Reductive amination of furfural to furfurylamine with high selectivity: effect of phase type of the nickel phyllosilicate precursor

Abstract

This study aimed to elucidate the importance of the phase type of catalysts on the catalytic performance of reductive amination. The typical precursors of nickel phyllosilicate, namely pimelite (denoted as Ni/SiO₂) and pecoraite (denoted as NiSi-T), were synthesized and utilized for the reductive amination of furfural (FAL) to produce furfurylamine (FAM). Ni/SiO₂ exhibited a significantly stronger metal–support interaction (SMSI), which diminished the reduction degree of nickel species and resulted in insufficient metallic Ni⁰ on the catalyst surface, consequently impairing its hydrogenation capability. In contrast, an optimal Ni⁰–Ni²⁺ equilibrium was achieved on the surface of the reduced NiSi-T with relatively weaker SMSI, which facilitated a synergistic catalytic effect between metal and acid sites. DFT calculations identified Ni²⁺ sites as the active centers for the adsorption and activation of imine intermediates, whereas Ni⁰ sites were responsible for H₂ dissociation. Under a NH₃/FAL ratio of 2.4 at 90 °C, a high FAM yield of 94.2% was attained, with effective suppression of side-products such as tetrahydrofurfurylamine (THFAM) and secondary amines. Investigation into various reaction parameters highlighted that the NH₃/FAL ratio had a pivotal role in determining the product distribution during reductive amination of FAL. Excess NH₃ effectively inhibited the hydrogenation of furan rings and Schiff base intermediates; however, the NH₃/FAL ratio should not exceed 2.4 to prevent unnecessary ammonia wastage. This work introduces a straightforward and efficient method for preparing highly active phyllosilicates and provides profound insights into the mechanism of furfural reductive amination, with potential applications in the preparation of catalysts for other catalytic domains.