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Alkylation of benzene with 2-chloropropane on chlorine-treated alumina
Applied
Catalysis, 56 (1989)
73-81
73
Elsevier Science Publishers B.V., Amsterdam -
Printed in The Netherlands
Alkylation of Benzene with 24hloropropane Chlorine-Treated Alumina
on
AKIMI AYAME* and KAZUHIRO IMANISHI Department Muroran,
of Industrial Hokkaido
Chemistry,
Muroran
Institute
of Technology, 27-1 Mizumoto,
050 (Japan)
(Received 27 February 1989, revised manuscript received 13 June 1989)
ABSTRACT The alkylation of benzene with chloropropanes on solid Lewis acid catalysts prepared by the chlorination of y-alumina at 1073 K was carried out at 293-473 K using a fixed-bed flow reactor. Products in the alkylation at 293 K were isopropylbenzene, m- andp-diisopropylbenzenes, 1,3,5tri-isopropylbenzene and 1,2,4,5-tetraisopropylbenzene. No chlorine-substituted benzenes were detected. The conversion of 2chloropropane to the substituted benzenes was about 14 times that on the alumina dehydrated in vacua, and the 1,3,5-triisopropylbenzene produced was about 70% of the liquid products. An increase in benzene-to-chloropropane ratio retarded the lowering of conversion with reaction time and increased the yield of isopropylbenzene. The reactivity of lchloropropane was about one-tenth of that of 2-chloropropane, but the isopropylbenzene content was greater. At the reaction temperature of 333 K, dehydrochlorination of 2-chloropropane began to take place, and above 373 K this was a main reaction. Also, the chlorination temperature dependence of the catalytic activities for alkylation and dehydrochlorination was similar to that for Lewis acidity with a maximum in the range 973 to 1073 K. The alkylation was explained to proceed through a Friedel-Crafts-type intermediate.
INTRODUCTION
It is well known that the development of methods for the preparation of solid Lewis acid catalysts is important as these catalysts are preferable to homogeneous Lewis acids for use in industrial processes [ 1,2]. The authors reported previously that such solid Lewis acids could be prepared by chlorinating a popular and cheap y-alumina above 973 K and that the chlorinated aluminas had Lewis acid sites of Ho < - 14.52, which were enhanced by a small amount of chlorine on its surface, and activated chloropropanes to yield isopropyl and x-ally1 cations [ 3,4]. The solid Lewis acids also activated benzene to yield a n-complex, and selectively isomerized cyclohexane and butane to methylcylopentane and isobutane, respectively, at 303 K [ 51. From these facts, the high temperature-chlorinated aluminas were concluded to be solid Lewis superacid. Thus, since the chlorinated alumina catalyst was a very strong Lewis acid, it
0166.9834/89/$03.50
0 1989 Elsevier Science Publishers B.V.
was presumed that the application to benzene alkylation with olefins and alcohols would be difficult. The present work reports one example of the application of the chlorinated alumina to a catalytic reaction. The gas-phase alkylation of benzene with chloropropanes was carried out at 293-473 K using a conventional fixed-bed flow reactor. EXPERIMENTAL
The alumina used was JRC-ALOin y-form with a surface area of 177 m2 g-l. This alumina was the Japan Reference Catalyst (JRC) supplied by The Catalysis Society of Japan. The bulk composition was A1203 99.7 wt.-%, Fe,O, 0.01 wt.-%, SiO, 0.01 wt.-% and Na,O 0.01 wt.-%. The cylindrical alumina was crushed with a very pure alumina ceramic mortar and pestle, sieved to obtain a loo-150 mesh fraction, this fraction being washed and then dried. The hightemperature chlorination was performed at 873-1173 K in the same way as reported elsewhere and the ALO- showed similar chlorination behaviour to JRC-ALO[4,5].The flow reactor used consisted of 0.4 cm I.D. Pyrex glass tubing 20 cm long, equipped with a 5 ml vertical cold trap. The catalyst bed (0.075 g) on a glass-wool support was 1.3 cm long. The reactant mixture of benzene and 2-chloropropane was prepared by passing two dry nitrogen streams through individual benzene and 2-chloropropane evaporators and mixing the streams in a mixer. The flow-rate of 2-chloropropane was 1 mmol h-l and the benzene-to-2-chloropropane ratio was controlled in the range 0.9 to 4.1. Prior to use, the benzene and chloropropanes were purified on Molecular Sieve 4A by vacuum distillation three times. The chlorinated aluminas were transferred into the reactor in vacua and/or in dry nitrogen. The products were analysed by gas chromatography and also identified with an IR spectrometer. RESULTS
AND DISCUSSION
Reaction temperature The typical results obtained on the ALO- chlorinated at 1073 K are shown in Table 1. At the reaction temperature 293 K, the alkylation of benzene proceeded very fast to result in a maximum yield of substituted benzenes. Products were isopropylbenzene (IPB ) , rneta- and para-diisopropylbenzene (m-di-IPB ) and p-di-IPB), 1,3,5-triisopropylbenzene (tri-IPB) and 1,2,4,5-tetraisopropylbenzene (tetra-IPB ). No chlorine-substituted benzenes were formed. All of the products were the same as those obtained by the Friedel-Crafts alkylation of benzene. The adsorbed species of 2-chloropropane (2-CP) on the chlorinated alumina have been ascertained to be mainly isopropyl cation [ 41. So, it is concluded that the alkylation proceeds through a Friedel-Crafts-type intermediate, that is,
75
‘:V”” H/
“‘Xl-
SL
where SL indicates possible strong Lewis acid site. The total conversion of 2-CP to the substituted benzenes over the chlorinated alumina was 14 times that of the sample dehydrated in vacua at the same temperature. Of the sample calcined in air, no reaction occurred. At 333 K propene formation was first observed, and above 373 K propene was a main product. When a large amount of propene was produced, the formation of a small amount of oligomers was observed. At 473 K, propene formation was preferential and the substituted benzenes were produced only in small quantities, but, however, 17 times more substituted benzenes were obtained over this sample than the dehydrated sample. The differences in the total conversions among three kinds of aluminas treated under different atmospheres were TABLE 1 Reaction data Alkylation of benzene in various atmospheres with l- or 2chloropropane at different reaction temp. on the ALO- chlorinated, dehydrated and cakined at 1073 K using a fixed-bed flow reaction” Product
Conversion of chloropropanes to each product
(%)
in chlorine
in vacua
in air
2chloropropane
1-chloropropane
2chloropropane
2chloropropane
293K
333K
293K
293K
293K
373K” 473K
m-di-IPB p-di-IPB tri-IPB tetra-IPB
5.2 0.7 1.7 33.3 5.2
1.5 0.3 0.2 3.3 1.0
2.9 1.2 0.5 3.5 0.0
4.1 0.5 0.2 0.3 0.0
1.9 0.03 0.2 2.7 0.0
3.8 1.1 0.5 4.3 0.0
5.9 1.0 0.4 0.1 0.0
1.1 0.0 0.01 0.5 1.7
2.2 0.1 0.1 1.2 0.0
0.3 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
0.07 0.0 0.0 0.0 0.0
total IPBs
46.1
6.3
8.1
5.1
4.83
9.7
7.4
3.31
3.6
0.3
0.0
0.0
0.07
0.0 0.0
0.1 0.0
15.2 0.2
71.6 0.0
0.0 0.0
17.1 17.3
78.6 0.0
0.0 0.0
15.9 0.2
85.9 0.0
0.0 0.0
1.5 0.0
86.1 0.0
46.1
6.4
23.5
76.7
4.83
44.1
86.0
3.31
19.7
86.2
0.0
1.5
86.17
IPB’
Propene CP isome@ Total
373Kb 473K
373K* 473K
373K* 473K
“Catalyst weight 0.075 g; flow rate of benzene 3 mmol h-i; flow rate of chloropropanes 1 mmol hh’ total flow rate 3 1b-i; reaction time 5 h ‘Reaction time 2 h ‘Isopropylbenzene ‘CP isomer means l-chloropropane for 2chIoropropane, and 2chloropropane for 1-chloropropane.
76
not so large. The conversion of 1-chloropropane (l-CP) to the substituted benzenes at 293 K was one-tenth of that of 2-CP. No n-propylbenzenes were formed and the IPB content of the liquid products obtained was higher than that obtained using a 2-CP alkylating agent. These results agree with the experimental evidence in the previous IR spectroscopic work in that l-chloropropane was adsorbed easily to form isopropyl cation on the chlorinated alumina [ 41. The hydrogen transfer reaction from secondary to primary carbon to form a secondary carbocation is fast even at low temperatures. At 373 K a fairly large isomerization of l-CP to 2-CP took place, in addition to dehydrochlorination and alkylation reaction. Reaction time The accumulated amount of products and the product composition change with reaction time were plotted using the data obtained in individual runs with different reaction times at 293 K (Fig. 1). Up to 2 h, the total amount of the substituted benzenes increased linearly with reaction time. During this period the colour of the catalyst was light yellow. At 373 K, benzene alkylation was almost complete within 20 min, because after this time the sum of accumulated amount of the substituted benzenes and the total number of the isopropyl groups incorporated into the substituted benzenes were together almost unchanged (Fig. 2). On the contrary, the dehydrochlorination proceeded steadily. The catalyst in this case also showed a yellow colour within 20 min. The yellow colour of the catalyst which was frequently observed while the alkylation proceeded demonstrated that an active reaction intermediate, i.e., n-complex, having a maximum absorption wavelength (A,,) of 294 nm, had been formed between benzene and the strong Lewis acid site [5]. In Fig. 1, a large amount of the tri-IPB was produced in the earlier part of the reaction. It is presently difficult to mechanistically interpret these results. But, further alkylations of IPB and m-di-IPB seem likely to proceed rapidly on chlorinated alumina. The increase in residence time of the IPB and m-di-IPB molecules due to the liquefaction of the products on the catalyst, which was observed only at 293 K, was one factor which promoted the consecutive alkylation reactions. Consequently, the maximum yield of the substituted benzenes was obtained at 293 K (Table 1 ), i.e., at a reaction temperature which was too low for the products to be removed from the catalyst surface. Benzene-to-2-chloropropane
(B-to-2-CP) ratio
The use of benzene in excess against the alkylating agent retarded the lowering of 2-CP conversion and increased the yield of IPB with increasing B-to2-CP ratio (Figs. 3 and 4). The gradual increases in the formation of IPB and m- and p-di-IPB with B-to-2-CP ratio seem to reflect a dynamic equilibrium
77
1
2
REACTION
3
TIME
4
5
( h )
Fig. 1 Reaction time dependence of the accumulated amount of the product composition at 293 K on the ALO- chlorinated at 1073 K rate of benzene 3 mmol h-i, flow rate of 2chloropropane 1 mmol Accumulated amount of the substituted benzenes = a. Composition di-IPB, 0 tri-IPB and 0 tetra-IPB.
substituted benzene6 and the (catalyst weight 0.075 g, flow h-‘, total flow rate 3 I h-i). = 0 IPB, a m-di-IPB, V p-
situation, that is, an increase in the mole ratio of partially activated benzene to the isopropyl cation. In general, the basicity of alkylbenzenes increases with an increase in the number of alkyl groups and branched-methyl groups and with an increase in carbon chain length. Consequently, the large proportion of the tri- and tetra-IPB in the product at low B-to-Z-CP ratio is considered to
I
I
0
1
2 REACTION
3 TIME
4
5
(h)
Fig. 2 Reaction time dependence of the accumulated amounts of the substituted benzenes and propene and the product composition at 373 K on the ALO- chlorinatedat 1073 K (other reaction conditions were as in Fig. 1):Accumulated amounts; 0 = the substituted benzenes, 0 = propene, and A = I-chloropropane: Composition; 0 = IPB, A = m-di-IPB, V =p-di-IPB, 0 = tri-IPB and O=propene.
cause the rapid deactivation of the catalyst, because the planar polysubstituted benzene molecules interact more strongly with the chlorine enhanced-strong Lewis acid sites than do the IPB and di-IPB, slowly poisoning the acid sites. The gradual decrease in the formation of the substituted benzenes with the increase in reaction time (Fig. 1) would be attributable to the same reason.
79
60
REACTION
TIME
(h )
Fig. 3 Dependences of the total conversion of 2-chloropropane on reaction time and benzene-to2-chloropropane ratio [catalyst weight 0.075 g, reaction temperature 293 K, reaction time 5 h, flow rate of 2-chloropropane 1 mmol h-i (constant), total flow rate 3 1h-i]. Benzene-to-2-chloropropane ratio: 0 = 0.9, @ = 1.8, l = 3.0 and @ = 4.1.
Fig. 4 Variations in product composition with the benzene-to-2-chloropropane ratio (the other reaction conditions were as in Fig. 3): 0 =IPB, A =m-di-IPB, V =p-di-IPB, 0 =tri-IPB and [7 = tetra-IPB.
Cakination temperature On the aluminas chlorinated below 873 K, basic sites and small amounts of hydroxyl groups remain and temperatures above 973 K are necessary to pre-
80
pare pure Lewis acid alumina surfaces [ 3,4]. The catalytic activities for alkylation and dehydrochlorination were greatest on the ALO- chlorinated in the range 973 to 1073 K (Fig. 5). The chlorination temperature dependence of the activities was similar to that for Lewis acidity as measured by the pyridine adsorption method [ 41. On the catalyst chlorinated at 573 K, dehydrochlorination was the main reaction. At a reaction temperature of 373 K, dehydro chlorination also proceeded on the ALO- dehydrated in vacua and calcined in air (Table 1) and even on the chlorinated ALO- after the alkylation was completely stopped (Fig. 2 1. Consequently, propene formation takes place even on Bronsted and weak Lewis acid sites. In this alkylation, hydrogen chloride generated has usually been in contact with the catalyst surface. Therefore, the possibility of aluminium trichloride formation and its dissolution into the liquid products was predicted [ 61. However, at least below 333 K aluminium trichloride was absent in the liquid condenced in the cold trap, which included benzene and a small amount of 2-chloropropane [ 71. In addition, the composition of the liquid product was unchanged for several days. Finally, compared to current industrial benzene alkylation processes using homogeneous Lewis acids [ 81, the reaction temperatures in this solid Lewis acid catalysed process are very much lower and the handling of the solid catalyst easier. If chloropropanes were adopted as alkylating agents, the advan-
16
673
a73
CHLORINATION
1073
TEMP.
1173
(K)
Fig. 5. Chlorination temperature dependence of the catalytic activities of the ALOto benzene alkylation, and dehydrochlorination and isomerization of 2-chloropropane (catalyst weight 0.075 g, reaction temperature 373 K, flow rate of benzene 3 mmol hP’, flow rate of 2-chloropropane 1 mmol h-r, total flow rate 3 I h-r): 0 =alkylation, 0 =dehydrochlorination and A =isomerization.
81
tages gained by the use of the chlorinated alumina solid Lewis acid catalyst would be expected to be large.
REFERENCES 1 G.A. Olah, G.K.S. Prakash and J. Sommer, Super-acids, Wiley, New York, 1985. 2 A.Mitsutani, Shokubai, 25 (1983) 86. 3 A. Ayame, K. Ohta, T. Izumizawa, G. Sawada, G. Zhang, H. Sato and H. Kakizaki, Shokubai, 30 (1988) 72. 4 A. Ayame, G. Sawada, H. Sato, G. Zhang, T. Ohta and T. Izumizawa, Appl. Catal., 48 (1989) 25. 5 A. Ayame, T. Izumizawa and H. Kakizaki, in K. Tanabe (Ed.), Acid-Base Catalysis (Proceedings of Acid-Base Catalysis International Symposium), Kodan-sha-VHC, Tokyo, 1989, p. 371. 6 K. Tanabe, T. Yamaguchi, K. Akiyama, A. Mitoh, K. Iwabuchi and K. Isogai, Proc. 8th Intern. Congr. Catal., Berlin, Verlag Chemie, 1984, V-601. 7 K. Imanishi and A. Ayame, unpublished results. 8 V.K. Weissermel and H.J. Arpe, Industriele Organ&he Chemie, Verlag Chemie GmbH, Weinheim, Germany, 1976.
Catalysis, 56 (1989)
73-81
73
Elsevier Science Publishers B.V., Amsterdam -
Printed in The Netherlands
Alkylation of Benzene with 24hloropropane Chlorine-Treated Alumina
on
AKIMI AYAME* and KAZUHIRO IMANISHI Department Muroran,
of Industrial Hokkaido
Chemistry,
Muroran
Institute
of Technology, 27-1 Mizumoto,
050 (Japan)
(Received 27 February 1989, revised manuscript received 13 June 1989)
ABSTRACT The alkylation of benzene with chloropropanes on solid Lewis acid catalysts prepared by the chlorination of y-alumina at 1073 K was carried out at 293-473 K using a fixed-bed flow reactor. Products in the alkylation at 293 K were isopropylbenzene, m- andp-diisopropylbenzenes, 1,3,5tri-isopropylbenzene and 1,2,4,5-tetraisopropylbenzene. No chlorine-substituted benzenes were detected. The conversion of 2chloropropane to the substituted benzenes was about 14 times that on the alumina dehydrated in vacua, and the 1,3,5-triisopropylbenzene produced was about 70% of the liquid products. An increase in benzene-to-chloropropane ratio retarded the lowering of conversion with reaction time and increased the yield of isopropylbenzene. The reactivity of lchloropropane was about one-tenth of that of 2-chloropropane, but the isopropylbenzene content was greater. At the reaction temperature of 333 K, dehydrochlorination of 2-chloropropane began to take place, and above 373 K this was a main reaction. Also, the chlorination temperature dependence of the catalytic activities for alkylation and dehydrochlorination was similar to that for Lewis acidity with a maximum in the range 973 to 1073 K. The alkylation was explained to proceed through a Friedel-Crafts-type intermediate.
INTRODUCTION
It is well known that the development of methods for the preparation of solid Lewis acid catalysts is important as these catalysts are preferable to homogeneous Lewis acids for use in industrial processes [ 1,2]. The authors reported previously that such solid Lewis acids could be prepared by chlorinating a popular and cheap y-alumina above 973 K and that the chlorinated aluminas had Lewis acid sites of Ho < - 14.52, which were enhanced by a small amount of chlorine on its surface, and activated chloropropanes to yield isopropyl and x-ally1 cations [ 3,4]. The solid Lewis acids also activated benzene to yield a n-complex, and selectively isomerized cyclohexane and butane to methylcylopentane and isobutane, respectively, at 303 K [ 51. From these facts, the high temperature-chlorinated aluminas were concluded to be solid Lewis superacid. Thus, since the chlorinated alumina catalyst was a very strong Lewis acid, it
0166.9834/89/$03.50
0 1989 Elsevier Science Publishers B.V.
was presumed that the application to benzene alkylation with olefins and alcohols would be difficult. The present work reports one example of the application of the chlorinated alumina to a catalytic reaction. The gas-phase alkylation of benzene with chloropropanes was carried out at 293-473 K using a conventional fixed-bed flow reactor. EXPERIMENTAL
The alumina used was JRC-ALOin y-form with a surface area of 177 m2 g-l. This alumina was the Japan Reference Catalyst (JRC) supplied by The Catalysis Society of Japan. The bulk composition was A1203 99.7 wt.-%, Fe,O, 0.01 wt.-%, SiO, 0.01 wt.-% and Na,O 0.01 wt.-%. The cylindrical alumina was crushed with a very pure alumina ceramic mortar and pestle, sieved to obtain a loo-150 mesh fraction, this fraction being washed and then dried. The hightemperature chlorination was performed at 873-1173 K in the same way as reported elsewhere and the ALO- showed similar chlorination behaviour to JRC-ALO[4,5].The flow reactor used consisted of 0.4 cm I.D. Pyrex glass tubing 20 cm long, equipped with a 5 ml vertical cold trap. The catalyst bed (0.075 g) on a glass-wool support was 1.3 cm long. The reactant mixture of benzene and 2-chloropropane was prepared by passing two dry nitrogen streams through individual benzene and 2-chloropropane evaporators and mixing the streams in a mixer. The flow-rate of 2-chloropropane was 1 mmol h-l and the benzene-to-2-chloropropane ratio was controlled in the range 0.9 to 4.1. Prior to use, the benzene and chloropropanes were purified on Molecular Sieve 4A by vacuum distillation three times. The chlorinated aluminas were transferred into the reactor in vacua and/or in dry nitrogen. The products were analysed by gas chromatography and also identified with an IR spectrometer. RESULTS
AND DISCUSSION
Reaction temperature The typical results obtained on the ALO- chlorinated at 1073 K are shown in Table 1. At the reaction temperature 293 K, the alkylation of benzene proceeded very fast to result in a maximum yield of substituted benzenes. Products were isopropylbenzene (IPB ) , rneta- and para-diisopropylbenzene (m-di-IPB ) and p-di-IPB), 1,3,5-triisopropylbenzene (tri-IPB) and 1,2,4,5-tetraisopropylbenzene (tetra-IPB ). No chlorine-substituted benzenes were formed. All of the products were the same as those obtained by the Friedel-Crafts alkylation of benzene. The adsorbed species of 2-chloropropane (2-CP) on the chlorinated alumina have been ascertained to be mainly isopropyl cation [ 41. So, it is concluded that the alkylation proceeds through a Friedel-Crafts-type intermediate, that is,
75
‘:V”” H/
“‘Xl-
SL
where SL indicates possible strong Lewis acid site. The total conversion of 2-CP to the substituted benzenes over the chlorinated alumina was 14 times that of the sample dehydrated in vacua at the same temperature. Of the sample calcined in air, no reaction occurred. At 333 K propene formation was first observed, and above 373 K propene was a main product. When a large amount of propene was produced, the formation of a small amount of oligomers was observed. At 473 K, propene formation was preferential and the substituted benzenes were produced only in small quantities, but, however, 17 times more substituted benzenes were obtained over this sample than the dehydrated sample. The differences in the total conversions among three kinds of aluminas treated under different atmospheres were TABLE 1 Reaction data Alkylation of benzene in various atmospheres with l- or 2chloropropane at different reaction temp. on the ALO- chlorinated, dehydrated and cakined at 1073 K using a fixed-bed flow reaction” Product
Conversion of chloropropanes to each product
(%)
in chlorine
in vacua
in air
2chloropropane
1-chloropropane
2chloropropane
2chloropropane
293K
333K
293K
293K
293K
373K” 473K
m-di-IPB p-di-IPB tri-IPB tetra-IPB
5.2 0.7 1.7 33.3 5.2
1.5 0.3 0.2 3.3 1.0
2.9 1.2 0.5 3.5 0.0
4.1 0.5 0.2 0.3 0.0
1.9 0.03 0.2 2.7 0.0
3.8 1.1 0.5 4.3 0.0
5.9 1.0 0.4 0.1 0.0
1.1 0.0 0.01 0.5 1.7
2.2 0.1 0.1 1.2 0.0
0.3 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
0.07 0.0 0.0 0.0 0.0
total IPBs
46.1
6.3
8.1
5.1
4.83
9.7
7.4
3.31
3.6
0.3
0.0
0.0
0.07
0.0 0.0
0.1 0.0
15.2 0.2
71.6 0.0
0.0 0.0
17.1 17.3
78.6 0.0
0.0 0.0
15.9 0.2
85.9 0.0
0.0 0.0
1.5 0.0
86.1 0.0
46.1
6.4
23.5
76.7
4.83
44.1
86.0
3.31
19.7
86.2
0.0
1.5
86.17
IPB’
Propene CP isome@ Total
373Kb 473K
373K* 473K
373K* 473K
“Catalyst weight 0.075 g; flow rate of benzene 3 mmol h-i; flow rate of chloropropanes 1 mmol hh’ total flow rate 3 1b-i; reaction time 5 h ‘Reaction time 2 h ‘Isopropylbenzene ‘CP isomer means l-chloropropane for 2chIoropropane, and 2chloropropane for 1-chloropropane.
76
not so large. The conversion of 1-chloropropane (l-CP) to the substituted benzenes at 293 K was one-tenth of that of 2-CP. No n-propylbenzenes were formed and the IPB content of the liquid products obtained was higher than that obtained using a 2-CP alkylating agent. These results agree with the experimental evidence in the previous IR spectroscopic work in that l-chloropropane was adsorbed easily to form isopropyl cation on the chlorinated alumina [ 41. The hydrogen transfer reaction from secondary to primary carbon to form a secondary carbocation is fast even at low temperatures. At 373 K a fairly large isomerization of l-CP to 2-CP took place, in addition to dehydrochlorination and alkylation reaction. Reaction time The accumulated amount of products and the product composition change with reaction time were plotted using the data obtained in individual runs with different reaction times at 293 K (Fig. 1). Up to 2 h, the total amount of the substituted benzenes increased linearly with reaction time. During this period the colour of the catalyst was light yellow. At 373 K, benzene alkylation was almost complete within 20 min, because after this time the sum of accumulated amount of the substituted benzenes and the total number of the isopropyl groups incorporated into the substituted benzenes were together almost unchanged (Fig. 2). On the contrary, the dehydrochlorination proceeded steadily. The catalyst in this case also showed a yellow colour within 20 min. The yellow colour of the catalyst which was frequently observed while the alkylation proceeded demonstrated that an active reaction intermediate, i.e., n-complex, having a maximum absorption wavelength (A,,) of 294 nm, had been formed between benzene and the strong Lewis acid site [5]. In Fig. 1, a large amount of the tri-IPB was produced in the earlier part of the reaction. It is presently difficult to mechanistically interpret these results. But, further alkylations of IPB and m-di-IPB seem likely to proceed rapidly on chlorinated alumina. The increase in residence time of the IPB and m-di-IPB molecules due to the liquefaction of the products on the catalyst, which was observed only at 293 K, was one factor which promoted the consecutive alkylation reactions. Consequently, the maximum yield of the substituted benzenes was obtained at 293 K (Table 1 ), i.e., at a reaction temperature which was too low for the products to be removed from the catalyst surface. Benzene-to-2-chloropropane
(B-to-2-CP) ratio
The use of benzene in excess against the alkylating agent retarded the lowering of 2-CP conversion and increased the yield of IPB with increasing B-to2-CP ratio (Figs. 3 and 4). The gradual increases in the formation of IPB and m- and p-di-IPB with B-to-2-CP ratio seem to reflect a dynamic equilibrium
77
1
2
REACTION
3
TIME
4
5
( h )
Fig. 1 Reaction time dependence of the accumulated amount of the product composition at 293 K on the ALO- chlorinated at 1073 K rate of benzene 3 mmol h-i, flow rate of 2chloropropane 1 mmol Accumulated amount of the substituted benzenes = a. Composition di-IPB, 0 tri-IPB and 0 tetra-IPB.
substituted benzene6 and the (catalyst weight 0.075 g, flow h-‘, total flow rate 3 I h-i). = 0 IPB, a m-di-IPB, V p-
situation, that is, an increase in the mole ratio of partially activated benzene to the isopropyl cation. In general, the basicity of alkylbenzenes increases with an increase in the number of alkyl groups and branched-methyl groups and with an increase in carbon chain length. Consequently, the large proportion of the tri- and tetra-IPB in the product at low B-to-Z-CP ratio is considered to
I
I
0
1
2 REACTION
3 TIME
4
5
(h)
Fig. 2 Reaction time dependence of the accumulated amounts of the substituted benzenes and propene and the product composition at 373 K on the ALO- chlorinatedat 1073 K (other reaction conditions were as in Fig. 1):Accumulated amounts; 0 = the substituted benzenes, 0 = propene, and A = I-chloropropane: Composition; 0 = IPB, A = m-di-IPB, V =p-di-IPB, 0 = tri-IPB and O=propene.
cause the rapid deactivation of the catalyst, because the planar polysubstituted benzene molecules interact more strongly with the chlorine enhanced-strong Lewis acid sites than do the IPB and di-IPB, slowly poisoning the acid sites. The gradual decrease in the formation of the substituted benzenes with the increase in reaction time (Fig. 1) would be attributable to the same reason.
79
60
REACTION
TIME
(h )
Fig. 3 Dependences of the total conversion of 2-chloropropane on reaction time and benzene-to2-chloropropane ratio [catalyst weight 0.075 g, reaction temperature 293 K, reaction time 5 h, flow rate of 2-chloropropane 1 mmol h-i (constant), total flow rate 3 1h-i]. Benzene-to-2-chloropropane ratio: 0 = 0.9, @ = 1.8, l = 3.0 and @ = 4.1.
Fig. 4 Variations in product composition with the benzene-to-2-chloropropane ratio (the other reaction conditions were as in Fig. 3): 0 =IPB, A =m-di-IPB, V =p-di-IPB, 0 =tri-IPB and [7 = tetra-IPB.
Cakination temperature On the aluminas chlorinated below 873 K, basic sites and small amounts of hydroxyl groups remain and temperatures above 973 K are necessary to pre-
80
pare pure Lewis acid alumina surfaces [ 3,4]. The catalytic activities for alkylation and dehydrochlorination were greatest on the ALO- chlorinated in the range 973 to 1073 K (Fig. 5). The chlorination temperature dependence of the activities was similar to that for Lewis acidity as measured by the pyridine adsorption method [ 41. On the catalyst chlorinated at 573 K, dehydrochlorination was the main reaction. At a reaction temperature of 373 K, dehydro chlorination also proceeded on the ALO- dehydrated in vacua and calcined in air (Table 1) and even on the chlorinated ALO- after the alkylation was completely stopped (Fig. 2 1. Consequently, propene formation takes place even on Bronsted and weak Lewis acid sites. In this alkylation, hydrogen chloride generated has usually been in contact with the catalyst surface. Therefore, the possibility of aluminium trichloride formation and its dissolution into the liquid products was predicted [ 61. However, at least below 333 K aluminium trichloride was absent in the liquid condenced in the cold trap, which included benzene and a small amount of 2-chloropropane [ 71. In addition, the composition of the liquid product was unchanged for several days. Finally, compared to current industrial benzene alkylation processes using homogeneous Lewis acids [ 81, the reaction temperatures in this solid Lewis acid catalysed process are very much lower and the handling of the solid catalyst easier. If chloropropanes were adopted as alkylating agents, the advan-
16
673
a73
CHLORINATION
1073
TEMP.
1173
(K)
Fig. 5. Chlorination temperature dependence of the catalytic activities of the ALOto benzene alkylation, and dehydrochlorination and isomerization of 2-chloropropane (catalyst weight 0.075 g, reaction temperature 373 K, flow rate of benzene 3 mmol hP’, flow rate of 2-chloropropane 1 mmol h-r, total flow rate 3 I h-r): 0 =alkylation, 0 =dehydrochlorination and A =isomerization.
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tages gained by the use of the chlorinated alumina solid Lewis acid catalyst would be expected to be large.
REFERENCES 1 G.A. Olah, G.K.S. Prakash and J. Sommer, Super-acids, Wiley, New York, 1985. 2 A.Mitsutani, Shokubai, 25 (1983) 86. 3 A. Ayame, K. Ohta, T. Izumizawa, G. Sawada, G. Zhang, H. Sato and H. Kakizaki, Shokubai, 30 (1988) 72. 4 A. Ayame, G. Sawada, H. Sato, G. Zhang, T. Ohta and T. Izumizawa, Appl. Catal., 48 (1989) 25. 5 A. Ayame, T. Izumizawa and H. Kakizaki, in K. Tanabe (Ed.), Acid-Base Catalysis (Proceedings of Acid-Base Catalysis International Symposium), Kodan-sha-VHC, Tokyo, 1989, p. 371. 6 K. Tanabe, T. Yamaguchi, K. Akiyama, A. Mitoh, K. Iwabuchi and K. Isogai, Proc. 8th Intern. Congr. Catal., Berlin, Verlag Chemie, 1984, V-601. 7 K. Imanishi and A. Ayame, unpublished results. 8 V.K. Weissermel and H.J. Arpe, Industriele Organ&he Chemie, Verlag Chemie GmbH, Weinheim, Germany, 1976.