Pyrolysis and catalysis of dimethyl ether in a flow reactor

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Pyrolysis and catalysis of dimethyl ether in a flow reactor ⁎

Dongxue Hana, Hongming Yina,b, , Enchao Qianb, Lili Yeb, Dajun Liub a b

College of Environmental Sciences and Engineering, Dalian Maritime University, Dalian 116026, PR China School of Science, Dalian Maritime University, Dalian 116026, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Dimethyl ether Pyrolysis Catalysis Methanol Hydrogen

Dimethyl ether (DME) was an interesting alternative fuel for diesel and HCCI engines with the global energy crisis and growing environmental concerns. The DME pyrolysis was investigated experimentally from 700 K to 1220 K in a flow reactor. The concentration of DME and products has been detected by a gas chromatography with a thermal conductivity detector. In addition to methane, carbon monoxide, ethylene and ethane, methanol has been detected in present study. A kinetic mechanism for DME pyrolysis has been studied to analyze the pathways of methanol formation. The main routes of methanol formation were the H-atom abstractions by methoxy radicals, in which the dominant one was H-abstractions from DME molecules. The catalytic pyrolysis and catalytic oxidation of DME over γ-Al2O3 catalyst were investigated in the flow reactor from 300 K to 840 K. A large amount of hydrogen has been detected from DME catalytic pyrolysis and catalytic oxidation, both in which hydrogen was the dominated final product. The excess O2 and the larger contact area with the catalyst contributed to the hydrogen production from DME catalytic oxidation.

1. Introduction As a colorless and odorless flammable gas, there has been a growing concern on the using of DME as an alternative fuel [1–2]. Because of the similar physical properties, DME has been considered as a substitution of liquefied petroleum gas (LPG) [3]. With a high cetane number more than 55 and a great property of compression ignition, DME has also been considered as an alternative fuel for diesel fuel [4]. Characteristics of DME fuel for atomization [5], ignition [6], combustion [7] and emission [8,9] have been extensively studied. Comparing combustion of DME with diesel, the emissions of carbon monoxide (CO) and hydrocarbons (HCs) from DME were lower than that from diesel, and DME combustion does not produce soot [10–13]. DME, available and lowcost, can be produced from natural gas and coal. For solving energy conservation and environmental concerns, DME as an alternative fuel has a wide application prospect. For being cleaner and more efficient energy, exploring the combustion characteristics of DME was necessary. The DME oxidation has been studied experimentally in flow reactor [14–17], jet-stirred reactor [18], shock tube [19] and burner [20] for laminar flame speed [21,22], ignition delay time [23,24], kinetic model [25] and oxidation with different equivalence ratios. As an alternative fuel, the study for the emission behavior of DME combustion was necessary. A series of species have been detected from DME oxidation, final products H2O and

⁎

CO2, as well as intermediate products hydrocarbons, peroxide, aldehydes and methyl formate [26,27]. The pyrolysis of DME was an essential process of DME combustion. There were a large number of previous studies about the pyrolysis of DME. Table 1 presents the researches for DME pyrolysis performed with experiments and kinetic models. The previous studies for DME pyrolysis were including low temperature (< 1000 K) and high temperature (> 1300 K). As shown in Table 1, the experimental studies for DME pyrolysis has been mostly performed in shock tubes, and the experimental conditions mostly based on high temperature and high pressure [28,29]. A series of products have been detected experimentally from the DME high temperature pyrolysis, carbon monoxide (CO), formaldehyde (CH2O) and hydrocarbons including methane (CH4), ethane (C2H6) and ethylene (C2H4) [28]. Many studies have developed kinetic models DME pyrolysis at high temperature to reproduce the experimental results in shock tube [30–32,37–39]. Hidaka et al. [30] have studied the high-temperature pyrolysis of DME behind reflected shock waves in the temperature range 900–1900 K at pressures in the range 0.83–2.9 atm. A tendency to form ethyne (C2H2) has been found at temperatures more than 1300 K [30]. The kinetic models for DME pyrolysis at medium-low temperature have been proposed and verified experimentally in the flow reactors and jet-stirred reactors, as listed in Table 1 [33–36]. Fischer et al. [35] have proposed a detailed chemical kinetic mechanism for DME pyrolysis to reproduce the fuel and

Corresponding author at: College of Environmental Sciences and Engineering, Dalian Maritime University, Dalian 116026, PR China. E-mail address: [email protected] (H. Yin).

https://doi.org/10.1016/j.fuel.2019.116700 Received 11 June 2019; Received in revised form 6 September 2019; Accepted 19 November 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Dongxue Han, et al., Fuel, https://doi.org/10.1016/j.fuel.2019.116700

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Table 1 The researches for DME pyrolysis performed with experiments and kinetic models. Type of research

Research condition

Refs

Shock tube

T = 1300–1600 K; P = 1.5 atm; T = 1500–2450 K;

Pyun et al. [28] Tranter et al. [29]

Model and shock tube

T T T T T

Hidaka et al. [30] Sivaramakrishnan et al. [31] Yasunaga Pacey et al. [37] Fernandes et al. [38] Cook et al. [39]

Model

T = 1063–1223 K; P = 1 atm T = 550–850 K; P = 12–18 atm

Aronowitz et al. [32] Curran et al. [34]

Model and jet-stirred reactor

T = 800–1300 K; P = 1, 10 atm

Curran et al. [36]

Model and flow reactor

T = 1118 K; P = 2.5 atm T = 980 K; P = 10 atm

Fischer et al. [35] Zhao et al. [33]

= = = = =

900–1900 K; P = 0.83–2.9 atm 1406–1764 K; P = 0.20–1.25 atm 900–1900 K; P = 1–4 atm 1270–1621 K; P = 0.35, 1.3 atm 1349–1790 K; P = 0.6–11.5 atm

was obtained that the DME hydrolysis was more likely to occur at a highly acidic OH· surface radical. Bondarenko et al. [45] suggested that the formation of methoxy groups in the interaction of DME and methanol with OH· surface of γ-Al2O3. Kipnis et al. [46] suggested that the main products of the surface reaction of DME over γ-Al2O3 were methanol and water, and elucidated the role of the OH· of γ-Al2O3 on methanol and water formation. Due to the outstanding catalytic performance, γ-Al2O3 also played a significant role in DME oxidation [47–51]. Ishikawa et al. [47] suggested the bifunctional pathways for the DME oxidation on Pt supported on γ-Al2O3, which arise from DME hydrolysis on γ-Al2O3 acid sites. The selective oxidation of DME to CH2O has been investigated over γ-Al2O3 supported VOx and MoOx catalysts [49–51]. Higher surface area of γ-Al2O3 led to a higher rates for catalyst mass and a significant improvement for catalytic productivity. The DME selective oxidation occurred only on the catalyst surface and CH2O was the preliminary product of the selective oxidation of DME. Quite more attention has been paid to the application of γ-Al2O3 as a catalyst carrier on the hydrogen production from DME [52–55]. γ-Al2O3 had strong acidic sites and was used as an acidic catalyst to increase the hydrogen yield. Park et al. [52] investigated the DME steam reforming on Cu/γAl2O3 catalyst mixing with three kinds of zeolites, in which the Cu10/γAl2O3 mixed with 10% MOR showed the highest hydrogen yield. Yu et al. [53] investigated the DME partial oxidation using the γ-Al2O3/Al catalyst with different contents of Rh, and the maximum hydrogen yield was obtained when the Rh amount was 0.5 wt%. γ-Al2O3 catalyst exhibited sufficient catalytic performance for DME hydrolysis to methanol as well as the steam reformation of DME to hydrogen. Therefore, γAl2O3 was an excellent catalyst, making it fit to be used as a catalyst for

intermediate-product species profiles. Zhao et al. [33] have revised several important reactions and combined with Fischer’s model. A comprehensive DME pyrolysis and oxidation model has been presented and a DME pyrolysis experiment was performed at 10 atm and 980 K to shown very good agreement with the model. In the previous researches, CH3OH has been mentioned in the model, while not obtained in the experiment. The previous experiments of DME pyrolysis mostly depended on shock tubes in high-temperature region. The studies in lowtemperature region were mainly based on the theoretical study with a lack of experimental data. It was necessary to provide experimental data for DME pyrolysis in the low and intermediate temperature region. Catalysts were characterized by the acceleration of the reaction rate, the change of the reaction pathway and the decrease of the reaction activation energy. Adding catalysts in the pyrolysis process could improve the pyrolysis efficiency, increase the yield of the pyrolysis products, and prepare a product which was not easily prepared by conventional pyrolysis. As the small particle size and the large specific surface area, γ-Al2O3 with a good heat resistance and stability was widely used as a catalyst or catalyst carrier. Table 2 has listed the researches about DME catalysis over γ-Al2O3. γ-Al2O3 catalyst has been used for the production of DME in one step from methanol dehydration [40]. γ-Al2O3 had the hydroxyl ions (OH–) and defect sites (Al+), the former adsorption the hydrogen atom of methanol hydroxyl and the latter adsorption the oxygen atom of methanol, which resulted in reaction of two methanol molecules and conversion into DME and water [41]. γ-Al2O3 was an active solid acid catalyst [42], and widely used on the DME hydrolysis for methanol production [42–46]. Hirunsit et al. [43] examined the catalytic behavior and surface species over γ-Al2O3 in DME hydrolysis, and a result

Table 2 The researches for DME catalysis over γ-Al2O3. Purpose

Catalyst and sizes

Refs

CH3OH dehydration to DME

γ-Al2O3, zeolites, ion exchange resins γ-Al2O3; 100–250 μm partical size γ-Al2O3; 280 m2/g surface area γ-Al2O3; 10–90 nm partical size anodic γ-Al2O3/Al; γ-Al2O3; 0.2–0.35 mm partical size γ-Al2O3; 200 m2/g surface area Pt/γ-Al2O3; 193 m2/g surface area γ-Al2O3-MO; 72.6, 54.5, 43.9 m2/g surface area VOx, MoOx/γ-Al2O3; 184.2, 162.1 m2/g surface area VOx, MoOx/γ-Al2O3; 101, 196 m2/g surface area MoOx/γ-Al2O3; 100 m2/g surface area Cu/γ-Al2O3; 1.7–300 nm partial size Rh/γ-Al2O3/Al; ~20 nm partial size H4SiW12O40/γ-Al2O3; 0.25–0.5 mm partial size Cu-Ni/γ-Al2O3; 7.46, 8.5, 8.81, 8.96, 9.23, 9.54 mm partial size

Hosseininejad et al. [40] Lee et al. [41] Semelsberger et al. [42] Hirunsit et al. [43] Fan et al. [44] Bondarenko et al. [45] Kipnis et al. [46] Ishikawa et al. [47] Zhou et al. [48] Huang et al. [49] Liu et al. [50] Cheung et al. [51] Park et al. [52] Yu et al. [53] Galvita et al. [54] Wang et al. [55]

DME hydrolysis

DME catalytic oxidation DME selective oxidation to CH2O

Hydrogen production from DME

2

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N2 and 0.00784 L/min for DME. In DME catalytic oxidation experiment, the DME flow rate was also 0.00784 L/min and air flow rate was 0.15 L/min.

the study of DME catalytic pyrolysis and oxidation. In the present study, the DME pyrolysis in the temperature range of 700–1220 K was investigated experimentally in a flow reactor. A gas chromatograph with a thermal conductivity detector was employed to detect the composition of exhaust gas on-line. The DME catalyst pyrolysis and oxidation over γ-Al2O3 were also investigated. The purpose of this paper was to investigate the products of DME pyrolysis and the catalysis of γ-Al2O3 on the DME consumption and the products emissions.

3. Results and discussion In the following discussion, the experimental results will be discussed in detail, focusing on the consumption of reactants and the formation of products. Specifically, section 3.1 aims to reveal the DME pyrolysis, while section 3.2 the DME catalytic pyrolysis and catalytic oxidation over γ-Al2O3 catalyst.

2. Experimental setup The overall experimental system was composed of gas supply, flow reactor and gas chromatograph detection, which was similar to that described in our previous study [56]. A difference was existed in the way of fuel supply between the current experimental setup and previous one. The fuel used in previous study was liquid, so a bubbler was employed for creating fuel vapor to supply fuel. However, the fuel DME in current experiment was gaseous, and the mixture of the DME and carrier gas was supply directly into the reactor. Therefore, the bubbler did not need to be installed in the current experimental setup, and the gas supply system was directly connected to the reactor. The gas supply system was responsible for providing DME, nitrogen (N2, purity greater than 99.999%) and air (purity greater than 99.999%), the flow rates of which were controlled by mass flow controllers with 0.20% accuracy and 2 L/min full range (calibrated by N2, the conversion coefficient of DME relative to N2 was 0.392). N2 was used as a shielding gas in the experiment of DME pyrolysis to insulate DME from surrounding air avoiding oxidization. To prevent airflow refluence, check valves were installed in the gas supply system. A homogeneous mixture was supplied into a flow reactor, which consisting of a tubular heating furnace, a reaction tube surrounded by the heating furnace body and a temperature control system. The reaction tube was a visible quartz tube with an inner diameter of 11 mm, outer diameter of 16 mm and a total length of 500 mm, of which the middle 180 mm was located at the heating zone. The temperature of reaction tube was measured and controlled by a temperature controller connecting with a K-type thermocouple (accuracy 0.25%) inserted from the middle of the furnace top. The effect of the air flow on the temperature of reaction tube was examined beforehand and was almost negligible. The flow reactor was connected to a gas chromatograph (FULI, GC9790). Exhaust gas was injected directly into the injection port of gas chromatograph, separated by a Porapak Q chromatographic column, and analyzed on-line by a thermal conductivity detector (TCD) for the composition and concentration. Identification of species was determined by the external standard method via injecting their standard sample into the gas chromatograph. Determination of species concentration was conducted by peak area normalization method. As a large volume expansion coefficient, the chromatographic peak of water cannot accurately reflect its concentration. Therefore, the content of water in the exhaust gas was not recorded in present paper. Three series of experiments were performed in the above experimental setup, the DME pyrolysis, DME catalytic pyrolysis and DME catalytic oxidation over γ-Al2O3 catalyst. A commercial γ-Al2O3 powder of 1 g with a particle size of about 100 nm was accurately weighed and placed in the center of the reaction tube. A breathable copper plug was placed at each end of the catalyst to hold it in place, forming a 30 mm long catalyst layer (about 2.85 × 103 mm3 of volume). The residence time of the reactants in the reaction tube was affected by the gas flow rates, and the change in residence time also affected the consumption of DME and the formation of products. In order to avoid the influence of the change in residence time caused by the gas flow rates on the experimental results, the total gas flow rates in each series of experiments were set to be the same. Both in DME pyrolysis and catalytic pyrolysis experiments, the gas flow rates were 0.15 L/min for

3.1. DME pyrolysis In current paper, the DME pyrolysis experiment has been carried out over the temperature range from 700 K to 1220 K in a flow reactor. A series of species have been detected from exhaust gas, including reactant DME and products CO, CH4, C2H6 and C2H4, which were also detected in previous studies [33,35]. Besides, methanol (CH3OH) was also detected in present study. In order to verify our experimental results, the chromatographic results of DME pyrolysis, catalytic pyrolysis and catalytic oxidation at appropriate temperature were selected for comparison with CH3OH standard samples. The chromatogram peaks of CH3OH from DME pyrolysis at 1040 K, from catalytic pyrolysis at 750 K, from catalytic oxidation at 630 K and the CH3OH standard sample (Sigma-Aldrich, HPLC, ≥99.9%) are displayed in Fig. 1. CH3OH was detected not only in DME pyrolysis, but also in DME catalytic pyrolysis and catalytic oxidation. The emission of CH2O was not detected in DME pyrolysis maybe because of too little production to cause an responsive detector. The emission of acetylene (C2H2) from DME pyrolysis has been detected by Hidaka et al. [30] above 1300 K. The reaction temperature in current study was over the range of 700–1200 K. The relatively lower temperature may be the reason why C2H2 was not detected in present experiment. Fig. 2 shows the mass fraction profiles of reactant and products from DME pyrolysis over the temperature range 700–1220 K. The reactant DME and major products including CO and CH4 are shown in Fig. 2(a), while minor products including C2H4, C2H6 and CH3OH in Fig. 2(b). The experimental error bars calculated by the relative error are also shown in Fig. 2. The simulation using Zhao’s [33] model with some modification based on our experimental conditions is shown in Fig. 2 with the dotted lines. The profiles of these species are basically in good agreement with zhao's [33] model. A difference is exist between the experimental results and simulation results of C2H6 and CH3OH. A wider temperature range of C2H6 and CH3OH is obtained in present experiment comparing to the model simulation. The temperature range of C2H6 and CH3OH is suggested to be improved in future model studies for DME pyrolysis. As shown in Fig. 2(a), the consumption of DME is initiated at 840 K through the scission of C-O bond. With the increasing temperature, the addition of other DME consumption pathways makes a contribution to DME decomposition, leading to a rapid reduction of the DME concentration from 920 K to 1100 K. For higher temperature, the DME consumption rate slows down again until DME is completely consumed at about 1140 K. CH4 and CO are the major final products of DME pyrolysis. The emission of CH4 appears as soon as the consumption of DME is observed. The emission of CO appears at a higher temperature than CH4. The appearance of CO emission corresponds to the starting of DME rapid decomposition. The mass fraction profile shapes of CH4 and CO shows a similar, increase with the increasing temperature. As the reduction of DME consumption rate, the increases in the concentrations of CO and CH4 slow down. As shown in Fig. 2(b), the other hydrocarbon products from DME pyrolysis are C2H4 and C2H6, whose concentrations both increase first and then decrease with the increasing temperature. The difference is that C2H4 obtains the maximum at 1140 K, while C2H6 at 1070 K. 3

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Fig. 2. The mass fraction profiles of the reactant and major species (a) as well as the minor species (b) from DME pyrolysis. The line with symbol represent the experimental data of present study and the dotted line represent the simulation by Zhao’s [33] model.

940 K and 1040 K, shown in Fig. 3(a) and (b), respectively. The percent of CH3OH production is the ratio of CH3OH production from a certain formation channel to that from all formation channels.

Fig. 1. The gas chromatogram peaks from 4.2 to 8.0 min from DME pyrolysis at 1040 K (a), from DME catalytic pyrolysis at 750 K (b), from DME catalytic oxidation at 630 K (c) and the CH3OH standard sample (d).

As shown in Fig. 2 (b), CH3OH has also been detected in the exhaust gas of DME pyrolysis over the temperature range from 940 K to 1180 K, which was not obtained by previous experimental studies between 700 K and 1220 K [28–30]. The kinetic mechanism of CH3OH was proposed in Zhao's [33] model, as shown by dotted line in Fig. 2(b). The evolution of mass fraction with temperature is basically consistent with that detected in this experiment. Its mass fraction increases and then decreases with the increase of temperature, and shows the highest CH3OH content of 0.54% at 1040 K. For better understand the pathways of CH3OH formation, a kinetic mechanism of DME pyrolysis was studied. According the current experiment and the previous modeling studies [33,35,36], a plug flow reactor model was used to simulate CH3OH formation from DME pyrolysis between 700 and 1300 K at atmosphere pressure. The mass fraction of CH3OH increases from the temperature of 940 K and reaches a maximum at 1040 K, so the temperature of the initial and maximum CH3OH is chosen for the mechanism analysis of CH3OH formation. The reaction path diagrams of CH3OH are drawn at

CH3 OCH3 = CH3 O·+CH3·

(R1)

CH3 OCH3 + CH3·=CH3 OCH2 ·+CH 4

(R2)

CH3 OCH3 + CH3 O·=CH3 OCH2 ·+CH3 OH

(R3)

CH3 O·+CHO·=CH3 OH + CO

(R4)

CH2 OH·+CHO·=CH3 OH + CO

(R5)

2CH2 OH·=CH3 OH + CH2 O

(R6)

The above shows the main channels related to CH3OH formation from DME pyrolysis. The DME molecule is decomposed though C-O bond scission forming methyl (CH3·) and methoxy radical (CH3O·), i.e. R1, which is the initiation reaction of DME pyrolysis. Other consumption pathways of DME are mainly dehydrogenated by CH3· to form CH3OCH2· and CH4, i.e. R2. As shown in Fig. 3, the proportion of R1 and R2 is almost equal at 940 K, while the proportion of R2 increases with temperature up to 1040 K, so the DME dehydrogenation reaction dominates. CH3O· reacts mainly in two ways, the dehydrogenation of CH3O· forming CH2O and the H-atom abstraction by CH3O· to form CH3OH. The latter is the main pathway of CH3OH formation, in which R3, the H-atom abstraction from DME molecule by CH3O·, is the dominated one. R3 is for about 99.42% of CH3OH production at 940 K, while about 96.99% of CH3OH production at 1040 K. The H-atom abstraction by CH3O· from CHO· (R4) also contribute to about 1.32% of CH3OH production at 1040 K. CH3OH is mainly consumed by dehydrogenation to form CH2OH·. The H-atom abstraction and dehydrogenation of CH2OH· form CH3OH and CH2O, respectively. At 940 K, 4

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Fig. 3. The main formation pathways of methanol from DME pyrolysis at 940 K (a) and 1040 K (b).

R1 exhibits a negative sensitivity. R3 shows a decreasing positive sensitivity in the overall temperature range. As the temperature increases, DME is gradually decomposed, and the promoting effect of R3 on CH3OH formation is gradually reduced. The pathways of CH2O formation from CH2OH· and CH3O· are show negative sensitivities, which means they hinder CH3OH formation. Two CH3OH consumption reactions with CH3O· and OH· both exhibit increasing negative sensitivity.

the H-atom abstraction reaction by CH2OH· from CHO· (R5) and another CH2OH· (R6) are respectively responsible for about 0.04% and 0.53% of CH3OH production, while for about 0.26% and 1.43% of CH3OH production at 1040 K, respectively. A small portion of the dehydrogenation of CH2OH· produces CH2O. Total absolute rate of production (ROP) for a species is calculated by subtracting its consumption rate from its production rate. Comparing to the ROP of DME pyrolysis at 940 K, CH3OH is 1.22 × 10−8 mol/cm3·s, and CH2O is 5.63 × 1010 mol/cm3·s. At 1040 K, CH3OH is 1.49 × 10-9 mol/cm3·s, and CH2O is 8.75 × 10-11 mol/cm3·s. The ROP of CH2O is more than 15 times less than that of CH3OH both at 940 K and at 1040 K, resulting in too little CH2O production to be detected. The sensitivity analysis for CH3OH formation from DME pyrolysis was investigated to further explore the influence of reactions on CH3OH formation. The sensitivity analysis was performed by multiplying the rate constant of a reaction by a factor of two (both forward and reverse rate constants) and then calculating the percent change in reactivity [36]. Fig. 4 shows the sensitivity coefficients for CH3OH formation from DME pyrolysis at 980 K, 1040 K and 1100 K, corresponding to the increase, maximum and decrease of CH3OH concentration, respectively. This plot shows that the formation of CH3OH is very sensitive to the decomposition of fuel molecule. R1 and R3 play an important role on CH3OH formation in the overall temperature range. With the temperature from 980 K up to 1040 K, R1 show a positive decreasing sensitivities. When DME is almost completely decomposed at 1100 K,

3.2. DME catalytic pyrolysis and oxidation on γ-Al2O3 catalyst The effects of catalyst and oxygen on DME decomposition and product formation are worth exploring. As a widely used catalyst, the application of γ-Al2O3 on the DME pyrolysis and oxidation was investigated. In current paper, the investigation for DME catalytic pyrolysis and catalytic oxidation over γ-Al2O3 with 100 nm particle size has been carried out over the temperature range from 300 to 840 K in the flow reactor. The exhaust emissions from the DME catalytic pyrolysis and catalytic oxidation have been detected using a gas chromatograph. The mass fraction profiles of CH4 and CO2 emitted from the DME catalytic oxidation are not drawn in present paper, because the appearance times of their gas chromatogram peaks are near, showing a overlapping peak, which leads a difficult quantification of their concentration. Fig. 5 shows the mass fraction profiles of DME and hydrocarbon products from the catalytic pyrolysis and catalytic oxidation of DME, and those from DME pyrolysis for comparison. The experimental error bars in Fig. 5 are calculated by the relative error. As shown in Fig. 5(a), the DME decomposition in catalytic pyrolysis starts at 480 K and ends at 840 K. The adsorption of DME by the active site on γ-Al2O3 catalyst surface reduces reaction activation energy, promoting the DME decomposition at a lower temperature. Compared to the DME pyrolysis and catalytic pyrolysis, with the addition of a catalyst, the starting temperature and ending temperature of DME decomposition are brought forward by 360 K and 340 K, respectively. The decomposition of DME in catalytic oxidation starts at 420 K. The co-existence of O2 and catalyst accelerates the DME decomposition which is completed at 690 K. Fig. 5(b), (c) and (d) show the hydrocarbon products, CH4, C2H4 and C2H6, respectively. Their profile shapes of mass fraction with temperature are similar in all experiments. The emissions of CH4 from DME catalytic pyrolysis are much less than that from DME pyrolysis. The emissions of C2H4 and C2H6 from DME catalytic pyrolysis are similar to those from DME pyrolysis, while those from DME catalytic oxidation are much less. In DME catalytic pyrolysis, the hydrocarbon products are produced at the temperatures of more than 660 K. The presence of a catalyst not only leads to a lower temperature for the decomposition of DME, but also increased the products. Compared

Fig. 4. Sensitivity coefficients for methanol formation from DME pyrolysis. 5

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Fig. 6. The mass fraction profiles of oxygenated products, CO (a), CH2O (b) and CH3OH (c), formed from the pyrolysis, catalytic pyrolysis and catalytic oxidation of DME.

and decreases after the maximum through an oxidation to CO2. As shown in Fig. 6(b), CH2O, not obtained from the DME pyrolysis, has been detected from the DME catalytic pyrolysis over the temperature range from 720 K to 810 K. It is very harmful on human health due to the irritation for the eyes and upper airways. As discussed in Fig. 3, the formation of CH2O is mainly derived from the dehydrogenation reaction of CH3O· formed from the C-O bond scission of DME. The oxygen and carbon atoms of the DME molecule are adsorbed by the Al + and OH· on γ-Al2O3 surface, which promotes C-O bond scission of DME formation CH3O·[45]. CH3OH is decomposed into CH2O on the γAl2O3 surface, which is also one of the formation pathways of CH2O [52]. Therefore, the presence of catalyst makes a contribution to the formation of CH2O from the DME catalytic pyrolysis. The formation of CH2O also has been observed from the DME catalytic oxidation over the temperature range of 600–810 K. The increase in CH2O emissions from DME catalytic oxidation can be observed significantly than that from catalytic pyrolysis, because the addition of O2 leads to an increase in the formation pathways of CH2O. Due to the addition of O2, DME goes through a series of oxidation, isomerization and decomposition to generate CH2O [57]. CH3OH have also been detected from the catalytic pyrolysis and catalytic oxidation of DME, and their mass fraction profiles are shown in Fig. 6(c). DME molecules interact with the OH· active sites on the γAl2O3 surface to produce CH3OH and CH3O·, i.e., CH3OCH3 + OH· = CH3OH + CH3O·[45], which is the main pathway for CH3OH formation. The H-atom abstraction of CH3O· is also

Fig. 5. The mass fraction profiles of DME (a) and hydrocarbon products, CH4 (b), C2H4 (c) and C2H6 (d), from the pyrolysis, catalytic pyrolysis and catalytic oxidation of DME.

to the products from the DME pyrolysis, there are not only CO, CH4, C2H4, C2H6 and CH3OH, but also hydrogen (H2) and formaldehyde (CH2O) in the products from the DME pyrolysis and oxidation over γAl2O3 catalyst. The gas chromatogram peaks of CH2O obtained from the DME catalytic pyrolysis at 750 K and from the DME catalytic oxidation at 630 K are supplied in the appendices as Fig. 1(b) and (c), respectively. The mass fraction profiles of oxygenated species, CO, CH2O and CH3OH produced from the pyrolysis, catalytic pyrolysis and catalytic oxidation of DME are shown in Fig. 6, respectively. The gas chromatogram peaks of H2 and CO formed from the DME catalytic pyrolysis at 750 K are shown in Fig. 6(a) as an illustration. The experimental error bars are calculated by the relative error and shown in Fig. 6. As shown in Fig. 6(a), the profile shapes of CO concentration from DME pyrolysis and catalytic pyrolysis are similar. The CO emissions from the DME catalytic pyrolysis are about half of those from the DME pyrolysis, which means that the emissions of other carbon-oxygencontaining species increase. Due to the presence of O2, a difference of the mass fraction profiles can be observed in the DME catalytic oxidation. The CO concentration from DME catalytic oxidation increases first 6

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Ref. [53]. The O2 supply in Ref. [53] was lean, while that in present study about 4 times of DME. The larger contact area with the catalyst and the excess O2 contribute to the catalytic oxidation of DME to produce H2 at a lower temperature.

responsible for the CH3OH formation. Therefore, CH3OH is formed from the DME catalytic pyrolysis once DME begins to decompose at 480 K. It also can be seen that the emissions of CH3OH from the DME catalytic pyrolysis is about 16 times more than that from the DME pyrolysis. That is to say that the presence of a catalyst is beneficial to the formation of CH3OH from DME pyrolysis. An almost amount of CH3OH emissions are also been detected from the DME catalytic oxidation. The DME is mainly oxidized by the adding of O2 to form H2O. A hydrolysis of DME is carried out on the acid catalyst γ-Al2O3 surface to form CH3OH [47], which is the dominant pathway of CH3OH formation from the DME catalytic oxidation. That is

CH3 OCH3 = H2 + CO + CH 4

CH3 OH = 2H2 + CO H·+H·=H2 These show the pathways of H2 formation from DME catalytic pyrolysis. As the existence of γ-Al2O3 catalyst, the decomposition of DME molecule and CH3OH are both responsible for the formation of H2. The hydrocarbons, such as CH4, C2H4 and C2H6, that are adsorbed onto the γ-Al2O3 surface are dehydrogenated by C–H bond scission to form free H·. The recombination of two H% is another pathway for H2 production. Therefore, the concentration of H2 is still slowly increasing as the temperature up to 780 K, when DME and CH3OH are almost completely decomposed. In DME catalytic oxidation, H2 is also the main final product, and begins to generate at 420 K. There are three main consumption pathways for DME, the oxidation, hydrolysis and steam reforming. The DME oxidation generates H2O which plays an important role on the DME hydrolysis and steam reforming. The DME hydrolysis on the γ-Al2O3 catalyst surface is performed to form CH3OH. The steam reforming of DME and CH3OH are the main two routes for the H2 production from DME catalytic oxidation. Another production route is the DME partial oxidation. All the pathways of H2 formation from DME catalytic oxidation are as follows.

CH3 OCH3 + 3O2 = 3H2 O+ 2CO2 CH3 OCH3 + H2 O= 2CH3 OH CH3OH is further oxidized to form CO, CO2 and so on, so that a gradual decrease on the CH3OH concentration can be seen above 690 K. A large amount of H2 emissions are detected from the DME catalytic pyrolysis and catalytic oxidation, while no H2 emissions are detected from the DME pyrolysis. Both in catalytic pyrolysis and catalytic oxidation of DME, H2 is the main final product. The mass fraction profiles of H2 from DME catalytic pyrolysis and catalytic oxidation are shown in Fig. 7, where the H2 yield from Park et al. [52] and Yu et al. [53] are also shown for comparison. H2 in Ref 52 are from the DME steam reforming (0.7% DME + 5% H2O) on Cu10/γ-Al2O3 + MOR10 catalyst in the presence of 5% CO2 and 1% O2. H2 in Ref 53 are from the DME partial oxidation over 0.5w% Rh/γ-Al2O3/Al. The error bars calculated by the relative error are shown in present experiment. In present study, the formation of H2 from DME catalytic pyrolysis begins at 540 K. Its concentration gradually increases with the increasing temperature, showing the highest H2 yield of 60.51% at 840 K. The highest H2 production from DME catalytic oxidation is about 61.67% obtained at 720 K. As shown in Fig. 7, the H2 yield from present study is almost the same with that from Ref 52, while is less than that from Ref 53. It is worth noting that the H2 from DME catalytic oxidation in present study is formed beginning at 450 K, which is lower about 120 K than that from Refs 52 and 53. Ref 52 focused on H2 production from DME steam reforming, with only 1% O2 presence. In present study, about 19.96% O2 is provided for DME catalytic oxidation. Sufficient oxygen makes for the H2 formation from DME catalytic oxidation moving to a low temperature. The catalyst used in Ref 53 was that 0.5w % Rh was dispersed on the γ-Al2O3 plate with 44 μm thick, and DME was flowed through its surface to catalyze. In the present study, DME flows through a 30 mm long γ-Al2O3 powder to catalyze, which resulted in a much larger contact area between DME and catalyst than that in

CH3 OCH3 + 3H2 O= 6H2 + 2CO2

CH3 OH + H2 O= 3H2 + CO2 CH3 OCH3 + 0.5O2 = 3H2 + 2CO As the temperature up to 720 K, DME is completely consumed and the CH3OH concentration is decreasing, leading to a maximum on H2 concentration. For higher temperature, since the further oxidation of H2 makes its consumption rate greater than formation rate, a reduction in H2 concentration can be observed. 4. Conclusion The DME pyrolysis was investigated experimentally in a flow reactor over the temperature range of 700–1220 K. The concentrations of reactant and products have been detected by using a gas chromatography with a thermal conductivity detector. The intermediate C2H4 and C2H6 as well as the final products CH4 and CO have been detected from the exhaust of DME pyrolysis. In particular, CH3OH also has been detected in DME pyrolysis. A kinetic mechanism of DME pyrolysis has been studied to analyze the pathways of CH3OH formation. The H-atom abstractions of CH3O· and CH2OH· were two ways of CH3OH formation, and the former was the dominated one. At 940 K, the H-abstractions by CH3O· from DME molecule was for about 99.42% of CH3OH production. At 1040 K, about 96.99% of CH3OH production was from the H-abstractions by CH3O· from DME molecule, and about 1.32% from the Habstractions of CH3O· from CHO·. The H-atom abstraction of CH2OH· from CHO· and another CH2OH· were responsible for about 0.26% and 1.43% of CH3OH production at 1040 K, respectively. The catalytic pyrolysis and catalytic oxidation of DME on γ-Al2O3 catalyst were also investigated in the flow reactor over the temperature range of 300–840 K. The presence of a catalyst not only led a lower temperature for DME decomposition, but also decreased the production of CO and CH4. The emissions of C2H4 and C2H6 from DME catalytic oxidation were also much less than that from DME pyrolysis and catalytic pyrolysis. The CH3OH productions from DME catalytic pyrolysis and catalytic oxidation were both about 16 times than that from DME pyrolysis. A small amount of CH2O and a large amount of H2 were

Fig. 7. The yield of H2 from the catalytic pyrolysis and catalytic oxidation of DME in present study (full symbols), comparing with the previous studies [52,53] (open symbols). 7

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produced in DME catalytic pyrolysis and catalytic oxidation. The existence of catalyst and oxygen helped an abundant H2 producing from DME oxidation on γ-Al2O3 surface between 450 K and 840 K, which led to a highest H2 production of 61.67%. Therefore, H2 was the dominated final product both in DME catalytic pyrolysis and catalytic oxidation.

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