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129Xe NMR study of ZSM-5 type zeolites Effect of cationic sites
MICROPOROUS MATERIALS ELSEVIER
Microporous Materials 8 ( 1997) 57-62
12’Xe NMR study of ZSM-5 type zeolites Effect of cationic sites Juha T. Timonen, Tuula T. Pakkanen * Departmentof Chemistry,Universityof Joensuu,P. 0. Box 111,SF-80101Joensuu,Finland Received26 January 1996;accepted19April 1996
Abstract A comprehensive‘*?Xe NMR spectroscopystudy on H-ZSM-5 zeoliteshaving different aluminum contents and on cation-exchangedZSM-5 zeolites is reported. The parent H-ZSM-5 zeolites were ion-exchangedwith Group I-III metal ions (K, Ca, Sr, Ba, Al, La) to varying degrees.The chemical shift of adsorbed ‘*%e is seen to be a function of the pentasilstructure of ZSM-5, of the numberof free Brernstedacid sitesand of the number of metal cationsin the framework. Differencesin the chemicalshift of ‘*?Xe are seenbetweencations due to their different polarizing forces againstxenon. The amount of cations has also an effect on the 6x,xc term in Fraissard’sequation that may be caused by changes in the diffusional characteristics of Xe atoms in the ZSM-5 framework. Keywords. ‘*‘Xe nuclear magnetic resonance; ZSM-5 zeolite; BrBnsted acid sites; Ion-exchange
1. Introduction ZSM-5 is a medium-pore zeolite which, owing to its unique acid properties and high thermal stability, is widely used as a shape-selective catalyst in oil refining and in the petrochemical industries. We have studied the modification of the catalytic properties of ZSM-5 with ion exchange. The modification was done with Group I-III metal ions [ 11. We were interested in finding out how the metal cations affect the structural properties of ZSM-5. The effects of cationic sites inside the zeolite pores, in particular, were targeted in the present study. Many studies [2] have shown that 129Xe NMR spectroscopy is a very sensitive method for examin* Corresponding author. Fax: + 358-73-1513344
ing the local environment inside zeolite channels. To our knowledge no comprehensive studies of the metal-ion-exchanged ZSM-5 zeolites have yet been done with 12’Xe NMR. In this work 12%e NMR spectroscopy was used to measure the distribution of metal ions in the ion-exchanged ZSM-5 zeolites.
2. Experimental 2.1. Samples
Samples of H-ZSM-5 with varying chemically analyzed Si/Al-ratios were synthesized by the Plank method [ 31. Calcination at 813 K was used to obtain the H-forms from the NH,-forms of the
0927-6513/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII SO927-6513(96)00058-2
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J. T. Timonen.
T. i? Pakkanenlhficroporous
zeolites. The distribution of aluminum atoms on framework and non-framework sites after calcination was studied with “Al MAS NMR on a Bruker AMX 400 spectrometer operating at 104.3 MHz. An exciting n/20 pulse with a repetition delay of 0.5 s was used, and 12,600 scans were collected. On the basis of spectral line deconvolution the proportion of non-framework aluminums was less than 1% in all H-ZSM-5 samples. Some of the H-ZSM-5 zeolites were further ion-exchanged from aqueous solutions of K, Ca, Sr, Ba-salts (AC-‘, Cl-) [l] and Al, La-salts (NO;) [4] to different degrees of exchange. The ion exchange was carried out at 80°C for two hours, and the solution was either stirred mechanically or sonicated. The products were washed with deionized water. The metal cation contents of the ionexchanged zeolites were determined with a Varian Spectra AA-400 spectrophotometer. Prior to any ‘?Xe NMR experiments the samples were dried overnight at 393 K and stored in a desiccator. 2.2. Xe adsorption isotherms A known amount of zeolite (typically 0.3 g) was loaded into a glass apparatus. Before measuring the isotherm the sample was evacuated to ca. 10e4 bar for one hour at ambient temperature. The temperature was raised to 323 K and kept there for one hour. The sample tube was then heated for three hours to 673 K, and this temperature was maintained for 6 h. After the activation of the zeolite sample, the tube was allowed to cool down to room temperature for four hours. A dynamic vacuum was kept on for the whole procedure. Adsorption isotherms were recorded following the pressure changes in constant volume with successive xenon (99.995%) additions. Isotherms were measured at room temperature (296+ I K), and the amount of adsorbed xenon was expressed as the number of xenon atoms per gram of zeolite. 2.3. “‘Xe NMR For NMR experiments, the activation of the samples was similar to that for the samples used in isotherm measurements. 12’Xe NMR spectra
Materials
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were recorded at 296 K on a Bruker AM 250 spectrometer operating at 69.19 MHz. Typically, 800-1600 scans were collected for each spectrum with a relaxation delay of one second between rf pulses. A 90” radio frequency pulse with a pulse width of 15 ps was used. All chemical shifts are expressed relative to the extrapolated chemical shift of bulk xenon gas at zero pressure.
3. Results and discussion The classical expression for the chemical shift of xenon in contact with porous media [5] is Fraissard’s equation: 6 = 6, + 6, + Bxbxe [ Xe] + 6, + (6,)
(1)
which states, that when xenon interacts with a zeolitic matrix, the observed chemical shift 6 is dependent on all interacting factors. 6, is the chemical shift of reference and has been fixed to zero, 6s is the part arising from collisions between xenon atoms and ‘zeolite walls’, Gxe-xeis pressuredependent and caused by Xe-Xe collisions, 6, is considered to arise from exchangeable cations, and 6, is the effect of possible paramagnetic species. 3.1. H-ZSM-5 Five different samples of H-ZSM-5 with chemically determined Si/Al-ratios between 38.6 and 62 3 were studied with 12’Xe NMR. Plotting the chemical shift of 12’Xe against the amount of adsorbed xenon disclosed a linear-start curve for all samples. The curves were linear and rising up to the loading of 4 x 10” Xc/g zeolite. Beyond this value, the slope of the S/[ Xe]-curve starts to grow. Gradients for the linear parts were constant 2.38 +0.14 ppm/( 102’ Xc/g) in the limits of statistical errors (Table 1). The four measurements with lowest Xe loadings in the range of l-4 x 102’ Xc/g zeolite were used in the extrapolation of the chemical shift to [Xe] = 0. A linear regression was applied, and only the series of spectra with the regression coefficient over 0.995 were accepted. Ryoo et al. [6] recently reported a minimum in the G/p,,-curve for H-ZSM-5 zeolites at low pres-
J. T. Timonen,
T. T Pakkanenlhficroporou
Table 1 Number of aluminum atoms per unit cell of the H-ZSM-5 samples and measured 6, and 6,, values
SW ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5
(A) (B) (C) (D) (E)
62.3 54.4 48.5 45.6 38.6
Wu.J 1.52 1.73 1.94 2.06 2.42
JS (mm)
6
105.2 (kO.4) 105.5 (kO.4) 106.3 (kO.4) 107.2 (kO.4) 107.6 (kO.4)
2.46 2.42 2.49 2.23 2.24
(So
ppm g-‘) (k0.14) (kO.14) (kO.14) (k0.14) (kO.14)
sures. They proposed this behavior to indicate the presence of paramagnetic solid state defects. In our experiments the low activation temperature (673 K) prevents the observation of this effect in the used pressure range. Alexander et al. [7] found an increase in 6, with the aluminum content of H-ZSM-5. A similar observation was done in the present work. Fig. 1 shows the plot of 12%e chemical shift extrapolated to zero xenon loading (6,) against the number of aluminum atoms in a H-ZSM-5 unit cell (u.c.). The same values are presented as numbers in Table 1. Our measurements show that, within the Si/Al ratios of the samples used in this study a linear equation that gives the proportion of (a) the zeolitic structure (6,) and (b) the Brarnsted acid sites (6,) to the i2%e NMR shift can be expressed in the form &=6,+6,
(2)
where 6, = 100.8 ppm and 6, = 2.8 ppm/(sites 04) x Wu.o.l~ This equation provides the possibility to predict
Fig. 1. S, versus the number of aluminum sites in a unit cell for H-ZSM-5 zeolites.
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59
the effect of acidic sites on 6s of i29Xe when the overall number of sites changes due to ion exchange. In Alexander’s work a full linearity over the range of 0-7.6[Al,,J was not obtained, and the use of Eq. (2) of our studies is also limited to a narrow region in [Al,,,] (Fig. 1). The observed effect of acidic sites is probably caused by the formation of transitory hydrogen bonds to xenon atoms. A similar interaction was recently found by Wakabayashi et al. [8,9] in a low temperature IR-study of H-mordenite and H-ZSM-5 with adsorbed gases. Such an influence of the Si/Alratio on the inert gas chemical shift has also been rationalized by simulations [lo]. 3.2. Cation-exchanged ZSM-5 zeoIites
The effect of cationic sites in ZSM-5 zeolites was studied with 12’Xe NMR for samples with a varying number of metal cations (K, Ca, Sr, Ba, Al, La). The number of cations in a unit cell was calculated from the metal contents measured with AAS [l]. An NMR procedure similar to that used for H-ZSM-5 zeolites was followed, and the extrapolated values of chemical shifts were examined more closely. Several investigators have reported a parabola-like 6 versus [ Xe]-curve with a minimum for metal-ion-exchanged Y-zeolites, especially for transition metals and hard cations like Ca [ 1 l131. In our experiments no such behavior was observed, and the linear regression could be applied to the whole NMR data. In a paper published by Liu et al. [ 131, a comprehensive study of cation-exchanged NaY-zeolites, a Langmuir type model is used to fit and explain xenon adsorption isotherms in order to correlate them with the lz9Xe NMR results. In the present work all the measured isotherms were tested with Langmuir, Temkin and Freundlich models. The closest approach to a fit was obtained with the Temkin type model, indicating the presence of strong adsorption sites. These sites along with the small zeolite channels, compared to Y-zeolite cavities, prevented the usage of the Langmuir model and thus the usage of Liu’s virial expansion model for ZSM-5-type zeolites. Neglecting the effect of paramagnetic species,
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J. T. Timonen,
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only the factors 6s and 6, have an effect on the numerical value of 6 at [ Xe] =O. In the cationexchanged samples some of the acidic protons have been replaced by metal cations. On the basis of the degree of cation-exchange, the number of remaining Bronsted acid sites in the unit cell can be calculated, and Eq. (2) can be used to estimate &. Subtracting the 6s part from the S values gives the proportion of BE. In Table 2 the cation-exchanged zeolites have been listed with their metal content and 6,. The cation exchange was carried out for two H-ZSM-5 zeolites with Si/Al-ratios of 62.3 (ZSM-5 (A)) and 45.6 (ZSM-5 (D)). These ratios gave the numbers of Bronsted acid sites in the unit cell, 1.52 and 2.06 for ZSM-5 (A) and ZSM-5 (D), respectively. The degree of ion exchange and the number of free Bronsted acid sites are calculated on the assumption that after activation each cation-containing species uses one acid site [14]. Also the 6 Xc-Xe -term from Fraissard’s equation is implemented and wilI be discussed later. When &, and bE are plotted versus [MU.,] for ZSM-5 zeolites (H, K, Ca, Sr, Ba, Al, La) in Fig. 2, the calculated points form four distinct groups, namely H+, Kf, M’+, and M3+. First three of these categories have points in linear formation starting from the origin with &, or & growing along with the number of cation sites per
Materials
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6
Csths
-2 1
per U.C.
Fig. 2. Calculated 6, and Ss values against the number of cations per U.C.for ion-exchanged and hydrogen forms of zeolites, respectively.
unit cell. The polarizing force towards xenon gets bigger in the order of Hf K > Ca > H. Together with our observation that 6 versus [Xe] is not a parabolic curve for Ca-ZSM-5 this means that the interaction between
Table 2 Number of metal cations, degree of ion exchange, measured 6,, calculated dE and the measured dxbxe of ion-exchanged ZSM-5 zeolites
K-ZSM-5-(A) K-ZSM-5-(D) Car-ZSM-5-(A) Ca-ZSM-5-(D) Car-ZSM-5-(A) Sr,-ZSM-5-(A) Sr,-ZSM-5-(A) Sr-ZSM-5-( D) Bar-ZSM-5-(A) Ba,-ZSM-5-(A) Ba-ZSM-5-(D) La,ZSM-5-(D) La,-ZSM-5-( D) Las-ZSM-5-( D) AI-ZSM-5-(D)
Degree of ion exchange
PLI
6s (mm)
SE (nm)
6?&xc ( 10ezo ppm g-r)
100% 100% 9.1% 7.5% 30.7% 9.3% 10.3% 11.9% 13.5% 46.8% 44.7% 5.0% 12.0% 19.3% 1.0%
1.52 2.06 0.138 0.155 0.467 0.141 0.157 0.245 0.205 0.711 0.921 0.103 0.247 0.398 0.021
107.3 (kO.4) 111.6(*0.4) 106.1 (kO.4) 108.1 (kO.4) 107.3 (kO.4) 104.8 (kO.4) 105.1 (kO.4) 107.6 (kO.4) 105.5 (hO.4) 106.6 (kO.4) 109.8 (kO.4) 106.3 (kO.4) 105.3 (kO.4) 105.2 (kO.4) 106.3 (kO.4)
6.5 (kO.8) 10.8 (kO.8) 1.4 (kO.8) 1.9 (kO.8) 3.5 (kO.8) 0.1 (kO.8) 0.4 (kO.8) 1.7 (kO.8) 1.0 (k0.8) 3.5 (kO.8) 5.8 (kO.8) 0.0 (kO.8) -0.6(+0.8) -0.3(*0.8) -0.2(kO.8)
2.29 (k0.14) 2.28 (kO.14) 2.37 (kO.14) 2.18 (k0.14) 1.99 (kO.14) 2.81 (k0.14) 2.57 (kO.14) 1.95 (kO.14) 2.29 (kO.14) 1.97 (kO.14) 1.82 (k0.14) 2.16 (k0.14) 2.28 (k0.14) 2.21 (kO.14) 2.25 (k0.14)
J. T. Timonen, T. T Pakkanen/Microporou
small metal cations and xenon is different in smallpore and wide-pore zeolites, possibly because of the lack of multilayer adsorption. Also electrostatic interactions may participate in case of highervalency cations. In the case of trivalent cations the situation is different. The & term is slightly negative for Aland La-ZSM-5 zeolites. This can be caused by a different mechanism of ion-exchange. Probably the trivalent metal cation displaces the lattice aluminum forming a new acid site with a polarizing force of the same magnitude as that of the original site. This result agrees well with our cracking studies, where the activities of these samples were close to that of the parent H-ZSM-5 zeolite [4]. Alexander et al. [7] showed that a fully linear correlation between the chemical shift of xenon and the aluminum content of H-ZSM-5 is not seen. However, using only a narrow distribution of aluminum contents, the proportions of the pentasil structure and free Brnrnsted acid sites to the “‘Xe NMR shift of adsorbed xenon can be separated. For modified ZSM-5 zeolites this can be further used to distinguish the part of polarization due to adsorbates. This practice of splitting the 6s to geometrical ‘wall’ and additional adsorbate interaction is probably limited to zeolites with small or medium cavities. In these systems the frequency of binary xenon-surface collisions is large enough to cause detectable changes in lzgXe NMR [15].
61
Materials 8 (1997) 5742
6 x+x,(E) is presented against [M,,,] in Fig. 3, where the zero point in the ordinate stands for H-ZSM-5. The calculated errors have quite high values compared to the variations of data points, but some trends can be seen. With divalent cations at a low ion exchange level the 6x6xe is larger than that for H-ZSM-5, indicating that some process concentrates adsorbed atoms. This can be due to cation-xenon-interactions that locally retard the diffusion of gas atoms. The crowding of the xenon atoms can also be caused by the hindered diffusion along these obstructed channels [ 161. The Gxbxe (E) diminishes drastically with increasing metal content. This drop can be attributed to increasing pore blockage by metal cations, hindering diffusion between unit cells. The concentrating effect seen at the beginning of the curve gives way to the blocking steric effect at this range. With a further increase in cation load, the curve turns upwards and obviously a limit, where changes in [MU.,.] have little or no effect on the diffusion hindering steric factor, is reached. On the other hand, the concentrating effect of metal sites increases with [MU.,]. The K-exchanged samples have graph points near the abscissa. As the adsorption mechanism of divalent cations and potassium is similar, the trend line should be drawn from divalent ions to K. At maximum exchange rate the Xe-Xe interaction is on the same level as in the parent zeolite.
3.3. 6x,xe term 05
The number of metal cations was also seen to have an effect on the gradient of the 6 versus [ Xe]curve. This 8xE-xc term in Fraissard’s equation describes the level of Xe-Xe-collisions in the zeolite network. Changes in 6x,x, may indicate that adsorbed gas atoms concentrate in limited areas. In H-ZSM-5 zeolites the 8xbxc terms (Table 1) were slightly different, but correlations with [Al,,,] were not found. In cation-exchanged zeolites some changes were seen. To study the effect of the cations alone on &+x0 the Gxbxe of the parent zeolite in Table 1 was subtracted from the &,xe data of the cation samples presented in Table 2. The subtraction result termed as
1T
4Cations
2.5
per u.c.
Fig. 3. The calculated 6,x, (E) for ion-exchanged ZSM-5 zeolites against the cation exchange degree.
62
J. T. Timonen,
T. i? PakkanenlMicroporous
It is quite reasonable to attribute the first, downward directing range of 6x,(E) versus [MU.,.] to the location of cations in the framework. In ZSM-5 the Bronsted acid sites are usually located near 3D intersections [17] of channels, which play an important role in internal pore channel diffusion, and the dramatic loss in 6x,,(E) between 0 and 0.5[M,.,] is caused by the blocking of these intersections. In case of trivalent cations the situation is again different. No noteworthy changes compared to the parent zeolite are seen, indicating that the difIusional characteristics for Xe in the framework are similar to H-ZSM-5. This also supports the hypothesis that Al- and La-cations go into the lattice, displacing aluminums atoms originally present.
4. Conclusions In the case of cation-exchanged ZSM-5, the overall number and type of cations are found to have an effect on the ‘%e NMR shift. In well established systems this can be used to predict the number of cations in the matrix. Also the mechanism of ion exchange has an effect towards xenon. The degree of ion exchange is correlated with the 6 xc-xc term in Fraissard’s equation. This behavior could stem from the bipartite nature of metal sites. First, the cations concentrate adsorbed gas around themselves, second, they hinder the long-range diffusion in zeolitic channels in which the location of cations is important.
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Acknowledgment Financial support of this research by the Technology Development Centre of Finland is gratefully acknowledged.
References [l] M. Ruotsalainen and T.T. Pakkanen, unpublished results. [2] P.J. Barrie and J. Klinowski, Prog. NMR Spectrosc., 24 ( 1992) 91, and references therein. [3] C.J. Plank, E.J. Rosinski and A.B. Schwartz, U.S. Pat. No. 3926782 (1975). [4] P. TynjIiluti and T.T. Pakkanen, unpublished results. [5] T. Ito and J. Fraissard, J. Chem. Phys., 76 (1982) 5225. [6] R. Ryoo, H. Ihee, J.H. Kwak, G. Seo and S.-B. Liu, Microporous Mater., 4 (1995) 59. [7] S.M. Alexander, J.M. Coddington and R.F. Howe, Zeolites, 11 (1991) 368. [S] F. Wakabayashi, T. Fujino, J.N. Kondo, K. Domen and C. Hirose, J. Phys. Chem., 99 (1995) 14805. [9] F. Wakabayashi, J.N. Kondo, K. Domen and C. Hirose, J. Phys. Chem., 100 (1996) 4154. [lo] C.J. Jameson and H.M. Lii., J. Chem. Phys., 103 (1995) 3885. [ 1 l] T. Ito and J. Fraissard, J. Chem. Sot. Faraday Trans. 1, 83 (1987) 451. [12] N. Bansal and C. Dybowski, J. Phys. Chem., 92 (1988) 2333. [13] S.-B. Liu, B.M. Fung, T.-C. Yang, E.-C. Hong, C.-T. Chang, P.-C. Shih, F.-H. Tong and T.-L. Chen, J. Phys. Chem., 98 (1994) 4393. [ 141 C.J. Plank, Proc. Third Int. Congr. Catalysis, Amsterdam, Vol. 1, 1964, p. 568. [ 151 F. Vignt-Maeder, J. Phys. Chem., 98 (1994) 4666. [ 161 J.C. Giddings, Dynamics of Chromatography, Part 1, Principles and Theory, Marcel Dekker Inc., New York, 1965, p. 244. [17] J.-C. Lin and K.-J. Chao, Zeolites, 11 (1991) 376.
Microporous Materials 8 ( 1997) 57-62
12’Xe NMR study of ZSM-5 type zeolites Effect of cationic sites Juha T. Timonen, Tuula T. Pakkanen * Departmentof Chemistry,Universityof Joensuu,P. 0. Box 111,SF-80101Joensuu,Finland Received26 January 1996;accepted19April 1996
Abstract A comprehensive‘*?Xe NMR spectroscopystudy on H-ZSM-5 zeoliteshaving different aluminum contents and on cation-exchangedZSM-5 zeolites is reported. The parent H-ZSM-5 zeolites were ion-exchangedwith Group I-III metal ions (K, Ca, Sr, Ba, Al, La) to varying degrees.The chemical shift of adsorbed ‘*%e is seen to be a function of the pentasilstructure of ZSM-5, of the numberof free Brernstedacid sitesand of the number of metal cationsin the framework. Differencesin the chemicalshift of ‘*?Xe are seenbetweencations due to their different polarizing forces againstxenon. The amount of cations has also an effect on the 6x,xc term in Fraissard’sequation that may be caused by changes in the diffusional characteristics of Xe atoms in the ZSM-5 framework. Keywords. ‘*‘Xe nuclear magnetic resonance; ZSM-5 zeolite; BrBnsted acid sites; Ion-exchange
1. Introduction ZSM-5 is a medium-pore zeolite which, owing to its unique acid properties and high thermal stability, is widely used as a shape-selective catalyst in oil refining and in the petrochemical industries. We have studied the modification of the catalytic properties of ZSM-5 with ion exchange. The modification was done with Group I-III metal ions [ 11. We were interested in finding out how the metal cations affect the structural properties of ZSM-5. The effects of cationic sites inside the zeolite pores, in particular, were targeted in the present study. Many studies [2] have shown that 129Xe NMR spectroscopy is a very sensitive method for examin* Corresponding author. Fax: + 358-73-1513344
ing the local environment inside zeolite channels. To our knowledge no comprehensive studies of the metal-ion-exchanged ZSM-5 zeolites have yet been done with 12’Xe NMR. In this work 12%e NMR spectroscopy was used to measure the distribution of metal ions in the ion-exchanged ZSM-5 zeolites.
2. Experimental 2.1. Samples
Samples of H-ZSM-5 with varying chemically analyzed Si/Al-ratios were synthesized by the Plank method [ 31. Calcination at 813 K was used to obtain the H-forms from the NH,-forms of the
0927-6513/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII SO927-6513(96)00058-2
58
J. T. Timonen.
T. i? Pakkanenlhficroporous
zeolites. The distribution of aluminum atoms on framework and non-framework sites after calcination was studied with “Al MAS NMR on a Bruker AMX 400 spectrometer operating at 104.3 MHz. An exciting n/20 pulse with a repetition delay of 0.5 s was used, and 12,600 scans were collected. On the basis of spectral line deconvolution the proportion of non-framework aluminums was less than 1% in all H-ZSM-5 samples. Some of the H-ZSM-5 zeolites were further ion-exchanged from aqueous solutions of K, Ca, Sr, Ba-salts (AC-‘, Cl-) [l] and Al, La-salts (NO;) [4] to different degrees of exchange. The ion exchange was carried out at 80°C for two hours, and the solution was either stirred mechanically or sonicated. The products were washed with deionized water. The metal cation contents of the ionexchanged zeolites were determined with a Varian Spectra AA-400 spectrophotometer. Prior to any ‘?Xe NMR experiments the samples were dried overnight at 393 K and stored in a desiccator. 2.2. Xe adsorption isotherms A known amount of zeolite (typically 0.3 g) was loaded into a glass apparatus. Before measuring the isotherm the sample was evacuated to ca. 10e4 bar for one hour at ambient temperature. The temperature was raised to 323 K and kept there for one hour. The sample tube was then heated for three hours to 673 K, and this temperature was maintained for 6 h. After the activation of the zeolite sample, the tube was allowed to cool down to room temperature for four hours. A dynamic vacuum was kept on for the whole procedure. Adsorption isotherms were recorded following the pressure changes in constant volume with successive xenon (99.995%) additions. Isotherms were measured at room temperature (296+ I K), and the amount of adsorbed xenon was expressed as the number of xenon atoms per gram of zeolite. 2.3. “‘Xe NMR For NMR experiments, the activation of the samples was similar to that for the samples used in isotherm measurements. 12’Xe NMR spectra
Materials
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were recorded at 296 K on a Bruker AM 250 spectrometer operating at 69.19 MHz. Typically, 800-1600 scans were collected for each spectrum with a relaxation delay of one second between rf pulses. A 90” radio frequency pulse with a pulse width of 15 ps was used. All chemical shifts are expressed relative to the extrapolated chemical shift of bulk xenon gas at zero pressure.
3. Results and discussion The classical expression for the chemical shift of xenon in contact with porous media [5] is Fraissard’s equation: 6 = 6, + 6, + Bxbxe [ Xe] + 6, + (6,)
(1)
which states, that when xenon interacts with a zeolitic matrix, the observed chemical shift 6 is dependent on all interacting factors. 6, is the chemical shift of reference and has been fixed to zero, 6s is the part arising from collisions between xenon atoms and ‘zeolite walls’, Gxe-xeis pressuredependent and caused by Xe-Xe collisions, 6, is considered to arise from exchangeable cations, and 6, is the effect of possible paramagnetic species. 3.1. H-ZSM-5 Five different samples of H-ZSM-5 with chemically determined Si/Al-ratios between 38.6 and 62 3 were studied with 12’Xe NMR. Plotting the chemical shift of 12’Xe against the amount of adsorbed xenon disclosed a linear-start curve for all samples. The curves were linear and rising up to the loading of 4 x 10” Xc/g zeolite. Beyond this value, the slope of the S/[ Xe]-curve starts to grow. Gradients for the linear parts were constant 2.38 +0.14 ppm/( 102’ Xc/g) in the limits of statistical errors (Table 1). The four measurements with lowest Xe loadings in the range of l-4 x 102’ Xc/g zeolite were used in the extrapolation of the chemical shift to [Xe] = 0. A linear regression was applied, and only the series of spectra with the regression coefficient over 0.995 were accepted. Ryoo et al. [6] recently reported a minimum in the G/p,,-curve for H-ZSM-5 zeolites at low pres-
J. T. Timonen,
T. T Pakkanenlhficroporou
Table 1 Number of aluminum atoms per unit cell of the H-ZSM-5 samples and measured 6, and 6,, values
SW ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5
(A) (B) (C) (D) (E)
62.3 54.4 48.5 45.6 38.6
Wu.J 1.52 1.73 1.94 2.06 2.42
JS (mm)
6
105.2 (kO.4) 105.5 (kO.4) 106.3 (kO.4) 107.2 (kO.4) 107.6 (kO.4)
2.46 2.42 2.49 2.23 2.24
(So
ppm g-‘) (k0.14) (kO.14) (kO.14) (k0.14) (kO.14)
sures. They proposed this behavior to indicate the presence of paramagnetic solid state defects. In our experiments the low activation temperature (673 K) prevents the observation of this effect in the used pressure range. Alexander et al. [7] found an increase in 6, with the aluminum content of H-ZSM-5. A similar observation was done in the present work. Fig. 1 shows the plot of 12%e chemical shift extrapolated to zero xenon loading (6,) against the number of aluminum atoms in a H-ZSM-5 unit cell (u.c.). The same values are presented as numbers in Table 1. Our measurements show that, within the Si/Al ratios of the samples used in this study a linear equation that gives the proportion of (a) the zeolitic structure (6,) and (b) the Brarnsted acid sites (6,) to the i2%e NMR shift can be expressed in the form &=6,+6,
(2)
where 6, = 100.8 ppm and 6, = 2.8 ppm/(sites 04) x Wu.o.l~ This equation provides the possibility to predict
Fig. 1. S, versus the number of aluminum sites in a unit cell for H-ZSM-5 zeolites.
Materials
8 (1997)
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59
the effect of acidic sites on 6s of i29Xe when the overall number of sites changes due to ion exchange. In Alexander’s work a full linearity over the range of 0-7.6[Al,,J was not obtained, and the use of Eq. (2) of our studies is also limited to a narrow region in [Al,,,] (Fig. 1). The observed effect of acidic sites is probably caused by the formation of transitory hydrogen bonds to xenon atoms. A similar interaction was recently found by Wakabayashi et al. [8,9] in a low temperature IR-study of H-mordenite and H-ZSM-5 with adsorbed gases. Such an influence of the Si/Alratio on the inert gas chemical shift has also been rationalized by simulations [lo]. 3.2. Cation-exchanged ZSM-5 zeoIites
The effect of cationic sites in ZSM-5 zeolites was studied with 12’Xe NMR for samples with a varying number of metal cations (K, Ca, Sr, Ba, Al, La). The number of cations in a unit cell was calculated from the metal contents measured with AAS [l]. An NMR procedure similar to that used for H-ZSM-5 zeolites was followed, and the extrapolated values of chemical shifts were examined more closely. Several investigators have reported a parabola-like 6 versus [ Xe]-curve with a minimum for metal-ion-exchanged Y-zeolites, especially for transition metals and hard cations like Ca [ 1 l131. In our experiments no such behavior was observed, and the linear regression could be applied to the whole NMR data. In a paper published by Liu et al. [ 131, a comprehensive study of cation-exchanged NaY-zeolites, a Langmuir type model is used to fit and explain xenon adsorption isotherms in order to correlate them with the lz9Xe NMR results. In the present work all the measured isotherms were tested with Langmuir, Temkin and Freundlich models. The closest approach to a fit was obtained with the Temkin type model, indicating the presence of strong adsorption sites. These sites along with the small zeolite channels, compared to Y-zeolite cavities, prevented the usage of the Langmuir model and thus the usage of Liu’s virial expansion model for ZSM-5-type zeolites. Neglecting the effect of paramagnetic species,
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only the factors 6s and 6, have an effect on the numerical value of 6 at [ Xe] =O. In the cationexchanged samples some of the acidic protons have been replaced by metal cations. On the basis of the degree of cation-exchange, the number of remaining Bronsted acid sites in the unit cell can be calculated, and Eq. (2) can be used to estimate &. Subtracting the 6s part from the S values gives the proportion of BE. In Table 2 the cation-exchanged zeolites have been listed with their metal content and 6,. The cation exchange was carried out for two H-ZSM-5 zeolites with Si/Al-ratios of 62.3 (ZSM-5 (A)) and 45.6 (ZSM-5 (D)). These ratios gave the numbers of Bronsted acid sites in the unit cell, 1.52 and 2.06 for ZSM-5 (A) and ZSM-5 (D), respectively. The degree of ion exchange and the number of free Bronsted acid sites are calculated on the assumption that after activation each cation-containing species uses one acid site [14]. Also the 6 Xc-Xe -term from Fraissard’s equation is implemented and wilI be discussed later. When &, and bE are plotted versus [MU.,] for ZSM-5 zeolites (H, K, Ca, Sr, Ba, Al, La) in Fig. 2, the calculated points form four distinct groups, namely H+, Kf, M’+, and M3+. First three of these categories have points in linear formation starting from the origin with &, or & growing along with the number of cation sites per
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6
Csths
-2 1
per U.C.
Fig. 2. Calculated 6, and Ss values against the number of cations per U.C.for ion-exchanged and hydrogen forms of zeolites, respectively.
unit cell. The polarizing force towards xenon gets bigger in the order of Hf
Table 2 Number of metal cations, degree of ion exchange, measured 6,, calculated dE and the measured dxbxe of ion-exchanged ZSM-5 zeolites
K-ZSM-5-(A) K-ZSM-5-(D) Car-ZSM-5-(A) Ca-ZSM-5-(D) Car-ZSM-5-(A) Sr,-ZSM-5-(A) Sr,-ZSM-5-(A) Sr-ZSM-5-( D) Bar-ZSM-5-(A) Ba,-ZSM-5-(A) Ba-ZSM-5-(D) La,ZSM-5-(D) La,-ZSM-5-( D) Las-ZSM-5-( D) AI-ZSM-5-(D)
Degree of ion exchange
PLI
6s (mm)
SE (nm)
6?&xc ( 10ezo ppm g-r)
100% 100% 9.1% 7.5% 30.7% 9.3% 10.3% 11.9% 13.5% 46.8% 44.7% 5.0% 12.0% 19.3% 1.0%
1.52 2.06 0.138 0.155 0.467 0.141 0.157 0.245 0.205 0.711 0.921 0.103 0.247 0.398 0.021
107.3 (kO.4) 111.6(*0.4) 106.1 (kO.4) 108.1 (kO.4) 107.3 (kO.4) 104.8 (kO.4) 105.1 (kO.4) 107.6 (kO.4) 105.5 (hO.4) 106.6 (kO.4) 109.8 (kO.4) 106.3 (kO.4) 105.3 (kO.4) 105.2 (kO.4) 106.3 (kO.4)
6.5 (kO.8) 10.8 (kO.8) 1.4 (kO.8) 1.9 (kO.8) 3.5 (kO.8) 0.1 (kO.8) 0.4 (kO.8) 1.7 (kO.8) 1.0 (k0.8) 3.5 (kO.8) 5.8 (kO.8) 0.0 (kO.8) -0.6(+0.8) -0.3(*0.8) -0.2(kO.8)
2.29 (k0.14) 2.28 (kO.14) 2.37 (kO.14) 2.18 (k0.14) 1.99 (kO.14) 2.81 (k0.14) 2.57 (kO.14) 1.95 (kO.14) 2.29 (kO.14) 1.97 (kO.14) 1.82 (k0.14) 2.16 (k0.14) 2.28 (k0.14) 2.21 (kO.14) 2.25 (k0.14)
J. T. Timonen, T. T Pakkanen/Microporou
small metal cations and xenon is different in smallpore and wide-pore zeolites, possibly because of the lack of multilayer adsorption. Also electrostatic interactions may participate in case of highervalency cations. In the case of trivalent cations the situation is different. The & term is slightly negative for Aland La-ZSM-5 zeolites. This can be caused by a different mechanism of ion-exchange. Probably the trivalent metal cation displaces the lattice aluminum forming a new acid site with a polarizing force of the same magnitude as that of the original site. This result agrees well with our cracking studies, where the activities of these samples were close to that of the parent H-ZSM-5 zeolite [4]. Alexander et al. [7] showed that a fully linear correlation between the chemical shift of xenon and the aluminum content of H-ZSM-5 is not seen. However, using only a narrow distribution of aluminum contents, the proportions of the pentasil structure and free Brnrnsted acid sites to the “‘Xe NMR shift of adsorbed xenon can be separated. For modified ZSM-5 zeolites this can be further used to distinguish the part of polarization due to adsorbates. This practice of splitting the 6s to geometrical ‘wall’ and additional adsorbate interaction is probably limited to zeolites with small or medium cavities. In these systems the frequency of binary xenon-surface collisions is large enough to cause detectable changes in lzgXe NMR [15].
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6 x+x,(E) is presented against [M,,,] in Fig. 3, where the zero point in the ordinate stands for H-ZSM-5. The calculated errors have quite high values compared to the variations of data points, but some trends can be seen. With divalent cations at a low ion exchange level the 6x6xe is larger than that for H-ZSM-5, indicating that some process concentrates adsorbed atoms. This can be due to cation-xenon-interactions that locally retard the diffusion of gas atoms. The crowding of the xenon atoms can also be caused by the hindered diffusion along these obstructed channels [ 161. The Gxbxe (E) diminishes drastically with increasing metal content. This drop can be attributed to increasing pore blockage by metal cations, hindering diffusion between unit cells. The concentrating effect seen at the beginning of the curve gives way to the blocking steric effect at this range. With a further increase in cation load, the curve turns upwards and obviously a limit, where changes in [MU.,.] have little or no effect on the diffusion hindering steric factor, is reached. On the other hand, the concentrating effect of metal sites increases with [MU.,]. The K-exchanged samples have graph points near the abscissa. As the adsorption mechanism of divalent cations and potassium is similar, the trend line should be drawn from divalent ions to K. At maximum exchange rate the Xe-Xe interaction is on the same level as in the parent zeolite.
3.3. 6x,xe term 05
The number of metal cations was also seen to have an effect on the gradient of the 6 versus [ Xe]curve. This 8xE-xc term in Fraissard’s equation describes the level of Xe-Xe-collisions in the zeolite network. Changes in 6x,x, may indicate that adsorbed gas atoms concentrate in limited areas. In H-ZSM-5 zeolites the 8xbxc terms (Table 1) were slightly different, but correlations with [Al,,,] were not found. In cation-exchanged zeolites some changes were seen. To study the effect of the cations alone on &+x0 the Gxbxe of the parent zeolite in Table 1 was subtracted from the &,xe data of the cation samples presented in Table 2. The subtraction result termed as
1T
4Cations
2.5
per u.c.
Fig. 3. The calculated 6,x, (E) for ion-exchanged ZSM-5 zeolites against the cation exchange degree.
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J. T. Timonen,
T. i? PakkanenlMicroporous
It is quite reasonable to attribute the first, downward directing range of 6x,(E) versus [MU.,.] to the location of cations in the framework. In ZSM-5 the Bronsted acid sites are usually located near 3D intersections [17] of channels, which play an important role in internal pore channel diffusion, and the dramatic loss in 6x,,(E) between 0 and 0.5[M,.,] is caused by the blocking of these intersections. In case of trivalent cations the situation is again different. No noteworthy changes compared to the parent zeolite are seen, indicating that the difIusional characteristics for Xe in the framework are similar to H-ZSM-5. This also supports the hypothesis that Al- and La-cations go into the lattice, displacing aluminums atoms originally present.
4. Conclusions In the case of cation-exchanged ZSM-5, the overall number and type of cations are found to have an effect on the ‘%e NMR shift. In well established systems this can be used to predict the number of cations in the matrix. Also the mechanism of ion exchange has an effect towards xenon. The degree of ion exchange is correlated with the 6 xc-xc term in Fraissard’s equation. This behavior could stem from the bipartite nature of metal sites. First, the cations concentrate adsorbed gas around themselves, second, they hinder the long-range diffusion in zeolitic channels in which the location of cations is important.
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Acknowledgment Financial support of this research by the Technology Development Centre of Finland is gratefully acknowledged.
References [l] M. Ruotsalainen and T.T. Pakkanen, unpublished results. [2] P.J. Barrie and J. Klinowski, Prog. NMR Spectrosc., 24 ( 1992) 91, and references therein. [3] C.J. Plank, E.J. Rosinski and A.B. Schwartz, U.S. Pat. No. 3926782 (1975). [4] P. TynjIiluti and T.T. Pakkanen, unpublished results. [5] T. Ito and J. Fraissard, J. Chem. Phys., 76 (1982) 5225. [6] R. Ryoo, H. Ihee, J.H. Kwak, G. Seo and S.-B. Liu, Microporous Mater., 4 (1995) 59. [7] S.M. Alexander, J.M. Coddington and R.F. Howe, Zeolites, 11 (1991) 368. [S] F. Wakabayashi, T. Fujino, J.N. Kondo, K. Domen and C. Hirose, J. Phys. Chem., 99 (1995) 14805. [9] F. Wakabayashi, J.N. Kondo, K. Domen and C. Hirose, J. Phys. Chem., 100 (1996) 4154. [lo] C.J. Jameson and H.M. Lii., J. Chem. Phys., 103 (1995) 3885. [ 1 l] T. Ito and J. Fraissard, J. Chem. Sot. Faraday Trans. 1, 83 (1987) 451. [12] N. Bansal and C. Dybowski, J. Phys. Chem., 92 (1988) 2333. [13] S.-B. Liu, B.M. Fung, T.-C. Yang, E.-C. Hong, C.-T. Chang, P.-C. Shih, F.-H. Tong and T.-L. Chen, J. Phys. Chem., 98 (1994) 4393. [ 141 C.J. Plank, Proc. Third Int. Congr. Catalysis, Amsterdam, Vol. 1, 1964, p. 568. [ 151 F. Vignt-Maeder, J. Phys. Chem., 98 (1994) 4666. [ 161 J.C. Giddings, Dynamics of Chromatography, Part 1, Principles and Theory, Marcel Dekker Inc., New York, 1965, p. 244. [17] J.-C. Lin and K.-J. Chao, Zeolites, 11 (1991) 376.