Catalysis by Transition Metal Oxides - American Chemical Society

1 Catalysis by Transition Metal Oxides JERZY HABER

Downloaded by MEMORIAL UNIV OF NEWFOUNDLAND on August 3, 2014 | http://pubs.acs.org Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch001

Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul.Niezapominajek, 30-239 Krakow, Poland

Catalytic oxidation reactions are divided into two groups: electrophilic oxidation proceeding through activation of oxygen and nucleophilic oxidation in which activation of the hydrocarbon molecule is the first step, followed by consecutive hydrogen abstraction and nucleophilic oxygen insertion. Properties of individual cations and their coordination polyhedra determine their behaviour as active centers responsible for activation of hydrocarbon molecules. A facile route for nucleophilic insertion of oxygen into such molecules by group V, VI and VII transition metal oxides is provided by the crystallographic shear mechanism, catalytic properties are thus dependent upon the geometry of the surface. The catalyst surface is in dynamic interaction with the gas phase, and changes of the latter may thus result in surface transformations and appearance of surface phases, which influence the selectivity of catalytic reactions. The vast majority of catalysts used in modern chemical industry are oxides. Because of their a b i l i t y to take part in the exchange of electrons, as well as i n the exchange of protons or oxide ions, oxides are used as catalysts i n both redox and acid-base reactions. They constitute the active phase not only i n oxide catalysts but also i n the case of many metal c a t a l y s t s , which in the conditions of c a t a l y t i c reaction are covered by a surface layer of a reactive oxide. Properties of oxides are also important in the case of preparation of many metal and sulphide c a t a l y s t s , which are obtained from an oxide precursor. Very often, highly dispersed metals are prepared by reduction of an appropriate oxide phase, and sulphide catalysts are formed from the oxide precursor in the course of the hydrodesulphurization by i n t e r a c t i o n with the reaction medium. Finally, oxides play an important role in c a r r i e r s for active metal or oxide phases, very often modifying strongly t h e i r c a t a l y t i c properties. The present paper concerns

0097-6156/85/0279-0003$06.00/0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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S O L I D STATE C H E M I S T R Y IN CATALYSIS

o n l y one a s p e c t o f the v a s t field of c h e m i s t r y o f o x i d e s , namely the c a t a l y s i s by t r a n s i t i o n m e t a l o x i d e s , w h i c h i s the b a s i s o f the s e l e c t i v e o x i d a t i o n of hydrocarbons. C a t a l y t i c o x i d a t i o n i s one of the most i m p o r t a n t types o f p r o c e s s e s , b o t h from a t h e o r e t i c a l and a p r a c t i c a l p o i n t o f v i e w . As e a r l y as 1918, the p r o d u c t i o n o f p h t h a l i c a n h y d r i d e by o x i d a t i o n o f n a p h t h a l e n e over ^2 5 introduced. The m i l e s t o n e i n the development o f modern p e t r o c h e m i c a l i n d u s t r y was the i n t r o d u c t i o n of the gas phase oxidation of propylene to a c r o l e i n and ammoxidation to a c r y l o n i t r i l e over b i s m u t h molybdate c a t a l y s t s , which p r o v i d e d i n the early s i x t i e s ' , an abundant s u p p l y o f new, inexpensive, and useful chemical intermediates(J_). Today, c a t a l y t i c o x i d a t i o n i s the basis o f the p r o d u c t i o n o f almost a l l monomers used i n the m a n u f a c t u r i n g of synthetic f i b e r s , p l a s t i c s , and many o t h e r p r o d u c t s . W i t h the i n c r e a s i n g c o s t o f energy and s h r i n k i n g s u p p l y o f cheap h y d r o c a r b o n s , much e f f o r t i s now b e i n g expended on the development o f new o x i d a t i o n p r o c e s s e s o f h i g h e r s e l e c t i v i t y and l o w e r energy consumption. S u b s t i t u t i o n o f the d e h y d r o g e n a t i o n by o x i d a t i v e p r o c e s s e s , as i n the p r o d u c t i o n o f s t y r e n e from e t h y l b e n z e n e , may be quoted as an example. Another i n c r e a s i n g l y important field of c a t a l y t i c application is the s e l e c t i v e o x i d a t i o n of p a r a f f i n s .


Q

w

a

s

Discussion E l e c t r o p h i l i c and n u c l e o p h i l i c o x i d a t i o n I n every o x i d a t i o n r e a c t i o n two r e a c t a n t s always t a k e p a r t : oxygen and the m o l e c u l e t o be o x i d i z e d . The r e a c t i o n may thus s t a r t e i t h e r by the a c t i v a t i o n o f the d i o x y g e n o r by the a c t i v a t i o n o f the h y d r o c a r b o n m o l e c u l e . At ambient o r moderate t e m p e r a t u r e s , an oxygen m o l e c u l e may be a c t i v a t e d by bonding i n t o an o r g a n o m e t a l l i c complex i n the l i q u i d phase. Depending on the type o f the c e n t r a l m e t a l atom and on the p r o p e r t i e s o f the l i g a n d s , s u p e r o x o - , p e r o x o - o r oxo-complexes may be formed: +M

+0o ->

M0

0

-> M0 M

> 2M0

O

I

ο 0 M

perox

superoxo

M

/

V

> MOM

Μ

μ-peroxo

Μ = 0

oxo

M/

\M

A*-oxo

I n the case o f the s u p e r o x o - c o m p i e x e s , an e l e c t r o p h i l i c a t t a c k o f a t e r m i n a l oxygen atom on the o r g a n i c r e a c t a n t o c c u r s r e s u l t i n g i n the f o r m a t i o n o f a ]U-peroxo-com p i e x , w h i c h decomposes i n t o the oxygenated p r o d u c t ( l o w e r l e f t p a r t o f F i g u r e 1 ) . I n the case o f perox complexes o f group I V , V and V I t r a n s i t i o n m e t a l s , a s t o i c h i o m e t r i c o x i d a t i o n takes p l a c e i f a v a c a n t c o o r d i n a t i o n s i t e e x i s t s a d j a c e n t to s i d e bonded oxygen and i s c a p a b l e o f b e i n g o c c u p i e d by the o r g a n i c r e a c t a n t . Its olefin bond i s then i n s e r t e d i n t o the



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Catalysis by Transition Metal Oxides

F i g u r e 1. Mechanism of the catalytic oxidation c a r b o n s . Reproduced with permission from Ref. 31.

of

hydro-



6


m e t a l - o x y g e n bond forming a p e r o x o m e t a l l o c y c l e w h i c h i s then decomposed i n t o the oxygenated h y d r o c a r b o n m o l e c u l e and oxo m e t a l complex C2)· In o r d e r to t r a n s f o r m the l a t t e r back i n t o the reduced m e t a l w h i c h c o u l d a g a i n form the peroxocomplexes w i t h a new oxygen m o l e c u l e , a c o r e d u c i n g agent i s r e q u i r e d , w h i c h may be a hydrogen donor o r a r e a c t a n t i t s e l f . It s h o u l d be b o r n i n mind that i n l i q u i d phase o x i d a t i o n r e a c t i o n s , the o r i g i n a l oxygen complex may be transformed i n t o o t h e r r e a c t i v e s p e c i e s which p l a y the r o l e o f a c t i v e intermediates. A superoxo-complex may be transformed i n t o an a l k y l p e r o x i d e or p e r a c i d complex, w h i c h i s the oxygen i n s e r t i n g i n t e r m e d i a t e ( 3 - 5 ) . _ At h i g h e r temperatures the p e r o x i d e 0^ and s u p e r o x i d e 0^ s p e c i e s may appear at the surface o f an o x i d e . Under these c o n d i t i o n s , the p e r o x i d e i o n i s _ u n s t a b l e and d i s s o c i a t e s forming the ion radical 0 . Both 0^ and 0 species are strongly e l e c t r o p h i l i c r e a c t a n t s w h i c h a t t a c k the o r g a n i c m o l e c u l e i n the region of i t s highest electron density. At variance w i t h t h e i r b e h a v i o u r i n the liquid phase, the p e r o x y - and epoxy-compiexes formed as the r e s u l t o f an e l e c t r o p h i l i c a t t a c k o f 0 ^οτ 0 species on the o l e f i n m o l e c u l e are intermediates w h i c h l e a d to the d e g r a d a t i o n o f the carbon s k e l e t o n under heterogeneous c a t a l y t i c r e a c t i o n c o n d i t i o n s (6^). Saturated aldehydes a r e formed i n the f i r s t stage (upper l e f t p a r t o f F i g u r e 1 ) . These are u s u a l l y much more r e a c t i v e than u n s a t u r a t e d aldehydes and at h i g h e r temperature undergo rapidly total oxidation. Indeed, experimental data c o l l e c t e d i n r e c e n t y e a r s c l e a r l y show t h a t e l e c t r o p h i l i c oxygen species i n heterogeneous p r o c e s s e s are responsible for total oxidation (7). When hydrocarbon m o l e c u l e s are activated, a v a r i e t y of reaction paths may be i n i t i a t e d , c o n s i s t i n g o f a s e r i e s of c o n s e c u t i v e o x i d a t i v e s t e p s , each o f them r e q u i r i n g a d i f f e r e n t a c t i v e c e n t e r t o be p r e s e n t at the c a t a l y s t s u r f a c e ( 8 - 1 0 ) . It s h o u l d be emphasized at t h i s point t h a t i t i s the c a t i o n s o f the c a t a l y s t w h i c h a c t as o x i d i z i n g agents i n some o f the c o n s e c u t i v e s t e p s o f the r e a c t i o n sequence, forming the a c t i v a t e d hydrocarbon species. These undergo subsequent s t e p s a n u c l e o p h i l i c a t t a c k by l a t t i c e oxygen i o n s 0 , w h i c h are n u c l e o p h i l i c r e a g e n t s w i t h no oxidizing properties. They are inserted i n t o the activated hydrocarbon m o l e c u l e by n u c l e o p h i l i c a d d i t i o n forming an oxygenated product, which a f t e r desorption leaves an oxygen vacancy a t the s u r f a c e o f the catalyst. Such v a c a n c i e s are then f i l l e d w i t h oxygen from the gas phase, s i m u l t a n e o u s l y r e o x i d i z i n g the reduced cations. I t s h o u l d be noted t h a t i n c o r p o r a t i o n o f oxygen from the gas phase i n t o the o x i d e s u r f a c e does not n e c e s s a r i l y take p l a c e a t the same s i t e from where s u r f a c e oxygen i s i n s e r t e d i n t o the h y d r o c a r b o n m o l e c u l e a f t e r b e i n g t r a n s p o r t e d t h r o u g h the l a t t i c e . In the case o f complex hydrocarbon m o l e c u l e s , the n u c l e o p h i l i c a d d i t i o n o f oxygen may take p l a c e a t d i f f e r e n t s i t e s o f the molecule. It will take p l a c e at a site w h i c h i s made most e l e c t r o p o s i t i v e by a p p r o p r i a t e bonding o f the m o l e c u l e at the a c t i v e c e n t e r o f the c a t a l y s t . When a d s o r p t i o n o f the hydrocarbon m o l e c u l e r e s u l t s i n the f o r m a t i o n o f a r a d i c a l , i n t e r a c t i o n between adsorbed m o l e c u l e s i s favoured and d i m e r i z a t i o n o r p o l y m e r i z a t i o n occurs. When the adsorbed species are negatively charged, 2



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1


i s o m e r i z a t i o n may be f a v o u r e d . T h i s type o f product o b t a i n e d depends on the type and p r o p o r t i o n o f d i f f e r e n t a c t i v e c e n t e r s at the c a t a l y s t s u r f a c e as w e l l as on the ratio o f the r a t e o f d e s o r p t i o n o f the p a r t i c u l a r intermediate product to the r a t e o f i t s t r a n s f o r m a t i o n i n t o the i n t e r m e d i a t e complex next i n the s e r i e s (upper r i g h t p a r t o f scheme i n F i g u r e 1 ) . These r a t e s may s t r o n g l y depend on the degree o f s u r f a c e r e d u c t i o n a t t a i n e d i n the course o f the r e a c t i o n , as i s the case w i t h the c a r b o x y l a t e complex, w h i c h i s an i n t e r m e d i a t e i n the o x i d a t i o n o f aldehydes t o c a r b o x y l i c a c i d s . On o x i d i z e d s u r f a c e s , t h i s complex d e s o r b s i n the form o f an a c i d , whereas on a reduced surface it undergoes decarboxylation, r e s u l t i n g i n the d e p o s i t i o n o f coke ( 1 1 ) . R e a c t i o n s o f c a t a l y t i c o x i d a t i o n may be thus d i v i d e d i n t o two g r o u p s : e l e c t r o p h i l i c o x i d a t i o n , p r o c e e d i n g t h r o u g h the a c t i v a t i o n of o x y g e n , and n u c l e o p h i l i c o x i d a t i o n , i n w h i c h a c t i v a t i o n o f the hydrocarbon molecule i s the first step, f o l l o w e d by c o n s e c u t i v e s t e p s o f n u c l e o p h i l i c oxygen i n s e r t i o n and hydrogen a b s t r a c t i o n . They may be c o n v e n i e n t l y s y s t e m a t i z e d according to the number o f elementary s t r u c t u r a l transformations i n t r o d u c e d i n t o the r e a c t i n g molecule (Table I ) . An a c t i v e and s e l e c t i v e c a t a l y s t f o r o x i d a t i o n o f hydrocarbons t o oxygenated products with retention o f double bonds or a r o m a t i c i t y s h o u l d thus have the f o l l o w i n g p r o p e r t i e s : - a c t i v a t i o n o f the h y d r o c a r b o n m o l e c u l e by m o d i f y i n g i t s bonds and generating at appropriate sites the electron distribution f a v o u r i n g the n u c l e o p h i l i c a t t a c k o f oxygen; - e f f i c i e n t i n s e r t i o n of the n u c l e o p h i l i c l a t t i c e oxygen i n t o the a c t i v a t e d hydrocarbon molecule; - r a p i d i n t e r a c t i o n w i t h gas phase oxygen to r e p l e n i s h the l a t t i c e oxygen and t r a n s p o r t it through the lattice to a c t i v e s i t e s , where the i n s e r t i o n t a k e s p l a c e ; - s h o u l d not g e n e r a t e e l e c t r o p h i l i c oxygen s p e c i e s . The fundamental q u e s t i o n a r i s e s as to how these p r o p e r t i e s a r e r e l a t e d t o the s o l i d s t a t e c h e m i s t r y o f o x i d e s . A c t i v a t i o n of organic molecule C l a s s i c a l s t u d i e s o f Adams (\2_, JL3) , u s i n g d e u t e r a t e d p r o p y l e n e and C^-Cg o l e f i n s , and of S a c h t l e r and de Boer (14) , w i t h C - l a b e l l e d p r o p y l e n e s , showed t h a t a c t i v a t i o n o f the o l e f i n m o l e c u l e c o n s i s t s o f the a b s t r a c t i o n o f α - h y d r o g e n and the f o r m a t i o n o f a symmetric a l l y l i c intermediate. Conclusions concerning the r o l e o f the c a t i o n i c and a n i o n i c s u b l a t t i c e s o f complex o x i d e c a t a l y s t s h a v i n g an o x y s a l t c h a r a c t e r , such as m o l y b d a t e s , t u n g s t a t e s , e t c . , i n the i n i t i a l α-hydrogen abstraction and the subsequent s t e p s o f the o x i d a t i o n p r o c e s s , were drawn by comparing the b e h a v i o u r o f o^3 and MoO^ f o r the r e a c t i o n of propylene and a l l y l i o d i d e ( 1 5 ) . When a l l y l i o d i d e was passed over Mo0~, p r a c t i c a l l y t o t a l c o n v e r s i o n was observed a l r e a d y at 310°C w i t h 98% s e l e c t i v i t y t o a c r o l e i n . Under the same c o n d i t i o n s , MoO^ was c o m p l e t e l y i n a c t i v e w i t h r e s p e c t t o propylene. On c o n t a c t i n g a l l y l i o d i d e w i t h B i 0 « , t o t a l c o n v e r s i o n a t 310°C was a l s o observed. However, i n t h i s case 70% of the products formed were 1,5-hexadiene w i t h p r a c t i c a l l y no a c r o l e i n being detected. 1,5-hexadiene was also the main product B i

9



2

2

1. With double bond f i s s i o n 1.1. oxidation of olefins to oxides 1.2. oxyhydratian of olefins to saturated ketones 2. With C-C bond f i s s i o n 2.r. oxidation of olefins to saturated aldehydes 2.2. oxidation of aromatics to anhydrides and acids with ring rupture 3 · Total oxidation to C0 +H 0

E l e c t r o p h i l i c Oxidation Reaction Type

2

5

3

4

2

Co 0 CuCo^ CuCr 0^

2°5 V 0 -Mo0

V

2

3

Sn0 -MoO^

Catalyst 1· Without introduction of heteroatom 1.1. oxidative dehydrogenation of alkanes and alkenes to dienes 1.2. oxidative dehydrodimerization and dehydrocyclization of alkenes 2. With introduction of heteroatom 2.1. introduction of heteroatom into hydrocarbon chain 2.1.1. introduction of oxygen a. oxidation of olefins to unsaturated aldehydes and ketones b. oxidation of alkylaromat i c s to aldehydes 2.1.2. introduction of nitrogen a. ammoxidation of olefins to n i t r i l e s 2.2. introduction of heteroatom into acyl group a. oxidation of~aldehydes to acids b. oxidation of alkylaroma t i c s to anhydrides

Nucieophilic Oxidation Reaction Type

Table I. Heterogeneous Oxidation of Hydrocarbons


2°5

2

3

2

3

3

2

2

5

v o -Tio

3

Ni0-Mo0

3

2

4

3

uo -sb o

2

Bi 0 -Mo0

2

3

3

Bi 0 -Mo0

3

Mo0 -Al 0

P

3

Bi 0 -Mo0 -

Catalyst

5

%

η

H

π χ m g

5

Ο r

00

1.

HABER


i n the r e a c t i o n o f p r o p y l e n e over B i 0 . These r e s u l t s c l e a r l y i n d i c a t e d t h a t a c t i v a t i o n o f the o l e f i n m o l e c u l e , w h i c h c o n s i s t s o f the a b s t r a c t i o n o f α - h y d r o g e n and the f o r m a t i o n o f an a l l y l i c s p e c i e , t a k e s p l a c e on c a t i o n i c a c t i v e c e n t e r s , whereas the MoO^ o r molybdate a n i o n i c s u b l a t t i c e i s responsible f o r the i n s e r t i o n o f oxygen i n t o the h y d r o c a r b o n m o l e c u l e . Indeed, quantum c h e m i c a l c a l c u l a t i o n s o f the s y s t e m , composed of c o b a l t i o n i n o c t a h e d r a l coordination of f i v e oxygen atoms and p r o p y l e n e m o l e c u l e as the s i x t h l i g a n d , have shown (J^o, 17) t h a t on approaching the p r o p y l e n e m o l e c u l e to the p l a n e o f the a c t i v e c e n t e r , the C-H bond i s continuously destabilized with an interaction appearing s i m u l t a n e o u s l y between t h i s hydrogen and the oxygen o f the a c t i v e c e n t e r and i n c r e a s i n g u n t i l the t o t a l energy a t t a i n e d a minimum and an i n t e r m e d i a t e complex i s formed. When the a l l y l specie i s removed, the energy r e q u i r e d f o r t h i s p r o c e s s i s much s m a l l e r than t h a t needed to remove the whole p r o p y l e n e m o l e c u l e . The OH bond i s further strengthened, and its energy attains the value c h a r a c t e r i s t i c o f a normal the OH g r o u p . Thus, i t may be c o n c l u d e d t h a t on c o n t a c t i n g p r o p y l e n e w i t h the s u r f a c e o f the t r a n s i t i t i o n m e t a l o x i d e , r e a c t i v e c h e m i s o r p t i o n takes p l a c e , i n w h i c h the C - H bond i s broken and an a b s o r p t i o n complex w i t h a l l y l s p e c i e s as one o f the l i g a n d s i s formed. C o n s i d e r a b l e charge t r a n s f e r takes p l a c e from the allyl species onto the t r a n s i t i o n metal orbitale, r e n d e r i n g the s p e c i e s p o s i t i v e , the charge d i s t r i b u t i o n depending on the type o f m e t a l , i t s v a l e n c e s t a t e and the l i g a n d f i e l d strength (18). E x p e r i m e n t s , w i t h azopropene and a l l y l a l c o h o l , c a r r i e d out by G r a s s e l l i e t a l . (19-22) , demonstrated t h a t a f t e r the f i r s t hydrogen a b s t r a c t i o n , i n s e r t i o n o f oxygen t a k e s p l a c e , and o n l y then the second hydrogen i s a b s t r a c t e d r e s u l t i n g i n the f o r m a t i o n o f the a c r o l e i n p r e c u r s o r . An i m p o r t a n t q u e s t i o n may be r a i s e d a t t h i s p o i n t as to what i s the s t r u c t u r e o f the c a t i o n i c a c t i v e c e n t e r a c t i v a t i n g the h y d r o c a r b o n m o l e c u l e . Can e v e r y c a t i o n s i t u a t e d a t the s u r f a c e o f the g i v e n o x i d e perform the r o l e o f the a c t i v e c e n t e r , o r must t h i s c a t i o n be l o c a l i z e d a t some s p e c i a l s i t e o f the s u r f a c e , and how does i t s a c t i v i t y depends on t h i s l o c a t i o n ? I n o r d e r to o b t a i n some r e l e v a n t i n f o r m a t i o n about t h i s q u e s t i o n , i s o l a t e d bismuth i o n s were supported at the surface o f Mo0~ ( 2 3 ) . Taking i n t o account the v e r y h i g h e f f i c i e n c y o f MoO^ f o r i n s e r t i n g oxygen i n t o a c t i v a t e d h y d r o c a r b o n m o l e c u l e s , i t might be assumed t h a t e v e r y p r o p y l e n e m o l e c u l e a c t i v a t e d a t the i s o l a t e d b i s m u t h i o n would p i c k up oxygen and be c o n v e r t e d to a c r o l e i n . The number o f a c r o l e i n m o l e c u l e s would thus be a measure o f the number o f p r o p y l e n e molecules activated by the known number o f b i s m u t h ions. Measurements o f the 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 as a f u n c t i o n o f the s u r f a c e c o n c e n t r a t i o n o f b i s m u t h i o n s , expressed as t h e i r number per s u r f a c e molybdenum atom, a r e shown i n F i g u r e 2 . When a l l y l i o d i d e was i n t r o d u c e d , the y i e l d o f a c r o l e i n was c o n s t a n t and independent o f the b i s m u t h coverage c o n f i r m i n g the assumption t h a t once a l l y l r a d i c a l s have been g e n e r a t e d they r a p i d l y undergo a n u c l e o p h i l i c a t t a c k by o x i d e i o n s from the l a t t i c e o f MoO^. On i n t r o d u c i n g a m i x t u r e on propylene and oxygen, the a c t i v i t y a t l o w b i s m u t h s u r f a c e coverages increased p r o p o r t i o n a l l y to the s u r f a c e c o n c e n t r a t i o n o f b i s m u t h , the turnover frequency per b i s m u t h i o n 2


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b e i n g thus c o n s t a n t . At h i g h e r b i s m u t h c o v e r a g e s , the a c t i v i t y l e v e l e d o f f because b i s m u t h i o n s became l o c a t e d too c l o s e to each o t h e r to o p e r a t e s i m u l t a n e o u s l y i n the r e a c t i o n . I t i s noteworthy t h a t the y i e l d o f a c r o l e i n observed at the p l a t e a u i s s i m i l a r to t h a t observed i n the case o f the Bi^CMoO^)^ phase. This c l e a r l y demonstrates t h a t the a b i l i t y t o a c t i v a t e the h y d r o c a r b o n m o l e c u l e i s r e l a t e d t o the i n d i v i d u a l bismuth c a t i o n s and t h e i r n e a r e s t neighbors. These a c t i v e c e n t e r s f u n c t i o n independent o f whether they a r e d i s t r i b u t e d randomly as a monolayer at the s u r f a c e of MoO^ o r form the s u r f a c e o f the b i s m u t h molybdate phase w i t h l o n g range order. I t s h o u l d a l s o be noted t h a t the amount o f C0« formed remains c o n s t a n t i n d i c a t i n g t h a t the s i d e r e a c t i o n or total o x i d a t i o n proceeds at some other sites, r e s u l t i n g from the p r o p e r t i e s o f MoO^ i t s e l f . I n s e r t i o n o f Oxygen As a l r e a d y m e n t i o n e d , e x p e r i m e n t s i n w h i c h a l l y l compounds were r e a c t e d w i t h complex o x i d e s , such as molybdatess or t u n g s t a t e s , showed t h a t i t i s MoO^, WO^, o r the corresponding anionic sublattices w h i c h perform the i n s e r t i o n o f oxygen i n t o the h y d r o c a r b o n m o l e c u l e . The q u e s t i o n may t h u s be r a i s e d as to w h i c h p r o p e r t i e s of these oxides are r e s p o n s i b l e f o r the v e r y h i g h a c t i v i t y and s e l e c t i v i t y i n the i n s e r t i o n o f oxygen. One o f the f e a t u r e s common to a l l group V , V I , and V I I t r a n s i t i o n m e t a l o x i d e l a t t i c e s , known t o be good c a t a l y s t s for s e l e c t i v e oxidation of hydrocarbons, i s t h e i r a b i l i t y t o form shear s t r u c t u r e s w h i c h relates to the facile p l a n a r rearrangement of coordination p o l y h e d r a and t h e i r p a r t i c u l a r s p a c i a l arrangement. I n the o c t a h e d r a l c o o r d i n a t i o n o f o x i d e i o n s , 2 3 i n w h i c h d sp h y b r i d i z e d o r b i t a l s a r e used by the m e t a l to form σ - b o n d s , the r e m a i n i n g d ,d and d o r b i t a l s o f group V , V I , and χζ yζ V I I m e t a l s extend f a r enough t o c o n s i d e r a b l y o v e r l a p w i t h π ρ o r b i t a l s o f o x y g e n , and the p o s i t i o n o f t h e i r redox p o t e n t i a l r e l a t i v e to the a n i o n v a l e n c e band edge are f a v o u r a b l e f o r the bond f o r m a t i o n ( 2 4 ) . As a r e s u l t , π - b o n d s w i t h oxygen i o n s a r e formed and the c a t i o n s become d i s p l a c e d from the c e n t e r s o f o c t a h e d r a towards t e r m i n a l oxygen atoms. Large displacement p o l a r i z a b i l i t i e s g i v e r i s e to h i g h r e l a x a t i o n e n e r g y , w h i c h d e c r e a s e s the c a t i o n c a t i o n r e p u l s i o n s opposing the formation of a structure with shorter metal-metal distance (25). Thus, removal of oxygen i o n s from the l a t t i c e o f t h e s e o x i d e s r e s u l t s i n the f o r m a t i o n o f ordered a r r a y s o f oxygen v a c a n c i e s , f o l l o w e d by a v e r y f a c i l e rearrangement o f the l a y e r s o f i n i t i a l l y c o r n e r - l i n k e d m e t a l oxygen o c t a h e d r a i n t o an arrangement of edge-linked octahedra, r e s u l t i n g i n the f o r m a t i o n o f a shear plane ( F i g u r e 3 ) . A h y p o t h e s i s was advanced (26) t h a t the easy e v o l u t i o n o f one oxygen i o n on the t r a n s f o r m a t i o n from c o r n e r - l i n k e d to e d g e - l i n k e d arrangement o f m e t a l - o x y g e n o c t a h e d r a may be one o f the f a c t o r s c r e a t i n g the a b i l i t y of these s t r u c t u r e s to i n s e r t oxygen i n t o the o r g a n i c molecule i n processes of s e l e c t i v e o x i d a t i o n of hydrocarbons. S t u d i e s o f a l l y l i o d i d e a c t i v i t y on d i f f e r e n t t u n g s t e n o x i d e s seem to confirm t h i s hypothesis (27). The experiments were c a r r i e d out w i t h two groups of tungsten oxides: those i n w h i c h shear


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s t r u c t u r e s a r e formed on r e d u c t i o n (WO^, ^ 2 0 ° 5 9 ^ those w h i c h do not show t h i s phenomenon (W. o 0 , , W C L ) , as may be seen from t h e i r structures i l l u s t r a t e d i n Figure 4. The s e l e c t i v i t y o f the a l l y l i o d i d e r e a c t i o n to a c r o l e i n as a f u n c t i o n o f the number o f p u l s e s i n t r o d u c e d i n t o the r e a c t o r are seen i n F i g u r e 5 . For comparison, r e s u l t s o b t a i n e d w i t h MoO^ a r e a l s o shown. I n the case o f 20^59 w h i c h , s i m i l a r l y t o MoO^, i s a b l e to g e n e r a t e shear p l a n e s on i n t e r a c t i o n w i t h the r e d u c i n g atmosphere, a c r o l e i n appears i n the p r o d u c t s a f t e r the f i r s t few p u l s e s . On the o t h e r hand, i n the case o f samples o f W . 0 ^ and WO-, w h i c h a r e u n a b l e to form shear s t r u c t u r e s , p r a c t i c a l l y no a c r o l e i n was p r e s e n t i n the p r o d u c t s o f the r e a c t i o n . L e t us now r e t u r n to t h g s u r f a c e o f a molybdate c a t a l y s t . Due to the d i s p l a c e m e n t o f Mo i o n s towards t e r m i n a l oxygens the b r i d g i n g oxygens become more b a s i c and thus more r e a c t i v e i n the nucleophilic attack on the a c t i v a t e d h y d r o c a r b o n m o l e c u l e . On r a i s i n g the temperature, the rearrangement o f the c o r n e r - l i n k e d m e t a l - o x y g e n p o l y h e d r a i n t o edge l i n k e d a r r a y s proceeds more and more r e a d i l y , and p r o v i d e s a f a c i l e and e f f i c i e n t r o u t e f o r the a d d i t i o n on a n u c l e o p h i l i c l a t t i c e oxygen to the h y d r o c a r b o n m o l e c u l e w i t h o u t the g e n e r a t i o n o f p o i n t d e f e c t s , w h i c h c o u l d be i n v o l v e d i n the f o r m a t i o n o f e l e c t r o p h i l i c oxygen s p e c i e s . Strong e v i d e n c e s u p p o r t i n g t h i s model i s p r o v i d e d by the ESR s t u d i e s of Mo0~ i n the c o u r s e o f i t s i n t e r a c t i o n w i t h d i f f e r e n t atmospheres ( 2 8 ) . As an example, F i g u r e 6 shows the ESR s p e c t r a o f MoO^ a f t e r o u t g a s s i n g the 430°C f o r 5 m i n . ( c u r v e A) and f o r 35 m i n . ( c u r v e B ) . A n a l y s i s o f thg v a l u e s o f the g - t e n s o r r e v e a l s the appearance o f two d i f f e r e n t Mo + c e n t e r s : type A , formed at an e a r l y stage of reduction, and c h a r a c t e r i z e d by r h o m b i c a l l y d i s t o r t e d square p y r a m i d a l s u r r o u n d i n g of n o n - a x i a l symmetry a l o n g the - d o u b l e bonded oxygen, and type B , o f d i s t o r t e d o c t a h e d r a l coordination, and appearing in strongly reduced samples. Comparison o f t h e s e r e s u l t s w i t h the s i t u a t i o n a t the s u r f a c e of Mo0« c r y s t a l l i t e s ( F i g u r e 7) l e a d s t o the c o n c l u s i o n t h a t the o n l y s u r f a c e oxygen i o n , w h i c h can be removed l e a v i n g reduced molydenum c a t i o n i n square p y r a m i d a l s u r r o u n d i n g w i t h double-bonded oxygen i n the o p p o s i t e apex, i s the surface oxygen b r i d g i n g two adjacent o c t a h e d r a i n the double s t r i n g o f e d g e - l i n k e d Mo-0 o c t a h e d r a . When c o n c e n t r a t i o n o f v a c a n c i e s i n c r e a s e s , c r y s t a l l o g r a p h i c shear t a k e s p l a c e ( F i g u r e 7 b ) , and Mo c a t i o n s assume the octahedral c o o r d i n a t i o n a l o n g the shear p l a n e s . It i s noteworthy t h a t on e x p o s i n g MoO^ t o a l l y l compounds o n l y the ESR spectrum o f type Β c e n t e r s a p p e a r s . T h i s i n d i c a t e s t h a t i n s e r t i o n o f oxygen i n t o the o r g a n i c m o l e c u l e i s accompanied by a s i m u l t a n e o u s rearrangement o f the c o o r d i n a t i o n o c t a h e d r a at the s u r f a c e o f Μο0~· Mo i o n s r e g i s t e r e d i n the ESR measurement c o n s t i t u t e o n l y ^ i s m a l l f r a c t i o n o f the reduced species, the m a j o r i t y b e i n g Mo i o n s , w h i c h as non-Kramers i o n s are not expected t o g i v e an ESR s i g n a l . As these i o n s a r e l o c a t e d i n the shear planes i n edgel i n k e d o c t a h e d r a , Mo-Mo bonds are formed as r e v e a l e d by the XPS s t u d i e s (_26, 2 9 ) . UV p h o t o e l e c t r o n spectra shown i n F i g u r e 8 indicate t h a t these c l u s t e r s of tetravalent molybdenum i o n s c o n s t i t u t e energy l e v e l s s i t u a t e d i n the f o r b i d d e n energy gap o f the o x i d e (_30). M o 0 , w h i c h has been o x i d i z e d at 4 0 0 ° C , shows the g


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v a l e n c e band w i t h two maxima at 5.0 and 7 . 6 eV ( c u r v e I A ) . Its edge i s o b s e r v a b l e a t a b i n d i n g energy o f about 2.8 eV. After o u t g a s s i n g ( c u r v e IB) l o c a l energy l e v e l s appear at about 0.9 eV and 2.0 eV. S i m i l a r l e v e l s are a l s o formed on o u t g a s s i n g the Bi^MoO^, as i n d i c a t e d by c u r v e I I B . These l e v e l s have a donor c h a r a c t e r as the s u b s t i t u t i o n o f h i g h e r v a l e n t i o n s i n the l a t t i c e s i t e s o f an o x i d e by l o w e r v a l e n t i o n s r e s u l t s i n the appearance o f η - t y p e semiconductor. A g e n e r a l c o n c l u s i o n may thus be f o r m u l a t e d t h a t the a b i l i t y o f group V , V I , and V I I t r a n s i t i o n m e t a l o x i d e s to form d i f f e r e n t types o f bonding between c o o r d i n a t i o n p o l y h e d r a p l a y s an i m p o r t a n t role i n determining t h e i r c a t a l y t i c properties by p r o v i d i n g a f a c i l e r o u t e f o r i n s e r t i o n o f oxygen i n t o an o r g a n i c m o l e c u l e . The mechanism of such i n s e r t i o n i s shown i n F i g u r e 9 ( 3 1 ) . Here i n c o n t r a s t to those o x i d e s i n w h i c h the d e s o r p t i o n o f an oxygenated product r e s u l t s i n the g e n e r a t i o n o f an oxygen vacancy at considerable expenditure of energy, the d e s o r p t i o n i s accompanied by the s i m u l t a n e o u s f a c i l e rearrangement o f o c t a h e d r a . A f t e r the g i v e n elementary s t e p o f the c a t a l y t i c r e a c t i o n has taken p l a c e w i t h the p a r t i c i p a t i o n o f the l a t t i c e oxygen and f o r m a t i o n o f a n u c l e u s o f the shear p l a n e , the a c t i v e c e n t e r at the s u r f a c e i s l e f t i n the reduced state. Before such an elementary s t e p can be r e p e a t e d , the a c t i v e c e n t e r must be r e o x i d i z e d . T h i s r e o x i d a t i o n can be r e a l i z e d e i t h e r by i n c o r p o r a t i o n o f oxygen from the gas phase o r by d i f f u s i o n o f oxygen i o n s from the b u l k . Depending on the r a t e o f such r e g e n e r a t i o n o f the a c t i v e c e n t e r , i t s "dead time" may be s h o r t o r l o n g . I n the case o f o x i d e s such as b i s m u t h m o l y b d a t e , the m o b i l i t y o f l a t t i c e oxygen i s h i g h ( 3 2 , 3 3 ) , and r e g e n e r a t i o n by d i f f u s i o n from the b u l k o p e r a t e s v e r y e f f i c i e n t l y because r e o x i d a t i o n o f the l a t t i c e may t a k e p l a c e at c e n t e r s d i f f e r e n t from those p a r t i c i p a t i n g i n the r e a c t i o n . Under such c o n d i t i o n s , the "dead time" of a c t i v e c e n t e r s i s v e r y s h o r t , the t u r n - o v e r frequency v e r y l a r g e , and the c a t a l y s t a c t i v i t y v e r y high. Thus, i t may be concluded t h a t parameters m o d i f y i n g the m o b i l i t y o f o x i d e i o n s o f a s o l i d l a t t i c e may s t r o n g l y i n f l u e n c e the c a t a l y t i c a c t i v i t y . The R o l e o f Surface Geometry As a l r e a d y mentioned i n t r a n s i t i o n m e t a l o x i d e s o f group V , V I , and V I I , the appearance o f a π - b o n d component o f the m e t a l - a n i o n bonding r e s u l t s i n the d i s p l a c e m e n t o f the c a t i o n from the c e n t e r of site symmetry w h i c h s t a b i l i z e s the l a y e r e d arrangement o f c o o r d i n a t i o n p o l y h e d r a and extended d e f e c t s shear and b l o c k structures. As the charge d e n s i t y and hence the acid-base character of oxide ions i s s t r o n g l y i n f l u e n c e d by the m e t a l - o x y g e n s e p a r a t i o n , the d i s p l a c e m e n t causes the d i f f e r e n t i a t i o n o f o x i d e i o n s i n v a r i o u s c r y s t a l l o g r a p h i c p o s i t i o n s i n r e s p e c t to t h e i r acid-base properties. Simultaneously, redox p r o p e r t i e s are m o d i f i e d as m a n i f e s t e d by the dependence o f the work f u n c t i o n on the type o f the c r y s t a l l o g r a p h i c p l a n e . I t may be c o n c l u d e d t h a t i n the case o f such o x i d e s the c a t a l y t i c p r o p e r t i e s w i l l depend on the g e o m e t r i c a l s t r u c t u r e o f the s u r f a c e . Indeed, experimental d a t a c o l l e c t e d i n the l a s t few y e a r s clearly indicate that




and B i M o 0 (II): A - o x i d i z e d at 470RC f o r 1 ~ΊΊΓ at 1 atm of oxygen; Β - o u t g a s s e d f o r 10 hr at 470RC; C - c o n t a c t e d w i t h p r o p y l e n e a t 440RC ( 2 0 ) . Reproduced w i t h p e r m i s s i o n from R e f . 30. C o p y r i g h t 1976, Academic P r e s s . 2

6

,;C=C_H

XDO

+

C H 0 3

4

Figure 9. Mechanism o f the insertion o f oxygen i n t o h y d r o c a r b o n m o l e c u l e on o x i d e c a t a l y s t s w i t h p o i n t d e f e c t s (a) and shear s t r u c t u r e s ( b ) .



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different c r y s t a l m o d i f i c a t i o n s o f o x i d e systems or d i f f e r e n t c r y s t a l planes o f g i v e n o x i d e c r y s t a l l i t e s may d i f f e r c o n s i d e r b l y in their catalytic properties. T a t i b o u e t and Germain ( 3 4 ) , u s i n g c r y s t a l l i t e s o f MoO^, prepared by s u b l i m a t i o n and a p p r o p r i a t e s e i v i n g showed t h a t on the b a s a l (010) p l a n e o f Mo0~, m e t h a n o l , i n presence o f o x y g e n , becomes dehydrogenated to formaldehyde, whereas on the (001) and (101) faces i t i s dehydrated t o d i m e t h y l e t h e r . This suggests that the (010) plane shows more pronounced redox p r o p e r t i e s , whereas the (001) and (101) planes behave as a c i d - b a s e surfaces. T h i s i s i n l i n e w i t h the r e s u l t o f e x t e n s i v e s t u d i e s o f V 0 and V 0 , - - T i 0 c a t a l y s t s (35). Namely, G a s i o r and Grzybowska ( 3 6 ; observed a d r a s t i c d e c r e a s e o f a c i d i t y o f V 0 ^ when i t was s u p p o r t e d on anatase a t s m a l l coverages n o t e x c e e d i n g a m o n o l a y e r . At h i g h e r l o a d i n g s , the a c i d i t y i n c r e a s e d w i t h V 0 ^ c o n t e n t , finally attaining t h a t o f the pure phase. Crystal structure a n a l y s i s o f V«0,- and anatase r e v e a l s a good c r y s t a l l o g r a p h i c f i t between the (001) c l e a v a g e p l a n e o f anatase and the (001) b a s a l plane o f V«0^ c o n t a i n i n g the V=0 groups s t i c k i n g out p e r p e n d i c u l a r t o the s u r f a c e . This structure may be assumed to p r e v a i l i n the monolayer o f V 0 ^ on a n a t a s e . However, such c a t a l y s t s show no acidity. T h e r e f o r e , i t may be concluded t h a t on pure V 0 _ the acid-base properties are located mainly at the s i d e p l a n e s \ 1 1 0 ) and ( 1 0 0 ) . T h i s has a d i r e c t b e a r i n g on the c a t a l y t i c p r o p e r t i e s . Thus, G a s i o r and Machej (37) s t u d i e d the c a t a l y t i c a c t i v i t y o f V 0 , samples o f d i f f e r e n t c r y s t a l h a b i t and found t h a t p l a t e - l i k e c r y s t a l l i t e s e x p o s i n g m a i n l y the b a s a l (001) planes w i t h V=0 groups show v e r y h i g h s e l e c t i v i t y i n the o x i d a t i o n o f o - x y l e n e to p h t h a l i c a n h y d r i d e , whereas i n the c a s e o f n e e d l e - l i k e c r y s t a l l i t e s w i t h predominance o f (110) and (100) side planes mainly t o t a l o x i d a t i o n to C0« i s o b s e r v e d . It is interesting t h a t a t v a r i a n c e w i t h these c o n c l u s i o n s , V o l t a e t . a l . ( 3 8 , 3 9 ) , s t u d y i n g o r i e n t e d samples o f MoO^, o b t a i n e d from i n t e r c a l a t i o n compounds o f MoCl^ - g r a p h i t e , c o n c l u d e d t h a t s e l e c t i v e o x i d a t i o n o f p r o p y l e n e to a c r o l e i n o r i s o b u t e n e t o m e t h a c r o l e i n i s m a i n l y c a t a l y s e d by the (100) s i d e f a c e s , whereas the (010) b a s a l r a c e i s r e s p o n s i b l e f o r t o t a l o x i d a t i o n . I t s h o u l d be remembered, however, t h a t selective oxidation i s a multistep p r o c e s s , c o n s i s t i n g of the c o n s e c u t i v e a b s t r a c t i o n s o f hydrogen atoms and i n s e r t i o n o f oxygen atoms. As a l r e a d y d e s c r i b e d , experiments w i t h a l l y l compounds (15) showed t h a t MoO^ l a t t i c e efficiently inserts n u c l e o p h i l i c l a t t i c e oxygen i o n s i n t o the a c t i v a t e d h y d r o c a r b o n m o l e c u l e but has o n l y a l i m i t e d a b i l i t y to a c t i v a t e the h y d r o c a r b o n , t h i s step being rate determining i n s e l e c t i v e o x i d a t i o n of o l e f i n s . Therefore, determination of c a t a l y t i c a c t i v i t y i n the o x i d a t i o n o f o l e f i n s can o n l y y i e l d l i m i t e d i n f o r m a t i o n about the mechanism o f the r e a c t i o n . Indeed, experiments on the i n t e r a c t i o n of Mo0~ c r y s t a l l i t e o f d i f f e r e n t c r y s t a l h a b i t w i t h a l l y l i o d i d e seems to i n d i c a t e (40) t h a t i t i s the (010) b a s a l plane w h i c h i s r e s p o n s i b l e f o r the i n s e r t i o n o f oxygen i n t o the o r g a n i c m o l e c u l e by the s h e a r i n g mechanism d e s c r i b e d i n the p r e c e d i n g s e c t i o n . 2

2

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Dynamics o f the C a t a l y s t

Surface

T r a n s i t i o n metal o x i d e s b e l o n g to n o n - s t o i c h i o r a e t r i c compounds, t h e i r c o m p o s i t i o n depending on the e q u i l i b r i u m between the l a t t i c e and i t s c o n s i t u t e n t s i n the gas phase, i . e . i t i s a f u n c t i o n of oxygen p r e s s u r e . Due to the c o n t r i b u t i o n o f the s u r f a c e f r e e e n e r g y , the compostion o f the surface o f the s o l i d d i f f e r s from t h a t o f the b u l k , and the system i n e q u i l i b r i u m i s composed o f b u l k crystallites, their surface and the gas phase. However, e q u i l i b r a t i o n o f the gas phase w i t h the b u l k o f c r y s t a l l i t e s takes p l a c e o n l y a f t e r a n n e a l i n g at h i g h t e m p e r a t u r e s , when d i f f u s i o n o f l a t t i c e c o n s t i t u t e n t s becomes s u f f i c i e n t l y r a p i d . When an o x i d e , w h i c h has been e q u i l i b r i a t e d at a h i g h temperature i n oxygen at a g i v e n p r e s s u r e , i s then heated under a d i f f e r e n t pressure at a low temperature at w h i c h the d i f f u s i o n o f d e f e c t s i n the l a t t i c e i s s l o w , the new e q u i l i b r i u m comprises o n l y the s u r f a c e l a y e r . When h y d r o c a r b o n m o l e c u l e s , w h i c h have r e d u c i n g p r o p e r t i e s , a r e a l s o p r e s e n t i n the gas phase, a c e r t a i n degree o f r e d u c t i o n o f the s u r f a c e i s reached c o r r e s p o n d i n g t o a s t e a d y s t a t e i n w h i c h the r a t e o f r e d u c t i o n o f the s u r f a c e by h y d r o c a r b o n m o l e c u l e s becomes e q u a l to the r a t e o f i t s r e o x i d a t i o n by gas phase oxygen. When the c o m p o s i t i o n o f the gas phase i s changed, a c o r r e s p o n d i n g change o f the s u r f a c e compostion o c c u r s which i n t u r n may r e s u l t i n changes of c a t a l y t i c a c t i v i t y (40, 41). However, s e v e r a l o t h e r phenomena may a l s o take p l a c e , such as o r d e r i n g of d e f e c t s a t the s u r f a c e , surface transformations, and p r e c i p i t a t i o n o f new b i d i m e n s i o n a l s u r f a c e phases. They may r e s u l t i n the appearance o f new t y p e s o f a c t i v e centers at the surface o f the catalyst, d i r e c t i n g the catalytic reaction along a new pathway and thus profoundly i n f l u e n c i n g the s e l e c t i t y (3J^, 4 1 - 4 4 ) . The c a t a l y s t s u r f a c e i s i n a dynamic i n t e r a c t i o n w i t h the gas phase. Depending on the p r o p e r t i e s o f the m i x t u r e o f r e a c t a n t s o f the c a t a l y t i c r e a c t i o n , d i f f e r e n t surface phases may be formed at the s u r f a c e o f the c a t a l y s t , d i r e c t i n g the r e c t i o n a l o n g d i f f e r e n t reaction paths. Thus, when the s t e a d y s t a t e c o n d i t i o n s o f the r e a c t i o n are changed, the s t r u c t u r e o f the c a t a l y s t s u r f a c e a l s o may change, m o d i f y i n g the a c t i v i t y and s e l e c t i v i t y o f the c a t a l y s t itself. T h i s means t h a t i n the r a t e e q u a t i o n i t i s not o n l y the c o n c e n t r a t i o n term w h i c h depends on the p r e s s u r e o f r e a c t a n t s , but a l s o the r a t e c o n s t a n t . C o n c l u d i n g Remark I n the catalytic r e a c t i o n o f o r g a n i c m o l e c u l e s w i t h gas phase o x i d a n t s ( e . g . oxygen, s u l p h u r , chlorine) either the o x i d a n t i s a c t i v a t e d and performs an e l e c t r o p h i l i c a t t a c k , o r the o r g a n i c m o l e c u l e s are a c t i v a t e d and the r e a c t i o n proceeds i n c o n s e c u t i v e s t e p s o f hydrogen a b s t r a c t i o n and n u c l e o p h i l i c oxygen i n s e r t i o n . R e a c t i o n s o f c a t a l y t i c o x i d a t i o n may be thus d i v i d e d i n t o two groups: a . e l e c t r o p h i l i c o x i d a t i o n , i n w h i c h e p o x i d e s are formed i n case o f l i q u i d phase r e a c t i o n and d e g r a d a t i o n o f the c a r b o n s k e l e t o n takes p l a c e under c o n d i t i o n s o f heterogeneous r e a c t i o n , r e s u l t i n g i n the t o t a l o x i d a t i o n , and b . n u c l e o p h i l i c o x i d a t i o n , i n which products o f the successive n u c l e o p h i l i c i n s e r t i o n of appropriate anionic l a t t i c e constituents i n t o the o r g a n i c m o l e c u l e



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are formed. T h i s i n s e r t i o n t a k e s p l a c e at the site o f the m o l e c u l e , w h i c h by i t s a p p r o p r i a t e bonding at the a c t i v e c e n t e r o f the catalyst, i s made most positive. The s t r u c t u r e o f the i n t e r m e d i a t e complex composed o f the r e a c t i n g m o l e c u l e and the a c t i v e c e n t e r thus d e t e r m i n e s the r e a c t i o n pathway and c o n s e q u e n t l y the s e l e c t i v i t y . The a b i l i t y t o a c t i v a t e the hydrocarbon m o l e c u l e i s r e l a t e d to the p r o p e r t i e s o f i n d i v i d u a l c a t i o n s and t h e i r n e a r e s t n e i g h b o u r s , constituting active centers. When d i s c u s s i n g the b e h a v i o u r o f an i n t e r m e d i a t e complex l o c a t e d a t the s u r f a c e of a s o l i d it i s n e c e s s a r y to take i n t o account the fact t h a t the occupancy o f d i f f e r e n t orbitals is determined by the c h e m i c a l p o t e n t i a l o f e l e c t r o n s i n the s o l i d , g i v e n by the p o s i t i o n o f the Fermi l e v e l . S h i f t i n g of t h i s p o s i t i o n e . g . , by i n t r o d u c t i o n o f a d d i t i v e s changes the o r b i t a l o c c u p a n c y , may i n t u r n change the r e a c t i v i t y o f bonds and modify the a c t i v i t y and s e l e c t i v i t y . T r a n s i t i o n m e t a l o x i d e s a r e n o n s t o i c h i o m e t r i c compounds. The n o n s t o i c h i o m e t r y may be i n t r o d u c e d e i t h e r by the g e n e r a t i o n o f p o i n t d e f e c t s o r by the change o f the mode o f l i n k a g e between the coordination polyhedra, r e s u l t i n g i n the f o r m a t i o n o f extended defects/shear structures. This latter way o f changing the s t o i c h i o m e t r y i s a c h a r a c t e r i s t i c f e a t u r e o f group V , V I , and V I I t r a n s i t i o n m e t a l o x i d e s and i s r e l a t e d to the presence o f the π - o r b i t a l component o f the m e t a l - o x y g e n bonds, r e s u l t i n g i n the d i s p l a c e m e n t o f the c a t i o n s from the c e n t e r o f s i t e symmetry, w h i c h s t a b i l i z e s the l a y e r e d arrangement o f c o o r d i n a t i o n p o l y h e d r a . As the charge d e n s i t y on o x i d e i o n s i s s t r o n g l y i n f l u e n c e d by the m e t a l - o x y g e n s e p a r a t i o n , the d i s p l a c e m e n t causes the d i f f e r e n t a t i o n o f o x i d e i o n s i n v a r i o u s c r y s t a l l o g r a p h i c p o s i t i o n s w i t h r e s p e c t to t h e i r redox and a c i d - b a s e p r o p e r t i e s . As a r e s u l t , the c a t a l y t i c p r o p e r t i e s s t r o n g l y depend on the g e o m e t r i c a l s t r u c t u r e o f the surface. D i f f e r e n t polymorphic m o d i f i c a t i o n s o r d i f f e r e n t c r y s t a l planes may d i f f e r c o n s i d e r a b l y i n t h e i r c a t a l y t i c behaviour. A g e n e r a l c o n c l u s i o n may be f o r m u l a t e d t h a t the a b i l i t y o f group V , V I and V I I t r a n s i t i o n m e t a l o x i d e s to form d i f f e r e n t t y p e s of bonding between c o o r d i n a t i o n p o l y h e d r a p l a y s an i m p o r t a n t r o l e i n determining t h e i r c a t a l y t i c properties by p r o v i d i n g a f a c i l e r o u t e f o r i n s e r t i o n o f oxygen i n t o an o r g a n i c m o l e c u l e . No oxygen v a c a n c i e s are formed and t h e g e n e r a t i o n o f e l e c t r o p h i l i c oxygen, w h i c h c o u l d i n i t i a t e the s i d e r e a c t i o n o f t o t a l o x i d a t i o n , i s thus eliminated. The c a t a l y s t s u r f a c e i s i n dynamic i n t e r a c t i o n w i t h the gas phase. Depending on the p r o p e r t i e s o f the m i x t u r e o f r e a c t a n t s o f the c a t a l y t i c r e a c t i o n , d i f f e r e n t surface phases may be formed at the s u r f a c e o f the c a t a l y s t , d i r e c t i n g the r e a c t i o n a l o n g d i f f e r e n t r e a c t i o n pathways. A change of the steady state-conditions i n f l u e n c e s the c a t a l y t i c r e a c t i o n , not o n l y d i r e c t l y t h r o u g h the c o n c e n t r a t i o n term i n the r a t e e q u a t i o n , but a l s o by m o d i f y i n g the p r o p e r t i e s o f the c a t a l y s t i t s e l f , i . e . the r a t e c o n s t a n t k . Thus, heterogeneous c a t a l y t i c systems s h o u l d not be t r e a t e d as two p h a s e s , but as t h r e e phase systems composed o f the gas phase, the s o l i d , and the surface region. The l a t t e r i s composed o f the s u r f a c e atoms o f the c a t a l y s t l a t t i c e i n t e r a c t i n g w i t h the adsorbed m o l e c u l e s o f the c a t a l y t i c r e a c t i o n .


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Literature 1.

2. 3.


4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

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21

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RECEIVED January 14, 1985


Catalysis by Transition Metal Oxides - American Chemical Society

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