High Selectivity of Cyclohexane Dehydrogenation for H2 Evolution Over Cu/SBA-15 Catalyst

Article abstract of DOI:10.1007/s10562-017-2033-5

Abstract: The effects of metal particle size on catalytic activities of Cu/SBA-15 with different Cu content were investigated for high selectivity of cyclohexane dehydrogenation. Overall, the smaller Cu nanoparticles exhibit higher hydrogen evolution rate or lower active energy barrier. But, even when the smaller CuO nanoparticles have formed on the catalyst with lower Cu content during the calcination, they would be more prone to sinter after reduction. An appropriate Cu content could lead to form amounts of stable and small Cu nanoparticles after high-temperature treatment with the space limitation by ordered channels of SBA-15.

Full text of DOI:10.1007/s10562-017-2033-5

Catal Lett (2017) 147:1295–1302
DOI 10.1007/s10562-017-2033-5
High Selectivity of Cyclohexane Dehydrogenation for H2
Evolution Over Cu/SBA-15 Catalyst
Zhijun Xia1 · Huayan Liu1 · Hanfeng Lu1 · Zekai Zhang1 · Yinfei Chen1
Received: 26 December 2016 / Accepted: 9 March 2017 / Published online: 29 March 2017
© Springer Science+Business Media New York 2017
Abstract The effects of metal particle size on catalytic
activities of Cu/SBA-15 with different Cu content were
investigated for high selectivity of cyclohexane dehydroge-
nation. Overall, the smaller Cu nanoparticles exhibit higher
hydrogen evolution rate or lower active energy barrier. But,
even when the smaller CuO nanoparticles have formed on
the catalyst with lower Cu content during the calcination,
they would be more prone to sinter after reduction. An
appropriate Cu content could lead to form amounts of stable
and small Cu nanoparticles after high-temperature treatment
with the space limitation by ordered channels of SBA-15.
Keywords Dehydrogenation · Cyclohexane · Selectivity ·
Copper · SBA-15
1 Introduction
Hydrogen has been regarded as a clean fuel. But its stor-
age remains a problem [1–3]. The organic hydride method
for hydrogen storage, transportation and supply has been
potential industrial candidates, due to its simple, secu-
rity, efficiency and COx free hydrogen [3–6]. In particular,
cyclohexane holds high hydrogen content about 56.0 g L−1
(volume capacity) and 7.1% (mass percent). Although the
endothermic energy requirement for cyclohexane dehydro-
genation is 68.8 kJ mol−1 of H2, this is much lower than
energy 248 kJ mol−1 of H2 oxidation reaction. Further-
more, the application of hydrogen can facilitate the saving
of a great amount of CO2 emissions by replacing gasoline
and diesel used in disperse or mobile facilities with H2 [6].
Moreover, the hydrogenation process from the dehydroge-
nation product benzene to cyclohexane is mature, and pro-
vides an additional advantage for the reversible catalytic
hydrogenation–dehydrogenation reactions. Thus, the inten-
sive research efforts have been done in cyclohexane dehy-
drogenation for hydrogen delivered [7–11].
Graphical Abstract
H2 evolution rate (mol-H2 h 1mol-Cu 1)
—
—
H2
ꢀ
2780
1Cu/SBA-15
5578
2Cu/SBA-15
3Cu/SBA-15
3071
3074
4Cu/SBA-15
Additionally, many attempts have been made to replace
precious Pt-based catalysts which have good performance
for the dehydrogenation reaction of cyclohexane [10, 11], in
order to reduce the cost of hydrogen storage [7–9]. Includ-
ing the earlier research of Sinfelt et al. [12], Ni–Cu bimetal-
lic catalysts have emerged good catalytic performance for
the dehydrogenation reaction of cyclohexane, and the Cu
addition into Ni-based catalysts improves the selectivity to
benzene. In fact, the monometallic Cu-based catalyst pre-
sents close to 100% selectivity to benzene even up to high
* Yinfei Chen
chfy@zjut.edu.cn
Zhijun Xia
xiazj919@163.com
1
Institute of Catalytic Reaction Engineering,
College of Chemical Engineering, Zhejiang
University of Technology, Hangzhou 310014,
People’s Republic of China
Vol.:(0123456789)
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Z. Xia et al.
temperature 375°C, although its conversion is less than that
of Ni-based catalyst [8]. Furthermore, because of that Cu
has lower melting point and Tammann temperature than
that of Ni, Cu is more prone to be migrated to the surface
of metallic particles [13]. Therefore, Cu plays important
roles on the selectivity of this reaction. However, a special
study on Cu-based catalysts for dehydrogenation reaction
of cyclohexane has not been developed yet.
samples were degassed at 280°C for 5 h. Specific surface
area, porosity and pore size distribution of all the samples
were calculated using BET (Brunaure, Emmett and Teller)
and BJH (Barrett, Joyner and Halenda).
Powder X-ray diffraction (XRD) patterns were collected
in the 2θ range of 10°–80° on a PANalytical X′Pert PRO
diffractometer with Cu Kα radiation (λ=0.154 nm).
Hydrogen temperature-programmed reduction (H2-TPR)
experiments were carried out on a FineSORB-3010 appa-
ratus. Firstly, 50 mg of sample was loaded into a U-shape
quartz reactor and pretreated with Ar at 200°C for 1 h.
Then, after cooling to 50°C under Ar, the gas flow was
switched to 5% H2 in Ar (30 mL min−1), and the sample
was heated to 800°C at 10°C min−1. The amount of hydro-
gen consumption was recorded with a thermal conductivity
detector.
The monometallic Cu-based catalysts have been used
frequently, due to its high activity and selectivity on hydro-
genation [14–16], dehydrogenation [17, 18] and water–gas
reforming reaction [19, 20]. The catalytic activity of Cu-
based catalyst would be dramatically improved as the par-
ticle size of Cu nanoparticles decreased [21]. However,
the preparation of highly dispersed Cu nanoparticles is
difficult, due to sintering easily for CuO or Cu nanopar-
ticles during thermal treatments [19]. Recently, SBA-15
with regular channels and high surface area has been used
widely as catalytic supporters to load Cu nanoparticles [14,
19, 22–24]. The sintering would be suppressed by the space
limitation of the ordered channels of SBA-15.
Diffuse reflectance UV–Visible (DR UV–Vis) spectra
were recorded on a UV-2600 (Shimadzu) spectrophotome-
ter, operating at room temperature in the wave length range
of 200–800 nm.
Raman data was collected with an inVia Raman micro-
scope (Renishaw) with 180 s in the wave-number range of
100–1200 cm−1.
In this work, in order to investigated the catalytic per-
formance of Cu-based catalysts, mesoporous SBA-15 was
employed to support and disperse metal Cu with different
content. Thereafter, a series of characterizations were per-
formed to reveal properties of catalysts and the relation-
ship between cyclohexane dehydrogenation and Cu-based
catalysts.
High-resolution transmission electron microscopy
(HRTEM) was performed with a Philips-FEI Tecnai G2
F30 S-Twin instrument operated at 300 kv.
2.3 Catalytic Reaction
Cyclohexane dehydrogenation reaction was carried out in
a fixed-bed microreactor (internal diameter 5 mm) with a
feed composition of C6H12: H2 =1:25 (molar ratio). Typi-
cally, after the sample reduced at 450°C with a H2 flow
(30 mL min−1) for 4 h and cooled to reaction temperature,
cyclohexane was introduced to the reactor by bubbling H2
at 0°C ice water bath, and the feed gas with a total flow
(30 mL min−1) was passed over 150 mg of sample (40–60
mesh), resulting in a gas hourly space velocity (GHSV)
12,000 mL g−1 h−1. The products were analyzed on-line
by peak area normalization method with an Agilent GC-
7890A, equipped with FID detector and a HP-INNOWax
capillary column. The conversion of cyclohexane, the
selectivity to benzene (or H2) and the evolution rate of H2
were calculated as:
2 Experimental
2.1 Catalyst Preparation
The Cu/SBA-15 catalysts were prepared by impregnation,
according to the recipe reported in document [25]. Briefly,
0.30, 0.60, 0.90 or 1.20 mmol Cu(NO3)2·3H2O (Sinop-
harm Chemical Reagent Co., 99% purity) was dissolved
into solvent mixed with 5.00 g hexane (Sinopharm Chemi-
cal Reagent Co., 99.5% purity) and 5.00 g alcohol (Sinop-
harm Chemical Reagent Co., 99.8% purity). Then, 1.00 g
mesoporous SBA-15 (JCNANO, 685.4 m2 g−1) was added
into the impregnation solution. After drying at room tem-
perature for 24 h and 60°C for 24 h, the samples were cal-
cined at 550°C for 4 h. Finally, the samples were obtained
and denoted as 1Cu/SBA-15, 2Cu/SBA-15, 3Cu/SBA-15
and 4Cu/SBA-15, respectively.
Moles of C6H12 reacted
Conversion of C6H12
Selectivity of C6H6 =
H2 evolution rate =
=
× 100%
Moles of C6H6 in feed
Moles of C6H6 in product
Moles of C6H6 reacted
× 100%
2.2 Catalyst Characterization
Nitrogen adsorption–desorption measurements were per-
formed at −196°C with a Micromeritics 3Flex Surface
Characterization Analyzer. Prior to measurement, the
3 × Moles of C6H6 in product
Moles of Cu in catalyst × Reaction time
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High Selectivity of Cyclohexane Dehydrogenation for H2 Evolution Over Cu/SBA-15 Catalyst
1297
3 Results and Discussion
3.1 Catalytic Activity
1.8
1.5
1Cu/SBA-15
2Cu/SBA-15
3Cu/SBA-15
4Cu/SBA-15
1.2
0.9
0.6
As shown in Fig. 1a, the conversion of cyclohexane on Cu/
SBA-15 catalysts in this work is no more than 25%, while
no any other product was detected which means that the
selectivity toward benzene is 100%, even up to 400°C.
However, more interesting is that the hydrogen evolution
rate at 350°C for per mole Cu atoms of 2Cu/SBA-15 is
about double of that in other catalysts. The small Cu par-
ticles provides a higher reaction rate than that of bulk Cu
[19], indicating its lower active energy barrier. The appar-
ent activation energy (Ea) was calculated from the slope
of Arrhenius plots of each catalyst (Fig. 2) [17, 26], and
used to evaluate catalytic activity. It is obvious that the Ea
(74.7 J mol−1 K−1) of 2Cu/SBA-15 in the temperature range
of 300–400°C is lower than that (80–82 J mol−1 K−1) of
other catalysts. Therefore, we deduce that amounts of small
Cu particles exit in the 2Cu/SBA-15 which result to higher
hydrogen evolution rate, compared with other catalysts.
0.3
y = 14.85ꢀ8.99 x
y = 15.19ꢀ9.62 x
0.0
R2=0.999
R2 =0.999
-0.3
-0.6
-0.9
-1.2
-1.5
-1.8
y = 15.63ꢀ9.87 x
R2=0.998
y = 15.28ꢀ9.63 x
R2=0.999
1.45 1.50 1.55 1.60 1.65 1.70 1.75
1
1
T— (10—3 K— )
Fig. 2 Arrhenius plots of Cu/SBA-15 catalysts for cyclohexane dehy-
drogenation
Cu, while the average pore size increases. However, the
micropores diminish, the pores about 3 nm appear, and the
pores in the size range of 5–7 nm decrease in the Cu/SBA-
15, compared with blank SBA-15. These imply that the
CuO precursor enters into the mesoporous channels, and
blocks partially the entrance, in according with the previ-
ous report [14].
3.2 Characterization of Catalysts
3.2.1 Textural Properties
Nitrogen adsorption–desorption measurements were used
to evaluate the textural properties of SBA-15 and calcined
samples (Fig. 3; Table 1). All the Cu/SBA-15 catalysts pre-
sent the same Type-IV isotherm and a H1 hysteresis loop
to the SBA-15. This means the mesoporous nature for the
catalysts after calcination is maintained. The BET surface
area as well as pore volume decrease with the loading of
3.2.2 Surface Analysis
Figure 4 shows the XRD patterns of calcined and spent cat-
alysts under ambient condition. The distinct peak of CuO
(PDF #48-1548) is observed for all catalysts after calcina-
tion (Fig. 4a). And the mean particle size (Table 1) calcu-
lated using Scherrer equation reduces as the Cu content
25
6000
5578
(b)
(a)
5000
4000
20
1Cu/SBA-15
2Cu/SBA-15
15
10
5
3Cu/SBA-15
4Cu/SBA-15
3071
3074
2780
3000
2000
1000
0
0
4Cu/SBA-15
2Cu/SBA-15 3Cu/SBA-15
1Cu/SBA-15
300
325
350
375
400
Temperature (oC)
Catalyst
Fig. 1 Catalytic activity of Cu/SBA-15 catalysts (150 mg, total 101 kPa, C6H12: H2 =1:25, GHSV=12,000 h−1). a Conversion of cyclohexane
and b H2 evolution rate at 350°C
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Z. Xia et al.
(b)
(a)
SBA-15
SBA-15
1Cu/SAB-15
1Cu/SBA-15
2Cu/SBA-15
2Cu/SBA-15
3Cu/SBA-15
3Cu/SBA-15
4Cu/SBA-15
0.2
4Cu/SBA-15
0
5
10
15
20
0.0
0.4
0.6
0.8
1.0
Pore Width (nm)
p/p0
Fig. 3 N2 adsorption–desorption isotherms (a) and pore size distribution (b) of SBA-15 and the calcined Cu/SBA-15 catalysts
Table 1 The physicochemical
properties of Cu/SBA-15
catalysts
Catalyst
Cu loadinga SBET (m2 g−1
(wt%)
)
Pore volume
Pore size (nm)
Crystal sizeb
(nm)
(cm3 g−1
)
CuO
Cu
SBA-15
0
685.4
517.0
483.2
458.8
451.0
0.95
0.72
0.70
0.66
0.67
5.54
5.61
5.81
5.80
5.90
–
–
1Cu/SBA-15
2Cu/SBA-15
3Cu/SBA-15
4Cu/SBA-15
1.88
3.70
5.45
7.13
20.1
26.7
28.0
32.5
33.7
42.2
39.6
33.9
aCu/(Cu+SBA-15) as determined according to the preparation of catalyst
bCalculated using Scherrer equation from XRD patterns
decreased, consistent with the result reported by Vargas-
Hernandez et al. [14]. Furthermore, it is larger than the
average size of pores inside of catalysts, indicating the sin-
tering of CuO particles on the exterior surface and in the
channels.
Figure 5 shows the TEM images of the spent Cu/SBA-
15 samples. The particles on the exterior surface of all
catalysts seem like lying on the face or being inside the
pores, similar with the previous reports [13, 19]. And no
obvious particles were observed outside the pores. These
would imply that the particles presented on the exterior
surface come from the internal surface of the pores, and
block the pore mouth. Nonetheless, the ordered channels
of SBA-15 were kept well, agreed with the result of N2
adsorption–desorption measurements.
For the spent catalysts after reaction for total 5 h (per
hour at each reaction temperature), their XRD patterns
show three sharp peaks at 2θ = 43.4°, 50.5° and 74.3°,
attributed to Cu (PDF #65-9743). The intensity of the
peaks becomes gradually weaker with the decrease of Cu
content, as the same trend with that of the calcined cata-
lysts. However, the mean particle size of metal Cu pre-
sents an increase trend in contrast, except for 1Cu/SBA-
15. This is mainly because of that the strong intensity
depends on not only crystallite size and crystallinity, but
also Cu content on the supporter. Moreover, the smaller
Cu crystal particles are more liable to gather and lead
to the formation of large Cu particles [14]. In addition,
the small Cu particles in the pores of SBA-15 could be
also aggregation, but their size is limited by the ordered
channels.
In the order to further investigate the surface species,
UV–vis DRS spectra (Fig. 6) and Raman spectra (Fig. 7) of
the spent catalysts were performed. In Fig. 6, the large band
centered at 250 nm in the range of 200–300 nm is attrib-
uted to SiO2 [27]. The shoulder peak at 370 nm is ascribed
to charge transfer bands of (O–Cu–O) [28], indicating the
existence of highly dispersive Cu species which was oxi-
dized easily and not observed in the XRD patterns. The
weak peak at 575 nm is attributed to Cun plasmon reso-
nance [28]. The intensity of these signals declines gradu-
ally with the decease of Cu content. In Fig. 7, the Raman
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High Selectivity of Cyclohexane Dehydrogenation for H2 Evolution Over Cu/SBA-15 Catalyst
1299
(a)
(b)
111
200
4Cu/SBA-15
220
4Cu/SBA-15
3Cu/SBA-15
3Cu/SBA-15
2Cu/SBA-15
1Cu/SBA-15
2Cu/SBA-15
1Cu/SBA-15
10
20
30
40
50
60
70
80
10
20
30
40
50
60
70
80
2θ (degree)
2θ (degree)
Fig. 4 XRD patterns of a calcined and b spent Cu/SBA-15 catalysts
Fig. 5 TEM images of the
spent catalysts. a 1Cu/SBA-15,
b 2Cu/SBA-15, c 3Cu/SBA-15
and d 4Cu/SBA-15
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Z. Xia et al.
0.10
0.08
0.06
0.04
0.02
250
II
III
I
4Cu/SBA-15
575
III
4Cu/SBA-15
3Cu/SBA-15
II
I
3Cu/SBA-15
370
I
III
II
I
2Cu/SBA-15
1Cu/SBA-15
2Cu/SBA-15
1Cu/SBA-15
100 150 200 250 300 350 400 450 500
Temprature (oC)
200 300 400 500 600 700 800
λ(nm)
Fig. 8 H2-TPR patterns of Cu/SBA-15 catalysts
Fig. 6 UV–vis DRS spectra of the spent catalysts
Cu. The correlation is much easier to be viewed by observ-
ing the angel between the two variable vectors [33]. And
the cosines of the angels were calculated in Table 3. Thus,
the H2 evolution rate has better correlation with the area
of peak I than that with the sum of area (or Cu content).
This indicates that the highly catalytic activity contributed
mainly to the smaller and highly dispersed Cu particles.
Furthermore, the temperature of reduction peak for 2Cu/
SBA-15 is the lowest of all, and the maximum H2 con-
sumption (about two times) in peak I. This explains that
more amounts of small Cu particles is existed in the 2Cu/
SBA-15, compared with that on the other catalysts [22, 34],
and just proves its lowest active energy barrier.
528 631
183 221
149
4Cu/SBA-15
3Cu/SBA-15
2Cu/SBA-15
1Cu/SBA-15
200
400
600
800
1000 1200
-1
Raman shift(cm )
In addition, the formation of large Cu particles for 3Cu/
SBA-15 and 4Cu/SBA-15 is not profit for the exposure of
active sites, and leads to low hydrogen evolution rate. For
the 1Cu/SBA-15 catalyst, a large reduction band was pre-
sented, due to the strong interaction with the support [15,
35]. Although small CuO particles on the surface of 1Cu/
SBA-15 were formed after calcination, they are more liable
to sinter in the reduction process. Thus, a suitable Cu con-
tent is needed for amounts of stable and small Cu particles
or active sites of cyclohexane dehydrogenation.
Fig. 7 Raman spectra of the spent catalysts
spectra present the similar trend with UV–vis DRS spec-
tra. The signals located at 149, 221, 528 and 631 cm−1 are
assigned to Cu2O [29–31]. And the 183 cm−1 band was
also observed in the Raman spectra of chrysocolla [32].
These indicate the existence of well-dispersed Cu species
which are small and prone to be oxidized after exposure to
air. Nonetheless, the surface species for all catalysts are no
difference.
3.2.4 Stability Test
The stability test of 2Cu/SBA-15 at 350 °C is shown in
Fig. 9. Although the catalyst after operating for 20 h
time-on-stream appeared a slight activity loss, the drop
in H2 evolution rate tended to vary gently with increas-
ing reaction time. This is attributed to the Cu sintering
or carbon formation caused by the high active of small
Cu nano-particles. However, these active sites are lim-
ited in quantity, and declined at such high temperature
and long time-on-stream. Therefore, the amounts of
3.2.3 Reduction Properties
The reducibility of all Cu/SBA-15 catalysts was investi-
gated by H2-TPR. Figure 8 displays the reduction plots, and
the result of analyses with peak fitting presents in Table 2.
Overall, the board H2 consumption band is located in the
temperature range of 125–350°C, which can be assigned to
the reduction of Cu2+ to Cu0. The sum of integral area for
each catalyst is in well harmony with the concentration of
1 3

High Selectivity of Cyclohexane Dehydrogenation for H2 Evolution Over Cu/SBA-15 Catalyst
1301
Table 2 H
2-TPR analysis of
Catalyst
Area
Center (°C)
Peak I
Cu/SBA-15 catalysts
Peak I
Peak II
Peak III
Sum
Peak II
Peak III
1Cu/SBA-15
2Cu/SBA-15
3Cu/SBA-15
4Cu/SBA-15
18,697
36,937
18,014
12,950
18,697
43,189
66,997
87,287
212.8
200.3
196.9
199.3
2339
9171
74,337
3913
39,812
14,149
221.5
214.5
241.0
262.5
240.1
261.3
Table 3 The cosines of the angel between the two variable vectors
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