Résumé
Layered double hydroxides (LDHs) with a hydrotalcite-like structure and containing Ni2+/Co2+/Mg2+/Al3+ cations in different amounts were prepared and activated in various conditions. Depending on the chemical composition and the calcination temperature, mixed-oxide and spinel-like phases of complex compositions are obtained. They lead to well-dispersed bimetallic phases of high metal loadings upon reduction. Temperature-programmed reduction by H2 showed that the introduction of Mg decreases the reducibility of metals and that most of the Ni and Co are together in bimetallic aggregates. These catalysts were tested in the gas-phase hydrogenation of acetonitrile between 350 and 450 K and with a H2/CH3CN molar ratio of ca. 33. The main product is ethylamine (MEA); secondary products are N-ethyl,ethylimine at low conversion, diethylamine and triethylamine at high conversion. The Ni-free catalyst is three orders of magnitude less active than the Ni-containing samples. The by-products are formed by condensation between “imine-” and “amine-like” adsorbed species on metal and acid sites (bifunctional mechanism) and on the metal sites alone as well. The tuned addition of Mg (Mg/(Mg+Ni+Co)≈0.25) lowers the surface acidity and the bifunctionalized formation of by-products consequently. A net increase in MEA selectivity is further reached thanks to the formation of bimetallic NiCo phases. It is proposed that by-product formation on the metal surface occurs by condensation at Ni0 sites between multibonded adsorbed species, which could be of the acimidoyl and aminomethylcarbene types. The first role of Co is the dilution of the Ni surface in small ensembles less prone to accomodate neighboring multibonded species. The IR spectroscopy of adsorbed CO provided evidences of the dilution of Ni by Co in bimetallic NiCo particles. A catalyst obtained from the Co/Ni/Mg/Al (0.27/0.26/0.22/0.25) LDH, calcined at 393 K and then reduced at 893 K, exhibits the highest selectivity to ethylamine, 98.2% at 10% CH3CN conversion.