Catalytic properties of system SnO2:Sb2O4 in the oxidative dehydrogenation of n-butenes to butadiene

CATALYTIC PROPERTIES OF SYSTEM SnO2:Sb204 IN THE OXIDATIVE DEHYDROGENATION OF n-BUTENES TO BUTADIENE * Y r . M. BAKSHI, R. N. GUI~'YANOVA, A. N. MAL'YAN a n d A. I. GEL'BSttTEIN L. ¥8. Karpov Physicochemical Institute

(Received 11 January 1966) Tg~. information contained in the patent literature [1-6] shows that it is possible to use various proportions of Sn02 with Sb~O4 as an oxidative hydrogenation catalyst of n-butenes to butadiene: C4H8-~-½0 2 --> C t H 6 ~ - H 2 0

(1)

The analysis of this information showed the activity and selective action indicated in reaction (1) to depend on the isomer composition of the starting butenes and on the temperature of preliminary heating of the catalysts. Earlier work by one of us dealt with the phase composition, eleetrophysical parameters (electron yield energy and electroconductanee) and catalytic properties of this system in the partial oxidation of propylene and isobutylene to the respective aldehydes, and in the ammoxidation of the first named to acrylonitrile, with respect to the composition of the system [7]. The lack of information about the relationship between catalytic properties of system SnO 2 : Sb~O~ and its chemical and phase composition in reaction (1) formed the basis of our study, which is reported here. Catalysts in this system with a variety of compositions were prepared from antimony pentoxide and metallic tin solutions in boiling, concentrated nitric acid, followed by drying and heating of the precipitate for 16 hr at 700°C. This mass was ground into a fine powder, pressed into tablets, these again heated for 10 hr at 700°C, then milled and the particle fraction 2-3 mm was used. Low-temperature nitrogen adsorption was used to determine the specific surface of the various samples before and after use in the reaction. The catalytic properties of the individual components and of mixtures of different proportion were determined by a circulation-flow (non-gradient) method at 450°C in reaction (1), using either but-l-ene (a fraction containing 99.6% v/v of but-l-ene), or a fraction having the composition: 29.6% but-l-ene, 22.6% cis-but-2-ene, 31.6% trans-but-2-ene, and 3.12% 2-methylpropene. The 13% * Neftekhimiya 7, No. 4, 537-543, 1967. 177

178

Yu. M. B~s]~_t et al-

of impurities consisted of ethane, propane, ethylene, propylene, butane, isobutane, acetylene and others. Chromatographic and chemical methods were used to analyse the reaction products. The chromatographs were types Kh.L.-3 and -4 having thermal conductivity detectors. The brick fragments were of type I. N. Z.-600 and of 0.25-0.5 mm din.; these were soaked with fl,fl'-hydroxypropionitrile and paraffinous oil, and placed into an (11 m long) column. The use of the above 2 stationary phases in certain proportions (9.5 m length of nitrile phase and 1.5 m with paraffinous oil) made it possible to carry out the analysis of the multi-component gas mixture containing ethylene, propylene, COe, acetylene, butane, isobutane, but-l-ene, 2-methylpropene, trans-, cis-but-2-ene, and butadiene in 20 rain at 25°C. The contents of O, N and CO were determined in a column packed with type 5A molecular sieves. A certain combination of stationary phases consisting of dioctyl sebaceate (1-7 m), polyethylene glycol adipate (0.3 m) and fl,fl'-diethyldieyan sulphide (3 m) in a 5 m long column made it possible to determine simultaneously a number of O-containing secondary products of reaction (1) at 80°C, amongst them furan, acrolein, methacrolein and acetaldehyde. B0 ~ r____C___, nl. firm-2

% ivlv ex

^

-180

8

<

oo

8nOz 80 L ~ 1

.

o

8o

~0

zO

t

I

I

3

1_.

1

lsn/Sb

~ 2o, ~

I

8b204,%wf. I

~

111 atomic1 ratio24 8 32

Fie. 1

.~

~ 8n°2

c'3

[

8

T

I

80

I

60

3

I 1

T

T

~o

8n/Sb atomic

~

I 1

e

2o

[ 1

Sb20,%wt. I 1

1"

/4 8 3 2

f~tio

FIG. 2

Fro. 1. Effect of catalyst composition on its catalytic properties in reaction (1)" 1--olefin/butene fraction (mixture of n-butenes); 2--original olefin/but-l-ene mixture. Fro. 2. ~o Conversion of n-butenes a n d sdectivitles as a function of chemical composition of catalyst: / - - s e l e c t i v i t y for butadiene; 2--selectivity for the oxides of carbon; 3 - - t o t a l n-butcnes conversion; 4 - - a s 3 to butadiene; ~--as 3 to oxides of carbon.

Catalytic properties of system SnO~: Sb=O4

179

Two methods were used to establish the dependence of the catalytic properties of the system SnO 2 : Sb204 on its chemical composition. According to the first the composition and ratio of components in the reaction mixture differed over various catalysts, even when the starting mixture fed into the flow-circulation system consisted of butenes, oxygen and nitrogen of identical composition. This was due to different activity- and efficiency levels of the catalyst samples. In the second method an approximately constant ratio of partial pressures of butene to oxygen was used over the catalyst b y adjusting the original mixture. The experimental results are compared in Tables 1 and 2, and also shown in Figs. 1-6; these lead to the following conclusions. The catalytic activity of the system can be characterized, independent of the method used to determine the catalytic properties, b y the ratio of total conversion rate of butenes (r), related to unit surface, to the partial pressures of butenes and oxygen. The use of this parameter leads to the conclusion that catalytic activity increased continuously when increasing the contents of the tin cations in the catalytic system; it was at a maximum where Sn02 was used alone (Tables 1, 2, Fig. 1).

5

1

8nO2 I

80 I

1

60 I

3

1-

~o l

f__

1 3n/Sb atomic r~tio

ro Sb=O~,%~. I

I 2

1

I f 4832

I

f

Fze. 3. Specific rate o£ n-butene conversion as a £unetion o£ chemical composition of the catalytic system: 1-- total conversion rate of n-butenes; 2--total conversion to butadiene; 3--to oxides of carbon. T h e conversion efficiency t o b u t a d i e n e on reacted butenes (Tables 1, 2, Fig. 2) was at a m a x i m u m w h e r e t h e Sn/Sb a t o m i c r a t i o was e q u a l or smaller

than 1. The selectivity of pure antimony oxide was lower than of mixtures with the indicated atomic ratio of components. The selected r / p c , ~ . l ~ o , for comparing the catalytic activities of various compositions was an arbitrary process, because we have no exact data on reaction kinetics with various compositions. The use of any other criteria however, is still less justified. A factor such as the total conversion of butenes does not take into account the

30.9

37'5 I 61"7

37.5168.4

5

4

4

62.2

53.3 61.8

9.1 18.3

SnO~ 9

32.0 41"8 54.6 51"5 47.7 41.5 45.3 31.9 26.9 21.3 10.8 5.78 4.17

14.1 18.3 37.7 38.0 20"21 19.6 18.7 16.0 14.0 18.2 10.3 15.0 15.3

°

~° ~ ®~.

SnO~ 9 3 4/3 1/2 1/3 1/4 1/6 1/8 1/12 1/32 1/64 Sb~0~

"'I°

~>>

46"0

53'2

51.2

26-3 36.5

40"3 52"3 63.3 64"0 69.0 75.5 66.7 73.3 70.1 72.5 65.6 57.0 10.1

o

~! ~ o

36'7

30'7

36.1

62.8 46"5

51.3 33.3 20.1 23.4 13.0 11.1 12.8 10.5 10.9 7.95 8.90 8.80 5.60

5.64

I

6'4]

0'85

I

0"4

I

]

I

2"2

2.1

3.7 1.3

11.7 5.0 2.4 4"4 0"91 0-79 0.87 0.79 0.79 0.75 0.76 0.78 0.77

SI

X~¢~

4-1510"94

5"22

Component ratio n-C~Hs:O~:N2= 1:1.8:4.2 5"20 I 6'6 ] 0"81 [ 0"3 I 0'64

0'43

5.26

Component ratio n-C,H~:0~:N~= 1:2-1:3.7 5-50 I 6"0 I 0"61 I 0.3 [ 0.4

Component ratio n-C,Hs:0~:N2= 1:2.8:2.8

6.39 5.45

. . 0.7 0.2 2"6 0.6 1.2 0.4 3.6 0.6 3.4 0.4 3"8 0.5 3.5 0.2 4.0 0.4 1 _Y"aldehyde 4.5 0.4 5.0 0'8 _~ aldehydes

Component ratio n-C~H~:O~:N~= 1:2.82:2.8 6.86 0.63 traces traces traces 8.13 1.8 1.2 0"7 1.3

.

8.52 8.22 6.00 6.74 7.02 7.68 7.37 9"54 10"3 11.3 12.7 13.9 14-1

. 2"7 6.4 5.5 6"4 3"6 7.2 3.7 5.3 ? 3.9 5.0 traces 0.8 2.4 1.0 4.7 4.1 5.6 6.3 6.3 12.9 12.8 18.0 34

5.10 7.25 4.50 3.83 2.44 2.41 2.76 2-50 2.75 1.94 3.00 3.00 2-60

Component ratio in starting mixture n-C~H=:O=:l== 1:1.33:4.33

8

~o v/v Conversion on reacted n-butenes t o _ _

450 °C, 0.6 see. contact time

I

[

3-74

3'36

4.05

11.8 7.05

4.43 4.58 3.23 3-19 4.73 4.71 4.92 4.35 4.73 2.87 2.61 0.99 0.73

~o : ' ~ ®I

X®~

2"05

2-68

3.15

10.8 3.05

7.10 3-47 2.15 3.06 0.875 0.625 0.784 0.376 0.351 0.169 0.123 0.040 0.028

TABLE 1. OXIDATIVE DEHYDROGENATION OF n-BUTENES OVER SYSTEM SnO2 : Sb204 HAVING DIFFERENT COMPOSITIONS

•

Catalytic properties of system SnO~: SbsO~

181

different specific surfaces of c a t a l y s t s o f different compositions, a l t h o u g h these are s u b s t a n t i a l (Table 1). U n a c c e p t a b l e is also a relative a c t i v i t y e v a l u a t i o n based on the specific r a t e of t h e t o t a l c o n s u m p t i o n o f b u t e n e s (Fig. 3), or of the p a r t i a l pressures o f reagents a n d o f r e a c t i o n p r o d u c t s o v e r catalysts of different compositions. T h e r e can be s u b s t a n t i a l differences, especially where t h e c a t a l y s t contains larger q u a n t i t i e s o f Sn cations. A b e t t e r assessment o f t h e a c t i v i t y difference will be possible o n l y where r e a c t i o n (1) is carried out o v e r catalysts with different compositions using t h e same ratio of t h e p a r t i a l pressures o f b u t e n e s a n d o x y g e n (Fig. 4). T h e results p l o t t e d in Figs. 1-4 show t h a t the relative activities of different composition catalysts will differ in a c c o r d a n c e w i t h the m e t h o d s used to e v a l u a t e t h e m . A more u n i f o r m picture is o b t a i n e d where the criterion for e v a l u a t i n g a c t i v i t y is r/pc,~/po ~. T h e analysis o f t h e e x p e r i m e n t a l findings indicated the presence of a significant d e p e n d e n c e of t h e o b s e r v e d reaction r a t e a n d selectivity on ratio TC,H,/Po~"T h e larger this value, the smaller will be the o b s e r v e d specific rate, and t h e larger t h e selective c a t a l y t i c a c t i v i t y . This correlation becomes m o r e p r o n o u n c e d with a larger c o n c e n t r a t i o n of Sn cations in t h e catalytic system. These facts h a v e t h e i r n a t u r a l e x p l a n a t i o n w h e n assuming t h a t t h e increase of

TABLE

2.

OXIDATIVE

DEItYI)ROGENATIOI~" OF B U T - I - E l S E

OVER

SYSTEM

She 2 :

Sb204

HAVING D I F F E R E N T COMPOSITIONS

450 °C ,0.43 sec. contact time % v/v Conversion on reacted but-l-ene to different products

o

o

SnOs* 9 3 4/3 1/3 1/12 1/64 Sb~O~

65.1 85.9 67.8 66-6 51.1 29.5 5.36 4.26

~

44.0 30.9 70.4 79.4 78.4 86.7 89.0 58.8

2.1 0.7 2.6 3.4 7.5 2.6 5.7 38-8

48.0 50'0 16"3 9.1 5.1 3.1 5.3 3"5

* T e s t with 0"048 sec. c o n t a c t time

2.7 16.2 5.0 4.9 6'0 4.7 --

0.6 1.1 2.5 2.5 2.6 3.0 ---

~ ~

~

°~

~

~.

~

::~

0.6 0.1 0"5 0.4 0.4 0.4 --

3.02 2.57 5.33 4.59 4.21 4-31 4.80 4.76

0.51 105 0.40 22.71 0-65 9"1 0.63 7"41 0.46 6.27 0.42 2.77 0.65 0.51 0.65 0.43

~

58.8 13-6 2.09 2.21 1.63 0.623 0.144 0-116

182

YI~. M. BAKSHI et al.

the last named ratio also increases the degree of reduction capacity of the catalyst surface (oxygen depletion), which creates a reduction of catalytic activity and an increase of selectivity. Such an interpretation of the discovered mechanisms finds further support on analysing the results of 2 series of tests on a catalyst having Sn/Sb = 1/3. The partial pressure of oxygen was changed in each test series while keeping t h a t of but-l-ene almost constant (pz). The 2 test series thus chiefly differed by the value of the partial pressure of butene. r,lO3,nl ~Zhr 100 80

GO

20

1 2 3 / / / I

sn o

80 I

9 T

60 3 T

•

J I

l

~o

I I

1_ I__ 1 2 2n/Sb ~tornic ratio

zo ] l 4

,~b20~,~wt. 1 1 8 32

Specific rate of but- 1-eneconvers;on to reaction products as a function of chemical composition of SnO2/Sb=04: /--total conversion rate; 2--conversion rate to butadiene; 3--conversion rate to the oxides of carbon. FIG. 4.

Changes of the oxygen concentration in the circulating mixture affected the reaction rate unequally as a function of but-l-ene concentration. The total conversion efficiency of but-l-ene at p 1 : 5 × 10 -2 arm depended practically on pO~ throughout the whole range of the changes of partial pressures of oxygen (Fig. 5). Where the but-l-ene concentration in the reaction zone was smaller (p1=0.6× 10 -~ arm) one could experimentally observe a range of oxygen pressures on which the latter dependence was slight (Fig. 6). A comparison of the two test series results, when referring to the same partial pressure of oxygen, but different partial pressures of but-l-ene, showed evidence of lower catalytic activity where the latter was larger. The capacity of catalysing the full oxidation of olefin and of its partial oxidation products increased with decreasing partial pressure of butene while t h a t of oxygen remained constant. The sum of all the above results permits the conclusion t h a t the mechanism of the studied reaction can be represented by a scheme which includes as stages

Catalytic properties of system SnO2: Sb20~

183

the alternation of reduction and oxidation of the lattice cations at the surface. 1) Z ( O ) + X ~ Z(OX)

2) z(ox) +nz(°)(n+l)Z( )+Y 3) Z( )+02 ~- Z(02) ~

2Z(O)

In the above Z (0) represents an adsorption centre, Z ( )--an oxygen vacancy at the surface, (O)--oxygen chemisorbed by a partly reduced surface, X - - t h e starting olefin, or its partial oxidation product capable of further oxidation, ¥ - - t h e sum of partly or fully oxidized products, also including the final reaction product. A completely selective oxidative dehydrogenation of butenes to butadiene will have factor n = 0 in stage 2. %v/v 60

".~

I

.,~

~

%v/v ~,

~,~,0

×

l'X-2

°

5

,0

,5

Po; 1o', ~t., FIG. 5

e0

e5

-

'

2'0

p, . 1o~, ~,tm 02

FIG. 6

Fro. 5. Partial pressure changes of oxygen (450°C; 0.15 sec; p1=5 x 10-2 atrn): /--total conversion of but-l-ene to all reaction products. Conversion efficiency of but-l-ene to: 2--butadiene; 3--furan; d--carbon oxides; 5--acrolein; 6--propylene; 7-- acetaldehyde. Fro. 6. Partial pressure changes of oxygen (450°C; 0.15 sec; p1=0.6 × 10-2 arm): /--total conversion of but-l-ene to all reaction products. Conversion efficiency of but-l-ene to: 2--butadiene; 3--furan; d--carbon oxides; 5--acrolein. According to the results of X-ray structural and thermographie phase analysis [7], carried out in the presence of one of us, there are three different zones present in system SnO 2 : Sb204 which can be isolated. The first with composition Sn/Sb smaller t h a n ½ is an SnO 2 solid solution in Sb204 with the structure of Sb204, the second, Sn/Sb greater t h a n 5, a Sb204 solid solution in SnO~ with a structure of SnO~, and the third a heterophase region containing

184

Ym M. BAKSHI et al.

both the above phases of solid solutions. Taking into account t h a t the two metal cations have a similar electron structure in the higher state of valency, it is possible to assume t h a t the adsorption centres marked as Z (0) in the above scheme are oxidised cations on which the oxygen atoms are chemisorbed. An increase of the selectivity of catalytic activity, when changing from Sn02 to compositions containing Sb-cations, can be apparently linked with an accompanying decrease of oxygen mobility in the lattice. The catalytic activity will also decrease in a similar change in the composition of the system. TABLE

3.

O X I D A T I V E D E H Y D R O G E N A T I O N OF n - B U T E N E S

OVER VARIABLE COMPOSITION

SnO~:P~0~ 450°C, 0"6 scc contact time; inital gas ratio l:l.33:4~n-C4Hs:02:N 2 SYSTEM

~ .£

% v/v Conversion of n-C4H8 to different oxidation products

~. ~

@

SnO, 24 4.9 2.4 1.0

~

~

14.1 35.9 25.0 4.0 0.8

32.0 38-7 / 39-1 / 25.8 3.12

~ 40-3 38.3 40.5 43.0 63.5

o 51.3 36.3 38.9 36.3 26.5

~

~.~

5.10 -8.52 10.5 2 . 0 0 7.40 16.6 2 . 4 6 6.45 15.1 5 . 2 0 7.20 10.0 10.0

~o

~m~

~

~

11.7 4.2 2.9 1.2 0.5

4.43 2.16 3.05 16.2 7.62

7.10 1.65 2.14 3.75 0.385

The relationship of the rates of stages, including a reduction or oxidation of the surface cations, is determined by the original composition of the Sn/Sb catalyst, but also by the ratio of concentrations of the reducing components, and of oxygen, in the reaction zone. The findings reported in Table 2 fully confirm this conclusion. The catalytic properties of SnO~, as influenced by tho presence of Sb, could also be expected from its analogue, i.e. phosphorus. A comparison of the data in Tables 1 and 3 shows this to be true. An increase of the P-content in system SnO~ : P~O s reduced the catalytic activity while the selectivity increased. SUMMARY

1. The influence of chemical composition in the mixed catalysts SnO~ : Sb~O, and SnO, :P,O~ on their catalytic properties was studied in the oxidative dehydrogenation of n-butenes to butadiene. An increase of the Sn-cations concentration in these systems increased their catalytic activity but their select i v i t y for butadiene decreased.

Catalytic properties of system SnO~: Sb~04

185

2. A n o p t i m u m c o m b i n a t i o n of a c t i v i t y a n d s e l e c t i v i t y in t h e s t u d i e d r e a c t i o n was possessed b y c a t a l y s t s in w h i c h t h e a t o m i c r a t i o of Sn/Sb w a s in t h e rangel:lto 1:4. 3. T h e a c t i v i t y a n d s e l e c t i v i t y is f u r t h e r m o r e influenced b y t h e r a t i o of r e a c t i o n c o m p o n e n t s o v e r t h e c a t a l y s t in a r e d u c e d a n d oxidized state. 4. T h e oxides of a n t i m o n y a n d p h o s p h o r u s h a d identical effects on t h e c a t a lytic p r o p e r t i e s of t i n oxide a n d this was s h o w n b y changing f r o m SnO~ to m i x t u r e s w i t h t h e s e oxides. Translated by K. A. ALLEN REFERENCES

1. 2. 3. 4. 5. 6. 7.

Brit. Pat. 968924, 9.09, 1964; Russk. zh. khim. 3, 3L244P, 1966 Brit. Pat. 945707, 8.01, 1964; Russk. zh. khim. 23, 23G188P, 1965 Brit. Pat. 967134, 19.08, 1964; Russk. zh. khim. 3 (II), 3L243P, 1966 French Pat. 1294907, 24.04, 1962; Russk. zh. khim. 19, 19H3P, 1963 Brit. Pat. 902952, 9.08, 1962; Russk. zh. khim. 16, 16H10P, 1963 Brit. Pat. 904602, 29.08, 1962; Russk. zh. khim. 12, 12L129P, 1963 S. S. STROYEVA, A. I. GEL'BSHTEIN et al. ~Teftekhimiya 6, No. 3, 412, 1966