Effect of Preparation Method on the Structure and Catalytic Performance of CuZnO Catalyst for Low Temperature Syngas Hydrogenation in Liquid Phase

Article abstract of DOI:10.1007/s10562-018-2328-1

Abstract: CuZnO catalysts were prepared by different methods, characterized by applying a combination of techniques, and the effect of preparation method on the structure and the catalytic performance for syngas hydrogenation at low temperature in liquid phase was investigated. The results showed that due to the difference of the interaction between Cu and Zn, the preparation method greatly affected the crystallinity of CuZnO. The crystallinity of CuZnO had a direct relation with the CO adsorption property. The total CO adsorption amount determined the catalytic activity, the weak adsorption of CO accounted for the methanol synthesis, while the strong adsorption of CO accounted for the ethanol synthesis. CuZnO prepared by the homogeneous precipitation with the lowest crystallinity exhibited the highest total carbon conversion of 68.6% with the methanol selectivity of 84.9%, while CuZnO prepared by the sol gel with the highest crystallinity exhibited the highest ethanol selectivity of 47.6%. Graphical Abstract: The preparation method of CuZnO had a significant effect on the CO adsorption property and then resulted in a substantial modification in the product distribution for the syngas hydrogenation at low temperature in liquid phase. The catalytic activity was determined by the total amount of CO adsorption, the weak CO adsorption accounted for the methanol synthesis and the strong CO adsorption accounted for the ethanol synthesis. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-018-2328-1

Catalysis Letters
https://doi.org/10.1007/s10562-018-2328-1
Effect of Preparation Method on the Structure and Catalytic
Performance of CuZnO Catalyst for Low Temperature Syngas
Hydrogenation in Liquid Phase
Huan Liu1,2 · Tong Chen1 · Gongying Wang1
Received: 17 November 2017 / Accepted: 5 February 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
CuZnO catalysts were prepared by different methods, characterized by applying a combination of techniques, and the effect
of preparation method on the structure and the catalytic performance for syngas hydrogenation at low temperature in liquid
phase was investigated. The results showed that due to the difference of the interaction between Cu and Zn, the preparation
method greatly affected the crystallinity of CuZnO. The crystallinity of CuZnO had a direct relation with the CO adsorp-
tion property. The total CO adsorption amount determined the catalytic activity, the weak adsorption of CO accounted for
the methanol synthesis, while the strong adsorption of CO accounted for the ethanol synthesis. CuZnO prepared by the
homogeneous precipitation with the lowest crystallinity exhibited the highest total carbon conversion of 68.6% with the
methanol selectivity of 84.9%, while CuZnO prepared by the sol gel with the highest crystallinity exhibited the highest
ethanol selectivity of 47.6%.
Graphical Abstract
The preparation method of CuZnO had a significant effect on the CO adsorption property and then resulted in a substantial
modification in the product distribution for the syngas hydrogenation at low temperature in liquid phase. The catalytic activity
was determined by the total amount of CO adsorption, the weak CO adsorption accounted for the methanol synthesis and
the strong CO adsorption accounted for the ethanol synthesis.
Keywords CuZnO · Ethanol · Methanol · Preparation method · Syngas hydrogenation
Extended author information available on the last page of the article
Vol.:(0123456789)
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H. Liu et al.
2.2 Catalyst Preparation
1 Introduction
CuZnO samples (Cu/Zn = 1, molar ratio) with different
structures were synthesized via homogeneous precipita-
tion, conventional co precipitation and sol gel, and named
as CuZnO-1, CuZnO-2 and CuZnO-3, respectively.
In the homogeneous precipitation method, an aqueous
solution of copper and zinc nitrates was mixed with urea.
The urea was hydrolyzed by heating the mixture to 90 °C
for 24 h to release the precipitant agent (hydroxide ions).
The precipitate was washed several times with deionized
water and dried at 120 °C for 12 h. Finally, the sample were
calcined at 350 °C for 3 h.
Syngas derived from coal, biomass or nature gas was widely
used to produce methanol. However, higher alcohols are
more desirable products than methanol as neat fuels, a
potential source of hydrogen for fuel cells and a feedstock
for the synthesis of variety of chemicals [1–6]. Considering
the energy shortage and the deterioration of global climate
change, the synthesis of higher alcohols from syngas has
attracted much attention [7, 8].
Currently, the synthesis of higher alcohols synthesis
from syngas was widely studied via the fixed bed reactor
under high temperature and high pressure (250–440 °C and
4.1–10.3 MPa) over the modified methanol synthesis cata-
lysts and the modified Fisher–Tropsch catalysts [1, 7, 9–15].
In order to enhance the carbon conversion and address ther-
modynamic limitation of syngas hydrogenation to alcohols
in conventional process, the process of a low tempera-
ture syngas hydrogenation in liquid phase was developed
[16–24]. For the low temperature process in liquid phase,
the catalytic system with Cu based catalyst and alcohol as
solvent has been widely investigated [20, 23–33]. Zn as an
effective promoter could not only stabilize the active phase
and also alter the interaction between components of the
catalyst [34–40]. Thus, CuZnO catalyst shows good catalytic
performance for the low temperature syngas hydrogenation
reaction in liquid phase, but methanol was the major alcohol
product [41, 42]. Previous studies about CuCo based cata-
lyst indicated that the preparation method greatly affected
the structure and then significantly influenced the catalytic
activity for CO hydrogenation in the conventional process
[43, 44]. However, few literatures reported the effect of the
structure of CuZnO catalyst on the product distribution and
catalytic activity for the low temperature syngas hydro-
genation reaction in liquid phase. Furthermore, the struc-
ture played an important role in the selective synthesis of
product.
In the conventional co precipitation method, 100 ml
mixed aqueous solution containing copper and zinc nitrates
(total metal concentration of 1.0 mol/l) and an aqueous
solution of Na2CO3 (1.0 mol/l) used as a precipitant were
simultaneously added to an amount of distilled water under
rapid stirring to generate precipitate. The temperature and
pH of the precipitation were controlled at 65 °C and 8.5,
respectively. The obtained precipitation was aged at 65 °C
for 4 h, followed by filtering and washing several times with
distilled water to remove residual sodium. Then, the pre-
cipitation was dried at 120 °C overnight and then calcined
at 350 °C for 3 h.
In the sol gel method, an aqueous solution of copper and
zinc nitrates was made to the desired composition. Then,
the citric acid solution was slowly added to the metal nitrate
solution under constant stirring. The resulting solution was
heated at 80 °C in a water bath to evaporate excess water and
a viscous gel was obtained. The gel was put into an oven at
100 °C and started boiling with frothing and foaming. Sub-
sequently, the precursor was put in a muffle furnace, heating
to 350 °C at 1 °C/min and holding at 350 °C for 3 h and form
a black powder.
2.3 Catalytic Activity Test
In this work, CuZnO samples with different structures
were prepared by different methods and characterized by
XPS, H2-TPR, XRD, SEM, TEM and CO-TPD. The relation-
ship between the structure and the catalytic performance for
the syngas hydrogenation at low temperature in liquid phase
was investigated.
The catalytic activity test was performed in a batch reac-
tor with an inner volume of 100 ml. Prior to the catalytic
activity test, the metallic oxide precursors of samples were
reduced at 250 °C for 8 h by a flow of 10% hydrogen in nitro-
gen. After reduction and cooling down to room temperature,
the sample was first added into the reactor with K2CO3 and
2-propanol. After the tightness of reactor was checked by
N2, the feed gas (H2/CO/CO2 =65/33/2) was introduced to
purge the inside gas. The test was conducted at 5.0 MPa and
170 °C, respectively. In case of the occurrence of the diffu-
sion-controlled regime between gas, liquid and solid phases,
the stir speed of the reactor was carefully checked and set at
1500 rpm. Liquid products were analyzed by SHIMADZU
GC-14B with capillary column via internal standard method,
2 Experimental Section
2.1 Materials
Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, K2CO3, Na2CO3, urea and
citric acid were of A. R. grade. All of materials were used
without further purification.
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Effect of Preparation Method on the Structure and Catalytic Performance of CuZnO Catalyst for…
and gas products were analyzed by SC-3000B with packed
column via external standard method.
2p3/2 of ~933.6 eV attributed to Cu2+ were detected [45, 46].
The peaks at the binding energies of ~962.5 and ~942.5 eV
were the satellite structures of Cu2+, which was due to the
shake-up transitions by ligand-to-metal 3d charge transfer
in the form of Cu2+ with the 3d9 configuration [47, 48]. The
The catalytic performance was evaluated by total carbon
conversion, the selectivity of products, which were calcu-
lated as follows:
Va
Vb
Total carbon conversion = CO conversion ×
+ CO2 conversion ×
Va + Vb
Va + Vb
where Va and Vb represented the volume concentration of
CO and CO2 in the feed gas, respectively.
binding energies of Zn 2p of Zn 2p1/2 of ~1044.7 eV and Zn
2p3/2 of ~1021.7 eV attributed to Zn2+ were observed [49].
For O 1s spectra, the binding energy of O 1s of ~530.0 eV
was ascribed to the metal oxide of CuO and ZnO, and the
binding energy of O 1s of ~ 531.7 eV was caused by the
surface hydration [5]. Apparently, the peaks of Cu 2p and
Zn 2p of 2p1/2 and 2p3/2 of CuZnO-3 shifted towards lower
binding energy by 0.6 eV, in contrast with that of CuZnO-1
and CuZnO-2. These results suggested that the preparation
method had no effect on the chemical valence states of Cu,
Zn and O, but affected the chemical environment of Cu and
Zn, which led to the difference of the interaction between Cu
and Zn. Thus, it was deduced that the interaction between
Cu and Zn in CuZnO-1 and CuZnO-2 was stronger than that
of CuZnO-3.
the mole number of carbon in Pi
Pi selectivity =
× 100%
the mole number of carbon conversed
where Pi was one of the products.
3 Results and Discussion
3.1 Catalyst Characterization
The XPS spectra of CuZnO samples prepared by different
methods at Cu 2p, Zn 2p and O 1s were shown in Fig. 1. It
can be seen from Fig. 1 that all samples exhibited the same
binding energies of Cu 2p, Zn 2p and O 1s. For Cu 2p spec-
tra, the binding energies of Cu 2p1/2 of ~953.6 eV and Cu
The H2-TPR profiles of CuZnO samples prepared by
various methods were displayed in Fig. 2. Since ZnO could
Fig. 1 The XPS spectra of CuZnO samples prepared by different methods. (a) CuZnO-1, (b) CuZnO-2, (c) CuZnO-3
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H. Liu et al.
Fig. 3 The pore size distribution of CuZnO samples prepared by dif-
ferent methods
Fig. 2 The H2-TPR profiles of CuZnO samples prepared by different
methods. (a) CuZnO-1, (b) CuZnO-2, (c) CuZnO-3
Table 1 The textural properties of CuZnO samples prepared by dif-
ferent methods
Samples
BET surface area Pore volume
Average pore
diameter (nm)
(m2/g)
(cm3/g)
CuZnO-1
CuZnO-2
CuZnO-3
53.7
48.4
16.4
0.26
0.25
0.07
19.68
21.31
17.26
not be reduced below 700 °C, all the reduction peaks in
Fig. 2 were attributed to the reduction of copper species
[50, 51]. The H2-TPR profiles of CuZnO-1 and CuZnO-2
showed similar reduction peaks, the low temperature reduc-
tion peak at around 183 °C corresponding to the reduction
of high dispersed copper species and the high temperature
reduction peak at around 196 °C corresponding to the reduc-
tion of copper species interacted with zinc species [52]. The
H2-TPR profile of CuZnO-3 displayed only a broad reduc-
tion peak at around 191 °C [8, 53]. Thus, it can be deduced
that comparing with the sol gel method, the precipitation
method was in favor of the enhancement of the interaction
between Cu and Zn, which was consistent with the result
of XPS.
Fig. 4 The XRD patterns of CuZnO samples prepared by different
methods. (a) CuZnO-1, (b) CuZnO-2, (c) CuZnO-3
the pore volume and average pore diameter were similar to
that of CuZnO-1, while the BET surface area was smaller
than that of CuZnO-1 by 9.8%. For CuZnO-3, the BET
surface area, pore volume and average pore diameter were
16.4 m2/g, 0.07 cm3/g and 17.26 nm, which were the lower
than that of two other samples. All the average pore diameter
of the samples was in the range of 2–50 nm, which owned to
the type of mesoporous. The results suggested that the large
surface area and pore volume may provide plenty of active
site and favor adsorption and diffusion of reactant gas.
The XRD patterns of CuZnO samples prepared by dif-
ferent methods were illustrated in Fig. 4. The characteristic
peaks of CuO and ZnO were observed from the XRD pat-
terns of the three samples. The intensities of the diffraction
The BET surface area, pore volume and average pore
diameter of CuZnO samples prepared by different methods
were summarized in Table 1. It can be found that the prepa-
ration methods significantly affected the textural properties
of CuZnO samples. For CuZnO-1, the BET surface area,
pore volume and average pore diameter were 53.7 m2/g and
0.26 cm3/g with mean pore diameter of 19.68 nm, which
was calculated from the pore distribution evaluated from the
desorption branch by BJH method (Fig. 3). For CuZnO-2,
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Effect of Preparation Method on the Structure and Catalytic Performance of CuZnO Catalyst for…
peaks of CuO and ZnO phases of CuZnO samples were in
the order of CuZnO-3>CuZnO-2>CuZnO-1. Among the
three CuZnO samples, CuZnO-3 showed the highest crys-
tallinity, that is, CuZnO-3 owned the largest particle size.
Combined with the results of H2-TPR, this result suggested
that the strong interaction between Cu and Zn in CuZnO-1
and CuZnO-2 inhibited the crystal particle growth. These
results suggested that the sol gel method was beneficial to
the growth of the crystal particle of CuO and ZnO.
that of CuZnO-2. Additionally, some agglomeration was
observed in Fig. 5c, which may be caused by the local high
temperature during the preparation process. The order of the
crystal size of the samples prepared by different method was
CuZnO-3 > CuZnO-2 > CuZnO-1. The results above were
in agreement with the BET surface area of Table 1 and the
analyses of XRD.
The transmission electron microscopy was used to inves-
tigate the exposed facets of CuZnO samples prepared by
different methods, and the TEM images were shown in
Fig. 6. The low resolution TEM images of CuZnO samples
in Fig. 6a, c and e displayed that the morphology of crys-
tals of CuZnO-1 was plate-like, the particles of CuZnO-2
were approximately spherical in shape with the size of about
8.3 nm, and the particle shape of CuZnO-3 was a mixture
of sphere and pate-like crystals with the size in the range of
18–89 nm. The results were consistent with the analyses of
XRD and SEM.
The morphologies of CuZnO samples prepared by dif-
ferent methods were analyzed by SEM and showed in
Fig. 5. CuZnO-1 displayed fluffy plate-like crystals, which
may provide high surface area for active sites and favored
the adsorption and diffusion of reactant molecular, while
CuZnO-2 exhibited the spongy morphology and relatively
uniform sphere-like particles, which could enhance the
syngas adsorption and diffusion. CuZnO-3 also showed the
spongy morphology, but the particle size was larger than
Fig. 5 The SEM images of the fresh CuZnO samples prepared by different methods. a CuZnO-1, b CuZnO-2, c CuZnO-3
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H. Liu et al.
Fig. 6 The TEM images of the fresh CuZnO samples prepared by different methods. Low resolution TEM images of a CuZnO-1, c CuZnO-2, e
CuZnO-3; High resolution TEM images of b CuZnO-1, d CuZnO-2, f CuZnO-3
The high resolution TEM images of three CuZnO sam-
ples were present in Fig. 6b, d and f. The observation of the
exposed facets of CuO and ZnO in the three CuZnO samples
demonstrated the polycrystalline nature. CuZnO-1 mainly
exhibited the facets of CuO of (1 1 1) and ZnO of (0 0 2),
(1 1 0). CuZnO-2 displayed the facets of CuO of (0 0 2), (1
1 1), (−1 1 0), ZnO of (0 0 2), (1 0 1), (1 0 0) and CuZn of
(3 1 0). Figure 6d of CuZnO-2 exposing much more planes
of CuO and ZnO than that of CuZnO-1 indicated the higher
crystallinity of CuZnO-2 than that of CuZnO-1. Figure 6f
of CuZnO-3 was fully covered by the facets of CuO and
ZnO, and besides the planes in Fig. 6b and d, CuO (−1 1 1)
was detected, which demonstrated the highest crystallinity
of CuZnO-3.
It was noteworthy that the presence of CuZn plane in
CuZnO-2 suggested the existence of the interaction between
Cu and Zn, which was also confirmed by H2-TPR. Due to the
similar reducibility of CuZnO-1 and CuZnO-2, the interac-
tion between Cu and Zn would also existence in CuZnO-
1, but no CuZn facet was observed in Fig. 6b, which may
own to the low crystallinity of CuZnO-1. However, no facet
of CuZn was distinguishable in Fig. 6f of CuZnO-3 with
the highest crystallinity, which implied that the interaction
between Cu and Zn in CuZnO-3 was weak.
The CO-TPD curves of CuZnO samples prepared by dif-
ferent methods were shown in Fig. 7. The CO-TPD profile
of CuZnO-1 displayed a broad desorption peak in the range
of 70–400 °C. For CuZnO-2, two desorption peaks in the
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Effect of Preparation Method on the Structure and Catalytic Performance of CuZnO Catalyst for…
Table 2 The effect of preparation method of CuZnO on the catalytic
activity and the product distribution for the low temperature conver-
sion of syngas to ethanol in liquid phase
Samples Total carbon Selectivity (%)
conversion
Methanol Ethanol Methyl
acetate
Iso-
(%)
propyl
formate
CuZnO-1 68.6
CuZnO-2 64.2
CuZnO-3 35.3
84.9
70.5
22.7
0
13.8
47.6
2.0
9.6
14.0
13.0
5.7
11.5
Reaction conditions: T=170 °C, P=5.0 MPa, H2/CO/CO2 =65/33/2;
2-propanol, 30 ml; sample, 2.0 g; K2CO3, 0.6 g; reaction time, 10 h
precipitation method strengthened the interaction between
Cu and Zn and inhibited the crystal particle growth, while
the sol gel method weakened the interaction and promoted
the crystal particle growth. Meanwhile, the difference of
CuZnO crystallization led to a substantial modification in
the CO adsorption property.
Fig. 7 The CO-TPD curves of CuZnO samples prepared by different
methods. (a) CuZnO-1, (b) CuZnO-2, (c) CuZnO-3
range of 100–450 °C were identified. The CO-TPD profile
of CuZnO-3 exhibited four overlapped desorption peaks in
the range of 115–550 °C.
3.2 Catalytic Performance
The CO desorption peak below 350 °C was regarded
as the low temperature desorption peak corresponding
to the weak CO adsorption, and the CO desorption peak
above 350 °C was regarded as the high temperature desorp-
tion peak corresponding to the strong CO adsorption. For
CuZnO-1, only the low temperature desorption peak was
observed. For CuZnO-2, the amount of CO desorption at
low temperature was much larger than that of CO desorp-
tion at high temperature. For CuZnO-3, the amount of CO
desorption at high temperature was larger than that of CO
desorption at low temperature.
The effect of the preparation method on catalytic perfor-
mance of CuZnO sample for the low temperature ethanol
synthesis from syngas in liquid phase was listed in Table 2.
As expected, the preparation method had a significant
effect on the catalytic activity and the product distribution.
CuZnO-1 exhibited the total carbon conversion of 68.6%
and the methanol selectivity of 84.9%, and methanol was the
only alcohol product. CuZnO-2 exhibited the total carbon
conversion of 64.2%, but ethanol was detected in the product
with the selectivity of 13.8%, and the methanol selectiv-
ity decreased to 70.5%. In comparison with CuZnO-1 and
CuZnO-2, the total carbon conversion over CuZnO-3 obvi-
ously decreased to 35.3%, but the selectivity towards ethanol
greatly increased to 47.6% with the selectivity of methanol
decreasing to 22.7%. Apparently, ethanol was synthesized
at the consumption of methanol, which may be due to that
the intermediate for the formation of methanol and ethanol
was same.
Comparing the CO desorption peak area with the same
sample loading, the difference of the area of the CO des-
orption peak in different temperature regions of the three
samples was obvious. The order of the total area of CO
desorption peaks was CuZnO-1 > CuZnO-2 > CuZnO-3,
the order of the area of the low temperature CO desorption
peak was CuZnO-1 > CuZnO-2 > CuZnO-3, and the order
of the area of the high temperature CO desorption peak was
CuZnO-3>CuZnO-2>CuZnO-1, respectively.
Based on the results of CO-TPD, it was found that the
total carbon conversion was associated with the total amount
of CO adsorption, the selectivity of methanol was linearly
related with the amount of the weak CO adsorption, and the
selectivity of ethanol was linearly related with the amount
of the strong CO adsorption. The results indicated that the
amount of CO adsorbed over the surface determined the
catalytic activity for the syngas hydrogenation, the weak
adsorption of CO accounted for the product of methanol,
while the strong adsorption of CO accounted for the product
of ethanol. That is, the stronger bond between CO and Cu
Combined with the analyses of XRD, it was found that
the beginning CO desorption temperature and the area of
the high temperature CO desorption peak were linearly
related with the crystallinity of CuZnO, which suggested
that high crystallinity strengthened the bond between CO
and Cu, and shifted some weak CO adsorption to the strong
CO adsorption.
Thus, the preparation method of CuZnO affected the
interaction between copper and zinc, which resulted in
the difference of the crystallinity of CuZnO samples. The
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H. Liu et al.
was beneficial to the carbon chain growth to promote the
ethanol synthesis.
activity, but favored the strong CO adsorption for the growth
of carbon chain to promote the ethanol synthesis.
Additionally, the catalytic activities of CuZnO-1 and
CuZnO-2 were much higher than that of CuZnO-3, while the
selectivity of ethanol of CuZnO-1 and CuZnO-2 were much
lower than that of CuZnO-3. Two factors may be responsible
for the phenomenon. First, the BET surface area of CuZnO-1
and CuZnO-2 were much larger than that of CuZnO-3,
which provided much more active sites for the adsorption
and diffusion of the reactant gas and then promoted the con-
version of syngas, which was also confirmed by CO-TPD
and SEM. Second, the strong interaction between Cu and Zn
in CuZnO-1 and CuZnO-2 samples created additional new
Cu-Zn bimetallic active sites for the syngas hydrogenation
[40, 54, 55]. Meanwhile, the strong interaction between Cu
and Zn inhibited the growth of crystal particle and decreased
the amount of the strong CO adsorption, which led to the
obvious decrease of the ethanol selectivity.
Compliance with Ethical Standards
Conflict of interest The authors declare no conflict of interest.
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H. Liu et al.
Affiliations
Huan Liu1,2 · Tong Chen1 · Gongying Wang1
2
* Tong Chen
University of Chinese Academy of Sciences, Beijing 100049,
People’s Republic of China
chentongw@sina.com.cn
1
Chengdu Institute of Organic Chemistry, Chinese Academy
of Sciences, Chengdu 610041, People’s Republic of China
1 3