Chemical Engineering Science 153 (2016) 10–20
Contents lists available at ScienceDirect
Chemical Engineering Science
journal homepage: www.elsevier.com/locate/ces
CuO-Fe2O3-CeO2/HZSM-5 bifunctional catalyst hydrogenated CO2 for
enhanced dimethyl ether synthesis
Xinhui Zhou a, Tongming Su a, Yuexiu Jiang a, Zuzeng Qin a,b,n, Hongbing Ji a,c,
Zhanhu Guo b,nn
a
School of Chemistry and Chemical Engineering, Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology,
Guangxi University, Nanning 530004, China
b
Integrated Composites Laboratory (ICL), Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, TN 37966, USA
c
Department of Chemical Engineering, School of Chemistry & Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
CuO-Fe2O3 catalyst modified with
different amounts of CeO2 were
prepared.
Addition of CeO2 led to the formation of a stable Cu-O-Ce solid solution.
CeO2 can modify the amount of acid
sites and acid type of CuO-Fe2O3
catalyst.
Cu-Fe-Ce/HZSM-5
catalyst
with
3.0 wt% CeO2 showed the optimal
catalytic activity.
art ic l e i nf o
a b s t r a c t
Article history:
Received 12 March 2016
Received in revised form
30 June 2016
Accepted 4 July 2016
Available online 5 July 2016
A series of CuO-Fe2O3-CeO2 catalysts with various CeO2 doping were prepared via the homogeneous
precipitation method, characterized and mechanically mixed with HZSM-5. Their feasibility and performance for the synthesis of dimethyl ether (DME) via CO2 hydrogenation in a one-step process were
evaluated. The formed stable solid solution after the CuO-Fe2O3 catalyst modified with CeO2 promoted
the CuO dispersion, reduced the CuO crystallite size, decreased the reduction temperature of highly
dispersed CuO, modified the specific surface area of the CuO-Fe2O3-CeO2 catalyst, and improved the
catalytic activity of the CuO-Fe2O3-CeO2 catalyst. The addition of CeO2 to CuO-Fe2O3 catalyst increased
the amount of Lewis acid sites and Brønsted acid sites, and enhanced the acid intensity of the weak acid
sites, which in turn promoted the catalytic performance of CO2 hydrogenation to DME. The optimal
introduced amount of Ce in the catalyst was determined to be 3.0 wt%. The CO2 conversion and DME
selectivity were 20.9%, and 63.1%, respectively, when the CO2 hydrogenation to DME was carried out at
260 °C, and 3.0 MPa with a gaseous hourly space velocity of 1500 mL gcat 1 h 1.
& 2016 Elsevier Ltd. All rights reserved.
Keywords:
Carbon dioxide hydrogenation
Dimethyl ether synthesis
Mixed oxide
Cu-Fe-Ce catalysts
Cerium oxide
1. Introduction
n
Corresponding author at: School of Chemistry and Chemical Engineering,
Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China.
nn
Corresponding author.
E-mail addresses: [email protected] (Z. Qin), [email protected] (Z. Guo).
http://dx.doi.org/10.1016/j.ces.2016.07.007
0009-2509/& 2016 Elsevier Ltd. All rights reserved.
Dimethyl ether (DME) possesses a high cetane number and
produces less NOx and SOx than do fossil fuels when combusted,
representing an environmental friendly renewable fuel (Frusteri
et al., 2015; García-Trenco and Martínez, 2012; Rutkowska et al.,
2015). DME can also be used as a chemical intermediate for
X. Zhou et al. / Chemical Engineering Science 153 (2016) 10–20
dimethyl sulphate, methyl acetate and low carbon olefin synthesis
(Ge et al., 1998). The fossil fuel usage has increased the amount of
CO2 in the atmosphere and the subsequent greenhouse effect
poses a threat to the environment. In addition, the energy crisis
facing the whole world is becoming increasingly urgent. Therefore,
converting CO2 into useful chemicals, such as DME, is an attractive
method to reduce greenhouse-gas emissions and to recycle
carbon.
The hydrogenation of CO2 to form DME mainly includes three
reactions, i.e., methanol synthesis reaction (Eq. (1)), methanol
dehydration reaction to form DME (Eq. (2)) and a reversed watergas shift reaction (Eq. (3)) (Bonura et al., 2014b; Yang Lim et al.,
2016). DME can be produced from CO2 via two routes. The first
route is a two-step process through two reactors (including methanol synthesis on a metallic catalyst and subsequent dehydration of methanol on an acid catalyst). The second route is a onestep process using a bifunctional catalyst in the same reactor to
simultaneously perform the two steps. However, methanol dehydration in the same reactor disturbs the equilibrium of synthesis
reaction. Therefore, one-step process is economically and thermodynamically favored (Jia et al., 2006; Vakili et al., 2011).
CO2 þ3H2⇋CH3OH þ H2O; ΔH298 ¼ 49.46 kJ mol 1
(1)
2CH3OH⇋CH3OCH3 þ H2O; ΔH298 ¼ 23.5 kJ mol 1
(2)
CO2 þH2⇋COþH2O; ΔH298 ¼41.17 kJ mol 1
(3)
The bifunctional catalysts for the one-step synthesis of DME via
CO2 hydrogenation include a Cu-base catalyst for the hydrogenation component and a solid acid, such as HZSM-5, γ-Al2O3 or HY
zeolite for the dehydration component, resulting in
CuO-ZnO-Al2O3/HZSM-5 (Liu et al., 2015), CuO-ZnO-Al2O3-ZrO2
/HZSM-5 (An et al., 2008), Cu-ZnO-ZrO2/HZSM-5 (Frusteri et al.,
2015), CuO-TiO2-ZrO2/HZSM-5 (Wang et al., 2009) or
CuO-Fe2O3-ZrO2/HZSM-5 (Liu et al., 2013; Qin et al., 2015). However, the conversion of stable CO2 molecule requires high temperature and pressure and only 10–30% was reported with the
DME selectivity of only 30–50% when the reaction was carried out
at 4–10 MPa and 200–300 °C. Due to the conversion of CO2 to
methanol was limited by the thermodynamic equilibrium, it was
more difficult to attain reasonable per-pass conversions for large
industrial scale CO2 treatment.
CeO2 is an effective co-catalyst with a hollow structure, and can
stabilize and disperse Cu and other precious metals (Bera et al.,
2003). CeO2 and CuO can form a stable solid solution (Gamarra
et al., 2007; Marbán and Fuertes, 2005) to improve both catalytic
activity and thermal stability of CuO-based catalysts. CeO2 has
been frequently selected as a component for use in complex oxides
or as a dopant to improve the catalyst performance. CeO2 has been
applied in the field of preferential CO oxidation (Hornés et al.,
2010; Jia et al., 2010) and methanol steam reforming (Tonelli et al.,
2011). For the dimethyl ether synthesis from CO2, the acid properties of the catalysts influenced the DME selectivity (GarcíaTrenco and Martínez, 2012; Yoo et al., 2007). For example, the
acidity of zeolite HY was enhanced by the addition of Ce, and it
possessed a high proportion of moderate strength acid sites and
were more stable for methanol dehydration to DME (Jin et al.,
2007); the presence of Ce on the HZSM-5 could modify the acidic
properties of HZSM-5 including the amount of acid sites and acid
type (Wang et al., 2007); and the addition of Ce could enhance the
acidity of MnOx/TiO2 (Wu et al., 2008). These indicate that Ce
played an important role on the catalysts acid properties.
In this work, a series of CuO-Fe2O3 catalysts modified using
different amounts of CeO2 were prepared via a homogeneous
precipitation method; characterized using X-ray diffractometer
11
(XRD) to determine the crystal structure of catalysts, Raman
spectroscopy for obtaining the vibrational and rotation mode of
the lattice and the molecules of catalysts, high-resolution transmission electron microscopy (HRTEM) to obtain the information
on the structure and composition of the particles, N2 adsorptiondesorption to determine the pore structures and the texture
properties of the catalysts, H2-temperature programmed reduction
(H2-TPR) to obtain the quantitative reduction information, thermal
analysis (TG-DTA) for determining the weight changes in the calcination process of the catalysts, Fourier transform infrared spectroscopy (FTIR) of adsorbed pyridine and temperature-programmed desorption of ammonia (NH3-TPD) for obtaining the
surface acidities of the catalysts, and temperature programmed
oxidation (TPO) to detect the coke formed during the reaction. The
catalysts were mixed with HZSM-5 to synthesize dimethyl ether
via CO2 hydrogenation in a one-step process.
2. Experimental
2.1. Catalyst preparation
The precursor for producing the CuO-Fe2O3-CeO2 catalyst was
prepared
via
a
homogeneous
precipitation
method.
Cu(NO3)2 3H2O and Fe(NO3)3 9H2O (Sinopharm Chemical Reagent Co., Ltd.) were weighed at Cu/Fe mole ratio of 3:2 (Liu et al.,
2013; Qin et al., 2015; Qin et al., 2016). Then, the amount of the
Ce(NO3)3 6H2O (Sinopharm Chemical Reagent Co., Ltd) was determined by the desired CeO2 content of 1.0, 2.0, 3.0 and 4.0 wt% in
CuO-Fe2O3-CeO2 in the final catalysts and was added to obtain a
metal nitrate solution (0.5 mol L 1). A urea (China Guangdong
Guanghua Sci-Tech Co., Ltd.) solution (5 mol L 1) was then added
to the metal nitrate solution, in which the urea to nitrates molar
ratio was 20. The mixture was stirred and reacted for 24 h at 90 °C.
The obtained precipitates were filtered and dried at 110 °C for 12 h,
ground through a 20–40 mesh, calcined at 400 °C for 4 h, and thus
the catalyst CuO-Fe2O3-CeO2 was obtained for methanol synthesis.
HZSM-5, using as the methanol dehydration component, with a
silica-alumina ratio of 300:1 (China Shanghai Novel Chemical
Technology Co., Ltd.) was mechanically mixed with the
CuO-Fe2O3-CeO2 oxide composites at a mass ratio of 1:1. For
comparison, a CuO-Fe2O3/HZSM-5 catalyst without CeO2 was
prepared via the same method.
2.2. Catalyst characterization
The XRD, N2 adsorption-desorption and H2-TPR catalyst characterizations can be referenced in previous studies (Liu et al.,
2013). Briefly, the X-ray diffraction (XRD) analysis was performed
using a Bruker D8 Advance X-ray diffractometer. The isotherm of
nitrogen adsorption and desorption was measured by an ASAP
2000 physical adsorption instrument (Micromeritics Instrument
Corporation). The catalyst surface area was calculated via Brunauer-Emmett-Teller (BET) method, and the pore size distribution
curve was determined using the Barrett-Joyner-Halenda (BJH)
model, which was based on the isotherm of desorption side. The
H2-TPR, NH3-TPD, and TPO experiments of catalyst samples were
taken in a PX200 multifunction catalyst analysis system (China
Tianjin Golden Eagle Technology Co., Ltd.).
The Raman spectra were obtained using Lab RAMHR800 Laser
Confocal Micro-Raman Spectroscopy (Horiba, Ltd.), including a
YAG laser with a 532 nm excitation wavelength. High resolution
transmission electron microscope (HRTEM) images of the catalysts
were obtained using a FEI tecnai G20 instrument with an acceleration voltage of 200 kV. A thermal analysis was performed using
a ZRY-1P thermal analyser (Techcomp Jingke Scientific
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X. Zhou et al. / Chemical Engineering Science 153 (2016) 10–20
Instruments (Shanghai) Co., Ltd.). About 8 mg catalyst was used for
the analysis, at a heating rate of 10 °C min 1 from 50 to 500 °C.
The FTIR of adsorbed pyridine was conducted using a Magna-IR
750 FTIR spectrometer (Thermo Nicolet Corporation). Self-supporting catalysts wafers (ca. 10 mg cm 2) were produced and
loaded into an IR cell. The wafers were evacuated at 150 °C for 2 h
to record the background spectrum and subsequently saturated
with pyridine and evacuated at 150 °C for 2 h. The Py-IR spectra
were recorded at a spectra resolution of 4 cm 1 after subtracting
the sample background.
2.3. Catalytic hydrogenation of CO2 to DME
The catalytic activity was investigated using a continuous fixedbed stainless steel reactor, which is detailed in the literature (Liu
et al., 2013). Briefly, 1.0 g hybrid catalyst was added to the fixed
bed reactor and reduced at 300 °C for 4 h with 30 mL min 1 of H2
(99.999%) at atmospheric pressure. A gas mixture composed of
20% CO2 and 80% H2 was then introduced and reacted at 240–
280 °C, 2.0–4.0 MPa with a gaseous hourly space velocity (GHSV)
of 1500–3500 mL gcat 1 h 1.
3. Results and discussion
3.1. XRD analysis
Fig. 1 illustrates the XRD patterns of the CuO-Fe2O3 catalysts
modified by different CeO2 amounts before H2 reduction. The
diffraction peaks at 2θ ¼35.49°, 38.69°, and 58.26° are associated
with monoclinic CuO (JCPDS No. 48-1548). The peaks at
2θ ¼ 30.24°, 35.63°, and 62.93° can be attributed to cubic crystal
Fe2O3 (JCPDS No. 39-1346). The CuO-Fe2O3 peaks are sharper than
those of CuO-Fe2O3 catalysts modified by CeO2, suggesting that
CuO-Fe2O3 possesses an enhanced crystallinity (Wang et al., 2016).
The CuO diffraction peaks become weakened and broadened as
the amount of CeO2 was increased, indicating that CuO dispersion
was improved by the addition of CeO2. The crystallite sizes of CuO
(111) for CuO-Fe2O3-CeO2 catalysts modified with 1.0, 2.0, 3.0 and
4.0 wt% CeO2 calculated by the Scherrer equation are 17.6, 14.7, 14.3
and 15.9 nm, respectively. However, the crystallite size of CuO
(111) for CuO-Fe2O3 catalyst is 22.5 nm, indicating that the modification with CeO2 can decrease the crystallite size of CuO and
promote the dispersion of catalysts. The XRD patterns suggest that
Fig. 2. Raman spectra of the CuO-Fe2O3-CeO2 catalysts modified by CeO2 content of
(a) 0, (b) 1.0, (c) 2.0, (d) 3.0, and (e) 4.0 wt% CeO2 calcined at 400 °C without H2
reduction.
CeO2 can significantly improve the CuO dispersion based on a
3.0 wt% CeO2. Further increasing the CeO2 amount did not improve
the CuO dispersion. The results reveal that appropriate CeO2 addition can inhibit the CuO crystals growth, resulting in small CuO
crystallites and increasing the catalysts dispersion. In addition, the
CeO2 x peaks at 2θ of 26.3°, 43.9° and 55.7° (JCPDS No. 49-1415)
were found in the CuO-Fe2O3-CeO2 catalysts, including that a
portion of smaller Cu2 þ ions (0.69 Å) and Fe3 þ ions (0.64 Å)
(Wang et al., 2014) entered to the CeO2 lattice (the radius of Ce4 þ
ionic is 0.97 Å) to form a solid solution (Li et al., 2011, 2014a;
Marbán and Fuertes, 2005).
3.2. Raman spectra
Fig. 2 shows the Raman spectra of the CuO-Fe2O3-CeO2 catalysts. The peaks of the CuO-Fe2O3 catalyst observed at 210, 276,
373, and 574 cm 1 are related to typical A1g and 2Eg Raman hematite modes, whereas the peak at 1288 cm 1 is associated with
hematite two-magnon scattering (Li et al., 2014b; Wang et al.,
2014). The peak at 276 cm 1 arises from CuO (Tsoncheva et al.,
2013). The peaks of the modified CuO-Fe2O3-CeO2 catalyst at
468 cm 1 can be attributed to the strong F2g mode of CeO2 (Qi
et al., 2012). The broad band between 550 and 700 cm 1 is related
to the oxygen vacancies after the introduction of cerium, suggesting the formation of a Cu-O-Ce or Fe-O-Ce solid solution (Chen
et al., 2010; Li et al., 2011; Wang et al., 2014). The substitution of
Cu2 þ , Fe3 þ for Ce4 þ can generate oxygen vacancies around
Cu2 þ -O-Ce4 þ , Fe3 þ -O-Ce4 þ to maintain charge neutrality (Bera
et al., 2002), respectively, which is consistent with the XRD result.
3.3. Transmission electron microscopy
Fig. 1. The XRD patterns of the CuO-Fe2O3-CeO2 catalysts modified by (a) 0, (b) 1.0,
(c) 2.0, (d) 3.0, and (e) 4.0 wt% CeO2 calcined at 400 °C without H2 reduction.
The atomic level morphology and lattice structure of the
CuO-Fe2O3 catalyst and CeO2 catalyst were investigated via HRTEM
measurements. The low-magnification TEM images in Figs. 3(a)
and 4(a) illustrate that the catalyst particles form disk-like shapes,
and the addition of Ce decreases the particle size. A smaller particle
size generally resulted in the catalysts with more edges, corners,
defects and oxygen vacancies, all of these could benefit the reaction
performance during the structure-sensitive reactions (Liu et al.,
2005). The HRTEM CuO-Fe2O3 images in Fig. 3(b,c) clearly illustrate
the presence of 0.232 and 0.252 nm interplanar spacing, corresponding to the characteristic (111) lattice plane of monoclinic CuO
X. Zhou et al. / Chemical Engineering Science 153 (2016) 10–20
13
Fig. 3. Typical TEM images (a), HRTEM images (b and c) and EDX (d) of the CuO-Fe2O3 catalyst calcined at 400 °C without H2 reduction, while the inset (b) represents a SAED
pattern.
Fig. 4. Typical TEM images (a), HRTEM images (b and c) and EDX (d) of the CuO-Fe2O3-CeO2 catalyst with 3.0 wt% CeO2 calcined at 400 °C without H2 reduction, while the
inset (b) represents a SAED pattern.
and characteristic (311) lattice plane of cubic crystal Fe2O3, respectively. The interplanar spacing of CuO (111) (0.2524 nm) and
Fe2O3 (311) (0.2518 nm) are nearly equal. Therefore, the 0.252 nm
interplanar spacing may also correspond to CuO (11 1). The
CuO-Fe2O3-CeO2 catalyst with 3.0 wt% CeO2 HRTEM image (Fig. 4
(b)) displays 0.252 and 0.295 nm interplanar spacing, corresponding
to the (311) and (220) planes of cubic crystal Fe2O3, respectively. The
magnified catalyst image in Fig. 4(c) is based on the square frame in
Fig. 4(b). This magnified image displays 0.187 and 0.271 nm interplanar spacing, corresponding to the (20 2) plane of monoclinic
14
X. Zhou et al. / Chemical Engineering Science 153 (2016) 10–20
Fig. 5. Nitrogen adsorption/desorption isotherms (A) and pore size distribution profiles (B) of CuO-Fe2O3-CeO2 with CeO2 content of (a) 0, (b) 1.0, (c) 2.0, (d) 3.0, and
(e) 4.0 wt%.
CuO and the (200) plane of CeO2, respectively. However, the (111)
plane of monoclinic CuO was not observed in any of the
CuO-Fe2O3-CeO2 (CeO2 ¼3.0 wt%) catalysts, suggesting that CuO
was relatively well-dispersed in the catalyst (Gamarra et al., 2007).
The selected area electron diffraction (SAED) patterns are given in
the inset of Figs. 3(b) and 4(b). The SAED patterns include diffraction
rings, indicating that the two catalysts are polycrystalline. In addition, the energy-dispersive X-ray spectrum (EDX) displays the Fe, Ce
and O signals, further proving that the mixed metal-oxide catalysts
have been fabricated.
the Ce structural relaxation, which consequently increases the
surface area and the concentration of defects, such as oxygen vacancies (Zhan et al., 2012). The largest surface area (67.34 m2 g 1)
was observed for the catalysts with 3.0 wt% CeO2. The larger
specific surface area can provide more absorption and/or reaction
sites for CO2 and H2, resulting in a higher catalytic activity (Huang
et al., 2016). However, the specific surface area was decreased
when the CeO2 loading was increased from 3.0 to 4.0 wt%, which
may be associated with the crystallite growth as the amount of
CeO2 increases above 3.0 wt%.
3.4. Nitrogen adsorption/desorption of catalysts
3.5. TG-DTA analysis of catalysts
The specific surface areas and pore size distribution curves of
the CuO-Fe2O3-CeO2 catalysts with different CeO2 amounts were
investigated using nitrogen adsorption-desorption isotherms,
Fig. 5. The isotherms of the catalysts are similar, Fig. 5A, exhibiting
IV type isotherms according to the IUPAC classification, suggesting
that all the catalysts possess the mesoporous features (Huang
et al., 2016; Zhang et al., 2014). Furthermore, the pore size distribution profiles (Fig. 5B) of the catalysts exhibited a narrow pore
size distribution, with a maximum of 3.0 nm. The BET method
was used to calculate the N2 adsorption-desorption isotherms and
obtain the specific surface areas of the catalysts, which are listed in
Table 1.
The specific surface areas of the CuO-Fe2O3-CeO2 catalysts were
50.32, 56.41, 62.26, 67.34 and 56.60 m2 g 1 for the CeO2 contents
of 0, 1.0, 2.0, 3.0 and 4.0 wt%, respectively, suggesting that the
addition of proper amount of CeO2 can increase the specific surface areas of the CuO-Fe2O3 catalysts. In addition, CeO2 can produce larger BET surfaces area than CuO (Tsoncheva et al., 2013; Xu
et al., 2016). Furthermore, the CeO2 particle size of the
CuO-Fe2O3-CeO2 catalyst will be reduced due to the 4f orbit and
Fig. 6 shows the precursor TG-DTA curves for different CeO2
amounts. Fig. 6A shows the TGA curves of the precursor. The
weight loss is observed to occur in three steps. The weight loss in
the range of 50–200 °C can be attributed to the desorption of
physically adsorbed surface water and the bound water from
precursors (Cao et al., 2008; Liu et al., 2015). The precursor weight
losses were 10.1%, 6.2%, 7.1%, 8.0% and 8.4% for the CeO2 amounts
of 0, 1.0, 2.0, 3.0 and 4.0 wt%, respectively. The weight loss from
200 to 400 °C corresponds to the thermal decomposition of
Cu(OH)2 (Eq. (4)) and Fe(OH)3 (Eq. (5)), forming CuO and Fe2O3,
respectively (Bonura et al., 2014a). The DTA curve (Fig. 6B) of the
corresponding precursors represents an endothermic peak. The
precursor weight losses in this step were 12.2%, 10.2%, 9.9%, 8.0%
and 11.3% for the CeO2 amount of 0, 1.0, 2.0, 3.0 and 4.0 wt%,
respectively.
Table 1
Textural properties of the CuO-Fe2O3-CeO2 catalysts with different CeO2 contents.
CeO2 content (wt%)
BET surface area (m2 g 1)
Average pore diameter (nm)
0
1.0
2.0
3.0
4.0
50.32
56.41
62.26
67.34
56.60
25.54
20.62
19.44
18.96
20.86
Cu(OH)2-CuO þH2O
(4)
2Fe(OH)3-Fe2O3 þ3H2O
(5)
The Ce2O3 conversion was unstable in air and a portion of the
Ce2O3 was oxidized to form CeO2 during the Ce(OH)3 to form
Ce2O3 step (Eq. (6)) (Li et al., 2014a; Mo et al., 2005). CeO2 and CuO
form a solid solution during the oxidation process. However, the
weight was increased from Ce2O3 to CeO2 (Eq. (7)). Therefore, the
weight loss of the precursors modified with CeO2 is less than that
of the CuO-Fe2O3 catalyst precursor.
2Ce(OH)3-Ce2O3 þ3H2O
(6)
2Ce2O3 þO2-4CeO2
(7)
No significant weight change was observed at temperatures
X. Zhou et al. / Chemical Engineering Science 153 (2016) 10–20
15
Fig. 6. (A) TGA and (B) DTA profiles for precursors of CuO-Fe2O3-CeO2 catalysts with CeO2 content of (a) 0, (b) 1.0, (c) 2.0, (d) 3.0, and (e) 4.0 wt%.
3.6. H2-TPR analysis of catalysts
Fig. 7. H2-TPR profiles for CuO-Fe2O3-CeO2 with CeO2 content of (a) 0, (b) 1.0,
(c) 2.0, (d) 3.0, and (e) 4.0 wt%. The solid curves are experimental curves and
broken curves are Gaussian multi-peak fitting curves.
Table 2
Temperatures and area distributions of the CuO-Fe2O3-CeO2 reduction peaks for
different CeO2 contents.a
CeO2 content (wt%)
0
1.0
2.0
3.0
4.0
Peak α
Peak β
Peak γ
Total area
T (°C)
Areab
T (°C)
Area
T (°C)
Area
172
156
167
155
170
0.89
2.31
1.51
3.02
2.23
219
178
178
177
182
7.30
2.39
2.30
2.67
0.93
247
195
198
187
203
3.26
3.34
1.47
3.05
7.73
11.45
8.04
5.28
8.74
10.89
a
The results were measured based on the H2-TPR profiles and the area distributions were calculated by integrating of the areas under the peaks.
b
The peak area unit is 104 units.
above 400 °C (Mo et al., 2005). Therefore, a 400 °C calcination
temperature was adopted to ensure a complete decomposition of
the catalyst precursors and minimize sintering phenomena.
The CuO-Fe2O3-CeO2 catalysts with different CeO2 amounts
were investigated via H2-TPR to study the redox abilities of the
catalysts and the effects of CeO2 on the reduction of Cu species.
The results of these analyses are displayed in Fig. 7. The H2-TPR
profiles peaks of the CeO2 modified CuO-Fe2O3 catalysts are similar but differ from those of the CuO-Fe2O3 catalysts. Three
Gaussian fitting peaks (α, β and γ) are shown for all of the catalysts
in the H2-TPR profiles. When the temperature was below 300 °C,
CuO and Fe2O3 had been reduced to Cu and Fe3O4, respectively.
The reductions of CeO2 often occur at high temperatures
(4300 °C), resulting in the formation of peaks (Maciel et al.,
2011). The three reduction peaks (α, β and γ) correspond to the
reductions of highly dispersed CuO, bulk CuO (Xu et al., 2016; Zeng
et al., 2013) and Fe2O3 (Cao et al., 2008), respectively (Avgouropoulos and Ioannides, 2003). The temperatures and areas of each
reduction peak are summarized in Table 2.
According to Table 2, the α hydrogen reduction peaks of the
CuO-Fe2O3-CeO2 catalysts are centered at 156 °C, 167 °C 155 °C and
170 °C for the catalyst with a CeO2 content of 1.0, 2.0, 3.0 and
4.0 wt%, respectively. These values are lower than the temperatures associated with the CuO-Fe2O3 catalyst (172 °C), indicating
that the CeO2 modification can decrease the reduction temperature and increase the reducibility of highly dispersed CuO to increase the reducibility of the catalysts. The observed lowest temperature of peak α at 155 °C, corresponded to the CuO-Fe2O3-CeO2
catalyst with 3.0 wt% CeO2, suggesting that this catalyst exhibited
an optimum reducing behavior. The ratios of the α reduction peak
area to the total area of the CuO-Fe2O3-CeO2 reduction peaks were
7.77%, 28.7%, 28.6%, 34.6% and 20.5%, for the CeO2 amount of 0, 1.0,
2.0, 3.0 and 4.0 wt%, respectively. These results suggest that the
proportion of highly dispersed CuO was increased due to the CeO2
modification. Moreover, the CuO-Fe2O3-CeO2 catalyst with 3.0 wt%
CeO2 possessed the largest proportion of highly dispersed CuO.
The total hydrogen consumption of CeO2 modified CuO-Fe2O3
catalysts was decreased compared to the CuO-Fe2O3 catalyst. The
XRD results suggest that a portion of the copper and iron embedded in ceria form a solid solution, and the solid solution becomes more difficult to be reduced (Han et al., 2013; Wang et al.,
2005). Therefore, the total hydrogen consumption of CeO2 modified CuO-Fe2O3 catalysts was decreased.
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X. Zhou et al. / Chemical Engineering Science 153 (2016) 10–20
Table 3
Effect of the CeO2 amount on the catalytic hydrogenation of CO2 to DME on
CuO-Fe2O3-CeO2/HZSM-5 catalysts.a
CeO2 content (wt%)
0
1.0
2.0
3.0
4.0
CO2 conversion
(mol%)
12.3
18.1
18.8
20.9
17.1
Products selectivity (mol%)
DME
CH3OH
CO
CH4
18.3
52.0
56.9
63.1
50.4
0.9
2.1
3.5
5.2
4.6
30.5
25.4
23.8
24.8
39.5
50.3
20.5
15.8
6.8
5.6
DME yield
(mol%)
2.3
9.4
10.7
13.2
8.6
a
Reaction conditions: T ¼260 °C, P¼ 3.0 MPa, GHSV¼1500 mL gcat 1 h 1, and
V(H2)/V(CO2) ¼ 4.
Fig. 8. FT-IR spectra of pyridine adsorbed on CuO-Fe2O3 (a) and CuO-Fe2O3-CeO2
catalysts with 3.0% wt% CeO2 (b) at a desorption temperature of 150 °C.
3.7. Surface acidity analysis
Fig. 8 shows the FTIR spectra of the adsorbed pyridine on the
CuO-Fe2O3 and CuO-Fe2O3-CeO2 (CeO2 ¼3.0 wt%) catalysts. The
spectra were normalized and recorded under identical operating
conditions. The two characteristic bands at ca. 1450 and 1540 cm 1
are associated with pyridine adsorption on Lewis acid sites and
Brønsted acid sites, respectively (Wang et al., 2007). The weak band
at 1442 cm 1 for CuO-Fe2O3 and the intense band at 1438 cm 1 for
CuO-Fe2O3-CeO2 (CeO2 ¼ 3.0 wt%) indicated that the modified of
CuO-Fe2O3 catalyst with Ce increased the amount of Lewis acid sites.
The Ce metal cations possess empty f orbits. The empty f orbit can
provide potential locations for Lewis acid sites (Wang et al., 2007).
Therefore, the empty f orbits modify the acidity of the CuO-Fe2O3
catalysts. Moreover, the shift towards lower wavenumbers can be
taken as an indicative of a lower acid strength of the Lewis acid sites
(García-Trenco and Martínez, 2012). The CuO-Fe2O3-CeO2 (CeO2
¼3.0 wt%) band at 1546 cm 1 demonstrates that the Brønsted acid
sites were formed after the Ce modification. This result indicates
that the acid is altered by the CeO2 modification. The observed band
at 1476 cm 1 for both catalysts has also been identified in the FTIR
spectra of the adsorbed pyridine on HY zeolites (Boréave et al.,
Fig. 9. NH3-TPD profiles of the CuO-Fe2O3-CeO2 catalysts modified by (a) 0, (b) 1.0,
(c) 2.0, (d) 3.0, and (e) 4.0 wt% CeO2 calcined at 400 °C without H2 reduction.
1997), and possibly results from the interactions between the adsorbed pyridine and metal oxides (Tang et al., 2010). The 1608 cm 1
band is associated with pyridine adsorption vibrations at Brønsted
acid sites (Stevens, 2003).
Fig. 9 shows the NH3-TPD profiles of the CuO-Fe2O3 catalysts
modified by different CeO2 amounts before H2 reduction. From
Fig. 9, three NH3 desorption peaks were detected at 50–200 °C
(peak α), 200-350 °C (peak β), and 350–670 °C (peak γ), indicating
the presence of three strength acid sites on the catalysts (Gao
et al., 2008). The α, β, and γ peaks corresponded to the desorption
of the adsorbed NH3 on weak, medium, and strong acid sites, respectively (Jin et al., 2007). It can be found that after different CeO2
amounts were added into the CuO-Fe2O3 catalyst, the intensities of
weak acid sites were enhanced, indicating increased weak acid
sites of the CuO-Fe2O3 catalysts modified by different CeO2
amounts and consistent with the Py-IR result. When the content of
CeO2 was 3.0 wt%, the acid intensity of weak acid site became the
strongest. However, the γ peak of the strong acid sites became
broadened after the Ce modification, implying that CeO2 decreased
the catalyst strong acidity.
3.8. Catalytic hydrogenation of CO2 to DME on CuO-Fe2O3-CeO2
/HZSM-5
3.8.1. Effect of the CeO2 content
The catalytic performances of the CuO-Fe2O3-CeO2/HZSM-5
catalysts with different amounts of CeO2 during DME synthesis
from CO2 hydrogenation are presented in Table 3. According to
Table 3, the CeO2 modifier can regulate the product distribution for
DME synthesis via CO2 hydrogenation. The CO2 conversion and
DME selectivity are significantly improved compared with the
non-modified CuO-Fe2O3/HZSM-5, and inhibited the production of
CO and CH4. The CeO2 addition caused the CH4 selectivity monotonously decrease from 50.3% to 5.6%, with a minimum value of
5.6% observed for the CuO-Fe2O3-CeO2/HZSM-5 catalyst with
4.0 wt% CeO2. Moreover, the CO2 conversion and the DME selectivity were increased when the CeO2 amount was increased
from 0 to 3.0 wt% and decreased when the CeO2 content was increased above 3.0 wt%. The CuO-Fe2O3-CeO2 catalyst with 3.0 wt%
CeO2 exhibited the maximum CO2 conversion and DME selectivity
values of 20.9% and 63.1%, respectively, indicating that specific
CeO2 additions can promote the catalytic performance.
The XRD analysis and Raman spectra showed that the CeO2
modification can increase the CuO dispersion and lead to the formation of a solid solution, which may affect the catalytic hydrogenation. The specific surface area was increased as the CeO2
amount was increased from 0 to 3.0 wt% during the nitrogen adsorption-desorption process, but declined when the CeO2 amount
was increased from 3.0 to 4.0 wt%. This indicates that the CeO2
modification can change the specific surface areas of the
CuO-Fe2O3-CeO2 catalysts. The larger specific surface areas can
X. Zhou et al. / Chemical Engineering Science 153 (2016) 10–20
17
Fig. 10. Effect of temperature on catalytic CO2 hydrogenation to DME for
CuO-Fe2O3-CeO2/HZSM-5. Reaction conditions: T ¼ 240–280 °C, P¼ 3.0 MPa,
GHSV¼1500 mL gcat 1 h 1, and V(H2)/V(CO2) ¼ 4.
Fig. 11. Effects of the reaction pressure on the catalytic CO2 hydrogenation to form
DME for CuO-Fe2O3-CeO2/HZSM-5. Reaction conditions: T ¼260 °C, P ¼2.0–4.0 MPa,
GHSV¼1500 mL gcat 1 h 1, and V(H2)/V(CO2) ¼ 4.
increase the absorption and/or the number of CO2 and H2 reaction
sites, which may be partially responsible for improving the catalytic hydrogenation process. Combined the H2-TPR results, the
reduction temperature of the highly dispersed CuO was decreased
after modified by CeO2, in addition, the proportion of highly dispersed CuO was increased, improving the activity of the catalyst.
The results of pyridine FTIR spectra and NH3-TPD suggest that
modifying the CuO-Fe2O3 catalyst with CeO2 can alter the acid
type, increase the amount of total acid sites (Lewis acid sites and
Brønsted acid sites), and enhance the acid intensity of weak acid
site, which can improve the CO2 hydrogenation to DME catalytic
performance.
increased from 2.0 to 3.0 MPa, but decreased when the reaction
pressure was increased from 3.0 to 4.0 MPa. The DME selectivity
also exhibited similar trends. This results imply that the direct
CO2-to-DME hydrogenation reaction was a volume decreasing
consecutive reaction, suggesting that high reaction pressure may
increase the DME selectivity. However, the CO2 conversion (from
20.9% to 17.2%) and DME selectivity (from 63.1% to 55.5%) were
decreased when the pressure was increased from 3.0 to 4.0 MPa.
This trend was most likely caused by the accumulation of water,
which cannot be removed from the fixed bed reactor during the
CO2 hydrogenation to form DME process and may decrease the
catalyst activity (Bonura et al., 2014b; Liu et al., 2015). The results
suggest that the optimal DME synthesis pressure is 3.0 MPa.
3.8.2. Reaction temperature effect
Fig. 10 shows the hydrogenation of CO2 to form DME on the
CuO-Fe2O3-CeO2/HZSM-5 bifunctional catalysts with 3.0 wt% CeO2
for temperatures ranging from 240 to 280 °C. Fig. 10 illustrates that
the CH3OH selectivity was increased with increasing the temperature. In addition, the CO2 conversion and DME selectivity were
increased when the reaction temperature rose from 240 to 260 °C,
reaching maximum values of 20.9% and 63.1% at 260 °C, respectively. However, further increasing the reaction temperature
caused the CO2 conversion and the DME selectivity to decrease.
According to the Arrhenius law, increasing the temperature during
the CO2 hydrogenation can enhance the reaction rate constant and
improve the reaction rate of the DME synthesis. However, the DME
synthesis from the CO2 hydrogenation is a reversible exothermic
reaction. Increasing the temperature will cause the reaction to
favor the reverse process, causing the DME selectivity to decrease.
Moreover, a higher temperature can cause the Cu species to sinter
and crystallize, decreasing the catalyst activity and coke formation,
provoking partial deactivation of the HZSM-5 catalyst (Zha et al.,
2013). Therefore, the CO2 conversion and DME selectivity initially
increase and subsequently decrease with increasing the temperature. As a result, the CO2 hydrogenation to form DME synthesis was investigated on the CuO-Fe2O3-CeO2/HZSM-5 bifunctional
catalysts at 260 °C.
3.8.3. Reaction pressure effect
Simulations were conducted for the pressures ranging from
2.0 to 4.0 MPa while all other variables were maintained constant.
As shown in Fig. 11, the CH3OH selectivity was increased as the
reaction pressure was increased from 2.0 to 4.0 MPa. In addition,
the CO2 conversion was increased when the pressure was
3.8.4. Space velocity effects
The CuO-Fe2O3-CeO2/HZSM-5 catalyst with 3.0 wt% CeO2 was
used to synthesize DME from CO2 and H2, and the space velocity
effect was investigated over a range of 1500–3500 mL gcat 1 h 1,
Fig. 12. The CO2 conversion and DME selectivity were decreased as
the space velocity was increased from 1500 to 3500 mL gcat 1 h 1.
However, the CH3OH selectivity was increased from 5.2% to 11.3%
as the space velocity was increased from 1500 to
Fig. 12. Effect of the space velocity on the catalytic CO2 hydrogenation to form DME
for CuO-Fe2O3-CeO2/HZSM-5. Reaction conditions: T¼ 260 °C, P¼ 3.0 MPa,
GHSV¼1500–3500 mL gcat 1 h 1, and V(H2)/V(CO2) ¼ 4.
18
X. Zhou et al. / Chemical Engineering Science 153 (2016) 10–20
Fig. 13. CO2 conversion (A) and DME selectivity (B) effects of CuO-Fe2O3/HZSM-5 and CuO-Fe2O3-CeO2/HZSM-5 ( 3.0 wt%) based on the time on stream stability. The TGA (C),
and the TPO profiles (D) of the used catalysts.
3500 mL gcat 1 h 1. The contact time between the CO2/H2 mixture
and the catalyst surface was decreased by increasing the space
velocity, leading to an insufficient contact between the CO2/H2
mixture and the active catalysts sites. Methanol could not
promptly dehydrate to DME on the HZSM-5 catalyst surface and
rapidly left the fixed bed reactor (An et al., 2008). Therefore, increasing the space velocity caused the DME selectivity to decrease
from 63.1% to 47.5% and the methanol selectivity to increase. These
findings suggest that the optimal space velocity for DME synthesis
is 1500 mL gcat 1 h 1.
3.8.5. Stabilities and coke analysis
The stabilities of the CuO-Fe2O3/HZSM-5 and 3.0 wt% CeO2
modified CuO-Fe2O3-CeO2/HZSM-5 catalysts were studied in the
context of DME synthesis via CO2/H2 at 260 °C and 3.0 MPa, with
GHSV¼1500 mL gcat 1 h 1 and V(H2)/V(CO2)¼ 4. The results were
recorded every 0.5 h, Fig. 13. The CO2 conversion and DME selectivity of the CuO-Fe2O3-CeO2/HZSM-5 catalyst with 3.0 wt%
CeO2 remained almost constant throughout the 15-h reaction at
19.9% and 58.9%, respectively. However, the CO2 conversion and
DME selectivity of the CuO-Fe2O3/HZSM-5 catalysts sharply declined with the time-on-stream, indicating that the CeO2 modification can improve the CuO-Fe2O3/HZSM-5 catalyst stability.
Modifying the CuO-Fe2O3 catalyst with CeO2 improved the CuO
dispersion, resulting in less amalgamated fine Cu crystal (Tan et al.,
2005) and increases the catalyst activity and stability.
Fig. 13C shows the TGA curves of the used catalysts. According
to the TGA curve of the used CuO-Fe2O3-CeO2/HZSM-5 (3.0 wt%)
catalyst, the weight loss at 400 °C is attributed to the coke combustion (Tonelli et al., 2015), indicating that slight carbon deposition occurred on the catalyst surface after the 15-h reaction.
Thus, DME selectivity and CO2 conversion slightly decreased as
time went on. Nevertheless, the used CuO-Fe2O3/HZSM-5 catalyst
exhibited a large weight loss above 400 °C after the 6-h reaction,
indicating that modifying the CuO-Fe2O3 catalyst with CeO2 can
inhibit the coke generation. Fig. 13D shows the TPO profiles of the
used catalysts, and three peaks (α, β and γ) are shown for the used
CuO-Fe2O3/HZSM-5 and CuO-Fe2O3-CeO2/HZSM-5 (3.0 wt%) catalyst. The α peak corresponded to the oxidation of Cu to CuO and
Fe3O4 to Fe2O3, and the β and γ peaks were the oxidation processes of the monatomic carbon and whisker carbon, respectively
(Hou et al., 2003). The peak β of the used CuO-Fe2O3-CeO2/HZSM5 (3.0 wt%) catalyst significantly became weakened compared to
the used CuO-Fe2O3/HZSM-5 catalyst, indicating that carbon deposition was suppressed because of the formation of solid solution
(Hou et al., 2003; Sierra et al., 2011) and then exhibited a high
activity and stability.
4. Conclusions
The CuO-Fe2O3-CeO2 catalysts with various CeO2 amounts were
successfully prepared using a homogeneous precipitation method.
The CuO-Fe2O3-CeO2 was mixed with HZSM-5 zeolite and used in
the CO2 hydrogenation to produce DME. The results showed that
the CeO2 modified CuO-Fe2O3 catalysts formed a stable solid
X. Zhou et al. / Chemical Engineering Science 153 (2016) 10–20
solution, which promoted CuO dispersion, decreased the CuO
crystallite size, decreased the reduction temperature of highly dispersed CuO, altered the specific surface areas of CuO-Fe2O3-CeO2
catalysts, and improved the catalytic activity of the CuO-Fe2O3-CeO2
catalyst. The addition of CeO2 to the CuO-Fe2O3 catalyst increased
the amount of Lewis acid sites and Brønsted acid sites, and enhanced the acid intensity of the weak acid sites, both of which increased the catalytic performance of the CO2 hydrogenation to form
DME. Reactions were conducted using a CuO-Fe2O3-CeO2/HZSM-5
catalyst with 3.0 wt% CeO2 at 260 °C and 3.0 MPa with a gaseous
hourly space velocity of 1500 m gcat 1 h 1, resulting in a 20.9% CO2
conversion and 63.1% DME selectivity.
Acknowledgement
This work was supported by National Natural Science Foundation of China, 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 (2015K004).
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