Low-temperature oxidation of sulfur dioxide by molecular oxygen in aqueous solutions of non-transition metals trifluoroacetates

Article abstract of DOI:10.1134/S0036024411050311

Oxidation of SO₂ was shown to occur in aqueous solutions of Li, Na, and K trifluoroacetates at room temperature. Oxidative activity of the non-transition metal trifluoroacetates increases on decreasing pH of solution and at equal pH values, it follows the sequence: Li < Na < K.

Full text of DOI:10.1134/S0036024411050311

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2011, Vol. 85, No. 5, pp. 777–779. © Pleiades Publishing, Ltd., 2011.
Published in Russian in Zhurnal Fizicheskoi Khimii, 2011, Vol. 85, No. 5, pp. 864–866.
CHEMICAL KINETICS
AND CATALYSIS
LowꢀTemperature Oxidation of Sulfur Dioxide by Molecular Oxygen
1
in Aqueous Solutions of NonꢀTransition Metals Trifluoroacetates
a
a
b
b
M. V. Vishnetskaya , Yu. V. Murtazina , I. Yu. Skrepleva , and M. Ya. Mel’nikov
a
Gubkin State University of Oil and Gas, Moscow, Russia
b
Faculty of Chemistry, Moscow State University, Leninskie Gory, Moscow, 119992 Russia
eꢀmail: mvvishnetskaya@mail.ru
Received May 13, 2010
Abstract—Oxidation of SO was shown to occur in aqueous solutions of Li, Na, and K trifluoroacetates at
2
room temperature. Oxidative activity of the nonꢀtransition metal trifluoroacetates increases on decreasing
pH of solution and at equal pH values, it follows the sequence: Li < Na < K.
Key words: molecular oxygen, active state, Br
ø
nsted acidity, oxidation, alkali metal, trifluoroacetates.
DOI: 10.1134/S0036024411050311
1
1
. INTRODUCTION
CF COOK were also obtained by reaction of the corꢀ
3
responding metal hydroxides (transition metals
Oxidation of organic substrates in the presence of
<
0.001 mg/kg) with TFA. Aqueous solutions of trifluꢀ
transitionꢀmetal salts in the media of trifluoroacetic
acid (TFA) is well documented in literature [1–3].
Such a behavior of TFA as a reaction media is believed
not only due to its fair oxidation resistance but also
because its ability to dissolve considerable amounts of
molecular oxygen which is characteristic of fluoroꢀ
containing compounds [2].
oroacetates were used in our experiments, pH of the
sample solutions was varied by adding appropriate
amounts of 0.1 M HCl (0.5–1 ml in 20 ml solutions of
trifluoroacetes) or glacial acetic acid (0.1–0.6 ml in
20 ml solutions of trifluoroacetes) and 0.1 M solutions
NaOH (for pH > 7).
Content of “active” dioxygen was estimated via
measuring the total amount of SO2 oxidized by passing
sulfur dioxide gas through a trifluoroacetate solution
till breakthrough of SO2 which was detected by the
color change of sodium bichromate solution. When
the reaction stopped, amount of the formed sulfate
ions was determined by potentiometric titration of 5–
In the earlier communications [4], we demonꢀ
strated that fluoroꢀcontaining acids, e.g., TFA and fluꢀ
orosulfonic acid, mediate the oxidation of various
organic and inorganic substrates on account of disꢀ
solved and activated dioxygen (*О2). When the disꢀ
solved oxygen was exhausted, the reaction stopped,
but it started again after saturation of the acids with O2.
However, it is not so clear whether the presence of
CF3ꢀgroup in a fluoroꢀcontaining acid or its own proꢀ
tic acidity (or both) are the key factors of dioxygen
activation.
20 ml of the solution by BaCl2.
3. RESULTS AND DISCUSSION
First, in neutral aqueous solutions trifluoracetates
In order to gain a better understanding of the reacꢀ
tion acceleration by fluoroꢀcontaining compounds,
we studied the catalytic action of nonꢀtransition metal
of Li, Na, and K were found to be inactive in SO oxiꢀ
2
dation. At the same time, the amounts of *О2 in trifluꢀ
oracetate solutions are remarkably larger than in TFA
solutions (see Fig. 1).
(
Li, Na, K, and Al) trifluoroacetates in their aqueous
solutions at various pH.
Notably, an increase in *О2 content in the trifluoꢀ
racetate solutions depends on cation nature. For
example, for TFALi and TFANa solutions, the
increase in the active oxygen concentration is twoꢀfold
on decreasing pH from 3.2 to 2.2, while for TFAK, this
change is about 5 times.
2
. EXPERIMENTAL
Al trifluoroacetate was prepared by reacting TFA
with aluminum hydroxide resulted from precipitation
of aluminum chloride by 25% aqueous ammonia.
Thus prepared Al(CF COO) was dried in air at 90°С
Trifluoroacetes CF COOLi
.
On the other hand, beside pH, oxidative activity of
3
3
,
CF COONa
,
and trifluoracetates depends on the acid nature. This is
clearly seen from the data in table where the results,
obtained for the two acids, are compared.
3
3
1
The article was translated by the authors.
777

778
VISHNETSKAYA et al.
m
Al
m
6
4
2
0
1
2
00
00
0
0
0
0
2
K
1
Na
Li
−1
1
3
5
7
9
pH
Fig. 1. Amounts of *O (mmole per mole of salt) in
Fig. 2. Amount of *O
(mmole per mole of salt) in 1 M
2
2
NaTFA (
1
) and TFA (
2
) solutions vs. pH values.
solutions of trifluoroacetates at pH 2.
[*O ], mmol/l
2
m
K+
20
8
0
0
0
Na+
pH 2
Li+
4
10
0
pH 3.2
0
.165
0.175
0.185
r
, nm
2
4
cNaTFA, M
Fig. 3. Amount of *O (mmole per mole of salt) in 1 M
2
Fig. 4. Amount of *O vs. NaTFA concentration at pH 2.3.
solutions of trifluoroacetates vs. the radius of hydrated ion.
2
These results can be explained so that CH COOH, salts. The similar results are also obtained for other pH
3
in contrast to HCl, upon reaction with dioxygen is able values.
to form the peroxoacid, acting as a strong oxidant.
Thus, trifluoroacetates of nonꢀtransition metals
Noteworthy, CH COOH itself can not activate dioxyꢀ catalyze liquid phase oxidations by molecular oxygen
3
gen and does not contain peroxogroups unless some in acidic solutions so that the smaller radius of
trifluoroacetate is present in solution.
hydrated ion [5] the higher *O
Fig. 3).
The dependence of active oxygen amount on the
concentration of aqueous salt solutions (Fig. 4) passes
through wellꢀdefined maximum and minimum at 1
and 3 M, correspondingly. Such a peculiar depenꢀ
dence is typical for the electroconductivity of aqueous
electrolite solutions where triple ion associates can
form [6].
2
concentrations
(
Figure 2 shows the *O concentration in trifluoroꢀ
2
acetate solutions at pH 2. It is clearly seen that more
than triple increase in *O amount for Al trifluoroaceꢀ
2
tate solution in comparison with that of alkali metal
Quantities of a formed sulfateꢀion at oxidation SO in 1 M
2
solution NaTFA in the presence of HCl and CH COOH
3
2
–
]
cacid, M
[SO4
, mmol/salt mol
4. CONCLUSION
pH
HCl
10–4
СН СООН
3
HCl
СН СООН
3
This work clearly demonstrated that alkali metal
trifluoroacetetes in aqueous solutions are able to oxiꢀ
1
.6
.0
0.45
0.12
114
69
576
216
dize SO at ambient temperatures. Oxiditive activity of
2
3
5
10–5
×
trifluoroacetets increases on decreasing pH value, at
Note: cacid, concentration of acid at which the preset value pH is close pH values, it changes in the order: Li < Na < K.
reached.
Beside pH, the oxidative activity of trifluoracetates is
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 85
No. 5 2011

LOWꢀTEMPERATURE OXIDATION OF SULFUR DIOXIDE BY MOLECULAR OXYGEN
779
affected by the nature of acid anion, in that presence
the reaction occurs. So that the question arises on the
mechanism of these specific oxidizing processes,
occurring in Fꢀcontaining environments. From possiꢀ
REFERENCES
1
. C. J. Moody and J. L. O’Connell, Chem. Commun.,
p. 1311 (2000).
2
. A. E. Gekhman, I. P. Stolyarov, N. V. Ershova, et al.,
Kinet. Katal. 45, 45 (2004) [Kinet. Catal. 45, 40
ble intermediates, performing such transformations, it
is possible to exclude singlet oxygen 1O2 as its generaꢀ
+
(2004)].
tion demands ~112 kJ/mol. Even though HO2 partiꢀ
cles have been observed in gas phase experimentally
3
. M. V. Kirillova, M. L. Kuznetsov, P. M. Reis, et al.,
J. Am. Chem. Soc. 129, 10531 (2007).
[
7] but its involving into the processes in solutions
4. I. A. Sidorenkova, I. Yu. Yakimova, M. V. Vishnetskaya,
and O. A. Ponomareva, Zh. Fiz. Khim., No. 8, 1526
seems to be little probable. Most likely, lowꢀtemperaꢀ
ture oxidation could result from the formation of
hydrogen peroxide. However, the formation of H O
(2005) [Russ. J. Phys. Chem. A 79, 1352 (2005)].
2
2
5. N. N. Greenwood and A. Earnshaw, Chemistry of the
Elements (Butterworth, Oxford, 1542).
and its involving into the process require the experiꢀ
mental confirmation.
6.
The Chemical Physics of Solvation, Pt. A, Ed. by
R. R. Dogonadze; pt C, Ed. by A. A. Kornyshev and
I. Ulstrup (Amsterdam, 1985).
ACKNOWLEDGMENTS
This work was supported by Russian Foundation
for Basic Research (no. 08ꢀ03ꢀ00388ꢀa).
7. S. A. Nizkorodov, D. Roth, R. V. Olkhov, et al., Chem.
Phys. Lett. 26, 278 (1997).
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 85
No. 5 2011