Noncatalytic Upgrading of Anisole in an Atmospheric DBD Plasma

Mar 5, 2014 - In this article, an atmospheric dielectric barrier discharge (DBD) plasma reactor was used as a novel tool for the upgrading of bio-oil ...
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Noncatalytic Upgrading of Anisole in an Atmospheric DBD Plasma Reactor: Effect of Carrier Gas Type, Voltage, and Frequency Hamed Taghvaei,† Mahsa Kheirollahivash,† Mohammad Ghasemi,†,§ Parisa Rostami,† and Mohammad Reza Rahimpour*,†,‡ †

Department of Chemical Engineering, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran Department of Chemical Engineering and Materials Sciences, University of California, Davis, California 95616, United States

‡

ABSTRACT: In this article, an atmospheric dielectric barrier discharge (DBD) plasma reactor was used as a novel tool for the upgrading of bio-oil using anisole as a model compound. The influences of different carrier gases (Ar, H2, and He) on the performance of the reactor were carefully studied. The results revealed that the conversion of anisole in He plasma is higher than that in Ar or H2 plasma. This may be attributed to the more stable and homogeneous discharge of He plasma. It is believed that in all of the experiments phenoxy radical was formed as the primary product of anisole dissociation via electron-attack reactions. Moreover, the most abundant product was phenol, which was formed by the free-radical reaction between phenoxy and H radicals. It was found that the upgrading of anisole involved demethylation, transalkylation, and hydrogenolysis reactions. In addition to phenol, 4-methylanisole, 2-methylphenol, benzene, 4-methylphenol, 2,6-dimethylanisole, and cyclohexane were also formed in the reactor. Furthermore, the effect of applied voltage and pulse frequency on the performance of He plasma were carefully investigated. As the voltage and frequency were increased, the quantity and quality of efficient collisions between active species and anisole molecules increased, resulting in an increase in anisole conversion and specific input energy of the discharge. The highest conversion of anisole was 72.7%, which was obtained in a He plasma at an applied energy of 9 kV and a pulse frequency of 20 kHz. Under these conditions, the average input power and specific input energy of the discharge were 71.2 W and 42.7 kJ/mL. The results imply that the DBD plasma reactor is a promising tool for the upgrading of anisole. produce lighter hydrocarbons and water.8 Accordingly, many studies have been carried out on catalytic HDO of compounds representative of bio-oil such as anisole,9−16 guaiacol,15−22 4methylanisole,16,23 and 4-methylphenol.24 The major goals of these studies are to introduce novel and efficient catalysts and also to investigate the chemistry of the upgrading process, as reviewed in a recent article by Saidi et al.8 Nonthermal plasma (NTP) processing is one of the most helpful technologies that can be used as an alternative to conventional and thermocatalytic processes of bio-oil upgrading. In NTP processing, electric power is implemented in order to break the chemical bonds of reactant molecules. The applied electric field transmits energy to the carrier gas molecules, which leads to the formation of a high flux of energetic electrons and active species. As a result of collisions between energetic electrons (or active species) and feed molecules, the chemical bonds can be broken and free radicals are formed.25,26 Because these intermediate radicals are chemically active, they tend to change into more stable components.27 Therefore, in an NTP reactor, the products are formed through decomposition of the reactants by both electron-attack and free-radical reactions. The dielectric barrier discharge (DBD) is one of the NTP generation devices that has attracted more attention because of its simple configuration compared to other devices.28 It can work at atmospheric pressure and has many applications in

1. INTRODUCTION The limitation of fossil fuels and the disastrous consequences of their consumption, including distribution of pollutants in addition to increases in greenhouse gases, particularly CO2, and consequently drastic changes of climate, altogether manifest the significance of using renewable energies.1−4 Lignocellulosic biomass is the most abundant source for the production of bioderived fuels and chemicals.5 This type of biomass consists of cellulose (40−50%), hemicellulose (25− 35%), and lignin (15−20%), with the composition depending on the origin.6 Lignin is markedly different in structure and composition from cellulose and hemicellulose, as it is highly aromatic and contains less oxygen. The relatively low oxygen content of lignin in comparison with cellulose and hemicellulose makes lignin a potentially attractive source.7 Lignin degradation processes such as fast pyrolysis produce a mixture of oxygen-containing compounds called bio-oil, which consists of a large number of compounds including phenol, anisole, guaiacol, cresol, and syringol. The crude bio-oil obtained from fast pyrolysis has a high oxygen content, typically more than 10 wt % and even as much as 45 wt %, which is responsible for chemical and thermal instability of the bio-oil and increases its tendency to polymerize. In addition, its high oxygen content, low heating value, high viscosity, and acidic nature render bio-oil essentially unusable for almost all of today’s fuel applications. Therefore, bio-oil needs to be upgraded before it can be used as a chemical feedstock and fuel. One of the most commonly investigated methods for upgrading of bio-oil is catalytic hydrodeoxygenation (HDO), which involves reactions with hydrogen that © 2014 American Chemical Society

Received: November 18, 2013 Revised: March 4, 2014 Published: March 5, 2014 2535

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treatment of biomaterials,29 functionalization and surface patterning,30 pollution and waste control,31 and hydrocarbon cracking.32−36 In our previous work, the upgrading of anisole was carried out in a catalytic DBD plasma reactor using various Al2O3-supported catalysts.37 The plasma technology was responsible for the progress of catalytic upgrading of anisole at room temperature. Moreover, deactivation of the catalysts was minimized by the plasma. In the present work, the noncatalytic upgrading of bio-oil using an atmospheric DBD plasma reactor was investigated at room temperature. Anisole (C6H5OCH3) was selected as a representative compound of lignin-derived bio-oil that incorporates a Caromatic−O−Cmethyl linkage.9,15 The major objectives of the present work were (1) to investigate the effect of different carrier gases (Ar, H2, and He) on the performance of the reactor and distribution of the products, (2) to identify the major products of plasma-chemical decomposition of anisole, and (3) to investigate the effects of applied voltage and pulse frequency on the performance of plasma He for anisole upgrading.

Table 1. Characteristics of the DBD Plasma Reactor inner electrode outer electrode dielectric gap distance annular cross-sectional area reactor length

stainless steel, 2.68 mm diameter brass sheet, 0.5 mm thickness quartz tube, 1.5 mm thickness, 10 mm i.d. 3.66 mm 0.729 cm2 20 cm

(Pearson 150, 2 A/V) were used to measure the applied voltage and current, respectively. Moreover, a high-speed oscilloscope (Tektronix TDS 1012B-SC, bandwidth = 100 MHz and time resolution = 1 GS/s) was used to record the voltage and current during the experiments. Details about the employed power supply and the electric behavior of the discharge have been presented in our previous works.32−36 Furthermore, the temperatures of the high-voltage (inner) and ground (outer) electrodes were measured using a radiative thermometer during the experiments. 2.2. Experimental Procedure. In each experimental run, the carrier gas (Ar, H2, or He with 99.99% purity) entered the reactor at ambient temperature (∼25 °C) and a constant flow rate of 100 mL/ min, which was controlled using an Alicat Scientific volumetric flow controller (MC-1 SLPM-D). The residence time of the carrier gas in the discharge zone of the reactor was 8.75 s. Anisole (99% purity, Merck, Darmstadt, Germany) was injected from the top of the DBD pulse reactor using an HPLC pump (SY-8100) at a constant flow rate of 0.1 mL/min. Before the start of the experiment, the carrier gas was flowed for 5 min in order to saturate the reactor chamber. After that, the gas flowing through the reactor was converted to plasma by application of the high voltage to the inner electrode. Then anisole was continuously introduced into the reactor and moved over the inner surface of the reactor. Once anisole entered the plasma zone, chemical reactions occurred through collisions between free electrons and anisole molecules, which transferred energy to anisole, contributing to the formation of ions, atoms, and free radicals. Recombination of free radicals led to the production of a range of liquid products, which left from the bottom of the reactor. After the steady-state condition was reached (3 min after entry of the anisole), the liquid product was taken from the sampling connection at the bottom of the reactor for further analysis. 2.3. Method of Analysis and Data Processing. A GC−MS (Shimadzu QP-5050, Kyoto, Japan) equipped with a SGE-BPX5 capillary column (30 m × 0.32 mm i.d., 0.25 μm film thickness) was used for qualitative analyses of liquid products. The carrier gas was helium (99.99%) at a flow rate of 50 mL/min. A temperature ramp method was developed, and components were recognized by matching the ion ratios with those in the Wiley mass spectral library. The liquid samples were mainly composed of unreacted anisole, phenol, 4methylanisole, 2-methylphenol, benzene, 4-methylphenol, and 2,6dimethylphenol. To determine the concentration of these components, a GC (Shimadzu 17A, Kyoto, Japan) equipped with an flame ionization detector and a SGE-BPX capillary column (30 m × 0.32 mm i.d., 0.25 μm film thickness) was used. The carrier gas was nitrogen (99.99%) at a flow rate of 20 mL/min. In addition to the mentioned components, 2-methylanisole, 2,5-dimethylphenol, 2,4dimethylphenol, 2,4,6-trimethylphenol, o-xylene, p-xylene, cyclohexane, methylcyclohexane, hexanal, decane, and methanol also existed in the liquid products, but because the amounts of these trace products were so small, they were not quantified. The conversion of anisole is defined as the molar concentration of consumed anisole divided by the total molar concentration of anisole fed (eq 1), and the selectivity of each component i in the liquid product (Si) was calculated from its concentration (analyzed by GC) using eq 2:

2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. In the present work, a continuous pulsed DBD plasma reactor was used for upgrading of anisole at room temperature and atmospheric pressure. A schematic diagram of the experimental system is presented in Figure 1. The coaxial cylindrical

Figure 1. Schematic diagram of the experimental setup. DBD plasma reactor was constructed from a stainless steel rod with a diameter of 2.68 mm as an inner electrode, which was centered in a tubular quartz glass (dielectric) with an outer diameter of 13 mm and a wall thickness of 1.5 mm. The outer electrode was a brass sheet with a length of 20 cm that was rolled around the outer surface of the quartz tube. The material and diameter of the inner electrode were chosen according to our previous experiences in the design and operation of this kind of reactor.36 A stainless steel rod with diameter of 2.68 mm has higher efficiency and lower energy consumption in comparison with other options.36 The reactor was oriented vertically with the gas and feed flow from top to bottom. The upgrading of anisole was carried out in the circular gap between the inner electrode and the dielectric (the discharge zone). The volume of the discharge zone was fixed in the present study. The detailed specifications of the DBD plasma reactor are summarized in Table 1. The simplified electrical circuit of the experimental setup is shown in Figure 1. As can be seen, the outer electrode was connected to the ground, while the inner electrode was connected to a high-voltage pulse generator, which produced pulses with voltages of up to 10 kV, rise and fall times of less than 80 ns, pulse widths of less than 50 ns, and an adjustable pulse repetition frequency of up to 20 kHz. A highvoltage probe (Tektronix P6015A, 1000:1) and a current probe

anisole conversion = 2536

C in − Cout × 100% C in

(1)

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moles of i produced × 100% moles of anisole converted

Table 3. Physicochemical Properties of Carrier Gases at Atmospheric Pressure39,41−43,48

(2)

where Cin and Cout are the concentrations of anisole in the feed and product, respectively. The specific input energy (SIE) of the discharge is defined as the ratio of the average plasma discharge power delivered to the reactor (Pavg) to the total feed flow rate of anisole (QA, here equal to 0.1 mL/min), as shown by eq 3: SIE (in kJ/mL) =

He Ar H2

(Pavg in W) × 60

dielectric constanta

ionization energy (eV)b

metastable energy level (eV)c

bond strength (eV)

0.15 0.18 0.5

24.6 15.8 15.2

19.82 11.55 11

− − 4.4

The ability of a substance to store electrical energy in an electric field. The energy required to remove an electron from a gaseous atom or molecule. cThe energy gap between the metastable state and the ground state. The metastable states of He, Ar, and H2 are He(23S1),42 Ar(3P2),41 and H2(2S1),43, respectively. a

(Q A in mL/min) × 1000

(3)

b

Pavg can be calculated using eq 4, n ⎛ V + Vi + 1 ⎞⎛ Ii + Ii + 1 ⎞ ⎟(t ⎟⎜ Pavg = f ∑ ⎜ i − ti) ⎝ ⎠⎝ 2 ⎠ i + 1 2 i=0

(4)

where f, Vi, and Ii are the pulse repetition frequency, instantaneous applied voltage, and current, respectively.

3. RESULTS AND DISCUSSION Here the influences of three important parameters, namely, the carrier gas type (Ar, H2, or He), the applied voltage, and the pulse frequency, on the conversion of anisole and distribution of products were studied. The experimental conditions under which the experiments were carried out are indicated in Table 2. It is important to note that each experiment was carried out two times and hence that each presented datum is the mean of two data points with a standard deviation of 4−10%. Table 2. Operating Conditions for Experimental Runs carrier gas type carrier gas flow rate (mL/min) feed flow rate (mL/min) applied voltage (kV) pulse frequency (kHz) temperature (°C) pressure (atm)

Ar, H2, He 100 0.1 6−9 5−20 25 1.0

Figure 2. Effect of different carrier gases on anisole conversion and SIE of discharge.

Ar and H2, He gives a higher anisole conversion. This is attributed to the physicochemical properties of the carrier gases presented in Table 3. In upgrading of anisole, the reactions take place by collisions of electrons and other active species with the anisole molecules in the discharge zone, and consequently, the stability and homogeneity of the discharge play key roles in performing reactions. Although the H2 plasma produces a high flux of H free radicals, because of the lower amount of dissociation of H2 compared with its metastable energy level, the more stable and homogeneous discharge of He contributes to the higher conversion of anisole.38 Figure 2 also shows the SIEs of discharge in the three carrier gases. In general, the energy consumption in monatomic gas plasma is lower than that in diatomic gas plasma.39 This can be explained by analysis of the electron collisions occurring in the process. If the energy of a given electron is less than the excitation energy of a monatomic molecule, the electron, which cannot excite the molecule, does not lose energy after collision because the collision is predominantly elastic. After that, the electron is accelerated in the electric field and hence gains more energy and can excite another molecule easily. However, as a result of a collision between an energetic electron and a diatomic molecule, the electron can lose energy as a result of the transfer of a fraction of its energy to many energy levels available in diatomic molecules, such as rotation, vibration, and dissociation, depending on the electron energy. Consequently, the energetic electron is deactivated and cannot gain energy effectively from electric field. Therefore, the energy of electrons in monatomic gases can be used more efficiently because there are no paths for energy loss to rotation, vibration, and dissociation.39 For this reason, the SIE of discharge in Ar

3.1. Effect of Carrier Gas Type. In a DBD plasma reactor, the type of carrier gas determines the effectiveness of metastable states and the stability of the discharge.38 Moreover, in a specific reactor configuration, the carrier gas identifies the gas breakdown voltage, which is defined as the minimum applied voltage to generate plasma when the electrical resistance of the gas between the electrodes becomes approximately zero. The breakdown voltages for pure Ar, H2, and He at atmospheric pressure were measured by increasing the applied voltage from a low value at which no discharge occurred until the discharge plasma was detected. The results demonstrated that the breakdown of Ar, H2, and He occurred at voltages of 4.2, 6.2, and 4.0 kV, respectively. As may be seen from the data presented in Table 3, the dielectric constant of H2 is relatively higher than those of Ar and He, and thus, a higher applied voltage is required for electrical breakdown of H2 compared with Ar and He in the same reactor configuration. The order of increasing breakdown voltage, He < Ar < H2, is consistent with the order of increasing dielectric strength of these gases. In order to examine the effect of different carrier gases on the performance capability of the DBD plasma reactor for upgrading of anisole, a number of experiments were carried out at a constant voltage of 9 kV and a pulse frequency of 20 kHz with Ar, He, or H2 as the carrier gas. The experimental results are shown in Figure 2. As may be seen, compared with 2537

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e− (or M) + C6H5OCH3 → C6H5OCH3* + e− (or M)

plasma (monatomic gas) is less than in H2 plasma (diatomic gas), as observed in Figure 2. However, the SIE of discharge in He plasma is a little bit more than in H2 plasma. This can be attributed to the high conversion of anisole in the presence of He. In He plasma, the stability of the discharge results in more successful collisions, transferring more energy to anisole molecules, and consequently, the consumption of energy increases. 3.2. Product Distribution and Reaction Pathway. In the present study, the range of applied voltages is higher than the breakdown voltages of the carrier gases at atmospheric pressure. This implies that electrical discharges occur, producing a great number of electrons in the discharge zone of the DBD plasma reactor. In general, the plasma discharge mechanism of DBD can be divided into four steps:40 (1) the application of an electric field; (2) the production of energetic electrons via ionization of carrier gas molecules in the discharge zone (i.e., initiation of the discharge channel); (3) the formation of metastable species through collisions of energetic electrons with atomic or molecular species; and (4) the occurrence of chemical reactions, including dissociation of metastable molecules and formation of products via free-radical reactions. The applied electric field leads to the formation of plasma, which produces a high flux of energetic electrons. These electrons impact the carrier gas molecules as explained in the following. In Ar plasma, the electrons collide with the Ar atoms (carrier gas), exciting them to the lowest metastable state, Ar(3P2) (denoted as Ar*), which has a metastable energy level of 11.55 eV.41

e− + Ar → Ar* + e−

(R4)

If the energy transferred to the anisole molecule is sufficient, the excited anisole molecule (C6H5OCH3*) can decompose via breaking of chemical bonds. Generally, in the DBD plasma reactor the mean energy of electrons is around 1−10 eV under an electric field of 0.1−100 kV/cm at atmospheric pressure. Since the dissociation energies of the chemical bonds in the anisole molecule are in the range of 2.7−5.4 eV (64−124 kcal/mol), the breaking of chemical bonds is possible. However, the weaker chemical bonds can be broken more easily than the stronger bonds. For this reason, the probability of breaking the weaker chemical bonds is much higher than the probability of breaking the stronger chemical bonds. Focusing on the strength of chemical bonds, as presented in Table 4, reveals that the C6H5O−CH3 bond is Table 4. Bond Dissociation Energies (De) in Anisole and Phenol

(R1)

(R2)

As H2 is a diatomic gas, the electron collision reaction with H2 may lead to dissociation of H2 into two H free radicals or excitation of H2. Because the dissociation energy of H2 (4.4 eV) is far less than the metastable energy level of the first excited state of H2 (11 eV),43 the electronic metastable states of H2 also dissociate into two H atoms, as shown here: e− + H 2 → H + H + e−

De (kcal/mol)

ref(s)

64 93 101.5 ≤101.5 ∼123.8 89 103 ≤103

45, 49 50 45, 49 51 52 53, 54 51 51

the weakest bond in the anisole molecule because of the resonance stabilization of the phenoxy radical (C6H5O),45 and it has a dissociation energy of 2.78 eV (64 kcal/mol). Therefore, the rate of bond breaking seems to be much higher for the C6H5O−CH3 bond than for other bonds, leading to dissociation of excited anisole molecules mainly to form phenoxy radicals. In the present study, phenol, 4-methylanisole, and 2methylphenol were detected as major products because of their relatively high selectivities in comparison with the minor products (i.e., benzene, 4-methylphenol, and 2,6-dimethylphenol). The distributions of major and minor products with different carrier gases are shown in Figure 3a,b, respectively. The experiments were carried out at a constant voltage of 9 kV and a pulse frequency of 20 kHz. As can be observed from the results, phenol was the most abundant product in all of the experiments, with selectivities of 49.7%, 44.4%, and 56.4% in Ar, H2, and He plasmas, respectively. The selectivity for phenol was much higher (nearly double) compared with the selectivity for 4-methylanisole in all of the experiments. For example, in He plasma the selectivities for phenol and 4-methylanisole were 56.4% and 23.1%, respectively. This clearly implies that the rate of breaking the C6H5O−CH3 bond is much higher than the rate of breaking the Caromatic−H bond. As may be seen from Figure 3b, the selectivity for benzene in H2 plasma is higher than those in Ar and He plasmas. This may be attributed to the relatively large amount of H free radicals in H2 plasma in comparison with other reactive radicals such as CH3, resulting in the formation of more benzene rather than methylphenols. In the DBD plasma reactor, the product molecules may be attacked by electrons (or M), forming the other subsequent products. For this reason, the products can be formed through

In He plasma, the major product of the electron collision reaction with He is its first electronic metastable state, He(23S1) (He*), which has a metastable energy level of 19.82 eV.42 e− + He → He* + e−

bond C6H5O−CH3 C6H5OCH2−H C6H5−OCH3 Caromatic−H in anisole Caromatic−Caromatic C6H5O−H C6H5−OH Caromatic−H in phenol

(R3)

Therefore, there are a great number of electrons, metastable species, and free radicals in the discharge zone of the DBD plasma reactor that may collide with anisole molecules, exciting them by energy transfer or reacting with them to form products. This high density of active reactants leads to more complex reaction mechanisms in the DBD reactor in comparison with those in a conventional reactor.44 Therefore, proposing a unique mechanism that can explain the reaction pathways in the DBD plasma reactor is very difficult. However, integration of the thermodynamic database of species with plasma discharge mechanisms helps us to understand the main reaction steps. As a result of collisions between anisole molecules (liquid phase) and electrons or temporarily excited collision partners (M), such as Ar*, He*, and H, the anisole molecules become excited to higher energy levels according to the following reaction: 2538

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The chemical structures of the major and minor products of anisole upgrading in the DBD plasma reactor are shown in Figure 4. From the chemical structures of the products, it can

Figure 4. Chemical structures of the major (red) and minor (blue) products of upgrading of anisole in the DBD plasma reactor.

be understood that three main reaction routes occur during the upgrading of anisole in the DBD plasma reactor: demethylation, hydrogenolysis, and transalkylation (methyl transfer). Since the C6H5O−CH3 bond is easier to break than the other bonds, demethylation of anisole is the primary reaction leading to formation of phenol, while a smaller amount of anisole is converted to 4-methylanisole via transalkylation. Further, benzene is formed via the hydrogenolysis of phenol or demethylation−hydrogenolysis of anisole. Anisole and phenol are converted to methylphenols (especially 2-methylphenol) via transalkylation. The appearance of a small amount of cyclohexane, especially in H2 plasma, indicates the hydrogenation of the aromatic ring in benzene. However, the hydrogenation of anisole and phenol to produce alicyclic hydrocarbons was negligible, as shown by the trace amounts of these products. 3.3. Effect of Applied Voltage. As discussed before, the conversion of anisole in He plasma is higher than that observed in Ar or H2 plasma (Figure 2). Therefore, to investigate the effect of applied voltage in the upgrading of anisole, a number of experiments were carried out in He plasma with applied voltages ranging from 6−9 kV at a constant pulse frequency of 20 kHz. The results are shown in Figure 5. As can be seen, the anisole conversion and SIE of discharge increase with increasing applied voltage. This behavior can be explained by analysis of the mechanism of the plasma-chemical process. When the applied voltage increases, the microdischarge current density increases, and stronger microdischarges are created in the reactor.46 This results in an increase in the amounts and energy levels of electrons and ions, leading to the accelerated formation of metastable active species via an increase in the number of efficient collisions. Consequently, more chemical bonds can be broken, which leads to an increase in conversion of anisole as well as improved process performance. In addition, with an increase in the applied voltage at constant pulse

Figure 3. Effect of different carrier gases on the distributions of (a) major products and (b) minor products.

several reaction routes, including dissociation of anisole or further electron-attack decomposition of other intermediate products (e.g., phenol). For instance, benzene can be formed by direct dissociation of anisole through breaking of the Caromatic−O bond or by breaking of the Caromatic−OH bond of phenol. In addition, chromatographic results revealed that some benzene aromatic rings react with free radicals of H, producing cyclohexane. The selectivity for cyclohexane in H2 plasma was higher than that in Ar or He plasma (1.2% in H2 plasma, 0.08% in Ar plasma, and 0.26% in He plasma). This could be attributed to the high density of H free radicals (formed via H2 dissociation) in H2 plasma compared with the small amount of H radicals in both He and Ar plasmas. Since there are a great number of electrons and active species in the discharge zone of the DBD plasma reactor, in addition to the major and minor products, some other trace products, including 2-methylanisole, methylcyclohexane, o-xylene, pxylene, trimethylphenol, 2,4-dimethylphenol, and 2,5-dimethylphenol, were formed by both electron-attack and free-radical reactions. The concentrations of these trace products were too small, so they were not analyzed quantitatively in this work. Moreover, only negligible amounts of linear hydrocarbons (e.g., decane and hexanal) were observed due to breaking of a limited number of aromatic rings. This result could be explained by the high strength of C−C bonds in aromatic rings compared with other C−H or C−O bonds (Table 4). Moreover, the free radicals generated in the discharge zone, such as CH3, CH2, CH, O, and H, may also react together to form gaseous products such as CH4, C2H2, C2H4, and H2 that leave the reactor with the effluent gas stream. 2539

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operating conditions such as the applied voltage. Figure 7a,b shows the distributions of the major and minor products,

Figure 5. Effects of applied voltage on the conversion of anisole and SIE of discharge in He plasma.

frequency, the average discharge power increases according to eq 4, leading to an increase in the SIE of discharge. The results demonstrate that increasing the applied voltage increased the anisole conversion, but because of the power generator limitations, further increases in the voltage were not applicable. Therefore, the highest anisole conversion was 72.7%, which was obtained at the highest applied voltage of 9 kV. The average discharge power and SIE were 71.2 W and 42.7 kJ/mL, respectively. It is worth noting that a fraction of the input power is always converted into heat, resulting in an increase in the temperature of both the high-voltage (inner) and ground (outer) electrodes. The variation of the temperatures of the high-voltage and ground electrodes, as measured using a radiative thermometer, with applied voltage is shown in Figure 6. As may be seen, the

Figure 7. Distributions of (a) major and (b) minor products as functions of applied voltage in He plasma.

respectively, in He plasma at different applied voltages. As may be seen from Figure 7a, the selectivity for phenol slightly increased as the applied voltage increased, while the selectivity for 2-methylphenol did not change significantly. Moreover, the selectivity for 4-methylanisole increased as the voltage changed from 6 to 7 kV; however, with a further increase in the voltage, it decreased as well. The selectivity for benzene increased from 3.7% to 6.5% when the applied voltage increased from 6 to 9 kV. This behavior may be attributed to the increase of microdischarges, leading to an increase in the number of efficient collisions, which resulted in decomposition of more phenol into benzene. Therefore, increasing the applied voltage is favorable for demethylation of anisole to phenol and hydrogenolysis of phenol to benzene. However, as can be seen from Figure 7, increasing the applied voltage has a complex effect on transalkylation of anisole and phenol to methylphenols. 3.4. Effect of Pulse Repetition Frequency. Figure 8 shows the effects of the pulse frequency on the conversion of anisole and SIE of discharge in He plasma at a constant applied voltage of 9 kV. As can be seen, increasing the pulse frequency resulted in an increase in both anisole conversion and SIE. In fact, the pulse frequency is defined as the number of pulses in a second, so increasing the pulse frequency means decreasing the ascent and descent times.47 Therefore, when the pulse frequency is increased, the ions and other active species produced in the former pulse discharge can promote the next

Figure 6. Temperature increases of the high-voltage and ground electrodes as functions of applied voltage in He plasma.

temperatures of the electrodes increased with increasing applied voltage. This implies that the amount of power loss increases with increasing voltage. However, because of the short contact time between the carrier gas and the high-voltage electrode (8.75 s), the temperature of the carrier gas approximately remained constant and was ∼28 °C at the exit of the reactor in all experiments. The distribution of products in the DBD plasma reactor depends on the quantity and quality of collisions between energetic electrons (or M) and reactants, which is influenced by 2540

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Figure 8. Effects of pulse frequency on the conversion of anisole and SIE of discharge in He plasma.

pulse discharge more effectively. This directly affects the number of efficient collisions between electrons (or M) and reactants, leading to an increase in the conversion of anisole. Moreover, an increase in pulse frequency at a constant voltage tends to increase both the average power, according to eq 4, and the SIE of discharge. It can be concluded that, as for applied voltage, increasing the pulse frequency improves the performance capability of the DBD plasma reactor for anisole upgrading. However, the limitations of the pulse generator prevented experiments at pulse frequencies higher than 20 kHz. Increasing the pulse frequency also increased the temperatures of both electrodes (as a result of increased power loss). When the pulse frequency increased from 5 to 20 kHz, the temperature of the high-voltage electrode increased from 91 to 195 °C, while the temperature of the ground electrode was affected less strongly, changing from 86 to 151 °C. The pulse frequency affects the microdischarges as well as the quality and quantity of active species, and hence, it can influence the plasma-chemical reactions. Therefore, the distribution of major and minor products changed with pulse frequency, as shown in Figure 9a,b, respectively. As may be observed, with an increase in the pulse frequency from 5 to 10 kHz, the selectivity for phenol decreased from 48.6% to 45.6%; however, with a further increase in the pulse frequency the selectivity for phenol increased, reaching 56.4% at 20 kHz. The variation trend of the selectivity for 4-methylanisole with pulse frequency is completely opposite to that for phenol. In other words, the selectivity for 4-methylanisole increased at first and then decreased after the frequency of 10 kHz. Furthermore, with an increase in the pulse frequency from 5 to 15 kHz, the selectivity for benzene increased from 4.1% to 6.5%. However, beyond the frequency of 15 kHz, increasing the pulse frequency had no significant influence on the selectivity for benzene. Moreover, the selectivity for 2-methylphenol remained almost constant. Therefore, it can be concluded that increasing the pulse frequency is more favorable for the demethylation of anisole to phenol than for the transalkylation of anisole to 4methylanisole and methylphenols. Moreover, the hydrogenolysis of phenol to benzene is improved by increasing the pulse frequency. To examine the effect of power consumption on the progress of anisole upgrading in He plasma, the variations of anisole conversion with power in two different cases are shown in Figure 10. In the first case, the power was increased from 30.2

Figure 9. Distributions of (a) major and (b) minor products as functions of pulse frequency in He plasma.

Figure 10. Effect of average input power on conversion of anisole. The power was increased as a result of (1) increasing the voltage from 6 to 9 kV at a constant frequency of 20 kHz or (2) increasing the frequency from 5 to 20 kHz at a constant voltage of 9 kV.

to 71.2 W as a result of increasing the voltage from 6 to 9 kV at a constant frequency of 20 kHz, while in the second case, the voltage was held constant at 9 kV and the power was increased from 20.1 to 71.2 W by increasing the frequency from 5 to 20 kHz. As can be seen, the conversion of anisole increased with increasing the power consumption. Moreover, increasing the power via increasing the voltage at first had a considerable effect on increasing the conversion of anisole, while further increasing the power had little effect. In the case of increasing 2541

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the frequency, the anisole conversion increased almost linearly as the power increased. The highest anisole conversion was 72.7%, which was obtained at a power of 71.2 W, equivalent to a voltage of 9 kV and a frequency of 20 kHz.

4. CONCLUSION An experimental investigation was conducted on the upgrading of anisole as a typical compound of lignin-derived bio-oil using an atmospheric DBD plasma reactor at room temperature. The influences of three different carrier gases (Ar, H2, and He) on the anisole conversion and distribution of products were carefully studied. The obtained conversion of anisole in He plasma was higher than that obtained in Ar or H2 plasma. This could be attributed to the relatively higher stability of discharge in the He plasma in comparison with the Ar or H2 plasma. The chromatographic results for the liquid products indicated that phenol, which formed via electron-attack dissociation of anisole, was the most abundant product of anisole upgrading in the DBD plasma reactor. It is believed that the relatively low dissociation energy of the chemical bond between the methyl group and oxygen in the anisole molecule corresponds to dissociation of anisole mainly to phenoxy radicals. Furthermore, it was found that the demethylation, transalkylation, and hydrogenolysis reactions are the main reactions performed in the discharge zone of the reactor. Moreover, the influences of applied voltage and pulse frequency on the performance of the He plasma for upgrading of anisole were investigated. It was found that increasing the power input by means of increasing the applied voltage or the frequency directly affects the quantity and quality of efficient collisions between active species and anisole molecules, resulting in increased anisole conversion and improved performance of the reactor. The highest anisole conversion, 72.7%, was obtained in He plasma at an applied voltage of 9 kV and a pulse frequency of 20 kHz. Under these conditions, the average power input and SIE of discharge were 71.2 W and 42.7 kJ/mL, respectively. The results imply that the characteristics of the DBD plasma reactor are favorable for upgrading of anisole.

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AUTHOR INFORMATION

Corresponding Author

*Tel.:+98-711-2303071. Fax: +98-711-6287294. E-mail: [email protected], [email protected]. Present Address

§ M.G.: Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York (SUNY), Buffalo, NY 14260, United States.

Notes

The authors declare no competing financial interest.

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