Upgrading of Anisole in a Dielectric Barrier ... - ACS Publications

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Upgrading of Anisole in a Dielectric Barrier Discharge Plasma Reactor Hamed Taghvaei,† Mahsa Kheirollahivash,† Mohammad Ghasemi,†,# Parisa Rostami,† Bruce C. Gates,‡ 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 Science, University of California, Davis, California 95616, United States

‡

ABSTRACT: A dielectric barrier discharge (DBD) plasma reactor for upgrading of anisole, a model compound representative of lignin-derived bio-oils, was investigated with helium as a carrier gas. The effects of carrier gas flow rate, liquid anisole feed flow rate, and reactor length on the reactor performance were investigated. As a result of the decomposition of anisole, the most prevalent free radical species that formed is inferred to have been phenoxy, resulting from the breaking of the Cmethyl−O bond. The residence times of reactive species and feed molecules are inferred to be key parameters affecting the conversion of anisole as well as the distribution of products. The three main classes of reaction of anisole were demethylation to give phenol, transalkylation, yielding 4-methylanisole and methylphenols, and hydrogenolysis of phenol to give benzene. The optimal experimental conditions were the carrier gas flow rate of 100 mL/min, the feed flow rate of 0.1 mL/min, and the outer electrode length of 20 cm; the anisole conversion and input power under these conditions were 72.7% and 71.2 W, respectively. cracking and waste treatment.27−29 A plasma is a partially ionized gas that contains reactive components (e.g., ions and electrons) that induce numerous reactions under mild conditions.29 Plasmas are classified as thermal and nonthermal plasmas. In a thermal plasma, the electric energy leads to increases in temperature of gas-phase reactants up to approximately 2000 K. In processes involving such plasmas, the charged and neutral species in reaction zones are nearly in thermal equilibrium.30 In contrast, in nonthermal plasmas (nonequilibrium plasmas), the electric energy results in ionization of carrier gas molecules to produce electrons, so that the temperature of the carrier gas and reactants is near room temperature, but the temperature of the electrons can be increased to a range of about 5 × 103−2 × 105 K, depending on the conditions.31 Among the several types of nonthermal plasma generation devices, such as those involving glow discharge or corona discharge, dielectric barrier discharge (DBD) devices have drawn much attention because of their simple configurations.32 Plasma cracking for the production of hydrogen and light hydrocarbons in DBD plasma reactors has therefore attracted attention, and excellent yields have been reported for shortcontact-time reactors operated at room temperature.33−40 In DBD plasma reactors, cracking occurs when excited electrons, which have energies in the range of 1−10 eV, transfer their energy through collisions with reactant molecules and break C−C and C−H bonds, which have dissociation energies of typically 3−4 eV.33−35 These collisions lead to the generation of exited radicals which recombine to produce light hydrocarbons. Nonthermal plasma processing can in prospect be used as an alternative to thermal and catalytic treatment of

1. INTRODUCTION The limits of conventional energy resources combined with the environmental problems they cause have encouraged researchers to focus on alternative energy resources1−3 that are both sustainable and environmentally friendly. Lignocellulosic biomass is thus a promising energy resource for the production of fuels and useful natural polymers.4 Lignocellulosic biomass can be converted into bio-oils by thermochemical methods such as pyrolysis, gasification, and liquefaction.5 However, the resultant crude bio-oils, especially those obtained in fastpyrolysis processes, are characterized by undesirable properties attributed in part to their high oxygen contents (10−45 wt %), including thermal instability, high viscosity, corrosiveness, low heating value, and immiscibility with petroleum-derived fuels.5 Therefore, bio-oils require upgrading before they can be used as transportation fuels. Most of the proposed upgrading routes involve catalysis, with reactions including hydrodeoxygenation, cracking, and steam reforming, and this topic has been well reviewed.6−12 Because of the complex nature of bio-oils, which are typically composed of hundreds of compounds, many researchers have chosen to work with prototypical bio-oil compounds to begin to unravel the chemistry of processes.13 Accordingly, many investigations have been focused on the catalytic upgrading of anisole,14−21 guaiacol,20−25 and 4-methylanisole21,26 as compounds representative of lignin-derived bio-oils. Although some catalysts give excellent yields and high selectivities to desired products (such as hydrocarbons), the reported catalysts seem not to be optimal, and the catalytic upgrading of bio-oils as practiced today requires high temperature and pressures, and catalyst deactivation is a major issue.12 Therefore, research continues to be directed at the discovery of better catalysts and at alternatives that do not involve catalysis. Plasma processing is one of these alternatives, having drawn attention for potential applications including hydrocarbon © 2014 American Chemical Society

Received: March 6, 2014 Revised: June 2, 2014 Published: June 2, 2014 4545

dx.doi.org/10.1021/ef500529r | Energy Fuels 2014, 28, 4545−4553

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biomass and bio-oils in processes such as pyrolysis, gasification, and upgrading to remove tars and other unwanted compounds.30,41−43 In previous work, we reported the upgrading of anisole by using a combination of plasma technology and catalysis (the catalysts were metals supported on Al2O3).44 The nonthermal plasma accelerated the anisole upgrading at room temperature and mitigated the catalyst deactivation. Extending this work, we investigated the effects of carrier gases (argon, hydrogen, and helium), voltage, and frequency in noncatalytic upgrading of anisole in a DBD plasma reactor.45 The results demonstrated that helium as a carrier gas resulted in relatively high conversions of anisole because the discharge was more stable and homogeneous than observed with argon and hydrogen. In the work reported here, we have investigated the effects of operating and design parameters such as carrier gas flow rate, feed flow rate, and reactor length on the performance of a DBD plasma reactor. Helium was used as the carrier gas45 and anisole as the prototypical reactant characteristic of the products of lignin pyrolysis.5,20

Table 1. Properties of DBD Plasma Reactor component

properties

inner electrode outer electrode dielectric gap annulus

stainless steel, 2.68 mm in diameter brass, 0.5 mm in thickness, variable in length quartz tube, 1.5 mm in thickness, 10 mm in inside diameter 3.66 mm 0.729 cm2 in cross sectional area

repetition frequency up to 20 kHz. The applied voltage and current were measured with a high-voltage probe (Tektronix, P6015A, 1000:1) and a current probe (Pearson 150, 2A/V), respectively. 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. As shown in Figure 2, during the rise of an applied pulse, the initial discharge began, and the secondary discharge started during the falling

2. EXPERIMENTAL SECTION 2.1. Experimental Equipment and Procedures. The apparatus, shown schematically in Figure 1, consists of a DBD plasma reactor, a

Figure 2. Typical voltage and current oscillograms at applied voltage of 9 kV, 50 ns per division. phase of the applied pulse. The amplitude of the discharge current rose to a value of the order of amperes. Wave changes resulted from variations in stray current, leakage inductance, and load impedance. Details of the power supply and electric behavior of the discharge are presented elsewhere.33−37 In a typical experiment, the carrier gas (helium with purity 99.99%) entered the reactor at a flow rate in the range of 100−900 mL/min, regulated with an Alicat Scientific volumetric flow controller (MC-1 SLPM-D). Anisole, with a purity 99% (Merck, Darmstadt, Germany), was introduced into the top of the reactor with an HPLC pump (SY8100) at room temperature. The feed flow rates varied over the range of 0.1−0.9 mL/min. After entering the reactor, the liquid anisole feed flowed over the inner surface of the reactor as a thin film, and the reactions proceeded in the discharge zone as a result of collisions between electrons and other exited elements (gas phase) with anisole molecules in the falling liquid film. After attainment of steady-state (3 min after the start of feed flow), liquid product for analysis was collected from the sampling port at the base of the reactor. 2.2. Product Analysis and Data Processing. To identify the components in the liquid product, the samples were analyzed with a GC−MS instrument (Shimadzu QP-5050, Kyoto, Japan) equipped with a SGE-BPX5 capillary column (30 m × 0.32 mm i.d., 0.25 μm film thickness). Helium was used as the carrier gas at a flow rate of 50 mL/min. The organic components were identified by matching the mass spectra with a standard Wiley mass spectral library. The concentrations of the major product components (i.e., phenol, 4methylanisole, 2-methylphenol, benzene, 4-methylphenol, 2,6-dimethylphenol, and unreacted anisole) were determined by injection of 2 μL of each sample into a GC (Shimadzu 17A, Kyoto, Japan) equipped with an FID and a SGE-BPX capillary column (30 m × 0.32 mm i.d.,

Figure 1. Schematic representation of equipment. high-voltage pulse generator, a high-pressure liquid chromatography pump, an oscilloscope, a high-voltage probe, and a current probe. The tubular reactor, made of quartz, with an outer diameter of 13 mm and a wall thickness of 1.5 mm, is equipped with inner and outer electrodes. The inner electrode, made of stainless steel with a diameter of 2.68 mm, is centered within the reactor and mounted on plexiglass supports. The outer electrode is a brass sheet rolled around the outer surface of the quartz tube with variable lengths in the range of 3−30 cm. The reactions took place in the annular discharge zone between the inner electrode and the outer electrode, which are separated by a dielectric, quartz. The volume of the discharged zone was varied by changing the length of the outer electrode. Figure 1 shows a crosssectional view of the reactor, and details of the design are summarized in Table 1. On the basis of our experience with this kind of reactor,37 a stainless steel rod with diameter of 2.68 mm was chosen as an inner electrode, giving higher efficiencies and lower energy consumptions than alternatives. As shown in Figure 1, the outer electrode is connected to ground and the inner electrode to a high-voltage pulse generator that is used to produce pulses up to 10 kV in voltage, with rise and fall times of less than 80 ns, pulse widths of less than 50 ns, and an adjustable pulse 4546


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0.25 μm film thickness). Nitrogen was used as the carrier gas at a flow rate of 20 mL/min. Some trace products, including methanol, cyclohexane, methylcyclohexane, o-xylene, p-xylene, 2-methylanisole, 2,4,6-trimethylphenol, 2,4-dimethylphenol, 2,5-dimethylphenol, hexanal, and decane, were detected by GC-MS analysis but not quantified. The conversion of anisole (X) and the selectivities (Si) and yields of individual components (Yi) were determined from the concentrations of components analyzed by GC according to the following relationships:

X (%) =

[anisole]in − [anisole]out × 100 [anisole]in

(1)

Si (%) =

moles of i produced × 100 moles of anisole converted

(2)

Yi (%) = X × Si × 100

contributing to the formation of ions, atoms, and free radicals (Figure 1). Recombination of these free radicals led to the production of a range of liquid products which were collected from the bottom of the reactor. In what follows, we summarize the results of anisole upgrading in the DBD plasma reactor and the influences of the aforementioned parameters on the reactor performance. 3.1. Upgrading of Anisole and Product Distribution. Because of the high density of electrons in the discharge zone of the DBD plasma reactor, a complex set of reactions takes place.46 The reaction pathways are sensitive to the population of reactive species.35 As shown in Figure 3, the products of anisole upgrading in the DBD plasma reactor indicate three main reaction routes:

(3)

where [anisole]in and [anisole]out are the concentrations of anisole in the feed and product, respectively, and i represents the individual component. The average discharge power (Pavg) of the reactor was calculated as follows: 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, the instantaneous applied voltage, and the current, respectively. In this investigation, each experiment was performed at an applied voltage of 9 kV and a pulse frequency of 20 kHz. The specific input energy (SIE) of the discharge was calculated with eq 5, where (QA) is the total feed flow rate of anisole: SIE (kJ/mL) =

Pavg(W ) × 60 Q A (mL/min) × 1000

(5)

Each experiment was carried out at least two times, and each reported datum is the mean of at least two values; typical standard deviations are estimated to be about 5−10%.

Figure 3. Structures of major (left column) and minor (right column) products of anisole upgrading in DBD plasma reactor. Transalkylation reactions are represented by black arrows, demethylation by a blue arrow, and demethylation followed by hydrogenolysis by a green arrow. CH3, H, and O radicals as reactants and products are omitted for simplicity.

3. RESULTS The operating conditions used for upgrading of anisole in this work are summarized in Table 2. The data determine the effects Table 2. Operating Parameters of Experimental Runs carrier gas flow rate (mL/min) feed flow rate (mL/min) outer electrode length (cm) applied voltage (kV) pulse frequency (kHz) temperature (°C) pressure (atm)

100−900 0.1−0.9 3−30 9 20 25 1.0

demethylation, transalkylation (methyl group transfer), and hydrogenolysis. Because the breaking of the C6H5O−CH3 bond is easier than breaking of the other bonds, demethylation of anisole is the principal reaction in the formation of phenol. Moreover, anisole is converted to 4-methylanisole and methylphenols via transalkylation. Furthermore, benzene is formed by demethylation followed by hydrogenolysis. The required H, CH3, and other radicals in the reactions shown in Figure 3 are provided in the discharge zone of the reactor, formed by decomposition of anisole and radicals such as methyl and phenoxy. The analytical data indicate a small amount of cyclohexane in the product, which indicates the hydrogenation of benzene. However, the hydrogenation of anisole and phenol to produce alicyclic hydrocarbons was negligible as indicated by the results showing only traces of these products. 3.2. Effect of Carrier Gas Flow Rate. Varying the flow rate of the carrier gas led to changes in the residence time of gas species in the discharge zone of the reactor (Table 3), and, consequently, the number of reactive species that were generated. The changes strongly affect the number of collisions of these species with anisole molecules and hence the product distribution.47

of the following operating parameters on the performance of the DBD helium plasma reactor: carrier gas (helium) flow rate, feed flow rate, and reactor length. The strategy of the experimentation was to vary one parameter at time, with the others kept constant. A goal of the investigation was to find the conditions that maximized anisole conversion within the limits of our equipment and to determine the distribution of products under various operating conditions. Before the start of an experiment, the carrier gas was allowed to flow for 5 min to purge the reactor. Then plasma was generated by applying a high voltage, and anisole was fed into the reactor. Once the feed entered the plasma zone, chemical reactions took place as a result of the collision of free electrons with anisole molecules, transferring energy to them and 4547


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Table 3. Average Residence Time of Carrier Gas in Reactor at Various Flow Rates helium flow rate (mL/min)

average residence time (s)

100 300 600 900

8.75 2.92 1.46 0.97

To explore the influence of carrier gas flow rate on the anisole conversion and product distribution, experiments were carried out with helium flow rates ranging from 100−900 mL/ min at a constant anisole feed flow rate of 0.1 mL/min and an outer electrode length of 20 cm. The results are shown in Figure 4. As the carrier gas flow rate increased, more reactive

Figure 5. Selectivities of products at various carrier gas flow rates: (a) major products, (b) minor products.

decreased selectivity for primary product, phenol. Moreover, the selectivities of 4-methylphenol and 2,6-dimethylphenol decreased slightly. However, the selectivity for benzene increased from 6.5 to 17.4% when the helium flow rate increased from 100 to 900 mL/min. In summary, we infer that increasing the carrier gas flow rate decreases the rate of demethylation of anisole to give phenol but increases the rate of hydrogenolysis of phenol to give benzene. Moreover, the carrier gas flow rate has different effects on the transalkylation reactions that lead to formation of 4methylanisole and methylphenols. For example, the yield of 4methylanisole has a peak at the carrier gas flow rate of 300 mL/ min, whereas the yield of 2-methylphenol was insensitive to the carrier gas flow rate. 3.3. Effect of Feed Flow Rate. Figure 6 shows the effect of feed flow rate on the conversion of anisole at a constant carrier gas flow rate of 100 mL/min and an outer electrode length of 20 cm. The data in this figure show that by increasing the feed flow rate from 0.1 to 0.9 mL/min, the anisole conversion decreased from 72.7 to 19.6%. This behavior is a consequence of decreasing the residence time of anisole molecules in the discharge zone (Table 4). Figure 6 further demonstrates that there is a dependence of the SIE of discharge on the feed flow rate. As a result of increasing the feed flow rate, the number of effective collisions between anisole molecules and electrons decreases, which leads to the transfer of less energy to the anisole molecules. Correspondingly, increasing the feed flow rate from 0.1 to 0.9 mL/min led to a decrease in the power consumption of the reactor from 71.2 to 42.0 W and a decrease in the SIE of discharge from 42.7 to 2.8 kJ/mL.

Figure 4. Effect of carrier gas flow rate on the conversion of anisole and SIE of discharge.

species formed in the discharge zone, but the residence times of these species decreased (Table 3), and consequently the conversion of anisole decreased markedly. As shown by the data presented in Table 3, increasing the carrier gas flow rate from 100 to 900 mL/min caused a decrease in the average gas residence time from 8.75 to 0.97 s, and the conversion of anisole correspondingly decreased from 72.7 to 27.4% (Figure 4). Therefore, the residence time of active species in the reactor plays an important role influencing the progress of the reactions by controlling the collision opportunities. The data show that the carrier gas flow rate for achieving the highest anisole conversion under the operating conditions of this investigation was about 100 mL/min. Moreover, increasing the flow rate of carrier gas leads to an increase in the electron density in the discharge zone as a consequence of an increase in the number of ionized helium atoms. Consequently, more discharge power was consumed, leading to an increase in the SIE of the discharge (Figure 4). A similar result was also obtained by our group37 for cracking of heavy naphtha, and by Xin et al.48 for the cracking of light hydrocarbons in a plasma reactor. Figure 5a,b shows the distribution of major and minor products as a function of carrier gas flow rate, respectively. As shown in Figure 5a, the selectivity for phenol decreased from 56.4 to 48.1% with increasing carrier gas flow rate from 100 to 900 mL/min. This behavior may be attributed to the decreased residence time of reactive species in the reactor, which led to a 4548


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Figure 6. Effect of feed flow rate on conversion of anisole and SIE of discharge.

Table 4. Average Residence Time of Anisole in Reactor at Various Feed Flow Rates feed flow rate (mL/min)

average residence time (s)

0.1 0.3 0.6 0.9

176.4 58.84 29.42 19.65

Figure 7. Selectivities of products at various feed flow rates: (a) major products, (b) minor products.

The product distribution in a DBD plasma reactor is a function of the number of collisions of exited electrons with reactants as well as the mean residence time of reactants in the plasma zone. Figure 7a,b shows the distributions of major and minor products as a function of feed flow rate, respectively. As shown in Figure 7a, an increase in the feed flow rate from 0.1 to 0.9 mL/min led to a decrease in the selectivity for phenol from 56.4 to 40.4%, whereas the selectivities for 4-methylanisole and 2-methylphenol increased. Moreover, the selectivity for major products decreased as well. According to these results, increasing the feed flow rate has negative effects on the demethylation of anisole to phenol and on the hydrogenolysis of phenol to benzene. On the other hand, transalkylation of anisole to give 4-methylanislole and 2-methylphenol increased with increasing feed flow rate. 3.4. Effect of Reactor Length. To investigate the effect of reactor length (length of the discharge zone) on the performance of the reactor in anisole upgrading, a number of experiments were carried out at a constant carrier gas flow rate of 100 mL/min and a feed flow rate of 0.1 mL/min, with outer electrode lengths ranging from 3 to 30 cm. The results are shown in Figure 8. Increasing the outer electrode length on the one hand led to an increase in the anisole conversion because of the increase in anisole residence time. On the other hand, the density of flux of electrons decreased as a consequence of the decrease in the intensity of the plasma field.43 As a result of these two contrasting effects, there was a maximum in anisole conversion at an outer electrode length of 20 cm, as shown in Figure 8. Moreover, the SIE rose from 25.5 to 43.2 kJ/mL as the outer electrode length increased from 3 to 30 cm. This increase may be attributed to an increase in power consumption (from 42.5 to 72.0 W) and has two main causes: (1) an increase in the residence time of reactants that leads to an increase in the number of ionized species, and, consequently,

Figure 8. Effect of outer electrode length on conversion of anisole and SIE of discharge.

an increase in the power consumption49 and (2) an increase in the amount of trapped air between the outer electrode and the dielectric (i.e., quartz) which increasingly consumes power.50 Figure 9, panels a and b, respectively, illustrates the distribution of major and minor products resulting from changes in the outer electrode length. When the reactor length increased, the residence time of reactants and active species increased, and the distribution of products changed. As was mentioned above, increasing the residence time of reactants in the reactor provides more opportunity for collisions of excited species. However, increasing the outer electrode length decreases the energy of electrons, which directly affects the 4549


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C6H5OCH3* → C6H5OCH 2 + H (De = 389.1154 )

(9)

C6H5OCH3* → C6H4OCH3 + H (De ≤ 424.6855)

(10)

C6H5OCH3* → C6H5 + OCH3 (De = 424.6852,53)

(11)

where De is the bond dissociation energy in units of kJ/mol. These reactions are all plausible, and we have a preliminary basis for predicting their relative rates, because the probability of breaking the weaker bonds is higher than that of breaking the stronger ones. Thus, considering the bond strengths summarized in Table 5, we suggest which free radicals are Table 5. Dissociation Energies, De, of Chemical Bonds in Anisole (kJ/mol) De

ref

C6H5O−CH3 C6H5OCH2−H C6H5−OCH3 Caromatic−H Caromatic−Caromatic

267.78 389.11 424.68 ≤424.68 ∼518

52, 53 54 52, 53 55 61

most readily formed. The C6H5O−CH3 bond is the weakest in the anisole molecule because of the resonance stabilization of the phenoxy radical,52 and the rate of breaking the C6H5O− CH3 bond (reaction 8) is thus expected to be much higher than that of the other bonds, leading to decomposition of excited anisole molecules mainly to form phenoxy radicals. Moreover, it is possible to break the C−H bonds of the methyl group and the aromatic ring (especially in the para position) providing highly reactive H atoms in the discharge zone of the reactor (reactions 9 and 10). However, the probability of breaking the bond between O and aromatic C (reaction 11) is low because of the relatively high bond strength. Furthermore, the probability of breaking the C−C bonds in the aromatic ring is negligible because of the high strength of C−C π-bond. The methyl radicals formed in reaction 8 may decompose into CH2, CH, and H radicals. Therefore, many free radicals are produced in the discharge zone of the reactor as a result of secondary reactions resulting from collisions with electrons (e−) or M, where by M we mean the temporary excited collision partner which has a higher energy level, such as He* or H*. Moreover, the radicals formed by dissociation of anisole molecules (phenoxy or other radicals) can decompose into smaller fragments. The relatively high rate of reaction 8 explains the formation of substantial numbers of phenoxy radicals, which can be attacked by electrons and other exited species, promoting the phenoxy species to higher energy levels. The unstable phenoxy radicals can decompose through the lowestenergy pathway involving the elimination of CO to yield C5H5, which in turn may react with a methyl radical to produce CH3C5H5 or subsequently decompose to give light hydrocarbon species such as C3H3 and C2H2.56−59 However, the more probable pathway for a phenoxy radicals involves reaction with free radicals in the discharge zone, such as H and CH3, to produce phenol and methylphenols.45 In the DBD plasma reactor, the mean energy of electrons produced is about 1−10 eV in an electric field of 0.1−100 kV/ cm at atmospheric pressure. This range of electron energies is sufficient to ionize the carrier gas and reactant molecules.31,46,60 Therefore, within the DBD plasma reactor, a large number of

Figure 9. Selectivities of products as a function of outer electrode length: (a) major products, (b) minor products.

effectiveness of collisions. Because of the complexity of influences of these two parameters, prediction of the variations in product selectivities is difficult. The selectivities for phenol and 2-methylphenol decreased, whereas the selectivity for 4methylanisole increased at first and then, after the outer electrode length of 20 cm was reached, remained approximately constant. The maximum selectivity for benzene at carrier gas and feed flow rates of 100 and 0.1 mL/min was 6.5%, obtained at a reactor length of 20 cm.

4. DISCUSSION The processes taking place in the DBD plasma reactor are too complex to allow the elucidation of mechanisms of the reactions on the basis of data as limited as ours, but the available thermodynamics data characterizing the species formed under plasma discharge conditions provide a basis for inferences about the main initiation reaction steps. The plasma produces a high flux of fast electrons that excite the carrier gas, neutral helium atoms, to the lowest metastable state of He(23S1), He*, which has a metastable energy level of 19.82 eV,45,51 according to the following reaction: (6) He + e− → He* + e− The electrons and metastable helium atoms (He*) collide with anisole molecules, exciting them to higher energy levels: C6H5OCH3 + e− (or He*) → C6H5OCH3* + e− (or He) (7)

The excited anisole (C6H5OCH3*) can decompose into fragments by means of breaking of chemical bonds. Plausible reactions are as follows: C6H5OCH3* → C6H5O + CH3 (De = 267.7852,53)

bond

(8) 4550


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5. CONCLUSIONS An experimental investigation of the upgrading of anisole, a model compound representing lignin-derived bio-oils, in a DBD plasma reactor at room temperature and atmospheric pressure determined the influences of the helium carrier gas flow rate, the anisole feed flow rate, and the reactor length on the reactor performance. The data demonstrate the following: (1) Anisole demethylation, transalkylation, and hydrogenolysis are the main three reactions that occurred during the upgrading the anisole in the DBD plasma reactor. (2) The residence times of active species and feed molecules are key parameters affecting the conversion of anisole. (3) An increase in the carrier gas flow rate or feed flow rate results in decreased conversion of anisole as a consequence of the decreased residence time of anisole molecules and reactive species formed by the plasma. (4) The power consumption and the SIE of discharge increased with increasing carrier gas flow rate, but these both decreased with increasing feed flow rate. (5) The highest anisole conversion of 72.7% was obtained with an optimal outer electrode length of 20 cm at a carrier gas flow rate and anisole liquid feed flow rate of 100 and 0.1 mL/ min, respectively. (6) The major reaction products were phenol, 4-methylanisole, and 2-methylphenol. (7) The DBD plasma reactor is a promising and easily operated system for upgrading of bio-oils, but much remains to be understood about the reaction chemistry and reactor operation.

free radicals are generated from the high and efficient collisions of electrons (and excited species) with reactants. After the initiation of the plasma reactions, the reactions between the radicals generate a range of products. In the present study, phenol, 4-methylanisole, and 2-methylphenol were detected as main products which are formed according to free-radical reactions. Moreover, benzene, 4-methylphenol, and 2,6-dimethylphenol were detected as minor products due to their relatively low yield in comparison with major products. The complex nature of plasmachemical reactions coupled with the variety of reaction pathways is responsible for different product distributions in plasma reactors in comparison with the previous anisole pyrolysis studies in other types of reactors. As may be observed from the data presented in Table 6, the Table 6. Product Yields of Components Observed in Anisole Conversiona

component

yield reported by Pecullan et al.56b

yield reported by Mackie et al.58c

yield observed in this workd

0.371 0.283e

0.15 0.09

0.776 0.148

0.04

0.042

0.093 0.371 0.213

0.03 0.12 0.02

0.089

0.077 0.088 0.038

0.11 0.048

phenol, C6H5OH 2-methylphenol, CH3C6H4OH 4-methylphenol, CH3C6H4OH benzene, C6H6 carbon monoxide, CO methylcyclopentadiene, C5H5CH3 methane, CH4 ethane, C2H6 cyclopentadiene, C5H6 benzaldehyde, C6H5CHO 4-methylanisole, CH3C6H4OCH3 2,6-dimethylphenol, (CH3)2C6H3OH

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AUTHOR INFORMATION

Corresponding Author

0.015 0.317

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

0.014

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.

a

The yields are expressed as fractions of moles of components to initial moles of anisole (eq 3). bData obtained under pyrolysis conditions at 1003 K and with a residence time of 98 ms in a flow reactor. cData obtained at 950 K with a residence time of 0.14 s in a stirred reactor. dData determined with a carrier gas flow rate of 100 mL/min, a feed flow rate of 0.1 mL/min, and a reactor length of 20 cm. eA summation of yields of methylphenols (cresols).

Notes

The authors declare no competing financial interest.

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studies by Mackie et al. (who used a stirred reactor in 950 K) and by Pecullan et al.56 (who used a flow reactor at 1003 K) show product distributions different from ours in a DBD plasma reactor at 298 K. Moreover, in addition to major and minor products, some other trace products, including 2-methylanisole, cyclohexane, methylcyclohexane, o-xylene, p-xylene, trimethylphenol, 2,4dimethylphenol, and 2,5-dimethylphenol, were produced in the discharge zone by colliding electrons and reactions involving other free radicals. The concentrations of these trace products were too small for quantitative analysis. We emphasize that the aromatic rings remained largely intact, and only very low yields of linear hydrocarbons (e.g., decane) were observed because of the limited breaking of the C−C bonds in the aromatic rings in anisole. The free radicals generated in the discharge zone, such as CH3, CH2, CH, O, and H, evidently also react with each other to form gas-phase products such as CH4, C2H2, C2H4, H2, and CO, which left the reactor in the effluent gas stream. 4551


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