J. Phys. Chem. 1986, 90, 4903-4905 Discussion The present results show the first REMPI detection of methoxy radicals. Although the complexity of these spectra causes them to appear noisy and congested, the sensitivity seems to be within an order of magnitude of that for hydroxymethyl radical. The absence of any detectable ion fragmentation during the REMPI process enhances sensitivity because all ion signal is concentrated into a single mass. Since the methoxy cations do not fragment, REMPI spectroscopy can conveniently resolve isotopically substituted radical analogues. Because of the limited spectral range viewed during these experiments, identity of the resonant electronic state that enhances the ionization cross section and generates the spectrum is uncertain. In fact, the present results cannot establish whether this resonant state resides at energies that are the sum of one or two (or more) photons. Any speculation of the identity of the resonant state and of the REMPI mechanism requires that we assume an ionization potential for C H 3 0 . The literature regarding this ionization potential is clouded because most ion processes initially attributed to the production of C H 3 0 + have been shown to originate from production of the more stable CH20H+.'"I6 However, elegant charge reversal expefiments by Burgers and Holmes" determined that AHro(CH30+X3Al) = 1034 (f20) kJ/mol which leads to ~~
~
~
~~
~
(14) Haney, M. A.; Patel, J. C.; Hayes, E. F. J . Chem. Phys. 1970, 53, 4105. (15) Dil1,'J. D.; Fischer, C. L.; McLafferty, F. W. J . A m . Chem. SOC. 1979, 101, 6531. (16) Bouma, W. J.; Nobes, R. H.; Radom, L. Org. Mass Spectrom. 1982, ,7
-..e
I / , 3IJ.
(17) Burgers, P. C.; Holmes, J. L. Org. Mass Spectrom. 1984, 19, 452.
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a derived ionization potential of 10.5 eV for CH30 (R2E).I8 In the present experiments a 10.5-eV ionization potential requires that methoxy radicals absorb three laser photons to ionize. On the basis of this ionization potential two REMPI mechanisms seem plausible. The first REMPI mechanism involves simultaneous two-photon absorption into the vibrational manifold of the 3 t 2 A 1Rydberg state. Rydberg radicals may ionize and become X3A1cations after they absorb one additional laser photon. To date, no Rydberg states of methoxy radical have been reported. The $ a n d REMPI mechanism involves one-photon preparation of the A2Al valezce state of the methoxy radical. The 0; band of the A2Al X2E electronic transition lies at -310 nm.' If the REMPI process involves absorption through the A2Al state, then the REMPI bands between 313 and 328 nm originate from Librationally hot methoxy radicals. Ionization from the methoxy A2AI state requir_esa minimum of two additional laser photons. But ionization of A2Al state radicals should produce electronically excited cations, e.g. 3E state cations. If these excited cation states lie greater than 11.2 eV relative to the methoxy radical, then ionization of methoxy radicals will require absorption of a total of four laser photons. Preliminary analysis shows that definitive state assignments are not possible from the present spectra. An analysis leading to firm state assignments requires data that encompass a larger wavelength range so that complete band progressions are identified. This work is in progress.
-
(18) For discussion purposes state symmetries are referenced to a C,, molecular symmetry. Evidence indicates that symmetry spoiling Jahn-Teller interactions in the methoxy radical are small. See ref 1 and Yarkony, D. R.; Schaefer 111, H. F.; Rothenberg, S. J . A m . Chem. SOC.1974, 96, 656.
Sorption of Dimethyl Ether in Zeolite H-rho Studied by Deuterium NMRt Zeev Luz* and Alexander J. Vega* Central Research and Development Department, E. I. du Pont de Nemours and Company, Experimental Station, Wilmington, Delaware 19898 (Received: April 28, 1986; In Final Form: July 11, 1986)
The sorption of dimethyl ether in zeolite H-rho was studied by deuterium NMR of the deuteriated sorbate. The line shapes depend strongly on the temperature and the loading. These changes are not due to dynamic line-broadening effects but rather to changing distributions of sites with different modes of motion. At low loading levels the molecules are rigidly bound and can be seen to undergo 180° flips about the C, axes. When the number of sorbed molecules exceeds that of the available acid sites (12 per unit cell) most of the molecules appear to be in a physisorbed state.
Introduction Considerable work has been published on the sorption capacity of zeolites,l but only recently has this problem been approached with techniques that provide information regarding the nature of the species formed on the molecular level. One of the most important of these techniques is nuclear magnetic resonance, in particular deuterium N M R of deuteriated sorbate^,^^^ because of its sensitivity to the structure and the dynamic characteristics of the binding complex formed between the sorbate and the zeolite. In the present letter we report on the deuterium N M R of perdeuteriated dimethyl ether (DME-d6) sorbed on the zeolite H-rho. This zeolite has a highly symmetric overall topology with all T sites occupying equivalent positions in the aluminosilicate lattice.4 Its unit cells consist of a-cages having 48 T sites of which on the average 12 are occupied by A1 atoms. Consequently there are also 12 hydroxyls per unit cell. The a-cages are connected via double-eight rings whose diameters range between 4 and 5 'Contribution No. 4019. f Permanent address: The Weizmann Institute of Science, Rehovot 76100, Israel.
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A, just large enough to accommodate molecules of the size of DME (-3.7 A). Experimental Section An acid form of this zeolite was prepared from ",-rho by a vacuum-calcination procedure that yields nearly perfect H-rho (1) See, e.g.: Flank, W. H., Ed. Adsorption and Ion Exchange with Synthetic Zeolites; American Chemical Society: Washington, DC, 1980; ACS Symp. Ser. No. 135. (2) Eckman, R. R.; Vega, A. J. J . A m . Chem. SOC.1983, 105, 4841; J . Phys. Chem. 1986, 90, 4679. (3) Gottlieb, H. E.; Luz, Z . J . Magn. Reson. 1983, 54, 257. (4) Robson, H. E.; Shoemaker, D. P.; Ogilvie, R. A.; Manor, P. C. In Molecular Sieves; Meier, W. M., Uytterhoeven, J. B., Eds.; American Chemical Society: Washington, DC, 1973; Adv. Chem. Ser. No. 121, p 106. Flank, W. H. In Molecular Sieves-II; Katzer, J. R., Ed.; American Chemical Society: Washington, DC, 1977; ACS Symp. Ser. No. 40, p 43. Barrer, R. M.; Barri, S.; Klinowski, J. Proc. Int. ConJ Zeolites, 5th 1980, 20. Parise, J. B.; Prince, E. Mater. Res. Bull. 1983, 18, 841. Parise, J. B.; Abrams, L.; Gier, T. E.; Corbin, D. R.; Jorgensen, J. D.; Prince, E. J. Phys. Chem. 1984, 88, 2303. Parise, J. B.; Gier, T. E.; Corbin, D. R.; Cox, D. E. J . Phys. Chem. 1984, 88, 1635. McCusker, L. B. Zeolites 1984, 4, 51. McCusker, L. B.; Baerlocher, C. Proc. Int. ConJ Zeolites, 6th 1984, 812.
0 1986 American Chemical Society
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The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 54Molec
uc
Letters
21 OMolec
uc
1 0 8 W
(d) Full Rotation
t ("C)
(b) 180° Flips
M
-40 0 40 Frequency (kHz)
Frequency (KFz)
Figure 1. Deuterium NMR quadrupole-echo spectra of H-rho samples sorbed with the indicated amounts of DME-d, at different temperatures.
with 0.9 bridging hydroxyls per AI site, as was determined by quantitative proton NMR.5 Three samples were studied with loading levels corresponding to 5.4, 10.8, and 21.0 DME-d, molecules per unit cell of H-rho. They were prepared by exposing weighed amounts of anhydrous H-rho to calibrated volumes of DME-d, in a vacuum system. The samples were sealed in 7-mm (0.d.) glass tubes of about 25-mm length. The N M R spectra were obtained with the quadrupole echo technique6 on a CXP300 Bruker spectrometer operating at 46 MHz. The echo sequence consisted of two quadrature 90' pulses of 4-1s duration with 30-1s separation. The delay time was between 0.05 and 5 s, adjusted to avoid saturation. Results The deuterium N M R spectra are shown in Figure 1. As may be seen, their line shapes depend strongly on the temperature and on the degree of loading, and, except at the highest temperatures recorded, they all exhibit features characteristic of rigidly bound molecules undergoing different types of restricted local motions.' Before proceeding to analyze these results in terms of the various modes of motions, we wish to emphasize that the line-shape changes are not due to dynamic line-broadening effects. This follows from the fact that the observed spectra do not conform to line shapes expected from simple dynamic models and more importantly from the absence of "echo distortion" effects in the echo experiments. Such dynamic processes should, in the intermediate-rate regime, result in spectra with a high degree of T, anisotropy and, consequently, conspicuous echo distortions when the interval between the i ~ / 2pulses is progressively increased.8 No such effects were observed in the spectra of the samples studied. We therefore resort to an alternative interpretation in which we assume a distribution of sites with different modes of motion. As the temperature is changed the population distribution of the sites changes, resulting in the observed variation in the spectral line shape. When the ensemble of sites encompasses a very broad distribution of correlation times, the quadrupole-echo spectra will be dominated by sites corresponding to very short and very long correlation times while sites corresponding to the intermediate regime will not be observed due to their very short relaxation times.9 Under such conditions the spectra only provide ~~
~~
( 5 ) Fischer, R. X.;Baur, W. H.; Shannon, R. D.; Staley, R. H.; Vega, A. J.; Abrams, L.; Prince, E. J . Phys. Chem. 1986, 90, 4414. (6) Davis, J. H.; Jeffrey, K. R.; Bloom, M.; Valic, M. J.; Higgs, T. P. Chem. Phys. Left. 1976, 42, 390. (7) Spiess, H. W.Colloid Polym. Sci. 1983, 261, 193; Adu. Polym. Sci. 1985, 66, 23. (8) Spiess, H. W.; Sillescu, H. J . Magn. Reson. 1981, 42, 381. Vega, A. J. Polym. P r e p . (Am. Chem. SOC.,Diu. Polym. Chem.) 1981, 22, 282. Schwartz, L . J.; Meirowitch, E.; Ripmeester, J. A.; Freed, J. H. J. Phys. Chem. 1983, 87, 4453. Baram, A. J . Phys. Chem. 1984, 88, 1695. (9) Vega, A. J. J . Magn. Reson. 1985. 65,252.
Figure 2. Deuterium NMR spectra of DME-d, calculated for four types of fast molecular motion: (a) no motion except internal methyl group rotation; (b) 180' flips about the C, axis; (c) fourfold rotational jumps about C, between orientationswith azimuthal angles & 2 5 O and 180' f 2 5 O ; (d) axially symmetric rotation about C,. The line shapes were calculated with a COC angle of 117'; e2qQ/h = 50 kHz for the rotating CD,; Lorentzian broadening of 2 kHz.
partial information on the site distribution.
Discussion With this background we proceed to interpret the results of Figure 1, starting with the sample loaded with 5.4 molecules per unit cell. The spectrum at the lowest temperature is dominated by a typical axially symmetric quadrupole tensor with e2qQ/h 50 kHz, which is typical for a rigid methyl group rapidly rotating about its C3axis.' We thus conclude that this spectrum corresponds to sites in which the molecules are firmly bound in the zeolite pores, and, except for the fast methyl group rotation, they do not undergo additional diffusion, libration, or reorientation to any significant extent. As the temperature is increased above -185 'C a weak doublet appears at the center of the spectrum that grows in intensity until it dominates the spectrum at around 0 'C. At this temperature the spectral line shape resembles that of a quadrupole tensor with asymmetry parameter, 9 = 0.69 f 0.03. Most likely this spectrum is due to sites in which the CD30CD3molecules undergo 180' , axis. Such motion results in an average biaxial flips about their C quadrupole tensor with components qx = -qo, qy = q0(2 - 3 X cos2 (8/2)), and qz = q0(3 cos2 (8/2) - l ) , where the z direction coincides with the C, axis, the x direction is perpendicular to the plane of the molecule, 8 is the COC angle, and qo = (3/8)e2qQ/h. If 8 were equal to the tetrahedral angle 109.5', this would be an 7 = 1 tensor with a single cusp in the center of the spectrum. For larger angles the line shape exhibits a central doublet with a splitting, 12q,l, which is quite sensitive to 8. A theoretically calculated line shape is shown in Figure 2b. Substitution of the experimental value 12qzl = 7 f 1 kHz gives 8 = 117' f 1'. This is larger than the COC angle of 11 1.5' determined by electron diffractionlo and microwave spectroscopy" but close to the angle of 120.6' between the methyl C3 axes which were found not to coincide with the O C bond directions.I0 The agreement between the experimental spectrum taken at 0 'C and the 9 = 0.69 line shape is however not perfect, the former having a reduced spectral intensity around the shoulders at f 18 kHz. This indicates that additional motion takes place around the C,, axis. This cannot be a complete 360' rotation, because that would lead to the narrow axially symmetric tensor spectrum with 7 kHz splitting shown in Figure 2d. Instead, we must assume that the DME molecules perform the additional rotations with restricted amplitudes, but since the spectra do not have the well-defined shape of a single tensor, these amplitudes differ among
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(10) Kimura, K.; Kubo, M. J . Chem. Phys. 1959, 30, 151. (11) Kasai, P. H.; Myers, R. J. J . Chem. Phys. 1959, 30, 1096
J . Phys. Chem. 1986,90, 4905-4908 molecules adsorbed at different sites. Despite this apparent dispersity of chemisorption sites, the prominent doublet at f3.5 kHz in the spectra taken below 0 "C shows that the averaged tensors of the majority of the molecules have principal components along the C,, axes. This can only result if the motions include , axis itself does not change its orientation. 180" flips and if the C It is difficult to precisely determine the angular range of the librations about C2,, but one may estimate an average amplitude by comparison with line shapes calculated for fourfold rotational jumps between directions with azimuthal angles of f t p and 180" f p. Such a jump model results in a narrowed tensor with components (q, cos2 p q,, sin2 /3), (qx sin2 p + q,, cos2 ,f3), and q2, where qx, q,,, and qr are the tensor components given above for single 180° flips. The line shape shown in Figure 2c of a tensor with components 11, -14.5, and 3.5 kHz, corresponding to /3 = 25", suggests that this angle could be considered as an average amplitude of the additional librations. By comparison with linear combinations of the line shapes of Figure 2, a and c, we estimate that at a loading of 5.4 molecules per unit cell the number of nonflipping DME molecules decreases from about 4 (80%) at -185 "C to about 1 (15%) at 0 O C . As the sample is heated above 0 "C the spectrum gradually transforms to a broad singlet that becomes narrower at higher temperatures. This line shape corresponds to sorbate molecules that undergo restricted isotropic motion, most likely jumps between different sites within as well as between the zeolite cavities. Due to the overall cubic symmetry of H-rho such a motion in the extreme fast limit will result in a sharp singlet. We may identify the species giving rise to this spectrum as weakly bound physisorbed molecules that undergo fast translational diffusion, while the spectra of the rigid powder may be identified with tightly bound chemisorbed species. The spectra in the central column, corresponding to 10.8 DME molecules per unit cell, are similar to those in the first column except for a large shift in the temperature scale. Now the lowtemperature spectrum (-170 "C) resembles the one observed at
+
4905
-100 "C in the 5.4 molecules per unit cell sample while the room-temperature spectrum resembles that observed above 100 "C of the latter. The estimated number of nonflipping molecules at -170 "C is 35% of 10.4, i.e., between 3 and 4 per unit cell. This is comparable to the number of rigid molecules found at that temperature when the zeolite contained 5.4 molecules per unit cell (see above), which indicates that increasing the number of sorbed molecules from half to full the number of acid sites does not affect the nature of the already existing strong chemisorption complexes. The additional molecules are chemisorbed at weaker sites, thus shifting the overall dynamic range to lower temperatures. When the sample is loaded with 21 molecules per unit cell the spectrum behavior (right column in Figure 1) is quite different. Over most of the temperature scale the spectrum is now dominated by a central peak due to different types of physisorbed molecules, including perhaps also molecules sorbed on the outer surfaces of the crystallites. These various physisorbed species gradually solidify below -80 "C, resulting in a spectrum characteristic of internally rotating but otherwise immobile methyl groups. These results show that for loading levels as high as 10.4 molecules per unit cell the DME molecules are mostly chemisorbed, while at higher loadings the physisorbed species dominate. This suggests that the number of chemisorbed molecules cannot exceed that of the available acid sites, which is close to 12 per unit cell, as mentioned above. The results of this study indicate the presence of a very wide distribution of both chemisorbed and physisorbed sites for DME in H-rho. We have found similar effects for other sorbates, in particular methanol in zeolite H-Y2 and methyl amines in H-rho. More extensive measurements performed on these systems as well as on NH4-rho and H-rho will .be published in forthcoming publications.
Acknowledgment. We thank D. R. Corbin, T. E. Gier, and R. D. Shannon for helping us with sample preparation and R. 0. Balback for his assistance in the experimental work.
Isotopic Oxygen Heteroexchange between Acetone and a Lattlce of Vanadium Oxide Supported on Silica: A New Method for the Determination of the Number of Surface V=O Species Tsunehiro Tanaka, Risa Tsuchitani, Masaharu Ooe, Takuzo Funabiki, and Satohiro Yoshida* Department of Hydrocarbon Chemistry and Division of Molecular Engineering, Kyoto University, Kyoto 606, Japan (Received: April 28, 1986; In Final Form: July 29, 1986)
It has been found that exchange of oxygen atoms takes place between acetone and V=O species of V205/Si02.The number of the V=O species, NV4, can be determined by utilizing this reaction. Dispersion, defined as the ratio of Nv4 to the number of vanadium ions, was consistent with the results of XRD analysis and hydrogen oxidation over two series of the catalysts prepared by different methods.
Introduction It is now well recognized that surface V=O bonds of vanadium oxide play an important role in catalysis, and the activity of the V=O species depends upon several factors. Of these, the effect of supports is important and many papers have been published on the characterization of supported vanadium oxide (1) Gellings, P. J. In Catalysis, Bond, G. C., Webb, G., Eds.; Royal Society of Chemistry: London, 1985; Spec. Period. Rep. Vol. 7, p 104. See references therein.
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In the study to correlate the catalytic activity to physicochemical properties of a catalyst, it is important to obtain information about (2) Yoshida, S.; Matsumura, Y.; Noda, S.; Funabiki, T. J . Chem. Soc., Faraday Trans. J 1981, 77, 2237. Yoshida, S.; Magatani, Y.; Noda, S.; Funabiki, T. J. Chem. Soc., Chem. Commun. 1982,601. Yoshida, S.;Tanaka, T.; Okada, M.; Funabiki, T. J . Chem. SOC.,Faraday Trans. J 1984,80, 119. Tanaka, T.; Ooe, M.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. J 1986, 82, 35. (3) Kera, Y.; Hirota, K. J . Phys. Chem. 1969, 73, 3973. Hirota, K.; Kera, Y.; Teratani, S. J . Phys. Chem. 1968, 72, 3133.
0 1986 American Chemical Society
