Selective Oxidative Coupling of Methane to Ethane and Ethylene

Selective Oxidative Coupling of Methane to Ethane and Ethylene

403 D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conuersion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in Th...

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403

D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak (Editors), Methane Conuersion 0 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

SELECTIVE OXIDATIVE COUPLING OF METHANE TO ETHANE AND ETHYLENE

KENJI ASAMI, SHIGERU HASHIMOTO, KAORU FUJIMOTO and HIRO-0 TOMINAGA Department o f S y n t h e t i c Chemistry, F a c u l t y o f Engineering, The U n i v e r s i t y o f Tokyo, Hongo 7-3-1,

Bunkyo-ku,

Tokyo 113

(Japan)

ABSTRACT

It has been found t h a t l e a d o x i d e c a t a l y s t s supported on b a s i c c a r r i e r s such as MgO o r @'-A1 O3 e x h i b i t e x c e l l e n t a c t i v i t y and s e l e c t i v i t y f o r t h e L a t t i c e oxygen i s proved t o be r e s p o n s i b l e f o r o x i d a t i v e c o u p l i n g o f methane. t h e f o r m a t i o n o f C2 hydrocarbon.

INTRODUCTION S y n t h e s i s o f ethane and e t h y l e n e b y t h e o x i d a t i v e c o u p l i n g o f methane i s a c u r r e n t s u b j e c t o f g r e a t s i g n i f i c a n c e t o a novel use o f n a t u r a l gas. c a t a l y s t s have been r e p o r t e d t o be a c t i v e f o r t h e r e a c t i o n ( r e f s .

Several

1-5).

The

p r e s e n t a u t h o r s have a l s o r e p o r t e d t h a t an MgO-supported PbO c a t a l y s t has h i g h a c t i v i t y and s e l e c t i v i t y f o r t h e f o r m a t i o n o f C2t hydrocarbons ( r e f .

6).

I n t h e p r e s e n t work, s e v e r a l supported PbO c a t a l y s t s have been t e s t e d f o r t h e i r r e a c t i v i t y and found t h a t b a s i c m a t e r i a l s such as MgO o r B"-A1203 a r e e x c e l l e n t c a r r i e r s f o r PbO which make C2 hydrocarbons f r o m methane.

The

d e t a i l s o f t h e c a t a l y t i c f e a t u r e s o f PbO/MgO and t h e r e a c t i o n mechanism have been s t u d i e d .

EX PER IMENTAL A l l c a t a l y s t s were prepared by i m p r e g n a t i n g t h e c a r r i e r m a t e r i a l s w h i c h had been c a l c i n e d a t 800

OC

f o r 2 h. w i t h l e a d n i t r a t e f r o m i t s aqueous s o l u t i o n ,

f o l l o w e d by d r y i n g i n a i r a t 120 O C f o r 12 h. wt%.

The standard PbO l o a d i n g was 20

They were a c t i v a t e d i n f l o w i n g a i r a t 750

OC

f o r 0.5-1

h.

Reactions were conducted w i t h a f l o w t y p e r e a c t i o n apparatus under a t m o s p h e r i c pressure. temperature:

750

OC.

The s t a n d a r d r e a c t i o n c o n d i t i o n s were as f o l l o w s : t i m e f a c t o r (W/F):

1.0 g'h/mol,

14%, 02 1.6%. N2 b a l a n c e , c a t a l y s t w e i g h t : 1.0 g.

feed gas c o m p o s i t i o n : CH4

404 RESULTS AND DISCUSSION Support e f f e c t on PbO c a t a l y s t

8

The r e l a t i o n s h i p between s e l e c t i v i t y and y i e l d o f C2 hydrocarbons o f a l l c a t a l y s t s t e s t e d ( F i g u r e 1) shows t h a t

6

t h e m a t e r i a l s which have b a s i c c h a r a c t e r . a r e t h e most e f f e c t i v e c a r r i e r s o f PbO f o r making

0 acidic A neutral

B"-Al203

.basic

U

Cz

hydrocarbons. The r e s u l t s a r e summarized as follows: (1) A c i d i c c a r r i e r s had h i g h a c t i v i t y and s e l e c t i v i t y t o carbon oxides.

0

(2) N e u t r a l and weakly a c i d i c o r b a s i c

20

40

60

C2 selectivity

c a r r i e r s showed l o w a c t i v i t y b u t

80

100

(C-mol%)

Fig. 1 E f f e c t o f c a r r i e r m a t e r i a l PbO l o a d i n g : 20 wt%. Temp.; 750 OC, W/F: 1.0 g'h/mol,

f a i r l y h i g h s e l e c t i v i t y t o C2 hydrocarbons.

(3) B a s i c c a r r i e r s had h i g h a c t i v i t y and s e l e c t i v i t y f o r C2 hydrocarbon formation. The reasons why a c i d i t y o r b a s i c i t y o f t h e supports s i g n i f i c a n t l y i n f l u e n c e d t h e r e a c t i o n w i l l be d i s c u s s e d i n a l a t e r paper. C h a r a c t e r i s t i c f e a t u r e s o f t h e PbO/MqO catalyst S i n c e an MgO-supported c a t a l y s t showed t h e h i g h e s t s e l e c t i v i t y f o r C2 hydrocarbon,

t h e d e t a i 1s o f i t s The e f f e c t s o f PbO

l o a d i n g a r e shown i n F i g u r e 2.

100

8

80

7 6

60

z 4

40

-

MgO,

3 0

hydrocarbon y i e l d ) .

Both the c a t a l y t i c

e

C

f r

E 0

C >

2

20

i t s e l f , showed some c a t a l y t i c a c t i v i t y

(0.8% o f C H 4 c o n v e r s i o n a n d 0.39% o f C2

-2 rl

e

0

c a t a l y t i c f e a t u r e s were studied. (i)PbO loading.

10

N

0

0

0 PbO loading

(wt%l

a c t i v i t y and s e l e c t i v i t y t o C2 hydrocarbon i n c r e a s e d w i t h i n c r e a s i n g PbO l o a d i n g r e a c h i n g a maximum a t 5 w t %

(72% C2 s e l e c t i v i t y ) and then decreased as t h e l o a d i n g increased.

Fig. 2

E f f e c t o f PbO l o a d i n g

~ ~ ~ p ) ' 0~~'=w~~:11iP69;h~~P:: ~57

405

800

OC

as demonstrated i n F i g u r e 3,

because t h e p r o m o t i v e e f f e c t o f

u 0

20

-

CH4

*-

-

I*

The i n i t i a l CH4 c o n v e r s i o n r a t e t o C2 hydrocarbons (4.0 mmol/g'h)

3 min.

-f:

. P

4 0

E

I 0)

d

e r(

Ll R

>C U

-n X

Fig. 4 T r a n s i e n t response o f methane c o n v e r s i o n on PbO/MgO c a t a l y s t PbO l o a d i n g : 20 wt%. Temp.: 750 OC, W/F: 4.3 g'h/mol.

,..

20 2

.

was

406

TABLE 1. Amount o f t h e consumed b u l k oxygen i n t h e c y c l i c r e a c t i o n . I n i t i a l amount o f PbO 0.896 (mmol) Temperature (OC)

Consumed oxygen (X) (mmol)

~

0.003

650

700

0.085 0.189 0.204

750

800

0.3

9.5 21.1 22.8

f a i r l y close t o t h a t i n the steady s t a t e r e a c t i o n (2.8 mmol/g'h).

whereas t h e

c o n v e r s i o n r a t e t o CO2 (0.2 mmol/g'h)

is

much l o w e r t h a n t h a t i n t h e s t e a d y s t a t e (1.4

mmol/g'h).

As gas phase oxygen

does n o t e x i s t i n t h i s r e a c t i o n system, l a t t i c e oxygen o f PbO must be responsible f o r t h e coupling reaction.

1

( C )

Table 1 shows t h e c a l c u l a t e d amounts

Pb

o f l a t t i c e oxygen o f PbO w h i c h were consumed d u r i n g t h e 12 m i n u t e s o f

I t s h o u l d be n o t e d t h a t t h e

reaction.

amount o f consumed l a t t i c e oxygen increased w i t h increased r e a c t i o n temperature,

w h i c h c o u l d be a t t r i b u t e d

t o t h e increased d i f f u s i o n r a t e o f t h e b u l k oxygen i n PbO. XRD p a t t e r n s o f a PbO/MgO c a t a l y s t

t

I

25

30 deg

under a v a r i e t y o f c o n d i t i o n s a t room

I

I

35

40

128

t e m p e r a t u r e show, as i l l u s t r a t e d i n F i g u r e 5,

t h a t both t h e fresh c a t a l y s t

and t h e used c a t a l y s t ,

w h i c h were

o x i d i z e d b y a i r a f t e r methane conversion,

gave s i m i l a r p a t t e r n s (a) o r

(b) i n w h i c h s t r o n g peaks o f PbO were

3

F'g.

5

a fresh

XRD p a t t e r n s o f PbO/MgO

b)after a i r oxidation i n the c y c l i c reaction ' ) a f t e r CHq c o n v e r s i o n i n t h e c y c l i c reaction.

407

observed.

P a t t e r n ( c ) was o b t a i n e d f r o m a c a t a l y s t a f t e r t h e c y c l i c methane

r e a c t i o n , showing weak peaks o f PbO and s t r o n g peaks o f m e t a l l i c Pb.

It

i n d i c a t e s t h a t PbO i s reduced t o Pb by r e a c t i n g w i t h methane and i s r e o x i d i z e d b y 02.

Thus t h e o x i d a t i v e c o u p l i n g o f methane i s i n f e r r e d t o proceed b y a

redox c y c l e between Pb(0) and Pb(I1).

as demonstrated i n e q u a t i o n s (1) and

(2). PbO

+

2

CHq-Pb

+

CH3CH3

t

H20

(1)

K i n e t i c s t u d y suggests s t r o n g l y t h a t r e a c t i o n (1) i s r a t e d e t e r m i n i n g .

The

s t u d y a l s o suggests t h e COX f o r m a t i o n i s m a i n l y c a t a l y z e d n o t b y l a t t i c e oxygen but by adsorbed oxygen on t h e PbO surface. REFERENCES 1

2

3 4 5 6

G.E.

K e l l e r , M.M. Bhasin, J. Catal., 73 (1982) 9-19. Hinsen, W. Bytyn, M. Boerns, Proceedings 8th I n t e r n a t i o n a l Congress on C a t a l y s i s , Vol. 3 (1984) 581-592. K. Otsuka, K. Jinno, A. Morikawa, Chem. Lett., (1985) 499-500. T. I t o , J.H. Lunsford. Nature, 314 (1985) 721-722. H. Imai, T. Tagawa, J. Chem. SOC., Chem. Commun., (1986) 52-53. K. Asami, S. H a s h i m o t o . T. S h i k a d a , K. F u j i m o t o , H. Tominaga, Chem. L e t t . , (1986) 1233-1236.

W.