129Xe NMR for Studying Surface Heterogeneity: Well-Known Facts

Mar 5, 1997 - 129Xe NMR of adsorbed xenon is shown to be useful for studying properties of zeolites and related materials such as pore structure (pore...
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1229

Xe NMR for Studying Surface Heterogeneity: Well-Known Facts and New Findings†

M. A. Springuel-Huet*, J. L. Bonardet, A. Ge´de´on, and J. Fraissard Laboratoire de Chimie des Surfaces, URA CNRS 1428, casier 196, Universite´ P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 05, France Received December 21, 1995. In Final Form: August 30, 1996X 129Xe NMR of adsorbed xenon is shown to be useful for studying properties of zeolites and related materials such as pore structure (pore dimensions, pore geometry, defects, intergrowth, etc.), multicharged cations (compensating cations or extraframework aluminum, etc.), and encumbering species (coadsorbed phases, coke deposits, etc.). The technique has been extended to the study of microporous polymers.

Introduction Xenon is an ideal probe for investigating the properties of microporous solids.1-4 It is an inert gas, and there are two xenon isotopes (129Xe and 131Xe) detectable by NMR spectroscopy, which is a technique particularly well adapted to the study of electron perturbation in rapidly moving molecules. The spin 1/2 129Xe (26.4% natural abundance, 2 × 10-2 sensitivity relative to 1H) is the more used, the spin 3/2 131Xe (21.2% natural abundance, 3 × 10-3 sensitivity relative to 1H), which has a quadrupolar moment, being more difficult to detect in adsorbed phases. The high polarizability of the xenon electron cloud makes it very sensitive to physical interactions with its environment. This leads to an extremely large chemical shift range in 129Xe NMR spectroscopy. More generally, all NMR parameters are affected by the local interactions, the symmetry, and the motion of the xenon atoms. When one xenon atom is adsorbed on microporous solids like zeolites, it interacts only with the pore surface. Fraissard et al. have shown that, in this case, the observed chemical shift (δS) is related to the dimensions and the shape of the pore. The larger these dimensions and /or the easier diffusion inside the pores, the smaller the δS value. The shape and the dimensions of the pores and the ease of xenon diffusion can be characterized by the mean free path l of xenon imposed by the pore structure. A correlation between δS and a parameter, l, has been made.5 When the xenon concentration, [Xe], increases, Xe-Xe interactions appear and the excess of chemical shift (δXe), due to these interactions, increases. The δ ) f([Xe]) plots are linear when Xe-Xe interactions are isotropic, which is the case of pores with large dimensions compared to the Xe atom diameter (4.4 Å), or have an increasing slope when Xe-Xe interactions are anisotropic, as for pores with dimensions hardly larger than the Xe diameter. If there is any surface heterogeneity in the microporous solid such as: different types of void volumes in the pore * To whom correspondence should be addressed: tel, (33) 1 44 27 55 37; fax, (33) 1 44 27 55 36; e-mail address, [email protected]. † Presented at the Second International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland/Slovakia, September 4-10, 1995. X Abstract published in Advance ACS Abstracts, February 15, 1997. (1) Dybowski, C.; Bansal, N.; Duncan, T. M. Annu. Rev. Phys. Chem. 1991, 42, 433. (2) Barrie, P. J. Klinowski, J. Prog. NMR Spectrosc. 1992, 24, 91. (3) Raftery, D. Chmelka, B. F. NMR 1994, 30, 111. (4) Springuel-Huet, M. A.; Bonardet, J. L.; Fraissard, J. Appl. Magn. Reson. 1995, 8, 427. (5) Springuel-Huet, M. A.; Demarquay, J.; Ito, T.; Fraissard, J. Stud. Surf. Sci. Catal. 1988, 37, 183.

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structure itself, mixture of several solids, structure defects, structure intergrowths, divalent (or more highly charged) cations, metal particles, etc., or encumbering species (coadsorbed phases, coke deposit, etc.), xenon atoms may suffer additional interactions which modify the NMR chemical shift. Then the chemical shift of xenon adsorbed in a porous solid can be written as the sum of different terms corresponding to the various types of interaction to which it is subjected5

δ ) δS + δXe + δSAS + δΕ + δΜ where δSAS is due to specific interactions with strong adsorption sites. δΕ and δΜ arise from the electric and magnetic fields, respectively, possibly created by cations. These different terms can be studied. We demonstrate here with a few examples. Experimental Section Samples are evacuated under vacuum (10-5 Torr) at 673 K for zeolites and related molecular sieves and at 300 or 330 K for polymers. Then xenon is adsorbed at equilibrium pressure ranging from a few Torr to 2 atm for zeolites to several atmospheres (3-15) for polymers. In the latter case a known amount is condensed, using nitrogen liquid, in the tube containing the sample which is then sealed. The reference of all the chemical shifts is the shift of gaseous xenon extrapolated to zero pressure.

Zeolites (Containing H+ or Na+ Cations) This large class of microporous solids consisting of crystallized silicoaluminates exists naturally. Many new structures are now synthesized with other elements such as phosphorus, gallium, germanium, etc. and even with transition metals like titanium, iron, molybdenum, etc. The framework is often negatively charged and, therefore, there are cations to maintain electroneutrality. These cations are more or less easily exchangeable. Usually the synthesis is performed in a basic medium (NaOH) so the as-synthesized zeolites contain Na+ cations which can be exchanged by NH4+ to obtain the acid form after decomposition of NH4+. The porous structures may be considered as follows: interconnected cages, as in Y zeolite, which is the most widely used, in A or Rho zeolite; interconnected channels as in ZSM-5, ZSM-11 and ferrierite (Figure 1); nonconnected channels as in SAPO-11 and AlPO4-11 where the channel section is elliptical, 3.9 × 6.7 Å (Figure 4), in mordenite where there are dead-end channels called sidepockets opening onto the main channels (cross section 6.5 × 7.0 Å) by elliptical windows of 3.6 × 5.2 Å; An important feature of 129Xe NMR spectroscopy is that when xenon is © 1997 American Chemical Society

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Figure 1. Schematic representation of ferrierite channels. Reprinted with permission from ref 7. Copyright 1989 Elsevier.

Figure 3. 129Xe NMR spectra, at room temperature, of Namordenite (A) and H-mordenite (B) and at 253 K of H-mordenite (C). The broad downfield line corresponds to xenon adsorbed in side-pockets. Reprinted with permission from ref 9. Copyright 1984 Elsevier.

Figure 2. 129Xe NMR spectrum of a mixture of CaA (42%) and NaY (58%). Reprinted with permission from ref 6. Copyright 1984 Elsevier.

adsorbed on a microporous solid, there are as many NMR lines in the spectrum as there are different types of void volumes. This statement is only true provided that there is no rapid exchange (on the NMR time scale) of xenon atoms between these volumes. If exchange occurs, one has to reduce the experimental temperature to slow down xenon motion. This property is clearly demonstrated in the case of a mixture of two different zeolites, NaY and CaA, for example. The spectrum contains two components corresponding to xenon adsorbed in each zeolite (Figure 2). Since NMR spectroscopy is quantitative, the line intensities give the composition of the mixture by comparison with standards under the same experimental conditions.6 The case of zeolites with different adsorption zones is similar. For instance, the spectrum of ferrierite shows two signals characteristic of the two types of channels.7 The spectrum of Na-mordenite also shows two signals at room temperature, arising from xenon adsorbed in the main channels and in the side-pockets.8,9 However, because of rapid exchange of xenon between these two regions, there is only one signal for H+-exchanged mordenite. One must decrease the experiment temperature (253 K) to obtain the signals corresponding to the two regions (Figure 3). Two signals have also been detected for H-Rho zeolite, but at about -50 °C to avoid rapid exchange of Xe atoms (6) Springuel-Huet, M. A.; Ito, T.; Fraissard, J. Stud. Surf. Sci. Catal. 1984, 18, 13. (7) Ito, T.; Springuel-Huet, M. A.; Fraissard, J. Zeolites 1989, 9, 68. (8) Ripmeester, J. A. J. Magn. Reson. 1984, 56, 247. (9) Ito, T.; de Menorval L. C.; Guerrier E.; Fraissard J. Chem. Phys. Lett. 1984, 111 (3), 271.

located in the octagonal prisms and in the larger cavities.10 In Cs-Rho only the signal corresponding to the cavities is detected, since all the prisms are occupied by Cs+ cations and then are inaccessible to xenon. Xe NMR study of this structure has been extended more thoroughly to a set of H-Cs Rho samples with variable Cs content11 and to Cd Rho.12 For a sample of ferrierite it has been possible to detect the presence of a mordenite intergrowth and to estimate its amount by comparing the intensity of the line corresponding to the side-pockets with that of a pure mordenite.7 The detection of intergrowths greatly depends on the size of monocrystalline domains and the ease of diffusion between these domains. Not only the position but also the shape of the line may be of interest. When Xe-surface interactions are anisotropic, the signal exhibits a chemical shift anisotropy reflecting the asymmetry of the xenon electron cloud interacting with the surface, provided exchange between regions with different chemical shift anisotropies is quenched. This is the case for an AEL type structure (SAPO-11 and AlPO4-11) where the small axis of the elliptical pore section (3.9 × 6.7 Å) induces strong Xesurface interactions in this direction and consequently a permanent deformation of the electron cloud, leading to an axial magnetic shielding tensor with two different components σ| and σ⊥, with σ| ) σxx and σ⊥ ) σyy ) σzz (see Figure 4).13 When [Xe] increases, Xe-Xe interactions increase mainly along the channel axis z, and the magnetic shielding tensor becomes nonsymmetric σxx * σyy * σzz. Finally, at high xenon pressure, the magnetic shielding tensor is axial again with σ| ) σyy and σ⊥ ) σxx ) σzz. (10) Ito,T.; Fraissard, J. Zeolites 1987, 7, 554. (11) Tsiao, C. J.; Kauffman, J. S.; Corbin, D. R.; Abrams, L.; Carroll, E. E., Jr.; Dybowski, C. J. Phys. Chem. 1991, 95, 5586. (12) Smith, M. L.; Corbin, D. R.; Abrams, L.; Dybowski, C. J. Phys. Chem. 1993, 97, 7793. (13) Springuel-Huet, M. A.; Fraissard, J. Chem. Phys. Lett. 1989, 154, 299.

129Xe

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Figure 4. Representation of the chemical shielding tensor (σi is the chemical shielding constant in the direction i) in the case of an asymmetric signal due to anisotopic chemical shift observed for example with AlPO4-11 structure (unidimensional channel with cross section of 3.9 × 6.3 Å). Reprinted with permission from ref 13. Copyright 1989 Elsevier.

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Figure 6. Chemical shift variation versus xenon concentration for MgY zeolite for various magnesium content: (() 47%; (2) 53%; (b) 62% ; (9) 71%. Reprinted with permission from ref 18. Copyright 1987 Royal Society of Chemistry.

If xenon adsorbs more strongly on some adsorption sites (SAS), a term δSAS, characteristic of this interaction, must be considered. Indeed these sites can be saturated while

the adsorption on the other sites continues to increase monotonically with [Xe]. At low Xe pressure the term δSAS predominates. Therefore, when the pressure increases, adsorption occurs on the weaker sites. The result is a decrease of the observed chemical shift which is a weighted average of δXe and δSAS. Afterward the chemical shift again increases due to important Xe-Xe interactions at high pressures. These characteristic δ ) f([Xe]) plots have been observed for X and Y zeolites with divalent cations such as Mg2+ (Figure 6), Ca2+, Sr2+, Zn2+, and Cd2+.18,19 When cations are able to create a sufficiently large average electric field (high charge and/or high concentration), there is a general displacement of the δ ) f([Xe]) curves toward the higher values of δ, due to the addition of a new term, δE, depending on this electric field. More precisely, this term is proportional to the square of the electric field at the nuclei of Xe atoms adsorbed on the cations. This interaction between cations and xenon has been described by simple models ranging from a high polarization of the electron cloud20 to an electronic transfer from the xenon atom to the cation.21 This sensitivity to interaction with highly charged cations is used to study the migration of cations between different sites accessible or inaccessible to Xe during heat treatment, e.g., dehydration. The problem is naturally more difficult in the case of paramagnetic cations (Ni2+, Co2+). One must consider another term, δM, which influences the chemical shift. This term may be very large and leads to δ values of several thousand parts per million. In this case, the oxidation state of this type of cations can be studied, in particular during reduction or oxidation of such samples. For

(14) Pellegrino, C.; Ito,T.; Gabelica, Z.; B’Nagy, J.; Derouane, E. G. Appl. Catal. 1990, 61, L1. (15) Ripmeester, J. A. J. Am. Chem. Soc. 1982, 104, 209. (16) Demarquay, J.; Fraissard, J. Chem. Phys. Lett. 1987, 136, 314. (17) Chen, Q. J.; Springuel-Huet, M. A.; Fraissard, J.; Smith, M. L.; Corbin, D. R.; Dybowski, C. J. Phys. Chem. 1992, 96, 10914.

(18) Ito, T.; Fraissard, J. J. Chem. Soc., Faraday Trans. 1 1987, 83, 451. (19) Ge´de´on, A.; Fraissard, J. Chem. Phys. Lett. 1994, 219, 440. (20) Fraissard, J.; Ito, T. Zeolites 1988, 8, 350. (21) Cheung, T. T. P.; Fu, C. M.; Wharry, S. J. Phys. Chem. 1988, 92, 5170.

Figure 5. δS variation versus Al content of the framework: (O) Na ZSM-5; (4) NaZSM-11. Reprinted with permission from ref 17.

Chemical shift anisoropy has been also observed in Nu-10 zeolite14 and Xe/clathrate systems.15 The chemical shift, δS, depends on several factors, the Xe-surface interaction itself, the frequency of collisions of Xe atoms against the pore surface, and their ability to diffuse. For chemically identical surfaces, the collision frequency is directly related to the pore size,16 but for a given pore structure, δS may reflect composition variations of the surface. For instance, δS increases linearly with the Al content of the framework in ZSM-5 and ZSM-11 zeolites.17 This variation shows a discontinuity at [Al] ) 2 atoms/unit cell for the two structures (Figure 5), which demonstrates that the Xe-surface interaction changes for this concentration. We conclude that Al is not randomly distributed in the lattice whatever [Al], but its distribution depends on its concentration. Influence of Divalent or Trivalent Cations

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Figure 7. 129Xe NMR chemical shift versus xenon concentration (Xe atoms per supercage) for Co15NaY: (a) Tt (K) (9) 300, (b) 323, (2) 373, ([) 423; (b) Tt (K) (9) 373, (b) 423, (4) 523, (2) 573, ([) 623, (0) 773. Full line: δ ) f([Xe]) curves for pure NaY zeolites treated at the same Tt than Co15NaY. Reprinted with permission ref 23. Copyright 1995 Elsevier.

example, Scharpf et al.22 have shown that nickel ions are removed from the supercages upon reduction at 643 K in a Ni-NaY zeolite. The reoxidation in 600 Torr of oxygen at various temperatures does not reverse the process of reduction. In the case of Co-NaY, with 15% of Na+ cations exchanged, Bonardet et al.23 have studied the dependence of the δM term on the pretreatment temperature Tt (Figure 7). For 300 e Tt e 423 K the chemical shift extrapolated to zero pressure, δ[Xe]f0, and the slopes of the δ ) f([Xe]) plots decrease, this corresponding to the departure of water molecules freeing the porosity. The small shift difference compared to NaY is due to the presence of Co(H2O)62+ partially blocking the pores. When Tt is higher than 423 K, the dramatic increase in chemical shift and the change in the shape of the δ ) f([Xe]) plots arising from paramagnetic Xe-Co2+ interactions proves that, despite the migration of Co2+ out of supercages, as shown by X-ray diffraction,24 Xe-Co2+ interactions are still detectable when water molecules are progressively eliminated from the coordination sphere of Co2+ cations. It has been proposed that Co2+ cations are located in SII’ sites of sodalite cages (Figure 8). Another interesting case is the unusual behavior of Ag+ and Cu+ cations. Ge´de´on et al have obtained small (compared to Na+ form), and even negative, chemical shifts for CuX and AgX zeolites, respectively.25-28 The adsorption of xenon in dehydrated AgX is much greater than that in NaX. Most remarkably, the shifts decrease with concentration down to negative values, -40 ppm for [Xe] f 0 (Figure 9). These results have been attributed to specific interactions of xenon with Ag+ cations in the supercages, especially Ag+ in SIII sites.29 This location of Ag+ allows closer contact with xenon than the SII sites. This could explain the greater efficiency of 4dπ-5dπ electron-donation from Ag+ to xenon involving the Ag 4d10 (22) Scharpf, E. W.; Crecely, R. W.; Gates, B. C.; Dybowski, C. J. Phys. Chem. 1986, 90, 9. (23) Bonardet, J. L.; Ge´de´on, A.; Fraissard, J. Stud. Surf. Sci. Catal. 1995, 94, 139. (24) Gallezot, P.; Imelik, B. J. Phys. Chem. 1973, 77, 2556. (25) Ge´de´on, A.; Burmeister, R.; Grosse, R.; Boddenberg, B.; Fraissard, J. Chem. Phys. Lett. 1991, 79, 191. (26) Grosse, R.; Burmeister, R.; Boddenberg, B.; Ge´de´on, A.; Fraissard, J. J. Phys. Chem. 1991, 95, 2443. (27) Ge´de´on, A.; Bonardet, J. L.; Fraissard, J. J. Phys. Chem. 1993, 97, 4254. (28) Ge´de´on, A.; Bonardet, J. L.; Lepetit, C.; Fraissard, J. Solid State NMR 1995, 5, 201. (29) Grosse, R.; Ge´de´on, A.; Watermann, J.; Fraissard, J.; Boddenberg, B. Zeolites 1992, 12, 909.

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and the xenon 5d0 orbitals, which is considered responsible for the observed low-frequency shifts. To confirm these results, we looked for a system where, even after dehydration and at low exchange level, there are still many Ag+ cations in the supercages, in direct contact with xenon. For this purpose we tried to prevent the migration of Ag+ cations out of the supercages. By cation exchange we prepared a sample, La64Y, where 64% of Na+ are replaced by La3+ so that all the cationic sites outside the supercages are occupied by La3+ after dehydration. Indeed it is known30,31 that after dehydration at 723 K under vacuum, the La3+ ions migrate irreversibly from the supercages to sodalite cages or hexagonal prisms. Starting from this LaY sample, silver exchange was performed to obtain a LaAg35Y where the remaining Na+ cations are now replaced by Ag+. The chemical shift variations for NaY, La64Y, Ag35Y, Ag50Y, and La64Ag35Y are reported in Figure 10. The linear variation obtained for La64Y shows that xenon does not interact with charged species; this confirms that La3+ ions have migrated out of the supercages. The shifts are very similar to those of NaY. For Ag+-exchanged zeolites, increasing Ag+ content displaces the δ ) f([Xe]) curves toward low shifts. On comparison of Ag35Y and Ag35LaY, Figure 10 shows that the chemical shift of Ag35LaY is lower than that of Ag35Y At low xenon concentration, the shifts are in the order δ(La64Ag35Y) ≈ δ(Ag50Y) , δ(Ag35Y). This observation proves that the low shift is due to specific interaction between xenon and Ag+ located in supercages. The higher the Ag+ content, the lower the chemical shift. The similarity between La64Ag35Y and Ag50Y shows that the Ag+ outside the supercages does not affect the chemical shift. Very close contact between xenon and Ag+ is needed to allow electron donation. For copper-exchanged X and Y zeolites, the parabolic form of the δ ) f([Xe]) curves, which was expected because of the presence of paramagnetic Cu2+, is not observed (Figure 11). During dehydration there is at the same time migration of Cu2+ from the supercages and partial autoreduction of Cu2+ to Cu+ inside the supercages. These very accessible Cu+ cations are able to behave like Ag+ and participate in 3dπ-5dπ electron donation. This behavior is not observed for Zn2+ and Cd2+ and seems not general among cations of nd10 configuration. It has been proposed that after dehydration Zn2+ and Cd2+ interact strongly with the matrix and are situated in SII sites, a position which prevents such dπ donation.17 The higher charge of the cation, responsible for a greater polarization of the electronic cloud leading to high chemical shift, can also be put forward and may compete with the electron donation. A common and crucial problem when dealuminating zeolites for catalytic purposes is that there are nonframework aluminum species (AlNF) in the pores and that these are difficult to extract. The catalytic properties depend greatly on the chemical nature of these species. A study of several dealuminated ZSM-5 samples has shown that the Xe interactions with AlNF are strongly dependent on the mean charge of AlNF rather than on their quantity.32 In Y zeolites, where the pores are bigger than in ZSM-5, the influence of the pore surface on the chemical shift is smaller at a given temperature. It is therefore necessary to decrease the experiment temperature, i.e., the xenon mobilty, to detect the AlNF. (30) Chen, Q. J.; Ito, T.; Fraissard, J. Zeolites 1991, 11, 239. (31) Kim, J. G.; Kompany, T.; Ryoo, R.; Ito, T.; Fraissard, J. Zeolites 1994, 14, 427. (32) Chen, Q. J.; Guth, J. L.; Serve, A.; Caullet, P.; Fraissard, J. Zeolites 1991, 11, 798.

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Figure 8. Schemes of Xe-Co2+ interactions in the supercages of Co15NaY zeolite (a) hydrated (b) dehydrated (hatched ball, xenon atom; white ball with gray center, Co(H2O)62+; gray ball, Co2+ fully dehydrated). Reprinted with permission from ref 23. Copyright 1995 Elsevier.

129Xe NMR chemical shift versus xenon concentration

Figure 9. for AgλX zeolites pretreated at 673 K: λ (%) (9) 0; (b) 20; (2) 40; ([) 80; (0) 100. Reprinted with permission from ref 26.

Coking These AlNF atoms play a major role in coke formation during hydrocarbon cracking reactions. This has been clearly shown in a study of cracking of n-heptane at 723 K in dealuminated HY zeolites. The curvature of the δ ) f([Xe]) plots at low Xe concentration, characteristic of Xe-AlNF interactions, disappears after a 3% (w/w) deposit of coke; this means that coke is first deposited on the AlNF (Figure 12). At the same time the zeolite loses more than half its catalytic activity, proving that these AlNF play an important role in the cracking activity of the zeolite. Examination of the δ ) f([Xe]) curves (Figure 13) shows that δ increases when the experiment temperature Tt decreases, but the slope is temperature independent and close to that of the straight section of the reference sample plot. Consequently, the free internal volume is hardly affected but xenon diffusion is restricted. These results show that at the beginning of coking, coke is initially

Figure 10. 129Xe NMR chemical shift versus xenon concentration for (0) NaY, (O) La64Y, (×) Ag35Y, (]) Ag50Y, (4) La64Ag35Y.

located at the windows between the supercages and that this is also the location of the AlNF species.33 At higher coke content (10.5%) the dδ/d[Xe] and δ[Xe]f0 values are practically independent of Tt. Therefore, the internal volume is reduced to channels whose diameter is close to that of xenon atom and the internal walls are covered with coke. This interpretation is confirmed by Xe adsorption isotherms which show that the free volume is divided by a factor of 2.4, a value which is closely similar to that of the slope ratio of the δ ) f([Xe]) plots. Finally, at high coke content (15%) we only observe an increase of δ[Xe]f0, indicating that the mean free path of xenon is dramatically reduced. Xenon diffusion is then restricted, many of the cavities being blocked by the coke. Moreover, at high xenon pressure two other signals appear: one, weakly shifted (≈40 ppm) and independent of xenon concentration, has been attributed to xenon adsorbed in micro- or mesoporous cavities of coke formed at the external surface of the catalyst; the second, at about (33) Barrage, M. C.; Bonardet, J. L.; Fraissard, J. Catal. Lett. 1990, 5, 143.

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Figure 11. 129Xe NMR chemical shift against xenon concentration for CuλY pretreated at 773 K: λ (%) (9) 0; (b) 15; (2) 35; (() 58; (0) 96. Reprinted with permission from ref 27.

Figure 13. 129Xe NMR chemical shift against xenon concentration for dealuminated HY zeolites: (A) 3% coke sample, (B) 10.5% coked sample; (1) 273 K, (b) 300 K, (9) 319 K, (2) 338 K.

levels, two pore aperture environments, one aperture with deposited coke and the other with little coke. Distribution of Coadsorbed Phases

Figure 12. 129Xe NMR chemical shift against xenon concentration for dealuminated HY zeolites at 300 K: (1) fresh sample; (b) 3% coke sample; (2) 10.5% coked sample; (9) 15% sample. Reprinted with permission from ref 33. Copyright 1990 Baltzer.

0 ppm, is due to xenon gas whose relaxation time has been shortened by interaction with paramagnetic centers of the graphite-like structure of coke. Miller et al., using Ar isotherms and 129Xe NMR for the study of the coking of HY zeolites after propylene adsorption followed by heating, showed that δ[Xe]f0 increases linearly with decreasing micropore volume.34 As low-temperature physical sorption of Ar is sensitive to the size of the pore aperture, they found, at high coke (34) Miller, J. T.; Meyers, B. L.; Ray, G. J. J Catal. 1991, 128, 436.

The first study of adsorbed molecules trapped in a zeolite was performed by Ge´de´on.35,36 He showed that it is easy, in a NaY zeolite, to differentiate between water adsorbed in regions accessible or not to xenon, to measure the volume of water in pores, and to determine the blockage of windows by water molecules. Figure 14 shows the variations of δ[Xe]f0 and of the slope, dδ/d[Xe], of the δ ) f([Xe]) curves with the water concentration, C, expressed in percentage of water content at saturation: the decrease of δ[Xe]f0 with C proves that right from the beginning of dehydration, the number of water molecules per supercage decreases. Furthermore, δ[Xe]f0 stops decreasing and remains constant when C e 0.15; this means that for C e 0.15 the mean free path, l, at zero concentration is constant. It may be deduced that for C e 0.15 the supercages and the windows between them are completely free of water. In other words, the residual water molecules are all in the sodalite cages and the prisms. This result is consistent with the fact that each sodalite cage can contain a maximum of four water molecules, corresponding to C ) 0.15. The slope, dδ/d[Xe], decreases with C down to C ) 0.4 and then is constant. Decreasing C from 0.4 to 0.15 causes δ[Xe]f0 to diminish but has practically no effect on dδ/d[Xe], i.e., no effect on the void volume. It is clear that the void space available to xenon remains constant, but there are modifications of the shape of the internal volume which lead to an increase in l. These results show that for 0.15 e C e 0.4 the water molecules are in the vicinity of the 8 Å apertures and then reduce the diffusion of xenon (35) Ge´de´on, A. Ph.D. Thesis, P. et M. Curie University, Paris, 1987. (36) Ge´de´on, A.; Ito, T.; Fraissard, J. Zeolites 1988, 8, 376.

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Figure 15. 129Xe NMR spectra of xenon adsorbed in solid EPDM: (A) before, and (B) after cross-linking reactions. Reprinted with permission from ref 44. Copyright 1990 Springer. Figure 14. Effect of the water concentration C on shifts δ[Xe]f0 and the slope dδ/d[Xe] of the δ ) f([Xe]) plots. The water concentration is expressed as a percentage wt/wt of anhydrous solid (upper) or as the relative concentration (lower). Reprinted with permission from ref 36. Copyright 1988 Elsevier.

between supercages. The departure of these molecules does not change the pore volume very much (i.e., dδ/d[Xe]) but leads to a much easier diffusion of xenon which greatly affects l and δ[Xe]f0. The last molecules removed are those located in sodalite cages where xenon cannot enter. This technique has been applied to the study of the adsorption of various compounds, such as benzene, trimethylbenzene, and hexane.37-39 Moreover, if 129Xe NMR spectra are recorded during the adsorption of these molecules, it is possible to study their diffusion properties.40,41 Microporous Polymers 129

Xe NMR has been used to study the heterogeneity of microporous polymers. To observe a signal for such solids, it is necessary to use relatively high pressures (3-15 bar) and pulse delays of several seconds. The first study was by Sefcik et al.42 in 1983. Sorption of xenon in PVC polymer leads to a broad line, shifted downfield (250 ppm), reflecting a continous distribution of sites due to the microscopic heterogeneity of the polymer matrix. More recently, Stengle and Williamson43 probed the amorphous phases in two types of polyethylene. In the case of linear low-density polyethylene (LLDP), the spectra of adsorbed xenon exhibit some structure, which proves that two subregions exist. (37) Ryoo, R.; Liu, S. B.; de Me´norval, L. C.; Takegoshi, K.; Chmelka, B.; Trecoske, M.; Pines, A. J. Phys. Chem. 1987, 91, 6575. (38) de Me´norval, L. C.; Raftery, D.; Liu, S. B.; Takegoshi, K.; Ryoo, R.; Pines, A. J. Phys. Chem. 1990, 94, 27. (39) Chmelka, B. F.; Pearson, J. G.; Liu, S. B.; Ryoo, R.; de Me´norval, L. C.; Pines, A. J. Phys. Chem. 1991, 95, 303. (40) Ka¨rger, J.; Pfeifer, H.; Wutscherk, T.; Ernst, S.; Weitkamp, J.; Fraissard, J. J. Phys. Chem. 1992, 96, 5059. (41) Springuel-Huet, M. A.; Nosov, A.; Ka¨rger, J.; Fraissard, J. J. Phys. Chem. 1996, 100, 7200. (42) Sefcik,M. D.; Schaefer, J.; Desa, A. E.; Yelou, W. B. Polym. Prepr. (Am. Chem. Soc., Div. Poym. Chem.) 1983, 24-1, 85. (43) Stengle, I. R.; Williamson, L. K. Macromolecules 1987, 20, 1428.

The effect of cross-linking on the amorphous structure of a solid EPDM (terpolymer of ethylene, propylene, and ethylidenenorbornene) was investigated by Kennedy.44 For the un-cross-linked sample this author decomposed the signal into four overlapping resonance lines between 198 and 201 ppm, attributed to four distinct amorphous regions with adsorption sites of different sizes. After crosslinking, only two peaks, slightly shifted downfield with respect to the previous sample, can be resolved (Figure 15). The diminution of the number of lines is attributed to a densification of the polymer due to cross-linking. At the same time the relative intensities of the peaks change, reflecting a change in the population of these environments. The small line widths indicate that exchange between the different amorphous domains is slow on the NMR time scale. The differences in the chemical shifts of different polymers can also be used to determine if a mixture of polymers forms a miscible blend or not. As observed by Miller45 the existence of two lines proves unambiguously the presence of two phases with different morphologies. If only one line is observed, there is a miscible blend. Because rapid diffusion between the two phases leads also to a single line, it is necessary to measure the temperature dependence before a conclusion can be reached. Very recently 2D NMR exchange experiments have been performed to study diffusion and exchange phenomena in polymer blends. For instance, Tomaselli46 showed that for a mixing time of 0.8 s, no exchange between the two phases of a PVC and PVME (poly(vinyl methyl ether)) mixture is observed; on the other hand, when the mixing time is as long as 8 s, a cross peak appears, showing clearly exchange is occurring. The 1D spectrum of Xe adsorbed in a mixture of 1/3 polypropylene and 2/3 polyethylene (Figure 16) shows three peaks at 0 (gaseous Xe), 200, and 220 ppm (xenon adsorbed in two different adsorption sites).47 For a mixing time of 5 s the 2D spectrum presents cross peaks between signals (44) Kennedy, G. J. Polym. Bull. 1990, 23, 605. (45) Miller, J. B.; Walton, J. H.; Roland, C. M. Polym. Sci. B 1992, 30, 327. (46) Tomaselli, M.; Meier, B. H.; Robyr, P.; Suter, V. W.; Ernst, R. R. Chem. Phys. Lett. 1993, 205, 145. (47) Mansfeld, M.; Veeman, W. S. Chem. Phys. Lett. 1993, 213, 153.

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between xenon adsorbed in the two polymer phases but no exchange can be observed with the free xenon gas: diffusion of the gas outside the polymer is strongly limited by the weak connectivity of the pores to the surface. We studied the location of Kelex in microporous Amberlite type polymers (one is a copolymer of divinylbenzene and ethylvinylbenzene, the other is a polymer of methacrylate ester) by classical 129Xe NMR. Kelex (7substituted 8-hydroxyquinoline) is a molecule which can extract Ga(III) and Ge(IV) from electrolytic solutions of zinc sulfate in the goal to valorize an impurity of great value. We observed that the 129Xe signal intensity decreases when the Kelex loading on the polymer increases. When the impregnation ratio is higher than 0.2 g/g, the resonance line disappears. At the same time nitrogen isotherms and B. J. H. pore distribution curves show that micropores are totally filled with Kelex. We can conclude that xenon adsorbs preferentially in the micropores rather than in the mesopores of the polymer. The very weak dependence (3-4 ppm) of the chemical shift on xenon pressure (in the range 0-1000 Torr) proves there is fast exchange between adsorbed atoms and free xenon gas; this indicates an open pore structure. Conclusion These few examples, by no means exhaustive, show that NMR is a very fruitful spectroscopic method for studying the microporosity and the heterogeneity of finely divided materials. First applied to zeolites, it has been then extended in recent years to amorphous systems, particularly to polymers. The development of techniques such as high-resolution NMR in solids or 2D NMR leads to an increase in the range of possible applications of 129Xe NMR. 129Xe

Figure 16. 129Xe NMR exchange spectrum of xenon adsorbed in a polymer blend (polypropylene with 21% copolymer, containing 1/3 polypropylene and 2/3 polyethylene) at room temperature. The spectrum is acquired in TPPI mode with a mixing time of 5 s. Reprinted with permission from ref 47. Copyright 1993 Elsevier.

at 200 and 220 ppm but no cross peak between gaseous and adsorbed Xe. This result proves that exchange occurs

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