The Gas-Phase Formation Mechanism of Dibenzofuran (DBF), Dibenzothiophene (DBT), and Carbazole (CA) from Benzofuran (BF), Benzothiophene (BT), and Indole (IN) with Cyclopentadienyl Radical.

Xuan Li,Yanhui Sun,Fei Xu,Yixiang Gao,Chenpeng Zuo,Qingzhu Zhang,Siyuan Zheng

doi:10.3390/ijms20215420

Abstract

Benzofuran (BF), benzothiophene (BT), indole (IN), dibenzofuran (DBF), dibenzothiophene (DBT), and carbazole (CA) are typical heterocyclic aromatic compounds (NSO-HETs), which can coexist with polycyclic aromatic hydrocarbons (PAHs) in combustion and pyrolysis conditions. In this work, quantum chemical calculations were carried out to investigate the formation of DBF, DBT, and CA from the reactions of BF, BT, and IN with a cyclopentadienyl radical (CPDyl) by using the hybrid density functional theory (DFT) at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. The rate constants of crucial elementary steps were deduced over 600−1200 K, using canonical variational transition state theory with a small-curvature tunneling contribution (CVT/SCT). This paper showed that the production of DBF, DBT, and CA from the reactions of BF, BT, and IN with CPDyl involved six elementary steps: the addition reaction, ring closure, the first H shift, C–C cleavage, the second H shift, and elimination of CH3 or H. The cleavage of the C–C bond was regarded as the rate-determining step for each pathway due to the extremely high barrier. The 1-methyl substituted products were more easily formed than the 4-methyl substituted products. The main products were DBF and 1-methyl-DBF, DBT and 1-methyl-DBT, and CA and 1-methyl-CA for reactions of BF, BT, and IN with CPDyl, respectively. The ranking of DBF, DBT, and CA formation potential was as follows: DBT and methyl-DBT formation > DBF and methyl-DBF formation > CA, and methyl-CA formation. Comparison with the reaction of naphthalene with CPDyl indicated that the reactions of CPDyl attacking a benzene ring and a furan/thiophene/pyrrole ring could be inferred to be comparable under high temperature conditions.

Highlights

Polycyclic aromatic compounds (PACs), including polycyclic aromatic hydrocarbons (PAHs) and heterocyclic aromatic compounds (NSO-HETs), have become a considerable threat to the environment and human health because of their acute toxicity, mutagenicity, photoinduced toxicity, and carcinogenic potential [1,2]
Benzofuran (BF), benzothiophene (BT), indole (IN), dibenzofuran (DBF), dibenzothiophene (DBT), and carbazole (CA) are typical NSO-HETs, which are composed of a benzene ring and a furan/thiophene/pyrrole ring
Dibenzo-NSO-HETs are widely found in creosote, coal tar, and crude oils, as well as in some high temperature processes of waste incineration, tobacco smoke, aluminum manufacturing, forest fire, rubber, petroleum, fossil fuel, and coal and wood combustion [7,14,15,16,17,18], and always coexist with other aromatic compounds, such as PAHs and benzo-NSO-HETs [15,19,20,21,22,23,24,25]

Summary

Introduction

Polycyclic aromatic compounds (PACs), including polycyclic aromatic hydrocarbons (PAHs) and heterocyclic aromatic compounds (NSO-HETs), have become a considerable threat to the environment and human health because of their acute toxicity, mutagenicity, photoinduced toxicity, and carcinogenic potential [1,2]. BF, BT, and IN (benzo-NSO-HETs) include one benzene ring and one heterocyclic five-membered ring; DBF, DBT, and CA (dibenzo-NSO-HETs) consist of two benzene rings fused together on either side of a furan/thiophene/pyrrole ring, respectively. They have been found to be toxic and mutagenic [7]. Dibenzo-NSO-HETs are widely found in creosote, coal tar, and crude oils, as well as in some high temperature processes of waste incineration, tobacco smoke, aluminum manufacturing, forest fire, rubber, petroleum, fossil fuel, and coal and wood combustion [7,14,15,16,17,18], and always coexist with other aromatic compounds, such as PAHs and benzo-NSO-HETs [15,19,20,21,22,23,24,25]. Three isomers of methyl-DBT (1-methyl-DBT, 2 or 3-metyl-DBT, and 4-methyl-DBT), were identified by Rebbert in crude oil, in which 4-methyl-DBT was the obtained with the highest precision at a concentration of 33.9 μg/g [28]

Methods

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Discussion

Conclusion