1113

STUDIES ON THE CATALYTIC OXIDATION OF PROPYLENE ON AMoO, TYPE MOLYBDATES

M.V.C. SASTRI, B. VISWANATHAN and C.V. BHUVANA Department o f Chemistry, I n d i a n I n s t i t u t e of Technology, Madras - 600 036, INDIA

ABSTRACT: The k i n e t i c s of c a t a l y t i c ox ida t ion o f propylene on d i v a l e n t t r a n s i t i o n metal molybdates have been s tudied w i t h a view t o evolving a u n i f i e d concept based on e f f e c t i v e bond-orders for p r e d i c t i o n of s e l e c t i v i t y of catalysts. The necessary condi- t i o n s imposed by the a p p l i c a t i o n of t h i s concept r e g a r d i n g the ad- sorbed s t a t e of propylene and oxygen have been e l u c i d a t e d by TPD and ESR s t u d i e s .

1. INTRODUCTION

The p a r t i a l o x i d a t i o n o f hydrocarbons over mixed oxide c a t a l y s t s has been t h e s u b j e c t of i n t e n s e i n v e s t i g a t i o n f o r t h e p a s t t e n y e a r s 1-3! I n s p i t e of many i n v e s t i g a t i o n s having been devoted e x c l u s i v e l y t o t h e o x i d a t i v e convers ions of hydrocarbons, a com- p l e t e theory t o e x p l a i n s e l e c t i v e o x i d a t i o n c a t a l y s i s has n o t evolved nor is t h e r e a u n i v e r s a l c o r r e l a t i o n t o p r e d i c t a ar ior i t h e a c t i v i t y of a g i v e n combination c a t a l y s t . This i s so, because t h e mechanism of t h e c a t a l y t i c o x i d a t i o n s h a s been descr ibed o n

t h e ( b ) s u r f a c e e n e r g e t i c s of oxygen a n d hydrocarbon, ( c ) acid-base p r o p e r t i e s of t h e C a t a l y s t and t h e r e a c t a n t , and ( d ) covalency con- c e p t s and t h e c o o r d i n a t i v e u n s a t u r a t i o n of t h e c a t i c n i c sites. On t h e basis of t h i s a n a l y s i s , c o r r e l a t i o n s have been obta ined with microscopic parameters l i k e s e m i c o n d u c t i ~ i t y ~ ~ width of t h e for- bidden gap or t h e p o s i t i o n o f the Fermi energy o f t he ~ a l i d ~ ’ ~ ? or

w i t h microscopic parameters l i k e the c o n f i g u r a t i o n of t h e adsorp- t i o n s i t e , metal-oxygen bond e n e r g i e ~ ~ ~ r e d u c i b i l i t y o f t h e m e t a l oxide‘, No-0 v i b r a t i o n frequenc1es’)and a c t i v i t y t o m r d s i s o t o p i c oxygen exchange and so on. It is c l e a r , t h e r e f o r e , that any r a t i o - n a l i z a t i o n of t h e r e a c t i v i t y p a t t e r n of a g iven s e r i e s of c a t a l y s t s

b a s i s of many concepts l i k e ( a ) ox ida t ion- reduct ion c y c l e s ,

1114 M.V.C. Sastri, B. Viswanathan, C.V. Bhuvana

i s poss ib le only when information on the mode of a c t i v a t i o n and r e a c t i v i t y of sur face hydrocarbon and oxygen spec ie s on them i s ava i lab le . The present paper attempts t o obtain this information on a s e r i e s of i s o s t r u c t u r a l U o 0 4 type o f molybdate c a t a l y s t s , where A i s a d iva len t t r a n s i t i o n metal ion , with a view t o r a t iona l i z ing the r e a c t i v i t y pa t t e rn observed on the basis of a uni f ied hypothesis.

2. EXPERIMENTAL

The molybdates of Mn, Co, N i , Cu and Zn were prepared e i t h e r by co-precipitation method or by soUd s t a t e r eac t ion between component oxides and were characterized by the usual methods. The k i n e t i c s tud ie s were car r ied out using a d i f f e r e n t i a l micro- r e a c t o r wi th on-line g a s chrornat ographic f a c i l i t y . were ca r r i ed out on ZnMo04 c a t a l y s t u s ing a V a r i a n E4-X-Band spectrometer while the TPD studies were car r ied out on the CoMo04 ca t a lys t . De ta i l s of experimental procedures employed have been described elsawherelo).

ZSR s tud ie s

3. RESULTS

The c a t a l y t i c oxidation of propylene w a s s tud ied on each of the c a t a l y s t s w i t h v a r i a t i o n of the p a r t i a l pressure of the r e - a c t a n t s and the temperature. On a l l the c a t a l y s t s studied, the r e a c t i o n w a s found t o be f i r s t order w i t h respec t t o propylene and zero order wi th respec t t o oxygen showing t h a t the processes occuring on a l l of t he c a t a l y s t s are mechanistically i d e n t i c a l . This enabled us t o make a d i r e c t c a p a r i s o n of the k ine t i c para- meters obtained on these ca t a lys t s , (given i n Table l ) , w i t h respeot t o the p a r t i a l o r crmplete canbustion reaction. The a c t i v i t i e s o f the c a t a l y s t s decrease i n the order CaYo04> NIMO04 3 CuMo04 ZnMo04 3 MnMo04. The s e l e c t i v i t y w i t h r e spec t t o p a r t i a l oxidation r eac t ion f o l l o w s the order K m l o 0 4 ~

ZnM004 > CuMo04 > l?iMo04 > CoMo04. I t w a s found that, t h e induction of C02 i n t o t h e r eac t an t stream d i d not a f f e c t e i t h e r the p a r t i a l or the t o t a l oxidation reac t ion . The presence of water vapor decreased the p a r t i a l oxidation a c t i v i t y alone, where- a s hydrogen decreased the t o t a l perce iltage conversion o f pro- pylene.

TA

BU

1

Val

ues

of A

rrhe

nius

par

amet

ers

for

the

oxid

atio

n of

pro

pyle

ne o

n M

oo4

type

rno

lybd

atea

fr

om i

nit

ial

rate

dat

a

Oxi

dati

on r

eact

ion

Acr

olei

n fo

rmat

ion

......

......

......

......

......

....

......

......

......

......

......

...

Ene

rgy

of

Freq

uenc

y ra

te c

onst

. Sn

ergy

of

Freq

uenc

y ra

te c

onat

. S

elec

tivi

ty for

Sat

alya

t ac

tvn.

E

fact

or ko

at 330 C

ac

tvn.

E

fact

or ko

at 330 C

ac

role

in

kcal

/mol

e m

in”

m-2

ah

-’

m-2

kcal

/mol

e m

in”

ma2

ah-’

m

2

MnM

o04

C0M

OO

4

NiM

a4

cuM

oo4

1

400

- 440 C

14

.6

3.2

x lo

6 1.

6 x

10

280

- 340 C

12

.2

5.9

x lo

6 2.

2 x

lo2 2

310 -

360

C

16.5

2.

0 x

lo

8 2.

1 x 1

0

300

- 350

C

11.4

2.

0 x

lo6

1.4

x lo

2

68.3

1

12.8

5.

0 lo

5 1.1

10

8- 9

8

.1

105

4.8

lo1

21.9

26.0

1

14.6

1.1

107

5.6

10

11.4

9.

5 105

7.0

10

51.4

1

znM

oo4

55 .o

0

10.7

7.0

104

3.5

10

1

360

- 400

C 13

.2

1.0

x lo

6 1.

7 x

10

1116 M.V.C. Sastri, B. Viswanathan, C.V. Bhuvana

On oxide sur faces one can expect the a c t i v a t i o n of the hydro carbon r eac t an t , the abs t r ac t ion o f e l ec t rons f rom the reac tan t and the r e a c t i v i t y of the ac t iva t ed r eac t an t t o depend upon the number and i n t r i n s i c s t rength of the ac id s i t e s present on the ca t a lys t . On the other hand, the a c t i v a t i o n of oxygen, t ha t i s the conversion of gaseous oxygen i n t o the r eac t ive oxygen i n thi adsorbed form o r i n t o the l a t t i c e oxygen, the r e a c t i v i t y of t he ac t iva t ed oxygen, that is, the i n t r i n s i c ox ida t ive power o f the surface oxygen species, w i l l depend on the na tu re and s t r eng th 6

bas ic s i t e s ava i l ab le on the surface. I n a t yp ica l oxidation r e - ac t ion proceeding through the p a r t i c l p a t i o n of l a t t i c e oxide ions, the s t rength of the metal-oxygen bonding decides the r e - a c t i v i t y of ac t iva t ed oxygen species and the number of such a c t i l a t t i c e oxygen sites determine the t o t a l ex ten t o f bas ic i ty . Similar arguments have been advanced by Ail’). On this basis,cm can formulate the following working hypothesis f o r c l a s s i f y i n g the oxidation c a t a l y s t s (1) A metal oxide has strong oxidising power when the number o f bas ic s i t e s is l a r g e as well as when t h i n t r i n s i c s t r eng th of the bas ic s i t e s i s high. These t w o para- meters a r e governed by the e lec t ron donating a b i l i t y of the meta ion concerned and/or the s t rength o f t he metal-oxygen bond. ( 2 ) A metal oxide has a weak oxidising power and therefore , pro- mote the p a r t i a l oxidation reac t ion when (i) i t has a l a rge number of a c i d s i t e s , ( i i ) the i n t r i n s i c a c i d s t r eng th o f the si is high enough t o a c t i v a t e the r eac t an t molecule, and ( i i i ) t h e metal-oxygen bond s t rength i s optimum with moderate i n t r i n s i c basic s t rength so t ha t the ac t iva t ed r eac t an t desorbs a s the par t i a l oxidation product before fu r the r oxygen incorporation takes place.

I n essence, a c a t a l y s t which is capable of a c t i v a t i n g the oxygen (highly bas ic ) would promote t o t a l oxidation reac t ion , while those systems which can adsorb the hydrocarbon i n a reacti form (ac id i c ) and a l s o provide reac t ive oxygen spec ies t o a l imited ex ten t (moderately bas i c ) would func t ion as a s e l e c t i v e oxidation c a t a l y s t . On the other hand, a c a t a l y s t which is N g h ac id i c can a c t i v a t e the r e a c t a n t and b r a transformations i n t i r eac t an t i t s e l f . Molybdates, antimonates and cuprous oxide pro, mote the p a r t i a l oxidation r eac t ion by v i r t u e of two cha rac t e r i t i c s , namely,

B32-”

Cxldatlon of P r o p y l e n e on AMOO, TFpe Molybdates 1117

1. t h e a c t i v e c a t i o n s i t e s possess enough a c i d i t y to a c t i v a t e t h e hydrocarbon r e a c t a n t i n a r e a c t i v e form and are s u f f i c i e n t i n number t o promote t h e p a r t i a l o x i d a t i o n r e a c t i o n p r e f e r e n t i e and 2. t h e IY-0 bond o r d e r i n these systems is always g r e a t e r than 1 due t o t h e a v a i l a b i l i t y of empty d o r b i t a l s on t h e a c t i v e c a t i o n .

However, t h e s imultaneous presence o f o t h e r i o n s l i K e Bi3+, Fe3+ e t c . , i n a l a t t i c e p o s i t i o n competing f o r t h e p e l e c t r o n s of oxygen, e f f e c t i v e l y d e c r e a s e s t h e Mo-0 bond order . An optimum bond s t r e n g t h o r bond o r d e r i s e s s e n t i a l so t h a t t h e oxide i o n s can p a r t i c i p a t e i n a r e a c t i o n with t h e r e a c t i v e hydrocarbon b u t s t i l l cannot y i e l d coffiplcte o x i d a t i o n products . That t h e p a r t i a l o x i d a t i o n r e a c t i o n r e q u i r e s a c a t a l y s t w i t h an optimum bond order i s c l e a r from the r e s u l t s g iven i n Pig. 1, which shows t h a t a c a t a l y s t wi th Mo-0 bond order arou:d 1.4 e x h i b i t s maximum p a r t i a l o x i d a t i o n a c t i v i t y . Therefore , i t a p p e a r s t h a t t h e o x i d a t i o n a c t i v i t y b p e n d s on M-0 bond energy and t h e r o l e of t h e second c a t i o n i s merely t o a d j u s t or moderate t h e #o-C bond energy t o t h e optimum value 80 t h a t the system is c a p a b l e of p r o v i d i n g enough b a s i c s i t e s of s u i t a b l e s t r e n g t h s o as t o p a r t i c i p a t e i n t h e p a r t i a l o x i d a t i o n r e a c t i o n . dhen t h e M-0 bond s t r e n g t h i s n o t c o n s t a n t bu t can be descr ibed by a d i s t r i b u t i o n f u n c t i o n , due t o v a r i a t i o n s i n bond l e n g t h s and bond o r d e r s , then t h e system can a l s o promote the complete o x i d a t i o n r e a c t i o n through p a r t i c i - p a t i o n o f l a t t i c e and/or adsorbed oxide i o n s . The presence v a r i o u s t y p e s o f a c t i v e oxygen i o n s i n y and p b i m u t h molybdate c a t a l y s t s have been i d e n t i f i e d by IR and Haman s p e c t r o s c o p i c s t u d i e s by Hoefs a t a l l 2 ) . a c t i v e oxide i o n s p a r t i c i p a t e i n t h e oxi*t ion r e a c t i o n s . This conclus ion i s i n conformity w i t h t h e p r o p o w d hypothes is . This working h y p o t h e s i s i s a p p l i c a b l e t o a c t u a l ox ida t ion c a t a l y s t s only when the fo l lowing c o n d i t i o n s o b t a i n :

i. The r e a c t a n t molecule h a s only one type o f binding state, yet w i t h a d i s t r i b u t i o n o f d i f fe rgr i t bindirig energ ies . Such a poss i - b i l i t y could be v e r i f i e d by TPD spectrum of the o l e f i n adsorbed on t h e c a t a l y s t s . e s t a b l i s h e d that propylene i s adsorbed i n only one a c t i v a t e d mode

hart from t h e normal r e v e r s i b l e mode o f a d s o r p t i o n ) and because o f t h e v a r i a t i o n s of t h e Mo-0 bond s t r e n g t h , the d e s o r p t i o n i s spread o v e r a wide ternpela t u r e range.

Phey have concladed t h a t a l l t h e s e

On a t y p i c a l Colo04 c a t a l y s t i t has been

7 1 1 8 M.V.C. Sastri. B. Viswanathan. C.V. Bhuvana

ii. The a c t i v a t i o n energy (ca. 42 icJ mole-’) f o r the d e s o r p t i o n of propylene is h i g h e r t h a n t h e va lue (34 LJ mole-’) far the o x i d a t i o n r e a c t i o n i t s e l f . This ahows that the t r a n s f o r m a t i o n o f the hydrocarbon on t h e sur face can take p l a c e i n a fac i le manner depending upon t h e o x i d i s i n g power of the s i te involved and t h i s determines whether p a r t i a l or c a n p l e t e o x i d a t i o n re- a c t i o n w i l l take p l a c e .

iii. The absence of a peak due t o d e s o r p t i o n of propylene a t h i g h e r h e a t i n g rates should be ascr ibed t o t h e occurrence of fast surface r e a c t i o n through the p a r t i c i p a t i o n o f the l a t t i c e oxide i o n s which h a s g iven rise t o d e t e c t i o n o f o x i d a t i o n pro- d u c t s only.

r e s u l t s presented i n Table 2. These conclus ions have been a r r i v e d a t on t h e b a s i s o f l!PD

TABLE 2

Summary of TPD r e s u l t s for the a d s o r p t i o n of propylene on c o b a l t molybdate

I n o r d e r t o e s t a b l i s h that t h e o x i d a t i o n of propylene on d i v a l e n t t r a n s i t i o n m e t a l molybdates proceeds through t h e p a r t i - c i p a t i o n of l a t t i c e oxide i o n s and r e q u i r e s t h e a c t i v a t i o n o f t h e hylrocarbon, a s e r i e s o f ESR experiments have been c a r r i e d Out under v a r i o u s exper imenta l c o n a i t i o n s . The r e s u l t s obtained a r e summarised i n Table 3. The assignments of the seven-line spec- trum t o s u r f a c e and bulk Mo5+, free s p i n s and adsorbed 0- and 02- s p e c i e s have been made on t h e b a a i s o f the observed g values .

The fol lowing p o i n t s emerge o u t from t h e ESi7 measurements of ZnXo04 c a t a l y s t a f t e r pre t rea tment w i t h oxygen o r propylene:

1. The s u r f a c e oxide i o n s possess a spectrum of bond s t r e n g t h s .

TABL

E 3

Sum

mar

y of

ESR

re

sult

s on

Zn

Mo0

4 ca

taly

st

Obs

erve

d ESR

sig

nal

s an

d th

eir

prob

able

as

sign

roen

t s

Inte

nsi

ty o

f th

e v

ario

us ESX

sig

nal

s

A 1.

928

1.96

7 B

1.92

7 1.

969

C 1.

928

1.97

0 D

1.92

2 1.

961

E 1.

925

1.96

9 F

1.92

9 1.

968

0 1.936

1.97

7 H

1.93

7 1.

977

I 1.

950

1.99

1 J

1.92

1 1.

961

1.99

8

1.99

1

1.99

7 2.

000

2.01

1 2.

020

1.99

5 -- -- --

2.01

5 2.

044

1.89

6 2.

012

0.038

1.89

4 --

--

1. g

oo

2.00

7 2.

034

1.88

9 --

2.

036

1.89

2 2.

018

2.04

5 1.

896

2.03

9 2.

066

1.91

7

2.01

0 2.

024

1,90

4 2.

025

2.05

4 1.

904

2.00

9 2.

039

1.89

1

1.87

4 10

2.6

62.4

1.

7 0.

1 0.

2 4.

5 0.

6 1.

871

156.

7 25

.6

- 0.

8 2.

5 5.

2 1.1

--

1.88

6 1.

868

1.87

6

113.

6 15

2.8

116.

9 1.

868

227.

4 1.

882

186.

6 1.

894

102.

7 1.

867

109.

6

410.

1

13.8

-

- -

6.6

- 13

.2

4.1

0.1

0.7

5.9

0.8

- 0.

5 6.1

0.7

27.7

-

ll.3

2

.3

0.4

0.4

5.7

1.3

15.6

2.

9 6.

9 12

.9

6.0

0.5

11.6

8.

8 0.

4 6.

9 5.

6 0.

4 6.

8 1.

7 0.

1 0.

4 3.

9 0.

3 12

.7

5.3

0.4

1.5

1.8

0.8

evac

uate

d a

t 45

0 C

trea

ted

wit

h ox

ygen

at 370

C tr

eate

d w

ith

oxy

gen

at

370

C,

quen

ched

an

d ev

acua

ted

at 30

C tr

eate

d w

ith

oxyg

en a

t 30 C

redu

ced

wit

h pr

opyl

ene

at 3

00 C

fo

r 30

min

. re

duce

d w

ith

pro

pyle

ne a

t 30

0 C

for

3 ho

urs

redu

ced

wit

h pr

opyl

ene

for

30 m

in.

at

300

C an

d th

en t

reat

ed w

ith

oxyg

en a

t 30

C tr

eate

d w

ith

pro

pyle

ne a

t 30

C,

prop

ylen

e is p

iunp

d ou

t a

t th

e sa

me

tem

pera

ture

an

d t

he

sam

ple is

trea

ted

wit

h o

xyge

n tr

eate

d w

ith

prop

ylen

e a

t 30

C

redu

ced

wit

h p

ropy

lene

at

300

C

and

trea

ted

wit

h o

xyge

n a

t 30

0 C

1120 M . V . C . S a s t r i , B . Viswanathan, C . V . Bhuvana

2. The adsorbed oxygen s p e c i e s a r e e s s e n t i a l l y 0- and 02- and t h e i r r e l a t i v e c o n c e n t r a t i o n s depend on t h e temperat we o f adsorp t ion . 3 . There e x i s t s a c e r t a i n number of l o o s e l y bound oxide i o n s which are e a s i l y removed by pumping a t 370°C but no t removed by pumping a t room temperature . 4. Treatment of t h e c a t a l y s t w i t h propylene a t JOO0C f o r shor t i n t e r v a l s causes c o n s i d e r a b l e r e d u c t i o n o f t h e c a t a l y s t , thereby account ing f o r t h e i n c r e a s e of MO5+ s i g n a l i n t e n s i t y . 5. Prolonged t r e a t m e n t of t h e c a t a l y s t wi th propylene l e a d s t o d i f f u s i o n of t h e oxide i o n s from t h e b u l k t o t h e s u r f a c e for p a r t i c i p a t i n g i n t h e r e a c t i o n as well as f o r o x i d i e i n g the sur- f a c e MO5+ species . e a r l i e r by Xeulks12). 6. The observa t ion t h a t t h e i n t e n s i t y of t h e f r e e s p i n s i g n a l decreased on t rea tment w i t h propylene shows that t h e a d s o r p t i o n of propylene takes p l a c e by charge t r a n s f e r t o the c a t a l y s t l a t t i c e 13s14). paper , the hydrocarbon molecules a r e a c t i v a t e d i n t h e &me mannw, though n o t e n e r g e t i c a l l y t o the same e x t e n t on the s u r f a c e , and y i e l d e i t h e r acrolein or carbon d ioxide depeni ing on the o x i d i s i n g a c t i v i t y o f the s i t e and t h e s t r e n g t h o f the l o 4 bond. The Mo i o n i n Mo-O-A c o n f i g u r a t i o n w i t h strcmg o x i d i s i n g power promotes t h e o x i d a t i o n r e a c t i o n while t h e tennin.31 Mo = 0 bond r e s u l t s i n t h e a c t i v a t i o n o f the hydrocarbon i n the same a c i d i c s i t e w i t h charge t r a n s f e r f rom t h e hydrocarbon s u r f a c e s p e c i e s t o t h e lat- t i c e . Depending on the c o n f i g u r a t i o n o f the a d s o r p t i o n s i t e , t h e o x i d i s i n g power of t h e s i t e involved and t h e temperature , t h e hydrocarbon molecule , a c t i v a t e d by donat ion o f charge d e n s i t y t o t h e l a t t i c e , undergoes r e a c t i o n t o y i e l d p a r t i a l / c o m p l e t e combus- t i o n products . 7. The uptake of oxygen af ter pretreat1nen-t of the c a t a l y s t wi th propylene a t 3OO0C takes p l a c e through t h e formation of O2 and 0- sur face s p e c i e s which u l t i m a t e l y turn i n t o l a t t i c e oxide ions.

T h i s r e s u l t In agreement wi th those r e p o r t e d

According t o t h e h y p o t h e s i s developed i a t h i s

-

4. CO~?CLUSIONS

The main conclus ions of t h e p r e s e n t work may be summarized a8 f o l l o w s :

1. C o r r e l a t i o n s e x i s t between a c t i v i t y and s e l e c t i v i t y and p h y s i c a l parameters l i k e Mo-0 bond order , Xo-0 bond s t r e n g t h and t h e a c i d i t y Of t h e c a t a l y s t s .

c

!: 80

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1122 M.V.C. Sastri, 9. Viswanathan, C.V. Bhuvana

b ) B. Viswanathan, C.V. Bhuvana, and M.V.C. Sastri,

c ) B. Viswanathan, C.V. Bhuvana and M.V.C. S a s t r i ,

z, 16 (1978).

Proc. Ind . Acad. Sci., 4878, 405 (1978).

Ind. J. Chem. ( In press). 11. M. Ai, J. Catal., &, 327, 318 (1975); 99, 305, 313 (1977); 12. E.V. Hoefs, J.R. Monmier and G.W. Keulks , J. Catal.,

13. L. Naccache. J. B a n d i e r a and M. Dufaux, J. Catal . , 21, 331 (1979). 3, 334 (1972). J. Mol. Catal., 2, 35 (1976).

14. J. Haber, M. Sochacka, 3. Grsylowska and A. Goleb iewsk i ,

DISCUSSION

T. Tagawa (Nagoya Univ . )

I have a s i m i l a r i d e a t o your h y p o t h e s i s conce rn ing t h e role of a c i d and b a s e c e n t e r s . We have s t u d i e d t h e o x i d a t i v e dehydro- gena t ion of e thy lbenzene on Na-treatedsilica-alumina, and good c o r r e l a t i o n s were o b t a i n e d between t h e a c i d i c and b a s i c proper- t i e s and a c t i v i t y of t h e c a t a l y s t s . Tha t is:

1) The amount of e thy lbenzene adsorbed on t h e ca t a lys t s t r o n g - l y depended on t h e amount of a c i d i c sites of s p e c i f i e d s t r e n g t h .

2 ) The t u r n o v e r f r equency ( r e a c t i o n r a t e / a c i d s i t e s mentioned above) s t r o n g l y depended on t h e amount of b a s i c sites of s p e c i - f i e d s t r e n g t h , and ESR s t u d i e s showed t h a t t h e s e b a s i c s i tes can a c t i v a t e t h e gaseous oxygen i n t o charged oxygen s p e c i e s . From t h e s e r e s u l t s were proposed t h e r e a c t i o n mechanismi) and t h i s a g r e e s s t r o n g l y w i t h your working h y p o t h e s i s mentioned on Page 9, t h e 3 r d of t h e c o n c l u s i o n s . P l e a s e let m e know i f you have any d a t a about t h e a c i d i t y and t h e b a s i c i t y of your ca ta ly- sts. 1) T . Tagawa, T. Hattori and Y . Murakami, Shokubai , z, 319 (1979).

M.V.C. Sast r i

I t i s g r a t i f y i n g t o n o t e t h a t s imi la r r e s u l t s have been o b t a i n e d on t h e o x i d a t i v e dehydrogenat ion of e thy lbenzene on Na t r e a t e d s i l i c a - a l u m i n a s u p p o r t i n g t h e working h y p o t h e s i s proposed i n t h i s pape r . We have t r i e d t o e v a l u a t e t h e a c i d i t y of t h e c a t a l y s t s by ammonia a d s o r p t i o n . However, we have n o t y e t made any measurements on t h e d e t e r m i n a t i o n of t h e b a s i c i t y of t h e c a t a l y s t s .

M. A i (Tokyo I n s t . Tech.)

1) How d i d you estimate t h e v a l u e s of bond o r d e r ? 2) Does t h e bond o r d e r r e p r e s e n t on ly bond s t r e n g t h or a com-

b ined c h a r a c t e r of b o t h ac id -base and bond s t r e n g t h ?

O x i d a t i o n of P r o p y l e n e on AMoO, T y p e M o l y b d a t e s 1123

3 ) I hope t o h a v e o n e or t w o o t h e r d a t a a t bond o r d e r s o f 1 . 3

t o 1 . 4 . D o you h a v e s u c h d a t a ?

M.V.C. S a s t r i

Bond o r d e r v a l u e s h a v e b e e n computed f rom t h e c o l l a t e d 0 - M and 0 - M - 0 bond d i s t a n c e s f o r t h e r e l e v a n t c r y s t a l s t r u c t u r e s For t h i s p u r p o s e t h e s y s t e m a t i c t r e a t m e n t o f S c h r o e d e r ( F . A .

S c h r o e d e r , A c t a . C r y s t . K , 2294 ( 1 9 7 5 ) ) f o r t h e Mo-0 bond l e n g t h bond o r d e r r e l a t i o n s h i p w a s made u s e o f . I t m e r e l y r e f l e c t s o n l y t h e a v e r a g e bond s t r e n g t h a n d may n o t r e f l e c t combined c h a r a c t e r of a c i d - b a s e and bond s t r e n g t h . We do n o t a t p r e s e n t h a v e d a t a on any o t h e r s y s t e m s t h a n t h o s e r e p o r t e d i n t h e p a p e r . I t is p o s s i b l e t o h a v e bond o r d e r v a l u e s be tween 1 . 4 a n d 1 . 3 i f t h e a v e r a g e Mo-0 bond d i s t a n c e i n t h e m o l y b d a t e s i s o f t h e o r d e r o f 1 . 8 0 2 t o 1 . 7 8 3 .

Yuan-Gen Yin, (Lanzhou I n s t . Chern. Phys., Ch ina)

D r . S a s t r i ' s e f f o r t t o c o r r e l a t e 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 of d i v a l e n t m e t a l m o l y b d a t e w i t h bond o r d e r is n o t e w o r t h y . However , t h e c a t a l y s t s w e r e compared on t h e b a s i s of ra te c o n s t a n t s , a t 3 3 O o C ( T a b . l), some o f them b e i n g c a l c u l a t e d by e x t r a p o l a t i o n q u i t e beyond t h e r a n g e of t e m p e r a t u r e s t u d i e d . T a k i n g any p o s s i b l e d e f l e c t i o n o f A r r h e n i u s p l o t beyond t h e t e m p e r a t u r e r a n g e s t u d i e d i n t o c o n s i d e r a t i o n , i t seems s a f e r a n d more r e a s o n a b l e t o compare t h e c a t a l y s t s w i t h o n t h e b a s i s

o f a c t i v a t i o n e n e r g i e s f o r o v e r a l l a n d p a r t i a l o x i d a t i o n . I f s o , t h e c a t a l y s t s would b e a r r a n g e d i n i n c r e a s i n g o r d e r o f a c t i v a t i o n e n e r g y , C o Zn < Mn < N i , w i t h r e s p e c t t o b o t h o v e r a l l and p a r t i a l o x i d a t i o n , e x c e p t i n g Cu m o l y b d a t e . Also, c o u l d t h e a u t h o r s i n t e r p r e t v a r i o u s e x p e r i m e n t a l e v i d e n c e on t h e c o n c e r t e d a c t i o n of d u a l metal i o n s , e s p e c i a l l y t h a t p ro- v i d e d by D r . Miura a n d o t h e r s ( c f . p a p e r C - 3 ) , i n l i g h t o f t h e i r p r o p o s e d c o n c e p t ?

M.V.C. S a s t r i

S i n c e many of t h e s e s y s t e m s e x h i b i t c o m p e n s a t i o n e f f e c t , it w a s f e l t t h a t c o m p a r i s o n o n t h e b a s i s o f e x t r a p o l a t e d v a l u e s of ra te c o n s t a n t s may b e s a f e r r a t h e r t h a n on t h e b a s i s of a c t i v a t i o n e n e r g y . However , it is p o s s i b l e t o draw c o r r e l a - t i o n o n t h e b a s i s of t h e v a l u e s o f a c t i v a t i o n e n e r g y f o r b o t h o v e r a l l and p a r t i a l o x i d a t i o n r e a c t i o n s . I t is o u r hope t h a t t h e c o n c e p t of c o n c e r t e d a c t i o n o f d u a l metal i o n s c a n b e u s e d s u c c e s s f u l l y t o i n t e r p r e t many o t h e r e x p e r i m e n t a l o b s e r v a t i o n s .

1124 M.V.C. Sastri, B. Viswanathan, C . V . Bhuvana

For example, Miura e t a1 observed t h a t , on La2bIoO6, o x i d a t i o n o f 1-butane does no t y i e l d b u t a d i e n e s e l e c t i v e l y , because t h e Mo-0 bond o r d e r i n t h e b r i d g e d c o n f i g u r a t i o n s cou ld be l o w ( n e a r l y 1) so as t o promote p r e f e r e n t i a l l y complete o x i d a t i o n

M. Iwamoto (Nagasaki Univ.)

I w a s s u r p r i s e d a t your a d d r e s s , because you s a i d t h a t you found t h e EPR spectrum o f 0- i o n s upon a d s o r p t i o n of oxygen molecu le s . The EPR s i g n a l s of 02- and 0- i o n s u s u a l l y have 3 l i n e s and 2 l i n e s , r e s p e c t i v e l y . However, your EPR spectrum o f oxygen an ion r a d i c a l s is no t i n agreement w i t h t h e s e . How do you e x p l a i n t h e s e d i f f e r e n c e s ? O r how d i d you a s s i g n your s p e c t r a ?

M . V . C . Sastri I t i s t r u e t h a t EPR s i g n a l s of 0; and 0- i o n s u s u a l l y have 3

l i n e s and 2 l i n e s r e s p e c t i v e l y and w e have a l s o obse rved t h r e e l i n e s f o r 0; w i t h g v a l u e s 2 . 0 0 3 , 2 . 0 1 5 , 2 .044 . two l i n e spectrum w i t h g v a l u e s 1.998 and 2.020 w a s obse rved f o r 0- s p e c i e s . s i g n a l s cou ld b e used f o r i n t e n s i t y measurements and d i s c u s s i o n of t h e r e s u l t s .

S i m i l a r l y , a

W e have c o n s i d e r e d o n l y t h e c l e a r l y r e s o l v e d

J . Haber ( Inst . o f Catalysis, Krakow) When look ing f o r c o r r e l a t i o n between k i n e t i c pa rame te r s l i k e

rate c o n s t a n t o r a c t i v a t i o n ene rgy , and o t h e r physicochemical p r o p e r t i e s , i t is n e c e s s a r y t o know t o which s t e p of t h e reac- t i o n are t h e s e k i n e t i c pa rame te r s r e f e r r i n g . I n t h e case of p ropy lene o x i d a t i o n , u s u a l l y a d s o r p t i o n of p ropy lene is t h e r a t e - d e t e r m i n i n g s t e p and t h e r e f o r e t h e r e is h a r d l y a c o r r e l a - t i o n w i t h oxygen bond s t r e n g t h . D o you have in fo rma t ion as t o which is t h e rate de te rmin ing s t e p i n your case?

M.V.C. Sastri I n o u r case t h e a d s o r p t i o n of p ropy lene h a s been shown t o be

t h e rate de te rmin ing s t e p . However, it is known t h a t t h e re- a c t i v i t y of t h e s u r f a c e oxygen s p e c i e s a l o n e c o n t r o l s t h e s e l e c t i v i t y . I t is known (D.G. K l i s s u r s h i , J . C a t a l . , 59, 448 ( 1 9 7 9 ) ) t h a t weakly bound and h i g h l y r e a c t i v e oxygen i n t h e s u r f a c e l a y e r s of t h e ca t a lys t s l e a d s t o t h e formation of com- p l e t e o x i d a t i o n p r o d u c t s , w h i l e lower r e a c t i v i t y of t h e s u r f a c e oxygen f a v o u r s p a r t i a l o x i d a t i o n . The proposed c o r r e l a t i o n between bond o r d e r and a c t i v i t y , though e m p i r i c a l , can b e u s e f u l

Oxidation of Propylene on AMoO* Type Molybdates 1125

i n c a t a l y s t s e l e c t i o n . F u r t h e r work a l o n e w i l l be a b l e t o e s t a b l i s h t h e physicochemical b a s i s of t h e e x i s t e n c e of such a c o r r e l a t i o n .