Selective oxidation of DDT by dioxygen on the dioxo-Mo(VI) complex anchored on a TiO2 under UV-irradiation

Article abstract of DOI:10.1016/j.catcom.2015.06.016

Oxidation of DDT (1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane) by the dioxo-Mo(VI) complex (dioxo-molybdenum(VI)-dichloro[4,4′-dicarboxylato-2,2′-bipyridine]), anchored on a TiO₂, in the presence of dioxygen under UV-irradiation (λ = 253.7 nm) in acetonitrile suspension has been examined. The reaction has catalytic character. Conversion of DDT (35% for 32 h reaction) to dicofol (2,2,2-trichloro-1,1-bis(4-chlorophenyl)ethanol, 21%) and a great number of other chlorinated and non-chlorinated products of the oxidative/reductive decomposition proceeds by the primary reaction of oxo-atom transfer from dioxo-Mo(VI) complex to bibenzylic-carbon atom of DDT. The catalytic cycle is performed via the re-oxidation of the reduced Mo(IV) into Mo(VI) by dioxygen in the "dark" reaction.

Full text of DOI:10.1016/j.catcom.2015.06.016

Selective Oxidation of DDT by Dioxygen on the Dioxo-Mo(VI) Complex
Anchored on a TiO2 Under UV-irradiation
R. Bakhtchadjian, L.A. Manucharova, L.A. Tavadyan
PII:
DOI:
Reference:
S1566-7367(15)30003-0
doi: 10.1016/j.catcom.2015.06.016
CATCOM 4366
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Catalysis Communications
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Accepted date:
10 March 2015
21 May 2015
22 June 2015
Please cite this article as: R. Bakhtchadjian, L.A. Manucharova, L.A. Tavadyan, Selective
Oxidation of DDT by Dioxygen on the Dioxo-Mo(VI) Complex Anchored on a TiO2 Un-
der UV-irradiation, Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.06.016
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ACCEPTED MANUSCRIPT
Selective Oxidation of DDT by Dioxygen on the Dioxo-Mo(VI) Complex
Anchored on a TiO2 Under UV-irradiation
R. Bakhtchadjian, L. A. Manucharova, L A. Tavadyan
Institute of Chemical Physics, National Academy of Sciences of the Republic of Armenia,
Paruyr Sevak st. 5/2, 0014 Yerevan, Republic of Armenia
 Corresponding author, email; robakh@hotmail.com
Abstract
Oxidation of DDT (1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane) by the dioxo-Mo(VI)
complex (dioxo-molybdenum(VI)-dichloro[4,4′-dicarboxylato-2,2′-bipyridine]), anchored on
a TiO2, in the presence of dioxygen under UV-irradiation (= 253.7nm) in acetonitrile
suspension has been examined. The reaction has catalytic character. Conversion of DDT
(35% for 32 h reaction) to dicofol (2,2,2-trichloro-1,1-bis-(4-chlorophenyl)ethanol, 21%) and
a great number of other chlorinated and non-chlorinated products of the oxidative/reductive
decomposition proceeds by the primary reaction of oxo-atom transfer from dioxo-Mo(VI)
complex to bibenzylic– carbon atom of DDT. The catalytic cycle is performed via the re-
oxidation of the reduced Mo(IV) into Mo(VI) by dioxygen in the “dark” reaction.
Keywords: catalytic oxidation; dioxo-Mo(VI)-complex anchored on a TiO2; DDT; dicofol
1. Introduction
Oxo-Mo complexes, bearing specific organic ligands, possess a capacity of the selective
oxidation of aryl alkanes, phosphines, olefins and alcohols [1-5]. It has been established that
the primary chemical process begins from the transfer of oxo-atom of these complexes to
substrates [4, 5]. Oxidizing agents, such as O2, H2O2, O3, DMSO and others, in certain
conditions, are able to re-oxidize the reduced metal atom, providing the catalytic cycle.
The catalytic activity increases, when these complexes are supported on the solid
substances or anchored on the solid matrix with covalent chemical bonds [6-9]. One of the
best examples of this is dioxo-Mo(VI)-dichloro[4,4′-dicarboxylato-2,2′-bipyridine], anchored
on TiO2 (P25) (Complex 1),
Fig. 1. Complex 1. Dioxo-molybdenum(VI)-dichloro[4,4′-dicarboxylato-2,2′-bipyridine],
anchored on TiO2.
the activity of which as a catalyst has been tested in oxidation of trimethylphosphine,
ethylbenzene, tetraline, diphenylmethan with O2 [7-10], as well as, in oxidation of chlorinated
hydrocarbons (1-chloro-4-ethylbenzene and 2,2-bis-(p-chloropheny)methane) [11, 12].
The selective oxidation of halogenated hydrocarbons are of interest in various areas, such
as chemical industry, green chemistry, organic syntheses [13]. Particularly DDT(1,1,1-
trichloro-2,2-bis-(4-chlorophenyl)ethane), mainly known as a pesticide and persistent
pollutant, also is a primary reactant for the production of dicofol (2,2,2-trichloro-1,1-bis-(4-
chlorophenyl)ethanol). In chemical industry the oxidation of DDT, directed to the production
of dicofol is performed as a multistep process, including: elimination of HCl from DDT,

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giving DDE (1,1-bis-(4-chlorophenyl)-2,2-dichloroethene); chlorination of DDE to tetramer
(dichlorodiphenyltetrachloroethane); hydrolysis of tetramer to dicofol, using para-toluene
sulfuric acid [14].
The aim of this work is to evaluate the feasibility of a direct oxidation of DDT with
dioxygen under “mild” conditions, by the application of Complex (1), under UV-irradiation,
in order to achieve the selective formation of oxygenated products, other than its total TiO2
induced photodegradation obtained in water.
2. Experimental
2.1 Experimental setup and materials
The specially designed experimental setup consisting of photochemical reactor (quartz, 30
mL) with a magnetic stirrer and optical junction for UV-irradiation (= 253.7nm), high
pressure Hg-lamp (DRT- 230, Russia), as well as the procedure of measurements were
described in [11].
DDT, dicofol, DDE, and nearly all other chemicals (Sigma-Aldrich) were used as received.
Acetonitrile (Merck, Germany) was distillated by an ordinary procedure, in argon atmosphere.
The Complex (1), was synthesized by the method described in [10, 15] and characterized
mainly by 13C CAP NMR method [10,16]. The amount of the anchored complex on the TiO2,
determined by TGA analysis (TG- 2950 Нi Re), was about 1,6 x10-4 mol per gram.
2.2. Procedures
Reaction mixture was a suspension of DDT (5,73 mg =1.6 x 10-5 mol ) in 10 mL acetonitrile,
containing 10 mg of the Complex (1). Each experimental cycle was consisted of two
consecutive periods: in the first period, the reaction mixture while stirring was exposed under
UV-irradiation, in argon atmosphere and in the absence of dioxygen during 6-7 h, then, in the
second period, the reaction was carried out without UV-irradiation and in the presence of
dioxygen (“dark” period, 2-2,5 h). Before the beginning of a new experimental cycle,
dioxygen was thoroughly removed from the reaction mixture during 1.5-2 h. All experiments
were carried out at the room temperature.
Identification and analysis of the products were carried out by gas-liquid chromatography
(electron capture detector), using column with SE-30 sorbent, the control analysis were
carried out by chrommass-spectrometer DANI Master DC TOF, using capillary column (30
m) coated by DB-ALC2. Analysis of Cl-ions, were performed by chemical methods
(argentometric way by the Mohr method).
3 . Results and Discussion
Experimental results of the consumption of DDT in 5 consecutive cycles are presented in
Fig. 2. The horizontal sections on the curve (a) correspond to the second periods, the aim of
which is re-oxidation of Mo(IV), formed in the first period, into Mo(VI) with dioxygen. The
curve (b) corresponds to the photochemical oxidation of DDT on the surface of “pure” TiO2
(without anchored complex on TiO2), in the presence of dioxygen. Note, the amount of the
“pure” TiO2 in reaction mixture was twice that of experiments with the Complex (1). The
simple comparison of two curves indicates a significant increase of the consumption of DDT
in the presence of the anchored complex.

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Fig.2. Time profiles of the consumption of DDT(C x10-5 mol) under UV- irradiation: (a) - in
the presence of the anchored Complex (1); (b) - in the presence of the “pure” TiO2 (in
amounts 2 times more than Complex (1)) and O2.
The turnover number, a molar ratio [DDT] / [anchored complex], at the last experimental
cycle is about 3,7 (corresponding to 35% conversion in 32 h reaction) (Table 1.). Under the
same conditions, but in the absence of the anchored complex, the photooxidation of DDT on
“pure” TiO2 is a relatively slow process (13% conversion of DDT for the same time period,
Fig. 2. b). The above facts indicate the catalytic character of the overall process in the
presence of the anchored Complex (1).
Table 1. Experimental data of the dependence of the turnover number [DDT]/[anchored
complex] and yield of dicofol on the number of experimental cycle (the reaction time (h) is
given in parentheses).
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Number of the experimental cycle
Turnover number
1
2
3
4
5
0.63 (5) 2.13(11) 2.58(17) 3.38(23)
3.68(32)
Yield of dicofol (%)
9.5

-----------------------------------------------------------------------------------------------------------------
The reaction products include large spectra of the chlorinated and non-chlorinated organic
compounds (C2-C14), as well as the products of the complete destruction, such as CO2, H2O,
HCl. Among them, taking into account the results of the chromatographic analyses and
overall amount of the analyzed Cl- ions, the sum of a dozen products of the complete
dechlorination was estimated as about 53-57%. Correspondingly, other products (43-47%) are
non-dechlorinated and partially dechlorinated compounds. The main product of the reaction
is dicofol (21 %), playing also the role of an intermediate for a number of other chlorinated
and non-chlorinated products. Note, under the same conditions, but in the absence of the
anchored complex ( “pure” TiO2), the formation of dicofol was very slow, and the reaction
gives a more complex mixture of other oxidative/reductive decomposition products, including
the products of the complete degradation in small amounts.
The second main product of the reaction is DDE (9%). All other products of the oxidative
decomposition of DDT, nearly are the same as observed in the photochemical decomposition
of dicofol on TiO2 in the presence of oxygen [17]. Among them 4,4'-dichlorobenzophenone
(3-4%), prevails over others in our experiments. However, the composition of products in the
presence of the anchored complex on a TiO2 has some obvious differences, than referred [17].
The reaction mixture contains also products of the reductive decomposition, such as DDD
(1,1-dichloro-2,2-bis-(p-chlorophenyl)ethane), DDM (2,2-bis-(p-chloropheny)methane),
diphenylmethane, and others, for a total 13-14%. One should note the similarity in the
composition of the reaction products between the enzymatic oxidation of DDT and the results
of the present work. For example: in biodegradation of DDT by so called White Rot Fungus

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Phanerochaete chrysosporium, the formation of the oxidative (2,2-dichloro-1,1-bis(4-
chlorophenyl)ethanol and 4,4'-dichlorobenzophenone), as well as reductive degradation
(DDD, DDM) were identified [18].
The formation of dicofol can be rationalized in terms of the oxygen atom transfer process
[4,5,9-12]. The suggested catalytic cycle is depicted in Scheme 1. A primary reaction between
DDT and Complex (1) gives dicofol and a reduced Mo(IV)-oxo Complex (2). In the second
period, in the “dark”, the Complex (2) is re-oxidized by O2 to give Mo(VI)-oxo-peroxo
Complex (3). The first period, of the second cycle under argon and UV-irradiation allows the
transfer of two oxygen atoms to DDT, one peroxo and one oxo, to give two molecule of
dicofol, regenerating Mo(IV)-oxo Complex (2) (reaction 3 and 1).
Scheme 1
(1)
where L is 4,4′-dicarboxylato-2,2′-bipyridine
(2)
(3)
An increase of the turnover number, as well as the yields of dicofol in the second and
further cycles (Table 1), probably, are related to the reactions of Complex 3, enable to transfer
twice more oxygen atoms to DDT, than Complex 1.
As an indirect evidence of the reactions (1-3) may be served a number of examples of the
analogous oxidation processes, of the chlorinated (1-chloro-4-ethylbenzene, 4,4'-
dichlorodiphenylmethane) [11,12] and non-chlorinated hydrocarbons (ethylbezene,
diphenylmethane) [9, 10], in same or similar conditions. In all mentioned cases, the oxo-atom
transfer to the benzylic or bibenzylic carbon atom is considered as the main primary reaction,
resulting the formation of the corresponding alcoholic compound.
4. Conclusions
The observed reaction of the direct photocatalytic oxidation of DDT with dioxygen opens
an attractive opportunity for the elaboration of an alternative method of the selective oxidation
of DDT under “mild” conditions, using the anchored complexes, containing the transition
metal oxo-atoms, on a TiO2, under UV-irradiation or sunlight. Apparently, it may be assumed
a complex mechanism of the oxidation of DDT with O2 in the presence of the Complex (1),
under UV-irradiation, including a combination of the primary transfer of oxo- or peroxo-atom
of oxygen from Mo(VI) to the substrate accompanied with a number of further
decomposition reactions.
Acknowledgments

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The authors are grateful to Dr. H. Arzoumanian (CNRS, Aix-Marseilles University, France)
for his valuable counsels, useful discussions and interest in the study.
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GRAPHICAL ABSTRACT
Reaction of O=MoVI(O)(Cl2)|[L]/TiO2 with DDT, L is 4,4′-dicarboxylato-2,2′-bipyridine.

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Highlights
. MoVIO2Cl2L/TiO2 complex is a catalyst for photooxidation of DDT by O2.
. The main product is dicofol (21%) and the conversion is 35% for 32 hours reaction.
. The oxo-atom transfer of MoVI to the bibenzylic-carbon of DDT is primary reaction.
. Catalytic cycle includes re-oxidation of MoIV to MoVI by O2 in the “dark” reaction.