Computational modelling of de novo synthesis of Dibenzofuran: oxidative pathways of Pyrene and Benzodibenzofuran

摘要

The dominating route to polychlorinated Dibenzo-p-dioxin and Dibenzofuran formation in the “cold zones” of flue gas cleaning systems of municipal solid waste incinerators is the so-called de novo synthesis, that is, carbonaceous matrix burnoff with simultaneous oxidation and chlorination reactions. Pyrene (1) and Benzodibenzofuran (2) were chosen as the model compounds of carbonaceous material present in fly ash. Possible routes of Dibenzofuran formation by oxidative pathways of compounds (1) and (2) were investigated by theoretical calculations at the density functional theory level. The key intermediate peroxy radical, formed by reaction with molecular oxygen, can follow three main paths leading to Dibenzofuran. In the kinetically favourite path, the highest energetic barriers (25–30 kcal mol?1) are encountered in the steps where CO molecules are released from ketene-like structures. These findings agree with previously reported temperature-programmed desorption results on CO desorption. Moreover, along this path, phenanthrene and biphenyl intermediates are formed, in agreement with the detection of these products in previously reported experimental Pyrene oxidation. Along the preferred path, different steric constraints in compounds (1) and (2) play a role in determining the relative stability of the intermediates, while they have less influence on the energetic barriers. As a consequence, compounds (1) and (2) should present similar kinetic behaviour as they present similar energetic barriers.