Upgrading of Anisole in a Catalytic Pulsed Dielectric Barrier Discharge

Nov 27, 2013 - The anisole conversion was measured with plasma alone and also in the presence of Al2O3 and various Al2O3-supported catalysts, leading ...
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Upgrading of Anisole in a Catalytic Pulsed Dielectric Barrier Discharge Plasma Reactor Mohammad Reza Rahimpour,*,†,‡ Abdolhossein Jahanmiri,† Parisa Rostami,† Hamed Taghvaei,† and Bruce C. Gates‡ †

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

‡

ABSTRACT: A new strategy for bio-oils upgrading with a catalytic pulsed dielectric barrier discharge (DBD) plasma reactor was investigated for the conversion of anisole, a compound representative of the bio-oils formed by pyrolysis of lignin. The anisole conversion was measured with plasma alone and also in the presence of Al2O3 and various Al2O3-supported catalysts, leading to the formation of benzene, phenol, 4-methylanisole, 2-methylanisole, 4-methylphenol, 2-methylphenol, 2,6-dimethylphenol, and cyclohexane. Results are presented in the form of specific input energy (SIE), anisole conversion, and product selectivities. The conversion decreased in the following order when these catalysts were used: Mo−Ni/Al2O3 > Pt/Al2O3 > Co−Mo/Al2O3 > Pt− Re/Al2O3 > Al2O3. The conversion increased with increasing mass of the Mo−Ni/Al2O3 catalyst and with increasing voltage; the highest anisole conversion (0.81) was observed with this catalyst, and the conversion under comparable conditions without a catalyst was 0.43. The predominant product was 4-methylanisole. Catalyst deactivation was minimized by the plasma. hexanone, and so on,16,17 and Ardiyanti et al.15 also used their catalysts to investigate the conversion of anisole, which is an important component of bio-oils formed from a major underutilized biomass resource, lignin. Anisole (C6H5OCH3) is considered to be a good representative of lignin-derived biooils, because, like most components of these bio-oils, it incorporates an aromatic ring and an ether linkage; it is one of the simplest compounds that represents these bio-oils at least moderately well. Consequently, anisole has been used in a number of investigations characterizing catalytic hydroprocessing with a focus on removal of oxygen to give hydrocarbons that are good candidate fuel components. A typical such reaction is classed in the broad category of hydrodeoxygenation (HDO), which gives water and products containing less oxygen, including hydrocarbons such as benzene. Li et al.18 investigated the HDO of anisole in the presence of Ni2P/SiO2, MoP/SiO2, and NiMoP/ SiO2 catalysts at 300 °C, changing the Ni/Mo ratios in the third of these catalysts. Runnebaum et al.16,19,20 investigated the conversion of anisole with H2 in the presence of a Pt/Al2O3 catalyst at 573 K, determining several competing reaction pathways and elucidating a reaction network. Runnebaum’s reaction network provides part of the foundation for interpreting the results of the investigation reported here, in which we have characterized the conversion of anisole under conditions expected to involve both free radicals and surface catalysis. We report scoping experiments characterizing anisole conversion in a plasma reactor under conditions expected to be very roughly comparable to those of catalytic pyrolysis of anisolebut at much lower temperatures. Our experiments

1. INTRODUCTION Today’s energy-environment challenges and the impact of greenhouse gas emissions1,2 have motivated extensive research on the conversion of biomass into fuels. Among the potential processes that could lead to economical production of renewable energy are (fast) pyrolysis and/or hydrolysis of biomass. The bio-oils formed by pyrolysis, which typically takes place at temperatures in the range of 400−500 °C, are not good candidate fuels themselves because of their thermal instability, wide boiling and viscosity ranges, distillation residues, high moisture contents, corrosiveness, low heating values, and incompatibility with petroleum-derived fuels. Consequently, much recent effort has been devoted to bio-oils upgrading, typically by catalytic processing, with one of the goals being the removal of oxygen to produce high-quality fuels.3−12 Some investigations of biomass pyrolysis have been carried out with solid catalysts present in the reactors. In such catalytic pyrolysis processes, one expects complex chemistry involving both free radical reactions in the gas phase and catalytic reactions on surfaces, but the chemistry is not well understood, and much work remains to elucidate the reaction pathways and begin to understand the interplay between the gas-phase and surface reactions. A first step toward analysis of the chemistry of catalytic pyrolysis might be to understand the chemistry of catalytic biooils conversion, and a substantial literature of that topic has already emerged, typically focused on reactions of the bio-oils with H2. For example, Xu et al.13,14 investigated the reactions of raw bio-oil produced by pyrolysis of pine sawdust in the presence of a series of nickel-based and also ruthenium catalysts. Ardiyanti et al.15 used Cu−Ni/γ-Al2O3 catalysts with various Ni/Cu ratios to investigate the hydroprocessing of fast pyrolysis oil. Bio-oils incorporate numerous aromatic compounds with oxygen-containing groups typified by anisole, guaiacol, cyclo© 2013 American Chemical Society

Received: July 16, 2013 Revised: November 16, 2013 Published: November 27, 2013 7424

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wrapped around the quartz tube. The length of the outer electrode that determined the boundaries of the discharge zone was 3 cm. A high-voltage pulse generator was connected to the inner electrode, supplying voltages from 0 to 9 kV with a pulsed waveform at a frequency of up to 20 kHz. It produced fall and rise times of less than 100 ns, a pulse length of less than 50 ns, and an adjustable pulse repetition frequency up to 20 kHz. The applied voltage was measured with a 1000:1 Tektronix P6015A high-voltage probe. A 2-AV−1 Pearson current probe was used for current measurements. The output signals were transmitted to a digitizing oscilloscope (Tektronix, TDS 1012B-SC). Further details have already been reported.29−33 2.2. Reactor Operation and Catalysts. In some experiments, the volume between the inner electrode and the quartz tube wall was filled with catalyst particles. In each of the reported experiments, helium was used as a carrier gas with a flow rate of 100 mL/min; the flow rate was controlled with an Alicat Scientific volumetric flow controller (MC-1 SLPM-D). Before a high voltage was applied between the electrodes, air was removed from the reactor by purging with helium for 5 min. In a typical experiment, the power supply was turned on, and after plasma generation, liquid anisole (98%, Merck) was injected from the top of the reactor using an SY-8100 HPLC pump at a flow rate of 0.1 mL/min. The duration of the upgrading process was fixed at 210 s in all experiments, and once the applied voltage was turned off, the reactions stopped immediately. Products in the form of liquid were collected from the bottom of the reactor for off-line analysis. The gasphase products were vented and not analyzed. Five different catalysts were used: platinum on alumina (Pt/Al2O3), Ni−Mo/Al2O3, Co−Mo/Al2O3, Re−Pt/Al2O3, and Al2O3. When catalyst was present in the reactor, the masses were as follows: Ni− Mo/Al2O3, 1.47 g; Pt/Al2O3, 2.39 g; Co−Mo/Al2O3, 1.71 g; Re−Pt/ Al2O3, 1.58 g; and Al2O3, 1.8 g. The BET surface areas and some properties of the catalysts are summarized in Table 1.

were carried out in a reactor in which plasma is generated by an electric current applied through a gas, resulting in ionization of the gas and production of reactive species that include free radicals, excited atoms, ions, and molecules that facilitate both the initiation and propagation of chemical reactions21−23 taking place in the presence of solid catalysts. Dielectric barrier discharge (DBD) is a nonuniform plasma discharge that operates at ambient temperature and pressure, forming stable discharges in various gases at high discharge powers, suggesting their possible suitability for industrial processes.24,25 An advantage of plasma processing is that it can be combined with other processing approaches, including catalysis by solids. Both chemical and physical properties of the plasma and catalyst can be modified by the presence of each other. This interaction can generate a synergistic effect, which might be expected to provide unique opportunities to enhance process efficiencies.26 For example, Górska et al.27 examined nonoxidative methane coupling to give higher hydrocarbons using a Cu/ZnO/Al2O3 catalyst in a DBD and finding that, when catalyst was added, the methane conversion decreased and the conversion to ethane increased. Production of methane from a mixture of CO and H2 was investigated with a DBD plasma reactor including Ni/alumina catalyst pellets, with the data showing that the nonthermal plasma significantly enhanced the catalytic conversion.28 We report data characterizing the conversion of anisole in a pulsed DBD plasma reactor in the presence of each of several catalysts.

2. EXPERIMENTAL SECTION

Table 1. Catalyst Physical Properties

2.1. Plasma Reactor. A schematic diagram of the equipment used to investigate the conversion of anisole in a DBD plasma reactor is shown in Figure 1. The reactor was a quartz tube with an outside diameter of 13 mm and a wall thickness of 1.5 mm that acted as a dielectric and separated two electrodes. A stainless steel rod with a diameter of 2.68 mm was placed axially in the center of the reactor, acting as a high-voltage electrode. This rod was held in place with two plexiglass rings. The outer (grounded) electrode was a copper foil

catalyst Al2O3 Pt/Al2O3 Mo−Ni/ Al2O3 Co−Mo/ Al2O3 Re−Pt/ Al2O3

commercial name

BET surface area (m2/g)

metal contents, wt %

RN-412

240 109 174

0.1% Pt (1−3)% Ni, (10−19)% Mo

DC-130

208

3.4% Co, 13.6% Mo

RG-682

202

0.3% Re, 0.4% Pt

2.3. Product Analyses. Liquid products were analyzed with a Shimadzu QP 50/50 GC-MS equipped with a SGE BPX5 capillary column (0.32 mm × 30 m × 0.25 μm) with helium (50 mL/min) as the carrier gas. Peaks in the chromatograms were identified on the basis of the mass spectra by matching to a Willy library. The identifications of the most abundant products were verified by comparison with the analyses of authentic standards purchased from Sigma Aldrich; these standards were benzene, phenol, 2-methylanisole, 4-methylanisole, 2-methylphenol, 4-methylphenol, 2,6-dimethylphenol, and cyclohexane. The products were analyzed quantitatively with a Shimadzu 17A GC (N2 carrier gas, 20 mL/min, capillary column: SGE BPX 0.25 mm × 30 m × 0.5 μm) equipped with a flame ionization detector (zero-air, 500 mL/min and H2 flowing at 75 mL/min). The products were quantified by comparison of the GC data with six-point calibration curves determined with the authentic standards.

3. DATA ANALYSIS The average discharge power (Pavg) delivered to the reactor was calculated from the following equation

Figure 1. Schematic diagram of equipment including plasma reactor and electrical circuit. 7425

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n ⎛ V + Vi + 1 ⎞⎛ Ii + Ii + 1 ⎞ ⎟(t ⎟⎜ Pavg = f ∑ ⎜ i − ti) ⎝ ⎠⎝ 2 ⎠ i + 1 2 i=0

4.1. Products of Anisole Conversion. The observed products formed from anisole (including the most abundant as well as trace components) were the following: benzene, cyclohexane, phenol, 2-methylphenol, 4-methylphenol, 2methylanisole, 4-methylanisole, 2,6-dimethylphenol, 2,4-dimethylphenol, o-xylene, p-xylene, trimethylphenol, methanol, hexanal, and decane. Some two-ring compounds were also identified. Molecular structures of the most abundant compounds identified in the anisole conversion are shown in Figure 2.

(1)

where f is the pulse repetition frequency and Vi is the instantaneous applied voltage; the terms Ii and t refer to current density and time, respectively. The specific input energy (SIE) of the discharge was computed by dividing the average consumed discharge power (Pavg) by the anisole feed flow rate. Anisole conversion (X) is defined here as the molar percent of anisole reactant consumed divided by the total molar percent of anisole fed. Component selectivities were calculated by dividing the product mole percentage to the mole percentage of consumed anisole.

4. RESULTS The main goal of this study was to determine the best type among the selected catalysts for the highest possible process efficiency of anisole upgrading through the catalytic DBD plasma reactor. When the reactor operation was started and high-voltage pulses applied, filamentary discharges (short-lived high-density currents) generated in the catalytic reactor-plasma zone were evident, being blue in color. These filaments were randomly distributed over the entire electrode and catalyst surfaces. After that, anisole was fed into the reactor. Once the feed entered into the plasma zone, chemical reactions were performed by collisions of free electrons and transfer of energy to anisole molecules, which contributes to the formation of ions, atoms, and free radicals. Recombination of these free radicals leads to formation of a range of liquid products, which were collected from the bottom of the reactor. We investigated various reaction conditions and collected conversion data systematically under conditions giving roughly optimum conversions to potentially valuable products. Once these conditions had been determined in preliminary experiments, they were held constant and referred to as standard conditions for collection of most of the data reported here. The standard conditions are summarized in Table 2.

Figure 2. Structures of most abundant compounds in products of anisole conversion (Me is methyl).

4.2. Results of Reactor Operation with and without Catalyst. The reactor design allowed operation with and without catalyst in the discharge gap, as shown schematically in Figure 3. Data were collected with each of the catalysts in the gap and with plasma alone. Reactor performance data (specific input energy (SIE) and anisole conversion) are summarized in Figures 4 and 5, providing evidence of the influence of the

Table 2. Properties of Reactor and Reactor Operating Parametersa parameter voltage (kV) frequency (kHz) outer electrode length (cm) outer electrode material inner electrode diameter (cm) inner electrode material carrier gas carrier gas flow rate (mL/min) anisole flow rate (mL/min) catalyst

value or component in standard experiment

value or component in other experiments

8 18 3

7, 8, 9 -

copper

-

2.68

-

stainless steel

-

helium 100

-

0.1

-

Ni−Mo/Al2O3

Pt/Al2O3, Ni−Mo/Al2O3, Co− Mo/Al2O3, Re−Pt/Al2O3, Al2O3

a

Middle column, standard values; right-hand column, ranges of variables in experiments with parameters other than standard values.

Figure 3. Schematic representation of DBD plasma reactor discharge gap: (a) without catalyst and (b) with catalyst. 7426

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catalyst composition and a comparison of the performances of the various catalysts.

Figure 6. Selectivities for formation of some products of anisole conversion in the absence of catalyst and in the presence of various catalysts (at 8 kV applied voltage).

Figure 4. Effect of plasma combined with various catalysts on specific input energy (at 8 kV applied voltage).

selectivity observed in the presence of catalyst (Al2O3) was 0.39. 4.3. Effect of Catalyst Mass. The data clearly demonstrate that the catalyst plays a role in the conversion, and one would expect the adsorption, desorption, and surface reaction processes to be influenced by the reactive gas-phase species.34−38 To test this hypothesis, we varied the catalyst mass under the same reaction conditions, checking for changes in the performance of the reactor. Four different masses (0.44, 0.81, 1.13, and 1.47 g) of Ni−Mo/Al2O3 catalyst were placed in the plasma discharge zone. The corresponding SIE and anisole conversion data are presented in Figure 7. Figure 5. Effect of plasma combined with various catalysts on anisole conversion (at 8 kV applied voltage).

The data show that the highest anisole conversion (81%) was achieved with the Ni−Mo/Al2O3 catalyst, and operation without a catalyst gave about half that conversion. Experimental results also confirm that the presence of Al2O3, the blank (carrier), increased the anisole conversion. It is possible that the Al2O3, without any active components, acted as a dielectric material and helped convert anisole. The carriers loaded with Pt, Co−Mo, and Re−Pt exhibited higher catalytic activities than the blank. According to the anisole conversion improvement, the activities of the catalysts could be ranked as follows: Ni− Mo/Al2O3 > Pt/Al2O3 > Co−Mo/Al2O3 > Pt−Re/Al2O3 > Al2O3 > plasma alone. Data showing the influence of the catalysts on the specific input energy (SIE) are given in Figure 4. The SIE value observed in the absence of a catalyst was found to be greater than those observed when the various catalysts were present The difference can be attributed to the reduction in useless electron-impact reactions as the electric field is increased. Selectivity data characterizing some of the most abundant products (benzene, phenol, 4-methylanisole, 2-methylphenol, and 4-methylphenol) are summarized in Figure 6 for reactions in the absence of a catalyst and for reactions with each of the catalysts. The selectivity for the formation of phenol in the absence of a catalyst (0.54) was greater than the values observed when a catalyst was present. The highest phenol

Figure 7. Effect of catalyst mass on SIE and anisole conversion (at 8 kV applied voltage).

These results show that increasing the catalyst mass reduced the SIE, from 18.9 kJ × mL−1 without catalyst to 15.12 kJ × mL−1 with 1.47 g of catalyst. The anisole conversion increased correspondingly from 0.43 to 0.81. The selectivity also changed depending on the catalyst mass, as shown in Figure 8. The data indicate that 4-methylanisole was formed with the highest selectivity, about 0.75 when 0.81 g of Ni−Mo/Al2O3 catalyst was used in the discharge gap. In the presence of 1.47, 1.13, 0.81, and 0.44 g of Ni−Mo/Al2O3 catalyst, the selectivities for formation of phenol were 0.18, 0.16, 0.15, and 0.31, respectively, and the selectivities for formation of other components were less than 0.11. These results are consistent 7427

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Figure 10. Effect of applied voltage on selectivities for formation of some products of anisole conversion (with 1.47 g of Ni−Mo/Al2O3 fresh catalyst).

Figure 8. Effect of catalyst mass on selectivities for formation of some products (at 8 kV applied voltage).

with the hypothesis stated above, but we emphasize that changes in the mass of catalyst led to changes in space velocity and the mean residence time of the reactants in the plasma zone, so that we lack the information for a more fundamental statement about the reactor performance. 4.4. Effect of Applied Voltage. Figure 9 indicates the effect of applied voltage on SIE and anisole conversion in the

Data showing the dependence of the inner and outer electrode temperatures on the applied voltage during the conversion of anisole catalyzed by Ni−Mo/Al2O3 are shown in Figure 11. As expected, the electrode temperatures increased

Figure 9. Effect of applied voltage on SIE and on anisole conversion (with 1.47 g of fresh Ni−Mo/Al2O3 catalyst).

Figure 11. Dependence of inner and outer electrode temperatures on applied voltage in the conversion of anisole catalyzed by Ni−Mo/ Al2O3.

presence of 1.47 g of Ni−Mo/Al2O3 catalyst. In these experiments, the voltage was increased in the range from 7 to 9 kV, while the other parameters were fixed (corresponding to the standard values summarized in Table 2). Increasing the applied voltage at a constant frequency increased both the SIE and the anisole conversion (the SIE and conversion values increased from the initial values of 9.12 kJ × mL−1 and 0.61 to 22.86 kJ × mL−1 and 0.81, respectively), but the increase in the anisole conversion as the voltage increased from 8 to 9 kV was very low. The selectivities for formation of benzene, phenol, 4methylanisole, 2-methylphenol, and 4-methylphenol as a function of voltage are shown in Figure 10. The selectivity for benzene formation at a voltage of 9 kV was approximately 0.17; the highest selectivity was that of 4-methylanisole (0.84 at a voltage of 7 kV).

when the voltage increased, corresponding to the increased power generation and consequent power loss. The main cause of power loss is energy reflection of the plasma associated with defective impedance matching between the reactor and circuit. The heat loss which is the releasing form of the reflected energy results in heating of the electrodes and catalyst. The warming of the catalyst particles led to their thermal activation and increased anisole conversions (Figure 9). 4.5. Catalyst Deactivation. Catalyst deactivation was investigated in experiments with 1.47 g of fresh Ni−Mo/ Al2O3. Four consecutive reaction tests were carried out at fixed voltage and frequency values with constant feed and carrier gas flow rates (Table 2). Product analyses led to the results shown in Figure 12, demonstrating the relationship between SIE and anisole conversion during 840 s of operation. Both the SIE and 7428

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anisole conversion declined as the catalyst underwent deactivation.

Figure 14. Dependence of product selectivities on time of operation of 1.47 g of fresh Ni−Mo/Al2O3 catalyst.

Figure 12. SIE and anisole conversion during 840 s of catalyst deactivation with 1.47 g of fresh Ni−Mo/Al2O3 catalyst.

products such as bio-oils. There are in prospect several advantages of this type of reactor relative to conventional catalytic reactors used for biomass upgrading. First, the plasma reactor works at atmospheric pressure and ambient temperature and minimizes the costs associated with handling of feeds and products under more severe conditions.39 Second, there is less catalyst deactivation in a plasma reactor than in a conventional one because of the reduced coke deposition. Third, corrosion is minimized at the low operating temperature and with the appropriate materials of reactor construction (e.g., quartz rather than metal). Fourth, even in the absence of hydrogen gas, operation in the plasma reactor results considerable anisole conversion. Furthermore, space velocities in the plasma reactor are likely to be high by comparison with those in conventional reactors.29,30 Further work to assess preliminary process economics would need to account for power consumption. Because of the complexity of the chemistry, the first step of analyzing the reaction chemistry in terms of reaction networks and the later steps of inferring reaction mechanisms are well beyond the scope of this work.40−43 Nonetheless, it is of value to compare the results with those observed for anisole conversion under conventional catalytic hydroprocessing conditions. Methoxy and hydroxy groups are the principal functional groups in lignin-derived bio-oils. These groups are reactive and involved in various reaction pathways that depend on the type of catalyst and the reaction conditions, including the presence of H2. Some of the observed products in conventional catalytic hydroprocessing of anisole16,18−20 are accounted for by transalkylation, hydrogenation, and hydrodeoxygenation (hydrogenolysis) reactions. The acidic function catalyzes transalkylation reactions leading to transfer of methyl groups to benzene rings, and the metals catalyze demethylation, hydrogenation, and hydrogenolysis reactions. Multiple transalkylation reactions lead to the transfer of methyl groups from the methoxy groups of anisole to the aromatic ring to produce methylanisole and phenol, for example.20 Subsequently, the transalkylation reaction products yield cresol and xylenol. Cyclohexanol is formed by hydrogenation of phenol and the cyclohexanol is subsequently dehydrated and hydrogenated to give cyclohexane. Hydrogenolysis converts anisole to methanol and benzene and phenol to water and benzene, for example.18,20

In DBDs, the input power characterizes the number of generated electrons and influences the chemical reactions. The observed reduction in anisole conversion suggests that less reactive species took part in the reactions and broke bonds in anisole and products formed from it. The consumed power and consequently SIE were reduced. The presumed decrease of consumed power would lead to less power loss, leading to the observed electrode temperature decreases shown in Figure 13.

Figure 13. Effect of catalyst operating time on inner and outer electrode temperatures with 1.47 g of fresh Ni−Mo/Al2O3 catalyst.

The selectivities for formation of the various products also changed as the catalyst underwent deactivation (Figure 14), and there is not a clear pattern in the results. The selectivity for formation of 4-methylanisole after 840 s was still high; the selectivities for formation of the dominant products phenol and benzene decreased.

5. DISCUSSION The data presented here, even though they are preliminary and do not lend themselves to a quantitative chemical reaction engineering analysis, show the potential value of using a plasma reactor for conversion of biomass and biomass-derived 7429

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Thus, the product distribution observed in our work is similar to those observed in catalytic reactions of anisole + H2 taking place in the presence of catalysts at the much higher temperatures mentioned in the Introduction.15,18,20 Specifically, for example, the observation of benzene as a product points to the occurrence of HDO (hydrogenolysis, as methanol was presumably formed with benzene) and the inference that hydrogen was supplied by the anisole itself or products of its conversion. The observation of methylanisole indicates the occurrence of transalkylation, another reaction that takes place during catalytic HDO when the catalysts include acidic components (such as an Al2O3 support).16,20 We emphasize that a fundamental interpretation of the results presented here is not realistic on the basis of the available data. The chemistry is complex. The nanosecond highvoltage pulses led to the formation of highly nonequilibrium discharges (with low total discharge power), generating high electric fields that are expected to have created highly reactive and short-lived particles, including free radicals, ions, and free electrons.44 As these species interacted with reactant and product molecules and with catalyst surfaces, they no doubt caused bond breaking and formation and led to complex product distributions.39 Around each contact point of a catalyst pellet, a strong electric field is generated, resulting in microdischarges among the pellets. As microdischarges are created within the catalyst pores, there is more discharge per volume, and the mean energy density of the discharge is increased.34,45 This increase of the mean energy density causes more energetic collisions between charged particles and reactant and product molecules and thus higher conversion. The dependence of SIE on the voltage is described elsewhere30,46−48 and re-emphasized by our own results. It is widely believed that when the applied voltage increases, the electric field becomes stronger,48−51 leading to an increase in micro discharge current density. Thus, more energetic electrons are generated, reacting with anisole and products of its conversion and breaking bonds in these molecules. We infer that increasing the applied voltage would increase the anisole conversion, but because of the limitations of our power generator, experiments at higher voltages than those we used were not carried out.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +98 711 2303071. Fax: +98 711 6473180. E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Hamelinck, C. N.; van Hooijdonk, G.; Faaij, A. P. Biomass Bioenergy 2005, 28, 384−410. (2) Sun, Y.; Jiayang, Ch. Bioresour. Technol. 2002, 83, 1−11. (3) Hew, K. L.; Tamidi, A. M.; Yusup, S.; Lee, K. T.; Ahmad, M. M. Bioresour. Technol. 2010, 101, 8855−8858. (4) Gayubo, A. G.; Aguayo, A. T.; Atutxa, A.; Aguado, R.; Bilbao, J. A. Ind. Eng. Chem. Res. 2004, 43, 2610−2618. (5) Gayubo, A. G.; Aguayo, A. T.; Atutxa, A.; Aguado, R.; Olazar, M.; Bilbao, J. Ind. Eng. Chem. Res. 2004, 43, 2619−2626. (6) Centeno, A.; Laurent, E.; Delmon, B. J. Catal. 1995, 154, 288− 298. (7) Ferrari, M.; Maggi, R.; Delmon, B.; Grange, P. J. Catal. 2001, 198, 47−55. (8) Peng, J.; Chen, P.; Lou, H.; Zheng, X. M. Bioresour. Technol. 2009, 100, 3415−3418. (9) Garcia, L.; French, R.; Czernik, S.; Chornet, E. Appl. Catal., A 2000, 201, 225−239. (10) Czernik, S.; French, R.; Feik, C.; Chornet, E. Ind. Eng. Chem. Res. 2002, 41, 4209−4215. (11) Dou, B. L.; Dupont, V.; Rickett, G.; Blakeman, N.; Williams, P. T.; Chen, H. S.; Ding, Y. L.; Ghadiri, M. Bioresour. Technol. 2009, 100, 3540−3547. (12) Rioche, C.; Kulkarni, S.; Meunier, F. C.; Breen, J. P.; Burch, R. Appl. Catal., B 2005, 61, 130−139. (13) Xu, Y.; Wang, T.; Ma, L.; Zhang, Q.; Wang, L. Biomass Bioenergy 2009, 33, 1030−36. (14) Xu, Y.; Wang, T.; Ma, L.; Chen, G. Energy Convers. Manage. 2012, 55, 172−77. (15) Ardiyanti, A. R.; Khromova, S. A.; Venderbosch, R. H.; Yakovlev, V. A.; Heeres, H. J. Appl. Catal., B 2012, 117, 105−17. (16) Runnebaum, R. C.; Nimmanwudipong, T.; Block, D. E.; Gates, B. C. Catal. Lett. 2011, 141, 817−820. (17) Branca, C.; Giudicianni, P.; Di Blasi, C. Ind. Eng. Chem. Res. 2003, 42, 3190−202. (18) Li, K.; Wang, R.; Chen, J. Energy Fuels 2011, 25, 854−63. (19) Runnebaum, R. C.; Nimmanwudipong, T.; Block, D. E.; Gates, B. C. Catal. Sci. Technol. 2012, 2, 113−118. (20) Runnebaum, R. C.; Nimmanwudipong, T.; Block, D. E.; Gates, B. C. Energy Fuels 2011, 25, 4776−85. (21) Istadi, I.; Amin, N. A. S. Chem. Eng. Sci. 2007, 62, 6568−81. (22) Harling, A. M.; Glover, D. J.; Whitehead, J. C.; Zhang, K. Environ. Sci. Technol. 2008, 42, 4546−50. (23) Liang, Y.; Li, X.; Tu, X.; Wang, Y.; Lu, S.; Yan, J. J. Phys. Chem. A 2009, 114, 360−68. (24) Gorry, A.; Whitehead, J. C.; Wu, J. Plasma Processes Polym. 2007, 4, 556−62. (25) Liu, Ch.-J.; Xu, G.-H.; Wang, T. Fuel Process. Technol. 1999, 58, 119−34. (26) Ayrault, C.; Barrault, J.; Blin-Simiand, N.; Jorand, F.; Pasquiers, S.; Rousseau, A.; Tatibouet, J. M. Catal. Today 2004, 89, 75−81. (27) Górska, A.; Krawczyk, K.; Jodzis, S.; Schmidt-Szałowski, K. Fuel 2011, 90, 1946−1952. (28) Mok, Y. S.; Kang, H. C.; Lee, H. J.; Koh, D. J.; Shin, D. N. Plasma Chem. Plasma Process. 2010, 30, 437−447. (29) Rahimpour, M. R.; Jahanmiri, A.; Shirazi, M. M.; Hooshmand, N.; Taghvaei, H. Chem. Eng. J. 2013, 219, 245−53. (30) Hooshmand, N.; Rahimpour, M. R.; Jahanmiri, A.; Taghvaei, H.; Shirazi, M. M. Ind. Eng. Chem. Res. 2013, 52, 4443−49. (31) Taghvaei, H.; Shirazi, M. M.; Hooshmand, N.; Rahimpour, M. R.; Jahanmiri, A. Appl. Energy 2012, 98, 3−10.

6. CONCLUSIONS Anisole conversion in a pulsed DBD plasma reactor in the presence of Ni−Mo/Al2O3, Pt/Al2O3, Co−Mo/Al2O3, Re−Pt/ Al2O3, and Al2O3 catalysts was observed to give products matching those observed in conventional catalytic hydroprocessing of anisole. The combination of plasma and the Ni− Mo/Al2O3, catalyst gave anisole conversions up to 0.81, and similar but lower conversions were observed with other catalysts, Pt/Al2O3, Co−Mo/Al2O3, Re−Pt/Al2O3, and Al2O3. In contrast, under the same conditions in the absence of a catalyst, the anisole conversion in the DBD plasma reactor was only 0.43. The results show that in all the experiments with catalysts the selectivity to 4-methylanisole was high and the selectivities for formation of other compounds, including benzene and phenol, for example, were lower. The DBD plasma reactor suggests a novel technology that offers potential advantages for conversion of biomass and bio-oils, possibly including high conversions at low temperatures and pressures. Plasma-assisted catalysis also has the potential of reducing catalyst poisoning, coking, and sintering. 7430

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(32) Jahanmiri, A.; Rahimpour, M. R.; Shirazi, M. M.; Hooshmand, N.; Taghvaei, H. Chem. Eng. J. 2012, 191, 416−425. (33) Taghvaei, H.; Jahanmiri, A.; Rahimpour, M. R.; Shirazi, M. M.; Hooshmand, N. Chem. Eng. J. 2013, 226, 384−392. (34) Amin, N. A. S. Fuel 2006, 85, 577−92. (35) Heintze, M.; Pietruszka, B. Catal. Today 2004, 89, 21−25. (36) Kizling, M. B.; Järås, S. G. Appl. Catal., A 1996, 147, 1−21. (37) Jung, S. H.; Park, S. M.; Park, S. H.; Kim, S. D. Ind. Eng. Chem. Res. 2004, 43, 5483−88. (38) Kraus, M.; Eliasson, B.; Kogelschatz, U.; Wokaun, A. Phys. Chem. Chem. Phys. 2001, 3, 294−300. (39) Whitehead, J. Ch. Pure Appl. Chem. 2010, 82, 1329−36. (40) Yan, J. H.; Bo, Z.; Li, X. D.; Du, C. M.; Cen, K. F.; Cheron, B. G. Plasma Chem. Plasma Process. 2007, 27, 115−26. (41) Indarto, A. J. Chin. Inst. Chem. Eng. 2008, 39, 23−28. (42) Mok, Y. S.; Nam, I. S. Chem. Eng. J. 2002, 85, 87−97. (43) Futamura, S.; Zhang, A. H.; Yamamoto, T. J. Electrostat. 1997, 42, 51−62. (44) Sobacchi, M. G.; Saveliev, A. V.; Fridman, A. A.; Kennedy, L. A.; Ahmed, S.; Krause, T. Int. J. Hydrogen Energy 2002, 27, 635−42. (45) Van Durme, J.; Dewulf, J.; Leys, C.; Van, H. Appl. Catal., B 2008, 78, 324−33. (46) Kriegseis, J.; Grundmann, S.; Tropea, C. J. Appl. Phys. 2011, 110, 013305−013305.9. (47) Subrahmanyam, C.; Magureanu, M.; Renken, A.; Kiwi-Minsker, L. Appl. Catal., B 2006, 65, 150−56. (48) Guo, Y.; Ye, D.; Chen, K.; He, J. Catal. Today 2007, 126, 328− 37. (49) Tang, S.; Lu, N.; Li, J.; Wu, Y. Purif. Technol. 2012, 95, 73−79. (50) Goujard, V.; Tatibouët, J. M.; Batiot-Dupeyrat, C. Appl. Catal., A 2009, 353, 228−35. (51) Bhoj, A. N.; Kushner, M. J. Plasma Sources Sci. Technol. 2008, 17, 035024−35.

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