5584
J. Phys. Chem. 1989, 93, 5584-5587 4
3
0.5
,-.
-
w
1
o
0
\
>c v
I
2
xo
2
-0.5
0
3u
-1
0
0.2
0.4
0.6
0.8
e
Figure 5. Apparent Arrhenius parameters for desorption as functions of coverages calculated at 550 K < T C 650 K.
is explained by repulsive lateral interactions between adsorbed particles. At intermediate coverages, 0.2 < 0 < 0.8, the desorption rate decreases with increasing coverage. The latter is explained by surface reconstruction (preferential occupation of metastable positions by surface atoms with an increase in adsorbate coverage). The adsorbate-induced changes in the surface are seen (Figure 4) to result in thermal desorption spectra that are characterized
by a shift of the peak maximum to higher temperatures with increasing initial coverage and by narrow peak widths. The order of desorption is about zero. Using the Arrhenius plots (see, e.g., Figure 3b), we have calculated the apparent Arrhenius parameters for desorption (Figure 5 ) . At intermediate coverages, reconstruction is seen to lead to an increase in desorption activation energy with increasing coverage. The coverage dependence of the preexponential factor for desorption is not strong (on the order of 10) and is in accordance with the compensation effect (the synchronous variation of the preexponential factor and the activation energy). In summary, we have analyzed the effect of adsorbate-induced surface reconstruction on the apparent Arrhenius parameters for desorption. The model presented here predicts first-order phase transition at low temperatures, T < T,. The desorption has been assumed to occur at T > T,. The reconstruction is shown to result in the compensation effect. The latter conclusion seems to be rather general because the opposite model of reconstruction3 also leads to the compensation effect. Of course, the compensation effect may be either weak (Figure 5 ) or strong (ref 3). Besides, the coverage dependence of the sticking coefficient can be weak. All these effects are frequently observed in real systems.12 Thus, the adsorbate-induced changes in the surface seem to play an important role in the kinetics of the surface rate processes. ( 1 1) Zhdanov, V. P. Surf. Sci. 1983, 133,469. (12) Seebauer, E. G.; Kong, A. C. F.; Schmidt, L. D. Surf. Sci. 1988,193, 417. Smith, A. H.; Barker, R. A,; Estrup, P. J. Surf. Sci. 1984, 136, 327. Estrup, P.J.; Greene, E. F.; Cardillo, M. J.; Tully, J. C. J . Phys. Chem. 1986, 90,4099.
Variable-Temperature Magic-Angte-Spinning Technique for Studies of Mobile Species in Solid-State NMR Tadashi Tokuhiro, Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39
Mark Mattingly, Bruker Instruments, Incorporated, Manning Park, Billerica, Massachusetts 01821
Lennox E. Iton, Materials Science Division, Argonne National Laboratory, 9700 S . Cass Avenue, Argonne, Illinois 60439
and Myong K. Ahn*st Department of Chemistry, Indiana State University, Terre Haute, Indiana 47809 (Received: January 20, 1989)
The utility of variablstemperature (VT) magic-angle-spinning (MAS) Fourier transform nuclear magnetic resonance (FT-NMR) spectroscopy is demonstrated for the study of mobile species in solids, especially when quadrupolar nuclei are being observed. A new VT-MAS probe is described for use in the temperature range 150-400 K, using spectrometers with high-field superconducting magnets. It has bzen utilized in studies of the bonding and dynamics of alkali-metal cations in hydrated zeolites at an applied field of 7.05 T. Results are presented for the cases of Cs/Na-A and Cs/Li-A zeolites, in which the Cs' ions exchange rapidly, at 293 K, between six-ring and eight-ring sites in the large cage, so that on the time scale of the NMR measurements the ions are indistinguishableand a single '33Csresonance is observed. Below 250 K, two well-resolved signals are observed in the Cs/Na-A zeolite, 107 ppm apart. These are easily assigned to the Cs' ions in the two sites, the large difference in chemical shifts being attributed to the stronger bonding of Cs' ions at the six-ring sites. Three sites are distinguished in the Cs/Li-A zeolite, the third site being assigned to a position near a four-ring in the large cage.
Introduction The magic.angle-spinning (MAS) techniquei in solid-state nuclear magnetic resonance (NMR) has been an extremely potent *Towhom correspondence should be addressed. Argonne National Laboratory Faculty Research Leave Appointee.
tool in the elucidation of silicon/aluminum distributions in the frameworks of aluminosilicate zeolites.* The %i NMR chemical (1) Schaefer, J.; Stejskal, E. 0. in Topics in Carbon-13N M R Spectroscopy; Levly, G . C., Ed.; Wiley-Interscience: New York, 1979; Vol. 3, p 283. (2) Klinowski, J. Prog. NMR Spectrosc. 1984, 16,237, and references therein.
0022-3654/89/2093-5584$01.50/00 1989 American Chemical Society
Mobile Species in Solid-state N M R shift is sensitive to the local geometry and the identities of the four tetrahedral atoms bridged (by oxygen atoms) to an individual silicon atom. The chemical shift of the 27AlN M R resonance is an easy diagnostic for tetrahedral (framework) and octahedral (nonframework) aluminum species and has been used to study the important problem of framework dealumination. One might expect that the MAS-NMR of cations could be used in an analogous manner to provide information on the site distribution of cations in the zeolites and the character of the cation bonding at a given ~ i t e . ~There - ~ are several reasons why this technique has not been as readily exploited. First, the range of chemical shifts is often very small for those cations commonly encountered in zeolites which are not paramagnetic and which do not have prohibitively small receptivities. Furthermore, large quadrupole moments pose the problem of broad line widths, except for measurements made at extremely high magnetic fields. The combination of these two factors makes it difficult for the site to be experimentally discriminated by chemical shift differences, even when the cation arrangement is static. An equally important difficulty arises from the dynamic state of the cations in the case of the hydrated zeolites. As illustrated by our 7Li N M R relaxation study of Li+ ions in the hydrated A, X, and Y zeolites, the dynamic state of the cations resembles that of cations in concentrated aqueous solution or in other associated liquids.6 Thus, the 6Li, 7Li, and 23NaN M R spectra in the various hydrated zeolites at room temperature exhibit nearly symmetric, narrowed line shapes without any structure, even without use of MAS. This appearance of equivalence among all the constituent ions is a consequence of rapid exchange of the cations between available sites in the crystal structure, on the time scale of the N M R experiment. This process is arrested upon dehydration of the zeolite, but the cation distribution is usually altered by this treatment. Our recent N M R study of 6Li, 7Li, and 23Nacations in dehydrated A zeolite has established' that the cations are immobile to at least 380 K. The resulting spectra are significantly broadened by a combination of nuclear magnetic dipole-dipole interactions and quadrupole interactions which are no longer motionally averaged. The observed line widths increase by factors of 12.6, 6.5, and 5.6 for 6Li, 7Li, and 23Na,respectively, relative to the line widths in the hydrated analogues. The MAS line width of the 27Alresonance, representing atoms that are localized in the framework, is insensitive to the dehydration treatment. N o cation site discrimination on the basis of chemical shift could be made in any of these systems. The site exchange of the cations in the hydrated zeolites can be arrested, or significantly slowed, by cooling the samples. This also results in line broadening, but the cations retain water molecules in the coordination shell, and less extreme electric field gradients are experienced by the localized hydrated cations than are present in the dehydrated materials. The resulting low-temperature spectra can be narrowed by use of variable-temperature (VT) MAS at high magnetic fields. In order then to resolve the resonances due to cations at nonequivalent crystallographic sites, a judicious choice is made of a cation isotope that exhibits a large range of chemical shifts relative to typical line widths. The 133Cs resonance is the ideal choice, and we demonstrate in this work the easy resolution and identification of Cs+ ion sites in the hydrated A-type zeolite obtained by the use of VT-MAS-NMR measurements. A new VT-MAS probe used in these measurements is described. The Cs+ ion site exchange dynamics have been extracted from the analysis of the spectral line shapes as a function of temperature, without use of MAS. These results are reported elsewhere.8 (3) West, G . W. Zeolites 1981, I , 150. (4) Freude, .; Hauser, A.; Pankau, H.; Schmiedel, H. Z . Phys. Chem. (Leipzig) 1972, 251, 13. ( 5 ) Freude, D.; Lohse, U.; Pfeifer, H.; Schirmer, W.; Schmiedel, H.; Stach, H. 2. Phys. Chem. (Leipzig) 1974, 255,443. (6) Tokuhiro, T.; Iton, L. E.; Peterson, E. M. J. Chem. Phys. 1983, 78, 7473. (7) Tokuhiro, T.; Iton, L. E.; Mattingly, M., to be published. (8) Ahn, M. K.; Iton, L. E., to be published.
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5585
TABLE I cesium chemical shift. m m vs CsCl Cs/Na-A 293 K
166 K -244.5
-219.2
Cs/Li-A 293 K -194.1
-137.7
222 K -266.8 -200.3 -1 5 1.7
site assient" eight-ring four-ring six-ring ~~
" Corresponding to low-temperature chemical shift value. Experimental Section 133CsN M R spectra utilizing the VT-MAS technique were obtained over the temperature range 165-295 K by use of a Bruker AM-300 N M R spectrometer equipped with a narrow-bore (Oxford Instruments) superconducting magnet. The resonance frequency of 133Csis 39.95 MHz, and the typical 90' pulse width is -10 ks. An important feature of the rf probe head is the structure of its spinning chamber. This double-bearing spinning chamber containing the rf coil is supported by gas-tight pivots, and thus it can be positioned at either 8 = 0' (vertical) or 54.735O. While in the vertical position, a sample rotor can be inserted or ejected from the top of the superconducting magnet by compressed air. Once a rotor is positioned, the tilting of the spinning chamber is achieved by use of a pneumatic piston which is mounted at the bottom of the rf probe. While the temperature of the nitrogen gas to the bearing can be varied to the desired value, the drive gas remains at room temperature. This makes spinning easy at any experimental temperature. The voltage output of a thermocouple near the pivot was calibrated by using the 'H chemical shifts of ethylene glycol and methanol over the range from 160 to 410 K. The rf circuits are doubly tuned to 'H and X nuclei. Variable-temperature 133CsN M R measurements were also made a t 26.24 MHz without MAS using a Bruker CXP-200 N M R spectrometer equipped with a standard, nonspinning high-power probe. The chemical shift values from the MAS spectra are reported with reference to solid CsCl (which has a shift of +225.5 ppm vs 0.1 M aqueous CsCl solution). The Cs/Na-A zeolite was prepared by conventional ion exchange at room temperature of Na-A zeolite with 0.1 M aqueous solution of cesium chloride. The replacement of sodium ions is partial under these conditions, and a 30% exchange level was attained. The final composition, as determined by conventional elemental analysis, corresponds to 3.8 Cs+ ions and 8.2 Na+ ions per unit cell of the A zeolite, designated as C S ~ . * N ~ ~ .where ~-A, the framework composition, A, is given by A112Si12048. The samples were equilibrated in a humidistat over water (100% relative humidity) or over a saturated sodium chloride solution (28%relative humidity) prior to making the N M R measurements. A dilute sample containing 1.8 Cs+ ions per unit cell was made by a technique of dry back-exchange of C S ~ , ~ N ~zeolite ~ , ~ -with A pure Na12-A eol lite.^ The Cs/Li-A zeolite was obtained by first ion-exchanging the Na-A zeolite repeatedly with 0.1 M LiCl solution at room temperature. This replaces 92%of the Na+ ions with Li' ions. The LillNa-A thus prepared was then exchanged once with the 0.1 M CsCl solution at room temperature. N o 23Na N M R signal could be observed in this exchanged zeolite.
Results and Discussion The essential results of the MAS measurements are summarized in the series of spectra presented in Figure 1 for the Cs/Na-A zeolite and in Figure 2 for the Cs/Li-A zeolite. These MAS Cs chemical shift results are tabulated with the respective assignments in Table I. CslNa-A Zeolite. As shown in Figure 1, the single resonance = -219.2 ppm) observed at 293 K for Cs/Na-A (chemical shift splits into two components below 270 K, which are very well resolved below 210 K. Assignments of these signals are made on the basis of the known site preference of Cs+ ions in this zeolite. The framework of A zeolite can be described in terms of cu(9) Iton, L. E.; Langston, M. A,; Ahn, M. K., to be published.
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The Journal of Physical Chemistry, Vol. 93, No. 14, 1989
Tokuhiro et al.
A
A
n
213
A Ai___
204
n
L
246
-,,,I
200
100
0
-100
I ' " ' ~ ' ' , * " ~ ~ ~ ' '
-200
.300
-400
PPM
Figure 1. "'Cs MAS-NMR spectra of hydrated Cs/Na-A zeolite at the indicated temperatures in kelvin. The spectra at 39.8 MHz (7.05 T) are obtained by Fourier-transforming the accumulation of 256 simple fid's with the sample M A S at 2.5 f 0.2 kHz. The spectrum at 166 K shows the clearly resolved peaks corresponding to the six- and eight-ring Cs' sites with the small spinning sidebands near -185 and -310 ppm. The high-temperature spectrum at 293 K shows a single sharp peak with the low-field base line showing the effect of incomplete averaging of the site-exchange narrowing process.
booctahedral sodalite cages (@-cages) consisting of six- and four-membered oxygen The larger a-cages are generated by connecting the @-cagesin cubic arrangements through the square faces. The eight-membered rings thus created on the faces of the cube form the large openings to the a-cage. A pseudo unit cell of A-type zeolite contains eight six-rings and three eight-rings. The large eight-ring sites are preferentially occupied by the large Cs+ ion, while the small Na+ ions favor the six-ring sites in the large a-cage of this zeolite structure. In the dehydrated zeolite, the pore structure becomes blocked at the level of three Cs+ ions per unit cell,I2 which corresponds to the number of eight-ring sites per unit cell. For Cs' ion exchange levels which exceed this, the six-ring sites (or other sites) will also be occupied by Cs+ ions. For an exchange level of 3.8 Cs+ ions per unit cell, it is clear that the predominant site occupied by the Cs+ ions must be the eight-ring site. This therefore corresponds to the more intense high-field resonance at fie = -244.5 ppm in Figure 1. This result is confirmed by the spectrum of hydrate Cs,,9Nalo,l-A zeolite which shows a single component (at -250 ppm) at low temperatures. The low-field resonance at fits = -137.7 ppm is assigned to Cs' ions in the six-ring sites. The actual ratio of intensities of the two components is 70:30, Le., about 2.7 Cs' ions per unit cell occupying eight-ring sites, which is less than the full complement of three. The claim that these shift values correspond (10) (a) Broussard, L.; Shoemaker,D. P.J. Am. Chem. SOC.1960, 82, 1041. (b) Gramlich, V.; Meier, W. M. Z . Kristallogr. 1971, 133, 134. (1 1) Breck, D. W. Zeolite Molecular Sieues; Wiley-Interscience: New York, 1974. (12) Fraenkel, D.; Shabtai, J. J . A m . Chem. SOC.1977, 99, 7074.
200
100
0
.io0
-200
-300
-400
PPM
Figure 2. MAS-NMR spectra of hydrated Cs/Li-A zeolite at the indicated temperatures in kelvin. These spectra are obtained at 39.8 MHz (7.05 T) by Fourier transformation of the 256 simple fid's with the sample MAS at 2.5 f 0.2 kHz. The spectrum at 222 K shows three resolved peaks corresponding to the six-, four-, and eight-ring Cs' ion exchanging among the three competing sites.
to real chemical shifts and are not influenced by quadrupole-induced shifts is confirmed by the nonspinning spectra recorded at lower frequency. The relative shifts of the components are unchanged, although the quality and resolution of the spectra are of course poorer. It is to be emphasized that the measurements at two frequencies, both with and without use of the MAS technique, are imperative in order to sustain the claim that true chemical shift distinction between sites is being made. The evidence here is unequivocal. A crystallographic study of Cs7Na5-A in both hydrated and dehydrated states has been published previo~s1y.l~The Cs+ ion to framework oxygen bond distances reported for the eight-ring sites (four 0 ( 1 ) at 3.401 A and four O(2) at 3.582 A) are substantially longer than those in the six-ring sites (three O(3) at 3.1 14 A). This indicates a stronger ion-framework interaction at the six-ring site. Although the detailed theory of the chemical shift in the alkali-metal nuclei is not well understood,'"'* a simple rationale for the observed relative shifts at the two sites can be offered. The large paramagnetic (low-field) shift of the ions in the six-ring sites is attributed to the stronger framework interaction at these sites. Back electron donation from the framework oxygen orbitals into the unoccupied 6p ortitals on the Cs+ ion is invoked to account for the paramagnetic shift, and this back-donation is weaker for ions at the eight-ring sites. This type of reasoning has been used to explain the observed correlations of chemical shifts (13) Vance, Jr., T. B.; Seff, K. J. Phys. Chem. 1975, 79, 2163. (14) Laszlo, P. Angew. Chem., Int. Ed. Engl. 1978, 17, 254. (15) Bloor, E. G.; Kidd, R. G. Can. J . Chem. 1968, 46, 3425. (16) Popov, A. I. Pure Appl. Chem. 1979, 51, 101. (17) Webb, G. A. In N M R of Newly Accessible Nuclei; Laszlo, P., Ed.; Academic Press: New York, 1983; Vol. 1, p 79. (18) Detellier, C. In N M R of Newly Accessible Nuclei; Laszlo, P., Ed.; Academic Press: New York, 1983; Vol. 2, p 105.
Mobile Species in Solid-state N M R with solvent electron donor abilities for the alkali-metal ions in solution.15~16 The positions of the water molecules were not determined in the published crystallographic study.13 The number of H 2 0 molecules coordinated by cations in the six-ring sites is reported to vary from one for Na+ in Nal2-Al0 to three for K+ in KIz-A.l9 It is reasonable to suggest that the Cs' ions in the large cage six-ring sites are each coordinated to one or two water molecules. Extraframework coordination of ions in eight-ring sites is even less well defined, but it is reasonable to suggest that the Cs+ ions at the centers of the eight-rings sites are each coordinated to two water molecules, one on each side of the ring. We are postulating that the chemical shift difference between the crystallographic sites is determined by the framework coordination and not by the extraframework ligands (Le., water). It has been proposedz0 that Cs+ ions begin to occupy six-ring sites inside the sodalite cages before beginning to occupy the six-ring sites in the large cages of the A zeolite. This suggestion was based on crystallographic studies of dehydrated Cs/Ca-A zeolite samples.z0 While it is possible that the Cs' ions can enter the sodalite cage upon dehydration, this is not expected in the mild conditions of conventional ion exchange since the Cs+ ionic diameter is 3.38 A, while the access diameter of the s h i n g window is only 2.2 A. This is clearly evidenced in the observed limit to Cs+ ion exchange of Na-Y zeolite, in which only the Na+ ions in the supercage are exchanged, while the Na+ ions in the sodalite cage remain inaccessible.21 On the basis of the N M R spectra observed in our hydrated zeolites, there is little doubt that the six-ring sites in the large cage of the A zeolite are occupied first by the entering Cs+ ions. Although the chemical shifts of the six-ring Cs' ions are expected to be similar whether they are inside or outside the sodalite cages (the ion is much too large to sit in the center of the six-ring), the site-exchange rate required for cation equivalence on the N M R time scale a t 293 K is too fast to involve a process in which the large Cs+ ion traverses the six-ring window in each migration between the sites involved in the exchange process. Cs/Li-A Zeolite. The situation is clearly more complicated in the case of the Cs/Li-A zeolite, since the low-temperature spectra show three components. A similar three-component spectrum with the same splittings between peaks was observed in the measurements at 26.4 M H z without MAS. Two components are readily assigned to ions occupying the eight-ring (acs = -226.8 ppm) and six-ring sites (bcs = -151.7 ppm), although a noticeable shift has occurred in the resonance position of each component as compared to the values measured in the Cs/Na-A zeolite. The high-field eight-ring site component dominantes the spectrum in a sample diluted in cesium by the dry back-exchange with Li-A zeolite (containing 1.9 Cs+ ions per unit cell); the six-ring site component is also observed, but the third component is not evident. We attribute the shift of the chzmical shift to the structural effect on the framework of this zeolite caused by the presence of Li+ ions. There is a reduction in the unit cell size from with 12.3 to 12.0 A when the Na+ ions are replaced by Li+ a corresponding contraction in the free diameter of the eight-ring (19) Leung, P. C. W.; Kunz, K. B.; Seff, K.; Maxwell, I. E. J . Phys. Chem. 1975, 79, 2157.
(20) Subramaniam, V.; Seff, K. J . Phys. Chem. 1980,84, 2928. (21) Reference 1 1 , p 543. (22) Melchior, M. T.;Vaughn, D. E. W.; Jacobson, A. J.; Pictroski, C. F. In Proceedings of the Sixth International Zeolite Conference;Olson, D., Bisio, A,, Eds.; Butterworths: Guilford, U.K., 1984; p 684.
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5587
from 4.2 to 4.0 Asz3 This contraction is pronounced even for a 50% Li+ ion content.22 If it is assumed that the primary Cs' ion site is still the center of the eight-ring, then the Cs-O( 1) bond distances will be shortened, enhancing the interaction between the Cs+ ion and the framework oxygen atoms. In the correlation of bonding and chemical shift being proposed in this discussion, this enhanced interaction would produce a shift to lower field, as is observed. The chemical shift of the Cs' ions in the six-ring sites shifts in the opposite direction, Le., to higher field, indicating a weakening of the Cs+ ion interaction with the framework at those sites. Since the Cs' ions are located away from the plane of the six-ring, this is an entirely possible consequence of the framework contraction. N o crystallographic data are available to confirm this inference. The third component in the low-temperature spectrum of the Cs/Li-A zeolite (6cS= -200.3 ppm) is assigned to hydrated Cs+ ions interacting to a moderate degree with the framework in sites in the large cage near the four-rings. The assignment is made on the basis of the very similar chemical shifts measuredu for Cs' ions in type I11 sites near the four-ring in the supercages of hydrated types X and Y zeolites. This site is not usually favored in the A-type zeolite, but its occupancy by three Cs+ ions and by two Cs' ions has been observed in the crystallographic studies by Seff and co-workers of highly exchanged A zeolites containing 10 [ C S , ~ N ~ ~ ( O H ) ~and - A11 ] Cs+ ions per unit cell, respecti~ely.~~ In the former case, the main framework interactions are with one 0 ( 1 ) oxygen at 3.00 A and two O(3) oxygens at 3.56 A. The single short bond would account for the framework interaction being intermediate between those found in the six-ring and eight-ring sites. In X zeolite, the Cs+ ions actually prefer the type I11 sites to type I1 six-ring sites which are similar to the six-ring ~ - small ~ ~ size of the Li+ ions in some sites in the A z e ~ l i t e . ~The of the six-rings must be the enabling factor facilitating occupation of these sites in the Cs/Li-A zeolite. Conclusion
The potential of the VT-MAS-NMR technique has been convincingly demonstrated in this study of cation siting in hydrated A zeolite samples containing various mixtures of alkali-metal cations. The exchange of the cations between sites is rapid at room temperature, but when the MAS-NMR spectra were recorded at low temperatures, unambiguous resolution of the resonances attributable to Cs+ ions in various sites in the porous crystal was achieved on the basis of their large differences in Ij3Cs N M R chemical shift. Unequivocal site assignments were made. The spectra reveal subtleties in the trend in bonding interactions not available by other types of measurements. Thus the VT-MASN M R technique provides a powerful new tool in studies of mobile species in the solid state. Acknowledgment. This work is supported by the U S . Department of Energy, BES-Materials Sciences, under Contract W-3 1-109-ENG-38, and the Indiana State University Research Committee. M.K.A. thanks Argonne National Laboratory, Division of Educational Programs, for sponsorship in the Faculty Research Leave program. We thank Professor Karl Seff for providing crystallographic data prior to publication. (23) Reference 1 1 , p 638. (24) Iton, L. E.; Ahn, M. K.; Tokuhiro, T.;Mattingly, M., to be published. (25) Heo, N. H.; Dejsupa, C.; Seff, K. J. Phys. Chem. 1987, 91, 3943. (26) Maxwell, I. E.; Baks, A. Adu. Chem. Ser. 1973, No. 121, 87.
