Microporous channel and crystallite surface effects on xenon atoms as studied by 129Xe NMR: shielding and exchange of xenon in SAPO-11 and AlPO4-11 molecular sieves

Microporous and Mesoporous Materials 46 (2001) 99±110

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Microporous channel and crystallite surface e€ects on xenon atoms as studied by 129Xe NMR: shielding and exchange of xenon in SAPO-11 and AlPO4-11 molecular sieves Tuomas Koskela, Mika Ylihautala 1, Jukka Jokisaari * Department of Physical Sciences, NMR Research Group, University of Oulu, P.O. Box 3000, FIN-90014 University of Oulu, Finland Received 13 November 2000; received in revised form 21 March 2001; accepted 21 March 2001

Abstract 129

Xe nuclear magnetic resonance (NMR) spectra of xenon gas adsorbed into silicoaluminophosphate (SAPO-11) and aluminophosphate (AlPO4 -11) molecular sieve samples were recorded at variable temperatures. From the variations of the anisotropic nuclear shielding of xenon atoms adsorbed inside the sieve channels it was possible to separate the direct temperature e€ect and the e€ect due to loading variation. It was found that in the AlPO4 -11 sample, unlike in SAPO-11, several resonance signals having di€erent temperature dependence arise from the non-adsorbed gas between the crystals. This behavior is shown to arise from the exchange between the external crystallite surfaces and the free intercrystallite atoms. The chemical shift variations of these signals are utilized to estimate the intercrystallite cavity sizes. Two-dimensional exchange spectra of xenon were recorded in order to reveal further the role of the intercrystallite cavities, surface physisorption and the contribution of the fast exchange of xenon to these spectral signals. Contributions of material di€erences and sample composition to the nuclear shielding and the e€ect of sample defects on the loading level estimations are also discussed. These results are useful when analyzing the spectrum of a probe atom in a microporous material sample as a whole, not only the spectral components from the adsorbed and the free gas. Furthermore, these ®ndings help to understand the peculiar temperature dependences of the NMR spectra, even in cases with imperfect sample quality or with variations in the packing degree, particularly with high gas amounts. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Aluminophosphate molecular sieves; Shielding anisotropy

129

Xe NMR; Temperature dependent nuclear magnetic shielding; Xenon exchange;

1. Introduction

* Corresponding author. Tel.: +358-8-553-1308; fax: +358-8553-1287. E-mail address: jukka.jokisaari@oulu.® (J. Jokisaari). 1 Present address: Marconi Medical Systems Finland, Inc.,  Ayritie 4, P.O. Box 185, FIN-01511 Vantaa, Finland.

Solid microporous materials ± such as molecular sieves ± are capable of adsorbing guest atoms (or molecules) with proper size and shape, like xenon. While adsorbed into the sieve material, a xenon atom experiences changes in its nuclear shielding tensor due to both Xe±Xe and Xe± molecular sieve channel wall interactions. These

1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 2 9 6 - 7

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changes can be detected through NMR spectroscopy. The 129 Xe NMR spectroscopy of adsorbed xenon in a variety of porous solids [1±5] and other substances as well [6] has become an ecient means to characterize these materials. This is due to the fact that the shielding of xenon is very sensitive to its environment [5,7,8]. Experiments have shown that the 129 Xe shielding of xenon in zeolites and molecular sieves depends on (i) the pore and channel dimensions [3,9±12], (ii) the cation number, distribution and location [13,14], (iii) coadsorbed molecules, (iv) dispersed metal atoms, (v) paramagnetic ions, (vi) blockage of pores, and (vii) domains of di€erent composition or crystallinity [15]. A number of molecular sieve properties have been studied with 129 Xe NMR; for example cavity sizes and shapes, and di€erent di€usion processes of adsorbates [4,16]. Intraparticle and interparticle xenon exchange and dynamics of adsorbed xenon have been studied by Moudrakovski et al. [17,18]. Silicoaluminophosphates and zeolites, both containing silicon atoms, exhibit framework charges, which are compensated by cations distributed in the framework. The location and size of the cations play an important role in catalytic activity, hydrothermal stability, and adsorptive properties of the materials [19± 23]. In the present paper, we have introduced xenon gas into two molecular sieves, SAPO-11 and AlPO4 -11. Especially the temperature-dependent nuclear shielding and the role of crystallite surface adsorption in xenon exchange were studied by means of 129 Xe NMR spectroscopy. Both materials have the same framework structure: straight 2 , channels with elliptical cross-section (3:9  6:3 A structure code AEL) and no interconnections [24]. Previous studies on these materials have revealed anisotropic shielding of the 129 Xe adsorbed inside the sieve channels [25,26]. The shielding was found to be more anisotropic in the SAPO-11 material in which the adsorptive ability is enhanced by the charge-compensating cations [25]. The loading dependence of the 129 Xe shielding anisotropy and asymmetry in AlPO4 -11 was explained by Ripmeester and Ratcli€e [26] with the existence of three types of xenon adsorption sites experiencing rapid exchange. This model was supported by a com-

puter simulation, in which AlPO4 -11 was modeled as a fully dynamic framework [27]. One reason for possible di€erences between the two materials ± as far as the 129 Xe NMR spectra arising from the xenon inside the sieve channels are concerned ± is the presence of cations in SAPO-11. Ab initio calculations of 13 CH4 suggested that the electrostatic ®eld from the charge-compensating cations in SAPO-11 is more probable reason than structural deformations of methane for the anisotropic 13 C shielding tensor [28]. However, for reliable comparison also the e€ect of channel loadings, i.e., the number of xenon±xenon collisions inside the channels, as well as the possible di€erences in the material quality (e.g., crystallinity and pore blockings) must be considered. 2. Experimental SAPO-11 was provided by UOP (Tarrytown, USA) and AlPO4 -11 by Laboratoire de Materiaux Mineraux (Mulhouse, France) in their as-synthesized forms. Both samples were calcined by keeping them at 870 K for at least 14 h [29]. The Si/Al ratio of 0.061 for SAPO-11 was determined by direct current plasma atomic emission spectrometry (DCP-AES) for the calcined material. The lack of Si atoms (<0.1 mg/g) in AlPO4 -11 was also con®rmed with DCP-AES. A total of three samples were made: one AlPO4 -11 sample and two SAPO-11 samples with di€erent amounts of xenon gas. A thick-wall Young valve tube was used for the high-pressure SAPO-11 sample, and thick-wall glass cells (outer diameter 8 mm, length 30 mm) were used for the other two samples. Water was removed from the sieves by connecting the sample tubes to a vacuum line (<1 mbar) and heating the samples gradually to 670 K and keeping them heated for approximately 14 h. Natural abundance xenon gas (Messer Griesheim; purity P 99:99 vol%) was transferred into the samples whereupon the glass cells were sealed with a ¯ame or the Young valve was closed. The initial loading of the samples in terms of total xenon gas/material amount was 1:3  10 3 mol/g (AlPO4 -11), 0:6  10 3 mol/g (SAPO-11, low pressure), and 1:6  10 3 mol/g (SAPO-11, high pressure).

T. Koskela et al. / Microporous and Mesoporous Materials 46 (2001) 99±110

One-dimensional 129 Xe NMR spectra were measured on a Bruker DSX300 spectrometer (resonance frequency 82.981 MHz). For the SAPO-11 low-pressure sample, the spectra were recorded using a 10 mm high-resolution probehead over the temperature range of 223±393 K applying 90° pulses, 1.0±1.5 s repetition time, and accumulating 3072±4096 scans. In addition, a spin-echo pulse sequence (90°-s-180°-s-Acq. [30]) was used in order to eliminate obscured baselines occurring in the spectra of SAPO-11 and AlPO4 -11 samples of higher gas amount. The delay between the 90° and 180° pulses was 1 ms, the repetition time 0.5±5.0 s, and 20±150k scans were accumulated over the temperature range of 165±375 K. A 10 mm wideline probehead was used for the AlPO4 -11 spectra at temperatures below 200 K. Sample temperatures were controlled and allowed to stabilize for variable times, which were dependent on the size of the temperature step, but at least ten minutes (much longer at low temperatures). The 129 Xe resonance signal from a bulk xenon gas sample (®xed to 0 ppm) was used as an external reference. The spectra were analyzed using the WIN-FIT program [31]. 129 Xe 2D exchange spectroscopy (EXSY) [32] NMR spectra for xenon in AlPO4 -11 were measured on a Bruker DRX500 spectrometer (resonance frequency 138.302 MHz). The spectra were recorded using a 10 mm high-resolution probehead at the temperatures 280 and 355 K using 100 ms mixing time, 2.0 s repetition time, and accumulating 1024 scans for each t1 increment with 64  256 (at 280 K) and 128  512 (at 355 K) data points sampled for each spectrum requiring over 38 h of spectrometer time. Both sieve materials and their crystallite dimensions were examined in the non-calcined form with the aid of ®eld emission scanning electron microscopy (FESEM) pictures. The average crystallite size was found to be <2 lm for SAPO-11, and a slightly lower for AlPO4 -11. The size and the shape variations of the crystallites are larger for SAPO-11. The average diameter of the large crystallite lumps for SAPO-11 is around 4±7 lm and around 3±5 lm for the AlPO4 -11 sample. In AlPO4 -11 the crystallite lumps are more foliaceous.

101

3. Results and discussion Fig. 1(a) displays the 129 Xe NMR spectra of xenon in the low-pressure SAPO-11 sample at variable temperatures. At low temperatures, only the chemical shift anisotropy (CSA) pattern of adsorbed xenon is observed. At higher temperatures, an additional resonance signal is detected at around 0 ppm, which arises from free intercrystallite gas (from now on referred to as FIG). The resonance frequency close to 0 ppm indicates that the e€ect of the sieve crystallite surfaces on the nuclear shielding of FIG atoms is small. This implies that the intercrystallite void spaces are relatively large and the bulk susceptibility e€ect of the sieve material is small. The intensity of the FIG signal diminishes with decreasing temperature, and ®nally vanishes at around 300 K. This is expected, because in thermal equilibrium, an increasing proportion of xenon should be adsorbed with decreasing temperature. The lineshape of the adsorbed gas signal approaches the ideal CSA powder pattern shape with decreasing temperature and increasing amount of channel loading. The more ideal lineshape might be a consequence of diminished exchange between the adsorbed and FIG xenon due to mutual blockage of the xenon di€usion routes to the channel entrances at higher loading. Fig. 1(a) also displays a small third resonance located near the CSA pattern. Interestingly, this signal becomes stronger with decreasing temperature. The shielding value in between the values of adsorbed and free gas could be an indication of an exchange signal. However, the temperature-independent frequency of this signal does not support such a conclusion, and therefore a possible explanation is that the sieve material contains a small amount of cavities with dimensions slightly larger than those of the channels. These cavities could be due to an imperfect crystallinity of the material or due to the formation of crystal ``lumps'' as shown by the FESEM pictures. Lineshape analyses of the asymmetric CSA m patterns reveal the tensor components rm 11 , r22 , and m r33 of the medium induced part of the 129 Xe shielding tensor in principal axis system, rm ii ˆ rii r0 , as the shielding of the reference (r0 ) is

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Fig. 1. Stack plot of the 129 Xe NMR spectra of xenon gas in the SAPO-11 sample at variable temperatures: (a) low-pressure sample, (b) high-pressure sample (recorded with spin-echo sequence). Note the use of shielding units on the x-axis.

close to the shielding of free xenon atom. The order of these tensor components is adjusted according to jr11 r0 j 6 jr22 r0 j 6 jr33 r0 j and presented as a function of temperature for the low pressure SAPO-11 sample in Fig. 2(a). In this m particular case, tensor components rm 11 and r22 intercross at around 325 K, and therefore the speci®ed order mentioned above is not valid at temperatures above 325 K. A closer look at the component values of the shielding tensor shows that the temperature behavior can be divided into two separate practically linear sections (see Fig. m m 2(a)). Above 293 K, rm 22 , r33 , and riso show a stronger temperature dependence than below 293 K. Table 1 lists the parameters of the linear leastsquares ®ttings. The e€ect of temperature on the shielding of xenon may be divided into direct and indirect effects. The direct temperature e€ect arises from the changes in the statistical weights (determined by the Boltzmann factor) of the di€erent locations of the adsorbate with respect to the intracrystallite space as well as with respect to the other adsorbates. In other words, the less probable highpotential energy intracrystallite locations of the adsorbate occur more frequently, and Xe atoms are able to get closer to each other when the temperature is increased. The indirect e€ect arises

from the fact that also the channel loading changes with temperature. The increase of the number of Xe atoms in the channels makes interactions of two and more xenon atoms more frequent, and thereby changes the average shielding. It is expected that the 129 Xe shielding contribution arising from the Xe±Xe interactions is directly proportional to the xenon loading in SAPO-11, as is the case with AlPO4 -11 [26]. Therefore, the linear variation of 129 Xe shielding indicates a linear change in the channel loading. This is indeed supported by the relative intensity variations of the free and adsorbed gas signals shown in Fig. 3. The change in the xenon loading was estimated from the integrated areas of the adsorbed and nonadsorbed spectral components with the help of the known amounts of xenon gas and sieve material in the sample tube. According to this estimation, the loading varies from 30% (100% loading corresponds to 4 atoms/unit cell [26]) at the highest temperature to 40% when all the xenon is adsorbed. However, the actual loadings as well as their variation are likely to be a little bit higher as the crystallinity of SAPO-11 is probably less than 100%, which was assumed in the estimation. At 293 K a major part of the xenon gas is adsorbed as indicated by the non-existing FIG signal, and thus the amount of adsorbed xenon is

T. Koskela et al. / Microporous and Mesoporous Materials 46 (2001) 99±110

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m Fig. 2. Average 129 Xe shielding tensor elements of xenon adsorbed in SAPO-11 and AlPO4 -11 as a function of temperature: rm 11 (N), r22 ( ) and rm (j). Also shown is the isotropic average of the shielding tensor ( ). (a) Low-pressure SAPO-11 sample, (b) high-pressure 33 SAPO-11 sample, and (c) AlPO4 -11 sample. Linear ®ttings are done separately for data points above and below temperatures (a) 293 K, (b) 300 K, and (c) 295 K (with the exception rm 11 for which ®tting was done with all the data points). The ®tting parameters are listed in Table 1.

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Table 1 Linear least-squares ®tting …rm ii ˆ a ‡ bT † parameters of Tensor element

129

Xe average shielding tensor elements at two temperature rangesa

a (ppm)

b (ppm/K)

SAPO-11 Low-pressure

AlPO4 -11 High-pressure

SAPO-11 Low-pressure

AlPO4 -11 High-pressure

High temperature rm 11 rm 22 rm 33 rm iso

127  2 220  4 273  6 208  3

120  8 302  6 352  13 257  2

110  8 306  7 305  6 246  3

0.018  0.005 0.267  0.010 0.30  0.02 0.188  0.007

0.04  0.03 0.43  0.02 0.49  0.04 0.289  0.006

0.08  0.03 0.41  0.02 0.31  0.02 0.233  0.009

Low temperature rm 11 rm 22 rm 33 rm iso

127  2 175  4 208  7 169  5

120  8 218b 234b 193b

110  8 200  9 217  12 170  12

0.018  0.005 0.117  0.014 0.08  0.03 0.06  0.02

0.04  0.03 0.15b 0.09b 0.08b

0.08  0.03 0.06  0.04 0.01  0.05 0.03  0.05

a b

m m The temperature ranges are ignored in the ®ttings of rm 11 . The ®ts of riso are done independently of rii ®ts. No error given because of small number of data points.

Fig. 3. Relative signal intensities of adsorbed (j) and nonadsorbed ( ) xenon gas as integrated from the 129 Xe NMR spectra of the low-pressure SAPO-11 sample at variable temperatures.

practically constant at temperatures below 293 K. Consequently, the exclusive reason for the shielding change must arise from the direct temperature e€ect: at decreasing temperatures xenon atoms spend more time in the potential energy minima locations created by the channel walls and local adsorption sites, thus sampling less uniformly the intracrystallite space. The absence of loading variation is seen as the parameters obtained from m m the linear ®ts of rm 22 , r33 and riso tensor compo-

nents show a 2±4 times smaller dependence on temperature (see Table 1). By contrast, rm 11 displays a negligible temperature dependence (see also Ref. [26]), and therefore the rm 11 ®ttings are done over the whole temperature range. In the case of the high-pressure SAPO-11 sample, a more reliable lineshape analysis than with conventional spectra was achieved with the spinecho spectra (Fig. 1(b)), as baseline distortions occurring in the conventional spectra are eliminated. As compared to the low-pressure sample, relatively more xenon atoms are adsorbed at high temperatures, which is also seen in the CSA patterns as higher absolute values of the shielding components. For example, the shielding components at around 375 K in the high-pressure SAPO-11 sample are close to the values in the low-pressure sample at around 320 K as revealed by Fig. 2(a) and (b). Again, the shielding components show a linear dependence on temperature. The slopes change similarly to the ones found in the lowpressure sample (see Table 1) indicating the presence of direct and indirect e€ects of temperature. The reduced temperature dependence of the shielding at the two lowest temperatures is almost the same as in the low-pressure sample at low temperatures, although this estimation su€ers from a lack of data points. This implies that the channel loading does not change considerably at low

T. Koskela et al. / Microporous and Mesoporous Materials 46 (2001) 99±110

temperatures, although the spin-echo spectra show changes in the free gas signal intensity. This, however, does not necessarily mean a change in the channel loading, because the signal intensities may be strongly a€ected by the relaxation rates, particularly spin±spin relaxation may a€ect the spin-echo amplitude. If this is the case, the only possibility for the stop of the adsorption of xenon with decreasing temperature is when the channels are fully occupied with xenon. An estimation based merely on the amount of xenon in the highpressure SAPO-11 sample would imply a loading of 90%, assuming all the xenon gas is adsorbed. As the estimation is based on the assumption of a perfectly crystalline material, the crystallinity of the material must be lower than 100% in order for the channels to be fully loaded before all the xenon is adsorbed. Fig. 4 displays the 129 Xe spin-echo NMR spectrum of xenon adsorbed in the AlPO4 -11 at variable temperatures. At temperatures above 200 K the CSA powder pattern arising from the adsorbed xenon gas can be found on the left side of the spectra and the FIG signal on the right. In addition, a relatively intensive third signal is seen between the two signals. At the lower temperatures, the intensity of the adsorbed gas signal relative to non-adsorbed gas signals decreases and ®nally vanishes when the temperature is below 200 K. Because the loading should not decrease with de-

Fig. 4. Stack plot of the 129 Xe NMR spin-echo spectra of xenon gas in the AlPO4 -11 sample at variable temperatures.

105

creasing temperature, the explanation must be the decrease of the spin±spin relaxation time (T2 ) of the adsorbed xenon with decreasing temperature, which causes signal vanishing in the spin-echo experiment. The decrease of T2 of 129 Xe is not so surprising when considering the fact that at low temperatures xenon atoms are more localized, and consequently dipolar interactions may enhance the spin±spin relaxation rate. The shielding components obtained from the lineshape analysis of the CSA pattern are shown in Fig. 2(c). As in SAPO-11, a separation of the temperature behavior in two approximately linearly varying secm m tions is seen. Above 295 K, rm 22 , r33 , and riso show a stronger dependence on temperature, whereas at lower temperatures, an even more dramatic drop in the temperature dependence than in SAPO-11 is seen (see Table 1). It is very likely that the reason for this is the same as in SAPO-11: the variation of channel loading dominates at high temperatures, whereas at lower temperatures only the direct temperature e€ect is present. However, because the 129 Xe spectra show considerable signals arising from Xe atoms not adsorbed in the channels, the reason for constant loading below 295 K must be the saturation of the AlPO4 -11 channels with xenon. A way to estimate channel loadings is to utilize the relation between the 129 Xe shielding tensor elements and channel loading at room temperature developed by Ripmeester and Ratcli€e [26]. This estimation gives a relatively high loading value (90%) at room temperature. Additional signals in between the free gas and CSA powder pattern are seen even more clearly in the conventional 129 Xe spectra (see Fig. 5). The shift of these signals toward higher frequencies (lower shielding) with decreasing temperature implies that the signals arise from xenon atoms fastly exchanging between a high- and a low-frequency state. Obviously, the low-frequency state arises from FIG. The high-frequency state may in turn correspond to either adsorbed atoms or atoms interacting with the crystallite surfaces. If the highfrequency state would arise from the adsorbed atoms, the exchange should take place between the free xenon and atoms adsorbed at the ends of the channels. However, in this case it is dicult to explain the quite di€erent behavior of the

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T. Koskela et al. / Microporous and Mesoporous Materials 46 (2001) 99±110

Fig. 5. Non-adsorbed gas resonances of the conventional 1D 129 Xe NMR spectra of xenon in the AlPO4 -11 sample at variable temperatures. The Roman numerals refer to di€erent resonance patterns described in Table 2.

exchange signals, that is, the average signals in the spectra shift di€erently towards higher frequencies with decreasing temperature. On the other hand, the exchange signal between the free xenon and the xenon on the surface should not show a strong temperature dependence, as long as the xenonsurface interaction depends only on the volume to surface area ratio of the intercrystallite void spaces. The situation changes if the external surfaces of the sieve crystallites physisorb the xenon atoms. Physisorption is enhanced at low temperatures, thus increasing the lifetime of adsorbates on the surface, and in these conditions the nuclear shielding of intercrystallite xenon atoms approaches the value it has on the surface. The e€ect of surface adsorption on the xenon (or any other adsorbate species) nuclear shielding can be estimated with the following simple model. Assume that xenon atoms reside in a cavity formed by the molecular sieve crystallites. If the crystallite surfaces have adsorptive properties, xenon atoms close to the crystallite surfaces have a lower potential energy than the atoms at large distances from the crystallite surfaces. As a crude model, one could approximate this situation with two values; potential energy on the surface Us , which is

the average value within a short distance from the crystallite surface, and potential energy in the free volume Uf , which is basically the same as for the free gas. According to Boltzmann statistics, these energies de®ne the distribution of xenon atoms between the two states. Now the probabilities to ®nd a xenon atom in either state can be evaluated, if the free volume Vf and the volume close to the surface Vs are known. The latter volume obviously depends on the surface area and the thickness of the adsorption layer determined by the range of the potential Us . These probabilities can be used instead of the average lifetimes of xenon atoms in the two states to evaluate the average nuclear shielding for exchanging atoms, because in equilibrium a kinetic balance has to exist between the adsorbed and the non-adsorbed atoms. Hence the observed nuclear shielding of xenon undergoing fast exchange is rex ˆ ps rs ‡ pf rf , where rs and rf refer to shielding components along the external magnetic ®eld, and ps and pf to the probabilities to ®nd a xenon atom on the surface and in the free volume, respectively. Strictly speaking, rs depends on the direction of the molecular sieve crystallite surface with respect to the external magnetic ®eld, because it is expected that the surface induces anisotropic distortion to the adsorbate shielding tensor. However, in (closely) spherically symmetric cavities the anisotropic contribution averages out if a xenon atom explores the whole cavity within the NMR time scale. Thus, the direction-independent isotropic part of the surface induced distortion gives a good approximation for rs . The complete equation for the medium induced nuclear shielding of exchanging atoms is   exp… DU † V f r ‡ rs kT Vs f  ; rex ˆ  …1† 1 ‡ exp… DU † Vf kT Vs where DU ˆ Uf Us . It is assumed that rs and rf are independent of temperature and xenon density on the surface and in the free volume. At least for the free gas both of these assumptions are quite justi®ed for reasonable temperature and density changes [7,8,33,34]. Aspects of this model are similar to those for micropores of increasing size developed by others [35±37].

T. Koskela et al. / Microporous and Mesoporous Materials 46 (2001) 99±110

Fig. 6. E€ect of molecular sieve crystallite surfaces on nonadsorbed gas signals. Non-linear ®ttings (see Eq. (1)) of the chemical shifts of three separate resonance patterns at variable temperatures. The dotted line indicates the case where the free space volume is far greater than the surface volume. The ®tting parameters are presented in Table 2. Roman numerals refer to di€erent resonance patterns described in Table 2 and in Fig. 5.

Fig. 6 shows least squares ®ts of Eq. (1) to the most clearly resolved exchange signals (in Fig. 5). In the ®ts, DU and rs were re®ned as common parameters and rf was ®xed to 0 ppm for all exchange signals (see Table 2). The only varied parameter from one ®t to another was the Vf =Vs ratio, arising from the varying intercrystallite cavity sizes existing in real samples. The behavior of all the exchange signals could be reproduced well with the present model. The value of adsorption energy DU (10 kJ/mol) is on the same order of magnitude as estimated for gases on solid surfaces (0.3±3 kJ/mol [38]). On the other hand, the value of rs … 254  22 ppm† is surprisingly large, even larger than the value found for xenon adsorbed in the channels Table 2 Non-linear ®tting parameters for non-adsorbed xenon 129 Xe average shieldings in the AlPO4 -11 sample at variable temperaturesa rs (ppm) 254  22

DU (J)

Resonance pattern

Vf =Vs

…1:73  0:05†  10 20

I

427  21

II III

472  23 1986  130

…10 kJ/mol† a

See Fig. 6 and Eq. (1).

107

of AlPO4 -11. In fact, the value is close to the value found for xenon in the liquid state [39]. This ®nding may be explained with a growing amount of Xe±Xe interactions on the surface due to increasing xenon density. From the di€erent Vf =Vs ratios one can determine approximate sizes of the intercrystallite cavities. Assuming that the cavities are spherical and that the thickness of the adsor (corresponding to one atom bate layer is 5 A layer), the smallest cavities have a diameter of 1 lm (corresponds to resonance patterns I and II in Fig. 5), the medium-sized cavities have diameter 6 lm (corresponds to resonance pattern III in Fig. 5), whereas the non-shifting gas signal arises from at least two orders of magnitude larger cavities. The crystallite size of AlPO4 -11, determined from FESEM pictures, is around 5 lm, which ®ts well to the sizes of the intercrystallite void spaces determined above. The smaller intercrystallite cavities exist in tightly packed regions of the sample, whereas the non-shifting signal arises from loosely packed regions. The role of the di€erent physical environments in exchange phenomena in AlPO4 -11 was further studied by the 2D EXSY method, which gives information on the slower exchange phenomena. Fig. 7 shows 129 Xe 2D EXSY spectra at two temperatures. The left-hand lower corner signal on the diagonal corresponds to the gas adsorbed in the channels of AlPO4 -11, whereas the other diagonal signals arise from the intercrystallite xenon gas. At 355 K (Fig. 7(a)) the intercrystallite gas signal forms a broad signal around 0 ppm. Only the gas residing in the smallest cavities of 1 lm diameter (see Fig. 5 and the component labelled ``I'') show a small distinct signal slightly shifted to higher frequencies. At 280 K (Fig. 7(b)) the smallest cavity gas signal is better resolved and also the lower frequency group of gas signals broadens due to a shift of the gas signal arising from the mediumsized intercrystallite cavities (resonance pattern III in Fig. 5). In both 2D exchange spectra, the correlation peaks are seen between the smallest cavity and the larger cavity signals, evidencing exchange of xenon gas between these cavities within the mixing time of 100 ms. In fact, there is exchange of xenon between all the di€erent size intercrystallite cavities as revealed by the broadening of the lower

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T. Koskela et al. / Microporous and Mesoporous Materials 46 (2001) 99±110

Fig. 7. 129 Xe 2D EXSY spectra of xenon gas in the AlPO4 -11 sample: (a) T ˆ 355 K and (b) T ˆ 280 K. The mixing time in both cases is 100 ms. The projections of columns in both the F1 and F2 dimensions are displayed as well.

frequency gas signal to the o€-diagonals. Such an exchange is not surprising as the gas phase selfdi€usion is fast enough allowing xenon atoms to travel considerably larger distances than the intercrystallite cavity sizes, even though the cavity boundaries (i.e., crystallite surfaces) impose some restrictions to the self-di€usion. More correlation peaks are observed at 355 K between the gas adsorbed in the channels and in the intercrystallite void spaces. This is evidence that the xenon atoms adsorbed at the ends of the channels are able to exchange their state with the non-adsorbed atoms as discussed in previous sections. The missing correlation peak between the adsorbed gas and the gas residing in the smallest intercrystallite cavities may imply that there is no exchange between these environments during the 100 ms mixing time. On the other hand, this may be ®ctitious because the correlation peak is probably buried by the background due to its small size, that is, the correlation signal, having only a fraction of the intensity of the small cavity signal on the diagonal, is distributed over the whole frequency range of the adsorbed gas signal. However, if this exchange signal is truly missing, it could be

an indication of the low crystallinity of the AlPO4 11 material as suggested above. In other words, the smallest molecular sieve particles forming the smallest cavities could be non-crystalline particles, which do not contain channels. Alternatively, the largest foliaceous AlPO4 -11 particles, as seen in the FESEM pictures, could be formed of non-crystalline particles merged together loosely, thus containing intraparticle cavities of the observed size. The fact that no such exchange signal as in AlPO4 -11 can be detected in the SAPO-11 samples may be explained by (i) a better sample quality (higher crystallinity and fewer channel blockages), (ii) the absence of small intercrystallite cavities due to fewer foliaceous crystallite lumps, which can be con®rmed with the FESEM pictures, or (iii) the surface of SAPO-11 crystallites do not adsorb xenon atoms in such a degree as AlPO4 -11, even if the channels in SAPO-11 do (according to adsorption isotherms, SAPO-11 adsorbs xenon slightly, but noticeably better than AlPO4 -11 [25]). In the low-pressure SAPO-11 sample, while the loading is changing, the 129 Xe shielding shows a smaller temperature dependence than in the high-

T. Koskela et al. / Microporous and Mesoporous Materials 46 (2001) 99±110

pressure SAPO-11 sample. Because the direct temperature dependence is almost the same for both SAPO-11 samples (see low temperature b values in Table 1) and the e€ect of loading is approximately linearly related to the 129 Xe shielding, the di€erence between the two samples most probably arises from the smaller change of loading and thus the decreasing number of Xe±Xe interactions in the low-pressure sample as a function of temperature. The isotropic average values of the 129 Xe shielding tensor in both SAPO-11 samples are slightly larger than the values in AlPO4 -11. The components of the average shielding tensor behave m quite similarly on both materials; both rm 22 and r33 increase linearly with increasing temperature, while rm 11 remains practically constant, which may mean that the 1-axis of the principal axis system is parallel to the short axis of the elliptical cross-section of the channel. Furthermore, while the channel occupations in the two sieve materials approach full loading towards lower temperatures, both rm 22 and rm 33 are approaching the same values (see Fig. 2(b) and (c)). This is due to an increasing contribution of Xe±Xe interactions to the 129 Xe shielding, which in both sieve materials have a similar e€ect due to the similar channel structures.

4. Conclusions The e€ect of temperature on the nuclear shielding of adsorbed xenon was found to be much smaller when the loading of the sieve channels is not changing. This direct e€ect of temperature is due to changes in the Xe-channel and Xe±Xe interactions. Usually, the condition of constant channel loading for variable temperature is not ful®lled, and thus the 129 Xe shielding shows a stronger temperature dependence. The extra contribution to the nuclear shielding arises from the change in the number of Xe±Xe interactions with the loading. The temperature dependence of 129 Xe shielding in AlPO4 -11 and the high-pressure SAPO-11 samples, being dependent on the loading, is quite similar, as revealed by the similar slope values at

109

the higher temperatures. This implies that the Xe± Xe interactions act similarly on the 129 Xe shielding on both samples, assuming that the loadings change identically as a function of temperature. In other words, a small amount of silicon atoms and cations in SAPO-11 does not signi®cantly a€ect the intracrystallite Xe±Xe interactions. Moreover, when the sieve channel loadings approach the saturated state, the 129 Xe shielding tensor components approach approximately the same values in both materials. The reason for this is that, at high loading levels, the Xe±Xe interactions dominate the shielding and therefore the e€ect of material di€erences remains relatively small. However, in order to con®rm these conclusions, measurements as a function of Xe loading at constant temperature should be performed (see previous works [25,26]). Beside the sieve channel framework structure and the overall gas pressure, other factors may have an in¯uence on the spectrum of xenon (or any other adsorbate). These include (i) crystallite structure and formation, (ii) surface adsorption strength, (iii) channel blockings, and in the case of strong surface adsorption, (iv) packing of the sample material. When the sample is based on small, tightly packed crystallites, the interaction of free intercrystallite xenon gas with crystallite surfaces may produce resonance signals, which can mistakenly be interpreted as adsorbed gas signals. Particularly, this may happen when surface adsorption occurs. This is evident for the AlPO4 -11 material used in the present study, where several intercrystallite cavity sizes were detected, unlike in SAPO-11.

Acknowledgements The authors are grateful to the Academy of Finland and the Neste Foundation for ®nancial support. T.K. is also grateful to the Faculty of Science of the University of Oulu for a grant. UOP is thanked for kindly providing SAPO-11. Likewise, Dr. Henri Kessler (Laboratoire de Materiaux Mineraux, Mulhouse, France) is thanked for kindly providing AlPO4 -11.

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