Selective amination of furfuryl alcohol to furfurylamine over nickel catalysts promoted by alumina encapsulation

Direct amination of bio-based furfuryl alcohol over metal catalysts provides an alternative route to produce furfurylamine without consuming expensive reducing reagents. The harsh and reductive condition of furfuryl alcohol amination, however, requires high durability and excellent resistance to side furan-ring hydrogenation for efficient catalysts, which is still substantially challenging to non-noble metals. We show here that alumina-encapsulated Ni catalysts prepared via a sol–gel method exhibited an outstanding and stable performance in selective amination of furfuryl alcohol to furfurylamine under mild NH3 and H2 partial pressures. Structural characterization unveiled that the alumina framework with appropriate mesopores effectively restrained the agglomeration of Ni nanoparticles and the side furan-ring hydrogenations. A volcano-type dependance of the intrinsic activity of Ni nanoparticles on their sizes was established with the maximum attained at about 5 nm, which correlated well with the redox ability of surface Ni sites. Kinetic assessments further indicated the kinetic relevance of furfural formation from furfuryl alcohol dehydrogenation, evidenced by quasi-zero and negative reaction orders on NH3 and H2 pressures, respectively. Combined with infrared spectroscopy, furfuryl alcohol was identified as the predominant adsorbed species with a nearly saturated coverage under reaction condition, consistent with the high susceptibility of furan-ring hydrogenations to H2 pressure. As a consequence, a slight amount of inert Ag (0.05 wt%) was introduced onto the alumina-encapsulated Ni catalysts to weaken the adsorption of furfuryl alcohol and its derivatives, which suppressed the side over-hydrogenation reactions to an extremely low level without sacrificing catalytic activity significantly.