Photoreductive dechlorination of chlorinated benzene derivatives catalyzed by
ZnS nanocrystallites
Yuji Wada,a Hengbo Yin,b Takayuki Kitamuraa and Shozo Yanagida*a
a Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
b Department of Chemical Engineering, Shenyang Institute of Chemical Technology, Shengyang, China
Received (in Cambridge, UK) 3rd August 1998, Accepted 28th October 1998
ZnS nanocrystallites effectively enhanced photo-reduction
of chlorinated benzene derivatives in the presence of
triethylamine as a sacrificial electron donor under UV
irradiation (l > 300 nm), leading to selective and stepwise
dechlorination to give benzene at the final stage.
The dechlorination proceeded at a relatively high rate even
without ZnS nanocrystallites. The absorption spectrum meas-
ured for the mixture of 1,4-dichlorobenzene and TEA showed
the appearance of a shoulder at the long wavelength edge of the
absorption of 1,4-dichlorobenzene, suggesting the formation of
the exciplex between the two compounds. The formation of the
exciplex should contribute to the photochemical dechlorination
without the catalyst.8,9 On the other hand, the photocatalytic
dechlorination in the presence of ZnS nanocrystallites should
mainly proceed through the excitation of ZnS, since ZnS
nanocrystallites absorb most light in this region when they are
present in the system.
Photochemical detoxification of halogenated compounds, such
as PCB, dioxin, and DDT, has been attracting much attention
because this can be regarded as a promising process to eliminate
C–Cl bonds under environmentally relevant and mild condi-
tions. In particular, TiO2-catalyzed photoprocesses involving
oxidation and reduction in aqueous systems have been ex-
tensively investigated.1 In such systems, however, photo-
oxidation proceeds through the formation of hydroxyl radical
(HO•), leading to unavoidable formation of unknown photo-
products especially from polychlorinated compounds.2,3 Fur-
ther, the rate of such photodegradation often slows in the case of
polychlorinated compounds because they are electron-deficient
molecules, i.e. their oxidation potentials are very positive in
nature, showing resistance to the electrophilic attack of HO•.4
Therefore, reductive processes with semiconductors should be
investigated practically for the purpose of detoxification of
chemicals.5,6
In our earlier report, ZnS nanocrystallites prepared in DMF
act as a photocatalyst for the two-electron reduction of CO2 to
HCOOH or CO in the presence of triethylamine.7 The photo-
generated electrons on ZnS nanocrystallites possess such a high
reducing power ( < 22.2 V vs. SCE) that the substrates are
subject to successive two-electron reduction, causing the
stepwise dechlorination. This fact suggests that the anion
radicals first formed should be readily dechlorinated to the
radicals, which may undergo further reduction and protonation.
The electrons can be supplied by photooxidation of TEA. The
resulting TEA+• works as
a
proton source giving
In this paper, we show that photoreduction induces selective
dehalogenation of chlorinated benzene derivatives, providing
more favorable dehalogenation processes than the photo-
oxidative ones. In particular, ZnS nanocrystallites stabilized in
N,N-dimethylformamide (DMF)7 catalyze photoreduction of
chlorinated benzene derivatives, leading to their step-wise
dechlorination under UV-irradiation at ambient temperature.
A DMF solution of ZnS nanocrystallites was prepared by the
reaction of Zn(ClO4)2 with H2S under cooling with ice and
water as described elsewhere.7 Photoreactions were carried out
under cooling with water by UV irradiation of a DMF solution
(2 ml) containing a substrate (25 mM), ZnS nanocrystallites (2.5
mM in diatomic concentration as ZnS) and triethylamine (TEA,
1 M) in a Pyrex glass tube using a 500 W high pressure mercury
lamp. The reaction mixtures were analyzed by gas chromatog-
raphy with a fused silica capillary column (HiCap-CBP20, 25 m
3 0.2 mm, Shimadzu) using dodecane as an internal stan-
dard.
[Et2NHCHCH3]• which can also function as a good electron
source in the system.7
In Table 1 are listed the conversions and the product
distributions for the photodechlorination of various chlor-
obenzene derivatives observed in the presence of ZnS nano-
crystallites with their reduction potentials.10,11 The distribution
of the products observed for the reactions of polychlor-
obenzenes well confirmed the successive and stepwise de-
chlorination reactions. For example, in the reaction of
1,2,3,4-tetrachlorobenzene, 1,2,4-trichlorobenzene formed
preferentially is dechlorinated at the 2 position giving 1,4-di-
Fig. 1 shows time conversion plots of the photoreaction of
1,4-dichlorobenzene. The concentration of 1,4-dichlorobenzene
decreased during irradiation with and without ZnS nano-
crystallites. However, the rate of the conversion in the presence
of ZnS nanocrystallites was twice that at the beginning and
several times faster at the later stages than that in the absence of
ZnS crystallites. In parallel with the consumption of 1,4-di-
chlorobenzene the formation of chlorobenzene was observed
and benzene was formed at the final stage, i.e. after the
induction period of 1 h with the decreased rate of formation of
chlorobenzene, indicating that 1,4-dichlorobenzene is de-
chlorinated successively to benzene through chlorobenzene.
Hydrogen was formed competitively but in a small amount (2–3
mmol in 5 h) in the presence of ZnS nanocrystallites. No other
byproducts, such as biphenyl derivatives and chlorophenol,
were detected by GC analysis, showing a consistent material
balance between the consumed substrate and the products.
Fig. 1 Time profile of the photocatalytic dechlorination of 1,4-di-
chlorobenzene by ZnS nanocrystallites. In the presence of ZnS: 1,4-di-
chlorobenzene (5), chlorobenzene (:), benzene (/), hydrogen (-); in the
absence of ZnS: 1,4-dichlorobenzene (2), chlorobenzene (Ω), benzene
(.), hydrogen (8).
Chem. Commun., 1998, 2683–2684
2683
Wada, Yuji
