4698
J. Phys. Chem. B 2001, 105, 4698-4708
Siting of Mixtures in Mordenite Zeolites:
19F
and
129Xe
NMR and Molecular Simulation
J.-H. Yang, L. A. Clark, G. J. Ray, Y. J. Kim, H. Du, and R. Q. Snurr* Department of Chemical Engineering, Northwestern UniVersity, EVanston, Illinois 60208 ReceiVed: October 4, 2000; In Final Form: March 8, 2001
19F
and 129Xe nuclear magnetic resonance spectroscopy (NMR) investigations were performed to validate predictions from Monte Carlo simulations for molecular siting in mordenite zeolites. For single-component adsorption, CF4 adsorbs preferentially in the mordenite main channel, while Xe adsorbs about equally in the main channel and the mordenite side pocket. The temperature and pressure dependences of the siting were studied, as well as the influence of zeolite cation exchange on molecular siting. 129Xe NMR studies of XeCF4 binary mixtures in Na-mordenite show that the main channel peak decreases with increasing concentration of CF4, implying that Xe prefers the side pocket while CF4 prefers the main channel in the binary system. Results show that 19F NMR of adsorbed CF4 can be used to characterize different zeolite environments in a manner similar to 129Xe NMR but with reduced sensitivity. The NMR results provide a stringent test of molecular simulation of adsorption by providing molecular-level siting that complements macroscopic experimental quantities such as the adsorption isotherm and heat of adsorption. Simultaneously predicting siting and macroscopic quantities is a difficult challenge for simulation. To improve the molecular model, an explicit polarization term was added to the usual Lennard-Jones potential.
Introduction Zeolites are crystalline aluminosilicates having pore structures with molecular size channels. They have high thermal and chemical stability and are widely used in catalysis, adsorption separations, and ion exchange.1 In a given zeolite, there may exist various environments for adsorption due to pores of different sizes and shapes, as well as defects and chargecompensating cations. Knowledge of where the molecules adsorb in these heterogeneous materials is essential to a fundamental thermodynamic description of adsorption. Such a description, along with the ability to predict or correlate multicomponent behavior, is important for the design of adsorption and catalytic processes and for the choice of optimal adsorbents.2 Siting of molecules and other thermodynamic and transport properties in zeolite systems are sensitive to subtle molecularlevel features such as small differences in pore diameter or pore shape. Because of this dependence on molecular-level structure, computer simulations based on an atomistic description of zeolite-sorbate interactions are a powerful tool for predicting and understanding adsorption in zeolites. Grand canonical Monte Carlo (GCMC) simulations3,4 can predict macroscopic properties such as the adsorption isotherm and heat of sorption, often in good agreement with experiment. The simulations also provide a great deal of information on the molecular-level structure, but there is usually little experimental data available on the molecular-level features for direct validation of simulation predictions. The general goal of this work is to use nuclear magnetic resonance (NMR) spectroscopy to validate simulation predictions of molecular siting in zeolites, focusing on mordenite. Mordenite (three-letter code MOR5) is an industrially useful zeolite containing two distinct environments for adsorption: a 12-ring main channel with smaller 8-ring side pockets off of * To whom correspondence should be addressed.
the main channel. Diffusion can only take place in one direction, through the main channels. The 12-ring channels have dimensions of 6.4 × 7.2 Å2 and the side pockets are 3.4 × 4.8 Å2. Clark et al.6 report relative volumes of 77.3% and 22.7% for the main channel and side pockets, respectively, on the basis of volumes accessible to an argon center of mass at potential energies less than 50 kJ/mol. The locations of the cations have been investigated by Mortier et al.7,8 and have been discussed recently by Macedonia et al.9 NMR studies using 129Xe to characterize zeolites were pioneered in the early 1980s by the groups of Ripmeester10-13 and Fraissard.14-19 Because the xenon chemical shift is so sensitive to its environment, this technique provides information on the dimensions of cavities and channels, the short- range crystallinity, the nature of structural defects, and the effect of cations in zeolites,17 as well as structural modifications during any dealumination process, steaming, or acid leaching.14 Fraissard represents the dependence of the chemical shift of adsorbed Xe in the form
δ ) δ0 + δσ + δ + δΜ + δXe where δ0 is the chemical shift of the reference, δσ is the shift due to collisions between Xe and the channel walls, δ is the shift due to the electric field created by the cations, δΜ is the shift that depends on the magnetic properties of the solid such as the paramagnetism of cations, and δXe is the shift caused by Xe-Xe collisions. The use of Xe as a probe molecule is now well established. 129Xe has a 1/2 spin and 26% natural abundance, and the range of Xe chemical shift is over 5000 ppm.20 It is chemically inert, nonpolar, spherical, and easily polarizable, with a large electron cloud sensitive to the pore structure. All of these features make it a convenient probe of nanoporous materials such as zeolites. Molecular siting has been investigated by van Santen and co-workers,21 who used 13C NMR to investigate siting of
10.1021/jp003626k CCC: $20.00 © 2001 American Chemical Society Published on Web 04/18/2001
Mixtures in Mordenite Zeolites n-alkanes in ferrierite, a zeolite consisting of 8-ring and 10ring structures. The 13C cross-polarized magic angle spinning (CP MAS) NMR results show that alkanes longer than pentane are size-excluded from the 8-ring pore structure of ferrierite, propane is preferentially adsorbed in the ferrierite 8-ring cage, and n-butane does not have any preferred site. The use of CF4 as a probe molecule also presents itself as an attractive possibility. 19F has a 1/2 spin and 100% natural abundance, giving a strong signal. It has a chemical shift range of 600 ppm,20 which, in theory, is sensitive enough to distinguish different adsorption environments. It is also chemically inert, nonpolar, basically spherical, and has been used as a probe molecule in diffusion studies in zeolites.22,23 Snurr et al.23 conducted 19F pulsed field gradient (PFG) NMR studies of the self-diffusion of CF4 during ethene conversion in H-ZSM-5 as a function of reaction time. In the course of these diffusion studies, they discovered that CF4 is fairly sensitive to its environment; they observed two peaks, one due to adsorbed CF4 and the other due to gas-phase CF4. Nakamura et al.24-27 have used CF4 and CnH2n+2 (n ) 1-4) to probe the nature of the surface potential inside Na-mordenite. Their approach was less direct than observing differences in chemical shift. Instead, the spin-lattice relaxation time, T1, was plotted vs reciprocal temperature for each probe molecule in Na-mordenite. They found that there were two T1 minima in the CH4 sample, while only a single T1 minimum appeared in the CF4, C2H6, C3H8, and C4H10 samples. In addition, from the Langmuir plots of the adsorption isotherm, the monolayer capacity for CH4 is 55 mL/g, while that for CF4 is only 23 mL/ g.27 Consequently, they concluded that CF4, C2H6, C3H8, and C4H10 were accommodated only in the main channel of Namordenite, while CH4 can access both the main channels and the side pockets. The goal of this work is to better understand molecular siting of CF4 and Xe in mordenite using 19F and 129Xe NMR, respectively, and to use NMR to test molecular siting predictions from atomistic simulations, especially for mixtures. The literature reports that Xe is a very sensitive probe molecule in zeolites, and a second goal of this paper is to show that CF4 can function similarly. The next section describes the experimental details. This is followed by a description of the GCMC simulations. The Results and Discussion section examines the use of CF4 and Xe as probe molecules and the sensitivity of CF4 in mordenite and faujasite with various types of cations. The NMR siting results are then used to test molecular-level predictions from GCMC simulations in both single component and binary systems. Experimental Section The synthetic zeolites Na-mordenite, siliceous mordenite, and Na-faujasite (zeolite Y) were used as starting materials. Namordenite and Na-faujasite were purchased from PQ Corporation (product codes: VALFOR CP500-11 and CBV 100), while the siliceous mordenite was provided by Prof. E. J. Maginn of the University of Notre Dame, who obtained the sample from Dow Chemical (product code: DOW 97000 16-60-3). It had been treated in steam to remove framework aluminum. H-, K-, and Ca-mordenite were prepared from Na-mordenite by ionexchange with excess aqueous salt solutions of NH4Cl, KCl, or CaCl2, respectively. K- and Ca-faujasite were prepared similarly.28 After each treatment, the zeolite was washed 7 times with distilled water to remove excess salt. Finally, the zeolite was air-dried overnight at 70 °C, followed by calcination. The zeolites were calcined in two steps to remove any organic remaining from the synthesis, first in nitrogen and then in air.
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4699 The final temperature was 500 °C,11 and the gas flow rate was 200 cm3/min. Both the nitrogen and air steps included holding the sample for 1 h at 100 °C to remove excess water and avoid self-steaming at higher temperatures. The bed depth was kept below 2 cm also to keep steaming and dealumination to a minimum. Detailed temperature programs are given elsewhere.28 The calcined samples were analyzed for Si, Al, and Na by atomic spectroscopy with an inductively coupled plasma (ICP) source (Atomscan 25, Thermo Instrument Systems, Inc.). Si/ Al ratios of the mordenite and faujasite samples were 5.5 ( 0.1 and 2.7 ( 0.1, respectively, except for siliceous mordenite (Si/Al > 2200). The Na/Al ratios of the cation-exchanged zeolites indicated that the Na cations were removed completely by the ion exchange procedure. Scanning electron microscope images28 of the samples showed Na-mordenite particle sizes of 1 to 3 µm and siliceous mordenite particles of 5 to 30 µm. The particles were composed of much smaller crystallites agglomerated together. The comparison of X-ray diffraction (XRD) of the zeolites in their initial state, after ion exchange, and after calcination indicated that the ion exchange and calcination conditions did not cause any apparent structural change. The zeolite samples were dehydrated and loaded with adsorbate in a home-built vacuum and heating apparatus equipped with a turbomolecular pump and an ion gauge. The system could be evacuated to a pressure on the order of 10-5 Torr, and the samples could be heated to 450 °C with temperature programming. Approximately 200 mg of zeolite powder was pushed down the tube and hand-packed with a glass rod. This process was repeated until the packed zeolite reached 3.5 cm in height, enough to overfill the NMR probe detection coil. The zeolite powder was packed for two reasons. First, higher density of the sample improved signal intensity of the adsorbate in the zeolite while reducing the signal intensity of the molecules in the gas phase. Second, packing keeps the zeolite powder from “bumping” during evacuation. The sample was slowly evacuated in the NMR tube at ambient temperature overnight reaching 10-2 Torr. Then the temperature was raised to 100 °C at 0.2 °C/min and held there for an hour for the pressure to reach 10-2 Torr again. Next, the temperature was raised to 450 °C at 0.5 °C/min and held there for 18 h reaching below 10-5 Torr. Finally, the sample was cooled to ambient temperature. Separate thermal gravimetric analysis (TGA) suggested that about 90% of the water in the zeolite was removed by holding the sample at 100 °C for an hour under the vacuum. The zeolite sample was exposed to high-purity gas (CF4, 99.97%, Air Products; Xe, 99.995%, Matheson) at the desired pressure. For binary component loading, specified gaseous mixtures were prepared in the vacuum manifold. We allowed approximately 10 min for the gases to mix thoroughly. Then, the zeolite sample was exposed to the mixtures for 20 min. After that, the zeolite sample was isolated from the vacuum manifold, which was then evacuated. A fresh charge of the same gaseous mixture was made in the manifold and the zeolite sample was again exposed to it for 20 min. We repeated this procedure three times to make sure that the adsorbed molecules were in equilibrium with the specified gas-phase compositions. This had a similar effect of exposing the zeolite sample to an infinitely large volume of a given gaseous mixture. Finally, the sample was isolated from the vacuum manifold, cooled with liquid nitrogen to reduce the pressure, and sealed off. Because of the experimental setup, gas from some additional dead volume was captured within the samples, causing the final pressures to be
4700 J. Phys. Chem. B, Vol. 105, No. 20, 2001 higher than the nominal loading pressures reported below. (See also the discussion of Table 3.) 19F and 129Xe spectra were obtained at 376.3 and 110.6 MHz, respectively, on a Varian INOVA 400 spectrometer (9.4 T) with a broadband Varian probe. In addition some spectra were acquired at different fields on an INOVA 500 spectrometer (11.7 T) and a VXR-300 spectrometer (7.0 T) for comparison. Spectral widths of 80 and 100 kHz were used for 19F and 129Xe, respectively. The delay time was set to at least 5 times the spinlattice relaxation time T1 of the adsorbate in mordenite. Spinlattice relaxation times were measured by the standard inversionrecovery method. T1 for CF4 ranges from 5 ms in the gas phase to 750 ms in the mordenite side pocket. T1 for Xe ranges from , 60 ms in the mordenite side pocket to 86 s29 in the gas phase. For 19F, the chemical shift is reported relative to CFCl3 at 0 ppm by means of an external reference of 500 Torr CF4 gas at -67 ppm.30 For 129Xe, the chemical shift is reported relative to the gas at infinite dilution by means of an external reference of 500 Torr Xe in NaY at 106 ppm. Typically, 64 and 10004000 scans were needed for 19F and 129Xe, respectively, to achieve reasonable signal-to-noise ratio. GCMC Simulations Adsorption simulations in both siliceous and sodium mordenite were performed with grand canonical Monte Carlo (GCMC) calculations,4,31 which sample the ensemble of constant chemical potential, volume, and temperature. The only assumptions that enter the calculations are an empirical potential model and the geometries of sorbate and sorbent. The potential models were primarily from the literature with some modifications as described in this section. For siliceous mordenite simulations, the zeolite lattice coordinates were taken from Alberti et al.32 as in our previous work.6 For sodium mordenite, the zeolite lattice coordinates were identical to those used by Macedonia et al.9 They used the Pbcn space group refinement of dehydrated sodium mordenite from Schlenker et al.33 to place all atoms except aluminum. The aluminum atom positions were then one possible set that is consistent with experimentally known cation placement and Si NMR data.9,34 To achieve a Si/Al ratio of 5.0, a supercell consisting of 4 unit cells of mordenite repeated along the c-axis was employed. The starting points for the development of the potentials were previously determined parameters for xenon35-37 and CF438 in siliceous MFI (silicalite), as well as experimental singlecomponent isotherms for xenon and CF4 in NaMOR.14,27 The strategy was to make minor adjustments, if necessary, to generally match the isotherms and then test how well the model performed in predicting the molecular-level siting. For comparison with NMR measurements in siliceous mordenite, and as a point of reference, we first performed simulations in siliceous mordenite. Xenon and CF4 were treated as single interaction centers, interacting with each other and with the oxygen atoms of the zeolite. The well-shielded tetrahedral atoms of the zeolite were neglected. The total potential energy was obtained by summing site-site Lennard-Jones interactions between all pairs of sites in the system. Parameters for CF4 were taken from Heuchel et al.’s work38 in silicalite with some adjustment of the CF4/oxygen epsilon parameter to bring the calculated isotherm into approximate agreement with the NaMOR experimental isotherm. For xenon, the two available parameter sets in silicalite underestimate35 or overestimate37 the
Yang et al. TABLE 1: Potential Parameters for Xenon and CF4 in Siliceous Mordenite interaction
/k (K)
σ (Å)
source
Xe-Xe CF4-CF4 Xe-CF4 Xe-oxygen CF4-oxygen
224.48 134.00 173.47 169.09 120.60
4.064 4.660 4.232 3.375 3.730
ref 37 ref 38 Lorentz-Berthelot average of refs 36 and 37 modified from ref 38
TABLE 2: Lennard-Jones Potential Parameters for Xenon and CF4 in Sodium Mordenite interaction
/k (K)
σ (Å)
source
Xe-Xe CF4-CF4 Xe-CF4 Xe-sodium CF4-sodium Xe-oxygen CF4-oxygen
224.48 134.00 173.47 46.22 46.22 84.50 85.00
4.064 4.400 4.232 3.734 4.047 3.375 3.630
ref 37 modified from ref 38 Lorentz-Berthelot ref 45 scaled from ref 45 see text see text
experimental heat of sorption (in silicalite) by about the same amount. We therefore took the simple approach of averaging the two parameter sets to form our siliceous potentials for mordenite. The potential parameters are given in Table 1. For simulations in NaMOR, Lennard-Jones interactions with the sodium atoms were added, as well as a polarization energy due to the electric field produced by the zeolite atoms. Point charges on the lattice atoms were identical to those used by Macedonia et al. (see Table 3 in their paper9), with one exception; a sodium charge of +0.7|e| rather than +1.0|e| was used in accordance with recent electronic structure calculations by Low for sodium mordenite. Low performed periodic density functional theory calculations (k ) 0) with a double numeric basis set, the Perdew-Wang 1992 local density functional, and a “xcoarse” integration grid.39 The DFT calculations were performed with the DMOL3 program.40 A charge of +0.7|e| was also used by Van Tassel et al. for sodium in zeolite A.41 In our GCMC simulations, Al charges were adjusted accordingly to maintain charge balance. The polarization energy for each sorbate molecule is given by -1/2RE2, where R is the molecular polarizibility and E is the electrostatic field at the position of the sorbate molecule. The electric field is calculated from the zeolite atomic charges using an Ewald summation.3 Note that the purely Lennard-Jones siliceous potentials implicitly carry a polarization part because some of the attraction in the real system is the result of polarization effects. Consequently, when the polarization term is added, the contribution from the Lennard-Jones term must be decreased. We chose to do this simply by scaling the sorbate/oxygen Lennard-Jones parameter with an adjustable factor. This adjustable factor was varied to get closer to the experimental isotherms in NaMOR. We also have chosen to decrease the CF4-CF4 σ slightly to improve agreement with the experimental isotherms.42 The polarizabilities were taken to be 4.04 Å3 and 3.838 Å3 for xenon and CF4, respectively.43,44 Xenon-sodium Lennard-Jones parameters were taken from Van Tassel et al.,45 and for the sake of simplicity, the CF4-sodium was taken to be identical to that of the xenon-sodium interactions. The CF4-sodium σ was obtained by scaling the xenon-sodium σ by the ratio of the CF4-CF4 σ to the xenon-xenon σ. The potential parameters for adsorption in NaMOR are given in Table 2. It should be noted that the highest electric fields are found near the cations and this can result in very large polarization energies. It is therefore quite important that the sodium σ values be large enough to avoid unrealistically close approach of sorbate molecules to the cations.
Mixtures in Mordenite Zeolites
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4701
Figure 1. 129Xe NMR spectra of Xe in Na-mordenite, loaded at 25 °C and various pressures.
Results and Discussion 129Xe
NMR results are presented first. This provides a wellestablished characterization of the zeolite samples and a comparison for the CF4 19F NMR results that follow. After showing that CF4 19F NMR can also provide useful information about molecular siting in mordenite, we present siting of Xe/ CF4 mixtures and finally a comparison of the experimental results with molecular simulations. 129Xe NMR of Adsorbed Xe in Zeolites. Variable pressure and variable temperature NMR spectra of Xe in Na-mordenite are plotted in Figures 1 and 2, respectively. Two peaks are present in each spectrum. It is well established that the higher frequency peak is due to side pocket adsorption, while the lower frequency peak is due to main channel adsorption;11 in general, Xe in a more confined environment gives rise to a higher frequency resonance. The chemical shift of the side pocket Xe remains nearly unchanged with varying pressure and temperature because only one Xe atom fits into the side pocket.14 However, the main channel peak in the variable pressure plot, Figure 1, shifts to higher frequency with increasing pressure and begins to merge with the side pocket peak at 1020 Torr and above. This trend has also been reported by Ripmeester.11 In Figure 2, with decreasing temperature the main channel peak again shifts to higher frequency. This shift is opposite to that reported by Ripmeester,11,13 indicating some differences in the mordenite samples. The fraction of adsorbed Xe in the side pockets and main channels as a function of temperature and pressure can be obtained by integrating the two peaks. The results of this analysis are presented below and compared with predictions from molecular simulation. A potential concern in interpreting these results is the possibility of exchange of main channel and side pocket xenon. To address this, Ray29 has done a saturation transfer experiment generated by the Dante pulse sequence to study the residence time of Xe in the side pocket and in the
Figure 2. Variable temperature 129Xe NMR spectra of Xe in Namordenite, loaded at 500 Torr, 25 °C.
main channel. The Dante pulse sequence46 is a train of 90° pulses that selectively inverts the intensity of xenon atoms at a particular frequencysin this case chosen to be Xe in the main channel or side pocket. After the Dante pulse sequence, an array of time delays followed by a 90° pulse is applied to study the time dependence of the signal intensity of the main channel and side pocket peaks. The overall signal intensity is due to a combination of spin-lattice relaxation and hopping of Xe atoms between the main channel and the side pocket.47 The residence times of Xe atoms in the main channel and side pocket were calculated by Ray to be 5.1 ms and 0.9 ms, respectively, at room temperature. This corresponds to a hopping frequency of 1 kHz for Xe in the side pocket. Since the peak separations in the spectra are about 5 or 6 kHz (50 ppm × 110 MHz), the exchange is considered slow on the NMR time scale. Ripmeester13 also claimed that there was a tight fit of Xe in the side pocket and that the two observed peaks could be unambiguously assigned to side pocket and main channel adsorption. The van der Waals diameter of Xe closely matches the available void space of the side pocket, and consequently, Ripmeester proposed that the side pocket was a strong site for Xe. Our recent molecular simulations also support this.6 In brief, the exchange of Xe between the side pocket and main channel is considered slow on the NMR time scale in Na-mordenite. 129Xe NMR studies were also performed in siliceous mordenite, as well as H-, K-, and Ca-mordenite samples prepared from the Na-mordenite sample by ion exchange. The spectra of Xe in H-mordenite at various temperatures are shown in Figure 3. At room temperature only a single peak is seen, but as the temperature is lowered a second peak appears as a broad
4702 J. Phys. Chem. B, Vol. 105, No. 20, 2001
Yang et al.
Figure 4. Variable temperature 129Xe NMR spectra of Xe in siliceous mordenite, loaded at 500 Torr, 25 °C.
Figure 3. Variable temperature 129Xe NMR spectra of Xe in Hmordenite, loaded at 500 Torr, 25 °C.
shoulder. We again attribute the peak at lower frequency to xenon in the main channels and the higher frequency peak to side pocket xenon. Thus the side pockets only fill at the lower temperatures in this H-mordenite sample. Note that the single peak seen at higher temperatures is not due to rapid exchange between the two sites. If that were the case, we should observe coalescence as temperature increases from the lower values. Instead the line width and chemical shift of the main channel peak remain almost constant throughout the temperature range. Exchange in H-mordenite was, however, observed by Ripmeester.10 In his paper, a single room-temperature exchange peak appears in the middle of the two low-temperature peaks. Again, this points to differences between his sample and our Na-mordenite and its ion-exchanged derivatives. For K- and Ca-mordenite our spectra show only single peaks even at -40 °C28 because the cations block the side pocket. Figure 4 shows the 129Xe NMR spectra of xenon in siliceous mordenite at 500 Torr. At -50 °C, two peaks are evident: the broad peak at 205 ppm is due to xenon in the side pocket and the relatively sharp peak at 152 ppm to main channel xenon. At room temperature, xenon atoms in the two different sites are in rapid exchange resulting in coalescence of the two peaks. The chemical shift of the side pocket Xe in dealuminated (siliceous) mordenite is at a lower frequency than in Namordenite, with H-mordenite falling between. A similar phenomenon has been observed previously11 and interpreted to mean that the individual side pockets are larger after dealumination.
Figure 5. 19F NMR spectra of CF4 in siliceous mordenite, loaded at 500 Torr, 25 °C. Zeolite sample was loosely packed in (a) and tightly packed in (b). Spectrum (c) shows the gas phase in equilibrium with (b). Spectrum (d) is pure CF4 gas (no zeolite) at 500 Torr. 19F NMR of Adsorbed CF in Zeolites. The results in this 4 section show that 19F NMR of adsorbed CF4 is also sensitive to the different sites in mordenite. Figure 5 presents the 19F NMR spectra of CF4 in siliceous mordenite at 500 Torr and 25 °C. The spectrum shown in Figure 5a was acquired for a loosely
Mixtures in Mordenite Zeolites
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4703
TABLE 3: NMR Results for CF4 in Mordenite and Faujasite Samples at 500 Torr and 25 °Ca chemical shift ((0.1 ppm)
line width at 11.7 T (Hz, (5%)
T1 at 7.0 T (ms, (10%)
sample
side
main
gas
side
main
gas
side
main
silic-MOR loosely packed silic-MOR packed H-MOR Na-MOR K-MOR Ca-MOR Na-Y K-Y Ca-Y CF4 gas
-54.5 -56.5
-64.5 -64.6 -63.7 -62.9 -63.0 -62.4 -64.7 -64.0 -64.3 n/a
-66.9 -66.9 (inverted)
3490 3070
325 286 1072 2930 2670 3660 84.2 212 197 n/a
245 120 (inverted)
890
20 24 17 21 55 34 62 35 n/a
n/a n/a n/a n/a
-66.9 -66.6
-67.0
n/a n/a n/a n/a
280
85.1
n/a n/a n/a n/a
gas 3.8 (inverted) 8.0
5.0
a
A blank entry indicates that no peak was seen, “-” indicates that the measurement was not performed, and n/a indicates that the entry does not apply to that system, e.g., Y zeolites have only large supercages with no side pockets.
packed zeolite sample, which leaves some gas-phase volume between the zeolite crystals. The spectrum shown in Figure 5b was acquired for a sample that was tightly packed to minimize the gas-phase volume in the detection region of the NMR spectrometer. Figure 5a shows a total of three peaks. By analogy to results above and in the literature10-19 for xenon in mordenite, the peak around -55 ppm can be inferred to be due to CF4 in a tight environment, namely, the side pocket. The remaining two peaks are assigned to main channel CF4 (-64.5 ppm) and gas-phase CF4 (-66.9 ppm). The gas-phase assignment agrees with the pure gas-phase sample at 500 Torr and 25 °C (no zeolite) shown in Figure 5d. Another way to see the CF4 gas peak is to invert the packed sample tube so that the probe coil detects only the gas phase that is in equilibrium with the zeolite. The spectrum of the inverted sample is shown in Figure 5c. The chemical shifts, line widths, and T1 values for these and other samples are collected in Table 3. For each property in the table there are three columns. The column on the left is CF4 in the side pocket, the column in the middle is CF4 in the main channel, and the column on the right is CF4 in the gas phase. The peak assignments based on the chemical shifts are supported by the T1 longitudinal relaxation values, which show an increasing trend as the molecular environment becomes more restricted.48 The line width of the gas peak in the loosely packed sample is larger than that of the pure gas sample (245 Hz vs 85 Hz, see Table 3). The reason for this is suspected to be exchange of CF4 between the main channel and the gas phase. The exchange phenomenon can be severe in the presence of mesopores, which may be created during steam dealumination of mordenite. Mesopores allow more rapid exchange between molecules in the main channels and the gas phase by creating more surface contact between the microporous regions and the outside gas. The fact that the spectrum in Figure 5b of the packed sample does not show a gas peak while the spectrum in Figure 5a of the loosely packed sample does show a gas peak is fair evidence that tightly packing the sample minimizes the amount of free gas to the point that it cannot be detected. Mordenite samples with various exchanged cations were examined to investigate the sensitivity of 19F NMR of CF4 to such differences and to shed further light on the exchange phenomenon. Figure 6 shows the 19F NMR spectra of CF4 in Ca-, K-, Na-, and H-mordenite loaded at 500 Torr and 25 °C; the spectrum of a pure CF4 gas sample prepared at the same conditions is included for comparison. Several observations can be made about Figure 6 and the corresponding rows in Table 3. First, the lines corresponding to CF4 in the main channel are generally broader in the cation-exchanged samples than in the dealuminated mordenite of Figure 5. Second, peaks correspond-
Figure 6. 19F NMR spectra of CF4 in Ca-, K-, Na-, and H-mordenite at 500 Torr, 25 °C. The spectrum of gas-phase CF4 at these conditions is also shown.
ing to the gas phase appear only for Ca-MOR and K-MOR, and these peaks are rather broad, presumably from exchange with adsorbed CF4. Whether or not this peak appears seems to be related to inevitable differences in sample packing within the NMR tube, as well as other possible factors. Finally, no side pocket peak around -55 ppm is seen in any of the spectra for these samples prepared from Na-mordenite. This inaccessibility of the side pocket by CF4 in Na-mordenite agrees with the previous result obtained from T1 measurements by Nakamura and co-workers.27 For comparison, we also recorded spectra of CF4 in Ca-, K-, and Na-faujasite (zeolite Y). We realized that cations could affect spectra for two reasons: (1) direct interaction of the
4704 J. Phys. Chem. B, Vol. 105, No. 20, 2001
Yang et al.
Figure 7. Variable temperature 19F NMR spectra of CF4 in siliceous mordenite, loaded at 500 Torr, 25 °C.
adsorbed molecules with the cations and (2) blocking effects where the cations restrict movement of adsorbed molecules such as blocking of the side pockets. The latter should not play a role in faujasite, which has a very open pore structure, so these spectra allow us to see the effect of the former. Results are given in Table 3. The chemical shifts varied by only a few tenths of a ppm, and only single sharp peaks were observed. 19F NMR of CF4 is thus apparently not very sensitive to differences in cations arising from direct molecule/cation interactions. Figure 7 is a stacked plot of CF4 in siliceous mordenite at 100, 25, and -50 °C. The sample was loaded at 500 Torr and 25 °C. Because the sample was packed to minimize free space, no gas-phase peak was observed. As the temperature is increased, the fraction of CF4 in the side pocket decreases. Figure 7 also shows that the chemical shift of the main channel peak decreases as the temperature rises. This is because CF4 desorbs as temperature increases and the contribution to the chemical shift due to CF4-CF4 interaction in the adsorbed phase becomes smaller. Comparing Figures 7 and 4, it is clear that the relative amount adsorbed in the side pockets is much larger for xenon than CF4. NMR of Xe-CF4 Binary Mixtures in Zeolites. The study of binary mixtures of Xe-CF4 in mordenite was one of our primary goals due to the importance of multicomponent mixtures in adsorption processes. Figure 8 shows a stacked plot of 129Xe NMR of Xe-CF4 binary mixtures in Na-mordenite. The samples were loaded at 760 Torr, 25 °C with the composition of the gas phase varying from 100% Xe to 10% Xe. The 129Xe spectra in Figure 8 show that the intensity of the main channel peak decreases as the concentration of CF4 increases, and it essentially disappears in the top two spectra, meaning that very few xenon atoms are in the main channels under these conditions. The 20% and 10% Xe spectra show only a single broad peak, which is generally in the region of the side pocket but slightly shifted toward the main channel peak found in samples of higher Xe concentrations. The shifting of the peak was suspected to
Figure 8. 129Xe NMR spectra of Xe-CF4 mixtures in Na-mordenite, loaded at 760 Torr, 25 °C. The gas-phase composition varied from 100% Xe to 10% Xe.
be due to the Xe-CF4 interaction. To study the chemical shift of Xe as a result of Xe-CF4 interactions, faujasite samples were prepared with pure Xe and a 50% Xe, 50% CF4 mixture. The zeolite faujasite was chosen because adsorbed Xe has a short spin-lattice relaxation T1 and faujasite has a large pore structure, which allows for many CF4-Xe interactions. We found that the chemical shift of the 100% Xe in faujasite sample was 106 ppm, and the chemical shift of the 50% Xe, 50% CF4 in faujasite sample decreased to 74 ppm. Because the Xe-CF4 interaction causes a decrease in Xe chemical shift, we are convinced that the broad peak observed in the 20% and 10% Xe in Namordenite samples in Figure 8 is due to Xe in the side pocket; the decrease in chemical shift is due to interactions with CF4, probably at the entrance of the side pocket. These changes in chemical shift with composition are consistent with gas phase work of Jameson et al.49 Figure 9 shows the analogous 19F spectra for the mixture samples of Figure 8, with the composition of the gas phase varying from 100% CF4 to 15% CF4. These spectra are more difficult to interpret. For example, the pure CF4 sample shows three peaks, but it is clear that none of them is due to side pocket adsorption, which would produce a peak around -55 ppm. The low-frequency peaks seen in many of the spectra may be attributed to gas-phase CF4, although evidently with some exchange causing line broadening. As discussed above, the presence of this peak seems to be dependent on the packing density within the NMR tube. The high-frequency peaks should be due to CF4 in the mordenite main channels according to our assignments above. What then causes the middle peak in the pure CF4 sample and the unusual broadness in the 80% and 90% CF4 samples? (This question is also related to finding the cause of the broadness in the Ca- and K-exchanged mordenite samples of Figure 6.)
Mixtures in Mordenite Zeolites
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4705
Figure 10. 19F NMR spectra of CF4 in Na-mordenite, loaded at 760 Torr, 25 °C. Results are shown for zeolite samples that were packed into the NMR tubes, zeolite not packed in the tubes, and zeolite pressed into granules.
Figure 9. 19F NMR spectra of Xe-CF4 mixtures in Na-mordenite, loaded at 760 Torr, 25 °C. The gas-phase composition varied from 100% CF4 to 15% CF4.
The right and left peaks are already assigned to molecules that spend most of their time in the zeolite or in the gas phase, respectively, on the NMR time scale. The middle peak, we then reasoned, is due to molecules that spend time in both the zeolite and the gas during the NMR experiment, e.g., molecules initially located near the zeolite/gas boundary.50 To test this hypothesis, we pressed some sodium mordenite into “granules,” which should reduce molecular transport between the zeolite boundaries and the gas phase. The granules were made by pressing powdered zeolite into wafers, which were then crushed and sieved to 8-10 mesh particles. The resulting CF4 spectrum is shown at the bottom of Figure 10. In accordance with our hypothesis, the middle peak disappears. Spectra are also shown in Figure 10 with the zeolite powder tightly packed into the NMR sample tube and with it just loosely placed in the tube. These two spectra both show again the “middle” peak, although the free gas peak is not seen in the top spectrum and is only a small shoulder in the unpacked sample. The peak for main channel CF4 in the 19F spectra of Figure 9 shifts to higher frequency as the xenon mole fraction increases. In Figure 8, the main channel xenon peak in the 129Xe spectra shifts to lower frequency with higher CF4 composition. These trends in Figures 8 and 9 can be explained by the replacement of Xe-Xe (in Figure 8) and CF4-CF4 (in Figure 9) interactions, respectively, with CF4-Xe interactions in the main channel of mordenite. The effect of intermolecular interactions on NMR
Figure 11. Comparison of experimental27 and simulated CF4 isotherms in Na-mordenite at 25 °C. The simulated isotherm in siliceous mordenite is also shown (open symbols).
chemical shifts was previously reported,20,49 and the trends here are consistent with that work. Comparison of NMR and Molecular Simulation. Before comparing siting results from simulation and NMR, we wish to know how well the simulations can predict the adsorption isotherms. Figures 11 and 12 compare experimental Na-MOR isotherms from the literature14,27 with those predicted from simulations in both siliceous mordenite and Na-mordenite. For xenon, the siliceous mordenite model, with no adjustment of parameters, agrees reasonably well with the Na-mordenite experimental isotherm. We made minor adjustments of the siliceous CF4 model to match the overall CF4 isotherm; the overestimated loading at higher pressures is consistent with a reduction in pore volume caused by the sodium ions in the experimental sample. The Na-MOR model predictions are better for CF4 but worse for Xe, where they consistently underestimate the loading in the saturation regime. This is the best agreement with experiment that can be achieved by adjusting only the
4706 J. Phys. Chem. B, Vol. 105, No. 20, 2001
Yang et al.
Figure 12. Comparison of experimental14 and simulated Xe isotherms in Na-mordenite at 25 °C. The simulated isotherm in siliceous mordenite is also shown (open symbols).
Figure 14. Temperature dependence of Xe siting in Na-mordenite at 500 Torr. Experimental results from Ripmeester13 are shown for comparison. Simulated results in siliceous mordenite are also displayed (open symbols).
Figure 13. Temperature dependence of CF4 siting in siliceous mordenite at 500 Torr. Experimental results are shown from both 400 and 500 MHz spectrometers. Simulated results in Na-mordenite are also shown for comparison (open symbols).
Figure 15. Pressure dependence of Xe siting in Na-mordenite at 25 °C. Simulated results in siliceous mordenite are also displayed (open symbols).
energy scaling () term in the Lennard-Jones potential, but adjustment of other parameters has not been fully explored. Generally, it seems that addition of the cations has a tendency to increase adsorption in the Henry’s law region and decrease the saturation loading. As discussed below, further improvement of the model would be worthwhile, particularly given molecularlevel NMR results, which could be used to test predictions of a model that matches the macroscopic isotherm data better. Integrating the side pocket peaks of the spectra in Figure 7, along with other unpublished spectra, yields the data shown in Figure 13 for the fraction of adsorbed CF4 in the side pocket as a function of temperature. Data obtained on 400 and 500 MHz spectrometers yield the same results within the uncertainty of the measurements. Figure 14 presents similar results for xenon in the side pockets as a function of temperature, and Figure 15 shows the variation with pressure. Quantitative evaluation of siting based on peak integration is susceptible to errors, especially in 129Xe spectra, due to the small signal-to-noise ratio, as well as an imperfect baseline typical when using very large spectral widths. The accuracy of the integration is estimated to be (5%. Ripmeester’s results are shown for comparison in Figure 14. While his data show a decreasing loading of xenon in the side pockets with increasing temperature, our results show little temperature dependence. There is little information avail-
able about Ripmeester’s Na-mordenite sample, however, and data presented above already indicated differences between his sample and ours. To verify the results, our variable temperature experiments have been repeated several times with the temperatures chosen in random order. Figures 13 through 15 also show simulation results in siliceous and sodium mordenite for comparison with experiment. In Figure 13, both simulation results capture the qualitative trend. The agreement with experiment for the NaMOR simulation is much better, however, even though the experiments used dealuminated mordenite; this must be viewed as somewhat fortuitous. For xenon, on the other hand (Figures 14 and 15), the siliceous model predicts qualitatively incorrect trends under both temperature and pressure variations; the NaMOR model agrees better with the experiments, which also used sodium mordenite. The improvements can be explained by noting that, relative to the siliceous model, the polarization term makes the 12-ring pores more favorable adsorption regions. In particular, it is largely the presence of the cations in the 12-ring channels that makes the polarization energy more favorable in that site. Figures 14 and 15 indicate that the addition of cations and an explicit polarization term in the potential model substantially improve the agreement of the simulations with the NMR siting results for NaMOR. However, it should be noted that no attempt was made to adjust the Lennard-Jones parameters for the
Mixtures in Mordenite Zeolites
Figure 16. Xe siting in Na-mordenite for Xe-CF4 mixtures at 760 Torr, 25 °C from NMR and simulation. The gas phase was a 50/50 mixture of Xe and CF4. Simulated results in siliceous mordenite are also displayed (open symbols).
interaction between xenon and the zeolite oxygen atoms of the siliceous model. Xenon siting results for binary xenon-CF4 mixtures are shown in Figure 16 as a function of gas-phase composition. As shown in Figure 8, the NMR results indicate that xenon is displaced from the 12-ring pore progressively as the CF4 mole fraction increases. From experiment, xenon occupies only the side pockets of the structure above 0.8 gas-phase mole fraction of CF4. The siliceous and NaMOR simulation models both show the same qualitative trend as the experiments. Correct prediction of the binary siting trends indicates that the relative interactions of CF4 and xenon with the zeolite are reasonable. Additionally, this agreement verifies previous predictions6 that the presence of one species can significantly affect the siting of another species. Here, the displacement of the xenon out of the main channel by the CF4 is an example of competitiVe adsorption in a binary system.6 Both species like to occupy the main channels, but the more strongly adsorbing species “pushes” the other out. Thus, the possibility of significant segregation in binary adsorption systems has been verified experimentally. Future efforts should seek to improve the agreement between experiment and simulation. First, it would be useful to have experimental adsorption isotherms for the same samples where the siting is determined by NMR. The single component isotherms used here to calibrate the potential models are over two different samples, while the NMR measurements were done on a third sample of NaMOR. It can be seen in Figure 14, by comparing our results with Ripmeester’s, that different samples may give significantly different results. Better control of the pressures within the NMR sample tubes could be achieved by another loading procedure. The potential model could also be improved upon by allowing the zeolite atoms and cations to move, by better treatment of the electrostatics, and by allowing for charge fluctuations. Conclusions 19F
and 129Xe NMR have been used to study adsorption of CF4 and Xe in mordenite zeolites. While past 129Xe NMR work on siting in mordenite has concentrated on single-component adsorption, we have here extended such studies to binary adsorption. This work has also demonstrated that 19F NMR of CF4 can be used to characterize zeolite micropores in a manner similar to 129Xe NMR but without the difficulties associated with long 129Xe spin-lattice relaxation times. Sometimes,
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4707 however, the 19F spectra are more difficult to interpret than 129Xe spectra, and exchange between adsorbed and gas-phase molecules complicates the observed spectra. In siliceous mordenite where no zeolite cations are present, CF4 and Xe can access both the main channels and the small side pockets, with the main channel being the preferred site for CF4. The fraction of CF4 molecules in the side pockets decreases with increasing temperature. In Na-mordenite no clear peak was observed for side pocket adsorption of CF4. The fraction of Xe in the side pockets of Na-mordenite is relatively constant as temperature is varied, and at room temperature Xe siting does not vary much over a pressure range of 30 to 200 kPa. For mixtures of Xe and CF4 in Na-mordenite, Xe adsorbs preferentially in the side pocket, especially at high concentrations of CF4. The NMR measurements provide a unique opportunity to compare GCMC simulations with experimental data for molecular-level siting and segregation in addition to the more common comparisons with macroscopic quantities such as the adsorption isotherms and heats of adsorption. A comparison was made with experiment for a purely siliceous model and a model that included cations and explicit polarization. Predicting the siting and isotherm simultaneously provides a stringent test of a potential model and may be helpful in further development of potential models for adsorption in zeolites. Acknowledgment. We thank Edward Maginn and Michael Olken for the siliceous mordenite sample and Cynthia Jameson and John Low for helpful discussions. Michael Macedonia and Edward Maginn are thanked for supplying the mordenite coordinates used in their simulations. Finanical support from the National Science Foundation CAREER program is gratefully acknowledged. References and Notes (1) van Bekkum, H., Flanigen, E. M., Jansen, J. C., Eds. Introduction to Zeolite Science and Practice; Elsevier: Amsterdam, 1991. (2) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley: New York, 1984. (3) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Clarendon Press: Oxford, 1987. (4) Snurr, R. Q.; Bell, A. T.; Theodorou, D. N. J. Phys. Chem. 1993, 97, 13742-13752. (5) Meier, W. M.; Olson, D. H. Atlas of Zeolite Structure Types; Butterworth-Heinemann: London, 1992. (6) Clark, L. A.; Gupta, A.; Snurr, R. Q. J. Phys. Chem. B 1998, 102, 6720-6731. (7) Mortier, W. J.; Pluth, J. J.; Smith, J. V. Mater. Res. Bull. 1975, 10, 1319-1326. (8) Mortier, W. J. Compilation of Extraframework Sites in Zeolites; Butterworth Scientific Ltd.: Guildford, Surrey, UK, 1982. (9) Macedonia, M. D.; Moore, D. D.; Maginn, E. J. Langmuir 2000, 16, 3823-3834. (10) Ripmeester, J. A. J. Am. Chem. Soc. 1982, 104, 289-290. (11) Ripmeester, J. A. J. Magn. Reson. 1984, 56, 247-253. (12) Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A. J. Am. Chem. Soc. 1998, 120, 3123-3132. (13) Ripmeester, J. A.; Ratcliffe, C. I. J. Phys. Chem. 1990, 94, 76527656. (14) Springuel-Huet, M. A.; Fraissard, J. P. Zeolites 1992, 12, 841845. (15) Ito, T.; De Menorval, L. C.; Guerrier, E.; Fraissard, J. P. Chem. Phys. Lett. 1984, 111, 271-274. (16) Fraissard, J.; Ito, T.; Springuel-Huet, M.; Demarquay, J. Stud. Surf. Sci. Catal. 1986, 28, 393-400. (17) Fraissard, J.; Ito, T. Zeolites 1988, 8, 350-361. (18) Springuel-Huet, M. A.; Fraissard, J. Chem. Phys. Lett. 1989, 154, 299-302. (19) Chen, Q.; Springuel-Huet, M. A.; Fraissard, J.; Smith, M. L.; Corbin, D. R.; Dybowski, C. J. J. Phys. Chem. 1992, 96, 10914-10917. (20) Jameson, A. K.; Jameson, C. J.; Gutowsky, H. S. J. Chem. Phys. 1970, 53, 2310-2321.
4708 J. Phys. Chem. B, Vol. 105, No. 20, 2001 (21) van Well, W. J. M.; Cottin, X.; de Haan, J. W.; Smit, B.; Nivarthy, G.; Lercher, J. A.; van Hooff, J. H. C.; van Santen, R. A. J. Phys. Chem. B 1998, 102, 3945-3951. (22) Snurr, R. Q.; Ka¨rger, J. J. Phys. Chem. B 1997, 101, 6469-6473. (23) Snurr, R. Q.; Hagen, A.; Ernst, H.; Schwarz, H. B.; Ernst, S.; Weitkamp, J.; Ka¨rger, J. J. Catal. 1996, 163, 130-137. (24) Nakamura, N.; Nakayama, H.; Eguchi, T.; Xu, Q. J. Chem. Soc., Faraday Trans. 1995, 91, 2949-2953. (25) Nakamura, N.; Nakayama, H.; Eguchi, T.; Xu, Q. J. Chem. Soc., Faraday Trans. 1996, 92, 1039-1042. (26) Nakamura, N.; Nakayama, H.; Eguchi, T.; Xu, Q. J. Chem. Soc., Faraday Trans. 1996, 94, 4601-4603. (27) Asanuma, T.; Nakayama, H.; Eguchi, T.; Nakamura, N. J. Chem. Soc., Faraday Trans. 1998, 94, 3521-3526. (28) Yang, J.-H. “19F and 129Xe NMR Studies of Molecular Siting in Zeolites.” M.S. Thesis, Northwestern University, 1999. (29) Ray, G. J. Unpublished results. (30) Hesse, M.; Meier, H.; Zech, B. Spektroskopische Methoden in der organischen Chemie; Thieme: Stuttgart, 1987. (31) Broadbelt, L. J.; Snurr, R. Q. Appl. Catal. A 2000, 200, 23-46. (32) Alberti, A.; Davoli, P.; Vezzalini, G. Z. Kristallogr. 1986, 175, 249. (33) Schlenker, J. L.; Pluth, J. J.; Smith, J. V. Mater. Res. Bull. 1979, 14, 751-758. (34) Takaishi, T.; Kato, M.; Itabashi, K. Zeolites 1995, 15, 21-32. (35) June, R. L.; Bell, A. T.; Theodorou, D. N. J. Phys. Chem. 1990, 94, 8232-8240. (36) June, R. L.; Bell, A. T.; Theodorou, D. N. J. Phys. Chem. 1991, 95, 8866-8878.
Yang et al. (37) Pickett, S. D.; Nowak, A. K.; Thomas, J. M.; Peterson, B. K.; Swift, J. F. P.; Cheetham, A. K.; den Ouden, C. J. J.; Smit, B.; Post, M. F. M. J. Phys. Chem. 1990, 94, 1233-1236. (38) Heuchel, M.; Snurr, R. Q.; Buss, E. Langmuir 1997, 13, 67956804. (39) Low, J. J. Unpublished results. (40) Delley, B. J. Chem. Phys. 1990, 92, 508-517. (41) Van Tassel, P. R.; Davis, H. T.; McCormick, A. V. J. Chem. Phys. 1993, 98, 8919-8928. (42) Decreasing sigma for CF4 is in line with the underprediction of the saturation loading by GCMC in silicalite,38 as well as the low diffusivities predicted by MD.22 (43) Miller, T. M.; Bederson, B. In AdVances in Atomic and Molecular Physics; Bates, D. R., Bederson, B., Eds.; Academic Press: New York, 1977; Vol. 13, pp 1-55. (44) Bose, T. K.; Sochanski, J. S.; Cole, R. H. J. Chem. Phys. 1972, 57, 3592-3595. (45) Van Tassel, P. R.; Davis, H. T.; McCormick, A. V. Mol. Phys. 1991, 73, 1107-1125. (46) Morris, G. A.; Freeman, R. J. Magn. Reson. 1978, 29, 433. (47) McConnel, H. M. J. Chem. Phys. 1958, 28, 430. (48) The T1 value of 5.0 ms for pure CF4 gas is higher than the literature value [Armstrong, R. L; Tward, E. J. Chem. Phys. 1968, 48, 332-334.], again indicating that the actual pressures in our sealed glass sample tubes are higher than the nominal loading pressures. (49) Jameson, C. J.; Jameson, A. K.; Cohen, S. M. J. Chem. Phys. 1976, 65, 3401-3406. (50) Jameson, C. J.; Jameson, A. K.; Gerald, R. E., II; Lim, H.-M. J. Phys. Chem. B 1997, 101, 8418-8437.
