J. Phys. Chem. B 2008, 112, 16721–16725
16721
Confinement of 1-Butyl-3-methylimidazolium Nitrate in Metallic Silver Marie-Alexandra Neouze* and Marco Litschauer Institute of Materials Chemistry, Vienna UniVersity of Technology, Getreidemarkt 9/165, 1060 Vienna, Austria ReceiVed: September 8, 2008; ReVised Manuscript ReceiVed: October 20, 2008
The structure of the composite material consisting of the ionic liquid 1-butyl-3-methylimidazolium nitrate (BMINO3) entrapped in a silver matrix was investigated. Entrapment is confirmed by combining thermal analysis and spectroscopic investigations and by comparing physicochemical properties of the genuine ionic liquid and the composite BMINO3@Ag. An organization of the ionic liquid molecules toward the silver surface was observed. Introduction Room temperature ionic liquids have attracted for about 10 years intensive interest thanks to their very peculiar features, like polarity or solvent properties as well as negligible vapor pressure.1-6 More recently, promising new composites or hybrid materials were developed based on the association of inorganic species with an ionic liquid.7,8 In some cases the ionic liquid was confined into an inorganic matrix.9-11 Understanding the structure and dynamics of a confined liquid (or gas) is of important fundamental interest.12 As a matter of fact, most of the features observed in bulk are different once the liquid is confined in a matrix.13,14 Ionic liquids confined in a silica matrix already found interesting applications as catalysts11 or as electrolyte.10 Even though bulk ionic liquids are still under intense investigation,15-17 the understanding of the confined state of ionic liquids is an exciting challenge.18 For example, simulation of an ionic liquid, 1,3-dimethylimidazolium chloride, confined between two noncorrugated walls, highlighted the faster diffusion of the ions under confinement compared to the bulk.19 Experiments confirmed those results and proved that molecular confinement is responsible for the stability at high temperatures without significant reduction of the liquidlike ion mobility.20 Moreover, through interaction with the surface, the ionic liquid tends to organize in a “layered system”. A proposed mechanism for the self-assembly of the confined ionic liquid is a π-π stacking, where interaction of the neighboring imidazolium rings plays a key role.21 This phenomenon was also described by means of sum-frequency vibrational spectroscopy.22 In a previous communication,23 we described the preparation of a new composite material, in which an ionic liquid, 1-butyl3-methylimidazolium nitrate (BMINO3), is entrapped in a matrix of metallic silver. The synthesis was carried out by precipitation of the silver in presence of the ionic liquid, following the procedure of Avnir at al.24,25 This work describes the investigation of the structural and physicochemical features of the confined ionic liquid in comparison with bulk or adsorbed 1-butyl-3-methylimidazolium nitrate. Experimental Part 1. Chemicals. All the starting materials were reagent grade and used as purchased. Silver nitrate was acquired from ABCR, * Corresponding author. E-mail:
[email protected].
and 1-bromobutane, N-methylimidazole, and sodium hypophosphite were obtained from Aldrich. 2. Characterization Methods. Nuclear Magnetic Resonance (NMR). Solution NMR spectra were recorded on a Bruker Avance 300 (1H at 300.13 MHz), equipped with a 5 mm inverse-broadband probe head and a z-gradient unit. Scanning Electron Microscopy (SEM) Measurements. The sample was not covered before measurement. SEM measurements were performed on a JEOL 5410 connected to an EDX ¨ NTEC. detector RO ThermograWimetric Analysis (TGA). The analyses were carried out with a Shimadzu TGA-50 at heating rates of 5 °C min-1 under air. Differential Scanning Calorimetry (DSC). For the analysis, on a Mettler Toledo DSC 823e, a few tenths of milligrams of sample was sealed in an aluminum crucible. The temperature program included a first cooling stage, from room temperature to -100 °C at the rate of -50 °C min-1, followed by a heating phase from -100 to 150 °C at the rate of +5 °C min-1. Fourier Transform Infrared (FT-IR) Spectra. The products were pelletized in KBr before measurement. The spectrometer is a Bruker Tensor-27-DTGS equipped with an Interferometer RockSolid and a DigiTect detector system, high-sensitivity DLATGS, using the OPUS software. N2 adsorption-desorption at 77 K was carried out on a Micromeritics ASAP 2020 analyzer. Before the measurement was carried out, the sample was degassed under vacuum at 150 °C overnight. For the Brunauer-Emmett-Teller (BET) calculation, the molecular cross-sectional area of nitrogen was considered as 0.162 nm2. 3. Synthesis. Synthesis of 1,3-Butylmethylimidazolium Bromide (BMIBr). Without solvent, 6.439 g (47 mmol) of butyl bromide was added dropwise under stirring at room temperature to 3.85 g of N-methylimidazole. After complete addition, the product was heated at 100 °C overnight. Synthesis of 1,3-Butylmethylimidazolium Nitrate (BMINO3)..26 1H NMR: (300 MHz, CD3CN): δ 8,54 (s, 1H, NCH-N); 7,70 (d, 1H, CH3-N-CH); 7,61 (d, 1H, CHN-CH2-); 3,93 (s, 3H, CH3-N); 4,23 (t, 2H, N-CH2-CH2); 1,82 (q, 2H, N-CH2-CH2-); 1,27 (s, 2H, -CH2-CH3); 0,89 (t, 3H, -CH2-CH3). Precipitation of SilWer in the Presence of the Ionic Liquid (IL@Ag). 125 mL of aqueous solution of 3.03 g (0.018 mol) AgNO3, 1.504 g (0.017 mol) of sodium hydrophosphite, and 0.0292 (0.14 mmol) of BMINO3 were mechanically stirred at
10.1021/jp8079606 CCC: $40.75 2008 American Chemical Society Published on Web 12/02/2008
16722 J. Phys. Chem. B, Vol. 112, No. 51, 2008
Neouze and Litschauer
Figure 1. SEM (left) and EDX (right) analysis of the hybrid material BMINO3@Ag.
room temperature for 2 days. After this period, a dark precipitate of metallic silver was observed. Extracting the Ionic Liquid from IL@Ag. An amount of 2 g of the composite material, BMINO3@Ag, was dispersed in 10 mL of acetonitrile, stirred at room temperature for 1 h, and then dipped in ultrasound bath for 10 min (2 s pulses, instantaneous power 12W). The solid was afterward centrifuged and washed 2 times with 5 mL of acetonitrile. Then the washing acetonitrile fractions were combined and concentrated under stirring at 70 °C overnight. Results and Discussion A previous communication described the synthesis and characterization of a new composite material, BMINO3@Ag. The composite results from the physical entrapment of an ionic liquid (IL) in a silver matrix.23 The entrapment method consists of reducing the silver precursor by sodium hydrophosphite (eq 1) in the presence of the organic species to be entrapped:24,25
Ag+ + H2PO2- + H2O + IL f IL@Ag + H2PO3 + 2H+
(1) The choice of the ionic liquid was based on two specific points. First, the ionic liquid had to be water soluble in order to start the reduction reaction from a homogeneous solution. The second criterion was to prevent the possibility of anion exchange between silver nitrate, and the ionic liquid anion. For both these reasons, the considered ionic liquid was 1-butyl-3methylimidazolium nitrate, BMINO3. The texture of the obtained composite material was the same as that of the silver precipitate without IL (Figure 1, left). The size of the silver crystallites was determined by the Scherrer equation at 80 ( 5 nm from XRD. Combining EDX and SEM analysis, the organic moiety was detected at low energy. The theoretical energy values for C, N, and O are 0.29, 0.36, and 0.51 keV, respectively. Consequently, the nitrogen and carbon atoms are not distinguishable from, both peaks overlapping (Figure 1, right). However, the EDX analysis was completed by TGA, which allows quantifying the amount of ionic liquid in the composite. BMINO3@Ag contains 1.5 wt % ionic liquid (Figure 2, line). When comparing the thermal behavior of the composite and the pure ionic liquid (Figure 2), it appeared that the entrapment of BMINO3 within the matrix enhances the onset temperature
Figure 2. Thermogravimetric analysis of (crosses) pristine BMINO3 and (line) BMINO3@Ag.
of its thermal decomposition. The shift in the decomposition’s onset is about 50 °C. In order to get an insight to the structure of the material, the composite was washed with acetonitrile to remove the entrapped ionic liquid, (BMINO3@Ag)extract (Scheme 1, left). The extraction of the ionic liquid from the composite IL@Ag was qualtitatively controlled by performing 1H NMR spectroscopy on the washing solvent, (acetonitrile)extract (Figure 3). BMINO3 was recognized in the spectrum, which proves the efficiency of the washing procedure. The two additional peaks at 1.94 and 2.27 ppm are due to an exchange between H and D atoms occurring between (acetonitrile)extract and deuterated acetonitrile used to lock the NMR signal. A thermogravimetric analysis of the washed solid product, (BMINO3@Ag)extract, was additionally carried out (Figure 4). At similar temperature as for BMINO3@Ag, between 250 and 350 °C, a weight loss of 0.07 ( 0.01 wt % can be detected (Figure 5, solid line). This indicates that a part of the entrapped ionic liquid is located in closed cavities and therefore not accessible to the solvent. The percentage of ionic liquid in closed cavities equals 4.3 mol % of the BMINO3 originally present in the composite. A BET calculation was performed using the N2 adsorption results obtained for (BMINO3@Ag)extract with relative pressure values (P/P0) in the range 0.05-0.35. In the given equation, n represents the specific amount of nitrogen adsorbed at the
Confinement of BMINO3 in Metallic Silver
J. Phys. Chem. B, Vol. 112, No. 51, 2008 16723
SCHEME 1: Extraction of the Ionic Liquid from the Composite (left); Adsorption of IL (right)
equilibrium pressure P. The linear (BMINO3@Ag)extract is reported in eq 2.
fit
( )
1 P ) -2.4 + 39.3 P0 P0 n -1 P
(
)
obtained
for
(2)
The correlation coefficient of the BET linear fit is only 0.94, and the intercept of the Y-axis is slightly negative (Yintercept ) -2.4 ( 1.5 g · cm-3). In consequence, a relatively large uncertainty is linked to this calculation. This leads to an average diameter for the open pores of 39 ( 10 nm.
Figure 3. 1H NMR spectrum of (acetonitrile)extract. Inset: numbering scheme of the ionic liquid cation.
Figure 4. Thermogravimetric analysis of (dashed line) BMINO3@Ag; (solid line) (BMINO3@Ag)extract.
Considering the physicochemical properties of the IL, a strong difference was also observed between neat (Figure 5, A) and entrapped (Figure 5, B) BMINO3 in FTIR spectra. Two sets of bands can be distinguished. Some vibration bands increase in intensity and were shifted toward higher energies, while other bands almost disappeared due to confinement of BMINO3. The band at 1563 cm-1 (in-plane C-C and C-N stretching vibrations of the imidazolium ring) was broadened and shifted toward 1639 cm-1 through confinement (Figure 5, stars). Similarly, the band at 1164 cm-1 (in plane C-H deformation vibration of the imidazolium ring) became stronger and is broadened (Figure 5, triangles). Moreover, the very small band at 858 cm-1 (C-H in plane vibration of the imidazolium ring) strong increased in intensity and was shifted toward 899 cm-1 (Figure 5, arrows). In parallel, the broadband centered at 1380 cm-1 attributed to the C-H bending vibrations of CH3 (Figure 5, solid circle) strongly decreased, and the two bands at 3095 and 2940 cm-1, corresponding to the C-H symmetrical stretching vibration of the imidazolium ring and the alkyl chain, almost disappeared (Figure 5, dashed circle). Obtaining an abnormal FT-IR spectrum for an ionic liquid due to physical entrapment was reported by Deng et al.11,27 This comparison of the FTIR spectra emphasizes an important point: the bands that became stronger due to the confinement belong to the imidazolium rings. This is a clear indication that the ions of the ionic liquid are partially ordered. This ordering of the ions toward a charged surface parallels the formation of a Stern layer next to a surface.28 Ion ordering was already reported for some dialkylimidazolium species confined in silica.21,22 Antonietti et al. described the structuration of an ionic liquid when entrapped. The FTIR signature of the imidazolium
Figure 5. FTIR spectrum of (A) neat ionic liquid BMINO3 and (B) composite BMINO3@Ag.
16724 J. Phys. Chem. B, Vol. 112, No. 51, 2008
Neouze and Litschauer SCHEME 2: Silver Matrix (Dark Filling) Containing Ionic Liquid (Hatched Filling) Directly in Contact or (Plain Light Filling) without Contact to the Silver Surface
Figure 6. Differential scanning calorimetry of (dashed line) genuine BMINO3 and (straight line) entrapped BMINO3.
moieties organization was a broadening and shifting of the vibration bands belonging to the imidazolium rings, induced by their rapprochement.21 The confinement influences also the transitions observed for BMINO3 by differential scanning calorimetry. The measurements were carried out in the temperature range -100 °C to +150 °C. In this temperature range, no degradation of the ionic liquid takes place. Heating pure silver also induced some slight variation of the measured heat flow. For this reason, in order to isolate the heat flow of the confined BMINO3 (entrapped BMINO3), the normalized influence of the silver matrix (Ag) was subtracted (eq 3):
heat flow (entrapped BMINO3) ) heat flow (BMINO3@Ag) - heat flow (Ag)(3) In agreement with reported studies on imidazolium ionic liquids,3,9 three transitions can be distinguished for the pure ionic liquid BMINO3. The second-order transition at -85 °C corresponds to a glass transition. The exothermic first-order transition at -19 °C corresponds to a crystallization phenomenon, and the endothermic first-order transition at +22 °C is referred to as a melting. The calculated heat flow variation of the entrapped BMINO3 (Figure 6, line with triangles) was compared to the one of the genuine ionic liquid (Figure 6, line with plain circles). For the entrapped ionic liquid, two sets of first-order transitions can be distinguished. A first set, from -100 to -50 °C, includes two sharp transitions, an exothermic event at -85 °C and an endothermic at -72 °C. Those transitions are very similar to the one observed at higher temperature for pure BMINO3. A lower shift of the transitions’ temperature was already reported for ionic crystals confined in silica matrix,29-31 as well as for ionic liquids confined in silica matrix.9 Gubbins et al. reported that the shift in the temperature toward either higher or lower temperature depends on the relative interactions within the system.32 The first-order transition’s temperature will be lowered if the interaction confined fluid/confined fluid is higher than the interaction wall/confined fluid. This indicates that for the composite BMINO3@Ag the organization of the imidazolium rings via confinement induces an IL/IL interaction higher than the IL/Ag interaction. Additionally, another set of first-order transitions was also observed, overlapping with the sharp transitions of the neat ionic liquid, i.e., a large exothermic peak at -19 °C and a large endothermic peak at +29 °C.
This phenomenon can be explained by the fact that the ionic liquid molecules next to the cavity walls are strongly influenced by the surrounding surface (Scheme 2, hatched filling). The transitions are thus shifted. This is also the case for moieties confined in small pores. Additionally, the ionic liquid molecules in the center of the cavities behave like pure ionic liquid (Scheme 2, plain filling). The FTIR study (Figure 5) showed large changes for the entrapped ionic liquid, which indicates that most of the entrapped molecules interact with the surface. A new issue arises from this observation: whether the interactions between the ionic liquid and the matrix originate from the entrapment in the silver matrix or from the adsorption on the silver surface. To clarify this point, a DSC measurement was carried out on the washed sample, (BMINO3@Ag)extract, to which pure ionic liquid BMINO3 was added ((BMINO3@Ag)extract + IL) (Scheme 1, right). To render this experiment comparable to the previous one (Figure 6), the molar ratio between added BMINO3 and silver (BMINO3@Ag)extract is the same as for the composite BMINO3@Ag. All the transitions already described for neat BMINO3 (Figure 6), viz., glass transition, crystallization, and melting, are observed anew for ((BMINO3@Ag)extract + IL) with a slight broadening of the peaks (Figure 7). Thus, when adsorbed onto the outer surface of the silver matrix, the ionic liquid behavior is completely different as the one observed for the composite BMINO3@Ag. Additionally, in the DSC curve of (BMINO3@Ag)extract, the first-order transitions, crystallization and melting, had disappeared (Figure 7, line with squares). Only a glass transition remained, onset at -85 °C as in the case of the neat ionic liquid (Figure 6). This glass transition revealed BMINO3 molecules located in closed cavities. Indeed, as already reported for
Figure 7. Differential scanning calorimetry of (straight line) (BMINO3@Ag)extract and (dashed line) ((BMINO3@Ag)extract + IL).
Confinement of BMINO3 in Metallic Silver confined ionic liquids, once the pores are becoming too small, only the glass transition remains observable.9 This observation, in combination with FTIR analysis and BET calculation, ensures unambiguously that the observed phenomena are due to entrapment of BMINO3 within the silver matrix and not simply adsorption of the BMINO3 on the surface of the solid. Conclusion The precipitation of silver from a solution containing 1-butyl3-methylimidazolium nitrate leads to the entrapment of the ionic liquid into the silver matrix. FTIR, TGA, and DSC analysis of the composite IL@Ag reveal strong changes of the ionic liquid’s physicochemical features due to the entrapment. Thus, the thermal stability of the IL in the composite is higher than of neat BMINO3, and the first-order transitions are shifted toward lower temperatures. The FTIR spectrum of the composite indicates an increased interaction between the imidazolium rings of the cation due to the confinement. DSC investigations also prove that the confinement is indeed physical entrapment and not adsorption on the outer surface. Acknowledgment. The authors thank Professor Ulrich Schubert and Professor David Avnir for their support to this project. The SEM measurements were carried out by Elisabeth Eitenberger (Institute of Chemical Technologies and Analytics, Vienna University of Technology). References and Notes (1) Debdab, M.; Mongin, F.; Bazureau, J. P. Synthesis 2006, 4046. (2) Endres, F., Abbott, A. P., MacFarlane D. R., Eds.; Electrodeposition from Ionic Liquids; Wiley: Weinheim, Germany, 2008. (3) Wasserscheid, P., Welton T., Eds.; Ionic Liquids in Synthesis; Wiley: Weinheim, Germany, 2003. (4) Antonietti, M.; Kuang, D.; Smarsly, B.; Zhou, Y. Angew. Chem. 2004, 43, 4988. (5) Abbott, A. P.; McKenzie, K. J. Phys. Chem. Chem. Phys. 2006, 8, 4265.
J. Phys. Chem. B, Vol. 112, No. 51, 2008 16725 (6) Luo, H.; Baker, G. A.; Dai, S. J. Phys. Chem. B 2008, 112, 10077. (7) Taubert, A. Acta Chim. SloVen. 2005, 52, 183. (8) Taubert, A.; Li, Z. Dalton Trans. 2007, 723. (9) Neouze, M.-A.; Le Bideau, J.; Gaveau, P.; Bellayer, S.; Vioux, A. Chem. Mater. 2006, 18, 3931. (10) Neouze, M.-A.; Le Bideau, J.; Leroux, F.; Vioux, A. Chem. Commun. 2005, 1082. (11) Shi, F.; Zhang, Q.; Li, D.; Deng, Y. Chem. Eur. J. 2005, 11, 5279. (12) Bras, A. R.; Dionisio, M.; Schonhals, A. J. Phys. Chem. B 2008, 112, 8227. (13) Naumov, S.; Valiullin, R.; Monson, P. A.; Kaerger, J. Langmuir 2008, 24, 6429. (14) Patil, S.; Matei, G.; Oral, A.; Hoffmann, P. M. Los Alamos National Laboratory, Preprint Archive, Condensed Matter, 2005; p 1. (15) Remsing, R. C.; Liu, Z.; Sergeyev, I.; Moyna, G. J. Phys. Chem. B 2008, 112, 7363. (16) Tsuzuki, S.; Arai, A. A.; Nishikawa, K. J. Phys. Chem. B 2008, 112, 7739. (17) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 16593. (18) Le Bideau, J.; Gaveau, P.; Bellayer, S.; Neouze, M. A.; Vioux, A. Phys. Chem. Chem. Phys. 2007, 9, 5419. (19) Pinilla, C.; Del Popolo, M. G.; Lynden-Bell, R. M.; Kohanoff, J. J. Phys. Chem. B 2005, 109, 17922. (20) Shimano, S.; Zhou, H.; Honma, I. Chem. Mater. 2007, 19, 5216. (21) Zhou, Y.; Schattka, J. H.; Antonietti, M. Nano Lett. 2004, 4, 477. (22) Fitchett, B. D.; Conboy, J. C. J. Phys. Chem. B 2004, 108, 20255. (23) Neouze, M.-A.; Litschauer, M. Aust. J. Chem. 2008, 61, 329. (24) Behar-Levy, H.; Avnir, D. Chem. Mater. 2002, 14, 1736. (25) Behar-Levy, H.; Avnir, D. AdV. Funct. Mater. 2005, 15, 1141. (26) Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Phys. Chem. Chem. Phys. 2001, 3, 5192. (27) Shi, F.; Deng, Y. Spectrochim. Acta A 2005, 62, 239. (28) Brinker, C.; Scherer, G. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA, 1990. (29) Fehr, C.; Dieudonne, P.; Primera, J.; Woignier, T.; Sauvajol, J. L.; Anglaret, E. Eur. Phys. J. E 2003, 12, S13. (30) Fehr, C.; Goze-Bac, C.; Anglaret, E.; Benoit, C.; Hasmy, A. Europhys. Lett. 2006, 73, 553. (31) Woignier, T.; Primera, J.; Lamy, M.; Fehr, C.; Anglaret, E.; Sempere, R.; Phalippou, J. J. Non-Cryst. Solids 2004, 350, 299. (32) Alba-Simionesco, C.; Coasne, B.; Dosseh, G.; Dudziak, G.; Gubbins, K. E.; Radhakrishnan, R.; Sliwinska-Bartkowiak, M. J. Phys.: Condens. Matter 2006, 18, R15.
JP8079606
