Selective Hydrogenation of Nitriles to Primary Amines on Metal-supported Catalysts

DARÍO J. SEGOBIA; ANDRÉS F. TRASARTI; CARLOS R. APESTEGUÍA

Louisville, Kentucky

Congreso; 23rd North American Catalysis Society Meeting; 2013

North American Catalysis Society

The hydrogenation of nitriles via homogeneous or heterogeneous catalysis is an important route to produce the corresponding amines that are widely used in industry as intermediates for funguicides, pesticides, pharmaceuticals, and other chemicals. Although saturated nitriles are initiallyhydrogenated to primary amines, the condensation reactions between the highly reactive imineintermediate and amines usually produce also secondary and tertiary amines. Figure 1 shows the reaction network of butyronitrile (BN) hydrogenation according to currently accepted reaction pathways. Unsupported Raney Co and Ni catalysts have been widely employed in liquidphase but supported-metal catalysts, specially using transition metals, have been lately studied in an attempt to overcome the difficulties of handling Raney catalysts. In this work, we have studied the liquid-phase hydrogenation of butyronitrile to n-butylamine on Ni, Co, Ru, Cu, Pd, and Pt metals supported on an inert single oxide (silica), using ethanol as solvent and without the presence of any additive such as ammonia in the reactor. Silica-supported catalysts were prepared by supporting Co, Ni, Cu, Pt, Pd, or Ru on a commercial SiO2 (Sigma-Aldrich G62) by incipient-wetness  impregnation at 303 K. Catalysts were characterized by a variety of physical and spectroscopic techniques. The metal dispersion (DM) was determined by H2 chemisorption or by titration with N2O. The main reaction products were n-butylamine (BA), dibutylamine (DBA), tributylamine (TBA), butylidene-butylamine (BBA), and N-ethylbutylamine (EBA). The initial reaction rate (r, mmol/h g), and BN conversion (XBN) and selectivities (Si) at the end of the runs at 373 K and 13 bar are presented in Table 1. The values show that the catalyst activity followed the order: Ni > Co > Pt > Ru > Cu > Pd. Pd/SiO2 and Cu/SiO2 did not form BA and produced essentially DBA, while Pt/SiO2 formed mainly DBA and minor amounts of BA and TBA. Ru/SiO2 formed  preponderantly BA but also produced significant amounts of DBA and BBA. The highest  selectivity to BA was obtained on Ni/SiO2 (SBA=84%). Co/SiO2 was also highly selective to BA giving 83 % BA after 90 min of reaction; then, the BA concentration diminished because it reacted with the solvent (ethanol) to form EBA. In an attempt to improve the BA yields we carried out additional catalytic tests changing the reaction conditions. Specifically, we evaluated the Ni/SiO2 and Co/SiO2 activity and selectivity for BN hydrogenation at a low temperature (343 K) and higher H2 pressure (25 bar). The selectivity to BA on Ni/SiO2 at 13 bar was 78%, slightly lower than the SBA value determined at 373 K (84%). In contrast, the BA selectivity on Co/SiO2  (93%) clearly increased as compared with that obtained at 373 K (74%). N-ethylbutylamine was not detected among the products on Co/SiO2 indicating that the ethanol/butylamine reductive amination reaction did not take place at 343 K. The initial BN conversion rate on Ni/SiO2 and Co/SiO2 clearly increased when PH2 was increased from 13 bar to 25 bar, showing that the reaction was order one with respect to H2. Regarding the effect of H2 pressure on catalyst selectivity, Table 2 shows that on Ni/SiO2 the selectivity to BA slightly diminished when PH2 was increased from 13 bar to 25 bar, probably because the order in H2 for hydrogenating butylimine to BA on Ni is lower in  comparison with the order for BN hydrogenation to butylimine . On the contrary, the selectivity (and the yield) to BA on Co/SiO2 at the end of reaction increased with PH2, from 93 % at 13 bar to 97 % at 25 bar, suggesting a positive effect of the H2 pressure on BA selectivity.