THOMAS R. HUQHESAND HARRYM. WHITE
2192
A Study of the Surface Structure of Decationized Y Zeolite by Quantitative Infrared Spectroscopy
by Thomas R. Hughes and Harry M. White Chevron Research Company, Richmond, Californk (Received December 16, 1966)
The intensities of infrared absorption bands of adsorbed amines and of two types of surface hydroxyl groups in decationized Y zeolite have been determined. From the intensities, the concentrations of these species were measured. Experiments with piperidine demonstrated that both OH groups are protonic acids and are accessible to molecules in the large intracrystalline channels. The concentration of these sites is much smaller than the ionexchange capacity. Results with pyridine revealed that the OH group with a band a t 3650 cm-l is a stronger Br$nsted acid than the group with a band at 3550 cm-'. Experiments with t-butyl alcohol showed that the lower frequency OH group is strongly hydrogen bonded to other oxygen atoms of the zeolite, whereaa the other OH group is not. Dehydration at elevated temperatures converts the Br@nstedsites to the Lewis form. Exposure to water converts some of the Lewis sites to Br6nsted sites but not necessarily to the original OH groups.
I. Introduction Infrared spectroscopy has been widely used for the qualitative and semiquantitative investigation of surface functional groups and adsorbed species on solids. During the past 2 years a number of quantitative infrared studies have also been In the present work, quantitative infrared spectroscopy was used to study the surface structure of decationized Y zeolite. The theoretical structure of metal ion-exchanged forms of the faujasitelike X- and Y-type zeolites does not include any hydroxyl groups. However, several infrared absorption bands have been reported in the OH region of the ~ p e c t r u m . ~ - ' Some ~ of these bands have been attributed to OH groups associated with the metal Others have been assigned to OH groups connected with the aluminosilicate portion of the z e ~ l i t e . ~ - ~ JThe ~ - ~present ~ work is concerned with the latter type of hydroxyl group. One of these OH groups has a sharp absorption band at 3740-3750 cm-l 1 0 ~ 1 2 ~ 1 3which closely resembles the band of isolated surface SiOH groups in silica" and noncrystalline sili~a-alumina.~~ In the zeolites, the 3740-3750-cm-l band is weak and the OH group reThe Journal of Physical Chsnrbtry
sponsible does not appear to play an important structural role. Infrared absorption bands due to two other OH groups have been observed in the ammonium forms ~~
(1) D. A. Seanor and C. H. Amberg, J. Chem. Phys., 42, 2967 (1965). (2) H.Heyne and F. C. Tompkins, Proc. Roy. SOC.(London), A292, 460 (1966). (3) J. D. Russell, Trans. Faraday SOC.,61, 2284 (1965). (4) M. R. Basila and T. R. Kantner, J. Phgs. Chem., 70, 1681 (1966). (5) J. A. Rabo, P. E. Pickert, D. N. Stamires, and J. E. Boyle, Actes Congr. Intern. Catalyse, 9,Pam's, 1960, 2055 (1961). (6) H.A. Ssymanski, D. N. Stamires, and G. R. Lynch, J. Opt. SOC.Am., 50, 1323 (1960). (7) 8. P. Zhdanov, A. V. Kiselev, V. I. Lygin, and T. I. Titova, DON.A M . Nauk SSSR, 150,584 (1963). ( 8 ) L.Bertsch and H. W. Habgood, J. Phya. Chem., 67, 1621 (1963). (9) 8. P.Zhdanov, A. V. Kiselev, V. I. Lygin, and T. I. Titova, Ruse. J. Phy8. Chem., 38, 1299 (1964). (10) J. L. Carter, P. J. Lucchesi, and D. J. C. Yates, J.Phyu. Chem., 68, 1385 (1964). (11) H. W.Habgood, ibid., 69, 1764 (1966). (12) J. B. Uytterhoeven, L. G. Christner, and W.K. Hall, aid., 69, 2117 (1965). (13) C. L. Angell and P. C. Schaffer, ibid., 69,3463 (1966). (14) R. S. McDonald, ibid., 62, 1168 (1958). (15) M.R. Basila, ibid., 66, 2223 (1962).
SURFACE STRUCTURE OF DECATIONIZED Y ZEOLITE
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the form of self-supporting wafers prepared by pressing of near faujasites after heating to temperatures high enough to decompose the ammonium i ~ n s . ~ J J ~ *20-50 ' ~ mg of fine powder in a 1-in. diameter die at 20,000 psi. Heat treatments of the wafers were performed in These OH groups presumably are formed by the combithe infrared cell. nation of oxygen atoms from the zeolite framework Wafers of NH4Y were pretreated a t 300' for at least and the protons left behind by the decomposition of 1 hr to decompose the ammonium ion prior to the exthe NH4+ ions.5 One of the bands, which is fairly periments in which the band intensities of pyridinium narrow, has been reported at 3655 cm-' in deammiion (PyB) and piperidinium ion (PiB) were determined. nated NHZ7 and a t 366012and 3640 cm-' la in. deamIn the experiments made to determine the effect of minated NH4Y. The other band, which is broader, has been reported at 3570 cm-' and at 3540 cm-l la pretreatment temperature on the concentrations of acid sites, the wafers were successively heated for 1 in deamminated NH4Yzeolite. In the present work the integrated absorption inhr at each pretreatment temperature studied. The tensities, accessibilities, relative acidities, and hydrodimensions and weights of the wafers were measured gen-bonding characteristics of both the 3650- and 3550a t the end of each experiment. The alumina aerogel (AI-600) and Nalco alumina cm-l OH groups were determined. The concentrations of both types of OH groups and of the Lewis (Nal-600) wafers were exposed to 200 torr of O2 at torr for sites formed by their dehydration were measured as 500' for 2 hr and then degassed at 2 X functions of pretreatment temperature. 2 hr at 600'. The wafer of Aerocat AAA was pretreated by heating for 4 hr at 500' in 200 torr of 02 and II. Experimental Section then for 16 hr at 500' and 2 X loF6 torr (SA-500). A . Materials. The ammonium form of Y-type The silica aerogel wafers were heated at 200' for 2 hr zeolite (Lot No. 12218-19-1) was supplied by the Linde at 2 X torr (Si-200). Co. This sample (NH4Y) contained 66.1% Si02, C. Infrared Spectrophotometer, Sample Cell, and 23.2% A1203, 8.3% (NH4)20,and 1.6% NazO on a dry Vacuum System. The infrared instrument used is an basis. The particles smaller than ca. 1 p , as measured altered version of the modified Beckman IR-7 ratiofrom electron micrographs, were removed by sedimenrecording spectrophotometer described by Keahl and tation in water in order t o minimize the fraction of sites Rea.'* They installed a 480-cps beam chopper and on the external faces of the crystals. The Ag+ exreplaced the thermocouple detector with fast-response change capacity was 3860 pmoles/g of dry, ammoniasemiconductor detectors. The design of Keahl and free Y zeolite. Rea included extra optics in both sample and reference An alumina gel was prepared in a methanolbeam paths in order to accommodate long, externally water solution by the method of Ziese,lBwashed with heated cells. The internally heated cell described methanol and then with ethyl ether, and converted below is short enough to permit a return to the IR-7 t o the aerogel form by removal of the ether above its optics with a more than fourfold improvement in signalcritical temperature.'? A silica gel was prepared by to-noise ratio. hydrolysis of ethyl orthosilicate in methanol-water The infrared cell, pictured in Figure 1, is in two parts. solution and converted to the aerogel in the same way The Pyrex cell body is permanently mounted in the as the alumina. spectrometer sample compartment. The removable A 25% alumina, 75% silica cracking catalyst (Aerosample holder carries a heater of Nichrome wire cat AAA) with a BET N2 surface area of 430 m2/g wrapped around a quartz cylinder axially aligned with was obtained from American Cyanamid. Nalco Chemthe infrared beam. The sample wafer is held in place ical Co. supplied a sample of alumina which had a surin the center of this cylinder by means of quartz face area of 328 m2/g after drying under vacuum at rings. The inside of the cylinder is lined with gold 450'. sheet in order to shield the wafer from direct radiation The pyridine used was Baker Analyzed reagent. from the heater. The heater is sheathed from the The piperidine was Eastman Practical grade redistilled vacuum system by the quartz envelope of the sample to a constant boiling range (106.7-107.0'). A puriholder. During high-temperature operation, the heater fied sample of t-butyl alcohol was prepared by five recrystallizations of Eastman White Label No. 820. (16) W. Ziese, Ber. Deut. Chem. Ges., 66, 1965 (1933). Each of the reagents was stored over Type 5A Linde (17) 5. 9. Kistler, Nature, 127, 741 (1931); J. Ph'ys. Chem., 36, 52 (1932). Molecular Sieve. (18) G . T. Keahl and D. G . Rea, presented at the 12th Pittsburgh B . Wafer Preparation and Pretreatments. All inConference on Analytical Chemistry and Applied Spectroscopy, frared measurements were made with adsorbents in Feb 1961. 5312
Volume 71, Number 7 June 1067
THOMAS R. HUGHESAND HARRYM. WHITE
2194
w
Figure 1. Infrared cell for catalyst studies.
is protected from oxidation by a stream of Nz in the sample holder envelope. The heater section of the sample holder is wrapped with a gold heat shield with access holes for the infrared beam and additional gold heat shields are placed just inside the windows. With this cell, spectra have been recorded from 1300 to 4000 cm-' at wafer temperatures up t o 760' and, with the sample holder envelope filled with coolant, down to -110'. A blank cell identical with the sample cell is placed in the reference beam and attached to the vacuum system. D . Measurement of Band Intensities and Cmcentrat i m s of Adsorbed Species. Band intensities of adsorbed species were measured by adding accurately known increments of adsorbate and recording the spectrum of the wafer 2.5-3 hr after each addition. Wafers were maintained at 150' during all of these calibrations except for Si-200, which was at 35'. Adsorbed species and the temperatures at which their spectra were measured are designated as follows: pyridinium ion at 150' (PyB-150), coordinately bonded pyridine at 150' (PyL-150), piperidinium ion at 150' (PiB-150), coordinately bonded piperidine at 150' (PiL-150), and hydrogen-bonded piperidine at 35' (PiH-35). For experiments in which the concentration of acid sites was determined as a function of the highest pretreatment, temperature, an excess of adsorbate was added to the wafer. After at least 0.5 hr, the pressure was reduced to 2 X torr and the spectrum was recorded. For experiments in which the effect of water was E,tudied, 15-20 torr of HzO was allowed The Journal of P h y a h l Chemistry
to contact the wafer a t 35' for 16 hr before the excess torr, usually for was removed by pumping to 2 X 2 hr. Band area rather than band height was used for the quantitative measurements. The former is a better measure of concentration than the latter when the half-width of an absorption band is not large compared to the spectral slit width.'@ Arbitrary but consistent procedures were used to construct the backgrounds for integration of the absorption bands of adsorbed pyridine and piperidine. For the pyridine bands at 1540, 1490, and 1450 cm-I, smooth curves were drawn tangent to the spectra on either side of the absorption bands (Figures 2B, 2C). A tangent background was also used for the bands of piperidine species a t 14501460 cm-I (Figures 3A, 3B, 3C). For the piperidine bands in the 1600-1650-~m-~ region, the spectrum of the wafer before addition of piperidine was used to construct the background (blank background). In the OH stretching region, approximate band areas
I
Wavenumber, cm-' Figure 2. Spectra of adsorbed pyridine species: A, hydrogen-bonded pyridine (PyH) on Si-200; B, coordinately bonded pyridine (PyL) on 4-600; C, pyridinium ion (PyB) on NHIY-300.
(19)
D.A. Ramsay, J . Am. Chem. Soc.,
74, 72 (1962).
SURFACESTRUCTUREOF DECATIONIZED Y ZEOLITE
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