Investigation on the promotional role of Ga2O3on the CuO-ZnO/HZSM-5 catalyst for CO2hydrogenation

Article abstract of DOI:10.1039/d0ra10849a

Dimethyl ether (DME) can be directly synthesized from carbon dioxide and hydrogen by mixing methanol synthesis catalysts and methanol dehydration catalysts. The activity and selectivity of the catalyst can be greatly affected by the promoter; herein, we presented a series of CuO-ZnO-Ga2O3/HZSM-5 hybrid catalysts, which were prepared by the coprecipitation method. The effect of the Ga2O3 content on the structure and performance of the Ga-promoted Cu-ZnO/HZSM-5 based catalysts was thoroughly investigated. The results showed that the addition of Ga2O3 significantly increased specific surface areas and Cu areas, decreased the size of Cu particles, maintained the proportion of Cu+/Cu0 on the surface of the catalyst, and strengthened the metal-support interaction, resulting in high catalytic performance. This journal is

Full text of DOI:10.1039/d0ra10849a

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Investigation on the promotional role of Ga O on
2
3
the CuO–ZnO/HZSM-5 catalyst for CO2
hydrogenation†
Cite this: RSC Adv., 2021, 11, 14426
Jie Du, Yajing Zhang, * Kangjun Wang,* Fu Ding, Songyan Jia,
and Limei Tan
Guoguo Liu
Dimethyl ether (DME) can be directly synthesized from carbon dioxide and hydrogen by mixing methanol
synthesis catalysts and methanol dehydration catalysts. The activity and selectivity of the catalyst can be
greatly affected by the promoter; herein, we presented a series of CuO–ZnO–Ga
catalysts, which were prepared by the coprecipitation method. The effect of the Ga
structure and performance of the Ga-promoted Cu–ZnO/HZSM-5 based catalysts was thoroughly
investigated. The results showed that the addition of Ga significantly increased specific surface areas
2
O
3
/HZSM-5 hybrid
2
O content on the
3
Received 26th December 2020
Accepted 17th March 2021
2 3
O
DOI: 10.1039/d0ra10849a
+
0
and Cu areas, decreased the size of Cu particles, maintained the proportion of Cu /Cu on the surface
rsc.li/rsc-advances
of the catalyst, and strengthened the metal–support interaction, resulting in high catalytic performance.
11
methanol dehydration catalysts, changing the preparation
process of catalysts, adopting a core–shell structure and
1
. Introduction
12
13
14
Recently, human beings mainly have two concerns, global mixing with new catalysts.
1
warming and energy shortage. CO
2
is one of the main green-
Previous studies have shown that appropriate promoters can
house gases and is considered the main contributor to global effectively improve the performance of Cu–ZnO/HZSM-5 with
2
warming. The hydrogenation of CO
(
2
to other chemicals optimized composition. Many studies have been performed on
to dimethyl ether, with
etc., aiming to increase the catalytic
hydrocarbons, methanol, and dimethyl ether (DME)) can catalysts for the hydrogenation of CO
2
15 7,16
effectively solve these problems. Particular attention has been carriers, promoters,
paid to the synthesis of DME. DME may be used as an alter- performance of catalysts.
native fuel and is widely used as feedstock for the production of
Ga , as a promoter, has been used by some researchers in
CO conversion catalysts.
hydrogenation to dimethyl ether combines two been reported on dimethyl ether catalysts
successive steps of methanol synthesis and methanol dehy- fewer researchers who have studied the inuence of Ga O
2 3
2 3
O
3,4
17–19
aerosols, refrigerants, foaming agents, etc.
One-step CO
2
In comparison, research has rarely
20,21
and there are
2
dration over a hybrid catalyst in a single reactor. The catalyst is content changes on catalysts. In the present work, we found that
5
composed of a methanol synthesis component and methanol Ga
O
can signicantly improve the selectivity and yield of
2 3
6
dehydration component together; in some cases, the catalyst is dimethyl ether; the effects of Ga
2
O
3
content on the performance
7,8
also called a bifunctional/hybrid catalyst. The Cu–ZnO/HZSM- and structure of the CuO–ZnO/HZSM-5 catalyst were carefully
based catalyst has been considered as an optimum combi- investigated in direct DME synthesis from a CO and H
nation of the two components, and the active temperature of mixture.
both components is within the same temperature range. At
present, however, the hydrogenation of CO to DME still has
a low CO conversion and a low selectivity to DME over the
catalyst. Various methods were employed to modify its
5
2
2
9,10
2
2
. Experimental section
2
2.1 Catalyst preparation
composition and structure, aiming to enhance the perfor- The catalysts were prepared by the oxalate co-precipitation
mance. For example, optimization methods include modifying method, and the mass ratio of the CuO–ZnO–Ga O3 compo-
2
nent to HZSM-5 (Catalyst Plant of Nankai University, SiO
2
/Al
38, molar ratio) was 2 : 1. First, a certain amount of
analytically pure nitrate precursors Cu(NO , Zn(NO , and
Ga(NO3)3 was dissolved into a certain amount of ethanol
2 3
O
16
¼
College of Chemical Engineering, Shenyang University of Chemical Technology,
Shenyang 110142, PR China. E-mail: yjzhang2009@163.com; angle_79@163.com;
Tel: +86-24-89383902
3
)
2
3 2
)
(
1
denoted as solution A; the total molar concentration is
†
Electronic supplementary information (ESI) available: Experimental section,
gures and tables given in the Results and discussion section. See DOI:
ꢀ
1
2 2 4 2
mol L ); H C O $2H O (200 wt% of metal nitrate) was also

ꢀ
1
1
0.1039/d0ra10849a
dissolved into ethanol (solution B, 1 mol L ). Second, parallel
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ꢁ
solutions A and B were slowly dropped into a beaker containing 0.5 h in Ar. Aer cooling to 40 C, CO
2
adsorption was switched
ꢁ
HZSM-5 ethanol solution and kept under stirring at 60 C. The to saturation, and then the catalyst was ushed with a He
suspension was sealed and aged for 2 hours and then the stream (30 mL min ) for 60 min, and nally it was heated from
ethanol was evaporated at 80 C to get a precipitate. Finally, the 50 to 900 C at a rate of 10 C min . The CO -TPD signal was
ꢀ1
ꢁ
ꢁ
ꢁ
ꢀ1
2
ꢁ
precipitate was dried at 120 C for 12 hours and calcined in air recorded.
ꢁ
at 400 C for 4 hours. The powder of the CZG
x
H catalyst was
X-ray photoelectron spectroscopy (XPS) and X-ray-induced
ground, pressed, pulverized and sieved to obtain particles of 20 Auger electron spectroscopy were performed on an ESCALAB
to 40 mesh before the activity was tested. The Ga modied 250Xi spectrometer. The monochromatized Al-Ka beam (1486.8
2 3
O
CuO–ZnO/HZSM-5 (CuO : ZnO ¼ 7 : 3, mass ratio) catalysts eV) was used as an X-ray source to conrm the characteristic
were abbreviated as CZG H (x stands for theoretical Ga O / peaks of Cu 2p, Zn 2p, Ga 2p, and C 1s with a beam resolution of
x
2 3
CZ wt%).
.2 Catalyst testing
0.1 eV. The characteristic C–C peak at 284.6 eV assigned to C 1s
was further used as a reference peak.
2
3
. Results and discussion
Catalyst testing was carried out in a continuous-ow xed-bed
reactor made of stainless steel (i.d. ¼ 10 mm). First, the cata-
3
.1 Catalytic performance of the catalysts
In Table 1, the catalytic performances of the CZG
displayed, along with the behaviour of a Ga
CZG H) taken as a reference. In this reaction, DME was the
main product, while methanol, CO and trace hydrocarbons
methane, ethane, etc.) were identied as by-products. Appar-
ꢁ
lyst was reduced with 10% H /N at 300 C for 3 h under
2
2
x
H catalysts are
2 3
O -free sample
ꢁ
atmospheric pressure. Then it was cooled to 50 C and reactant
gas ow was introduced, raising the pressure to 3.0 MPa, and
(
0
ꢁ
the reaction temperature was 260 C. The exit line was heated to
prevent condensation. The products were analyzed on line with
a gas chromatograph (SP-3420) equipped with both a TCD (for
(
ently, the catalytic performance is signicantly improved with
the addition of Ga O compared to the Ga O -free catalyst
CO and CO
2
, GDX-101 connected with Porapak T column) and
2
3
2
3
a FID (for CH , CH OH and CH OCH , Porapak Q column).
4
3
3
3
under the same reaction conditions. All the Ga
lysts exhibited higher CO
selectivity to DME (Fig. S2†). On the other hand, it is also
observed that the CO conversion, DME selectivity and DME
yield show a trend of increase initially, followed by a decrease
2
O
3
-added cata-
Conversion and selectivity values were calculated by the internal
standard and mass conservation method.
2
conversion (Fig. S1†) and higher
2
2.3 Catalyst characterization
2 3
XRD measurements were performed on a Rigaku D/max 2500pc with Ga O addition; the attained better catalytic performances
ꢀ
X-ray diffractometer with Cu-Ka radiation (l ¼ 1.54156 A) at were respectively 22.3%, 62.6% and 14.0% over the CZG H
5
ꢁ
ꢀ1
a scan rate of 4 min at 50 kV and 250 mA. The crystallite size catalyst. By comparing the catalysts in this paper with similar
was calculated using Scherrer's equation. Copper surface areas material catalysts with similar ratios in other literature under
(SCu) were measured by a nitrous oxide titration method as almost the same test conditions, although there is no signi-
23
22
described elsewhere.
cant difference between the catalysts with promoters Ga
adsorption at ZrO in terms of the carbon dioxide conversion, the selectivity to
modied catalyst was signicantly
modied catalyst, so the yield of
2 3
O and
BET surface areas were measured by N
2
2
ꢁ
ꢀ
196 C using a Quantachrome Autosorb 1-C. Before the dimethyl ether of the Ga
2
O
3
absorption–desorption measurements, samples were degassed higher than that of the ZrO
2
ꢁ
under vacuum at 300 C for 3 h. The specic BET (SBET) was dimethyl ether is improved accordingly. The catalytic perfor-
estimated from the linear part of the Brunauer–Emmett–Teller mance results showed that an appropriate amount of Ga O3
2
(
BET) plot.
could effectively convert CO
2
into dimethyl ether, but not in
on the
H
2
-TPR of catalysts was performed on a chemisorption excess. In order to evaluate the inuence of Ga O
2 3
analyser (ChemBET 3000). Before reduction, 0.02 g of sample catalysts, the catalysts were further characterized in the
ꢁ
was preheated with owing He at 400 C for 60 min, and then following sections.
cooled down to room temperature. Subsequently, the temper-
Turnover frequency (TOF) of CO
/Ar (50 mL min ) at a ramp rate of molecular number of the transferred CO
C min to 400 C. H2 consumption was detected with surface Cu atom, has been calculated from the SCu for various
a TCD. catalysts. As shown in Fig. 1, to better understand the role of
NH
2
, which represents the
ꢀ1
ature was raised in 10% H
1
2
2
per second per
ꢁ
0
ꢀ1
ꢁ
3
-TPD was conducted on a ChemBET 3000. 300 mg metallic Cu in the process of hydrogenation of CO
catalyst was heated to 400 C for 30 min, and then cooled to plotted versus SCu of the CZG
2
, TOF is also
ꢁ
x
H catalysts. It can be seen that the
ꢁ
ꢀ3 ꢀ1
5
0
C; following that 6 vol% NH
3
/Ar was introduced for values of TOF varied in the range of 0.67–0.75 ꢂ 10
s . TOF is
adsorption (60 min). Then, the catalyst was ushed with a He not a xed value, which indicates that the catalytic activity is not
ꢀ1
stream (30 mL min ) for 60 min to remove the weakly adsorbed only related to SCu, but also affected by such factors as the
ꢁ
24
NH
1
3
, and nally it was heated from 50 to 800 C at a rate of interaction between copper and zinc oxide. This observation
ꢁ
ꢀ1
0 C min . The NH -TPD signal was recorded.
indicates a good correlation of structure sensitive characters of
3
25
CO -TPD was also conducted on a ChemBET 3000. 50 mg DME from CO2. In addition, combining the values of TOF with
2
ꢁ
ꢀ1
catalyst was rst reduced at 300 C in a H
for 1 h, purged with puried Ar, and further treated at 450 C for addition of Ga
2
ow of 30 mL min
the results of Ga
2
O
3
content in Table 1 indicates that the
ꢁ
2
O
3
can increase the value of TOF.
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Table 1 Physicochemical properties and catalytic performances of the catalysts
b
Selectivity (%)
2
ꢀ1
a
a
2
ꢀ1
a
b
b
Catalyst
S
BET (m g
)
Dispersion (%)
S
Cu (m g
)
d
Cu (nm) CO
2
conversion (%) CO
C
2
H
4
MOH DME DME yield (%)
CZG
CZG
CZG
CZG
0
1
3
5
H
H
H
H
123.7
146.1
154.2
163.8
164.2
13.2
14.5
15.0
15.5
15.3
33.4
36.6
37.9
39.1
38.5
7.6
6.9
6.7
6.5
6.6
18.6
19.9
20.8
22.3
21.3
49.5 1.9
41.4 2.1
34.8 2.0
30.0 2.0
31.5 2.1
5.0
3.7
4.9
5.5
5.8
43.7
52.8
58.3
62.6
60.7
8.1
10.5
12.1
14.0
13.0
CZG10
H
a
b
ꢁ
Determined by the nitrous oxide titration method. Reaction conditions: T ¼ 260 C, P ¼ 3.0 MPa, weight ¼ 1 g, CO
2
: H
2
: N
2
¼ 3 : 9 : 1, GHSV ¼
ꢀ
1
1
500 h .
27
appeared in other works, which shows that the embedded
dispersion of CuO and ZnO are relatively uniform. The ne
particles of CuO and ZnO which may exist in an amorphous or
more disordered form, this will promote the synergistic effect,
28
thus resulting in the intensied CuO–ZnO interaction. This
interaction between CuO and ZnO is benecial to the increases
2
in the activity for CO conversion, which exactly coincides with
catalytic performance results.
The Cu surface area, Cu dispersion, and Cu particle sizes are
greatly affected by the concentrations of Ga O , as listed in
2
3
Table 1. When the content of Ga O increased, the change of the
2
3
peak shape rst increased and then decreased appeared in the
Cu surface area and Cu dispersion. The Cu surface area corre-
lated well with the copper dispersion; the results showed higher
values of SCu and DCu for CZG
maximum values were found to 39.1 (m
5
H than other catalysts, and the
2
ꢀ1
g
) and 15.5%,
Fig. 1 The relationship between the TOF of reacted CO
surface copper areas (SCu).
2
and the
respectively, while the particle size trend was opposite. The
small CuO particles and improved copper dispersion can be
ascribed to the character of the structural promoter of Ga O3
2
3
.2 The structure of the catalysts
increasing the resistance of CuO to sinter during the thermal
treatment, which consequently results in a larger Cu surface
30
The X-ray diffraction results are shown in Fig. 2. For all the
ꢁ
ꢁ
ꢁ
ꢁ
catalysts, the diffraction peaks at 2q ¼ 35.5 , 38.7 , 48.6 , 58.3 ,
ꢁ ꢁ
68.1 , and 75.2 can be indexed to the CuO phase (tenorite,
ꢁ
ꢁ
ꢁ
ꢁ
JCPDS 48-1548), the peaks at 2q ¼ 31.8 , 34.4 , 36.3 , 47.6 ,
ꢁ ꢁ ꢁ ꢁ
56.6 , 62.9 , 66.4 , and 72.6 can be ascribed to the ZnO phase
(
JCPDS 65-3411), and the diffraction peaks observed in the 2q
ꢁ
range of 21–25 can be attributed to HZSM-5 (JCPDS 44-0003).
2 3
However, diffraction peaks belonging to Ga O or other new
binding products are not observed for the catalysts with the
promoter of Ga O added, indicating that Ga O species are
2
3
2 3
present in either the amorphous state or highly dispersed in the
26
catalysts and cannot be detected by XRD. Strong CuO peak
intensities were detected in all the catalysts, which conrmed
that they had been completely crystallized. With the increase of
2 3
Ga O content to 5%, the intensities of the CuO diffraction
peaks decreased gradually, whereas the peak width broadened
slightly; it can be inferred from Scherrer's formula that the
average particle size of CuO decreases, indicating that the
crystallinity of CuO decreased and the dispersion of copper
increased with the addition of Ga O . Some diffraction peaks of
2
3
ꢁ
ꢁ
ꢁ
CuO (2q ¼ 35.5 ) and ZnO (2q ¼ 34.4 and 36.3 ) were severely
0 1 3
Fig. 2 XRD patterns of the catalysts: (a) CZG H; (b) CZG H; (c) CZG H;
superimposed on each other, and similar phenomena had
(
d) CZG H; (e) CZG10H.
5
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2
Table 2 Peak temperature, peak area and distribution ratio of H -TPR
A
A
a
/(A
) (%)
a
+
Catalyst
T
a
T
b
A
a
+ A
b
b
CZG
CZG
CZG
CZG
0
1
3
5
H
H
H
H
236
238
241
246
248
269
264
270
272
277
6282
53
57
63
69
30
14463
15694
15849
8107
CZG10
H
observed were only related to the CuO reduction. The H
results are presented in Fig. 4. The occurrence of two Gaussian
tting peaks (a and b) is observed for all of the H -TPR proles,
2
-TPR

2
which has been related to the reduction of different CuO species
in the catalysts. Previous authors have observed similar reduc-
Fig. 3 Relationship between the CO
2
conversion and Cu surface area. tion peaks for Cu based catalysts and have regarded the
ꢁ
a reduction peak at about 220–250 C as the surface reduction
peak of dispersed amorphous CuO, whereas the b reduction
ꢁ
area of the CZG
5
x
H catalysts. Decreasing the Ga
2 3
O
loading below peaks around 270–300 C can be classied as the reduction
31
% or increasing the Ga O loading over 5% also resulted in peaks of CuO in the bulk phase with large grains.
2 3
a decrease in Cu surface area and Cu dispersion, and an increase
in Cu particle size, which is consistent with the XRD results. The added catalysts shied to the higher temperature region,
2 3
In Table 2, it is notable that the reduction peaks of all Ga O -
32
results conrmed the conclusion that adding excessive Ga
longer benecial to improve the dispersion of Cu on the catalyst existence of CuO particles with different sizes and the different
surface. As displayed in Fig. 3, a plot of CO
conversion as a func- interaction strength between CuO particles and other oxides in
tion of Cu surface area is depicted, and there is a linear relation- the catalyst. As the content of Ga
ship between them. Perhaps an increase in the SCu resulted in of the two Gaussian peaks also changes. Among them, A
more dissociatively adsorbed hydrogen and its migration to ZnO or + A ) of CZG H is the largest, the proportion of easily reduced
Ga O through hydrogen spillover, which improved the activity of CuO on the surface is up to 69% in this catalyst system, and
2 3
O
is no consistent with other literature studies. This is due to the
2
1
7
2
O
3
changes, the area ratio
/(A
a
a
b
5
2
3
the catalyst. Therefore, Cu surface area is a crucial factor for the the more CuO on the surface is benecial to the hydrogena-
high yield of DME by hydrogenation of CO
tion of CO . Whether the amount of Ga is too large or too
small, the value of A /(Aa + A ) will decrease. Moreover, it
2
.
2
O
2 3
a
b
could be observed that the addition of Ga
cantly increased the peak areas as compared to the reduction
peak area of the Ga -free catalyst, indicating that more
CuO in the catalysts is reduced to produce more hydrogen
consumption. However, adding excess Ga (10%) leads to
a lower peak area; this suggests that Ga O species may be
2 3
O (<5%) signi-
3
.3 The reducibility of the catalysts
2 3
O
It has been reported that ZnO and Ga
the H
2 3
O
are not reduced under
2
-TPR conditions; therefore, the H reduction peaks
2
2
O
3
2
3
incorporated into CuO or may be deposited on the surface of
CuO, in accordance with previous observations.
3.4 Surface acidity of the catalysts
Fig. 5 shows the NH -TPD proles of the catalysts. There are
3
ꢁ
ꢁ
three desorption peaks near 110–150 C (peak a), 330–400 C
(
ꢁ
peak b) and 400–500 C (peak g) on the surface of each catalyst,
which respectively correspond to the weak acid center, medium
acid center and strong acid center on the surface of the cata-
33
2 3
lyst. In Table 3, compared with the Ga O -free catalyst, peak
a moves slightly to higher temperature, while peaks b and g
shi to higher temperature more obviously, suggesting the
increased strength, implying that Ga O modication increased
2 3
the catalyst medium and strong acidity obviously. During the
preparation process, the amount of oxalic acid as a precipitator
2 3
will be changed with the addition of Ga O , which may modify
Fig. 4
2 0 1
H -TPR profiles of the catalysts: (a) CZG H; (b) CZG H; (c)
CZG
3
H; (d) CZG H; (e) CZG10H.
5
the medium and strong acid sites. Comparing the total amount
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3 2 3
Fig. 5 NH -TPD spectrum of catalysts with different Ga O contents:
(
a) CZG H; (b) CZG
0
1
H; (c) CZG H; (d) CZG H; (e) CZG10H.
3
5
0 5
Fig. 6 Spectra of the prepared hybrid catalysts of CZG H and CZG H:
(A) Cu 2p; (B) Zn 2p; (C) Ga 2p.
of acid with the increase of Ga
peak areas of NH desorption peaks rst increase and then
decrease, indicating that the surface acid amounts change 3.6 X-ray photoelectron spectroscopy analysis
2 3
O content in the catalyst, the
3
34
correspondingly. It can be seen that the peak area of CZG
5
H is
As shown in Fig. 6, the surface chemical states of the Cu, Zn and
Ga on the prepared catalysts were analyzed by X-ray photo-
electron spectroscopy (XPS). As representative results, the XPS
the largest, and the content of weak acids is large, which is
benecial to the improvement of DME selectivity.
17,33
Comparing the NH -TPD and the previous catalytic perfor-
3
0
spectra of Cu 2p for the ex situ prepared catalysts of CZG H and
mance results (Table 1), the CZG H with different Ga O
2 3
x
CZG H are shown in Fig. 6A. It can be observed that the binding
5
contents has similar methanol selectivity, which indicates that
the acidity of the catalyst is enough to convert the generated
methanol to DME. The dimethyl ether selectivity is signicantly
different. The results suggest that an appropriate acid content
and acid strength of the catalyst are favorable for the selectivity
energies (BE) of the main characteristics peaks at about 932.9 eV
and 953.0 eV are assigned to Cu 2p and Cu 2p1/2, respec-
3
/2
35
tively, and two shake-up satellite peaks appeared at 940–
45 eV, indicating that the copper species in the catalyst surface
9
36
existed in the form of CuO. In addition, the binding energy
values of Cu 2p3/2 and cu 2p1/2 increased slightly with the
to DME in the reaction of CO hydrogenation to dimethyl ether.
2
addition of Ga
and Ga was enhanced, which was consistent with the results of
H -TPR. Both of the spectra contained two peaks at about 1022
2 3
O , indicating that the interaction between Cu
37
3
.5 Adsorption and desorption performance of catalysts for
CO
CO TPD proles of reduced CZG H and CZG H catalysts are
2
and 1045 eV (Fig. 6B), which are assigned to Zn 2p and Zn 2p1/
3/2
2
38
2
peaks of ZnO, respectively, with a spin energy separation of
2
0
5
2
3 eV. This shows that zinc atoms are present in the catalyst in
shown in Fig. S3.† There are three CO desorption peaks. With
2
39
the form of ZnO. The binding energies of Zn 2p1/2 and Zn 2p3/2
Ga
2
O
3
addition, the proportion of weak CO
2
adsorption (<300
2 3
become larger with the addition of Ga O . Similarly, as shown
in Fig. 6C, the double peaks of 1117.4 eV and 1144.5 eV corre-
spond to the binding energies of Ga 2p3/2 and Ga 2p1/2 of
ꢁ
ꢁ
C) and strong CO
2
adsorption (>500 C) increased, while the
ꢁ
proportion of medium adsorption (300–450 C) decreased. It
can be seen that the addition of Ga
2 3
O did not promote the
absorption of CO (Fig. S3 and Table S5†).
2
a
2 3
Table 3 The acid properties of catalysts with different Ga O contents
Medium acidic
amount
Strong acidic
amount
Total acidic
amount
Catalyst
Weak acidic amount
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
CZG
CZG
CZG
CZG
0
1
3
5
H
H
H
H
0.13 (140 C)
0.32 (354 C)
0.19 (407 C)
0.64
0.71
0.92
1.00
0.79
0.13 (144 C)
0.34 (355 C)
0.25 (409 C)
ꢁ
ꢁ
ꢁ
0.20 (148 C)
0.47 (356 C)
0.25 (415 C)
ꢁ
ꢁ
ꢁ
0.22 (146 C)
0.44 (362 C)
0.34 (415 C)
ꢁ
ꢁ
ꢁ
CZG10
H
0.19 (152 C)
0.39 (373 C)
0.21 (425 C)
a
5
The amount of acidity of CZG H was assigned as 1.0, and compared with that of the other samples. The temperature in parentheses is Tmax.
14430 | RSC Adv., 2021, 11, 14426–14433
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+
0
methanol, it is generally believed that Cu and Cu contribute to
19
the catalytic activity.
3.7 Effect of reaction temperature on the catalysts
Reaction temperature also has a signicant effect on CO
2
conversion, product selectivity and yield. Under different
temperature conditions, the changes of conversion and yield
are shown in Fig. 8. It has been reported in the literature that
Cu/ZnO-based catalyst activity will be reduced when the
0 5
Fig. 7 Spectra of the reduced and recovered CZG H and CZG H
hybrid catalysts: (A) Cu 2p; (B) Cu (LMM) Auger; (a) the reduced CZG
b) the recovered CZG H; (c) reduced CZG H; (d) the recovered
CZG H.
0
H;
(
0
5
ꢁ
temperature exceeds 300 C; the deactivation is caused by the
5
45
aggregation of Cu crystals. Therefore, the maximum temper-
ꢁ
ature we tested was controlled at 300 C. Comparing the curves
reduced CZG
oxidation state of Ga .
The XPS spectra of reduced and recovered CZG
5
H, respectively, indicating that Ga
2
O
3
is in the
at different temperatures, it is concluded that the optimal
3+
40
ꢁ
reaction temperature is 260 C. At low temperature, there is
0
H and
competitive adsorption of the reaction product CO and reactant
CO2 on metal active sites. Methanol is mainly prepared by
hydrogenation of CO produced by the reverse reaction of water
vapor, so the conversion of carbon dioxide is low. Since the
reaction is exothermic, with the increased temperature and
reaction time of HZSM-5, there will be carbon deposition on the
outer surface and in the channel, leading to a decrease in the
DME selectivity. In addition, higher temperatures will cause
sintering and crystallization of copper species, causing catalyst
coking and thereby reducing the conversion rate of carbon
CZG H hybrid catalysts are shown in Fig. 7. The signal peaks of
Cu that appeared at 933 eV and 952 eV are assigned to Cu 2p3/2
5
and Cu 2p1 , respectively, which correspond to the reduction
/2
0
+
41
sites (Cu and/or Cu ) (Fig. 7A). Compared with Fig. 6A, the Cu
p XPS spectra of the catalyst aer H reduction show that the
2
2
satellite peak disappeared and the Cu 2p peak moved to lower
binding energy at the same time, indicating that copper was
+
0
42
almost reduced to Cu or Cu . The enhanced peak intensity
indicates that the copper part of the bulk phase migrates to the
surface during reduction. Sun et al. proposed that the electronic
interaction between Cu and Zn will produce the active center
15
ꢁ
dioxide. The yield of DME reached the maximum at 260 C,
which means that due to the exothermic reaction, the conver-
+
Cu –O–Zn, which provides a synergistic effect between Cu and
2
sion of CO is thermodynamically restricted, or may be Cu
43
ZnO for the production of dimethyl ether. Aer the reaction,
the XPS intensity of Cu 2p changed slightly, but no obvious
changes in the position and shape were observed. It can be seen
that the valence state of copper has not changed aer the
catalyst is used (10 hours). The comparison shows that the
valence states of Cu at the main Cu 2p3/2 and Cu 2p1/2 on the
catalyst surface are basically unchanged aer reduction and
reaction, indicating that the catalyst has good chemical stability
as a whole. In the XPS spectra of Cu 2p3/2, the binding energies
46
sintering. Relatively the selectivity of CO gradually increased,
which is due to the reverse water vapor reaction (RWGS) of
47
CO
2 2
. In view of these results, we investigated the CO hydro-
genation activity of the CuO–ZnO–Ga O /HZSM-5 catalyst at the
2
3
ꢁ
optimal temperature of 260 C.
In view of these observations, each metal component has
a unique effect on the system studied in this paper. As stated
above, CuO is the active center of methanol reaction in the
catalyst of carbon dioxide hydrogenation to dimethyl ether,
which causes carbon dioxide hydrogenation to form
+
0
2
of Cu and Cu are almost the same, and those of Cu O and Cu
metal are 932.2 eV and 932.4 eV, respectively. Therefore, the
kinetic energies (KE) of the reduced catalyst were studied by Cu
LMM X-ray auger electron spectroscopy (XAES) spectrum anal-
ysis to further distinguish the valence state of Cu on the catalyst
surface. The line positions in the Cu (LMM) Auger spectra of the
reduced and recovered CZG
0 5
H and CZG H catalysts indicate
that the two asymmetrically located peaks near 916.5 eV and
+
0
18
9
18.7 eV correspond to Cu and Cu species, respectively, and
+
Cu is the main copper species detected on the catalyst surface
Fig. 7B). Based on the corresponding Cu LMM peaks, the Cu /
Cu area ratios of the two reduction catalysts are obtained by
+
(
0
calculation. The area ratio of the CZG H catalyst was higher
5
than that of the CZG H catalyst, which indicated that the ratio
0
+
0
2 3
of Cu /Cu can be maintained better with the addition of Ga O
and the activity of the catalyst may be determined by Cu species
rather than Cu species. It is worth noting that this XPS test is
not performed in situ, and this may be subject to some errors.
Although there is still controversy about the active center of
+
0
44
Fig. 8 Effect of reaction temperature on the catalytic performance: (a)
CZG H; (b) CZG H; (c) CZG H; (d) CZG H; (e) CZG10H.
0 1 3 5
copper-based catalysts in the process of CO hydrogenation to
2
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2021 The Author(s). Published by the Royal Society of Chemistry
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4
8
methanol. ZnO can effectively improve the degree of copper
dispersion to increase the specic surface area of Cu and
4 T. H. Fleisch, A. Basu and R. A. Sills, J. Nat. Gas Sci. Eng.,
2012, 9, 94–107.
5 V. N. Ipatieff and G. S. Monroe, J. Am. Chem. Soc., 1945, 67,
2168–2171.
6 D. M. Sung, Y. H. Kim, E. D. Park and J. E. Yie, Catal.
Commun., 2012, 20, 63–67.
49
increase the stability of the catalyst. The formation of the Zn–
CuO alloy from ZnO and CuO is the main active point of
methanol synthesis, thus increasing the conversion rate of
50
2 2 3
CO . The Ga O promoted bifunctional catalyst shows a good
dispersion of copper with a small particle size, which leads to
a high surface area of copper and an increase in the acidity of
7 S. J. Ren, X. Fan, Z. Y. Shang, W. R. Shoemaker, L. Ma,
T. P. Wu, S. G. Li, N. B. Klinghoffer, M. Yu and X. H. Liang,
dehydrated components. Ga
promoter to adjust the optimal ratio of Cu /Cu in the catalyst.
2
O
3
as is used as an electronic
2
J. CO Util., 2020, 36, 82–95.
+
0
29
8 I. Miletto, E. Catizzone, G. Bonura, C. Ivaldi, M. Migliori,
E. Gianotti, L. Marchese, F. Frusteri and G. Giordano,
Materials, 2018, 11, 2275.
9 J. Abu-Dahrieh, D. Rooney, A. Goguet and Y. Saih, Chem. Eng.
J., 2012, 203, 201–211.
All of these lead to a high rate of methanol formation and
continuous dehydration to DME, thus increasing the selectivity
51
to DME.
1
0 S. Hassanpour, F. Yaripour and M. Taghizadeh, Fuel Process.
Technol., 2010, 91, 1212–1221.
4
. Conclusions
In conclusion, Ga
catalysts for dimethyl ether synthesis from CO
were prepared and the catalytic performances were evaluated.
2
O
3
promoted CuO–ZnO–Ga
2
O
3
/HZSM-5 11 H. Bahruji, R. D. Armstrong, J. R. Esquius, W. Jones,
hydrogenation
M. Bowker and G. J. Hutchings, Ind. Eng. Chem. Res., 2018,
57, 6821–6829.
2
An appropriate amount of Ga O beneted smaller Cu particles, 12 R. Khoshbin, M. Haghighi and P. Margan, Energy Convers.
2
3
higher copper surface areas, intensied CuO–ZnO interaction,
Manage., 2016, 120, 1–12.
+
and proper acid amounts, maintained the proportion of Cu / 13 M. Sanchez-Contador, A. Ateka, A. T. Aguayo and J. Bilbao,
0
Cu on the surface of the catalyst and acid site distribution,
therefore leading to higher CO conversion and DME selectivity. 14 T. Witoon, P. Kidkhunthod, M. Chareonpanich and
Among the investigated catalysts, the hybrid system CuO–ZnO– J. Limtrakul, Chem. Eng. J., 2018, 348, 713–722.
Ga
/HZSM-5 showed the best performance when the content 15 Q. Zhang, Y. Z. Zuo, M. H. Han, J. F. Wang, Y. Jin and F. Wei,
of Ga O was 5 wt% in the direct conversion of CO to DME. The Catal. Today, 2010, 150, 55–60.
CZG H catalyst showed the maximum CO conversion, DME 16 Y. B. Hu, Y. J. Zhang, J. Du, C. Y. Li, K. J. Wang, L. D. liu,
Fuel Process. Technol., 2018, 179, 258–268.
2
2 3
O
2
3
2
5
2
selectivity and DME yields of 22.3%, 62.6% and 14.0%,
X. R. Yu, K. Wang and N. Liu, RSC Adv., 2018, 8, 30387–
respectively, under the reaction conditions of the optimal
30395.
ꢁ
reaction temperature of 260 C, the space velocity of GHSV 1500 17 W. J. Cai, Q. Chen, F. G. Wang, Z. C. Li, H. Yu, S. Y. Zhang,
ꢀ1
h
and the reaction pressure of 3.0 MPa.
L. Cui and C. M. Li, Catal. Lett., 2019, 149, 2508–2518.
8 J. Słoczy n´ ski, R. Grabowski, P. Olszewski, A. Kozłowska,
J. Stoch, M. Lachowska and J. Skrzypek, Appl. Catal., A,
1
Conflicts of interest
2006, 310, 127–137.
The authors declare no conict of interest.
19 S. Natesakhawat, J. W. Lekse, J. P. Baltrus, P. R. Ohodnicki,
B. H. Howard, X. Y. Deng and C. Matranga, ACS Catal.,
2
012, 2, 1667–1676.
Acknowledgements
2
0 G. Bonura, C. Cannilla, L. Frusteri and F. Frusteri, Appl.
This work was supported by the National Natural Science
Catal., A, 2017, 544, 21–29.
Foundation of China (51301114), Natural Science Foundation of 21 H. Ham, S. W. Baek, C. H. Shin and J. W. Bae, ACS Catal.,
Liaoning Province (2019-ZD-0077), Natural Science Foundation
2019, 9, 679–690.
of Shenyang University of Chemical
Technology 22 Q. Jiang, Y. F. Liu, T. Dintzer, J. J. Luo, K. Parkhomenko and
XXLQ2019002), Science Research Foundation of Education
A. C. Roger, Appl. Catal., B, 2020, 269, 118804.
Department of Liaoning Province (LZ2019003), and Liaoning 23 Y. Q. Zhao, J. X. Chen and J. Y. Zhang, J. Nat. Gas Chem.,
(
Revitalization Talents Program (XLYC1907029).
2007, 16, 389–392.
2
2
4 Q. Sun, Y. L. Zhang, H. Y. Chen, J. F. Deng, D. Wu and
S. Y. Chen, J. Catal., 1997, 167, 92–105.
5 Y. J. Lee, M. H. Jung, J. B. Lee, K. E. Jeong, H. S. Roh,
Y. W. Suh and J. W. Bae, Catal. Today, 2014, 228, 175–182.
References
1
2
3
O. I. Awad, O. M. Ali, R. Mamat, A. A. Abdullah, G. Naja,
M. K. Kamarulzaman, I. M. Yusri and M. M. Noor, 26 J. Toyir, P. R. de la Piscina, J. L. G. Fierro and N. Homs, Appl.
Renewable Sustainable Energy Rev., 2017, 69, 1232–1242.
Catal., B, 2001, 29, 207–215.
F. A. Rahman, M. M. A. Aziz, R. Saidur, W. A. W. A. Bakar, 27 K. W. Jun, W. J. Shen, K. S. R. Rao and K. W. Lee, Appl. Catal.,
M. R. Hainin, R. Putrajaya and N. A. Hassan, Renewable
Sustainable Energy Rev., 2017, 71, 112–126.
Z. Azizi, M. Rezaeimanesh, T. Tohidian and
M. R. Rahimpour, Chem. Eng. Process., 2014, 82, 150–172.
A, 1998, 174, 231–238.
28 Y. Q. Sun, X. H. Han and Z. K. Zhao, Catal. Sci. Technol.,
2019, 9, 3763–3770.
14432 | RSC Adv., 2021, 11, 14426–14433
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
2
3
3
3
3
3
3
3
3
3
3
4
9 J. Toyir, P. R. de la Piscina, J. L. G. Fierro and N. Homs, Appl. 41 L. M. He, X. R. Li, W. W. Lin, W. Li, H. Y. Cheng, Y. C. Yu,
Catal., B, 2001, 34, 255–266.
0 P. B. Sanguineti, M. A. Baltanas and A. L. Bonivardi, Appl.
Catal., A, 2015, 504, 476–481.
1 C. J. Xu, X. Y. Hao, M. Y. Gao, H. Q. Su and S. H. Zeng, Catal.
Commun., 2016, 73, 113–117.
2 S. Kuhl, J. Schumann, I. Kasatkin, M. Havecker, R. Schlogl
and M. Behrens, Catal. Today, 2015, 246, 92–100.
S. I. Fujita, M. Arai and F. Y. Zhao, J. Mol. Catal. A: Chem.,
2014, 39, 143–149.
42 P. H. Matter, D. J. Braden and U. S. Ozkan, J. Catal., 2004,
223, 340–351.
43 K. P. Sun, W. W. Lu, F. Y. Qiu, S. W. Liu and X. L. Xu, Appl.
Catal., A, 2003, 252, 243–249.
44 K. Klier, Adv. Catal., 1982, 31, 243–313.
3 S. Asthana, C. Samanta, R. K. Voolapalli and B. Saha, J. 45 G. Celik, A. Arinan, A. Bayat, H. O. Ozbelge, T. Dogu and
Mater. Chem. A, 2017, 5, 2649–2663. D. Varisli, Top. Catal., 2013, 56, 1764–1774.
4 L. Y. Li, D. S. Mao, J. Xiao, L. Li, X. M. Guo and J. Yu, Chem. 46 S. J. Ren, W. R. Shoemaker, X. F. Wang, Z. Y. Shang,
Eng. Res. Des., 2016, 111, 100–108.
5 R. W. Liu, Z. Z. Qin, H. B. Ji and T. M. Su, Ind. Eng. Chem.
Res., 2013, 52, 16648–16655.
N. Klinghoffer, S. G. Li, M. Yu, X. Q. He, T. A. White and
X. H. Liang, Fuel, 2019, 239, 1125–1133.
47 G. Bonura, M. Migliori, L. Frusteri, C. Cannilla, E. Catizzone,
6 C. M. Li, X. D. Yuan and K. R. Fujimoto, Appl. Catal., A, 2014,
2
G. Giordano and F. Frusteri, J. CO Util., 2018, 24, 398–406.
4
69, 306–311.
7 F. C. F. Marcos, J. M. Assaf and E. M. Assaf, Mol. Catal., 2018,
58, 297–306.
48 S. Kattel, P. J. Ram ´ı rez, J. G. Chen, J. A. Rodriguez and P. Liu,
Science, 2017, 355, 1296–1299.
49 J. Nakamura, T. Uchijima, Y. Kanai and T. Fujitani, Catal.
Today, 1996, 28, 223–230.
50 T. Fujitani and J. Nakamura, Appl. Catal., A, 2000, 191, 111–
129.
4
8 F. P. Li, M. Ao, G. H. Pham, J. Sunarso, Y. P. Chen, J. Liu,
K. Wang and S. M. Liu, Small, 2020, 16, 1906276.
9 V. Deerattrakul, P. Dittanet, M. Sawangphruk and
P. Kongkachuichay, J. CO
0 A. M. Hengne, D. J. Yuan, N. S. Date, Y. Saih, S. P. Kamble,
2
Util., 2016, 16, 104–113.
51 S. H. Kang, J. W. Bae, H. S. Kim, G. M. Dhar and K. W. Jun,
Energy Fuels, 2010, 24, 804–810.
C. V. Rode and K. W. Huang, Ind. Eng. Chem. Res., 2019,
58, 21331–21340.
©
2021 The Author(s). Published by the Royal Society of Chemistry
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