Selective cleavage of lignin and lignin model compounds without external hydrogen, catalyzed by heterogeneous nickel catalysts

Article abstract of DOI:10.1039/c9sc00691e

Selective hydrogenolysis of the C_aryl-O bonds in lignin is a key strategy for the generation of fuels and chemical feedstocks from biomass. Currently, hydrogenolysis has been mainly conducted using hydrogen, which is flammable and not sustainable or economical. Herein, an external hydrogen-free process for aryl ethers hydrogenolysis in lignin models and dioxasolv lignin over nickel nanoparticles supported on Al₂O₃, is reported. Kinetic studies reveal that the transfer hydrogenolysis activity of the three model compounds decreased in the following order: benzyl phenyl ether (α-O-4), 2-phenylethyl phenyl ether (β-O-4) and diphenyl ether (4-O-5), which linearly corresponds to their binding energies and the activation energies. The main reaction route for the three model compounds was the cleavage of the ether bonds to produce aromatic alkanes and phenol, and the latter was further reduced to cyclohexanol. Dioxasolv lignin depolymerization results exhibit a significant C_aryl-O decrease over the Ni nanoparticles supported on Al₂O₃ with iso-propanol as the hydrogen source through 2D-HSQC-NMR analysis, which confirmed the transfer hydrogenolysis conclusion in the model study. This work provides an economical and environmentally-friendly method for the selective cleavage of lignin and lignin model compounds into value-added chemicals.

Full text of DOI:10.1039/c9sc00691e

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Selective cleavage of lignin and lignin model
compounds without external hydrogen, catalyzed
Cite this: DOI: 10.1039/c9sc00691e
by heterogeneous nickel catalysts†
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
bc
b
Liang Jiang, ‡a Haiwei Guo,
‡
Changzhi Li,
Peng Zhoua
*
a
*
and Zehui Zhang
Selective hydrogenolysis of the Caryl–O bonds in lignin is a key strategy for the generation of fuels and
chemical feedstocks from biomass. Currently, hydrogenolysis has been mainly conducted using
hydrogen, which is flammable and not sustainable or economical. Herein, an external hydrogen-free
process for aryl ethers hydrogenolysis in lignin models and dioxasolv lignin over nickel nanoparticles
supported on Al2O3, is reported. Kinetic studies reveal that the transfer hydrogenolysis activity of the
three model compounds decreased in the following order: benzyl phenyl ether (a-O-4), 2-phenylethyl
phenyl ether (b-O-4) and diphenyl ether (4-O-5), which linearly corresponds to their binding energies
and the activation energies. The main reaction route for the three model compounds was the cleavage
of the ether bonds to produce aromatic alkanes and phenol, and the latter was further reduced to
cyclohexanol. Dioxasolv lignin depolymerization results exhibit a significant Caryl–O decrease over the Ni
nanoparticles supported on Al2O3 with iso-propanol as the hydrogen source through 2D-HSQC-NMR
analysis, which confirmed the transfer hydrogenolysis conclusion in the model study. This work provides
an economical and environmentally-friendly method for the selective cleavage of lignin and lignin model
compounds into value-added chemicals.
Received 8th February 2019
Accepted 22nd February 2019
DOI: 10.1039/c9sc00691e
rsc.li/chemical-science
bond in lignin due to the high C–O bond energy (218–
Introduction
314 kJ molꢀ1).9
Both homogeneous and heterogeneous catalysts have been
explored for the cleavage of aryl ethers in liquid phase using
hydrogen. Homogeneous catalysts including Ru, V, and Ni
complexes were investigated for the cleavage of the CAr–O
bond,10–12 which were performed under relatively mild condi-
With the increasing crises of shrinking fossil fuel supplies and
global climate change, great effort has been devoted to the
development of new routes for producing chemicals and liquid
fuels from renewable sources.1,2 Biomass, the only carbon con-
taining renewable resource, is undoubtedly regarded as the best
candidate to provide sustainable chemicals and energy for our
society. Lignin is the second most abundant component in
lignocellulose with 20–30 weight percentage.3 It is a three-
dimensional bio-polymer composed of various C–O linkages
connected with methoxylated phenyl-propane units.4 Therefore,
the depolymerization of lignin could produce diverse aryl ether
fragments (CAr–O bond) either in the form of a-O-4 or b-O-4
linkages.5–8 However, it is a great challenge to cleave the CAr–O
ꢁ
tions (80–135 C, 1 bar H2) due to the exibility of the homo-
geneous catalysts, allowing them to contact the CAr–O bonds
freely without high steric limitation. However, these catalytic
systems demonstrated several drawbacks such as: difficulty in
recycling the homogeneous catalysts, the purication of prod-
ucts, the requirement of complex ligands, and the use of air-
sensitive organic bases.10 In addition, the use of air-sensitive
bases such as t-BuOK increases the production cost water has
to be removed from raw biomass or the solvents.
From the viewpoint of green and suitable chemistry,
heterogeneous catalysts are a better choice for the aqueous-
phase conversion of aryl ethers. Either unsupported nano-
particles Fe,13 RuNi,14 AuNi,15 or supported metal catalysts16–24
have been used for the cleavage of lignin derived aryl ethers to
a mixture of aromatics and phenolics/cycloalcohols (Scheme
1a). Particularly, nickel catalysts have recently received great
interest for this transformation because they are readily avail-
able and low cost.18–20 Due to the inherent lower catalytic activity
in comparison with noble-metal catalysts, the heterogeneous Ni
aKey Laboratory of Catalysis and Materials Sciences of the Ministry of Education,
South-Central University for Nationalities, Wuhan, 430074, China. E-mail:
zehuizh@mail.ustc.edu.cn; Fax: +86-27-67842572; Tel: +86-27-67842572
bState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian 116023, China. E-mail: licz@dicp.ac.cn
cUniversity of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9sc00691e
‡ These authors contributed equally to this work.
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conditions.29 However, the process of the preparation of
RANEY®-Ni is non-environmentally friendly and RANEY®-Ni is
very sensitive to air, requiring special treatment for both storage
and application. Therefore, it is highly desirable to develop
supported non-noble metal catalysts for the transfer hydroge-
nation of lignin model compounds as well as lignin to produce
value added chemicals.
Inspired by the fact that some supported nickel catalysts are
capable of promoting the transfer hydrogenation and the
release of hydrogen from hydrogen donors,30,31 we report the
transfer hydrogenation of CAr–O bonds in lignin model
compounds using biomass-derived iso-propanol over supported
nickel catalysts (Ni/Al2O3). The Ni/Al2O3 catalysts were prepared
by the calcination of Ni–Al layered double hydroxide
compounds, followed by reduction under a hydrogen atmo-
sphere. High activity and selectivity for the hydrogenation of
CAr–O bonds was observed during transfer hydrogenation with
iso-propanol under mild conditions. More importantly, the
developed method is also effective for the transfer hydrogena-
tion cleavage of actual lignin to produce valuable chemicals.
Our developed methods could overcome the drawbacks as
mentioned above. Not only does our method not require the use
of high-pressure H2, but the catalysts are stable and do not
require cautious treatment. More importantly, our transfer
hydrogenation processes were performed under mild condi-
tions with comparable or even lower temperatures in compar-
ison with reported methods using H2 and other hydrogen
donors. Thus, this work provides an economical and
environmentally-friendly method for the selective cleavage of
lignin model compounds, as well as lignin, into value-added
chemicals.
Scheme 1 Methods for the cleavage of lignin model compound
diphenyl ether.
catalyst systems were previously performed at high temperature
18
and high hydrogen pressure.21–23 Subsequently, Ni/SiO2 and
Ni/TiN19 catalysts were observed to promote the cleavage of
lignin derived aryl ethers under relatively mild conditions (120–
150 ꢁC, and 6–12 bar H2) at the expense of high Ni loading
(Scheme 1b and c). Recently, a heterogeneous Ni catalyst
formed by the in situ reduction of a homogeneous Ni complex,
was found to be effective for this transformation with low
content of Ni (0.005–0.25 equiv.).24 However, the excessive use of
a moisture-sensitive base (2.5–10 equiv. t-BuONa), the low
availability of the Ni complex, a long reaction time (up to 24–96
h), and high reaction temperature (up to 180 ꢁC) limit its
practical application (Scheme 1d).
Experimental
Materials
All chemicals were purchased from Aladdin Chemicals Co. Ltd.
(Beijing, China). All solvents were purchased from Sinopharm
Chemical Reagent Co., Ltd. (Shanghai, China), and used
directly.
Methods for the cleavage of the CAr–O bond in aryl ethers
usually require hydrogen. While the use of hydrogen for the
cleavage produces water as the oxidation product, the use of
hydrogen at high temperature and high pressure requires
specialist equipment, and has potential safety issues.25 In
addition, caution is needed to transport and store hydrogen gas
which increases production costs. Therefore, developing new
catalytic systems for the cleavage of CAr–O bonds selectively and
environmentally is highly desirable. Compared with the
hydrogenation reactions involving hydrogen, transfer hydroge-
nation using hydrogen donors such as alcohols and formic acid
are safer and easier to handle.26 In fact, transfer hydrogenation
of the lignin model compounds, even lignin, has been reported
in recent years.27 However, the reactions were performed under
harsh conditions (supercritical methanol, 300–320 ꢁC).28
RANEY®-Ni could promote transfer hydrogenation of the lignin
model compounds as well as wood biomass under mild
Catalyst preparation
NiAl-LDH with the formula [Ni2Al(OH)6](NO3)$0.6H2O was
prepared according to the published procedure.30 Typically,
0.20 mol of Ni(NO3)2$6H2O, 0.10 mol of Al(NO3)3$9H2O and
0.70 mol of urea were added into 100 mL of deionized water,
and then the mixture was stirred at room temperature until
a clear solution was observed. Aer that, the mixture was stirred
at 140 ꢁC for 9 h in an autoclave. Aer cooling to room
temperature, the solid slurry was ltered, washed several times
ꢁ
with deionized water and nally dried at 60 C under vacuum.
Preparation of Ni/Al2O3-T catalysts
Ni/Al mixed oxide (NiO–Al2O3) was rst prepared by calcining
the NiAl-LDH precursor at 500 ꢁC for 5 h under static air. Then,
the NiO–Al2O3 sample was reduced under a H2 ow at 400 ꢁC for
30 min with a heating rate of 2 ꢁC minꢀ1, and increased the
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reduction temperature to 500, 550, 600, 650 or 700 ꢁC for 30 min
with a heating rate of 2 ꢁC minꢀ1 (denoted as Ni/Al2O3-T, T
refers to the reduction temperature). Aer being cooling to
room temperature in a continuous H2 ow, the Ni/Al2O3-T was
passivated in a 1.0 mol% O2/N2 mixture for 1 h, and stored
under air for subsequent catalytic evaluation.
General procedures for the transfer cleavage of lignin
The catalytic conversion of lignin was conducted in a stainless
steel autoclave (50 mL) with an initial N2 pressure of 1 MPa. In
a typical process, the catalyst (30 mg), lignin (100 mg), and iso-
propanol (20 mL) were charged in the autoclave and the reactor
was stirred at 800 rpm for 12 h at 170 ꢁC. Aer reaction, the
mixture was cooled to room temperature and was ltered. The
ꢁ
ltrate was distilled at 45 C under reduced pressure to obtain
Catalyst characterization
light-yellow liquid oil. The liquid oil was weighed and was then
diluted with methanol to 1.5 mL, the monomer products in the
liquid oil were qualitatively determined with a HP 5973 GC-MS
(HP-5 column, 30 m ꢂ 0.32 mm ꢂ 0.25 mm) and were quantied
with GC-FID (HP-5 column, 30 m ꢂ 0.32 mm ꢂ 0.25 mm) by an
internal standard method using mesitylene as the standard.
NMR spectra were recorded on a Bruker AVANCE III HD 700
MHz spectrometer at 25 ꢁC. The Bruker standard pulse program
hsqcetgpsi was used for HSQC acquisition. Fiy milligrams of
lignin was dissolved in 0.5 mL d6-DMSO and the solvent peak
was used as the internal reference. HSQC cross-signals were
analyzed and assigned by comparing with the published
literature.41
Transmission electron microscopy (TEM) images of the samples
were obtained on an FEI Tecnai G2-20 instrument. The samples
were rst dispersed in ethanol under ultrasonication and
dropped onto copper grids for observation. Mesopore surface
area and pore size measurements were performed with N2
adsorption/desorption isotherms at 77 K on a V-Sorb 2800P
instrument. Before the measurements, the samples were
degassed at 100 ꢁC for 12 h. X-ray powder diffraction (XRD)
patterns of samples were determined with a Bruker advanced
D8 powder diffractometer (Cu Ka). All XRD patterns were
collected with a scanning rate of 0.016ꢁ sꢀ1. The nickel content
in the Ni/Al2O3-T catalysts was quantitatively determined by
inductively coupled-atomic emission spectrometer (ICP-AES) on
an IRIS Intrepid II XSP instrument (Thermo Electron
Corporation).
Results and discussion
Catalyst preparation and characterization
General procedure for the hydrogenation of aryl ethers
Ni–Al layered double hydroxide compound (NiAl-LDH) is a class
of two dimensional (2D) anion-intercalated materials, which
was prepared by traditional co-precipitation.32 Powder X-ray
diffraction patterns (Fig. S1†) illustrated the crystalline nature
of NiAl-LDH, which can be indexed to a characteristic feature of
the layered NiAl-LDH phase.33 The XRD pattern of NiAl-LDH
presents the basal spacing (003), which is characteristic of the
layered structures.33 Aer calcination at 500 ꢁC in air, the basal
reections of NiAl-LDH disappear, and 3 new peaks with 2q ¼
37.3ꢁ, 43.5ꢁ and 63.2ꢁ appear in the XRD patterns of NiO–Al2O3,
corresponding to (111), (200), and (220) reections of the NiO
(Fig. 1).34 However, no reection peaks of Al2O3 in the XRD
patterns of NiO–Al2O3 were detected, indicating that an amor-
phous phase of Al2O3 is present. These results suggest that the
The catalytic reactions were carried out in a 50 mL autoclave
reactor. In a typical experiment, the lignin model compound of
aryl ethers (1.0 mmol), Ni/Al2O3-600 (20 mg), and iso-propanol
(10 mL) were added into the autoclave reactor. The air in the
reactor was removed by N2, and nally the autoclave was pres-
surised with 10 bar N2. Then the reactor was heated at room
temperature to 150 ꢁC within 10 min, the reaction started at
150 ꢁC and was le for the desired time with a stirring speed of
1000 rpm. The reaction was quenched at ambient temperature
using ice-water.
The organic products were analyzed with a 7890F gas chro-
matograph system (GC) with a ame ionization detector, both
equipped with HP-5 capillary columns (30 m ꢂ 0.32 mm ꢂ 0.4
mm). The temperature of the column was initially kept at 80 ꢁC
for 3 min, and then increased at a rate of 20 ꢁC minꢀ1 to 220 ꢁC.
Products were identied by comparison of the retention time of
the unknown compounds with those of standard compounds,
and quantied based on the internal standard method. Ethyl-
benzene and diphenyl ether were used as the internal standard
to calibrate the liquid product concentrations when the reaction
substrate was benzyl phenyl ether; ethylbenzene and benzyl
alcohol were used as the internal standard when the reaction
substrate was diphenyl ether; toluene and diphenyl ether were
used as the internal standard when the reaction substrate was 2-
phenylethyl phenyl ether. These internal standards were added
aer the reaction had nished. Conversion is dened as the
amount of change in raw materials during the reaction, divided
by the total amount of starting material, multiplied by 100%.
The selectivity is dened as the mole percentage of the target
products with the total amount of starting material.
ꢁ
calcination of NiAl-LDH in air at 500 C generates NiO and an
Fig. 1 XRD patterns of NiAl-MMO and Ni/Al2O3-T catalysts.
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amorphous Al2O3 phase. The NiO–Al2O3 sample was then
reduced at 5 representative temperatures of 500, 550, 600, 650
and 700 ꢁC. NiO was the dominant phase for the Ni–Al-500
catalyst, while new peaks corresponding to metallic Ni were
clearly observed in the XRD patterns of the Ni–Al-550 catalyst.
Further increasing the reduction temperature to 600 ꢁC,
metallic Ni became the main phase for the NiO–Al2O3-600
catalyst. Three peaks with 2q ¼ 44.2ꢁ, 51.6ꢁ and 76.2ꢁ clearly
appeared in the XRD patterns of the NiO–Al2O3-600, NiO–Al2O3-
650 and NiO–Al2O3-700 catalysts, which were assigned to (111),
(200), and (220) reections of metallic Ni (PDF number 01-087-
0712). The XRD results indicate that the NiO phase gradually
transformed into metallic Ni with the increase of the reduction
temperature. The particle size of metallic Ni calculated by the
Scherrer equations (Table S1†) increased from 5.5 nm for the
Ni/Al2O3-500 catalyst to 10.2 nm for the Ni/Al2O3-700 catalyst,
suggesting the growth of Ni nanoparticles with the increase of
reduction temperature. Ni loadings in the Ni/Al2O3-500, Ni/
Al2O3-550 and Ni/Al2O3-600 catalysts determined by ICP-AES
slightly increased with the increase of the reduction tempera-
ture (Table S1†). The ICP-AES results were consistent with the
XRD results. XRD results reveal that NiO in the NiO–Al2O3
sample were gradually reduced to metallic Ni by increasing the
ꢁ
ꢁ
reduction temperature from 500 C to 600 C, and thus the Ni
loading in the Ni/Al2O3-T catalysts increased in the order of Ni/
Al2O3-500, Ni/Al2O3-550 and Ni/Al2O3-600.
The BET surface areas and average pore diameter of the as-
prepared Ni/Al2O3-T catalysts are listed in Table S1.† All the
Ni/Al2O3-T catalysts display a type IV isotherm shape (Fig. 2a),
characteristic of the mesoporous structure and wide mesopore
size distribution (Fig. 2b). Interestingly, the surface area of the
Ni/Al2O3-T catalysts gradually decreased with the increase of the
reduction temperature (Table S1†). The average size of the Ni/
Al2O3-T catalysts increased with the increase of the reduction
temperature, which is contrary to the effect on the surface area
of the Ni/Al2O3-T catalysts (Table S1†).
Fig. 2 (a) N2 adsorption/desorption isotherms of Ni/Al2O3-T catalyst
and, (b) corresponding Barrett–Joyner–Halenda (BJH) pore-size
distribution curve.
large-sized nanoparticles at high reduction temperatures
ꢁ
beyond 600 C.
X-ray photoelectron spectroscopy (XPS) was carried out to
detect the valence states of surface Ni of the representative Ni/
Al2O3-600 catalyst (Fig. S2†). The peaks of binding energy at
Catalyst screen
ꢃ851.7 (Ni 2p3/2), ꢃ854.3/872.1 (Ni 2p1/2), and ꢃ860.1/878.7 eV Firstly, the activity of the Ni/Al2O3-T catalysts was evaluated by
can be assigned to metallic Ni, divalent valence state Ni2+ and the transfer hydrogenation of benzyl phenyl ether (a-O-4) as the
the satellite peak, respectively.35 Obviously, most of the nickel model reaction with iso-propanol as the hydrogen donor as well
nanoparticles were present in the oxidation state as detected by as the solvent. The hydrogenation of benzyl phenyl ether did not
XPS, albeit the XRD results of the Ni/Al2O3-600 catalyst reveal occur without the catalyst (Table 1, entry 1). Similarly, no
that the nickel species presented mainly in the metallic state. conversion of benzyl phenyl ether was observed in the presence
The crystal phase detected by XRD was metallic state, while the of Al2O3, suggesting that Al2O3 was inactive for this trans-
surface of the metallic nickel nanoparticle was highly oxidized formation (Table 1, entry 2). To our delight, all the Ni/Al2O3-T
into its oxidation state by oxygen either from the passivated catalysts were active for the transfer hydrogenation of benzyl
process or air storage. Nickel nanoparticles were clearly phenyl ether, with iso-propanol as the hydrogen donor (Table 1,
observed and well dispersed on the surface of the Ni/Al2O3-T entries 3–7). The catalytic activity of Ni/Al2O3-T greatly increased
catalysts (Fig. 3). The average size of the nickel nanoparticle with the increase of reduction temperature from 500 to 600 ꢁC.
slightly increased to 5.6, 6.9 and 7.2 nm for the Ni/Al2O3-500, Ni/ The conversions of benzyl phenyl ether were 15.4%, 22.0% and
Al2O3-550 and Ni/Al2O3-600 catalysts, respectively. However, its 55.3% for the Ni/Al2O3-500, Ni/Al2O3-550 and Ni/Al2O3-600,
size greatly increased to 14.9 and 16.5 nm for the Ni/Al2O3-650 respectively (Table 1, entries 3–5). Comparing the results in
catalyst and Ni/Al2O3-700 catalyst with a large size distribution. entries 3–5, shows that the active phase for the transfer
These results suggest that the nickel nanoparticles grow into hydrogen from iso-propanol was metallic Ni. The highest
conversion of benzyl phenyl ether was attained in 58.9% over
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For all the reactions, the C–O–C bond was selectively cleaved
at the position of the aliphatic carbon, producing phenol and
toluene. In addition, the gas phase was also investigated with
gas chromatography, and hydrogen was not detected. These
results suggest that the cleavage of benzyl phenyl ether was
a real hydrogen-transfer process by iso-propanol over the Ni/
Al2O3-T catalysts. Although the Ni/Al2O3-600 catalyst showed
a similar catalytic activity as the Ni/Al2O3-650 catalyst, the
former demonstrated stronger ability to promote the subse-
quent hydrogenation of phenol into cyclohexanol (Table 1,
entries 5 vs. 6). Therefore, the Ni/Al2O3-600 catalyst was used for
the following studies.
Transfer hydrogenation of benzyl phenyl ether at different
temperatures
The effect of the reaction temperature on the transfer hydro-
genolysis of benzyl phenyl ether was also examined over the Ni/
Al2O3-600 catalyst (Table 2). Interestingly, the Ni/Al2O3-600
catalyst was even active for the cleavage of the C–O–C bond at
a low temperature of 100 ꢁC (Table 2, entry 1). To the best of our
knowledge, there has been no such report on the transfer
hydrogenation of benzyl phenyl ether by iso-propanol under
such low temperatures over a heterogeneous non-noble metal
catalyst. Prolonging the reaction time to 48 h, 100% conversion
of benzyl phenyl ether was attained with toluene and phenol as
the main product (Table 2, entry 2). The transfer hydrogenolysis
of benzyl phenyl ether at low temperature without external
hydrogen shows huge potential for the large-scale utilization of
lignin to produce valuable chemicals as these processes are
environmentally friendly, easy to handle, and economical. It
was noted that both the cleavage of the benzyl phenyl ether
bond and the subsequent hydrogenation of phenol are sensitive
to the reaction temperature. The conversion of benzyl phenyl
ether increased with the increase of the reaction temperature
from 4.3% at 100 ꢁC to 100% at 150 ꢁC aer 3 h (Table 2, entries
1–6). The selectivity of toluene remained 100% at different
Fig. 3 TEM images of the as-prepared Ni/Al2O3-T catalysts and
particle size distribution of the Ni nanoparticles ((a) Ni/Al2O3-500; (b)
Ni/Al2O3-550; (c) Ni/Al2O3-600; (d) Ni/Al2O3-650; (e) Ni/Al2O3-700).
Table 1 The hydrogenation of benzyl phenyl ether with different
catalystsa
Yield (%)
Entry
Catalyst
Con (%)
1
2
3
4
1
2
3
4
5
6
7
—
Al2O3
0
0
—
—
—
—
—
—
—
—
—
Table 2 The effect of reaction temperature on the reduction of benzyl
phenyl ethera
Ni/Al2O3-500
Ni/Al2O3-550
Ni/Al2O3-600
Ni/Al2O3-650
Ni/Al2O3-700
15.4
22.0
55.3
58.9
33.1
15.4
22.0
55.3
58.9
33.1
14.4
20.9
50.5
57.8
32.6
1.0
1.1
4.3
0.9
0.5
—
0.5
0.2
—
a
Reaction conditions: benzyl phenyl ether (185 mg, 1 mmol), 130 ꢁC,
3 h, catalyst (20 mg), iso-propanol (10 mL), stirring at 1000 rpm.
Yield (%)
Temperature
Entry
(ꢁC)
Con (%)
1
2
3
4
1
100
100
120
130
140
150
160
4.3
4.3
4.3
—
—
12.2
1.9
the Ni/Al2O3-650 catalyst (Table 1, entry 6). However, further
increasing the pyrolysis temperature to 700 C, the conversion
2b
3
100
25.5
55.3
88.5
100
100
100
25.5
55.3
88.5
100
100
87.5
23.3
50.5
75.5
22.4
6.8
0.3
0.2
0.5
1.1
1.6
0.9
ꢁ
of benzyl phenyl ether signicantly decreased to 33.1% (Table 1,
entry 7). Although Ni/Al2O3-700 has the largest content of
metallic nickel, its lower catalytic activity should be due to its
large particle size, as metal nanoparticles with smaller size have
been generally considered to provide more active sites for
chemical reactions.
4
5
6
7
4.3
11.9
73.1
92.3
a
Reaction conditions: benzyl phenyl ether (185 mg, 1 mmol), Ni/Al2O3-
b
600 (20 mg), iso-propanol (10 mL), 3 h, stirring at 1000 rpm. The
reaction time was 48 h.
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temperatures, while phenol could be further reduced to cyclo-
hexanol, especially at high temperatures of 140 to 160 ꢁC, (Table
2, entries 5–7). For example, the selectivity of cyclohexanol
signicantly increased from 13.4% at 140 ꢁC, to 92.3% at 160 ꢁC
aer 3 h. Unlike the transfer cleavage of the C–O–C bond in
benzyl phenyl ether, the selectivity of cyclohexanol was more
sensitive to the reaction temperature suggesting that the
transfer hydrogenation of phenol into cyclohexanol was more
difficult than the transfer cleavage of the C–O–C bond in benzyl
phenyl ether. Cyclohexanone, the intermediate of the hydroge-
nation of phenol into cyclohexanol, remained at low levels
irrespective of the reaction temperature, suggesting that cyclo-
hexanone could be readily reduced to cyclohexanol.
Fig. 4 The relationship between apparent Ea and BDE for the three
model compounds.
Catalytic transfer hydrogenation of benzyl phenyl ether with
different alcohols
phenyl ether (b-O-4, Ea ¼ 130 kJ molꢀ1) and diphenyl ether (4-O-
5, Ea ¼ 147 kJ molꢀ1).
Furthermore, other common alcohols were tested for the
transfer hydrogenolysis of benzyl phenyl ether at 160 ꢁC.
Compared with iso-propanol, the transfer hydrogenolysis of
benzyl phenyl ether in primary alcohols such as ethanol, n-
propanol and n-butanol produced much lower conversions
(Table S2,† entries 1–3 vs. 4), due to the difficulty of eliminating
b-hydrides from primary alcohols on the surface of the cata-
lyst.36 In addition, 2-butanol was inactive for the transfer
hydrogenolysis of benzyl phenyl ether due to the larger steric
hindrance in comparison with iso-propanol, which makes
attack by the metal sites difficult (Table S2,† entries 4 vs. 5).
Kinetics and reaction pathways of the transfer hydrogenolysis
of the three model compounds
The time course of the products distribution was studied for the
transfer hydrogenolysis of benzyl phenyl ether over the Ni/
ꢁ
Al2O3-600 catalyst at 150 C (Fig. 5a). Benzyl phenyl ether was
fully consumed aer 2 h at 150 ꢁC. Interestingly, the slope of the
conversion curve between the 4 reaction points (0–15 min, 15–
30 min; 30–45 min; 45–60 min) became larger, indicating that
a higher transfer hydrogenolysis rate was obtained. Generally,
a higher reaction rate was observed at the earlier reaction stage
because of the higher concentration of benzyl phenyl ether at
Comparison of the activity of the three representative model
compounds
The transfer hydrogenolysis of three model compounds – benzyl
phenyl ether (a-O-4), 2-phenylethyl phenyl ether (b-O-4) and
diphenyl ether (4-O-5) – were studied. The bond dissociation
energy (BDE) of the aliphatic ether bonds of a-O-4
(218 kJ molꢀ1) is lower than the aliphatic ether bonds of b-O-4
(289 kJ molꢀ1) and the aryl ether bond (314 kJ molꢀ1) of 4-O-
5.18 As listed in Table S3,† the reaction rate at substrate
conversions below 20% decreased greatly in the order: benzyl
phenyl ether (7.2 mmol gcata.ꢀ1 hꢀ1), 2-phenylethyl phenyl ethꢀe1r
ꢀ1
(2.2 mmol gcata. hꢀ1), and diphenyl ether (1.6 mmol gcata.
hꢀ1). This correlates well with the bond dissociation energies of
these bonds (Table S3,† entries 1–3).18 Kinetic studies of the
three model compounds were further studied (Fig. S3†), and
their apparent activation energies were calculated to be 103, 130
and 147 kJ molꢀ1 for the cleavage of the benzyl phenyl ether (a-
O-4), 2-phenylethyl phenyl ether (b-O-4) and diphenyl ether (4-
O-5), respectively. The apparent activation energies for the
cleavage of a-O-4, b-O-4, and 4-O-5 bonds increased almost
linearly with their C–O BDEs (Fig. 4), indicating that the
cleavage barrier of the aryl C–O bonds was heavily inuenced by
the C–O BDE. As shown in Table S3,† the lowest BDE of a-O-4
Fig. 5 (a) The time course of the product distributions for the
conversion of benzyl phenyl ether (a-O-4) over the Ni/Al2O3-600
catalyst. Reaction conditions: benzyl phenyl ether (a-O-4) (185 mg, 1
mmol), Ni/Al2O3-600 (20 mg), solvent (10 mL), 150 ꢁC, stirring at
1000 rpm. (b) Reaction pathway for the cleavage of benzyl phenyl
(218 kJ molꢀ1) corresponds well with the highest reaction rate
ꢀ1
(7.2 mmol gcata.
hꢀ1) and the lowest apparent activation
energy (Ea ¼ 103 kJ molꢀ1). By comparison, much higher
apparent activation energy was observed for the 2-phenylethyl ether.
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the earlier reaction stage. The possible reason could be that NiO
in the Ni/Al2O3-600 catalyst was gradually reduced to metallic
nickel by iso-propanol within 60 min, which is the active site for
transfer hydrogenation.
Phenol and toluene are the major products early in the
reaction. The further hydrogenation of the aromatic ring in
toluene into methylcyclohexane was not observed, but phenol
was further reduced to cyclohexanol with cyclohexanone as the
intermediate. It has been reported that the hydrogenation of
phenol into cyclohexanol does not occur with direct hydroge-
nation of the aromatic ring,38 cyclohexanone is the interme-
diate. According to the reported results,38 the –OH group
possibly promotes the facile formation of cyclohexanone via
keto–enol tautomerism as depicted in Fig. 5b. Therefore, the
hydroxyl group in phenol is crucial for its hydrogenation, while
toluene is stable under the reaction conditions.
The selectivity of cyclohexanol reached 100% aer 4.5 h at
150 ꢁC. The deep hydrogenolysis of cyclohexanol into cyclo-
hexane was not observed, which is promoted by acid–metal
bifunctional catalysts involving the acid-catalyzed dehydration
of cyclohexanol into cyclohexene and the metal-catalyzed
hydrogenation of cyclohexene into cyclohexane.37 Benzene and
benzyl alcohol were not detected during the reaction process,
which revealed that the C–O–C bond of benzyl phenyl ether was
selectively cleaved at the position of the aliphatic carbon oxygen
bond (Caliphatic–O) on the Ni surface, due to the much lower C–O
bond dissociation energy barrier of the Caliphatic–O bond (Table
S3,† entry 1), producing phenol and toluene (Fig. 5b). Given the
above results, the plausible reaction route for the transfer
hydrogenolysis of benzyl phenyl ether is depicted in Fig. 5b.
Hydrogenolysis was the dominant and initial reaction pathway
for the cleavage of benzyl phenyl ether (a-O-4) on the surface of
Ni nanoparticles, affording toluene and phenol as the primary
products (Fig. 5b). The as-formed phenol underwent the
sequential hydrogenations to produce cyclohexanol.
Compared with the hydrogenolysis of benzyl phenyl ether (a-
O-4), the hydrogenolysis conversion of diphenyl ethers was
much more challenging.23 Fortunately, the transfer hydro-
genolysis of diphenyl ether could be performed under mild
reaction conditions (120–150 ꢁC, Fig. S3b†). The time course of
the products distribution for the hydrogenolysis of diphenyl
ether over the Ni/Al2O3-600 catalyst at 150 ꢁC is recorded in
Fig. 6a. Six products were detected for the hydrogenolysis of
benzyl phenyl ether. As shown in Fig. 6a, benzene (33.6%
selectivity), phenol (28.7% selectivity) and cyclohexyl phenyl
ether (32.2% selectivity) were detected as the three major
products within 1 h, suggesting the transfer hydrogenolysis of
diphenyl ether underwent a different reaction pathway in
comparison with that of benzyl phenyl ether (a-O-4). Cyclohexyl
phenyl ether should be generated from the hydrogenation of
one aromatic ring in benzyl phenyl ether (Fig. 6b, route B). Aer
7 h, the conversion of diphenyl ether reached 100%, and the
yield of benzene gradually increased to 76.3% aer 7 h, and
nally remained stable from 7 h to 12 h. These results suggest
that benzene is generated from the direct cleavage of the
diphenyl ether (4-O-5) as depicted in route A, while phenol was
simultaneously produced and was further hydrogenated into
Fig. 6 (a) The time course of the product distributions for the
conversion of diphenyl ether (4-O-5) over the Ni/Al2O3-600 catalyst.
Reaction conditions: diphenyl ether (4-O-5) (170 mg, 1 mmol), Ni/
Al2O3-600 (20 mg), solvent (10 mL), 150 ꢁC, stirring at 1000 rpm. (b)
Reaction pathway for the cleavage of diphenyl ether.
cyclohexanol via cyclohexanone as the intermediate as dis-
cussed above in the reaction pathway for the hydrogenolysis of
benzyl phenyl ether (a-O-4) in Fig. 6b. The stable benzene yield
from 7 h to 12 h also revealed that the cyclohexyl phenyl ether
did not undergo the pathway to produce benzene and cyclo-
hexanol, and the deep hydrogenation of benzene into cyclo-
hexane did not occur under our reaction conditions. The
selective cleavage of the aliphatic C–O bond in cyclohexyl
phenyl ether generated the cyclohexane and phenol, and phenol
was nally hydrogenated into cyclohexane (Fig. 6b). The
depicted reaction pathways in Fig. 6b clearly explain our results.
At the end of the reaction time point, the yield of cyclohexanol
was 100%, and the yields of benzene and cyclohexane were
74.9% and 25.1%, respectively. Cyclohexanol could be produced
from route A and route B in Fig. 6b, therefore the 100% yield is
achieved. However, benzene could only be produced from route
A, and cyclohexane only from route B. Thus, the total yield of
benzene and cyclohexane is 100%.
Finally, the time course of the product distribution of the
transfer hydrogenation of 2-phenylethyl phenyl ether (b-O-4)
was also studied (Fig. 7a). Phenol and ethylbenzene were the
main products within 1 h, generated from the cleavage of the
aliphatic C–O bond in 2-phenylethyl phenyl ether (b-O-4) (route
A, Fig. 7b). Phenol was again gradually hydrogenated into
cyclohexanol via cyclohexanone as the intermediate, and the
yield of ethylbenzene increased during the reaction process.
These results indicate that ethylbenzene is stable. In fact, the
further hydrogenation of ethylbenzene did not occur under the
same reaction conditions. Similar to the hydrogenolysis of the
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C
aromatic–O–Caromatic linkage in diphenyl ether. At the same
time, the hydrogenation of the aromatic rings was not observed
in benzyl phenyl ether. However, the hydrogenation of aromatic
rings in 2-phenylethyl phenyl ether and diphenyl ether occurred
at higher reaction temperatures. The partial hydrogenation
products of 2-phenylethyl phenyl ether and diphenyl ether also
underwent the cleavage of Caliphatic–O to generate the hydro-
genolysis products. Phenol was further hydrogenated into
cyclohexanol, while the aromatic rings in benzene, toluene, and
ethylbenzene inert under our reaction conditions.
Catalytic transfer hydrogenolysis cleavage of lignin
Given the excellent results in model compounds and the kinetic
study, dioxasolv beech lignin was further used to prove the
transfer hydrogenolysis activity of Ni/Al2O3-600 by iso-propanol.
The depolymerisation results of total oil yield and identied
monomers yield are shown in Table S4.† 69.2% of aromatic
liquid oil with 13.4% of identied monomers (i.e., product 1–9,
Table S4†) was obtained over Ni/Al2O3-600, demonstrating that
the lignin could also be effectively depolymerized without
external hydrogen.
2D HSQC NMR was employed to better estimate the struc-
ture change of lignin before and aer reaction. The linkage
region of lignin and the corresponding lignin oil are illustrated
in Fig. 8a and c. It was found that all the A (b-O-4), B (b-5/a-O-4),
C (b–b) and lignin-bound Hibbert’s ketone (LBHK) in lignin oil
experienced notable decrease in area aer reaction compared
with that in the lignin sample (Fig. 8c vs. Fig. 8a), indicating the
versatility of our catalytic system. In particular, a signicant
decrease in area of A and LBHK linkages was observed. In detail,
45 per C9 units was attributed to A in lignin before reaction
(Fig. 8a), while the datum was 12 in lignin oil aer reaction
(Fig. 8c), suggesting that major b-O-4 bonds in lignin were
cleaved over Ni/Al2O3-600. A similar trend was obtained in
LBHK linkage, the number of which fell to 3 in lignin oil from
13 in lignin. The above phenomenon implies that all the link-
ages could be attacked by the Ni/Al2O3-600 catalyst, while A and
LBHK linkages to be destructed due to the relatively lower BDE,
as described above compared with B and C linkages. No clear
difference of S : G ratio was observed in Fig. 8b and d. The above
results further conrm the high transfer hydrogenolysis of our
catalytic system in lignin depolymerisation.
Fig. 7 (a) The time course of the product distributions for the
conversion of 2-phenylethyl phenyl ether (b-O-4) over the Ni/Al2O3-
600 catalyst. Reaction conditions: 2-phenylethyl phenyl ether (b-O-4)
(198 mg, 1 mmol), Ni/Al2O3-600 (20 mg), solvent (10 mL), 170 ꢁC,
stirring at 1000 rpm, (b) reaction pathway for the cleavage of 2-phe-
nylethyl phenyl ether.
diphenyl ether (4-O-5), the transfer hydrogenation of the
aromatic ring in 2-phenylethyl phenyl ether without the
cleavage of the ether (b-O-4) was also observed, generating
cyclohexyl phenethyl ether (route B, Fig. 7b) and 2-cyclo-
hexylethyl phenyl ether (route C, Fig. 7b), but the degree of the
hydrogenation of the aromatic ring in 2-phenylethyl phenyl
ether (b-O-4) was very low. Ethylcyclohexane as one of the
products was also observed, and its yield increased during the
reaction process. Ethylcyclohexane could be generated from the
hydrogenolysis of 2-cyclohexylethyl phenyl ether, and phenol
was simultaneously produced followed by the further hydroge-
nation into cyclohexanol (route B, Fig. 7b). The cleavage of
cyclohexyl phenethyl ether produced ethylbenzene and cyclo-
hexanol (route C). The proposed reaction pathway in Fig. 7b is
consistent with our experiments. Cyclohexanol can be produced
in three routes in Fig. 7b, thus it could be attained at 100%
yield. The total yield of ethylbenzene and ethylcyclohexane
should be 100% according to Fig. 7b, which is consistent with
our experimental results.
Mechanisms of the cleavage of the C–O bond for the three
model compounds
Although it is difficult to get an accurate mechanism for the
transfer hydrogenolysis of C–O bonds in lignin model
compounds, we would like to provide further insights into this
transformation. As we know, the hydrogen in the hydroxyl
group (–OH) of iso-propanol is active (H+), and can move freely.
It is reported that Ni/Al2O3 catalyst contains acid and basic sites
(Nid+–Odꢀ or Ald+–Odꢀ),39 and the H+ in the O–H group of iso-
propanol can be combined with the basic sites (Odꢀ, Scheme
2). In fact, Odꢀ containing material such as a-cyclodextrin (a-
CD) or molecular sieves have been reported to serve as basic
sites (combing with H+) to promote the heterolytic cleavage of
In summary, the Caliphatic–O–Caromatic linkages (a-O-4, benzyl
phenyl ether and b-O-4, 2-phenylethyl phenyl ether) are mainly
cleaved on the Caliphatic–O side due to the weaker bond energy of
C
aliphatic–O bonds. The Caromatic–O–Caromatic linkage in the
diphenyl ether underwent transfer hydrogenolysis as well. In
addition, the cleavage of the Caliphatic–O in benzyl phenyl ether
(a-O-4) was much easier due to its lower activation energy than
those of the Caliphatic–O bonds in 2-phenylethyl phenyl ether and
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Scheme 2 Proposed mechanisms for cleavage of C–O bonds in the
a-O-4, b-O-4, and 4-O-5 model compounds.
Fig. 9 Catalytic cycling tests of Ni/Al2O3-600 catalyst during the
hydrogenation of benzyl phenyl ether. Reaction conditions: benzyl
phenyl ether (a-O-4) (185 mg, 1 mmol), Ni/Al2O3-600 (20 mg), solvent
(10 mL), 150 ꢁC, stirring at 1000 rpm, 5 h.
phenylethlium) to generate benzene, toluene and ethylbenzene,
respectively. Similarly, the hydrogenation of phenol by the H+
and Hꢀ species generated from iso-propanol over the Ni/Al2O3-
600 catalyst produced cyclohexanol via cyclohexanone as the
intermediate.
Fig. 8 2D HSQC NMR spectra of beech lignin in DMSO-d6 before
reaction (a and b); and lignin oil after reaction (c and d). Note: the
percentage of linkages A, B, C and LBHK was calculated based on per
100 C9 unit, see Fig. S4† for the detailed integration process.
Catalyst stability
As a heterogeneous catalyst, ability to recycle is important.42,43
The recycling of the Ni/Al2O3-600 catalyst was studied during
the transfer hydrogenation of benzyl phenyl ether at 150 ꢁC. One
important advantage is that the catalyst is easily separated from
the reaction mixture owing to the magnetic properties of Ni
particles, which simplies the tedious recycling procedures via
centrifugation and ltration for non-magnetic catalysts. As
shown in Fig. 9, the Ni/Al2O3-600 catalyst is stable and shows no
obvious decrease in activity and selectivity aer ve catalytic
cycles. ICP measurements show that there is no leaching of Ni
into the reaction solution. These results suggest that the Ni/
Al2O3-600 catalyst is highly stable during the reaction process
without loss of its activity.
H2 molecules.40 According to the above experiments and anal-
ysis, a plausible mechanism is proposed for the transfer
hydrogenolysis of an ether bond. The dissociated H+ would
combine with the oxygen atom in the ether bond (C–O), and
then the phenol molecule breaks away. Three carbon cations
(phenylium, phenylmethylium, phenylethlium) are hence
generated. Our results in Table 1 conrm that the active sites
are on the metallic nickel, and the metallic nickel activates the
C–H bond connected by the hydroxyl group to generate the
active species (Ni–Hꢀ). Then the hybrid (Hꢀ) would combine
with the carbon cations (phenylium, phenylmethylium,
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Conclusions
In this study, we developed an effective method for the transfer
cleavage of the C–O bond in model compounds as well as beech
lignin using iso-propanol as the hydrogen donor over Ni/Al2O3-T
catalysts. The catalytic activity is greatly inuenced by the
reduction temperature of the NiAl-layered double hydroxide
precursor, which greatly affects the nickel nanoparticle size as
well as the metallic nickel composition. The Ni/Al2O3-600
catalyst shows the highest catalytic activity for the transfer
hydrogenolysis reactions, as it demonstrates an appropriated
nanoparticle size and a relatively high ratio of metallic nickel.
Kinetic studies reveal that the transfer hydrogenolysis activity of
the three model compounds decreases in the following order:
benzyl phenyl ether (a-O-4), 2-phenylethyl phenyl ether (b-O-4)
and diphenyl ether (4-O-5), which linearly correspond to their
binding energy and the activation energies. A plausible mech-
anism is proposed for the transfer hydrogenolysis of ether
bonds over the Ni/Al2O3-600 catalyst by the use of iso-propanol
as the hydrogen donor, in which the proton (H+) and hybrid
(Hꢀ) were the active sites for the cleavage of the C–O bonds.
Reaction pathway studies revealed that aromatic alkanes and
cyclohexanol are the major products for the transfer cleavage of
the lignin model compounds, and a whole reaction pathway is
depicted for each model compound. The present work provides
a deep understanding of the excellent activity of Ni/Al2O3 in
cleaving both lignin models and lignin without external
hydrogen. This study may also inspire research on the use of
non-noble metal catalysts for the production of other value-
added chemicals from biomass-derivatives by transfer
hydrogenation.
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Conflicts of interest
21 V. M. Roberts, V. Stein, T. Reiner, X. Li, A. Lemonidou, X. Li
and J. A. Lercher, Chem.–Eur. J., 2011, 17, 5939–5948.
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There are no conicts to declare.
Acknowledgements
This project was supported by the Special Fund for Basic
Scientic Research of Central Colleges, South-Central Univer-
sity for Nationalities (CZR18001 and YCZW15100), the Natural
Science Foundation of Hubei Province (No. 2017CFB432) and
the National Natural Science Foundation of China (No.
21872175, 21690080, 21690083, 21878288).
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