RSC Advances PAPER View Article Online View Journal | View Issue Formation pathways of polycyclic aromatic hydrocarbons (PAHs) in butane or butadiene Cite this: RSC Adv.,2021,11,5629 flames† Tingting Zhang,a Guizhi Mu,a Shourong Zhangb and Jialin Hou *a The reaction pathways from phenyl radicals to phenanthrene (A3) and pyrene (A4) via C2H3 and C4H4 additions were investigated using the G3(MP2, CC) method. Rate constants of elementary reactions were calculated. The influence of additions, H-abstraction ways and reactive sites on the reaction rates were considered. These polycyclic aromatic hydrocarbon (PAH) formation pathways were used to improve the combustion chemistry model for C4 fuels, and the results from the improved model and the original model were compared with experimental data. H atoms are important for PAH formation owing to their influential roles in the production of aromatic radicals and stable aromatic structures. C2H3 and C4H4 addition reactions can occur at low temperature, and need less energy than C2H2 addition. The PAH Creative Commons Attribution 3.0 Unported Licence. formation pathways determined from G3 calculations, which were used to improve the model, were Received 14th October 2020 effective in promoting PAH formations in this model. Comparison of PAH formation in butane and Accepted 17th January 2021 butadiene flames showed both the C2H3 and C4H4 addition pathways included in this work can improve DOI: 10.1039/d0ra08744k the formation of PAHs in butadiene and butane flames. C4H4 addition pathways in a butane flame were rsc.li/rsc-advances better for PAH formation than C2H3 addition. 1. Introduction abstraction/vinyl radical addition (HAVA) mechanism may be the best complement to the HACA mechanism, and the phenyl This article is licensed under a Polycyclic aromatic hydrocarbons (PAHs), which mainly come addition/cyclization (PAC) mechanism can promote the growth from the incomplete combustion of fuel, are extremely harmful of molecular mass and ring number of PAHs without the limit 10,17 to humans and the environment. So far, the most commonly- of reactive sites. Hence, the additions of C2H3,C4H4 and used addition for PAH growth is C2H2, and the most popular C6H5 contribute greatly to PAHs formation, and more research Open Access Article. Published on 02 February 2021. Downloaded 10/2/2021 7:19:52 AM. PAH formation mechanism is H-abstraction-C2H2-addition on PAHs formation from C2H3,C4H4 and C6H5 additions is (HACA).1–5 However, the HACA mechanism underestimates the needed. concentrations of PAHs and soot compared with the experi- Butane and butadiene, two light hydrocarbons, are both mental results.6–9 This underestimation may be explained by benecial to the environment and can relieve energy shortage. two main reasons. Firstly, there are many non-acetylene organic In this research, PAHs formation routes via C2H3,C4H4 and compounds (e.g. CH3,C3H3,C3H4,C3H6,C4H2,C4H4,C4H6 and C6H5 additions within all reaction rates of elementary reactions C5H8) that also contribute to PAH growth in various ame in the modi ed Arrhenius equation were studied and coupled to congurations.8–13 Secondly, many PAH formation reactions in a chemical kinetic model for butadiene and butanol. The effects the HACA mechanism are reversible.14 Hence, more additions of additions, reaction types and reaction sites were considered in PAH formation should be considered to perfect the PAH during PAHs formation. formation mechanism. Many non-acetylene additions have been proved experi- 2. Methods mentally to be important for PAHs formation. C4H4 addition 15 can effectively lead to PAHs formation. C2H3 addition is crit- All molecular structures involved in the reactions under study ical for the formation of PAHs and soot.16 The hydrogen were optimized by using the hybrid B3LYP functional with the 6- 311++G(d,p) basis set.18–20 Vibrational frequency was calculated at the same level to identify the optimized structures as local a School of Mechanical and Electrical Engineering, Shandong Agricultural University, minimum or rst-point saddle points and to provide their zero- Taian 271018, P. R. China. E-mail: [email protected] point vibrational energy (ZPE). Intrinsic reaction coordinate bDepartment of Traffic Engineering, Shandong Transport Vocational College, Taian 271000, P. R. China calculations were implemented to ensure that the transition † Electronic supplementary information (ESI) available. See DOI: states connect to relevant reactants and products correctly. The 10.1039/d0ra08744k nal single-point energies of all species were determined using © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2021,11,5629–5642 | 5629 View Article Online RSC Advances Paper the G3(MP2, CC) method,21,22 which is expected to generate species (CS) and transition states (TS) are shown in ESI.† The relative energies of various species within the accuracy of 1– G3(MP2, CC) energies are calculated as follows: À 2 kcal mol 1 (ref. 23) and is extensively used to study PAHs growth mechanisms.21,24–28 The intermediate species and tran- E[G3(MP2, CC)] ¼ E[CCSD/6-311++(d,p)] + DE(MP2) sition structures found in this study were all closed shell +DE(SO) + DE(HLC) + E(ZPE) (1) singlets or open shell doubles. Molecular properties of chemical where DE(MP2) ¼ E[MP2/6-311G++(3df,2p)] À E[MP2/6- 311G++(d,p)] is the basis set correction; DE(SO) is the spin–orbit Fig. 1 Phenanthrene formation pathways from phenyl radical via C H and C H additions. Creative Commons Attribution 3.0 Unported Licence. 6 5 2 3 This article is licensed under a Open Access Article. Published on 02 February 2021. Downloaded 10/2/2021 7:19:52 AM. Fig. 2 Phenanthrene formation pathways from naphthyl radical via C4H4 addition. Fig. 3 Pyrene formation pathways from phenanthryl radical via C2H3 addition. 5630 | RSC Adv.,2021,11,5629–5642 © 2021 The Author(s). Published by the Royal Society of Chemistry View Article Online Paper RSC Advances Creative Commons Attribution 3.0 Unported Licence. This article is licensed under a Fig. 4 Pyrene formation pathways via C4H4 addition onto different sites of 1-ethynylnaphthyl radical. Open Access Article. Published on 02 February 2021. Downloaded 10/2/2021 7:19:52 AM. Table 1 Barrier heights and reaction energies for steps involved in correction, included for atomic species only. For molecules, phenanthrene formation pathways from phenyl radical via C H and D ¼ À À 6 5 higher level correction (HLC) is E(HLC) Anb B(na nb) C2H3 additions computed at the G3(MP2, CC) level with A ¼ 9.279 mHartrees and B ¼ 4.471 mHartrees, and for atoms, is DE(HLC) ¼Cn À D(n À n ) with C ¼ 9.345 Barriers/ Reaction heats/ b a b À À ¼ No. Reaction kcal mol 1 kcal mol 1 mHartrees and D 2.021 mHartrees, where na and nb are the numbers of a and b valence electrons, respectively. All calcula- Carbon addition tions were performed on Gaussian 09.29 ¼ À R1 2CS1 CS2 14.3 119.7 Based on the calculated potential energy surface (PES) and R3 CS3 + C H ¼ CS4 3.8 À116.2 2 3 molecular characteristics, Rice–Ramsperger–Kassel–Marcus Hydrogen abstraction (RRKM) and transitional state theory (TST) were used to deter- Ra CS2 + O ¼ CS3 + OH 292.3 16.7 mine reaction rates by ChemRate program.30 Noticeably, none ¼ Rb CS2 + OH CS3 + H2O 436.5 0.5 of the rate constants were arbitrarily changed or intuitively Rc CS2 + C2H3 ¼ CS3 + C2H4 18.1 7.4 ¼ estimated to match the computed and the experimental results. Rd CS2 + C6H5 CS3 + C6H6 14.4 3.0 ¼ The calculated PAHs formation pathways were used to update R2 CS2 + H CS3 + H2 22.4 13.1 31 R4 CS4 + H ¼ CS5 + H2 23.6 13.0 Hansen's mechanism which consists of 216 species connected via 1028 reactions. Then the updated mechanism was used to Ring formation simulate the premixed 1,3-butadiene or butane ame and to ¼ À R5 CS5 CS6 7.3 39.7 discuss the roles of our pathways in PAHs formation under 1,3- Hydrogen loss and (or) disproportionation butadiene or butane ame. R6 CS6 ¼ CS7 + H 27.9 16.5 © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2021,11,5629–5642 | 5631 View Article Online RSC Advances Paper 3. Results and discussion (C6H5) and twice additions of C2H3 onto phenyl radical; (ii) formation of phenanthrene via once addition of C4H4 onto 3.1 PAHs formation pathways naphthyl radical; (iii) formation of pyrene via once addition of Four PAHs formation pathways were studied (Fig. 1–4): (i) C2H3 onto phenanthryl radical; (iv) formation of pyrene via once formation of phenanthrene via once addition of phenyl radical addition of C4H4 onto 1-ethynylnaphthyl radical. Comparisons Creative Commons Attribution 3.0 Unported Licence. This article is licensed under a Open Access Article. Published on 02 February 2021. Downloaded 10/2/2021 7:19:52 AM. Fig. 5 H-abstractions via the assistance of H, O, OH, C2H3 or C6H5 radicals. Fig. 6 C2H2,C6H5 and C2H3 addition reactions. 5632 | RSC Adv.,2021,11,5629–5642 © 2021 The Author(s). Published by the Royal Society of Chemistry View Article Online Paper RSC Advances Creative Commons Attribution 3.0 Unported Licence. Fig. 7 Phenanthrene formation initiated by C4H4 addition onto naphthyl radical. Table 2 Barrier heights and reaction energies for steps involved in formation of CS2, biphenyl. This thermodynamically and via pyrene formation pathway from phenanthrene C2H3 addition kinetically favorable process is highly exothermic computed at the G3(MP2, CC) level À À (119.7 kcal mol 1) with a barrier of 14.3 kcal mol 1, leading to This article is licensed under a Barrier/ Reaction heat/ a quick increase of ring number.