The promotional effect of surface Ru decoration on the catalytic performance of Co-based nanocatalysts for guaiacol hydrodeoxygenation

Article abstract of DOI:10.1016/j.mcat.2020.111224

In the present work, carbon-supported Ru-decorated Co-based nanocatalysts were fabricated via a layered double hydroxide/carbon composite precursor approach and applied to the efficient hydrodeoxygenation (HDO) of guaiacol to produce cyclohexanol. It was demonstrated that uniform and highly dispersed Co nanoparticles could be formed on the carbon matrix, and the decoration of a small amount of Ru on the surface of Co nanoparticles could introduce stronger hydrogenolysis active sites. Furthermore, the reduction temperature for catalyst precursors could tune the size of Co-containing nanoparticles and regulate the density of surface oxygen vacancies originating from CoO_x species. Under the mild reaction conditions (200 ℃ and 1.0 MPa hydrogen pressure), as-fabricated Ru-Co/C catalyst obtained at the reduction temperature of 600 °C showed excellent catalytic activity in the HDO of guaiacol, with a high cyclohexanol yield of ～94 %, which was attributable to surface exposure of highly dispersive Ru° sites and the formation of abundant defective oxygen vacancies. The present results provide a new approach for designing high-performance Co-based HDO nanocatalysts by both the surface decoration of small amounts of precious metals and the introduction of surface defective structures.

Full text of DOI:10.1016/j.mcat.2020.111224

Molecular Catalysis 497 (2020) 111224
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
The promotional effect of surface Ru decoration on the catalytic
performance of Co-based nanocatalysts for guaiacol hydrodeoxygenation
Qiangqiang Xu, Yisheng Shi, Lan Yang*, Guoli Fan, Feng Li*
State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical
Technology, Beijing, 100029, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
In the present work, carbon-supported Ru-decorated Co-based nanocatalysts were fabricated via a layered double
hydroxide/carbon composite precursor approach and applied to the efficient hydrodeoxygenation (HDO) of
guaiacol to produce cyclohexanol. It was demonstrated that uniform and highly dispersed Co nanoparticles could
be formed on the carbon matrix, and the decoration of a small amount of Ru on the surface of Co nanoparticles
could introduce stronger hydrogenolysis active sites. Furthermore, the reduction temperature for catalyst pre-
cursors could tune the size of Co-containing nanoparticles and regulate the density of surface oxygen vacancies
Surface Ru decoration
Highly dispersed Co nanoparticles
Surface defects
Hydrodeoxygenation
Guaiacol
originating from CoO
x
species. Under the mild reaction conditions (200 ℃ and 1.0 MPa hydrogen pressure), as-
◦
fabricated Ru-Co/C catalyst obtained at the reduction temperature of 600 C showed excellent catalytic activity
in the HDO of guaiacol, with a high cyclohexanol yield of ~94 %, which was attributable to surface exposure of
◦
highly dispersive Ru sites and the formation of abundant defective oxygen vacancies. The present results pro-
vide a new approach for designing high-performance Co-based HDO nanocatalysts by both the surface decoration
of small amounts of precious metals and the introduction of surface defective structures.
1
. Introduction
In the recent decades, because accelerated consumption of fossil
used in the HDO processes of lignin-derived phenols [9–11]. Besides,
other relatively cheap transition metals, such as Mo, Ni, Fe, Co, Cu, and
W [12–16], are also widely explored, despite their relatively low HDO
activities. Among them, a large amount of research has focused on
cobalt-based catalysts, since the flexible change in chemical valence
states of Co species can affect the adsorption/activation of oxygen atoms
in phenolic compounds, thus causing unique catalytic HDO activities of
these Co-based catalysts [17–21]. For instance, Liu and coworkers re-
ported that the particle size and the dispersion of Co were strongly
dependent on the support [22]. Besides, the introduction of a second
transition metal would improve their performance [23–26]. It was re-
leads to major global concerns with respect to the finite reserves and
environmental pollution, new and effective “energy strategy” for sus-
tainable development of society and economy has become an urgent
need in the world [1]. In this regard, there is an immediate need for
sustainable energy sources and corresponding conversion technologies
to substitute the conventional nonrenewable petroleum-based fuels [2].
For instance, as a renewable carbon source, the efficient catalytic con-
version of a wide range of biomass sources into diverse high value-added
chemical feedstocks and fuels has sprouted widely varied research
attention [3,4]. In addition to the cellulosic fraction of biomass, the
lignin fraction is also a useful but more challenging resource to produce
high-value aromatic compounds [5]. Due to abundant carbon-oxygen
ported that compared with monometallic Co-based catalyst, γ-Al
2 3
O
supported bimetallic NiCo one with a Ni/Co molar ratio of 1:3 could
◦
afford higher guaiacol conversion up to 96.1 % at 200 C, along with the
production of cyclohexanol as the main product, despite severe reaction
conditions including 5.0 MPa hydrogen pressure and long reaction time
[27]. Additionally, the addition of few Re into Co-based catalysts was
beneficial to the HDO of phenol, which was attributed to the improved
reducibility of Co species and the generation of additional active hy-
drogenation sites [28]. Therefore, for cobalt-based catalysts, the high
dispersion of Co species and the introduction of small amounts of
bonds in lignin-based compounds, selective C O cleavage through
–
hydrodeoxygenation (HDO) processes becomes more critical [6]. As one
of the main phenolic monomers obtained from lignin depolymerization,
guaiacol with both types of carbon-oxygen bonds is a typical model
molecule for the HDO research [7,8].
In earlier studies, noble metals (e.g. Ru, Pd, Pt and Re) are commonly
*
Corresponding authors.
E-mail addresses: yanglan@mail.buct.edu.cn (L. Yang), lifeng@mail.buct.edu.cn (F. Li).
https://doi.org/10.1016/j.mcat.2020.111224
Received 2 July 2020; Received in revised form 11 September 2020; Accepted 13 September 2020
Available online 25 September 2020
2
468-8231/© 2020 Elsevier B.V. All rights reserved.

Q. Xu et al.
Molecular Catalysis 497 (2020) 111224
Fig. 1. XRD patterns of (A) LDH/C composite precursors and (B) different supported Co-based catalysts obtained at different reduction temperatures: (a) Co/C-500,
b) Co/C-600, (c) Ru-Co/C-500, (d) Ru-Co/C-600, (e) Ru-Co/C-700.
(
precious metals should be two crucial factors promoting their catalytic
HDO activity for lignin-based phenolics.
(6.25 mmol) were ultrasonically dispersed in deionized water (120 ml)
containing an appropriate amount of RuCl to get a clear solution. Then,
an 80 ml of basic solution containing NaOH (40.0 mmol) and Na CO
3
On the other side, two-dimensional layered double hydroxides
2
3
(
(
LDHs) are a family of layered compounds with different metal cations
(6.23 mmol) was used to titrate the above salt solution with stirring until
pH was around 9.0. After aging at 90 ℃ for 18 h under nitrogen atmo-
sphere, the CoAlRu-LDH precipitate was washed and dried overnight at
70 ℃. In addition, CoAl-LDH was obtained without the addition of
2
+
3+
M
and M ) uniformly distributing within the brucite Mg(OH) -like
2
layers [29]. Interestingly, through the heating treatment and the
following reduction process, LDHs can be topologically transformed into
a broad range of supported metal-based catalysts with highly dispersive
active metal sites and high specific surface areas [30]. Interestingly,
LDH-derived supported Ni-based catalysts with the highly dispersed Ni
nanoparticles (NPs) of 3 ~ 10 nm in size were reported to catalyze the
breaking of Caromatic-O bond in anisole at 280 ℃ [31]. And, some
Cu-containing LDHs were utilized to precisely prepare Cu-based cata-
lysts applied in the HDO of biomass-derived compounds to gain higher
catalytic performances [32,33].
3
RuCl .
CoAlRu-LDH/C composites were assembled by a glucose-induced
hydrothermal method [35]. Briefly, CoAlRu-LDH (3.59 g) and glucose
(4.22 g) was added into a Teflon-lined stainless-steel reactor with 50 ml
deionized water. Then, the resulting suspension was aged at 150 ℃ for
10 h. The CoAlRu-LDH/C composite precipitate was filtered, washed,
and dried under vacuum at room temperature. For comparison,
CoAl-LDH/C and ZnAlRu-LDH/C composite precursors were assembled
under the same procedure as for CoAlRu-LDH/C composite.
Inspired by highly active Ru-based catalysts in the upgrading of
lignin-derived phenolic compounds, in this study, we mainly focus on
developing new high-performance carbon-supported Ru-decorated Co-
based nanocatalysts through combining a small amount of Ru with a
cheaper first-row transition metal Co to induce surface exposure of
CoAl-LDH/C, CoAlRu-LDH, and ZnAlRu-LDH/C composites was
ꢀ 1
reduced under pure H
respectively, and held for 3 h. Then, the resulting samples were sub-
jected to the passivation under N flow with 1% O for 1 h at room
2
flow (100 ml min ) at 500, 600, or 700 ℃,
2
2
◦
highly dispersive Ru sites. The results demonstrated that the surface
temperature. The obtained supported metal samples are denoted as Ru-
Co/C–T, Co/C–T, and Ru/C-600, where T represents the applied
reduction temperature. At last, the obtained samples were switched into
in a sealed sample quartz tube filled with nitrogen gas for further
characterization and catalytic tests.
decoration of a small amount of Ru on Co NPs could significantly
improve the catalytic performance of Co-based catalysts, and the as-
fabricated Ru-Co/C catalyst obtained at a reduction temperature of
◦
6
00 C showed excellent catalytic performance for the guaiacol HDO
process to generate cyclohexanol under the mild reaction conditions
200 ℃ and 1.0 MPa hydrogen pressure), with a high cyclohexanol yield
(
2
.2. Sample characterization
of ~94 %. Further, the high catalytic efficiency of Ru-Co/C catalysts was
found to be closely associated with the surface synergy between highly
X-ray diffraction (XRD) patterns of samples were recorded on Shi-
◦
◦
dispersive active Ru /Co sites and abundant defective oxygen
vacancies.
madzu XRD-6000 diffractometer using the Cu K
metals was analyzed on Shimadzu ICPS-7500 inductively coupled
plasma atomic emission spectroscopy (ICP-AES). adsorption-
α
source. The content of
N
2
2
. Experimental section
desorption isotherms were obtained from Micromeritics ASAP 2460
sorptometer apparatus. The specific surface area was calculated by the
BET method, and the date of pore structure were obtained according to
the BJH method through desorption isotherms. Transmission electron
microscopy (TEM) and high-angle annular dark-field scanning TEM-
energy-dispersive X-ray spectroscopy (HAADF-STEM-EDX) were used
to obtain the microstructural property of samples on JEOL JEM-2100
2
2
.1. Catalyst preparation
.1.1. Synthesis of samples
CoAlRu-LDH precursors were prepared by a coprecipitation route
[
34]. Typically, Co(NO
3
)
2
⋅6H
2
O
(12.5 mmol) and Al(NO
3
)
3
⋅9H
2
O
2

Q. Xu et al.
Molecular Catalysis 497 (2020) 111224
Table 1
Composition and textural properties of different samples.
a
a
a
b
c
d
Samples
Co
wt %)
Al
Ru
S
BET
2
D
p
V
p
ꢀ 1
3
ꢀ 1
g )
(
(wt %)
(wt %)
(m
g
)
(nm)
(cm
Co/C-500
26.5
31.5
32.7
35.5
37.3
6.4
6.4
7.5
6.7
7.2
0
218
259
228
232
263
7.82
6.16
7.85
10.14
7.10
0.10
0.14
0.15
0.20
0.28
Co/C-600
0
Ru-Co/C-500
Ru-Co/C-600
Ru-Co/C-700
0.59
0.48
0.53
a
Determined by ICP-AES analysis.
BET specific surface area.
Average pore diameter.
b
c
d
Total volume of pores.
in the sealed sampler of the X-ray photoelectron spectrometer, in order
to prevent further oxidation of samples at exposed air. Raman spectra
were recorded through Mono Vista 2560 spectrometer using an excita-
tion wavelength of 532 nm. Micromeritics ChemiSorb 2920 instrument
2 2
was utilized to test H temperature programmed desorption (H -TPD) of
samples. In situ Fourier transform infrared (FT-IR) spectra for anisole
adsorption were collected from Thermo Nicolet 380 FT-IR spectrometer.
Before measurement, the sample was pretreated in Ar at 200 ℃ for 1 h.
The powder sample was exposed to an atmosphere richer in anisole at
room temperature for 30 min. Surface physically adsorbed anisole was
removed by the Ar flow for 10 min.
Fig. 2. XRD patterns of ZnAlRu-LDH/C composite precursor (a) and Ru/C-
6
00 (b).
transmission electron microscope and HITACI S-5500 instrument. X-ray
photoelectron spectroscopy (XPS) was collected through Thermo VG
ESCALAB250 XPS spectrometer at the Al K
α
radiation. Before XPS
2
.3. Catalytic HDO test
measurement, the sample was transferred to a glove box filled with inert
gas to make a testing sample, and then the tested sample was packaged
The catalytic HDO reaction of guaiacol was conducted in a stainless-
Fig. 3. TEM images (a and b), HRTEM image (c), and the particle size distribution histogram (d) of Ru-Co/C-600 sample.
3

Q. Xu et al.
Molecular Catalysis 497 (2020) 111224
Fig. 4. TEM images and particle size distribution histograms of Ru-Co/C-500 (a,d), Ru-Co/C-700 (b,e), and Co/C-600 (c,f).
steel reactor (100 ml), where guaiacol (0.4 ml), n-decane (12 ml) and
the catalyst (15 mg) were added. Firstly, the reactor was purged with
nitrogen for five times. Then, the reactant was heated to a certain
temperature and hydrogen gas was charged through continuous supply
from the gas cylinder. Afterward, the reaction was initiated through
introduction of a small amount of Ru species in CoAl-LDH can promote
the reduction of Co species, probably due to the strong interactions
between Ru and Co atoms. In all cases, no metallic Ru and other
Ru-containing species are detected [39]. Furthermore, XRD patterns of
Ru/C-600 reference sample derived from ZnAlRu-LDH/C precursor also
reveal only the formation of ZnO phase (Fig.2). The above results sug-
gest that for Ru-containing reduced samples, no formation of Ru dif-
fractions in XRD patterns is probably contributed to the much small Ru
loading amount (~0.5 wt %) (Table 1).
2
vigorous magnetic stirring (800 rpm) at a certain H pressure. After a
desired time, the reactor was rapidly cooled down. At last, the products
were analyzed by a gas chromatograph (Agilent GC7890B) equipped
with DB-WAX capillary column (30.0 m x 250
μ
m × 0.25
μ
m) using
flame ionization detector. In each case, the carbon balance was more
than 95 %.
Fig. 3 shows the typical TEM image of the representative Ru-Co/C-
600. It is clearly seen that many small NPs are evenly distributed on a
thin carbon sheet, and these NPs with an average size of about 11.1 nm
present a narrow distribution of particle sizes. And, there are three types
of the lattice fringes of interplanar spacings of 0.204, 0.244 and
3
. Results and discussion
0
.286 nm, respectively, which match well with those of Co (111), Co
3 4
O
3
.1. Structure analysis of samples
(
311), and Co (220) planes [40,41], confirming that Co species co-
3 4
O
exists in zero valence state and oxidation states. Meanwhile, TEM im-
ages of other Ru-Co/C samples depict similar results, despite the
increasing size of NPs with the elevated reduction temperature (Fig. 4).
As we know, XRD characterization can confirm the crystalline
structure of samples. It is noted from Fig. 1A that XRD patterns of CoAl-
LDH/C and CoAlRu-LDH/C samples show seven well-defined charac-
◦
In all cases, however, no lattice fringes related to the metallic Ru phase
teristic diffractions for
2
a representative highly crystalline Co Al
can be observed [42], suggesting that Ru species may be homogeneously
deposited onto the surface of Co particles because of the miscibility of
two metals in bulk. In this regard, the formation of Co NPs with the
larger average particle size (17.9 nm) in the case of Co/C-600 suggests
that the surface decoration of Ru species may probably restrain the
growth of metallic Co particles, thus inducing the formation of
small-sized Co NPs. Further, HAADF-STEM images of representative
Ru-Co/C-600 with elemental mapping and EDX line scanning spectra
reveal a clear overlapping of surface Co and Ru species and a separation
between Al and Co/Ru elements (Fig.5), minoring the similar surface
distributions of Co and Ru elements, as well as their same positions on
the surface. The above results confirm that Ru species can be homoge-
neously distributed over the surface of Co NPs, considering that the
much lower content of Ru than that of Co.
(
OH) (CO O phase (JCPDS 56-0954) [36]. No diffractions related
6
3
)
0.5⋅H
2
to Ru-containing species and carbon materials can be observed, sug-
gesting that Ru species may be successfully incorporated into the lattice
of LDH precursor and carbon species can be combined with LDH phase
in an amorphous form. After reduction of CoAlRu-LDH/C and
CoAl-LDH/C samples, Ru-Co/C samples present a broad diffraction peak
◦
of (111) plane for metallic Co phase (JCPDS 89-7039) at about 44.4
(
Fig. 1B), while the broad (311) diffraction for Co
3 4
O spinel phase
(
JCPDS 42-1467) and the weak (200) diffraction for CoO phase (JCPDS
◦ ◦
3-1004) can be observed at 36.9 and 42.4 [37,38], respectively. The
4
above diffraction peaks corresponding to metallic Co and Co oxides are
broad, indicative of the character of small-sized Co-containing particles.
And, no diffraction peaks assigned to crystalline Al
demonstrating that Al species should exist in amorphous alumina form.
Interestingly, different from Ru-Co/C-500, CoO and Co are the only
two crystalline phases in the Co/C-500, strongly implying that the
2 3
O are detected,
XPS of Co 2p3/2 and O 1s regions was gained to determine surface
electronic structures of Co and O elements on different samples (Fig.6).
3 4
O
4

Q. Xu et al.
Molecular Catalysis 497 (2020) 111224
Fig. 5. HAADF-STEM images (a and c) of Ru-Co/C-600, the elemental EDX mapping (b) of O-K, Al-K, Co-K and Ru-K, and EDX line spectra of elements (d) along the
red line in (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Table 2
The surface element valence distribution of different supported Co-based cata-
lysts obtained at different reduction temperatures.
Samples
Co
Co
Co(II)
Co(II)/Co
O
II
/
H
2
a
a
a
b
b
c
(
0)
at
(III)
(III)
(O
I
+OII
)
uptake
(
(at %)
(at %)
ratio
ratio
(mmol/g)
%
)
Co/C-600
Ru-Co/C-
18.1
9.5
38.2
48.3
43.7
42.2
1.14
0.87
0.75
0.72
0.0386
0.1560
5
00
Ru-Co/C-
00
Ru-Co/C-
00
10.4
16.7
42.6
37.4
47.0
45.9
1.10
1.23
0.75
0.82
0.0822
0.0427
6
7
a
b
c
Surface fractions of different Co species determined by XPS.
Determined by XPS of Co 2p3/2 or O1s region.
Determined by H
2
-TPD profiles in the range of 50ꢀ 400 ◦C.
◦
2+
3+
elevated reduction temperature from 500 to 700 C, surface Co /Co
intensity ratio increases gradually from 0.87 to 1.23 in the cases of
Ru-containing samples (Table 2), while surface Co2 /Co intensity
ratio for Co/C-600 is almost equal to that for Ru-Co/C-600. Meanwhile,
there are three deconvoluted contributions in the fine O 1s region, which
+
3+
Fig. 6. XPS of Co 2p3/2 and O 1s regions for Co/C-600 (a), Ru-Co/C-500 (b),
Ru-Co/C-600 (c), Ru-Co/C-700 (d) samples.
are assigned to three types of O species: O
I
(530.0–530.9 eV), OII
(
531.7–532.3 eV), and OIII (533.5–533.9 eV), which are mainly associ-
There are four deconvoluted peaks (778.7, 780.3, 782.7 and 786.5 eV)
◦
3+
ated with the lattice oxygen species in Co-containing oxides, the
in the fine Co 2p3/2 spectra, which are associated with surface Co , Co
,
2
+
2+
adsorbed oxygen species on defective sites (i.e. oxygen vacancies, Vo) or
Co species and shake-up Co satellite [43], respectively. With the
5

Q. Xu et al.
Molecular Catalysis 497 (2020) 111224
Fig. 7. Sputtering-XPS spectra of (a) Co 2p3/2 and (b) C 1s with Ru-Co/C-600.
Fig. 9. H
2
-TPD profiles of different Co-based catalysts after reducing at
different temperature.
hydroxyl species, and the surface oxygen species bonded to carbon
atoms [40,44], respectively. As gathered in Table 2, the surface
O
II/(O
I
+OII) intensity ratio slightly increases from Ru-Co/C-500 to
Ru-Co/C-600 (Co/C-600) and Ru-Co/C-700. The above results imply
that the formation of surface oxygen vacancies on as-fabricated
Co-based samples in the form of Co2 -V
+
2+
o
-Co defective structure, and
that the higher reduction temperature is beneficial to the generation of
more defective structures.
Based on the XPS results, in all cases, the surface fraction of metallic
◦
Co in total Co species is below 20 %, which is completely different from
◦
the XRD result that metallic Co is the main bulk crystalline phase in
x
reduced samples. Therefore, one can suspect that surface CoO -deco-
rated metal NPs may form on the surface of reduced samples. To prove
this picture of such surface structure, Ru-Co/C-600 was tested by sput-
tering XPS analysis. As shown in Fig. 7, with the prolonging of sputtering
◦
time, the peak intensity of Co 2p3/2 core level gradually exceeds that of
cobalt oxide and becomes dominant. However, the 3d5/2 signal of Ru
species cannot be detected at about 280 eV during XPS sputtering
(
Fig.7), due to the low Ru content. Meanwhile, the absence of Ru3d5/2
signal also can rule out the possibility that Ru species are wrapped inside
the core of Co particles or CoO species. The above results imply the
formation of the core-shell structured Ru-Co@CoO configuration on the
Fig. 8. Raman spectra of Ru-Co/C-500 (a), Co/C-600 (b), Ru-Co/C-600 (c), and
Ru-Co/C-700 (d).
x
x
Fig. 10. (A) Catalytic performance for the HDO of guaiacol over different Co-based catalysts (200 ℃, 1.0 MPa hydrogen pressure, and 0.5 h) and the change in the
conversion and product selectivities with the reaction time over the Ru-Co/C-600 (B) (200 ℃ and 1.0 MPa hydrogen pressure).
6

Q. Xu et al.
Molecular Catalysis 497 (2020) 111224
Table 3
To get more insight into the dissociation hydrogen capacity of
a
Catalytic performance for hydrodeoxygenation over different reactants .
reduced samples, H
of H
-TPD was conducted. As shown in Fig. 9, the amount
2
◦
uptake in the range of 50ꢀ 400 C decreases gradually in the
Selectivity (%)
2
Reactants
Conv. (%)
following order: Ru-Co/C-500 > Ru-Co/C-600 > Ru-Co/C-700 > Co/C-
Benzene
CHOL
CHON
CHA
MCHDOL
6
00. Before TPD measurements, all Co-based samples were obtained by
anisole
72.4
97.8
100
7.2
0.7
0.5
22.5
97.3
99.2
0.5
1.8
0
26.9
1.3
42.9
0
◦
the reduction of H
and CO
2
above 500 C. Therefore, the possible release of CO
phenol
◦
2
from carbon supports in the range of 50ꢀ 500 C during H
2
-TPD
cyclohexanone
0.3
0
process for reduced Co-based samples can be neglected. Here, the
increased amount of H uptake demonstrates the improved capacity of
dissociation hydrogen, which originates from the enhanced dispersion
a
Reaction condition: 200 ℃; 1.0 MPa hydrogen pressure; 1.5 h.
2
◦
◦
of active Ru /Co sites, consistent with the decrease in the size of Co or
Ru-decorated Co NPs.
3
.2. Catalytic HDO performance of Ru-decorated Co-based catalysts
The catalytic HDO reactions over different supported Co-based cat-
◦
alysts were conducted under 1.0 MPa hydrogen pressure at 200 C. As
presented in Fig. 10A, the catalytic activities of Ru-Co/C catalysts are
remarkably better than that of Co/C-600 catalyst under the same reac-
tion conditions. Especially, Ru-Co/C-600 catalyst delivers a guaiacol
conversion of 79.2 % after a reaction of 0.5 h, with a cyclohexanol
(
CHOL) selectivity of 78.4 %. In striking contrast, Co/C-600 catalyst
delivers much lower conversion of guaiacol (24.3 %) and cyclohexanol
selectivity (59.8 %). Apparently, the introduction of a small amount of
Ru (0.48 wt %) can greatly improve the catalytic HDO activity of Ru-Co/
C-600, which mainly originates from the increased amount of active
metallic sites responsible for the dissociation of molecular hydrogen.
Meanwhile, compared with Ru-Co/C-600, Ru-Co/C-700 catalyst ex-
hibits a slightly reduced activity, probably due to the presence of larger
metal particles. On the other side, over the Ru-Co/C-600, the conversion
and product selectivities changed with the reaction time (Fig.10B). The
guaiacol conversion rapidly increases within 90 min. Meanwhile, the
cyclohexanol selectivity increases gradually, and the phenol selectivity
decreases to zero. After a reaction of 3 h, the guaiacol conversion and
the cyclohexanol selectivity reach about 99.6 % and 94.1 %, respec-
tively. It demonstrates that the processes from guaiacol to phenol and
from phenol to cyclohexanol are consecutive reactions. In general, even
though previously reported Ru-containing catalysts with much higher
loading amounts (above 5%) are effective for the HDO of guaiacol [48,
Fig. 11. The change of TOF value of guaiacol converted over different cata-
◦
lysts. Reaction conditions: 200 C, 1.0 MPa hydrogen pressure, and 5 min.
4
0
9], our Ru-Co/C catalyst systems with the much lower Ru loading of
.48 % exhibit high activity, limiting the use of precious metal by
incorporating with a cheaper transition metal Co.
Further, anisole was used as the substrate to study the HDO reaction
process over the present Ru-Co/C-600 catalyst (Table 3, entry 1). It is
seen that no phenol is produced at 200 ℃ and 1.0 MPa after a reaction of
1
.5 h, along with a small cyclohexanol selectivity of 22.5 %. However, as
a result of hydrogenation-isomerization and dehydration reactions
promoted by weak acidic sites on the catalyst surface [50], anisole also
can be converted to a large amount of (cis- and trans-)1-methyl-1,
2
-cyclohexanediol (MCHDOL) or cyclohexanone (CHON). Meanwhile,
the product distributions using phenol as the substrate is consistent with
those using guaiacol as the substrate (Table 3, entry 2). In two cases of
anisole and guaiacol as substrates, phenolic hydroxyl can be hydro-
deoxygenated to produce benzene. Furthermore, cyclohexanone used as
the substrate can be easily converted into cyclohexanol under the same
reaction conditions (Table 3, entry 3). The above results illustrate that
the direct removal of the methoxy group from guaiacol is the first step of
the HDO process.
Fig. 12. In situ FT-IR spectra of anisole adsorbed on different Ru-Co/C catalysts
at room temperature.
surface of Ru-Co/C-600 sample.
In order to further confirm the presence of surface defective struc-
tures, Raman spectra of supported Co-based samples were analyzed. As
displayed in Fig. 8, five Raman peaks (186, 470, 511, 599, and
70 cmꢀ ) are assigned to F 12 g, E2 g
1
2
3
6
F2 g, F2 g, and A1g vibrations of Co O
3 4
1
In addition, the turnover frequency (TOF) for guaiacol conversion
was calculated based on the moles of converted guaiacol per mole sur-
in spinel structure [45], respectively. Specifically, two F2 g and A1g vi-
brations are broadened and red-shift with the elevated reduction tem-
perature, as well as the introduction of Ru, clearly demonstrating the
face metallic sites determined by the H
min, in order to identify the intrinsic catalytic HDO activity of
different catalysts. Notably, the TOF values over Ru-containing catalysts
2
-TPD results after a reaction of
5
3 4
enhanced defective structure of Co O phase [46], due to the formation
of more oxygen vacancies. Such surface oxygen vacancies are thought to
be conducive to the adsorption and activation of oxygen-containing
functional groups in phenolic compounds in the HDO reaction [47].
ꢀ 1
are larger than that over the Co/C-600 (0.35 s ) (Fig. 11). Especially,
the TOF value over the Ru-Co/C-700 catalyst reaches as high as 4.416
ꢀ 1
ꢀ 1
s
, which is much higher than those over Ru-Co/C-500 (0.517 s ) and
7

Q. Xu et al.
Molecular Catalysis 497 (2020) 111224
Fig. 13. Effect of reaction temperature (A) and hydrogen pressure (B) on the HDO of guaiacol over the Ru-Co/C-600 after 1.5 h. Reaction conditions: (a) 1.0 MPa
hydrogen pressure; (b): 200℃.
Ru-Co/600 (2.619 s 1). Such significant TOF difference between these
catalysts demonstrates that surface active metallic species are not the
sole active reaction sites.
ꢀ
Table 4
a
Catalytic performance for hydrodeoxygenation over different reaction time .
Selectivity (%)
To get more information on the adsorption and activation of guaiacol
on Ru-Co/C catalysts, in situ FT-IR spectra for adsorbed anisole on
samples were collected (Fig. 12). The two absorption bands near 1600
Time
Conv. (%)
Benzene
CHOL
CHON
CHA
5
1
min
99.8
99.8
1.4
0.2
73.8
20.5
3.7
0.2
18.8
77.0
ꢀ 1
–
and 1497 cm are correlated with the C
benzene ring. Meanwhile, in the cases of Ru-Co/C-600 and Ru-Co/C-
–
C stretching vibration in the
80 min
a
Reaction condition: 260 ℃; 2.0 MPa hydrogen pressure.
7
00, the asymmetric and symmetric bending vibrations of the
ꢀ 1
methoxy group of adsorbed anisole appear at 1472 and 1453 cm
,
respectively. The above results imply that there is stronger chemical
adsorption between the methoxy group of anisole and the surface of Ru-
Co/C-600 and Ru-Co/C-700 through the interaction between surface Vo
x
sites in CoO species and the lone electron pair of oxygen in methoxy
group in anisole, by which the Cring-O bond may be weakened. In our
present Ru-Co/C catalyst system, highly dispersed surface Ru sites
combined with metallic Co sites may more easily dissociate molecular
hydrogen, and as-generated active hydrogen species can further rapidly
transfer to the catalyst surface. In this case, surface Ru sites also may act
as hydrogen transfer centers to significantly promote the HDO process
x
through the hydrogen spillover from Co NPs to defective CoO . Mean-
while, a large amount of surface oxygen vacancies may act as adsorp-
tion/activation sites for guaiacol. Therefore, the surface synergy
◦
◦
between highly dispersed Ru /Co sites and oxygen vacancies greatly
improves the instinct activity of Ru-Co/C catalysts, especially Ru-Co/C-
7
00 with more surface oxygen vacancies.
The catalytic HDO performance of Ru-Co/C-600 catalyst as a func-
tion of the reaction temperature was assessed (Fig.13a). With the
extended reaction time up 1.5 h, the guaiacol conversion can reach as
high as 99.6 % at 200 ℃, with a high cyclohexanol selectivity of 91.3 %.
With the decreasing reaction temperature from 200 ℃ to 180 ℃, the
guaiacol conversion decreases, while the selectivity to 1-methyl-1,2-
Fig. 14. Reusability of Ru-Co/C-600. Reaction condition: 200 ℃, 1.0 MPa
hydrogen pressure, and 1.5 h.
cyclohexanediol and phenol increase to 12.8
% and 9.5 %,
Scheme 1. Plausible reaction pathways and mechanism for the HDO of guaiacol over Ru-Co/C catalysts.
8

Q. Xu et al.
Molecular Catalysis 497 (2020) 111224
respectively. On the contrary, upon increasing temperature from 180 to
CRediT authorship contribution statement
2
60 ℃, the amount of 1-methyl-1,2-cyclohexanediol or phenol is
decreased sharply, while the selectivity of cyclohexanone, benzene,
bicyclohexane and cyclohexane (CHA) gradually increases to 7.8, 7.7,
Qiangqiang Xu: Investigation, Writing - original draft. Yisheng Shi:
Investigation, Methodology. Lan Yang: Conceptualization, Writing -
original draft. Guoli Fan: Visualization. Feng Li: Conceptualization,
Formal analysis, Writing - review & editing.
2
.4 and 28.5 %, respectively. Nevertheless, the sharply increased
cyclohexane selectivity is accompanied by the enhanced formation of
cyclohexanone, benzene and bicyclohexane at the expense of cyclo-
hexanol production, indicating that the as-fabricated Ru-Co/C catalysts
can catalyze deoxygenation process of cyclohexanol at higher temper-
atures. To further explain the above results, the HDO reaction was also
conducted at 260 ℃ and 2 MPa hydrogen pressure (Table 4). It is found
that the main product is cyclohexanol with a yield of about 74.0 % after
a reaction of 5 min, and the cyclohexane yield reaches 77 % after a re-
action of 3 h. It demonstrates that higher reaction temperatures can
favor the formation of cyclohexane. Different from the reaction tem-
perature, the increase in the hydrogen pressure from 1.0–2.0 MPa
cannot remarkably affect guaiacol conversion and product selectivities
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
We thank gratefully the financial support from National Natural
Science Foundation of China (21776017; 21991102; 21521005) and the
Fundamental Research Funds for the Central Universities (XK1802-6).
(
Fig.13b). At the pressure of 0.5 MPa, a large amount of phenol (29.4 %)
can be produced. Clearly, the higher hydrogen pressure promotes both
the ring-hydrogenation and the hydrogenolysis of the methoxy group in
guaiacol to 1-methyl-1,2-cyclohexanediol, despite the constant selec-
tivity of cyclohexanol. The reusability and stability of catalysts are quite
important properties for heterogeneous catalysts. It is found that Ru-Co/
C-600 still retains high catalytic activity even after five cycles (Fig.14),
indicating the excellent reusability of the catalyst.
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