Thermal dechlorination of PCB-209 over Ca species-doped Fe2O3

Abstract

h i g h l i g h t s• 1% mol Ca species enhanced the activity of Fe 2 O 3 in the degradation of DecaCB.• Dominant Nona-MonoCB isomers were determined during hydrodechlorination reaction.• Thermodynamic and/or steric effects drive conversion.• Steric effect prevented the hydrodechlorination at ortho-positions. a r t i c l e i n f o b s t r a c tDegradation reaction of decachlorobiphenyl (PCB-209) was investigated over the synthesized Ca speciesdoped Fe 2 O 3 at 300°C. The 1%Ca-Fe 2 O 3 exhibited the highest activity among the four catalysts prepared with the pseudo-first order reaction at k obs = 0.103 min −1 . PCB-207, PCB-197, PCB-176, PCB-184, PCB-150, PCB-136, PCB-148, PCB-104, PCB-96, PCB-54, PCB-19, PCB-4 and PCB-1 were identified as the dominant isomers in their respective nonachlorobiphenyl (NonaCB) to monochlorobiphenyl (MonoCB) homologue groups. Analysis of the hydrodechlorination products indicated that dechlorination was much more favored on meta-and para-than on ortho-positions. The formation of significantly predominant NonaCB and octachlorobiphenyl (OctaCB) isomers was attributed to lower energy principles and to the 90°dihe-dral angles of two aromatic rings which prevented the hydrodechlorination at ortho-positions. When the number of chlorine atoms is not more than 7, the steric effect supports the formation of predominant PCB isomers having chlorines at four ortho-positions. During the dechlorination of tetrachlorobiphenyl (TetraCB) formed to generate monochlorobiphenyl (MonoCB) isomers, the chlorine atoms fully substituted at the ortho-positions have to be successively removed, with the first two dechlorinations preferentially occurring at the two different benzene rings. This is dissimilar to that of octachloronaphthalene in which the hydrodechlorination reaction happened preferentially at ortho-position due to the existence of steric effects. The opposite roles of the steric effect in ortho-position between PCB-209 and PCN-75 might be due to the difference of the π -conjugated plane caused by the dihedral angle of 90°and 0°of the two aromatic rings.

Mechanistic Considerations on the Hydrodechlorination Process of Polychloroarenes

“…Along with metallic Ca, other Ca compounds were tested in a hydrodechlorination process: Ca(OH) 2 [92], CaO [93][94][95] or CaSiO 3 [95]. Although most of these studies favored a radicalic mechanism, Gao and coworkers [95] discussed alternative pathways that involved either an electron transfer (with subsequent formation of a radical-anion that expels the chlorine anion and form the aryl radical) or a direct hydrogen transfer (in an S RN 1-type mechanism).…”

Synthesis of three crystalline forms of Al2O3 featuring rod-like fibers and their effect on the gaseous degradation of 1-chloronaphthalene

“…It has been reported that competition between hydrodechlorination and oxidation reactions often occurs during the degradation of chlorinated aromatic hydrocarbons over metal oxides. 10,61,62 Several studies have reported that the surface oxygen of metal oxides can attack the aromatic ring, thereby resulting in oxidation reactions. 8,12 Based on the characteristics of the XPS spectra, reactive oxygen species, such as O 2 In order to further determine the oxidation products, the produced organic acid products after the degradation of CN-1 over γ-Al 2 O 3 were detected by IC.…”