J. Phys. Chem. C 2008, 112, 10899–10908
10899
Evidence of Multiple Cation Site Occupation in Zeolite NaY with High Si/Al Ratio Laurent Gueudre´, Anne Agathe Quoineaud, Gerhard Pirngruber, and Philibert Leflaive* IFP-Lyon, Institut Franc¸ais du Pe´trole, BP n° 3, 69390 Vernaison, France ReceiVed: April 8, 2008; ReVised Manuscript ReceiVed: April 30, 2008
23Na
MAS NMR and 23Na 2D MQMAS NMR spectra of dehydrated NaY zeolite with different Si/Al ratios were analyzed. Particularly, the cation distribution was examined in both as synthesized NaY with a Si/Al ratio of 2.7 and a dealuminated NaY with a framework Si/Al ratio of 8.6, prepared by ion-exchange of a dealuminated NH4Y zeolite CBV712. This is, to the best of our knowledge, the first experimental investigation of the cation distribution in dealuminated NaY. Careful analysis of 23Na MAS NMR and 23Na 2D MQMAS NMR spectra of dealuminated NaY revealed the occupation of at least three different sites, contrary to what was expected from previous computational studies where site II was the only occupied site. Moreover, two new features could be observed. First, a split of the contribution of site I into two relatively close and similar narrow peaks in the MQMAS NMR spectra, and second a new narrow signal, interpreted in terms of fast cation movement inside the sodalite cage. This work also allows a clear identification of the contribution of site II to the 23Na MAS and MQMAS NMR spectra of as synthesized NaY (Na52Y). This was hardly possible in previous 23Na MAS NMR studies of NaX or NaY due to strong overlap of the broad quadrupolar pattern of site II with the one of the cations located in site I′. Introduction Understanding the factors that control separation or catalytic properties in zeolites requires a detailed knowledge of the structure of the material. In aluminosilicate zeolites, the presence of aluminum atoms introduces negative charges, which are compensated with nonframework cations. Zeolites are characterized by the fact that the number of potential cation sites in the structure usually exceeds the number of cations needed to ensure electroneutrality of the material. The different cation sites in X and Y zeolites as derived from X-ray and neutron diffraction studies are illustrated in Figure 1 along the cubic 3-fold (1,1,1) axis. Knowledge concerning the positions of the extraframework cations is critical to the understanding of the adsorption, separation, and catalytic properties of cation-exchanged zeolitic materials. The nature and positions of the cations will control the electrostatic field present within the zeolite pores, which strongly influences the adsorption and reactivity of the sorbed molecules. However, the precise location of supercage cations in monovalent zeolites by conventional diffraction techniques is rendered difficult because of partial occupancies affecting low symmetry sites. Typically, since the initial diffraction work by Eulenberger,1 a significant number of studies has been dedicated to the reinvestigation of the location of cations in as synthesized NaY (i.e., with a Si/Al ratio ranging from 2.4 to 2.7), using conventional powder or single crystal diffraction refinements,2–5 NMR studies,6–9 and computational approaches.10–13 In NaY zeolite, i.e., a sodium-exchanged zeolite with a faujasite structure and a Si/Al ratio above 1.5, the cations may occupy one or more of the extra-framework positions shown in Figure 1, depending on the framework silicon-to-aluminum ratio. The population of the different sites by the sodium ion as a function of the Si/Al ratio has been described by Monte Carlo * Corresponding author: E-mail:
[email protected]. Fax: + 33 4 78 02 20 66. Tel: + 33 4 78 02 28 34.
Figure 1. Cation sites in faujasites X and Y zeolites as derived from X-ray and neutron diffraction studies along the cubic 3-fold (1,1,1) axis.
simulations. The Monte Carlo simulations predict (and experimental data confirm) that for NaY zeolite with less than 52 cations (Si/Al ratio above 2.7) only sites SII, SI, and SI′ are occupied (see Table 1). Sites SI and SI′ are respectively located in the hexagonal prisms which connect the sodalite cages and inside the sodalite cages facing site I. Sites II are located in the supercage near six-membered oxygen rings (see Figure 2). For faujasites with less than 32 cations, the Monte Carlo simulations predict that exclusively sites II are occupied.12 This arrangement minimizes the electrostatic repulsion between the cations. Unfortunately, this prediction has until now not been experimentally verified since no experimental data is available for NaY with a Si/Al ratio above 3, i.e., with less than 48 cations. Solid-state 23Na MAS NMR is very sensitive to the sodium local environment and has been widely used to characterize the sodium cations in zeolite NaY. However, the interpretation of the 23Na MAS NMR spectra is not always straightforward due to the overlap of the resonances, as a result of the second-order quadrupolar interaction. This interaction is due to the dispersion of the electron cloud around the atomic nucleus, which is represented by the quadrupolar coupling constant (Qcc). In case of a spherical symmetry Qcc ) 0 MHz. When geometry becomes distorted, Qcc increases. Na+ ions in the center of the hexagonal prism (site I) are surrounded by six framework oxygens which
10.1021/jp803037u CCC: $40.75 2008 American Chemical Society Published on Web 06/26/2008
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TABLE 1: Cation Site Occupancy for a Dehydrated NaY Zeolite with a Si/Al Ratio Ranging from 2.35 to 2.7 (a Reference is Given for Each) method
Eulenberger1 XRD
Fitch2 ND
Jira´k4 XRD
Marra5 XRD
Grey7 XRD
Buttefey10 GCMC
Jaramillo11 GCMC
I I′ II other sites total
8.00 18.88 30.08 0.04 57.00
7.1 18.6 32.2 0 56
4.00 17.60 32.00 1.40 55.00
9.30 13.70 25.30 3.50 51.80
4.3 18.9 32 0 53
8 16 32 0 56
7 17 25 7 56
places them in a nearly octahedral coordination. According to Koller et al.14 the contribution of site SI to the overall 23Na MAS spectra is, therefore, limited to a single, rather narrow line shape toward -15 ppm. Its quadrupolar coupling constant Qcc is small as a result of the octahedral symmetry.14 Nevertheless, recent studies using two-dimensional multiple-quantum MAS (2D MQMAS)15,16 reported that Qcc value for the sodium cations located at the site I position in NaY is 1.2 MHz, which is higher than the value obtained by MAS and DOR NMR spectroscopy (0.2 MHz). It was proposed that the position of site I is displaced from the center of the hexagonal prism, resulting in the large Qcc value.17 Still, the contribution of site I is characterized by a comparatively narrow Gaussian-like line shape in 23Na MAS spectra of dehydrated “as synthesized” NaY and there is a general agreement on its assignment in the literature. Contrarily to the cations situated in site I, the cations present in the sodalite cage (SI′) and in the supercage (SII) have only three oxygen atoms in the vicinity, resulting in a symmetry close to C3V as shown on Figure 2. The 23Na MAS NMR signals of sodium cations located in theses sites in dehydrated NaY zeolite are strongly broadened, due to second-order quadrupolar effects. The corresponding quadrupolar coupling constants Qcc were found to be ranging between 3.9 and 6.0 MHz.8,15,18 Koller et al.14 reported that beside the peak of site I, the other two remaining maxima in 23Na MAS NMR spectra of NaY (Si/ Al ratio of 2.5) correspond to the cations present in sites I′ and II, but they could not be clearly attributed. The application of high-resolution solid-state NMR spectroscopy methods is required to improve the resolution of 23Na NMR spectra. For instance, two-dimensional multiple-quantum MAS (2D MQMAS) has already been applied to study the sodium cations in dehydrated NaY zeolite. These investigations demonstrated the suitability of this technique to improve the resolution of 23Na NMR spectra. Applying a three pulse or a RIACT sequence, Lim and Grey8 were able to obtain 23Na MQMAS NMR spectra of dehydrated NaY showing large quadrupolar coupling constants for sodium cations located at SII and SI′ positions. The
authors nevertheless indicate that the signals from the large Qcc sites (II and I′) are significantly broadened in the isotropic dimension of the 2D MQMAS NMR spectra, which results in a considerable amount of uncertainty for the NMR parameters extracted from the MQMAS NMR spectra for these species. The authors also found evidence of a hydrated species, not observed by others, resulting in a peak located between those of sites I and II in the 23Na MAS NMR spectra. A more recent study19 reported the presence of an additional line shape at -18 ppm attributed to Na+ cations located at sites SIII′ in the framework. Since the occupancy of site SIII′ in NaY zeolites has never been reported using X-ray diffraction or any other characterization method, this assignment can be considered as doubtful. The use of off-resonance RIACT for 23Na triple quantum MAS NMR significantly improves the overall quality of the spectra of NaY,19 but still yields spectra with overlapping SI′ and SII signals. The information obtained by the twodimensional 23Na ORIACT triple-quantum MAS NMR may be considered as a good starting point for a quantitative simulation of the one-dimensional 23Na MAS NMR spectra. Nevertheless, simulation and fitting of the one pulse 23Na MAS NMR spectra is still necessary to calculate the contribution of site II and site I′ to the overall spectra. From this analysis, it can be clearly seen that despite the use of the most sophisticated NMR techniques, the clear contribution of site I′ and site II to the one-dimensional 23Na MAS NMR and the 2D MQMAS NMR spectra of the dehydrated NaY zeolite is still under debate. The spectral superimposition of site I′ and site II contributions in both spectra renders the assignment of the sodium cation resonances ambiguous. In the present work, we try to determine the accurate contribution of both site II and site I′ in the 23Na MAS NMR spectra of sodium faujasite. The characterization of sodium sites includes an estimation of NMR parameters using a method based on 23Na MAS and MQMAS spectra simultaneous decomposition. This topic is also tackled by determining the 23Na MAS NMR of a dealuminated Y zeolite containing sodium atoms (framework Si/Al ratio of
Figure 2. Position and coordinence of extra-framework crystallographic sites in faujasite.
Zeolite NaY with High Si/Al Ratio
Figure 3.
23
J. Phys. Chem. C, Vol. 112, No. 29, 2008 10901
Na MAS NMR (a) and 2D MQMAS NMR (b) spectra of dehydrated Na52Y zeolite.
8.6). At low cation occupancy (Si/Al > 5) the only filled sites are expected to be sites II. Thus, the 23Na MAS NMR of a dealuminated NaY should only exhibit the contribution of those sites. The contribution of site I′ should then be extracted from the overall spectrum with the knowledge of site I and II contributions. The 23Na MAS NMR study of a dealuminated NaY also provides, to our knowledge, the first investigation of the cation distribution in sodium faujasite with low aluminum content, as no experimental data is available for NaY with a Si/Al ratio above 3. Experimental Section Sample Preparation. Zeolite NaY with framework Si/Al ratio of 2.70 (52 cations per unit cell) is a commercial material of Zeolyst Corp. This sample is referred to as Na52Y. Zeolite NaY with framework Si/Al ratio of 8.6 (20 cations per unit cell) was prepared by ion exchange of zeolite NH4Y (CBV712 from Zeolyst) with a 4-fold excess of a 0.9 M aqueous solution of NaCl at a temperature of 90 °C over a period of 4 h. The sample was then filtered, washed by distilled water,
and dried at 110 °C in an oven over a period of 12 h to remove excess water. The overall exchange and drying procedure was repeated four times to ensure full sodium exchange. This sample is referred to as Na20Y. All materials were characterized by X-ray diffraction, atomic emission spectroscopy with inductively coupled plasma (ICPAES), 29Si and 27Al MAS NMR. The framework Si/Al ratios of Na52Y (2.7) and Na20Y (8.6) were determined both from unit cell parameter from XRD and 29Si MAS NMR spectra. 27Al MAS NMR of Na20Y revealed the presence of extra-framework aluminum due to the dealumination of the zeolite framework needed for the synthesis of CBV712. Analysis of the parent CBV712 indicated that no more dealumination occurred during ion-exchange. Prior to the 23Na NMR investigations, the powder materials were heated in vacuum with a rate of 20 °C/h up to the final temperature of 450 °C. At this temperature, the samples were dehydrated for 12 h at a pressure below 10-2 Pa. Finally, the
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Figure 4. MAS and MQMAS spectra of dehydrated Na52Y zeolite calculated from δiso, Qcc, and the relative intensities estimated from experimental MAS and MQMAS spectra simultaneous decomposition.
TABLE 2: Average NMR Parameters of the Various Cation Sites in the Dehydrated Na52Y Zeolite Obtained from 2D MQMAS NMR estimated from MQMAS site Na+1 Na+2 Na+3
assignment I I′ II
calculated from the decomposition of MAS and MQMAS by dmfit2007
δISO (ppm)
Qcc (MHz)
δISO (ppm)
Qcc (MHz)
I (%)
-2.3 -10.6 -15.8
1.5-2 ∼4 >5
-10.5 -12.6 -20.2
1.2 2.6 3.8
9 31 61
samples were filled into the 4 mm MAS NMR rotors under dry nitrogen using a glovebox and tightly sealed with Kel-F rotor caps. NMR Measurements. 23Na NMR experiments were performed on a Bruker Avance 400 spectrometer at a resonance frequency of 105.8 MHz with a sample spinning rate between ca. 12-15 kHz using a 4 mm probe optimized for 23Na MQMAS experiments. 23Na MAS experiments were carried out under selective and quantitative conditions using a single pulse excitation corresponding to a π/12 flip angle at a 35 kHz radiofrequency field and with a 0.5 s relaxation delay. The number of scans is adapted to the amount of sodium cation in the zeolite. 23Na MAS spectra
were thus recorded using 640 and 2560 scans for Na52Y and Na20Y samples, respectively. 23Na
MQMAS NMR experiments were obtained using the z-filter MQMAS sequence synchronized on the rotor spinning rate proposed by Frydman et al.20 and Amoureux et al.21 Processing was performed using the shearing procedure reported by Medek et al.22 The spinning speed of the sample was 15 kHz. The two exciting conversion multiple quanta pulses were performed at a 210 kHz radiofrequency field corresponding to 5.1 and 1.7 µs and a selective 9.8 µs z-filter pulse at 35 kHz radiofrequency field for both Na52Y and Na20Y zeolites. The experiments were performed using
Zeolite NaY with High Si/Al Ratio
J. Phys. Chem. C, Vol. 112, No. 29, 2008 10903
Figure 5. (a) 23Na MAS spectra of dehydrated Na20Y (bold) and Na52Y (dashed). (b) 2D MQMAS NMR spectrum of dehydrated Na20Y zeolite.
22 920 and 56 208 scans for Na52Y and Na20Y samples, respectively, with a 0.5 s relaxation delay. All 23Na NMR chemical shifts are referenced to the solid NaCl. If a 1 M NaCl solution would have been used as in refs 5 and 16, chemical shifts would expected to be 7 ppm shifted from values reported in this study. Two methods were applied to obtain NMR parameters of the different sodium contributions. The first one was based on a direct estimation of the isotropic chemical shift (δiso) and the quadrupolar coupling constant (Qcc) from the 23Na MQMAS spectra. The isotropic chemical shift was estimated as the center of gravity of the signal obtained from the chemical shift projections in the isotropic and the MAS dimensions of the MQMAS spectrum. The quadrupolar coupling constant was estimated from the composite quadrupolar parameter PQ and the asymmetry parameter ηQ. The composite quadrupolar parameter PQ is given by the isotropic chemical shift and the chemical shift projection in the isotropic dimension of the MQMAS spectrum and the asymmetry parameter ηQ was assumed to be equal to 0.5. This first method is frequently used
in the literature6–9 but may result, for large Qcc sites, in an imprecise estimation of the NMR parameters and an inaccurate estimation of the signal intensity (overestimated for weak quadrupolar signals and underestimated for strong quadrupolar signals). The second method is an alternative method23–25 based on a computational estimation of the isotropic chemical shift and the quadrupolar coupling constant via the simultaneous decomposition of both 23Na selective and quantitative MAS and 23Na MQMAS spectra using the dmfit2007 program26 taking the distribution of the interaction parameters into account. Due to the quadrupolar character of the sodium nucleus, Gaussian lineshapes could not be used to model sodium cations signals. As a consequence, Czjzek’s line shape was used.27,28 The isotropic chemical shift δiso, the quadrupolar coupling constant Qcc and asymmetry parameter ηQ were adjusted from the MQMAS spectrum. Then, the intensity of each signal was calculated from the MAS spectrum. Computational Methodology. Plots of the potential energy within the unit cell of faujasite Y were generated by the code
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Figure 6. MAS and MQMAS spectra of dehydrated Na20Y zeolite calculated from δiso, Qcc, and the relative intensities estimated from simultaneous decomposition of experimental MAS and MQMAS spectra.
TABLE 3: Calculated Average NMR Parameters of the Various Cation Sites in the Dehydrated Na20Y Zeolite Obtained from 2D MQMAS NMR (Determined by dmfit) sites
assignment
δISO (ppm)
Qcc (MHz)
I (%)
Na+1* Na+2* Na+3* Na+4*
I I sodalite II
-8.4 -13.3 -25.3 -18.9
1.1 1.2 1.6 3.5
3 3 15 78
GIBBS (version 7.3). The faujasite host structure was taken from the experimental neutron diffraction studies of Fitch et al.2 The average charges of the framework atoms were interpolated from the EEM (electronegativity equalization method) results of Uytterhoven et al.29 Al and Si were not distinguished in the simulation. An average T atom charge was used for both. The cation-framework potential energy plots are the sum of two contributions, a repulsion-dispersion term that acts between the cation and the oxygen atoms of the faujasite and a Coulombic term that acts between the cation and both the oxygen and T atoms of the framework using partial charges on all atoms (qT ) +1.514, qNa ) +1, qO ) -0,809). The repulsion-dispersion termisdescribedbyaLennard-JonespotentialandLorentz-Berthelot combination rules. Oxygen parameters (σO ) 3.0 Å and εO )
93.53 K) were taken from Pascual et al.,30 and Na+ values (σNa+ ) 2.584 Å, σO-Na+ ) 2.792 Å) used are those reported by Dang in.31 The Lennard-Jones potential reproduces the cation distribution obtained in earlier work12 that used a Buckingham potential11 to describe the NasO repulsion-dispersion term. The potential energy grid was calculated with a resolution of 128 points along each unit cell direction. Plots of the potential energy were extracted along the (111) axis, which runs through the hexagonal prism, the sodalite cage and then the supercage. Results and Discussion Na52Y. Figure 3 represents 23Na MAS NMR (a) and 2D MQMAS NMR (b) spectra of Na52Y zeolite dehydrated at 450 °C corresponding to the NaY zeolite with 52 cations per unit cell. The one-dimensional 23Na MAS NMR spectrum exhibits three maxima at -15, -35, and -65 ppm and a shoulder at -85 ppm in accordance with previously reported spectra. On the basis of the different reported studies, the narrow resonance at -15 ppm is attributed to site I because of its spherical geometry and its weak quadrupolar coupling constant. The two other maxima at -35 and -65 ppm may be assigned to the two overlapping resonances of the Na+ cations in sites I′ and
Zeolite NaY with High Si/Al Ratio
Figure 7. Bottom: potential energy profile of a single sodium cation in the empty faujasite host structure [Si140Al52O384]52- and in a faujasite host with all sites II occupied by Na+ [Na32Si140Al52O384]20-, both along the axis (111). Top: sketch of the positions of site I, I′, and II in the faujasite host.
II. These signals show a strong broadening due to second-order quadrupolar effects. The presence of other additional sodium species cannot be excluded. The strong second-order quadrupolar effect makes a detailed identification of 23Na sites in the 1D NMR spectrum difficult. Therefore, 23Na MQMAS NMR experiments are performed to improve the resolution and also the identification of sodium species. Although the signal/noise ratio of the 2D MQMAS NMR spectrum of dehydrated Na52Y is low, at least three species (referred to as Na+1, Na+2, Na+3) can be clearly observed in the 2D spectrum. 23Na MQMAS NMR spectrum exhibits chemical shift distributions and quadrupolar parameters which can be expected for a disordered material. This disorder can result from different Na positions in cavities but also from different local cation surroundings. The Qcc parameters extracted from the spectra are therefore average values. The broadening along dipolar and quadrupolar axes on the MQMAS spectrum is represented by a line broadening parameter (Em) in the dmfit program.26 The isotropic chemical shifts (δiso) and quadrupolar coupling constants (Qcc) are estimated from the projections of lineshapes in the two dimensions according to the first method (Table 2). The simultaneous decomposition of MAS and MQMAS leads to different values of δiso and Qcc, We believe that the simultaneous fit of the MAS and the MQMAS spectra leads to a more reliable determination of the NMR parameters than the visual estimation of the center of gravity of the signals in the MQMAS spectrum alone. Moreover, the relative intensity of each sodium site can be determined (Table 2). A decomposition of the 23Na MQMAS spectrum of Na52Y using NMR parameters
J. Phys. Chem. C, Vol. 112, No. 29, 2008 10905 reported in the literature was also tested. No good match could be obtained comparing the 2D spectrum obtained using the dmfit program and reported parameters with the experimental MQMAS spectrum. By comparing the relative intensity of each site with other experimental data (Table 1), we can suggest an assignment of the NMR signals. Na+1, Na+2, and Na+3 correspond to site I, I′, and II, respectively. Both the isotropic chemical shifts and quadrupolar coupling constants are in agreement with the values proposed by Lim et al.,8 but sites I′ and II are reversed. Na20Y. In order to confirm the attribution of the different lineshapes in the dehydrated Na52Y zeolite, a sample of NaY containing 20 cations Na+ per unit cell was analyzed. This sample should only exhibit the peak of site II, this site being the only expected occupied site from previous computational studies.12 The 23Na MAS and MQMAS spectra of dealuminated and dehydrated Na20Y zeolite are represented by Figure 5, panels a and b, respectively. In the 23Na MAS spectrum, three maxima are observed at -15, -35, and -55 ppm. On the basis of the attribution of the analogous line shape in the Na52Y zeolite, the maximum at -15 ppm is attributed to sodium located in site I. Using the 23Na MQMAS, four sodium species are identified on the spectrum of dehydrated Na20Y referred to as Na+1*, Na+2*, Na+3*, and Na+4*. As for Na52Y, 23Na MQMAS NMR spectrum exhibits large chemical shift distributions and quadrupolar parameters. This effect is attributed to local disorder of the surrounding of the sodium sites. The broadening is represented by the Em parameter in the dmfit program.26 Decompositions are presented in Figure 6 and results are reported in Table 3. Although Monte Carlo simulations indicated that cations in zeolite Na20Y should only be located in sites II, the 23Na NMR results show that not only sites II, but also sites I and others are occupied by Na+ cations in the dealuminated zeolite. This multiple cation site occupation is however in agreement with the calculations of Van Dun et al.32 who stated that for a NaHY containing 22 cations (Na22H10.5Al32.5Si159.5O384) sites I, I′, and II should be partially occupied. The site population fractions at 350 °C were respectively calculated to be 0.03(I), 0.19(I′), and 0.48(II). Taking into account the relative occupancy and shape of signal Na+4*, we can presume that it corresponds to site II. In comparing species Na+4* and Na+3 of Na52Y 23Na NMR spectra, we can see that they have a similar isotropic chemical shifts (δiso) and quadrupolar coupling constants (Qcc). It can be thus deduced that Na+2 species on Na52Y 23Na NMR is assigned to site I′ (Table 2). The Na+2 signal observed in the 23Na MQMAS of Na52Y zeolite is no more present in the 23Na MQMAS of Na20Y zeolite. This can be attributed to the very low occupancy of site I′ as calculated by Van Dun et al.32 On the other hand, two new signals referred to as Na+2* and Na+3* are observed for Na20Y zeolite. The line shape of Na+2* in the 23Na MQMAS spectrum of Na20Y presents a very low quadrupolar character and an isotropic chemical shift (δiso) close to the one of Na+1*. Due to this similarity, we can assume that Na+2* species also corresponds to a cation occupying site I, the difference being due to a different local environment for the two species. In site I, Na+ cations are surrounded by 6 oxygen atoms, each of them being linked to two T atoms of the hexagonal windows. The charge of the oxygen depends on the T atoms they are linked with. The local environment of site I, will depend on the number of Si and Al atoms that are in the two hexagonal windows forming the prism. In the Na52Y, the most encountered situation should be by far one or two aluminum atoms per hexagonal
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Figure 8. 2D MQMAS NMR spectra of dehydrated Na52Y zeolite (a) and of dehydrated Na20Y zeolite (b).
window, the Si/Al ratio of the zeolite being 2.7. In Na20Y, the average hexagonal window contains less than one aluminum (0.625 for a Si/Al ratio of 8.6). One can easily imagine that a cation in an hexagonal prism containing a window with no aluminum atom should have a different position (off-center) and chemical environment than a cation in a prism with both windows containing one or more aluminum atoms, yielding thus to a separation of the two contributions in the 23Na MQMAS spectrum of the Na20Y zeolite. Na+1* and Na+2* can be thus both attributed to site I. The line shape of Na+3* in the 23Na MQMAS spectrum of Na20Y also presents a small quadrupolar character. Nevertheless the isotropic chemical shift (δiso) and the quadrupolar constant (Qcc) are quite different from those of Na+1* and Na+2*. It is thus unlikely that Na+3* also corresponds to another configuration of site I. The Na+3* signal at -25.3 ppm can be associated to a sodium species which presents an electronic environment close to a spherical symmetry and a homogeneous chemical neighborhood. This could be site U as reported by Kirschhock et al. for NaY and NaX in refs 33 and 34 site U being the only site which presents an almost spherical symmetry (in the center of the sodalite cage). Even if partial occupancy of this site has been reported for Na+ in sodium faujasites, this attribution seems
unlikely, this site being probably rather unstable for small size monovalent cation such as sodium, the cation being too far from the framework oxygens. Figure 7 shows the potential energy grid of a single Na+ cation in the negatively charged faujasite host Y52 (containing 52 Al atoms but without extra-framework cations) along the axis (111). One can see that the most stable cation site is site I′, followed closely by site II′ at the opposite side of the sodalite cage, which has a similar potential energy. The center of the sodalite cage (site U) is indeed not a stable position for Na+. Also site II is only discerned as a weak shoulder in the potential energy profile and is a priori not the most preferred location for the cation. However, when the faujasite host is filled with more than one cation per unit cell, occupation of site II allows the minimization of the cation-cation repulsion. That explains why site II is usually fully occupied. For comparison, Figure 7 shows the potential energy profile (Na32Y52) of a single Na+ cation in a faujasite host where all 32 sites II are already occupied by Na+. In that case site II′ is not stable any more, due the electrostatic repulsion of the cation in site II. Sites I and I′ are now the most preferred locations, i.e. the additional cations try to maximize their distance from site II. The lesson we can learn from these energy profiles is that site II′ is, in
Zeolite NaY with High Si/Al Ratio principle, a favorable location for the cation, but only when the neighboring site II is empty. In Na20Y not all sites II are occupied. In the sodalite cage next to an unoccupied site II, sites I′ and II′ have almost the same potential energy. The barrier for passing from site I′ to II′ along the axis (111) is high. In reality, however, the cation would not take the direct pathway via the center of the cage, but rather move along the wall of the cage. A three-dimensional view of the sodalite cage shows that site I′ and site II′ of neighboring hexagonal windows of the sodalite cage are very close to each other. It should then be easy for the cation to jump from site I′ to the neighboring site II′. Molecular dynamics calculations confirmed that cations can move easily along the wall of a ring.35 Having this in mind, the new Na3+* line shape in the 23Na MQMAS of Na20Y can be interpreted in terms of fast sodium cation intersite mobility from the I′ to the II′ position. The cation rapid exchange between two cationic positions gives a weak quadrupolar line shape in the NMR spectrum. It yields an overall quasi-symmetric environment of such cations as if they were located in the average position site U. In dehydrated Na52Y zeolite, such an intersite cation mobility is not possible due to the full occupancy of sites II, which is responsible for strong Coulombic repulsion between cations located in site II and II′. The global attribution of the four contributions in the 23Na MQMAS spectrum of the Na20Y zeolite is resumed in Table 3. We would like to note that the Na20Y is not a “clean” sample. It contains extraframework aluminum, which may occupy ion exchange positions and thereby influence the distribution of Na+. Yet, the considerations concerning the assignment of the sharp 23Na NMR peak made in the previous paragraph will hold true for any cation distribution where site II is not occupied, even if extraframework Al is present as a cocation. Final Attribution. 23Na 2D MQMAS NMR spectra with the corresponding sites of both as synthesized NaY with a Si/Al ratio of 2.7 and a dealuminated NaY with a framework Si/Al ratio of 8.6 are presented in figure 8. In comparing the values of Table 3 with those of Table 2, one can see that apart from the splitting of the Na+1 peak into Na+1* and Na+2*, the removal of cations through dealumination does not have a large influence on the isotropic chemical shift (δiso) and the quadrupolar constant (Qcc) of the peaks that correspond to the site that remain in the zeolite, i.e., site I and site II. The quadrupolar constant (Qcc) values reported for site I, I′, and II in the Na52Y zeolite in table 2 agree well with literature values given in refs 5 and 15 for sites I and II but is much lower for site I′ (2.6 MHz instead of 4.8). Discrepancies between values reported in this manuscript and those previously reported in the literature may be attributed to the use of a different method for the extraction of chemical shifts and quadrupolar parameters from the experimental spectra. In previous papers, isotropic chemical shifts were directly estimated as the center of gravity of the signal obtained from the chemical shift projections in the isotropic and the MAS dimensions of the MQMAS spectrum and quadrupolar coupling constant were estimated from the composite quadrupolar parameter PQ and the asymmetry parameter ηQ. In the case of very broad peaks (i.e., in the case of high Qcc values), this method results in a considerable uncertainty for the extracted NMR parameters. An alternative method23–25 based on a computational estimation of the isotropic chemical shift and the quadrupolar coupling constant via the simultaneous decomposition of both 23Na selective and quantitative MAS and 23Na MQMAS spectra using the dmfit2007 program26 was therefore used for this study.
J. Phys. Chem. C, Vol. 112, No. 29, 2008 10907 Although this method is not fundamentally different from the first one, the simultaneous fit of the one- and twodimensional spectra increases the confidence in the extracted NMR parameters. The results of dmfit provide a good agreement of experimental and simulated spectra for both MAS and 23Na MQMAS. With the previously reported NMR parameters, a good fit of the MQMAS spectrum could not be obtained (although the fit of the MAS spectrum was acceptable). Conclusion 23Na
MAS NMR and 23Na 2D MQMAS NMR spectra of both as synthesized NaY with a Si/Al ratio of 2.7 and a dealuminated NaY with a framework Si/Al ratio of 8.6, were analyzed. Careful analysis of 23Na MAS NMR and 23Na 2D MQMAS NMR spectra of Na20Y revealed the occupation of at least three different sites, contrary to what was expected from previous computational studies where site II was the only occupied site. Moreover, two new features could be observed. First, a split of the contribution of site I into two relatively close and similar narrow peaks in the MQMAS NMR spectra, and second a new narrow signal, interpreted in terms of fast cation movement inside the sodalite cage. Combined attribution of site contributions for both zeolites allowed a clear deconvolution of the Na52Y spectra. Quadrupolar constant (Qcc) values reported here for this zeolite agree well with literature values for sites I and II but is much lower for site I′ (2.6 MHz instead of 4.8). References and Notes (1) Eulenberger, G. R.; Shoemaker, D. P.; Keil, J. G. J. Phys. Chem. 1967, 71, 1812. (2) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986, 90, 1311. (3) Mortier, W. J.; Bosmans, H. J. J. Phys. Chem. 1971, 75, 3327. (4) Jirak, Z.; Vratislav, V.; Bosacek, V. J. Phys. Chem. Solids 1980, 41, 1089. (5) Marra, G. L.; Fitch, A. N.; Zecchina, A.; Salvalaggio, M.; Bordiga, S.; Lamberti, C. J. Phys. Chem. B 1997, 101, 10653. (6) Hannus, I.; Kircsi, I.; Lentz, P.; Nagy, J. B. Colloids Surf. 1999, 158, 29. (7) Grey, C. P.; Poshni, F. I.; Gualtieri, A. F.; Norby, P.; Hanson, J. C.; Corbin, D. R. J. Am. Chem. Soc. 1997, 119, 1981. (8) Lim, K. H.; Grey, C. P. J. Am. Chem. Soc. 2000, 122, 9768. (9) Feuerstein, M.; Hunger, M.; Engelhardt, G.; Amoureux, J. P. Solid State NMR 1996, 7, 95. (10) Buttefey, S.; Boutin, A.; Mellot-Draznieks, C.; Fuchs, A. H. J. Phys. Chem B 2001, 105, 9569. (11) Jaramilllo, E.; Auerbach, S. J. Phys. Chem B 1999, 103, 9589. (12) Beauvais, C.; Boutin, A.; Fuchs, A. H. C. R. Chim. 2005, 8, 485. (13) Beauvais, C.; Boutin, A.; Fuchs, A. H. ChemPhysChem. 2004, 5, 1791. (14) Koller, H.; Burger, B.; Schneider, A. M.; Engelhardt, G.; Weitkamp, J. Microporous Mater. 1995, 5, 219. (15) Hu, K. N.; Hwang, L. P. Solid State NMR 1998, 12, 211. (16) Hunger, M.; Sarv, P.; Samoson, A. Solid State NMR 1997, 9, 115. (17) Engelhardt, G. Microporous Mater. 1997, 12, 369. (18) Zhu, J.; Huang, Y. Stud. Surf. Sci. Catal. 2005, 158, 1057. (19) Caldarelli, S.; Buchholz, A.; Hunger, M. J. Am. Chem. Soc. 2001, 123, 7118. (20) Frydman, L.; Harwood, J. S. J. Am. Chem. Soc. 1995, 117, 5367. (21) Amoureux, J. P.; Fernandez, C.; Steuernagel, S. J. Magn. Reson., Ser. A 1996, 123, 116. (22) Medek, A.; Harwood, J. S.; Frydman, L. J. Am. Chem. Soc. 1995, 117, 12779. (23) Fernandez, C.; Quoineaud, A. A.; Montouillout, V.; Gautier, S.; Lacombe, S. Stud. Surf. Sci. Catal. 2001, 135, 183. (24) Quoineaud, A. A.; Montouillout, V.; Gautier, S.; Lacombe, S.; Fernandez, C. Stud. Surf. Sci. Catal. 2002, 142, 391. (25) Pires, R.; Nunes, T. G.; Abrahams, I.; Hawkes, G. E.; Morais, C. M.; Fernandez, C. J. Mater. Sci.: Mater. Med. 2004, 15, 201. (26) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve´, S.; Alonso, B.; Durand, J.-O.; Bujoli, B.; Gan, Z.; Hoatson, G. Magn. Reson. Chem. 2002, 40, 70.
10908 J. Phys. Chem. C, Vol. 112, No. 29, 2008 (27) Czjzek, G. Phys. ReV. B 1982, 25, 4908. (28) Czjzek, G.; Fink, J.; Go¨tz, F.; Schmidt, H.; Coey, J. M.; Rebouillat, J. P.; Lie´nard, A. Phys. ReV. B 1981, 23, 2513. (29) Uytterhoeven, L.; Bompas, D.; Mortier, W. J. J. Chem. Soc., Faraday Trans. 1992, 88, 2753. (30) Pascual, P.; Ungerer, P.; Tavitian, B.; Pernot, P.; Boutin, A. Phys. Chem. Chem. Phys. 2003, 5, 3684. (31) Dang, L. X. J. Am. Chem. Soc. 1995, 117, 6954.
Gueudre´ et al. (32) Van Dun, J. J.; Dhaeze, K.; Mortier, W. J.; Vaughan, D. E. W. J. Phys. Chem. Solids 1989, 50, 469. (33) Kirschhock, C.; Fuess, H. Stud. Surf. Sci. Catal. 1994, 84, 843. (34) Kirschhock, C.; Fuess, H. Zeolites 1996, 17, 381. (35) Plant, D. F.; Maurin, G.; Jobic, H.; Llewellyn, P. L. J. Phys. Chem. B 2006, 110, 14372.
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