Role of Active Oxygen Forms and Acidity in Oxidative Conversion of Ethane on Zeolites

G. Centi and F. Trifiro' (Editors), New Developments in Selective Oxidation 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

483

ROLE OF ACTIVE OXYGEN FORMS AND ACIDITY I N OXIDATlVE CONVERSION OF ETHANE ON ZEOLITES

S. N . VERESHCHAGIN, N . N . SHISHKINA, A. G. ANSHITS I n s t i t u t e of C h e m i s t r y and Chemical Technology, S i b e r i a n Branch of t h e USSR Academy of S c i e n c e . K a r l M a r k s s t . , 42, K r a s n o y a r s k . 680049. USSR SUMMARY C a t a l y t i c o x i d a t i o n of e t h a n e w i t h n i t r o u s o x i d e a n d oxygen h a s been i n v e s t i g a t e d over ZSM-5 a n d m o r d e n i t e . Selective c o n v e r s i o n of e t h a n e t o e t h y l e n e a n d p r o p y l e n e w a s believed to i n v o l v e s u r f a c e oxygen atomic s p e c i e s which w e r e formed by t h e i n t e r a c t i o n of n i t r o u s o x i d e w i t h z e o l i t e s . C a t a l y t i c a c t i v i t y w a s p r o p o r t i o n a l t o t h e number of N20-decomposition sites a n d d i d n o t

depend upon t h e t o t a l number of s t r o n g l y acidic sites measured by ammonia c h e m i s o r p t i o n .

I NTRODUCTI ON Zeolites

have

attracted

c o n s i d e r a b l e a t t e n t i o n as

catalysts

for m o r e t h a n t w o d e c a d e s a s a r e s u l t of t h e i r h i g h a c t i v i t y and unusual

selectivity

for

acid-catalyzed

reactions.

The

lack

of

i n t e r e s t i n z e o l i t e s as c a t a l y s t s for s e l e c t i v e o x i d a t i o n may be

of poor s e l e c t i v i t i e s and h i g h r e a c t i o n t e m p e r a t u r e s .

a result However

has

it

c o n v e r s i o n of HZSM-5-type Crefs.

recently

been

reported

methane t o h y d r o c a r b o n s ,

that

catalytic

the

i n c l u d i n g aromatics o v e r

z e o l i t e i n t h e p r e s e n c e of N 0 or O2 w a s o b s e r v e d 2 Differences in the product distribution and

1,2>.

catalytic

activity

mechanism is n o t t h e

with

two

the

oxidants

indicate

that

the

s a m e for N 0 a n d 02. Although t h e mechanism

is unknown i t is s u g g e s t e d

2 that initial

r e a c t i o n is t h e p r o t o n a t i o n of

s t e p of

t h e catalytic

methane by s u p e r a c i d sites Cref.

3>. I t h a s been r e p o r t e d t h a t O2 a n d N 0 h a v e e x t r e m e l y d i f f e r e n t 2 a c t i v i t i e s and a l s o t h e o x i d e r a d i c a l i o n 0-. which c a n be generated

primarily

r e a c t i v i t y t h a n 02,

in

the

decomposition

2-, 03, 0 i o n s

Crefs.

s t u d i e d t h e o x i d a t i o n of e t h a n e b y N 2 0

of NgO.

4-63.

shows

higher

W e have t h e r e f o r e

a n d O2 over

HZSM-5 a n d

m o r d e n i t e which w i l l b e of s i g n i f i c a n t i n t e r e s t i n t h e c h e m i s t r y

of a c t i v e oxygen s p e c i e s and a c i d - b a s e p r o p e r t i e s of t h e s u r f a c e .

484

EXPERIMENTAL.

Materials.

High

purity

g r a d e C99.8+%3, e t h a n e ,

oxygen

n i t r o u s o x i d e w e r e used without f u r t h e r p u r i f i c a t i o n .

and

Helium w a s

p u r i f i e d by p a s s i n g t h r o u g h CaA-liquid n i t r o g e n t r a p . The a c i d i c f o r m of

and mordenite w a s

t h e 234-5

e x c h a n g i n g t h e N a c a t i o n s w i t h NH C1 1 . 0 N a t QO'C 4

c a l c i n i n g i n a i r a t 550°C. impregnation

w a s o b t a i n e d by w e t

1.5%Na-HZSM-SC413

HZSM-SC41>

of

with

aqueous

o b t a i n e d by

a n d f u r t h e r by

solution

NaOH.

of

Exper i ment s h a v e been p e r f or med on sampl es w i t h Si 0 2 / A l 203 r a t i 0s equal

to

38.

41,

60.

90,

H a - 8 .

148 for

and

I1

HM.

for

Na

c o n t e n t w a s as l o w as 0.1%. Zeol i tes

Equi pment a n d C a t a l y s t Eva1 u a t i o n . 60-80 mesh for u s e i n t h e c a t a l y t i c r u n .

were

A m i x t u r e of

si e v e d C2H6.

to

NZO.

CO > w a s p a s s e d t h r o u g h a f i x e d bed i n a t u b u l a r f l o w reactor a t 2 atmospheric p r e s s u r e . The c a t a l y t i c r u n w a s carried o u t under the

following

conditions:

p r e s s u r e s of e t h a n e 370 kPa. respectively,

catalyst

of NgO C 0 2 >

weight

0.3

g.

partial

37 kPa and of H e 606 kPa

r e a c t i o n t e m p e r a t u r e 387OC.

t h e e x t e n t of

ethane

c o n v e r s i o n w a s u p t o 5%. a n d t h a t of N 2 0 CO > u p t o 20%. 2 P r o d u c t s a n a l y s e s were c a r r i e d o u t b y o n - l i n e g a s chromatography u s i n g

flame-ionization

detector

liquid

and catharometer

and t w o columns: Porapak Q a n d 5 A m o l e c u l a r s i e v e . A c i d i t v measurements.

After

a c t i v a t i o n a t 5 5 O 0 C under h e l i u m

ammonia w a s a d s o r b e d on t o t h e c a t a l y s t a t 1 0 0 ° C .

r a t e 17°/minl

i n d i c a t e d t h e number

TF'D

Cheating

of a c i d sites. g i v e n as t h e

m i 11i mol es of ammonia chemi sor bed p e r gram of c a t a l y s t . $0 the

decomposition.

reactor

under

He

Catalyst C40 s c c m

s a m p l e s w e r e h e a t e d CEjSO°C> i n at

1 atmosphere> for

3 hours.

P u l s e s of p u r e N 0 C 0 . 2 sccm3 i n H e w e r e i n t r o d u c e d a t 347-C. 2 The amount of oxygen h e l d by t h e s u r f a c e No w a s d e t e r m i n e d as

N o = N

- E N N2 O2 and N were t h e amount of n i t r o g e n a n d oxygen r e l e a s e d where N N2 O2 r e s p e cti v e l y . RESULTS AND D I S C U S I O N Upon p a s s i n g t h e r e a c t a n t s over

a c i d i c f o r m of z e o l i t e s t h e

C H a n d H 2 0 were d e t e c t e d . 2 4 The n a t u r e of t h e o x i d a n t u s e d had a s i g n i f i c a n t effect o n t h e

p r o d u c t s CO. CO,.

e t h a n e c o n v e r s i o n r a t e and p r o d u c t

f o r m a t i o n s e l e c t i v i t y Clable

l>.

With O2 as t h e o x i d a n t t h e main p a r t of e t h a n e u n d e r g o e s t h e

Table 1

C a t a l y t i c o x i d a t i o n of e t h a n e by n i t r o u s o x i d e over

ZSM-5 and rnordenite at 3 8 7 * C

Catalyst

O x i dant

R a t e of e t h a n e conver si on, 1 0 a mol ec /c g . $33

HZSM-SC383 HZSM-SC601

O2 N2° O2 N2°

HZSM-SCQO1 HZSM-SC1483

Oz N2° Oz N2°

HM-1 I

O2

HZSM-SC413

N2° N20

S e l e c t i v i t y to,%

C02

*h-peak

C3H6

0.5

48

58

-

6.3

2

88

8

0.3

so

50

-

4.4

2

88

10

0.2

60

40

7

3. 6

1

90

0.2

75

2s

-

0.4

3

8Q

6

total

h-peak

*

0.72

0.39

0.30

0.18

0.24

0.14

0.21

0.13

0. 86

0.34

0.3

59

39

-

2.8

13

85

I

9.7

1

91

7

0.74

0. 38

1

91

6

0.98

0. 08

I .5% N a -

HZSM-SC413 N20

C2H4

A c i d i t y , mmol /g

13.0

- t h e amount of ammonia which i s desorbed above 300*C.

486 deep o x i d a t i o n and no t r a c e of propylene and butenes w a s f o r m e d . By changing Si02/A1203 r a t i o f r o m 38 t o 1 4 8 c a t a l y t i c a c t i v i t y of z e o l i t e s w a s reduced by t h r e e t i m e s . The

ethane

HZSM-SC383

conversion

rate

for

over

N20-C2H6

w a s by an o r d e r of magnitude g r e a t e r

Upon i n c r e a s i n g SiO /A1203

02-C2Hs.

reaction

than t h a t

for

ratio catalytic activity was

2

reduced by a f a c t o r of 10. Ethylene and propylene w e r e found t o be t h e major p r o d u c t s with CO,.

methane and C -hydrocarbons 4

minor amounts. The d i f f e r e n t

of c a t a l y t i c

level

a c t i v i t y for

02-C2H6

in and

t h a t t h e r e a r e t w o r e a c t i o n mechanisms,

N20-C2H6

can i n d i c a t e .

caused

by t h e d i f f e r e n t

c o n t r i b u t i ons of

aci d i c and

oxi d a t i ve

pathways of e t h a n e conversion.

To e l u c i d a t e t h e r o l e of t h e c a t a l y s t

acid-base

a c t i v i t y w a s compared with a c i d i t y ,

catalytic

properties

e v a l u a t e d by TPD

s p e c t r a of ammoni a . conversion for r e a c t i o n of

Upon comparing t h e r a t e s of C2H6 02-C2He

it is evident.

t h a t t h e d e c r e a s e of c a t a l y t i c a c t i v i t y

and t h e amount of s t r o n g a c i d i c sites t a k e s p l a c e c o n c u r r e n t l y . The 0 -C 2

correlation H

2 6

indicates

that

the

activity

of

zeolites

for

conversion can be caused by t h e a c i d i t y of t h e s u r f a c e .

With N 0 as t h e oxidant t h e v a r i a t i o n of z e o l i t e s a c t i v i t y is 2 r a n g i n g f r o m 0.2.10iB t o 13.0.10'* m o 1 e c . C H / C g * s > and does n o t 2 6 c o r r e l a t e with t h e amount of ammonia adsorbed. The r a t e of N 0-C H conversion over HZSM-SCQOI w a s 10 times g r e a t e r t h a n t h e 2 2 6 r a t e observed over HZSM-5C1483, t h e s e samples having t h e equal

amount of s t r o n g a c i d sites. The sodium f o r m of ZSM-SC413 was not a c t i v e i n o x i d a t i v e conversion

of e t h a n e i n N20-C2H6

mixture.

D e c a t i o n i z a t i o n of samples l e d t o t h e appearence of s t r o n g a c i d sites.

These

reaction.

samples

exhibited

also

high

activity

I n t r o d u c t i o n of 1.5%N a i n t o HZSM-SC413

temperature

ammonia

c o n v e r s i o n over

adsorption

form,

1.5% Na-HZSM-SC413

but

the

in

the

suppresed high rate

of

w a s even 1 . 3 t i m e s

ethane

greater.

t h a n over a c i d i c form HZSM-5C413. Therefore. can n o t

be

t h e high l e v e l of c a t a l y t i c a c t i v i t y f o r N20-C e x p l a i n e d by t h e

connected with s p e c i f i c NzO

a c i d i t y of

activation.

samples

and

could

H

2 6

be

T h i s is c o n s i s t e n t with

t h e e a r l i e r s t u d y concerning t h e decomposition of n i t r o u s o x i d e

a t 45O0C over H-mordenite Cref. 73. To check

between

t h e p o s s i b i l i t y of

nitrous

oxide

and

N20

activation

zeolites

was

the interaction

studied

at

347-C.

487

E x p e r i m e n t s d e m o n s t r a t e d t h a t p u r e N 0 decomposed u n d e r s t u d i e d 2 c o n d i ti o n s t o g i v e g a s e o u s n i t r o g e n a n d n o a p p r e c i a b l e evol u t i on of

The N 0 c o n v e r s i o n w a s h i g h e s t i n t h e 2 13, b u t a f t e r 10-19 p u l s e s d e c o m p o s i t i o n d i d

oxygen w a s o b s e r v e d .

f i r s t p u l s e CFig. not occur.

E v o l u t i o n of

oxygen a l s o d i d n o t

a t 347OC i n

occur

f l o w i n g h e l i u m d u r i n g a n hour p e r i o d . Hence. t h e amount of oxygen

by

held

species

decomposed calculation z e o l i t es .

N

and

the the

surface nitrogen

to

equal

is

released.

the

This

amount

fact

of

N20

allows

the

of t h e number of N 20 d e c o m p o s i t i o n sites for each

I F i g 1. The amount of n i t r o u s o x i d e decomposed

-

I

on

HZSM-SC603.

2 - HZSM-8C1483

a s a f u n c t i o n of t h e p u l c e number

n.

P u l c e volume 0 . 2 c c m , T=347-C.

Number of p u l c e s . n The r a t e of C2H6 c o n v e r s i o n as a f u n c t i o n of N 2 0 d e c o m p o s i t i o n sites is shown i n F i g . samples l i n e a r

2.

I t is e v i d e n t t h a t for a l l

correlation exists,

, which h a s d i f f e r e n t s e l e c t i v i t i e s

HM-I1

examined

is t h e case e v e n f o r

that

and C3H6

t o C02, C2H4

f o r m a t i on. The

determining

role

of

surface

oxygen

species

c o n v e r s i o n is a l s o c o n f i r m e d b y p u l s e e x p e r i m e n t s . oxygen p r o d u c e d b y N 2 0

pretreatment

of

HZSM-SC38)

Upon i n c r e a s i n g t h e number of C2H6 p u l s e s e t h a n e

w a s s h a r p l y r e d u c e d t o z e r o a f t e r 5-6 p u l s e s . i'

e t h a n e c o n v e r t e d w a s 5.0.10

m o 1 e c . C 2H6 / g .

C2H6

reacted with

e t h a n e t o f o r m e t h y l e n e as a major p r o d u c t w i t h minor C02.

in

The s u r f a c e amount of conversion

The total amount of Taking i n t o a c c o u n t

t h e s e l e c t i v i t y of e t h y l e n e and C02 f o r m a t i o n i t is p o s s i b l e t o c a l c u l a t e t h e amount of equal

t o 5.3.10''

s u r f a c e oxygen consumed.

a t o m O/g

that

T h i s v a l u e is

c o r r e s p o n d s t o 804 of

initial

488

5

10

-

6

5 -

/7

8

4

12

Number of s i t e s , No*lO-is Fig. 2 .

R a t e of C2H6

decomposition sites N

conversion r i n N20-CeH6

0

0

sites/g

surface N 0 2 r e a c t i o n a t 387OC on z e o l i t e s : v e r s u s number of

I -NaZSM-SC 41 > , 2 - N a Z S M - S C 383 , 3-HZSM-SC 1483, 4-HZSM-SC QO> , 5-HM-11, 6-HZSM-5C 603 , 7-HZSM-SC 38>, 8-HZSM-SC 41 3 , 9-1.5% Na-HZSM-SC 41 3.

a t o m O/g for HZSM-SC38>>.

oxygen c o v e r a g e C 6 . 8 . 1 0 "

agreement between t h e amount of the

consumed

composition observed. coverage

oxygen

, as well dur i ng

o b t a i ned

These

results

determines

oxygen h e l d by t h e s u r f a c e and

as

the

pulse

and

c l e a r l y show

high

Hence, a good

degree

similarity f 1o w

that

of

the

paramagnetic

resonance

studies

product

oxygen

catalytic

e x c e l l e n t s e l e c t i v i t y t o C2-C3 o l e f i n e s f o r N20-C2HE Electron

of

exper i ments

have

was

surface

activity

and

reaction. demonstrated

t h a t t h e r e are s t r o n g r e d o x sites on HZSM-5 and m o r d e n i t e c a p a b l e to

ion-radical

form

cation-radicals w i t h HZSM-5

of

organic olefines

Cref.

species.

me

w a s observed d u r i n g t h e i n t e r a c t i o n of

appearence

8 3 , t h e anion-radical

w a s d e t e c t e d by EPR

90;

on HM d u r i n g a d s o r p t i o n of s u l f u r d i o x i d e C r e f . 9). Decomposition of

nitrous

oxide

can

be

a

result

of

N 0 2

s t r u c t u r a l d e f e c t s on t h e z e o l i t e framework 3+ m e t a l i o n s . for example Fe .

interaction

Cref.

with

73 o r impure

As t o p o s s i b l e oxygen s p e c i e s r e s p o n s i b l e f o r t h e r e a c t i o n t h e f o l l o w i n g o n e s may be mentioned: t o work

Cref.

10>

O-..-O-type

0- and atomic oxygen. sites

can

be

formed

According by

a

high

489

temperature being

t r e a t m e n t of

equal

to

lo"

CSi02/A1203=70-1403, t h e number

HZSM-5

spin/g

as

measured

by

EPR

spectroscopy.

W i t h i n a n o r d e r of magnitude i t c o r r e s p o n d s t o t h e number of NEO decomposition

sites which

HZSM-SC148>.

The

was

surface

found

oxygen

to

be

5.10''

species

sites/g

formed

by

for N20

d e c o m p o s i t i o n d o e s n o t e x h i b i t a n EPR s i g n a l . I t may b e c o n c l u d e d that

they

are l i k e l y t o be uncharged

forms

having

an

atomic

c h a r a c t e r as p r o p o s e d for 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 e t h a n e by n i t r o u s oxi d e o v e r c o b a l t -doped magnesi um oxi d e C r e f 11>. REFERENCES 1.

2.

S. S. S h e p e l e v . C19631 319. S. S.

K . G. I o n e .

Shepelev.

K . G.

React.

Ione.

React.

Kinet. Kinet.

Catal

Cata .

Lett. ,

23

L e t t . , 23

6.

Cl9833 323. S. K o w a l a k , J . B. Moffat , A p p l i e d C a t a l y s i s , 3 6 C I -23 C 19883 139. K . A i k a . J . H . L u n s f o r d . J . Phys. Chem., 81 C19773 1393. M. I w a m o t o . J . H. L u n s f o r d , J . Phys. Chem. 84 C19801 3078. M. I w a m o t o , T. Taga. S.K a g a w a . Chem. L e t t . , ClQ823 1469.

8. 9

19C43. C19783 Q22. S. J . S h i h , J . C a t a l . , 79 ClQ833 390. A. A . S l i n k i n . A. V. Kucherov, D. A. K o n d r a t j e v ,

3. 4.

5.

.

7. A . A . S l i n k i n .

T. K . Lavrovskaya,

I . V. Mishin.

Kinet.

Katal. ,

T. N. Bondarenko. A.M.Rubinstein. Kh. M. Minachev. K i n e t . K a t a l . . 22 (1-3 156. V. A. Poluboyarov. V. F. Anufrienko. N. G. K a l i n i n a . S. N. Vosel , 10. K i n e t . Katal . , 28 C19851 751. 11. K . A i k a . M.Isobe. K.Kido, T.Mariyama. T . O n i s h i . J . Chem. SOC. F a r a d a y T r a n s . -1, 83 C18873 3139.