α-elimination and hydrogenolysis to ethane (Fennelly and Roberts 1998; Song and
Carraway 2005). Degradation of COCs in a surface-mediated process results in the
formation of transition species like hydrocarbon moieties, in particular alkyls,
carbenes, carbenoids, and carbynes (Baltruschat et al. 1993; Bent 1996; Müller
et al. 1997). Thus, Lien and Zhang (2005) proposed that the formation of surface
ethylidyne on Pd/Fe particles can explain the direct reduction of 1,1,1-TCA in
ethane.
Due to the presence of an α,β-pair of chlorine atoms, 1,1,2-TCA and 1,2-TCA are
susceptible to react mainly via β-elimination. However, production of vinyl chloride
from 1,1,2-TCA has not been observed with iron nanoparticles, so vinyl chloride
must react rapidly to form ethane. Similarly, production of ethane from 1,2-TCA has
not been proven (Song and Carraway 2005). However, these two pathways are
strongly suspected to occur with iron particles, as a same mechanism have been
reported with zinc particles (Arnold et al. 1999).
Reduction of chlorinated ethylenes undergoes successive hydrogenolysis pathways. PCE is successively transformed into TCE, DCE isomers (1,1-DCE, cis-1,2DCE or trans-1,2-DCE), then vinyl chloride and finally ethylene (Vogel et al. 1987;
Orth and Gillham 1996). Roberts et al. (1996) highlighted the reductive elimination
of trans- and cis-1,2-DCE, resulting in the formation of acetylene as an intermediate
of the production of ethylene, and small amounts of vinyl chloride and ethane have
been observed. Conversely, 1,1-DCE and vinyl chloride only react to ethylene and
ethane. Later, Arnold and Roberts (2000) have shown that PCE and TCE can also
react by β-elimination to dichloroacetylene and chloroacetylene, respectively. The
authors proposed that the production of dichloroacetylene results from a
mono-σ-bonded vinyl intermediate at iron surface, whereas the formation of TCE
results from a mono-σ-bonded alkyl intermediate.
Dichloroacetylene reacts then via successive hydrogenolysis to chloroacetylene
and acetylene, with possible formation of ethylene and ethane as final products
(Arnold and Roberts 2000). Dechlorination by bimetallic particles involving indirect
reduction with adsorbed atomic hydrogen on the catalyst surface results in a higher
formation of saturated hydrocarbons instead of unsaturated hydrocarbons, and less
accumulation of chlorinated by-products. Therefore, ethane is the major degradation
end product of reduction of chlorinated ethylenes by bimetallic particles (Schreier
and Reinhard 1995; Lien and Zhang 2001; Schrick et al. 2002; Tee et al. 2005; Lien
and Zhang 2007).
Coupling reaction of acetylene, dichloroethyl radicals or carbenoids in C 4 compounds, and traces of C 3 , C 5 and C 6 compounds, have also been reported (Fennelly
and Roberts 1998; Arnold and Roberts 2000).
A summary of the general pathway of reduction of chlorinated benzenes is shown
in Fig. 6.4. Reduction of chlorinated benzenes by ZVI undergoes hydrogenolysis.
Hexachlorobenzene
is
firstly
transformed
into
pentachlorobenzene.
Pentachlorobenzene reacts to form 1,2,3,4-TeCB and 1,2,3,5-TeCB, and the end
product is 1,2,4-TCB. No other chlorobenzenes have been observed by Shih et al.
(2009). In a later study, the authors have shown the production of three
tetrachlorobenzene isomers, i.e., 1,2,3,4-TeCB, 1,2,3,5-TeCB and 1,2,4,5-TeCB,
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
327
Carraway 2005). Degradation of COCs in a surface-mediated process results in the
formation of transition species like hydrocarbon moieties, in particular alkyls,
carbenes, carbenoids, and carbynes (Baltruschat et al. 1993; Bent 1996; Müller
et al. 1997). Thus, Lien and Zhang (2005) proposed that the formation of surface
ethylidyne on Pd/Fe particles can explain the direct reduction of 1,1,1-TCA in
ethane.
Due to the presence of an α,β-pair of chlorine atoms, 1,1,2-TCA and 1,2-TCA are
susceptible to react mainly via β-elimination. However, production of vinyl chloride
from 1,1,2-TCA has not been observed with iron nanoparticles, so vinyl chloride
must react rapidly to form ethane. Similarly, production of ethane from 1,2-TCA has
not been proven (Song and Carraway 2005). However, these two pathways are
strongly suspected to occur with iron particles, as a same mechanism have been
reported with zinc particles (Arnold et al. 1999).
Reduction of chlorinated ethylenes undergoes successive hydrogenolysis pathways. PCE is successively transformed into TCE, DCE isomers (1,1-DCE, cis-1,2DCE or trans-1,2-DCE), then vinyl chloride and finally ethylene (Vogel et al. 1987;
Orth and Gillham 1996). Roberts et al. (1996) highlighted the reductive elimination
of trans- and cis-1,2-DCE, resulting in the formation of acetylene as an intermediate
of the production of ethylene, and small amounts of vinyl chloride and ethane have
been observed. Conversely, 1,1-DCE and vinyl chloride only react to ethylene and
ethane. Later, Arnold and Roberts (2000) have shown that PCE and TCE can also
react by β-elimination to dichloroacetylene and chloroacetylene, respectively. The
authors proposed that the production of dichloroacetylene results from a
mono-σ-bonded vinyl intermediate at iron surface, whereas the formation of TCE
results from a mono-σ-bonded alkyl intermediate.
Dichloroacetylene reacts then via successive hydrogenolysis to chloroacetylene
and acetylene, with possible formation of ethylene and ethane as final products
(Arnold and Roberts 2000). Dechlorination by bimetallic particles involving indirect
reduction with adsorbed atomic hydrogen on the catalyst surface results in a higher
formation of saturated hydrocarbons instead of unsaturated hydrocarbons, and less
accumulation of chlorinated by-products. Therefore, ethane is the major degradation
end product of reduction of chlorinated ethylenes by bimetallic particles (Schreier
and Reinhard 1995; Lien and Zhang 2001; Schrick et al. 2002; Tee et al. 2005; Lien
and Zhang 2007).
Coupling reaction of acetylene, dichloroethyl radicals or carbenoids in C 4 compounds, and traces of C 3 , C 5 and C 6 compounds, have also been reported (Fennelly
and Roberts 1998; Arnold and Roberts 2000).
A summary of the general pathway of reduction of chlorinated benzenes is shown
in Fig. 6.4. Reduction of chlorinated benzenes by ZVI undergoes hydrogenolysis.
Hexachlorobenzene
is
firstly
transformed
into
pentachlorobenzene.
Pentachlorobenzene reacts to form 1,2,3,4-TeCB and 1,2,3,5-TeCB, and the end
product is 1,2,4-TCB. No other chlorobenzenes have been observed by Shih et al.
(2009). In a later study, the authors have shown the production of three
tetrachlorobenzene isomers, i.e., 1,2,3,4-TeCB, 1,2,3,5-TeCB and 1,2,4,5-TeCB,
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
327