Selective hydrogenation of nitriles to secondary imines over silica-supported metal catalysts

MILTON AGÜERO; DARÍO J. SEGOBIA; NICOLÁS M. BERTERO; ANDRÉS F. TRASARTI

Buenos Aires

Congreso; 11th World Congress of Chemical Engineering; 2023

Asociación Argentina de Ingenieros Químicos - World Chemical Engineerign Council

Secondary imines and their derivatives are intermediates in the synthesis of valuable products such as fragrances, fungicides, agrochemicals, and inks. The main processes reported in the literature for obtaining secondary imines are: 1) condensation of amines with ketones or aldehydes, 2) oxidative self-coupling of amines, and 3) oxidative amination of alcohols. These methods involve the use of a primary amine as starting material, which has a relatively high cost. As an alternative, it is proposed to obtain secondary imines through the reduction of nitriles, less expensive raw materials.The catalytic hydrogenation of benzonitrile to secondary imine (benzylidenebenzylamine) was studied in this work over silica-supported Co, Ni, Ru, Cu, Pd, Pt and Ir catalysts. These samples were prepared by incipient wetness impregnation method, followed by drying, calcination in air flow and activation in hydrogen flow. The catalytic tests were performed in a commercial stirred tank reactor (Parr), operated in batch mode. BN hydrogenation was carried out in liquid phase at 373 K and 10 bar (H2), employing cyclohexane as solvent, under intense stirring (600 rpm). The concentration of reactant and products was determined during the reaction using a gas chromatograph (GC Agilent 6850) and employing n-dodecane as internal standard.The catalytic activity and selectivity strongly depended on the nature of the metal used in the catalyst formulation. As shown in the Table, the decreasing activity pattern (based on r0 values) resulted Ni > Co > Pt > Ir > Pd >> Ru >> Cu. Ni and Co-based catalysts produced secondary imine (SI) in a high yield. At the end of the tests, a mixture of primary (PA) and secondary amine (SA) (benzyl and dibenzylamine) was obtained. Pt, Pd and Ir showed good activity but were not selective to secondary imine. Finally, Ru and Cu showed poor activity in the benzonitrile hydrogenation. Besides, Ir and Pt produced toluene (TOL) as byproduct.Although Ni was more active than Co, the latter allowed obtaining secondary imine in a higher yield. In agreement with the observations of Volf and Pasek [1], on cobalt catalyst the hydrogenation of the SI to SA did not take place until total BN conversion was achieved, thus giving a higher yield to the desired product.It was concluded that it is possible to obtain valuable benzylidenebenzylamine through the hydrogenation of benzonitrile in liquid phase using Ni and Co catalysts supported on silica, reaching a maximum product yield between 52.8 and 72.5 %.