Small Particle Size Magnesium in One-pot Grignard-Zerewitinoff-like Reactions under Mechanochemical Conditions: On the Kinetics of Reductive Dechlorination of Persistent Organic Pollutants (POP's)

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Small particle size base metals are produced in a ball mill mechanochemical (MC) reactor. During ongoing milling they react at room temperature simultaneously and in a single process step, to rapidly, and highly selectively reduce by dechlorination persistent polychlorinated organic pollutants, such as polychlorinated biphenyls (PCBs). In the presence of a low acidic hydrogen donor, reduction is to the parent hydrocarbons. Whether in a purified form or as mixtures, the polychlorinated organic pollutants can be destroyed both effectively and in an environmentally friendly manner. This non-incineration and non-thermal process extends even to contaminated materials, such as soils, filter dusts and the like. A kinetic study utilizing dichloro- and monochlorobenzene (DCB, MCB) as the model pollutants and excess magnesium and n-butylamine as reagents in a laboratory centrifugal ball mill at 25 °C, was carried out in order to investigate the reaction mechanisms for MC dehalogenation reactions employing magnesium and slightly acidic hydrogen donors. The focus of the study was on polyhalogenated aromatic POPs, such as PCBs or hexachlorobenzene which have chlorine-aromatic ring carbon bonds, with higher bond strength than chlorine-aliphatic carbon bonds. Analysis of the findings strongly suggested that one-pot, consecutive Grignard-Zerewitinoff-like reactions occur: first, formation of the Grignard intermediates from DCB or MCB, respectively, and then, in a stepwise manner, their protonation to monochlorobenzene or benzene, respectively, by the amine. Furthermore, a rationale is derived for the observed complete reductive dechlorinations to benzene, formed at approximately 100 % yield (mole/mole, based on DCB or MCB). The rate constants for the formation of intermediates 3-chlorophenylmagnesium chloride and phenylmagnesium chloride were calculated. These correspond well to similar data previously reported elsewhere. The time-dependent development of the actual reactive surface of the small-sized magnesium particles during grinding was assessed.
Original languageEnglish
Title of host publicationEnvironmental Applications of Nanoscale and Microscale Reactive Metal Particles
EditorsC. L. Geiger, K. M. Carvalho-Knighton
Number of pages16
PublisherACS Publications
Publication date20.12.2009
ISBN (Print)9780841269927
ISBN (Electronic)9780841224674
Publication statusPublished - 20.12.2009