Thomas R. Hughes
2192
and
Harry M. White
A Study of the Surface Structure of Decationized Y Zeolite by
Quantitative Infrared Spectroscopy
by Thomas R. Hughes and Harry M. White
J. Phys. Chem. 1967.71:2192-2201. Downloaded from pubs.acs.org by LA TROBE UNIV on 01/02/19. For personal use only.
Chevron Research Company, Richmond, California
(Received December 15, 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 at 3650 cm-1 is a stronger Br0nsted acid than the group with a band at 3550 cm-1. Experiments with ¿-butyl alcohol showed that the lower frequency OH group is strongly hydrogen bonded to other oxygen atoms of the zeolite, whereas the other OH group is not. Dehydration at elevated temperatures converts the Br0nsted sites to the Lewis form. Exposure to water converts some of the Lewis sites to Br0nsted 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 published.1-4 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 spectrum.5-13 Some of these bands have been attributed to OH groups associated with the metal ion.7-10 Others have been assigned to OH groups connected with the aluminosilicate portion of the zeolite.5-7’10-13 The 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-1 which closely resembles the band of isolated surface SiOH groups in silica14 and noncrystalline silica-alumina.15 In the zeolites, the 3740-3750-cm-1 band is weak and the OH group re10·I.
**********12’13
The Journal of Physical Chemistry
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. Phys. Chem., 70, 1681 (1966) (5) J. A. Rabo, P. E. Pickert, D. N. Stamires, and J. E. Boyle, Actes Congr. Intern. Catalyse, Paris, 1960, 2055 (1961). (6) . A. Szymanski, D. N. Stamires, and G. R. Lynch, J. Opt. Soc. Am., 50, 1323 (1960). (7) S. P. Zhdanov, A. V. Kiselev, V. I. Lygin, and T. I. Titova, DoM. Akad. Nauk SSSR, 150, 584 (1963). .
(8) L. Bertsch and H. W. Habgood, J. Phys. Chem., 67, 1621 (1963). (9) S. P. Zhdanov, A. V. Kiselev, V. I. Lygin, and T. I. Titova, Russ. J. Phys. Chem., 38, 1299 (1964). (10) J. L. Carter, P. J. Lucchesi, and D. J. C. Yates, J. Phys. Chem., 68, 1385 (1964). (11) H. W. Habgood, ibid., 69, 1764 (1965).
(12) J. B. Uytterhoeven, L. G. Christner, and W. K. Hall, ibid., 69, 2117 (1965). (13) C. L. Angelí and P. C. Schaffer, ibid., 69, 3463 (1965). (14) R. S. McDonald, ibid., 62, 1168 (1958). (15) M. R. Basila, ibid., 66, 2223 (1962).
Surface Structure
of
Decationized Y Zeolite
of
near faujasites after heating to temperatures high enough to decompose the ammonium ions.6,7·12,13 These OH groups presumably are formed by the combination of oxygen atoms from the zeolite framework and the protons left behind by the decomposition of the NH4+ ions.6 One of the bands, which is fairly narrow, has been reported at 3655 cm-1 in deamminated NH4X7 and at 366012 and 3640 cm-113 in. deamminated NH4Y. The other band, which is broader, has been reported at 3570 cm-15·12 and at 3540 cm-113 in deamminated NH4Y zeolite. In the present work the integrated absorption intensities, accessibilities, relative acidities, and hydrogen-bonding characteristics of both the 3650- and 3550cm-1 OH groups were determined. The concentrations of both types of OH groups and of the Lewis sites formed by their dehydration were measured as functions of pretreatment temperature.
.
Experimental Section A. Materials. The ammonium form of Y-type zeolite (Lot No. 12218-19-1) was supplied by the Linde Co. This sample (NH4Y) contained 66.1% Si02, 23.2% AI2O3, 8.3% (NH4)20, and 1.6% Na20 on a dry basis. The particles smaller than ca. 1 µ, as measured from electron micrographs, were removed by sedimentation in water in order to minimize the fraction of sites on the external faces of the crystals. The Ag+ exchange capacity was 3860 /¿moles/g of dry, ammoniafree Y zeolite. An alumina gel was prepared in a methanolwater solution by the method of Ziese,16 washed with methanol and then with ethyl ether, and converted to the aerogel form by removal of the ether above its critical temperature.17 A silica gel was prepared by hydrolysis of ethyl orthosilicate in methanol-water solution and converted to the aerogel in the same way as the alumina. A 25% alumina, 75% silica cracking catalyst (Aerocat AAA) with a BET N2 surface area of 430 m2/g was obtained from American Cyanamid. Nalco Chemical Co. supplied a sample of alumina which had a surface area of 328 m2/g after drying under vacuum at 450°.
The pyridine used was Baker Analyzed reagent. The piperidine was Eastman Practical grade redistilled to a constant boiling range (106.7-107.0°). A purified sample of ¿-butyl alcohol was prepared by five recrystallizations of Eastman White Label No. 820. Each of the reagents was stored over Type 5A Linde Molecular Sieve. B. Wafer Preparation and Pretreatments. All infrared measurements were made with adsorbents in
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the form of self-supporting wafers prepared by pressing 20-50 mg of fine powder in a 1-in. diameter die at 20,000 psi. Heat treatments of the wafers were performed in the infrared cell. Wafers of NH4Y were pretreated at 300° for at least 1 hr to decompose the ammonium ion prior to the experiments in which the band intensities of pyridinium ion (PyB) and piperidinium ion (PiB) were determined. In the experiments made to determine the effect of pretreatment temperature on the concentrations of acid sites, the wafers were successively heated for 1 hr at each pretreatment temperature studied. The dimensions and weights of the wafers were measured at the end of each experiment. The alumina aerogel (Al-600) and Nalco alumina (Nal-600) wafers were exposed to 200 torr of 02 at 500° for 2 hr and then degassed at 2 X 10-6 torr for 2 hr at 600°. The wafer of Aerocat AAA was pretreated by heating for 4 hr at 500° in 200 torr of 02 and then for 16 hr at 500° and 2 X 10_e torr (SA-500). The silica aerogel wafers were heated at 200° for 2 hr at 2 X 10~6 torr (Si-200). C. Infrared Spectrophotometer, Sample Cell, and Vacuum System. The infrared instrument used is an altered version of the modified Beckman IR-7 ratiorecording spectrophotometer described by Keahl and Rea.18 They installed a 480-cps beam chopper and replaced the thermocouple detector with fast-response semiconductor detectors. The design of Keahl and Rea included extra optics in both sample and reference beam paths in order to accommodate long, externally heated cells. The internally heated cell described below is short enough to permit a return to the IR-7 optics with a more than fourfold improvement in signalto-noise ratio. The infrared cell, pictured in Figure 1, is in two parts. The Pyrex cell body is permanently mounted in the spectrometer sample compartment. The removable sample holder carries a heater of Nichrome wire wrapped around a quartz cylinder axially aligned with the infrared beam. The sample wafer is held in place in the center of this cylinder by means of quartz rings. The inside of the cylinder is lined with gold sheet in order to shield the wafer from direct radiation from the heater. The heater is sheathed from the vacuum system by the quartz envelope of the sample holder. During high-temperature operation, the heater (16) W. Ziese, Ber. Deut. Chem. Gee., 66, 1965 (1933). (17) S. S. Kistler, Nature, 127, 741 (1931); J. Phya. Chem., 36, 52 (1932). (18) G. T. Keahl and D. G. Rea, presented at the 12th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Feb 1961.
Volume 71, Number 7
June 1967
2194
Thomas R. Hughes
and
Habby M. White
to contact the wafer at 35° for 16 hr before the excess was removed by pumping to 2 X 10 ~6 torr, usually for 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.19 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-1, 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 at 14501460 cm-1 (Figures 3A, 3B, 3C). For the piperidine bands in the 1600-1650-cm-1 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 is protected from oxidation by a stream of N2 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-1 at wafer temperatures up to 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 Concentrations 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 After at least 0.5 hr, the preswas added to the wafer. sure was reduced to 2 X 10 ~6 torr and the spectrum was recorded. For experiments in which the effect of water was studied, 15-20 torr of H2O was allowed The Journal of Physical Chemistry
Figure 2. Spectra of adsorbed pyridine species: A, hydrogen-bonded pyridine (PyH) on Si-200; B, coordinately bonded pyridine (PyL) on Al-600; C, pyridinium ion (PyB) on NH
