2
90
BAJ AND CHROBOK
◦
to the long-alkyl chain (C8) in the anion of the IL. In
other ILs, the solubility of cumene varies from 5% to
[bmim]OSO3Oc) at 130 C was performed. In this pro-
cess, [bmim]OSO3Oc also does not influence the oxi-
dation rate compared to similar process carried out in
t-BuPh (entries 12 and 13 in Table I).
To confirm the thesis that some ILs do not influence
therate rox, theoxidationofcumenein[bmim]OSO3Oc
was examined by the method proposed by Russell
32% v/v. The amount of cumene applied for oxidation
processes in this study allowed for the preparation of
homogeneous solutions with ILs at experimental tem-
peratures.
[
6]. The studies were performed for several solu-
tions of different concentrations of cumene in ILs
25–100 v/v%) with constant ACHN molar concen-
Autooxidation Processes Carried Out
in Ionic Liquids
(
3
tration (0.002 mol/dm ). The linear dependence of the
rate of oxidation on the cumene concentration con-
firmed the inert nature of ILs in this process (Fig. 3).
Exactly the same results were obtained for the same
experiments with t-BuPh as solvent. For all experi-
ments, good correlations (r = 0.9980.999) between the
measured loss of oxygen and the time of oxidation
were observed.
One of the reasons for the differing behavior of
cumene in ILs than in t-BuPh could be the influence of
ILs on the mechanism of the autooxidation process. As
known from the reaction mechanism [7], hydroperox-
ides are the main intermediate products of the oxidation
process in the liquid state with molecular oxygen. Un-
der experimental conditions, they can be decomposed
to free radicals that, in turn, generate new kinetic chains
and thus the process becomes autocatalytic. That is
why acceleration of CHP decomposition during the
oxidation process can result in acceleration of the oxi-
dation rate. A similar effect involving the influence of
tetra-alkylammonium salts (used as phase-transfer cat-
alysts) on oxidation reactions has been demonstrated in
the literature [8]. It was shown that organic hydroper-
oxides were decomposed efficiently by these salts, ac-
celerating the oxidation process.
The kinetic data of the autooxidation process of
cumene (10% v/v solution in selected ILs) with molec-
ular oxygen carried out in a gasometric apparatus with
ACHN (0.02 mole/dm ) at 100 C are shown in Table II.
The method of rox determination is described in the Ex-
perimental section. The advantage of this method is a
short measuring time (10–15 min) and consequently
low degree of conversion (0.03%). The influence of
the oxidation products on the course of the reaction
can be eliminated.
3
◦
The rates of cumene oxidation in experiments car-
ried out in some ILs (entries 2 and 3 in Table II) were
equal to the rate obtained in the same reaction with
tert-butylbenzene (t-BuPh) as a solvent (rox = 7.15 ×
−6
3
10
mol/(dm s)). This solvent is described in the lit-
erature as a classical representative of inert solvents. In
the case of other ILs, the acceleration of the reaction
−6
−6
3
rates (rox = 8.89 × 10 to 14.12 × 10 mol/dm s)
was observed.
In addition, the oxidation of n-decane as a repre-
sentative of n-alkyl linear hydrocarbons (15% v/v in
Table II Rate of Cumene Oxidation rox in the Presence
of ACHN (0.02 mol/dm ) at 100 C
For this reason, we have decided to perform some
additional tests checking the stability of CHP in ILs
at the reaction temperature. To begin, the mixture of
3
◦
−
6
rox× 10
◦
3
CHP and IL (10:90 v/v) was stirred at 100 C for 3 h
Entry
Solvent
t-BuPh
[bmim]OSO3Oc
[empd]OSO3Et
[bmp]NTf2
[bmim]NTf2
[tmba]NTf2
[bmim]BF4
(mol/dm s)
(Table III). After this time, the concentration of CHP
1
2
3
4
5
6
7
8
9
7.06
7.05
7.00
10.26
9.33
12.05
12.72
8.89
14.12
14.30
13.23
13.02
13.10
was measured.
In the case of t-BuPh, 10% of the CHP was decom-
posed after 3 h in what can only be a result of thermal
◦
decomposition. Similar behavior of CHP at 100 C was
observed in ILs based on alkylsulfate anions (entries
−
2
and 3 in Table III) and NTf anions (entries 4–6 in
2
[bmim]PF6
[bmim]OAc
[bmim]HSO4
Table III). A different conclusion can be drawn from
experiments with other ILs (entries 7–10 in Table III).
These ILs influence the CHP decomposition process.
Taking into account data concerning the influ-
ence of ILs on the stability of CHP and rox values
a
10
11
12
13
[hmim]PF3(C2F )3
5
b
t-BuPh
b
[bmim]OSO3Oc
(Table II), an interesting dependence can be observed.
a
b
ILs based on alkylsulfate anions do not influence the
decomposition of CHP and do not accelerate rox in
5
1
% v/v of cumene.
5% v/v of n-decane at 130 C.
◦
International Journal of Chemical Kinetics DOI 10.1002/kin