3.3.1. Proposal of Reaction Pathways for Resorcinol Pyrolysis
Scheme 1 depicts the mechanisms of various reaction pathways that lead to CO and CO2
formation during the pyrolysis of resorcinol, R. The reaction pathways present in the pyrolysis of m-semiquinone radical M1 have been mapped out, where the aromatic ring contracts to form a fused bicyclic compound containing a five-membered and a three-membered ring, followed by breaking of a C−C bond in the three-three-membered ring and subsequent generation of both a hydroxycyclopentadienyl radical and CO [18,19]. On the basis of this theoretical study, reaction pathways 1 and 2 for resorcinol pyrolysis were proposed, as shown in Scheme 1. However, previous studies did not touch on the indicated reaction pathways following the formation of m-benzoquinone involving M6 with triplet state and M6 (Singlet) with singlet state, which was formed by the O−H bond dissociation of M1.
The possible pathways are as follows: One is along with the singlet-state pathway following the formation of M6 (Singlet); the other is along with the triplet-state pathway following the formation of m-benzoquinone M6. Kayembe et al. [28] theoretically determined the most stable bicyclo[3.1.0]quinone generated from singlet-ground state m-benzoquinone M6 (Singlet); while using the B3LYP/6-311+G(2d,p) method, Roithová et al. [29] considered the reaction pathways for the decarbonylation of m-benzoquinone that lead to CO release. By applying these theoretical approaches in identifying the thermal decomposition routes for CO release, we proposed the singlet-state pathway (pathways 4, Scheme 1) leading to CO formation. Analogous to a study on the decomposition of the phenoxyl radical [30,31], we proposed the triplet-state reaction pathway 5, where the direct cleavage of a C−C bond of the six-membered ring in M6 occurs to give M8, which undergoes cyclization to form the five-membered ring intermediate M11.
Moreover, on the basis of experimental product distributions detected by online gas chromatography, Yang et al. [15] reported resorcinol pyrolysis pathways leading to CO2
formation. Therefore, we proposed two additional triplet-state pathways involving CO2
release (reaction pathways 6 and 7), as shown in Scheme 1. Following the formation of M11, the ketone radical combines with the carbon atom of the aldehyde radical group to form 6-oxabicyclo[3.2.0]hepta-2,4-dien-7-one M12. M13 is then generated by C−C bond breakage in the four-membered ring, resulting in the release of CO2.
More specifically, hydroxycyclopentadienyl radical M5 and cyclopenta-2,4-dienone M10 can be decomposed to form ethyne or vinylacetylene with the concurrent release of CO [32,33]. In addition, the five-membered ring diradical M14 is expected to capture a H atom in the gas phase, and this is followed by the formation of either naphthalene or
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cyclopentadiene [34,35]. As expected, C2, C4, and C5 hydrocarbons and naphthalene were indeed detected experimentally [15].
Although the other reaction pathways (e.g., bimolecular decomposition) might exist, we assume that the unimolecular decomposition reactions proceed along the proposed Scheme 1.
3.3.2. Optimized Resorcinol Configurations
We then considered the optimized configurations of resorcinol, R. Various experimental [36−39] and theoretical studies [40−42] have been carried out to determine the configuration of R and, in particular, the relative orientation of the two hydroxyl groups.
Figure 1 shows the optimized configurations at the B3LYP/6-311G(2d,d,p) level of theory along with the relative energies at the CBS-QB3 level for the three possible conformers.
Relative energies of these three configurations were within computational accuracy of 1 kcal/mol. We herein selected the anti−anti conformer of R for all subsequent calculations.
3.3.3. Potential Energy Surfaces in the Pyrolysis Process for Each Reaction Pathway Two possible pathways exist for cleavage of the O−H bond in R, M1, and M3, as depicted in Scheme 1. These pathways are fission of the O−H bond or H-abstraction by a H atom generated during pyrolysis. The possibility of either process taking place depends on the H atom concentration in the gas phase. For O−H bond fission, the PES along the seven proposed reaction pathways calculated at the CBS-QB3 level are described in Figures 2 (reaction pathways 1−5, CO release) and 3 (pathways 6, 7, CO2 release).
According to Figure 2, the ring contraction of the m-semiquinone radical M1 in step 2 (see Scheme 1) had a significantly lower activation energy (37.5 kcal/mol) than the O−H bond cleavage in step 8. Comparison of the energy barriers of steps 4 and 7 shows that the activation energy of step 7 was 65.1 kcal/mol higher than that of step 4. Thus reaction pathways 1 and 2 were kinetically more favorable than the other reaction pathways, and their steps requiring the highest energy correspond to the initial O−H bond hemolysis of R (step 1), which had an energy barrier of 86.2 kcal/mol (experimental value = 89.0 ± 2.0 kcal/mol [27]). Comparison of the energy of steps 8 and 9 demonstrated that the triplet-state pathways were more kinetically favorable than the singlet-triplet-state pathways. From the data shown in Figure 3, it appears that reaction pathway 6, with a lower activation energy in step 2, was a kinetically favorable route to CO2 expulsion, with the step requiring the highest energy corresponding to step 1 (as for pathways 1 and 2).
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3.3.4. High-Pressure Limiting Rate Constants 3.3.4.1. Rate Constants for Step 1, 7, 8, and 9
Comparison of the activation energies of the various reaction steps shown in Figure 2 and 3 indicates that steps 1, 7, 8, and 9 had the high-energy barriers (i.e., 86.2, 69.6, and 86.2 kcal/mol, respectively, experimental value for step 8 = 90.0 ± 3.6 kcal/mol [27]). Steps 1, 8, and 9 have comparable high O−H bond dissociation energies, whereas step 7 has a smaller one probably due to the destabilization of strained five-membered ring system in M3 [43]. The high-pressure limiting rate constants of steps 1, 7, 8, and 9 were calculated between 300 and 1500 K at intervals of 100 K (temperature ranging from 650 to 950 °C in pyrolysis calculated by using variational transition state theory (VTST) based on the calculated potential energy curves as a function of the O−H bond distance (see Figure S1 in the Appendix B). Figure 4 demonstrates that step 1 is the rate-determining
reaction at all temperatures among reaction pathways 1−3 and 5−7, whereas step 9 is at reaction pathway4.
As shown in Figures 2 and 3, the homolytic cleavage of O−H bond could be the initial reaction channel and result in H radical generation. O−H bond dissociation of R, M1, and M3 would be expected to occur by abstraction reactions by H atoms formed in bond scission reactions. We then consider step 1, 7, and 8 in terms of the H-abstraction. Using the B3LYP/6-311G(2d,d,p) level of theory, structures of transition state for these steps were determined and are depicted in Figure 5. In addition, Table 1 provides the calculated activation energies both with and without H-abstraction, evaluated at the CBS-QB3 level.
Abstraction of the hydroxyl H atom is associated with lower activation energies and might be compatible to steps 1, 7, and 8, which depends on H radical concentration. Figure 6 provides the calculated rate constants at high-pressure limit for these three H-abstraction steps.
3.3.4.2. Rate Constants for Steps 2 and 8
To evaluate if M1 is mainly consumed via the M2 or the M6 reaction pathway, we compared the high-pressure limiting rate constants between steps 2 and 8, as shown in Figure 7. The formation of M2 was expected to be the major exit channel for M1 because step 2 had a larger rate constant than step 8 at all temperatures, whereas with H-abstraction step 8 tended to take place as discussed in Section 3.3.4.1.
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3.3.4.3. Rate Constants for Steps 14 and 15
The rate constants of step 14 decomposing into CO and step 15 into CO2 were calculated as shown in Figure 8. The rate constant of step 14 was significantly larger than step 15 at all temperatures, and thus CO formation was dominant within the proposed unimolecular decomposition pathways in Scheme 1.
As the other possible pathways responsible for CO2 formation, the produced CO and M10 might recombine to generate M13 through step 21, as shown in Scheme 2. The reverse reactions of step 14 and 15 are also considered as the CO and M10 recombination. When comparing the rate constant for step 21 with those of the reverse reactions, as shown in Figure 9, step 21 had a lower rate constant at all temperatures and was likely the minor reaction channel. CO2 formation routes remain unresolved theoretically, and further insight into this aspect is left to future work. Modified Arrhenius parameters for all steps (step 1 to 22) are given in Table S1 in the Appendix B for future research of reaction kinetic modeling. Additionally, the possible reaction steps for the H-addition reaction with resorcinol were depicted in Figure S2 in the Appendix B, and the corresponding Modified Arrhenius parameters are given in Table S2 in the Appendix B.