Formulation of Reaction Kinetics for Cyclohexanone Ammoximation

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Formulation of Reaction Kinetics for Cyclohexanone Ammoximation Catalyzed by a Clay-Based Titanium Silicalite-1 Composite in a Semibatch Process Alex C. K. Yip and Xijun Hu* Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong

bS Supporting Information ABSTRACT: This article focuses on the kinetic modeling of cyclohexanone ammoximation catalyzed by the clay-based titanium silicalite-1 (TS-1) composite. With the use of computational tools, modeling was carried out systematically, and a rational interpretation of the reaction mechanism was successfully achieved. By fitting the experimental and calculated data, it was found that cyclohexanone ammoximation over the clay-based TS-1 composite follows the EleyRideal mechanism in which the reaction takes place between adsorbed H2O2 and other reactants in the free state. It is concluded that a full understanding of the mechanism and kinetics of the reaction allows one to engineer the overall cyclohexanone ammoximation process by influencing elementary steps. The pre-exponential factor and activation energy of the reaction were also determined as a reference for industrial reactor design.

1. INTRODUCTION The liquid-phase cyclohexanone ammoximation, catalyzed by titanium silicalite-1 (TS-1), in the presence of H2O2 and ammonia has great industrial value not only because the oxime product from this reaction is the key intermediate for the production of caprolactam,18 but also because of the elimination of numerous reaction steps required in conventional routes, which involve the inevitable use of hazardous chemicals and generation of pollutants.9,10 Previously, we published articles to present a novel design of TS-1 catalytic composite (called the clay-based TS-1 composite) that greatly enhances the durability of the TS-1 catalyst by supporting the TS-1 crystals on a bentonite clay substrate. With this design, catalyst deactivation in industrial process due to particle agglomeration can be significantly reduced.11,12 In a separate study, issues related to the specific catalytic reaction promoted by TS-1 composite were discussed in detail, and the optimum reaction conditions for the maximum oxime yield were also identified.13 To further establish the clay-based TS-1 composite in industrial practice, it is of vital importance to understand the reaction mechanism. For this reason, this article aims to obtain fundamental information on cyclohexanone ammoximation over the clay-based TS-1 composite through the formulation of likely reaction mechanisms. These mechanisms were proposed to follow LangmuirHinshelwood or EleyRideal kinetics as suggested in our previous work.13 By using computational tools, it is possible to search for the kinetic parameters in the rate model that best suit the experimental results and, thereby, to obtain a rate expression. The best-fitted rate expression with kinetically significant parameters then verifies mechanistic postulates. This fundamental information could be useful for industrial reactor design. Moreover, kinetic analysis is most useful if it can be related to fundamental physical and chemical principles, such as through estimation of the pre-exponential factor and activation energy of the catalytic process using the catalyst developed. r 2011 American Chemical Society

Therefore, another objective of this article was to measure these parameters to serve as fundamental information for reactor design. It is also hoped that, with the kinetic information acquired, one can conduct thought experiments with reaction mechanisms to extrapolate knowledge beyond the experimentally measured regions of operation.

2. EXPERIMENTAL SECTION 2.1. Preparation of Clay-Based TS-1 Composite. The claybased TS-1 catalytic composite was synthesized by hydrothermal treatment based on the mixed alkoxide route described elsewhere.12 First, 0.77 g of tetrabutyl orthotitanate (TBOT) was added to 13.2 g of tetraethyl orthosilicate (TEOS) in a glass beaker under mild stirring. The reactant mixture was then stirred for 15 min at room temperature until it turned from a milky color to a clear homogeneous ester mixture. In a separate beaker, the structure-directing solution was prepared by mixing 22.07 g of aqueous tetrapropylammonium hydroxide (TPAOH) solution with one-half the quantity of additional water required (1.73 mL). The dilute TPAOH solution was then added to the ester mixture by a metric pump at a rate of 0.1 mL/min under moderate stirring. After all of the structure-directing agent had been added, the total mixture was stirred for an additional 1 h to ensure that a homogeneous phase was obtained. The remaining stoichiometrically required water (1.73 mL) was added, so that the chemical composition of the final gel was TPAOH/SiO2/TiO2/H2O/ EtOH/BuOH = 0.36:1.00:0.04:18.81:4.00:0.16, where ethanol (EtOH) and butanol (BuOH) were the products resulting from Received: July 9, 2011 Accepted: November 16, 2011 Revised: November 1, 2011 Published: November 16, 2011 13703

dx.doi.org/10.1021/ie201467u | Ind. Eng. Chem. Res. 2011, 50, 13703–13710

Industrial & Engineering Chemistry Research

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Figure 1. Schematic of the reactor setup for cyclohexanone ammoximation.

the hydrolysis of TEOS and TBOT, respectively. Bentonite was then added to the resulting gel, and the mixture was left stirring overnight to ensure sufficient contact. The claygel synthesis mixture was transferred to a Teflon-lined autoclave and heated to 175 °C (5 °C/min ramping) for 48 h. Then, the solid material was recovered from the mother liquid by centrifugation and washed with distilled deionized water. The product was dried at 110 °C and finally calcined at 550 °C for 6 h at ramp rate of 5 °C/min to remove the remaining organic material. 2.2. Acquisition of Kinetic Data. Because of the presence of the two liquid phases, it was experimentally impossible to take representative samples of the reaction mixture during reaction. Therefore, no real kinetic data could be collected, and the cyclohexanone, hydrogen peroxide (H2O2), and ammonia (NH3) concentrations were measured from several experiments after different reaction durations (i.e., 5, 10, 15, 25, 30, 35, 45, 55, 60, 65, 75, 85, 90, 95, 105, 115, 120, 125, 135, 145, and 150 min). This approximation is acceptable in view of the good trends obtained for the experimental rates of concentration change. The general procedure of cyclohexanone ammoximation was carried out as follows: In a standard run, defined amounts of cyclohexanone (99%) and the water/t-butanol solvent (1:1 g/g) were first added to a glass reactor (STEM Omni-Reacto Station) equipped with a water-circulated condenser and a built-in magentic stirrer. Then, the clay-based TS-1 composite (0.33 g/g cyclohexanone)

was added to the reactor under mixing. The reaction mixture was then slowly heated to 80 °C by a temperature-controlled heater. After the temperature had reached 80 °C, the required amount of hydrogen peroxide (H2O2, 30%) was introduced into the reaction mixture in dropwise fashion by a metric pump over the reaction period. On the other hand, ammonia solution (NH3 3 H2O, 28%) was added equally to the reaction mixture at 0, 30, 60, 90, and 120 min so that the overall reagent composition was cyclohexanone/H2O2/NH3 3 H2O = 1:1.6:2.5. The standard reaction period was 2.5 h timed as soon as H2O2 was first added to the mixture. After the reaction had completed, cyclohexanone oxime and the remaining unreacted cyclohexanone in the upper liquid were extracted with toluene. The concentrations of cyclohexanone and cyclohexanone oxime in the extracted solution were analyzed by gas chromatography (Hewlett-Packard 5890 Series II; AT-wax column, 30 m 0.25 mm i.d.; flame ionization detector) without pretreatment. Standard solutions with known concentrations of cyclohexanone and cyclohexanone oxime were used to construct a calibration curve for concentration measurements. A schematic of the reactor setup is shown in Figure 1. The H2O2 and NH3 concentrations were measured using classic titration. The H2O2 concentration was determined by titrating the reaction solution with a 0.02 M potassium permanganate (KMnO4) solution prepared in 4.5 M sulfuric acid (H2SO4). To determine the NH3 concentration, the reaction 13704

dx.doi.org/10.1021/ie201467u |Ind. Eng. Chem. Res. 2011, 50, 13703–13710


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solution was titrated with a strong acid titrant (H2SO4) of known concentration to the pale lavender end point of methyl red/ methylene blue indicator. The dye indicator was prepared by dissolving 0.2 g of methyl red and 0.1 g of methylene blue in 100 and 50 mL, respectively, of 95% isopropyl alcohol. The solution was stirred for a period to ensure that all dye contents were completely dissolved. 2.3. Computational Method. Numerical computational programs, namely, Polymath 5.1 and Excel, were used for parameter estimation in this study. Polymath 5.1 was used to perform numerical differentiation of the concentration data obtained in the experiment. Models with initial guessed parameters were then input into an Excel spreadsheet together with the experimentally determined concentrations and the Polymath-estimated experimental rates. The “solver” function in Excel was used to adjust the values (parameters) in the changing cells specified until the sum of squared error (SSE) between the experimental rate and the calculated rate was a minimum, with sum of squared error ðSSEÞ ¼

n

∑ ðycal yexp Þ2 i¼1

written in the following forms dCCyclo dt

CCyclo F VR

dCH2 O2 FCH2 O2 λ 1 C H2 O2 F ¼ R kd CH 2 O2 VR dt VR

ð2Þ ð3Þ

where F is the H2O2 supply rate, VR is the reactor volume at time t, R is the term representing the disappearance by reaction, kd is the rate constant of H2O2 decomposition, and λ1 is the order of decomposition. For these expressions, it is important to note that cyclohexanone was added before the reaction was started, whereas H2O2 was added in a dropwise (assumed continuous) fashion and NH3 was injected in five equal doses. Because NH3 was added evenly in five equal doses (ca. 0.01638 mol of NH3/ dose) at 0, 30, 60, 90, and 120 min during the reaction period, the material balance on NH3, which takes NH3 evaporation at high temperature into account, is given by the following expressions

ð1Þ

VR ¼ Ft þ V0 þ VC ðN 1Þ

ð4Þ

For 0 e t e 30, N = 1, giving

where n represents the nth set of data, ycal is the value calculated using the models, and yexp is the experimental value. Specific constraints were given to the solver so that the parameters obtained were kinetically meaningful. The models were then analyzed individually after the parameters had successfully been found.

3. RESULTS AND DISCUSSION Cyclohexanone ammoximation catalyzed by the clay-based TS-1 catalyst was carried out in a semibatch process for a few important reasons. The most apparent one is that cyclohexanone ammoximation involves the use of volatile reagents such as H2O2 and NH3 at high temperature. Therefore, feeding the reagents to the reactor in a controlled manner significantly improves the product yield and process efficiency. Another important reason for using semibatch operation in this study is that it is often preferable to use the semibatch mode as opposed to continuous or even batch processes in chemical manufacturing sites. This is of safety concern because overheating can occur through the evolution of heat from exothermic reactions, which can result in progressive increases in the rate of heat generation and, hence, thermal runaway. With one of the reactants being charged to the reactor in the beginning and the others being metered in, the semibatch process has the advantage of good temperature control and the capability of minimizing unwanted side reactions by maintaining a low concentration of one of the reactants. It therefore allows control of the rate of reaction by controlling the concentration of the reaction mixture. After the reaction has proceeded to the required extent, the reaction mixture is withdrawn from the reactor, and appropriate solvent extraction is applied to obtain the desired product. A disadvantage of semibatch processes perhaps is the substantial increase in the volume of reaction mixture during the operation, making the process difficult to analyze mathematically. Therefore, semibatch operation should be appropriately modeled. 3.1. Mass Balances. Considering that cyclohexanone ammoximation over the clay-based TS-1 catalytic composite was performed in a semibatch reactor as shown in Figure 1, where the reaction was carried out by the controlled addition of H2O2 and NH3, the material balances of cyclohexanone and H2O2 can be

¼ R

dCNH3 λ2 CNH3 F , ¼ R ke CNH 3 dt VR C0, NH3 ≈ 0:8 mol=L

ð4aÞ

For 30 < t e 60, N = 2, giving dCNH3 λ2 CNH3 F , ¼ R ke CNH 3 dt VR CNH3 jt ¼ 30 VR jt ¼ 30 þ 0:01638 C0, NH3 ¼ VR

ð4bÞ


ð4cÞ


ð4dÞ

For t > 120, N = 5, giving dCNH3 λ2 CNH3 F , ¼ R ke CNH 3 dt VR CNH3 jt ¼ 120 VR jt ¼ 120 þ 0:01638 C0, NH3 ¼ VR

ð4eÞ

In these equations, ke is the evaporation rate constant of NH3, and λ2 is the order of NH3 evaporation. Because the absence of mass- (and heat-) transfer limitations was confirmed under the optimum reaction conditions,13 the intrinsic kinetics can be modeled by directly incorporating the mechanistic model into the above mass balance equations (eqs 24) as R. 13705



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3.2. Reaction Scheme. Experimental data can be correlated to a rate expression derived from a reaction sequence invoking a rate-determining step (RDS) on an ideal surface. The approach associated with this assumption is typically referred to as a LangmuirHinshelwood (LH) process, which is subject to the same assumptions as are associated with the Langmuir isotherm. It is important to note, however, that the number of sites measured by adsorption under nonreacting conditions might not be identical to the number of active sites described by the site balance in the Langmuir isotherm. The LH model was applied to cyclohexanone ammoximation over the clay-based TS-1 catalyst, considering that the surface reactions between the adsorbed species are the RDSs and the other elementary steps are quasiequilibrated (fast). By assuming that the generation rate of the intermediate, namely, NH2OH, is equal to the consumption rate and that one RDS is much faster than the other, no accumulation of NH2OH is assumed in the reaction sequence. Therefore, the mechanism based on the LH model can be expressed as k1

½A þ H2 O2 r s f½A 3 H2 O2 k1

k6

½A 3 H2 O2 þ NH3 sf½A 3 NH2 OH þ H2 O k7

ð12Þ

½A 3 Prod sf½A þ Prod

ð13Þ

½A 3 NH2 OH þ Cyclo sf½A 3 Prod þ H2 O k8

0

k6

½A 3 H2 O2 þ NH3 þ Cyclo sf Prod þ ½A þ 2H2 O

ð14Þ

We also proposed a second model based on the EleyRideal mechanism, the ER-2 model, in which the reaction sequence requires the free hydroxylamine (NH2OH) intermediate to contact and react with a cyclohexanone molecule adsorbed on the surface of the catalytic composite to form the oxime product. This model is given by k1

ð5Þ

s f½A 3 H2 O2 ½A þ H2 O2 r

ð5Þ

ð6Þ

½B þ Cyclo r s f½B 3 Cyclo

ð7Þ

½A 3 H2 O2 þ NH3 sf NH2 OH þ ½A þ H2 O

k1 k3

k2

s f½A 3 NH3 ½A þ NH3 r k2

ð7Þ

k3

k9

k3

½B þ Cyclo r s f½B 3 Cyclo k3

k10

½B 3 Cyclo þ NH2 OH sf½B 3 Prod þ H2 O

k4

½A 3 H2 O2 þ ½A 3 NH3 sf½A 3 NH2 OH þ ½A þ H2 O

k11

½B 3 Prod sf½B þ Prod

ð8Þ

k5

½A 3 NH2 OH þ ½B 3 Cyclo sf Prod þ ½A þ ½B þ H2 O ð9Þ

where [A] is the Ti active site in the clay-based catalytic composite, [B] is the adsorption site on the inner and outer surface of the catalytic composite, Cyclo is cyclohexanone, and Prod represents cyclohexanone oxime (i.e., the desired product). The above model is henceforth referred to as the LH model. Another possibility is that a free molecule inside the TS-1 channel reacts with an adsorbed molecule without itself adsorbing on the surface. Such a case is denoted as an EleyRideal (ER) process. We considered two models based on the ER mechanism. The first, referred to as the ER-1 model, considered the Ti active site as the only position where reaction takes place. According to the experimental observation, an additional assumption can be made about the sorption of oxime product on the clay-based TS-1 catalyst. As experimental tests showed that the adsorption of cyclohexanone oxime on the catalyst is extremely weak, immediate desorption of the oxime product was assumed; that is, the desorption step of the oxime (eq 13) was assumed to be irreversible. Therefore, by defining the surface reaction as the RDS, the ER-1 model can be expressed as k1

ð16Þ ð17Þ

0

0

k9

k4

k1

ð15Þ

Considering that eqs 15 and 16 are the RDSs and assuming immediate release of NH2OH and cyclohexanone oxime from sites [A] and [B], respectively, eqs 1517 can be combined to give ½A 3 H2 O2 þ ½B 3 Cyclo þ NH3 sf Prod þ ½A þ ½B þ 2H2 O

½A 3 H2 O2 þ ½A 3 NH3 þ ½B 3 Cyclo sf Prod þ 2½A ð10Þ þ ½B þ 2H2 O

s f½A 3 H2 O2 ½A þ H2 O2 r

ð11Þ

ð5Þ

ð18Þ

3.3. Kinetic Model. The rate law that describes the intrinsic kinetic based on the LH mechanism is 0

R ¼

k4 K1 K2 K3 CH2 O2 CNH3 CCyclo ð1 þ K1 CH2 O2 þ K2 CNH3 Þ2 ð1 þ K3 CCyclo Þ ð19Þ

(See the Supporting Information for the development of the rate laws.) The corresponding rate models derived from the ER-1 and ER-2 proposals are 0

R ¼

k6 K1 CH2 O2 CNH3 CCyclo ð1 þ K1 CH2 O2 Þ

ð20Þ

0

R ¼

k9 K1 K3 CH2 O2 CCyclo CNH3 ð1 þ K1 CH2 O2 Þð1 þ K3 CCyclo Þ

ð21Þ

The intrinsic kinetics, R, of cyclohexanone ammoximation over the clay-based TS-1 catalyst is then included in the mass balance equations of the semibatch process (eqs 24). 13706



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Table 1. Calculated Parameters of the LH, ER-1, and ER-2 Models k04 (mol.L1 3 min1)

k06 (L 3 mol 1 3 min1)

k09 (min1)

K1 (L 3 mol1)

K2 (L 3 mol1)

K3 (L 3 mol1)

LH

0.1608

7.8761

0.0420

233.4186

ER-1

0.0020

2.1340

ER-2

0.0098

2.1487

0.1480

The values of dCCyclo/dt, dCH2O2/dt, and dCNH3/dt obtained from these mass balance equations are considered as the calculated rates and are compared with the measured experimental rates. By minimizing the objective function, the sum of squared error (SSE) between the two sets of value, all parameters were estimated (Table 1). Comparison between the experimental and calculated rates for the three tested model are presented in Figures 2 and 3. To analyze the models obtained, two criteria for the parameters of the kinetic model were applied: (1) the parameters should give relevant meaning of the elementary steps (i.e., the reaction rate constants and adsorption equilibrium constants must be positive) and (2) the SSE should be sufficiently small. Comparing the three tested models, statistical analysis using the SSE in Table 2 shows that the LH model, which is based on the assumptions that H2O2 and NH3 are both adsorbed on the Ti active site and that the NH2OH intermediate thus formed reacts with the cyclohexanone adsorbed on the Si surface, has the largest SSE, indicating that the calculated data are most inconsistent with the experimental data among the three tested models, even though all of the SSE values are of the same order of magnitude. This result might suggest that one or more reactants are not chemisorbed directly at the catalyst surface during the reaction. The disagreement is particularly obvious in the rate of cyclohexanone as shown in Figure 2a. Moreover, whereas all of the estimated parameters are positive, the adsorption equilibrium constant (K3) of cyclohexanone seems to be too large compared with that of H2O2. Therefore, it is suggested the cyclohexanone ammoximation over the clay-based TS-1 catalyst is unlikely to follow a LangmuirHinshelwood mechanism. This further indicates that the treatment of Langmuir kinetics on uniform surfaces cannot be extended to reactions that occur on two types of sites in close proximity as in the case of the clay-based TS-1 catalyst. On the other hand, both the ER-1 and ER-2 models, which were developed on the basis of the assumption that one reactant interacts with another that is in adsorbed phase, show very small SSE values compared with that of the LH model and give positive parameters. Therefore, from a statistical point of view, the EleyRideal mechanism represents the intrinsic kinetics of cyclohexanone ammoximation catalyzed by the clay-based TS-1. The calculated data for the ER-1 model fit the experimental rate data best (Figure 3ac). In fact, the rate expression based on the ER-2 model can be simplified to the ER-1 form. Based on the experimental results that the adsorption of cyclohexanone on the clay-based TS-1 catalyst is rather weak, the value of K3CCyclo should be very small and can be ignored, that is, K3CCyclo , 1. Also, the product of k09 and K3 in the numerator is 0.0015 L 3 mol1 3 min1, which is close to the value of k06 (0.0020 L 3 mol1 3 min1) in the ER-1 model. Taking these two mathematical comparisons, the ER-1 model seems to be more reasonable from an experimental perspective. Even so, it should be kept in mind that, at this stage, the two ER models cannot be completely discriminated because the difference in

Figure 2. Experimental versus calculated reaction rates of (a) cyclohexanone, (b) H2O2, and (c) NH3 based on the LH model. 13707



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Figure 3. Experimental versus calculated reaction rates of (a,d) cyclohexanone, (b,e) H2O2, and (c,f) NH3 based on the (ac) ER-1 and (df) ER-2 model.

SSE is not large enough to draw a firm conclusion. A detailed microkinetic analysis should be applied to further elucidate the catalytic surface chemistry of cyclohexanone ammoximation. In the meantime, the ER-1 model appears to be a more generalized form of kinetics that describes cyclohexanone ammoximation best. 3.4. Determination of the Pre-exponential Factor and Activation Energy. In addition to H2O2 decomposition and NH3 evaporation, many other side reactions occur in addition to the main reaction of cyclohexanone ammoximation when the

reaction conditions are not optimal. A reaction parameter that influences the selectivity of the desired oxime is the reaction temperature. For example, the reaction carried out at 60 and 70 °C gives oxime selectivities of 62% and 92%, respectively.13 Under such conditions, noncatalytic homogeneous or heterogeneous parallel and consecutive side reactions occur in the system and yield at least 10 trace byproducts in the reaction product. It has been suggested in the literature that these side reactions might include oxidation and hydrolysis of cyclohexanone 13708



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Table 2. Sum of Squared Error (SSE) Values for the Three Tested Models model

SSE

LH

7.6620 105

ER-1 ER-2

2.8967 105 3.0565 105

Table 3. Estimated Kinetic Parameters temperature (°C) 80 70 60

α

β

γ

1.24 1.80 0.34

k (L2.38 3 mol2.38 3 min1)

SSE

2.050 10

3

6.5639 104

7.568 10

4

4.4883 104

2.712 10

4

3.6823 104

oxime, ammonia oxidation, hydroxylamine oxidation, cyclohexanone condensation, and cyclohexanone reaction with t-butanol and perhaps with ammonia, among others.14 Therefore, kinetic modeling of cyclohexanone ammoximation at the elementary level under other-than-optimal reaction conditions is extremely difficult, because it requires precise and detailed information on the complete reaction framework. For this reason, the approach for determining the pre-exponential factor and activation energy of cyclohexanone ammoximation was different from what was described in the previous section. Here, instead of applying the rate expression derived from the LH or ER proposals, the intrinsic kinetics was presented by a power-law model, which is an empirical kinetic expression given by R ¼ kCCycloα CH2 O2β CNH3γ

ð22Þ

where k is the reaction rate constant, which can be related to the pre-exponential factor and activation energy through the Arrhenius equation, and α, β, and γ are reaction orders. This empirical kinetic model was applied to the mass balance equations in the same manner as the LH and ER models, and the expressions were solved with the experimental data obtained at 60, 70, and 80 °C by minimizing the SSE between the calculated and experimental rates data. Because the reaction order is independent of reaction temperature, the reaction orders estimated at 80 °C remain unchanged and were considered valid for reaction temperatures of 60 and 70 °C. The calculated and experimental rates data are plotted in the Supporting Information. The reaction order and the rate constant, k, calculated at different temperatures are reported in Table 3. The temperature dependence of the reaction constants is expressed by the linearized form of the Arrhenius equation Ea 1 ð23Þ ln k ¼ ln k0 R T where k is the rate constant of the reaction, T is the absolute temperature, and R is the gas constant. The parameter Ea is the activation energy with dimensions of energy per mole, and k0 is the pre-exponential factor, which has the same units as k. The temperature dependence of the pre-exponential factor is weak compared with that of the exponential term. A plot of ln k versus 1/T gives Ea from the slope of the straight line and allows k0 to be determined from the intercept (Supporting Information). The activation energy (Ea) and pre-exponential factor (k0) were

determined to be 9.883 104 J/mol and 8.590 1011 L2.38 3 mol2.38 3 min1, respectively.

4. CONCLUSIONS As a catalytic process moves from the laboratory scale to the pilot-plant level and finally to industrial units, kinetics is one of the most important research tools to screen and optimize newly found catalysts. This article formulates the reaction mechanism of cyclohexanone ammoximation over the clay-based TS-1 catalyst by fitting the experimental data to data calculated using three mechanistic proposals, namely, the LangmuirHinshelwood (LH) mechanism and two forms of the EleyRideal (ER) mechanism. Because the reaction studied was carried out in semibatch mode, the rate of reaction was not only governed by the mechanism of the reaction, but also depended on the manner of addition of reactants. For this reason, the kinetic modeling in this article originated from the development of mass balance equations for cyclohexanone, H2O2, and NH3, depending on their mode of addition in the reaction. After the absence of masstransfer limitations had been verified, model fitting suggested that the ER-1 model, which is based on the assumption that ammoximation takes place between H2O2 adsorbed on Ti active sites and NH3 and cyclohexanone in the free state, is most consistent with the experimental data. This was determined from a statistical analysis based on a comparison between the SSE values of different models and, more importantly, from the quality of the parameters obtained. With a power-law model, which is an empirical kinetic expression, the activation energy of the reaction and the pre-exponential factor were found to be 9.883 104 J/mol and 8.590 1011 L2.38 3 mol2.38 3 min1, respectively. Moreover, the kinetic modeling presented in this work is very likely to be rewarding in terms of industrial use of mechanistic rate expression during the process design phase. For instance, if an applicable reaction model is available, it might be possible to discover improved operating conditions by early simulation of the initial reactor design. ’ ASSOCIATED CONTENT

bS

Supporting Information. Comparison of experimental and calculated rates at 80 °C (Figures S1S3), 70 °C (Figures S4S6), and 60 °C (Figures S7S9); development of the rate law (eqs S1S11); and Arrhenius plot (Figure S10). This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: (852) 2358-7134. Fax: (852) 2358-0054.

’ ACKNOWLEDGMENT The authors acknowledge financial support from the Research Grants Council of Hong Kong under Grant 605108. ’ NOMENCLATURE CCyclo = concentration of cyclohexanone at time t, mol/L CH2O2 = concentration of H2O2 at time t, mol/L CNH3 = concentration of ammonia at time t, mol/L 13709


Industrial & Engineering Chemistry Research Ea = activation energy of cyclohexanone ammoximation catalyzed by the clay-based TS-1 composite, J/mol F = supply rate of H2O2 k0 = pre-exponential factor, L2.38 3 mol2.38 3 min1 kd = decomposition rate constant of H2O2, L 3 mol1 3 min1 ke = evaporation rate constant of NH3, min1 ki = rate constant of forward reaction i, i = 111 ki = rate constant of backward reaction i, i = 13 k04 = rate constant of surface reaction in the LH model, mol. L1 3 min1 0 k6 = rate constant of surface reaction in the ER-1 model, L 3 mol1 3 min1 0 k9 = rate constant of surface reaction in the ER-2 model, min1 K1 = adsorption equilibrium constant of H2O2, L 3 mol1 K2 = adsorption equilibrium constant of NH3, L 3 mol1 K3 = adsorption equilibrium constant of cyclohexanone, L 3 mol1 N = Nth dose of NH3 (N = 15) R = gas constant, 8.314 J 3 K1 3 mol1 T = reaction temperature, K V0 = initial volume at t = 0 min; including required amount of cyclohexanone, t-butanol:water mixture and the first dose of NH3, L VC = volume of each NH3 dose, L VR = volume at time t, L ycal = calculated values based on the models yexp = experimental values

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(11) Yip, A. C. K.; Lam, F. L. Y.; Hu, X. A Heterostructured Titanium Silicalite-1 Catalytic Composite for Cyclohexanone Ammoximation. Microporous Mesoporous Mater. 2009, 120, 368–374. (12) Yip, A. C. K.; Lam, F. L. Y.; Hu, X.; Li, P.; Yuan, W. K. Study on the Synthesis of Clay-Based Titanium Silicalite-1 Catalytic Composite. Ind. Eng. Chem. Res. 2009, 48, 5266–5275. (13) Yip, A. C. K.; Hu, X. Catalytic Activity of Clay-Based Titanium Silicalite-1 Composite in Cyclohexanone Ammoximation. Ind. Eng. Chem. Res. 2009, 48, 8441–8450. (14) Li, Y.; Wu, W.; Min, E. Z.; Sun, B.; Shu, X. Intrinsic Kinetic Modeling of Cyclohexanone Ammoximation over Titanium Silicate Molecular Sieves. Stud. Surf. Sci. Catal. 2004, 154, 2661–2667.

Greek Letters

α = reaction order of cyclohexanone β = reaction order of H2O2 γ = reaction order of NH3

’ REFERENCES (1) LeBars, J; Dakka, J.; Sheldon, R. A. Ammoximation of Cyclohexanone and Hydroxyaromatic Ketones over Titanium Molecular Sieves. Appl. Catal. A: Gen. 1996, 136, 69–80. (2) Mantegazza, M. A.; Cesana, A.; Pastori, M. Ammoximation of Ketones on Titanium Silicalite: A Study of the Reaction Byproducts. Top. Catal. 1996, 3, 327–335. (3) Perego, C.; Carati, A.; Ingallina, P.; Mantegazza, M. A.; Bellussi, G. Production of Titanium Containing Molecular Sieves and Their Application in Catalysis. Appl. Catal. A: Gen. 2001, 221, 63–72. (4) Rao, P.; Thangaraj, A.; Ramaswamy, A. Oxidative Dehydrogenation of Aqueous Ethanol over the Molecular-Sieve TS-1. J. Chem. Soc., Chem. Commun. 1991, 16, 1139–1140. (5) Ratnasamy, P.; Srinivas, D.; Knozinger, H. Active Sites and Reactive Intermediates in Titanium Silicate Molecular Sieves. Adv. Catal. 2004, 48, 1–169. (6) Sooknoi, T.; Chitranuwatkul, V. Ammoximation of Cyclohexanone in Acetic Acid using Titanium Silicalite-1 Catalyst: Activity and Reaction Pathway. J. Mol. Catal. A: Chem. 2005, 236, 220–226. (7) Wu, P.; Komatsu, T.; Yashima, T. Ammoximation of ketones over titanium mordenite. J. Catal. 1997, 168, 400–411. (8) Zecchina, A.; Spoto, G.; Bordiga, S.; Geobaldo, F.; Petrini, G.; Leofanti, G.; Padovan, M.; Mantegazza, M.; Roffia, P.; Kaliaguine, S.; Krauss, H. L.; Sivasanker, S. Ammoximation of Cyclohexanone on Titanium Silicalite—Investigation of the Reaction Mechanism. Stud. Surf. Sci. Catal. 1993, 75, 719–729. (9) Mantegazza, M. A.; Petrini, G.; Spano, G.; Bagatin, R.; Rivetti, F. Selective Oxidations with Hydrogen Peroxide and Titanium Silicalite Catalyst. J. Mol. Catal. A 1999, 146, 223–228. (10) Roffia, P.; Padovan, M.; Moretti, E.; De Alberti, G. Catalytic Process for Preparing Cyclohexanone-Oxime.European Patent 208311, 1987. 13710


Formulation of Reaction Kinetics for Cyclohexanone Ammoximation

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