Effect of Ethylenediamine on Co2+ Ion Exchange in Zeolite ZSM-5

Langmuir 1998, 14, 6539-6544

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Effect of Ethylenediamine on Co2+ Ion Exchange in Zeolite ZSM-5 M. Solache-Rı´os,† I. Garcı´a,† F. de M. Ramı´rez,† P. Bosch,†,‡ and S. Bulbulian*,† Instituto Nacional de Investigaciones Nucleares, Departamento de Quı´mica, A. P. 18-1027, Col. Escando´ n, Delegacio´ n Miguel Hidalgo, C.P. 11801, Me´ xico, D. F., Mexico, and Universidad Auto´ noma Metropolitana, Iztapalapa, Departamento de Quı´mica, A. P. 55-532, Michoaca´ n Esq. Purı´sima, Iztapalapa, C.P. 09340, Me´ xico, D. F., Mexico Received December 10, 1997. In Final Form: August 25, 1998 The uptake behavior of Co2+ was studied in zeolite ZSM-5 samples. The Co2+ uptake decreased in this zeolite as ethylenediamine was added, as opposed to the results found in previous experiments in which the Co2+ uptake increased in zeolite Y and NaA. Spectroscopic and magnetic studies suggested the presence of two high-spin species in Co-ZSM-5 and a high-spin and radical species, mainly in Co-ZSM-5, which was exchanged in the presence of ethylendiamine.

Introduction The effect of ethylenediamine (en) on Co2+ ion exchange at room temperature from aqueous cobalt chloride solution with NaY and NaA zeolites has been studied in previous papers.1,2 It was found that Co2+ uptake values in these zeolites were altered by the en addition. A considerable increase in the Co2+ uptake was observed at about 40% in the first case and 25% for the second case. The increase of the cobalt content in the bulk of zeolite is important for catalysis with the application of metal ion containing zeolites. Indeed, it was found by Li and Armor3,4 that Co2+ in some zeolites, e.g., ZSM-5 and mordenite, is an active catalyst for NOx reduction, and the activity is proportional to the level of Co2+ exchanged into ZSM-5. However, the authors3,4 found that Co-exchanged Y zeolite, which had much more Co2+ exchange capacity, is much less active compared with Co-ZSM-5 or Co-mordenite. This means that the catalytic capacity may be related to the zeolite structure. The spectroscopic5-11 and magnetic techniques12,13 have been important tools in the elucidation of sorbed species in zeolites. They have permitted us to know the intake and the geometry of the species in zeolites. Besides, they have been very useful in the understanding of the cationoxygen linkage in the skeleton, water linkage, and sites * To whom correspondence should be addressed. † Instituto Nacional de Investigaciones Nucleares. ‡ Universidad Auto ´ noma Metropolitana, Iztapalapa. (1) Solache-Rı´os, M.; Garcı´a, I.; Martı´nez-Miranda, V.; Bosch, P.; Bulbulian, S. J. Radioanal. Nucl. Chem. 1995, 191, 89. (2) Garcı´a, I.; Solache-Rı´os, M.; Bosch, P.; Bulbulian, S. Langmuir 1996, 12, 4474. (3) Li, Y.; Armor, J. N. Appl. Catal., B 1993 2, 239. (4) Li, Y.; Armor, J. J. Catal. 1994, 150, 376. (5) Wichterlova´, B.; Jiru, P.; Curinova´, A. Z. Phys. Chem. 1974, 88, 180. (6) Hutta, P. J.; Lunsford, J. H. J. Chem. Phys. 1977, 66, 4716. (7) Hoser, H.; Krzyzanowski, S.; Trifiro´, F. J. Chem. Soc., Faraday Trans. 1975, 665. (8) Klier, K.; Kellerman, R.; Hutta, P. J. J. Chem. Phys. 1974, 61, 4224. (9) Howe, R. F.; Lunsford, J. H. J. Chem. Phys. 1975, 79, 1836. (10) Sun, T.; Trudeau, M. L.; Ying, J. Y. J. Chem. Phys. 1996, 100, 13662. (11) Lamberti, C.; Bordiga, S.; Salvalaggio, M.; Spoto, G.; Zecchina, A.; Geobaldo, F.; Vlaic, G.; Bellatreccia, M. J. Chem. Phys. 1997, 101, 344. (12) Howe, R. F.; Lunsford, J. H. J. Am. Chem. Soc. 1975, 97, 5156. (13) Herron, N. Inorg. Chem. 1986, 25, 4714.

of exchange in zeolites X, Y, A, and ZSM-5 mainly with Co2+, Cu+, and Cu2+ ions. However, there are only a few papers1,9,13 related to cobalt exchange in X, Y, and A zeolites, in which ethylenediamine is added. It seems that the zeolite framework determines the metal coordination. In this paper the cobalt uptake by ZSM-5 zeolite and the effect of ethylenediamine addition during the process were studied. Spectroscopic and magnetic studies were performed to elucidate the behavior of sorption. Experimental Section (A) Materials and Equipment. (i) Zeolite and Reagents. Zeolite ZSM-5 (sample OZ) from Air Products & Chemicals, Inc., was kindly provided by J. N. Armor. The chemical composition was as follows: Al2O3, 7.5%; SiO2, 85.1%; Na2O, 6.5%; K2O, 0.14%; MgO, 0.07%; CaO, 0.01%; TiO2, 0.07%; Fe2O3, 0.05%; atomic Si/ Al ratio ) 20.0, which corresponds to a typical ZSM-5.14 It was studied in its hydrated and dehydrated forms. Analytical reagents were used for both analyses and ion exchange processes; a 0.06 N CoCl2 solution was utilized for ion exchange. Ethylenediamine from Merck was distilled before use. (ii) Neutron Activation Analysis. Cobalt uptake and sodium content in the solids were determined by neutron activation analysis. The photopeaks of 1.17 and 1.33 MeV, due to 60Co produced by 59Co(n,γ)60Co reaction, and 1.37 MeV, due to 24Na produced by 23Na(n,γ)24Na reaction, were measured with a Ge/hyperpure solid-state detector coupled to a 2048-channel pulse height analyzer. (B) Co2+-Exchanged Zeolite (Samples CoZ). Cobalt exchange was calculated in milliequivalents of Co2+ ion uptake per gram of hydrated zeolite. Each point of the uptake curve was obtained by introducing 200 mg of the zeolite in an Erlenmeyer flask and adding 20 mL of 0.06 N CoCl2 solution. The solids and the cobalt solutions were kept in contact for different times, which were 0.25, 0.50, 1, 2, 4, 8, 12, 16, 20, 24, 52, 120, and 144 h. After this the liquids were separated from the solids by centrifugation. The solids, having been washed with deionized water and dried at 80 °C were then left in a humid medium to equilibrate with water. Then they were analyzed by neutron activation. The experiments were carried out at pH 7.4. The results were plotted in milliequivalents of Co2+ per gram of the hydrated zeolite vs contact time between the cobalt solution and the zeolite. The resulting samples were called samples CoZ, Figure 1. (14) Breck, D. Zeolite Molecular Sieves; John Wiley and Sons: New York, 1974.

10.1021/la971357s CCC: $15.00 © 1998 American Chemical Society Published on Web 10/09/1998

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Figure 1. Scheme of cobalt sorption experiments. (C) Co2+-Exchanged Zeolites in the Presence of Ethylenediamine (Samples CoenZ). Three series of Co uptake experiments (CoenZ.1, CoenZ.Ar2, CoenZ.3) were performed in the presence of ethylendiamine. Samples CoenZ.1. In the first series the points from 0 to 4.0 h were determined as mentioned in section B. After a contact period of 4 h between the solid and the cobalt solution, the liquid was separated from the solid by centrifugation. Then 5 mL of ethylenediamine was added to each sample. The organic compound was left in contact with the zeolite for 15 min and then separated from the solid by centrifugation. The solid, once washed twice with 20 mL of the CoCl2 solution, was left in contact with 20 mL of CoCl2 solution from 4.5 to 144 h according to the point analyzed. The zeolite samples were washed with deionized water and dried at 80 °C and finally left in a humid medium. The samples were analyzed by neutron activation. When en was added to the zeolite, the pH went up to 11.8, and the pH dropped to 7.4 again when the en excess was removed. The results were plotted in milliequivalents of Co2+ per gram of zeolite vs contact time. The resulting zeolites were called samples CoenZ.1 Samples CoenZ.Ar2. The second series of experiments was performed the same as the first but this time in an Ar atmosphere. The zeolite was previously dehydrated at 200 °C under vacuum (10-3 Torr) for 1 h, and all liquids were degasified in order to eliminate oxygen during the uptake process (before adding ethylenediamine). After the uptake and before analysis, the samples were handled in an air atmosphere. Only the points corresponding to 4, 9, and 20 h were determined. The resulting zeolites were called samples CoenZ.Ar2, Figure 1. Samples CoenZ.3. The third series of experiments was performed as described above (section C). The only difference was that the excess of ethylenediamine was not eliminated from the zeolite samples, but they were left in contact with 20 mL of CoCl2 solutions after the separation of ethylenediamine. The resulting zeolites were called samples CoenZ.3, Figure 1. (D) Characterization. The zeolite samples were studied by X-ray diffraction, infrared (IR), ultraviolet-visible-near-infrared (UV-vis-near-IR), and electron spin resonance (EPR) spectroscopies and magnetic susceptibility. CoCl2‚6H2O and ethylenediamine (en) were also studied by spectroscopic and magnetic techniques. en sorption was studied by gravimetric and thermogravimetric methods. A Siemens D500 diffractometer with a copper anode X-ray tube was used for X-ray diffraction analysis. The KR radiation was selected with a diffracted beam monochromator. IR spectra of KBr pellets in the range 4000-450 cm-1 were recorded on a Perkin-Elmer/1600 FTIR spectrometer; UV-visnear-IR spectra for solids were obtained by using a Cary-5E. EPR measurements were carried out with a Bruker ER200DSRC; the spectra were recorded at 290 and 77 K at the X-band frequency. All the EPR samples were recorded from 50 to 9950 G. The magnetic susceptibility measurements were carried out on a Johnson Matthey Balance at room temperature. The semiquantitative determination of carbon, nitrogen and metal was carried out by EDAX ZAF by using a Phillips XL30 scanning electron microscope.

Figure 2. Co2+ uptake curves: (a) Co uptake in cobaltexchanged zeolite (CoZ); b) Co uptake in cobalt-exchanged zeolite with en (CoenZ1). A DuPont 2000 TA Instrument, model TGA 51, was used to determine the weight loss, up to 1073 K.

Results and Discussion (A) Original Zeolite ZSM-5 (Sample OZ). Neutron activation analyses of Na showed that the original zeolite ZMS-5 contained 1.52 ( 0.06 mequiv of Na+/g of hydrated zeolite and 9.6% of water, determined by thermogravimetric analysis. (B) Effect of Ethylenediamine on Cobalt Uptake Curves. Samples CoZ and CoenZ.1. Pale pink cobaltexchanged ZSM-5 (sample CoZ) turned out to be brownish yellow under the effect of ethylenediamine (sample CoenZ.1). Curves a and b of Figure 2, comparing the Co2+ uptake in hydrated zeolite samples, showed the different behavior of Co2+ uptake (sample CoZ) both when no ethylenediamine was present (curve a) and when ethylenediamine was added (sample CoenZ.1) (curve b). In this case excess en was removed. Curve a shows a slow sorption of Co2+ until uptake raised a value of approximately 0.9 mequiv Co2+/g. When ethylenediamine was added (sample CoenZ.1) (curve b), at first there was a fast Co2+ uptake to 0.5 mequiv of Co/g. After en addition, an additional increase to 1.0 mequiv Co2+/g was observed. Co2+ was then desorbed to 0.6 mequiv of Co2+/g, and finally the Co2+ ion uptake attained an equilibrium value of 0.8 mequiv/g. Sodium Content in Samples CoZ and CoenZ.1. Figure 3, curves a and b, shows, respectively, the sodium content in the zeolite after cobalt solution came into contact with samples CoZ and CoenZ.1, i.e., without and with ethylenediamine. In sample CoZ, the sodium content decreased to approximately 0.3 mequiv of Na+/g of zeolite, thus showing that Na+ ions were exchanged by Co2+ ions. On the other hand, in sample CoenZ.1, i.e, when ethylenediamine was added, sodium decreased slowly, finally reaching a very low content of Na+ ions (0.1 mequiv of Na+/g), which were not compensated by Co2+ ions. The charge of the zeolite was compensated by the hydronia produced by hydrolysis; this effect has been reported elsewhere.15 Samples CoenZ.Ar2. Table 1 shows the results from samples CoenZ.Ar2. The values of this table show that most of the Na+ ions were exchanged by Co2+ ions. (15) Franklin, K. R.; Townsend, R. P. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2155.

Ion-Exchanger in Zeolite

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Figure 3. Sodium content in zeolites: (a) in cobalt-exchanged zeolite (CoZ); (b) in cobalt-exchanged zeolite with en (CoenZ.1).

Figure 5. X-ray diffraction patterns of (a) cobalt-exchanged zeolite (CoZ) and (b) cobalt-exchanged zeolite with en (CoenZ.1). (hkl) indexes were assigned according to the reported reflections of calcined ZSM-5.

Figure 4. (a) Co2+ uptake in cobalt-exchanged zeolite with en (CoenZ.3). (b) Na+ content in cobalt-exchanged zeolite with en, excess of en was removed (CoenZ.3). Table 1. Co and Na Content in ZSM-5 during 4, 9, and 20 h of Ion Exchange in an Argon Atmosphere (Fourth Series of Experiments) element

4 h, mequiv/g ZSM-5

9 h, mequiv/g ZSM-5

20 h, mequiv/g ZSM-5

Co Na

1.82 0.071

1.56 0.066

1.09 0.070

Sample CoenZ.3. Figure 4, curve a, shows the Co2+ uptake when ethylenediamine was not eliminated completely after its separation from the solid. In this case, the cobalt uptake was always low, showing that when ethylenediamine was present cobalt was washed out from the zeolite lattice. Curve b shows the sodium content of the zeolite in this experiment. The solid samples were characterized by spectroscopic and magnetic techniques in order to understand the cobalt uptake behavior. (C) Characterization. (a) X-ray Diffraction. Since the decrease of cobalt uptake could be due to the collapse of the zeolite structure, the samples were analyzed by X-ray diffraction (Figure 5). All samples were found to be crystalline both before and after cobalt sorption and ethylenediamine addition. The X-ray diffraction peaks were all attributed to zeolite ZSM-5. As expected, due to the exchange process, the relative intensities of the diffraction peaks varied depending on the cation present in the zeolite network. (b) IR Spectroscopy. (i) Sample OZ. The IR spectrum of the original ZSM-5 zeolite, Figure 6a, showed

Figure 6. IR spectra of (a) ZSM-5 zeolite (OZ), (b) cobaltexchanged zeolite (CoZ), (c) cobalt-exchanged zeolite with en (CoenZ.1), and (d) cobalt-exchanged zeolite with en in an Ar atmosphere (CoenZ.Ar2).

several bands. At 1218 cm-1, a well-defined and intense vibration band was observed, which was associated with the asymmetric internal tetrahedron vibrations (T-O) of SiO4. Vibration frequencies at 546, 792, and 1086 cm-1 were assigned to double-ring, symmetric, and asymmetric stretching of the external linkages, respectively. Two

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bands at 3448 and 1630 cm-1 were present, corresponding to stretching vibrations of the lattice water in the zeolite. (ii) Sample CoZ. The IR spectra of ZSM-5 in contact with the aqueous solution of cobalt for 30 min, 4 h, and 144 h were recorded. The more important observations were as follows: (1) the band at 1218 cm-1 increased in intensity with contact time; thus, at 144 h, Figure 6b, it increased up to 30%, and the band observed at 1086 cm-1 in OZ was wider and slightly shifted to higher energy in CoZ. (2) Four bands corresponding to O-H vibrations were observed. Those at 3434 and 1636 cm-1 were assigned to water coordinated to cobalt ions (the first band was shifted 14 cm-1 to lower frequency and the second one 6 cm-1 if compared with those of the OZ). The shoulder at 3293 cm-1 and the unresolved band at 3607 cm-1 may be due to OH vibrations of water molecules bonded to cobalt ion. However, a possible protonation of the oxygen of the internal-bridged -Si-(O-)-Al sites during contact time cannot be ruled out.11 (iii) CoenZ.1 Samples. The IR spectra of CoenZ.1 samples after being in contact with the aqueous cobalt solution for 3, 20, and 144 h were also recorded. The intensity of the bands due to OH vibration frequencies of coordinated water in the CoZ sample diminished and shifted to lower energies, since water molecules were eliminated from the zeolite as ethylenediamine formed species with cobalt already present in the zeolite. The IR spectrum of the sample, whose contact time was 144 h, Figure 6c, showed a band centered at 3424 cm-1 and another one at 1624 cm-1. As has been previously established,14 these bands are due to OH vibrations of molecular water. However, they appeared at lower energies than those mentioned above; then they can be associated with water molecules coordinated to the cobalt ion but in a different environment. These species could be located on the exterior zeolite crystallite surfaces because of the compound size, which is larger than the pore (∼6 Å) of ZSM-5. Although the vibration bands of the ethylenediamine ligand were weak, after coordination with the cobalt ion, some of the typical vibrations of the N-H and -CH2- groups of primary amines between 3500 and 2700 cm-1, as well as a very weak band around 1460 cm-1 attributed to C-N- vibrations, were observed9,16,17 The region corresponding to the vibration bands of the zeolite itself in this sample was quite similar to that of the OZ sample without cobalt. In some way this was consistent with the presence of the species formed with en and water, which were finally located on the surface of the zeolite and not in its framework. (iv) Samples CoenZ.Ar2. In this sample (Figure 6d), the regions from 3700 to 2600 cm-1 and from 1800 to 1300 cm-1 showed several additional bands which were not observed in the samples discussed above. To elucidate the presence of the additional bands, ethylenediamine was kept in contact with nondehydrated zeolite for 9 h without cobalt, and the spectrum indicated that lattice water was partially removed and ethylenediamine sorbed as expected,9 but in the region from 1800 to 1300 cm-1 some weak bands were additionally observed. This result indicated that ethylenediamine was slightly decomposed to some ethylene imine groups, which could be NH2CH2CHdNH and NHdCHCHdNH.9 Thus, in the case of the washed CoenZ.Ar2 sample the number of bands in that (16) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; John Wiley and Sons: New York, 1986. (17) Williams, D. H.; Fleming, I. Spectroscopic Methods in Organic Chemistry, 3rd ed.; McGraw-Hill Book Company, 1986.

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Figure 7. UV-vis-near-IR spectra at 298 K of (a) ZSM-5 zeolite (OZ), (b) cobalt-exchanged zeolite (CoZ), (c) cobaltexchanged zeolite with en (CoenZ.1), and (d) cobalt-exchanged zeolite with en in an Ar atmosphere (CoenZ.Ar2).

region increased, which is indicative of a higher decomposition of en by the cobalt already present in the dehydrated zeolite. The absorption of en in the hydrated ZSM-5, being determined by gravimetric and thermogravimetric methods, was found to be 0.05 cm3/g of ZSM-5 zeolite. It was detected, as well, by IR spectroscopy that the bands due to pore opening (sensitive to the structure) in the region from 350 to 400 cm-1 were shifted to higher energies after en was sorbed. The band at 369 cm-1 was shifted to 379 cm-1 and that at 394 cm-1 was shifted to 396 cm-1, indicating that en was incorporated to the network.14,18 (c) Electronic Spectroscopy. (i) Samples OZ. The UV-vis-near-IR spectrum (Figure 7a) of OZ sample showed two complex bands in the near-infrared region, with their maxima at 6974 and 5181 cm-1. In zeolite Y, these two sets of bands were assigned by Hutta et al.6 to electronic transitions of molecular water in the zeolite; therefore, we expect that in OZ zeolite a similar assignment is valid. (ii) Samples CoZ. The UV-Vis-near-IR spectrum (Figure 7b) of cobalt-exchanged CoZ sample obtained after 144 h of contact time showed not only a band centered at 19 608 cm-1 with a shoulder at 21 552 cm-1 but also a weak band at 25 063 cm-1. On the basis of the literature,14,16-21 we propose that these bands correspond to octahedral-like species. The bands already assigned to water molecules of the zeolite were shifted about 34 cm-1 and diminished in intensity if compared to the original zeolite. These results are consistent with those found by IR. (iii) Samples CoenZ.1. Figure 7c shows the UV-visnear-IR spectrum of the cobalt exchanged with en sample CoenZ.1. The region from 8000 to 4348 cm-1 corresponding to the water molecules of the zeolite was substantially modified. Although the intensity of the main signals at 5181 and 6974 cm-1 is very low in this sample, the bands are not shifted. The complex water bands observed in the OZ sample are better defined in this sample. A band at 6402 cm-1 and a medium broad band centered at 4679 cm-1 were also observed. Cobalt species bands are usually (18) Giannetto, P. G. Zeolitas, Caracterı´sticas, Propiedades y Aplicaciones Industriales; Editorial Innovacio´n Tecnolo´gica: Caracas, 1990. (19) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press, Inc.: New York, 1990. (20) Egerton, T. A.; Hagan, A.; Stone, F. S.; Vickerman, J. C. J. Chem. Soc., Faraday Trans. 1972, 68, 723. (21) Migita, K; Chikira, M. J. Chem. Soc., Dalton Trans. 1983, 2281.

Ion-Exchanger in Zeolite

found19-22 in the region from 33 333 to 13 333 cm-1. An intense band at 27 624 cm-1 with an important shoulder at 23 365 cm-1 was observed. All these features are indicative of cobalt ethylenediamine species interacting with the zeolite surface. The high intensity of the band at 27 624 cm-1, associated with electron transfer between the ligands and cation ion,22 indicates that the ethylenediamine is coordinated to the cobalt ion and not merely adsorbed on the zeolite. (iv) Samples CoenZ.Ar2. The UV-vis-near-IR (Figure 7d) spectrum of dehydrated cobalt exchanged OZ with en was complex. The first important observation is that the chemical environment of the cobalt species is different from that of CoenZ.1. The bands at 20 408 and 14 837 cm-1 were attributed to a d-d transition of cobalt chloride complexes. The bands centered at 26 738 and 16 340 cm-1 are probably associated with the mentioned ethylenediamine species. (d) Magnetic Susceptibility. (i) Sample OZ. The magnetic susceptibility and EPR measurements of OZ sample showed that ZSM-5 was slightly paramagnetic, which is a consequence of the 0.12% of Fe2O3 and TiO2 present in the sample. However, after the exchange with cobalt solutions, this paramagnetism diminished considerably. (ii) Sample CoZ. The pale pink cobalt-exchanged CoZ sample presented a surprisingly effective magnetic moment of 10.06 µBM, which indicated the existence of two high-spin species which will be discussed later. The two cobalt species in CoZ catalyst, located in different sites, were reported recently by Sun et al.,10 who establish that the only oxidation state of this species is 2+. (iii) Sample CoenZ.1. The brownish yellow CoenZ.1 sample presented an effective magnetic moment of µBM ) 6.40, which suggests the presence of two species, a highspin species of about 4.8 µBM and a low-spin oxygen radical species (approximately 1.6 µBM) or a low-spin cobalt species (1.8 µBM). The CoZ sample plays an important role in the formation of a Co2+-ethylenediamine (oxygen) species with a brownish-yellow color. The IR spectrum of this sample presented no band around 1900 cm-1, which is indicative of Co3+ superoxides formed by Co2+ with O2. We can then suggest that since this radical did not strongly interact with the cobalt cation, magnetic coupling did not occur between the two paramagnetic species coexisting in the same species but in different coordination spheres. (iv) Sample CoenZ.Ar2. The yellowish brown CoenZ.Ar2 sample presented an effective magnetic moment of µBM ) 8.20, which can be correlated with the presence of more than two cobalt species, a high spin (4.8 µBM), corresponding to ethylenediamine, and a low spin (1.8 µBM), attributed to CoCl2.xH2O and finally a 1.6 µBM due to radical species. (e) EPR Spectroscopy. (i) Sample CoZ. The EPR spectrum at 77 K (Figure 8a) of CoZ sample showed a small structural signal centered at g ) 2.0130, corresponding to one of the two high-spin cobalt(II) species which survived at 77 K. Hence, the two species did not have the same relaxation time. We suggest then that one of the species was in an octahedral-like geometry and the other in a tetrahedral-like geometry.10,11,19,22 Probably the tetrahedral species was one that survived. One of the species could be [Co(H2O)6]2+ and the other the high-spin cobalt coordinated to zeolite oxygen and to water molecules, which finally is stabilized in a tetrahedral-like (22) Huheey, J. E.; Keiter, E. A.; Keiter R. L. Inorganic Chemistry, (Principles of Structure and Reactivity), 4th ed.; Harper Collins College Publishers: New York, 1993.

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Figure 8. EPR spectra of (a) cobalt-exchanged zeolite (CoZ) at 77 K, (b) cobalt-exchanged zeolite with en(CoenZ.1) at 291 K, and (c) cobalt-exchanged zeolite with en(CoenZ.1) at 77 K.

geometry. This interpretation agrees with the results mentioned above and those reported by Sun et al.10 on the low symmetry exhibited by Co2+ in OZ sample. Lamberti et al.11 have given ample evidence to indicate the presence of two different families of tetrahedra in ZSM-5, the first one exposing three oxygens in the channel and the second one only two oxygens. Unfortunately we do not have enough evidence to propose which of the two possible sites interacted with cobalt. (ii) Sample CoenZ.1. The EPR spectrum at 291 K (Figure 8b) of CoenZ.1 sample showed two independent signals, one centered at g1 ) 2.2867 and the other centered at g2 ) 1.9996. They were separated by 487 G. The features of the spectrum between these two values suggest that in one species two paramagnetic centers could be interacting. However, at 77 K (Figure 8c) one axial signal was observed only with g| ) 2.1206 and g⊥ ) 2.01632. As only the axial signal was observed at 77 K, the other species at 291 K was a high-spin species and probably in an octahedral geometry. In this Co-exchanged ethylenediamine zeolite, only one real species was present, the cobalt being bonded to the ethylenediamine in the first coordination sphere and, to a lesser extent, to the radical located in its second coordination sphere, forming the adduct [Co(ethylenediamine)3]O2-, in which external electron transfer could occur at 291 K. At 77 K only the signal of the radical was clearly observed. Therefore, on one hand, the Co-(ethylenediamine)3 was of high spin and octahedrally coordinated and tetrahedral cobalt(II) complexes had to be discarded since they are very poor oxygen binders. On the other hand, the interaction between the oxygen radical and the Co-(ethylenediamine) species was not direct. This proposal is consistent with the result previously reported for Co-ethylenediamine-treated zeolite.9,12 Moreover, it is well-known that some metal complexes provide the delicate balance required to form adducts with dioxygen, without the irreversible oxidation of the metal or the ligands.10,11,23,24

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We propose in this paper that Co-ZSM-5 promoted the stabilization of this adduct (where the oxidation state of cobalt is 2+) in the presence of ethylenediamine at normal oxygen pressure. This proposal is supported by several papers: Howe and Lunsford reported9,12 that NaY zeolite containing adsorbed ethylenediamine presented no EPR signal after exposure to oxygen, nor did the CoNaY, which contained no adsorbed ethylenediamine. However, two overlapping EPR signals were observed, if containing adsorbed ethylenediamine Co-Y was exposed to an oxygen stream. Additional studies suggest the association of one of the signals (with hyperfine structure) to the low-spin [Co3+(en)2O-2]2+ species and the other one to an O2- species formed only in the presence of Co(en)32+ (without hyperfine structure). Sun et al.10 have also given evidence of the high stability of the oxidation state of Co2+ ions in CoZSM-5 sample under oxygen treatment at 723 K. This is also valid for the CoenZ.1 sample. (iii) Sample CoenZ.Ar2. The EPR spectrum, Figure 9a, of cobalt-exchanged ZSM-5 dehydrated at 291 K presented two signals centered at g1 ) 2.0692 and at g2 ) 2.0198. At 77 K the spectrum (Figure 9b) was similar to the one found above (Figure 8c), but an axial signal was observed with g| ) 2.1292 and g⊥ ) 2.0240. This result suggests the presence of [Co-(ethylenediamine)3]O2-, already discussed in the previous section. According to the data obtained in this paper, we propose that the cobalt uptake in ZSM-5 zeolite and the cobalt species formed depend on zeolite structure. In CoZSM-5, the spectroscopic and magnetic studies gave evidence which suggests two high-spin cobalt species, one in an octahedral and the other in a tetrahedral-like geometry. It was found that the cobalt uptake decreased when en was present during the ion exchange. In this case, a highand a low-spin species were found, as shown by the magnetic moments at 297 K. These two species interacted only to some extent, since magnetic coupling did not occur between the two paramagnetic species. This indicates that the compound formed with cobalt is the adduct ([Co(en)3]2+)O2•-, in which the cationic complex is the highspin octahedral cobalt species (the first coordination (23) Cockle, S. A.; Hill, H. A. O.; Williams, R. J. P. Inorg. Nucl. Chem. Lett. 1970, 6, 131. (24) Jones, R. D.; Summerville, D. A.; Basolo, F. Chem. Rev. 1979, 79, 139.

Solache-Rı´os et al.

Figure 9. EPR spectra of (a) cobalt-exchanged zeolite with en in an Ar atmosphere (CoenZ.Ar2) at 291 K and (b) cobaltexchanged zeolite with en in an Ar atmosphere (CoenZ.Ar2) at 77 K.

sphere) and the superoxide(O2•-) is the low-spin one (the second coordination sphere). According to the results we can suggest that due to the pore size of ZSM-5, this adduct is not located in the zeolite framework but stabilized on the zeolite surface because of its negative charge density at basic pH. In the case of the dehydrated sample, the studies reveal the presence of two paramagnetic species (the formed adduct) and a third one, probably CoCl2, coordinated to zeolite framework oxygens. Conclusions The maximum cobalt uptake in zeolite ZSM-5 was about 0.9 mequiv of Co/g of zeolite, which decreased to about 0.8 mequiv of Co/g of zeolite when en was added. This cobalt uptake behavior of ZSM-5 in the presence of ethylenediamine during the ion-exchange process depends on the structure of this zeolite, which promotes the migration of the adduct ([Co(en)3]2+)O2•- to the surface. The stability of the adduct on the surface can be explained only by the presence of negative charge density on it. LA971357S