Synthesis of Dimethyl Ether from CO2 and H2 Using a Cu–Fe–Zr

Nov 4, 2013 - Recent advances in the photocatalytic reduction of carbon dioxide. Tong-ming Su , Zu-zeng Qin , Hong-bing Ji , Yue-xiu Jiang , Guan Huan...
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Synthesis of Dimethyl Ether from CO2 and H2 Using a Cu−Fe−Zr/HZSM‑5 Catalyst System Rui-wen Liu,† Zu-zeng Qin,*,† Hong-bing Ji,*,†,‡ and Tong-ming Su† †

School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, People’s Republic of China School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China

‡

ABSTRACT: The CuO−Fe2O3−ZrO2/HZSM-5 bifunctional catalyst was prepared and used for the direct synthesis of dimethyl ether (DME) from CO2 and H2. The results revealed that doping the CuO−Fe2O3 catalyst with ZrO2 might increase the specific surface area and change the chemical combination state of CuO by decreasing the outer-shell electron density of Cu via an obvious change in the interaction between CuO and Fe2O3. Addition of ZrO2 to the catalyst strongly affects the hydrocarbon selectivity. When using the CuO−Fe2O3−ZrO2/HZSM-5 bifunctional catalyst system, both the conversion of CO2 and the yield of DME were much higher than those obtained using CuO−Fe2O3/HZSM-5 as catalysts. Reactions carried out at 260 °C and 3.0 MPa with a gaseous hourly space velocity of 1500 mL·gcat−1·h−1 using CuO−Fe2O3−ZrO2/HZSM-5 with 1.0 wt % ZrO2 as the hydrogenation catalyst provided a 28.4% conversion of CO2 with 64.5% selectivity for DME.

1. INTRODUCTION With the sharp increase in the world’s population, as well as economic development and the expanding degree of industrialization, excess CO2 emission is a gradual threat to nature.1 However, it is undeniable that CO2 is a possible carbon source. To utilize CO2 and solve the environmental problems caused by CO2, the key issue is to develop technologies able to capture, store, and use CO2.2,3 As for utilizing CO2, the technology of hydrogenating CO2 to dimethyl ether (DME) is an efficient method.4 With a high cetane number and excellent properties, DME is not only a widely used chemical product but is also clean energy. DME is regarded as one of new energy sources in the 21st century and as an environmentally friendly fuel with low NOx and no SOx.5,6 Presently, there are two main processes to synthesize DME using CO2 and H2 as raw materials: a two-step process and a one-step process. The two-step process consists of a synthetic step providing methanol, followed by a dehydration step to produce DME. The one-step process integrates the synthesis of methanol and its dehydration to DME into a single step. The emphasis of the current research is on developing the one-step process to synthesize DME from CO2 for two main reasons. (1) The simultaneous production of methanol and DME in a one-step process reduces the buildup of the methanol intermediate in the reactor that usually impedes the synthesis of methanol due to reaching the thermodynamic equilibrium; therefore, the effect is an increased driving force for the first step and thus the overall process. (2) From the perspective of equipment investment, a one-step process is more advantageous than a two-step process because the synthesis and dehydration of methanol requires only one reactor.7,8 The activation of CO2 is achieved mainly by chemical catalysis.2,9 In a one-step process, the bifunctional catalyst for the synthesis of DME can create synergy by combining a hydrogenation component with a dehydration component. Therefore, the key point is to prepare an efficient catalyst for © 2013 American Chemical Society

the activation of CO2. Until now, bifunctional catalysts for the synthesis of DME have relied on CuO−ZnO as the hydrogenation component and a solid acid, such as Al2O3, or HZSM-5, for the dehydration component of the catalyst, which shows higher CO2 conversion and higher DME selectivity than single-metal oxide catalysts for CO2 hydrogenation.10,11 However, the reaction producing DME generates more water than the reaction producing methanol, and the activity and stability of the catalyst could be affected by the hydrophilic properties of ZnO.12,13 Iron catalysts have a significant role, and iron and the heat and formation of its oxides usually occur as promoters in supported metal catalysts in the form of isolated ions or nanosized oxide crystallites, even under reduction or reaction conditions.14 Its properties of preventing active centers from sintering and dispersing active centers’ crystallites are welldocumented in the literature.15,16 Incorporation of Fe into catalysts has been used widely to improve the stability of CuO catalysts by inhibiting the deactivation of CuO surfaces during the reaction.17 Introduction of Fe into a Cu-based catalyst can also promote the catalytic hydrogenation activity due to the strong interaction between Cu and Fe, which facilitates strong CO2 and H2 absorption. Due to these properties, CuO−Fe2O3 shows excellent catalytic activity for CO2 conversion. The effectiveness of these catalytic functions has been reflected by its use in the Fischer−Tropsch process and the gas-cleaning process in the synthesis of ammonia. However, for the catalytic hydrogenation of CO2, CuO−Fe2O3 is mainly applied to synthesize some low value-added products, such as CO and CH4, with low DME and alcohol content. The reaction carried out requires high pressure (12−15 MPa), and the conversions of CO and CO2 are typically very low. Thus, if CuO−Fe2O3 Received: Revised: Accepted: Published: 16648

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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 the desorption side. A Thermo ESCALAB 250X multifunction imaging electron spectrometer (American Thermo Fisher Scientific Co., Ltd.) was used to obtain the X-ray photoelectron spectrum of catalysts, using the Al Kα ray as the X-ray source. The H2 temperature programmed reduction (H2-TPR) experiments of catalyst samples were taken in a PX200 multifunction catalyst analysis system (China Tianjin Golden Eagle Technology Co., Ltd.). 2.3. Catalytic Hydrogenation of CO2 to DME. The DME synthesis from CO2 and H2 was carried out in a fixed bed reactor, consisting of a stainless steel reaction tube with a 10 mm inner diameter and a 300 mm tube length. A total of 1.0 g of CuO−Fe2O3−ZrO2/HZSM-5 catalyst was taken into the reactor and reduced at 300 °C for 4 h accompanied by H2 (99.999%) at a flowing rate of 30 mL/min. After the catalyst was reduced, the inflow of the H2 gas was replaced with a mixture of H2 and CO2 gas (5:1 mol ratio), and the catalytic hydrogenation of CO2 to DME was reacted at 240−280 °C and 2.0−3.5 MPa with gaseous hourly space velocity (GHSV) of 1500−3000 mL·gcat−1·h−1. When the reaction temperature and GHSV of the fixed bed reactor were kept constant, the products of DME synthesis were analyzed using an online Agilent 4890D gas chromatograph with a thermal conductivity detector (TCD). The CO2 conversion and the DME selectivity were calculated using a peak area normalization method.

could be used to produce DME and methanol, it could have broad potential applications. Furthermore, in a reverse water gas shift reaction, there are competitive phenomena between the formation reaction of CH4 and the CO2 hydrogenation process in the reactions forming methanol and DME.18,19 Due to the strong absorption of CO2 and H2 to Fe, CO2 is quickly reduced to CO and CH4 as the DME synthesis byproduct. While Cu species have been proven to be the active center for the production of DME and methanol, it should be possible to find a way to improve the activity of Cu that would weaken the absorption of CO2 onto Fe. Doping Zr into the CuO−Fe2O3 catalyst would provide a ternary mixed metal oxide that might improve the conversion of CO2 and selectivity of DME. The synthesis of multicomponent oxides is an effective way to improve the activation of catalysts by creating plentiful compositions, large surface areas, and pore volume.20,21 ZrO2 can be described as a good type of textural promoter.10,22 Studies on improving the performance of CuObased catalysts in CO2 hydrogenation reactions by doping ZrO2 are numerous. By adding ZrO2, there would be some variations in microstructure and porosity features, such as surface area, pore size distribution, and the reducibility of active centers.23,24 The activation of Cu species would likely be improved and the hydrocarbon selectivity would be strongly affected as a result of being able to use mild reaction conditions, such as lower temperature and lower pressure. In this work, a new type of bifunctional catalyst was developed for the DME synthesis by using CuO−Fe2O3−ZrO2 for the methanol synthesis component and HZSM-5 for the methanol dehydration component.

3. RESULTS AND DISCUSSION 3.1. Crystallization of Catalysts. Figure 1 shows the XRD patterns of CuO−Fe2O3−ZrO2 catalysts with different ZrO2

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalyst. The CuO−Fe2O3−ZrO2/ HZSM-5 bifunctional catalyst was made for the methanol synthesis and dehydration components of the DME process. The CuO−Fe2O3−ZrO2 mixed oxide catalyst, which was used as a methanol synthesis component, was prepared by a coprecipitation method. Cu(NO3)2·3H2O and Fe(NO3)3·9H2O were weighed according to the Cu/Fe mole ratio of 3:2, which was the optimal proportion of the Cu/Fe mole ratio observed in the preliminary experiments. This mixture was dissolved by deionized water followed by the addition of Zr(NO3)4 solution. The amount of the Zr(NO3)4 used depended on the desired ZrO2 content of 1.0, 2.0, and 3.0 wt % in CuO−Fe2O3−ZrO2. Under medium-speed stirring, these nitrate solutions and a 1.0 mol/L solution of Na2CO3 were added to a beaker using a parallel flow co-precipitating method while maintaining pH 10 and a reaction temperature of 70 °C for 2 h. After being aged for 1 h, the mixture was filtered and dried at 110 °C for 12 h, ground to 20−40 mesh, and calcined at 400 °C for 4 h, and the CuO−Fe2O3−ZrO2 catalyst was obtained. For the methanol dehydration transformation, HZSM-5 with a silica−alumina ratio of 300:1 (China Shanghai Novel Chemical Technology Co., Ltd.) was mechanically mixed with the CuO−Fe2O3−ZrO2 composite oxides at a 1:1 mass ratio. Thus the CuO−Fe2O3− ZrO2/HZSM-5 bifunctional composite catalyst was obtained. For comparison, a CuO−Fe2O3/HZSM-5 catalyst without Zr was prepared using the same method. 2.2. Characterization of the Catalyst. 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 Corp.). The catalyst surface area was calculated via

Figure 1. Diffraction patterns from JCPDS card nos. 80-1916 (CuO), 39-1346 (Fe2O3), and 24-1164 (ZrO2). XRD patterns of calcined CuO−Fe2O3−ZrO2 composites doped with 0, 1.0, 2.0, and 3.0 wt % ZrO2 without H2 reduction.

contents. By indexing the XRD patterns, CuO−Fe2O3−ZrO2 catalysts with various ZrO2 contents mainly consisted of monoclinic CuO (JCPDS No. 48-1548) and Fe2O3 in the form of cubic crystal (JCPDS No. 39-1346). At 2θ = 35.49 and 38.69°, obvious CuO features were observed, such as diffracting peaks corresponding to planes (−111) and (111) of monoclinic CuO; these peaks were observed in all of the catalysts. The 16649

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Figure 2. Nitrogen adsorption/desorption isotherms (a) and pore size distribution profiles (b) of CuO−Fe2O3−ZrO2 composites with different ZrO2 contents.

Table 1. Textural Properties of CuO−Fe2O3−ZrO2 Catalysts with Different ZrO2 Contents

characteristic peaks of Fe2O3 can be found at 2θ = 30.24, 35.63, and 62.93°. However, CuO and Fe2O3 characteristic peaks were overlapping at 2θ = 32.47, 35.49, 48.6, and 68.00°. On the basis of the data of the line broadening at half the maximum intensity (full width at half-maximum, FWHM) and the Bragg angle (θ), the Sherrer equation was used to calculate the mean crystallite sizes of the CuO and Fe2O3. The crystallite sizes of CuO (111) and Fe2O3 (311) were approximately 34 and 31 nm, respectively. The ionic radii of Cu2+ and Fe3+ (0.069 and 0.064 nm, respectively) are almost the same, which might result in the same crystalline phases between monoclinic CuO substituted by Fe and pure CuO during the co-precipitation process.25,26 However, no clear diffraction peak of ZrO2 was observed in these XRD patterns, which most likely implies that the incorporation of ZrO2 was low or was in an amorphous form in the catalysts.10,27 3.2. Nitrogen Adsorption/Desorption of Catalysts. Figure 2 shows the nitrogen adsorption/desorption isotherms and pore size distribution profiles of CuO−Fe2O3−ZrO2 catalysts. On the basis of IUPAC classification, the nitrogen adsorption/desorption isotherms of all CuO−Fe2O3−ZrO2 catalysts (seen in Figure 2a) belonged to a IV-type isotherm.28 In the low-pressure region (P/P0 = 0.0−0.4), the adsorption quantity of N2 was gently increased, which indicated that the absorption of the N2 molecule shifted from monolayer to multilayer.29 In the high-pressure region (P/P0 = 0.4−1.0), evident hysteresis loops are observed and are likely due to a capillary condensation phenomenon. From Figure 2a, the CuO−Fe2O3−ZrO2 catalysts exhibit an H4-type hysteresis, indicating that all of the catalysts are mesoporous structures.30 Furthermore, the pore size distribution profiles (Figure 2b) gave a description that the pore size distribution of the catalyst became wider after Zr doping, indicating that the tacticity of the pore size was reduced. In other words, Zr doping can change the catalyst pore size. On the basis of the data of nitrogen adsorption/desorption isotherms, the BET method was used to calculate the specific surface area of the catalysts, and these results are presented in Table 1. The specific surface areas of the CuO−Fe2O3−ZrO2 catalysts were 94.24, 107.22, 102.04, and 92.62 m2·g−1 for the ZrO2 contents of 0, 1.0, 2.0, and 3.0 wt %, respectively. These results illustrate that doping with ZrO2 can provide increased

ZrO2 content (wt %)

BET surface area (m2·g−1)

av pore diam (nm)

0 1.0 2.0 3.0

94.24 107.22 102.04 92.62

6.54 6.28 5.29 6.28

specific surface area. The largest surface area observed was 107.22 m2·g−1 corresponding to the CuO−Fe2O3−ZrO2 catalyst with 1.0 wt % ZrO2. However, the specific surface area decreased with the increasing of Zr-doped contents past 1.0 wt %, which can be ascribed to the increased crystallinity values of the CuO and Fe2O3 species.31 For all of the catalysts, the average pore diameters were in a range of 5.2 to 6.5 nm, meaning that the catalysts were mesoporous materials. 3.3. Results of X-ray Photoelectron Spectra. Analysis using X-ray photoelectron spectroscopy (XPS) was applied to study the oxidation states of the elements on the surface of the CuO−Fe2O3 catalyst doped with ZrO2; the results are shown in Figure 3. In the Cu 2p spectrum, the binding energy of Cu 2p3/2 in the pure CuO−Fe2O3 catalyst was 933.57 eV along with the featured satellite peaks of approximately 942.23 eV, and the binding energy of Cu 2p1/2 was 953.43 eV along with the featured satellite peaks of approximately 962.08 eV, suggesting that Cu occurred as the form of Cu2+ in CuO.32 These satellite peaks were shown in the Cu 2p spectrum of CuO due to the shakeup transitions by ligand to metal 3d charge transfer.33 The Cu 2p3/2 peak was located at 933.79 eV and the Cu 2p1/2 peak was located at 953.85 eV after 1.0 wt % ZrO2 doping. For the Fe 2p spectrum in Figure 3, the Fe 2p3/2 binding energy (711.03 eV) of the catalyst after ZrO2 doping was similar to the catalyst without ZrO2 (711.01 eV). The Fe 2p1/2 binding energy of the CuO−Fe2O3−ZrO2 catalyst (731.41 eV) and CuO−Fe2O3 catalyst (731.50 eV) resembled each other. Clearly featured satellites can be observed on the high-energy side of all the major Fe peaks, suggesting that the chemical valence of Fe was Fe3+ in Fe2O3. From the Zr 3d spectrum in Figure 3, specific peaks are observed at the binding energies of 182.01 and 184.36 eV, which belong to Zr 3d5/2 and Zr 3d3/2, respectively; therefore, the form of Zr was Zr4+ in 16650

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Figure 3. XPS spectra of Cu 2p, Fe 2p, and Zr 3d regions for pure CuO−Fe2O3 composite and CuO−Fe2O3−ZrO2 composite with 1.0 wt % ZrO2.

ZrO2.34 After doping with Zr, XPS analysis showed that the Cu 2p3/2 and Cu 2p1/2 major peaks shifted by 0.22 and 0.42 eV, respectively, toward higher binding energies, indicating that there was a small effect in the chemical combination state of CuO as a result of doping with Zr. This effect might be caused by a decrease in the outer-shell electron density of Cu.35 Considering that the chemical state of CuO was closely associated with the performance in CO2 hydrogenation to DME, doping Zr might affect the activity of the CuO−Fe2O3 catalyst. However, compared with the binding energies of the main Fe peaks before and after Zr doping, it indicated that there was no obvious effect on the chemical environment of Fe3+ by Zr doping.36,37 3.4. H2-TPR Analysis of Catalyst. Cu species were supposed to be the main center of activity for the methanol synthesis process.38,39 An H2-TPR analysis was made to evaluate the reducibility of the CuO−Fe2O3−ZrO2 catalysts and effect of the presence of ZrO2 on the reduction of Cu species and the CuO−Fe2O3 interaction. The results are shown in Figure 4. Despite doping the catalyst with different amounts of ZrO2, the peak shapes for all of the catalysts in H2-TPR profiles were similar. Three Gaussian fitting peaks (α, β, and γ) are shown within a temperature range of 110−300 °C, which induced the total reducing processes of CuO, while the peaks corresponding to the reduction of Fe2O3 and ZrO2 are at higher temperatures. These three reducing peaks matched with the reducing processes of CuO species; i.e., peak α corresponded to the reducing process of a highly dispersed CuO, peak β corresponded to the reducing process of the larger grains of bulk CuO, and peak γ was a shoulder peak of peak β.40 Compared with the pure CuO−Fe2O3 component, the β and γ peaks of the CuO−Fe2O3−ZrO2 catalysts were wider, which indicated that there was an obvious change in the interaction between CuO and Fe2O3 due to the presence of ZrO2.41 The temperatures and areas of each reducing peak are summarized in Table 2. The reducing temperature of highly dispersed CuO was related to the reducibility of the CuO-based catalyst. According to Table 2, the hydrogen reduction peaks, peak α, of the CuO−Fe2O3−ZrO2 catalyst with ZrO2 of 1.0, 2.0, and 3.0

Figure 4. H2-TPR profiles for CuO−Fe2O3−ZrO2 with different contents of ZrO2: 0, 1.0, 2.0, and 3.0 wt %. The solid curves are experimental curves, and broken curves are Gaussian multipeak fitting curves.

Table 2. Temperatures and Area Distributions of Reduction Peaks of CuO−Fe2O3−ZrO2 with Different Contents of ZrO2a peak α ZrO2 content (wt %)

T (°C)

0 1.0 2.0 3.0

209 198 206 205

peak β

areab

T (°C)

2.24 2.07 2.53 2.61

249 242 249 247

peak γ

area

T (°C)

area

total area

2.42 3.21 3.19 2.94

272 268 273 272

0.87 1.03 0.96 0.93

5.52 6.31 6.68 6.48

a The results were measured from H2-TPR profiles and the area distributions calculated by integration of the area under the peaks. b The unit of the peak area is ×104 units.

wt % are centered at 198, 206, and 205 °C, respectively. Each of these was lower than that of the temperature associated with the CuO−Fe2O3 catalyst (209 °C), thus indicating that the 16651

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of the catalyst is improved. During the process of DME synthesis, CuO−Fe2O3−ZrO2/HZSM-5 catalysts with 1.0 wt % ZrO2 demonstrated the best catalytic activity with the CO2 conversion of 28.4% and the DME selectivity of 64.5%. While the ZrO2 content was increased from 1.0 to 3.0 wt %, the CO2 conversion and the DME selectivity were decreased. Therefore, in the following study, the CuO−Fe2O3−ZrO2/HZSM-5 catalyst with 1.0 wt % ZrO2 was chosen in a further investigation of other effect factors involved in the catalytic hydrogenation of CO2 into DME. 3.5.2. Effect of Reaction Temperature. CuO−Fe2O3− ZrO2/HZSM-5 catalyst with 1.0 wt % ZrO2 was used to synthesize DME from CO2 and H2, and the effect of the reaction temperature at a range of 240−280 °C in the process was investigated. The results are shown in Figure 5. There was

reducibility of highly disperse CuO benefits from ZrO2 doping. The lower reducing temperature is beneficial because it avoids the formation of Cu crystal grains during the hydrogenation process, and more Cu is exposed on the surface of the catalyst, which can increase the reducibility of catalyst.42,43 Of all of the CuO−Fe2O3−ZrO2 catalysts, the lowest temperature peak α was observed when the ZrO2 content in the catalyst was 1.0 wt %, suggesting that this catalyst exhibits optimum reducing behavior. The total areas of the reducing peaks for the CuO− Fe2O3−ZrO2 catalysts were 14.0−21.0% higher than those of the CuO−Fe2 O3 catalyst, revealing that the hydrogen consumption of the CuO−Fe2O3 catalyst was enhanced by doping with ZrO2 and that ZrO2 doping most likely improves the reducibility of the catalyst. 3.5. Catalytic Hydrogenation of CO2 to DME on CuO− Fe2O3−ZrO2/HZSM-5. 3.5.1. Effect of ZrO2 Content. The results of the catalytic hydrogenation of CO2 to DME on CuO−Fe2O3−ZrO2/HZSM-5 catalysts at a Cu/Fe atomic ratio of 3:2 with different contents of ZrO2 are shown in Table 3, Table 3. Effect of ZrO2 Content on the Catalytic Hydrogenation of CO2 to DME on CuO−Fe2O3−ZrO2/ HZSM-5 Catalystsa selectivity of products (mol %) ZrO2 content (wt %)

conversion of CO2 (mol %)

DME

CH3OH

CO

CH4

yield of DME (mol %)

0 1.0 2.0 3.0

6.3 28.4 15.2 11.2

24.0 64.5 60.7 46.1

0.8 14.8 3.2 3.2

36.8 7.8 20.3 25.1

38.4 12.9 15.8 25.6

1.5 16.8 9.2 5.2

Reaction conditions: T = 260 °C, P = 3.0 MPa, GHSV = 1500 mL· gcat−1·h−1, and V(H2)/V(CO2) = 5.

a

Figure 5. Effect of reaction temperature on the catalytic hydrogenation of CO2 to DME over CuO−Fe2O3−ZrO2/HZSM-5. Reaction conditions: T = 240−280 °C, P = 3.0 MPa, GHSV = 1500 mL· gcat−1·h−1, and V(H2)/V(CO2) = 5.

and CuO−Fe2O3/HZSM-5 is used as a comparison. In the process of DME synthesis via CO2/H2, DME and CH3OH were the main products and CO and CH4 were the byproducts. Compared with CuO−Fe2O3/HZSM-5, the conversion of CO2, the selectivity of DME, and the selectivity of CH3OH were improved together with maintaining byproducts to low amounts by simply doping the catalyst with ZrO2. This finding suggests that ZrO2 doping might play a significant role in improving the synthesis of DME. The improvement might be obtained due to forming ternary mixed oxides, namely, CuO− Fe2O3−ZrO2, using a co-precipitation method. From a combination of nitrogen adsorption/desorption results, the specific surface area was found to increase when the ZrO2 content was increased from 0 to 1.0 wt % and then decline when the ZrO2 content was increased from 1.0 to 3.0 wt %, which was in accordance with the results of the CO2 conversion and the DME selectivity. Therefore, the increase in the specific surface area by doping ZrO2 might be partially responsible for the improved effect on the catalytic hydrogenation process. XPS analysis showed that the internal structure of the CuO− Fe2O3−ZrO2 catalyst was changed as a result of modulating the peripheral electron of Cu2+ after doping with ZrO2, which may affect the catalytic hydrogenation process. In addition, with the results of H2-TPR, it was found that the reducing temperature of highly dispersed CuO was decreased and that the hydrogen consumption of the CuO−Fe2O3−ZrO2 catalyst was increased by doping with ZrO2. These phenomena indicated that CuO− Fe2O3−ZrO2 might be reduced more easily and that the activity

an increase on the selectivity of CH3OH from 14.6 to 29.8%, with the increasing reaction temperature from 240 to 280 °C. The selectivity of DME was increased slightly in the present temperature range, and the selectivity of DME reached the maximum value of 64.5% at 260 °C. While the reaction achieved a temperature of 280 °C, a dramatic decrease in the selectivity of DME was observed (47.5%). On the one hand, this process of DME synthesis is a reversible reaction, and the heat released by the reaction is restricted by the thermodynamic equilibrium, and increasing the temperature will make the present reaction more favorable in the backward process, causing the DME yield to decrease. On the other hand, the CuO−Fe2O3−ZrO2 catalyst, especially the species of Cu, is likely to sinter and crystallize at high temperature,23 which might also lead to the decreased yield of DME. These findings suggested that the optimal temperature in the process for the synthesis of DME is 260 °C. 3.5.3. Effect of Reaction Pressure. With reaction pressures ranging from 2.0 to 3.5 MPa, the catalytic hydrogenation results converting CO2 to DME with the CuO−Fe2O3−ZrO2/HZSM5 bifunctional catalyst and 1.0 wt % ZrO2 are shown in Figure 6. The overall formation of DME via CO2 hydrogenation is a reversible reaction accompanied by a reduction of volume. Therefore, high reaction pressure in the fixed bed reactor might result in improved DME yields. Figure 6 illustrates that the conversion of CO2 was increased from 17.6 to 28.4% when the 16652

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decreased the conversion of CO2 from 27.9 to 22.3% but also reduced the selectivity of DME. An increase of space velocity might possibly shorten the contact time between the surface of the catalyst and the CO2/H2 mixing gases, leading to insufficient contact between the CO2/H2 gases and the active sites of catalysts. Increasing the space velocity caused the selectivity of DME to decrease from 64.6 to 58.5% and caused the selectivity of methanol to increase from 14.8 to 20.0%. Compared with the effects of reaction temperature and pressure as shown in sections 3.5.2 and 3.5.3, the effects of changing the space velocity from 1500 to 3000 mL·gcat−1·h−1 were minimal on the overall performance of the CuO−Fe2O3− ZrO2/HZSM-5 catalyst in DME synthesis, and 1500 mL·gcat−1· h−1 was chosen as an optimal space velocity. 3.5.5. Stability of CuO−Fe2O3−ZrO2/HZSM-5. The stability of CuO−Fe2O3−ZrO2/HZSM-5 with 1.0 wt % ZrO2 was studied for the reaction of DME synthesis via CO2/H2 at 260 °C and 3.0 MPa with GHSV = 1500 mL·gcat−1·h−1 and V(H2)/ V(CO2) = 5. The stability results were recorded every 0.5 h and are shown in Figure 8. During a 16 h reaction process, the CO2

Figure 6. Effect of reaction pressure on the catalytic hydrogenation of CO2 to DME over CuO−Fe2O3−ZrO2/HZSM-5. Reaction conditions: T = 260 °C, P = 3.0−3.5 MPa, GHSV = 1500 mL·gcat−1·h−1, and V(H2)/V(CO2) = 5.

reaction pressure rose from 2.0 to 3.0 MPa. The catalyst presented a relatively low activity with CO2 conversion of 22.0% at 3.5 MPa, whereas the selectivity of CH3OH was increased with increasing pressure. At a pressure greater than 3.0 MPa, the conversion of CO2 was reduced, which cannot be fully explained by the thermodynamic pressure effect that indicates that an increase in pressure is predicted to cause an enhancement in the reaction rate. The decrease in CO2 conversion is most likely caused by the accumulation of water that cannot be removed in time in the fixed bed reactor during the DME production process, therefore perhaps shifting the conversion reaction of CO2 in the negative direction.11,44,45 As a result, the optimal reaction pressure of catalytic hydrogenation of CO2 to DME on CuO−Fe2O3−ZrO2/HZSM-5 hybrid catalyst was 3.0 MPa. 3.5.4. Effect of Space Velocity. The CuO−Fe2O3−ZrO2/ HZSM-5 catalyst with 1.0 wt % ZrO2 was applied in DME synthesis from CO2 and H2 using various space velocities, including 1500, 2000, 2500, and 3000 mL·gcat−1·h−1, to investigate the effects of space velocity on the process. The results are shown in Figure 7. Increasing the space velocity of the feed gas from 1500 to 3000 mL·gcat−1·h−1 not only

Figure 8. Effects of the conversion of CO2 and the selectivity of DME/ CH3OH with CuO−Fe2O3−ZrO2/HZSM-5 with time on stream.

conversion and DME/CH3OH selectivity remained almost constant, which indicated that the CuO−Fe2O3−ZrO2/HZSM5 catalyst with 1.0 wt % ZrO2 was a stable catalyst for DME synthesis, providing a CO2 conversion of approximately 23%, a DME selectivity of approximately 65%, and a CH3OH selectivity of approximately 16%. However, over time there was still a slight decrease on the selectivity of DME, which might be a consequence of the water production. Water can not only shift the equilibrium of DME synthesis backward46 but also it can presumably deactivate the CuO−Fe2O3−ZrO2 catalyst.47 3.5.6. Suggestion Mechanism of Catalytic Hydrogenation of CO2 to DME. During the integrated process of direct DME synthesis via CO2/H2 over Cu−Fe−Zr/HZSM-5 catalyst, DME and CH3OH were identified as the major carbonaceous products from the experiments above. CO and CH4 were the two byproducts. To investigate the mechanism of catalytic hydrogenation of CO2 to DME, four reactions were considered: hydrogenation of CO2:20,48

Figure 7. Effect of space velocity on the catalytic hydrogenation of CO2 to DME with CuO−Fe2O3−ZrO2/HZSM-5. Reaction conditions: T = 260 °C, P = 3.0 MPa, GHSV = 1500−3000 mL·gcat−1·h−1, and V(H2)/V(CO2) = 5.

CO2 + 3H 2 ⇌ CH3OH + H 2O (ΔH298 = −49.46 kJ ·mol−1) 16653

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catalyst provided higher catalytic activity compared to that of the CuO−Fe2O3/HZSM-5 catalyst in the catalytic hydrogenation of CO2 to DME. When the CuO−Fe2O3−ZrO2/ HZSM-5 catalyst with 1.0 wt % ZrO2 was used in the catalytic hydrogenation of CO2 to DME at 260 °C and 3.0 MPa with GHSV = 1500 mL·gcat−1·h−1, the CO2 conversion was 28.4%, the DME selectivity was 64.5%, and the catalysts were stable for 16 h.

reverse water gas shift reaction: CO2 + H 2 ⇌ CO + H 2O

(ΔH298 = 41.17 kJ ·mol−1) (2)

dehydration of methanol to DME: 2CH3OH ⇌ CH3OCH3 + H 2O (ΔH298 = −23.51 kJ ·mol−1)

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(3)

formation of CH4: CO2 + 4H 2 ⇌ CH4 + 2H 2O (ΔH298 = −165.00 kJ·mol−1)

AUTHOR INFORMATION

Corresponding Authors

*(Z.Q.) Tel: +86 771 32372702. Fax: +86 771 3233718. Email: [email protected]. *(H.J.) Tel: +86 20 84113658. Fax: +86 20 84113654. E-mail: [email protected].

(4)

Among these four reactions, reactions 1 and 3 belonged to a chain reaction; thus, methanol is a significant intermediate product in the process of DME synthesis, so that the reaction rate of methanol synthesis might be a strong influence in DME synthesis. Reactions 1, 2, and 4 are parallel reactions, indicating that methanol synthesis might be affected by the amount of CO, CH4, and H2O from a reverse water gas shift reaction and a CH4 formation reaction.11,44,45 Methanol synthesis, reverse water gas shift reactions, and the formation of CH4 take place on the metallic catalyst, specifically the Cu species, the main active center for methanol synthesis supposedly being Cu(0).38,39 A considerable amount of the adsorbing and activating of CO2 and H2 occurs on Cu(0). For the CO2 catalytic hydrogenation, the most strongly supported mechanism involves the successive additions of adsorbed hydrogen atoms to an adsorbed CO2 on Cu species, followed by the formation of carbonate, formate species,24,49 or a ringtype ester50 and a methanol synthesis reaction. For methanol dehydration to DME, the reaction occurs between two adjacent molecules of methanol adsorbed onto the HZSM-5 catalyst according to the Langmuir−Hinshelwood mechanism.51 The modification of Cu species in the metallic functional catalyst is therefore an effective way to improve the DME synthesis. When 1.0 wt % ZrO2 was introduced into CuO−Fe2O3/ HZSM-5, the properties and the performance of hydrogenation were affected. In detail, using CuO−Fe2O3−ZrO2/HZSM-5 with 1.0 wt % ZrO2 instead of CuO−Fe2O3/HZSM-5 in the catalytic hydrogenation of CO2 to DME, the CO and CH4 selectivity decreased from 36.8 and 38.4% to 7.8 and 12.9%, respectively, and the DME selectivity increased from 24.0 to 64.5%, as shown in Table 2. The yields of CO and CH4 were decreased and CO2 mainly shifted to CH3OH product and then to DME. The improvement was presumably due to the changes in the CuO−Fe2O3−ZrO2 catalyst, especially the Cu species, due to doping with ZrO2, as illustrated by the characterization analysis of the catalyst. The effect of Zr incorporation was aimed at changing the outer-shell electrons of Cu, enhancing the specific surface area and the reducing behavior of CuO, and increasing the number of active sites for the catalytic hydrogenation of CO2 to DME.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21006013).

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4. CONCLUSION The CuO−Fe2O3−ZrO2/HZSM-5 bifunctional catalyst was developed using CuO−Fe2O3−ZrO2 for the methanol synthesis component and HZSM-5 for the methanol dehydration component in the synthesis of DME. The outer-shell electron density of Cu2+, the specific surface area of CuO−Fe2O3−ZrO2 catalysts, and the reducing behavior of CuO species were all affected by Zr doping, and the CuO−Fe2O3−ZrO2/HZSM-5 16654

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