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The influence of water treatment on the relation between the adsorptive and catalytic properties of ScY zeolite
Surface Technology, 9 (1979) 435 - 442
435
© Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
T H E I N F L U E N C E OF WATER T R E A T M E N T ON T H E R E L A T I O N BETWEEN THE A D SO R PT I V E AND C A T A L Y T IC P R O P E R T I E S OF ScY Z E O L I T E
M. M. SELIM and G. A. EL-SHOBAKY Laboratory of Surface Chemistry and Catalysis, National Research Centre, Dokki, Cairo (Egypt)
E. M. EZZO Chemistry Department, Faculty of Women, Ain Shams University, Cairo (Egypt)
(Received April 23, 1979)
Summary Th e high t e m p e r a t u r e adsorption of c u m e n e as well as its cracking on ScY zeolite were investigated using a pulse chromatographic technique. 12.3% o f sodium ions were substituted by scandium ions. T he effect of water on b o th the adsorptive and catalytic properties was investigated. T r e a t m e n t o f 20 mg o f catalyst with a small a m o u n t of water (10 -2 ml) was f o u n d to decrease the catalytic activity considerably. The degree of deactivation was d e p e n d e n t on bot h the t e m p e r a t u r e of t r e a t m e n t with water and the m o d e o f activation of each catalyst sample. In contrast, neither the heat o f adsorption n o r the r e t e n t i o n volume of c u m e n e was f o u n d to vary appreciably with water t r e a t m e n t . These results m a y p o i n t to the absence of a direct relation between adsorption and catalysis in this case.
1. I n t r o d u c t i o n Zeolites are c o m m o n l y used in several catalytic reactions. Their activity and selectivity can be altered by a variety of m e t h o d s which depend generally on the silica:alumina ratio [1 - 3 ] , substitution of t he sodium ions by o t h e r cations [1 - 6] and the t e m p e r a t u r e and m o d e o f activation [4, 7, 8]. Cracking o f h y d r o c a r b o n s has been studied by several authors on zeolites in which the sodium was substituted with different cations such as Ca 2÷, Ni 2÷ and Sc a+ [1 - 9 ] . Th e catalytic activity in cum e ne cracking was f o u n d to increase on increasing the e x t e n t of substitution o f sodium ions with foreign ions
[6, 9]. In the case o f scandium zeolite it has been shown by one o f the authors [6] t h a t the catalytic activity o f ScY zeolite in cum ene cracking is proportional to the e x t e n t o f substitution, reaching m a x i m u m activity when a b o u t 66% o f the sodium ions are substituted by scandium ions. A ny f u r t h e r
436 increase in scandium c o n t e n t is accompanied by a marked decrease in the zeolite activity. In a previous investigation [ 10] it has been shown that the substitution of about 65% of the sodium by a m m o n i u m ions produces an active catalyst in cumene cracking that has the same crystal structure as the original NaY zeolite. Moreover water treatment of decationated Y zeolite greatly decreases its catalytic activity owing to degradation of the crystallinity. In the present investigation a small proportion of the sodium ions of NaY zeolite were substituted by scandium ions. The effect of water treatment on the catalytic and adsorptive properties was studied using a pulse microcatalytic technique.
2. Experimental
2.1. Catalysts Linde Molecular Sieve Catalyst Base SK 40 (a molecular sieve type Y) was provided by Union Carbide Corporation, U.S.A. A typical chemical composition of the anhydrous zeolite is SiO2 63.5 wt.%, A1203 23.5 wt.%, Na20 13 wt.% and the silica:alumina ratio is 4.6. The average particle size is 5 pm and the ultimate crystal size 0.5 - 1.5 gm. The exchange of the sodium ions in the starting NaY zeolite by scandium was carried out by shaking 5 g of NaY zeolite powder with 60 ml 0.1 N scandium chloride solution at room temperature for 20 min. The sodium c o n t e n t in the resulting sample was determined by flame p h o t o m e t r y . A 20 mg sample was used in each catalytic experiment; in the case of adsorption 100 mg zeolite was mixed with 3 ml inert quartz fragments (2 mm) and was charged in the reactor. All catalyst samples were activated by heating for 2 h with dry air at 450 °C and then for 1 h in a helium stream at the same temperature. 2.2. Apparatus and techniques 2.2.1. Cracking o f cumene The catalytic measurements were carried out in a pulse micro-catalytic system. The reactant under investigation (cumene) was introduced in microquantities (1 × 10 -3 ml = 0.718 × 10 -5 mol), with the aid of a microsyringe and as a pulse, into a microreactor containing a small quantity of the catalyst (20 mg). The reaction products were transferred directly by an inert gas carrier (helium) at a flow rate of 50 ml min -1 to a gas-liquid chromatographic column for their separation and determination. 2.2.2. High temperature adsorption of cumene The adsorption of cumene was studied at various temperatures between 200 and 400 °C using the gas chromatographic method. The apparatus for cumene cracking was used for the adsorption study with the chromatographic column disconnected. The reactor was used as the chromatographic column, which was filled with 100 mg zeolite mixed with 3 cm 3 inert packing in the
437
form of 2 mm quartz fragments. The adsorbate (cumene) was introduced by microsyringe in 1 X 1 0 - 3 ml doses in a 50 ml min -1 flow of helium. The retention times of cumene were corrected with respect to the retention time of a non-adsorbable gas. The retention volumes VR were calculated from the corresponding retention times using an equation given elsewhere [ 11, 12]. The heats of adsorption of cumene of the samples were determined by plotting In VR versus 1/T. After the first adsorption run had been performed with cumene the zeolite sample was cooled to room temperature in a flow of air, water was injected in 5 pulses each of 1 X 10 -3 ml (5 X 10 -3 ml) and then the solid was dehydrated at 450 °C for 1.5 h in an air current and then in a helium current for 0.5 h. This process of h y d r a t i o n - d e h y d r a t i o n was repeated after each adsorption run.
2.2.3. X-ray investigation The X-ray diffraction patterns of all the zeolite samples were investigated at room temperature using a Rich Seifert X-ray diffractometer and Cu Ks radiation. 3. Results
3.1. High temperature adsorption of cumene The pulse chromatographic technique was f o u n d to be a more practically useful m e t h o d for studying adsorption at high temperature than the conventional static m e t h o d . In the static m e t h o d the long contact time involved m a y lead to decomposition of the adsorbates [13 - 15]. In the present investigation this technique was used to follow up the effect of water treatm e n t on the adsorptive properties of scandium zeolite. The data on retention volumes at 320 °C and the heat of adsorption as functions of water injections are given in Table 1. TABLE 1 T h e e f f e c t o f successive a d d i t i o n s o f w a t e r o n t h e r e t e n t i o n v o l u m e a n d h e a t of a d s o r p t i o n o f c u m e n e o n s c a n d i u m zeolite No. o f H 2 0 i n j e c t i o n s
V R at 3 2 0 °C (ml g - 1 )
Q (kcal mo1-1 )
0 1 2 3 4 5 6 7 8 9
417 398 398 355 346 339 316 288 288 251
20.5 20.1 20 19.7 19.8 19.9 20 20 20.6 20
438 Table 1 shows t h a t successive additions of water to scandium zeolite lead to a slight decrease in r e t e n t i o n volume f rom 417 to 251 ml g-1 (at 320 °C). This decrease was obtained after 9 additions of water each of 5 X 10 -3 ml. In contrast, the heat of adsorption of cumene remains almost c o n s tan t ( a b o u t 20 kcal mo1-1); preliminary experiments showed that the heat o f adsorption of cum e ne on NaY zeolite was 20 kcal mo1-1 . F r o m the results in Table 1 and in Fig. 1 it can be seen t hat the addition o f water to the zeolite containing 87.7% sodium ions slightly affects its adsorptive properties in cum e ne adsorption. This slight modification in the adsorptive properties of the zeolite may have an influence on its catalytic activity.
3.2. Cracking o f cumene 3.2.1. The influence of water treatment on the catalytic cracking o f cumene in contact with Sc Y zeolite It is accepted t hat the c u m e n e cracking reaction is a model reaction, and it is a convenient measure for evaluating the catalytic activity of different catalysts in cracking reactions. Most authors have f o u n d t hat cumene cracking proceeds mainly via a c a r boni um ion mechanism. Moreover, the functional h y d r o x y l groups of the catalyst m a y play a decisive role in its activity. T h e y m a y be varied by different m e t h o d s such as changing the silica:alumina ratio or treating the catalyst with water. Thus it is of interest to study the e f f e c t of water on the catalytic activity of ScY zeolite in cumene cracking. 3.2.1.1. The effect o f successive additions of water to the catalyst in a helium stream without regeneration. The results o f the cum ene cracking in c o n t a c t with scandium zeolite and the effect of water on its catalytic activity are graphically represented by Fig. 2, curve a. T he reaction of cum ene cracking was carried o u t at a c o n s t a n t t e m p e r a t u r e of 320 °C with 1 X 10 -3 ml c umen e in a cons t a nt flow of helium. After separation and det ect i on o f the reaction p r oduct s 2 X 10 -3 ml of water was injected. A b o u t 10 successive injections of water into the reactor at 320 °C were carried out. Th e percentage conversion was calculated in each case. F r o m Fig. 2, curve a, it can be seen t hat the percentage conversion decreases gradually fr o m 63% to a co n s t a nt value of 35% after a b o u t 9 - 10 pulses. F u r t h e r addition o f water to the catalyst was f o u n d to have no effect on the conversion. 3.2.1.2. The influence o f regeneration-water addition cycles at 450 °C. In this set o f experiments the scandium zeolite was used catalytically at 320 °C. The catalyst was then regenerated at 450 °C for 30 min. A pulse of 2 X 10 3 ml o f water was injected at 450 °C followed by regeneration. The reaction, water injections and regenerations were carried out in a stream of helium of flow rate 50 ml min 1. The results obtained are graphically represented in Fig. 2, curve b. It can be seen from this figure t hat only 4 injections o f water lead to a sudden decrease in the percentage conversion from 63 to 20%. F u r t h e r t r e a t m e n t with water has no influence on the conversion.
439
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I 1.7
i
1.18 118 IO00/T
i
1 2
i
i
i
i
i
i
,
,
J
3 4 5 6 7 8 9 10 No water pulses
Fig. 1. P l o t s of log VIi vs. 1 / T f o r fresh a n d w a t e r - t r e a t e d catalysts. Fig. 2. T h e p e r c e n t a g e c o n v e r s i o n as a f u n c t i o n o f n u m b e r o f w a t e r pulses for c a t a l y s t s t r e a t e d w i t h w a t e r : curve a, c a t a l y s t t r e a t e d w i t h w a t e r at r e a c t i o n t e m p e r a t u r e 3 2 0 °C w i t h o u t r e g e n e r a t i o n ; curve b, c a t a l y s t t r e a t e d w i t h w a t e r a t 4 5 0 °C a n d t h e n h e a t e d in a h e l i u m s t r e a m at t h e s a m e t e m p e r a t u r e ; curve c, c a t a l y s t t r e a t e d w i t h w a t e r at r o o m t e m p e r a t u r e a n d t h e n h e a t e d at 4 5 0 °C in a c u r r e n t o f d r y air.
3.2.1.3. The effect o f regeneration by air and water treatment on the percentage conversion o f the catalyst. For the fresh catalyst the activity was tested at 320 °C with cumene in a helium flow. Water additions were carried out at room temperature in a stream of air. The zeolite was subjected to regeneration for 30 min in air at 450 °C before and after each water injection. The results obtained are illustrated in Fig. 2, curve c, which shows that only 4 pulses of water each of 2 X 10 -3 ml caused a marked decrease in percentage conversion from 63 to 7.5%. This low conversion remains almost constant on further treatment with water.
4. Discussion It has been observed in a previous study [16] that the successive h y d r a t i o n - d e h y d r a t i o n cycles of ScY zeolite increase considerably the number of h y d r o x y l groups characterized by a narrow line in nuclear magnetic resonance (NMR) spectra. It is therefore expected that this increase in the concentration of h y d r o x y l groups will be accompanied by an increase in the catalytic activity of zeolite in reactions proceeding via a carbonium ion mechanism [1, 7]. From the results presented above it is clear t h a t the addition of water to the zeolite decreases its catalytic activity in cumene cracking. The degree of deactivation of the catalyst depends on the conditions of water treatment. The m a x i m u m effect of water on the catalytic activity of scandium zeolite was observed when the catalyst was treated with water at room temperature and heated in air at 450 °C before and after water addition. The effect of water was m i n i m u m when the catalyst was treated with water at the reaction temperature in a flow of helium w i t h o u t regeneration. An intermediate effect was observed when water was injected at 450 °C in a flow of helium.
440
In the first case, during the thermal treatment of the catalyst, oxygen oxidizes severely the condensation products formed on the catalyst surface, converting them into gaseous products and leaving a clean surface accessible for further water attack; it thus enhances the process of deactivation by water molecules. The accumulation of condensation products on the surface at the reaction temperature (320 °C) forms a certain protective layer, thus preventing further decrease in activity by the addition of water (Fig. 2, curve a). The activation of the catalyst by helium at 450 °C is not sufficient to remove all the condensation products formed on the zeolite surface, unlike the case of activation by air. In a previous paper [ 12] we have shown that the substitution of 65% of the sodium ions of Y zeolite with a m m o n i u m ions greatly modified the adsorption characteristics of cumene, since the zeolite was treated with water at room temperature. The heat of adsorption of cumene was found to decrease from 21 to 4.3 kcal mo1-1 ; the retention volume was also decreased considerably. In the present investigation the substitution of 12.3% of the sodium ions of the same zeolite sample by scandium, however, was n o t accompanied by important changes in the high temperature adsorption of cumene. When the catalyst is treated with water under the same conditions, the heat of adsorption remains almost constant while the retention volume decreases slightly. It appears that the adsorption of cumene depends mainly on the sodium c o n t e n t of the zeolite. Substitution of a small fraction (12.3%) of sodium ions with scandium ions might be expected to have a small influence on cumene adsorption. In fact it was shown that NaY zeolite adsorbs cumene with a heat of adsorption 20 kcal mo1-1 ; almost the same value was obtained for ScY zeolite. Increasing the extent of substitution of sodium by scandium ions may lead to a decrease in both h e a t of adsorption and retention volume of cumene with a simultaneous collapse of structure. A lower degree of substitution restricts us since it ensures that the crystallinity of the ionexchanged zeolite remains almost unchanged [ 17] ; any modification in the crystallinity of the zeolite will be due to water treatment. It is known that the catalytic activity of NaY zeolite in cumene cracking is undetectable; however, substitution of some of its sodium ions by multivalent cations such as scandium ions creates an active catalyst for this reaction. The catalytic activity is proportional to the degree of substitution of sodium ions by multivalent cations so long as the crystal structure of scandium is conserved. This structure can also be modified in a controlled manner by treating ScY zeolite with different amounts of water at room temperature or at elevated temperatures. NaY zeolite resists any structural collapse by such treatment, as confirmed experimentally by X-ray and NMR studies [16]. In contrast, the fraction of zeolite which contains scandium is capable of undergoing structural modification. The X-ray investigation, however, reveals t h a t the t r e a t m e n t of ScY zeolite by water does n o t produce any important modification in its crystaUinity (Fig. 3). These results can be attributed to the low scandium c o n t e n t and the insensitivity of the X-ray technique used to the slight modification in the crystallinity of the zeolite.
441
40
30
S
20
10
Fig. 3. X-ray diffraction patterns of NaY and ScY zeolites: ScY-(1), untreated ScY zeolite; ScY-(2), catalyst treated with water at room temperature in air and then used in cumene cracking at 320 °C; ScY-(3), catalyst treated with water-cumene at 320 °C in a helium stream.
It is to be concluded that substitution of 12.3% of sodium ions by scandium ions greatly increases the catalytic activity of NaY zeolite in cumene cracking. The activity of the ScY zeolite prepared was found to decrease on water treatment. The e x t e n t of deactivation was greatly influenced by the experimental conditions, e.g. the temperature of treatment with water, the temperature of activation of the catalyst and the presence of air or an inert carrier gas such as helium in contact with the catalyst during the activation process. Water t r e a t m e n t of ScY zeolite under the experimental conditions employed in this investigation was f o u n d to exert only a small influence on the high temperature adsorption of cumene. These results may point to the absence of a direct relation between adsorption and catalysis in this case.
References 1 K. V. Topchieva, B. V. Romanovski, L. I. Piguzova, Ho Shi Tuang and E. V. Bezre, Proc. 4th Int. Congr. on Catalysis, Moscow, Vol. 2, 1968, p. 135. 2 K. Tsutsumi and H. Takahashi, J. Catal., 24 (1) (1972) 1. 3 B. V. Patzelova, V. Aybl and C. Tavaruzkova, J. Catal., 36 (3) (1975) 371. 4 P. B. Venuto and P. S. Landis, Adv. Catal., 18 (1968) 259. 5 Ho Shi Tuang, B. V. Pomanovski and K. V. Topchieva, Dokl. Akad. Nauk SSSR, 168 (5) (1966) 1114. 6 M. M. Selim and A. I. Kukina, Vestn. Mosk. Univ., Khim., 5 (1972) 586.
442 7 8 9 10
11 12 13 14 15 16 17
A Report on Molecular Sieve Catalysts, Union Carbide Corporation, 1964. J. W. Ward, J. Catal., 11 (1968)259. L. I. Piguzova and A. S. Vitukhina, Khim. Tekhnol. Topl. Masel, 6 (1963) 17. M. M. Selim, G. A. E1-Shobaky and E. M. Ezzo, J. Res. Inst. Catal., Hokkaido Univ., 26 (2) (1979), in the press. A. B. Littlewood, C. S. Phillips and D. T. Price, J. Am . Chem. Soc., 77 (1955) 1480. M. M. Selim, G. A. E1-Shobaky and E. M. Ezzo, Surf. Technol., 9 (4) (1979) 279. D. E. Eberly, Trans. Farad. Soc., 57 (1961) 1169. P. E. Eberly, J. Phys. Chem., 66 (1962) 812. M. I. Yanovski and G. A. Gaziev, Probl. Kinet. Katal. (1968) 277. M. M. Selim, S. P. Habuda and Z. V. Gryaznova, Surf. Technol., 7 (3) (1978) 195. A.M. Youssef, M. M. Selim and Th. E1-Nabarawy, Surf. Technol., 8 (1) (1979) 67.
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© Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
T H E I N F L U E N C E OF WATER T R E A T M E N T ON T H E R E L A T I O N BETWEEN THE A D SO R PT I V E AND C A T A L Y T IC P R O P E R T I E S OF ScY Z E O L I T E
M. M. SELIM and G. A. EL-SHOBAKY Laboratory of Surface Chemistry and Catalysis, National Research Centre, Dokki, Cairo (Egypt)
E. M. EZZO Chemistry Department, Faculty of Women, Ain Shams University, Cairo (Egypt)
(Received April 23, 1979)
Summary Th e high t e m p e r a t u r e adsorption of c u m e n e as well as its cracking on ScY zeolite were investigated using a pulse chromatographic technique. 12.3% o f sodium ions were substituted by scandium ions. T he effect of water on b o th the adsorptive and catalytic properties was investigated. T r e a t m e n t o f 20 mg o f catalyst with a small a m o u n t of water (10 -2 ml) was f o u n d to decrease the catalytic activity considerably. The degree of deactivation was d e p e n d e n t on bot h the t e m p e r a t u r e of t r e a t m e n t with water and the m o d e o f activation of each catalyst sample. In contrast, neither the heat o f adsorption n o r the r e t e n t i o n volume of c u m e n e was f o u n d to vary appreciably with water t r e a t m e n t . These results m a y p o i n t to the absence of a direct relation between adsorption and catalysis in this case.
1. I n t r o d u c t i o n Zeolites are c o m m o n l y used in several catalytic reactions. Their activity and selectivity can be altered by a variety of m e t h o d s which depend generally on the silica:alumina ratio [1 - 3 ] , substitution of t he sodium ions by o t h e r cations [1 - 6] and the t e m p e r a t u r e and m o d e o f activation [4, 7, 8]. Cracking o f h y d r o c a r b o n s has been studied by several authors on zeolites in which the sodium was substituted with different cations such as Ca 2÷, Ni 2÷ and Sc a+ [1 - 9 ] . Th e catalytic activity in cum e ne cracking was f o u n d to increase on increasing the e x t e n t of substitution o f sodium ions with foreign ions
[6, 9]. In the case o f scandium zeolite it has been shown by one o f the authors [6] t h a t the catalytic activity o f ScY zeolite in cum ene cracking is proportional to the e x t e n t o f substitution, reaching m a x i m u m activity when a b o u t 66% o f the sodium ions are substituted by scandium ions. A ny f u r t h e r
436 increase in scandium c o n t e n t is accompanied by a marked decrease in the zeolite activity. In a previous investigation [ 10] it has been shown that the substitution of about 65% of the sodium by a m m o n i u m ions produces an active catalyst in cumene cracking that has the same crystal structure as the original NaY zeolite. Moreover water treatment of decationated Y zeolite greatly decreases its catalytic activity owing to degradation of the crystallinity. In the present investigation a small proportion of the sodium ions of NaY zeolite were substituted by scandium ions. The effect of water treatment on the catalytic and adsorptive properties was studied using a pulse microcatalytic technique.
2. Experimental
2.1. Catalysts Linde Molecular Sieve Catalyst Base SK 40 (a molecular sieve type Y) was provided by Union Carbide Corporation, U.S.A. A typical chemical composition of the anhydrous zeolite is SiO2 63.5 wt.%, A1203 23.5 wt.%, Na20 13 wt.% and the silica:alumina ratio is 4.6. The average particle size is 5 pm and the ultimate crystal size 0.5 - 1.5 gm. The exchange of the sodium ions in the starting NaY zeolite by scandium was carried out by shaking 5 g of NaY zeolite powder with 60 ml 0.1 N scandium chloride solution at room temperature for 20 min. The sodium c o n t e n t in the resulting sample was determined by flame p h o t o m e t r y . A 20 mg sample was used in each catalytic experiment; in the case of adsorption 100 mg zeolite was mixed with 3 ml inert quartz fragments (2 mm) and was charged in the reactor. All catalyst samples were activated by heating for 2 h with dry air at 450 °C and then for 1 h in a helium stream at the same temperature. 2.2. Apparatus and techniques 2.2.1. Cracking o f cumene The catalytic measurements were carried out in a pulse micro-catalytic system. The reactant under investigation (cumene) was introduced in microquantities (1 × 10 -3 ml = 0.718 × 10 -5 mol), with the aid of a microsyringe and as a pulse, into a microreactor containing a small quantity of the catalyst (20 mg). The reaction products were transferred directly by an inert gas carrier (helium) at a flow rate of 50 ml min -1 to a gas-liquid chromatographic column for their separation and determination. 2.2.2. High temperature adsorption of cumene The adsorption of cumene was studied at various temperatures between 200 and 400 °C using the gas chromatographic method. The apparatus for cumene cracking was used for the adsorption study with the chromatographic column disconnected. The reactor was used as the chromatographic column, which was filled with 100 mg zeolite mixed with 3 cm 3 inert packing in the
437
form of 2 mm quartz fragments. The adsorbate (cumene) was introduced by microsyringe in 1 X 1 0 - 3 ml doses in a 50 ml min -1 flow of helium. The retention times of cumene were corrected with respect to the retention time of a non-adsorbable gas. The retention volumes VR were calculated from the corresponding retention times using an equation given elsewhere [ 11, 12]. The heats of adsorption of cumene of the samples were determined by plotting In VR versus 1/T. After the first adsorption run had been performed with cumene the zeolite sample was cooled to room temperature in a flow of air, water was injected in 5 pulses each of 1 X 10 -3 ml (5 X 10 -3 ml) and then the solid was dehydrated at 450 °C for 1.5 h in an air current and then in a helium current for 0.5 h. This process of h y d r a t i o n - d e h y d r a t i o n was repeated after each adsorption run.
2.2.3. X-ray investigation The X-ray diffraction patterns of all the zeolite samples were investigated at room temperature using a Rich Seifert X-ray diffractometer and Cu Ks radiation. 3. Results
3.1. High temperature adsorption of cumene The pulse chromatographic technique was f o u n d to be a more practically useful m e t h o d for studying adsorption at high temperature than the conventional static m e t h o d . In the static m e t h o d the long contact time involved m a y lead to decomposition of the adsorbates [13 - 15]. In the present investigation this technique was used to follow up the effect of water treatm e n t on the adsorptive properties of scandium zeolite. The data on retention volumes at 320 °C and the heat of adsorption as functions of water injections are given in Table 1. TABLE 1 T h e e f f e c t o f successive a d d i t i o n s o f w a t e r o n t h e r e t e n t i o n v o l u m e a n d h e a t of a d s o r p t i o n o f c u m e n e o n s c a n d i u m zeolite No. o f H 2 0 i n j e c t i o n s
V R at 3 2 0 °C (ml g - 1 )
Q (kcal mo1-1 )
0 1 2 3 4 5 6 7 8 9
417 398 398 355 346 339 316 288 288 251
20.5 20.1 20 19.7 19.8 19.9 20 20 20.6 20
438 Table 1 shows t h a t successive additions of water to scandium zeolite lead to a slight decrease in r e t e n t i o n volume f rom 417 to 251 ml g-1 (at 320 °C). This decrease was obtained after 9 additions of water each of 5 X 10 -3 ml. In contrast, the heat of adsorption of cumene remains almost c o n s tan t ( a b o u t 20 kcal mo1-1); preliminary experiments showed that the heat o f adsorption of cum e ne on NaY zeolite was 20 kcal mo1-1 . F r o m the results in Table 1 and in Fig. 1 it can be seen t hat the addition o f water to the zeolite containing 87.7% sodium ions slightly affects its adsorptive properties in cum e ne adsorption. This slight modification in the adsorptive properties of the zeolite may have an influence on its catalytic activity.
3.2. Cracking o f cumene 3.2.1. The influence of water treatment on the catalytic cracking o f cumene in contact with Sc Y zeolite It is accepted t hat the c u m e n e cracking reaction is a model reaction, and it is a convenient measure for evaluating the catalytic activity of different catalysts in cracking reactions. Most authors have f o u n d t hat cumene cracking proceeds mainly via a c a r boni um ion mechanism. Moreover, the functional h y d r o x y l groups of the catalyst m a y play a decisive role in its activity. T h e y m a y be varied by different m e t h o d s such as changing the silica:alumina ratio or treating the catalyst with water. Thus it is of interest to study the e f f e c t of water on the catalytic activity of ScY zeolite in cumene cracking. 3.2.1.1. The effect o f successive additions of water to the catalyst in a helium stream without regeneration. The results o f the cum ene cracking in c o n t a c t with scandium zeolite and the effect of water on its catalytic activity are graphically represented by Fig. 2, curve a. T he reaction of cum ene cracking was carried o u t at a c o n s t a n t t e m p e r a t u r e of 320 °C with 1 X 10 -3 ml c umen e in a cons t a nt flow of helium. After separation and det ect i on o f the reaction p r oduct s 2 X 10 -3 ml of water was injected. A b o u t 10 successive injections of water into the reactor at 320 °C were carried out. Th e percentage conversion was calculated in each case. F r o m Fig. 2, curve a, it can be seen t hat the percentage conversion decreases gradually fr o m 63% to a co n s t a nt value of 35% after a b o u t 9 - 10 pulses. F u r t h e r addition o f water to the catalyst was f o u n d to have no effect on the conversion. 3.2.1.2. The influence o f regeneration-water addition cycles at 450 °C. In this set o f experiments the scandium zeolite was used catalytically at 320 °C. The catalyst was then regenerated at 450 °C for 30 min. A pulse of 2 X 10 3 ml o f water was injected at 450 °C followed by regeneration. The reaction, water injections and regenerations were carried out in a stream of helium of flow rate 50 ml min 1. The results obtained are graphically represented in Fig. 2, curve b. It can be seen from this figure t hat only 4 injections o f water lead to a sudden decrease in the percentage conversion from 63 to 20%. F u r t h e r t r e a t m e n t with water has no influence on the conversion.
439
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~,/~//2
80
t r e a t m e ~ z / ~ 2.8
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2.0
°®
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20 1.6 I 1,6
I 1.7
i
1.18 118 IO00/T
i
1 2
i
i
i
i
i
i
,
,
J
3 4 5 6 7 8 9 10 No water pulses
Fig. 1. P l o t s of log VIi vs. 1 / T f o r fresh a n d w a t e r - t r e a t e d catalysts. Fig. 2. T h e p e r c e n t a g e c o n v e r s i o n as a f u n c t i o n o f n u m b e r o f w a t e r pulses for c a t a l y s t s t r e a t e d w i t h w a t e r : curve a, c a t a l y s t t r e a t e d w i t h w a t e r at r e a c t i o n t e m p e r a t u r e 3 2 0 °C w i t h o u t r e g e n e r a t i o n ; curve b, c a t a l y s t t r e a t e d w i t h w a t e r a t 4 5 0 °C a n d t h e n h e a t e d in a h e l i u m s t r e a m at t h e s a m e t e m p e r a t u r e ; curve c, c a t a l y s t t r e a t e d w i t h w a t e r at r o o m t e m p e r a t u r e a n d t h e n h e a t e d at 4 5 0 °C in a c u r r e n t o f d r y air.
3.2.1.3. The effect o f regeneration by air and water treatment on the percentage conversion o f the catalyst. For the fresh catalyst the activity was tested at 320 °C with cumene in a helium flow. Water additions were carried out at room temperature in a stream of air. The zeolite was subjected to regeneration for 30 min in air at 450 °C before and after each water injection. The results obtained are illustrated in Fig. 2, curve c, which shows that only 4 pulses of water each of 2 X 10 -3 ml caused a marked decrease in percentage conversion from 63 to 7.5%. This low conversion remains almost constant on further treatment with water.
4. Discussion It has been observed in a previous study [16] that the successive h y d r a t i o n - d e h y d r a t i o n cycles of ScY zeolite increase considerably the number of h y d r o x y l groups characterized by a narrow line in nuclear magnetic resonance (NMR) spectra. It is therefore expected that this increase in the concentration of h y d r o x y l groups will be accompanied by an increase in the catalytic activity of zeolite in reactions proceeding via a carbonium ion mechanism [1, 7]. From the results presented above it is clear t h a t the addition of water to the zeolite decreases its catalytic activity in cumene cracking. The degree of deactivation of the catalyst depends on the conditions of water treatment. The m a x i m u m effect of water on the catalytic activity of scandium zeolite was observed when the catalyst was treated with water at room temperature and heated in air at 450 °C before and after water addition. The effect of water was m i n i m u m when the catalyst was treated with water at the reaction temperature in a flow of helium w i t h o u t regeneration. An intermediate effect was observed when water was injected at 450 °C in a flow of helium.
440
In the first case, during the thermal treatment of the catalyst, oxygen oxidizes severely the condensation products formed on the catalyst surface, converting them into gaseous products and leaving a clean surface accessible for further water attack; it thus enhances the process of deactivation by water molecules. The accumulation of condensation products on the surface at the reaction temperature (320 °C) forms a certain protective layer, thus preventing further decrease in activity by the addition of water (Fig. 2, curve a). The activation of the catalyst by helium at 450 °C is not sufficient to remove all the condensation products formed on the zeolite surface, unlike the case of activation by air. In a previous paper [ 12] we have shown that the substitution of 65% of the sodium ions of Y zeolite with a m m o n i u m ions greatly modified the adsorption characteristics of cumene, since the zeolite was treated with water at room temperature. The heat of adsorption of cumene was found to decrease from 21 to 4.3 kcal mo1-1 ; the retention volume was also decreased considerably. In the present investigation the substitution of 12.3% of the sodium ions of the same zeolite sample by scandium, however, was n o t accompanied by important changes in the high temperature adsorption of cumene. When the catalyst is treated with water under the same conditions, the heat of adsorption remains almost constant while the retention volume decreases slightly. It appears that the adsorption of cumene depends mainly on the sodium c o n t e n t of the zeolite. Substitution of a small fraction (12.3%) of sodium ions with scandium ions might be expected to have a small influence on cumene adsorption. In fact it was shown that NaY zeolite adsorbs cumene with a heat of adsorption 20 kcal mo1-1 ; almost the same value was obtained for ScY zeolite. Increasing the extent of substitution of sodium by scandium ions may lead to a decrease in both h e a t of adsorption and retention volume of cumene with a simultaneous collapse of structure. A lower degree of substitution restricts us since it ensures that the crystallinity of the ionexchanged zeolite remains almost unchanged [ 17] ; any modification in the crystallinity of the zeolite will be due to water treatment. It is known that the catalytic activity of NaY zeolite in cumene cracking is undetectable; however, substitution of some of its sodium ions by multivalent cations such as scandium ions creates an active catalyst for this reaction. The catalytic activity is proportional to the degree of substitution of sodium ions by multivalent cations so long as the crystal structure of scandium is conserved. This structure can also be modified in a controlled manner by treating ScY zeolite with different amounts of water at room temperature or at elevated temperatures. NaY zeolite resists any structural collapse by such treatment, as confirmed experimentally by X-ray and NMR studies [16]. In contrast, the fraction of zeolite which contains scandium is capable of undergoing structural modification. The X-ray investigation, however, reveals t h a t the t r e a t m e n t of ScY zeolite by water does n o t produce any important modification in its crystaUinity (Fig. 3). These results can be attributed to the low scandium c o n t e n t and the insensitivity of the X-ray technique used to the slight modification in the crystallinity of the zeolite.
441
40
30
S
20
10
Fig. 3. X-ray diffraction patterns of NaY and ScY zeolites: ScY-(1), untreated ScY zeolite; ScY-(2), catalyst treated with water at room temperature in air and then used in cumene cracking at 320 °C; ScY-(3), catalyst treated with water-cumene at 320 °C in a helium stream.
It is to be concluded that substitution of 12.3% of sodium ions by scandium ions greatly increases the catalytic activity of NaY zeolite in cumene cracking. The activity of the ScY zeolite prepared was found to decrease on water treatment. The e x t e n t of deactivation was greatly influenced by the experimental conditions, e.g. the temperature of treatment with water, the temperature of activation of the catalyst and the presence of air or an inert carrier gas such as helium in contact with the catalyst during the activation process. Water t r e a t m e n t of ScY zeolite under the experimental conditions employed in this investigation was f o u n d to exert only a small influence on the high temperature adsorption of cumene. These results may point to the absence of a direct relation between adsorption and catalysis in this case.
References 1 K. V. Topchieva, B. V. Romanovski, L. I. Piguzova, Ho Shi Tuang and E. V. Bezre, Proc. 4th Int. Congr. on Catalysis, Moscow, Vol. 2, 1968, p. 135. 2 K. Tsutsumi and H. Takahashi, J. Catal., 24 (1) (1972) 1. 3 B. V. Patzelova, V. Aybl and C. Tavaruzkova, J. Catal., 36 (3) (1975) 371. 4 P. B. Venuto and P. S. Landis, Adv. Catal., 18 (1968) 259. 5 Ho Shi Tuang, B. V. Pomanovski and K. V. Topchieva, Dokl. Akad. Nauk SSSR, 168 (5) (1966) 1114. 6 M. M. Selim and A. I. Kukina, Vestn. Mosk. Univ., Khim., 5 (1972) 586.
442 7 8 9 10
11 12 13 14 15 16 17
A Report on Molecular Sieve Catalysts, Union Carbide Corporation, 1964. J. W. Ward, J. Catal., 11 (1968)259. L. I. Piguzova and A. S. Vitukhina, Khim. Tekhnol. Topl. Masel, 6 (1963) 17. M. M. Selim, G. A. E1-Shobaky and E. M. Ezzo, J. Res. Inst. Catal., Hokkaido Univ., 26 (2) (1979), in the press. A. B. Littlewood, C. S. Phillips and D. T. Price, J. Am . Chem. Soc., 77 (1955) 1480. M. M. Selim, G. A. E1-Shobaky and E. M. Ezzo, Surf. Technol., 9 (4) (1979) 279. D. E. Eberly, Trans. Farad. Soc., 57 (1961) 1169. P. E. Eberly, J. Phys. Chem., 66 (1962) 812. M. I. Yanovski and G. A. Gaziev, Probl. Kinet. Katal. (1968) 277. M. M. Selim, S. P. Habuda and Z. V. Gryaznova, Surf. Technol., 7 (3) (1978) 195. A.M. Youssef, M. M. Selim and Th. E1-Nabarawy, Surf. Technol., 8 (1) (1979) 67.