Promotional Effect of Water on Direct Dimethyl Ether Synthesis from

The equilibrium CO conversion in the direct DME synthesis is higher than ... Surprisingly, we find a promotional effect of water on the DME production...
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Promotional Effect of Water on Direct Dimethyl Ether Synthesis from Carbon Monoxide and Hydrogen Catalyzed by Cu−Zn/Al2O3 Kaoru Takeishi,*,† Yutaro Wagatsuma,‡ Hiroko Ariga,§ Kenichi Kon,§ and Ken-ichi Shimizu§ †

Department of Engineering, Graduate School of Integrated Science and Technology, Shizuoka University, 3-5-1, Jouhoku, Naka-ku, Hamamatsu-shi, Shizuoka-ken 432-8561, Japan ‡ Department of Materials Science, Faculty of Engineering, Shizuoka University, 3-5-1, Jouhoku, Naka-ku, Hamamatsu-shi, Shizuoka-ken 432-8561, Japan § Institute for Catalysis, Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan ABSTRACT: Cu−Zn/Al2O3 was prepared by using the sol−gel method and employed for the direct synthesis of dimethyl ether (DME) from synthesis gas (syngas) in a continuous-flow reactor. We studied the effect of water concentration in the feed on the formation rates of various products. With an increase in the amount of water, the formation rate of DME initially increased and then decreased. Co-feeding of 0.09% water was co-fed resulting in the highest formation rate of DME, 486 μmol gcat−1 h−1 (8.0% yield of DME), at 220 °C under 0.9 MPaG. Structural studies by XANES, EXAFS, XPS, and infrared spectroscopies showed that the treatment of the catalyst with 0.66% water at 240 °C under atmospheric pressure resulted in oxidation of the metallic copper species and increased the number of surface Cu+ sites. Combined with the catalytic results, we conclude that the increase in the surface Cu+ sites is responsible for the promotion effect of steam on the DME production. KEYWORDS: Dimethyl ether, DME, Catalyst, Carbon monoxide, Hydrogen, Sol−gel method

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INTRODUCTION Dimethyl ether (DME) has attracted much attention as a clean alternative fuel or hydrogen carrier.1−4 DME can be used as a clean diesel fuel because it shows a high cetane number as well as no soot emission and less NOx emission in the exhaust gas from a diesel engine. DME can be an alternative to liquefied petroleum gas (LPG) because physical properties of DME are similar to those of LPG. DME is used as a propellant for aerosol, sprays, gas dusters, and so on. Recently, DME is considered to be a hydrogen carrier for a fuel cell system. DME infrastructures may be settled more rapidly than those of hydrogen because the current LPG infrastructures can be used for DME. Natural gas, coal, coal bed methane, and biomass can be converted to DME via syngas (CO/H2 mixture). In China, there are many DME plants with a capacity of greater than 10,000 tons/year. In 2013, the annual total DME production capacity was ∼13.5 million ton/year, and the production was 4.5 million tons/year.5 The produced DME is mixed with LPG, and the mixed DME/LPG is used for cooking and heating as domestic fuel. Currently, DME is produced from syngas by a two-step process, where syngas is converted to methanol by a Cu-based catalyst followed by dehydration of methanol to DME by a solid acid catalyst such as γ-alumina and zeolite. From the viewpoints of thermodynamics, sustainable chemistry, and the economy, the current two-step process should be replaced by a one-step process (direct DME synthesis from syngas), where methanol synthesis and its dehydration are performed in one reactor. The equilibrium CO conversion in the direct DME synthesis is higher than that in the methanol synthesis. © 2017 American Chemical Society

Compared to the methanol synthesis and the two-step DME synthesis, the direct synthesis of DME (eq 1) is more suitable to CO-rich gas.6,7 3CO + 3H 2 → CH3OCH3 + CO2

(1)

This method consists of three reaction steps: methanol synthesis (eq 2), methanol dehydration (eq 3), and the water− gas shift reaction (eq 4). Copper-based catalysts are used for the methanol synthesis. The copper catalysts in the same reactor may catalyze the water−gas shift reaction, where water, produced by the methanol dehydration step, reacts with CO to give H2 and CO. CO + 2H 2 → CH3OH

(2)

2CH3OH → CH3OCH3 + H 2O

(3)

H 2O + CO → H 2 + CO2

(4)

Combined with the fact that syngas from sustainable resources, such as biomass gasification gas or syngas from coelectrolysis of H2O and CO2, contains water vapor, the effect of the water on the catalytic efficiency of the direct DME synthesis is an important research target. Special Issue: Asia-Pacific Congress on Catalysis: Advances in Catalysis for Sustainable Development Received: December 2, 2016 Revised: March 16, 2017 Published: March 31, 2017 3675

DOI: 10.1021/acssuschemeng.6b02915 ACS Sustainable Chem. Eng. 2017, 5, 3675−3680

Research Article

ACS Sustainable Chemistry & Engineering Previously, the direct DME synthesis has been studied by using a physical mixture of a Cu-based methanol synthesis catalyst with a methanol dehydration catalyst.8−18 Slurry reactors and fixed-bed reactors with the physical mixture catalysts have been developed for this process by some companies.6,7,19−27 We have developed an effective direct DME synthesis catalyst (Cu−Zn/Al2O3) prepared by the sol− gel method.3,28−35 We report herein the effect of water concentration on the catalytic properties of the Cu−Zn/ Al2O3 catalyst prepared by the sol−gel method for the direct DME synthesis from syngas. Surprisingly, we find a promotional effect of water on the DME production. Spectroscopic characterizations of the fresh and the water-treated catalysts are shown to discuss a possible reason for the promotional effect.

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Figure 1. Effect of water concentration on the production rate of DME for H2 + CO reaction by Cu−Zn/Al2O3 (1.0 g) at atmospheric pressure.

EXPERIMENTAL SECTION

Catalyst Preparation. Aluminum isopropoxide (AIP, 95% purity), ethylene glycol (EG, 99.5% purity), 60% HNO3 solution, Cu(NO3)2· 3H2O (99% purity), and Zn(NO3)2·9H2O (99% purity) were manufactured by Wako Pure Chemical Industries, Ltd. The Cu−Zn/ Al2O3 catalyst (15 wt % Cu; 15 wt % Zn) was prepared by a consecutive sol−gel method as follows. AIP (29.5 g), crushed by a mortar, was dissolved in ∼500 mL hot water (∼70 °C), and then EG (25 g) was added to the mixture, followed by heating the mixture at ∼70 °C for ∼30 min under stirring. Then, an aqueous solution of HNO3 (1 mL, 20%) was added to the mixture every 15 min several times to lower the pH of the mixture to 1−2. During the HNO3 addition, a clear-sol of boehmite was formed. A mixed aqueous solution of Cu(NO3)2·3H2O (5.76 g) and Zn(NO3)2·9H2O (6.89 g) was added into the clear-sol, followed by heating the mixture at ∼70 °C until its volume decreased to 5 MPa). However, we have conducted the reaction under the absolute pressure of 1.0 MPa by considering the High Pressure Gas Safety Law in Japan. Two online gas chromatographs (GC) were used for the product analysis. One GC, a Shimadzu GC-14 equipped with a conductivity detector (TCD), and a methanizer (for CO analysis), MS-5A stainless column (60−80 mesh, 5m long, i.d. Three mm) using N2 carrier gas, was used for the quantitative analysis of H2, Ar (as internal standard for GC analysis), CH4, and CO. The other GC, a Shimadzu GC-14 equipped with a TCD, and an FID in series, a Porapak T stainless column (60−80 mesh, 2m long, i.d. Three mm) using He carrier gas, was used for the quantitative analysis of CH4, CO2, DME, H2O, methanol, and hydrocarbons. XANES and EXAFS. X-ray absorption near-edge structures (XANES) and X-ray absorption fine structure (EXAFS) at Cu Kedge were measured in transmission mode at the BL01B1 in the SPring-8 (Proposal No. 2015B1246) in a transmittance mode. The

Figure 2. Effect of water concentration on the production rate of CO2 for H2 + CO reaction by Cu−Zn/Al2O3 (1.0 g) at atmospheric pressure. storage ring was operated at 8 GeV. A Si(111) double-crystal monochromator was used to obtain a monochromatic X-ray beam. The Cu−Zn/Al2O3 catalyst powder was re-reduced in a flow of 100% H2 (20 cm3 min−1) for 0.5 h at 450 °C, cooled to room temperature under H2, and sealed in the cells made of polyethylene under N2. Then, the X-ray absorption spectrum of the sealed sample was taken at room temperature. The EXAFS analysis was performed using the REX version 2.5 program (RIGAKU). The parameters for the Cu−O and Cu−Cu shells were provided by FEFF6. XPS. X-ray photoelectron spectroscopy (XPS) was measured by an Omicron EA 125 X-ray photoelectron spectrometer equipped on a home-modified UHV chamber with a Mg Kα (1253.6 eV) radiation source. Binding energies were calibrated with O 1s peak energy of Al2O3 at 532.0 eV. The measured two Cu−Zn/Al2O3 catalysts (0.5 g) were reduced by flowing H2 (99.99%, 10 mL min−1) at 450 °C for 10 h, followed by heating in the presence of Ar gas (10.0 mL min−1) with and without 0.66% water at 240 °C for 5 h under atmospheric pressure. Catalysts were transferred to a XPS chamber through N2 atmosphere without exposing to air. IR Study of CO Adsorption. IR spectra were recorded using a JASCO FT/IR-4200 equipped with an MCT detector. A closed IR cell surrounded by the Dewar vessel was connected to an evacuation system. During the IR measurement, the IR cell with CaF2 windows was cooled by liquid nitrogen in the Dewar vessel, and the thermocouple near the sample showed −150 ± 1 °C. Spectra were measured accumulating 15 scans at a resolution of 4 cm−1. The Cu− Zn/Al2O3 catalyst was reduced by flowing H2 (99.99%, 10 mL min−1) at 450 °C for 10 h, followed by heating in the presence of Ar gas (10.0 mL min−1) with and without 0.66% water at 240 °C for 5 h under atmospheric pressure. Then, two of the Cu−Zn/Al2O3 samples were pressed into 40 mg of a self-supporting wafer (ϕ = 2 cm) under the ambient conditions and mounted into the IR cell. After evacuation, 3676

DOI: 10.1021/acssuschemeng.6b02915 ACS Sustainable Chem. Eng. 2017, 5, 3675−3680

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optimum water concentration for DME production was where water concentration of 0.66% gave the highest value at 220 and 240 °C. At 220 °C, the production rate with 0.66% water was 9.8 μmol gcat−1 h−1, while the rate with low water concentration (0.07 mol %) was 4.2 μmol gcat−1 h−1. This indicates that water vapor promotes the direct DME synthesis. However, at higher water concentration (from 0.95% to 1.2%), the DME production rates decreased with the water concentration. In contrast, the formation rate of CO2 linearly increased (from 65 to 493 μmol gcat−1 h−1) with an increase in the water concentration (from 0.07% to 1.2%) at 220 °C (Figure 2). The CO2 production rates at 240 and 260 °C also increased linearly with an increase in the steam concentration in the reaction gas. This indicates that the water gas shift reaction occurred in the presence of an excess amount of water in the feed. Temperature dependences of the formation rates of DME, methanol, CH4, C2 hydrocarbons, and CO2 without and with 0.66% water in the reaction gas are shown in Figure 3(a) and (b). In the absence of co-fed water (Figure 3(a)), the main product was methane at a temperature range of 240 °C−280 °C. The methane production rate increased from 1.7 to 321 μmol gcat−1 h−1 with an increase in the temperature from 200 to 280 °C. The DME production rates were in a range from 0.38 to 4.7 μmol gcat−1 h−1, and the methanol production rate varied from 0.19 to 0.46 μmol gcat−1 h−1. CO2 is also produced, and its production rate varied from 36 to 65 μmol gcat−1 h−1. In the presence of 0.66% water in the gas feed (Figure 3 (b)), the main product was CO2, and the formation rates of CO2 were nearly constant (239 to 288 μmol gcat−1 h−1) in the temperature range of 200 to 280 °C. This indicates that water gas shift reaction (H2O + CO → H2 + CO2) is the main reaction in this condition. DME, methanol, and methane were also produced. The DME production rates ranged from 0.53 to 9.8 μmol gcat−1 h−1. The methanol production rates ranged from 0.19 to 1.2 μmol gcat−1 h−1, and the methane rates ranged from 0.55 to 18 μmol gcat−1 h−1. A comparison between the results with and without the steam indicates that the rates of methanol formation were increased by the steam. Considering the stoichiometry of methanol dehydration and methane formation for the reaction at 220 °C in the presence of 0.66% water, the concentration of water produced by the reaction is estimated to be 0.08%, which is much lower than the co-fed water (0.66%). This indicates that the excess amount of co-fed water plays an important role in the promotion of methanol formation. Figure 4 shows the effect of the steam concentration on the methane production rate. Methane production rate was decreased rapidly by increasing the steam concentration. In summary, it is found that the co-feed of steam decreases the methane production rate, and the co-feed of a moderate concentration of steam (0.66%) increases the production rates of DME and methanol at low temperatures (200−220 °C). XAFS. Figure 5 compares the Cu K-edge XANES spectra of the as-prepared Cu−Zn/Al2O3 and a reference compound for a bulk Cu metal (Cu foil). The XANES result (Figure 5(a)) shows that the spectral features for Cu−Zn/Al2O3 are similar to those for Cu foil, which indicates that the copper species in the bulk of the Cu−Zn/Al2O3 catalyst is mostly metallic Cu. The EXAFS (Figure 5(b)) of the Cu−Zn/Al2O3 catalyst consists of two shells: the first Cu−O shell and the second Cu−Cu shell. The curve fitting analysis of the EXAFS result (Table 1) shows a Cu−O bond at 1.85 Å with a coordination number of 0.6 and a Cu−Cu bond at 2.54 Å with a coordination number of 5.8.

Figure 3. Temperature dependence of products formation rates for H2 + CO reaction by Cu−Zn/Al2O3 (1.0 g) at atmospheric pressure (a) without and (b) with co-feed of 0.66% water.

Figure 4. Effect of water concentration on the production rate of CH4 for H2 + CO reaction by Cu−Zn/Al2O3 (1.0 g) at atmospheric pressure. followed by re-reduction of the wafer at 450 °C for 0.5 h under 1 atm H2, and by evacuation, a reference spectrum of the sample disc was measured at −150 ± 1 °C. Then, the sample was exposed to 65 Pa of 10% CO/He at −150 ± 1 °C for 600 s. Then, a differential IR spectrum, with respect to the reference spectrum, was recorded at −150 ± 1 °C.

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RESULTS AND DISCUSSION Effect of Water on Catalytic Properties. The direct synthesis of DME from CO/H2 was carried out with different water concentration in the feed gas at 220−280 °C under atmospheric pressure. Figure 1 shows the relationship between the steam concentration and the DME production rate under atmospheric pressure at representative temperatures. The 3677

DOI: 10.1021/acssuschemeng.6b02915 ACS Sustainable Chem. Eng. 2017, 5, 3675−3680

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Figure 5. Cu K-edge (a) XAFS and (b) EXAFS Fourier transforms.

Table 1. Curve-Fitting Analysis of Cu K-edge EXAFS sample

shell

Na

R (Å)b

σ (Å)c

Rf (%)d

Cu−Zn/Al2O3

O Cu Cu

0.9 5.8 11.7

1.85 2.54 2.54

0.020 0.082 0.085

0.9

Cu foil a

Coordination number. Residual factor.

d

b

Bond distance.

c

0.7

Debye−Waller factor.

The Cu−Cu bond length of 2.54 Å is the same as that for Cu foil, which indicates the presence of metallic Cu particles in the catalyst. The lower Cu−Cu coordination number (5.8) than the bulk Cu metal suggests that the size of the Cu metal particles of the Cu−Zn/Al2O3 catalyst is quite small. The presence of the Cu−O bond together with the lower Cu−Cu coordination number (5.8) than the bulk Cu metal (11.7) indicates that copper oxides are also included in the Cu−Zn/ Al2O3 catalyst as a minor Cu species. XPS. XPS spectra of the catalyst treated with and without 0.66% steam at 240 °C for 5 h are shown in Figure 6. The binding energy of the Cu 2p3/2 main peak was shifted from 933.2 to 933.8 eV by steam treatment (Figure 6(a)). Moreover, the satellite peak around 944 eV was observed for the steamed catalyst. The B.E. shift of main peaks and the appearance of a weak satellite peak indicate that part of Cu0 was oxidized to Cu2+.36,37 The positions of the Zn 2p3/2 peaks of the catalysts with (1023.0 eV) and without (1023.3 eV) the steam treatment are close to each other. IR Study. The XPS analysis mentioned above shows the oxidation states of copper species on the surface as well as in the subsurface of the catalyst. The oxidation states of the surface copper species on the Cu−Zn/Al2O3 catalysts treated with and without 0.66% steam at 240 °C for 5 h were analyzed by FT-IR experiments for CO adsorption on the Cu−Zn/Al2O3 at low temperature (−150 °C). Figure 7 compared the IR spectra of CO adsorbed on the Cu−Zn/Al2O3 catalysts treated with and without 0.66% steam. It should be noted that weights of the two samples were the same (40 mg), so the absorbance in the spectra corresponds to the relative amount of the CO species on the samples. The spectrum for the catalyst without steam treatment showed the peaks due to CO adsorbed linearly on Cu+ sites (Cu+−CO) at 2110 cm−138 and CO adsorbed on Cu2+ sites at 2150 and 2184 cm−1.38 These peaks were observed for the steam-treated catalyst, but the intensity of the band due to Cu+−CO (2110 cm−1) was relatively high. This indicates that the number of the surface Cu+ sites is increased by the steam treatment at 240 °C. Considering the XPS results that the copper species on the surface and in the subsurface of the catalyst is oxidized by the steam treatment, the following discussion could account for the XPS and IR results. The steam

Figure 6. (a) Cu 2p and (b) Zn 2p X-ray photoelectron spectroscopy pattern of the Cu−Zn/Al2O3 catalysts treated under flowing Ar with or without 0.66% steam at 240 °C for 5 h.

Figure 7. IR spectra of CO adsorbed on the Cu−Zn/Al2O3 catalysts treated under Ar flowing with and without 0.66% steam at 240 °C for 5 h. The samples were placed under 65 Pa of 10% CO/He for 600 s at −150 °C, and the spectra were obtained at −150 °C.

treatment oxidizes the metallic copper species in the as-reduced Cu−Zn/Al2O3 catalyst. The subsurface of the steam-treated catalyst consists mainly with Cu2+ species, but the surface of the catalyst consists with Cu+ sites. Combined with the catalytic result in Figure 1, it is suggested that the promotion of the DME by steam is due to the increase in the Cu+ sites as catalytically important sites for the CO hydrogenation. Similar effects of the co-fed water or water treatment were reported by 3678

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ACS Sustainable Chemistry & Engineering Kobori et al.39 and Yano et al.40 for the CO hydrogenation to methanol over ruthenium catalysts. Direct DME Synthesis under 0.9 MPaG. The relation between the steam concentration and the DME production rate for CO hydrogenation by Cu−Zn/Al2O3 under 0.9 MPaG (absolute pressure 1.0 MPa) at 200−240 °C is shown in Figure 8. The rate increased with the water concentration up to 0.09%

sites. Combined with the catalytic results, the promotional effect of steam on the DME formation can be ascribed to the increase in the surface Cuδ+ sites by the steam.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kaoru Takeishi: 0000-0003-3350-2165 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by JST-CREST, Japan Science and Technology Agency − Core Research for Evolutional Science and Technology. Our project title is “Co-electrolysis of CO2 and H2O for DME synthesis” in the project of “Development of Innovative Technology for Energy-Carrier Synthesis using Novel Solid Oxide Electrolysis Cell”.

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Figure 8. Effect of water concentration on the production rate of DME for H2 + CO reaction by Cu−Zn/Al2O3 (1.0 g) at 0.9 MPaG.

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and then decreased with water concentration. Under the optimal conditions with 0.09% steam at 220 °C, the highest rate of DME production is 486 μmol gcat−1 h1, which corresponds to a DME yield of 8.0%. The 8% yield under relatively low pressure and low temperature compared well with the catalytic performance reported by some companies.3,19−27 Promotional and Suppressive Roles of Water. Thermodynamically, water in the reaction system undergoes a water−gas shift reaction (H2O + CO → H2 + CO2), which decreases the CO concentration and consequently decreases the yield of methanol, and additionally, the excess water suppresses methanol dehydration to DME. On the other hand, our experimental results showed that water assisted oxidation of Cu0 species to Cu+ and Cu2+ species during the reaction, and these oxidized copper species promoted methanol synthesis and DME synthesis. The negative and positive effects of water mentioned above can explain the optimal water concentration (0.66% under atmospheric pressure and 0.09% under 0.9 MPa).

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CONCLUSIONS We studied the effect of water concentration in the feed on the formation rates of various products for the CO hydrogenation by Cu−Zn/Al2O3 prepared by using the sol−gel method. With an increase in the water concentration, the production rate of DME initially increased and then decreased. The highest production rates of DME were observed when 0.66% and 0.09% of water were co-fed at 220 °C under atmospheric pressure and under 0.9 MPaG, respectively. Under the optimal conditions under 0.9 MPaG, the DME production rate of 486 μmol gcat−1 h−1 and the DME yield of 8.0% were achieved. XANES/EXAFS analysis of the as-prepared catalyst showed that the catalysts consist with small Cu metal particles as major Cu species with copper oxides as minor species. XPS analysis and CO adsorption IR studies showed that the steam treatment of the as-prepared catalyst resulted in oxidation of the metallic Cu species at the surface/subsurface region accompanied by an increase in the number of the surface Cuδ+ (Cu+ and Cu2+) 3679

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DOI: 10.1021/acssuschemeng.6b02915 ACS Sustainable Chem. Eng. 2017, 5, 3675−3680