Heterogeneous Catalysis - American Chemical Society

25

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Selective Oxidation by Heterogeneous Catalysis ROBERT K. GRASSELLI The Standard Oil Company, Research Center, Cleveland, OH 44128 Selective oxidation of hydrocarbons by heterogeneous catalysis is a versatile approach to commercial production of many important monomers such as acrylonitrile, acrylic acid, acrylates, ethylene oxide, maleic anhydride, and phthalic anhydride. Over the past twenty-five years the development of efficient catalysts for selective oxidation resulted in a new generation of commercial processes which utilize inexpensive olefinic and paraffinic feeds, replacing more reactive and costly raw materials. The catalysts are complex solid metal oxide systems which selectively activate hydrocarbons. Olefins, in particular, are activated via an allylic intermediate formation. The catalysts contain facile solid state redox couples which allow for efficient electron and lattice oxygen transport between reactant, adsorption and surface active site, and the surface reoxidation site which is then reconstituted by gaseous oxygen. Historically, the key discoveries were based on an understanding of the important features of oxidation catalysis in terms of oxygen bond strength, site isolation, and redox mechanisms. Further advancement of fundamental catalyst science will continue to serve as a basis for the design of even more efficient catalyst systems of the future. The r e l a t i o n s h i p between the petroleum and chemical indust r i e s i s one of d i r e c t interdependence. About 85% of the primary organic chemicals produced today are derived from p e t r o leum and n a t u r a l gas sources. Thus, r a p i d l y changing s u p p l i e s of petroleum have a double impact by a f f e c t i n g both energy and chemical i n d u s t r i e s . This i s r e f l e c t e d , f o r example, i n the automotive i n d u s t r y , by the trend toward production of more

0097-615 6/ 83/0222-0317$08.25/0 © 1983 American Chemical Society In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


318

HETEROGENEOUS CATALYSIS

e n e r g y - e f f i c i e n t engines and lightweight bodies, to extend the dwindling petroleum s u p p l i e s . In the chemical s e c t o r , i t becomes necessary to d i s c o v e r more e f f i c i e n t processes which produce, with higher s e l e c t i v i t y , the d e s i r e d u s e f u l products to the e x c l u s i o n of waste i n order to maintain petroleum as an a t t r a c t i v e primary source. This w i l l become i n c r e a s i n g l y important, s i n c e the percentage of the a v a i l a b l e petroleum which i s used f o r chemicals i s expected to increase as petroleum s u p p l i e s decrease. A comparison of petroleum u t i l i z a t i o n by the energy and chemical i n d u s t r i e s shows that the major p o r t i o n (90-92%) of every b a r r e l of o i l i s used f o r f u e l s , i . e . , energy production v i a t o t a l combustion to carbon d i o x i d e and water. In c o n t r a s t to t h i s u n s e l e c t i v e use of petroleum, o x i d a t i o n r e a c t i o n s which are c a r r i e d out on the much smaller f r a c t i o n devoted to the chemicals i n d u s t r y r e s u l t i n conversion to u s e f u l products v i a s e l e c t i v e processes which stop short of t o t a l combustion. The s e l e c t i v e nature of nearly a l l of the o x i d a t i o n r e a c t i o n s of i n d u s t r i a l s i g n i f i c a n c e i s made p o s s i b l e by the use of a c a t a l y s t , which lowers the a c t i v a t i o n energy f o r the s e l e c t e d process and provides a f a c i l e path by which u s e f u l products can form. Thus, the key to both the d i s c o v e r y of new routes to u s e f u l chemicals and improvements i n e x i s t i n g i n d u s t r i a l processes l i e s i n c a t a l y s i s . The m a j o r i t y of today's important i n d u s t r i a l organic chemi c a l s i s produced by c a t a l y t i c r e a c t i o n s . Based on a comparison of the weight of end-product produced, f i v e major r e a c t i o n types account f o r about 91% of the top twenty organic chemicals produced i n the United States by c a t a l y t i c processes (Figure 1 ) . These are, aromatic a l k y l a t i o n (24%), heterogeneous o x i d a t i o n (23%), dehydrogenation (15%), methanol s y n t h e s i s (16%), and homogeneous o x i d a t i o n (13%). Styrene, an important monomer f o r the manufacture of t h e r m o p l a s t i c s , i s produced v i a a two-step route which u t i l i z e s both aromatic a l k y l a t i o n (propylene + ethylene -> ethylbenzene) and dehydrogenation (ethylbenzene -> styrene + hydrogen). Methanol, a product of the c a t a l y t i c conv e r s i o n of synthesis gas (a mixture of carbon monoxide and hydrogen), and phenol, which i s produced from the homogeneous c a t a l y t i c o x i d a t i o n of cumene, are both used e x t e n s i v e l y i n the production of important i n d u s t r i a l r e s i n s . A d i p i c a c i d , used i n the manufacture of nylon 6,6, i s a l s o produced from a homogeneous c a t a l y t i c o x i d a t i o n , which involves the two-step o x i d a t i o n of cyclohexane, f i r s t to a mixture of cyclohexanol and cyclohexanone. C a t a l y t i c r e d u c t i o n and h y d r a t i o n account f o r the remaining 9% of the t o t a l which i s not produced by one of these f i v e major r e a c t i o n c l a s s e s . Cyclohexane production from c a t a l y t i c hydrogénation of benzene, and the a c i d catalyzed hydration of ethylene to ethanol, which i s used e x t e n s i v e l y as a paint solvent i n the form of e t h y l a c e t a t e , are important examples of these r e a c t i o n s .

In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


C6H6

F i g u r e 1. I m p o r t a n t t o p 20 i n d u s t r i a l o r g a n i c c h e m i c a l s by c a t a l y s i s . ( R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 6. C o p y r i g h t 1981, Adv. C a t a l . )


VO

»—*

to


320


S e l e c t i v e o x i d a t i o n by heterogeneous c a t a l y s i s (hetero geneous o x i d a t i o n i n Figure 1) r e f e r s to those processes where organic feeds are converted i n the vapor phase to u s e f u l products c o n t a i n i n g the same number of carbon atoms using s o l i d phase c a t a l y s t s . Of the o x i d a t i o n processes by which important (top twenty) organic chemicals are produced by heterogeneous c a t a l y s i s i n the U.S., s e l e c t i v e o x i d a t i o n of hydrocarbons accounts f o r 79% of the t o t a l supply (Figure 2). This type of o x i d a t i o n may be d i v i d e d i n t o four c l a s s e s : Allylic o x i d a t i o n , epoxidation, aromatic o x i d a t i o n and p a r a f f i n oxida t i o n (Table 1). A l l y l i c o x i d a t i o n of o l e f i n s g i v e s , α,Β-unsaturated n i t r i l e s from o l e f i n s i n the presence of NH^ and 0^, but can a l s o produce, i n the absence of ammonia, aldehydes, a c i d s , dienes, and a l l y l acetates. The epoxidation of o l e f i n s gives epoxides, while aromatic s i d e chain o x i d a t i o n and p a r a f f i n o x i d a t i o n y i e l d c a r b o x y l i c a c i d s and anhydrides. The c a t a l y t i c o x i d a t i o n of methanol to formaldehyde, an important chemical i n the manufacture of r e s i n s , accounts f o r the remaining 24% of the important organic chemicals produced by heterogeneously catalyzed processes.

Figure 2. Important organic chemicals by s e l e c t i v e heterogeneous o x i d a t i o n . (Reproduced with permission from Ref. 6. Copyright 1981, Adv. Catal.)



ALLYLIC

EPOXIDATION

AROMATIC

PARAFFINIC

1.

2.

3.

4.

OXIDATION CLASS

Table

2

3

UNSATURATED POLYESTER RESINS FUMARIC ACID, INSECTICIDES AND FUNGICIDES

H

4 2°3

4

MALEIC ANHYDRIDE

6

C

H

4 10

3)2

C

C H

n-BUTANE

H

6 4(

POLYESTERS, PLASTICIZERS, FINE CHEMICALS

C

PHTHALIC ANHYDRIDE C H (C 0 )

ETHYLENE GLYCOL, ANTIFREEZE POLYESTERS, SURFACTANTS

o-XYLENE

4

ETHYLENE OXIDE C H 0

2

ACRYLIC FIBERS, RESINS, RUBBERS ADIPONITRILE

END USE

Reactions.

ETHYLENE C H 4

C3H3N

2

ACRYLONITRILE

H

3 6

PROPYLENE

PRODUCT

C

STARTING MATERIAL

I Examples o f S e l e c t i v e H e t e r o g e n e o u s C a t a l y t i c O x i d a t i o n


0.5

5.8

MILLIONS TON (WORLD)

322



Epoxidation The most important example of t h i s r e a c t i o n i s the format i o n of ethylene oxide (Eqn. 1), over A g - c a t a l y s t s which d i s placed the two-step chlorohydrine route (Eqn. 2 ) . Ethylene oxide i s used i n the production of ethylene g l y c o l , a n t i f r e e z e , p o l y e s t e r s and s u r f a c t a n t s , and accounts f o r 18% of U.S. ethylene consumption (Figure 3 ) . ^ S i l v e r i s unique i n i t s a b i l i t y to c a t a l y z e the r e a c t i o n forming a molecular O2 adsorbed species which r e a c t s with ethylene to form ethylene oxide (Scheme 1). Absorbed atomic oxygen [Ag(0)ads], a by-product of t h i s process, i s r e s p o n s i b l e f o r waste formation. From the stoichiometry of the mechanism, the maximum y i e l d of s e l e c t i v e product (ethylene oxide) i s 80%.

Ag 5CH2=CH2

• 50

2

—v4CH

2

-

CH2 • 2C02

+ 2H20

ETHYLENE

ETHYLENE

OXIDE

Equation 1.

CH2=CH2

• C l

2

• H20 —H0CH2CH2C1

•

HCl

Ca(0H>,

CHo \2

Equation

CH0 / 2

•

1/2 C a C l

2

• H20

2.



25.

GRASSELLi

323

Selective Oxidation

F i g u r e 3. E t h y l e n e o x i d e

utilization.

Scheme 1« E t h y l e n e e p o x i d a t i o n mechanism.


324



Aromatic Oxidation A process a k i n to the a l l y l i c o x i d a t i o n i n a c t i v a t i o n i s aromatic s i d e chain o x i d a t i o n to produce acids or anhydrides. P h t h a l i c anhydride, an important intermediate i n production of p o l y e s t e r s , p l a s t i c i z e r s , and f i n e chemicals s y n t h e s i s , can be produced v i a s e l e c t i v e o x i d a t i o n of ^-xylenes using vanadium oxide c a t a l y s t s (Eqn. 3). This process today accounts f o r over 85% of the p h t h a l i c anhydride produced worldwide, and has l a r g e l y d i s p l a c e d the p a r t i a l l y w a s t e f u l and more expensive naphthalenebased route (Eqn. 4 ) , by which n e a r l y a l l PA was produced i n 1960 (Figure 4 ) . ^ Nearly a l l of the p h t h a l i c anhydride produced today i s used f o r manufacturing v i n y l p l a s t i c i z e r s , with a much smaller a p p l i c a t i o n i n the f i n e chemicals i n d u s t r y .

o-XYLENE

PHTHALIC

ANHYDRIDE

Equation 3.

0

NAPHTHALENE

Equation 4.

Oh

100 h

1960

1970

1980

Figure 4. Change i n feedstock for p h t h a l i c anhydride production.


25.

GRASSELLi

325

Selective Oxidation


P a r a f f i n Oxidation C a t a l y s t s which can s e l e c t i v e l y a c t i v a t e the normally unr e a c t i v e p a r a f f i n s have been developed i n recent years. The production of maleic anhydride from butane over vanadiumphosphorous-oxide c a t a l y s t s has received much a t t e n t i o n (Eqn. 5 ) , and i s beginning to r e p l a c e the more w a s t e f u l produc t i o n of maleic anhydride from benzene (Eqn. 6 ) which i s s t i l l the major feedstock. M a l e i c anhydride production from butene or butadiene i s a l s o p o s s i b l e (Eqn. 7 ) , but cannot compete with the cheaper butane feed. M a l e i c anhydride i s mainly used i n the manufacture of unsaturated p o l y e s t e r r e s i n s , fumaric a c i d manu f a c t u r e , i n s e c t i c i d e s , and f u n g i c i d e s (Figure 5 ) . ^

V/P/Ox

0

η-BUTANE

MALEIC ANHYDRIDE

E q u a t i o n 5.

0

BENZENE

0

E q u a t i o n 6. C a t a l y s t s : V 0 / M o 0 , V ^ / S b ^ , 2

5

!,2-BUTENES, BUTADIENE

3

^ °5 2° ,?

2

0



326


F i g u r e 5. M a l e i c a n h y d r i d e

utilization.


25.


Allylic

327

Selective Oxidation

GRASSELLi

Oxidation

In a l l y l i c o x i d a t i o n r e a c t i o n s (Eqns. 8-13), an o l e f i n (usually propylene) i s a c t i v a t e d by the a b s t r a c t i o n of a hydrogen α to the double bond to produce an a l l y l i c intermediate. This intermediate can be i n t e r c e p t e d by c a t a l y s t l a t t i c e oxygen to form a c r o l e i n or a c r y l i c a c i d ( o x i d a t i o n ) , l a t t i c e oxygen i n the presence of ammonia to form a c r y l o n i t r i l e (ammoxidation), HX to form an a l l y l - s u b s t i t u t e d o l e f i n (e.g. a c e t o x y l a t i o n ) , or i t can dimerize to form 1,5-hexadiene. I f an o l e f i n c o n t a i n i n g 3-hydrogens i s used, l o s s of Η from the a l l y l i c intermediate occurs f a s t e r than l a t t i c e oxygen i n s e r t i o n , to form a diene with the same number of carbons, e.g. butadiene from butene (oxydehydrogenation). In a l l of these processes a common a l l y l i c intermediate i s formed.

CH =CH-CH 2

+

3

NH

+

3

3/2 0

Equation 8.

CH =CH-CH 2

+

3

0

* CH =CHCN

2

3H 0 2

Ammoxidation

>CH =CHCHO

2

+

2

+

2

H 0 2

Equation 9. Oxidation

CH =CH-CH 2

3

+

3/2 0

2

>CH =CHC0 H 2

2

+

H 0 2

Equation 10. Oxidation

2(CH =CH-CH ) 2

+

3

1/2 0

> CH =CH-CH CH -CH=CH

2

2

2

2

2

+

H 0 2

Equation 11. Dimerization

CH =CH-CH CH 2

2

3

+

1/2 0

2

> CH =CH-CH=CH 2

2

+

H 0 2

Equation 12. Oxydehydrogenation

CH =CH-CH 2

3

+

1/2 0

2

+

HOAc

>CH =CH-CH OAc 2

2

+

Equation 13. A c e t o x y l a t i o n


H 0 2

328



A l l y l i c o l e f i n s of higher molecular weight than propylene can a l s o be c o n v e r t e d t o t h e c o r r e s p o n d i n g α-β u n s a t u r a t e d n i t r i l e s , a l d e h y d e s , and d i e n e s by c a t a l y t i c v a p o r phase o x i d a t i o n and a m m o x i d a t i o n . Examples i n c l u d e t h e c o n v e r s i o n o f i s o b u t h y l e n e t o m e t h a c r y l o n i t r i l e ( e q . 14) o r m e t h a c r o l e i n ( e q . 16 ) , α - m e t h y l s t y r e n e t o a t r o p o n i t r i l e ( e q . 15 ) o r a t r o p o l d e h y d e ( e q . 1 7 ) , and 2 - m e t h y l b u t e n e t o i s o p r e n e ( e q . 1 8 ) .

E q u a t i o n 14. A m m o x i d a t i o n

CH =CCH 2

3

+ NH

3

+ 3/2 0

-CH =CCN + 3H 0

2

2

2

E q u a t i o n 15. A m m o x i d a t i o n

E q u a t i o n 16. O x i d a t i o n

CH =CCH 2

3

+ 0

*CH =CCHO *

2

2

H 0 2

E q u a t i o n 17. O x i d a t i o n

CH =CCH CH 2

2

CH

3

3

+ 1/20

2

*CH =CCH=CH 2

CH

2

+ H 0 2

3

E q u a t i o n 18. O x y d e h y d r o g e n a t i o n


25.

Ammoxidation


329

Selective Oxidation

GRASSELLi

of Propylene to A c r y l o n i t r i l e

The most i n d u s t r i a l l y s i g n i f i c a n t and w e l l - s t u d i e d a l l y l i c o x i d a t i o n r e a c t i o n i s the ammoxidation of propylene ( eq. 8 ) which accounts f o r v i r t u a l l y a l l of the 8 b i l l i o n pounds of a c r y l o n i t r i l e produced annually world-wide. The r e l a t e d oxida t i o n r e a c t i o n produces a c r o l e i n ( eq. 9 ), another important monomer. Although ammoxidation r e q u i r e s high temperatures, the c a t a l y s t s are, i n general the same f o r both processes and i n clude bismuth molybdates, uranium antimonates ( ϋ ^ β Ο ^ ) , i r o n antimonates, and bismuth molybdate based multicomponent systems. The l a t t e r category includes many of todays h i g h l y s e l e c t i v e and a c t i v e commercial c a t a l y s t systems. The ammoxidation process ( eq. 8 ) d i s p l a c e d the more expensive acetylene-HCN-based route i n the e a r l y 1960 s (eq. 20). Other obsolete processes a l s o i n v o l v e more expensive reagents (e.g. ethylene oxide, eq. 19, and acetaldehyde, eq. 21) and oxidants (e.g. NO, eq. 22). The impact of the i n t r o d u c t i o n of the ammoxidation process i n 1960 was an immediate d r a s t i c reduction i n a c r y l o n i t r i l e p r i c e and g r e a t l y increased produc t i o n which made p o s s i b l e many of today's high-volume a p p l i c a tions of a c r y l o n i t r i l e (Figure 6A). The production of a c r y l o n i t r i l e , which accounts f o r 17% of the t o t a l U.S. propylene consumption, i s used e x t e n s i v e l y i n f i b e r s , p l a s t i c s and r e s i n s (ARS/SA) and rubber i n d u s t r i e s , with a growing number of miscellaneous a p p l i c a t i o n s , i n c l u d i n g the e l e c t r o - h y d r o d i m e r i z a t i o n process f o r a d i p o n i t r i l e production (Figure 6B). 1

CH2

CH2

-H 0 2

+

HCN

HO-CH CH CN

BASE CAT.

2

2

200°

* CH =CHCN 2

E q u a t i o n 19.

Ο

H-C=C-H

+

HCN

80-90° C

CH =CHCN 2

CuCINH CI 4

E q u a t i o n 20. -H 0 2

CH3CHO + HCN

> CH =CHCN

CH3CHCN OH

2

600-700° C

E q u a t i o n 21. Ag 0/Si0 2

C H = C H C H + 3/2NO 2

3

2

CH =CHCN + 3/2H 0 + 1/4 N 2

2

E q u a t i o n 22.


2

330



F i g u r e 6A. U.S. with permission

p r o d u c t i o n and p r i c e o f a c r y l o n i t r i l e . ( R e p r o d u c e d f r o m R e f . 6. C o p y r i g h t 1981, Adv. C a t a l . )



25.

GRASSELLI

Selective Oxidation

331

F i g u r e 6B. W o r l d a c r y l o n i t r i l e c o n s u m p t i o n by end u s e . ( R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 6. C o p y r i g h t 1981, Adv. C a t a l . )



332


Development o f S e l e c t i v e A m m o x i d a t i o n and O x i d a t i o n C a t a l y s t s The d e v e l o p m e n t o f t h e S o h i o a m m o x i d a t i o n p r o c e s s was b a s e d on t h e t h e o r y t h a t l a t t i c e o x y g e n f r o m a s o l i d m e t a l o x i d e w o u l d s e r v e a s a more s e l e c t i v e and v e r s a t i l e o x i d a n t ^ ^ t h a n w o u l d m o l e c u l a r oxygen-*, and h a s r e c e n t l y been r e v i e w e d . * When a p p l i e d t o t h e a m m o x i d a t i o n o f p r o p y l e n e ( e q . 8 ) , a c r y l o n i t r i l e i s formed i n t h e c a t a l y s t r e d u c t i o n s t e p , w h i l e m o l e c u l a r oxygen s e r v e s t o r e c o n s t i t u t e l a t t i c e oxygen v a c a n c i e s ( F i g . 7). F o r t h e o x i d a t i o n o f p r o p y l e n e t o a c r o l e i n , l a t t i c e oxygen i s u s e d t o f o r m a c r o l e i n i n c a t a l y s t r e d u c t i o n s t e p ( F i g . 8, eq. 23), and r e o x i d a t i o n o f m e t a l o x i d e by gaseous O2 r e c o n s t i t u t e s t h e a c t i v e o x i d a n t ( e q . 24). The m e t a l o x i d e becomes a c a t a l y s t when t h e p r o c e s s c a n b e c a r r i e d o u t i n t h e p r e s e n c e of m o l e c u l a r o x y g e n , t h e s t o i c h i o m e t r i c o x i d a n t ( e q . 9).

HYDROCARBON

SELECTIVE OXIDATION PRODUCT

F i g u r e 7. C a t a l y t i c s e l e c t i v e o x i d a t i o n - r e d u c t i o n c y c l e . ( R e p r o d u c e d w i t h p e r m i s s i o n from Ref. 6. C o p y r i g h t 1981, Adv. C a t a l . )

n

+

2M( ) O

x

+

CH =CHCH 2

F i g u r e 8. C a t a l y t i c r e d o x

CH =CHCH 2

Propylene

3

n

^

3

C

A

2

T

H + C H = CHCHC=0+ H 0

+

x

cycle.

+ 0 (g)

2

2M( " ) O .

1

2

2

Reduction of metal

> C H = CHC=0 2

+ Oxygen

Acrolein

+

oxide.

H 0 2

+ Water

E q u a t i o n 23. C a t a l y s i s .

2M(n-2)+O . x

1

+

0 (g) 2

n

+

2M< ) O

E q u a t i o n 24. R e o x i d a t i o n o f m e t a l

x

oxide.


25.

GRASSELLi

Selective Oxidation

333


In an e a r l y embodiment of t h i s r e a c t i o n , known as the oxidant process ( F i g . 9 ) ^ , hydrocarbon i s fed to a r e a c t o r f i l l e d with metal oxide at high temperature to produce the d e s i r e d s e l e c t i v e o x i d a t i o n products. The r e s u l t i n g reduced metal oxide i s then l i f t e d to a separate regeneration v e s s e l where a i r i s also f e d , which r e o x i d i z e s the c a t a l y s t before i t i s

REGENERATOR

OXIDATION PRODUCTS

-AIR

REACTOR FILLED WITH M E T A L OXIDE (OXIDANT)

HYDROCARBON FEED

LIFT

Figure 9. Oxidant

process.



334

HETEROGENEOUS

CATALYSIS

returned to the r e a c t o r . This process, while t h e o r e t i c a l l y p o s s i b l e , i s not p r a c t i c a l due to the l a r g e quantity of c a t a l y s t which must be c y c l e d (400 pounds per pound of u s e f u l product produced). Since n o n - s e l e c t i v e , deep o x i d a t i o n processes are even more thermodynamically f a v o r a b l e than the s e l e c t i v e ones (Table 2 ) , i t i s necessary to i n t e r c e p t the d e s i r e d products k i n e t i c a l l y . C a t a l y s t s , t h e r e f o r e , must be designed which lower the a c t i v a t i o n energy of the d e s i r e d r e a c t i o n s and thus allow the process to operate at lower temperatures than f o r n o n - c a t a l y t i c e q u i l i b r i u m l i m i t e d processes. In t h i s manner undesirable waste formation (deep oxidation) i s minimized.

Table II Thermodynamics

of Oxidation

427 C (KCAL/MOLE)

REACTIONS (A) C H 3

+ 0 —* CH =CHCHO + H 0

6

2

Reactions.

2

-80.92

2

(B) C H

6

+ 3/2 0 —^CH =CHCOOH + H 0

-131.42

(C) C H

6

+ 3 0 ^ 3 C O + 3H 0

-304.95

(D) C H

6

+ 9/2 0 ^ > 3 C 0

3

3

3

2

2

2

2

2

2

2

+ 3H 0

-463.86

2

(E) C H 3

6

+ N H + 3/2 0 —>CH =CHCN + 3 H 0

(F)

C H

6

+ 3/2 N H + 3/2 0 ^ 3 / 2 C H C N + 3 H 0

-142.31

+ 3NH + 30 —>3HCN + 6 H 0

-273.48

3

(G) C H 3

3

2

3

6

3

2

2

2

-136.09

2

3

2

2

R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 6. C o p y r i g h t 1981, Adv. C a t a l .


25.

GRASSELLi

335

Selective Oxidation

From these fundamental concepts of l a t t i c e oxygen u t i l i z a t i o n , evolved two b a s i c requirements of a heterogeneous s e l e c tive oxidative c a t a l y s t : (1) Oxygen atoms must be d i s t r i b u t e d on the s u r f a c e of a 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 t i n an arrange ment which provides f o r l i m i t a t i o n of the number of a c t i v e oxygen atoms i n various i s o l a t e d groups ( s i t e i s o l a t i o n ) ; (2) Metal-oxygen bond energy of the a c t i v e oxygen atoms at the conditions of r e a c t i o n , must be i n a range where r a p i d removal (hydrocarbon o x i d a t i o n ) and a d d i t i o n (regeneration by oxygen) i s assured ( c h a r a c t e r i s t i c , unique M-K) bond s t r e n g t h ) . The i s o l a t i o n of a c t i v e s i t e s can be accomplished by the p a r t i a l removal of l a t t i c e oxygen, i . e . r e d u c t i o n , of a metal oxide. Using s t a t i s t i c a l (Mote Carlo) methods, the d i s t r i b u t i o n of oxygen atoms and vacancies on a surface at various stages of r e d u c t i o n , represented by a surface oxide a c i d (Fig. 1 0 ) , can be c a l c u l a t e d , as w e l l as the corresponding density of oxygen c l u s t e r s c o n t a i n i n g 1-5 oxygen atoms and > 5 oxygen atoms ( F i g . I I ) . Based on the assumption that c l u s t e r s of 1-5 oxygen atoms w i l l r e s u l t i n s e l e c t i v e product, the r e l a t i v e propylene conversion to s e l e c t i v e product ( a c r o l e i n ) should be maximized at about 65% reduction ( F i g . 12) . In f a c t , the experimentally determined dependence of s e l e c t i v e product y i e l d on % reduction f o r CuO ( F i g . 13) i s very s i m i l a r to the c a l c u l a t e d curve ( F i g . 12), but with the maximum s h i f t e d to ^25% reduction. While CuO provides an e x c e l l e n t i l l u s t r a t i o n of the impor tance of a c t i v e s i t e i s o l a t i o n i n s e l e c t i v e o x i d a t i o n over heterogeneous c a t a l y s t s , i t i s not a p r a c t i c a l c a t a l y s t s i n c e the small range of p a r t i a l l y reduced s t a t e s , required f o r s e l e c t i v i t y , i s d i f f i c u l t to maintain. A metal oxide c a t a l y s t which would operate s e l e c t i v e l y i n a f u l l y or n e a r l y f u l l y o x i d i z e d s t a t e would be a much more p r a c t i c a l system. One a l t e r n a t i v e to p a r t i a l reduction f o r s e l e c t i v i t y i s a c t i v e s i t e i s o l a t i o n chemically by m o d i f i c a t i o n of s o l i d s t a t e structure. The zig-zag chain s t r u c t u r e of ^2®5 ( S * l ^ ) provides inherent s t r u c t u r a l i s o l a t i o n , which can be enhanced by the a d d i t i o n of a l k a l i metal oxide. The dependence of conversion to s e l e c t i v e product on K2O/V2O5 r a t i o i s s i m i l a r to the e f f e c t of reduction ( F i g . 15), and gives a maximum at ^0.25 K/V. B i f u n c t i o n a l systems, i n which the components i n d i v i d u a l l y have very low a c t i v i t y , are among the most e f f i c i e n t c a t a l y s t s . Among the most w e l l studied and commercially s u c c e s s f u l ^ i n t h i s c l a s s are the bismuth molybdates, Β ΐ 2 θ ο . η Μ ο Ο ο , where n~ (a), 2 (β) and 1(γ). The B±2^oJ)^2 (α-phase) has a S c h e e l i t e -derived s t r u c t u r e ( F i g . 16) i n which the Mo atoms are t e t r a h e d r a l - l i k e , and one c a t i o n vacancy i s present per 2 B i atoms. The Bi2Mo0^ (γ-phase) however, has a layered s t r u c t u r e of a l t e r n a t i n g corner-shared octahedral M0O2 l a y e r s and B12O2 l a y e r s ( F i g . 17). The mechanism of bismuth molybdate c a t a l y z e d o x i d a t i o n has been e x t e n s i v e l y s t u d i e d . In the c a t a l y t i c c y c l e , the f u n c t i o n


5

5

5

Fi



336


F i g u r e 10. R e d u c t i o n of an o x i d i z e d s u r f a c e g r i d , ( a ) 80%, (b) 60%, ( c ) 20% o x i d i z e d . ( R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 5. C o p y r i g h t 1963, Α. I . Ch. Ε. J o u r n a l . )



25.

GRASSELLi

Selective Oxidation

337

F i g u r e 11. S i t e p o p u l a t i o n as a f u n c t i o n o f s u r f a c e c o v e r a g e Oxygen r e g e n e r a t i o n o f r e d u c e d g r i d . ( R e p r o d u c e d w i t h p e r m i s s i o n from R e f . 5. C o p y r i g h t 1963, Α. I . CH. Ε. J o u r n a l . )




338

X SURFACE REDUCTION

F i g u r e 12. R e l a t i v e p r o p y l e n e c o n v e r s i o n as a f u n c t i o n o f o x i d a t i o n s t a t e - Oxygen r e g e n e r a t i o n o f r e d u c e d g r i d . ( R e p r o d u c e d w i t h p e r m i s s i o n from R e f . 5. C o p y r i g h t 1963, Α. I . Ch. Ε. J o u r n a l . )


25.

339

Selective Oxidation

GRASSELLi


1.0

\\

0.9

0.8

ο ω

0.7

s g

0.6

/

ο ο •J

ν

\

\ \

f

/

Ο

/ 1

0.1

ν 10

20

ACR01 -Ε IN +

y

/

0.4

0.2

2

ΐ \

0.5

0.3

^ Ο Χ Ι Ε )ES OF CARBOtI + H C)

30

40

D

\ 50

Κ 60

70

80

90

100

% REDUCTION

F i g u r e 13. E x p e r i m e n t a l p r o p y l e n e o x i d a t i o n a c t i v i t y v s . c a t a l y s t o x i d a t i o n s t a t e - Copper o x i d e c a t a l y s t , 300 °C r e a c t i o n t e m p e r a t u r e . ( R e p r o d u c e d w i t h p e r m i s s i o n from Réf.. 5. C o p y r i g h t 1963, Α. I . Ch. Ε. J o u r n a l . )



340


F i g u r e 14. C r y s t a l s t r u c t u r e o f V^O^ - s i d e v i e w . w i t h p e r m i s s i o n f r o m R e f . 9. C o p y r i g h t 1961, Z e i t

(Reproduced Kristall.)


GRASSELLi

Selective Oxidation


25.


341

342

CATALYSIS


HETEROGENEOUS

M o 0 Oyer 2

Bi 0 2

2

loyer

corner shored

F i g u r e 17. L a y e r e d s t r u c t u r e o f Bi^MoO^. ( R e p r o d u c e d p e r m i s s i o n from R e f . 11.)

with



25.

GRASSELLi

Selective Oxidation

343

of the oxygens associated with B i i s to perform the r a t e determining α-hydrogen a b s t r a c t i o n step, while those i n Mo polyhedra are s i t e s f o r o l e f i n chemisorption and 0 - i n s e r t i o n (F.18). The a c t i v e and s e l e c t i v e s i t e i s composed of Bi:Mo p a i r s , repre sented by s t r u c t u r e 1 ( F i g . 19) i n which the α-hydrogen a b s t r a c t i n g bismuthyl (Bi=0) i s bonded through a b r i d g i n g oxygen to the o l e f i n chemisorption/oxygen-insertion Mo-dioxo f u n c t i o n ality. I n i t i a l chemisorption, and a-H a b s t r a c t i o n produces Ο-π-allyl 2 and subsequently Ο-σ-complex 3, and then a c r o l e i n a f t e r a 2nd H-abstraction. Formation of the analogous N-bonded species (4 and 5) followed by a 3rd Η-abstraction produces a c r y l o n i t r i l e when ammonia i s present, a f t e r i n i t i a l NH^ a c t i v a t i o n by formation of a Mo-di imido species 6. Ammoxidation of Propylene

in Practice

In a t y p i c a l bench s c a l e experiment a f i r s t generation BigPMo-^2^52 c a t a l y s t produces 65.2% per pass propylene conver s i o n to a c r y l o n i t r i l e with 4.1% HCN, 4.0% a c e t o n i t r i l e , 0.1% a c r o l e i n , and 16.8% C0 (Table 3). Y i e l d s of u s e f u l products have been g r e a t l y improved with newer generation c a t a l y s t systems. Advancement of new c a t a l y s t systems progresses i n s e v e r a l stages, eventually r e s u l t i n g i n a 30 m i l l i o n - f o l d scale-up from a 5-gram l a b o r a t o r y microreactor to a 26 f t . d i a meter commercial r e a c t o r (Table 4). 2

Future Trends The trend i n chemical feedstocks i s towards l e s s expensive, more a v a i l a b l e ones and away from the expensive, more r e a c t i v e feeds. For example, the extensive use of acetylene as a feed stock i n the 1930-1940 s has been replaced i n the 1960-1980 s by o l e f i n s and d i o l e f i n s . The f u t u r e trend appears to be towards p a r a f f i n s and synthesis gas (Figure 20). Continued developments i n fundamental c a t a l y s t science w i l l serve as the b a s i s f o r the design of the c a t a l y t i c s i n g l e - s t e p processes of the f u t u r e which w i l l e f f i c i e n t l y u t i l i z e inexpensive, r e a d i l y a v a i l a b l e feeds. f

f

Acknowledgments I should l i k e to acknowledge my e a r l y mentor, Dr. F r a n k l i n Veatch, my long time f r i e n d and c o l l a b o r a t o r , Dr. James L. Callahan, and my many co-workers, i n p a r t i c u l a r , Dr. Dev D. Suresh, Dr. James D. Burrington, and Dr. James F. B r a z d i l .



344


M

1

= a-H ABSTRACTION

M

2

= OLEFIN

CHEMISORPTION/O-INSERTION

F i g u r e 18. A l l y l i c o x i d a t i o n mechanism. ( R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 6. C o p y r i g h t 1981, Adv. C a t a l . )


GRASSELLi

Selective Oxidation

345


25.

F i g u r e 19. Mechanism f o r s e l e c t i v e o x i d a t i o n and a m m o x i d a t i o n of p r o p y l e n e . ( R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 12. C o p y r i g h t 1980, J . C a t a l . )



2

2

3

12

CATALYST)

95.69 95.6 8.2

65.2 4.0 0.09 4.1 5.5 16.8

2.2

0.255 3.355 0.282 0.0253

566 G 470° C ATMOSPHERIC 9.1 S E C .

R e p r o d u c e d w i t h p e r m i s s i o n from R e f . 5. C o p y r i g h t 1963, Α. I . Ch. Ε. J o u r n a l .

9

(50% B i P M o 0 5 2 - 50% S I 0

2

(C-ATOM BASIS) TO:

TOTAL % CARBON BALANCE % UNREACTED N H

MOLE % EXCESS 0 % PER PASS CONVERSION ACRYLONITRILE ACETONITRILE ACROLEIN HCN CO C0

RESULTS

3

REACTORS 1-1/2 INCH PIPE FLUID BED CATALYST CHARGE TEMPERATURE PRESSURE, PSIG C O N T A C T TIME MOLES FEED/HOUR PROPYLENE AIR NH PROPYLENE WT. HOURLY SPACE VELOCITY

CONDITIONS

T a b l e I I I B e n c h - S c a l e A c r y l o n i t r i l e Run


25.

GRASSELLi

347

Selective Oxidation


T a b l e I V S c a l e up o f C a t a l y s t s .

MICROREACTOR A

1.5 GMS

(0.003 LBS)

MICROREACTOR Β

5.0 GMS

(0.01 LBS)

1-1/2 INCH REACTOR

550 GMS

(1.2 LBS)

3 INCH REACTOR

2,000 GMS

(4.4 LBS)

18 INCH REACTOR

249,700 GMS

(550 LBS)

24 INCH REACTOR

681,000 GMS

(1,500 LBS)

26 FT. COMMERCIAL REACTOR

136,200,000 GMS

(300,000 LBS)

PARAFFINS $ SYNTHESIS GAS

(CO+H2)

1950

F i g u r e 20. T r e n d s i n c h e m i c a l f e e d s t o c k . ( R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 6. C o p y r i g h t 1981. Adv. C a t a l . )

American Chemical Society Library 1155 16th St. N. W. In Heterogeneous Catalysis; B., et al.; Washington, 0. C.Davis, 20096 ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


348

Literature Cited 1. 2. 3.


4. 5. 6. 7. 8. 9. 10. 11. 12.

Chem. and Eng. News, June 8, 1981. Hydrocarbon Processing, October 1979, 59, 123. Weissermel, Κ., and Arpe, H . J . , "Industrial Organic Chemistry," trans. Mullen, Α., Verlag Chemie, New York, 1978. Synthetic Organic Chemicals, U.S. Production and Sales, U.S. Fundamental Trade Commission; Chemical Marketing Reputes. Callahan, J.L., and Grasselli, R.K., Α.I.Ch. Ε. Journal 1963 9, 755 Grasselli, R.K., and Burrington, J.D., Adv. Catal. 1981, 30, 133. Grasselli, R.K., Burrington, J.D., and Brazdil, J.F., J. Chem. Soc. Faraday Soc. Disc., 1981, 72, 203. Callahan, J.L., and Grasselli, R.K., Milberger, E . C . , and Strecker, H.A., I & E.C. Prod. Res. Dev., 1970, 9, 134 Bachman, H.G., et.al., Zeit. Kristall, 1961, 115, 110. Jeitschko, W., Acta. Cryst. 1973, B29, 2074. Gates, B.C., Katzer, J . A . , and Schuit, G.C.A., "Chemistry of Catalytic Processes," McGraw-Hill, New York, 1979. Burrington, J . D., Kartisek, C. T., and Grasselli, R. Κ., J. Catal., 63, 235 (1980) (Figure 19).

RECEIVED

November 29, 1982


Heterogeneous Catalysis - American Chemical Society

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