Ind. Eng. Chem. Res. 1992,31, 2069-2074
2069
KINETICS AND CATALYSIS Kinetic Modeling of Oxidative Carbonylation of Aniline over Pd/C-NaI Cata1yst+ Sunil P.Gupte a n d R a g h u n a t h V. Chaudhari* Chemical Engineering Division, National Chemical Laboratory, Pune 41 1 008, India
Kinetics of oxidative carbonylation of aniline to N,”-diphenylurea has been investigated using the Pd/C-NaI catalyst system. The effect of the concentration of aniline, catalyst loading, partial pressures of CO and 02,and temperature on the rate of reaction has been studied. The rate varied linearly with catalyst and with 1.5 order with respect to aniline concentration. With an increase in CO partial pressure, the rate first increased and then was independent of pco at higher pressures. The rate vs PO, plot passed through a maximum indicating substrate inhibition a t higher po,. On the basis of these rate data, a rate equation has been proposed and kinetic parameters have been evaluated. The activation energy was found to be 82.7 kJ/mol. Introduction Oxidative carbonylation of amines is important in the synthesis of alkyl and aryl carbamates having applications in agrochemical and polymer industries. A new process for methylenediphenyl diisocyanate (MDI) has been developed on the basis of this non-phosgene route (Fukuoka et al., 1984a). An important step in this process involves catalytic oxidative carbonylation of aniline to phenylurethane. The stoichiometric reactions are 2ArNH2 + CO + f/202 CON A r + HzO (1) -Nf-%phen$urea ArNHCONHAr + ROH Ar HC OR + ArNH2 (2) pheny car amate The previous work on this reaction relates to the reports on synthesis of phenylurethane using different types of catalysts (Fukuoka and Chono, 1984; Fukuoka et al., 1984b). Recently, Kelkar et al. (1992) investigated selectivity behavior in oxidative carbonylation of methylamine to methyl N-methylcarbamate, Similarly, the role of catalyst pretreatment and selectivity behavior in oxidative carbonylation of aniline using supported Pd catalyst has been reported in our earlier paper (Gupte and Chaudhari, 1988). Being a case of complex multiphase catalytic re action, understanding of the kinetics of oxidative carbonylation of aniline is also very important, but this aspect has not been addressed in any of the previous studies. Therefore, the aim of the present work was to undertake a detailed investigation on the kinetics of oxidative carbonylation of aniline to diphenylurea (reaction 1) and develop a rate equation. Such a study would be useful in view of the industrial importance of this reaction.
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Experimental Section The details of materials, analytical methods, and experimental setup, etc., were similar to that described in our earlier paper (Gupte and Chaudhari, 1988). Experiments for the kinetic study were performed in a 3 X 1044-m3stirred batch autoclave reactor (Parr Instrument
* To whom correspondence should be addressed. N.C.L. Communication No. 5057.
Co., Moline, IL), a schematic of which is given in Figure 1.
Kinetic Study. In a typical experiment, known quantities of aniline, 5% Pd/C catalyst, NaI, and solvent (DMF) were charged into the autoclave, and the contents were flushed with nitrogen. The autoclave was heated to a desired temperature, and, after attaining the set temperature, the system was pressurized with a CO/02 mixture (in a molar ratio of 2:l; proper safety precautions against any explosion hazards were employed during this work) to the level required. At this stage, the experiment was started by switching the stirrer on. In order to maintain constant pressure in the reactor, the CO/02 mixture was fed through a constant pressure regulator from a reservoir vessel. The pressure drop in the reservoir vessel was recorded by means of a pressure transducer connected to a strip chart recorder. Since the consumption of CO/02 by the reaction is expected to be in a ratio of 2:l as per the stoichiometry (reaction l), this procedure ensured maintenance of not only a constant pressure in the reactor but also a constant ratio of CO:02 as preset at the beginning of the reaction. At the end of the reaction, gas samples were analyzed for CO, 02,and COP In each kinetic run, initial and final liquid samples were also analyzed for concentrations of reactants and products to check the gas- and liquid-phase material balance. Initial rates of aniline reacted were calculated from the experimentally observed pressure drop vs time data. For this purpose, the amount of CO + O2 consumed in the ratio of 2:l was calculated from the pressure drop in the reservoir vessel. Following this procedure, the effect of various parameters on the rate of reaction was studied. Results and Discussion Preliminary Experiments. Some preliminary results on oxidative carbonylation of aniline to N,”-diphenylurea using 5% Pd/C-NaI catalyst, reported earlier (Gupte and Chaudhari, 1988),established that reaction 1is catalytic in nature, while reaction 2 is noncatalytic. Therefore, only reaction 1needs to be considered for kinetic analysis. A typical concentration profile of gaseous reactants CO + O2 and aniline consumed as a function of time is shown in
0888-5885/92/2631-2069$03.00/00 1992 American Chemical Society
2070 Ind. Eng. Chem. Res., Vol. 31, No. 9, 1992 L
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Figure 2. Typical concentration profile of oxidative carbonylation of aniline: temperature, 323 K; total pressure, 4.82 X lo3 kPa; 5% Pd/C, 5.0 kg/ms; NaI, 8.7 X kmol/m3; stirrer speed, 16.7 Hz; solvent, DMF,0.9 X lo-* m3.
Figure 2 for a temperature of 323 K. For the range of conditions chosen in this work, it was o b ~ e ~ that e d almost no side reactions were found to occur. Only those experiments in which the material balance agreed as per the stoichiometry in reaction 1to the extent of 95% and above were considered for the initial rate calculations. Reproducibility of rate measurement was within 5-7% error. In order to ensure that the activity of catalyst was constant during a run,catalyst recycle experiments were carried out, which indicated constancy of the catalyst activity even after four recycles. Earlier investigations showed that activity of the catalyst system Pd/C-NaI depends on the concentration ratio of NaI to catalyst (Gupte and Chaudhari, 1988). In the present investigation, this ratio was kept constant at 0.26. Another important aspect that deserves consideration in this specific case is the effect of catalyst pretreatment. In our earlier work, it has been shown that pretreatment
of Pd/C catalyst with all the reactants resulted in a decrease in the initial activity compared to that without any pretreatment (Gupte and Chaudhari, 1988). Pretreatment with O2in particular resulted in a significant decrease in the activity of the catalyst. The selectivity of formation of N,”-diphenylurea was, however, not found to be affected. The decrease in catalyst activity due to pretreatment was found to be reversible, as the catalyst regains most of its activity at the end of the reaction (Gupte and Chaudhari, 1988). In order to have consistency in the rate data for kinetic analysis, the kinetic experiments were carried out without any catalyst pretreatment. Initial Rate Data. The effects of aniline concentration, temperature, partial pressures of CO and 02,and catalyst loading on the initial rate of reaction were studied. The effects of individual parameters on the initial rate are discussed below. Effect of Catalyst Loading. The effect of catalyst loading on the initial rate of oxidative carbonylation of
Ind. Eng. Chem. Res., Vol. 31,NO. 9,1992 2071
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Figure 5. Effect of oxygen partial pressure on reaction rate of aniline: aniline, 1.08 kmol/m3;5% Pd/C, 5.0 kg/m3;NaI, 8.7 X kmol/m3;pco, 3.21 X 103 !&a'; stirrer speed, 16.7 Hz; solvent, DMF, 0.9 x lo-' m3.
aniline at 323,333,and 343 K is shown in Figure 3. The rate showed linear dependence on catalyst loading at all temperatures. This observation suggests that gas-liquid mass-transfer resistance may not be important under these conditions. Effect of Partial Pressure of Carbon Monoxide. The effect of partial pressure of CO on the initial rate is shown in Figure 4 for 323,333,and 343 K at a constant O2pressure of 1.61 X lo3kPa. In the lower range of pco, (
