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Mechanistic and reaction engineering aspects of nitrile hydrogenation Proceedings of European Congress of Chemical Engineering (ECCE-6) Copenhagen, 16-20 September 2007

Mechanistic and reaction engineering aspects of nitrile hydrogenation
Peter Schärringer, Thomas E. Müller, Johannes A. Lercher
Department Chemie, Lehrstuhl II für Technische Chemie, Technische Universität München,Lichtenbergstr. 4, 85747 Garching, Germany, Tel. +49 89 28912827, Fax +49 89 28913544, E-Mail: thomas.mueller@tum.de

1. Abstract Liquid phase hydrogenation of nitriles is an important method for the production of primary amines, which find a variety of applications as intermediates in chemical and pharmaceutical industry. Raney-Co or supported Cobalt catalysts are frequently used due to the relativelyhigh selectivity to primary amines. However, selectivities in excess of 95% can only be achieved, when ammonia is used as solvent. Thereby, thermodynamic control of the reaction is achieved as condensation reactions, where ammonia is released, are suppressed. However, liquid ammonia is difficult to handle and it is highly interesting to avoid, or at least to minimise, the addition of ammonia. Thisrequires kinetic control of the reaction by optimizing catalyst properties and process conditions. Keywords: Multi-phase reaction, Trickle-bed reactor, Hydrogenation, Cobalt catalyst, Nitrile 2. Introduction Primary amines are often used as feedstock in the production of, e.g., of fibres for textiles and surface-active compounds. One important industrial process for their manufacture is thehydrogenation of the corresponding nitriles over transition metal catalysts, 1 which is usually accompanied by the formation of secondary and tertiary amines as undesired by-products.2 However, in certain applications even very small quantities of the by-products result in poor quality of the final product.3, 4 Understanding the development of by-products from a mechanistic point of view is considered asan essential prerequisite for further optimization of catalysts and consequently higher selectivity. Already in 1923 it was suggested that the side reactions proceed via reactive aldimine intermediates 2 and ever since numerous mechanistic discussions were based on von Braun’s proposal.5-7 As direct observation of the aldimine had not been reported 5 other possible surface intermediates, such ascarbenes and nitrenes, were included in the discussion.8-10

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Peter Schärringer et al.

A widely used class of catalysts are skeletal Raney catalysts based on Co or Ni.11 Compared to other transition metals (e.g. Ni and Ru) Co is known to exhibit the highest selectivity to primary amines but generally provides relatively low activity.12 A present study in our group aims at establishingstructure-selectivity correlations for Raney-Co13-15 with the aim to generate information on how to enhance the selectivity on highly active hydrogenation catalysts. For studying the sorption of hydrogen containing molecules on metal surfaces Inelastic Neutron Scattering (INS) has proven to be a useful tool.16-18 As the cross-section of hydrogen is 10-100 times larger than that of all otherelements19, INS is particularly sensitive for vibrations involving hydrogen atoms during the sorption of hydrogen and nitriles on Raney catalysts.13, 20-22 In this work, special emphasis was given to the identification of the surface intermediates formed during the hydrogenation of nitriles over a Raney-Co catalyst to unravel the elementary steps on the metal surface. The co-adsorption of acetonitrile-d3(CD3CN) and hydrogen on Raney-Co was investigated as a model reaction. The choice of this model is based on the idea that during the hydrogenation of the CN triple bond the C-H vibrations of the resulting intermediates and the product will be much more pronounced in the INS spectrum compared to the C-D vibrations of the reactant. Thus, a better differentiation between the reactant and surface...