Superacids as catalysts of the oxidation of inorganic substrates

Article abstract of DOI:10.1134/S0036024406020087

The oxidation of sulfur dioxide, nitrogen dioxide, carbon monoxide, etc., in anhydrous trifluoroacetic acid (TFA) at room temperature was observed. The oxidative ability of anhydrous TFA in reactions with inorganic substances was shown to be related to molecular oxygen dissolved in it. The dissolved molecular oxygen endowed TFA with the properties of a superacid with pK _a ? -12. Mechanisms of active oxygen reactions with inorganic substrates were suggested. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S0036024406020087

ISSN 0036-0244, Russian Journal of Physical Chemistry, 2006, Vol. 80, No. 2, pp. 173–175. © Pleiades Publishing, Inc., 2006.
Original Russian Text © M.V. Vishnetskaya, I.Yu. Yakimova, I.A. Sidorenkova, 2006, published in Zhurnal Fizicheskoi Khimii, 2006, Vol. 80, No. 2, pp. 236–238.
CHEMICAL KINETICS
AND CATALYSIS
Superacids as Catalysts of the Oxidation of Inorganic Substrates
M. V. Vishnetskaya, I. Yu. Yakimova, and I. A. Sidorenkova
Gubkin State University of Oil and Gas, Leninskii pr. 65, Moscow, 117917 Russia
E-mail: Merkyrij@mail.ru
Received February 4, 2005
Abstract—The oxidation of sulfur dioxide, nitrogen dioxide, carbon monoxide, etc., in anhydrous trifluoroace-
tic acid (TFA) at room temperature was observed. The oxidative ability of anhydrous TFA in reactions with
inorganic substances was shown to be related to molecular oxygen dissolved in it. The dissolved molecular oxy-
gen endowed TFA with the properties of a superacid with K ꢀ –12. Mechanisms of active oxygen reactions
‡
with inorganic substrates were suggested.
DOI: 10.1134/S0036024406020087
INTRODUCTION
Transformations of nitrogen(IV) oxide were studied
similarly. The reaction was considered complete when
The catalytic activity of fluorine-containing super-
NO appeared at the exit from the system. The presence
2
acids such as HF–SbF , FSO H, and FSO H–SbF in
5
3
3
5
of nitrate(V) ions in the reaction mixture was estab-
lished by the “brown ring” reaction [4]. The amount of
nitrogen(V) oxide formed was estimated from the
amount of nitrogen(IV) oxide absorbed until it began to
flow through the reactor unchanged. Experiments with
the oxidation of SO and NO were performed so as to
hydrocarbon transformations is usually related to the
high mobility of protons capable of forming carbonium
ions [1]. It was found in [2] that anhydrous trifluoroace-
tic acid (TFA) also exhibited the properties of superac-
ids. It was suggested that this was related to dissolved
molecular oxygen in an active state different from the
ground triplet state. We therefore deemed it interesting
to study the chemical nature of active oxygen dissolved
in TFA in more detail and find out if high oxidative
activity is a general property of classic superacids. In
this work, we studied the oxidation of a wide range of
inorganic substances in anhydrous TFA as compared
with their similar transformations in fluorosulfonic acid
2
2
attain 100% conversion of the initial substrate into
compounds with higher oxidation degrees.
The oxidation of CO was also conducted at room
temperature. Carbon monoxide prepared by treating
formic acid with concentrated ç SO was bubbled
2
4
through anhydrous TFA. The distillation of anhydrous
TFA with absorbed CO was accompanied by the evolu-
tion of carbon dioxide, which reacted with a solution of
(
FSA), one of the strongest superacids.
barium nitrate to form a white LJëé precipitate. Dur-
3
ing distillation, ëé evolved was not completely
2
EXPERIMENTAL
absorbed by the solution of barium nitrate. We were
therefore unable to determine the degree of substrate
conversion in this reaction.
The oxidation of sulfur(IV) oxide was studied at
room temperature by passing a flow of SO at a rate of
2
2
0 ml/min through anhydrous TFA or FSA (100 ml) in
Parallel experiments with TFA were always per-
a bubbler. Experiments were continued until SO began formed after removing dissolved oxygen by oxalic acid
2
to get through the reactor, as was established from admixtures until the absence of oxidizers was estab-
changes in the color of an aqueous solution of potas- lished by the “benzene ring” reaction with potassium
sium bichromate. Sulfur(VI) oxide formed was deter- iodide. Several oxidation experiments were also per-
mined gravimetrically. For this purpose, a solution formed with glacial acetic acid.
sample (20 ml) was taken from the bubbler, a 20% solu-
tion of Ba(NO ) was added to it in excess, and the mix-
3
2
RESULTS AND DISCUSSION
ture was heated at 50°ë for 20 min. Heating was neces-
sary for the destruction of Caro’s acid anhydride and its
The suggestion was made in [5] that peroxide
dimers, which we found to be the primary products of groups could form in TFA. For this reason, we first
the oxidation of SO in TFA; these substances did not experimentally checked this hypothesis. All qualitative
2
form precipitates with barium nitrate. The BaSO pre- reactions for peroxide groups in TFA were, however,
4
cipitate was separated from the solution, washed with indicative of their absence. It follows that the oxidative
distilled water, dried until its weight ceased to change, ability of TFA was related exclusively to active oxygen
and weighed [3]. With fluorosulfonic acid, sulfur con- dissolved in it. The reactivity of this active oxygen was
tained in FSA itself was taken into account.
demonstrated in reactions of TFA with many sub-
1
73

1
74
VISHNETSKAYA et al.
stances capable of redox transformations, including oxidation. Trifluoroacetic acid can therefore be consid-
chromium(III) chloride and carbon(II), sulfur(IV), ered a catalyst of the oxidation of SO in the presence
2
nitrogen(II), and nitrogen(IV) oxides.
of dissolved molecular oxygen.
Chromium(III) compounds can be oxidized to chro-
The role played by trifluoroacetic acid as a catalyst
mium +6 derivatives under the action of oxidative melts of oxidation is the activation of oxygen dissolved in it.
and in aqueous solutions, always with the formation of It was shown in [6] that fluorine-containing compounds
oxo compounds. In acid media, oxidation to chro- formed very strong complexes with molecular oxygen.
mium(VI) can only be caused by strong oxidizers. Mix- The formation of these complexes changes the elec-
ing equal volumes of TFA and an aqueous solution of tronic configuration of the dioxygen molecule, likely
CrCl caused green solution coloration to change into into that of the reactive singlet state. The resulting elec-
3
blue, which was evidence of the formation of the Cré5 tronic configuration of the oxygen molecule corre-
peroxo complex. As has been mentioned, TFA does not sponds to its singlet state, which may be the reason for
contain peroxo groups. It was therefore the interaction the anomalously strong oxidizing properties of oxygen
3
+
of Cr with dissolved active oxygen (*é ) that was in TFA and the strength of bonds between dissolved
2
responsible for the formation of the Cré peroxo com- oxygen and TFA molecules.
5
plex,
All superacids contain fluorine atoms. It is therefore
O
logical to assume that they can also dissolve molecular
oxygen with its transition into the active state. Fluoro-
O
O
O
O
Cr3+ + *O2
Cr
sulfonic acid HF · SO is the strongest among all acids
3
known; its K equals –13.5. It is most extensively used
‡
Importantly, no such peroxo complex was formed
under the action of molecular oxygen in the triplet state.
It follows that active oxygen in TFA is a stronger oxi-
dizer than potassium bichromate.
The oxidation of sulfur dioxide in the absence of a
catalyst is a very slow reaction. Performing oxidation
requires the transition of oxygen into an active state.
as a superacidic catalyst. The only direct proof of the
presence of dissolved active oxygen in it would be the
oxidation of SO to SO , because the reduction of
2
3
molecular oxygen contained in fluorosulfonic acid
changes the degree of oxidation of the central atom and
thereby destroys superacid molecules.
We found that the behavior of FSA differed from
that of TFA by the absence of peroxo groups and a
much larger (by a factor of 4) amount of absorbed and
converted SO . The larger solubility of SO in FSA
Trifluoroacetic acid absorbs SO in an 8.8 : 1 molar
2
ratio. The resulting reaction mixture contains peroxo
groups and sulfur(VI), whereas sulfur(IV) is com-
pletely absent,
2
2
compared with FTA is related to donor–acceptor and
dative interactions between sulfur and oxygen atoms of
a neighboring molecule, which results in the formation
of trimers and polymers (SO ) very characteristic of
O
O
O
O
S + *O2
S
,
O
O
3
n
sulfur(VI) oxide. Possibly, it is the higher concentration
of SO in HF · SO that explains the absence of peroxo
O
O
O
O
S
O
O
O
O
S +
S
+
S
2
3
O
O
compounds as intermediate oxidation products,
O
O
SO + SO
2SO3.
The presence of peroxo groups and sulfur(VI) corre-
sponds to the formula unit of Caro’s acid anhydride
2
4
The hypothesis of the stabilization of active singlet
oxygen in fluorine-containing superacids allows us to
consider two variants of their interaction with sub-
strates after one-electron oxidation and the formation
of ion–radical pairs or charge transfer complexes:
(
SO ). Indeed, hydrolysis under weak heating gave
4
ç SO and ç é ,
2
4
2
2
O
O
O
O
O OH
S
+ H2O
S
,
O
O
OH
OH
–
+
. .
A + *O ⋅ F–R
A O ⋅ F–R ,
O
O
O OH
O
2
2
S
+ H O
S
+ H O2
2
2
–
OH
O
OH
+
.
.
–
+
. .
1
A + O ⋅ F–R
2
A O ⋅ F–R
These results directly prove the presence of dis-
2
2
solved active oxygen in TFA; this oxygen is consumed
AO + F–R
2
to oxidize SO . The question arises of whether this con-
2
–
sumption can be compensated for by absorbing molec-
ular oxygen. After the oxidation of sulfur(IV) to sul-
fur(VI) was complete, when all dissolved oxygen was
If the interaction energy between the O radical anion
2
and fluorine-containing fragments is higher than the
+
.
used up, a flow of air was passed through TFA. After energy of its interaction with radical cation A , charge
this, the oxidation of sulfur(IV) resumed. It follows that separation occurs, and the radical cation goes out of the
a virtually unlimited amount of SO can be oxidized by complex (path 1). Its further transformations depend on
2
alternating the saturation of TFA with air oxygen and its intrinsic nature and medium acidity [7]. However, if
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 2 2006

SUPERACIDS AS CATALYSTS OF THE OXIDATION OF INORGANIC SUBSTRATES
175
–
removed, the same products were formed, but the con-
version decreased to 70%. This leads us to conclude
that the reaction with methylphenylcyclopropane in
TFA follows path 1.
REFERENCES
1
. K. Tanabe, Catalysts and Catalytic Processes (Mir, Mos-
cow, 1993) [in Russian].
As with sulfur dioxide, the oxidation of NO in
2. M. V. Vishnetskaya, B. V. Romanovskii, I. G. Bolesov,
2
and M. Ya. Mel’nikov, Dokl. Akad. Nauk SSSR 330 (1),
anhydrous TFA involves the formation of nitrogen(V)
peroxo compounds, which decompose under the action
of water and when heated to produce nitric acid. Calcu-
lations show that the number of active oxygen mole-
2
04 (1993).
3. V. P. Vasil’ev, Analytical Chemistry: Gravimetric and
Titrimetric Methods of Analysis (Vysshaya Shkola, Mos-
cow, 1989) [in Russian].
cules that participate in the oxidation of NO is much
2
larger (O : TFA = 1 : 12) than with the oxidation of SO
4. V. A. Alekseev, A Course in Qualitative Semimicroanal-
ysis (Gos. Nauch.-Tekhn. Izd. Khim. Lit., Moscow,
2
2
(
1 : 17). This is likely related to a lower ionization
1
958) [in Russian].
potential of NO2 (9.78 eV) compared with SO2
12.34 eV). The oxidation of substances with still
larger ionization potentials, for instance, carbon mon-
oxide (14 eV), also occurs in anhydrous TFA. In the lat-
ter reaction, no peroxide compounds were detected in
the reaction mixture.
(
5. A. E. Gekhman, N. I. Moiseeva, and I. I. Moiseev, Izv.
Akad. Nauk, Ser. Khim., No. 4, 605 (1995).
6. A. L. Buchachenko, Usp. Khim. 54 (2), 195 (1985).
7. M. V. Vishnetskaya and B. V. Romanovskii, Ross. Khim.
Zh. 38 (6), 12 (1994).
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 2 2006