Study of the Adsorption of Toluene in Zeolite LiNaâ Y by Solid-State

J. Phys. Chem. C 2007, 111, 13427-13436

13427

Study of the Adsorption of Toluene in Zeolite LiNa-Y by Solid-State NMR Spectroscopy Jianfeng Zhu,† Nick Mosey,† Tom Woo,‡ and Yining Huang*,† Department of Chemistry, UniVersity of Western Ontario, 1151 Richmond Street, London, Ontario, Canada N6A 5B7, and Department of Chemistry, UniVersity of Ottawa, D’lorio Hall, Ottawa, Ontario, Canada K1N 6N5 ReceiVed: January 25, 2007; In Final Form: June 15, 2007

The adsorption of toluene inside partially Li+-exchanged zeolite Y (LiNa-Y) was examined by solid-state NMR spectroscopy. The environments of Li+ and Na+ ions at different sites in the framework before and after adsorption were characterized by 7Li and 23Na magic-angle-spinning (MAS) NMR. The information on the dynamic behavior of guest molecules inside the supercage of the zeolite was obtained from wide-line 2H NMR spectra. The cation-sorbate interactions were directly probed by heteronuclear dipolar coupling based double-resonance experiments such as 7Li{1H} and 23Na{1H} rotational-echo double-resonance (REDOR) experiments. Molecular modeling was also performed to assist in interpreting NMR results. The 7Li and 23Na MAS results indicate that during ion exchange the incoming Li+ ions fill the SI′ sites first and then the SII sites, and the remaining Na+ ions stay mostly at SII sites. The MAS and REDOR results show that the toluene molecules are facially coordinated to the cations at SII sites in the supercage to form a π-complex, resulting in a significant change in chemical shift of the Li+ ions and a dramatic reduction in the quadrupolar coupling constant (QCC) of the Na+ ions. The 2H NMR spectra indicate that at room temperature some toluene molecules reorient around the molecular long axis (two-site flip) and others undergo multiple-site jump motion, but at low temperature only two-site flip, at a much slower rate, remains. Both 2H and REDOR results show that the molecular dynamics of the guest species is also affected by the cation composition.

Introduction Among the hundreds of zeolites known to date, zeolites with faujasite (FAU) structure including zeolite Y (Si/Al > 1.5) and zeolite X (Si/Al e 1.5) are of great interest due to their industrial importance. The structure of faujasite zeolite is shown in Figure 1. It contains so-called sodalite cages. A sodalite cage is a truncated octahedron containing both four- and six-membered rings built from corner-sharing SiO44- and AlO45- tetrahedra. The sodalite cages are connected through a hexagonal prism or double six-membered ring (D6R) to form the supercage (sc). Because of the charge difference between Al and Si, each AlO4 tetrahedron in the framework carries a negative charge which has to be balanced by a nonframework cation. The chargecompensating cations can take five different positions in the framework (Figure 1): SI in the hexagonal prism; SI′ in the sodalite cage facing the six-membered ring of the hexagonal prism; SII in the supercage facing the six-membered ring of the sodalite cage; SIII and SIII′ in the supercage facing the fourmembered ring. Because of the close proximity of SI′ sites to SI sites, the two positions may not be occupied simultaneously. The cations play an important role in determining the adsorption, separation, and catalytic properties of the zeolites. For example, most alkali metal ion-exchanged FAU zeolites can catalyze the side-chain alkylation of aromatic compounds,1 while the acidic forms can be used to selectively catalyze ring alkylation.2 The mechanism of the side-chain alkylation of toluene with methanol over basic FAU zeolites has been widely * Corresponding author. Telephone: (519) 661-2111, ext 86384. Fax: (519) 661-3022. E-mail: [email protected]. † University of Western Ontario. ‡ University of Ottawa.

Figure 1. Schematic drawing of the faujasite structure with extraframework cation sites.

studied using NMR,3-5 IR,6-8 and other9,10 methods such as quantum chemical calculation. It is found that the selectivity of the catalysts for alkylation reactions is dependent on the number and nature of the cation in the zeolites. For example, the Li-FAU catalyzes only ring alkylation and the K-, Rb-, and Cs-exchanged FAU zeolites selectively catalyze side-chain alkylation, whereas the Na form can catalyze both reactions.6,9,10 FAU zeolites can also be utilized to separate C8 aromatics (xylene isomers and ethylbenzene).1 It is found that Na-Y prefers to adsorb m-xylene, K-Y to adsorb p-xylene, and Rb-X and Cs-X to adsorb ethylbenzene. To better understand the above-mentioned processes, detailed information on the locations of the different cations, the adsorption sites of the sorbates, and the interactions between cations and aromatic molecules is desired. A large number of studies11-24 have been performed on the benzene adsorption (a model system) inFAU zeolites.Therearealsostudies3,4,6,10,14,15,25-27 of toluene adsorption in FAU zeolites. Early IR work,27 NMR5

10.1021/jp0706275 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/22/2007

13428 J. Phys. Chem. C, Vol. 111, No. 36, 2007 work, and a recent simulation study25 indicated that each toluene is coordinated to a cation in the supercage with its phenyl ring parallel to the surface of the supercage.25,27 Other studies mainly focused on the reaction mechanism of the side-chain alkylation of toluene with methanol. 13C magic-angle-spinning (MAS) NMR was applied to follow the alkylation process.3,4 Despite these excellent studies, the understanding of the interactions of toluene molecules with different charge-balancing cations in FAU zeolites is still incomplete. In the present study, we have investigated the adsorptive behavior of toluene in partially Li+-exchanged zeolite Y, and for comparison the adsorption in zeolite Na-Y was also examined. Although Li+ ion has a relatively large neutron scattering cross section, determination of the Li+ sites by neutron diffraction is still difficult due to the mobility of the Li+ ions and low occupancy of certain sites.28 Solid-state NMR spectroscopy is the method of choice for our study since both 7Li and 23Na have relatively high resonance frequency (155 and 105 MHz at a field strength of 9.4 T, respectively) and high natural abundance (92.58 and 100%, respectively). Li has two NMR active isotopes, 6Li (I ) 1) and 7Li (I ) 3/2). 7Li has a much higher natural abundance (92.58 vs 7.42% for 6Li) and also a higher resonance frequency (155.5 vs 58.9 MHz for 6Li at 9.4 T). Both 6Li and 7Li nuclei possess a rather small nuclear quadrupole moment (Q), with the Q for 6Li being 50 times smaller than that for 7Li.29 A disadvantage of Li NMR is its very small chemical shift range. Several 7Li NMR studies28,30-33 were performed on dehydrated Li-X. Three resonance signals were observed in the 7Li MAS NMR spectra and assigned to Li+ ions at sites SI′ (δiso ) 0.4 ppm), SII (δiso ) -0.3 ppm), and SIII (δiso ) -0.7 ppm). The assignments are in agreement with previous X-ray34 and neutron35 diffraction results. Blasco and co-workers36,37 studied dehydrated LiNa-Y and found that the incoming Li+ ions tend to occupy the SI′ and SII sites and that, upon the adsorption of pyrrole molecules, a high field shift was observed for the signal originating from Li+ ions at the SII site. In the present work, 7Li and 23Na MAS spectra were acquired to obtain information on the local environment of lithium and sodium ions in the framework. 7Li{1H} and 23Na{1H} rotationalecho double-resonance (REDOR) techniques were applied to directly probe the cation-sorbate interactions. Molecular dynamics of the guest species inside the zeolite cavity were examined by analyzing the line shape of static 2H NMR spectra. Monte Carlo simulations were also performed to assist in interpreting the NMR results. Experimental Section Materials. Zeolite Na-Y (Si/Al ) 2.35) was purchased from Strem Chemicals. Its identity and crystallinity were checked by powder X-ray diffraction. Toluene methyl-d3 (98%) and ringd5 (98%) were obtained from Cambridge Isotope Laboratories. All the aromatic compounds were dried by dehydrated 3A molecular sieves (Caledon) before use. The low Li content sample was prepared by stirring Na-Y in 1 M LiCl solutions at 363 K for 48 h once. The high Li content sample was made by exchanging with 1 M LiCl for 24 h first and then with 6 M LiCl for 24 h (three times) at 363 K.28 The exchanged samples were recovered by filtering and washed with about 1000 mL of distilled water, and dried in air at ambient temperature. Na contents in the exchanged samples were determined by X-ray fluorescence (XRF) analysis, and Li contents were then calculated. The compositions of the unit cell are Li33Na24Y and Li45Na12Y for low- and high-Li-content samples, respectively.

Zhu et al. Sample Preparation. The zeolites were dehydrated at 673 K for 48 h under dynamic vacuum, and proton NMR spectra (not shown) indicated that water was completely removed. The procedure of toluene adsorption was the following: first, the dehydrated zeolites were accurately weighted and transferred into glass tubes in a dry glovebox, and then precisely measured volumes of toluene were added to the zeolite powder. The tubes were then sealed and placed in an oven at 373 K overnight, allowing the sorbate molecules to disperse uniformly throughout the sample. The maximum loading of toluene in our LiNa-Y zeolites is 3.4 molecules/supercage determined by thermogravimetric analysis (TGA). Therefore, we carried out both experiments and simulations at 3 molecules/supercage (a loading close to the maximum but not overloaded). The toluene-loaded sample was packed into an NMR rotor in a glovebox under N2 atmosphere and was also under N2 atmosphere during all NMR experiments. NMR Measurement. All NMR experiments were performed on a Varian Chemagnetics Infinityplus 400 WB spectrometer with three rf channels. The resonance frequencies were 399.9, 61.4, 155.3, and 105.8 MHz at the field strength of 9.4 T for 1H, 2H, 7Li, and 23Na, respectively. The shifts of 1H, 7Li, and 23Na were referenced to TMS, 0.1 M LiCl, and 1 M NaCl aqueous solution, respectively. The 1H and 23Na MAS NMR spectra of dehydrated zeolite Na-Y were obtained by using a 3.2-mm double-tuned MAS probe with a spinning speed in the range of 15-21 kHz. A 2 µs (π/4) pulse and 1 µs (π/8) pulse were used for 1H and 23Na excitation, respectively. The recycle delay was 2 s for both nuclei. A 7.5-mm triple-tuned T3 MAS probe was used for tolueneloaded Na-Y samples. A 1.5 µs (π/8) pulse and recycle delays of 2 and 0.5 s were used for 1H and 23Na MAS experiments, respectively. The 23Na{1H} REDOR experiments were conducted using the standard REDOR pulse sequence38 and the 23Na and 1H π-pulse lengths were both 12 µs. The samples were spun at 6 kHz ( 2 Hz. The low-temperature experiments were performed at 153 K. The toluene-loaded LiNa-Y samples were examined by using a 4.0-mm triple-tuned probe. A 1 µs (π/4) pulse and recycle delays of 2, 5, and 0.5 s were used for 1H, 7Li, and 23Na MAS experiments, respectively. π pulses of 8, 8.3, and 9.5 µs were used for 1H, 7Li, and 23Na in the 7Li{1H} and 23Na{1H} REDOR experiments. The samples were spun at 10 kHz ( 5 Hz. The low-temperature experiments were carried out at 163 K. The 2H powder patterns of the static samples were acquired by using a 5-mm horizontal wide-line probe. The spectra were obtained with a quadrupole-echo pulse sequence.39 A π/2 pulse width of 3.0 µs and a spin-echo delay of 20 µs were used. The echo was collected prior to the echo maximum and shifted to ensure that the signal used in Fourier transformation began exactly at the echo maximum. Simulations of MAS spectra were carried out with the WSOLIDS software package provided by Prof. R. E. Wasylishen (University of Alberta). Fitting REDOR curves was performed using the SIMPSON (version 1.1.0) package40 on an LG computer with 1.2 GHz Pentium 4 processors running Red Hat Linux. The evolution of the density matrix under timedependent internal Hamiltonians is calculated for incrementally small steps, during which the Hamiltonian is considered timeindependent. The maximum time step (max dt) was set to 1 µs for our simulations. Simulations were carried out based on the standard REDOR pulse sequence.38 The start and detect operators were set to I1x and I1p, respectively. Iterative fitting of the

Adsorption of Toluene in Zeolite LiNa-Y

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13429 rates, rf fields, and spectrum width, were set equal to those employed in experiments.

Figure 2. 23Na MAS NMR spectra. (A) Dehydrated Na-Y: observed (top) and simulated (bottom) spectra. (B) Toluene/Na-Y complex at a loading of 3 molecules/sc: observed (top) and simulated (bottom) spectra. / denotes spinning sidebands.

experimental REDOR data was carried out until convergence to obtain dipolar coupling constants. For the two-spin NaLi system, a powder averaging scheme using 18 γ angles and the rep320 crystal file was chosen. Due to the very large size of the density matrix resulting from ISn (n > 1) multispin systems, much smaller crystal files, rep30 and rep10, were used for LiH3 (toluene-d5/LiNa-Y) and LiH5 (tolene-d3/LiNa-Y) spin systems, respectively. To simplify the simulation, the geometric arrangements of the dipolar vectors in LiH3 and LiH5 spin systems were ignored; i.e., the dipolar angles were set to 0 (see text for more details). All other parameters, including spinning

Molecular Modeling. Monte Carlo simulations were performed to investigate the sorbate-zeolite host interactions assuming fixed zeolite structure and fixed sorbate geometry. The simulations were carried out with the commercial program Cerius2 4.9 supplied by Accelrys. The atomic positions of the zeolite structures were taken from the X-ray diffraction data of zeolite Y.34 The geometry of the sorbate toluene molecule was taken from the optimized B3LYP/6-311++G** structure obtained with the Gaussian 98 package.41 Approximate site locations of the Na+ and Li+ nonframework cations were determined from the MAS NMR experiments (vide infra) and then optimized. As an example, for Li33Na24Y the following cation locations were determined from the MAS NMR data: 1 Na+ at SI site, 23 Na+ at SII, 25 Li+ at SI′, and 8 Li+ at SII in the unit cell. Na+ and Li+ cations were placed in these site locations followed by optimization whereby the zeolite framework was fixed. Numerous permutations of cation site locations that fit the experimentally determined site assignments were tested. The lowest energy optimized structure was used for the Monte Carlo simulations. Lennard-Jones parameters for the noncation zeolite atoms were taken from the Burchart42 zeolite force field provided in Cerius2. Lennard-Jones parameters for Na+, Li+, and the atoms of the sorbate molecules were taken from the universal force field of Goddard and co-workers.43 The partial charges of the zeolite atoms were taken from the literature44 and modified for the specific Si/Al ratio. More specifically, the charges of Si, Al, and O were +1.43, +1.23, and -0.833, respectively. Charges for Na+ and Li+ were taken as +1.0. MK45 electrostatic potential fitted partial atomic charges from a B3LYP/6-311++G** calculation were used for toluene. An interaction cutoff of 12 Å was applied. The maximum translation value and rotation value were set to 12 Å and 50°, respectively, which provided an acceptance ratio of approximately 35-40%. A typical Monte Carlo run involved 10 000 000 steps. The NVT ensemble sampled at a temperature of 298 K was used. The simulation of the toluene/M-Y system was carried out at the same toluene loading of 3 molecules/supercage as in the experiments. Similar empirical parameters and simulations have been used to examine biphenyl/M-Y (M ) Na+, K+, Cs+)44 and aromatics/Na-Y (aromatics ) benzene, toluene, xylene).46 To verify the simulation method and the reliability of the force field used in this study, the adsorptions of benzene in zeolite Si-Y and Na-Y were simulated. For benzene/SiY, the same adsorption sitessabove the six-membered ring, in the 12-membered ring window, and above the four-membered ringswere found for benzene molecules as in a previous simulation study.17 For the benzene/Na-Y system, our simulation gave two adsorption sites: “capping” on Na+ ions at the SII sites and in the 12-membered ring window, with the former

TABLE 1: Simulation Parameters of 23Na MAS Spectra of Dehydrated Na-Y and Toluene/Na-Y Complexa

a

samples

loading (molecules/sc)

Na site

NNa/u.c.a

QCC (MHz)

η

δiso (ppm)

dehydrated Na-Y

0

SI SII SI′ SIII′

12 32 7 6

1.2 3.9 4.8 2.3

0 0.1 0.1 0.1

-1.5 -8.0 -9.0 -18.0

toluene/NaY

3

SI PA PB

12 40 5

1.4 1.6 3.0

0 0.1 0.1

-1.2 -14.4 -8.0

NNa refers to the number of Na+ ions at each site; u.c. stands for unit cell.

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Figure 3. Experimental and simulated 23Na MAS NMR spectra. Zeolite Li33Na24Y: (A) dehydrated and (C) toluene-loaded samples. Zeolite Li45Na12Y: (B) dehydrated and (D) toluene-loaded samples. The loading levels are both 3 molecules/supercage / denotes spinning sidebands. (Insert table: numbers of Na+ ions at each site for different zeolites.)

being preferred. These results are in good agreement with both experimental11,46 and simulation46 results reported in the literature. Results and Discussion Cation Locations and Adsorption Sites. Previous structural studies on dehydrated Na-Y by X-ray47,48 and neutron8 diffraction have shown that the Na+ ions are located at sites SI, SII, and SI′. After exchanging Na+ for Li+, X-ray34 and neutron35 diffraction results showed that, in zeolite Y, Li+ ions occupy the SI′ and SII sites, and in zeolite X additional Li+ will enter site SIII′. The unexchanged Na+ ions remain at SII sites. Since the occupancy of each site depends on the exact Si/Al ratio and the nature of the cation, we first acquired 23Na and 7Li MAS spectra to determine the locations and populations of the cations in the parent Na-Y used in this study with a Si/Al of 2.35 and the ion-exchanged LiNa-Y, and then to probe the effect of toluene adsorption on these cations. The 23Na MAS spectrum (Figure 2A, top) of dehydrated parent Na-Y (unloaded) exhibits several broad overlapping signals, which can be fit by four components. The simulated spectrum is shown in Figure 2A (bottom), and the four resonances can be assigned to the Na+ ions located at SI, SII, SIII′, and SI′ sites based on previous studies.36,49,50 The quadrupolar parameters and the number of ions at each site are obtained through simulation (Table 1). After partially exchang-

ing Na+ for Li+ ions, 23Na MAS spectra were acquired (Figure 3A,B). For both LiNa-Y samples, the spectra are dominated by the broad signal originating from Na+ ions at SII sites. In Li33Na24Y (Figure 3A), there is also a small fraction of Na+ ions remaining at SI sites. Upon loading (3 toluene molecules/sc), the 23Na MAS spectrum changes remarkably (Figure 2B, top). The simulated spectrum and the parameters used in the simulation are shown in Figure 2B (bottom) and Table 1, respectively. Since no significant shift occurs upon toluene adsorption, the peak at -4 ppm is assigned to Na+ ions at SI sites. The quadrupolar parameters (Table 1) of the Na+ ions at this site are almost identical to those before toluene adsorption, which indicates that adsorption of toluene induces very little change in the environment of the Na+ at this site. This is understandable since the cations at this site are located in the center of the double sixmembered ring (D6R), which is not accessible for toluene. The other two resonances are denoted as PA and PB (Figure 2B, bottom). On the basis of its intensity and previous reports on similar sorbates such as benzene20 and pyrrole36 adsorbed in Na-Y, the resonance PA is mainly due to the Na+ ions at site SII; it also includes a very small number of Na+ initially at site SIII′. The large reduction in the QCC from 3.9 to 1.6 MHz for the Na+ ions at SII site upon adsorption (Table 1) implies that the adsorbed guest molecules mainly interact with the Na+ ions at SII site. As for the last component PB, it is assigned to the


J. Phys. Chem. C, Vol. 111, No. 36, 2007 13431

Figure 4. 7Li MAS NMR spectra. Zeolite Li33Na24Y: (A) dehydrated and (C) toluene-loaded samples. Zeolite Li45Na12Y: (B) dehydrated and (D) toluene-loaded samples. The loading levels are both 3 molecules/supercage. (Insert table: numbers of Li+ ions at each site for different zeolites.)

Na+ ions located at SI′ site before adsorption, but now shifted slightly within the β-cages upon loading. The adsorption of toluene over Na-Y probably induces the migration of Na+ ions originally at site SI′ in the sodalite cage to a new location closer to the supercage.48,49 The 23Na MAS spectra of the toluene-loaded LiNa-Y samples are shown in Figure 3C,D. A strong, narrow peak at about -20 ppm due to the Na+ ions at SII sites was observed for both samples. Similar to toluene adsorbed in Na-Y, the significant change in the line shape of 23Na MAS spectra before and after toluene adsorption suggests that the toluene molecules are adsorbed on the Na+ ions at site SII. In Li33Na24Y (Figure 3C), there is also a small fraction of Na+ ions remaining at site SI, which is consistent with the results from the 23Na MAS spectrum of dehydrated sample (Figure 3A). The numbers of Na+ ions at each site obtained from the intensity of each signal for both samples are given in Figure 3. The changes in the population of Na+ ions at each site compared to those of parent Na-Y indicate that Li+ ions replace the Na+ ions originally at SI and SI′ sites almost completely and that a portion of the Na+ ions at site SII are also replaced by Li+. To directly probe the location of the Li+ ions, 7Li MAS spectra were acquired (Figure 4), which are indicative of two Li sites. Since the QCC of 7Li in zeolite Y is very small (only about 200 kHz), it behaves more like a spin 1/2 nucleus. By integration of the peak areas, the numbers of Li+ ions at each site were obtained. Comparing these numbers with those from X-ray and neutron diffraction studies,34,35 the two peaks at -0.1 and -0.5 ppm in the dehydrated sample and -0.2 and -2.7 ppm in the toluene-adsorbed sample are assigned to Li+ ions at SI′ and SII sites, respectively. The fact that for Li33Na24Y and Li45Na12Y the numbers of Li+ ions at SI′ sites are almost equal, but the number of Li+ ions at SII sites increases with increasing Li content, indicates that Li+ ion prefers to fill the SI′ sites first and then SII sites, which is consistent with literature results.35 It is also noticed that the peaks originating from Li+ ions at SII sites shift significantly from -0.5 to -2.7 ppm upon toluene adsorption. This suggests that the toluene molecules are adsorbed on Li+ ions at this site. The observed high field shift

upon adsorption may result from that the Li+ ions at SII site slightly move out of the six-membered ring to interact with the π electrons of the phenyl ring, and these π electrons make the Li+ ions more shielded.36 Unlike Li+ ions at SII site, the signals from Li+ ions at SI′ site do not have an obvious shift upon adsorption, which implies that the environment of the cations is not affected significantly by the adsorbent due to the fact that the Li+ ions at SI′ sites are not accessible. 23Na and 7Li MAS NMR data suggest that for Li Na Y there 33 24 are 1 Na+ at SI site, 23 Na+ at SII, 25 Li+ at SI′, and 8 Li+ at SII in the unit cell. Similarly, for Li45Na12Y, 12 Na+ ions remain at SII, 26 Li+ ions take position at SI′ site, and 19 Li+ occupy the SII site. Since there are eight supercages in one unit cell, each supercage, on average, has 1 Li+ ion and 3 Na+ ions at site SII for Li33Na24Y and 2.5 Li+ ions and 1.5 Na+ ions at SII for Li45Na12Y (this may correspond to the situation where four supercages have 2 Na+ and 2 Li+; another four have 1 Na+ and 3 Li+). In order to support the 23Na and 7Li NMR results, Monte Carlo simulations were performed to investigate the preferred adsorption sites of toluene molecules in zeolite LiNa-Y. As described in the Experimental Section, locations of the nonframework Na+ and Li+ cations used for the simulations were based upon the above-discussed 23Na and 7Li MAS NMR results. The simulations were carried out at a fixed toluene loading of 3 molecules/supercage. Mass cloud plots and other analysis of the resulting ensemble configurations generated from the Monte Carlo simulations revealed that there was a strong preference for the toluene molecules to “cap” the cations at the SII sites with their six-membered rings. In other words, the toluene ring prefers to lie almost parallel to the local sixmembered ring of the zeolite framework in which the Na+ or Li+ cation is located. Typical adsorption modes of toluene in the supercage of Li33Na24Y and Li45Na12Y are shown in Figure 5. 23Na{7Li} REDOR technique was utilized to obtain the NaLi distance information and therefore the information on the possibility of cation relocation. The REDOR38 method is a rotor synchronized double-resonance MAS technique with two sepa-


Zhu et al.

Figure 5. Typical orientation of toluene molecules in the supercage of (A) Li33Na24Y and (B) Li45Na12Y predicted by molecular simulation (loading: 3 molecules/sc).

rate experiments. The first one (the control experiment) is a normal spin-echo experiment performed on the observing nucleus (23Na). In the second (REDOR) experiment, a number of 180° pulses are applied to 7Li channel (dephasing nucleus) during the spin-echo on 23Na. A nonzero average of dipolar coupling results in a decrease in echo intensity (Sf) in the second experiment compared to the normal echo (S0) in control experiment. The REDOR difference spectrum (∆S) is obtained by subtracting the dephasing spectrum (Sf) from the normal spin-echo spectrum (S0) and provides the measure of dipolar coupling. Figure 6A shows the 23Na{7Li} REDOR curves [plots of REDOR faction (∆S/S0) as a function of dephasing time (τ)] of the Li45Na12Y samples at 298 and 163 K. Upon toluene adsorption, the initial slope of toluene/Li45Na12Y complex is almost the same as that of unloaded Li45Na12Y, indicating that the average Na-Li distance does not change significantly upon adsorption. This suggests no significant movement of the Na+ ions at site SII and Li+ ions at sites SI′ upon loading. The result is consistent with the study by Fitch and co-workers11 on the adsorption of benzene in Na-Y. Their work showed that the Na+ ions at site SII only move slightly (0.09 Å) to interact with the adsorbent. Unlike the situation in toluene/Na-Y where the Na+ ions at site SI′ shifted slightly upon adsorption, no significant movement of Li+ ions at site SI′ was observed. This is likely due to the stronger interaction of the smaller Li+ ion, with a much larger charge density, with the framework oxygen, preventing the relocation of the Li+ ions. A Na-Li dipolar interaction of 63 Hz is obtained by fitting the REDOR curve using the SIMPSON40 program (Figure 6B). The corresponding Na-Li distance is 5.8 Å, compared to the distance of 4.9 Å between Na+ at SII and Li+ at SI′ determined by XRD in a

Figure 6. (A) 23Na{7Li} REDOR fraction (∆S/S0) as a function of dephasing time for dehydrated zeolite Li45Na12Y and toluene-loaded samples (3 molecules/sc) at 298 and 163 K. (B) Fitting curve of the experimental data of toluene-loaded sample at 163 K by the SIMPSON program.

FAU zeolite with a composition of H4Na12.8Li39.2Si136Al56O384‚ 263H2O.34 The difference is due to the slight difference in unit cell composition and more likely the degree of hydration. Dynamics of Toluene Molecules. Deuterium NMR spectroscopy is a suitable tool for studying molecular motion, as it is sensitive to motions ranging from the second to the nanosecond time scale.51 2H NMR studies of molecular motion of guest species in zeolites have been reviewed.12 We acquired static 2H NMR spectra of deuterated toluene molecules (C5D5CH3) to examine the dynamic behavior of toluene in LiNa-Y. Variable temperature 2H spectra of C5D5CH3/Na57Y at the loading of 3 molecules/supercage (sc) are shown in Figure 7A. Using NMR-WEBLAB developed by Spiess and co-workers,52 the experimental spectra can be best simulated by a two-site flip with a flip angle of 105° (Figure 7B,C). When lowering the temperature, the rate of the flip motion decreases as indicated by the simulated 2H spectra. The motion still persists at 133 K. A careful inspection reveals a very weak central peak in the spectrum recorded at 298 K, indicating that a very small fraction of the toluene molecules undergo multiple-site jump inside the supercage. The jump ceases at low temperature, resulting in the disappearance of the central peak. The variable temperature 2H spectra of C5D5CH3/Li45Na12Y and C5D5CH3/Li33Na24Y samples (3 molecules/sc) are shown in parts D and E, respectively, of Figure 7. The two sets of



Figure 7. 2H static spectra of C6D5CH3/Na-Y: (A) observed and (B) calculated spectra. (C) Model of two-site flip motion of toluene. Variable temperature 2H static spectra of C6D5CH3 in (D) Li45Na12Y and (E) Li33Na24Y.

spectra are very similar. At 298 K, there are two components: a narrow central component and a broad feature underneath. When temperature is lowered, the broad component gradually becomes dominant. At very low temperature, i.e., 143 K, the line shape looks more similar to that observed in Na-Y. Overall, the evolution of the broad component as a function of temperature is very similar to the trend seen in the parent Na-Y. Therefore, we suggest that the broad feature results from a twosite flip motion. The sharp central component is likely due to the toluene molecule jumping among four SII sites tetrahedrally distributed within the supercage. A similar situation where the guest species undergo different motions in a zeolite was also observed in systems such as benzene/Na-Y13 and p-xylene/ Na-ZSM-5.53 Comparing the three spectra at 298 K, we find that the higher the Li+ content, the stronger and narrower the central peak. This indicates that at higher Li+ content the number

of toluene molecules undergoing multiple-site jump is greater and the jump rate is higher as well. The Monte Carlo simulations suggest that the toluene molecules are coordinated to the cations at SII sites (Figure 5) of the zeolite. However, the simulations also suggest that the adsorption geometry is slightly different for Na+ or Li+. Due to its small size, the Li+ cation lies almost within the plane of the six-membered ring of the SII site framework (Figure 8A), whereas the larger Na+ cation lies significantly above the plane (Figure 8B). As a result, the toluene molecule coordinated to the Li+ ion is about 1 Å closer to the framework compared to the toluene molecule bound to a Na+ ion. Consequently, the supercage of LiNa-Y with a higher Li content is less crowded. This results in a larger free space in the supercage that facilitates toluene for multiple-site jumping, leading to a strong and narrow central peak in the 2H spectrum at room temperature. At 143


Figure 8. Optimized geometry of the toluene molecule adsorbed on the site SII (A) Li+ ion and (B) Na+ ion from molecular simulation. Only the site SII six-membered ring of the zeolite is shown for clarity.

K, the line shapes of toluene adsorbed in the framework are very similar regardless of the Li content. This is because at this temperature the multiple-site jump has ceased and only the local reorientation about a fixed axis remains. Cation-Sorbate Interactions. Previous work has shown that the cations in FAU zeolites can directly interact with π electrons in aromatic compounds.54 Such interaction between a toluene molecule and a cation (Li+, Na+) means the presence of dipolar interaction between the protons of toluene and the cations, which should be able to be detected by 7Li{1H} and 23Na{1H} REDOR experiments. Figure 9 illustrates the selected 7Li spin-echo (S0), REDOR (Sf), and REDOR difference (∆S) spectra of C6H5CD3/ Li45Na12Y (3 molecules/sc) at both 298 and 163 K. Observing the peaks due to Li+ ions at both SI′ and SII sites in the REDOR difference spectra indicates that the Li+ ions at both sites were dipolar-coupled to ring protons. The signals in the difference spectra are much stronger at 163 K than those at 298 K, indicating a stronger dipolar coupling at low temperature. This is because the dipolar interaction is averaged by fast motions of the toluene molecule at room temperature, whereas at low temperature the motions are more restricted, leading to a stronger

Zhu et al. dipolar interaction. It is noticed that, at 163 K, the relative intensity of Li+ ions at site SII is stronger than the Li+ at site SI′ in the difference spectrum. This is due to a shorter average distance between Li+ ions at site SII and the ring protons since the toluene is directly coordinated to the Li+ at SII. For REDOR experiments, it is well-established that, in multiple-spin (ISn) systems such as ours, the initial part of a REDOR curve (∆S/S0 less than 0.2) only depends on the strength of I-S dipolar interaction (i.e., the larger the slope of the initial curve, the stronger the dipolar coupling) and is independent of the exact geometry involved.55-57 The 7Li{1H} REDOR curves of C6H5CD3/Li45Na12Y and C6D5CH3/Li45Na12Y samples (loading: 3 molecules/sc) are shown in Figure 10. For C6H5CD3/ Li45Na12Y sample, the initial REDOR plots (Figure 10A) suggest that the protons on the aromatic ring are coupled more strongly to the Li+ ions located at SII than those at SI′ sites, indicating qualitatively that the average distance from a ring proton to Li+ at SII is shorter. Similarly, Figure 10B illustrates that at low temperature the dipolar interaction of the methyl protons with the Li+ ions at SII site is also stronger than that of the protons with the Li+ ions at SI′ site. To obtain semiquantitative information, the REDOR data were fitted by using the SIMPSON40 program without considering the geometric arrangement of the spins. For toluene-d5/LiNa-Y complex (an IS3 system), the simulated REDOR curves match the experimental data well not only in the region where ∆S/S0 is less than 0.2, but also in the region with higher ∆S/S0 values (Figure 10B). This is because the Li+ ion is relatively far away from the three methyl H’s and therefore the difference in geometric arrangement between the Li+ ion and the three methyl H’s is less significant. For toluene-d3/LiNa-Y, an IS5 system (Figure 10A), the simulated curve for the Li+ ion at SI′ is in reasonably good agreement with the experimental data especially in the region where ∆S/S0 values are less than 20%. However, the fit for Li+ at site SII is not as good as that for the Li+ ion at SI′ even for the initial 20% of the ∆S/S0 value. As mentioned in the Experimental Section, only a rather small crystal file is used due to the large size of the density matrix of the LiH5 system, which may result in poor powder averaging. In addition, since the toluene molecule is directly coordinated to the Li+ at site SII, the five ring protons are much closer to the Li+ ion. As a result, the fit is more sensitive to the geometric arrangement of these spins during the SIMPSON simulation. For the LiH5 system, there are five Li-H spin pairs and the simulation yields five very similar Li-H dipolar coupling constants, which have been averaged to give an average dipolar

Figure 9. Selected (A) 298 K and (B) 163 K 7Li spin-echo (S0), REDOR (Sf), and REDOR difference (∆S) spectra of C6H5CD3 in Li45Na12Y (3 molecules/sc) with a dephasing time of 5 ms.


Figure 10. 7Li{1H} REDOR fraction (∆S/S0) as a function of dephasing time for (A) C6H5CD3 and (B) C6D5CH3 in Li45Na12Y (loading: 3 molecules/sc) at 298 and 163 K. Red curves in (A) and (B) show the fitting curves of the experimental data by the SIMPSON program.

coupling constant. For toluene-d3/LiNa-Y, the fitting (Figure 10A) yields average H-Li dipolar couplings of 492 and 272 Hz for Li+ ions at site SII and site SI′, respectively. The result is consistent with the MAS data discussed earlier that toluene is directly adsorbed on Li+ at SII site and is not accessible to Li+ at SI′ sites. For toluene-d5/LiNa-Y, the fitting results in average dipolar interactions of 167 and 110 Hz for Li+ ions at SII and SI′ sites, respectively. The REDOR fractions (∆S/S0) are very small at room temperature for both Li+ sites in both samples, because the dipolar interactions are efficiently averaged by the fast motion at room temperature. To investigate the effect of cation composition on dipolar interaction, 23Na{1H} and 7Li{1H} REDOR curves of C6H5CD3 adsorbed in the zeolites with different Li contents are compared in Figure 11. At room temperature, the strength of average Na-H dipolar coupling (Figure 11A) increases with increasing Na content. At low temperature, the dipolar coupling is, however, independent of cation composition. 7Li{1H} REDOR curves (Figure 11B) show a very similar trend: i.e., at room temperature, the smaller the Li content (corresponding to a higher Na content), the stronger the Li-H dipolar interaction. At low temperature, the strength of dipolar coupling does not depend on the ion composition. These observations can be explained by the dynamic behavior of toluene molecules discussed earlier. As mentioned earlier, the selectivity of the catalysts for alkylation reactions is dependent on the cation in the FAU


Figure 11. (A) 23Na{1H} and (B) 7Li{1H} REDOR fractions (∆S/S0) as a function of dephasing time for toluene in zeolite Y with different Na (Li) contents at 298 and 163 K.

zeolites: the Li form catalyzes only ring alkylation, the K, Rb, and Cs forms selectively catalyze side-chain alkylation, and the Na form can catalyze both reactions.3,9,10 The effect of the cations on the mobility of the toluene molecules may be one of the reasons why faujasite zeolites containing larger size alkali metal cations (e.g., Rb+, Cs+) are the better catalysts for sidechain alkylation of toluene since the restricted motion of the molecule makes the methyl group easy to be attacked by other reactants such as methanol. Conclusions We have examined the adsorption of toluene in zeolite LiNa-Y by solid-state NMR spectroscopy. The locations of Na+ and Li+ in Li45Na12Y and Li33Na24Y were determined by 23Na and 7Li MAS experiments. Li+ ions tend to occupy sites SII and SI′, and the Na+ ions mostly are located at site SII. Upon toluene adsorption, 23Na and 7Li MAS spectra suggest that the toluene molecules were adsorbed on the cations at SII site, resulting in a reduction in the QCC of the Na+ ions and a significant change in chemical shift of the Li+ ions. The Li+ ions at SI′ site were not affected significantly upon toluene adsorption. 2H NMR spectra show that, although the interaction between cation and the sorbate exists, the toluene molecules undergo a two-site flip motion and multiple-site jump at room temperature. The degree of the motion is a function of cation composition. As indicated by molecular simulation, upon

13436 J. Phys. Chem. C, Vol. 111, No. 36, 2007 adsorption of toluene, the free space of the supercage with higher Li content is larger, and consequently the guest species has a larger degree of motional freedom compared to the zeolite Y with a higher Na composition. When the temperature was lowered, the motions of the toluene molecules slowed markedly. Using the 23Na{1H} and 7Li{1H} REDOR experiments, dipolar couplings between the cations (both Na+ and Li+) at SII sites and the ring protons were detected, confirming that toluene molecules are coordinated to the cations at SII sites, and that both the ring and the methyl protons are also weakly coupled to the Li+ ions at SI′ sites via dipolar interaction. Upon cooling, the strength of dipolar interactions increases significantly. In addition to temperature, the REDOR effect was also affected by the ion composition. The adsorption of toluene in these zeolites was also studied by molecular modeling, and the results were consistent with the conclusions based on NMR data. Acknowledgment. We acknowledge financial support from NSERC of Canada for research grants and CFI for an equipment grant. Funding from the Canada Research Chair and Primer’s Research Excellence Award programs is also gratefully acknowledged. We thank Dr. C. W. Kirby for assistance in NMR experiments and Prof. R. E. Wasylishen for providing the software WSOLIDS for spectral simulation. References and Notes (1) (2) (3) (4) (5)

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