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Articles Variation of Electron-Donating Ability of Smectites as Probed by Photoreduction of Methyl Viologen Norishige Kakegawa,† Toshimitsu Kondo,‡ and Makoto Ogawa*,‡ Graduate School of Science and Engineering, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan, and Department of Earth Sciences, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan Received September 5, 2002. In Final Form: November 15, 2002 The preparation of methyl viologen-smectite (synthetic saponite, Sumecton SA; synthetic hectorite, Laponite XLG; and natural montmorillonite, Kunipia F) intercalation compounds and the photoinduced electron transfer from smectite to methyl viologen were investigated. UV-vis and electron spin resonance spectra revealed that the methyl viologen intercalated in Sumecton SA and Laponite were photoionized by UV irradiation while photochemical reduction of methyl viologen did not occur in the interlayer space of Kunipia F, indicating that Sumecton SA and Laponite acted as electron donors.
Introduction Methyl viologen has received much attention as a redox catalyst and for electron relays for solar energy conversion and storage.1,2 Photoreduction of viologen embedded on various solid matrixes such as silica gels,3,4 zeolites,5-9 clay minerals,10-12 layered transition metal oxides,13-19 and layered zirconium phosphonates20,21 has been studied in order to construct a photoredox system. It was shown * Corresponding author. Tel: 81-3-5286-1511. Fax: +81-3-32074950. E-mail:
[email protected]. † Graduate School of Science and Engineering. ‡ Department of Earth Sciences. (1) Sliwa, W.; Bachowska, B.; Zelichowicz, N. Heterocycles 1991, 32, 2241-2273. (2) Monk, P. M. S. The viologens: Physical properties, synthesis and applications of the salts 4,4′-bipyridine; Monk, P. M. S., Ed.; Wiley: New York, 1998. (3) Mao, Y.; Breen, N. E.; Thomas, J. K. J. Phys. Chem. 1995, 99, 9909-9917. (4) Xiang, B.; Kevan, L. J. Phys. Chem. 1994, 98, 5120-5124. (5) Macmanus, H. J. D.; Finel, C.; Kevan, L. Radiat. Phys. Chem. 1995, 45, 761-764. (6) Yoon, K. B.; Kochi, J. K. J. Am. Chem. Soc. 1988, 110, 65866588. (7) Alvaro, M.; Garcia, H.; Marquez, F.; Scaiano, J. C. J. Phys. Chem. B 1997, 101, 3043-3051. (8) Ranjit, K. T.; Kevan, L. J. Phys. Chem. B 2002, 106, 1104-1109. (9) Park, Y. S.; Um, S. Y.; Yoon, K. B. J. Am. Chem. Soc. 1999, 121, 3193-3200. (10) Miyata, H.; Sugahara, Y.; Kuroda, K.; Kato, C. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1851-1858. (11) Villemure, G.; Kodama, H.; Detellier, C. Can. J. Chem. 1985, 63, 1139-1142. (12) Detellier, C.; Villemure, G. Inorg. Chim. Acta 1984, 86, 19-20. (13) Nakato, T.; Kuroda, K.; Kato, C. Catal. Today 1993, 16, 471478. (14) Nakato, T.; Kuroda, K.; Kato, C. Chem. Mater. 1992, 4, 128132. (15) Nakato, T.; Sugahara, Y.; Kuroda, K.; Kato, C. Mater. Res. Soc. Symp. Proc. 1991, 233, 169-173. (16) Nakato, T.; Kuroda, K.; Kato, C. J. Chem. Soc., Chem. Commun. 1989, 1144-1145. (17) Nakato, T.; Miyata, H.; Kuroda, K. React. Solids 1988, 6, 231238. (18) Miyata, H.; Sugahara, Y.; Kuroda, K.; Kato, C. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2677-2682. (19) Bose, A.; He, P.; Liu, C.; Ellman, B. D.; Twieg, R. J.; Huang, S. D. J. Am. Chem. Soc. 2002, 124, 4-5. (20) Vermeulen, L. A.; Thompson, M. E. Nature 1992, 358, 656-658.
that photoreduction of viologen and the stability of the formed viologen radical cation were affected by the hostguest and guest-guest interactions. However, the dominant controlling factors in the photochemistry of viologens are yet to be investigated. Therefore, further studies on photoreduction in solids are worth investigating for controlling the photochemical reactions and for understanding the role of matrixes in the photochemistry of viologens. Smectite, which is a group of layered clay minerals, is composed of negatively charged silicate layers and exchangeable cations, which are located in the interlayer space.22 Smectite possesses large surface area and ionexchange properties, which can be utilized for organizing guest species, including organic dyes, in the twodimensional expandable interlayer space.23 A variety of oxidation-reduction reactions in smectite-organic systems have been investigated. Charge transfer from adsorbed aromatic molecules such as benzidine to smectite has been reported, and the charge transfer was affected by the chemical composition or the crystal edge of smectites.22,24 In contrast, there are few studies on charge transfer from smectite to organic species except for the studies on reduced clays, whose ferric ions were reduced prior to the complexation.25 Charge transfer from benzidine to smectites results in metachromasy of benzidine.22,24,25 Metachromasy of acridine orange and tetracyanoethylene adsorbed on smectites was reported for charge transfer from smectites to adsorbed organic dyes.26 It has been proposed that the charge transfer from smectites to adsorbed organic species is affected by isomorphous substitution in tetrahedral sheets and whether the sample is di- or tri-octahedral.26,27 (21) Vermeulen, L. A.; Snover, J. L.; Sapochak, L. S.; Thompson, M. E. J. Am. Chem. Soc. 1993, 115, 11767-11774. (22) Theng, B. K. G. The Chemistry of Clay Organic Reactions; Adam Hilger: London, 1974. (23) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399-438. (24) Theng, B. K. G. Clays Clay Miner. 1971, 19, 383-390. (25) Stucki, J. W.; Bailey, G. W.; Gan, H. Appl. Clay Sci. 1996, 10, 417-430. (26) Garfinkel-Shweky, D.; Yariv, S. J. Colloid Interface Sci. 1997, 188, 168-175.
10.1021/la020763v CCC: $25.00 © 2003 American Chemical Society Published on Web 04/04/2003
Electron-Donating Ability of Smectites
Langmuir, Vol. 19, No. 9, 2003 3579 Table 1. Host Materials Used in This Study
host
chemical compounds
Na-montmorillonite (Kunipia F) Na-saponite (Sumecton SA) Na-hectorite (Laponite XLG)
(Na0.53Ca0.09)+0.71(Si7.65Al0.35)(Mg0.43FeIII/II0.31Al3.28)O20(OH)4-0.71 (Na0.49Mg0.14)+0.77(Si7.20Al0.80)(Mg5.97Al0.03)O20(OH)4-0.77 (Na0.70)+0.70Si8.00(Mg3.50Li0.30)O20(OH)4-0.70
host
cation exchange capacity (mequiv/100 g clay)
Na-montmorillonite (Kunipia F) Na-saponite (Sumecton SA) Na-hectorite (Laponite XLG) a
119b 71c 63
diameter of particlesa g1.00 µm g0.01 µm g0.02 µm
Reference 30. b Reference 28. c Reference 29.
In this paper, we demonstrate photoinduced electron transfer from smectites (synthetic saponite, Sumecton SA; synthetic hectorite, Laponite XLG; and natural montmorillonite, Kunipia F) to methyl viologen. Photoreduction of viologen adsorbed on smectites has been conducted previously in the presence of added electron donors10-12 such as poly(vinylpyrrolidone).10 There are few papers on photoinduced chemical reactions between smectites and organic species such as pyrene and dioxin adsorbed on smectites.28-31 Electron-donating ability of smectites has not been reported, while zeolites are known to act as electron donors and photoinduced electron transfer from alkali ion-exchanged zeolites to methyl viologen has been reported.5-9 It was presumed that the electron-donating ability of smectite was affected by the structure and particle size of smectites and found that the methyl viologen intercalated in Sumecton SA and Laponite XLG was photoionized by UV irradiation while photochemical reduction of methyl viologen did not occur in the interlayer space of Kunipia F. Experimental Section Materials. The following host materials were used in this study (Table 1). Sodium-montmorillonite (Kunipia F, Kunimine Industries Co.; reference clay sample of The Clay Science Society of Japan), sodium-saponite (Sumecton SA, Kunimine Industries Co.; reference clay sample of the Clay Science Society of Japan), and sodium-hectorite (Laponite XLG, Rockwood Additives Ltd.) were used as the host materials. Methyl viologen, N,N′-dimethyl4,4′-bipyridinium (abbreviated as MV2+) dichloride (Tokyo Kasei Ind. Co.), was used as received. Preparation of Methyl Viologen-Smectite Intercalation Compounds. Smectites were dispersed in methyl viologen aqueous solution, and the suspensions were stirred for 24 h. The smectite/water weight ratio was 1/100. The amounts of added methyl viologen dichloride were just equal to the cation exchange capacity of the smectites. After the ion exchange, the product was washed with deionized water repeatedly until a negative AgNO3 test was obtained. The suspensions were centrifuged at 4000 rpm for 15 min, and the precipitates were dried under reduced pressure. Photochemical Reactions. Methyl viologen-smectite intercalation compounds (abbreviated as MV2+-Sumecton SA, MV2+-Laponite XLG, and MV2+-Kunipia F) were irradiated using a 500 W high-pressure mercury lamp (USH-500D, Ushio Inc.) under an air atmosphere at room temperature. The distance between the lamp and the sample was 15 cm. During the UV irradiation, the powdered products were stirred to ensure homogeneous irradiation over the whole sample body. Characterization. X-ray powder diffraction (XRD) patterns of the products were recorded on a Rigaku RAD-IA diffractometer using monochromatized Cu/KR radiation. Thermogravimetric(27) Yariv, S. Int. Rev. Phys. Chem. 1992, 11, 345-375. (28) Iu, K.-K.; Liu, X.; Thomas, J. K. Chem. Phys. Lett. 1991, 186, 198-203. (29) Liu, X. S.; Thomas, J. K. Langmuir 1991, 7, 2808-2816. (30) Liu, X. S.; Iu, K. K.; Thomas, J. K. Langmuir 1992, 8, 539-545. (31) Mao, Y.; Pankasem, S.; Thomas, J. K. Langmuir 1993, 9, 15041512.
Figure 1. XRD patterns of Sumecton SA (a), MV2+-Sumecton SA (b), Laponite XLG (c), MV2+-Laponite XLG (d), Kunipia F (e), and MV2+-Kunipia F (f). differential thermal analysis (TG-DTA) curves were obtained on a Rigaku TAS-2000 instrument at the heating rate of 10 °C min-1 under an air atmosphere and with R-alumina as the standard. Diffuse reflectance UV-vis absorption spectra of the products were recorded on a Shimadzu UV-3100PC spectrophotometer. The amounts of adsorbed methyl viologen were determined by CHN analysis (Yanaco MT-3). Electron spin resonance (ESR) spectra were recorded at room temperature at 9.4 GHz using a JEOL JES-TE200 spectrometer with 100 kHz field modulation. The yields of methyl viologen radical cation (abbreviated as MV•+) were determined by double integration of the ESR spectra using the JES-TE200 software. 4-Hydroxyl-TEMPO benzene solution was used as a standard sample for quantitative analysis for ESR spectra.
Results and Discussion Preparation of Methyl Viologen-Smectite Intercalation Compounds. The XRD patterns of the smectites and the products prepared by the reactions between smectites and MV2+ are shown in Figure 1. The basal spacings were 1.3, 1.3, 1.3, 1.3, 1.2, and 1.3 nm for Sumecton SA, MV2+-Sumecton SA, Laponite XLG, MV2+Laponite XLG, Kunipia F, and MV2+-Kunipia F, respectively. The thicknesses of the interlayer space of MV2+smectite intercalation compounds were determined to be 0.3 nm by subtracting the thickness of the silicate layer (ca. 1.0 nm) from the observed basal spacings. This value was close to the values reported for both the MV2+smectites35-38 and hydrated Na+-smectites (Figure 1).
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Figure 2. Diffuse reflectance UV-vis absorption spectra of MV2+-Sumecton SA before (a) and after (b) UV irradiation.
Kakegawa et al.
Figure 4. Diffuse reflectance UV-vis absorption spectra of MV2+-Kunipia F before (a) and after (b) UV irradiation.
Figure 3. Diffuse reflectance UV-vis absorption spectra of MV2+-Laponite XLG before (a) and after (b) UV irradiation.
The products and those of hydrated smectites were heated at 180 °C under reduced pressure. The basal spacing of MV2+-smectites did not change just after the heat treatment (1.3 nm), while that of the Na+-smectites decreased to 1.0 nm showing dehydration. These results confirmed the intercalation of MV2+ in the interlayer space. The carbon contents of the products, which were obtained by CHN analysis, were 4, 4, and 7 mass % for MV2+-Sumecton SA, MV2+-Laponite XLG, and MV2+Kunipia F, respectively. The amounts of the adsorbed methyl viologen cation were determined by the content to be C wt %, and the smectite wt % was 66, 61, and 116 mequiv/100 g of clay for MV2+-Sumecton SA, MV2+Laponite XLG, and MV2+-Kunipia F, respectively. The amounts of adsorbed MV2+ agreed with the ion-exchange capacity of smectites (Table 1), indicating a quantitative ion-exchange reaction between sodium ions and MV2+. Diffuse reflectance spectra of the MV2+-smectites are shown in Figures 2-4. In the spectra of the MV2+smectites (Figures 2a, 3a, and 4a), absorption bands due to the π-π* transition of MV2+ were observed at 275 nm. This absorption band is much broader and shifted from that observed for MV2+ in aqueous solution (257 nm). On the other hand, λmax values were the same irrespective of the host (Figures 2a, 3a, and 4a). This red shift reflects that the clay surface provides a different polarity environment compared with the water solvent. The other (32) Sugahara, Y.; Kuroda, K.; Kato, C. J. Am. Ceram. Soc. 1984, 67, C-247-248. (33) Ogawa, M.; Nagafusa, Y.; Kuroda, K.; Kato, C. Appl. Clay Sci. 1992, 7, 291-302. (34) Miyamoto, N.; Kawai, R.; Kuroda, K.; Ogawa, M. Appl. Clay Sci. 2000, 16, 161-170. (35) Rytwo, G.; Nir, S.; Margulies, L. Soil Sci. Soc. Am. J. 1996, 60, 601-610. (36) Raupach, M.; Emerson, W. W.; Slade, P. G. J. Colloid Interface Sci. 1979, 69, 398-408. (37) Weber, J. B.; Perry, P. W.; Upchurch, R. P. Soil Sci. Soc. Proc. 1965, 29, 678-688. (38) Knight, B. A. G.; Denny, P. J. Weed Res. 1970, 10, 40-48.
Figure 5. ESR spectra of MV2+-smectites before (a,c,e) and after (b,d,f) UV irradiation.
reason for the red shift is the difference in the planarity of the rings, and/or molecular orbital distortions owing to the confinement in a restricted space.35,39-42 When the molecule is more planar, the number of functional π electrons is maximized, causing a red shift in the π-π* transitions.43 Taking into account the gallery height of the products (0.3 nm), the thickness of the pyridine ring (0.3 nm), and the spectral red shift, methyl viologen dications were intercalated in the interlayer space of smectites and their pyridine rings were arranged parallel to the silicate layer.35 Formation of MV•+ by UV Irradiation. MV2+smectites were colorless (Figures 2a, 3a, and 4a) and ESR silent (Figure 5a,c,e). After UV light irradiation, MV2+Kunipia F was still colorless (Figure 4b) and ESR silent (39) Villemure, G.; Detellier, C.; Szabo, A. G. Langmuir 1991, 7, 1215-1221. (40) Villemure, G.; Detellier, C.; Szabo, A. G. J. Am. Chem. Soc. 1986, 108, 4658-4659. (41) Hayes, M. H. B.; Pick, M. E.; Toms, B. A. Residue Rev. 1975, 57, 1-25. (42) Haque, R.; Lilley, S.; Coshow, W. R. J. Colloid Interface Sci. 1970, 33, 185-188. (43) Berlmann, B. I. J. Phys. Chem. 1970, 74, 3085-3093.
Electron-Donating Ability of Smectites
(Figure 5e). On the contrary, MV2+-Sumecton SA changed its color to dark brown-green (Figure 2b) and showed a symmetric single ESR signal (Figure 5b) with g ) 2.004 and a peak-to-peak line width of 8.0 G after the UV irradiation. MV2+-Laponite XLG changed its color to yellow (Figure 3b) and showed a weak symmetric single ESR signal (Figure 5d) compared with the ESR signal of MV2+-Sumecton SA after the UV irradiation. This ESR signal was typical of the methyl viologen radical cation.2 Although the diffuse reflectance absorption spectrum of the MV•+-Sumecton SA is not identical to that of the typical MV•+, the new band seen at 600-700 nm in the spectrum of the MV2+-Sumecton SA after UV irradiation (Figure 2b) is similar to that of MV•+ in water detected by the transient absorption spectrum.44 The λmax values of MV•+ in the transient absorption spectrum were 388 and 644 nm. Relative to the spectrum of MV•+ in methanol solution, the absorption band appearing at 388 nm is blueshifted, and the absorption band appearing at 644 nm is red-shifted.44 This result suggested that water molecules affected the electronic states of MV•+. The half-life of ESR intensity for MV•+ adsorbed on Sumecton SA was 4 months, when kept in the dark at room temperature after UV irradiation. The absorption maximum of the new band seen at 600-700 nm in the spectrum of the MV2+Sumecton SA after UV irradiation was decreased with the ESR intensity. The new bands seen at 300-500 nm in the spectra of the MV2+-smectites after the UV irradiation (Figures 2b and 3b) were attributed to oxides of MV2+ 45 or the protonated form of MV•+.46 Especially, UV-irradiated MV2+-Sumecton SA (Figure 2b) emitted visible light (520 nm) using UV light (351 nm), indicating the formation of the 1′,2′-dihydroxo-1′,1′-dimethyl-2′-oxo-4,4′-bypyridium cation.45 Electron-Donating Ability of Smectites. UV-vis and ESR spectra revealed that methyl viologen intercalated in Sumecton SA and Laponite XLG was photoionized by UV irradiation while photochemical reduction of methyl viologen did not occur in the interlayer space of Kunipia F. The amount of MV•+ was determined by ESR to be 0.73 mmol/100 g clay on Sumecton SA, which is larger than that (0.16 mmol/100 g clay) on Laponite XLG. Taking into account the adsorbed methyl viologen amount determined by the content to be C mass %, the molar ratios of formed MV•+ to adsorbed MV2+ were 1.1 and 0.3% for MV2+Sumecton SA and MV2+-Laponite XLG, respectively. These results suggested that the electron-donating ability of smectites was affected by the structures of the smectites, and we proposed that two electron donor sites exist on the smectite surface (Figure 6). The substitution in the tetrahedral sites (Si4+ f Al3+) of the framework of the clays may generate an electrondonating site. A similar isomorphous substitution in zeolites has been thought to be an electron donor of MV2+.5-9 Bridging Si-O-Al oxygens have the higher electron density sites of the framework of Sumecton SA. Hence the excited MV2+ abstracted one electron from a bridging Si-O-Al oxygen lone pair. Sumecton SA has a lot of substitution in the tetrahedral sites compared with other hosts (Table 1), so that the amount of formed MV•+ adsorbed on Sumecton SA was the largest of any tested (44) Peon, J.; Tan, X.; Hoerner, J. D.; Xia, C.; Luk, Y. F.; Kohler, B. J. Phys. Chem. A 2001, 105, 5768-5777. (45) Bahnemann, D. W.; Fischer, C.-H.; Janata, E.; Henglein, A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2559-2571. (46) Solar, S.; Solar, W.; Getoff, N. J. Chem. Soc., Faraday Trans. 1 1982, 78, 2467-2477.
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Figure 6. Proposed electron-donating sites of smectite.
samples. This result suggested that bridging Si-O-Al oxygens act as electron donors. However, the MV2+ in the interlayer space of the Kunipia F did not form radical cations despite the existence of a considerable amount of tetrahedral Al in Kunipia F (Table 1). It was thought that the octahedral Fe in Kunipia F acted as a quencher to the excited MV2+. It is known that the excited states of [Ru(bpy)3]2+ and MV2+ were quenched by Fe in the framework of smectite.39,47 The fluorescence intensity of the MV2+ monomer adsorbed on Sumecton SA was several times stronger than that of MV2+ adsorbed on Kunipia F. Hence, the excited MV2+ adsorbed on Kunipia F was quenched by the Fe. Another possible electron-donating site is the crystal edges of smectites. Particle sizes of Sumecton SA and Laponite XLG were much smaller than those of Kunipia F, so that these smectites have a lot of dangling bonds on their crystal edges. Although the structure of the crystal edges of smectites is not clear, charge transfer from adsorbed aromatic molecules such as benzidine to smectite was affected by the crystal edge of smectites.22,24 Pretreatment of smectites with polyphosphate which specifically adsorbed at crystal edge surfaces caused a marked reduction in the color intensity of the charge-transfer complexes with montmorillonite and hectorite.48 Although these results suggested that crystal edges of smectites act as electron acceptors, we supposed that crystal edges acted as electron-donating sites too. It was reported that excitation of strongly oxidizing species on SiO2 (silica gel) leads to electron abstraction from SiO2 to form the radical anion of the species and a positive hole on the SiO2.3,49 Since methyl viologen is a strongly oxidizing species (electron acceptor), we thought that the differences in ESR and UV-vis results reflected the number of dangling bonds and dangling bonds acted as electron-donating sites. Conclusions The preparation of methyl viologen-smectite (synthetic saponite, Sumecton SA; synthetic hectorite, Laponite XLG; and natural montmorillonite, Kunipia F) intercalation compounds and the photoinduced electron transfer from (47) Habti, A.; Keravis, D.; Levitz, P.; Damme, H. V. J. Chem. Soc., Faraday Trans. 2 1984, 80, 67-83. (48) Michaels, A. S. Ind. Eng. Chem. 1958, 50, 951-958. (49) Thomas, J. K.; Ellison, E. H. Adv. Colloid Interface Sci. 2001, 90, 195-238.
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smectite to methyl viologen were investigated. The methyl viologen intercalated in Sumecton SA and Laponite XLG was photoionized by the UV irradiation, while photochemical reduction of methyl viologen did not occur in the interlayer space of Kunipia F. Sumecton SA and Laponite could behave as electron donors.
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This result suggested that a bridging Si-O-Al oxygen lone pair in tetrahedral sites in Sumecton SA or dangling bonds of Sumecton SA and Laponite acted as electrondonating sites. LA020763V
