Langmuir 1988, 4 , 1044-1048
1044
the micellar and hexagonal phases.
Acknowledgment‘ Thanks are due to R*G* and Gamble Company, Cincinnati, for a of the kind gift of the surfactant used in the present study and to G. Karlstrom and H. Wennerstrom for valuable discussions. The Swedish Natural Science Research Council and Comitate Teccnologico (C*N.R-,Italy) are thanked for financial support. Finally, we would like to thank Bruce Springsteen for providing suitable acoustical background in our NMR room. Appendix this Appendix we will presenta simple argument in favor of a situation where the main components of the electric field gradient, V,,, and thus the quadrupole coupling constants, x, in C12TACl and C2,AH are equal, whereas the asymmetry parameters, 7, are not equal at the
site of the nitrogen in the two surfactants. Let us assume that the contribution to the field gradient on the nitrogen from the hydrocarbon chain does not depend on the length. Moreover, let us arbitrarily set this contribution equal to zero. To the contribution from the methyl partial charges are positioned at the sites of these groups, and the contribution from one such charge to the field gradient on the nitrogen is arbitrarily set equal to It is now easy to show that for C12TACl vz,= while = o, and the z-axis of the principal axis system of the field gradient tensor coincides k t h the N-a-CH2 bond. In the C-&H case, V,, = Vowhile t = 1, and the z-axis of the Principal axis system is Perpendicular to the CH2-N-CH2 plane; the axis with the field gradient component equal toO‘- is Perpendicular to the CH3-N-CH3 plane.
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Registry No. C,&H, 72861-47-3.
Mossbauer Spectroscopic Studies of Fe(I1)-Y, Fe(11)-Mordenite, and Fe(11)-ZSM-5 Zeolites Luis M. Aparicio and J. A. Dumesic” Chemical Engineering Department, University of Wisconsin-Madison, 1415 Johnson Drive, Madison, Wisconsin 53706 Received January 26, 1988. I n Final Form: April 12, 1988 Mossbauer spectroscopy was used to study the room temperature adsorption of dioxygen and carbon dioxide on a high %/A1 ratio Fe(I1)-Y zeolite and of carbon monoxide on Fe-mordenite and Fe-ZSM-5. All of these adsorbates were found to interact with cations that contribute Fe(II) doublets with small splittings to the spectra (i.e., inner doublets). In each case, the adsorption process increased the coordination of the cations, decreasing the spectral area of the Fe(I1) inner doublet and forming a new doublet with a larger quadrupole splitting. The adsorption of CO on Fe-mordenite and Fe-ZSM-5 removed the inner doublet completely, whereas the same adsorbate does this only partially for Fe(I1)-Y. The assignments made for the two doublets present in the spectra of these zeolites were tested theoretically by calculating temperature dependences of the quadrupole splitting for Fe(1I) cations in various exchange sites in Y-zeolite. The calculations suggested that the differences in Mossbauer spectroscopy parameters of cations giving the two doublets may arise from differences in the length and degree of covalency of Fe-0 bonds in the two types of coordination.
Introduction Mossbauer spectroscopy has been used in a number of studies to characterize divalent iron exchange into zeolites. The zeolites covered by these studies have included Y~ e o l i t e , l -m~~ r d e n i t e ,A-zeolite,6-12 ~ L-zeolite,13J4 and ZSM-5.15 In general, the Mossbauer spectra of these materials are composed of two Fe(I1) quadrupole doublets. One of these, generally denoted as the “outer doublet”, has a relatively high isomer shift (1.0-1.3 mm/s with respect to metallic Fe) and a large, temperature-dependent quadrupole splitting (2.1-2.4 mm/s at room temperature). The other doublet, generally denoted as the “inner doublet”, has a lower isomer shift (0.8-1.0 mm/s with respect to metallic Fe) and a small, temperature-indedpendent quadrupole splitting (0.5-1.0 mm/s). The outer doublet has been assigned to coordinatively saturated Fe cations, while the inner doublet has been assigned to coordinatively unsaturated cations. These assignments were originally made for Y-zeolite based on an empirical correlation between isomer shift and coordination number and
* Author to whom all correspondence should be addressed. 0743-7463/88/2404-1044$01.50/0
on observed interactions between the cations responsible for the doublets and adsorbate gases of different ~ i z e . ~ . ~ Assignments in other zeolites have been made primarily from the similarities between their spectra and the spectrum of Fe(II)-Y.5,6J3 (1)Morice, J. A.;Rees, L. V. C. Trans. Faraday SOC.1968,64,1388. (2)Delgass, W.N.;Garten, R. L.; Boudart, M. J. Phys. Chem. 1969, 73,2970. (3)Garten, R. L.; Delgaas, W. N.; Boudart, M. J. Catal. 1970,18,90. (4)Aparicio, L. M.; Dumesic, J. A.; Fang, S.M.; Long, M. A.; Ulla, M. A.; Millman, W. S.; Hall, W. K. J. Catal. 1987,104,381. (5)Garten, R. L.; Gallard-Nechtschein, J.; Boudart, M. Ind. Eng. Chem. Fundam. 1973,12,299. ( 6 ) Dickson, B. L.; Rees, L. V. C. J. Chem. SOC.,Faraday Trans. 1 1974. 70. 2038. ( 7 ) Dickson, B. L.; Rees, L. V. C. J. Chem. SOC.,Faraday Trans. 1 1974,70,2051. (8)Dickson, B. L.; Rees, L. V. C. J. Chem. SOC., Faraday Trans. 1
-.
1974. 70. - -, 20130.
(9)Gao, Z.;Rees, L. V. C. Zeolites 1982,2, 72. (10)Gao, Z.;Rees, L. V. C. Zeolites 1982,2,79. (11)Gao, Z.;Rees, L. V. C. Zeolites 1982,2,205. (12)Gao, Z.;Rees, L. V. C. Zeolites 1982,2, 222. (13)Fitch, F. R.; Rees, L. V. C. Zeolites 1982,2, 33. (14)Fitch, F. R.; Rees, L. V. C. Zeolites 1982,2,279. (15)Petrera, M.; Gennaro, A.; Gherardi, P.; Gubitosa, G.; Pemicone, N. J. Chem. SOC.,Faraday Trans. 1 1984,80, 709.
0 1988 American Chemical Society
Langmuir, Vol. 4,No. 4,1988 1045
Mossbauer Spectroscopy of Fe-Zeolites
Upon partial hydration or adsorption of gases on Fe(II)-zeolites, cations that in the absence of adsorbates give rise to the inner doublet have been observed to become coordinatively saturated, giving doublets with parameters similar to the outer doublet. Such an effect has been observed clearly when Fe(I1)-A has been partially hydrated or exposed to methanol or ethanol!v8J2 when Fe(I1)-L has been partially hydrated or exposed to ethanol, isopropyl alcohol, tert-butyl alcohol, methylamine, or triethylamine,13J4and when Fe(I1)-Y samples with high Si/Al ratios have been partially hydrated or exposed to C0.4 In the case of Fe(I1)-Y zeolites having low Si/Al ratios, the effect is also believed to occur; however, because the spectra are dominated by the outer doublet, the disappearance of the inner doublet can only be detected when these zeolites are exposed to piperidine, ethanol, tert-butyl alcohol, and CS2.2 In the work reported here, Mossbauer spectroscopic studies of the interaction of Fe(I1)-zeolites with adsorbate gases were extended to the room temperature adsorption of O2 and C 0 2on high %/A1 ratio Fe-Y, as well as to the room temperature adsorption of CO on Fe-mordenite and Fe-ZSM-5. In addition, the assignments of the outer and inner doublets to cations with different coordination environments were tested by performing calculations of the temperature dependence of the quadrupole splitting for Fe(I1) cations in various exchange sites in Y-zeolite.
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Experimental Sections The samples for this study were prepared by ion exchangeof iron into sodium Y-zeolite, mordenite, and ZSM-5. The starting materials for the Y-zeolite and mordenite were Linde SK-40 and Linde LZ-M5, respectively. Sodium ZSMd was prepared by the method of F1anigen,l6and its crystallinity was checked by X-ray diffraction, using methods discussed previously!J7 The high Si/M ratio Y-zeolite sample was prepared by using the method of Skeels and Breck,l*where ("4)2SiF6 is reacted in an aqueous suspension of the zeolite. The details of the iron-exchangeprocedure have been summarized el~ewhere.~J~ The Mossbauer spectra were recorded with an Austin Science Associates Model S-600 Mossbauer spectrometer controller and a Tracor-Northern Model N6-900 multichannel analyzer. All isomer shifts are with respect to metallic iron at room temperature. The details of the experimental procedure have been outlined previ~usly.~ Results and Discussion Figure 1 shows spectra of an Fe(I1)-Y sample with a Si/Al ratio of 8.9 before and after room temperature exposure to 100 kPa of O2 and COP. The figure shows that both of these gases increase the coordination of cations in this zeolite that were originally unsaturated, decreasing the spectral area of the inner doublet while simultaneously increasing that of the outer doublet. The inner doublet in this sample does not disappear completely upon adsorption of the gases. This is an indication that some of the coordinatively unsaturated cations (presumablyin sites I' and/or 11" in the sodalite units) are not accessible to O2 and COP,while other cations (presumably in site I1 in the supercage) are accessible to these adsorbates. An essentially identical result was obtained for this same sample upon exposure to CO in a previous s t ~ d y . ~ In Figures 2 and 3, spectra of Fe(I1)-mordenite and Fe(II)-ZSM-5 are shown before and after exposure to 100 (16) Flanigen, E. M.; Grose, R. W. U.S.Patent 4061724, 1977. (17) Aparicio, L.M.; Hall, W. K.; Fang, S. M.; Ulla, M. A.; Millman, W. S.;Dumesic, J. A. J. Catal. 1987, 108,233. Breck, D. W. R o c . of the Sixth Inter. Zeol. Conf.; (18) Skeels, G.W.; Olson, D., Bisio, A., Eds.; Butterworths: London, 1984; p 87.
Figure 1. Mossbauer spectra of Fe(I1)-Y @/A1 = 8.9): (A) evacuated, (B) after exposureto 100 kPa of 02,(C)after exposure to 100 kPa of C02.
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