J. Phys. Chem. 1993,97, 9695-9702
9695
Low-Frequency Raman Spectra of Dehydrated Faujasitic Zeolites Claude Brbmard' and Marielle Le Maire Luboratoire de Spectrochimie Infrarouge et Raman, CNRS UPRA 2631L, Bat.C5, Universitd des Sciences et Technologies de Lille, 59655- Villeneuve d'Ascq Cedex. France Received: March 2, 1993; In Final Form: May 19, 1993'
Significant Raman spectra of the fully dehydrated zeolites of the cubic faujasitic family have been obtained particularly in the low-frequency region, at room and low temperatures. The experimental Si/Al ratios (44, 32,20, 3.8, 2.49, and 1.26) correspond to the dealuminated zeolites Na3-DY, Nas-DY, and N a p D Y and the as-synthesized zeolites Nadl-Y, Nasa-Y, and Nass-X, respectively. The incorporation of aluminum atoms at low level in the zeolite framework broadens the sharp Raman lines observed a t 298, 312, 492, and 510 cm-l for the purely siliceous zeolite. This broadening is attributed to the disorder in the AI distribution. Upon incorporation of aluminum atoms at high level, marked changes in intensity as well as slight frequency shifts were observed for the most Raman-active bands. Significant changes in position and intensity of the bands attributed to the framework vibrations were observed according to the nature of the extraframework cation of zeolites obtained by exchange of the Na+ cations of Nas6-Y and Nass-X with H+,Li+, K+,Rb+, Cs+, T1+, Ca2+, and Mg2+ cations. The Raman scattering spectroscopy enables us to assign some bands below 120 cm-1 to translational motions of the intrazeolite charge-balancing extraframework cations. The dependence of the Raman spectrum on the water adsorption was found to be weak for the framework bands, except those between 400 and 300 cm-l, whereas the broadening of the bands corresponding to the translational motions of thecations, below 120 cm-l, is in good agreement with the change in the cation distribution and coordination of water molecules which occurs in the hydrated zeolites.
Introduction The structural properties of crystalline aluminosilicates,such as zeolites, have been under intense investigation for at least four decades.' While the most definitive insights into the structure of zeolite frameworks come from X-ray and neutron diffraction measurements,2-4many salient features of zeolite dynamics can be deduced from vibrational spectroscopic data. The faujasitic family of zeolites plays an important role in a wide variety of separation,chemical, and petrochemical processes. Their thermal stability is important to their function as catalyst, and dealumination is an established procedure for the improvement of thermal ~ t a b i l i t y .Direct ~ ~ conventional synthesis of cubic faujasitic-like zeolites is only possible for Si/Al ratios of the framework between 1 and 3.3 The zeolites with Si/A1 ratios around 1 or 3 are of the X or Y type, respectively. To achieve higher Si/Al ratios, dealumination techniques that remove A1 from the framework are required. These faujasite-like zeolites are of the DY (dealuminated) type. It should be noted that a new direct route to faujasitic zeolites with high Si/Al ratios has been recently proposed.8 The charge-balancing extraframework cations of the aluminated zeolites control or significantly affect the chemical and physical properties of the zeolite and hence its possible applications.gJ0 For a structure with the same Si/Al ratio, the Na+ cations can be exchanged to varying degrees by other cations without altering the structure. All the zeolites under study can be in aquated, partially hydrated, and fully dehydrated forms. The vibrational spectroscopies, infrared absorption,ll and inelastic neutron scatteringI2are techniques that have been shown to be efficient tools to resolve some key problems in zeolite science because of their use as a sensitive and direct means of obtaining useful information regarding the zeolitic framework and extraframework cations. Particularly, far-infrared spectroscopy yields some frequencies and intensities of extraframework cation translatory absorptions.13-18 The inelastic neutron scattering technique is particularly devoted to the study of compounds Abstract published in Advance ACS Abstracts, September 1, 1993.
0022-3654/93/2091-9695$04.00/0
containing hydrogen atoms because of the high sensitivity of this technique toward the hydrogen vibrational motions.*2 Despite the fact that Raman spectra of hydrated zeolites have been reported for at least 20 years,l9-Uonly recently has the dependence of the Raman spectrum of the hydrated forms of the faujasitic and A zeolites on the Si/Al ratio been carefully examined.23.24 However, before zeolites can be used for applications, it is necessary to evacuate the water molecules from the porous framework. High-quality Raman spectra of fully dehydrated forms of zeolites are difficult to obtain2s29 because of the weak Raman cross sections of the vibrational motions and intense fluorescence background under laser excitation with visible radiations. In the present work, wereport a Ramanstudy, carried out using both the dispersive and Fourier transform techniques, of the dehydrated forms of X,Y, and DY faujasitic zeolites with several framework Si/AI ratios (1.26,2.49,3.8,20, 32,44) and the subsequent exchanged X and Y zeolites. The Na+ cations of the Nags-X and Nas6-Y zeolites (Si/Al = 1.26, 2.49) have been exchanged by the H+, Li+, K+,Rb+, Cs+, TI+, Ca*+, and Mg2+cations. The motivation behind this study is twofold. First, there is considerableinterest in developing Raman spectroscopic data on zeolite frameworks, because the off-resonance and resonance Raman spectroscopies are efficient tools for directly probing organic, organometallic, or inorganic compounds entrapped in the porous system of zeolites.m2 Second,there are an increasing number of calculationson zeolite frameworkand extraframework vibrations,43-56 the veracity of which can only be verified by comparison with high-quality experimentaldata, particularly for dehydrated faujasitic zeolites in the low-frequency region.
Experimental Section Materials. The dealuminated DYsH20 zeolites (Si/Al = 44, 32, and 20) obtained by hydrothermal treatment were kindly provided by G. Descat and S.Tretjak (Grande ParoisseSA).The unit cell compositionsof thedealuminated zeolites DY were found to be H1.6Na2sAL3Sil~7.~0~~nH20, HzsNazaA15.8Sila620384.nH20, and HgNa1A19Si183038~nH20for DY(Si/A1=44), DY(Si/ Q 1993 American Chemical Society
9696 The Journal of Physical Chemistry, Vol. 97, No. 38, 199'3
Al=32), and DY(Si/Al=20), respectively. TheSi/Al ratios were verified using 29SiMAS-NMR spectroscopy. A part of the DY zeolite samples was stirred in an aqueous NaOH, NaCl mixture. After 24 h, the solid was separated by filtration, rapidly washed, and dried in air at 400 K; this procedure was repeated 3 times. The unit cell compositionswere found to be Na3Al3Si18g0wnH20, Na5A15Si1870384.nH20, and Na9Al&ils30~4-nH20,respectively. The Na41-Y sample (Si/Al = 3.8) was kindly provided by J. M. Manoli (Laboratoire de r6activitC et structure, Universit6 Pierre et Marie Curie, Paris). This sample was synthesized as reported recently8 without any dealumination procedure. The unit cell composition of Nadl-Y was found to be Na41AhlSi1s10384.nH20. The sodium form of Y zeolite (LZ Y-52) used in the present work, abbreviated hereafter as Nas6-Y-nH20 (Si/Al = 2.49), was obtained from Union Carbide. The sodium form of X zeolite NasrX-nHzO (Si/Al = 1.26) was purchased from Strem Chemicals Inc. The exchanged Y zeolites, Li~oNacY.nH20,KslNarY.nH20, Rb52Na4-Y-nH20,Cs48NarY.nH20, T151Na5-Y.nH20, Ca24NarY-nHzO, and Mg25Nao~-Y.nH20, were obtained by stirring Nas6-Y with 1 M LiCl, KCl, RbC1, CsCl, TlNO3, CaC12, and MgC12 at room temperature. After 24 h, the solid was filtered off, dried at 600 K, and then stirred again with fresh solution. This procedure was repeated 6 times. The exchanged X zeolites, Li78NarX.nH20, K7~Nal~-X*nH20,Rb7~Na7-X.nH20, cs72Na13-Y.nH20, and T179Na6-X.nH20, were obtained according to analogous procedures. The unit cell compositions of the X and Y zeolites under study were found to be in good agreement with the above formula. The Na56-Y starting material was ion exchanged using an aqueous solution of NH4Cl. After 24 h, the solid was separated, dried in air at 350 K, and stirred again. This procedure was repeated 6 times. The zeolite has the following composition: H4(NH4)4,Na5A156Si1370384'nH20.The sample was dried in air at 400 K and then calcinated at 630 K under vacuum. The composition of the H48-Y zeolite was found to be H48Na~A153Si1390384-nH20. Dehydration. The powdered hydrated zeolite sample was introduced into a cell and connected to a vacuum line. The sample was dried under vacuum ( l b 3 Pa) then heated stepwise to 700 K. O2was then admitted into the cell. After 6 h, the sample was held under vacuum and then cooled to room temperature. Then, the powder was transferred under dry argon into a cylindrical thin glass tube which was sealed off for the dispersive Raman experiments. The powder was transferred under dry argon in a quartz glass Suprasil cuvette for FT-Raman experiments. The X-ray diffraction patterns indicated that the 02-treated zeolite maintains its structural integrity. Instrumentation. The Raman spectra obtained by using the dispersive technique were recorded at 300 and 77 K, in the 50-4000-cm-l wavenumber range, on a triple-monochromator spectrometer DILOR Model RTI, equipped with accumulation of spectra. The Raman scattering was excited using argon or krypton laser lines (457.9, 488.0, 514.5, 568.2, 647.9 nm), and a laser power of 50-400 mW was used. With the backscattering geometry used, the Raman spectrum of the glass tube is reduced to a negligible level. Slit widths were typically 4 cm-l, and the sloping background in the Raman spectra was not corrected. To reduce the strong "Rayleigh band" in the lower wavenumber region, narrower slit widths were used. The cause of this intense "Rayleigh band" arises mostly from the particles with size in the micron range, which leads to strongdiffuse reflectanceand intense plasma lines of the laser. These parasite lines can interfere with Raman bands of the sample. In order to detect the significant Raman bands of the sample, the Raman spectra were recorded with all the available exciting radiations and carefully compared in the Stokes and anti-Stokes regions, at 300 and 77 K.
BrCmard and Le Maire The Raman spectra using the FT technique were recorded at room temperature on a Bruker spectrometer Model IFS 88. The use of a NIR Nd3+:YAG laser at 1064 nm as an excitationsource drastically reduces the fluorescence background without chemical treatment. The FT-Raman instrument collects light from a 1-mm patch on the sample. The laser power of 40&700 mW was used. The spectra were recorded at 4-cm-1 resolution using 200 scans. Removal of theParasite Photoemission. As reported previously, usable Raman spectra of hydrated X, Y, and DY zeolites can be obtained with visible exciting radiations.23 Using near-infrared exciting radiation (1064 nm), some photoemission level remains.26.70 In contrast, if the sampleswere heated to 700 K under vacuum or under an argon atmosphere for several hours, all the dehydrated zeolite samples became excessively fluorescent under visible laser radiations, and the strong photoemission obscures the Raman scattering of the zeolite. All these photoemissions are a functionof the exciting radiation wavelength value, and the background decreases significantly when red exciting radiations are used. Usable Raman spectra were recorded in the 350&100-~m-~ range using the FT-Raman technique and in the 1064-nm exciting radiation without any chemical treatment. In order to record Raman spectra at very low wavenumber, it is necessary to use a triple monochromator which is more operative in the UV-vis region. Thus, it is necessary both to use visible exciting radiations and to reduce the photoemission to acceptable levels beforeany spectroscopicstudiesonzeolitescanbe attempted. Reacting the activated zeolite at 700 K with pure oxygen for 6 h reduces the fluorescence emission drastically for all the samples used. Nevertheless, for the X zeolites a residual weak emission remains around 600 nm. This emission has been attributed previously to mineral impurities25 such as iron(II1) salts, which cannot be quenched by the procedures described above. The zeolite samples, once activated and reacted with oxygen, show no increase in emission levels upon reheating if they are stored in a clean environment in the hydrated form. This indicates that the strong emission is caused by impurities but is not an intrinsic property of the zeolite itself. Hydrocarbon impurities have often been suggested as the causes of the emission.25 The problem of the purity of the samples is crucial in Raman spectroscopy of zeolites. All the elementary chemical analyses are in good agreement with the expected chemical composition of the zeolites under study. However, the Raman spectra of three different Na56-Y samplesincluding two commercialsamples were recorded, and a supplementary intense peak was observed at 143 cm-l for one commercial sample. This band was not removed after several washings and after cation exchanges. No attempt was undertaken to identify this impurity at low level with a very strong Raman cross section. Therefore, the Raman data concerning this zeolite samplewere excluded from the present work. To verify the veracity of all the Raman features, the exchanged Y and X zeolites were exchanged once more by Na+ cation, and the original Raman spectra were retrieved. Crystal Structures and Factor Group Analyses The first X-ray diffraction determination for the structure of the dealuminated faujasite, with the Fd3m (oh7)space group, was obtained by H ~ e u . 5The ~ topology of the completely siliceous zeolite framework can be described in terms of finite component units that are specific arrays of identical Si04 tetrahedra with four different Si-0 distances. Different structural units, tetramer and hexamer rings, hexagonal prisms, and sodalite cages are characteristic units of the array of Si04 moieties. The sodalite cages are interconnected to a three-dimensionalsystemof channels by 12-membered-ringwindows. These sodalite cages form the building block for numerous minerals and for synthetic zeolites. Previously published papers concerning the molecular clusters H4Si04,48H4Si8012,56 and high-silica sodalite46 provide experimental and theoretical information on the building blocks of faujasites.
Low-Frequency Raman Spectra of Zeolites
The Journal of Physical Chemistry, Vol. 97,No.38, 1993 9697
TABLE I: Occupation Numbers for Extraframework Cation Sites I, I f , 11, and IIP h Faujasite Zeolites
cations of dehydrated zeolites, some key experimental problems must be resolved. Taking into account the weak Raman scattering of the aluminosilicate framework, the parasite photoemission, zeolite I (16c) I’ (32e) I1 (32e) 111’ (96g) ref the Raman features of the glass tube, and the plasma lines of the Nas-DY 1 2 60 laser have been minimized using careful experimentalconditions Na3l-Y 1 12 18 60 (see Experimental Section). Na56-Y 7.1 18.6 32 62 The Raman data are presented in three main ways. First, the Na56-Y.nHzO 2.6 11 8 70 Lis5-Y 24 24 65 Raman spectra of the faujasitic zeolites with Na+ cations were Kss-Y 5.4 18.1 26.8 63 studiedas a functionof the Si/Al ratio of the framework. Second, Ca27-Y 14 2 11 69 the Raman spectra of the exchangedY and X zeolites were studied Nass-X 3.8 32.3 30.8 7.9 61 as a function of the nature of the extraframework cation. Third, Nass-X.nH20 8.6 11.2 21.7 19.6 61 the influence of the hydration level upon the Raman spectra was With the YSi48096nstructure unit ( o h factor group), the evaluated. contributing symmetry species at the center of the Brillouin zone Si/Al Ratio of the Framework. The high-frequency Raman are the following:ss 10 AI, + 8 A2, 18 E, + 26 F1, + 28 F2, spectra of the dehydrated siliceous zeolites Na~-Dy(Si/Al=44) 7 AI, 11 Azu 11 E,,+ 29 Flu 25 Fzu. Among them, only and Nas-DY(Si/Al=32) are very analogous to the spectrum of the 10 Al, 18 E, + 28 F2, modes are Raman active, and 28 the hydrated purely siliceous faujasite.23 However, in the lower Flu are infrared active. The predicted Raman-active stretching frequency region, two supplementary bands were detected near modes are 24 (4 AI, + 8 E, 12 Fz,), whereas 12 Fluinfrared100 cm-1. The higher frequency bands are weak, and we are not active modes are expected in the S i 4 stretching region. able to detect any fine component in the broad bands around All the X-ray or neutron diffraction determinations of structures 1190, 1040, and 820 cm-l, where 24 modes are predicted in this of powdered aluminated faujasites were performed assuming a Si-0 stretching region.& In the bending region, two sharp complete Si/A1 disorder and Fd3m space group. However, the doublets are observed with a bandwidth on the order of 8 cm-1 larger value of the cell parameter for X zeolites compared to the at room temperature. It should be noted that the Raman spectrum Y ones arises from the higher A1 content in X zeolite compared of the siliceous faujasite has no special analogy with the Raman to Y zeolite. AI atom exhibits a covalent radius (0.53 A) greater spectra of the octa(hydridosi1a)sesquioxane H&%OIZ, which could than that of Si (0.40 A). be considered as a cluster of siliceous faujasite. The number of bands of the framework predicted at the center Lattice calculations46 provide a simulated Raman spectrum of of the Brillouin zone does not depend on the Si/Al compo~ition.~~ dealuminated faujasite which has a good resemblance to our T (Si or Al) is used as a designationfor the framework aluminum experimental spectrum, Figure 3, despite an overestimation of or silicon atoms in the correlation charts, and there is the implicit the Raman intensities in the stretching region. Particularly, the assumption that the particular site is populated by either Si or calculation predicts two bands in the region between 475 and 500 A1 atom and that the two types of atoms are considered to be cm-l, in excellent agreement with the observed data in Figure 1 equivalentfor the purpose of the factor group analysis. With the and Table 111. The relative intensities of the simulated spectrum “(Al,Si)48096” structure units and oh factor group, the number are better reproduced with a normal coordinate analysis using and type of contributing symmetry species are the same as those the generalized valence force field approximation and approximate found for the siliceous faujasite. Raman intensity ~alculations.~3However, it should be noted The aluminum atoms of the framework implicate extraframethat the sharp doublets observed around 500,300, and 100 cm-l work cations to compensate the charges. X-ray and neutron were simulated as single bands near 500, 300, and 100 cm-l, diffraction datas7~59-68indicatethree positions for extraframework respectively. Molecular dynamics (MD) calculation71 with a cations in the dehydrated Y zeolites: sites I and I’ in the sodalite suitable potential set reproduces well the main Raman features cage and site I1 in the super cage. The site preferences are I > including frequency, intensity, and band shape, although the I1 > I’ and I > 1’> I1 for monovalent and divalent cations, splittings of the bands near 500, 300, and 100 cm-l are not respectively. Four positions have been found for the extraframereproduced. work cations: sites I, 1’, 11, and 111’ in the supercage for the The character of the Raman bands near 800 and 1200 cm-l dehydrated X ~ e o l i t e s . ~ lThere - ~ ~ are no special indications of is mainly s t r e t ~ h i n g . ~They ~ . ~can ~ be described as symmetrical significant deviations of the framework geometry from the average and asymmetrical stretching modes, v,(Si-0) and v,,(Si-0), characteristics as a consequence of the hydration state of the respectively. The intense bands around 500 cm-1 are mainly of zeolite.69 Nevertheless, upon hydration, a redistribution of the 0 S i - O bending character S(Si-Mi), whereas the bands at extraframework cations is observed, in which the population of lower frequency can be assigned to normal modes which can be sites I, 1’, 11, and 111’ decreases and the number of unlocated described in terms of 043-0 and Si-O-Si bending modes as cations in the supercage in~reases.6~The factor group analyses well as S i - 0 4 - 0 deformation modes, Table 111. of the supplementary degrees of freedom introduced by the The Si/Al ratios of the purified dealuminated zeolite samples extraframework cations of X and Y zeolites are realized, taking reported in Figure 1and Table I11 were deduced from elementary into account the full occupancy of the sites, Tables I and 11. analyses. The Si/Al ratios of the DY samples are found to be in good agreement with the 29SiNMR data. The remaining Results extraframework impurities introduced by the hydrothermal process appear negligible in the purified samples. It should be Before significant Raman spectra can be obtained based only noted that in the case of crude samples the analytical data are on the vibrational motions of the framework and extraframework TABLE E. Factor Group Analyses of the Translational Motions of the Extraframework Cations in the Aluminated X and Y Zeolites (Smce GrouD F&m = Ob7)
+
+
+
+ +
+
+
~~
contributing symmetry species and optical activity site I (16c) D3d = 3m factor group oh
Bremard and Le Maire
9698 The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
TABLE IIk Observed Frequencies (cm-l) in the Raman Spectra of Zeolites of the Faujasite Type at 300 K as a Function of Si/Al Ratio, with Relative Intensities N a r D Y Na5-DY Nag-DY Na4,-Y N a r y N a a r X (Si/AI = 44) 60 w 70 w
(32) 60w 70w
(20) 60w 70w
95 w 405w
95 w 405 w
95 w 405 w
298m 312m 360vw
298m 312m 360vw 460vw 493 s 510vs
170vw 298m 312m 360vw 460vw 493s 510vs
493s 510vs 820wb 1090wb 1180wb
(3.8) 60w
(2.49) 60w
75m
75s 82 sh 90vw llOw 120w 170vw 290s
90vw 405 w 116sh 170vw 296s 312sh 375vw 44Ovw
480sh 502vs
820wb 820wb 820wb 1090wb 1090wb 1060wb 1180wb 1180wb 1150wb
375vw 440vw 480sh 504vs 800wb 820wb 1020wb ll2Owb
(1.26)
assign.#
60w b(T-O-T) b(T-0-T) 77m transNa+ b(T-O-T) 90vw b(T-0-T) 112w b(T-0-T) 120s transNa+ 150b b(0-T-O) 280b b(0-T-O) 6(0-T-O) 380b b(0-T-O) 440sh b(0-T-O) 480sh b(0-T-O) 505vs b(0-T-O) 800wb v,(T-O) 4T-O) 980wb v"(T-0) 1090wb v"(T-0)
0 vs, very strong; s, strong; m, medium; w, weak; vw, very weak; b, broad; sh, shoulder. T: Si, AI.
100
300
500
WAVENUMIIER(CM-~)
I
I 100
300
500
WAVENUMRER(CM-~)
Figure 1. (A, top) Ramanspactra (300K) of thedehydrateddealuminated
faujasites as a function of the Si/Al ratio: (a) Nag-DY (20), (b) NasDY (32),(c) NarDY (44). Excitationradiation: 568.2nm. (B,bottom) Raman spectra (300 K) of the dehydrated as-synthesized faujasites as a function of the Si/AI ratio: (a) Nags-X (1.26), (b) Nas6-Y (2.49), (c) Na41-Y (3.8). Excitation radiation: 568.2 nm. not found to be in agreement with the 29Si NMR and Raman results. However, no clear Raman evidence was found for extraframework compounds. The width of all the bands increases progressively from Si/Al = 44 to 3.8. However, no other important feature was detected, except weak frequency shifts. Upon cooling to 77 K, no significant narrowing of the Raman bands was observed. At higher aluminum content the intensities of some minor bands increase, and the most striking feature is the appearance of supplementary bands in the low-frequency range, near 75 and 120 cm-l, which could be assigned to some translatory motions of the extraframe-
work Na+ cations (see below). These bands become clearly apparent for Na56-Y and Naa5-Y zeolites, Table 111. As was mentioned above, the number of bands of the aluminosilicate framework predicted by the factor group analysis at the center of the Brillouin zone does not depend on the Si/A1 composition of the zeolite framework. However, the relative concentration of Si and A1 in site T affects the frequencies, the intensities, the width, and the shape of the Raman bands. Some bands, ~ ~ ( 0 T-O) region, shift markedly to low-frequency values as the aluminum content increases, whereas some hardly move to lower or higher wavenumber values. The frequency shifts arise from combined effects of structural and electronic parameters, although correlations can be established between the wavenumber of some characteristic bands and Si/Al ratiovalues or the crystallographic cell parameters or the average T-0-T angle values. However, more indicative is the bandwidth of the most prominent peaks (near 500 cm-l) as a function of the Si/Al ratio of the framework, which reflects the aluminum content at low level of dealuminated samples. ExtraframeworkCations. Earlier, it was shown by far-infrared spectroscopy that some extraframeworkcation modes of the fully dehydrated X and Y zeolites occur below about 200 The supplementary degrees of freedom introduced by the translation motions of the charge-balancing cations in sites ,'1 11, and 111' are expected to be seen by Raman scattering according to the factor group analysis, Table 11. The translation motions of the cations located in site I are not Raman active-they are infrared active. Because of their weak Raman cross sections, the Na+vibrational motions are only apparent in the spectra of Na41Y,Na56-Y,andNass-X. It should benotedthattheexactlocation of the cationic sites as well as the occupation depends on the aluminum content.59 At first, the replacement of the Na+ cations by H+, Li+, K+, Rb+, Cs+, T1+, Ca2+,and Mg2+produces significant changes in the position and intensity of the most prominent bands, near 500 and 300 cm-1 and assigned to framework motions (Figures 2 and 3, Tables I V and V). Nevertheless, depending on the nature and the number of extraframework cations, all the low-frequency bands assigned to the frameworkvibrations hardly shift, whereas the relative intensities vary markedly, Figures 2 and 3. I n contrast, the low-frequency bands, which are the most sensitive in frequency to the cation exchange, are assigned to the translational motions of the extraframework cations against the framework, Tables I V and V. No band can be reasonably detected in the wavenumber region attributed to the infrared-active translational motions according to previous w0rks.~3-'5 Rea-
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9699
Low-Frequency Raman Spectra of Zeolites
II
100
300
500
WAVENUMRER(CM.') I I
I 1
1
100
I I
In the case of H48-Y zeolite, no clear evidence was obtained by Raman spectroscopy concerningthe 0-Hvibrationalmotions. The in-plane and out-of-plane 6(OH) and ?(OH)deformation modeswerepreviouslyobservedat1089and419cm-1,respive1y, by inelastic neutron scattering (INS) and near-infrared spect r o s ~ o p i e s . ~The ~ . ~v(0H) ~ modes were clearly detected at 3640 and 3550 cm-l by infrared spectroscopy. The lower frequency region of the Raman spectrum has a good resemblance with the spectra of the dealuminated zeolites (Table IV) and is representative mainly of the framework deformation motions. The frequency shifts can be attributed to slight structural changes of the framework upon exchange of the extraframework cations. However, noclear correlation was obtained between the position of the bands and the T-O-T angle values deduced by diffraction techniques.5*96245 Particularly, the splitting of the band near 500 cm-l, Figure 2 and Table IV, cannot be explained through the structural parameters only. Hydration. The Raman features corresponding to liquid water were detected as weak bands at 3600 and 1660 for all the fully hydrated samples. It should be noted that the H48-Y.nH20 sample exhibits a supplementary intense band at 1630 cm-l. This band is assigned to the deformation motion of HsO+ cation. The spectrum of the fully hydrated siliceous material was essentially the same as that of the dehydrated form, except in the lower frequency region. Nevertheless, there are significant and clear changes in the spectral region between 300 and 500 cm-l upon hydration (and dehydration) for all the aluminated zeolites. Indeed, the minor and broad band centered near 375 cm-1 for the dehydrated Na56-Y zeolite is shifted to 355 cm-l, whereas the band at 290 cm-l is shifted to 308 cm-l. The frequency shifts depend on the hydration level. There are no special indications of significant deviations of the framework geometry of faujasitic zeolites from the X-ray powder diffraction data70as a consequence of the hydration state of the zeolite. Weak deviations of the mean interatomic distances and bond angles have been noted in the order of 0.01 A and 7 O , respectively. In contrast, the influence of the adsorption of water on cation location was found to be significant by X-ray diffraction meth0ds.m For the Nas6-Y sample, a comparison of the structure of hydrated and dehydrated zeolites at room temperature is possible, Table I. If the positions of the cation sites are analogous in the hydrated and dehydrated forms, the cation distribution on the different sites is found to be very different.70 Many solvated cations are delocalized over the supercages. The low-frequency Raman spectra of the hydrated form of the aluminated zeolites indicate that the bands attributed to the translational motions of cations decrease in intensity and broaden considerably. This result is in good agreement with the redistribution of the cation locationand coordination with water molecules after hydration.
I
300
500
WAVENUMIIER(CM-~)
Figure 2. (A, top) Raman spectra (300 K) of the dehydrated exchanged Excitation Yzeolites: (a)Cs-Y,(b)RCY,(c)K-Y,(d)Na-Y,(e)Li-Y. radiation: 568.2 nm. (B, bottom) Raman spectra (300 K) of the dehydrated exchanged X zeolites: (a) Cs-X, (b) R C X , (c) K-X, (d) Na-X. Excitation radiation: 568.2 nm.
sonably, only one or two Raman features corresponding to the translational motions of cations can be distinguished in the lowfrequency region of each spectrum of Y and X zeolites (Figure 4, Tables IV and V), whereas numerous bands are expected from the factor group analysis for cations in sites 1', 11, and 111'. From the Raman data only, it is not possible to assign unambiguously the Raman features to any cationic site. However, it is tempting to assume that the most intense bands correspond to the most populated sites. Particularly, in the case of Ca-Y zeolite the low-frequency Raman features are weak because of the Raman inactivity of the highly populated site I.
Discussion The main Raman data of the present work as functions of the Si/Al ratio, extraframework cation nature, and hydration level can be compared to the inelastic neutron scattering'2J4J5 and the mid-infrared11.76 and far-infrared's results previously obtained with analogous zeolite samples. There are important differences between the infrared absorption, Raman scattering, and INS concerning the selection rules. The comparison between the observables (IR, INS, Raman spectra) and models (energy minimization, normal modes analysis, molecular dynamics) provides an insight into the vibrational dynamics of the framework of the siliceous faujasite. These results give the foundationof the assignment of the frameworkvibrations for all the faujasitic zeolites used in the present work. The number of predicted modes by the factor group analysis is considerably greater than the number of discernible featurestaking into account the signal-to-noise ratio and the resolution of IR, INS, and Raman spectra, respectively. Nevertheless, the width of the most
9700
Brbmard and Le Maire
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 I
100
200
300
400
500
600
700
800
900
1000
1100
WAVENUMBER( CM-1)
Figure 3. Raman spectra (300 K) of the dehydrated exchanged TI-Y and T1-X zeolites: (a) T1-X, (b) T1-Y. Excitation radiation: 568.2 nm.
TABLE IV: Observed Frequencies ("1) in the Raman Spectra of Dehydrated Zeolites of Y Faujasite type (SUA1 = 2.49) as a Function of the Nature of the Extraframework Cations, with Relative Intensities Li-Y Na-Y K-Y Rb-Y cs-Y TI-Y Ca-Y H-Y assign.' trans Cs+,TI* 60 sh 55 sh 65 sh 60 w b(T-0-T) 65 w 65 sh 60 w 60 w 60 w 75 w
75 s
82 w 90 w
82 sh 90 sh
110 w 120 w 170 vw 300 s 360 vw 392 m 440 sh 480 sh 516 vs
800 wb 830 wb 1120wb a vs,
70 w 75 sh
70 m 75 sh 80 s
75 sh
80 m
90 sh
82 sh 90 w
llOw 120 m 170 vw 290 s 375 vw 400 vw 440 sh 480 sh 504 vs
83 w 90 sh 105 s llOsh 120 sh 170 w 285 s 370 vw 400 vw 440 sh 475 s 493 vs
llOsh 120 vw 170 m 285 s 370 vw 400 vw 440 sh 470 s 490 vs
llOsh 120 vw 170 m 300 s 370 w 400 vw 440 sh 480 s 500 vs
800 wb 830 wb 1020 wb 1120 wb
800 wb 830 wb 1000 wb 1140 wb
800 wb 830 wb 1000 wb 1140 wb
800 wb 1120wb
150 sh 300 m 353 w 476 sh 498 vs 594 w 715 wb 780 wb 1000 wb lllOwb
90 w
90 w
llOsh 120 w 170 w 300 s 385 w 400 vw 450 w
llOw
800 wb
170 w 300 s 360 w 400 w 440 w 465 sh 485 s 600 sh 800 wb
1120wb
1000 wb 1120 wb
525 vs
trans K+, CaZ+ trans Na+ trans Rb+, Ca2+ b(T-0-T) b(T-0-T) trans K+ b(T-0-T) trans Na+ b(0-T-O)
b(0-T-O) S(0-T-O) b(0-T-O) b(0-T-O) b(0-T-O)
b(0-T-O) 4T-O) ~s(T-0) ~0-0) vu(T-0) vu(T-0)
very strong; s, strong; m, medium; w, weak; vw, very weak; b, broad; sh, shoulder. T: Si, Al.
prominent Raman bands corresponding to the siliceous faujasite is small even at room temperature. The increase in bandwidth is a reflection of the increase of disorder as A1 atoms substitute for Si, even at low level. Concerning the Si/Al distribution over the faujasite framework, the so-called Loewenstein rule is now accepted as a crystallochemical criterion that holds for natural and synthetic materials. Several short-range-order parameters related to the Al/Si distribution of dealuminated zeolites were deduced from 29Si NMR data77 and Monte Carlo calculation^^^ by means of an effective interaction between second-neighboratoms. However, the Raman line width does not provide clear evidence concerning the short-range ordering in dealuminated faujasite. In addition, we are unable to provide any Raman or IR information about the order4isorder transition for 64 A1 atoms in the unit cell, as well
as a long-range-ordered Si/AI distribution for 80 A1 atoms per unit ce11,78 even though the Raman spectrum of theNas6-Y zeolite (56 Al) is markedly different from the spectrum of Na85-X (85 Al) . A recent molecular dynamics simulation of zeolite Na4*-Y reproduces quite well the infrared spectrum: most of the main peaks appear in the calculated spectrum, although the peaks near 800 cm-1 are not r e p r o d ~ c e d . ~ The ~ low-frequency Raman features near 1OOcm-1 (attributed to theframeworkdeformations) are in agreement with the calculated spectral density of the diameter fluctuationsof the window of the supercagewhich occurs between 30 and 100 ~ m - ' .These ~ ~ modes are not observed on the FIR spectrum. The exploration of the low-frequency region of the IRIS and Raman spectra of aluminated faujasitesrevealsadditional features
Low-Frequency Raman Spectra of Zeolites
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9701
TABLE V: Observed Frequencies (cm-') in the Raman Spectra of Dehydrated Zeolites of X Fau'asite type (Si/Al = 1.26) as a Function of the Nature of the kxtraframework Cations, with Relative Intensities Na-X 60w 77m 90vw 112vw 120s l5Ovw 280b 380b 440sh
480sh 505vs 800wb 990 wb 1090 wb
K-X 60w 77w 90vw 105 bm 112vw 120w 140vw 280b 370b 440sh 460s 490vs
Rb-X 72w 90vw llOvw 140vw 300sh 370vw
Tl-X
Cs-X
60 sh
55 m 60 sh 77w 90vw
94s
ll2vw 120 vw 135m 295 b 370b
110s 157s 286m 367m
440 sh 455w 507s
460m 51Ovs
790wb
800wb
1090 wb
1090 wb
485sh 500vs 580w 660wb 700wb 980wb 1100 wb
assign.0 trans Cs+, TI+ b(T-0-T) trans Na+, Rb+ b(T-0-T) trans K+ b(T-0-T) trans Na+ b(0-T-O) b(0-T-O) b(0-T-O) b(0-T-O) 8(0-T-O) b(0-T-Q) u(T-0)
v,(T-O) u,(T-O) v,(T-O) v"(T-0)
vs, very strong; s, strong; m, medium; w, weak; vw, very weak; b, broad; sh, shoulder. T: Si, Al.
Conclusions
The bandwidths of the sharp Raman bands of the dealuminated faujasites are sensitive to the aluminum content at low level. The incorporation of aluminum content at high level induces marked changes in frequency and intensity. The lower frequency region exhibits Raman bands in good agreement with framework deformation motions, such as supercage window opening. Significant changes in position and intensity of the bands assigned to the framework vibrations were observed according to the nature of the extraframework charge-balancing cation of exchanged zeolites. The Raman scattering spectroscopy enables us to assign some bands below 120 cm-I to translational motions of cations. The hydration of the samples induces the broadening of the translational motions of the cations, whereas the dependence of the Raman bands correspondingto thevibrationsof the framework was found to be weak, particularly for thedealuminated faujasites. These results are in good agreement with the change in the cation distribution and weak deformation of the framework upon hydration. The Raman scattering spectroscopy has been shown to be an efficient tool, in addition to infrared and inelastic neutron scattering spectroscopy, for obtaining useful information concerning the framework and extraframework cations of faujasites. Acknowledgment. We thank Dr. D. Bougeard, Pr. Dr. P. Bopp, and Dr. K.Smirnov for helpful comments and discussions. We are very grateful to Dr. A. Lorriaux and Dr. B. Sombret for their assistance and advice while using dispersive and FT-Raman instruments. References and Notes
100
200
WAVENUMIIER(CM-~)
Figure 4. Low-frequency Raman spectra (300 K) of the dehydrated exchanged Y zeolites: (a) Cs-Y, (b) Rb-Y,(c) K-Y, (d) Na-Y, (e) Li-Y. Excitation radiation: 568.2 nm. The arrows indicate the bands assigned to the translational motions of the extraframework cations.
that are sensitive more than anything else to the nature of the charge-balancing cation. These features are assigned to the translational motions. It is expected and observed that the centrosymmetric space group Fd3m induces mutual exclusion between Raman and IR activities. From the vibrational density of states obtained by MD calculation^,^^ it turns out that the sodium motions are involved in frequencies up to 400 cm-'; the main amplitudes, however, appear at 110 and 200 cm-'. This finding is in reasonable agreement with the FIR measurements previously reported,IS as well as the present Raman results. Nevertheless, the simulation was based on a full occupancy of sites I and I1 by 48 sodium ions to overcome the problem of partial occupancy in the Nas6-Y sample.
(1) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974. (2) von Ballmos, R. Collection of simulated XRD powder patterns for zeolites; Butterworths: Surrey, 1985. (3) Newsam, J. M.Science 1986, 231, 1093. (4) Mortier, W. J. Compilation of Extra-framework sites in Zeolites; Butterworths: Guildford, 1982. ( 5 ) Mc Daniel, C. V.; Maher, P.K.In Zeolite chemistry and catalysis; Rabo, J. A., Ed.; ACS Monograph 171; American Chemical Society: Washington, DC, 1976; p 285. (6) Scherzer, J. In Catalytic Materials, Relationship between Structure and Reactivity; Derouane et al., Eds.; ACS SymposiumSeries 248; American Chemical Society: Washington, DC, 1984; p 157. (7) Corma, A. In Zeolites, Facts, Figures, Future; Jacobs, P.A., van Santen, R. A., Eds.; Studies in Surface Science and Catalysis Vol. 49; Elsevier: Amsterdam, 1989; p 49. (8) Delprato, F.; Delmotte, L.; Guth, J. L.; Huve, L. Zeolites 1990,10, 546. (9) Ozin,G. A.; Baker, M.D.;Godber, J.; Gil, C. J.J. Phys. Chem. 1989, 93, 2899. (10) Ramamurthy, V.;Eaton, D. F.; Caspar, J. V. Acc. Chem. Res. 1992, 25, 299. (1 1) Flanigen, E. M.;Khatami, H.; Szymanski, H. A. Adv. Chem. Ser. 1971, 101, 201. (12) Jobic, H. Spectrochim. Acta 1992, 48A, 293. Angell, C. L.; Risen, Jr., W. M.J. Phys. Chem. 1977, (13) Butler, W. M.; 81, 2061. (14) Tabourier,P.; Carru, J.C.; Wacrenier, J. M . J . Chem.Soc.,Furaday Trans. 1 1983, 79, 779. (15) Baker, D. M.;Ozin, G. A.; Godber, J. Catal. Rev. Sci. Eng. 1985, 27, 591. (16) Baker, D. M.;Ozin,G. A.; Godber, J.J. Phys. Chem. 1985,89,305. (17) Ozin,G.A.;Baker,M.D.;Godber,J.;Shihua,W.J.Am.Chem.Soc. 1985, 107, 1995. (18) Carru, J. C. Thesis, Lille, France, 1986. (19) Angell, C. L. J. Phys. Chem. 1973, 77, 222. (20) Roozeboom, F.; Robson, H. E.; Chan, S.S.Zeolites 1983, 3, 321. (21) Dutta, P.K.;Shieh, D. C.; Puri, M. J . Phys. Chem. 1987,91,2332. (22) Miecznikowski, A.; Hanuza, J. Zeolites 1987, 7, 249. (23) Dutta, P. K.;Twu, J. J. J. Phys. Chem. 1991, 95, 2408. (24) Dutta, P. K.;Del Barco, B. J . Phys. Chem. 1988,92, 354. (25) Dutta, P. K.;Zaykoski, R. E. Zeolites 1988,8, 179. (26) Salzer, R.; Steinberg,K. H.; Klaebw, P.; Schrader, B. Zeolites 1991, 11, 694. (27) Saperstein, D. D.; Rein, A. J. J. Phys. Chem. 1977, 81, 2134. (28) Pilz, W.; Kriegsmann, H. 2.Phys. Chem. 1987,268, 215. (29) Boulet, H.; BrCmard, C.; Le Maire, M.J . Roman Spectrosc. 1992, 23, 421.
9702
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
(30) De Kanter, J. J. P. M.; Maxwell, I. E.; Trotter, P. J. J . Chem. Soc., Chem. Commun. 1972, 723. (31) Egerton,T. A.; Hardin, A. H.; Kozirovski, Y.; Sheppard, N. J. Catal. 1974, 32, 343. (32) Dutta, P. K.; Zaykoski, R. E.;Thomson, M. A. Zeolites 1986,6,423. (33) Buch, R.; Passingham, C.; Warnes, G. M.; Rawlence, D. J. Spectrochim. Acta 1990, 46A, 243. (34) Rubin, J. C.; Sala, 0. J . Raman Spectrosc. 1981, 11, 320. (35) Cooney, R. P.; Tsai, R. P. J . Raman Spectrosc. 1979, 8, 195. (36) Tsai, P.; Cooney, R. P. Chem. Phys. Lett. 1978, 59, 510. (37) Bartlett, J. R.; Cooncy, R. P. Spectrochim. Acta 1987, 43A, 1543. (38) Bremard, C.; Denneulin, E.; Depecker, C.; Legrand, P. Structure and Reactivity of Surfaces; Mortera, C., Ed.; Elsevier: Amsterdam, 1989; p 219. (39) BrCmard, C.; Des Grousilliers, H. J . Raman Spectrosc. 1991, 22, 125. (40) Dutta, P. K.; Mohana Rao, K.; Yul Park, J. J. Phys. Chem. 1991, 95, 6654. (41) Buckley, R. G.; Deckman, H. W.; Witzke, H.; Mc Henry, J. A. J . Phys. Chem. 1990,94,8384. (42) Jakupca, M. R.; Dutta, P. K. AMI. Chem. 1992, 64, 953. (43) No, K. T.; Bae, D. H.; Jhon, M. S . J. Phys. Chem. 1986,90, 1772. (44) No, K.T.; Seo, B. H.; Park, J. M.; Jhon, M. S. J . Phys. Chem. 1988, 92, 6783. (45) No, K. T.; Huh, Y. Y. J. Phys. Chem. 1989, 93,6413. (46) De Man, A. J. M.; Van Beest, B. W. H.; Leslie, M.; Van Santen, R. A. J . Phys. Chem. 1990, 90, 13 11. (47) Van Beest, B. W. H.; Kramer, G. J.; van Santen. R. A. Phys. Rev. Lett. 1990, 64, 1955. (48) Kramer, G . J.; Farragher, N. P.; van Beest, B. W. H.; van Santen, R. A. Phys. Rev. 1991,43, 5068. (49) Van Santen, R. A.; de Man, A. J. M.; Jacobs, W. P. J. H.;Teunissen, E. H.; Kramer, G. J. Catal. Lett. 1991, 9, 273. (50) Nicholas, J. B.; Hopfinger, A. J.; Trouv, F. R.; Iton, L. E. J. Am. Chem. Soc. 1991, 113,4792. (51) Shin, J. M.; No, K. T.; Jhon, M. S. J . Phys. Chem. 1988,92,4533. (52) Boumiz, A.; Cartigny, J.; Cohen de Lara, E. J . Phys. Chem. 1992, 96, 5419. (53) de Man, A. J. M.; van Santen, R. A. Zeolites 1992, 12, 269. (54) Blackwell, C. S. J . Phys. Chem. 1979,83, 3251. ( 5 5 ) Blackwell, C. S. J . Phys. Chem. 1979, 83, 3257.
Br6mard and Le Maire (56) Bornhauser, P.; Calzaferri, G. Spectrochim. Acta 1990,46A, 1045. (57) Schrader, B.; Hoffmann, A.; Keller, S . Spectrochim. Acta 1991,17A, 1135. (58) Hseu, T. Ph.D. Thesis, University of Washington, University Microfilms No. 73-13835, Ann Arbor, MI, 1972. (59) Maroni, V. A. Appl. Spectrosc. 1988, 42, 487. (60) Lievens, J. L.; Mortier, W. L.; Chao, K.-J. J . Phys. Chem. Solids 1992, 53, 1163. (61) Mortier, W. J. Compilation of Extra-Framework Sites in Zeolites; issued by the commission of the international Zeolite Association, 1981. (62) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986, 90, 1311. (63) Eulenberger, G. R.; Schoemaker, D. P.; Keil, J. G. J . Phys. Chem. 1967, 71, 1812. (64) Czjzek, M.; Jobic, H.; Fitch, A. N.; Vogt, T. J. Phys. Chem. 1992, 96, 1535. (65) Forano, C.; Slade, R. C. T.; Krog Andersen, E.; Krog Andersen, I. G.; Prince, F. J. Solid State Chem. 1989,82, 95. (66) Van Dun, J. J.; Dhaeze, K.; Mortier, W. J. J. Phys. Chem. 1988,92, 6747. (67) Van Dun, J. J.; Mortier, W. J. J. Phys. Chem. 1988, 92, 6740. (68) Smolders, F.; Van Dun, J. J.; Mortier, W. J. J. Phys. Chem. 1991, 95, 9908. (69) Bennett, J. M.; Smith, J. B. Mater. Res. Bull. 1968, 3, 633. (70) Mortier, W. J.; Van den Bossche, F.; Uytterhoeven, J. B. Zeolites 1984, 4, 41. (71) Smimov, K. S.; Bougeard, D. J. Raman Spectrosc. 1993, 24, 255. (72) Godber, J.; Ozin, G. A. J. Phys. Chem. 1988, 92,2841. (73) Ozin, G. A.; Baker, M. D.; Parnis, J. M. Angew. Chem. Suppl. 1983, 1075. (74) Jacobs, W. P. J. H.; Jobic, H.; van Wolput, J. H. M. C.; van Santen, R. A. Zeolites 1992, 12, 315. (75) Jobic, H. J . Catal. 1991, 131, 289. (76) Jacobs, W. P. J. H.; van Wolput, J. H. M. C.; van Santen, R. A. J . Chem. Soc., Faraday Tram. 1993,89, 1271. (77) Engelhardt, G.; Michel, D. High Resolution solid state NMR of Silicates and Zeolites; Wiley: New York, 1987. (78) Herrero, C. P.; Utrera, L.; Ramirez, R. Phys. Rev. B 1992,46,787. (79) Schrimpf, G.; Schlenkrich, M.; Brickmann, J.; Bopp, P. J . Phys. Chem. 1992,96, 7404.
