Multiphase Catalytic Reactions have wide ranging applications in both bulk and fine chemical industries with a significant impact in evolving new, environmentally benign processes (Mills and Chaudhari, 1997). Many of these processes are combinations of catalytic hydrogenation with condensation, alkylation and amination reactions, which lead to one pot or tandem synthesis of valuable chemicals. For example, conversion of cyclohexanone or substituted cyclohexanones to aromatic amines with specialty applications are carried out such that single catalyst facilitates hydrogenation of intermediate imine and dehydrogenation of amino cyclohexane derivatives (Semikolenov et al, 1989). In other example, reductive alkylation (Roy and Chaudhari, 2005) reaction involves a combination of non-catalytic equilibrium reaction with catalytic hydrogenation. Understanding of the process chemistry as well as reaction engineering aspects is very much important for such tandem catalytic synthesis. In this paper, a detailed case study on kinetic modeling of reductive alkylation of aromatic amines followed by modeling of slurry and trickle bed reactors will be presented.

Reductive alkylation of amines is commercially practiced in a variety of industrial processes for the manufacture of higher (secondary and tertiary) amines, which find applications as an intermediate in fine chemicals (Lehtonen et al, 1998), antioxidant in rubber and petroleum industries (Hayes et al, 2001) and intermediate in dyestuff industries (Mylorie et al, 1999). The reaction involves condensation between an amine compound or its precursor and a carbonyl compound or alcohol to form an imine – a Shiff‘s Base, which is hydrogenated in the presence of a supported metal catalyst to N-alkylated products. A schematic of the reductive alkylation reaction is shown in Scheme I for aniline and acetone where the intermediate N-IPA is the product of industrial interest.

Scheme-I

While the condensation step is a homogeneous non-catalytic equilibrium reaction, the hydrogenation step is catalytic. There are not many studies on the evaluation of intrinsic kinetics and reactor modeling of the reductive alkylation reactions.  Therefore, our laboratory work on intrinsic kinetic studies and modeling of different reactors for different amine compounds are highlighted in this presentation. First, intrinsic kinetics of reductive alkylation of aniline with acetone using 3% Pd/Al₂O₃ catalyst in a semi batch slurry reactor for a temperature range of 378-408K is presented. Further, kinetic modeling to a more complex reaction system with p-phenylenediamine (PPDA) as the amine substrate, wherein two amine functionalities are available for the alkylation reaction will be discussed. The kinetics have been studied using methyl ethyl ketone (MEK) as the alkylating agent, 3% Pt/Al₂O₃ as the catalyst, which was observed to be the best among several supported transition metal catalysts for the diamine substrates. A detailed reaction scheme (Scheme –II) for reductive alkylation of PPDA with MEK is shown below. The effect of relative positions of two amine functionalities in the aromatic ring on activity and selectivity to Di-amine, ortho-, meta- and pera- phenylenediamine (OPDA, MPDA and PPDA respectively) will also be discussed.

Scheme-II

Several rate equations were considered to fit the batch slurry reactor data and rate models based on competitive adsorption of hydrogen (dissociative) and the reactive substrates as the rate limiting catalytic steps were found to represent the experimental data. The kinetic parameters were evaluated by fitting the integral batch reactor data at different temperatures.

Based on these detailed kinetic modeling, a theoretical analysis of non-isothermal trickle bed and bubble column slurry reactors has also been developed. For the trickle bed studies, the model predictions on selectivity of N-IPC as a function of liquid velocity at different aniline concentrations are shown in Figure 1 along with experimental data. These results can be useful in not only understanding the performance behavior of multiphase reactors for such complex reaction systems but also useful in design and selection of suitable reactor types.

Figure 1. Effect of liquid velocity on selectivity to N-IPC in trickle bed reactor at different aniline inlet concentrations

Reaction conditions: P_H2: 4 MPa; Ug: 2.14x10^-3 m/s; temperature: 393K

References

Hayes, K.S. Industrial processes for manufacturing amines. App. Cat.-A General, 221, (2001), 187.

Lehtonen, J.; Salmi, T.; Vaori, A.; Tirronen, E. On the principles of modeling of homogeneous-heterogeneous reactions it the production of fine chemicals. A case study: Reductive alkylation of aromatic amines. Org. Process Res. Dev., 2(2), (1998), 78.

Mills P. L.; Chaudhari R. V., Multiphase catalytic reactor engineering and design for pharmaceuticals and fine chemicals. Catalysis Today, 37, (1997), 367.

Mylorie, V. L. Reductive alkylation process to prepare tertiary aminoaryl cyan dye-transfer intermediates. US 5861535, (1999).

Roy D.; Jaganathan R.; Chaudhari R. V. Kinetic modeling of reductive alkylation of aniline with acetone using 3% Pd/Al₂O₃ catalyst in a batch slurry reactor. Ind. Eng. Chem. Res., 44, (2005), 5388.

Semikolenov, V. A.; Boldyreva, M. E.; Shmidt, Yu. V.; Stepanov, A. G. Formation of 2,6-dimethylaniline from 2,6-dimethylphenol with catalysts obtained via anchoring colloidal palladium compounds on carbon supports. J. Mol. Catal., 55(1-3), (1989) 415-28.