Dimethyl ether and catalyst development for production from syngas

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Special Report

Dimethyl ether and catalyst development for production from syngas Biofuels (2010) 1(1), 217–226

Kaoru Takeishi† Dimethyl ether (DME) is manufactured from syngas using an indirect DME-synthesis method (two‑step process), that consists of a methanol synthesis and dehydration process. The price of DME for this method depends on the methanol price; therefore, an economical process should be developed. A direct DME‑synthesis method (one-step process) has been developed and the catalysts consist of methanol-synthesis catalysts and methanoldehydration catalysts. Syngas, the raw material for DME, is produced from natural gas, coal and biomass for example. Currently, DME can be produced from biomass, so DME is in the spotlight as a potential biofuel. We have developed new catalysts and the Cu–Zn/Al2O3 catalysts prepared using the sol–gel methods, which are very effective for direct DME synthesis. Even if the raw gas is contaminated with oxygen, DME is effectively produced over these sol–gel catalysts. We hope that these catalysts will be widely used for economical DME synthesis from biomass, helping to solve environmental problems.

Dimethyl ether (DME) is the smallest ether; its molecular formula is CH3OCH3 and its molecular weight is 46.07. The boiling point of DME is -25°C and, under atmospheric pressure and room temperature, DME exists as a gas. It is easy to liquefy by being pressurized to 0.53 MPa at 20°C (room temperature) and by being cooled to less than -25°C under 0.1 MPa (atmospheric pressure) [1] . As the physical properties of DME resemble those of propane and butane (which are the main ingredient of liquefied petroleum gas [LPG]), storage and handling of DME can apply the technology used for LPG, and the existing infrastructure of LPG can be used for DME by improving the sealing materials. In addition, DME is a clean fuel that does not contain sulfur or any nitrogen compound, and does not corrode metal. DME decomposes to CO2 and H2O by a photochemical reaction within 3–30 h in the atmosphere, so the greenhouse effect and the ozone-depletion effect are not accepted. DME is a dim, sweet-smelling gas that is colorless at room temperature and atmospheric pressure; the gas density is heavier than air and has high solubility for some chemical products. DME is chemically and thermally

stable, and has moderate vapor pressure and very low toxicity in the human body. Therefore, DME is now used as a spray agent for aerosol, a propellant, like LPG. Dimethyl ether has a molar heat capacity under standard pressure and temperature (Cp; 298 K) of 60.71 J/mol/K. This value is higher (by ~65%) than that of methane (natural gas) and is higher by approxmately 40% than that of methanol. Compared with LPG, the value of DME is approximately 65% less than that of LPG due to the difference in the chemical structure. However, DME can be stored as a material with approximately 90% of the LPG heat capacity in a tank of the same size because the liquid density of DME is higher than that of LPG. Moreover, DME is excellent as a clean fuel for diesel engines compared with the light oil currently used as a diesel fuel. The cetane number of DME for self-ignition is 55–60; this value is the same level as light oil. DME burns without particulate matter (PM), such as dark smoke, because oxygen is contained and there is no C–C bonding in the DME (CH3OCH3) unit. DME also burns with no generation of sulfur oxides (SOx) because it has no sulfur content.

Department of Materials Science & Chemical Engineering, Faculty of Engineering, Shizuoka University, 5‑1, Jouhoku 3-choume, Naka-ku, Hamamatsu-shi, Shizuoka-ken, 432-8561, Japan; E-mail: [email protected]

†

future science group

10.4155/BFS.09.16 © 2010 Future Science Ltd

ISSN 1759-7269

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Special Report Takeishi In addition, the generation of nitrogen oxides (NOx) can be decreased by 20–30%; therefore, there is great hope for DME as a diesel engine substitution fuel. In Japan, China, Bio-DME: DME synthesized from syngas or methanol produced from biomass Korea and Sweden, some DME Syngas: Synthesis-mixed gas of diesel engine trucks and buses have hydrogen and carbon monoxide been being demonstrated and/or Direct DME synthesis: DME produced used on public roads. Moreover, it from syngas directly through a is a useful fuel for household, heater, single process turbine and power-generation plant Hydrogen: H2 is the cleanest fuel and is gas for these LPG infrastructures. If used for fuel cells DME is used for these fuels rather Methanol: CH3OH; methyl alcohol, a than coal, heavy oil and LPG, the liquefied syngas emission of PM, NOx, SOx and Dehydration: Elimination or loss of CO2 will decrease. In addition, water from molecules DME is strongly favored as a hydroCatalyst: A substance that increases the gen carrier and hydrogen source of rate of a reaction fuel cells. From the recent research Sol–gel method: Method of making on DME steam-reforming catalysts, material: chemical solution → sol → gel hydrogen will be produced effectively from DME. It is speculated that the DME steam-reforming hydrogen generator can be a smaller hydrogen supply system compared with the high-pressure hydrogen gas cylinder (70 MPa) and hydrogen storage alloy (3 wt%), and DME is the most promising material for hydrogen storage and as a hydrogen carrier [2–4] . Therefore, DME is a clean fuel for the 21st Century [5] . Approximately 150,000 tons of DME is used around the world as a propellant per year, but it is expected that the demand will expand rapidly from the possibility of wide usage, as aforementioned. In China, 24 DME plants were constructed in recent years, and the DME production capacity in 2008 is 3.6 million tons/year [6] . The capacity will be increased to approximately 7.0 million tons/year at the end of 2009. DME is used as a domestic fuel mixed with LPG in China. In Japan, an 80,000-tons/year DME plant for fuel (not for chemicals) was constructed in 2008, and some of the produced DME has been used for DME boilers. For DME expansion, economical, ecologically friendly and environmentally friendly DME production methods and systems should be developed. For the most part, DME is synthesized from methanol, and methanol is produced from syngas (synthesis gas; mixed gas with hydrogen and carbon monoxide). Syngas is produced from natural gas, coal, coal-bed methane, biomass and others. Therefore, DME is a multi-use and multisource fuel. It can be produced from biomass, and is therefore in the spotlight as a biofuel. In Sweden, The bio-DME Project will produce DME from black liquor gasification gas, and the bio-DME produced will be used for diesel trucks. This project will continue until 2013 Key terms

Dimethyl ether: CH3OCH3; the simplest ether and a clean fuel for the 21st Century

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and is organized by Volvo, Haldor Topsoe and others. Volvo presented that bio-DME from black liquor is the most effective fuel for transport work per hectare from bioethanol, biodiesel and rapeseed methyl ester; the value of bio-DME is approximately 11000 km/ha/year and that of bioethanol is 4000–5000 km/ha/year [7] . Methods for DME synthesis Dimethyl ether is mainly produced by dehydration of methanol and is a byproduct of the methanol manufacturing process. However, DME and new technologies for DME production have recently drawn attention as contributors to creating a stable energy supply and demand in the future. They also have environmental benefits because DME is produced through syngas from multiple sources. There are two methods of industrialized and demonstrated new technologies for DME production: indirect DME synthesis (a two-step process) and direct DME synthesis (a one-step process). For the indirect-synthesis method, methanol is produced from syngas and the produced methanol is dehydrated to DME; these two processes are carried out in different reactors. In the direct-synthesis method, DME is produced from syngas directly in a reactor. Indirect DME-synthesis method (a two‑step process)

Methanol is produced by a methanol-synthesis process from syngas (Equation 1) . CO + 2H 2 $ CH 3 OH

Equation 1

Purified methanol manufactured by the methanolsynthesis process or raw methanol separated from syngas in the methanol-synthesis process are converted into DME by the dehydration process (in this case, etherification) (Equation 2) . Equation 2

2CH 3 OH $ CH 3 OCH 3 + H 2 O

This method uses an existing methanol-synthesis and dehydration process, and so there is less need for the development of technologies and catalysts. For methanol synthesis, existing methanol-synthesis catalysts are used and the catalysts are usually copper based. For methanol dehydration, solid catalysts such as g-alumina, zeolite and sulfuric acid are used. Therefore, costs for development are also reduced. The rationalized industrial plant can be installed promptly. However, the system will be complicated because the products are a mixture of DME, methanol and water, and thus distillation, purification and recycling processes will be needed. The price of DME depends on the price of methanol, with the DME price usually approximately twice that of methanol. This is a big problem for this method.


Dimethyl ether & catalyst development for production from syngas Special Report There are some disadvantages, as discussed previously, but all industrial DME plants in the world use this indirect DME-synthesis method. In China, there are 24 DME plants and the total production capacity in 2008 was 3.6 million tons/year. In Japan, there is an 80,000-tons/year DME plant. Almost all plants use solidtype acid catalysts (e.g., g-alumina) and the dehydration process is performed in the gas phase. However, some plants (e.g., Zhangjiangang Plant of Jiutai Energy Group in China) use sulfuric acid, and the dehydration process is operated in the liquid phase. The reaction mechanism is: Equation 3

CH 3 OH + H 2 SO 4 $ CH 3 HSO 4 + H 2 O Equation 4

CH 3 HSO 4 + CH 3 OH $ CH 3 OCH 3 + H 2 SO 4

The sulfuric acid is used several times for the dehydration process and weakened sulfuric acid is used as a raw material for ammonium sulfate for fertilizer. The manufacturing cost of DME plants using the liquidphase dehydration can be reduced in half compared with the cost of the gas-phase dehydration plants. DME can also be manufactured one third cheaper. However, sometimes, less sulfuric acid will remain in the product. Direct DME-synthesis method (a one-step process)

The merit of the direct synthesis method is that the manufacturing process is economical because the methanol synthesis and dehydration processes are performed in one reactor. The equilibrium conversion of syngas is much higher compared with the conversion of the methanol synthesis, and it is possible to reduce the unreacted gas ratio so that it is very low. The reaction equation of the direct synthesis method is shown in Equation 5 ; copper-based catalysts are used for this method as they are very effective in the water–gas shift reaction. Therefore, this method is more suitable for use with CO-rich gas than methanol synthesis and indirect DME synthesis.

Equation 7

2CH 3 OH $ CH 3 OCH 3 + H 2 O Equation 8

H 2 O + CO $ H 2 + CO 2

From this reaction mechanism, a mixture of the methanol-synthesis catalysts, methanol-dehydration catalysts and water–gas shift reaction catalysts is used. Slurry reactors and fixed-bed reactors have been developed for this process by some companies [8,9] . In Japan, a consortium under the leadership of JFE Holdings, Inc. demonstrated a 100-tons/day plant from 2002 to 2006 as a national project. This project achieved the target values and was finished successfully. The reaction temperature was 260°C, the reaction pressure was 5.0 MPa and the gas velocity was 40 cm/s. The total conversion was 96%, the DME production rate was 109 tons/day and the DME purity was 99.8%. The reactor was a slurry reactor, because this reaction produces much heat and the slurry dispersed the reaction heat [10–12,101,102] . In Korea, Korea Gas Corp. (KOGAS) demonstrated a 10-tons/day direct DME-synthesis plant. The reaction temperature was 260°C, the reaction pressure was 6.0 MPa and the space velocity of reaction gas was 2000/h. The reactor was a fixed-bed reactor and was cooled by 230°C hot water. KOGAS is targeting commercial 3000-tons/year plants [13–15] . Catalysts for DME synthesis As previously mentioned, methanol is dehydrated to DME by acid catalysts such as g-Al 2O3, zeolite and, sometimes, by sulfuric acid. These dehydration catalysts Aluminum isoproxide Al[OCH(CH3)2]3

Water H2O

+

Nitrate solution, for example (e.g., Cu(NO3)2, Zn(NO3)2)

+

Ethylene glycol HOCH2CH2OH

70°C heating, being stirred Dilute aqueous HNO3

pH: 1–2

Equation 5 Alumina clear sol

3CO + 3H 2 $ CH 3 OCH 3 + CO 2

This method is known as the one-step process, but actually consists of three reaction steps: methanol synthesis (Equation 6) , methanol dehydration (Equation 7) and the water–gas shift reaction (Equation 8) . For methanol synthesis, copper-based catalysts are usually used. The copper-based catalysts work for the water–gas shift reaction, which occurs by H 2O produced from the methanol dehydration and CO in the same reactor. Equation 6

CO + 2H 2 $ CH 3 OH


Evaporation Dried gel Example: Crushing under 150 µm Calcination: 500°C × 5 h Reduction: 450°C × 10 h Cu–Zn/Al2O3 catalyst

Figure 1. The sol–gel catalyst preparation method.

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Intensity (a.u.)

100

350 300

Cu–Zn (25–25 wt.%) /Al 2O3 (imp.)

80

250

60

Al 2O2 (sol.)

Cu (10 wt.%) /Al 2O3 (sol.)

40

200 150 100

20

50

0

10

20

30

40 50 2θ (°)

60

70

0

80

Figure 2. X-ray diffraction patterns of Al2O3 and Cu (10 wt%)/Al2O3 prepared using the sol–gel method and Cu–Zn (25–25 wt%)/Al2O3 prepared using the impregnation method with a commercial alumina.

are almost fully developed, but some companies have been improving them. The g-Al 2O3 catalysts sometimes have additives, such as SiO2 or metals, in order to change the surface acidity and reduce the number of byproducts. The methanol-synthesis catalysts are also almost developed. The main content of the methanolsynthesis catalysts is copper and the second content is zinc oxide. Al2O3 and Cr2O3 are sometimes added to improve the activity and catalyst stability. For indirect DME synthesis, these catalysts are used in the methanol-synthesis reactor and the methanol-dehydration reactor, respectively. As mentioned previously, because the reaction consists of mainly three reaction steps (the methanol-synthesis reaction [Equation 6] , the methanol-dehydration

Rate of dimethyl ether production (C.µmol/g-cat /h)

30 Me 2O (sol–gel)

25

Me 2O (N211)

20 15

10 5 0

140

180

220

260

300

340

380

Reaction temperature (°C)

Figure 3. Dimethyl ether production rate of CO hydrogenation over the Cu–Zn(36–4 wt%)/Al2O3 (Sol ) catalyst prepared using the sol–gel method and the mixed catalyst (N211) composed of a methanolsynthesis catalyst (N211) and a methanol-dehydration catalyst (Al2O3) under atmospheric pressure. Catalyst: 0.5 g; reaction gas: H2/CO/N2 = 7.5/7.5/1.5 ml/min; space velocity: 2.2 x 103 h-1.

220


reaction [Equation 7] and the water–gas shift reaction [Equation 8] ), direct DME synthesis catalysts are a mixture of methanol-synthesis catalysts, methanoldehydration catalysts and water–gas shift reaction catalysts. Copper used in the methanol-synthesis catalysts is effective for water–gas shift reaction catalysts, which are usually copper–zinc catalysts like methanol-synthesis catalysts, so the water–gas shift reaction catalysts are sometimes neglected for direct DME-synthesis catalysts. For fixed-bed reactors, each catalyst is ground to small particles and each ground catalyst is mixed and molded into a pellet shape. For slurry reactors, each catalyst is ground to a small particle size so that it is easy to suspend in the slurry. Then, each ground catalyst is mixed in the slurry reactor. In the case of Air Products and Chemicals, Inc., the catalysts are a mixture of methanol-synthesis and methanol-dehydration catalysts (ratio: 19.3/1.0) and the reactor is a slurry reactor [16] . In the case of KOGAS, the catalysts are a mixture of Cu–ZnO/Al 2O3 and additives with g-Al 2O3, the catalyst size is a 5‑mm tablet and the reactor is fixed-bed reactor [14] . In the case of the JFE group, the reactor is a slurry reactor and the catalyst is a mixture of methanol catalyst, methanol-dehydration catalyst and water–gas shift reaction catalyst [101,102] . Details about the companies’ catalyst development are not well known, but each catalyst (methanolsynthesis catalysts, methanol-dehydration catalysts and water–gas shift reaction catalysts) is developed in order to improve the catalytic activity and durability, and each developed catalyst is mixed for the direct DME-synthesis catalysts. The direct DME-synthesis method is more advantageous, because the equilibrium conversion rate of methanol is higher than in the indirect method. However, there is a problem in that the load of the catalyst grows and the sintering of the catalysts is promoted by the water generated in the reactor. It is reported that such sintering can be eased by adding a small amount of SiO2 to the methanol-synthesis catalysts [17] . On the other hand, we have been developing direct DME-synthesis catalysts by putting the active sites of methanol-synthesis (water–gas shift reaction) and methanol-dehydration reactions (Equations 6–8) very close together on the surface of one particle of the catalyst. These combined sites are well dispersed in one particle of the catalyst. The new catalysts are prepared using the sol–gel method. The catalysts have a high activity at lower reaction temperatures and a high stability on thermal treatment without mixing with other catalysts. The catalyst can produce DME well from syngas without mixing with other catalysts, even if syngas is contaminated by oxygen gas.


Dimethyl ether & catalyst development for production from syngas Special Report

Equation 9

CH 3 OCH + 3H 2 O $ 6H 2 + 2CO 2 Equation 10

CH 3 OCH 3 + H 2 O $ 2CH 3 OH 2CH 3 OH + 2H 2 O $ 6H 2 + 2CO 2

From this reaction mechanism, DME steam-reforming catalysts are usually a mixture of DME hydrolysis and methanol steam-reforming catalysts. We have been developing DME steam-reforming catalysts by putting active sites (g-Al 2O3) for DME hydrolysis and active sites (copper) for methanol steam reforming very close together on the surface of one particle of the catalysts. These combined sites are well dispersed in one particle of the catalyst. The new catalysts are prepared using the sol–gel method and have high activity for lower reaction temperatures, high stability for thermal treatment. Furthermore, the catalysts can produce H 2 from DME efficiently, without mixing with other catalysts. A flow chart of the preparation method is given in Figure 1, and details of the procedure can be found elsewhere [18–23,103] . The Brunauer–Emmett–Teller (BET) surface area of the catalysts is approximately 200 m 2 /g and the active metal is well dispersed; therefore, x-ray diffraction (XRD) is not usually useful. Figure 2 shows XRD patterns of Al 2O3 prepared using

10 µm

Me 2O (sol–gel, O2)

100

B Cu

10 µm

Me 2O (N211, O2)

80 60 40 20

0 Equation 11

A SEM image

120 Rate of dimethyl ether production (C.µmol/g-cat /h)

Cu–Zn/Al2O3 catalysts prepared using the sol–gel method We have already developed DME steam-reforming catalysts that efficiently produce hydrogen from DME [18–23,103] . The DME steam-reforming reaction (Equation 9) consists of two reaction steps: DME hydrolysis (Equation 10) and methanol steam reforming (Equation 11) .

200

250

300

350

400

Reaction temperature (°C)

Figure 4. Dimethyl ether production rate of CO hydrogenation with oxygen gas (0.43 vol%) over the Cu–Zn(36–4 wt%)/Al2O3 (Sol ) catalyst prepared using the sol–gel method and the mixed catalyst (N211) composed of a methanol-synthesis catalyst (N211) and a methanoldehydration catalyst (Al2O3) under atmospheric pressure. Catalyst: 2.0 g; reaction gas: H2/CO/N2/O2 = 30/30/6.0/0.28 ml/min; space velocity: 2.2 x 103 h-1.

the sol–gel method, Cu(10 wt%)/Al2O3 prepared using the sol–gel method and Cu–Zn(25–25 wt%)/Al 2O3 prepared using an impregnation method with a commercial Al 2O3. The impregnation catalyst has sharp peaks of Cu, but the sol–gel catalysts have no sharp peaks. Al 2O3 prepared using the sol–gel is g-alumina. These sol–gel catalysts were applied to direct DME synthesis from syngas and can produce DME with higher activity at lower reaction temperature than the usual mixed catalysts of methanol-synthesis catalysts and methanol-dehydration catalysts [24,104] . Figure 3 shows the temperature dependence for DME synthesis from syngas (H2 /CO = 1) over two catalysts. The C Al

10 µm

Figure 5. Scanning electron microscope–energy dispersive x-ray spectrometer (SEM–EDS) analysis for the Cu–Zn catalyst prepared using the sol–gel method. (A) SEM image of the surface of one particle of the catalyst, (B) the copper mapping photo by EDS analysis and (C) the aluminum mapping photo by EDS analysis.



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Special Report Takeishi before mixing. The other catalyst is Cu–Zn(36–4 wt%)/Al2O3 prepared by the sol–gel method, and the cataH2 + CO lyst was ground into less than 150 µm and used for DME synthesis Al O Cu 2 3 Cu–Zn Al2O3 H2O + CO singly (without mixing with other catalysts). For the mixed catalyst, the optimum temperature for H2 + CO2 H2 + CO2 H2O + CO DME synthesis was approximately 310°C and the DME production rate was 9 C.µmol/g-cat /h (carbon Figure 6. Direct DME-synthesis mechanisms over the mixed catalysts and the Cu–Zn/Al2O3 base micromolar production per 1 catalyst prepared using the sol–gel method. (A) General direct DME-synthesis catalysts g catalyst and per 1 h reaction time). (mixture of methanol-synthesis, methanol-dehydration and water–gas shift reaction catalysts). In the case of the sol–gel Cu–Zn/ (B) Direct DME-synthesis catalyst prepared using the sol–gel method (single use of Cu/Al2O3 Al 2O3 catalyst in single use, the [sol–gel]). Cu/Al2O3 catalysts prepared using the sol–gel method are better for direct DME optimum temperature is the lower synthesis than mixed commercial and impregnation catalysts. The active sites for each reaction reaction temperate of approximately exist close together and, therefore, the reactions progress more smoothly over the single 250°C and the DME production Cu/Al2O3 catalyst prepared using the sol–gel method than over the mixed catalysts. The single rate is the 2.7-times higher rate of catalyst is very effective for direct DME synthesis. 25 C µmol/g‑cat/h. weight of the used catalyst was 0.5 g and the reaction This sol–gel catalyst was tested for DME syntheconditions were as follows: reaction gas: H 2 /CO/ sis from syngas contaminated with oxygen [25,105] . N2 = 7.5/7.5/1.5 ml/min; space velocity: 2.2 × 103/h. Syngas manufactured from biomass and coal, for One such catalyst is the physically mixed catalyst example, sometimes contains O2 gas, and this O2 gas of methanol-synthesis catalyst (CuO/ZnO [weight is removed from the syngas by a purification process. ratio: 1], N211 manufactured by Nikki Chemical However, Cu–Zn/Al2O3 catalysts prepared using the Co., Ltd. [now, JGC Catalysts and Chemicals Ltd.]) sol–gel method are very effective when used in an and methanol-dehydration catalyst (g-alumina; K-105 economical DME‑synthesis process with the eliminamanufacture by Sumitomo Chemical Co., Ltd.). tion of the O2-removal process. Figure 4 illustrates the These catalysts were ground into less than 150 µm temperature dependence of DME synthesis from syngas (H 2 /CO = 1) with O2 gas (0.43 vol%) over the 45 450 aforementioned two catalysts. For the mixed cataMe 2O lyst, the optimum temperature for DME synthesis is 40 400 approximately 350°C and the DME production rate CH4 is 27 C.µmol/g-cat/h. In the case of the sol–gel Cu–Zn/ 35 350 MeOH Al2O3 catalyst in single use, the optimum temperature 30 300 is a lower reaction temperate of approximately 240°C and the DME production rate is approximately four25 250 times higher at 114 C.µmol/g-cat/h. The electrical conditions of the catalysts have not yet been determined 20 200 using x‑ray photoelectron spectroscopy; however, it is 15 150 considered that the copper may be oxidized by O2 gas and the Cu+/Cu0 ratio may increase. The Cu+ sites are 10 100 very effective for methanol synthesis, and methanol is produced well. Much synthesized methanol will be 5 50 dehydrated to DME. Surrounding alumina prepared 0 0 using the sol–gel method will interrupt the excessive 0.0 0.5 1.0 1.5 oxidation and help the Cu+ to remain. Therefore, a small Absolute pressure (MPa) amount of O2 improves DME synthesis; however, excessive amounts of O2 gas will change the copper to CuO Figure 7. Effect of pressure on the activity of CO hydrogenation over the and the catalyst activity will decrease. Cu–Zn(36–4 wt%)/Al2O3 catalyst prepared using the sol–gel method at Cu–Zn/Al2O3 catalysts prepared using the sol–gel 200°C. Catalyst: 2.0 g; reaction gas: H2/CO/N2/O2 = 30/30/6.0/0.28 ml/min; method in single use produce DME more effectively 3 space velocity: 2.2 x 10 /h. at a lower reaction temperature than physically mixed CH3OCH3 + H2O

222

B

CH3OH

CH3OCH3 + H2O

Rate of methanol or methane production (C.µmol/g-cat /h)

CH3OH


A H + CO 2



100

20

90

18

80

Me 2O

16

70

CH4

14

MeOH

60

12

50

10

40

8

30

6

20

4

10

2

0 0.0

0.5

1.0

1.5

Selectivity for methanol or methane production ( C.%)

catalysts of methanol-synthesis and methanol-dehydration catalysts. This is the reason why Cu, for methanol synthesis and water–gas shift reaction, and Al2O3, for methanol dehydration, coexist closely and are well-dispersed on the catalyst surface. In the case of the mixed catalysts, larger distances exist between the methanolsynthesis catalyst (and water–gas shift reaction) and methanol-dehydration catalyst than those of the two sites on the Cu–Zn/Al2O3 catalyst prepared using the sol–gel method. In order to confirm this speculation, scanning electron microscope (SEM) energy dispersive x-ray spectrometer (EDS) analysis was performed on the surface of the Cu–Zn/Al2O3 catalysts (Figure 5) . As shown in Figure 5, Al (alumina) exists on the whole in the catalyst surface, with Cu dotted within the same. If Al2O3 is compared with seawater, Cu is observed to be exposed from sea level (Al2O3 surface) like an iceberg and dotted on the surface of the sea. Al2O3 and Cu coexist and are distributed with sufficient balance, as expected. Methanol synthesis from syngas is considered to occur on the Cu sites, and methanol dehydration to DME takes place immediately on the Al2O3 very near Cu. In the case of the mixed catalyst, the presence of some distance between the methanol-synthesis catalyst (water–gas shift reaction catalyst) and methanol-dehydration catalyst makes it difficult to carry out these three reactions immediately. It is concluded that the Cu–Zn/Al2O3 catalyst prepared using the sol–gel method with single use allows these three reactions to occur sequentially and efficiently, and thus a swift DME production rate is obtained. Figure 6 is a conceptual figure of this speculation. The sol–gel catalyst was tested for reaction temperature dependence for DME synthesis and the results of the production rates are shown in Figures 7 & 8. Figure 7 shows the pressure dependence for the activity and Figure 8 shows the pressure dependence of selectivity. The production rates of the byproducts methanol and methane are not increased by increasing the reaction pressure, but the DME production rate is rapidly increased by increasing the reaction pressure. From these phenomena,

Selectivity for dimethyl ether production ( C.%)


0

Absolute pressure (MPa)

Figure 8. Effect of pressure on the selectivity of CO hydrogenation over the Cu–Zn(36–4 wt%)/Al2O3 catalyst prepared using the sol–gel method at 200°C. Catalyst: 2.0 g; reaction gas: H2/CO/N2/O2 = 30/30/6.0/0.28 ml/min; space velocity: 2.2 × 103 /h.

the selectivity for DME is increased by raising the reaction pressure and the selectivity approaches 96% under 1.4 MPa. The experiments were carried out in our university laboratory, so the reaction pressure was restricted by The High Pressure Gas Safety Law of Japan. Therefore, we cannot operate DME synthesis under higher reaction pressures, but nevertheless this selectivity value is very high under milder reaction conditions compared with other companies’ data [8,10,14] . We also tested the pressure dependence for direct DME synthesis over the sol–gel catalyst from the pure syngas. The comparison data are listed in Table 1. The space velocity of our process is 2.2 × 103/h and that of the KOGAS process is 2000/h. Values of other processes are not published and the JFE process is carried out using the slurry reactor. Our experiment was performed on the laboratory scale and the others

Table 1. Comparison of direct DME-synthesis technologies. Catalyst

Single-type Cu–Zn/Al2O3 prepared using the sol–gel method

Developer H2/CO ratio Reactor type Reaction temperature (°C) Reaction pressure (MPa) One-pass conversion (%) DME/(DME plus methanol) (%)

Shizuoka University 1.0 1.0 (with 0.42 vol.% O2) Fixed-bed reactor Fixed-bed reactor 220 200 1.6 1.4 5–15 5–15 98 96

Mixed (hybrid)-type methanol synthesis, dehydration condensation and water–gas shift reaction catalysts JFE 1.0 Slurry reactor 250–280 5–6 55–60 90

Air Products 0.7 Slurry reactor 250–280 5–10 33 30–80

Haldor Topsoe 2 Fixed-bed reactor 210–290 7–8 18 60–70

KOGAS 1.0 Fixed-bed reactor 240–260 5–6 ? 85–95

DME: Dimethyl ether. Data from [8,10].



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C1

MeOH

Selectivity for CH 4 or MeOH (C.%)

Rate of methanol production (C.µmol/g-cat /h)

Rate of dimethyl ether production (C.µmol/g -cat /h)

0

0

5

10

15

20

25

30

35

40

0

Zn-loading percentage (wt%)

Figure 11. Effect of the Zn-loading percentage for activity on direct DME synthesis by CO hydrogenation under atmospheric pressure over some Cu–Zn (the whole loading percentage = 40 wt%)/Al2O3 catalysts prepared using the sol–gel method. Catalyst: 0.5 g; reaction gas: H2/CO/N2 = 7.5/7.5/1.5 ml/min.

224

50 45

Rate of MeOH or CH 4 production (C.µmol/g -cat /h)

Me 2O

Selectivity for dimethyl ether (C.%)


are on the plant-scale, and each space velocity value may be different; therefore, there are few meaningful data 400 40 to compare. However, we have devel350 35 oped excellent catalysts that have 300 30 high selectivity for DME. This sol– 250 25 gel catalyst was used for a durability 200 20 test for approximately 200 h (Figures 9 150 15 & 10) . The activity slightly decreased 100 10 with reaction time (Figure 9) and there 50 5 was approximately a 20% decrease 0 0 0 20 40 60 80 100 120 140 160 180 200 between the first and last measured Reaction time (h) activities, but the activity continued and so there is not significant deactiFigure 9. Time course analysis of the activity of CO hydrogenation over the Cu–Zn(36–4 wt%)/ vation. In the case of the selectivity Al2O3 catalyst at 220°C under 1.1 MPa. Catalyst: 0.5g; H2/CO/Ar = 7.5/7.5/1.5 ml/min. (Figure 10) , the value is almost stable or slightly increases from 93 to 96%. The catalysts must be further 100 10 improved for practical use, but these 80 8 sol–gel copper–alumina catalysts are Me 2O C1 MeOH very useful for direct DME synthesis. 60 6 They produce DME effectively, even 40 4 if the syngas is contaminated with O2 gas [26,106] , and will be useful for eco2 20 nomical DME manufacturing from biomass and coal, for example. 0 0 0 20 40 60 80 100 120 140 160 180 200 Furthermore, we have recently Reaction time (h) examined the Cu/Zn ratio dependence for DME synthesis and have Figure 10. Time course analysis of the selectivity of CO hydrogenation over the Cu–Zn(36–4 wt%)/ achieved a more effective catalyst Al2O3 catalyst at 220°C under 1.1 MPa. Catalyst: 0.5g; H2/CO/Ar = 7.5/7.5/1.5 ml/min. for direct DME synthesis than the Cu–Zn(36–4 wt%)/Al2O3 catalyst prepared using the sol–gel method. Parts of the results 200 20 are shown in Figures 11 & 12. The production rate of DME Me O 2 180 18 over the sol–gel Cu–Zn(19–21 wt%)/Al2O3 and Cu– MeOH Zn(15–25 wt%)/Al2O3 catalysts is approximately four160 16 times higher than that of the Cu–Zn(36–4 wt%)/Al2O3 140 14 catalyst. The selectivity of the sol–gel Cu–Zn(19–21 120 12 wt%)/Al2O3 and Cu–Zn(15–25 wt%)/Al2O3 catalysts 100 is much higher than that of the sol–gel Cu–Zn(36–4 10 wt%)/Al2O3 catalyst. Therefore, we will be able to obtain 80 8 more effective catalysts for direct DME synthesis by cata60 6 lyst development. We continue to research these catalysts 40 4 for high-pressure reactions and durability tests and will develop these for practical use for DME manufacture. 20 2 500 450


Future perspective The economical DME-synthesis process and the direct DME-synthesis method have been successful in Japan and Korea. The bio-DME project has started in Sweden. I hope that these projects will promote the cleanliness of DME and that it will eventually be used widely. DME is a very clean fuel for environmental preservation and should be used for power generation, domestic fuels,


100

20

90

Me 2O

18

80

MeOH

16

70

14

60

12

50

10

40

8

30

6

20

4

10

2

0

0

5

10

15

20

25

30

35

40

Selectivity for methanol production (C.%)

diesel fuel and as a hydrogen source. If the supply and demand increases, the price of DME will become lower, which will in turn increase the amount of DME usage. An effective DME synthesis process should be developed and catalysts for DME (e.g., developed catalysts for DME mass production, our sol–gel Cu–Zn/Al2O3 catalysts) will be important. Moreover, if the developed process and catalysts can help to manufacture DME at a lower price, it can be economically manufactured by omitting the purification process of removing oxygen. If the price of DME becomes much lower, it will be easy to use DME and systems related to DME will be well-developed and necessary. When the points mentioned above are wellcirculated, industries related to DME will improve and DME catalysts will play important roles. Approximately 7 million tons of DME will be produced in China this year. This will be consumed by mixing with LPG, because the price of DME is lower than LPG. However, in Japan and other countries, DME faces a ‘chicken and egg’ dilemma: if big users do not appear, then DME will not be produced in large quantities. This contradiction should be solved for the environment. By reducing the DME price and increasing ecofriendly DME production, mass-production of DME using the direct DME-synthesis method could be widely used and the raw material, syngas, could be produced from biomass and CO2. For this purpose, effective processes, systems and catalysts should be developed and widely used. Sol–gel catalysts are usually difficult to mass-produce; however, for effective DME production in the future, the sol–gel catalysts that we have produced must be further developed.

Selectivity for dimethyl ether production (C.%)


0

Zn-loading percentage (wt%)

Figure 12. Effect of the Zn-loading percentage for selectivity on direct DME synthesis by CO hydrogenation under atmospheric pressure over some Cu–Zn(the whole loading percentage = 40 wt%)/Al2O3 catalysts prepared using the sol–gel method. Catalyst: 0.5 g; reaction gas: H2/CO/N2 = 7.5/7.5/1.5 ml/min. Financial & competing interests disclosure The author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties. No writing assistance was utilized in the production of this manuscript.

Executive summary Dimethyl ether (DME) is a multisource and multi-use fuel. It is called a ‘clean fuel for the 21st Century’. The combustion gas of DME is very clean and, thus, DME should be widely used to help solve environmental concerns. An economical DME‑synthesis process has been developed and the direct DME-synthesis method (a one-step process) produces DME without restriction of methanol price, because DME is produced from syngas directly. The catalysts for the process consist of methanol-synthesis catalysts (usually Cu-Zn catalysts) and methanol-dehydration catalysts (usually alumina catalysts), according to the reaction mechanism. JFE and KOGAS are currently developing the process and the catalysts. We have also developed new catalysts (sol–gel Cu–Zn/Al2O3 catalysts). Our catalysts are prepared using the sol–gel method and the catalysts will be used singly without mixing with other catalysts. The catalysts are very effective for direct DME synthesis, even if raw syngas is contaminated with oxygen gas. For the benefit of the environment, I hope these catalysts will be widely used for economical DME synthesis from biomass.

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1

n

2

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3

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21

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