Mn(III) active site in hydrotalcite efficiently catalyzes the oxidation of alkylarenes with molecular oxygen

Article abstract of DOI:10.1016/j.mcat.2020.111276

Developing efficient heterogeneous catalytic systems based on easily available materials and molecular oxygen for the selective oxidation of alkylarenes is highly desirable. In the present research, NiMn hydrotalcite (Ni₂Mn-LDH) has been found as an efficient catalyst in the oxidation of alkylarenes using molecular oxygen as the sole oxidant without any additive. Impressive catalytic performance, excellent stability and recyclability, broad applicable scope and practical potential for the catalytic system have been observed. Mn³⁺ species was proposed to be the efficient active site, and Ni²⁺ played an important role in stabilizing the Mn³⁺ species in the hydrotalcite structure. The kinetic study showed that the aerobic oxidation of diphenylmethane is a first-order reaction over Ni₂Mn-LDH with the activation energy (E_a) and pre-exponential factor (A₀) being 85.7 kJ mol^?1 and 1.8 × 10⁹ min^?1, respectively. The Gibbs free energy (ΔG^≠) was determined to be -10.4 kJ mol^-1 K^-1 for the oxidation based on Eyring-Polanyi equation, indicating the reaction is exergonic. The mechanism study indicated that the reaction proceeded through both radical and carbocation intermediates. The two species were then trapped by molecular oxygen and H₂O or hydroxyl species, respectively, to yield the corresponding products. The present research might provide information for constructing highly efficient and stable active site for the catalytic aerobic oxidation based on available and economic material.

Full text of DOI:10.1016/j.mcat.2020.111276

Molecular Catalysis 499 (2021) 111276
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Mn(III) active site in hydrotalcite efficiently catalyzes the oxidation of
alkylarenes with molecular oxygen
Anwei Wang, WeiYou Zhou*, Zhonghua Sun, Zhong Zhang, Zhihui Zhang, MingYang He*,
Qun Chen
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou University, 213164 Changzhou, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Developing efficient heterogeneous catalytic systems based on easily available materials and molecular oxygen
Alkylarenes oxidation
NiMn Hydrotalcite
Kinetic study
2
for the selective oxidation of alkylarenes is highly desirable. In the present research, NiMn hydrotalcite (Ni Mn-
LDH) has been found as an efficient catalyst in the oxidation of alkylarenes using molecular oxygen as the sole
oxidant without any additive. Impressive catalytic performance, excellent stability and recyclability, broad
Aerobic oxidation
Mn(III) cation
3
+
applicable scope and practical potential for the catalytic system have been observed. Mn species was proposed
to be the efficient active site, and Ni2 played an important role in stabilizing the Mn species in the hydro-
+
3+
talcite structure. The kinetic study showed that the aerobic oxidation of diphenylmethane is a first-order reaction
ꢀ
1
over Ni
2
ꢀ 1
Mn-LDH with the activation energy (E
a
) and pre-exponential factor (A
0
) being 85.7 kJ mol and 1.8 ×
9
∕=
-1 -1
1
0
min , respectively. The Gibbs free energy (ΔG ) was determined to be -10.4 kJ mol
K for the oxidation
based on Eyring-Polanyi equation, indicating the reaction is exergonic. The mechanism study indicated that the
reaction proceeded through both radical and carbocation intermediates. The two species were then trapped by
2
molecular oxygen and H O or hydroxyl species, respectively, to yield the corresponding products. The present
research might provide information for constructing highly efficient and stable active site for the catalytic
aerobic oxidation based on available and economic material.
1
. Introduction
would limit its wide application. Palladium NCN and CNC pincer com-
2
plexes [20], Pd(OAc) /bis-triazole system [21] and an enzyme-like
Efficient and selective activation of
α
-C–H bonds of alkylarenes is one
system based on 1,2,4-triazole-type ligands and nickel(II) bromide
[22] have been described by Urgoitia’s group, but NaOAc was required
as additive in all catalytic systems. Moreover, these noble metal-based
catalysts are not cost effective. Monjezi, et al. [23] have developed a
nano-sized Co-Mn catalyst and investigated its performance in the aer-
obic oxidation of diphenylmethane. Under high pressure, the conversion
and the selectivity reached 87 % and 90 %, respectively. The catalytic
system also suffered the drawbacks of laborious preparation and poor
of the most important and challenging research topic, and has attracted
much attentions from chemists. Development of efficient and practical
catalytic systems for the reaction is of both academic and practical in-
terests. Much effort has been devoted to the oxidative activation of
alkylarenes, and many catalytic systems have been developed using
peroxides, such as TBHP (tert-butyl hydroperoxide) [1–6], H
2
O
2
[7–10],
ozone [11,12], NaIO [13,14] and mCPBA (m-chloroperoxybenzoic
4
acid) [15], and so on. Compared with peroxides, molecular oxygen
obviously has the advantages of being less costly, environmentally
friendly, practically safe and easily controllable.
2
stability. A MnO @wool complex has been found to be able to catalyze
the selective aerobic oxidation of alkylarenes [24]. In the aerobic
oxidation of diphenylmethane, qualitative yield of the corresponding
ketone could be obtained in 9 h. However, the catalyst was produced
A variety of aerobic systems have been developed, but radical initi-
ators were usually required [16–18]. Supported gold nanoparticles (Au
NPs@3D-(N)GFs) synthesized by Kazerooni et al. [19] exhibited excel-
lent performance in the oxidation of alkylarenes. However, the stability
of the catalyst was not satisfactory enough, and the use of a noble metal
4
through the oxidation of wool with KMnO , which is an environmentally
unfriendly process. In addition, the catalytic efficiency still needs to be
improved. Ji and coworkers [25] have developed a catalytic system
including metalloporphyrins as the catalysts and cumene as
a
*
Corresponding authors.
E-mail addresses: zhouwy426@126.com (W. Zhou), hemy_cczu@126.com (M. He).
https://doi.org/10.1016/j.mcat.2020.111276
Received 13 September 2020; Accepted 18 October 2020
Available online 6 November 2020
2
468-8231/© 2020 Elsevier B.V. All rights reserved.

A. Wang et al.
Molecular Catalysis 499 (2021) 111276
2
Fig. 1. XRD pattern (A) and SEM image (B) of Ni Mn-LDH.
co-substrate. The yield of benzophenone reached 96 % in the aerobic
oxidation of diphenylmethane under the selected conditions, but the
process consumed a large amount of cumene, leading to the difficulty of
purification and high costs. The vapor phase oxidation of diphenyl-
for the transformation has been tried to be determined based on the
results of series of controlled experiments and characterization of the
catalyst. Furthermore, the possible reaction pathways were proposed
according to the intermediate analysis and isotope labeled experiments.
◦
methane in air over CeAlPO-5 catalysts under high temperature 325 C
has also been reported [26]. MN(SiMe
3
)
2
[M = K, Na or Li] has been
2. Experimental
found effective in the aerobic oxidation of diarylmethanes [27], but
excess base was required as the reactant, and the yields of the corre-
sponding products were not attractive. Mn- or Cr-containing meso-
porous molecular sieves have also been found effective in the catalytic
2.1. Preparation of LDHs catalyst
Ni
[39]. Firstly, A solution of NaOH (0.06 mol, 2.4 g) and NH
wt%, 4.2 g) in 150 mL deionized water was prepared in a 500 mL
three-neck round bottom flask. Then, a mixture of Ni(NO ‧6H O (0.06
mol, 17.4 g) and MnCl (0.03 mol, 3.8 g) in 150 mL deionized water was
2
Mn-LDH was obtained according to our previous reported method
oxidation of alkylarenes to ketones using O
2
as the sole oxidant [28,29],
3
‧H
2
O (25ꢀ 28
but the conversion of substrate was quite low. Heterogeneous catalysts
based on Mn complexes or metal-organic framework have been prepared
by Kuwahara and the coworkers, and moderate yields of the carbonyl-
ation products could be obtained [30,31]. Recently, Yamaguchi’s group
developed a series of transition-metal-containing manganese oxides
3
)
2
2
2
slowly added into the flask with mechanical stirring at 30 ℃ under air
atmosphere. After the titration, the resulting suspension was digested at
60 ℃ for 24 h. Finally, the formed precipitate was separated and washed
(
M–MnO
x
) through a low-temperature reduction method [32], and good
has been observed in the aerobic
catalytic performance for the Ni–MnO
x
with water and dried at 120 ℃ over night. The Ni
obtained in a brown powder. CuMgAl-LDH, Ni Al-LDH, Co
Mn Al-LDH were prepared through similar procedures using the corre-
2
Mn-LDH sample was
oxidation of alkylarenes. To sum up, the present catalytic systems for the
aerobic oxidation of alkylarenes suffer some drawbacks, including using
expensive noble metal or ligands, low efficiency, requirement of addi-
tives, higher reaction temperature or high pressure of oxygen. In addi-
tion, some of these are homogeneous, which would result in difficulty in
recycling of catalysts and purification of product. From the viewpoint of
sustainable chemistry, it is highly desirable to search for efficient het-
erogeneous catalytic systems originated from easily available materials
and molecular oxygen for the selective oxidation of alkylarenes.
LDHs (layered double hydroxides) has a hydrotalcite-like structure
2
2
Al-LDH and
2
sponding nitrates as the precursors.
2.2. Characterization of LDH samples
The identification of the crystallization of the prepared samples were
confirmed through Powder X-ray diffraction (XRD) using a Rigaku D/
max 2500 PC X-ray diffractometers with Cu-K (1.5402 Å) radiation.
α
Surface morphology was analyzed by scanning electron microscopy
(SEM) with a JEOL JSM-6360LA scanning electron microscope. The
compositions of samples were determined by inductively coupled
plasma analysis (ICP) using a Varian Vista-AX device. X-ray photoelec-
tron spectroscopy (XPS) measurement was analyzed on a Thermo Sci-
entific ESCALAB 250Xi instrument. Porosity and surface area data were
collected on a micromeritics ASAP2010C apparatus, using nitrogen as
the adsorbate at liquid nitrogen temperature. The surface area was
calculated using the BET method and the pore size distributions were
deduced through the BJH method.
and are represented by the general formula of [M(II)(1-x)
M
x+ nꢀ
(
III)
x
(OH)
2
]
[A
]x/n⋅mH
2
O [33,34]. It has been an important catalyst
due to its adjustability of the compositions [35,36], and further their
physic-chemical properties. Actually, LDHs has been widely employed
in various organic transformations based on its characteristic properties
of basicity and the redox functionality. Jana, et al. have employed Ni
hydrotalcite in the aerobic oxidation of diphenylmethane, but only a 62
conversion could be obtained even under high reaction temperature
5
Al
%
ꢀ
1
[
37]. MnO
4
-exchanged Mg–Al hydrotalcite also was prepared and
tested in the transformation [38], but the conversion was quite low.
In our continuous research on the development of efficient catalytic
systems for the oxidative activation of C–H bonds, LDHs with different
compositions have been tested using the aerobic oxidation of diphe-
2.3. Catalytic aerobic oxidation of alkylarenes
The oxidation was carried out in a carousel reaction tube to which
nylmethane as the model reaction. We found that Ni
2
Mn-LDH could
diphenylmethane (1.0 mmol), Ni Mn-LDH (0.2 g), dodecane (2 mL) and
2
smoothly accelerate the oxidation and exhibited the highest catalytic
activity under the selected conditions (Fig. S1 and Table S1). Based on
chlorobenzene (0.5 mmol, used as the internal standard reference) were
added. The mixture was then magnetically stirred at 120 ℃ under ox-
ygen atmosphere (1 atm). The reaction was sampled periodically and
analyzed through a gas chromatograph (Shimadzu GC-2010AF). After
the reaction, the catalyst was separated via filtration, washed with ethyl
acetate, dried at 120 ℃ for 4 h and introduced into a new reaction to
recycle the catalyst. The conversion of the diphenylmethane and the
the findings, the selective aerobic oxidation of
α-C–H bonds of arylenes
under the catalysis of Ni Mn-LDH has been studied in detail in the
2
present study. The effects of conditions on the reaction, relationship of
structure and activity, reaction kinetics and thermodynamics, and
recyclability were systematically investigated. The active catalytic site
2

A. Wang et al.
Molecular Catalysis 499 (2021) 111276
2
Fig. 2. (A) Full-scan XPS spectrum of the Ni Mn-LDH sample. Fine-scan XPS spectra of (B) O 1s, (C) Ni 2p, and (D) Mn 2p catalysts.
selectivity of benzophenone were calculated using the internal standard
method according to the GC analysis with no allowance for the back-
ground. The turnover frequency (TOF) value was calculated based on
the mol of diphenylmethane converted per hour per mol of Mn on the
surfaces of catalysts. The catalytic oxidations of other substrates were
conducted with similar procedure, and the conversion and selectivity
were calculated using the normalization method. Some of the products
were isolated using flash chromatography analyzed through a NMR
spectrometer.
spacing distances of LDHs are actually determined by the size of anions
between the layers and the value can be calculated from the reflection
2
(003). A value of 7.89 Å was obtained for the prepared Ni Mn-LDH
sample, generally equal to the reported results [42], indicating the an-
ions in the brucite are mainly CO2
–
and OH–.
Mn-LDH were then character-
ized. The atomic concentrations of the Ni Mn-LDH sample and the
3
The surface chemical properties Ni
2
2
chemical environments of the nickel, oxygen and manganese were
investigated by XPS, and the spectra can be found in Fig. 2. Full-
spectrum XPS (Fig. 2A) indicates the presence of Ni, Mn, O and C in
.4. 18O-labeled H
18
2
the Ni Mn-LDH. The chemical shifts in the O 1s spectra can be used to
2
O experiments
2
identify the species in the sample. Analysis of O 1s spectra (Fig. 2B) for
2
Ni Mn-LDH shows that the sample is composited of metal hydroxides,
To a mixture of diphenylmethane (1.0 mmol), Ni
2
Mn-LDH (0.2 g),
namely that of Ni and Mn, in consistent with the XRD pattern [43]. For
the XPS spectra of the Ni 2p3/2 (Fig. 2C), only one peak can be fitted at a
binding energy of ca. 855.5 eV, and a spin-energy separation of about
dodecane (2 mL), chlorobenzene (0.5 mmol, used as the internal stan-
1
8
dard reference) in a carousel reaction tube, H
2
O [40 mg (2 mmol), 97 %
O enriched, Aladdin Chemical Co.] was added. The reaction mixture
was stirred for 120 min at 120 ℃ under oxygen atmosphere (1 atm) and
1
8
1
2
7.6 eV is observed, which refer to Ni(OH) according to the reported
1
6
18
results in literatures [43–45].
Unlike the Ni 2p3/2, the XPS spectra of the Mn 2p3/2 can be fitted by
three peaks (Fig. 2D), and binding energies of ca. 637.5 eV, ca. 642.1 eV
then directly analyzed by GC–MS. The O and O composition in
benzophenone were determined by the relative abundance of mass
1
6
18
peaks at m/z = 182 for O and m/z = 184 for O.
and ca. 646.5 eV are identified to Mn2 , Mn and Mn , respectively
+
3+
4+
2
+
3+
4+
[
46–48], indicating that the oxidation of Mn to Mn and Mn took
place during the synthesis of NiMn hydrotalcite. It is obvious that the
content of Mn3 was significantly higher than Mn and Mn , and
about a value of 78 % could be observed. The observation is consistent
with the XRD analysis, because the trivalent cation is essential for the
formation of hydrotalcite structure [49,50]. In the previous reports
3
. Results and discussion
+
2+
4+
3
2
.1. Characterization of Ni Mn-LDH
The XRD pattern of the Ni
2
Mn-LDH depicted in Fig. 1A exhibits
intense typical reflections located at the angles of a hydrotalcite-like
3
+
◦
◦
about the Mn-contained catalysts for oxidation transformation, Mn
phase. The sharp and asymmetrical diffraction peaks at 11 , 23 and
3
+
◦
has been proposed as a key catalytic site [32,51]. Therefore, Mn in the
Ni Mn-LDH might be responsible for its high catalytic performance. To
the best of our knowledge, there has been no report for effectively
constructing the stable Mn3 . Some materials containing Mn have
3
5 are related to (003), (006) and (009), respectively; broad and
◦
2
asymmetrical peak for (018) located at about 40 [40,41]. No diffrac-
tions for impurities can be found, indicating the good purity of the
+
3+
◦
sample. The distinguished peaks at 60ꢀ 61 for the (110) and (113)
been reported, such as hierarchical brush-like
α
-MnO
2
and Co
3
O
O
4
2
suggest its good crystallization [35,36]. The SEM image of Ni Mn-LDH
nanoarrays [52] and magnetic composites of biochar and MnFe
2
3
4
obviously shows the plate-like agglomerated crystals, also demon-
strating the formation of hydrotalcite structure (Fig. 1B). The basal
+
(
MnFe
2
O
4
/MX) [53], but only 51.2 % and 33 % content of the Mn
,
3

A. Wang et al.
Molecular Catalysis 499 (2021) 111276
Table 1
3.2. Catalytic performance of Ni
2
Mn-LDH
Effect of solvents on the diphenylmethane oxidation.
The catalytic performance of Ni
2
Mn-LDH in the aerobic oxidation of
alkylarenes using diphenylmethane as the model substrate was then
systematically investigated. The effects of reaction conditions on the
oxidation were firstly examined and optimized.
Entry
Solvent
Conv. (%)
Sel. (%)
Various solvents, including p-xylene, benzyl benzoate, diphenyl
ether, polyethylene glycol-200 (PEG-200), dimethylformamide (DMF),
γ-valerolactone, diglycol, mesitylene and dodecane, have been investi-
1
2
3
4
5
6
7
8
9
p-xylene
48
6
>99
54
benzyl benzoate
diphenyl ether
polyethylene glycol-200
γ-valerolactone
diglycol
9
>99
>99
97
2
gated in the diphenylmethane oxidation. Using Ni Mn-LDH as the
5
catalyst, most of the selected solvents exhibited excellent selectivities of
benzophenone. On the whole, solvents with weak polarity were superior
to polar solvents, and the dodecane with the lowest polarity exhibited
the highest conversion and yield (Table 1, entry 9). Although PEG-200
gave good result in the oxidation over Pd NCN and CNC pincer com-
plexes [20], it provided negligible result in the present catalytic system.
5
4
>99
67
DMF
22
47
80
mesitylene
>99
>99
dodecane
Reaction conditions: diphenylmethane 1 mmol, catalyst 0.2 g, sol-
◦
vent 2 mL, 100 C, 6 h, O
2
(1 atm).
The results of the effect of temperature on the catalytic reaction are
displayed in Fig. 3. It is obviously observed that high temperature
evidently improved the catalytic reactivity. The reaction finished within
1
00 min. under a reaction temperature of above 120 ℃, while only
about 32 % conversion could be obtained at 80 ℃. In spite of this, no
significant by-product was observed in the reaction. Under the opti-
mized conditions, quantitative yield of benzophenone could be effi-
ciently obtained in the catalytic aerobic oxidation of diphenylmethane
over Ni
Reaction conditions: diphenylmethane 1 mmol, Ni
dodecane 2 mL, O (1 atm).
2
Mn-LDH.
2
Mn-LDH 0.2 g,
2
To compare the present catalytic system with known catalysts, some
typical reported results for the aerobic oxidation of diphenylmethane to
benzophenone are collected in Table 2. The catalytic systems based on
noble metals (entries 1–3) exhibited excellent reactivity in the oxida-
tion. However, the additive NaOAc was required in the cases of Pd
pincer complexes. Gold nanoparticles supported on three-dimensional
nitrogen-doped graphene-based frameworks (Au NPs@3D-(N)GFs)
gave a 94 % conversion of the substrate in 5 h, but the stability was not
good enough [20]. In addition, the high costs of the noble metal based
Fig. 3. Effect of temperature on the diphenylmethane oxidation.
respectively, could be obtained. The low content of the catalytic site
should limit its catalytic performance. Therefore, the present study
might provide an effective method to construct Mn3 on the basis of
hydrotalcite structure, which will be helpful to prepare efficient cata-
lysts with high catalytic performance.
2
catalysts limit their large-scale application. The NiBr complexes have
+
also been investigated by the same research group (entries 4–5), but the
catalytic efficiency was obviously low in comparison with Pd. MnTPPCl
combined with cumene as the co-substrate could efficiently catalyze the
transformation, but a large amount of cumene added in the system
should result in difficulty of the separation [25]. The above homoge-
neous catalytic systems generally suffered by the low efficiency of
recyclability of the catalyst. Some heterogeneous catalysts have also
Table 2
Some typical catalytic results for the aerobic oxidation of diphenylmethane.
ꢀ
1
Entry
Catalyst
Additive
Reaction time (h)
TOF (h
)
Conv. (%)
Sel. (%)
Ref.
a
1
2
3
4
5
6
7
8
9
Pd NCN pincer complexes
Pd CNC pincer complexes
Au NPs@3D-(N)GFs
NaOAc
24
24
5
–
98
[20]
[20]
[19]
[22]
[22]
[25]
[23]
[36]
[35]
[29]
[28]
[28]
[28]
[49]
[24]
[30]
This work
a
NaOAc
94
–
–
–
–
–
94
>99
a
NiBr
NiBr
2
/ 1,2,4-triazole
NaOAc
48
48
12
3
84
a
2
/ bis-triazolyl ligand
NaOAc
97
MnTPPCl
cumene
96
87
27.4
62
51
4
100
90
Co/Mn-BSO0.01 nanocatalyst
–
–
–
–
–
–
–
–
–
–
–
ꢀ
1
MnO
4
-exchanged Mg–Al hydrotalcite
5
8.0
–
–
12
16
17
100
96.9
98
Ni Al-LDH hydrotalcite
5
5
10
11
12
13
14
15
16
17
CrMCM-41
24
6
MnS-1(10 bar)
100
100
100
MnMCM-41(10 bar)
MnMCM-48(10 bar)
6
14
15
6
a
PC-MnO
MnO @wool
Ni-MnO
Ni Mn-LDH
2
6
95
a
2
9
–
–
100
a
x
5
>99
2
1.7
12.6
>99
>99
a
Yield of benzophenone.
4

A. Wang et al.
Molecular Catalysis 499 (2021) 111276
Table 3
2
Catalytic aerobic oxidation of various alkylarenes under Ni Mn-LDH.
a
Entry
1
Substrate
Product
Reaction time (h)
Yield (%)
1.7
>99
2
3
10.5
93
1.5
>99
4
5
1.5
2.5
98
92
6
7
3.5
4
>99
>99 (91)
8
9
2.5
3.5
97
81 (63)
1
1
0
1
12
12
83 (69)
79 (66)
2
b
1
2
40
3
1
3
4
20
20
19
5
1
2
1
5
1
>99
2 2
Reaction conditions: substrate 1 mmol, Ni Mn-LDH 0.2 g, dodecane 2 mL, 120 ℃, O (1 atm).
a
Calculated via the normalization method based on the GC–MS analysis, and isolated yield in brackets.
b
◦
8
0
C.
5

A. Wang et al.
Molecular Catalysis 499 (2021) 111276
Table 4
Catalytic aerobic oxidation of some other substrates with ꢀ CH
2
– in different environments.
a
Entry
Substrate
Product
Reaction time (h)
Yield (%)
1
6.5
95
b,c
b,c
2
4
83
3
16
78
b
4
48
8
trace
84
d,e
5
d,e
6
10
66
57
43
d,e
7
4
d,e
8
10
1
8
3
d
9
4
5
d
1
0
24
8
2 2
Reaction conditions: substrate 1 mmol, Ni Mn-LDH 0.2 g, dodecane 2 mL, 120 ℃, O (1 atm).
a
Calculated via the normalization method based on the GC–MS analysis, and isolated yield in brackets.
b
c
DMSO was used as the solvent.
◦
8
0
C.
d
e
◦
Mesitylene was used as the solvent, 160 C.
It was difficult to isolate the product even using flash chromatography, therefore, these products were analyzed by GC–MS.
been developed. Supported Co-Mn catalyst (Co/Mn-BSO0.01 nano-
catalyst) could give good result for the reaction, but high reaction
temperature and pressure of oxygen were required (entry 6). Under the
mild conditions, most of the heterogeneous catalytic systems based on
non-noble metals could not give satisfied catalytic reactivity in the
oxidation, although the selectivity was high enough (entries 7–12).
2
been calculated for the Ni Mn-LDH catalytic system based on the mol
number of Mn on the surface of catalyst, which was estimated based on
the BET and XPS data (the details for the calculation can be found in
ꢀ 1
Supporting Information). And a TOF value of 12.6 h was obtained for
the present catalytic system. Although it is on the same level as reported
results (entries 8, 11–13), these catalytic systems can not give satisfied
oxidation yields.
2
Porous chitosan and wool supported MnO catalysts [24,54] and
Ni-MnO [32] exhibited excellent catalytic activities in the aerobic
x
oxidation of diphenylmethane, however, the preparing processes were
quite complicated. In comparison with these results, the present
2
3.3. Catalytic activity of Ni Mn-LDH for other alkylarenes oxidation
Ni
2
Mn-LDH catalyst can be conveniently prepared by simple copreci-
With the interesting finding for the excellent catalytic performance
of Ni Mn-LDH in the aerobic oxidation of diphenylmethane, we tried to
pitation method. And the catalytic system can provide excellent cata-
lytic performance and almost qualitative yield in the oxidation of
diphenylmethane under relatively milder conditions. The TOF has also
2
investigate the scope of catalytic system, and various alkylarenes were
employed as substrates under the optimized reaction conditions
6

A. Wang et al.
Molecular Catalysis 499 (2021) 111276
(
2
Table 3). The CH group of all the alkylbenzene substrates was
– –
oxidized to carbonyl with excellent selectivity (>99 %), showing high
chemoselectivity in the transformation. Diphenylmethane and its ana-
logs could be well tolerated in the catalytic system, and excellent yield of
the corresponding oxidative products could be obtained (entries 1–3).
Electronic effect can be obviously observed in the reaction, and the
substrates with electro-donating group located in the aryl rings have the
higher activity than the ones with electron-withdrawing substituents.
When xanthene was used as the substrate, the reaction could finish in
1
.5 h, shorter than that for diphenylmethane (entry 4); in contrast, a
prolonged time was required for bis(4-fluorophenyl)methane (entry
–3), which should be due to the deactivation of the F groups. Similar
2
–
results were also found in the cases of 2-benzylpyridine and 4-benzylpyr-
idine (entries 5–7). Fluorene and its derivatives could also be well
tolerated over the catalytic system (entries 8–11), and good to excellent
yields for the carbonylation products were obtained. Substrates with
electron-withdrawing substituents dramatically decrease the reactivity,
which can be deduced from the reaction time, in consistent with the
above observations. Ethylbenzene and its derivatives could not give the
corresponding carbonylation products even under high temperature and
prolonging time, which should be ascribed to the low activation of
methyl group (entries 12–14). When 9,10-dihydroanthracene was
introduced as the substrate, dehydrogenative aromatization product
with excellent selectivity was obtained (entry 15). No oxygenation
product, namely anthracene-9,10-dione, could be detected, quite
different from that under the catalysis of Pd NCN or CNC pincer com-
plexes [20] and metalloporphyrins [55].
Fig. 4. The hot-filtration experiment.
the chemical structure of alkylarenes, it can be concluded that the pre-
pared Ni Mn-LDH catalyst provides an efficient and simple method for
the transformation of alkylarenes to carbonyl compounds.
2
3
.4. The stability and recyclability of Ni
2
Mn-LDH
The stability for the catalyst was then investigated under the selected
Furthermore, some other substrates with
2
CH in different envi-
– –
conditions. To examine the leaching of metallic ions, a hot-filtration
experiment has been conducted. When the reaction proceeded about
ronments have also been tested in the catalytic system (Table 4). The
aerobic oxidation of 1,2-diphenylethan-1-one could proceed smoothly in
3
0 min (about 40 % conversion), the catalyst was filtered off, and the
2
the Ni Mn-LDH catalytic system, and excellent yield of the 1,2-dicar-
filtration was continuously stirred under the same reaction conditions.
The plots for the conversion depicted in Fig. 4 show that no further
transformation took place without catalyst, suggesting that no leaching
of the active site happened.
bonyl compound could be obtained (entry 1). Interestingly, when 3,4-
dihydronaphthalen-2(1H)-one was used as the substrate, 2-naphthol
was formed in an excellent 83 % yield (entry 2), suggesting that dehy-
drogenative aromatization took place in the case. Although some
methods have been reported for the dehydrogenative aromatization of
cyclohexenones [56,57], the present catalytic system will provide a
simple process to synthesize naphthol from naphthalenone based on
heterogeneous catalyst and using molecular oxygen as the sole oxidant.
Benzylic ethers were also introduced as substrate to synthesis the cor-
Reaction conditions: diphenylmethane 1 mmol, Ni
dodecane 2 mL, 120 ℃, O (1 atm).
The recyclability of the Ni Mn-LDH catalyst was subsequently tested
2
Mn-LDH 0.2 g,
2
2
with a reaction time of 80 min. Almost no reduction of activity could be
observed for the catalyst even after eight runs (Fig. S2). XRD pattern for
the reused catalyst indicates that the layered structure was completely
preserved after several reuses (Fig. 5A). ICP analysis of regenerated
catalyst revealed that Ni and Mn contents almost did not change after
2
responding benzoate esters. Under the catalysis of Ni Mn-LDH, benzyl
methyl ether could transform to methyl benzoate in good yield (entry 3),
significantly higher than the reported result [58]. However, the oxida-
tion of isochroman could not happened (entry 4). Isoquinolones, which
are important synthetic intermediates for biologically active molecules
the reaction. XPS spectra for the Mn 2p suggest that the distribution of
n+
Mn
2
active site remained unaltered (Fig. 5B and Table S3). N -
adsorbtion/desorption also demonstrate no significant variation of the
physic-chemical properties (Fig. 5C, D and Table S3). These observa-
tions strongly indicate the excellent structurally and catalytic stability of
[
59–61], could be obtained in moderate to good yield when isoquino-
lines were used as the substrates using mesitylene as the solvent (entries
–8). We speculated that tandem reaction sequence of
5
2
the Ni Mn-LDH catalyst during the aerobic oxidation of diphenyl-
dehydrogenation-oxidation happened in the catalytic system. Electronic
effect could also be observed in these cases. The substrate with
electron-withdrawing substituent in the phenyl reduced the reactivity,
while electron-donating group improved the reaction rate. Although
some methods have been reported for the synthesis of isoquinolones
with various starting materials [62–64], the present protocol has the
advantages of simple starting material, heterogeneous catalysis and mild
reaction conditions. However, for the N,N-dimethylbenzylamine, only a
methane under the present conditions.
3.5. Kinetic and thermodynamic analysis for the catalytic oxidation of
diphenylmethane over Ni Mn-LDH
2
Kinetic study is an important aspect for one catalytic reaction and it
has also been conducted in the present research. After elimination of
internal and external diffusion (the calculation details can be found in
the Supporting Information) [33,39,65,66], the experiments were
investigated under different temperatures to study the kinetic aspects of
the aerobic oxidation of diphenylmethane. The fitting results indicate
that the oxidation is a first order reaction (Fig. 6).
1
8 % yield of the oxygenation product could be obtained (entry 9), and
benzaldehyde was found as the main by-product. When N-benzyl-N-e-
thylaniline was introduced into the reaction, only the imine could be
observed by GC–MS in about 8% yield, probably resulted from
de-ethylation and dehydrogenation. No oxygenation product for the
substrate could be observed, which might be due to the steric-hindrance.
Reaction conditions: diphenylmethane 2 mmol, Ni Mn-LDH 0.2 g,
2
Based on the aforementioned pieces of results, the Ni
2
Mn-LDH cat-
dodecane 3 mL, O (1 atm).
2
alytic system has exhibited excellent activities in the oxidation of
alkylarenes, and high yields were obtained for most of the studied
substrates. Although the reaction rate and reaction conditions depend on
The k values can be correlated by an Arrhenius-type expression, and
the parameters of Arrhenius equation (Eq. 2) can be deduced via the plot
of lnkto the 1/T.
7

A. Wang et al.
Molecular Catalysis 499 (2021) 111276
2 2
Fig. 5. XRD pattern (A), Mn 2p XPS spectra (B) and N adsorption/desorption isotherms (C and D) of the recycled Ni Mn-LDH.
Fig. 6. First order kinetics fit of aerobic oxidation of diphenylmethane under
different temperatures (X
A
is the value of diphenylmethane conversion).
Fig. 7. Arrhenius plot for the aerobic oxidation of diphenylmethane over
Ni Mn-LDH.
2
lnk = lnA
0
ꢀ E
a
/RT
(2)
∕
ꢀ 1 ꢀ 1
K
where k is the rate constant at temperature T, ΔH (kJ mol
) is the
change in enthalpy of activation, ΔS (J mol K ) is the change in
entropy of activation, k , h and R are Boltzmann, Planck, and universal
is the energy of activation (kJ mol 1) and A
ꢀ
is the pre-
where E
a
0
∕=
-1 -1
ꢀ 1
exponential factor (min ).
b
A multiple regression analysis with this expression of the constants
against temperature was depicted in Fig. 7. The slope and the intercept
of the equation can be obtained as -10303.29808 and 21.31649,
gas constant, respectively.
The Eyring-Polanyi plot for the aerobic oxidation of diphenyl-
methane over Ni
2
Mn-LDH is shown in Fig. S4. The slope and the inter-
respectively. And the values of the apparent activation energy (E
a
) and
cept of the equation can be obtained as -9905.4 and 14.3303,
the pre-exponential factor (A ) can be calculated through Eq. (2), and
0
∕=
∕=
respectively. And the values of the ΔH and ΔS can be calculated
ꢀ 1
9
ꢀ 1
they are 85.7 kJ mol and 1.8 × 10 min , respectively.
ꢀ 1 -1
ꢀ 1
ꢀ 1 -1
through Eq. (3) to be ꢀ 82.4 kJ mol
K
and ꢀ 183.2 J mol K ,
Thermodynamic characteristics of the catalytic oxidation were
respectively. A value of ꢀ 10.4 kJ mol K-1 for the change in Gibbs
calculated according to the Eyring-Polanyi equation (Eq. 3) [67–69].
∕=
energy (ΔG ) can be determined by the Eq. (4), indicating the reaction is
∕=
∕=
exergonic.
ln(k/T) = ꢀ ΔH /RT + ln(k
b
/T) + ΔS /R
(3)
8

A. Wang et al.
Molecular Catalysis 499 (2021) 111276
Table 5
2 2 2
active site for the oxidation, commercial MnCl .4H O and MnO , having
2
+
4+
The results for the controlled experiments.
Mn and Mn cations, respectively, have been tested in the reaction
for comparison. Quite low conversions were obtained (entries 7–8),
suggesting that the active site for the aerobic diphenylmethane oxida-
Entry
Catalyst
Additive
Conv. (%)
Sel. (%)
1
Ni
Ni
Ni
–
2
2
2
Mn-LDH
Mn-LDH
Mn-LDH
–
–
–
–
–
–
–
–
>99
56
11
2
>99
>99
>99
>99
>99
>99
78
3
+
2+
a
tion might be Mn , which is not stable and prone to transform to Mn
2
b
and Mn4 . In the Ni
+
Mn-LDH sample, Ni can stabilize the Mn cat-
2+ 3+
3
2
4
5
6
7
8
9
ions in the hydrotalcite structure through hybridization of the Mn(3d)
orbitals with the Ni(3d) orbitals to yield an average d electron config-
uration per metal atom greater than 4 [34].
Ni
2
Al-LDH
3
MnCO
MnCl
MnO
3
5
2
⋅4H
2
O
3
To further study the catalytic activity of the Mn cations with different
2
8
>99
>99
48
c
2+
Ni
Ni
2
Mn-LDH
Mn-LDH
BHT
46
94
valences in the oxidation, the investigation of catalytic activity of Mn
,
c
3+
4+
1
0
2
TEMPO
Mn and Mn has been tried using MnCO
3
, Ni
2
Mn-LDH and MnO
2
as
the represents, respectively. Although the evaluation methodology is
rough, it can reflect the relative catalytic activity of these species to
Reaction conditions: diphenylmethane 1 mmol, Ni
mL, 100 min, 120 ℃, O (1 atm).
2
Mn-LDH 0.2 g, dodecane 2
2
a
2+
4+
Air atmosphere (1 atm).
some extent. The contribution of Mn and Mn has been subtracted in
the case of Ni Mn-LDH, because it contains all the three species. From
the curves of conversion of diphenylmethane against reaction time in
b
c
Nitrogen atmosphere (1 atm).
2
2
Equiv. of substrate.
3
+
Fig. 8, it is obvious that Mn shows significantly high catalytic activity
than Mn2 and Mn in the oxidation. The fitting results for the kinetic
profiles indicate that the oxidations under the three Mn species are all
first order reactions (inserted plots in Fig. 8). The reaction rate constants
+
4+
2
+
3+
4+
ꢀ 4 -1
for Mn , Mn and Mn were calculated to be 1.5 × 10 s , 210 ×
ꢀ
0
4
s-1 and 0.51 × 10 s , respectively, further verifying the highest
ꢀ 4 -1
1
3
+
catalytic activity of Mn . The result should be related to the higher
3
+/2+
4+/2+
standard reduction potential of Mn
(~1.54 V) than Mn
~1.22 V), that is, Mn3 species has a stronger oxidation ability toward
diphenylmethane [51,70].
Reaction conditions: diphenylmethane 1 mmol, Mn (n = 2, 3 and
) 0.47 mmol, dodecane 2 mL, 120 ℃, O (1 atm).
The reaction mechanism for the oxidation over Ni
+
(
n+
4
2
2
Mn-LDH was then
investigated in detail. The possibility of involvement of a radical inter-
mediate was examined by introduction of BHT (2,6-di-tert-butyl-4-
methylphenol) and TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl),
which are common radical scavengers, in the catalytic reaction. BHT
could significantly suppress the reaction, while the selectivity almost did
not change (Table 4, entry 9). In contrast, the conversion decreased
slightly in the case of TEMPO, but the selectivity decreased to only 48 %
(
Table 4, entry 10). In addition, small amount of 1,1,2,2-tetraphenyl-
Fig. 8. The catalytic activity of Mn species with different valences and the first
order kinetics fit of the oxidation of diphenylmethane.
ethane was observed in the reaction by GC–MS analysis (compound I).
These results suggest that radical intermediate might be involved in the
catalytic reaction, and 1,1,2,2-tetraphenylethane might form through
coupling of two diphenylmethane radical.
ΔG = ΔH ∕= ꢀ TΔS ∕=
∕=
(4)
3
.6. Discussion of reaction pathways for the catalytic oxidation of
diphenylmethane over Ni Mn-LDH
2
To get further insight into the catalytic system and structure-activity
relationship, some controlled experiments were performed and the re-
sults are summarized in Table 5.
When the oxygen was replaced by air or nitrogen, significantly
decreased conversions were obtained, indicating the necessity of the
molecular oxygen (entries 2–3). Without catalyst, only a 2% conversion
was observed, suggesting that the catalyst is critical for the oxidation of
diphenylmethane (entry 4). To distinguish the effect of Ni and Mn spe-
cies, we have tried to prepare NiAl and MnAl hydrotalcites through
coprecipitation method. From the XRD pattern for the two samples
To further elucidate the possibility of the single-electron transfer
(
SET) mechanism in the activation of diphenylmethane in the catalytic
system, the aerobic oxidation 4-ethylanisole (compound II) and 4-iso-
propylanisole (compound III) were conducted under the catalysis of
2
Ni Mn-LDH to distinguish the SET and hydrogen-atom transfer (HAT)
mechanism. The difference in reactivity between the above two sub-
strates can reflect the reaction pathways. For the SET reactions, the
(
Fig. 1), it is observed that Ni
but only MnCO phase formed during the coprecipitation of MnCl
Al(NO
2
Al-LDH has been successfully synthesized,
3
2
and
◦
◦
◦
◦
relative rate of C–H oxidation is 2 >3 , whereas it is 3 >2 for HAT
reactions [32,71,72]. As shown in Fig. S5, the initial rate for the
oxidation of 4-ethylanisole was apparently higher than that for 4-isopro-
pylanisole. Therefore, the aerobic oxidation of alkylarenes over
3
)
3
solutions. The two samples only gave ≤5% conversions in the
oxidation of diphenylmethane (entries 5–6), implying that Mn species in
hydrotalcite structure is essential for the catalytic function. The results
also demonstrated that Ni played a key role in forming Mn-contained
hydrotalcite compound in the present system, suggesting that Ni and
Ni
1,72].
In the aerobic oxidation of alkylarenes, Mars–van Krevelen mecha-
nism and oxidative nucleophilic substitution are also possibly involved
2
Mn-LDH is suggested to be initiated mainly by the SET process [32,
7
Mn in Ni
cording to the XPS analysis of the prepared Ni
the LDH structure existed in different valences (Fig. 2). To determine the
2
Mn-LDH might have synergistic effects in the reaction. Ac-
2
Mn-LDH, the Mn cation in
9

A. Wang et al.
Molecular Catalysis 499 (2021) 111276
In the above discussed reaction pathways, either through radical
intermediate or carbocation intermediate, alcohol should be produced
as intermediate product. To further check the formation of alcohol, the
alcohol and ketone were monitored during the oxidation under the
optimized conditions, and the plot of their concentrations against the
reaction time were depicted in Fig. 9. It could be found that the con-
centration of alcohol was quite low, and it almost could not be detected
after 70 min. However, the observation does not exclude the formation
of alcohol as the intermediate, because the rate of diphenylmethanol
oxidation might be significantly higher than that of diphenylmethane
oxidation under the selected conditions. To examine the speculation, a
mixture of diphenylmethane and diphenylmethanol were introduced as
the reactants under the selected conditions. The result in Fig. S7 obvi-
ously indicates that the oxidation rate of diphenylmethanol is evidently
higher than that of the oxidation of diphenylmethane, suggesting that
diphenylmethanol formed as the intermediate is reasonable in the
oxidation of diphenylmethane to benzophenone. In addition, the alcohol
might come from two pathways, namely the decomposition of peroxides
Fig. 9. The variation of the concentration of diphenylmethane and products in
the reaction.
in the reaction. To distinguish the process, the aerobic oxidation of
diphenylmethane was conducted under the selected conditions in the
presence of H1
8
O. As a result, O-lablled product increased at the first 5
min., and then decreased to a constant value as the reaction advanced
18
2
(
Fig. S6). The observation is significantly different from the Mars–van
18
Krevelen-type mechanism, where the O-lablled product would in-
crease from zero at the low turnover number [32,72,73]. In contrast, the
oxidative nucleophilic substitution mechanism is probably involved in
and the nucleophilic attack of H O or hydroxyl species to the cations.
2
Reaction conditions: diphenylmethane 1 mmol, Ni Mn-LDH 0.2 g,
2
dodecane 2 mL, 120 ℃, O (1 atm).
2
the reaction, because the attack of H
2
O molecules to the benzylic cation
Finally, plausible reaction pathways are suggested for the Ni Mn-
2
should be fast. The formation of diphenylmethanol detected by GC–MS
also verified the speculation. In addition, the reaction was also tested in
the presence of 2 equivalent of acetic acid as an alternative nucleophile
under the selected reaction conditions. The corresponding benzhydryl
acetate (compound IV) was observed by GC–MS, further elucidating the
formation of benzyl cations in the reaction. In a standard reaction, the
formed carbocation intermediate could be trapped by active molecular
LDH-catalyzed alkylarenes oxidation with molecular oxygen based on
the above experimental evidences and discussions (Scheme 1). Initially,
a cation radical species is formed by a SET process from an alkylarenes
substrate to Mn3 species in the catalyst. Then, deprotonation of the
+
cation radical species occurs to form an alkyl radical species, which
might be accelerated by the Ni Mn-LDH for its known surface basicity
2
[35,36]. The radical species is then trapped by O2 (path I) to form an
H
2
O or hydroxyl species, which are generally contained in the in-
alkylperoxy radical. The alkylperoxy radical can abstract a hydrogen
from substrate to lead to a hydroperoxide, and then decomposes to ke-
tone and alcohol. The formed alcohol can be further quickly oxidized to
terlayers of hydrotalcite material.
ketone under the catalysis of Ni
2
Mn-LDH. Alternatively, a second SET of
3
+
the radical species in combination with the transformation of Mn to
2
Scheme 1. The plausible reaction mechanism for the aerobic oxidation of diphenylmethane over Ni Mn-LDH.
1
0

A. Wang et al.
Molecular Catalysis 499 (2021) 111276
Mn2 may occur to form a carbocation species, followed by nucleophilic
+
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