Cyanide-Free and Green Synthesis of Hexamethylenediamine from the Catalytic Amination of 1,6-Hexanediol over Hydrotalcite-Derived Ni/AlO _<i>x</i> and Co/AlO _<i>x</i> Catalysts

Hexamethylenediamine (HMDA) is a key monomer for producing high-performance nylon-66 and nylon-610. Developing an efficient, eco-friendly industrial process for HMDA synthesis is therefore essential. HMDA was efficiently prepared through the amination of 1,6-hexanediol (HDO) and its intermediates (6-amino-1-hexanol (AHO) and hexamethylenimine (HMI)) using Ni/AlOx and Co/AlOx catalysts derived from hydrotalcite. Physicochemical characterization of Ni/AlOx and Co/AlOx catalysts before and after reaction revealed the nature and evolution of metallic active sites, linking the structure directly to function. Density functional theory (DFT) calculations quantified the adsorption energies of key intermediates on representative surface sites and the Gibbs free energy of the main pathway. The Ni/AlOx catalyst achieved 72.0% HDO conversion and 99.0% selectivity to amines (HMI + AHO + HMDA) under the optimized conditions. The Co/AlOx catalyst achieved 13.5% HMI conversion and 99.3% selectivity to HMDA and 14.3% AHO conversion with 86.3%/97.5% selectivity to HMDA/amines. Atomic utilization reached 57.4%. Catalyst deactivation is primarily attributed to reversible pore blockage, and the catalyst exhibits excellent regenerability. DFT calculations reveal a stepwise Ni(111) surface mechanism: hydroxyl-containing species (R–OH) dehydrogenate to aldehydes at bridge sites; aldehydes undergo nucleophilic amination to imines at top sites; and imines hydrogenate to primary amines at bridge sites.