Résumé
Layered double hydroxides with a hydrotalcite-like structure and containing Ni
2+/Mg
2+/Al
3+ cations in different amounts were prepared and activated under various conditions. These catalysts were tested in the liquid-phase hydrogenation of adiponitrile with the aim of producing aminocapronitrile (ACN). The reaction was carried out in a batch reactor at 323–353 K and 2.5 MPa H
2 pressure with catalysts reduced at 823 K. The products were ACN, hexamethylenediamine (HMDA), azacycloheptane (ACH), and C
12 compounds. The ACH and C
12 byproducts are formed by condensation between “imine-” and “amine-like” adsorbed species on metal and acid sites (bifunctional mechanism), and on the metal sites as well. The tuned addition of Mg (Mg/(Mg+Ni)=0.20) allows us to reach the highest selectivity and yield in ACN (66% selectivity at 70% conversion, 50% yield at 85% conversion). The IR spectroscopy of adsorbed CO provided evidence of the presence of smaller Ni
0 ensembles on the sample with Mg/(Mg+Ni)=0.20, as well as larger back-donation from Ni
0 sites to the 2π* orbitals of CO. On that account, it is proposed that the lower formation of HMDA, ACH, and C
12 byproducts may be mainly due to (i) a faster desorption of ACN from the Ni
0 surface before deeper hydrogenation and (ii) the decrease of transimination reactions which need large Ni
0 ensembles to proceed. The correlation between acidity and condensation reactions is not obvious, since upon Mg substitution for Ni, the number of acid sites increases but their strength decreases concurrently, as shown by temperature-programmed desorption of NH
3.