Ce4+ Redox

Double 6-Ring as a Unique Cation Site for the Ce3+/Ce4+ Redox Couple in Zeolites ... unit among the cation sites in seven different zeolite structures...
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J. Phys. Chem. B 2001, 105, 11961-11963

11961

Double 6-Ring as a Unique Cation Site for the Ce3+/Ce4+ Redox Couple in Zeolites Suk Bong Hong* DiVision of Chemical Engineering, Hanbat National UniVersity, Taejon 305-719, Korea ReceiVed: May 31, 2001; In Final Form: August 23, 2001

Luminescence spectroscopy reveals that the intrazeolitic redox process of Ce3+ T Ce4+ takes place only inside the double 6-ring (D6R) unit among the cation sites in seven different zeolite structures studied here. This unexpected phenomenon can be rationalized in terms of the spatial constraints in D6Rs imposed by the location of Ce3+ ions larger than Ce4+ ions, leading to a considerable decrease in the potential barrier of the Ce3+/Ce4+ couple.

Introduction

Experimental Section

The rare-earth-exchanged zeolites are widely used as commercial catalysts in many petrochemical processes such as isomerization, alkylation, and cracking of hydrocarbons.1 Since the degree of exchange and the location of trivalent cations in zeolites directly affect their hydrothermal stability and catalytic activity, detailed information on the intrazeolitic distribution and chemical environment of rare-earth ions is crucial to the preparation of rare-earth-exchanged zeolites with improved catalytic properties. A phenomenon repeatedly encountered in the study of this important class of zeolites is the migration of rare-earth ions to internal sites of zeolites during the thermal treatment step that is essential for further ion exchange and/or for the catalyst activation. For example, the rare-earth ions initially exchanged into the supercages of Na-Y are known to migrate to β-cages upon dehydration around 80 °C.2,3 More complications arise when the oxidation state of rare-earth ions in zeolites is changed from +3 to +2 or +4, depending on the type of exchanged cations and subsequent thermal treatment conditions. Despite the industrial importance of rare-earth-exchanged zeolites, however, there is little known on the relationship between the intrazeolitic location of rare-earth ions and the change in their oxidation states. Luminescence spectroscopy has provided valuable information on the coordination environment and oxidation state of rareearth ions in zeolites.4 In addition, this technique is known to be useful for monitoring thermal effects on the intrazeolitic location of rare-earth ions, especially for determining their migration temperatures to specific internal sites.5-7 The Ce3+/ Ce4+ couple is one of the most noted various oxidation states for rare-earth ions. Here we report the preliminary results obtained from luminescence measurements on the Ce3+ ions exchanged into a number of zeolites with different framework structures followed by thermal treatments at 25-500 °C in order to provide clear evidence that, among the possible cation sites in the zeolite structures studied here, the double 6-ring (D6R) unit is the sole cation site in which the redox process of Ce3+ T Ce4+ can take place.

Fourteen zeolites including seven different framework topologies (i.e., LTA, FAU, EMT, RHO, CHA, MFI, and MOR) in the Si/Al ratio range 1.0-22.0 were synthesized and converted to their sodium form according to the procedures that are given elsewhere.8 The Ce3+ ion exchange was performed by stirring zeolite powder in 0.01-0.05 M CeCl3 solutions (2 g soild/100 mL solution) at room temperature for 24 h. The Ce3+-exchanged zeolites were heated under flowing air (50 cm3 min-1) at the desired temperatures for 4 h and kept over saturated NH4Cl solution at room temperature for 2 days, to avoid possible broadening of luminescence spectra arising from dehydration. Crystallinity and phase purity of all zeolites prepared here were determined by powder X-ray diffraction (XRD) using a Rigaku Miniflex diffractometer (Cu KR radiation). Chemical analysis was carried out by a Jarrell-Ash Polyscan 61E inductively coupled plasma (ICP) spectrometer together with a Perkin-Elmer 5000 atomic absorption spectrophotometer. Emission and excitation spectra were recorded at room temperature on a Perkin-Elmer LS-50B luminescence spectrometer equipped with a 150-W Xe flash lamp and a Monk-Gillieson type monochromator. All of the luminescence spectra obtained were automatically corrected for the instrumental response (determined with calibrated fluorescent lamps).

* Phone: +82-42-821-1549. Fax: +82-42-821-1593. E-mail: sbhong@ hanbat.ac.kr.

Results and Discussion Powder XRD patterns of all zeolites prepared here show that they have the corresponding structures and no impurity phases are detected. Also, the crystallinity of each material was found to remain unchanged during the Ce3+ ion exchange and thermal treatment steps. The chemical compositions of representative zeolites after Ce3+ ion exchange are listed in Table 1. These data reveal that the larger pore size the zeolite has, the higher percentage of Na+ exchanged with Ce3+ it has. However, Table 1 shows that when the zeolites used in Ce3+ ion exchange possess the same framework topology, the percentage of Na+ exchanged with Ce3+ is almost independent of the framework Si/Al ratio. This trend is consistent with that found in our previous work on Tb3+-exchanged LTA and FAU zeolites.5-7 Figure 1 shows changes in the emission spectrum of CeNa-X zeolites heated at different temperatures. The sample dried at

10.1021/jp012088o CCC: $20.00 © 2001 American Chemical Society Published on Web 11/03/2001

11962 J. Phys. Chem. B, Vol. 105, No. 48, 2001

Hong

TABLE 1: Chemical Compositions of Representative Zeolites Prepared in This Study sample

IZA code

anhydrous unitcell composition

Si/Al ratio

% exchanged

CeNa-A CeNa-ZK-4 CeNa-X CeNa-Y CeNa-EMC-2 CeNa-chabazite CeNa-rho CeNa-mordenite CeNa-ZSM-5

LTA LTA FAU FAU EMT CHA RHO MOR MFI

Ce4.3Na82.1Al95Si97O384 Ce2.1Na51.4Al57.7Si134.3O384 Ce14.4Na37.8Al81Si111O384 Ce10.1Na24.0.2Al54.5Si137.5O384 Ce3.9Na10.1Al21.8Si74.2O192 Ce0.2Na9.4Al10.0Si26.0O72 Ce0.2Na6.3Al6.9Si31.1O98 Ce0.4Na6.8Al8.0Si40.0O96 Ce0.2Na6.0Al6.6Si89.4O192

1.02 2.33 1.37 2.52 3.40 2.60 4.51 5.0 13.5

14 11 53 56 54 6 9 15 9

room temperature after Ce3+ ion exchange exhibits a broad band at 367 nm and a shoulder at 356 nm, assigned to the 2T2g f 2F 2 2 7/2 and T2g f F5/2 transitions between 4f and 5d levels for the Ce3+ ion, respectively.9 The band positions of these two transitions remain almost unchanged with the Ce3+ content, which is in good agreement with the result previously reported.10 As seen in Figure 1, however, their positions were found to depend highly on the pretreatment temperature. When CeNa-X was heated at 100 °C, for example, the 2T2g f 2F7/2 band at 367 nm shifts to a higher energy region, i.e., 361 nm. With elevating the treatment temperature to 150 °C, by contrast, this band appears at 375 nm. Finally, no noticeable emission bands are observed from the sample heated at 200 °C. According to the ideal point symmetries for cation sites in the FAU topology given by Schoonheydt,11 site I in D6Rs has the ideal Oh symmetry, while both sites I′ and II′ in β-cages have the ideal point symmetry of C3V. In addition, it is well established that the relative energy of the lower d orbital among the five degenerate d levels by crystal field splitting in Oh is lower than that in C3V, due to the higher crystal field stabilization energy (CFSE).12 In the case of CeNa-A and CeNa-ZK-4, where no D6Rs but R- and β-cages exist, comparison of the emission spectra (not shown) for the respective materials dried at room temperature and heated at 100 °C after Ce3+ ion exchange, respectively, reveals a noticeable shift of the 2T2g f 2F7/2 transition to a higher energy region. Therefore, changes in the emission spectrum of CeNa-X found in Figure 1 strongly suggest that most of the Ce3+ ions exchanged into the supercages of Na-X migrate first to β-cages at 100 °C and in turn to D6Rs at 150 °C, which is based on the fact that the band positions of 4fn f 4fn-1 5d transitions for the intrazeolitic rare-earth ions are very sensitive to the site symmetry around cations.4-7,10 The validity of our emission interpretation for CeNa-X can be further supported by the excitation spectra of Figure 1, in which the two transitions for the Ce3+ ion are well resolved compared

to the emission spectra. Since the Ce4+ ion is not luminescence active, in addition, it is clear that the oxidation state of the cations migrated to D6Rs changes from +3 to +4. All changes in the location and oxidation state of Ce3+ ions in Na-X presented so far, which were caused by thermal treatments at different temperatures, are actually consistent with those observed from the X-ray crystallographic studies of CeNa-X zeolites reported by Olson et al.13 and Hunter and Scherzer.14 Finally, it should be noted that upon exposure to H2 at 400 °C the CeNa-X sample heated at 200 °C yields essentially the same luminescence spectra as those of the sample heated at 150 °C, indicating the reduction of Ce4+ to Ce3+ in D6Rs. Figure 2 compares the excitation spectra of CeNa-Y, CeNaZK-4, CeNa-chabazite, and CeNa-rho zeolites pretreated at 25-500 °C. When CeNa-Y was dried at room temperature after Ce3+ ion exchange, the 2F7/2 f 2T2g band is observed at 299 nm, which is shorter in wavelength by 5 nm than that (304 nm) from unheated CeNa-X. With elevating the treatment temperature to 200 °C, this band shifts first to 291 nm and then to 304 nm. We also note that the band at 304 nm, as well as the 2F5/2 f 2T2g transition appearing at a higher energy region, is completely missing in the excitation spectrum of CeNa-Y

Figure 1. (a) Emission and (b) excitation spectra of CeNa-X zeolites pretreated at different temperatures. The excitation and emission wavelengths used are 300 and 370 nm, respectively.

Figure 2. Excitation spectra of (a) CeNa-Y, (b) CeNa-ZK-4, (c) CeNa-chabazite, and (d) CeNa-rho zeolites pretreated at different temperatures. The emission wavelength used is 370 nm.

A Unique Cation Site for the Ce3+/Ce4+ Redox Couple

J. Phys. Chem. B, Vol. 105, No. 48, 2001 11963

Figure 3. Excitation spectra of (a) CeNa-EMC-2, (b) CeNa-mordenite, and (c) CeNa-ZSM-5 zeolites pretreated at different temperatures. The emission wavelength is 370 nm.

heated at 300 °C. Thus, the Ce3+ oxidation temperature was found to be higher in Na-Y than in Na-X. This is not unexpected because the migration temperature of rare-earth ions to a specific internal site in zeolites becomes higher with decreasing Al content in the zeolite.5-7 Unlike that of unheated CeNa-Y, on the other hand, the excitation spectrum of unheated CeNa-ZK-4 is characterized by a strong band at 261 nm together with a very weak band around 295 nm, revealing that differences in the zeolite structure are sufficient to give matrix effects on the Ce3+ excitation. An important observation obtained from Figure 2 is that CeNa-ZK-4 still exhibits two excitation bands around 280 and 260 nm even after heating at 500 °C, while heating the CeNachabazite (CHA) sample at 400 °C causes the excitation intensity to disappear. Notice that the LTA topology contains no D6Rs, since this structure is built from β cages linked in a cubic array via double 4-ring (D4R) units. However, the CHA topology is characterized by a series of interconnected D6Rs forming a single type of open cage.15 Thus, it is clear that the intrazeolitic oxidation of Ce3+ to Ce4+ occurs inside the D6Rs rather than inside the more spacious cation sites. This can be further supported by several lines of observations given below. First, two bands at 300 and 253 nm are clearly found even in the excitation spectrum of CeNa-rho (RHO) heated at 500 °C (Figure 2). The RHO topology consists of R cages linked in a cubic array via double 8-ring (D8R) units. Second, thermal treatment of CeNa-EMC-2 containing D6Rs as the structural unit at 500 °C gave rise to disappearance of all excitation bands, as seen in Figure 3. Third, no significant changes in the excitation band intensity are observed from CeNa-mordenite and CeNa-ZSM-5 in which all of the cation sites are located on channels with g8-rings, even after heating at 500 °C. These results taken in total lead us to believe that the oxidation of Ce3+ to Ce4+ in sites with more than 6 rings is energetically unfavorable. Apparently, the metal cations located at the so-called site I with the ideal Oh symmetry in D6Rs are so “buried” that they cannot be easily attacked by H2/O2. Thus, these cations have been considered much more difficult to be reduced/oxidized than those located in more spacious sites such as β- or supercages.11 Since the Ce4+ ion has no 4f electrons, on the other hand, the oxidation of Ce3+ to Ce4+ in D6Rs found here cannot be explained by considering CFSE. Rather, we believe that this unexpected phenomenon is due to the spatial restrictions imposed by D6Rs with a maximum free dimension of 2.7 Å.16 The effective radius (1.15 Å) of Ce3+ for six-coordination is fairly larger than that (1.01 Å) of Ce4+ with the same

coordination number.17 In our view, the difference (ca. 0.14 Å) in the effective cation radius is sufficiently large to influence the spatial constraints inside the D6Rs and eventually to decrease the potential barrier of the Ce3+/Ce4+ couple. If such is the case, the Ce3+ oxidation in buried D6Rs could be possible in the presence of O2 at elevated temperatures. The importance of the size of cation sites in the intrazeolitic oxidation of Ce3+ can be further evidenced by the fact that the excitation features of CeNa-rho are retained even after heating at 500 °C (Figure 2), although this zeolite contains D8Rs with a maximum free dimension of 4.4 Å where the cation site has the same symmetry as that (Oh) of site I in D6Rs. To the best of our knowledge, in conclusion, our study is the first example where the intrazeolitic oxidation state of metal ions can differ according to the structural feature of zeolites. In addition, the overall results presented here reveal that the luminescence technique is simple but very powerful for monitoring changes in the intrazeolitic location and oxidation state of rare-earth ions. Acknowledgment. The author thanks the Korea Research Foundation for financial support (KRF-1999-E00346). He also extends his thanks to Prof. C. W. Park at HNU for helpful discussion. References and Notes (1) Chen, N. Y.; Garwood, W. E.; Dywer, F. G. Shape SelectiVe Catalysts in Industrial Applications; Marcel Dekker: New York, 1989. (2) Bennett, J. M.; Smith, J. V. Mater. Res. Bull. 1969, 4, 343. (3) Lee, E. D. F.; Rees, L. V. Zeolites 1987, 7, 143. (4) Tanguay, J. F.; Suib, S. L. Catal. ReV.-Sci. Eng. 1987, 29, 1. (5) Hong, S. B.; Seo, J. S.; Pyun, C.-H.; Kim, C.-H.; Uh, Y. S. Catal. Lett. 1995, 30, 87. (6) Hong, S. B.; Shin, E. W.; Moon, S. H.; Pyun, C.-H.; Kim, C.-H.; Uh, Y. S. J. Phys. Chem. 1996, 99, 12274. (7) Hong, S. B.; Shin, E. W.; Moon, S. H.; Pyun, C.-H.; Kim, C.-H.; Uh, Y. S. J. Phys. Chem. 1996, 99, 12278. (8) International Zeolite Association, Synthesis Commission, http:// www.iza-online.org. (9) Blasse, G. Prog. Solid State Chem. 1988, 18, 79. (10) Kynast, U.; Weiler, V. AdV. Mater. 1994, 6, 937. (11) Schoonheydt, R. A. Catal. ReV.-Sci. Eng. 1993, 35, 129. (12) Krishnamurthy, R.; Schaap, W. B. J. Chem. Educ. 1969, 46, 799. (13) Olson, D. H.; Kokotailo, G. T.; Charnell, J. F. J. Colloid Interface Sci. 1968, 28, 305. (14) Hunter, F. D.; Scherzer, J. J. Catal. 1971, 20, 246. (15) Meier, W. W.; Olson, D. H.; Baerlocher, Ch. Atlas of Zeolite Structure Types, 4th ed.; Elsevier: New York, 1996. (16) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic: London, 1982; p 24. (17) Huheey, J. E. Inorganic Chemistry: Principles of Structure and ReactiVity, 2nd ed.; Harper: New York, 1978.