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.