A SPECTROSCOPIC STUDY OF
THE
SURFACE OF ZEOLITEY
4211
A Spectroscopic Study of the Surface of Zeolite Y.
11. Infrared
Spectra of Structural Hydroxyl Groups and Adsorbed Water on Alkali, Alkaline Earth, and Rare Earth Ion-Exchanged Zeolites by John W. Ward U n i o n Oil Company of California, U n i o n Research Center, Brea, California 08681
(Received M a y 51, 1068)
Infrared spectra of alkali, alkaline earth, and rare earth Y zeolites have been observed during dehydration and subsequent rehydration at various temperatures. No structural hydroxyl groups were observed for the alkali cation Y zeolites. Readsorbed water simply physically adsorbed by association with the exchangeable cations. No new structural hydroxyl groups were formed. The frequency of the absorption bands of the adsorbed water varied with the cation radius and hence with the calculated electrostatic field and potential and with the ionization potential. The results were similar to those found for alkali cation X zeolites by Habgood. An examination of NaX zeolite confirmed the results of Habgood in most respects. The adsorption of water on the alkali cation zeolites is highly reversible. Barium Y zeolite dehydrated and rehydrated analogous to the alkali cation forms. Spectra of magnesium, calcium, and strontium Y zeolites all showed structural hydroxyl groups. The absorption bands near 3640 and 3540 cm-l are considered to be the same as observed in decationized zeolites and represent silanol groups. An absorption band in the 3600-3560-cm-l region is identified with hydroxyl groups associated with the divalent cations, while a band near 3690 cm-l is due to adsorbed undissociated water. Rehydration produces a strong band near 3690 cm-l, which, when the zeolite is heated above 200°, disappears simultaneously with the growth of bands near 3640 and 3600-3560 om-1. The latter band is very sensitive to the hydration level, probably indicating that the 310" species is readily converted to the oxide. Confirmation of this band representing attachment to the cation is shown by the regular variation of its frequency with cation. Spectra of the rare earth Y zeolite showed two dominant hydroxyl bands at 3640 and 3522 cm-l assigned to silanol groups and MOH groups. Water adsorbed in a highly reversible manner. No new structural hydroxyl groups were formed.
Introduction Synthetic zeolites of the faujasite type have now become important adsorbants and catalysts. I n spite of their frequent use as drying agents and as catalysts in nonwater-free systems, the nature of the surface structural groups and of adsorbed water is far from being resolved. Recently, several studies of the X- and Ytype zeolites have been reported concerning the decationated zeolites and the cation-exchanged zeolites. Studies of the decationated Y zeolites have shown that three absorption bands occur at 3745, 3650, and 3545 cm-l attributable to structural hydroxyl groups.1-8 The 3745-cm-1 band has generally been attributed to silica-type hydroxyl groups present as an impurity or terminating the giant crystal lattice. The bands a t 3650 and 3545 c1n-l have been attributed to structural silanol groups introduced by the deamination of the ammonium Y zeolite. There is, however, some doubt as to their exact Location in the The spectra of' group IA X zeolites have been studied by Bertsch and H a , b g ~ o d .After ~ calcination at 500" they observed 110 structural hydroxyl groups. I n a subsequent study, Habgood'O detected small concentrations of structural hydroxyl groups. On readsorption of small amounts of water, a sharp band between 3720 and 3648 cm-l depending on cation and broad bands
near 3400 and 3200 cm-I were o b ~ e r v e d . Because ~ the frequency depended on the cation and the band intensities depended on the degree of hydration, these bands were considered to represent water directly bonded to the cation. I n an investigation of various cationic forms of X zeolite, Carter, Lucchesi, and Yatesll observed structural hydroxyl groups in all cases, the frequencies and number of bands varying with cation. The bands were attributed to hydroxyl groups attached t o silicon and aluminum. The band near 3750 cm-' was attributed to silica-like silanol groups. A band in the region of 3715-3685 cm-l was attributed to AlOH (1) J. E. Uytterhoeven, L. G. Christner, and W. K. Hall, J. Phys. Chem., 69,2117 (1965). (2) C. L. Angel1 and P. C. Schaffer, ibid., 69, 3463 (1965). (3) B. V. Liengme and W. K. Hall, Trans. Faraday SOC.,62, 3229 (1966). (4) P. E. Eberly, Jr., J . P h y s . Chem., 71, 1717 (1967). (5) J. L. White, A. W. Jelli, J. M. Andre, and J. J. Fripiat, Trans. Faraday Soc., 63,461 (1967). (6) T. R. Hughes and H. M. White, J . Phys. Chem., 71, 2192 (1967). (7) J. W. Ward, ibid., 71, 3106 (1967). (8) J. W. Ward, J . Catal., 9, 225 (1967). (9) L. Bertsch and H. W. Habgood, J . Phys. Chem., 67, 1621 (1963). (10) H . W. Habgood, ibid., 69, 1764 (1965). (11) J. L. Carter, P. J. Lucchesi, and D. J. C . Yates, ibid., 68, 1386 (1964). Volume 73, Number 18 November 1968
4212
JOHN W. WARD
groups. In most cases, a band was observed near 3650 was attributed to hydroxyl groups similar to those in cm-1. Although this was not definitely assigned by hydrogen Y zeolite and those near 3590 cm-I to MOH+ Carter, et ~ 1 . ~ 1it1 probably represents the same type of groups. Hall, et a1.,19 has reported observing no hyhydroxyl group as found in the decationated Y zeolites. droxyl groups on group I1 zeolites but reported their This is supported by the zeolites used having a cation prescence on group IIA zeolites. Recently, Olson has deficiency and confirmed by Habgood, who deliberately reported X-ray evidence for residual water in divalent introduced a cation deficiency into an NaX zeolite.1° cation X and Y zeolitesbZ0 I n the multivalent cation forms, it is probably due to Although structural hydroxyl groups on zeolites have structural silanol groups, like those present in the been suggested as important active catalytic sites7~13~17~21 decationized zeolite, introduced during or subsequent to and small amounts of water have been shown to prothe exchange.12-14 Analogous to Bertsch and Habmote catalytic r e a c t i o n ~ 1 9and ~ ~the ~ ~formation ~ ~ ~ ~ ~ of goodjg Zhdanov, Kiselev, Lygin, and T i t o ~ oba ~ ~ Brgnsted ~ ~ ~ acid sites on ~ e o l i t e s , * ~ ~ ~ there ~ ' ~ - 1is 9 conserved a band near 3700 cm-' for adsorbed water on X siderable doubt as to whether hydroxyl groups are part zeolites which varied in frequency with the cation. of the structure of pure zeolites or whether they are I n contrast to X zeolites little attention has been introduced as impurities. Furthermore, although some given to Y zeolites. Angell and Schaffe? studied a work has been conducted with sodium X zeolites, little number of mono- and divalent cation Y zeolites (as well work has been undertaken with Y zeolites and then only as Na and CaX zeolites) after calcination up to 585". after calcination at 500". Only a cursory study of They observed several hydroxyl groups on each zeolite. adsorbed water has been made.2 Bands at 3640 and 3540 cm-l were assigned to hydroxyl I n this paper, a study of the surface structural groups groups, arising from the cation deficiency, similar to of various Y zeolites as a function of dehydration temthose on decationated Y zeolite. The band near perature and of the adsorption of small amounts of 3691-3673 cm-l was attributed to AlOH by analogy water is reported. With the exception of the study of with the results of Carter, et al.," while the band at sodium X zeolite by Carter, et ai.," and Zhdanov, 3745 cm-' was assigned t o silicalike silanol groups. et al.,15 no observations have been reported of the On readdition of water, they observed results similar to dehydration of cationic forms of zeolites as a function of Bertsch and Habgoodg with NaX and Nay, but with temperature, and, apart from the work of Bertsch and CaX only a broad band appeared near 3480 cm-1. Habgoodg and Angell and Schaffer,2 no studies of the With divalent cation Y zeolites, a broad band appeared readsorption of water have been reported. at 3540 cm-I with little effect on the 3640-cm-l band. Experimental Section The hydroxyl groups exchanged with heavy water at Materials. The samples of ion-exchanged Y zeolites room temperature. Recently, EberlyI3 reported obwere those described in previous ~ t u d i e s . ~l8~ ~Details *' serving no absorption bands on Na and CaY after of their preparation and analytical data were given. dehydration. On addition of water to Cay, bands were The rare earth zeolite was prepared by ion exchange observed at 3650 and 3550 cm-l. Wardl48l7has also with a 10% solution of mixed rare earth chloride recently reported detection of no hydroxyl groups on (REC1) obtained from American Potash and Chemical group IA zeolites but observed a number of structural Co., Rare Earth Division. A typical analysis of the hydroxyl groups on group IIA zeolites after calcinamixture for major components was: CeOz, 47.8; tion at 500". I n general frequencies reported were similar to those found by Angell and Schaffer.2 Bands observed near 3740, 3645, and 3540 cm-I were assigned (12) W. K. Hall, Chemical Engineering Progress Symposium Series, Vol. 63, American Institute of Chemical Engineers, 1967, No. 73, p to silanol groups similar to those observed in hydrogen 68. Y zeolites, while a band at 3690 cm-l was assigned to (13) P. E. Eberly, J . Pkys. Chem., 72, 1042 (1968). AlOH groups by analogy with Carter, et al." A band (14) J. W. Ward, J . Colloid Interfac. Sci., in press. was also observed in the region 3560-3582 cm-l but was (15) S. P. Zhdanov, A. V. Kiselev, V. I. Lygin, and T. I. Titova, Rum. J . Phgs. Chem., 38, 1299 (1964). not assigned. Similar to the report of Angell and (16) 5. P. Zhdanov, A. V. Kiselev, T. I. Lygin, M. E. Ovsepyan, Schaff er, the hydroxyl groups underwent deuterium and T. I. Titova, ibid., 39, 1309 (1965). exchange readily at room temperature. Wardla has (17) J. W. Ward, J . Catal., 10, 34 (1968). confirmed the results of Bertsch and Habgoodg and (18) J. W. Ward, ibid., in press. Angell and Schaffer2 for the addition of water to cal(19) L. G. Christner, B. V. Liengme, and W. K. Hall, Trans. Faraday Soc., 64, 1679 (1968). cined sodium zeolite. A sharp band was observed a t (20) D. H. Olson, J . Phys. Chem., 72, 1400 (1968). 3694 cm-l and broad bands were observed at 3550 and (21) P. B. Venuto, L. A. Hamilton, and P. S. Landis, J . Catal., 5 , 3340 cm-l. Upon addition of small amounts of water 484 (1966); (b) C. J. Plank, comment on paper by P. E. Pickert, J. A. Rabo, E. Dempsey, and V. Schomaker, Proc. Int. Congr. Catal., to magnesium and calcium Y zeolites followed by reSrd, Amsterdam, 1964, 1, 727 (1965). moval of excess by evacuation at 150", strong bands (22) H. A. Benesi, J . Catal., 8 , 368 (1967). were formed at 3642 and near 3590 cm-l (3584 cm-' (23) H. W. Habgood and Z. M. George, "Molecular Sieves," Society for Ca and 3595 cm-l for Mg). The band at 3642 cm-l of Chemical Industry, London, 1968, p 130. The Journal of Physical Chemistry
A SPECTROSCOPIC STUDY OF
THE
SURFACE OF ZEOLITEY
Laz08, 24.2; Ndz03, 18.2; Sme03, 2.8, GdeOa, 0.9. No cation deficiency was detected in the samples. A sample of X zeolite (SiOz:AI&& ratio of 2.3 : 1; Na content, 14.7%) was also studied. The analyses are given in Table I. Heavy water, reagent grade, was used obtained from Stuart Oxygen Co., San Francisco, Calif. It was 99.8% pure. The sampling technique, cells, and vacuum equipment were the same as described previously.l4 A Cary-White 90 infrared spectrometer was used. The spectra resolution was 3 cm-' and a scan speed of 1 cm-' sec-' was used. The instrument was calibrated frequently against the water vapor spectrum, The frequencies of the sharp bands are accurate to &0.5 cm-l. All spectra were measured at room temperature. Thermogravimetric analysis data were obtained with an Aminco Thermograv. The measurements were made in flowing helium with a temperature program of l"/min.
Xa
K Rb
cs
Mg Ca Sr Ba
RE NaX
Figure 1. Spectra of the hydroxyl stretching and water bending vibrations of NaY zeolite: (a) evacuated overnight a t looo, ( b ) evacuated 2 hr a t 225O, (c) evacuated 2 hr a t 380", ( d ) evacuated 2 hr a t 480", (3) 6 pmol of water readsorbed, (f) 12 pmol of water readsorbed.
It is seen that at low dehydration temperatures physi-
Table I : Analysis of Samples
Li
4213
Surface area,
%
% Na
rnl g-'
exchanged
1.87 10.24 0.02 1.09 2.22 2.3 0.66 1.09 1.48 1.80
826 901 899
81.7
... 937 801 810 810 876 801 876
...
99.8 89.4 78.3 77.5 93.6 89.4 85.6 82.8
Procedure. The sample of zeolite (50 mg) was dehydrated under vacuum at a series of increasing temperatures. Initially, the sample was evacuated overnight at 90" and the spectrum was recorded. Dehydration was then carried out for 2 hr at a series of increasing temperatures. After dehydration near 500", small amounts of water (6-20 pmol) mere added back to the sample and the spectrum was recorded. The removal of the readsorbed water was then studied as a function of temperature. Some experiments were also conducted in which excess water was adsorbed. Experiments were also carried out in which heavy water was added back to the dehydrated zeolite. In a second series of experiments the zeolites were calcined at lower temperatures and then the readsorption-desorption of water was studied.
Results Alkali Cation Zeolites. The spectra of sodium Y zeolite as a function of dehydration temperature in the 4000-3000 cm-' (hydroxyl stretching) and 1700-1500 em-' (hydroxyl bending) regions are shown in Figure 1.
cally adsorbed water is present, as indicated by the absorption band at 1643 cm-l. After evacuation a t 300°, this band is not observed, indicating that most of the physically adsorbed water has been removed. Thermogravimetric analysis also shows that most of the physically adsorbed water is removed by 250". I n the 4000-3000-~m-~region, marked changes occur in the spectrum as the dehydration temperature is raised. Up to lOO", only broad, partially resolved absorption bands are observed. These rapidly disappear until after evacuation at 225" four broad bands are observed. All absorption bands except two very weak bands at 3730 and 3695 cm-' are removed at 380". These two remaining absorption bands are not observed after evacuation at 500". The spectra differ from that reported by Angell and Schaffe? in that no absorption band is detected near 3650 cm-l. When small amounts of water (6-18 pmol) are readded to the system, new bands appear in the hydroxyl regions, as shown in Figure 1. Bands are detected a t 3735, 3694, 3460, 3360, 3242, and 1638 cm-'. These bands appear similar to those reported by Bertsch and Habgoodg and by Angell and Schaffe? for NaX and NaY , respectively. However, probably because of their poorer spectral conditions, less detail was reported, and the lower frequency bands were ignored. The 3 6 9 4 - ~ m - band ~ is sharp, typical of isolated hydroxyl groups, while the 3460-, 3380-, and 3242-cm-' bands are broad and incompletely resolved. I n Figure 2, the peak height of the 3694-cm-' band is plotted against the peak heights of the 3460-, 3360-, and 3243and 1643-cm-l bands. The absorption bands were progressively removed by evacuation at increasing temperatures until after evacuation at 250" no bands were detected. No new absorption bands were formed upon heating, in contrast to the alkaline earth forms. Volume 72, Number 18 November 1968
JOHN W. WARD
4214 Table IT: Hydroxyl Stretch Frequencies (om-') of Various Cation Y Zeolites as a Function of Temperature Temp, ')C
100 225 360 480 Ambient 250
Li
Na
K
Rb
cs
3740,3718,3670,3560, 3420,3280 3740,3718,3660,3560, 3400,3280 3740,3715,3580,3450, 3270 3740,3714,3600-3300 3740,3714,3628,3550, 3420,3230 3740,3714,3600-3300
3735,3690,3600,3380, 3280 3735,3690,3635,3544
3740,3610,3540,3440, 3290 3740,3590,3520,3400, 3290 3740,3700-3400
3740,3620,3584,3520, 3440,3280 3740,3640-3400
3740,3610,3520,35003200 3740,3656-3200
3740,3650-3400
3740-3200
3738,3695,3588
...
...
...
3735,3694,3460,3360, 3242
3668,3490,3420,3255
3654,3580,3420,3260
3740 3640,3450,3270
Temp,
Mg
O C
110 230
3740,3690,3640,3548, 3460-3300 2740,3688,3642,3550
350
3740,3688,3642,3540
480
3740,3688,3642
Ambient
3740,3700,3690,3550
105
3740,3688,3642,3545
225
3740,3688,3642,3540
350
3740,3688,3642,3540
a
... 2
3800-3100 3740,3690,3640,3590, 3560,3520 3740,3690,3640,3590, 3520 3738,3690,3640,3585, 3540 3740,3683,3640,3550 3740,3700,3640,3560, 3400 3740,3690,3640,3585, 3520 3740,3690,3640,3585
1643cm-l BAND
13460crn-1 BAND
1
I
30
Sr
Ca
3740,3690,3638,3610, 3540,3460 3740,3690,3639,3570, 3500-3200 3738,3700,3639,3570
3740,3700-3100
3740,3620,3560
3740,3700-3100
3740,3630,3618,3655
3740,3690-3300
3740,3636,3540
3740,3690,3640
3740
3740,3640,3522
3735,3679,3639,35903200 3735,3680,3639,3572, 3500-3200 3735,3690,3639,3570
3740,3682,3630,3430
3740,3636,3610,35803520 3740,3636,3610,35803520 3740,3636,3610,35803520 3740,3640,3528
1
//
I
I
I
I
50 70 PEAK HEIGHT 3694 cm-l BAND
I
Figure 2. Peak heights of the 3694-cm-l band us. peak heights of 3460-, 3360-, 3243-, and 1643-cm-l bands as a function of water content.
On adsorption of heavy water, deuteroxyl group frequencies were observed. The absorption bands were removed by evacuation at 125" for 2 hr. The ratio of the OH frequency to the OD frequency was 1:36, which is close to 1:37 found for free hydroxyl groups. I n a further examination, the sodium zeolite was dehydrated at lower temperature. At 208", physically adsorbed water was still present, as indicated by the band at 1643 cm-I. Absorption bands, presumably The Journal of Physical Chemistry
RE
Ba
...
3740,3682,3500,3260 3740,3682,3500,3200
...
due to physically adsorbed water, were also observed at 3630 and 3540 cm-l. However, after evacuation a t 330" no 1643-cm-I band was observed, and only weak bands were observed at 3738 and 3694 cm-l. These temperatures are much lower than those at which structural degradation occurs.24 Readsorption of a small amount of water on the 200 and 300" evacuated samples produced absorption bands at 3694, 3460, 3360, and 3250 cm-l as found for the 500" dehydrated samples. Spectra obtained during desorption were also similar to those obtained for the 500" sample. I n further experiments in which excess water was readded, the spectra were again similar to those formed after addition of small amounts of water and were unlike the spectra observed in the initial dehydration. All of the readsorbed water could be removed by evacuation at 250'. Results of dehydration and rehydration studies on lithium potassium, rubidium, and cesium Y zeolites were similar apart from differences in band frequencies and in the temperature at which the adsorbed water was removed. In general, the larger the cation, the more easily the water was removed. The frequencies observed are tabulated in Table I1 and typical spectra are shown in Figure 3. I n particular, it should be noted that the 3694-cm-' band of readsorbed water (6 (24)
J. L. McAtee, J. Catal., 9, 289 (1967).
A SPECTROSCOPIC STUDYOF
THE
SURFACE OF ZEOLITEY
4215
Figure 3. Spectra of Li, K, Rb, and CsY: (a) evacuated overnight at 120°, (b) evacuated 2 hr a t 215", (c) evacuated 2 hr a t 360°, (d) evacuated 2 hr a t 480°, (e) 10 pmol of water readsorbed, ( f ) reevacuated a t 150'.
1
3720
36201
36001
X
FREQUENCY VI FIELO
fREQUENCY
OLi
VI
POTENTIAL
0.5 1.0 1.5 2.0 ELECTROSTATIC FIELD [ V / i ) ELECTROSTATIC P O T E N T I A L ( $ 1
Figure 4. Frequency of the 3715-3640-em-' band us. electrostatic field and potential as a function of the cation.
I
f
u
4 5 6 IONIZATION POTENTIAL, ev
Figure 5. Frequency of the 3715-3640-em-' band us. ionization potential as a function of the cation.
pmol) for NaY occurs a t 3714 cm-l for Li, 3665 cm-l for K, 3654 cm-l for Rb, and 3640 cm-l for Cs. I n Figure 4, the frequency of this sharp absorption band is plotted against electrostatic field and potential, cal-
culated from the data of Rabo, Angell, Kasai, and SchomakerZ5& and Dempsey. 25b The same bands are plotted against the ionization potential of the cation in Figure 5. All of the samples have three broad bands in their spectra between 3500 and 3200 cm-', but only the sodium zeolite has a sharp band near 3695 cm-l. The lithium sample also has a fairly sharp band near 3625 cm-1 not found in the other ion-exchanged forms. Using the band near 1640 cm-I as an indicator, physically adsorbed water was removed from lithium by 550", sodium by 370") potassium by 360") rubidium by 220", and cesium by 210". As previously shown by Angell and Schaff er,2the "1640" cm-l band frequency varied with the cation. The frequencies for various cations are listed in Table 111. It also varied with extent of hydration. As with the sodiumY zeolite, readsorption of water on and desorption from lithium and rubidium Y zeolites after dehydration at 200 and 300" were similar to the 500" dehydrated samples. Addition of heavy water caused the appearance of OD bands displaced by a factor of 1.36 from the respective OH absorption bands in all cases. The dehydration and rehydration of sodium X were also investigated for comparison with the results of Bertsch and Habgoodg and Carter, Lucchesi, and Y at es. After dehydration overnight at 225", weak absorption bands were seen at 3655 and 3543 cm-'. The spectra were much weaker than those reported by Carter, et uZ.,ll for 150 and 300". On further dehydration at 360°, the bands became weaker until after dehydration at 500" they were no longer detected. On rehydration (6 pmol) a sharp band was formed at
(26) (a) J. A. Rabo, C. L. Angell, P. H. Kasai, and V. Schomaker, Discussions Faraday SOC.,41, 328 (1966); (b) E. Dempsey, Molecular Sieves," Society of Chemical Industry, London, 1968, p 293.
Volume 72.Number 12 November 1068
JOHN W. WARD
4216
Table I11 : Frequency of Adsorbed Water Bending Vibration on Zeolites Frequency, om -1
Frequency,
om -1
Li Na K Rb
cs
1655 1643 1658 1657 1652
1% Ca
Sr Ba
RE
1638 1635 1642 1648 1438
3688 cm-l and broad bands appeared at 3400-3330 and 3230 cm-l. A weak band was also detected at 3590 cm-' and the water bending vibration at 1649 cm-l. i The spectra are very similar to but somewhat better resolved than those reported by Bertsch and H a b g o ~ d . ~ 3600 3200 1700 1600 FREQUENCY, cm-1 The sharp absorption band in the spectrum of adsorbed water is about 7 cm-l lower than that observed for Figure 6. Spectra of calcium Y zeolite: (a) evacuated overnight a t 130°, (b) evacuated 2 hr at 245O, (c) evacuated sodium Y. The spectrum also differs from that of 2 hr a t 325O, (d) evacuated 2 hr at 470°, (e) 10 pmol water sodium Y in that no absorption band was observed near added, ( f ) evacuated a t 200", (g) evacuated a t 470', (h) 10 3400-3500 cm-l in the spectrum of adsorbed water. pmol water added, (i) evacuated a t 200'. Alkaline Earth Cation Zeolites. The spectra of these divalent cation zeolites were much more complex than broad band near 3590 cm-l. Upon heating t o 225", the those of the alkali cations. Whereas for the alkali 3685-cm-' band weakens while that at 3640 cm-1 cations no absorption bands attributable to hydroxyl grows. The spectrum now resembles that reported groups were detected after dehydration at 500", all of by Angell and Schaffe? very closely. On evacuation the alkaline earth forms had absorption bands after 500" at 470", the spectrum is similar to 6d again. Figures dehydration which are most probably due to structural Gh and i show that the sampIe can undergo further hydroxyl groups. Marked differences were observed dehydration-hydration cycles, possibly with some difbetween different alkaline earth cation forms. ferences in the distribution of hydroxyl groups. In Figure 6, the spectra of calcium Y at various When 20 pmol of water was introduced instead of 10 stages of dehydration are shown. In dehydration at pmol, somewhat different results were obtained. 26 go", poorly resolved absorption bands are observed After evacuation at 250", strong bands were formed at between 3800 and 3100 cm-l. On further dehydration 3640 and 3585 cm-li the 3640-cm-l band being 1.5 at 240", a strong sharp absorption band is observed at times as intense as the 3585-cm-' band. Chemisorp3640 cm-l together with weaker, less well resolved tion of pyridine did not affect the 3585-cm-' band but bands a t higher and lower frequencies as given in decreased the 3640-cm-' band to one-eighth of its Table 11. Further dehydration at higher temperaoriginal value.26 These observations suggest that the tures results in more clearly defined absorption bands, 3585 cm-1 band type hydroxyl groups are nonacidic, as shown in Figure 6; until after dehydration at 470", while those represented by the 3640-cm-l band are distinct absorption bands are seen at 3738, 3690, 3640, acidic, like the hydroxyl groups of hydrogen Y zeolite. 3585, and about 3540 cm-l. The spectrum observed Dehydration and rehydration spectra for magnesium, after the 325" dehydration is very similar to that restrontium, and barium are shown in Figure 7. The ported by Angell and Schaffer2for 500" vacuum activaresemblence of the spectra of barium Y zeolite to tion. They only reported bands at 3746 and 3645 cm-', those of the alkali cation forms and the difference despite a band being clearly present in their spectra from the other alkaline earth zeolites are readily seen. near 3590 cm-l. The spectra in the 1700-1400-cm-1 No distinct, well-resolved absorption bands are obregion showed that most of the physically adsorbed served. Dehydration simply removes the broad bands water had been removed by 300". Even after dehybetween 3700 and 3100 cm-I until after evacuation dration at 500", an absorption band was still observed at 360", only a weak band at 3740 cm-' is detected. near 1630 cm-l. A similar band has been reported by Unlike magnesium and calcium, no strong absorption Eberly for Mg and CaY.13 band is observed near 1630 cm-l after dehydration. I n Figure 6, spectra during rehydration are also Readdition of water gives rise to a spectrum similar shown. Figure 6e is the spectrum after addition of to those observed for alkali cations having absorption 10 bmol of water to the zeolite at room temperature. It is seen, in particular, that bands at 3693 and 3640 cm-l increase markedly in intensity, together with a (26) J. W. Ward, J. Phys. Chem., 72, 2689 (1968). 1
The Journal of Physical Chemistry
1
-
A SPECTROSCOPI~C STUDYOF
I
3600
3200
I
3600
THE
I
SURFACE OF
\
1
3200
/
3600
ZEOLITE Y
1
1
3200
FREQUENCY, cm-I
Figure 7. Spectra of magnesium, strontium, and barium Y zeolite: (a) evacuated overnight a t l l O o , (b) evacuated 2 hr a t 215O, (c) evacuated 2 hr a t 350', (d) evacuated 2 hr a t 480", (e) 12 pmol of' water readsorbed, (f) evacuated a t 15O0, (g) evacuated a t 3310".
bands a t 3740, 3682, 3630, 3430, and 3256 cm-1 (Figure 7e). Evacuation at increasing temperatures simply resulted in desorption. I n particular, unlike the magnesium, calcium, and strontium forms, no new hydroxyl bands were formed. When heavy water was adsorbed, analogous spectra were obtained, displaced to a lower frequency. Spectra of rehydrated barium Y which had been dehydrated a t 200 and 300" were similar to the rehydration of 500" dehydrated samples. The 200" dehydrated sample showed weak absorption bands in the spectrum a t 3742, 3688, 3645, 3590, and 1648 cm-l. The spectrum was intermediate between Figures 7c and d. The 300" overnight dehydrated sample had a spectrum resembling that of the 500" dehydrated sample. Magnesium and strontium Y zeolites behaved more analogous to calcium than to the alkali cation forms. There appeared to be a gradation in behavior from magnesium to barium, strontium resembling calcium in some aspects and barium in others. After dehydration at 350", the strontium zeolite had absorption bands at 3738, 3695, 3638, and 3570 cm-' (Figure 7). After evacuation at 450", only very weak absorption bands were detected a t lower dehydration temperatures, the band at 3695 and a band a t 3610 cm-' were more noticeable. Most of the physically adsorbed water had been removed by 300°, but an absorptialn band remained at 1647 cm-l similar to that in magnesium and calcium Y even after dehydration at 450". Readdition of water yielded this spectrum in Figure 7e, with bands at 3679, 3639, 3610, 3550, and 3480 cm-l. This spectrum is very similar to that of water adsorbed in barium Y zeolite. Heating at 120" removed most of the 3679-cm-l absorption band, while heating at 250" caused the formation of a band at 3639 cm-l analogous to calcium Y zeolite. Addition of heavy water produced absorp-
4217 tion bands similar to those formed on addition of water but displaced in frequency by the factor 1.36. When water was added back to the zeolite after dehydration at 250 and 350",similar effects were seen, although they were more noticeable and the behavior was more similar to that of magnesium and calcium Y zeolites. Typical spectra are shown in Figure 8. The growth of the 3679-cm-' band, without influencing the 3639-cm-' band upon addition of water is readily seen. There is also some indication for the formation of a band at about 3560 cm-l. On more extensive removal of the physically adsorbed water by evacuation at 350", this band appears well defined at 3570 cm-' and is probably the strontium analog of the band seen at 3585 cm-l with calcium. The spectra observed during the dehydration of magnesium Y are very similar to those seen for calcium Y apart from the precise frequency. Spectra are shown a t various degrees of dehydration in Figure 7. After dehydration at 210", absorption bands are detected at 3739, 3688, 3642, and 3540 cm-'. Further dehydration at higher temperatures reduces the intensity of the absorption bands at 3688 and 3540 cm-l, while after dehydration at 490" the spectrum shown in Figure 7d was obtained. Observation of the 1635-cm-l band showed that the physically adsorbed water was removed by 250". Readdition of water produced a strong band at 3705 cm-l and a broad adsorption between 3600 and 3300 cm-l. Upon desorption at 150", the 3705-cm'-l band decreases and a well-resolved band appears a t 3688 cm-'. Simultaneously, the 3642 cm-l increases in intensity, and the broad band between 3600 and 3300 cm-l is replaced by a somewhat narrower band at 3545 cm-l. Further dehydration a t 200" caused no significant changes in the spectrum.
IC. d
h,
Sr
I
-,U3800
3600 I
3400I
3800 I
3600 I 3400
FREQUENCY, cm-1
Figure 8. Spectra of calcium and strontium Y: (a) after dehydration a t 350", (b) 10 pmol water added, (c) evacuated a t 150°, (d) evacuated a t 250'. Volume 79,Number 13 November 1968
JOHN W. WARD
4218
and t o that reported by Rabo, et aLZ4 Upon readdition of water (10 pmol), absorption bands appeared at about 3636, 3610, and 3580-3520 cm-I. Upon evacuation at 105", the absorption bands near 3610 and 3580-3520 cm-l were weakened. Upon heating to 225') these bands further decreased in intensity. Further evacuation at 360" removed the 3610-cm-l band, and the 3540-cm-l band shifted to 3528 cm-l. This latter spectrum is similar to that found after dehydration at 450" (Figure 9). Previous studies of pyridine chemisorptionz5have shown the 3520-cm-1 band to be nonacidic. Upon addition of a small amount of heavy water, absorption bands were observed at 2662 and 2622 cm-l. These frequencies differ by factors of 1.37 and 1.35 from the corresponding OH stretching frequencies. Similar results were obtained when water was added back to the zeolite after calcination at 330".
Discussion
3800
3600 3400 FREQUENCY, cm-I
3201
Figure 9. Spectra of rare earth Y zeolite: (a) evacuated overnight a t loo', (b) evacuated 2 hr a t 225", (c) evacuated 2 hr a t 330', ( d ) evacuated 2 hr at 450", (e) 12 pmol of water added, ( f ) evacuated a t 150", (g) evacuated a t 450'.
When excess water was adsorbed on the zeolite followed by desorption at 250") a strong band was observed in the spectrum near 3595 cm-l as reported previously.z6 Treatment with heavy water resulted in the formation of deuteroxyl bands corresponding to the hydroxyl groups in the light-water system. Spectra of readsorbed water on the magnesium Y zeolite after dehydration a t 300" showed no differences from the 490" dehydrated sample (Figure 8). Rare Earth Zeolite. The rare earth zeolite is one of the few zeolites containing trivalent ions which are stable and readily prepared. It is also of great interest catalytically. Figure 9 shows spectra of the zeolite during dehydration and rehydration. As shown by the absorption band at 1638 cm-l, the physically adsorbed water was removed by 225". At this stage of dehydration, hydroxyl stretching bands were observed at 3740, 3630, 3618, and 3555 cm-l. The spectrum is very similar to that reported by Hall for cerium Y ze01ite.l~ Further dehydration at 330' removed the 3615 cm-1, while the 3630-cm-1 band moved to 3636 cm-l and the 3555-cm-' band moved to 3540 cm-1. Dehydration at 450" reduced the band intensities further. The 3555-cm-1 band moved to 3522 cm-1 and the 3636-cm-l band moved to 3640 cm-l. This spectrum is also similar to that reported for cerium Y after 460" dehydrationlS The Journal of Physical Chemistry
Alkali Cation Zeolites. A consideration of the zeolite structure would lead to the expectation of a complete absence of structural hydroxyl groups with the exception of those necessary to terminate the giant lattice. This is borne out by the observations shown in Figures 1 and 3, which show that, with the exception of lithium, only a weak hydroxyl band is observed at 3740 cm-l in the spectra of the various zeolites. This band probably represents SiOH groups which terminate the lattice1j2I" or in siliceous material are present as an impurity.z As observed by Angell and Schaffer,2this band undergoes deuterium exchange readily. The absence of other structural hydroxyl groups is in marked contrast to the results for X zeolite of Carter, et aZ.,ll and of Angell and Schaffer2 for Y zeolite. However, the zeolites of these workers were cation deficient. Since it has been shown by HabgoodlO that the introduction of cation deficiency introduces hydroxyl groups, the bands observed by these workers are most likely due to this cause. Similar experiments which we have carried out with cation-deficient zeolites resulted in spectra showing structural hydroxyl groups. If the absence of structural hydroxyl groups is accepted in the zeolites studied, all the absorption bands observed must be due to physically adsorbed water. The spectra observed on readdition of water are very dependent intensitywise on the amount of water admitted. The data of Figure 2 show that the hydroxyl bands are all formed at about the same rate despite their obviously different band frequencies and shapes. I n their study of X zeolites, Bertsch and Habgoodg reported spectra very similar to those shown in Figure 1 and Figure 3 and assigned the sharp band for KaX at 3695-3688 cm-I to water associated with the cation such that at least one of the hydrogens is free. They considered the broad band near 3500-3400 cm-l to represent the second OH of the adsorbed water bound to the lattice oxygens. They did not interpret the band observed near 3200 cm-l.
A SPECTROSCOPIC STUDY OF THE SURFACE OF ZEOLITEY The results for NaY in this study are very similar, except that three broad bands are detected. The bands in the 4000-3000-~m-~region are always accompanied by the water bending band at 1640 em-l. The variation of the band intensities with the amount of water added and extent of subsequent rehydration strongly support Bertsch and Habgood's assignmentg of the sharp band to bonded water as shown Na-
--0/H
i
".. Al;o-si
The three broad bands probably are due to hydrogen being bound to crystallographically different oxygens which results in different strengths of interaction. Alternatively, these bands could represent adsorption associated with cations located in different crystallographic positions. The parallel growth of intensity of the three broad bands suggests that none is formed preferentially. Habgoodg found a similar parallel growth of the 369.5, 3400-, and 1 6 5 0 - ~ m -bands ~ for NaX zeolite as a function of water content. These observations all suggest that the absorption bands are due to adsorbed water and not to structural hydroxyl groups. As suggested previously,2 the ease of removal of the absorption bands by evacuation shows that they are of completely different origin than the bands found near these frequencies in hydrogen Y zeolite.2rs Furthermore, the band at 3695 cm-' is probably not due to AlOH groups as previously suggested2!" but is due to the cation-water interaction. Absorption bands previously reported near this frequency for sodium zeolites" are probably due to incomplete dehydration or readsorption of small amounts of water. The results of Olson support this conclusion.20 These band assignments are confirmed by the results obtained with the other alkali cation zeolites. None of these have an absorption band near 3695 em-'. I n all cases except lithium, no absorption bands, other than at 3740 cm-l, are detected after dehydration at temperatures near 500". Readsorption of water (Figure 3) produces a sharp band and two or three broad bands at lower frequency, similar to those observed for sodium Y zeolite. The broad bands vary in frequency with the cation but in no regular manner. The sharp band decreases in frequency as the cation radius increases. The variation of the frequency with electrostatic fieldz5and electrostatic potential are shown in Figure 4. For sodium to cesium a linear relationship holds, indicating stronger interaction as the field and potential increase with decreasing radius. A similar relationship for Li, Ea, and K X zeolite is to be found in the data of Bertsch and Habg0od.O The observed frequency for lithium is mucli lower than would be predicted but is close to that observed for X zeolite.9 This deviation
4219 could be expected, since it is possible that lithium does not exchange into a zeolite as the simple ion. However, when the frequency was plotted against the ionization potential, a linear correlation was obtained for all ionic forms (Figure 5 ) . Correlations of cationic zeolite properties with ionization potential (electron affinity) have been made previously.1g~27This observation would tend t o suggest that the assumptions made in calculating the electrostatic field and potential are not applicable to the small cations. Lithium Y zeolite was the only zeolite which showed hydroxyl bands after dehydration at 500". The higher temperature, compared with other alkali cation forms, a t which hydroxyl groups are retained could be expected from the smaller cation radius of lithium. Lithium also exhibited a hydroxyl band at 3628 cm-1 after rehydration, in contrast to the other alkali cation forms. Although the origin of this band is not clear, it could possibly represent hydroxyl groups associated with the cation, since lithium hydroxide hydroxyl stretch occurs at 3678 cm-1.28 The rehydration of the zeolites with heavy water resulted in similar spectra, except the frequencies were displaced by the factor of 1.36. The calculated shift factor from the change in mass would be 1.37. For lithium, sodium, and rubidium, the rehydration experiments carried out after dehydration at about 200 and 350" indicated that the rehydration occurred in the same manner as for the higher temperature dehydrated samples. Potassium and cesium would be expected to behave similarly. The spectra during the initial dehydration are different from those observed during rehydration and subsequent dehydration. I n particular, no sharp band is observed in the 3700-3550-cm-l region. Mainly broad, poorly resolved bands are observed between 3700 and 3200 em-l. These spectra suggest that in the presence of excess water extensive hydrogen bonding of water molecules to the surface and to other water molecules or changes occur in position and/or coordination of the cations as discussed e1sewhe1-e.~~Under conditions used in this study, the readsorbed water on alkali cation Y zeolites was distinctly different from the initial water. The results of the dehydration and rehydration of sodium X zeolite are similar to those previously rep ~ r t e d . ~ - l lIt is of interest to note that the sharp absorption band for adsorbed water is at a lower frequency (8 cm-l) on the X zeolite than on the Y zeolite, as would be expected from the lower electrostatic field on the X zeolite (Figure 10). The reason for the absence of the band occurring in Y zeolites near (27) J. T. Richardson, J . Catal., 9, 172 (1967). (28) K. Nakamoto, "Infrared Spectra of Inorganic and Coordination Compounds," John Wiley & Sons, Inc., New York, N. Y., 1963, p 73. (29) E. Goldish and J. W. Ward, to be submitted for publication. Volume 72, Number 19 November 1968
4220
JOHN W. WARD
3800
3600
3400
3200
FREQUENCY, cm-I
Figure IO. Spectra of sodium X: (a) evacuated 2 hr a t room temperature, (b) evacuated a t 1250' overnight, (c) evacuated 2 hr a t 210°, ( d ) evacuated 2 hr a t 360°, (e) evacuated 2 hr a t 510°, ( f ) 6 pmol of water added, (f) 24 Mmol of water added.
3400-3500 cm-l from the spectra of sodium X zeolite is not apparent at this time. Similarly, the origin of the 3590-em-' band is not clear. Habgood'O also observed a band in the region 3600-3580 em-l but made no assignment. Alkaline Earth Y Zeolites. The results of this study show that the dehydration-rehydration phenomena of the group IIA cations are markedly different from those of group IA and also that there is a large change in properties within the group with cation size. These differences are not entirely unexpected, since the chemisorption of carbon monoxide and pyridine and the catalytic properties of Y zeolites have been shown to be greatly dependent on the cation nature.14~17~30~31 Furthermore, it has been shown for X zeoliteP that carbon dioxide adsorbs on group IA and barium zeolites differently from the other group IIA zeolites. I n general, the dehydration and rehydration of Bay, apart from the precise frequencies, is analogous to the group IA zeolites. I n particular, during dehydration, no strong absorption band near 3640 cm-l is observed as in the other alkaline earth forms. Also, on the initial adsorption, a sharp band and two broad bands are formed at frequencies comparable with those observed for the alkali cation forms. I n the dehydration, no well-defined structural hydroxyl group frequencies are observed analogous to the alkali cation forms but difT h e Journal of Phgsical Chemistry
ferent from the other alkaline earth forms. The reasons for these differences will be discussed at a later point. However, it seems reasonable to conclude that analogous to the alkali cation forms water simply adsorbs on the barium Y zeolite with a high degree of reversibility. Little evidence is found for the formation of structural hydroxyl groups. Such groups must, though, be formed to a limited extent, since it has been shown that the barium Y zeolite exhibits Br#nsted acid sites whereas the alkali cation forms do not.")" It is difficult to compare the results of this study of barium Y zeolite with those by Angell and Schaffer,2 since they showed no spectra and gave no indication of the strength of the two bands they observed at 3744 and 3647 em-' after calcination at 500". In a sample incompletely dehydrated, as by heating at 200", weak bands were observed at these frequencies together with bands at 3688, 3590, and 1648 cm-'. The observation of the latter band suggests some of the higher frequency bands could be due to physically adsorbed water. The spectrum observed by Carter, et aZ.,ll for barium X zeolite has no resemblence to that found for barium Y. They observed three distinct hydroxyl groups after dehydration at 450" with frequencies of 3750, 3695, and 3620 cm-'. Even after partial dehydration or rehydration, no spectra resembling that of Carter, et al.," are obtained. Hall, et aZ.,19in their study found no bands after evacuation at 460" but found a weak band after 250" evacuation at 3645 cm-'. However, spectra observed in this study resembled Hall's 250" specimen at no stage. The differences must be attributed to differences in samples and/or experimental techniques. The other three alkaline earth zeolites showed distinct hydroxyl group absorption frequencies even after dehydration at 500". Although the precise frequency and intensity of the bands varied with the cation, in general bands were observed near 3740, 3640, and 3540 cm-l. Weak bands were also observed near 3690 and 3590 em-l. The precise frequencies are listed in Table 11. The 3740-cm-l band is the commonly observed hydroxyl band of zeolites and probably represents silicatype hydroxyl groups as discussed by many worke r ~ . The ~ ~absorption ~ ~ ~ bands , ~ ~at 3640 and 3540 cm-l are at frequencies similar to those observed in the decationized zeolites1-* and hence probably of the same type. Further evidence for the similarity is found from pyridine adsorption studies, in which it has been shown that these hydroxyl groups are able to protonate pyridine just as the hydroxyl of decationized zeolites do. l 4 < l 7 The concentration of these hydroxyl groups is, however, (30) C. L. Angell and P. C. Schaffer, J. Phys. Chem., 70, 1413 (1966).
(31) J. T. Richardson, J . Catal., 9, 182 (1967). (32) J. W. Ward and H. W. Habgood, J . Phys. Chem., 70, 1178 (1966).
A SPECTROSCOPIC STUDYOF
THE
SURFACE OF ZEOLITE Y
4221
formation of the 3640-cm-' band is readily explained much less than in the decationized zeolites but is much and its interaction with pyridinel4>l7 is expected. The greater than that observed for the alkali cation zeolites into which a cation deficiency has been i n t r ~ d u c e d . ~ ~detection of absorption bands due to MOH+ might also be expected. The frequency of bands associated with It would therefore seem that the presence of these the MOH+ group might be expected to vary with the groups is not due to cation deficiency. A plausible cation, RI. I n a previous article,26 in which excess explanation has been put forward previously in which water was added to the dehydrated zeolites and then it is considered that a simple divalent cation cannot removed by evacuation a t 250°, strong absorption satisfy the charge-distribution requirements of the bands were detected a t 3585 cm-l for calcium and zeolite lattice in the absence of much water. 12,14,17,21,34 3595 cm-1 for magnesium zeolites. These hydroxyl During dehydration the multivalent cation becomes groups were found to be nonacidic to pyridine. These localized and its associated electrostatic field may inobservations were repeated and confirmed in this study. duce dissociation of coordinated water molecules to For strontium, the band was observed at 3570 cm-1. produce iClOH+ and H + species. The proton would KO comparable absorption band was detected for barthen interact with a lattice oxygen at the second exium. The decrease in frequency of the band due to change site to produce the type of hydroxyl groups MOH+ groups is in agreement with what would be present in the decationized zeolites. predicted from consideration of the electrostatic field in M(OH2)n" the vicinity of the cation-the smaller the cation, the o\si/ 0\a/O\\si' 0\si/ 0\-&/ 0 stronger the field and hence the higher the frequency. A similar trend in the hydroxyl stretch frequencies of / \ I \ I \ / I / \ 0 0 0 001 0 0 0 0 0 alkaline earth hydroxides is observed.Z8 The failure to observe the formation of the 3640-cm-' band and a MOH+ BI band in the 3590-3550-cm-' region for barium suggests 0 0 0 0 0 that the electrostatic field associated with the barium si/ \si/ \si/ 'IA \ / \ / \ / \ cation is insufficient to bring about the dissociation of 0 0 0 0 0 0 O P O adsorbed water to any appreciable extent. When only a small amount of water was added back to the zeolites Such a reaction forming SiOH groups accounts for the (6-12 pmol), only weak bands were observed near 3640- and 3540-cm-' bands. 3590-3560 cm-l, as shown in Figures 6 and 7. These The remainder of the bands observed in the spectrum observations suggest that the formation of MOH+ of the alkaline earth zeolites are not found in the spectra species is very sensitive to the hydration level of the of the alkali cation or the decationized zeolites. Hence system. Upon mild dehydration it is possible that the they are probably specific to the presence of alkaline alkaline earth oxide is formed. Reversal of the reearth cations. The assignment of these other bands action --SiO;LI(OHp)2++ MOH+ SiOH is will now be considered in conjunction with the influence unlikely since the 3640-cm-' band does not decrease of adsorbed water, when water is added back to the proportionately in intensity with the MOH+ band. dehydrated zeolite, if the sample is not reheated above The formation of the strong band near 3690 cm-l loo", a band near 3690 cm-l is observed. This band upon addition of water to the alkaline earth zeolites is always accompanied by the water bending vibration casts further doubt on the assignment of a band at near 1640 cm-l. Vpon heating to 200", the 3690-cm-' about this frequency to AlOH Unless the band and the 1640-cm-' band decrease markedly in inAlOH band fortuitously occurs at this frequency, it is tensity and new bands are formed. These observahighly feasible that the absorption band assigned tions suggest that the 3690-cm-' band in the alkaline previously is due to incomplete dehydration. If earth zeolites is due to physically adsorbed water. AlOH groups are formed in zeolites, they would be Since the band is not observed with alkali cation forms, expected in multivalent and decationized zeolites except sodium, it must represent the interaction of rather than in the monovalent forms. No evidence water with the divalent cation rather than with the (see above) was found for AlOH groups in the alkali zeolite lattice. However, since the band shows no cation forms. The possibility of incomplete dehydramarked change in frequency with change in cation, as tion is further suggested by the study of Angel1 and the sharp band of adsorbed water on alkali cation zeolites does, the interaction with the cation must be Schaffera2 They observe bands in the region 36913673 cm-' in magnesium, strontium, nickel, and zinc Y very weak. zeolites but not in calcium, barium, manganese, and The decrease of the 3690-cm-I band upon heating cobalt Y zeolites and in calcium X zeolites. There is above 200" together with the formation of new hydroxyl groups (see Figures 6 and 7) suggests that the adsorbed water undergoes reaction. The 3640-cm-1 band is produced in all cases. If the water associated with the (33) J. W.Ward, unpublished data. cation dissociates as shown schematically above, the (34) A. E. Hirsohler, J. Cutul., 2 , 428 (1963).
\a/
+
+
Volume 78, iVumber 18 November 1968
4222
JOHN W. WARD
3670 i1630
I
4
>-
U
z
d
,
I
I
,
I
I
,
6 8 10 12 14 IONIZATION POTENTIAL
16
2 E 1650-
I
.
I
0 I
10
I
I
I
I
I
1640
I
1630
0
1 2 3 4 5 ELECTROSTATIC FIELD 0 B POTENTIAL X I
I
I
I
I
11 12 13 14 15 IONIZATION POTENTIAL A
Figure 11. Variation of hydroxyl stretch frequencies with cation.
no obvious reason why AlOH groups should be found in some divalent zeolites and not in others. The frequencies observed for magnesiun Y are essentially the same as those observed by Angell and Schaffer2 and Hall, et aZ.19 The observations for barium Y are similar to those of Hall, et ~ 1 . ~ 1for 9 dehydration at 450". However, at 250", the results are dissimilar to those of Hall, et aZ.,14 and Angell and Schaffera2 I n particular, no discrete band is observed at 3645 cm-'. Again for calcium and strontium, different surface species are observed compared with Angell and Schafferj2several additional bands being detected. For magnesium, calcium, and strontium, there is a small lowering of the absorption frequency with increasing cation radius. These frequencies are plotted against electrostatic field and potential and ionization potential in Figure 11. Hall19 has previously made a similar plot for various zeolites as a function of electron affinity. The changes in frequency with cation are quite small when compared with the changes with alkali cations, but they vary in a similar predictable manner. However, the dependence of the hydroxyl stretch frequency on cation properties is completely different from that reported by Hall, et aZ.19 I n the present study, magnesium, calcium, and strontium decrease with decreasing electrostatic potential and field and decreasing ionization potential. Hall, et aZ.,l9 reported a decrease of frequency with increasing electron affinity. The trends found in this study are the same as found for other adsorbed molecules on zeolites,14~17~25~a2 for water adsorbed on the alkali cation Y zeolites (see above) and for the frequencies of the hydroxyl stretch in alkali and alkaline earth hydroxide. The frequency of the water bending vibration follows a similar trend to that reported by Hall, et aZ.,19 that is, a decrease in frequency with increasing electrostatic field, potential, and ionization The Journal of Physical Chemistry
I
0
I
\
I
I
1 2 3 4 5 ELECTROSTATIC FIELD 0 ELECTROSTATIC POTENTIAL X
Figure 12. Variation of water deformation frequency with cation.
potential. However, as shown by Figure 12, there are several anomalous points, suggesting that the interaction of undissociated water with the zeolite is far from being described by this simple picture. Rare E a r t h Y Zeolites. The spectra of structural hydroxyl groups on the rare earth zeolite are distinctly different from those of the alkaline earth zeolites both in the anhydrous and hydrous states. The spectra of the dehydrated system is relatively simple, having only two absorption bands at 3640 and 3522 cm-l along with the characteristic 3740-cm-' band. The rare earth ion forms can most likely undergo a hydrolytic reaction similar to the alkaline earth formsz1 RE(OH2)23+
+
RE(OH)(H20)'+ H+ RE(OH)z+
+ 2H+
Hence the 3640-cm-1 band is almost certainly due to silanol-type hydroxyl groups similar to those present in the alkaline earth and decationized zeolites. The band at 3522 cm-1 has been previously assigned to REOH Since a band near the frequency is not observed in the region 3595-3570 cm-l for which a band was assigned to RIOH groups in the alkaline earth forms, the 3522-cm-' band assignment seems justified. Again in marked contrast to the other zeolites, addition of water does not give rise to a band near 3690 cm-l but at 3610 and near 3550-3560 cm-l. The latter band is very broad and ill defined. The adsorbed water behaves similarly to that adsorbed on the alkali cation forms. Subsequent dehydration simply removes the water reversibly, and no new hydroxyl groups appear to be formed. These results suggest that the electrostatic forces associated with the rare earth cations in the form in which they are present in the zeolite lattice are
A SPECTROSCOPIC STUDY OF
THE
SURFACE OF ZEOLITE Y
insufficient to dissociate readsorbed water. The observed frequency for the readsorbed water of 3610 cm-l compared with about 3690 cm-I for the alkaline earth also suggests weaker interactions. The 3640-em-1 band is several times stronger than that observed in any of the alkaline earth forms. This greater intensity is probably accounted for by the cation hydrolysis being represented by
RE(OHz)z3+
RE(OH)z+
+ 2H+
Because of the large distance between the three exchange centers compared with the distance between the two centers in the divalent forms, the equilibrium is probably more to the right. Thus the possibility exists that in the rare earth form, for every six exchange sites, four silanol groups can potentially be formed, whereas in the divalent cation forms, a maximum of three silanol groups can be formed. The differences in the position of the equilibrium probably further accounts for thle differences. I n previous studies, the rare earth forms have been shown to be about twice as acidic as the most acid alkaline earth forms.a5 This greater acidity can be understood in terms of the greater 3640 cm-’ type hydroxyl group content of the rare earth zeolite. It has also been shown previously that the 3522 cm-’ band type of hydroxyl groups are nonacidic to pyridine. By comparison with the nonacidic hydroxyl groups on alkali and alkaline earth zeolites, the nonacidity of these groups further supports their assignment to REOH groups. I n previous studies, 13217 the presence of hydroxyl groups with frequencies near 3640 cm-1 have been related to the catalytic activity of divalent zeolites.
4223
It was shown that the presence of the 3640 cm-1 coincided with the presence of Br@nsted acidity and cracking activity. Other studiesa2have shown a similar relationship for the rare earth zeolites. Further showed that Brflnsted acid sites of various multivalent zeolites increased upon addition of small have shown amounts of water, and Plank21band that the catalytic activity increases in the presence of a small amount of water. The present studies show that adsorbed water at elevated temperatures forms structural hydroxyl groups of the type responsible for acidic sites by dissociation. These studies also confirm that all the zeolites which are Brflnsted acids have the 3640 cm-I band type of hydroxyl groups. Proposed theories for the catalytic activity of multivalent zeolites have suggested that the electrostatic fields associated with the cation are able to polarize adsorbed hydrocarbons sufficiently to promote reaction. 37 The present results confirm the previously proposed concept that the electrostatic field causes dissociation of adsorbed water with production of acidic hydroxyl groups;” the stronger the field, the greater the zeolite acidity. The acidic hydroxyl groups are similar to those formed on decationized zeolites but less in number. The presence of acidic hydroxyl groups in the presence of adsorbed water could be significant catalytically, since most reactor feeds contain traces of water. (35) J. W. Ward, Division of Colloid and Surface Chemistry, 164th National Meeting of the American Chemical Society, Chicago, Ill., 1967. (36) J. W. Ward, J. Catal., in press. (37) P. E. Pickert, J. A. Rabo, E. Dempsey, and V. Schomaker, Proc. Int. Congr. Catal., $rd, Amsterdam, 1964, 1, 714 (1965).
Volume 72, Number 12 November 1968
