Ci Jl

CßHgClg lindane benzene

Figure 5. Transformation pathway of lindane to benzene. Reprinted from Applied Catalysis B: Environmental, Vol. 18, Schtlth and Reinhard, "Hydrodechlorination and Hydrogenation of Aromatic Compounds over Palladium on Alumina in Hydrogen-Saturated Water," pp. 219, Copyright 1998, with permission from Elsevier Science.

The chlorinated alkenes are likely to follow one of three basic reaction pathways.

Pathway 1: formation and reaction of radical ethyl species Pathway 2: dehalogenation, followed by hydrogenation of unsaturated

Pathway 3: hydrogenation of the double bond, followed by dehalogenation.

Pathway 1 is implied by Muftikian's observation of butane and hexane from a saturated aqueous TCE solution with Pd/Fe. (Muftikian et al. 1995) However, these products were only observed in this singular case, indicating that the radical reaction may occur only under special conditions (e.g. saturated solution).

Pathway 2 (shown below in Figure 6) is probable for most cases, given that chlorinated ethanes (i.e. from Pathway 3) have not been observed. In addition, Schuth reports that hydrodechlorination generally occurs faster than hydrogenation for halogenated aromatics (Schuth and Reinhard 1998); if the same holds true for halogenated alkenes, Pathway 2 would be kinetically favorable over Pathway 3. This is corroborated by Schreier, who found that the reaction rate of 1,1,2,2-TCA was an order of magnitude slower than that of PCE (Schreier 1996); this indicates that saturation of the double bond is not the first step in the transformation of PCE to ethane. The common observation of ethylene as a reactive intermediate is further evidence for Pathway 2. From the two cases where DCE and/or VC were observed (with Pd/Fe and pure Pd powder), it is clear that sequential dehalogenation associated with the second pathway does occur under some conditions. However, Lowry noted that, in the case of metallic Pd powder, DCE and VC were produced and disappeared simultaneously; this implies multiple dechlorination of TCE to both DCE and VC, rather than sequential

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