Depolymerization and hydrodeoxygenation of lignin to aromatic hydrocarbons with a Ru catalyst on a variety of Nb-based supports

Article abstract of DOI:10.1016/S1872-2067(19)63317-6

Efficient conversion of lignin to aromatic hydrocarbons via depolymerization and subsequent hydrodeoxygenation is important. Previously, we found that NbO_x species played a key role in the activation and cleavage of C–O bonds in lignin and its model compounds. In this study, commercial niobic acid (HY-340), niobium phosphate (NbPO-CBMM) and lab-made layered niobium oxide (Nb₂O₅-Layer) were chosen as supports to study the effect of Br?nsted and Lewis acids on the activation of C–O bonds in lignin conversion. A variety of Ru-loaded, Nb-based catalysts with different Ru particle sizes were prepared and applied to the conversion of p-cresol. The results show that all the Ru/Nb-based catalysts produce high mole yields of C₇–C₉ hydrocarbons (82.3–99.1%). What's more, Ru/Nb₂O₅-Layer affords the best mole yield of C₇–C₉ hydrocarbons and selectivity for C₇–C₉ aromatic hydrocarbons, of up to 99.1% and 88.0%, respectively. Moreover, it was found that Lewis acid sites play important roles in the depolymerization of enzymatic lignin into phenolic monomers and the cleavage of the C–O bond of phenols. Additionally, the electronic state and particle size of Ru are significant factors which influence the selectivity for aromatic hydrocarbons. A partial positive charge on the metallic Ru surface and a smaller Ru particle size are beneficial in improving the selectivity for aromatic hydrocarbons.

Full text of DOI:10.1016/S1872-2067(19)63317-6

Chinese Journal of Catalysis 40 (2019) 609–617
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Depolymerization and hydrodeoxygenation of lignin to aromatic
hydrocarbons with a Ru catalyst on a variety of Nb-based supports
Di Ma†, Shenglu Lu†, Xiaohui Liu *, Yong Guo, Yanqin Wang #
Shanghai Key Laboratory of Functional Materials Chemistry, Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering,
East China University of Science and Technology, Shanghai 200237, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Efficient conversion of lignin to aromatic hydrocarbons via depolymerization and subsequent hy-
drodeoxygenation is important. Previously, we found that NbOx species played a key role in the
activation and cleavage of C–O bonds in lignin and its model compounds. In this study, commercial
niobic acid (HY-340), niobium phosphate (NbPO-CBMM) and lab-made layered niobium oxide
(Nb2O5-Layer) were chosen as supports to study the effect of Brönsted and Lewis acids on the acti-
vation of C–O bonds in lignin conversion. A variety of Ru-loaded, Nb-based catalysts with different
Ru particle sizes were prepared and applied to the conversion of p-cresol. The results show that all
the Ru/Nb-based catalysts produce high mole yields of C7–C9 hydrocarbons (82.3%–99.1%). What’s
more, Ru/Nb2O5-Layer affords the best mole yield of C7–C9 hydrocarbons and selectivity for C7–C9
aromatic hydrocarbons, of up to 99.1% and 88.0%, respectively. Moreover, it was found that Lewis
acid sites play important roles in the depolymerization of enzymatic lignin into phenolic monomers
and the cleavage of the C–O bond of phenols. Additionally, the electronic state and particle size of Ru
are significant factors which influence the selectivity for aromatic hydrocarbons. A partial positive
charge on the metallic Ru surface and a smaller Ru particle size are beneficial in improving the se-
lectivity for aromatic hydrocarbons.
Received 20 December 2018
Accepted 25 January 2019
Published 5 April 2019
Keywords:
Lignin
Aromatic hydrocarbons
Nb-based supports
Lewis acid sites
Ru
© 2019, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
renewable energy source for aromatic compounds [8,9]. Thus,
converting lignins into aromatic hydrocarbon compounds is
Lignocellulosic biomass consists of lignin, cellulose, and
hemicellulose. Effective utilization of lignocellulosic biomass is
ineluctable and necessary, due to the accelerating consumption
and dwindling reserves of petroleum-based chemicals [1–6]. It
is well known that aromatic hydrocarbons are ideal additives
for aviation fuel, due to their high energy density and other
properties [7]. Lignin is rich in aromatic rings and is the only
one of the most promising processes in biomass utilization,
which usually include lignin depolymerization and subsequent
hydrodeoxygenation steps [10–14].
Recently, NbOx species have been found to play a key role in
the activation and cleavage of Caliphatic–O and Caromatic–O bonds in
both lignin and its model compounds [15–18]. For example,
p-cresol, a typical lignin model compound, can be efficiently
* Corresponding author. E-mail: xhliu@ecust.edu.cn
# Corresponding author. E-mail: wangyanqin@ecust.edu.cn
† These authors contribute equally.
The work was supported by the National Natural Science Foundation of China (21832002, 21872050, 21808063) and the Natural Science Foundation
of Shanghai (18ZR1408500).
DOI: S1872-2067(19)63317-6 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 40, No. 4, April 2019

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Di Ma et al. / Chinese Journal of Catalysis 40 (2019) 609–617
converted into toluene over Ru/Nb2O5–SiO2 catalysts using
2-PrOH as the hydrogen source [17]. Reaction pathway studies
show that toluene was produced via the direct deoxygenation
(DDO) route (C–O cleavage without saturation of the aromatic
ring). Furthermore, niobium-based catalysts have shown great
promise in converting lignin into arenes, despite the robust and
complex lignin structure. Shao et al. [18] reported the direct
conversion of lignin over Ru/Nb2O5 catalytic systems in water,
with a high yield of liquid hydrocarbons (a mass yield of 35.5
wt%) and an exceptionally high arene selectivity for C7–C9 hy-
drocarbons (71 wt%). The Ru/Nb2O5 catalyst in this process
not only depolymerized the raw lignin to phenolic monomers
via the effective cleavage of Caliphatic–O–C bonds in the lignin
network, but also promoted the hydrodeoxygenation of phe-
nolics to arenes.
h, reduced at 400 °C for 4 h under 10 vol% H2/Ar flow, and
then purged with N2 for 2 h until they reached ambient tem-
perature.
Additionally, Ru-loaded Nb-based catalysts with specific Ru
particle sizes were prepared by adsorption of colloidal Ru na-
noparticles (NPs) onto the Nb2O5-Layer, HY-340, and
NbPO-CBMM
supports,
which
were
designated
Ru@Nb2O5-Layer, Ru@HY-340, and Ru@NbPO-CBMM, respec-
tively [20]. Colloidal Ru NPs with mean sizes of 1.2 and 1.8 nm
were first prepared by the reduction of RuCl3 with ascorbic
acid, in the aqueous phase, at 80 °C, for 1 and 5 min, respec-
tively. Then, the support was added to the colloidal Ru particle
solution and the mixture was stirred for 24 h. Finally, the solid
product was collected by filtration, washed with deionized wa-
ter, dried at 50 °C in a vacuum oven, and solidified at 300 °C for
4 h under a N2 flow. The initial content of Ru NPs was 2.0 wt%.
In this study, we compared the structures and properties of
different commercial niobium-based catalysts and our
lab-made layered Nb2O5 (Nb2O5-Layer), and subsequently
studied the catalytic performance for both lignin and p-cresol
conversion to arenes over these Nb-based carrier supported Ru
catalysts. As Companhia Brasileira de Metalurgia e Mineração
(CBMM) is the world’s leading niobium manufacturer, we chose
HY-340 and niobium phosphate produced by CBMM as sup-
ports and compared them with lab-made layered Nb2O5. It was
found that the Ru/Nb2O5-Layer catalyst had the best perfor-
mance and durability. Moreover, the relationship between the
catalytic performance in the depolymerization and hydrode-
oxygenation of lignin to aromatic hydrocarbons and the struc-
tural properties of the Ru catalysts supported on Nb-based
materials was clarified by powder X-ray diffraction (XRD),
Brunauer-Emmett-Teller (BET) analysis, thermogravimetry
(TG), pyridine-absorbed Fourier transform infrared spectros-
copy (Py-FTIR), CO-chemisorption, and X-ray photoelectron
spectroscopy (XPS).
2.3. Characterization
The powder XRD patterns were recorded with a Rigaku
D/max-2550VB/PC diffractometer, using Cu K (λ = 0.15406
nm) radiation, operating at 40 kV and 40 mA.
The quantities of metals in the catalysts were determined
using inductively coupled plasma atomic emission spectrosco-
py (ICP-AES) on a Perkin-Elmer Optima 2100 DV spectrometer.
Samples were digested in a mixture of HF and H2SO4.
X-ray photoelectron spectroscopy (XPS) was performed us-
ing a Thermo Scientific ESCA LAB-250 spectrometer with
monochromatic Al Kα radiation.
Nitrogen adsorption-desorption isotherms of the catalysts
were measured at 196 C on a Micromeritics ASAP 2020 M
sorption analyzer. Prior to N2 adsorption, the catalysts were
pre-treated under vacuum at 200 C for 6 h to remove moisture
and volatile impurities. The surface area was calculated using a
multipoint Brunauer-Emmett-Teller (BET) method.
2. Experimental
Thermogravimetry-differential thermal analysis (TG-DTA)
of the catalyst was performed on PerkinElmer Pyris Diamond
differential scanning calorimeter, under air, with a heating rate
of 10 °C/min.
2.1. Materials
RuCl3∙3H2O was purchased from Heraeus Materials Tech-
nology Shanghai Co. Ltd. p-cresol was purchased from J & K Co.
Ltd. HY-340 and niobium phosphate (NbPO) were obtained
from CBMM. Enzymatic lignin was purchased from Shandong
Longlive Bio-Technology Co., Ltd. This lignin is obtained by the
enzyme-catalyzed hydrolysis of cellulose from corncobs. The
distribution and quantities of the lignin monomers are shown
in Table S1. Layered Nb2O5 (Nb2O5-Layer) was synthesized
using a hydrothermal method following the literature proce-
dure [19]. Specifically, we used lab-synthesized niobium oxa-
late as the niobium precursor.
Infrared (IR) spectra of the pyridine adsorption were rec-
orded on a Nicolet iS50 spectrometer, with 32 scans at an effec-
tive resolution of 4 cm−1. 20 mg of sample was pressed into a
self-supporting wafer. The disc was mounted in a quartz IR cell
equipped with a CaF2 window and a vacuum system. Prior to
adsorption, the samples were pretreated at 400 C under vac-
uum, then cooled to 30 C, and pyridine vapor was introduced
into the cell. The samples were heated under vacuum at 100C
and spectra were recorded at room temperature. The quantifi-
cation of acid sites was performed using the expressions re-
ported by Emeis [21]:
π
r2
ꢀL = KL ꢁ1450
=
ꢂ
ꢃ ꢁ1450
2.2. Catalyst preparation
IMECL
w
π
r2
The catalysts (Ru/Nb2O5-Layer, Ru/HY-340, and
Ru/NbPO-CBMM) were prepared by the incipient wetness im-
pregnation method, using RuCl3∙3H2O as the precursor. After
impregnation, the resulting catalysts were dried at 50 °C for 12
ꢀB = K B ꢁ1540
=
ꢂ
ꢃ ꢁ1540
IMECB
w
CL and CB are the concentrations of Lewis and Brönsted acid
sites; A1450 and A1540 are the integrated areas of the bands at

Di Ma et al. / Chinese Journal of Catalysis 40 (2019) 609–617
611
1450 and 1540 cm−1 in the original FTIR data, as shown in the
Results and discussion section. KL and KB are the molar extinc-
tion constants for the Lewis and Brönsted acid sites; IMECL and
IMECB are the integration molar extinction coefficients, 2.22
and 1.67 cm·μmol/g for Lewis and Brönsted acids, respectively;
r is the radius; and w is the weight of the self-supporting cata-
lyst disk.
(001)
(002)
Nb2O5-Layer
Ru dispersion was measured by the CO chemisorption
method with the Micromeritics ASAP 2020 system, CO/Ru = 1
and a surface area of 8×10−20 m2 per Ru atom were set as
standard. The sample was initially pretreated at 200 C under
hydrogen and then purged with helium. Then, the catalyst was
cooled to room temperature and CO pulses were injected
through a calibrated on-line sampling valve. CO adsorption was
assumed to be complete after three successive peaks showed
the same peak areas. The number of Ru atoms at the external
surface was calculated using the total quantity of CO adsorbed,
obtained from the thermal conductivity detector (TCD) signal.
The dispersion of Ru is equal to the number of Ru atoms at the
external surface divided by the total number of Ru atoms.
HY340
NbPO-CBMM
10
20
30
40
50
2 (o)
60
70
80
Fig. 1. The XRD patterns of the niobium-based materials.
3.1.1. XRD
XRD analysis was initially performed to confirm the
phase-structure of the different supports (Fig. 1). The XRD pat-
terns of both HY-340 and NbPO-CBMM show two broad peaks
in the 2θ ranges of 15–40 and 40–60, indicating the amor-
phous nature of the samples. However, Nb2O5-Layer displays
diffraction peaks at 2θ = 22.7° and 46.2°, attributed to the (001)
and (002) diffraction planes, respectively [19]. This result hints
at the probable layer-structure of our lab-made Nb2O5-Layer
sample, which is verified by the high-resolution transmission
electron microscope (HR-TEM) images (Fig. S1).
2.4. Reaction procedure and product analysis
The reaction for the hydrodeoxygenation of lignin was per-
formed in a 50 mL stainless-steel autoclave as follows: lignin
(0.1 g), Ru/support catalyst or Ru@support catalyst (0.2 g),
deionized water (10 mL), and cyclohexane (5 mL) were
charged into the autoclave. Then, the reactor was sealed,
purged three times with hydrogen, and charged to an initial
pressure of 0.5 MPa with H2. Finally, the reaction was carried
out at 250 °C, with magnetic stirring, for 20 h.
3.1.2. BET
The reactions for the hydrodeoxygenation of model com-
pounds were also performed in a 50 mL stainless-steel auto-
clave. A typical experiment was carried out as follows: p-cresol
(0.2 g), Ru@support (0.04 g) and deionized water (15 mL)
were put into the autoclave. After being charged to an initial
pressure of 0.5 MPa with H2, the reaction was conducted at 250
°C, under magnetic stirring, for 1 h.
After the reaction, the reactor was quenched in an ice bath,
and the organic products were extracted using ethyl acetate
and qualitatively analyzed by gas chromatography-mass spec-
trometry (GC-MS) (Agilent 7890A-5975C) and quantitatively
analyzed by a GC equipped with a flame ionization detector
(GC-FID) (Agilent 7890B). Tridecane was added as an internal
standard. After the first run, the catalysts were washed with a
methanol and ethyl acetate solution, dried in a vacuum oven at
50 C for 12 h, then used for a second cycle. The mole yields of
hydrocarbons were calculated based on the moles of lignin
monomers in enzyme lignin, determined by the nitrobenzene
oxidation method (NBO) (Table S1) [22].
The nitrogen adsorption-desorption isotherms of the
Nb-based materials are shown in Fig. 2. All the Nb-based mate-
rials exhibit Type IV isotherms with hysteresis loops, which are
intermediate between the H1 and H2-types in the P/P0 range
from 0.4 to 0.8, which is attributed to capillary condensation
within the mesopores. The BET surface areas, pore diameters,
and pore volumes of these materials are summarized in Table
1. The BET surface area and pore volume of Nb2O5-Layer are
200
150
Nb2O5-Layer
100
NbPO-CBMM
50
HY-340
Mole yield of C7–C9 hydrocarbons
⁄
moles of C7 − C9 hydrocarbons produced (ꢄmol (g LigB))
=
⁄
moles of C7 − C9 hydrocarbons determined by NBO method (ꢄmol (g LigB))
0
0.0
0.2
0.4
0.6
0.8
1.0
3. Results and discussion
Relative pressure (P/P0)
Fig. 2. N2 adsorption-desorption isotherms of Nb-based materials.
3.1. Characterization of supports

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Di Ma et al. / Chinese Journal of Catalysis 40 (2019) 609–617
Table 1
Summary of physicochemical properties of the Nb-based materials.
BET surface area
(m2/g)
Pore size
(nm)
Pore volume
(cm³/g)
1540
Nb2O5-Layer
HY340
Sample
Nb2O5-Layer
HY-340
NbPO-CBMM
197.1
144.6
177.2
4.6
3.5
5.4
0.29
0.17
0.28
197.1 m2/g and 0.29 cm3/g, respectively. Those of HY-340 and
NbPO-CBMM are 144.6 m2/g and 0.17 cm3/g, and 177.2 m2/g
and 0.28 cm3/g, respectively. It is evident that Nb2O5-Layer has
a higher specific surface area and smaller pore size relative to
those of the commercial Nb-based materials.
NbPO-CBMM
1600
1560
1520
1480
1440
1400
Wavenumber (cm-1)
3.1.3. TG
Fig. 4. Py-FTIR spectra of the Nb-based materials collected after treat-
ment at 400 C, under vacuum.
Fig. 3 shows the results of thermogravimetric analysis for
the Nb-based materials. The step observed on the TG curve at
approximately 100 °C, for all samples, is associated with dehy-
dration. The observed weight loss in this step was 5.10% for
Nb2O5-Layer, 6.88% for NbPO-CBMM, and 17.54% for HY-340.
For Nb2O5-Layer and NbPO-CBMM, it is caused by the desorp-
tion of physisorbed water. HY-340, a type of niobic acid, is rich
in crystalline water and surface hydroxyl groups, so a higher
weight loss occurred.
NbPO-CBMM, and 147.5 μmol/g for HY-340) than Nb2O5-Layer,
which possesses merely 2.2 μmol/g. Since the Nb2O5-Layer
support contains a significantly larger number of Lewis acid
sites, Nb2O5-Layer exhibits a lower Brönsted/Lewis acid ratio
(0.004) than both HY-340 (0.3) and NbPO-CBMM (1.1).
3.2. Catalytic conversion of lignin and p-cresol
3.1.4. Py-FTIR
The catalytic activities were initially studied in the direct
depolymerization and hydrodeoxygenation of lignin to hydro-
carbons over the Ru/Nb-based catalysts and the results are
summarized in Table 3. Four phenolic monomers were found
after reactions over the Ru/HY-340 (15.6%) and
Ru/NbPO-CBMM (5.5%) catalysts, while none of these oxygen-
ated intermediates were detected during the catalytic test over
Ru/Nb2O5-Layer. It is well known that lignin is first depoly-
merized to phenolic monomers and that the resultant phenolic
monomers are subsequently converted to arenes and cycloal-
kanes [24]. This is consistent with our results. Moreover, it can
be seen from Table 3 that the mole yields of arenes and cyclo-
alkanes resulting from the hydrodeoxygenation of phenolic
monomers over Ru/Nb2O5-Layer (91.1%) are higher than those
over Ru/HY-340 (67.5%) and Ru/NbPO-CBMM (69.2%). Thus,
the catalytic performance results over the Ru/Nb2O5-Layer
catalyst highlight its excellent hydrodeoxygenation activity,
which results in the rapid conversion of phenolic monomers
into arenes and cycloalkanes, once the lignin has been depoly-
merized. Based on the Py-FTIR spectra, Lewis acid sites are the
predominant component of the total acid sites on the surface of
the Nb2O5-Layer support (only a few Brönsted acid sites were
To investigate the different types of acidic sites on the sup-
port surfaces, the Py-FTIR spectra were recorded and the re-
sults are shown in Fig. 4, Table 2, and Table S2. In Fig. 4, the
intense bands at 1445 and 1540 cm–1 are assigned to Lewis and
Brönsted acid sites, respectively [21]. Moreover, the band at
1490 cm–1 is attributed to the adsorption of pyridine, due to a
synergic effect of the co-existence of Brönsted and Lewis acid
sites in the supports [23]. Brönsted and Lewis acid sites in the
three Nb-based samples were quantified using the Py-FTIR
spectra (Table 2). The NbPO-CBMM sample possesses higher
acidity (921.8 μmol/g) than the other samples; 629.0 μmol/g
for HY-340, and 568.7 μmol/g for Nb2O5-Layer, respectively. In
particular, as we expected, the commercial niobium materials
possess more Brönsted acid sites (473.5 μmol/g for
100
98
Nb2O5-Layer
96
94
NbPO-CBMM
92
90
88
Table 2
Quantities of Brönsted and Lewis acid sites on the Nb-based materials,
quantified by Py-FTIR spectra, after evacuation at 400 C.
86
HY-340
84
82
80
Brönsted
acid
(μmol/g) (μmol/g)
Lewis
acid
Total
acid
(μmol/g)
568.7
Ratio of
Brönsted acid
/Lewis acid
0.004
Nb-based
material
0
100 200 300 400 500 600 700 800
Temperature (oC)
Nb2O5-Layer
HY-340
2.2
566.5
481.5
448.3
147.5
473.5
629.0
921.8
0.3
1.1
Fig. 3. TG profiles of the Nb-based materials.
NbPO-CBMM

Di Ma et al. / Chinese Journal of Catalysis 40 (2019) 609–617
613
Table 3
The direct hydrodeoxygenation of lignin over the Ru/Nb-based catalysts.
Yield of products (mol%)
C7–C9 cycloalkanes
C7–C9 arenes
Total C7–C9
hydrocarbons
Selectivity for
aromatic
Phenolic
monomers *
hydrocarbons
Time
Entry
Catalyst
(h)
1
2
3
Ru/Nb2O5-Layer
Ru/HY-340
Ru/NbPO-CBMM
10
10
10
18.6
8.3
7.4
47.7
28.1
26.0
15.4
14.2
10.5
2.3
1.8
3.8
5.7
13.1
14.5
1.4
2.0
7.0
91.1
67.5
69.2
89.7
74.9
63.4
–
15.6
5.5
Reaction conditions: 0.1 g lignin, 0.2 g catalyst, 10 mL H2O, 5 mL cyclohexane, 250 C, 0.5 MPa H2, 10 h. The Ru loading is 5 wt%.
* The mole yields of phenolic monomers are calculated based on the moles of lignin monomers in lignin; the phenolic monomers consist of the fol-
lowing:
observed). In addition, the quantity of Lewis acid sites over the
three Nb-based supports is in the following sequence:
Nb2O5-Layer > HY-340 > NbPO-CBMM. The quantity of Lewis
acid sites on the Nb-based supports shows a strong positive
correlation with the depolymerization of lignin, i.e. the total
mole yields of depolymerized products (including phenolic
monomers). This result suggests that increasing the quantity of
Lewis acid sites on the catalyst might be a promising strategy
for the design of a better Nb-based lignin-conversion catalyst.
Moreover, the selectivity for C7–C9 aromatic hydrocarbons
over the Ru/Nb2O5-Layer catalyst was up to 89.7%. Those over
Ru/HY-340 and Ru/NbPO-CBMM were 74.9% and 63.4%, re-
spectively, with more cycloalkanes detected. This indicates that
the hydrogenation of benzene rings is easier over both
Ru/HY-340 and Ru/NbPO-CBMM catalysts than over the
Ru/Nb2O5-Layer catalyst. Generally speaking, Ru species are
widely-accepted to act as the active sites in the hydrogenation
processes [25–27], and the characteristics of the metal species
(particle size or electronic state) over supports may also affect
the reaction performance [28–31].
the Ru/HY-340 and Ru/NbPO-CBMM catalysts showed similar
Ru dispersions (33.3% and 34.0%, respectively). In other
words, the particles of Ru are smallest on the Ru/Nb2O5-Layer
catalyst. A small ruthenium particle is not favorable for the
formation of cycloalkanes. According to our previous studies
[18], arenes are generated by the direct cleavage of the Car-
omatic–OH bond in phenolic monomers, while cycloalkanes are
formed by the removal of –OH after the hydrogenation of the
benzene ring in phenolic monomers, which are competitive
routes. Moreover, Nb species adsorb phenol via the oxygen
atom of the Caromatic–O bond, not via the benzene ring, thus fur-
ther activating the Caromatic–O bond. When the Ru particle size is
too small to adsorb the benzene ring of the phenolic monomers,
the Nb2O5 species first activates the Caromatic–O bond, then
arenes are generated. When the Ru particle size is large enough
to adsorb the benzene ring of the phenolic monomers, the hy-
drogenation of the benzene ring of the phenolic monomers on
Ru takes place and cycloalkanes are formed [27,32]. Thus, more
aromatic
hydrocarbons
were
obtained
over
the
Ru/Nb2O5-Layer catalyst due to the smaller Ru particles. The
differences in selectivity for aromatics over the
Ru/Nb2O5-Layer, Ru/HY-340, and Ru/NbPO-CBMM catalysts
demonstrate the size effect of Ru on hydrogenation.
First, to determine the particle size of Ru over the niobium
supports, measurements of Ru content and dispersion of Ru
over the three Nb-based catalysts were carried out (Table 4).
The Ru was best dispersed on the Nb2O5-Layer support
(66.5%), probably due to its higher specific surface area, while
The valence state of Ru in the different catalysts was studied
using XPS analysis (Fig. 5). The binding energy at 280.2 eV is
characteristic of the Ru0 species (metallic Ru). The shift in the
Ru 3d5/2 peak to a higher value (281.2 eV) can be attributed to
Ru4+ species, which most likely exist in the form of RuO2
[33,34]. Surprisingly, the surface Ru species over the
Ru/Nb2O5-Layer and Ru/HY-340 catalysts had a 3d5/2 binding
energy (BE) of 280.5 eV, which indicates that they are partially
Table 4
Ru contents and dispersions in the Ru/Nb-based catalysts.
Ru content a Dispersion b Particle diameter c
Catalyst
(wt%)
4.2
4.6
(%)
66.5
33.3
34.0
(nm)
0.9
1.9
Ru/Nb2O5-Layer
Ru/HY-340
Ru/NbPO-CBMM

+
positively charged species (Ru ). In the case of the
4.3
1.8
Ru/NbPO-CBMM catalyst, the relevant Ru species is the metal-
lic state (280.2 eV, Ru0). The differences in aromatic selectivi-
ties over the Ru/NbPO-CBMM and Ru/HY-340 catalysts, which
a
The metal loading was 5 wt% and the actual Ru contents of the cata-
lysts were measured by ICP-AES analysis.
b The Ru dispersions were calculated using CO chemisorption.
c TheRu particle diameters were calculated using CO chemisorption.

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Di Ma et al. / Chinese Journal of Catalysis 40 (2019) 609–617
of 1.2 and 1.8 nm were investigated. The catalytic performanc-
281.2
281.2
280.5
es for the hydrodeoxygenation of p-cresol over the
Ru@Nb-based catalysts are shown in Table 5. It was found that
p-cresol conversions over the three catalysts were significantly
different (56.8% for Ru@Nb2O5-Layer, 35.8% for Ru@HY-340,
and 25.1% for Ru@NbPO-CBMM), while the toluene selectivi-
ties were very similar (approximately 63.0%) (Table 5, entries
1–3), when the Ru particle size was fixed at 1.2 nm. This indi-
cates that the conversion of p-cresol is most effective over the
Ru/Nb2O5-Layer catalyst and the properties of the Nb-based
supports have the predominate influence on the conversion of
p-cresol.
Next, we used the Nb2O5-Layer as a support for colloidal Ru
with a different particle size to study the effect of Ru particle
size on the hydrodeoxygenation of p-cresol. As shown in Table
5, the selectivity for toluene decreases from 62.0 % to 53.9 %
with the increase in Ru particle size from 1.2 to 1.8 nm (Table 5,
entries 1 and 4). This indicates that the size of Ru particles can
affect the selectivity for toluene and that small Ru particles are
advantageous for high toluene selectivity. Additionally,
Nb-based supports with different Lewis acids affect the conver-
sion of p-cresol, and the quantity of Lewis acid sites on the
Nb-based supports showed a strong positive correlation with
the conversion of p-cresol. These trends are also consistent
with those in enzymatic lignin conversion reactions.
Ru/Nb2O5-Layer
Ru/HY-340
280.5
280.9
280.2
Ru/NbOPO4
279
283
282
281
280
278
Bingding energy (eV)
Fig. 5. Ru 3d5/2 XPS spectra of the Ru/Nb-based catalysts.
have similar Ru dispersions, show the influence of electronic
density on hydrogenation activity. The metallic Ru is beneficial
for the hydrogenation of the benzene ring in phenolic mono-
mers [35]. Hence, aromatic hydrocarbons were preferentially
formed over Ru/ Nb2O5-Layer and Ru/HY-340, which contain
more positively charged metallic Ru.
These results show that the ability of Ru to hydrodeoxygen-
ate the depolymerized products of lignin is affected by both
particle size and electronic density. This provides direction for
the design of Ru catalysts for lignin conversion with high aro-
matic selectivities.
3.3. Stability tests
Since the stability of the catalyst is of significant importance
from an industrial standpoint, the catalytic durability of the
three catalysts was investigated. The results are shown in Table
6. The selectivity for aromatic hydrocarbons showed a slight
change over the spent Ru/NbPO-CBMM catalyst, and the mole
yield of C7–C9 hydrocarbons decreased, in comparison with the
fresh catalyst. This is probably due to a slight decrease in spe-
cific surface area and a small loss of Ru (Table 7). A significant
decrease in both the selectivity for aromatic hydrocarbons
(from 74.2% to 61.4%) and the yield of C7–C9 hydrocarbons
(from 95.4% to 76.5%) was observed over once-used
Ru/HY-340 catalyst. Comparing the properties of the fresh and
spent Ru/HY-340 catalysts, it is apparent that the structure of
In order to investigate the roles of the Nb-based supports
and Ru in the catalytic hydrodeoxygenation process, p-cresol
was chosen as a candidate to exclude the complexity of lignin.
Moreover, to rule out changes in Ru state caused by support,
supported colloidal Ru-catalysts on the three Nb-based carriers
were used to conduct the hydrodeoxygenation of p-cresol. The
interaction between Ru and Nb-based supports is relatively
weak in catalysts synthesized by the colloid method and the Ru
particle size is well-defined. Therefore, the actions of Ru and
the Nb-based support can be distinguished. Moreover, the par-
ticle size of Ru on Nb-based catalysts synthesized by the im-
pregnation method ranged from 0.9 to 1.9 nm, according to the
dispersion results (Table 4). Accordingly, Ru colloidal particles
Table 5
The hydrodeoxygenation of p-cresol over the Ru@Nb-based catalysts.
Selectivity (%)
Conv.
Entry
Catalyst
Carbon Balance (%)
(%)
56.8
35.8
25.1
55.7
1
2
3
4
13.2
9.6
10.9
16.9
5
1
2
3
4
Ru@Nb2O5-Layera
Ru@HY-340a
Ru@NbPO-CBMMa
Ru@Nb2O5-Layer b
62.0
62.7
62.8
53.9
8.5
9.7
6.3
9.2
3.0
2.7
5.1
3.1
2.7
2.5
2.1
1.6
94.0
95.4
96.7
91.4
a Reaction conditions: substrate 0.2 g, catalyst 0.04 g, H2O 15 ml, 250 C, H2 0.5 MPa, 1 h. The mean size of the Ru NPs is 1.2 nm (Fig. S4(a)).
b The mean size of the Ru NPs is 1.8 nm (Fig. S4(b)).

Di Ma et al. / Chinese Journal of Catalysis 40 (2019) 609–617
615
Table 6
The direct hydrodeoxygenation of lignin over used catalysts.
Entry
Catalyst
Yield of products (mol%)
C7–C9 cycloalkanes
C7–C9 arenes
Total C7–C9
hydrocarbons
Selectivity for
aromatic
Others*
hydrocarbons
1
2
3
4
5
Ru/Nb2O5-Layer-1st use
Ru/Nb2O5-Layer-2nd use
Ru/Nb2O5-Layer-3rd use
Ru/HY-340-spent
22.1
21.7
20.5
7.4
50.5
50.2
48.9
29.0
20.4
14.6
13.4
14.8
10.6
10.3
2.9
3.1
2.4
3.9
6.4
7.3
7.9
9.4
20.3
15.7
1.7
2.2
2.1
5.3
4.2
99.1
98.5
98.1
76.5
65.2
88.0
86.6
85.8
61.4
59.8
0.6
0.7
0.9
0.5
2.4
Ru/NbPO-CBMM-spent
8.2
Reaction conditions were: 0.1 g lignin, 0.2 g catalyst, 10 mL H2O, 5 mL cyclohexane, 250 C, H2 0.5 MPa, 20 h.
* The mass yields of C10–C15 hydrocarbons were calculated based on the mass of lignin.
The others consist of the following:
HY-340 transformed from an amorphous state to an ortho-
rhombic crystalline structure, accompanied by a large decrease
in specific surface area (Fig. S6(b)); furthermore, an obvious
leach of Ru species was confirmed by ICP analysis (Table 7).
These results indicate that the Ru/HY-340 catalyst has poor
stability and does not effectively retain catalytic activity in this
system. However, the Ru/Nb2O5-Layer catalyst shows excellent
recycling stability under the harsh reaction conditions, as the
mole yield of C7–C9 hydrocarbons and selectivity for C7–C9
arenes remained at approximately 98% and 85%, respectively,
within the three runs (Table 6), despite slight Ru leaching
(from 4.2 to 3.7 wt%) and an insignificant decrease in the sur-
face area (from 149.3 to 141.9 m2/g) (Table 7). Our results
clearly suggest that the Ru/Nb2O5-Layer catalytic system is a
promising candidate for the catalytic conversion of lignin to
arenes.
cluding commercial niobic acid (HY-340), niobium phosphate
(NbPO-CBMM) and lab-made Nb2O5 (Nb2O5-Layer) as supports
for Ru and investigated the influence of the properties of the
support and Ru on the conversion of enzymatic lignin to aro-
matic hydrocarbons. It was found that the Lewis acid sites, ra-
ther than the Brönsted acid sites, of the support play an im-
portant role in the conversion of enzymatic lignin and p-cresol.
The electronic density and size of the Ru particles have a con-
siderable influence on the selectivity for aromatic hydrocar-
bons. In this study, the Ru/Nb2O5-Layer catalyst was found to
have a large quantity of Lewis acid sites, high specific surface
area, and good dispersion of Ru, which led to a mole yield of
99.1% for C7–C9 hydrocarbons and a selectivity of 88.1% for
C7–C9 aromatic hydrocarbons in the reaction of enzymatic lig-
nin to produce aromatic hydrocarbons. Moreover, the
Ru/Nb2O5-Layer catalyst showed good catalytic stability and
retained a high catalytic performance even after three reaction
cycles.
4. Conclusions
In summary, we choose a variety of niobium materials, in-
Acknowledgments
Table 7
The research was sponsored financially by the National
Natural Science Foundation of China (21832002, 21872050,
21808063) and the Natural Science Foundation of Shanghai
(18ZR1408500).
Characterization of fresh and used Ru/Nb-based catalysts.
Metal element
analysis * (%)
Catalyst
ABET (m2/g)
Ru/Nb2O5-Layer
Ru/Nb2O5-Layer-3rd use
Ru/HY-340
Ru/HY-340-1st use
Ru/NbPO-CBMM
Ru/NbPO-CBMM-1st use
149.3
141.9
68.5
13.0
122.6
113.9
4.2
3.7
4.6
3.1
4.3
3.8
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616
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木质素在不同Ru/Nb基催化剂上解聚和脱氧加氢制芳烃化合物
马 迪†, 陆圣璐†, 刘晓晖*, 郭 勇, 王艳芹#
华东理工大学化学与分子工程学院, 工业催化研究所, 上海市功能材料化学重点实验室, 上海200237
摘要: 木质素高效转化为芳烃是木质素利用的一个非常重要的过程, 一般通过解聚和脱氧加氢反应来实现. 我们曾发现
NbOx物种在木质素及其模型化合物的C–O键活化和断裂的过程中发挥了至关重要的作用. 本文分别选择两种商业的铌基
材料(HY-340和NbPO-CBMM)和实验室自制的具有层状结构的氧化铌材料(Nb2O5-Layer)为载体, 制备了负载型Ru催化剂,
将其用于木质素及其模型化合物的催化转化. 同时, 为了尽量避免Ru与铌基载体相互作用的影响, 制备了较为均匀的Ru纳
米胶粒并吸附于铌基载体上, 得到Ru@铌基催化剂, 并用于木质素模型化合物—对甲酚的催化转化反应中. 研究表明, 木
质素在所有的Ru/铌基催化剂上都可以得到比较高的C7–C9碳氢化合物的收率. 其中, 在Ru/Nb2O5-Layer催化剂上C7–C9碳
氢化合物的摩尔收率为99.1%, 选择性为88.0%. 采用X射线衍射、N2吸附-脱附、热重分析、吡啶吸附红外光谱(Py-FTIR)、
X射线光电子能谱(XPS)以及CO化学吸附等技术表征了Ru/铌基催化剂的性能与铌基材料的性质、金属Ru颗粒大小及其表
面电子状态之间的关系.
Py-FTIR结果表明, Nb2O5-Layer材料上几乎没有Brönsted酸, 但含有最多的Lewis酸, 而NbPO-CBMM上的Lewis酸量最
低. 结合催化性能数据发现, 单体产物收率与铌基载体的Lewis量成正相关关系, 其中以Ru/Nb2O5-Layer催化剂上最高.
CO化学吸附和XPS结果表明, 不同的铌基载体负载的金属Ru的分散度和电子状态都有差异. 在Ru/Nb2O5-Layer上Ru
的分散度最好, 颗粒尺寸最小, 木质素转化得到的芳烃选择性最好；在Ru/HY-340和Ru/NbPO-CBMM上, 虽然Ru的分散度
相近, 但其表面电子状态不同, Ru/HY-340上的金属态Ru带有更多的正电荷δ+, 其得到芳烃的选择性高于Ru/NbPO-CBMM.
由此可见, Ru的颗粒尺寸和表面电子状态会影响芳烃选择性.
对甲酚催化转化反应结果表明, Ru颗粒大小相同时, 铌基载体性质会影响对甲酚转化率；而同一铌基载体上, Ru颗粒
大小则影响芳烃选择性, 较小的Ru颗粒有利于芳烃的生成.
关键词: 木质素; 芳烃化合物; 铌基载体; 路易斯酸; 钌
收稿日期: 2018-12-20. 接受日期: 2019-01-25. 出版日期: 2019-04-05.
*通讯联系人. 电子信箱: xhliu@ecust.edu.cn
#通讯联系人. 电子信箱: wangyanqin@ecust.edu.cn
†
共同第一作者.
基金来源: 国家自然科学基金(21832002, 21872050, 21808063); 上海市自然科学基金(18ZR1408500).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).