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A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma
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A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma Lu Chen a, Dang-guo Cheng a,b,*, Fengqiu Chen a,b, Xiaoli Zhan a,b a
Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, 310027, China b Institute of Zhejiang University-Quzhou, 78 Jiuhua Boulevard North, Quzhou, 324000, China
highlights DFT method is used to explore the decomposition pathways of PAHs in hydrogen plasma. The participation of hydrogen radicals makes the dissociation of naphthalene easier. C2H2 and H2 are the major products in hydrogen plasma pyrolysis.
article info
abstract
Article history:
Tar is the byproduct of fuel in the pyrolysis or gasification process. Hydrogen plasma could
Received 19 August 2019
effectively promote the decomposition of tar into acetylene and hydrogen, but the detailed
Received in revised form
cracking mechanism is difficult to detect. The DMol3 calculations, based on density func-
14 October 2019
tional theory (DFT), have been employed to explore the pyrolysis pathways of naphthalene.
Accepted 26 October 2019
Naphthalene is chosen as the model compound for polycyclic aromatic hydrocarbons
Available online xxx
(PAHs), which are the main components of tar. Our calculations investigate that the energy barriers required for the reactions are greatly reduced due to the participation of active
Keywords:
hydrogen atoms. Naphthalene is easily converted into acetylene, hydrogen and carbon
Plasma
black through two main routes. This is in good agreement with the experiment results.
Tar
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Density functional theory Pyrolysis Acetylene Hydrogen
Introduction Pyrolysis and gasification are promising techniques to convert fuels, such as coal, petroleum and renewable biomass into
fuel gas (CO and H2) for industrial use, where tar is an inevitable byproduct [1e5]. Tar is a mixture of heavy organic compounds - mainly of aromatic nature. At low temperature, it is prone to polycondensation to form soot, which easily blocks and fouls process equipment like fuel lines, filters and
* Corresponding author. Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, 310027, China. E-mail address: [email protected] (D.-g. Cheng). https://doi.org/10.1016/j.ijhydene.2019.10.208 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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engines [3,6,7]. At present, traditional thermochemical methods for removing tar include chiefly thermal cracking and catalytic cracking [3,8e11]. The thermal cracking of tar imposes high energy cost in production process since the conversion of aromatic hydrocarbons of tar, like benzene and naphthalene, into smaller gas molecules requires temperatures over 1100 C. Meanwhile, catalytic cracking method reduces the reaction temperature to some extent, at the expense of high catalyst cost, low lifetime, sulfur poisoning and carbon deposition [9]. Therefore, an efficient and economical cracking method is urgently desired. Plasma methods can overcome the above problems due to the presence of reactive species such as electrons, ions, radicals, and etc. And it appears to be economic and energy efficient. Basic research on non-thermal plasmas such as pulsed plasma [12], sliding arc plasma [13e15] and microwave-assisted plasma [16,17] for tar destruction has been studied. Among them, the electrodeless microwave plasma has recently received great attention, due to the avoidance of electrode contamination and low cost of magnetrons, whose construction is simple and compact. Wnukowski et al. achieved 98% tar conversion using microwave plasma method with the initial tar concentration of 10 g/Nm3, the nitrogen gas flow rate being 30 L/min and the steam-tocarbon ratio equal to 3. It has been revealed that the main products of the tar decomposition were acetylene, carbon black, and benzene derivatives [18]. Their amounts were significantly decreased in favor of CO, CO2, and H2 by steam addition. Notably, carbon black produced in the former will greatly affect the gas discharge efficiency, while the latter CO2 inevitably aggravates the greenhouse effect. However, non-thermal plasma has disadvantages in the scale-up in practical applications, hindering its further development in tar disposition. Ultra-high temperature and high heating rate of thermal plasma promote pyrolyzing fuels and generating light hydrocarbons (methane, ethane, acetylene, and etc.) and a large amount of H2 and CO. These products can be obtained by means of rapid quenching [19,20]. Thus, when using H2 as plasma working gas, plasma-assisted pyrolysis is a hydrogen self-sufficient process. Applying thermal plasma technology to produce acetylene is a special and progressive route, while its industrial commercialization at home and abroad is basically mature [21]. In terms of experimental researches, Cheng et al. decomposed coal tar into acetylene and other light gases in thermal plasma, wherein the acetylene yield could reach up to 45% when using Ar plasma [22]. They also summarized that increased hydrogen concentration in plasma working gases enhanced the coal tar conversion and the yield of acetylene, and also reduced coking, which was generated by vapor deposition of carbon black and condensation reactions [23]. Huang et al. explored the cracking pathways of coal in hydrogen plasma, and it was investigated by theoretical calculation that the addition of hydrogen radicals made the dehydrogenation and pyrolysis reactions more favorable [24,25]. The thermal decomposition of tar is a process in which chemical covalent bonds are broken into active radical fragments, whose detection is hardly achieved by modern means [26]. Hence, utilizing theoretical calculation to probe its decomposition mechanism is necessary and imperative.
In order to explore the cracking paths of tar in hydrogen plasma, providing theoretical guidance for the effective removal of tar, density functional theory (DFT) is used in this work. As the main components of tar [19,27], polycyclic aromatic hydrocarbons (PAHs) attach great importance to tar pyrolysis in hydrogen plasma because of the high aromaticity. Naphthalene is the simplest PAH, which has been selected as a model compound of polycyclic aromatic hydrocarbons in many references [15,19,28e34]. The optimal pathways of naphthalene in hydrogen plasma will be well explored and analyzed in this study.
Method of calculation Density functional theory (DFT) is applied through the interface of the Dmol3 module using Material Studio software, which has been widely applied in the field of quantum chemistry in recent years [35,36]. In the geometric optimization of reactants and products and the search for the transition states, the vibration frequencies of these molecules need to be calculated to obtain zero-point energy (ZPE) corrections. The frequencies of the reactants and products are guaranteed to be positive, while the transition state has one and only one imaginary frequency. PW91 functional of GGA, with the basic set of DNP, is employed to optimize geometries. The complete LST/QST method is used to search the transition states. The convergence criteria of geometric optimization include Energy change, Max. force and Max. displacement, while root mean square (RMS) of the gradients is the convergence criterion of transition state search. Based on the overlap matrix in the population analysis, the calculation of the Mayer bond order could measure atomic bond strength, providing guidance for determining where bonds break [37].
Results and discussion Initiation of naphthalene pyrolysis After the plasma generator is started, an arc is generated between the anode and cathode, where the hydrogen gas is decomposed into highly active hydrogen radicals, with reaction enthalpy being 424.0 kJ/mol [24,25]. In the plasma reactor, the arc exotherm could raise the temperature of the system to 103 Ke104 K. The initial step of naphthalene pyrolysis mainly involves in the break of both CeH bonds and CeC bonds at the ultra-high temperature of hydrogen plasma. The bond order and reaction energy values analysis help us judge the priority of different reaction paths. Fig. 1 summarizes the Mayer bond order distribution of the intermediates required for naphthalene pyrolysis process. For naphthalene, its CeH bonds have a bond order of 0.98, which is much smaller than that of all CeC bonds, and it means the CeH bonds are most susceptible to breakage. The hydrogen radical with extremely high reactivity and energy readily captures a hydrogen atom to generate hydrogen at the same time of breaking CeH bonds, hence two reaction paths exist in the above hydrogen loss process. Notably, there are two types of CeH bonds in naphthalene,
Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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Fig. 1 e Mayer bond distribution of species required during naphthalene cracking.
and hydrogen loss may occur in both positions. For the CeC bonds, it can be viewed on the bond order distribution of naphthalene that the bond orders between atomic number 4a and 8a and between 1 and 8a are relatively small, and the two positions are prone to breakage. In general, there are six possible initial paths for cracking: C10H8 / 2-C10H7∙ þ H∙
(1-1)
C10H8þH∙ / 2-C10H7∙ þ H2
(1e2)
C10H8 / 1-C10H7∙ þ H∙
(1e3)
C10H8þH∙ / 1-C10H7∙ þ H2
(1e4)
C10H8 / c-C10H8:
(1e5)
Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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C10H8 / o-C6H4∙-1,3-C4H4∙
(1e6)
Fig. 2 shows the potential energy diagram of the initiation step of naphthalene, including calculated and reference values (inside the bracket) of the energy required for the above paths [28,38e40]. 2-C10H7∙ is obtained both in reaction (1-1) and reaction (1e2). Reaction (1-1) is a CeH bond breaking process, and the energy required is 451.1 kJ/mol [28]. With the participation of an active hydrogen atom, reaction (1e2) has a small activation energy barrier of 43.3 kJ/mol, and its transition state is TS1. The total energy consumption of reaction (1e2) required is 33.0 kJ/mol. The processes of reaction (1e3) and reaction (1e4) to form 1-C10H7∙ are consistent with the above two, with the difference in the position of hydrogen loss. The energy required for direct CeH bond breaking in reaction (1e3) is 451.8 kJ/mol, which is slightly higher than that of reaction (1-1) [38]. Reaction (1e4) is slightly endothermic by 35.4 kJ/mol, with a small activation energy barrier of 43.5 kJ/mol, and its transition state is TS2. The above calculated values are similar to the values in references [28,39]. In reaction (1e5), the breaking process of the CeC bond converts naphthalene into a diradical macrocyclic molecule (c-C10H8), which requires an energy input of 241.6 kJ/mol [41]. Reaction (1e6) requires 657.1 kJ/mol of energy through the TS3 transition state, with reaction enthalpy being 537.5 kJ/mol. In summary, reaction (1e2) and (1e4) are the main pathways for the initiation of naphthalene cracking. Also, the ultra-high temperature in hydrogen plasma could provide enough energy required for other reactions, which will not be eliminated.
The decomposition of 2-C10H7∙ 2-C10H7∙ is the product of reaction (1e2), one of the main pathways for the initiation of naphthalene cracking. In this section, our focus is its further decomposition.
Referring to the calculation of the bond orders of 2-C10H7∙ listed in Fig. 1, the bond order between carbon number 1 and 8a is small, and the product o-C6H4∙-n-C4H3 (o-buta-1-ne-3yne-phenylene) formed after the fracture of the CeC bond has a new CeC triple bond. This path requires an endotherm of 241.0 kJ/mol [42,43]. TS4 is the transition state of it, with an active barrier of 253.2 kJ/mol.
Fate of o-C6H4∙-n-C4H3 There are three possible pathways for the decomposition of oC6H4∙-n-C4H3, which are presented in Fig. 3. The first pathway is the direct cleavage of CeC single bond in o-C6H4∙-n-C4H3. After bond order analysis and energy calculation, it is known that the direct decomposition of o-C6H4∙-n-C4H3 into orthobenzyne and n-C4H3∙-cis requires the lowest energy of 381.2 kJ/mol. The second pathway is the isomerization of oC6H4∙-n-C4H3 to oeC6H4eC2HeC2H2∙, whose conversion is mentioned in section The decomposition of 1-C10H7∙ below. The CeC bond combination in the intermediate process is an exothermic reaction, which is not easy to occur at superhigh temperature. In the third pathway, o-C6H4∙-n-C4H3 is readily isomerized to o-C6H4∙-i-C4H3, which is then decomposed into the phenyl radical and diacetylene via an intermediate ceC6H5eC4H2-2∙. The latter two conversion pathways of o-C6H4∙n-C4H3 draw on that of phenyl acrylonitrile (PhAN) radical [44]. The only difference in the structure of them is the element at 1 position. Based on the potential energy described in Fig. 3, the third of the three pathways should be the easiest to occur, and the ultra-high temperature in hydrogen plasma makes the first path possible. The above calculations indicate that c-C6H5∙ and C4H2 are the main products in the primary decomposition of 2-C10H7∙ and the pathway is as follows: 2-C10H7∙ / o-C6H4∙-n-C4H3 / ceC6H5eC4H2-2∙ / cC6H5∙ þ C4H2
Fig. 2 e The reaction pathways and energies of initiation reactions. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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Fig. 3 e The reaction pathways and energies of o-C6H4∙-n-C4H3 decomposition.
Further decomposition of the phenyl radical generated by the third path is described in detail by Huang et al. [24]. Their calculations implied that the main products of phenyl radical decomposition in hydrogen plasma are C2H2, H2, C4H2 and carbon black. The product C4H2 will continue to react with the active hydrogen atom under hydrogen-rich conditions, which plays an important role in the production of C2H2. The reaction path with calculated enthalpy value could be expressed as: C4H2 þ H∙ / C2H∙ þ C2H2
DH ¼ 150.0 kJ/mol
According to the experimental results in the literature, C2H2 is mainly formed in the temperature ranging from 1100 to 2000 K [45], which is close to the temperature of the thermal plasma.
hexa-3-ne-1,5-diyne), which will be further decomposed into n-C4H3∙-cis and C2H∙ at the ultra-high temperature in hydrogen plasma. The rupture positions of these CeC bonds are determined with the help of bond order analysis, and the specific values are listed in Fig. 1. Unlike the first three paths where CeC bond cleavage occurs, path 4 is a dehydrogenation process. Due to the participation of active hydrogen radical, the four hydrogen atoms in ortho-benzyne are easily removed one after the other to form o-C6, which is one of the sources of carbon black. Paths 2, 3, and 4 are reflected in Fig. 4(a)e(c), respectively. It could be concluded that path 4 is most likely to occur, followed by path 1, so the decomposition of o-C6H4 mainly produces C4H2, C2H2, o-C6 and H2. The main paths can be summarized as: o-C6Hn$ þ H∙ / o-C6Hn-1$ þ H2
(n ¼ 4,3,2,1)
Fate of o-C6H4 In this work, the conversion of ortho-benzyne produced in the first pathway above will be elaborated carefully. The potential energy diagram for the conversion of the o-C6H4 is illustrated in Fig. 4. There are four different pathways for it [46e49]. Path 1 is a retro-Diels-Alder reaction and a concerted, C2v symmetric bond fracture of o-C6H4 happens, which undergoes an activation energy barrier of 365.7e370.7 kJ/mol to form C4H2 and C2H2. In path 2, o-C6H4 is first converted into an open-chain species (1,3-C6H4-1,4∙) through a transition state of TS10. This species is unstable and easily isomerized to the intermediate (IM1), whose subsequent decomposition produces C4H2 and C2H2. Both path 1 and path 2 require an energy input of 269.6 kJ/mol to generate diacetylene and acetylene, while path 2 experiences more intermediate processes and higher activation energy barriers. Path 3 mainly describes the isomerization process of o-C6H4, o-C6H4/m-C6H4:/p-C6H4:. After the rearrangement, p-C6H4: could undergo a Bergman fragmentation to enediyne (l-C6H4,
o-C6H4 / C4H2 þ C2H2
The decomposition of 1-C10H7∙ 1-C10H7∙ is the product of reaction (1e4), another major pathway for the initiation of naphthalene cracking. The cracking process of 1-C10H7∙ is analogous to that of 2-C10H7∙ [40,50]. According to the minimum energy principle and bond order analysis of 1-C10H7∙, the CeC bond between carbon atom number 2 and 3 has the greatest chance of breaking. 1C10H7∙ is converted to oeC6H4eC2HeC2H2∙ by absorbing heat of 245.4 kJ/mol. TS20 is the transition state, with an active barrier of 257.0 kJ/mol oeC6H4eC2HeC2H2∙ is easily isomerized to IM2, which will remove a C2H2 to generate o-C6H4∙C2H through the transition state of TS22. Fig. 5 is the potential energy diagram of the above cracking process, which can be succinctly expressed as:
Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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Fig. 4 e The reaction pathways and energies of o-C6H4 decomposition: (a) path2, (b) path3, and (c) path4. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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Fig. 5 e The reaction pathways and energies of the decomposition of 1-C10H7∙.
1-C10H7∙ / oeC6H4eC2HeC2H2∙ / IM2 / o-C6H4∙C2H þ C2H2 The three paths for the initial cracking of o-C6H4∙-C2H are shown in Fig. 6 [44]. In path 1, o-C6H4∙-C2H is directly decomposed into o-C6H4 and C2H∙ by breaking the CeC bond between carbon atom number 1 and 8a, requiring an energy consumption of 466.8 kJ/mol. Both path 2 and 3 are processes
in which benzene ring is opened, except that the CeC bond is broken at different positions. Path 2 generating 1,3-C6H4∙-C2H requires the energy of 282.0 kJ/mol. TS23 is the transition state, with an active barrier of 290.9 kJ/mol 1,3-C6H4∙-C2H will successively remove an acetylene and ethynyl radical to form diacetylene. The decomposition path of 1,3-C6H4∙-C2H is presented in Fig. 7. Path 3 producing 1,3-C8H5-1∙ has an energy barrier of 252.9 kJ/mol, and it is endothermic by 235.1 kJ/mol. The third of the three cracking paths requires the lowest
Fig. 6 e The reaction pathways and energies of the decomposition of o-C6H4∙-C2H. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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Fig. 7 e The reaction pathways and energies of the decomposition of 1,3-C6H4∙-C2H. reaction enthalpy and activation energy, and is believed to be the most favorable pathway. Of course, the possibility of other paths occurring is also considered at superhigh temperature. The conversion of 1,3-C8H5-1∙ generated by the third path above can be attributed to two different paths, which is illustrated in Fig. 8. In one path, IM4 is rearranged from 1,3C8H5-1∙ and is easily decomposed into n-C2H2 and n-C6H3∙. Referring to n-C4H3∙-cis, both the CeC bond and CeH bond of n-C6H3∙ have the possibility of breaking [51]. It can be seen from the potential energy diagram that the fracture of the CeH bond requires less energy, and the resulting C6H2 (hexa-
1,3,5-triyne) is also an important source of carbon black. Likewise, isomerization from 1,3-C8H5-1∙ to 3,5-C8H5∙-3∙ occurs at the beginning of another path, passing through a higher activation energy barrier of 29.3 kJ/mol 3,5-C8H5∙-3∙ will be converted into n-C4H3∙-cis and C4H2 by breaking the CeC bond. In short, the previous path is more likely to occur at superhigh temperature, which explains why carbon black is readily formed after naphthalene dissociation in plasma [19]. Through the above analysis, we can summarize its main cracking path as:
Fig. 8 e The reaction pathways and energies of the decomposition of 1,3-C8H5-1∙. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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Fig. 9 e The reaction pathways and energies of the decomposition of c-C10H8:.
o-C6H4∙-C2H / 1,3-C8H5-1∙ / IM4 / n-C6H3∙ þ C2H2 / C6H2 þ C2H2 þ H∙
Fate of c-C10H8 Reaction (1e5) in Section The decomposition of 2-C10H7∙ requires relatively low energy among the six initiation paths. The product c-C10H8: will be transformed from a cyclic hydrocarbon to a linear open-chain species 1,3,5,7-C10H8-1,4∙ by
breaking the CeC bond between carbon atom number 7 and 8, where the value of the bond order is smallest. This progress requires a high energy input of 526.9 kJ/mol, with an energy barrier of 548.1 kJ/mol. The resulting double-radical chain species 1,3,5,7-C10H8-1,4∙ is easily converted into IM5, whose energy is close to it through a small energy barrier. The chain length of IM5 will be shortened through the removal of acetylene. After bond order analysis and energy calculation, disconnecting the CeC bond between atomic number 4 and 4a requires the lowest energy, and the reaction producing 3,5C8H6 and C2H2 is exothermic by 87.8 kJ/mol. Subsequently, (octa-3,5-dine-1,7-diyne) will be uniformly 3,5-C8H6
Fig. 10 e The reaction pathways and energies of the decomposition of o-C6H4∙-1,3-C4H4∙. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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dissociated into two n-C4H3∙-trans, Consistent with n-C4H3∙cis, the decomposition of n-C4H3∙-trans has two paths,
could be achievable in hydrogen plasma. The whole process can be described as follows:
c-C10H8: / 1,3,5,7-C10H8-1,4∙ / IM5 / 3,5-C8H6 þ C2H2 /2n-C4H3∙-trans þ C2H2/ 2H∙ þ 2C4H2 þ C2H2
namely, breaking the CeH bond and breaking the CeC bond. The former pathway generating diacetylene and a hydrogen atom is more likely to occur, judging from the energy required. The cracking paths and energy values mentioned above are shown in Fig. 9 and the bond order distribution required is listed in Fig. 1. In general, the energy required for opening the ring of cC10H8: and cracking into smaller hydrocarbons is high, but it
The decomposition of o-C6H4∙-1,3-C4H4 Among the six initial pathways of naphthalene cracking, reaction (1e6) consumes the highest energy. Although the production of o-C6H4∙-1,3-C4H4∙ is uneasy, it could not be eliminated considering plasma condition. o-C6H4∙-1,3-C4H4∙ isomerizes into IM6 through the transition state of TS35. IM6 will successively lose two acetylene molecules and becomes
Fig. 11 e Main reaction routes of naphthalene decomposition in hydrogen plasma. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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benzyne, whose cracking pathway is detailed in Section Fate of o-C6H4. The disconnection position of the CeC bond is judged by the bond order shown in Fig. 1 and energy consumption of the intermediates. Fig. 10 illustrates the potential energy diagram for the dissociation process of o-C6H4∙-1,3C4H4∙. All the optimized geometries of species involved in above figures are listed in Table S1 of the Supporting information.
Termination of reaction For the pyrolysis reaction of naphthalene under hydrogen plasma conditions, it is worth noting that the quenching agent is used to terminate the reaction, avoiding further conversion of acetylene into carbon black and hydrogen [20]. Under quenching conditions, the combination of free radicals with molecules occurs, with heat being released, and the paths are mainly as follows: C2H∙ þ H2 / C2H2 þ H∙
DH ¼ 127.2 kJ/mol
(6e1)
C2H∙ þ C2H∙ / C4H2
DH ¼ 701.2 kJ/mol
(6e2)
Reaction (6e1) is responsible for the production of acetylene from C2H∙ [52], and our calculated reaction endothermicity is similar to the reported one [53] (DHref ¼ 125.1 kJ/ mol). Also, the C2H∙ will continue to remove a hydrogen atom in the presence of active hydrogen, and the resulting diradical product $C≡C$ is susceptible to polycondensation at lower temperatures, i.e. n$C≡C$/½$C≡Cn . This may be the cause of generating soot during quenching [54].
General discussion At normal high temperatures, naphthalene readily condenses into hydrocarbons with higher aromaticity, which is one of the sources of soot. This has no benefit in the decomposition and utilization of naphthalene. The ultra-high temperature in hydrogen plasma and high activity of hydrogen radicals facilitate the opening of the aromatic ring of naphthalene to generate small molecular gases. Fig. 11 summarizes six decomposition pathways for C10H8 in plasma, where the major pathways are bolded. As shown, the two main paths can be described as: Path 1: C10H8 þ H∙ / 2-C10H7∙ þ H2
11
In the first major pathway, the carbon elements in the 2C10H7∙ are transferred to acetylene and phenyl radical, which will be decomposed to acetylene or dehydrogenated into o-C6. It indicates that the presence of hydrogen plasma can reduce the extent to which naphthalene is transferred to carbon black. The main product C6H2 in the second main route is one of the precursors of carbon black. Both paths are highly likely to occur at the same time, suggesting that naphthalene is more likely to form carbon black than benzene [24]. This is inconsistent with the conclusion drawn by thermodynamic analysis that higher effective mass ratio of C/H would have a more positive effect on the concentration and yield of C2H2 [55]. We hypothesize that both the C/H mass ratio and chemical structure of the molecule affect the yield and concentration of acetylene, which is subject to further validation by experiments. In general, our calculations indicated that naphthalene is mainly decomposed into C2H2, H2 and carbon black in hydrogen plasma. C2H2 is derived from the further decomposition of naphthyl radicals after ring opening, while the dehydrogenation reaction in the naphthalene pyrolysis attaches great importance to the production of H2 and inevitable carbon black. This agrees with the experimental results of naphthalene pyrolysis in plasma [15,18,19].
Conclusions In our work, the decomposition pathways of PAHs in hydrogen plasma are explored using the DFT method. Naphthalene is selected as the model compound for PAHs. The calculation results manifest that the participation of hydrogen free radical in the reaction makes the dissociation of naphthalene easier. The major products of naphthalene pyrolysis in hydrogen plasma are C2H2, H2 and carbon black, whose existence is inevitable. The calculation results in this paper could provide meaningful guidance for removing tar and reducing carbon black production, mainly derived from PAHs.
Acknowledgements The support from the National Key R&D Program of China (2016YFB0301800) is greatly appreciated.
Appendix A. Supplementary data 2-C10H7∙ / o-C6H4∙-n-C4H3 / c-C6H5∙ þ C4H2 c-C6H5∙ / C4H2 þ C2H2 þ H∙
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.208.
C4H2 þ H∙ / C2H2 þ C2H∙
references
o-C6Hn∙ þ H∙ / o-C6Hn-1∙ þ H2 (n ¼ 5,4,3,2,1) Path 2: C10H8 þ H∙ / 1-C10H7∙ þ H2 1-C10H7∙ / oeC6H4eC2HeC2H2∙ / o-C6H4∙C2H þ C2H2 o-C6H4∙-C2H/1,3-C8H5-1∙ / C6H2 þ C2H2 þ H∙
[1] Kabe T, Godo M, Otsuki S, Ishihara A, Qian WH. Reactivity of naphthalene in pyrolysis of coal tar using the 14C tracer method. Energy & Fuels 2005;11(6):1299e302. [2] Zhang YL, Luo YH, Wu WG, Zhao SH, Long YF. Heterogeneous cracking reaction of tar over biomass char, using naphthalene as model biomass tar. Energy & Fuels 2014;28(5):3129e37.
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[3] Han J, Kim H. The reduction and control technology of tar during biomass gasification/pyrolysis: an overview. Renew Sustain Energy Rev 2008;12(2):397e416. [4] Bernabei M, Reda R, Galiero R, Bocchinfuso G. Determination of total and polycyclic aromatic hydrocarbons in aviation jet fuel. J Chromatogr A 2003;985(1):197e203. [5] Kim SC, Lim MS, Chun YN. Hydrogen-rich gas production from a biomass pyrolysis gas by using a plasmatron. Int J Hydrogen Energy 2013;38(34):14458e66. [6] Yang J, Lu M, Chai M. The study of 1-ethylnaphthalene pyrolysis in a flow reactor. J Anal Appl Pyrolysis 2009;86(2):287e92. [7] Zhu FS, Li XD, Zhang H, Wu AJ, Yan JH, Ni MJ, et al. Destruction of toluene by rotating gliding arc discharge. Fuel 2016;176:78e85. [8] Yan M, Li JY, Hantoko D, Kanchanatip E, Grisdanurak N, Cai Y, Gao ZL. Hydrogen-rich syngas production by catalytic cracking of tar in wastewater under supercritical condition. Int J Hydrogen Energy 2019;44(36):19908e19. [9] Li LH, Wen GD, Yang QY, Zhang M, Xing HB, Su BG, et al. Advance in the treatment methods of biomass tar. Chem Ind Eng Prog 2017;36(7):2407e16. [10] Gai C, Dong YP, Lu ZC, Zhang ZL, Liang JC, Liu YT. Pyrolysis behavior and kinetic study of phenol as tar model compound in micro fluidized bed reactor. Int J Hydrogen Energy 2015;40(25):7956e64. [11] Rhyner U, Edinger P, Schildhauer TJ, et al. Experimental study on high temperature catalytic conversion of tars and organic sulfur compounds. Int J Hydrogen Energy 2014;39(10):4926e37. [12] Nair SA, Pemen AJM, Yan K, van Gompel FM, van Leuken HEM, van Heesch EJM, et al. Tar removal from biomass-derived fuel gas by pulsed corona discharges. Fuel Process Technol 2003;84(1):161e73. [13] Chun YN, Kim SC, Yoshikawa K. Decomposition of benzene as a surrogate tar in a gliding Arc plasma. Environ Prog Sustain Energy 2013;32(3):837e45. [14] Yan X, Li XD, Zhu FS, Kong XZ, Yan JH. Decomposition of naphthalene as tar model compound from the gasification of municipal solid waste by rotating gliding arc plasma. Chem Ind Eng Prog 2018;37(3):1174e80. [15] Nunnally T, Tsangaris A, Rabinovich A, Nirenberg G, Chernets I, Fridman A. Gliding arc plasma oxidative steam reforming of a simulated syngas containing naphthalene and toluene. Int J Hydrogen Energy 2014;39(23):11976e89. [16] Medeiros HS, Pilatau A, Nozhenko OS, da Silva Sobrinho AS, Petraconi Filho G. Microwave air plasma applied to naphthalene thermal conversion. Energy & Fuels 2016;30(2):1510e6. [17] Sun J, Wang Q, Wang WL, Song ZL, Zhao XQ, Mao YP, et al. Novel treatment of a biomass tar model compound via microwave-metal discharges. Fuel 2017;207:121e5. z P, Kordylewski W, Wnukowski M. Microwave plasma [18] Jamro application in decomposition and steam reforming of model tar compounds. Fuel Process Technol 2018;169:1e14. [19] Fourcault A, Marias F, Michon U. Modelling of thermal removal of tars in a high temperature stage fed by a plasma torch. Biomass Bioenergy 2010;34(9):1363e74. [20] Li X, Wu C, Han J. Quenching experiment study on thermal plasma pyrolysis process of coal tar. Plasma Chem Plasma Process 2016;36(3):869e80. [21] Chen LW, Meng YD, Shen J, Shu XS, Fang SD, Xiong XY. Coal pyrolysis to acetylene using dc hydrogen plasma torch: effects of system variables on acetylene concentration. J Phys D Appl Phys 2009;42(5):055505. [22] Cheng Y, Yan BH, Li TY, Cheng Y, Li X, Guo CY. Experimental study on coal tar pyrolysis in thermal plasma. Plasma Chem Plasma Process 2015;35(2):401e13.
[23] Li X, Han JT, Wu CN, Guo Y, Yan BH, Cheng Y. Coal tar pyrolysis to acetylene in thermal plasma. Ciesc J 2014;65(9):3680e6. [24] Huang XY, Cheng DG, Chen FQ, Zhan XL. The decomposition of aromatic hydrocarbons during coal pyrolysis in hydrogen plasma: a density functional theory study. Int J Hydrogen Energy 2012;37(23):18040e9. [25] Huang XY, Cheng DG, Chen FQ, Zhan XL. A density functional theory study on the decomposition of aliphatic hydrocarbons and cycloalkanes during coal pyrolysis in hydrogen plasma. J Energy Chem 2015;24(1):65e71. [26] Zhang XR, Liu ZY, Chen ZZ, Xu T, Liu QY. Bond cleavage and reactive radical intermediates in heavy tar thermal cracking. Fuel 2018;233:420e6. [27] Tursun Y, Xu SP, Wang GY, Wang C, Xiao YH. Tar formation during co-gasification of biomass and coal under different gasification condition. J Anal Appl Pyrolysis 2015;111:191e9. [28] Lu M, Mulholland JA. PAH Growth from the pyrolysis of CPD, indene and naphthalene mixture. Chemosphere 2004;55(4):605e10. [29] Mimura K, Madono T, Toyama S, Sugitani K, Sugisaki R, Iwamatsu S, et al. Shock-induced pyrolysis of naphthalene and related polycyclic aromatic hydrocarbons (anthracene, pyrene, and fluoranthene) at pressures of 12-33.7 GPa. J Anal Appl Pyrolysis 2004;72(2):273e8. [30] Jess A. Mechanisms and kinetics of thermal reactions of aromatic hydrocarbons from pyrolysis of solid fuels. Fuel 1996;75(12):1441e8. [31] Ji BY, Wu TT, Zhou K, Yan YY, Li S. Progresses in research for hydrocracking mechanism and catalysts of polycyclic aromatic hydrocarbons. Petrochem Technol 2016;45(10):1263e71. [32] Vostrikov AA, Dubov DY, Psarov SA. Naphthalene oxidation in supercritical water. Russ Chem Bull 2001;50(8):1481e4. [33] Santos RC, Agapito F, Gonc¸alves EM, Martinho Simoes JA, Borges dos Santos RM. Energetics of H-atom addition to naphthalene: a thermochemical cycle from tetralin to naphthalene. J Chem Thermodyn 2013;61(61):83e9. [34] Ferella F, Stoehr J, De Michelis I, Hornung A. Zirconia and alumina based catalysts for steam reforming of naphthalene. Fuel 2013;105:614e29. [35] Lu¨ R, Cao Z, Wang S. Density functional study on models of interaction between phosphorus species and HZSM-5. J Mol Struct 2008;865(1e3):1e7. [36] Zhang M, Yang B, Yu Y. A density functional theory study on the conversion of ethylene to carbon monomer on Pd Au (1 0 0) surface. Appl Surf Sci 2015;356:1252e61. [37] Yuan S, Li J, Zhou ZJ, Wang FC. Mechanisms of HCN and NH3 formation during rapid pyrolysis of pyridinic nitrogen containing substances. J Fuel Chem Technol 2011;39(6):413e8. [38] Solano EA, Mayer PM. A complete map of the ion chemistry of the naphthalene radical cation? DFT and RRKM modeling of a complex potential energy surface. J Chem Phys 2015;143(10):104305. [39] Wang H, Frenklach M. A detailed kinetic modeling study of aromatics formation in laminar premixed acetylene and ethylene flames. Combust Flame 1997;110(1e2):173e221. [40] Mebel AM, Georgievskii Y, Jasper AW, Jasper AW, Klippenstein SJ. Temperature- and pressure-dependent rate coefficients for the HACA pathways from benzene to naphthalene. Proc Combust Inst 2017;36(1):919e26. [41] Liu YX, Xu JW, Feng L, Zhang WX. Selective ring opening of naphthalene by halogen bombardment. Comput Appl Chem 2015;32(10):1168e72. [42] Dyakov YA, Ni CK, Lin SH, Lee YT, Mebel AM. Ab initio and RRKM study of photodissociation of azulene cation. Phys Chem Chem Phys 2006;8(12):1404e15.
Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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[43] Van der Hart WJ. Density functional calculations on the loss of acetylene from the naphthalene radical cation. Int J Mass Spectrom 2002;214(2):269e75. [44] Dubnikova F, Tamburu C, Lifshitz A. A deep insight into the details of the inter-isomerization and decomposition mechanism of o-quinolyl and o-isoquinolyl radicals. Quantum chemical calculations and computer modeling. J Phys Chem A 2016;120(38):7538e47. [45] Hidaka Y, Henmi Y, Ohonishi T, Okuno T, Koike T. Shocktube and modeling study of diacetylene pyrolysis and oxidation. Combust Flame 2002;130(1e2):62e82. [46] Ghigo G, Maranzana A, Tonachini G. o-Benzyne fragmentation and isomerization pathways: a CASPT2 study. Phys Chem Chem Phys 2014;16(43):23944e51. [47] Zhang X, Maccarone AT, Nimlos MR, Kato S, Bierbaum VM, Ellison GB, et al. Unimolecular thermal fragmentation of ortho-benzyne. J Chem Phys 2007;126(4):044312. [48] Li Q, Yao L, Lin SH. Anharmonic effect of the unimolecular isomerization/decomposition of ortho-benzyne. J Chin Chem Soc 2015;62(4):355e65. [49] Moskaleva LV, Madden LK, Lin MC. Unimolecular isomerization/decomposition of ortho-benzyne: ab initio MO/statistical theory study. Phys Chem Chem Phys 1999;1(17):3967e72.
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[50] Richter H, Benish TG, Mazyar OA, Green WH, Howard JB. Formation of polycyclic aromatic hydrocarbons and their radicals in a nearly sooting premixed benzene flame. Proc Combust Inst 2000;28(2):2609e18. [51] Landera A, Krishtal SP, Kislov VV, Mebel AM, Kaiser RI. Theoretical study of the C6H3 potential energy surface and rate constants and product branching ratios of the C2H(2Sþ )þC4H2(1Sgþ ) and C4H(2Sþ )þC2H2(1Sgþ ) reactions. J Chem Phys 2008;128(21):214301. [52] Zhao MJ, Fang JJ, Zhang L, Dai Z, Yao ZW, Zhao MY. Present state of coal pyrolysis to acetylene in plasma. Chem Eng 2017;45(01):45e9. [53] Chen L, Shao K, Chen J, Yang MH, Zhang DH. Fulldimensional quantum dynamics study of the H2þC2H/HþC2H2 reaction on an ab initio potential energy surface. J Chem Phys 2016;144(19):194309. [54] Gai C, Dong YP, Yang S, Zhang ZL, Liang JC, Li JD. Thermal decomposition kinetics of light polycyclic aromatic hydrocarbons as surrogate biomass tar. RSC Adv 2016;6(86):83154e62. [55] Yan B, Xu P, Jin Y, Cheng Y. Understanding coal/hydrocarbons pyrolysis in thermal plasma reactors by thermodynamic analysis. Chem Eng Sci 2012;84(52):31e9.
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A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma Lu Chen a, Dang-guo Cheng a,b,*, Fengqiu Chen a,b, Xiaoli Zhan a,b a
Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, 310027, China b Institute of Zhejiang University-Quzhou, 78 Jiuhua Boulevard North, Quzhou, 324000, China
highlights DFT method is used to explore the decomposition pathways of PAHs in hydrogen plasma. The participation of hydrogen radicals makes the dissociation of naphthalene easier. C2H2 and H2 are the major products in hydrogen plasma pyrolysis.
article info
abstract
Article history:
Tar is the byproduct of fuel in the pyrolysis or gasification process. Hydrogen plasma could
Received 19 August 2019
effectively promote the decomposition of tar into acetylene and hydrogen, but the detailed
Received in revised form
cracking mechanism is difficult to detect. The DMol3 calculations, based on density func-
14 October 2019
tional theory (DFT), have been employed to explore the pyrolysis pathways of naphthalene.
Accepted 26 October 2019
Naphthalene is chosen as the model compound for polycyclic aromatic hydrocarbons
Available online xxx
(PAHs), which are the main components of tar. Our calculations investigate that the energy barriers required for the reactions are greatly reduced due to the participation of active
Keywords:
hydrogen atoms. Naphthalene is easily converted into acetylene, hydrogen and carbon
Plasma
black through two main routes. This is in good agreement with the experiment results.
Tar
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Density functional theory Pyrolysis Acetylene Hydrogen
Introduction Pyrolysis and gasification are promising techniques to convert fuels, such as coal, petroleum and renewable biomass into
fuel gas (CO and H2) for industrial use, where tar is an inevitable byproduct [1e5]. Tar is a mixture of heavy organic compounds - mainly of aromatic nature. At low temperature, it is prone to polycondensation to form soot, which easily blocks and fouls process equipment like fuel lines, filters and
* Corresponding author. Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, 310027, China. E-mail address: [email protected] (D.-g. Cheng). https://doi.org/10.1016/j.ijhydene.2019.10.208 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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engines [3,6,7]. At present, traditional thermochemical methods for removing tar include chiefly thermal cracking and catalytic cracking [3,8e11]. The thermal cracking of tar imposes high energy cost in production process since the conversion of aromatic hydrocarbons of tar, like benzene and naphthalene, into smaller gas molecules requires temperatures over 1100 C. Meanwhile, catalytic cracking method reduces the reaction temperature to some extent, at the expense of high catalyst cost, low lifetime, sulfur poisoning and carbon deposition [9]. Therefore, an efficient and economical cracking method is urgently desired. Plasma methods can overcome the above problems due to the presence of reactive species such as electrons, ions, radicals, and etc. And it appears to be economic and energy efficient. Basic research on non-thermal plasmas such as pulsed plasma [12], sliding arc plasma [13e15] and microwave-assisted plasma [16,17] for tar destruction has been studied. Among them, the electrodeless microwave plasma has recently received great attention, due to the avoidance of electrode contamination and low cost of magnetrons, whose construction is simple and compact. Wnukowski et al. achieved 98% tar conversion using microwave plasma method with the initial tar concentration of 10 g/Nm3, the nitrogen gas flow rate being 30 L/min and the steam-tocarbon ratio equal to 3. It has been revealed that the main products of the tar decomposition were acetylene, carbon black, and benzene derivatives [18]. Their amounts were significantly decreased in favor of CO, CO2, and H2 by steam addition. Notably, carbon black produced in the former will greatly affect the gas discharge efficiency, while the latter CO2 inevitably aggravates the greenhouse effect. However, non-thermal plasma has disadvantages in the scale-up in practical applications, hindering its further development in tar disposition. Ultra-high temperature and high heating rate of thermal plasma promote pyrolyzing fuels and generating light hydrocarbons (methane, ethane, acetylene, and etc.) and a large amount of H2 and CO. These products can be obtained by means of rapid quenching [19,20]. Thus, when using H2 as plasma working gas, plasma-assisted pyrolysis is a hydrogen self-sufficient process. Applying thermal plasma technology to produce acetylene is a special and progressive route, while its industrial commercialization at home and abroad is basically mature [21]. In terms of experimental researches, Cheng et al. decomposed coal tar into acetylene and other light gases in thermal plasma, wherein the acetylene yield could reach up to 45% when using Ar plasma [22]. They also summarized that increased hydrogen concentration in plasma working gases enhanced the coal tar conversion and the yield of acetylene, and also reduced coking, which was generated by vapor deposition of carbon black and condensation reactions [23]. Huang et al. explored the cracking pathways of coal in hydrogen plasma, and it was investigated by theoretical calculation that the addition of hydrogen radicals made the dehydrogenation and pyrolysis reactions more favorable [24,25]. The thermal decomposition of tar is a process in which chemical covalent bonds are broken into active radical fragments, whose detection is hardly achieved by modern means [26]. Hence, utilizing theoretical calculation to probe its decomposition mechanism is necessary and imperative.
In order to explore the cracking paths of tar in hydrogen plasma, providing theoretical guidance for the effective removal of tar, density functional theory (DFT) is used in this work. As the main components of tar [19,27], polycyclic aromatic hydrocarbons (PAHs) attach great importance to tar pyrolysis in hydrogen plasma because of the high aromaticity. Naphthalene is the simplest PAH, which has been selected as a model compound of polycyclic aromatic hydrocarbons in many references [15,19,28e34]. The optimal pathways of naphthalene in hydrogen plasma will be well explored and analyzed in this study.
Method of calculation Density functional theory (DFT) is applied through the interface of the Dmol3 module using Material Studio software, which has been widely applied in the field of quantum chemistry in recent years [35,36]. In the geometric optimization of reactants and products and the search for the transition states, the vibration frequencies of these molecules need to be calculated to obtain zero-point energy (ZPE) corrections. The frequencies of the reactants and products are guaranteed to be positive, while the transition state has one and only one imaginary frequency. PW91 functional of GGA, with the basic set of DNP, is employed to optimize geometries. The complete LST/QST method is used to search the transition states. The convergence criteria of geometric optimization include Energy change, Max. force and Max. displacement, while root mean square (RMS) of the gradients is the convergence criterion of transition state search. Based on the overlap matrix in the population analysis, the calculation of the Mayer bond order could measure atomic bond strength, providing guidance for determining where bonds break [37].
Results and discussion Initiation of naphthalene pyrolysis After the plasma generator is started, an arc is generated between the anode and cathode, where the hydrogen gas is decomposed into highly active hydrogen radicals, with reaction enthalpy being 424.0 kJ/mol [24,25]. In the plasma reactor, the arc exotherm could raise the temperature of the system to 103 Ke104 K. The initial step of naphthalene pyrolysis mainly involves in the break of both CeH bonds and CeC bonds at the ultra-high temperature of hydrogen plasma. The bond order and reaction energy values analysis help us judge the priority of different reaction paths. Fig. 1 summarizes the Mayer bond order distribution of the intermediates required for naphthalene pyrolysis process. For naphthalene, its CeH bonds have a bond order of 0.98, which is much smaller than that of all CeC bonds, and it means the CeH bonds are most susceptible to breakage. The hydrogen radical with extremely high reactivity and energy readily captures a hydrogen atom to generate hydrogen at the same time of breaking CeH bonds, hence two reaction paths exist in the above hydrogen loss process. Notably, there are two types of CeH bonds in naphthalene,
Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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Fig. 1 e Mayer bond distribution of species required during naphthalene cracking.
and hydrogen loss may occur in both positions. For the CeC bonds, it can be viewed on the bond order distribution of naphthalene that the bond orders between atomic number 4a and 8a and between 1 and 8a are relatively small, and the two positions are prone to breakage. In general, there are six possible initial paths for cracking: C10H8 / 2-C10H7∙ þ H∙
(1-1)
C10H8þH∙ / 2-C10H7∙ þ H2
(1e2)
C10H8 / 1-C10H7∙ þ H∙
(1e3)
C10H8þH∙ / 1-C10H7∙ þ H2
(1e4)
C10H8 / c-C10H8:
(1e5)
Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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C10H8 / o-C6H4∙-1,3-C4H4∙
(1e6)
Fig. 2 shows the potential energy diagram of the initiation step of naphthalene, including calculated and reference values (inside the bracket) of the energy required for the above paths [28,38e40]. 2-C10H7∙ is obtained both in reaction (1-1) and reaction (1e2). Reaction (1-1) is a CeH bond breaking process, and the energy required is 451.1 kJ/mol [28]. With the participation of an active hydrogen atom, reaction (1e2) has a small activation energy barrier of 43.3 kJ/mol, and its transition state is TS1. The total energy consumption of reaction (1e2) required is 33.0 kJ/mol. The processes of reaction (1e3) and reaction (1e4) to form 1-C10H7∙ are consistent with the above two, with the difference in the position of hydrogen loss. The energy required for direct CeH bond breaking in reaction (1e3) is 451.8 kJ/mol, which is slightly higher than that of reaction (1-1) [38]. Reaction (1e4) is slightly endothermic by 35.4 kJ/mol, with a small activation energy barrier of 43.5 kJ/mol, and its transition state is TS2. The above calculated values are similar to the values in references [28,39]. In reaction (1e5), the breaking process of the CeC bond converts naphthalene into a diradical macrocyclic molecule (c-C10H8), which requires an energy input of 241.6 kJ/mol [41]. Reaction (1e6) requires 657.1 kJ/mol of energy through the TS3 transition state, with reaction enthalpy being 537.5 kJ/mol. In summary, reaction (1e2) and (1e4) are the main pathways for the initiation of naphthalene cracking. Also, the ultra-high temperature in hydrogen plasma could provide enough energy required for other reactions, which will not be eliminated.
The decomposition of 2-C10H7∙ 2-C10H7∙ is the product of reaction (1e2), one of the main pathways for the initiation of naphthalene cracking. In this section, our focus is its further decomposition.
Referring to the calculation of the bond orders of 2-C10H7∙ listed in Fig. 1, the bond order between carbon number 1 and 8a is small, and the product o-C6H4∙-n-C4H3 (o-buta-1-ne-3yne-phenylene) formed after the fracture of the CeC bond has a new CeC triple bond. This path requires an endotherm of 241.0 kJ/mol [42,43]. TS4 is the transition state of it, with an active barrier of 253.2 kJ/mol.
Fate of o-C6H4∙-n-C4H3 There are three possible pathways for the decomposition of oC6H4∙-n-C4H3, which are presented in Fig. 3. The first pathway is the direct cleavage of CeC single bond in o-C6H4∙-n-C4H3. After bond order analysis and energy calculation, it is known that the direct decomposition of o-C6H4∙-n-C4H3 into orthobenzyne and n-C4H3∙-cis requires the lowest energy of 381.2 kJ/mol. The second pathway is the isomerization of oC6H4∙-n-C4H3 to oeC6H4eC2HeC2H2∙, whose conversion is mentioned in section The decomposition of 1-C10H7∙ below. The CeC bond combination in the intermediate process is an exothermic reaction, which is not easy to occur at superhigh temperature. In the third pathway, o-C6H4∙-n-C4H3 is readily isomerized to o-C6H4∙-i-C4H3, which is then decomposed into the phenyl radical and diacetylene via an intermediate ceC6H5eC4H2-2∙. The latter two conversion pathways of o-C6H4∙n-C4H3 draw on that of phenyl acrylonitrile (PhAN) radical [44]. The only difference in the structure of them is the element at 1 position. Based on the potential energy described in Fig. 3, the third of the three pathways should be the easiest to occur, and the ultra-high temperature in hydrogen plasma makes the first path possible. The above calculations indicate that c-C6H5∙ and C4H2 are the main products in the primary decomposition of 2-C10H7∙ and the pathway is as follows: 2-C10H7∙ / o-C6H4∙-n-C4H3 / ceC6H5eC4H2-2∙ / cC6H5∙ þ C4H2
Fig. 2 e The reaction pathways and energies of initiation reactions. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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Fig. 3 e The reaction pathways and energies of o-C6H4∙-n-C4H3 decomposition.
Further decomposition of the phenyl radical generated by the third path is described in detail by Huang et al. [24]. Their calculations implied that the main products of phenyl radical decomposition in hydrogen plasma are C2H2, H2, C4H2 and carbon black. The product C4H2 will continue to react with the active hydrogen atom under hydrogen-rich conditions, which plays an important role in the production of C2H2. The reaction path with calculated enthalpy value could be expressed as: C4H2 þ H∙ / C2H∙ þ C2H2
DH ¼ 150.0 kJ/mol
According to the experimental results in the literature, C2H2 is mainly formed in the temperature ranging from 1100 to 2000 K [45], which is close to the temperature of the thermal plasma.
hexa-3-ne-1,5-diyne), which will be further decomposed into n-C4H3∙-cis and C2H∙ at the ultra-high temperature in hydrogen plasma. The rupture positions of these CeC bonds are determined with the help of bond order analysis, and the specific values are listed in Fig. 1. Unlike the first three paths where CeC bond cleavage occurs, path 4 is a dehydrogenation process. Due to the participation of active hydrogen radical, the four hydrogen atoms in ortho-benzyne are easily removed one after the other to form o-C6, which is one of the sources of carbon black. Paths 2, 3, and 4 are reflected in Fig. 4(a)e(c), respectively. It could be concluded that path 4 is most likely to occur, followed by path 1, so the decomposition of o-C6H4 mainly produces C4H2, C2H2, o-C6 and H2. The main paths can be summarized as: o-C6Hn$ þ H∙ / o-C6Hn-1$ þ H2
(n ¼ 4,3,2,1)
Fate of o-C6H4 In this work, the conversion of ortho-benzyne produced in the first pathway above will be elaborated carefully. The potential energy diagram for the conversion of the o-C6H4 is illustrated in Fig. 4. There are four different pathways for it [46e49]. Path 1 is a retro-Diels-Alder reaction and a concerted, C2v symmetric bond fracture of o-C6H4 happens, which undergoes an activation energy barrier of 365.7e370.7 kJ/mol to form C4H2 and C2H2. In path 2, o-C6H4 is first converted into an open-chain species (1,3-C6H4-1,4∙) through a transition state of TS10. This species is unstable and easily isomerized to the intermediate (IM1), whose subsequent decomposition produces C4H2 and C2H2. Both path 1 and path 2 require an energy input of 269.6 kJ/mol to generate diacetylene and acetylene, while path 2 experiences more intermediate processes and higher activation energy barriers. Path 3 mainly describes the isomerization process of o-C6H4, o-C6H4/m-C6H4:/p-C6H4:. After the rearrangement, p-C6H4: could undergo a Bergman fragmentation to enediyne (l-C6H4,
o-C6H4 / C4H2 þ C2H2
The decomposition of 1-C10H7∙ 1-C10H7∙ is the product of reaction (1e4), another major pathway for the initiation of naphthalene cracking. The cracking process of 1-C10H7∙ is analogous to that of 2-C10H7∙ [40,50]. According to the minimum energy principle and bond order analysis of 1-C10H7∙, the CeC bond between carbon atom number 2 and 3 has the greatest chance of breaking. 1C10H7∙ is converted to oeC6H4eC2HeC2H2∙ by absorbing heat of 245.4 kJ/mol. TS20 is the transition state, with an active barrier of 257.0 kJ/mol oeC6H4eC2HeC2H2∙ is easily isomerized to IM2, which will remove a C2H2 to generate o-C6H4∙C2H through the transition state of TS22. Fig. 5 is the potential energy diagram of the above cracking process, which can be succinctly expressed as:
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Fig. 4 e The reaction pathways and energies of o-C6H4 decomposition: (a) path2, (b) path3, and (c) path4. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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Fig. 5 e The reaction pathways and energies of the decomposition of 1-C10H7∙.
1-C10H7∙ / oeC6H4eC2HeC2H2∙ / IM2 / o-C6H4∙C2H þ C2H2 The three paths for the initial cracking of o-C6H4∙-C2H are shown in Fig. 6 [44]. In path 1, o-C6H4∙-C2H is directly decomposed into o-C6H4 and C2H∙ by breaking the CeC bond between carbon atom number 1 and 8a, requiring an energy consumption of 466.8 kJ/mol. Both path 2 and 3 are processes
in which benzene ring is opened, except that the CeC bond is broken at different positions. Path 2 generating 1,3-C6H4∙-C2H requires the energy of 282.0 kJ/mol. TS23 is the transition state, with an active barrier of 290.9 kJ/mol 1,3-C6H4∙-C2H will successively remove an acetylene and ethynyl radical to form diacetylene. The decomposition path of 1,3-C6H4∙-C2H is presented in Fig. 7. Path 3 producing 1,3-C8H5-1∙ has an energy barrier of 252.9 kJ/mol, and it is endothermic by 235.1 kJ/mol. The third of the three cracking paths requires the lowest
Fig. 6 e The reaction pathways and energies of the decomposition of o-C6H4∙-C2H. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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Fig. 7 e The reaction pathways and energies of the decomposition of 1,3-C6H4∙-C2H. reaction enthalpy and activation energy, and is believed to be the most favorable pathway. Of course, the possibility of other paths occurring is also considered at superhigh temperature. The conversion of 1,3-C8H5-1∙ generated by the third path above can be attributed to two different paths, which is illustrated in Fig. 8. In one path, IM4 is rearranged from 1,3C8H5-1∙ and is easily decomposed into n-C2H2 and n-C6H3∙. Referring to n-C4H3∙-cis, both the CeC bond and CeH bond of n-C6H3∙ have the possibility of breaking [51]. It can be seen from the potential energy diagram that the fracture of the CeH bond requires less energy, and the resulting C6H2 (hexa-
1,3,5-triyne) is also an important source of carbon black. Likewise, isomerization from 1,3-C8H5-1∙ to 3,5-C8H5∙-3∙ occurs at the beginning of another path, passing through a higher activation energy barrier of 29.3 kJ/mol 3,5-C8H5∙-3∙ will be converted into n-C4H3∙-cis and C4H2 by breaking the CeC bond. In short, the previous path is more likely to occur at superhigh temperature, which explains why carbon black is readily formed after naphthalene dissociation in plasma [19]. Through the above analysis, we can summarize its main cracking path as:
Fig. 8 e The reaction pathways and energies of the decomposition of 1,3-C8H5-1∙. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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Fig. 9 e The reaction pathways and energies of the decomposition of c-C10H8:.
o-C6H4∙-C2H / 1,3-C8H5-1∙ / IM4 / n-C6H3∙ þ C2H2 / C6H2 þ C2H2 þ H∙
Fate of c-C10H8 Reaction (1e5) in Section The decomposition of 2-C10H7∙ requires relatively low energy among the six initiation paths. The product c-C10H8: will be transformed from a cyclic hydrocarbon to a linear open-chain species 1,3,5,7-C10H8-1,4∙ by
breaking the CeC bond between carbon atom number 7 and 8, where the value of the bond order is smallest. This progress requires a high energy input of 526.9 kJ/mol, with an energy barrier of 548.1 kJ/mol. The resulting double-radical chain species 1,3,5,7-C10H8-1,4∙ is easily converted into IM5, whose energy is close to it through a small energy barrier. The chain length of IM5 will be shortened through the removal of acetylene. After bond order analysis and energy calculation, disconnecting the CeC bond between atomic number 4 and 4a requires the lowest energy, and the reaction producing 3,5C8H6 and C2H2 is exothermic by 87.8 kJ/mol. Subsequently, (octa-3,5-dine-1,7-diyne) will be uniformly 3,5-C8H6
Fig. 10 e The reaction pathways and energies of the decomposition of o-C6H4∙-1,3-C4H4∙. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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dissociated into two n-C4H3∙-trans, Consistent with n-C4H3∙cis, the decomposition of n-C4H3∙-trans has two paths,
could be achievable in hydrogen plasma. The whole process can be described as follows:
c-C10H8: / 1,3,5,7-C10H8-1,4∙ / IM5 / 3,5-C8H6 þ C2H2 /2n-C4H3∙-trans þ C2H2/ 2H∙ þ 2C4H2 þ C2H2
namely, breaking the CeH bond and breaking the CeC bond. The former pathway generating diacetylene and a hydrogen atom is more likely to occur, judging from the energy required. The cracking paths and energy values mentioned above are shown in Fig. 9 and the bond order distribution required is listed in Fig. 1. In general, the energy required for opening the ring of cC10H8: and cracking into smaller hydrocarbons is high, but it
The decomposition of o-C6H4∙-1,3-C4H4 Among the six initial pathways of naphthalene cracking, reaction (1e6) consumes the highest energy. Although the production of o-C6H4∙-1,3-C4H4∙ is uneasy, it could not be eliminated considering plasma condition. o-C6H4∙-1,3-C4H4∙ isomerizes into IM6 through the transition state of TS35. IM6 will successively lose two acetylene molecules and becomes
Fig. 11 e Main reaction routes of naphthalene decomposition in hydrogen plasma. Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
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benzyne, whose cracking pathway is detailed in Section Fate of o-C6H4. The disconnection position of the CeC bond is judged by the bond order shown in Fig. 1 and energy consumption of the intermediates. Fig. 10 illustrates the potential energy diagram for the dissociation process of o-C6H4∙-1,3C4H4∙. All the optimized geometries of species involved in above figures are listed in Table S1 of the Supporting information.
Termination of reaction For the pyrolysis reaction of naphthalene under hydrogen plasma conditions, it is worth noting that the quenching agent is used to terminate the reaction, avoiding further conversion of acetylene into carbon black and hydrogen [20]. Under quenching conditions, the combination of free radicals with molecules occurs, with heat being released, and the paths are mainly as follows: C2H∙ þ H2 / C2H2 þ H∙
DH ¼ 127.2 kJ/mol
(6e1)
C2H∙ þ C2H∙ / C4H2
DH ¼ 701.2 kJ/mol
(6e2)
Reaction (6e1) is responsible for the production of acetylene from C2H∙ [52], and our calculated reaction endothermicity is similar to the reported one [53] (DHref ¼ 125.1 kJ/ mol). Also, the C2H∙ will continue to remove a hydrogen atom in the presence of active hydrogen, and the resulting diradical product $C≡C$ is susceptible to polycondensation at lower temperatures, i.e. n$C≡C$/½$C≡Cn . This may be the cause of generating soot during quenching [54].
General discussion At normal high temperatures, naphthalene readily condenses into hydrocarbons with higher aromaticity, which is one of the sources of soot. This has no benefit in the decomposition and utilization of naphthalene. The ultra-high temperature in hydrogen plasma and high activity of hydrogen radicals facilitate the opening of the aromatic ring of naphthalene to generate small molecular gases. Fig. 11 summarizes six decomposition pathways for C10H8 in plasma, where the major pathways are bolded. As shown, the two main paths can be described as: Path 1: C10H8 þ H∙ / 2-C10H7∙ þ H2
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In the first major pathway, the carbon elements in the 2C10H7∙ are transferred to acetylene and phenyl radical, which will be decomposed to acetylene or dehydrogenated into o-C6. It indicates that the presence of hydrogen plasma can reduce the extent to which naphthalene is transferred to carbon black. The main product C6H2 in the second main route is one of the precursors of carbon black. Both paths are highly likely to occur at the same time, suggesting that naphthalene is more likely to form carbon black than benzene [24]. This is inconsistent with the conclusion drawn by thermodynamic analysis that higher effective mass ratio of C/H would have a more positive effect on the concentration and yield of C2H2 [55]. We hypothesize that both the C/H mass ratio and chemical structure of the molecule affect the yield and concentration of acetylene, which is subject to further validation by experiments. In general, our calculations indicated that naphthalene is mainly decomposed into C2H2, H2 and carbon black in hydrogen plasma. C2H2 is derived from the further decomposition of naphthyl radicals after ring opening, while the dehydrogenation reaction in the naphthalene pyrolysis attaches great importance to the production of H2 and inevitable carbon black. This agrees with the experimental results of naphthalene pyrolysis in plasma [15,18,19].
Conclusions In our work, the decomposition pathways of PAHs in hydrogen plasma are explored using the DFT method. Naphthalene is selected as the model compound for PAHs. The calculation results manifest that the participation of hydrogen free radical in the reaction makes the dissociation of naphthalene easier. The major products of naphthalene pyrolysis in hydrogen plasma are C2H2, H2 and carbon black, whose existence is inevitable. The calculation results in this paper could provide meaningful guidance for removing tar and reducing carbon black production, mainly derived from PAHs.
Acknowledgements The support from the National Key R&D Program of China (2016YFB0301800) is greatly appreciated.
Appendix A. Supplementary data 2-C10H7∙ / o-C6H4∙-n-C4H3 / c-C6H5∙ þ C4H2 c-C6H5∙ / C4H2 þ C2H2 þ H∙
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.208.
C4H2 þ H∙ / C2H2 þ C2H∙
references
o-C6Hn∙ þ H∙ / o-C6Hn-1∙ þ H2 (n ¼ 5,4,3,2,1) Path 2: C10H8 þ H∙ / 1-C10H7∙ þ H2 1-C10H7∙ / oeC6H4eC2HeC2H2∙ / o-C6H4∙C2H þ C2H2 o-C6H4∙-C2H/1,3-C8H5-1∙ / C6H2 þ C2H2 þ H∙
[1] Kabe T, Godo M, Otsuki S, Ishihara A, Qian WH. Reactivity of naphthalene in pyrolysis of coal tar using the 14C tracer method. Energy & Fuels 2005;11(6):1299e302. [2] Zhang YL, Luo YH, Wu WG, Zhao SH, Long YF. Heterogeneous cracking reaction of tar over biomass char, using naphthalene as model biomass tar. Energy & Fuels 2014;28(5):3129e37.
Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
12
international journal of hydrogen energy xxx (xxxx) xxx
[3] Han J, Kim H. The reduction and control technology of tar during biomass gasification/pyrolysis: an overview. Renew Sustain Energy Rev 2008;12(2):397e416. [4] Bernabei M, Reda R, Galiero R, Bocchinfuso G. Determination of total and polycyclic aromatic hydrocarbons in aviation jet fuel. J Chromatogr A 2003;985(1):197e203. [5] Kim SC, Lim MS, Chun YN. Hydrogen-rich gas production from a biomass pyrolysis gas by using a plasmatron. Int J Hydrogen Energy 2013;38(34):14458e66. [6] Yang J, Lu M, Chai M. The study of 1-ethylnaphthalene pyrolysis in a flow reactor. J Anal Appl Pyrolysis 2009;86(2):287e92. [7] Zhu FS, Li XD, Zhang H, Wu AJ, Yan JH, Ni MJ, et al. Destruction of toluene by rotating gliding arc discharge. Fuel 2016;176:78e85. [8] Yan M, Li JY, Hantoko D, Kanchanatip E, Grisdanurak N, Cai Y, Gao ZL. Hydrogen-rich syngas production by catalytic cracking of tar in wastewater under supercritical condition. Int J Hydrogen Energy 2019;44(36):19908e19. [9] Li LH, Wen GD, Yang QY, Zhang M, Xing HB, Su BG, et al. Advance in the treatment methods of biomass tar. Chem Ind Eng Prog 2017;36(7):2407e16. [10] Gai C, Dong YP, Lu ZC, Zhang ZL, Liang JC, Liu YT. Pyrolysis behavior and kinetic study of phenol as tar model compound in micro fluidized bed reactor. Int J Hydrogen Energy 2015;40(25):7956e64. [11] Rhyner U, Edinger P, Schildhauer TJ, et al. Experimental study on high temperature catalytic conversion of tars and organic sulfur compounds. Int J Hydrogen Energy 2014;39(10):4926e37. [12] Nair SA, Pemen AJM, Yan K, van Gompel FM, van Leuken HEM, van Heesch EJM, et al. Tar removal from biomass-derived fuel gas by pulsed corona discharges. Fuel Process Technol 2003;84(1):161e73. [13] Chun YN, Kim SC, Yoshikawa K. Decomposition of benzene as a surrogate tar in a gliding Arc plasma. Environ Prog Sustain Energy 2013;32(3):837e45. [14] Yan X, Li XD, Zhu FS, Kong XZ, Yan JH. Decomposition of naphthalene as tar model compound from the gasification of municipal solid waste by rotating gliding arc plasma. Chem Ind Eng Prog 2018;37(3):1174e80. [15] Nunnally T, Tsangaris A, Rabinovich A, Nirenberg G, Chernets I, Fridman A. Gliding arc plasma oxidative steam reforming of a simulated syngas containing naphthalene and toluene. Int J Hydrogen Energy 2014;39(23):11976e89. [16] Medeiros HS, Pilatau A, Nozhenko OS, da Silva Sobrinho AS, Petraconi Filho G. Microwave air plasma applied to naphthalene thermal conversion. Energy & Fuels 2016;30(2):1510e6. [17] Sun J, Wang Q, Wang WL, Song ZL, Zhao XQ, Mao YP, et al. Novel treatment of a biomass tar model compound via microwave-metal discharges. Fuel 2017;207:121e5. z P, Kordylewski W, Wnukowski M. Microwave plasma [18] Jamro application in decomposition and steam reforming of model tar compounds. Fuel Process Technol 2018;169:1e14. [19] Fourcault A, Marias F, Michon U. Modelling of thermal removal of tars in a high temperature stage fed by a plasma torch. Biomass Bioenergy 2010;34(9):1363e74. [20] Li X, Wu C, Han J. Quenching experiment study on thermal plasma pyrolysis process of coal tar. Plasma Chem Plasma Process 2016;36(3):869e80. [21] Chen LW, Meng YD, Shen J, Shu XS, Fang SD, Xiong XY. Coal pyrolysis to acetylene using dc hydrogen plasma torch: effects of system variables on acetylene concentration. J Phys D Appl Phys 2009;42(5):055505. [22] Cheng Y, Yan BH, Li TY, Cheng Y, Li X, Guo CY. Experimental study on coal tar pyrolysis in thermal plasma. Plasma Chem Plasma Process 2015;35(2):401e13.
[23] Li X, Han JT, Wu CN, Guo Y, Yan BH, Cheng Y. Coal tar pyrolysis to acetylene in thermal plasma. Ciesc J 2014;65(9):3680e6. [24] Huang XY, Cheng DG, Chen FQ, Zhan XL. The decomposition of aromatic hydrocarbons during coal pyrolysis in hydrogen plasma: a density functional theory study. Int J Hydrogen Energy 2012;37(23):18040e9. [25] Huang XY, Cheng DG, Chen FQ, Zhan XL. A density functional theory study on the decomposition of aliphatic hydrocarbons and cycloalkanes during coal pyrolysis in hydrogen plasma. J Energy Chem 2015;24(1):65e71. [26] Zhang XR, Liu ZY, Chen ZZ, Xu T, Liu QY. Bond cleavage and reactive radical intermediates in heavy tar thermal cracking. Fuel 2018;233:420e6. [27] Tursun Y, Xu SP, Wang GY, Wang C, Xiao YH. Tar formation during co-gasification of biomass and coal under different gasification condition. J Anal Appl Pyrolysis 2015;111:191e9. [28] Lu M, Mulholland JA. PAH Growth from the pyrolysis of CPD, indene and naphthalene mixture. Chemosphere 2004;55(4):605e10. [29] Mimura K, Madono T, Toyama S, Sugitani K, Sugisaki R, Iwamatsu S, et al. Shock-induced pyrolysis of naphthalene and related polycyclic aromatic hydrocarbons (anthracene, pyrene, and fluoranthene) at pressures of 12-33.7 GPa. J Anal Appl Pyrolysis 2004;72(2):273e8. [30] Jess A. Mechanisms and kinetics of thermal reactions of aromatic hydrocarbons from pyrolysis of solid fuels. Fuel 1996;75(12):1441e8. [31] Ji BY, Wu TT, Zhou K, Yan YY, Li S. Progresses in research for hydrocracking mechanism and catalysts of polycyclic aromatic hydrocarbons. Petrochem Technol 2016;45(10):1263e71. [32] Vostrikov AA, Dubov DY, Psarov SA. Naphthalene oxidation in supercritical water. Russ Chem Bull 2001;50(8):1481e4. [33] Santos RC, Agapito F, Gonc¸alves EM, Martinho Simoes JA, Borges dos Santos RM. Energetics of H-atom addition to naphthalene: a thermochemical cycle from tetralin to naphthalene. J Chem Thermodyn 2013;61(61):83e9. [34] Ferella F, Stoehr J, De Michelis I, Hornung A. Zirconia and alumina based catalysts for steam reforming of naphthalene. Fuel 2013;105:614e29. [35] Lu¨ R, Cao Z, Wang S. Density functional study on models of interaction between phosphorus species and HZSM-5. J Mol Struct 2008;865(1e3):1e7. [36] Zhang M, Yang B, Yu Y. A density functional theory study on the conversion of ethylene to carbon monomer on Pd Au (1 0 0) surface. Appl Surf Sci 2015;356:1252e61. [37] Yuan S, Li J, Zhou ZJ, Wang FC. Mechanisms of HCN and NH3 formation during rapid pyrolysis of pyridinic nitrogen containing substances. J Fuel Chem Technol 2011;39(6):413e8. [38] Solano EA, Mayer PM. A complete map of the ion chemistry of the naphthalene radical cation? DFT and RRKM modeling of a complex potential energy surface. J Chem Phys 2015;143(10):104305. [39] Wang H, Frenklach M. A detailed kinetic modeling study of aromatics formation in laminar premixed acetylene and ethylene flames. Combust Flame 1997;110(1e2):173e221. [40] Mebel AM, Georgievskii Y, Jasper AW, Jasper AW, Klippenstein SJ. Temperature- and pressure-dependent rate coefficients for the HACA pathways from benzene to naphthalene. Proc Combust Inst 2017;36(1):919e26. [41] Liu YX, Xu JW, Feng L, Zhang WX. Selective ring opening of naphthalene by halogen bombardment. Comput Appl Chem 2015;32(10):1168e72. [42] Dyakov YA, Ni CK, Lin SH, Lee YT, Mebel AM. Ab initio and RRKM study of photodissociation of azulene cation. Phys Chem Chem Phys 2006;8(12):1404e15.
Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208
international journal of hydrogen energy xxx (xxxx) xxx
[43] Van der Hart WJ. Density functional calculations on the loss of acetylene from the naphthalene radical cation. Int J Mass Spectrom 2002;214(2):269e75. [44] Dubnikova F, Tamburu C, Lifshitz A. A deep insight into the details of the inter-isomerization and decomposition mechanism of o-quinolyl and o-isoquinolyl radicals. Quantum chemical calculations and computer modeling. J Phys Chem A 2016;120(38):7538e47. [45] Hidaka Y, Henmi Y, Ohonishi T, Okuno T, Koike T. Shocktube and modeling study of diacetylene pyrolysis and oxidation. Combust Flame 2002;130(1e2):62e82. [46] Ghigo G, Maranzana A, Tonachini G. o-Benzyne fragmentation and isomerization pathways: a CASPT2 study. Phys Chem Chem Phys 2014;16(43):23944e51. [47] Zhang X, Maccarone AT, Nimlos MR, Kato S, Bierbaum VM, Ellison GB, et al. Unimolecular thermal fragmentation of ortho-benzyne. J Chem Phys 2007;126(4):044312. [48] Li Q, Yao L, Lin SH. Anharmonic effect of the unimolecular isomerization/decomposition of ortho-benzyne. J Chin Chem Soc 2015;62(4):355e65. [49] Moskaleva LV, Madden LK, Lin MC. Unimolecular isomerization/decomposition of ortho-benzyne: ab initio MO/statistical theory study. Phys Chem Chem Phys 1999;1(17):3967e72.
13
[50] Richter H, Benish TG, Mazyar OA, Green WH, Howard JB. Formation of polycyclic aromatic hydrocarbons and their radicals in a nearly sooting premixed benzene flame. Proc Combust Inst 2000;28(2):2609e18. [51] Landera A, Krishtal SP, Kislov VV, Mebel AM, Kaiser RI. Theoretical study of the C6H3 potential energy surface and rate constants and product branching ratios of the C2H(2Sþ )þC4H2(1Sgþ ) and C4H(2Sþ )þC2H2(1Sgþ ) reactions. J Chem Phys 2008;128(21):214301. [52] Zhao MJ, Fang JJ, Zhang L, Dai Z, Yao ZW, Zhao MY. Present state of coal pyrolysis to acetylene in plasma. Chem Eng 2017;45(01):45e9. [53] Chen L, Shao K, Chen J, Yang MH, Zhang DH. Fulldimensional quantum dynamics study of the H2þC2H/HþC2H2 reaction on an ab initio potential energy surface. J Chem Phys 2016;144(19):194309. [54] Gai C, Dong YP, Yang S, Zhang ZL, Liang JC, Li JD. Thermal decomposition kinetics of light polycyclic aromatic hydrocarbons as surrogate biomass tar. RSC Adv 2016;6(86):83154e62. [55] Yan B, Xu P, Jin Y, Cheng Y. Understanding coal/hydrocarbons pyrolysis in thermal plasma reactors by thermodynamic analysis. Chem Eng Sci 2012;84(52):31e9.
Please cite this article as: Chen L et al., A density functional theory study on the conversion of polycyclic aromatic hydrocarbons in hydrogen plasma, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.208