CHANGES IN SURFACE PROPERTIES OF KAOLIN MINERAL IN DEALUMINATION PROCESS BY SULFURIC ACID TREATMENT

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1 Clay Science 5, (1978) 163 CHANGES IN SURFACE PROPERTIES OF KAOLIN MINERAL IN DEALUMINATION PROCESS BY SULFURIC ACID TREATMENT MIN CHE CHON Chon Engineering Co., Ltd., Seoul Korea AND TATSURO TSURU AND HIROSHI TAKAHASHI Institute of Industrial Science, The University of Tokyo, Tokyo, Japan (Received May 19, 1978) ABSTRACT To understand the pore structure and the surface properties of the dealuminated kaolin mineral, adsorption studies of ammonia and benzene were made, and the distributions of acid strength of these samples were measured by using an amine-titration method. Isomerization of 1-butene was chosen as the test reaction. The products distributions were measured and the characteristics of acid site of the dealuminated kaolin mineral were discussed. INTRODUCTION In the previous work, we have investigated the changes in the pore structure of the halloysite mineral due to dealumination by nitrogen adsorption-desorption technique. The surface area of the dealuminated samples generally increased with increasing extent of aluminum extraction by sulfuric acid. For certain samples, however, the surface area decreased as the extent of aluminum extraction increased. In this work, studies are extended to ammonia adsorption to understand more of the pore structure and the surface properties of the dealuminated samples. Adsorption of ammonia also serves as a method to determine the surface acidity. The surface acidity of the dealuminated samples as well as 1-butene isomerization reaction using pulse microcatalytic technique are studied to find the correlation between the catalytic activity of the acid catalyst and surface acidity. EXPERIMENTAL Kaolin mineral used in this study was from the Sung-ju area. Aluminum was extracted from the kaolin sample by treating with sulfuric acid. Details of the preparation of aluminum extracted samples are described in the

2 164 M. C. Chon, T. Tsuru and H. Takahashi previous paper (Chon, Tsuru and Takahashi, 1978). Table 1 shows the nitrogen BET surface area for various aluminum extracted samples reported in the previous paper. Adsorption of benzene and ammonia has been studied volumetrically. Fig. 1 shows the apparatus used for the adsorption measurements. Prior to the adsorption measurements, samples were treated at 200 Ž for 3 hours in air and 1 hour in vacuum. The amount of benzene adsorbed was read at 18 Ž and p/p0=0.55. For ammonia adsorption at 18 Ž, the adsorbed amount was read at 20 mmhg. The amounts of ammonia adsorbed at 100 Ž and 200 Ž were obtained by subtracting the amount desorbed when the temperature was raised to 100 Ž and 200 Ž respectively from the amount adsorbed at 18 Ž. TABLE 1. Percentage of Al-extraction and BET surface area of dealuminated kaolin mineral FIG. 1. Volumetric adsorption apparatus.

3 Surface Properties of Acid Treated Kaolin 165 The acidity of the samples was measured by Benesi method (Benesi, 1957) grams of samples treated in air for 3 hours at 200 Ž were placed in glass vials and titrated with 0.01 N n-butylamine benzene solution. After the samples had been placed in a desiccater overnight the end points were determined by 0.01% benzene solution of Hammett indicators of various pka values. The indicators neutral red (pka=6.8), methyl red (pka=4.8) and butter yellow (pka=3.3) were Fisher A.C.S. grade. Benzene-azodiphenylamine (pka=1.5), dicinnal aceton (pka=-3.0) and benzene acetophenone (pka=-5.6) were Tokyo Kasei GR. n-butylamine was Fisher A.C.S. grade and WACO Sp. grade benzene was passed through Linde 4A molecular sieve before use. The catalytic activities of the samples for the 1-butene isomerization were compared using pulse microcatalytic technique. Reaction products were analyzed using dioctyl phthalate column of 10 feets (20% dioctyl phthalate on 60/80 mesh chromosorb W) operated at 293 Ž. 0.2 grams of samples were pelleted into 4.4 ~1.6 mm cylinder form and placed in the center portion of reactor tube which was a stainless steel tubing of 1/4 inch O.D. Catalyst samples were pretreated in He stream (30 ml/min.) for 3 hours at 200 Ž prior to the activity measurements. 1-butene was Matheson C. P. Grade and He Airco Product (99.995%). RESULTS In Table 1 are shown the amount of benzene adsorbed at 18 C and p/p0= 0.55 for various samples. Except for samples A-2, A-4 and A-7 the amount TABLE 2. Ammonia adsorption of dealuminated kaolin mineral at 18, 100 and 200 Ž (m mole /g) TABLE 3. Acid strength distribution of dealuminated kaolin mineral

4 166 M. C. Chon, T. Tsuru and H. Takahashi TABLE 4. Composition of reaction product from 1-butene isomerization at 100, 150 and 200 Ž of benzene adsorbed increases with increasing degree of aluminum extraction. This is exactly the same tendency observed for the nitrogen adsorption. In Table 2 is shown the amount of ammonia adsorbed at 18 Ž, 100 Ž and 200 Ž. As is seen from the table a sharp increase in the amount of ammonia adsorbed is observed starting from sample A-2. In contrast to the nitrogen and benzene adsorption, large ammonia adsorption is observed for samples A-2, A-4 and A-7. In Table 3 are given the acid strength distribution for the samples. Tables 4a, 4b and 4c show the compositions of the reaction products when the pulses of 1-butene were introduced into the microreactor at 100, 150 and 200 Ž, respectively. Sample A-0 shows almost no activity at 100 Ž. Cis/ trans ratio is about 1 at 100 Ž except for samples A-4 and A-7 for which the ratio is close to 1.2. At 200 Ž except for sample A-0 the reaction product ratio is close to the equilibrium ratio (Kilpatrick et al., 1946). DISCUSSION Fig. 2 shows the adsorption of nitrogen, benzene and ammonia on the samples of different degree of aluminum extraction. The amounts of ammonia and benzene adsorbed is measured at 18 Ž and the amount of nitrogen adsorbed is equal to its BET monolayer volume. Both for nitrogen and benzene the adsorbed amounts increase with increasing percentage of aluminum extraction except for samples A-2, A-4 and A-7. It may be due to its smaller size and strong adsorption on the surface acid sites that ammonia molecules can diffuse into the primary micropores produced by the

5 Surface Properties of Acid Treated Kaolin 167 FIG. 2. Adsorption of nitrogen, benzene and ammonia on kaolin mineral different in degree of aluminum extraction. extraction of aluminum. The fact that samples A-2, A-4 and A-7 show a decrease in the amount of both nitrogen and benzene adsorbed deviates from the general tendency of increased adsorption with increasing aluminum extraction. Nevertheless ammonia shows large adsorption on samples A-2, A-4 and A-7. These suggest that they possess the large amount of primary micropores produced by the elimination of aluminum where ammonia molecules can penetrate and are adsorbed but nitrogen and benzene molecules can not. Among the methods of determining the acidity of the solid, the method involving titration with base in nonaqueous solvent and the method based on surface adsorption of gaseous bases are commonly used. With a relatively simple procedure, the acid site density and acid strength can be determined by the titration method. The titration method has disadvantages, not being able to make determination under the working conditions of the catalyst, difficult to determine the end points, and taking a long time to reach equilibrium. The method based on the surface adsorption of gaseous bases has the advantage that it can be applied at the working temperature of the catalyst. There is, however, uncertainty as to whether adsorption is only on acid centers. Also the amount adsorbed may be different depend-

6 168 M. C. Chon, T. Tsuru and H. Takahashi ing on the size of the molecules employed. If we take the acid site density at pka=6.8 and the amount of ammonia adsorbed at 18 Ž as the measure of the surface acidity, the acid amount obtained by the titration method runs parallel with the nitrogen and benzene adsorption showing a decrease from samples A-7, to A-4 and A-2 while the acid amount taken as the amount of ammonia adsorbed shows larger values for those samples. n-butylamine molecule may not be able to diffuse into the micropores where the acid site may be present, while ammonia molecules can diffuse into such micropores, as discussed before. Surface acidity is an important factor in determining the activities of the solid catalyst for the acid catalyzed reactions. The result of the amine titration shows that the surface acid points are primarily weak ones. Isomerization of 1-butene was chosen as the test reaction because the reaction is catalyzed by relatively weak acid and its mechanism and kinetics have been extensively studied. 1-Butene isomerization involves 1, 2 isomerization and cis-trans isomerization. Both reactions seem to proceed by a conventional carbonium ion mechanism with a common intermediate, the secondary butyl carbonium ion (Haag and Pines. 1960; Hightower and Hall, 1967). FIG. 3. Percentage conversion of 1-butene to 2-butene at 100 Ž, the amount of ammonia adsorbed at 100 Ž and the amount of acid (pka=6.8) obtained from the amine titration of various dealuminated samples:

Surface Properties of Acid Treated Kaolin 169 For the cis-trans isomerization stronger acid points are required than for the 1, 2 isomerization.

7 Surface Properties of Acid Treated Kaolin 169 For the cis-trans isomerization stronger acid points are required than for the 1, 2 isomerization. Our results show that the cis/trans ratio of the reaction product at 100 Ž is about 1, at 150 Ž 0.65, and at 200 Ž 0.55 which is close to the equilibrium value. It seems that at 100 Ž the reaction was primarily the 1, 2 isomerization and as the temperature was raised the cis-trans isomerization becomes active. Fig. 3 shows the percentage conversion of 1- butene at 100 Ž together with the amount of ammonia adsorbed at 100 Ž and the acid amount (pka=6.8) obtained from the amine titration for the various samples. It can be seen that there is a parallelism between the conversion of 1-butene at 100 Ž and the amount of ammonia adsorbed. The acid amount determined by the adsorption of ammonia at the temperature of the 1-butene reaction seems to be the adequate measure of the active sites for the reaction of 1-butene. REFERENCES BENESI, H. A. (1957) J. Phys. Chem., 61, 970. CHON, M. C., TSURU, T. and TAKAHASHI, H.,(1978) Clay Sci. 5, 155. HAAG, W. O. and PINES, H. (1960) J. Am. Chem. Soc., 82, HIGHTOWER, J. W. and HALL, W. K. (1967) J. Phys. Chem., 71, KILPATRICK, J. E., PROSEN, E. J., PITZER, K. S. and ROSSINI, F. D. (1946) J. Res. Nat. Bur. Standards, 36, 559.

Oxidative Dehydrogenation of Olefin*

Surface Heterogenity of Bismuth-Molybdate Catalyst in Oxidative Dehydrogenation of Olefin* by Toru Watanabe** and Etsuro Echigoya** Summary: The oxidative dehydrogenation of C4, C5 olefins over bismuth