Aerobic Alcohol Oxidation by a Zeolitic Octahedral Metal Oxide based on Iron Vanadomolybdates Under Mild Conditions

Article abstract of DOI:10.1002/cctc.202001696

Zeolitic octahedral metal oxides are fully-inorganic crystalline materials with the multi-component redox property and the intrinsic microporosity. The redox active zeolitic iron vanadomolybdate acts as a catalyst for aerobic oxidation of primary aromatic alcohols. Molecular oxygen is able to be activated in the material due to the confinement effect of the micropore and the unique redox property of the framework. Different primary aromatic alcohols are oxidized by the material, and high alcohol conversion and aldehyde selectivity are achieved. The material is stable and able to be reused for 5 times without loss of activity.

Full text of DOI:10.1002/cctc.202001696

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Aerobic Alcohol Oxidation by a Zeolitic Octahedral Metal
Oxide based on Iron Vanadomolybdates Under Mild
Conditions
[a]
[a]
[a]
[a]
[a]
Qianqian Zhu, Shanshan Yin, Mengyuan Zhou, Jie Wang, Chaomin Chen,
Panpan Hu, Xizhuo Jiang, Zhenxin Zhang,* Yanshuo Li,* and Wataru Ueda
[a]
[a]
[a]
[a]
[b]
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Zeolitic octahedral metal oxides are fully-inorganic crystalline
materials with the multi-component redox property and the
intrinsic microporosity. The redox active zeolitic iron vanadomo-
lybdate acts as a catalyst for aerobic oxidation of primary
aromatic alcohols. Molecular oxygen is able to be activated in
the material due to the confinement effect of the micropore
and the unique redox property of the framework. Different
primary aromatic alcohols are oxidized by the material, and
high alcohol conversion and aldehyde selectivity are achieved.
The material is stable and able to be reused for 5 times without
loss of activity.
Introduction
materials, MoÀ V based metal oxides are interesting because the
materials are noble metal free materials.
Selective alcohol oxidation is important not only for green
organic synthesis but also for industrial procedures, because
the products such as esters, carboxylic acids, ketones, and
aldehydes, are widely used in the fields of fine chemicals,
commercial products, and polymer industries. Molecular oxy-
gen is one of the best oxidants for alcohol oxidation, because
molecular oxygen is an inexpensive, abundant, and easily
available oxidant. Although molecular oxygen is an ideal
oxidant for oxidation, its activation is difficult due to the high
stability. Therefore, to realize the activation of molecular oxygen
is a key step for selective alcohol oxidation.
Zeolitic transition metal oxides are microporous crystalline
metal oxides composed of metal-oxygen octahedra, which are
also called zeolitic octahedral metal oxides (ZOMOs). ZOMOs
are new fully-inorganic porous materials, and the construction
of the framework is similar to that of metal organic framework
(MOF), which is formed by assembly of metal-oxygen clusters as
building blocks and inorganic ions as linkers (Figure 1a). The
micropores are occupied by guest molecules and are able to be
opened. ZOMOs have both advantages of transition metal
oxides and microporous materials, showing not only the multi-
electron redox property but also the intrinsic microporosity.
Combined with the redox property and the confinement effect
of the micropores, the materials show application potentials in
many fields particularly in catalytic oxidations.
[1]
There are catalysts being used for heterogeneous aerobic
[2]
oxidation of alcohol, such as supported noble metals, Ru-
[
3]
[4]
based metal oxides, inorganic-organic hybrids, other metal
[5]
[6]
oxides, and MoÀ V based oxides. Among the reported
However, the examples of ZOMOs are not many, and the
synthesis is difficult. The assembly of polyoxometalates (POMs)
that act as ideal building blocks is a good methodology for
[7]
obtaining ZOMOs. There are different ZOMOs being synthe-
sized by this approach for catalytic oxidation, such as pentagon
[
a] Prof. Q. Zhu, S. Yin, M. Zhou, J. Wang, C. Chen, P. Hu, X. Jiang,
Prof. Z. Zhang, Prof. Y. Li
[
8]
[9]
unit based MoVO, hexagon unit based WPO, cubane unit
School of Materials Science and Chemical Engineering.
Ningbo University
Fenghua road 818
[10]
[11]
based WVO, and ɛ-Keggin POM based material.
Among these ZOMOs, the materials based on ɛ-Keggin
POMs are formed by assembly of ɛ-Keggin POM with inorganic
ions in a tetrahedral manner (Figure 1). The materials show high
elemental diversity and is able to incorporate different elements
such as Mo, V, Bi, Zn, Mn, Fe, and Co in the frameworks and to
incorporate alkali metal ions and alkali earth metal ions as
Ningbo
Zhejiang, 315211 (P.R. China)
E-mail: zhangzhenxin@nbu.edu.cn
liyanshuo@nbu.edu.cn
[b] Prof. W. Ueda
Faculty of Engineering
Kanagawa University
Rokkakubashi
[12]
cation species,
forming various iso-structural compounds
Kanagawa-ku
Yokohama-shi
Kanagawa, 221-8686 (Japan)
with different elemental compositions. Among these iso-
II
II
VI
V
structural materials, (NH ) Fe
H
11.7[Fe 2.0Mo 1.1Mo O ] and
4
2
0.6
10.9 40
II
II
VI
V
IV
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202001696
(NH ) Fe H [Fe 1.0Mo 2.7Mo 8.9V O ], denoted as NH MoFeO
4
2
0.6 8.5
1.4 40
4
and NH MoFeVO, are interesting. The Fe and Mo based ZOMOs
4
This publication is part of a Special Collection on “Supported Nanoparticles
and Single-Atoms for Catalysis: Energy and Environmental Applications”.
Please check the ChemCatChem homepage for more articles in the collec-
tion.
[13]
can activate molecular oxygen at room temperature. There-
fore, we assume that the material based on the similar
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Figure 1. a) schematic representation of the assembly of ZOMOs by POM unit and linker. Structure modes of b) ɛ-Keggin POM unit, c) connection of ɛ-Keggin
POM unit with metal ion linker, and d) the micropore system of the material, Mo or V (blue), Fe (purple), O (red).
[
14]
[15]
compositions are active for the low temperature catalytic
oxidation of organic compounds.
MoFeVO and MoFeO, showing peaks at 234.6 and 231.4 eV
V
V
for Mo (3d ) and Mo (3d5/2), respectively (Figure 2a,c). Based
3
/2
Herein, we reported the redox active zeolitic iron vanado-
on XPS analysis, Mo in the as-synthesized MoFeVO and MoFeO
molybdate
(MoFeVO)
based
on
ɛ-Keggin
was in the reduced state. Fe in the materials was in the mixed
III
II
V
V
IV
12.38À
40
[16]
[Fe 1.82Fe 0.18Mo 11.5V 0.3V O ]
with Fe and V linkers and
oxidation states with the peak positions at 710.8
and
0.7
[17]
III
II
the unique redox property led to the catalytic performance for
the aerobic oxidation of aromatic alcohols under mild con-
709.0 eV ascribing to Fe (2p ) and Fe (2p3/2). The ratios of
3/2
III
II
Fe :Fe were 10 and 4 for MoFeVO and MoFeO, respectively
V
IV
V
IV
ditions. O adsorbed in the micropores of the material at room
(Figure 2e,g). V and V coexisted in MoFeVO, and V :V was
2
[18]
[19]
V
IV
temperature, which was confined and activated, causing
oxidation of the material. V improved the redox activity of the
material and increased the O adsorbed amount. The material
catalyzed the oxidation of the aromatic alcohols to the
aldehydes using air as an oxidant. The material was able to be
recovered and reused for 5 times without loss of activity.
0.4 with the peaks at 517.2 and 515.6 eV for V and V
respectively (Figure 2i,k). There was no V peak being observed
in MoFeO (Figure 2i). XPS demonstrated that Mo and V were
reduced. The chemical formulae of the materials were esti-
mated in Table S1.
2
Fourier transform infrared spectroscopy (FTIR) showed that
the characteristic peaks of the as-synthesized (Figure S1Ba,b)
À 1
MoFeVO and MoFeO at 970, 754, 548, and 522 cm . The FTIR
Results and Discussion
spectra slightly changed with new shoulder peaks near ca.
À 1
7
54 cm after incorporation of V, which indicating that V
Material synthesis
incorporation slightly changed the molecular structure.
The building unit of MoFeVO was the ɛ-Keggin
III
II
V
V
IV
12.38À
40
MoFeVO and MoFeO were synthesized by the hydrothermal
[Fe 1.82Fe 0.18Mo 11.5V 0.3V O ]
unit (Figure 1a), and the
0.7
method, which was basically the same to the reported method
POM unit was connected with Fe or V in a tetrahedral manner
to form the framework (Figure 1c). Based on the structural
analysis, there were intrinsic micropores in MoFeVO and
MoFeO (Figure 1c). The cavities were connected by the
channels in a tetrahedral manner, and the channels were
formed by six oxygen atoms with the diameter of ca. 3.4 Å. The
cavity was composed of ten POM units with the size of ca. 7.7 Å
[13]
with slight modification. Herein, Na MoO ·2H O and NaVO
3
2
4
2
were used as the Mo source and the V source, respectively, to
+
produce the resulting materials with Na as the cation. The as-
synthesized MoFeVO and MoFeO were characterized by
powder X-ray diffraction (XRD) (Figure S1Aa,b), which demon-
strated that the crystal structures of the materials were basically
the same. They were typical iso-structural materials.
+
(Figure 1c). The micropores were originally occupied by Na
Elemental analysis showed that Na:Mo:Fe:V=1:11.5:2:1
and 1.5:12:2.5:0 for MoFeVO and MoFeO, respectively, which
indicated that V substituted both Mo and Fe in MoFeVO,
because Mo and Fe ratios of MoFeVO decreased compared
with those of MoFeO. Oxidation states of Mo from X-ray
and water, which were opened by heat-treatment. N adsorp-
2
tion-desorption measurements were conducted for the materi-
als (Figure S2a,b). The surface areas calculated by the BET
2
method were 25 and 29 m /g for MoFeVO and MoFeO,
respectively, and the external surface areas calculated by the t-
V
2
photoelectron spectroscopy (XPS) analysis were Mo for both
plot method were 19 and 19 m /g for both materials, which
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Figure 2. XPS profiles of Mo in a) MoFeO, b) MoFeO after O
2
adsorption, c) MoFeVO, d) MoFeVO after O
2
adsorption, Fe in e) MoFeO, f) MoFeO after O
adsorption,
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adsorption, g) MoFeVO, h) MoFeVO after O adsorption, V in i) MoFeO, j) MoFeO after O
2
2
adsorption, k) MoFeVO, and i) MoFeVO after O
2
were the same. Pore size distribution based on the SF method
showed that the micropore size of MoFeVO was 0.61 nm and of
MoFeO was 0.57 nm, respectively (Figure S2c).
MoFeVO and MoFeO were characterized by scanning
electronic microscopy (SEM) and energy dispersive X-Ray
spectroscopy (EDX) (Figure S3). As shown in the SEM-EDX
elemental mapping images, V distributed in a MoFeVO particle,
while there was no V in MoFeO being observed (Figure S3b,f).
Mo and Fe uniformly distributed in MoFeVO and MoFeO
and MoFeO after O adsorption showed new peaks appeared at
798, 702, and 638 cm compared with the fresh materials. This
indicated that the molecular structure changed after the O2
adsorption by oxidation of the material (Figure S1B). The XRD
patterns showed that the basic structure of the materials did
not change (Figure S1A).
There were three major factors for the unique O adsorption
property, chemical composition, microporosity, and valence.
Chemical compositions of the materials played an important
2
À 1
2
(Figure S3c,d,g,h).
role. Fe was critical for O activation at room temperature. Only
2
SEM showed that MoFeVO and MoFeO were polyhedral
Fe incorporated material showed the unique O2 adsorption
isotherm. Sodium zinc molybdate (NaMoZnO) had the similar
structure of MoFeVO, but no O2 adsorption was observed
(Figure S4). Furthermore, the adsorbed amount of O2 in
MoFeVO (ca. 8.8 cm /g, 0.39 mmol/g) was higher than that in
MoFeO (ca. 7.4 cm /g, 0.33 mmol/g) (Figure S4b). V improved
crystalline particle (Figure 3). Size distribution showed that the
particle size of both materials was in the range of 20–160 nm.
The distribution maximum was at 60–80 nm, which indicated
that the size of the material was in nanoscale.
3
3
the redox activity of the material and caused more Mo to be
oxidized during the O adsorption (Figure 2d). XPS showed the
Oxygen activation
2
VI
V
[20]
peaks at 235.6 and 234.4 eV for Mo (3d ) and Mo (3d3/2) and
232.4 and 231.2 eV for Mo (3d ) and Mo (3d5/2),
3
/2
[13]
VI
V
[21]
NH MoFeO was able to activate O at room temperature. O2
in both
4
2
5/2
VI
V
adsorbed in NH MoFeO with oxidation of Mo and Fe during the
materials (Figure 2b,d). The ratios of Mo :Mo in MoFeVO and
MoFeO was 3 and 0.75 (Table S1). Compared with XPS of the
as-synthesized materials, the valence of Mo increased after the
O adsorption, indicating that Mo was oxidized by O . More Mo
4
adsorption. The local structure of NH MoFeO changed with
4
changing the distance of Mo in the POM unit. However, the
detailed factors on the O adsorption and activation were not
2
2
2
investigated.
was oxidized in MoFeVO than in MoFeO, which indicated that
V incorporation promoted the redox capability during the O2
adsorption. Furthermore, Fe in both materials showed only a
single peak at 710.8 eV for Fe after O adsorption, and V in
MoFeVO was fully oxidized to V , which demonstrated that
both Fe and V were oxidized. The chemical formulae of
To understand the critical effects for O activation in the
2
ZOMOs. O2 adsorption-desorption measurements were con-
III
ducted. MoFeVO showed a unique O adsorption-desorption
2
2
V
isotherm. O was adsorbed in the material, which was not able
2
to be desorbed from the material, forming an irreversible
adsorption-desorption isotherm (Figure S4). FTIR of MoFeVO
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Figure 3. a) SEM image of MoFeO, b) size distribution of MoFeO, c) SEM image of MoFeVO, and d) size distribution of MoFeVO.
MoFeVO and MoFeO after O adsorption were estimated in
2
Table S1.
Microporosity was another factor for O2 activation and
adsorption. MoFeVO before heat-treatment showed low O2
adsorption capacity (Figure S4), because the micropore was not
fully opened. Furthermore, MoFeVO was heated at 400°C in N .
2
The resulting material was denoted as MoFeVONC400. The
structure of the material was damaged by the high temperature
heat-treatment, as the XRD patterns and the FTIR spectra of the
material changed after the heat-treatment (Figure S1A,Bh),
Figure 4. A) TPO profiles and B) TPR profiles of a) MoFeO and b) MoFeVO.
while the valence of Mo might not change as the material was
heated in N . N adsorption-desorption measurement confirmed
2
2
there was no microporosity in MoFeVONC400 (Figure S2c). The
material did not adsorb O (Figure S4).
oxidation peaks at 200°C and 350°C, which might be attributed
to the easily oxidized metal species and the hardly oxidized
metal species. After incorporation of V in the structure, the peak
area increased, which indicated that MoFeVO became more
active to be oxidized. The reduction ability of the materials was
investigated by TPR (Figure 4). Before the TPR measurement,
2
The O adsorption behavior was dependent on the valence
2
of metal in the material. When MoFeVO was pre-treated in O2
at 200°C, which was denoted as MoFeVOAC200, XRD showed
that the crystal structure did not change (Figure S1 Ag). FTIR
À 1
spectra showed that the peak at 754 cm for the reduced
material disappeared while the peaks at 798, 702, and 638 cm
À 1
the material was oxidized by O at 200°C. TPR showed that
2
for the oxidized material appeared (Figure S1B), which indicated
that the material was oxidized. N2 adsorption-desorption
measurement showed that the material still had microporosity
with the pore diameter of 0.63 nm (Figure S2c). The oxygen
adsorption isotherm showed that MoFeVOAC200 did not
there was a single reduction peak starting at 200°C with the
peak maximum at 400°C. The reduction peak of MoFeVO
increased compared with that of MoFeO, which indicated that
V increased the reduction ability.
adsorb O anymore (Figure S4).
2
The redox properties of the materials were investigated by
temperature programmed oxidation (TPO) and temperature
programmed reduction (TPR). As shown in Figure 4, the
materials started to be oxidized at 100°C. There were two main
Catalytic reaction
MoFeVO and MoFeO were used as heterogeneous catalysts
with tetrabutyl ammonium bromide (TBAB) and triethyl ammo-
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[
13]
nia (TEA) as the co-catalysts for the aerobic oxidation of the
alcohols at 110°C in toluene for 24 h. Before the reaction,
MoFeVO and MoFeO were heated in air at 80°C for 2 h to
would enable the material for the O activation. Herein, we
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found that introducing Fe by the ion-exchange process would
enhance the catalytic activity of the material. Compared with
the catalytic activity of NH MoZnOAC80 (Table 1, entry 8), the
oxidize the materials, and the resulting materials were denoted
as MoFeVOAC80 and MoFeOAC80. The structures of the
materials did not change during the treatment (Figure S1e). The
reaction was carried out using p-methoxybenzyl alcohol as a
model substrate. p-Methoxybenzyl alcohol was converted to p-
methoxybenzaldehyde with 92% of selectivity and 99% of
conversion of the alcohol (Table 1, entry 1).
4
Fe exchanged NH MoZnO (FeÀ NH MoZnOAC80) was more
4
4
active (Table 1, entry 9). However, the catalytic activity of
FeÀ NH MoZnOAC80 was still lower than that of MoFeVOAC80
4
and MoFeOAC80. This might be due to the lower Fe amount
incorporated by the ion-exchange process and also the different
positions of Fe in the materials. Fe was only in the cation site of
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Chemical composition affected the catalytic activity. The
iso-structural ZOMOs based on different ɛ-Keggin POMs were
used for p-Methoxybenzyl alcohol oxidation. Zinc molybdate
FeÀ NH MoZnOAC80 and not in the framework.
4
The catalytic activity of MoFeVO changed with different
pre-treatment conditions (Table S2, entry 1). The activity of the
as-synthesized MoFeVO was low (Table S2, entry 2). The materi-
al became active when it was heat at 80–200°C in air, which
indicated that oxidation of the material would increase the
catalytic activity of the material (Table S2, entries 2–4). Further
heating at 400°C would damage the crystal structure of the
material and the catalyst became inactive (Table S2, entry 5).
The co-catalysts, TBAB and TEA, were important for the
reaction. Without the co-catalysts the conversion of the alcohol
was still high, but the selectivity to the aldehyde was low
(Table S3). Gas Chromatograph Mass (GC-MS) confirmed that
some by-products were derived from alkylation catalyzed by
acids (Figure S5), and therefore adding basic compound (TEA)
could effectively suppress the side-reactions. However, almost
no aldehyde was obtained only using the co-catalysts.
[12]
(
(
NaMoZnO and NH MoZnO),
MoMnO), and cobalt molybdate (MoCoO) were synthe-
manganese molybdate
4
[12]
[22]
sized. All ZOMOs were heated at 80°C for 2 h before the
reaction, and the resulting materials were denoted as NaMoZ-
nOAC80, MoMnOAC80, MoCoOAC80, and NH MoZnOAC80. As
4
shown in Table 1, MoFeVOAC80 showed a high catalytic
activity, and high alcohol conversion and aldehyde selectivity
were achieved. Furthermore, V improved the catalytic activity.
Compared with MoFeVOAC80, MoFeOAC80 showed a slightly
lower alcohol conversion with similar aldehyde selectivity
(
Table 1, entries 1, 2). Cation affected the catalytic activity of the
+ +
catalysts. When the cation was changed from Na to NH4 , the
catalytic activity decreased (Table 1, entries 3, 4). Fe was critical
for the high activity of the reaction, and only the Fe
incorporated materials, MoFeOAC80 and MoFeVOAC80, were
active for the reaction (Table 1). When ZOMOs without Fe, such
as MoZnOAC80, MoMnOAC80, and MoCoOAC80, were used
for the reaction the activity was low, which demonstrated that
Fe was the key element for the selective oxidation (Table 1,
entries 1–8). Simple metal oxides, such as MoO , V O , and
Solvent effect of MoFeVOAC80 was investigated. As shown
in Table S4, toluene was a good solvent for this reaction. High
conversion of the alcohol and selectivity to the aldehyde were
obtained (Table S4, entry 1). In other aromatic hydrocarbon
solvents, the selectivity to the aldehyde was high while the
alcohol conversion was low (Table S4, entries 2–5). The reaction
in the polar solvent, N-methyl pyrrolidone (NMP) showed a
lower conversion and selectivity (Table S4, entry 6).
Other factors of the reaction were also investigated. The
dosage of the catalyst affected the catalytic activity. Catalytic
activity increased with the catalyst amount increasing. The
conversion of the alcohol reached to 99% with >90% of
selectivity using 0.04 g of the catalyst (Figure S6a). When the
reaction was run at a temperature below 55°C, no conversion
was observed (Figure S6b). When the reaction temperature
increased to 110°C, the reaction became active. The conversion
of the alcohol increased with the reaction time prolonged, and
the selectivity to the aldehyde kept most the same and above
3
2
5
Fe O , were not active for the reaction (Table 1, entries 10–12).
3
4
Without the catalyst and the co-catalysts no product was
obtained (Table 1, entry 11).
Furthermore, ion-exchange was able to replace the cation
species of the material, which would change the property of
the material. According to the previous study, Fe-exchange
Table 1. p-Methoxybenzyl alcohol oxidation catalyzed by different cata-
[
a]
lytic materials with air.
Entry
Catalyst
Conv. [%]
Yield [%]
Sel. [%]
1
2
3
4
5
6
7
8
9
MoFeVOAC80
MoFeOAC80
99
88
91
72
12
21
15
8
29
80
99
0
92
87
67
66
12
10
6
92
99
74
92
99
48
40
99
93
25
69
0
9
0% (Figure S6c).
NH MoFeVOAC80
4
MoFeVOAC80 was able to oxidize a variety of different
primary aromatic alcohols with air as the oxidant in toluene at
4
NH MoFeOAC80
NaMoZnOAC80
MoMnOAC80
MoCoOAC80
1
10°C. High conversion of the alcohol and high selectivity to
the aldehyde were obtained, such as p-methoxybenzyl alcohol,
4-methylbenzyl alcohol, and 3-methylbenzyl alcohol, benzyl
alcohol, and 4-bromo benzyl alcohol (Table 2, entries 1–5).
Furfuralcohol, as a heterocyclic aromatic alcohol, was able to be
converted to form furfural (Table 2, entry 6). MoFeVOAC80
showed almost no catalytic activity for the secondary aromatic
alcohol such as α-phenylethanol, which might be due to the
weaker interaction of the compound with the catalysts (Table 2,
NH
4
MoZnOAC80
MoZnOAC80
8
FeÀ NH
4
27
25
69
0
1
0
MoO
V2O5
3
11
1
1
2
3
Fe O
3 4
–
2
0
–
[
0
a] Reaction conditions: catalyst: 0.04 g, p-methoxybenzyl alcohol:
.8 mmol, TBAB: 0.01 g (0.0311 mmol), TEA: 0.01 mL (0.0721 mmol),
toluene: 0.5 mL, decane: 0.05 mL, temperature: 110°C, time: 24 h.
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[
a]
kept over 90%. The reused catalyst for 5 times was character-
ized by FTIR, which indicated that the material was oxidized, as
the FTIR peaks at 798, 702, and 638 cm for the oxidized
Table 2. Oxidation of different alcohols catalyzed by MoFeVO with air.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
Entry Substrate
Product
Conv. Y. [%] Sel. [%]
%]
À 1
[
material was observed (Figure S1B and Figure S7). XPS of
MoFeVOAC80 before and after the reaction also showed that
Mo and V were oxidized after the reaction (Figure S8). The
material after the reaction still showed the microporosity
1
2
99
62
92
62
92
99
(Figure S2). The XRD patterns of the material showed that the
basic structure of the material did not change after the reaction,
which indicated that the material was stable (Figure S7).
MoFeVOAC80 was tested by the filtration experiment. As
shown in Table S5, the reaction was conducted with the catalyst
for 5 h. The catalyst was then removed by centrifugation. The
filtrate was heated at 110°C for the rest 19 h, showing that the
conversion of the alcohol was 28%. Compared with the reaction
with the catalyst for 24 h, the conversion of the alcohol after
removal of the catalyst was remarkably lower that of the
reaction with the catalyst, which indicated that MoFeVO was a
heterogeneous catalyst (Figure 3).
The conversion of the alcohol was plotted against the time
at different temperatures. When the reaction time increased the
conversion of the alcohol increased. As shown in Figure S6c,
there was a linear relationship between the conversion and the
reaction time, which demonstrated that the apparent order of
the alcohol was close to 0. Thus, we assumed that the oxidation
was a surface reaction. The adsorption of the alcohol molecule
on the surface of the catalyst was a fast step. The apparent
activation energy of the reaction was calculated based on the
kinetic data using the Arrhenius equation to be 94 kJ/mol
(Figure S9).
Because of the surface reaction, the interaction of the
alcohol molecule and the catalyst surface was investigated. The
surface active site would catalyze the reaction. Transmission
electron microscopy (TEM) image showed that the external
surface of the material was the (1 1 1) plane (Figure 6a), which
was constructed by the ɛ-Keggin POM units with linkers
(Figure 6b). The cavity and channel would also exist on the
external surface. The surface cavity formed by 12 oxygen atoms
with the size of ca. 7.7 Å in the diameter. The channel was
surrounded by 6 oxygen atoms with the diameter of ca. 3.4 Å
3
81
80
99
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
4
5
6
84
99
72
78
93
53
93
93
74
7
0
0
–
8
9
0
0
0
0
–
–
[
(
a] Reaction conditions: MoFeVO: 0.04 g, alcohol: 0.8 mmol, TBAB: 0.01 g
0.0311 mmol), TEA: 0.01 mL (0.0721 mmol), toluene: 0.5 mL, decane:
0
.25 mmol, temperature: 110°C, time: 24 h.
entry 7), and it was discussed in a later part. Besides the
secondary aromatic alcohol. The material also could not oxidize
alkanol and cyclic alcohol (Table 2, entries 8,9).
Stability of MoFeVOAC80 was evaluated by the recycling
experiment (Figure 5). After the reaction, the catalyst was
separated by centrifugation (see the experimental part) and
dried for the next cycle directly. The catalyst showed a stable
catalytic activity, which was reused for 5 times without loss of
the catalytic activity. The yield and selectivity to the aldehyde
(Figure 6b). The size of benzyl alcohol was estimated to be
6
.5 Å, which was smaller than the cavity but larger than the
channel. The surface cavity was open and accessible to the
alcohol while the channel was too small for the alcohol to enter
the bulk. Therefore, we assumed that the (1 1 1) plane on the
surface interacted with the alcohol and catalyzed the reaction.
The Monte-Carlo simulation showed that the aromatic
alcohol adsorbed on the external surface of the catalyst, which
located in the cavity on the surface and not enter the bulk of
the material. The hydroxymethyl group interacted with the
cation species on the external surface, and the cation located in
the cage on the surface of the material (Figure 6c). Using benzyl
alcohol and α-phenylethanol as probe molecules caused differ-
ent results. The system energy of benzyl alcohol based system
Figure 5. Reusability of MoFeVO for alcohol oxidation, Reaction conditions:
MoFeVO: 0.04 g, p-methoxybenzyl alcohol: 0.8 mmol, TBAB: 0.01 g
0.0311 mmol), TEA: 0.01 mL (0.0721 mmol), toluene: 0.5 mL, decane:
.25 mmol, temperature: 110°C, time: 24 h.
(
À 18.56 kcal/mol) was lower than that of α-phenylethanol
(
0
based system (À 17.07 kcal/mol), which indicated that the
material had a stronger interaction with benzyl alcohol than α-
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
Figure 6. a) Polyhedral representation of unit cell of the material, inserted image: TEM image of the material showing the (1 1 1) plane of the material, b)
polyhedral representation (up) and CPK representation (down) of the (1 1 1) plane of the material, c) benzyl alcohol adsorbed on the (1 1 1) plane of the
material, and d) α-methylbenzyl alcohol adsorbed on the (1 1 1) plane of the material, Mo (blue), Fe (purple), O (red), C (black), H (white), Na (green).
phenylethanol. The larger volume of hydroxyethyl group of α-
phenylethanol might cause the weaker interaction with the
cage. The weaker interaction of the secondary aromatic alcohol
would make its reactivity low, which would be the reason for
the low activity for α-phenylethanol (Table 2, entry 7). Further-
temperature even room temperature (Figure S4), while the
alcohol oxidation did not occur at a temperature below 55°C
(Figure S6b). The re-oxidation of the catalyst might be faster
than the oxidation of the alcohol. Therefore, we assumed that
the slow step of the reaction was the oxidation of the adsorbed
alcohol to form the aldehyde by the catalyst.
+
more, Na in the catalyst and H in the alcohol were positively
charged and O in the catalyst and the alcohol was negatively
charged. There was electrostatic interaction between the
catalyst and the alcohol, which would be important for the
Based on the above discussion, the reaction pathway of the
alcohol oxidation catalyzed by MoFeVOAC80 was proposed.
Firstly, the alcohol adsorbed on the external cavity of the
material. Then, the adsorbed alcohol was oxidized by the
material to form the aldehyde and the reduced material. Finally,
the reduced material was re-oxidized by O2 at a certain
temperature without change of the structure for the next cycle.
+
+
reaction. After Na
(MoFeVO) was changed to NH4
(
NH MoFeVO), the catalytic activity decreased (Table 1, en-
4
tries 8, 9), which indicated that the interaction of the cation and
the alcohol was important.
When the alcohol was adsorbed on the surface of the
material the redox reaction occurred. The adsorbed alcohol was
oxidized by MoFeVOAC80. The reaction was carried out without Conclusion
the existence of O at 110°C for 24 h. The result showed that
2
there a small amount of the alcohol converted to the aldehyde
ca. 9%), which indicated that the material oxidized the alcohol.
In summary, zeolitic iron vanadomolybdates was synthesized as
(
the catalyst for the aerobic oxidation of alcohols under a mild
condition. Oxygen activation in the micropore of the material.
Oxygen oxidized Mo in the material. V incorporation improved
the redox reaction. More Mo was oxidized in MoFeVO. The
material was used as one catalyst for the oxidation of alcohols
using air as an oxidant. Different primary aromatic alcohols can
be oxidized to form corresponding aldehydes with high
conversion and selectivity. The material can be recovered and
reused without losing activity. The material showed different
activities for primary alcohols and secondary alcohols, which
Furthermore, the resulting MoFeVOAC80 was characterized by
FTIR (Figure S10). The peak for oxidized material (798 and
À 1
7
(
02 cm ) decreased and the peak for the reduced material
À 1
754 cm ) increased compared with FTIR of MoFeVOAC80 and
MoFeVO (Figure S10a,b), which indicated that MoFeVOAC80
was reduced.
After oxidation of the alcohol, MoFeVOAC80 was reduced.
The reduced MoFeVOAC80 was re-oxidized by O for the next
2
catalytic cycle. MoFeVO was able to be oxidized by O at a low
2
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might be due to the active site confine the molecules interacted
with material.
600°C with a ramp of 10°C/min under O
flow (10% of O in He,
2 2
0 mL/min). The signal was monitored by TCD. TPR measurements
were carried out on a BelcatII. Before the measurements, the
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
2
material was pre-treated at 200°C for 1 h under O flow (10% of O
2
2
in He, 50 mL/min). The temperature was increased from 80
°
C to
Experimental Section
6
2
00°C with a ramp of 10°C/min under H flow (10% of H in Ar,
2
2
0 mL/min). The signal was monitored by TCD.
Material synthesis
Synthesis of MoFeO and MoFeVO: Na
MoO
·2H
O (8.75 mmol
2
4
2
Catalytic reactions
based on Mo for MoFeO and 7.88 mmol based on Mo for MoFeVO)
and NaVO3 (0.87 mmol based on V only for MoFeVO), were
dissolved in 40 mL of distilled water. Metal Mo (0.150 g, 1.56 mmol)
The reaction was carried out on the flask with condenser. Typically,
catalyst (0.04 g), TBAB (0.01 g, 0.031 mmol), TEA (0.01 mL,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
0.072 mmol), p-methoxybenzyl alcohol (0.1 mL, 0.8 mmol), decane
and Fe O4 (0.194 g, 0.83 mmol) were added to the mixture
3
(0.05 mL, 0.25 mmol), and toluene (0.5 mL) were added to the flask.
The mixture was heated at 110°C for 24 h. After the reaction
finished and the mixture had been cooled under, the solution was
analyzed by GC-FID, and conversion and selectivity were calculated.
sequentially. The mixture was acidified by 6 mL of sulfuric acid
(10 g of sulfuric acid to 100 mL of water). The mixture was
introduced into a 50-mL of a stainless-steel autoclave with a Teflon
liner. The autoclave was fixed in an oven with a mechanical rotation
system. Hydrothermal synthesis was performed at 175°C with
tumbling (4 rpm) for 48 h. After the autoclave had been cooled
down, the crude solid was moved to a 100 mL-beaker and 60 mL of
water was added. The un-reacted Fe O was completely removed
For cycle-recovery experiment, the catalyst was recovered by
centrifugation (10000 rpm, 5 min) after the reaction. The catalyst
was washed with ethanol for 3 times, which was dried at room
temperature under high vacuum overnight for the next cycle.
3
4
by a magnet. The resulting suspension was centrifuged (3500 rpm,
0 min), and solids on the bottom of the centrifugation tube were
3
collected. The collected solid was washed with water by dispersing
in 10 mL of water and subsequent centrifugation (3500 rpm,
Simulation
3
0 min). After the washing process was repeated two more times,
Monte-Carlo simulation was performed to predict the adsorbed
structure of the alcohol molecule on the surface of the catalyst.
Before the Monte-Carlo simulation, the structure of the material
was optimized based on the previous study using NaMoZnO,
which was an iso-structural material of MoFeVO, as a model
structure by geometry optimization using the DMol program in
the Materials studio package. The Perdew-Burke-Ernzerhof (PBE)
generalized gradient functional and DND basis set were used for
calculation. The calculated Mulliken atomic charge was applied for
the Monte-Carlo simulation. The structures of the framework of the
material, benzyl alcohol, and phenethyl alcohol, were optimized
before the Monte-Carlo simulation. The Monte-Carlo simulation was
carried out with the adsorption locator program in the Materials
studio package. The guest molecules were introduced into the
framework of the material one by one.
the obtained solid was dried. The resulting 0.995 g and 0.509 g of
MoFeO and MoFeVO, were obtained with the yields of 62% and
36% based on Mo, respectively. Elemental analysis for
[
23]
NaH11.38[Fe Mo V O ]·12H O Calc Na, 1.09; Mo, 52.40; Fe, 5.31; V,
2
11.5
1
40
2
3
[24]
2
.24, found Na 1.16, Mo 53.03, Fe 4.95, V 1.99. Elemental analysis for
Na H [Fe Mo O ]·8.5H O Calc Na, 1.62; Mo, 54.05; Fe, 6.56,
1
.5 11.5
2.5
12 40
2
found Na, 1.46; Mo, 54.40; Fe, 7.05.
Synthesis of other materials
Synthesis of other materials based on ɛ-Keggin POM unit, such as
[
12]
[12]
[22]
NaMoZnO, NH MoZnO, MoMnO, and MoCoO, were on the
4
basis of the previous methods. Fe exchange experiment was carried
out at room temperature in aqueous solution for NH MoZnO,
4
[
13]
which was also according to the previous method.
Acknowledgements
Characterizations
XRD patterns were recorded on a Bruker, D8 Advance with CuÀ Kα
radiation (tube voltage: 40 kV, tube current: 40 mA). FT-IR was
conducted on a Bruker Vertex 70. XPS was performed on Thermo
Scientific K-Alpha. The spectrometer energies were calibrated using
the C1s peak at 284.7 eV. SEM image and EDX were obtained with
Nova Nano 450. TEM image was taken with a 200 kV TEM (JEOL
JEM-2100F). Elemental analysis was measured in the analysis center
of School of Material Science and Chemical Engineering, Ningbo
University. GC-MS was carried out on an Agilent 7890B with Agilent
5977B MSD.
This work was financially supported by the National Natural
Science Foundation of China (Grant No. 21801141, 21622607,
2
1761132009, 22075153, and 22006077), the Natural Science
Foundation of Ningbo (Grant No. 2019A610020 and
019A610452), National Natural Science Foundation of Zhejiang
2
(No. LR18B060002), Major Special Projects of the Plan “Science
and Technology Innovation 2025” in Ningbo (No. 2018B10016),
and the K. C. Wong Magna Fund in Ningbo University.
Gas adsorption-desorption measurements were performed on
MoFeO and MoFeVO by a Micromeritics 3flex sorption analyzer.
The samples were evacuated at 200°C for 2 h before the measure-
Conflict of Interest
ment. N adsorption-desorption measurements were carried out at
2
The authors declare no conflict of interest.
À 196°C, and O adsorption-desorption measurements were carried
2
out at 25°C.
Keywords: zeolitic transition metal oxide · redox property ·
selective oxidation · microporosity
TPO measurements were carried out on a BelcatII. Before the
measurements, the material was pre-treated at 200°C for 1 h under
He flow (50 mL/min). The temperature was increased from 80°C to
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