A Stable Polyoxometalate-Based Metal-Organic Framework as Highly Efficient Heterogeneous Catalyst for Oxidation of Alcohols

Article abstract of DOI:10.1021/acs.inorgchem.8b03589

A novel copper-containing 3D polyoxometalate-based metal-organic framework (POMOF), H[Cu₅^ICu^II(pzc)₂(pz)_4.5{P₂W₁₈O₆₂}]·6H₂O (HENU-1, HENU = Henan University; Hpzc = pyrazine-2-carboxylic acid, pz = pyrazine), was successfully isolated by a one-step hydrothermal method. In this compound, the {P₂W₁₈} polyanion acts as a seven-connected linker bridging adjacent 2D double-layer networks, as well as a template to induce the formation of the desired 3D framework. Particularly, the pz ligands are generated from pzc ligands in situ during the reaction process. HENU-1 exhibits not only good stability in air but also tolerance to acidic and basic media. It was first employed as a highly efficient heterogeneous catalyst for the oxidation of 1-phenylethanol into acetophenone, which shows 97% yield using tert-butyl hydroperoxide as oxidant with a turnover frequency of up to 9690·h^-1, and was reused for at least five cycles without significant catalytic activity loss. No POM leaching or framework decomposition was observed in our study.

Full text of DOI:10.1021/acs.inorgchem.8b03589

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
A Stable Polyoxometalate-Based Metal−Organic Framework as
Highly Efficient Heterogeneous Catalyst for Oxidation of Alcohols
Dandan Li, Qiaofei Xu, Yingguang Li, Yueting Qiu, Pengtao Ma, Jingyang Niu,* and Jingping Wang*
Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, College of Chemistry and
Chemical Engineering, Henan University, Kaifeng, 475004, Henan, P. R. China
S
* Supporting Information
ABSTRACT: A novel copper-containing 3D polyoxometalate-based metal−
organic framework (POMOF), H[Cu5 CuΙΙ(pzc)2(pz)4.5{P2W18O62}]·6H2O
Ι
(HENU-1, HENU = Henan University; Hpzc = pyrazine-2-carboxylic acid, pz
= pyrazine), was successfully isolated by a one-step hydrothermal method. In
this compound, the {P2W18} polyanion acts as a seven-connected linker
bridging adjacent 2D double-layer networks, as well as a template to induce
the formation of the desired 3D framework. Particularly, the pz ligands are
generated from pzc ligands in situ during the reaction process. HENU-1
exhibits not only good stability in air but also tolerance to acidic and basic
media. It was first employed as a highly efficient heterogeneous catalyst for
the oxidation of 1-phenylethanol into acetophenone, which shows 97% yield
using tert-butyl hydroperoxide as oxidant with a turnover frequency of up to
9690·h−1, and was reused for at least five cycles without significant catalytic
activity loss. No POM leaching or framework decomposition was observed in
our study.
solving the above problems.24−29 Specifically, the accessible
ordered porous structure of POMOFs can facilitate substrate
INTRODUCTION
■
The oxidation of alcohols into carbonyl compounds is a
frequently important research topic in the laboratory, for
aldehyde/ketone products take up the key position in the
diffusion at the molecule level to realize the homogeneity of
the heterogeneous catalysis.30−32 More importantly, the spatial
distances between the POM anions can be significantly
chemical industry including the fine chemical and pharma-
ceutical industry.1,2 Traditionally, various stoichiometric
increased via the immobilization of functional POMs into
MOFs, thereby improving the dispersity of anion clusters and
amounts of oxidizing agents (e.g., MnO2, CrO3, SeO2,
exposing more active sites.33,34 Inspired by that, catalytically
KMnO4, K2Cr2O7, etc.) have been widely used in industry3
but under severe conditions such as high pressure or
active POM precursors were screened first, to be assembled
temperatures and/or with strong mineral acids.4 Also, a mass
with low-cost transition metals and organic ligands under
hydrothermal conditions, so as to directly obtain the POMOF
of environmentally harmful toxic inorganic salts is usually
generated from these chemical transformations.5 Hence, it is
still a challenge to design and construct an effective,
material employed as an efficient and stable heterogeneous
catalyst for the oxidation of alcohols. Preliminary studies
indicated that Dawson-type phosphotungstates exhibit better
sustainable, and eco-friendly oxidative catalytic platform.
catalytic activities on the selective oxidation of 1-phenylethanol
Polyoxometalates (POMs), as a subset of metal oxides, have
into acetophenone among several potential active POM
been employed rather extensively as catalysts for various
species (Table S1). Considering from the synthetic perspec-
organic reactions over the past few decades, because of their
unique redox properties and tunable acidity.6−12 Recently,
tive, however, the immobilization of functional Dawson anions
into MOFs is not simple owning to their larger steric
many POM-based catalyst systems have been developed for
hindrance, compared with Keggin clusters.35−44 The mixed-
ligand systems consisting of two linkers have proven an
impactful approach in directing the construction of an open
framework to encapsulate Dawson anions directly.35,36,40
Especially, the bifunctional pyrazine carboxylic acid ligand is
a good candidate because of its capacity for pyrolysis in situ to
give a secondary ligand under certain hydrothermal conditions.
the oxidation of inactive alcohols;13−20 however, among them,
only lots of noble metals-containing (Ru, Pd, Au) catalysts
exhibit high efficiency on the alcohol oxidation reac-
tion,17,21−23 which is not suitable for the mass preparation of
cheaper fine chemicals. Also, the inherent shortcomings of
poor stability and low exposure of active sites impeded the
further application of POM-based catalysts.
In this regard, POM-based metal−organic frameworks
(POMOFs), which maximize the superiority of POMs and
MOFs, have already proven to be an effective strategy for
Received: December 24, 2018
© XXXX American Chemical Society
A
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The Procedure of the Catalysis for the Oxidation of
Alcohols. The alcohols (1 mmol), tert-butyl hydroperoxide
(TBHP) (3 mmol) catalyst (0.2 mol %), and CH3CN (1 mL) were
added into a tube at room temperature. The mixture was further
stirred under the prescribed conditions on the parallel reactor. All of
target products were identified by GC−MS, and the yields were
monitored by GC with methylbenzene as internal standard. The
catalyst was recovered by filtration when the reaction mixture was
cooled to room temperature at the end of each cycle, and then washed
thoroughly (at least three times) with acetonitrile, and dried in an
oven at 120 °C overnight to be reused for the next round.
X-ray Crystallography. A suitable crystal of HENU-1 was
selected and placed on a Bruker Apex-II CCD diffractometer. The
diffraction intensity data were collected at a temperature of 296.15 K
using a graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) as
the radiation source. Using Olex2,48 the structure was solved with the
ShelXT structure solution program using Intrinsic Phasing and refined
with the ShelXL refinement package49 using Least Squares
minimization on F2. In the last refinement cycles, all the heavy
atoms were refined anisotropically and the remaining non-hydrogen
atoms were isotropically refined. The lattice water molecules were
located by using a Fourier map and further determined by TGA
results. The hydrogen atoms of the organic ligands were placed in
calculated positions, while those H atoms from water molecules were
added into the formula directly. A summary of the crystallographic
data and structural determination of HENU-1 is listed in Table 1, and
Besides, taking into consideration that the copper sites are
generally regarded as one of the active centers in most
oxidation reactions,13,45,46 we chose copper acetate, {P2W18}
anions, and bifunctional ligand, pyrazine-2-carboxylic acid
(Hpzc), as the reactants to construct a novel POMOF catalyst
for the oxidation of alcohols.
In this work, we reported a copper-containing 3D POMOF
based on Dawson-type phosphotungstates and in situ-
Ι
generated ligands, namely, H[Cu5 CuΙΙ(pzc)2(pz)4.5{P2W18-
O62}]·6H2O (HENU-1; Hpzc = pyrazine-2-carboxylic acid,
pz = pyrazine), where the pzc ligands are partly pyrolyzed to
generate the pz secondary ligands (Scheme 1), and both of
Scheme 1. Schematic Diagram of Ligands Decomposition in
Situ
them participate in the construction of the framework.
Particularly, HENU-1 exhibits not only good stability in air
but also tolerance to acidic and basic media. Furthermore, it
was employed as an efficient heterogeneous catalyst for the
oxidation of 1-phenylethanol into acetophenone, which has
shown 97% yield with a high turnover frequency (TOF) of
9690 h−1 under certain conditions, and reused for at least five
runs without obvious decrease in catalytic performance.
Table 1. Crystal Data and Structure Refinement for HENU-
1
compound
empirical formula
formula weight
T/K
HENU-1
C28H24Cu6N13O67P2W18
5367.08
296.15
EXPERIMENTAL SECTION
crystal system
space group
a/Å
triclinic
■
Materials and Methods. All chemical reagents were purchased
from the commercial supplier and used without further purification,
except for the precursor K6P2W18O62·15H2O was prepared following
ref 47 and further identified by IR spectrum. The IR spectrum was
obtained on a Bruker VERTEX 70 IR spectrometer through KBr
pellet in the range of 400−4000 cm−1 region. Elemental analyses (C,
H, and N) were performed on an Elementar VarioElcube CHNS
analyzer. ICP-OES analyses (Cu, P, and W) were performed with a
Perkin-Elemer Optima 2100DV spectrometer. The thermal gravi-
metric analyses (TGA) were measured in a flowing N2 atmosphere on
a NETZSCH STA 449 F5 Jupiter thermal analyzer from 35 to 970 °C
with a heating rate of 10 °C min−1. The powder X-ray powder
diffraction (PXRD) patterns were carried out on a Bruker D8
Advance instrument with Cu Kα radiation (λ = 1.5418 Å). X-ray
photoelectron spectra (XPS) were recorded with an Axis Ultra X-ray
photoelectron spectrometer. GC analyses were performed on a Bruker
450-GC with a flame ionization detector equipped with a 30 m
column (GsBP-5, 0.25 mm internal diameter and 0.25 μm film
thickness) with nitrogen as carrier gas.
P1
̅
13.136(4)
b/Å
17.177(5)
c/Å
20.598(7)
α/deg
89.288(6)
β/deg
87.601(6)
γ/deg
V/Å3
70.447(6)
4376(2)
Z
2
ρcalc/g cm−3
μ/mm−1
4.073
25.114
crystal size/mm3
2θ range/deg
index ranges
reflns collected
independent reflns
0.22 × 0.17 × 0.14
3.198−50.198
−14 ≤ h ≤ 15, −19 ≤ k ≤ 20, −20 ≤ l ≤ 24
22717
15409
data/restraints/parameters 15409/2/665
I
GOF on F2
1.003
Synthesis of Compound H[Cu5 CuII(pzc)2(pz)4.5{P2W18O62}]·
6H2O (HENU-1). A mixture of K6P2W18O62·15H2O (0.906 g, 0.20
mmol) and Cu(OAc)2·2H2O (0.256 g, 1.28 mmol) was dissolved in 5
mL of distilled water under stirring. Then, pzc (0.164 g, 1.32 mmol)
was added into the resulting solution and stirred for another 30 min.
After that, the pH value was adjusted to 1.98 by adding 4 M HCl
dropwise and the suspension was sealed in a 25 mL Teflon-lined
autoclave and further heated at 160 °C for 4 days under autogenous
pressure. After slow cooling to room temperature at a rate of 10 °C
h−1, black block crystals of HENU-1 were isolated and further
collected by washing with distilled water, and then air-dried to give a
yield of 40% (based on W). Elemental analysis (%) calcd for
C28H24Cu6N13O67P2W18 (Mr = 5367.08): Cu, 7.10; P, 1.15; W, 61.66;
C, 6.27; H, 0.451; N, 3.39. Found: Cu, 7.02; P, 1.05; W, 61.72; C,
6.39; H, 0.404; N,3.48. IR (KBr, cm−1): 3462 (w), 1635 (s), 1603 (s),
1425 (w), 1356 (w),1161 (w), 1088(s), 961 (w), 912 (w), 794 (s).
R1, wR2 [I ≥ 2σ(I)]
R1, wR2 [all data]
R1 = 0.0895, wR2 = 0.2099
R1 = 0.1597, wR2 = 0.2605
the selected bond lengths and angles are given in Table S2. CCDC
1869003 contains the supplementary data for this paper. These data
can be obtained free of charge from the Cambridge Crystallographic
Data Centre.
RESULTS AND DISCUSSION
Structural Description. Single-crystal structural analyses
showed that HENU-1 crystallizes in the triclinic P1
■
̅
space
group. In this compound, the Dawson-type POMs, {P2W18}, as
guests, have been introduced into a 2D Cu/pz/pzc double-
B
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layered net and further bridged the adjacent networks to give a
3D POMOF.
Specifically, the asymmetry unit of HENU-1 is composed of
one [P2W18O62]6− polyanion, five Cu+, one Cu2+, two
deprotonated pzc ligands, four and a half pz ligands, and six
lattice water molecules (Figure S1). For the purpose of charge
balance, one proton was added directly to give the final
chemical formula of H[Cu5 CuΙΙ(pzc)2(pz)4.5{P2W18O62}]·
Ι
6H2O. The {P2W18} anions show a typical Dawson-type
polyanions structure which offers more profuse coordination
sites to capture transition metals, compared with Keggin-type
POMs. Bond valence sum (BVS)50 calculations indicated that
the P center and W atoms are all conventionally in +5 and +6
oxidation states, while copper atoms are in mixed-valence
states where Cu1, Cu2, Cu3, Cu5, Cu6 are all +1 valences
apart from that the Cu4 is +2 oxidation state, exceptionally
(Table S3). The presence of mixed-valence copper is further
evidenced by XPS (Figure S7a).
In HENU-1, there are six crystallographically independent
copper atoms (Cu1, Cu2, Cu3, Cu4, Cu5, and Cu6), which
exhibit two kinds of coordination modes (Table S3). The bond
lengths of Cu−O (1.85−2.83 Å) and Cu−N (1.84−2.02 Å)
are within a reasonable range. The O/N−Cu−N/O bond
angles vary from 81.4(12)° to 177.8(11)° (Table S2). It is
worth noting that the secondary ligands pz are generated from
pzc ligands in suit through pyrolysis under certain condition.
Each linear pz ligand bridges two Cu atoms (Cu1, Cu2, Cu3,
Cu5, and Cu6) in a μ2-coordination fashion (Figure S2a).
Two deprotonated pzc ligands in a central symmetric position
are bridged by Cu4 atom and further attached to two Cu
atoms (Cu3, Cu1; Cu5, Cu6), respectively (Figure S2b).
In this coordination mode, two linear pz ligands and one
tetra-connected pzc ligand alternately interconnect two
adjacent copper atoms to form a [Cu8(pz)4(pzc)2] macrocycle
(A) which possesses a 14.45 × 12.15 Å window (Figure S3a).
Furthermore, the macrocycle A fused with another two
neighboring ones by edge-sharing to give a 1D loop containing
chain (Figure 1a). Beyond that, a similar alternating mode is
employed for the rest of the organic ligands, three pz ligands
and one pzc ligand, to link adjacent copper atoms. As a result, a
novel [Cu10(pz)6(pzc)2] macrocycle (B) is generated. And the
window size of macrocycle B is about 18.67 × 12.92 Å (Figure
S3b). Differently, each macrocycle B is linked with another six
identical ones to give a 2D layer (Figure 1b). Such two parallel
2D layers are bridged by the 1D loop containing chains
mentioned above in a vertical direction via N4-Cu4-N5 and
O63-Cu4-O65 linkages. Thus, a 3-connected Cu/pz/pzc
double-layered net with the new {6.72}{62.7}{7.82}2{72.8}
topology is formed (Figure 1c). It should be pointed out that,
from the a-axis direction, the 2D bilayer structure possesses a
super large Z-shaped void with a size range of 17.18 × 15.13 Å
which is precisely accessible for the encapsulation of Dawson-
type POMs (12.3 × 10.5 Å). The {P2W18} anion, as a seven-
connected inorganic template, is incorporated into the host
double-layered network (Figure 1d). Two adjacent polyanions
are linked by two O2−Cu3−O4 bonds to form a dimer, which
fits the large void exactly. In addition, the introduction of
POMs joints two identical double-layered networks to give a
resulting 3D complicated POMOF structure through the Cu-O
linker, which stabilizes the entire framework to some extent
(Figure 1e). From the topology view, if the {P2W18}
polyanions are considered as 7-connected nodes, Cu1, Cu2,
Cu3, Cu5, and Cu6 are considered as 4-connected nodes, and
Figure 1. (a) 1D loop containing chain in 2D host double-layered
framework; (b) 2D layer based on edge-sharing large rings; (c) 2D
host double-layered framework; (d) coordination mode of {P2W18}
cluster; and (e) 3D framework of HENU-1.
pzc ligands are considered as 3-connected nodes, the whole
structure of HENU-1 can be simplified as a 3D (3,4,7)-
connected framework with a new topology {3.4.5.62.7}{3.6.-
73.8}{3.62.73}{3.63.72}{32.4.5.67.74.83.93}{62.7}2{65.8} (Figure
2). The solvent-accessible void in HENU-1 is 13.1% calculated
by using the PLATON program.51
Figure 2. (a) The topology network of the 2D host double-layered
net and (b) 3D framework of HENU-1.
Stability Tests. The phase purity was first certified by
PXRD measurement. As can be seen from Figure 3, the
experimental value is in good agreement with the simulated
value, which proves that the collected samples are pure. In
addition, HENU-1 exhibits good thermal stability before 320
°C. According to the TG curve, HENU-1 shows the first
weight loss step of 2.1% within 200 °C, corresponding to the
removal of six lattice water molecules (Figure S8a). At this
moment, the framework of HENU-1 still remains intact, which
was further confirmed by PXRD after heating the sample in an
oven at different temperatures for 12 h (Figure S8b). Starting
from 320 °C, the skeleton starts to collapse. The observed
second weight loss step occurred in the range of 320−740 °C
of 11.4%, which is ascribed to the decomposition of organic
molecules, and the third weight loss step between 740 and 950
°C corresponds to the partial collapse of {P2W18} clusters.
Additionally, the framework is air-stable with good crystal-
linities maintained after several months. Anything else, the
solution stability of HENU-1 was further investigated. It is
steadily undissolved in water and common organic solvents,
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Figure 3. PXRD patterns of HENU-1 after 24 h immersion in (a) acidic and (b) basic aqueous solutions with the pH range of 1−12.
such as ethanol, acetonitrile, toluene, methanol, dichloro-
methane, and octane. Remarkably, this POMOF also exhibits
excellent tolerance in acidic and basic aqueous solutions with
the pH range of 1−12, which can be further demonstrated by
PXRD after 24 h immersion (Figure 3). It is rare for MOFs to
remain stable in both acidic and basic aqueous solutions,
especially for POMOFs, which are always disassembled in the
alkaline environment for the inherent acidity of POMs. In a
word, good chemical stability is essential for the further study
of heterogeneous catalytic properties.
Catalytic Tests. The successful encapsulation of functional
Dawson anions and good chemical stability offer the HENU-1
promising potential for further catalytic studies. Concretely,
the catalytic activity of as-synthesized HENU-1 was evaluated
in the oxidation of 1-phenylethanol into acetophenone using
TBHP as the oxidant under mild conditions. And the results of
parallel tests have demonstrated that HENU-1 possessed
better catalytic activity for the oxidation of 1-phenylethanol
compared with the catalytically active precursors. As shown in
Table 2, running the experiment without any catalyst at 60 °C
for 0.5 h resulted in the formation of the desired product
acetophenone only with a yield of 11% (Table 2, entry 1).
However, great promotion in yields (11% to 84%) was
observed distinctly when HENU-1 employed as heterogeneous
catalysts were added (Table 2, entry 8), which indicates its
superior catalytic activity in the oxidation of 1-phenylethanol.
Compared to HENU-1, only 24% yield of acetophenone could
be obtained in the presence of the POM precursor (K6P2-
W18O62·15H2O). Notably, Cu(OAc)2 (Table 2, entry 3)
behaved more active than other raw materials including the
separate {P2W18} precursor and dual organic ligands (Table 2,
entries 2, 4, and 5), which implied that copper atoms played a
vital role in this transformation. In addition, the catalytic
activity of the P2W18/Cu mixture is better than that of any
single constituent, indicating that there would be the synergic
effects between POM units and copper atoms in HENU-1
(Table 2, entry 6). The P2W18/Cu/pz/pzc mixture has no
superior catalysis than P2W18/Cu, suggesting the ligands pz,
pzc do not have an obvious contribution to the system (Table
2, entry 7). Furthermore, compared with the P2W18/Cu/pz/
pzc mixture, the higher catalytic efficiency of HENU-1
demonstrates that the formation of the framework, which
possesses large solvent accessible voids, may have a positive
effect on the process.
Optimization of Catalytic Oxidation of 1-Phenyl-
ethanol with HENU-1. In order to find the optimal reaction
conditions for the oxidation of 1-phenylethanol with HENU-1,
the influence of various reaction parameters was studied in
detail. Initially, the dosage of catalyst came into the first
research object. As shown in Table 3, with the increasing
amount of catalyst (from 0.05 to 0.2 mol %), the acetophenone
yields rose from 94% to 97% (Table 3, entries 1−3),
correspondingly. It is obvious that reaction activity was
positively correlated with catalyst dosage in a certain range.
Similarly, a higher dosage of the oxidants is also beneficial to
the conversion of this reaction (Table 3, entries 3−5). Keeping
the other parameters constant, when the temperature goes up
from 50 to 60 °C, the yield increases somewhat (about 4%). As
the temperature continues to rise, however, the production rate
is fixed at 97% without improvement (Table 3, entries 3, 6, and
7). Therefore, the optimal reaction temperature can be locked
at 60 °C. In addition, the influence of solvent on catalytic
activity is also an important factor to be considered. For the
template reaction of the oxidation of 1-phenylethanol to
acetophenone, it is observed that there is already a yield of 69%
in the solvent-free system (Table 3, entry 12). When some
organic solvents, such as toluene, octane, and methanol, were,
respectively, introduced into the catalyst system, the yields
were significantly reduced to 56%, 59%, 46% (Table 3, entries
8−10). In contrast, when acetonitrile and dichloromethane
Table 2. Catalytic Oxidation of 1-Phenylethanol with
a
Different Catalysts
b
entry
cat.
reaction system
yield /%
1
2
3
4
5
6
7
8
11
24
58
16
14
74
72
84
P2W18
Cu(OAc)2
pz
homogeneous
homogeneous
homogeneous
homogeneous
homogeneous
homogeneous
heterogeneous
pzc
P2W18/Cu
P2W18/Cu/pz/pzc
HENU-1
a
Reaction conditions: 1-phenylethanol (1 mmol), TBHP (3 mmol),
b
catalyst (0.2 mol %), CH3CN (1 mL), T = 60 °C, t = 0.5 h. GC
yields for target product acetophenone were based on methylbenzene
as internal standard. All of the products were identified by GC−MS
spectra and GC spectra.
D
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acetophenone is not significantly reduced after at least five
cycles, which fully indicates the stability and recyclability of the
catalyst. Meanwhile, PXRD patterns and IR spectrum proved
that the framework of catalyst HENU-1 remains intact after
catalysis (Figure S9). Furthermore, the ICP results also
showed that there was no W element detected in the recycled
filtrate, further demonstrating the stability of catalyst HENU-1
during five contiguous runs.
Table 3. Optimization of the Different Reaction Conditions
for the Catalytic Oxidation of 1-Phenylethanol with HENU-
a
1
b
entry
cat./mol %
TBHP/equiv
T/°C
solvent
yield /%
Scope of Substrate. To evaluate the applicability of
HENU-1, various alcohols were used as substrates under the
optimal reaction conditions, including electron-donating and
electron-withdrawing groups substituted 1-phenylethanol
derivatives, aliphatic alcohols, and cyclopentanol, which was
carried out in the presence of catalyst HENU-1. The obtained
results are listed in Table 4. Both electron-withdrawing and
electron-donating 1-phenylethanol derivatives can be effec-
tively oxidized to the corresponding products using HENU-1
as catalyst. For instance, a high yield (96%) can be
simultaneously observed in the transformation of methyl/
methoxyl or bromo substituted 1-phenylethanol into aceto-
phenone derivatives (Table 4, entries 1−3). However, the
substrate with large steric hindrance could not achieve a good
yield of related product under the same conditions (85%;
Table 4, entry 7). In terms of aliphatic alcohols substrates, the
low yields of corresponding products were obtained due to the
inherent inert reactivity of substrates (Table 4, entries 8−10).
Kinetic Study. To clarify the kinetic state of the catalytic
oxidation of 1-phenylethanol, a series of catalytic experiments
were conducted at different temperatures and monitored at
intervals. As shown in Figure S10b, the yield and C/C0(C0 −
C) are plotted against the reaction time at optimal 60 °C,
respectively, in which C0 and C represent the corresponding
initial and transformed concentration. The linear fitting of the
data proves that the catalytic process conforms to second-order
kinetics (R2 = 0.9992), and the value of the reaction rate
constant k was calculated to be 0.14332 mol L−1 min−1
according to eq 1. The same linear fittings were performed
for the catalytic reaction data at other temperatures (50 °C, 70
°C). The results showed that the catalytic experiments were all
consistent with the kinetic characteristics of the second-order
reaction within the temperature range tested (Figure S10a,c),
and the corresponding rate constants k were 0.10846 mol L−1
min−1, and 0.18595 mol L−1 min−1, respectively (Figure S10e).
As shown in Figure S10d, the yield of product displayed a
gradual upward trend at different temperatures within the 60
1
2
3
4
5
6
7
8
9
0.05
0.1
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
3
3
3
2
1
3
3
3
3
3
3
3
60
60
60
60
60
50
70
60
60
60
60
60
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
toluene
octane
94
95
97
91
78
93
97
56
59
46
71
69
10
11
12
MeOH
CH2Cl2
a
Reaction conditions: substrate (1 mmol), t = 3 h, solvent (1 mL or
b
none). GC yields for target product acetophenone were based on
methylbenzene as internal standard. All of the products were
identified by GC−MS spectra and GC spectra.
were used as solvents, severally, the yields were visibly
improved (Table 3, entries 3, 11). Especially, the catalyst in
acetonitrile exhibits the best performance for the process
where an excellent yield of 97% has been obtained (Table 3,
entry 3). In particular, compared with the reported POM-
based catalysts, the catalyst HENU-1 has lower requirements
on reaction conditions but possesses superior catalytic
performance for the oxidation of 1-phenylethanol (Table S4).
Recyclability and Stability Tests. For the purpose of
verifying the heterogeneous system of the catalyst HENU-1, a
hot filtration test was performed. The reaction mixture was
filtered to remove HENU-1 after 5 min reaction, while the
filtrate continued to react for another 25 min. And the 1-
phenylethanol conversion finally reached 55.7%, demonstrating
that the oxidation reaction does not proceed after the removal
of the catalyst (Figure 4a). In addition, the cyclability of
catalyst HENU-1 for the oxidation of 1-phenylethanol was
studied under the optimal conditions. The catalyst can be
easily separated and reused in the next round of experiments.
As presented in Figure 4b, the yield of target product
Figure 4. (a) Hot filtration test for the HENU-1 catalyst. (b) Recyclability of the HENU-1 catalyst.
E
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Inorganic Chemistry
Article
a
Table 4. Catalytic Oxidation of Different Substrates
a
b
Reaction conditions: substrate (1 mmol), TBHP (3 mmol), catalyst (0.2 mol %), CH3CN (1 mL), T = 60 °C, t = 3 h; GC yields for target
product were based on methylbenzene as internal standard. All of the products were identified by GC−MS spectra and GC spectra.
Scheme 2. Mechanism for Oxidation of 1-Phenylethanol Catalyzed by HENU-1
min detection time range when HENU-1 was used in the
system. In addition, increasing the reaction temperature from
50 to 70 °C leads to a promotion in the yield from 87% to
92%, indicating that the activity of this catalytic reaction was
significantly affected by the temperature. On this basis,
activation energy (Ea) of the oxidation of 1-phenylethanol
catalyzed by HENU-1 under optimal conditions was calculated
according to the Arrhenius law [eq 2] for 24.829 kJ mol−1
(Figure S10f). Besides, the activation enthalpy was determined
at ΔH⧧ = 22.060 kJ mol−1 [eq 3], the activation entropy at
ΔS⧧ = −228.624 J mol−1 K−1 [eq 4], and the Gibbs energy at
ΔG⧧ = 97.049 kJ mol−1 [eq 5]. The negative entropy indicates
that there is a correlation process for the rate-limiting step.52 In
particular, the conversion of the substrate in 1 min reached
32% at 70 °C; that is, the TOF is as high as 9690 h−1, which
showed impressive catalytic performance among the POM-
based catalysts used for the oxidation of alcohols (Table S4).
Encouraged by the outstanding catalytic performance of
HENU-1, enlarged-scale test of the 1-phenylethanol substrate
(from 1 to 10 mmol, 0.2 mol % catalyst, 5 mL of CH3CN, 60
°C) was further conducted, and turnover number (TON)
reaches 4000 after 3 h with an acetophenone yield of 80%.
C/[C0(C0 − C)] = kt
(1)
ln k = −Ea/RT + ln A
(2)
ΔH⧧ = Ea − RT
(3)
F
DOI: 10.1021/acs.inorgchem.8b03589
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Inorganic Chemistry
Article
⧧
⊖
⊖
⧧
⊖
⊖
Accession Codes
i
j
y
z
i
j
y
z
kBT
h
Δ S (c )
Δ H (c )
r
m
r
m
j
j
z
z
j
z
z
j
k =
(c⊖ )1−nexp
exp −
CCDC 1869003 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
j
j
z
z
j
j
z
z
R
RT
(4)
(5)
k
{
k
{
ΔG = ΔH⧧ − TΔS⧧
Mechanism Study. For the purpose of the determination
of catalytic mechanism, a radical capture experiment was
further carried out, and the result showed that the catalytic
activity for the oxidation of 1-phenylethanol was significantly
decreased after the addition of the radical scavenger butylated
hydroxytoluene (BHT), demonstrating the nature of the
radical reaction process (Table S5, entry 4). Whereafter, this
catalytic system was proceeded under O2 and N2 atmospheres,
respectively, but no obvious difference could be observed,
which indicated that O2 molecules did not participate in the
process (Table S5, entries 1−3). On the basis of the
observations above, a possible reaction mechanism in Scheme
2 was proposed according to the literatures.4 Initially, TBHP
(tBuOOH) reacted with Cu sites of POM-Cu(Ι) catalyst in a
so-called Haber−Weiss catalytic mechanism, resulting in a
transformation from tBuOOH into tert-butoxyl (tBuO·) radical
and hydroxyl ion (OH−).53,54 Subsequently, accompanied by
the leaving of a tBuOH, the generated tBuO· interacted with 1-
phenylethanol to form the radical (1), which would convert
into the intermediate (2) with the effect of Cu(ΙΙ) sites in
HENU-1. Afterward, the corresponding acetophenone was
formed along with the departure of water molecules and the
regeneration of active sites Cu(Ι), which was supported by the
XPS data of Cu element in the HENU-1 after catalysis (Figure
S7b). The formation of both tBuOH and H2O molecules has
been detected during the transformation, which experimentally
supported the proposed mechanism.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: jyniu@henu.edu.cn (J.N.).
*E-mail: jpwang@henu.edu.cn (J.W.).
■
ORCID
Jingyang Niu: 0000-0001-6526-7767
Author Contributions
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the Natural
Science Foundation of China (Grants 21571050, 21573056,
21771053, 21771054).
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CONCLUSION
■
In summary, we reported a novel copper-containing 3D
POMOF based on Dawson-type phosphotungstate and in situ-
generated ligands, HENU-1, which was employed as a
heterogeneous catalyst for oxidation of alcohols, exhibiting
superior catalytic activity. The Cu-based POMOF displays a
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ASSOCIATED CONTENT
■
Wang, J.; Niu, J. The Polyoxovanadate-Based Carboxylate Derivative
K6H[VV
V
12(OH)4O60(OOC(CH2)4COO)8]·nH2O: Synthesis,
S
IV
* Supporting Information
17
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b03589.
Crystal Structure, and Catalysis for Oxidation of Sulfides. Inorg.
Chem. 2017, 56, 14053−14059.
(12) Lu, J.; Ma, X.; Singh, V.; Zhang, Y.; Wang, P.; Feng, J.; Ma, P.;
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Tetracarbonyl Metal Selenotungstate Derivate [{Mn(CO)3}4(Se2W11-
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BVS, PXRD, IR, XPS, TG, and catalytic properties of
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G
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