Catalytic hydroprocessing of furfural to cyclopentanol over Ni/CNTs catalysts: Model reaction for upgrading of bio-oil

Article abstract of DOI:10.1007/s10562-013-1149-5

A series of nickel-based catalysts with HNO₃- pretreated CNTs as support (x% Ni/CNTs, x represents the Ni loading amount) were synthesized using impregnation method, which were successfully applied for the upgrading of model compound (furfural) in bio-oil. Effects of nickel loading amount, reaction temperature, reaction time and hydrogen pressure on conversion of furfural as well as selectivity for cyclopentanol were investigated systematically. The conversion of furfural over 30 wt% Ni/CNTs was up to 96.5 % with a yield of 83.6 % toward cyclopentanol, when the reaction was carried out at 140 °C with a initial H2 pressure of 5.0 MPa. The features of the Ni/ CNTs catalysts were investigated via XRD, XPS, TEM. Springer Science+Business Media New York 2013.

Full text of DOI:10.1007/s10562-013-1149-5

Catal Lett (2014) 144:235–241
DOI 10.1007/s10562-013-1149-5
Catalytic Hydroprocessing of Furfural to Cyclopentanol Over
Ni/CNTs Catalysts: Model Reaction for Upgrading of Bio-oil
•
Minghao Zhou Hongyan Zhu Lei Niu
•
•
•
Guomin Xiao Rui Xiao
Received: 30 August 2013 / Accepted: 18 October 2013 / Published online: 1 November 2013
Ó Springer Science+Business Media New York 2013
Abstract A series of nickel-based catalysts with HNO3-
pretreated CNTs as support (x% Ni/CNTs, x represents the
Ni loading amount) were synthesized using impregnation
method, which were successfully applied for the upgrading
of model compound (furfural) in bio-oil. Effects of nickel
loading amount, reaction temperature, reaction time and
hydrogen pressure on conversion of furfural as well as
selectivity for cyclopentanol were investigated systemati-
cally. The conversion of furfural over 30 wt% Ni/CNTs
was up to 96.5 % with a yield of 83.6 % toward cyclo-
pentanol, when the reaction was carried out at 140 °C with
a initial H2 pressure of 5.0 MPa. The features of the Ni/
CNTs catalysts were investigated via XRD, XPS, TEM.
oxygenated compounds (such as aldehydes, ketones, acids
and so on), making it inappropriate as a transportation fuel.
So, the quality of bio-oil should be improved by partial or
total elimination of the oxygenated functionalities [1, 3].
Mild hydrogenation is one of the promising routes for the
upgrading of bio-oil, which could eliminate the most
reactive oxygenated groups. Crocker et al. [4] investigated
the upgrading of a model bio-oil containing 4 wt% furfural
(FFR) over Pt catalysts, which was confirmed to be suitable
for the upgrading of bio-oil. And it’s reasonable to choose
FFR for initial screening, while leaving the investigation of
other bio-oil components (such as guaiacol, acetol, etc.) the
subject of further research.
FFR is an important starting material, which can be
hydrogenated to prepare furfuryl alcohol, methyl FFR and
even longer carbon chain compounds [5, 6], catalyzed by
Cu–Cr, nickel based catalysts or noble metal catalysts [1,
7–9]. And cyclopentanol, mainly prepared by catalytic
vapor-phase cyclization of 1,6-hexanediol [10] or by liquid
phase oxidation of cyclopentene [11], is widely applied for
preparation of fungicides, fragrance chemicals. To the best
of our knowledge, there has been no report demonstrating
the direct hydrogenation of FFR to prepare cyclopentanol
using non-noble metal catalysts with satisfactory conver-
sion and selectivity.
Keywords Ni/CNTs ꢀ Furfural ꢀ Hydrogenation ꢀ
Cyclopentanol
1 Introduction
Recently the development of new catalytic processes for
preparation of bio-fuel and chemicals using renewable
materials has been attracting much attention, due to the
rapid consumption of fossil resource [1–3]. Bio-oil is an
attractive bio-fuel considering the development of renew-
able energy sources [1]. As bio-oil contains many kinds of
Nickel-based catalysts are widely used in hydrogena-
tion, hydrodesulfurization and steam reforming [12, 13].
Although Ni-based catalysts are of lower catalytic activity
than noble metals (e.g. Pt, Ru, Rh) [14–16], which are not
common in applications for high cost. So it is of significant
importance to evaluate nickel-based catalysts with high
activity.
M. Zhou ꢀ H. Zhu ꢀ L. Niu ꢀ G. Xiao (&)
School of Chemistry and Chemical Engineering, Southeast
University, Nanjing 211189, People’s Republic of China
e-mail: xiaogmseu@gmail.com; xiaogm@seu.edu.cn
R. Xiao
Carbon nanotubes (CNTs) was confirmed to be suitable
support [17–20], owing to its intrinsic properties such as
chemical inertness, thermal stability [21–23]. It was
Key Laboratory of Energy Thermal Conversion and Control of
Ministry of Education, Southeast University, Nanjing,
People’s Republic of China
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M. Zhou et al.
2.3 Catalyst Characterization
The prepared Ni/CNTs catalyst was characterized by XRD
on a Rigaku D/max-A instrument with a Cu Ka radiation at
50 kV and 30 mA with a scan speed of 0.02°/min. The
microstructure of the catalyst was examined with trans-
mission electron microscopy (TEM, Tecnai G2 20). The
powder sample was dispersed in ethanol and kept in an
ultrasonic bath for 2 h, then the sample was deposited onto
a carbon-covered Cu supporting grid and dried at 25 °C for
TEM analysis. The XPS measurements were performed on
an ESCALAB-250 (Thermo-VG Scientific, USA) spec-
trometer with Al Ka (1486.6 eV) irradiation source. The
elemental concentration and peak’s positions were deter-
mined with C(1s) peak at 284.6 eV as calibration standard.
The Brunauer–Emmett–Teller (BET) surface area were
evaluated from the N2 adsorption–desorption isotherms
using a COULTER SA 3100 analyzer. TG studies were
carried out under an air flow rate of 30 ml/min with ana-
lyzers by using 10–15 mg sample and a 20 °C/min tem-
perature increasing. The IR spectroscopy was applied to
detect the functional groups on the surface of CNTs.
Scheme 1 Possible reaction paths for hydrogenation of furfural in
water [24]
reported that ruthenium supported on CNTs exhibited high
selectivity toward unsaturated alcohol in the hydrogenation
of cinnamaldehyde [17]. However, to develop CNTs-sup-
ported non-noble metal catalysts with high catalytic
activity remains a challenge. The main aim in this work is
to prepare CNTs supported Ni catalysts for the conversion
of FFR to cyclopentanol (see Scheme 1). The focus is on
the Ni loading amount, reaction time, reaction temperature
and hydrogen pressure on conversion of FFR and selec-
tivity for cyclopentanol.
2 Experimental
2.4 Catalytic Hydrogenation of Furfural in Water
2.1 Chemicals
All the aqueous-phase catalytic hydrogenations of FFR
were carried out in a 200 ml stainless autoclave equipped
with an electromagnetic driven stirrer. For each run, 95 ml
of water, 5 ml of FFR and 1.5 g catalyst were added to the
reactor vessel, then the hydrogenation reaction was carried
out in a temperature range of 110–150 °C to evaluate the
effect of reaction temperature, for a reaction time of 4–14 h
to evaluate the effect of reaction time. Additionally, other
influencing factors such as Ni-loading amount and hydro-
gen pressure were also taken into consideration. After
displacing air, the hydrogen pressure was raised to a certain
value. Then the reactor was heated to desired temperature
and the stirring speed fixed to 600 rpm to eliminate the
diffusion effects. Finally, the reactor was quickly cooled
down, and the reaction products were separated from the
catalysts by centrifugation. The aqueous phase was ana-
lyzed using gas chromatography (Ouhua GC 9160)
All of the reagents except CNTs are of analytic purity
grade, and were purchased from local Sinopharm Chemical
Reagent. CNTs was purchased from Chengdu Organic
Chemicals Co., LTD (purity [97 %), which was synthes-
ised from natural gas by catalytic cracking method cata-
lyzed by copper-based catalysts.
2.2 Catalyst Preparation
Raw CNTs pretreated with concentrated HNO3 was
selected as support. CNTs was firstly refluxed in 68 wt%
HNO3 for 12 at 363 K, to increase the concentration of
oxygen-containing functional groups on the surface. The
mixture was filtered and washed with deionized water, then
centrifugated. Each run, the centrifugation process lasted
for 5 min under a rotation speed of 9,000 r/min, then
washed with water. The process repeated as needed.
Finally, the obtained catalysts were dried at 343 K
overnight.
equipped
with
a
SE-54
capillary
column
(30 m 9 0.32 mm 9 0.5 lm). The main products were
cyclopentanol and cyclopentanone, and the product yield
and selectivity were calculated and defined as follows:
Ni/CNTs catalyst was prepared by impregnation
method. The support was immersed into an aqueous solu-
tion of Ni(NO3)2 under stirring for 24 h. Then, the obtained
solutions were slowly evaporated and the residues were
dried at 353 K for 12 h. The obtained solid was calcined at
673 K for 4 h in N2. After grind, it was loaded in a tubular
reactor and reduced under H2 at 673 K for 4 h. Then it was
cooled down and Ni/CNTs catalysts were obtained.
n0FFR ꢁ nFFR
ni
n0FFR
XFFR
¼
ꢂ 100 %; Yi ¼
ꢂ 100 %;
n0FFR
Yi
Si ¼
ꢂ 100 %;
XFFR
where i represents the product cyclopentanol, cyclopenta-
none or pentadiol in the reaction; n0FFR and nFFR depict the
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Model Reaction for Upgrading of Bio-oil
237
amounts of FFR before and after reaction, respectively, in
3.1.2 Chemical and Physical Properties of Ni/CNTs
Catalysts
mol; and ni is the amount of product i, in mol.
Table 1 shows the fundamental chemical and physical
properties for the CNTs-supported Ni catalysts, including
the Ni loading contents, BET surface area and averaged Ni
size. The BET surface areas for the Ni/CNTs catalysts
range from 107.6 to 145.6 m2/g. The averaged crystallite
size of Ni/CNTs catalysts (estimated from TEM image)
ranges from 8.3 to 20.4 nm, which closely associated with
the Ni loading content. The increase of Ni content of the
Ni/CNTs catalysts led to the decrease of the BET surface
areas [27]. The decreased BET surface areas was mainly
caused by the coverage of Ni on the CNTs surface, which
can be seen from the TEM test results (Fig. 3). Generally,
the increase of Ni content would inevitably lead to the
aggregation of particle on the CNTs surface, finally lead to
particle growth of Ni. The changes of the catalysts prop-
erties would inevitably lead to significant influence on the
catalyst activity when increasing the Ni loading content,
which were confirmed by the following experiments.
3 Results and Discussion
3.1 Catalyst Characterization
3.1.1 Acid Treatment of Raw CNTs
CNTs is a kind of hydrophobic support, which was
reported to be difficult to adsorb metal on its surface [25].
Concentrated acids can be used to introduce functional
groups (e.g.C=O, COOH and OH) on the surfaces of CNTs
[26]. Therefore, CNTs pre-treated with concentrated nitric
acids can help to effectively support active metals. The
acid treatment cannot only make CNTs form functional
groups but can also remove impurities on the surface of
CNTs; the results can be analyzed by TGA. From TGA
analysis in Fig. 1a, we found that the commercial CNTs
contains the impurities about 2.5–3.5 wt%, and acid
treatment could help to remove the impurities in the
commercial CNTs (see Fig. 1a). The nature of the surface
functional groups was investigated by IR spectroscopy, as
shown in Fig. 1b. The peak at 1,425/cm was connected
with the O–H bending deformation in carboxylic acids.
Higher peaks at 1,690 and 3,390/cm were connected with
C=O and OH functional groups respectively, which con-
firmed the presence of C=O and OH functional groups in
the acid-treated CNTs. In comparison of the IR results of
raw CNTs and acid treated CNTs, it can be found that acid
treated CNTs contains relatively large amount of C=O,
COOH or OH functional groups, which would help to
adsorb active metal on the surface.
Table 1 Chemical and physical properties of Ni/CNTs catalysts
Catalyst
Ni content
(wt%)a
Ni content
(wt%)b
dTEM
(nm)c
dXRD
(nm)d
10 % Ni/CNTs
20 % Ni/CNTs
30 % Ni/CNTs
40 % Ni/CNTs
a
10.28
19.64
30.36
40.52
145.6
132.4
121.8
107.6
8.3
10.2
13.6
20.4
8.0
9.5
11.7
18.5
Measured by AAS analysis
b
c
d
Evaluated from N2 adsorption–desorption isotherms
Measured by TEM analysis
Calculated by Scherrer equation from the peak of XRD
Fig. 1 TGA spectra and IR
spectra of raw CNTs and acid
treated CNTs. a TGA results,
b IR results
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M. Zhou et al.
3.1.3 XRD and XPS Characterization
Ni. The used 30 % Ni/CNTs showed sharp and symmetric
peaks with little decrease in intensity, which indicated that
the catalyst was of good crystal form and stability before or
after reaction (see Fig. 2a).
Figure 2a shows the XRD patterns of five different sam-
ples, including 10–40 % Ni/CNTs, all reduced by H2 at
673 K for 4 h, and the used 30 % Ni/CNTs after FFR
hydrogenation at 413 K for 10 h. The sharp and symmetric
peaks clearly indicate that the samples were well crystal-
lized. The peak at 26.5° could be assigned to the diffraction
peaks of the (002) planes of graphite-like tube-wall of the
CNTs [17–19]. Besides the CNTs feature, the presence of
metallic Ni in the reduced Ni–CNTs catalysts is clearly
revealed. As can be seen from the XRD results, only one
kind of Ni-based phase was detected for Ni/CNTs catalysts.
Peaks at 44.5°, 51.9°, and 76.4° could be assigned to the
diffraction of the (111), (200), and (220) planes of metallic
XPS detection was taken to analyze the composition of
prepared Ni/CNTs. Absorption peaks were identified for C,
O, and Ni. Their mass concentrations were 60.91, 10.71
and 28.38 wt%, respectively. Seen from Fig. 2b, as for the
high amount of O, it was caused by the increase of oxygen-
containing groups during the treatment of HNO3. For
another thing, the oxidation of Ni/CNTs catalyst before or
during the XPS detection also contributed to the high
amount of O. Seen from Fig. 2d, binding energies at 852.9
and 856.3 eV were observed for 30 % Ni/CNTs, corre-
sponding to Ni0 (2p3/2) and Ni2? (2p3/2), respectively. The
Fig. 2 a XRD spectra for Ni/CNTs with different Ni content. b XPS pattern of HNO3-treated CNTs. c General XPS spectra of 30 % Ni/CNTs.
d XPS pattern of Ni2p of 30 % Ni/CNTs
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Model Reaction for Upgrading of Bio-oil
239
Fig. 3 TEM images a 30 %
Ni–CNTs reduced, b particle
size distribution histogram of
30 % Ni–CNTs reduced, c 30 %
Ni–CNTs used d 40 % Ni–
CNTs reduced
3.2 Aqueous-Phase Catalytic Hydrogenation
of Furfural
binding energies at 872.3 and 874.8 eV correspond to the
main lines of Ni0 (2p1/2) and Ni2? (2p1/2) [27]. It can be
found that partial of metallic Ni was oxidised before or
during XPS detection, which contributed to the under-
standing of the high amount of O element.
3.2.1 Effect of Ni Loading Amount on the Conversion
and Product Distribution
As seen in Fig. 4a, the conversion of furfural changes little
within all the range of the Ni loading investigated in this
work, which suggests there is little effect of the loading
amount on the conversion. However, the selectivities
toward cyclopentanol (CPOL) and cyclopentanone
(CPON) vary greatly, which means the Ni loading has great
effect on the product distribution. It can be explained that
when adding the Ni amount, the amount of catalytic active
site increased for Ni is the most important active site in the
reaction. And the varied selectivities were due to the suf-
ficient catalytic active site, which would help to lead to the
further hydrogenation to give CPOL with the combined
effect of hydrogen in the reaction. Additionally, increase in
the loading amount would lead to the decrease in surface
area (see Table 1), and cause particle accumulation (see
3.1.4 TEM Characterization
As can be seen in Fig. 3, most of Ni particles display
outside the CNTs. Thus, the Ni particle size and its dis-
tribution mainly depend on loading content. An obvious
particle accumulation was observed when Ni-loading
increased from 30 to 40 % (see Fig. 3a–d). The histogram
of 30 % Ni/CNTs was given in Fig. 3b, in which the mean
size was 13.6 nm, smaller than 40 % Ni/CNTs (20.4 nm).
TEM result of used 30 % Ni/CNTs (see Fig. 3c) showed
little change compared with reduced one, which were in
accordance with the XRD result (see Fig. 2). In the fol-
lowing section, it was found smaller Ni particle size with a
higher dispersion should be favor to the hydrogenation
reaction.
123

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M. Zhou et al.
Fig. 4 Effect of different influencing factors. a Ni loading amount, b reaction temperature, c H2 pressure, d reaction time
Fig. 3), which would influence the catalyst activity
[27].The selectivity toward CPOL reaches the maximum
catalyzed by 30 % Ni/CNTs (up to 82.5 %). So, the opti-
mum Ni loading is 30 wt%.
the selectivity toward CPOL changes greatly as a result of
the change of temperature. The selectivity toward CPOL
arrives the maximum of 88.7 % at 140 °C. When the
reaction temperature was higher than 140 °C, by-product
pentanediol (POL) began to appear as a result of the further
conversion of CPOL (Fig. 4b). So, considering the energy
consumption and the yield of CPOL, the optimum reaction
temperature is 140 °C.
3.2.2 Effect of Temperature on the Conversion
and Product Distribution
Figure 4b shows the effect of reaction temperature on the
conversion of furfural and the reaction selectivity. As seen
in Fig. 4b, the conversion of furfural increases from 68.8 to
94.4 % when the temperature increases from 110 to 150 °C
and the conversion reaches its maximum at 140 °C (about
96.5 %). Then the conversion decreases a little with the
further increase of the temperature. It also can be seen that
3.2.3 Effect of Hydrogen Pressure on the Conversion
and Product Distribution
Figure 4c shows the effect of the hydrogen pressure on the
conversion of furfural and the reaction selectivities. As seen
from Fig. 4c, the conversion increases with the increase of
123

Model Reaction for Upgrading of Bio-oil
241
the hydrogen pressure.When the hydrogen pressure reaches
5 MPa, the conversion is up to 96.5 %.With the further
increase of the hydrogen pressure, there is almost no change
in the conversion. However, selectivity toward CPOL
improved greatly as a result of the increase of hydrogen
pressure, which was up to 86.3 % when the hydrogen pres-
sure is 5 MPa. And further increase of the hydrogen pressure
shows no obvious improvement in selectivity. The increased
CPOL selectivity when increasing hydrogen pressure could
be explained by the pushing effect of excess hydrogen on the
reaction balance. The reaction balance could also explain
why there did not show a continuous increase when continue
to increase the hydrogen pressure when exceed 5 MPa. So
the optimum hydrogen pressure is 5 MPa.
83.6 % catalyzed by 30 wt% Ni/CNTs under 140 °C and
the initial H2 pressure of 5.0 MPa and reaction time for
10 h. And the possible mechanism for the hydrogenation of
furfural to prepare CPOL was briefly investigated. The Ni/
CNTs composite particles exhibit excellent catalytic per-
formance for the hydrogenation of furfural in water, which
was confirmed to be suitable for initial screening of bio-oil
upgrading, while leaving the investigation of other bio-oil
components the subject of further research.
Acknowledgments Authors are grateful for the financial support
from the National Hi-tech Research and Development Program of
China (863 Program) (2012AA051801) and the National Basic
Research Program of China (973 Program) (2010CB732206).
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4 Conclusion
Ni/CNTs catalysts were prepared by impregnation method.
Furfural could be converted to CPOL with the yield up to
123