Concurrent Hydrogenation of Aromatic and Nitro Groups over Carbon-Supported Ruthenium Catalysts Patrick Tomkins, † Ewa Gebauer-Henke, † Walter Leitner, ‡ and Thomas E. Mü ller* ,† † CAT Catalytic Center, RWTH Aachen University, Worringerweg 2, 52074 Aachen, Germany ‡ Institut fü r Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany * S Supporting Information ABSTRACT: The concurrent hydrogenation of aromatic and nitro groups poses particular challenges due to the highly differing adsorption strengths of the two chemical moieties on the surfaces of metal catalysts. In a study of the hydrogenation of nitrobenzene as a model reaction, catalysts of ruthenium supported on carbon nanotubes (Ru/CNT) provided an ideal compromise, allowing for hydrogenation of both the aromatic ring and the nitro group. The use of methyl-labeled substrates enabled tracking the pathway of specific substrates and obtaining insight into the relative rates for the hydrogenation of nitrobenzene and intermediates. Together with findings on the coadsorption of nitrobenzene and aniline on the Ru/CNT catalyst, an advanced mechanistic model for the hydrogenation of nitrobenzene emerges. KEYWORDS: nitrobenzene hydrogenation, amine synthesis, heterogeneous catalysis, reaction network, competitive adsorption, Ru/CNT catalyst ■ INTRODUCTION Many important fine chemicals, 1 agrochemicals, 2,3 pharmaceut- icals, 4 and polymer building blocks have been characterized by the presence of multiple functional groups. Selective hydro- genation of corresponding precursor molecules bearing several unsaturated moieties is frequently applied to generate these functional groups. 5-10 However, with heterogeneous catalysts, the presence of a strongly coordinating functional group poses particular challenges for the hydrogenation of more weakly adsorbing groups. Further complexity arises when starting materials and intermediates compete with each other for adsorption on the metal surface. An industrially relevant example is the hydrogenation of nitroaromatic compounds to the corresponding cycloaliphatic primary amines. 11-13 In such nitroaromatic compounds, the aromatic ring is electron deficient due to the strong electron- withdrawing effect of the nitro group. 14 Consequently, the aromatic ring coordinates only weakly to metals typically employed in hydrogenation reactions. 15 In contrast, the nitro group is strongly coordinating. 16 Due to the competing adsorption modes, 17 the nitro group has a high propensity to being hydrogenated first (Scheme 1, path A/B). However, the aromatic moiety can also be hydrogenated first (path C/D), as reported for substrates where the aromatic ring and nitro group are not directly linked. 18 Also conceivable is a direct hydrogenation pathway that does not involve desorption of an intermediate from the catalyst surface. When a Langmuir- Hinshelwood-type mechanism applies, the prevailing pathway is ruled by the propensity of the particular group to be chemisorbed on the metal surface. 7,19-23 Received: August 2, 2014 Revised: November 9, 2014 Published: November 18, 2014 Scheme 1. Analysis of Possible Reaction Pathways for the Hydrogenation of Nitrobenzene and Methyl-Substituted Analogues Research Article pubs.acs.org/acscatalysis © 2014 American Chemical Society 203 dx.doi.org/10.1021/cs501122h | ACS Catal. 2015, 5, 203-209