Formation of trichlorinated dibenzo-p-dioxins from 2,4-dichlorophenol and 2,4,5-trichlorophenolate: a theoretical study

Transition Metal-Decorated Biphenylene Sheet for Dioxin Detection: A First-Principles Investigation

Detecting highly toxic environmental pollutants, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is crucial for environmental safety. However, current sensors often lack the necessary sensitivity and efficiency for rapid detection and reusability. In this study, we explore the potential of biphenylene (BPh), a 2D carbon allotrope, for TCDD detection by using density functional theory (DFT). To enhance the performance of BPh monolayers, we introduced metal decoration using Sc, Co, and Pd. Our analysis includes geometric structure, adsorption energy, partial density of states (PDOS), charge transfer mechanisms, and work function evaluation. The results reveal that while TCDD weakly adsorbs onto pristine BPh, metal decoration significantly improves the adsorption strength by enhancing charge transfer between TCDD and the BPh monolayer, leading to stronger orbital hybridization and more stable adsorption configurations. The Co-decorated BPh system exhibits the highest adsorption energy at -1.95 eV, followed by Pd (-1.77 eV) and Sc (-1.48 eV), with Co and Pd systems showing strong interactions but limited reusability due to prolonged desorption times. In contrast, the Sc-decorated BPh monolayer balances strong adsorption and efficient desorption, making it the most practical candidate for sensing applications. The recovery time for the Sc-decorated system at 500 K was calculated to be 1.59 s, and under UV light, it reduced to 89.2 ms, indicating optimal desorption efficiency. These findings suggest that Sc-decorated BPh monolayers hold promise for developing practical, reusable sensors for TCDD detection, providing both high sensitivity and reusability.

Quantum chemical pathways for the formation of 2,3,7,8-tetrachloro dibenzo-p-dioxin (TCDD) from 2,4,5-trichlorophenol: a mechanistic and thermo-kinetic study

CONTEXT

Dioxins, specifically 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (TCDD), are highly toxic dioxins known for their severe health impacts and persistent environmental pollutants. This study focuses on understanding the formation pathways of TCDD from its precursor molecule 2,4,5-trichlorophenol (2,4,5-TCP). In our exploration of reaction pathways from 2,4,5-trichlorophenol (TCP), we delve into three reaction mechanisms: free-radical, direct condensation, and anionic. Our findings highlight the significance of the radical mechanism, particularly propagated by H radicals, with a notable increase in dioxin formation around 900 K. These results are consistent with experimental observations indicating an increase in the conversion of trichlorophenol from 600 to 900 K in the non-catalytic gas phase reaction. Thermodynamic parameters (∆H, ∆S, and ∆G), reaction barriers, and rate constants (k) were calculated across a temperature range of 300-1200 K to support the findings and provide insights into the optimal temperature range for controlling dioxins during the incineration process.

METHOD

In this study, quantum chemical calculations were conducted using density functional theory (DFT) with the B3LYP functional and the 6-311 +  + G(d,p) basis set in Gaussian 16 software. Stationary points, including transition states (TS), were confirmed with frequency calculations. Intrinsic reaction coordinate (IRC) calculations ensured minimum energy paths between TS and products, visualized in GaussView 6.0 Program. Single-point energy calculations utilized a more precise basis set, 6-311 +  + G(3df,2p), for enhanced energy accuracy, incorporating zero-point vibrational energy (ZPE) and other energy corrections. These calculations were repeated over a temperature range of 298.15-1200 K at 1 atm pressure. Finally, rate constant (k) expressions associated with TCDD formation were determined using transition state theory (TST).

The role of CuCl on the mechanism of dibenzo-p-dioxin formation from poly-chlorophenol precursors: A computational study

A computational study is performed for the elucidation of the role played by CuCl in the condensation of two polychlorophenol molecules to yield PCDDs. The mechanism found consists of six sequential steps, which allow the final recuperation of the CuCl molecule, and applies for phenol molecules with an ortho chlorine. In the temperature range of 453-473 K (previously reported as adequate to diminish PCDDs formation in the post-combustion area), CuCl is able to softly retain chlorophenol molecules, mainly those less chlorinated. After a first HCl release, Cu(I) remains bonded to phenol oxygen atom, thus avoiding the formation of phenoxy radicals and the subsequent radical processes. A temperature raise up to 1200 K destabilizes the initial CuCl-chlorophenol complexes and causes that the rate limiting step change from the formation of the first oxygen bridge to HCl elimination. It has been checked that tetra and penta-chlorophenols undergo essentially the same reaction process of 2-chlorophenol. In view of our results and trying to arrive at a practical way to diminish the rate of formation of PCDDs, we propose that an extra addition of powdered CuCl to the post-combustion zone, cooled down to temperatures lower than 473 K, could act as an inhibitor in the formation of these pollutants.

New insight into the formation mechanism of PCDD/Fs from 2-chlorophenol precursor

Chlorophenols are known as precursors of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). The widely accepted formation mechanism of PCDD/Fs always assumes chlorophenoxy radicals as key and important intermediates. Based on the results of density functional theory calculations, the present work reports new insight into the formation mechanism of PCDD/Fs from chlorophenol precursors. Using 2-chlorophenol as a model compound of chlorophenols, we find that apart from the chlorinated phenoxy radical, the chlorinated phenyl radical and the chlorinated α-ketocarbene also have great potential for PCDD/F formation, which has scarcely been considered in previous literature. The calculations on the self- and cross-coupling reactions of the chlorophenoxy radical, the chlorinated phenyl radical and the chlorinated α-ketocarbene show that the formations of 1-MCDD, 1,6-DCDD, 4,6-DCDF, and 4-MCDF are both thermodynamically and kinetically favorable. In particular, some pathways involving the chlorinated phenyl radicals and the chlorinated α-ketocarbene are even energetically more favorable than those involving the chlorophenoxy radical. The calculated results may improve our understanding for the formation mechanism of PCDD/Fs from chlorophenol precursors and be informative to environmental scientists.