1 INTRODUCTION
With the increasing demand for energy and the depletion of fossil fuels, the utilization of biomass as a renewable source for energy has drawn worldwide attention. Lignin is the most abundant renewable aromatic biopolymer on earth and has the potential to serve as a feedstock in the production of fuels and aromatic compounds1, 2. However, the highly functionalized structure and robust chemical bonds of lignin greatly hamper its depolymerization for downstream utilization. It is known that lignin is composed of phenylpropane units linked by C-O and C-C bonds. The C-O bonds account for two-thirds of the total linkages between the repeating units, and have lower dissociation energy than that of the C-C bonds. Thus, a catalyst with high activity for C-O bond cleavage is the key to the successful conversion of lignin into value-added products3-6.
Many feasible methods, including hydrogenolysis, oxidation, pyrolysis, two-step strategy and photocatalysis have been proposed for lignin depolymerization7. From the perspective of selectivity and green chemistry, hydrogenolysis is one of the most efficient approaches8. Various metal catalysts based on Pd9-11, Ru4,12,13, Pt14-16, Ni17,18 and Re19-21, etc., have been reported for the hydrogenolysis of lignin, among which noble metal catalysts have shown good hydrogenation performance. However, undesirable over-hydrogenation is hard to avoid in noble metal-catalyzed transformation. Moreover, the high costs of the noble metals restrict their large-scale application. In light of the much lower cost and considerable catalytic activity for H2 dissociation, Ni has been used in the hydrogenolysis of lignin. As an elegant example, Sergeev et al 22 reported the hydrogenolysis of lignin model compound using a soluble nickel carbene complex under a mild condition (80-120 °C and 1 bar H2) by virtue of the advantages of homogenous catalysis. To address the issues of catalyst separation and recycling, heterogeneous Ni-based catalysts such as monometallic Ni/C23, bimetallic NiM (M=Ru, Rh, and Pd)24, and Ni-Fe alloy25 have also been employed in the conversion of lignin by modulation the electronic structure of Ni atoms and improving synergistic effect between Ni and the second metal species, or Ni and the support to increase catalytic efficiency. Notwithstanding, due to the inherent lower catalytic activity of Ni than that of noble metal catalysts, the hydrogenolysis of lignin over heterogenous Ni-based catalyst is usually carried out at usually high reaction temperature and high pressure, leading to side reactions such as over-hydrogenation, or fast deactivation due to the aggregation or leaching of active species.
Single-atom catalysts (SACs), with atomically dispersed metals onto the support surface, have the advantages of both “isolated sites” of homogeneous catalysts and the stability and reusability of heterogeneous catalysts, and thus are emerged as a promising frontier to bridge hetero- and homogeneous catalysis26-29. The utilization of SACs would be potentially superior alternative to the traditional hetero- and homogeneous catalysts in biomass conversion30-34. In another scenery, recent developments in metal-coordinated N-doped carbon catalysts have shown promise in selective hydrogenation35. The strong electronic interaction between metal atoms and N atoms accelerates the electron transfer in the catalytic system, resulting in improved activity and stability of the catalyst36. In view of the advantages of SACs, and the fact that the electronic structure of active sites is the key factor for affecting the hydrogenolysis activity of a catalyst in lignin depolymerization23, it is assumed that design of atomically dispersed Ni on N-doped carbon material with electronic interaction between Ni and N atoms might be promising catalysts for lignin decomposition.
Herein, a facile chelation-anchored strategy is developed for the construction of a single-atom Ni@N-C catalyst with a high-Ni loading via a two-stage pyrolysis of a mixture of D-glucosamine hydrochloride, nickel acetate and melamine, which are individually served as chelating agent, metal precursor and soft-template, respectively. This catalyst exhibits much higher catalytic activity and durability than the commercial Pd/C catalyst and N-doped carbon supported Ni nanoparticles (Ni@NC) in the hydrogenolysis of lignin into aromatic compounds, demonstrating the application potential of SACs in the conversion of complex biopolymers.