J. Phys. Chem. 1992, 96,7493-7496
7493
Institute of Technology, under contract to the National Aeronautics and Space Administration. Y.L.Y. was supported by NASA Grant NAGW-413 to the California Institute of Technology. References and Notes
NUMBER DENSITY ( ~ m - ~ ) Figure 3. Model calculations of the vertical profiles of 02C10N02,C10, and C10N02 in the antarctic stratosphere during noontime conditions (second week of spring). Vertical profiles of 02CION02labeled A and B reflect variations of i 2 kcal mol-' in the estimated bond energy (see text).
been carried out on the bond energy of 02C10N02. Curve A (Figure 3) shows the results for 02C10N02if the bond energy is 2 kcal mol-' smaller than the standard value. Curve B gives the corresponding results if the bond energy is 2 kcal mol-' larger. This large range of uncertainty is the principal source of uncertainty in the modeling of 02C10N02. The derived column densities of 02C10N02are significantly less than those obtained for C10N02 and C10. Consideration of other reactions which form 02C10N02may increase the atmospheric abundance, however. For example, we propose that the reaction of C10, with NO2 will produce 02C10N02: C103 + NO2 + M -.+ 02ClON02 + M (9)
Currently we are investigatingatmosphericpathways for formation of c10,. Acknowledgment. Part of the research described in this report was carried out at the Jet Propulsion Laboratory, California
(1) de Zafra, R. L.; Jaramillo, M.; Parrish, A.; Solomon, P.; Conner, B.; Barrett, J. Nature 1987, 328, 408. (2) Brune, W. H.; Anderson, J. G.; Chan, K. R. J . Geophys. Res. 1989, 94. 16649. (3) Solomon, S.; Mount, G. H.; Sanders, R. W.; Schmeltekopf, A. L. J . Geophys. Res. 1987, 92, 8329. (4) Molina, L. T.; Molina, M. J. J . Phys. Chem. 1987, 91, 433. (5) Sander, S. P.; Friedl, R. R.; Yung, Y. L. Science 1989, 249, 1095. (6) Christe, K. 0.;Wilson, W. W.; Wilson, R. P. Inorg. Chem. 1989, 28, 675. (7) Friedl, R. R.; Sander, S. P. J . Phys. Chem. 1987, 91, 2721. (8) Lang, V. I.; Sander, S. P.; Friedl, R. R. J . Mol. Spectrosc. 1988, 132, 89. (9) Becker, E.; Wille, U.; Rahman, M. M.; Schindler, R. N. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1173. (10) Biggs, P.; Harwood, M. H.; Paar, A. D.; Wayne, R. P. J. Phys. Chem. 1991, 95. 7746. (11) Schack, C. J.; Pilipovich, D. Inorg. Chem. 1970, 9, 1387. (12) Christe, K. 0.;Schack, C. J.; Curtis, E. C. Inorg. Chem. 1971, 10, 1589. (13) Burkholder, J. 8.; Orlando, J. J.; Howard, C. J. J . Phys. Chem. 1990, 94, 687. (14) Jansen, M.; Tobias, K. M.; Willner, H. Natunvissenschaften 1986, 73, 734. (15) Witt, J. D.; Hammaker, R. M. J. Chem. Phys. 1973, 58, 303. (16) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Molina, M. J.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R. JPL Publication 90-1; Jet Propulsion Laboratory: Pasadena, CA, 1990. (17) Troe, J. J . Phys. Chem. 1979,83, 114. (18) Colussi, A. J. J . Phys. Chem. 1990, 94, 8922. (19) Computer analysis performed using program QCMPO67 supplied by the Quantum Chemistry Program Exchange, Indiana University. The input geometrical parameters and force constants for 02CION02were estimated from CIONO, (Miller, R. H.; Bernitt, D. L.; Hisatsune, I. C. Spectrochim. Acta 1967, 23A, 223) and CI2O3(author's unpublished data). (20) Allen, M.; Yung,Y. L.; Waters, J. J . Geophys. Res. 1981,86,3617. (21) Froidevaux, L.; Allen, M.; Yung, Y. L. J . Geophys. Res. 1985, 90, 12999. (22) Toon, G. C.; Farmer, C. B.; Lowes, L. L.; Schaper, P. W.; Blavier, J. F.; Norton, R. H. J . Geophys. Res. 1990, 94, 16571. (23) The maximum concentrations of CIO, BrO, and NO, in the model are !S.ppbv, 5 pptv, and 10 pptv, respectively. The model results are for the beginning of September.
Polarized Fourier Transform Infrared Microscopy as a Tool for Structural Analysis of Adsorbates in Molecular Sieves F.Schiith Institut fur Anorganische Chemie und Analytische Chemie der Johannes Gutenberg-Universitat, Mainz, Becherweg 24, 6500 Mainz, Germany (Received: May 26, 1992; In Final Form: July 28, 1992)
Using FTIR microscopy with polarized IR radiation on silicalite I single crystals fully loaded with p-xylene, the existence of an ordered adsorbate could be proven for the first time by IR spectroscopy. By analyzing the polarized absorption bands the orientation of the p-xylene molecules relative to the host structure could be determined. The results agree well with structural data obtained from X-ray diffraction experiments. These first results suggest that polarized IR microscopy could develop into a powerful tool for the analysis of adsorbate structures, assisting in complete structure resolution by diffraction techniques.
Introduction Polarized IR spectroscopy is a well-known technique for the analysis of orientated molecules since about 50 years ago.' It can be used either for assignment of IR bands to certain modes, if the orientation of molecules with respect to the IR beam is known (see, for instance, the work of Zelei and D o h on para-substituted benzene d e r i ~ a t i v e s ~ - or ~ ) ,for structural analysis, if bands have already been assigned unanimously to certain modes, e.g., for orientation in polymer^,^ for analysis of details in the crystal 0022-3654/92/2096-7493$03.00/0
structure6. To our best knowledge, however, this method has until now never been applied to analyze the orientation of adsorbates in molecular sieves. Usually for this purpose X-ray diffraction is used.' Occasionally also neutron diffraction* or one of these methods in combination with NMR spectroscopy have been a p plied.9 Since experiments and data analysis in these experiments can be difficult and time consuming, it would be very advantageous to use a method which allows a relatively simple determination of the orientation of molecules in a crystal. This would exclude 0 1992 American Chemical Society
7494 The Journal of Physical Chemistry, Vol. 96, No. 19. 1992
Letters 2.51
TABLE I: Polarized r-Xylene &ads between 3500 rad 1300 cm-'" position (cm-I) 3141 3111* 3052 3033c 3025d 3003" 29Mb 2923 2867 2730 1888 1795 1705 1518 1455 1418 1375
(100) oriented I
11 II I
(010) oriented
I
II II I I
sym species ? ? ? ?
direction of transition moment ? ? ? ?
11
II /I
Blu B2U E
I I
I
A1
z z
I
I
II
I/
B2" Blu
Y
I 11
I
:\
Z
Y XY Z
I
I
I/
?
?
ll /I
II I II II
BIU E B2u
Z XY
Y
I
I
AI
z
I
Z
2assignment ? ? ? ? 20a 20b V,(CH) v&H) 26,,(CH) 26,(CH) 5 17a 10a + 17a lattice 19a d,(CW 19b 6,(CH)
2
several structural models and facilitate the analysis of X-ray data appreciably. Such a method could be polarized IR spectroscopy. These experiments, however, can only be performed on single crystals or orientated polycrystalline material. Since zeolite single crystals are at most several hundred micrometers long, such measurements require the use of an infrared microscope. Infrared microscopy is a relatively novel technique in the field of zeolite chemistry: It was first used by van Bekkum et al.IOto analyze the elemental distribution of boron in a B-ZSM-5. Later on also Lercher and his group"J2 applied a microscope to analyze the decomposition of the template in ZSM-5 and to study the diffusion of pxylene in single crystals. Polarized IR measurements, however, have not yet been performed on zeolites to the best of our knowledge. To determine whether this method could be useful in the analysis of the adsorbate structure in zeolites, we performed measurements on p-xylene-loaded single crystals of silicalite I. This system has two advantages for testing the potential of the method: (1) It is well characterized by X-ray diffraction; (2)the vibration modes of para-substituted benzenes are well-known.
Experimental Section The samples studied were large silicalite I crystals with dimensions of approximately 380 r m X 70 r m synthesized by u. Muller, following a procedure described elsewhere.I3 The crystals were calcined at 823 K in air for 12 h. After activation in vacuo at 573 K for 2 h, the samples were exposed to p-xylene vapor (Aldrich, 99+%) at room temperature for 16 h. Spectra of the loaded samples were recorded immediately after loading with a Nicolet SSXB FTIR spectrometer equipped with a SpectraTec Research IR-PLAN microscope. Polarization of the IR radiation was achieved with a ZnSe wire grid polarizer (SpectraTec). For recording the spectra, a crystal was selected by means of two adjustable apertures. Spectra of several crystals were recorded, orientated with the (100) or (010)direction, respectively, parallel to the IR beam axis. Spectra of different crystals, orientated in the same direction, were almost identical. The polarization was scanned in 10' intervals from 8 = 0' (beam perpendicular to the crystal c axis) over 8 = 90° (beam parallel to c axis) to 8 = NOo. Over the duration of a typical experiment, scanning the whole range between 0 and 180° (10h), a loss of about 30% pxylene was observed, indicated by the decrease in intensity of thepxylene absorption bands. This, however, did not alter the principle features of the spectra: All the strongly polarized bands retained
'tl
w
0.5
iu 0
1.5-
A
+
a 11 and I with respect to crystal c axis. Symmetry species of ring those of CH, group vibration based on C,, vibrations based on D2*, local symmetry. The z axis passes through methyl groups, the x axis perpendicular to aromatic ring. bShifts to 3103 cm-' over 10 h after removing crystal from xylene atmosphere. CDecreasesstrongly in intensity after removing crystal from xylene atmosphere. Develops within about 1 h after removing crystal from xylene atmosphere.
I
b
9)
I
0
I
3300 3200 3100 3000 2900 2800 2700 Wavenumber [cm-11 4
3 9)
0 C
m e
8
2
9 1
8 IO 1900 1800 1700 1600 1500 1400 1 DO Wavenu mber [cm-1] Figure 1. Polarized IR spectra of pxylene loaded silicalite I. 8 = Oo corresponds to polarization perpendicular to the crystal's c axis. Arrows marking xylene bands. Spectra recorded with crystal orientated with (100) parallel to beam axis (see inset).
their dichroic ratios which shows that the orientation of the molecules remaining in the pore system is not changed. Only a minor fraction of the polarized bands was changed in relative intensity compared to the remaining bands (indicated in Table I). The spectra presented in this paper were recorded 1 h after removing the samples from the xylene atmosphere. Other groups observed that at room temperature the adsorbate is stable for times between 10 hI4 up to 6 months.Is Although in the present work slight changes of the spectra were observed within the first 10 h, these findings can in principle be confirmed, especially since in the IR microscope the sample heats up to temperatures around 310 K.
Results and Discussion Figure 1 shows the polarized spectra of pxylena-loaded crystals of silicalite I recorded with the crystal orientated with the u axis parallel to the IR beam, as shown in the inset. The spectra with the crystal b axis parallel to the beam look very similar. The spectral regions shown are those where absorption bands of the p-xylene (arrows) can be observed. At lower wavenumbers the absorption of the crystals is too strong to clearly determinepxylene absorption bands. Polarized bands, however, are present in the region between 600 and 800 cm-I as well. There is, for instance, a very strongly polarized band at 692 cm-l which couid a m p o n d to vibration mode 4 of the pxylene molecule. This vibration, however, belongs to the 4 species,and should thus be IR inactive. IR activity could be induced, though, by interaction with the zadite lattice or with neighboringpxylene m o l d e s which would reduce the symmetry of the molecule. Further investigation of this effect is in progress. As can be seen in the figure, most of the pxylene absorption bands which can clearly be attributed are strongly polarized.
Letters Scanning stepwise the orientation of the electric field vector of the polarized light with respect to the zeolite crystal, it could be shown that the maximum and minimum intensities of the polarized bands coincide with the main axes of the crystal. Table I gives the position of the adsorption bands together with the polarization, the band assignments, the symmetry species, and the direction of the transition moment. The bands were assigned following Green.I6 The C2axis through the two methyl groups was chosen as z axis,the x axis is Perpendicular to the benzene plane, assuming Dlh symmetry for the benzene ring vibrations, with the methyl groups amsidered as mass points. C, local symmetry was assumed for the methyl group CH vibrations. There is vast literature concerning the assignment of aromatic methyl groups" with respect to free or hindered rotation. Since the bands discussed in thisrespect, however, cannot be analyzed in the present study (due to superposition of the very strong u,(CH)), this question will not be considered here. The assignment of the bending vibrations and the bands at 2923 and 2867 cm-I, which show very strong polarization, are unanimously accepted. The rather high intensity (for an overtone) of the 2867-m-' band can be explained by Fermi resonance with the u,(CH) fundamental at 2923 cm-'.'* The assignment of the aromatic CH valence vibrations (20a and 20b) is somewhat ambiguous. GreenI6gives values of 3044 (20a) and 3017 cm-' (20b). No bands could be detected at these positions. On the basis of the polarization behavior of the bands above 3000 cm-' (see below), the 3025- and the 3003-cm-' bands were identified with the 20a and 20b modes, respectively. Assignment of the remaining bands above 3000 cm-' (combination bands) was not attempted. All bands (except for the 6,(CH) modes) in the spectral range above 1300 cm-l are associated with transition moments in the benzene ring plane. The out-of-plane modes of the benzene ring absorb at wavenumbers below 1000 cm-I. Clear identification of xylene bands is not possible in this spectral region as stated above. It might be possible, however, to also analyze the outof-plane bands by using smaller silicalite crystals. With smaller crystals the silicalite bands do not blank out whole regions of the spectra. From Figure 1 and Table I it can be seen that all bands with a transition moment in the z direction have maximum intensity with the polarization perpendicular to the crystal c axis and almost no intensity with the polarization parallel to c. This observation also leads to the assignment of the aromatic CH valence vibration above 3000 cm-I. The 20a vibration has a transition moment in the z direction and thus should show the same polarization behavior as the 1519 cm-', for instance. A similar argument holds for the 20b vibration. From the polarized spectra it can be immediately inferred, that the xylene molecules are basically orientated with the long axis (z) in the crystal a or b direction. Moreover, from the time dependence of the band intensities it can be deduced that the loss of p-xylene over the duration of an experiment occurs predominantly from the straight channels. With a crystal orientated parallel (100) (the long axis of molecules in the straight channels is monitored), band intensities decreased by about 45% over the experiment time, while for a crystal orientated along (010) (monitoring of the long axis of molecules in the sinusoidal channels) a decrease of about 25% is recorded. The dichroic ratios, however, remain unaffected. This shows that the orientation of molecules is not dependent on loading over a fairly large range. The analysis of the orientation is in good agreement with X-ray data from Mentzen14and the detailed analysis of van Koningsveld et al.,15wherepxylene could be localized in two different positions: One molecule is located in the channel intersection, orientated with the long axis parallel to (100) at an angle of about 7.5O with respect to (010). The long axis of the other molecule deviates about 5 . 5 O from (100) and is parallel to (OlO).I3 The polarized spectra can be analyzed in more detail following the orientated gas model, assuming no vibrational coupling of different xylene molecules with each other or with the zeolite lattice. This is most probably true in first approximation, since pxylene is not adsorbed very strongly in the zeolite and can usually
The Journal ofPhysica1 Chemistry, Vol. 96, No. 19, 1992 1495
be pumped off at temperatures not much above room temperature. A relatively good fit for the dicroic ratios of the polarized bands was obtained assuming the following orientations of the different xylene molecules in the unit cell (four molecules in the channel intersections, four molecules in the sinusoidal channel): For xylene 1 (channel intersection) the long axis deviates by 18O from the (010) direction and is parallel to the (100) direction, and the benzene plane is tilted by 41 O against the bc plane. For xylene 2 (sinusoidal channels) the long axis deviates also by 18O from the (100) direction and is parallel to (010). The benzene plane of xylene 2 is tilted by 37' against the ac plane. These results agree reasonably well with the X-ray data of van Koningsveld et aI.l3 The deviations of about 10' in some of the angles between their analysis and this work could be due to uncertainties in the determinationof the integral absorbance of the absorption bands, which are fairly high, especially for low values, the errors in the orientation of the crystals which is certain to be about So. A more detailed analysis of the data, with complete deconvolution of all bands, is being carried out at the moment. However, it should be mentioned that a xylene structure with only one xylene orientation in the framework could explain the polarized infrared results as well. A xylene molecule with the long axis pointing in the crystal (1 10) direction and a benzene plane tilted by about SOo against the ab plane would yield similarly good fits of the experimental data. Thus, polarized IR spectroscopy alone would not be sufficient to resolve the structure. Also one other problem arises in the analysis of the data. Although the crystals exhibit a perfect morphology with the (100) and (010) surfaces being absolutely smooth in the SEM, in the light microscope two diagonal lines are seen if the crystal is orientated in the (100) direction. The sections cut out by these lines exhibit a different extinction behavior when inspected with visible polarized lights through crossed polarizers. About 50 different samples, also samples synthesized by other 'groups, were inspected which all exhibited the described pattern. There are several possible explanations for this effect: It could be a stress-induced pattern, a zonation as sometimes observed for quartz crystals, or a twinning of crystals. If it were indeed a twinning, the relatively similar orientation of the p-xylene molecules in the straight and sinusoidal channels, respectively, could be due to a superposition of the orientations in the different specimen of the twins. If this were the case, only statements concerning the orientation of the xylene in the ab plane with respect to the c axis were possible. Work is under way to find out whether the crystals are twinned and to elucidate how the different parts are exactly orientated, if they are actually twins. First analyses with optical and X-ray methods did not yet yield conclusive results. An additional hint that the crystals are not completely homogeneous was obtained by analyzing the crystals spatially resolved using polarized IR microscopy. Figure 2 shows the spectra in the 3300-2700-cm-' region for the middle of an (010) orientated crystal and the end, respectively, as shown in the insets. As can be seen from the spectrum of the end of the crystal, only little intensity is observed for the bands corresponding to transition moments in direction of the long axis of thepxylene. This means that the p-xylene molecules in this part of the crystal are orientated basically along the crystal b axis. Spectra of the end of crystals orientated with the (100) direction along the beam axis, however, show intensive polarized bands for transition moments in the direction of the crystal b axis, supporting the conclusion drawn above. These results suggest that less p-xylene is present at the ends of the crystals compared to the middle section. The reason for this effect could be the above mentioned inhomogeneity of the large crystals. For a detailed analysis of these effects a highvacuum cell is under construction which will allow the analysis of crystals in equilibrium with a defined pressure of the adsorbed gas. Conclusion Using polarized FTIR microscopy on p-xylene-loaded single crystals, the existence of an ordered adsorbate could be proven.
Letters
7496 The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 1.21
I
't4 3300 3200 3100 3000 2900 2800 2700 Wavenumber [cm-11 1.21
ICh
'1 ! I !
It can be expected that further analysis of smaller crystals will give additional information, since then, probably, also the bands in the spectral regions which are now ohscured by the zeolite lattice vibrations can be analyzed. With a high-vacuum cell for the microscope which is presently being built, it should also be possible to investigate the low-coverage p-xylene silicalite. Analyzing the transient behavior, one might even be able to obtain information on the loading/unloading process, somewhat similar to the investigations of the diffusion of pxylene performed with unpolarized light by Lercher et a1.'* Another possible application of polarized IR microscopy could be assistance in assigning lattice vibrations. First results of experiments on unloaded crystals show that also part of the lattice vibrations below 1000 cm-' are strongly polarized. The results of this work, and the points addressed in the conclusion suggest that polarized IR microscopy could develop into a valuable tool for the analysis of adsorbates in zeolites and, possibly, also the zeolites themselves. One prerequisite, however, has to be fulfilled: Single crystals of at least some 10 pm are needed to obtain spectra with a sufficiently high signal-to-noise ratio. This restricts the applicability of the method to only a smaller fraction of the known molecular sieve structures, one of which, however, is the important ZSM-5. Acknowledgment. Financial support of the DFG under Grant Number Schu744/4-1 is gratefully acknowledged. I would like to thank K. Unger, W. Liptay, C. Weidenthaler, and B. Rtidinger for helpful discussions.
References and Notes
%oo
3ioo 3ioo 3600 2600 2600 2700 Wavenumber [cm-11
Figure 2. Spatially resolved polarized spectra of a p-xylene loaded silicalite I. 8 = Oo corresponds to polarization perpendicular to the crystal's c axis. Crystal orientated with (010) parallel to beam axis. Top: middle of crystal. Bottom: end of crystal (see inset).
Assignments of most of the polarized bands was possible. From the analysis of the dichroic behavior of some of the absorption bands, information on the orientation of the p-xylene with respect to the host lattice could be obtained. The results are in fairly g o d accordance with literature data. The results are, however, still somewhat ambiguous about the exact orientation, since the crystals investigated might be twinned in a rather complex way. If this is the case, it is not possible to separate between the p-xylene orientation with respect to the crystallographic u and b axes. Only information about the orientation of the p-xylene molecules against the c axis can be obtained in such a case. A detailed analysis of the possible twinning is under way to solve this problem. However, even considering theac difficulties, it has clearly been demonstrated that the analysis of polarized spectra of zeolites loaded with an adsorbate can contribute to the structural analysis of the adsorbate.
(1) Ambrose, E. J.; Elliott, A.; Temple, R. B. Proc. R. SOC.London, A 1951, 206, 192 and reference therein. (2) Zelei, B.; D o h , S.;Righini, R. Spectrochim. Acta 1978, 34A, 343. (3) Zelei, B.; Dobos, S . Spectrochim. Acra 1979, 35A, 915. (4) Dobos, S.;Szabo, A.; Zelei, B. Spectrochim. Acra 1976, 32A, 1401. (5) Bradley, D. D. C.; Friend, R. H.; Hartmann, T.; Marseglia, E. A,; Sokolowski, M.M.; Townsend, P. D. Synth. Mer. 1987, 17, 437. (6) Giermanska, J.; Smtak, M. M.; Kowala, W. W. J . Mol. Struct. 1990, 222, 285. (7) For instance: Baerlocher, C. In Olson, D. H., Bisio, A., Eds.; Proc. 6th IZC; London 1984,823. van Koningsveld, H.; Tuinstra, F.; van Bekkum, H.; Jansen, J. C. Acta. Crystallogr. 1989, B45,423. Mentzen, B. F. Mater. Res. Bull. 1987, 22, 489. (8) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986,90, 1311. (9) Fyfe, C. A.; Strobl, H.; Gies, H.; Kokotailo, G. T. Can. J. Chem. 1988, 66, 1942. (10) Jansen, J. C.; de Ruiter, R.; Biron, E.; van Bekkum, H.In Jacobs,
P. A,, van Santen, R. A., Eds.;Zeolites: Facts, Figures, Future; Amsterdam, 1989; p 679. (11) Nowotny, M.; Lercher, J. A.; Kessler, H. Zeolites 1991, 11, 454. (12) Narbeshuber, T.; Lercher, J. A.; Presentation at the 4th German Workshop on Zeolite Chemistry, Mainz, March, 1992. (13) Muller, U.; Unger, K. Zeolites 1988, 8, 154. (14) Mentzen, B. F. C. R. Acad. Sci. 1987, 305, 581. (15) van Koningsveld, H.; Tuinstra, F.; van Bekkum, H.; Jansen, J. C. Acra Crystallogr. 1989, B45, 423. (16) Green, J. H. S . Spectrochim. Acta 1970, 26A, 1503. (17) For instance: Dempster, A. B.; Powell, D. B.; Sbeppard, N. Spectrochim. Acta 1972, 28A, 373. Kanesaka, I.; Satozaki, Y.; Kiyoyasu, K.J. Chem. Phys. 1982, 76, 3953; Dempster, A. B.; Powell, D. B.; Sheppard, N. Spectrochim. Acta 1975, A31, 245; Tramer, A.; Tomczak, Z . Spectrochim. Acta 1968, 24A, 2051. Forel, M. T. Spectrochim. Acta 1968, 24A, 311. (18) Dempster, A. B.; Powell, D. B.; Sheppard, N. Spectrochim. Acra 1972, 28A, 373.
