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1.7 The Role of Alcohols on Lactam Selectivity in the Vapor-Phase Beckmann Rearrangement Reaction
1.7 The Role of Alcohols on Lactam Selectivity in the Vapor-Phase Beckmann Rearrangement Reaction M. Kitamura and €1. Ichihashi Organic Synthesis Research Laboratory, Sumitomo Chemical Co., Ltd. Takatsuki-City, Osaka 569, Japan Abstract For the vapor-phase Beckmann rearrangement reaction of cyclohexanone oxime over high-silicious pentasil zeolites, the selectivity to &-caprolactam increased when methanol was supplied with cyclohexanone oxime into the reaction system. FT-IR measurements showed that the neutral silanol groups of the zeolite were methylated by the alcohol. This indicates that the neutral silanol is not the active site for producing c-caprolactam. 1.INTRODUCTION E-caprolactam is an important starting material for the production of nylon-6 polymers. It is commercially synthesized by the Beckmann rearrangement reaction of cyclohexanone oxime using fuming sulfuric acid in the liquid phase. The reaction in this system proceeds with a high selectivity to e-caprolactam. The process is considered to be disadvantageous, however, because it produces a large amount of ammonium sulfate as a bysilicious product. Recently Sato et al. reported that a highly pentasil zeolite treated with trimethylchlorosilane had the effect of greatly increasing the selectivity to lactam. [ l - 2 1 2.EXPERIMENTAL METHOD 2.1. Preparation of Catalysts
Tetraethylorthosilicate, 1 0 % aqueous tetra-n-propylammonium hydroxide solution, and wat,er were charged in an autoclave. A hydrothermal reaction was conducted at l 0 5 O C for 96 hours. Whjte solids were filtered out, washed, and dried. The crystals were then calcined at 530°C. The ion-exchange treatment by 5% aqueous ammonium chloride solution was carried out at 60°C for 1 hour. This treatment was repeated four times. 2.2. Reaction Procedure The reaction was carried out using a fixed-bed type reactor at 350°C. atmospheric pressure. The reaction temperature was Cyclohexanone oxime was fed as a benzene solution. The reaction products were analyzed by G . C .
61
68
M. KITAMURA and H. ICHIHASHI
2.3. FT-IR Measurement A diffuse reflection type FT-IR was used. The catalyst was pretreated at 3 5 0 C for 1 hour in a helium atmosphere. The spectra were measured at 3 5 0 C in an atmosphere O F helium gas
containing methanol vapor. 3.RESLLTS AND DISCUSSION 3.1. Effects of Additives
Table 1 shows the ePfects of addltives on catalytic per-formance. When oxime was f e d with methanol, the selectivlty to lactam increased to more than 9 0 % . 'Pie same effect was observed when oxime was fed with other alcohols. The longer the length of the alkyl chain, however, the larger the decline of catalytic activity. The other solvents, for example, phenol, acetaldehyde, and acetic acid, did not increase the selectivity to lactam. In addition , another interesting point was discovered. The methanol used as an additive was recovered quantitatively. Generally speaking, methanol converts to hydrocarbons on a ZSM-5 type zeolite, but in this case, this reaction did not occur, A s shown in Table 2, when some kinds of other catalysts were used instead of highly silicious pentasil zeolite, the cFfect O F methanol to increase the selectivity to lactam was not observed. When borontrioxide was used as a catalyst, the reaction could not be continued, because the boron is lost from the catalstst as a methyl ester.
T a b l e 1.
E f f e c t
a d d I t i v e s
B e n z e n e CH,OH CZHSOH n-C,H,OH
o f
A d d i t i v e s
c o n v . I
1 1 9 9
0 0 0 0 9 0 6. 6
-
(%)
9 8 99. 97 9 2.
6 6
0 6
(1)
s e I 7 9 9 9
(%)
0 2 2 1
i so-C.H70H C.H.OH CHnCHO
9 9 0-94. 3 40. 5-2 1 4
9 1 7 9
99. 6-94.
2
7 9
CHsCOOH
1 0 0 -95.
2
7 3 - 6 3
* C a t a l y s t : P e n t a s i I z e o l i t e ( S i / A I = l 4 7 0 0 0 ) * O x i m e / S o l v e n t = 3 5-4.0 (mo I r a t i 0 ) *W/F=41-43 ( 1 - c a t / m o l - o x i m e / h r ) *1) 1 s t h o u r *2) 6 t h h o u r
Table2.Effectofadditives
catalyst
(2)
L a c t a m a e l . (%) ~B e n z e n e _Mathano1 ~
Pentaoil zeolite (S i /A I = 1 4 7 000) Silica-Alumina (A I2O. 1 3 w t W Silica-Alumina (AI,O, 2 8 w t W
70
92
66
50
52
60
Si I ica
29
30
Boron t r i o x i d o
90
-
*Oxime/Solvent=3.5-4 0 (mol r a t i o ) *W/F=41-43 (a-cat/mol-oxime/hr)
In order to identify precisely the role o f a l c o h o l s on lactam selectivity, we studied the effect between the selectivity and the methanol/oxime weight ratio. The results are shown in Fig. 1. Up to a methanol/oxime weight ratio of one, the selectivity to lactam increases gradually. And over the methanol/oxime weight ratio of one, the selectivity to lactam reaches a constant value of over SO%, but the conversion of oxime decreases.
Role of Alcohols in Beckmann Rearrangement
69
100
Oxirne c o n v . Lactarn select.
70
t
351
,
,
0
1
2
),
12
Methanol/Oxirne (weight r a t i o ) F i g . l . E f f e c t ofMethanol/Oxirne
Ratio
This result indicates that the interaction between methanol and the catalyst i s very important. In the range of the lower methanol/oxime weight ratio, the stronger the interaction between methanol and the zeolite, the greater the selectivity to lactam. On the other hand, in the range of the higher methanol/oxime weight ratio, strong adsorption o f methanol on the catalyst surface occurs, decreasing the conversion. 3.2. Modification of the Catalyst Surface by Alcohols
To completely understand the interaction between methanol and catalyst, the FT-IR spectra of catalysts treated with methanol were measured by a diffuse reflection type FT-IR. Fig. 2 shows the FT-IR spectra of the catalyst: one is treated with methanol and the other is not. Tke non-treated catalyst has a sharp IR absorption at 3740 cm- , which is attributed to the neutral silanol. In the case of the catalyst treated with methanol, ho ever, this peak decreases notably and a new peak at is clearly observed. This is attributed to the C-H 2970cm-' stretching of the methyl group. These data indicate that the neutral silanol is methylated by methanol, and we believe this phenomenon to be the main reason enhancing the selectivity to lactam (Scheme 1). ( S c h erne 1)
OH -S
I
I
i-
OCHs
+ CHsOH C - S
I i-
I
-I-H20
70
M. KITAMURA and H. ICHIHASHI
This result shows that the neutral silanol groups on the zeolite are not the active sites for converting oxime to lactam; on the contrary, it disturbs the smooth rearrangement reaction. The loss of catalytic activity is quick when the alcohols with a long alkyl chain are fed into the reaction system. It is suggested that the long alkyl chains, which modify the neutral silanol o f the catalyst, cover the active sites and block the smooth rearrangement reaction.
Non-Treated 0)
Treatedwithmethanol, 0
0
C
0 C
n
n
m
(II
L
L
0
0
In
u)
n
n U
U
I
3600 3 0 0 0 2 5 0 0
w a v e n u m b e r (cm-'1
w a v e n u m b e r (cm-')
Fig.2. FT-IRSpectra
of Zeolite
REFERENCES 1. H. Sato, N . Ishii, K. Hirose, and S. Nakamura, Proc. 7th Int. Zeolite Conference, 1986, 755 2 . H. Sato, K. Hirose, M. Kitamura, and Y. Nakamura, Proc. 8th Int. Zeolite Conference, 1989, 1213
Tetraethylorthosilicate, 1 0 % aqueous tetra-n-propylammonium hydroxide solution, and wat,er were charged in an autoclave. A hydrothermal reaction was conducted at l 0 5 O C for 96 hours. Whjte solids were filtered out, washed, and dried. The crystals were then calcined at 530°C. The ion-exchange treatment by 5% aqueous ammonium chloride solution was carried out at 60°C for 1 hour. This treatment was repeated four times. 2.2. Reaction Procedure The reaction was carried out using a fixed-bed type reactor at 350°C. atmospheric pressure. The reaction temperature was Cyclohexanone oxime was fed as a benzene solution. The reaction products were analyzed by G . C .
61
68
M. KITAMURA and H. ICHIHASHI
2.3. FT-IR Measurement A diffuse reflection type FT-IR was used. The catalyst was pretreated at 3 5 0 C for 1 hour in a helium atmosphere. The spectra were measured at 3 5 0 C in an atmosphere O F helium gas
containing methanol vapor. 3.RESLLTS AND DISCUSSION 3.1. Effects of Additives
Table 1 shows the ePfects of addltives on catalytic per-formance. When oxime was f e d with methanol, the selectivlty to lactam increased to more than 9 0 % . 'Pie same effect was observed when oxime was fed with other alcohols. The longer the length of the alkyl chain, however, the larger the decline of catalytic activity. The other solvents, for example, phenol, acetaldehyde, and acetic acid, did not increase the selectivity to lactam. In addition , another interesting point was discovered. The methanol used as an additive was recovered quantitatively. Generally speaking, methanol converts to hydrocarbons on a ZSM-5 type zeolite, but in this case, this reaction did not occur, A s shown in Table 2, when some kinds of other catalysts were used instead of highly silicious pentasil zeolite, the cFfect O F methanol to increase the selectivity to lactam was not observed. When borontrioxide was used as a catalyst, the reaction could not be continued, because the boron is lost from the catalstst as a methyl ester.
T a b l e 1.
E f f e c t
a d d I t i v e s
B e n z e n e CH,OH CZHSOH n-C,H,OH
o f
A d d i t i v e s
c o n v . I
1 1 9 9
0 0 0 0 9 0 6. 6
-
(%)
9 8 99. 97 9 2.
6 6
0 6
(1)
s e I 7 9 9 9
(%)
0 2 2 1
i so-C.H70H C.H.OH CHnCHO
9 9 0-94. 3 40. 5-2 1 4
9 1 7 9
99. 6-94.
2
7 9
CHsCOOH
1 0 0 -95.
2
7 3 - 6 3
* C a t a l y s t : P e n t a s i I z e o l i t e ( S i / A I = l 4 7 0 0 0 ) * O x i m e / S o l v e n t = 3 5-4.0 (mo I r a t i 0 ) *W/F=41-43 ( 1 - c a t / m o l - o x i m e / h r ) *1) 1 s t h o u r *2) 6 t h h o u r
Table2.Effectofadditives
catalyst
(2)
L a c t a m a e l . (%) ~B e n z e n e _Mathano1 ~
Pentaoil zeolite (S i /A I = 1 4 7 000) Silica-Alumina (A I2O. 1 3 w t W Silica-Alumina (AI,O, 2 8 w t W
70
92
66
50
52
60
Si I ica
29
30
Boron t r i o x i d o
90
-
*Oxime/Solvent=3.5-4 0 (mol r a t i o ) *W/F=41-43 (a-cat/mol-oxime/hr)
In order to identify precisely the role o f a l c o h o l s on lactam selectivity, we studied the effect between the selectivity and the methanol/oxime weight ratio. The results are shown in Fig. 1. Up to a methanol/oxime weight ratio of one, the selectivity to lactam increases gradually. And over the methanol/oxime weight ratio of one, the selectivity to lactam reaches a constant value of over SO%, but the conversion of oxime decreases.
Role of Alcohols in Beckmann Rearrangement
69
100
Oxirne c o n v . Lactarn select.
70
t
351
,
,
0
1
2
),
12
Methanol/Oxirne (weight r a t i o ) F i g . l . E f f e c t ofMethanol/Oxirne
Ratio
This result indicates that the interaction between methanol and the catalyst i s very important. In the range of the lower methanol/oxime weight ratio, the stronger the interaction between methanol and the zeolite, the greater the selectivity to lactam. On the other hand, in the range of the higher methanol/oxime weight ratio, strong adsorption o f methanol on the catalyst surface occurs, decreasing the conversion. 3.2. Modification of the Catalyst Surface by Alcohols
To completely understand the interaction between methanol and catalyst, the FT-IR spectra of catalysts treated with methanol were measured by a diffuse reflection type FT-IR. Fig. 2 shows the FT-IR spectra of the catalyst: one is treated with methanol and the other is not. Tke non-treated catalyst has a sharp IR absorption at 3740 cm- , which is attributed to the neutral silanol. In the case of the catalyst treated with methanol, ho ever, this peak decreases notably and a new peak at is clearly observed. This is attributed to the C-H 2970cm-' stretching of the methyl group. These data indicate that the neutral silanol is methylated by methanol, and we believe this phenomenon to be the main reason enhancing the selectivity to lactam (Scheme 1). ( S c h erne 1)
OH -S
I
I
i-
OCHs
+ CHsOH C - S
I i-
I
-I-H20
70
M. KITAMURA and H. ICHIHASHI
This result shows that the neutral silanol groups on the zeolite are not the active sites for converting oxime to lactam; on the contrary, it disturbs the smooth rearrangement reaction. The loss of catalytic activity is quick when the alcohols with a long alkyl chain are fed into the reaction system. It is suggested that the long alkyl chains, which modify the neutral silanol o f the catalyst, cover the active sites and block the smooth rearrangement reaction.
Non-Treated 0)
Treatedwithmethanol, 0
0
C
0 C
n
n
m
(II
L
L
0
0
In
u)
n
n U
U
I
3600 3 0 0 0 2 5 0 0
w a v e n u m b e r (cm-'1
w a v e n u m b e r (cm-')
Fig.2. FT-IRSpectra
of Zeolite
REFERENCES 1. H. Sato, N . Ishii, K. Hirose, and S. Nakamura, Proc. 7th Int. Zeolite Conference, 1986, 755 2 . H. Sato, K. Hirose, M. Kitamura, and Y. Nakamura, Proc. 8th Int. Zeolite Conference, 1989, 1213