Improvement of Ethylene Epoxidation in a Parallel Plate Dielectric

Feb 14, 2014 - Department of Chemical Engineering, Thammasat University, ... Srinakharinwirot University, Innovative Learning Center, Sukhumvit Rd., 1...
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Improvement of Ethylene Epoxidation in a Parallel Plate Dielectric Barrier Discharge System by Ethylene/Oxygen Separate Feed and Ag Catalyst Thitiporn Suttikul,†,‡ Bunphot Paosombat,† Malee Santikunaporn,§ Malinee Leethochawalit,∥ and Sumaeth Chavadej*,†,⊥ †

The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12, Phyathai Road, Pathumwan, Bangkok 10330, Thailand ‡ Department of Chemical Process Engineering Technology, King Mongkut’s University of Technology North Bangkok, Tambon Nonglalok, Amphur Bankhai, Rayong 21120 Thailand § Department of Chemical Engineering, Thammasat University, Pathum Thani 12121, Thailand ∥ Srinakharinwirot University, Innovative Learning Center, Sukhumvit Rd., 10110 Bangkok, Thailand ⊥ Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University, Bangkok 10330, Thailand ABSTRACT: This work investigated ethylene epoxidation performance using a parallel plate dielectric barrier discharge (DBD) system with a separate ethylene/oxygen feed in order to produce oxygen active species prior to reaction with ethylene. The highest ethylene oxide (EO) selectivity of 73% was obtained when the combined catalytic−DBD system was operated at an ethylene feed position of 0.5, an O2/C2H4 feed molar ratio of 0.2:1, and an Ag loading of 10 wt %. The presence of a glass platesupported Ag catalyst provided considerably higher EO selectivity, twice as much as that of the DBD system alone. As compared to the DBD system with an Ag catalyst supported on glass beads, the glass plate dielectric barrier was found to be a more efficient support for ethylene epoxidation. This is because the glass plate system provided plasma that was more uniform than that provided by the glass bead system. treatment,7−9 environmental remediation,10 reforming processing,11−14 pollution emission control,15−18 and chemical reactions.19−21 Electrons are emitted from the surface electrodes when the electrodes receive sufficient electrical potential to overcome the potential barrier of the system. The generated electrons then directly collide with the gaseous components present in the plasma zone to create various active species, which initiate subsequential reactions. One of the main characteristics of nonthermal plasma is that the generated electrons in the plasma zone have a very high energy, corresponding to an extremely high temperature (approximately 104−105 K),22 while the bulk gas temperature is relatively low (close to ambient temperature). Consequently, the low-temperature plasma can be applied to various chemical reactions at ambient temperature and atmospheric pressure, leading to low energy consumption. The catalyst problems at high operating temperatures, i.e., catalyst deactivation, catalyst regeneration, and catalyst replacement, are eliminated when the plasma is combined with a catalyst. The electrical properties of the catalyst surface can be altered under an electric field in the plasma zone.23 Consequently, chemical reactions can take place at low operating temperatures. Many research works have reported that plasma-assisted catalyst systems result in the

1. INTRODUCTION Partial oxidation of ethylene to ethylene oxide, or ethylene epoxidation, has been of great interest globally. Ethylene oxide (C2H4O, EO) is used commercially as an intermediate or a feedstock in the production of various chemicals. For example, EO is mainly used in the production of ethylene glycol.1 EO can be polymerized to form polyethylene glycol or polyethylene oxide, which is very useful as a nontoxic and water-soluble polymer. Ethylene glycol is primarily used in the production of polyester polymers and is also commonly known for its use as an automotive coolant and antifreeze. EO is also important in the manufacture of surfactants by a process called ethoxylation. Additionally, it is used as a sterilizer for foodstuffs, solvents, antifreezes, adhesives, and sterilized medical supplies.2 Generally, EO is produced by catalytic processes using Ag catalysts, especially using a low surface area α-alumina support (Ag/ (LSA) α-Al2O3), providing reasonably high efficiency in terms of high selectivity. Many studies have reported that EO selectivity is significantly enhanced by applying Ag catalysts with promoters, such as Cs, Cu, and Re.3−6 In practice, the catalytic process requires a high temperature and pressure operation, leading to high energy consumption and catalyst deactivation. The deactivation of the catalysts is a result of coke deposits, the deposition of reactant materials on the catalyst surface, and the agglomeration of catalyst particles, resulting in a reduction of catalytic activity over time. Therefore, a new technique is required to solve these limitations. Nonthermal plasma, a kind of electric discharge, is a promising technique for several applications, such as biofilm © 2014 American Chemical Society

Received: Revised: Accepted: Published: 3778

August 14, 2013 February 5, 2014 February 14, 2014 February 14, 2014 dx.doi.org/10.1021/ie402659c | Ind. Eng. Chem. Res. 2014, 53, 3778−3786

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Figure 1. (a) Schematic of experimental setup of DBD plasma system for the ethylene epoxidation reaction and (b) the configuration of the parallel plate DBD reactor.

loading, on the activity of ethylene epoxidation, were examined. Finally, the ethylene epoxidation performances of three DBD systems, parallel DBD alone, parallel DBD with 20 wt % Ag/ glass beads, and parallel DBD with 10 wt %/ glass plate, were comparatively studied.

enhancement of reaction rates, selectivity, and yield, as well as energy utilization efficiency.23−26 In our previous work,27 ethylene epoxidation was investigated in a parallel plate dielectric barrier discharge (DBD) system with and without either Al2O3- or SiO2- supported Ag catalysts. The Al2O3 and SiO2 particles of 3−4 and 2 mm diameters, respectively, were used as catalyst supports. The use of Ag catalysts in the DBD system was found to significantly increase EO selectivity, as compared with the DBD system alone. Interestingly, the Ag catalyst on the glass bead support (SiO2) provided higher EO selectivity than that on the Al2O3 support because the glass beads had lower electrical resistivity, leading to more stable and uniform discharges. The results revealed that the electrical resistivity significantly affected the ethylene epoxidation activity in the combined DBD−Ag catalyst system. Therefore, it was hypothesized that the reduction of electrical resistivity, by replacing Ag catalysts on glass beads by Ag catalysts coated on a glass plate dielectric barrier, could improve ethylene epoxidation performance. The main objective of this work was to investigate the combined catalytic−parallel plate DBD system for ethylene epoxidation by using Ag catalysts coated on a glass plate dielectric barrier. The effects of various operating parameters, including ethylene feed position, O2/C2H4 molar ratio, and Ag

2. EXPERIMENT 2.1. Chemicals and Gases. Silver nitrate (AgNO3) with 99.9% purity, supplied by Italmar, was used as an Ag catalyst precursor. A 2 mm thick glass plate was employed as a dielectric barrier in the studied DBD reactor and as the catalyst support. In this work, polyoxyethylene octyl phenyl ether (Triton-x-100, t-Oct-C6H4-(OCH2CH2)xOH, x = 9−10), a nonionic surfactant, supplied by Sigma Aldrich, was used to improve the wetability of the AgNO3 solution on the glass plate. Helium (99.995% purity), 40% ethylene in helium (±1% uncertainty), and 97% oxygen in helium (±1% uncertainty) were used to produce a feed gas for this study. Ethylene oxide in helium (30%; ± 1% uncertainty) was used as a standard gas for EO analysis. All gases were supplied by Thai Industrial Gas Co., Ltd. 2.2. Catalyst Preparation Procedure. The incipient wetness impregnation technique was used to prepare all supported Ag catalysts. The glass plate, used as the dielectric 3779

dx.doi.org/10.1021/ie402659c | Ind. Eng. Chem. Res. 2014, 53, 3778−3786

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The operating pressure of the DBD system was fixed at 1 atm via a needle valve. The studied DBD system was first operated under base conditions: an O2/C2H4 feed molar ratio of 1/4, an electrode gap distance of 7 mm, an input frequency of 500 Hz, an applied voltage of 19 kV, and a total feed flow rate of 50 cm3/min.27 The O2, balanced with He, was fed through the whole plasma volume of the DBD reactor, while the C2H4 stream was separately injected at different C2H4 feed positions (0−1) to find the optimum C2H4 feed position to trade off the two effects of the residence time of C2H4 and the electron collision to C2H4 (Figure 1b). Next, the effect of the feed molar ratio of O2/C2H4 was investigated after obtaining the optimum C2H4 feed position. Finally, the influence of various Ag catalyst loadings on the glass plate on the ethylene epoxidation performance was investigated. The moisture in the effluent gas was removed using a water trap before the gas entered an online gas chromatograph (GC). For all studied conditions, a trace amount of yellowish liquid inside the DBD system, with lower than 3% carbon balance, was negligible. There was not any acetaldehyde or formaldehyde product detected by GC under the studied conditions. The description of the GC was given in previous work.27 At a steady state, the product gas was sampled every 20 min for GC analysis. The experimental data, from at least 3 runs with less than 5% error, were averaged to evaluate the process performance of the studied DBD system. The C2H4 and O2 conversions, product selectivities, including H2, CO, CO2, EO, CH4, C2H6, C2H2, and C3H8, and the EO yield were calculated using the equations described in our previous work.27

barrier and catalyst support surface, was cleaned with distilled water and acetone. The Ag catalyst precursor was prepared by mixing 0.05 mL of the Triton-x-100 solution with 3 mL of different AgNO3 aqueous solutions (0.58, 1.23, 1.95, and 2.77 g/mL to achieve various Ag loadings of 5, 10, 15, and 20 wt %, respectively). Next, the prepared AgNO3 solution was applied with a dropper to completely cover the surface of the cleaned glass plate. The AgNO3-loaded glass plates were then dried in air overnight at 110 °C and finally calcined at 500 °C for 5 h. 2.3. Catalyst Characterization Techniques. The elemental compositions of the glass beads and glass plate were measured by X-ray photoelectron spectroscopy (XPS, Kratos, AXIS Ultra DLD series). A monochromatic Al source was used as the X-ray source. The samples were analyzed under the analysis chamber pressure of about 4 × 10−7 Pa. All the energies are calibrated with contaminant carbon (C1s = 284.6 eV) as a reference. All prepared Ag catalysts before the reaction and the 10 wt % Ag catalyst were scraped from the glass plates for characterization. A BET surface area analyzer (Quantachrome, Autosorb I) was used to determine the specific surface area of all the prepared catalysts, using nitrogen adsorption at a liquid nitrogen temperature of −196 °C. The catalyst sample was outgassed under vacuum at 150 °C for 10 h to remove humidity and any volatile components adsorbed on the catalyst surface prior to analysis. The crystalline phases of the Ag catalysts were analyzed by X-ray diffraction (XRD) using a Rigaku RINT 2000 diffractometer equipped with a Ni filtered Cu Kα radiation source (λ = 1.542 Å) of 40 kV and 30 mV. The catalyst sample was scanned in the 2θ range from 5° to 90° in the continuous mode at a rate of 5°/min. The mean crystallite Ag size was calculated by the Scherrer equation,28 using the full line width at half-maximum of intensity and the 2θ value. The catalyst surface morphologies were investigated by a field emission scanning electron microscope (FE-SEM; JEOL 5200-2AE). The catalyst samples were coated with Pt to improve conductivity before analysis. 2.4. Experiment Setup and Reaction Activity Experiments. The experiment of the ethylene epoxidation reaction over the supported Ag catalysts in a parallel dielectric barrier discharge (DBD) reactor was carried out at ambient temperature (25−27 °C) and atmospheric pressure. The schematic of the experiment setup of the DBD system for the ethylene epoxidation reaction is shown in Figure 1a. The DBD reactor was made of an acrylic plate and consisted of two stainless steel plate electrodes. As shown in Figure 1b, the reactor was 1.5 cm thick, 5.5 cm wide, and 17.5 cm long for the inner dimensions and 3.9 cm thick, 9.5 cm wide, and 21.5 cm long for the outer dimensions. The gap distance between the two plate electrodes was 7 mm, and the thickness of the glass plate was 2 mm. There were five different C2H4 feed positions, as a length fraction along the electrode length, as indicated at 0, 0.2, 0.5, 0.75, and 1 (Figure 1b). The Ag catalyst coated on the glass plate was used as a dielectric barrier. A three-step power supply system was used to convert domestic AC power (200 V, 50 Hz) to highvoltage AC, and a power analyzer (Extech Instruments Corporation, 380801) was used to measure the low-side voltage, power, and current of the DBD system, as described in our previous work.27,29 The flow rates of ethylene (C2H4), oxygen (O2), and helium (He, as a balance gas) were controlled by electronic mass flow controllers before the reactants were fed into the parallel plate DBD system. Each reactant line had a 7 μm in-line filter before the mass flow controller in order to trap any foreign particles.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization Results. The surface composition of the glass beads and glass plates consisted of Si2p, Si2s, O1s, O2s, and a trace amount of C1s (Table 1). The element composition of the glass beads was not much different from the glass plates. Table 1. Percentage of Mass Concentration of Si2p, Si2s, O1s, O2s, and C1s Elements from Wide Scan Analyses for Glass Bead and Glass Plate type of catalyst support

Si2p

Si2s

O1s

O2s

C1s

glass bead glass plate

13.60 15.02

13.15 14.81

37.41 36.55

34.58 32.20

1.26 1.42

Table 2 shows the specific surface area of Ag particles supported on the glass plate at various Ag loadings. The specific surface area slightly increased from 0.85 to 5.27 m2/g with increasing Ag loads from 5 to 20 wt %, which agreed well with our previous work.27 The low specific surface area was hypothesized to be good for the ethylene epoxidation reaction.30 Table 2. Specific Surface Area and Mean Ag Crystallite Size of All Glass Plate-Supported Silver Catalysts catalyst/ support Ag/glass plate

3780

Ag loading (wt %)

specific surface area (m2/g)

mean Ag crystallite size (nm)

5

0.85

16.72

10 15 20

1.65 3.84 5.27

17.47 19.12 21.96

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The XRD patterns of the Ag catalysts on the glass plate support at different Ag loadings are shown in Figure 2. The

Figure 2. XRD patterns of glass plate-supported Ag catalysts at different Ag loadings: (a) 5 wt %, (b) 10 wt %, (c) 15 wt %, and (d) 20 wt %.

only dominant peaks of metallic Ag were found at 2θ of approximately 38°, 44°, 64°, and 77°, while there was no Ag2O peak, suggesting that the prepared Ag catalysts were mostly in metallic form. The XRD results showed that the Ag crystalline size increased in the range of 17−22 nm with increasing Ag loads from 5 to 20 wt % (Table 2). The SEM images of the glass plate supported Ag catalysts at different Ag loadings and the spent 10 wt % Ag catalyst are shown in Figure 3. At the 5 wt % Ag loading, the Ag surface looked very smooth with well-dispersed tiny particles (Figure 3a). When Ag loading increased to 10 wt %, the Ag particles became larger, displaying different sizes and irregular shapes. Beyond 10 wt % Ag loading, the number of Ag particles significantly increased with greater size differences (Figure 3b− d). As compared to the fresh catalyst (Figure 3b) and the spent catalyst (Figure 3e), the Ag particles were found to agglomerate to become larger; the average particle size increased from 0.6 μm before the reaction to 1.5 μm after the activity testing of the combined catalytic−DBD system. The increase in Ag particle size after the reaction may be responsible for the drop in the ethylene epoxidation activity after a long reaction time, as described below. 3.2. Possible Chemical Reactions. Possible chemical pathways that may occur in the combined Ag catalytic−DBD system are expressed (in which the system was operated at the feed separation mode in order to maximize electron activation on O2 molecules and to minimize electron activation on C2H4 molecules) as the following reaction equations. Active oxygen formation: O2 + 2e− → 2O + 2e−

Figure 3. SEM images of glass plate-supported Ag catalysts at different Ag loadings: (a) 5 wt %, (b) 10 wt %, (c) 15 wt %, (d) 20 wt % before the reaction, and (e) 10 wt % after the reaction at 36 h.

1 O2 → C2H4O 2 Partial and complete oxidation:

(7)

C2H4 +

C2H4O + 5O → 2H 2O + 2CO2

(9)

C2H4 + e → 2C + 4H + e

(10)

C2H4 + e → 2CH 2 + e

(11)

CH 2 + e → CH + H + e

(12)

CH + e → C + H + e

(13)

Coupling: 3CH 2 + 2H → C3H8

(14)

C2H4 + CH 2 + 2H → C3H8

(15)

CH + CH 2 + 3H → C2H6

(16)

Hydrogenation:

Ethylene epoxidation: (6)

(8)

Cracking:

(5)

C2H4 + O → C2H4O

C2H4O + 3O → 2H 2O + 2CO

C2H4 + H 2 → C2H6

(17)

2H → H 2

(18)

CH 2 + 2H → CH4

(19)

3.3. Reaction Activity Performance. 3.3.1. Effect of Ethylene Feed Position. The effect of the C2H4 feed position on the C2H4 and O2 conversions is shown in Figure 4a. The O2 conversion varied insignificantly with increasing ethylene feed position. However, the C2H4 conversion slightly increased with increasing C2H4 feed position from 0 to 0.5. At a C2H4 feed 3781

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Figure 4. (a) C2H4 and O2 conversions, (b) EO selectivity and EO yield, (c) other product selectivities, and (d) power consumption as a function of C2H4 feed position (an O2/C2H4 feed molar ratio of 0.25:1, an applied voltage of 19 kV, an input frequency of 500 Hz, and a total feed flow rate of 50 cm3/min).

Figure 5. (a) C2H4 and O2 conversions, (b) EO selectivity and EO yield, (c) other product selectivities, and (d) power consumption as a function of O2/C2H4 feed molar ratio (a C2H4 feed position of 0.5, an applied voltage of 19 kV, an input frequency of 500 Hz, and a total feed flow rate of 50 cm3/min).

and reached maximum levels at a C2H4 feed position of 0.5. Beyond the feed position of 0.5, they decreased with further increase in C2H4 feed position from 0.5 to 1. The results can be explained by the fact that the C2H4 feed position can directly affect both important process parameters, residence time of C2H4 molecules in the plasma zone and the opportunity of C2H4 molecules to be activated by the generated electrons in the plasma zone. As described in our previous study,31 the

position higher than 0.5, the C2H4 conversion tended to decrease, and it reached a minimum at a C2H4 feed position of 0.75. The O2 conversion was much higher than the C2H4 conversion. This is because the system was operated under an O2 lean condition. As shown in Figure 4b, an increase in the C2H4 feed position significantly affects both EO selectivity and yield. They increased with increasing C2H4 feed position from 0 to 0.5 3782

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Figure 6. (a) C2H4 and O2 conversions, (b) EO selectivity and EO yield, (c) other product selectivities, and (d) power consumption as a function of Ag loading on SiO2 support (a C2H4 feed position of 0.5, an O2/C2H4 feed molar ratio of 0.2:1, an applied voltage of 19 kV, an input frequency of 500 Hz, and a total feed flow rate of 50 cm3/min).

conditions) to minimize all undesirable reactions, especially partial and complete combustion reactions to produce CO and CO2. The other operating parameters were fixed at a C2H4 feed position of 0.5 (the optimum value obtained from the previous experiment), an applied voltage of 19 kV, an input frequency of 500 Hz, and a total feed flow rate of 50 cm3/min . As shown in Figure 5a, an increase in O2/C2H4 feed molar ratio slightly affects both the C2H4 and O2 conversions, and the highest conversions of both reactants are obtained at the highest O2/ C2H4 feed molar ratio of 0.5:1. The explanation is that under O2 lean conditions, an increase in the molar ratio of O2/C2H4 resulted in more O2 being available to react with C2H4 molecules, leading to increased C2H4 and O2 conversions. As shown in Figure 5b, both EO selectivity and yield increased with an increase in O2/C2H4 feed molar ratio from 0.17:1 and reached maximum values at the O2/C2H4 feed molar ratio of 0.2:1. However, they rapidly decreased with further increases in the O2/C2H4 feed molar ratio beyond 0.2:1. An increase in the O2/C2H4 feed molar ratio in the range of 0.17:1−0.2:1 also provided an enhancement of the H2 selectivity. After that, the H2 selectivity decreased dramatically with further increases in O2/C2H4 feed molar ratios higher than 0.2:1 (Figure 5c). The C3H8 and C2H6 selectivities tended to decrease moderately with increasing O2/C2H4 feed molar ratio; whereas the CH4 selectivity varied slightly in the entire O2/ C2H4 feed molar ratio range. The CO and CO2 selectivities increased substantially when the O2/C2H4 feed molar ratio increased throughout the studied range of O2/C2H4 feed molar ratio of 0.17:1−0.5:1. Under the studied conditions, the main products were EO and H2 with significant amounts of CH4, C3H8, and C2H6. The largest hydrocarbon of C3H8 was produced in a small fraction. The results reveal that both complete and partial oxidation reactions and ethylene epoxidation are mainly governed by the ratio of oxygen in the feed. An oxygen lean condition is an essential factor for ethylene epoxidation. The decreases in the selectivities for these hydrocarbons and H2 and the increases in the selectivities for

higher the C2H4 feed position, the lower the reaction rates of all chemical reactions including ethylene epoxidation as a result of lower electron activation of C2H4 molecules, leading to fewer undesirable chemical reactions. Hence, there must be an optimum C2H4 feed position to maximize the EO formation with fewer undesirable reactions. Under the studied conditions, the optimum C2H4 feed position of 0.5 provided the highest EO selectivity and yield with reasonably low undesirable reactions. The C2H6 and C3H8 selectivities tended to decrease slightly, while the CH4 selectivity tended to increase slightly with increasing C2H4 feed position (Figure 4c). An increase in the C2H4 feed position played a significant role in the decrease in H2 selectivity while increasing C2H4 feed position from 0 to 0.5, but the H2 selectivity increased with further increase in C2H4 feed position over 0.5. The CO and CO2 selectivities slightly fluctuated in the range of 0.4−0.7% and 0.5−0.6%, respectively. However, both CO and CO2 selectivities were very low as compared with other product selectivities. From the results as shown in Figure 4b,c, ethylene epoxidation is a considerably dominant reaction under the separate feed of C2H4 and O2, and the ethylene epoxidation reaction can be maximized at the C2H4 feed position of 0.5. Figure 4d shows the relation between power consumption and C2H4 feed position. The power consumption per molecule of converted C2H4 reached a minimum when the C2H4 feed position increased to 0.25, and then tended to increase with a further increase in C2H4 feed position. However, the power consumption per molecule of produced EO reached a minimum at a C2H4 feed position of 0.5. Hence, a C2H4 feed position of 0.5 was selected for further investigation because it provided the highest EO selectivity and yield with the lowest values of both H2 selectivity and power consumption per molecule of EO produced. 3.3.2. Effect of O2/C2H4 Feed Molar Ratio. To determine the influence of the O2/C2H4 feed molar ratio, the O2/C2H4 feed molar ratio was varied from 0.17:1 to 0.5:1 (O2-lean 3783

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Table 3. Comparison of the Plasma Systems on the Ethylene Epoxidation Performance conversion (%)

power consumption (Ws/ molecule)

plasma system (conditions)

C2H4

O2

EO selectivity (%)

parallel DBD alone (mixed feed, O2/C2H4 molar ratio of 1/1, a gap distance of 1 cm, an applied voltage of 19 kV, an input frequency of 500 Hz, and a total feed flow rate of 50 cm3/min) parallel DBD/20 wt % Ag/glass beads27 (mixed feed, O2/C2H4 molar ratio of 0.25/1, a gap distance of 0.7 cm, an applied voltage of 19 kV, an input frequency of 500 Hz, and a total feed flow rate of 50 cm3/min) parallel DBD/10 wt % Ag/glass plate (this work) (separate feed [C2H4 feed position of 0.5], O2/C2H4 molar ratio of 0.2/1, a gap distance of 0.7 cm, an applied voltage of 19 kV, an input frequency of 500 Hz, and a total feed flow rate of 50 cm3/min)

91.0

93.7

6.2

5.6

0.4 × 10−16

6.1 × 10−16

7.0

64.4

30.6

2.1

4.6 × 10−16

15.3 × 10−16

1.6

83.0

72.1

1.3

23.0 × 10−16

16.6 × 10−16

32

EO yield (%)

C2H4 converted

EO produced

dielectric barrier of the DBD system can enhance the ethylene epoxidation activity in terms of EO selectivity but has a negative effect on EO yield. The effect of Ag loading on the selectivities for other products is illustrated in Figure 6c. Interestingly, there was no CO2 production at all Ag loadings in the combined catalytic and DBD system. The CH4, C3H8, and C2H6 selectivities tended to increase with increasing Ag loadings from 0 to 10 wt %, and decreased with further increasing Ag loadings from 10 to 20 wt %. The presence of Ag catalyst seemed to have a negative effect on both CO and H2 selectivities, especially at the Ag loading of 5 wt %. However, at the highest Ag loading of 20 wt %, both CO and H2 selectivites were more or less the same as those of the DBD system alone. The results suggest that the presence of Ag catalyst loaded on the glass plate dielectric barrier exhibits the effects on both C2H4 conversion and EO selectivity more than the other process performance parameters. Figure 6d shows the power required to convert a C2H4 molecule and to produce an EO molecule at different Ag loadings. The power consumption per molecule of converted C2H4 substantially increased when the Ag loading increased and reached a maximum at the highest Ag loading of 20 wt %. However, the power consumption per molecule of produced EO slightly increased with increasing Ag loading from 5 to 10 wt %, and it sharply increased with further increase in Ag loading from 10 to 20 wt %. Therefore, the Ag loading of 10 wt % was an optimum value for the ethylene epoxidation reaction. For a catalyst stability study, the DBD system packed with the 10 wt % Ag catalyst was first operated about 3 h under the optimum conditions. After that, the system was rerun under the same conditions without any catalyst reactivation. As compared to the first-run results, the ethylene epoxidation activity significantly decreased for the second run in terms of the EO selectivity, from 72.1 to 69.7%, and the EO yield, from 1.3 to 0.8%. In addition, the C2H4 conversion slightly decreased from 1.6 to 1.2%, while O2 conversions also decreased from 83 to 74.3%. Interestingly, in a comparison between the two sequential runs, the C2H6 and C3H8 selectivities were found to increase from 23.5 to 28.1% and from 23.7 to 28.7%, respectively. The combined 10 wt % Ag catalyst−DBD system was also performed under optimum conditions for up to 36 h for testing the long-term stability of the system. The EO selectivity was found to decrease steadily from 69.75% at 3 h operation time to 36.3% at 36 h operation time. Apart from the agglomeration of Ag catalyst particles after the reaction test, as mentioned before (Figure 3e), the thin layer of liquid products as well as coke

CO and CO2 with increasing oxygen fractions in the feed clearly reveal that the oxidative dehydrogenation and coupling reactions occur unfavorably under O2-rich conditions, as expected. The power required to convert a C2H4 molecule and to produce an EO molecule at different O2/C2H4 feed molar ratios is shown in Figure 5d. The power consumption per molecule of produced EO reached a minimum at an O2/C2H4 molar ratio of 0.2:1, which corresponded well with the highest EO selectivity and yield. However, the power consumption per molecule of converted C2H4 remained almost unchanged in the O2/C2H4 feed molar ratio range of 0.2:1−0.25:1, but then slightly decreased with a further increase in O2/C2H4 feed molar ratio to 0.5:1, where the lowest EO selectivity was observed. The O2/C2H4 feed molar ratio of 0.2:1 was considered to be an optimum condition for ethylene epoxidation in terms of the highest EO selectivity and yield, as well as the lowest power consumption per molecule of produced EO. 3.3.3. Effect of Ag loading. To investigate the influence of Ag catalyst existence, Ag loading on the glass plate support was varied in the range of 5−20 wt % while the other operating parameters were fixed at the optimum conditions: a C2H4 feed position of 0.5, an O2/C2H4 feed molar ratio of 0.2:1, an applied voltage of 19 kV, an input frequency of 500 Hz, and a total feed flow rate of 50 cm3/min. As shown in Figure 6a, the C2H4 conversion decreased substantially with increasing Ag loadings from 0 to 5 wt %, whereas the O2 conversion slightly increases. Beyond the Ag loading of 5 wt %, both C2H4 and O2 conversions slightly decreased with further increases in Ag loading from 5 to 20 wt %. The results can be explained by the fact that an increase in Ag loading made the loaded Ag surface rougher (Figure 3), leading to less uniform plasma. As a result, both C2H4 and O2 conversions tended to decrease with increasing Ag loading. The EO selectivity and yield in relation to the Ag loading are shown in Figure 6b. The increased Ag loading greatly affected both EO selectivity and EO yield. The EO selectivity increased with increasing Ag loadings from 0 to 10 wt % and reached a maximum level at an Ag loading of 10 wt %. Beyond an Ag loading of 10 wt %, the EO selectivity decreased sharply with any further increasing Ag loading to 20 wt %. In contrast, the EO yield tended to decrease precipitously with increasing Ag loading when the Ag loading was in the 0−5 wt % range. The EO yield almost mirrored the C2H4 conversion, except at the Ag loading of 10 wt %, showing a slight increase in the EO yield as compared to the Ag loading of 5 wt %. These results imply that a proper amount of Ag catalyst loaded on the glass plate 3784

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deposit on the Ag catalyst surface might be other factors affecting the poor long-term stability of the Ag catalyst. 3.3.4. Performance Comparisons of the DBD System Alone and Combined Ag Catalyst−DBD Systems. Comparisons of the ethylene epoxidation performance of all DBD systems are shown in Table 3. The DBD system with unloaded Ag catalyst on glass beads unexpectedly provided an EO selectivity significantly higher than that with unloaded Ag catalyst on glass plate (10% vs 6%), as described in our previous works.27,32 The results can be explained by the fact that the system with unloaded Ag catalyst on glass beads was operated under an oxygen/ethylene molar ratio lower than that with unloaded Ag catalyst on glass plate (0.25:1 vs 1:1). As mentioned above, a leaner oxygen condition promotes the ethylene epoxidation with lower CO and CO2 formations. The Ag catalysts significantly improved the EO selectivity when they were applied in both the DBD systems (previous27 and present works). The DBD with the 10 wt % Ag/glass plate (the present work) exhibited better ethylene epoxidation performance in terms of considerably higher EO selectivity while EO yield was slightly lower, as compared to the DBD with 20 wt % Ag/glass beads. The explanation is that the glass beads with Ag catalysts caused the reduction of electrical resistivity of the DBD, leading to alterations in the plasma behavior (less uniformity); however, there was no electrical resistivity effect on the Ag catalyst loaded on the glass plate, which was used in this work. Therefore, the DBD system with the 10 wt % Ag/glass plate generated more stable and uniform discharges, resulting in the enhancement of all chemical reactions, including ethylene epoxidation. The separate C2H4/O2 feed was proven to reduce the possibility of all undesired reactions, including C2H4 cracking, dehydrogenation, oxidation, and coupling reactions, as compared to the mixed feed.31 This resulted in an increase in EO selectivity. However, the DBD system with the 10 wt % Ag/glass plate produced both lower C2H4 conversion and EO yield. This is because C2H4 molecules under the separate C2H4/O2 feed had lower residence time in the plasma zone, leading to less collision opportunities between the C2H4 molecules and high energetic electrons. Unfortunately, the DBD system with the 10 wt % Ag/glass plate required more power. It is important to note that no C2H2 was produced in the 10 wt % Ag/glass plate−DBD system and only a tiny trace of CO (0.2%) was produced. In addition, the DBD system with the 10 wt % Ag/glass plate produced a lower H2 selectivity of 31.0% (an undesired product), which was half (61.6%) that of the DBD system with 20 wt % Ag/glass bead−DBD system.27

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +66-2-218-4139. Fax: +66-2-218-4139. E-mail: sumaeth. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the postdoctoral research fellowship under the Ratchadaphiseksomphot Endowment Fund, Chulalongkorn University, Thailand; the Dudsadeepipat Scholarship from Chulalongkorn University, Thailand; and the Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University, Thailand.

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4. CONCLUSIONS Ethylene epoxidation was investigated in a parallel plate DBD system with a focus on the effects of C2H4 feed position, O2/ C2H4 feed molar ratio, and Ag loading. The optimum C2H4 feed position of 0.5 significantly enhanced EO selectivity and yield because of the reduction of cracking as well as complete and partial oxidation reactions. The highest EO selectivity of 72.1% was obtained when the combined catalytic−DBD system was operated at a C2H4 feed position of 0.5, an O2/C2H4 feed molar ratio of 0.2:1, and an Ag loading of 10 wt %. As compared to the EO selectivity range of 70−80% for commercial Ag catalysts, the studied DBD system proved to be a promising alternative for industrial applications. Further development is needed to increase both EO selectivity and yield. 3785

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