Al2O3 Catalyst

Apr 26, 2005 - Guo, J. W.; Cui, Y. D.; Mi, Z. T.; Zhang, X. W. Synthesis of adamantane by hydrogenation and isomerization of dicyclopentadiene. Chin. ...
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Ind. Eng. Chem. Res. 2005, 44, 3846-3851

KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetics of Dicyclopentadiene Hydrogenation over Pd/Al2O3 Catalyst Guozhu Liu, Zhentao Mi,* Li Wang, and Xiangwen Zhang School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P.R. China

The intrinsic kinetics of dicyclopentadiene (DCPD) hydrogenation into endo-tetrahydrodicyclopentadiene (endo-THDCPD) over Pd/Al2O3 catalyst was investigated using stirred semibatch reactors in the absence of transport limitations over ranges of temperature (358.15-438.15 K) and hydrogen pressure (0.5-3 MPa). A Langmuir-Hinshelwood type model was proposed to fit the experimental data. The kinetic parameters were regressed by numerically integrating the differential mass balance equations for the stirred semibatch reactors with fourth-order RungeKutta method and then minimizing the objective function describing the deviations between the model-predicted values and the experimental data with the Levernberg-Marquardt algorithm. The activation energy for the first and second step reaction is 7.8945 and 12.2068 kJ/mol, respectively. It was found that the developed model was accurate to predict the experimental results with the average relative error (ARE) less than 12.7%. Introduction The catalytic hydrogenation of dicyclopentadiene (DCPD) to endo-tetrahydrodicyclopentadiene (endoTHDCPD) is a very important reaction to synthesize an organic intermediate adamantine and a high-energy jet fuel JP-10, which are the products of the isomerizations reactions of endo-THDCPD.1,2,3 The reaction consists of typical consecutive steps, in which DCPD hydrogenation into dihydrodicyclopentadiene (DHDCPD) occurs first and then endo-THDCPD is obtained by further hydrogenation of DHDCPD. There are two isomers of DHDCPD, i.e., 1,2-dihydrodicyclopentadiene (1,2-DHDCPD) and 9,10-dihydrodicyclopentadiene (9,10-DHDCPD). In fact, the principal product of the first step reaction is 9,10-DHDCPD due to the steric hindrance effect. Scheme 1 shows the reactions of DCPD hydrogenation into endoTHDCPD. In view of the scientific research, the reaction proceeds at relatively mild conditions and presents both conversion and selectivity issues. The hydrogenation of DCPD into endo-THDCPD is highly exothermic and thus significant temperature rise can be observed,4 moreover, the hydrogenation of DCPD must be carried out below 453.15 K to avoid the pyrolysis of DCPD into cyclopentdiene. Owing to those advantages, Silveston and Hanika5 indicated that the hydrogenation of DCPD was an excellent model reaction to study the reactor performance (including conversion and selectivity) and heat behavior of periodically operated trickle-bed reactors (TBR) for the highly exothermic complex reactions, which could lead to an effective strategy to improve reactor performance and to control the local hot spots by periodic modulation of feed flow rate, concentration, and temperature. Obviously, it is indispensable to know the intrinsic kinetics of DCPD hydrogenation for further * To whom correspondence should be addressed. Tel/fax: 86-22-27402604. E-mail: [email protected].

understanding of reactor behavior under periodic operation. Chou et al.4 reported the intrinsic kinetics of DCPD hydrogenation into DHDCPD over nickel catalyst supported on γ-Al2O3 using a differential reactor and excluding external and internal mass transfer limitations, and indicated that the reaction is first-order with respect to hydrogen and zero-order with respect to DCPD concentrations. Our group previously presented kinetics for the continuous hydrogenation of DCPD into endo-THDCPD over nickel catalyst supported on γ-Al2O3 using an integral reactor, and provided a rate equation for the formation of endo-THDCPD, but the intermediate product DHDCPD was neglected.6,7 Up to now there is no information available in the literature on the intrinsic kinetics of DCPD hydrogenation including twostep reactions over Pd/Al2O3 catalyst. The objective of this work is to investigate DCPD hydrogenation into endo-THDCPD in the presence of Pd/Al2O3 catalyst in stirred semibatch reactors in the absence of transport limitations. A Langmuir-Hinshelwood model is also formulated to fit the experimental data. A set of differential mass-balance equations for the stirred semibatch reactors is then developed for the regression of the L-H model parameters and then used to simulate the experimental results. The accuracy of the model is further confirmed by comparison with experimental data and the activation energy reported previously. The developed kinetic model provides a useful tool for further investigations of DCPD hydrogenation reactor simulation and design. Experimental Section Materials. DCPD with a purity of >98.8% was purchased form Yangli Petrochemical Inc., Hangzhou, China. For all runs hydrogen with a purity of >99.99% was purchased from Chenxi Idurstrial Gas Co., Ltd (Tianjin, China). Analytic reagent-grade n-hexane was purchased from Sinopharm Chemical Reagent Co., Ltd

10.1021/ie0487437 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/26/2005

Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 3847 Scheme 1. Dicyclopentadiene Hydrogenation into Tetrahydrodicyclopentadiene

(Shanghai, China) and used as the solvent for DCPD. It should be noted that initial reactant concentrations of DCPD in the n-hexane were kept at less than 1 mol/L to maintain isothermal conditions. The self-prepared catalyst was 2.5-mm-diameter spherical pellet of Pd/Al2O3 with an eggshell distribution (ca. 0.10-mm thickness) to minimize intraparticle diffusion resistance. The pellets were crushed into the particle size range of 40-90 µm to further decrease intraparticle diffusion limitations. The palladium content of 0.29 wt % was measured using Vista MPX inductively coupled plasma-optical emission spectrometers (ICP-OES) (Varian, Inc., Palo Alto, CA). A BET surface area of 134.12 m2/g, an average pore diameter of 1.66 µm, and a pore volume of 0.33 cm2/g were measured by N2 adsorption using CHEMBET-3000 (Quantachrome Instruments, Boynton Beach, FL). Experimental Apparatus. All the reactions were carried out in a 1000-mL FC Series stirred reaction vessel (Pressure Products Industries, Inc., Warminster, PA) equipped with a PPI Dyna/Mag magnetic mixer and a heating mantle. The temperature was controlled by a PID temperature controller with a heater percent power adjustment that controlled the reactor temperature to (1 °C, while the speed of agitation were measured by a digital tachometer. All the pipes and valves including the gas inlet, gas release valve, cooling water feed line, sample pipe, and valve were make of 316 stainless steel and situated on the top of the reaction vessel. Experimental Procedures. In each experiment, the fine particle of catalyst was first added to the reaction vessel, which was then completely purged with nitrogen at 433.15 K to ensure an inert atmosphere excluding air and aqueous vapor. The catalyst was in situ reduced by the hydrogen at 433.15 K for 3 h and then kept at ambient temperature for 8 h protected with nitrogen atmosphere to stabilize and thus ensure the same activity of catalysis. Then 520 mL of reaction mixture was added via syringe pump. Hydrogen was then used to purge the nitrogen, and the solution was heated to the desired temperature under agitation speed lower than 150 rpm. When the speed of agitation was adjusted to a predetermined value, hydrogen from the cylinder was then sparged into the liquid phase directly beneath the impeller at the desired partial pressure of hydrogen. This was considered initial time for the reaction. As the reaction preceded the hydrogen pressure was maintained at a constant by continuously replenishing the consumed gas. Samples were withdrawn at different time intervals after sufficient flushing of the sample line. The total amount of samples was less than 25 mL to avoid greater errors. When the reaction was terminated, the system was cooled to ambient temperature. Products Analysis. The samples of the reaction mixtures were analyzed using a HP4890 series (HewlettPackard, Palo Alto, CA) with a flame ionization detector (FID) and capillary column HP-5 (30 m × 0.32 mm ×

0.25 µm, Hewlett-Packard). Nitrogen was used as the carried gas. The temperature of the column was kept at 403.15 K, while the injector and detector were maintained at 453.15 and 523.15 K, respectively. The area normalization method was used to quantitatively determine the concentrations of the components. The reproducibility of the results was checked, and the error in the most unfavorable cases was less than 1.5%. Taking the experimental error into account, the mass balance was fulfilled satisfactorily. Results General Considerations. In all the experiments of this work both 1,2-DHDCPD and 9,10-DHDCPD were also observed and confirmed by using GC-MS (HP5890II-HP5917A), but the concentration of the former is invariably less than 0.15 wt %, which approaches the lower detection limit of the GC analysis. Therefore, during the kinetics analysis, we neglected the appearance of 1,2-DHDCPD, as shown in Scheme 1. A complete conversion of DCPD was achieved in every experiment with the yields of endo-THDCPD exceeding 90%. Verification of Kinetic Regime. DCPD hydrogenation in the presence of Pd/Al2O3 catalyst is a gasliquid-solid three-phase reaction. Therefore, it is necessary for the intrinsic kinetics runs to be carried out in the absence of both the external mass transfer and intraparticle diffusion resistances. Because the first step reaction rate is very high and generally terminates in 2 or 3 min at higher temperature and pressure, the relationship of endo-THDCPD yield with time under different agitation speed is used to verify the external mass transfer limitations. Figure 1 presents the experi-

Figure 1. Effect of agitation speed on the yield of THDCPD (T ) 438.15 K, pH2 ) 3.5 MPa, C0A ) 0.5133 mol/L, mc ) 0.6 g)

ments performed at 438.15 K under 3.5 MPa under different agitation speeds. It is clear that the agitation speed has no effect on the reaction rate when it exceeds 850 rpm, which indicates the absence of the external mass transfer limitations. In this work all the kinetics

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Figure 2. Effect of catalyst loading on the initial reaction rate (T ) 363.15 K, pH2 ) 0.65 MPa, C0A ) 0.65 mol/L)

experiments were carried out at lower temperature and pressure but with an agitation speed of 1100 rpm. This conclusion was also supported by the following theoretical calculations under an agitation speed of 1100 rpm. Gas-liquid and solid-liquid mass-transfer coefficients for H2 and DCPD were first estimated using the Yawalkar et al.8 and Jadhav and Pangarkar9 correlations, respectively. The mass-transfer rates were then calculated and found to be at least 28 times larger than the observed reaction rates, indicating that the masstransport resistances across phase boundaries were negligible at the experimental conditions studied. Since it is difficult to verify the effect of internal diffusion limitations by changing the diameter of catalyst particles, a classical Weisz-Prater criterion is used as suggested by Jere et al.10 For all the experimental conditions ηφ2for both hydrogen and DCPD are less than 10-3 indicating the absence of intraparticle diffusion limitations.

robsFcdp2 (ηφ )DCPD < (ηφ )H2 ) < 1 × 10-3 , 1 (1) 4CH2De,H2 2

2

Initial Rate Study. It is necessary to carry out the initial rate study for obtaining some insights into the reaction kinetics. What should be noted is that because the first step reaction is very fast and generally terminates in several minutes, the effects of the catalyst loading and the hydrogen partial pressure on the initial rates are only carried out at 363.15 K. The effect of catalyst loading on the initial rate is presented in Figure 2, from which the initial rate is found to vary linearly with the catalyst loading, suggesting the absence of gas-liquid mass transfer resistance and that the reaction maybe involved a single adsorbed species.11 From Figure 3 the first step of hydrogen addition is expected to be first-order in hydrogen, assuming an elementary reaction. A previous study by Chou et al.4 on the kinetics of DCPD hydrogenation into DHDCPD over nickel catalyst also observed a similar relationship. Kinetic Experiments Results. In this work 25 runs of DCPD hydrogenation were performed with the following conditions: temperature from 358.15 to 438.15 K, hydrogen pressure from 0.5 to 3 MPa, initial DCPD concentration from 0.3 to 0.9 mol/L, and catalyst loading from 0.3 to 1.2 g. The results of those experiments used to determine parameters of the kinetic model below are reported in part by the data points to investigate the

Figure 3. Effect of hydrogen pressure on the initial reaction rate (T ) 363.15 K, mc ) 0.6 g, C0A ) 0.65 mol/L)

Figure 4. Experimental and predicted composition profiles at different hydrogen pressure (T ) 373.15 K; C0A ) 0.5133 mol/L; mc ) 0.6 g; (0) experimental at 0.76 MPa; (+) experimental at 1.26 MPa; (∆) experimental at 1.76 MPa; (O) experimental at 2.26 MPa; (s) solid line, model predicted at the corresponding pressures.)

effects of pressure, temperature, and DCPD concentration on the reaction rates. The effect of pressure on DCPD hydrogenation is illustrated in Figure 4. It is evident that hydrogen pressure has a strong influence on the reaction rates of both the first and second steps. For the first reaction step the reaction rate increases directly with the increased hydrogen pressure, whereas for the second reaction step the effect of hydrogen pressure on the reaction rate is significant at lower pressures and this influence sharply decreases with the increase of pressure. In detail, the DHDCPD concentrations variations are more significant when hydrogen pressure increases from 0.76 to 1.26 MPa than from 1.76 to 2.26 MPa. In general, DCPD hydrogenation is favored at hydrogen pressure higher than 1.26 MPa. However, the pressure has almost no effect on the maximum yield of DHDCPD, which can be interpreted by the Langmuir-Hinshelwood (L-H) model developed in the next section. The temperature effect is illustrated in Figure 5. The figure reveals that the temperature has a relatively mild influence on the reaction rates of both the first and second steps in comparison with the influence of hydrogen pressure. DCPD hydrogenation is favored at high temperatures, whereas the maximum yield of DHDCPD increases with the decrease of temperature. The influences of initial DCPD concentrations on the kinetics are given in Figure 6. It is clear that the initial

Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 3849

r1 ) mck1KACApH2θ

(2)

r2 ) mck2KBCBpH2θ

(3)

where

1 1 + KACA + KBCB + KCCC

(4)

ki ) ki,0 exp(-Ei/RT) (i ) 1, 2)

(5)

KJ ) KJ,0 exp(-EJ/RT) (J ) A, B, C)

(6)

θ)

Figure 5. Experimental and predicted composition profiles at different temperatures (pH2 ) 1.26 MPa; C0A ) 0.5133 mol/L; mc ) 0.6 g; (0) experimental at 358.15 K; (+) experimental at 373.15 K; (∆) experimental at 388.15 K; (O) experimental at 408.15 K; (s) solid line, model predicted at the corresponding temperatures.)

Then, the differential mathematic model describing the concentrations variations of liquid-phase organic reactants in the stirred semibatch reactors can be written as

dCA ) -r1 dt

(7)

dCB ) r1 - r2 dt

(8)

dCC ) r2 dt

(9)

t ) 0, CA ) C0A, CB ) CC ) 0

(10)

with I. C.

Figure 6. Experimental and predicted composition profiles at different initial DCPD concentrations (T ) 373.15 K; pH2 ) 1.26 MPa; mc ) 0.6 g; (0) experimental at 0.9 mol/L; (+) experimental at 0.7 mol/L; (∆) experimental at 0.5133 mol/L; (O) experimental at 0.3 mol/L; (s) solid line, model predicted at the corresponding DCPD concentrations.)

DCPD concentrations have no significant effect on the initial rate of DCPD hydrogenation, suggesting that the hydrogenation of DCPD into DHDCPD is zero-order with respect to the DCPD concentrations. This result is in accordance with the results of Chou et al.4 It is also observed that the time needed to reach the maximum selectivity of DHDCPD increases with the increase of DCPD concentrations. Kinetic and Stirred Reactor Model. At first, a power-law empirical model was used to fit the experimental data collected above, as well as the other 15 runs including the initial rate runs and 6 runs carried out at higher pressure (up to 3 MPa) or temperature (up to 438.15 K). It is found that the power-law model brought an average relative error (ARE) higher than 40% between the experimental and predicted values. For this reason various Langmuir-Hinshelwood models are proposed to gain a better fit of experimental data. The comparison of single- and dual-site adsorption of organic species with and without the disassociation adsorption of hydrogen further shows that the Rideal type mechanism for dissolved hydrogen is more appropriate. The following kinetic expressions are obtained for the experimental data with the assumption that surface reactions are the rate-controlling steps:

Determination of Kinetic Parameters. The experimental data used to determine the kinetic parameters in the model are collected from 25 experiments including 694 data points over specified ranges of temperature (358.15-438.15 K) and hydrogen pressure (0.5-3 Mpa). To carry out the calculation, an initial set of kinetic parameters is chosen. For given initial values of the above kinetics parameters, the concentration vs. time curves are first predicted by numerically integrating the differential mass balance equations (7-10) with fourth-order Runge-Kutta method, and then the objective function Q that described the deviations between the computed values and the experimental data is minimized using the LevernbergMarquardt algorithm. The error function is defined as below L M N

Q)

∑l ∑ ∑ m n

(

)

m,n Cm,n exp - Ccalc

Cm,n exp

2

(if Cexp * 0)

(11.1)

(if Cexp) 0)

(11.2)

l

L M N

Q)

m,n 2 (Cm,n ∑l ∑ ∑ exp - Ccalc)l m n

where L is the number of experiments involving M species, with concentrations measured at N reaction times. The regressed kinetics parameters are summarized in Table 1. Chou et al.4 studied hydrogenation of DCPD into DHDCPD using the differential reactor and excluding both external and internal mass transfer limitations, and obtained that the reaction activation energy is 7. 9632 kJ/mol, which is well in accordance with the value of 7.8945 kJ/mol reported in this work.

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Table 1. Kinetics Parameters Regressed From Experimental Data parameter

k1

k2

KA

KB

KC

preexponential coefficenta activation energyb

1.9473

0.9342

85.6515

43.2287

46.8726

7.8945

12.2068

0.3007

1.0160

2.1130

a The units for k and K are mol/L‚g ‚ MPa‚min and L/mol, i J cat respectively. b The unit for active energy is kJ/mol.

Figure 7. Comparison between the predicted and experimental concentrations for all the experimental points.

The model-predicted reactants concentrations profiles to the experimental data are provided in Figures 4-6. Furthermore, a comparison of all the experimental points and their corresponding values predicted by this model is also plotted in Figure 7. As can be seen from the figures, the proposed model describes the experimental data reasonably well with the average absolute error (AAE) less than 0.02 mol/L and ARE less than 12.7%. The higher accuracy of model prediction indicates that this model can be used for the DCPD hydrogenation reactor analysis and design. Concluding Remarks The kinetics of DCPD hydrogenation into endoTHDCPD over Pd/Al2O3 catalyst excluding external and internal transport limitations was studied in stirred slurry reactors. A Langmuir-Hinshelwood model for the single-site adsorption of all the organic species was developed. The kinetic parameters were regressed by minimizing the objective function describing the deviations between the model-predicted values and the experimental data with the Levernberg-Marquardt algorithm. The activation energies for the first and second steps of the reaction are 7.8945 and 12.2068 kJ/mol, respectively. The activation energy values are shown to be consistent with those reported in the literature. It was shown that the developed model could be used to accurately predict the experimental results over a range of system parameters including catalyst mass, temperature, initial reactant concentrations, and hydrogen pressure with ARE less than 12.7% and AAE less than 0.02 mol/L. The developed kinetic model provides a valuable tool for the further investigations of DCPD hydrogenation reactor simulation and design. Acknowledgment Financial support from the State Key Development Program for Basic Research of China grant G2000048005

and SINOPEC grant X503023 is gratefully acknowledged. Special thanks are given to Dr. Z. Q. Xiong, Dr. A. L. Zhang, and Mr. X. Y. Tang of Tianjin University for their kind assistance and helpful academic discussions. Nomenclature AAE ) average absolute error of experimental and calculated values ARE ) average relative error of experimental and calculated values C ) molar concentration, mol/L De ) effective diffusivity, m2/s dp ) diameter of catalyst particle, m Ei ) activation energy of the ith step reaction, kJ/mol EJ ) adsorption activation energy of the reactant J, kJ/ mol ki ) rate constant for the ith step reaction, mol/L‚gcat‚ MPa‚ min ki,0 ) preexponential coefficient for the ith step reaction, mol/L‚gcat‚ MPa‚min KJ ) adsorption rate constant for species J, L/mol KJ,0 ) preexponential coefficient of adsorption rate constant for species J, L/mol L ) number of experiments performed mc ) catalyst mass, g M ) number of species N ) number of samples per experiment pH2 ) hydrogen partial pressure, MPa Q ) objective function defined by eqs 16.1 and 16.2 ri ) rate of the ith step reaction, mol/L‚min r0 ) initial reaction rate, mol/L‚min robs ) observed reaction rate, mol/gcat‚min R ) ideal gas constant, kJ/mol‚K t ) time, min T ) temperature, K Greek Letters θ ) surface concentration of the free active site ηφ2 ) observable modulus for hydrogen and DCPD Fc ) density of the catalyst, kg/m3 Subscripts A ) dicyclopentadiene B ) dihydrodicyclopentadiene C ) endo-tetrahydrodicyclopentadiene calc ) calculated value exp ) experimental value i ) the ith step of reactions J ) the reactant A, B, or C l ) the lth experiments performed Superscripts 0 ) initial value m ) the mth species

Literature Cited (1) Navratilova, M.; Sporka, K. Synthesis of adamantane on commercially available zeolitic catalysts. Appl. Catal. A Gen. 2000, 203 (1), 127-132. (2) Chung, H. S.; Chen, C. S. H.; Kremer, R. A.; Boulton, J. R.; Burdette, G. W. Recent developments in high-energy density liquid hydrocarbon fuels. Energy Fuels 1999, 13 (3), 641-649. (3) Guo, J. W.; Cui, Y. D.; Mi, Z. T.; Zhang, X. W. Synthesis of adamantane by hydrogenation and isomerization of dicyclopentadiene. Chin. J. Catal. 2001, 22 (3), 271-274.

Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 3851 (4) Chou, S. H.; Chen, S. C.; Tan, C. S.; Wang, W. H. Hydrogenation of dicyclopentadiene in trickle-bed reactor. J. Chin. Inst. Chem. Eng. 1997, 28 (3), 175-182. (5) Silveston, P. L.; Hanika, J. Challenges for the periodic operation of trickle-bed catalytic reactors. Chem. Eng. Sci. 2002, 57, 3373-3385. (6) Yang, J.; Guo, J.; Mi, Z.; Liu, S. Studies on the continuous catalytic hydrogenation of DCPD. Gongneng Cailiao 1998, 29 (4), 475-479. (7) Mi, Z.; Yang, J.; Li, J.; Xu, Y. Studies on catalysts for fixedbed hydrogenation of dicyclopentadiene. Ranliao Huaxue Xuebao 1997, 25 (6), 492-497. (8) Jadhav, S. V.; Pangarkar, V. G. Particle-Liquid Mass Transfer in Mechanically Agitated Contactors. Ind. Eng. Chem. Res. 1991, 30, 2496.

(9) Yawalkar, A. A.; Heesink, A. B. M.; Versteeg, G. F.; Pangarkar, V. G. Gas-liquid mass transfer coefficient in stirred tank reactors. Can. J. Chem. Eng. 2002, 80 (5), 840-848. (10) Jere, F. T.; Jackson, J. E.; Miller, D. J. Kinetics of the aqueous-phase hydrogenation of L-Alanine to L-Alaninol. Ind. Eng. Chem. Res. 2004, 43, 3297-3303. (11) Wilhite, B. A.; McCready, M. J.; Varma, A. Kinetics of Phenylacetylene Hydrogenation over Pt/γ-Al2O3 Catalyst. Ind. Eng. Chem. Res. 2002, 41 (14), 3345-3350.

Received for review December 27, 2004 Revised manuscript received March 10, 2005 Accepted April 5, 2005 IE0487437