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Adsorption Characteristics and Polymerization of Pyrrole on Y-Zeolites Akihiko Matsumoto,* Tsutomu Kitajima, and Kazuo Tsutsumi Department of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8580, Japan Received March 10, 1999. In Final Form: June 16, 1999 Adsorption characteristics and polymerization of pyrrole on Y-zeolites of different cation types (NaY, HY, and CuY) have been investigated in connection with adsorption behavior, in situ IR spectroscopy, and EPR spectroscopy. Adsorption of pyrrole on NaY is physisorption giving no significant changes in IR and EPR spectra. In the adsorption on HY and CuY, the formation of pyrrole oligomers or polymers is observed. Pyrrole oligomer formed on HY is a nonconjugated one, which gives no EPR signal. In the case of CuY, EPR signal assigned to polaron of polypyrrole was observed at g ) 2.008 by the pyrrole adsorption. The relationship between the amount of the spin of g ) 2.008 and the adsorbed amount of pyrrole was linear even at the number of pyrrole exceeding that of Cu2+, which suggests that the polymerization giving conjugated polypyrrole would take place on Cu2+ sites and the polypyrrole of aromatic form would be oxidized to quinoid form to give polaron on CuY surface.
Conductive polymers have attracted considerable interest from the viewpoint of their electrochemical aspects and have been investigated for application in novel devices such as solid electrolyte cells and molecular electronic devices.1 Polypyrrole doped with anions is one of the conductive conjugated polymers, and considerable work on synthesis,2,3 electrochemical nature,1,4-6 and application to electronic devices7-9 has been reported. The doped polypyrrole was usually prepared by an electrochemical procedure2 and charge-transfer polymerization in the presence of iodine or iron(III) chloride3 as thin films or spongy aggregates. However, these polymers contain defects and the ideal single chains of conjugated system with long-range order have not been synthesized. Microporous zeolites are crystalline aluminosilicates with regular pore arrays. Taking note of the porosity of the zeolites, the preparation of polypyrrole in the zeolite cavities with control of the polymer structure and the characterization of the polymer have been studied. Bein and Enzel reported the synthesis of polypyrrole in microporous cavities of Cu2+-exchanged zeolites to prepare shape-controlled polypyrrole in the nanodimension, called “molecular wire”.10 It is considered that Cu2+ ions in zeolite cavity oxidize pyrrole monomers to initiate polymerization10 and also oxidize the polymer into the conducting
form by polaron formation.11 Polypyrrole formation on Cu2+-exchanged Y-zeolite was also studied by Uehara et al., and they suggested the formation of bipolaron in polypyrrole as well as polaron by EPR measurements.12 Miller et al. studied the reaction of pyrrole in Cu2+- and Ni2+-exchanged mordenite by use of X-ray photoelectron (XPS) and photoacoustic infrared spectroscopies to reveal the oxidizing properties of the cations, and it was found that polypyrrole was observed only in Cu2+-exchanged mordenite.13 Furthermore, in a continuation of the work, McCann et al. studied the polymerization in Cu2+- and proton-exchanged mordenite with EPR and UV-vis spectroscopy, and estimated the average length of polymer chain associated with a radical center as 15-20 monomer units.11 The polymerization of heterocyclic monomer such as pyrrole or thiophene in zeolites of different cation species (Fe3+, Ni2+, Cr2+) and crystal types (mordenite, ZSM-5, AlPO4) cavities was attempted.13-17 In the case of adsorption on zeolite micropores of which the pore width is less than ca. 2 nm, the interactions between adsorbate molecules and surface of micropores are enhanced by overlapping of the potential field of pore walls thereby promoting strong adsorption.19-21 In particular, the anion-cation pair in the zeolite cavity forms strong electrostatic fields and the cation exists in a quite different condition from solution or bulk salt, which interacts strongly with the electronic structure of adsor-
(1) Wegner, G. Angew. Chem., Int. Ed. Engl. 1981, 20, 361. (2) For example: Diaz, A. F.; Kanazawa, K. K.; Gardini, G. P. J. Chem. Soc., Chem. Commun. 1979, 635. Diaz, A. F.; Castillo, J. I. J. Chem. Soc., Chem. Commun. 1980, 397. Kanazawa, K. K.; Diaz, A. F.; Geiss, R. H.; Gill, W. D.; Kwak, J. F.; Logan, J. A.; Rabolt, J. F. J. Chem. Soc., Chem. Commun. 1979, 854. (3) Kang, E. T.; Tan, T. C.; Neoh, K. G.; Ong, Y. K. Polymer 1986, 27, 1958. (4) McNeill, R.; Siudak, R.; Wardlaw, J. H.; Weiss, D. E. Aust. J. Chem. 1963, 16, 1056. (5) Kanazawa, K. K.; Diaz, A. F.; Gill, W. D.; Grant, P. M.; Street, G. B.; Gardini, G. P.; Kwak, J. F. Synth. Met. 1979/80, 1, 329-336. (6) Yamamoto, T.; Ito, T.; Sanechika, K.; Hishinuma, M. Synth. Met. 1988, 25, 103. (7) Kittlesen, G. P.; White, H. S., Wrighton, M. S. J. Am. Chem. Soc. 1984, 106, 7389. (8) Murao, K.; Suzuki, K. J. Chem. Soc., Chem. Commun. 1984, 238. (9) Murao, K.; Suzuki, K. Appl. Phys. Lett. 1985, 47, 724. (10) Bain T.; Enzel, P. Angew. Chem., Int. Ed. Engl. 1989, 28, 1692.
(11) McCann, G. F.; Millar, G. J.; Bowmaker, G. A.; Cooney, R. P. J. Chem. Soc., Faraday Trans. 1995, 91, 4321. (12) Uehara, H.; Miyake, M.; Matsuda, M.; Sato, M. J. Mater. Chem. 1998, 8, 2133. (13) Miller, G.; McCann, G.; Hobbis, C. M.; Bowmaker, G. A.; Cooney, R. P. J. Chem. Soc., Faraday Trans. 1994, 90, 2579. (14) Enzel, P.; Bain, T. J. Chem. Soc., Chem. Commun. 1989, 1326. (15) Esnouf, S.; Mory, J.; Zuppiroli, L.; Enzel, P.; Bain, T. J. Chim. Phys. 1992, 89, 1137. (16) Bain, T. Stud. Surf. Sci. Catal. 1996, 102, 295. (17) Bain, T.; Enzel, P.; Beuneu, F.; Zupprioli, L. Adv. Chem. Ser. 1990, 226, 433. (18) Miyake, M.; Uehara, H.; Miyake, K.; Sato M.; Matsuda, M. J. Porous Mater. 1996, 3, 227. (19) Gregg, S. J.; Sing, K. S. W. Adsorption Surface Area and Porosity; Academic: London, 1982; Chapter 4. (20) Stoeckli, F. Helv. Chim. Acta 1974, 57, 237. (21) Everett, D. H.; Powl, J. C. J. Chem. Soc., Faraday Trans. 1 1976, 72, 619.
Introduction
10.1021/la990286p CCC: $18.00 © 1999 American Chemical Society Published on Web 08/31/1999
Polymerization of Pyrrole on Y-Zeolites
Langmuir, Vol. 15, No. 22, 1999 7627 Table 1. Y-Zeolites Used in This Study
zeolites
composition
cation-exchange ratio/%
BET surface area/m2 g-1
pore volume/mL g-1
CuY-1.4 CuY-5.9 CuY-9.8 CuY-13.6 CuY-16.3 NaY HY
Cu1.4Na48.4Al51.2Si140.8O384 Cu5.9Na39.4Al51.2Si140.8O384 Cu9.8Na31.6Al51.2Si140.8O384 Cu13.6Na24.0Al51.2Si140.8O384 Cu16.3Na18.6Al51.2Si140.8O384 Na51.2Al51.2Si140.8O384 H51.2Al51.2Si140.8O384
5.5 23.0 38.3 53.1 63.7 -
710 690 680 650 620 710 730
0.38 0.37 0.36 0.35 0.34 0.38 0.39
bate molecules22-24 and brings about reaction such as polymerization of heterocyclic compounds.10-18 Therefore, in the case of polymerization of heterocyclic compounds in zeolites, it is important to consider the adsorption behavior affected by cation species and pore structure of zeolite as well. However, the studies mentioned above were focused mainly on the reaction mechanisms and the characterization of reaction products,10-18 and adsorption properties of pyrrole on zeolites of different cation species were not discussed enough. In this study, Y-zeolites of different cation types, Na+, +, and Cu2+, were prepared and characterized by several H techniques. The adsorption behavior and polymerization of pyrrole on these zeolites were examined in connection with the EPR spectral change with adsorption property. Experimental Section Samples. NaY-zeolite (Toso Corp., HSZ-320NAA, denoted NaY) of Si/Al ) 2.75 was used as original zeolite. Cu2+-exchanged NaY-zeolites of different Cu2+ contents (CuY) were prepared by ion-exchange technique from copper nitrate solution of Cu2+ contents of 0.04-5 equivalent to the Na+ amount in NaY-zeolite at 298 K. Concentrations of the solution were adjusted from 3 to 44 mmol/L. The Cu zeolites were rinsed with distilled water and dried at 283 K. H-type zeolite (HY) was also prepared by ion exchange of the NaY-zeolite from ammonium nitrate solution followed by calcination at 733 K in vacuo. Characterization. Chemical compositions of these zeolites were determined by inductively coupled plasma (ICP) analysis. Crystallinity of the ion-exchanged zeolites was checked by powder X-ray diffraction (XRD) using Cu KR radiation. The adsorption isotherm of nitrogen on zeolites was measured volumetrically at 77 K. The samples were evacuated at 773 K and 1 mPa for 12 h before measurements. Pyrrole Adsorption. The adsorption isotherms of pyrrole were measured gravimetrically using a quartz balance at 298 K. Pyrrole (Tokyo Kasei, Reagent grade) was purified by several freeze-thaw cycles. IR spectral changes of the zeolites before and after pyrrole adsorption were measured in situ at 298 K. The sample was pressed at 150 kg/cm2, and the self-supporting wafer (diameter 13 mm) of 7 mg/cm2 concentration was prepared. Electron paramagnetic resonance (EPR) spectra before and after pyrrole adsorption at 298 K were measured in situ by an X-band spectrometer (JEOL JES-FE1XG). The g value of EPR signals was corrected by a signal position of 1,1-diphenyl-2-picrylhydrazyl (DPPH) solution at g ) 2.0036, and the spin concentration was determined by comparing the peak area of integration signal of the first derivative spectrum with that of a signal of CuSO4‚ 5H2O or violanthrone diluted in silica powder at 298 K. Zeolite samples were heated at 773 K and 1 mPa for 10 h prior to each adsorption experiment. Solid-state 13C MAS NMR spectra of pyrrole adsorbed in zeolite were measured on an FT-NMR spectrometer (Varian, VNMR400P) at the Lamor frequency of 100.534 MHz using an MAS probe (Doty Scientific). (22) Barrer, R. M. J. Colloid Interface Sci. 1966, 21, 415. (23) Kiselev, A. V. Adv. Chem. 1971, 102, 37. (24) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic: London, 1978; Chapter 4.
Figure 1. Adsorption isotherms of nitrogen on Y-zeolites at 77 K.
Results and Discussion CuY-Zeolites. The CuY-zeolites of different Cu2+ contents (denoted CuY-n; n stands for the number of Cu2+ per unit cell of the Y-zeolite) are tabulated in Table 1. The number of Cu2+ was controlled from 1.4 to 16.3 per unit cell by regulating ion exchange conditions. Figure 1 shows adsorption isotherms of nitrogen on CuY of different Cu2+ contents. Isotherms on NaY- and HY-zeolites are also shown. Each isotherm was of type Ia in the BDDT classification, which was a typical type in adsorption on micropores.26 The apparent surface area and pore volume of the samples were estimated by the BET plot and by the uptake at point B,19 respectively, and tabulated in Table 1. The specific surface area and pore volume decreased slightly from 710 to 620 m2/g and 0.38 to 0.34 mL/g, respectively, with an increase in Cu2+ amount from 0 to 16 per unit cell. The copper nitrate solution used in the ion-exchange process was acidic, so that zeolite structures would be partially collapsed and the porosity decreased. No clear changes in XRD patterns were observed among these samples. Pyrrole Adsorption. Figure 2 shows the adsorption isotherms of pyrrole on CuY, NaY, and HY. Each isotherm was of type I regardless of cation species and the number of Cu2+. This result suggests that pyrrole can be adsorbed strongly in micropores of zeolites. In the case of NaY, no color change was observed by pyrrole adsorption. However, the color of samples was changed from pale blue to dark gray or black in CuY and from white to pale yellow in HY, respectively, by pyrrole adsorption. These color changes, as well as the IR results shown below, suggest that chemisorption and oligomerization of pyrrole take place on Cu2+ and H+ sites.10-18 The saturated adsorption amount of pyrrole on NaY estimated from a Langmuir plot was 3.2 mmol/g. However, the saturated adsorptions (25) Matsumoto, A.; Tsutsumi, K. J. Chem. Soc., Faraday Trans. 1995, 91, 1707. (26) Sing, K. S. W. J. Porous Mater. 1995, 2, 9.
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Figure 4. Possible mechanism of the formation of nonconjugated pyrrole oligomer. Figure 2. Adsorption isotherms of pyrrole on Y-zeolites at 298 K.
Figure 3. Changes in nitrogen adsorption isotherms before/ after pyrrole adsorption followed by evacuation at various temperatures.
on CuY and HY were slightly smaller because the porosity (surface area and pore volume) was decreased by the Cu2+ exchange. Furthermore, in the case of HY and CuY, since the interactions between pyrrole molecules and Cu2+ and H+ sites were strong, the reaction should take place. The reaction products on Cu2+ and H+ sites near the pore entrance would block further diffusion of pyrrole molecules; therefore, the saturated adsorption amount of pyrrole would decrease. Strong interactions between pyrrole and Cu2+ or H+ sites rather than Na+ ones in the zeolite pores will be discussed later. Figure 3 shows the adsorption isotherms of nitrogen on NaY, HY, and CuY-16.3 at 77 K before and after pyrrole adsorption. The adsorption isotherms after pyrrole adsorption were measured after saturated adsorption of pyrrole followed by evacuation at 383 K (denoted “383 K treatment”) or after further evacuation at 473 K (denoted “473 K treatment”). The adsorption amount of nitrogen on each sample decreased drastically after pyrrole adsorption. In the case of NaY, the apparent pore volume decreased from 0.38 to 0.1 mL/g after the 383 K treatment, but recovered to almost the initial value (the value before pyrrole adsorption) of 0.36 mL/g after the 473 K treatment. However, in the cases of CuY-16.3 and HY, the apparent pore volume decreased from 0.34 to 0.01 mL/g and from 0.38 to 0.01 mL/g, respectively, after the 383 K treatment and did not recover to the initial value even after the 473 K treatment. In the case of CuY-16.3, adsorbed pyrrole
molecules reacted on Cu2+ sites and gave rise to conjugated polymer.10-18 Therefore, the pore volume did not recover after the 473 K treatment. Since the proton site in HY has acidic character, it is more chemically active than the Na+ site and induces polymerization reactions.25 Pyrrole oligomerization takes place by an acid-catalyzed mechanism that gives nonconjugated oligomer as shown in Figure 4.27 Similarly, in the present case, pyrrole molecules interacted strongly with H+ sites and gave nonconjugated oligomers. Consequently, the pore volume of HY did not recover after the 473 K treatment. On the other hand, the interaction between adsorbed pyrrole molecules and univalent Na+ sites was relatively weak and nonreactive, so that adsorbed pyrrole began to be desorbed after the 383 K treatment and the pore volume recovered after the 473 K treatment. IR Changes with Pyrrole Adsorption. Figure 5 shows the IR spectra of NaY, HY, and CuY-16.3 before and after pyrrole adsorption. Significant differences in spectra between CuY-16.3 and other CuY samples were not observed. Therefore, only the spectrum of CuY-16.3 is shown in the figure. Before pyrrole adsorption, sharp bands were observed at 3550 and 3620 cm-1 in the spectra of HY and CuY-16.3 as shown in Figure 5a. In the case of HY, the absorption band at 3550 cm-1 is attributed to the OH stretching vibration in the inaccessible bridge positions located between two sodalite units of Y-zeolite.28 The 3650 cm-1 band is ascribed to the stretching mode of OH groups located inside large cavities near six-membered oxygen rings.29 In the case of CuY-16.3, the bands at 3550 and 3620 cm-1 are assigned to the stretching mode of OH groups formed by dissociation of adsorbed water strongly polarized by the electrostatic field of bivalent Cu2+.30,31 These absorption bands due to surface hydroxyl groups were not observed in the case of NaY. The absorption bands between 1700 and 2000 cm-1 observed in each zeolite are known to be overtones of crystal lattice vibrations although these bands cannot be assigned to specific group vibrations.32 As shown in Figure 5b, new bands were observed at 3400 cm-1, 3150 cm-1, and the 1600-1300 cm-1 region in (27) Katritzky, A. R.; Rees, C. W. In Comprehensive Heterocyclic Chemistry; Pergamon: Oxford, 1984; Vol. 4, Part 3, Chapter 3.05. (28) van Hooff, J. H. C.; Roelofsem, J. W. Stud. Surf. Sci. Catal. 1991, 58, 241. (29) Eberly, Jr., P. E. J. Phys. Chem. 1967, 71, 1717. (30) Breck, D. W. Zeolite Molecular Sieves; Krieger: Malabar, 1984; Chapter 6. (31) Ward, J. W. J. Phys. Chem. 1968, 72, 4211. (32) Eberly, Jr., P. E.; Laurent, S. M.; Robson, H. E. U.S. Patent 3,506,400, 1970.
Polymerization of Pyrrole on Y-Zeolites
Langmuir, Vol. 15, No. 22, 1999 7629
Figure 6. 13C MAS NMR spectra of bulk pyrrole (a) and pyrrole adsorbed on CuY-16.3 (b).
Figure 5. IR spectral changes with pyrrole adsorption on Y-zeolites. Before pyrrole loading (a); pyrrole adsorbed followed by evacuation (b). Transmittance axis is displaced for each run.
each zeolite upon dosing with 1.1 kPa of pyrrole followed by evacuation of vapor; these bands were assigned to N-H stretching, aromatic C-H stretching, and conjugated Cd C and CdN ring stretching vibrations (skeletal bands) of adsorbed pyrrole, respectively.33 The position of the 3400 cm-1 band depends on the degree of hydrogen bonding and shifts to a lower wavelength by the formation of hydrogen bonding.33 In the case of HY and CuY-16.3, a weak base pyrrole would adsorb on acidic sites, and hence the 3400 cm-1 band became broadened at lower wavelength and the bands at 3650 and 3550 cm-1 reduced in their intensities. Besides these changes, new bands consisting of broad bands centered around 1460 cm-1 were observed in the spectra of HY and CuY-16.3. These broad bands are assigned to pyrrole polymer.10,11 The pyrrole molecules react with each other to give conjugated polymer on CuY, and nonconjugated polymer on HY, respectively. However, the spectral differences between HY and CuY16.3 were not detected clearly. These results also show the formation of pyrrole oligomer or polymer on HY and (33) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectroscopic Identification of Organic Compounds, 4th ed.; Wiley: New York, 1981; Chapter 3.
CuY zeolite. Several sharp bands at 1600-1300 cm-1 observed in CuY-16.3 were due to skeletal bands of pyrrole monomer remaining on the surface. 13 C NMR Spectrum of Pyrrole Adsorbed on CuY. In the 13C NMR spectrum of pyrrole, sharp signals assigned to C-2 (R-carbon) and C-3 (β-carbon) are observed at 118 and 108 ppm, respectively as shown in Figure 6. Street et al. reported that neutral bulk polypyrrole gives signals at 123 and 105 ppm due to R- and β-carbons, respectively.34 Due to a decrease in π-electron density of pyrrole by polymerization, the resonance signals of pyrrole polymer were shifted slightly to lower magnetic field compared to those of pyrrole monomer.34 The pyrrole molecule tends to polymerize by R-R′ bonding rather than by R-β bonding because of the difference in stability of transitional states. Figure 6 also shows the solid-state 13C NMR spectrum of pyrrole adsorbed on CuY-16.3. The spectrum gave wide signals centered at 125 and 108 ppm which were positions similar to those assigned to the R- and β-carbons of bulk pyrrole. Therefore, the NMR results also suggest the polymerization of pyrrole on CuY-zeolite. EPR Spectral Changes with Pyrrole Adsorption. In the case of NaY and HY, no EPR signals were observed after evacuation at 423 K. However, EPR signals were observed in CuY regardless of Cu2+ content after evacuation at 423 K. Figure 7 shows EPR spectra of CuY of differing Cu2+ content. In the spectrum of CuY-1.4, several signals due to Cu2+ species were observed with ESR parameters, g| ) 2.332, A| ) 157 × 10-4 cm-1, and g ) 2.064, which coincided well with other works on Cu2+exchanged Y-zeolite35,36 and other zeolites.11,37-39 The most intense signals was observed in the g region at about 0.32 T with poor resolution of hyperfine structure due to small coupling constant (A) of (9.3-18.7) × 10-4 cm-1. On the other hand, despite weak intensities, the hyperfine structures were better resolved in the g| region. These signals became broader with increasing number of Cu2+ (34) Street, G. B.; Clarke, T. C.; Kronishi, M.; Kanazawa, K.; Lee, V.; Pfluger, P.; Scott, J. C.; Weiser, G. Mol. Cryst. Liq. Cryst. 1982, 83, 253. (35) Chao, C. C.; Lunsford, J. H. J. Phys. Chem. 1972, 76, 1546. (36) Schoonheydt, R. A. Catal. Rev.sSci. Eng. 1993, 35, 129. (37) Anderson, M. W.; Kevan, L. J. Phys. Chem. 1987, 91, 4174. (38) Giamello, E.; Murphy, D.; Magnacca, G.; Morterra, C.; Shioya, Y.; Nomura, T.; Anpo, M. J. Catal. 1992, 136, 510. (39) Sendoda, Y.; Ono, Y. Zeolites 1986, 6, 209.
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Figure 7. EPR spectra of CuY.
Matsumoto et al.
Figure 9. Change in the amount of spin at g ) 2.008 with relative pressure of pyrrole. Table 2. Estimate of the Number of Radical Spins Measured by EPR no. of radical spins at the saturated adsorption
sample CuY-1.4 CuY-5.9 CuY-9.8 CuY-13.6 CuY-16.3
Figure 8. EPR of spectral change with pyrrole adsorption on CuY-1.4 at different uptakes.
in the zeolite, CuY-5.9 to CuY-16.3, due to the magnetic dipole interactions between the Cu2+ centers in EPR.11,35,40 No EPR signals were observed after the pyrrole adsorption on NaY and HY because no oligomerization giving nonconjugated oligomer took place. However, EPR spectral changes were observed by pyrrole adsorption on CuY. Figure 8 shows the EPR spectral change with pyrrole adsorption on CuY-1.4 at different adsorption uptakes. When pyrrole was adsorbed on CuY-1.4, a new sharp symmetrical signal appeared at g ) 2.008. The new signal increased in its intensity with an increase in the adsorbed amount of pyrrole, and at the same time, that of Cu2+ decreased and diminished at the adsorption saturation of pyrrole. Such a sharp EPR signal close to the g value of free electron, g ) 2.0023, is ascribed to less spin-orbital interaction, which is typical in organic radicals or polaron formed on conducting polymers.11,16 The same spectral changes were also observed in the adsorption of pyrrole on the ferric form of Y-zeolites and the copper form of mordenite.11,14 Cu2+ ions located where pyrrole molecules (40) De Tavernier, S.; Schoonheydt, R. A. Zeolites 1991, 11, 155.
per 10-4 no. of adsorbed per 10-2 per 10-3 (adsorbed pyrrole molecules per radical/103 (unit cell)-1 (Cu2+)-1 pyrrole)-1 0.04 0.75 1.45 2.23 2.95
0.31 1.27 1.48 1.64 1.81
0.13 2.23 4.30 6.90 9.23
80 4.5 2.3 1.4 1.1
can be accessible, such as site II(G) and/or III′(I) suggested by Goldfarb and Zukerman, and Packet and Schoonheydt.35,41,42 In the present case, Cu2+ ion took an electron from pyrrole and gave rise to the reactive pyrrole radical which would undertake further polymerization reaction described below. Consequently, the sharp signal at g ) 2.008 would be assigned to pyrrole radical or polaron which is observed in a conducting polypyrrole. The similar tendency was observed in other CuY-zeolites of differing Cu2+ content (not shown here), whereas the signals due to Cu2+ were broader than those in the CuY-1.4. The change in the amount of spins of g ) 2.008 with relative pressure of pyrrole, called “spin isotherm”, is shown in Figure 9. The shape of the spin isotherm of each sample was similar to the adsorption isotherm of pyrrole shown in Figure 2. However, the saturated spin amount of each CuY was changed largely from 4 × 10-4 to 2.95 × 10-2 spins/(unit cell) with an increase in the number of Cu2+ cations from 1.4 to 16.3 per unit cell as tabulated in Table 2, although the saturated adsorption amount of pyrrole did not show such a large difference of the order of 102. The difference in saturated spin amount among CuY-zeolites suggests that the Cu2+ cations in CuY play an important role in the formation of radical giving such a sharp signal at g ) 2.008. In the case of polymerization of pyrrole in solution, the following reaction sequence is proposed.1 Initially, the radical cation of pyrrole monomer is formed by a redox (41) Goldfarb, D.; Zukerman, K. Chem. Phys. Lett. 1990, 171, 167. (42) Packet, D.; Schoonhydt, R. A. In New Developments in Zeolite Science and Technology; Murakami, Y., Iijima, A., Ward, J. W., Eds.; Kodansha: Tokyo, 1986; p 385.
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Figure 11. Amount of radical spins plotted against adsorbed amount of pyrrole on CuY.
Figure 10. Possible mechanism of the formation of conjugated polypyrrole on CuY.
reaction, which is further dimerized to the transitional state having the structure of a dication, and is stabilized by deprotonation to give the stable pyrrole dimer. The subsequent oxidation of the dimers gives rise to radical cations which are easily combined with each other, forming radical oligomers. Miller et al. mentioned the similar mechanism of pyrrole polymerization in Cu2+-exchanged mordenite, and proposed the redox reaction, viz. oxidation of pyrrole and pyrrole oligomer by Cu2+.13 In the case of CuY, the Cu2+ reacts in the same manner as summarized in Figure 10. Namely, one Cu2+ reacts with a pyrrole to give a pyrrole radical. Two pyrrole radicals bond to each other and form one dimer. The elongation of pyrrole oligomer proceeds with oxidation of the oligomer and/or monomer of pyrrole with Cu2+. Oxidation of one copper ion and release of two protons are involved in this polymerization mechanism. The released H+ in the oligomerization process would compensate the charge of zeolite structure and react with other pyrrole molecules by the acid-catalyzed mechanism mentioned above. The amount of spins of g ) 2.008 is plotted against the adsorbed amount of pyrrole on CuY and shown in Figure 11. The amount of the spins increased linearly with an increase in the adsorbed amount of pyrrole through the whole range of adsorption. There must be enough Cu2+ cations to interact with diffused pyrrole in cavities at the initial stage of adsorption where the number of adsorbed pyrrole molecules is less than that of Cu2+. Pyrrole molecules in the cavities could easily reach the Cu2+ sites and react with the cations to give pyrrole radicals at this stage. In this case, the amount of spins of g ) 2.008 would (43) Blanking, J. R.; Miller, G. J.; Bowmaker, G. A.; Cooney, R. P. J. Raman Spectrosc. 1993, 24, 523.
be identical or close to the adsorption uptake of pyrrole. However, the amount of the spin per one pyrrole molecule, which corresponds to the slope of Figure 10, is only 1.2 × 10-5 - 9.1 × 10-3 spins/molecule. Furthermore, the amount of the spin increased linearly even after the adsorption uptake became greater than the number of Cu2+. Considering these results and the high reactivity of pyrrole radical, the EPR signal at g ) 2.008 would not be assigned to pyrrole radical. On the other hand, polypyrrole easily loses an electron and forms a polaron which gives an EPR signal around g ) 2.10,43 Consequently, the EPR signal at g ) 2.008 would be assigned to a polaron formed on polypyrrole. It is possible that zeolite surface of low Si/Al ratio gives partially oxidized polypyrrole having quinoidal structure.16,43 The quinoidal structure is associated with the charge carriers, polarons. Therefore, once polypyrrole is formed in zeolite cavities, the oxidation of polypyrrole would take place. Thus polypyrrole formation would bring about increasing the number of polarons, so that the spin amount increased linearly with an increase of adsorbed amount of pyrrole through the whole range of adsorption. Conclusions Pyrrole adsorption on Y-zeolites of different cation types, HY, NaY, and CuY, were studied by adsorption measurement and in situ measurements of IR and EPR. Adsorption on NaY was of a physical character, giving no changes in IR and EPR spectra. In the case of adsorption on HY and CuY, adsorbed pyrrole reacted with H+ or Cu2+ sites and gave an oligomer. Pyrrole oligomerization on HY takes place by the acid-catalyzed process, and the oligomer would be nonconjugated, giving no polaron. In the case of CuY, the EPR signal assigned to polaron was observed at g ) 2.008 by pyrrole adsorption. The relationship between the amount of the spin of g ) 2.008 and the adsorbed amount of pyrrole was linear even when the number of pyrrole molecules exceeded that of Cu2+. Acknowledgment. Financial support by a Grant-inAid from the Ministry of Education, Science, Sports and Culture, Japan, is greatly appreciated. LA990286P
