Ind. Eng. Chem. Res. 1996, 35, 2091-2095
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetic Modeling of the Hydrogenation of 1,5,9-Cyclododecatriene on Pd/Al2O3 Catalyst Including Isomerization M’hamed Benaissa, Galo Carillo Le Roux, Xavier Joulia, Raghunath V. Chaudhari,† and Henri Delmas* Laboratoire de Ge´ nie Chimique, UMR 5503 CNRS ENSIGC/INPT, 18 chemin de la loge, 31078 Toulouse Cedex, France
The kinetic modeling of the hydrogenation of 1,5,9-cyclododecatriene has been studied using a Pd/Al2O3 catalyst in a slurry reactor on the basis of a complete isomers analysis. A reaction scheme involving multistep hydrogenation and reversible isomerization reactions has been considered and a kinetic model of Langmuir-Hinshelwood type proposed. The kinetic parameters were estimated from experimental batch slurry reactor data. The model predictions of cyclododecene selectivity at different pressures agree well with the experimental data. Introduction Hydrogenation of polyunsaturated hydrocarbons is important in several industrial processes for the synthesis of organic intermediates as fine chemicals and pharmaceuticals. An important example is the selective hydrogenation of 1,5,9-cyclododecatriene (CDT) to cyclododecene (CDE). The product cyclododecene is a key intermediate for 12-laurolactam and 1,10-decanedicarboxylic acid used in the pharmaceutical industry. Other applications are in the production of dyes, chemicals for plant protection, freeze resisting plasticizers, and synthetic lubricants. Several types of supported metal catalysts consisting of Raney-Ni, Pd, Pt, Rh, and Ru have been proposed (Barinov et al., 1974a), but supported Pd catalysts are generaly used in industry (McAlister, 1968). This reaction is an example of a complex multistep catalytic reaction involving several parallel and consecutive reaction steps, and hence, selectivity of partial hydrogenation to cyclododecene is an important aspect. The selectivity of the intermediate would depend on the optimum choice of a catalyst as well as the kinetics of the different reaction steps. Being a three-phase catalytic reaction, the overall rate of hydrogenation and selectivity would also depend on the interphase and intraparticle diffusion effects. As a first step in understanding the selectivity behavior of such a complex multiphase reaction, it is necessary to investigate the intrinsic kinetics of the reaction. In a previous work, McAlister (1968) reported the optimum conditions for selective hydrogenation of 1,5,9cyclododecatriene using Pd/C catalyst but indicated that several isomers are formed during the hydrogenation depending on the reaction conditions. Barinov et al. (1974a, 1974b) studied the activity and selectivity of Rh/ Al2O3, Pd/Al2O3, Ru/Al2O3, and Pt/Al2O3 and found that the activity and selectivity of hydrogenation catalysts decreased in the order, Rh g Pd > Ru > Pt. Hanika et al. (1980) investigated the kinetics of the reaction using * Author to whom correspondence should be addressed. † Present address: Chemical Engineering Division, NCL, Pune 411008, India.
S0888-5885(96)00011-5 CCC: $12.00
Figure 1. Reaction scheme for the catalytic hydrogenation of 1,5,9-CDT with lumped isomers.
supported Pd catalysts, at atmospheric pressure of hydrogen and 300 K, using a simplified first order kinetics. These authors also reported concentrationtime profiles with some selected isomeric substrates. Stu¨ber et al. (1995) reported a detailed kinetic modeling using 0.5% Pd/Al2O3 catalyst. The effects of hydrogen pressure, catalyst particle size, and agitation speed on the concentration-time profiles were studied over a temperature range 413-453 K. The kinetic modeling of the successive hydrogenation reactions of 1,5,9-CDT using a batch reactor model was based on a lumped reaction scheme (see Figure 1) without discriminating the isomers. A detailed evaluation of different types of Langmuir-Hinshelwood models was carried out. While dissociative hydrogen adsorption is certainly occurring, no significant effect of hydrogen adsorption competing with unsaturated hydrocarbons was observed from discrimination of different rate models, due to the very high hydrocarbon concentration. Then the following model was proposed
ri )
Ri mcatkiKjCjPH 2 3
(1 +
(1)
KjCj) ∑ j)1
where i represents the number of the reaction and j the reactant species [1, cyclododecatriene (CDT); 2, cyclododecadiene (CDD); and 3, cyclododecene (CDE)]. It was reported that the effect of hydrogen pressure on the selectivity could be described only by considering a higher reaction order for the hydrogenation of CDE to cyclododecane (CDA). Though this leads to a good fit of the data, the arbitrary change in the reaction order does not conform to any reaction mechanism or catalytic phenomena. © 1996 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996
Figure 2. A schematic setup for kinetic studies.
In none of the previous studies has a detailed consideration of the simultaneous isomerization and hydrogenation reactions been given, which may be important to understand the selectivity behavior. The aim of this work was to study a detailed kinetics incorporating the various parallel and consecutive reactions and propose a more rational kinetic model. Experimental Section The hydrogenation experiments were carried out in a stirred batch autoclave (500 cm3 capacity) reactor, supplied by Autoclave Engineers. A schematic of the experimental setup is shown in Figure 2. This reactor was provided with automatic temperature control, arrangement for sampling of liquids, and variable agitation speeds. The experiments were carried out at constant H2 pressure by continuous supply of H2 from the reservoir vessel through a constant pressure regulator. The observation of H2 pressure drop as a function of time in the reservoir allowed monitoring of the progress of the reaction. The liquid samples were also withdrawn at different time intervals to analyze for the various reactants and products. The analysis of the liquid samples was carried out using a HP 5890 gas chromatograph with a HP-FFAP capillary column (25 m × 0.2 mm × 0.33 µm). The various isomers of the triene, diene, and monoene were identified by a GCNMR analysis. Product Distribution and Reaction Scheme The isomerization of olefins during hydrogenation of polyunsaturated hydrocarbons is known to occur, but most of the published information is qualitative. McAlister (1968) and Hanika et al. (1981) have observed formation of several isomerization products during hydrogenation of 1,5,9-cyclododecatriene, but a complete quantitative description of the concentration-time profiles and a kinetic analysis of these were not available from these studies. The substrate 1,5,9-CDT used in
Figure 3. A detailed reaction scheme for the hydrogenation of 1,5,9-CDT including isomerization steps.
this work itself was found to contain three isomers, 97% cis,trans,trans (ctt), 2% trans,trans,trans (ttt), and 0.5% cis,cis,cis (ccc). During the hydrogenation in the presence of Pd/Al2O3 catalyst, the composition of the isomers varied significantly and 14 different isomers were detected by GC-NMR analysis. These include four CDT, three CDD, and two CDE geometrical isomers. Furthermore, two CDT and three CDD positional isomers were also observed. However, the concentration of the positional isomers was negligible. On the basis of these observed product distribution profiles, a reaction scheme has been proposed for simultaneous isomerization and hydrogenation of 1,5,9-CDT, as shown in Figure 3. Typical results of concentration-time behavior in a batch slurry reactor at a constant hydrogen pressure are shown in Figure 4. An important observation was that, with an initial decrease in ctt isomer, a sharp increase in the concentration of other isomers of CDT was found along with the hydrogenation products. For example, ttt isomer to the extent of 12% was formed (see Figures 4 and 5). The effect of H2 pressure on the variation of the tt-CDD isomer profile was found to be very significant while its effect on the cc-CDD isomer profile was negligible (see Figure 6). Moreover, cc-CDD forms and disappears faster than tt-CDD, as also evident from the observed maxima, which occur later for tt-CDD, proving that cc-CDD is more reactive. These results clearly indicate that the approach of
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Figure 4. Concentration-time profile of CDT and CDD isomers: Comparison of experimental data with model predictions. (a) >0.2 kmol/m3 CDT and (b)
