Cobalt/N-Hydroxyphthalimide(NHPI)-Catalyzed Aerobic Oxidation of Hydrocarbons with Ionic Liquid Additive

Article abstract of DOI:10.1016/j.mcat.2018.01.006

A highly efficient and solvent-free system of cobalt/NHPI-catalyzed aerobic oxidation of hydrocarbons was developed using imidazolium-based ionic liquid (IL) as an additive. These amphipathic ILs were found self-assemble at the interface between the organic hydrocarbons and the aqueous phase of catalyst combination (Co/NHPI), with forming a solution of reversed multilamellar vesicles for catalysis. The initial reaction rate was influenced by both the composition of microdomains and the structure of IL launched. Consequently, a proper water content (χ_H2O) of wet IL was requisite to reach the optimum reactivity. Besides, the interfacial boundary between aqueous and organic phase composed by C2-alkylated imidazolium ILs, such as [bdmim]SbF₆ and [C₁₂dmim]SbF₆, not only has ternary aggregates (hydrocarbons/IL/H₂O) of higher stability but renders O₂ a faster diffusion rate and higher concentration, thereby offering a high reactivity of the protocol towards hydrocarbon oxidation.

Full text of DOI:10.1016/j.mcat.2018.01.006

Molecular Catalysis 447 (2018) 90–96
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Cobalt/N-Hydroxyphthalimide(NHPI)-Catalyzed Aerobic Oxidation of
Hydrocarbons with Ionic Liquid Additive
Sajid Mahmooda ,ab ,,b,B⁎ ao-Hua Xua,b,⁎, Tian-Lu Ren , Zhi-Bo Zhangc,1, Xiao-Min Liua,
a
Suo-Jiang Zhang
a
Beijing Key Laboratory of Ionic Liquids Clean Process State Key Laboratory of Multiphase Complex Systems Institute of Process Engineering, Chinese Academy of Sciences,
Beijing 100190, PR China
b
School of Chemistry and Chemical Engineering University of Chinese Academy of Sciences Beijing 100049, PR China
Department of Chemical and Biochemical Engineering, Center for BioProcess Engineering, Building 229, Technical University of Denmark, DK-2800 Kgs, Lyngby,
c
Denmark
A R T I C L E I N F O
A B S T R A C T
Keywords:
A highly efficient and solvent-free system of cobalt/NHPI-catalyzed aerobic oxidation of hydrocarbons was
developed using imidazolium-based ionic liquid (IL) as an additive. These amphipathic ILs were found self-
assemble at the interface between the organic hydrocarbons and the aqueous phase of catalyst combination (Co/
NHPI), with forming a solution of reversed multilamellar vesicles for catalysis. The initial reaction rate was
influenced by both the composition of microdomains and the structure of IL launched. Consequently, a proper
water content (χH2O) of wet IL was requisite to reach the optimum reactivity. Besides, the interfacial boundary
Aerobic oxidation
NHPI catalysis
Ionic liquid
Self-assemble
Hydrocarbons
between aqueous and organic phase composed by C2-alkylated imidazolium ILs, such as [bdmim]SbF
6
and
[
C
12dmim]SbF , not only has ternary aggregates (hydrocarbons/IL/H O) of higher stability but renders O a
6
2
2
faster diffusion rate and higher concentration, thereby offering a high reactivity of the protocol towards hy-
drocarbon oxidation.
1. Introduction
Since then, a combination of NHPI with either transition metal salts
[
14] or various non-metal co-catalysts [15] was successively employed
Aerobic oxidation and functionalization of hydrocarbons is a pro-
to accelerate the autoxidation efficiency. However, NHPI as a catalyst
suffers two major shortcomings of low solubility in nonpolar organic
medium and low thermo stability [16]. To address this issue, the focus
was turned to the molecular design of novel structures of nitroxyl ra-
dical precursor with increased lipophilicity and stability [17]. It’s also
requisite to render the OeH group thereof proper bond dissociation
energy (BDE) for the high reactivity towards hydrocarbons [18]. Al-
ternatively, the search for new reaction medium as solvent/phase
transfer catalyst (PTC) was also effective and attracted attentions [19].
Ionic liquids (ILs) were initially introduced as alternative green
reaction medium because of their unique chemical and physical prop-
erties, such as nonvolatility, nonflammability, thermal stability and
controlled miscibility [20]. Quite a lot of aerobic oxidations were ex-
amined in ILs, which exhibited enhanced activities as compared with
that in the conventional organic solvents [21]. In such a system, ILs
cannot be considered inert since the formation and transfer rate of ra-
dical species generated in situ should be strongly affected by the
mising subject in industrial chemistry [1]. Many fundamental chemicals
such as terephthalic acid [2], KA oil (a mixture of cyclohexanone and
cyclohexanol) [3], phenol accompanied by acetone [4], benzoic acid
[
2
5], are produced by homogeneous liquid-phase oxidations with O as
the external oxidant. However, hydrocarbon oxidations suffer a major
limitation imposed by their intrinsic nature: the reaction products are
generally more reactive than the starting precursors [6]. In the past two
decades, many endeavours had been put to explore novel strategies
with better selectivity under milder reaction conditions. Consequently,
homogeneous catalysis includes transition-metal [7], dye-related [8],
enzymatic [9], biomimetic [10], and halogen-based [11] catalysis were
developed. Pioneered by Grochowski’s group, N-hydroxyphthalimide
NHPI) was recognized as a mediator for free-radical reactions [12]. In
995, Ishii and co-workers reported alkanes and alcohols could be ef-
fectively oxidized to the corresponding carbonyl compounds, which
opened up a history of hydrocarbon oxidations catalyzed by NHPI [13].
(
1
⁎
Corresponding authors at: Beijing Key Laboratory of Ionic Liquids Clean Process State Key Laboratory of Multiphase Complex Systems Institute of Process Engineering, Chinese
Academy of Sciences, Beijing 100190, PR China.
E-mail addresses: bhxu@ipe.ac.cn (B.-H. Xu), sjzhang@ipe.ac.cn (S.-J. Zhang).
On leave to Institute of Process Engineering.
1
https://doi.org/10.1016/j.mcat.2018.01.006
Received 31 October 2017; Received in revised form 9 January 2018; Accepted 10 January 2018
2468-8231/ © 2018 Elsevier B.V. All rights reserved.

S. Mahmood et al.
Molecular Catalysis 447 (2018) 90–96
−2
presence of an ionic environment. Noteworthy, mechanistic studies of
autoxidation in the presence of NHPI and cobalt salt demonstrated both
regeneration of nitroxyl radicals and addition of O to carbon radical
2
are diffusion-controlled process [16b]. Consequently, there should be
pronounced solvent effect on the reaction efficiency if IL solution was
launched. Closely related, Compton and coworkers conducted sys-
IL (χH2O = nH2O/nIL) under study was controlled less than 4.8 × 10
determined by Karl Fischer titration. In addition, the performance in
general media, like acetic acid (HAc) [27], was also studied for com-
parison (entries 9–11). All oxidations were carried out in the presence
2 2
of Co(OAc) (0.5 mol%) and NHPI (10 mol%) under O (1 atm) at
100 °C for 15 h, with generating benzoic acid (2a) as the major product
together with traces of benzaldehyde (3a). As indicated by entries 1–4,
the reaction efficiency decreases with an increasing acidity of C2 − H in
the imidazolium cation, albeit with distinction being far less than we
expected. The relatively higher yield of 2a obtained in the presence of
[bmim]Br ([bmim]: butyl metylimidazolium) (entry 5) is likely attrib-
tematic researches on the electron reduction of oxygen (O
of ILs [22]. The diffusion rate of O and O %
2 2
2
) in a series
were found strongly
−
depend on the specific structure of ILs using ultramicroelectrode (UME)
technology. Besides, the micelles or higher aggregates presented in IL or
multicomponent system containing IL have been recognized [23],
wherein the distribution of substrates and transiency species could be
designed to reach high reaction efficiency. For instance, the extensively
−
uted to the cooperative role of Br in the cobalt-catalyzed aerobic
oxidation [28]. The imidazolium cation was then functionalized with
stronger HBD groups such as carboxylic acid with the aim to further
optimize the oxidation efficiency through accelerating the electron
explored binary system of IL/H
oxidative transformation of aliphatic alcohols to carbonyl compounds
24]. The high efficiency obtained was attributed to the local con-
2
O was recently launched in the aerobic
[
2
transfer rate from Co(II) to O . Unfortunately, the corresponding re-
centration experienced by the reactants in the assembly is higher than
in a bulk solution. Finally, from a viewpoint of molecular level, func-
tionalization of ILs with hydrogen-bonding donor (HBD) groups was
supposed beneficial to accelerate electron transfer rate from metal
complexes to the oxygen molecule (O ), a requisite step in the initial
2
stage of metal-catalyzed aerobic oxidation. Evidently, Jacobsen [25]
and Song [26] disclosed imidazolium-based ILs exhibited a dramatic
action proceeded sluggishly (entry 6), with leading to an overall yield
of products upon oxidation in less than 34%. Instead, significant im-
provements were obtained when C2 − H was blocked with an alkyl
group (entries 7 and 8). Besides, it is beneficial to reach a relatively
high yield of 2a by introducing a longer carbon chain at N1-position. In
comparison, the performance is poor in the presence of HAc (entry 10)
as an additive. A comparable yield of 2a could be attained in HAc when
its amount was increased up to a solvent scale (5 mL, entry 10).
The impact of catalyst species on the autoxidation rate of 1a was
subsequently studied by kinetic profiles of several experiments, using
different catalyst combinations at 80 °C (Fig. 1). Evidently, the ternary
acceleration effect on the electron transfer from metal complexes to O
2
,
which was ascribed to the stabilization of the oxygen radical anions
−
2
(O % ) by coordinating with the acidic C2-H of imidazolium-based ILs.
We, therefore, envisioned the efficiency of Co/NHPI-catalyzed aerobic
oxidation of hydrocarbons could be effectively tuned by using different
IL as an additive or solvent medium. The readily modified structure of
ILs may render it possible to explore the important structure-perfor-
mance relationship therein.
Co/NHPI/[C12dmim]SbF
dazolium) (b) is more reactive than the dual combinations of either Co/
[C12dmim]SbF (d) or NHPI/[C12dmim]SbF (c). In addition, we found
6
([C12dmim] = 1-dodecyl-3, 3-dimethylimi-
6
6
it’s requisite to attain high reaction efficiency through loading the wet
IL of a proper χH2O, despite the aerobic oxidation process thereof ac-
companied with production of H O. As it depicted, the oxidative con-
2
2. Results and discussion
version of 1a to 2a underwent apparently faster within 50 min when
−
3
χ
H2O in [C12dmim]SbF
.7 × 10
6
was increased from ca. 5.5 × 10
(a) to
We started with screening various imidazolium-based ILs as an ad-
ditive to the Co/NHPI catalytic system for the aerobic oxidation of to-
luene (1a) (Table 1, entries 1–8). Notably, moisture content in each wet
−
2
7
(b). On the other hand, the initial oxidation rate was
−
1
slightly reduced when χH2O was increased up to 1.8 × 10
(e).
We, therefore, can postulate these amphipathic ILs self-assemble at
the interface between the apolar hydrocarbons, such as 1a, and the
aqueous phase of catalyst combination (Co/NHPI), with forming a so-
lution of reversed multilamellar vesicles. In such a system, IL additive
functions as a kind of surfactant or even phase transfer catalyst during
the initial stage of oxidation. Consequently, the reaction activity was
influenced by not only the composition of microdomains but also the
Table 1
Co/NHPI-catalyzed aerobic oxidation of toluene (1a) in the presence of imidazolium-
based IL as an additive .
a
a
b
7
5
4
0
6
2
c
d
e
Entry
Additive
Conv. (%)
Yield (%)b
2
a
3a
1
2
3
4
5
6
7
8
9
[bmim]SbF
[bmim]PF
[bmim]NTf
[bmim]BF
[bmim]Br
6
55
52
49
44
50
34
72
80
56
82
53
50
47
42
48
32
71
78
54
80
2
N.D.
2
1
1
1
1
1
1
c
6
2
28
4
14
[HOOC
4
mim]SbF
6
[bdmim]SbF
6
0
[C12dmim]SbF
HAc
6
0
50
100
150
200
250
d
1
0
HAc
2
a
Reaction condition: 1a (2 mM), NHPI (10 mol%), Co(OAc)
(1 atm), 100 °C, 15 h.
Yield determined by GC through using areas of peak normalization method.
N.D.: not detected.
2
(0.5 mol%), additive (5
Fig. 1. Time-course plots for the aerobic oxidation of 1a to 2a. General conditions: 1a
mol%), O
2
b
(2 mmol), O
(5 mol%), χH2O (5.5 × 10 ); b) Co(OAc)
2
(1 atm), 80 °C; a) Co(OAc)
2
(1.0 mol%), NHPI (10 mol%), [C12dmim]SbF
(1.0 mol%), NHPI (10 mol%), [C12dmim]SbF
H2O (7.7 × 10 ); c) NHPI (10 mol%), [C12dmim]SbF (5 mol%),
6
6
−3
c
2
−2
d
(5 mol%),
(
χ
6
χ
−3
H2O
Additive (5 mL). [bmim]: butyl metylimidazolium; [bdmim]: 3-butyl-1,2-dimethyl-
5.5 × 10 3); d) Co(OAc)
−
(1.0 mol%), [C12dmim]SbF (5 mol%), χH2O (5.5 × 10 ); e)
2 6
1
H-imidazol-3-ium; [C12dmim]: 1-dodecyl-3,3-dimethylimidazolium; [HOOC
4
mim]: 3-
−1
Co(OAc) (1.0 mol%), NHPI (10 mol%), [C12dmim]SbF (5 mol%), χH2O (1.8 × 10 ).
2
6
butyl-2-carboxy-1-methyl-1H-imidazol-3-ium.
91

S. Mahmood et al.
Molecular Catalysis 447 (2018) 90–96
Table 2
2 6
Intermolecular energy for ternary system of H O/IL/tolene (A: [bmim]SbF ; B: [bdmim]
7
6
5
4
0
0
0
0
6 2
SbF ; Ca: Cation; An: anion; W: H O; Tol: toluene).
Type
IL Electrostatic (kJ/mol) Lennard-Jones (kJ/
mol)
Intermolecular
energy (kJ/mol)
Ca-Ca
Ca-An
An-An
Ca-W
A
B
A
B
A
B
A
B
A
B
A
B
A
B
4603.41 ± 9.9
3474.13 ± 5.6
−4060.30 ± 15.0
−3821.74 ± 7.3
−5965.26 ± 14.0
−6774.18 ± 5.4
−681.582 ± 2.2
−593.78 ± 1.2
−435.11 ± 1.8
−539.91 ± 3.0
126.36 ± 0.4
543.11 ± 24.9
−347.61 ± 12.9
−29645.90 ± 51.0
−30448.78 ± 26.4
3513.00 ± 17.2
3046.66 ± 8.7
−1108.26 ± 5.5
−992.15 ± 8.6
−2903.47 ± 6.9
−2824.24 ± 19.2
−5082.73 ± 47.4
−5316.52 ± 22.8
−3929.36 ± 31.0
−3891.76 ± 13.0
−23680.60 ± 37.0
−23674.60 ± 21.0
4194.58 ± 15.0
3640.44 ± 7.5
−673.15 ± 3.7
−452.24 ± 5.6
−3029.83 ± 6.5
−2952.81 ± 17.0
512.51 ± 4.4
[
[
bmim]SbF
6
bdmim]SbF
6
0
.00
0.02
0.04
0.06
0.08
0.10
0.12
An-W
Ca-Tol
An-Tol
128.57 ± 2.2
−5595.24 ± 43.0
−5873.37 ± 21.0
−1997.67 ± 16.0
−1941.04 ± 6.6
556.86 ± 1.8
−1931.69 ± 15.0
−1950.72 ± 6.4
Fig. 2. Plots of the yield of 2a from Co/NHPI-catalyzed aerobic oxidation of 1a with IL
additive as a function of χH2O presented in the wet IL launched. General conditions: 1a
2 mmol), NHPI (10 mol%), Co(OAc) (0.5 mol%), IL (5 mol%), O (1 atm), 80 °C, 8 h.
2 2
(
imposed by the hydrogen-bonding interaction between C2 − H and
O.
Evidence of the self-assembly structures of ILs, such as [bmim]SbF
and [bdmim]SbF
H2O less than 2.5 × 10
structure of surface-active molecule (IL) employed. This conjecture was
further supported by examination of H O content in two different ILs to
reach an optimum yield of 2a, respectively (Fig. 2). It demonstrated
that the efficiency varied significantly with χH2O in the case of C2 − H
6
blocked bdmimSbF ([bdmim] = 1-butyl-2, 3-dimethyl imidazolium),
H
2
2
6
6
, and their transition in the micelles consisting of
−
1
χ
was obtained by surface tension measure-
2
ments towards a combination of IL and H O at 80 °C (Fig. 4). There are
but was kept much steady using the non-blocked [bmim]SbF
identical conditions.
6
under
two transition points for these systems within the range of test de-
monstrated by the plots of surface tensions against χH2O, which are
corresponding to two different self-assembly structures of IL mixed with
H O. It’s likely that a proper surface tension between the aqueous and
2
IL’s phase, representing a specific shape, is crucial to the initial reaction
To gain insights into the differences of spontaneous aggregation
between [bmim]SbF and [bdmim]SbF in a combination of 1a and
O, molecular dynamic simulation was performed. These systems
started from random of toluene, ILs and H O with a molar ratio of 20:1:
H2O (χH2O = 0.6), of which the initial configurations were generated
by Packmol package. It should be noted that χH2O was set up nearly
6
6
H
2
2
rate. The relatively higher value of surface tension in the case of
χ
[
bdmim]SbF
6 6
as compared with that of [bmim]SbF is ascribed to a
−3
stronger interaction between [bmim]SbF6 and H O.
3
–540 folds of those in experiments (ca. χH2O = 6 × 10 –0.2).
Spherical or quasi-spherical micelles were observed for both systems of
bmim]SbF and [bdmim]SbF in the simulation with H O dispersing in
2
We next examined medium effects with the aim to point out the
structure-performance relationship of ILs in regard to a catalytic cycle.
To address this aspect, various constituents of the vesicles in the pos-
sible locales, namely the aqueous phase, the interfacial boundary, and
the organic phase were schematically displayed in Scheme 1. The cat-
alytic pathway and key intermediates thereof were assumed according
to the literature [16b] and their relative amounts in the distinct locales
depend on their hydrophobicities and amphiphilicities. Consequently,
[
6
6
2
the discontinuous IL phase presented in the solution of reversed mul-
tilamellar vesicles (Fig. 3). Intermolecular energies for the above ve-
sicles are shown in Table 2. It is noticeable that the total interaction
energy of both systems (Ca-Ca + Ca-An + An-An + Ca-W + An-
W + Ca-Tol + An-Tol) is negative in the solution state. However, the
system consisting of [bdmim]SbF
bmim]SbF (2160.79 kJ/mol decreased in total interaction energy). As
indicated by the results, the stability was mainly attributed to stronger
interactions of Ca–Ca and Ca-Tol in the case of [bdmim]SbF . Herein,
the relatively stronger interaction of H O with [bmim] (−1108.26 kJ/
mol) as compared to with [bdmim] (−992.15 kJ/mol) probably
6
is much more stable than that of
2
the hydrophilic species (Co(OAc) and NHPI) are confined in the aqu-
[
6
eous droplets, whereas the lipophilic hydrocarbon substrates and
carbon centered radicals are located in the organic phases. The inter-
facial boundary may be considered as a pseudo-phase, constituted of
amphiphilic ILs in equilibrium between the aqueous and the organic
6
2
phase. For clarity, only the transport of O
2
, NHPI and transient oxygen-
[
[
bmim
]SbF
6
30
29
28
27
26
25
24
bdmim
]
SbF
6
0.00
0.05
0.10
0.15
0.20
0.25
Fig. 3. Snapshots for cluster of ions and water molecules. %H
are omitted.
2
O; — IL; toluene molecules
Fig. 4. Plots of the surface tension of water in two ILs ([bmim]SbF
a function of χH2O at 80 °C.
6 6
and [bdmim]SbF ) as
92

S. Mahmood et al.
Molecular Catalysis 447 (2018) 90–96
Scheme 1. Schematic display the localization of hydrocarbons (i.e. 1a),
transient species and products in the aqueous microdomain, in the organic
phase and at the interface of the microemulsion; for clarity, the distribu-
tion of species for the termination stage was not shown.
centred radical species across the interfacial boundary into aqueous and
organic phases is portrayed in the form of equilibrium arrows because
evidently the whereabouts of these reacting components of the vesicles
will be relevant for the mechanistic rationalization of the results.
Herein, the distribution of species at the initiation and propagation
stage was focused, whereas that of the termination stage was not
shown.
On the basis of literatures elucidating the mechanism of Co/NHPI-
catalyzed autoxidation [24], we envisioned that different diffusion
rates of active species at the interfacial boundary composed by different
amphiphilic ILs should take a crucial role on the oxidation efficiency,
which are strongly IL’s structure dependent. The diffusion coefficient
A
40
3
5
0
3
25
(a)
b)
(
2
0
5
1
10
5
0
(
D) of O
2
and NHPI could be measured without knowing their con-
0.0
0.5
1.0
1.5
2.0
2.5
3.0
centration and the number of electrons, n, involved by measuring both
transient (I) and steady-state currents (Iss) in the diffusion-controlled
system at an ultramicroelectrode (UME) [29]. The normalized current
1/2
/t
1
1
.7
B
−
1/2
1.6
I/Iss – t
relationship shown in eq 1 is a good approximation for the
time range considered:
1
1
1
1
1
.5
.4
.3
.2
.1
I/Iss = 0.7854 + 0.8862a/2D1
/2 1/2
t
+ …
(1)
(c)
In Eq. (1), a is the radius of the Pt UME and t is the measurement time.
These i-t curves were obtained in O -saturated and NHPI-contained IL
solutions, respectively, correcting for background in N -saturated IL
plots, which were obtained for
each IL solution, can be used to evaluate the diffusion current of O and
(d)
2
2
−
1/2
(
Fig. S5, A and B). The I/Iss vs. t
2
−1/2
NHPI molecule from the slope of the line. Representative I/Iss vs. t
1.0
curves in [bmim]SbF
Fig. 5.
6 6
and [bdmim]SbF are shown in Table 3 and
0
.0
0.2
0.4
0.6
0.8
1.0
From this set of data it appears that the transport of both O
NHPI in [bdmim]SbF are significantly faster than those in [bmim]
SbF , but in general, O diffuses slower when compared to NHPI. The
smaller D values obtained in [bmim]SbF demonstrated the diffusion of
electron-donor solute was inhibited in the presence of C2 − H subunit
at the imidazolium cation, which caused not only a hydrogen-bonding
2
and
−
1/2
Fig. 5. i/iss vs t
2
curve in O saturated (A) and NHPI-contained (B) ILs conducted at Pt
6
UME (diameter, 25 μm). Dots or cycles are experimental data; lines indicate simulated
results of transient currents. (a) IL: [bdmim][SbF
stepped to −0.45 V; (b) IL: [bmim][SbF
to −0.65 V; (c) IL: [bmim][SbF
(d) IL: [bdmim][SbF ] χ
6
2
−
5
6
] χH2O 2.00 × 10 ; the potential was
−5
6
6
] χH2O 2.00 × 10 ; the potential was stepped
−5
6
] χH2O 2.51 × 10 ; the potential was stepped to 0.96 V;
−5
6
H2O
2.51 × 10 , the potential was stepped to 0.98 V.
interaction between O
2
/NHPI and IL but also a high viscosity of the IL
medium itself [30]. On the basis of literatures [19c,22a,31], it’s most
and PINO, imposed by the structure of IL medium are correlated with
those of their precursors, albiet simulation was not conducted to obtain
the precise value. Besides, we found the solvation property of [bdmim]
−
likely the differences in diffusion rates of active species, such as O
2
%
Table 3
SbF
6
towards O
2
is better than that of [bmim]SbF
6
by comparing re-
Steady-State Current (Iss) and Diffusion Coefficient (D) in Different ILs.
duction potentials of O
in the cyclic voltammetry (CV) analysis at 40 °C
2
(
(
Fig. S6) [32]. Evidently, in the extremely dried [bmim]SbF
6
IL
Solute
Potential (V)
I
ss (nA)
D (cm2s−1
)
−5
χ
H2O = 2.0 × 10 ), the presence of O
2
showed a reduction peak at
−
−
−
−
9
7
7
6
[
[
[
[
bmim]SbF
bdmim]SbF
bmim]SbF
bdmim]SbF
6
O
O
2
−0.45
−0.65
0.96
1.00
1.57
1160
1450
1.19 × 10
6.56 × 10
6.15 × 10
4.37 × 10
approximately −0.86 V and an oxidation peak at −0.67 V vs. SCE. In
comparison, a more positive value of reductive wave at −0.54 V vs.
SCE featured with irreversible peak was observed in the dry [bdmim]
6
6
2
6
NHPI
NHPI
0.98
−5
SbF
6
(χH2O = 2.5 × 10 ). Collectively, the interfacial boundary
93

S. Mahmood et al.
Molecular Catalysis 447 (2018) 90–96
Scheme 2. Aerobic oxidation of various hydrocarbons catalyzed by the
[a].
2 6
ternary system Co(OAc) /NHPI/[C12dmim]SbF .
[
[
a] Substrate (2 mmol), NHPI (10 mol%), Co(OAc)
2
(1.0 mol%),
5.0 mol%, χH2O (7.7 × 10 2), O
−
(1 atm), 80 °C, 15 h,
C
12dmim]SbF
6
2
isolated yield. [b] Yield determined by GC–MS through using areas of peak
normalization. [c] The yield of dicarboxylic acid was estimated as di-
methyl ester after esterification with an excess amount of methanol (see
Supporting Information for the details); 5o: 2-(carboxymethyl)benzoic
acid; 5p: 2,2′-(1,2-phenylene)diacetic acid; 5q: adipic acid. [d] N.O.: not
observed.
consisting of C2-alkylated imidazolium-based ILs, such as [bdmim]SbF
6
the oxidation of toluene derivatives. For instance, ortho-substituted
toluene leads to slightly decreased yields as compared with those
bearing para- or meta-substituent (2b vs. 2c and 2j vs. 2k). In addition,
toluene substituted with electron-donating groups performs relatively
better than those with electron-withdrawing groups (2b – e vs. 2f – l).
To our surprise, the oxidation of 1e also proceeded smoothly, which
generates the corresponding acid 2e in a high yield of 80%. In Sasson’s
condition, the same reaction underwent sluggishly.1 It was therein
attributed to the formation of p-cresol (1f) upon hydrolysis of 1e, which
brought the process to a standstill. In fact, the oxidative acidification of
1f in this study was found to give a satisfactory yield of 2f under
standard conditions. Besides, 4-methylpyridine (1m), as a typical het-
eroaromatic hydrocarbon, was tolerated in such a catalytic system and
afforded the carboxylic acid in a higher yield than the conventional
binary Co/NHPI system. Various substrates with secondary carbons
(1n–1q) were also examined, which offered the corresponding ketones
and [C12dmim]SbF
6
rather than [bmim]SbF
6
, not only holds ternary
aggregates (hydrocarbons/IL/H
2 2
O) of higher stability but renders O a
faster diffusion rate and higher concentration, thereby offering a high
initial reaction rate of the protocol towards hydrocarbon oxidation.
Finally, further optimizations with respect to the loading of catalyst
constituents, such as NHPI and [C12dmim]SbF
well were performed towards the aerobic oxidation of 1a (Fig. S7).
Herein, the value of χH2O in the [C12dmim]SbF was controlled ap-
6
, and temperature as
9b
6
−
2
proximately at 7.7 × 10 . Under the optimized conditions, 1a was
smoothly converted to 2a in a high yield of 91%. We then turned our
attention to the substrate tolerance of such a protocol. A series of
substituted toluene were examined (Scheme 2, 1b–1l), which was
converted into a wealth of acids in yields that certainly rival those
conducted in HOAc without operational disadvantages and chemical
waste production. Remarkable steric and electronic effect was found in
94

S. Mahmood et al.
Molecular Catalysis 447 (2018) 90–96
and acids in poor to moderate yields.
calibration of tensiometer as mentioned in the manual provided by the
manufacturer.
3. Conclusions
4.2.3. Cyclic voltammetry (CV) and amperometry analysis
In this paper we studied cobalt/NHPI-catalyzed aerobic oxidation of
CV measurements were performed on a CH Instruments electro-
hydrocarbons in the presence of imidazolium-based IL additive func-
tioning as a kind of surfactant or phase transfer catalyst. The reaction
activity was influenced by not only the composition of microdomains
but also the structure of ILs launched. Evidently, the initial oxidation
rate was varied with water content contained in the wet IL loaded and
faster in the presence of C2-alkylated imidazolium ILs as compared to
that bearing C2 − H group. The structure-performance relationship of
ILs in regard to a catalytic cycle at the interfacial boundary was pre-
liminarily explored. The interfacial boundary between aqueous and
organic phase composed by C2-alkylated imidazolium ILs, such as
chemical analyzer with CHI 660D software. A three-electrode system
was used, consisting of a platinum ultra-micro disk (25 μm diameter)
working electrode, platinum wire (0.5 mm diameter and 6 cm length)
counter electrode, and saturated calomel electrode (SCE) as reference
electrode. The scan rate 50 mV/s was used for very dry and wet cases.
The final potential for amperometric i-t curve was taken from the oxi-
dation potential of oxygen from the cyclic voltammetry. The working
electrodes were repeatedly polished with 0.3 and 0.05 mm alumina
powder by using microcloth wetted with Milli-Q water before use. The
electrodes were also sonicated in Milli-Q water for 5–10 min and rinsed
[
bdmim]SbF
6
and [C12dmim]SbF
6
, not only has ternary aggregates
a faster dif-
2 4
with Milli-Q water. To clean Pt electrode 0.05 M H SO solution was
used and the potential scan was repeated between −0.2 and +1.3 V at
(
hydrocarbons/IL/H
2
O) of higher stability but renders O
2
fusion rate and higher concentration, thereby offering a high reactivity
of the protocol towards hydrocarbon oxidation. The present results may
hold promise for futher development of Co/NOH-catalyzed non-chela-
tion-assisted aerobic oxidation of hydrocarbons. Further studies to-
wards the optimization of ternary Co/NHPI/IL system for selective
cross-coupling of hydrocarbon with other organic substrates are cur-
rently underway.
0.1 V/s for 20 cycles. Theses pretreated electrodes were dried with dry
nitrogen before use. O
2
was bubbled directly into the ILs samples to
obtain an O -saturated solution. Cyclic voltammetry and amperometry
2
measurements were performed at room temperature.
Acknowledgements
Financial support by National Basic Research Program of China
(973 Program, 2015CB251401), National Natural Science Foundation
of China (21476240), Instrument Developing Project of the Chinese
Academy of Sciences (YZ201521), CAS (Chinese Academy of Sciences)
4
. Experiments
4.1. General procedure for oxidation of hydrocarbons
100-Talent Program (2014) and Key Research Program of Frontier
A mixture of [C12dmim]SbF
10 mol%), and Co(OAc) (1 mol%) was added into an autoclave (steel
type double jacketed pressure reactor with 20 mL teflon cylinder filled
with O at 1 atm). The reaction was heated by thermostated paraffin oil
6
(5.0 mol%), substrate (2 mmol), NHPI
Sciences, CAS (QYZDY-SSW JSC011) is gratefully acknowledged.
(
2
Appendix A. Supplementary data
2
at 80 °C and stirred for 15 h. Afterwards, it was cooled down to room
temperature and extracted with diethyl ether (10 mL × 3). Upon con-
centration on a rotation machine, it was analyzed directly by GC and
the yield of product was obtained through using areas of peak nor-
malization measurement.
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.mcat.2018.01.006.
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