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5 Oxidation of Organic Compounds by Sulfur Dioxide under Pressure
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A. J . SHIPMAN Imperial Chemical Industries, Ltd., Mond Division, Runcorn, Cheshire,
England
Alkyl substituted aromatic compounds are oxidized to the corresponding carboxylic acids by sulfur dioxide under pressure in the absence of catalysts or solvents. For example, toluene is oxidized to benzoic acid, xylenes to phthalic acids, and pseudocumene to trimellitic acid. The overall reaction may be represented by the equation: RCH + 1½ SO --> RCOOH + 1½ S + H O, sulfur and water being by-products. In the presence of excess sulfur dioxide, yields of 80-90% carboxylic acid may be achieved. The effects of reaction variables upon the reaction rate and yield have been studied. The reaction is catalyzed by compounds of bromine and iodine and inhibited by metallic copper and silver. Several other novel reactions of sulfur dioxide have been observed. 3
2
2
Q u r i n g a n i n v e s t i g a t i o n i n t o the reactions of s u l f u r d i o x i d e w i t h some s i m p l e o r g a n i c c o m p o u n d s u n d e r pressure, several n o v e l effects w e r e f o u n d w h i c h , so f a r as is k n o w n , h a v e n o t b e e n r e p o r t e d p r e v i o u s l y . T h e most i m p o r t a n t is t h e o x i d a t i o n of a l k y l s u b s t i t u t e d a r o m a t i c c o m p o u n d s to t h e c o r r e s p o n d i n g c a r b o x y l i c a c i d b y s u l f u r d i o x i d e i n the absence of b o t h a solvent o r a catalyst. T h e r e a c t i o n m a y b e i l l u s t r a t e d b y t h e o x i d a t i o n o f toluene to b e n z o i c a c i d at temperatures a r o u n d 250°—300° C . a n d a pressure of 300 a t m : CeH CK + 1 7 S0 -» C H COOH + V/ S + H 0 t h e c o p r o d u c t s b e i n g s u l f u r a n d w a t e r ; t h e h e a t of r e a c t i o n is — 5 0 k c a l . / m o l e a n d t h e c h a n g e i n free e n e r g y is — 3 4 k c a l . / m o l e . I n t h e presence of excess s u l f u r d i o x i d e , t h e c o n v e r s i o n of t h e h y d r o c a r b o n is c o m p l e t e , a n d y i e l d s of 9 0 % h a v e b e e n a c h i e v e d . T h e reactions of s u l f u r d i o x i d e w i t h a w i d e r a n g e of o r g a n i c c o m p o u n d s h a v e b e e n s t u d i e d q u a l i 5
2
s
2
6
5
2
52 Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
2
S.
SHIPMAN
Organic Compounds by Sulfur Dioxide
53
t a t i v e l y , a n d the oxidations of toluene, xylenes, a n d p s e u d o c u m e n e
to
benzoic, phthalic, a n d trimellitic acids, respectively, have been studied quantitatively i n m u c h more detail.
T h e p u r p o s e of this p a p e r is b r i e f l y
to r e v i e w the e x p e r i m e n t a l results d e r i v e d f r o m these studies.
Previous Work
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T h e earliest o x i d a t i o n effected b y a s u l f u r c o m p o u n d w a s c a r r i e d o u t · b y W i l l g e r o d t i n 1887. H e s h o w e d t h a t a r y l m e t h y l ketones are c o n v e r t e d to a r y l acetamides b y aqueous a m m o n i u m p o l y s u l f i d e (26). Ο II Ar—G—GH
Aqueous 3
Ο || > Ar—GH —G—NH 2
(NH4) Sx200 -300° C.
2
o
2
T h e W i l l g e r o d t r e a c t i o n has b e e n w i d e l y s t u d i e d a n d w a s r e v i e w e d i n 1946 b y C a r m a c k a n d S p i e l m a n (4)
who, i n addition, indicated that early
attempts to o x i d i z e a s a t u r a t e d h y d r o c a r b o n side c h a i n o n a n a r o m a t i c n u c l e u s b y aqueous p o l y s u l f i d e h a d f a i l e d . C a l i f o r n i a R e s e a r c h C o r p . (19)
S u b s e q u e n t l y , T o l a n d at the
a n d N a y l o r at d u P o n t ( 8 )
found that
u n d e r m o r e v i g o r o u s c o n d i t i o n s , alkanes a n d a r a l k y l h y d r o c a r b o n s o x i d i z e d to a c i d d e r i v a t i v e s b y a w i d e v a r i e t y of s u l f u r c o m p o u n d s aqueous
solution.
F o r e x a m p l e , T o l a n d h e a t e d the h y d r o c a r b o n
are in and
s u l f u r c o m p o u n d together at 300° C . i n the presence of a l a r g e excess of w a t e r at pressures a r o u n d 170 a t m .
H e u s e d s u l f u r d i o x i d e alone
(20)
or i n c o n j u n c t i o n w i t h h y d r o g e n sulfide, or a n a l k a l i or a m m o n i u m sulfite, or a n a l k a l i s u l f i d e / s u l f a t e m i x t u r e (21).
I n other processes, e l e m e n t a l
s u l f u r has b e e n u s e d i n w a t e r either alone or w i t h a base ( 22, 23, 24, 25 ). G o o d y i e l d s are d e s c r i b e d f o r the p r e p a r a t i o n of b e n z o i c a n d p h t h a l i c acids.
It is l i k e l y t h a t t h e o x i d a n t i n these reactions is w a t e r - s o l u b l e
i n o r g a n i c polysulfides (10,11 ) p r o d u c e d b y b a s e - c a t a l y z e d d i s p r o p o r t i o n a t i o n reactions of s u l f u r or s u l f u r anions. T h e m e c h a n i s m of t h e r e a c t i o n is r e v i e w e d b y P r y o r ( 9 ) w h o c o n c l u d e s t h a t the effective species is the p o l y s u l f e n y l r a d i c a l a n d suggests the f o l l o w i n g s c h e m e : Y S 2
a
+
b
,
» YS
k l
a
+
YS
Y
b
=
H , N a , A r C H , etc. 2
kt YS (or YS ) + a
ArCH
b
Ar G H . + 2
[Ar G H
2
S Y] +
a
2
a
fast
YS
a
[Ar G H ( S Y ) ] 2
YS
+
^—> slow
3
fast a
fast
ArCH -
a
> [Ar G H
a
YS
YS H +
2
2
S Y]
> [Ar G H
a
2
> [Ar G H I φ
(S Y) ] a
2
2
(S.Y),] Rapid Hydrolysis
Ar G0 H 2
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
SELECTIVE OXIDATION PROCESSES
54
S t r i c k l a n d a n d B e l l of E a s t m a n K o d a k ( 1 7 ) h a v e o x i d i z e d t o l u e n e a n d xylenes w i t h s u l f u r d i o x i d e o v e r v a n a d i u m p e n t o x i d e catalysts a t a t m o s p h e r i c pressure.
T o l u e n e , at 410° C . w i t h a contact t i m e of five seconds,
gave a c o n v e r s i o n of 2 8 % a n d 7 5 % y i e l d of b e n z o i c a c i d w h i l e a l o w e r t e m p e r a t u r e gave p r e d o m i n a n t l y b e n z a l d e h y d e .
This method
is n o t
a p p l i c a b l e to t h e p r e p a r a t i o n of p h t h a l i c a c i d s ; v e r y l o w y i e l d s (less t h a n 5 % ) w e r e o b t a i n e d p r e s u m a b l y b e c a u s e these acids a r e n o t sufficiently
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v o l a t i l e to leave the fixed b e d r e a c t i o n zone.
Experimental F o r t h e p r e l i m i n a r y q u a l i t a t i v e e x p e r i m e n t s , h i g h pressure reactors w i t h a n o m i n a l c a p a c i t y o f 100 m l . w e r e u s e d w h i c h w e r e l i n e d w i t h , o r m a d e of stainless steel a n d s t i r r e d b y m a g n e t i c a l l y o p e r a t e d r e c i p r o c a t i n g stirrers. H e a t i n g w a s effected b y a n e x t e r n a l , c o n t r o l l e d e l e c t r i c f u r n a c e . W o r k i n g w i t h s u c h s m a l l q u a n t i t i e s of reagents t o r e d u c e t h e h a z a r d , p r o d u c t r e c o v e r y w a s p o o r , a n d i t w a s n o t possible to d e r i v e m e a n i n g f u l reaction yields. I n m o r e d e t a i l e d q u a n t i t a t i v e studies of specific reactions, reactors of 1-liter a n d 2 5 - l i t e r c a p a c i t y w e r e u s e d . T h e e x p e r i m e n t a l a r r a n g e m e n t w a s s i m i l a r t o the s m a l l reactors except that t h e 2 5 - l i t e r reactor w a s fitted w i t h a n i n t e r n a l c o o l i n g c o i l to assist i n t h e c o n t r o l o f t h e e x o t h e r m i c reaction. T h e e x p e r i m e n t a l p r o c e d u r e most c o m m o n l y u s e d w a s to c h a r g e a k n o w n w e i g h t of t h e o r g a n i c c o m p o u n d to t h e reactor, assemble i t i n t o the f u r n a c e i n a b l a s t - p r o o f reactor b a y , heat to r e a c t i o n t e m p e r a t u r e , a n d pressurize w i t h s u l f u r d i o x i d e . A s t h e r e a c t i o n p r o c e e d e d , t h e pressure f e l l o w i n g to s u l f u r dioxide's b e i n g c o n s u m e d , a n d m o r e w a s a d d e d i n t e r m i t t e n t l y f r o m a compressor t o m a i n t a i n t h e r e a c t i o n pressure. A t t h e e n d of t h e r e a c t i o n , t h e p r o d u c t s c o u l d either b e r e m o v e d f r o m t h e reactor b y b l o w i n g off" v i a a d i p p i p e a t a r o u n d 200° C . o r b y c o o l i n g t h e reactor, p e r h a p s to — 8 0 ° C , a n d d i s m a n t l i n g . T h e best p r o d u c t r e c o v e r y w a s o b t a i n e d b y the latter p r o c e d u r e . F o r those c a r b o x y l i c acids w h i c h a r e s o l u b l e i n w a t e r , a s i m p l e w a t e r e x t r a c t i o n w a s sufficient t o g i v e p r i m a r y s e p a r a t i o n f r o m t h e s u l f u r c o p r o d u c t . M e t h o d s w e r e d e v e l o p e d to r e d u c e r e s i d u a l s u l f u r c o n t a m i n a t i o n i n b e n z o i c a c i d a n d t r i m e l l i t i c a n h y d r i d e , f o r e x a m p l e , d o w n to t h e o r d e r of parts p e r m i l l i o n . F o r w a t e r - i n s o l u b l e a c i d s , i t is necessary to a v o i d s t r o n g a l k a l i s , s u c h as c a u s t i c soda, w h i c h dissolve s u l f u r ; e x t r a c t i o n w i t h s o d i u m acetate s o l u t i o n w a s f o u n d to b e a p p l i c a b l e to t e r e p h t h a l i c acid. Example 1. Preparation of Benzoic A c i d from Toluene. I n t o a l i t e r r e a c t o r w e r e p l a c e d 260 grams of t o l u e n e , h e a t e d to 300° C . a n d p r e s s u r i z e d t o 300 a t m . b y a d d i n g a p p r o x i m a t e l y 590 g r a m s o f s u l f u r d i o x i d e . T h e reaction was completed i n t w o a n d a half hours, during w h i c h a f u r t h e r 150 grams s u l f u r d i o x i d e w e r e a d d e d to m a i n t a i n r e a c t i o n pressure. T h e reactor w a s c o o l e d , excess s u l f u r d i o x i d e v e n t e d , a n d after d r y i n g t h e p r o d u c t w e i g h e d 469 grams. A f t e r e x t r a c t i o n w i t h w a t e r , 306 grams of b e n z o i c a c i d w e r e p r e c i p i t a t e d c o r r e s p o n d i n g to a y i e l d o f 9 1 % c a l c u lated o n toluene.
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
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5.
SHIPMAN
Organic Compounds by Sulfur Dioxide
55
Example 2. Preparation of Trimellitic A c i d from Pseudocumene. I n a s i m i l a r e x p e r i m e n t , 263 grams of p s e u d o c u m e n e a n d 700 g r a m s o f s u l f u r d i o x i d e w e r e h e a t e d together at 275° C . a n d 300 a t m . D u r i n g a r e a c t i o n t i m e of 11 h o u r s , a f u r t h e r 2 0 0 g r a m s o f s u l f u r d i o x i d e w a s a d d e d . 773 grams of d r i e d p r o d u c t w a s o b t a i n e d w h i c h , o n e x t r a c t i o n w i t h w a t e r , gave 394 grams o f t r i m e l l i t i c a c i d of a p p r o x i m a t e l y 9 4 % p u r i t y , c o r r e s p o n d i n g to a y i e l d of 8 1 % c a l c u l a t e d o n p s e u d o c u m e n e . S u l f u r d i o x i d e i s , i n g e n e r a l , a g o o d solvent f o r o r g a n i c c o m p o u n d s ; toluene, x y l e n e , a n d p s e u d o c u m e n e are m i s c i b l e i n a l l p r o p o r t i o n s w i t h s u l f u r d i o x i d e . T h e r e a c t i o n t e m p e r a t u r e s u s e d f o r t h e o x i d a t i o n reactions are a b o v e the c r i t i c a l temperatures o f t h e b i n a r y phase, a n d t h e i n i t i a l r e a c t i o n a t least is i n t h e v a p o r phase. A t t h e e n d o f t h e r e a c t i o n , t h e c a r b o x y l i c a c i d is s o l u b l e i n s u l f u r d i o x i d e , a n d s u l f u r is i n s o l u b l e . I f t h e p r o d u c t of a r e a c t i o n s u c h as E x a m p l e 1 w e r e t a k e n o u t of t h e reactor c a r e f u l l y , i t w a s f o u n d t o consist o f t w o l a y e r s , t h e l o w e r b e i n g m a i n l y s u l f u r , containing about 5 - 1 0 % benzoic acid a n d the upper layer containing a b o u t 8 5 % a c i d . I n t h e p u r e state t h e m u t u a l s o l u b i l i t i e s i n t h e m o l t e n systems b e n z o i c a c i d / s u l f u r , t r i m e l l i t i c a n h y d r i d e / s u l f u r a r e a r o u n d 1 % only.
Results E x p l o r a t o r y R e a c t i o n s . I n g e n e r a l , o n l y t h e m o r e o b v i o u s results of these s c o u t i n g experiments h a v e b e e n n o t e d . I n most cases, n o a t t e m p t has b e e n m a d e t o o p t i m i z e t h e y i e l d o r s t u d y t h e r e a c t i o n o v e r a w i d e range of c o n d i t i o n s . T a b l e I s u m m a r i z e s t h e results of reactions o f s u l f u r d i o x i d e w i t h m a n y different o r g a n i c c o m p o u n d s . T h e o x i d a t i o n o f t h e m e t h y l g r o u p s u b s t i t u t e d i n a n a r o m a t i c r i n g gives a h i g h y i e l d of t h e c o r r e s p o n d i n g c a r b o x y l i c a c i d i n t h e presence o f excess s u l f u r d i o x i d e (12). I f the m e t h y l g r o u p is f u r t h e r s u b s t i t u t e d to e t h y l o r i s o p r o p y l (e.g., i n c u m e n e ) , the y i e l d of b e n z o i c a c i d is v e r y r e d u c e d . A s t h e n u m b e r of m e t h y l g r o u p s s u b s t i t u t e d i n t h e b e n z e n e r i n g is i n c r e a s e d , t h e y i e l d of t h e c a r b o x y l i c a c i d decreases as s h o w n i n T a b l e I I . T h e rate o f o x i d a t i o n r e a c t i o n is n o t affected b y t h e a d d i t i o n o f freer a d i c a l initiators o r i n h i b i t o r s , o x y g e n i n s m a l l a m o u n t s , s u l f u r o r i n o r g a n i c c o m p o u n d s of s u l f u r . T h e r e a c t i o n is c a t a l y z e d b y b r o m i n e a n d i o d i n e a n d t h e i r c o m p o u n d s (13), those c o n t a i n i n g b r o m i n e b e i n g t h e m o r e effective. T h i s w i l l b e d i s c u s s e d i n m o r e d e t a i l i n this p a p e r . B e n z a l d e h y d e is o x i d i z e d to b e n z o i c a c i d i n g o o d y i e l d . T h e r a t e of this r e a c t i o n is a b o u t 10 times as great as t h e o x i d a t i o n o f t o l u e n e u n d e r c o m p a r a b l e c o n d i t i o n s . T h i s suggests that i f t h e r e a c t i o n p r o c e e d s v i a t h e a l d e h y d e , t h e i n i t i a l step is rate d e t e r m i n i n g . I f t h e o x i d a t i o n of a n a r o m a t i c h y d r o c a r b o n is s t o p p e d b e f o r e c o m p l e t i o n , s m a l l a m o u n t s o n l y of aldehydes m a y be f o u n d i n the product. T h e chlorotoluenes a r e o x i d i z e d to t h e a c i d s , b u t n i t r o t o l u e n e s are decomposed even under m i l d conditions, Y i e l d s of n a p h t h a l e n e c a r -
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
SELECTIVE OXIDATION PROCESSES
56 boxylic acids were generally rather l o w .
T h e activated methylene group
i n d i p h e n y l m e t h a n e w a s o x i d i z e d to a ketone.
Phenol and naphthols d i d
not react u p to 300° C , b u t a n i l i n e d e c o m p o s e d w i t h explosive v i o l e n c e w i t h s u l f u r d i o x i d e at 200° C . Table I.
Reactions of Organic Compounds with Sulfur Dioxide Temp., ° C.
Pressure, atm.
Toluene
250-400
50-2500
Ethylbenzene Gumene o-Xylene Tw-Xylene /^-Xylene
275 300 275-300 300 250-320
1000 300 1000 300 100-500
Mesitylene Pseudocumene Durene a-Methylnaphthalene 2,6- a n d 2 , 7 - D i m e t h y l naphthalenes Acenaphthene Tetrahydronaphthalene Decahydronaphthalene Dibenzyl Stilbene Styrene Diphenylmethane Methylcyclohexane Gyclohexane Gyclohexene Benzene Naphthalene Anthracene o-, m-, / > - C h l o r o b e n z a l dehyde ο-, m-, />-Chlorotoluene />-Dichlorotoluene Benzaldehyde G i n n a m i c acid Benzyl alcohol Phenol a-Naphthol Aniline Gyclohexanol Nitrobenzaldehyde Nitrotoluene 2-Nitro-/>-xylene Pyridine
275 250-310 250 300 280
1000 100-500 300 1000 300
300 250 250 300 275 300 300 300 257 250 450 500 500 275
300 300 300 400 1000 1000 1000 300 300 900 900 300 300 300
275
300
275 300 300 250 300 275 200 200 200 200 200 400
300 300 1000 1000 300 1000 1000 300 300 300 300 300
a-Picolene
300
300
Ethylene Propylene Butene Decene E t h y l e n e oxide M e t h y l - b u t y l alcohols E t h y l e n e glycol Acetic acid
250 250 250 250 250 250 250 300
300 300 300 300 500 300 300 300
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Compound
Product Benzoic acid 9 0 % yield, dibenzyl, polybenzylidenes Benzoic acid—moderate yield Benzoic a c i d — l o w yield o-Toluic acid, phthalic acid m - T o l u i c a c i d , isophthalic a c i d />-Toluic a c i d , terephthalic a c i d 9 0 % y i e l d polyxylyenes Trimesic acid Trimellitic acid 8 0 % yield Pyromellitic acid, low yield α-Naphthoic acid N a p h t h a l e n e m o n o - a n d d i c a r b o x y l i c acids Naphthalic acid Naphthalene Naphthalene Benzoic acid Benzoic acid Benzoic acid Benzophenone Benzoic acid D i p h e n y l , sulfide, etc. D i p h e n y l , sulfide, etc. D i p h e n y l , sulfide, etc. N o reaction N o reaction G h l o r o b e n z o i c acids C h l o r o b e n z o i c acids j&-Dichlorobenzoic a c i d Benzoic acid, good yield Benzoic acid Benzoic acid N o reaction N o reaction D e c o m p o s i t i o n w i t h explosive violence A d i p i c acid, low yield Decomposition Decomposition Decomposition Addition compound pyridine—sulfur dioxide A d d i t i o n c o m p o u n d picolene—sulfur dioxide Polysulfone, sulfolane Polysulfone, m e t h y l sulfolane Decomposition Decomposition E t h y l e n e sulfite E t h e r s , g o o d yields 1.4 D i o x a n N o reaction
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
5.
SHIPMAN
57
Organic Compounds by Sulfur Dioxide Table II
Hydrocarbon
Product
Yield
Temp., °C.
Pressure, atm.
Time, hrs.
Toluene /^-Xylene Pseudocumene Durene
Benzoic acid Terephthalic acid Trimellitic acid Pyromellitic acid
>90% 90% 80% low
300 280 270 250
300 300 300 300
3 6 8
U n s u b s t i t u t e d a r o m a t i c c o m p o u n d s are c o m p a r a t i v e l y u n r e a c t i v e to sulfur dioxide.
B e n z e n e r e a c t e d a b o v e 450° C . to g i v e d i p h e n y l , d i p h e n y l
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sulfide etc., a n d n a p h t h a l e n e a n d a n t h r a c e n e d i d n o t react u p to 500° C . Neither p y r i d i n e nor picolines undergo oxidation or coupling reactions.
U n d e r m i l d e r conditions, a p y r i d i n e / s u l f u r dioxide addition c o m -
p o u n d is f o r m e d , a n d u n d e r m o r e v i g o r o u s c o n d i t i o n s extensive c a r b o n i z a t i o n takes p l a c e .
I t is p o s s i b l e that t h e strong a t t r a c t i o n b e t w e e n t h e
aromatic nitrogen atom a n d sulfur dioxide precludes the possibility of attack at a n o t h e r p a r t of t h e m o l e c u l e .
A similar result was obtained
with pyridine hydrochloride. Alicyclic compounds compounds.
are, i n general, dehydrogenated
to a r o m a t i c
T h i s is a w e l l k n o w n r e a c t i o n of s u l f u r b u t h a s n o t p r e v i -
ously been described for sulfur dioxide.
Cyclohexane a n d cyclohexene
g a v e d i p h e n y l a n d d i p h e n y l sulfides, s i m i l a r to results d e s c r i b e d p r e v i ously
( 5 ) ; methylcyclohexane
decalin gave naphthalene.
gave some b e n z o i c
acid.
Tetralin and
C y c l o h e x a n o l g a v e a s m a l l y i e l d of a d i p i c
a c i d , this b e i n g t h e o n l y case of those e x a m i n e d w h e r e a n a l i p h a t i c a c i d was derived from a sulfur dioxide oxidation reaction. F o r t h e o x i d a t i o n of a n a r a l k y l c o m p o u n d to t h e c a r b o x y l i c a c i d to p r o c e e d to c o m p l e t i o n , i t is necessary to m a i n t a i n a n excess of s u l f u r d i o x i d e present.
I f t h e h y d r o c a r b o n is present i n excess o v e r s u l f u r d i o x i d e ,
o x i d a t i v e c o u p l i n g t h r o u g h t h e m e t h y l groups takes p l a c e . d i b e n z y l was formed from toluene a n d compounds
F o r example,
of h i g h molecular
w e i g h t ( u p to ~ 6 0 0 ) w h i c h w e r e p r o b a b l y p o l y b e n z y l i d i n e s c o n t a i n i n g sulfur links:
T h e r e a c t i o n of xylenes w i t h s u l f u r d i o x i d e g a v e p o l y x y l y l e n e s . example, from p-xylene l o w molecular weight polymers were w h i c h c o n t a i n e d a m o l a r r a t i o of a b o u t 6 : 1 x y l e n e to s u l f u r .
For
formed
These are
w e l l k n o w n reactions o f s u l f u r , b u t t h e y h a v e n o t p r e v i o u s l y b e e n d e scribed for sulfur dioxide. O t h e r n o v e l reactions of s u l f u r d i o x i d e h a v e b e e n
found which,
w h i l e n o t o x i d a t i o n reactions, are o f interest.
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
SELECTIVE OXIDATION PROCESSES
58 ETHYLENE.
I t is w e l l k n o w n that e t h y l e n e w i l l c o p o l y m e r i z e i n excess
s u l f u r d i o x i d e i n t h e p r e s e n c e of a free r a d i c a l i n i t i a t o r to f o r m e t h y l e n e polysulfone
(—CH —CH —S0 ) 2
2
2
n
(-Z6).
I f a n excess of e t h y l e n e
over
s u l f u r d i o x i d e t h e n is u s e d w i t h a free r a d i c a l i n i t i a t o r , a s m a l l y i e l d of s u l f o l a n e ( t e t r a m e t h y l e n e s u l f o n e ) is o b t a i n e d (14). i n a similar manner.
Propylene behaves
A t t e m p t s to o x i d i z e p r o p y l e n e a n d i s o b u t e n e
to
acrolein a n d acrylic a c i d derivatives were unsuccessful i n b o t h b a t c h a n d c o n t i n u o u s flow r e a c t i o n s . O L E F I N OXIDES.
S u l f u r d i o x i d e w i l l react w i t h olefin oxides i n t h e
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presence of a base s u c h as p y r i d i n e to f o r m c y c l i c sulfites—e.g., e t h y l e n e sulfite—in g o o d y i e l d
(15).
ALIPHATIC ALCOHOLS.
E t h e r s are f o r m e d i n g o o d y i e l d b y t h e r e a c -
tions of the l o w e r a l i p h a t i c alcohols w i t h s u l f u r d i o x i d e p r e s u m a b l y b y a r e a c t i o n analogous to t h a t w i t h s u l f u r i c a c i d .
E t h y l e n e g l y c o l is c o n v e r t e d
to 1.4 d i o x a n . T o s u m m a r i z e , i t has b e e n f o u n d t h a t s u l f u r d i o x i d e m a y r e a c t i n t h e following ways w i t h organic compounds: 1. 2. 3. 4. 5. 6.
T o o x i d i z e a n a l k y l s u b s t i t u t e d a r o m a t i c c o m p o u n d to t h e c o r r e sponding carboxylic acid. T o b r i n g a b o u t o x i d a t i v e c o u p l i n g of a l k y l s u b s t i t u t e d a r o m a t i c c o m pounds a n d benzene. T o dehydrogenate or aromatize alicyclic compounds. T o f o r m c y c l i c sulfones w i t h olefins. T o f o r m c y c l i c sulfites w i t h olefin oxides. T o f o r m ethers f r o m a l i p h a t i c a l c o h o l s . O x i d a t i o n of A l k y l Substituted Benzenes.
T h e reactions of
most
c o m m e r c i a l interest w e r e those l e a d i n g to the s i m p l e r a r o m a t i c a c i d s . to
benzoic,
p h t h a l i c , a n d t r i m e l l i t i c a c i d s h a v e b e e n s t u d i e d i n some d e t a i l .
The
oxidations
of
toluene,
xylenes,
and
pseudocumene
T h e ob-
ject of this w o r k w a s to d e t e r m i n e the effect of t h e r e a c t i o n v a r i a b l e s u p o n the rate a n d p r o d u c t y i e l d a n d h e n c e find t h e o p t i m u m c o n d i t i o n s . s e c t i o n s u m m a r i z e s the d e t a i l e d results o b t a i n e d .
This
K i n e t i c a l l y , t h e three
reactions a r e s i m i l a r ; w h e r e a p p a r e n t differences w e r e f o u n d , t h e y c o u l d b e r e l a t e d to t h e different heats of reaction—about methyl group.
T h e o x i d a t i o n of p s e u d o c u m e n e
50 k c a l . / m o l e
was studied i n
per most
d e t a i l u p to a r e a c t o r size of 25 liters, t o l u e n e a n d xylenes b e i n g s t u d i e d u p to a r e a c t o r s i z e of 1 l i t e r . A
t y p i c a l r e a c t i o n w a s c a r r i e d o u t as f o l l o w s .
T h e reactor
was
c h a r g e d w i t h reactants a n d b r o u g h t to t h e r e q u i r e d t e m p e r a t u r e a n d p r e s sure as q u i c k l y as p o s s i b l e .
W h e n t h e r e a c t i o n b e g a n , t h e pressure f e l l
o w i n g to c o n s u m p t i o n of s u l f u r d i o x i d e , the pressure w a s a l l o w e d to f a l l , say 5 - 1 0 % , t h e pressure d r o p m e a s u r e d a n d t h e n r e s t o r e d b y a d d i n g m o r e sulfur dioxide from a compressor
(Figure 1 ( A ) ) .
I n this w a y a n i n t e -
g r a t e d pressure d r o p vs. t i m e c u r v e c a n b e p l o t t e d ( F i g u r e 1 ( B ) ).
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
For
5.
SHIPMAN
5 to £
300 275 250225L
Organic Compounds by Sulfur Dioxide
59 (A)
[2 5 cc
F
(B)
100
INDUCTION PERIOD
\ \
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200
Q-
o ο
300
400
OC ZD
to
£ 500
REACTION TIME
Ou
600,
0-5
_L 1-0
1-5 2-0 TIME t HOURS
2-5
30
Figure 1. (A) Pressure curve; (B) Pressure drop curve for typical SO reaction
3-5
pseudocumene-
B
c o m p a r i s o n , e i t h e r t h e slope o f t h e ( n e a r l y ) l i n e a r p o r t i o n o f t h e c u r v e dP/dt
d i v i d e d b y the t o t a l pressure d r o p Δ Ρ , o r t h e t i m e f o r h a l f t h e
pressure d r o p t o o c c u r , t h e t i m e o f h a l f - c h a n g e f measures o f t h e r e a c t i o n rate.
l / f
, h a v e b e e n u s e d as
I n some e x p e r i m e n t s , p a r t i c u l a r l y i n t h e
smallest reactor, a n i n d u c t i o n p e r i o d w a s sometimes o b s e r v e d b u t this r a r e l y h a p p e n e d i n t h e l a r g e r reactors.
T h e "tail" o n the curve was
t h o u g h t to b e c a u s e d b y the final stages o f the reaction's t a k i n g p l a c e i n a m o r e viscous m e d i u m . Effect of R e a c t i o n V a r i a b l e s u p o n t h e R a t e .
TEMPERATURE.
The
a c t i v a t i o n energies f o r the o x i d a t i o n o f t o l u e n e , xylenes, a n d p s e u d o c u m e n e are a l l w i t h i n the r a n g e 42 db 2 k c a l . / m o l e (see F i g u r e 2, f o r e x a m p l e ) . CONCENTRATION.
T h e o v e r a l l r e a c t i o n rate is a p p r o x i m a t e l y p r o p o r
t i o n a l t o t h e s q u a r e o f t h e h y d r o c a r b o n c o n c e n t r a t i o n (see F i g u r e 3, f o r example). with
i n c r e a s i n g pressure.
S o m e results f o r a fixed w e i g h t o f p s e u d o c u m e n e
are s u m m a r i z e d i n
PRESSURE.
T h e r e a c t i o n rate decreases
Table III. I n these reactions t h e pressure is g e n e r a t e d b y c o m p r e s s i n g s u l f u r d i o x i d e i n t o t h e reactor, a n d a n increase i n pressure c o r r e s p o n d s t o a r e d u c t i o n i n t h e m o l e f r a c t i o n of t h e h y d r o c a r b o n .
It has already been
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
SELECTIVE OXIDATION PROCESSES
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60
Ι·60
170
±
y
| f t
3
1-80
Figure 2. Arrhenius plots for oxidation of pseudocumene
Figure 3. Effect of pseudocumene concentration on reaction rate
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
1-90
5.
SHIPMAN
Organic Compounds by Sulfur Dioxide
61
Table III Ratio of Reaction Rates Pressure, atm.
Pseudocumene, mole fraction
Reaction rate 7/h/2 Χ 70
200 300 400
0.170 0.157 0.148
2.27 1.51 1.25
2
Observed
Concentration Effect (Calculated)
Net Pressure Effect
1.0 0.85 0.76
1.0 0.79 0.72
1.0 0.67 0.55
stated t h a t a t a g i v e n pressure t h e r e a c t i o n r a t e is p r o p o r t i o n a l to t h e
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s q u a r e of t h e h y d r o c a r b o n m o l e f r a c t i o n , a n d a c o r r e c t i o n m u s t b e m a d e f o r this.
I n T a b l e I I I , t h e ratios o f t h e o b s e r v e d r e a c t i o n rates m a y b e
compared
w i t h those e x p e c t e d
from
variation i n concentration.
The
o b s e r v e d decrease i n rate is greater t h a n c o u l d b e a c c o u n t e d f o r b y a concentration
change,
i n d i c a t i n g t h a t there
is a n e t r e t a r d i n g
effect
c a u s e d b y pressure. Experiments have been carried out using
PURITY OF REACTANTS.
c o m m e r c i a l a n d s p e c i a l l y p u r i f i e d samples of e a c h r e a c t a n t , b u t n e i t h e r the r e a c t i o n rate n o r t h e p r o d u c t v a r i e d s i g n i f i c a n t l y w i t h t h e p u r i t i e s o f the reactants. ADDITIVES.
T h e r e a c t i o n w a s n o t affected b y t h e p r e s e n c e o f a f e w
atmospheres ( s a y 5 a t m . i n 300 a t m . ) of o x y g e n o r a i r .
I n the search for
p o s s i b l e catalysts, m a n y c o m p o u n d s w e r e f o u n d to b e w i t h o u t effect, f o r e x a m p l e , free r a d i c a l i n i t i a t o r s a n d i n h i b i t o r s , m e t a l salts (sulfates, s u l fites, sulfides o f a l k a l i a n d t r a n s i t i o n m e t a l s ) , m e t a l oxides, a n d c h l o r i n e compounds. T h e r e a c t i o n is c o m p l e t e l y i n h i b i t e d b y t h e p r e s e n c e of m e t a l l i c s i l v e r or c o p p e r .
F o r e x a m p l e , t h e r e a c t i o n w o u l d n o t p r o c e e d i n a 100 m l .
reactor w h i c h w a s e i t h e r l i n e d w i t h s i l v e r o r h a d a s i l v e r stirrer.
When
s m a l l pieces of s i l v e r w e r e a d d e d to a 1 l i t e r r e a c t o r a n d a r e a c t i o n c a r r i e d o u t , there w a s i n i t i a l l y a v e r y l o n g i n d u c t i o n p e r i o d after w h i c h the r e a c t i o n p r o c e e d e d at t h e rate a p p r o p r i a t e to t h e c o n d i t i o n s S i l v e r sulfide w a s r e c o v e r e d behave similarly.
from the product.
used.
C o p p e r w a s f o u n d to
I t appears t h a t these metals i n h i b i t t h e r e a c t i o n , b u t
the m e t a l sulfides d o not. T h e r e a c t i o n is c a t a l y z e d b y c o m p o u n d s o f b r o m i n e a n d i o d i n e , a s m a l l catalyst c o n c e n t r a t i o n g i v i n g a l a r g e increase i n rate ( F i g u r e 4 ) . C a t a l y s i s b y 1% H B r ( c a l c u l a t e d c n t h e h y d r o c a r b o n )
has b e e n s h o w n t o
l o w e r t h e a c t i v a t i o n e n e r g y f r o m 4 2 k c a l . / m o l e to a b o u t 2 1 k c a l . / m o l e , a n d t h e increase i n rate is p r o p o r t i o n a l to l o g [ H B r ] ( F i g u r e 5 ) .
Both
o r g a n i c a n d i n o r g a n i c c o m p o u n d s of b r o m i n e a n d i o d i n e h a v e b e e n f o u n d to c a t a l y z e t h e r e a c t i o n , t h e effect of b r o m i n e c o m p o u n d s b e i n g g e n e r a l l y greater t h a n t h a t o f i o d i n e .
I t is w e l l k n o w n t h a t the b r o m i n e a t o m is a b l e
to abstract a h y d r o g e n a t o m f r o m a m e t h y l g r o u p s u b s t i t u t e d o n t o a n aromatic ring:
R
C
H
8
+
B
r
>f]RCH - + HBr ?
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
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62
SELECTIVE OXIDATION PROCESSES
oc
Ol 270
I
I
I
I
I
L_
280
290 300 310 320 330 TEMPERATURE °C Figure 4. Effect of additives upon reaction rate (80 ml.)
S u c h a mechanism c o u l d account for the l o w e r i n g of the activation energy of t h e o v e r a l l o x i d a t i o n r e a c t i o n b y catalysis of d i e i n i t i a l step.
Chlorine
c o m p o u n d s a r e ineffective as catalysts b e c a u s e of t h e h i g h r e d o x p o t e n t i a l r e q u i r e d to f o r m c h l o r i n e atoms. SOLVENTS.
I t w a s t h o u g h t that i n t r o d u c i n g a solvent t o a c t as a n i n e r t
diluent m i g h t h e l p the reaction b y r e d u c i n g the viscosity i n the latter stages a n d h e n c e , a i d m i x i n g . zene, acetic a c i d , a n d w a t e r .
Experiments were carried out w i t h benT h e q u a l i t y of the product was not signifi-
c a n t l y i m p r o v e d , a n d thus solvents w e r e n o t g e n e r a l l y u s e d .
W i t h water
a n d a c e t i c a c i d , v e r y h e a v y c o r r o s i o n of t h e stainless steel r e a c t o r took place.
I n t h e absence of a d d i t i v e s , t h e r e a c t i o n m i x t u r e attacks stainless
steel ( 1 8 . 1 0 . M o . T i . )
a t a r a t e o f a b o u t 0.1 i n c h p e r y e a r u n d e r n o r m a l Table Volume
Radius
of Reactor V, liters
of Reactor, cm.
Surface Area A Volume cm.-
0.08
1.3
1.55
1.20
4.5
0.56
9.5
0.72»
1
Ratio of A/V 2.8 1.3
25.0
W i t h
cooling coil.
Uncooled
(estimated).
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
5.
SHIPMAN
63
Organic Compounds by Sulfur Dioxide
reaction conditions.
A d d i t i o n of hydrogen
bromide
considerably i n
creases c o r r o s i o n . REACTOR SIZE.
T h i s t o p i c is r e l e v a n t t o t h e g e n e r a l p r o b l e m of s c a l
i n g u p t h e size of t h e reactor.
T a b l e I V s u m m a r i z e s s o m e results.
T h e t h r e e reactors w e r e r o u g h l y s i m i l a r g e o m e t r i c a l l y ; t h e o n e of 25-liter c a p a c i t y w a s fitted w i t h a n i n t e r n a l c o o l i n g c o i l . agitation was comparable i n each.
T h e degree of
A l l h a d r e c i p r o c a t i n g stirrers w h i c h
w e r e s l o w a n d r a t h e r inefficient, a n d i t is a s s u m e d t h a t t h e effect o f t h e stirrer o n the t e m p e r a t u r e d i s t r i b u t i o n i n e a c h vessel is s i m i l a r .
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T h e m a x i m u m o p e r a t i n g t e m p e r a t u r e is t h a t at w h i c h i t w o u l d just b e p o s s i b l e t o c a r r y o u t a r e a c t i o n at a c o n t r o l l e d r a t e f o r a m a x i m u m c h a r g e
TEMPERATURE
275 ° C
PRESSURE
300 ATM.
TWO DIFFERENT PSEUDOCUMENE CONCENTRATIONS USED.
Θ
ft
Ν >ο
'
Θ
ι
1
I
20
-10
LOG 10 (% HBr)
Figure 5. Effect of HBr catalyst concentration
on reaction rate
IV Maximum Operating Temperature, °C. 310
Time of HalfChange 285° C. min. 260
280
113
290» 250 »
122
Ratio of Reaction Rates 2.3 1.1
Time of HalfChange 285° C, min.
Ratio of Reaction Rates
115 70 91
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
1.7 1.3
64
SELECTIVE OXIDATION PROCESSES
of p s e u d o c u m e n e .
T h e t e m p e r a t u r e m e a s u r e d corresponds a p p r o x i m a t e l y
to t h a t of t h e reactor w a l l , T . T h e r e is a c l e a r r e l a t i o n s h i p b e t w e e n this 0
t e m p e r a t u r e a n d t h e reactor r a d i u s as s h o w n i n F i g u r e 6. mum
A t the m a x i
o p e r a t i n g t e m p e r a t u r e , t h e t e m p e r a t u r e d i s t r i b u t i o n across t h e r e
actor is just o n t h e p o i n t of b e c o m i n g u n s t a b l e b e c a u s e t h e rate o f heat g e n e r a t i o n p e r u n i t v o l u m e b y t h e r e a c t i o n is greater t h a n t h e rate at 320
- Λ ·
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310
Η—100 ml VESSEL
300 ο ο
290 280
5 — 1 LITER VESSEL
270 260
y/
250
25 U T E R I " VESSEL
240 230 1
1
1
1
1
I
I
I
I
I
ι
10 9 8 7 6 5 4 3 2 1 0 1 RADIUS OF VESSEL
ι
ι
ι
ι
ι
ι
ι
2 3 4 5 6 7 (cm)
ι
8 9 10
Figure 6. Temperature profile for 25-liter reactor with profiles for the smaller reactors indicated at maximum operating temperature w h i c h h e a t c a n b e lost b y c o n d u c t i o n to the vessel w a l l .
T h e results g i v e n
i n T a b l e I V s h o w a r e l a t i o n s h i p b e t w e e n t h e r e a c t i o n rate a n d t h e surface a r e a t o v o l u m e r a t i o f o r t h e vessel.
T h e h e a t o f r e a c t i o n causes t h e
t e m p e r a t u r e w i t h i n t h e r e a c t i o n to b e greater t h a n at t h e r e a c t o r w a l l .
A
t e m p e r a t u r e d i s t r i b u t i o n is e s t a b l i s h e d across t h e reactor s i m i l a r to that s h o w n d i a g r a m m a t i c a l l y i n F i g u r e 7.
T h e o b s e r v e d r e a c t i o n rate i n e a c h
case w i l l c o r r e s p o n d to a t e m p e r a t u r e w h i c h is a n average o f T
l9
m u m temperature a n d T the measured temperature. 0
the maxi
T h e rates w i l l f o r m
a series s u c h t h a t : rate i n 100 m l < rate i n 1 l i t e r < rate 25 liters u n c o o l e d T h e effect o f c o o l i n g t h e 2 5 - l i t e r reactor w i t h a n i n t e r n a l c o i l is effectively to r e d u c e T as s h o w n b y t h e b r o k e n c u r v e i n F i g u r e 7. 0
T h i s is w h y t h e
m e a s u r e d rates i n the 1-liter a n d c o o l e d 25-liter reactors are s i m i l a r . I n this context, i t is of interest to c o m p a r e t h e m a x i m u m o p e r a t i n g t e m p e r a t u r e s of s i m i l a r charges of t o l u e n e , p - x y l e n e , a n d p s e u d o c u m e n e
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
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5.
SHIPMAN
Organic Compounds by Sulfur Dioxide
65
Figure 7. Increase in reactor internal temperature with increase in reactor radius; T constant 0
i n the 1-liter reactors.
T h e y w e r e 335°, 310°, a n d 280° C , r e s p e c t i v e l y , i n
the o r d e r e x p e c t e d f r o m t h e i r i n c r e a s i n g heats o f r e a c t i o n . M u c h of t h e b e h a v i o r o f these r e a c t i o n systems m a y b e e x p l a i n e d q u a l i t a t i v e l y o n t h e basis of t h e F r a n k - K a m e n e t s k i i s t a t i o n a r y t h e o r y of t h e r m a l e x p l o s i o n ( 6 ) , p a r t i c u l a r l y t h e effects of size o f reactor, s t i r r i n g , pressure, solvents, etc. u p o n r e a c t i o n rate. PRODUCT YrELD.
T h e y i e l d from the sulfur dioxide oxidation reaction
seems p r i n c i p a l l y to d e p e n d u p o n t h e average r e a c t i o n rate p e r u n i t v o l ume.
F i g u r e 8 shows t h e v a r i a t i o n of y i e l d o f t r i m e l l i t i c a c i d w i t h r e a c -
t i o n rate f o r p s e u d o c u m e n e i n a 1-liter reactor.
F r o m this i t c a n b e seen
that at rates b e l o w 10 X 10~ g m . m o l e s . / m l . t h e y i e l d m a y e x c e e d 8 0 % 6
w h i l e at a rate h i g h e r t h a n 30 X 10~ g m . m o l e s . / m l . / m i n . , t h e y i e l d m a y 6
f a l l to 5 0 % .
M a x i m u m y i e l d s i n excess of 9 0 % h a v e b e e n o b t a i n e d f o r t h e
o x i d a t i o n of t o l u e n e a n d x y l e n e . T h e y i e l d m a y b e l i m i t e d b y the formation of intermediate p o u n d s , tars, a n d d e g r a d a t i o n to c a r b o n .
com-
E v e n i n t h e presence o f excess
s u l f u r d i o x i d e , w h e n t h e r e a c t i o n is a p p a r e n t l y t a k e n t o c o m p l e t i o n , a s m a l l p e r c e n t o f i n t e r m e d i a t e acids m a y b e present i n t h e p r o d u c t .
If the
r e a c t i o n is s t o p p e d b e f o r e c o m p l e t i o n , t h e n s m a l l q u a n t i t i e s o f a l d e h y d e s m a y also b e f o u n d .
T h e major by-product from p-xylene was p-toluic a c i d
and from pseudocumene was 4-methylphthalic acid.
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
SELECTIVE OXIDATION PROCESSES
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66
30120'
0
aKJ
1
1
5
10
REACTION RATE
1
"
1
15
20
9m. molt ml" min. χ ιο· 1
25
30
-1
Figure 8. Variation of crude yield with reaction rate in 1-liter reactor D e g r a d a t i o n w a s n o t a serious p r o b l e m i n the 1-liter reactor, b u t i t w a s a m o r e serious p r o b l e m f r o m p s e u d o c u m e n e l i m i t i n g the y i e l d to a b o u t 7 5 % .
i n the 2 5 - l i t e r r e a c t o r
F o r m a t i o n of c a r b o n i n the p r o d u c t w a s
a c c o m p a n i e d b y the f o r m a t i o n o f c a r b o n d i o x i d e i n the gas phase.
Small
a m o u n t s o f t a r r y m a t e r i a l s w e r e sometimes f o u n d i n the p r o d u c t s .
From
t h e i r i n f r a r e d s p e c t r a a n d analyses, i t seems l i k e l y t h a t t h e y w e r e p o l y m e r i c , h a v i n g b e e n f o r m e d b y o x i d a t i v e c o u p l i n g reactions as d e s c r i b e d earlier.
Discussion T h e p r i m a r y objective of this w o r k w a s to d e t e r m i n e t h e o p t i m u m c o n d i t i o n s f o r those reactions w h i c h m i g h t b e c o m m e r c i a l l y v i a b l e for m a n u f a c t u r i n g s i m p l e b e n z e n e c a r b o x y l i c acids.
N o serious a t t e m p t has
b e e n m a d e to e l u c i d a t e t h e m e c h a n i s m of the r e a c t i o n .
Because initially
the reactants are o n l y s u l f u r d i o x i d e a n d the h y d r o c a r b o n , i t is u n l i k e l y that t h e r e a c t i o n m e c h a n i s m is i d e n t i c a l to that for o x i d a t i o n b y a l k a l i polysulfides i n the presence of a l a r g e excess of w a t e r w h i c h w a s o u t l i n e d i n the i n t r o d u c t i o n . T o s u m m a r i z e the e x p e r i m e n t a l observations: 1. I n the presence of excess s u l f u r d i o x i d e the o x i d a t i o n m a y p r o c e e d s t o i c h i o m e t r i c a l l y a c c o r d i n g to E q u a t i o n 1.
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
s.
SHIPMAN
Organic Compounds by Sulfur Dioxide R C H
+
8
S0
lVi
— R ·G O O H
2
+
IV2S +
H
2
0
67 (1)
2. I f t h e r e a c t i o n is s t o p p e d b e f o r e c o m p l e t i o n , a l d e h y d e s a n d h y d r o g e n sulfide are f o u n d i n significant q u a n t i t i e s suggesting that t h e r e a c t i o n p r o c e e d s v i a several steps s u c h a s : R C H
+
3
S0
R-GHO R C H
+
3
H S
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2
R C H O
2
VîSOt
+
S0
+
V2SO2
+
H
0 +
R ·G O O H
— R ·G O O H
2
2
+
+
IV2S +
H
S
(2)
S
(3)
H S
(4)
0
(5)
2
2
T h e rate of o x i d a t i o n of a n a l d e h y d e — R e a c t i o n 3—has b e e n s h o w n to b e r a p i d c o m p a r e d w i t h t h e o v e r a l l R e a c t i o n 1, suggesting t h a t t h e o x i d a t i o n of m e t h y l to a l d e h y d e is t h e r a t e - c o n t r o l l i n g step. R e a c t i o n 5 is k n o w n to b e fast. 3. I n t h e presence of excess h y d r o c a r b o n , o x i d a t i v e c o u p l i n g r e a c tions m a y occur. 2RGH
+
3
S0
2
—> R G H 2 G H 2 R or R G H 2 S G H 2 R
(6)
4. T h e r e a c t i o n appears to b e h o m o g e n e o u s a n d n o t affected b y surface catalysis. 5. T h e r e a c t i o n rate is p r o p o r t i o n a l to t h e s q u a r e o f t h e h y d r o c a r bon concentration. 6. T h e r e a c t i o n rate decreases w i t h i n c r e a s i n g pressure. 7. T h e r e a c t i o n is c a t a l y z e d b y b r o m i n e , i o d i n e , a n d c o m p o u n d s of these elements. 8. T h e r e a c t i o n is i n h i b i t e d b y s i l v e r a n d c o p p e r metals. 9. T h e r e a c t i o n is n e i t h e r c a t a l y z e d b y s t a n d a r d f r e e - r a d i c a l i n i t i ators n o r i n h i b i t e d b y s t a n d a r d free r a d i c a l i n h i b i t o r s ( t h o u g h i t is p o s s i b l e that t h e catalysts a n d i n h i b i t o r s u s e d m i g h t h a v e b e e n i n a c t i v a t e d b y the r e a c t i o n c o n d i t i o n s u s e d ) . 10. A t t e m p t s to o x i d i z e olefins s u c h as e t h y l e n e , p r o p y l e n e , butènes, etc. w i t h s u l f u r d i o x i d e so f a r h a v e b e e n unsuccessful. A n y r e a c t i o n o t h e r t h a n a n a d d i t i o n r e a c t i o n , p r o c e e d e d to c a r b o n i z a t i o n . N o m o r e t h a n traces of o x i d a t i o n p r o d u c t s h a v e b e e n f o u n d . I t is suggested that one possible r e a c t i o n m e c h a n i s m f o r this t y p e o f r e a c t i o n is a free r a d i c a l c h a i n r e a c t i o n b a s e d o n t h e f o l l o w i n g s c h e m e w h i c h is analogous to t h e a c c e p t e d m e c h a n i s m f o r the o x i d a t i o n of h y d r o carbon b y oxygen Initiation
(2,3) RGH
(RGH
Propagation
— RCH -
3
2
3
+
χ
+
H -
RCH -
(RGH3 +
B r - -> R G H S0
+
RCH S0 -
+
or R C H S 0 -
+
2
2
2
2
+
2
RCH 2
(7)
2
Hx)
(8)
+ HBr)
2
(9)
RGH S0 2
RGH
RCH S0 H
3
RGH
(10)
2
2
3
2
+
RGH S0 GH R 2
2
2
RGH 2
+
H
Termination
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
(11) (12) (13)
SELECTIVE OXIDATION PROCESSES
68 Subsequent reactions
2
RCH S0 CH R -> 2 R G H O + H S
(15)
RGHO + V2SO1 — RCOOH + S
(16)
2
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(14)
RCH2SO2H — RCOOH + H S
2
2
2
It is known that hydrogen abstraction reactions are catalyzed b y bromine (Reaction 9 ) . Indeed, this forms the basis of the bromidecatalyzed air oxidation process ( 7 ) , and the benzyl radical so formed is stable ( 1 8 ) . The probability of forming benzyl radicals by thermal dis sociation (Reaction 7) at around 250-300° C . is very low, and it is more likely that i n the absence of a halogen, a sulfur species designated b y χ i n Equation 8 is responsible. Absorption of χ b y copper or silver could cause the inhibition of the reaction. It is unlikely that elemental sulfur is the initiator because the addition of sulfur to the reactants d i d not affect the reaction rate. It may be noted that the initiation step may involve a net increase i n volume and may be retarded by an increase i n pressure. Sulfur dioxide w i l l copolymerize with olefins i n the presence of free radical initiators to form 1:1 copolymers (16). F o r example, ethylene polysulfone ( — C H — C H — S 0 — ) may be prepared under conditions similar to those used for aromatic oxidations. A t higher temperatures, the polysulfones decompose to the olefins and sulfur dioxide. The reaction between styrene and sulfur dioxide proceeds differently ( I ) . T h e primary product is a 1:1 complex C6H5CH2CH2SO2" which then copolymerizes with another styrene molecule and not sulfur dioxide. T h e re sulting polysulfone contains a 2:1 molecular ratio of styrene:sulfur d i oxide. O n the basis of this, the addition of a benzyl radical to sulfur dioxide and the subsequent reaction with the hydrocarbon is postulated (Reactions 10,11,12). The subsequent rearrangement of the sulfinic acid or diaryl sulfone may occur to give oxidation products w i t h a net increase i n v o l u m e Reactions 14 and 15—reactions which again might be retarded b y an i n crease of pressure. O n the basis of experimental evidence so far available, it is not possible to propose such a reaction mechanism as anything more than a tentative hypothesis. In a full analysis, the contributions to the reaction of all the species present at various stages such as sulfur, hydrogen sulfide, organic intermediates, and less stable compounds would a l l have to be taken into account. 2
2
2
n
Literature Cited (1) Barb, W . G., Proc. Roy. Soc. A.212, 66, 177 (1953). (2) Bateman, L . , Quart. Rev. 8, 147 (1954). (3) Bolland, J. L . , Quart. Rev. 3, 1 (1949). (4) Carmack, M., Spielman, Μ. Α., "Organic Reactions," edited by R. Adams. Vol. III, John Wiley & Sons Inc., New York, 1946.
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
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5.
SHIPMAN
Organic Compounds by Sulfur Dioxide
69
(5) Danford, J. D., Bender, M. J., Ind. Eng. Chem. 46, 1701 (1954). (6) Frank-Kamenetskii, D . Α., "Diffusion and Heat Transfer in Chemical Kinetics," Princeton University Press, Princeton, 1955. (7) Mid-Century Corp., B.P. 807,091 (1959). (8) Naylor, Μ. Α., U.S.P. 2,610,980 (1952); U.S.P. 2,640,077 (1953). (9) Pryor, W . Α., "Mechanism of Sulphur Reactions," McGraw-Hill, New York, 1962. (10) Pryor, W . Α., J. Am. Chem. Soc. 80, 6481 (1958). (11) Pryor, W . Α., Ferstandig, L. L . , J. Am. Chem. Soc. 82, 283 (1960). (12) Shipman, A . J., B.P. 926,019. (13) Shipman, A . J., B.P. 952,524. (14) Shipman, A . J., B.P. 956,624. (15) Shipman, A . J., B.P. 898,630. (16) Snow, R. D., Frey, F . E., Ind. Eng. Chem. 30, 176 (1938). (17) Strickland, T. H . , Bell, Α., Chem. & Eng. News 1959, Sept. 21, p. 36; Ind. Eng. Chem. 53, 7 (1961); U.S.P. 2,821,552 (1958); U.S.P. 2,982,879 (1960). (18) Szwarc, M., J. Chem. Phys. 16, 128 (1948); Chem. Rev. 47, 75 (1950). (19) Toland, W . G., U.S.P. 2,531,172 (1950); U.S.P. 2,587,666 (1952). (20) Toland, W . G., U.S.P. 2,900,412 (1959). (21) Toland, W . G., J. Am. Chem. Soc. 82, 1911 (1960). (22) Toland, W . G., U.S.P. 2,903,480 (1959). (23) Toland, W . G., Hagmann, D . L., Wilkes, J. B., Brutschy, F . J., J. Am. Chem. Soc. 80, 5423 (1958). (24) Toland, W . G., U.S.P. 2,722,548 (1955); U.S.P. 2,734,079; U.S.P. 2,762,839. (25) Toland, W . G., J. Org. Chem. 26, 2929 (1961). (26) Willgerodt, C., Ber. 20, 2467 (1887); 21, 534 (1888). RECEIVED October 15, 1964.
Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
