J Mol Model (2014) 20:2127 DOI 10.1007/s00894-014-2127-6
ORIGINAL PAPER
ReaxFF molecular dynamics simulations of the initial pyrolysis mechanism of unsaturated triglyceride Zhiqiang Zhang & Kefeng Yan & Jilong Zhang
Received: 2 June 2013 / Accepted: 26 December 2013 # Springer-Verlag Berlin Heidelberg 2014
Abstract To understand the impact of C=C double bonds in acyl chains of unsaturated triglycerides on the reaction mechanism and product composition in their initial pyrolysis process, ReaxFF molecular dynamics simulations were carried out using a molecular model, trilinolenin, at temperatures of 2000, 2250, and 2500 K. Analyses indicated that the observed pyrolysis mechanisms of unsaturated triglyceride are nearly identical to the saturated triglyceride, and the pyrolysis products also include alkanes, alkenes, alkadienes, aromatics, oxygenated species, CO2, and H2. The formation of intermediates and products is a sequential process. Three C–O bonds in trilinolenin molecule are usually successive dissociated first, leading to the formation of unsaturated C3H5· radical and straight-chain C18H29O2· (RCOO·) radicals. Following that, the deoxygenated alkenyl chain is produced through decarboxylation of RCOO·radicals with consequent release of CO2. The resulting hydrocarbon radicals undergo a variety of disproportionation, isomerization, and hydrogen-transfer reactions, yielding straight and branched-chain hydrocarbons. It was found that the scission of C–O bond and
decarboxylation should preferentially occur before the cleavage of the C–C bond β to the C=C bond in the initial decomposition process of unsaturated trilinolenin. In addition, the formation of cyclic hydrocarbons could proceed through intramolecular cyclization mechanisms, including non-radical electrocyclic, biradical cyclization and cyclization of alkenyl radical, which are inconsistent with previously proposed bimolecular Diels–Alder addition mechanisms. More rapid pyrolysis of trilinolenin would occur at higher temperatures without significantly affecting the apparent reaction mechanisms of trilinolenin pyrolysis in the considered temperature range. Aromatic ring structures are observed to be stable after formation and do not decay within the 500 ps simulation period.
Electronic supplementary material The online version of this article (doi:10.1007/s00894-014-2127-6) contains supplementary material, which is available to authorized users.
Hydrocarbons are used in large quantities in a wide range of industrial applications. Traditionally, hydrocarbons are produced from petroleum sources by thermal cracking technology, which is a very well known process in the petroleum industry. In thermal cracking, energy is used in order to break chemical bonds, which forms free radicals. Reactions of hundreds of radicals lead to the various products. Therefore, at high temperatures, large organic molecules break up during pyrolysis, usually into smaller and more useful hydrocarbons. The major problem associated with using petroleum for hydrocarbons is that petroleum is a non-renewable resource, which has motivated many researchers to find an alternative source of renewable energy from other sources such as triglyceride based on plant oils or animal fats [1]. Apart from hydrotreating and transesterification, previous researchers
Z. Zhang (*) : J. Zhang (*) College of Mining Engineering, Taiyuan University of Technology, No.79 West Yingze Street, Taiyuan, Shanxi, People’s Republic of China 030024 e-mail: [email protected] e-mail: [email protected] K. Yan Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China K. Yan Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China
Keywords Mechanism . Molecular dynamics . Pyrolysis . Unsaturated triglyceride
Introduction
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have shown that triglycerides are also amenable to pyrolysis processes for transforming them into hydrocarbons for industrial use under the right processing conditions. Many studies of pyrolysis carried out in the absence of a catalyst have been done using vegetable oil and fat as raw materials [1]. Generally, plant oil contains unsaturated C=C double bonds in the acyl chains of triglyceride, which significantly affects the reaction mechanism and product composition. It has also found that C-C bond cleavage for unsaturated and saturated molecules results in different products. Schwab et al. [2] believed that unsaturated sites enhance cleavage at the allylic positions. This cleavage is a dominant reaction, which would provide unsaturated polyenes for the production of aromatics via Diels–Alder reaction. Idem et al. [3] studied the thermal cracking of canola oil and proposed a reaction scheme for the thermal cracking of an oil composed of both saturated and unsaturated fatty acids to various products. They found that the degree of unsaturation of the triglyceride has a significant effect on the cracking behavior. If the triglyceride is unsaturated, the cleavage most likely occurs before the decarboxylation and decarbonylation. However, this proposal has been brought into question by Cheng et al. [4]. Through thermodynamic calculation, they suggested that cleavage of the C-O bond takes place at lower temperature while the scission of the C-C bond at position β to the C=C bond occurs at higher temperature. In addition, in the pyrolysis product of triglyceride, a significant fraction of cyclohydrocarbon (including benzene and toluene) could be detected among the products [5]. It is found that aromatics in the product mixture rise with increasing number of double bonds [6]. Although the mechanisms involving thermal cracking of saturated and unsaturated triglycerides have been studied, the formation mechanism of cyclohydrocarbons is still under debate. A commonly proposed reaction pathway for the formation of cyclic hydrocarbons is Diels–Alder reaction (involving a diene and an alkene), followed by further dehydrogenation of the formed cycloalkene [2]. While this route was appealing on the grounds of simplicity and the clear relationship between reactants and products, radical-based routes have acquired more prominence. A proposed mechanism is intramolecular cyclization of alkenyl radical formed as a result of triglyceride cracking [5]. Both Diels–Alder reaction and intramolecular cyclization of alkenyl radical require unsaturated C=C bonds in the precursor for formation of cyclic hydrocarbons. However, our recent reactive molecular dynamics computations [7] suggested that in the process of thermal cracking of saturated triglycerides, five-membered cyclohydrocarbons could be formed via intramolecular biradical cyclization mechanism. These studies show the complexity of pyrolysis mechanism of triglycerides. At present, the influence of the unsaturated C=C bond is still not well understood. The variety of reaction pathways and short-lived and highly reactive intermediates
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make it difficult to describe the reaction mechanism experimentally. Molecular dynamics simulation has been proven to be a powerful tool for examining complex reactive systems. Using ReaxFF reactive force field [8], the initial reaction mechanisms and kinetics associated with hydrocarbon [9], soot [10], and coal [11] pyrolysis processes have been published. Recently, we have also reported the decomposition reactions occurring in the process of saturated triglyceride pyrolysis at high temperature using tripalmitin as a model molecule [7]. To gain insight into the role of the unsaturated bond in the process of triglyceride pyrolysis, in this work we performed molecular dynamics simulation employing ReaxFF reactive force field using trilinolenin as a model molecule. Modeling details To simulate the detailed initial mechanism of the pyrolysis process of unsaturated triglycerides, we use trilinolenin (C57H92O6), an omega-3 polyunsaturated fat, as the model molecule. This study uses H/C/O ReaxFF parameters as reported by Chenoweth et al. [12], which were also used in the study of the pyrolysis of saturated triglyceride [7]. To prepare the initial structure, three trilinolenin model molecules were first placed in a cubic periodic box with a 17.13 Å side length. Then the energy of the system was minimized. Figure 1(a) shows one initial configuration of the calculation system. In order to study the kinetics of the thermal decomposition reactions, constant volume–temperature (NVT) molecular dynamics simulations were performed for 500 ps at a target temperature of 2000 K. In addition, in order to evaluate the temperature dependence of reaction mechanisms, NVT molecular dynamics were also performed at 2250 and 2500 K. Temperature was controlled using a Nose-Hoover thermostat [13] with a damping constant of 10 fs. The time step was set to 0.1 fs in order to describe the pyrolysis process correctly [14]. To improve the statistical accuracy, all these simulations were performed with three independent starting structures (I, II, III) for 2000, 2250, and 2500 K. Five trajectories (a, b, c, d, and e) were simulated at 2000 K from each of these three starting structures by setting the initial velocities of all the atoms according to a random Gaussian distribution. System configurations were saved at every 50 fs, and these data were then used to analyze the initial chemical reactions during the pyrolysis process. Analysis of products formed during the simulation was performed with a 0.3 bond-order cutoff for all atom pairs to recognize molecular and radical species. The results of the molecular identification procedure were also confirmed through visual examination of representative simulation snapshots (e.g., Fig. 1(b)). To study the system-size dependence, we also performed 500 ps simulation on a cubic cell with a 34.27 Å side length containing 24 trilinolenin molecules at 2000 K. It is found that the reaction mechanisms
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Fig. 1 (a) Snapshot of an initial system containing three trilinolenin molecules. (b) Final configuration of the system after 500 ps NVT molecular dynamics simulation at 2000 K. Black, gray and white spheres represent O, C and H atoms, respectively
and the sequence of intermediates and products formation during the simulation agree well with what is obtained from that of small cubic simulation cell (17.13 Å side length), without depending significantly on system size. Therefore, in this study we present only the results of small system simulations. All molecular dynamics simulations were carried out with the Reax package as implemented in LAMMPS software [15]. Details of simulation methods are described elsewhere [7].
Results and discussion In a conventional pyrolysis temperature regime (about 600– 1100 K), it will take from several seconds to hours to complete the whole pyrolysis process of triglyceride. Although reactive force fields are much faster than first-principle methods, their computational cost is still a significant factor when using them to model reaction processes. In order to observe the detailed chemical reaction in ps time scale, ReaxFF molecular dynamics must be performed at higher temperature. In previous study, we found that only at an onset temperature of 2000 K, some reaction products such as cyclic hydrocarbon and H2 could be formed during the 500 ps simulation period [7].
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Thus, in this study we perform reactive molecular dynamics simulations at 2000, 2250, and 2500 K. Table 1 lists the pyrolysis products of trilinolenin of three representative runs out of overall 15 runs after 500 ps simulation at 2000 K (see the detailed product information of all 15 runs in the Supplementary materials). From Table 1, it could be found that the pyrolysis products of unsaturated trilinolenin consists of alkanes, alkenes, alkadienes, aromatic, oxygenated species, CO2, and H2. In addition, the main product evolution of triglyceride pyrolysis is also presented in Fig. 2. From Fig. 2 (a), it could be found that the depletion of trilinolenin is very fast. In the pyrolysis process of trilinolenin, the produced molecules and radicals (C39H64O4·, C21H34O2, C18H29O2· and C17H29·) are also only the reaction intermediates (Fig. 2 (b)). They are all consumed in succession during 500 ps reactive molecular dynamics simulation. However, both C2H4 and CO2 are stable reaction products, which still survived after the 500 ps simulation period. At the end of 500 ps simulation, C2H4 molecules become the most abundant products as shown in Table 1. In addition, it could also be found that almost all the oxygen atoms in trilinolenin have been transformed into CO2 rather than to other oxygen-containing products (Table 1 and Fig. 1(c)). Detailed structural analysis shows that the main pyrolysis pathway observed for unsaturated trilinolenin at 2000 K is fairly consistent with saturated triglyceride. The decomposition of trilinolenin is caused by the release of C18H29O2· Table 1 Chemical composition observed after 500 ps of NVT simulation at 2000 K for trilinolenin model 2000 K 1 C4− hydrocarbons
2 CH4 12 C2H4 1 C2H5 1 C3H4 1 C4H6 1 C4H8 (ring)
2
1 CH4 2 C2H2 1 C2H3 18 C2H4 1 C2H6 1 C3H5 2 C3H6 4 C4H6 C4-15 hydrocarbons 1 C6H8 2 C5H8 1 C7H10 (ring) 1 C5H9 1 C5H10 1 C11H15 1 C11H16 1 C6H6 (ring) 1 C12H19 1 C9H13 1 C11H16 1 C13H19 1 C15H26 C17+ hydrocarbons 1 C18H27 1 C20H34 1 C34H58 Oxygenated hydrocarbons 1 C19H29O2 Others 9 CO2 8 CO2 2 H2 1 H2
3 1 CH4 1 C2H2 11 C2H4 1 C3H4 4 C3H6 1 C4H6 1 C4H7 1 C4H8 1 C5H6 1 C5H10 1 C6H8 1 C9H14 1 C11H16 1 C12H18 1 C12H20 1 C18H28 1 C19H32 1 C14H23O2 8 CO2 1 H2
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b
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a Number of trilinolenin molecules
Fig. 2 Distribution of reactant (a) and main intermediates (b) and products (c) of trilinolenin pyrolysis obtained from ReaxFF molecular dynamics at 2000 K (averaged over 15 independent simulation runs)
J Mol Model (2014) 20:2127 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
C39H63O4 C21H34O2 C18H29O2 C17H29
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c 14 12
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radicals. Deoxygenation is found to proceed via removal of the carboxyl group in the C18H29O2· radical structure as CO2, leading to the formation of C17H29· radical, which is consistent with that of saturated triglycerides. Subsequent thermal cracking of C17H29· results in a straight and branched-chain hydrocarbon mixture with prominent alkane and olefin series through disproportionation, isomerization, and hydrogentransfer reactions. The large amount of C2H4 results from accumulation during its successive elimination from hydrocarbon radicals according to the C–C bond β-scission during secondary cracking, which is also known to be the most important degradation pathway of the fuel molecules [16]. Unlike saturated triglyceride, in unsaturated triglyceride due to the existence of an unsaturated C=C bond, the initial decomposition of triglylcerides might take place with the cleavage of the C–O bond or the C-C bond at position β to C=C bond, controversial in the literature [3, 4]. In this study, we observe C–O bond cleavage of trilinolenin and subsequent loss of CO2 at first in all independent simulations at 2000 K. For example, in one simulation, we found that at about 1.2 ps, the first CO2 molecule and C17H29· radical form via the decarboxylation of C18H29O2·, which comes from the C–O bond cleavage of trilinolenin. The resulting C17H29· starts to decompose and release the first ethylene (C2H4) by C–C bond β-scission at about 11.8 ps. The first scission of β C-C bond to C = C bond occurs at 31.45 ps. At that time, all three trilinolenin molecules have been decomposed via C–O bond scission and seven CO2 molecules have been formed via decarboxylation. Thus, the present work suggests that C–O
bond scission and decarboxylation should occur before the C– C bond cleavage. This finding also confirms the results of Cheng et al. obtained by thermodynamics calculation [4]. Actually, we find that triene in trilinolenin is relatively stable during the whole pyrolysis process. Most unsaturated triene remains in the form of polyene until the end of 500 ps simulation at 2000 K. In addition, eight cyclic hydrocarbons including cycloalkanes, cycloalkenes and aromatic hydrocarbons (shown in Fig. 3), obtained via multiple formation mechanisms, were observed in seven out of 15 simulations after 500 ps runs at 2000 K in the present study. Among them, Ia1, I-b, III-b, and III-e in Fig. 3 are found to be formed by intramolecular electrocyclic reactions. Figure 4 gives the formation pathway of a cycloolefin molecule, 5-methyl-1,3cyclohexadiene (I-a1, the formation pathway of I-b, III-b, and III-e are shown in the supplementary Fig. S1–S3). First, a trilinolenin molecule converts to C17H29· radical after 17.50 ps run via C–O bond cleavage and subsequent decarboxylation (Fig. 4 a-c). Then a selective addition of C17H29· radical, which comes from another pyrolytic trilinolenin molecule, to a C atom of the C=C bond would produce a radical in neighboring C atom at 18.25 ps (Fig. 4 d). After that, intramolecular rotation and radical attack cause a five-membered ring formation at 19.30 ps (Fig. 4 e). However, due to the existence of an unstable radical within the five-membered ring, the ring immediately cleaves in another position, which causes isomerization of the carbon chain (Fig. 4 f). Following that, an inherently strained three-membered ring formation
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a
b
a
b
b
e
d
Fig. 3 Observed cyclic hydrocarbons in the pyrolysis products of trilinolenin from three independent starting structures (I, II, and III) after 500 ps ReaxFF molecular dynamics simulations. Each of these starting
structures was used in five (a, b, c, d, and e) independent runs through changed random velocities of all the atoms according to Gaussian distribution at the temperature of 2000 K
and cracking lead to the chain isomerization into a diene at 25.00 ps (Fig. 4g and h). After detachment of C17H29· radical and abstraction of H· radical, a 1, 3, 5-heptatriene molecule forms (Fig. 4 i and j). After a period of 166.60 ps, the triene can come close enough to close the ring by forming a bond (Fig. 4k) and then give 5-methyl-1, 3-cyclohexadiene at 381.70 ps via electrocyclic reaction (Fig. 4l) [17], which survives until the end of the 500 ps simulation. Through its continued dehydrogenation, this cyclohexadiene most likely serves as an intermediate that further transforms into toluene or subsequent condenses to form polycyclic aromatic hydrocarbons. Our previous computations suggested that saturated fivemembered cyclohydrocarbon could be formed via intramolecular biradical cyclization in the thermal cracking of saturated triglycerides [7]. We found that cyclic hydrocarbons could also be formed via biradical cyclization mechanism in the pyrolysis of unsaturated trilinolenin. Two cyclic hydrocarbons (I-a2 and II-a as shown in Fig. 3) are found in the reaction products at the end of 500 ps simulations at 2000 K, both of
which form via a biradical cyclization mechanism. Figure 5 displays the formation pathway of a benzene molecule (II-a). The formation pathway of methylcyclopropane (I-a2) can be found in supplementary Fig. S4. From Fig. 5, it can be seen that the origin of the six carbon atoms of benzene could be followed back to two different trilinolenin molecules. Through a series of cleavage and recombination of these two trilinolenin molecules (Fig. S5), two hydrocarbon radical fragments are formed at 237.15 and 301.10 ps, separately (Fig. 5 a and b). At 306.10 ps, these two radicals come close enough to recombine into a resonantly stabilized polyene hydrocarbon. Following that, this polyene forms a biradical species under release of H2 in a stepwise mechanism and loss of a branched chain (Fig. 5 d–f), which further forms the pseudo six-membered ring conformation via isomerization and double bond shift (Fig. 5g and h). This intermediate undergoes biradical cyclization, leading to the formation of a six-membered ring. Finally, the benzene molecule is eventually formed at 489.35 ps, accompanied by successive C2H4 and C2H3· loss.
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a
b c
d e
f g
h i
j l
k
Fig. 4 Example of the observed six-membered ring (I-a1) formation event via intramolecular electrocyclic reaction
In addition, we also find the intramolecular alkenyl radical cyclization process in the pyrolysis process. Two cyclic hydrocarbons, II-b and II-d, are found in the final products of two independent simulations at 2000 K. Figure 6 shows the formation pathway of II-b (see Fig. S6 for the formation pathway of II-d). The pathways of initial pyrolysis pathway of this trilinolenin molecule are analogous to those observed for other trilinolenin molecules, which decompose into radical fragments via scission and recombination (Fig. 6 a-f). As a result, an alkenyl radical occurs after 249.75 ps. This intermediate then
further undergoes intramolecular cyclization of alkenyl radical to form a methylcyclopentadiene radical (II-b). It is well known that five-membered ring is a relatively stable species and commonly could be found in the pyrolysis products [5]. If a radical addition reaction occurs in the methyl radical position of fivemembered ring, this five-membered ring would be a stable unsaturated cyclic hydrocarbon. Therefore, the intramolecular cyclization of alkenyl radicals should play a candidate role in the formation mechanism of cyclic hydrocarbon in the pyrolysis process of unsaturated triglyceride.
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a
b
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g h
i
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j
Fig. 5 Example of the observed benzene (II-a) formation event via intramolecular biradical cyclization
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b c d e
g
f
Fig. 6 Example of the observed methylcyclopentadiene radical (II-b) formation event via intramolecular cyclization of alkenyl radical
chemical systems tend to be more reactive and more pyrolysis fragments are formed at the end of 500 ps simulation (Fig. 7) at higher temperatures. Despite the higher pyrolysis temperatures, it is found that the thermal decomposition mechanisms at 2250 and 2500 K are the same as that at 2000 K. In the process of reaction, six-membered aliphatic cyclic structures could also be formed. Analysis of simulation trajectories show that some of them further transform into aromatic rings, while some of them decompose to radical species shortly after their formation. It is also confirmed that aromatic rings are resistant 65
Number of molecules (fragments)
A commonly proposed reaction pathway in the pyrolysis of unsaturated triglyceride is that the aromatic ring should be formed via intermolecular Diels–Alder reaction (involving a diene and an alkene), followed by further dehydrogenation of the formed cycloalkene. In the pyrolysis process of saturated triglyceride, there is nearly no formation of unsaturated conjugated C=C bonds apart from C2H4 resulted from β scission of alkyl radical [7]. Thus, the cyclic compounds do not form through a Diels–Alder addition mechanism. However, in a trilinolenin molecule, there are three conjugated trienes. In the process of pyrolysis, most of the C=C bonds also survive after 500 ps of simulation. As a result, the abundant presence of alkene (e.g., C2H4) and conjugated polyenes (e.g., C4H6 and other polyene) is observed in the reaction products (Table 1). Therefore, the existence of these species should provide enough opportunity for Diels–Alder addition reaction in the pyrolysis process of trilinolenin. However, this type of reaction has not been observed in any of the 15 independent simulations. Thus, in the pyrolysis process of unsaturated triglyceride, Diels–Alder cyclization should not play an important role in the ring-formation process at high pyrolysis temperature, which is similar to that of saturated triglyceride. ReaxFF pyrolysis simulations of trilinolenin at temperatures of 2250 and 2500 K were also performed to evaluate the effects of temperature on the final product distribution (see the detailed product information of all six runs in the supplementary materials). Figure 7 gives an overview of the kinetics of degradation of trilinolenin model for NVT molecular dynamics at 2000, 2250, and 2500 K for 500 ps. As expected, the
60 55 50 45 40
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35 30 25 20 15 10 5 0 0
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Time (ps) Fig. 7 Number of the total fragments observed in the NVT molecular dynamics simulation of the pyrolysis of trilinolenin at temperatures of 2000, 2250, and 2500 K
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to thermal stress. Once the aromatic ring is formed, no degradation of it was observed at 2250 and 2500 K during the 500 ps simulation period.
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ring structures are observed to be stable after formation and survived to the end of the 500 ps simulation period.
Conclusions References Using the ReaxFF reactive force field and trilinolenin molecule model, we studied the pyrolysis of unsaturated triglyceride at 2000, 2250, and 2500 K. We find that the observed pyrolysis mechanisms of unsaturated and saturated triglyceride are nearly identical. The pyrolysis products of unsaturated trilinolenin consist of alkanes, alkenes, alkadienes, aromatics, oxygenated species, CO2 and H2. In general, the modelcompound decomposition is initiated by C–O cleavage to form one unsaturated C3H5· radical and three straight-chain C18H29O2· radicals, followed by the decarboxylation of the latter to form C17H29· radicals. The resulting hydrocarbon radicals would undergo a variety of secondary thermal degradation and rearrangement process, leading to different alkanes and alkenes. It was confirmed that in unsaturated trilinolenin, the scission of C–O bond and decarboxylation should occur before the cleavage of β C–C bond to C=C bond. As opposed to previously proposed bimolecular Diels–Alder addition, the formation of cyclic hydrocarbons via intramolecular nonradical electrocyclic, biradical cyclization, and alkenyl radical cyclization are observed. Increasing thermal stress accelerates the overall pyrolysis reaction consistent with expectations. However, the apparent reaction mechanisms are not changed over the temperature range considered. In addition, aromatic
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ORIGINAL PAPER
ReaxFF molecular dynamics simulations of the initial pyrolysis mechanism of unsaturated triglyceride Zhiqiang Zhang & Kefeng Yan & Jilong Zhang
Received: 2 June 2013 / Accepted: 26 December 2013 # Springer-Verlag Berlin Heidelberg 2014
Abstract To understand the impact of C=C double bonds in acyl chains of unsaturated triglycerides on the reaction mechanism and product composition in their initial pyrolysis process, ReaxFF molecular dynamics simulations were carried out using a molecular model, trilinolenin, at temperatures of 2000, 2250, and 2500 K. Analyses indicated that the observed pyrolysis mechanisms of unsaturated triglyceride are nearly identical to the saturated triglyceride, and the pyrolysis products also include alkanes, alkenes, alkadienes, aromatics, oxygenated species, CO2, and H2. The formation of intermediates and products is a sequential process. Three C–O bonds in trilinolenin molecule are usually successive dissociated first, leading to the formation of unsaturated C3H5· radical and straight-chain C18H29O2· (RCOO·) radicals. Following that, the deoxygenated alkenyl chain is produced through decarboxylation of RCOO·radicals with consequent release of CO2. The resulting hydrocarbon radicals undergo a variety of disproportionation, isomerization, and hydrogen-transfer reactions, yielding straight and branched-chain hydrocarbons. It was found that the scission of C–O bond and
decarboxylation should preferentially occur before the cleavage of the C–C bond β to the C=C bond in the initial decomposition process of unsaturated trilinolenin. In addition, the formation of cyclic hydrocarbons could proceed through intramolecular cyclization mechanisms, including non-radical electrocyclic, biradical cyclization and cyclization of alkenyl radical, which are inconsistent with previously proposed bimolecular Diels–Alder addition mechanisms. More rapid pyrolysis of trilinolenin would occur at higher temperatures without significantly affecting the apparent reaction mechanisms of trilinolenin pyrolysis in the considered temperature range. Aromatic ring structures are observed to be stable after formation and do not decay within the 500 ps simulation period.
Electronic supplementary material The online version of this article (doi:10.1007/s00894-014-2127-6) contains supplementary material, which is available to authorized users.
Hydrocarbons are used in large quantities in a wide range of industrial applications. Traditionally, hydrocarbons are produced from petroleum sources by thermal cracking technology, which is a very well known process in the petroleum industry. In thermal cracking, energy is used in order to break chemical bonds, which forms free radicals. Reactions of hundreds of radicals lead to the various products. Therefore, at high temperatures, large organic molecules break up during pyrolysis, usually into smaller and more useful hydrocarbons. The major problem associated with using petroleum for hydrocarbons is that petroleum is a non-renewable resource, which has motivated many researchers to find an alternative source of renewable energy from other sources such as triglyceride based on plant oils or animal fats [1]. Apart from hydrotreating and transesterification, previous researchers
Z. Zhang (*) : J. Zhang (*) College of Mining Engineering, Taiyuan University of Technology, No.79 West Yingze Street, Taiyuan, Shanxi, People’s Republic of China 030024 e-mail: [email protected] e-mail: [email protected] K. Yan Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China K. Yan Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China
Keywords Mechanism . Molecular dynamics . Pyrolysis . Unsaturated triglyceride
Introduction
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have shown that triglycerides are also amenable to pyrolysis processes for transforming them into hydrocarbons for industrial use under the right processing conditions. Many studies of pyrolysis carried out in the absence of a catalyst have been done using vegetable oil and fat as raw materials [1]. Generally, plant oil contains unsaturated C=C double bonds in the acyl chains of triglyceride, which significantly affects the reaction mechanism and product composition. It has also found that C-C bond cleavage for unsaturated and saturated molecules results in different products. Schwab et al. [2] believed that unsaturated sites enhance cleavage at the allylic positions. This cleavage is a dominant reaction, which would provide unsaturated polyenes for the production of aromatics via Diels–Alder reaction. Idem et al. [3] studied the thermal cracking of canola oil and proposed a reaction scheme for the thermal cracking of an oil composed of both saturated and unsaturated fatty acids to various products. They found that the degree of unsaturation of the triglyceride has a significant effect on the cracking behavior. If the triglyceride is unsaturated, the cleavage most likely occurs before the decarboxylation and decarbonylation. However, this proposal has been brought into question by Cheng et al. [4]. Through thermodynamic calculation, they suggested that cleavage of the C-O bond takes place at lower temperature while the scission of the C-C bond at position β to the C=C bond occurs at higher temperature. In addition, in the pyrolysis product of triglyceride, a significant fraction of cyclohydrocarbon (including benzene and toluene) could be detected among the products [5]. It is found that aromatics in the product mixture rise with increasing number of double bonds [6]. Although the mechanisms involving thermal cracking of saturated and unsaturated triglycerides have been studied, the formation mechanism of cyclohydrocarbons is still under debate. A commonly proposed reaction pathway for the formation of cyclic hydrocarbons is Diels–Alder reaction (involving a diene and an alkene), followed by further dehydrogenation of the formed cycloalkene [2]. While this route was appealing on the grounds of simplicity and the clear relationship between reactants and products, radical-based routes have acquired more prominence. A proposed mechanism is intramolecular cyclization of alkenyl radical formed as a result of triglyceride cracking [5]. Both Diels–Alder reaction and intramolecular cyclization of alkenyl radical require unsaturated C=C bonds in the precursor for formation of cyclic hydrocarbons. However, our recent reactive molecular dynamics computations [7] suggested that in the process of thermal cracking of saturated triglycerides, five-membered cyclohydrocarbons could be formed via intramolecular biradical cyclization mechanism. These studies show the complexity of pyrolysis mechanism of triglycerides. At present, the influence of the unsaturated C=C bond is still not well understood. The variety of reaction pathways and short-lived and highly reactive intermediates
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make it difficult to describe the reaction mechanism experimentally. Molecular dynamics simulation has been proven to be a powerful tool for examining complex reactive systems. Using ReaxFF reactive force field [8], the initial reaction mechanisms and kinetics associated with hydrocarbon [9], soot [10], and coal [11] pyrolysis processes have been published. Recently, we have also reported the decomposition reactions occurring in the process of saturated triglyceride pyrolysis at high temperature using tripalmitin as a model molecule [7]. To gain insight into the role of the unsaturated bond in the process of triglyceride pyrolysis, in this work we performed molecular dynamics simulation employing ReaxFF reactive force field using trilinolenin as a model molecule. Modeling details To simulate the detailed initial mechanism of the pyrolysis process of unsaturated triglycerides, we use trilinolenin (C57H92O6), an omega-3 polyunsaturated fat, as the model molecule. This study uses H/C/O ReaxFF parameters as reported by Chenoweth et al. [12], which were also used in the study of the pyrolysis of saturated triglyceride [7]. To prepare the initial structure, three trilinolenin model molecules were first placed in a cubic periodic box with a 17.13 Å side length. Then the energy of the system was minimized. Figure 1(a) shows one initial configuration of the calculation system. In order to study the kinetics of the thermal decomposition reactions, constant volume–temperature (NVT) molecular dynamics simulations were performed for 500 ps at a target temperature of 2000 K. In addition, in order to evaluate the temperature dependence of reaction mechanisms, NVT molecular dynamics were also performed at 2250 and 2500 K. Temperature was controlled using a Nose-Hoover thermostat [13] with a damping constant of 10 fs. The time step was set to 0.1 fs in order to describe the pyrolysis process correctly [14]. To improve the statistical accuracy, all these simulations were performed with three independent starting structures (I, II, III) for 2000, 2250, and 2500 K. Five trajectories (a, b, c, d, and e) were simulated at 2000 K from each of these three starting structures by setting the initial velocities of all the atoms according to a random Gaussian distribution. System configurations were saved at every 50 fs, and these data were then used to analyze the initial chemical reactions during the pyrolysis process. Analysis of products formed during the simulation was performed with a 0.3 bond-order cutoff for all atom pairs to recognize molecular and radical species. The results of the molecular identification procedure were also confirmed through visual examination of representative simulation snapshots (e.g., Fig. 1(b)). To study the system-size dependence, we also performed 500 ps simulation on a cubic cell with a 34.27 Å side length containing 24 trilinolenin molecules at 2000 K. It is found that the reaction mechanisms
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Fig. 1 (a) Snapshot of an initial system containing three trilinolenin molecules. (b) Final configuration of the system after 500 ps NVT molecular dynamics simulation at 2000 K. Black, gray and white spheres represent O, C and H atoms, respectively
and the sequence of intermediates and products formation during the simulation agree well with what is obtained from that of small cubic simulation cell (17.13 Å side length), without depending significantly on system size. Therefore, in this study we present only the results of small system simulations. All molecular dynamics simulations were carried out with the Reax package as implemented in LAMMPS software [15]. Details of simulation methods are described elsewhere [7].
Results and discussion In a conventional pyrolysis temperature regime (about 600– 1100 K), it will take from several seconds to hours to complete the whole pyrolysis process of triglyceride. Although reactive force fields are much faster than first-principle methods, their computational cost is still a significant factor when using them to model reaction processes. In order to observe the detailed chemical reaction in ps time scale, ReaxFF molecular dynamics must be performed at higher temperature. In previous study, we found that only at an onset temperature of 2000 K, some reaction products such as cyclic hydrocarbon and H2 could be formed during the 500 ps simulation period [7].
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Thus, in this study we perform reactive molecular dynamics simulations at 2000, 2250, and 2500 K. Table 1 lists the pyrolysis products of trilinolenin of three representative runs out of overall 15 runs after 500 ps simulation at 2000 K (see the detailed product information of all 15 runs in the Supplementary materials). From Table 1, it could be found that the pyrolysis products of unsaturated trilinolenin consists of alkanes, alkenes, alkadienes, aromatic, oxygenated species, CO2, and H2. In addition, the main product evolution of triglyceride pyrolysis is also presented in Fig. 2. From Fig. 2 (a), it could be found that the depletion of trilinolenin is very fast. In the pyrolysis process of trilinolenin, the produced molecules and radicals (C39H64O4·, C21H34O2, C18H29O2· and C17H29·) are also only the reaction intermediates (Fig. 2 (b)). They are all consumed in succession during 500 ps reactive molecular dynamics simulation. However, both C2H4 and CO2 are stable reaction products, which still survived after the 500 ps simulation period. At the end of 500 ps simulation, C2H4 molecules become the most abundant products as shown in Table 1. In addition, it could also be found that almost all the oxygen atoms in trilinolenin have been transformed into CO2 rather than to other oxygen-containing products (Table 1 and Fig. 1(c)). Detailed structural analysis shows that the main pyrolysis pathway observed for unsaturated trilinolenin at 2000 K is fairly consistent with saturated triglyceride. The decomposition of trilinolenin is caused by the release of C18H29O2· Table 1 Chemical composition observed after 500 ps of NVT simulation at 2000 K for trilinolenin model 2000 K 1 C4− hydrocarbons
2 CH4 12 C2H4 1 C2H5 1 C3H4 1 C4H6 1 C4H8 (ring)
2
1 CH4 2 C2H2 1 C2H3 18 C2H4 1 C2H6 1 C3H5 2 C3H6 4 C4H6 C4-15 hydrocarbons 1 C6H8 2 C5H8 1 C7H10 (ring) 1 C5H9 1 C5H10 1 C11H15 1 C11H16 1 C6H6 (ring) 1 C12H19 1 C9H13 1 C11H16 1 C13H19 1 C15H26 C17+ hydrocarbons 1 C18H27 1 C20H34 1 C34H58 Oxygenated hydrocarbons 1 C19H29O2 Others 9 CO2 8 CO2 2 H2 1 H2
3 1 CH4 1 C2H2 11 C2H4 1 C3H4 4 C3H6 1 C4H6 1 C4H7 1 C4H8 1 C5H6 1 C5H10 1 C6H8 1 C9H14 1 C11H16 1 C12H18 1 C12H20 1 C18H28 1 C19H32 1 C14H23O2 8 CO2 1 H2
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b
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a Number of trilinolenin molecules
Fig. 2 Distribution of reactant (a) and main intermediates (b) and products (c) of trilinolenin pyrolysis obtained from ReaxFF molecular dynamics at 2000 K (averaged over 15 independent simulation runs)
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C39H63O4 C21H34O2 C18H29O2 C17H29
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c 14 12
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radicals. Deoxygenation is found to proceed via removal of the carboxyl group in the C18H29O2· radical structure as CO2, leading to the formation of C17H29· radical, which is consistent with that of saturated triglycerides. Subsequent thermal cracking of C17H29· results in a straight and branched-chain hydrocarbon mixture with prominent alkane and olefin series through disproportionation, isomerization, and hydrogentransfer reactions. The large amount of C2H4 results from accumulation during its successive elimination from hydrocarbon radicals according to the C–C bond β-scission during secondary cracking, which is also known to be the most important degradation pathway of the fuel molecules [16]. Unlike saturated triglyceride, in unsaturated triglyceride due to the existence of an unsaturated C=C bond, the initial decomposition of triglylcerides might take place with the cleavage of the C–O bond or the C-C bond at position β to C=C bond, controversial in the literature [3, 4]. In this study, we observe C–O bond cleavage of trilinolenin and subsequent loss of CO2 at first in all independent simulations at 2000 K. For example, in one simulation, we found that at about 1.2 ps, the first CO2 molecule and C17H29· radical form via the decarboxylation of C18H29O2·, which comes from the C–O bond cleavage of trilinolenin. The resulting C17H29· starts to decompose and release the first ethylene (C2H4) by C–C bond β-scission at about 11.8 ps. The first scission of β C-C bond to C = C bond occurs at 31.45 ps. At that time, all three trilinolenin molecules have been decomposed via C–O bond scission and seven CO2 molecules have been formed via decarboxylation. Thus, the present work suggests that C–O
bond scission and decarboxylation should occur before the C– C bond cleavage. This finding also confirms the results of Cheng et al. obtained by thermodynamics calculation [4]. Actually, we find that triene in trilinolenin is relatively stable during the whole pyrolysis process. Most unsaturated triene remains in the form of polyene until the end of 500 ps simulation at 2000 K. In addition, eight cyclic hydrocarbons including cycloalkanes, cycloalkenes and aromatic hydrocarbons (shown in Fig. 3), obtained via multiple formation mechanisms, were observed in seven out of 15 simulations after 500 ps runs at 2000 K in the present study. Among them, Ia1, I-b, III-b, and III-e in Fig. 3 are found to be formed by intramolecular electrocyclic reactions. Figure 4 gives the formation pathway of a cycloolefin molecule, 5-methyl-1,3cyclohexadiene (I-a1, the formation pathway of I-b, III-b, and III-e are shown in the supplementary Fig. S1–S3). First, a trilinolenin molecule converts to C17H29· radical after 17.50 ps run via C–O bond cleavage and subsequent decarboxylation (Fig. 4 a-c). Then a selective addition of C17H29· radical, which comes from another pyrolytic trilinolenin molecule, to a C atom of the C=C bond would produce a radical in neighboring C atom at 18.25 ps (Fig. 4 d). After that, intramolecular rotation and radical attack cause a five-membered ring formation at 19.30 ps (Fig. 4 e). However, due to the existence of an unstable radical within the five-membered ring, the ring immediately cleaves in another position, which causes isomerization of the carbon chain (Fig. 4 f). Following that, an inherently strained three-membered ring formation
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a
b
a
b
b
e
d
Fig. 3 Observed cyclic hydrocarbons in the pyrolysis products of trilinolenin from three independent starting structures (I, II, and III) after 500 ps ReaxFF molecular dynamics simulations. Each of these starting
structures was used in five (a, b, c, d, and e) independent runs through changed random velocities of all the atoms according to Gaussian distribution at the temperature of 2000 K
and cracking lead to the chain isomerization into a diene at 25.00 ps (Fig. 4g and h). After detachment of C17H29· radical and abstraction of H· radical, a 1, 3, 5-heptatriene molecule forms (Fig. 4 i and j). After a period of 166.60 ps, the triene can come close enough to close the ring by forming a bond (Fig. 4k) and then give 5-methyl-1, 3-cyclohexadiene at 381.70 ps via electrocyclic reaction (Fig. 4l) [17], which survives until the end of the 500 ps simulation. Through its continued dehydrogenation, this cyclohexadiene most likely serves as an intermediate that further transforms into toluene or subsequent condenses to form polycyclic aromatic hydrocarbons. Our previous computations suggested that saturated fivemembered cyclohydrocarbon could be formed via intramolecular biradical cyclization in the thermal cracking of saturated triglycerides [7]. We found that cyclic hydrocarbons could also be formed via biradical cyclization mechanism in the pyrolysis of unsaturated trilinolenin. Two cyclic hydrocarbons (I-a2 and II-a as shown in Fig. 3) are found in the reaction products at the end of 500 ps simulations at 2000 K, both of
which form via a biradical cyclization mechanism. Figure 5 displays the formation pathway of a benzene molecule (II-a). The formation pathway of methylcyclopropane (I-a2) can be found in supplementary Fig. S4. From Fig. 5, it can be seen that the origin of the six carbon atoms of benzene could be followed back to two different trilinolenin molecules. Through a series of cleavage and recombination of these two trilinolenin molecules (Fig. S5), two hydrocarbon radical fragments are formed at 237.15 and 301.10 ps, separately (Fig. 5 a and b). At 306.10 ps, these two radicals come close enough to recombine into a resonantly stabilized polyene hydrocarbon. Following that, this polyene forms a biradical species under release of H2 in a stepwise mechanism and loss of a branched chain (Fig. 5 d–f), which further forms the pseudo six-membered ring conformation via isomerization and double bond shift (Fig. 5g and h). This intermediate undergoes biradical cyclization, leading to the formation of a six-membered ring. Finally, the benzene molecule is eventually formed at 489.35 ps, accompanied by successive C2H4 and C2H3· loss.
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a
b c
d e
f g
h i
j l
k
Fig. 4 Example of the observed six-membered ring (I-a1) formation event via intramolecular electrocyclic reaction
In addition, we also find the intramolecular alkenyl radical cyclization process in the pyrolysis process. Two cyclic hydrocarbons, II-b and II-d, are found in the final products of two independent simulations at 2000 K. Figure 6 shows the formation pathway of II-b (see Fig. S6 for the formation pathway of II-d). The pathways of initial pyrolysis pathway of this trilinolenin molecule are analogous to those observed for other trilinolenin molecules, which decompose into radical fragments via scission and recombination (Fig. 6 a-f). As a result, an alkenyl radical occurs after 249.75 ps. This intermediate then
further undergoes intramolecular cyclization of alkenyl radical to form a methylcyclopentadiene radical (II-b). It is well known that five-membered ring is a relatively stable species and commonly could be found in the pyrolysis products [5]. If a radical addition reaction occurs in the methyl radical position of fivemembered ring, this five-membered ring would be a stable unsaturated cyclic hydrocarbon. Therefore, the intramolecular cyclization of alkenyl radicals should play a candidate role in the formation mechanism of cyclic hydrocarbon in the pyrolysis process of unsaturated triglyceride.
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a
b
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g h
i
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j
Fig. 5 Example of the observed benzene (II-a) formation event via intramolecular biradical cyclization
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b c d e
g
f
Fig. 6 Example of the observed methylcyclopentadiene radical (II-b) formation event via intramolecular cyclization of alkenyl radical
chemical systems tend to be more reactive and more pyrolysis fragments are formed at the end of 500 ps simulation (Fig. 7) at higher temperatures. Despite the higher pyrolysis temperatures, it is found that the thermal decomposition mechanisms at 2250 and 2500 K are the same as that at 2000 K. In the process of reaction, six-membered aliphatic cyclic structures could also be formed. Analysis of simulation trajectories show that some of them further transform into aromatic rings, while some of them decompose to radical species shortly after their formation. It is also confirmed that aromatic rings are resistant 65
Number of molecules (fragments)
A commonly proposed reaction pathway in the pyrolysis of unsaturated triglyceride is that the aromatic ring should be formed via intermolecular Diels–Alder reaction (involving a diene and an alkene), followed by further dehydrogenation of the formed cycloalkene. In the pyrolysis process of saturated triglyceride, there is nearly no formation of unsaturated conjugated C=C bonds apart from C2H4 resulted from β scission of alkyl radical [7]. Thus, the cyclic compounds do not form through a Diels–Alder addition mechanism. However, in a trilinolenin molecule, there are three conjugated trienes. In the process of pyrolysis, most of the C=C bonds also survive after 500 ps of simulation. As a result, the abundant presence of alkene (e.g., C2H4) and conjugated polyenes (e.g., C4H6 and other polyene) is observed in the reaction products (Table 1). Therefore, the existence of these species should provide enough opportunity for Diels–Alder addition reaction in the pyrolysis process of trilinolenin. However, this type of reaction has not been observed in any of the 15 independent simulations. Thus, in the pyrolysis process of unsaturated triglyceride, Diels–Alder cyclization should not play an important role in the ring-formation process at high pyrolysis temperature, which is similar to that of saturated triglyceride. ReaxFF pyrolysis simulations of trilinolenin at temperatures of 2250 and 2500 K were also performed to evaluate the effects of temperature on the final product distribution (see the detailed product information of all six runs in the supplementary materials). Figure 7 gives an overview of the kinetics of degradation of trilinolenin model for NVT molecular dynamics at 2000, 2250, and 2500 K for 500 ps. As expected, the
60 55 50 45 40
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35 30 25 20 15 10 5 0 0
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Time (ps) Fig. 7 Number of the total fragments observed in the NVT molecular dynamics simulation of the pyrolysis of trilinolenin at temperatures of 2000, 2250, and 2500 K
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to thermal stress. Once the aromatic ring is formed, no degradation of it was observed at 2250 and 2500 K during the 500 ps simulation period.
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ring structures are observed to be stable after formation and survived to the end of the 500 ps simulation period.
Conclusions References Using the ReaxFF reactive force field and trilinolenin molecule model, we studied the pyrolysis of unsaturated triglyceride at 2000, 2250, and 2500 K. We find that the observed pyrolysis mechanisms of unsaturated and saturated triglyceride are nearly identical. The pyrolysis products of unsaturated trilinolenin consist of alkanes, alkenes, alkadienes, aromatics, oxygenated species, CO2 and H2. In general, the modelcompound decomposition is initiated by C–O cleavage to form one unsaturated C3H5· radical and three straight-chain C18H29O2· radicals, followed by the decarboxylation of the latter to form C17H29· radicals. The resulting hydrocarbon radicals would undergo a variety of secondary thermal degradation and rearrangement process, leading to different alkanes and alkenes. It was confirmed that in unsaturated trilinolenin, the scission of C–O bond and decarboxylation should occur before the cleavage of β C–C bond to C=C bond. As opposed to previously proposed bimolecular Diels–Alder addition, the formation of cyclic hydrocarbons via intramolecular nonradical electrocyclic, biradical cyclization, and alkenyl radical cyclization are observed. Increasing thermal stress accelerates the overall pyrolysis reaction consistent with expectations. However, the apparent reaction mechanisms are not changed over the temperature range considered. In addition, aromatic
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