CES 2016.Pdf

Chemical Engineering Science 153 (2016) 10–20

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Chemical Engineering Science

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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

HIGHLIGHTS GRAPHICAL ABSTRACT

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. article info abstract

Article history: A series of CuO-Fe2O3-CeO2 catalysts with various CeO2 doping were prepared via the homogeneous Received 12 March 2016 precipitation method, characterized and mechanically mixed with HZSM-5. Their feasibility and per- Received in revised form formance for the synthesis of dimethyl ether (DME) via CO2 hydrogenation in a one-step process were 30 June 2016 evaluated. The formed stable solid solution after the CuO-Fe2O3 catalyst modified with CeO2 promoted Accepted 4 July 2016 the CuO dispersion, reduced the CuO crystallite size, decreased the reduction temperature of highly Available online 5 July 2016 dispersed CuO, modified the specific surface area of the CuO-Fe2O3-CeO2 catalyst, and improved the Keywords: catalytic activity of the CuO-Fe2O3-CeO2 catalyst. The addition of CeO2 to CuO-Fe2O3 catalyst increased Carbon dioxide hydrogenation the amount of Lewis acid sites and Brønsted acid sites, and enhanced the acid intensity of the weak acid Dimethyl ether synthesis sites, which in turn promoted the catalytic performance of CO hydrogenation to DME. The optimal Mixed oxide 2 introduced amount of Ce in the catalyst was determined to be 3.0 wt%. The CO conversion and DME Cu-Fe-Ce catalysts 2 Cerium oxide selectivity were 20.9%, and 63.1%, respectively, when the CO2 hydrogenation to DME was carried out at 1 1 260 °C, and 3.0 MPa with a gaseous hourly space velocity of 1500 mL gcat h . & 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Dimethyl ether (DME) possesses a high cetane number and

n produces less NO and SO than do fossil fuels when combusted, Corresponding author at: School of Chemistry and Chemical Engineering, x x Guangxi Key Laboratory of Petrochemical Resource Processing and Process In- representing an environmental friendly renewable fuel (Frusteri tensiﬁcation Technology, Guangxi University, Nanning 530004, China. et al., 2015; García-Trenco and Martínez, 2012; Rutkowska et al., nn Corresponding author. 2015). DME can also be used as a chemical intermediate for 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. X. Zhou et al. / Chemical Engineering Science 153 (2016) 10–20 11 dimethyl sulphate, methyl acetate and low carbon oleﬁn synthesis (XRD) to determine the crystal structure of catalysts, Raman (Ge et al., 1998). The fossil fuel usage has increased the amount of spectroscopy for obtaining the vibrational and rotation mode of

CO2 in the atmosphere and the subsequent greenhouse effect the lattice and the molecules of catalysts, high-resolution trans- poses a threat to the environment. In addition, the energy crisis mission electron microscopy (HRTEM) to obtain the information facing the whole world is becoming increasingly urgent. Therefore, on the structure and composition of the particles, N2 adsorption- converting CO2 into useful chemicals, such as DME, is an attractive desorption to determine the pore structures and the texture method to reduce greenhouse-gas emissions and to recycle properties of the catalysts, H2-temperature programmed reduction carbon. (H2-TPR) to obtain the quantitative reduction information, thermal

The hydrogenation of CO2 to form DME mainly includes three analysis (TG-DTA) for determining the weight changes in the cal- reactions, i.e., methanol synthesis reaction (Eq. (1)), methanol cination process of the catalysts, Fourier transform infrared spec- dehydration reaction to form DME (Eq. (2)) and a reversed water- troscopy (FTIR) of adsorbed pyridine and temperature-pro- gas shift reaction (Eq. (3))(Bonura et al., 2014b; Yang Lim et al., grammed desorption of ammonia (NH3-TPD) for obtaining the

2016). DME can be produced from CO2 via two routes. The ﬁrst surface acidities of the catalysts, and temperature programmed route is a two-step process through two reactors (including me- oxidation (TPO) to detect the coke formed during the reaction. The thanol synthesis on a metallic catalyst and subsequent dehydra- catalysts were mixed with HZSM-5 to synthesize dimethyl ether tion of methanol on an acid catalyst). The second route is a one- via CO2 hydrogenation in a one-step process. step 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 2. Experimental reaction. Therefore, one-step process is economically and ther- modynamically favored (Jia et al., 2006; Vakili et al., 2011). 2.1. Catalyst preparation

1 CO2 þ3H2⇋CH3OHþH2O; ΔH298¼49.46 kJ mol (1) The precursor for producing the CuO-Fe2O3-CeO2 catalyst was prepared via a homogeneous precipitation method. ⇋ þ Δ ¼ 1 2CH3OH CH3OCH3 H2O; H298 23.5 kJ mol (2) Cu(NO3)2 3H2O and Fe(NO3)3 9H2O (Sinopharm Chemical Re- agent Co., Ltd.) were weighed at Cu/Fe mole ratio of 3:2 (Liu et al., 1 CO2 þH2⇋COþH2O; ΔH298 ¼41.17 kJ mol (3) 2013; Qin et al., 2015; Qin et al., 2016). Then, the amount of the Ce(NO3)3 6H2O (Sinopharm Chemical Reagent Co., Ltd) was de- The bifunctional catalysts for the one-step synthesis of DME via termined by the desired CeO2 content of 1.0, 2.0, 3.0 and 4.0 wt% in CO hydrogenation include a Cu-base catalyst for the hydrogena- 2 CuO-Fe2O3-CeO2 in the final catalysts and was added to obtain a γ 1 tion component and a solid acid, such as HZSM-5, -Al2O3 or HY metal nitrate solution (0.5 mol L ). A urea (China Guangdong zeolite for the dehydration component, resulting in Guanghua Sci-Tech Co., Ltd.) solution (5 mol L1) was then added CuO-ZnO-Al2O3/HZSM-5 (Liu et al., 2015), CuO-ZnO-Al2O3-ZrO2 to the metal nitrate solution, in which the urea to nitrates molar /HZSM-5 (An et al., 2008), Cu-ZnO-ZrO2/HZSM-5 (Frusteri et al., ratio was 20. The mixture was stirred and reacted for 24 h at 90 °C. 2015), CuO-TiO2-ZrO2/HZSM-5 (Wang et al., 2009)or The obtained precipitates were filtered and dried at 110 °C for 12 h, CuO-Fe2O3-ZrO2/HZSM-5 (Liu et al., 2013; Qin et al., 2015). How- ground through a 20–40 mesh, calcined at 400 °C for 4 h, and thus ever, the conversion of stable CO2 molecule requires high tem- the catalyst CuO-Fe2O3-CeO2 was obtained for methanol synthesis. perature and pressure and only 10–30% was reported with the HZSM-5, using as the methanol dehydration component, with a DME selectivity of only 30–50% when the reaction was carried out silica-alumina ratio of 300:1 (China Shanghai Novel Chemical – – ° at 4 10 MPa and 200 300 C. Due to the conversion of CO2 to Technology Co., Ltd.) was mechanically mixed with the methanol was limited by the thermodynamic equilibrium, it was CuO-Fe2O3-CeO2 oxide composites at a mass ratio of 1:1. For fi more dif cult to attain reasonable per-pass conversions for large comparison, a CuO-Fe2O3/HZSM-5 catalyst without CeO2 was industrial scale CO2 treatment. prepared via the same method. CeO2 is an effective co-catalyst with a hollow structure, and can stabilize and disperse Cu and other precious metals (Bera et al., 2.2. Catalyst characterization 2003). CeO2 and CuO can form a stable solid solution (Gamarra et al., 2007; Marbán and Fuertes, 2005) to improve both catalytic The XRD, N2 adsorption-desorption and H2-TPR catalyst char- activity and thermal stability of CuO-based catalysts. CeO2 has acterizations can be referenced in previous studies (Liu et al., been frequently selected as a component for use in complex oxides 2013). Briefly, the X-ray diffraction (XRD) analysis was performed or as a dopant to improve the catalyst performance. CeO2 has been using a Bruker D8 Advance X-ray diffractometer. The isotherm of applied in the field of preferential CO oxidation (Hornés et al., nitrogen adsorption and desorption was measured by an ASAP 2010; Jia et al., 2010) and methanol steam reforming (Tonelli et al., 2000 physical adsorption instrument (Micromeritics Instrument 2011). For the dimethyl ether synthesis from CO2, the acid prop- Corporation). The catalyst surface area was calculated via Bru- erties of the catalysts influenced the DME selectivity (García- nauer-Emmett-Teller (BET) method, and the pore size distribution Trenco and Martínez, 2012; Yoo et al., 2007). For example, the curve was determined using the Barrett-Joyner-Halenda (BJH) acidity of zeolite HY was enhanced by the addition of Ce, and it model, which was based on the isotherm of desorption side. The possessed a high proportion of moderate strength acid sites and H2-TPR, NH3-TPD, and TPO experiments of catalyst samples were were more stable for methanol dehydration to DME (Jin et al., taken in a PX200 multifunction catalyst analysis system (China 2007); the presence of Ce on the HZSM-5 could modify the acidic Tianjin Golden Eagle Technology Co., Ltd.). properties of HZSM-5 including the amount of acid sites and acid The Raman spectra were obtained using Lab RAMHR800 Laser type (Wang et al., 2007); and the addition of Ce could enhance the Confocal Micro-Raman Spectroscopy (Horiba, Ltd.), including a acidity of MnOx/TiO2 (Wu et al., 2008). These indicate that Ce YAG laser with a 532 nm excitation wavelength. High resolution played an important role on the catalysts acid properties. transmission electron microscope (HRTEM) images of the catalysts

In this work, a series of CuO-Fe2O3 catalysts modiﬁed using were obtained using a FEI tecnai G20 instrument with an accel- different amounts of CeO2 were prepared via a homogeneous eration voltage of 200 kV. A thermal analysis was performed using precipitation method; characterized using X-ray diffractometer a ZRY-1P thermal analyser (Techcomp Jingke Scientiﬁc 12 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 min1 from 50 to 500 °C. The FTIR of adsorbed pyridine was conducted using a Magna-IR 750 FTIR spectrometer (Thermo Nicolet Corporation). Self-sup- porting catalysts wafers (ca. 10 mg cm2) 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 cm1 after subtracting the sample background.

2.3. Catalytic hydrogenation of CO2 to DME

The catalytic activity was investigated using a continuous fixed- bed stainless steel reactor, which is detailed in the literature (Liu et al., 2013). Briefly, 1.0 g hybrid catalyst was added to the fixed 1 bed reactor and reduced at 300 °C for 4 h with 30 mL min of H2 (99.999%) at atmospheric pressure. A gas mixture composed of

20% CO2 and 80% H2 was then introduced and reacted at 240– ﬁ 280 °C, 2.0–4.0 MPa with a gaseous hourly space velocity (GHSV) Fig. 2. Raman spectra of the CuO-Fe2O3-CeO2 catalysts modi ed by CeO2 content of 1 1 (a) 0, (b) 1.0, (c) 2.0, (d) 3.0, and (e) 4.0 wt% CeO2 calcined at 400 °C without H2 of 1500–3500 mL gcat h . reduction.

fi 3. Results and discussion CeO2 can signi cantly improve the CuO dispersion based on a 3.0 wt% CeO2. Further increasing the CeO2 amount did not improve 3.1. XRD analysis 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 Fig. 1 illustrates the XRD patterns of the CuO-Fe2O3 catalysts CeO2x peaks at 2θ of 26.3°, 43.9° and 55.7° (JCPDS No. 49-1415) modified by different CeO2 amounts before H2 reduction. The diffraction peaks at 2θ¼35.49°, 38.69°, and 58.26° are associated 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) with monoclinic CuO (JCPDS No. 48-1548). The peaks at 4 þ 2θ¼30.24°, 35.63°, and 62.93° can be attributed to cubic crystal (Wang et al., 2014) entered to the CeO2 lattice (the radius of Ce ionic is 0.97 Å) to form a solid solution (Li et al., 2011, 2014a; Fe2O3 (JCPDS No. 39-1346). The CuO-Fe2O3 peaks are sharper than Marbán and Fuertes, 2005). those of CuO-Fe2O3 catalysts modified by CeO2, suggesting that CuO-Fe O possesses an enhanced crystallinity (Wang et al., 2016). 2 3 3.2. Raman spectra The CuO diffraction peaks become weakened and broadened as the amount of CeO was increased, indicating that CuO dispersion 2 Fig. 2 shows the Raman spectra of the CuO-Fe O -CeO cata- was improved by the addition of CeO . The crystallite sizes of CuO 2 3 2 2 lysts. The peaks of the CuO-Fe O catalyst observed at 210, 276, (111) for CuO-Fe O -CeO catalysts modified with 1.0, 2.0, 3.0 and 2 3 2 3 2 373, and 574 cm1 are related to typical A and 2E Raman he- 4.0 wt% CeO calculated by the Scherrer equation are 17.6, 14.7, 14.3 1g g 2 matite modes, whereas the peak at 1288 cm1 is associated with and 15.9 nm, respectively. However, the crystallite size of CuO hematite two-magnon scattering (Li et al., 2014b; Wang et al., (111) for CuO-Fe O catalyst is 22.5 nm, indicating that the mod- 2 3 2014). The peak at 276 cm 1 arises from CuO (Tsoncheva et al., ification with CeO2 can decrease the crystallite size of CuO and 2013). The peaks of the modified CuO-Fe2O3-CeO2 catalyst at promote the dispersion of catalysts. The XRD patterns suggest that 1 468 cm can be attributed to the strong F2g mode of CeO2 (Qi et al., 2012). The broad band between 550 and 700 cm1 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 Ce4þ 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

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, corre- 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. sponding 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, respec- (b)) displays 0.252 and 0.295 nm interplanar spacing, corresponding tively. The interplanar spacing of CuO (111) (0.2524 nm) and to the (311) and (220) planes of cubic crystal Fe2O3, respectively. The

Fe2O3 (311) (0.2518 nm) are nearly equal. Therefore, the 0.252 nm magniﬁed catalyst image in Fig. 4(c) is based on the square frame in interplanar spacing may also correspond to CuO (111). The Fig. 4(b). This magniﬁed image displays 0.187 and 0.271 nm inter-

CuO-Fe2O3-CeO2 catalyst with 3.0 wt% CeO2 HRTEM image (Fig. 4 planar spacing, corresponding to the (202) 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) the Ce structural relaxation, which consequently increases the plane of monoclinic CuO was not observed in any of the surface area and the concentration of defects, such as oxygen va- 2 1 CuO-Fe2O3-CeO2 (CeO2¼3.0 wt%) catalysts, suggesting that CuO cancies (Zhan et al., 2012). The largest surface area (67.34 m g ) was relatively well-dispersed in the catalyst (Gamarra et al., 2007). was observed for the catalysts with 3.0 wt% CeO2. The larger The selected area electron diffraction (SAED) patterns are given in speciﬁc surface area can provide more absorption and/or reaction the inset of Figs. 3(b) and 4(b). The SAED patterns include diffraction sites for CO2 and H2, resulting in a higher catalytic activity (Huang rings, indicating that the two catalysts are polycrystalline. In addi- et al., 2016). However, the speciﬁc surface area was decreased tion, the energy-dispersive X-ray spectrum (EDX) displays the Fe, Ce when the CeO2 loading was increased from 3.0 to 4.0 wt%, which and O signals, further proving that the mixed metal-oxide catalysts may be associated with the crystallite growth as the amount of have been fabricated. 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 Fig. 6 shows the precursor TG-DTA curves for different CeO2 the CuO-Fe2O3-CeO2 catalysts with different CeO2 amounts were amounts. Fig. 6A shows the TGA curves of the precursor. The investigated using nitrogen adsorption-desorption isotherms, weight loss is observed to occur in three steps. The weight loss in Fig. 5. The isotherms of the catalysts are similar, Fig. 5A, exhibiting the range of 50–200 °C can be attributed to the desorption of IV type isotherms according to the IUPAC classification, suggesting physically adsorbed surface water and the bound water from that all the catalysts possess the mesoporous features (Huang precursors (Cao et al., 2008; Liu et al., 2015). The precursor weight et al., 2016; Zhang et al., 2014). Furthermore, the pore size dis- losses were 10.1%, 6.2%, 7.1%, 8.0% and 8.4% for the CeO2 amounts tribution profiles (Fig. 5B) of the catalysts exhibited a narrow pore of 0, 1.0, 2.0, 3.0 and 4.0 wt%, respectively. The weight loss from size distribution, with a maximum of 3.0 nm. The BET method 200 to 400 °C corresponds to the thermal decomposition of was used to calculate the N2 adsorption-desorption isotherms and Cu(OH)2 (Eq. (4)) and Fe(OH)3 (Eq. (5)), forming CuO and Fe2O3, obtain the specific surface areas of the catalysts, which are listed in respectively (Bonura et al., 2014a). The DTA curve (Fig. 6B) of the Table 1. corresponding precursors represents an endothermic peak. The

The speciﬁc surface areas of the CuO-Fe2O3-CeO2 catalysts were precursor weight losses in this step were 12.2%, 10.2%, 9.9%, 8.0% 2 1 50.32, 56.41, 62.26, 67.34 and 56.60 m g for the CeO2 contents and 11.3% for the CeO2 amount of 0, 1.0, 2.0, 3.0 and 4.0 wt%, of 0, 1.0, 2.0, 3.0 and 4.0 wt%, respectively, suggesting that the respectively. addition of proper amount of CeO2 can increase the speciﬁc sur- Cu(OH)2-CuOþH2O (4) face areas of the CuO-Fe2O3 catalysts. In addition, CeO2 can produce larger BET surfaces area than CuO (Tsoncheva et al., 2013; Xu 2Fe(OH)3-Fe2O3 þ3H2O (5) et al., 2016). Furthermore, the CeO2 particle size of the

CuO-Fe2O3-CeO2 catalyst will be reduced due to the 4f orbit and 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 Table 1 form a solid solution during the oxidation process. However, the

Textural properties of the CuO-Fe2O3-CeO2 catalysts with different CeO2 contents. weight was increased from Ce2O3 to CeO2 (Eq. (7)). Therefore, the weight loss of the precursors modiﬁed with CeO2 is less than that 2 1 CeO2 content (wt%) BET surface area (m g ) Average pore diameter (nm) of the CuO-Fe2O3 catalyst precursor.

0 50.32 25.54 2Ce(OH)3-Ce2O3 þ3H2O (6) 1.0 56.41 20.62 2.0 62.26 19.44 3.0 67.34 18.96 2Ce2O3 þO2-4CeO2 (7) 4.0 56.60 20.86 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

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 si-

milar 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 (Avgour- opoulos 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 °C155°C and

170 °C for the catalyst with a CeO2 content of 1.0, 2.0, 3.0 and

Fig. 7. H2-TPR profiles for CuO-Fe2O3-CeO2 with CeO2 content of (a) 0, (b) 1.0, 4.0 wt%, respectively. These values are lower than the tempera- (c) 2.0, (d) 3.0, and (e) 4.0 wt%. The solid curves are experimental curves and tures associated with the CuO-Fe2O3 catalyst (172 °C), indicating broken curves are Gaussian multi-peak fitting curves. that the CeO2 modification can decrease the reduction temperature and increase the reducibility of highly dispersed CuO to in- Table 2 crease the reducibility of the catalysts. The observed lowest tem- Temperatures and area distributions of the CuO-Fe2O3-CeO2 reduction peaks for a perature of peak α at 155 °C, corresponded to the CuO-Fe O -CeO different CeO2 contents. 2 3 2 catalyst with 3.0 wt% CeO2, suggesting that this catalyst exhibited α β γ CeO2 content (wt%) Peak Peak Peak Total area an optimum reducing behavior. The ratios of the α reduction peak

T (°C) Areab T (°C) Area T (°C) Area 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, 0 172 0.89 219 7.30 247 3.26 11.45 2.0, 3.0 and 4.0 wt%, respectively. These results suggest that the 1.0 156 2.31 178 2.39 195 3.34 8.04 proportion of highly dispersed CuO was increased due to the CeO 2.0 167 1.51 178 2.30 198 1.47 5.28 2 fi 3.0 155 3.02 177 2.67 187 3.05 8.74 modi cation. Moreover, the CuO-Fe2O3-CeO2 catalyst with 3.0 wt% 4.0 170 2.23 182 0.93 203 7.73 10.89 CeO2 possessed the largest proportion of highly dispersed CuO. fi a The total hydrogen consumption of CeO2 modi ed CuO-Fe2O3 The results were measured based on the H2-TPR profiles and the area distributions were calculated by integrating of the areas under the peaks. catalysts was decreased compared to the CuO-Fe2O3 catalyst. The b The peak area unit is 104 units. XRD results suggest that a portion of the copper and iron em- bedded in ceria form a solid solution, and the solid solution be- above 400 °C(Mo et al., 2005). Therefore, a 400 °C calcination comes more difficult to be reduced (Han et al., 2013; Wang et al., temperature was adopted to ensure a complete decomposition of 2005). Therefore, the total hydrogen consumption of CeO2 mod- the catalyst precursors and minimize sintering phenomena. ified CuO-Fe2O3 catalysts was decreased. 16 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 a CuO-Fe2O3-CeO2/HZSM-5 catalysts.

CeO2 con- CO2 conversion Products selectivity (mol%) DME yield tent (wt%) (mol%) (mol%)

DME CH3OH CO CH4

0 12.3 18.3 0.9 30.5 50.3 2.3 1.0 18.1 52.0 2.1 25.4 20.5 9.4 2.0 18.8 56.9 3.5 23.8 15.8 10.7 3.0 20.9 63.1 5.2 24.8 6.8 13.2 4.0 17.1 50.4 4.6 39.5 5.6 8.6

a 1 1 Reaction conditions: T¼260 °C, P¼3.0 MPa, GHSV¼1500 mL gcat h , and

V(H2)/V(CO2)¼4.

1997), and possibly results from the interactions between the adsorbed pyridine and metal oxides (Tang et al., 2010). The 1608 cm1 band is associated with pyridine adsorption vibrations at Brønsted acid sites (Stevens, 2003). Fig. 8. FT-IR spectra of pyridine adsorbed on CuO-Fe2O3 (a) and CuO-Fe2O3-CeO2 Fig. 9 shows the NH3-TPD profiles of the CuO-Fe2O3 catalysts catalysts with 3.0% wt% CeO2 (b) at a desorption temperature of 150 °C. modified by different CeO2 amounts before H2 reduction. From Fig. 9, three NH desorption peaks were detected at 50–200 °C 3.7. Surface acidity analysis 3 (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 Fig. 8 shows the FTIR spectra of the adsorbed pyridine on the et al., 2008). The α, β, and γ peaks corresponded to the desorption CuO-Fe O and CuO-Fe O -CeO (CeO ¼3.0 wt%) catalysts. The 2 3 2 3 2 2 of the adsorbed NH on weak, medium, and strong acid sites, re- spectra were normalized and recorded under identical operating 3 spectively (Jin et al., 2007). It can be found that after different CeO conditions. The two characteristic bands at ca. 1450 and 1540 cm 1 2 amounts were added into the CuO-Fe O catalyst, the intensities of are associated with pyridine adsorption on Lewis acid sites and 2 3 weak acid sites were enhanced, indicating increased weak acid Brønsted acid sites, respectively (Wang et al., 2007). The weak band sites of the CuO-Fe2O3 catalysts modified by different CeO2 at 1442 cm1 for CuO-Fe O and the intense band at 1438 cm1 for 2 3 amounts and consistent with the Py-IR result. When the content of CuO-Fe2O3-CeO2 (CeO2¼3.0 wt%) indicated that the modified of CeO2 was 3.0 wt%, the acid intensity of weak acid site became the CuO-Fe2O3 catalyst with Ce increased the amount of Lewis acid sites. strongest. However, the γ peak of the strong acid sites became The Ce metal cations possess empty f orbits. The empty f orbit can broadened after the Ce modification, implying that CeO2 decreased provide potential locations for Lewis acid sites (Wang et al., 2007). the catalyst strong acidity. Therefore, the empty f orbits modify the acidity of the CuO-Fe2O3 catalysts. Moreover, the shift towards lower wavenumbers can be 3.8. Catalytic hydrogenation of CO2 to DME on CuO-Fe2O3-CeO2 taken as an indicative of a lower acid strength of the Lewis acid sites /HZSM-5 (García-Trenco and Martínez, 2012). The CuO-Fe2O3-CeO2 (CeO2 ¼ 1 3.0 wt%) band at 1546 cm demonstrates that the Brønsted acid 3.8.1. Effect of the CeO2 content fi sites were formed after the Ce modi cation. This result indicates The catalytic performances of the CuO-Fe2O3-CeO2/HZSM-5 fi that the acid is altered by the CeO2 modi cation. The observed band catalysts with different amounts of CeO2 during DME synthesis 1 fi at 1476 cm for both catalysts has also been identi ed in the FTIR from CO2 hydrogenation are presented in Table 3. According to spectra of the adsorbed pyridine on HY zeolites (Boréave et al., 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 mono- tonously 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 hydro-

genation. The speciﬁc surface area was increased as the CeO2 amount was increased from 0 to 3.0 wt% during the nitrogen ad-

sorption-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 Fig. 9. NH3-TPD profiles of the CuO-Fe2O3-CeO2 catalysts modified by (a) 0, (b) 1.0, ° fi (c) 2.0, (d) 3.0, and (e) 4.0 wt% CeO2 calcined at 400 C without H2 reduction. CuO-Fe2O3-CeO2 catalysts. The larger speci c 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 Fig. 11. Effects of the reaction pressure on the catalytic CO2 hydrogenation to form ¼ ° ¼ – CuO-Fe2O3-CeO2/HZSM-5. Reaction conditions: T¼240–280 °C, P¼3.0 MPa, DME for CuO-Fe2O3-CeO2/HZSM-5. Reaction conditions: T 260 C, P 2.0 4.0 MPa, 1 1 ¼ 1 1 ¼ GHSV¼1500 mL gcat h , and V(H2)/V(CO2)¼4. GHSV 1500 mL gcat h , and V(H2)/V(CO2) 4.

increase the absorption and/or the number of CO2 and H2 reaction increased from 2.0 to 3.0 MPa, but decreased when the reaction sites, which may be partially responsible for improving the cata- pressure was increased from 3.0 to 4.0 MPa. The DME selectivity lytic hydrogenation process. Combined the H2-TPR results, the also exhibited similar trends. This results imply that the direct reduction temperature of the highly dispersed CuO was decreased CO2-to-DME hydrogenation reaction was a volume decreasing after modiﬁed by CeO2, in addition, the proportion of highly dis- consecutive reaction, suggesting that high reaction pressure may persed CuO was increased, improving the activity of the catalyst. increase the DME selectivity. However, the CO2 conversion (from

The results of pyridine FTIR spectra and NH3-TPD suggest that 20.9% to 17.2%) and DME selectivity (from 63.1% to 55.5%) were modifying the CuO-Fe2O3 catalyst with CeO2 can alter the acid decreased when the pressure was increased from 3.0 to 4.0 MPa. type, increase the amount of total acid sites (Lewis acid sites and This trend was most likely caused by the accumulation of water, Brønsted acid sites), and enhance the acid intensity of weak acid which cannot be removed from the ﬁxed bed reactor during the site, which can improve the CO2 hydrogenation to DME catalytic CO2 hydrogenation to form DME process and may decrease the performance. 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 3.8.4. Space velocity effects CuO-Fe2O3-CeO2/HZSM-5 bifunctional catalysts with 3.0 wt% CeO2 The CuO-Fe2O3-CeO2/HZSM-5 catalyst with 3.0 wt% CeO2 was for temperatures ranging from 240 to 280 °C. Fig. 10 illustrates that used to synthesize DME from CO2 and H2, and the space velocity – 1 1 the CH3OH selectivity was increased with increasing the tem- effect was investigated over a range of 1500 3500 mL gcat h , perature. In addition, the CO2 conversion and DME selectivity were Fig. 12. The CO2 conversion and DME selectivity were decreased as 1 1 increased when the reaction temperature rose from 240 to 260 °C, the space velocity was increased from 1500 to 3500 mL gcat h . reaching maximum values of 20.9% and 63.1% at 260 °C, respec- However, the CH3OH selectivity was increased from 5.2% to 11.3% tively. However, further increasing the reaction temperature as the space velocity was increased from 1500 to 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 Fig. 12. Effect of the space velocity on the catalytic CO2 hydrogenation to form DME reaction pressure was increased from 2.0 to 4.0 MPa. In addition, for CuO-Fe2O3-CeO2/HZSM-5. Reaction conditions: T¼260 °C, P¼3.0 MPa, ¼ – 1 1 ¼ the CO2 conversion was increased when the pressure was GHSV 1500 3500 mL gcat h , and V(H2)/V(CO2) 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.

1 1 3500 mL gcat h . The contact time between the CO2/H2 mixture catalyst, the weight loss at 400 °C is attributed to the coke com- and the catalyst surface was decreased by increasing the space bustion (Tonelli et al., 2015), indicating that slight carbon de- velocity, leading to an insufficient contact between the CO2/H2 position occurred on the catalyst surface after the 15-h reaction. mixture and the active catalysts sites. Methanol could not Thus, DME selectivity and CO2 conversion slightly decreased as promptly dehydrate to DME on the HZSM-5 catalyst surface and time went on. Nevertheless, the used CuO-Fe2O3/HZSM-5 catalyst rapidly left the fixed bed reactor (An et al., 2008). Therefore, in- exhibited a large weight loss above 400 °C after the 6-h reaction, creasing the space velocity caused the DME selectivity to decrease indicating that modifying the CuO-Fe2O3 catalyst with CeO2 can from 63.1% to 47.5% and the methanol selectivity to increase. These inhibit the coke generation. Fig. 13D shows the TPO profiles of the findings suggest that the optimal space velocity for DME synthesis used catalysts, and three peaks (α, β and γ) are shown for the used 1 1 is 1500 mL gcat h . 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

3.8.5. Stabilities and coke analysis Fe3O4 to Fe2O3, and the β and γ peaks were the oxidation pro- The stabilities of the CuO-Fe2O3/HZSM-5 and 3.0 wt% CeO2 cesses of the monatomic carbon and whisker carbon, respectively modified CuO-Fe2O3-CeO2/HZSM-5 catalysts were studied in the (Hou et al., 2003). The peak β of the used CuO-Fe2O3-CeO2/HZSM- context of DME synthesis via CO2/H2 at 260 °C and 3.0 MPa, with 5 (3.0 wt%) catalyst significantly became weakened compared to 1 1 GHSV¼1500 mL gcat h and V(H2)/V(CO2)¼4. The results were the used CuO-Fe2O3/HZSM-5 catalyst, indicating that carbon de- recorded every 0.5 h, Fig. 13. The CO2 conversion and DME se- position was suppressed because of the formation of solid solution lectivity of the CuO-Fe2O3-CeO2/HZSM-5 catalyst with 3.0 wt% (Hou et al., 2003; Sierra et al., 2011) and then exhibited a high CeO2 remained almost constant throughout the 15-h reaction at activity and stability. 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 mod- 4. Conclusions ification can improve the CuO-Fe2O3/HZSM-5 catalyst stability. Modifying the CuO-Fe2O3 catalyst with CeO2 improved the CuO The CuO-Fe2O3-CeO2 catalysts with various CeO2 amounts were dispersion, resulting in less amalgamated fine Cu crystal (Tan et al., successfully prepared using a homogeneous precipitation method.

2005) and increases the catalyst activity and stability. The CuO-Fe2O3-CeO2 was mixed with HZSM-5 zeolite and used in Fig. 13C shows the TGA curves of the used catalysts. According the CO2 hydrogenation to produce DME. The results showed that to the TGA curve of the used CuO-Fe2O3-CeO2/HZSM-5 (3.0 wt%) the CeO2 modified CuO-Fe2O3 catalysts formed a stable solid X. Zhou et al. / Chemical Engineering Science 153 (2016) 10–20 19 solution, which promoted CuO dispersion, decreased the CuO Huang, Y., Long, B., Tang, M., Rui, Z., Balogun, M.-S., Tong, Y., Ji, H., 2016. Bifunctional crystallite size, decreased the reduction temperature of highly dis- catalytic material: An ultrastable and high-performance surface defect CeO2 fi nanosheets for formaldehyde thermal oxidation and photocatalytic oxidation. persed CuO, altered the speci c surface areas of CuO-Fe2O3-CeO2 Appl. Catal. B: Environ. 181, 779–787. catalysts, and improved the catalytic activity of the CuO-Fe2O3-CeO2 Jia, A.P., Jiang, S.Y., Lu, J.Q., Luo, M.F., 2010. Study of catalytic activity at the CuO-CeO interface for CO oxidation. J. Phys. Chem. C 114, 21605–21610. catalyst. The addition of CeO2 to the CuO-Fe2O3 catalyst increased 2 Jia, G.X., Tan, Y.S., Han, Y.Z., 2006. A comparative study on the thermodynamics of the amount of Lewis acid sites and Brønsted acid sites, and en- dimethyl ether synthesis from CO hydrogenation and CO2 hydrogenation. Ind. hanced the acid intensity of the weak acid sites, both of which in- Eng. Chem. Res. 45, 1152–1159. creased the catalytic performance of the CO2 hydrogenation to form Jin, D., Zhu, B., Hou, Z., Fei, J., Lou, H., Zheng, X., 2007. Dimethyl ether synthesis via fi – DME. Reactions were conducted using a CuO-Fe O -CeO /HZSM-5 methanol and syngas over rare earth metals modi ed zeolite Y and dual Cu 2 3 2 Mn–Zn catalysts. Fuel 86, 2707–2713. ° catalyst with 3.0 wt% CeO2 at 260 C and 3.0 MPa with a gaseous Li, J., Han, Y., Zhu, Y., Zhou, R., 2011. Purification of hydrogen from carbon monoxide 1 1 fi – hourly space velocity of 1500 m gcat h ,resultingina20.9%CO2 for fuel cell application over modi ed mesoporous CuO CeO2 catalysts. Appl. – – conversion and 63.1% DME selectivity. Catal. B: Environ. 108 109, 72 80. Li, L., Song, L., Chen, C., Zhang, Y., Zhan, Y., Lin, X., Zheng, Q., Wang, H., Ma, H., Ding, L., Zhu, W., 2014a. Modified precipitation processes and optimized copper

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