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Selective Oxidation of Ammonia to Hydroxylamine with Hydrogen Peroxide on Titanium Based Catalysts
V. Cortis Corberin and S. Vic Bell6n (Editors), New Deveiopnienls in Seleclive O x i d d o n If 0 1994 Elsevier Science B.V. All rights reserved.
SELECTIVE O X I D A T I O N OF AMMONIA TO HYDROXYLAMINE W I T H PEROXI DE ON T I TAN1 UM BASED CATALYSTS M . A . M A N T E G A Z Z A ~ , G. LEO FAN TI^, Z E C C H I N A ~ , s. B O R D I G A ~
G.
P E T R I N I ~ , M.
'ENICHEM ANIC, Centro Ricerche d i Bollate, 20021 B o l l a t e ( M I ) , I T A L Y
541
HYDROGEN
PA DO VAN^,
V i a S.
Pietro
A.
50,
2 D i p a r t i m e n t o d i Chimica I n o r q a n i c a , Chimica F i s i c a e Chimica d e i M a t e r i a l i d e l l ' U n i v e r s i t i i d i T o r i n o , V. P. G i u r i a 7 , 10125 T o r i no, I T A L Y
ABSTRACT
.
The s y n t h e s i s of h y d r o x y l a m i n e by o x i d a t i o n of ammonia w i t h h y d r o g e n p e r o x i d e on t i t a n i u m b a s e d c a t a l y s t s i s r e p o r t e d , Titanium s i l i c a l i t e i s t h e b e s t c a t a l y s t f o r t h e r e a c t i o n . The i n f l u e n c e of some r e a c t i o n p a r a m e t e r s on t h e main and s i d e r e a c t i o n s i s d i s c u s s e d and t h e r e a c t i o n n e t w o r k i s p r o p o s e d . The r o l e o f t i t a n i u m i s p o i n t e d o u t by r e s u l t s of s p e c t r o s c o p i c studies.
1. I N T R O D U C T I O N Hydroxylamine i s of g r e a t i n d u s t r i a l i m p o r t a n c e a s i n t e r m e d i a t e . More t h a n 9 5 % of hydroxylamine p r o d u c t i o n i s used t o p r o d u c e c y c l o h e x a n o n e oxime i n t h e caprolactam process. I n d u s t r i a l p r o d u c t i o n of h y d r o x y l a m i n e i s c a r r i e d o u t by t h e r e d u c t i o n of nitrogen oxides with s u l f u r dioxide o r by c a t a l y t i c hydrogenation ( r e f . 1 ) . I n a l l cases t h e product i s a n aqueous s o l u t i o n of a s a l t , r a t h e r t h a n f r e e hydroxylamine. Titanium s i l i c a l i t e (TiS) ( r e f . 2,3) i s a very s e l e c t i v e catalyst i n o x i d a t i o n r e a c t i o n s w i t h hydrogen peroxide, p a r t i c u l a r l y i n t h e l i q u i d p h a s e ammoximation o f cyclohexanone t o c y c l o h e x a n o n e oxime ( r e f . 4 ) . I n a p r e v i o u s work w e d e m o n s t r a t e d t h a t t h e ammoximation r e a c t i o n proceeds v i a t h e hydroxylamine i n t e r m e d i a t e ( r e f . 5 , 6 ) . I n t h e f i r s t s t e p , c a t a l y z e d by T i S , ammonia a n d h y d r o g e n p e r o x i d e r e a c t t o g i v e hydroxylamine which t h e n r e a c t s w i t h c y c l o h e x a n o n e t o g i v e t h e oxime. further In this communication w e r e p o r t t h e r e s u l t s of
542
i n v e s t i g a t i o n s on t h e c a t a l y t i c o x i d a t i o n of ammonia to hydroxylamine. T h i s r e a c t i o n i s i m p o r t a n t from a n i n d u s t r i a l p o i n t of view, because f r e e hydroxylamine can be o b t a i n e d d i r e c t l y from ammonia w h i t h o u t f o r m a t i o n of ammonium s a l t s .
2. E X P E R I MENTAL C a t a l y s t s samples w e r e s y n t h e s i z e d a c c o r d i n g t o r e f . 3. A l l samples were c h a r a c t e r i z e d by e l e m e n t a l a n a l y s i s , XRD,
N2 a d s o r p t i o n a t 7 7 K and s p e c t r o s c o p i c t e c h n i q u e s . The main f e a t u r e s of T i S samples were h i g h c r y s t a l l i n i t y and absence of e x t r a - framework T i . The ammonia o x i d a t i o n was c a r r i e d o u t under He atmosphere i n a j a c k e t e d g l a s s r e a c t o r equipped w i t h a mechanical s t i r r e r and a condenser. Aqueous hydrogen p e r o x i d e (30 w t % ) was f e d t o t h e s l u r r y o b t a i n e d by d i s p e r s i n g t h e c a t a l y s t i n an aqueous ammonia ( 1 5 w t % ) - s o l v e n t mixture (1: 1 v / v ) h e a t e d a t t h e A t the end, a f t e r c o o l i n g , t h e gaseous d e s i r e d temperature. r e a c t i o n p r o d u c t s were a n a l y z e d by gas chromatography. The c a t a l y s t w a s f i l t e r e d o f f and t h e hydroxylamine, a f t e r r e a c t i o n w i t h cyclohexanone, was determined as cyclohexanone oxime by g a s chromatography. N i t r i t e s and n i t r a t e s were d e t e r m i n e d by HPLC a n a l y s i s . The hydroxylamine o x i d a t i o n was c a r r i e d o u t i n t h e same way; aqueous s o l u t i o n s of NH2OH ( 5 0 w t % ) and H202 were f e d s e p a r a t e l y t o t h e aqueous ammonia/catalyst s y s t e m .
3. RESULTS AND DISCUSSION 3. 1 C a t a l y s t e v a l u a t i o n The r e s u l t s of ammonia o x i d a t i o n on d i f f e r e n t c a t a l y s t s , t - b u t a n o l ( T B A ) , a r e shown i n Table 1. A l l t h e y i e l d d a t a based on hydrogen p e r o x i d e .
in are
Table 1 O x i d a t i o n of ammonia o n d i f f e r e n t c a t a l y s t s Catalyst
Catalyst (wt %)
Silicalite
Ti02 Ti02/Si02 TiS ~~
1. 0 1. 8 4. 8 21. 3 63. 7
-
8 3
60
8 2 ~
NH20H y i e l d (mol%)
Ti
(wt%)
6 2 ~~
~~
Reaction conditions: s o l v e n t TBA; t e m p e r a t u r e 80' C; molar r a t i o 30; r e a c t i o n t i m e 0 . 5 hour.
NH3/H202
543 I n t h e absence of a c a t a l y s t t h e r e a c t i o n does n o t t a k e p l a c e . The h y d r o x y l a m i n e y i e l d i s n e g l i g i b l e a n d t h e main p r o d u c t i s oxygen r e s u l t i n g from h y d r o g e n p e r o x i d e d e c o m p o s i t i o n . With s i l i c a l i t e a l m o s t t h e same r e s u l t s a r e o b t a i n e d . Only T i - c o n t a i n i n g c a t a l y s t s s u c c e e d i n t h e o x i d a t i o n of ami s more e f monia t o h y d r o x y l a m i n e . Amorphous s i l i c a - t i t a n i a f i c i e n t t h a n t i t a n i u m o x i d e , b u t t h e y i e l d i s s t i l l low. With b o t h c a t a l y s t s t h e main r e a c t i o n p r o d u c t i s n i t r o g e n . T i S i s t h e most s e l e c t i v e c a t a l y s t a n d h y d r o x y l a m i n e i s t h e main r e a c t i o n product. The d a t a show t h a t t h e h y d r o x y l a m i n e y i e l d i s r e l a t e d t o t h e n a t u r e o f T i s p e c i e s r a t h e r t h a n t o i t s amount i n t h e c a t a l y s t . T h i s i s a n o t h e r example o f t h e p e c u l i a r i t y of isolated framework t i t a n i u m atoms i n TiS. 3. 2 R e a c t i o n parameters The i n f l u e n c e of s e v e r a l r e a c t i o n p a r a m e t e r s o n T i S c a t a l y z e d o x i d a t i o n o f ammonia i s d i s c u s s e d below. Solvent e f f e c t The r e s u l t s o f ammonia oxidation i n different s o l v e n t s , m i s c i b l e and non-miscible w i t h water, a r e shown i n T a b l e 2. I t has been found t h a t
Table 2 S o l v e n t e f f e c t on ammonia o x i d a t i o n Solvent
NH3/H202
molar r a t i o
NH20H y i e l d
(mol%)
32 H20 t h e r e a c t i o n c a n be p e r formed i n many s o l v e n t s : n-BuOH 16 t -BuOH 12 w a t e r , a l c o h o l s , amides, aromatic hydrocarbons. MeOH 16 The h y d r o x y l a m i n e y i e l d Toluene 11 r a n g e s from a b o u t 30% t o CH3CONH2 13 5 0 % , based on hydrogen peroxide. Good y i e l d s R e a c t i o n c o n d i t i o n s : temp. have been obtained a l s o 1. I w t % ; r e a c t i o n t i m e 0. 5 i n s o l v e n t s non-miscible w i t h w a t e r , l i k e t o l u e n e and n-butanol.
53. 5 39. 2 50. 1 46. 1 51. 0 32. 1
80' C;
hour.
TiS
E f f e c t of NH3/H307 molar r a t i o The r e s u l t s o f ammonia o x i d a t i o n i n TEA a t d i f f e r e n t N H 3 / H 2 0 2 m o l a r r a t i o s a r e r e p o r t e d i n T a b l e 3. The i n c r e a s e o f t h e N H 3 / H 2 0 2 r a t i o r e s u l t s i n a n i n c r e a s e of the hydroxylamine yield. At ratios higher than 100, h y d r o x y l a m i n e c a n be o b t a i n e d w h i t h y i e l d s a s h i g h a s 7 0 % . R e a c t i o n b y p r o d u c t s a r e n i t r o g e n , n i t r o u s o x i d e , n i t r i t e s and n i t r a t e s . N i t r o g e n i s t h e main b y p r o d u c t .
544 Table 3 E f f e c t of NH,/H202 Reaction
t i m e (h)
molar r a t i o on ammonia o x i d a t i o n Yield (mol%)
NH 3/H202 molar r a t i o
0. 5
7
0.3 0. 6 0.5 0.3
12 17 31 120
Reaction conditions: solvent c o n c e n t r a t i o n 1. 7 w t % .
TBA;
NH20H
N2
39. 0 50. 1 57. 1 63. 7 70. 3
49. 2 36. 5 32. 4 16. 4 11. 8
temperature
80' C;
TiS
E f f e c t of c a t a l y s t c o n c e n t r a t i o n The r e s u l t s of ammonia o x i d a t i o n a t i n c r e a s i n g c a t a l y s t c o n c e n t r a t i o n a r e shown i n Table 4. The hydrogen p e r o x i d e c o n v e r s i o n i s always complete e x c e p t f o r t h e t e s t w i t h o u t t h e c a t a l y s t (55 m o l % ) . By i n c r e a s i n g t h e c a t a l y s t c o n c e n t r a t i o n i ) t h e hydroxylamine y i e l d promptly i n c r e a s e s and r e a c h e s a p l a t e a u , ii) the n i t r o g e n and n i t r o u s o x i d e y i e l d q u i c k l y r e a c h e s a maximum and t h e n d e c r e a s e s s l i g h t l y , i i i ) t h e n i t r i t e s and n i t r a t e s t r e n d i s s i m i l a r t o t h e n i t r o g e n one b u t t h e i r d e c r e a s e i s more pronounced, I V ) t h e oxygen, t h e main p r o d u c t i n t h e a b s e n c e of TiS, becomes negligible even a t the lowest catalyst concentration. Table 4 E f f e c t of T i S c o n c e n t r a t i o n on ammonia o x i d a t i o n Yield ( m o l % )
T iS
(wt%) -
0.
a
1. 1 1. 7 3. 3
NH2OH 1. 0 29. a 59. 2 57. 9 62. 9
N2 2. 3 34. 2 28. 1 30. 0 28. 5
N20 7. 4 5. 5
5. 4 3. 6
NO; 1. 4 19. 5 3. 5 2. 9 3.0
NO;
02
0. 7
30. 0
9. 6 2. 1 1. a 1. 9
2. 2 1. 4 1. 0 0.7
R e a c t i o n c o n d i t i o n s : s o l v e n t TBA; t e m p e r a t u r e 80' C; NH3/H202 molar r a t i o 17; r e a c t i o n t i m e 0.5 hour.
545 E f f e c t of temperature The r e s u l t s of ammonia o x i d a t i o n a t d i f f e r e n t t e m p e r a t u r e s a r e shown i n T a b l e 5. A t t e m p e r a t u r e s from 6 0 ' t o 8 O ' C t h e hydroxylamine y i e l d is a l m o s t c o n s t a n t a n d t h e h y d r o g e n p e r o x i d e c o n v e r s i o n i s comp l e t e . A t 5 0 ' C t h e c a t a l y s t a c t i v i t y and t h e hydroxylamine y i e l d decrease. The b e h a v i o u r of b y p r o d u c t s i s q u i t e d i f f e r e n t . By d e c r e a s i n g t h e t e m p e r a t u r e n i t r o g e n and n i t r o u s o x i d e d e c r e a s e w h i l e n i t r i t e s , n i t r a t e s a n d oxygen i n c r e a s e . Table 5 E f f e c t o f t e m p e r a t u r e on ammonia o x i d a t i o n Temperature ( "C)
H202 conv. (mol%) ~
80 70 60 50
NH20H ~~
99 98 91
~
39. 0 41. 8 39. 1 21. 7
46
Yield (mol%) N2 N20 NOS
NO;
O2
0.4 0.4 1. 4 4. 3
0. 5 0. 3 0.2 1. 3
~
49. 2 42. 2 40. 6 a. 6
5. 4 5. 4 4.
2. 1 3. 7 5. 2
7
a.
1. I
1
R e a c t i o n c o n d i t i o n s : s o l v e n t TBA; NH3/H202 m o l a r r a t i o 7; c o n c e n t r a t i o n 1. 7 w t % ; r e a c t i o n t i m e 0. 5 h o u r .
TiS
E f f e c t of hydrogen p e r o x i d e f e e d r a t e The r e s u l t s o f ammonia o x i d a t i o n a t d i f f e r e n t h y d r o g e n p e r o x i d e f e e d r a t e a r e r e p o r t e d i n T a b l e 6. N o i n f l u e n c e on t h e hydroxylamine y i e l d i s o b s e r v e d by v a r y i n g t h e f e e d r a t e from 190 m l / h t o 4 ml/h. The h y d r o g e n p e r o x i d e f e e d r a t e a f f e c t s m a i n l y t h e n i t r i t e s , n i t r a t e s a n d n i t r o g e n y i e l d s . N i t r i t e s and n i t r a t e s d e c r e a s e while n i t r o g e n i n c r e a s e s almost p r o p o r t i o n a l l y . Table 6 E f f e c t of H202 f e e d r a t e on ammonia o x i d a t i o n H202 feed
rate (ml/h)
time (min)
190 9 4
1 21 51
~~
NH20H 53. 8 52. 1 53. 0
Yield ( m o l % ) N2 N20 27. 4 36. 5 38. 7
3. 4 3. 6 3. 7
NOS
a.
4 3. 1 2. 1
NO3
O2
2. 9 0.7 0.7
0.6 0.
a
0.9
~
R e a c t i o n c o n d i t i o n s : s o l v e n t TBA; t e m p e r a t u r e 80' C; m o l a r r a t i o 1 2 ; T i S 1. 7 w t % ; r e a c t i o n t i m e 1 h o u r .
NH3/H202
546
3 . 3 Hydroxylamine o x i d a t i o n w i t h hydrogen p e r o x i d e The r e a c t i v i t y of hydroxylamine towards hydrogen p e r o x i d e , w i t h and w i t h o u t TiS, has been i n v e s t i g a t e d i n o r d e r t o v e r i f y t h e s t a b i l i t y of hydroxylamine i n t h e r e a c t i o n medium. The r e s u l t s a r e r e p o r t e d i n Table 7 . Hydroxylamine i s o x i d i z e d i n b o t h n o n - c a t a l y z e d (homogeneous) and c a t a l y z e d ( h e t e r o g e n e o u s ) r e a c t i o n s . I n t h e a b s e n c e of t h e c a t a l y s t t h e r e a c t i o n i s n o n - s e l e c t i v e . N i t r o g e n , n i t r o u s o x i d e and n i t r i t e s a r e formed i n a l m o s t t h e same y i e l d . The f o r m a t i o n of a r e l e v a n t amount of oxygen p o i n t s o u t t h a t a l s o t h e hydrogen p e r o x i d e decomposition t a k e s p l a c e . With TiS t h e r e a c t i o n i s v e r y s e l e c t i v e and n i t r o g e n i s t h e main p r o d u c t .
Table 7 Hydroxylamine o x i d a t i o n w i t h hydrogen p e r o x i d e Yield ( m o l % )
TiS
(wt%) -
1. 7
3
N2
N20
NO:
NO
21
25 9
25 1
2 1
86
02
15 1
Reaction c o n d i t i o n s : N H 4 0 H ( 7 . 4 w t % ) 100 m l ; temperature molar r a t i o 0. 9; r e a c t i o n t i m e 2 . 5 hours.
8O'C;
NH20H/H202
3. 4 R e a c t i o n network The r e s u l t s p r e s e n t e d p o i n t o u t t h a t t h e ammonia o x i d a t i o n t o hydroxylamine i s c a t a l y z e d by TiS. The r e a c t i o n i s v e r y f a s t . The hydroxylamine produced can f u r t h e r r e a c t w i t h hydrogen nitrogen, nitrous p e r o x i d e to g i v e more o x i d i z e d p r o d u c t s : o x i d e , n i t r i t e s and n i t r a t e s . A s e v i d e n c e d i n Table 7 , n i t r o g e n and n i t r o u s o x i d e d e r i v e b o t h from n o n - c a t a l y z e d r e a c t i o n s i n t h e a b s e n c e of TiS and from c a t a l y z e d r e a c t i o n s i n t h e p r e s e n c e of T i S . A s r e p o r t e d i n Table 3, a l a r g e e x c e s s of ammonia i n c r e a s e s t h e hydroxylamine y i e l d and c u t s down t h e n i t r o g e n r e s u l t i n g from t h e c a t a l y z e d c o n s e c u t i v e hydroxylamine o x i d a t i o n , wich i s c o m p e t i t i v e w i t h t h e ammonia o x i d a t i o n . N i t r i t e s and n i t r a t e s a r e produced by n o n - c a t a l y z e d oxidation of t h e hydroxylamine, which i s c o m p e t i t i v e w i t h c a t a l y z e d r e a c t i o n s when t h e c a t a l y s t a c t i v i t y i s low, f o r i n s t a n c e a t low c a t a l y s t c o n c e n t r a t i o n ( s e e Table 4) and low t e m p e r a t u r e ( s e e Table 5 ) . The o v e r a l l r e a c t i o n network can be r e p r e s e n t e d by t h e f o l l o w i n g equations:
547
NH3
+
H202
1 H20
+
T iS
TiS,
NH2OH
c
H202
H202
N2
+
N20
O2
3 . 5 S p e c t r o s c o p i c c h a r a c t e r i z a t i o n of t h e a c t i v e T i c e n t r e s T i S h a s b e e n i n v e s t i g a t e d by s e v e r a l s p e c t r o s c o p i c t e c h n i q u e s t o e l u c i d a t e t h e i n t i m a t e mechanism o f t h e c a t a l y t i c r e a c t i o n b e t w e e n ammonia a n d h y d r o g e n p e r o x i d e , t h e r e a s o n why T i i n T i S i s s o a c t i v e a n d how i t c a r r i e s on i t s c a t a l y t i c a c t i v i t y . Under vacuum c o n d i t i o n s t h e most i m p o r t a n t s p e c t r o s c o p i c f e a t u r e s a s s o c i a t e d w i t h T i c a n be summarized a s f o l l o w s : 1 ) I R band a t 9 6 0 cm-' a s s o c i a t e d w i t h a S i - 0 s t r e t c h i n g mode i n [ S i 0 4 ] u n i t s p e r t u r b e d by a d j a c e n t [ T i 0 4 ] u n i t s or w i t h a s t r e t c h i n g mode o f [ T i 0 4 ] u n i t s ( r e f . 3 , 7 ) ; 2 ) o p t i c a l t r a n s i t i o n a t 48000 cm-l w i t h l i g a n d t o m e t a l c h a r g e t r a n s f e r ( C T ) c h a r a c t e r i n t e t r a c o o r d i n a t e d and i s o l a t e d T i ( I V ) ( r e f . 3, 8 ) ; 3 ) X-ray a b s o r p t i o n i n T i K p r e - e d g e r e g i o n , w i t h p e a k p o s i t i o n , f u l l w i d t h h a l f maximum (FWHM) and i n t e n s i t y i n d i c a t i n g t h a t T i i s t e t r a c o o r d i n a t e d and i n a symmetry v e r y c l o s e t o a perfect tetrahedron (ref. 9). The w h o l e r e s u l t s p o i n t o u t t h a t i n a w e l l s y n t h e s i z e d T i S a n i s o m o r p h o u s s u b s t i t u t i o n o f framework S i atoms by T i o n e s t a k e s p l a c e a n d t h e n a l l T i atoms a r e i s o l a t e d and i n t e t r a h e d r a l coordination. A f t e r t h e a d s o r p t i o n of molecules ( f o r instance water o r ammonia) t h e f o l l o w i n g c h a n g e s h a v e b e e n o b s e r v e d : 1 ) a s h i f t o f T i ( 1 V ) CT band t o l o w e r f r e q u e n c y d u e t o T i ( 1 V ) h e x a c o o r d i n a t e d complexes formed by l i g a n d a d d i t i o n ( r e f . 5 ) 2 ) a d i s a p p e a r a n c e of t h e p r e - e d g e p e a k r e p l a c e d by a new w e a k e r a b s o r p t i o n c h a r a c t e r i z e d by v e r y l a r g e FWHM, c l e a r l y i n d i c a t i n g t h e f o r m a t i o n of d i s t o r t e d o c t a h e d r a l s p e c i e s (ref. 9). These r e s u l t s , t o g e t h e r w i t h t h o s e o b t a i n e d by v o l u m e t r i c a d s o r p t i o n m e a s u r e m e n t s , d e m o n s t r a t e t h a t framework T i atoms i n T i S a r e a b l e t o a d s o r b up t o two l i g a n d s t o r e a c h t h e i r t y p i c a l hexacoordinated s t a t u s . The main s p e c t r o s c o p i c e v i d e n c e o f t h e i n t e r a c t i o n o f T i S w i t h h y d r o g e n p e r o x i d e i s t h e a p p e a r a n c e of i ) a band a t 43000 cm-' d u e t o t h e i n s e r t i o n of two w a t e r m o l e c u l e s i n t h e T i ( 1 V ) c o o r d i n a t i o n s p h e r e and i i ) a s t r o n g band a t 2 6 0 0 0 cm-' a s s o c i a t e d w i t h a CT from a h y d r o p e r o x o - t y p e s p e c i e s ( r e f . 5 ) .
548 When ammonia i s dosed on t h e preformed p e r o x o - s p e c i e s the band a t 26000 cm-l s h i f t s t o 2 7 5 0 0 cm-' and t h e n d e c l i n e s givaqueous i n g t h e same s p e c t r u m o b t a i n e d by a d i r e c t d o s a g e of hydroxylamine s o l u t i o n on TiS. S i m i l a r l y t h e ammonia d o s a g e on p r e a d s o r b e d hydrogen p e r o x i d e a band a t 1590 cm-I i n d i c a f o l l o w e d by I R s p e c t r o s c o p y g i v e s t i n g t h e hydroxylamine f o r m a t i o n ( r e f . 5 ) . I n c o n c l u s i o n t h e c a t a l y t i c s t e p of f o r m a t i o n of hydroxylamine c a n be r e p r e s e n t e d as f o l l o w s : 03Ti-OH
H20
0 3 T i OH) ( H 2 0 I 2
1
NH3
H2°2
03Ti ( OH) ( H 2 0 ) 2
NH 3
-
03Ti(OH) (NH3) ( H 2 0 )
1
H2°2
03Ti(00H)(NH3)(H20)
-c
03Ti(OH)(NH20H)(H20) REFERENCES
J. N. A r m o r i n " C a t a l y s i s of Organic R e a c t i o n s " , J . R. Kosak ( e d . 1 , Dekker, New York, 1984, 4 0 9 M. Taramasso, G. Perego, B. N o t a r i , U.S. P a t . 4. 410. 5 0 1 (1983) A. Zecchina, G. Spoto, S. Bordiga, A. F e r r e r o , G. Petrini, G. L e o f a n t i , M. Padovan i n Z e o l i t e Chemistry and C a t a l y s i s , P . A . J a c o b s e t a l . ( e d s . ) , E l s e v i e r , Amsterdam, 1991, 251 Roffia, G. Leofanti, A. Cesana, M. Mantegazza, M. 4 ) P. Padovan, G. Petrini, S. Tonti, P. Gervasutti i n "New Developments i n S e l e c t i v e O x i d a t i o n " , G. Centi et al. ( e d s . ) , E l s e v i e r , Amsterdam, 1990, 4 3 Petrini, 5 ) A. Zecchina, G. Spoto, S. Bordiga, F. Geobaldo, G. G. Leofanti, M. Padovan, M.A. Mantegazza, P. Roffia, P r o c e e d i n g s of t h e 1 0 t h I n t . Congr. on C a t a l y s i s - P a r t A, L. Guzci e t a l . ( e d s ) , Akademiai Kiadb, Budapest, 1993, 719 M . A . Mantegazza, M . Padovan, G. P e t r i n i , P. R o f f i a , I t . P a t . Appl. M I 91A001915 ( 1 9 9 1 ) A. Zecchina, G. Spoto, S. Bordiga, M. Padovan, G. Leofanti, G. G. P e t r i n i i n " C a t a l y s i s and A d s o r p t i o n by Z e o l i t e s " , Ohlman e t a l . ( e d s . ) , E l s e v i e r , Amsterdam, 1992, 671 F. Geobaldo, S. Bordiga, A. Zecchina, E. Giamello, G. L e o f a n t i , G. P e t r i n i , C a t a l y s i s L e t t e r s 1 6 ( 1 9 9 2 ) 109 S. Bordiga, F. Boscherini, S. Coluccia, F. Genoni, G. L e o f a n t i , L. Marchese, G. P e t r i n i , G. V l a i c , A. Zecchina, i n press
549
G. CENT1 (Dip. C h i m . I n d . e M a t e r i a l i , Bologna, I t a l y ) : I n t h e k i n e t i c network f o r hydroxylamine s y n t h e s i s and t r a n s f o r m a t i o n you proposed t h a t N 2 0 i s formed t o g h e t e r w i t h N2 by o x i d a t i o n of NH20H. However, i n s e v e r a l c a s e s you r e p o r t e d t h a t t h e N2 y i e l d i s a b o u t one o r d e r of magnitude h i g h e r t h a n t h a t of N20. My q u e s t i o n i s t h e r e f o r e whether t h e N 2 0 f o r m a t i o n may d e r i v e from a d i f f e r e n t s i d e r e a c t i o n , l i k e t h e r e a c t i o n of n i t r a t e s p e c i e s w i t h ammonia, o r N20 i s an i n t e r m e d i a t e i n t h e a b e t t e r u n d e r s t a n d i n g of o x i d a t i o n of NH20H t o N2. I n f a c t , t h e mechanism of N 2 0 f o r m a t i o n may a l l o w t o l i m i t t h i s byproduct which f o r m a t i o n may be p r o b l e m a t i c i n commercial ammoximation r e a c t i o n s . (ENICHEM, C e n t r o Ricerche B o l l a t e , Bollate For what concerns t h e N 2 0 f o r m a t i o n from a s i d e r e a c t i o n between n i t r a t e s and ammonia w e have v e r i f i e d t h a t b o t h ammonium n i t r a t e and n i t r i t e a r e s t a b l e i n ammonia s o l u t i o n i n t h e r e a c t i o n c o n d i t i o n s , even i n t h e p r e s e n c e of the catalyst. I n o u r r e s u l t s t h e r e i s no e v i d e n c e of a r e l a t i o n between N2O and N2 l i k e betweeen a n i n t e r m e d i a t e and a f i n a l p r o d u c t , as you s u g g e s t e d . The b e h a v i o u r of t h e s e p r o d u c t s i s s i m i l a r , i. e. when n i t r o g e n i n c r e a s e s also n i t r o u s o x i d e i n c r e a s e s , s o w e b e l i e v e t h a t N 2 0 i s a f i n a l p r o d u c t and n o t a n i n t e r m e d i a t e i n t h e NH20H oxidation. W e a g r e e t h a t a b e t t e r u n d e r s t a n d i n g of t h e mechanism of N20 f o r m a t i o n c o u l d be h e l p f u l a l s o f o r t h e ammoximation r e a c t i o n . Anyway i n t h e ammoximation of cyclohexanone t h e p r o d u c t i o n of N 2 0 i s negligible.
M. A. MANTEGAZZA ITALY):
F. T R I F I R O ' (Dip. Chim. Ind. e M a t e r i a l i , Bologna, I t a l y ) : O n t h e b a s i s of t h e work you have done on ammonia o x i d a t i o n you a r r i v e t o p r o p o s e t h a t t h e mechanism of ammoximation r e a c t i o n i s t h r o u g h NH2OH i n t e r m e d i a t e . However s c i e n t i s t s ( J i r u and a l . 1 claimed t h a t t h e main i n t e r m e d i a t e c a n be cyclohexanone imine. My q u e s t i o n i s : do you have a l s o k i n e t i c e v i d e n c e t h a t NH20H i s t h e t r u e i n t e r m e d i a t e i n ammoximation r e a c t i o n ? M. A. MANTEGAZZA: We h a v e n ' t done any k i n e t i c s t u d y on t h e ammoximation r e a c t i o n . Anyway t h e e v i d e n c e t h a t k e t o n e s w i t h l a r g e m o l e c u l a r s i z e , unable t o d i f f u s e i n t o t h e c a t a l y s t , g i v e good y i e l d s i n ammoximation (ref. 5 ) rules out t h e hypothesis of the c y c l ohexanone i m i ne i n t e r m e d i a t e . The ammoximation mechanism v i a NH20H i n t e r m e d i a t e i s based o n t h e following evidencies ( r e f . 5 ) : i ) there i s a r e l a t i o n
550
between t h e k e t o n e r e a c t i v i t y i n t h e ammoximation and i n the o x i m a t i o n r e a c t i o n : i n t h e c a s e of k e t o n e s w i t h low r e a c t i v i t y , t h e o x i m a t i o n i s t h e r a t e d e t e r m i n i n g s t e p i n t h e ammoximation r e a c t i o n i i ) t h e i n t e r m e d i a t e NH20H has been i s o l a t e d i n y i e l d c l o s e t o t h e ammoximation y i e l d .
SELECTIVE O X I D A T I O N OF AMMONIA TO HYDROXYLAMINE W I T H PEROXI DE ON T I TAN1 UM BASED CATALYSTS M . A . M A N T E G A Z Z A ~ , G. LEO FAN TI^, Z E C C H I N A ~ , s. B O R D I G A ~
G.
P E T R I N I ~ , M.
'ENICHEM ANIC, Centro Ricerche d i Bollate, 20021 B o l l a t e ( M I ) , I T A L Y
541
HYDROGEN
PA DO VAN^,
V i a S.
Pietro
A.
50,
2 D i p a r t i m e n t o d i Chimica I n o r q a n i c a , Chimica F i s i c a e Chimica d e i M a t e r i a l i d e l l ' U n i v e r s i t i i d i T o r i n o , V. P. G i u r i a 7 , 10125 T o r i no, I T A L Y
ABSTRACT
.
The s y n t h e s i s of h y d r o x y l a m i n e by o x i d a t i o n of ammonia w i t h h y d r o g e n p e r o x i d e on t i t a n i u m b a s e d c a t a l y s t s i s r e p o r t e d , Titanium s i l i c a l i t e i s t h e b e s t c a t a l y s t f o r t h e r e a c t i o n . The i n f l u e n c e of some r e a c t i o n p a r a m e t e r s on t h e main and s i d e r e a c t i o n s i s d i s c u s s e d and t h e r e a c t i o n n e t w o r k i s p r o p o s e d . The r o l e o f t i t a n i u m i s p o i n t e d o u t by r e s u l t s of s p e c t r o s c o p i c studies.
1. I N T R O D U C T I O N Hydroxylamine i s of g r e a t i n d u s t r i a l i m p o r t a n c e a s i n t e r m e d i a t e . More t h a n 9 5 % of hydroxylamine p r o d u c t i o n i s used t o p r o d u c e c y c l o h e x a n o n e oxime i n t h e caprolactam process. I n d u s t r i a l p r o d u c t i o n of h y d r o x y l a m i n e i s c a r r i e d o u t by t h e r e d u c t i o n of nitrogen oxides with s u l f u r dioxide o r by c a t a l y t i c hydrogenation ( r e f . 1 ) . I n a l l cases t h e product i s a n aqueous s o l u t i o n of a s a l t , r a t h e r t h a n f r e e hydroxylamine. Titanium s i l i c a l i t e (TiS) ( r e f . 2,3) i s a very s e l e c t i v e catalyst i n o x i d a t i o n r e a c t i o n s w i t h hydrogen peroxide, p a r t i c u l a r l y i n t h e l i q u i d p h a s e ammoximation o f cyclohexanone t o c y c l o h e x a n o n e oxime ( r e f . 4 ) . I n a p r e v i o u s work w e d e m o n s t r a t e d t h a t t h e ammoximation r e a c t i o n proceeds v i a t h e hydroxylamine i n t e r m e d i a t e ( r e f . 5 , 6 ) . I n t h e f i r s t s t e p , c a t a l y z e d by T i S , ammonia a n d h y d r o g e n p e r o x i d e r e a c t t o g i v e hydroxylamine which t h e n r e a c t s w i t h c y c l o h e x a n o n e t o g i v e t h e oxime. further In this communication w e r e p o r t t h e r e s u l t s of
542
i n v e s t i g a t i o n s on t h e c a t a l y t i c o x i d a t i o n of ammonia to hydroxylamine. T h i s r e a c t i o n i s i m p o r t a n t from a n i n d u s t r i a l p o i n t of view, because f r e e hydroxylamine can be o b t a i n e d d i r e c t l y from ammonia w h i t h o u t f o r m a t i o n of ammonium s a l t s .
2. E X P E R I MENTAL C a t a l y s t s samples w e r e s y n t h e s i z e d a c c o r d i n g t o r e f . 3. A l l samples were c h a r a c t e r i z e d by e l e m e n t a l a n a l y s i s , XRD,
N2 a d s o r p t i o n a t 7 7 K and s p e c t r o s c o p i c t e c h n i q u e s . The main f e a t u r e s of T i S samples were h i g h c r y s t a l l i n i t y and absence of e x t r a - framework T i . The ammonia o x i d a t i o n was c a r r i e d o u t under He atmosphere i n a j a c k e t e d g l a s s r e a c t o r equipped w i t h a mechanical s t i r r e r and a condenser. Aqueous hydrogen p e r o x i d e (30 w t % ) was f e d t o t h e s l u r r y o b t a i n e d by d i s p e r s i n g t h e c a t a l y s t i n an aqueous ammonia ( 1 5 w t % ) - s o l v e n t mixture (1: 1 v / v ) h e a t e d a t t h e A t the end, a f t e r c o o l i n g , t h e gaseous d e s i r e d temperature. r e a c t i o n p r o d u c t s were a n a l y z e d by gas chromatography. The c a t a l y s t w a s f i l t e r e d o f f and t h e hydroxylamine, a f t e r r e a c t i o n w i t h cyclohexanone, was determined as cyclohexanone oxime by g a s chromatography. N i t r i t e s and n i t r a t e s were d e t e r m i n e d by HPLC a n a l y s i s . The hydroxylamine o x i d a t i o n was c a r r i e d o u t i n t h e same way; aqueous s o l u t i o n s of NH2OH ( 5 0 w t % ) and H202 were f e d s e p a r a t e l y t o t h e aqueous ammonia/catalyst s y s t e m .
3. RESULTS AND DISCUSSION 3. 1 C a t a l y s t e v a l u a t i o n The r e s u l t s of ammonia o x i d a t i o n on d i f f e r e n t c a t a l y s t s , t - b u t a n o l ( T B A ) , a r e shown i n Table 1. A l l t h e y i e l d d a t a based on hydrogen p e r o x i d e .
in are
Table 1 O x i d a t i o n of ammonia o n d i f f e r e n t c a t a l y s t s Catalyst
Catalyst (wt %)
Silicalite
Ti02 Ti02/Si02 TiS ~~
1. 0 1. 8 4. 8 21. 3 63. 7
-
8 3
60
8 2 ~
NH20H y i e l d (mol%)
Ti
(wt%)
6 2 ~~
~~
Reaction conditions: s o l v e n t TBA; t e m p e r a t u r e 80' C; molar r a t i o 30; r e a c t i o n t i m e 0 . 5 hour.
NH3/H202
543 I n t h e absence of a c a t a l y s t t h e r e a c t i o n does n o t t a k e p l a c e . The h y d r o x y l a m i n e y i e l d i s n e g l i g i b l e a n d t h e main p r o d u c t i s oxygen r e s u l t i n g from h y d r o g e n p e r o x i d e d e c o m p o s i t i o n . With s i l i c a l i t e a l m o s t t h e same r e s u l t s a r e o b t a i n e d . Only T i - c o n t a i n i n g c a t a l y s t s s u c c e e d i n t h e o x i d a t i o n of ami s more e f monia t o h y d r o x y l a m i n e . Amorphous s i l i c a - t i t a n i a f i c i e n t t h a n t i t a n i u m o x i d e , b u t t h e y i e l d i s s t i l l low. With b o t h c a t a l y s t s t h e main r e a c t i o n p r o d u c t i s n i t r o g e n . T i S i s t h e most s e l e c t i v e c a t a l y s t a n d h y d r o x y l a m i n e i s t h e main r e a c t i o n product. The d a t a show t h a t t h e h y d r o x y l a m i n e y i e l d i s r e l a t e d t o t h e n a t u r e o f T i s p e c i e s r a t h e r t h a n t o i t s amount i n t h e c a t a l y s t . T h i s i s a n o t h e r example o f t h e p e c u l i a r i t y of isolated framework t i t a n i u m atoms i n TiS. 3. 2 R e a c t i o n parameters The i n f l u e n c e of s e v e r a l r e a c t i o n p a r a m e t e r s o n T i S c a t a l y z e d o x i d a t i o n o f ammonia i s d i s c u s s e d below. Solvent e f f e c t The r e s u l t s o f ammonia oxidation i n different s o l v e n t s , m i s c i b l e and non-miscible w i t h water, a r e shown i n T a b l e 2. I t has been found t h a t
Table 2 S o l v e n t e f f e c t on ammonia o x i d a t i o n Solvent
NH3/H202
molar r a t i o
NH20H y i e l d
(mol%)
32 H20 t h e r e a c t i o n c a n be p e r formed i n many s o l v e n t s : n-BuOH 16 t -BuOH 12 w a t e r , a l c o h o l s , amides, aromatic hydrocarbons. MeOH 16 The h y d r o x y l a m i n e y i e l d Toluene 11 r a n g e s from a b o u t 30% t o CH3CONH2 13 5 0 % , based on hydrogen peroxide. Good y i e l d s R e a c t i o n c o n d i t i o n s : temp. have been obtained a l s o 1. I w t % ; r e a c t i o n t i m e 0. 5 i n s o l v e n t s non-miscible w i t h w a t e r , l i k e t o l u e n e and n-butanol.
53. 5 39. 2 50. 1 46. 1 51. 0 32. 1
80' C;
hour.
TiS
E f f e c t of NH3/H307 molar r a t i o The r e s u l t s o f ammonia o x i d a t i o n i n TEA a t d i f f e r e n t N H 3 / H 2 0 2 m o l a r r a t i o s a r e r e p o r t e d i n T a b l e 3. The i n c r e a s e o f t h e N H 3 / H 2 0 2 r a t i o r e s u l t s i n a n i n c r e a s e of the hydroxylamine yield. At ratios higher than 100, h y d r o x y l a m i n e c a n be o b t a i n e d w h i t h y i e l d s a s h i g h a s 7 0 % . R e a c t i o n b y p r o d u c t s a r e n i t r o g e n , n i t r o u s o x i d e , n i t r i t e s and n i t r a t e s . N i t r o g e n i s t h e main b y p r o d u c t .
544 Table 3 E f f e c t of NH,/H202 Reaction
t i m e (h)
molar r a t i o on ammonia o x i d a t i o n Yield (mol%)
NH 3/H202 molar r a t i o
0. 5
7
0.3 0. 6 0.5 0.3
12 17 31 120
Reaction conditions: solvent c o n c e n t r a t i o n 1. 7 w t % .
TBA;
NH20H
N2
39. 0 50. 1 57. 1 63. 7 70. 3
49. 2 36. 5 32. 4 16. 4 11. 8
temperature
80' C;
TiS
E f f e c t of c a t a l y s t c o n c e n t r a t i o n The r e s u l t s of ammonia o x i d a t i o n a t i n c r e a s i n g c a t a l y s t c o n c e n t r a t i o n a r e shown i n Table 4. The hydrogen p e r o x i d e c o n v e r s i o n i s always complete e x c e p t f o r t h e t e s t w i t h o u t t h e c a t a l y s t (55 m o l % ) . By i n c r e a s i n g t h e c a t a l y s t c o n c e n t r a t i o n i ) t h e hydroxylamine y i e l d promptly i n c r e a s e s and r e a c h e s a p l a t e a u , ii) the n i t r o g e n and n i t r o u s o x i d e y i e l d q u i c k l y r e a c h e s a maximum and t h e n d e c r e a s e s s l i g h t l y , i i i ) t h e n i t r i t e s and n i t r a t e s t r e n d i s s i m i l a r t o t h e n i t r o g e n one b u t t h e i r d e c r e a s e i s more pronounced, I V ) t h e oxygen, t h e main p r o d u c t i n t h e a b s e n c e of TiS, becomes negligible even a t the lowest catalyst concentration. Table 4 E f f e c t of T i S c o n c e n t r a t i o n on ammonia o x i d a t i o n Yield ( m o l % )
T iS
(wt%) -
0.
a
1. 1 1. 7 3. 3
NH2OH 1. 0 29. a 59. 2 57. 9 62. 9
N2 2. 3 34. 2 28. 1 30. 0 28. 5
N20 7. 4 5. 5
5. 4 3. 6
NO; 1. 4 19. 5 3. 5 2. 9 3.0
NO;
02
0. 7
30. 0
9. 6 2. 1 1. a 1. 9
2. 2 1. 4 1. 0 0.7
R e a c t i o n c o n d i t i o n s : s o l v e n t TBA; t e m p e r a t u r e 80' C; NH3/H202 molar r a t i o 17; r e a c t i o n t i m e 0.5 hour.
545 E f f e c t of temperature The r e s u l t s of ammonia o x i d a t i o n a t d i f f e r e n t t e m p e r a t u r e s a r e shown i n T a b l e 5. A t t e m p e r a t u r e s from 6 0 ' t o 8 O ' C t h e hydroxylamine y i e l d is a l m o s t c o n s t a n t a n d t h e h y d r o g e n p e r o x i d e c o n v e r s i o n i s comp l e t e . A t 5 0 ' C t h e c a t a l y s t a c t i v i t y and t h e hydroxylamine y i e l d decrease. The b e h a v i o u r of b y p r o d u c t s i s q u i t e d i f f e r e n t . By d e c r e a s i n g t h e t e m p e r a t u r e n i t r o g e n and n i t r o u s o x i d e d e c r e a s e w h i l e n i t r i t e s , n i t r a t e s a n d oxygen i n c r e a s e . Table 5 E f f e c t o f t e m p e r a t u r e on ammonia o x i d a t i o n Temperature ( "C)
H202 conv. (mol%) ~
80 70 60 50
NH20H ~~
99 98 91
~
39. 0 41. 8 39. 1 21. 7
46
Yield (mol%) N2 N20 NOS
NO;
O2
0.4 0.4 1. 4 4. 3
0. 5 0. 3 0.2 1. 3
~
49. 2 42. 2 40. 6 a. 6
5. 4 5. 4 4.
2. 1 3. 7 5. 2
7
a.
1. I
1
R e a c t i o n c o n d i t i o n s : s o l v e n t TBA; NH3/H202 m o l a r r a t i o 7; c o n c e n t r a t i o n 1. 7 w t % ; r e a c t i o n t i m e 0. 5 h o u r .
TiS
E f f e c t of hydrogen p e r o x i d e f e e d r a t e The r e s u l t s o f ammonia o x i d a t i o n a t d i f f e r e n t h y d r o g e n p e r o x i d e f e e d r a t e a r e r e p o r t e d i n T a b l e 6. N o i n f l u e n c e on t h e hydroxylamine y i e l d i s o b s e r v e d by v a r y i n g t h e f e e d r a t e from 190 m l / h t o 4 ml/h. The h y d r o g e n p e r o x i d e f e e d r a t e a f f e c t s m a i n l y t h e n i t r i t e s , n i t r a t e s a n d n i t r o g e n y i e l d s . N i t r i t e s and n i t r a t e s d e c r e a s e while n i t r o g e n i n c r e a s e s almost p r o p o r t i o n a l l y . Table 6 E f f e c t of H202 f e e d r a t e on ammonia o x i d a t i o n H202 feed
rate (ml/h)
time (min)
190 9 4
1 21 51
~~
NH20H 53. 8 52. 1 53. 0
Yield ( m o l % ) N2 N20 27. 4 36. 5 38. 7
3. 4 3. 6 3. 7
NOS
a.
4 3. 1 2. 1
NO3
O2
2. 9 0.7 0.7
0.6 0.
a
0.9
~
R e a c t i o n c o n d i t i o n s : s o l v e n t TBA; t e m p e r a t u r e 80' C; m o l a r r a t i o 1 2 ; T i S 1. 7 w t % ; r e a c t i o n t i m e 1 h o u r .
NH3/H202
546
3 . 3 Hydroxylamine o x i d a t i o n w i t h hydrogen p e r o x i d e The r e a c t i v i t y of hydroxylamine towards hydrogen p e r o x i d e , w i t h and w i t h o u t TiS, has been i n v e s t i g a t e d i n o r d e r t o v e r i f y t h e s t a b i l i t y of hydroxylamine i n t h e r e a c t i o n medium. The r e s u l t s a r e r e p o r t e d i n Table 7 . Hydroxylamine i s o x i d i z e d i n b o t h n o n - c a t a l y z e d (homogeneous) and c a t a l y z e d ( h e t e r o g e n e o u s ) r e a c t i o n s . I n t h e a b s e n c e of t h e c a t a l y s t t h e r e a c t i o n i s n o n - s e l e c t i v e . N i t r o g e n , n i t r o u s o x i d e and n i t r i t e s a r e formed i n a l m o s t t h e same y i e l d . The f o r m a t i o n of a r e l e v a n t amount of oxygen p o i n t s o u t t h a t a l s o t h e hydrogen p e r o x i d e decomposition t a k e s p l a c e . With TiS t h e r e a c t i o n i s v e r y s e l e c t i v e and n i t r o g e n i s t h e main p r o d u c t .
Table 7 Hydroxylamine o x i d a t i o n w i t h hydrogen p e r o x i d e Yield ( m o l % )
TiS
(wt%) -
1. 7
3
N2
N20
NO:
NO
21
25 9
25 1
2 1
86
02
15 1
Reaction c o n d i t i o n s : N H 4 0 H ( 7 . 4 w t % ) 100 m l ; temperature molar r a t i o 0. 9; r e a c t i o n t i m e 2 . 5 hours.
8O'C;
NH20H/H202
3. 4 R e a c t i o n network The r e s u l t s p r e s e n t e d p o i n t o u t t h a t t h e ammonia o x i d a t i o n t o hydroxylamine i s c a t a l y z e d by TiS. The r e a c t i o n i s v e r y f a s t . The hydroxylamine produced can f u r t h e r r e a c t w i t h hydrogen nitrogen, nitrous p e r o x i d e to g i v e more o x i d i z e d p r o d u c t s : o x i d e , n i t r i t e s and n i t r a t e s . A s e v i d e n c e d i n Table 7 , n i t r o g e n and n i t r o u s o x i d e d e r i v e b o t h from n o n - c a t a l y z e d r e a c t i o n s i n t h e a b s e n c e of TiS and from c a t a l y z e d r e a c t i o n s i n t h e p r e s e n c e of T i S . A s r e p o r t e d i n Table 3, a l a r g e e x c e s s of ammonia i n c r e a s e s t h e hydroxylamine y i e l d and c u t s down t h e n i t r o g e n r e s u l t i n g from t h e c a t a l y z e d c o n s e c u t i v e hydroxylamine o x i d a t i o n , wich i s c o m p e t i t i v e w i t h t h e ammonia o x i d a t i o n . N i t r i t e s and n i t r a t e s a r e produced by n o n - c a t a l y z e d oxidation of t h e hydroxylamine, which i s c o m p e t i t i v e w i t h c a t a l y z e d r e a c t i o n s when t h e c a t a l y s t a c t i v i t y i s low, f o r i n s t a n c e a t low c a t a l y s t c o n c e n t r a t i o n ( s e e Table 4) and low t e m p e r a t u r e ( s e e Table 5 ) . The o v e r a l l r e a c t i o n network can be r e p r e s e n t e d by t h e f o l l o w i n g equations:
547
NH3
+
H202
1 H20
+
T iS
TiS,
NH2OH
c
H202
H202
N2
+
N20
O2
3 . 5 S p e c t r o s c o p i c c h a r a c t e r i z a t i o n of t h e a c t i v e T i c e n t r e s T i S h a s b e e n i n v e s t i g a t e d by s e v e r a l s p e c t r o s c o p i c t e c h n i q u e s t o e l u c i d a t e t h e i n t i m a t e mechanism o f t h e c a t a l y t i c r e a c t i o n b e t w e e n ammonia a n d h y d r o g e n p e r o x i d e , t h e r e a s o n why T i i n T i S i s s o a c t i v e a n d how i t c a r r i e s on i t s c a t a l y t i c a c t i v i t y . Under vacuum c o n d i t i o n s t h e most i m p o r t a n t s p e c t r o s c o p i c f e a t u r e s a s s o c i a t e d w i t h T i c a n be summarized a s f o l l o w s : 1 ) I R band a t 9 6 0 cm-' a s s o c i a t e d w i t h a S i - 0 s t r e t c h i n g mode i n [ S i 0 4 ] u n i t s p e r t u r b e d by a d j a c e n t [ T i 0 4 ] u n i t s or w i t h a s t r e t c h i n g mode o f [ T i 0 4 ] u n i t s ( r e f . 3 , 7 ) ; 2 ) o p t i c a l t r a n s i t i o n a t 48000 cm-l w i t h l i g a n d t o m e t a l c h a r g e t r a n s f e r ( C T ) c h a r a c t e r i n t e t r a c o o r d i n a t e d and i s o l a t e d T i ( I V ) ( r e f . 3, 8 ) ; 3 ) X-ray a b s o r p t i o n i n T i K p r e - e d g e r e g i o n , w i t h p e a k p o s i t i o n , f u l l w i d t h h a l f maximum (FWHM) and i n t e n s i t y i n d i c a t i n g t h a t T i i s t e t r a c o o r d i n a t e d and i n a symmetry v e r y c l o s e t o a perfect tetrahedron (ref. 9). The w h o l e r e s u l t s p o i n t o u t t h a t i n a w e l l s y n t h e s i z e d T i S a n i s o m o r p h o u s s u b s t i t u t i o n o f framework S i atoms by T i o n e s t a k e s p l a c e a n d t h e n a l l T i atoms a r e i s o l a t e d and i n t e t r a h e d r a l coordination. A f t e r t h e a d s o r p t i o n of molecules ( f o r instance water o r ammonia) t h e f o l l o w i n g c h a n g e s h a v e b e e n o b s e r v e d : 1 ) a s h i f t o f T i ( 1 V ) CT band t o l o w e r f r e q u e n c y d u e t o T i ( 1 V ) h e x a c o o r d i n a t e d complexes formed by l i g a n d a d d i t i o n ( r e f . 5 ) 2 ) a d i s a p p e a r a n c e of t h e p r e - e d g e p e a k r e p l a c e d by a new w e a k e r a b s o r p t i o n c h a r a c t e r i z e d by v e r y l a r g e FWHM, c l e a r l y i n d i c a t i n g t h e f o r m a t i o n of d i s t o r t e d o c t a h e d r a l s p e c i e s (ref. 9). These r e s u l t s , t o g e t h e r w i t h t h o s e o b t a i n e d by v o l u m e t r i c a d s o r p t i o n m e a s u r e m e n t s , d e m o n s t r a t e t h a t framework T i atoms i n T i S a r e a b l e t o a d s o r b up t o two l i g a n d s t o r e a c h t h e i r t y p i c a l hexacoordinated s t a t u s . The main s p e c t r o s c o p i c e v i d e n c e o f t h e i n t e r a c t i o n o f T i S w i t h h y d r o g e n p e r o x i d e i s t h e a p p e a r a n c e of i ) a band a t 43000 cm-' d u e t o t h e i n s e r t i o n of two w a t e r m o l e c u l e s i n t h e T i ( 1 V ) c o o r d i n a t i o n s p h e r e and i i ) a s t r o n g band a t 2 6 0 0 0 cm-' a s s o c i a t e d w i t h a CT from a h y d r o p e r o x o - t y p e s p e c i e s ( r e f . 5 ) .
548 When ammonia i s dosed on t h e preformed p e r o x o - s p e c i e s the band a t 26000 cm-l s h i f t s t o 2 7 5 0 0 cm-' and t h e n d e c l i n e s givaqueous i n g t h e same s p e c t r u m o b t a i n e d by a d i r e c t d o s a g e of hydroxylamine s o l u t i o n on TiS. S i m i l a r l y t h e ammonia d o s a g e on p r e a d s o r b e d hydrogen p e r o x i d e a band a t 1590 cm-I i n d i c a f o l l o w e d by I R s p e c t r o s c o p y g i v e s t i n g t h e hydroxylamine f o r m a t i o n ( r e f . 5 ) . I n c o n c l u s i o n t h e c a t a l y t i c s t e p of f o r m a t i o n of hydroxylamine c a n be r e p r e s e n t e d as f o l l o w s : 03Ti-OH
H20
0 3 T i OH) ( H 2 0 I 2
1
NH3
H2°2
03Ti ( OH) ( H 2 0 ) 2
NH 3
-
03Ti(OH) (NH3) ( H 2 0 )
1
H2°2
03Ti(00H)(NH3)(H20)
-c
03Ti(OH)(NH20H)(H20) REFERENCES
J. N. A r m o r i n " C a t a l y s i s of Organic R e a c t i o n s " , J . R. Kosak ( e d . 1 , Dekker, New York, 1984, 4 0 9 M. Taramasso, G. Perego, B. N o t a r i , U.S. P a t . 4. 410. 5 0 1 (1983) A. Zecchina, G. Spoto, S. Bordiga, A. F e r r e r o , G. Petrini, G. L e o f a n t i , M. Padovan i n Z e o l i t e Chemistry and C a t a l y s i s , P . A . J a c o b s e t a l . ( e d s . ) , E l s e v i e r , Amsterdam, 1991, 251 Roffia, G. Leofanti, A. Cesana, M. Mantegazza, M. 4 ) P. Padovan, G. Petrini, S. Tonti, P. Gervasutti i n "New Developments i n S e l e c t i v e O x i d a t i o n " , G. Centi et al. ( e d s . ) , E l s e v i e r , Amsterdam, 1990, 4 3 Petrini, 5 ) A. Zecchina, G. Spoto, S. Bordiga, F. Geobaldo, G. G. Leofanti, M. Padovan, M.A. Mantegazza, P. Roffia, P r o c e e d i n g s of t h e 1 0 t h I n t . Congr. on C a t a l y s i s - P a r t A, L. Guzci e t a l . ( e d s ) , Akademiai Kiadb, Budapest, 1993, 719 M . A . Mantegazza, M . Padovan, G. P e t r i n i , P. R o f f i a , I t . P a t . Appl. M I 91A001915 ( 1 9 9 1 ) A. Zecchina, G. Spoto, S. Bordiga, M. Padovan, G. Leofanti, G. G. P e t r i n i i n " C a t a l y s i s and A d s o r p t i o n by Z e o l i t e s " , Ohlman e t a l . ( e d s . ) , E l s e v i e r , Amsterdam, 1992, 671 F. Geobaldo, S. Bordiga, A. Zecchina, E. Giamello, G. L e o f a n t i , G. P e t r i n i , C a t a l y s i s L e t t e r s 1 6 ( 1 9 9 2 ) 109 S. Bordiga, F. Boscherini, S. Coluccia, F. Genoni, G. L e o f a n t i , L. Marchese, G. P e t r i n i , G. V l a i c , A. Zecchina, i n press
549
G. CENT1 (Dip. C h i m . I n d . e M a t e r i a l i , Bologna, I t a l y ) : I n t h e k i n e t i c network f o r hydroxylamine s y n t h e s i s and t r a n s f o r m a t i o n you proposed t h a t N 2 0 i s formed t o g h e t e r w i t h N2 by o x i d a t i o n of NH20H. However, i n s e v e r a l c a s e s you r e p o r t e d t h a t t h e N2 y i e l d i s a b o u t one o r d e r of magnitude h i g h e r t h a n t h a t of N20. My q u e s t i o n i s t h e r e f o r e whether t h e N 2 0 f o r m a t i o n may d e r i v e from a d i f f e r e n t s i d e r e a c t i o n , l i k e t h e r e a c t i o n of n i t r a t e s p e c i e s w i t h ammonia, o r N20 i s an i n t e r m e d i a t e i n t h e a b e t t e r u n d e r s t a n d i n g of o x i d a t i o n of NH20H t o N2. I n f a c t , t h e mechanism of N 2 0 f o r m a t i o n may a l l o w t o l i m i t t h i s byproduct which f o r m a t i o n may be p r o b l e m a t i c i n commercial ammoximation r e a c t i o n s . (ENICHEM, C e n t r o Ricerche B o l l a t e , Bollate For what concerns t h e N 2 0 f o r m a t i o n from a s i d e r e a c t i o n between n i t r a t e s and ammonia w e have v e r i f i e d t h a t b o t h ammonium n i t r a t e and n i t r i t e a r e s t a b l e i n ammonia s o l u t i o n i n t h e r e a c t i o n c o n d i t i o n s , even i n t h e p r e s e n c e of the catalyst. I n o u r r e s u l t s t h e r e i s no e v i d e n c e of a r e l a t i o n between N2O and N2 l i k e betweeen a n i n t e r m e d i a t e and a f i n a l p r o d u c t , as you s u g g e s t e d . The b e h a v i o u r of t h e s e p r o d u c t s i s s i m i l a r , i. e. when n i t r o g e n i n c r e a s e s also n i t r o u s o x i d e i n c r e a s e s , s o w e b e l i e v e t h a t N 2 0 i s a f i n a l p r o d u c t and n o t a n i n t e r m e d i a t e i n t h e NH20H oxidation. W e a g r e e t h a t a b e t t e r u n d e r s t a n d i n g of t h e mechanism of N20 f o r m a t i o n c o u l d be h e l p f u l a l s o f o r t h e ammoximation r e a c t i o n . Anyway i n t h e ammoximation of cyclohexanone t h e p r o d u c t i o n of N 2 0 i s negligible.
M. A. MANTEGAZZA ITALY):
F. T R I F I R O ' (Dip. Chim. Ind. e M a t e r i a l i , Bologna, I t a l y ) : O n t h e b a s i s of t h e work you have done on ammonia o x i d a t i o n you a r r i v e t o p r o p o s e t h a t t h e mechanism of ammoximation r e a c t i o n i s t h r o u g h NH2OH i n t e r m e d i a t e . However s c i e n t i s t s ( J i r u and a l . 1 claimed t h a t t h e main i n t e r m e d i a t e c a n be cyclohexanone imine. My q u e s t i o n i s : do you have a l s o k i n e t i c e v i d e n c e t h a t NH20H i s t h e t r u e i n t e r m e d i a t e i n ammoximation r e a c t i o n ? M. A. MANTEGAZZA: We h a v e n ' t done any k i n e t i c s t u d y on t h e ammoximation r e a c t i o n . Anyway t h e e v i d e n c e t h a t k e t o n e s w i t h l a r g e m o l e c u l a r s i z e , unable t o d i f f u s e i n t o t h e c a t a l y s t , g i v e good y i e l d s i n ammoximation (ref. 5 ) rules out t h e hypothesis of the c y c l ohexanone i m i ne i n t e r m e d i a t e . The ammoximation mechanism v i a NH20H i n t e r m e d i a t e i s based o n t h e following evidencies ( r e f . 5 ) : i ) there i s a r e l a t i o n
550
between t h e k e t o n e r e a c t i v i t y i n t h e ammoximation and i n the o x i m a t i o n r e a c t i o n : i n t h e c a s e of k e t o n e s w i t h low r e a c t i v i t y , t h e o x i m a t i o n i s t h e r a t e d e t e r m i n i n g s t e p i n t h e ammoximation r e a c t i o n i i ) t h e i n t e r m e d i a t e NH20H has been i s o l a t e d i n y i e l d c l o s e t o t h e ammoximation y i e l d .