1629
J. Phys. Chem. 1993,97, 1629-1633
Location and Molecular Motion of Hexamethylbenzene in Zeolite Na-Y Suk Bong Hong,t Herman M. Cho,#*fand Mark E. Davis'$+ Department of Chemical Engineering and A. A . Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91 125 Received: May 14, 1992; In Final Form: October 22, 1992
Hexamethylbenzene (HMB) adsorbed into the supercages of zeolite Na-Y is characterized by thermogravimetric analyses, proton NMR, 13C NMR, proton multiple-quantum NMR, and UV reflectance and emission spectroscopies. Room temperature, proton multiple-quantum N M R results reveal that the intracrystalline distribution of HMB is primarily pairwise at bulk concentrations of 0.5, 1.O, and 2.0 molecules per supercage. Results from emissionspectroscopysuggest that the HMB pairs are uniformly dispersed among the intracrystalline supercages of Na-Y.
Introduction
Results and Discussion
In our accompanying report we illustrate the use of proton multiple-quantumNMR spectroscopy to count the proton cluster size of organic templates occluded during the crystallization process in the intracrystalline voids of the cubic (FAU) and hexagonal (EMT) polytypes of faujasite and ZSM-18, as well as organic species adsorbed within the supercages of Na-Y.' At a bulk concentration of one hexamethylbenzene(HMB) molecule per supercage of Na-Y, the multiple-quantum NMR measurements reveal a proton cluster size indicativeof two HMB molecules per supercage. The purpose of this work is to investigate a series of HMB-containing Na-Y samples with different bulk concentrations and to elucidate the spatial location and motion of the HMB molecules.
X-ray Analyses. X-ray powder diffraction patterns of all three HMB-containing Na-Y samples prepared by the vapor phase impregnation method show that the structure of Na-Y remains intact after heat treatment during the adsorptionstep. The color of the HMB-containing Na-Y samples is white like pure Na-Y. TbermogravimetricAnalyses. Figure 1 shows the TGA patterns for theNa-Y samplescontaining0.5,l.O,and2.0HMBmolecules per supercage. The TGA of the Na-Y sample with an average bulk concentrationof 2.0 HMB molecules per supercageexhibited a very strongexotherm even at a heating rate Iower than 5 K-min-1. Thus, the TGA were performed with a heating rate of 1 K-min-l. The TGA patterns from the Na-Y samples containing HMB are characterized by two distinct stages of weight loss: 25-200 OC and 200-450 OC. The first loss is due to the desorptionof water since the samples are fully rehydrated beforeTGA measurements. The second loss is from HMB. The TGA pattern of pure HMB gave a 100% weight loss below 100 OC at a heating rate of 1 K-min-l. These results indicate that most, if not all, the HMB molecules are adsorbed into the supercagesof Na-Y. Using the second weight loss as a measure of the HMB content, the average numbers of HMB molecules per supercage are approximately 0.5, 1.0, and 2.0, respectively, within experimental error. An interesting result is obtained from the Na-Y sample containing 2.0 HMB molecules per supercage; a small weight gain is observed in the temperature region of 200-250 OC. This trend is more apparent when the TGA is taken at a higher heating rate. The exact reason for this observationis not clear. However, it is most likely that this weight gain is caused by uptake of oxygen as HMB is oxidized at high temperature during, the TGA measurements, which were performed in air. This behavior has also been observed with other organics occluded in zeolite^.^ '3c a d Proton NMB Me8surement.s. The MAS 13CNMR spectra of the three HMBcontaining Na-Y samples displayed in Figure 2 show the presence of undecomposed HMB and the absence of extraneous hydrocarbon impurities. The static *)C NMR spectra of the three samples are shown in Figure 3. The peak due to the aromatic carbon atoms appears at 180-190 ppm, which is different from that oberved with MAS, Also, the lint shape and line width we asymmetric and much bro8det; respectively, in the static 13CNMR spectra as compared to the MAS spectra. The static I3CNMRspectrum of pure HMB has been reported by several groups.4-6 The 13C line width of the aromatic resonance from HMB in Na-Y is considerably narrower than that observed in the low-temperature investigations afpure HMB. For reasons given elsewhere, the line shape and @ion of the aromatic carbon peaks strongly indicate that the H N B molecules are spinning rapidly in place about their six-fold
Experimental Section Three HMB-containing Na-Y samples with bulk ooncentrations of 0.5,1.O,and 2.0 molecules per supercage were prepared by a vapor phase impregnation meShod.I.* Therqogravimetric analyses (TGA) were performed on a Dupont 950 thermogravimetric analyzer. Approximately 8 mg of sample was used with a heating rate of 1 K - m i d . The proton NMR, 13CNMR, and proton multiple-quantum NMR measurements employed b r e are the same as given in our apxmpanying paper.', VV diffuse reflectance spectra were obtained on spectrometer system constructed from an EG&G PAR diode array (1024 element Si) detector and a high-radiance Orid deuterium lamp. Light from the deuterium lamp was directed upon ca. 10 mg of powdered sample with a fused silica lens fiber optic assembly and detected a t 90° geometry with a second fiber optic assembly which directed the light to the polychrometer before the diode array. The spectra were ratioed to a reference spectrum obtained from powdered MgO. Emission spectra were obtained using a homebuilt instrument. The 366-nm line of a 200-W Hg/Xe arc lamp was used as an excitation source. This excitation beam, with a resolution of 1 nm, was focused with fused silica lenses onto thesample. Emitted light was collected and dispersed with a Spex 1870.5-m grating monochrometer lfl6.9) equipped with 1411SW and 1811SW entrance and exit slits. The dispersed l i e t was detected with a Hamamatsu R955 photomultiplier tube in a Precision Instruments Model 3377D dry ice-cooled housing. To whom correspondence should be addressed. Department of Chemical Engineering. 8 Presentaddress: Molecular Science Research Center MSIN K2-20, Pacific Northwest Laboratory, Richland, WA 99352. 8 A. A. N o y a Laboratory of Chemical Physics. +
0022-3654/93/2097-1629$04.00/0@ - 1993 American Chemical Society
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1630 The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 100
90 wt. K a0
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Fipn1. Thermogravimetric analyses of Na-Y containing (A) 0.5, (B) 1.O, and (C) 2.0 HMB molecules per supercage. 100
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Figure 4. Variable temperature static IT NMR spectra of Na-Y containing 1.O HMB molecule per supercage: (A) 300 K, (B) 260 K, and (C) 210 K.
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Figure2. MAS I3CNMR spectra of Na-Y containing (A) 0.5, (B) 1.0, and (C) 2.0 HMB molecules per supercage. Spinning side bands are marked by asterisks.
(C)
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Figure 5. Room temperature proton NMR spectra of Na-Y containing (A) 0.5, (B) 1.0, and (C) 2.0 HMB molecules per supercage.
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Fipn3. Static IT NMR spectra of Na-Y containing (A) 0.5, (B) 1.0, and (C) 2.0 HMB molecules per supercage.
symmetry axis in the Na-Y supercages.I This hypothesis is supported by the room temperature proton NMR measurements illustrated in Figure 5 . The resonance from the aromatic carbons in HMB is not observed in the room temperature, static I3CNMR spectrum of Na-Y with 1.0 HMB molecule per supercage. However, the resonance appears in the spectra of Na-Y containing 0.5 or 2.0 HMB molecules per supercage. When the static 13C NMR spectrum of Na-Y with 1.O HMB molecule per supercage was recorded at 210 K, this peak was barely visible (Figure 4). Theroom temperatureprotonNMRspcctraofthethree HMBcontaining Na-Y samples are illustrated in Figure 5. There is a slight but visible dissimilarity in the three spectra shown in Figure 5. One obvious reason why these spectra differ may be that fundamental changes occur in the way the HMB molecules are distributed in the supercages as more HMB molecules are
adsorbed in the host. Alternatively, and more likely, if a fraction of the proton signal is assigned to water in some fixed concentration, then thedifference in the three spectra may instead simply be a result of the change in the ratio of the HMB NMR signal intensity to the NMR signal intensity of the water. Proton Mdtipk-QluntruaNMR R d t s . To more accurately quantify the distribution of HMB molecules in the Na-Y supercages, we have performed multiplequantum NMR counting experiments on the three HMB-containing Na-Y samples. The specific procedures for acquiring the multiple-quantum NMR magnitude spectra are given e1sewherc.I Figures6 and 7 illustrate the representative multiple-quantum magnitude NMR spectra for the Na-Y samples containing 2.0 and 0.5 HMB molecules per supercage, respectively. The calculated least-squares fits are denoted by the open circles. The best least-squares fits to these spectra were obtained with the assumption that there were two different sized clusters in the sample. The proton multiplequantum magnitude NMR spectra obtained from the Na-Y sample with 1.O HMB molecule per supercage are the same as those reported in our accompanying paper. The hydrogen cluster sizes of three HMB-containing Na-Y samples with bulk concentrations of 0.5, 1.0, and 2.0 molecules per supercage have been determined by acquiring a series of multiplequantum NMR spectra with increasingpreparation time period. If the number of hydrogen nuclei in the cluster is finite, then the cluster size, Nj will converge to limits corresponding to the presumptive hydrogen cluster sizes as the preparation time period increases. Plots of N,vs preparation period are shown in Figure 8 and give values for NI and NZof approximately 6 and
The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 1631
Hexamethylbenzene in Na-Y Zeolite
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i 10
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Fiyre6. Proton multiple-quantumNMR magnitude spectra with leastsquares fits of Na-Y containing 2.0 HMB molecules per supercage. The spectra were acquired at different radio frequency preparation times: (A) 132, (B) 396, and (C) 660 ps. Each open circle represents the fitted n-quantum line intensity to the experimentally measured intensity.
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Figure 7. Proton multiple-quantum NMR magnitude spectra with leastsquares fits of Na-Y containing 0.5 HMB molecule per supercage. The preparation times are the same as those of Figure 6 . Each open circle represents the fitted n-quantum line intensity to the experimentally
measured intensity.
36 for both Na-Y samplescontaining 2.0 and 0.5HMB molecules per supercage. These results match those obtained from Na-Y containing 1.O HMB molecule per suercage. The hydrogen cluster sizeof 36demonstrates that a significantfractionof thesupercages in all three samples contains exactly two HMB molecules. This appears to be true even for the Na-Y sample containingan average of 0.5 HMB molecules per supercage. These results suggest that there is a tendency for HMB moleculesto associate in pairs during the adsorption step into the Na-Y supercages even at relatively !ow average HMB concentrations. Proton multiple-quantum NMR experiments showing that
0
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t
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Figure 8. n-Quantum intensity vs preparation time from the proton multiple-quantumNMR data for Na-Y containing 2.0 HMB molecules per supercage (A) and Na-Y containing0.5 HMBmoleculepersupercage (B).
HMB occurs singly and not in pairs when prepared in bulk concentrations of less than 1.O molecule per supercage have been reported by The discrepancy with our results appears to be attributable to differences in the procedure by which the respective samples have been synthesized. The HMB adsorption procedure employed here differs from that of previous work in that the tube containing the physical mixture of HMB and dehydrated Na-Y was sealed after evacuation at 77 K. Sealing the sample tube both protects the contents of the tube from exposure to air and assures a constant pressure within the tube during the heat treatment at 573 K. We have previously demonstrated that it is important that the pressure of the guest molecules inside the sample tube be kept as high and as constant as possible for successful vapor phase impregnation.* The analytical tests we have performed confirm the success of the impregnationand the absence of extracrystalline HMB molecules. The relatively small number of hydrogens in the second cluster and the absence of the '3CNMR signal from any hydrocarbon species except the HMB (Figure 2) are consistent with the supposition that crystalline water is present in groups of two or three water molecules per site. This hypothesis was investigated on a test sample prepared by heating a Na-Y sample overnight at 673 K under high vacuum, as was done for the earlier samples. Despitethis attempt to extract all impurities, a weak protonNMR signal consistent in position and line shape with the resonance found in the HMB-containing Na-Y samples was observed. The conclusion we draw from this observation is that hydrogencontaining impurities persist in the Na-Y at high enough concentrations to be detected by NMR, even after the pretreatment procedure, and may be the source of the second multiplequantum signal. UV Reflectance and Emission Spectra. The UV reflectance spectra for the three HMB-containing Na-Y samples and crystalline HMB in the region of 250-430nm are given in Figure 9. The spectrum of crystalline HMB was obtained from the physical mixture of HMB with MgO.' The electronic absorption spectrum of neat HMB at low temperatures is reported to show three electronic transitions centered at 200,230,and 270 nm and corresponds to analogous transitions in b e n ~ e n e . ~The , ' ~ band at 270 nm is assigned to the lBZu 'AI, transition mode in the aromatic ring of H M k 9 All the spectra shown in Figure 9 give one broad absorption band in the region of 270-280 nm. This implies that no observable energy differences exist between the excited states of crystalline HMB and HMB molecules adsorbed in the supercages of Na-Y. The emission spectra of the three dehydrated HMB-containing Na-Y samples, which were sealed in sample tubes without exposure to air prior to the emission measurements, are very similar toone another. These dehydrated HMB-containingNa-Y samples show one emission band at 460 nm with a full width at half-maximum (fwhm) of approximately 145 nm (Na-Y does
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Hong et al.
1632 The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 2.0
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0.
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Figure 9. UV reflectance spectra of Na-Y samples containing (A) 0.5, (B) 1.0, and (C) 2.0 HMB molecules per supercage and of (D) crystalline HMB. (A) 1.2
.-
1 .o
Figure 11. Schematic diagrams of possible spatial HMB distributions within the supercages of a single Na-Y crystallite at bulk concentrations of (A) 2.0, (B) 1.O, and (C) 0.5 HMB molecules per supercage. Theopen
D-A
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circles represent empty supercages, and solid circles are the supercages containing two HMB molecules.
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Figure 10. Emission spectra of hydrated Na-Y samples containing (A) 0.5, (B) 1.O, and (C) 2.0 HMB molecules per supercage and of (D)neat, crystalline HMB.
TABLE k Emission Data for Hydrated Na-Y Samples with Mfferent Bull HMB Concentrations molecules per A,, fwhm," AE, sample neat HMB 2.0 HMB/Na-Y 1.O HMB/Na-Y 0.5 HMB/Na-Y a
supercage
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440 465 485 490
75 130 130 120
-1.46 -2.52 -2.17
Full width at half-maximum of the emission band.
not give an emission band). The emission band position and fwhm of neat, crystalline HMB are 440 and 75 nm, respectively. The values for neat, crystalline HMB are the same as those obtained from HMB dissolved in cyclohexane solution at the same concentration as Na-Y containing 0.5 molecules per supercage of Na-Y. This indicates that the HMB molecules adsorbed in Na-Y may have different lifetimes than the HMB molecules in the crystalline or liquid state. An interesting observation is obtained from the emission spectra for the HMBcontaining Na-Y samples fully rehydrated prior to the emission measurements (Figure 10). Emission data for hydrated Na-Y samples with different bulk HMB concentrations are listed in Table I. The emission spectrum of the rehydrated Na-Y sample containing 2.0 HMB molecules per supercage is very similar to that obtained from the dehydrated Na-Y sample with the same HMB concentration. However, the emission band positions are red-shifted with decreasing bulk concentration of HMB adsorbed in the supercagesof Na-Y. Also, the fwhmvalues of the emissicn bands of the three rehydrated HMB-containing Na-Y samples
are slightly narrower than those obtained from the dehydrated HMB-containing Na-Y samples. The existence of coadsorbed water is reported to influence the photophysical properties of the guest molecules trapped in the crystalline voids of the zeolite.I1-l3 For example, the tris(bipyridine)ruthenium(II) (Rubp) trapped in the dehydrated Na-Y exhibits an emission maximum (Amax) at 586 nm, while the A,, is at 621 nm in the hydrated Na-Y." As illustrated in TGA patterns of Figure 1, the three HMBcontaining Na-Y samples have different water contents after the rehydration in air. The higher the concentration of HMB in the sample, the smaller the amount of water it contains. Therefore, it is most likely that the shift of emissionband positions illustrated in Figure 10 is due to the differences in the amount of coadsorbed water of the three rehydrated HMB-containing Na-Y samples. This suggests that the interaction between the HMB pairs and the zeolite framework is dependent on water content. There is currently little theoretical understanding of the adsorbatezeolite interactionsor of the states of adsorbates located in the cavities of the zeolite hosts. However, the observation illustrated in Figure 10 can be used to understand the intraparticle distribution of the pairs of HMB in Na-Y. Figure 11 shows the schematic diagrams of possible spatial HMB distributions within the supercages of a single Na-Y crystallite. If the intraparticle distribution of HMB molecules in Na-Y containing 1.0 HMB molecule per supercage is not uniform (e.g., 'egg-shell" type distribution), a portion of the crystals must have the same water concentration as that of Na-Y containing 2.0 HMB molecules per supercage. If such is the case, the emission spectrum should be the same as that of Na-Y containing 2.0 HMB molecules per supercage regardless of the bulk water content. This would be true even for the case of the Na-Y sample containing 0.5 HMB molecule per supercage. However, in the case in which the spatial distributionof HMB pairs is uniform in the three HMB-containing Na-Y samples, the average number of the empty supercages adjacent to the supercageoccupied by the HMB pair must increase with decreasing bulk HMB concentration. Also, the empty supercages have more space for water than the HMB-containing ones. This suggests that the more empty supercages the hydrated zeolite has, the stronger the interaction between pairs of HMB and water. Therefore, it can be concluded from the emission
Hexamethylbenzene in Na-Y Zeolite spectra illustrated in Figure 10that the spatial distribution of the pairs of HMB molecules in Na-Y appears uniform among the intracrystalline supercagesof Na-Y. Finally, if our speculation is true, the diffusivity and adsorption kinetics of HMB molecules may not be crucial factors influencing their spatial distribution in a single Na-Y crystallite, at least under the adsorption condition employed in this study. We do not believe that the pairs of the HMB molecules can be directly adsorbed into the supercages because of the 7.4-hsupercage aperture of Na-Y. Rather, we believe that the formation of the pairs of HMB molecules occurs inside the Na-Y crystals by the migration of the singly adsorbed HMB molecules fromonesupercageto another one. This suggests that the state of the pairs of the HMB molecules is more thermodynamically stable than that of the singly adsorbed HMB molecules. However, the precise reason why the paired HMB moleculesare more favorable than a single HMB in the supercages of Na-Y still remains unknown. Further study is necessary to elucidate the factors influencing HMB pair formation. In conclusion, it is observed from the room temperature, proton multiple-quantum NMR measurements that HMB molecules associate in pairs with the intracrystalline voids of Na-Y for samples with bulk concentrations of 0.5, 1.0, and 2.0 molecules per supercage. Results from emission spectroscopy imply that
The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 1633 HMB pairs are uniformly dispersed among the intracrystalline supercages of Na-Y at all adsorption levels.
Acknowledgment. Support of this work was provided by the
NSF Alan T.Waterman Award to M.E.D. We thank Professor G. R. Rossman of the Geology Department at Caltech for assistance in obtaining UV reflectance spectra.
References and Notes (1) Hong, S.B.;Cho, H.M.; Davis, M. E. J . Phys. Chem., preceding paper in this-issue. (2) Hong, S. B.; Mielczaski, E.; Davis, M. E. J . Cutul. 1992, 134, 349. (3) Annen. M. J.: Young. - D.: Arhancet. J. P.: Davis. M. E.: Schramm. S. Zeolites 1960, 10, 546. (4) Pausak, A.; Tegenfeldt, J.; Waugh, J. S. J . Chem. Phys. 1974, 61, 1338. (5) Wemmer, D. E.; Ruben, D. J.; Pines, A. J . Am. Chem. Soc. 1981, 103, 28. (6) Pines, A.; Gibby, M. G.;Waugh, J. S. Chem. Phys. L r r r . 1972, IS, 373. (7) Ryoo, R.; Liu, S.-B.; de Menorval, L. C.; Takegoshi, K.; Chmclka, B. F.; Trecoske, M.; Pines, A. J . Phys. Chem. 1987, 91, 6575. (8) Chmelka, B. F.; Pearson. J. G.;Liu, S.-B.;Ryoo, R.; de Menorval. L. C.; Pines, A. J. Phys. Chem. 1991, 95, 303. (9) Craig, D. P.; Lyous, L. E. Nature 1952, 169. 1102. (IO) Schnepp, 0. J . Chem. Phys. 1957, 26.83. ( 1 1 ) Incavo, J. A.; Dutta, P. K. J . Phys. Chem. 1990. 94, 3075. (12) Liu, X.; Iu, K. K.; Thomas, J. K. J . Phys. Chem. 1989, 93, 4120. (13) Iu, K. K.; Thomas, J. K. Lungmuir 1990,6, 471.
