The Journal of
Physical Chemistry
Registered i n U.S. P a t e n t Office
0 Copyright, 1981, by t h e American Chemical Society
VOLUME 85, NUMBER 26
DECEMBER 24, 1981
LETTERS Exchange of Oxygen between Water and a Partially Copper( 11)-Exchanged Y Zeolite Masakazu Iwamoto,* ShuJI Morlta, and Shulchl Kagawa" Department of Industrial Chemistty, Faculty of Engineering, Nagasaki University, Nagasaki 852, Japan (Received: July 6, 198 1; I n Final Form: August 5, 198 1)
Thermal desorption experiments of H2180adsorbed on a copper(I1)-exchangedY zeolite indicated the existence of at least two kinds of water adsorbates, one of which can reversibly exchange its oxygen with an oxygen atom in the zeolite framework.
Zeolites are active catalysts in an extraordinary variety of oxidation reactions of hydr0carbons.l Their catalytic activity is probably due in part to their high capacity for activating oxygen. The ESR2 and temperature-programmed desorption (TPD)3-5techniques have been applied to study oxygen species adsorbed on various metal-ion-exchanged zeolites. On the other hand, there are very few investigationsk7 on the mobility of framework oxygen in zeolites,which are potentially important to understand the role of lattice oxygen in catalytic oxidations on zeolites. In this Letter, the framework oxygen atoms in a partially Cu(I1)-exchanged Y-type zeolite (CuNa-Y) have been (1)For example, P. A. Jacobs, "Carboniogenic Activity of Zeolites", Elsevier, Amsterdam 1977,p 15;Kh.M. Minachev and Ya. I. Isakov, ACS Monogr., No.171, 552 (1976). (2)For example, K. M. Wong and J. H. Lunsford, J. Phys. Chem., 75, 1165 (1971);Y. Ono, K. Suzuki, and T. Keii, ibid., 78, 214 (1974). (3)M. Iwamoto, K. Maruyama, N. Yamazoe, and T. Seiyama, J.Phys. Chem., 81, 622 (1977). (4) P. A. Jacobs and H. K. Beyer, J. Phys. Chem., 82, 1174 (1979). (5)M. Iwamoto, S.Morita, S. Kagawa, and T. Seiyama, J.Chem. SOC., Chem. Commun., 842 (1980). (6)J. B. Peri, J.Phys. Chem., 79, 1582 (1975);C. Gensse, T.F. Anderson, and J. J. Fripiat, ibid., 84, 3562 (1980). (7) G.V. Antoshin, Kh. M. Minachev, E. N. Sevastjanov, D. A. Kodratjev, and C. Z. Newy, Adu. Chem. Ser., No. 101, 514 (1971). 0022-3654/81/2085-3955$01.25/0
found to participate in the reversible exchange with the oxygen of water as well as adsorbed oxygen5s7and carbon dioxide! by means of an isotopic tracer method combined with the TPD technique. The sample of CuNa-Y with an exchange level of 73%, the apparatus used, and the sample conditioning applied were essentially the same as previously r e p ~ r t e d .Na-Y ~,~ as a starting material was obtained from Union Carbide (SK-40, Lot No. 968079002-1). After sample conditioning, H2180(2.7 kPa, l80atomic fraction = 67%) was introduced onto the sample at a desired temperature for 30 min and then evacuated a t the same temperature for 1 h. The sample was cooled to an ambient temperature under static vacuum and programmed heating was started at a rate of 5 Kemin-l. A typical result is shown in Figure 1A where Hz180was adsorbed at 348 K. The respective chromatograms of H2l8Oand H2160desorbed from CuNa-Y showed welldefined single peaks separated by ca. 30 K. It is clear that the distribution of isotopic species as widely changed during the TPD process. Here, the extent of exchange of oxygen in water, E , was calculated from E = (0.67 - a ) / 0.67, where CY is the atomic fraction of l8O in the desorbed water molecules. The hypothetical value E = 1 would @ 1981 American Chemical Society
Letters
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spectively. If we assume that each oxygen atom of the two OH groups has equal probability to desorb as a water molecule irrespective of the origin of the oxygen, the value of the exchange level of oxygen in a desorbed water molecule would be 0.5. The small deviation of the observed value (0.55) from the theoretical one (0.5) is probably due to a readsorption of the water desorbed once from zeolite surface. These suggestions may be supported by the observations of other workersg-ll in which they measured proton NMR relaxation timeseJOor diffuse reflectance spectral' in zeolites and found two or three kinds of water adsorbates including surface OH groups. The above discussion would be in connection with earlier observations that Cu"Na-Y underwent autoreduction to Cu'Na-Y with some loss of oxygen.12 Following the autoreduction, the charge deficiency of the CUI ions are suggested to be compensated by oxygen vacancies. The resultant defects might act as active sites for the observed oxygen exchange as follows:
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1 Temperature I K Flgure 1. Temperature-programmed desorption chromatogram of water following the absorption of H,'*O (67% purity) on a CuNa-Y sample at 348 K. E represents the extent of exchange of oxygen between water and zeolite lattice.
H
0
mean that after a pure Hz180adsorption the water leaving the surface due to the thermal desorption procedure would be H21e0only. The value of E was plotted in Figure 1B. It was evident that E was nearly zero at the initial stage of desorption, increased gradually with increasing desorption temperature, and attained a constant value of approximately 0.55 around 570-623 K. This indicates that 55% of the water molecules desorbed between 573 and 773 K originated from the interaction of lattice oxygen atoms with water adsorbates. Upon raising the adsorption temperature of HZl80to 523 K, the value of E in the desorbed water was maintained at ca. 0.55-0.60 throughout the TPD treatment. I t is noteworthy that this value is essentially the same as the constant value in Figure 1B. In a separate experiment the amount of water desorbed in the range 873-1173 K was confirmed to be less than one-fiftieth of that desorbed in the range 373-873 K.8 This excludes the possibility that the observed oxygen exchange might be due to exchange between water desorbed below 873 K and water still retained in the zeolite framework. On the basis of the above findings, we suggest the existence of at least two kinds of water adsorbates on CuNa-Y. One water adsorbate has no ability for oxygen exchange reaction with the zeolite lattice. This species desorbs in the lower temperature region below 573 K. Another water adsorbate can easily exchange its oxygen with a lattice oxygen and desorbs between 473 and 773 K. The value of exchange level of ca. 0.55 strongly suggests the following adsorption-desorption cycle: Hz180(g) adsorption +-------t
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+
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Si A1 Si A1 Si 4b Structure 4a is identical with 2, while in 4b the defect migrates to the position adjacent to the original. The latter process would contribute to the exchange of oxygen in water. Finally two comments are worth noting. First, the oxygen-18 atoms introduced into the zeolite lattice through the oxygen exchange between water and framework oxygen could be released again as desorbed water molecules upon subsequent adsorption of H2160,or as oxygen molecules in a TPD experiment of adsorbed oxygen like that reported previ~usly.~J'This indicates oxygen circulation in the exchange of adsorbed water framework oxygen + adsorbed oxygen. Second, the amount of water adsorbed on a pure Na-Y zeolite at 523 K was very small13 in comparison to that on CuNa-Y but the value of E in the water desorbed from Na-Y was ca. 0.4. The difference between the values of E , 0.55 (CuNa-Y) and 0.4 (Na-Y), was not very large. It follows that defects similar to those depicted in structure 2 are very few in number but surely exist on Na-Y, and that oxygen exchange analogous to that on
H2l80(g) f ''O(s)
{w
H Z l 6 0 ( g )+ "O(s)
Here any electronic charge on the species is neglected, and (g) and (s) represent gas phase and zeolite surface, re(8) This is well supported by the results of Lai and Rees. P. P. Lai and L. V. C. Rees, J. Chem. SOC.,Faraday Trans. I , 72, 1840 (1976).
(9) W. D. Basler, J. Phys. Chem., 81, 2102 (1977). (10) J. S. Murday, R. L. Patterson, H. A. Resing, J. K. Thompson, and N. H. Turner, J.Phys. Chem., 79, 2674 (1975). (11) J. H. Shen, A. C. Zettlemoyer, and K. Klier, J.Phys. Chem., 84, 1453 (1980). (12) P. A. Jacobs, W. de Wilde, R. Schoonheydt, J. B. Uytterhoeven, and H. Beyer, J. Chem. SOC.,Faraday Trans. I , 72, 1221 (1976). (13) The amount of water adsorbed on Na-Y at 523 K was less than one-tenth of that on CuNa-Y.
J. Phys. Chem. 1981, 85, 3957-3960
CuNa-Y proceeds also on Na-Y. The Cu(I1) ions introduced into zeolite framework would make it easy to generate the defects, and hence CuNa-Y has much more reactive oxygen than Na-Y. The mobility of protons in intrazeolitic water has so far been fairly well investigated, but that of oxygen has never
3957
been reported. The results in the present work would also be useful in a study on the properties of zeolitic water.
Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research, No. 56750569, from the Ministry of Education, Science and Culture of Japan.
Ultrafast Intersystem Crossing in Some Intramolecular Heteroexcimers Tadashl Okada,’ Ichiro Karakl, Eli1 Matsuzawa, Noboru Mataga,” Yoshiteru Sakata,t and Solchl Misumit Department of Chemisity, Faculty of Engineering Science, Osaka University, Toyonaka 560, Osaka Japan and The Instltute of Scleniific and Industrlal Research, Osaka University, Suiia, Osaka 565, Japan (Receive& September 30, 198 1)
Picosecond laser photolysis studies upon intramolecular heteroexcimer systems, (l-pyrenyl)-(CH.Jn-(amine), have been carried out uncovering the very fast generation of the triplet state localized in the pyrene moiety via the intramolecular heteroexcimer state. The intersystem crossing rate depends rather strongly upon the mutual configuration of donor and acceptor groups as well as the solvent polarity.
Introduction The generation of triplet states from the charge transfer (CT) excited state of EDA complexes is a well-known phen0mena.l It has been observed in some EDA complexes that intersystem crossing (isc) from the S1 (CT) state to the T1state of the donor or acceptor is enhanced by CT interaction even in the absence of heavy atom or paramagnetic species in the comp1ex.l The mechanism for the enhancement of isc in EDA complexes, however, is not yet completely elucidated. On the other hand, the charge-transfer quenching of fluorescence also frequently leads to efficient production of the T1state of the donor or acceptor. The problem of triplet formation in heteroexcimer systems has been a subject of recent lively investigations by means of the nanosecond laser photolysis methode2 There have been some controversies concerning the “fast” isc of the excited CT system.2 It has been suggested that, in some typical heteroexcimer systems, the crossing to the triplet precedes the generation of the relaxed heteroexcimer state in nonpolar solvent^.^ However, such a “fast” isc mechanism has been rejected on the basis of the results of picosecond and nanosecond laser photolysis studies in the case of anthracene- and pyrene-diethylaniline heteroexcimers in nonpolar solvent^.^ In polar solvents, however, where photodissociation into solvated ion radicals is predominant, relatively fast triplet generation within ca. 10 ns is possible owing to geminate re~ombination.~ The formation of triplet geminate pairs from the singlet pair within ca. 10 ns seems to be caused by the hyperfine coupling mechanism between unpaired electron spins and nuclear spins of the radical pain6 This mechanism has been confirmed by the magnetic field effect upon triplet generation from the geminate ion pair.7 We have found by means of picosecond laser spectroscopy extremely fast triplet generations from the CT state of some intramolecular heteroexcimer systems as indicated in I-IV. Isc within 30 ps has been observed in the case of the most rapid generation. Results of these investigations will be described in the following together with those + T h e Institute of Scientific and Industrial Research.
of picosecond laser photolysis studies upon the relatively fast isc of some intermolecular heteroexcimer systems such as pyrene-secondary amine and pyrene-Dabco systems. We have obtained quite similar results also for the intramolecular heteroexcimer systems of anthracene and amine with structures analogous to I-IV, although these results are not included in the present report. Experimental Section Time-resolved absorbance spectra in the picosecond and nanosecond time regions were obtained with a mode-locked ruby laser system.8 A pulsed nitrogen gas laser photolysis systemgset up in this laboratory was used for measurement (1) McGlynn, S. P.; Azumi, T.;Kinoshita, M. “Molecular Spectroscopy of the Triplet State”;Prentice Hall: New York, 1969; pp 284-325. (2) Mataga, N.; Ottolenghi, M. ”Molecular Association Including Molecular Complex”;Foster, R., Ed.; Academic Press: New York, 1979; Vol. 11, pp 65-71. (3) Orbach, N.; Nomos, J.; Ottolenghi, M. J. Phys. Chern. 1973, 77, 2831-6. (4) Nishimura,T.;Nakashima,N.; Mataga, N. Chem. Phys. Lett. 1977, 46, 334-8. (5) Orbach, N.; Ottolenghi, M. “The Exciplex”;Gordon, M.; Ware, W. R., Ed.; Academic Press: New York, 1975; pp 75-111. (6) Brocklehurst, B. Chem. Phys. Lett. 1974, 28, 357-60. (7) Schulter, K.; Staerk, H.; Weller, A.; Werner, H.-J.; Nickel, B. 2. Phys. Chern. (Frankfurt am Main) 1976, 101, 371-90. (8) Okada, T.; Migita, M.; Mataga, N.; Sakata, Y.; Misumi, S.J . Am. Chem. SOC. In press. (9) Yasoshima, S.;Masuhara, H.; Mataga, N.; Suzaki, H.; Uchida, T.; Minami, S. J. Spectrosc. SOC.Jpn. 1981, 30, 98-100.
0022-3654/81/2085-3957$01.25/00 1981 American Chemical Society
