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R. R. Bailey and J. F’. Wightman, J. Colloid Interface Sci. 70, 112 (1979).
(16) W. Stober, KolloidZ., 145, 17 (1956). (17) L. T. Zhuravlev and A. V. Kiselev in “International Symposium on
L. G. Berry, Ed., “Selected Powder Diffraction Data for Minerals”, Joint Committee on Powder Diffraction Standards, Swarthmore, Pa., 1974, p 42). J. . W. ..Edmonds, .~ W. W. Henslee, and R. E. Guerra, Anal. Chem., 49,
Surface Area Determination”, D. H. Everett and R. H. Ottewiii, Ed., Butterworths, London, 1970,p 155. (18) J. W. Whalen and P. C. Hu, J. ColloaInterface Sci., 65, 460 (1978). (19) A. V. Kiselev, Proc. Int. Congr. Surf. Act. Znd, 7957, 309 (1957). (20) (a) R. K. Iier, “The Chemistt-of Silica”, Wiley, New York, 1979. (b) Dr. Iler suggested these experiments to LSI at the 52nd National Colloid and Surface Science Symposium. (21)J. W. Whaien Adv. Chem. Ser., No. 33, 281 (1961). (22) R. E. Day, 0. D. Parfitt, and J. Peacock, Progr. i a c . Microbalance Techno/., 2, 61 (1973). (23) A. Nonaka and E. Ishizaka, J. CollkdInterface Sci., 83, 381 (1977). (24)J. W. Whalen, J . Phys. Chern., 65, 1676 (1961). (25) S.P. Zhdanov, mi.Mad. Nauk SSR(€ngl. Trans/.),123, 833 (1958). (26) A. V. Kiselev and V, I. Lygin, “Infrared Spectra of Surface Compounds”, Related Press, New York, 1967,p 103. (27) M. Tschapek, S.G. de Bussetti, and G. P. Ardiui, Ekctroanal. Chem. Interf. Electrochem., 52,304 (1974). (28) T. A. Carlson, “Photoelectron and Auger Spectroscopy”, Plenum Press, New York, 1975.
2196 (197’7). E. G. Rochow in “Comprehensive Inorganic Chemistry, Voi. l”, A. F. Trotman-Dlckensen, Ed., Pergamon Press, Oxford, 1973,p 1393. I H. W. Van der Marcel and H. Beutelsparker, ”Atlas of Clay Minerals and Their Admixtures”, Eisevier, Amsterdam, 1976,p 328. I S. S. Gregg and K. S. W. Sing, “Adsorption Surface Area and Porosity”, Academic Press, New York, 1697,p 36 ff. 1 L. Robert, Bull SOC. Chim. Fr., 2309 (1967). I P. Thorne, Ph.D., Dissertation, University of Bristoi (U.K.), 1974. I S. Partyka, F. Rouquerol, and J. Rouqueroi, J. Co//o~Interface Scl., 68, 21 (1979). (13) J. H. Scofieid, Livermore Laboratory Report UCRL-51826, 1973. (14) V. I. Nefedov and ‘Ya. V. Saiyn, J . Electron Spectrosc. Relat. Phenom., 10, 121 (1977). (15) A. C. Zettiemoyer in “Hydrophobic Surfaces”, F. M. Fowkes, Ed., Academic Press, New York, 1969,p 1.
Water on Silica and Silicate Surfaces. VI. Sodium Form of Type-Y Synthetic Zeolite J. H. Shen, A. C. Zettlemoyer, and K. Kller” Center for Surface and Coatings Research, Lehigh University, Bethlehem, Pennsylvanla 180 15 (Received July 20, 1979) Publication costs assisted by Lehigh University
The binding and association of water in the Na-Y zeolite was studied by diffuse reflectance spectroscopy in the near infrared at coverage ranging from a fraction of a molecule per unit cell to saturation. Three distinct species, a “monomer” with a H20 ( u + 6) band at 5320 cm-l, intrasodalite water (5240 cm-l), and ion and hydrogen bound water (5120 cm-l), were identified, they were quantitatively analyzed as a function of the adsorbed amount, and their rotational relaxation times were determined. The f i s t two species have the longest rotational relaxation times (>10-l2s) ever observed for adsorbed water, indicating their precise location and orientation in the zeolite framework. On the basis of this spectroscopic investigation and earlier crystallographic studies by other authors, a structure is proposed in which first water molecules bind to the Na+ ions in the SI1 sites, immediately followed by binding of a second water oxygen-down to the H20-Na+ complex. At occupations higher than 64 H20per unit cell the interactions among the water molecules result in an increased rotational mobility and a poorer definition of water locations, as documented by abrupt changes in the line shapes, line widths, and intensities of the H 2 0 ( v + 6) and H20(2v) bands as well as by an earlier observed sudden increase of the dielectric constant of the water-Na-Y system. In all stages of sorption, surface O H groups play only a minor role as centers for binding water, as their concentration does not exceed 200 mmol per unit cell.
Introduction The properties of intrazeolitic water are of interest in ion binding, ion exchange, and diffusion processes in fully and partially hydrated zeolites. Although crystallographic investigations have advanced to determine the relatively precise location and distribution of the exchangeable cations, zeolite water has so far been characterized more by the measurements of its physicochemical properties such as heats of sorption,l dielectric constants,2diffusion rates,3 and vibrational spectra4i6than by structural parameters. The purpose of the present work is to demonstrate from the measurements of vibrational frequencies and rotational relaxation times that there is indeed a range of partial coverages of the zeolite cavities in which the water molecules occupy well-defined positions. This behavior was evident already from an earlier investigation of Szymanski et al.;4in the present work some assignments by Szymanski et al. are reexamined and the various molecular water species and hydroxyls are analyzed quantitatively. The present results are also compared with our earlier infrared spectroscopic investigation of the water-
Na-A system5 to show the profound differences in water binding in zeolites type A and Y.
Experimental Section Sample Preparation and Spectra Measurements. The zeolite used in this study was the sodium form of Linde Y synthetic zeolite of the formula Na56A156Si13603Md320 per unit cell with n equal to 240-260 for the fully hydrated form. The structure of this zeolite consists of a well-known framework aluminosilicate skeleton with eight sodalite units and eight supercages per unit cell, each arranged in a diamond lattice, and a cation distribution, the details of which are summarized below (cf. Discussion). The zeolite powder was dehydrated in a greaseless vacuum system by increasing the temperature stepwise to 350 “C and then maintaining this temperature for 2 h under constant pumping. The dehydrated zeolite was transferred under vacuum into a thermostated reflectometric cell6 connected to a volumetric apparatus which allowed simultaneous measurement of the adsorbed amount and of the infrared spectra. Spectra measure-
0022-3654/80/2084-1453$01.0010@ 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 12, 1980
Shen, Zettlemoyer, and Klier
BULK WRTER HT 30'C ( R I G H T HANU S C A L E )
i
N A - Y - Z E O L I T E WflTER-FREE N A - Y - Z E O L I T E + WATER [11a6-100 CC/GI -SATURATED SAMPLE [LEFT HflVD SCALE)
--
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Figure 1. The H,O(u+6) bands in the progress of water sorption in the Na-Y zeolite at 25 O C . The H20(v+6) band of bulk liquid water at 30 O C is shown for comparison (0).The water-free zeolite is represented by the baseline marked by crosses. Ordinates: The SKM function F(R,) for sorbed water (left-hand scale) and the absomtion coefficient for bulk liquid water (right-hand scale). Abscissae: wave numbers between 4500 and 0000 em-'. 'Adsorbed amounts are glven'ln Table I.
ments, standards, data processing, and transforms were described earliera5 As in our previous studies of water adsorption, the spectral bands chosen here were the combination band ( ~ 1 ~ 2 = ~ 3(0 ) 0 0) (011)at 5100-5300 cm-' ) 0 0) (10 denoted as H20(v+6), the band ( ~ 1 ~ 2 ~=3 (0 1)at 6900-7200 cm-l of molecular water denoted as HzO(2v), and the surface hydroxyl first stretching overtone at 7241 cm-l, denoted as SiOH(2v). The advantage of this choice is an easy distinction between hydroxyls and molecular water and reliable quantitative interpretation of the measured intensities. The limits of detection of the absorbance function (1- RJ2/2Rm were between 1X and 2 X when the spectra were multiply scanned. For the determination of rotational relaxation times, those infrared bands which were composed of several distinct peaks were resolved into their Gaussian components, the half-width (AwlI2) of which relates to the rotational relaxation time (7)as T = 4(ln 2)1/2/A~1/2.7For strongly overlapping or merging bands the relaxation times were not determined from the line widths but a comparison of their line shapes with those studied earlier by determining the time correlation functions8 still permitted a semiquantitative estimate of the rotational relaxation times to be made.
-
-
Results The properties of water sorbed in the Na-Y zeolite were followed by simultaneous measurements of the sorbed amounts and of the spectra in the v + 6 and 2v regions.
The dehydrated zeolite displayed a spectrum with no detectable H20(v+6)and H20(2v)bands and a sharp (width 80 cm-') OH(2v) peak at 7241 cm-l of intensity (1 RJ2/(2R,) = 3.3 X This residual hydroxyl intensity corresponds to approximately 0.16 OH per unit cell. Upon addition of first small amounts of water the intensity of the OH(2v) peak was reduced in a similar fashion as in the formation of the SiOH-.0H2 surface complex on hydroxylated silicas8 and accompanied by an appearance of the H20(v+6) and H20(2v)bands. Water vapor was then dosed into the reflectometric cell at 25 O C in small increments. The adsorbed amount from each dose was measured volumetrically and the spectra were taken in the range 4500-8500 cm-l after each incremental adsorption. The H20(v+6) bands appearing between 4500 and 6000 cm-I at coverages between zero and saturation are shown in Figure 1. In the initial stages of sorption there are three H20(v+6) bands at 5320, 5240, and approximately 5120 cm-l. The shape of the H20(2v)band, not shown here, is very similar to that of H20(v+6) with subbands at 7220, 7050, and 6900 cm-l. In water-saturated Na-Y zeolite the H20(v+6) has a maximum at 5225 cm-I and the H20(2u) band at 7042 cm-l. In a desorption experiment the fully hydrated zeolite was subject to evacuation at stepwise increasing temperatures between 25 and 450 "C, the desorbed water was collected in a freezing trap cooled by liquid nitrogen, and its amount was measured after each desorption by warming the cold trap and measuring the pressure of the vapor in
The Journal of Physical Chernhtry, Vol. 84, No. 12, 1980 1455
Water on Silica and Silicate Surfaces
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ENERGY I WFIVENOS 1x10 Flgure 2. The H20(v+6) bands in the progress of water desorption from the Na-Y zeolite. The ordinates and abscissae are the same as In Figure 1. Adsorbed amounts and temperatures of desorption are given in Table 11.
a closed calibrated volume. Spectra in the same range as before were recorded after each desorption step. The H20(v+6) bands so measured are shown in Figure 2. The subband maxima were in identical positions as during the adsorption experiment, although their relative intensities were different. The significance of this observation will be discussed later. Once again the relative intensities within the H20(2v) band followed closely those in the HzO(v+6) band and appeared at the same frequencies as in the adsorption experiment. At low water coverages the individual subbands of the H20(v+6) and H20(2v)bands appeared to be symmetric peaks, the shape of which was close to Gaussian. Where the H20(v+6) band showed distinct subbands, it was deconvoluted into three Gaussian components centered at 5320 (half-width 43. cm-l), 5240 (half-width 67 cm-l), and 5120 cm-l (half-width 210 cm-l). The intensities X I ' of these subbands expressed as the product of half-width with the value of the absorbance (1- RJ2/(2R,) a t maximum (I,,,=), XI' = ImexA~1/2 are presented along with the adsorbed amounts and conditions of adsorption and desorption in Tables I and 11. The total intensities of the H20(vS6) bands were also determined by computes integration, and their values are graphically represented in Figure 3 as a function of adsorbed and desorbed amounts, u,. The sums of the three subband intensities, expressed as integrals under Gaussian curves (1/2)zI'(?r/ln 2)ll2 = 1.064CI', were found to be close to the total integrated intensities of the whole HzO(v+6)band. In the course of adsorptian there is an initial lag in the devel-
TABLE I: Intensities ZZ'of the H,O(v + 6 ) Bands at 5320, 5240, and 5120 cm-I and Amounts of Water Sorbed in Na-Y at 25 "Cduring the Adsorption Experiment adsorbed am0unt.b molecules of H,O/UC
zZ' (cm-I) of band a t 5320 5240 5120 cm-' cm-I cm-' ~ 1cm-' , ~ 0 0 0 0 0 1.2(-'3) 0 0 1.28(-3) 13.52 1.96(-1) 5.03(-2) 1,23(-1) 3.93(-1) 17.92 7.88(-1) 3.39(-1) 7.67(-1) 2.02 9.88(-1) 3.67(-1) 20.48 7.67(-1) 2.26 23.04 1.144 4:38(-1) 9.64(-1) 2.71 26.96 1.60 6.17(-1) 1.40 3.85 33.92 2.39 9.48(-1) 2.39 6.10 38.40 3.98 1.79 4.78 11.23 46.08 4.46 1.99 12.51 5.31 51.60 5.16 2.52 6.67 15.27 57.92 5.31 2.68 6.67 15.60 62.32 5.79 2.84 8.40 18.12 65.6 6.80 4.10 11.34 23.66 68.8 7.09 4.84 12.35 25.83 75.2 6.66 6.07 14.67 29.04 80.5 6.70 7.56 18.04 34.37 86.5 7.14 9.93 16.30 35.51 94.0 merging bands merging bands 98.9 106.2 merging bands 114.2 merging bands 122.0 merging bands satd merging bands X I = [ 2 Z ' ( 5320 cm-') t XI'( 5240 cm-I) t ZZ'(5120 cm-I)] X 1.064 to obtain total intensity under the three Gaussian peaks. UC denotes unit cell.
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The Journal of Physical Chemistty, Vol. 84, No. 12, 1980
TABLE 11: Intensities zZ' of the H,O(vt6 ) Bands at 5320,5240, and 5120 cm-I and Amounts of Water Sorbed in Na-Y at 25 "C Following Desorption at 25-450 "C
I
60-
adsorbed amount, molecules of H,O/UC satd 127.0 123.0 104.5 91.2 79.0 73.5 54.3 30.7 21.1 8.84 3.24 2.30 1.58 0.40 0.00
XZ' (cm-l) of band at
5320 cm-'
5240 5120 cm-' cm-I merging bands merging bands merging bands merging bands 5.21 13.13 nd 6.36 7.73 15.81 7.05 6.54 14.09 5.49 3.63 8.40 3.10 2.34 4.12 2.03 2.18 2.96 0.514 1.60 1.09 0.096 0.448 0.222 0.005
