Directed regulation mechanism on chlorine substitution of PCDD/Fs isomers based on quantum chemical computation

Chlorine substitution has been considered as one of the key steps of polychlorinated dibenzodioxin/furan (PCDD/Fs) generation. The introduction of oxygen carriers (OCs), especially in chemical looping combustion (CLC), provides the platform of directed regulation for the chlorine substitution process.

Density functional theory (DFT) calculations with code VASP 5.4 were employed to investigate the free energy of PCDD/Fs adsorption on different surfaces. 12378-PCDD, which is the product of a one-step chlorine substitution for toxic 2378-PCDD, has been selected as the calculation case, and the regulation mechanisms on the inter-isomeric conversion of 12378-PCDD were identified by calculating the energy barrier and action angle.

It was found that the chlorine substitution of 12378-PCDD, particularly in 4# position, 9# position, and 6# position, emerged a tendency to increase the difficulty in turn, which conforms to the principle of distal preference. Besides, the influence from CaO adsorption on the crystalline surface of the iron-based oxygen carrier (OC) has been analyzed and it was verified that CaO adsorption can significantly increase the energy barrier for the chlorine substitution of 12378-PCDD. Meanwhile, the action angle was proposed to evaluate the parameters of adsorption process, and the adsorption of CaO can not only change the action angle between the 12378-PCDD molecule and the lattice surface, but also can modulate the energy barrier order of chlorine substitution among PCDD isomers. In addition, the loading component modulation was carried out to further confirm the feasibility of modulating the chloride substitution pathway, which proved the influence degree of loading component. And accordingly, the stretching analysis of the inactive component provides a theoretical basis for the subsequent study of the directional regulation of the PCDDs isomer generation pathway. Finally, the chlorine substitution rules and directed regulation mechanisms of PCDD/Fs isomers were obtained, which provides a modification direction for the structural components of OCs.