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\begin{document}

\title{Constructing Single-atom Ni on N-doped Carbon Via Chelation-anchored
Strategy for the Hydrogenolysis of Lignin}



\author[1]{Li Tianjin}%
\author[1]{Chen Bo}%
\author[1]{Cao Meifang}%
\author[1]{Ouyang Xinping}%
\author[2]{Xueqing Qiu}%
\author[1]{Li Changzhi}%
\affil[1]{Affiliation not available}%
\affil[2]{South China University of Technology}%


\vspace{-1em}



  
  \date{May 17, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
The utilization of lignin remains a great challenge due to its complex
non-repetitive structure and the lack of efficient catalyst. Herein, a
single-atom catalyst Ni@N-C was designed via a facile chelation-anchored
strategy. Ni atoms were immobilized on the N-doped carbon carrier by a
two-stage pyrolysis of a mixture of D-glucosamine hydrochloride, nickel
acetate and melamine. D-glucosamine hydrochloride as a chelating agent
prevented the aggregation of Ni2+, and melamine provided enough N to
anchor Ni by forming Ni-N4 structure. Ni@N-C gave a 31.2\% yield of
aromatic compounds from lignin hydrogenolysis, which was twice higher
than that achieved by Ni cluster catalyst. Based on the experimental and
DFT calculation results, the higher activity of Ni@N-C was attributed to
its lower H2 dissociation energy and the reduced energy barriers of the
transition states. The strategy described opens an efficient green
avenue for preparing single-atom catalyst that possesses outstanding
activity in lignin depolymerization.%
\end{abstract}%



\sloppy


\textbf{Constructing Single-atom Ni on N-doped Carbon Via
Chelation-anchored Strategy for the Hydrogenolysis of Lignin}

Tianjin Li\textsuperscript{1,4}, Bo Chen\textsuperscript{1}, Meifang
Cao\textsuperscript{1}, Xinping Ouyang\textsuperscript{1}*, Xueqing
Qiu\textsuperscript{2}, Changzhi Li\textsuperscript{3}*

1. School of Chemistry and Chemical Engineering, Guangdong Provincial
Key Lab of Green Chemical Product Technology, South China University of
Technology, Guangzhou 510640, P. R. China.

\textsuperscript{2} School of Chemical Engineering and Light Industry,
Guangdong University of Technology, Guangzhou 510006, P. R. China.

\textsuperscript{3}CAS Key Laboratory of Science and Technology on
Applied Catalysis, Dalian Institute of Chemical Physics, Dalian 116023,
P.R. China.

\textsuperscript{4}Shandong Provincial Key Laboratory of Biomass
Gasification Technology, Energy Research Institute, Qilu University of
Technology (Shandong Academy of Sciences), Jinan 250014, P.R China.

*Authors to whom any correspondence should be addressed.

E-mail: ceouyang@scut.edu.cn (Xinping Ouyang); licz@dicp.ac.cn (Changzhi
Li).

\textbf{ABSTRACT}

The efficient utilization of lignin remains a great challenge due to its
complex non-repetitive structure and the lack of efficient catalyst.
Herein, a single-atom catalyst Ni@N-C was designed via a facile
chelation-anchored strategy. Ni atoms were immobilized on the N-doped
carbon carrier by a two-stage pyrolysis of a mixture of D-glucosamine
hydrochloride, nickel acetate and melamine. D-glucosamine hydrochloride
as a chelating agent prevented the aggregation of
Ni\textsuperscript{2+}, and melamine provided enough N to anchor Ni by
forming Ni-N\textsubscript{4}structure. Ni@N-C gave a 31.2\% yield of
aromatic compounds from lignin hydrogenolysis, which was twice higher
than that achieved by Ni cluster catalyst. Based on the experimental and
DFT calculation results, the higher activity of Ni@N-C was attributed to
its lower H\textsubscript{2} dissociation energy and the reduced energy
barriers of the transition states. The strategy described opens an
efficient green avenue for preparing single-atom catalyst that possesses
outstanding activity in lignin depolymerization.

\textbf{KEYWORDS}

single atom catalyst, lignin, hydrogenolysis,
Ni-N\textsubscript{4}structure, aromatic compound

\textbf{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 compounds\textsuperscript{1, 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 products\textsuperscript{3-6}.

Many feasible methods, including hydrogenolysis, oxidation, pyrolysis,
two-step strategy and photocatalysis have been proposed for lignin
depolymerization\textsuperscript{7}. From the perspective of selectivity
and green chemistry, hydrogenolysis is one of the most efficient
approaches\textsuperscript{8}. Various metal catalysts based on
Pd\textsuperscript{9-11}, Ru\textsuperscript{4,12,13},
Pt\textsuperscript{14-16}, Ni\textsuperscript{17,18} and
Re\textsuperscript{19-21}, 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
H\textsubscript{2} dissociation, Ni has been used in the hydrogenolysis
of lignin. As an elegant example, Sergeev \emph{et al}
\textsuperscript{22} reported the hydrogenolysis of lignin model
compound using a soluble nickel carbene complex under a mild condition
(80-120 \selectlanguage{ngerman}°C and 1 bar H\textsubscript{2}) 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/C\textsuperscript{23}, bimetallic NiM (M=Ru, Rh, and
Pd)\textsuperscript{24}, and Ni-Fe alloy\textsuperscript{25} 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 catalysis\textsuperscript{26-29}. The
utilization of SACs would be potentially superior alternative to the
traditional hetero- and homogeneous catalysts in biomass
conversion\textsuperscript{30-34}. In another scenery, recent
developments in metal-coordinated N-doped carbon catalysts have shown
promise in selective hydrogenation\textsuperscript{35}. 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 catalyst\textsuperscript{36}. 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 depolymerization\textsuperscript{23}, 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.

\textbf{2 MATERIALS AND METHODS}

\textbf{2.1 Materials}

All chemicals used were analytical grade without further purification.
Glucosamine hydrochloride, melamine,
Ni(OAc)\textsubscript{2}\selectlanguage{english}*4H\textsubscript{2}O, active carbon (AC), Raney
nickel and Ru/C were purchased from Aladdin Chemistry Co. Ltd (China).
2-phenoxy-1-phenethanol, phenethoxybenzene, phenol, ethylbenzene,
phenylethyl alcohol were purchased from Alfa Aesar Co. Inc. (China).
Birch wood was purchased from Shandong wood Co. Ltd. (China).

\textbf{2.2 Synthesis of Ni@N-C SAC}

A total of 1 g Ni(OAc)\textsubscript{2}\selectlanguage{english}*4H\textsubscript{2}O was
combined with 20 g melamine and D-glucosamine hydrochloride followed
with thorough mixing. The mixture was then placed in a ceramic boat and
calcined with a two-stage pyrolysis procedure: The temperature was
raised to 600 degC with a heating rate of 2.0 degC/min and hold for 1 h;
After that, the temperature was further raised to 800 degC with the same
heating rate; Finally, the sample was cooled down to room temperature.
For comparison, Ni@NC was synthesized by the above method without the
addition of melamine. Similarly, 1 g D-glucosamine hydrochloride was
directly calcined at 800 degC for 1h, the obtained sample was co-pyrolyzed
with Ni(OAc)\textsubscript{2}\selectlanguage{english}*4H\textsubscript{2}O and melamine using
the same process as the Ni@N-C SAC and the resultant sample was named as
Ni@C. The contrastive samples were synthesized in parallel by the same
method as that for Ni@N-C SAC except for the chitosan/lignin instead of
D-glucosamine hydrochloride.

\textbf{2.3 Synthesis of Ni NPs deposited on AC}

The catalyst of Ni@AC was prepared by the wetness impregnation method.
Ni(OAc)\textsubscript{2}\selectlanguage{english}*4H\textsubscript{2}O (0.1106 g) was first
dissolved in 20 mL deionized water, and then 0.3 g of AC was added to
the Ni solution. The resulting slurry was stirred for 24 h and then
dried in oil bath at 80 degC. The dried slurry was subjected to a heating
process in a tube furnace with an
H\textsubscript{2}/N\textsubscript{2}(1:9) gas flow at a flow rate of
100 mL/min. The tube furnace was programmed as follows: raise to 120 degC
at 2 degC/min and hold for 20 min; then further raise to 400 degC at 1
degC/min, cool down to room temperature.

\textbf{2.4 Characterization of Catalysts}

X-ray diffraction (XRD) was collected on a Rigaku diffractometer (D/MAX,
3 kW) equipped with Cu K\selectlanguage{greek}α \selectlanguage{english}radiation (40 kV, 30 mA, \selectlanguage{greek}λ\selectlanguage{english}=0.1543 nm) (Bruker,
Germany) and the patterns were recorded in the 10-90\selectlanguage{ngerman}º. X-ray
photoelectron spectroscopy (XPS) was carried out on a Kratos Axis Ultra
DLD system. Transmission electron microscopy (TEM) was performed using a
JEOL Model 2100F electron microscopy at 200 kV. Aberration-corrected
scanning STEM images were taken using a Cs-corrected FEI Titan G2 60-300
Microscope operated at 300 KV using an HAADF detector. Field emission
scanning electron microscopy (FFSEM, Merlin, Germany) was performed to
measure the morphology of the samples. The X-ray absorption spectra were
collected on the beamline BL01C1 in NSRRC. The radiation was
monochromatized by a Si (111) double-crystal monochromator. X-ray
absorption near-edge structure (XANES) and extended X-ray absorption
fine structure (EXAFS) data reduction and analysis were processed by
Athena software. N\textsubscript{2} adsorption isotherms of catalysts at
77K were measured on a Tristar II 3020, USA. The samples were pretreated
at 200 °C for 12 h under vacuum. The metal content of the catalysts was
confirmed by Perkin-Elmer TJA RADIAL IRIS 1000.

\textbf{2.5 Decomposition of Lignin Model Compounds}

0.1 g (0.467 mmol) lignin model compound, 0.02 g catalyst and 20 mL
water/ethanol (1:1) were added into the 100 mL stainless steel autoclave
(Beijing Shijisenlang Chemical Machinery Co., Ltd., China). The
autoclave was purged with N\textsubscript{2} for three times and the
pressure was increased to 1MPa hydrogen. And the catalytic reaction was
carried out at 200 ºC for 5 h. After reaction, the mixture was filtered
to remove catalyst. The organic products were extracted with ethyl
acetate and analyzed by gas chromatography-mass spectrometry (GC-MS).
The conversion of model compounds and the yield of products were
calculated using the following Eq. (1) and Eq. (2):

Conversion (\%) = W\textsubscript{R}/W\textsubscript{S}×100\% (1)

Yield of product \emph{i} (\%) =\emph{n\textsubscript{i}} /\emph{n}
\textsubscript{m}×100\% (2)

Where W\textsubscript{R} is the weight of reacted after the
hydrogenolysis of lignin model compound; W\textsubscript{S} is the
weight of substrate (0.1 g); where \emph{n\textsubscript{i}}
and\emph{n\textsubscript{m}} are the moles of product \emph{i} and
initially added compound (0.467 mmol), respectively.

\textbf{2.6 Lignin Hydrogenolysis}

The hydrogenolysis of birch organosolv lignin was carried out in 100 mL
stainless steel autoclave (Beijing Shijisenlang Chemical Machinery Co.,
Ltd., China). Typically, 0.3 g birch organosolv lignin 0.06 g catalyst
and 20 mL water/ethanol (1:1) were added into the autoclave vessel. The
vessel was purged three times with nitrogen before sealed and pressured
to 1 MPa with hydrogen. The reaction was carried out at 280 ºC for 2 h.
After reaction, the reaction vessel was cooled to room temperature. The
reaction mixture was filtered to remove catalyst and the filtrate was
extracted with ethyl acetate to obtain lignin oil. And then, the lignin
oil was analyzed by GC-MS, gel permeation chromatography (GPC) and
two-dimensional heteronuclear single quantum correlation (2D HSQC). The
yield of the aromatic monomers was calculated using the following Eq.
(3):

Yield of the aromatic monomers (\%) =
W\textsubscript{A}/W\textsubscript{F}×100\% (3)

Where W\textsubscript{A} is the weight of aromatic monomers after the
hydrogenolysis of lignin; W\textsubscript{F} is the weight of feed
lignin.

\textbf{2.7 Analysis of Products}

GC-MS was performed with an Agilent 5975C-7890A equipped with a
capillary column (HP-5MS 30 m×mm×0.25 \selectlanguage{greek}μ\selectlanguage{english}m) and EI ion source (70 eV).The
following operating condition was used: a column temperature program of
50 \selectlanguage{ngerman}ºC (3 min), 7 ºC/min to 150 ºC, 150 ºC for 10 min, 20 ºC /min to 300
ºC, 300 ºC for 20 min. GPC analysis was performed with a Waters
1515/2414 liquid chromatography system (Waters 1515/2414, Waters Co.,
USA). The tetrahydrofuran (THF) was used as the eluent. The lignin and
depolymerized products were dissolved in THF at a concentration of 5 mg
mL\textsuperscript{-1}.The 2D HSQC was performed on a Bruker AV 600 MHz
spectrometer. And 100 mg of lignin and lignin oil were dissolved in 0.5
mL DMSO-d6.

\textbf{2.8 DFT Calculations}

The present first principle DFT calculations are performed with the
projector augmented wave (PAW) method\textsuperscript{37,38} in Vienna
ab initio simulation package (VASP). The exchange-functional is treated
using the generalized gradient approximation (GGA) of
Perdew-Burke-Ernzerhof (PBE)\textsuperscript{37} functional. The cut-off
energy of the plane-wave basis is set at 450 eV for optimize
calculations of atoms and cell optimization. The vacuum spacing in a
direction perpendicular to the plane of the catalyst is at least 12 Å.
The Brillouin zone integration is performed using 4×4×1 Monkhorst-Pack
k-point sampling for a primitive cell\textsuperscript{39}. The
self-consistent calculations apply a convergence energy threshold of
10\textsuperscript{-5} eV. The equilibrium lattice constants are
optimized with maximum stress on each atom within 0.03 eV/Å. The Hubbard
U (DFT+U) corrections for 3d transition metal by setting according to
the literature\textsuperscript{40}. The binding energy is calculated
using the following Eq (4):

E\textsubscript{ads}=E\textsubscript{total}-(E\textsubscript{m}+E\textsubscript{1})
(4)

where the E\textsubscript{total} is the energy of structure with
adsorbed structure, E\textsubscript{m} is the energy of N-doped carbon
structure, E\textsubscript{1} is the energy of the single atom or
clusters.

The free energy was calculated using the following Eq (5):

G=E+ZPE-TS (5)

where G, E, ZPE and TS are the free energy, total energy from DFT
calculations, zero point energy and entropic contributions (T was set to
be 298K), respectively.

\textbf{3 RESULTS}

\textbf{3.1 Chelation-anchored strategy for constructing Ni@N-C SAC}

The Ni@N-C SAC was prepared by the mean of a two-stage pyrolysis of a
mixture of D-glucosamine hydrochloride, nickel acetate and melamine,
which were individually served as chelating agent, metal precursor and
soft-template, respectively (Figure 1a). In the first stage
(\textless{}600 ºC), the thermal condensation of melamine produced
graphite carbon nitride (g-C\textsubscript{3}N\textsubscript{4}), and
thermal degradation of D-glucosamine hydrochloride formed a carbon
skeleton between
g-C\textsubscript{3}N\textsubscript{4}layers\textsuperscript{41}. In the
second stage, g-C\textsubscript{3}N\textsubscript{4} began thermolysis
at a temperature above 700 ºC and finally formed N-doped graphene-like
nanosheets, providing a nitrogen source for the formation of
Ni-N\emph{\textsubscript{x}} structure when the temperature reached 800
ºC. The morphology of Ni@N-C SAC was characterized by SEM and TEM.

As shown in Figure 1b and 1c, Ni@N-C SAC is composed of wrinkled
graphene-like nanosheets. The TEM image of Ni@N-C SAC only shows one
atom lattice structure (d=3.52 Å), which is attributed to graphite
carbon (Figure 1d). The XRD spectrum of the Ni@N-C SAC has two broad
peaks at 26.3º and 41.2º, which are corresponding to 002 peak and 100
peak of graphitic carbon\textsuperscript{42}, while diffraction peaks of
Ni are not detected (Figure S1). However, aberration-corrected
high-angle annular dark field scanning transmission electron microscopy
(HAADF-STEM) shows that Ni single atoms are homogeneously distributed
across the wrinkled graphene-like nanosheets (Figure 1e-f). And the
average diameter of the spots is 1.67 ± 0.29 Å based on the statistical
analysis of over 100 bright spots (the top right corner of Figure 1f).
In addition, the EDS-mapping analysis reveals the homogeneous
distribution of Ni, N and C atoms across the whole graphene-like
nanosheets (Figure 1g). These above results indicate that Ni is
atomically distributed on the carrier.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{FIGURE} 1 Synthesis and characterizations of Ni@N-C SAC: (a)
Schematic illustration of the synthesis process of Ni@N-C SAC. (b) SEM
image. (c) TEM image. (d) HRTEM image. (e, f) Aberration corrected
HAADF-STEM image. (g) EDX elemental mapping.

\textbf{3.2 Electronic structure of Ni atoms in Ni@N-C SAC}

The XPS was used to characterized the chemical composition and valence
state of Ni@N-C SAC (Figure S2). The binding energy of the Ni
2p\textsubscript{3/2} peak is 855.1 eV, which is higher than that
reported for Ni\textsuperscript{0} (853.4 eV) and lower than that for
Ni\textsuperscript{2+} (855.7 eV) \textsuperscript{43, 44}. The
high-resolution XPS N 1s spectrum can be deconvoluted into pyridinic N
(~398.1 eV), Ni-N (~399.2 eV), pyrrolic N (~400.4 eV), graphite N
(~401.3 eV) and N-oxidized (~403.0 eV)\textsuperscript{45}. Based on the
XPS data (Table S1), the atomic concentration of Ni in Ni@N-C SAC is
8.01 wt\%.

To further study the electronic structures of Ni species in Ni@N-C SAC,
the analysis of XANES and EXAFS was conducted. Figure 2a shows the Ni
K-edge XANES spectra of Ni@N-C SAC and two references: Ni foil and NiO.
In the first derivative XANES spectra, the intensity lines (absorption
edges) of Ni@N-C SAC lie between those of Ni foil and NiO, demonstrating
the unique electronic structure of
Ni\selectlanguage{greek}\textsuperscript{δ+}\selectlanguage{english}(0\textless{}\selectlanguage{greek}δ\selectlanguage{english}\textless{}2) in Ni@N-C SAC, where
the valence of the Ni species is between Ni\textsuperscript{0} and
Ni\textsuperscript{2+}. This observation is consistent with the
aforementioned XPS results. The Fourier transform of the
k\textsuperscript{3}-weighted Ni K-edge EXAFS spectrum of Ni@N-C SAC
exhibits a dominant peak at 1.41 \selectlanguage{ngerman}Å, corresponding to the Ni-N
coordination, which is shorter than both the Ni-O peak in the NiO
spectrum and the Ni-Ni peak in the Ni foil spectrum (Figure 2b). The
wavelet transform was also employed to assess the Ni K-edge EXAFS
oscillations of Ni@N-C SAC. As shown in Figure 2c, the wavelet transform
analysis of Ni@N-C SAC displays only one intensity maximum at 5.0
Å\textsuperscript{-1}, which is ascribed to the Ni-N coordination. The
above results further prove the presence of atomically dispersed Ni in
Ni@N-C SAC. The EXAFS fitting analysis was applied to investigate the
configuration of Ni atoms in Ni@N-C SAC. The calculated coordination
number of Ni is \emph{ca} 4 and the mean bond length of Ni@N-C SAC is
1.88 Å (Table S2). Hence, each Ni atom is confined in the N-doped carbon
matrix by coordinating with four N (Ni-N\textsubscript{4} structure,
Figure S3).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{FIGURE 2} Atomic structure analysis of Ni@N-C SAC by XAFS: (a)
Ni K-edge XANES spectra. (b) Fourier transform of Ni k-edge EXAFS
spectra. (c) wavelet transform of \emph{k} \textsuperscript{3}-weighted
EXAFS data of Ni@N-C SAC and reference samples (NiO and Ni foil).

\textbf{3.3 Mechanism and universality of chelation-anchored strategy
for preparing Ni@N-C SAC}

The above-described morphology and electronic structure studies
demonstrate the successful synthesis of Ni@N-C SAC. To investigate the
role of chelating agent in synthesizing Ni@N-C SAC, D-glucosamine
hydrochloride was first treated at 800 \selectlanguage{ngerman}°C to remove its amino/hydroxyl
groups, then the generated sample was co-pyrolyzed with nickel acetate
and melamine by a standard process used in the synthesis of Ni@N-C SAC
(Figure 3a), obtaining a catalyst denoted as Ni@C. SEM image of Ni@C
suggests that carbon nanotubes are obtained probably via a Ni-catalyzed
growth during the decomposition of
melamine\textsuperscript{46,47}(Figure 3b). Moreover, TEM images show
that a large proportion of Ni atoms aggregate to clusters rather than
atomically dispersed Ni (Figure 3 c, d). The diffraction peaks of Ni@C
in XRD profile at 2\selectlanguage{greek}θ \selectlanguage{english}of 43.2\selectlanguage{ngerman}°, 44.5°, 49.6°, 51.8° and 76.4° are
attributed to NiO (012), Ni (111), Ni\textsubscript{2}O\textsubscript{3}
(112), Ni (200), Ni (220), respectively (Figure S4). The above results
indicate that the abundant amino/hydroxyl groups of D-glucosamine
hydrochloride play crucial roles to coordinate with Ni ions, which
prevent the aggregation of Ni for obtaining single atom Ni.

Meanwhile, a comparison catalyst denoted as Ni@NC was prepared without
the addition of melamine (Figure 3e). The SEM characterization showed
that the morphology of Ni@NC (Figure 3f) was non-porous. Contrarily,
Ni@N-C SAC (Figure 1b) and Ni@C (Figure 3b) display the wrinkled and
porous morphology. The XRD spectrum of Ni@NC (Figure S4) shows Ni peaks
obviously, and its TEM images (Figure 3 g, h) reveal that Ni clusters
are formed on the carbon support.

Relying on the above studies relating the roles of chelating agent and
soft-template, it could be deduced that the pyrolysis of D-glucosamine
hydrochloride formed a carbon skeleton in the interlayer of the
g-C\textsubscript{3}N\textsubscript{4} at the lower temperature stage of
pyrolysis (600 °C), The layer-by-layer stacking structure similar to
sandwiches may inhibit the growth of Ni particles NPs. The further
pyrolysis at a higher temperature (800 °C) produced volatile gases from
the decomposition of the g-C\textsubscript{3}N\textsubscript{4} to form
wrinkled and porous morphology. Meanwhile, decomposition of
g-C\textsubscript{3}N\textsubscript{4} resulted in the formation of
active N radicals, doping into carbon skeleton and inducing the
coordination of Ni with N to form Ni-N\textsubscript{4} structure. It
should be noted that although every molecule of D-glucosamine
hydrochloride contains one amino group, it cannot provide enough N to
chelate Ni for hindering the aggregation of Ni in the absence of
melamine. For this reason, the addition of melamine is also essential
for preparing the atomically dispersed Ni species. Therefore, during the
process of forming Ni SAC, the chelating agent prevents the aggregation
of Ni\textsuperscript{2+}, and the soft template provides enough N to
coordinate and anchor Ni by forming Ni-N\textsubscript{4} structure.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{FIGURE 3} Synthesis and characterizations of control samples:
(a) Scheme of the formation of Ni@C. (b) SEM images. (c, d) TEM images
of Ni@C. (e) Scheme of the formation of Ni@NC. (f) SEM images. (g, h)
TEM images of Ni@NC.

It is well known that lignin and chitosan are two of the most abundant
biomass resources, which have rich hydroxyl groups or/and amine groups
in their structures (Figure S5), suggesting the possibility that they
can be used as chelating agents instead of D-glucosamine hydrochloride
to synthesize Ni@N-C SAC. The aberration-corrected HAADF-STEM images of
the two prepared catalysts using lignin and chitosan as chelating agents
shown in Figure 4 reveal the atomically dispersed Ni sites supported on
both carriers. Other characterizations of the catalysts, including SEM
images (Figure S6), EDS analysis (Figure S7), together with XRD and XPS
analysis (Figure S8) further reveal that Ni is atomically dispersed on
the carrier of catalysts, regardless of the chelating agent is lignin or
chitosan. ICP analysis indicates that Ni loading of the catalysts
obtained from lignin and chitosan are 8.1 wt\% and 7.5 wt\%,
respectively. The above results indicate that two readily available
waste biomass resources are successfully used as chelating agents to
synthesize Ni@N-C SAC, demonstrating that the chelation-anchored
strategy is a universal method.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{FIGURE 4} Microstructure and morphology characterizations of
Ni@N-C SACs prepared by using lignin and chitosan as chelating agents:
(a) TEM images. (b) Corresponding EDX elemental mapping. and (c)
Aberration-corrected HAADF-STEM images of Ni@N-C SAC prepared with
lignin. (d) TEM images. (e) Corresponding EDX elemental mapping and (f)
Aberration-corrected HAADF-STEM images of Ni@N-C SAC prepared with
chitosan.

\textbf{3.4 Catalytic Performance on C-O Bond Cleavage}

The catalytic performances of the synthesized catalysts were evaluated
by using a typical lignin \selectlanguage{greek}β-\selectlanguage{english}O-4 model compound, 2-phenoxy-1-phenethanol
(PPE). Table 1 summarizes the PPE hydrogenolysis results over different
catalysts. The reaction does not occur in the absence of catalyst (entry
1) and the NC support also has no catalytic activity in the reaction
(entry 2). Ni@NC shows a 31\% PPE conversion and a 16\% phenol yield
(entry 6). In sharp comparison, Ni@N-C SAC gives a much higher PPE
conversion (99\%) with excellent phenol yield of 92\% and ethylbenzene
yield of 78\%. As a comparison, the commercial Raney nickel exhibits a
PPE conversion of 39\%, accompanying with phenol and ethylbenzene yields
of 24\% and 18\%, respectively. The above results demonstrate clearly
that Ni@N-C SAC possesses high activity in catalyzing PPE
hydrogenolysis, during which the C-O bond is cleaved in the presence of
H\textsubscript{2}. Table 1 also shows that the yield of byproduct
cyclohexanol (37\%) in the reaction over Pd/C catalyst is much higher
than that over Ni@N-C SAC (only 1\%), indicating that Ni@N-C SAC
possesses high selectivity in cleaving C-O bonds without over
hydrogenation of aromatic rings.

\textbf{TABLE 1} Catalytic performance of various catalyst for PPE
depolymerization\textsuperscript{a}.

\par\null\selectlanguage{english}
\begin{longtable}[]{@{}llllllll@{}}
\toprule
Entry & Catalyst & Conversion (\%) & Yield (mol\%) & Yield (mol\%) &
Yield (mol\%) & Yield (mol\%) & Yield (mol\%)\tabularnewline
& & & B & C & D & E & F\tabularnewline
1 & Blank & 0 & ND & ND & ND & ND & ND\tabularnewline
2 & NC & 0 & ND & ND & ND & ND & ND\tabularnewline
3 & Raney Nickel\textsuperscript{b} & 39 & 24 & 18 & 3 & 2 &
8\tabularnewline
4 & Ni@N-C SAC & 99 & 92 & 78 & 8 & 1 & ND\tabularnewline
5 & Ni@C\textsuperscript{c} & 30 & 22 & 10 & 2 & 2 & 5\tabularnewline
6 & Ni@NC\textsuperscript{d} & 31 & 16 & 5 & 3 & 2 & 8\tabularnewline
7 & Ni@AC\textsuperscript{e} & 28 & 14 & 10 & 2 & 1 & 10\tabularnewline
8 & Pd/C\textsuperscript{f} & 91 & 56 & 28 & 5 & 37 & 5\tabularnewline
\bottomrule
\end{longtable}

a: Reaction conditions: PPE (100 mg), 200 \selectlanguage{ngerman}ºC, 5 h, catalyst (20 mg),
isopropanol (20 ml), stirring at 400 rpm. b: Raney nickel was commercial
purchased. c: Ni@C was prepared by pyrolyzing the D-glucosamine
hydrochloride at 800 °C, the generated sample was co-pyrolyzed with
nickel acetate and melamine. d: Ni@NC was prepared by the same method as
the Ni@N-C SAC except that melamine was absent. e: Ni@AC was prepared by
immobilizing Ni species on the commercial purchased AC via a wet
impregnation method. f: Pd/C was commercial purchased and the Pd loading
was 5 wt\%. ND: No detected.

The DFT and molecular dynamic stimulation were employed to investigate
the reason why Ni@N-C SAC is superior to Ni@NC in cleavage of the C-O
bond. Based on the XANES and EXAFS results, the optimized models of
Ni@N-C SAC and Ni@NC were established using VASP software (Figure 5).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{FIGURE 5} Optimized configurations of different catalysts: (a)
Ni@N-C SAC. (b) Ni@NC. (c) the H\textsubscript{2} dissociation of energy
of Ni@N-C SAC and Ni@NC.

The selective hydrogenolysis begins with the adsorption and activation
of H\textsubscript{2}\textsuperscript{48}, and then the activated
hydrogen species react with PPE molecules resulting in the cleavage of
the C-O bond. The calculated Ni adsorption energies were -3.17 eV for
Ni@N-C SAC and -1.21 eV for Ni@NC, respectively (Figure 5a and 5b). The
lower adsorption energies of Ni in the Ni@N-C SAC model suggests that Ni
atoms prefer to chelate with the N atoms of the N-doped carbon support
to form single-atom sites (Ni@N-C SAC) rather than aggregate with each
other to form Ni clusters (Ni@NC). Moreover, the dissociation energy of
molecular hydrogen on the surface of Ni@N-C SAC and Ni@NC are 1.21 eV
and 1.69 eV, respectively (Figure 5c), indicating that Ni@N-C SAC
contributes to more active hydrogen species, and hence exhibits higher
catalytic activity.

The time course of the product distribution of PPE hydrogenolysis was
monitored (Figure S9). Results shows that PPE is almost quantitatively
converted to phenol, ethylbenzene and phenethoxybenzene within 150 min.
Phenethoxybenzene increases initially but decreases with the prolonging
of time. Therefore, phenethoxybenzene should be an intermediate for the
transformation, which might be produced from the dehydration of PPE on
the OH group of C\selectlanguage{greek}\textsubscript{α}, \selectlanguage{english}and it can be further hydrogenated
to produce phenol and ethylbenzene by the cleavage of the aliphatic C-O
bond (Figure 6a, route A). Despite there is another possible dehydration
path between C\selectlanguage{greek}\textsubscript{α}-\selectlanguage{english}OH and C\selectlanguage{greek}\textsubscript{β}-\selectlanguage{english}H, the vinyl
ether intermediate was not detected yet even at low temperature, we thus
exclude this dehydration pathway. Phenylethyl alcohol was a minor
product of the PPE hydrogenolysis, which was probably produced through
the cleavage of the aliphatic C-O bond (Figure 6a, route B). The yield
of phenol and ethylbenzene increases with the prolonging of the reaction
time, which indicates that route A is the main reaction path of PPE
hydrogenolysis.

According to the hydrogenolysis reaction pathway A, the DFT calculation
of the mechanism with free energy profile was also shown in Figure 6.
The reaction energy barrier diagram indicates that the two dissociated
hydrogen atoms combine with the oxygen atom on C\selectlanguage{greek}\textsubscript{α}-\selectlanguage{english}OH
(energy barrier: 0.979 eV) and the C\selectlanguage{greek}\textsubscript{α} \selectlanguage{english}(energy barrier:
0.354 eV), respectively, to produce phenethoxybenzene, which is further
cleaved to generate phenol and ethylbenzene by overcoming the energy
barriers of 0.493 eV and 0.264 eV. For Ni@NC, it was found that the
energy barriers of the four transition states (TS1, TS2, TS3, and TS4)
are lower than those in the Ni@N-C SAC-catalyzed PPE hydrolysis.
Therefore, Ni@N-C SAC exhibits higher catalytic activity than Ni@NC in
the cleavage of C-O bonds.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{FIGURE 6} Proposed rection path and the reaction energy barrier
diagrams: (a) Proposed reaction path of PPE hydrogenolysis. (b) Reaction
energy barrier of PPE hydrogenolysis by Ni@N-C SAC and Ni@NC.

To explore the range of the Ni@N-C SAC catalyst, two more \selectlanguage{greek}β-\selectlanguage{english}O-4 model
compounds and one \selectlanguage{greek}α-\selectlanguage{english}O-4 model compound were evaluated as the substrates.
It is found that benzyl phenyl ether (\selectlanguage{greek}α-\selectlanguage{english}O-4) and 2-phenylethyl phenyl
ether (\selectlanguage{greek}β-\selectlanguage{english}O-4) over the Ni@N-C SAC are nearly completely converted,
providing the corresponding phenols and arenes in high yields ([?] 80\%,
Table S3, entries 1-2). It is known that \selectlanguage{greek}β-\selectlanguage{english}O-4 dimeric model compound
with C\selectlanguage{greek}\textsubscript{γ}-\selectlanguage{english}OH is structurally closer to the true lignin.
Gratefully, such a model compound was also completely converted (Table
S3, entry 3), affording guaiacol and 4-propylguaiacol in yields of 80\%
and 53\%, respectively. These data demonstrated the versatility of
Ni@N-C SAC in selective cleavage of the C-O bond in different lignin
model compounds.

The recyclability of the Ni@N-C SAC was explored by carrying out the
hydrogenolysis of PPE as listed in Figure 7a. After each run, the used
catalyst was washed with ethanol following by vacuum drying. It was
found that the conversion of PPE in Ni@N-C SAC-catalyzed hydrogenolysis
remains unchanged after three runs. Characterizations of the used
catalyst were conducted by XRD, XPS, HRTEM and aberration-corrected
HAADF-STEM. There is no Ni metal diffraction peak in XRD pattern (Figure
7b). The binding energy of Ni 2p3/2 peak (855.13 eV) of used catalyst is
almost the same as that of original Ni@N-C SAC (Figure 7c). No Ni
clusters or particles are observed on the surface of the used catalysts
(Figure 7d, e). Aberration-corrected HAADF-STEM characterizations show
that Ni atoms were still atomically dispersed with no aggregation
observed (Figure 7 f). These results demonstrate the excellent stability
of Ni@N-C SAC in the hydrogenolysis systems.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\textbf{FIGURE 7} Recycling of Ni@N-C SAC in the catalytic
hydrogenolysis of PPE and characterization of the used catalysts: (a)
Recycling numbers (reaction conditions: 100 mg PPE, 20 mg catalyst, 20
ml isopropanol, stirring at 400 rpm, 200 \selectlanguage{ngerman}ºC, 5 h). (b) XRD pattern. (c)
XPS Ni 2p. (d, e) TEM images. (f) Aberration-corrected HAADF-STEM image.

\textbf{3.5 Catalytic Hydrogenolysis of Lignin}

The high catalytic activity of Ni@N-C SAC toward the hydrogenolysis of
lignin model compounds encouraged us to explore the application of
Ni@N-C SAC in real lignin decomposition. In this work, birch organosolv
lignin was employed as the substrate in the reaction. The yield of the
aromatic monomers of Ni@N-C SAC is 31.2\% with propylguaiacol and
propysyringol as the main products (Table S4), which is about twice
higher than those of Ni@NC (14.9\%) and Raney nickel catalyst (12.3\%),
indicating Ni@N-C SAC is an efficient catalyst for lignin
depolymerization. Figure 8 shows the 2D HSQC spectra of birch organosolv
lignin and the corresponding oily products. The signals in the 2D HSQC
spectra are labeled and summarized in Table S5. It is found that almost
all the signals of A (\selectlanguage{greek}β-\selectlanguage{english}O-4), B (\selectlanguage{greek}β-\selectlanguage{english}5) and C (\selectlanguage{greek}β-β\selectlanguage{english}) in lignin oil are
disappeared after catalyzed reaction (Figure 8a vs Figure 8c) compared
with that in lignin. In detail, the relative content of \selectlanguage{greek}β-\selectlanguage{english}O-4 linkages
in birch organosolv lignin was 42/100Ar, while the signals of
A\selectlanguage{greek}\textsubscript{α}\selectlanguage{english}and A\selectlanguage{greek}\textsubscript{β} \selectlanguage{english}corresponding to benzylic
alcohols and secondary alkyl protons of \selectlanguage{greek}β-\selectlanguage{english}O-4 linkages almost completely
disappeared, implying most \selectlanguage{greek}β-\selectlanguage{english}O-4 linkages have been cleaved after
reaction. Moreover, the signals of B (\selectlanguage{greek}β-\selectlanguage{english}5) and C (\selectlanguage{greek}β-β\selectlanguage{english}) in oily products
are no longer observed, indicating that the stubborn C-C bonds are also
cleaved after reaction. It is therefore interesting that Ni@N-C SAC
shows high activity not only in cleaving C-O bonds, but also in cracking
C-C bonds in lignin. It is also found that G and S units are observed
with the amounts of 29\% and 71\% in fresh lignin. After reaction, the
corresponding amounts were similarly 31\% and 69\%, respectively,
suggesting that most of the aromatic units are not destroyed after
reaction. As a comparison, Ni@NC exhibits lower depolymerization
efficiency according to Figure 8b, and Figure 8e. Of which, the signals
of A (\selectlanguage{greek}β-\selectlanguage{english}O-4), B (\selectlanguage{greek}β-\selectlanguage{english}5) and C (\selectlanguage{greek}β-β\selectlanguage{english}) in lignin oil are still calculated
21\%, 2\% and 4\%, respectively, and the corresponding amounts of the
primary units (S and G) are also similarly to that in original lignin.
The above results further confirm the higher catalytic activity of
Ni@N-C SAC than Ni@NC in lignin depolymerization. GPC analysis gives
another evidence of the excellent activity of Ni@N-C SAC. The GPC
spectrum of birch organosolv lignin (Figure S10 and Table S6) shows a
board molecular weight distribution with an average \emph{M}
\textsubscript{W} of 4150 g/mol. After reaction, the average molecular
weight of the two lignin oil samples catalyzed by Ni@N-C SAC and Ni@NC
are 590 g/mol and 1870 g/mol, respectively, which are much lower than
that of birch organosolv lignin.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{FIGURE 8} 2D HSQC NMR spectra of birch organosolv lignin and the
corresponding oily products: (a, d) birch organosolv lignin in
DMSO-\emph{d} 6 before reaction. (b, e) lignin oil after reaction with
Ni@NC and (c, f) Ni@N-C SAC.

\textbf{4 DISCUSSION}

In this study, we demonstrate a chelation-anchored strategy for the
synthesis of Ni@N-C SAC with a Ni loading up to 7.8 wt\%. In the
preparation of Ni@N-C SAC, Ni ions are first well dispersed by chelating
with precursor containing OH or NH\textsubscript{2} groups, and then a
two-step pyrolysis process not only restrains the growth of Ni particles
but also introducing active N species into C carrier to form atomically
dispersed Ni sites by providing N coordination site between the Ni and N
species. The active site of Ni@N-C is a Ni-N\textsubscript{4} structure
according to X-ray absorption fine structure analysis and computational
modeling. Compared to Ni nano-cluster, the atomically dispersed Ni
contributes to lower adsorption energies of Ni on the carrier, lower
H\textsubscript{2}dissociation energy and energy barriers of the
transition states in the hydrogenolysis process. Therefore, Ni@N-C SAC
exhibits high catalytic activity in the hydrogenolysis of a lignin model
compound and birch lignin. The 2 D HSQC analysis of lignin and oily
products reveals that Ni@N-C SAC shows high activity not only in
cleaving C-O bonds, but also in cracking C-C bonds in lignin. This
methodology opens up a facile strategy for the design of Ni@N-C SAC
catalyst featuring high Ni loading and excellent catalytic activity in
hydrogenolysis of both C-O and C-C bonds in lignin, highlighting the
application potential not only in hydrocracking of large biomass
molecules, but also in other catalytic reactions.

\textbf{ACKNOWLEDGMENT}

We thank financially supported by National Key Research and Development
Plan of China (2019YFC1905301), the National Natural Science Foundation
of China (No. 21776108, 22078115,21690083).

\textbf{AUTHOR CONTRIBUTIONS}

The manuscript was written through contributions of all authors. All
authors have given approval to the final version of the manuscript.

\textbf{DATA AVAILABILITY STATEMENT}

The data that support the findings of this study are available from the
corresponding author upon reasonable request.

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\textbf{SUPPORTING INFORMATION}

The Supporting Information is available for this paper. Extraction of
birch lignin, the results of catalytic tests, catalyst characterization
and identification of aromatic monomers.

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