Selective Oxidation Processes

4 Catalytic Oxidation of Isobutylene to Methacrolein over Copper Oxide Catalysts RANVEER S. MANN and DENIS J . ROULEAU

1

Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0051.ch004

Chemical Engineering

Department,

University of Ottawa, Ottawa 2, Canada

The air oxidation of isobutylene was investi gated over supported and unsupported copper oxide catalysts in an isothermal integral flow reactor. The effect of several variables—ratio of oxygen to isobutylene in the feed, process temperatures, copper concentrations of the cat alysts, and reciprocal of space velocity on the conversion and yield—were determined. Though several mechanisms were postulated, the rate of reaction was most satisfactorily correlated by a mechanism which assumes the rate-controlling step as the surface reaction be tween strongly adsorbed isobutylene and gas eous (weakly adsorbed) oxygen. The rate ex -r=

αKC H 4

1

+

PC H

8

4

KC H PC H 4

8

4

8

PO

2

8

+

KC H OPC H O 4

8

4

6

pression fitted the data very well.

Q w i n g to t h e l i k e l y o c c u r r e n c e of c o m p l e t e o x i d a t i o n to c a r b o n d i o x i d e a n d w a t e r , c o m p a r a t i v e l y f e w studies h a v e b e e n c a r r i e d o u t o n t h e v a p o r phase c a t a l y t i c o x i d a t i o n o f olefins o n m e t a l oxides. T h e selective o x i d a t i o n o f e t h y l e n e to e t h y l e n e oxide ( 7 ) , p r o p y l e n e to a c r o l e i n (18), a n d v a r i o u s a r o m a t i c s t o a r o m a t i c d i c a r b o x y l i c acids o r a n h y d r i d e s r e p r e sent cases of c o n s i d e r a b l e i n d u s t r i a l i m p o r t a n c e . T h o u g h s e v e r a l patents ( 1, 3, 4, 5, 6, 8, 9,14 ) h a v e a p p e a r e d d u r i n g t h e past f e w years d e s c r i b i n g the use of v a r i o u s m e t a l oxides i n t h e p a r t i a l o x i d a t i o n of i s o b u t y l e n e to m e t h a c r o l e i n , scientific l i t e r a t u r e d e s c r i b i n g t h e effects of v a r i o u s o p e r a t i n g v a r i a b l e s o n t h e c o m p o s i t i o n o f p r o d u c t s is q u i t e l i m i t e d . T h e gas p h a s e o x i d a t i o n of i s o b u t y l e n e has b e e n e x a m i n e d b y a static m e t h o d b e t w e e n 1

Present address:

Canadian Celanese Co., Drummondville, Quebec, Canada.

40 In Selective Oxidation Processes; Fields, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.

4.

MANN AND ROULEAU

41

Isobutylene to Methacrolein

252° a n d 320° C . b y S k i r r o w a n d W i l l i a m s ( 1 7 ) . T h e y o b t a i n e d c a r b o n dioxide,

carbon

monoxide,

water,

propylene,

acetone,

formaldehyde,

a c e t a l d e h y d e , i s o b u t y l a l d e h y d e , m e t h a c r o l e i n , p r o p y l e n e oxide, m e t h a n e , a n d acetic a c i d as r e a c t i o n p r o d u c t s .

T h e formation of the more i m -

p o r t a n t p r o d u c t s has b e e n a c c o u n t e d f o r i n terms o f a r a d i c a l a d d i t i o n m e c h a n i s m , i n i t i a t e d b y a d d i t i o n of a h y d r o x y l r a d i c a l t o t h e t e r m i n a l CH

2

g r o u p a n d f o l l o w e d b y p e r o x y r a d i c a l f o r m a t i o n a n d its subsequent

decomposition.

Bretton, W a n , a n d D o d g e

e x p e r i m e n t a l v a r i a b l e s , contact

( 2 ) s t u d i e d t h e effects of

time, b e d temperature, a n d a i r / h y d r o -


c a r b o n ratios o n t h e c o n v e r s i o n a n d y i e l d s of p r o d u c t f o r t h e o x i d a t i o n of h y d r o c a r b o n s c o n t a i n i n g f o u r c a r b o n atoms o n V 0 2

5

supported on Alfrax.

I n t h e case o f i s o b u t y l e n e o x i d a t i o n , t h e y o b t a i n e d acetic a c i d , m e t h a c r o lein, a n d formaldehyde water.

besides c a r b o n d i o x i d e , c a r b o n m o n o x i d e , a n d

T h e f o r m a t i o n of t h e r e a c t i o n p r o d u c t s has b e e n e x p l a i n e d b y a

s c h e m e o f a t o m i c d e h y d r o g e n a t i o n a n d p e r o x i d a t i o n s i m i l a r to that s u g gested b y W a t e r s ( 2 0 ) , a n d b y p e r o x i d e d e c o m p o s i t i o n .

Popova, M i l m a n ,

a n d L a t y s h e v a ( 1 5 ) s t u d i e d t h e o x i d a t i o n o f i s o b u t y l e n e to m e t h a c r o l e i n at 3 5 0 ° - 7 0 ° C . i n t h e presence of a 0 . 1 - 1 . 5 % c o p p e r o x i d e ( 7 0 % C u 0 2

+

3 0 % C u O ) catalyst s u p p o r t e d o n a Suite base, u s i n g a n i s o b u t y l e n e /

o x y g e n r a t i o of 5.6:1.

O n o x i d a t i o n , 3 . 8 % of t h e i s o b u t y l e n e w a s c o n -

v e r t e d to c a r b o x y l c o m p o u n d s ,

8 2 . 5 % of w h i c h was methacrolein, the

balance being propionaldehyde, acetaldehyde, and acrolein. T h e a i r o x i d a t i o n of i s o b u t y l e n e w a s i n v e s t i g a t e d o v e r several s u p p o r t e d a n d u n s u p p o r t e d c o p p e r oxide catalysts alone o r m i x e d w i t h other m e t a l oxides i n a n i s o t h e r m a l i n t e g r a l flow reactor.

T h e effect o f several

variables—ratio of o x y g e n to i s o b u t y l e n e i n the' feed, process t e m p e r a t u r e , c o p p e r c o n c e n t r a t i o n , r e c i p r o c a l of space v e l o c i t y — o n t h e c o n v e r s i o n a n d yield was determined.

Based o n experimental data, the rate-controlling

step a n d the rate e q u a t i o n f o r the p a r t i a l a i r o x i d a t i o n of i s o b u t y l e n e w e r e determined.

Experimental T h e e x p e r i m e n t a l a p p a r a t u s u s e d to s t u d y t h e r e a c t i o n is s h o w n s c h e m a t i c a l l y i n F i g u r e 1. T h e reactants, d r y a i r , a n d c h e m i c a l l y p u r e i s o b u t y l e n e ( m i n . 9 9 % p u r i t y ) w e r e o b t a i n e d f r o m h i g h pressure, d i a p h r a g m t y p e regulators, w h e r e b y t h e pressure o f a i r a n d i s o b u t y l e n e w e r e r e d u c e d to 20 p.s.i.g. a n d m e t e r e d o v e r B r o o k s rotameters. T h e gases w e r e m i x e d a n d p r e h e a t e d b e f o r e e n t e r i n g t h e reactor. T h e p r e h e a t i n g section, c o n s i s t i n g of a 3 foot l o n g / inch—o.d., 316 stainless steel t u b i n g w o u n d a r o u n d the reactor, w a s i m m e r s e d i n a constant t e m p e r a t u r e m e t a l b a t h . T h e reactor w a s h e a t e d i n a l i q u i d m e t a l ( 5 0 % b i s m u t h , 5 0 % l e a d ) bath, w h i c h i n t u r n was situated i n a n electrically heated furnace, whose temperature w a s controlled to w i t h i n ± 3 ° C . b y a H o n e y w e l l Pyrovane t e m p e r a t u r e c o n t r o l l e r . T h e catalyst b e d consisted of c o p p e r o x i d e s u p 1

8

In Selective Oxidation Processes; Fields, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.


42

SELECTIVE OXIDATION PROCESSES

Figure

1.

Flow diagram for the air oxidation

of

isobutylene

p o r t e d alone o r m i x e d w i t h a n o t h e r o x i d e , p l a c e d i n a 0.5 i n c h — i . d . 304 stainless steel reactor, h e l d i n p l a c e b y a stainless steel p o r o u s p l a t e i n t h e b o t t o m a n d steel w o o l at t h e t o p . T h e r m o c o u p l e s p l a c e d a t the center o f the catalyst b e d i n the reactor, a n d i n t h e m e t a l b a t h s u r r o u n d i n g i t , r e c o r d e d t h e same temperatures at h i g h flow rate of gases, i n d i c a t i n g t h a t t h e p r e h e a t i n g section w a s l o n g e n o u g h to heat t h e e n t e r i n g gases u p to t h e d e s i r e d t e m p e r a t u r e . H o w ever, d u r i n g the course of c a t a l y t i c r e a c t i o n , these t h e r m o c o u p l e s r e c o r d e d a difference of a b o u t 7 ° C , w h i c h c o u l d b e c o n s i d e r e d as n e g l i g i b l e . T h e r e a c t i o n has therefore b e e n t r e a t e d as v i r t u a l l y t a k i n g p l a c e u n d e r i s o thermal conditions, considering the h i g h l y exothermic nature of the reaction. P r o c e d u r e . F o r t h e start of t h e r u n , a i r w a s s l o w l y passed t h r o u g h the reactor, w h i l e t h e catalyst w a s b r o u g h t t o t h e r e q u i r e d t e m p e r a t u r e . W h e n t h e r e q u i r e d t e m p e r a t u r e w a s a t t a i n e d , i s o b u t y l e n e a n d a i r ratios w e r e a d j u s t e d to t h e d e s i r e d values a n d m a i n t a i n e d at t h e specified rates over a p e r i o d of 15 m i n u t e s , a n d t h e reactor w a s b r o u g h t to steady state. T h e steady state r u n w a s c o n t i n u e d f o r 3 0 - 6 0 m i n u t e s b y c o l l e c t i n g t h e l i q u i d s a m p l e f r o m t h e exit gases i n a n i c e c o o l e d t r a p ( T r a p I ) . T h e n o n c o n d e n s e d gases w e r e passed t h r o u g h a s a m p l i n g v a l v e l e a d i n g t o a F i s h e r G a s P a r t i t i o n e r . T h e off gases f r o m t h e s a m p l i n g v a l v e w e r e passed through a l i q u i d air trap ( T r a p I I ) . Part of the aldehydes w h i c h d i d not c o n d e n s e i n T r a p I c o n d e n s e d i n T r a p I I . A n a l y t i c a l . T h e t o t a l a c i d content w a s o b t a i n e d b y t i t r a t i n g t h e condensate f r o m i c e - c o o l e d t r a p w i t h 0.055IV K O H s o l u t i o n . V e r y l i t t l e a c i d w a s present i n t h e condensate a n d w a s c o n s i d e r e d t o o i n s i g n i f i c a n t to affect the m a t e r i a l b a l a n c e ; as s u c h , i t w a s n o t i n c l u d e d i n t h e analysis of t h e p r o d u c t s . T h e condensate f r o m T r a p I c o n t a i n e d m i n o r q u a n t i t i e s of a l d e h y d e s .



4.

MANN AND ROULEAU

43


GASES. T h e i n l e t f e e d gas a n d exit p r o d u c t gases w e r e a n a l y z e d f o r isobutylene, carbon monoxide, carbon dioxide, oxygen, a n d nitrogen b y p e r i o d i c i n j e c t i o n o f 0.5 m l . s a m p l e i n t o G a s P a r t i t i o n e r c o n t a i n i n g a 6-foot h e x a m e t h y l p h o s p h o r a m i d e c o l u m n a n d a 13-foot 13 X m o l e c u l a r sieve c o l u m n c o n n e c t e d i n series. N i t r o g e n w a s o b t a i n e d b y difference. ALDEHYDES. T h e sample from the l i q u i d air trap w a s diluted w i t h approximately 8 m l . of d i e t h y l ether a n d p l a c e d i n a refrigerator w h i c h w a s k e p t at 5 ° C . O n c e t e m p e r a t u r e e q u i l i b r i u m w a s r e a c h e d , .005 m l . o f the m i x t u r e w a s i n j e c t e d i n t o a 1 5 4 D P e r k i n - E l m e r gas c h r o m a t o g r a p h , c o n t a i n i n g t w o 4-meter c o l u m n s o f C a r b o w a x 1500 o n T e f l o n . T h e c o l u m n c o u l d separate acetone, a c e t a l d e h y d e , p r o p i o n a l d e h y d e , c r o t a n a l d e h y d e , a c r o l e i n , m e t h a c r o l e i n , a n d w a t e r . O n l y m e t h a c r o l e i n , w a t e r , a n d traces of acetone a n d a c e t a l d e h y d e w e r e o b t a i n e d w i t h a l l catalysts except catalyst V I I I a n d catalyst I X . Catalysts. Catalyst I , a pumice-supported copper oxide catalyst (16 w t . % of c o p p e r ) w a s p r e p a r e d b y i m p r e g n a t i n g 2 0 - 4 0 m e s h c r u s h e d p u m i c e stone w i t h c o p p e r n i t r a t e s o l u t i o n . C a t a l y s t I I ( 2 % c o p p e r ) , Catalyst I I I ( 8 % copper), and Catalyst I V ( 3 2 % copper) were prepared b y impregnating pumice w i t h the required amount of copper nitrate solutions. C a t a l y s t V , a n u n s u p p o r t e d c o p p e r o x i d e c a t a l y s t , w a s p r e p a r e d b y c a l c i n i n g c o p p e r n i t r a t e f o r t w o h o u r s a t 200° C . a n d f o u r h o u r s at 550° C . C a t a l y s t V I , a m o l y b d e n u m o x i d e c a t a l y s t s u p p o r t e d o n p u m i c e (8% M o 0 ) was prepared b y impregnating pumice with an ammonium paramolybdate solution. Catalyst V I I , a copper-molybdenum oxide c a t a l y s t ( 7 5 % C u O + 2 5 % M0O3) w a s p r e p a r e d b y i m p r e g n a t i n g t h e pumice w i t h a copper nitrate plus a m m o n i u m paramolybdate solution, c o n t a i n i n g c o p p e r o x i d e a n d m o l y b d i c o x i d e i n r e q u i r e d ratios. C a t a l y s t V I I I , a copper oxide supported o n a l u m i n a ( 1 6 % copper) was prepared by impregnating alumina w i t h copper nitrate solution. Catalyst I X , a m i x e d o x i d e c a t a l y s t ( 8 % oxides, M0O3, V 0 , P 0 i n a r a t i o o f 5 : 1 : 0 . 6 ) was prepared b y impregnating alumina w i t h a n a m m o n i u m paramolybdate-ammonium vanadate-ammonium phosphate solution containing r e q u i r e d ratios o f oxides. A l l t h e catalysts except C a t a l y s t V w e r e d r i e d o v e r n i g h t at 105° C . a n d c a l c i n e d at 550° C . f o r six h o u r s . E a c h c a t a l y s t w a s a c t i v a t e d b y p l a c i n g i t i n t h e flow r e a c t o r a n d p a s s i n g a s l o w s t r e a m of a i r o v e r i t f o r 12 h o u r s a t 400° C . 3

2

5

2

5

Results and Discussion T h e effect of o x y g e n ( i n t h e a i r ) / i s o b u t y l e n e i n t h e f e e d , t h e operati n g temperature, a n d the copper concentration i n the catalyst o n the c o n v e r s i o n of i s o b u t y l e n e a n d t h e y i e l d of m e t h a c r o l e i n w e r e i n v e s t i g a t e d . W h i l e c o n v e r s i o n is r e f e r r e d to as t h e m o l e s of i s o b u t y l e n e

consumed

( r e a c t e d ) p e r h o u r to t h e m o l e s of i s o b u t y l e n e f e d p e r h o u r , t h e r a t i o o f the moles of m e t h a c r o l e i n p r o d u c e d p e r h o u r t o t h e m o l e s o f i s o b u t y l e n e r e a c t e d p e r h o u r h a s b e e n d e f i n e d as y i e l d o f m e t h a c r o l e i n .

F o r carbon

d i o x i d e a n d c a r b o n m o n o x i d e , t h e y i e l d has b e e n d e f i n e d as o n e - f o u r t h o f the r a t i o o f t h e m o l e s of c a r b o n m o n o x i d e o r c a r b o n d i o x i d e f o r m e d t o the moles of i s o b u t y l e n e r e a c t e d .

T h e ratio of grams of catalyst to t h e



44 Table I.

Conversions and Yields for Different Catalysts

τ = 4oo° α, Ν = ι, W/F = u i fields


Catalyst

Conversion of Isobutylene

CO

C0

Acids

Other Aldehydes

0.296 0.333 0.308 0.274 0.069

0 .688 0 .651 0 .678 0 717 0 840

0.009 0.013 0.012 0.009 0.098

— — — — —

— — — — —

0.301 0.181 0.273

0 .682 0 532 0 .383

0.009 0.171 0.083

0.010 0.046

4

0.400 0.272 0.371 0.422 0.467 negligible 0.386 0.526 0.618

I II III IV V VI VII VIII IX

C HsO

2

—

—

0.106 0.215

.70

.60

*

X

°

y

C H 4

8

C H 0 4

6

.50

9 LJ

>ο ζ
Ζ

ο ο

.20

.10

c

80

100

Effect of copper concentration conversion and yield

on the

20

40

60

% COPPER Figure

2.

g r a m m o l e of i s o b u t y l e n e f e d p e r h o u r is defined as W / F . T h e r a t i o of W/F w a s v a r i e d b y v a r y i n g t h e a m o u n t of catalyst w h i l e k e e p i n g t h e a m o u n t o f f e e d constant, a n d v i c e - v e r s a . E f f e c t o f V a r i o u s C a t a l y s t s o n O x i d a t i o n . T h e effect o n t h e c o n v e r sion a n d y i e l d f o r several catalysts at 400° C , a W/F r a t i o of 1.71, a n d a n



4.

MANN AND ROULEAU

ΟI


ι

0

ι

1

I

1

1 2 3 4 MOLES OXYGEN/MOLES C H IN FEED 4

Figure

45

5

8

3. Effect of oxygen/isobutylene in feed (Ν) on conversion and yield

o x y g e n / i s o b u t y l e n e r a t i o o f 1.0 is g i v e n i n T a b l e I .

ratio

T h o u g h catalyst I X

g a v e t h e h i g h e s t c o n v e r s i o n , catalyst I w a s c o n s i d e r e d t h e best f o r t h e detailed kinetic study.

It h a d negligible activity towards the formation

of o r g a n i c c o m p o u n d s other t h a n m e t h a c r o l e i n a n d a v e r y h i g h s e l e c t i v i t y for m e t h a c r o l e i n p r o d u c t i o n at a h i g h rate o f c o n v e r s i o n .

T h i s catalyst

w a s u s e d therefore t h r o u g h o u t t h e i n v e s t i g a t i o n of t h e r e a c t i o n k i n e t i c s . Effect of C o p p e r Concentration.

T h e o x i d a t i o n of i s o b u t y l e n e w a s

c a r r i e d o u t o v e r s e v e r a l c o p p e r o x i d e catalysts c o n t a i n i n g v a r y i n g a m o u n t s of c o p p e r o x i d e / s u p p o r t

ratios.

F i g u r e 2 shows

t h e effect o f c o p p e r

c o n c e n t r a t i o n i n t h e catalyst o n t h e c o n v e r s i o n of i s o b u t y l e n e at 400° C , a W/F r a t i o o f 1.71 a n d a n Ν — 3.0. I t i n c r e a s e d f a i r l y r a p i d l y u p t o a c o n c e n t r a t i o n of 1 0 % c o p p e r b y w e i g h t .

F o r h i g h e r concentrations, t h e

effect o n t h e c o n v e r s i o n w a s m u c h less p r o n o u n c e d , t h o u g h i t i n c r e a s e d s t e a d i l y u p to a v a l u e o f 0.467 f o r u n s u p p o r t e d c o p p e r oxide.

A n increase

i n c o p p e r content of the catalyst, o n t h e other h a n d , a d v e r s e l y affected t h e y i e l d of methacrolein.

T h e decrease i n t h e y i e l d o f m e t h a c r o l e i n seems

to b e d i r e c t l y p r o p o r t i o n a l t o a n increase i n c o p p e r content o f t h e catalyst.



46 Effect

of Oxygen/Isobutylene

b u t y l e n e r a t i o (N)

Ratio.

T h e effect of

oxygen/iso-

i n t h e f e e d o n t h e c o n v e r s i o n a n d y i e l d f o r a W/F

r a t i o o f 1.0 o v e r c o p p e r o x i d e s u p p o r t e d o n p u m i c e ( C a t a l y s t I ) a t 400° C . is s h o w n i n F i g u r e 3.

T h e c o n v e r s i o n of i s o b u t y l e n e i n c r e a s e d r a p i d l y

w i t h reactant ratios u p t o a n Ν of 2 a n d t h e n s e e m e d to b e c o m e i n d e p e n d e n t of the r e a c t a n t ratios. E f f e c t of T e m p e r a t u r e .

T h e influence o f t h e t e m p e r a t u r e a n d y i e l d

over Catalyst I was investigated i n the temperature range 350°-450° C .


.35|

—I

* ο

X

C H 4

8

V y

CH0 4

6

.20· 350 Figure

4.

375 400 425 450 TEMPERATURE °C Effect of temperature on conversion and yield

F i g u r e 4 shows t h e effect of t e m p e r a t u r e o n t h e c o n v e r s i o n a n d y i e l d at a W/F r a t i o of 1.2 a n d Ν — 2.0. W i t h i n c r e a s i n g t e m p e r a t u r e f r o m 350° to 450° C , t h e c o n v e r s i o n i n c r e a s e d b u t the y i e l d decreased. E f f e c t o f W/F R a t i o s . F i g u r e 5 shows t h e effect of v a r i o u s W/F ratios o n t h e c o n v e r s i o n of i s o b u t y l e n e a n d t h e y i e l d of m e t h a c r o l e i n a n d c a r b o n d i o x i d e at 400° C . a n d a n Ν of 1.0 over C a t a l y s t I . I t is seen that t h o u g h t h e f r a c t i o n of i s o b u t y l e n e c o n s u m e d (% c o n v e r s i o n ) i n c r e a s e d



4.

MANN AND ROULEAU

Isobutylene to

Methacrolein

47

r a p i d l y u p to a W/F r a t i o o f 2, t h e r e is n o t m u c h c h a n g e i n t h e c o n v e r s i o n of i s o b u t y l e n e f o r W/F e x c e e d i n g 2. F i g u r e s 2, 3, a n d 4, s h o w t h a t w h i l e t h e m e t h a c r o l e i n y i e l d decreases w i t h i n c r e a s e d c o p p e r concentrations, t e m p e r a t u r e o r o x y g e n / i s o b u t y l e n e ratio i n the feed, the y i e l d of carbon dioxide, carbon monoxide, a n d water i n c r e a s e d . T h e s e results suggest t h a t m e t h a c r o l e i n u n d e r w e n t f u r t h e r o x i d a t i o n t o c a r b o n d i o x i d e a n d m o n o x i d e . T h e a b o v e results a r e i n l i n e w i t h t h e findings of Isaev, M a r g o l i s , a n d S a z a n o v a (12) a n d V o g e , W a g n e r , a n d Stevenson (19), w h o o x i d i z e d p r o p y l e n e over c u p r o u s o x i d e catalysts. T h e y e s t a b l i s h e d , b y means o f k i n e t i c a n d i s o t o p i c d a t a , t h a t most of t h e c a r b o n d i o x i d e f o r m e d i n t h e process results f r o m t h e o x i d a t i o n of a c r o l e i n . A k i n e t i c analysis o f t h e e x p e r i m e n t a l d a t a (16) w a s m a d e u s i n g t h e H o u g e n - W a t s o n (11) a p p r o a c h , w h e r e b y v a r i o u s m e c h a n i s m s w h i c h m i g h t c o n t r o l t h e r e a c t i o n rate are p o s t u l a t e d , a n d t h e rate expressions consistent w i t h these hypotheses are d e r i v e d . T h e r a t e - c o n t r o l l i n g m e c h a -

A. C. S. Editorial Library In Selective Oxidation Processes; Fields, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.

48



n i s m of t h e s o l i d - c a t a l y z e d gas r e a c t i o n m a y b e t h e mass transfer o f t h e reactants o r p r o d u c t s o r c h e m i c a l r e a c t i o n o f a d s o r b e d m o l e c u l e s at the catalyst surface. T h e rate expressions thus d e r i v e d are t h e n fitted to t h e e x p e r i m e n t a l d a t a . T h e rate expression w h i c h gives t h e best fit is r e t a i n e d ; the others are d i s c a r d e d . T h e eifects of diffusion a n d mass transfer w e r e k e p t at a m i n i m u m b y u s i n g a h i g h v e l o c i t y of gas t h r o u g h the catalyst b e d . T h e use of inert p o r o u s s u p p o r t a n d its s m a l l p a r t i c l e size ( 2 0 - 4 0 m e s h ) a n d surface area (less t h a n 0.1 s q . m e t e r / g r a m ) r u l e d o u t the p o s s i b i l i t y of t h e d i f f u s i o n i n the pores c o n t r o l l i n g t h e r e a c t i o n . A p l o t of i n i t i a l rates against p a r t i a l pressure of i s o b u t y l e n e i n d i c a t e d that the d e s o r p t i o n of t h e p r o d u c t s w a s definitely n o t rate c o n t r o l l i n g . S i n c e d e s o r p t i o n of p r o d u c t s , mass transfer, a n d d i f f u s i o n a l effects w e r e n o t rate c o n t r o l l i n g , t h e p o s s i b i l i t y of t h e a d s o r p t i o n of reactants a n d surface r e a c t i o n b e t w e e n a d s o r b e d m o l e c u l e s as rate c o n t r o l l i n g w a s e x a m i n e d . W r i t i n g t h e s t o i c h i o m e t r i c e q u a t i o n f o r the r e a c t i o n as G H 4

8

+

0

2

ç± C H 0 + 4

6

H 0 2

a n d f o l l o w i n g the u s u a l p r o c e d u r e of d e r i v a t i o n ( J O ) , n e g l e c t i n g t h e t e r m ( ^ C

4

H

6

O

K ^ H O , ) / ^

S

I

N

C

E

Κ

( e q u i l i b r i u m constant)

o x i d a t i o n of i s o b u t y l e n e w a s v e r y l a r g e

(In Κ >

f o r t h e gas phase 2 0 ) , equations f o r

v a r i o u s p o s t u l a t e d m e c h a n i s m s r e l a t i n g t h e rate of f o r m a t i o n of m e t h a c r o l e i n a n d w a t e r w e r e d e r i v e d ( T a b l e I I ) a n d tested f o r t h e best fit of experimental data. T h e d a t a w a s most satisfactorily c o r r e l a t e d b y m e c h a n i s m V I , w h i c h assumes

that t h e r a t e - c o n t r o l l i n g step is a surface

reaction

Table II.

between

Rate Equations

Type of Reaction Mechanism I :

Surface reaction between adsorbed isobutylene a n d adsorbed oxygen, adsorption of isobutylene controlling.

M e c h a n i s m II :

Surface r e a c t i o n between adsorbed isobutylene a n d oxygen i n gas phase, a d s o r p t i o n o f isobutylene c o n t r o l l i n g .

M e c h a n i s m III :

Surface reaction between adsorbed isobutylene a n d adsorbed oxygen, adsorption of oxygen c o n t r o l l i n g .

Mechanism I V :

Surface reaction between adsorbed oxygen a n d isobutylene i n gas phase, adsorption o f oxygen controlling.

Mechanism V :

Surface reaction between adsorbed isobutylene a n d adsorbed oxygen phase

Mechanism V I :

Surface reaction between adsorbed isobutylene a n d oxygen i n gas phase

Mechanism V I I :

Surface reaction between adsorbed oxygen a n d isobutylene i n gas phase


4.

MANN AND ROULEAU


49

s t r o n g l y a d s o r b e d i s o b u t y l e n e a n d gaseous ( o r v e r y w e a k l y a d s o r b e d ) oxygen.

T h e a d s o r p t i o n e q u i l i b r i u m constant f o r m e t h a c r o l e i n

(K

C 4 H e 0

w a s a s s u m e d t o b e t h e same as f o r i s o b u t y l e n e since a c h a n g e i n K h a d l i t t l e effect o n t h e c o r r e l a t i o n .

)

C 4 H e 0

T h o u g h the numerical value o f the

a d s o r p t i o n e q u i l i b r i u m constant f o r m e t h a c r o l e i n c o u l d n o t b e e s t i m a t e d p r e c i s e l y , t h e best fit t o t h e e x p e r i m e n t a l d a t a w a s o b t a i n e d w h e n K w a s o f the^eame

o r d e r o f m a g n i t u d e as K

C 4 H ( J

.

C 4 H e 0

T h e r e a c t i o n rate f o r

the o x i d a t i o n o f i s o b u t y l e n e t o m e t h a c r o l e i n h a s b e e n satisfactorily e x


pressed b y the f o l l o w i n g e q u a t i o n :

Selective Oxidation Processes

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