H2O2-based selective oxidations by divanadium-substituted polyoxotungstate supported on nitrogen-doped carbon nanomaterials

Article abstract of DOI:10.1016/j.cattod.2019.03.060

In this work, we present a new methodology for the preparation of highly active, selective, leaching-tolerant and recyclable catalysts on the basis of polyoxometalates (POM) and nitrogen-doped carbon nanomaterials. A divanadium-substituted γ-Keggin phosphotungstate [γ-PW₁₀O₃₈V₂(μ-O)(μ?OH)]^4? (PV₂), was immobilized on two types of supports – N-doped carbon nanofibers (N-CNFs) having herring-bone packing of graphite layers and bamboo-like N-doped carbon nanotubes (N-CNTs). Two series of catalysts have been prepared and characterized by elemental analysis, N₂ adsorption, TEM, XPS and FTIR techniques. Their catalytic performance was assessed in the liquid-phase selective oxidation of two representative organic substrates, 2,3,6-trimethylphenol and cyclohexene, with aqueous H₂O₂ as the green oxidant. The presence of nitrogen in the supports ensures strong binding and quasi-molecular dispersion of POM on the carbon surface, which is crucial for the catalytic performance and catalyst stability. The catalysts reveal truly heterogeneous nature of the catalysis and can be easily recovered and reused without loss of the catalytic performance. The morphology of the support has a significant impact on the catalytic performance: the supported PV₂ catalysts prepared using N-CNTs are more active and, in general, more selective than the catalysts prepared with N-CNFs.

Full text of DOI:10.1016/j.cattod.2019.03.060

Accepted Manuscript
Title: H2O2-based selective oxidations by
divanadium-substituted polyoxotungstate supported on
nitrogen-doped carbon nanomaterials
Authors: Vasiliy Yu. Evtushok, Olga Yu. Podyacheva, Arina
N. Suboc, Nataliya V. Maksimchuk, Olga A. Stonkus, Lidiya
S. Kibis, Oxana A. Kholdeeva
PII:
S0920-5861(18)31441-X
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2019.03.060
CATTOD 12082
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
26 December 2018
1 March 2019
25 March 2019
Please cite this article as: Evtushok VY, Podyacheva OY, Suboc AN, Maksimchuk NV,
StonkusOA, KibisLS, KholdeevaOA, H2O2-basedselectiveoxidationsbydivanadium-
substituted polyoxotungstate supported on nitrogen-doped carbon nanomaterials,
Catalysis Today (2019), https://doi.org/10.1016/j.cattod.2019.03.060
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

H2O2-based selective oxidations by divanadium-substituted polyoxotungstate
supported on nitrogen-doped carbon nanomaterials
Vasiliy Yu. Evtushok,a,b Olga Yu. Podyacheva,a,b Arina N. Suboch,a,b
Nataliya V. Maksimchuk,a,b Olga A. Stonkus,a,b Lidiya S. Kibis,a,b and Oxana A. Kholdeevaa,b,*
a Boreskov Institute of Catalysis, Lavrentieva ave. 5, Novosibirsk, 630090, Russia
b Novosibirsk State University, Pirogova st. 2, Novosibirsk, 630090, Russia
Fax: (+7)-(383)-330-9573; phone: (+7)-(383)-326-9433; e-mail: khold@catalysis.ru
Graphical abstract
Highlights
 N-doped carbon nanomaterials are prospective supports for immobilization of
polyoxometalates.
 A divanadium-substituted phosphotungstate PV2 supported on N-CNMs retains its
structure.
 The presence of N in N-CNMs ensures strong binding and quasi-molecular dispersion of
PV2.
1

 PV2/N-CNMs are truly heterogeneous and recyclable catalysts for selective oxidations
with H2O2.
Abstract
In this work, we present a new methodology for the preparation of highly active, selective,
leaching-tolerant and recyclable catalysts on the basis of polyoxometalates (POM) and nitrogen-
doped carbon nanomaterials. A divanadium-substituted γ-Keggin phosphotungstate [γ-
PW10O38V2(μ-O)(μ-OH)]4- (PV2), was immobilized on two types of supports – N-doped carbon
nanofibers (N-CNFs) having herring-bone packing of graphite layers and bamboo-like N-doped
carbon nanotubes (N-CNTs). Two series of catalysts have been prepared and characterized by
elemental analysis, N2 adsorption, TEM, XPS and FTIR techniques. Their catalytic performance
was assessed in the liquid-phase selective oxidation of two representative organic substrates,
2,3,6-trimethylphenol and cyclohexene, with aqueous H2O2 as the green oxidant. The presence of
nitrogen in the supports ensures strong binding and quasi-molecular dispersion of POM on the
carbon surface, which is crucial for the catalytic performance and catalyst stability. The catalysts
reveal truly heterogeneous nature of the catalysis and can be easily recovered and reused without
loss of the catalytic performance. The morphology of the support has a significant impact on the
catalytic performance: the supported PV2 catalysts prepared using N-CNTs are more active and,
in general, more selective than the catalysts prepared with N-CNFs.
Keywords: selective oxidation; polyoxometalates, hydrogen peroxide; N-doped carbon
nanotubes; N-doped carbon nanofibers
1. Introduction
2

The construction of highly active, selective, easily recyclable and truly heterogeneous catalysts is
a challenging goal of liquid phase oxidation catalysis [1-3]. Early-transition-metal oxygen nano-
size anionic clusters or polyoxometalates (POMs) possess a unique combination of properties
that includes inorganic nature, metal oxide-like structure, thermodynamic stability to oxidation
and hydrolysis over a wide pH range, fairly good thermal stability, tunable composition,
solubility, and acidity [4-7]. Transition-metal-substituted POMs (M-POMs) which comprise
heterometal center(s) M strongly bound to multidentate POM ligand provide wide opportunities
for an activation of green oxidants, such as molecular oxygen and aqueous hydrogen peroxide
[8-13]. All these make M-POMs promising active components to design heterogeneous catalysts
for green selective oxidations and energy applications [14-19]. The development of strategies for
converting catalytically active M-POMs to solid leaching-tolerant materials that would preserve
catalytic activity and selectivity of the molecular POMs catalysts but could be easily recovered
and reused remains, therefore, a hot research topic.
Traditionally, methodologies for immobilization of POMs involve elaboration of
+
insoluble (preferably, porous) salts using Cs+, Ag+, K+, NH4 or some specific organic
countercations [20-23], entrapment within silica during sol-gel synthesis [15,24-27], electrostatic
attachment to silica modified with cationic functional groups [28-32], anion exchange with
layered double hydroxides [33-35] or metal-organic frameworks (MOFs) [36-38], anchoring by
means of dative linkage between a coordinatively unsaturated heterometal of POM and surface
functional groups [39,40], and finally, covalent attachment to the surface [41-43] or inclusion of
POMs into the regular structure of MOFs through covalent bonding [44-46]. In the last decade,
some novel approaches that involve combinations of various types of interactions between POM
and surface [47-49] have been developed and successfully used for the preparation of stable
POM-based selective oxidation catalysts.
In recent year, carbon nanomaterials (CNMs) have received increasing attention for the
preparation of hybrid inorganic materials for electronics, energy conversion and storage,
3

molecular sensors, and catalysis [50]. Composites based on POMs and CNMs have been widely
used as electrocatalysts for the splitting of water and redox reactions in fuel cells [51-56].
Immobilization of POMs on the carbon surface was accomplished by simple adsorption
[53,55,57] or impregnation [58,59], as well as using more sophisticated methodologies, such as
electrostatic attachment by means of cationic functional groups preliminary introduced into the
carbon material [51] or covalent attachment of organo-functionalized POMs [56,60]. However,
very few materials have been tested for selective oxidation reactions. A catalyst prepared by
deposition of Cs2.5H0.5PW12O40 on carbon nanotubes (CNTs) was used for the oxidative removal
of dibenzothiophene with hydrogen peroxide, with a desulfurization efficiency of up to 100%
[59].
Doping of carbon nanomaterials with nitrogen results in the formation of different types
of surface and bulk N species, e.g. pyridine-like (NPy), pyrrole-like (NPyr) and quaternary (NQ)
ones, which provide additional opportunities for immobilization of catalytically active
components and, moreover, may affect hydrophilic and electronic properties of the supported
catalysts [61,62]. So far, N-CNMs have been widely employed as supports for metal
nanoparticles [63-68]. For example, Pd nanoparticles supported on N-CNTs turned out to be
effective catalysts for selective oxidation of benzyl alcohol [63], while wet air oxidation of
phenol was realized with Ru nanoparticles supported on N-doped carbon nanofibers (N-CNFs)
[64]. Few publications were devoted to the immobilization of metal oxide nanoparticles on N-
CNMs and studies on their physicochemical properties [67,69].
Heteropolyanion [γ-PW10O38V2(μ-O)(μ-OH)]4- (PV2) comprising a dinuclear vanadium
core is known as a highly efficient homogeneous catalyst for H2O2-based selective oxidations of
various organic substrates [70], including alkanes [71], alkenes [72], alkylbenzenes [74,75],
alkylphenols [76], and methoxyarenes [79]. Studies by experimental and computational
techniques suggested that the active species responsible for electrophilic oxygen atom transfer is
a peroxo complex [γ-PW10V2(-2:2-O2)O38]3- formed upon dehydration of a protonated
4

hydroperoxo complex [γ-PW10O38V2(μ-OH)(μ-OOH)]3− [70-72,74,77]. The latter is, most likely,
generated upon protonation of the hydroperoxo complex [γ-PW10O38V2(μ-O)(μ-OOH)]4−, which
in turn, is formed upon interaction of the parent PV2 with H2O2 [77]. Since the protonation state
of PV2 is critical for the selective oxidation catalysis, one might anticipate that immobilization
techniques that lead to PV2 deprotonation would result in a significant loss of catalytic activity
and selectivity.
Recently, we found that PV2 can be irreversibly adsorbed onto the surface of bamboo-like
N-CNTs from a MeCN solution, provided mineral acid is added [78]. While immobilization
using conventional approaches, such as embedding into silica, electrostatic attachment to amine-
modified SiO2 or metal-organic framework MIL-101, and adsorption on the active carbon
Sibunit [15] resulted in a significant loss of catalytic activity and selectivity of PV2, use of N-
CNTs as supports has led to elaboration of highly efficient catalysts for selective oxidation of
alkylphenols to corresponding alkyl-p-benzoquinones with aqueous H2O2 as the terminal oxidant
[78].
The aim of the present work was to immobilize PV2 on the surface of another type of
carbon nanomaterials, N-CNFs having herring-bone packing of graphite layers and to compare
the catalytic performances of PV2/N-CNFs and PV2/N-CNTs catalysts in H2O2–based selective
oxidation of two representative organic substrates – 2,3,6-trimethylphenol (TMP) and
cyclohexene (CyH) (Scheme 1).
Scheme 1. Oxidation of TMP and CyH with hydrogen peroxide over PV2/N-CNMs.
5

2. Experimental
2.1. Materials
2,3,6-Trimethylphenol (97+%) was obtained from Fluka. Cyclohexene was purchased
from Aldrich and passed through neutral alumina prior to use. Acetonitrile (Panreac, HPLC
grade) was dried and stored over activated 4 Å molecular sieves. All other reagents and solvents
were the best reagent grade and were used without further purification. The concentration of
H2O2 (ca. 30 wt.% in water) was determined iodometrically prior to use.
2.2. Catalysts and supports
Acid tetrabutylammonium salt TBA4H[γ-PW10O38V2(μ-O)(μ-OH)] was prepared
following the protocol reported previously [79]. The purity of the compound was confirmed by
FTIR, 51V and 31P NMR spectroscopic techniques.
Syntheses of N-CNTs and N-CNFs were carried out in flow-type quartz reactors by
decomposition of C2H4/NH3 mixtures using Fe-Ni-Al2O3 and Ni-Cu-Al2O3 catalysts,
respectively [80,81]. N-free CNTs and CNFs were prepared by similar procedures using pure
C2H4. To remove the catalyst from N-CNMs, the solids were treated with concentrated HCl
several times for several weeks at room temperature and then additionally boiled in 2 M HCl for
12 h. Then the materials were washed with distilled water until no chloride ions were detected in
the rinsing liquids. These severe treatments reduced the content of the initial catalyst to 1–2
wt.%. The remaining catalyst particles were encapsulated within the carbon framework, as
evidenced by the inertness of the N-free CNTs and CNFs in a decomposition of H2O2 (vide
infra) [78]. The thoroughly washed samples were dried in an Ar flow at 170 °C.
Prior to PV2 immobilization and adsorption studies, all carbon nanomaterials were extra
dried in vacuum at 100 °C. Adsorption/desorption measurements were carried following the
procedure described earlier [78]. Immobilization of PV2 was carried out from MeCN solutions at
6

room temperature. To enhance the adsorption capacity, HClO4 was added (1 mol equiv. with
respect to POM). The completion of the adsorption process was controlled by UV–vis (λ = 342
nm). The resulting solid was separated by filtration, thoroughly washed with MeCN and dried in
vacuum at 60 °C prior to use in catalytic experiments.
2.3. H2O2 decomposition
A support (10 mg) was placed into a glass reactor containing 5 mL of MeCN, and then 50
μL of 11 M H2O2 was added. The reaction mixture was vigorously stirred at 50 °C, and samples
(300 μL) were taken periodically by a syringe. The concentration of H2O2 was determined by
titration with KMnO4. Initial rates of H2O2 decomposition were evaluated from initial portions of
kinetic curves.
2.4. Catalytic oxidation and product analyses
Catalytic experiments were carried out in thermostated glass vessels under vigorous
stirring (500 rpm). Typical reaction conditions for TMP oxidation were as follows: TMP 0.1
mmol, H2O2 0.35–0.45 mmol, catalyst 1.5 µmol PV2, MeCN 1 mL, 60 °C. CyH oxidation
reactions were performed under the following reaction conditions: CyH 0.1 mmol, H2O2 0.1–0.2
mmol, catalyst 1.3 µmol PV2, MeCN 1 mL, 30 °C. All reactions were started with the addition of
H2O2. The reaction products were identified by GC-MS and quantified by GC using biphenyl as
an internal standard. Before reuse, the catalyst was separated by filtration, washed with
acetonitrile and dried in vacuum at 60 °C.
2.5. Instrumentation
GC analyses were performed using a gas chromatograph Chromos GC-1000 equipped
with a flame ionization detector and a quartz capillary column BPX5 (30 m × 0.25 mm). 31P and
51V NMR spectra were recorded on a Brüker AVANCE-400 spectrometer at 161.67 and 105.24
7

MHz, respectively. Chemical shifts for P and V, δ, were determined relative to 85% H3PO4 and
VOCl3, respectively. Infrared spectra were collected in KBr pellets on an Agilent 660 FTIR
spectrometer. UV-vis spectra were recorded on a Varian Cary 60 UV-vis spectrophotometer. X-
ray photoelectron spectra were collected on a KRATOS ES300 photoelectron spectrometer with
non-monochromatic AlKα radiation (hv 1486.6 eV). The Au4f7/2 and Cu2p3/2 core-level lines
with binding energies 84.0 eV and 932.7 eV, respectively, were used for the spectrometer
calibration. The nitrogen content in N-CNTs and N-CNFs was defined as N/C ratio (at.%) from
quantitative estimations using XPS data. High-resolution transmission electron microscopy
(HRTEM) was obtained using a JEM-2200FS (JЕОL Ltd., Japan) electron microscope operated
at 200 kV for obtaining TEM images. Images with a high atomic number contrast were acquired
using a high angle annular dark field (HAADF) detector in Scanning-TEM (STEM) mode. The
samples for the TEM study were prepared on perforated carbon film mounted on a copper grid.
Nitrogen adsorption measurements were carried out at 77 K using an ASAP-2400 instrument. The
content of vanadium in the filtrate was determined by ICP–OES using a PerkinElmer Optima–
430 DV instrument.
3. Results and Discussion
3.1. Catalysts synthesis and characterization
To evaluate the potential of different types of carbon nanomaterials as supports for
immobilization of PV2, we first compared adsorption of this POM on N-CNFs and N-CNTs from
MeCN, the solvent where the TBA salt of PV2 is fully soluble. The corresponding adsorption
curves obtained in the presence and in the absence of acid are shown in Figure 1. As one can see,
the maximal amount of PV2 adsorbed on N-CNFs is ca. 2 times lower as compared to N-CNTs
and the addition of HClO4 is crucial for both types of the support. The highest amount of
irreversibly adsorbed PV2 (i.e., the amount of POM that could not be washed out with
acetonitrile) was also lower for N-CNFs than for N-CNTs (15 vs 22 wt.%).
8

N-CNTs + HClO4
CNTs + HClO4
0.30
0.25
0.20
0.15
0.10
0.05
0.00
N-CNFs + HClO4
N-CNTs
CNTs
N-CNFs
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
[PV2], mM
Figure 1. Adsorption of PV2 on N-CNFs (5.5 at.% N), N-CNTs (4.8 at.% N), and CNTs from
MeCN at 25 °C. HClO4 (if any) 1 equiv.
We may tentatively assume that the ability of carbon nanomaterials to adsorb POM
correlates with their capability of adsorbing water. The water capacity of N-CNFs achieves the
value of ca. 1 cm3/g, which is typical of mesoporous carbons, and the dependence on the
nitrogen content is rather weak (Figure 2). On the contrary, we observed a marked increase in
water capacity (up to ca. 6 cm3/g) upon increasing nitrogen content in N-СNTs. Such an
anomalous value may indicate that water molecules are able to penetrate inside the nanotube
channels through the holes in the arches or on the walls of the tubes by means of capillarity, that
is favored by the hydrophilic character of the tube walls due to the nitrogen centers [82,83].
9

6
4
2
0
N-CNTs
N-CNFs
0
2
4
6
8
N content, at.%
Figure 2. Effect of nitrogen content on water capacity of N-CNTs and N-CNFs.
The crucial role of HClO4 in stabilization of POM on the carbon surface was confirmed
by the data of XPS experiments on the carbon supports treated by the acid. Regardless the type
-
of the support, the Cl2p spectra (Figure 3) showed the presence of ClO4 anions (the peak at
~207 eV [84]), thus pointing to the adsorption of the acid on the surface of both N-free and N-
-
doped CNMs. Taking into account that the signal of ClO4 disappeared in the XPS spectrum of
the supported catalyst PV2/N-CNTs (Figure 3b, spectrum 3), we may suppose that the HClO4-
treated supports serve as an anion exchanger and the attachment of POM to the carbon surface
proceeds via anion exchange. However, given that PV2 could not be re-extracted into MeCN
solution using 1 M TBAClO4, we cannot exclude a contribution of other types of non-covalent
interactions between the POM and N-CNMs.
Importantly, IR spectroscopy confirmed the retention of the PV2 structure after
immobilization on both N-CNTs and N-CNFs (Figure 4). While the position of characteristic
bands attributed to W=O, W-O-W and W-O-V vibrations remained nearly intact, the 700 cm-1 IR
feature attributed to V-O-V stretching vibrations was moved to ca. 735 cm-1 in the supported
catalyst. A higher energy shift (to 750-800 cm-1) of this band was previously observed upon
partial deprotonation of the V-(OH)-V bridge in acid TBA-salts of PV2 [71,79]. Therefore, we
10

may suggest that POM binding to the surface of N-CNTs results in some delocalization of proton
between the polyanion and, most likely, pyridine nitrogen (NPy) of the support.
207.1
207.1
Cl2p
Cl2p
(a)
(b)
3
2
2
1
1
195
200
205
210
195
200
205
210
Binding energy, eV
Binding energy, eV
Figure 3. The Cl2p spectra of (a) CNFs (1), N-CNFs (2) and (b) CNTs (1), N-CNTs (2), PV2/N-
CNTs (3) (initial carbon materials - black curves, carbon materials treated by HClO4 (0.15 M in
MeCN) - red curves).
A
V-O-V
865
700
W-O-V
W=O 950
B
C
800
W-O-W
735
D
1400
1200
1000
800
600
Wavenumber, cm-1
Figure 4. FT-IR spectra of (A) PV2 (bulk), (B) 8 wt.% PV2/N-CNTs, (C) 8 wt.% PV2/N-CNTs,
and (D) 15 wt.% PV2/N-CNFs after TMP oxidation. In B and С, the spectrum of N-CNTs (4.8
at.% N) was subtracted. In D, the spectrum of N-CNFs (2.2 at.% N) was subtracted.
11

Elemental analysis data and textural characteristics of representative carbon supports and
immobilized PV2 catalysts are given in Table 1. The surface area of N-CNTs slightly reduced
after immobilization of PV2, while mesopore volume and mean pore diameter remained nearly
the same. On the other hand, some decrease of all the textural characteristics was observed for
N-CNFs after deposition of the active component, which is quite typical for this type of carbon
nanomaterials [64].
Table 1. Textural properties and elemental analysis data for N-CNFs and N-CNTs and supported
PV2 catalysts.
b
c
d
Support
N a
POM
wt.%
SBET
Vpore
Dpore
nm
at.%
m2/g
cm3/g
N-CNFs
N-CNTs
N-CNTs
N-CNTs
N-CNTs
N-CNTs
N-CNTs
N-CNFs
N-CNFs
0
-
-
194
167
132
170
145
157
147
230
195
0.68
0.86
0.64
0.59
0.64
0.53
0.53
0.72
0.49
13
20
19
14
15
14
14
12
10
0
0
15
-
1.8
1.8
4.8
4.8
5.5
5.5
15
-
8
-
8
a Determined by XPS.
b Surface area.
c Pore volume.
d Average pore diameter.
Figure 5 presents HRTEM images of N-CNTs and N-CNFs with bamboo-like and
herring-bone packing of graphite layers, respectively, containing 8 wt.% of PV2. The presence of
PV2 on the carbon surface results in a local change of the image contrast. This is best seen when
12

POM molecules are located on the nanotube or nanofiber boundary in a given projection, while
in the background contrast with graphene layers, regions of greater intensity are barely
distinguishable. Images obtained in the HAADF-STEM mode where POM particles give a bright
light contrast (Figure 5 b-c and e-f) reveal that a quasi-molecular distribution of PV2 on the
carbon surface is realized. Indeed, individual particles of about 1 nm in size, which is close to the
diameter of the Keggin polyanion, can be easily distinguished. Therefore, we may conclude that
the morphology of N-CNMs does not play a critical role in the POM immobilization and its
distribution on the carbon surface and that both N-doped CNFs and CNTs can be considered as
prospective carriers for immobilizing PV2.
Figure 5. HRTEM (a, d) and HAADF-STEM (b, c, e, f) images of 8 wt.% PV2/N-CNTs with 4.8
at.% N (a-c) and 8 wt.% PV2/N-CNFs with 5.5 at.% N (d-f)
13

3.2. Comparison of catalytic performance of PV2/N-CNFs and PV2/N-CNTs
The catalytic activity of N-CNTs in a range of oxidation reactions due to the presence of
nitrogen species on the surface has been documented [85-87]. Indeed, our previous work showed
that N-CNTs themselves are able to catalyze TMP oxidation with H2O2 but, in this case,
selectivity toward the target product TMBQ is negligible and the main oxidation product is
2,2’,3,3’,5,5’-hexamethyl-4,4’-bisphenol (BP) [78], the formation of which is typical of radical
oxidation mechanisms [88]. A similar result was obtained for N-CNFs: TMP conversion attained
50%, but TMBQ selectivity was below 1%. The catalytic scenario changed drastically when PV2
was supported on the N-CNTs: with a catalyst containing 15 wt.% of PV2, nearly 100%
selectivity of TMBQ was reached at complete TMP conversion [78]. Therefore, we compared
the catalytic performance of catalysts obtained by deposition of 15 wt.% of PV2 on N-CNTs and
N-CNFs with different amount of nitrogen. The results are presented in Figure 6.
(a)
TMP conversion
TMBQ selectivity
(b)
TMP conversion
TMBQ selectivity
100
100
80
60
40
20
0
80
60
40
20
0
0
1.8
Nitrogen content, at.%
4.8
0
2.2
Nitrogen content, at.%
5.5
Figure 6. Effect of N content in (a) N-CNTs and (b) N-CNFs on TMP oxidation to TMBQ over
supported PV2 catalysts (15 wt.% PV2). Reaction conditions: TMP 0.1 M, H2O2 0.45 M, catalyst
40 mg, MeCN 1 mL, 15 min, 60 °C.
One can notice that while nearly 100% TMP conversion can be achieved with all these
catalysts, the effect of N content in the support on TMBQ selectivity is opposite for N-CNTs
(Figure 6a) and N-CNFs (Figure 6b). Whereas quinone selectivity reached its maximum value
14

(>99%) with N-CNTs having 1.8 at.% of N and then it remained constant, in the case of carbon
nanofibers, the best TMBQ selectivity (ca. 90%) was surprisingly realized over the N-free
support and then it tended to decrease with increasing N content. Given that the presence of
nitrogen in both N-CNTs and N-CNFs prevented POM aggregation and ensured quasi-molecular
dispersion of PV2 on the carbon surface, as evidenced by the HRTEM studies (see above), the
observed difference in the catalytic performance is not completely clear. We may tentatively
assume that it may be related to specific sorption properties of N-doped CNTs and CNFs with
regards to the phenolic substrate and hydrogen peroxide. Since N-CNTs are more hydrophilic
than N-CNFs (see Figure 2), this may result in enhanced adsorption of the polar phenolic
substrate on the former. A comparison of the rates of hydrogen peroxide decomposition in the
absence of any organic substrate showed that PV2 supported on N-CNTs is more active than PV2
supported on N-CNFs and, in both cases, the activity increased with increasing N content in the
support (Figure 7).
6
4
N-CNTs
N-CNFs
2
0
0
2
4
6
8
10
Nitrogen content, at.%
Figure 7. Effect of N content in N-CNTs and N-CNFs on the initial rate of H2O2 decomposition
(10 mg of support, 0.11 M H2O2, 5 mL MeCN, 50 °C).
Catalytic performance of PV2 supported on N-CNFs and N-CNTs with a similar N
content as well as N-free CNTs was also evaluated in alkene oxidation with aqueous H2O2 using
15

CyH as a model substrate and MeCN as a solvent. The main results are presented in Table 2. For
all the supported catalysts, the principle products were cyclohexene epoxide, 1,2-trans-
cyclohexane diol and its overoxidation product, 2-hydroxycyclohexane-1-one (ketol). Although
the ratio of diol and ketol to epoxide increased for the immobilized PV2 relative to homogeneous
PV2, such set of products with carbon mass balance in the range of 93-100% certainly indicates a
heterolytic mechanism of CyH oxidation, because a homolytic mechanism would lead to a
significant ratio of allylic oxidations products.
Table 2. CyH oxidation with H2O2 in the presence of supported PV2 catalysts.a
Entry Catalystb
H2O2, H2O2 eff.,c CyH conv., TOF,d Product selectivity,e,f %
mmol
0.1
%
%
h-1
Epoxide Diol
Ketol
1
1
2
3
4
5
6
PV2
94
44
35
34
24
84
93
39
60
32
42
82
341
147
200
30
94
51
22
33
18
79
5
PV2/N-CNTs 0.1
PV2/N-CNTs 0.2
PV2/N-CNFs 0.1
PV2/N-CNFs 0.2
33
55
55
61
19
13
18
8
58
14
2
PV2/CNTs
0.1
310
a Reaction conditions: CyH 0.1 mmol, H2O2 0.1–0.2 mmol, catalyst 1.3 µmol PV2, 30 °C, MeCN
1 mL, 2 h.
b For all supported catalysts, 15 wt.% PV2; N-CNFs (2.2 at.% N) and N-CNTs (1.8 at.% N).
c H2O2 utilization efficiency = total yield of products based on the oxidant consumed.
d TOF = (moles of substrate consumed)×(moles of POM)-1×time-1.
e Based on substrate consumed.
f
Minor amounts of allylic oxidation products (cyclohexenyl hydroperoxide, 2-cyclohexene-1-ol,
and 2-cyclohexene-1-one) were also found.
As one can judge from the TOF values given in Table 2, again PV2/N-CNTs was more
active than PV2/N-CNFs. However, in contrast to the TMP oxidation, both PV2/N-CNTs and
PV2/N-CNFs were less active than homogeneous PV2 and moderate CyH conversions (32–39%)
16

were attained with 1 equiv. of H2O2 over all PV2/N-CNMs catalysts. The reason for that might
be higher rates of the unproductive decomposition of H2O2 relative to the rates of the alkene
oxidation, i.e. lower efficiency of the oxidant utilization over the supported catalysts. Indeed,
CyH conversion could be increased up to 60% by using 2 equiv. of the oxidant (Table 2,
compare entries 2 and 3). Another reason for the lower activity of the immobilized PV2 could be
a reallocation of the activating protons between PV2 anion and nitrogen centers of N-CNTs and
N-CNFs. Previously, it was demonstrated that protons play a vital role in generating active
peroxo species from PV2 [71,72,77,89]. Indeed, when PV2 was supported on nitrogen-free
CNTs, both activity (TOF) and CyH conversion were similar to those attained with
homogeneous PV2 (Table 2, compare entries 1 and 6).
3.3. Catalyst stability and reusability
Both PV2/N-CNTs and PV2/N-CNFs behaved as truly heterogeneous catalysts in the
H2O2-based oxidations, as proved by hot filtration tests [90] (a representative experiment is
shown in Figure 8a) and elemental analysis data for the filtrate. This agrees well with strong
adsorption and molecular dispersion of PV2 on the surface of the two N-CNMs studied.
The IR spectrum of the catalyst recovered from the reaction mixture was very similar to
the spectrum of immobilized PV2 (Figure 4, compare spectra B and C). Both spectra revealed the
main vibrations characteristic of bulk PV2 (Figure 4, spectrum A).
On the other hand, hot filtration tests performed for PV2/CNTs showed minor substrate
conversion in the filtrate after removal of the solid catalyst (Figure 8b), which implies some
leaching of the active metal species into solution. Indeed, the amount of vanadium in the filtrate
determined by ICP-OES was 7 ppm. Therefore, we may conclude that doping of carbon
nanomaterials by nitrogen is essential not only for molecular dispersion of PV2 on the carbon
surface but also for strong binding of PV2 to ensure lack of leaching under turnover conditions of
liquid-phase oxidation with aqueous H2O2.
17

Reusability of the catalyst with the composition optimal for TMP oxidation to TMBQ (15
wt.% PV2, 1.8 at.% N in CNTs) was studied in several consecutive operation cycles. As one can
judge from Figure 9, kinetic curves for TMBQ accumulation practically coincided during, at
least four reuses, which implies fairly good stability and productivity of the heterogeneous
catalyst. Indeed, the total turnover number (TON) could reach 250-300 due to the catalyst
recycling.
100
80
60
40
20
0
100
80
60
40
20
0
(a)
(b)
Filtration of the catalyst
Filtration of the catalyst
0.0
0.5
1.0
1.5
2.0
0.0
0.1
0.2
0.3
0.4
Time, h
Time, h
Figure 8. Hot catalyst filtration test for (a) TMP oxidation with H2O2 over 15 wt.% PV2/N-CNTs
(1.8 at.% N) and (b) CyH oxidation with H2O2 over 15 wt.% PV2/CNTs. Reaction conditions: (a)
TMP 0.1 mmol, H2O2 0.35 mmol, catalyst 1.5 µmol POM, MeCN 1 mL, 50 °C and (b) CyH 0.1
mmol, H2O2 0.1 mmol, catalyst 1.3 µmol POM, MeCN 1 mL, 30 °C.
0.04
0.03
Run:
0.02
0.01
0.00
1
2
3
4
0
10
20
30
40
Time, min
18

Figure 9. Recycling performance of PV2/N-CNTs in TMP oxidation to TMBQ. Reaction
conditions: TMP 0.1 mmol, H2O2 0.35 mmol, catalyst 1.5 µmol POM, MeCN 1 mL, 50 °C.
4. Conclusions
The present study demonstrated that nitrogen-doped carbon nanomaterials are prospective
supports for immobilization of polyoxometalates, in particular PV2, which requires retention of
its structure and protonation state to perform heterolytic activation of H2O2. The catalysts
prepared using bamboo-like N-CNTs are more active and, in general, more selective than the
catalysts prepared with “herring-bone” N-CNFs. The presence of nitrogen in the supports is
necessary to ensure strong attachment and quasi-molecular dispersion of POM on the carbon
surface, which in turn, affect catalytic performance and catalyst stability under turnover
conditions. The N-CNTs-supported PV2 can be employed as highly efficient catalyst for
selective oxidation of 2,3,6-TMP to TMBQ (vitamin E precursor) using aqueous H2O2 as green
oxidant. With the optimal catalyst PV2/N-CNTs containing 15 wt.% PV2 and 1.8 at.% N, TMBQ
can be produced in a nearly quantitative yield and 80% oxidant utilization efficiency, thereby
retaining characteristics of homogeneous PV2. In the oxidation of less polar substrates, e.g. CyH,
the product yield and H2O2 efficiency decrease relative to homogeneous PV2. Further catalyst
optimization may lead to improvement of these characteristics. In contrast to PV2 immobilized
on N-free CNTs, the PV2/N-CNTs catalysts do not suffer from metal leaching under the turnover
conditions and demonstrate truly heterogeneous nature of the catalysis and excellent recycling
performance.
Acknowledgements. The help of T.Ya. Efimenko in N2 adsorption measurements is appreciated.
This work was partially supported by the Ministry of Science and Higher Education of the
Russian Federation (project АААА-А17-117041710080-4) and the Russian Foundation for
Basic Research (grant N 18-33-00764).
19

References
[1] F. Cavani, G. Centi, S. Perathoner, F. Trifiro (Eds.), Sustainable Industrial Processes, Wiley-
VCH, Weinheim, 2009.
[2] M. G. Clerici, O. A. Kholdeeva (Eds.), Liquid Phase Oxidation via Heterogeneous Catalysis:
Organic Synthesis and Industrial Applications, Wiley, Hoboken, New Jersey, 2013.
[3] D. Duprez, F. Cavani (Eds.), Handbook of Advanced Methods and Processes in Oxidation
Catalysis, Imperial College Press, London, 2014.
[4] M. T. Pope, Heteropoly and Isopoly Oxometalates, Springer-Verlag, Berlin, 1983.
[5] C. L. Hill (Guest Ed.), Thematic issue on polyoxometalates, Chem. Rev. 98 (1998) 1–390.
[6] I. A. Weinstock (Guest Ed.), Thematic issue “Frontiers in Metal Oxide Cluster Science”,
Israel J. Chem. 51 (2011) 169–302.
[7] L. Cronin, A. Müller (Guest Eds.), Themed collection “Polyoxometalate Cluster Science”,
Chem. Soc. Rev. 41 (2012) 7325–7648.
[8] R. Neumann, in: M. Beller, C. Bolm (Eds.), Transition Metals for Organic Synthesis, 2nd
ed.,Wiley-VCH, Weinheim, 2004, Vol. 2, pp. 415–426.
[9] C. L. Hill, in: A. G. Wedd (Ed.), Comprehensive Coordination Chemistry II; Elsevier
Science, New York, 2004, Vol. 4, pp. 679–759.
[10] R. Neumann, Inorg. Chem. 49 (2010) 3594–3601.
[11] N. Mizuno, K. Kamata, Coord. Chem. Rev. 255 (2011) 2358-2370.
[12] S.-S. Wang, G.-Y. Yang, Chem. Rev. 115 (2015) 4893−4962.
[13] I. A. Weinstock, R. E. Schreiber, R. Neumann, Chem. Rev. 118 (2018) 2680−2717.
[14] N. Mizuno, K. Kamata, S. Uchida, K. Yamaguchi, in: N. Mizuno, (Ed.), Modern
Heterogeneous Oxidation Catalysis: Design, Reactions and Characterization, Wiley–VCH,
Weinheim, 2009, pp 185–217.
[15] O. A. Kholdeeva, N. V. Maksimchuk, G. M. Maksimov, Catal. Today 157 (2010) 107−113.
[16] Y.-F. Song, R. Tsunashima, Chem. Soc. Rev. 41 (2012) 7384–7402.
20

[17] C. L. Hill, O. A. Kholdeeva, in: M. G. Clerici, O. A. Kholdeeva (Eds.), Liquid Phase
Oxidation via Heterogeneous Catalysis, Wiley, Hoboken, 2013, p. 263.
[18] M. Carraro, G. Fiorani, A. Sartorel, M. Bonchio, in: D. Duprez, F. Cavani (Eds.), Handbook
of Advanced Methods and Processes in Oxidation Catalysis, Imperial College Press, London,
2014, Ch 21, p. 586.
[19] Y. Zhou, G. Chen, Z. Long, J. Wang, RSC Adv. 4 (2014) 42092−42113.
[20] J. T. Rhule, W. A. Neiwert, K. I. Hardcastle, B. T. Do, C. L. Hill, J. Am. Chem. Soc. 123
(2001) 12101–12102.
[21] L. Plault, A. Hauseler, S. Nlate, D. Astruc, J. Ruiz, S. Gatard, R. Neumann, Angew. Chem.
Int. Ed. 43 (2004) 2924–2928.
[22] N. Mizuno, K. Kamata, K. Yamaguchi, in: R. Richards (Ed.), Surface and Nanomolecular
Catalysis, CRC Press LLC, Boca Raton, Fla, 2006, pp. 463-492.
[23] N. Mizuno, S. Uchida, K. Kamata, R. Ishimoto, S.;Nojima, K. Yonehara, Y. Sumida, Angew.
Chem. Int. Ed. 49 (2010) 9972−9976.
[24] Y. Izumi, Res. Chem. Intermed. 24 (1998) 461-471.
[25] T. Okuhara, Chem. Rev. 102 (2002) 3641-3666.
[26] O. A. Kholdeeva, M. P. Vanina, M. N. Timofeeva, R. I. Maksimovskaya, T. A. Trubitsina,
M. S. Melgunov, E. B. Burgina, J. Mrowiec-Bialon, A. B. Jarzebski, C. L. Hill, J. Catal. 226
(2004) 363−371.
[27] N. V. Maksimchuk, M. S. Melgunov, J. Mrowiec-Białon, A. B. Jarzębski, O. A. Kholdeeva,
J. Catal. 235 (2005) 175−183.
[28] R. Neumann, H. J. Miller, Chem. Soc., Chem. Commun. (1995) 2277−2278.
[29] A. Bordoloi, F. Lefebvre, S. B. Halligudi, J. Catal. 247 (2007) 166−175.
[30] K. Yamaguchi, C. Yoshida, S. Uchida, N. Mizuno, J. Am. Chem. Soc. 127 (2005) 530–531.
[31] N. V. Maksimchuk, M. S. Melgunov, Yu. A. Chesalov, J. Mrowiec-Białoń, A. B. Jarzębski,
O. A. Kholdeeva, J. Catal. 246 (2007) 241–248.
21

[32] J. Kasai, Y. Nakagawa, S. Uchida, K. Yamaguchi, N. Mizuno, Chem. Eur. J. 12 (2006) 4176 –
4184.
[33] S. K. Yun, T. Pinnavaia, J. Inorg. Chem. 35 (1996) 6853−6860.
[34] S. K. Jana, Y. Kubota, T. Tatsumi, J. Catal. 255 (2008) 40−47.
[35] S. Omwoma, W. Chen, R. Tsunashima, Y.-F. Song, Coord. Chem. Rev. 258–259 (2014)
58–71.
[36] N. V. Maksimchuk, M. N.; Timofeeva, M. S. Melgunov, A. N. Shmakov, Yu. A. Chesalov,
D. N. Dybtsev, V. P. Fedin, O. A. Kholdeeva, J. Catal. 257 (2008) 315−323.
[37] N. V. Maksimchuk, K. A. Kovalenko, S. S. Arzumanov, Y. A. Chesalov, M. S. Melgunov,
A. G. Stepanov, V. P. Fedin, O. A. Kholdeeva, Inorg. Chem. 49 (2010) 2920–2930.
[38] C. M. Granadeiro, A. D. S. Barbosa, S. Ribeiro Cunha−Silva, I. C. M. S. Santos, B. de Castro, L.
Cunha-Silva, S. S. Balula, Catal. Sci. Technol. 4 (2014) 1416−1425.
[39] R. Neumann, M. Cohen, Angew. Chem. Int. Ed. 36 (1997) 1738−1740.
[40] B. J. S. Johnson, A. Stein, Inorg. Chem. 40 (2001) 801−808.
[41] Y. Liu, K. Murata, M. Inaba, Green Chem. 6 (2004) 510−515.
[42] A. Proust, R Thouvenot, P. Gouzerh, Chem. Commun. (2008) 1837–1852
[43] A. Proust, B. Matt, R. Villanneau, G. Guillemot, P. Gouzerh, G. Izzet, Chem. Soc. Rev. 41
(2012) 7605–7622.
[44] P. Zhao, Y. Leng, M. Zhang, J. Wang, Y. Wu, J. Huang, Chem. Commun. 48 (2012) 5721−5723.
[45] Q. Han, C. He, M. Zhao, B. Qi, J. Niu, C. J. Am. Chem. Soc. 135 (2013) 10186–10189.
[46] D.-Y. Du, J.-S. Qin, S.-L. Li, Z.-M. Su, Y.-Q. Lan, Chem. Soc. Rev. 43 (2014) 4615-4632.
[47] W. Qi, Y. Wang, W. Li, L. Wu, Chem. Eur. J. 16 (2010) 1068–1078.
[48] S. Nojima, K. Kamata, K. Suzuki, K. Yamaguchi, N. Mizuno, ChemCatChem 7 (2015)
1097–1104.
[49] Y. Wang, K.Kamata, R. Ishimoto, Y. Ogasawara, K. Suzuki, K. Yamaguchi, N. Mizuno Catal.
Sci. Technol. 5 (2015) 2602−2611.
22

[50] D. Eder, Chem. Rev. 110 (2010) 1348–1385.
[51] F. M. Toma, A. Sartorel, M. Iurlo, M. Carraro, P. Parisse, C. Maccato, S. Rapino, B. R.
Gonzalez, H. Amenitsch, T. Da Ros, L. Casalis, A. Goldoni, M. Marcaccio, G. Scorrano, G.
Scoles, F. Paolucci, M. Prato, M. Bonchio, Nat Chem. 2 (2010) 826–831.
[52] S.-X. Guo, Y. Liu, C.-Y. Lee, A. Bond, M. J. Zhang, Y. V. Geletii, C. L. Hill, Energy
Environ. Sci. 6 (2013) 2654–2663.
[53] D. Pan, J. Chen, W. Tao, L. Nie, S. Yao, Langmuir 22 (2006) 5872–5876.
[54] Z. Cui, C. M. Li, S. P. Jiang, Phys. Chem. Chem. Phys. 13 (2011) 16349–16357.
[55] N. Kawasaki, H. Wang, R. Nakanishi, S. Hamanaka, R. Kitaura, H. Shinohara, T.
Yokoyama, H. Yoshikawa, K. Awaga, Angew. Chem. Int. Ed. 50 (2011) 3471–3474.
[56] Y. Ji, L. Huang, J. Hu, C. Streb, Y.-F. Song, Energy Environ. Sci. 8 (2015) 776–789.
[57] A. Giusti, G. Charron, S. Mazerat, J.-D. Compain, P. Mialane, A. Dolbecq, E. Riviere, W.
Wernsdorfer, R. N. Biboum, B. Keita, L. Nadjo, A. Filoramo, J.-P. Bourgoin, T. Mallah, Angew.
Chem. Int. Ed. 48 (2009) 4949–4952.
[58] H. Salavati, S. Tangestaninejad, M. Moghadam, V. Mirkhani, I. Mohammadpoor-Baltork,
Ultrasonics Sonochemistry 17 (2010) 453–459.
[59] R. Wang, F. Yu, G. Zhang, H. Zhao, Catal. Today 150 (2010) 37−41.
[60] W. Chen, L. Huang, J. Hu, T. Li, F. Jia and Y.-F. Song, Phys. Chem. Chem. Phys. 16 (2014)
19668–19673.
[61] O. Yu. Podyacheva, Z. R. Ismagilov, Catal. Today 249 (2015) 12−22.
[62] R. Arrigo, M. E. Schuster, Z. Xie, Y. Yi, G. Wowsnick, L. L. Sun, K. E. Hermann, M.
Friedrich, P. Kast, M. Hävecker, A. Knop-Gericke, R. Schlögl, ACS Catal. 5 (2015) 2740–2753.
[63] Arrigo, R.; Wrabetz, S.; Schuster, M. E.; Wang, D.; Villa, A.; Rosenthal, D.; Girsgdies, F.;
Weinberg, G.; Prati, L.; Schlögl, R.; Su, D. S. Phys. Chem. Chem. Phys. 2012, 14, 10523−10532.
23

[64] A. B. Ayusheev, O. P. Taran, I. A. Seryak, O. Yu. Podyacheva, C. Descorme, M. Besson, L.
S. Kibis, A. I. Boronin, A. I. Romanenko, Z. R. Ismagilov, V. N. Parmon, Appl. Catal. B:
Environ. 146 (2014) 177–185.
[65]W. Xia Catal. Sci. Technol. 6 (2016) 630–644.
[66] M. Li, F. Xu, H. Li, Y. Wang, Catal. Sci. Technol. 6 (2016) 3670–3693.
[67] Y. Cao, S. Mao, M. Li, Y. Chen, Y. Wang, ACS Catal. 7 (2017) 8090−8112.
[68] D. A . Bulushev, M. Zacharska, E. V Shlyakhova, A. L. Chuvilin, Y. Guo, S. Beloshapkin,
A. V. Okotrub, L. G. Bulusheva, ACS Catal. 6 (2016) 681−691.
[69] O. Y. Podyacheva, A. S. Lisitsyn, L.S. Kibis, A.I. Stadnichenko, A.I. Boronin, E.M.
Slavinskaya, O.A. Stonkus, S.A. Yashnik, Z.R. Ismagilov, Catal. Today 301 (2018) 125-133.
[70] N. Mizuno, K. Kamata, Coord. Chem. Rev. 255 (2011) 2358−2370.
[71] K. Kamata, K. Yonehara, Y. Nakagawa, K. Uehara, N. Mizuno, Nat. Chem. 2 (2010)
478−483.
[72] K. Kamata, K. Sugahara, K. Yonehara, R. Ishimoto, N. Mizuno, Chem. Eur. J. 17 (2011)
7549−7559.
[73] T. Yamaura, K. Kamata, K. Yamaguchi, N. Mizuno, Catal. Today 203 (2013) 76−80.
[74] K. Kamata, T. Yamaura, N. Mizuno, Angew. Chem. Int. Ed. 51 (2012) 7275−7278.
[75] O. V. Zalomaeva, V. Y. Evtushok, G. M.; Maksimov, O. A. Kholdeeva. J Organomet.
Chem. 793 (2015) 210−216.
[76] I. D. Ivanchikova, N. V. Maksimchuk, R. I. Maksimovskaya, G. M. Maksimov, O. A.
Kholdeeva, ACS Catal. 4 (2014) 2706−2713.
[77] I. Y. Skobelev, V. Yu. Evtushok, O. A. Kholdeeva, N. V. Maksimchuk, R. I.
Maksimovskaya, J. M. Ricart, J. M. Poblet, J. J. Carbó, ACS Catal. 7 (2017) 8514–8523.
[78] V. Yu. Evtushok, A. N. Suboch, O. Yu. Podyacheva, O. A. Stonkus, V. I. Zaikovskii, Yu.
A. Chesalov, L. S. Kibis, O. A. Kholdeeva, ACS Catal. 8 (2018) 1297−1307.
24

[79] O. V. Zalomaeva, V. Y. Evtushok, G. M. Maksimov, R. I. Maksimovskaya, O. A.
Kholdeeva, Dalton Trans. 46 (2017) 5202−5209.
[80] A. N. Suboch, S. V. Cherepanova, L. S. Kibis, A. D. Svintsitskiy, O. A. Stonkus, A. I.
Boronin, V. V. Chesnokov, A. I. Romanenko, Z. R. Ismagilov, O. Yu. Podyacheva, Fullerenes,
Nanotub. Carbon Nanostructures 24 (2016) 520–530.
[81] O. Yu. Podyacheva, S. V. Cherepanova, A. I. Romanenko, L. S. Kibis, D. A. Svintsitskiy,
A. I. Boronin, O. A. Stonkus, A. N. Suboch, A. V. Puzynin, Z. R. Ismagilov, Carbon 112 (2017)
475-483.
[82] K. Chizari, I. Janowska, M. Houllé, I. Florea, O. Ersen, T. Romero, P. Bernhardt, M.J.
Ledoux, C. Pham-Huu, Appl. Catal. A 380 (2010) 72−80.
[83] K. V. Kumar, K. Preuss, Z. X. Guo, M. M. Titirici, J. Phys. Chem. C 120 (2016)
18167−18179.
[84] A. G. Wren, R.W. Phillips, L.U. Tolentino. J. Colloid Interface Sci. 70 (1979) 544−557.
[85] J. Long, X. Xie, J. Xu, Q. Gu, L. Chen, X. Wang, ACS Catal. 2 (2012) 622−631.
[86] J. Luo, F. Peng, H. Wang, H. Yu, Catal. Commun. 39 (2013) 44−49.
[87] H. Watanabe, S. Asano, S. Fujita, H. Yoshida, M. Arai, ACS Catal. 5 (2015) 2886−2894.
[88] O. A. Kholdeeva, O. V. Zalomaeva, Coord. Chem. Rev. 306 (2016) 302–330.
[89] N. Mizuno, K. Kamata, K. Yamaguchi, Catalysis Today 185 (2012) 157–161
[90] R. A. Sheldon, M. Wallau, I. W. C. E. Arends, U. Schuchardt, Acc. Chem. Res. 31 (1998)
485−493.
25