Hydrogenation of CO2 to dimethyl ether on La-, Ce-modified Cu-Fe/HZSM-5 catalysts

Article abstract of DOI:10.1016/j.catcom.2015.12.010

Cu-Fe-La/HZSM-5 and Cu-Fe-Ce/HZSM-5 bifunctional catalysts were prepared and applied for the direct synthesis of dimethyl ether (DME) from CO₂ and H₂. The catalysts were characterized by X-ray diffraction (XRD), N₂ adsorption-desorption, H₂-temperature programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The results showed that La and Ce significantly decreased the outer-shell electron density of Cu and improved the reduction ability of the Cu-Fe catalyst in comparison to the Cu-Fe-Zr catalyst, which may increase the selectivity for DME. The Cu-Fe-Ce catalyst had a greater specific surface area than the Cu-Fe-La catalyst. This promoted CuO dispersion and decreased CuO crystallite size, which increased both the DME selectivity and the CO₂ conversion. The catalysts were stable for 15 h.

Full text of DOI:10.1016/j.catcom.2015.12.010

Catalysis Communications 75 (2016) 78–82
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Catalysis Communications
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Short communication
Hydrogenation of CO2 to dimethyl ether on La-, Ce-modified
Cu-Fe/HZSM-5 catalysts
a
a
a
Zu-zeng Qin a, , Xin-hui Zhou , Tong-ming Su , Yue-xiu Jiang , Hong-bing Ji
⁎
a,b,⁎⁎
a
School of Chemistry and Chemical Engineering, Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University,
Nanning 530004, PR China
b
School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 October 2015
Received in revised form 4 December 2015
Accepted 12 December 2015
Available online 13 December 2015
Cu–Fe–La/HZSM-5 and Cu–Fe–Ce/HZSM-5 bifunctional catalysts were prepared and applied for the direct
synthesis of dimethyl ether (DME) from CO2 and H2. The catalysts were characterized by X-ray diffraction
(XRD), N2 adsorption–desorption, H2-temperature programmed reduction (H2-TPR), and X-ray photoelectron
spectroscopy (XPS). The results showed that La and Ce significantly decreased the outer-shell electron density
of Cu and improved the reduction ability of the Cu–Fe catalyst in comparison to the Cu–Fe–Zr catalyst,
which may increase the selectivity for DME. The Cu–Fe–Ce catalyst had a greater specific surface area than the
Cu–Fe–La catalyst. This promoted CuO dispersion and decreased CuO crystallite size, which increased both the
DME selectivity and the CO2 conversion. The catalysts were stable for 15 h.
Keywords:
CO2 hydrogenation
Cu based catalysts
DME synthesis
Mixed oxide
© 2015 Elsevier B.V. All rights reserved.
Bifunctional catalyst
1. Introduction
Al2O3/HZSM-5 [7,8], CuO–ZnO–Al2O3–ZrO2/HZSM-5 [9], and Cu-ZnO-
ZrO2/HZSM-5 [10], along with non-Cu-based catalysts including Pd-
With the economic development and the expansion of industria-
lization, the combustion of coal, gasoline, natural gas, and other hydro-
carbons increases the CO2 content in the atmosphere every year,
causing the earth's temperature to rise [1]. However, CO2 is also a poten-
tial carbon resource. To utilize CO2 and solve the environmental prob-
lems caused by CO2, the key issue is to develop technologies to
capture, store and use CO2 [2,3]. One way to do this is to effectively
translate CO2 into hydrocarbon fuels by, for example, CO2 hydrogena-
tion to dimethyl ether (DME) [4].
Presently, there are two main processes for synthesizing DME — a
two-step process and a one-step process — using CO2 and H2 as the
raw materials. The one-step process combines methanol synthesis and
methanol dehydration catalysts in the same reactor to directly syn-
thesize DME from CO2 and H2. This method has received considerable
attention, as it is thermodynamically and economically more advanta-
geous than the traditional two-step process. However, the one-step pro-
cess still remains in the laboratory exploration stage. The catalysts used
in the one-step process to synthesize DME include Cu-based catalysts
such as CuO–TiO2–ZrO2/HZSM-5 [5], Cu-ZnO/Al2O3 [6], CuO–ZnO–
Pd2Ga [11,12]. Due to the stability of CO2, its activation is a bottleneck
problem that is difficult to solve, and the CO2 hydrogenation reaction
is conducted at 4–8 MPa and 250–350 °C, leading to the deactivation
of the catalyst. Even when used Cu–Fe–Zr/HZSM-5 was used as catalyst
[13,14], which decreased the pressure and temperature to 3–4 MPa and
220–260 °C, respectively, the conversion of CO2 was still only 25%–30%
and the selectivity of DME was only 40%–50% [13,15], limiting the in-
dustrial application of this method.
La and Ce have been added to catalysts to promote the dispersion of
metal, decrease the reduction temperature and the crystallite size, and
improve the thermal stability of the catalyst [16–18]. Based on a previ-
ous Cu–Fe–Zr catalyst [13,14], the Cu–Fe–La, and Cu–Fe–Ce catalysts
were synthesized in this work using a homogeneous precipitation
method. The catalysts were then characterized by X-ray diffractometer
(XRD), N2 adsorption-desorption, H2-temperature programmed re-
duction (H2-TPR), and X-ray photoelectron spectrometer (XPS), and ap-
plied in the hydrogenation of CO2 to DME.
2. Experimental
2.1. Preparation of catalyst
⁎
Correspondence to: Z.-z Qin, School of Chemistry and Chemical Engineering, Guangxi
University, Nanning 530004, PR China.
⁎⁎ Correspondence to: H.-b Ji, School of Chemistry and Chemical Engineering, Sun Yat-
sen University, Guangzhou 510275, PR China.
The precursor of the methanol synthesis catalyst was prepared
by homogeneous precipitation. Cu(NO3)2·3H2O and Fe(NO3)3·9H2O
were weighed according to the Cu/Fe molar ratio of 3:2. Likewise,
E-mail addresses: qinzuzeng@gmail.com (Z. Qin), jihb@mail.sysu.edu.cn (H. Ji).
http://dx.doi.org/10.1016/j.catcom.2015.12.010
1566-7367/© 2015 Elsevier B.V. All rights reserved.

Z. Qin et al. / Catalysis Communications 75 (2016) 78–82
3. Results and discussion
79
3.1. XRD analysis
Fig. 1 shows the XRD patterns of the Cu–Fe, Cu–Fe–Zr, Cu–Fe–La and
Cu–Fe–Ce catalysts calcined at 400 °C without H2 reduction.
The characteristic peaks of monoclinic CuO (JCPDS No. 48-1548) at
2θ = 35.49°, 38.69°, and 58.26° and of cubic crystalline Fe2O3 (JCPDS
No. 39-1346) at 2θ = 30.24°, 35.63°, and 62.93° are found in the XRD
patterns of Cu–Fe–La and Cu–Fe–Ce. Compared with Cu–Fe and Cu–
Fe–Zr, the diffraction peaks of CuO and Fe2O3 in the Cu–Fe–La and Cu–
Fe–Ce patterns are broadened, and the peak intensities are weakened,
indicating that the crystallite size of CuO is smaller. Smaller crystallites
correspond to better copper dispersion. The crystallite sizes of CuO
(111) in the Cu–Fe–La and Cu–Fe–Ce catalysts were calculated using
the Sherrer equation and determined to be 19.1 and 17.6 nm, respec-
tively. However, the crystallite sizes in the Cu–Fe and Cu–Fe–Zr catalysts
are 22.5 and 19.3 nm, respectively, indicating that the modification of
La2O3 and CeO2 decreased the crystallite size of CuO and promoted the
dispersion of CuO [19]. The crystallite size of the Cu–Fe–Ce catalyst is
clearly the smallest among the catalysts. The diffraction peaks at 2θ =
29.9°, 46.1°, and 52.1° were attributed to the La2O3 phase (JCPDS No.
05-0602). Peaks corresponding to CeO2 − x at 2θ = 26.3o, 43.9o and
55.7o (JCPDS No. 49-1415) were found. These results suggested that
some of the smaller-sized Cu2+ ions (ionic radius of 0.79 Å compared
to 0.92 Å for Ce4+) entered the CeO2 lattice to form a Ce1 − xCuxO2 − x
solid solution [20].
Fig. 1. XRD patterns of Cu–Fe (a), Cu–Fe–Zr (b), Cu–Fe–La (c) and Cu–Fe–Ce (d) calcined at
400 °C without H2 reduction.
Zr(NO3)4·5H2O, La(NO3)3·6H2O, and Ce(NO3)3·6H2O were weighed
based on the corresponding ZrO2, La2O3, and CeO2 contents of 1.0 wt%
in the ternary metal oxides. After mixed oxide Cu–Fe–Ce, Cu–Fe–La
and Cu–Fe–Zr (i.e., the methanol dehydration components) were pre-
pared via homogeneous precipitation, they were mechanically mixed
with HZSM-5(Shanghai Novel Chemical Technology Co., Ltd.) with a sil-
ica–alumina ratio of 300:1 in a 1:1 mass ratio. The details of the prepa-
ration method are given in the supporting information.
3.2. Nitrogen adsorption/desorption of catalysts
Fig. 2 shows the N2 adsorption–desorption isotherms and pore size
distribution profiles of the Cu–Fe, Cu–Fe–Zr, Cu–Fe–La, and Cu–Fe–Ce
catalysts calcined at 400 °C without H2 reduction.
2.2. Characterization of the catalyst
Based on the IUPAC classification, the N2 adsorption–desorption iso-
therms of the Cu–Fe–La and Cu–Fe–Ce catalysts (Fig. 2A) belonged to
XRD, XPS, N2 adsorption–desorption and H2-TPR were used to char-
acterize the catalysts following previously reported procedures [13].
IV-type isotherms. In the low- and medium-pressure region (P/P0 =
0.0–0.8), the amount of adsorbed N2 gently increased, indicating that
the adsorption of N2 on the internal surfaces of catalyst pores shifted
2.3. Catalytic hydrogenation of CO2 to DME
from monolayer to multilayer. In the high-pressure region (P/P0 =
0.8–1.0), an H3-type hysteresis loop was generated by capillary conden-
sation, suggesting that the Cu–Fe–La and Cu–Fe–Ce catalysts were
mesostructured materials [21]. The pore size distribution profiles
(Fig. 2B) indicate that the majority of mesopores had diameters of
approximately 3 nm. The specific surface areas of the Cu–Fe, Cu–
Fe–Zr, Cu–Fe–La, and Cu–Fe–Ce catalysts were calculated using
the Brunauer–Emmett–Teller (BET) equation according to the N2
The reaction process for DME synthesis from CO2 and H2 can be
found in the literature [13]. The feed gas was a mixture of H2 and CO2
gas (4:1 mol ratio) after reduction, and the catalytic hydrogenation of
CO2 to DME was performed at 260 °C and 3.0 MPa with a gaseous hourly
space velocity (GHSV) of 1500 mL·g−cat1·h−1. The details of the catalytic
hydrogenation of CO2 to DME are found in the literature [13].
Fig. 2. Nitrogen adsorption/desorption isotherms (A) and pore size distribution profiles (B) of Cu–Fe (a), Cu–Fe–Zr (b), Cu–Fe–La (c) and Cu–Fe–Ce (d) catalysts calcined at 400 °C without
2 reduction.
H

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Z. Qin et al. / Catalysis Communications 75 (2016) 78–82
Table 1
Textural properties of Cu-Fe, Cu-Fe-Zr, Cu–Fe–La and Cu–Fe–Ce catalysts.
Catalyst
BET surface area (m2·g−1
)
Average pore diameter (nm)
Cu–Fe
50.32
52.37
52.55
56.41
25.54
20.86
23.44
20.62
Cu–Fe–Zr
Cu–Fe–La
Cu–Fe–Ce
adsorption isotherms, and the results are shown in Table 1. The
specific surface areas of the Cu–Fe, Cu–Fe–Zr, Cu–Fe–La and Cu–
Fe–Ce catalysts were 50.32, 52.37, 52.55, and 56.41 m2·g−1, respec-
tively. Based on the 4f electron orbit and structural relaxation of Ce,
the CeO particle size will be reduced, consequently increasing the
surface area and the concentration of defects, such as oxygen vacan-
cies [22].Similar results were obtained after modification with La
and Zr.
Fig. 4. H2-TPR profiles for Cu–Fe, Cu–Fe–Zr, Cu–Fe–La and Cu–Fe–Ce catalysts. The solid
curves are experimental curves, and broken curves are Gaussian multipeak fitting curves.
3.3. Results of X-ray photoelectron spectroscopy
XPS was applied to study the oxidation states of the elements on the
surfaces of the Cu–Fe, Cu–Fe–Zr, Cu–Fe–La, and Cu–Fe–Ce catalysts
(Fig. 3).
indicating that Ce modification has the greatest effect on the outer elec-
tron density of Cu. Considering the XRD results, this likely occurs be-
cause CuO and CeO2 form a solid solution.
In Fig. 3, the binding energy of Cu 2p3/2 in the Cu–Fe–Ce catalyst was
933.97 eV, with featured satellite peaks at approximately 941.37 eV. The
binding energy of Cu 2p1/2 was 953.97 eV, with featured satellite peaks
at approximately 962.42 eV. Likewise, the binding energy of Cu 2p3/2 in
the Cu–Fe–La catalyst was 933.77 eV, with featured satellite peaks at ap-
proximately 941.57 eV, and the binding energy of Cu 2p1/2 was
953.82 eV, with featured satellite peaks at approximately 962.52 eV.
Hence, it can be determined that Cu occurred in the form of Cu2+ in
CuO [23]. After modification with Zr, La, and Ce, XPS analysis showed
that the Cu 2p3/2 and Cu 2p1/2 major peaks shifted by 0.18 and 0.31 eV
[13], 0.17 and 0.37 eV, and 0.37 and 0.52 eV, respectively, towards
higher binding energies. These data suggest that Zr, La, and Ce can ex-
change electrons with CuO, decreasing the outer-shell electron density
of Cu and slightly affecting the chemical combination state of CuO
[24], finally influencing the catalytic activities of the catalysts. Further-
more, after modification with Ce, the binding energies of Cu 2p3/2 and
Cu 2p1/2 show a maximum red shift among the three kinds of modifiers,
In Fig. 3, the binding energies of Fe 2p3/2 and Fe 2p1/2 in the Cu–Fe–La
and Cu–Fe–Ce catalysts were 711.07 eV and 724.62 eV, and 711.17 eV
and 724.47 eV, respectively, suggesting that the chemical valence of Fe
was Fe3+ in Fe2O3 [25]. Seven peaks can be fit in the XPS spectrum of
La 3d in the Cu–Fe–La catalyst; the solid curves correspond to La 3d5/2
and the broken curves correspond to La 3d3/2, suggesting that the chem-
ical valence of La was La3+ [26]. Fig. 3 shows the XPS spectrum of Ce 3d
in the Cu–Fe–Ce catalyst. Due to the hybridization between Ce 4f and O
2p, the explanation of Ce 3d was more complex [27]. Two sets of spin-
orbit multiplets were observed: U and V correspond to Ce 3d3/2 and Ce
3d5/2 contributions, respectively. The Ce 3d spectrum contains three
main Ce 3d5/2 features at 883.1 (V1), 889.5 (V2), and 898.5 (V3) eV and
three main Ce 3d3/2 features at 902.6 (U1), 912.3 (U2), and 917.1 (U3)
eV, indicating that Ce exists in Cu–Fe–Ce catalyst as Ce3+ and Ce4+
[28–30].
Fig. 3. XPS spectra of Cu 2p, Fe 2p, La 3d and Ce 3d regions for Cu–Fe (a), Cu–Fe–Zr (b), Cu–Fe–La (c) and Cu–Fe–Ce (d) catalysts.

Z. Qin et al. / Catalysis Communications 75 (2016) 78–82
81
Table 2
the sintering of the Cu active species and the formation of Cu crystal
grains during the hydrogenation process. Furthermore, more Cu
would be exposed on the surface of the catalyst, which could increase
the reducibility of the catalyst [33,34]. The reduction properties of high-
ly disperse CuO corresponding to peak α are closely related to the re-
duction properties of the Cu-based catalyst [35]; thus, modifying the
CuO–Fe2O3 catalyst with La or Ce may improve the reducibility of the
catalyst.
The temperature of peak α of the Cu–Fe–Ce catalyst was the lowest,
suggesting that the Cu–Fe–Ce catalyst exhibited optimum reducing be-
havior. Moreover, the area of the reducing peaks α in the Cu–Fe–Ce cat-
alyst was 20.9% higher than that in the Cu–Fe–La catalyst. At the same
time, the areas of the reducing peaks α in Cu–Fe–La and Cu–Fe–Ce
were 15.3% and 28.7% of the total area of the reducing peaks, respective-
ly; the corresponding value for Cu–Fe was 7.77%. This results suggested
that the proportion of highly disperse CuO was greatly increased by
modifying with La and Ce. In addition, the total hydrogen consumption
of Cu–Fe–Ce was decreased compared to that of the Cu–Fe catalyst. Con-
sidering the XRD results, this might be because CuO and CeO2 formed a
solid solution.
Temperatures and areas of the reduction peaks of Cu–Fe, Cu–Fe–Zr, Cu–Fe–La, and Cu–Fe–
Ce catalysts.a
Catalyst
Peak α
T (°C)
Peak β
T (°C)
Peak γ
T (°C)
Total area
Areab
Area
Area
Cu–Fe
172
168
161
156
0.89
3.33
1.91
2.31
219
193
182
178
7.30
1.89
5.98
2.39
247
213
209
195
3.26
6.78
4.57
3.34
11.45
11.97
12.46
8.04
Cu–Fe–Zr
Cu–Fe–La
Cu–Fe–Ce
a
The results were measured from H2-TPR profiles, and the areas were calculated
by integrating the areas under the peaks.
b
The unit of peak area is ×104 units.
Table 3
DME synthesis by CO2 hydrogenation over Cu–Fe based catalysts.a
Catalyst
Conversion of Selectivity of products
Yield of DME
(mol%)
CO2 (mol%)
(mol%)
DME CH3OH CO
CH4
Cu-Fe/HZSM-5
12.3
18.3
39.9
51.3
52.0
0.9
1.9
1.5
2.1
30.5 50.3 2.3
21.3 36.9 6.9
30.3 16.9 8.8
25.4 20.5 9.4
Cu-Fe-Zr/HZSM-5 17.3
Cu-Fe-La/HZSM-5 17.2
Cu-Fe-Ce/HZSM-5 18.1
3.5. Catalytic hydrogenation of CO2 to DME
a
Reaction conditions: V(H2) / V(CO2) = 4, T = 260 °C, P = 3.0 MPa, GHSV = 1500
mL·g−cat1·h−1
.
The Cu–Fe–La/HZSM-5 and Cu–Fe–Ce/HZSM-5 bifunctional catalysts
were used for the direct synthesis of DME from CO2 and H2. The catalytic
activities of the two catalysts were compared with those of Cu-Fe/
HZSM-5 and Cu-Fe-Zr/HZSM-5, and the results are shown in Table 3.
As shown in Table 3, the conversions of CO2 and selectivity of DME
were 12.3%, 17.3%, 17.2%, and 18.1% and 18.3%, 39.9%, 51.3%, and
52.0%. Compared with the Cu-Fe/HZSM-5 catalyst, the catalysts modi-
fied with ZrO2, La2O3, and CeO2 exhibited improved the CO2 conversion
and the DME selectivity along with reduced selectivity for CO and CH4.
These results suggest that ZrO2, La2O3 and CeO2 can improve the catalyt-
ic activity of the Cu–Fe catalyst in the catalytic hydrogenation of CO2 to
DME. Combined with the XPS and H2-TPR results, modifying the Cu–Fe
catalyst with Ce decreased the Cu outer-shell electron density and im-
proved the reduction ability of Cu–Fe, which increased the CO2 conver-
sion and DME selectivity. Furthermore, the Cu–Fe–Ce catalyst has a
greater specific surface area than the Cu–Fe–La catalyst. The Cu–Fe–Ce
catalyst also promotes CuO dispersion and decreases CuO crystallite
size. Thus, the catalytic activity (determined as the CO2 conversion
and DME selectivity) of the Cu–Fe–Ce catalyst is higher than that of
Cu–Fe–La. When the Cu–Fe–Ce/HZSM-5 catalyst with 1.0 wt.% CeO2
was used in the catalytic hydrogenation of CO2 to DME at 260 °C and
3.0 MPa with GHSV = 1500 mL·g−cat1·h−1, the CO2 conversion was
18.1%, and the DME selectivity was 52.0%.
3.4. H2-TPR analysis of catalyst
H2-TPR was used to analyze the effects of La2O3 and CeO2 on the re-
duction properties of Cu–Fe–La and Cu–Fe–Ce catalysts, and the results
are shown in Fig. 4.
The peak shape for the Cu–Fe–La catalyst is similar to that of the
CuO–Fe2O3 catalyst, whereas the peak shape of the Cu–Fe–Ce catalyst
differs from that of the CuO–Fe2O3 catalyst. Three Gaussian fitting
peaks (α, β and γ) are shown for the Cu–Fe–La and Cu–Fe–Ce catalysts.
In addition, the three reduction peaks (α, β and γ) correspond to the re-
duction process of highly dispersed CuO, small particles of CuO, and
larger grains of bulk CuO, respectively [13,31]. The temperatures and
areas of each reducing peak are summarized in Table 2.
The hydrogen reduction peaks (α), of the Cu–Fe–La and Cu–Fe–Ce
catalysts are centered at 161 °C and 156 °C, respectively; these peaks
were 16 °C and 11 °C lower than those of the CuO–Fe2O3 catalyst and
12 °C and 7 °C lower than those of the Cu–Fe–Zr catalyst. This indicated
that the reducibility of highly dispersed CuO is enhanced by La and Ce
modification. Cu is the active component in the catalytic hydrogenation
of CO2 to DME [32], and a lower reducing temperature will help to avoid
Fig. 5. Effects of the conversion of CO2 (A) and the selectivity of DME (B) with Cu–Fe, Cu–Fe–Zr, Cu–Fe–La, Cu–Fe–Ce with time on stream.

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Z. Qin et al. / Catalysis Communications 75 (2016) 78–82
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Acknowledgments
This work was supported by National Natural Science Foundation of
China (21366004, 21425627), Guangxi Zhuang Autonomous Region
special funding of distinguished experts, and Open Project of Guangxi
Key Laboratory of Petrochemical Resource Processing and Process
Intensification Technology.
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