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High temperature adsorption of cumene on decationated zeolite as influenced by the adsorbent crystallinity
Surface Technology, 9 (1979) 279 - 284 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
279
HIGH T E M P E R A T U R E A D SO R PT I O N OF CUMENE ON D E C A T I O N A T E D Z E O L I T E AS I N F L U E N C E D BY THE A D S O R B E N T C R Y S T A L L I N I T Y
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 Girls, Ain Shams University, Cairo (Egypt) (Received November 9, 1978)
Summary The adsorption o f c u m e n e at 200 - 400 °C on decationated zeolite was carried o u t using a dynam i c micropulse technique. T he crystallinity o f the zeolite sample was modified by treating it with water at r o o m t e m p e r a t u r e and then heating it in a cur r ent of air at 450 °C. The degree o f crystallinity was f o u n d to decrease considerably when the a m o u n t o f water brought into c o n t a c t with the zeolite sample reached 2.5 X 10 -2 mol g-1. The ret ent i on volumes o f cu me ne at 200 - 400 °C on zeolite treated with different amounts o f water were calculated. The data obtained permitted us to c o m p u t e the initial heat o f adsorption o f cumene. T he heat of adsorption o f c u m e n e was f o u n d to decrease gradually when the n u m b e r o f h y d r a t i o n - d e h y d r a t i o n cycles was increased. It was 21 kcal mo1-1 for a fresh sample and 4.3 kcal mo1-1 for the water-treated sample. These results are explained in terms o f the collapse o f the crystal structure o f zeolite on being treated with water.
1. Introduction The catalytic and adsorptive properties o f zeolites are greatly influenced by the c o n c e n t r a t i o n o f their structural h y d r o x y l groups. These OH groups seem to play a decisive role in the adsorption and interactions occurring on the zeolite surface. It has been shown [1, 2] t hat the a m o u n t of base needed fo r complete poisoning o f the dealkylation activity corresponds to the n u m b e r o f OH groups. However, Boreskova et al. [3] f o u n d t hat for a c o m p l e t e blocking o f active sites an a m o u n t o f quinoline was needed t hat was several times higher than the OH concentration. Generally, the concentration and the stability o f these functional groups depend on the chemical composition o f the zeolite, the calcination t e m p e r a t u r e and water t reat m ent . NaHX zeolite was f o u n d to be thermally stable if the Na:Al atomic ratio was
280 at least 0.6 [ 4 ] . The structural stability of this t y p e of zeolite to recalcination after water sorption was also f o u n d to be a funct i on of the Na:A1 ratio. The c o n t a c t o f water with zeolite at high temperatures may cause hydrolysis and extraction o f lattice aluminium ions which appear at cationic positions. At the same time outgassing at higher t e m p e r a t u r e may produce a structural rearrangement o f faujasite, leading to recrystallization involving silica migration f r o m regions with structural breakdown [ 5 - 7 ]. In this work, the effect of water on the adsorptive p r o p e r t y of HY zeolite at high tem per a t ur e was studied using a dynamic micropulse t echni que; the modification of the crystallinity resulting from water t r e a t m e n t was examined using X-ray diffraction.
2. Experimental
2.1. A d s o r b e n t NaY zeolite SK-40 was supplied by the Lind Company; its chemical c o m p o s itio n is given in Table 1. Th e decationated sample was prepared by exchanging the sodium ions with a m m o n i u m ions from a m m o n i u m chloride solution for a period of 20 rain at r o o m t e m p e r a t u r e . The exchanged zeolite was then washed w i t h distilled water until chloride ions were no longer present and was dried at 100 °C. Flame p h o t o m e t r i c analysis revealed that 65% of the sodium ions were substituted by a m m o n i u m ions. The zeolite was activated by heating at 450 °C in a current of dry air for 3 h and t he n in a current of helium for 1 h before adsorption measurements were c o n d u c t e d . 2.2. A d s o r p t i o n measurements Cumene was injected into the reactor in pulses of 1 × 10 -3 ml at a flow o f 50 ml min -1 helium. This rate was f ound to be convenient to avoid any apparent d e c o m p o s i t i o n of cumene. The system consisted of an electrically heated silica tu b e r eact or of length 30 cm and internal diameter 0.5 cm. The t e m p e r a t u r e was regulated and controlled within + 0.5 °C by means of a c h r o m e l - a l u m e l t h e r m o c o u p l e and p o t e n t i o m e t e r . The reactor contained 100 mg o f the zeolite sample diluted with 5 ml of quartz fragments of size 2-3mm. The high t e m p e r a t u r e adsorption of c umene at 200 - 400 °C on zeolite was measured. The corrected r et ent i on volumes were calculated from the corresponding r e t e n t i o n times using the equation [8, 9]
vR-
teEm
T~
3
(Pi/Po) 2 -- 1
W
Tm
2
(Pi/Po) 3 - 1
where Vn is the cor r eet ed r e t e n t i o n volume per gram zeolite, tc is the corrected r e t e n t i o n time in seconds, Fr, is the flow rate at r o o m t e m p e r a t u r e in millilitres per second, W is the weight of the sample in grams, Tin, Tc are the
281
TABLE
1
Composition
of NaY zeolite
SK-40
(wt.%) 63.5 23.5 13.0 0.05 0.05 4.6
SiO2 M2O3
NazO clFSiOz/A102
room and column temperature in kelvins and Pi,Po are inlet and outlet pressures in torr. The initial heats of adsorption were then determined according to the equation [lo] In v, = --+
+c
where Q is the initial heat of adsorption in kilocalories per mole, R is the universal gas constant expressed in calories and C is a constant which is a function of the entropy of adsorption, dimension of the column and the carrier gas flow rate. By plotting In V, uersus l/T a straight line is obtained whose slope gives Q/R.
3. Results and discussion The adsorptive properties of solids at high temperatures are conveniently studied by allowing a pulse of adsorbate to be transported through a packed column of adsorbent by an inert carrier gas stream. This method is more convenient than the static one since the long contact time involved may lead to decomposition of the adsorbates [ 11 - 131. In this investigation this technique was employed to monitor the effect of successive additions of water on the adsorptive properties of decationated HY zeolite. The results obtained are presented graphically in Fig. 1. The data for retention volumes at 250 and 320 “C and the heat of adsorption as a function of the number of water injections are given in Table 2. It can be seen from these results that both the retention volumes and the heats of adsorption decrease gradually on increasing the number of hydration-dehydration cycles. Five pulses of water (25 X 10e3 mol g-l) caused a marked decrease in the heat of adsorption from 21 to 4.4 kcal mol-I. The last value corresponds to physisorption rather than chemisorption. The retention volumes also decrease; however, the rate of decrease with increase in the number of water injections is more pronounced at higher temperatures.
282
o
6//o/ 30
a
o
o
o
22 1.8
d°~
1!6
11.7
I
I
I
I
I
I
1
1.8
1.6
1.7
1.8
1.6
1.7
1.8
3.0 2.6
1:8
d
1.6 '
e
17 ! 1,6 ,1.'8, 1.7
'
1.8
f
"
'
1.9
1.'7
1.8 '
o
1.9 2.0 ' ' I000/I
Fig. 1. Log V R vs. l I T for zeolite s a m p l e s : curve a, fresh s a m p l e ; c u r v e s b - f, samples t r e a t e d w i t h w a t e r . Doses (X 10 - 3 m o l g-1 ): curve b, 5 ; curve c, 10 ; curve d, 15 ; curve e, 20 ; curve f, 25.
It is worth mentioning that the substitution of sodium ions by hydrogen does not influence the heat of adsorption of cumene, as indicated from the almost similar values of 19 - 21 kcal tool -1 [14]. It is difficult to believe that the drastic decrease in the heat of adsorption of cumene produced by water treatment of the adsorbent is a result of blocking of the most active sites by H20(ads.), simply because the solid was preheated at a high temperature of 450 °C before and after addition of water. Such deterioration of the adsorption energy of cumene on the watertreated zeolite can thus be explained in terms of an important modification of its crystal structure. This conclusion was verified by the X-ray investigation of the water-treated zeolite. Figure 2 represents the X-ray diffraction pattern TABLE 2 T h e e f f e c t o f w a t e r t r e a t m e n t o n t h e r e t e n t i o n v o l u m e s and h e a t o f a d s o r p t i o n o f c u m e n e o n zeolite Number of water injections
0 1 2 3 4 5
V R (ml g-1 )
Q (keal tool - 1 )
250°C
320°C
2260 2178 2200 1955 1818 1737
350 230 175 125 80 55
21.0 17.3 14.6 11.3 8.1 4.3
283
4'0
3'o
~o
i'o
Fig. 2. X-ray diffraction patterns for NaY, HY and water-treated HY zeolites.
of NaY and HY zeolites as influenced by water treatment. It can be seen that the substitution of 65% of Na ÷ ions by NH~ ions has no influence on the degree of crystallinity of the zeolite sample. In contrast, the addition of water (25 × 10 -3 mol g-l) to HY zeolite at room temperature and subsequent heating at 450 °C in a current of air produced an almost amorphous solid. The complete disappearance of crystallinity of the zeolite on being treated with water indicates the collapse of its crystal structure, leading to the formation of free silica and alumina. This hypothesis was confirmed by determining the heat o f adsorption of cumene on both silica and alumina. The values 4.0 and 2.5 kcal mo1-1 were calculated for silica and alumina respectively. Taking into consideration that the silica:alumina ratio in Y zeolite is 4.6 (cf. Table 1), the value of 4.3 kcal tool -1 obtained for watertreated zeolite corresponds to the heat of adsorption of cumene on a silicaalumina mixture, thus indicating the decomposition of zeolite into similar components. Such modification of the crystal structure of zeolite, although leading to an increase in the concentration of the h y d r o x y l groups [15], greatly decreased its catalytic activity in the cracking reaction [16]. From study of the high temperature adsorption of cumene and the X-ray investigation it is concluded t h a t treating HY zeolite with water leads to the decomposition of the zeolite into the amorphous phase. This phase is characterized by a high concentration of h y d r o x y l groups and a reduced adsorptive capacity; in other words, the adsorptive capacity of zeolite cannot be determined simply by the concentration of its h y d r o x y l groups.
284 References 1 J. Turkevich, F. Nozaki and D. N. Stamires, Proc. 3rd Int. Congr. on Catalysis, Amsterdam, 1964, North-Holland, Amsterdam, 1965, p. 586. 2 J. Turkevich and Y. Ono, Adv. Chem. Ser., 102 (1971) 315. 3 E. G. Boreskova, K. V. Topchieva and L. I. Piguzova, Kinet. Katal., 5 (1964) 792. 4 G. H. Kfihl and A. E. Schweizen, J. Catal., 38 (1975) 469. 5 J. Scherzer and J. L. Bass, J. Catal., 28 (1973) 101. 6 J. Scherzer and J. L. Bass, Abstr. 2nd North A m. Meet. Catal. Soc., Houston, 1971, p. 44. 7 A. K. Maher, F. D. Hunter and J. Scherzer, Adv. Chem. Ser., 101 (1971) 266. 8 A. B. Littlewood, G. S. Phyllips and D. T. Price, J. Am . Chem. Soc., 77 (1955) 1480. 9 H. Matsumoto, H. Futami, F. Koto and Y. Morito, Bull. Chem. Soc. Jpn, 44 (1971) 317. 10 S. A. Greene and H. Pust, J. Phys. Chem., 62 (1958) 55. 11 P. E. Eberly, Trans. Faraday Soc., 57 (1961) 1169. 12 P. E. Eberly, J. Phys. Chem., 66 (1962) 812. 13 M. I. Yanovskii and G. A. Gaziev, Probl. Kinet. Katal., 5 (1968) 277. 14 M.M. Selim and A. I. Kukina, Vestn. Mosk. Univ. Set. Chem., 5 (1972) 586. 15 M. M. Selim, P. S. Habuda and Z. V. Gryaznova, Surf. Technol., 7 (3) (1978) 195. 16 G. A. E1-Shobaky, M. M. Selim and E. M. Ezzo, J. Res. Inst. Catal., Hokkaido Univ., 26 (3) (1978) 163 - 170.
279
HIGH T E M P E R A T U R E A D SO R PT I O N OF CUMENE ON D E C A T I O N A T E D Z E O L I T E AS I N F L U E N C E D BY THE A D S O R B E N T C R Y S T A L L I N I T Y
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 Girls, Ain Shams University, Cairo (Egypt) (Received November 9, 1978)
Summary The adsorption o f c u m e n e at 200 - 400 °C on decationated zeolite was carried o u t using a dynam i c micropulse technique. T he crystallinity o f the zeolite sample was modified by treating it with water at r o o m t e m p e r a t u r e and then heating it in a cur r ent of air at 450 °C. The degree o f crystallinity was f o u n d to decrease considerably when the a m o u n t o f water brought into c o n t a c t with the zeolite sample reached 2.5 X 10 -2 mol g-1. The ret ent i on volumes o f cu me ne at 200 - 400 °C on zeolite treated with different amounts o f water were calculated. The data obtained permitted us to c o m p u t e the initial heat o f adsorption o f cumene. T he heat of adsorption o f c u m e n e was f o u n d to decrease gradually when the n u m b e r o f h y d r a t i o n - d e h y d r a t i o n cycles was increased. It was 21 kcal mo1-1 for a fresh sample and 4.3 kcal mo1-1 for the water-treated sample. These results are explained in terms o f the collapse o f the crystal structure o f zeolite on being treated with water.
1. Introduction The catalytic and adsorptive properties o f zeolites are greatly influenced by the c o n c e n t r a t i o n o f their structural h y d r o x y l groups. These OH groups seem to play a decisive role in the adsorption and interactions occurring on the zeolite surface. It has been shown [1, 2] t hat the a m o u n t of base needed fo r complete poisoning o f the dealkylation activity corresponds to the n u m b e r o f OH groups. However, Boreskova et al. [3] f o u n d t hat for a c o m p l e t e blocking o f active sites an a m o u n t o f quinoline was needed t hat was several times higher than the OH concentration. Generally, the concentration and the stability o f these functional groups depend on the chemical composition o f the zeolite, the calcination t e m p e r a t u r e and water t reat m ent . NaHX zeolite was f o u n d to be thermally stable if the Na:Al atomic ratio was
280 at least 0.6 [ 4 ] . The structural stability of this t y p e of zeolite to recalcination after water sorption was also f o u n d to be a funct i on of the Na:A1 ratio. The c o n t a c t o f water with zeolite at high temperatures may cause hydrolysis and extraction o f lattice aluminium ions which appear at cationic positions. At the same time outgassing at higher t e m p e r a t u r e may produce a structural rearrangement o f faujasite, leading to recrystallization involving silica migration f r o m regions with structural breakdown [ 5 - 7 ]. In this work, the effect of water on the adsorptive p r o p e r t y of HY zeolite at high tem per a t ur e was studied using a dynamic micropulse t echni que; the modification of the crystallinity resulting from water t r e a t m e n t was examined using X-ray diffraction.
2. Experimental
2.1. A d s o r b e n t NaY zeolite SK-40 was supplied by the Lind Company; its chemical c o m p o s itio n is given in Table 1. Th e decationated sample was prepared by exchanging the sodium ions with a m m o n i u m ions from a m m o n i u m chloride solution for a period of 20 rain at r o o m t e m p e r a t u r e . The exchanged zeolite was then washed w i t h distilled water until chloride ions were no longer present and was dried at 100 °C. Flame p h o t o m e t r i c analysis revealed that 65% of the sodium ions were substituted by a m m o n i u m ions. The zeolite was activated by heating at 450 °C in a current of dry air for 3 h and t he n in a current of helium for 1 h before adsorption measurements were c o n d u c t e d . 2.2. A d s o r p t i o n measurements Cumene was injected into the reactor in pulses of 1 × 10 -3 ml at a flow o f 50 ml min -1 helium. This rate was f ound to be convenient to avoid any apparent d e c o m p o s i t i o n of cumene. The system consisted of an electrically heated silica tu b e r eact or of length 30 cm and internal diameter 0.5 cm. The t e m p e r a t u r e was regulated and controlled within + 0.5 °C by means of a c h r o m e l - a l u m e l t h e r m o c o u p l e and p o t e n t i o m e t e r . The reactor contained 100 mg o f the zeolite sample diluted with 5 ml of quartz fragments of size 2-3mm. The high t e m p e r a t u r e adsorption of c umene at 200 - 400 °C on zeolite was measured. The corrected r et ent i on volumes were calculated from the corresponding r e t e n t i o n times using the equation [8, 9]
vR-
teEm
T~
3
(Pi/Po) 2 -- 1
W
Tm
2
(Pi/Po) 3 - 1
where Vn is the cor r eet ed r e t e n t i o n volume per gram zeolite, tc is the corrected r e t e n t i o n time in seconds, Fr, is the flow rate at r o o m t e m p e r a t u r e in millilitres per second, W is the weight of the sample in grams, Tin, Tc are the
281
TABLE
1
Composition
of NaY zeolite
SK-40
(wt.%) 63.5 23.5 13.0 0.05 0.05 4.6
SiO2 M2O3
NazO clFSiOz/A102
room and column temperature in kelvins and Pi,Po are inlet and outlet pressures in torr. The initial heats of adsorption were then determined according to the equation [lo] In v, = --+
+c
where Q is the initial heat of adsorption in kilocalories per mole, R is the universal gas constant expressed in calories and C is a constant which is a function of the entropy of adsorption, dimension of the column and the carrier gas flow rate. By plotting In V, uersus l/T a straight line is obtained whose slope gives Q/R.
3. Results and discussion The adsorptive properties of solids at high temperatures are conveniently studied by allowing a pulse of adsorbate to be transported through a packed column of adsorbent by an inert carrier gas stream. This method is more convenient than the static one since the long contact time involved may lead to decomposition of the adsorbates [ 11 - 131. In this investigation this technique was employed to monitor the effect of successive additions of water on the adsorptive properties of decationated HY zeolite. The results obtained are presented graphically in Fig. 1. The data for retention volumes at 250 and 320 “C and the heat of adsorption as a function of the number of water injections are given in Table 2. It can be seen from these results that both the retention volumes and the heats of adsorption decrease gradually on increasing the number of hydration-dehydration cycles. Five pulses of water (25 X 10e3 mol g-l) caused a marked decrease in the heat of adsorption from 21 to 4.4 kcal mol-I. The last value corresponds to physisorption rather than chemisorption. The retention volumes also decrease; however, the rate of decrease with increase in the number of water injections is more pronounced at higher temperatures.
282
o
6//o/ 30
a
o
o
o
22 1.8
d°~
1!6
11.7
I
I
I
I
I
I
1
1.8
1.6
1.7
1.8
1.6
1.7
1.8
3.0 2.6
1:8
d
1.6 '
e
17 ! 1,6 ,1.'8, 1.7
'
1.8
f
"
'
1.9
1.'7
1.8 '
o
1.9 2.0 ' ' I000/I
Fig. 1. Log V R vs. l I T for zeolite s a m p l e s : curve a, fresh s a m p l e ; c u r v e s b - f, samples t r e a t e d w i t h w a t e r . Doses (X 10 - 3 m o l g-1 ): curve b, 5 ; curve c, 10 ; curve d, 15 ; curve e, 20 ; curve f, 25.
It is worth mentioning that the substitution of sodium ions by hydrogen does not influence the heat of adsorption of cumene, as indicated from the almost similar values of 19 - 21 kcal tool -1 [14]. It is difficult to believe that the drastic decrease in the heat of adsorption of cumene produced by water treatment of the adsorbent is a result of blocking of the most active sites by H20(ads.), simply because the solid was preheated at a high temperature of 450 °C before and after addition of water. Such deterioration of the adsorption energy of cumene on the watertreated zeolite can thus be explained in terms of an important modification of its crystal structure. This conclusion was verified by the X-ray investigation of the water-treated zeolite. Figure 2 represents the X-ray diffraction pattern TABLE 2 T h e e f f e c t o f w a t e r t r e a t m e n t o n t h e r e t e n t i o n v o l u m e s and h e a t o f a d s o r p t i o n o f c u m e n e o n zeolite Number of water injections
0 1 2 3 4 5
V R (ml g-1 )
Q (keal tool - 1 )
250°C
320°C
2260 2178 2200 1955 1818 1737
350 230 175 125 80 55
21.0 17.3 14.6 11.3 8.1 4.3
283
4'0
3'o
~o
i'o
Fig. 2. X-ray diffraction patterns for NaY, HY and water-treated HY zeolites.
of NaY and HY zeolites as influenced by water treatment. It can be seen that the substitution of 65% of Na ÷ ions by NH~ ions has no influence on the degree of crystallinity of the zeolite sample. In contrast, the addition of water (25 × 10 -3 mol g-l) to HY zeolite at room temperature and subsequent heating at 450 °C in a current of air produced an almost amorphous solid. The complete disappearance of crystallinity of the zeolite on being treated with water indicates the collapse of its crystal structure, leading to the formation of free silica and alumina. This hypothesis was confirmed by determining the heat o f adsorption of cumene on both silica and alumina. The values 4.0 and 2.5 kcal mo1-1 were calculated for silica and alumina respectively. Taking into consideration that the silica:alumina ratio in Y zeolite is 4.6 (cf. Table 1), the value of 4.3 kcal tool -1 obtained for watertreated zeolite corresponds to the heat of adsorption of cumene on a silicaalumina mixture, thus indicating the decomposition of zeolite into similar components. Such modification of the crystal structure of zeolite, although leading to an increase in the concentration of the h y d r o x y l groups [15], greatly decreased its catalytic activity in the cracking reaction [16]. From study of the high temperature adsorption of cumene and the X-ray investigation it is concluded t h a t treating HY zeolite with water leads to the decomposition of the zeolite into the amorphous phase. This phase is characterized by a high concentration of h y d r o x y l groups and a reduced adsorptive capacity; in other words, the adsorptive capacity of zeolite cannot be determined simply by the concentration of its h y d r o x y l groups.
284 References 1 J. Turkevich, F. Nozaki and D. N. Stamires, Proc. 3rd Int. Congr. on Catalysis, Amsterdam, 1964, North-Holland, Amsterdam, 1965, p. 586. 2 J. Turkevich and Y. Ono, Adv. Chem. Ser., 102 (1971) 315. 3 E. G. Boreskova, K. V. Topchieva and L. I. Piguzova, Kinet. Katal., 5 (1964) 792. 4 G. H. Kfihl and A. E. Schweizen, J. Catal., 38 (1975) 469. 5 J. Scherzer and J. L. Bass, J. Catal., 28 (1973) 101. 6 J. Scherzer and J. L. Bass, Abstr. 2nd North A m. Meet. Catal. Soc., Houston, 1971, p. 44. 7 A. K. Maher, F. D. Hunter and J. Scherzer, Adv. Chem. Ser., 101 (1971) 266. 8 A. B. Littlewood, G. S. Phyllips and D. T. Price, J. Am . Chem. Soc., 77 (1955) 1480. 9 H. Matsumoto, H. Futami, F. Koto and Y. Morito, Bull. Chem. Soc. Jpn, 44 (1971) 317. 10 S. A. Greene and H. Pust, J. Phys. Chem., 62 (1958) 55. 11 P. E. Eberly, Trans. Faraday Soc., 57 (1961) 1169. 12 P. E. Eberly, J. Phys. Chem., 66 (1962) 812. 13 M. I. Yanovskii and G. A. Gaziev, Probl. Kinet. Katal., 5 (1968) 277. 14 M.M. Selim and A. I. Kukina, Vestn. Mosk. Univ. Set. Chem., 5 (1972) 586. 15 M. M. Selim, P. S. Habuda and Z. V. Gryaznova, Surf. Technol., 7 (3) (1978) 195. 16 G. A. E1-Shobaky, M. M. Selim and E. M. Ezzo, J. Res. Inst. Catal., Hokkaido Univ., 26 (3) (1978) 163 - 170.