Highly dispersed Co and Ni nanoparticles encapsulated in N-doped carbon nanotubes as efficient catalysts for the reduction of unsaturated oxygen compounds in aqueous phase

Article abstract of DOI:10.1039/c8cy01488d

N-Doped carbon nanotube-encapsulated metal nanoparticles are of great interest in heterogeneous catalysis owing to their improved mass transfer ability and superior stability. Herein, a facile one-pot pyrolysis approach using melamine as the carbon and nitrogen source was developed to fabricate metal nanoparticles embedded in bamboo-like N-doped carbon nanotubes (named as Co@NCNTs-600-800 and Ni@NCNTs-600-800). The optimized Co@NCNTs-600-800 catalyst exhibited outstanding activity in furfural (FAL) selective hydrogenation to furfuryl alcohol (FOL) or cyclopentanone (CPO) in aqueous media. High yields of FOL (100%) and CPO (75.3%) were achieved at 80 °C and 140 °C, respectively. Besides, this cobalt catalyst showed very good stability and recyclability during the reaction. The synergistic effect between metallic cobalt and N-doped carbon nanotubes was systematically investigated. In addition, the as-prepared Ni@NCNTs-600-800 catalyst also exhibited remarkable activity. Under optimal conditions (100 °C and 4 MPa H₂ pressure), a maximum tetrahydrofurfuryl alcohol (THFOL) yield (99.5%) was obtained in the aqueous-phase hydrogenation of FAL. The research thus highlights new perspectives for non-noble metal-based N-doped carbon nanotube catalysts for biomass transformation.

Full text of DOI:10.1039/c8cy01488d

View Article Online
View Journal
Catalysis
Science &
Technology
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: W. Gong, C.
Chen, H. Zhang, G. Wang and H. Zhao, Catal. Sci. Technol., 2018, DOI: 10.1039/C8CY01488D.
This is an Accepted Manuscript, which has been through the
Volume 6 Number 1 7 January 2016 Pages 1–308
Royal Society of Chemistry peer review process and has been
accepted for publication.
Catalysis
Science &
Technology
Accepted Manuscripts are published online shortly after
www.rsc.org/catalysis
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
ISSN 2044-4753
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
rsc.li/catalysis

Please do not adjust margins
Catalysis Science & Technology
Page 1 of 9
View Article Online
DOI: 10.1039/C8CY01488D
Journal Name
ARTICLE
Highly dispersed Co, Ni nanoparticles encapsulated in N-doped
carbon nanotubes as efficient catalysts for reduction of
unsaturated oxygen compounds in aqueous-phase
Received 00th January 20xx,
Accepted 00th January 20xx
Wanbing Gong,a Chun Chen,*a Haimin Zhang,a Guozhong Wanga and Huijun Zhao*ab
DOI: 10.1039/x0xx00000x
www.rsc.org/
N-doped carbon nanotubes encapsulated metal nanoparticles are of great interest in heterogeneous catalysis owing to
their improved mass transfer ability and superior stability. Herein, a facile one-pot pyrolysis approach using melamine as
carbon and nitrogen source was developed to fabricate metal nanoparticles embedded in bamboo-like N-doped carbon
nanotubes (named as Co@NCNTs-600-800 and Ni@NCNTs-600-800). The optimized Co@NCNTs-600-800 catalyst exhibited
outstanding activity in furfural (FAL) selective hydrogenation to furfuryl alcohol (FOL) or cyclopentanone (CPO) in aqueous
media. The high yields of FOL (100%) and CPO (75.3%) were achieved at 80 oC and 140 oC, respectively. Besides, this cobalt
catalyst showed very good stability and recyclability during the reaction. The synergistic effect between metallic cobalt and
N-doped carbon nanotubes was systematically investigated. In addition, the as-prepared Ni@NCNTs-600-800 catalyst also
exhibited remarkable activity. Under the optimal conditions (100 oC and
4 MPa H2 pressure), a maximum
tetrahydrofurfuryl alcohol (THFOL) yield (99.5%) was obtained in the aqueous-phase hydrogenation of FAL. The research
thus highlights new perspectives for non-noble metal-based N-doped carbon nanotubes catalysts for biomass
transformation.
hydrogenation of FAL to these chemicals is carried out either in the
gas-phase or in the liquid-phase.10 The gas-phase hydrogenation
1. Introduction
Due to the decrease of fossil fuel sources and the increase of
environmental pollution, the catalytic conversion of bio-refinery
platform molecules to fuels and value-added chemicals has been
the subject of intense research in the recent years.1-3 Especially,
furfural (FAL) is one of these promising platform molecules, and it is
produced easily and commercially by combined acid-catalyzed
can suffer from poor selectivity of target products or catalyst
deactivation.11 To overcome the mentioned shortcomings, it is
attractive to design an efficient catalysis system for FAL
hydrogenation in liquid-phase. Strikingly, the conventional liquid-
phase hydrogenation is not eco-friendly because it inevitably
requires organic solvents (methanol, ethanol or 2-propanol) or
produces toxic wastes.12, 13 From the standpoint of green chemistry,
the use of water as reaction medium is preferred.14 Despite the
number of investigations in liquid-phase hydrogenation of FAL,
there are not much references about the selective hydrogenation of
FAL in aqueous-phase.6 Recently, several research groups have
done some pioneering works in this regard. For example, Pd–Cu
supported on MgO or Mg(OH)2 exhibited complete conversion and
higher than 98% selectivity at 110 oC and 0.6 MPa of hydrogen in
aqueous-phase hydrogenation of FAL to FOL.15 Chen et al. also
reported complete conversion and high selectivity (> 99%) in water
was observed for g-C3N4 nanosheets supported Pt catalysts at 100
oC after 5 h reaction.14 Yang et al. found the FAL was almost
completely converted to THFOL over Ni/Ba–Al2O3 in water under
optimized conditions (140 oC, 4 MPa H2 and 4 h).16 At present, more
and more research groups have focused on the study of aqueous-
phase hydrogenation of FAL to CPO. About 98% FAL conversion with
92.1 mol% CPO yield were early reported under an optimal
hydrolysis and the dehydration of hemicellulosic xylans in
4, 5
lignocellulose
.
FAL can be further converted into many valuable
chemicals, such as furfuryl alcohol (FOL), tetrahydrofurfuryl alcohol
(THFOL), 2-methylfuran (2-MF), cyclopentanone (CPO) and
cyclopentanol (CPL) by the hydrogenation or hydrodeoxygenation
way.6, 7 Among these products, the as-synthesized alcohols have
been extensively used in industrial processes as green solvents or
intermediates for fine chemical synthesis, such as resins, fibers and
wetting agents.7, 8 For instance, CPO is a versatile compound used
for the synthesis of fuels, fungicides, pharmaceuticals, rubber
9
chemicals, and flavor and fragrance chemicals.5, The catalytic
a.Key Laboratory of Materials Physics, Centre for Environmental and Energy
Nanomaterials, Anhui Key Laboratory of Nanomaterials and Nanotechnology, CAS
Centre for Excellence in Nanoscience, Institute of Solid State Physics, Chinese
Academy of Sciences, Hefei 230031, P. R. China. E-mail: h.zhao@griffith.edu.au,
chenchun2013@issp.ac.cn.
b.Centre for Clean Environment and Energy, Gold Coast Campus, Griffith University,
Queensland 4222, Australia
o
condition (160 C, 3 MPa of hydrogen) over Pd–Cu catalyst.17 Later,
Guo et al. demonstrated that a 62% yield of CPO was obtained at
the optimized condition (150 °C, 4 MPa H2, 6 h) over CuZnAl
†Electronic Supplementary
10.1039/x0xx00000x
Information
(ESI)
available.
See
DOI:
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 2 of 9
View Article Online
DOI: 10.1039/C8CY01488D
ARTICLE
Journal Name
catalyst.18 Recently, 95% FAL conversion with 85% CPO yield was 2.2. Catalyst preparation
achieved over a CuZn/CNT catalyst in a mild reaction condition (140
oC, 4 MPa H2).19 Although some promising works have been done on
the hydrogenation of biomass-derived FAL to some valuable
chemicals using water as solvent, further studies are still needed in
developing non-noble metal based catalysts with excellent catalytic
performance under mild reaction condition.
In a typical synthesis, 0.003 mol of Co(NO3)2·6H2O was first
dissolved in an appropriate amount of ethanol. Then, 0.012 mol of
melamine was added into the above solution and continually stirred
for 0.5 h. Next, the homogeneous mixture was stirred at room
temperature until ethanol was fully evaporated and dried at 60 oC
overnight. Afterward, the mixture was transferred into a ceramic
boat and heated to 600 °C for 2 h with a ramp rate of 2 oC min−1
under a N2 flow (first step heat treatment), and the temperature
further rose to desired temperature for 2 h with a ramp rate of 4 oC
min−1 under a N2 atmosphere (second step heat treatment). Finally,
the sample was gained by cooling down to room temperature. The
obtained sample was denoted Co@NCNTs-600-x (x stands for the
temperature in the second step heat treatment). For comparison,
Co@NC-600 was also synthesized without high temperature
treatment in the second step. Similarly, the Ni@NCNTs-600-x
samples were also prepared by the identical synthesis strategy in
which the metal salt was substituted by Ni(NO3)2·6H2O.
In recent years, tremendous efforts have been made to develop
novel N-doped carbon based materials due to their desirable
physical and chemical properties.20, 21 As catalysts or supports, N-
doped carbon based materials have shown superior activities in
many applications covering a wide range of heterogeneous catalytic
reactions.22, 23 Among them, metal nanoparticles encapsulated in N-
doped carbon nanotubes (NCNTs) are widely used to improve the
performance of the catalysts because of high electron transfer
performance and abundant active sites.24-26 For example, Cao et al.
developed a facile synthesis method for N-doped bamboo-like
carbon nanotubes (CNTs) with Co nanoparticles encapsulated at the
tips. The uniform catalyst showed better ORR catalytic activity and
higher stability in alkaline solutions compared with commercial Pt/C
and comparable catalytic activity to Pt/C in acidic media.27 Recently,
Lin et al. reported the facile synthesis of cobalt nanoparticles
encapsulated into graphitic N-doped carbon nanotube materials.
The composites showed excellent catalytical performance for
selective oxidation of ethylbenzene under solvent-free and
molecular oxygen conditions.28 However, to the best of our
knowledge, metal nanoparticles encapsulated in N-doped carbon
nanotubes are rarely applied to selectively catalytic aqueous-phase
hydrogenation reactions.
2.3. Catalyst characterization
Field emission scanning electron microscopy (FESEM) images of
the samples were taken on a FESEM (SU8020) operated at an
accelerating voltage of 10.0 kV. Transmission electron microscopy
(TEM, JEOL-2010) images operated at an acceleration voltage of 200
kV. Scanning transmission electron microscopy (STEM) images of
the samples were recorded by a high resolution TEM (Philips
TecnaiG2 F20) operated at an acceleration voltage of 200 kV.
Powder X-ray diffraction (XRD) patterns were analyzed on a Philips
X-Pert Pro X-ray diffractometer with using the Ni-filtered
monochromatic Cu Kα radiation (λKα1 = 1.5418 Å) at 40 keV and 40
mA. The surface area and porosity of samples were measured at 77
K using a Surface Area and Porosity Analyzer (Autosorb iQ Station
2). XPS analysis was performed on an ESCALAB 250 X-ray
photoelectron spectrometer (Thermo, USA) equipped with Al Kα1,2
monochromatized radiation at 1486.6 eV X-ray source. Raman
spectra of the samples were recorded on a LabRAM HR800 confocal
microscope Raman system (Horiba Jobin Yvon) using an Ar ion laser
operating at 632 nm. The metal content in the composite structure
was determined by the inductively coupled plasma spectroscopy
after microwave digestion of the samples (ICP 6300, Thermo Fisher
Scientific).
Herein, we report
a facile approach to fabricate metal
nanoparticles embedded bamboo-like N-doped carbon nanotubes
(named as Co@NCNTs-600-800/ Ni@NCNTs-600-800) via
thermolysis
of
melamine
and
corresponding
nitrates
(Co(NO3)2·6H2O or Ni(NO3)2·6H2O). The samples have been
characterized by SEM, TEM, STEM, XRD, XPS, Raman and BET. The
Co@NCNTs-600-800 exhibited an excellent catalytic performance
for selective hydrogenation of FAL to FOL and CPO in aqueous-
phase under mild conditions, respectively. The excellent stability of
catalyst was also proved. Besides, the Ni@NCNTs-600-800 was
demonstrated to be an effective catalyst for aqueous-phase
hydrogenation of FAL to THFOL.
2.4. Catalytic tests
2. Experimental
The catalytic hydrogenation of FAL was carried out in a 25 mL
stainless steel autoclave with a mechanical stirrer, a pressure gauge
2.1. Materials and Chemicals
and automatic temperature control apparatus. In
a typical
Co(NO3)2·6H2O, Ni(NO3)2·6H2O and melamine were purchased
from Sinopharm Chemical Reagent Co. Ltd. FAL (99%), FOL (98%),
THFOL (98%), CPO (99.5%), CPL (99.5%), n-octanol (99.5%) and ethyl
acetate (99%) were purchased from Aladdin Reagent Company.
Other analytical grade solvents including ethanol were
commercially available.
experiment, the reaction solutions of FAL (1 mmol), catalyst (30
mg), and H2O (10 ml) were loaded into the reactor. The reactor was
sealed, purged three times with N2 at 1 MPa, and then pressurized
with H2 to a setting point. The reactor was then heated to the
desired temperature and the stirring speed fixed to 700 rpm which
eliminates the diffusion effects. After reaction, the autoclave was
cooled down quickly. The autoclave contents were transferred to a
centrifuge tube, and the catalyst was separated by centrifugation.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 3 of 9
View Article Online
DOI: 10.1039/C8CY01488D
Journal Name
ARTICLE
The products were extracted by ethyl acetate from the liquid and briefly illustrated in Fig. 1a, where the melamine acts as carbon and
then analyzed.
nitrogen source. The synthesis started with the formation of
Co@NC-600 by heat treatment at 600 oC, which was consisted of
irregular short rods (Fig. 1b). Then, the Co@NC-600 was subjected
to high-temperature pyrolysis at 800 oC under N2 atmosphere to
yield the desired Co@NCNTs-600-800. Finally, the N-doped carbon
nanotubes with lengths of more than several micrometers was
formed, a closer look showed that white dots were found at the tip
of N-doped carbon nanotubes, indicating that the incorporation of
Co nanoparticles when the sample was further treated at high
temperature (Fig. 1c). The TEM image (Fig. 1d) of Co@NCNTs-600-
800 observes the typical bamboo-like N-doped carbon nanotubes
with diameters ranging from 30 to 40 nm were formed, and cobalt
nanoparticles were encapsulated at the tip of nanotubes. The
HRTEM (Fig. 1e) and HAADF-STEM images (Fig. 1f) further confirm
Co nanoparticles were coated by thin graphitic carbon layers, and
the wall thickness was only about 2-4 nm. The lattice fringe of the
core was consistent with the Co crystal structure having (111)
lattice fringes with an interspacing of 0.204 nm. The corresponding
elemental mapping from HAADF-STEM image (Fig. 1g) shows that
Co, O, N and C were homogeneously distributed on the catalyst.
The liquid product was identified by gas chromatography–mass
spectrometry (GC-MS, Thermo Fisher Scientific-TXQ Quntum XLS,
column-TG-WAXMS, 30 m × 0.25 mm × 0.25 µm), and was
quantitatively analyzed by GC (Shimadzu, GC-2010 Plus), equipped
with FID and a (30 m × 0.25 mm × 0.25 µm) KB-WAX capillary
column (Kromat Corporation, USA) using n-octanol as an internal
standard. The carbon balance was checked in every run and it was
found to be higher than 94%.
During the catalyst stability test, the catalyst was reused without
any further treatments. More specifically, the reaction mixture was
filtered to recover the catalyst, which was washed first with
acetone then water followed by drying under vacuum oven at 60 oC
and employed for stability test.
3. Results and discussions
The preparation procedure for the Co@NCNTs-600-800 catalyst is
Fig. 1 (a) Schematic illustration of the synthetic procedure of the Co catalysts; (b) SEM image of as-prepared Co@NC-600;
(c) SEM image, (d) TEM image, (e) HRTEM image, (f) HAADF-STEM image, (g) EDX mapping, (h) XRD pattern, (i) Co 2p
spectrum, (j) N 1s spectrum, (k) N2 adsorption and desorption isotherm and BJH pore size distribution plot (inset) of as-
prepared Co@NCNTs-600-800.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 4 of 9
View Article Online
DOI: 10.1039/C8CY01488D
ARTICLE
Journal Name
The XRD diffraction pattern (Fig. 1h) displays three peaks at 44.2°, and porous structures of samples. The sample exhibited type-IV
51.5°, and 75.9°, corresponding to (111), (002), and (022) of cubic isotherms with hysteresis loops, indicating the presence of
Co (JCPDS 96-901-0969), respectively. XPS characterization is mesoporous structures (Fig. 1k). The specific surface area and total
carried out to identify the surface electronic states of the catalyst. pore volume were 200.7 m2 g−1 and 0.588 cm3 g−1 (Table S1†),
The surface XPS survey spectrum (Fig. S1a†) shows the sample was respectively. The ICP data shows the Co content was 40.3 wt% for
mainly composed of C (92.2 at.%), O (3.31 at.%), N (3.83 at.%) and Co@NCNTs-600-800 (Table S1†). The high content of metallic Co
Co (0.67 at.%). The low Co content was due to that Co nanoparticles can enhance the catalytic activities for N-doped porous carbon
were tightly encapsulated by thin carbon layers in the catalyst.27 materials. Based on the above results, the metallic Co nanoparticles
The characteristic Co 2p3/2 and Co 2p1/2 peaks related to the spin- encapsulated by N-doped carbon nanotubes play an important role
orbit splitting are observed from the high resolution Co 2p in the remarkable catalytic activity for the aqueous-phase reactions.
spectrum (Fig. 1i), and Co 2p3/2 was fitted into Co0, Co3+, Co2+ and
satellites peaks, which confirmed the mixture of Co0 and CoOx on
The selective aqueous-phase hydrogenation of FAL to CPO is
the surface of catalyst.28 In the meantime, the high resolution N 1s
chosen as the model reaction to evaluate the catalytic performance
of as-prepared Co@NCNTs-600-800 catalyst. Fig. 2a demonstrates
spectrum (Fig. 1j) was fitted into two peaks at 398.7 eV for pyridinic
that FAL conversion gradually increased from 22.9% at 60 oC to
N (N1) and 401.3 eV for graphic N (N3), indicating the successful
almost 100% at 130 oC at 4 MPa H2 pressure after 5 h reaction. The
incorporation of N into the carbon nanotubes.29 The high N doping
main products were FOL, CPO and CPL. At the low temperature
range (60-80 oC), FOL was sole product. After that, it is found that
content (3.83 at.%), high percentage of pyridinic N (67.5 %), and
certain percentage of graphitic N (32.5 %) in the Co@NCNTs-600-
CPO selectivity raised from 0.8% to 75.3% with reaction
temperature increased from 90 to 140 oC. But then the CPO
800 were beneficial to improving the aqueous-phase hydrogenation
activity (Table S1†). Nitrogen adsorption–desorption isotherms selectivity decreased to 75.1% at 160 oC because of the deep
hydrogenation of CPO to CPL. In addition, the reaction time
measured at 77 K are used to evaluate the specific surface areas
Fig. 2 Aqueous hydrogenation of FAL over Co@NCNTs-600-800. Reaction conditions: (a) Catalyst = 30 mg, FAL = 1 mmol,
Water =10 mL, H2 pressure = 4 MPa, Reaction time = 5 h. (b) Catalyst = 30 mg, FAL = 1 mmol, Water =10 mL, Reaction
temperature = 140 oC, H2 pressure = 4 MPa. (c) Catalyst = 30 mg, FAL = 1 mmol, Water =10 mL, Reaction temperature = 140
oC, Reaction time = 5 h. (d) Catalyst = 30 mg, FAL = 1 mmol, Water =10 mL, Reaction temperature = 80 oC, H2 pressure = 4
MPa, Reaction time = 3 h.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 5 of 9
View Article Online
DOI: 10.1039/C8CY01488D
Journal Name
ARTICLE
dependence of FAL hydrogenation on Co@NCNTs-600-800 was also
surveyed. Firstly, the FAL conversion gradually increased from 7.6%
o
within 1 h to 49.5% within 5 h at the low temperature of 80 C (Fig.
S2†). When the reaction time was extended to 8 h, almost complete
conversion was definitively obtained. Fig. 2b further shows that the
selectivity of FOL decreased and CPO/CPL selectivity increased as
reaction time went on at the high temperature of 140 oC. After 1
hour reaction, almost 100% FAL conversion with 97.3% FOL
selectivity and 1.5% CPO selectivity were achieved. Then CPO
selectivity increased from 14.1% to 75.3% as reaction time
increased from 2 h to 5 h. After 7 h reaction, the CPO selectivity
decreased to 60.9%. It is attributed to the overreaction in
hydrogenation of CPO to CPL, which causes a decrease of CPO
selectivity but increase of the corresponding CPL selectivity. Based
on these data, in order to gain high CPO selectivity in the aqueous-
phase hydrogenation of FAL, the reactant is first transformed to FOL
and then the intermediate is further hydrogenated to CPO rather
than THFOL in the presence of water reaction medium. The result is
consistent with the previous studies.9, 17 To further study structural
change of the catalyst, the XRD patterns of the catalyst with
different reaction time were demonstrated (Fig. S3†). It reveals the
structure of the catalyst remained unchanged, but intensity of the
metal peaks gradually weakened with the prolonging of reaction
time, indicating the catalyst could be oxidized gradually in medium
of water under reaction condition. To provide more insights into
the aqueous-phase hydrogenation of FAL into CPO, the rate
constants k at different temperatures can be calculated over the
Co@NCNTs-600-800 catalyst and the results are shown in Fig. S4†.
The aqueous-phase hydrogenation of FAL showed pseudo-first-
order kinetics for all studied temperatures at the condition of low
conversion. The rate constants k were 0.0117 min-1, 0.0172 min-1,
0.0202 min-1 and 0.0259 min-1 for temperatures of 120 oC, 130 oC,
140 oC and 150 oC, respectively. Moreover, the rate constants for
the hydrogenation of FAL to FOL are much higher than that for
generating CPO, indicating that the step of FOL to CPO is the rate-
determining step. According to the Arrhenius plots at the low
conversion of FAL, the activation energy (Ea) was calculated to be
34 kJ mol−1 over the Co@NCNTs-600-800 catalyst (Fig. S5†).
Moreover, the TOF values were also calculated to investigate the
intrinsic catalytic activity in a low conversion. The calculated TOF
values were 231 h-1, 285 h-1, 339 h-1 and 428 h-1 for temperatures of
Fig. 3 (a-b) TEM images, (c) XRD patterns and (d) Co 2p
spectrums of the as-synthesized and reused Co@NCNTs-600-
800 catalysts. The inset in (a-b) shows the distribution of
corresponding particle size.
increased from 30.4 nm to 35.1 nm after six runs (Fig. 3a-b). The
intensities of cobalt peaks decreased slightly after six runs, which
could be attributed to active sites were gradually oxidized (Fig. 3c).
For deeply confirming the changes of Co catalyst in reaction, the
chemical state of reused catalyst was determined by XPS analysis
(Fig. 3d). Comparing with the fresh catalyst, the content of cobalt
was basically the same (0.66 at.%), and cobalt oxides (CoOx) have
been detected obviously after six runs. It indicates that the metallic
Co NPs on the surface of support could be oxidized under reaction
conditions.
The above results demonstrate superior aqueous-phase
hydrogenation catalytic performances of the as-synthesized
Co@NCNTs-600-800 catalyst. To further understand the
o
120 C, 130 oC, 140 oC and 150 oC over the Co@NCNTs-600-800
catalyst, respectively (Table S2†). Fig. 2c shows the catalytic
performance of aqueous-phase FAL hydrogenation by varying H2
pressure. At relatively low H2 pressure (1 MPa), the FAL conversion
was 89.1%, and the selectivity of FOL and CPO were 98.1% and
1.5%, respectively. When the H2 pressure increased to 4 MPa, FAL
was completely converted and CPO selectivity was 75.3%.
Lastly, we evaluated the recyclability of Co@NCNTs-600-800
catalyst at low conversion (~ 25 %), and the results are shown in Fig.
2d. The FAL conversion remained almost unchanged after six cycles,
indicating the good stability of the catalyst. The reused catalyst had
been characterized by ICP, TEM, XRD and XPS. As determined by ICP
technique, the Co content in the reused catalyst is 17.0 wt%, which
is closed to 17.2 wt% in the as-synthesized catalyst, and only trace
amount of Co has been detected in the reaction mixture after six
cycle runs. is known that leaching or sintering of active metal is a
key obstacle for heterogeneous metal catalysts in a water medium.
The obtained Co@NCNTs-600-800 catalyst displayed high stability
due to the protecting effect of N-doped carbon shell. The cobalt
particles had a certain aggregation and the average particle size
Fig. 4 Co 2p (a-c) and N 1s (d-f) XPS spectrum of Co@NCNTs-
600-700, Co@NCNTs-600-800 and Co@NCNTs-600-900
catalysts.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 6 of 9
View Article Online
DOI: 10.1039/C8CY01488D
ARTICLE
Journal Name
Fig. 6 Characterization and evaluation of Ni@NCNTs-600-800:
(a) SEM image; (b) TEM image; (c) effect of reaction time on the
hydrogenation of FAL. (Reaction conditions: Catalyst = 30 mg,
FAL = 1 mmol, Water =10 mL, Reaction temperature = 100 oC, H2
pressure = 4 MPa.)
Fig. 5 Hydrogenation of FAL over different Co catalysts. 1,
Co@NC-600; 2, Co@NCNTs-600-700; 3, Co@NCNTs-600-800; 4,
Co@NCNTs-600-900. (Reaction conditions: Catalyst = 30 mg,
FAL = 1 mmol, Water =10 mL, Reaction temperature = 140 oC,
H2 pressure = 4 MPa, Reaction time = 5 h.)
relationship between the structure and activity of the as- graphitic N became the dominant phase in the Co@NCNTs-600-900
synthesized cobalt catalysts, different Co based catalysts were catalyst. It can be inferred from Fig. 4 that the conversion of
synthesized and evaluated. On the basis of XRD analysis (Fig. S6†), it pyridinic N and pyrrolic N (400.8 eV, N2) to graphitic N can occur
clearly shows that the diffraction peaks of metallic cobalt were gradually with the increase of calcination temperature. Some
apparently more intense and sharp with the increase of calcination studies had also confirmed that pyridinic
N can effectively
temperature, indicating a higher crystallization and crystallite size at contribute the carbon material surface with Lewis base property,
an elevated temperature. The mean size of Co nanoparticles were possibly being favourable for the adsorption of acidic functional
about 21.4 nm, 28.6 nm and 52.1 nm (based on Scherrer equation) groups or precursors on material surface, thus improving catalytic
for Co@NCNTs-600-700, Co@NCNTs-600-800 and Co@NCNTs-600- performance.31, 32 Moreover, graphitic N in carbon based catalysts
900 catalysts, respectively (Table S1†). Besides, the Raman can effectively improve their electron transfer capability, thus
spectrum (Fig. S7†) shows that two peaks located at about 1350 resulting in high catalytic performance in hydrogenation.33 In our
cm−1 and 1580 cm−1 can be assigned to typical D- and G-bands of previous work, we suggested that pyridinic N can make carbon with
carbon, respectively.30 The relative intensity ratio of D-band to G- Lewis basic sites to increase the dispersion of catalyst in solvent and
band (ID/IG) was decreased with the increase of calcination adsorption of FAL. Meanwhile, graphitic
N could effectively
temperature, indicating a higher degree of graphitization. The SEM increased the electron transfer property of catalyst, thus improving
images (Fig. S8†) show that tubular morphology and structure the catalytical hydrogenation performance.34 The O 1s peak (Fig.
became more and more uniform with the increase of calcination S1b-d†) suggests the presence of three main O-containing species:
temperature from the 700 oC to 800 oC. But irregular and broken the lattice oxygen species (O1, ~ 530.2 eV), surface oxygen species
tubular morphology has also been found when the temperature (O2, ~ 531.8 eV) and adsorbed oxygen species (O3, ~ 533.2 eV).35, 36
was further increased to 900 oC, which suggests that 800 oC is It can be found that relative amount of surface defects (i.e. oxygen
optimum calcination temperature to form highly ordered tubular vacancies) were existed on the surface of catalyst. These results
morphology. Besides, the diameters of the nanotubes increased provide basic clues concerning the interaction between the Co and
with calcination temperature (Fig. S9†). The specific surface area N-doped carbon nanotubes.
and total pore volume increased with the increasing of calcination
temperature from 700 oC to 800 C. It can be attributed to the
formation of N-doped carbon nanotubes at elevated temperature.28
However, when the temperature was further increased to 900 oC,
the larger Co nanoparticles and broken tubular morphology were
inevitably formed (Table S1 and Fig. S10, †). The average pore
diameter was basically the same (~ 3.9 nm) for all samples (Fig.
S10†). In terms of the chemical environment and electronic
structure of surface, Fig. 4 shows Co 2p and N 1s XPS spectrum of
different Co catalysts. According to the fitting results, with the
increasing calcination temperature from 700 oC to 900 oC, the
proportion of metallic Co increased, indicating that the extent of
reduction of cobalt ions was higher on the surface of catalysts
(Table S1†). Moreover, we can find that the N doping content
increased first then decreased with the calcination temperature
(Table S1†). In terms of N types, pyridinic N was the dominant
phase in the Co@NCNTs-600-700 and Co@NCNTs-600-800, but
Several supported Co catalysts were screened for the aqueous-
phase hydrogenation of FAL, and their activity results are shown in
detail. First, FOL was identified as sole product obtained from FAL
hydrogenation at the low temperature of 80 oC after 5 h reaction
o
(Fig. S11†), indicating that the cobalt is
a truly selective
hydrogenation catalyst for the C=O bond of unsaturated
38
aldehydes.37,
Besides, we can find that the FAL conversion
increased first then decreased with the calcination temperature
increased from 600 oC to 900 oC. When using Co@NCNTs-600-800
as catalyst, the highest FAL conversion (49.5%) to FOL can be
achieved at 80 oC after 5 h reaction. Next, the FAL can be further
converted to CPO through furan ring rearrangement at high
temperature in aqueous medium generally.18, 39 Fig. 5 exhibits FAL
o
can be hydrogenated to CPO at the high temperature of 140 C. In
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 7 of 9
View Article Online
DOI: 10.1039/C8CY01488D
Journal Name
ARTICLE
all cases, the full conversion was obtained after 5 h reaction under 4
MPa H2 pressure. Due to the unaffected C=C bond and the easier
hydrogenation of the C=O bond to an alcohol, the major product
was consisted of the FOL and CPO. With the increase of calcination
temperature, the CPO selectivity increased first then decreased.
And a 75.3% CPO selectivity with complete conversion was realized
within 5 h using Co@NCNTs-600-800 as catalyst. These experiments
directly confirm that Co@NCNTs-600-800 is the optimum catalyst
for the hydrogenation of FAL to FOL and CPO, respectively.
Based on the above experimental results and previous
discussions, the possible main reasons for their high activity and
selectivity in this work could be attributed to the synergistic effect
of following points: (1) abundant graphited carbon nanotubes with
high surface area can give rise to abundant active sites; (2) high N-
doping content and appropriate N-doping type can increase
adsorption of FAL and electron transfer; (3) highly dispersed Co
nanoparticles can increase the activation of H2 and FAL; (4) the
synergy between Co nanoparticles and graphited N-doped carbon
nanotubes can enhance the hydrogenation of FAL. After etching
with 2 M HCl solution, the intensities of metallic Co peaks
decreased sharply (Fig. S12†), suggesting the metallic Co NPs on the
surface of support have been effectively removed. After that, the
catalytic performance of FAL aqueous-phase hydrogenation over
the acid treated Co@NCNTs-600-800 catalysts has also been
investigated and compared with untreated catalyst (Table S3†). It
can be found that the activity of treated Co@NCNTs-600-800
catalyst decreased obviously due to the decrease of active Co
species.
Fig. 7 Reaction pathway for aqueous-phase hydrogenation of
FAL over (Co, Ni) based catalysts.
decreased correspondingly. It also reveals that the hydrogenation
of FAL to THFOL was a consecutive two-step hydrogenation process
via the FOL intermediate. Striking, a 99.5% THFOL selectivity with
complete conversion could be achieved after 7 h reaction at 100 oC.
Similarly, the step of converting FOL to THFOL could be regarded as
the rate-determining step in the aqueous-phase hydrogenation of
FAL over the Ni@NCNTs-600-800 catalyst (Fig. S16†). Meanwhile,
the TOF values of Ni@NCNTs-600-800 were 27 h-1, 88 h-1, 118 h-1
and 160 h-1 for temperatures of 90 oC, 100 oC, 110 oC and 120 oC,
respectively (Table S4†).
Based on the above experimental results, we suggest a possible
pathway for the catalytic aqueous-phase hydrogenation of FAL over
Co- and Ni-based catalysts, which is exhibited in Fig. 7. As it shows,
FOL is an intermediate in both cases. For the Co-based catalyst, FOL
can further convert to CPO via hydrogenation and rearrangement
reactions. Meanwhile, the Ni-based catalyst usually converts FOL to
THFOL, because metallic Ni nanoparticles are too active and prone
to taking place furan ring hydrogenation. The results are in
accordance with previous studies.40, 41
Different metal catalytic sites usually undergo different reaction
pathways, which leads to the different product in the
hydrogenation process. To probe the effect of active sites on
hydrogenation activity, Ni@NCNTs-600-800 catalyst is synthesized
and characterized. Interestingly, we did
a
series of
4. Conclusions
characterizations, including SEM, TEM, XRD and Raman test (Fig. 6,
Fig. S13 and Fig. S14, †). The morphology and microstructure of Ni
catalyst was similar to the Co catalyst, indicating that the synthesis
strategy can be widely applied to synthesize N-doped carbon
nanotubes catalysts with different metal sites. The three sharp
peaks at 44.4°, 51.8°, and 76.3° are ascribed to the (111), (002), and
(022) of cubic Ni (JCPDS 96-210-0647), respectively (Fig. S13†).
Unlike Co-based catalyst, the hydrogenation of furan ring took place
over the Ni catalyst, leading to a mixture of FOL, THFOL, and CPO in
the product (Fig. S15†). And the FAL conversion and product
distribution were high correlated with reaction temperature. FAL
In summary, N-doped carbon nanotubes encapsulated metal (Co,
Ni) nanoparticles have been successfully fabricated by a facile
pyrolysis approach using melamine as carbon and nitrogen sources.
The catalysts showed excellent catalytic performance in aqueous-
phase hydrogenation of FAL to some valuable chemicals (FOL,
THFOL and CPO). When Co@NCNTs-600-800 was a model catalyst,
high yields of FOL (100%) and CPO (75.3%) can be obtained at 80 oC
and 140 oC, respectively. The catalyst was reused for six cycles
without obvious loss of catalytic activity. In addition, a 99.5% THFOL
o
yield can also be attained under 100 C reaction temperature and 4
o
conversion gradually raised from 25.8% at 60 C to almost 100% at
MPa H2 pressure within 7 h reaction using Ni@NCNTs-600-800 as
100 oC. FOL was major product at the low temperature range (60-70
oC). The FOL selectivity decreased but THFOL selectivity increased
catalyst. Hence, it provides
a universal strategy to construct
heterogeneous catalysts for efficient transforming bio-derived
platform molecules into important and valuable chemicals.
o
o
with reaction temperature increased from 80 C to 120 C. Further
increasing reaction temperature, part of FOL was converted to CPO.
For instance, the CPO selectivity was 49.4% and the THFOL
selectivity was 30.2% when reaction temperature was 150 oC.
Additionally, the time course of reaction was also explored at 100
oC and 4 MPa H2 pressure. As shown in Fig. 6c, THFOL selectivity
obviously increased with the reaction time, but FOL selectivity
Conflicts of interest
The authors declare no competing financial interest.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 8 of 9
View Article Online
DOI: 10.1039/C8CY01488D
ARTICLE
Journal Name
30 Z. Wei, J. Wang, S. Mao, D. Su, H. Jin, Y. Wang, F. Xu, H. Li and Y.
Wang, ACS Catal., 2015, 5, 4783-4789.
Acknowledgements
31 E. Yoo, J. Nakamura and H. Zhou, Energy Environ. Sci., 2012, 5,
6928-6932.
32 D. H. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo and J.
Nakamura, Science, 2016, 351, 361-365.
33 Z. Li, J. Li, J. Liu, Z. Zhao, C. Xia and F. Li, ChemCatChem, 2014, 6,
1333-1339.
This work was financially supported by the National Natural Science
Foundation of China (Grant No. 51502297, 51871209, 51372248,
and 51432009), Instrument Developing Project of the Chinese
Academy of Sciences (Grant No.yz201421).
34 W. Gong, C. Chen, H. Zhang, Y. Zhang, Y. Zhang, G. Wang and H.
Zhao, Mol. Catal., 2017, 429, 51-59.
Notes and references
35 Z. Gao, L. Yang, G. Fan, F. Li, ChemCatChem, 2016, 8, 3769-3779.
36 A. Rokicińska, P. Natkański, B. Dudek, M. Drozdek, L. Lityńska-
Dobrzyńska and P. Kuśtrowski, Appl. Catal., B, 2016, 195, 59-68.
37 M. G. Dohade and P. L. Dhepe, Green Chem., 2017, 19, 1144-
1154.
38 M. Audemar, C. Ciotonea, K. De Oliveira Vigier, S. Royer, A.
Ungureanu, B. Dragoi, E. Dumitriu and F. Jerome,
ChemSusChem, 2015, 8, 1885-1891.
39 X.-L. Li, J. Deng, J. Shi, T. Pan, C.-G. Yu, H.-J. Xu and Y. Fu, Green
Chem., 2015, 17, 1038-1046.
40 Y. Su, C. Chen, X. Zhu, Y. Zhang, W. Gong, H. Zhang, H. Zhao and
G. Wang, Dalton T., 2017, 46, 6358-6365.
1
2
3
4
M. Besson, P. Gallezot and C. Pinel, Chem. Rev., 2014, 114,
1827-1870.
D. M. Alonso, S. G. Wettstein and J. A. Dumesic, Chem. Soc. Rev.,
2012, 41, 8075-8098.
J. C. Serrano-Ruiz, R. Luque and A. Sepulveda-Escribano, Chem.
Soc. Rev., 2011, 40, 5266-5281.
K. Yan, G. Wu, T. Lafleur and C. Jarvis, Renew. Sust. Energ. Rev.,
2014, 38, 663-676.
X. Li, P. Jia and T. Wang, ACS Catal., 2016, 6, 7621-7640.
R. Mariscal, P. Maireles-Torres, M. Ojeda, I. Sadaba and M. L.
Granados, Energy Environ. Sci., 2016, 9, 1144-1189.
D. Sun, S. Sato, W. Ueda, A. Primo, H. Garcia and A. Corma,
Green Chem., 2016, 18, 2579-2597.
S. Bhogeswararao and D. Srinivas, J. Catal., 2015, 327, 65-77.
Y. Liu, Z. Chen, X. Wang, Y. Liang, X. Yang and Z. Wang, ACS
Sustain. Chem. Eng., 2017, 5, 744-751.
5
6
7
41 B. Chen, F. Li, Z. Huang and G. Yuan, Appl. Catal., A, 2015, 500,
23-29.
a
8
9
10 Y. Nakagawa, M. Tamura and K. Tomishige, ACS Catal., 2013, 3,
2655-2668.
11 M. M. Villaverde, N. M. Bertero, T. F. Garetto and A. J. Marchi,
Catal. Today, 2013, 213, 87-92.
12 M. Ghashghaee, S. Shirvani and M. Ghambarian, Appl. Catal., A,
2017, 545, 134-147.
13 C. P. Jiménez-Gómez, J. A. Cecilia, D. Durán-Martín, R. Moreno-
Tost, J. Santamaría-González, J. Mérida-Robles, R. Mariscal and
P. Maireles-Torres, J. Catal., 2016, 336, 107-115.
14 X. Chen, L. Zhang, B. Zhang, X. Guo and X. Mu, Sci. Rep., 2016,
6, 28558.
15 K. Fulajtárova, T. Soták, M. Hronec, I. Vávra, E. Dobročka and M.
Omastová, Appl. Catal., A, 2015, 502, 78-85.
16 Y. Yang, J. Ma, X. Jia, Z. Du, Y. Duan and J. Xu, RSC Adv., 2016, 6,
51221-51228.
17 M. Hronec, K. Fulajtárová, I. Vávra, T. Soták, E. Dobročka and M.
Mičušík, Appl. Catal., B, 2016, 181, 210-219.
18 J. Guo, G. Xu, Z. Han, Y. Zhang, Y. Fu and Q. Guo, ACS Sustain.
Chem. Eng., 2014, 2, 2259-2266.
19 M. Zhou, J. Li, K. Wang, H. Xia, J. Xu and J. Jiang, Fuel, 2017, 202,
1-11.
20 M. Li, F. Xu, H. Li and Y. Wang, Catal. Sci. Technol., 2016, 6,
3670-3693.
21 M. Li, M. Tang, J. Deng and Y. Wang, Nano Res., 2016, 9, 3129-
3140.
22 D. Su, G. Wen, S. Wu, F. Peng and R. Schloegl, Angew. Chem. Int.
Ed., 2017, 56, 936−964.
23 E. Lam and J. H. T. Luong, ACS Catal., 2014, 4, 3393-3410.
24 X. Zou, X. Huang, A. Goswami, R. Silva, B. R. Sathe, E.
Mikmekova and T. Asefa, Angew. Chem. Int. Ed., 2014, 53,
4372-4376.
25 J. Meng, C. Niu, L. Xu, J. Li, X. Liu, X. Wang, Y. Wu, X. Xu, W.
Chen, Q. Li, Z. Zhu, D. Zhao and L. Mai, J. Am. Chem. Soc., 2017,
139, 8212-8221..
26 S. Fu, C. Zhu, H. Li, D. Du and Y. Lin, J. Mater. Chem. A, 2015, 3,
12718-12722.
27 T. Cao, D. Wang, J. Zhang, C. Cao and Y. Li, Chem., 2015, 21,
14022-14029.
28 X. Lin, Z. Nie, L. Zhang, S. Mei, Y. Chen, B. Zhang, R. Zhu and Z.
Liu, Green Chem., 2017, 19, 2164-2173.
29 Y.-Z. Chen, G. Cai, Y. Wang, Q. Xu, S.-H. Yu and H.-L. Jiang, Green
Chem., 2016, 18, 1212-1217.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 9
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C8CY01488D
N-doped carbon nanotubes encapsulate (Co, Ni) nanoparticles exhibit excellent
catalytic performance for selective reduction of unsaturated oxygen compounds in
aqueous-phase