Insight into the synergetic solvent effect of H2O-ethanol on the adiponitrile hydrogenation

Abstract

The Co@NC catalyst exhibits significant protic solvent preference for hydrogenation of nitriles to primary amines. However, the effect of mixed protic solvents on catalytic hydrogenation has received little attention. Herein, the synergetic solvent effect has been proposed to accelerate the hydrogenation of adiponitrile (ADN) to hexamethylenediamine through H₂O-ethanol hydrogen bond networks on Co@NC catalyst. Experimental screenings on solvents showed that ADN conversion in H₂O-ethanol was 1.6 ~ 5.1 times greater than in single solvents. Kinetic models in H₂O/ethanol (v_W = 0.6), H₂O, and ethanol showed that the solvents effected on H₂ transformation dominated the reaction. Isotope labelling and kinetic experiments revealed that H₂O and ethanol acted as co-catalysts through exchanging and transferring hydrogen via hydroxyl groups. Density functional theory calculations confirmed that the energy barrier for proton transfer mediated by H₂O–ethanol was reduced by 0.18 eV compared to proton transfer mediated by H₂O–H₂O dimers.