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Langmuir 1996, 12, 4474-4475
Effect of EDA on Co2+ Ion Exchange in Zeolite 4A I. Garcı´a,† M. Solache-Rı´os,† P. Bosch,†,‡ and S. Bulbulian*,† Departamento de Quı´mica, Instituto Nacional de Investigaciones Nucleares, A. P. 18-1027, Col. Escando´ n, Delegacio´ n Miguel Hidalgo, C. P. 11801, Me´ xico, D. F., and Departamento de Quı´mica, Universidad Auto´ noma Metropolitana, Iztapalapa, A. P. 55-532, Michoaca´ n Esq. Purı´sima, Iztapalapa, C. P. 09340, Me´ xico, D. F. Received October 23, 1995. In Final Form: June 14, 1996
Introduction At room temperature, Co2+ ion exchange from an aqueous cobalt chloride solution with NaY zeolite was studied in a previous work.1 Upon dehydration, Co2+ ions are distributed in the Y zeolite network.2-7 The effect of ethylenediamine (EDA) during the ion exchange has been studied as well,2 and it was found that Co2+ uptake values in the zeolite were altered by EDA additions. A considerable increase of cobalt uptake was observed, in 600 °C dehydrated zeolite. It was then concluded that the hexagonal prisms and the sodalite cages are almost sealed with the Co2+ complexes formed with EDA which interact with the oxygens of the network within the large cavity of the zeolite. The increase of cobalt content in the bulk of the zeolite is of great importance to catalysis using metal ion containing zeolites. Indeed the turnover frequency for selective reduction of NOx by methane in Co-ferrierite increases with increasing Co2+ level.8 To confirm such propositions, the study of zeolite A under the same conditions should be most enlighting: Although zeolite A has sodalite cages, it does not have hexagonal prisms, and the large cavities in this zeolite are smaller. If specific sites of Y zeolite determine the cobalt uptake behavior, the curves obtained in zeolite A should be very different. Experimental Section Zeolite A was studied in its hydrated and dehydrated form. Zeolite A was left for 8 days in a 5 N NaCl solution. A 0.05 N CoCl2-0.05 N NaCl mixed solution (cobalt solution) was utilized for ion exchange. EDA from Merck was used without further purification. 60Co was obtained by neutron irradiation of Co(NO3)2‚6H2O, as in the previous work.2 Cobalt exchange was studied under various conditions. Each uptake curve was obtained as stated before.2 Results are given in milliequivalents of Co2+ uptake per gram of hydrated zeolite. Cobalt exchange at room temperature was studied in hydrated and dehydrated zeolites at 150 and 600 °C in vacuum, as in the previous work.2 In the first series of experiments EDA was added to the dehydrated zeolite before Co2+ exchange. In the second series EDA was added to the hydrated zeolite 4 h after the initial contact between the zeolite and the solution, and finally in the last series, EDA was added after a contact time of 4 h between cobalt solution * To whom correspondence should be addressed. † Instituto Nacional de Investigaciones Nucleares. ‡ Universidad Auto ´ noma Metropolitana, Iztapalapa. (1) Garcı´a, I.; Solache-Rı´os, M.; Bosch, P.; Bulbulian, S. J. Phys. Chem. 1993, 97, 1249. (2) Solache-Rı´os, M.; Garcı´a, I.; Martı´nez-Miranda, V.; Bosch, P.; Bulbulian, S. J. Radioanal. Nucl. Chem. Articles 1995, 191, 87. (3) Wichterlova, B.; Jire, P.; Curirova, A. Z. Chemie 1974, 88, 180. (4) Drakonov, S. S.; Kiselev, A. V.; Kuzmenko, N. M.; Ligin, V. I.; Zh. Fiz. Khim. 1975, 49, 3173. (5) Huta, P.; Lunsford, J. H. J. Chem. Phys. 1977, 49, 3173. (6) Egerton, T. A.; Hagan, A.; Stone, F. S.; Vickerman, D. C. J. Chem. Soc., Faraday Trans. 1972, 14, 723. (7) Fraenkel, D.; Shabtai, J. J. Am. Chem. Soc. 1977, 99, 7074. (8) Li, Y.; Armor, J. J. Catal. 1994, 150, 376.
S0743-7463(95)00919-X CCC: $12.00
Figure 1. Co2+ uptake curves in hydrated zeolite A: (A) no EDA added samples; (B) EDA added after 4 h of contact time between the zeolite and the cobalt solution.
Figure 2. Co2+ uptake in 150 °C dehydrated zeolite: (A) no EDA added samples; (B) EDA added after 4 h of contact time between the zeolite and the cobalt solution. and dehydrated zeolite A at 150 and 600 °C (in vacuum). All experiments were performed in the pH range 5-6.5. The Co2+ species in solution were determined by visible spectroscopy using a Beckman, model 35, spectrophotometer. Co2+ ion exchange with sodium ions from zeolite A was determined by γ spectroscopic analyses of 60Co. It was measured either with a NaI(Tl) well detector coupled to a monochannel Picker analyzer or with a Ge/hiperpure solid-state detector coupled to a 2048 channel pulse height analyzer. All zeolite samples were studied by X-ray diffraction with a Siemens D500 diffractometer with a copper anode X-ray tube. The K-R radiation was obtained with a diffracted beam monochromator.
Results The analytical data of the hydrated zeolite 4A treated with NaCl solution are summarized in the following formula: Na12Al12Si12O48(H2O)27 This zeolite contains 5.5 mequiv of cationic sites per gram. By visible spectroscopy the cobalt present in the solution was found to be in the form of the octahedral aquo complex [Co(H2O)6]2+, as expected. Curves A and B (Figures 1-3) compare the Co2+ uptake behavior (A) without and (B) with EDA, in hydrated (Figure 1), dehydrated at 150 °C (Figure 2), and dehydrated at 600 °C (Figure 3) zeolite samples. Curve A shows a fast sorption of Co2+ until uptake raised to a constant © 1996 American Chemical Society
Notes
Langmuir, Vol. 12, No. 18, 1996 4475
corresponds to 2 cobalt atoms per unit cell, one of them probably in the sodalite cage and the other in the large cavity.9 Such was not the case in Y zeolite, whose Co2+ uptake depended strongly on the hydration level and on the dehydration temperature. Furthermore, in Y zeolite the shape of the curve varied; after a fast cobalt sorption uptake in which Na+ ions were replaced by Co2+ ions, a cobalt desorption process was observed. In the present case, for zeolite A, no cobalt desorption process was observed. We concluded therefore that in zeolite A cobalt species are trapped in the sodalite cage due to the smaller size of the windows; furthermore, the proposed reaction with Cl- ions in the solution does not occur.1 The reactions that may occur in the CoNaA zeolite as in the CoNaY zeolite10 when EDA is added at room temperature are the following: Figure 3. Co2+ uptake in 600 °C dehydrated zeolite: (A) no EDA added samples; (B) EDA added after 4 h of contact time between the zeolite and the cobalt solution; (C) EDA added before starting the ion exchange with the cobalt solution.
value of approximately 2 Co2+ mequiv/g of zeolite. The maximum uptake in curve B was raised to 2.5 Co2+ mequiv/g of zeolite, showing a clear effect of EDA addition. These results were reproduced when the zeolite was dehydrated at 150 °C (Figure 2) or at 600 °C (Figure 3). Figure 3 describes also the Co2+ uptake behavior for a 600 °C dehydrated sample when EDA was added before starting the ion exchange with the cobalt solution (curve C). To summarize, EDA addition was crucial in the experiments, as the Co2+ maximum uptake value (2.5 mequiv/g) increased 20% in all cases. On the other hand all zeolite samples followed the same behavior. When EDA was not added, the hydrated or dehydrated at 150 or 600 °C samples reached similar maximum plateau values (2 mequiv/g). Therefore the hydration level and the dehydration temperature are not determining parameters. All samples were found to be crystalline before and after cobalt sorption and EDA addition. The X-ray diffraction peaks were all attributed to zeolite A, and no other compounds were found. As expected, due to the exchange process, the relative intensities of the diffraction peaks varied depending on the cation present in the zeolite network. Discussion The maximum cation capacity of the studied zeolite was 5.5 mequiv/g; however, under our working conditions only 2 mequiv/g of Na+ were exchanged with Co2+. This amount
[Co(H2O)6]2+ + 3EDA a [Co(EDA)3]2+ + 6H2O [Co(H2O)6]2+ + 2EDA a LCo(EDA)2(H2O) L ) H2O, zeolite oxygen, or monodentate ethylenediamine The addition of EDA to Y zeolite increased the amount of cobalt uptake as much as 50%. The sodalite cages and the hexagonal prisms seemed to be sealed by the Co2+ complexes formed with EDA.2 Instead, in zeolite A, the amount of cobalt retained in the presence of EDA was smaller probably due to the lack of hexagonal prisms. Conclusions The following points emerge from this study: When EDA was added before or after the ion exchange, the maximum plateau value increased up to about 2.5 mequiv/g. This amount corresponded to an increase of one Co2+ per two unit cells. This behavior was independent of the hydration level or pretreating temperature. Hydrated or dehydrated zeolite samples follow the same sorption behavior; they reach similar maximum plateau values of 2 mequiv/g of zeolite A. The increase of Co2+ uptake can be attributed to the formation of a cobalt complex in the zeolite. Less cobalt is retained in zeolite A than in zeolite Y, as the former has no hexagonal prisms. LA950919+ (9) Breck, D. Zeolite Molecular Sieves; John Wiley & Sons: New York, London, Sidney, Toronto, 1974. (10) Hutta, P. J.; Lunsford, J. H. J. Am. Chem. Phys. 1977, 66, 4716.
