Highly Stable and Recyclable Graphene Layers Protected Nickel-Cobalt Bimetallic Nanoparticles as Tunable Hydrotreating Catalysts for Phenylpropane Linkages in Lignin

Nickel-cobalt bimetallic nanoparticles coated with several layers of graphene were developed through direct heating treatment of bimetallic oxide precursor prepared by the modified Pechini-type sol-gel method. These nanomaterials were demonstrated to be versatile catalysts for lignin depolymerization. The catalysts showed unexpectedly tunable selectivity that directly depends on the composition of bimetallic nanoparticles. Dimeric lignin model compounds can be converted totally and the hydrogenolysis selectivities above 85% over Ni-Co@C (Ni:Co = 1:3). During the recycling test, the nanocatalyst showed excellent recyclability in the ten-batch investigation. The deposition of graphene layers over bimetal nanoparticles fosters a subtle balance between protecting effects and surface accessibility to catalytic reactions and significantly improves their stability to air and moisture. Ni-Co@C catalysts were readily separated from the liquid mixtures with high recycling ratio due to their magnetic properties. Ni-Co bimetallic nanoparticles are coated with graphene layers. Graphene layers over the nanoparticles protect them from deactivation. Ni-Co@C shows tunable selectivity in the hydrogenolysis of dimeric lignin linkage. The non-precious metal catalyst showed excellent recyclability and can be reused ten times without significant loss of activity.