C.L. Angel1
222
Raman Spectroscopic Investigation of Zeolites and Adsorbed Molecules
Union Carbide Corporation, Tarrytown Technical Center, Tarrytown, New York, 10591 (Received June 79, 1972) Pubiication costs assisted by Union Carbide Corporation
The Raman spectra of quartz, sodalite, and 16 natural and synthetic zeolites have been determined. All these materials give weak Raman spectra. Although these spectra are not as informative as the infrared framework frequencies, they still show differences between zeolite structures. In nearly all cases, the samples gave rise to an excessive background and it was necessary to use special high-purity materials to eliminate this background. Even when the background was decreased by prolonged exposure to the laser, the intensity of the Raman scattering depended on the presence of impurities. Activation at higher temperatures always led to increased bqckground. The Raman spectra of adsorbed molecules (carbon dioxide, propylene, acetonitrile, acrolein) on several types of zeolites were obtained. It was possible to record the complete spectra of the adsorbates, in contrast to the infrared where only the part of the spectra over 1200 c u i - I can be obtained.
Introduction This paper deals with two closely related subjects: the Raman spectra of zeolates and the Raman spectra of molecules adsorbed on zeolites. Infrared spectra of a large number of zelolites have been reported by Flanigen, Khatarni, and §zymanski,l who have observed a number of strong bands in the region 1200-200 cm-l. They were able to assign these bands either to the vibration of the silica tetrahedra or to external linkages between these tetrahedra Characteristic of individual zeolite structures. They could identify certain spectral regions with characteristic building units of the zeolite structure, such as double rings or various pore openings. Although the Raman spectra of a number of ailiceous minerals have been reported,2 no Raman spectrum of a zeolite is available in the literature, to our knowledge. In the present work we examined a large number of trasious natural and synthetic zeolites. Such examinations are full of experimental difficulties due to the weakness of the Raman spectra and the strong fluorescent background. We were able to circumvent some of these difficulties and obtain good Raman spectra of about 16 different types of zeolites. There is a vast literature on the infrared spectra of molecules adsorbed on a variety of high surface area porous solids but there are only a few papers on the Raman spectra of adsorbed m o l e c ~ l e s Most . ~ of this work was done on silica gels or related materials and no Raman spectra of molecules adsorbed on zeolites have been reported so €ar. The purpose of such @dies is mainly to expand the range of the observable spectrum from the rather limited region of the infrared spectra. Most of the substrates show very strong infrared bands; for example, in the spectra of molecules adsorbed on zeolites it is impossible to observe anything below 1200 c n ~ -because -~ of the practically total absorption of the zeolite. Since siliceous materials, including zeolites, exhibit very weak Raman spectra, this would eliminate the background due to the substrate and allow us to observe the vibrational spectra of adsorbed molecules down to a p p r ~ x ~ K ~ a t100 e l y cm-I. The other purpose of such investigations i b that Raman scattering obeys different selection rules from infrared absorption. The two kinds of spectra are ofren complementary, and in the case of adsorbed molecules obtaining the Raman spectra in adThe Journal of Physical Chemistry, Voi. 77, No. 2, 7973
dition to the infrared spectra would allow the observation of all possible vibrations (except those that are inactive both in the infrared and the Raman) and complete the vibrational assignment of such molecules. Ultimately the hope would be to observe homonuclear mdecules such as hydrogen, oxygen, or nitrogen in the adsorbed state. In the present work we have shown that under favorable circumstances it is possible to obtain very good quality Raman spectra of adsorbed molecules. While these studies at the moment are limited to certain high-purity zeolite samples, they show that despite certain experimental difficulties it is still possible to obtain Raman spectra of molecules adsorbed on zeolites. Experimental Section The spectra were obtained on a Jarrel-Ash Model 25-300 Raman spectrometer equipped with a photon counting detector. The laser used was a Coherent Radiation Model 52 argon-krypton mixed gas laser. The wavelength used was 4880 with the power output of approximately 300 mW. Samples were examined at 90" illumination. Powders were pressed into 0.5-in. diameter pellets at less than 4000 lb/in.z pressure. For studies 011 adsorbed molecules, parts of such pellets were placed in a small glass vacuum cell with a flat window through which the laser beam and the scattered light could pass. Zeolite samples were either natural crystals or synthetic materials; some samples were commercial materials and others were special high-purity materials as described later. Natural crystals used were sodalite, basalt mineral from Bancroft, Ontario; chabazite, basalt mineral from Kagawa, Japan; gmelinite, basalt mineral from Nova Scotia; and erionite, sedimentary mineral from Pine Valley, Nev. The first three were available in clusters of crystals of several millimeters in size, while the erionite was a powder of less than 0.1-mm particle size. The other samples were synthetic zeolites prepared in Union Carbide laboratories; they had particles in the micron or submicron ranges. A brief description of the composition of the E M Flanigen, H Khatarni, and H A Szymariski, Advan Chem Ser , No. 101,201 (1971) (2) W p. Grifflth, J Chem SOC A, 1372 (1969) (3) For references see P J Hendra, J R Horder, and E J Loader, J Chem SOC A, 1766 (1971)
(1)
Zeolites and Adsorbed Molecules
223
SODAUTE
1
ri
X
A
I
f:
'i
503
I I
II
It
B
FREQUENCY (em -l
FREQUENCY (cm"' )
Figure 1. Raman spectra of quartz, sodalite, and
zeolite samples can be found in Table I1 of ref 1. We observed very early in our experiments that zeolites give rise t o very weak Raman scattering. This is not surprising since it has 'been noted in the literature4 that crystals like quartz lor sapphire also give very weak Raman spectra compared to diamond, for example. The main difficulty in the observation of our spectra was the considerably high background of most of the samples. This background (or ,fluorescence)has been noted by many authors3.5 in the examination of metal oxide materials. They have also reported, as we have found in the present work, that prolonged exposure to the laser beam (so-called burnout) generally decreased this background. However, in the case of many samples even this decrease did not reduce the background! sufficiently for the observation of Raman spectra. It has been suggested5 previously that the cause of this high background could be extremely small particle size of some of the materials examined, for example, Cab0-Si1 Silica,6 We found that some zeolites that come in extremely small (submicron) particle size (e.g., Q ) did not show a flulorescent background any stronger than did a large crystal of sodalite specimen which was ground and
I
some zeolites obtained with 4880-a excitation. therefore contained much larger (of' th.e order of hundred of microns) particles. Another possible reason for the excessive background could be flurorescence due to transition metal, especially iron impurities. We have been able to obtain a high-purity zeolite Y preparation which contained less than 17 ppm iron. This material showed much less of a fluorescent background initially; on exposure to the laser this background decayed very little. Eventually it reached the same level as the initially high background of some other zeolites which decreased considerably on exposure to the laser. The intensity-frequency profiles of the fluorescent backgrounds were very similar in all the zeolites; therewas a very broad maximum at about 3400 em-I. Other high-purity materials available were samples of zeolite X, zeolite A, and an ammonium exchanged zeolite Y. All these high-purity materials gave rise to a relatively strong Raman spectrum. One o f the most interesting observaS. P. S. Porto and R. S. Krishnan, J. Chem. Phys., 47,1009 (1967). (5) P. J. Hendra and E. J. Loader, Trans. Faraday Soc., 67, 828 (I971). (6) Cab-0-Si1 is a trademark of the Cabot Co. (4)
The Journal of Physical Chemistry, Voi. 77, No. 2, 7973
C. L. Angel1
224
TABLE I: Observed Frequencies in the Raman Spectra of Zeolites and Some Other Materials Matorial
Quartz Sodalite
1160~
A
IlOOw
x
ioaow 1065w 1037w 1075w
CaX Y B
aiow 988a 977w 99Ow 99ow
~ O Q W
700w
505s 51 Os
llO0w 1lOOW
502s 482s 490s 480s
1;1
Chabazite
11oow
Gmelinite
1115w
s Offretite (synthetic) T
Erionite L
1080m 1140w
605w
C (analcime type) Zeolon wi
Band positions in cm-' 466s 403w 463s 410w 490s
490s 490s 488s 485s 490s 498s
635s
1085w
tions of this work was that samples which initially gave a very high background gave much weaker Raman spectra, even after the background decayed on exposure to the laser to a comparable level with the high-purity compounds. For example, seven zeolite Y samples from different sources were examined under identical conditions: same pellet preparation, position in the spectrometer, laser power, and gain conditions on the amplifier. Samples that initially gave high background eventually finished with a low background but there was a variation by a factor of 20 between the strength of the bands observed in the various spectra. In addition, when adsorbed molecules were studied, the Raman spectra of the adsorbed species on zeolites with a strong zeolite spectrum were strong, while on zeolites that gave rise to weak zeolite spectra the bands of the adsorbed species were weak. Apparently the impurity present that gives rise to the high background of the zeolite itself in some way interferes with the Raman scattering phenomenon. The mechanism for this is unknown, but it does not seem to be likely that the interference is with the Raman scattering phenomenon but must be some way hindering the propagation of the Raman rsidiation coming out of the sample. When samples were activated under various conditions the background invariably got much higher, and in some cases it was impo,&ble to make any spectral measurements at all. This phenomenon has been noted by other authors in the case of oxide material^.^ Sheppard and coworkers7 have reported that in the case of silica and silica-alumina materials the fluorescent background can be burnt out by heating the materials in oxygen at a high temperature (about 500") Unfortunately, activation of zeolites under comparable conditions (in a pure oxygen stream a t 500" for 6 hr or in U ~ C U Oat 500" for 2 hr, or in air a t 500" for 2 hr) alwaya leads to considerably higher backgrounds. Where spectra of adsorbed species could be studied, materials were activated at an intermediate temperature of 200 or 250" for a prolonged period. The increase of background was not overwhelming and exposure to the laser managed to reduce it to a useable level. The Journal of Physical C h e m i s t r y , Vol. 77, No. 2, 1973
512m 485s
340w 375w 360w 350w
2 6 8 ~ 294w 280w
210s 263 w
2aow 270w 290w
240w
425w 463sh 452sh 41 5m 41 2w 430m 430w 423w 472w 440w
480s 810w
355m
465m 440m 420w
370w 330w 31 5m 330w
370w
317m 31 5w 275w 305w
390w 395s
31 Ow
220w
It is difficult to imagine what causes the increase of the background on activation. One possible suggestion is that when the cavities of the zeolitic materials are emptied of water, this creates a series of discontinuities in the material which could cause scattering of the light. Indeed, it has been observed that when activated materials were saturated with some adsorbates, such as carbon dioxide or propylene, the background decreased somewhat but nowhere near the original level of the unactivated zeolite. It is difficult to see how the discontinuities represented by the empty cavities in the zeolite can interfere with the light since the dimensions are so completely different. Zeolitic cavities are approximately 10 A in diameter while the light we are using is about 5000 A; therefore normal scattering phenomena are impossible. Another explanation for the increase in background on activation could be the formation of color centers on heating. It is known that some zeolites develop a color on activation, either in air or i n uacuo. The samples we have examined, however, did not show any color at all. It was observed that ammoniumexchanged samples where activation involves not just the removal of the water but the removal of ammonia and modification in the structure of the zeolite always caused the worst background problems. In no case were we able to examine adsorbed molecules on rn ammonium-exchanged sample activated by heating even when the activation temperature was kept as low as 200". Discussion A. Raman Spectra of the Zeolite Frameworks. Observed frequencies are collected in Table I together with the spectra of quartz and sodalite, while typical Raman spectra are given in Figures 1 and 2. The Raman spectra of both a and p quartz have been reported by several aut h o r ~ who ~ , ~ assigned all of the observed bands. The strongest band in the spectrum of quartz at 466 cm-1 is (7) T. A. Egerton, A. H. Hardin, Y . Kosirovski, and N. Sheppard, Chem. Commun., 888 (1971). ( 8 ) J. F. Scott and S. P.S. Porto, Phys. Rev., 161, 903 ('1967). (9) J. B. Bates and A. S.Quist, J . Chem. Phys., 56, 1528 (1972).
Zeolites and Adsorbed Molecules
225
GHABAZITE 477
GMELlNmE
r
i i -*Lrunuld.-
1200
1000
BCO
600
635
400
MO
il”’” 1-
FREQUENCY ( c m - ’ )
Figure 2. Rarnan spectra of some natural and synthetic zeolites obtained with 4880-A excitation.
due to a silicon- oxygen angle bending vibration.1° It is also known from the infrared spectra of various silicate materials1 that the asymmetric stretch is in the 1200900-cm -1 region while the symmetric stretch (when it does appear) has been assigned to the 800-600-~m-~ region. Significantly, the Raman spectra of zeolites show little or no bands in the region where extremely strong asymmetric stretching bands appear in the infrared spectra. The strongest b,and in the Raman spectra of the zeolites is always, with the exception of Zeolon, in the 510-480-~m-~ region. Sodalite itself gives a band in practically the same position as quartz (463 cm-I) but the other zeolites have this band ai a somewhat higher frequency. It was ieporteat that several of the in€rared bands showed a regular dependence on the silica-alumina ratio of the zeo1ites.l No such correlation was observed in the behavior of the 5 0 0 - t ~ n -band ~ in the Raman spectra. For example, zeolite X with a silica-alumina ratio of 2.5 and zeolite Y with a silica-alumina ratio of 5 both give a band a t 505 em This band is very sharp in the spectra of
quartz, sodalite, and zeolites A, X, and V. All these zeolites have a unit cell of high symmetry, cubic11 En other zeolites this band shows definite shoulders, the most pronounced being in the case of ehabazite where this band is definitely split into at least three components. Chabazite has a much lower unit cell symmetry; it is trigonal. Analcime again has cubic symmetry and a very sharp 480cm -1 band. Below 500 cm-l a number o f small bands appear in the spectra of most zeolites. Their positions are quite reproducible in the cases where several samples of the same zeolite were run. They also seem GO be characteristic of the individual zeolites. In the infrared spectroscopic work this region was identified with typical pore openings of the zeolite structures.l No such definite correlation could be established in the Raman spectra. The case of %eolon,12a mordenite-lype synthetic zeo(10) J. B. Bates, J. Chem. Phys., 56, 1910 (1972). (11) For a summary of the unit cell symmetries, see ref 1. (12) Zeolon is a trademark for the Norton Co. The Journai of Physical Chemistry, Vol. 77, &lo. 2, 1973
C. L. Angell
226 PROPYLENE ON NaX
1625
2925
I
i
TABLE II: Observed Frequenciesin the Raman Spectra of Molecules Adsorbed on Some Zeolites (cm-?) Propylene
Acetonitrile Liquida
w
~~~~
3000
:lo00
380 921
P 1500 500 1375 FREQUENCY (cm')
Raman spectrum of propylene adsorbed on NaX zeolite. The Zeolite, pressed into a pellet and activated in vacuo at 500°, was exposed to 100 Torr of propylene. (The discrepancies between frequencies of this spectrum and the values given in Table I I are due to the fact that this is an individual spectrum while the listed values are means of a number of spectra observed.) Figure 3.
2252 2293 2944 3000
Adsorbed on Y or X 388 505-508d 923 1373 2263 2298 2942 3003
Adsorbed Liquidb on A or X or Y 432 920 1297 1415 1648 2890 2924 2990 3087
428 490-508d 913 1296
1415 1635 2895 2925 2995 3073
-_
Acrolein
LiquidC
Adsorbed on Y
333 568 990 1153 1277 1360 1420 1618 1690 2995 3035 31 00
338 5 78 990 1165 1278 1362 1424 1612 1685 2995 3034 31 02
*
lite, is interesting. Although it has a very high silica content, the silica-alumina ratio being 10, its spectrum is completely different from that of other zeolites. The strongest barid LS at 395 em-l, the second strongest a t 634 cm- I, while >inthe region of the characteristic strong band of the other zeolites, it only has a medium band, at 521 cm-I. In the cases of the other zeolites the general appearance of thc' Raman spectra did not depend on the symmetry of the unit cells. Therefore, the fact that Zeolon has the lowest symmetry (orthorhombic) does not seem to explain the great difference between the spectrum of ZeoIon and those of the other zeolites. It is interesting to note that in sapphire, which is an aluminum oxide, there are strong bands at 380 and 640 crn-l. These bands therefore are characteristic of vibration modes between aluminum (in octahedral coordination) and oxygen. The close correspondence bcltweea these and the bands for zeolon could be just a coiiicidence since the aluminum in Zeolon is tetrahedral. Neither the infrared spectral nor the Raman spectra of the other zeolites make it possible to differentiate between vibrations of silicon-oxygen and aluminumoxygen bonds; therefore, they are referred to as T-O bonds (T for tetrahedral atom). But it is possible that due to the nature of the Zeolon structure, the silicon-oxygen and aluminum-oxygen vibrations have become sufficiently different that they give rise to different frequencies. It s e e m that such separation could only occur if there was a large difference in polarizability of the A10 and S i 0 bonds in the Zeolon framework. Because zeolites give rise to very weak Raman scattering, the Raman spectra provide an excellent method for detecting small amounts of certain inorganic ions. The tetramethylammonium ion, for example, gives very strong characteristic bands (for example, see the spectrum of offretite or omega). The band at 985 em-l in sodalite is due to the sulfate ion.1i3Similarly, sulfate (left by insufficient washing after cation elrchange with a sulfate salt) could be detected in other zeohte spectra, including some cases where the actual sulfate content was less than 1%.In a few experiment?) involving sodium nitrate containing zeolites, the nitrate band at 1040 cm -1 could be easily detected even at a concentration of only 0.4 nitrate ions per unit cell. B Rumun Spectra of Molecules Adsorbed on Zeolites As mentioned in the Experimental Section, several zeolite samples, in particular an A, an X, a Y, and an ammoThe Journal of Physical Chemrstry, Vol. 77,No. 2, 1973
From ref 5 G. Herzberg, "Infrared and Raman Spectra of Polyatomic Molecules," Van Nostrand, New York, N.Y.. 1945, 11 355 Present work Zeolite band
nium-exchanged Y sample, gave rise to relatively strong bands due to the zeolite structure. In each caseJhey also showed bands due to substances adsorbed on them. Adsorptions were carried out on samples activated in vacuum at 250" for 3-6 hr (except for the NH4Y which was pumped at room temperature). Frequencies observed in spectra of adsorbed acetonitrile, propylene, carbon dioxide,14 and acrolein are given in Table 11, while an example of the spectra is presented in Figure 3. In each case the spectrum resembled the spectrum of the liquid state, indicating that in all of these .cases only physical adsorption has occurred. However, there are enough differences between the spectrum of the liquid and the adsorbed species (for example the carbon-carbon double bond stretching in the case of propylene or the carbon-nitrogen triple bond stretching vibration in the case of acetonitrile) that it was quite clear that we were not looking at either the gas phase or some condensation in the pores. In the case of propylene, acetonitrile, and acrolein the samples were pumped at room temperature for at least 10 rnin and the spectrum of the adsorbed species did not change significantly. This also provides a confirmation that we are looking at the adsorbed species in each case. It has been reported in many studies by infrared spectroscopyl5 that the sodium forms of the zeolites usually give rise to only physical adsorption and very little change occurs in the spectra of the adsorbed species. It is unfortunate therefore that so far we have only been able to obtain the spectra of adsorbed species on the sodium forms. The ammonium-exchanged Y zeolite used was only pumped at room temperature; therefore it was still in the ammonium ion form. However, these experiments show that it is possible, in favorable cases, to obtain very good quality Raman spectra of adsorbed nmlecules. We are continuing experiments to find other suitable cation exchanged forms of zeolites where it would be possible to obtain the Raman spectra d molecules adsorbed on the zeolites. (13) Sodalite contains chloride as the anion but we believe that we could detect a rather small amount of sulfate impurity. (14) Carbon dioxide adsorbed on X or Y gave bands at 1278 and 1383 cm-', thegas phase frequencies being 1285 and 1388 cm-'. (15) Eg., C. L. Angell and M. V. Howell, J. Phys. Chem., 73, 2551 (1969).
Microwave Spectrum af Trimethylgermane
227
Acknowledgment. The author wishes to express his gratitude to Miss IS. Flanigen for supplying the large majority of the zeolite samples used in this work and for
helpful suggestions to the discussion. He is also grateful to M. J. O'Hara and R. W. Larson for help in the experimentalwork.
.
Spectra and Struct ure of 0rga nagermanes XV.la Microwave Spectrum of Trimethylgermane
. C)urig,* M. M. Chen,lbY. S. Li, Depattment of Chemfstry, University of South Carolina, Columbia, South Carolina 29208
anid J. B. Turner Ueprfmenf of Chemistry, Augusta College, Augusta, Georgia 30904 (Received June 5, 1972)
The rotational spectrum of trimethylgermane has been recorded from 18.0 to 40.0 GHz. The ground-state rota,tional constants have been determined for ten different isotopic species. An r, value of 1.532 0.001 has been determined for the GeH bond distance. With this value of the GeH distance and an assumed structure for the methyl group, the following two structural parameters were obtained: r(GeC) = 1.947 i 8.006 A and LCGeH = 109.3 f 0.1". The determined structural parameters are compared to the corresponding ones for other methyl-substituted germanes.
Introduction Microwave spectra and structures of propane2a and isobutane2b have been studied by Lide who made a comparison in their structures with that of ethane and concluded that the C-C bond distance is quite c0nstan.t in the simple hydrocarbon series. The CCC angle (111.15") in isobutarie was found to be -1" smaller than the corresponding angle in propane (112.4") whereas the tertiary C-H bond distance (1.108 A) was found to be slightly longer than that in the CH2 group of propane. The microwave spectra and structures of a series of methyl-substituted silanes, which included methyl~ilane,~ dimethylsilane* and t r i ~ e t h y l s i l a n e ,have ~ been studied. It was found that the Si-C bond distance remained essentially constant with a value of 1.867 A in these three molecules while the CEX angle in trimethylsilane (110" 10') is smaller than that in dimethylsilane (110" 59') and the tertiary Si--H bond distance (1.489 A) can be regarded as slightly longer than those in methylsilane (1.485 A) and dimethylsilane (1.483 A ) ~These two series of comparisons seem to provide consistent results about the constant 6-M (M = C or Si) bond distance, the smaller CMC angle, and the longer M-H bond distance in tertiary compounds than those in primary and secondary compounds. An investigation of the microwave spectrum and structure of trimethylgermane is expected to provide another series o f interesting comparisons with the methyl-substituted germanes since the microwave spectra and structures of methylgermane arid dimethylgermane have been studied by Laurie6 and Thomas and L a ~ r i erespectively. ,~ The present paper gives the results of our study of the microwave spectrum and structure of trimethylgermane. Isotopic species investigated have included (CH&GeH
and (CH&GeD with the naturally abundant isotopic species of the germanium atom. Experimental and Results Samples of (CH3)GeH and (CHg)&eD were prepared by the reduction of (CH3)3GeBr with LiAlH4 and LiAlD4, respectively, in benzene at -5" followed by distillation at -0" and vacuum line distillation at -90". The products were checked by their mid-infrared spectra in the gaseous state. The microwave spectrum of trimethylgermane was investigated with a Hewlett-Packard model 8460A MRR spectrometer in the K-band (18.0-26.5 GHz) and R-band (26.5-40.0 GHz) frequency ranges. Frequency measurements were carried out while the Stark cell was covered with Dry Ice. Frequency accuracy is estimated to be 0.05 MWz for the stronger lines, but the uncertainty is larger for the weaker lines. For all of the isotopic species studied the 3 2 and 4 3 transitional frequencies were observed in the R-band frequency range and behaved like those for a rigid symmetric rotor. Assignment was made based on the relative frequency spacing and the relative intensities expected for the different isotopic species of naturally occurring abundance. The assignment was fur-
-
-
(1) (a) For part XiV, see J. Phys. Chem., 76, 1558 (1972). (b) Taken in part from the thesis' of M . M. C. which is to be submitted to the Department of Chemistry in part.ial fulfillment of the Master of Science degree. (2) (a) D. R. Lide. Jr., J. Chem. Phys., 33, 15.14 (1960); (b) ibid., 33, 1519 (1960). (3) R. W. Kilb and L. Pierce, J. Chem. Phys., 27, 108 (1957). (4) L. Pierce, J. Chem. Phys., 31, 547 (1959). (5) L. Pierce and 0.W. Petersen, J. Chem. Phys., 33, 907 (1960). (6) V. W. Laurie, J. Chem. Phys., 50, 1210 (1959). (7) E. C . Thomas and V. W. Laurie, J. Ghem, Phys., 50,3512 (1969).
The Journal of Physical Chemistry, V d , 77, No, 2, 1973
