- Email: [email protected]
Intrinsic kinetic modeling of cyclohexanone ammoximation over titanium silicate molecular sieves
Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.
2661
INTRINSIC KINETIC M O D E L I N G OF C Y C L O H E X A N O N E A M M O X I M A T I O N O V E R TITANIUM SILICATE M O L E C U L A R SIEVES Li, Y.*, Wu, W., Min, E.Z., Sun, B. and Shu, X. Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China. *Corresponding author: E-mail: [email protected]
ABSTRACT An intrinsic kinetic of the cyclohexanone ammoximation in the liquid phase over titanium silicate molecular sieves is investigated in an isothermal slurry reactor at different initial reactant concentrations, catalyst loading, and reaction temperature. The rate equations are developed by analyzing data of kinetic measurement. More than 10 kinds of side reactions are found. H202 decomposition must be considered and others side reactions can be neglected in the kinetic modeling. The predicted values of reaction rates based on the kinetic models are almost consistent with experimental ones. The models have guidance to selection of reactor types and they are useful to the design and operation of reactor. Keywords: kinetics; Titanium silicalite; Cyclohexanone; Ammoximation; Cyclohexanone oxime
INTRODUCTION Cyclohexanone oxime is the key intermediate in the manufacture of e-caprolactam. Conventional product routes involve numerous steps and the use of hazardous chemicals like oleum, halides, and oxides of nitrogen. In addition, large quantities of the low value by-product ammonium sulfate and considerable waste are also produced. An alternative method of cyclohexanone oxime synthesis is the reaction of cyclohexanone with ammonia and hydrogen peroxide in the liquid phase over titanium silicate molecular sieves[l]. Since the main byproduct of the reaction is water and the reaction takes place under mild condition, it is an environmental friendly, clear and efficient process technology. Since the discovery of the catalytic effect of titanium silicate in the process[2], the literature has mostly deals with either the identification and characterization of active sites or the experimental determination of conversion and selectivity data under specific conditions[36]. However, only a limited number of studies have focused so far on the mechanism and the kinetics of this reaction over titanium silicate sieves and using diluted hydrogen peroxide[7,8]. The kinetic study would nevertheless be necessary for the rational choice of industrial operating conditions and the design of an optimized large-scale catalytic reactor. This reaction is actually conducted in a multiphase reaction medium with a complex parallel-consecutive reaction scheme. The objectives of the present study are to examine the reaction framework of cyclohexanone ammoximation by a more detailed kinetic analysis. More complete kinetic models including hydrogen peroxide decomposition are also developed. EXPERIMENTAL
Materials The titanium silicate catalysts used in this study were synthesized hydrothermally. The gel was prepared from an appropriate mixture of tetraethylorthosilicate (TEOS), tetrabutylorthotitanate (TBOT), and sodium-free tetrapropyl ammonium hydroxide as organic template. The hydrothermal crystallization was performed in a stainless steel kettle at higher temperature. The solid was then filtered, washed, dried and calcined at 823K[9]. SEM observation revealed that these catalyst samples were spherical particle in the range of 0.1-0.3ktm. The specific interfacial area of the catalyst and its porous volume are 429m2/g and 0.497ml/g, respectively. The materials used in this study were cyclohexanone, hydrogen peroxide (27.5wt% hydrogen peroxide in water), ammonia, t-butyl alcohol, and they were obtained from commercial suppliers (China).
2662
Apparatus and procedures The powder titanium silicate was used in this study. A certain amount of catalyst was weighted and pretreated by leaving it in an oven at 373K overnight to remove moisture from the catalyst. A desire amount of cyclohexanone and H202 was placed into a slurry reactor consisting 250ml three-necked flask fitted with a condenser in the central opening. 1-t202 was used as oxidant and t-butyl alcohol as the solvent. The mixture was stirred and heated up to a desired temperature by circulating hot water through the jacket. The reaction was started by adding the catalyst and ammonia into the reaction mixture. Liquid samples of 3ml were taken to measured concentrations of cyclohexanone, cyclohexanone oxime, t-butyl alcohol, H20, H202, and ammonia at different reaction time. The reactions were carried out at different temperatures and molar ratios of NH3/C6H100 and H202/C6H100 under atmospheric pressure. The concentrations of titanium silicate sieves in liquid were maintained in 0.5-~1.0wt%. For every kinetic run, the temperature, with a precision of +0.5~ was kept constant. It is noted that the liquid samples taken from the reactor were small compared to the total liquid volume. The reaction products of cyclohexanone ammoximation were analyzed using a gas chromatograph (CE8000) equipped with a capillary column (OV-I) and a flame ionization detector (FID). In view of the large differences in the response factors for the different components of the products, standard calibration mixtures were used to estimate their response factors accurately. The H202 concentration in reaction mixtures was determined by iodometric titration. Using acid alkali titration obtained the ammonia concentration. In this study the initial rates method was used for kinetic analysis. They were obtained by fitting the concentration/time data by Marquardt technique. The gauss-Newton method was used to estimate model parameters by fitting the kinetic data.
Experimental conditions In order to fulfill the requirements for cyclohexanone ammoximation scale-up, appropriate kinetic experimental conditions should be chosen. The reaction pressure is to maintain the reaction mixture at the liquid phase. The solvent of t-butyl alcohol is used to keep one liquid phase in this system. The effects of the two operating parameter on reaction results are small. However, temperature, the mole ratios of H202/C6H1oO and NH3/C6HI00 must be considered. The scheme of four levels and three factors were used in this study. Table 1 gives the experimental conditions. The orthogonal method was used to arrange these experimental points, and L32(49) as the orthogonal list. Table 1. Factors and levels. temperature
(K)
H202/C6H!o0
NH3/C6H100
levels
1 2 3 4
323.2 333.2 343.2 353.2
(mol/mol) 0.9 1.0 1.1 1.2
(moi/mol) 1.0 1.5 2.0 2.5
REACTION FRAMEWORK
ANALYSIS
The ammoximation of cyclohexanone was found to proceed with good cyclohexanone conversion and selectivity to oxime over the titanium silicate sieves. The major product of the reactions is cyclohexanone oxime, while some by-products are also produced. The identification of the various compounds was carried out by matching gas chromatograph retention times with pure compounds and also by isolating and matching the IR and mass spectra with those of the known compound. We have found that more than ten trace by-products appeared in the reaction product[l~ It is also noted from the analysis of the tail gas that ammonia oxidation and H202 decomposition occurred. Therefore, in addition to the ammoximation of cyclohexanone main reaction, homogenous or heterogeneous parallel and consecutive side reactions exist in the system. Most organic side reactions are non-catalytic homogenous reactions. The results of quantitative analysis show that only 1-t202 decomposition side reaction need to be considered in the kinetic models.
2663 The ammoximation of cyclohexanone .TS
~O+
--
NH3 + H202
NOH + 2H20
(1) H202 decomposition 2 H202
-- 2 H 2 0 + 02
(2)
Side reactions also include cyclohexanone oxime oxidation, ammonia oxidation, cyclohexanone oxime hydrolysis, hydroxylamine oxidation, the condensation of cyclohexanone, the formation of peroxydicyclohexylamine and cyclohexanone azine, and the reaction of cyclohexanone with t-butyl alcohol and ammonia. These side reactions may be expressed by the following equations. ==NOH
+ H202
~~--NO
2 + H20
(3)
~
=NOH
+ H202
-- ~ : = N O H
+ 2 H20 (4)
2 ~ ~ = N O H + H202
--2 ~ = O +
N20 +2 H20
4 H202 + 2 NH 3
-- N20 + 7 H20
3 H20 2 + 2 NH 3
-- N2 + 6 H20
~
=NOH+ H20
(6) (7)
-- ~ - ~ / = O + NH2OH X.....__/
2 NH2OH + 2 H202
(5)
(8)
--N20 + 5 H20
(9)
o
2.
.
~
or
+ H20 (10)
2~ ~ = O +
2C~=O
NH3 +H202
+2 NH3 + 5H202
-- C
~
~
+ 2H20
-- O = = ~ , = N - - N ~ C ~ O
(11)
+ 10 H20 (12)
C~O
+ (CH3)3COH + NH3
~
~-
NH 2
+
O--C(CH3)3
H20 (13)
The titanium silicate sieves can irreversible adsorb some by-products of the reaction resulting in blockage of active sites, which is one of the most important reasons causing the deactivation of titanium silicate catalyst[l~ The analysis of the reaction framework is useful to understanding the deactivation mechanism of the catalyst and increasing the catalyst stability.
2664 RESULTS AND DISCUSSION
Effect of internal diffusion resistance The powder titanium silicate molecular sieve, with the average particle size about 0.2gm, was used directly as catalyst in this study. It is difficult to vary the particle size of this catalyst for determining internal diffusion resistance. However, the other method, which can be used to estimate the effect of internal diffusion resistance in a single particle size, is called Weize-prater criterion. That is
l l.Z= (rA)~
<< I_I_
(14)
I'I
D CA s
where n is the total order of reaction, ~ is modified Thiele modulus, (r)obs is observed rates of reaction, De is effective diffusivity for transport in a pellet, CAs is molar concentration of reacting component at the surface of solid and L is the particle diameter of the titanium silicate molecular sieve. When Eq. (14) can be contented, the effect of internal diffusion resistance in pellets on the reaction rate may be ignored. In this reaction system, the magnitude order of the apparent reaction rate, particle size, effective diffusivity and the concentration on particle surface are 10-1-10 "3, 10 5, 10-4--~10"5, 10-2--~100, respectively, then O<
Effect of external mass transfer resistance The effect of external mass transfer of catalyst was studied by varying stirring speeds. Figure 1 shows the relationship between the conversion of cyclohexanone and the stirring speed. It is found that the conversion increase with increasing speed and, finally, it has almost no increase at the speed of more than 700 rpm. Hence, the effect of external mass transfer was negligible at the stirring speed higher than 700 rpm. In all kinetics runs, a speed of 900rpm was used to ensure no external mass transfer limitations.
SLit $ p e ~ d t r p m
051
~=~ cO
--~"-
0~
!
500 700
r-
r
8 o co
100
- - e - . . - ~100
900
O~
o
~.)
o2
OD,, 0
i 20
.
i
,
"0
i 60
, 80
R e a c t i o n t i m 9 1 rni n
Figure 1. Cyclohexanone convesion on reaction time at diffenent stirrer speed HzOz/C6HloO (mol)=l.1, NH3/C6HjoO(mol)=2.0,T = 353K, Ccat=l.0%wt.
Initial rate determination The kinetic experiments can obtain a series of reactant and product concentrations vs. time data at different initial reaction conditions. Designating XA and XB as the cyclohexanone conversion and the total conversion of H202, respectively, an efficient way to estimate the initial rates of cyclohexanone ammoximation and H202 decomposition are first to fit the time evolution of the cyclohexanone and H202 conversion by an empirical equation X = L[1 - e x p ( - ~ t ) ]
(15)
2665 The values of the empirical parameters )~ and ~ are obtained by the Marquardt technique for the minimization of the following objective function M
= ~ w j ( X E - X c)2
(16)
j=l
where wj is a weighting factor, M is the total number of experimental points, XE and Xc are the experimental values and calculated values using the modeling, respectively. The calculated initial reaction rates of cyclohexanone ammoximation and 1-1202 decomposition are obtained by the derivative of Eq. (15) at t = 0, that is: (dX / dt)l,__0 = ~ . Thus, the initial rate is No r0 = ~ ' ) W
(17)
W cat
Where r0 is the initial reaction rate (mol'g-l-min-1), No is the initial amount of a reactant (mol) and Wearis the amount of catalyst loading (g),)~ and W are the empirical parameters in Eq. (15). The initial unselective reaction rate of H202 is therefore obtained by the following equation rB2 0 --" rB0 - - rA0
(18)
where rBo is the total initial reaction rate of H202, and rAo are the initial rates of cyclohexanone ammoximation. On the basis of the above reaction framework analysis, the difference between the initial unselective reaction rate of H202 and the initial decomposition rate of H202 is negligible.
Kinetic models The main objective of the present work is to study the intrinsic kinetics of the catalytic liquid-phase ammoximation of cyclohexanone with H202 and ammonia over the catalyst. Based on the analysis of the reaction framework above, the reaction equations (1) and (2) will be considered in this kinetic study. Both the conversion of cyclohexanone and the selectivity to oxime are dependent on the reaction temperature, H202/C6H100 molar ratio, NHa/C6H100 molar ratio, and catalyst concentration. Hence, the effects of these four reaction parameters on the initial rate of ammoximation and H202 decomposition are discussed. The rates of cyclohexanone ammoximation and H202 decomposition may be expressed by the power law kinetic model, respectively. E A
rA = k Aoexp(- ~-~--)C ~.C ~ C E
B2
v
rB2 - k B:Oexp(- -Z-~ )C B
(19)
(20)
K I
where R is gas constant (J-moll.K-~), k0 is the preexponential factor, T is reaction temperature (K), E is the activation energy (J.mol-1), C is reactant concentration (mol/L), ct, 13, 7 and v are reaction order, Subscript A, B, and C denotes cyclohexanone, hydrogen peroxide and ammonia, respectively.
Evaluation of kinetic parameters The initial rates of cyclohexanone ammoximation and H202 decomposition at different operating conditions are presented in Table 2. Based on thirty-four data sets of the initial reaction rate in table 2, the rate parameters in Eqs. (19) and (20) are evaluated by the gauss-Newton method. The objective function of the parameter estimation is as follows M
S = ~--'(rE -- rc) z j=l
where rE is experimental values and rc is predicted values based on the models.
(21)
2666
The results o f estimated kinetic parameters o f Eqs. (19) and (20) are s u m m a r i z e d in Table 3. Table 2. Kinetics data of Cyclohexanone ammoximation and H202 decomposition over titanium silicate catalyst. Exp. No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
C C6HloO L-1
Tempture /K 323.2 323.2 323.2 323.2 323.2 323.2 323.2 323.2 333.2 333.2 333.2 333.2 333.2 333.2 333.2 333.2 343.2 343.2 343.2 343.2 343.2 343.2 343.2 343.2 353.2 353.2 353.2 353.2 353.2 353.2 353.2 353.2 343.2 353.2
C
/mol"
'
0.536 0.428 0.342 0.514 0.615 0.269 0.353 0.411 0.424 0.507 0.338 0.623 0.556 0.402 0.327 0.298 0.420 0.503 0.335 0.667 0.287 0.456 0.412 0.314 0.331 0.414 0.497 0.562 0.359 0.427 0.654 0.782 0.552 0.453
H202 /mol'L -I 0.482 0.386 0.411 0.615 0.671 0.301 0.354 0.413 0.380 0.453 0.409 0.750 0.609 0.449 0.330 0.301 ~
C
.
'
' 0.557 0.333 0.658 0.263 0.405 0.489 0.369 0.360 0.449 0.493 0.565 0.436 0.512 0.586 0.705 0.606 0.458
NH3 /molL l 0.538 0.645 0.515 0.512 1.228 0.674 0.882 0.821 0.845 1.262 0.835 1.249 0.551 0.598 0.494 0.295 0.423 0.749 0.507 0.671 0.578 1.135 1.039 0.621 0.656 1.028 1.241 1.130 0.358 0.641 0.981 0.779 0.843 0.896
.
i t i I
rA0 /xl04mol.g-l.min -I 6. 658 5. 435 4. 746 5.875 12.50 3.949 5.812 5.823 20. 06 26.89 15.61 35.14 20.06 16.71 11.08 9.229 35.32 64.36 29.69 74.19 33.23 65.69 51.31 32.44 94.38 164.3 177.1 196.6 83.50 117.4 228.1 218.8 72.97 137.6
rB20
,
/• 104 mol.g-t.min -1 4.082 2.998 2.937 5.171 6.103 1.840 2.589 2.988 7. 504 9.224 7.556 18.99 13.34 8.355 5.518 5.166 19.54 28.41 I2.71 40. 72 I0.06 20.35 22.90 14.93 30.52 41.99 56.54 68.16 42.16 51.54 75.51 I10.7 31.71 45.49
Table 3. Estimated kinetic parameters. Eq. 19 20
Preexponential factor kAo=4.351 • kB2o=2.286• 101~
Activation energy EA=9.551 • 104 Et32=8.212• 104
C~=0.76
reaction order 13=0.19 V w 1.55
~'= 0.45
The results o f comparing the experimental and predicated rates data are shown in Fig. 2. These data clearly show a good agreement between the model predictions and the observed results. The average relative errors between predicted values and experimental values for Eqs.(19) and (20) are 5.21% and 6.25%, respectively.
2667 120
:;~.0
o/o/
290
100
160
;= '~
o
120
o
L_ L. "0
= 0
aO
~10
120
16
i 200
0
, 2~0
0
20.
.
.
AO .
r~cxlO 4
go
r~10
.
~;
.
100 ,
.
120
4
Figure 2. Predicted vs. experimental of reaction rates. CONCLUSION The intrinsic kinetics of the cyclohexanone ammoximation in the liquid phase over titanium silicate molecular sieves has been studied in a slurry reactor. On the basis of these experimental data under the reaction conditions investigated, the power law rate equations have been developed and the kinetic parameters are evaluated by simulation the initial rate data. The reactions are between zero and unity order with respect to reactant concentrations. The rate equations show that cyclohexanone ammoximation is no more sensitive than H202 decomposition with respect to H202. The activation energy is found to be 95kJ/mol and 82kJ/mol for the ammoximation of cyclohexanone and H202 decomposition, respectively. These data clearly show a good agreement between the model prediction and experimental data under the present operating conditions also. The kinetic models are useful to the design and operation of reactor. ACKNOWLEDGEMENT The authors acknowledge the financial support provided by the Major State Basic Research Development Program of China REFERENCES 1. Thangaraj, A., Sivasankers, S., Ratnasamy, P., J. Catal., 131 (1991), 394-400. 2. Taramosso, M., Perego, G., Notari, B., US Patent 4 410 501.1983 3. Roffia, P., Padovan, M., Leofanti, G. Mantegazza, M. A., De Alberti, G., Tauszik, G. R.,US 4 794 198. 1988. 4. Roffia, P., Paparatto, G., Cesana, A., Tauszik, G., EP 0 301 486. 1989. 5. Bars, J. Le, Dakka, J., Sheldon. R. A., Appl Catal, 136 (1996), 69-80. 6. Perego, G., Bellussi, G., Corno, C., Stud SurfSci Catal, 28 (1986), 129-136. 7. Kul'kova, N. V., Kotova, V. G., Kvyatkovskaya M. Yu., Murzin, D. Yu, Kinetics and Catalysis,20 (1997), 43-46. 8. Murzin, D. Yu, Kul'kova, N. V., Kotova, V. G., Kvyatkovskaya M. Yu. Kinetics and Catalysis,39 (1998), 763-766. 9. Lin, M., Shu, X., Wang, X., Petroleum Processing and petrochemicals, 30(1999), 1-4. 10. Liu, Y., Li, Y., Wu, W., Min, Enz., Petroleum Processing and petrochemicals, 33(2002), 41-45.
2661
INTRINSIC KINETIC M O D E L I N G OF C Y C L O H E X A N O N E A M M O X I M A T I O N O V E R TITANIUM SILICATE M O L E C U L A R SIEVES Li, Y.*, Wu, W., Min, E.Z., Sun, B. and Shu, X. Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China. *Corresponding author: E-mail: [email protected]
ABSTRACT An intrinsic kinetic of the cyclohexanone ammoximation in the liquid phase over titanium silicate molecular sieves is investigated in an isothermal slurry reactor at different initial reactant concentrations, catalyst loading, and reaction temperature. The rate equations are developed by analyzing data of kinetic measurement. More than 10 kinds of side reactions are found. H202 decomposition must be considered and others side reactions can be neglected in the kinetic modeling. The predicted values of reaction rates based on the kinetic models are almost consistent with experimental ones. The models have guidance to selection of reactor types and they are useful to the design and operation of reactor. Keywords: kinetics; Titanium silicalite; Cyclohexanone; Ammoximation; Cyclohexanone oxime
INTRODUCTION Cyclohexanone oxime is the key intermediate in the manufacture of e-caprolactam. Conventional product routes involve numerous steps and the use of hazardous chemicals like oleum, halides, and oxides of nitrogen. In addition, large quantities of the low value by-product ammonium sulfate and considerable waste are also produced. An alternative method of cyclohexanone oxime synthesis is the reaction of cyclohexanone with ammonia and hydrogen peroxide in the liquid phase over titanium silicate molecular sieves[l]. Since the main byproduct of the reaction is water and the reaction takes place under mild condition, it is an environmental friendly, clear and efficient process technology. Since the discovery of the catalytic effect of titanium silicate in the process[2], the literature has mostly deals with either the identification and characterization of active sites or the experimental determination of conversion and selectivity data under specific conditions[36]. However, only a limited number of studies have focused so far on the mechanism and the kinetics of this reaction over titanium silicate sieves and using diluted hydrogen peroxide[7,8]. The kinetic study would nevertheless be necessary for the rational choice of industrial operating conditions and the design of an optimized large-scale catalytic reactor. This reaction is actually conducted in a multiphase reaction medium with a complex parallel-consecutive reaction scheme. The objectives of the present study are to examine the reaction framework of cyclohexanone ammoximation by a more detailed kinetic analysis. More complete kinetic models including hydrogen peroxide decomposition are also developed. EXPERIMENTAL
Materials The titanium silicate catalysts used in this study were synthesized hydrothermally. The gel was prepared from an appropriate mixture of tetraethylorthosilicate (TEOS), tetrabutylorthotitanate (TBOT), and sodium-free tetrapropyl ammonium hydroxide as organic template. The hydrothermal crystallization was performed in a stainless steel kettle at higher temperature. The solid was then filtered, washed, dried and calcined at 823K[9]. SEM observation revealed that these catalyst samples were spherical particle in the range of 0.1-0.3ktm. The specific interfacial area of the catalyst and its porous volume are 429m2/g and 0.497ml/g, respectively. The materials used in this study were cyclohexanone, hydrogen peroxide (27.5wt% hydrogen peroxide in water), ammonia, t-butyl alcohol, and they were obtained from commercial suppliers (China).
2662
Apparatus and procedures The powder titanium silicate was used in this study. A certain amount of catalyst was weighted and pretreated by leaving it in an oven at 373K overnight to remove moisture from the catalyst. A desire amount of cyclohexanone and H202 was placed into a slurry reactor consisting 250ml three-necked flask fitted with a condenser in the central opening. 1-t202 was used as oxidant and t-butyl alcohol as the solvent. The mixture was stirred and heated up to a desired temperature by circulating hot water through the jacket. The reaction was started by adding the catalyst and ammonia into the reaction mixture. Liquid samples of 3ml were taken to measured concentrations of cyclohexanone, cyclohexanone oxime, t-butyl alcohol, H20, H202, and ammonia at different reaction time. The reactions were carried out at different temperatures and molar ratios of NH3/C6H100 and H202/C6H100 under atmospheric pressure. The concentrations of titanium silicate sieves in liquid were maintained in 0.5-~1.0wt%. For every kinetic run, the temperature, with a precision of +0.5~ was kept constant. It is noted that the liquid samples taken from the reactor were small compared to the total liquid volume. The reaction products of cyclohexanone ammoximation were analyzed using a gas chromatograph (CE8000) equipped with a capillary column (OV-I) and a flame ionization detector (FID). In view of the large differences in the response factors for the different components of the products, standard calibration mixtures were used to estimate their response factors accurately. The H202 concentration in reaction mixtures was determined by iodometric titration. Using acid alkali titration obtained the ammonia concentration. In this study the initial rates method was used for kinetic analysis. They were obtained by fitting the concentration/time data by Marquardt technique. The gauss-Newton method was used to estimate model parameters by fitting the kinetic data.
Experimental conditions In order to fulfill the requirements for cyclohexanone ammoximation scale-up, appropriate kinetic experimental conditions should be chosen. The reaction pressure is to maintain the reaction mixture at the liquid phase. The solvent of t-butyl alcohol is used to keep one liquid phase in this system. The effects of the two operating parameter on reaction results are small. However, temperature, the mole ratios of H202/C6H1oO and NH3/C6HI00 must be considered. The scheme of four levels and three factors were used in this study. Table 1 gives the experimental conditions. The orthogonal method was used to arrange these experimental points, and L32(49) as the orthogonal list. Table 1. Factors and levels. temperature
(K)
H202/C6H!o0
NH3/C6H100
levels
1 2 3 4
323.2 333.2 343.2 353.2
(mol/mol) 0.9 1.0 1.1 1.2
(moi/mol) 1.0 1.5 2.0 2.5
REACTION FRAMEWORK
ANALYSIS
The ammoximation of cyclohexanone was found to proceed with good cyclohexanone conversion and selectivity to oxime over the titanium silicate sieves. The major product of the reactions is cyclohexanone oxime, while some by-products are also produced. The identification of the various compounds was carried out by matching gas chromatograph retention times with pure compounds and also by isolating and matching the IR and mass spectra with those of the known compound. We have found that more than ten trace by-products appeared in the reaction product[l~ It is also noted from the analysis of the tail gas that ammonia oxidation and H202 decomposition occurred. Therefore, in addition to the ammoximation of cyclohexanone main reaction, homogenous or heterogeneous parallel and consecutive side reactions exist in the system. Most organic side reactions are non-catalytic homogenous reactions. The results of quantitative analysis show that only 1-t202 decomposition side reaction need to be considered in the kinetic models.
2663 The ammoximation of cyclohexanone .TS
~O+
--
NH3 + H202
NOH + 2H20
(1) H202 decomposition 2 H202
-- 2 H 2 0 + 02
(2)
Side reactions also include cyclohexanone oxime oxidation, ammonia oxidation, cyclohexanone oxime hydrolysis, hydroxylamine oxidation, the condensation of cyclohexanone, the formation of peroxydicyclohexylamine and cyclohexanone azine, and the reaction of cyclohexanone with t-butyl alcohol and ammonia. These side reactions may be expressed by the following equations. ==NOH
+ H202
~~--NO
2 + H20
(3)
~
=NOH
+ H202
-- ~ : = N O H
+ 2 H20 (4)
2 ~ ~ = N O H + H202
--2 ~ = O +
N20 +2 H20
4 H202 + 2 NH 3
-- N20 + 7 H20
3 H20 2 + 2 NH 3
-- N2 + 6 H20
~
=NOH+ H20
(6) (7)
-- ~ - ~ / = O + NH2OH X.....__/
2 NH2OH + 2 H202
(5)
(8)
--N20 + 5 H20
(9)
o
2.
.
~
or
+ H20 (10)
2~ ~ = O +
2C~=O
NH3 +H202
+2 NH3 + 5H202
-- C
~
~
+ 2H20
-- O = = ~ , = N - - N ~ C ~ O
(11)
+ 10 H20 (12)
C~O
+ (CH3)3COH + NH3
~
~-
NH 2
+
O--C(CH3)3
H20 (13)
The titanium silicate sieves can irreversible adsorb some by-products of the reaction resulting in blockage of active sites, which is one of the most important reasons causing the deactivation of titanium silicate catalyst[l~ The analysis of the reaction framework is useful to understanding the deactivation mechanism of the catalyst and increasing the catalyst stability.
2664 RESULTS AND DISCUSSION
Effect of internal diffusion resistance The powder titanium silicate molecular sieve, with the average particle size about 0.2gm, was used directly as catalyst in this study. It is difficult to vary the particle size of this catalyst for determining internal diffusion resistance. However, the other method, which can be used to estimate the effect of internal diffusion resistance in a single particle size, is called Weize-prater criterion. That is
l l.Z= (rA)~
<< I_I_
(14)
I'I
D CA s
where n is the total order of reaction, ~ is modified Thiele modulus, (r)obs is observed rates of reaction, De is effective diffusivity for transport in a pellet, CAs is molar concentration of reacting component at the surface of solid and L is the particle diameter of the titanium silicate molecular sieve. When Eq. (14) can be contented, the effect of internal diffusion resistance in pellets on the reaction rate may be ignored. In this reaction system, the magnitude order of the apparent reaction rate, particle size, effective diffusivity and the concentration on particle surface are 10-1-10 "3, 10 5, 10-4--~10"5, 10-2--~100, respectively, then O<
Effect of external mass transfer resistance The effect of external mass transfer of catalyst was studied by varying stirring speeds. Figure 1 shows the relationship between the conversion of cyclohexanone and the stirring speed. It is found that the conversion increase with increasing speed and, finally, it has almost no increase at the speed of more than 700 rpm. Hence, the effect of external mass transfer was negligible at the stirring speed higher than 700 rpm. In all kinetics runs, a speed of 900rpm was used to ensure no external mass transfer limitations.
SLit $ p e ~ d t r p m
051
~=~ cO
--~"-
0~
!
500 700
r-
r
8 o co
100
- - e - . . - ~100
900
O~
o
~.)
o2
OD,, 0
i 20
.
i
,
"0
i 60
, 80
R e a c t i o n t i m 9 1 rni n
Figure 1. Cyclohexanone convesion on reaction time at diffenent stirrer speed HzOz/C6HloO (mol)=l.1, NH3/C6HjoO(mol)=2.0,T = 353K, Ccat=l.0%wt.
Initial rate determination The kinetic experiments can obtain a series of reactant and product concentrations vs. time data at different initial reaction conditions. Designating XA and XB as the cyclohexanone conversion and the total conversion of H202, respectively, an efficient way to estimate the initial rates of cyclohexanone ammoximation and H202 decomposition are first to fit the time evolution of the cyclohexanone and H202 conversion by an empirical equation X = L[1 - e x p ( - ~ t ) ]
(15)
2665 The values of the empirical parameters )~ and ~ are obtained by the Marquardt technique for the minimization of the following objective function M
= ~ w j ( X E - X c)2
(16)
j=l
where wj is a weighting factor, M is the total number of experimental points, XE and Xc are the experimental values and calculated values using the modeling, respectively. The calculated initial reaction rates of cyclohexanone ammoximation and 1-1202 decomposition are obtained by the derivative of Eq. (15) at t = 0, that is: (dX / dt)l,__0 = ~ . Thus, the initial rate is No r0 = ~ ' ) W
(17)
W cat
Where r0 is the initial reaction rate (mol'g-l-min-1), No is the initial amount of a reactant (mol) and Wearis the amount of catalyst loading (g),)~ and W are the empirical parameters in Eq. (15). The initial unselective reaction rate of H202 is therefore obtained by the following equation rB2 0 --" rB0 - - rA0
(18)
where rBo is the total initial reaction rate of H202, and rAo are the initial rates of cyclohexanone ammoximation. On the basis of the above reaction framework analysis, the difference between the initial unselective reaction rate of H202 and the initial decomposition rate of H202 is negligible.
Kinetic models The main objective of the present work is to study the intrinsic kinetics of the catalytic liquid-phase ammoximation of cyclohexanone with H202 and ammonia over the catalyst. Based on the analysis of the reaction framework above, the reaction equations (1) and (2) will be considered in this kinetic study. Both the conversion of cyclohexanone and the selectivity to oxime are dependent on the reaction temperature, H202/C6H100 molar ratio, NHa/C6H100 molar ratio, and catalyst concentration. Hence, the effects of these four reaction parameters on the initial rate of ammoximation and H202 decomposition are discussed. The rates of cyclohexanone ammoximation and H202 decomposition may be expressed by the power law kinetic model, respectively. E A
rA = k Aoexp(- ~-~--)C ~.C ~ C E
B2
v
rB2 - k B:Oexp(- -Z-~ )C B
(19)
(20)
K I
where R is gas constant (J-moll.K-~), k0 is the preexponential factor, T is reaction temperature (K), E is the activation energy (J.mol-1), C is reactant concentration (mol/L), ct, 13, 7 and v are reaction order, Subscript A, B, and C denotes cyclohexanone, hydrogen peroxide and ammonia, respectively.
Evaluation of kinetic parameters The initial rates of cyclohexanone ammoximation and H202 decomposition at different operating conditions are presented in Table 2. Based on thirty-four data sets of the initial reaction rate in table 2, the rate parameters in Eqs. (19) and (20) are evaluated by the gauss-Newton method. The objective function of the parameter estimation is as follows M
S = ~--'(rE -- rc) z j=l
where rE is experimental values and rc is predicted values based on the models.
(21)
2666
The results o f estimated kinetic parameters o f Eqs. (19) and (20) are s u m m a r i z e d in Table 3. Table 2. Kinetics data of Cyclohexanone ammoximation and H202 decomposition over titanium silicate catalyst. Exp. No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
C C6HloO L-1
Tempture /K 323.2 323.2 323.2 323.2 323.2 323.2 323.2 323.2 333.2 333.2 333.2 333.2 333.2 333.2 333.2 333.2 343.2 343.2 343.2 343.2 343.2 343.2 343.2 343.2 353.2 353.2 353.2 353.2 353.2 353.2 353.2 353.2 343.2 353.2
C
/mol"
'
0.536 0.428 0.342 0.514 0.615 0.269 0.353 0.411 0.424 0.507 0.338 0.623 0.556 0.402 0.327 0.298 0.420 0.503 0.335 0.667 0.287 0.456 0.412 0.314 0.331 0.414 0.497 0.562 0.359 0.427 0.654 0.782 0.552 0.453
H202 /mol'L -I 0.482 0.386 0.411 0.615 0.671 0.301 0.354 0.413 0.380 0.453 0.409 0.750 0.609 0.449 0.330 0.301 ~
C
.
'
' 0.557 0.333 0.658 0.263 0.405 0.489 0.369 0.360 0.449 0.493 0.565 0.436 0.512 0.586 0.705 0.606 0.458
NH3 /molL l 0.538 0.645 0.515 0.512 1.228 0.674 0.882 0.821 0.845 1.262 0.835 1.249 0.551 0.598 0.494 0.295 0.423 0.749 0.507 0.671 0.578 1.135 1.039 0.621 0.656 1.028 1.241 1.130 0.358 0.641 0.981 0.779 0.843 0.896
.
i t i I
rA0 /xl04mol.g-l.min -I 6. 658 5. 435 4. 746 5.875 12.50 3.949 5.812 5.823 20. 06 26.89 15.61 35.14 20.06 16.71 11.08 9.229 35.32 64.36 29.69 74.19 33.23 65.69 51.31 32.44 94.38 164.3 177.1 196.6 83.50 117.4 228.1 218.8 72.97 137.6
rB20
,
/• 104 mol.g-t.min -1 4.082 2.998 2.937 5.171 6.103 1.840 2.589 2.988 7. 504 9.224 7.556 18.99 13.34 8.355 5.518 5.166 19.54 28.41 I2.71 40. 72 I0.06 20.35 22.90 14.93 30.52 41.99 56.54 68.16 42.16 51.54 75.51 I10.7 31.71 45.49
Table 3. Estimated kinetic parameters. Eq. 19 20
Preexponential factor kAo=4.351 • kB2o=2.286• 101~
Activation energy EA=9.551 • 104 Et32=8.212• 104
C~=0.76
reaction order 13=0.19 V w 1.55
~'= 0.45
The results o f comparing the experimental and predicated rates data are shown in Fig. 2. These data clearly show a good agreement between the model predictions and the observed results. The average relative errors between predicted values and experimental values for Eqs.(19) and (20) are 5.21% and 6.25%, respectively.
2667 120
:;~.0
o/o/
290
100
160
;= '~
o
120
o
L_ L. "0
= 0
aO
~10
120
16
i 200
0
, 2~0
0
20.
.
.
AO .
r~cxlO 4
go
r~10
.
~;
.
100 ,
.
120
4
Figure 2. Predicted vs. experimental of reaction rates. CONCLUSION The intrinsic kinetics of the cyclohexanone ammoximation in the liquid phase over titanium silicate molecular sieves has been studied in a slurry reactor. On the basis of these experimental data under the reaction conditions investigated, the power law rate equations have been developed and the kinetic parameters are evaluated by simulation the initial rate data. The reactions are between zero and unity order with respect to reactant concentrations. The rate equations show that cyclohexanone ammoximation is no more sensitive than H202 decomposition with respect to H202. The activation energy is found to be 95kJ/mol and 82kJ/mol for the ammoximation of cyclohexanone and H202 decomposition, respectively. These data clearly show a good agreement between the model prediction and experimental data under the present operating conditions also. The kinetic models are useful to the design and operation of reactor. ACKNOWLEDGEMENT The authors acknowledge the financial support provided by the Major State Basic Research Development Program of China REFERENCES 1. Thangaraj, A., Sivasankers, S., Ratnasamy, P., J. Catal., 131 (1991), 394-400. 2. Taramosso, M., Perego, G., Notari, B., US Patent 4 410 501.1983 3. Roffia, P., Padovan, M., Leofanti, G. Mantegazza, M. A., De Alberti, G., Tauszik, G. R.,US 4 794 198. 1988. 4. Roffia, P., Paparatto, G., Cesana, A., Tauszik, G., EP 0 301 486. 1989. 5. Bars, J. Le, Dakka, J., Sheldon. R. A., Appl Catal, 136 (1996), 69-80. 6. Perego, G., Bellussi, G., Corno, C., Stud SurfSci Catal, 28 (1986), 129-136. 7. Kul'kova, N. V., Kotova, V. G., Kvyatkovskaya M. Yu., Murzin, D. Yu, Kinetics and Catalysis,20 (1997), 43-46. 8. Murzin, D. Yu, Kul'kova, N. V., Kotova, V. G., Kvyatkovskaya M. Yu. Kinetics and Catalysis,39 (1998), 763-766. 9. Lin, M., Shu, X., Wang, X., Petroleum Processing and petrochemicals, 30(1999), 1-4. 10. Liu, Y., Li, Y., Wu, W., Min, Enz., Petroleum Processing and petrochemicals, 33(2002), 41-45.