ABSTRACT
Abstract Supported Co, Ni, Ru, Rh, Pd and Pt as well as Raney Ni and Co catalysts were used for the hydrogenation of dodecanenitrile to amines in stirred SS autoclaves both in cyclohexane and without a solvent. The reaction temperature and the hydrogen pressure were varied between 90-140 °C and 10-80 bar, respectively. Over Ni catalysts NH3 and/or a base modifier suppressed the formation of secondary amine. High selectivity (93-98 %) to primary amine was obtained on Raney nickel, Ni/Al2O3 and Ru/Al2O3 catalysts at complete nitrile conversion. With respect to the effect of metal supported on alumina the selectivity of dodecylamine decreased in the order: Co~Ni~Ru>Rh>Pd>Pt. The difference between Group VIII metals in selectivity can be explained by the electronic properties of d-band of metals. High selectivity to primary amine was achieved on base modified Raney Ni even in the absence of NH3. Introduction Catalytic hydrogenation of nitriles is an industrially important route for the manufacture of amines [1]. The amines have wide ranging applications such as solvents and intermediates of agrochemicals and pharmaceuticals. Fatty amines are used as components of different chemical products, e.g. emulsifiers, softeners, corrosion inhibitors. The hydrogenation of nitriles typically gives a mixture of primary, secondary and tertiary amines. This hydrogenation is usually carried out on heterogeneous catalysts under high pressure and temperatures of 55-150 °C. The catalysts most often used are Group VIII metals on Al2O3, SiO2 or MgO, Raney nickel or Raney cobalt [2]. While the desired product of the hydrogenation of nitriles is often the primary amines, the proportion of primary/secondary/tertiary amines in the product is strongly affected by the nature of metal. In the hydrogenation of nitriles on Group VIII metals, the selectivity of primary amine decreases in the order: Co>Ni>Ru>Rh>Pd>Pt [1]. The difference between Group VIII metals in selectivity to primary amine is explained by the difference in the electronic
properties of the metals [3]. Pd and Pt preferentially form tertiary amines while Cu and Rh are selective towards secondary amines and Co, Ni and Ru have high selectivity for primary amines [4]. For instance, in the hydrogenation of acetonitrile, the selectivity to primary amine decreased in the order of Ru>Ni>Rh>Pd>Pt. Ru is highly selective for primary amine and Pt for tertiary amine. The selectivity of the metals is related to their propensity for multiple bond formation. The metal known for the highest propensity to form multiple bonds has the highest selectivity to primary amine [5]. Furthermore, the inhibition of the sites responsible for condensation reactions resulting in secondary and tertiary amines can be affected by the addition NH3 or base compounds, e.g. alkalis [1]. In this study Al2O3 supported Co, Ni, Ru, Rh, Pd and Pt, as well as Raney Ni and Co catalysts were used for the hydrogenation of dodecanenitrile (RCN) to amines in cyclohexane and without a solvent. The effect of metal, reaction conditions and modifiers on the selectivity was investigated. Experimental Section Catalysts - A commercial Raney nickel (RNi-C) and a laboratory Raney nickel (RNi-L) were used in this study. RNi-C was supplied in an aqueous suspension (pH < 10.5, Al < 7 wt %, particle size: 0.012-0.128 mm). Prior to the activity test, RNi-C catalyst (2 g wet, 1.4 g dry, aqueous suspension) was washed three times with ethanol (20 ml) and twice with cyclohexane (CH) (20 mL) in order to remove water from the catalyst. RCN was then exchanged for the cyclohexane and the catalyst sample was introduced into the reactor as a suspension in the substrate. RNi-L catalyst was prepared from a 50 % Ni-50 % Al alloy (0.0450.1 mm in size) by treatment with NaOH which dissolved most of the Al. This catalyst was stored in passivated and dried form. Prior to the activity test, the catalyst (0.3 g) was treated in H2 at 250 °C for 2 h and then introduced to the reactor under CH. Raney cobalt (RCo), a commercial product, was treated likewise. Alumina supported Ru, Rh, Pd and Pt catalysts (powder) containing 5 wt. % of metal were purchased from Engelhard in reduced form. Prior to the activity test, catalyst (1.5 g) was treated in H2 at 250 °C for 2 h and then introduced to the reactor under solvent. 10 % Ni and 10 % Co/γ-Al2O3 (200 m2/g) catalysts were prepared by incipient wetness impregnation using nitrate precursors. After drying the samples were calcined and reduced at 500 °C for 2 h and were then introduced to the reactor under CH. Activity tests - Catalytic activity tests were carried out in 70 or 300 ml stainless steel stirred autoclaves (Parr Co.) at a stirring rate of 1000 rpm. Reaction conditions are given in the Tables. Analysis of reaction products - Liquid reaction products were analyzed by GC using a capillary column CP-Sil-8CB (WCOT Fused Silica - stationary phase: 5% phenyl-methyl-polysiloxane, length: 50 m, ID: 0.32 mm, OD: 0.45 mm, film
thickness: 0.25 µm). Liquid samples were diluted with ethanol in ratio sample/ethanol = 1/15. Hexadecane was used as an internal standard in the quantitative analysis. Results and Discussion Raney-nickel catalysts - The effect of NH3 and base modifier on the activity and selectivity of RNi-C catalyst is shown in Table 1. The addition of NH3 significantly decreased the pseudo first-order rate constants, the conversion of RCN and the selectivity to R2NH. Upon increasing the reaction time (t) on Table 1 Hydrogenation of RCN without solvent on RNi-C catalyst at T= 125 °C.
