Effect of oxygen on the oxidative dehydrogenation of butenes to butadiene over a bismuth-molybdenum catalyst

EFFECT OF OXYGEN ON THE OXIDATIVE DEHYDROGENATION OF BUTENES TO BUTADIENE OVER A BISMUTH-MOLYBDENUM CATALYST* R. A. ZIMI~r, S. Z. I~oGI~CSKII and M. I. YA~OVSKII Institute of Chemical Physics, U.S.S.R. Academy of Sciences (Received 19 June 1965)

P ~ E ~ S dealing with the oxidative dehydrogenation of butenes to butadiene examined the effect of the butene : oxygen ratio on the selectivity and activity of the catalyst. One of these showed [1] t h a t the rate of but-l-ene oxidation did not change when changing the C4H s : (02-bAr) ratio by a factor of 5 (argon was used here as diluent). Other authors stated [2] t h a t the state of phase of the catalyst and its activity greatly depend on the C4H, : 02 ratio because there can be a decrease as well as an increase of the oxygen content of metal oxides. The C4Hs yield was shown to be only 6.7% if no oxygen was present. The same paper [2] also published the finding t h a t the oxygen content was very small in the exhaust gas up to an 02 : C4Hs----1 while there was a continuous increase of the C02 and C4H s content. The selectivity passes through a m a x i m u m when the 02 : C4Hs ratio is increased. The authors [2] stated t h a t the presence of oxygen in the outlet gases ensured the high activity of the catalyst. Experiments on catalyst stability with different component ratios had shown an activity decrease being due to a low oxygen content which caused the valency of Bi and Mo to change. The activity decrease was found not to be due to carbonization of the catalyst surface. Another paper [3] also showed the butadiene yield to pass through a m a x i m u m when plotted as a function of 02 : C4Hs ratio; this was displaced at high temperatures towards a larger ratio. The oxygen adsorption on a Bi-Mo catalyst was also studied using different catalyst compositions [4]. I t was found t h a t MoO3 absorbed most of the oxygen. The same authors also measured the activation energy of oxygen adsorption; this was found to be 20 and 14 kcal for ~ o 0 3 and Bi203 respectively. These seem to be averages because the activation energy normally changes with the amount of absorbed oxygen. This paper reports the results of impulse chromatography, which made a more detailed study of the effect of oxygen on oxidative dehydrogenation possible. * Neftekhimiya 6, No. 3, 374-379, 1966. 114

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EXPERIMENTAL

The method and apparatus used in this chromatographic study was the same as r e p o r t e d earlier [6]. T h e m i x t u r e of n - b u t e n e s used in t h e e x p e r i m e n t a l o x i d a t i o n s h a d t h e following c o m p o s i t i o n (~o): 82.2 a-CiHs, 7.6 fl-trans-C4Hs, 10.2 fl-cis-C4Hs. The o x i d a t i o n a g e n t was a technical grade o x y g e n supplied from a cylinder. The s t a r t i n g m i x t u r e o f n - h u t c h e s was p r e p a r e d in a syringe a n d fed into the r e a c t o r as impulses of a certain f r e q u e n c y . The c a t a l y s t b e d h a d a d e p t h o f 12-13 m m , t h e q u a r t z r e a c t o r a 7 m m dia. The w e i g h t of t h e c a t a l y s t was 0.5-0.7 g, its surface 1.2-1.7 m2/g [5]. The p r o d u c t analysis was carried o u t on two in-line columns w i t h a t h e r m a l c o n d u c t i v i t y d e t e c t o r b e h i n d each column. The t o t a l 0 2 + C O c o n t e n t was d e t e r m i n e d in the first column, also CO2 a n d t h e s e p a r a t e Ca h y d r o c a r b o n fractions; t h e second column, p a c k e d w i t h a c t i v a t e d carbon, was used to s e p a r a t e 02 from CO. The carrier gas was helium. A r g o n was used in a few cases o f h y d r o g e n analysis. The c o n t a c t t i m e was calculated f r o m t h e flow rate o f helium in t h e r e a c t o r at a given pressure a n d t e m p e r a t u r e . RESULTS AND THEIREVALUATION The Table below lists t h e results of s t u d y i n g t h e d e p e n d e n c e of p r o d u c t c o m p o s i t i o n on t h e o x y g e n c o n t e n t o f t h e fed p o r t i o n of m i x t u r e a t 423°C. The sample was p r e p a r e d as follows: a v o l u m e o f 1 ml o f m i x t u r e was t a k e n u p into t h e syringe, t h e n a certain v o l u m e o f oxygen, t h e t o t a l v o l u m e being m a d e u p to 3 ml w i t h helium. T h e Table shows t h a t a n increase of 02 c o n t e n t COMPOSITION

OF

THE

PRODUCTS

OF

OXIDATIVE

DEHYDROGENATION

AS

A

FUNCTION

OF

C4H8 : O3 RATIO Temp. 423°C, 2.3 aim. pressure; total volume 3 cm 3 of which 1 cm s butenes, 0.5 sec contact time Ca-hydrocarbon fraction 02 content Butadiene Absolute yields, 02 of sample composition cm a yield on utilization, fed o/ 7o, but-l- trans- cis-but- butaO/ butene, em a /0 but-2CO2 CO V/V. erie 2-ene diene % erie 0.2 0.4 0.8 1.0 1-2 1.6 2"0

6"66 13.20 26.40 33.30 39.96 53.30 66.60

24"2 23"6 17"4 22"0 23"4 18"3 19"0

9'1 5"6 23"0 25.4 24.7 23-1 24-0

25"0 25"0 21'9 13'7 13"5 19"3 20"8

20"7 25"8 37"7 38"9 33"4 39"3 36.2

22"6 26"2 25.2 24.4 23-2 24.6 26.1

0.06 0.10 0.28 0.34 0-47 0.56 0.76

0"02 0.03 0.07 0.09 0.10 0.16 0.23

85'5 91"5 90'5 90"0 89"0 92'5 92"0

in t h e initial m i x t u r e f r o m 6.66 t o 66.6~/o (equivalent to a change o f C4H s : O3 ratio f r o m 10 : 2 to 1 : 2) g a v e a b u t a d i e n e yield w h i c h was p r a c t i c a l l y indep e n d a n t of o x y g e n c o n t e n t a t a 0.5 sec. c o n t a c t time; its c o n c e n t r a t i o n in

116

R . A . ZIlgIN et

al.

the C4 fraction increased at the same time. This is taken to indicate t h a t the butadiene is the most resistant to oxidation under these conditions. There was a linear increase of CO Sand CO yields and their rate of formation was the same. A similar mechanism was found to exist for the three studied contact times (0.5, 1.0 and 2.0 sac.). The degree of oxygen utilization was large in all cases. The experimental series was followed by one in which oxygen was fed into the reactor at reaction temperature and the peaks were determined at the outlet. The difference between the introduced amount of oxygen and t h a t found at the outlet was in this case about 1 ml. Such treatment of the catalyst appeared to cause C02 not to be formed, as it could not be detected. It therefore follows t h a t carbonization and tar formation does not take place at a large initial oxygen content of the mixture. The second experimental series was carried out by feeding the butenes into the reactor without oxygen. The catalyst was treated before the test with oxygen until the oxygen peak area was identical with t h a t obtained when oxygen was fed directly into the chromatographic column, indicating t h a t the catalyst was saturated. Figure la shows the chromatogram of reaction products on a fresh catalyst after passage of an n-butane mixture without oxygen. One can see t h a t two reactions take place, i.e. an isomerization and dehydrogenation, the rate of the former being the larger. Figure 1 also shows the chromatograms after the 12th, 22nd, 32nd, 42nd, and 52nd introduction of separate doses of mixture into the reactor. There is a rapid change of the reaction rate with increasing number of doses. The rate of isomerization decreases continuously and becomes practically zero at the end of the test. The rate of butadiene formation (dependent on the number of doses) passed through a maximum. The catalyst was found to be capable of taking up 10 cm 3 02 after the experiments; about 4 cm 3 C02 were produced during oxygen absorption. This type of catalyst regeneration restored the original properties of the catalyst and the chromatogram would again be identical with t h a t shown in Fig. la. I t should be noted t h a t the oxygen of the surface area will be utilized even with a large excess of oxygen present in the reaction mixture. The composition of the doses used in one of the test series was 1 cm a C4Hs+2 cm 3 02, i.e. there was a 4-fold excess of 02 present compared with the stoichiometric quantity. After metering the 50th dose into the reactor containing 0.7 g Bi-Mo catalyst the ehemisorption of oxygen was studied by chromatography. The catalyst absorbed 0.8 cm a 02, apparently to replenish t h a t having come off the surface. A second series of tests, in which the amount of 02 present was stoichiometric (0.5 cm 3 02-~-1 em a C4Hs) , gave a catalyst absorption capacity of 2.0 cm a 02 after the 50th dose. We should like to emphasize t h a t the 03 absorption capacity seems to be linked with the reaction because the treatment of the catalyst with helium for 7 hr at reaction temperature did not produce a n y extra oxygen absorption capacity.

Oxidative dehydrogenation of butenes over a bismuth-molybdenum catalyst

117

These findings show that the feed of purely the mixture of butenes will lead to part of the oxygen bound to the catalyst surface being utilized in the dehydrogenation and for the production of CO~ and CO. Oxygen thus has a dual function, i.e. to maintain the activity of the oxidized form of catalyst and to prevent the catalyst from becoming covered with a surface film of polymer or carbon.

2

rnL/

4 3

~Z~l f

5 /5

/0

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1

f 15

5

10

m/o

5

C

d 4

I 15

18

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3

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1 md,

5 I

2

8

f

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FIG. 1. Changes of product composition after introduction of n-butenes without oxygen, a--f--Chromatograms after 1st, 12~h, 22rid, 32nd, 42nd and 52nd dose of sample: 1--CO~; 2--a-butene; 3--trans.but-2-ene; 4--cis-but-2-ene; 5--butadiene.

X-ray diffraction pictures were taken of the catalyst at different stages of the process; these are shown in Fig. 2, where changes of the state of phase are shown to occur after treatment with only the butenes for various times, these changes being completely reversible by treatment with oxygen. Figure 3 shows the gradual development of the catalyst with a 1 : 5 ratio of O~ : Call s at 490°C, the catalyst having been treated with oxygen before the experiment. One can see that severe oxidation takes place in the early stages and that the reaction intensity is great; this is indicated by the large and diffuse CO~ peak. Subsequently the C,Ha yield increases somewhat and the C02 peak decreases, also becoming more distinct. It must be assumed that the fresh catalyst contained less rigidly sorbed oxygen utilized during the

118

R.A.

ZIMI~¢ et al.

FIG. 2. X - r a y diffraction pictures of the Bi-Mo catalyst: a - - O r i g i n a l sample, composition Bi2082MoOa; b - - c a t a l y s t treated with the butene mixture without oxygen (Fig. 1 f ) ; c - - c a t a l y s t sample as in b after saturation with oxygen; d - - c a t a l y s t sample with m a x i m u m butadiene activity without oxygen present (Fig. 1 c); e-catalyst sample after prolonged t r e a t m e n t with a 1 : 2 m i x t u r e of C4Hs:O~. mV 6

~z

5

4

_L~!

3 YO

5 mio

2

5

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0 min FIG. 3. Development of catalyst i n a 5 : 1 m ix tu r e of C4H8 : O~ at 490 °C. a, b, c-- 1st, 2nd and 4th dose respectively: 1 - - O , + C O ; 2--CO~; 8 - - b u t - l - e n e ; 4 - - t r a n s . b u t . 2-ene; 5 - - c i s - b u t - 2 - e n e ; 6--butadiene. 20

5

m/n

Oxidative dehydrogenation of butenes over a bismuth-molybdenum catalyst

119

t r e a t m e n t of the catalyst. I t s s t a t e o f phase a f t e r r e d u c t i o n b y b u t e n e s (Fig. 2b) has n o t b e e n established so far. T h e tests a t 470°C were m a d e to find out w h e t h e r h y d r o g e n is l i b e r a t e d during t h e d e h y d r o g e n a t i o n ; argon was used as the carrier gas for this purpose. A dose c o n t a i n i n g a 1 : 1 m i x t u r e of C4Hs : 02 showed no h y d r o g e n present. This was followed b y t h e feed of o x y g e n with h y d r o g e n into the reactor, w i t h o u t butenes, a n d t h e h y d r o g e n was not oxidized at the reaction t e m p e r a t u r e . Similar results were o b t a i n e d w h e n an H2d-O2d-C4H s m i x t u r e was introduced. I t t h e r e f o r e follows t h a t h y d r o g e n is not one of the i n t e r m e d i a t e s p r o d u c e d o v e r a B i - ~ o catalyst; this is in a g r e e m e n t w i t h t h e findings of others [ 1, 2] who studied t h e r e a c t i o n u n d e r d y n a m i c conditions. The cause of t h e peculiar a c t i v i t y differences of the c a t a l y s t w h e n o x y g e n is p r e s e n t or a b s e n t has n o t been finally established. I t could be t h a t t h e largest a c t i v i t y is r e a c h e d at a c e r t a i n Me/O ratio [7, 8]. This is s u p p o r t e d b y t h e tests m a d e w i t h o u t oxygen. W e were able to o b t a i n a 9 5 - 1 0 0 % conversion of b u t e n c s a t such a ratio a n d a 9 3 - 9 5 % efficiency on b u t a d i e n e . SUMMARY

1. T h e b u t a d i e n e yield reaches a peak, which depends on t h e O2: C4Hs ratio a n d t h e t e m p e r a t u r e . 2. O x y g e n i n t r o d u c e d in a m i x t u r e w i t h t h e b u t e n e s is utilized to oxidize t h e l a t t e r b u t also in t h e c o m b u s t i o n o f t h e c a r b o n a n d p o l y m e r film otherwise deposited on t h e c a t a l y s t surface. A certain q u a n t i t y of the o x y g e n is utilized in the o x i d a t i o n of the c a t a l y s t p a r t l y r e d u c e d b y t h e butenes during reaction. 3. X - r a y diffraction studied showed t h a t the state o f phase of the catalyst will change in a m e d i u m deficient of oxygen. This change is c o m p l e t e l y reversible a n d t h e c a t a l y s t will be r e s t o r e d to t h e original state b y t r e a t m e n t w i t h oxygen. Translated by K. A. ALLE~r REFERENCES 1. C. R. ADAMS, I-I. H. VOGE, C. Z. MORGAN and W. ARMSTRONG, J. Catalysis 3,

379, 1964 2. V. A. KOLOBIKHIN, and Ye. I. YEMEL'YANOV, Neftekhimiya 4, No. 6, 829, 1964 3. I. I. YUKEL'SON and E. A. BOGUSLAVSKII, Neftekhimiya 4, No. 6, 834, 1964 4. A. I. GEL'BSHTEIN, N. V. K U L ' K O V A , S. S. STROYEVA, Yu. M. BAKSHI, B. L. LAPIDUS, I. B. VASIL'EVA and N. G. SEVAST'YANOV, in: Scientific Basis of the Selection and Production of Catalysts. Novosibirsk. Izdat. Akad. Nauk SSSR, 201, 1964. 5. I. K. KOLCHIN, S. S. BOBKOV and L. Ya. MARGOLIS, Neftekhimiya 4, No. 2, 301, 1964 6. S. Z. ROGINSKII, M. I. YANOVSKII and G. A. GAZIEV, Kinetika i kataliz 3, 529, 1962 7. C. R. ADAMS, Third Internat. Congr. Catalysis, Amsterdam I, 240, 1964 8. W. M. SACHTLER and N. tt. BOER, As under 7, p. 252