Catalytic transfer hydrogenation of 4-O-5 models in lignin-derived compounds to cycloalkanes over Ni-based catalysts

Article abstract of DOI:10.1002/jccs.202000360

There is an urgent need to develop a selective hydrogenolysis of C_aryl-O bonds in lignin to produce valued-added chemicals and fuels. Recently, hydrogen has been used in the hydrogenation reaction, which hides inevitable danger and is not economical. Therefore, isopropanol, as a hydrogen-donor solvent, is employed for aryl ether hydrogenolysis in lignin models over nickel supported on a carbon nanotube (CNT). Except for aromatic ether (4-O-5), the Ni/CNT catalyst is also found to be suitable for alkyl-aryl ether (α-O-4 and β-O-4) cleavage in control experiments. The physicochemical characterizations were carried out by means of H₂-temperature-programmed reduction, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy analyses. The catalyst can be magnetically recovered and efficiently reused for five consecutive recycling tests in the transfer hydrogenation of aromatic ethers. A mechanism study indicated that the hydrogenolysis cleavage of the ether bond is the first step in the reaction process, and hydrogenation of aromatic rings is only a successive step in which phenol and benzene are intermediate states and are then further hydrogenated. Furthermore, it has been demonstrated that aryl groups play an important role in the hydrogenation of phenol in the competitive catalytic hydrogenation reaction of phenol.

Full text of DOI:10.1002/jccs.202000360

Received: 21 August 2020
Revised: 13 October 2020
Accepted: 23 October 2020
DOI: 10.1002/jccs.202000360
A R T I C L E
Catalytic transfer hydrogenation of 4-O-5 models in
lignin-derived compounds to cycloalkanes over
Ni-based catalysts
Changzhou Chen1,3 | Peng Liu1,3 | Haihong Xia1,3 | Minghao Zhou1,2,3
Jianchun Jiang1,3
|
1Institute of Chemical Industry of Forest
Abstract
Products, Chinese Academy of Forestry,
Key Laboratory of Biomass Energy and
Material, National Engineering
Laboratory for Biomass Chemical
Utilization, Key and Open Laboratory on
Forest Chemical Engineering, SFA,
Nanjing, China
2School of Chemistry and Chemical
Engineering, Yangzhou University,
Yangzhou, China
3Co-Innovation Center of Efficient
Processing and Utilization of Forest
Resources, Nanjing Forestry University,
Nanjing, China
There is an urgent need to develop a selective hydrogenolysis of Caryl-O bonds
in lignin to produce valued-added chemicals and fuels. Recently, hydrogen has
been used in the hydrogenation reaction, which hides inevitable danger and is
not economical. Therefore, isopropanol, as a hydrogen-donor solvent, is
employed for aryl ether hydrogenolysis in lignin models over nickel supported
on a carbon nanotube (CNT). Except for aromatic ether (4-O-5), the Ni/CNT
catalyst is also found to be suitable for alkyl-aryl ether (α-O-4 and β-O-4) cleav-
age in control experiments. The physicochemical characterizations were car-
ried out by means of H2-temperature-programmed reduction, X-ray
diffraction, transmission electron microscopy, and X-ray photoelectron spec-
troscopy analyses. The catalyst can be magnetically recovered and efficiently
reused for five consecutive recycling tests in the transfer hydrogenation of aro-
matic ethers. A mechanism study indicated that the hydrogenolysis cleavage of
the ether bond is the first step in the reaction process, and hydrogenation of
aromatic rings is only a successive step in which phenol and benzene are inter-
mediate states and are then further hydrogenated. Furthermore, it has been
demonstrated that aryl groups play an important role in the hydrogenation of
phenol in the competitive catalytic hydrogenation reaction of phenol.
Correspondence
Minghao Zhou, Institute of Chemical
Industry of Forest Products, Chinese
Academy of Forestry, Key Laboratory of
Biomass Energy and Material, National
Engineering Laboratory for Biomass
Chemical Utilization, Key and Open
Laboratory on Forest Chemical
Engineering, SFA, Nanjing 210042, China.
Email: zhouminghao@yzu.edu.cn
K E Y W O R D S
Funding information
National Natural Science Foundation of
China, Grant/Award Number: 31700645;
Fundamental Research Funds, Grant/
Award Number: CAFYBB2018QB007
aromatic ether bonds, catalytic transfer hydrogenation, isopropanol, lignin, Ni/CNT
1 | INTRODUCTION
aggravate greenhouse pollution. Lignocellulosic biomass,
especially lignin, has received great attention as it can be
Upgrading biomass to obtain value-added fuels, energies,
and chemicals has been recognized as a green and novel
approach to the use of renewable carbon resources, com-
plementing the traditional chemical industry that heavily
depended on fossil fuels.[1–4] In comparison, biomass con-
tains peer carbon, and the use of biomass will not
potentially utilized to produce functionalized aromatic
and cycloalkane derivatives.[5–7] Owing to the complex
natural aromatic three-dimensional network structure, it
is difficult to achieve the depolymerization of lignin into
small-molecule compounds.[8,9] As is well known, a vari-
ety of ether bonds is present in the lignin, containing
J Chin Chem Soc. 2020;1–10.
http://www.jccs.wiley-vch.de
© 2020 The Chemical Society Located in Taipei & Wiley-VCH GmbH
1

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CHEN ET AL.
FIGURE 1 The dissociation energies of different types of C-O
bonds in lignin
β-O-4, α-O-4, and 4-O-5.[10] Among these types of ether
bonds, α-O-4 is the most vigorous, while 4-O-5 is the
most robust,[11] resulting in more challenges in the cleav-
age of the 4-O-5 ether bond (Figure 1).
Thus, selective cleavage of these ether bonds is of great
significance for the valorization of lignin and its derived
compounds. In the past decades, noble metal and nonnoble
metal catalysts (i.e., Ni,[12] Cu,[13] Pt,[14] Ru,[15] Pd[16]) have
been widely investigated for the hydrogenolysis of the
above substrates. For example, Gao et al reported a hetero-
geneous catalyst Ni/C that showed satisfying conversion
for cleaving the aromatic ether bond of 4-O-5 linkage
under very demanding conditions (high temperature, long
reaction time, excessive use of moisture-sensitive base, and
1 bar H2) with 86.5% of cyclohexanol and 86.5% of ben-
zene.[17] To overcome the shortcomings, Hua et al explored
the first work on the transfer hydrogenolytic cleavage of
diphenyl ether over Ru/C using iPrOH as the hydrogen
resource (avoiding the use of H2) under relatively milder
conditions (120ꢀC and 10 hr) compared to Gao et al.[18]
However, some cases still added sodium tert-butoxide to
give impetus to the reaction. Although sodium tert-
butoxide has the ability to promote the efficiency of the cat-
alytic transfer hydrogenation, its recyclability is still a diffi-
cult problem that needs to be dealt with. Therefore,
Ru/Hydroxyapatite was designed to achieve an efficient
cleavage of aromatic ether bonds in the absence of
moisture-sensitive bases from the same group.[19] Although
noble metal catalysts have been proven to have a strong
ability to transfer hydrogen, which has the advantage in
the hydrogenolysis to the aromatic ether bonds, the hydro-
genation of aromatic rings occurs and leads to a poor selec-
tivity toward cycloalkanes. Hence, the exploration of novel
and efficient catalysts is still ongoing.[20,21]
FIGURE 2 XRD of Ni/CNT catalysts with different
proportions. (a) 5% Ni/CNT, (b) 10% Ni/CNT, (c) 15% Ni/CNT,
(d) 20% Ni/CNT, and (e) 25% Ni/CNT
various research groups have successfully achieved the
efficient transfer hydrogenolysis of lignin and model mol-
ecule conversion to value-added chemicals containing
aromatics and cycloalkanes.[25,26]
Carbon nanotubes (CNTs) are interesting materials for
carrying out chemical reactions in confined spaces, and sev-
eral organic reactions have been conducted widely.[27–29] In
this paper, CNT-supported nickel was developed by a simple
and economic impregnation method to test its catalytic per-
formance in the hydrogenolysis of aromatic compounds in
iPrOH in the absence of hydrogen. We initially chose
diphenyl ether as the model molecule to optimize the
hydrogenolysis reaction over a 20% Ni/CNT catalyst. Then,
the catalyst was also found to be efficient in the transforma-
tion of other diphenyl ether derivatives to corresponding
cycloalkanes and cyclic alcohols. All of the catalysts studied
were characterized by H2-temperature-programmed reduc-
tion (TPR), transmission electron microscopy (TEM), X-ray
diffraction (XRD), and X-ray photoelectron spectroscopy
(XPS) analyses, and the mechanism was proposed at the end.
Catalytic transfer hydrogenation shows an efficient
and green alternative for hydrogen in sustainable cataly-
sis, in that hydrogen donor molecules such as iso-
propanol can be easily obtained from renewable
resources.[22,23] Furthermore, the use of hydrogen donor
molecules can reduce security risks and cut the costs
associated with the transportation and storage of hydro-
gen. Hence, the application of catalytic transfer hydroge-
nation in lignin depolymerization and ether bond
cleavage have been known as efficient and novel alterna-
tives to traditional hydrogenolysis procedures.[24] So far,
2 | RESULTS AND DISCUSSION
2.1 | Catalyst characterization
Figure 2 shows the XRD patterns of the Ni/CNT catalyst
with different metal proportions, including 5% Ni/CNT,
10% Ni/CNT, 15% Ni/CNT, 20% Ni/CNT, and 25%
Ni/CNT catalysts, in the system for the hydrogenolysis of
diphenyl ether. The peak at 26.5ꢀ could be assigned to
the diffraction peaks of the (002) planes of the graphite-

CHEN ET AL.
3
like tube wall of the CNT.[30–32] Apart from the CNT fea-
ture, the presence of metallic Ni in the reduced Ni/CNT
catalyst was clearly demonstrated. As could be clearly
seen from the XRD results, only one kind of Ni-based
phase was observed for the Ni/CNT catalyst. Peaks at
44.5ꢀ, 51.9ꢀ, and 76.4ꢀ could be assigned to the diffraction
of the (111), (200), and (220) planes of metallic Ni,[33]
indicating that only metallic nickel species were observed
in reduced Ni/CNT catalysts.
As shown in Figure 3, all Ni/CNT catalysts exhibit a
wide peak in the temperature range of 400–700ꢀC, which
would be assigned to the CNTs' methanation process.
Methanation occurs in the case of the reduction process,
starting at 390ꢀC and ending at about 800ꢀC.[34,35] It can
be seen from TPR profiles of the Ni/CNT catalysts that a
peak occurs at 100–200ꢀC, which could most likely be
attributed to H2 consumption of bulk NiO particles. The
amount of superficial NiO species increased when the Ni-
loading amount increased from 5 to 25%. To some extent,
a higher temperature is required for the reduction of
more NiO particles.
As can be seen in Figure 5, most Ni particles were dis-
played uniformly outside the CNT. However, the Ni par-
ticle size and its distribution varied from each other,
mainly owing to the loading amount of Ni on the surface
of the CNT. We selected the optimal catalyst in our cata-
lytic system (20% Ni/CNT catalyst) as an example. Ni par-
ticles were nearly evenly distributed on the surface of the
CNTs in Figure 5a, while aggregation of nickel metal
occurred when the loading amounts increased to 25%
Ni/CNT (Figure 5b). Taken together, nanoparticle Ni on
the surface of a CNT demonstrated an excellent activity
in the catalytic transfer hydrogenation of diphenyl ether.
2.2 | Optimization of the reaction
condition
Transfer hydrogenolysis of diphenyl ether, a model mole-
cule of the 4-O-5-type linkage in lignin, is of great
XPS detection was carried out to test the composition
of the optimal Ni/CNT catalyst (20% Ni/CNT), and the
absorption peaks were identified for C, O, and Ni. As we
can clearly see in Figure 4, binding energies at 852.9 and
856.3 eV were observed for 20% Ni/CNT, which cor-
responded to Ni0 (2p3/2) and Ni2+ (2p3/2), respectively.
The binding energies at 872.3 and 874.8 eV corresponded
to the main lines of Ni0 (2p1/2) and Ni2+ (2p1/2), respec-
tively. It could be seen that a small amount of metallic Ni
was oxidized before or during the XPS testing, which con-
tributed to the understanding of Ni2+ detection in the
XPS spectra.
FIGURE 4 XPS of the 20% Ni/CNT catalyst
FIGURE 3 TPR of Ni/CNT catalyst with different
FIGURE 5 TEM of the 20% Ni/CNT catalyst (a) and 25%
proportions. 5% Ni/CNT ~ 25% Ni/CNT
Ni/CNT catalyst (b)

4
CHEN ET AL.
TABLE 1 Optimization of different catalysts for the hydrogenolysis of diphenol ether 1aa
Yield (%)b
Entry
Catalyst
Solvent
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
T. (ꢀC)/t. (h)
240/4
Con. (%)b
2a
0
3a
0
1
None
0
2
CNT
240/4
8
<5
88
72
70
68
<5
16
8
<5
82
60
58
54
<5
12
6
3
20% Ni/CNT
15% Ni/CNT
10% Ni/CNT
5% Ni/CNT
20% Fe/CNT
20% Co/CNT
20% Mo/CNT
20% Ce/CNT
240/4
100
100
95
90
<5
28
22
<5
4
240/4
5
240/4
6
240/4
7
240/4
8
240/4
9
240/4
10
240/4
<5
<5
Abbreviations: Con, conversion; t, reaction time; T, reaction temperature.
aReaction conditions: 1a (500 mg), Catalyst (50 mg), Solvent (10 ml).
bThe conversion and yields were determined by GC/MS with n-dodecane as the internal standard.
importance and is also a challenging step for the valoriza-
tion of lignin owing to the high stability of the diaryl
ether bonds, compared to alkyl-aryl ether bonds
(Figure 1). Hence, diphenyl ether was chosen as a model
molecule to explore the transfer hydrogenolysis with iso-
propanol as the hydrogen resource. Various catalysts
were screened to show the catalytic activity for the trans-
fer hydrogenolysis of diphenyl ether at 240ꢀC within a
reaction time of 4 hr. No reaction was determined in the
absence of any catalysts and when employing a solo CNT
as the catalyst, which presented a poor conversion of
diphenyl ether (Table 1, entries 1–2). The highest conver-
sion (up to 100%) was achieved with the Ni/CNT catalyst
in iPrOH after 4 hr, while the corresponding yields of
cyclohexane and cyclohexanol varied with the different
loading amounts of nickel over CNT. The 5% Ni/CNT
exhibited a perfect conversion of diphenyl ether, but only
68 and 54% yields of cyclohexane and cyclohexanol were
obtained, respectively. Further analysis by Gas Chroma-
tography/Mass Spectroscopy (GC/MS) demonstrated that
aromatic chemicals, including benzene and phenol, were
detected with very low yields. When the loading amounts
of nickel over the CNT were gradually increased from
5 to 20% at an interval of 5%, the change of cyclohexane
and cyclohexanol yields presented the same trend, and in
the end, the 20% Ni/CNT catalyst was the most suitable
catalyst to yield 88% cyclohexane and 82% cyclohexanol
(Table 1, entries 3–6). In comparison, different metal-
supported catalysts (i.e., 20% Fe/CNT, 20% Mo/CNT, 20%
Co/CNT, and 20% Ce/CNT) were also synthesized to
investigate the catalytic performance during the
hydrogenolysis of diphenyl ether. However, it was disap-
pointing to find that Fe, Mo, Co, and Ce showed poor
performance on the hydrogenolysis of the 4-O-5 model
compound, and large amounts of diphenyl ether were
recovered (Table 1, entries 7–10). In summary, the com-
bination of nickel and CNT played a highly important
role in the hydrogenolysis of diphenyl ether to produce
cyclohexane and cyclohexanol in iPrOH at 240ꢀC with a
reaction time of 4 hr.
In addition, the effect of time and temperature on the
hydrogenolysis of diphenyl ether was explored subse-
quently. As was presented in Table 2, the conversion of
diphenyl ether was nearly up to 100%, and yields of cyclo-
hexane and cyclohexanol had no change (up to 80%)
within the error range when the reaction time increased
from 2 to 6 hr in intervals of 2 hr, which indicated that
time played an unimportant role in the reaction process
at a temperature of 240ꢀC (Table 2, entries 1–3). Hence,
temperature was another key factor to the hydrogenolysis
of 4-O-5 model compounds. The conversion was found to
be completely achieved at 100% when the temperature
remained over 210ꢀC. When the temperature decreased
from 210ꢀC to 190ꢀC, the yields of cyclohexane and
cyclohexanol decreased, which was because part of the
diphenyl ether is recovered, and generated benzene and
phenol were detected in the reaction process (Table 2,
entries 4–8).

CHEN ET AL.
5
TABLE 2 Optimization of time and temperature for the hydrogenolysis of diphenol ether 1aa
Yield (%)b
Entry
Catalyst
Solvent
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
T. (ꢀC)/t. (h)
240/2
Con. (%)b
100
2a
85
88
80
83
84
88
56
42
3a
82
82
78
79
81
82
52
38
1
2
3
4
5
6
7
8
20% Ni/CNT
20% Ni/CNT
20% Ni/CNT
20% Ni/CNT
20% Ni/CNT
20% Ni/CNT
20% Ni/CNT
20% Ni/CNT
240/4
100
240/6
100
230/4
100
220/4
100
210/4
100
200/4
78
190/4
56
Abbreviations: Con, conversion; t, reaction time; T, reaction temperature.
aReaction conditions: 1a (500 mg), Catalyst (50 mg), Solvent (10 ml).
bThe conversion and yields were determined by GC/MS with n-dodecane as the internal standard.
With the optimal catalyst, temperature, and reaction
time (20% Ni/CNT, 210ꢀC and 4 hr) in hand, the reaction
conditions for the hydrogenolysis of diphenyl ether are
still being researched. It was found that solvents played a
crucial role in the depolymerization of lignin based on
chemical properties of different solvents. Hence, polar
solvents (i.e., iPrOH, methanol, ethanol, dioxane, H2O,
and THF) and apolar solvents (i.e., n-hexane and cyclo-
hexane) were chosen in the Ni/CNT-catalyzed
hydrogenolysis of the 4-O-5 model compound. Apolar
solvents were expected to present poor activities for the
hydrogenolysis of diphenyl ether, resulting in 100%
recovery in the starting substrate and no corresponding
products detected in the reaction process (Table 3, entries
1–2). Hydrogen-donor solvents (iPrOH, methanol, and
ethanol) were believed to favor the ether bond
hydrogenolysis reaction in lignin. However, almost no
reaction was unexpectedly detected in the Ni/CNT-
catalyzed hydrogenolysis reaction when methanol and
ethanol were assessed as solvents, while iPrOH showed a
highly efficient activity in the transformation of diphenyl
ether to yield 88% cyclohexane and 82% cyclohexanol
(Table 3, entries 3–5). This is probably because the sec-
ondary alcohol isopropanol could give higher activity
than those primary alcohols[33]; 24% yield of cyclohexane
and 20% yield of cyclohexanol were obtained in tetrahy-
drofuran (THF) under the catalytic system, which indi-
cated that THF had a poor ability to convert diphenyl
ether to expected products (Table 3, entries 7). As an
environmentally friendly nature resource, water has been
employed in a variety of chemical reactions. In spite of
the 41% conversion of diphenyl ether, relatively lower
yields of cyclohexane (22%) and cyclohexanol (16%) were
achieved. Taking all the factors into consideration, the
optimal reaction condition was finally described (20%
Ni/CNT, 210ꢀC, 4 hr, iPrOH).
2.3 | Scope of the substrates
Encouraged by the satisfactory results above, further study
on the catalytic activity of five 4-O-5 model compounds,
one α-O-4 dimer, and two β-O-4 dimers over 20% Ni/CNT
were investigated under the optimal reaction conditions.
As was shown in Table 4, the dominant products in the
first five reactions were cyclohexane and cyclohexanol
derivatives, which suggested that the hydrogenolysis of
diphenyl ether derivatives occurred. Diphenyl ether could
be completely transferred to yield cyclohexane (88%) and
cyclohexanol (82%) (Table 4, entries 1), while the sub-
strates containing different substituent groups could be
transferred to different corresponding products over 20%
Ni/CNT. In these cases, the conversions and yields dimin-
ished, indicating that the steric hindrance played a signifi-
cant factor in this reaction. When diphenyl ethers were
substituted with methyl and hydroxyl groups on the para-
position, 100% conversion of substrates was achieved, and
the yields of corresponding cycloalkanes and cyclic alco-
hols were relatively higher (Table 4, entries 2–3). In com-
parison (entries 2 and 3), only one side methyl-substituted
diphenyl ether was employed in our reaction process.
Although 1-methyl-3-phenoxybenzene was fully con-
sumed, small amounts of cyclohexane (50%) and
3-methylcyclohexan-1-ol (43%) were achieved, which

6
CHEN ET AL.
TABLE 3 Optimization of different solvents for the hydrogenolysis of diphenol ether 1aa
Yield (%)b
Entry
Catalyst
Solvent
n-hexane
Cyclohexane
iPrOH
T. (ꢀC)/t. (h)
210/4
Con. (%)b
2a
0
3a
0
1
2
3
4
5
6
7
8
20% Ni/CNT
20% Ni/CNT
20% Ni/CNT
20% Ni/CNT
20% Ni/CNT
20% Ni/CNT
20% Ni/CNT
20% Ni/CNT
0
210/4
0
0
0
210/4
100
0
88
0
82
0
Methanol
Ethanol
Dioxane
THF
210/4
210/4
0
0
0
210/4
0
0
0
210/4
31
41
24
22
20
16
H2O
210/4
Abbreviations: Con, conversion; t, reaction time; T, reaction temperature.
aReaction conditions: 1a (500 mg), Catalyst (50 mg), Solvent (10 ml).
bThe conversion and yields were determined by GC/MS with n-dodecane as the internal standard.
suggested that the symmetrical structure with substituent
groups was beneficial to the hydrogenolysis of 4-O-5
lignin-derived compounds under the Ni/CNT-catalyzed
system (Table 4, entries 4). In addition to the diphenyl phe-
nol derivatives, this catalytic system was also well suited
for aryl-alkyl ether, in which phenyl methyl ether could be
transferred to cyclohexanol with a yield of 62% (Table 4,
entry 5).
In comparison with entries 1, 6, and 7 in Table 4, an
interesting phenomenon was discovered: Even though the
yields of corresponding cycloalkanes were up to 60%, the
yields of cyclic alcohols varied (entry 1 > entry 6 > entry 7).
Hence, a competitive catalytic hydrogenation reaction was
carried out, in which we mixed phenol with benzene, tolu-
ene, and ethylbenzene in an equimolar feed composition to
simulate the catalytic hydrogenation of diphenyl ether, phe-
nyl methyl ether, and phenyl ethyl under the optimal condi-
tions. At the same time, a blank experiment including solo
phenol hydrogenation was tested. As is demonstrated in
Figure 6, the hydrogenation yields of cyclohexanol
decreased by degrees in the presence of benzene,
methylbenzene, and ethylbenzene. So, the addition of differ-
ent aromatic chemicals contributed to an impairment of
phenol hydrogenation activity. Related coordination chemis-
try of benzene and toluene on nickel (111), (110), and (100)
surfaces has already been reported in previous literature.[36]
Moreover, an experiment involving the conversion of
one α-O-4 model compound was also investigated
(Table 4, entry 6). It was gratifying to find that benzyl
phenol ether underwent selective carbon–oxygen bond
cleavage and hydrogenolysis of aromatic rings over the
treatment with the 20% Ni/CNT catalyst, and there was
no negative effect on the conversion and yield during the
reaction process. As the β-O-4 bond is predominant in
the lignin structure, phenethoxybenzene and 2-(2-meth-
oxyphenoxy)-1-phenylethan-1-ol were chosen to investi-
gate the catalytic activity of Ni/CNT, and the perfect
performance on the catalytic transfer hydrogenation sys-
tem is shown in Table 4. When methoxy substitution of
the O-terminus aryl ring led to a decreased yield of
2-methoxycyclohexan-1-ol (45%), compared to no substit-
uent group on the O-terminus aryl ring (Table 4, entries
7–8), it was worth noting that almost no aromatic com-
pounds were observed, indicating that the Ni/CNT cata-
lyst demonstrated the perfect ability to hydrocrack the
carbon–oxygen bond and dearomatize the benzene rings
in one step. The results above proved that Ni/CNT was
highly efficient for the catalytic transfer hydrogenation of
a variety of diphenyl ether derivatives.
2.4 | Possible reaction mechanism and
the recycling of catalyst
Comprehending the reaction pathway is of great impor-
tance for the valorization of lignin to produce value-
added chemicals. In general, we proposed two possible
reaction pathways for the hydrogenolysis of diphenyl
ether. In the first potential pathway (pathway one in
Figure 7, red one), benzene and phenol were directly
obtained from the cleavage of an aromatic ether bond in
which the formation of a coordinative bond between the

CHEN ET AL.
7
TABLE 4 Cleavage of various 4-O-5, α-O-4, and β-O-5 model compounds over 20% Ni/CNTa
Abbreviations: t, reaction time; T, reaction temperature.
aReaction conditions: 1a–1h (500 mg), Catalyst (50 mg), iPrOH (10 ml).
bThe conversion and yields were determined by GC/MS with n-dodecane as the internal standard.
phenoxide group and nickel (oxidative addition step) was
subsequently hydrogenated in the presence of iPrOH.
After this, the produced benzene and phenol were further
hydrogenated to afford cyclohexane and cyclohexanol.
On the other hand, intermediate product cyclohexyl phe-
nyl ether was hydrogenated from diphenyl ether by active
hydrogen. Owing to the dissociation energy of the
C(sp2)-O bond being higher than the C(sp3)-O bond, the

8
CHEN ET AL.
generated cyclohexyl phenyl ether above could easily be
transformed to cyclohexane and phenol. Subsequently,
phenol was hydrogenated to cyclohexanol under our
reaction
system.
However,
only
cyclohexane,
cyclohexanol, benzene, and phenol were observed in the
reaction process. Hence, pathway one could be regarded
as the dominate reaction pathway.
Finally, the recyclability of the 20% Ni/CNT catalyst,
based on successive recycling tests at 210ꢀC for 4 hr, was
subsequently explored using diphenyl ether conversion
as the model reaction. The 20% Ni/CNT retained its activ-
ity after five consecutive runs, and minor changes in
product selectivity was observed, giving prominence to
the good stability of the catalyst. Except for this, the cata-
lyst could be recovered by a magnet and could be used in
the next run after simple processing (washing with iPrOH
and drying at 105ꢀC) (Figure 8).
3 | EXPERIMENTAL
3.1 | Materials
FIGURE 6 The competitive catalytic hydrogenation reaction
of phenol with different aromatics at equimolar mixture under the
optimal reaction condition (20% Ni/CNT, 210ꢀC, 4 hr, iPrOH)
Nickel nitrate hexahydrate (Ni(NO3)2ꢁ6H2O) was pur-
chased from Aladdin Industrial Inc. Shanghai, China; the
CNT was provided by Sigma-Aldrich; diphenyl ether
(>99.99%),
dimethyl 4,40-oxydibenzoate (>99.99%), 4,40-oxydiphenol
(>99.99%), 1-methyl-3-phenoxybenzene (>99.99%),
4,40-oxybis(methylbenzene)
(>99.99%),
anisole (>99.99%), 9H-xanthene (>99.99%), (benzyloxy)
benzene (>99.99%), and all the solvents were obtained
from commercial sources and used without further purifica-
tion.
2-phenoxy-1-phenylethan-1-ol
and
2-(2-meth-
oxyphenoxy)-1-phenylethan-1-ol were synthesized according
to literature method.[37]
3.2 | General procedure for Ni/CNT-
catalyzed DPE
In a typical catalytic reaction, 500 mg of diphenyl ether
(DPE) or another derivate, 50 mg of catalyst, and 10 ml
FIGURE 7 Possible pathways of the catalytic transfer
hydrogenation of diphenyl ether over Ni/CNT
FIGURE 8 The recycling runs for
Ni/CNT catalyst in the catalytic transfer
hydrogenation and magnetic recovery of
Ni/CNT catalyst

CHEN ET AL.
9
of isopropanol were placed in a stainless Parr autoclave
(25 ml), and any trace of air present in the reactor was
eliminated by fluxing H2 (99.99%) thrice. The reactions
were subsequently conducted at the desired temperature
and stirred (600 rpm). At the end of the reaction, the
reactor was cooled to room temperature, and the pres-
sure was released; the mixture was filtered to collect the
catalyst; and the filtrate was analyzed by the gas chro-
matograph/mass spectrometer using the n-dodecane as
an internal standard. The collected catalyst was washed
with isopropanol thrice and dried at 100ꢀC to explore
the cycle test under the optimal reaction condition. The
conversion and product yield in the liquid phase were,
respectively, calculated according to the following
formulas:
4 | CONCLUSION
We described a mild and efficient strategy, which
proceeded through the selective cleavage of aromatic
ether bonds and a hydrogenolysis of aromatic rings. The
Ni/CNT catalyst not only exhibited the perfect ability to
catalyze the transfer hydrogenolytic cleavage of aromatic
ether bonds in various compounds, employing iPrOH as
a hydrogen resource, but also could be easily recovered
magnetically from the reaction process for the next five
recycling tests. The competitive catalytic hydrogenation
reaction indicated that the yields of cyclic alcohols in the
transfer hydrogenolysis of diphenyl ether, benzyl phenol
ether, and phenethoxybenzene were deeply affected by the
nature of the aryl groups, in which the more arene deriva-
tives substituted, the lower yields of cyclohexane achieved.
The circular diagram of the mechanism provided two path-
ways to form cycloalkanes and cyclic alcohols, and the
main pathway can be concluded as follows: direct cleavage
of aromatic ether bonds and dearomatization subsequently.
This work may inspire more research on the depolymeriza-
tion of real lignin using nonnoble metal catalysts in the
absence of hydrogen.
model of DPE reacted
total mole of DPE
Conversion =
× 100%
model of cyclohexane
total mole of DPE
Yied of cyclohexane
Yied of cyclohexane
× 100%
× 100%
model of cyclohexane
total mole of DPE
ACKNOWLEDGMENTS
The authors are grateful for the financial support from
the National Key R&D Program of China
(No.2018YFB1501405-05) and the National Natural Sci-
ence Foundation of China (31700645).
3.3 | Catalyst preparation
In a typical process, the 20% Ni/CNT catalyst was pre-
pared by impregnating the CNT (2 g) with an aqueous
solution of Ni(NO3)2ꢁ6H2O (1.5 g) in 30 ml of deionized
water. Then, the suspension was stirred for 24 hr at
room temperature to ensure high dispersion. Subse-
quently, the suspension was dried at 110ꢀC for 5 hr, and
the obtained solid sample was calcined in a furnace at
500ꢀC for 5 hr. The desired 20% Ni/CNT catalyst was
finally achieved after reducing at 500ꢀC for 3 hr under
hydrogen flow. Different M/CNT catalysts (i.e., Fe/CNT,
Mo/CNT, Co/CNT, and Ce/CNT) were synthesized by
the same method.
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How to cite this article: Chen C, Liu P, Xia H,
Zhou M, Jiang J. Catalytic transfer hydrogenation
of 4-O-5 models in lignin-derived compounds to
cycloalkanes over Ni-based catalysts. J Chin Chem
Soc. 2020;1–10. https://doi.org/10.1002/jccs.
202000360