937
Langmuir 1991, 7, 937-940
Dioxygen Complexes of Co(tetren) in Zeolites A and Y Influence of Zeolitic Architecture Prabir K. Dutta' and Chris Bowers Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 Received August 13,1990. In Final Form: November 2, 1990
The formation of dioxygen complexes of Co(I1) complexed to tetraethylenepentamine inside the supercages of zeolite A and Y have been investigated. Various spectroscopic probes show that the size constraint of the cages allows for the formation of the superoxo complex in zeolite A and p-peroxo complex in zeolite Y. The superoxo complex loses the O2 in an atmosphere of N2 and can recombine with 02 at an equilibrium constant of 2.2 X 103 M-1. The rapid and efficient formation of these complexes in an aqueous environment under mild conditions,yet constrained by the zeolite architecture to make different complexes, makes this an interesting system. Also, the pentadentate nature of the ligand does not require the addition of an axial base, limiting problems of diffusion through the zeolite. Introduction There has been considerable interest in the synthesis of reversible cobalt-dioxygen complexes since Tsukami's discovery relating to Co(salen) many decades ago.' The analogy with important biological oxygen carrier systems as well as a route for an efficient method for separating dioxygen from air has been the motivating forces in this research. The most significant problems associated with the design of synthetic oxygen carriers are the decomposition of the complex via autoxidation of the metal center and/or the ligand and the formation of peroxo-bridged dimers.2 In the quest to synthesize the optimal dioxygen carrier, elegant synthetic schemes of ligands based upon the concepts of inclusion chemistry have been developed.9t4 The electronic properties of ligands has also been examined in order to hinder major degradation pathway^.^ Another approach has been to encapsulate the metal complexes in a variety of matrices, with the goal of preventing contact between oxygen carrier molecules.6 Zeolites offer an attractive medium toward this goal, since there exist frameworks in which cages are separated from each other by windows with dimensions smaller than the cages, thus allowingfor ready encapsulation. There have been several reports in the literature on complexation of dioxygen by Co(I1) complexes in zeolite cavities. Lunsford and coworkers, showed, on the basis of electron paramagnetic resonance (EPR)spectroscopy that superoxo complexes of cobalt(II)-ethylenediamine could be formed in zeolite cages? Schoonheydtand co-workers investigated the same system with diffuse reflectance spectroscopy and indicated the presence of peroxo complexes.s Resonance Raman spectroscopy has also provided information on the structure of the superoxo and peroxo complexes in this ~ y s t e m . ~ Lunsford et al. have also synthesized five-coordinate mixed bipyridine and terpyridine superoxo complexes in zeolite Y and showed that the material was capable of separating (1) Tsukami, T. Bull. Chem. SOC.Jpn. 1938, 13, 252. (2) Niederhoffer, E. C.; Timmona, J. H.; Martell, A. E.Chem. Reu.
02from Nz.lo The yields of these mixed ligand complexes were low, and even though they were thermally stable until 343 K, they were susceptible to deactivation from water absorption. Herron reported on the synthesis of Co(salen) in zeolite Y and found that upon addition of pyridine as an axial base, the complex could reversibly bind oxygen." The low binding propensity in this case was attributed to crowding in the zeolite cage, which hindered the binding of both axial base pyridine and dioxygen. The synthesis of an anionic cobalt(I1) cyanide complex in zeolite Y is another example of a zeolite-encapsulated reversible oxygen binder.I2 This complex exhibits reversible binding in the presence of water vapor. The major difficulty with this system is that the active oxygen binding component represents only 1% of the totalcobalt content. In this study, we examine the use of tetraethylenepentamine (tetren) as the ligand for a zeolite-encapsulated cobalt complex. Tetren is a pentadentate ligand (shown below) and its use eliminates the need for addition of an axial base, makiig the binding of oxygen a one-step process. Also, since tetren is a liquid that is readily soluble in water, the synthesis can take place in an aqueous slurry, which greatly increases the mobility of the ligand in the zeolite as compared to a solid-state synthesis. We have examined the formation of Co(tetren) in the cages of zeolites Y and A and their binding with dioxygen.
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Experimental Section
Am. Chem. Soc. 1979,101,1622. (4) J m m n , G. B.; Molinaro, F. S.;Ibere, J. A.; Collmann,J. P.; Braumnn, J. I.; Row, E.; Suslick, K. S. J. Am. Chem. SOC.1978,100,6769. (5) Kurokura,K.;Okawa, H.;Kida, S. Bull. Chem. SOC.Jpn. 1978,54,
Materials. NaA and NaY samples were shaken overnight in 0.1 M NaCl, filtered, and rinsed until chloride free. CoNaA and CoNaY zeolites were prepared via ion exchange with a solution of cobalt acetate. Tetraethylenepentaminewaa obtained from Aldrich and used without further purification. The Co loading levels were one ion per unit cell.
(6) Smith, T. D.; Pilbrow, J. R. Coord. Chem. Reo. 1981,39, 295. (7) Howe, R. F.; Lunsford, J. H. J. Phys. Chem. 1975, 79, 1836. (8) Schoonheydt, R. A.;Pelgrim, J.J. Chem. Soc.,Dalton Trans. 1981, 916. (9) Dutta, P. K.; Zaykoeki, R. E. J. Phys. Chem. 1989,93, 2603.
(10) Imamura, S.; Luneford, J. H.Langmuir 1986, 1, 326. (11) Herron, N.Inorg. Chem. 1986,25,4714. (12) Taylor, R. J.; Drago, R. S.; George, J. E.J . Am. Chem. Soc. 1989, 111, 6610.
1984.84.137.
(3) fkh-mmel, W. P.;Merter, K. S. B.; Chrietoph, G.; Buach, D. H. J.
2036.
0 1991 American Chemical Society
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