Carbon nanospheres as novel support in the nickel catalyzed gas phase hydrogenation of butyronitrile

Abstract

Three nickel catalysts supported on carbon nanospheres have been prepared by deposition-precipitation with urea (Ni/CNS_DP) and standard impregnation (Ni/CNS_IMP-1 and Ni/CNS_IMP-2). The CNS support was characterized by scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD), Raman spectroscopy and N₂ adsorption/desorption, exhibiting a graphitic structure and limited porosity. Ni/CNS catalysts were characterized by temperature-programmed reduction (TPR), XRD, TEM and N₂ adsorption/desorption. Surface area weighted mean Ni particle diameters (post activation at 623 K) were comparable in the case of Ni/CNS_DP and Ni/CNS_IMP-1 (10.4–12.7 nm) and shifted to 36.6 nm in the case of Ni/CNS_IMP-2. The three catalysts were tested in the gas phase hydrogenation of butyronitrile where 463 K < T < 583 K. All the catalysts exhibited a temporal drop of activity an approached the steady state after 12 hours-on-stream. Extracted specific reaction rates followed the sequence: Ni/CNS_DP < Ni/CNS_IMP-1 < Ni/CNS_IMP-2, where those Ni particles presenting a lower electron density conducted to a stronger adsorption of the reactant (via CN bond) making it more difficult to hydrogenate. Selectivity to the primary amine was favoured in those catalysts presenting a higher electron density, while a lower electron density enhanced condensation reactions leading to higher amines. The three catalysts presented an activity maximum in the temperature interval studied, associated to a change in reactant surface coverage.

Introduction

Carbon materials, with demonstrated resistance to acid/basic media, controllable porosity and surface chemistry and easy burning off (allowing the recovery of the metal) are well established catalyst supports [1], [2]. Activated carbon (amorphous), as a conventional carbon structure, has been successfully employed as metal support in multitude of hydrogenation [3], oxidation [4], hydrodesulphuration [5], hydrodenitrogenation [6] and Fischer–Tropsch [7] reactions among others. Graphite (structured) is another standard carbon material widely employed in catalytic hydrodechlorination [8], hydrogenation [9], oxidation [10] or ring transformation [11] reactions. The discovery of novel carbon nanostructures has meant a step forward in carbon related catalysis, both in their synthesis and potential applications [12], given their high mechanical strength, thermal resistance and appreciable surface area, pointing out to a promising combination of the most interesting properties of the classic ones. In this sense, filamentous nanostructures, such as carbon nanotubes (CNT) and nanofibers (CNF) have been employed in a number of catalytic reactions, such as hydrogenation [9], [13], Fischer–Tropsch [14] and ammonia synthesis [15] reactions. Another novel carbon nanostructure: carbon nanospheres (CNS) has only now started to attract significant research activity. In its spherical arrangement, the graphene sheets are not closed shells but rather waving flakes that follow the curvature of the sphere at different depths, creating many open edges at the surface. These unclosed graphitic flakes provide reactive “dangling bonds” that are proposed to enhance surface reactions, establishing CNS as good candidates for catalytic applications [16]; however, there is a dearth of studies dealing with their use in catalysis. In a previous work, Ni supported on CNS by the deposition-precipitation technique (DP) was successfully employed in the gas phase hydrogenation of nitrobenzene [17]. In this work, two more Ni/CNS catalysts, with varying metal loading introduced by standard impregnation (IMP) were prepared in order to modify the metal particle size/degree of interaction of Ni with the CNS support. These catalysts were tested in the gas phase hydrogenation of butyronitrile (BTN), process that conducts to its primary amine, mono-n-butylamine (MBA) as main reaction product. It can also lead to the corresponding secondary and tertiary amines; di-n-butylamine (DBA) and tri-n-butylamine (TBA), respectively. The production of aliphatic amines has an enormous interest, given their wide spectrum of industrial applications, being involved in the manufacture of fungicides, chelating agents, surfactants or fine chemicals [18]. According to von Braun's mechanism for the hydrogenation of nitriles [19], shown in Fig. 1, MBA is formed via hydrogenation of BTN, passing through the intermediate butylidenimine. Condensation reactions conducting to secondary and tertiary amines occur via the reactive butylidenimine, which is nucleophilicaly attacked by n-butylamine or di-n-butylamine. Subsequent elimination of ammonia yields N-butylidene-butylamine (BBA) or but-1-enyl-dibutylamine as condensation products, to be finally hydrogenated to DBA or TBA, respectively. While the role of the active metal is well recognized for the hydrogenation step, formation of higher amines is still under debate. Some authors have claimed the active metal being the most important factor determining selectivity in hydrogenation of nitriles [20], [21], other studies, however, point to a support effect, where reactions conducting to higher amines may occur in the acid sites of the support [22], [23], [24], [25]. Moreover, metal-support boundaries have been proposed as new active sites that enhance the catalyst activity [26]. The exposed above suggests a complex reaction mechanism, where both metal and support act in tandem, being both activity and selectivity strongly dependent on the catalytic system. The aim of the present work is to test the behaviour of a novel catalyst support (CNS), whose potential surface properties may stimulate surface reactions. Furthermore, this work constitutes the first reported study of nanostructured carbon based catalysts used in the hydrogenation of a nitrile feed.