A New Method for the Dealumination of Faujasite-Type Zeolites

B. Imelik et al. (Editors), Catalysis by Zeolites © 1980 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

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A NEW METHOD FOR THE DEALUMINATION OF FAUJASITE-TYPE ZEOLITES HERMANN K. BEYER and ITA BELENYKAJA Central Research Institute for Chemistry of the Hungarian Academy of Sciences, Budapest (Hungary)

INTRODUCTION In the last few years highly siliceous zeolites with Si0 2!A1 ratios 20 3 over 30 have been synthesized, e.g. ZSM-5 (ref. 1) and Nu-l (ref. 2). These materials possess unique catalytic properties and offer the possibility of new catalytic processes, especially the conversion of methanol to gasoline (ref. 3). Knowing the relation between the Si0 2!A1 20 3 ratio of a zeolite and the acid strength of its hydrogen from (ref. 4) it is obvious that highly siliceous H-zeolites are extremely strong Br6nsted acids. In order to decide whether the unique catalytic behaviour is connected to the acid strength or rather to shape-selectivity effects it would be of interest to study other zeolite structures with high Si0 2!A1 20 ratios, especially the well 3 known and from catalytic viewpoints also intensively studied faujasite-type zeolites. However, no methods are known for the synthesis of faujasite with extremely high silicon contents. Consequently, the preparation of highly siliceous faujasite by dealumination of synthetic faujasite-type zeolites is of great importance. It has been reported that during hydrothermal treatment of ammoniumexchanged Y zeolites part of the framework aluminium migrates into lattice cation positions (ref. 5), nevertheless framework vacancies are not or only scarcely formed because of the migration of silicon and oxygen atoms under these experimental conditions (ref. 6, 7). In the last years methods have been developed for the preparation of faujasite-type zeolites with Si0 2!A1 20 ratios over 100 basing on repeated ammonium-exchange and hydro3 therma» treatment combined with acid extraction of the nonframework aluminium (ref. 8, 9). However, these procedures are time- and labour-consuming and result in the formation of a mesopore-system probably due to the elimination of part of the cubooctaeders (ref. 10). This paper deals with the dealumination of faujasite-type zeolites basing on a completely different chemical reaction. Starting from any cation form the dealumination process is performed in only one step and in absence of water steam.

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MATERIAL AND METHODS Na-Y zeolite used in most of the experiments was obtained from VEB Elektrochemisches Kombinat Bitterfeld, GDR (Na[All.OSi2.52]-Y,FAU). From this sample a (NH 4,Na)-Y zeolite was prepared by ion exchange with NH 4Cl solution (degree of exchange: 6~ %). Further experiments were carried out with Na-Y, Na-X and L-type zeolite from Union Carbide (Linde Division) and Na-mordenite from Norton. Calcium-exchanged X zeolite and ammonium-exchanged L zeolite were also studied. X-ray diffraction pattern of the original and dealuminated zeolites were taken on a Phillips diffractometer PW 1130100. The mid-infrared transmission spectra were recorded with a Nicolet 7199 FT-IR spectrophotometer using the KBr pellet technique. DEALUMINATION PROCEDURE Already more than 100 years ago it has been reported that alumina reacts at "red heat" with silicon tetrachloride forming volatile A1C13 and Si0 2 (ref. 11, 12). Inspired by this early observation we treated Y zeolites at high temperatures and we obtained the expected but nevertheless surprising result that under certain experimental conditions aluminium is substituted in the framework by silicon. This reaction may be formally described by the equation Ml/ n[A10 2·(Si02)x] + SiC1 4 ---+ lin MCl n + A1C13 + [(Si0 2)x+l] (1) in which M is some lattice cation. The zeolites are placed in powder form in a vertical quartz tube reactor (bed height about 3 cm) and dehydrated for two hours at about 650 K in dry nitrogen streaming through the zeolite bed. After dehydration the nitrogen stream is saturated at room temperature with silicon tetrachloride and the temperature of the reactor is rised at a constant rate of 4 Komin- l till a choosen final level between 730 and 830 K. Using Y zeolites as starting material a white smoke of aluminium trichloride appears in the effluent gas at temperatures around 730 K which indicates the beginning of the substitution reaction. The silicon tetrachloride treatment is carried on for two hours at the final temperature. Then the product is purged with dry nitrogen, washed until complete disappearence of any chlorides in the washing water and dried at about 400 K. High-crystallinity faujasite-type structures are obtained. However, if dehydrated Y zeolite is first heated to temperatures over 750 K and then brought into contact with silicon tetrachloride vapour a violent exothermal reaction proceeds resulting in the formation of an amorphous product. This indicates that the incorporation of silicon into framework vacancies left by dealumination is a slower process than the formation of A1C1 Consequently, at high dealumination rates the con3• centration of vacancy sites reaches a level where the structure becomes unstable.

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The degree of dealumination depends mainly on the reaction time and less on the final temperature. Treatment of Y zeolites in the temperature range from 780 till 830 K for 2 hours leads to products with Si0 2/A1 20 3 ratios from about 40 till 100. however, starting from Na-Y zeolite only part of the aluminium leaving framework positions is escaping in form of volatile aluminium chloride. That is easy to explain. Equation (1) shows that sodium lattice cations are transformed to NaCl. On the other hand it has been found long ago that Na[A1C1 4] is formed by melting NaCl with A1C13 (ref. 13) or by passing gaseous A1C1 over heated NaCl (ref. 14). 3 It is obvious that part of the A1C1 3 reacts with the formed sodium chloride to Na A1C1 4 which is not volatile under the reaction conditions but can be removed by washing. Indeed, the washing water contains always aluminium and it is acidic due to the hydrolytic decomposition of the complex. If the aluminium would escape quantitatively as A1C1 3 the progression of the dealumination reaction could be followed by gravimetric measurements using a suitable thermobalance or a special McBain balance. Unfortunately, the secondary reaction of part of the A1C1 with the formed 3 NaCl excludes the application of this simple method. At present we don't see any possibility to study the kinetics of the dealumination of metal cation containing zeolites. However, it should be possible to study the dealumination of completely ammonium-exchanged Y zeolite by gravimetric methods. It should be noted that Y zeolites can be also dealuminated using trichlorosilane as dealuminating agent. This compound is, however, less thermostable than silicon tetrachloride. It is necessary to carry out the process in a hydrogen atmosphere but even observing all precautions it was not possible to avoid completely the decomposition of trichlorosilane. The dealuminated products were always contaminated with traces of metallic silicon. (NH 4,Na)-Y zeolite can be dealuminated under the same conditions as the sodium Y. However, attemps to dealuminate L-type zeolite and its NH 4form as well as Na-mordenite and Na-X failed. (Ca,Na)-x zeolite could be partially dealuminated under the conditions given above for Na-Y. It is obvious that the described method is not applicable for the dealumination of zeolites with narrow pore systems in which silicon tetrachloride cannot enter. We suppose that in the case of X zeolites the framework is shielded from the attack of the silicon tetrachloride by lattice cations present in higher concentration as in Y zeolites. CHARACTERIZATION AND PROPERTIES OF DEALUMINATED Y ZEOLITES The dealuminated samples have very good crystallinity as reflected by the X-ray diffractograms (fig. 1). As expected the diffraction peaks are shifted towards higher 2e values indicating the contraction of the unit cell. Unit cell size and residual aluminium content of the dealuminate~ samples fit exactly the relation given by BRECK and FLANIGEN (ref. 15).

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d (nrn) All diffraction peaks appearing in the X-ray diffractogram of the original Na-Y zeolite are also present in the diffractograms of the dealuminated samples but remarkable intensity differences are observed. The peaks belonging to crystal planes with higher indices are more intense in the diffr,actogram of Na-Y. On the contrary, the (111), (220), (311) and (331) reflections are much more intense in the case of dealuminated samples. The structure factors calculated from the intensity of these peaks are in good agreement with the contribution of the framework to the corresponding structure factors of hydrated Na-Y zeolite given by VARGA et al. (ref. 16). The obtained X-ray results are consistent with the substitution of aluminium by silicon in the zeolite framework. As for the intensity of the individual diffraction peaks Y zeolite dealuminated by hydrothermal treatment combined with acid extraction (fig. lb) is situated between the original Na-Y zeolite (fig. lc) and the samples dealuminated by silicon tetrachloride (fig. la) though the acid extracted sample has the highest dealumination degree. That indicates structural differences between the products obtained by the two fundamentally different dealumination procedures. A detailed structure factor analysis may give more information. It has been reported that aluminium removal from the framework of (NH 4,Na)-Y zeolites and elimination of lattice vacancies created by dealumination shift the framework vibration bands in the mid-infrared spectrum toward higher frequencies and result in an increase of the bandsharpness (ref. 6, 10). The frequency shift can be explained by the shorter average T-O bond distances in the aluminium-free structure. The increasing band sharpness points to a higher "degree of ordering". Itcan be seen from the spectra given in fig. 2 that the dealumination of Y zeolite by reaction with silicon tetrachloride results also in sharper bands and shifts the absorption maxima to higher frequencies. The

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Fig. 2. Mid-infrared spectra of (a) Na-Y zeolite, Si0 2!A1 20 3 ratio = 5.04; (b) Y zeolite dealuminated by silicon tetrachloride, Si0 2!A1 20 3 ratio = 46.

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spectrum (b) of the dealuminated product is quite similar to the spectra of Y zeolites after hydrothermal treatment without (ref. 6) and with (ref. 10) acid extraction of the formed non framework aluminium. However, the spectrum of the sample dealuminated with silicon tetrachloride is much better resolved, it reveals more details and the bands are considerably sharper. Consequently, the infrared spectrophotometric results indicate a higher "degree of ordering" in this sample. Zeolites dealuminated by silicon tetrachloride show an extremely high thermal stability and resistance to mineral acids. As reflected by X-ray diffractograms and adsorption data the crystallinity of a dealuminated sample with a Si0 2!A1 20 3 ratio of about 50 remains unchanged even after heating at 1370 K. The structure collapses only at 1450 K as indicated by an exothermal DTA peak appearing at this temperature. The crystallinity is also fUlly maintained after boiling in 6 N hydrochloric acid for two hours. In this respect the samples dealuminated by silicon tetrachloride or under hydrothermal conditions behave in the same way (ref. 6, 10). The dealuminated zeolites do not contain sodium and aluminium in equimolar amounts. It is assumed that part of the sodium is exchanged during the washing process. Acid treatment of samples dealuminated by silicon tetrachloride results in a further removal of framework aluminium remaining after the substitution reaction. For example the Si0 2!A1 20 3 ratio of a dealuminated Y sample could be increased from 27 to 550 by repeated treatment with 1 N hydrochloric acid at 353 K. It is well known that the adsorption capacity of Na-Y zeolite for water and ammonia amounts to about 0.33 em3 liquid adsorbate per 9 adsorbent and that it is reached already at very low relative pressures. In contrast to the preference of zeolite surfaces for polar molecules, highly dealumina-

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ted Y zeolites have an extremely low selectivity for the adsorption of water and ammonia (fig. 3) and behave in this respect like silicalite, the aluminium-free homologue of ZSM-5 zeolite (ref. 17).

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The isotherms for the adsorption of organic compounds (n-butane, benzene, n-hexane) on dealuminated ¥ zeolite (fig. 3) show the near rectilinear shape typical for the volume filling of micropores and known from the adsorption on zeolites. The adsorption capacity of dealuminated zeolites is only slightly lower than that of the original zeolite (see "a" curves in fig. 3). A more realistic Qomparison of the adsorption capacities is possible if the adsorbed amount is given in molecules per unit cell (fig. 4). As can be seen from the adsorption isotherms in fig. 4 the dealuminated ¥ samples adsorb about 15 % less molecules in the unit cell than Na-¥ zeolite. About half of this difference is due to the diminuation of the unit cell volume caused by the lattice contraction during the substitution of framework aluminium by silicon. It is not yet clear whether the remaining difference of 7-8 % in the adsorption capacity reflects a crystallinity loss or a less dense packing of the molecules in the supercages. This difference, however, is not enough pronounced to attribute to it a fundamental significance. As for the adsorption of n-hexane (and other hydrocarbons) zeolite ¥ dealuminated under hydrothermal conditions shows quite another behaviour

209

(see fig. 4. curve b) which points to a bidisperse pore system (ref. 10). The volume of the faujasite-type micropore system amounts to 75 % compared with Na-Y. Secondary pores formed during the dealumination process have radii in the range from 1.5-1.9 nm. It is assumed that silicon atoms migrating into the framework vacancies left by dealumination corne from positions inside the crystal in such a manner that structural units, e.g. whole sodalite units, disappear (ref. 10). CONCLUSION Y zeolites can be dealuminated by reaction with silicon tetrachloride without collapse of the crystal structure. X-ray diffraction pattern and mid-infrared spectra point to a high degree of ordering in the framework of the dealuminated products and adsorption of hydrocarbons reveals no significant differences in the pore system compared with Na-Y. In contrast to the described new dealumination method, dealumination under hydrothermal conditions changes the pore system and the structure of the framework to a certain degree. Y zeolites dealuminated by silicon tetrachloride may offer new application possibilities as hydrophobic adsorbents. It can be expected that the hydrogen form of partly dealuminated zeolite Y is an active catalyst for hydrocarbon reactions with excellent thermal and hydrothermal stability. ACKNOWLEDGEMENT The authors are indebted to the X-Ray Diffraction and Optical Spectroscopy Groups of the Central Research Institute for Chemistry of the Hungarian Academy of Sciences for X-ray diffraction and infrared measurements, respectively. Technical assistance of Mrs. I. Szanisz16 and Mr. I. Csorba is gratefully acknowledged. REFERENCES 1 G.T. Kokotailo, S.L. Lawton, D.H. Olson and W.M. Meier, Nature, 272 (1978) 437-438. 2 M.S. Spencer and T.V. Whittam, Acta Phys. Chern. (Szeged), 24 (1978) 307-311. 3 S.L. Meisel, J.P. McCullough, C.H. Lechthaler and P.B. Weisz, Chemtech, 6 (1976) 86-89. 4 D. Barthomeuf, Acta Phys. Chern. (Szeged), 24 (1978) 71-75. 5 G.T. Kerr, J. Phys. Chern., 71 (1967) 4155-4156. 6 J. Scherzer and J.L. Bass, J. Catalysis, 28 (1973) 101-115. 7 P. Gal1ezot, R. Beaumont and D. Barthomeuf, J. Phys. Chern., 78 (1974) 1550-1553. 8 J. Scherzer, J. Catalysis, 54 (1978) 285-288. 9 U. Lohse, E. Alsdorf and H. Stach, Z. anorg. allg. Chern., 447 (1978) 64-74. 10 U. Lohse, H. Stach, H. Tharnrn, W. Schirmer, A.A. Isirikjan, N.!. Regent and M.M. Dubinin, Z. anorg. al1g. Chern., 460 (1980) 179-190. 11 M. Daubree, Comt. Rend., 34 (1854) 135-140. 12 L. Troost and P. Hautefeuil1e, Compt. Rend., 75 (1872) 1819-1821. 13 F. Wahler, Pogg. Ann., 11 (1827) 155. 14 H. Rose, Pogg. Ann., 96 (1855) 157.

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D.W. Breck and E.M. Flanigen, Molecular Sieves, The Chemical Society, London, 1968, pp. 47-60. K. Varga, I. Kiricsi and Gy. Argay, Acta Phys. Chern. (Szeged), 25 (1979) 69-77. E.M. F1anigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton and R.M. Kirchner, Nature, 271 (1978) 512-516.