Pyrolysis Mechanisms of Thiophene and Methylthiophene in

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Pyrolysis Mechanisms of Thiophene and Methylthiophene in Asphaltenes Xinli Song and Carol A. Parish* Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond Virginia 23173, United States (USA)

bS Supporting Information ABSTRACT: The pyrolysis mechanisms of thiophene in asphaltenes have been investigated theoretically using density functional and ab initio quantum chemical techniques. All of the possible reaction pathways were explored using B3LYP, MP2, and CBS-QB3 models. A comparison of the calculated heats of reaction with the available experimental values indicates that the CBS-QB3 level of theory is quantitatively reliable for calculating the energetic reaction paths of the title reactions. The pyrolysis process is initiated via four different types of hydrogen migrations. According to the reaction barrier heights, the dominant 1,2-H shift mechanism involves two competitive product channels, namely, C2H2 þ CH2CS and CS þ CH3CCH. The minor channels include the formation of CS þ CH2CCH2, H2S þ C4H2, HCS þ CH2CCH, CS þ CH2CHCH, H þ C4H3S, and HS þ C4H3. The methyl substitution effect was investigated with the pyrolysis of 2-methylthiophene and 3-methylthiophene. The energetics of such systems were very similar to that for unsubstituted thiophene, suggesting that thiophene alkylation may not play a significant role in the pyrolysis of asphaltene compounds.

’ INTRODUCTION The situation in the energy field suggests that, in the near future, conventional oil will no longer be available as a source of sustainable energy in the quantities known today. Oil shale is considered one of the promising options for supplementing conventional fuels such as light sweet crude oil. Thiophene (C4H4S) and its derivatives are one of the basic constituents of asphaltenes contained in oil shale and as such are expected to play an important role in the combustion and processing of these alternative fuels.1 Thiophene has been implicated as a significant species in the pyrolysis of fossil fuels2 and has attracted considerable interest as an organosulfur prototype in the investigation of the desulfurization mechanism.3 Moreover, thiophene and its polymerized derivatives are widely used in organic electronic devices and molecular electronics because of the particular stabilization and excellent transport properties of the oligomers4 and nanocomposites.5,6 Extensive studies have been performed to determine the spectroscopic and structural parameters of thiophene.715 However, very few studies have been reported on the thermal decomposition of thiophene and the detailed pyrolysis mechanism of thiophene is still unclear. Memon et al.16 investigated the pyrolysis of thiophene over the temperature range 15982022 K using a shock-tube technique. In their work, it was suggested that the principal hydrocarbon product at all temperatures is acetylene and the pyrolysis of thiophene is initiated by a CS bond fission. In contrast, r 2011 American Chemical Society

Winkler et al.17 concluded that CH fission is the major initial mechanism based on the observation of secondary thiophene condensation products in a continuous flow pyrolysis experiment in the temperature range 7731373 K. In addition, this previous report also suggested that the amount of H2S increases significantly at temperatures above 1123 K. Cullis et al.18 also detected a large quantity of H2S as one of the major products of the pyrolysis of thiophene at 1323 K. In 2004, Hore et al.19 undertook a laser pyrolysis study of the thermal decomposition of 5-membered rings. They suggested that 1,2-H transfer is the most probable initiation step, which requires an activation energy of 300 kJ/mol. Evidently, the initial mechanism for the thermal decomposition of thiophene remains unclear. Theoretically, the pyrolysis of furan (C4H4O) and pyrrole (C4H4N), which are both analogous to thiophene, have been well-established.2022 However, to date, there are only two DFT studies of the pyrolysis mechanism of thiophene in coal.23,24 In each case, the formation mechanism of H2S was studied using relatively low levels of theory (e.g., B3LYP/6-31G*23 and PW91/DND24). Each study produced differing results with Chong and co-workers concluding that the CS bond is weakest and will be dissociated first, whereas Wang Received: December 13, 2010 Revised: January 31, 2011 Published: March 16, 2011 2882

dx.doi.org/10.1021/jp1118458 | J. Phys. Chem. A 2011, 115, 2882–2891

The Journal of Physical Chemistry A

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and co-workers suggested that the first step in pyrolysis is an R-H migration to the S atom. A rigorous theoretical investigation on the detailed reaction mechanism for the pyrolysis of thiophene would help further the mechanistic understanding of this important reaction. The purpose of this work is 3-fold: (1) to clarify the pyrolysis mechanism of thiophene with an emphasis on the initial reaction routes and the distribution of all of the possible products, (2) to clarify the methyl substituent effect, and (3) to compare with the analogous pyrolysis reactions of furan and pyrrole. Therefore, we set out to characterize the energetic reaction routes for the pyrolysis of thiophene and methyl thiophene using various highlevel ab initio quantum chemistry methods. The theoretical data presented here will be useful for building a realistic kinetic model for pyrolysis of thiophene and shed new light on the combustion of asphaltenes.

’ COMPUTATIONAL DETAILS All ab initio and density functional calculations were carried out using the Gaussian 03 package.25 All possible intermediates and transition states were first calculated at the UB3LYP/ 6-31G(d, p) level of theory.26 Local minima and transition states were confirmed to have all real frequencies and only one imaginary frequency, respectively. The connection of the transition state with the designated reactants and products was confirmed using intrinsic reaction coordinate (IRC) calculations.27 Subsequently, all species were also geometry optimized using either UB3LYP or the second-order MollerPlesset (UMP2)28 theory with the more flexible 6-311þþG(2d, p) basis set. The zero-point energies (ZPEs) were calculated at the same level of theory. To further improve the energetic data, two composite ab initio methods, namely, CBS-QB329 and G3MP2B3,30 were also employed to refine the relative energies for all of the reactants and products. In brief, both of these composite methods are based on the additivity of several correction terms that use larger basis sets at lower levels of theory and smaller basis sets at higher levels of theory. G3MP2B3 energies were calculated on the basis of the B3LYP/6-311þþG(2d,p) optimized geometries, whereas the CBS-QB3 energies were determined using B3LYP/CBSB7 geometries. Scaled zero point energy corrections are included in all CBS-QB3 energies reported herein. ’ RESULTS AND DISCUSSION Twelve product channels for the thermal decomposition of thiophene have been found in this work, namely, C4 H4 S f C2 H2 þ CH2 CS ðthioketeneÞ

ðR1Þ

f CS þ CH3 CCH ðpropyneÞ

ðR2Þ

f CS þ CH2 CCH2 ðalleneÞ

ðR3Þ

f CS þ CH2 CHCH ðvinylmethyleneÞ

ðR4Þ

f H2 S þ C4 H2 ðbutadiyneÞ

ðR5Þ

f CH2 C þ CH2 CS

ðR6Þ

f HCS þ CH2 CCH

ðR7Þ

Figure 1. Thiophene geometry determined using various levels of theory and in comparison to the experimental values (units: Å,).

f HS þ C4 H3

ðR8Þ

f H þ 2-C4 H3 S

ðR9Þ

f H þ 3-C4 H3 S

ðR10Þ

f SCHCHCHCH ðlinear C4 H4 S, formed by C S ruptureÞ

ðR11Þ

f CHCHSCHCH ðlinear C4 H4 S, formed by C C ruptureÞ

ðR12Þ

Before we present a detailed discussion of the decomposition mechanism, it is worth commenting on the quality of the calculated data in this work. Figure 1 illustrates the thiophene geometries determined using a variety of methods in comparison with experimental values. From this data, we can conclude that all of the computational methods employed here can describe well the structure of thiophene. The geometries optimized at the B3LYP/CBSB7 level are summarized in Figure S1 of the Supporting Information along with the relative CBS-QB3 energies determined at the B3LYP/CBSB7 geometry for each species involved in the pyrolysis of thiophene (Table S2 of the Supporting Information). In most cases, the MP2 geometries are very similar to the ones at B3LYP/CBSB7 level. The uncertainty of the energies can be estimated from the data in Table 1, which lists the heats of reaction (at 298.15 K) for the twelve product channels calculated at different levels of theory. A comparison, where available, to the experimental data determined using the enthalpies of formation suggests that the CBS-QB3 method is most accurate at describing these systems with an average deviation of less than 2.8 kcal/mol. To further support this conclusion, our CBS-QB3 results for the analogous reactions of furan are also in good agreement with experimental values with an average deviation of less than 1 kcal/mol. Results for furan are shown in Table S3 of the Supporting Information. As shown in Table S3, the CBS-QB3 results for furan are in even better agreement with experiment than previous studies20 obtained with the G2MP2 or CASPT2 methods. It is hard to estimate the uncertainty of the barrier heights because of the lack of experimental data. However, it is reasonable to assume that they have similar or slightly worse accuracy as the heats of reaction, as indicated by the kinetic calculations of furan.20 Therefore, all of the relative energies and geometries in the following 2883

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Table 1. CBS-QB3 Heats of Reaction (kcal/mol) for All Pathways (R1R12) of the Decomposition of Thiophene at 298.15 K C2H2 þ CH2CS (R1)

MP2/6-311þþG(2d,p)

B3LYP/6-311þþG(2d,p)

CBS-QB3

G3MP2B3

exptla

68.88

60.37

75.27

78.23

73.10b

CS þ CH3CCH (R2)

79.56

75.30

84.96

85.46

83.05

CS þ CH2CCH2 (R3)

83.83

72.79

85.73

89.61

84.49

CS þ CH2CHCH(S) (R4)

147.01

132.82

147.29

155.65

CS þ CH2CHCH(T) (R4)

141.30

121.77

138.61

145.27

H2S þ HCCCCH (R5)

67.11

61.08

79.0

84.69

CH2C þ CH2CS (R6)

116.81

99.42

116.69

127.08

HCS þ CH2CCH (R7) HS þ C4H3 (R8)

138.14 147.09

104.24 117.01

125.20 138.45

146.99 154.53

H þ 2-C4H3S (R9)

124.21

108.47

118.39

126.20

H þ 3-C4H3S (R10)

121.69

105.53

115.93

123.99

SCHCHCHCH (R11)

107.42

75.43

89.55

CHCHSCHCH (R12)

149.55

120.36

133.67

IM4

50.47

48.05

53.27

IM8

44.27

39.34

48.00

76.88 119.10

a

Heats of reaction determined at 298.15 K using the enthalpies of formation found in Goos, Burcat, Ruscic, Third Millennium Ideal Gas and Condensed phase Thermochemical Database for Combustion.33 b Because there is no experimental enthalpy of formation available for CH2CS, the calculated G2(MP2) value of 193.6 kJ from ref 34 was used.

Figure 2. Initial decomposition pathways of thiophene.

discussion are electronic energies at the CBS-QB3 level unless otherwise noted. 3.1. Initial Processes. In principle, thermal decomposition of thiophene can take place via four initiation processes, namely, CH rupture (reactions R9 and R10 above), CS (R4/R11) or CC cleavage (R12), or hydrogen migration (R1-R3, R5-R8). The structural details of these initial reaction steps are illustrated in Figure 2. In this work, the CH bond strength is calculated to be 116.87 (2-position) and 114.44 (3-position) kcal/mol. Surface scans of increasing CH bond distances indicates that CH rupture in thiophene is an ever increasing function converging to a maximum value at well separated H þ C4H3S species. When corrected to 298.15 K, the CH bond strength is calculated to be 118.39 kcal/mol at the 2-position and 115.93 kcal/mol at the 3-position suggesting hydrogen loss from the 3-position is

favored over the 2-position by 2.46 kcal/mol. These results are in good agreement with the CBS-Q values for thiophene reported previously (119.2 and 117.0 kcal/mol at 298.15 K for the 2- and 3-positions, respectively).31 The corresponding preference for dissociation from the 3-position over the 2-position of furan and pyrrole are 0.4 and 0.4 kcal/mol at CBS-Q level, respectively.31 It should be noted that the experimental work of Winkler et al.17 proposed CH homolysis as the initial step because of the large number of secondary condensation products they observed in this work we are comparing the 0 K energetics of the primary pyrolysis steps and therefore have limited ability to directly account for the experimentally observed products from that study. Analogous to furan and pyrrole, the ring scission of thiophene via CS or CC bond cleavage forms the corresponding linear 2884



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Figure 3. The initial process of hydrogen migration. For reactive intermediates IM1 IM4 singlet (S) and triplet (T) state energies are shown. For transition states TS1TS4 only singlet energies are provided. All energies are in kcal/mol.

biradical intermediates of which energies are significantly higher than that of thiophene. For instance, CS bond fission (R11) generatesa linear-thiophenebiradicalintermediate, 3 SCHCHCHCH 3 . The triplet state of this biradical lies 88.77 kcal/mol above thiophene (Figure S2 of the Supporting Information). (It should be noted that only the unrestricted triplet states of these diradicals were evaluated. Spin contamination was minimal judging by the ÆS2æ value. Work is currently underway utilizing highly correlated multireference approaches to characterize the open-shell singlet states of these species.) The triplet product of CC bond cleavage (R12), 3 CHCHSCHCH 3 , lies 133.67 kcal/ mol higher in energy with respect to thiophene. Moreover, the subsequent decomposition of the 3 SCHCHCHCH 3 form of linear thiophene via reaction pathway R4 yields highly endothermic final products CS and vinylmethylene. These products lie 136.71 kcal/mol higher than thiophene as shown in Figure S2 of the Supporting Information. Therefore, initial pathways based upon CH rupture, CS cleavage and CC scission can be considered to be negligible based on our CBS-QB3 energetic results. The bulk of this work will therefore focus on reaction channels that are initiated by hydrogen migration reactions. It is conceivable that hydrogen migration along the conjunction skeleton of thiophene should be the dominant mechanism for the thermal decomposition. In total, there are four parallel and distinct pathways for hydrogen shifting, as illustrated in Figure 3. In consideration of the C2V symmetry of thiophene, it is obvious that there are two types of hydrogen migration, namely, R-H migration and β-H migration. As for R-H shifting, the R-H can migrate to the C3 site via transition state TS1 or move to the S site via TS3, producing intermediates IM1 and IM3, respectively. Accordingly, the β-H could transfer to either C2 or C4 leading to IM2 and IM4, respectively. All four pathways are intramolecular 1,2-H migration. For completeness, efforts were also expended trying to locate the transition states for 1,3-H migration and 1,4-H migration with both MP2 and DFT theory; however, neither 1,3-H or 1,4 H migration were located. Such processes for furan or pyrrole are also not reported in the literature. In previous studies,23,24 subsets of the four possible

Figure 4. Thiophene decomposition reaction channels resulting from the initial 1,2 R-H migration from C2 to C3 resulting in either the formation of thioketene and acetylene (R1) or but-3-ynethial (IM4). As shown in Figures 8 and 9 below, but-3-ynethial decomposes to HCS þ CH2CCH radicals (Figure 9) or SH þ CHCHCCH via IM8 (Figure 8). Energies are calculated at 0 K and reported in kcal/mol relative to thiophene. Geometry optimized structures can be found in the Supporting Information.

1,2-H migrations were considered; here we give a complete and detailed comparison of the four parallel hydrogen migration processes. In what follows, we present each pathway ordered according to the energetic feasibility of the reaction. 3.2. 1,2 r-Hydrogen Migration Path I: The Pathway for Formation of Thioketene and Acetylene (R1). This pathway is initiated via a 1,2-hydrogen migration via transition state TS1 that leads to the R-cyclic carbene intermediate IM1. The potential energy surface for the product channels initiated by this reaction is shown in Figure 4. TS1 was calculated to be 66.84 kcal/mol above thiophene. This is in good agreement with the laser pyrolysis results of Hore et al. that suggested a 1,2-H initiation step requiring 71.70 kcal/mol.19 As seen in Figure 5 below and in Figure S1 of the Supporting Information, the migrating hydrogen in TS1 is nearly perpendicular to the plane of 2885



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Figure 5. B3LYP/CBSB7 optimized geometry of IM1, TS1, and TS5. (Unit: Å, ).

the ring and located closer to the C3 atom (C2H distance = 1.390 Å; C3H distance = 1.247 Å). A similar transition state for the analogous reaction of furan was reported to have a barrier of 66.92 kcal/mol at CASPT2 and 64.53 kcal/mol at G2MP2 level of theory, respectively.20 The R-cyclic intermediate IM1 has Cs symmetry with 1-A electronic state. IM1 has a carbene-like nature, with a lone pair of electrons located on C2. The lone pair of electrons on S are likely delocalized through the CS bond into the formally empty π orbital on carbon C2. Evidence for this can be found in the Mulliken atomic charge analysis (Figure S3 of the Supporting Information), which shows that the charge on S becomes less negative, whereas the charge on C2 becomes more negative in IM1 relative to thiophene. Additionally, the C2S bond shortens to 1.657 Å in IM1 (from 1.727 Å in thiophene) indicative of double bond character. Correspondingly, the C5S bond in IM1 elongates to 1.808 Å. Further evidence for the carbene-like nature of IM1 can be seen in the HOMO-1 shown in Figure S3 of the Supporting Information. Also, the bond angle of — SC2C3 decreases to 106.8 in IM1, which is consistent with the 1-A singlet state of a carbene, similar to the — OC2C3 bond angle of 105.8 for the carbene-like structure of furan.20 The — C2C3C4 is 112.7 in IM1 nearly the same as the angle in thiophene. Similar cyclic carbene intermediates have also been reported for furan and pyrrole decomposition.20,22 By comparison with the singlet state of IM1, the triplet was calculated to be 33.47 kcal/mol above the singlet. The triplet does not show the same delocalization of electron density from sulfur in that the SC2C3 bond angle in the triplet is 10.68 bigger than in the singlet and the C2S bond is 1.725 Å, similar to the corresponding bond length in thiophene. Starting from IM1 and following pathway R1, the five membered thiophene ring opens via concerted C5S and C3C4 bond cleavage through transition state TS5 (99.39 kcal/mol), producing C2H2 þ CH2CS (Figure 4). The breaking C5S and C3C4 bond distances of TS5 are 2.652 and 2.080 Å, respectively. This is a new, previously unreported decomposition pathway for IM1. Ling et al24 have reported an alternative IM1 decomposition pathway, but with a higher barrier (103.33 kcal/mol) corresponding to a second 1,2-hydrogen shift (from C5 to the S) forming a 2,5 didehydro, diradical species (IM7 in ref 24). The new pathway we are reporting here, via TS5, is only 41.06 kcal/mol higher than IM1. Liu et al21 found a similar concerted transition state lying 80.3 kcal/mol (QCISD(T)/6-311þþG**) above furan, which is only ∼19 kcal/mol lower than TS5. However, Sendt et al20 suggested a two-step process for ring-opening in furan. Their mechanism begins from a carbene intermediate very similar to our IM1 species which undergoes OC bond rupture to form a diradical intermediate, 3 C(dO)CH2CHdC(H) 3 followed by CC bond cleavage to form ketene and acetylene. They argued that the biradical intermediate can only be located using multiconfigurational methods using at least a two-determinant reference state. We tried to locate

Figure 6. Thiophene decomposition reaction channels resulting from the initial 1,2 β-H migration from C3 to C2. Energies are calculated at 0 K and reported in kcal/mol relative to thiophene. Geometry optimized structures can be found in the Supporting Information.

similar linear diradical intermediates for thiophene decomposition from IM1 using CASSCF theory with different active spaces but were unsuccessful. However, even in the case for furan, the barrier to OC bond rupture is only 1.2 kcal/mol lower than the energy of the resulting diradical intermediate, that is the intermediate formed after the first step of the two step process lies in a very shallow well suggesting that the ring-opening may be a concerted process. Alternatively, IM1 can also undergo a second 1,2-hydrogen migration from C3 to C4 via transition state TS6. TS6 was calculated to be 85.91 kcal/mol higher in energy relative to thiophene and leads to IM5 which is 17.88 kcal/mol less stable than IM1. IM5 undergoes a concerted hydrogen shift and CS bond rupture via transition state TS7 to form acyclic but-3-ynethial (IM4). The subsequent decomposition of but-3-ynethial will be discussed in path IV (vide infra). The critical energy barrier associated with R1 was calculated to be 99.39 kcal/mol whereas the barrier to forming but-3-ynethial is a comparable 96.90 kcal.mol. However, as will be shown below, the complete decomposition of but-3-ynethial to HCS þ CH2CCH is a relatively high energy process. Overall, in this work, the decomposition reaction of thiophene to thioketene and acetylene is calculated to be 73.16 kcal/mol endothermic at 0 K. This is much higher than the analogous reaction of furan, which was calculated to be 49.24 kcal/mol endothermic at the G2MP2 level.20 However, hydrogen migration followed by ring-opening to but-3ynethial requires only 52.27 kcal/mol of energy. 3.3. 1,2 β-Hydrogen Migration Path II: The Pathway for Formation of CS þ Propyne (R2) and CS þ Allene (R3). This pathway is initiated by a 1,2-H migration from C3 to C2 with simultaneous C2S bond fission. This proceeded via transition state TS2 (Figure 7) and produced an acyclic intermediate IM2. As shown in Figure 6, TS2 was calculated to be 74.59 kcal/mol above thiophene. Although TS2 seems to be leading to a ring structure, relaxation of the geometry along the intrinsic reaction coordinate toward the product resulted in a linear intermediate IM2. A singlet β-cyclic intermediate was not located at either the MP2/6-311þþG(2d,p) or CBS-QB3 levels of theory, however, a triplet cyclic intermediate was identified lying 33.22 kcal/mol above the acyclic IM2 singlet with a SC2 bond elongation of 1.887 Å (relative to 1.727 Å in thiophene and 1.657 Å in the 2886



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Figure 7. B3LYP/CBSB7 optimized geometries of several important transition states involved in hydrogen migration pathway II. (Unit: Å, ).

Figure 8. Thiophene decomposition reaction channels resulting from the initial 1,2 R-H migration from C2 to S. Energies are calculated at 0 K and reported in kcal/mol relative to thiophene. (TS16 was not located at the CBS-QB3 level but was found on the MP2/6-311þþG(2d,p) surface 126.58 kcal/mol above thiophene.) Geometry optimized structures can be found in the Supporting Information.

R-cyclic intermediate IM1). Similar to the transition state found in path I, the shifting hydrogen in TS2 is near-perpendicular to the plane of the ring. The breaking and forming CH bond distances of TS2 are 1.384 and 1.243 Å, respectively. At the same time, the C2S bond is stretched to 1.805 Å. IM2 has Cs symmetry with a planar backbone of heavy atoms and is thermodynamically the most stable intermediate from among the four possible intermediates formed from initial hydrogen shifting. It readily isomerizes to the trans conformer (IM2a), as the barrier for the cistrans rearrangement was determined to be only 5.83 kcal/mol. The trans conformer (IM2a) is 2.71 kcal/ mol more stable than the cis (IM2) conformer. On the CBS-QB3 surface, IM2a will undergo a two-step process to form CS þ propyne (R2). A 1,4-hydrogen migration via transition state TS9 leads first to intermediate IM6, which lies in a relatively deep minimum 13.88 kcal/mol lower in energy than the subsequent transition state TS10. IM6 undergoes a CC bond fission via transition state TS10 resulting in the final products. In TS9, the migrating hydrogen is 1.677 Å from C5 and 1.239 Å from C2 respectively, and the C2C3C4 bond angle compresses from 180 in IM2a to 113.1 (Figure 7.) Previous results for furan suggest a concerted one step process for decomposition to form CO and propyne.20 The furan transition state for the concerted process is much lower in energy (66.68 kcal/mol G2MP2) and structurally distinct from TS9. The shifting hydrogen is approximately equidistant from C5 and C2 (∼1.4 Å) in the concerted furan transition state and possesses a fairly long breaking CC bond of

1.78 Å. The CC bond in TS10 is elongated to 1.966 Å. TS10 has a relatively lower barrier 10.2 kcal/mol lower than TS9. Therefore, it is clear that the 1,4-hydrogen migration step is the ratedetermining step for the decomposition of IM2a. Starting from IM2, a 1,2-H transfer migration from C5 to C4 is also possible via TS11 leading to intermediate IM7. TS11 is approximately 13 kcal/mol higher than TS9, which means that 1,4-H transfer is relatively easier than 1,2-H transfer starting from IM2a. IM7 is 88.68 kcal/mol above thiophene, and can readily decompose to the final product CS þ allene (R3) via transition state TS12. In summary, the products CS þ propyne and CS þ allene are 82.97 and 83.84 kcal/mol endothermic at 0 K, respectively. IM2 and IM2a can also decompose to final products HCS þ CH2CCH (R7) directly without a barrier. This process is highly endothermic by 122.78 kcal/mol, which could be neglected safely at low temperature. 3.4. 1,2 r-Hydrogen Migration Path III: The Pathway for Formation of H2S þ Butadiyne (R5) and CH2C þ CH2CS (R6). A recent DFT study by Ling et al.24 examined two of the four possible H-migration pathways for the decomposition of thiophene (corresponding to paths I and III in this work.) They concluded that the most favorable path includes an R-H migration to S first; followed by SC2 bond cleavage, isomerization and then a second CS bond cleavage resulting in the formation of H2S and butadiyne. The rate determining step in this pathway was reported to be the second CS bond scission with a barrier of almost 131 kcal/mol relative to thiophene. Initially, we utilized 2887



Figure 9. Thiophene decomposition reaction channels resulting from the initial 1,2 β-H migration from C3 to C4. Energies are calculated at 0 K and reported in kcal/mol relative to thiophene. Geometry optimized structures can be found in the Supporting Information.

the MP2 method to explore these and other mechanisms initiated by a 1,2 R-H migration from C2 to S. We were able to obtain results in good agreement with the DFT data and concurred with their conclusion that many of the species in the pathways that they considered were too high in energy to play an important role. Instead, following initial 1,2 R-H migration from C2 to S, we obtained the alternative pathways illustrated in Figure 8. The first transition state that we obtain, TS3, corresponds structurally and energetically to the first step in the Ling DFT mechanism (CBS-QB3 energy of this work = 85.84 kcal/ mol, PW91/DND energy of Ling et al. = 84.04 kcal/mol24) and yields a cyclic intermediate IM3. Like other transition states for initial hydrogen migration, the ring maintains a near-planar structure, whereas the migrating hydrogen is nearly perpendicular to the ring plane. In TS3, the shifting hydrogen is 1.679 Å from C2 and 1.414 Å from the S atom, respectively. There is concomitant increase of the C2S bond in TS3, which is 1.995 Å. Although both TS1 and TS3 are R-H migration, TS3 is 19 kcal/mol higher than TS1 and the corresponding IM3 is 22.79 kcal/mol higher than IM1. In this pathway, hydrogen migration disrupts the aromaticity of the thiophene ring. As seen in Figure S5 of the Supporting Information, the nucleus independent chemical shift values (NICS)35 for thiophene are much more negative than for IM3 and IM1 suggesting a decrease in the aromatic character of the cyclic intermediates. Starting from IM3, a concerted transition state TS13 leads to acyclic IM8. This process is a β-H migration with simultaneous SC2 bond fission. TS13 is calculated to be 93.41 kcal/mol in good agreement with the results of Ling et al.24 Both IM8 and IM8a are planar chain structures with Cs symmetry, and they can interconvert readily via transition state TS14, which lies only 3.63 or 4.84 kcal/mol higher in energy than IM8 and IM8a, respectively. From IM8a to IM9 is also a hydrogen migration process, which needs to cross a high energy barrier via transition state TS15. Because the relative energy of TS15 is so high, this pathway can be safely neglected at the lower temperatures studied here but must be considered in any future kinetic modeling conducted at the higher temperatures typical of experimental pyrolysis studies.

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This pathway may become particularly important at higher temperatures as it is the one channel identified in this work that leads to the production of H2S in agreement with experimental measurements at temperatures above ∼1100 K.17,18 Moreover, there is another path beginning at IM3. IM3 can isomerize to intermediate IM10 via 1,2-H migration. IM10 rearranges to IM11 by CS bond breaking via TS18. However, the subsequent decomposition of IM11 needs to overcome a significant barrier height, 125.48 kcal/ mol, suggesting that products derived from IM3 are not important. In light of the comparable barrier heights (TS13 and TS17, TS15 and TS19), the two parallel decomposition pathways R5 and R6 starting from IM3 could be competitive with each other. In the thermodynamic view the R5 pathway, that is formation of products H2S þ C4H2 is more favorable because it has the lowest overall heat of reaction compared to R6 and R8. Although both IM8 and IM8a decompose into HS and CHCCHCH radicals directly without a barrier, this process is highly endothermic, which can likely be safely neglected in any future kinetic models. Therefore, we conclude that the product channels in path III are not important. 3.5. 1,2 β-Hydrogen Migration Path IV: The Pathway for Formation of HCS þ CH2CCH (R7) radicals. This pathway involves an alternative route to HCS þ CH2CCH radicals. This decomposition was considered previously in path II (Figure 6). In this alternate pathway, thiophene can undergo a β-H migration from C3 to C4 with simultaneous CS bond fission via transition state TS4. In TS4, the breaking and forming CH bonds are 1.45 and 1.196 Å, respectively. This concerted process leads to the acyclic Cs symmetric intermediate IM4 (IM4 is also an intermediate in the path I/R1 mechanism) with a planar backbone of heavy atoms. Like IM2 in path II, IM4 can readily isomerize into a trans conformation, namely, IM4a. There is only a 1.99 kcal/mol barrier separating the two conformers. Both IM4 and IM4a can lead to CC bond fission, producing HCS and CH2CCH radicals. TS4 was calculated to be 86.72 kcal/mol above thiophene, which is just 1.59 kcal/mol higher than the similar transition state for the analogous reaction of furan at the G2MP2 level.20 Like TS2 in path II, TS4 seems to be leading to a ring structure, but the intrinsic reaction coordinate confirmed that it leads to a linear IM4 species. However, CBS-QB3 results indicate that the triplet state of IM4 is a cyclic structure, which is 38.61 kcal/mol higher than the acyclic singlet. The four pathways considered in this work are not isolated from each other. For instance, IM4 can isomerize to IM8 via a 1,3-H migration transition state TS21 with a relative energy of 97.39 kcal/mol. As described above, both IM4 and IM8 are thermodynamically stable intermediates and separated from the final products by significant barriers; it may be possible to trap the species that result from hydrogen migrations. 3.6. Summary of All the Pyrolysis Pathways for Thiophene. Four distinct decomposition pathways have been discussed above. The most favorable pathway is path I, which has the lowest initial barrier height corresponding to hydrogen migration (TS1) and lowest overall barrier (TS5) for producing the final product C2H2 þ CH2CS. Overall barrier heights associated with pathways R7 (TS7, 96.90) and R8 (TS13, 93.41 and TS21, 97.39) are energetically competitive with TS5 but lead to very high energy products. The second dominant product channel is the R2 pathway forming CS and propyne in path II. For this pathway, no matter the barrier (TS2) for initial hydrogen shifting or the rate-determining step (TS9) for producing the product, path II species are only 8 kcal/mol higher than the corresponding species in path I. Thus, we can conclude that 2888


The Journal of Physical Chemistry A the pyrolysis of thiophene has two dominant product channels, namely, to forming C2H2 þ CH2CS and CS þ CH3CCH, respectively. For furan, Sendt et al.20 calculated that the dominant pathway is CO þ propyne and that C2H2 þ ketene is a minor channel. However, it is commonly agreed that pyrolysis of furan has two major decomposition channels yielding C2H2 þ CH2CO and CO þ CH3CCH.32 The corresponding product channels of thiophene are much more endothermic than furan. The reason for this could be explained in two respects. First, a comparison of the thiophene and furan heats of reaction, corresponding to one of the major decomposition channels, indicates that thiophene decomposition is a more endothermic process. For instance, for C4H4X f CX þ CH3CCH where X is

Figure 10. CBS-QB3 potential energy surface (kcal/mol) for the formation of CH2CS þ CH3CCH. This pathway is analogous to R1 above for thiophene. Energies are reported in kcal/mol relative to 2-methylthiophene (bottom number) and 3-methylthiophene (top number), respectively.

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either O or S, ΔHrxn = ΣΔHf(prods) ΣΔHf(rcts) using standard enthalpies of formation for CH3CCH (þ184.9 kJ/mol), CO (110.53 kJ/mol), furan (34.16 kJ/mol), CS (þ278.55 kJ/mol), and thiophene (þ115.96 kJ/mol)33 yields ΔHrxn= 108.9 and 347.9 kJ/mol for furan and thiophene, respectively. Second, even though thiophene and furan have similar HOMO and LUMO properties (Figure S3 of the Supporting Information), the HOMOLUMO gap of thiophene is slightly larger than for furan. Minor product channels in the decomposition of thiophene include the formation of CS þ allene and H2S þ butadiyne, but these may not play an important role in the overall kinetics due to the high barrier heights. The specific reaction branching ratios need to be calculated with a kinetic model, work which is currently underway. A note in closing about error analysis: The barrier heights for the initial transition state of each channel range between 66.84 (R1) and 88.77 (R7) kcal/mol. The energetic differences between the barriers are, in all but the highest energy channels (paths III and IV), significantly greater than the 12 kcal/mol error associated with the CBS-QB3 method. The smallest initial ΔΔEa is 0.88 kcal/mol corresponding to the difference between the initial barriers TS3 and TS4 on the higher energy path III. In cases such as these, with very small ΔΔEa, our results are not able to distinguish these pathways and it is very possible that multiple pathways are equally likely. 3.7. Pyrolysis of 2,3-Methylthiophene. Asphaltenes are known to contain thiophene with long alkyl chains.1 To gauge the effect of such alkylation on the pyrolysis mechanism, we extended our study to 2-, and 3-methylthiophene using the same theoretical methods as described above. As far as we know, there has not been a previously published report on the pyrolysis of methylthiophene. The introduction of a weakly electron-donating methyl group does not significantly change the ring structure of thiophene as shown in Figure S4 of the Supporting Information. However, the ring carbons (C2 or C3) connected to the

Figure 11. Potential energy surface for the formation of CH2CS þ CH3CCH and CS þ CH3CCCH3 at 0 K (pathway shown at the top). This later pathway is analogous to R2 above for thiophene. (Energies are reported in kcal/mol relative to 2-methylthiophene (bottom number) or 3-methylthiophene (top number), respectively; except for IM6 and IM6a in which case IM6 results from the decomposition of 2-methylthiophene and IM6a from the decomposition of 3-methylthiophene.) 2889


The Journal of Physical Chemistry A methyl group have gained a much more positive charge compared to thiophene possibly as a result of σπ conjugation. Whereas methyl groups are weak sigma donors, the Mulliken atomic charges of 2 and 3-methylthiopene (shown in the Supporting Information) suggest that π electron density has moved from the C2 or C3 ring atoms to the C atom in the methyl group. Although there is some change in geometry and charge distribution in the thiophene ring due to the methyl group substituent, the dominant pathway for decomposition of 2- and 3-methylthiophene is the same as for thiophene, as shown in Figure 10. No matter the transition state (TS1) for hydrogen migration, or the cyclic carbene intermediates (IM1), or the concerted transition state (TS5) for forming the final products CH3CCH þ CH2CS, the relative energy differences between thiophene and 2, 3methylthiophene are only about 1 kcal/mol and the heats of reaction differ by approximately 2 kcal/mol. The second dominant pathway for the decomposition of 2- and 3-methylthiophene is summarized in Figure 11. First, the β-H migrates to C2, with a concerted SC bond cleavage via transition state TS2. The barrier height for this process is nearly the same as for thiophene; the corresponding barriers differ by less than 1 kcal/mol. TS2 leads to an acyclic intermediate IM2, which readily isomerizes to IM2a. The decomposition of IM2a proceeds through a 1,4-H migration, leading to two different intermediates IM6 and IM6a as shown in Figure 11. Both IM6 and IM6a undergo CC bond fission to form the final products. 3-methylthiophene will form CS þ CH3CCCH3 which is analogous to the R2 process of thiophene. This process is endothermic by 81.58 kcal/mol. The dominant product channel for 2-methylthiophene in Figure 11 provides an alternative route to the CH2CS þ CH3CCH products also described in Figure 10. A comparison of the decomposition surfaces of thiophene, 2-methylthiophene and 3-methylthiophene suggests that methyl substitution plays very little role in the energetics or mechanism for the pyrolysis of thiophene.

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(IM2 and IM4). The subsequent decomposition products C2H2 þ thioketene and CS þ propyne are the two dominant pathways, which are likely to be competitive with each other. The analogous reaction of furan has two similar major product channels CO þ propyne and C2H2 þ ketene. Compared to furan, the major decomposition products of thiophene are much more endothermic. (4) Both 2-methylthiophene and 3-methylthiophene have very similar dominant decomposition pathways as thiophene. It was shown that the methyl substituent does not have a significant effect on the thermal decomposition mechanism.

’ ASSOCIATED CONTENT

bS

Supporting Information. CBS-QB3 geometries, energies, triplet energy profile for CS rupture, Mulliken atomic charges, molecular orbitals and NICS values. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], phone (804) 484-1548, fax (804) 287-1897.

’ ACKNOWLEDGMENT The work was supported by the Department of Energy (Grant CHE-0239664), NSF RUI (Grant CHE-0809462), and Henry Dreyfus Teacher Scholar Award program, as well as the Floyd D. and Elisabeth S. Gottwald Endowment. Support is also acknowledged from the Donors of the American Chemical Society Petroleum Research Fund. ’ REFERENCES

’ CONCLUSIONS The gas-phase pyrolysis of thiophene has been investigated at the CBS-QB3 level. The results show that: (1) Initiation of decomposition by direct fission of CC or CS bonds leads to a biradical linear thiophene. It was revealed that this process demands very high energy. For instance, direct CC cleavage requires as much as 133.67 kcal/mol of energy. CS bond cleavage requires 88.77 kcal/mol to produce the triplet linear thiophene. The analogous reaction of CO in furan and CN in pyrrole was calculated to be 79.11 and 93.93 kcal/mol at G2MP2 level, respectively. (2) Direct CH rupture results in cyclic free radicals. The preference for loss from the 3 position over the 2 position in thiophene is 2.46 kcal/mol. The bond energy of CH was calculated to be 118.39 kcal/mol at the 2-position. Because of the high energy demands of direct scission of CH, CC, or CS bonds, we therefore conclude that direct ring-bond fission is not likely to be the most favorable initial step at low temperature. (3) 1,2-H migration is the lowest energy pathway for the initiation process. Four distinct pathways result from 1,2-H migration. Among them, R-H migration results in two kinds of cyclic carbene intermediates (IM1 and IM3), whereas β-H migration leads to two linear intermediates

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Pyrolysis Mechanisms of Thiophene and Methylthiophene in

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