Theoretical Mechanistic and Kinetic Studies on Homogeneous Gas-Phase Formation of Polychlorinated Naphthalene from 2-Chlorophenol as Forerunner

Abstract

Polychlorinated naphthalenes (PCNs) are dioxins-like compounds and are formed along with polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in thermal and combustion procedures. Chlorophenols (CPs) are the most important forerunners of PCNs. A comprehensive comprehension of PCN formation procedure from CPs is a precondition for reducing the discharge of PCNs. Experiments on the formation of PCNs from CPs have been hindered by PCN toxicity and short of precise detection methods for active intermediate radicals. In this work, PCN formation mechanism in gas-phase condition from 2-chlorophenol (2-CP) as forerunner was studied by quantum chemistry calculations. Numbers of energetically advantaged formation routes were proposed. The rate constants of key elementary steps were calculated over 600–1200 K using canonical variational transition-state theory (CVT) with small curvature tunneling contribution (SCT) method. This study illustrates formation of PCNs with one chlorine atom loss from 2-CP is preferred over that without chlorine atom loss. In comparison with formation of PCDFs from 2-CP, PCN products are less chlorinated and have lower formation potential.

Quantum Chemical and Kinetic Study on Polychlorinated Naphthalene Formation from 3-Chlorophenol Precursor

“…The error correction of the kinetic calculation may be mainly from the SCT method [ 51 , 52 , 53 , 54 ]. The reliability and accuracy of the CVT/SCT method was verified in our recent studies [ 47 , 48 , 49 , 50 , 55 , 56 , 57 , 58 ]. For example, the CVT/SCT rate constants of C 6 H 5 OH + H → C 6 H 5 O + H 2 and C 6 H 5 OH + OH → C 6 H 5 OH + H 2 O are in good agreement with the corresponding experimental values [ 49 , 50 ], respectively.…”

“…Comparison of the formation of PCNs from the 3-CP with our previous studies of PCN formation from 2-CP and 4-CP precursors [ 47 , 48 ] shows that the position of chlorination affects each elementary step involved in the PCN formation. On the one hand, the isomer patterns of PCNs products are completely different from the three CPs.…”

“…In Kim’s mechanism, the chloro-dihydrofulvene occurs intramolecular rearrangement of 1,5-sigmatropic shifts of H/Cl to the ortho -carbon of cyclopentadiene ring before ortho -carbon H/Cl atom is abstracted by H, OH and Cl radicals. However, this deduction has been refused in our previous study on PCN formation from 2-CP and 4-CP [ 47 , 48 ], owning to the high potential barrier of 1,5-sigmatropic shift step. Similarly, theoretical calculation of 1,5-sigmatropic H shift mechanism of IM10 is shown in Figure 5 .…”

“…By In addition, dimerization of CPRs is barrierless and is the initial step of PCN formation. The dimerization of 3-CPRs (−23.42, −23.25 and −23.60 kcal/mol) is approximately 10 kcal/mol more exothermic than those of 2-CPRs (−17.63 and −10.29 kcal/mol) and 4-CPRs (−16.20 kcal/mol) [ 47 , 48 ]. Thus, 3-CP have stronger PCN formation potential than that of 2-CP and 4-CP [ 46 , 47 ], which is consistent with the experimental result wherein 3-CP produced the highest PCN yield [ 32 , 33 ].…”

“…Due to the high toxicity of PCNs and the lack of efficient detection schemes for intermediate radicals, the specific formation mechanism of PCNs from CPs remains unclear. Our previous studies have systematically investigated the homogeneous gas-phase formation mechanism of PCNs from 2-CP and 4-CP precursors [ 47 , 48 ]. As a ongoing study of this field, it is important to continue study the PCN formation mechanism from 3-CP in order to compare the contributions and effects of ortho -, meta- , and para- chlorine of monochlorophenol on the formation potential and isomer distributions of PCN products.…”