De novo synthesis of polychlorinated dibenzo-p-dioxins and dibenzofurans - Proposal of a mechanistic scheme

This paper presents a chemical reaction scheme for de novo synthesis of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) from carbon in fly ash. The reaction scheme consists of the following mechanistic steps: (1) gaseous O-2 chemisorbs upon metallic sites: O-2 + 2Cu --> 2CuO; (2) the oxygen is transferred to a free site in the carbon structure and the carbon is oxidized to a gaseous product: C-f + CuO --> CO + Cu; (3) in the course of carbon gasification, the decomposition of the carbon structure also produces some small aromatic compounds: C (graphitic layer) + aO(2) --> bCO(2) + cCO + dAr. These aromatic compounds include PCBz, PCPh and PCB which then undergo the Ullmann coupling reactions leading to PCDD/F formation. Alternatively, PCDD/F could be formed directly from the decomposition of the carbon structure as a consequence of the in-plane oxygen complexes: C (graphitic layer) + aO(2) --> bCO(2) + cCO + dAr + e PCDD/F; (4) halogenation/dehalogenation on carbon structure, aromatic compounds and DD/F structure may occur concurrently in the above chemical transformations; and (5) decomposition of PCDD/Fs. Major experimental observations in de novo synthesis can be rationalized using the reaction scheme. Experimental techniques for further investigations are discussed. Copyright (C) 1996 Elsevier Science B.V.