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The photoreduction of methylviologen via electron transfer from aluminosilicate zeolites
Radiat. Phys. Chem. Vol.45, No. 5, pp. 761 764. 1995
Pergamon
0969-806X(94)00095-6
Copyright ~" 1995ElsevierScienceLtd Printed in Great Britain.All rights reserved 0969-806X/95 $9.50+ 0.00
THE PHOTOREDUCTION OF METHYLVIOLOGEN VIA ELECTRON TRANSFER FROM ALUMINOSILICATE ZEOLITES HUGH J. D. McMANUS, CHRISTOPHF FINEL and LARRY KEVANt Department of Chemistry, University of Houston, Houston, TX 77204-5641, U.S.A. (Accepted 16 June 1994)
Abstract--The photoreduction yields of methylviologen and various alkylated homologues in zeolite-X, Y and A were determined at 77 K in the absence of any reducing counteranion. Upon irradiation at 77 K, dehydrated samples turned light blue and a single line ESR spectrum was observed with a g-factor identical to that of the methylviologen radical cation. Samples that were not dehydrated produced ESR spectra about 10% as intense as those that had the water removed. The photoreduction yield was largest in zeolite Y, less in zeolite X which has more cations and hence more crowding in the cages, and very small in zeolite A which has much smaller cages. The electron donor in these systems is believed to be the anionic aluminosilicate framework of the host zeolite.
INTRODUCTION Methylviologen is widely studied as an electron relay in model systems for photochemical research (Matsuo, 1982). When an anion such as chloride, sulfate or phosphate is present, dimethylviologen is readily reduced via one-electron charge transfer from its counterion (Hopkins et al., 1969; McKellar and Turner, 1971; Ebbesen and Ferraudi, 1983). In the presence of catalysts, such as colloidal platinum or hydrogenase, this photoreduced viologen cation may be used to produce hydrogen through the oxidation of water (Kalyanasundaram and Porter, 1978). The photoreduction of dimethylviologen dichloride has been studied in both vesicular (Lukac and Harbour, 1983; Colaneri et al., 1987) and micellar suspensions (Colaneri et al., 1987; Colaneri et al., 1989). More recently this molecule has been used as an electron accepter in zeolites (Yoon and Kochi, 1988; Yoon and Kochi, 1989; Sankaraman et al., 1991). Zeolites are cavernous inorganic polymers composed of a three-dimensional framework of aluminum oxide and silicon oxide tetrahedra (Turro, 1986). These frameworks contain pores, channels and cages which possess an anionic charge. This charge is neutralized by the presence of counter cations, often sodium. The cations are easily exchangeable, hence it is possible to replace sodium with an organic cation such as methylviologen to study the photochemistry of this organic species within a restricting geometry. Other recent work has focused on the photochemistry of molecules such as ruthenium tris(bipyridine) within a zeolite (Tuberville et al., 1992; Dutta and Tuberville, 1992; f'Author to whom all correspondence should be addressed. RPC4~5--F
Kim and Mallouk, 1992). In most photochemical research in zeolites, an assumption is made that the aluminosilicate framework is photochemically inert. Recently, it has been shown that excited methylviologen constrained in micelles and vesicles is readily reduced by the anionic headgroups of micelles and vesicles (McManus et al., 1992). The reduction of excited methylviologen in constrained environments has here been extended to inorganic microstructures. In this study, we show that photoinduced electron transfer can occur between the anionic surface of zeolites X or Y and the MV 2+ cation. The yield of the photoreduced viologen is measured using electron spin resonance (ESR) at 77 K. EXPERIMENTALSECTION Materials
Dimethylviologen dichloride hydrate (MV -'÷) and diheptylviologen dibromide were purchased from Aldrich Chemical Company. Zeolite X, Y and A were obtained from Linde. Lithium, sodium, potassium and cesium chloride were purchased from Aldrich Chemical Co. N - h e x y I - N ' - m e t h y l - 4 , 4 " - b i p y r i d i n i u m dichloride (C6 V-'+), N-octyI-N'-methyl-4,4'-bipyridinium dichloride (C~ V2~ ), N-decyl-N '-methyl-4,4'bipyridinium dichloride (Cl0V-' ~ ), N-dodecylN'-methyl-4,4'-bipyridinium dichloride (CI_,V-'+), N-hexadecyl-N '-methyl-4,4'-bipyridiniumdichloride (Cl6 V-~+) were kindly donated by D. H. P. Thompson and J. K. Hurst of the Oregon Graduate Research Center. The deuterium oxide (D_,O, 99.9 atom% D) used to prepare the stock alkylviologen solutions was obtained from Aldrich Chemical Co. All chemicals were used as received. Only deionized water was employed. The deionization was carried out in a
761
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Millipore Milli-Q reagent grade water system; the resistivity of the resulting water was greater than 18 M ~ cm. Sample preparation Stock 0.5 M solutions of the four alkali metal salts were prepared (Li, Na, K, and Cs). The solutions were prepared in deionized water which was purged with nitrogen gas for 1 h. After preparation, the solutions were purged for a further 20 min followed by storage in the dark at 3°C. To ensure consistent conditions for all experiments, each zeolite was ion exchanged by placing 1 g of the aluminosilicate in 10 ml of the required stock solution. This mixture was agitated and placed in an oven at 73 ___4°C for 6 h. Excess liquid was decanted off and another 10 ml of stock alkali metal salt solution added. Once again, this mixture was placed in an oven at 73 + 4"C for 6 h. This procedure was carried out three times for each zeolite. The zeolite was stored under a dry, nitrogen atmosphere at 3°C. A stock 25mM solution of dimethylviologen dichloride (MV) in oxygen-free, deionized water was prepared. MV was exchanged into the zeolite by placing 200 mg of the aluminosilicate into 5 ml of the stock viologen solution contained in a centrifuge tube. This mixture was then placed in an oven at 73 ± 40C for 10 h. Following exchange, the centrifuge tube was placed into an IEC centrifuge and the mixture was spun at 700 rpm for 10 min. Immediately after centrifugation, the excess liquid was carefully removed with a 25 ml syringe. The zeolite was then washed with 5 ml of boiling water. This dispersion was agitated and subjected to a further 10min of centrifugation. The zeolite was washed four times in this way. After the fourth step, the optical spectrum of the decanted water was taken and no methylviologen was observed. The procedure adopted for experiments involving N-alkylmethylviologens (C.V) was identical in most respects to that described above, but with the following differences. The stock solutions used were 3 mM in C°V (D20). Further, 1 ml of this stock solution was added to zeolite-Y and this mixture was maintained in a dark environment at 80 + 4'~C for 12 h. The impregnated zeolite was washed twice with boiling water, then dried in vacuo (10 5 torr) for 24 h at 80 + I~C. The MV-impregnated zeolite sample was then placed in a 7 cm by 3 mm o.d. Suprasil quartz tube which was flame sealed at one end. About 1 cm of glass wool was placed in the tube 0.5 cm above the zeolite sample. The quartz tube was attached to a vacuum line through a Cajon Ultra-torr connector pumped down to 10 5torr. The tube was then removed and flame sealed. For samples that required dehydration, the zeolites were dried under vacuum at 90 _+ 1 C for 12h before sealing the tube. All samples were labeled and stored at 77 K. ESR experiments were performed within 24 h of sample preparation.
Optical absorbance experiments were carried out in a Suprasil cuvette with a 0.5 cm path length using a Varian Techtron Model 635 UV-vis spectrophotometer. The absorption maximum was observed at 256 nm which is consistent with previous reports on this system (Watanabe and Honda, 1982). Optical photolysis Photoirradiation of all the samples was carried out for 3 min at either room temperature or 77 K in a quartz irradiation Dewar (Wilmad Glass Co.). A 300-W Cermax Xenon lamp (LX300 UV) was used in all experiments. The power supply used was from ILC Technology which was operated at 50% (10 A) of its rated maximum output power. During photoirradiation the Dewar was rotated at 4 rpm to ensure complete illumination of the sample. The irradiation light passed through a filter combination consisting of a 10cm water filter and a Corning glass filter, No. 7-54 which passed light at the maximum absorbance of methylviologen (MV2+: 2,,~x= 257 nm). A YSI Kettering model 65 radiometer was used to measure the average radiant power incident upon the sample which was 1.1 + 0.1 × 103W m 2. Magnetic resonance experiments ESR spectra were recorded at X-band using a Bruker ESP 300 spectrometer with 100kHz field modulation. The irradiated sample cell was placed in a quartz ESR dewar (Wilmad Glass Co.) which was filled with liquid nitrogen and secured in a TErn2 cavity. The finger of the dewar was placed at the microwave magnetic field maximum. The microwave power was maintained at 1.97 mW, and the microwave frequency was measured with a HewlettPackard 5350B frequency counter. The magnetic field was monitored with a Bruker ER 032M Hall effect field controller. A coal sample was used as a g-factor reference before and after all experiments. RESULTS
A single line ESR spectrum was obtained from most samples following photoirradiation at 77 K. The g-factor, line width of the singlet (g = 2.0033; AHrp = 1.7 mt), and pale blue color of the irradiated sample agree with previous reports on this system (Sakaguchi and Kevan, 1991; Sakaguchi and Kevan, 1989). ESR signals were an order of magnitude weaker in hydrated zeolites X and Y compared to those which had been dehydrated. In zeolite A, however, the ESR spectra obtained from hydrated and dehydrated samples were identical and was about 16 times less intense as that obtained from zeolite Y. Note that the yield is about 3 times greater in zeolite Y than in zeolite X. Also, no ESR spectrum attributable to methylviologen was observed from any sample which was irradiated at room temperature. Figure I shows the ESR spectra at 77 K obtained from photoirradiation at 77 K of MV/zeolite-Y,
Photoreduction of methylviologen
/
(A)
Figure 2 shows a plot of the photoyield observed for a series of N-alkylviologens in zeolite-Y. The photoyield increases slightly up to C~0V after which it decreases. The experiment for methyl viologen was repeated twice. The figure contains the value obtained from determining the arithmetic mean of three experiments. The standard deviation is 0.03. Exchanging Na ÷ by Li ÷ , K + or Cs + in zeolites X or Y had no observable effect on the intensity of the MV radical cation ESR spectrum. However, in zeolite-A no ESR spectrum was observed from hydrated Cs-exchanged samples. In zeolites X, Y and A, ESR signals were obtained from all samples that were dehydrated in the manner described above. Both zeolites X and Y were impregnated with diheptylviologen dibromide (CTV). The conditions used in these experiments were identical to those obtained during work with methylviologen. The photoreduction yield obtained for CTV in zeolite Y was about 30% greater than for MV.
/
(c)
2 mT
I
763
2.0033
I
= DISCUSSION
Fig. 1. The ESR spectra at 77 K obtained from photoirradiation at 77 K of methylviologen impregnated (A) zeolite Y, (B) zeolite X and (C) zeolite A each dehydrated for 12 h at 90°C. MV/zeolite-X and MV/zeolite-A dehydrated samples. When the samples were allowed to warm to room temperature, the pale blue color disappeared and the samples were no longer ESR active.
1.00'
0.90'
W e-
O _c 0.80 al r-
._m (n 0.70 cc (n uJ 0.60
0.50
0
5
1'0
1'5
Alkyl Chain Length
Fig. 2. A plot of the double integrated ESR signal intensity at 77 K in zeolite Y as a function of alkyl chain length for a series of N-alkylviologens. Each sample was irradiated for 3 min at room temperature. The signals are normalized to that obtained from C,~V.
Electron spin resonance can be used to confirm the existence and identification of radical species. In this study ESR was used to detect the radicals present following the photo-irradiation of methylviologen impregnated zeolites. The g-factor, line width and color of these irradiated samples indicate that the only species present following photo-irradiation is the methylviologen radical cation. This radical is produced in solution through electron transfer from a reducing counter anion (Barnett et al., 1973; Ebbesen et al., 1982). In recent work involving methylviologen embedded in anionic organic aggregates such as micelles or vesicles, it was shown that the charged polar headgroups of these surfactant assemblies can act as an electron source for the photoreduction chemistry (McManus et al., 1992). In this study, no inorganic counter anion is present. Although methylviologen is available as a dichloride, the anionic zeolitic framework is known to selectively exchange cationic species only (Maxwell, 1982). Further, experiments were conducted which involved the dibromide salt of diheptylviologen. In this case, the photoreduction yield was even greater than that observed for dimethylviologen dichloride in zeolite Y. Since bromide is a much weaker reducing anion than is chloride for dialkylviologens (Ebbesen and Ferraudi, 1983), this result cannot be explained on the basis of electron transfer from the different halogen anions. This implicates the aluminosilicate framework of the zeolite as the electron source in the photoreduction of dialkylviologens in zeolites. This shows that zeolites X and Y are not photochemically inert when impregnated with dialkylviologens. The large difference in photoreduction yields between zeolites Y and A is most probably due to the
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Hugh J. D. McManus et al.
difference in pore sizes. Zeolite Y has an opening of 0.74 nm compared to the much smaller aperture of 0.42 nm present in zeolite A (Kevan, 1987). Using a space-filling model based on the covalent radii of the constituent atoms, the dimensions of methylviologen are 1.35 x 0.65 × 0.35 nm. A molecule of this size should be readily exchanged for Na ÷ into zeolites X and Y, but not in zeolite A. Hence, this difference in photoyield is most probably due to the inability of MV to exchange into the interior of zeolite A. The several times greater yield in zeolite Y versus X correlates with a lower cation density in zeolite Y. This probably reflects less viologen incorporation into zeolite X due to cation crowding. The differential photoyield observed between methyl and heptylviologen is most probably a result of the restricted diffusion for the larger molecule. Once in the zeolitic framework, the heptyl substituents reduce the mobility of the viologen moiety compared to the dimethyl analog. The results obtained with the alkylmethylviotogen derivatives support this hypothesis. However, the results in Fig. 3 indicate that the yield does not grow monotonically with increasing alkyl chain length and that a maxim u m is reached for C~0. Since longer chained alkylviologens are more likely to generate micelles in water (Wolszczak and Stradowski, 1989), the formation of such aggregates will lower the rate of diffusion of the alkylviologen into the zeolite and lead to a diminished photoyieid. Also with a sufficiently long alkyl chain it is expected that the rate of diffusion of the alkylviologen into the zeolite interior will be reduced. CONCLUSIONS Dialkylviologen dications readily diffuse into the zeolitic framework of X and Y zeolites. Once within the aluminosilicate, these organic cations can be photoreduced by electron transfer from the zeolite framework to produce the viologen cation radical. Experiments using chloride and bromide reducing anions external to the zeolite show no difference in the photoyield even though bromide is a much less efficient reducing anion than is chloride. Thus these
halide anions are not the electron donor. This supports that the dialkylviologen cations are inside the zeolite and that the zeolitic framework is the electron donor in this system. Acknowledgement--This research was supported by the
Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy. REFERENCES
Barnett J. R., Hopkins A. S. and Ledwith A. (1973) J. Chem. Soc. Perkin Trans. 2, 80. Colaneri M. J., Kevan L. and Schmehl R. (1989) J. Phys. Chem. 93, 397. Colaneri M. J., Kevan L., Thompson D. H. P. and Hurst J. K. (1987) J. Phys. Chem. 91, 4072. Dutta P. and Tuberville W. J. (1992) J. Phys. Chem. 96, 9410. Ebbesen T. W. and Ferraudi G. (1983) J. Phys. Chem. 87, 3717. Ebbesen T. W., Levey G. and Patterson L. K. (1982) Nature 298, 545. Hopkins A. S., Ledwith A. and Stam M. F. (1969) Chem. Comm. 259. Kalyanasundaram K. and Porter G. (1978) Proc. R. Soc. London Set. A. 364, 29. Kevan L. (1987) Acc. Chem. Res. 20, 1. Kim Y. I. and Mallouk T. E. (1992) J. Phys. Chem. 96, 2879. Lukac S. and Harbour J. R. (1983) J. Am. Chem. Soc. 105, 4248. Matsuo T. (1982) Pure Appl. Chem. 54, 1693. Maxwell I. E. (1982) Adv. Catal. 31, 1. McKellar J. F. and Turner P. H. (1971) Photochem. Photobiol. 13, 439. McManus H. J. D., Kang Y. S. and Kevan L. (1992) J. Phys. Chem. 96, 2274. Sakaguchi M. and Kevan L. (1991) J. Phys. Chem. 95, 5996. Sakaguchi M. and Kevan L. (1989) J. Phys. Chem. 83, 6039. Sankaraman S., Yoon K. B., Yabe T. and Kochi J. K. (1991) J. Am. Chem. Soc. 113, 1419. Tuberville W., Robins D. S. and Dutta P. K. (1992) J. Phys. Chem. 96, 5024. Turro N. J. (1986) Pure Appl. Chem. 56, 1219. Watanabe T. and Honda K. (1982) J. Phys. Chem. 86, 2617. Wolszczak M. and Stradowski Cz. (1989) Radiat. Phys. Chem. 33, 355. Yoon K. B. and Kochi J. K. (1989) J. Am. Chem. Soc. 111, 1128. Yoon K. B. and Kochi J. K. (1988) J. Am. Chem. Soc. I10, 6568.
Pergamon
0969-806X(94)00095-6
Copyright ~" 1995ElsevierScienceLtd Printed in Great Britain.All rights reserved 0969-806X/95 $9.50+ 0.00
THE PHOTOREDUCTION OF METHYLVIOLOGEN VIA ELECTRON TRANSFER FROM ALUMINOSILICATE ZEOLITES HUGH J. D. McMANUS, CHRISTOPHF FINEL and LARRY KEVANt Department of Chemistry, University of Houston, Houston, TX 77204-5641, U.S.A. (Accepted 16 June 1994)
Abstract--The photoreduction yields of methylviologen and various alkylated homologues in zeolite-X, Y and A were determined at 77 K in the absence of any reducing counteranion. Upon irradiation at 77 K, dehydrated samples turned light blue and a single line ESR spectrum was observed with a g-factor identical to that of the methylviologen radical cation. Samples that were not dehydrated produced ESR spectra about 10% as intense as those that had the water removed. The photoreduction yield was largest in zeolite Y, less in zeolite X which has more cations and hence more crowding in the cages, and very small in zeolite A which has much smaller cages. The electron donor in these systems is believed to be the anionic aluminosilicate framework of the host zeolite.
INTRODUCTION Methylviologen is widely studied as an electron relay in model systems for photochemical research (Matsuo, 1982). When an anion such as chloride, sulfate or phosphate is present, dimethylviologen is readily reduced via one-electron charge transfer from its counterion (Hopkins et al., 1969; McKellar and Turner, 1971; Ebbesen and Ferraudi, 1983). In the presence of catalysts, such as colloidal platinum or hydrogenase, this photoreduced viologen cation may be used to produce hydrogen through the oxidation of water (Kalyanasundaram and Porter, 1978). The photoreduction of dimethylviologen dichloride has been studied in both vesicular (Lukac and Harbour, 1983; Colaneri et al., 1987) and micellar suspensions (Colaneri et al., 1987; Colaneri et al., 1989). More recently this molecule has been used as an electron accepter in zeolites (Yoon and Kochi, 1988; Yoon and Kochi, 1989; Sankaraman et al., 1991). Zeolites are cavernous inorganic polymers composed of a three-dimensional framework of aluminum oxide and silicon oxide tetrahedra (Turro, 1986). These frameworks contain pores, channels and cages which possess an anionic charge. This charge is neutralized by the presence of counter cations, often sodium. The cations are easily exchangeable, hence it is possible to replace sodium with an organic cation such as methylviologen to study the photochemistry of this organic species within a restricting geometry. Other recent work has focused on the photochemistry of molecules such as ruthenium tris(bipyridine) within a zeolite (Tuberville et al., 1992; Dutta and Tuberville, 1992; f'Author to whom all correspondence should be addressed. RPC4~5--F
Kim and Mallouk, 1992). In most photochemical research in zeolites, an assumption is made that the aluminosilicate framework is photochemically inert. Recently, it has been shown that excited methylviologen constrained in micelles and vesicles is readily reduced by the anionic headgroups of micelles and vesicles (McManus et al., 1992). The reduction of excited methylviologen in constrained environments has here been extended to inorganic microstructures. In this study, we show that photoinduced electron transfer can occur between the anionic surface of zeolites X or Y and the MV 2+ cation. The yield of the photoreduced viologen is measured using electron spin resonance (ESR) at 77 K. EXPERIMENTALSECTION Materials
Dimethylviologen dichloride hydrate (MV -'÷) and diheptylviologen dibromide were purchased from Aldrich Chemical Company. Zeolite X, Y and A were obtained from Linde. Lithium, sodium, potassium and cesium chloride were purchased from Aldrich Chemical Co. N - h e x y I - N ' - m e t h y l - 4 , 4 " - b i p y r i d i n i u m dichloride (C6 V-'+), N-octyI-N'-methyl-4,4'-bipyridinium dichloride (C~ V2~ ), N-decyl-N '-methyl-4,4'bipyridinium dichloride (Cl0V-' ~ ), N-dodecylN'-methyl-4,4'-bipyridinium dichloride (CI_,V-'+), N-hexadecyl-N '-methyl-4,4'-bipyridiniumdichloride (Cl6 V-~+) were kindly donated by D. H. P. Thompson and J. K. Hurst of the Oregon Graduate Research Center. The deuterium oxide (D_,O, 99.9 atom% D) used to prepare the stock alkylviologen solutions was obtained from Aldrich Chemical Co. All chemicals were used as received. Only deionized water was employed. The deionization was carried out in a
761
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Hugh J. D. McManus et al.
Millipore Milli-Q reagent grade water system; the resistivity of the resulting water was greater than 18 M ~ cm. Sample preparation Stock 0.5 M solutions of the four alkali metal salts were prepared (Li, Na, K, and Cs). The solutions were prepared in deionized water which was purged with nitrogen gas for 1 h. After preparation, the solutions were purged for a further 20 min followed by storage in the dark at 3°C. To ensure consistent conditions for all experiments, each zeolite was ion exchanged by placing 1 g of the aluminosilicate in 10 ml of the required stock solution. This mixture was agitated and placed in an oven at 73 ___4°C for 6 h. Excess liquid was decanted off and another 10 ml of stock alkali metal salt solution added. Once again, this mixture was placed in an oven at 73 + 4"C for 6 h. This procedure was carried out three times for each zeolite. The zeolite was stored under a dry, nitrogen atmosphere at 3°C. A stock 25mM solution of dimethylviologen dichloride (MV) in oxygen-free, deionized water was prepared. MV was exchanged into the zeolite by placing 200 mg of the aluminosilicate into 5 ml of the stock viologen solution contained in a centrifuge tube. This mixture was then placed in an oven at 73 ± 40C for 10 h. Following exchange, the centrifuge tube was placed into an IEC centrifuge and the mixture was spun at 700 rpm for 10 min. Immediately after centrifugation, the excess liquid was carefully removed with a 25 ml syringe. The zeolite was then washed with 5 ml of boiling water. This dispersion was agitated and subjected to a further 10min of centrifugation. The zeolite was washed four times in this way. After the fourth step, the optical spectrum of the decanted water was taken and no methylviologen was observed. The procedure adopted for experiments involving N-alkylmethylviologens (C.V) was identical in most respects to that described above, but with the following differences. The stock solutions used were 3 mM in C°V (D20). Further, 1 ml of this stock solution was added to zeolite-Y and this mixture was maintained in a dark environment at 80 + 4'~C for 12 h. The impregnated zeolite was washed twice with boiling water, then dried in vacuo (10 5 torr) for 24 h at 80 + I~C. The MV-impregnated zeolite sample was then placed in a 7 cm by 3 mm o.d. Suprasil quartz tube which was flame sealed at one end. About 1 cm of glass wool was placed in the tube 0.5 cm above the zeolite sample. The quartz tube was attached to a vacuum line through a Cajon Ultra-torr connector pumped down to 10 5torr. The tube was then removed and flame sealed. For samples that required dehydration, the zeolites were dried under vacuum at 90 _+ 1 C for 12h before sealing the tube. All samples were labeled and stored at 77 K. ESR experiments were performed within 24 h of sample preparation.
Optical absorbance experiments were carried out in a Suprasil cuvette with a 0.5 cm path length using a Varian Techtron Model 635 UV-vis spectrophotometer. The absorption maximum was observed at 256 nm which is consistent with previous reports on this system (Watanabe and Honda, 1982). Optical photolysis Photoirradiation of all the samples was carried out for 3 min at either room temperature or 77 K in a quartz irradiation Dewar (Wilmad Glass Co.). A 300-W Cermax Xenon lamp (LX300 UV) was used in all experiments. The power supply used was from ILC Technology which was operated at 50% (10 A) of its rated maximum output power. During photoirradiation the Dewar was rotated at 4 rpm to ensure complete illumination of the sample. The irradiation light passed through a filter combination consisting of a 10cm water filter and a Corning glass filter, No. 7-54 which passed light at the maximum absorbance of methylviologen (MV2+: 2,,~x= 257 nm). A YSI Kettering model 65 radiometer was used to measure the average radiant power incident upon the sample which was 1.1 + 0.1 × 103W m 2. Magnetic resonance experiments ESR spectra were recorded at X-band using a Bruker ESP 300 spectrometer with 100kHz field modulation. The irradiated sample cell was placed in a quartz ESR dewar (Wilmad Glass Co.) which was filled with liquid nitrogen and secured in a TErn2 cavity. The finger of the dewar was placed at the microwave magnetic field maximum. The microwave power was maintained at 1.97 mW, and the microwave frequency was measured with a HewlettPackard 5350B frequency counter. The magnetic field was monitored with a Bruker ER 032M Hall effect field controller. A coal sample was used as a g-factor reference before and after all experiments. RESULTS
A single line ESR spectrum was obtained from most samples following photoirradiation at 77 K. The g-factor, line width of the singlet (g = 2.0033; AHrp = 1.7 mt), and pale blue color of the irradiated sample agree with previous reports on this system (Sakaguchi and Kevan, 1991; Sakaguchi and Kevan, 1989). ESR signals were an order of magnitude weaker in hydrated zeolites X and Y compared to those which had been dehydrated. In zeolite A, however, the ESR spectra obtained from hydrated and dehydrated samples were identical and was about 16 times less intense as that obtained from zeolite Y. Note that the yield is about 3 times greater in zeolite Y than in zeolite X. Also, no ESR spectrum attributable to methylviologen was observed from any sample which was irradiated at room temperature. Figure I shows the ESR spectra at 77 K obtained from photoirradiation at 77 K of MV/zeolite-Y,
Photoreduction of methylviologen
/
(A)
Figure 2 shows a plot of the photoyield observed for a series of N-alkylviologens in zeolite-Y. The photoyield increases slightly up to C~0V after which it decreases. The experiment for methyl viologen was repeated twice. The figure contains the value obtained from determining the arithmetic mean of three experiments. The standard deviation is 0.03. Exchanging Na ÷ by Li ÷ , K + or Cs + in zeolites X or Y had no observable effect on the intensity of the MV radical cation ESR spectrum. However, in zeolite-A no ESR spectrum was observed from hydrated Cs-exchanged samples. In zeolites X, Y and A, ESR signals were obtained from all samples that were dehydrated in the manner described above. Both zeolites X and Y were impregnated with diheptylviologen dibromide (CTV). The conditions used in these experiments were identical to those obtained during work with methylviologen. The photoreduction yield obtained for CTV in zeolite Y was about 30% greater than for MV.
/
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I
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2.0033
I
= DISCUSSION
Fig. 1. The ESR spectra at 77 K obtained from photoirradiation at 77 K of methylviologen impregnated (A) zeolite Y, (B) zeolite X and (C) zeolite A each dehydrated for 12 h at 90°C. MV/zeolite-X and MV/zeolite-A dehydrated samples. When the samples were allowed to warm to room temperature, the pale blue color disappeared and the samples were no longer ESR active.
1.00'
0.90'
W e-
O _c 0.80 al r-
._m (n 0.70 cc (n uJ 0.60
0.50
0
5
1'0
1'5
Alkyl Chain Length
Fig. 2. A plot of the double integrated ESR signal intensity at 77 K in zeolite Y as a function of alkyl chain length for a series of N-alkylviologens. Each sample was irradiated for 3 min at room temperature. The signals are normalized to that obtained from C,~V.
Electron spin resonance can be used to confirm the existence and identification of radical species. In this study ESR was used to detect the radicals present following the photo-irradiation of methylviologen impregnated zeolites. The g-factor, line width and color of these irradiated samples indicate that the only species present following photo-irradiation is the methylviologen radical cation. This radical is produced in solution through electron transfer from a reducing counter anion (Barnett et al., 1973; Ebbesen et al., 1982). In recent work involving methylviologen embedded in anionic organic aggregates such as micelles or vesicles, it was shown that the charged polar headgroups of these surfactant assemblies can act as an electron source for the photoreduction chemistry (McManus et al., 1992). In this study, no inorganic counter anion is present. Although methylviologen is available as a dichloride, the anionic zeolitic framework is known to selectively exchange cationic species only (Maxwell, 1982). Further, experiments were conducted which involved the dibromide salt of diheptylviologen. In this case, the photoreduction yield was even greater than that observed for dimethylviologen dichloride in zeolite Y. Since bromide is a much weaker reducing anion than is chloride for dialkylviologens (Ebbesen and Ferraudi, 1983), this result cannot be explained on the basis of electron transfer from the different halogen anions. This implicates the aluminosilicate framework of the zeolite as the electron source in the photoreduction of dialkylviologens in zeolites. This shows that zeolites X and Y are not photochemically inert when impregnated with dialkylviologens. The large difference in photoreduction yields between zeolites Y and A is most probably due to the
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difference in pore sizes. Zeolite Y has an opening of 0.74 nm compared to the much smaller aperture of 0.42 nm present in zeolite A (Kevan, 1987). Using a space-filling model based on the covalent radii of the constituent atoms, the dimensions of methylviologen are 1.35 x 0.65 × 0.35 nm. A molecule of this size should be readily exchanged for Na ÷ into zeolites X and Y, but not in zeolite A. Hence, this difference in photoyield is most probably due to the inability of MV to exchange into the interior of zeolite A. The several times greater yield in zeolite Y versus X correlates with a lower cation density in zeolite Y. This probably reflects less viologen incorporation into zeolite X due to cation crowding. The differential photoyield observed between methyl and heptylviologen is most probably a result of the restricted diffusion for the larger molecule. Once in the zeolitic framework, the heptyl substituents reduce the mobility of the viologen moiety compared to the dimethyl analog. The results obtained with the alkylmethylviotogen derivatives support this hypothesis. However, the results in Fig. 3 indicate that the yield does not grow monotonically with increasing alkyl chain length and that a maxim u m is reached for C~0. Since longer chained alkylviologens are more likely to generate micelles in water (Wolszczak and Stradowski, 1989), the formation of such aggregates will lower the rate of diffusion of the alkylviologen into the zeolite and lead to a diminished photoyieid. Also with a sufficiently long alkyl chain it is expected that the rate of diffusion of the alkylviologen into the zeolite interior will be reduced. CONCLUSIONS Dialkylviologen dications readily diffuse into the zeolitic framework of X and Y zeolites. Once within the aluminosilicate, these organic cations can be photoreduced by electron transfer from the zeolite framework to produce the viologen cation radical. Experiments using chloride and bromide reducing anions external to the zeolite show no difference in the photoyield even though bromide is a much less efficient reducing anion than is chloride. Thus these
halide anions are not the electron donor. This supports that the dialkylviologen cations are inside the zeolite and that the zeolitic framework is the electron donor in this system. Acknowledgement--This research was supported by the
Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy. REFERENCES
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