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Oxidative dehydrogenation products obtained from n-butenes over a bismuth-molybdenum catalyst
OXIDATIVE DEHYDRO6~ENATION PRODUCTS OBTAINED FROM n-BUTENES OVER A BISMUTH-MOLYBDENUM CATALYST* A. L. TSAXLIIffGOL'D, •. S. I~ILIPEIqKO, G. A. STEPANOV and I. YA. TYURYAYEV Synthetic l~ubber Monomer Research I n s t i t u t e
(Received 27 March 1965)
O~¢E Of the outstanding characteristics of the oxidative dehydrogenation process of n-butenes to butadiene over Bi-Mo catalysts is the large selectivity. An 80% conversion can thus give up to 90 mole% butadiene yields on reacted butenes [1, 2]. Secondary reaction products are C02 and other oxygen containing compounds, b u t very little has been published on the composition of the latter so far. The knowledge of the composition of the reaction products, and also of the mechanism of formation is essential for an understanding of the process mechanism and to the solution of a number of practical problems such as the selection of optimum process conditions, isolation and purification of the butadiene, prevention of corrosion, etc. This paper reports the results of studying the general principles of the formation of these secondary products made up of C02, aldehydes (formaldehyde, acetaldehyde), unsaturated aldehydes, acids (maleie amongst them) and furan. EXPERIMENTAL
The main work of studying the reaction products and of the mechanisms of formation was carried out on a laboratory-scale, flow t y p e reactor with a fixed bed catalyst. The quartz reactor (14 mm int. din.), fitted with a thermocouple sleeve (8 mm ext. din.), was contained in a fluidized sand bath to obtain a practically constant temperature throughout the reactor and fixed bed catalyst. The temperature was measured with a chromel-alumel thermocouple and was maintained within ~ 2 ° C of requirement with the aid of a potentiometer. The flow of gases was determined with calibrated flowmeters, the water feed with an automated dosimeter. The reaction products were released into the atmosphere after passage through a water condenser and a gasmeter. Each experiment lasted 2 hr and the contact gas was analysed at the end of each hour. The aqueous condensate was collected throughout the experiment. * Neftekhimiya 6, No. 3, 367-373, 1966. 107
108
A. L. TSAILINGOL'D et al.
The composition of the catalyst was a 1 : 1 (atomic) mixture of Bi with Mo, 25% of active mass being contained on silica gel promoted with 0.5% P205; the silica gel was of 0.25-0.5 mm mesh. The butene had the following composition (% v/v): 0.5 C8 total hydrocarbons, 1.4 butane, 34.5 but-l-ene, 34.6 trans-but-2-ene, 26 ci&but-2-ene, 1.7 C4H6 dienes; the total C5 hydrocarbon content was 1.3. A certain proportion of the tests was carried out with but-l-cue obtained b y dehydrating primary butanol (anal. purity) b y a method described b y other authors [3]. This gas had the following composition (% v/v): 95.5 but-l-ene, 1.8 transbut-2-ene, 2.5 ci&but-2-ene, 0-2 C4H6 dienes. The butadiene oxidation tests were made with a butadiene distillate of 99.4% purity and with oxygen from air. The contact gas was chromatographed and the CO2, aldehyde, furan, and aerolein contents were determined using n-butyrate of ethylene glycol as sorbent and hydrogen as the carrier gas. The C02, O3 and sum of unsaturated compounds were determined b y using the Ors apparatus. The oxidate contained in the aqueous condensate was checked b y polarography and potentiometric titration. The formaldehyde, acetaldehyde, maleie acid and sum of unsaturated aldehydes (as methaerolein) contents were determined b y polarography on instrument OIg-101 using the standard addition method of compounds to be analysed. Two base electrolytes were used, an alkaline (0.05 ~ of (CHa)4NOH) in the determination of carbonyl compounds, and an acid (a 0.1 N solution of H2SOa) in the determination of maleic acid. The latter and the total acids, such as formic and acetic (determined as acetic) were also determined b y potentiometric titration of the sample in acetone using a 0.1 alcoholic solution of K O H . The yield of reaction products was worked out on computer "Minsk-l" on the basis of supplied hydrocarbons as the ration of converted original hydrocarbon to that supplied (as %); the yield on reacted hydrocarbon was given as the ratio of hydrocarbon, converted to that product, to the sum of converted original hydrocarbon (not taking into account any isomerization). The effects of temperature and contact time on product composition were studied at atmospheric pressure and at a constant proportion of n-butenes, oxygen and steam present in the starter gas. We had shown earlier [4] that the Bi-Mo catalysts show a constant activity for a long time at higher than 3% oxygen content in the reaction products (in the range 410-490°C). The total conversion rate of n-butenes and that of COs formation will be independent of oxygen concentration under these conditions. B y using molar proportions of 1 : 1.5 : 5 of C4Hs : 02 : H~O, the oxygen concentration remained above 3 mole~o in the reaction products in all tests. The yields of p r o d u c t s obtained with an industrial butene fraction, which was practically an equilibrium mixture of n-butenes, on fed starting material is shown as a function of W / F for 5 temperatures ranging from 390 to 470°C
Oxidative dehydrogenation products from n-butenes
109
in Fig. 1 ( W = m a s s o f catalyst, g; F = t o t a l r a t e o f gas flow, moles/hr). T h e results were o b t a i n e d a t c o n s t a n t flow r a t e o f gas, W / F was v a r i e d b y changing t h e a m o u n t o f c a t a l y s t used. T h e r e was a c o n t i n u o u s increase o f C02 a n d f u r a n yields w h e n W / F was increased (up to W / F : 3 0 ) a t all t e m p e r a t u r e s .
% 1o0 ~-
Z_A.~,5
5
75
20 ~ 0
t I0
t 20
t
r 30
3
[]
"
19
,,
2
iz 5
,
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, ¢
1°o
5
I0,
, 25 , 30 , 15, 20 W/F;,g ,hr/rno/e
5
8
lO
o i o
15
20 25 30 W/F, g~hp/mole
,,4
4 2 0
3 70
20
G
2 30
3
20
10
4
4
2
5
30 3
2 ~
5
70
/5
20 25 30 W/F,,g./;r/mote
FIG. 1. Effect of temperature and contact time on the conversion of n-butenes (I) to reaction products (on fed hydrocarbons): II--butadiene; I I I - - C O ~ ; IV--furan; V--total carbonyl compounds; VI--total acids. Temp. °C- 1--390; 2--410; 3--430; 4--450; 5--470; equilibrium mixture of n-butcnes; - - - but- 1-ene.
A t W / F = 3 0 a n d 470°C t h e yield o f CO s was 20.5%, t h a t o f f u r a n 6%. T h e yields of t o t a l c a r b o n y l c o m p o u n d s a n d o f acids t a k e n as a f u n c t i o n o f W / F r e a c h e d e x t r e m e values a t high t e m p e r a t u r e s . F o r example, the e a r b o n y l yield r e a c h e d a m a x i m u m o f 7 % a t 470°C a t W / F = 2 0 while t h a t o f acids a t the same t e m p e r a t u r e was a t W / F = 1 3 . 5 . A t e m p e r a t u r e r e d u c t i o n shifted
110
A.
L.
TSAILINGOL'D
et
al.
t h e m a x i m u m acid yield t o w a r d s larger W / F values a n d t h e yields increased a t t h e s a m e t i m e . T h e l a r g e s t acid yield of 30//0 was a t 450°C ( c o m p a r e d w i t h 1.9% a t 470°C) a n d W/F~--20. T h e e x p e r i m e n t a l n a t u r e of t h e s e relationships is o b v i o u s l y due to t h e severe o x i d a t i o n of t h e c a r b o n y l c o m p o u n d s to C02 and water. T h e d e p e n d e n c e of t h e m a i n p r o d u c t yield, b u t a d i e n e , is also d i s t i n c t l y of a n e x p e r i m e n t a l n a t u r e . I t s yield a t 470°C was 6 3 % a t W / F : 2 0 and then decreased to 59% at W/F:30. T h e m a i n a n d p r a c t i c a l l y all s e c o n d a r y r e a c t i o n s were p u r e l y c a t a l y t i c u n d e r t h e s t u d i e d conditions. T h i s was p r o v e d in t e s t s o f t h e t h e r m a l oxidat i o n of n - b u t e n e s a n d of b u t a d i e n e in a n e m p t y reactor. U s i n g a m o l a r r a t i o of 1 : 1 - 5 : 5 o f h y d r o c a r b o n s ; 0 2 : H 2 0 a t 490°C a n d a 3 sec. residence t i m e , t h e n - b u t e n e s a n d b u t a d i e n e c o n v e r s i o n s did n o t exceed 1 % in t h e t h e r m a l o x i d a t i o n tests. T h e a b o v e T a b l e gives several e x p e r i m e n t a l results w i t h t h e i r p r o d u c t yields. As one can see, t h e c o m p o s i t i o n o f t h e r e a c t i o n p r o d u c t s g r e a t l y d e p e n d e d on t e m p e r a t u r e a n d c o n t a c t t i m e . A n increase o f W / F f r o m 4.7 to 30 a t 470°C c a u s e d t h e b u t a d i e n e yield to decrease f r o m 83.7 to 63.5% while t h e CO, yield increased f r o m 5.3 to 21.8%. T h e r a t i o of C02 t o all o t h e r O - c o n t a i n i n g p r o d u c t s t h e n b e i n g 32.5%, it i n c r e a s e d t o 5 9 . 8 % a t W/F----30. A similar r e l a t i o n also exists for a n increase of t e m p e r a t u r e . T h e largest q u a n t i t y p r e s e n t COMPOSITION
OF R E A C T I O N n-BUTENES
PRODUCTS AND
OF OXIDIZING
BUT-1-ENE
AN EQUILIBRIUM
MIXTURE
OF
O V E R A B1-~¢~O C A T A L Y S T
Hydrocarbon: 02 : I-I20 molar ratio ~ 1 : 1-5 : 5 Product yield on decomposed hydrocarbons %
? Catalyst weight : total flow rate of gas g × hr/mole -~
O ¢2 o O
0
~9
410 470 470
28.0 4.7 30.0
470 470
4.6 18.2
410 470 470
30-9 4.6 28.0
Equilibrium mixture of n-butenes 65-0 69.1 16.5 3"7 2.0 49-0 83"7 5.3 3"0 0'8 93-5 6 3 " 5 21"8 6.5 1"8 69.0 93-7 21.1 13.0 41-5
86.2 67.7
3"4 2"2 2-2
1"4 3"2 2"7
0'8 0"6 0'2
3"1 1"2 1"3
But-l-ene 4-1 3-6 16.9 7.5
1.0 1.0
1"4 1"5
1"9 4"1
0"4 0"1
1"4 1"2
Butadiene 51.0 18.5 36.0 30-9 60-7 20.9
3"0 4"5 2"0
2"2 3"0 3"6
13"3
12"0 3"1 0'7
7"2 2"9
15"3 9"4
m
Oxidative dehydrogenation products from n-butenes
111
amongst the oxygen containing reaction products, besides C02, was furan; its yield reached 6.5% on decomposed butenes. Slightly more of the unsaturated aldehydes was produced amongst the carbonyl compounds at 470°C. The lower acids were dominant amongst the acids; their content decreased with increasing temperature. The Table gives also the results of but-l-ene and butadiene oxidation; the oxidation of all three starting materials gave the same O-containing products. Figure 2 shows the yields of oxidation products as a function of W / F for but-l-ene and butadiene at 470°C; the diagram also gives the results of oxidizing an equilibrium mixture of n-butenes for comparison. One can see t h a t the relationship between carbonyls and acids yield, and W / F is practically the same, which m a y be taken as proof of the same rates of their formation from butadiene and different n-butene isomers. The furan yield was much larger when butadiene was oxidized; this could mean t h a t the furan obtained by oxidative dehydrogenation of n-butanes could be mainly produced from butadiene. The rate of COn formation is also greater when butadiene is oxidized. The :nature of the dependence of C02 yields on the one hand, and of the combined %
%
% ~/00
1
8
6
24 20
4
2
o
80 4O
18 12 5
4
8 4 ~-l
L
10
I
I
I
20
I
30 W/F,g.hr/mote
FIO. 2
0
1.0 02:C4Hs molaPpatio FIG. 3
2.0
Fro. 2. Yields of oxygen containing products as a function of contact time at 470°C. 1--Furan; 2--CO,; 3--total carbonyls; 4--total acids; /--equilibrium mixture of n-butenes;//--but-l-ene; II/--butadiene. FIG. 3. Effect of O, :C4tI8 ratio on process characteristics. /--Selectivity; 2--conversion of n-butenes; 3--butadiene yield; g--CO, yield; 5--furan yield; 6--total carbonyls and acids yield; I--500°C; II--460°C;
112
A.L.
TSAILII~GOI~'D et al.
carbonyl and acid yields on the other, speaks of a parallel-consecutive oxidation mechanism of butadiene. The C02 and furan yield is slightly larger from but-l-ene than from an equilibrium mixture of n-butenes under the same conditions. This can be explained as due to the higher rate of butadiene formation from but-l-ene (Fig. 1) and its subsequent oxidation. The effect of the oxygen : n-hutches ratio was studied in a quartz reactor (42 mm dia.) with a fluid bed catalyst. The apparatus used, method of analysis and calculations did not differ from those outlined earlier. Each experiment lasted 30 minutes, after which the catalyst was regenerated with an Mr/steam mixture. The gas was sampled for analysis on the 10th and 30th minute, the experiments being carried out at W / F = 2 3 , 460 and 500°C using a 7 : 1 molar dilution with steam. The reactor contained 275 c.c. of catalyst of 0.250.5 mm mesh. The Oz : C4I-Is ratio was varied from 0.5 : 1 to 2 : 1, the flow rate of gas remained constant (0.1 m/see, as a result of diluting the reaction mixture with nitrogen. The relationship of n-butenes conversion with product yields is shown as a function of O2 : C4I-Is ratio in Fig. 3. There was no potentiometric titration of the aqueous condensate contents in this test series, b u t the lack of information on the low mol.w, acid content could not have had any great effect on the butadiene yields. The l~igure shows that an increase of the 02 : C4I-Is ratio to 1 : 1 increased the conversion of n-butenes, which remained practically constant on increasing it to 2 : 1. The butadiene yield on fed n-butenes is a function of an extreme nature, the maximum yield at different temperatures being obtainable with different O~:C4Hs ratios. I t was 70% at 500°C and 0-75 : 1, 65% at 460°C and 1.5 : 1. The C02, furan, carbonyls and maleic acid yields increased continuously on increasing the 02 ; C4Its ratio to 2 : 1. It should be noted that a lower than 1.5 : 1 ratio gives a lower than 1 °/o v / v oxygen concentration in the contact gas, which made it impossible to work the process without periodic regeneration of the catalyst [4]. On the basis of the established general principles, one could illustrate the mechanism of formation of the oxygen containing compounds in the above process b y the following simplified scheme:
n.
C4H6
~Hs
CorbongZ oompounda+ ecid3
Furan
Oxidative dehydrogenation products from n-butenes
113
The established correlations indicate the means of obtaining maximum butadiene yields on supplied and converted n-butenes. Without catalyst regeneration, i.e. with more t h a n 3~o oxygen present in the contact gas, one can increase the butadiene yield by shortening the contact time at relatively high temperatures (above 470°C). Where the contact is longer t h a n 5 sec., it will be essential to work with a fluid bed reactor and high butadiene yields will be obtainable with relatively small oxygen to n-butenes ratios, although the catalyst will have to be regenerated periodically. The analytical method for the oxygen-containing products present in the aqueous condensate was developed by Ya. I. Turyan and L. ~ . Kozina, the chromatographic method with the assistance of A. G. Pankov and V. D. Loshchilova SUMMARY
1. The composition of the oxidative dehydrogenation products of n-butenes was studied using a bismuth-molybdenum catalyst. 2. The establihed mechanisms of the rate of formation of the reaction products suggest a parallel-consecutive mechanism of formation of the oxygen containing products and the possible means of improving the butadiene yields are discussed. Translated by K. A. ALLE~ REFERENCES 1. I. Ya. TYURYAYEV, A. L. TSAILINGOL'D, V. V. MASHTAKOV, and V. A. KOLOBIKHIN, Neftekhimiya 4, No. 2, 190, 1964
2. C. R. ADAMS, H. H. VOGE, C. L. MORGAN and W. C. ARMSTRONG, J. Catalysis 3, 379, 1964 3. I. I. PIS'MAN, M. A. DALIN, V. V. KAS'YANOV and E. S. MAMEDOVA, Azerb. khim. Zhur., No. 1, 31, 1963 4. A. L. TSAILINGOL'D, I. YA. TYURYAYEV, F. S. PILIPENKO, M. Ye. BASNER, V. V. DOSHCHATOV and G. A. STEPANOV, Khim. Prom., No. 9, 647, 1965
(Received 27 March 1965)
O~¢E Of the outstanding characteristics of the oxidative dehydrogenation process of n-butenes to butadiene over Bi-Mo catalysts is the large selectivity. An 80% conversion can thus give up to 90 mole% butadiene yields on reacted butenes [1, 2]. Secondary reaction products are C02 and other oxygen containing compounds, b u t very little has been published on the composition of the latter so far. The knowledge of the composition of the reaction products, and also of the mechanism of formation is essential for an understanding of the process mechanism and to the solution of a number of practical problems such as the selection of optimum process conditions, isolation and purification of the butadiene, prevention of corrosion, etc. This paper reports the results of studying the general principles of the formation of these secondary products made up of C02, aldehydes (formaldehyde, acetaldehyde), unsaturated aldehydes, acids (maleie amongst them) and furan. EXPERIMENTAL
The main work of studying the reaction products and of the mechanisms of formation was carried out on a laboratory-scale, flow t y p e reactor with a fixed bed catalyst. The quartz reactor (14 mm int. din.), fitted with a thermocouple sleeve (8 mm ext. din.), was contained in a fluidized sand bath to obtain a practically constant temperature throughout the reactor and fixed bed catalyst. The temperature was measured with a chromel-alumel thermocouple and was maintained within ~ 2 ° C of requirement with the aid of a potentiometer. The flow of gases was determined with calibrated flowmeters, the water feed with an automated dosimeter. The reaction products were released into the atmosphere after passage through a water condenser and a gasmeter. Each experiment lasted 2 hr and the contact gas was analysed at the end of each hour. The aqueous condensate was collected throughout the experiment. * Neftekhimiya 6, No. 3, 367-373, 1966. 107
108
A. L. TSAILINGOL'D et al.
The composition of the catalyst was a 1 : 1 (atomic) mixture of Bi with Mo, 25% of active mass being contained on silica gel promoted with 0.5% P205; the silica gel was of 0.25-0.5 mm mesh. The butene had the following composition (% v/v): 0.5 C8 total hydrocarbons, 1.4 butane, 34.5 but-l-ene, 34.6 trans-but-2-ene, 26 ci&but-2-ene, 1.7 C4H6 dienes; the total C5 hydrocarbon content was 1.3. A certain proportion of the tests was carried out with but-l-cue obtained b y dehydrating primary butanol (anal. purity) b y a method described b y other authors [3]. This gas had the following composition (% v/v): 95.5 but-l-ene, 1.8 transbut-2-ene, 2.5 ci&but-2-ene, 0-2 C4H6 dienes. The butadiene oxidation tests were made with a butadiene distillate of 99.4% purity and with oxygen from air. The contact gas was chromatographed and the CO2, aldehyde, furan, and aerolein contents were determined using n-butyrate of ethylene glycol as sorbent and hydrogen as the carrier gas. The C02, O3 and sum of unsaturated compounds were determined b y using the Ors apparatus. The oxidate contained in the aqueous condensate was checked b y polarography and potentiometric titration. The formaldehyde, acetaldehyde, maleie acid and sum of unsaturated aldehydes (as methaerolein) contents were determined b y polarography on instrument OIg-101 using the standard addition method of compounds to be analysed. Two base electrolytes were used, an alkaline (0.05 ~ of (CHa)4NOH) in the determination of carbonyl compounds, and an acid (a 0.1 N solution of H2SOa) in the determination of maleic acid. The latter and the total acids, such as formic and acetic (determined as acetic) were also determined b y potentiometric titration of the sample in acetone using a 0.1 alcoholic solution of K O H . The yield of reaction products was worked out on computer "Minsk-l" on the basis of supplied hydrocarbons as the ration of converted original hydrocarbon to that supplied (as %); the yield on reacted hydrocarbon was given as the ratio of hydrocarbon, converted to that product, to the sum of converted original hydrocarbon (not taking into account any isomerization). The effects of temperature and contact time on product composition were studied at atmospheric pressure and at a constant proportion of n-butenes, oxygen and steam present in the starter gas. We had shown earlier [4] that the Bi-Mo catalysts show a constant activity for a long time at higher than 3% oxygen content in the reaction products (in the range 410-490°C). The total conversion rate of n-butenes and that of COs formation will be independent of oxygen concentration under these conditions. B y using molar proportions of 1 : 1.5 : 5 of C4Hs : 02 : H~O, the oxygen concentration remained above 3 mole~o in the reaction products in all tests. The yields of p r o d u c t s obtained with an industrial butene fraction, which was practically an equilibrium mixture of n-butenes, on fed starting material is shown as a function of W / F for 5 temperatures ranging from 390 to 470°C
Oxidative dehydrogenation products from n-butenes
109
in Fig. 1 ( W = m a s s o f catalyst, g; F = t o t a l r a t e o f gas flow, moles/hr). T h e results were o b t a i n e d a t c o n s t a n t flow r a t e o f gas, W / F was v a r i e d b y changing t h e a m o u n t o f c a t a l y s t used. T h e r e was a c o n t i n u o u s increase o f C02 a n d f u r a n yields w h e n W / F was increased (up to W / F : 3 0 ) a t all t e m p e r a t u r e s .
% 1o0 ~-
Z_A.~,5
5
75
20 ~ 0
t I0
t 20
t
r 30
3
[]
"
19
,,
2
iz 5
,
5
, ¢
1°o
5
I0,
, 25 , 30 , 15, 20 W/F;,g ,hr/rno/e
5
8
lO
o i o
15
20 25 30 W/F, g~hp/mole
,,4
4 2 0
3 70
20
G
2 30
3
20
10
4
4
2
5
30 3
2 ~
5
70
/5
20 25 30 W/F,,g./;r/mote
FIG. 1. Effect of temperature and contact time on the conversion of n-butenes (I) to reaction products (on fed hydrocarbons): II--butadiene; I I I - - C O ~ ; IV--furan; V--total carbonyl compounds; VI--total acids. Temp. °C- 1--390; 2--410; 3--430; 4--450; 5--470; equilibrium mixture of n-butcnes; - - - but- 1-ene.
A t W / F = 3 0 a n d 470°C t h e yield o f CO s was 20.5%, t h a t o f f u r a n 6%. T h e yields of t o t a l c a r b o n y l c o m p o u n d s a n d o f acids t a k e n as a f u n c t i o n o f W / F r e a c h e d e x t r e m e values a t high t e m p e r a t u r e s . F o r example, the e a r b o n y l yield r e a c h e d a m a x i m u m o f 7 % a t 470°C a t W / F = 2 0 while t h a t o f acids a t the same t e m p e r a t u r e was a t W / F = 1 3 . 5 . A t e m p e r a t u r e r e d u c t i o n shifted
110
A.
L.
TSAILINGOL'D
et
al.
t h e m a x i m u m acid yield t o w a r d s larger W / F values a n d t h e yields increased a t t h e s a m e t i m e . T h e l a r g e s t acid yield of 30//0 was a t 450°C ( c o m p a r e d w i t h 1.9% a t 470°C) a n d W/F~--20. T h e e x p e r i m e n t a l n a t u r e of t h e s e relationships is o b v i o u s l y due to t h e severe o x i d a t i o n of t h e c a r b o n y l c o m p o u n d s to C02 and water. T h e d e p e n d e n c e of t h e m a i n p r o d u c t yield, b u t a d i e n e , is also d i s t i n c t l y of a n e x p e r i m e n t a l n a t u r e . I t s yield a t 470°C was 6 3 % a t W / F : 2 0 and then decreased to 59% at W/F:30. T h e m a i n a n d p r a c t i c a l l y all s e c o n d a r y r e a c t i o n s were p u r e l y c a t a l y t i c u n d e r t h e s t u d i e d conditions. T h i s was p r o v e d in t e s t s o f t h e t h e r m a l oxidat i o n of n - b u t e n e s a n d of b u t a d i e n e in a n e m p t y reactor. U s i n g a m o l a r r a t i o of 1 : 1 - 5 : 5 o f h y d r o c a r b o n s ; 0 2 : H 2 0 a t 490°C a n d a 3 sec. residence t i m e , t h e n - b u t e n e s a n d b u t a d i e n e c o n v e r s i o n s did n o t exceed 1 % in t h e t h e r m a l o x i d a t i o n tests. T h e a b o v e T a b l e gives several e x p e r i m e n t a l results w i t h t h e i r p r o d u c t yields. As one can see, t h e c o m p o s i t i o n o f t h e r e a c t i o n p r o d u c t s g r e a t l y d e p e n d e d on t e m p e r a t u r e a n d c o n t a c t t i m e . A n increase o f W / F f r o m 4.7 to 30 a t 470°C c a u s e d t h e b u t a d i e n e yield to decrease f r o m 83.7 to 63.5% while t h e CO, yield increased f r o m 5.3 to 21.8%. T h e r a t i o of C02 t o all o t h e r O - c o n t a i n i n g p r o d u c t s t h e n b e i n g 32.5%, it i n c r e a s e d t o 5 9 . 8 % a t W/F----30. A similar r e l a t i o n also exists for a n increase of t e m p e r a t u r e . T h e largest q u a n t i t y p r e s e n t COMPOSITION
OF R E A C T I O N n-BUTENES
PRODUCTS AND
OF OXIDIZING
BUT-1-ENE
AN EQUILIBRIUM
MIXTURE
OF
O V E R A B1-~¢~O C A T A L Y S T
Hydrocarbon: 02 : I-I20 molar ratio ~ 1 : 1-5 : 5 Product yield on decomposed hydrocarbons %
? Catalyst weight : total flow rate of gas g × hr/mole -~
O ¢2 o O
0
~9
410 470 470
28.0 4.7 30.0
470 470
4.6 18.2
410 470 470
30-9 4.6 28.0
Equilibrium mixture of n-butenes 65-0 69.1 16.5 3"7 2.0 49-0 83"7 5.3 3"0 0'8 93-5 6 3 " 5 21"8 6.5 1"8 69.0 93-7 21.1 13.0 41-5
86.2 67.7
3"4 2"2 2-2
1"4 3"2 2"7
0'8 0"6 0'2
3"1 1"2 1"3
But-l-ene 4-1 3-6 16.9 7.5
1.0 1.0
1"4 1"5
1"9 4"1
0"4 0"1
1"4 1"2
Butadiene 51.0 18.5 36.0 30-9 60-7 20.9
3"0 4"5 2"0
2"2 3"0 3"6
13"3
12"0 3"1 0'7
7"2 2"9
15"3 9"4
m
Oxidative dehydrogenation products from n-butenes
111
amongst the oxygen containing reaction products, besides C02, was furan; its yield reached 6.5% on decomposed butenes. Slightly more of the unsaturated aldehydes was produced amongst the carbonyl compounds at 470°C. The lower acids were dominant amongst the acids; their content decreased with increasing temperature. The Table gives also the results of but-l-ene and butadiene oxidation; the oxidation of all three starting materials gave the same O-containing products. Figure 2 shows the yields of oxidation products as a function of W / F for but-l-ene and butadiene at 470°C; the diagram also gives the results of oxidizing an equilibrium mixture of n-butenes for comparison. One can see t h a t the relationship between carbonyls and acids yield, and W / F is practically the same, which m a y be taken as proof of the same rates of their formation from butadiene and different n-butene isomers. The furan yield was much larger when butadiene was oxidized; this could mean t h a t the furan obtained by oxidative dehydrogenation of n-butanes could be mainly produced from butadiene. The rate of COn formation is also greater when butadiene is oxidized. The :nature of the dependence of C02 yields on the one hand, and of the combined %
%
% ~/00
1
8
6
24 20
4
2
o
80 4O
18 12 5
4
8 4 ~-l
L
10
I
I
I
20
I
30 W/F,g.hr/mote
FIO. 2
0
1.0 02:C4Hs molaPpatio FIG. 3
2.0
Fro. 2. Yields of oxygen containing products as a function of contact time at 470°C. 1--Furan; 2--CO,; 3--total carbonyls; 4--total acids; /--equilibrium mixture of n-butenes;//--but-l-ene; II/--butadiene. FIG. 3. Effect of O, :C4tI8 ratio on process characteristics. /--Selectivity; 2--conversion of n-butenes; 3--butadiene yield; g--CO, yield; 5--furan yield; 6--total carbonyls and acids yield; I--500°C; II--460°C;
112
A.L.
TSAILII~GOI~'D et al.
carbonyl and acid yields on the other, speaks of a parallel-consecutive oxidation mechanism of butadiene. The C02 and furan yield is slightly larger from but-l-ene than from an equilibrium mixture of n-butenes under the same conditions. This can be explained as due to the higher rate of butadiene formation from but-l-ene (Fig. 1) and its subsequent oxidation. The effect of the oxygen : n-hutches ratio was studied in a quartz reactor (42 mm dia.) with a fluid bed catalyst. The apparatus used, method of analysis and calculations did not differ from those outlined earlier. Each experiment lasted 30 minutes, after which the catalyst was regenerated with an Mr/steam mixture. The gas was sampled for analysis on the 10th and 30th minute, the experiments being carried out at W / F = 2 3 , 460 and 500°C using a 7 : 1 molar dilution with steam. The reactor contained 275 c.c. of catalyst of 0.250.5 mm mesh. The Oz : C4I-Is ratio was varied from 0.5 : 1 to 2 : 1, the flow rate of gas remained constant (0.1 m/see, as a result of diluting the reaction mixture with nitrogen. The relationship of n-butenes conversion with product yields is shown as a function of O2 : C4I-Is ratio in Fig. 3. There was no potentiometric titration of the aqueous condensate contents in this test series, b u t the lack of information on the low mol.w, acid content could not have had any great effect on the butadiene yields. The l~igure shows that an increase of the 02 : C4I-Is ratio to 1 : 1 increased the conversion of n-butenes, which remained practically constant on increasing it to 2 : 1. The butadiene yield on fed n-butenes is a function of an extreme nature, the maximum yield at different temperatures being obtainable with different O~:C4Hs ratios. I t was 70% at 500°C and 0-75 : 1, 65% at 460°C and 1.5 : 1. The C02, furan, carbonyls and maleic acid yields increased continuously on increasing the 02 ; C4Its ratio to 2 : 1. It should be noted that a lower than 1.5 : 1 ratio gives a lower than 1 °/o v / v oxygen concentration in the contact gas, which made it impossible to work the process without periodic regeneration of the catalyst [4]. On the basis of the established general principles, one could illustrate the mechanism of formation of the oxygen containing compounds in the above process b y the following simplified scheme:
n.
C4H6
~Hs
CorbongZ oompounda+ ecid3
Furan
Oxidative dehydrogenation products from n-butenes
113
The established correlations indicate the means of obtaining maximum butadiene yields on supplied and converted n-butenes. Without catalyst regeneration, i.e. with more t h a n 3~o oxygen present in the contact gas, one can increase the butadiene yield by shortening the contact time at relatively high temperatures (above 470°C). Where the contact is longer t h a n 5 sec., it will be essential to work with a fluid bed reactor and high butadiene yields will be obtainable with relatively small oxygen to n-butenes ratios, although the catalyst will have to be regenerated periodically. The analytical method for the oxygen-containing products present in the aqueous condensate was developed by Ya. I. Turyan and L. ~ . Kozina, the chromatographic method with the assistance of A. G. Pankov and V. D. Loshchilova SUMMARY
1. The composition of the oxidative dehydrogenation products of n-butenes was studied using a bismuth-molybdenum catalyst. 2. The establihed mechanisms of the rate of formation of the reaction products suggest a parallel-consecutive mechanism of formation of the oxygen containing products and the possible means of improving the butadiene yields are discussed. Translated by K. A. ALLE~ REFERENCES 1. I. Ya. TYURYAYEV, A. L. TSAILINGOL'D, V. V. MASHTAKOV, and V. A. KOLOBIKHIN, Neftekhimiya 4, No. 2, 190, 1964
2. C. R. ADAMS, H. H. VOGE, C. L. MORGAN and W. C. ARMSTRONG, J. Catalysis 3, 379, 1964 3. I. I. PIS'MAN, M. A. DALIN, V. V. KAS'YANOV and E. S. MAMEDOVA, Azerb. khim. Zhur., No. 1, 31, 1963 4. A. L. TSAILINGOL'D, I. YA. TYURYAYEV, F. S. PILIPENKO, M. Ye. BASNER, V. V. DOSHCHATOV and G. A. STEPANOV, Khim. Prom., No. 9, 647, 1965