Oxidative coupling of methane in a corona discharge ...

Oxidative coupling of methane in a corona discharge plasma reactor using HY zeolite as a catalyst Saeed Delavari, Nor Aishah Saidina Amin & Hossein Mazaheri

Reaction Kinetics, Mechanisms and Catalysis ISSN 1878-5190 Reac Kinet Mech Cat DOI 10.1007/s11144-014-0741-z

1 23

Your article is protected by copyright and all rights are held exclusively by Akadémiai Kiadó, Budapest, Hungary. This e-offprint is for personal use only and shall not be selfarchived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”.

1 23

Author's personal copy Reac Kinet Mech Cat DOI 10.1007/s11144-014-0741-z

Oxidative coupling of methane in a corona discharge plasma reactor using HY zeolite as a catalyst Saeed Delavari • Nor Aishah Saidina Amin Hossein Mazaheri

•

Received: 1 February 2014 / Accepted: 28 June 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014

Abstract Oxidative coupling of methane in the presence of corona discharge plasma has been studied for the production of higher hydrocarbons under the conditions of ambient temperature and atmospheric pressure. The corona discharge was created by applying 3 kV (DC) between a tip and a plate electrode, 1.5 and 2.5 mm apart, in a tubular reactor. The effects of variables such as methane to oxygen ratio, total flow rate, electric current and more importantly, electrode gap distance were investigated. The electrode gap distance affected the electric field strength and subsequent plasma reactions. Five electrodes shaped like needles in the tip plate were applied to create more discharge intensity. At CH4/O2 ratio = 4, total flow rate = 4 mL/min, electric current = 10 mA and electrode gap distance = 1.5 mm, 31.9, 58 and 55 % of C2 yield, C2 selectivity and methane conversion, respectively were achieved. The main products were ethane, ethylene, acetylene, while CO and CO2 were also observed. The corona discharge interaction with HY-zeolite catalyst has led to low temperature methane conversion. Thus, the effect of surface modified zeolite catalyst was also examined. At a CH4/O2 ratio of 4 and total flow rate of 4 mL/min, 59.4, 69.7 and 85.3 %, of C2 yield, C2 selectivity and methane conversion, respectively were achieved. Experimental results revealed that corona discharge techniques, in the presence of HY-zeolite catalyst, has potentials for improving methane conversion, C2 selectivity and C2 yield. Keywords Corona discharge Plasma reactor Needle electrode NH4Y-zeolite Modified zeolites

S. Delavari N. A. S. Amin (&) H. Mazaheri Chemical Reaction Engineering Group (CREG), Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia e-mail: [email protected]

123

Author's personal copy Reac Kinet Mech Cat

Introduction Oil and natural gas are not only the main sources of energy, but also raw materials for a variety of hydrocarbon products. As the standards of living for the growing population rises, so is the demand for energy amid depleting fossil-fuel resources. Therefore, the search for renewable energy resources or suitable methods to decrease greenhouse gas emission and air pollution is inevitable. Despite methane being the main part of natural gas and an initial compound of chemical and petrochemical industries, it is also a dangerous greenhouse gas. Thus, at present, most of the research focus on converting methane to petrochemical intermediates and liquid fuels due to excess methane available [1–5]. Most research activities have focused on converting methane to fuels and chemicals (such as methanol) by indirect and direct routes. In the indirect reaction, methane is first converted to synthesis gas (mixture of CO and H2 gas), and subsequently converted to fuel products by steam reforming. It is a highly energy intensive process that represents the main cost in chemicals and fuel productions. However, methane is converted to synthesis gas in the presence of water vapor, and then methanol (as fuel and chemical product) is produced after hydrogenation of CO according to the following reactions: CH4 þ H2 O ! CO þ 3H2

ð1Þ

CO þ 2H2 ! CH3 OH

ð2Þ

In the direct conversion, methane is directly converted to hydrocarbons and chemicals by oxidative coupling of methane (OCM) in the chemical plasma process [6–10]. It is demonstrated that the plasma technology can create a large number of free radicals that play an important role in the oxidative and non-oxidative reactions. In fact, in the OCM reaction mechanism, the activation of C–H bonds occurred by homogenous and heterogeneous cleavage of bonds [11]. In the homogenous cleavage method, at the catalyst surface, the methane molecule combines with activated oxygen (O-, O2-) and it is converted to methyl radical. Then, methyl radical combines with similar radicals and it is converted to acetylene, ethylene, ethane, and heavier alkanes (such as, C3, C4, C5, C6) [12–14]. In the OCM reaction, methane combines with oxygen in the presence of catalysts and produces ethylene, ethane, COx, and water according to equations (3)–(5) [15]: 4CH4 þ 3=2O2 ! C2 H6 þ C2 H4 þ 3H2 O

ð3Þ

CH4 þ 2O2 ! CO2 þ 2H2 O

ð4Þ

CH4 þ 3=2O2 ! CO þ 2H2 O

ð5Þ

The main hindrance of developing the OCM process is low yield, poor selectivity of C2 products, and high processing temperature (higher than 700 °C). Today, electrochemical manufacturing methods such as plasma has been considered for direct chemical conversion. Lately, the catalytic plasma technique has attracted

123


more attention for converting the natural gas to fuel because natural gas is cheap, clean and abundantly available in many countries [16–19]. The reactions in the plasma field are complex and consists of a large number of elementary reactions of gas molecules such as excitation, dissociation and ionization. The main elementary reaction that takes place in the presence of plasma is listed in Table 1 [20–23]. Plasma at atmospheric pressure that is generated by negative and positive corona discharges has some advantages such as low investment costs and higher reaction rates. For generating and growing corona discharge, a high voltage must be applied [24–27]. In fact, the electrical discharge of the DC corona leads to electrons with 6 eV energy but this is not enough for methane ionization, as an ionization energy higher than 12 eV is required [28], because the ground state electronic structure of the methane molecule in Td symmetry is (1a1)2, (2a1)2 (inner valence), and (1t2)6 (outer valence). The methane ionization potentials of the valence electrons occur with the presence of incident photon energy range of 10–24 eV. Therefore, in this energy range, most of the ionization and superexcitation processes of the valence electrons are expected to contribute to the photoabsorption, photoionization and neutral dissociation cross sections [29]. As a result, this energy is sufficient to start the reactions to create oxygen negative ions. In the gas phase, corona discharge produces the bulk of negative oxygen ions (such as O-, O2-) by electron attacking on oxygen molecules. It seems that methane is activated by separating hydrogen, due to the effect of oxygen ions on methane according to the following reaction equations (6–17). The C2 products are produced in the homogenous reactions such as reaction between two methyl radicals and carbon oxides or reaction between methyl radicals and oxygen [30, 31]. CH4 þ e ! CH3 þ H þ e0

ð6Þ

CH3 þ CH3 ! C2 H6

ð7Þ

C2 H6 þ e ! C2 H5 þ H þ e0

ð8Þ

C2 H5 þ C2 H5 ! C4 H10

ð9Þ

C2 H5 þ e ! C2 H4 þ H þ e0

ð10Þ

CH3 þ e ! CH2 þ H þ e0

ð11Þ

CH3 þ H ! CH4

ð12Þ

C2 H5 þ H ! C2 H6

ð13Þ

C2 H5 þ CH3 ! C3 H8

ð14Þ

C4 H10 þ e ! C4 H9 þ H þ e0

ð15Þ

C2 H5 þ C4 H9 ! C6 H14

ð16Þ

C4 H9 þ CH3 ! C5 H12

ð17Þ

123

Author's personal copy Reac Kinet Mech Cat Table 1 Gas phase reactions involving electrons and heavy species Name

Reaction

Excitation of atoms or molecules

e ? A2 ? A*2 ? e e ? A ? A* ? e

De-excitation

e ? A*2 ? A2 ? e ? hv

Ionization

e ? A2 ? A? 2 ? e

Dissociation

e ? A2 ? 2A ? e

Dissociative attachment

e ? A2 ? A? ? A ? e

Dissociative ionization

e ? A2 ? A ? e

Volume recombination

e?A?B?A?B

Penning dissociation

M* ? A2 ? 2A ? M

Penning ionization

M* ? A ? A? ? M ? e

Charge exchange

A? ? B ? B? ? A

Recombination of ions

A- ? B? ? AB

Electron–ion recombination

e ? A? 2 ? M ? A2 ? M

Ion–ion recombination

A? ? B- ? M ? AB ? M

Such charged catalytic activity gained from the corona discharge interaction with a zeolite catalyst resulted in low temperature methane conversion. The gas discharges obviously modify the properties of the zeolite. As a matter of fact, the thermal catalytic OCM comprises both homogeneous and heterogeneous reactions. The methane activation at high temperatures by catalyst active sites led to formation of methyl radicals that respond homogeneously to form ethane. When a corona discharge occurs within or near a catalyst bed or catalyst layer, both heterogeneous and homogeneous reactions are influenced. Generally, the conversion of methane at low temperature over zeolites advances in two ways: one is the modification of supported metal properties; the other is the modification of the performance of support zeolite. Some studies have also presented mechanistic investigates [32–35]. The zeolites properties associated with their specific electronic structures are relevant to these investigations. ˚ in the zeolite The intensity of the natural Coulombic electric field reaches 1 V/A microporous structure that may result in a selectivity based on charge in zeolites. Probably, the catalytic properties of the zeolite are changed if it is charged electrically. Generally, it is assumed that proton conduction happens consistently with the polyatomic ion migration such as NH4? mechanisms. This conduction type is also seen in NH4Y-zeolites. Its applicable temperature range is more extensive and protonic conductivity is high. Consequently, NH4Y-zeolite can be potentially used in discharge process as a solid electrolyte [36]. Liu et al. [37] concentrated on the oxidative conversion of methane in atmospheric pressure in the corona discharge reactor with 30 Hz, 5 kV (rms) input power. They reported higher C2 yields and methane conversion at 21 and 43.3 %, respectively, while C2 selectivity increased to 48.3 % at a flow rate of 100 cm3/min. A small amount of acetylene was produced in the presence of zeolite catalysts in the cold plasma reactor. Horvath et al. [38] demonstrated that acetylene was the main product in the corona discharge plasma. Lu¨ et al. [31] investigated the conversion of methane to C2 hydrocarbons in the

123


non-equilibrium plasma technology at ambient temperature and pressure. In their work, methane conversion and C2 selectivity of hydrocarbons increased to 47 and 40 %, respectively at a voltage between 4 and 8 kV. Investigations on methane partial oxidation to aromatics over zeolites have been performed [32]. Further investigation of direct partial oxidation of methane over metal-containing ZSM-5 catalysts has also been reported. Wang et al. [39], Szo¨ke et al. [33] and Chen et al. [34] applied Mo/ZSM-5 or Mo/HZSM-5-based catalysts for enhancing the stability and activity for non oxidative conversion of methane to aromatic and C2 hydrocarbons [39]. Low temperature non oxidative activation of methane over H-galloaluminosilicate (MFI) zeolite was reported. The methane conversion of 45 %, by using alkenes as additives was reported [40]. The number of needles on the negative electrode and the electrode gap distance also affected the potential difference and strength or magnitude of the electric field. There have been no reports on the effect of electrode gap distance on methane conversion so far. Therefore, it is of considerable importance to fill this void by studying the effect of the gap distance on methane conversion and C2 selectivity in the corona discharge reactor. In addition, the effect of different parameters like methane to oxygen ratio, total flow rate, electric current on the C2 based selectivity as well as the yield of OCM reaction in a corona discharge plasma in the absence and presence of surface modified zeolite catalyst is also investigated.

Methods and experiments A simple and efficient method was used in this study to prepare the surface modified zeolite as catalyst. Surface modified zeolite was prepared from NaY-zeolite by room temperature chemical treatment with NH4Cl to produce NH4Y-zeolite. First, 20 g of NaY-zeolite was mixed with 200 mL 2 N NH4Cl for 24 h, then residual NH4Cl solution was removed by filtration and washing with distilled water. The procedures of modification and cleaning were repeated twice. Washing continued until low residual of NH4? was confirmed by trace amount from the reaction with AgNO3. The produced NH4Y-zeolite was collected and dried in the oven at 100 °C for 24 h, then calcined in a furnace at 450 °C and atmospheric pressure for 6 h. The Na? ion concentration was reduced from 8,300 to 560 ppm with 93.2 % Na? removal was achieved. By analysis of IR, XRD spectra and FESEM image, the crystalline phase of the produced zeolite as well as any structural changes were identified [41, 42]. The schematic diagram of the experimental setup are illustrated in Fig. 1. The corona reactor was made of a quartz tube, with 8 and 9.2 mm internal and external diameters and consisted of five needle-shaped electrodes fabricated from stainless steel. These electrodes were vertically positioned inside the reactor and connected to a DC power supply. The gap distance between the electrodes could be adjustable. The gaseous feed to the reactor was a mixture of oxygen and methane. The outlet gaseous products were analyzed online by a gas chromatograph (Carl 400 AGC) with methanizer FID detector. The reactor temperature was increased by exothermic heat and electrical discharge at atmospheric pressure.

123

Author's personal copy Reac Kinet Mech Cat Feed System

CV-1

Plasma Generation System Vent-1

3Way Valve-1

MFC-1

Reactor System Product Collection System 3Way Valve-2 Bypass

CV-2

CH4 O2

CV-3

Mixing Chamber

Vent-2

MFC-2

3Way Valve-3

Insulated Seal MFC-3

_

He

+

Resistor

GC

Tipped Electrode Corona Dischrge Catalyst Bed

DC Power Supply

Lower Plate Electrode DC – HVPS (High Voltage Power Supply)

Computer

Corona Reactor Condenser

Vent-3

3Way Valve-4

Voltage Regulator

Fig. 1 Schematic diagram of experimental setup

In the electrical circuit, some electrical changeable resistances (MX) were arranged with plasma in series for current adjustment as depicted in Fig. 1. By changing the electrical resistance, the currents between needles and plate electrodes will inadvertently be changed. It should be noted that the circuit voltage was about 3 kV (DC), while the voltage of plasma tips was 1.04 kV (DC). In this investigation, the needles and plate electrodes were connected to negative and positive potential respectively to create negative corona. The methane conversion, C2 products selectivity and yield were calculated by the following formulas:: CH4 conversionð%Þ ¼ ½ðmoles of CH4 consumedÞ=ðmoles of CH4 introducedÞ 100 ð18Þ Selectivity of C2 H6 ð%Þ ¼ ½2 ðmoles of C2 H6 producedÞ=ðmoles of CH4 consumedÞ 100 ð19Þ Selectivity of C2 H4 ð%Þ ¼ ½2 ðmoles of C2 H4 producedÞ=ðmoles of CH4 consumedÞ 100 ð20Þ Selectivity of C2 H2 ð%Þ ¼ ½2 ðmoles of C2 H2 producedÞ=ðmoles of CH4 consumedÞ 100 ð21Þ Yield of C2 hydrocarbons ð%Þ ¼ h i X ðSelectivities of C2 H6 ; C2 H4 ; C2 H2 Þ 100 CH4 conversion

123

ð22Þ


Atomic balance calculation for carbon and hydrogen was performed in each experiment.

Results and discussion IR spectroscopy of surface modified zeolite On the completion of the zeolite surface modification, IR-spectroscopy and XRD were conducted to confirm chemical structure and composition. A shoulder peak in IR spectra in the 900–1,000 cm-1 range occurs due to the elimination of Na? and substitution with other ions inside the skeletal structure of Si–O–Si or Si–O–Al [43]. Fig. 2 shows a shoulder peak in the IR spectrum at 908 cm-1 due to residual H? ions bound on the zeolite surface after calcination. This represents the character of a surface modified zeolite. The peaks from 1,300–500 cm-1 confirmed a typical zeolite spectrum with regular structure, suitable for the purpose of this study [44– 46]. XRD spectra and FESEM micrograph of surface modified zeolite The reduced peak heights in the calcined modified zeolite XRD spectrum compared to the NaY-zeolite XRD spectrum, shown in Fig. 3, confirmed that surface modification made the zeolite less crystalline. However, the peak positions for both original and modified zeolite are identical. These results demonstrated that the zeolite structure was not changed by NH4Cl treatment followed by calcination at 450 °C. Field emission scanning electron microscopy (FESEM) was used to determine morphology of the catalyst. Fig. 4a and b exhibit the FESEM images of NaY-zeolite and modified zeolite (HY-zeolite) structure. It was observed that the modified zeolite and NaY-zeolite possess the same uniformity in morphology of their particles indicating that the NH4Y-zeolite structure remained unchanged in surface modified zeolites as also observed in XRD analysis. However, the particle size

100

Fig. 2 IR-spectra of NaYzeolite and surface modified zeolite

90

T (%)

80 70 60 50 NaY-zeolite Surface modified zeolite

40 30 1400

1200

1000

800

600

400

Wavenumber (cm-1)

123

Author's personal copy

Intensity (a.u.)

Reac Kinet Mech Cat

Surface modified zoelite

NaY-zeolite

10

20

30

40

50

60

2-Theta Fig. 3 XRD-spectra of NaY-zeolite and surface modified zeolite

seems to be larger, it may indicate that the smaller zeolite crystals are agglomerated to the larger particles. Effect of total flow rate and electrode gap distance Methane conversion, C2 selectivity and C2 yield with various total flow rates at electrode gap distances of 1.5 and 2.5 mm are shown in Fig. 5. The C2 selectivity for 1.5 mm electrode gap distance was higher than that for 2.5 mm. Besides, C2 selectivity increased slightly in both cases as the total flow rate increased from 4 to 27 mL/min. The methane conversion in the case of 1.5 mm electrode gap distance decreased significantly compared with 2.5 mm when the total flow rate was gradually increased between 4 and 16 mL/min. Comparing the results in Fig. 5, the conversions for 1.5 and 2.5 mm electrode gap distance were found to decrease by nearly the same amount as the total flow rate increased from 16 to 27 mL/min. In addition, the result also showed that the C2 yield obtained from the OCM reaction with 2.5 mm electrode gap distance was less than with 1.5 mm electrode gap distance treatment. From Fig. 5, it can be seen that at both 1.5 and 2.5 mm electrode gap distance, there were many similar trends such as decreasing conversion and yield Comparatively, the selectivity for both cases was increased as the total flow rate increased from 4 to 16 mL/min after which it remained almost constant with further increment of total flow rate up to 27 mL/min. In addition, by increasing the electrode gap distance from 1.5 to 2.5 mm, the potential difference and strength or magnitude of electric field between the electrodes decreased. Thus, the conversion, yield, and C2 selectivity for the case of 2.5 mm electrode gap distance is less than the case of 1.5 mm electrode gap distance due to less free radical available.

123


Fig. 4 Field emission scanning electron microscopy (FESEM) images of a NaY-zeolite and b surface modified zeolite

Fig. 6 indicates that at any electrode gap distance, 1.5 and 2.5 mm, the ethylene and ethane selectivity increased proportionally with increasing total flow rate. On the contrary, the results show that acetylene selectivity decreased dramatically. In fact, the results presented that acetylene was produced in relatively high concentrations and less acetylene was observed as the total flow rate increased. It is clear from Fig. 6 that the acetylene selectivity decreased when the total flow rate increased from 4 to 25 mL/min regardless of the electrode gap distance. Furthermore, Fig. 6 reveals that the minimum acetylene selectivity occurred at the total flow rate of 25 mL/min and remained almost constant with a further increment of total flow rate from 25 to 27 mL/min. The acetylene production in the case of 2.5 mm electrode gap distance was less than the case of 1.5 mm electrode

123


80

80

60

60 Conversion (1.5 mm) Conversion (2.5 mm) C2 Yield (1.5 mm) C2 Yield (2.5 mm) C2 Selectivity (1.5 mm) C2 Selectivity (2.5 mm)

40

40

20

20

0

C2 Selectivity (%)

CH4 Conversion and C2 Yield (%)

Reac Kinet Mech Cat

0 0

5

10

15

20

25

30

35

Total Flow Rate (mL/min) Fig. 5 Effect of total flow rate and electrode gap distance on conversion, yield and C2 selectivity (electric current: 10 mA, CH4/O2 ratio: 4, voltage: 3 kV DC)

gap distance due to the lowering of electric field strength between the electrodes. It is apparent that the dehydrogenation of ethylene to acetylene required more energy than that of ethane to ethylene due to the relatively higher dissociation energy of CH2CH–H [47]. In addition, ethylene plays an intermediate role between hydrocarbon products, but the ethylene formation rate was more obvious in the lower electric field [37]. From the obtained data, it is seen that by increasing the residence time or decreasing the total flow rate, a moderate increase in CO is accompanied by a fall in the CO2 values, whereas the increase in the acetylene is accompanied by a drop in the ethane and ethylene values. These results may propose that the COx species is formed in parallel paths while the C2 hydrocarbon is formed in consecutive paths. CH4 or CHn or C þ O ! CO þ CO2

ð23Þ

CH4 ! CH3 ! C2 H6 ! C2 H4 ! C2 H2

ð24Þ

Effects of methane to oxygen ratio Effects of the CH4/O2 ratio (between 2.9 and 19) on methane conversion, yield, and C2 selectivity at the 1.5 mm electrode gap distance, and 4 mL/min as total flow rate is depicted in Fig. 7. Methane conversion increased significantly from 15 to 55 % with the increment of the CH4/O2 ratio from 2.9 to 4 while C2 yield increased from 8.1 to 31.9 %. It seems that with the increment of CH4/O2 ratio from 2.9 to 4 at a constant total flow rate, free radical production increased and led to higher C2 production. Methane conversion and C2 yield gradually decreased from 55 to 31.9 to 20 and 16 % as the CH4/O2 ratio increased from 4 to 10. This is because decreasing oxygen ion concentration due to increasing CH4/O2 ratio reduced the conversion of methane and also yield. As can be seen from Fig. 7, the methane conversion and yield were found to be nearly the same as the CH4/O2 ratio increased to 19. It is probably due to the increasing percentage of methane molecules in

123

Author's personal copy Reac Kinet Mech Cat 70

Fig. 6 Effect of total flow rate and electrode gap distance on C2 products selectivity (electric current: 10 mA, CH4/O2 ratio: 4, voltage: 3 kV DC)

Product Selectivity (%)

C 2H2 (1.5 mm)

60

C 2H2 (2.5 mm) C 2H4 (1.5 mm)

50

C 2H4 (2.5 mm) C 2H6 (1.5 mm)

40

C 2H6 (2.5 mm)

30 20 10 0 0

5

10

15

20

25

30

35

60

100

50

80

40

Conversion C2 Yield

30

C2 Selectivity

60 40

20

C2 Selectivity (%)

Conversion and C2 Yield (%)

Total Flow Rate (mL/min)

20 10

0 0

5

10

15

20

CH 4 / O 2 Ratio Fig. 7 Effect of CH4/O2 ratio on conversion, yield and C2 selectivity (electric current: 10 mA, total flow rate: 4 mL/min, electrode gap distance: 1.5 mm, voltage: 3 kV DC)

collisions and the energy of electrons for participating in radical chain reactions enriched. In the presence of excess oxygen, C2 products were converted to CO and CO2 which explained the low C2 product selectivity at lower ratio. In the OCM reactions of corona electric discharge, initially, C2 selectivity increased from 54 to 58 % when the CH4/O2 ratio increased from 2.9 to 4 and then increased significantly to 90 % as CH4/O2 ratio increased to 19, consistent with results of Larkin et al. [48, 49]. From the obtained data, it can be seen that at high oxygen concentrations, ethane was the main hydrocarbon product. At this point, the dehydrogenation of ethane was less since higher oxygen concentration led to the increase in CO concentration due to C2 oxidation. Ethane dehydrogenation was faster than other C2 products as the oxygen partial pressure decreased while ethylene and acetylene production were enhanced at CH4/O2 ratio of 4. As the CH4/O2 ratio increased from 4 to 10 (i.e. decreasing oxygen), COx production decreased due to the decline of oxidation activity. It has been demonstrated by Liu et al. [37], that the O-species generated, by

123


100

100

80

80

60

60

40

40 Conversion C 2 Yield

20

C2 Selectivity

CH 4 Conversion and C 2 Yield (%)

Reac Kinet Mech Cat

20

C 2 Selectivity

0 8

10

12

14

16

18

20

22

Electric Current (mA) Fig. 8 Effect of electric current on conversion, yield and C2 selectivity (total flow rate: 4 mL/min, CH4/ O2 ratio: 4, electrode gap distance: 1.5 mm, voltage: 3 kV DC)

interaction of O2 with electrons in the plasma field, play a role similar to corresponding surface oxygen anion to activation of methane. These species are formed during dissociative attachment of oxygen. Despite the continuous reduction of oxygen ion, at CH4/O2 ratio higher than 10, the higher percentage of methane molecules collided to free electrons and cooperated in the radical chain reaction. Under these conditions, in addition to the growing number of methyl radicals, acetylene production was increased due to dehydrogenation of ethane and ethylene. The other considerable point is that in the OCM reaction, the selectivity of CO2 production was less than CO selectivity since in the presence of hydrogen, the reverse water–gas shift reaction occurred as reported by Larkin et al. [49]. Effect of electric current The methane conversion, yield and C2 selectivity with various electric currents at the electrode gap distance 1.5 mm are shown in Fig. 8 at the different electric current between 10 and 20 mA, CH4/O2 ratio of 4, and total flow rate of 4 mL/min. The methane conversion, yield and C2 selectivity increased when the electric current increased from 10 to 18 mA and then the methane conversion and yield indicated a relative rise as the current went up to 20 mA. It seems that increasing the electric current in the presence of oxygen caused to incorporate more methane molecules directly in radical reactions. In fact, in the OCM reaction of corona discharge with increasing electric current, the radiated electron energy is increased, which probably increased the effective collision and active speices formation such as oxygen ions. Thus, methane conversion is increased due to increasing methyl radical concentration. From the obtained data, it can be seen the CO selectivity decreased while the electric current increased. It can be seen that C2H2 selectivity gradually increased although the C2H6 selectivity decreased slightly with an increment of electric current. This is because acetylene is produced by dehydrogenation of ethane, and

123

Author's personal copy Reac Kinet Mech Cat 100

Percent

80

C 2 Yield C 2 Selectivity Conversion

60

40

20

0

Plasma

Plasma + Catalyst

Fig. 9 Effect of surface modified zeolite catalyst on conversion, yield and C2 selectivity (total flow rate: 4 mL/min, CH4/O2 ratio: 4, electrode gap distance: 1.5 mm, voltage: 3 kV DC)

ethylene plays an intermediate product role. Effect of catalyst In order to investigate the effect of catalyst on the OCM in corona discharge reactor, the methane conversion, yield and C2 selectivity in the presence and absence of surface modified zeolite as catalyst were particularly investigated at 4 mL/min total flow rate, CH4/O2 ratio of 4 and 1.5 mm electrode gap distance at 3 kV (DC) applied voltage. As stated before, in this investigation, the needles and plate electrodes were connected to negative and positive potential, respectively, to create negative corona. The catalyst bed (28 mg of catalyst in powder form) is held on the lower plate electrode, and thus is between the electrodes. Fig. 9 reveals that when the surface modified zeolite was added to the OCM reaction in the corona discharge reactor, the methane conversion and yield can reach up to 85.3 and 59.4 %, respectively. In fact, the methane conversion and yield in the presence of modified zeolite as a catalyst were more than 1.5 and 1.9 times, respectively, to that of the OCM corona discharge reaction without catalyst. As shown in Fig. 9, there is a slow rise in C2 selectivity in the presence of modified zeolite catalyst. It is clear that the gaseous phase in the cold corona consisted of natural species, electrons, and existed ions due to high energy electron of corona electrical discharge. In fact, these natural species, electrons, and existed ions affected the catalyst particles and enhanced electrostatic potential variation of the catalyzed surface. The effects of these variations on the methane conversion depended on catalyzed surface concentration and existed species. The presence of surface modified zeolite as a catalyst in the corona discharge plasma reactor, improved the acetylene and carbon dioxide selectivity. The surface modified zeolite catalyst has inverse effects on the carbon monoxide, ethylene and ethane selectivity. It seems that the catalyst in the corona OCM reactions caused ethane and ethylene dehydrogenation to increase acetylene production. Table 2

123

123

Methane conversion (%)

85.3

43.3

36.5

39.3

52.6

54.9

38.9

53.0

47

Reactor

Corona plasma

Corona plasma

Microwave plasma

Microwave plasma

Microwave plasma

Microwave plasma

Microwave plasma

Microwave plasma

Cold plasma

40

91.8

90.9

89.7

88.7

78.9

87.1

48.3

69.7

C2 selectivity (%)

18.8

48.6

35.4

49.2

46.6

31

31.8

21

59.4

C2 yield (%)

4

7.5

7.5

7.5

7.5

7.5

7.5

5

3

Voltage (kV)

Table 2 Reported methane conversion, C2 selectivity and yield of OCM reaction in different discharge plasma reactors

No catalyst

Cu–ZSM-5

Ni–ZSM-5

Co–ZSM-5

Fe–ZSM-5

H–ZSM-5

No catalyst

No catalyst

Surface modified zeolite

Catalyst

[31]

[15]

[15]

[15]

[15]

[15]

[15]

[37]

This study

Reference



tabulates the methane conversion, C2 selectivity and C2 yield of OCM reaction in different discharge plasma reactors from previous studies. It can be seen when methane and oxygen were fed to the reactor at a ratio of CH4/O2 = 4 over the surface modified zeolite at 3 kV, the methane conversion, C2 selectivity and C2 yield were determined to be 85.3, 69.7 and 59.4 %. The methane conversion and C2 yield in the present study were much higher than in previous studies, while the applied voltage was lower than others.

Conclusions The use of corona discharge plasma has made possible the direct conversion of methane with the OCM reaction under the conditions of ambient temperature and atmospheric pressure. In this paper, we have tested the OCM reactions by applying 3 kV between a tip and a plate electrode to create corona discharge plasma. In this corona, five needle-shaped electrodes were applied in needle-plate to give more discharge intensity. The variation in C2 selectivity as well as methane conversion with total flow rate, methane to oxygen ratio, electric current and electrode gap distance in the absence and presence of surface modified zeolite (HY-zeolite) catalyst were explored. In the plasma, the OCM reaction at CH4/O2 ratio of 4 and total flow rate of 4 mL/min, the C2 selectivity can reach up to 58 % and the C2 yield can reach up to 31.9 %. Experimental results confirmed that by increasing the electrode gap distance from 1.5 to 2.5 mm, the potential difference and discharge intensity between the electrodes decreased. C2 selectivity increased slightly in both cases as the total flow rate increased, but methane conversion and yield changed in the converse way. The CO selectivity decreased slightly when the total flow rate increased. The C2 selectivity increased with increasing CH4/O2 ratio. Methane conversion, C2 yield and C2 selectivity increased when the electric current increased. The methane conversion and C2 yield in the presence of surface modified zeolite catalyst were 1.5 and 1.9 times more, respectively to that of the OCM corona discharge reaction without catalyst. Acknowledgments The authors would like to extend their deepest appreciation to the Ministry of Higher Education (MOHE), Malaysia and Universiti Teknologi Malaysia (UTM) for the financial support of this research under LRGS (Long-term Research Grant Scheme;) and RUG (Research University Grant). One of the authors (SD) gratefully acknowledged the financial support received in the form of a fellowship (Ref. No.: UTM.J10.02.00/13.14/1/125-073) from UTM.

References 1. Li M, Xu G, Tian Y, Chen L, Fu H (2004) Carbon dioxide reforming of methane using DC corona discharge plasma reaction. J Phys Chem A 108:1687–1693 2. Khassin A, Pietruszka B, Heintze M, Parmon V (2004) The impact of a dielectric barrier discharge on the catalytic oxidation of methane over Ni-containing catalyst. Reac Kinet Mech Cat Lett 82:131–137 3. Wan YJ, Fan X, Zhu TL (2011) Removal of low-concentration formaldehyde in air by DC corona discharge plasma. Chem Eng J 171:314–319 4. Janda M, Morvova M, Machala Z, Morva I (2008) Study of plasma induced chemistry by DC

123

Author's personal copy Reac Kinet Mech Cat discharges in CO2/N2/H2O mixtures above a water surface. Orig Life Evol Biosph 38:23–35 5. Song S-H, Park J, Chang T-S, Suh J-K (2013) Enhancing effects on the catalytic performance during the preparation of nickel supported lanthanum–alumina catalyst for the catalytic carbon dioxide dry reforming of methane. Reac Kinet Mech Cat 108:161–171 6. Li D, Li X, Bai M, Tao X, Shang S, Dai X et al (2009) CO2 reforming of CH4 by atmospheric pressure glow discharge plasma: a high conversion ability. Int J Hydrog Energy 34:308–313 7. Indarto A, Choi JW, Lee H, Song HK (2008) Decomposition of greenhouse gases by plasma. Environ Chem Lett 6:215–222 8. Ghose R, Hwang HT, Varma A (2014) Oxidative coupling of methane using catalysts synthesized by solution combustion method: catalyst optimization and kinetic studies. Appl Catal A Gen 472:39–46 9. Vandenbulcke L, de Persis S, Gries T, Delfau JL (2012) Molecular beam mass spectrometry and kinetic modelling of CH4–CO2–H2O plasmas for syngas production. J Taiwan Inst Chem Eng 43:724–729 10. Linga Reddy E, Biju VM, Subrahmanyam C (2012) Production of hydrogen and sulfur from hydrogen sulfide assisted by nonthermal plasma. Appl Energy 95:87–92 11. Benlounes O, Cheknoun S, Mansouri S, Rabia C, Hocine S (2011) Catalytic activation of C–H bonds of hydrocarbons by heteropolycompounds. J Taiwan Inst Chem Eng 42:132–137 12. Lee MR, Park MJ, Jeon W, Choi JW, Suh YW, Suh DJ (2011) PLS-based kinetics modeling and optimization of the oxidative coupling of methane over Na(2)WO(4)/Mn/SiO(2) catalyst. Korean J Chem Eng 28:2142–2147 13. Y-R Zhu, Z-H Li, Y-H Zhou, Lv J, Wang H-T (2005) Plasma treatment of Ni and Pt catalysts for partial oxidation of methane. Reac Kinet Mech Cat Lett 87:33–41 14. Yao S, Ouyang F, Nakayama A, Suzuki E, Okumoto M, Mizuno A (2000) Oxidative coupling and reforming of methane with carbon dioxide using a high-frequency pulsed plasma. Energy Fuels 14:910–914 15. Cho W, Baek Y, Moon S-K, Kim YC (2002) Oxidative coupling of methane with microwave and RF plasma catalytic reaction over transitional metals loaded on ZSM-5. Catal Today 74:207–223 16. Li L, Chen DR (2011) Performance study of a DC-corona-based particle charger for charge conditioning. J Aerosol Sci 42:87–99 17. Otto AJ, Reader HC (2011) HVDC corona space charge modeling and measurement. IEEE Trans Power Deliv 26:2630–2637 18. Liu C, Eliasson B, Xue B, Li Y, Wang Y (2001) Zeolite-enhanced plasma methane conversion directly to higher hydrocarbons using dielectric-barrier discharges. Reac Kinet Mech Cat Lett 74:71–77 19. Indarto A (2008) Hydrogen production from methane in a dielectric barrier discharge using oxide zinc and chromium as catalyst. J Chin Inst Chem Eng 39:23–28 20. Song HK, Lee H, Choi JW, Na B (2004) Effect of electrical pulse forms on the CO2 reforming of methane using atmospheric dielectric barrier discharge. Plasma Chem Plasma Process 24:57–72 21. Wang F, Tang X, Yi H, Li K, Wang J, Wang C (2014) NO removal in the process of adsorption nonthermal plasma catalytic decomposition. RSC Adv 4:8502–8509 22. Li M-W, Xu G-H, Tian Y-L, Chen L, Fu H-F (2004) Carbon Dioxide reforming of methane using DC corona discharge plasma reaction. J Phys Chem A 108:1687–1693 23. Xi-Zhen L, Wang J-G, Liu C, He F, Eliasson B (2003) Partial oxidation of methane to syngas over Ni–Fe/Al2O3 catalyst with plasma enhanced activity. Reac Kinet Mech Cat Lett 79:69–76 24. Dedov AG, Makhlin VA, Podlesnaya MV, Zyskin AG, Loktev AS, Tyunjaev AA, Nipan GD, Koltsova TN, Ketsko VA, Kartasheva MN et al (2010) Kinetics, mathematical modeling, and optimization of the oxidative coupling of methane over a LiMnW/SiO(2) catalyst. Theor Found Chem Eng 44:1–11 25. Yu Q, Liu T, Wang H, Xiao L, Chen M, Jiang X, Zheng X (2012) Cold plasma-assisted selective catalytic reduction of NO over B2O3/c-Al2O3. Chin J Catal 33:783–789 26. Su J, Zhou J, Liu C, Wang X, Guo H (2010) Gas phase epoxidation of propylene with TS-1 and in situ H2O2 produced by a H2/O2 plasma. Chin J Catal 31:1195–1199 27. Branco JB, Ferreira AC, do Rego AMB, Ferraria AM, Lopes G, Gasche TA (2014) Oxidative coupling of methane over KCl–LnCl3 eutectic molten salt catalysts. J Mol Liq 191:100–106 28. Lee H, Lee CH, Choi JW, Song HK (2007) The effect of the electric pulse polarity on CO2 reforming of CH4 using dielectric barrier discharge. Energy Fuels 21:23–29 29. Kameta K, Kouchi N, Ukai M, Hatano Y (2002) Photoabsorption, photoionization, and neutraldissociation cross sections of simple hydrocarbons in the vacuum ultraviolet range. J Electron

123

Author's personal copy Reac Kinet Mech Cat Spectrosc Relat Phenom 123:225–238 30. Wang Y, Liu C, Zhang YP (2005) Plasma methane conversion in the presence of dimethyl ether using dielectric-barrier discharge. Energy Fuels 19:877–881 31. Lu¨ J, Li Z (2010) Conversion of natural gas to C2 hydrocarbons via cold plasma technology. J Nat Gas Chem 19:375–379 32. Xu Y, Liu S, Guo X, Wang L, Xie M (1994) Methane activation without using oxidants over Mo/ HZSM-5 zeolite catalysts. Catal Lett 30:135–149 33. Szo¨ke A, Solymosi F (1996) Selective oxidation of methane to benzene over K2MoO4/ZSM-5 catalysts. Appl Catal A Gen 142:361–374 34. Chen L, Lin L, Xu Z, Zhang T, Li X (1996) Promotional effect of Pt on non-oxidative methane transformation over Mo-HZSM-5 catalyst. Catal Lett 39:169–172 35. Liu C, Eliasson B, Xue B, Li Y, Wang Y (2001) Zeolite-enhanced plasma methane conversion directly to higher hydrocarbons using dielectric-barrier discharges. Reac Kinet Mech Cat Lett 74:71–77 36. Liu CJ, Mallinson R, Lobban L (1999) Comparative investigations on plasma catalytic methane conversion to higher hydrocarbons over zeolites. Appl Catal A Gen 178:17–27 37. Liu C, Marafee A, Hill B, Xu G, Mallinson R, Lobban L (1996) Oxidative coupling of methane with ac and dc corona discharges. Ind Eng Chem Res 35:3295–3301 38. Horvath G, Zahoran M, Mason NJ, Matejcik S (2011) Methane decomposition leading to deposit formation in a DC positive CH4–N2 corona discharge. Plasma Chem Plasma Process 31:327–335 39. Wang L, Xu Y, Wong S-T, Cui W, Guo X (1997) Activity and stability enhancement of MoHZSM-5based catalysts for methane non-oxidative transformation to aromatics and C2 hydrocarbons: effect of additives and pretreatment conditions. Appl Catal A Gen 152:173–182 40. Guczi L, Sarma KV, Borko´ L (1996) Non-oxidative methane coupling over Co–Pt/NaY bimetallic catalysts. Catal Lett 39:43–47 41. Yilmaz B, Trukhan N, Mu¨ller U (2012) Industrial outlook on zeolites and metal organic frameworks. Chin J Catal 33:3–10 42. Yoon S-J, Lee YH, Cho W-J, Koh I-O, Yoon M (2007) Synthesis of TiO2 entrapped EFAL-removed Y-zeolites: novel photocatalyst for decomposition of 2-methylisoborneol. Catal Commun 8:1851–1856 43. Min G-H, Yim T, Lee HY, Huh DH, Lee E, Mun J, Oh SM, Kim YG (2006) Synthesis and properties of ionic liquids: imidazolium tetrafluoroborates with unsaturated side chains. J Korean Chem Soc 27:847–852 44. Kerrya´ Thomas J (1993) Preparation, characterization and photoreactivity of titanium(IV) oxide encapsulated in zeolites. J Chem Soc Faraday Trans 89:1861–1865 45. Cho WJ, Seo Y, Jung SJ, Lee WG, Kim BC, Mathieson G et al (2013) Removal of Na from ionic liquids by zeolite for high quality electrolyte manufacture. Bull Korean Chem Soc 34:1693–1697 46. Jwa E, Lee S, Lee H, Mok Y (2013) Plasma-assisted catalytic methanation of CO and CO2 over Ni– zeolite catalysts. Fuel Process Technol 108:89–93 47. Zhu A, Gong W, Zhang X, Zhang B (2000) Coupling of methane under pulse corona plasma (I)—in the absence of oxygen. Sci China Ser B Chem 43:208–214 48. Larkin DW, Lobban LL, Mallinson RG (2001) Production of organic oxygenates in the partial oxidation of methane in a silent electric discharge reactor. Ind Eng Chem Res 40:1594–1601 49. Larkin DW, Zhou L, Lobban LL, Mallinson RG (2001) Product selectivity control and organic oxygenate pathways from partial oxidation of methane in a silent electric discharge reactor. Ind Eng Chem Res 40:5496–5506

123