J. Phys. Chem. 1992, 96, 1809-1814 with vanadium is also indicated in the greater decrease in the B value compared with that one with adsorbed water. From the comparison of the ESR parameters and corresponding bonding coefficients for V20S-H-ZSM-5 mixtures and those of the ion-exchanged V02+,H-ZSM-5zeolite, as well as a decrease in the amount of both kinds of OH groups in the zeolite mixtures depending on vanadium content, it follows that after the hightemperature interaction of vanadium oxide with the zeolite at least two V02+ complexes with C, symmetry of various geometry are present. One type of site for V02+placement could be the zeolite terminal Si-OH groups on the outer zeolite crystal and/or on a small amount of amorphous material (cf. IR spectrum of HZSM-5, band at 3745 cm-I). The second type of site is evidently the zeolite cationic site, where V02+replaces the zeolite structural protons and sharply decreases in their number already at low V,O, content in the oxide-zeolite mixture. The same conclusion results from the bonding coefficients for hydrated and dehydrated oxi d m l i t e mixtures,which at the lowest vanadium content exhibit lower values than those for the ion-exchanged V02+ zeolite and the increase in the vanadium concentration causes their approaching to values for V02+,H-ZSM-5. However, formation of vanadyl complexes in the second vanadium "surface" layers, similar to those found on inorganic carriers, cannot be excluded at higher vanadium oxide loadings. The character of the V=O bond, important for the catalytic function, varies with the VOz+complex geometry at various sites. The V = O bond at cationic sites is
1809
slightly weaker and, therefore, longer than the V - 0 bond in the V02+ complex bound via terminal Si-OH groups. This finding could be important for catalytic processes occurring on these systems. Conclusion It can be summarized that the high-temperature (720 K) interaction of vanadium pentoxide with the H-ZSM-5 zeolite leads to the formation of V02+ species on/in the zeolite that partly replace the zeolite strong acid protons (bridging O H groups, roughly 408)and terminal Si-OH groups located on the surface of the zeolite crystals. The V02+complexes in the dehydrated vanadium oxide-zeolite mixture exhibit square pyramid ligand field symmetry while after adsorption of water or ammonia square bipyramid coordination has been observed, accompanied by the weakening (lengthening) of the V = O bond. Even though the number of V02+ formed exceeds the number of bridging O H groups available in the zeolites not all of the strong acid protons are replaced by these cations. It indicates that at higher vanadium-loading considerable number of V02+ is connected via the surface Si-OH or other not specified sites.
Acknowledgment. This work was supported by Grant No. 44003 of the Czechoslovak Academy of Sciences. We thank the referees for offering helpful suggestions. Registry No. V205,1314-62-1; NH3, 7664-41-7; H20, 7732-18-5.
Application of the 12'Xe NMR Technique. 1. Variable-Temperature 12'Xe NMR Study of Xenon Adsorbed in Zeolites Q. J. Chen and J. Fraissard* Laboratoire de Chimie des Surfaces, associC au CNRS URA 1428, Universite Pierre et Marie Curie, 75252 Paris Cedex 05, France (Received: January 25, 1991; In Final Form: October 20, 1991)
Variable-temperature Iz9XeNMR has been used to study the adsorption state of xenon inside zeolites. It is shown that the way xenon behaves inside a zeolite depends much upon the experimental temperature as well as on the amount of xenon adsorbed. At room temperature, adsorbed xenon can move quite freely in all the space where it is adsorbed, which proves the validity of the fast exchange model.
Introduction It has been a decade since the first publications on the application of 129XeNMR to the study of the void space in During this time many advances have been made in the application of the technique: It is now possible to use the technique to probe structural particularities of the void space where xenon is confiied. These particularities include the size and shape of the void space and the short-range crystallinity of some porous materials. Other applications include the study of electrical and magnetic effects of cations and the migration processes of cations in It is also used to study occluded species such as metal clusters,8 adsorbed phase~,~JO and coke deposits inside zeolites.lI (1) Ito, T.; Fraissard, J. In Proc. 5th Int. ConJ on Zeolites, Naples; Ed: Rees, L., Ed.; Heyden: London, 1980; p 510. (2) Ito. T.; Fraissard, J. J. Chem. Phys. 1982, 76, 5225. (3) Ripmeester, J. A. J . Am. Chem. SOC.1982, 104, 289. (4) Fraissard, J.; Ito, T. Zeolites 1988, 8, 350 and references therein. (5) Gedcon, A.; Bonardet, J. L.; Ito, T.; Fraissard, J. J . Phys. Chem. 1989, 93, 2563. (6) Bansal, N.; Dybowski, C. J . Phys. Chem. 1988, 92, 2333. (7) Chen, Q.J.; Ito, T.; Fraissard, J. Zeolites 1991, 11. 239. (8) Fraissard, J.; Ito, T.; de Menorval, L. C. In Proc. 8th Int. ConJ on Catalysis., Berlin; Verlag Chemie: Dechema, 1984. (9) Gedeon, A.; Ito, T.; Fraissard, J. Zeolires 1988, 8, 376. (10) de Menorval, L. C.; Raftery, D.; Liu, S.B.;Takegoahi, K.; Ryoo, R.; Pines, A. J . Phys. Chem. 1990, 94, 27.
The information about the void space where xenon is confined is obtained from the analysis of chemical shift variation 6 =fixe]. Fraissard et a1.lV2 hqve shown that the chemical shift of xenon adsorbed in a zeolite can be expressed by the following equation: 6 60 6s 6E 6M 6xtxc (1) where a0 is the reference; bE and ahi express the electrical and magnetic effects of cations within the zeolite; axex, represents the Xe-Xe interaction which is proportional to the local xenon density inside the zeolite; 6s characterizes the specific physical interactions between xenon and the wall of the void space where the xenon is adsorbed. This last term depends on the structure of the void space. The importance of eq 1 lies in the fact that if the zeolite contains only Na+, Li+, or H+ cations, there is no magnetic effect and the influence of the electric field is rather small at room temperature? Therefore, 6, can readily be obtained by extrapolation of the 6 = A x e ] plot to [Xe] = 0. Several empirical correlations have been made between 6, and the structure of the void space where xenon is adsorbed. By assuming that the 6, value measured at 27 OC is the average value of the shift of xenon rapidly exchanging between the surface (A) and the free space (V) of the cavity or channel, Fraissard et al.I2
+ + + +
(1 1) Ito, T.; Bonardet, J. L.; Fraissard, J.; BNagy, J.; Andre, C.; Gabelica, 2.;Derouane, E. G. Appl. Coral. 1988, 43, 1.
0022-365419212096-1809$03.00/0 0 1992 American Chemical Society
1810 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992
Chen and Fraissard
(n
established a relationship between 6, and the mean free path of xenon imposed by the zeolite structure. In another study, Derouane and B"agy13 correlated 6, with the surface curvature effect influencing the physisorption energy. In their model the adsorbed xenon always stays near the surface, in contrast to the model proposed by Fraissard et al. Johnson and Giffiths14 found a linear relationship between 6, and the surface area to volume ratio of the void space. Their article implies the same idea as that of Fraissard about the influence of the dimensions of the void space on the chemical shift of xenon. Ripmeester3 and Cheung et al.I5 proposed another totally different way of determining void volume by low-temperature xenon NMR. In their method, xenon is considered as a hard sphere and the number of xenon atoms needed to fill the zeolite pore is used as a criterion. In principle, this method can be used to determine only the total volume but not the precise size of the void. Cheung et al.I5J6 even extended the fast site exchange idea to low temperature (-129 "C). The two assumptions regarding the adsorbed state of xenon inside zeolite, made by Fraissard and Derouane, are totally empirical. Until now no direct experimental evidence has been given. Although theoretically the cage wall is the energetically most favorable position for adsorption, one must bear in mind that the real position must also depend greatly upon the thermal energy of the adsorbed species. The fact that the chemical shift of xenon adsorbed in a zeolite depends also upon pore blockage9J1seems to support Fraissard's idea, but further evidence is still needed. NMR is a powerful technique for studying adsorbed phases. It has been used to study both intra- and interzeolite crystallite xenon diffusion.17J8 By using xenon NMR, Cheung et al.I5 observed a gas-liquid phase transition of xenon adsorbed in Y zeolite at a temperature (-129 "C) below the normal freezing point of xenon (-1 12 "C). In the present work, the state of adsorbed xenon under different conditions is investigated by variable-temperature NMR.
Experimental Section 1. Samples. Two types of zeolite structure, Y and ZSM-5, previously used for intercrystallite xenon diffusion studies,'* were chosen. The ZSM-5 zeolite used was a laboratory-synthesized sample with Si/A1 = 100. Three different Y zeolite samples were used. They are referred to as Y-A, Y-B, and Y-C. Samples Y-A ( N a y , LZY-52) and Y-B (HY, LZY-82) are commercial products from Union Carbide. According to Union Carbide, LZY-82 was obtained by NH4+exchange with LZY-52 and then dealuminated by deepbed steaming. The steamed zeolite is then exchanged again with NH4+ and calcined. 27AlN M R shows that this sample contains extraframework aluminum. Sample Y-C is supplied by Beyer.I9 It is a SiC14dealuminated Y zeolite with Si/Al= 54 and contains a very small amount of extraframework aluminum. As the influence of intercrystalline xenon diffusion on xenon NMR is small for well-crystallized pure samples,'* all the samples used in the present study are in the powder form. 2. Pretreatment. A known amount of sample is introduced into an NMR tube. The sample is evacuated at room temperature to a pressure of about lo4 Torr and then heated slowly to 400 OC during 7 h. It is kept at this temperature for 8 h (-lov5 Torr) and then cooled to room temperature. 3. Xenon Adsorption. Xenon adsorption isotherms are measured on a volumetric apparatus. To work at -75 "C, the sample is immersed in a bath of dry ice and absolute ethanol. The amount ~
~
(12) Demarquay, J.; Fraissard, J. Chem. Phys. Lett. 1987, 136, 314. Springuel-Huet, M.; Demarquay, J.; Ito, T.; Fraissard, J. Stud. Surf. Sci. Catal. 1988, 37, 183. (13) Derouane, E. G.; B'Nagy, J. Chem. Phys. Lett. 1987, 137, 341. (14) Johnson, D. W.; Griffiths, L. Zeolites 1987, 7, 484. (15) Cheung, T. T. P.; Fu, C. M.; Wharry, S . J . Phys. Chem. 1988, 92, 5170. (16) Cheung, T. T. P.; Fu, C. M. J . Phys. Chem. 1989, 93, 3740. (17) Karger, J.; Pfeifer, H.; Stallmach, F.; Spindler, H. Zeolites 1990, 10, 288. (18) Chen, Q.J.; Fraissard, J. J . Phys. Chem., following paper in this issue. (19) Beyer, H. K.; Belenykaja, I. M.; Hange, F. J. Chem. SOC.,Faraday Trans 1 1985, 81, 2889.
A
Xe atomslg Figure 1. 6 = .flNs] variations for Y-A a t temperatures T ("C):(0) -100; ('I-80; ) (0) -60; (A) -40;(A)-20; ( 0 )0; (V)27; (X) 50; (m) 80; (+)loo. For the sake of clarity, one-third of the experimental points are put in the figure.
of xenon adsorbed is expressed either as the number of xenon atoms per gram of anhydrous solid (Ng) or the number of xenon atoms per supercage (N,). The isosteric heat of adsorption Q,, is calculated from isotherms at two temperatures using the equation
= [ N T I ~ ' ~ ) / -( TT2)l I In ( P I / P ~ ) (2) For high-temperature N M R experiments (0 IT I 100 "C), xenon is adsorbed at the temperature of the NMR experiment. The quantity adsorbed is determined by weighing. Gas-phase xenon above the sample is subtracted. For low-temperature NMR measurements (-100 IT < 0 "C), xenon is first adsorbed at 0 "C and the tube is then sealed. Helium (2 Torr) is coadsorbed in order to get better heat conduction. The gas-phase xenon condensed upon cooling is corrected. To minimize the error in the adsorption correction, the dead volume of the tube is designed to be as small as possible (ANg/Ng 52.5%). Cheung et al. have studied the xenon concentration range right up to s a t u r a t i ~ n . ' ~InJ ~the present study, we shall pay particular attention to the low xenon concentration region (N, I2). Special attention is paid to the temperature dependence of 6,, the chemical shift value obtained by extrapolating the 6 = A x e ] variation to zero xenon concentration, as it reflects the Xe-wall interactions. 4. 129XeNMR Measurement. N M R measurements are performed on a Bruker CXP-100 Fourier transform pulse spectrometer operating at 24.9 MHz with a recycle time of 1 s. The reference signal for the chemical shift, 6, is that of xenon gas extrapolated to zero pressure. The spectrometer is calibrated at room temperature. The error in 6 measurement is between 0.5 and 1 ppm. It increases with decreasing temperature. The temperature (-100 to 100 "C;accuracy f l "C) is varied by heating or cooling the sample in a stream of nitrogen gas. For the low-temperature experiments the cooling rate is about 0.5 "C/min. Data acquisition begins after the sample has been at the desired temperature for 20 min. Qst
Results 1. Y Zeolite. 1.1. Sample Y-A. Within the temperature and xenon concentration ranges studied, the xenon NMR signals are always symmetrical. Signal width increases with decreasing temperature. The chemical shift variation with xenon concentration at different temperatures is illustrated in Figure 1. For a given xenon Ng(or N,) 6 increases with decreasing temperature. Figure 1 can be divided into two regions, I and 11, corresponding to the xenon concentration ranges 0 IN, I1 and 1 < N, I2, respectively. When T > 0 "C,6 varies linearly with Ng in both regions. The slope of the 6 = ANg] variation decreases with
Application of the Iz9XeNMR Technique. 1
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1811
%
6s
t
E
P E
90
70
0
-40
40
80 T("C)
Figure 2. Relative Xe-Xe interaction change S(?')/S(lOO)with temperature T (0)xenon in N a y , (+) xenon gas.
50 A.
t.
. 1 M
I
-80 0 80 T("C) Figure 4. 6, variations against temperature T for samples: (0)Y-A; (0) Y-B; (V) Y-C; (A)ZSM-5.
'
102 f
c
2 1020
u
-loo 10 100 PxC Torr Figure 3. Xenon adsorption isotherms on Y-A at T ("C):(0)-75; (0) 21. 1
increasing temperature. When T I 0 OC, 6 = A N g ] is still linear and its slope decreases slightly with temperature in region I1 (line AB). On the contrary, in region I, the 6 =/mr,] variation changes from linear to concave. More precisely, at low Ng,the chemical shift value is constant (line CD, Figure 1) up to a certain value of the xenon concentration, which we call the 'threshold loading" N, then increases with xenon concentration. The N,value depends on the experiment temperature. The lower the temperature T, the higher N,. At -100 OC, N, is about 0.5 Xe atom/supercage. Taking N, = 1 as the limit, we calculate the slope S of the 6 = A N s ] variation at point B. S increases with decreasing temperature. Its relative variation S( T)/S(100) with temperature is shown in Figure 2. In the same figure, the relative temperature dependence of the second virial coefficient of chemical shielding of xenon gas, determined by Jameson et is also illustrated. The two curves fit very well. When the xenon concentration goes through N,= 1, the 6 = A N s ] variation changes abruptly. A singular point occurs at N, = 1. Note that the xenon adsorption isotherms (Figure 3), expressed on double logarithmic coordinates, and the 6 = A N s ] variations are comparable. For example, at T = 27 OC, the two are linear functions. At T = -75 O C , the singular point of 6 = ANs] at Ns= 3.8 X 1020 (or N, = 1) corresponds to the singular point on the adsorption isotherm. The temperature dependence of a,( Tj is illustrated in Figure 4. 6, depends very much upon experiment temperature. For example, the difference between 6,(-100) and 6,(100) is about 37.5 ppm. The temperature dependence of the isosteric heat of adsorption Qs,,o at zero xenon coverage is shown in Figure 5. The absolute value of Q,,,odecreases with increasing temperature. The temperature effect on Q40 is less and less important as the temperature increases. 1.2 sample Y-B.At low loading only a single resonance signal is observed, regardless of the temperature. When N, is close to 1 and the temperature is below 0 OC,a shoulder appears downfield.
T("C) 100
50
Figure 5. Variations of the initial isosteric heat of adsorption Q,,,owith temperature T for samples: (0)Y-A; (0)Y-B.
160
80
120
40
PPI11
Figure 6. Xenon N M R spectrum of Y-B at -80 OC and Ns = 1.
The shoulder increases with decreasing temperature and finally two not very well resolved peaks can be observed (Figure 6). As for sample Y-A, at high temperature ( T 1 27 "C),the 6 = A N s ] variation is a straight line whose slope decreases slightly with increasing temperature. At lower temperatures (TI0 "C) and for Ns < 3.8 X 1020 (N, < 1 Xe atoms/cage), the 6 = A N s ] variation is concave. The 6, value at T = 100 O C is not so far from that of sample Y-A (-49 ppm), but the rate of increase of 6, with decreasing temperature, -d6,( T)/dT, is greater than that of Y-A (Figure 4). At high temperature, the absolute value of Qc0 is smaller than that of sample A, but it increases more rapidly with decreasing temperature. 1.3. Sample Y-C.Only the low-temperature range ( T I27 "C)is studied. The xenon resonance signals are always symmetric. Figure 7 shows the 6 = A N , ] variation at different temperatures. These variations have one feature in common with that illustrated in Figure 1 for Y-A, Le., a singular point at N, = 1. Comparing Figures 1 and 7,one can find that there are several differences between the two. Firstly, for Y-C, the 6 = A N , ] variation at -80 OC shows a shallow minimum for Ns about 2 X lozoXe atoms/g, which is not observed for Y-A. Second, for Ns (20) Jameson, C. J.; Jameson, A. K.; Cohen, S. M. J . Chem. Phys. 1975, 62, 4224.
1812 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992
5u
Chen and Fraissard we shall see how xenon behaves inside zeolite crystallites under different conditions. First look at sample Y-A. As shown in Figures 1 and 4,the chemical shift 6 of adsorbed xenon depends very much on temperature. The influence of temperature is more important when it is low (T < 27 "C). A priori, the two models1*J3are not incompatible with the results obtained. In fact, the oscillation of xenon on the potential curve must increase with T (Derouane's model). On the other hand, in Fraissard's model it is assumed that adsorbed xenon exchanges rapidly between sites A (surface) and V (free space); then the chemical shift is the average of the shift of xenon on these two sites:
t
1
1
1.
0
6 140
1020
-
atoldcage
. .
5x1OZo
.
I,,
Ng
/'
1
0 1020 Figure 8. 8 = f [ N 8 ]variations for 27; (X) 50; (m) 80; (+) 100.
(3)
2 N,
-
where 6,, (6,), r,, and r, are the chemical shifts and the mean residence time of xenon atom on the surface (A) and in the free space (V), respectively. Both 6, and (6,) are zeolite-structure dependent. It is evident that the residence time of xenon on the surface and, therefore, 6 vary with T. In the fast exchange model, it is worth noting that 6, expresses the Xe-wall interaction. (6,) depends on 6,. It is an average 6 value between two succesive collisions with the surface and is smaller when the travel distance of xenon is longer. Consider now the simplest case: as Nxe 0, (6) = 6,. According to statistical mechanics, r,/r, = n,/n, (n, and n, are the xenon atoms on each site). By the same formulation as depicted in ref 12, eq 3 can be transformed to
-
1
l o z 1 Xe atomdg
ZSM-5at T ("C): (0) -100 C T C
> 2 X 1020,the 6 value of Y-C is always lower than that of Y-A under the same conditions. Finally, at low temperature, the increase of 6,(Y-C) with decreasing temperature is slightly greater than for 6,(Y-A) (Figure 4). 2. ZSM-5.The xenon resonance signals are almost symmetric ( N , S loz1)as reported by Cheung for silicalite.21 Sometimes they are very slightly asymmetric at low temperature. Between -100 and 27 OC, the chemical shift at very low xenon concentration is almost independent of temperature. At high xenon loading, the chemical shift decreases slightly with temperature (maximum 2 ppm). Above 27 OC, the chemical shift decreases with increasing temperature for a given xenon concentration (Figure 8). At low loadings, the slope of the 6 = f l N J variation seems to increase very slightly with temperature. The 6, variation with temperature is shown in Figure 4. The difference in 6, between -100 and 100 OC is about 6 ppm, which is much less than in the case of Y zeolite. Discussion The fast intercrystalline site exchange of adsorbed xenon has recently been recognized.I8 This kind of site exchange shows that, in spite of the deep potential well imposed by the zeolite structure for xenon adsorption, adsorbed xenon can still diffuse in and out of the potential well easily at room temperature, especially when it is adsorbed in zeolites with an open structure like faujasite. Reducing the temperature decreases the thermal energy of adsorbed xenon which in turn increases the residence time of xenon inside the zeolite crystallites. When the temperature is as low as -80 "C, xenon is relatively fmed in the potential well. Of course, this kind of site exchange only shows the high intercrystalline mobility of xenon but does not really provide us information about the adsorbed state of xenon inside the zeolite. In what follows (21) Cheung, T.T.P.J . Phys. Chem. 1990, 94, 376.
where Dxc is the xenon diameter and pa and pv are the xenon densities in the adsorbed state and in the free space, respectively. A and V are the surface area and the volume of the void space, respectively. K (K = p,/p,) is the reduced probability ratio of xenon residence on the surface (A) and in the free space (V). It depends upon the zeolite structure and the temperature. C (C = V/ADxe)is a constant for a given system. To see whether the fast site exchange model is valid, and if it is valid, then in what temperature range, we rearrange eq 4 to (5)
Furthermore
Here AE is the energy difference between the xenon in the two states A and V. It is a constant for a given system. Therefore, if the model is valid, then the {ln (6, - 6,)/(6, - (I?,))} versus 1 / T plot should be a straight line. To get the experimental (In (6, - 6,)/(6, - (6,))} versus 1/T plot, we need to know 6, and (6,). As the 6, value measured at -100 OC and the value reported by CheungIs at -129 OC are about 86 ppm, we take 6, 90 ppm. On the other hand, for an open structure like faujasite we assume (6,) = 0 ppm. We thus obtain the experimental plot illustrated in Figure 9. One can see that at high temperature ( T > 30 "C) the plot is a straight line which means the model is valid. At lower temperature, the variation starts to be nonlinear. In this case the xenon atom cannot sample the void space uniformly. Only when the temperature is as low as --lo0 OC Derouane's model may be valid, but then the cation effect is not negligible. Let us note that the AE value calculated from the slope of Figure 9 is about 5.5 kJ mol-', which is in good agreement with the value given in ref 28 (case R 10 A). Xenon site exchange between A and Vis also reflected in the isosteric heat of adsorption dependence on temperature. Strictly
-
-
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1813
Application of the lz9Xe NMR Technique. 1
u
I
I
I
,
,
,
I
I
,
I
\
2.5
3.5
4.5
lOOO/T ( U K ) Figure 9. Experimental In ((6, - 6,)/(6, - 6,)) versus 1/T plot for sample Y-A.
speaking, a zeolite is a heterogeneous adsorption system. It consists of two kinds of energetically different positions, A and V. Therefore, adsorption in a zeolite should be divided into two parts (we assume surface A is homogeneous). One is absorption (from gas phase G to site V) and the other is adsorption on the surface (from V to A ) . The Q,, measured by the volumetric method is an average: na(Ea - EG)+ &(E, - EG) Qst = na
+ nv
where E,, E,, and EG are the energies of the adsorbate in different states ( E , - E, = AE). Its value depends upon the relative populations of adsorbate on the positions A and V. The lower the temperature, the higher n,/(n, + a), and hence the greater the absolute value of Q,,. This is in good agreement with the experimental observations (Figure 5). When the temperature is high enough, the relative population does not depend much on temperature, which is the same for Qst. Consider now the case of sample Y-B. The 6, =AT)variations of Y-B and Y-A are slightly different (Figure 4). When T < 27 "C, 6,(Y-B) is higher than b,(Y-A). At higher temperatures the 6, values of the two samples are inversed. These indicate that the surface and free space properties of the two samples are not the same. These differences are due to the fact that steaming causes dealumination and destruction of the zeolite. It has been shown that extraframework aluminum (AINF)can be a charged species (attractive centre for xenon adsorption), and its average charge depends on the AINFcontent.22 Decreasing the temperature increases the residence time of xenon on the wall and therefore the influence of the AINFcharges on the measured chemical shift. Another possible explanation is that dealumination leads to the formation of cages with different sizes.16923324Smaller pores may exist due to the extraframework species which reduces the size of the supercage. On the other hand, larger pores may be created by destroying some of the cage walls separating the cages. It is well-known that at low temperature, gas molecules will adsorb into smaller pores first due to the overlaps of force fields in smaller pores. In this case, the xenon chemical shift is expected to be larger in dealuminated zeolites. On the contrary, at high temperature, adsorbed xenon can diffuse rapidly in all the free space. Due to the defects created by dealumination, the effective free space in Y-B is greater than in Y-A. Therefore the xenon chemical shift is slightly lower in Y-B. The existence of defects is revealed by the presence of a second xenon NMR signal at low temperature (Figure 6). The = AT) variation of Y-B (Figure 5) also shows the interaction of xenon with centers of different attractive forces. The differences in the Qst,O =AT) variations of Y-A and Y-B (22) Chen, Q.J.; Guth, G. L.; Seive, A.; Caullet, P.; Fraissard, J. Zeolites, in press. (23) Fraissard, J.; Springuel-Huet, M. A.; Demarquay, J. Proc. 7th Int. Zeolite Con/. New developments in Zeolite Science and Technology, Tokyo; Kodanska, Elsevier: Amsterdam, 1986; p 393. (24) Ripmeester, J. A. J . Magn. Reson. 1984, 56, 247.
indicate that the total number of attractive centers in Y-B is smaller but that they are stronger than those in Y-A. In the case of the SiC1,-dealuminated sample Y-C, both the total aluminum and AINFcontents are low ([Si/AlId = 54). The destruction of the structure is minimized by careful preparation and almost all the extraframework species are removed by thorough washing.19 Therefore, the presence of a minimum on the 6 = A N ] variation at low temperature and xenon concentration can %ea proof that the AINFplays the role of cation.2s That is why under these conditions 6(Y-C) is higher than 6(Y-A). On the contrary, at higher temperature and xenon concentrations, 6(Y-C) < 6(Y-A) due to the lower total aluminum content in the sample.2 While the differences in surface properties of the three samples can be discerned easily by working at low temperatures, the chemical shifts at T 1 27 "C are not much different. This proves the correctness of our previous conclusion: the adsorbed xenon a t room temperature exchanges rapidly between A and V. Adsorbed xenon can sample the total void space under these conditions, in contrast to the model proposed by D e r 0 ~ a n e . l ~ Now look at the case of ZSM-5 zeolite. Its 6 does not depend very much upon temperature (Figure 8). The variation of 6, when T increases from -100 to 100 "C is only about 6 ppm. Compare the results of ZSM-5 with that of Y zeolite: at -100 "C, 6,(ZSM-5) is comparable to 6,(Y-B) (i.e., the 6, of the two samples are almost the same), but a t 100 "C, 6,(Y-B) is 50 ppm lower than 6,(ZSM-5). This stems from the influence of the mean free path, I, on (6,). When 7 is large, (6,) is small compared to 6,. The influence of 7 on (6) increases with decrease in the xenon residence time on the surface. On the contrary, when 7 is small, (6,) is very close to 6,. Consequently, the (6) value does not change much with temperature. These results precise the experimenta126927and theoreticalz ones already published. Therefore, the mean free path (7) of xenon imposed by the zeolite structure plays a vital role in the determination of the xenon chemical shift. Of course the limit of this model corresponds to the case where the channel or the cage diameter is about the same size as the xenon atom. Experiment temperature affects not only the Xe-wall interaction but also the Xe-Xe interaction. The 6 =AN,] variation of Y-A is concave a t low temperatures ( T < 0 "C) and N, < 1 (Figure 1). When T i s below -80 "C, 6 is almost independent of Ng for Ng < N, (=0.5 Xe atom/supercage), which indicates that the probability of Xe-Xe collision is negligible when there is less than 1 atom/2 supercages. The probability of Xe-Xe collision increases with temperature. The slope ratio S( T)/S(loo), which characterizes the Xe-Xe interaction in the range from 100 to T "C, increases with decreasing temperature between Nc and NB(= l). The value at point B is very close to that of gas-phase xenon at the same temperature as determined by Jameson et al.,20 which indicates the high mobility (compared to that of xenon gas at the same temperature) of xenon adsorbed in Y zeolite. Cheung15 has claimed that the Xe-Xe collision in Y zeolite at low temperature (-129 "C) is the same as for xenon gas at room temperature. However if one looks more carefully into the results presented in their article, this conclusion is seen to be erroneous. They have neglected the steps in the 6 =AN,] variation and simply drawn a straight line through the concentration range 0 < Ns < 4 Xe atoms/supercage. The Xe-Xe interaction of adsorbed xenon in zeolites depends not only on temperature but also on the quantity of xenon adsorbed. At low temperatures ( T I-20 "C), when N, goes through 1 Xe atomlcage, an inflection point B can be observed on the 6 (25) van Broekhoven, E. H.; Daamcn, S.; Smeink, R. G.; Wijngaards, H.; Nieman, J. Stud. Surf. Sci. Catal. 1989, 49, 1291. (26) Barrage, M. C.; Bonardet, J. L.; Fraissard, J. Catal. Lett. 1990, 5, 143-154. (27) Chen, Q.J.; Springuel-Huet, M. A.; Fraissard, J. Proc. 'ZEOCAT 90" Leipzig, August 1990; Catalysis and Adsorption by Zeolites; Elsevier: Amsterdam, 1991; pp 219-232. (28) Ripmeester, J. A.; Ratcliffe, C. I. J . Phys. Chem. 1990, 94, 7652-7656.
1814
J . Phys. Chem. 1992, 96, 1814-1819
= f [ N J variation (Figures 1 and 7). The position of point B corresponds exactly to the inflection point on the xenon adsorption isotherm at -75 OC (Figure 3). This point in the xenon adsorption isotherm at low temperatures has been observed by several aut h o r ~ . ~ ~It, ~is* an indication of strong adsorbate-adsorbate interaction. To explain the N M R results, we can imagine the following: When N, < NB(=l), there is statistically a maximum of 1 Xe atom/supercage, as if there were a special adsorption site in each supercage; this is in accordance with the proposal made by CheungIs and Anderson.'O In this case, the probability of a three-body xenon collision is negligible. Addition of more xenon changes the whole Xe-Xe interaction inside the zeolite. The presence of a strong Xe-Xe attraction force, as indicated by the adsorption isotherm (Figure 3), increases the probability of a threebody xenon collision which causes the abrupt change of slope at point B. In the case of ZSM-5, the X e X e interaction decreases slightly with temperature. This can be due to the strong van der Waals (29) Aristov, B. G.; Basacek, V.; Kiselev, A. V. Trans. Faraday Soc. 1%7, 63. 2057. (30) Anderson, M. W.; Klinowski, J.; Thomas, M. J. Chem. Soc.,Faraday Trans. I 1986,82, 2851.
force exerted on xenon by the zeolite structure which immobilizes the adsorbed xenon upon cooling.
Conclusion NMR is a powerful technique for the study of adsorbed species. Variable-temperature 129XeN M R shows that both the motional state and the mutual interaction of xenon inside zeolite depend greatly upon experiment temperature as well as upon the amount of xenon adsorbed. The variation in temperature changes the relative residence time of xenon on the surface and in the free space, which in turn influences the xenon chemical shift. The temperature dependence of chemical shift depends also on the openness of the zeolite structure. The more open the structure, the greater the temperature dependence of chemical shift. As for the application of the 129XeNMR technique, it has been shown that xenon is quite mobile a t room temperature in all the void space where it is adsorbed. Therefore, routine '29Xe NMR at room temperature should provide us with more information about the free volume than the surface. A higher temperature may sometimes also be desireable to detect subtle changes in free volume. On the other hand, low-temperature experiments are most helpful for the study of surface properties. Registry No. Xe, 7440-63-3.
Application of the lnsXeNMR Technique. 2. lngXeNMR Study of Xenon Diffusion between Zeolite Crystallites Q. J. Chen and J. Fraissard* Laboratoire de Chimie des Surfaces, associ6 au CNRS URA 1428, Universite Pierre et Marie Curie, 75252 Paris Cedex 05, France (Received: February 7, 1991; In Final Form: October 20, 1991)
129XeNMR has been used to study the diffusion of adsorbed xenon between zeolite crystallites. This intercrystallite xenon diffusion depends much upon zeolite structure. Other factors such as zeolite dilution, compression and experiment temperature can also greatly influence the xenon NMR results. Details of these effects are discussed in the present work. This information is of vital importance from the point of view of the application of the xenon NMR technique to the study of industrial zeolitic catalysts.
Introduction In the past few years, xenon has been widely used as a probe for studying the pore structure of zeolites' and other porous materials2p3by means of N M R spectroscopy. The xenon-129 nucleus, with a spin of and a relative high natural abundance (26.6%), has a gyromagnetic ratio comparable to that of carbon-13. Thus it can readily be observed with conventional FTNMR spectrometers. The advantages of xenon in comparison with other adsorbates are due also to its chemical inertness and high polarizability, which render it very sensitive to the physical interactions (detectable by NMR spectroscopy) with its surroundings. With the advent of this technique and owing to its good applicability, 129XeNMR is now being applied as a routine technique in both academic and industrial laboratories. In this technique, the relevant information is deduced from an analysis of the positions (sometimes the forms also4) of the NMR lines and their dependence on the xenon concentration. The distinct spectral lines are attributed to certain states or regions of the xenon atoms within the adsorbattadsorbent This
TABLE I: Sample Characteristics sample Si/AI morphology NaY (U.C.) 2.5 quasi-spheric NaA NaZSM-5
1 100
cubic quasi-spheric
av size, pm 1.5 f 0.5 3.5 & 0.5 1.5 f 0.5
implies that the site exchange rate between these states or regions is less than the difference in the Larmor frequencies of these lines. While the fast site exchange of xenon between the adsorbed and the gas phases has been observed on amorphous adsorbent systems,* little attention has been paid to zeolite systems. That this idea holds may be due mainly to the result from xenon adsorbed in a mixture of NaY and CaA zeolites which exhibits two lines.' Each signal corresponds to a component and the intensities represent the composition of the mixture. In fact, 6, measurement by extrapolating the 6 = f(Pxc)variation to Px, = 0 is based on this assumption too, i.e., that site exchange between the adsorbed phase and the gas phase is very slow or even negligible. However, in ref 8 and 9, signals between two mechanically mixed Y zeolites
(1) Fraissard, J.; Ito, T. Zeolites 1988, 8, 350 and references therein.
(2) Weist, E. L.; Comer, W. C.; Ito, T.; Fraissard, J. J. Phys. Chem. 1989, 93, 4138. (3) Bansal, N. Doctoral Thesis, University of Delaware, 1988. (4) Springuel-Huet, M. A.; Fraissard, J. Chem. Phys. Lett. 1989,154,299. (5) Ito, T.; Fraissard, J. Zeolites 1987, 7, 554.
(6) Ripmeester, J. A. J . Magn. Reson. 1984, 56, 247. (7) Springuel-Huet, M. A,; Ito, T.; Fraissard, J. Stud. Surf. Sci. Catal. 1984, 18, 13. (8) Chen, Q. J.; Fraissard, J. Chem. Phys. Lett. 1990, 169, 595. (9) Shoemaker, R.; Apple, T. J . Phys. Chem. 1987, 91, 4024.
0022-365419212096-1814$03.00/00 1992 American Chemical Society
