to the reaction with BmImBr, the Keggin anions are also isolated and their polarizing power decreased causing the blue shift (from 330 to 304 nm) in the hybrid molecular material [26,27].
Fig. 7. Near IR spectra of (a) H4 SiW12 O40 ·nH2 O, and (b) [MmIm]4 [SiW12 O40 ] hybrid molecular material.
T. Rajkumar, G. Ranga Rao / Materials Chemistry and Physics 112 (2008) 853–857
overtones and combination bands in the absorption region between 1100 and 2500 nm of water and structural hydroxyl groups [15,28]. Thus the near IR spectra of parent polyoxometalate and the IL-SiWA hybrid materials are recorded by diffuse reflectance mode in the range of 1200–2500 nm, which are shown in Fig. 7. The two prominent bands observed at 1460 and 1961 nm in the near IR spectrum show the presence of crystallization water in H4 SiW12 O40 ·nH2 O. These bands are attributable to the first overtone of hydroxyl groups (2O–H ), and combination bands corresponding to hydroxyl stretching and bending (2O–H + ıH2 O) vibrational modes, respectively, of the water present in silicotungstic acid. The characteristic overtones and combination bands due to H2 O are absent in the IL-SiWA hybrid material (Fig. 7b). The IR peak at 1630 cm−1 due to water is also negligible in the IR spectrum of the hybrid molecular material (Fig. 1c). This corroborates the elimination of water molecules associated with SiWA when reacted with the BmIm counter cations forming the molecular material. 4. Conclusions The hybrid molecular material formed between 1-butyl 3methyl imidazolium bromide ionic liquid and silicotungstic acid has been studied. IR spectrum shows the presence of both 1-butyl 3-methyl imidazolium cations and Keggin anions. The substitution of bulky BmIm cation in place of Br− ion blue shifts the electronic transition from 330 to 304 nm and splits the imidazolium ring C–H stretching modes which confirms the interaction between the organic cations and inorganic Keggin anions in the material. Near IR shows no water present in the hybrid molecular material [BmIm]4 [SiW12 O40 ]. Acknowledgment The DRDO research grant No. ERIP/ER/0300231/M/01/791 is gratefully acknowledged.
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis and characterization of hybrid molecular material prepared by ionic liquid and silicotungstic acid T. Rajkumar, G. Ranga Rao ∗ Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e
i n f o
Article history: Received 5 December 2007 Received in revised form 4 June 2008 Accepted 21 June 2008 Keywords: Ionic liquid Silicotungstic acid Keggin unit Near IR Hybrid molecular material
a b s t r a c t A white colour IL-SiWA hybrid molecular material has been synthesized using 1-butyl 3-methyl imidazolium bromide ionic liquid and silicotungstic acid. The material is found to dissolve in polar organic solvent such as DMSO, but not in water. It is characterized by CHN analysis, FTIR, XRD, UV–vis DRS, TGA and SEM. The FTIR spectra indicate the formation of a hybrid compound showing fingerprint vibrational bands of both Keggin ions of silicotungstic acid and imidazolium ions. There is a strong evidence of interaction between the Keggin anions and imidazolium cations from the UV–vis DR spectra and the splitting of imidazolium C–H bands in the IR spectrum of the IL-SiWA. The near IR shows loss of crystallization water from heteropoly acid when it reacts with ionic liquid forming the hybrid molecular material. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Synthesis of hybrid materials using polyoxometalates has been a subject of immense interest in solid-state materials chemistry [1–10]. The new polyoxometalate composite materials offer rich structural chemistry and physicochemical properties of interest in catalysis [7], photochromism [3,8,9], proton conductors [10,11]. The pairing of polyoxometalate (POM) anions with organic cations and ionic liquids (ILs) continue to yield interesting materials, including a recent report on the synthesis of ambient temperature POMbased ionic liquids [12]. The POM-based materials may retain Keggin anion structure and show hybrid properties of both polyoxometalates and the organic part. Inorganic polyoxometalates are solid acid materials having Brønsted acidity and contain Keggin anions such as [XM12 O40 ]ı−8 (X = P5+ or Si4+ and M = W6+ , Mo6+ ), which are interconnected by hydrogen-bonded water molecules (maximum 29). These large metal oxide framework anions (size ∼1 nm) can react with room temperature ionic liquids forming new organic–inorganic hybrid composite materials [12,13]. The purpose of synthesizing the POM-based hybrid molecular salt is to explore the potential applications in POM-based catalysis and catalytic materials chemistry [14–16]. It is also important to understand the nature of interactions between the Keggin anions and organic cations and their stability in the molecular material [17].
∗ Corresponding author. Tel.: +91 44 2257 4226; fax: +91 44 2257 4202. E-mail address: [email protected] (G. Ranga Rao). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.06.046
Here we report for the fist time the synthesis and characterization of a molecular compound based on 1-butyl 3-methyl imidazolium (BmIm) cation and silicotungstic Keggin anion (SiWA). Near IR, IR and UV–vis DR spectroscopy methods show that the molecular compound contains Keggin anions paired with BmIm cations. The split in the C–H stretching region (3050–3200 cm−1 ) and the redshift of the 332 nm transition to 305 nm in the UV–vis DR spectra show strong electrostatic interactions between Keggin anions and BmIm cations. 2. Experimental 2.1. Synthesis of ionic liquid 1-Butyl 3-methyl imidazolium bromide ionic liquid was synthesized using materials as received (Fluka) and following procedure reported [18]. The IL was prepared by taking 1:1 mole ratio of 1-methyl imidazole and 1-bromobutane. In a typical preparation, 33.29 g of liquid 1-bromobutane was added to 20 g of liquid 1methyl imidazole in a round bottom flask fitted with reflux condenser under stirring conditions at 80 ◦ C for 24 h. The top phase of the two phases formed contains unreacted starting material which can be removed by washing twice with ethyl acetate. This also ensures the removal of the unreacted material in the bottom phase containing IL product. The yellowish IL product of 1-butyl 3-methyl imidazolium bromide (BmImBr) liquid was further heated to 50 ◦ C to eliminate any dissolved ethyl acetate solvent and its formation was confirmed by FTIR, 1 H and 13 C NMR [18,19]. 2.2. Synthesis of 1-butyl 3-methyl imidazolium silicotungstic hybrid For the synthesis of 1-butyl 3-methyl imidazolium silicotungstic hybrid molecular material, 1-butyl 3-methyl imidazolium bromide and silicotungstic acid (H4 SiW12 O40 ·nH2 O) were taken in 4:1 mole ratio in order to prepare one mole of 1-butyl 3-methyl imidazolium silicotungstate. Ionic liquid was added drop wise to the solution containing silicotungstic acid (SiWA) under constant stirring at room
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T. Rajkumar, G. Ranga Rao / Materials Chemistry and Physics 112 (2008) 853–857
temperature. The white precipitate obtained was washed with distilled water and dried at 80 ◦ C for overnight. It was characterized by CHN analysis which shows that one mole of Keggin anions of [SiW12 O40 ]4− consume four moles of 1-butyl 3-methyl imidazolium cations to form the molecular material. The formula estimated from the C–H–N elemental analysis of the sample is C32.13 H55.85 N8.08 SiW12 O40 which gives an empirical structure of [BmIm]4 [SiW12 O40 ]. This is consistent with 4:1 mole ratio of BmIm cation to [SiW12 O40 ]4− used. The white colour hybrid material has surface area of 0.7 m2 g−1 and has no porous structure. The hybrid material can be dissolved readily in dimethyl sulphoxide (DMSO) and re-crystallized. However, it scarcely dissolves in other standard organic solvents and water. 2.3. Characterization The samples were analysed by X-ray diffraction employing Shimadzu XD-D1 diffractometer using Cu K␣ radiation ( = 1.5418 Å). The IR spectra of samples as KBr pellets were recorded using PerkinElmer infrared spectrometer with a resolution of 4 cm−1 and in the range of 400–4000 cm−1 . Thermogravimetric analysis (TGA) of IL-SiWA hybrid molecular material was conducted in pure N2 gas (30 ml min−1 ) at a heating rate of 20 ◦ C per min on a PerkinElmer TGA-7 instrument. The UV–vis NIR spectra were recorded in diffuse reflectance mode on Jasco V-570 UV-vis NIR spectrophotometer equipped with an integrating sphere in the spectral range of 200–2500 nm. The spectra are presented as F(R), Kubelka–Munk function versus incident photon wavelength. Scanning electron microscopy (SEM) images were taken using a FEI Quanta 200 microscope operating at 30 kV. The sample powders were deposited on a carbon tape before mounting on a sample holder. Fig. 1. FTIR spectra of (a) 1-butyl 3-methyl imidazolium bromide (BmImBr) ionic liquid, (b) H4 SiW12 O40 ·nH2 O and (c) [MmIm]4 [SiW12 O40 ] hybrid molecular material.
3. Results and discussion 3.1. FTIR analysis The FTIR spectra are useful to study the skeletal modes appearing between 700 and 1100 cm−1 of Keggin anions present in the IL-SiWA hybrid molecular material. The parent [SiW12 O40 ]4− Keggin structure shows characteristic bands due to W–O–W vibrations of edge- and corner-sharing WO6 octahedra linked to the central SiO4 tetrahedra [1,20]. Accordingly, the stretching modes of edge sharing (W–Oe –W) and corner sharing (W–Oc –W) units appear between 790 and 890 cm−1 , whereas the Si–O stretching mode appears at 926 and 1014 cm−1 (Fig. 1). The IR spectrum of BmImBr ionic liquid is also presented in Fig. 1, which is consistent with the IR spectrum reported by Wu et al. [19]. It shows the characteristic IR modes at 590–690, 1080–1600 and 2800–3200 cm−1 (alkyl and imidazole ring C–H stretch). In addition to these characteristic bands, both the parent compounds (BmImBr and SiWA) are hydrophilic and show the presence of large amount of water at ∼3450 cm−1 (Table 1) [14,15,19]. However, the water content is reduced significantly in the IL-SiWA hybrid molecular compound, which is discussed in the near IR study in Section 3.5. The IL-SiWA hybrid molecular material is hydrophobic and insoluble in water. The main characteristic vibrations of various modes of organic and inorganic moieties in the IL-SiWA molecular salt are given in Table 1.
The basic structure and geometry of Keggin anions entrapped in the BmIm cations are preserved in the hybrid molecular material. However, a closer examination of IR spectra in the imidazolium ring C–H stretch (3000–3250 cm−1 ) and imidazolium ring stretch (1500–1620 cm−1 ) in Fig. 2 reveals that there is a strong electrostatic interaction between BmIm cation and large Keggin anion [21–23]. Due to this interaction, the two C–H stretching peaks of the imidazolium ring in BmImBr are split into five vibrational peaks in the [MmIm]4 [SiW12 O40 ] molecular salt Fig. 2(A). The imidazolium ring vibration is also split with intensity variations Fig. 2(B). The origin of such C–H split is clearly attributed to specific interaction of BmIm+ ion with bulky Keggin anion. This type of interaction is demonstrated by Jerman and co-workers in the case of MPIm+ I− x compounds [23]. The SEM picture of the [BmIm]4 [SiW12 O40 ] hybrid molecular material is given in Fig. 3 which shows powdery morphology with small clusters of adhered IL-SiWA particles. 3.2. Powder XRD In this work, the bulky anion [SiW12 O40 ]4− , is substituted in place of Br− in the imidazolium-based ionic liquid template. This has lead to the formation of organic–inorganic hybrid solid material. Fig. 4 shows the powder X-ray diffraction (PXRD) pattern of pure silicotungstic acid and IL-SiWA hybrid molecular material.
Table 1 IR bands of BmImBr, H4 SiW12 O40 ·nH2 O and [BmIm]4 [SiW12 O40 ] hybrid molecular material Wavenumber (cm−1 )
Vibration
BmImBr
SiWA
[BmIm]4 [SiW12 O40 ]
3450 3147, 3090 2959, 2936, 2873 1630 1573, 1562 1168 – – – – 840 752 620
3450 – – 1630 – – 1018, 926 980 878 781 – – –
– 3086, 3104, 3112, 3141, 3157 2958, 2933, 2871 – 1562, 1570, 1573 1164 1014, 926 976 883 795 – – 621
Water in the material Imidazole ring (C–H) Aliphatic (C–H) O–H bending Imidazole (ring stretching) Imidazole H–C–C & H–C–N bending (Si–O) (W O) (W–Oc –W) (W–Oe –W) In-plane imidazole ring bending Out-of-plane C–H bending of imidazole ring Imidazole C2 –N1 –C5 bending
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Fig. 2. Imidazole ring C–H stretch (panel A) and imidazole ring stretch (panel B) regions of (a) 1-butyl 3-methyl imidazolium bromide (BmImBr) ionic liquid and (b) [MmIm]4 [SiW12 O40 ] hybrid molecular material.
Using Scherrer’s formula, the average size of the IL-SiWA cluster is estimated to be 18.5 nm. The silicotungstic acid undergoes structural transformation where the four acidic protons and H2 O molecules are replaced by 1-butyl 3-methyl imidazolium cations. The PXRD pattern of IL-SiWA hybrid material reveals different structure compared to pure SiWA salt. The PXRD pattern suggests the existence of host–guest interactions between silicotungstic acid and 1-butyl 3-methyl imidazolium cations in the absence of sandwiched water molecules, which are normally present in the bulk polyoxometalates.
weight loss at 400–580 ◦ C and a minor loss at 580–700 ◦ C. Thermal analysis of polyoxometalates generally shows water loss below 300 ◦ C and the Keggin anion decomposition in the temperature range of 350–600 ◦ C [24,25]. Pure silicotungstic acid decomposes to the constituent oxides around 500 ◦ C which falls in the major decomposition region of IL-SiWA at 400–580 ◦ C. This region is
3.3. TG analysis The thermogravimetric behaviour of IL-SiWA hybrid molecular material is shown in Fig. 5. The TGA curve shows a major
Fig. 3. SEM image of [MmIm]4 [SiW12 O40 ] hybrid molecular material.
Fig. 4. Powder XRD pattern of (a) H4 SiW12 O40 ·nH2 O, and (b) [MmIm]4 [SiW12 O40 ] hybrid molecular material.
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Fig. 5. TGA of [MmIm]4 [SiW12 O40 ] hybrid molecular material.
therefore attributed to the total decomposition of both organic part and the inorganic Keggin anions in the hybrid molecular material. There is no water loss seen below 300 ◦ C indicating that the bonding between BmIm and [SiW12 O40 ]4− eliminates water and four acidic protons associated with bulk silicotungstic acid. The minor loss around 640 ◦ C is attributed to the formation of defective WO3 [25].
Fig. 6. UV–vis DR spectra of (a) BmImBr ionic liquid, (b) H4 SiW12 O40 ·nH2 O and (c) [MmIm]4 [SiW12 O40 ] hybrid molecular material.
3.5. Near IR analysis Near IR spectroscopy can be used to detect water content in polyoxometalates and the hybrid molecular material by studying
3.4. UV–vis DRS analysis UV–vis diffuse reflectance method is often employed to study solids such as dispersed metal oxides, polyoxometalates, layered clays and hybrid materials by measuring d–d, f–d and oxygen–metal ion charge transfer transitions [8,14,15,26,27]. The polyoxometalates in their non-reduced form are generally characterized by ligand to metal charge transfer (LMCT) bands which appear in the UV region between 200 and 400 nm [8,26,27]. When hybrid materials based on polyoxometalates are irradiated with UV light, electrons are excited from the low-energy electronic states (O 2p orbitals) to the high-energy states (metal d orbitals). The energy of the electronic transitions in polyoxometalates depends on the size of the cluster and counter-cation [26]. The UV–vis DR spectra of IL-SiWA hybrid material recorded using integrating sphere are shown in Fig. 6. The electronic spectrum of silicotungstic acid shows three absorption bands at 217, 259 and 330 nm due to LMCT transitions in the Keggin units and there is no electronic transition related to BmImBr ionic liquid in this region. The diffuse reflectance spectrum of the IL-SiWA salt shows the same three transitions in spectrum (c) in Fig. 6. However the position of the bands at 217 and 259 nm remains unchanged while the lowest energy absorption band at 330 nm has become sharp and blue shifted to lower wavelength at 304 nm. This indicates again that there is an interaction between Keggin ions and BmIm cations. The blue shift is attributed to large size of the BmIm cations substituted in place of Br− ions. Due to the reaction with BmImBr, the Keggin anions are also isolated and their polarizing power decreased causing the blue shift (from 330 to 304 nm) in the hybrid molecular material [26,27].
Fig. 7. Near IR spectra of (a) H4 SiW12 O40 ·nH2 O, and (b) [MmIm]4 [SiW12 O40 ] hybrid molecular material.
T. Rajkumar, G. Ranga Rao / Materials Chemistry and Physics 112 (2008) 853–857
overtones and combination bands in the absorption region between 1100 and 2500 nm of water and structural hydroxyl groups [15,28]. Thus the near IR spectra of parent polyoxometalate and the IL-SiWA hybrid materials are recorded by diffuse reflectance mode in the range of 1200–2500 nm, which are shown in Fig. 7. The two prominent bands observed at 1460 and 1961 nm in the near IR spectrum show the presence of crystallization water in H4 SiW12 O40 ·nH2 O. These bands are attributable to the first overtone of hydroxyl groups (2O–H ), and combination bands corresponding to hydroxyl stretching and bending (2O–H + ıH2 O) vibrational modes, respectively, of the water present in silicotungstic acid. The characteristic overtones and combination bands due to H2 O are absent in the IL-SiWA hybrid material (Fig. 7b). The IR peak at 1630 cm−1 due to water is also negligible in the IR spectrum of the hybrid molecular material (Fig. 1c). This corroborates the elimination of water molecules associated with SiWA when reacted with the BmIm counter cations forming the molecular material. 4. Conclusions The hybrid molecular material formed between 1-butyl 3methyl imidazolium bromide ionic liquid and silicotungstic acid has been studied. IR spectrum shows the presence of both 1-butyl 3-methyl imidazolium cations and Keggin anions. The substitution of bulky BmIm cation in place of Br− ion blue shifts the electronic transition from 330 to 304 nm and splits the imidazolium ring C–H stretching modes which confirms the interaction between the organic cations and inorganic Keggin anions in the material. Near IR shows no water present in the hybrid molecular material [BmIm]4 [SiW12 O40 ]. Acknowledgment The DRDO research grant No. ERIP/ER/0300231/M/01/791 is gratefully acknowledged.
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