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Catalytic Hydrogenation Reactions in Fine Chemicals: Applications in Tandem Synthesis and Relevant Reaction Engineering Issues

R. V. Chaudhari1, Narendra G. Patil1, Amit S. Chaudhari1, and Patrick L. Mills2. (1) Homogeneous Catalysis Division, National Chemical Laboratory, Pune-411008, India, (2) Department of Chemical and Natural Gas Engineering, Frank H. Dotterweich College of Engineering, Texas A & M University-Kingsville, 700 University Blvd,MSC 188, Kingsville, TX 78363-8202

Catalytic hydrogenation 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). Several new applications are emerging (Chaudhari and Mills, 2004) in which the combination of catalytic hydrogenation with condensation, alkylation and amination reactions have led to one pot synthesis of valuable chemicals. For example, reductive alkylation (Roy and Chaudhari, 2005) and amination reactions involve a combination of equilibrium reactions with catalytic hydrogenation. In other examples, 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. Such catalytic synthesis involves not only design of a selective catalyst but also the understanding of reaction engineering implications of coupling of endothermic and exothermic reactions. In this paper catalytic hydrogenation reactions with such complexities will be reviewed with detailed presentation of a case study on reductive alkylation of p-phenylenediamine using Pt/Al2O3 catalyst.

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). An important example of this class of reactions is the reductive alkylation of p-phenylenediamine to N, N'-Di-sec alkyl p-phenylenediamine, which is an intermediate in the manufacture of inhibitor sweetening agents, (Symon et al, 1979), antioxidant in rubber and petroleum industries (Hayes et al, 2001), intermediate in dyestuff industries (Mylorie et al, 1999) etc. The reductive alkylation reaction goes through a condensation reaction between an amine compound or its precursor and a carbonyl compound or alcohol to form an imine – a Shiff‘s Base - (Ege et al, 1994), which is hydrogenated in the presence of a metallic catalyst to N-alkylated products. Reductive alkylation of nitro or amine compounds has been investigated using a wide range of alkylating agents and catalysts. Most commonly used alkylating agents are ketones and aldehydes. Aldehydes give N, N-dialkylated products, while ketones give N-monoalkylated compounds due to its relatively higher molecular size and lower activity. Sometimes alcohols have also been used for the alkylation; these alcohols first dehydrogenate to the corresponding carbonyl compounds and then initiate the condensation reaction with amine functionality. Two major side reactions: ring hydrogenation and hydrogenation of the carbonyl compounds to the corresponding alcohols were significant in case of ruthenium and rhodium. It has been observed that platinum tends to become more effective than palladium as the molecular weight of ketone and amine increases.

In addition to being industrially important, the reductive alkylation reaction is an interesting reaction system comprising simultaneous homogeneous and heterogeneous reaction steps. While the condensation step is a homogeneous equilibrium reaction, the hydrogenation step is catalytic. There are not many studies on the evaluation of intrinsic kinetics of the reductive alkylation reactions. Hence, the objective of this paper was to evaluate the intrinsic kinetics of reductive alkylation p-phenylenediamine with methyl ethyl ketone using 3% Pt/Al2O3 catalyst in a semi batch slurry reactor. The overall reaction involves a combination of consecutive and parallel reactions with catalytic and non-catalytic steps. The experiments were carried out in a batch slurry reactor. The progress of the reaction was followed by recording the concentration of the reactants and products in the liquid phase as well as the hydrogen consumption as a function of time. The homogeneous condensation reaction was studied separately to evaluate the rate parameters for the condensation step. For the purpose of kinetic modeling of the overall reaction, the effects of p-phenylenediamine concentration, catalyst loading, agitation effect, temperature and H2 partial pressure were studied on concentration-time and hydrogen consumption–time profiles in a temperature range of 373-413 K. Different rate equations have been proposed considering all the elementary steps involving catalytic and non-catalytic reactions, which lay stress on the different types of interactions occurring between the reactants, products and the active catalyst particles. The best rate model has been chosen by rigorous optimization and model discrimination techniques by simulating the experimental concentration-time data. For the best model the agreement between the model predictions and the experimental data was found to be very good over a wide range of operating conditions.


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