Langmuir 1999, 15, 5937-5941
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Interaction of Water with Alkali-Metal Cation-Exchanged X Type Zeolites: A Temperature-Programmed Desorption (TPD) and X-ray Diffraction Study† B. Hunger,*,‡ O. Klepel,‡ C. Kirschhock,§ M. Heuchel,‡ H. Toufar,| and H. Fuess⊥ Wilhelm-Ostwald-Institute for Physical and Theoretical Chemistry, Faculty of Chemistry and Mineralogy, University of Leipzig, D-04103 Leipzig, Germany, Center for Surface Chemistry and Catalysis, KU-Leuven, B-3001 Leuven (Heverlee), Belgium, Zeosorb GmbH, D-06749 Bitterfeld, Germany, and Department of Material Science, Technical University of Darmstadt, D-64287 Darmstadt, Germany Received September 17, 1998. In Final Form: January 21, 1999 We have investigated the interaction of water with alkali-metal cation-exchanged X type zeolites with different Si/Al ratios by means of temperature-programmed desorption (TPD). The nonisothermal desorption of water shows, depending on the type of cation (Li+, Na+, K+, Rb+, and Cs+) and the exchange degree, differently structured desorption profiles. Using a numerical regularization method, desorption energy distribution functions have been calculated. The width of the distributions of about 30-50 kJ mol-1 shows clearly the energetic heterogeneity of the water-zeolite interaction. All desorption energy distributions exhibit two characteristic ranges. Beside a small part at lower energy values the distribution functions have a pronounced main peak between 55 and 65 kJ mol-1 and a second range at higher energy values (65-95 kJ mol-1). The systematic changes with respect to position and intensity of the peaks at higher energy values show clearly the influence of the respective cation on the water desorption at higher temperatures. This is also supported by the fact that the position on the energy scale of the cation-specific peak correlates linearly with the softness of the cations. To determine the localization of these water molecules X-ray powder diffraction studies were carried out. It was found that in NaX, KX, and CsX at low loadings the water was exclusively localized between sodium ions on the SIII and SII* positions.
Introduction Water is an important guest molecule in the pore system of natural and synthetic zeolites.1 The water content of zeolites plays a decisive role in their adsorption and catalytic properties. Information about the interaction of water is also important with respect to their application for energy storage using adsorption methods (e.g. ref 2). Temperature-programmed desorption (TPD) has been frequently used for investigations concerning the interaction of water with zeolites of different type and with different cations (e.g. refs 3-12). However, until now no systematic investigation into the way cations influence the adsorption behavior of water on one type of zeolite † Presented at the Third International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland, August 9-16, 1998. ‡ University of Leipzig. § Center for Surface Chemistry and Catalysis. | Zeosorb GmbH. ⊥ Technical University of Darmstadt.
(1) Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry, and Use; John Wiley & Sons: New York, 1974. (2) Aiello, R.; Nastro, A.; Colella, C. Thermochim. Acta 1984, 79, 271. (3) Dima, E.; Rees, L. V. C. Zeolites 1987, 7, 219. (4) Kulkarni, S. J.; Kulkarni, S. B. Thermochim. Acta 1982, 54, 251. (5) Kulkarni, S. J.; Kulkarni, S. B. Thermochim. Acta 1982, 56, 93. (6) Wolf, B.; Siegel, H.; Scho¨llner, R.; Dyer, A. Thermochim. Acta 1985, 87, 117. (7) Dondur, V.; Vucelic, D. Thermochim. Acta 1983, 68, 91. (8) Dondur, V. In Thermal Analysis; Zivkovic, Z. D., Ed.; Bor, Yugoslavia, 1984; p 251. (9) Sa´nchez, J. R.; Palermo, A.; Aldao, C. M. Langmuir 1996, 12, 36. (10) Hoffmann, J.; Hunger, B.; Dombrowski, D.; Bauermeister, R. J. Therm. Anal. 1990, 36, 1487. (11) Hunger, B.; Heuchel, M.; Matysik, S.; Beck, K.; Einicke, W.-D. Thermochim. Acta 1995, 269/270, 599. (12) Hunger, B.; Matysik, S.; Heuchel, M.; Geidel, E.; Toufar, H. J. Therm. Anal. 1997, 49, 553.
existed. In addition, the methods for analysis of TPD experiments developed in recent years13-16 allow meanwhile detailed information on the energetic heterogeneity of solids. Until now, these methods were applied to only a few adsorbate systems. It is, therefore, the aim of this contribution to characterize in detail the interaction of water with alkali-metal cation-exchanged X type zeolites. To determine the localization of water molecules at low loadings additionally X-ray powder diffraction studies were carried out. Experimental Section Zeolites. Two commercial NaX zeolites (Chemie AG, Bitterfeld/Wolfen, Germany) with a Si/Al ratio of 1.18 and 1.35 were used as starting material for the cation-exchanged samples. Ion exchange was carried out with aqueous solutions of KCl, RbCl, and CsCl at 353 K for 8 h. The degree of cation exchange was determined by means of ICP-AES. Furthermore, LSX zeolites with Li+, K+, and Na+ cations, produced by ZeoSorb GmbH, Bitterfeld, Germany, were studied. The micropore volumes were determined by nitrogen adsorption at 77 K. The characteristics of all zeolites are summarized in Table 1. Temperature-Programmed Desorption (TPD). The temperature-programmed desorption (TPD) was carried out in a flow apparatus with helium as carrier gas (3 L h-1). For evolved gas detection both a thermal conductivity detector (TCD) and a (13) Karge, H. G.; Dondur, V. J. Phys. Chem. 1990, 94, 765. (14) Bischke, S. D.; Chemburkar, R. M.; Brown, L. F.; Travis, B. J. In Fundamentals of Adsorption-Proceedings of the Third International Conference on Fundamentals of Adsorption; Mersmann, A. B., Scholl, S. E., Eds.; Engineering Foundation, New York, and Deutsche Vereinigung fu¨r Chemie- und Verfahrenstechnik, Frankfurt/Main, Germany, 1991; p 145. (15) Hunger, B.; von Szombathely, M.; Hoffmann, J.; Bra¨uer, P. J. Therm. Anal. 1995, 44, 293. (16) Koch, K.; Hunger, B.; Klepel, O.; Heuchel, M. J. Catal. 1997, 172, 187.
10.1021/la981284s CCC: $18.00 © 1999 American Chemical Society Published on Web 04/17/1999
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Figure 2. Desorption profiles of water on LSX zeolites with different cations (10 K min-1). Figure 1. Desorption profiles of water on X type zeolites with different cations and CsX zeolites with different exchange degrees (10 K min-1). Table 1. Zeolite Characteristics zeolite
Si/Al
chemical composn
micropore vol, cm3 g-1
NaX KX RbX CsX(5) CsX(30) CsX(45) Li-LSX Na-LSX K-LSX
1.18 1.18 1.35 1.18 1.18 1.18 1.01 1.02 1.05
Na88.1[Al88.1Si103.9O384] K66.1Na22[Al88.1Si103.9O384] Rb57.8Na23.9[Al81.7Si110.3O384] Cs4.4Na83.7[Al88.1Si103.9O384] Cs26.4Na61.7[Al88.1Si103.9O384] Cs39.6Na48.5[Al88.1Si103.9O384] Li93.1Na2.4[Al95.5Si96.5O384] Na95[Al95Si97O384] K86.2Na7.5[Al93.7Si98.3O384]
0.295 0.263 0.217 0.231 0.221 0.194 0.309 0.290 0.231
quadrupole mass spectrometer (Leybold, Transpector CIS System) with a capillar-coupling system were used. The zeolites were equilibrated with water vapor over a saturated Ca(NO3)2 solution in a desiccator. For each experiment 50 mg of the waterloaded zeolite was used in a mixture with 1 g of quartz of the same grain size (0.2-0.4 mm). At first all samples were flushed with helium at room temperature for a period of 1 h. Then the linear temperature program (10 K min-1) was started. Some experiments were carried out using different flow rates of helium (2-6 L h-1) and different zeolite amounts (3.5-50 mg of the hydrated sample). The adsorbed water amounts after equilibration were determined using a simultaneous thermal analysis apparatus (TG-DTA-QMS, Netzsch, System STA-QMS 409/403). XRD. Diffractograms of the dehydrated powder samples were measured on a Stoe Stadi P diffractometer (Debye Scherrer geometry, Cu KR radiation) between 5 and 80° in steps of 0.02°. Rietveld refinement using the GSAS software package served for determination of the structure. To circumvent difficulties with the profile function at low angles, simultaneous refinements between 5 and 15° and between 9 and 80° were carried out. Difference Fourier analysis was applied to localize residual water molecules. Refinement was performed until the Rf value was better than 10% and Rp and Rwp were better than 5%. The number of cations during refinement was held fixed, thereby enabling the identification of the kind of cation on a given site.
Results and Discussion Figures 1 and 2 show the desorption curves for X type zeolites with different cations, CsX zeolites with different degree of cation exchange, and the LSX zeolites. Reversibility of the processes can be assumed, because several desorption experiments carried out on one probe showed identical courses of desorption. All desorption curves (X
Table 2. Water Amounts
zeolite
water uptake in the desiccator, mmol g-1
NaX KX RbX CsX(5) CsX(30) CsX(45) Li-LSX Na-LSX K-LSX
18.1 14.5 10.9 15.0 12.4 10.5 19.3 16.8 14.1
nonisothermal desorption mmol g-1 H2O/uc 13.7 12.1 9.5 12.8 11.6 9.2 14.3 15.3 11.6
185 176 161 179 190 164 173 208 174
and LSX zeolites) have a pronounced maximum at about 400 K. The desorption behavior at higher temperature (>420 K) shows a distinct dependence on the kind of cation and the degree of the cation exchange with Cs+ ions. It is a characteristic of these zeolites that the highest desorption temperature decreases in the sequence LiLSX, NaX, KX, RbX, and CsX(45) (Li-LSX, about 700 K; CsX(45), about 520 K). The desorbed amounts of water are given in Table 2. Both the water uptake in the desiccator and the amount desorbed nonisothermally decrease clearly with increasing cation radius and degree of cation exchange. All desorption curves are structured. This demonstrates that different adsorption sites and adsorbate structures exist which differ with regard to the strength of the interaction. For an interpretation of TPD experiments one has always to consider that the course of desorption on porous solids may depend significantly on the experimental conditions (flow rate of the carrier gas, probe amount).17,18 Therefore, additional experiments with different flow rates of the carrier gas and different amounts of zeolite have been carried out. The normalized presentation of appropriate desorption (rn ) rd/rd,max, Tn ) T/Tmax) in Figures 3 and 4 shows clearly that width and shape of the desorption curves do not change with respect to these experimental parameters. Therefore, the observed shift of the maximum of the desorption curve with respect to the flow rate of the carrier gas and the input amount (not shown here) should be caused, foremost, by mass transport (17) Demmin, R. A.; Gorte, R. J. J. Catal. 1984, 90, 32. (18) Rieck, J. S.; Bell, A. T. J. Catal. 1984, 85, 143.
Interaction of Water with X Type Zeolites
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Figure 3. Normalized desorption profiles on NaX for different flow rates of the carrier gas.
Figure 5. Desorption energy distributions for water on X type zeolites with different cations and CsX zeolites with different exchange degrees.
Figure 4. Normalized desorption profiles on NaX for different zeolite amounts.
in the gas phase. In the other case, when processes in the pores of the zeolite (desorption, readsorption, and diffusion) are influenced by experimental conditions (carrier gas flow, sample amount), then the shape of the desorption profile ought to be changed with respect to these experimental parameters.18 This interpretation is also supported by the observation that the shift of the maximum of NH3TPD spectra on a HZSM-5 in dependence on the amount of the zeolite showed no influence on the shape of the loading dependence of the effective desorption energy.19 Only the value of the effective preexponential factor was changed. To obtain detailed information about the desorption process, a deeper analysis of the TPD curves has been carried out. For that a desorption process of first order with a distribution function f(E) of the effective desorption energy E was considered:15,16,20 Emax
rd(T) ) -
∫
dθ ) A θloc(E,T) exp(-E/RT) f(E) dE dt E min
Here rd(T) is the observed overall rate of desorption, θ is the average coverage, and A is an effective preexponential factor. θloc is the coverage of adsorption sites with desorption energy E, and Emin and Emax are the limits of the range of desorption energy. The calculations were (19) Hunger, B.; Hoffmann, J.; Heitzsch, O.; Hunger, M. J. Therm. Anal. 1990, 36, 1379. (20) von Szombathely, M.; Bra¨uer, P.; Jaroniec, M. J. Comput. Chem. 1992, 13, 17.
Figure 6. Desorption energy distributions for water on LSX zeolites with different cations.
carried out by means of the program INTEG, which involves a regularization method for solving this integral equation.16,20 For NaX zeolites the distribution functions of the desorption energy were calculated by using several desorption curves with different heating rates (2-20 K min-1) and an extended integral equation.16 From that an effective preexponential factor of 2.5 × 107 min-1 was calculated. The assumption of an uniform effective preexponential factor seems useful, because for water desorption similar adsorbate structures can be assumed on all investigated zeolites and equal experimental conditions have been used. Therefore, the value A ) 2.5 × 107 min-1 was used for all calculations. Figures 5 and 6 show the calculated desorption energy distributions. The width of these distributions of about 30-50 kJ mol-1 shows clearly the energetic heterogeneity of the water-zeolite interaction. The energy range from about 45 kJ mol-1 up to about 75-95 kJ mol-1 corresponds well with microcalorimetrically determined differential heats of adsorption for different amounts of water on
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Table 3. Desorbed Water Amounts of the Different Peaks of the Distribution Functions peak 1 zeolite NaX KX RbX CsX(5) CsX(30) CsX(45) Li-LSX Na-LSX K-LSX
Emax, kJ 51 51 52 49 51 50 50 51 51
mol-1
peak 2 H2O/uc 21 8 33 28 39 27 3 9 16
Emax, kJ
mol-1
58 58 59 57 58 58 58 58 58
Figure 7. Desorption energy distribution for water on a mixture of NaX (18.9 mg) and CsX(45) (21.0 mg).
comparable X type zeolites.21,22 Therefore the desorption energy distributions represent an adequate quantitative measure to characterize the strength distribution of the water interaction. All desorption energy distributions show a pronounced maximum at about 58 kJ mol-1. At lower energy values they have a further range with lower energy (desorption energy maximum at about 50 kJ mol-1). Both ranges seem to show no dependence on the Si/Al ratio, the kind of cation, and the exchange degree with Cs+ ions. Significant differences appear at the higher energy values. The LiLSX zeolite shows two peaks at about 67 and 85 kJ mol-1 (see Figure 6). The NaX and Na-LSX display two peaks in this energy range, at 68 and 80 kJ mol-1. After exchange of Na+ with K+, Rb+, and Cs+ ions the intensity of the peak at 80 kJ mol-1 is decreased and the intensity of the respective cation-dependent peak maximum at about 6668 kJ mol-1 is increased (see Figure 5). These peaks of the distribution functions also appear for desorption of water on a mixture of two different zeolites (18.9 mg of NaX and 21.0 mg of CsX(45)) as it is shown in Figure 7. For the CsX zeolites the intensity of the peak at about 66 kJ mol-1 increases with increasing degree of exchange, but the position of the maximum is not shifted (see Figure 5). These systematic changes with respect to position and intensity of the peaks at larger energy values show clearly the influence of the respective cation on the water desorption at higher temperature. This is also supported by the fact that the position on the energy scale of the cation specific peak correlates linearly with the softness of the cationswhich is a measure for their polarizability and thus for the strength of interaction with water23 (see Figure 8). To determine the number of water molecules which correspond to the different energy ranges the desorption (21) Dzhigit, O. M.; Kiselev, A. V.; Mikos, K. N.; Muttik, G. G.; Rahmanova, T. A. Trans. Faraday Soc. 1971, 67, 458. (22) Dubinin, M. M.; Isirikyan, A. A.; Rachmatkariev, G. U.; Serpinsky, V. V. Izv. Akad. Nauk SSSR, Ser. Khim. 1973, 4, 934. (23) Pearson, R. G. Inorg. Chem. 1988, 27, 734.
peak 3 H2O/uc 111 116 76 88 97 73 117 128 77
Emax, kJ 68 71 68 64 66 66 69 68 69
mol-1
peak 4 H2O/uc
Emax, kJ mol-1
H2O/uc
33 45 41 45 54 63 23 24 70
80 83 75 79
21 7 11 17
85 79 84
29 47 11
Figure 8. Relationship between the energy value of the cationspecific peak of the desorption energy distribution and the cation softness.
Figure 9. Gaussian fit for the desorption energy distribution for water on NaX and CsX(45): (b) calculated distribution using the program INTEG; (s) fitted distribution by four Gaussian peaks; (- - -) single Gaussian peaks.
energy distributions were fitted to three or four Gaussian functions. Examples are shown in Figure 9. For all zeolites Table 3 summarizes the position of the respective maximum in kJ mol-1 and the number of water molecules which can be attributed to the peaks. Peak 2 has the same width (standard deviation: 2.9 ( 0.2 kJ mol-1) for all zeolites. From this result it may be concluded that these water molecules have similar adsorbate structures in all zeolites under study. Table 3 shows further that for X type zeolites the desorption of about 50-60 water molecules per unit cell (peaks 3 and 4) are influenced by the cations. This amount is relatively independent of the kind of the cations and the exchange degree. For LSX type zeolites (except
Interaction of Water with X Type Zeolites
Li-LSX) the number is 70-80 water molecules per unit cell. This result shows clearly the higher cation content of the LSX type zeolites (see Table 1). To localize those water molecules which desorption was influenced by cations X-ray powder diffraction studies were carried out. The cations in the investigated samples occupy traditional cation sites (SI, SI′, SII′, SII, SII*, SIII). We found that the water is situated between sodium ions on SIII and SII*. In most cases this results in a coordination of the sodium ions on SII* in a distorted octahedron.24,25 In NaX about 11 of these cation-water arrangements per unit cell were detected. When large amounts of potassium ions are present, only the SII* positions are occupied by sodium ions. In such a sample almost no water molecules were found between SII* (Na+) and SIII (K+). When only very few sodium ions are exchanged by Cs+ (see Figure 10), the same cation-water formation as in NaX is encountered (11 per unit cell). Four Cs+ ions per unit cell reside on position SI′. Increase of Cs+ content leads to occupation of SIII by the heavy alkali-metal ions beside a small occupation of SI′. Despite this finding the same number of water-cation complexes is present concluding in an almost 100% occupation of the SIII position by Cs+ and Na+. It is surprising that, at lower coverage of these zeolites, water is bounded only by Na+ ions, because the desorption behavior of water at higher temperatures on the zeolites, exchanged with K+ and Cs+ ions, is significantly different (24) Kirschhock, C.; Fuess, H. Zeolites 1996, 17, 381. (25) Kirschhock, C.; Hunger, B.; Fuess, H. Publication in preparation.
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Figure 10. Localization of water molecules in CsX(45) at low loading.
from the behavior on NaX. It is possible that ion exchange with K+ and Cs+ ions influences the properties of the remaining Na+ ions in such a way that a weaker bonding of water molecules is developed in the adsorbate complexes. For a detailed interpretation further investigations are necessary, most importantly with varying water loadings. Acknowledgment. The authors gratefully acknowledge the financial support of the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft, Graduate College “Physical Chemistry of Interfaces”. LA981284S
