DFT Research on Benzothiophene Pyrolysis Reaction Mechanism

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

DFT Research on Benzothiophene Pyrolysis Reaction Mechanism Tianshuang Li, Jie Li, Hongliang Zhang, Kena Sun, and Jin Xiao J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b09882 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 3, 2019

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The Journal of Physical Chemistry

DFT Research on Benzothiophene Pyrolysis Reaction Mechanism

Tianshuang LI, Jie LI, Hongliang ZHANG *, Kena SUN, Jin XIAO (School of Metallurgy and Environment, Central South University, Hunan Province, Changsha 410083, China. *E-mail:[email protected])

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Abstract: Thiophene sulfur is the most stable organic sulfur species in petroleum coke, among which benzothiophene accounts for a significant portion. Removal of benzothiophene will help to gain ultra-low desulfurization. In this work, a density function theory (DFT) method was adopted to investigate benzothiophene pyrolysis mechanism. It was found that the most possible pyrolysis reaction of benzothiophene is triggered by α-H migration to β-position. The dominating products are S radical and ethenethione, which could explain benzothiophene pyrolysis experiments well. Converting thiophene fused on aromatic to a thiol group could help to promote desulfurization. As a contrast, thiophene pyrolysis reaction was also calculated at the same level. The initial pyrolysis temperature of benzothiophene and thiophene may be close, but the pyrolysis rate of thiophene is higher than benzothiophene. The implication of benzothiophene pyrolysis mechanism may be beneficial for the development of new desulfurization technology.


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1 Introduction Petroleum accounts for more than 41% of total world energy1. In recent years, low sulfur crude supplies have been decreased, more crude is with higher sulfur content. Sulfur will cause emission of SOx gases, which lead to serious air pollution. To minimize the negative effects, worldwide countries are adopting stricter regulations to reduce sulfur emissions by enforcing ultra-low sulfur concentration in fuels2-8. Petroleum coke is the residue product from petroleum refining process. It is also the raw material for aluminum electrolysis carbon anode9. Influenced by high-sulfur crude oil supply, the sulfur concentration of petroleum coke is getting higher. So it happens to carbon anode10,11. In modern aluminum industry, the theoretical minimum consumption of carbon anode is 334 kg per ton aluminum. Considering huge aluminum annual output, the amount of SOx released from aluminum electrolysis industry is beyond our imagination if there is no effective treatment. Unfortunately, the sulfur concentration in aluminum electrolysis exhaust gas is too lower for industrial desulfurization. Hence, desulfurization of petroleum coke or anode is the most economical and effective way. Sulfur in petroleum coke appears in two forms, inorganic and organic. Inorganic sulfur mainly consists of sulfate and sulfide. Organic sulfur refers to a wide range of compounds, such as thiophenes, sulfones, thiols. Most sulfurs can be removed during petroleum coke calcination or carbon anode thermal treatment. However, a significant amount organic sulfur still remains. To promote desulfurization, raising the thermal treatment temperature is the common way12-15. In some researches, the temperature 3 ACS Paragon Plus Environment


even reached 1600℃16. Taking the cost and energy consumption into consideration, temperature could not be raised without limit. Thiophene sulfur is the most abundant and stable organic sulfur species in petroleum coke17-19. Reducing thiophene sulfur content in petroleum coke will help to gain ultra-low desulfurization. To remove thiophene sulfur economically, research must be conducted to understand the pyrolysis behavior of thiophene sulfur. Thiophene sulfur consists of thiophenes, benzothiophene and dibenzothiophenes. Among these, benzothiophenes account for a significant portion17,20. Removal of benzothiophene is with great significance in petroleum coke ultra-low desulfurization. Many studies about benzothiophene desulfurization have been conducted. Liu et al.21 showed that the decomposition temperature of benzothiophene is lower during pyrolysis under an oxidative atmosphere than under inert and hydrogen atmospheres. Guo et al.22 indicated that SO2, not H2S, is generated when acetone exists in the pyrolysis process. Xing23 found that the amount of benzothiophene and dibenzothiophene release behavior is related with temperature, the release rate increases during 600℃~800℃ then decreased till 1200℃~1300℃. Jie Shan24 proved that thiophene reaction with alkali tends to generate an equal amount of sulfate and sulfide. Liu et al.25 declared that gaseous H2 could easily react with the surface S to form H2S and the S on the surface is much lower than under N2. Investigations on benzothiophene thermal decomposition are mainly performed by experiments. The detailed pyrolysis mechanism remains unclear. It's known that quantum chemical calculation plays a major role in chemical reaction mechanism study. 4 ACS Paragon Plus Environment

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Earlier studies, relying on experiment, suggested that pyrolysis of thiophene is triggered by the C-S fission26-28. However, recent studies using quantum chemical calculation, indicated that α-H migration to S site is more likely to take place compared with C-S fission29-34. Hence quantum chemical research on benzothiophene pyrolysis behavior is necessary to promote the understanding of pyrolysis mechanism. In this work, a density function theory (DFT) method was adopted to conduct detailed research on benzothiophene pyrolysis mechanism. The initial reaction step and followed reaction processes were investigated. As a contrast, thiophene pyrolysis reactions were also calculated at the same level. The pyrolysis mechanism of multiple aromatic rings fused benzothiophene and dibenzothiophene will be the focus in our post study. We hope our results would shed a light on benzothiophene pyrolysis mechanism and provide guidance for the development of new desulfurization technology.

2 Computational Details All DFT calculations were performed with the program package of Gaussian0935. All intermediates(IM) geometry optimization, frequency and transition states (TS) search were calculated at UB3LYP/6-311G++(d,p) level. Intermediates were confirmed with all real frequencies and transition states were confirmed by only one imaginary frequency. Instrinsic reaction coordinate(IRC)36 calculation were also employed to validate the connection between reactants and products. Thermodynamic data and Gibbs free energy change were obtained from Gaussian frequency calculation results. To test the reliability of UB3LYP/6-311G++(d,p), thiophene and benzothiophene 5 ACS Paragon Plus Environment


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were optimized at a variety of basis sets. The results are shown in Table 1 and Table 2.

Table 1. Thiophene bond length, bond angle, α-H migration energy barrier

S-C1 C1-C2 C2-C3 C1-H C2-H ∠C1-S-C5

6-311++ 6-311++ G(d,p) G(2d,2p) Geometry(unit: Å and ° ) 1.733 1.727 1.366 1.364 1.428 1.424 1.079 1.076 1.081 1.079 91.49 91.68

6-311++ G(df,pd)

CBSQB3

1.729 1.364 1.425 1.078 1.081 91.51

1.727 1.366 1.426 1.079 1.082 91.74

H-migration energy barrier( unit: kcal/mol) from C1 to C2 69.21 69.11 70.11 from C1 to S 89.17 88.32 89.90 Single point energy(relative to thiophene, unit: kcal/mol) TIM1 58.86 58.38 60.08 58.31 TIM4 84.46 82.61 85.53 76.19 TIM1 is formed by C1-H migration to C2. TIM4 is formed by C1-H migration to S.

Table 2. Benzothiophene bond length, bond angle, α-H migration energy barrier. 6-311++ 6-311++ 6-311++ G(d,p) G(2d,2p) G(df,pd) Geometry(∠S is ∠C8-S-C1, unit: Å and °) S-C1 1.749 1.745 1.746 S-C8 1.756 1.751 1.752 C1-C2 1.356 1.353 1.354 C2-C3 1.439 1.436 1.436 C3-C4 1.406 1.404 1.404 C3-C8 1.416 1.413 1.413 C7-C8 1.397 1.394 1.395 C1-H 1.079 1.077 1.079 C2-H 1.082 1.079 1.082 90.92 90.94 90.94 ∠S

CBSQB3 1.743 1.750 1.356 1.439 1.406 1.416 1.397 1.080 1.082 91.11

H-migration energy barrier(unit: kcal/mol) From C1 to S 86.49 86.74 87.51 From C1 to C2 67.72 67.72 68.54 From C2 to C1 72.60 73.39 73.50 Single point energy(relative to benzothiophene, unit: kcal/mol) IM1 80.62 78.68 81.30 77.42


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IM2 54.18 53.76 55.34 53.50 IM3 61.56 62.33 62.54 66.85 IM1, IM2 and IM3 is the corresponding intermediate formed by H-migration C1 to S, C1 to C2 and C2 to C1.

From Table 1 and Table 2, it can be found that 6-311G++(d,p), 6-311G++(2d,2p) and 6-311g++(df,pd) have a nearly the same results on describing the geometry and energy of benzothiophene and thiophene. And also, the deviation between 6311G++(d,p) and CBS-QB337 is acceptable. Further, CBS-QB3 energy of all intermediates was calculated

using

B3LYP/CBSB7 optimization to comment on the energy deviation. The results can be found in Supporting Information Part D. The energy deviation of singlet intermediates between B3LYP/6-311G++(d,p) and CBS-QB3 is 0.46kcal/mol. However, energy calculated at B3LYP/6-311G++(d,p) for biradical intermediates and products is not that accurate as singlet intermediates. This study is focused on benzothiophene pyrolysis process, so it makes little difference to the mechanism. It can be concluded that B3LYP/6-311G++(d,p) has a good performance on thiophene and benzothiophene calculation. It should be noted that all B3LYP/6-311G++(d,p) energy is reported at 0K energy without ZPE correction herein.

3 Results and discussions 3.1 Benzothiophene initial reaction steps As illustrated in the introduction, earlier studies suggested that pyrolysis of thiophene is triggered by the C-S fission and quantum chemical calculation indicated that α-H migration to S site is more likely to occur compared to C-S fission29-34. In this 7 ACS Paragon Plus Environment


paper, this discussion was also introduced in benzothiophene pyrolysis mechanism. The possible initial reaction steps of benzothiophene pyrolysis can be mainly divided into two groups: bond rupture, H(proton)-migration. Seven H-migration initial steps could take place in benzothiophene pyrolysis. The details of this seven steps are illustrated in Figure 1.

Figure 1.

H-migration initial reaction steps for benzothiophene pyrolysis.

All the seven H-migration steps are directly related with benzothiophene pyrolysis. The H-migrations on benzene ring are not included, since it has little influence on thiophene ring reactions. In previous study33, it was confirmed that the energy barrier of H atom migration with single electron is higher than H proton migration process. Hence, all the H-migration steps appeared in this paper is proton migration and no single electron will be generated in this process. So, IM1~IM7 are benzothiophene carbenes. Another group of initial steps is triggered by bond rupture. The rupture of four 8 ACS Paragon Plus Environment

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bonds of thiophene ring will generate five intermediates, IM8~IM12. Details of this five steps could be found in Figure 2. It should be noted that none benzene C-C bond rupture is included, for the bond energy is really high for initial step.

Figure 2.

Bond rupture initial reaction steps for benzothiophene pyrolysis.

Biradical intermediates IM8~IM12 are all formed by two processes. First process is bond rupture. The bond is stretched to form corresponding biradical intermediate (BIM1~BIM4). Followed the process is ortho- or meta-bond rotation. The distance of two radical atom increases and the BIM transforms to the stable biradical intermediate. By scanning energy versus bond distance, it was found that the first process is an energy increasing process and the energy converges to a maximum value (BIM1~BIM4 energy) which means BIM1~BIM4 are unstable and will be transformed back to benzothiophene without any barrier. In the second process, the energy firstly increases 9 ACS Paragon Plus Environment


then decreases along with bond rotation which means biradical intermediates BIM1~BIM4 transform to stable biradical BIM5~BIM9 via a transition state. The Potential Energy Surface (PES) of H-migration steps is illustrated in Figure 3. For H-migration initial steps, the lowest energy barrier is 67.72kcal/mol located at the step RE-IM2 which is α-H migratation to β-postion process. The other last two processes are RE-IM3(H migration from C1 to S) and RE-IM1 (H migration from C1 to C2) and the energy barriers are 72.60kcal/moland 86.49kcal/mol, respectively.

Figure 3. PES of benzothiophene H-migration initial steps (illustrated in Figure 2.) Details of TS and IM geometry can be found in Supporting Information. The PES of bond rupture initial steps is shown in Figure 4. For bond rupture initial steps, the energies for C8-S, S-C1 and C2-C3 bond cleavage to form corresponding biradical intermediate are 74.17kcal/mol, 80.14kcal/mol and 124.13kcal/mol, respectively. Particularly, C1-C2 bond cleavage energy is the highest in all bond rupture initial steps, as high as 165.07kcal/mol. Four single electrons will be generated in this step. 10 ACS Paragon Plus Environment

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Figure 4. PES of benzothiophene bond rupture initial steps (illustrated in Figure 2.) Details of TS, IM and BIM geometry can be found in Supporting Information. From Figure 4, it is obvious that energy for C8-S bond rupture to form BIM1 is the lowest among four bonds. Taking the whole two processes into consideration, the easiest two bond rupture initial steps are RE-IM9 and RE-IM10, the energy barriers are 86.10kcal/mol and 87.56kcal/mol. It is worth mentioning that some isomerization or direct dissociation initial steps were also found. The intermediates are listed in Figure 5. However, the energy barriers of these initial step are much higher than H-migration and two body bond rupture initial steps. Therefore, isomerization initial steps have little influence on benzothiophene dominated reaction pathway. So, these initial steps will not be introduced in detail.



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Figure 5. Isomerization initial steps for benzothiophene pyrolysis. Details of TS and IM geometry can be found in Supporting Information. Energy barriers and reaction energy of all initial reaction steps are listed in Table 3. Table 3.

Energy barriers and reaction energies of benzothiophene initial steps.

(unit: kcal/mol) Initial Step RE-IM1 RE-IM2 RE-IM3 RE-IM4 RE-IM5 RE-IM6 RE-IM7 RE-IM8 RE-IM9 RE-IM10 RE-IM11 RE-IM12 RE-I1 RE-I2 RE-I3 RE-I4 RE-I5 RE-I6 RE-I7

Energy barrier 86.49 67.72 72.60 121.11 94.52 97.47 97.17 95.72 86.10 87.56 165.78 127.21 117.9443 143.5086 149.1106 106.9239 103.6639 106.018 155.8648


Reaction energy 80.62 54.18 61.56 98.63 92.76 97.07 97.00 76.61 78.36 80.58 164.22 123.52 112.8459 29.26197 112.5629 83.77075 79.24408 94.89059 109.9194

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RE-I8

112.8746

32.80389

From the discussion above, it can be concluded that RE-IM2, RE-IM3, RE-IM1, RE-BIM6 and RE-BIM7 are the five initial steps most likely to take place. Hence, REIM2, RE-IM1, RE-IM3, RE-BIM6 and RE-BIM7 are selected as the five main reaction channels for benzothiophene pyrolysis. The channel is named by its start IM. In the following, the authors will follow this five initial steps to find out how benzothiophene evolves to products. Although the energy barrier of RE-IM5 (TS5, 94.52 kcal/mol above RE), RE-IM7 (TS7, 97.17kcal/mol above RE) and RE-IM8 (TS8, 95.72 kcal/mol above RE) are close to RE-IM1 (TS1, 86.53kcal/mol above RE), they are nearly 30kcal/mol higher than the energy barrier of RE-IM2.

3.2 Channel IM2 reaction process IM2 is α-carbene of benzothiophene, formed by 1,2-H migration. In channel IM2, four products could be generated, thioketene(SC2H2) and m-benzyne(1,3-C6H4), S atom and phenylacetylene (C6H5CCH), H2S and 1,2-C6H3CCH radical, phenyl radical(C6H5) and SCCH radical. The reaction pathways of this four products have been illustrated in Figure 6.



Figure 6.

Sketch of Channel IM2 reaction pathway. P2, P3, P4 and P5 are short for

Product 2, Product 3, Product 4 and Product 5. Two main reaction pathways are included in Channel IM2. Each one will lead to two products formation. One reaction pathway is initiated with IM14 and the products are P3(sulfur free atom and C6H5CCH), P5(C6H5 radical+SCCH radical). The other pathway is through IM15 and product is P2 (SC2H2 and 1,3-C6H4). It should be noted that sulfur free atom is free neutral atom and octet-deficient. IM14 is formed from IM2 by C2-H migration to C3 via transition state TS14 (42.98kcal/mol above IM2). The PES of the first reaction pathway IM2-IM14 is shown in Figure 7.


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Figure 7. PES of IM2-IM14 reaction pathway in Channel IM2. Details of TS and IM geometry can be found in Supporting Information. From IM14, two possible intermediates IM16 and IM17 could be formed. IM16 is formed by C2-C3 bond dissociation and triple bond formation between C1-C2 meanwhile. IM17 is generated from IM14 by S-C1 cleavage, accompanied by backmigration of H from C2-C1. Both the two processes will result in a triple C-C bond formation. However, the energy barriers are quite different, 6.26kcal/mol for IM14IM16 and 40.66kcal/mol for IM14-IM17. For following process from IM16 to P5, energy of 66.46kcal/mol is needed for C8S rupture. IM23 is formed preferentially from IM17. It should be pointed out that IM23 is 2-ethynylbenzenethiol, same with IM10. The formation of IM23 is favorable because the six member ring re-aromatizes. The reaction process will be illustrated in Chapter 3.6. Another reaction pathway for IM2 is through IM15 which is from IM2 by C2-C3 bond cleavage and C3-C1 bond formation. The energy barrier is 42.36kcal/mol. The PES of IM15 is shown in Figure 8. 15 ACS Paragon Plus Environment


Figure 8. PES of IM2-IM15 reaction pathway in Channel IM2. Details of TS and IM geometry can be found in Supporting Information. IM15 is transformed to IM19 by S-C1 cleavage via TS19A (31.96kcal/mol above IM15). IM19-IM20 is a process of H-migration from C7 to S site with an energy barrier of 67.09kcal/mol. IM20-IM21 process is the migration of the thiol group from C8 to C1. From IM21, two products can be generated, P2 and P4. P2 is generated from IM22, which is formed from IM21 by H migration from S to C8 via TS22 (14.69kcal/mol above IM21). Then IM22 dissociates to P2 and the energy barrier of IM22-P2 is 50.25kcal/mol. P4 is generated directly from IM21 dissociation with H migration from C1 to S site via TSP4 (50.25kcal/mol). The PES of whole Channel IM2 reactions is shown in Figure 9. Four desulfurization products can be generated, thioketene(P2), S atom (P3), H2S molecule (P4) and SCCH radical(P5).


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Figure 9.

PES of Channel IM2.

From RE to P3~P5, the highest energy barrier is 67.72kcal/mol in RE-IM2 process. For RE-P2, the highest energy barrier is 71.08kcal/mol located at IM21-P2. So, S atom (P3), H2S molecule (P4) and SCCH radical(P5) are more likely to generate during benzothiophene pyrolysis than thioketene(P2). From IM2 to P3~P5, the highest energy barrier of IM2-P3 is 57.96kcal/mol, lower than IM2-P4 and IM2-P5. Hence, in Channel IM2, the dominant reaction pathway for benzothiophene pyrolysis is to form 2-ethynylbenzenethiol which will dissociate to S atom and C6H5CCH. All these four products are direct pyrolysis products, not the final desulfurization products. They will react with other free radical in the environment to form final sulfur compounds. Under a hydrogen-rich condition, S atom may combine with hydrogen to produce H2S. In oxygen-rich atmosphere, maybe SO2 will be the final product. Benzothiophene pyrolysis to S radical could explain experiments more reasonably.



3.3 Channel IM3 reaction Process IM3 is β-carbene of benzothiophene, formed by H migration from C2 to C1. The reaction energy and reaction energy barrier of RE-IM3 are a bit higher than RE-IM2, of which the deviation is 4.88kcal/mol and 7.38kcal/mol, respectively. The reaction pathway of Channel IM3 is shown in Figure 10.

Figure 10.

Sketch of Channel IM3 reaction pathway.

The reaction pathway for IM3 is to form IM20 and IM23 which have appeared in Chanel IM2 already. In this part, two different formation reaction pathways of IM20 and IM23 are taken into discussion. IM19 is formed by C1-S bond cleavage via TS19B (4.49kcal/mol above IM3). It's followed by two possible reaction pathways. One is to form IM20 by H migration from C7 to S, the other is to form IM23 by H migration from C1 to S. The energy barriers of this two reactions are 67.09kcal/mol and 21.56kcal/mol, respectively. The PES of Channel IM3 is shown in Figure 11.


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Figure 11.

PES of Channel IM3. Details of TS and IM geometry can be found in

Supporting Information. The pyrolysis behavior of IM20 and IM23 has been illustrated in Chapter 3.2. Whether IM20 and IM23 formed through channel IM3 or not has no influence on the final products, but it is significant in understanding real reaction mechanism. Therefore, the two different formation reaction pathways of IM20 and IM23 are compared below. IM19 can be formed through IM2-IM15-IM19 or IM3-IM19. IM23 can also be generated by the process of IM2-IM14-IM17-IM23 and IM3-IM19-IM23. The PESes of two different formation pathways of IM19 and IM23 are illustrated in Figure 12.



Figure 12.

PES of IM19 and IM23 formation pathways through Channel IM2 and

Channel IM3. From Figure 12, it is obvious that IM3-IM19 reaction is more likely to occur than IM2-IM19. However, the energy barrier of RE-IM2 is lower than RE-IM3. From the aspect of energy barrier, RE-IM2-IM15-IM19 reaction pathway is more likely to take place. It is the same with IM23. Considering the little energy difference between REIM2 and RE-IM3, the two reaction pathways may both exist in the pyrolysis process.

3.4 Channel IM1 reaction process IM1 is generated from RE by H migration from C1 to S. The energy barrier of this process is 86.26kcal/mol. Compared with RE-IM2, the reaction energy and barrier of RE-IM1 is 26.44kcal/mol and 18.77kcal/mol higher, respectively. Hence, Channel IM3 reactions are side reactions of benzothiophene pyrolysis. In Channel IM3, three products can be generated from IM1 through three reaction pathways. The products contain benzene(C6H6) and SC2 radical(triplet), S atom and 20 ACS Paragon Plus Environment

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phenylacetylene(C6H5CCH), S atom and benzocyclobutene(C6H4C2H2). The reaction pathway of IM1 is shown in Figure 13. Two main reaction pathways exist. First is to form IM29, 1-thiol-benzocyclobutene. Second is to form IM25, 2benenyl-thioketene.

Figure 13.

Sketch of Channel IM1 reaction pathway.

In the first reaction pathway, IM25 is formed by H migration from S to C8 with C8-S bond cleavage accompanied. The energy barrier of this process is 18.49kcal/mol. The PES of IM1-IM25 reaction pathway is shown in Figure 14.



Figure 14.

PES of IM1-IM25 reaction pathway in Channel IM1. Details of TS and

IM geometry can be found in Supporting Information. From IM25, two parallel reactions exist. One is to form IM28 via TS28( 78.87kcal/mol above IM25) and then P1 is yielded by IM28 dissociation. The other is to form IM26 by H-migration from C2 to S. IM26 is transformed to IM27 overwhelming a barrier of 33.42kcal/mol. IM27 has a three-member-ring with S bonded with two adjacent C atom. Subsequently, IM27 dissociates to P3 directly. In the second reaction pathway, IM1 is transformed to IM29 by C8-S bond cleavage and C1-C8 bond formation. Details of second reaction pathway are shown in Figure 15.


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Figure 15.

PES of IM1-IM29 reaction pathway. Details of TS and IM geometry

structure can be found in Supporting Information. IM29 has a four-member carbon ring with a thiol group bonded on. The most likely reaction pathway is to form IM30, in which the S atom is bonded with two adjacent C atom. The energy barrier of this step is 74.21kcal/mol. In the following, S leaves the structure as a free atom. The PES of whole Channel IM1 reaction pathway is shown in Figure 16.

Figure 16.

PES of Channel IM1 reaction pathway. 23 ACS Paragon Plus Environment


Two sulfur-containing products, S atom and SC2 radical, can be generated through three pathways. The highest energy barrier of IM1-P6 is 74.21kcal/mol in IM29-IM30 process and for IM1-P1, it is 78.87kcal/mol(TS28). For IM1-P3, the highest energy barrier is 54.11kcal/mol(IM25-IM26). All this three highest energy barriers are lower than the energy barrier of RE-IM1 which means if IM1 is formed. The three reaction pathways may exist either.

3.5 Channel BIM6 reaction pathway BIM6 is a biradical intermediate formed by benzothiophene bond rupture initial step. The energy barrier for RE-BIM6 is 86.10kcal/mol, nearly the same with RE-IM1. BIM6 is a stable triplet intermediate. During pyrolysis, BIM6 can transform to singlet intermediate, IM9, with a little lower energy. The details of this process are illustrated in Figure 17.

Figure 17.

PES of BIM6 reaction.

First, a triplet intermediate BIM10 is formed from BIM6 with an energy barrier of 44.05kcal/mol. Along with two single electrons pairing, BIM10 converts to IM9. From 24 ACS Paragon Plus Environment

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IM9, Three products can be generated. S atom and phenylacetylene (P3), S atom and C6H4CCH2(P7), cyclized SCHCH and o-benzyne (1,2-C6H4). Details of IM9 pyrolysis reactions are shown in Figure 18 and Figure 19.

Figure 18.

Sketch of IM9 pyrolysis reactions.

Figure 19. PES of IM9 pyrolysis reactions. Details of TS and IM geometry can be found in Supporting Information. From Figure 19, it can be concluded that the most possible reaction pathway for IM9 is IM9-P7 and IM9-P3. The sulfur containing compound for product P3 and P7 is S free atom. 25 ACS Paragon Plus Environment


For IM9-P7, IM9 will first convert to its isomer by the C1-C2 bond rotation. Then a cyclized C-S-C formed by H-migration from C2 to C1. Subsequently, S leaves the structure as a free atom(P7). In the process of IM9-P3, a cyclized C-S-C is formed either in IM27. Hence, the dominating reaction pathway for BIM6 is through cyclized C-S-C ring. And dominated product is S atom.

3.6 Channel BIM7 reaction pathway BIM7 is another stable triplet intermediate formed by benzothiophene bond rupture initial steps. During pyrolysis, BIM7 will convert to IM23(2-ethynylbenzenethiol) firstly. The details are shown in Figure 20.

Figure 20.

The PES of BIM7 reaction pathway.

For BIM7, the neutral H atom on C2 firstly migrates to S leading a triplet intermediate formation, BIM11 with a thio group bonded. The energy of BIM11 will decrease by the pairing of two lone electrons on branch carbon atoms. The pyrolysis reactions of IM23 are partly same with IM26. The PESes of IM23 and IM26 pyrolysis reactions are shown in Figure 21 and Figure 22, respectively. 26 ACS Paragon Plus Environment

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Figure 21.

The PES of 2-ethynylbenzenethiol pyrolysis reaction.

Figure 22.

The PES of 2-phenylethyne-1-thiol pyrolysis reaction.

The pyrolysis of this two intermediates is to form a three member ring structure (IM24 and IM27) with an S atom bonding two adjacent C atom and subsequently dissociates to S atom and phenylacetylene. The energy barrier of this two pyrolysis process is relatively lower than other reaction processes. For 2-ethynylbenzenethiol, the highest barrier is 57.96kcal/mol. For 2-phenylethyne-1-thiol, the energy needed for dissociation is 37.85kcal/mol. Hence, converting benzothiophene into 2-phenylethyne1-thiol or 2-ethynylbenzenethiol could help to promote benzothiophene desulfurization. 27 ACS Paragon Plus Environment


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3.7 Thiophene pyrolysis reactions As a contrast, thiophene pyrolysis reactions are also calculated at the same level. Considering the paper size, details of thiophene initial reaction steps were not covered. All thiophene products appeared in this paper are evolved from α-carbene intermediate which formed by 1,2-H migration. The energy barrier of 1,2-H migration step is 69.21kcal/mol, which is the lowest in all thiophene first steps. In reference30, the 1,2-H migration energy barrier is 66.84kcal/mol and it is 67kcal/mol in reference34. The pyrolysis reaction pathway of thiophene is illustrated in Figure 23.

Figure 23. Sketch of thiophene pyrolysis reaction pathway. TIM is short for thiophene intermediate. TP is short for thiophene product. Five products can be generated in the thiophene pyrolysis process through five possible reaction pathways, ethyne(C2H2) and ethenethione(SCCH2), ethyne(C2H2) and ethynethiol(SHCCH),

carbon

monosulfide(CS)

and

allylene(CH3CCH),

vinylacetylene( CH2CHCCH ) and S atom, H2S and C4H2(butadiyne), respectively. The formation pathway of C2H2 and SCCH2 (TP1) is the same with reference 30 28 ACS Paragon Plus Environment

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and reference 34. However, the formation pathways of other products are different from the reported data. In previous studies, CS(TP2) is from β-carbene of thiophene30,34, H2S(TP5) is evolved from TIM4 formed by C1-H migration to S30,34, S atom(TP4) is generated from β-carbene of thiophene34, no ethynethiol products were found. In this study, all products are generated form α-carbene. The energy barrier of hydrogen migration along the conjunction skeleton of thiophene is found to be much lower than other processes. TIM2 is formed from TIM1 by 2,3-H migration. TIM3 is formed from TIM2 by 3,4-H migration. TIM4 is formed from TIM3 by C4-H migration to S. TIM2, TIM3, TIM4 are all carbene-intermediate. TP2-TP5 are all generated through corresponding carbene-intermediate. The PES of TP1 and TP3 formation processes is shown in Figure 24.

Figure 24. PES of thiophene TP3 and TP1 formation pathway. TTS is short for thiophene transition state. Details of TTS and TIM geometry can be found in Supporting Information. TP1 can be directly generated by thiophene α-carbene dissociation, of which the energy barrier is 40.54kcal/mol. TP3 is generated by S-C4 and C2-C3 bond rupture. 29 ACS Paragon Plus Environment


TIM4 is formed from TIM2 through two H-migration steps along conjunction skeleton. The highest energy barrier in TIM1-TP3 is 45.26kcal/mol located at TTS4, much lower than 1,5-H migration which is 89.16kcal/mol. Hence, TIM4 is more likely to be formed from thiophene by four H-migration steps than direct H migration from C1 to S. H2S and C4H2 (TP5) is generated from TIM4. The PES is shown in Figure 25.

Figure 25. PES of TP5 formation pathway. Details of TTS and TIM geometry can be found in Supporting Information.(Value in bracket is the energy calculated at CBSQB3 level) TIM10 (vinylacetylene thiol) is formed from TIM4 by C1-S cleavage accompanied by back H migration from C2 to C1. TIM10 fragments to H2S and CHCCCH (diacetylene) by C4-H migration to S, C3-H migration to C4 with an energy barrier 82.33kcal/mol to overwhelm (TTSP5). TP2 and TP4 are all evolved from TIM6 which formed by S-C4 bond cleavage of TIM3. The formation pathways of TP2 and TP4 are presented in Figure 26.


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Figure 26. PES of thiophene TP2 and TP4 formation pathway. Details of TTS and TIM geometry can be found in Supporting Information. Two reaction pathways exist for TIM6. One reaction pathway is to form TIM9 by C2-H migration to C1 accompanied by C1-C3 bond formation. Subsequently, TIM9 fragments to CS and CH3CCH. The other reaction pathway is TIM6-TIM7-TIM8-TP4. TIM6 is transformed to its chiral intermediate TIM7 firstly, and then C2-H migrates back to C1 resulting in a C-S-C three-member ring formation. S leaves the structure as a free atom. Both the two reaction pathways have a high energy barrier to overwhelm. For TP4, the energy barrier of TIM7-TIM8 is 53.2kcal/mol (TTS8). For TP2, it is 70.42kcal/mol (TTS9) located at TIM6-TIM9 process. The whole PES of thiophene pyrolysis reactions is shown in Figure 27.



Figure 27.

PES of thiophene pyrolysis reactions.

The highest energy barriers of TIM1-TP1, TIM1-TP2, TIM1-TP3, TIM1-TP4 and TIM1-TP5 are 40.54kcal/mol, 70.42kcal/mol 45.26kcal/mol, 53.2kcal/mol and 82.33kcal/mol, respectively. TP1, TP2, TP3 are more likely to be generated from αcarbene. The formations of TP2 and TP5 require more strict condition. It can be concluded that the dominating products of thiophene pyrolysis are C2H2, CH2CS (thioketene), SHCCH (ethynethiol), S atom and CH2CHCCH (vinylacetylene). This is consistent with reference and results. In reference 30, to form C2H2+CH2CS and CS+ CH3CCH are the two dominant channels of thiophene pyrolysis. In reference 34, all this five products were observed in thiophene decomposition experiment.

3.8 Comparison of Benzothiophene and Thiophene In this part, the most two possible reaction pathways of benzothiophene and thiophene pyrolysis processes are taken for comparison. Meanwhile, the Gibbs free energy changes of benzothiophene and thiophene pyrolysis reactions are calculated. 32 ACS Paragon Plus Environment

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Experimenalt data are also used to obtain experiment result. The results are presented in Table 4 and Table 5. Table 4. Standard Gibbs free energy change(△Gϴ) of benzothiophene pyrolysis reactions. (unit:kcal/mol) Benzothiophene UB3LYP/ CBS-QB3 Exptl. Reactions 6-311G++(d,p) P1 SC2(triplet)+C6H6 92.99 111.10 P2 P3 P4 P5 P6 P7

SC2H2+1,3-C6H4 105.99 118.45 S atom+C6H5CCH 80.07 94.02 90.8885 H2S+C6H3CCH 102.76 109.68 SC2H +C6H5 100.25 119.90 113.1947 S atom+C6H4C2H2 105.67 117.02 114.5261 S atom+C6H4C2H2 116.15 132.78 The experiment values are calculated based on Goos, Burcat, Ruscic reported data38

Table 5. Standard Gibbs free energy change(△Gϴ) of thiophene pyrolysis reactions. (unit:kcal/mol) UB3LYP/ Thiophene Reactions CBS-QB3 Exptl. 6-311++ G (d,p) TP1 C2H2+SC2H2 55.80 65.37 TP2 CS+CH3CCH 66.24 71.93 70.19 TP3 C2H2+SHCCH 74.53 80.16 75.49 TP4 CH2CHCCH+S atom 85.72 99.26 95.44 TP5 H2S+ C4H2 58.36 68.20 64.79 The experiment values are calculated based on Goos, Burcat, Ruscic reported data38.

From table 4 and Table 5, it can be concluded that CBS-QB3 is more accurate on describing Gibbs free energy change. The value calculated at UB3LYP/6-311G++ (d,p) level has the same variation trend with CBS-QB3. For benzothiophene, the △Gϴ of benzothiophene pyrolysis to S atom and C6H5CCH

is the minimum in all experiment or DFT calculation △Gϴ data. On the consideration

of the energy barrier, it is indicated that benzothiophene is most likely to decompose to S atom and C6H5CCH, H2S and C6H3CCH. For thiophene, pyrolysis to H2S+C4H2 is



lowest △Gϴ in all experiment value. However, a high energy barrier is needed to

overwhelm. Combined with the energy barrier, the most possible products for thiophene are C2H2 and SHCCH, C2H2 and SC2H2. In the following, the two possible pyrolysis reaction will be taken into comparison, as shown in Figure 28 and Figure 29.

Figure 28.

PES of benzothiophene dominating pyrolysis process.

Figure 29.

PES of thiophene dominating pyrolysis process.

Both for benzothiophene and thiophene, the dominating pyrolysis reaction pathway are by transformation to α-carbene which is the highest energy barrier step. However, 34 ACS Paragon Plus Environment

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the highest energy barrier of benzothiophene and thiophene are nearly the same, 67.72kcal/mol and 69.21kcal/mol. The highest energy barrier of following reaction steps from α-Carbene is different. Among this four reaction pathways, the highest two are benzothiophene reaction pathways,

IM1-P5(66.46kcal/mol)

and

IM1-P3(57.96kcal/mol),

respectively.

Compared with benzothiophene, the two reaction pathways of thiopehene are with lower highest energy barrier, 40.54kcal/mol(TIM1-TP1) and 45.26kcal/mol (TIM1TP3).

Given

the

situation

that

the

α-Carbene(benzothiophene)

and

α-

Carbene(thiophene ) are generated at the same rate, α-carbene(benzothiophene) are more easily transformed to products than α-Carbene(thophene). Hence, benzothiophene is difficult to remove than thiophene.

4 Conclusion Detailed DFT calculation on benzothiophene and thiophene pyrolysis reactions carried out for further understanding of petroleum coke desulfurization process. The main results are as followings: (1) All the pyrolysis process of benzothiophene calculated are with a high energy barrier, which indicates that benzothiophene can withstand high temperature treatment. The pyrolysis reactions of benzothiophene is started by α-carbene(benzothiophene) formed by proton migration from α-position to β-position. This is the highest energy barrier step during pyrolysis. The dominating pyrolysis products are S free atom and thioketene. Some H2S and radical and SC2(triplet) radical may be generated when the temperature is elevated higher. S radical will combine other radicals existing in the 35 ACS Paragon Plus Environment


environment to generate final desulfurization products. (2) 2-phenylethyne-1-thiol and 2-ethynylbenzenethiol were found to be important intermediates for benzothiophene pyrolysis. For this two intermediates can be desulfurized more easily than benzothiophene. Converting benzothiophene to benzothiophene or 2-ethynylbenzenethiol could be a potential desulfurization method. (3) The pyrolysis of thiophene are through its α-carbene. Ethyne(C2H2) and thioketene (SCCH2), ethynethiol(SHCCH) are the major products. Ethyne and thioketene can be directly generated by α-carbene dissociation. Ethyne and ethynethiol are evolved through multi step hydrogen migration along the conjunction skeleton of thiophene. S atom and vinylacetylene(CH2CHCCH), carbon monosulfide(SC) and allylene could be generated during thiophene pyrolysis. H2S can hardly be produced for the high energy barrier. The rate limiting step is the first step, α-Carbene(thiophene) formation step. (4) Pyrolysis of benzothiophene and thiophene are mainly initiated by forming corresponding α-carbene. S atom, thioketene and a certain amount of H2S are the dominating products for benzothiophene. While for thiopehene, the dominating products are thioketene, ethynethiol, S atom and SC. H2S could be hardly generated. For both benzothiophene and thiophene, the highest energy barrier step is the initial step which is α-carbene formation step. The highest energy barrier for both are almost the same value (the difference is 1.149kcal/mol) which indicates that the initial pyrolysis temperature of thiophene and benzothiophene could be very close. For the subsequent pyrolysis process from α-carbene, energy barrier of thiophene is lower than 36 ACS Paragon Plus Environment

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benzothiophene which means thiophene is easier to yield products than benzothiophene. The pyrolysis rate of thiophene is higher than benzothiophene. Hence, benzothiophene is with a relatively higher thermal stability.

Acknowledgement The authors acknowledge the financial support of the National Key R&D Program of China (2017YFC0210401), the National Natural Science Foundation of China (51574289, 51874365, 61751312, and 61621062) ，the Natural Science Foundation of Hunan Province, China(2018JJ2521) and the Fundamental Research Funds for the Central Universities of Central South University (2017zzts118). And also we acknowledge the software support of National Supercomputing Center in Shenzhen.

Supporting information All intermediate and transition state geometries, frequency and infrared of transition states, CBS-QB3 energy of all IMs.

Author information ORCID Tianshuang Li: 0000-0003-0081-4269. Hongliang Zhang: 0000-0003-2878-8138.

Reference (1) Vimal, C.S. An Evaluation of Desulfurization Technologies for Sulfurremoval from Liquid Fuels. RSC Adv. 2012, 2, 759-783.



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(20) Ayten, A.; Gisele, A.; Kihyok, C.; Willian, H.G.; Michael, T.T. The Role of Catalyst in Supercritical Water Desulfurization. Appl. Catal. B 2014, 147, 144-155. (21) Liu, F.R.; Xie, L.L.; Guo, H.Q.; Xue, M.; Hu, R.S.; Hu, H.Q. Sulfur Release and Transformation Behaviors of Sulfur-containing Model Compounds during Pyrolysis under Oxidative Atmosphere. Fuel 2014, 115, 596-599. (22) Guo, H.Q.; Xie, L.L.; Wang, X.L.; Liu, F.R.; Wang, M.J.; Hu, R.S. Sulfur Removal and Release Behaviors of Sulfur-containing Model Compounds during Pyrolysis under Inert Atmosphere by TG-MS Connected with Py-GCJ. Fuel Chem. Technol. 2014, 42, 1160-1166. (23) Mengwen, X.; Jiao, K.; Jie, D.; Haili, J.; Fan, L. Thiophenic Sulfur Compounds Released During Coal Pyrolysis. Environ. Eng. Sci. 2013, 30, 273-279. (24) Jie, S.; jiejie, H.; Jiazhou, L.; Yitian, F. Insight into Transformation of Sulfur Species during KOH Activation of High Sulfur Petroleum Coke. Fuel 2018, 215, 258265. (25) Fenrong,L.; Wen, L.; Haokan, C.; Baoqing, L. Uneven Distribution of Sulfurs and Their Transformation during Coal Pyrolysis. Fuel 2007, 86, 360-366. (26) William, H.C. Investigation of Organic Sulfur-containing Structures in Coal by Flash Pyrolysis Experiments. Energy & Fuels 1987, 1, 59-64. (28) Martin, B.; Vacav, V.; Jozef, B.; Plet, A.L.; Jacques, A.R. Steam Cracking of Hydrocarbons. 5. Effect of Thiophene on Reaction Kinetics and Coking. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 741-745. (29) Haffeez, U.R.M.; Alan, W.; Paul, T.W. Shock Tube Pyrolysis of Thiophene. Int. J. 40 ACS Paragon Plus Environment

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frequencies. J. Chem.Phys. 1999,110, 2822-2827. (38) Goos, E.; Burcat, A.; Ruscic, B. Third Millennium Ideal Gas and Condensed Thermochemical Database for Combustion, September 2010. (http://garfield.chem.elte.hu/Burcat/hf.doc) (Accessed: 6/10/2018, Beijing Time)

TOC


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H

H

H H

1,9-H Migration

S

H

4,3-H Migration

S H

H

H H

H

IM1

H H

IM7

H

H

S

H H

H

1,2-H Migration H

H

H

H

IM2（α-Carbene）

H H

H H

S

H

2,1-H Migration

H

S

H

2,3-H Migration

H H H H

H

IM6

H

H

S

H

IM3（β-Carbene）

H

H H

H S

7,8-H Migration

H H H

IM4

H

4,2-H Migration

S H

H H

ACS Paragon Plus Environment

H

IM5


Page 44 of 74

H H

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

C1-C2 Rotation

H

H

S

H

C8-S Rupture

BIM5

S

H H

H

H H

H

H

H

C2-C3 Rotation

H

H

H

BIM1

H

H

S

BIM6 H H

H

H

C1-S Rupture

S

H

S

H

C2-C3 Rotation

H H

H

H

H

H

H

H H

H

BIM2

H

S

BIM7

H H

S

H

S-C8 Rotation H

H

S

H

C H

H

H H

H

H

S

H

H

S-C1 Rotation

H H

H

BIM8

BIM3 C2-C3 Rupture

C

H

C1-C2 Rupture H

H

H

S

H H

H H


BIM4

BIM9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


140

Energy kcal/mol

Page 45 of 74

TS4 121.11

120 100

TS6 97.47 TS7 97.17 TS5 94.52 TS1 86.49 TS3 72.60

80

TS2 67.72

60 40 20 0 -20

RE(Benzothiophene) ACS Paragon Plus Environment

IM4 98.63 IM6 97.07 IM7 97.00 IM5 92.76 IM1 80.62 IM3 61.56 IM2 54.18

The Journal of Physical Chemistry TS11 BIM3 165.78 165.07

180

kcal/mol

160

Energy

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

140

BIM4 124.13

TS12 127.21

Page 46 of 74

BIM8 164.22 BIM9 123.52

120 100

BIM2 80.14

80

BIM1 74.27

60

TS8 95.72 TS10 87.56 TS9 86.10

40 20 0

RE(Benzothiophene) -20


BIM7 80.58 BIM6 78.36 BIM5 74.61

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H

H C

S

H

S

H

C

H

H

H H

C H

H

H

H

I5

I1 H

H H

S

H

S

H

C H

H H

H

H

H

I2

H

I6

S H

H H

S

H

H

H

H H

H

H

H

H

H

H

S H

H H

I7 H

S

H

H

S C

H

H

H

H

I4

HC CH

H

I3 H

＋


I8

C H H

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 H 17 18 19 H 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 48 of 74

H

H S

H

H

H

S C

C H H

H H

H

S

H

H S

C

H

C

H

C H

H

H

IM20

H

H

S

C

H

H S

C

IM14

H

H

H H

H

C C

C

H

C H H

IM17

H

H

H

H

S

＋ C CH

H

H

S

IM22

2

P2

C H

S

H

H

H

IM23

H C C H

H

H S

H C C H

H


C CH

P5 H

S

H H

＋S

H

H

IM16 H H H

H

H

H

H

H

IM21

H

H

P4

C

H

H

H

H

H

H S H

IM2

H

H

H

H

H

＋

IM21

S

H

H2S

C H

C H H

H

IM19

IM15

H

H

H

H

C

H

H

C C H

H H

H

IM24

＋ H

S(atom) P3

Page 49 of 74


170

Energy kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53

H

150

H

S

H

C

H

H H

H

130

S

H

90 70

H H

S H

H H

H

C

IM2 54.18

30

H

10

H

H

TS24 100.25

C H

H

C H

H

IM23 42.29 C C H

-10

H


P5

115.92

P3

92.44

H

S

H H

S H

H H

＋

H

C

C

H

H C C H

H H

IM16 49.46

S H

H H

RE

C C H

H

S

S C C H

H

IM24 76.32

H H

H S

H

IM17 73.67

S

H

H

C C H H

H

H

H

H

H

H

S

H

TS23 111.96

H

H H H H

H

C C H H

S

H

50

S

H

H

H

H

H H

H C C

H

＋

H

S

H

TS14 H 97.16 IM14 TS16 78.22 95.25

TS2 67.72

H

TS17 121.72

H H H

110

H

H


H

H

Page 50 of 74

S

170 H

H

150

H

C

H

H

130

H

H

C H

H

H

H

H

90 70

H

H

C H

S

TS15 96.54

H

H H

S

H

C C H H

H

H H

10

H

RE -10

IM15 32.80

S

H

C H

H

H

H

H

S C

H H

C H H

H

H H

C H H

S

H

C

C H

H


H

C C

TSP2 H

H S

H

C H

C H

S C C

H

＋

P2 134.01 H

S H

H

TS22 108.22

H

H

H

H

H

H

IM19 54.11 H

H

C H

IM21 93.53

H S

H

TS19A 64.76

IM2 54.18

30

IM20 107.94

H

TS2 67.72

50

H

TS20 121.20

S

H

TS21 132.31

C H

TSP4 164.61

H C

C

H

S

C H

S

H

110

H

H

Energy kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

S

H

IM22 83.76

H H

H

H

C C

H

H

H H

126.09 123.73H H

H2S

H

H

P4

C C H

H

＋

H

H S H

Page 51 of 74

Energy kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58


H

170

S

H

150 H

TS2 67.72

H

H

H H

RE -10

H

C

TS20 121.20 TS23 111.96

IM15 H32.80 S C

H H

C H H

H

C

C H

H

H

H

H

C C

TS22 108.22

H

H

TSP2 134.01 S H

IM22 83.76 H S

S

C C H H

C H

IM16 49.46 H C C H

H H

S

H H

H


H

H H

P4 123.73 P5 115.92

H

H H

IM23 42.29

P2 126.09

P3 92.44

IM24 76.32

H

H

C H

IM20 TS24 IM21 107.94 100.25 93.53

H

S

C H H

TSP4 164.61

H S

TS21 132.31

H

H

H H

IM19 54.11

S

H

10

TS17 121.72

IM2 H54.18

30

H H

TS14 97.16 IM14 TS16 78.22 TS15 95.25 96.54 IM17 TS19A 73.67 64.76

90

50

H

H

S

H H H

110

70

H

H

130

C H

C H

H

H S

C

C

H

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

H

H S

H

C C H H

H

H H

S

H H

H H

IM3

IM20

H H

S

H

C H

C H

H

IM19

H

S H C C H

H H

H

IM23 ACS Paragon Plus Environment

Page 52 of 74

Page 53 of 74

Energy kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59


170

150

TS21 132.31

TS20 121.20

130

110

S

H

90

H

S

H H

TS3 72.60

TS19B 66.05

70

IM3 61.56 H

50 H

S

S

H

IM19 54.11 H

H

C

IM23 42.29

S

H

H

C H

RE

H

H

H

H

10

C

H

H

H n

-10

H

H H

30

IM20 107.94 TS24 H 100.25 IM21 H 54.18 TS23B IM24 75.69 H 76.32 H

H

C H

S H

H

H

C C H H Plus Environment HACS Paragon

P1 146.46

TSP2 134.01

P2 126.09 P4 123.73

H H

TSP4 164.61 TSP1 158.19

TS22 108.22

P3 92.44 IM22 83.76


130

Energy kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

Page 54 of 74

TS17 121.72

H H

110

TS14 97.16 H

90

H

TS3 72.60

S

H

S

H H

H H

IM14 95.25

TS15 96.54

H

70

TS2 67.72

H

TS19B 66.05

10

H

IM23 42.29

H

IM15 32.80 H

H

H

H ACS Paragon Plus Environment

H

S

H

C H

S C

H

RE

TS23 75.39

IM17 73.67 IM19 54.11

30 H

C H

H

TS19A 64.76

S

H

C H H

IM3 61.56

S

H

IM2 54.18 H

50

-10

H

H

H

TS23 111.96

C H H

C H

H

H

S

H C C H

H H

H

Page 55 of 74 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 H 16 17 18 19 H 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


H

H

H H

S

H

H

H

H

H

H

H

S H

H

H

H

IM30

IM29

H

H

H

H

H

＋ S(atom) P6

S H

H

H

H

H

H C C S

IM1

H

H

H

C S

H H

H

P1

IM28

S

H C C H

H

H

IM25

H

H C

H

H

H

H

H

＋C

H

H

H

H

S

C C S H

H H

H


IM26

C C

H H

H H

＋ H

H

S(atom)

IM27

P3


Page 56 of 74

150

Energy kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

H

H H H

130 H H

H

70

H H

H

TS28 102.98

50

H

H

C

H

H

H

C C

H H

P3

＋

H

S

H

H

H

S

H C C SH

H H ACS Paragon Plus Environment

H

C C H

H C C H

H

H

H

H

H

H

H

H

92.44

H

IM27 54.49

IM26 39.91

H

H

RE

S

S

H

H

H

TS27 73.33

IM28 H71.55

IM25 H24.11 S

H

C C S H

H

H H

H

H

30

H

H

H

H

TS26 78.22

H

H

10

H

H

H

H

C S

P1 106.70 C C S

S

H

H

TS1 86.49 IM1 80.62

90

＋C

H

H

TS25 102.19

H

H

H C C S

H

H

H

H

-10

H

S

C

H S

110

H

H

H

Page 57 of 74

Energy kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59


150

TS29 140.25 H

130

H

H

H

TS30 135.92

S C

H

H S

H

H

H

H

S

110

H

H

P6 P6 117.08 117.08

H H

H

H

H

TS1 86.49 IM1 80.62

90

70

H H

50

H

H

IM29 61.17 H

H H

H

H

IM30 52.64

S

H H

30

H

H H

H

H

H H

H H

H H

H S

H

S

10

H H

S

H

-10

＋

H

H

RE

H

H


S


150

Energy kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

Page 58 of 74

H H

TS29 140.25

TS30 135.92

130

＋S

H

H

H

H

H

H

P6 117.08 110

TS1 86.49 IM1 80.62

90

H

H

70

H

H

H

TS26 78.22

H

H

IM28 71.55

H

IM29 61.17

H H

IM30 52.64

＋

TS27 73.33 H

S IM27 54.49

S H

H H

IM25 24.11 H

H

10

H

H

-10

H H

H

C

H H

H

S

H

C C SH H

H


C C

H

H

H

H

S H

RE

H

H S

IM26 39.91

H

H

H

H C C H

H H

H

30

H

＋ C C S

P3 92.44

H

H

H

H

C C S

S

H

H

H

H

H

H

S

50

H

H

H

P1 106.70

TS28 102.98

TS25 102.19

H

H

Page 59 of 74 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


140

H

H H C

S C H H

H H

120

S C H

H C

TSB10 122.41

H

H

BIM10 119.20

H

100

80

BIM6 77.74

IM9 77.74

H H H H

60

H

H S

S C H

H C

H


H


H

＋

H H H

H

H

H

S C

H H

H

C C

H

H

H

H

H

H C

C H

H

H

H

S

P8

H S C

C H

H

Page 60 of 74

C S

C H

IM31

C

H

H

H

H

H

IM32

IM9 H

P7

H

H

S C C

H H

H

H

IM33

H

＋ S atom

C C H

H H


＋S atom

H

H

P3

Page 61 of 74

H


＋H C C H

H 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

S

H

160

H

TSP8 152.93

P8 150.25

140

P7 128.60

TS33 125.48

120

H

TS32 118.55

H

H

C

H

H C

100

H

H

C S

C

＋S atom

H

H H

H

H

80

IM9 77.74 H

60

TS31 80.01 C H C

H

40

H

IM31 75.08

S

H

IM32 79.84

H H

H S C H

C H

H

H H

H

C C


IM27 S 79.84

H

H

H

P3 92.44 H

H C C H

H H

H

＋

S atom


H 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

120

S

H C C H

H H

100

H

P3 92.44

TS24 100.25

80

Page 62 of 74

IM24 76.32 H S

60 40

H

20 0

S H C C H

H H

C C H H

H


H

H C C H

H

H

H

IM23 42.29

H

H

＋

H

S atom

Page 63 of 74 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


S

H

140

H

C H

H

120

H

H

C H

H

TSB11 119.45

100 80

40

H C

H

C

H

H

BIM11 109.78 H

BIM7 80.58

S H C C H

H H

60

S

H

S

H

C C

H

H

H

IM23 42.29



Page 64 of 74

120 H

100

H C C

H H

80

S

P3 92.44 H

H

H

H C C H

H

TS27 73.33

60 40

IM26 39.91

0

H

H

H C C SH

H H

C C

H H


H

＋

H

S atom

H S

H

20

IM27 54.49

H

Page 65 of 74 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


HC CH

＋S

HC CH

C CH2

TP1

S

H C

S H

H

H S

C

H

H

TIM1

H

S

H

TIM4

S

S C

H H

TIM2

H

C C CH

TP5

TIM10

H H

＋CH

H2S

H

H H

C CH

TP3

H H

H

＋HS

H

H

S C C

TIM3 H H H

TIM6

H

C C H

C

＋C

HC

H

C CH3

TP2

TIM9

H

C C

H

H

H 2C C

S

H

C C S

H

CH C

H

H

S

H

H

＋

S(atom)


TIM7

TIM8

H

TP4

H


Energy kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

H

130

H

110 90 70

H

S C

H H

H H

50 30

H

H

H

-10

＋ HC CH

H

TP1 69.90

S

H

TIM3 54.50

H

H

H

S

H H

H

HC CH

S H

10

RE(Thiophene)

HS C CH

TP3 90.28

TIM4 84.86

H

H

H

H

S

H

S

H

S

H

H

TTS4 99.76

TTSP1 H 99.40 TTS3 H TTS2 90.72 H H 90.46 TTS1 69.21 TIM2 77.85 S

TIM1 58.86

TTSP3 121.77

S

Page 66 of 74


＋ C CH2

Page 67 of 74 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


140 120 100 80 60 40 20

S

H

H

S H

H

TTSP5 126.60 (127.96)

C C

H

H

H

TTS5 89.17 (85.84)

TIM4 84.86 (81.12) H

H

C

H

C C C H

TP5 74.28 (76.45)

TIM10 44.73 H (46.84) C

S

H

H

S

TTS10 93.34 (93.41)

H

H S

C

H

H

0 -20

H

H

RE(Thiophene) ACS Paragon Plus Environment

H 2S

CH

＋ C C

CH


Energy kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

Page 68 of 74

S

130 H

110 90 70 50 30

H H

H

H

S

H

H

H

TTS3 90.72

H

H H

H

H

H

H

TIM6 30.49

H H

10 H

RE(Thiophene) -10

S

H

H


S C C

H

C C S

H

H

TTS7 35.33 TIM7 28.49 H

H

C C S

H

H

C S

H

TIM8 58.94 TIM9 43.67

HC S

C H H

H

TP2 82.22

C C

H

H

H

C

TP4 98.40

H

H

S H

C

H

TTS8 81.69

TIM3 54.50

H

C

C

H

TTS6 69.54

S

H

H

C

H C

S＋H2C

S

TTS9 100.91

S

H

TIM2 77.85

S H

H

TTS2 90.46

TTS1 69.21

H

H C C C H H

S

H

H

H

TIM1 58.86

C

H

S

CH

C C H

C

H

＋

C CH3

Page 69 of 74

Energy kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


130

90 70

TTS3 90.72

TTS2 90.46

TTS1 69.21

S

30 H

10 S

H

H

RE

H

H H

H

TIM3 54.50

H H

H

TIM6 30.49 H

H

H

S C C

TP4 98.40 TP3 90.28 TP2 82.22

TTS8 81.69

TP1 69.90 TIM8 58.94 TIM9 43.67 S

TIM7 28.49 H

H

H

H

TTS7 35.33

S

H ACS Paragon Plus Environment H

TTS9 100.91

S

H

C C

H

H

TIM1 58.86 H

H

S

H

TIM4 TTS6 84.86 69.54

TIM2 77.85 H

50

-10

TTS4 H 99.76

TTSP1 99.40

TTSP3 121.77

S

H

110

H

H

C C S

H

H

C H H

C C H

C

H


180

Energy kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

160 H

140 H H

120

TS15 IM14 96.54 95.25

TS2 67.72

60

IM2 H54.18

40 H

20

S

H H

H

H

IM17 TS19A 73.67 64.76 IM19 54.11

H

Benzothiophene H

H

H H

C H

C H H

C H

C H

H

H

C H

C

H

H S

H

H

H


H

92.44 H C C H

H H

H C C H

H

C C H

H

IM24 76.32 H

IM23 42.29 S

H H S

H

S

H

C H H

123.73

P3

93.53 H

H S C

H

P4

H S

H

H

H

TS24 TS23 IM20 100.25 111.96 107.94 IM21

IM15 32.80H

-0 -20

TS20 121.20

H

H

TS14 97.16

100 80

H H

TS17 121.72

H2S

TS21 132.31

C H

C H

H

S

＋

H

S

H

H

TSP4 164.61

Page H 70 of 74

H

＋ S

Page 71 of 74

Energy kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

H


H

130

H

110 90 70

H

S C

H H

H H

50 30

H

H

-10

＋ HC CH

H

TP1 69.90

S

H

TIM3 54.50

H

H

H

H

S

H H

H

HC CH

S H

10

Thiophene

HS C CH

TP3 90.28

TIM4 84.86

H

H

H

H

S

H

S

H

S

H

H

TTS4 99.76

TTSP1 H 99.40 TTS3 H TTS2 90.72 H H 90.46 TTS1 69.21 TIM2 77.85 S

TIM1 58.86

TTSP3 121.77

S


＋ C CH2


H 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 72 of 74

S H C C H

H H

H

H

H C C H

H H

H H

S H H

＋

S atom

H H

H

S

H H

H H

HC CH S

H

H

S H


＋ C CH2

Page 73 of 74 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fig in table 1



fig in table 2


Page 74 of 74

DFT Research on Benzothiophene Pyrolysis Reaction Mechanism

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