J . Phys. Chem. 1991, 95, 9416-9419
9416
Variable-Temperature
NMR Studies of a Pillared Montmorillonite
Patrick J. Barrie? Graham F. McCann, Ian Gameson, Trevor Raymen&* and Jacek Klinowski Department of Chemistry, University of Cambridge, Lensfeld Road, Cambridge CB2 I E W,U.K. (Received: December 13, 1990; In Final Form: June 20, 1991) 129XeNMR spectroscopy is shown to be useful for investigating the phase transitions of xenon adsorbed inside a layered microporous material. Results obtained at low temperatures with different xenon loadings indicate the initial formation of xenon clusters and a phase transition when the interlayer space is completely filled with xenon. Unusual behavior with temperature of xenon adsorbed on the external surface of the clay crystallites has also been discovered.
Introduction
Pillaring of clays by large inorganic cations is one method of increasing their thermal stability and surface area, leading to materials of potential catalytic importance. We have previously examined the adsorption of krypton and xenon on pillared montmorillonite by X-ray diffraction and thermodynamic measurements.' Two maxima in the isosteric heats of the adsorption of xenon were found, and it was suggested that they are due to phase transitions associated with the formation of a monolayer and, at full coverage (0 = 1 ,O),a bilayer of xenon. There was also evidence for a phase change from xenon surface area measurements between 134 and 136 K.' The low-temperature XRD results could be interpreted as due to a close-packed single layer of xenon, but the highest coverage studied was 8 = 0.3 (the large X-ray absorption coefficient of xenon hindered XRD studies at higher xenon concentrations).' Here we report results of variable-temperature IBXe N M R experiments on the same sample to determine more precisely the nature of the previously observed phase changes. '29Xe N M R spectroscopy is a useful structural probe for microporous materials, since the chemical shift of xenon is very sensitive to the environment.2 The chemical shift is affected by xenon-surface interactions, which depend heavily on pore shape and size, and by xenon-xenon colli~ions.~-~ There may also be other effects due to the effect of xenon adsorption on polarizing cations and the influence of paramagnetic species."s Some, or all, of these interactions will have a temperature dependence. While there is much published work on room-temperature 129Xe NMR of zeolites,2 there have been comparatively few studies at low temperatures,s-'O and these have not been accompanied by complementary adsorption and XRD results. This is, to our knowledge, the first detailed paper reporting 129XeN M R results on a layered material. Experimental Section
The starting material was Gelwhite L (English China Clays International, St. Austell, U.K.), which is a naturally occurring montmorillonite. It contains Ca2+ and Na+ as the major exchangeable cations in approximately equal amounts and only traces of Fe. The initial surface area was determined by a BET analysis of nitrogen adsorption and found to be 46 m2 g-I. Gelwhite was pillared by treatment with an aqueous solution of aluminum chlorohydrate at 80 OC for 2 h with stirring, followed by calcination in air at 500 OC for 4 h. This results in the insertion of polymeric [A11304(OH)24]7+ cations between the layers, which decompose irreversibly to form alumina pillars upon calcination. There was no evidence in the XRD pattern for any residual unpillared material in the treated sample.' The pillared clay has a BET surface area for nitrogen of about 240 m2 g-l; the surface area for xenon adsorption is 183 m2 g-l. The pillared material has an interlayer spacing of 8.1 A, which may be compared to the xenon atom free diameter of 4.2 A at 170 K. This makes it possible for xenon to form a correlated 'Current address: University College London, Christopher lngold L a b ratories, 20 Gordon Street, London, WClH OAJ, U.K.
0022-3654/91/2095-9416$02.50/0
close-packed bilayer between the layers. We have previously estimated from diffraction datal that the coherence length of adsorbed xenon is 18 f 4 A (at 0 = 0.3), giving a mean separation between the pillars of about 30 A. Xenon is unable to adsorb inside unpillared montmorillonite, which when dehydrated has negligible spacing between layers, but may adsorb on the external surface of crystallites. Variable-temperature 129XeN M R spectra were recorded between 110 and 298 K on the same sample of pillared montmorillonite with different xenon contents. The xenon loadings corresponded to coverages of 8 = 0.15,0.30,0.37,0.45,0.55,0.64, 0.73,0.88, 1.07, and 1.12. The pillared clay was dehydrated at 300 OC under vacuum, loaded with a known amount of xenon, and sealed in a glass tube. The spectra were measured at 110.7 MHz on a Bruker MSL-400 spectrometer using 10-ps radiofrequency pulses with a OS-s repetition time. Chemical shifts are quoted relative to xenon gas extrapolated back to zero pressure, using the equation obtained by Jameson et ai." Relaxation time measurements on the sample with 8 = 0.88 showed that the spin-lattice relaxation time, T I ,was in the region of 40-50 ms and independent of temperature within experimental error. The low-temperature experiments were acquired in 15-20 K steps using cooling rates of 3-5 K/min between temperatures, with a 2-min equilibration time at each step. The acquisition time was 10-20 min for each temperature, depending on sample loading. It is expected that there might be a small temperature gradient along the sample, but this will only have a small consequence on the effects observed here. Virtually the same NMR spectra were obtained during a warming cycle as during a cooling cycle, and the small differences could be explained on the basis of the small temperature gradient in the samples. This indicates that the cooling rate was sufficient for sample equilibration at each temperature. Chemical shifts measurements are within A2 ppm, except for the broadest peaks at very low temperatures, when the expected error reaches up to 6 ppm in the worst cases. For comparison purposes some spectra were also recorded on xenon adsorbed on the external surface of a dehydrated sample of unpillared Gelwhite L.
Results and Discussion We found no initial decrease in chemical shift with xenon pressure at any temperature, indicating that the sample contains no strong preferential adsorption sitess and the chemical shift is not influenced greatly by the presence of cationic charge on the pillars.6 At room temperature (295 K), a featureless plot of '29Xe ( I ) Gameson, I.; Stead, W.J.; Rayment, T. J . Phys. Chem. 1991,95, 1727. (2) Fraissard, J.; Ito, T. Zeolifes 1988, 8, 350. (3) Ito, T.; Fraissard, J. J . Chem. Phys. 1982, 76, 5225. (4) Ito, T.;de Menorval, L. C.; Guerrier, E.; Fraissard, J. Chem. Phys. Left. 1984, 111, 271. (5) Demarquay, J.; Fraissard, J. Chem. Phys. Leu. 1987, 136, 314. ( 6 ) Ito, T.; Fraissard, J. J . Chem. Soc., Faraday Trans. I 1987,83,451. (7) Bansal, N.; Dybwski, C. J . Phys. Chem. 1988, 92, 2333. (8) Cheung, T. T. P.; Fu, C. M.; Wharry, S. J . Phys. Chem. 1988, 92, 5170. (9) Cheung, T. T. P. J . Phys. Chem. 1990, 94, 376. (10) Cheung, T. T. P.; Fu, C . M. J . Phys. Chem. 1989, 93, 3740. ( 1 1 ) Jameson, A. K.; Jameson, C. J.; Gutowsky, H.S. J . Chem. Phys. 1970, 53, 2310.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9417
129XeN M R Studies of a Pillared Montmorillonite
A
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L
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Figure 1. Plot of Iz9Xechemical shift against xenon pressure for xenon adsorbed on pillared montmorillonite at room temperature (295 K).
chemical shift against coverage is obtained (see Figure 1). The intercept of this pattern extrapolated back to zero xenon content is 94 (f4) ppm and is due solely to xenon interactions with the surface. Ito and Fraissard3 found that in zeolites the intercept is independent of framework composition, although it may be affected by cations;6 Demarquay and FraissardS established an empirical correlation between the value of the intercept and the mean free path of adsorbed xenon. In our case their equation gives an estimated xenon mean free path of 3.3 A. The correlation is not necessarily valid for layered materials, however, and indeed has been found wanting for some zeolitic materials as We have calculated the mean free path of xenon in the sample to be about 4.3 A (by computer simulation of random motion of a xenon atom within the pillared clay), which is rather higher than the value obtained from the equation of Demarquay and Fraissard. This may be due to differences in the mean free path modeling procedure, the effect of cationic charge on the pillars, or the limitations in the correlation of Demarquay and Frai~sard.~ Their equation does, however, allow a very rough estimate of mean free path to be made in the absence of other structural data. The 129Xespectra for all coverages at all temperatures show a single peak with the exception of those with 8 > 1 where an extra weak peak appears as a shoulder as discussed below, The set of spectra at varying temperatures for the 8 = 0.30 case on the absolute intensity scale is shown in Figure 2 and is typical of our results. The increase in absolute intensity on cooling the sample is due partly to more xenon being adsorbed from the dead volume of the NMR tube into the sample and partly to the Boltzmann factor affecting the population of the two energy levels. We can take into account the fact that N M R is a quantitative technique: the NMR signal intensity as measured by the area under the peak should be proportional to the number of xenon atoms in the coil and inversely proportional to temperature. This enables an estimate of the number, N , of xenon atoms within the coil to be obtained as a function of temperature. This enables an accurate estimate of xenon content within the sample, as all spectra for each particular loading were recorded under identical conditions. A typical NMR adsorption curve obtained by using this method, given in Figure 3, shows that the bulk of the xenon has adsorbed below temperatures of ca. 180 K. Similar results were obtained for the other sample loadings investigated. Measurements at these lower temperatures should therefore accurately reflect the chemical shift variation with xenon concentration. Figure 4 shows the (12) Chcn, Q. J.; Springuel-Huct, M. A,; Fraissard, J. Chem. fhys. Len. 1989, 159, 117. (13) Barric, P. J. Ph.D. Thesis, University of Cambridge, 1990.
A
260K
273K
A
300
295K
0
100
200
PPm
Figure 2. 129XeNMR spectra of xenon adsorbed on pillared montmorillonite as a function of temperature for 6 = 0.30.
I
I
Temperature
(K)
Figure 3. Amount of xenon adsorbed with temperature calculated from NMR intensity measurements for B = 0.30.
change of chemical shift as a function of 8 at 130, 150, 170, and 200 K,where the 8 scale assumes that all xenon has been adsorbed from the dead volume at these temperatures. The chemical shift of the main '29Xepeak in all the samples increases as expected with reduction in temperature. The value of the intercept of chemical shift extrapolated to zero xenon pressure is temperature dependent. This has also been observed for some zeolitic structure^.^,'^ There is no major change in the overall pattern at a particular loading with temperature so the results do not detect any change in the structure of the pillared clay. Thus, NMR does not detect the minor change in surface area of the sample previously observed' from adsorption data between 134 and 136 K. There is a general increase in chemical shift with coverage resulting from the greater frequency of Xe-Xe collisions with increasing xenon concentration. Superimposed upon this trend are a number of distinct features.
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991
9418
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Barrie et al.
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e
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,-", 0.0
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, 0.2
.
, 0.4
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, 0.6
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, 0.8
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,
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Figure 4. 129Xechemical shift versus coverage at 130, 150, 170, and 200
K.
At coverages less than 0.35 the chemical shift increases monotonically, but the slope seems to become small between 0 z 0.35 and 0.55. There is a significant jump of chemical shift around 0 = 0.55,which is followed by a region between 0 = 0.64 and 0 = 0.88 in which the shift changes little. Finally, there is a large change in chemical shift in the vicinity of 0 = 1 (30-50 ppm, between 0 = 0.88 and 0 = 1.07 depending on temperature). The presence of plateaus following regions of greater slopes (or ujumps" given the finite number of data points) is compatible with the growth of domains or clusters. In the early stage of growth the average coordination number, and hence lZ9Xechemical shift, will increase rapidly; however, as the clusters grow larger the addition of xenon atoms to the edge will have a negligible effect upon the overall xenon environment. Hence, the chemical shift will appear to level off as the coverage increases. Since growth occurs within the interlamellar spaces, the shape of the clusters is likely to be that of patches or layers. This accounts for the behavior up to 0 = 0.55. The repetition of this feature between 0 = 0.55and 0.88 coincides with the region of "bilayer" formation identified in our previous studies of adsorption. Addition of more xenon atoms to the monolayer necessarily increases the average coordination number in the clusters, bringing about a change of slope followed by a plateau as the bilayer is completed. The large jump in chemical shift in the vicinity of 0 = 1 corresponds to the situation of complete filling of the interlamellar space. Previous adsorption data also indicated the existence of a phase transition at this loading. The large increase in chemical shift will probably be associated with not only an increase in the number of xenon nearest neighbors but also a sudden change in the xenon diffusional and dynamical behavior as the interlamellar region is completely filled. At the highest xenon concentrations (with 0 > l), an additional minor peak appears in the spectra below 220 K as a shoulder on the main peak as shown in Figure 5 . This extra peak is assigned to xenon adsorbed on the exterior surface of the crystallites. The behavior of the peak with temperature is highly unusual, a s it shows a reduction in chemical shift position at the lowest temperatures (see Figure 6 ) . It is interesting to note that the additional peak attributed to xenon on the external surface does not resonate at the frequency of bulk xenon. Gaseous ImXe resonates at ca.0-10 ppm, while bulk solid xenon resonates at ca. 300 ppm (depending on temperature). The extra peak position is in contrast to the results of Cheung et a].,* who found a lz9Xesignal at 304 ppm for excess xenon adsorbed on the external surface of zeolite Y at 144 K, which does agree with the figure for bulk solid xenon. It seems that in the pillared clay the excess xenon does not condense onto other xenon atoms (at least at the loadings investi-
400
500
ZOO
100
PPm
Figure 5. Iz9XeNMR spectra as a function of temperature for 0 = 1.07 showing the appearance of the additional weak peak.
100
150
Temperature
200
250
(K)
Figure 6. Behavior of the additional Iz9Xe NMR peak for B > 1.
gated), but rather spreads out over the surface. In an attempt to confirm the assignment of this extra peak, some Iz9Xe NMR experiments on xenon adsorbed on the external surface of an unpillared gelwhite (also dehydrated a t 300 "C) were also performed. The results on unpillared gelwhite, with a low and a very high xenon loading, are shown in Figure 7 . The low coverage corresponds to a coverage of approximately half a xenon monolayer on the outer surface. At higher temperatures the samples also show a peak between 0 and 15 ppm due to unadsorbed xenon gas. Below about 220 K, the low xenon concentration sample shows a peak between 150 and 170 ppm, which shows only a small temperature variation. The higher coverage sample has an intense
Iz9Xe N M R Studies of a Pillared Montmorillonite 340
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9419
I m Lwrmv(0.S)
zoo-
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E. 1w
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140
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: -E
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r
nI
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w , \
I
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Figure 7. 129Xechemical shift versus temperature for xenon adsorbed on unpillared montmorillonite.
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Figure 9. Variation of the line width (measured as the full width at half-maximum) with coverage for Xe in the pillared clay at 130 and 170 K.
the external surface. We attribute the small decrease in chemical shift of this peak with reduction of temperature below about 160 K to a reduction in the number of nearest xenon neighbors. This means that xenon must be leaving the external surface upon cooling, either to adsorb inside the pillared clay or to form bulk solid xenon in the case of the unpillared material. Line-width measurements do not clearly show the presence of the phase transitions suggested from the chemical shift measurements above. There is, however, a significant increase in peak width at the lower temperatures around 0 = 1 (see Figure 9). While it might be expected that a phase transition would cause a decrease in peak width, as seen for example by Cheung et al.,8*9 it seems that in this case the formation of the fully packed bilayer of xenon between the layers is accompanied by a reduction in xenon mobility, thus increasing the line width. It should be pointed out that the linewidths of IZ9Xein our sample are very large, compared to low-temperature IZ9Xespectra on zeolitic materials.8*9313This demonstrates that the system is not very homogeneous: xenon may be in close proximity to one or all of the interior surface, pillars, and other xenon atoms.
'
& & & & r ; o r o o K , PPm
Figure 8. IZ9XeN M R spectra of xenon on unpillared montmorillonite, corresponding to a xenon coverage of approximately 3.8 monolayers.
peak, whose shift depends heavily on temperature. At the lowest temperatures, it corresponds to bulk solid xenon as expected. However, the sample also has a weaker intensity peak showing similar anomalous temperature behavior as the additional peak obtained in the pillared samples with 0 > 1. Spectra for this sample are shown in Figure 8. The extra peak for the pillared sample occurs at chemical shifts between those obtained for the low-coverage unpillared sample and the weaker peak of the high-coverage unpillared sample. This confirms our assignment of the extra peak for the pillared samples with 8 > 1 to xenon on
Conclusions Variable-temperature Iz9XeN M R is a useful probe of layered microporous systems and may be used to investigate xenon phase transitions. However, there are a wide variety of factors influencing the observed Iz9Xe chemical shifts, and it is difficult to interpret complicated trends on the basis of Iz9XeN M R without the use of complementary techniques such as detailed adsorption and XRD measurements. The low-temperature 129XeN M R results in this paper have been assigned on the basis of various phase transitions. The unusual behavior with temperature of the chemical shift of Iz9Xe adsorbed on the external surface of clay crystallites further illustrates the potential applications of this technique to characterizing pore volumes. Acknowledgment. P.J.B. and I.G. acknowledge financial support from the SERC for a studentship and a fellowship, respectively. G.F.M. acknowledges the support of SERC and Unilever Research, Port Sunlight, for a CASE award. Registry No. Gelwhite L, 1318-93-0; Xe, 7440-63-3; aluminum chlorohydrate, 1327-41-9; alumina, 1344-28-1.
