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J . Phys. Chem. 1988, 92, 14-15
Comparative Electron Spin Echo Study of Aluminum Modulation Associated with Cu2+ in Ca-ZSM-5 and K-ZSM-5 Zeolites Candace E. Sass and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: October 16, 1987)
Electron spin echoes (ESE) were recorded from fully hydrated samples of CuCa-ZSM-5 and CuK-ZSM-5. In the case of CuCa-ZSM-5, modulation from 27Aland 'H was observed. In contrast, the ESE spectrum of CuK-ZSM-5 only contains protium modulation. These results indicate that the exchange location for cupric ions in Ca-ZSM-5 is nearer to an aluminum nucleus than in K-ZSM-5. Thus, the major cation valency seems to control the exchange locations of catalytically active ions.
Introduction Zeolites containing cupric ions are used as catalysts for a wide variety of chemical Electron spin resonance (ESR) has been employed in conjunction with electron spin echo modulation (ESEM) spectroscopy to characterize many of these Cu*+-doped zeolites including zeolites A, X, and Y.3-5 ESR studies2 provide valuable information about the stereochemistry of the Cu2+complexes formed in zeolites. ESEM studies allow one to determine the number of adsorbate molecules that are coordinated to Cu2+,as well as the distance between Cu2+ and the ligand.5 This information is critical for understanding the basis of their catalytic activity. In a typical ESEM study, modulation from specifically deuteriated adsorbates is analyzed to determine the coordination of Cu2+ in the zeolite. However, modulation from 27Alcontained in the zeolite framework has proven to be useful in examining the position of Cuz+ in zeolites such as types X6 and ZK4.7 A recent ESEM study of Cu2+ in NaH-ZSM-S8 did not show any 27Al modulation. This indicates that the exchanged cupric ion site is approximately 0.45 nm or further from an aluminum nucleus based on model calculations. In this communication, the results of an ESEM study of aluminum modulation from Cu2+-doped K-ZSM-5 and Ca-ZSM-5 are compared and it is found that 27Al modulation is observed in Ca-ZSM-5 which demonstrates an important cocation effect on the Cu2+ siting.
Experimental Section Zeolite ZSM-5 was prepared following the method of Araya and L ~ w e . ~The zeolite was synthesized in the Na-tetrapropylammonium form, which was subsequently calcined at 500 OC for 8 h to yield the N a H form of ZSM-5. The Ca2+and K+ forms were prepared by repeatedly exchanging NaH-ZSM-5 with 0.1 M solutions of CaCl, and KC2H30,respectively. The samples were doped with Cu2+by exchanging 10 mL of M Cu(N03), with 1 g of either Ca-ZSM-5 or K-ZSM-5 and 100 mL of triply distilled water. This procedure resulted in an exchange of approximately 1 Cu2+per 40 unit cells. The samples were washed thoroughly with hot, triply distilled water and allowed to air-dry. The %/A1 ratio and cation compositions of each of the samples were determined by commercial atomic absorption analysis. The samples were loaded into 3 mm 0.d. Suprasil quartz tubes and (1) Kevan, L. Rev. Chem. Intermed. 1987, 8, 53. (2) Buzon.J.; Guichard, N.; Lebbe, J.; Prevot, A,; Serpinet, J.; Tranchant, J. Coord. Chem. Rev. 1970, 5'3, 142. (3) Anderson, M. W.; Kevan, L. J . Phys. Chem. 1986, 90, 6452. (4) Anderson, M. W.; Kevan, L. J . Phys. Chem. 1987, 91, 2926. (5) Ichikawa, T.;Kevan, L. J. A m . Chem. SOC. 1981, 103, 5355. (6) Ichikawa, T.;Kevan, L. J . A m . Chem. SOC.1983, 105, 402. (7) Anderson, M. W.; Kevan, L. J . Phys. Chem. 1986, 90, 3206. (8) Anderson, M. W.; Kevan, L. J . Phys. Chem. 1987, 91, 4174. (9) Araya, A,; Lowe, B. M. Zeolites 1986, 6, 111
0022-3654/88/2092-0014$01.50/0
exposed to the saturated rmm temperature vapor pressure of water (-20 Torr) before sealing. Electron spin echo spectra were recorded at 4 K on a home-built spectrometer that has been described.1° Both two-pulse and three-pulse echo experiments were carried out. A complete description of the theory of ESE is given elsewhere.''
Results and Discussion Figure l a shows the three-pulse electron spin echo spectrum that was obtained from a fully hydrated sample of CuCa-ZSM-5. Two distinct modulation frequencies can be observed in this spectrum. The shallow protium modulation arises from an interaction of Cuz+with hydrogens in H20. The deeper modulation has a frequency of 3.55 MHz, which coincides with the free nuclear frequency of 27Al at the magnetic field used. This modulation can only arise from a direct interaction between Cu2+ and an aluminum contained in the zeolite framework. In contrast, only protium modulation can be observed in the ESE spectrum of CuK-ZSM-5, which is shown in Figure lb. This indicates that no direct interaction occurs between Cu2+ and the zeofitic aluminum in K-ZSM-5. This result is consistent with the result of a previous study of CuNaH-ZSM-5, where no modulation due to the framework aluminum was detected.8 The ESEM spectra for CuK-ZSM-5 and CuCa-ZSM-5 that were obtained in a two-pulse experiment are shown in Figure 2. The lack of 27Almodulation in the spectrum of CuK-ZSM-5 is readily apparent and confirms the results of the three-pulse ESEM studies. However, modulation from 27Alnuclei can be observed in the spectrum of CuCa-ZSM-5, as was noted in the three-pulse experiments. These results are consistent with the three-pulse experimental results and support the conclusion that a direct interaction between Cu2+and 27Aloccurs in CuCa-ZSM-5, but not in CuK-ZSM-5. The difference in the observed ESE spectrum of cupric ions in CuCa-ZSM-5 compared to those of the other ZSM-5 zeolites studied can be attributed to a difference in the charge of the major cation. These cations compensate for the negatively charged aluminum tetrahedra of the zeolite lattice. One monovalent cation such as K+ can balance the charge of one aluminum center by coordinating at a site close to the negatively charged aluminum. Ion exchange removes 2K+ for each Cu2+that enters the zeolite. The single cupric ion must then neutralize the charge of two negative aluminum centers. Since Cu2+ must compensate two aluminum centers from a single site, one can envision that the Cu2+ is not located close to one aluminum center, but is approximately equidistant between the two aluminum nuclei. Such a location for Cu2+is clearly indicated by the ESEM results. The lack of aluminum modulation in Na-ZSM-5 and K-ZSM-5 in(10) Ichikawa, T.; Kevan, L.; Narayana, P. A. J . Phys. Chem. 1979, 83, 3378. (1 1) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L.; Schwartz, R. N., Eds.; Wiley-Interscience: New York, 1979; Chapter 8.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 1 , 1988 15
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Three-pulse ESEM spectrum of fully hydrated CuK-ZSM-5 recorded at 4 K. The arrows denote where the 27Alperiod would be ifaluminum modulation were observed in this sample. dicates that the cupric ions are not directly interacting with the framework aluminums. Model simulations of echo modulation show that one or two aluminum nuclei that are located further than 0.45 nm from Cu2+do not generate observable modulation, since the effective magnetic field is too weak at this distance." The ZSM-5 samples that were used in this study had an average Si/Al ratio of 26/ 1. If we assume that the aluminum distribution is random, then the average AI-A1 distance in the main channel is about 2.8 nm. A cupric ion located midway between aluminums would be beyond the distance at which modulation is observable. When the cation contained in the zeolite is divalent, as in CuCa-ZSM-5, the negative charge of two alumina tetrahedra is balanced by a single cation. However, the location of the cation relative to the position of the aluminum is not known. At the low level of exchange employed here (approximately 1 Cu2+ for 70 Ca2+), it appears that the most easily exchanged sites are those located less than 0.5 nm from an aluminum center. While quantitative simulations of the aluminum modulation were not
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undertaken, since the quadrupole contribution is unknown, model calculations neglecting the quadrupole interaction of 27Alwere carried out. For one aluminum, modulation was present until a distance of 0.35 nm was reached. Modulation from two alumin u m was detectable up to distances of 0.45 nm. Weak quadrupole interactions usually damp the echo modulation, so these model calculations do demonstrate that the Cuz+ must be located less than 0.5 nm for modulation to be observable. The results obtained in this study show the position of Cu2+ in ZSM-5 is influenced by the major cation. By selecting the proper cation, it may be possible to control the position of catalytically active ions in ZSM-5 zeolites.
Acknowledgment. This research was supported by the National Science Foundation (CHM-8514108), the Robert A. Welch Foundation, and the Texas Advanced Technology Research Program. We thank Michael Anderson for critical assistance at the beginning of this work.
