INFRARED INVESTIGATIONS OF ZEOLITES AND ADSORBED MOLECULES
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Infrared Spectroscopic Investigations of Zeolites and Adsorbed Molecules.
I.
Structural OH Groups'
by C. L. Angel1 and Paul C. Schaffer Union Carbide Research Institute, Tarrytown, New York
(Received A p r i l 88, 1966)
The infrared spectra of mono- and divalent cations containing X- and Y-type zeolites show several bands in the OH stretching region which cannot be removed even a t 585' although their number and size depend on the activation (dehydration) technique. The band a t 3744 cm.-l has the same frequency and intensity in all samples and does not interact with adsorbed molecules. Its assignment to crystal surface groups, occluded impurities, or defective sites is discussed. The bands at 3640 and 3540 cm.-l are assigned to cation deficiency OH groups, the latter being due to association. Changes in these bands on the addition of water, benzene, ammonia, trimethylamine, hydrogen chloride, and hydrogen cyanide are discussed.
Introduction The crystal structures of synthetic zeolites have recently been reviewed by Breck.2 The theoretical structure of X- and Y-type zeolites does not include any OH groups; still, several have reported bands in the OH stretching region in the infrared spectra and have given a number of different, often inconsistent, interpretations for them. Bertsch and Habgood5 reported that all of the OH bands found were due to adsorbed water, each molecule of which was both attached to the exchangeable cations and hydrogen-bonded to the surface oxygens. Carter, Lucchesi, and Yates6 assigned the three OH bands in the 3-p region to surface hydroxyl groups, the band at 3750 cm.-l to Si-OH groups, the band a t 3700 cm.-l to Al-OH groups, and the band around 3650 cm.-l to some OH groups which were influenced by the exchangeable cations. I n the present work, further evidence is presented on this controversial issue. The availability of special materials was a factor contributing to the significance of this work. (1) All previous authors have used material of normal particle size such that the light scattering in the 3-p region is very considerable, causing serious loss in transmission and forcing their inst,ruments considerably beyond their normal performance. I n the present work special trouble was taken to obtain an especially fine-grained material. With this material, trans-
mission losses were very much reduced, and normal resolution of the infrared spectrometers used was achieved. (2) A much larger variety of cation-exchanged samples was available, including a series of Y zeolites with different degrees of cation deficiency. Several OH bands were found in the 3-p region. Some of these bands varied with the zeolitic cation, and some bands were related to the cation deficiency. Deuterium-exchange experiments were carried out also, and the effects of a variety of adsorbed molecules on the OH bands were observed.
Experimental Section Materials. The Y-type zeolite samples were prepared from one original, specially fine-grained sodium Y sample with a silicon/aluminum ratio of 2.35. (This sample was found to be 98% Y zeolite by X(1) The major portion of this work was presented at the 49th Annual Meeting of the Optical Society of America, New York, N. y., Oct. 6-9, 1964. (2) D.W.Breck, J. Chem. Educ., 41, 678 (1964). (3) (a) J. A. Rabo, P. E. Pickert, D. N. Stamires, and J. E. Boyle, Actes Congr. Intern. Catalyse, 8e Paris, 1960, 2055 (1961); (b) H. A. Szymanski, D. N. Stamires, and G. R. Lynch, J. Opt. SOC.Am., 50, 1323 (1960). (4) S. P. Zhdanov, A. V. Riselev, V. I. Lygin, and T. I. Titova, Dokl. Alead. Nauk SSSR,150, 584 (1963). (6) L. Bertsch and H. W. Habgood, J. Phgs. Chem., 67, 1621 (1963). (6) J. L. Carter, P. J. Lucchesi, and D. J. C. Yates, ibid., 68, 1385 (1964).
Volume 69, Number 10
October 1966
C. L.
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ray examination.) All the other Y-zeolite samples were prepared from this by cation exchange (the decationized samples by NH, exchange, see ref. 3); during the exchange special care was taken to keep the p H of the solutions above 4.0 to avoid partial decomposition. Each sample was analyzed by wet chemical methods for AlZO3,SiOn, NazO, and metal oxide. The samples were also subjected to an X-ray examination, and, in most cases, the intensity and sharpness of the lines indicated that the samples had retained a very high degree of crystallinity during the cation exchange. A few samples showed weakened lines or increased background, but this was to be expected because of absorption or fluorescence by the particular cations present. The small-particle-size sodium Y, obtained from the Linde Division, had a size range, estimated from electron micrographs, from 0.04 to 1.2 p , with an average of 0.4 to 0.8 p. Unfortunately, it is not the average particle size that counts for the scattering since even small amounts of larger particles can cause serious scattering. Therefore, the material was further fractionated by suspending it in water and shaking ultrasonically for about 3 hr. The fraction, about 0.1 of the original sample, that stayed in suspension after 24 hr. of settling was collected by centrifugation. The particle size of this fraction was not measured directly, but comparison of the infrared spectrum in the 3-p region with the spectra of materials of known particle size indicated a size less than 0.1 /I. (A number of samples of X and Y zeolites of varying particle size was made available from the Linde Division (Union Carbide Corp., Tonawanda, N. Y.); the transmission in the 2 - 3 5 , region seems to be inversely proportional to the average particle size.) This treatment was successful : the resulting materials gave very good transmission in the 2-3.5-p region. Sodium X and calcium X samples were available in extremely small particle size (
