5694
J. Phys. Chem. C 2007, 111, 5694-5700
Probing Xe Exchange in Delaminated Zeolites by Hyperpolarized
129Xe
NMR
M.-A. Springuel-Huet,*,† F. Guenneau,† A. Ge´ de´ on,† and A. Corma‡ Laboratoire SIEN, UniVersite´ P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 05, France ReceiVed: September 20, 2006; In Final Form: December 22, 2006
MCM-22 and ferrierite zeolites and the corresponding delaminated materials, ITQ-2 and ITQ-6, respectively, have been studied by 129Xe NMR of adsorbed xenon and by nitrogen adsorption at 77 K. One-dimensional 129 Xe NMR spectra of delaminated materials show additional lines compared with those of the parent zeolites. For ITQ-2, these new lines are attributed to Xe exchanging between the adsorbed phase and the intercrystallite gaseous phase, with the extent of exchange depending on the particle size, the distribution of which is discrete. For ITQ-6, the additional lines are attributed to the presence of mesopores in the particles. Two-dimensional NMR spectra of ITQ-6 samples under magic angle spinning conditions and continuous flow of hyperpolarized Xe allowed us to investigate the exchange between different sites and to obtain information on the delamination process.
Introduction In heterogeneous catalysis, the success of zeolites as oxidation and acid catalysts1 has favored the development of new silicate structures, with larger pores able to process bulky reactant molecules.2-8 Nevertheless, the limited dimensions of the micropores (below 1 nm) of these structures are still not sufficient for many fine chemicals or pharmaceuticals. A new approach to expanding the use of zeolites as catalysts for reactions involving large molecules has been developed.9 It consists of the synthesis of layered zeolite precursors which are swollen (by exchange with bulky cations) and then exfoliated, making the potentially active catalytic sites accessible through the external surface. For large reactants, delaminated zeolites show higher activity than conventional large-pore acid zeolites, AlMCM-41, or amorphous silica-alumina.10 The first member of the class of delaminated zeolites is ITQ2, obtained by exfoliation of the MCM-22 layered precursor.11 The structure of ITQ-2 is deduced from that of MCM-22. Basically, it is made up of thin sheets, 2.5 nm thick, with an extremely high external surface area (g700 m2 g-1). Each sheet consists of a hexagonal array of “cups” that penetrate into the sheets on both sides. A 10-membered ring (10 MR) channel system runs between the cups inside the sheets.6 Another delaminated material, ITQ-6, is obtained by delamination of a laminar precursor of ferrierite (PREFER).12,13 ITQ-6 is believed to consist of sheets containing one or, at most, a very small number of (100) ferrierite layers. Preliminary 129Xe NMR studies of these two types of delaminated materials and their precursors have already been reported.14 New 129Xe NMR experiments and N2 adsorptiondesorption results lead us to refine the interpretation of the 129Xe NMR spectra. * To whom correspondence should be addressed. E-mail:
[email protected]. † Laboratoire SIEN, Universite ´ P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 05, France. Tel.: 33 1 44 27 55 37. Fax: 33 1 44 27 55 36. ‡ Inst. de Tecnologı´a Quı´mica, UPV-CSIC, Av. de los Naranjos, s/n, 46022 Valencia, Spain. Tel.: 34 96 3877800. Fax: 34 96 3877809.
Figure 1. Schematic of the preparation of MCM-22 and ITQ-2. Detailed information on the crystallographic structure of MCM-22 can be found in the atlas of zeolites (code MWW) at http://www.izastructure.org/databases/.
Experimental Section Materials. An MCM-22 sample with a Si/Al ratio of 15 was synthesized following the procedure given in ref 11. A ferrierite (FER) sample was obtained by the calcination of a PREFER material synthesized according the procedure of ref 12. ITQ-2 and ITQ-6 were prepared by swelling and exfoliating the corresponding precursors, MCM-22(P) and PREFER, respectively. The procedure consists of heating the precursors at 368 K for 16 h in an aqueous solution of cetyltrimethylammonium bromide and tetrapropylammonium hydroxide. The resulting solid, containing cetyltrimethylammonium (CTMA+) as the charge-compensating cation, is subsequently delaminated by ultrasound irradiation (50 W/40 kHz) for 1 h. The solid is collected, washed thoroughly with distilled water, and calcined at 853 K for 7 h (Figure 1). The characteristics of the samples are given in Table 1. Apparatus and Procedure. The nitrogen adsorption isotherms were performed at 77 K with an automatic Micromeretics ASAP 2010 apparatus. The thermally polarized one-dimensional (1D) 129Xe NMR spectra were recorded on a Bruker MSL 400 spectrometer
10.1021/jp066163c CCC: $37.00 © 2007 American Chemical Society Published on Web 03/27/2007
Xe Exchange in Delaminated Zeolites by HP
129Xe
NMR
J. Phys. Chem. C, Vol. 111, No. 15, 2007 5695
TABLE 1: Sample Characteristics sample
Si/Al
MCM-22 ITQ-2 (loose powder) ITQ-2 (compressed) FER ITQ-6
≈15 ≈50 ≈50 ≈30
specific surface area (m2 g-1)
external surface area (m2 g-1)
567a 688a 415a 254 865a
44b 308b 143b 62 353
micropore volume (cm3 g-1) total 0.21b 0.21b 0.13b
ext. 0.02d 0.17d 0.08d
0.21
int. 0.19e 0.04e 0.05e
micropore surface area (m2 g-1) 523c 380c 272c 210 31
Determined by the BET (Brunauer, Emmet, Teller) method. b Determined from the Rs-plot calculated using the nitrogen adsorption isotherm data at 77 K on LiCrospher Si-1000 silica as the reference solid;15 see Figure 5. c Calculated as the difference between specific surface area and external surface area. d Volume of cups calculated from a simple geometrical model, using a compact arrangement of cylindrical cups on the surface and from the external surface area obtained from the Rs-plot. e Volume of all internal pores (channels and cavities) calculated as the difference between total micropore volume and volume of cups. a
Figure 2.
129
Xe NMR spectra of MCM-22 (A) and in ITQ-2 (B) at 293 K for various xenon pressures.
operating at 110.688 MHz. A 90° pulse (ca. 10 µs) was used. Typically, 1000-50000 scans, with a repeat time of 1 s, were recorded. The chemical shifts, given with an accuracy of 0.5 ppm, are referenced to that of gaseous xenon extrapolated to zero pressure (0 ppm). The laser-polarized 129Xe NMR spectra were recorded on a Bruker AMX 300 spectrometer operating at 83.02 MHz under continuous gas flow using a home-built system.16 The 2D exchange experiments were performed under magic angle spinning (MAS) conditions using a home-modified commercial probe described in ref 16. The use of hyperpolarized Xe allowed us to perform 2D experiments in a reasonable period of time. The spectra obtained with hyperpolarized (HP) and thermally polarized 129Xe were identical. The intensity scale of each spectrum presented in the figures is arbitrary and was generally chosen to emphasize the signal of the adsorbed species. NMR spectroscopy of hyperpolarized Xe is not quantitative. In addition to the T1 relaxation time of 129Xe in the different phases, the line intensities also depend on the exchange rate between depolarized 129Xe (the hyperpolarization is destroyed by a radio frequency pulse) and hyperpolarized 129Xe, which itself depends on the gas flux inside the hyperpolarization device and on the Xe diffusion rate inside the pore network. At low temperature, the amount of adsorbed Xe increases but Xe diffusion is reduced and depolarized Xe is replaced by hyperpolarized Xe more slowly. If the T1 value of adsorbed Xe is sufficiently long, this can be compensated by increasing the repeat time. In the case of spectra showing several lines, the Xe concentration corresponding to each site is determined from the NMR intensity obtained with thermally polarized 129Xe and from the adsorption isotherm.
Before xenon adsorption to a pressure between 1.3 × 103 and 2.1 × 105 Pa, all samples were evacuated overnight at 673 K. The adsorption isotherms were determined by pressure monitoring. Results and Discussion MCM-22 and ITQ-2. The MCM-22 zeolite consists of two non-interconnected types of pores: oblate cavities with a 12membered ring cross-section (12 MR) (0.71 nm diameter and 1.82 nm long) connected by six 10 MR windows (0.40 × 0.55 nm) situated in their middle and a bidimensional system of sinusoidal channels with at least a 10 MR cross-section (0.41 × 0.51 nm) presenting small cages (0.64 × 0.69 nm) between two 10 MRs (Figure 1). The diameter of the channel intersections is 0.56 nm.11 Unexpectedly, in the pressure range 2-144 × 103 Pa, the 129Xe-NMR spectra of thermally polarized Xe adsorbed in MCM-22 show only a single line (Figure 2A) due to the similarity of the mean free paths of Xe in the two types of pores. Following our discussion in ref 14, we definitely think that this signal should not be attributed to Xe adsorbed in cavities alone, Xe adsorbing in the channels only at very high pressure, as was originally proposed by Chen et al.17 At high Xe loadings, above 8 × 1020 Xe atoms g-1, the signal broadens. This could be due to an overlapping of two signals attributable to the two types of pores, where the Xe-Xe interactions differ because of distinct Xe diffusion processes. This is also consistent with the broadening observed at high temperature (spectra not shown). Usually, the signal broadens at low temperature because Xe spends more time interacting with the internal pore surface and is, therefore, more sensitive to surface heterogeneity, whereas at high temperature it preferentially samples the pore volume. The line
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Figure 3. Chemical shift variations against Xe pressure (A) and against Xe local concentration (B) for MCM-22 (b), compressed ITQ-2 powder (2), loose ITQ-2 powder, line b ([), and loose ITQ-2 powder, line c (9). (The experimental data for line a of loose ITQ-2 powder with constant shift are not shown.)
broadening at high temperature reveals the presence of voids of different shapes, which may be attributed here to the two types of pores. For the delaminated sample, ITQ-2, the single asymmetric line observed at low Xe pressure splits into two lines (noted b, c) at high pressure and room temperature (Figure 2B). A small signal, labeled a, appears for pressures above ca. 200 Torr (2.6 × 104 Pa). The low-intensity line at 0 ppm is due to Xe in the gas phase (used as the chemical shift reference). The chemical shift variations of Xe adsorbed at room temperature in MCM-22 and ITQ-2 vs Xe pressure (δ-P) and vs Xe concentration (δ-n) are reported in Figure 3. The δ-P curves corresponding to lines a, b, and c of ITQ-2 lie below that of MCM-22. The chemical shift, δa, of line a is practically constant and equal to 74 ppm. In contrast, δb and δc increase with PXe, but the slopes decrease when PXe increases. For lines c and b of ITQ-2, the estimated values at zero loading are 98 and 94 ppm, respectively, and 104 ppm for the single line of MCM-22. The δ-n curves are quite different. The slope increases for MCM-22 but is practically constant for the various lines of ITQ-2 and much higher for lines b and c than for MCM-22. The Xe adsorption isotherms (not shown) show that the amount of Xe adsorbed at a given pressure is smaller for ITQ-2 than for MCM-22, reflecting the decrease in micropore volume measured by Xe adsorption. The higher slopes of the δ-n curves observed for the ITQ-2 lines b and c are understandable if we consider that, at a given total Xe concentration, the local concentration (Xe atoms per unit pore volume, for example) is higher for ITQ-2 than for MCM-22 since the ITQ-2 micropore volume is smaller. Therefore, the higher Xe-Xe interactions inside the micropores lead to an increase in the chemical shift. Small values of the chemical shift at zero loading, δ0, usually indicate larger pores or easier Xe diffusion inside the pore structure. In the case of ITQ-2, which is supposed to partly retain the pore structure of MCM-22, the lower δ0 values of lines b and c, compared with that of MCM-22, are attributable not to smaller pore size but to exchange between adsorbed and external gaseous Xe. The ITQ-2 particles, originating from the delamination of the MCM-22 zeolite crystals, are much smaller than those of MCM-22. The influence of exchange between adsorbed and gaseous Xe is all the more important since the particles are small. The presence of two lines (b and c), showing similar δ-n curves, just indicates a similarity of the corresponding pore structures; the fact that there are two curves means that there are particles of two sizes. Line a, with constant chemical shift (74 ppm) at room temperature, is attributed to Xe adsorbed in the cups at the
Figure 4. Static hyperpolarized 129Xe NMR spectra of Xe adsorbed in ITQ-2 (loose powder) at variable temperature for P(Xe) ≈ 1.33 × 103 Pa.
external surface of the particles. Indeed, there is no Xe-Xe interaction in these sites which can accommodate only one Xe atom. This line is only observed for ITQ-2, showing that the number of cups is much larger in ITQ-2 than in MCM-22. We also performed variable-temperature experiments using hyperpolarized Xe (Figure 4). At low temperature (193 K), the main line is broad, reflecting the distribution of surface heterogeneities. When the temperature is increased, we first observe a narrowing of the signal. Above 253 K, it splits into two lines, then into three lines at 353 K. The chemical shift of line a decreases from 105 ppm (at 193 K) to 49.4 ppm (at 393 K), expressing the increase in the rate of exchange between Xe adsorbed in the cups and gaseous Xe. Line a is always very weak. The splitting of the main line corresponding to Xe adsorbed inside the particles with increase in temperature, that is, with increase in Xe diffusivity, is consistent with the influence of an exchange between adsorbed and gaseous Xe which depends on the particle size. Line d, at 103 ppm, observed at 393 K (Figure 4) corresponds to the greater ITQ-2 particles which cannot be distinguished from average-sized particles at room temperature even at high Xe pressure (Figure 2B). At 393 K, it can be seen that the gas line near 0 ppm is also split into two lines at 1.1 and 0 ppm; the line at 1.1 ppm corresponds to the fraction of Xe gas (certainly the interparticle Xe gas) exchanging with adsorbed Xe. To confirm this “exchange” interpretation, the ITQ-2 sample was compressed at 510 MPa to reduce the interparticle space and, consequently, exchange with the interparticle gaseous Xe. The NMR spectra then show a single line whose chemical shift
Xe Exchange in Delaminated Zeolites by HP
129Xe
NMR
Figure 5. Rs-plots of N2 adsorption isotherms at 77 K of MCM-22 (b), loose ITQ-2 powder ([), and compressed ITQ-2 powder (2) using LiCrospher Si-100 silica as the solid reference.
variation with pressure, δ-P, is practically identical to that of MCM-22 (Figure 2B). This observation clearly shows that ITQ-2 has the same pore structure as MCM-22 and that the spectral differences are not due to differences in the internal pore structure but to a discrete distribution of particle sizes, which can be detected by Xe exchange between adsorbed and gaseous Xe phases. The δ-n curve of compressed ITQ-2 is superimposed on that of MCM-22 at very low Xe loading, but the slope increases rapidly with the concentration, n, and the curve meets that of a noncompressed ITQ-2 sample at high concentration. As mentioned above, the increase in chemical shift with Xe loading is due to Xe-Xe interactions inside the pores, which depend on the effective pore volume that is not modified by compression. To check this last conclusion, nitrogen adsorptions at 77 K were performed; the data are reported in Table 1. We used transformed isotherms (Rs-plot) to analyze the porosity of the materials. An Rs-plot compares the adsorption isotherm of a solid to a standard adsorption isotherm chosen as that of a nonporous solid with the same chemical surface. Comparison is made through a common coordinate, Rs, corresponding to the quantity adsorbed at a given relative P/P0 value (with P0 being the saturation pressure of N2 at 77 K) compared to the quantity adsorbed at P/P0 ) 0.4. This allows us to highlight the different types of porosities, to determine surface areas, from the slope of the different linear sections, and pore volumes, from the extrapolation of these linear sections.18 Analysis of the Rsplots gives several interesting results. The total micropore volume, calculated from the values obtained by extrapolation of the linear section corresponding to adsorption on the external surface only, that is, for R above 1 (corresponding to P/P0 > 0.4) (Figure 5), is practically the same for MCM-22 and loose ITQ-2 powder (see Table 1). Delamination simply cuts the supercages in half, which leads to cups, about 0.71 nm deep and 0.71 nm in diameter, on the external surface of the small particles of ITQ-2.6 These cups are also micropores, and they are roughly half the size of the initial cavities. Consequently, the total micropore volume should not be changed by delamination. Nevertheless, the relative volumes of cavities and cups must change, but this cannot be observed by N2 adsorption. As expected, the external surface area increases considerably since the particles are much smaller in the delaminated material. When the delaminated material is compressed, part of the external surface is inaccessible, and both the external surface area and the micropore volume are smaller, since the cups are also inaccessible.
J. Phys. Chem. C, Vol. 111, No. 15, 2007 5697 However, one can ask why the micropore surface areas are different for MCM-22 and ITQ-2 whereas the volumes, determined from the R-plot, are identical (Table 1). One reason may be that the micropore surface area is determined from the BET surface area, which is not applicable to microporous solids. Indeed, it is based on multilayer N2 adsorption that could not occur within pores of the same size as the N2 molecule itself. Nevertheless, the BET surface area is widely used although the value calculated for micropores has no real meaning. The smaller the pore size, the more the BET surface area is underestimated. It can be noticed that extrapolation to zero of the linear region of the R-plot, in the range 0.3 < R < 0.9, gives similar volumes for the loose and compressed ITQ-2 powders. This volume is attributed to N2 adsorbed in the channels running inside the ITQ-2 sheets and is not modified by compression. Calculated pore volumes are 0.07 and 0.06 cm3 g-1 for the loose and compressed powders, respectively. The 0.3 < R < 0.9 region corresponds to the filling of micropores larger than the channels, that is, the supercages or the cups. For the loose ITQ-2 powder, there is a “jump” of adsorption around P/P0 ) 0.3, which is also visible in the R-plot for R around 0.85. At this relative pressure, 2-2.5 nm pores are filled. This type of pore is not detected in the compressed powder. They may come from partial delamination of MCM22 particles, that is, the sheets might not separate over their entire length. This would be consistent with the observation that the laminar precursors of MCM-22 with lower Si/Al ratios are more difficult to delaminate, and delamination occurs to a lesser extent.19 Owing to this partial delamination, mesopores may be formed in the space between two adjacent sheets. When compressed, the sheets stick together and the mesopores disappear. It has been shown that the chemical shift variation with Xe concentration, n, is linear when the Xe-Xe interactions are isotropic and that the slope increases with n when they are anisotropic.20 The increase in slope observed for MCM-22 shows that the Xe atoms are in an anisotropic environment. This is no longer the case when there is exchange between adsorbed and gas phases. In the gas phase, the Xe-Xe interactions are evidently isotropic. As a result, for the loose ITQ-2 powder sample, it appears that the average Xe-Xe interaction is rather isotropic, since the chemical shift variations are quasi-linear. For the compressed powder, the slope of the chemical shift variation increases with n, as in MCM-22, since the influence of exchange has been suppressed. We can then conclude that, from the point of view of Xe NMR, the laminar precursor of MCM-22 with a Si/Al ratio of 15 is only partially delaminated. When the sheets are partially separated, some supercages are opened forming mesopores, which results in an increase in the number of accessible cups. When the resultant powder is compressed, the sheets that were not fully separated stick together, decreasing the mesopore volume. Ferrierite and ITQ-6. The pore structure of ferrierite consists of a bidimensional channel network: cylindrical c channels (10 MR elliptical cross section, 0.42 × 0.54 nm) and b channels. In fact, the b channels can be considered as pseudo-cavities (largest diameter, ca. 0.6-0.7) with two opposed windows (8 MR, 0.35 × 0.48 nm) opening onto two adjacent c channels. In the following, they will be referred to as “b cavities”.21 At room temperature, the spectra of hyperpolarized 129Xe adsorbed in ferrierite (Figure 6) show two lines (b and c) corresponding to the two types of pores.22 The intensity of the gas-phase line at 0 ppm is high due to the nuclear hyperpolar-
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Figure 8. Static hyperpolarized (HP) 129Xe NMR spectra of xenon adsorbed on ITQ-6 at various Xe pressures and T ) 293 K. Figure 6. Hyperpolarized (HP) 129Xe NMR static spectra of xenon adsorbed on ferrierite at 293 K and various Xe pressures.
Figure 7. Hyperpolarized 129Xe NMR static spectra of Xe adsorbed in ferrierite at variable temperature for P(Xe) ) 1.5 × 104 Pa.
ization and the long relaxation time, T1, of Xe gas. An additional line, a, at high field, is due to Xe exchanging between the adsorbed phase and the gas phase in the intercrystallite space. Its chemical shift decreases when the Xe pressure increases because the relative amount of Xe gas, whose chemical shift is 0 ppm, increases. The chemical shift of line b, due to Xe in the b cavities, is constant whatever the pressure, because the b cavities can accommodate only one Xe atom. On the contrary, the chemical shift of line c increases with pressure due to possible Xe-Xe interactions in the c channels. The relative intensity of line c increases with pressure, indicating preferential adsorption in the b cavities at low pressure. The changes in the widths and positions of lines b and c observed under MAS conditions with a continuous flow of HP 129Xe show that these two lines are anisotropic, which reflects the anisotropy of the environment (Figure not shown). In the MAS spectrum, line a disappears because the powder is compressed both mechanically and by the centrifugal force in the rapidly spinning rotor. The intercrystallite space is, therefore, reduced and the fraction of Xe exchanging with the gas phase in the intercrystallite space becomes negligible. Variable-temperature experiments are shown in Figure 7. When the temperature increases above 297 K, the Xe atoms acquire enough energy to pass rapidly through the 8 MR
windows between b cavities and c channels, and the corresponding lines begin to coalesce. At 373 K, a single asymmetric line is observed around 120 ppm. Between 297 and 233 K, the position of line b is constant while line c moves toward higher chemical shifts. Below 233 K, the chemical shifts of both lines increase, as is generally observed. There are two reasons for this and they are as follows: the increase in the amount of adsorbed Xe (at least for line c) and the increase in the residence time of Xe on the pore surface. The “exchange” line a broadens and also moves toward high chemical shifts when the temperature decreases, because exchange is slowed down, with the residence time of Xe in the adsorbed state increasing. The spectra of ITQ-6 show the typical lines (b and c) of ferrierite (Figure 8), which proves that there are particles containing a few ferrierite layers so that c and b channels can be formed. Additionally, there are two lines, d and d ′, around 60 and 79 ppm, respectively, whose relative intensities increase with Xe pressure. The chemical shift of line d ′ is almost constant while that of line d decreases slightly. As for ferrierite, there is a broad “exchange line”, a, around 30 ppm, whose relative intensity becomes negligible at high pressure. As mentioned before, the intensity of this line depends markedly on the degree of compression of the powder in the NMR tube. The line almost disappears when the sample is compressed manually in an NMR rotor. The two lines, d and d ′, are attributed to Xe adsorbed in mesopores formed during delamination. Nitrogen adsorptiondesorption experiments at 77 K confirm the presence of two types of mesopores (Figure 9) with average sizes centered at 3.4 and 1.5 nm. These values should be regarded as estimations since we used DFT (density functional theory) with a slit pore model for carbon materials and not for silica-alumina (not available on our apparatus). The relative intensities of lines b and c compared with those of lines d and d ′ decrease when the Xe pressure increases. At low pressure, Xe adsorbs preferentially in the micropores rather than in the mesopores. However, since the micropore volume is much smaller than the mesopore one, line d is the strongest at high pressure. Variable-temperature experiments are shown in Figure 10. At high temperature, the intensities of lines b and c, corresponding to Xe adsorbed in micropores, are larger than that of Xe adsorbed in mesopores. As the temperature decreases, the adsorption of Xe in the micropores rapidly reaches saturation while mesopores continue to adsorb. Thus, the intensities of lines b and c become negligible compared with those of lines d and d ′. The chemical shifts of all lines increase because the
Xe Exchange in Delaminated Zeolites by HP
129Xe
NMR
Figure 9. Pore size distribution obtained from N2 adsorption isotherm at 77 K using density functional theory (DFT) with slit pore geometry.
Figure 10. Static hyperpolarized 129Xe NMR spectra of Xe adsorbed on ITQ-6 at variable temperature for P(Xe) ) 3.8 × 103 Pa.
J. Phys. Chem. C, Vol. 111, No. 15, 2007 5699 The intensity of the exchange line a is high at 373 K due to rapid diffusion of Xe that favors the exchange. For the same reason, the line disappears for temperatures below 273 K. 2D exchange NMR experiments were performed with MAS of the ITQ-6 sample with a continuous flow of HP Xe. These 2D experiments reveal changes in NMR frequencies occurring on a time scale ranging from milliseconds to a few seconds. This is achieved by monitoring frequencies before and after a so-called mixing time during which spin exchange and/ or molecular motions can occur.23 Changes in the NMR frequencies appear as off-diagonal intensities (or cross-peaks) which depend on the mixing time. In the 2D exchange MAS spectrum shown in Figure 11, the cross-peaks reveal several types of exchange within the mixing time of 50 ms used in the pulse sequence. There is Xe exchange between the mesopores and the gas phase, between the b cavities and the mesopores, between the b cavities and the c channels but not between the c channels and the mesopores. This means that there are b cavities opening onto the mesopores. This observation can be explained by an incomplete delamination of the PREFER particles. During cation exchange, the CTMA+ cations do not migrate as far as the core of the particles. The subsequent ultrasonic treatment only separates the layers on the particle edge, as sketched in Figure 11. This suggests that delamination occurs along yz planes, that is, perpendicular to the [100] direction, based on the PREFER structure proposed by Schreyeck et al.12 Upon calcination, the untouched core of the delaminated particles forms ferrierite. It may be noted that exchange between b cavities and some mesopores would explain the line at ca. 98 ppm seen in the static spectra for Xe pressures above 2.9 × 104 Pa. The elongated shape of lines b and c in the diagonal spectrum shows that there is no exchange between different b cavities and different c channels. In contrast, the round shape of line d shows that Xe exchanges between different mesopores. Conclusions
Figure 11. 2D exchange MAS NMR spectrum of HP 129Xe adsorbed on ITQ-6 (mixing time 50 ms). Sample was rotated at 3 kHz And P(Xe) ≈ 1.06 × 103 Pa. For each of the 256 increments, 8 scans were acquired with a 2 s delay. A sketch of partially delaminated ITQ-6 particles is also shown.
amount of adsorbed Xe and its residence time on the pore surface increase. In particular, at low temperature, Xe cannot distinguish the two types of mesopores and lines d and d ′ merge into a single signal.
The pore structure of MCM-22, with a Si/Al ratio of 15, ferrierite and the corresponding delaminated materials have been studied by 129Xe NMR of both thermally and hyperpolarized adsorbed xenon. Whereas the two types of pores in ferrierite give rise to two signals which can be clearly assigned, the two independent pore systems of MCM-22 give a single line, showing that the mean free path of a Xe atom is similar in the two environments. For ITQ-2 (i.e., delaminated MCM-22), the spectra show several lines which have been attributed to a discrete distribution of particle sizes. In particular, ITQ-2 particles, obtained by delaminating MCM-22, are necessarily much smaller than MCM-22 particles, and the chemical shift of Xe adsorbed in the former material is lowered due to exchange with the interparticle gas phase. The smaller the particles are, the greater that the chemical shifts decrease. When the sample is compressed, the interparticle spaces are drastically reduced and exchange between the adsorbed phase and the interparticle gas phase becomes negligible. Indeed, the spectra of thoroughly compressed ITQ-2 powder approach that of MCM-22. This result indicates that MCM-22 samples with a low Si/Al ratio of 15 are only partially delaminated, in the sense that the sheets do not separate over their entire length. Nevertheless, the achieved separation increases the mesoporosity, the number of cups formed, and, consequently, the accessibility of molecules to internal sites.
5700 J. Phys. Chem. C, Vol. 111, No. 15, 2007 For ITQ-6 (i.e., delaminated ferrierite), there is no direct effect of exchange between the adsorbed phase and the interparticle gas phase. In fact, the diffusion of Xe in the ferrierite pores is restricted due the small dimensions of the channels and windows. The ITQ-6 spectra show the same signals as ferrierite. Additional lines have been attributed to mesopores formed by delamination. 2D exchange NMR spectra show different types of exchange; in particular, there is exchange between the mesopores and the b channels but not between the mesopores and the c channels. This observation is explained by a partial delamination of ferrierite that occurs at the edge of the particles, with the ferrierite structure being maintained in the core of the material. References and Notes (1) Corma, A. J. Catal. 2003, 216, 298. (2) Balkus, K. J.; Gabrielov, A. G.; Sandler, N. Mater. Res. Soc. Symp. Proc. 1995, 368, 359. (3) Burton, A.; Elomori, S.; Chen, C. Y.; Medrud, R. C.; Chan, I. Y.; Bull, L. M.; Kibby, C.; Harris, T. V.; Zones, S. I.; Vittoratos, E. S. Chem.Eur. J. 2003, 9, 5737. (4) Pailland, J. L.; Harbuzaru, B.; Patarı´n, J.; Bats, N. Science 2004, 304, 990. (5) Corma, A.; Dı´az-Caban˜as, M. J.; Rey, F.; Nicolopoulus, S.; Boulahya, K. Chem. Commun. 2004, 12, 1356. (6) Cheetman, T.; Fjellvag, H.; Gier, T. E.; Kongshang, K. O.; Lillerud, K. P.; Stucky, G. D. Stud. Surf. Sci. Catal. 2001, 135, 788. (7) Strohmaier, K. G.; Vaughan, D. E. J. Am. Chem. Soc. 2003, 125, 16035.
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