Ni/Co@NDC Modulates the Highly Selective Conversion of Vanillin in Flow Reactor under Mild Conditions

Vanillin is currently the most abundant aromatic molecule produced from lignin on an industrial scale and one of the few renewable resources that can be readily catalyzed to obtain alkanes. The hydrogenation of vanillin as a lignin model has been a hot research topic in the catalytic community. Vanillin has the potential to be upgraded to value-added compounds by catalytic hydrogenation, and its valuable hydrogenation products are usually vanillinol (VA) and 4-methylguaiacol (MMP). The difficulty of this reaction lies in the selectivity control of the hydrogenation products. Complete conversion of both products cannot be achieved with either noble or non-precious metal catalysts in the same catalytic system. For the hydrogenation of vanillin, the usual catalysts can achieve complete conversion of one product or partial conversion of both products. In our work, we developed a novel bimetallic catalyst encapsulated in nitrogen-rich carbon to achieve, for the first time, the selective and regulated conversion of two products, i.e., 96.06% vanillin and 99.99% MMP, in the same system. The use of nitrogen-doped carbon (NDC) as a carrier for Ni nanoparticles allows the charge transfer from Ni to the carrier, i.e., NDC, resulting in the oxidation potential is corrected. Combined with the more favorable deoxidation activity of Co, our catalysts are inexpensive, simple to prepare, stable in performance, easy to separate, high in activity, and have the advantage of being tunable to the target product. Due to the synergistic effect of Ni and Co bimetals, the vanillin hydrogenation reaction can be almost completely stopped at the step of VA generation and the catalytic activity of the active center can be adjusted by controlling the temperature in a continuous flow to achieve the conversion of the other product MMP at 99.99%, which has never been reported before.