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Investigation of dehydrocyclization of paraffins with an alumina-chromium oxide-potassium oxide catalyst using a tracer technique
INVESTIGATION OF DEHYDROCYCLIZATION OF PARAFFINS WITH AN ALUMINA-CHROMIUM OXIDEPOTASSIUM OXIDE CATALYST USING A TRACER TECHNIQUE*
V. G. LII'OVICH, O. I. SHMIDT, )/~. A. LUI~'E a n d I. V. KA_hECHITS Institute of Petroleum and Carbon-chemical Synthesis, A. A. Zhdanov State University, Irkutsk (Received 4 February 1969)
IN SPITE of numerous studies carried out in the field of dehydrocyclization of paraffins [1, 2] there is no uniform opinion concerning the role of olefins in this process and the dependence of the rate of aromatization on the location of the double bond [3-9]. To elucidate these problems we investigated the conversion of n-heptane-n-heptene (1 : 1) mixtures, of which one component was labelled with 14C radioactive carbon, in a circulatory apparatus with an alumina-chromium oxide-potassium oxide catalyst. Initial specific radioactivities of heptane, hept-l-ene and hept-2-ene-hept-3-ene mixture were 286, 314 and 325 pulses/min.mg BaC03. The compositions of catalysts prepared and the distribution of radioactivity between components were determined with KhL-4, Khrom-2 chromatographs and a radiochromatograph. The catalyst was regenerated in air for 5 hr, increasing temperature from 250 to 550 °, then reduced for 30 min with hydrogen at 487 ° and treated for 70 min at this temperature, passing heptane through at a space velocity of 2 hr -1. I t was found that, after this treatment, catalytic activity remained practically constant for 1 hr when heptane was passed through at a velocity of 2 hr-L Experiments with a heptane-hept-l-ene mixture established that, with a contact time of 4 sec and an increase in temperature (Fig. 1) from 450 to 550 °, toluene yield increases from 15 to 42 wt.%. The amount of benzene formed by dealkylation of toluene and by dehydrocyclization of hexanes produced by the separation of raw material, increases at the same time. It was shown by the tracer technique [10] that, on dehydrocyclization on an alumina-chromium oxide-potassium oxide catalyst, olefins are converted into paraffins by hydrogenation or disproportionation of hydrogen at a high * l~eftekhimiya 9, No. 5, 661-665, 1969. 179
V. G. LrPovic~ et al.
180
rate and the ratio of reaction rates of dehydrocyclization and hydrogenation for h e p t - l - e n e considerably decreases w i t h a change in c o n t a c t t i m e from 4 to 1.1 sec. F u r t h e r , it was established t h a t h e p t - l - e n e is c o n v e r t e d into h e p t a n e a t a lower r a t e t h a n h e p t - 2 e n e s a n d hept-3-enes, so t h a t the difference in velocities increases in p a r t i c u l a r on lowering c o n t a c t time. I t should be n o t e d t h a t m i g r a t i o n o f the double b o n d in olefins takes place a t a considerably higher r a t e t h a n a r o m a t i z a t i o n o v e r a fairly wide r a n g e o f t e m p e r a t u r e a n d c o n t a c t time, since e v e n w i t h a negligible toluene yield a practically equilibrium olefin m i x t u r e is f o r m e d [11]. I t follows f r o m Fig. 2 t h a t olefins are the m a i n sources of a r o m a t i c h y d r o carbon f o r m a t i o n for a wide r a n g e o f c o n t a c t t i m e a n d w i t h c o n t a c t t i m e s lower t h a n 3.3 sec initial paraffin c o n t e n t remains practically u n c h a n g e d . I t m a y be assumed f r o m these results t h a t d e h y d r o g e n a t i o n of paraffins is a limiting stage in a r o m a t i z a t i o n . Results o f e x p e r i m e n t s w i t h r a d i o a c t i v e h y d r o c a r b o n s (Tables 1-2) confirm t h a t t h e r a t e o f a r o m a t i z a t i o n o f olefins is higher t h a n t h a t o f a r o m a t i z a tion o f paraffins. w-if. o,
wt.%
e~
-
2
2
1
~ 3o 3
060
090
20
:FIG. 1
1
550 °C
~o ~ 30
1
3
5
7
9
11
Contect time, sec Fro. 2
FIG. 1. Dependence of the composition of catalysed products of n-heptane-hept-l-ene on temperature with a contact time of 4 sec: 1--n-heptane; 2--toluene; 3--total of n-heptenes; 4--total of products of separation; 5--benzene. Notation is the same in :Fig. 2. FIG. 2. Dependence of the composition of catalysed products of n-heptane-hept-l-ene on contact time at a temperature of 487 °. W h e n using a h e p t a n e - h e p t - l - e n e m i x t u r e as r a w m a t e r i a l (14C)* w i t h a c o n t a c t t i m e o f 4 sec t o l u e n e contains a n average o f 22-3% of t h e overall $ The (14C) carbon atom was in position 4 in every case.
~181
D e h y d r o c y e l i z a t i o n of paraffins
radioactivity, whereas heptane is the radioactive component of the mixture in the proportion of 8"9~/o in all. This relation is also reflected in experiments with a heptane-hept-2-ene and hept. 3-ene mixture (14C) and contact time of 1.1 see. The use of radioisotopes confirms the higher rate of dehydrocycliza-
TABLE 1. DISTRIBUTIO1W OF RADIOACTIVITY" B E T W E E N T H E C O M P O N E N T S OF C A T A L Y S E D :PRODUCTS
C o n t a c t t i m e 1.1 see, t e m p e r a t u r e 487°C. A is c o n t e n t , wt.~/o calculated for t h e initial m i x t u r e ; B is specific r a d i o a c t i v i t y , pulses/rain, m g BaCOa: C is ~o of t h e overall radioa c t i v i t y of t h e c a t a l y s e d p r o d u c t
n-Heptanen - H e p t a n e (14C)n-hept-l-ene n - h e p t - 1-erie, (14C),* specific specific a c t i v i t y a c t i v i t y of h e p t of h e p t a n e 1-ene 314 286
Composition of t h e catalysed product
B
C
A
B
6.5 2-1 37.7 48"1 • 4"9
95 259 281 21 164
4.6 4.0 78.1 7'4 5"9
11-0 7.3 28.0 41"3 11"6 0"8 --
52 84 58 288 66 66
I
~1-C4
C~-Cs a-Heptenes a-Heptane Toluene Benzene
CI-C, Cs-C~
0"7
--
--
6-9
96
4.3 2.8 76.7
--
!
41.6
216 280
--
i
45.1
32
9.5
--!
--
4"0
226 211
6.0 0-8
---
---
1.9
n-Heptenes n-Heptane Toluene Benzene *
A
0"5
C
,
--
The carbon atom in position 4 acts as tracer
---
in
n-I~eptanen-hept-2-enes n - H e p t a n e (14C)a n d n-hept-3n-hept-2-enes enes (14C), spea n d n - h e p t - 3eifie a c t i v i t y ! cries, specific acof h e p t - 2-enes t i v i t y of h e p t a n d hept-3-enes ane 286 325 A
B
C
67 297 291 34 274
3.9 4.7 73.9 10.1 7-4
63 201 231 50.0 / 40 4.5 230
3.6 5"1 66.6 16.3 8-4
9.0 i 2.4, 39.0i 45-4 I 4.2i 7.0 3.1
!
A
B
C
10.0 9-6 26.0 50.8 2.0 1.6
52 70 52 252 62 54
3-3 4.3 8.7 82.3 0.8 0.6
every case.
tion of 1-olefins compared with 2-olefins and 3-olefins [11]. In fact, with a contact time of 4 sec and hept-l-ene (14C)-heptane, approximately 10-15% toluene is formed with 19-26% overall radioactivity, whereas from a hept-2-ene and hept-3-ene-heptane mixture (14C) only about 6% toluene was obtained in the product with 10-11% overall radioactivity. If dehydrocyclization of paraffins was stepwise, it could be expected that specific radioactivities of toluene, when using heptane-heptene (14C) as initial mixtures, would be higher than those of olefins of the product, since at the initial moment toluene is formed from the initial undiluted radioactive olefin and vice versa, for a heptane (laC)-
182
V.G. LIPOVICH et al.
heptene initial mixture the specific radioactivities of toluene should be lower t h a n those of heptenes in the product. Information obtained (Table 1) indicates that with a contact time of 1.1 sec the ratio of toluene/heptene specific radioactivities is less than one for initial heptane-heptene (14C) mixtures and greater than one for initial heptane (14C)-heptenes mixtures. This indicates t h a t part of the heptane is converted to toluene without volumetric discharge of intermediate compounds into the volume. With a contact time of 4 sec, specific activities of toluene and heptencs are equalized (Table 2), which m a y be due to a particular ratio of reaction rates of dehydrogenation of heptane to heptenes and dehydrocyclization to toluene or to the equilibrium between rates of desorption and olefin adsorption. With a contact time of 4 sec heptenes may, apparently, displace heptane to a considerable extent from the catalyst surface, consequently, the role of the catalyst in the formation of toluene is considerably lower. It can be seen from Tables 1 and 2 that, under given conditions, aromatization of hept-l-ene takes place much more quickly than aromatization of hept-2-enes and hept-3-enes. I t has also been established [9, 12] that under intermittent conditions according to rates of dehydrocyclization, olefins occupy the order: cis-hept- 2-ene ~ cis-hept-3-ene > trans-hept-3-ene > trans-hept- 2-ene > helot- 1-ene.
This apparent contradiction is because pulse reactors enable us to study primary conversion of the raw material whereas, in circulatory plants and industrial reactors, relations are observed, which are typical of catalysts partially deactivated by initial and intermediate products. In addition, it is known t h a t adsorption by oxide catalysts has a donor character [13, 14], therefore hept-l-ene should have a lower adsorption coefficient and consequently, reduced liability to coke formation on these catalysts [15], as confirmed by previous studies [7, 9, 12]. It is of interest to note that hept-2-enes and hept-3-enes not only undergo dehydrocyclization at a lower rate than hept-l-ene, but delay dehydrocyclization of n-heptane to an even greater extent [10]. This m a y be due to the varying degree of deactivation of the dehydrogenating and ring forming centres of the catalyst, according to the structure of olefins adsorbed. Kazanskii, Rozengart and )5ortikov [8, ]2, 16] pointed out that dehydrocyclization of heptane is a multi-stage sequential process, n-Heptenes, n-heptadienes, n-heptatrienes and methylcyclohexadienes observed in the catalysed product are intermediates. The authors expressed the view t h a t heptadienes can be converted to toluene without volumetric desorption of intermediate products. It is not excluded, however, that the conversion of heptane to toluene m a y also take place via the stage of cyclic complex formation with the catalyst; unfortunately, however, there is no information available at the present time to evaluate the structure of these complexes.
Dehydroeyclization of paraffins TABLE 2.
DISTRIBUTION
OF
RADIOACTIVITY
BETWEI~I~
183
COMPONENTS
OF
CATALYSED
PRODUCTS
Contact time 4 sec, t e m p e r a t u r e 487°C. A is content, w t . ~ , calculated for the initial mixture; B is specific radioactivity, pulses/rain.rag BaCO3; C is ~/o of overall radioactivity of the catalysed product
n-I-Ieptenes n-}-Ieptane Toluene Benzene Cx-C4 C5-C6 n-l~eptenes n-Heptane Toluene Benzene
42.1 14"9 2"8 10.0 5"9 26.9 45"8 10"1 1"3
236
8"9 25"8
126 237 258 34 254
9"2 10.2 50.7 11.0 18"8
29
20"0 36"8 14.4 1"9 5"0 13"8 19"4 38.4 21"8 1"6
64 287 63 85 67 122 75 253 82 85
8"7 71"3 6"1 1'1 2"2 11"2 9.6 64"3 11.8 0"9
[0"8 4"2 14"9 L3"2 6"2 0"7 5"0 3"2 15"6 i0'4 5"8
251 67 250 244 60 262 250 28 258
58"5 19"3 10'4 1"2 2"3 6"4 68"8 10.9 11.6
* The carbon a t o m in p o s i t i o n 4 acts as tracer in every case.
SUMMARY
1. E x p e r i m e n t s w i t h n - h e p t a n e - n - h e p t e n e mixtures show that with an a l u m i n a - c h r o m i u m o x i d e - p o t a s s i u m o x i d e c a t a l y s t a t a t e m p e r a t u r e o f 487 ° a n d w i t h c o n t s c t t i m e s o f 1-1 a n d 4.0 s e c p a r t o f n - h e p t a n e is c o n v e r t e d t o toluene without volumetric desorption of intermediate products. 2. T h e p r o p o r t i o n o f t o l u e n e f o r m e d f r o m n - h e p t e n e s u n d e r t h e s e c o n d i t i o n s is 71-84°/o . 3. T J n d e r t h e c o n d i t i o n s s t u d i e d n - h e p t - l - e n e is c o n v e r t e d t o t o l u e n e m o r e rapidly than hept-2-ene and hept-3-enes. REFERENCES
1. A. F. PLATE, K a t a l i t i e h e s k a y a aromatizatsiya parafinovykh uglevodorodov (Catalytic Aromatization of Paraffinic Hydrocarbons). Izd. A N SSSR, Moscow, 1948 2. V. G. LIPOVICH and O. I. SHlYIIDT, Izv. N I I nefte- i uglekhimicheskogo sinteza pri I r k u t s k o m gos. un-te (Report of the Scientific Research I n s t i t u t e of Petroleum and Carbon-Chemical Synthesis, I r k u t s k State University). I X , p. 1, I r k u t s k , 1967 3. H. HOOG, I. VERIIEUS and F. S. ZUIDERWEG, Trans. F a r a d a y Soc. 35, 993 1939 4. E. F. G. HERRIN{~TON, E . K . RIDEAL, Proc. Roy. Soc. A184, 437, 447, 1945 5. A. F. PLATE and G. A. TARASOVA, Zh. obshch, khimii 22, 1702, 1955
184
V . G . LIPOWCH et cal.
6. Yu. N. USOV a n d N. V. SIDOROVA, Zh. obsch, khimii 25, 1702, 1955 7. M. I. ROZENGART, Ye. S. MORTIKOV and B. A. KAZANSKII, Dokl. AN SSSR 158, 911, 1964 8. M. I. ROZENGART, Dokt. diss., I n - t organ, khimii A N SSSR, Moscow, 1967 9. M. I. ROZENGART, Ye. S. MORTIKOV and B. A. KAZANSI~H, Neftekhimiya 9, No. 1, 41, 1969 10. V. G. LIPOVICH, O. I. SHMIDT, M. A. LUR'E and I. V. KALECHITS, Izv. N I I noftei uglekhimicheskogo sinteza pri Irkutskom gos. un-te (Report of the Scientific Research I n s t i t u t e of Petroleum and Carbon-Chemical Synthesis, I r k u t s k State University). IX, p. 1, Irkutsk, 1967 11. V. G. LIPOVICH, R. I. SIDOROV, M: P. IVANOVA, O. I. SHMIDT, M. A. LUR'E. Yu. M. ZHOROV and I. V. KALECHITS, Izv. N I I nefte- i uglel~himicheskogo sinteza pri Irkutskom gos. un-te (Report of the Scientific Research I n s t i t u t e of Petroleum and Carbon-Chemical Synthesis, I r k u t s k State University). IX, p. 1, Irkutsk, 1967 12. Ye. S. MORTIKOV, K a n d . diss., I n - t organ, khimii, AN SSSR, Moscow, 1965 13. E. Kh. YENIKEYEV, L. Ya. MARGOLIS a n d S. Z. ROGINSKII, Dokl. AN SSSR 124, 606, 1959 14. E. Kh. YENIKEYEV, L. Ya. MARGOLIS and O. V. ISAYEV, Kinetika i kataliz 1, 431, 1960 15. JOSHIHIKO MORO-OKA, ATSUMU OZAKI, J. Amer. Chem. Soe. 89, No. 20, 5124, 1967 16. M. I. ROZENGART, Ye. S. MORTIKOV and B. A. KAZANSKII, Dokl. AN SSSR 166, 3, 1966
V. G. LII'OVICH, O. I. SHMIDT, )/~. A. LUI~'E a n d I. V. KA_hECHITS Institute of Petroleum and Carbon-chemical Synthesis, A. A. Zhdanov State University, Irkutsk (Received 4 February 1969)
IN SPITE of numerous studies carried out in the field of dehydrocyclization of paraffins [1, 2] there is no uniform opinion concerning the role of olefins in this process and the dependence of the rate of aromatization on the location of the double bond [3-9]. To elucidate these problems we investigated the conversion of n-heptane-n-heptene (1 : 1) mixtures, of which one component was labelled with 14C radioactive carbon, in a circulatory apparatus with an alumina-chromium oxide-potassium oxide catalyst. Initial specific radioactivities of heptane, hept-l-ene and hept-2-ene-hept-3-ene mixture were 286, 314 and 325 pulses/min.mg BaC03. The compositions of catalysts prepared and the distribution of radioactivity between components were determined with KhL-4, Khrom-2 chromatographs and a radiochromatograph. The catalyst was regenerated in air for 5 hr, increasing temperature from 250 to 550 °, then reduced for 30 min with hydrogen at 487 ° and treated for 70 min at this temperature, passing heptane through at a space velocity of 2 hr -1. I t was found that, after this treatment, catalytic activity remained practically constant for 1 hr when heptane was passed through at a velocity of 2 hr-L Experiments with a heptane-hept-l-ene mixture established that, with a contact time of 4 sec and an increase in temperature (Fig. 1) from 450 to 550 °, toluene yield increases from 15 to 42 wt.%. The amount of benzene formed by dealkylation of toluene and by dehydrocyclization of hexanes produced by the separation of raw material, increases at the same time. It was shown by the tracer technique [10] that, on dehydrocyclization on an alumina-chromium oxide-potassium oxide catalyst, olefins are converted into paraffins by hydrogenation or disproportionation of hydrogen at a high * l~eftekhimiya 9, No. 5, 661-665, 1969. 179
V. G. LrPovic~ et al.
180
rate and the ratio of reaction rates of dehydrocyclization and hydrogenation for h e p t - l - e n e considerably decreases w i t h a change in c o n t a c t t i m e from 4 to 1.1 sec. F u r t h e r , it was established t h a t h e p t - l - e n e is c o n v e r t e d into h e p t a n e a t a lower r a t e t h a n h e p t - 2 e n e s a n d hept-3-enes, so t h a t the difference in velocities increases in p a r t i c u l a r on lowering c o n t a c t time. I t should be n o t e d t h a t m i g r a t i o n o f the double b o n d in olefins takes place a t a considerably higher r a t e t h a n a r o m a t i z a t i o n o v e r a fairly wide r a n g e o f t e m p e r a t u r e a n d c o n t a c t time, since e v e n w i t h a negligible toluene yield a practically equilibrium olefin m i x t u r e is f o r m e d [11]. I t follows f r o m Fig. 2 t h a t olefins are the m a i n sources of a r o m a t i c h y d r o carbon f o r m a t i o n for a wide r a n g e o f c o n t a c t t i m e a n d w i t h c o n t a c t t i m e s lower t h a n 3.3 sec initial paraffin c o n t e n t remains practically u n c h a n g e d . I t m a y be assumed f r o m these results t h a t d e h y d r o g e n a t i o n of paraffins is a limiting stage in a r o m a t i z a t i o n . Results o f e x p e r i m e n t s w i t h r a d i o a c t i v e h y d r o c a r b o n s (Tables 1-2) confirm t h a t t h e r a t e o f a r o m a t i z a t i o n o f olefins is higher t h a n t h a t o f a r o m a t i z a tion o f paraffins. w-if. o,
wt.%
e~
-
2
2
1
~ 3o 3
060
090
20
:FIG. 1
1
550 °C
~o ~ 30
1
3
5
7
9
11
Contect time, sec Fro. 2
FIG. 1. Dependence of the composition of catalysed products of n-heptane-hept-l-ene on temperature with a contact time of 4 sec: 1--n-heptane; 2--toluene; 3--total of n-heptenes; 4--total of products of separation; 5--benzene. Notation is the same in :Fig. 2. FIG. 2. Dependence of the composition of catalysed products of n-heptane-hept-l-ene on contact time at a temperature of 487 °. W h e n using a h e p t a n e - h e p t - l - e n e m i x t u r e as r a w m a t e r i a l (14C)* w i t h a c o n t a c t t i m e o f 4 sec t o l u e n e contains a n average o f 22-3% of t h e overall $ The (14C) carbon atom was in position 4 in every case.
~181
D e h y d r o c y e l i z a t i o n of paraffins
radioactivity, whereas heptane is the radioactive component of the mixture in the proportion of 8"9~/o in all. This relation is also reflected in experiments with a heptane-hept-2-ene and hept. 3-ene mixture (14C) and contact time of 1.1 see. The use of radioisotopes confirms the higher rate of dehydrocycliza-
TABLE 1. DISTRIBUTIO1W OF RADIOACTIVITY" B E T W E E N T H E C O M P O N E N T S OF C A T A L Y S E D :PRODUCTS
C o n t a c t t i m e 1.1 see, t e m p e r a t u r e 487°C. A is c o n t e n t , wt.~/o calculated for t h e initial m i x t u r e ; B is specific r a d i o a c t i v i t y , pulses/rain, m g BaCOa: C is ~o of t h e overall radioa c t i v i t y of t h e c a t a l y s e d p r o d u c t
n-Heptanen - H e p t a n e (14C)n-hept-l-ene n - h e p t - 1-erie, (14C),* specific specific a c t i v i t y a c t i v i t y of h e p t of h e p t a n e 1-ene 314 286
Composition of t h e catalysed product
B
C
A
B
6.5 2-1 37.7 48"1 • 4"9
95 259 281 21 164
4.6 4.0 78.1 7'4 5"9
11-0 7.3 28.0 41"3 11"6 0"8 --
52 84 58 288 66 66
I
~1-C4
C~-Cs a-Heptenes a-Heptane Toluene Benzene
CI-C, Cs-C~
0"7
--
--
6-9
96
4.3 2.8 76.7
--
!
41.6
216 280
--
i
45.1
32
9.5
--!
--
4"0
226 211
6.0 0-8
---
---
1.9
n-Heptenes n-Heptane Toluene Benzene *
A
0"5
C
,
--
The carbon atom in position 4 acts as tracer
---
in
n-I~eptanen-hept-2-enes n - H e p t a n e (14C)a n d n-hept-3n-hept-2-enes enes (14C), spea n d n - h e p t - 3eifie a c t i v i t y ! cries, specific acof h e p t - 2-enes t i v i t y of h e p t a n d hept-3-enes ane 286 325 A
B
C
67 297 291 34 274
3.9 4.7 73.9 10.1 7-4
63 201 231 50.0 / 40 4.5 230
3.6 5"1 66.6 16.3 8-4
9.0 i 2.4, 39.0i 45-4 I 4.2i 7.0 3.1
!
A
B
C
10.0 9-6 26.0 50.8 2.0 1.6
52 70 52 252 62 54
3-3 4.3 8.7 82.3 0.8 0.6
every case.
tion of 1-olefins compared with 2-olefins and 3-olefins [11]. In fact, with a contact time of 4 sec and hept-l-ene (14C)-heptane, approximately 10-15% toluene is formed with 19-26% overall radioactivity, whereas from a hept-2-ene and hept-3-ene-heptane mixture (14C) only about 6% toluene was obtained in the product with 10-11% overall radioactivity. If dehydrocyclization of paraffins was stepwise, it could be expected that specific radioactivities of toluene, when using heptane-heptene (14C) as initial mixtures, would be higher than those of olefins of the product, since at the initial moment toluene is formed from the initial undiluted radioactive olefin and vice versa, for a heptane (laC)-
182
V.G. LIPOVICH et al.
heptene initial mixture the specific radioactivities of toluene should be lower t h a n those of heptenes in the product. Information obtained (Table 1) indicates that with a contact time of 1.1 sec the ratio of toluene/heptene specific radioactivities is less than one for initial heptane-heptene (14C) mixtures and greater than one for initial heptane (14C)-heptenes mixtures. This indicates t h a t part of the heptane is converted to toluene without volumetric discharge of intermediate compounds into the volume. With a contact time of 4 sec, specific activities of toluene and heptencs are equalized (Table 2), which m a y be due to a particular ratio of reaction rates of dehydrogenation of heptane to heptenes and dehydrocyclization to toluene or to the equilibrium between rates of desorption and olefin adsorption. With a contact time of 4 sec heptenes may, apparently, displace heptane to a considerable extent from the catalyst surface, consequently, the role of the catalyst in the formation of toluene is considerably lower. It can be seen from Tables 1 and 2 that, under given conditions, aromatization of hept-l-ene takes place much more quickly than aromatization of hept-2-enes and hept-3-enes. I t has also been established [9, 12] that under intermittent conditions according to rates of dehydrocyclization, olefins occupy the order: cis-hept- 2-ene ~ cis-hept-3-ene > trans-hept-3-ene > trans-hept- 2-ene > helot- 1-ene.
This apparent contradiction is because pulse reactors enable us to study primary conversion of the raw material whereas, in circulatory plants and industrial reactors, relations are observed, which are typical of catalysts partially deactivated by initial and intermediate products. In addition, it is known t h a t adsorption by oxide catalysts has a donor character [13, 14], therefore hept-l-ene should have a lower adsorption coefficient and consequently, reduced liability to coke formation on these catalysts [15], as confirmed by previous studies [7, 9, 12]. It is of interest to note that hept-2-enes and hept-3-enes not only undergo dehydrocyclization at a lower rate than hept-l-ene, but delay dehydrocyclization of n-heptane to an even greater extent [10]. This m a y be due to the varying degree of deactivation of the dehydrogenating and ring forming centres of the catalyst, according to the structure of olefins adsorbed. Kazanskii, Rozengart and )5ortikov [8, ]2, 16] pointed out that dehydrocyclization of heptane is a multi-stage sequential process, n-Heptenes, n-heptadienes, n-heptatrienes and methylcyclohexadienes observed in the catalysed product are intermediates. The authors expressed the view t h a t heptadienes can be converted to toluene without volumetric desorption of intermediate products. It is not excluded, however, that the conversion of heptane to toluene m a y also take place via the stage of cyclic complex formation with the catalyst; unfortunately, however, there is no information available at the present time to evaluate the structure of these complexes.
Dehydroeyclization of paraffins TABLE 2.
DISTRIBUTION
OF
RADIOACTIVITY
BETWEI~I~
183
COMPONENTS
OF
CATALYSED
PRODUCTS
Contact time 4 sec, t e m p e r a t u r e 487°C. A is content, w t . ~ , calculated for the initial mixture; B is specific radioactivity, pulses/rain.rag BaCO3; C is ~/o of overall radioactivity of the catalysed product
n-I-Ieptenes n-}-Ieptane Toluene Benzene Cx-C4 C5-C6 n-l~eptenes n-Heptane Toluene Benzene
42.1 14"9 2"8 10.0 5"9 26.9 45"8 10"1 1"3
236
8"9 25"8
126 237 258 34 254
9"2 10.2 50.7 11.0 18"8
29
20"0 36"8 14.4 1"9 5"0 13"8 19"4 38.4 21"8 1"6
64 287 63 85 67 122 75 253 82 85
8"7 71"3 6"1 1'1 2"2 11"2 9.6 64"3 11.8 0"9
[0"8 4"2 14"9 L3"2 6"2 0"7 5"0 3"2 15"6 i0'4 5"8
251 67 250 244 60 262 250 28 258
58"5 19"3 10'4 1"2 2"3 6"4 68"8 10.9 11.6
* The carbon a t o m in p o s i t i o n 4 acts as tracer in every case.
SUMMARY
1. E x p e r i m e n t s w i t h n - h e p t a n e - n - h e p t e n e mixtures show that with an a l u m i n a - c h r o m i u m o x i d e - p o t a s s i u m o x i d e c a t a l y s t a t a t e m p e r a t u r e o f 487 ° a n d w i t h c o n t s c t t i m e s o f 1-1 a n d 4.0 s e c p a r t o f n - h e p t a n e is c o n v e r t e d t o toluene without volumetric desorption of intermediate products. 2. T h e p r o p o r t i o n o f t o l u e n e f o r m e d f r o m n - h e p t e n e s u n d e r t h e s e c o n d i t i o n s is 71-84°/o . 3. T J n d e r t h e c o n d i t i o n s s t u d i e d n - h e p t - l - e n e is c o n v e r t e d t o t o l u e n e m o r e rapidly than hept-2-ene and hept-3-enes. REFERENCES
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