Copper and iron hydroxides as new catalysts for redox reactions in aqueous solutions

Article abstract of DOI:10.1070/MC2001v011n01ABEH001379

Supported or colloidal Cu^II and Fe^III hydroxides catalyse the oxidation of catechol to a muconic acid derivative and of benzene to phenol in aqueous hydrogen peroxide solutions at ambient temperature.

Full text of DOI:10.1070/MC2001v011n01ABEH001379

,
2001, 11(1), 15–16
Copper and iron hydroxides as new catalysts for redox reactions in aqueous
solutions
Galina L. Elizarova,* Lyudmila G. Matvienko, Andrei O. Kuzmin, Elena R. Savinova and Valentin N. Parmon
G. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation.
Fax: +7 3832 34 3056; e-mail: geliz@catalysis.nsk.su
1
0.1070/MC2001v011n01ABEH001379
II
III
Supported or colloidal Cu and Fe hydroxides catalyse the oxidation of catechol to a muconic acid derivative and of benzene to
phenol in aqueous hydrogen peroxide solutions at ambient temperature.
Metal hydroxides are widely used as ion exchangers, adsorb-
ents and collectors for impurities. However, their application as
catalysts is rather scarce. Meanwhile, they are very attractive for
catalysis: they exhibit a polynuclear structure and a high surface
area, and they can be prepared in various forms (supported
species, colloidal solutions and bulk materials). We found re-
cently that copper and iron hydroxides can be successfully used
2
.5
2.0
1.5
1.0
0.5
8
7
6
1,2
as catalysts for the oxidation of water and light hydrocarbons
8
,9
5
and for H O decomposition in aqueous solutions. In this
2
2
†
work, we found that copper and iron hydroxides catalyse ben-
zene and catechol oxidation with H O ; in the latter case, their
2
2
4
catalytic performance is similar to that of dioxygenase enzymes.
2
Recently, we found that H O oxidises methane and ethylene
3
2
2
to formic acid in the presence of a Cu(OH) /SiO catalyst in
2
2
2
aqueous solution. To clarify whether the oxidation of methane
proceeds via the consecutive formation of methanol and form-
aldehyde, these compounds were used as substrates. Methanol
was not oxidised, whereas formaldehyde was oxidised very
slowly under the conditions of methane oxidation. This led us
to an assumption that formic acid is formed from hydrocarbons
via the transfer of two oxygen atoms to the substrate, likewise
the operation of dioxygenases. To prove this assumption, cate-
chol oxidation with H O in the presence of copper and iron
1
0
.0
190
240
290
340
390
l/nm
Figure 1 UV-VIS absorption spectra of the reaction mixture during the
oxidation of 2×10 M catechol with H O (7×10 mol dm ) in the pres-
ence of colloidal 1×10 mol dm Cu(OH) stabilised with 0.5% starch in
.1 M NaOH at 295 K. Reaction time/min: (1) 2; (2) 5; (3) 10; (4) 15; (5)
–
3
–3
–3
2
2
–
3
–3
2
0
20; (6) 25; (7) 30 and (8) 40.
2
2
hydroxides was studied.
Catechol oxidation was carried out in Ar at 295 K with
the kinetic curves of the catechol consumption (ln Ccat vs. time)
were linear in the concentration range from 0.005 to 0.02 M,
thus proving the first reaction order with respect to the substrate.
vigorous stirring in the presence of either SiO -supported
2
III
II
hydroxides containing 2% Fe or Cu ions or in their colloidal
solutions. We found in a special experiment that the catalytic
H O decomposition followed by O formation begins only
The influence of the concentrations of the substrate, H O2
2
and NaOH and the amount of a catalyst on the rate of catechol
oxidation was studied. Note that the behaviours of Fe- and Cu-
containing catalysts were almost identical, except for a higher
activity of the latter. An increase in the NaOH concentration
from 0.001 to 0.2 M accelerated the reaction by a factor of
about two; the reaction rate remained nearly constant upon a
further increase in the alkali concentration. In a neutral medium,
the reaction was very slow. A change in the catalyst amount
from 25 to 100 mg in 20 ml of the reaction mixture lead to a
less than twofold increase in the oxidation rate. When the
concentration of either catechol (0.005–0.05 M) or H O (0.01–
2
2
2
after catechol oxidation.
To monitor the kinetics in the presence of supported catalysts,
the samples taken from the reaction mixture were diluted with
0
.1 M H SO to stop the oxidation reaction and centrifuged;
2 4
then, the spectra were measured. The semilogarithmic plots of
Table 1 Concentrations of phenol in aqueous benzene (ca. 0.02 M) solu-
tions after 1–2 h of vigorous stirring at 295 K.
–3
4
–3
Catalyst
pH
[H O ]/mol dm
[PhOH]/10 mol dm
2
2
2
2
0
.3 M) was varied, the reaction order changed from 1 to 0 for
both of the reactants. Such orders are typical of nonradical
oxidation reactions with H O .
FeO(OH)/SiO2
12
7
0.025
0.025
0
7
in situ FeO(OH)
12
7
7
0.025
0.025
0.01
0
5
3
2
2
A study of catechol oxidation in the presence of starch-
stabilised hydroxide colloids supplemented these results. The
reaction was studied spectrophotometrically in 0.05–0.1 M NaOH
solutions in the presence of 1–5 mM catalyst. The molar ratio
Colloid FeO(OH) 12
7
0.025
0.025
0
0
between catechol and a metal varied from 0.5 to 3.0, H O2
Cu(OH) /SiO
12
7
0.1
0.025
0
2
2
2
traces
concentrations changed from 0.005 to 0.025 M for each ratio.
The reaction orders with respect to catechol and a catalyst
equalled 1; the order with respect to H O varied from 1 to 0 (in
in situ Cu(OH)2
13
7
0.1
0.01
0
3
2
2
the same way as for the supported catalysts).
Figure 1 demonstrates the UV-VIS spectra of the reaction
mixture during the catalytic oxidation of catechol in colloidal
†
Materials and methods. A UVIKON 923 spectrophotometer (USA),
and a Kristall 2000M gas chromatograph with FID (Russia) were used;
H O was determined using Ti . Catechol absorption bands at 275 and
IV 3
2
2
Cu(OH) . The absorption bands at 230 and 290 nm correspond
2
2
90 nm were used for its detection in acidic and alkaline media, respec-
tively.
Supported FeO(OH)/SiO , Cu(OH) /SiO and starch-stabilised colloidal
to catechol absorption. The evolution of the absorption at 250 nm
originates from the formation of a reaction product. The trans-
formation of the catechol spectrum into the spectrum of the
product through isosbestic points is indicative of the absence of
stable intermediates. It is remarkable that an o-benzoquinone
2
2
2
1,2
hydroxides were prepared as described elsewhere. In some experi-
ments, hydroxides were obtained in situ by adding NaOH to copper and
iron nitrates solutions.
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15 –

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2001, 11(1), 15–16
related product of di-tret-butylcatechol oxidation was observed
previously.5 Thus, copper and iron hydroxides catalyse the
cleavage of a benzene ring and the transfer of two oxygen
atoms, similar to catechol dioxygenases. Chemical systems
modelling these enzymes have been reported.5 However, this
work gives the first example of the simplest inorganic catalysts
acting in aqueous solutions similarly to catechol dioxygenases.
1
,6
1
0
0
0
.0
.8
.6
.4
–7
2
1
0
0
.0
.6
.2
Previously,8 we found that H O2 decomposition in the
,9
2
presence of Cu and Fe hydroxides proceeds via a nonradical
mechanism. This is due to the formation of peroxo complexes
IV
of these hydroxides and Fe upon the interaction with H O .
2
2
The rates of formation and consumption of peroxo complexes
IV
and Fe are strongly influenced by pH, reactant concentrations
and the presence of an organic substrate.8
–10
It is likely that
depending on the substrate nature and/or the reaction conditions
different catalyst–H O intermediates may react with the sub-
2
2
strate and transfer either one or two oxygen atoms to it. It was
thus interesting to study the products of benzene oxidation in
the presence of the hydroxide catalysts.
1
0
20
30
t/min
We found that benzene is oxidised to phenol in aqueous
solutions, i.e., the reaction proceeds via one oxygen atom
transfer from the oxidant to the substrate. Table 1 lists the
concentrations of phenol formed in 1–2 h in aqueous benzene
(ca. 0.02 M) solutions under different experimental conditions.
Phenol concentrations were determined either from the absorp-
tion at l 286 nm or by GC. It is remarkable that phenol (as well
as ethanol and methanol) is not oxidised in the systems under
study. This is another evidence for the nonradical oxidation
reaction catalysed by transition metal hydroxides.
Figure 2 Kinetics of (1) the substrate and (2) H O consumption during
2
2
–3
catechol oxidation in a colloidal FeO(OH) solution; 2×10 M catechol,
–
2
–3
1
×10 M H O , 1×10 M FeO(OH), 0.1 M NaOH, 295 K.
2 2
4
absorption band at 380 nm was never observed. Figure 2 shows
the intensity of the catechol absorption at 290 nm and the H O2
2
concentration. Induction periods are observed in the kinetic
curves, which increased with catechol concentration and de-
creased with H O2 concentration. At a threefold excess of
2
catechol over the catalyst and at a low H O concentration, the
2
2
oxidation rate was close to zero if catechol was added to the
reaction mixture before H O , but it was rather high [especially
This study was supported in part by the Russian Foundation
for Basic Research (grant no. 98-03-32410). We are grateful to
L. A. Kozhanova for performing HPLC analysis.
2
2
in the case of Cu(OH) ] with the reverse order of the reagent
2
addition. This suggests a competition between the substrate and
the oxidant for the sites at the catalyst active centre. The per-
oxide intermediate formation is likely to play a key role.
References
Note that the catechol absorption at 290 nm increases during
the induction period (Figure 2, curve 1); this can result from the
accumulation of a complex between catechol and the catalyst.
At the same time, the H O concentration does not change, the
1
2
3
4
G. L. Elizarova, G. M. Zhidomirov and V. N. Parmon, Catal. Today,
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2
2
2
product is not formed and the spectra of the reaction mixture do
not cross the isosbestic points (dotted lines in Figure 1). We sup-
pose that an active intermediate is formed via the coordination
of the substrate to either a peroxo complex or metal ions in high
1967.
5
6
H. Weiner and R. G. Finke, J. Am. Chem. Soc., 1999, 121, 9831.
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Kinet. Katal., 2000, 41, 366 [Kinet. Catal. (Engl. Transl.), 2000, 41,
332].
IV
III
oxidation states (Fe or Cu ).
The UV-VIS spectrum of the product does not contain the
absorption band at 270–290 nm characteristic of a benzene
ring, but contains a very intense absorption band at 250 nm
7
8
4
3
–1
–1
(
e > 10 dm mol cm ), which is characteristic of muconic
4
acid and its lactone known as the products formed under the
action of catechol dioxygenases. In acidic solutions, the product
absorption band shifts to 225 nm, the addition of an alkali
restores its position. This reversible shift of the absorption band
with pH proves that the product is really an acid. Liquid chro-
matography (HPLC) of the working solutions and IR spectra of
the samples obtained after filtration of the supported catalyst
and careful drying of the supernatant evidence that the reaction
product is γ-lacton of α-hydro-β-hydroxymuconic acid. The
9
G. L. Elizarova, L. G. Matvienko and V. N. Parmon, Kinet. Katal.,
2
000, 41, 839 (in Russian).
10 G. L. Elizarova, L. G. Matvienko, A. O. Kuzmin, E. R. Savinova and
V. N. Parmon, Dokl. Ross. Akad. Nauk, 1999, 367, 640 [Dokl. Chem.
(
Engl. Transl.), 1999, 367, 188].
Received: 26th September 2000; Com. 00/1705
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