Unimolecular Decomposition Pathway for the Vapor-Phase Cracking

Article pubs.acs.org/EF

Unimolecular Decomposition Pathway for the Vapor-Phase Cracking of Eugenol, A Biomass Tar Compound Elmer B. Ledesma,*,† Jennifer N. Hoang,† Quan Nguyen,‡ Valeria Hernandez,† Mitchell P. Nguyen,† Shuhsien Batamo,‡ and Carla K. Fortune‡ †

Department of Chemistry and Physics, University of St. Thomas, Houston, Texas 77006, United States Houston Community College, Central Campus, Houston, Texas 77004, United States

‡

ABSTRACT: To design optimal thermochemical processes for the conversion of biomass into chemicals, fuels, and electrical power, an understanding of the mechanisms for the secondary vapor-phase cracking of tar compounds is crucial. Despite the many studies examining the homogeneous secondary cracking of biomass tar existing in the literature, its thermal decomposition reaction pathways are not completely understood. Much of this lack of understanding is due to the complex, heterogeneous nature of biomass tar. A useful approach is to examine the pyrolysis of model-fuel compounds that are actual components or are representative of compounds found in biomass tar. In this study, we focus on eugenol, a model-fuel compound representative of the lignin-derived components found in biomass tar. We conduct pyrolysis experiments at temperatures of 300−900 °C and one second residence time using a non-isothermal laminar-flow reactor system. We report the variation in the experimental yield of light product gases as functions of the reactor temperature. We examine a reaction pathway for the unimolecular decomposition of eugenol with consideration of the experimental product distributions and analogous reactions based on established decomposition mechanisms of similar compounds. We examine the detailed energetics of the unimolecular decomposition route using computational chemistry calculations at the B3LYP/6-311G+(d,p) level of theory. The results presented in this study would be of relevance to the pyrolysis, gasification, and combustion of biomass.

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INTRODUCTION Lignocellulosic biomass, such as wood, agricultural and forestry waste products, and energy crops, is an inexpensive and abundant plant material that can be used for the production of chemicals, transportation fuels, and electrical power generation.1,2 Thermochemical processes, such as fast pyrolysis, gasification, and combustion, provide economically viable routes for the conversion of lignocellulosic biomass into these globally important commodities.3 The process of pyrolysis (application of heat in the absence of oxygen) is common in thermochemical conversion processes and is the initial step in both gasification and combustion.4,5 The pyrolysis step initially involves the primary thermal cracking of the biomass structure, producing high-molecular-weight volatile tar compounds and solid char. The amounts of tar and char produced are dependent upon the heating rate, where high heating rates as employed in fast pyrolysis, gasification, and combustion favor the production of tar.4−6 Once produced, the tar compounds undergo secondary thermal cracking to produce lower molecular weight tar compounds and gases. Because of the high heating rates used in fast pyrolysis, gasification, and combustion, tar compounds and their subsequent secondary vapor-phase cracking reactions are of significance. Although tar compounds produced from fast pyrolysis can be catalytically upgraded to chemicals and fuels,6−8 they can be detrimental in gasification and combustion. In gasification, tar compounds cause coking, fouling, and catalyst deactivation.5,9,10 In combustion, the secondary cracking of tar compounds is a source of soot and the precursors that lead to its formation.4,11,12 To design optimal thermochemical processes for the conversion of lignocellulosic biomass into chemicals, © 2013 American Chemical Society

fuels, and electrical power, it is crucial to understand the secondary vapor-phase cracking mechanisms of the tar compounds. Although several studies examining the homogeneous secondary cracking of biomass tar exist in the literature,13−21 the thermal decomposition mechanisms of biomass tar compounds are not completely understood. Much of this lack of understanding is due to the complex, heterogeneous composition of biomass tar, which is comprised of a variety of chemical compounds: acids, esters, alcohols, sugars, furans, and phenols, such as guaiacols and syringols.6,8,22−24 To study in detail the vapor-phase cracking mechanisms of tar compounds, a useful approach is to examine the pyrolysis of model compounds that are actual components or are representative of compounds found in biomass tar. There are a number of studies in the literature pertaining to the pyrolysis of model biomass tar compounds, such as furan,25,26 phenol,27−30 anisole,31−33 catechol,34−38 guaiacols,24,39−42 and syringols.42 In this study, we have used a flow reactor system and computational chemistry calculations to examine a unimolecular decomposition pathway for the vapor-phase cracking of the model compound, eugenol (4-allyl-2-methoxyphenol). Eugenol is an alkyl-substituted guaiacol and has been observed as a product from the primary pyrolysis of wood.13,14,24 Eugenol thus serves as a model compound to investigate the secondary tarcracking process, because its structure is representative of the tar components, especially the lignin-derived components, obtained Received: August 31, 2013 Revised: October 8, 2013 Published: October 29, 2013 6839

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thermal decomposition is the scission of the C−O bond in the methoxy functional group. Eugenol is similar in structure to both vanillin and ortho-guaiacol, and all molecules are derived from anisole. We therefore expect the thermal decomposition mechanism of eugenol to be similar to these compounds, with the initiation reaction governed by the scission of the C−O bond in the methoxy functional group.

from the primary pyrolysis of biomass. To the best of our knowledge, no study in the literature has yet examined the unimolecular thermal cracking mechanism of eugenol. In this study, we report the variation in light gas products as functions of the reactor temperature. We discuss the reaction pathway for the unimolecular decomposition of eugenol in terms of the experimental gas product distribution and computational chemistry calculations. Our present study complements our previous eugenol pyrolysis study43 that examined the distribution of condensable tar compounds as functions of the temperature and residence time.

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EXPERIMENTAL SECTION

A description of our flow reactor system, product collection, and analytical instrumentation is discussed elsewhere.43 We briefly describe the system here. A non-isothermal laminar-flow reactor system is used for the vapor-phase cracking experiments. The flow-reactor system consists of a fuel vaporizer, a quartz tube heated by a single-zone tube furnace, and a Tedlar gas bag for the collection of light gas products. Because of the nonlinear axial temperature profile in the reactor, the furnace set point temperatures are used to report the results in this study. To conduct the vapor-phase cracking experiments, a small amount of eugenol is first vaporized inside the vaporizer, which consists of a glass impinger held fixed inside a gravity convection oven kept at a constant temperature of 90 °C. The glass impinger is charged with eugenol. Nitrogen gas is continuously dispersed into the eugenol within the impinger by means of a mass flow controller. Upon exiting the vaporizer, the eugenol/nitrogen feed stream enters the laminar-flow reactor with an inlet concentration in eugenol of 0.12 mol %. Pyrolysis experiments are performed at furnace set point temperatures between 300 and 900 °C in 50 °C increments at a residence time of 1 s. Such operating parameters are typical to those found in industrial pyrolysis, gasification, and combustion processes.1 Pyrolysis experiments at each set point temperature are performed in triplicate, and the mean value of each experiment is used to report the results presented in this study. Exiting the reactor, gas products are collected in a Tedlar gassampling bag and analyzed via gas chromatography using a HP5890 series II gas chromatograph equipped with flame ionization detection for analysis of C1−C6 hydrocarbons and thermal conductivity detection for carbon monoxide analysis. Separation of hydrocarbon gases is achieved using an alumina-BOND/Na2SO4 PLOT column, and carbon monoxide is separated using a molecular sieve PLOT column. Products are identified by matching the retention time of each product to those of reference standard compounds.

Table 1 displays computed bond dissociation energies of probable bond scission reactions in eugenol. Because no Table 1. Bond Dissociation Energies Computed at the B3LYP/6-311G+(d,p) Level of Theory

experimental values for bond dissociation energies are available for eugenol, we have computed the equivalent methoxy C−O bond dissociation energies in anisole and ortho-guaiacol to assess the accuracy of the B3LYP/6-311G+(d,p) level of theory adopted in this study. Our calculated values are 62.1 kcal mol−1 for anisole and 53.5 kcal mol−1 for ortho-guaiacol. The corresponding experimental values41 are 63.4 and 57.5 kcal mol−1, respectively. The good agreement between our calculated values to the experimental values shows that the B3LYP/6-311G+(d,p) level of theory is a reasonable method for investigating the energetics of the thermal decomposition of eugenol. The results in Table 1 clearly demonstrate that the lowest bond dissociation energy is indeed the C−O bond in the methoxy functional group. The computed value at the B3LYP/ 6-311G+(d,p) level of theory for the methoxy C−O bond dissociation energy in eugenol obtained in this study compares well to the 50.7 kcal mol−1 activation energy obtained in our previous experimental study that examined the global kinetics of eugenol decomposition.43 The initiation reaction results in the formation of two radical species: methyl and 5-allyl-2-hydroxyphenoxyl, labeled R in eq 1. Once produced, methyl radicals can abstract hydrogen atoms from the fuel molecule to produce methane. The methyl radicals can also undergo radical recombination to generate ethane. Both methane and ethane were observed as hydrocarbon gas products in our experiments. Figure 1 shows the experimental fractional yield data of both methane and ethane as functions of the temperature. In this study, the fractional yield of a product is defined as the ratio of its outlet mass flow rate to the eugenol mass flow rate in the reactor feed stream. As the figure illustrates,

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COMPUTATIONAL CHEMISTRY METHOD Computational chemistry calculations were conducted to examine the relative potential energies of the species involved in the unimolecular thermal decomposition pathway for the vapor-phase cracking of eugenol. All calculations were performed using the Gaussian 09 software suite.44 Geometries of all reactants, products, and transition states were optimized at the B3LYP/6-311G+(d,p) level of theory.45 Vibrational frequencies and zero- and single-point energies of all reactants, products, and transition states were computed at this same level of theory. Transition-state structures were located using the combined quadratic synchronous transit and quasi-Newton (STQN) method46,47 and confirmed by possessing one imaginary frequency, whose vibrational motions yield reactants in one direction and products in the other.

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RESULTS AND DISCUSSION Previous studies on the pyrolysis of anisole,31−33 vanillin,39 and ortho-guaiacol40,41 show that the initial step leading to their 6840

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in eugenol as carbon monoxide gas, a major light gas product observed in our experiments. The energetics of the reaction steps illustrated in eqs 2 and 3 are summarized in the potential energy diagram of Figure 2. All energy values calculated are relative to ground-state R at 0 K.

Figure 1. Fractional yields of methane (●) and ethane (▽) as functions of the temperature.

Figure 2. Potential energy diagram of the unimolecular decomposition of R to P3 and CO. T1−T3 indicate the transition states of each reaction step.

the methane yield increases as the temperature increases, obtaining a fractional yield of 0.13 at 900 °C. This high yield makes methane one of the major light gas products observed at the highest temperature examined. Ethane yields are much lower than methane at all temperatures, attaining a maximum yield of 0.046 at 800 °C. The formation of the 5-allyl-2-hydroxyphenoxyl radical is a key route that leads to the complete breakdown of the aromatic ring in eugenol. Pyrolysis studies of molecules related to eugenol, such as catechol,35,37,38 vanillin,39 and ortho-guaiacol,41 have also postulated the significant role of substituted phenoxyl radicals in the thermal decomposition mechanisms. The formation of the unsubstituted phenoxyl radical has also been shown by Liu et al.30 to be a major pathway leading to the thermal decomposition of phenol. By analogy to the unimolecular decomposition mechanism proposed for the phenoxyl radical by Liu et al.,30 the 5-allyl-2-hydroxyphenoxyl radical undergoes an isomerization reaction over an energy barrier of 62.2 kcal mol−1 to produce P1, an allyl-substituted bicyclic radical. This is then followed by fission of a C−C bond in P1 to generate P2, a five-carbon ring radical, over an activation energy barrier of 7.3 kcal mol−1. The steps are depicted in eq 2 below.

In their study on toluene oxidation, Emdee et al.48 suggested that, on the basis of thermochemical considerations, an unsubstituted cyclopentadienol-lyl radical would unimolecularly decompose to produce cyclopentadienone and a hydrogen atom. By analogy to this mechanism suggested by Emdee et al.,48 the O−H bond in P3, the allyl-substituted cyclopentadienol-lyl radical, dissociates to generate P4, 3-allylcyclopentadienone, and a hydrogen atom.

Our B3LYP/6-311G+(d,p) computations revealed that the reaction shown in eq 4 has an activation energy of 52.0 kcal mol−1, with the products lying 49.0 kcal mol−1 higher in energy than the reactant P3. Wang and Brezinsky49 showed that elimination of carbon monoxide from unsubstituted cyclopentadienone yields cyclobutadiene via an oxobicyclopentene intermediate. Because of the close similarity between P4, an allyl-substituted cyclopentadienone, and cyclopentadienone, we expect P4 to unimolecularly decompose in an analogous fashion to that proposed by Wang and Brezinsky.49 The reaction sequence is depicted in eq 5.

A C−C single bond in P2 dissociates with an activation energy of 3.6 kcal mol−1 to produce P3, an allyl-substituted cyclopentadienol-lyl radical, as shown in eq 3 below. The step depicted in eq 3 also results in the release of the first oxygen atom 6841

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require an activation energy of 50.3 kcal mol−1. Unimolecular decomposition of P7 over an energy barrier of 48.5 kcal mol−1 then yields two products: allylacetylene, P8, and acetylene, one of the major hydrocarbon products observed in our experiments. Equation 6 summarizes the reaction sequence from P6 to P8 and acetylene and is analogous to the mechanism suggested by Mebel et al.50 The energetics of the reaction steps illustrated in eq 6 are summarized in the potential energy diagram of Figure 5. All energy values calculated are relative to ground-state P6 at 0 K.

The first step in eq 5 generates an allyl-substituted oxobicyclopentene intermediate, P5, over an energy barrier of 66.7 kcal mol−1. Elimination of carbon monoxide in P5 involves an activation energy of 27.1 kcal mol −1 and then produces P6, 1-allylcyclobutadiene. The energetics of the reaction steps illustrated in eq 5 are summarized in the potential energy diagram of Figure 3. All energy values calculated are relative to ground-state P4 at 0 K.

Figure 3. Potential energy diagram of the unimolecular decomposition of P4 to P6 and CO. T5 and T6 indicate the transition states of each reaction step.

Figure 5. Potential energy diagram of the unimolecular decomposition of P6 to P8 and C2H2. T7 and T8 indicate the transition states of each reaction step.

Figure 4 shows the fractional yield of carbon monoxide as a function of the temperature. The experimental results in Figure 4

An alternative route from P6 to acetylene and allylacetylene is also possible. The second pathway is similar to that depicted in eq 6 but with the other C−C single bond in the cyclobutadiene ring of P6 breaking to produce a diradical species. The 63.1 kcal mol−1 energy barrier calculated for this process is higher than that for the route P6 → P7 shown in eq 6. Moreover, the diradical produced in the alternate route has all of its radical centers as primary, whereas in P7, it has a primary radical center and a secondary radical center. Our calculations indeed show that P7 is 5.4 kcal mol−1 more stable. Thus, because of the thermodynamic stability of P7 and the lower activation energy to produce it from P6, the sequence given by eq 6 would be favored over the alternative route. Scission of the C−C single bonds in allylacetylene produces the ethynyl, allyl, vinyl, and propargyl radicals.

Figure 4. Fractional yield of carbon monoxide as function of the temperature.

show that carbon monoxide increases in yield as the temperature increases. The fractional yield value of 0.23 at the highest temperature investigated in this study makes carbon monoxide one of the major light gas products from the vapor-phase cracking of eugenol. The sequence of reactions from eqs 1 to 5 appears to be a viable reaction pathway for the high yields of carbon monoxide observed in our experiments. Scission of a C−C single bond in the cyclobutadiene ring of P6 produces a diradical species, P7. This process was computed to 6842

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Our calculations show that the products of eq 7 are 26.0 kcal mol−1 higher in energy than the products of eq 8. As such, it is expected that the favored decomposition of P8 would proceed via eq 8. The ethynyl, allyl, vinyl, and propargyl radicals produced in eqs 7 and 8 are likely sources for the production of the other hydrocarbon products observed in our experiments. We observed that ethylene and acetylene were two major hydrocarbon gas products from the vapor-phase cracking of eugenol, with high yields especially at high temperatures. Figure 6 shows

Figure 7. Fractional yields of propylene (●), propane (▽), and propadiene (■) as functions of the temperature. Yield values of propane have been scaled by a factor of 10.

undergone by the vinyl and ethynyl radicals to produce ethylene and acetylene, respectively, the allyl radical can abstract a hydrogen atom from the fuel molecule or other stable products to generate propylene. A route toward propane formation first involves the addition reaction between a methyl radical and ethylene to produce the propyl radical, CH3 + C2H4 → n-C3H7.51 This step is then followed by hydrogen abstraction of eugenol or other stable species to give propane. A likely source for propadiene is the unimolecular decomposition of the ally radical, C3H5 → C3H4 + H.52 This reaction pathway would be in competition with the hydrogen abstraction reaction of eugenol by the allyl radical to produce propylene. The much higher yields of propylene compared to propadiene at all temperatures examined suggest that the bimolecular reaction between the allyl radical and eugenol, which is in abundant supply in the pyrolysis environment, is favored over the decomposition of the allyl radical to give propadiene. Figure 8 illustrates the fractional yields of 1-butene and 1,3butadiene as functions of the temperature. Above 450 °C, yields of 1-butene increase sharply with the temperature, exhibiting a maximum yield of 0.097 at 650 °C. This is then followed by a steep decrease in yields at higher temperatures. Its peak yield at 650 °C makes 1-butene the highest yield hydrocarbon at temperatures below 700 °C. In comparison to 1-butene, fractional yields of 1,3-butadiene show a slower increase in the yield above 500 °C. It peaks at 750 °C with a yield of 0.028, followed by a slow decrease in the yield at higher temperatures. A source for the high yields of 1-butene at temperatures below 700 °C is the recombination reaction between the methyl radical and the allyl radical, CH3 + C3H5 → C4H8.53 1,3-Butadiene can be produced via the recombination reaction between the methyl radical and propargyl radical, CH3 + C3H3 → C4H6.53 Isomers of 1-butene were also experimentally observed. Figure 9 shows the fractional yields of trans-2-butene, cis-2-butene, and isobutylene as functions of the temperature. Their low yields show that these 1-butene isomers are minor products from the vaporphase cracking of eugenol. The experimental results show that all of their yields peak at 750 °C, with a maximum value of 0.0029 for trans-2-butene, 0.0023 for cis-2-butene, and 0.0019 for isobutylene. trans-2-Butene and cis-2-butene can be formed via the isomerization

Figure 6. Fractional yields of ethylene (●) and acetylene (▽) as functions of the temperature.

the fractional yields of ethylene and acetylene as functions of the temperature. The experimental results presented in Figure 6 illustrate that, for both gases, their fractional yields increase with the temperature, especially above 450 °C for ethylene and above 600 °C for acetylene. At 900 °C, the fractional yields of ethylene and acetylene are 0.15 and 0.12, respectively. A source for the formation of ethylene is the abstraction of a hydrogen atom from the fuel molecule (or other stable product) by the vinyl radical. Acetylene was shown above in eq 6 to be directly produced from the decomposition of P6. A source of acetylene derives from the hydrogen abstraction of the fuel molecule or other stable product by the ethynyl radical. In addition to ethylene and acetylene, the C3 hydrocarbons, propane, propylene, and propadiene, were also observed as products from our vapor-phase cracking experiments. Figure 7 shows the experimental fractional yields of propane, propylene, and propadiene as functions of the temperature. The experimental results in Figure 7 show that propylene was the highest yielding C3 hydrocarbon, with a maximum fractional yield of 0.075 at 750 °C. Propadiene achieved a maximum yield of 0.0094 at 800 °C. Yields of propane were found to be much lower than those for propylene and propadiene at all temperatures examined. For better clarity, the results for propane presented in Figure 7 have been scaled by a factor of 10. The results show that propane fractional yields never go above 0.0020 at all temperatures investigated. The allyl radical produced in eq 7 is the most likely source for propylene. Similar to the hydrogen abstraction reactions 6843

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sources for both acetylene and methane production. The vinyl radical can combine with the ethynyl radical to generate acetylene (C2H3 + C2H → 2C2H2) or with the allyl radical to give ethylene and propadiene (C2H3 + C3H5 → C2H4 + C3H4). Decomposition of the allyl radical can lead to the production of ethylene (C3H5 → C2H4 + CH) and acetylene and methyl radical (C3H5 → C2H2 + CH3) that can lead to further methane production. The reaction between the allyl radical and the ethynyl radical can produce acetylene (C3H5 + C2H → C2H2 + C3H4) or the vinyl and propargyl radicals (C3H5 + C2H → C2H3 + C3H3). The formation of coke or soot is an alternate and important reaction pathway involving the ethynyl, allyl, vinyl, and propargyl radicals. Reactions between the coke surface and methyl radicals may also be responsible for some of the methane production observed in this study. Previous studies on anisole pyrolysis have shown that such surface reactions account for some of the methane produced.31,32 There have been extensive studies investigating the mechanisms for the formation of coke or soot during the pyrolysis of hydrocarbons and solid fuels.60−65 These studies show that aromatic ring growth is accomplished through a successive ring build-up sequence involving the successive additions of small hydrocarbon species, such as the ethynyl, allyl, vinyl, and propargyl radicals. These radicals are likely sources for the formation of benzene observed in this study. Figure 10 portrays the experimental fractional yield of benzene as a function of the temperature. The results in the figure

Figure 8. Fractional yields of 1-butene (●) and 1,3-butadiene (▽) as functions of the temperature.

Figure 9. Fractional yields of trans-2-butene (●), isobutylene (▽), and cis-2-butene (■) as functions of the temperature.

of 1-butene to 2-butene.54 Isobutylene can be formed through a sequence starting with propadiene reacting first with the methyl radical to produce the isobutenyl radical, C3H4 + CH3 → i-C4H7.54 Hydrogen abstraction by the isobutenyl radical then produces isobutylene.54 The ethynyl, allyl, vinyl, and propargyl radicals may also participate in reactions among themselves, thereby providing alternate routes to generate some of the hydrocarbon gases observed in this study. These reactions are well-documented in the literature and include self-reactions, recombination reactions, and decomposition reactions.52−59 Three such reactions have already been discussed above for the formation of 1-butene, 1,3butadiene, and isobutylene. Another example is the self-reaction of vinyl radicals to produce the observed products ethylene and acetylene, 2C2H3 → C2H2 + C2H4. Decomposition of the vinyl radical (C2H3 → C2H2 + H) and its reaction with the methyl radical (C2H3 + CH3 → C2H2 + CH4) provides additional

Figure 10. Fractional yield of benzene as function of the temperature.

demonstrate that benzene yields increase with the temperature, especially above 550 °C. A direct route toward benzene involves the self-reaction of the propargyl radical.56 Once the first aromatic ring is built, larger aromatic units and eventually coke or soot can be produced through successive additions of ethynyl, allyl, vinyl, and propargyl radicals.66−70 Although we did not quantitatively examine coke or soot in our experiments, we did observe carbonaceous deposits on the walls of our reactor after experiments at all temperatures above 700 °C. Moreover, our previous eugenol pyrolysis study revealed the presence of polycyclic aromatic hydrocarbons as tar products at temperatures above 700 °C.43 Our main focus in this study has been on the reaction pathway for the unimolecular decomposition of eugenol. Although we 6844

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gratefully acknowledge helpful comments and recommendations of the reviewers for this manuscript.

have discussed a number of reactions that lead to the formation of experimentally observed products, it must be pointed out that other pathways are also possible that could contribute to the formation of these observed products. Examples include pathways stemming from the products of hydrogen abstraction on the fuel molecule by hydrocarbon radicals. Subsequent reactions of these products will result in the formation of light gas products. To investigate the relative importance of these alternate routes to the unimolecular route, a detailed kinetic modeling study needs to be performed, and such a study is the subject of our currently ongoing work.

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CONCLUSION A reaction pathway for unimolecular decomposition during the vapor-phase cracking of eugenol, a model-fuel compound representative of the lignin-derived components found in biomass tar, was investigated both experimentally and computationally. Vapor-phase cracking experiments at temperatures of 300−900 °C and 1 s residence time were conducted in a nonisothermal laminar-flow reactor operated at atmospheric pressure. Analysis of the light gas products by gas chromatography with flame ionization and thermal conductivity detections revealed that the major products were methane, ethylene, acetylene, propylene, 1-butene, benzene, and carbon monoxide. Minor products were ethane, propane, propadiene, trans-2butene, cis-2-butene, and isobutylene. Computational chemistry calculations at the B3LYP/6-311G+(d,p) level of theory was used to examine the unimolecular decomposition reaction pathways. The computations demonstrated that the cracking process is initiated by the scission of the C−O bond in the methoxy group. Methane can result from the hydrogen abstraction of eugenol by the methyl radical produced in the initiation step. A 5-allyl-2-hydroxyphenoxyl radical (R) is also produced in the initiation step. Through a series of isomerization steps (R → P1 → P2 → P3 + CO), R decomposes, resulting in the production of an allyl-substituted cyclopentadienol-lyl radical (P3) and carbon monoxide. P3 then decomposes to P4, an allylsubstituted cyclopentadienone, which, through a series of isomerization steps (P4 → P5 → P6 + CO), decomposes to generate 1allylcyclobutadiene (P6) and carbon monoxide. Rupture of P6 via the diradical P7 results in the production of allylacetylene (P8) and acetylene. Decomposition of P8 results in the production of four radicals: ethynyl, allyl, vinyl, and propargyl. Reactions of these radicals with eugenol, other stable products, or among themselves produce the other experimentally observed hydrocarbon products. In addition, these radicals are likely sources for aromatic ring growth that eventually lead to the formation of coke or soot.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Telephone: +1-713-831-7810. Fax: +1-713-942-3460. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the U.S. Department of Education, Office of Postsecondary Education, Hispanic-Serving Institutions, STEM Articulation Grant P031C110128 and CCRAA Grant P031C080184. This work was also funded in part by the Welch Foundation Grant AV-0024 and the Dunn Foundation Grant. The authors also 6845

dx.doi.org/10.1021/ef401760c | Energy Fuels 2013, 27, 6839−6846

Energy & Fuels

Article

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