Application of a dioxo-molybdenum(VI) complex anchored on TiO2 for the photochemical oxidative decomposition of 1-chloro-4-ethylbenzene under O2

Article abstract of DOI:10.1007/s11243-011-9547-2

The catalytic activity of dioxo-molybdenum(VI)- dichloro[4,4'- dicarboxylato-2,2'-bipyridine] covalently anchored through the carboxylate function to the surface of TiO₂ has been tested for the oxidative degradation of 1-chloro-4- ethylbenzene in MeCN solution under argon and UV irradiation (λ = 254 nm). After 4-5 h of photochemical reaction, the Mo complex was reoxidized in the presence of O₂ in the dark, and then the reaction was continued under argon. The reaction proceeds by the intermediate formation of 40-chloroacetophenone that undergoes further decomposition to chlorobenzene, plus small amounts of oxygen-containing organochlorine compounds, CO₂ and H₂O. Similar results were obtained for the decomposition of 4'-chloroacetophenone under the same conditions, which also gave chlorobenzene as one of the main products. The ratio of [final product]/ [Mo complex] increases during the decomposition of 1-chloro-4-ethylbenzene (up to 350-400% for 30-35 h of reaction), which provides evidence of a catalytic process. The probable photochemical reactions are discussed. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11243-011-9547-2

Transition Met Chem (2011) 36:897–900
DOI 10.1007/s11243-011-9547-2
Application of a dioxo-molybdenum(VI) complex anchored
on TiO2 for the photochemical oxidative decomposition
of 1-chloro-4-ethylbenzene under O2
•
Robert Bakhtchadjian Samvel Tsarukyan
•
•
Joel Barrault Fernando O. Martinez
•
•
Levon Tavadyan Nelson J. Castellanos
Received: 28 June 2011 / Accepted: 27 September 2011 / Published online: 15 October 2011
Ó Springer Science+Business Media B.V. 2011
Abstract The catalytic activity of dioxo-molybdenum(VI)-
dichloro[4,40-dicarboxylato-2,20-bipyridine] covalently anc-
hored through the carboxylate function to the surface of TiO2
has been tested for the oxidative degradation of 1-chloro-4-
ethylbenzene in MeCN solution under argon and UV irradi-
ation (k = 254 nm). After 4–5 h of photochemical reaction,
the Mo complex was reoxidized in the presence of O2 in the
dark, and then the reaction was continued under argon. The
reaction proceeds by the intermediate formation of 40-chlo-
roacetophenone that undergoes further decomposition to
chlorobenzene, plus small amounts of oxygen-containing
organochlorine compounds, CO2 and H2O. Similar results
were obtained for the decomposition of 40-chloroacetophe-
none under the same conditions, which also gave chloroben-
zene as one of the main products. The ratio of [final product]/
[Mo complex] increases during the decomposition of
1-chloro-4-ethylbenzene (up to 350–400% for 30–35 h of
reaction), which provides evidence of a catalytic process. The
probable photochemical reactions are discussed.
Introduction
The oxidative degradation of persistent organic pollutants
(POP’s) including organochlorine compounds may be real-
ized by different methods [1–3], including photocatalytic
[4, 5] and non-catalytic [6, 7] processes. Usually, oxidants
such as ozone, hydrogen peroxide, potassium permanganate,
dichromate or Fenton reagent, with or without different
combinations of UV or visible irradiation, are used [8–10].
Among the metal oxide catalysts, TiO2 in combination with
UV or visible light presently seems the most effective for
catalytic oxidative decomposition of organochlorine com-
pounds, due to its ability to mediate multielectron transfer
processes, stability, high surface area, low toxicity, avail-
ability etc. [11–13]. However, to our knowledge, there is no
widely applied catalytic method for the oxidative destruction
of chlorinated POP’s by direct use of molecular oxygen as a
non-expensive direct oxidant. Recently, TiO2 was success-
fully used as a support material for the syntheses of cova-
lently grafted organometallic entities such as dioxo-Mo(VI)-
dihalo[4,40-dicarbixylato-2,20-bipyridine] complexes [13–15].
The oxo, dioxo, peroxo or l-oxo Mo complexes with different
ligands are known as oxygen transfer agents under homoge-
neous conditions, in various oxidation processes of organic
molecules [16, 17]. The primary oxidant could be DMSO, N2O
or O2. However, the catalytic process, especially with dioxy-
gen, may be limited because of the rapid formation of dimer
l2-dioxo-Mo(V) complexes that are totally inert for reoxida-
tion [18]. Fortunately, this problem was eliminated when
the dioxo-Mo complexes with different ligands were grafted
on the surface of a solid matrix with covalent bonds. In
this context, the recently reported dioxo-molybdenum(VI)-
dichloro[4,40-dicarboxylato-2,20-bipyridine] complex cova-
lently anchored on the surface of TiO2 (Scheme 1) shows
attractive oxidative properties.
R. Bakhtchadjian (&) ꢀ S. Tsarukyan ꢀ L. Tavadyan
Nalbandian Institute of Chemical Physics, National Academy
of Sciences of Armenia, Paruyr Sevak st., 5/2, 0014 Yerevan,
Republic of Armenia
e-mail: rbakh@free.fr
J. Barrault ꢀ F. O. Martinez
Laboratoire de Catalyse en Chimie Organique (LACCO),
UMR 6503, CNRS, 40, Av. du recteur Pineau, 86022,
Poitiers, France
F. O. Martinez ꢀ N. J. Castellanos
´
Escuela de Quimica, Centro de Investigacion en Catalisis
(CICAT), Universidad Industrial de Santander,
´
´
Km 2 vıa El Refugio, Piedecuesta, Santander, Colombia
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Transition Met Chem (2011) 36:897–900
O
O
O
Cl
Mo
Cl
N
N
O
TiO2
O
O
Scheme 1 Dioxo-molybdenum(VI)-dichloro[4,40-dicarboxylato-2,20-
bipyridine] complex covalently anchored on the surface of TiO2
This material was fully characterized by NMR, FTIR,
elemental analysis, etc. [15]. It is advantageous for oxidation
processes in comparison with non-grafted complexes in two
aspects. Firstly, covalent linking of the complex isolates the
metal centers and avoids the possibility of formation of the
l2-dioxo-Mo(V) dimer, and secondly, it promotes photo-
electron transfer to molybdenum via the TiO2 solid matrix.
Its catalytic activity was tested for oxidation of substrates
such as triphenylphosphine, tetraline and ethylbenzene
[13–15].
Fig. 1 Schematic diagram of the experimental installation: 1 metallic
microreactor, 2 quartz reaction vessel, 3 water jacket, 4 UV lamp, 5
quartz window, 6 manometer, 7 tubes, 8 thermostat
dehydrated with acetone and characterized by FTIR, 1H and
13C NMR and 13C MAS NMR spectroscopic methods and
shown to be identical with the previously reported com-
pound [15, 19].
In the present work, the catalytic activity of this material
was tested in the photochemical oxidative decomposition
of 1-chloro-4-ethylbenzene under UV irradiation, using
molecular oxygen as oxidant. This photochemical oxidation
of 1-chloro-4-ethylbenzene under UV and visible light may
be considered as a model reaction for the degradation of
persistent organochlorine compounds (pesticides and other
POP’s). We hoped also to throw light on the mechanism of
oxo-transfer interactions during the photochemicalprocesses.
The method for anchoring of 2,2-bipyridine-4,40-dicar-
boxylic acid on TiO2 has been recently reported [15]. The
procedure involves reaction of TiO2 and hexamethyldisi-
lazane in order to obtain a silylated TiO2 surface that is
then transesterificated with the diacid. The ligand anchored
onto the matrix was characterized by 13C MAS NMR.
The reaction of MoO2Cl2 with 4,40-dicarboxylato-2,20-
bipyridine anchored on TiO2 was done by treating the
anchored complex with a THF solution containing the cal-
culated amount of MoO2Cl2. The characterization was done
with 13C MAS NMR, and the loading was estimated by TGA
analysis.
Experimental
Commercially obtained 1-chloro-4-ethylbenzene and 40-
chloroacetophenone (Aldrich) were kept under argon. Dis-
tilled MeCN (Merck) was dried using thermally activated
molecular sieves. TiO2 (Degussa, P25) with a specific area
of 45 m2/g, average primary particle size of 21 nm and
crystalline structures of 80% anatase and 20% rutile was
dehydrated before use at 60 °C and 5 9 10-4 bar for 48 h.
Experiments were carried out using a specially designed
metallic microreactor consisting of a quartz reaction vessel
(30 mL) equipped with a quartz window and an optical
junction for the UV and visible light irradiation, as shown in
Fig. 1. The source of the UV irradiation was a high-pressure
Hg lamp (LPT-230, 22 W, Russia) with distilled water
cooler, emitting a wavelength mainly at k = 254 nm.
2,20-bipyridine-4,40-dicarboxylic acid was prepared
according to a reported procedure [19] by the K2Cr2O7
oxidation of 4,40-dimethyl-2,20-bipyridine in concentrated
sulfuric acid (98.8%). The product (98% yield) was
All 13C MAS NMR spectroscopic data obtained from
our samples were identical with the previously reported
compounds [15].
Catalytic reactions
The reaction mixture consisted of 10 mL of deoxygen-
ated MeCN containing 5.96 9 10-3 mmol of 1-chloro-4-
ethylbenzene, plus 10 mg of the TiO2-anchored dioxo-
molybdenum(VI)-dichloro[4,40-dicarbixylato-2,20-bipyridine]
corresponding to 1.6 9 10-3 mmol of complex. The reac-
tion mixture was irradiated under UV light for 4–5 h with
stirring under Ar, and then O2 was flowed through the
reaction mixture in the dark for 2 h for regeneration of the
initial Mo(VI) complex. The irradiation was then restarted
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Transition Met Chem (2011) 36:897–900
899
under Ar. The procedure was repeated periodically until
completion of the experiment.
about 3.5 mol of chloroethylbenzene per mole of Mo
complex (Fig. 2).
In experiments with 40-chloroacetophenone (4 9 10-3
mmol in 10 mL of MeCN), the reaction conditions and
procedures were identical with those of 1-chloro-4-ethyl-
benzene and the total duration of the experiment was 30 h.
The reaction products were analyzed by gas chroma-
tography using a KXM-80 chromatograph (manufactured in
Russia) with the automatic integrator B-02 (flame ioniza-
tion detector) and a Varian 3860 (electron capture detector)
with a capillary column E2-GRTIP (30 m). Cl- ions and
organic peroxides were analyzed by chemical methods.
EPR spectra were recorded with a Varian-104 spectrometer
in order to test for Mo(V) ion-radical species in the solid-
state samples both before and after the reactions.
As shown in Fig. 2, a period 1 under UV irradiation and
Ar was followed by a period of 2 h under O2 in the dark
during which no apparent increase in chlorobenzene was
observed. When the resulting mixture was replaced under
Ar and irradiated (period 3), the chlorobenzene concen-
tration increased again, whereas the amount of ketone
remained low and constant. This sequence was repeated
twice more, exhibiting identical behavior each time.
The time profile depicted in Fig. 2 resembles that
reported for the oxidation of ethylbenzene under visible
light [15]. This strongly suggests that under UV irradiation,
an analogous prior oxidation to 40-chloroacetophenone is
plausible, followed by a heterolytic transformation into
chlorobenzene. Supporting evidence was obtained by sub-
mitting 40-chloroacetophenone to the same reaction con-
ditions and observing the formation of chlorobenzene as a
main product of its decomposition.
Results and discussion
Before addition of the supported dioxo-Mo complex, the
solution of 1-chloro-4-ethylbenzene in MeCN was thor-
oughly deoxygenated by bubbling Ar in the dark at 3 °C
for 2 h. After the addition of the complex, the reaction
mixture was kept under UV irradiation at an Ar pressure
of 1.2 Atmospheres at room temperature. The reaction
products were analyzed periodically both before and after
replacing Ar by O2 or O2 by Ar. After 7 h of UV irradiation
under Ar, chromatographic and chemical analyses showed
the formation of chlorobenzene, accompanied by smaller
quantities of ClC6H4C(O)Me, ClC6H4CH(OH)Me, CO2,
H2O, traces of ClC6H4CH(OOH)Me and other chlorinated
aromatic oxygen-containing organic compounds. Tests for
chloride ions in the reaction media were negative. The total
amount of transformed starting material corresponds to
The overall catalytic mechanism can be summarized as
follows. Under argon and UV irradiation, a stoichiometric
four-electron oxygen atom transfer occurs on the benzylic
carbon, necessitating 2 moles of dioxo-Mo complexes in
two distinct steps, the first one giving the benzyl alcohol
and the second giving the ketone. The ketone is then rap-
idly transformed into chlorobenzene by photocatalysis,
Fig. 2 Time evolution profiles of products: PhCl (filled circle),
ClC6H4C(O)Me (open square), ClC6H4CH(OH)Me (filled triangle)
on decomposition of 1-chloro-4-ethylbenzene. 1; 3; 5; 7: periods
under UV irradiation and Ar; 2; 4; 6: periods of O2 flow in the dark.
a = ([final product]/[Mo complex]) 9 100 (%)
Scheme 2 Proposed catalytic mechanism consistent with experimen-
tal data for the UV photochemical oxidative decomposition of 1-chloro-
4-ethylbenzene
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Transition Met Chem (2011) 36:897–900
Acknowledgments This work has been supported by the bilateral
CNRS-SCS Franco-Armenian Scientific program. The authors are
grateful to Dr. S. Minassian and Dr. L.Manucharova from Armenia
for their help in chromatographical analyses.
while in the dark and under O2, the reduced Mo(IV)-oxo
species is reoxidized to a Mo(VI)-oxo-peroxo entity. This
latter when placed under Ar and irradiated transfers two
oxygen atoms to give 1 mole of ketone per mole of
molybdenum complex. This mechanism is summarized in
Scheme 2.
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