Design of an Autoreduced Copper in Carbon Nanotube Catalyst to Realize the Precisely Selective Hydrogenation of Dimethyl Oxalate

Article abstract of DOI:10.1002/cctc.201601503

An autoreduced catalyst that comprised Cu nanoparticles encapsulated inside the nanochannels of carbon nanotubes (Cu@CNTs) was designed and prepared. As a result of the interaction of Cu species with the electron-deficient interior surface of the CNTs, calcination could realize the autoreduction of copper oxide directly with CNTs as the reductant. In the hydrogenation of dimethyl oxalate (DMO), the autoreduced Cu@CNTs catalyst, which did not need to be prereduced, exhibited an excellent catalytic activity, high target product selectivity, and high catalytic efficiency. Furthermore, the effect of the calcination temperature on the autoreduction degree of Cu@CNTs and the product selectivity in DMO hydrogenation were investigated in detail. The results showed that the autoreduction degree could be tuned easily by changing the calcination temperature, and the highest selectivity of ethanol could be obtained over the catalyst calcined at 500 °C. The findings obtained will inspire the development of other autoreduced catalysts, the reduction degree and catalytic performance of which can be tuned as desired.

Full text of DOI:10.1002/cctc.201601503

Accepted Article
Title: Designing auto-reduced Cu@CNTs catalysts to realize the
precisely selective hydrogenation of dimethyl oxalate
Authors: Peipei Ai, Minghui Tan, Yuki Ishikuro, Yuta Hosoi, Guohui
Yang, Yoshiharu Yoneyama, and Noritatsu Tsubaki
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Designing auto-reduced Cu@CNTs catalysts to realize the
precisely selective hydrogenation of dimethyl oxalate
Peipei Ai,[a] Minghui Tan,[a] Yuki Ishikuro,[a] Yuta Hosoi,[a] Guohui Yang,[a] Yoshiharu Yoneyama,[a] and
Noritatsu Tsubaki*[a]
Abstract: An auto-reduced Cu@CNTs catalyst with copper (Cu)
nanoparticles encapsulated inside the nano-channels of carbon
nanotubes (CNTs) was designed and prepared. Due to the
interaction of Cu species with electron deficient interior surface of
CNTs, calcination process could directly realize the auto-reduction of
Cu oxide with CNTs itself as reductant. In dimethyl oxalate (DMO)
hydrogenation reaction, the auto-reduced Cu@CNTs catalyst
without pre-reduction exhibited excellent catalytic activity, high target
product selectivity and catalytic efficiency. Furthermore, the effect of
calcination temperature on the auto-reduction degree of Cu@CNTs
and the product selectivity in DMO hydrogenation were investigated
in detail. The reaction results disclosed that the auto-reduction
degree could be tuned facilely by changing the calcination
temperature, and the highest selectivity of ethanol could be obtained
over the Cu@CNTs-500 catalyst. The findings obtained in this study
will inspire the development of other auto-reduced catalysts whose
reduction degree and catalytic performance can be tuned facilely as
desired.
was that the transition metal oxide, such as Fe, Co, Ni,
encapsulated inside the CNTs could be auto-reduced to its
[15-17]
metallic state if being calcined in an inert gas atmosphere.
[15]
Chen et al.
presented direct experimental evidence of facile
reduction of Fe2O3 nanoparticles located inside the nano-
channels of CNTs at 600 °C. This is the first example of
transition metal obtained through direct reduction of metal oxide
by the CNTs support in helium atmosphere. Recently, two
Co/CNTs catalysts activated respectively in nitrogen atmosphere
and hydrogen atmosphere were prepared and tested for NH3
[18]
decomposition reaction.
The catalytic activity of Co/CNTs
catalyst reduced by CNTs support in nitrogen atmosphere was
almost 1.2 times higher than that of Co/CNTs catalyst reduced in
hydrogen atmosphere. The auto-reduced behavior of the CNTs-
based catalyst provides a facile way to reduce active metal
species with lower cost and simplified process, which is
considerably significant for many catalytic reactions.
Introduction
Carbon nanotubes (CNTs), since being first reported in 1991,
has drawn increasing attention for being used as novel catalyst
[1-3]
support.
Many studies have shown the advantages of CNTs
as catalyst support for some chemical reactions, such as
ammonia decomposition/synthesis, Fischer-Tropsch synthesis
[4-7]
and hydrogenation reaction.
This special one-dimensional
Scheme 1. Alternative synthesis approach for EG or ethanol from syngas via
DMO hydrogenation
carbon nanomaterial exhibits many unique features, including
well-defined nano-channel, graphene-like tube wall and sp2
hybridized carbon framework, making it an intriguing catalyst
[8, 9]
support.
As catalyst support, the CNTs can eliminate the
Ethanol and ethylene glycol (EG) are attractive products and
versatile feedstocks for the synthesis of various products (e.g.,
chemicals, fuels and polymers), which are mainly produced from
petroleum-derived ethylene in the present industrial approach.
intraparticle mass transfer in the reaction medium, offer strong
metal-support interaction and promote the reducibility of the
[10-12]
metals loaded inside its channels.
In addition, the well-
defined nano-channels of CNTs were constructed by curved
graphene layers. As a result, the deformed sp2 hybridization in
[19-21]
With the decline of crude oil reserve, developing an
alternative synthesis method for EG or ethanol from syngas
(mixture of CO and H2) via dimethyl oxalate (DMO)
hydrogenation has attracted increasing attention (Scheme 1).
DMO production from syngas is already commercialized by UBE
Industries Ltd. of Japan using Pd-based catalyst and NOx. For
DMO hydrogenation reaction, the copper (Cu)-based catalyst is
extensively studied, since its outstanding catalytic activity and
low cost if being used for industrial application. [22-24] Copper is a
the graphene walls shifts π electron density from the inner
[8, 13, 14]
surface to the outer surface of CNTs,
therefore changing
the physical, chemical and catalytic properties of the CNTs-
based catalysts. An interesting phenomenon arising with CNTs
[a]
P. Ai, Dr. M. Tan, Y. Ishikuro, Y. Hosoi, Dr. G. Yang, Dr. Y.
Yoneyama, Dr. N. Tsubaki
typical transition metal, and its oxides have attracted extensive
[25-28]
Department of Applied Chemistry, School of Engineering
University of Toyama
Gofuku 3190, Toyama 930-8555 (Japan)
Fax: +(81)76-445-6846
attention in many reactions,
e.g., hydrogenation, coupling
reaction, methanol synthesis and water gas shift reaction. Prior
to application for reactions, almost all the Cu-based catalysts
needed to be reduced in hydrogen atmosphere, since the
E-mail: tsubaki@eng.u-toyama.ac.jp
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10.1002/cctc.201601503
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catalytic active sites are generally Cu0 or Cu+. Nevertheless, this
additional reduction step is not beneficial for industrialization
from the practical viewpoint of reducing cost and simplifying
process. Therefore, it is significant that the high reducibility of
CNTs can be utilized to prepare an auto-reduced Cu-based
catalyst. Moreover, although the common and effective catalysts
for DMO hydrogenation were silica supported Cu catalysts in
most researches, the silica support could be a fatal flaw in DMO
Cu@CNTs-350 catalyst, the TEM images of Cu nanoparticles
encapsulated inside the nano-channels of CNTs were presented
in Figure 1. The CNTs had opened tips, and majority of the Cu
nanoparticles (87%) with the size of 10 to 20 nm dispersed well
inside the nano-channels of CNTs, indicating that the sonication-
assisted impregnation method successfully afforded a catalyst
with Cu nanoparticles encapsulated inside CNTs. A similar result
has also been reported by Wang et al. [30] It can be explained by
the capillary force that effectively helped to suck the metal salt
solution into the inner channels of CNTs. For comparison, a
Cu/CNTs-350 reference catalyst with Cu supported on the outer
surface of CNTs was also prepared by the same method of
Cu@CNTs-350, except adding a specific pre-filling procedure on
CNTs before the Cu(NO3)2 solution impregnation process. For
Cu/CNTs-350 reference catalyst, Cu nanoparticles had been
deposited evenly on the outer surface of CNTs observed from
TEM, as shown in Figure S1. Cu nanoparticles loaded outside of
CNTs was attributed to the fact that xylene was pre-filled in the
CNTs channels at the assistance of ultrasound, inhibiting the
entrance of Cu(NO3)2 solution into CNTs channels. Furthermore,
TEM micrographs of Figure 1 indicated that the inner diameter
and wall thickness of CNTs were about 20-30 nm and 10-20 nm,
respectively. The physicochemical properties of Cu@CNTs-350
and reference catalyst were listed in Table S1. The average
pore diameter (23.4 nm) of Cu@CNTs-350 calculated from N2
adsorption-desorption data was consistent with the TEM results.
hydrogenation process because of the leaching of the silica
under the gas phase reaction condition containing methanol
[29]
.
Thus, the development of the non-silica catalysts for the DMO
hydrogenation will have increased interests in both academia
and industry. Many studies have shown the advantages of CNTs
as catalyst support for some chemical reactions. However, to the
best of our knowledge, CNTs-encapsulated Cu catalyst used for
DMO hydrogenation to synthesize EG and ethanol was rarely
reported until now. Moreover, the interaction of Cu species with
electron deficient interior surface of CNTs during the calcination
process was barely investigated, and the auto-reduced
Cu@CNTs catalyst without pre-reduction has never been used
for DMO hydrogenation.
In this report, by combining the advantages of CNTs and the
reduction requirement of Cu-based catalysts, an auto-reduced
Cu@CNTs catalyst consisting of Cu nanoparticles encapsulated
inside the nano-channels of CNTs was designed and prepared
for the selective hydrogenation of DMO. The auto-reduction
behavior and catalytic performance for DMO hydrogenation of
Cu@CNTs catalyst were investigated. By changing the catalyst
calcination temperature, we could facilely tune the auto-
reduction degree of Cu@CNTs catalysts. The effect of varied
calcination temperature on the reaction direction controlling in
DMO hydrogenation reaction over the Cu@CNTs catalysts was
studied in detail.
▼
▼ C ◆ CuO ▲ Cu2O ★ Cu
▲
▲
◆
◆
★
▲
(c)
◆
◆
◆
Results and Discussion
▲
★
▲
▲
(b)
(a)
◆
Physicochemical features of Cu@CNTs-350 and reference
catalysts
10
20
30
40
50
60
70
80
2Theta (degree)
(a)
(b)
Figure 2. XRD patterns of (a) Cu/AC-350, (b) Cu/CNTs-350 and (c)
Cu@CNTs-350
Since the Cu nanoparticles were encapsulated inside the
nano-channels of CNTs, the crystal phase of Cu nanoparticles of
Cu@CNTs catalyst would be changed during calcination
process. The effect of CNTs support on the crystal phase of Cu
nanoparticles after calcination could be confirmed by XRD.
Figure 2 showed the XRD patterns of Cu@CNTs-350 and the
reference catalysts, Cu/AC-350 and Cu/CNTs-350, only after
calcination in argon atmosphere without further reduction by
hydrogen. For the XRD patterns of Cu@CNTs-350 and
Cu/CNTs-350 catalysts, a series of weak diffraction peaks
ascribed to the CuO (JPCDS 44-0706), Cu2O (JCPDS 34-1354)
and Cu phase (JCPDS 01-1241) were observed clearly,
whereas only the peaks assigned to CuO phase could be
detected on Cu/AC-350 catalyst. Comparing the XRD pattern of
Cu@CNTs-350 catalyst with that of Cu/CNTs-350 catalyst, more
metallic Cu and monovalent Cu could be detected on the
(c)
(d)
CNTs
CNTs
Cu
Cu
Figure 1. TEM images of Cu@CNTs-350: (a) and (b) low magnification, (c)
and (d) high magnification
TEM is a convenient and powerful technique that provides direct
information about the microstructure of samples. For the
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Cu@CNTs-350 catalyst. These results indicated that the auto-
reduction of Cu nanoparticles encapsulated inside CNTs
channels was easier to realize than those loaded outside of
CNTs channels and other carbon materials.
in Figure 3(c). Compared with the pure CNTs, the weight loss
line of Cu@CNTs-350 decreased sharply. The CO2 and CO
desorption curves obtained from TPD-MS were shown in Figure
o
3(d). The CO2 desorption peak around 200 C overlapped with
Calcination of Cu@CNTs precursor is vitally important in
catalyst preparation process, which directly reflects the
interaction between Cu species and CNTs support. In order to
investigate the interaction of Cu species with CNTs support
during the calcination process, TG and TPD-MS were used to
study the thermodynamic behavior of the process through the
thermal weight loss and desorption.
that of the pure CNTs (Figure 3(b)), mainly attributed to the
decomposition of carboxyl groups on the surface of CNTs. Due
to the presence of Cu oxide, the CO2 desorption became very
complicated. Considering the fact that there was almost no CO
desorption for pure CNTs below 650 oC, therefore the CO
desorption could be utilized to investigate the interaction
between Cu species and CNTs support. The CO desorption
curve in Figure 3(d) exhibited three major desorption peaks in
the range of 179-200 oC, 200-240 oC and 243-316 oC,
respectively. It was reported that the reduction temperature of
highly dispersed CuxO species was lower than that of bulk CuxO
To eliminate the interference from CNTs support, the TG
curve of the pretreated pure CNTs calcined in argon atmosphere
was shown in Figure 3(a). For pure CNTs, a 18% weight loss
could be detected when the sample was heated to 800 oC. A
same temperature-rising program was also applied for TPD-MS
on the pure CNTs, and the outlet gas was detected by mass
spectrometry. Two CO2 desorption peaks at 209 and 655 oC
were observed respectively, while only one CO desorption peak
[34]
species due to their relatively higher exposed surface area.
o
Therefore, the first CO desorption peak in 179-200 C could be
attributed to the reduction of highly dispersed CuO to Cu2O, and
the CO desorption peak at 200-240 oC was assigned to the
partial reduction of bulk CuO to Cu2O. With increasing the
temperature, a portion of highly dispersed Cu2O was reduced to
metallic Cu, as suggested by the emergence of CO peak
between 243 and 316 oC. Moreover, as given by Figure 3(e),
there were no desorption of CO and CO2 when the temperature
was kept at 350 oC for 3h, indicating that the Cu species have no
obvious change in this constant calcination process.
o
could be detected at the temperature higher than 650 C (Figure
3(b)). CO2 desorption at 209 oC could be attributed to the
decomposition of carboxyl groups chemically bonded on the
surface of CNTs, [31] and the CO2 and CO desorption above 650
oC were ascribed to the decomposition of phenol, carbonyl and
ether groups. [32, 33]
The reduction of Cu oxide could be attributed to the
interaction of Cu species with interior surface of CNTs. Due to
the shift of π electron density from the inner surface to the outer
surface of CNTs, the interior surface of CNTs was in an
electron-deficient state. [8, 13, 14] Chen et al. have reported that the
interaction between the electron-deficient CNTs interior surface
and the metallic oxide could lead to weakened bond strength of
100
209
655
CO
90
80
70
60
50
40
2
CO
(b)
(a)
metal-oxygen and hence lower the activation energy for its
100 200 300 400 500 600 700 800
100 200 300 400 500 600 700 800
o
o
Temperature ( C)
[15]
Temperature ( C)
reduction.
Therefore, the Cu oxide could be facilely reduced
through its interaction with CNTs support in inert atmosphere,
and the oxygen atoms of Cu oxide would react with carbon
atoms to realize the auto-reduction of Cu@CNTs.
100
90
CO
2
80
70
(c)
(d)
CO
60
205
177
50
100
150
200
250
300
350
50
100
150
200
250
C)
300
350
o
Temperature (o
Temperature ( C)
(c)
261
CO
CO
209
2
(b)
(e)
193
0
20 40 60 80 100 120 140 160 180
253
Time (min)
(a)
Figure 3. TG and TPD-MS curves during calcination process of (a, b) CNTs,
(c, d) Cu@CNTs-350 and the TPD-MS curve of (e) Cu@CNTs-350 thermal
treatment at 350 oC for 3h
100
200
300
400
600
700
800
500o
Temperature ( C)
Figure 4. H2-TPR profiles of catalysts (a) Cu/AC-350, (b) Cu/CNTs-350 and
(c) Cu@CNTs-350
To investigate the calcination process of Cu@CNTs-350, the
operating conditions of TG and TPD-MS were identical for the
calcination process of Cu@CNTs-350. The sample before
analysis was only dried at 120 oC without calcination. The TG
curve of Cu@CNTs-350 under argon atmosphere was displayed
The reduction of Cu@CNTs-350 catalyst in hydrogen
atmosphere also revealed the reducibility of Cu species
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encapsulated inside CNTs enhanced by CNTs support. Figure 4
showed the H2-TPR profiles of Cu@CNTs-350 and the reference
catalysts, Cu/AC-350 and Cu/CNTs-350. It was reported that the
lower reduction temperature peak was attributed to the reduction
Table 1. Catalytic activity of different catalysts for DMO hydrogenation
Sel.(%)[c]
of highly dispersed CuxO, and the higher reduction temperature
Reduction
Conv.
(%)
Catalyst[a]
[34]
[b]
was originated from the reduction of bulk CuxO.
For
Ethanol
MG
EG
Others
Cu@CNTs-350 catalyst, the both reduction peaks emerged at
lower temperature than those of reference catalysts,
demonstrating its enhanced reducibility due to the existence of
close interaction between Cu species and CNTs support.
Moreover, it is worth to be noted that the decreasing hydrogen
consumption of the first peak on the Cu@CNTs-350 and
Cu/CNTs-350 catalysts, compared with that on Cu/AC-350
catalyst, was observed, which further confirmed the partial
reduction of the well dispersed CuxO by CNTs support during
catalyst calcination process under argon atmosphere.
Cu/ACs-
350
Cu/ACs-
350H
Cu/CNTs-
350
Cu/CNTs-
350H
Cu@CNTs-
350
Cu@CNTs-
350H
×
√
×
√
×
√
6.2
7.2
7.6
7.0
4.4
7.9
22.8
80.4
46.1
20.6
8.6
12.4
41.0
62.2
76.6
87.3
71.6
0.0
99.8
84.0
98.9
99.4
99.5
5.3
10.2
10.4
3.9
0.9
4.1
1.5
[a] Reaction conditions: 240 oC, 2.5 MPa, LHSV = 0.2 h−1, H2/DMO = 200,
reaction time= h, 15 wt% DMO/1, 4-dioxane as feed. [b] Reduction
Catalytic activity of Cu@CNTs-350 and reference catalysts
6
conditions: the catalysts were pre-reduced in a 5%H2-95%N2 atmosphere at
300 oC for 3 h. [c] MG: methyl glycolate, EG: ethylene glycol, Others mainly
consist of methyl methoxyacetate, methyl acetate, 1, 2-propanediol and 1, 2-
butanediol.
DMO conversion
EG selectivity
Ethanol selectivity
100
80
60
40
20
0
The low catalytic activity of Cu/AC-350 and Cu/CNTs-350
catalysts showed in Figure 5 was mainly due to the insufficient
surface area of Cu0 and Cu+ active sites (Figure 2). Therefore,
the catalytic activity experiments on the Cu/AC-350H, Cu/CNTs-
350H and Cu@CNTs-350H catalysts with addition of
a
conventional reduction step were also conducted. As shown in
Table 1, although the DMO conversion of Cu/AC-350H and
Cu/CNTs-350H catalysts was obviously enhanced, but their EG
selectivity was still lower than that of Cu@CNTs-350 catalyst.
These results also demonstrated that the auto-reduced
Cu@CNTs catalyst even without pre-reduction exhibited
excellent catalytic activity and high target product selectivity,
much better than other reference Cu-based catalysts with or
without reduction. In addition, considering Ag was frequently
used for DMO hydrogenation, an Ag@CNTs-350 catalyst was
also prepared and utilized in DMO hydrogenation to EG. As
shown in the XRD pattern of Ag@CNTs-350 catalyst (Figure S2),
the auto-reduction of Ag oxide could also realize in calcination
process with CNTs itself as reductant. Compared with
Cu@CNTs-350 catalyst, the catalytic activity of Ag@CNTs-350
and Ag@CNTs-350H catalysts was lower than that of
Cu@CNTs-350 catalyst, especially for ethylene glycol selectivity
(Table S2). The main product of DMO hydrogenation over
Ag@CNTs-350 and Ag@CNTs-350H catalyst at 240 oC was MG.
This result was mainly due to the relatively weak hydrogenation
0
0
0
35
s-
Ts-35
/AC-35
/CNT
Cu
@CN
Cu
Cu
Figure 5. Catalytic activity of Cu@CNTs-350 and reference catalysts.
Reaction conditions: 240 oC, 2.5 MPa, liquid hourly space velocity (LHSV) =
0.2 h−1, H2/DMO = 200, reaction time= 6 h
The catalytic performances of Cu@CNTs-350, the reference
catalysts of Cu/AC-350 and Cu/CNTs-350 for DMO
hydrogenation were evaluated in a fixed-bed reactor at 240 C
o
and 2.5 MPa as in Figure 5. It has been reported that the main
active sites for Cu-based catalyst in DMO hydrogenation were
Cu0 and Cu+.
Thus, a reduction step was supposed to be
[35, 36]
indispensable prior to catalytic test. As suggested by XRD and
TPD-MS measurements (Figure 2 and Figure 3), Cu oxide could
be reduced by CNTs support under argon atmosphere in
calcination process. Therefore, the auto-reduced Cu@CNTs-350
catalyst without any further reduction was directly used for DMO
hydrogenation, and the same operation was adopted for the
reference catalysts Cu/AC-350 and Cu/CNTs-350. The selective
hydrogenation of DMO was based on the following consecutive
reaction scheme: DMO hydrogenation to methyl glycolate (MG),
then MG hydrogenation to EG, and followed by deep
hydrogenation of EG to ethanol (Scheme 1). As compared in
Figure 5, the DMO conversion and EG selectivity of Cu@CNTs-
350 catalyst were 99.4% and 87.3% separately, obviously
superior to those of reference catalysts. The highest catalytic
performance of Cu@CNTs-350 may be ascribed to the
existence of abundant Cu0 and Cu+ formed by auto-reduction
behavior of CNTs support.
property of the Ag active sites compared to Cu [37-40]
.
The catalytic efficiency of catalyst is important for selective
hydrogenation of DMO from the practical viewpoint. Thus, the
catalytic efficiency of auto-reduced Cu@CNTs-350 catalyst was
evaluated at different LHSV and shown in Figure 6(a). Although
increasing the LHSV from 0.2 h-1 to 1.6 h-1, the catalyst activity
for DMO hydrogenation was still stable. In addition, to our
surprise, the DMO conversion (99.9%) and EG selectivity
(96.0%) were significantly higher than those reported in the
literature even at a relatively high LHSV of 1.6 h-1. [41, 42] The high
catalytic efficiency here could be attributed to the larger inner
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10.1002/cctc.201601503
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diameter of CNTs to some extent, which provided an enough
reaction space for DMO hydrogenation. The channel of CNTs
with special tubular structure could act as a microreactor. When
DMO and the intermediate (MG) diffused through the channels
of CNTs, they would be in further contact with other Cu active
sites loaded inside the channels of CNTs to produce more EG
(Scheme 2).
Effect of calcination temperature on auto-reduction degree
of Cu@CNTs catalysts
Since the auto-reduction behavior proceeded in the calcination
process, it is reasonable to postulate that the auto-reduction
degree of Cu@CNTs catalyst could be tuned by altering the
calcination temperature.
▲ Cu O
★ Cu
■ C
★
2
Cu
■
◆ CuO
★
★
H2
(e)
(d)
(c)
(b)
H2
★
■
■
★
▲
H2
H2
H2
Cu
▲
▲
▲
▲
★
★
★
▲
▲
★
▲
★
★
Scheme 2. DMO hydrogenation in the channel of Cu@CNTs catalyst
▲
★
▲
◆
Moreover, the stability of catalyst is of great importance
besides catalytic activity and selectivity. The catalytic activity as
a function of time on stream for the Cu@CNTs-350 catalyst was
investigated. As demonstrated in Figure 6(b), the Cu@CNTs-
350 catalyst retained its high catalytic activity even after 200 h.
Therefore, the auto-reduced Cu@CNTs catalyst developed in
this report had great potential for industrial application as a
highly efficient and stable catalyst for DMO hydrogenation
without pre-reduction step.
◆
▲
▲
★
(a)
10
◆
★
20
30
40
50
60
70
80
2Theta (degree)
Figure 7. XRD patterns of (a) Cu@CNTs-350, (b) Cu@CNTs-450, (c)
Cu@CNTs-500, (d) Cu@CNTs-550 and (e) Cu@CNTs-650
XRD characterization was carried out to investigate and
compare the phase evolution for the auto-reduced Cu@CNTs as
a
function of calcination temperature (Figure 7). The
(a)
DMO
characteristic diffraction peaks of CuO on the Cu@CNTs-450
catalyst disappeared totally, which indicated the full reduction of
CuO into Cu2O and Cu by CNTs support at temperature above
450 C. There was no obvious change on the valence state of
Cu species when the calcaination temperature was increased
from 450 oC to 550 oC. However, elevating the calcination
100
80
60
40
20
0
100
80
60
40
20
0
EG
o
o
o
temperature from 450 C to 550 C made the diffraction peak of
metallic Cu gradually stronger, while the diffraction peak of Cu2O
became weaker, indicating the step-by-step reduction of Cu2O to
Cu with increasing calcination temperature. Further increasing
the calcination temperature to 650 oC resulted in the complete
reduction of Cu2O to Cu, as evidenced by the disappearance of
characteristic diffraction peaks of Cu2O. This phenomenon
suggested that the auto-reduction degree of Cu oxide strongly
depended on the calcination temperature in argon atmosphere.
Based on the XRD results, therefore, the auto-reduction degree
of Cu@CNTs catalyst could be facilely tuned by tuning the
calcination temperature.
The auto-reduction degree of Cu@CNTs at varied
calcination temperature was also investigated by H2-TPR. In
Figure 8, the position, intensity and shape of the reduction peaks
of Cu@CNTs catalysts were significantly different at varied
calcination temperature. With increasing the Cu@CNTs catalyst
calcination temperature, the reduction temperature of Cu
species shifted toward lower temperature. In addition, the
hydrogen consumption area decreased obviously with
increasing the calcination temperature to 550 oC, because a
fraction of CuO had been auto-reduced to Cu2O or Cu by CNTs
support. This result was consistent with the above XRD findings.
For Cu@CNTs-650, since complete reduction of CuO or Cu2O to
Cu, thus there was no hydrogen consumption peak on its TPR
Ethanol
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
LHSV (h-1)
(b)
100
80
60
40
20
0
Conversion over DMO
Selectivity over EG
0
25
50
75
100
125
150
175
200
Time on stream (h)
Figure 6. Catalytic (a) efficiency and (b) stability of Cu@CNTs-350
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profile. The interaction between Cu oxide and CNTs support was
believed to be responsible for the change of auto-reduction
degree of Cu@CNTs at varied calcination temperature. The
elevated calcination temperature here could enhance the
interaction of Cu oxide with CNTs support, finally reaching
higher reduction degree of Cu@CNTs catalyst.
respectively. As shown by Figure 9(a), the weight loss increased
gradually with elevating the calcination temperature. Compared
with Cu@CNTs-350 catalyst, the CO desorption curve of
Cu@CNTs-450 catalyst showed four major peaks. The emerged
CO desorption peak at 316-419 oC is mainly due to the reduction
of bulk CuO to bulk Cu2O and highly dispersed Cu2O to metallic
Cu. Although no new CO desorption peak emerged when the
temperature was gradually increased to 650 oC, the CO
desorption curve was still in upward trend, which indicated the
continuous reduction of Cu2O to metallic Cu. The complete
reduction of Cu2O to metallic Cu was accomplished at the
temperature of 650 oC, which had also been demonstrated by
XRD (Figure 7) and H2-TPR (Figure 8) analysis results.
(e)
131
(d)
145
179
(c)
151
170
Table 2. Physicochemical features of Cu@CNTs calcined at different
temperature
(b)
177
205
Cu loading (wt%)[a]
SCu0 (m2/g)[b]
SCu+ (m2/g)[c]
(a)
Catalyst
Cu@CNTs-350
Cu@CNTs-450
Cu@CNTs-500
Cu@CNTs-550
Cu@CNTs-650
14.3
14.8
15.0
15.3
15.9
4.6
3.0
5.3
3.4
2.8
0
100
200
300
400
500
600
700
o
Temperature ( C)
15.6
18.8
22.1
23.3
Figure 8. H2-TPR profiles of (a) Cu@CNTs-350, (b) Cu@CNTs-450, (c)
Cu@CNTs-500, (d) Cu@CNTs-550 and (e) Cu@CNTs-650
(a)
100
90
100
90
[a] Cu loading determined by ICP. [b] The N2O pulse adsorption was
employed to measure the exposed surface area of Cu0 (SCu0). [c] The
exposed surface area of Cu+ (SCu+) was determined from CO chemisorption
isotherms.
80
80
70
70
Cu@CNTs-450
Cu@CNTs-500
60
60
100
200
300
400
100
200
300
400
500
Temperature (o
Temperature (o
C)
C)
100
90
100
90
In DMO hydrogenation reaction,
a cooperative effect
80
80
between Cu0 and Cu+ of Cu-based catalyst played a key role on
70
70
[43]
products selectivity as proposed by Gong et al.
According to
Cu@CNTs-650
Cu@CNTs-550
60
60
our studies above, the calcination temperature of Cu@CNTs
catalyst could effectively tune the auto-reduction degree of Cu
species, which changed the surface area of Cu0 and Cu+ active
100
200
300
400
500
600
100
200
300
400
500
Temperature (o
Temperature (o
C)
C)
(b)
Cu@CNTs-450
sites. Table 2 illustrated the physicochemical features of
Cu@CNTs-500
Cu@CNTs calcined at different temperature, wherein the
exposed surface area of Cu0 and Cu+ was determined by N2O
pulse adsorption and CO chemisorptions, respectively. From
N2O pulse adsorption results, large Cu0 surface area was
exhibited in the auto-reduced Cu@CNTs catalysts, and the
surface area gradually increased with increasing calcination
temperature, demonstrating the higher reduction degree of
Cu@CNTs catalyst under the higher calcination temperature. In
addition, the Cu+ surface area first increased and then
decreased with the increasing calcination temperature, and the
largest Cu+ surface area was obtained over the Cu@CNTs-450
catalyst. With the increase of calcination temperature from 450
to 650 oC, the Cu+ surface area decreased simultaneously,
possibly due to the reduction of more Cu+ to Cu0 by CNTs
support under elevated temperature. These findings indicated
that the auto-reduction degree of Cu@CNTs catalyst strongly
depended on the calcination temperature, as also supported by
the XRD (Figure 7), TPR (Figure 8) and TPD-MS (Figure 9)
analysis results. In addition, the Cu loading in the calcined
Cu@CNTs-x catalysts determined by ICP was listed in Table 2.
100
200
300
400
500
100
200
300
400
Temperature (o
o
C)
Temperature ( C)
Cu@CNT-650
Cu@CNTs-550
100
200
300
400
500
100
200
300
400
500
600
o
Temperature (o
Temperature ( C)
C)
Figure 9. (a) TG and (b) TPD-MS of Cu@CNTs-x (x=450, 500, 550, 650)
More detailed information on the interaction between Cu
oxide and CNTs support at different calcination temperature in
argon atmosphere could be further disclosed by TG and TPD-
MS given in Figure 9(a) and Figure 9(b). A same temperature-
rising program to that of calcination process was used for TG
and TPD-MS analysis. The temperature was increased from 50
oC to 450, 500, 550 or 650 oC at a ramping rate of 3 oC/min in Ar,
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The Cu loading of Cu@CNTs-350 and Cu@CNTs-450 was
slightly lower than the theoretical value (15%), which might be
due to the presence of residual Cu in the solution that was not
adsorbed by the catalysts. With increasing the calcination
temperature, the Cu loading was increased gradually with
elevating calcination temperature, due to the different weight
loss of CNTs support in calcination processing.
Table 3. The catalytic performance of Cu@CNTs calcined at different
temperature.
Sel.(%)[b]
Catalyst[a]
Conv. (%)
Ethanol
MG
EG
Others
Cu@CNTs-350
Cu@CNTs-450
Cu@CNTs-500
Cu@CNTs-550
Cu@CNTs-650
99.4
99.5
99.5
99.5
99.7
7.9
1.8
2.2
0.9
3.0
1.1
87.4
61.1
17.5
60.5
66.7
2.9
2.6
3.7
2.4
3.5
Catalytic performance of Cu@CNTs-x catalysts for DMO
hydrogenation
34.1
77.9
34.1
28.7
The calcination temperature of Cu@CNTs catalyst could
effectively tune the auto-reduction degree of Cu specie and the
surface area of Cu0 and Cu+ active sites, which play an
important role in the DMO hydrogenation. To further uncover the
effect of calcination temperature on the catalytic performance of
Cu@CNTs-x catalysts, the DMO hydrogenation over the auto-
reduced Cu@CNTs-x was examined. DMO hydrogenation can
be used to produce EG or ethanol, and the product selectivity
are greatly influenced by the reaction conditions and the catalyst
active sites composition. The catalytic activities of the
Cu@CNTs-x catalysts calcinated in different temperature for the
hydrogenation of DMO to EG were evaluated at 240 oC. As
shown in Table S3, the DMO conversion and EG selectivity had
almost no obvious changes over Cu@CNTs-x catalysts
calcinated in different temperature. From thermodynamic
perspective, the selective synthesis of products from DMO
hydrogenation is primarily affected by the reaction temperature
[44]. Therefore, it was a reasonable speculation that the relatively
low reaction temperature (such as 240 oC) was favorable for the
formation of EG, whereas inhibited the intramolecular
dehydration of EG to ethanol which was thermodynamically
favorable at high temperature (such as 260 oC-280 oC). This
may be the reason why the EG selectivity almost had no obvious
changes over Cu@CNTs-x catalysts. Ethanol formation through
DMO hydrogenation involves many processes, such as the
cleavage of C-O bonds, C=O bonds hydrogenation and
subsequent activation of O-H bands, which requires the catalyst
owning well-optimized catalytic activity. Therefore, the ethanol
selectivity in DMO hydrogenation can be adopted to evaluate the
catalytic performance of Cu@CNTs-x catalysts.
[a] Reaction conditions: 270 oC, 2.5 MPa, LHSV = 0.2 h−1, H2/DMO = 200,
reaction time= 6 h, 15 wt% DMO/1, 4-dioxane as feed. [b] MG: methyl
glycolate, EG: ethylene glycol, Others mainly consist of methyl
methoxyacetate, methyl acetate, 1, 2-propanediol and 1, 2-butanediol.
The catalytic performances of Cu@CNTs-x calcined at
different temperature for DMO hydrogenation to ethanol were
shown in Figure 10 and Table 3. All the auto-reduced
Cu@CNTs-x catalysts exhibited excellent catalytic activity with
DMO conversion higher than 99%. Along with the increase of
calcination temperature of the Cu@CNTs-x catalysts, the
selectivity of ethanol first increased and then decreased, and the
highest selectivity of ethanol was obtained over the Cu@CNTs-
500 catalyst. This result indicated that the calcination
temperature of Cu@CNTs catalysts had significant effect on the
ethanol selectivity, via tuning the surface area of Cu0 and Cu+
active sites to optimize the ethanol selectivity. However, no
consensus has been reached on the precise roles of Cu0 and
Cu+. In relation to the catalytic functions of Cu0 and Cu+ active
sites in DMO hydrogenation, it is generally accepted that Cu0
sites dissociatively absorb hydrogen molecules and absorb
intermediates through the hydroxyl group to facilitate the
dehydration reaction, and that Cu+ sites strongly bind and
activate the C-O bonds and C=O bonds [43, 44]. In addition, Cu+
active sites can also function as Lewis acidic sites to activate O-
H via the electron long pair in oxygen [41]. On this account, both
Cu0 and Cu+ active sites play indispensable role in the
hydrogenation of DMO to ethanol. As in Table 2 and Figure 10,
with the increased Cu0 surface area determined by the
increased Cu@CNTs calcination temperature from 350 oC to
650 oC, the selectivity of ethanol was enhanced and reached the
100
DMO
100
99
80
98
Ethanol
60
o
97
96
95
40
20
0
maximum at Cu@CNTs calcination temperature of 500 C. This
trend means that Cu0 was not the only active sites for this
consecutive reaction. It is referred that Cu+ acted as Lewis acid
sites to catalyze EG to ethanol as dehydration catalytic site,
besides Cu0 site contribution. For Cu@CNTs-550 and
Cu@CNTs-650 catalysts, both had very high Cu0 surface area
but very low Cu+ surface area. Consequently, EG had no
enough Cu+ Lewis acid sites to be converted to ethanol on these
two catalysts, resulting in decreased ethanol selectivity.
Correlating the ethanol selectivity and surface area of Cu0 and
Cu+ active sites, we observed that the ethanol selectivity linearly
increased with increasing Cu0 surface area from 4.6 to 18.8 m2/g,
whereas it no longer remained this trend when the Cu0 surface
350
400
450
500
550
600
650
o
Temperature ( C)
Figure 10. The catalytic performance of Cu@CNTs-x (x=350, 450, 500, 550,
650). Reaction conditions: 270 oC, 2.5 MPa, LHSV = 0.2 h−1, H2/DMO = 200,
reaction time= 6 h.
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under argon atmosphere for 3 h (heating rate: 3 oC/min), respectively.
The obtained catalysts were denoted as Cu@CNTs-x, where the “x”
stands for the calcination temperature. For comparison, two reference
catalysts, Cu/AC-350 and Cu/CNTs-350 (Cu-outside-CNTs), were also
prepared by the same method to that of Cu@CNTs-350. For Cu/CNTs-
350, a specific pre-filling procedure on CNTs was used before the
Cu(NO3)2 solution impregnation process. Briefly, the CNTs was first pre-
impregnated with 1 ml xylene at the assistance of ultrasound, inhibiting
the entrance of Cu(NO3)2 solution into its inner wall. The Cu theoretical
loading amount on all the catalysts was 15 wt%.
area further increasing from 18.8 to 23.2 m2/g but primarily
decreased with the decreased Cu+ surface area. According to
the results obtained above, a hypothesis was proposed as that
when the Cu0 surface area is below a certain value, the ethanol
selectivity linearly increases with increasing Cu0 surface area,
whereas it is primarily affected by the Cu+ surface area. This
result is in agreement with a recent report from Ma et al., who
suggested that the Cu0 and Cu+ active sites competed against
each other to exert a dominant effect on the catalytic activity for
hydrogenation of methyl acetate [45]. In summary, the calcination
temperature has strong influence on the auto-reduction degree
and products distribution, especially for ethanol selectivity, over
the Cu@CNTs catalysts.
Catalyst characterization
Transmission electron microscopy (TEM) was performed on a JEM-
2100UHR (JEOL) at an acceleration voltage of 120 kV. X-ray powder
diffraction (XRD) spectra for the calcined catalysts were recorded by a
Rigaku D/max-2550 V diffractometer employing Cu Kα radiation (λ= 1.54
Å), 40 kV tube voltage and 20 mA current. The scanning rate was 0.02
o/s with a scanning angle ranging from 10o to 80o.
Conclusions
In this study, an auto-reduced Cu@CNTs catalyst without further
reduction process was successfully prepared via a convenient
sonication-assisted impregnation method and exhibited excellent
catalytic performance for selective hydrogenation of DMO.
Characterization on the Cu@CNTs catalyst disclosed that the
Cu oxide encapsulated inside the nano-channels of CNTs could
be auto-reduced by CNTs support during the calcination in
argon atmosphere. The interaction of Cu species with the
interior surface of CNTs contributed to the auto-reduction of
Cu@CNTs catalyst. The auto-reduced Cu@CNTs catalyst
exhibited high catalytic activity on DMO hydrogenation. Varied
catalyst calcination temperature had obvious effect on the auto-
reduction degree of Cu@CNTs catalyst, by which to facilely tune
the products distribution, especially ethanol selectivity in DMO
hydrogenation reaction. The highest ethanol selectivity of 77.9%
was obtained on the Cu@CNTs-500 catalyst which had a
relatively high surface area of both Cu0 and Cu+. The findings in
this report have great potential for ethanol synthesis though
DMO hydrogenation and will also inspire the development on
other auto-reduced catalysts where the reduction degree and
catalytic performance can be tuned facilely as demanded.
Thermogravimetric analysis (TG) was carried out on the TG-
DTA2010SA-TD23 (NETZSCH, Japan) using argon as the purge gas.
The flow rate of purge gas was 30 ml/min and the heating rate was 3
oC/min. N2 adsorption-desorption isotherms were measured on
a
Quantachrome NOVA 2200 sorptometer. The specific surface area was
calculated from the isotherms through the method of Brunauer-Emmett-
Teller (BET).
The interaction of Cu oxide with CNTs support in calcination process was
studied by a catalyst analyzer BELCAT-B-TT (BEL Japan Co. Ltd.)
equipped with
a thermal conductivity detector (TCD) and a mass
spectrometry (BEL-mass). The catalyst precursor, without calcination
process, was first pretreated in argon flow at 120 oC for 1 h to remove the
water and then cooled to 50 oC. CO and CO2 desorption were recorded
along with heating catalyst precursor from 50 oC to 350, 450, 500, 550 or
650 oC at the heating rate of 3 oC/min under argon atmosphere,
respectively. The desorbed products, CO and CO2, were detected by
mass spectrometry.
Temperature-programmed reduction (TPR) profiles of the catalysts were
recorded using a catalyst analyzer BELCAT-B-TT (BEL Japan Co. Ltd.)
equipped with
a thermal conductivity detector (TCD). Prior to
measurement, the calcined catalyst was pretreated with argon gas flow
at 120 oC for 1 h to remove traces of water and then cooled to 50 oC.
TPR analysis on catalyst was performed using 5% H2/Ar with a flow rate
of 30 mL/min, and the temperature was raised from 50 to 800 oC with the
heating rate of 10 oC/min.
Experimental Section
Catalyst preparation
The metallic Cu surface area of catalyst was measured by the
decomposition of N2O at 90 oC using a pulsed method. The consumption
of N2O was detected by a TCD detector, and the surface area of metallic
Cu was calculated from the total consumption amount of N2O with 1.46 ×
1019 Cu atoms per m2.
In this report, we selected two kinds of catalyst support, multiple wall
CNTs (inner diameter: 20-30 nm; length: 1-10 μm; Chengdu, China) and
activated carbon (AC, 40-60 mesh, Kanto Chemical Co. Inc., Japan), for
Cu-based catalysts preparation. In order to remove the residual metals or
impurities in the CNTs and AC supports, the CNTs was pretreated with a
65 wt% HNO3 solution at 120 oC for 14 h, and the AC was pretreated with
a 30 wt% HNO3 solution at 60 oC for 10 h. Both supports were then
washed with deionized water until pH = 7, followed by vacuum drying for
12 h at 60 oC. The obtained CNTs and AC were used for the Cu-based
catalysts preparation in the following section.
CO chemisorption experiments were performed to quantify the surface
area of Cu+ by using an automatic chemisorption analyzer
(Quantachrome), and the CO adsorption stoichiometry was assumed as
Cu:CO = 1:1. [43]
Catalytic performance test of catalysts
The auto-reduced Cu@CNTs catalyst was prepared by a convenient
sonication-assisted impregnation method. In a typical procedure,
a
The catalytic performance of catalysts was measured with a stainless
steel fixed bed reactor. Typically, 0.125 g catalyst mixed with 3 g quartz
sand was placed in the middle of the reactor with quartz wool packed in
both sides of the catalyst bed. Then, the system was heated to reaction
temperature and pressured to 2.5 MPa. A 15 wt% DMO solution (1, 4-
dioxane as solvent) was continuously pumped into the reactor with co-
certain amount of Cu(NO3)2·3H2O was dissolved in deionized water at
ambient temperature and then dripped into the pretreated CNTs, stirred
with the assistance of ultrasonic for 30 min. Subsequently, a calculated
amount of deionized water was dripped, sonicated for another 30 min.
After vacuum drying for 1 h, the obtained dark product was dried
overnight at 120 oC and then calcined at 350, 450, 500, 550 or 650 oC
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condensed in a cold trap and then analyzed using a gas chromatography
instrument equipped with a flame-ionization detector, in which 2-propanol
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Entry for the Table of Contents
FULL PAPER
An auto-reduced Cu@CNTs catalyst
without further reduction process
was successfully prepared and
exhibited excellent catalytic
performance for hydrogenation of
DMO.
Peipei Ai, Minghui Tan, Yuki Ishikuro,
Yuta Hosoi, Guohui Yang, Yoshiharu
Yoneyama, Noritatsu Tsubaki*
DMO
Page No. – Page No.
Auto-reduced Cu@CNTs catalyst
Designing auto-reduced Cu@CNTs
catalysts to realize the precisely
selective hydrogenation of dimethyl
oxalate
EG
H2
Ethanol
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