Thermal Oxidation Degradation of 2,2′,4,4′-Tetrabromodiphenyl Ether over Li_αTiO_<i>x</i>Micro/Nanostructures with Dozens of Oxidative Product Analyses and Reaction Mechanisms

Abstract

Flowerlike Li_αTiO_x micro/nanostructures were successfully synthesized to degrade 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) at 250-350 °C. The pseudo-first-order kinetics rate constant of the reaction at 300 °C was in the range of 0.034-0.055 min^-1. The activation energy was as low as 39.9-48.1 kJ/mol. The excellent performance attained over Li_αTiO_x was attributed to Li dopant having the electron-donating effect, which enhanced the oxygen species mobility. The oxidative reaction was believed to be the dominant degradation pathway following the Mars-van Krevelen mechanism, being accompanied by the weak hydrodebromination occurrence generating the trace mono- to tri-BDEs. More than 70 types of oxidation products containing diphenyl ether backbone, single-benzene rings, and ring-opened products were detected by GC-MS with derivatization, ESI-FT-ICR-MS, and ion chromatography. An increase in the number of ring-cracked oxidative products under prolonged reaction was observed by ESI-FT-ICR-MS analysis according to the van Krevelen diagram. In the oxidative reaction, a series of oxidative products, such as OH-tri-BDEs and OH-tetra-BDEs, first formed via the nucleophilic O^2- attack and subsequently transformed into dibromophenol, tribromophenol, and benzenedicarboxylic and benzoic acids, etc. They could be further attacked by electrophilic O₂^- and O^- and completely cracked to small molecules such as formic, acetic, propionic, and butyric acids.