Catalyst deactivation of high silica metallosilicates in beckmann rearrangement of cyclohexanone oxime

Catalyst deactivation of high silica metallosilicates in beckmann rearrangement of cyclohexanone oxime

Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved. 431...

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Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.

431

Catalyst deactivation of high silica metallosilicates in Beckmann rearrangement of cyclohexanone oxime Takashige Takahashi, Takami Kai and M.N.A.Nasution Department of Applied Chemistry & Chemical Engineering, Faculty of Engineering, Kagoshima University, Kagoshima 890 Japan The vapor phase Beckmann rearrangemem was carried out over high silica ZSM-5 type metallosilicates to elucidate the effect of acid strength on ~ -caprolactam selectivity and catalyst deactivation rate. It was found that the indiosilicate which had the lowest acid strength was the best catalyst among the metallosilicates. When carbon dioxide and methanol were used as diluent gas and diluent solvent of cyclohexanone oxime, respectively, the deactivation rate decreased over indiosilicate. Furthermore, when the indiosilicate was modified with a precious metal, the catalyst deactivation significantly decreased. It was considered that the oxidation of coke on the surface was accelerated by the precious metal. 1. INTRODUCTION Although the vapor phase Beckmann rearrangement of cyclohexanone oxime (CHO) is known to be an attractive process to prepare ~ -caprolactam (CL) as the starting compound of nylon-6, the industrial process was not achieved due to the low selectivity to CL and rapid catalyst deactivation of the solid catalysts. Recently, some investigations were carried out to improve the CL selectivity. Sato et al. reported that a high silica HZSM-5 zeolite (SiO2/A120 3 ratio = 3200) was effective for the rearrangement reaction and the selectivity was 90 % or more [ 1]. They also reported that when an alcohol was used as the diluent solvent of CHO, the selectivity and catalyst life were significantly improved [2]. Furthermore, when the CHO vapor diluted with carbon dioxide was fed into the catalyst layer, the selectivity to the CL was higher than that diluted with nitrogen [3]. It was reported that the catalyst life and the selectivity to CL also increased by use of boria supported HZSM-5 zeolite [4, 5], MEL type zeolite [6], femerite zeolite [7] and bona deposited on an alumina or silica by a CVD method [8, 9]. The CL selectivity gradually increased by use of the new type catalysts, but the catalyst deactivation was not overcome so far. Many questions on the reaction mechamsm of the Beckmann rearrangement over the zeolites and on the deactivation mechanism of the zeolites still remained.

432 In the presem study, the rearrangement has been camed out over a ZSM-5 type metallosilicates including iron, gallium or radium in the crystal lattice to elucidate the effect of acid strength on the catalyst deactivation rate. We have also examined the effects of diluent solvent and diluent gas of CHO on the deactivation rate and CL selectivity. Furthermore, the rearrangement was camed out over an mdiosilicate modified with precious metals to decrease the catalyst deactivation. 2. EXPERIMENTAL

A high silica ZSM-5 type metallosilicate with gallium, iron or indium in the crystal lattice was synthesized by modifying the method described in a previous paper [ 10]. In the present study, the ratio of silica/metal changed from 500 to 3200. After the powder of the silicates was compressed into a small die, it was crushed to 32 -~ 48 mesh. The surface area was measured by a nitrogen adsorption method, in which Langmuir equation was used for calculation. The acidity and acid strength distribution were measured using ammonia temperature programmed desorption method. The impregnation method was used for the precious metal modification of zeolites with silica/metal ratio = 500. The vapor phase reaction of CHO has been carried out at atmospheric pressure in a flow type system. The reactor assembly was essentially similar to that reported in a previous paper [4]. 3.RESULTS AND DISCUSSION It has been reported that the catalytic activity of zeolites decreased with time on stream due to the deposition of coke on the strong acid sites in the reaction of CHO. Figure 1 shows the relationship between CHO conversion and time on stream over Z5(Ga)-500H and Z5(Ga)-1000H. Z5(Ga)-500H means proton exchanged gallosilicate whose Si/Ga ratio is 500. Since similar relationships between CHO conversion and time on stream were obtained over the metallosilicates, the effect of time on stream on CHO conversion is represemed by Equation (1). x(t)---x(O) 9exp(-b 9t)

(1)

where x(t) and x(0) are CHO conversion at any time on stream and initial conversion, respectively, b is deactivation coefficient and t is time on stream. The deactivation coefficients of various proton exchanged metallosilicates with the same Si/M ratio (500) are shown in Table 1. This table also shows the strong acid concentration and the maximum temperature of strong acid sites of the silicates measured by ammonia TPD method. The maximum

433

1.0 0.8 0.6

~

17 O tn

!

0.4 -

k.,

Reaction temp. = 623 K WlFA0 = 113 kg.s/mot Diluent gas = H2 Diluent solvent = Benzene

c 0.2

O Z5(Ga)-1OOOH 9 Z5(Ga)- 500H

0 U

0.1 ~ 0

10

~

~ 20 30 40 50 Tim~ on stream [ m i n i

60

Figure 1. Catalyst deactivation of Z5(Ga)-500H and Z5(Ga)-1000H. Table 1 Strong acid concentration, deactivation factor(b) and maximum temperature of ammonia TPD Catalyst Z5(A1)500-H Z5(Fe)500-H Z5(Ga)500-H Z5(In)500-H

Strong acid conc. 1) [mmol/g] 0.058 0.046 0.044 0.039

Deact. factor [I/s] 2.7 x 10-4 2.4 x 10-4 1.8 • 10-4 4.0 • 10-5

Max. temp. [K] 758 749 733 706

1) Strong acid means the acid sites can retain ammonia at >680 K. Reaction temperature=623 K, Diluent gas=H 2, Diluent solvent=Benzene temperature of aluminosilicate is highest among the silicates. At the same time, the deactivation coefficiem is also the highest. On the other hand, the deactivation coefficient of the indiosilicate with the lowest maximum temperature is smallest among the silicates. This result indicates that the catalyst deactivation can decrease to control the acid strength of the strong acid sites m the metallosilicate. It was found that when the change of acid strength distribution of the used silicate was examined by ammonia TPD method, the strong acid concentration did not reduce as expected from the decrease in CHO conversion. Furthermore, the surface area of the used silicate was found to be almost the same as that of the fresh silicate. These results suggest that the coke on the surface should be removed on heating for 1 h at 773 K in a nitrogen stream or evacuating for 2 h at 473 K ~mder 1 torr. If the coke was removed from the

434

1.0 A

9

"

t

7_>_0.8 m 0.6 - Catalyst 9 Z 5 ( I n ) - 500H =Reaction temp. = 623 K O Diluent gas = H2 solvent = Benzene O.4 -Diluent W/FAo = 120 kg .s/mo[ O ~

(J

0.2. 0

I 1

I 2

~- Evacuated at 473 K f o r 2h CHO Conversion

~

CL Selectivity

I I I I 5(0) I 2 3 4 T i m e on st ream [ h ]

I 3

Figure 2. Effect of evacuation on recovery of catalytic activity. silicate surface, the catalytic activity would be recovered by the evacuatton. Figure 2 demonstrates the effect of the evacuation of the used silicates on the CHO conversion. The catalytic activity decreases with the time on stream as shown in Figure 2. The reaction was terminated after 5 h of time on stream. The silicate taken out from a reactor was evacuated for 2 h at 473 K under 1 torr. After the silicate was recharged in the reactor, the reaction was started under the same conditions. Although the deactivation rate of the treated silicate is higher than that of the ti'esh silicate,the catalytic activity is recovered as expected. On the other hand, when the used mordenite was treated by the same conditions,the activity did not increased. This result indicates that the carbonaceous deposit on the metallosilicate should be low molecular weight. Recently, Ichihashi et al.[2] reported that when the solvent of CHO changed from benzene to methanol or other alcohols,the CL selectivity and the catalyst life of the high silica aluminosilicate were simultaneously improved. They also reported that when the carrier gas changed from hydrogen to carbon dioxide, the CL selectivity was improved [3]. The reaction of CHO was carried om under the same diluent gas and solvent over Z5(In)500-H. Figure 3 demonstrates the relationship between CHO conversion and CL selectivity and time on stream. The lactam selectivity and catalyst life are significantly improved under the reaction conditions. These results suggest that the oxygen atom in alcohol or carbon dioxide would be importm~t for increasing the catalyst performances. If the coke is removed with oxygen on the silicate surface, much higher selectivity and longer catalyst life will be expected on Beckmann rearrangement over the mdiosilicate modified by platinum. Figure 4 shows the relationship between CHO conversion and CL selectivity on time on stream over the

435

1.0

>" 0.95

.,=,..

O CHO Conversion

>

.m,.

u

9 CL Selectivity

-~ 0.90 U3

E O L_

O U

Catalyst : Z 5 ( I n ) - 5 O O R ~ O"'%M~O.." Reaction temp.= 623 K 0.85 -Diluent gas = CO2 Diluent solvent = n - Prol)anot WlFA0 ~ 118 kgis/mo{ 1 0.80 0 2 4 6 8 T i m e on stream [ h ]

10

Figure 3. Relationship between CHO conversion and CL selectivity over Z5(In)-500H.

1.00

0 CliO Conversion

I

,

i....i

>, 0.98

I

CL Selectivity

> o ~ .

u 0-96 151 m

- 0.94 -

Reaction temp. = 623 K Diluent gas = CO2 _ Diluent solvent = n - Propanot ~- 0.92 > W/FAo = 118 kg.s/mot

E O

. ~

E O

u 0.90

0

I

I

2

4

I

6 Time on st ream

I 8

10

[hi

Figure 4. Relationship between CHO conversion and CL selectwity over PtZ5(In)-500H. PtZ5(In)-500H. The selectivity to CL was constant throughout time on stream of 10 h. The CHO conversion gradually decreases with time on stream, but the deactivation coefficient calculated from Figure 4 is 1.29 • 10"6s. This value lS about two orders of

436 magnitude less than the aluminosilicate shown in Table 1. When CHO rearrangement was carded out over ruthenium or palladium modified indiosilicate under the same reaction conditions as shown in Figure 4, the CHO conversion and the selectivity to CL were almost same as the results shown in Figure 4. These results indicate that the precious metals with oxidation activity are effective to reduce the coke deposited on the metallosilicate surface. It is well known that an alcohol is easily dehydrated over acid catalysts. In this reaction system, since the metallosilicates with strong acid sites were used as the catalysts, the dehydration of n-propanol used as the diluent would occur. However, an ether or an olefin which would be produced by the dehydration was not detected by a gas chromatograph in this study, because the production rate would be too small to be measured. When CHO mixed with a small amount of water was fed into the reactor the catalyst deactivation rate decreased up to 0.3 wt% of water. This result suggests that water produced from n-propanol should play an important role in decreasing catalyst deactivation. The pentasil type indiosilicate with high silica ratio modified with precious metals could be effective catalyst for the vapor phase Beckmann rearrangement of CHO. Furthermore, when n-propanol and carbon dioxide were used as the diluent solvent and diluent gas, respectively, the catalyst deactivation rate was significantly depressed. The further investigations are required to develop a high CL selective catalyst with long catalyst life. REFERENCES

1. 2.

H. Sato, K. Hirose and M. Kitamura, Nippon Kagaku Kaishi, 1989 (1989) 548. M. Kitamura and H. Ichihashi, Prep. Acid-Base Catalysis II, The Organizing Committee of International Symposium Acid-Base, Sapporo (1993) p. 217. 3. H. Sato, N. Ishii, K. Hirose and S. Nakamura, Proc. 7th Int. Zeolite Conf., Y. Murakami, A. Iijima and J.W. Ward eds., Elsevier, Amsterdam, 1986, p. 755. 4. T. Takahashi, K. Ueno and T. Kai, Can. J. Chem. Eng., 69 (1991) 1096. 5. T. Takahashi, M. Nishi, Y. Tagawa and T. Kai, Microporous Materials, 3 (1995) 467. 6. J.S. Reddy, R. Ravishankar, S. Sivasanker and P. Ratnasamy, Catal. Lett., 17 (1993) 139. 7. K. Miura, T. Komatsu, S. Namba and T. Yashima, Prep. 64th Autumn Meeting of Chem. Soc. Japan (1992) p. 477. 8. H. Sato, S. Hasebe, H. Sakurai, K. Urabe and Y. Izumi, Appl. Catal., 29 (1987) 107. 9. H. Sato, K. Urabe and Y. Izumi, J. Catal., 102 (1986) 99. 10. T. Takahashi and X.Y. Yun, Research Report of Faculty of Engineering, Kagoshima University No. 26 (1984) 119.