Green Oxidation of Cyclohexanone to Adipic Acid over Phosphotungstic Acid Encapsulated in UiO-66

Article abstract of DOI:10.1007/s10562-019-02764-0

Abstract: A very stable catalyst, phosphotungstic acid (PTA) encapsulated in metal–organic framework UiO-66, was prepared by a simple one-pot solvothermal method. Characterization results show that UiO-66 is quite stable in the catalyst preparation process, and PTA is encapsulated in the cavities of UiO-66 with good dispersity. The as-synthesized composite material exhibited good catalytic activity and excellent reusability for the green oxidation of cyclohexanone to adipic acid (AA). Under mild reaction conditions, the isolated yield of AA was as high as 80.3% without the introduction of any organic solvent or phase transfer agent. The excellent immobilization effect of UiO-66 for PTA is mainly because UiO-66 has a well matched window size to confine PTA molecule in its nanocages. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02764-0

Catalysis Letters
https://doi.org/10.1007/s10562-019-02764-0
Green Oxidation of Cyclohexanone to Adipic Acid
over Phosphotungstic Acid Encapsulated in UiO‑66
1
1
1
Jian Feng · Min Li · Xiaojing Meng
Received: 16 January 2019 / Accepted: 22 March 2019
Springer Science+Business Media, LLC, part of Springer Nature 2019
©
Abstract
A very stable catalyst, phosphotungstic acid (PTA) encapsulated in metal–organic framework UiO-66, was prepared by a
simple one-pot solvothermal method. Characterization results show that UiO-66 is quite stable in the catalyst preparation
process, and PTA is encapsulated in the cavities of UiO-66 with good dispersity. The as-synthesized composite material
exhibited good catalytic activity and excellent reusability for the green oxidation of cyclohexanone to adipic acid (AA). Under
mild reaction conditions, the isolated yield of AA was as high as 80.3% without the introduction of any organic solvent or
phase transfer agent. The excellent immobilization eꢀect of UiO-66 for PTA is mainly because UiO-66 has a well matched
window size to conꢁne PTA molecule in its nanocages.
Graphical Abstract
Keywords Phosphotungstic acid · Metal–organic frameworks · Cyclohexanone · Adipic acid
Abbreviations
AA
Adipic acid
DMF
FT-IR
N,N-dimethylformamide
Fourier transform infrared spectroscopy
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02764-0) contains
supplementary material, which is available to authorized users.
ICP-AES Inductively coupled plasma-atomic emission
spectroscopy
*
Jian Feng
MOFs
PTA
Metal–organic frameworks
Phosphotungstic acid
jianfeng@cqust.edu.cn
Min Li
SEM
TEM
TGA
Scanning electron microscope
Transmission electron microscopy
Thermogravimetric analysis
limin1406@163.com
1
School of Chemistry and Chemical Engineering, Chongqing
University of Science and Technology, Chongqing 401331,
China
Vol.:(0
1
123456789)
3

J. Feng et al.
TOF
Turnover frequency
X-ray diꢀraction
reversible nature of coordination bonds [23]. The regenera-
tion and reuse of MOFs-based catalysts is still far from that
of the more robust and stable inorganic zeolites [26]. Con-
siderable eꢀort has been devoted to preparing MOFs with
improved stability [29]. Gratifyingly, a very stable Zr-based
MOF, UiO-66, was constructed by Cavka et al. and cow-
orkers [30]. The structure of UiO-66 exhibits exceptional
thermal, chemical and mechanical stability beyond most of
other present MOFs. It has been reported to be stable up to
540 °C, and also stable to water and many organic solvents
[30]. To the best of our knowledge, there are no reports on
the oxidation of cyclohexanone to AA over a catalyst derived
from UiO-66. Considering the high activity of PTA for the
target reaction, we envisage using UiO-66 to encapsulate
PTA for improved performances. In this paper, we report
the encapsulation of PTA in the pore of UiO-66 by a one-
pot solvothermal method. The composite material (PTA@
UiO-66) was used as a catalyst for the oxidation of cyclohex-
anone to AA, which exhibited good AA yield and excellent
reusability.
XRD
1
Introduction
Adipic acid (AA) is an important commodity chemical with
huge demand worldwide. The primary use of AA is served
as a precursor for the synthesis of Nylon-6,6. At present,
the industrial process for the production of AA involves
the oxidation of cyclohexanone and/or cyclohexanol using
excess HNO [1, 2]. A serious problem of the current oxida-
3
tion process is the generation of nitrogen oxides, especially
N O, which is generally considered as a nuisance causing
2
global warming and ozone depletion [3]. Therefore, it is in
urgent need to explore an environmental friendly oxidant to
replace HNO .
3
Hydrogen peroxide is one of the most promising green
oxidants because the only byproduct from it is water. In
particular, the oxidation of cyclohexanone to AA by H O
2
2
can be achieved in the presence of various catalysts [3–15].
Among the reported catalysts, phosphotungstic acid (PTA)
exhibits very high activity, but the application of PTA is
limited due to its low stability under the catalytic condition
and high solubility in the reaction mixture [16, 17]. These
properties of PTA will bring many troubles to the product
separation and catalyst recycling. A feasible strategy to
improve the performance of PTA as well as other heteropoly-
acids is loading them onto solid supports like silica [18, 19],
molecular sieves [20, 21], and metal oxides [22]. Neverthe-
less, the use of conventional support materials shows several
disadvantages such as PTA leaching, PTA agglomeration,
and deactivation by water [22]. Hence the stabilization of
PTA using a suitable new-type support is highly desirable.
Metal–organic frameworks (MOFs), emerging as a new
class of crystalline porous materials, have been regarded as
potential and eminent candidates for the catalyst supports
2 Experimental
2.1 Catalyst Preparation
UiO-66 samples were prepared following a recipe described
previously [31] with some modiꢁcations. Brieꢂy, ZrCl
4
(0.7 g) and terephthalic acid (0.5 g) were dissolved in 60 mL
N,N-dimethylformamide (DMF) by using ultrasound for
20 min. Then, the modulator acetic acid (3 mL) was added
into the solution and dispersed by ultrasound for 1 min. The
mixture was sealed in a Teꢂon-lined autoclave and kept in an
oven at 120 °C under static conditions for 24 h. White pre-
cipitates were produced, and were isolated by centrifugation
after cooled down to room temperature. The solids were suc-
cessively washed with DMF and methanol to remove unre-
acted precursors and residual solvent. Finally, the obtained
samples were dried at 160 °C overnight.
[
23–25]. MOFs possess a series of unique properties such
as high surface area, well-deꢁned and tunable pore struc-
ture, and intriguing functionality [23, 26]. In recent years,
the encapsulation of PTA in the pore of MOFs has attracted
much attention because MOFs can act as hosts to conꢁne
and stabilize PTA in their cavity, thereby facilitating the
product separation and catalyst recycling. For example, PTA
was successfully encapsulated in the nanocages of MIL-101,
and this composite material showed good catalytic activity
in the oxidative desulfurization, although PTA leaching was
observed in the catalyst recycling test [27]. MIL-100(Fe)
was also used as a host to conꢁne PTA, which exhibited
satisfying performances for the oxidative desulfurization of
gasoline [17] or the acetalization of benzaldehyde [28].
Despite the fact that MOFs have many advantages as cata-
lyst supports, most MOFs show low stability owing to the
The encapsulation of PTA in UiO-66 was performed by a
one-pot solvothermal method following the same procedure
as that for pure UiO-66, besides PTA was added to the DMF
solution during the synthesis process. The PTA weight con-
tent was 28.7% based on the measurement using inductively
coupled plasma-atomic emission spectroscopy (ICP-AES,
Thermo iCAP 6300). The resulting catalyst was denoted as
PTA@UiO-66.
2.2 Characterization
The powder X-ray diꢀraction (XRD) patterns of the samples
were recorded on a Shimadzu XRD-7000 diꢀractometer with
Cu Kα radiation (λ=0.1541 nm), which operated at 40 kV and
1
3

Green Oxidation of Cyclohexanone to Adipic Acid over Phosphotungstic Acid Encapsulated in…
6
0 mA. Fourier transform infrared spectroscopy (FT-IR) char-
3 Results and Discussion
acterization was performed on a Bruker Tensor-37 spectrom-
−1
eter in the range of 4000–400 cm . The spectra of samples
were collected with diluting by KBr. The nitrogen adsorption
and desorption were performed on a Quantachrome Autosorb-
IQ-C apparatus. The samples were degassed under vacuum at
3.1 Catalyst Characterization
The XRD patterns of pure UiO-66 and PTA@UiO-66 are
shown in Fig. 1. The diꢀraction pattern of the as-synthesized
UiO-66 is fairly consistent with the reported standard pat-
terns [30, 31], indicating that the crystalline structure of
UiO-66 is well constructed by the preparation method in this
work. The diꢀraction pattern of PTA@UiO-66 is almost the
same as that of pure UiO-66, suggesting that UiO-66 has a
robust structure as a host for PTA nanoparticles. It should be
noted that the characteristic diꢀraction peaks of PTA are not
observed after the encapsulation, which indicates that PTA is
evenly dispersed in the nanocages of UiO-66 [17]. The XRD
pattern of the recycled catalyst is also presented in Fig. 1,
and this will discuss in the section of catalyst reusability.
The FT-IR spectra of the catalyst samples are illustrated
1
50 °C for 6 h before the adsorption measurements. The spe-
ciꢁc surface areas were evaluated using the BET method. The
total pore volume was calculated from the amount of nitro-
gen adsorbed at a relative pressure of 0.99. Scanning electron
images were taken on a JEOL JSM-7800F scanning electron
microscope (SEM). All the samples were mounted on a carbon
tape and coated with a thin ꢁlm of gold prior to measure-
ment. Transmission electron microscopy (TEM) studies were
performed on a JEOL JEM-2100F microscope operating at
2
00 kV. The samples were ultrasonically suspended in ethanol
and deposited on carbon-coated copper grids. The thermo-
gravimetric analysis (TGA) measurements were carried out
on a Hengjiu HCT-4 thermoanalyzer in air atmosphere with a
−1
in Fig. 2. For UiO-66, the broad band centered at 3420 cm
can be ascribed to physisorbed water condensed inside the
−1
ꢂow rate of 50 mL min .
−
1
crystal cavities [32]. A small band around 1658 cm is
associated with the C=O asymmetric stretch of the residual
DMF within the pores of UiO-66, indicating that DMF is
very diꢃcult to completely eliminated [32]. The two intense
2
.3 Catalytic Reaction
The oxidation of cyclohexanone was carried out in a round-
bottom ꢂask ꢁtted with a water condenser and a magnetic
stirrer. In a typical reaction procedure, required amounts of
catalyst and hydrogen peroxide (30 wt%) were mixed. After
vigorous stirring for 10 min, cyclohexanone (5.9 g, 60 mmol)
was introduced. The mixture in the ꢂask was placed in a pre-
heated oil bath at the reaction temperature, and stirred for 2
to 16 h. Then, the catalyst was separated by ꢁltration. The
−
1
bands around 1585 and 1395 cm represents the in- and
out-of-phase stretching modes of the carboxylate group in
−
1
terephthalic acid. A sharp band at 1508 cm corresponds
to the vibration of C=C in the benzene ring. At lower fre-
quencies, the bands (mainly around 820, 746, 659, 555 and
−1
478 cm ) due to OH and C–H bending are mixed with Zr–O
modes. The spectra of PTA@UiO-66 and PTA@UiO-66
after the reaction cycles show similar bands to the UiO-66
ꢁ
ltrate was allowed to stand at 4 °C for 12 h, and the result-
ing precipitate was separated by suction ꢁltration and washed
with a small amount of cold water. Generally, the product was
isolated as pure white crystals, whose FT-IR spectroscopy
and melting point (152 °C) were consistent with the standard
sample [6, 7]. In some cases, the product can be puriꢁed by
recrystallization if necessary. The AA yield is calculated as
the ratio of the isolated AA mass to the theoretical AA mass.
To test if further conversion occurs by the leached PTA
from the catalyst, a hot ꢁltration experiment was carried out.
After reaction for 4 h, the catalyst was removed by a syringe
c
ꢁ
6
ꢁ
lter, and the obtained reaction solution was stirred for another
h. In the recycling experiment, the catalyst obtained from
ltration was washed thoroughly with methanol and dried in
b
air overnight before the reusability test.
a
10
20
30
40
50
60
o
2
θ ( )
Fig. 1 XRD patterns of (a) UiO-66, (b) PTA@UiO-66, and (c)
PTA@UiO-66 after the reaction cycles
1
3

J. Feng et al.
82 825
600
500
400
300
200
100
0
9
c
b
a
b
a
1
080
890
0.0
0.2
0.4
0.6
0.8
1.0
4000
3500
3000
2500
2000
1500
1000
500
Relative Pressure (P/P0)
-1
Wavenumber (cm )
Fig. 3 N adsorption–desorption isotherms of (a) UiO-66 and (b)
2
Fig. 2 FT-IR spectra of (a) UiO-66, (b) PTA@UiO-66, and (c)
PTA@UiO-66
PTA@UiO-66 after the reaction cycles
of intergrown aggregates rather than individual nanocrys-
tals, which is in line with the result of a previous study
[16]. Similar SEM micrograph was observed for the pure
UiO-66 support, suggesting that the crystalline framework
can be retained after PTA incorporation. The TEM images
show the morphology more clearly (Fig. 4b, c). No con-
glomeration of PTA particles was observed, further verify-
ing the good dispersity of PTA on the support as that has
been demonstrated by XRD.
support, suggesting that the crystalline structure of UiO-66
is stably retained after the treatments. These FT-IR results
are in good agreement with the XRD results. The successful
incorporation of PTA can be veriꢁed from the four typical
infrared bands of PTA with Keggin structures. As labeled in
−1
Fig. 2, the band at 1080 cm is attributed to the asymmetric
vibrations of the P–O bond in PTA, while the bands at 982,
−
1
8
90, and 825 cm are ascribed to the terminal W=O , edge-
a
sharing W–O –W, and corner-sharing W–O –W, respec-
Figure 5 shows the TGA curves of UiO-66 and PTA@
UiO-66. The two curves display a similar proꢁle, mainly
including two stages of weight loss. The ꢁrst weight loss
between 50 and 350 °C is due to the release of adsorbed
water and residual DMF solvent. The second weight loss in
the range of 450 to 540 °C can be attributed to the thermal
decomposition of organic ligand, leading to the forma-
b
c
tively [17]. For the PTA@UiO-66 sample, although the two
−
1
bands at 890 and 825 cm are overlapped with the skeletal
−
1
modes of pure UiO-66, the bands at 1080 and 982 cm are
very obvious, indicating that PTA is deꢁnitely encapsulated
in UiO-66.
The N adsorption–desorption isotherms of UiO-66 and
2
PTA@UiO-66 are presented in Fig. 3, and the corresponding
textural properties are listed in Table 1. The UiO-66 sam-
ple basically exhibits a type-I isotherm, demonstrating the
microporous feature of this material. The isotherm of PTA@
tion of ZrO [30]. For PTA@UiO-66, the decomposition
2
of the PTA Keggin structure into simple oxides may be
accompanied with the decomposition of organic ligand
[16]. When the temperature is above 550 °C, no weight
loss was observed. According to the TGA results, the UiO-
66 support is really thermal stable as reported [30], and
the incorporation of PTA has no obvious inꢂuence on the
thermal stability.
UiO-66 is similar to that of UiO-66, but the volume of N
2
adsorbed decreased, further indicating the entry of PTA into
the pores of UiO-66. As shown in Table 1, the BET surface
2
−1
area of UiO-66 reaches 1054 m g , and the pore volume
3
−1
is 0.83 cm g , which are in accord with the preparation
method we consulted [31]. After PTA loading, a decrease of
the BET surface area and pore volume are observed, which
can be attributed to the occupation of nanocages by PTA
nanoparticles.
Table 1 Some textural properties of the catalyst samples
Catalysts
BET surface area (m g−1) Pore
2
Figure 4 provides the SEM and TEM images of PTA@
UiO-66. As can be seen from the SEM picture in Fig. 4a,
the morphology of the composite catalyst is not very
regular, showing near octahedral shapes with passivated
edges. The obtained PTA@UiO-66 catalyst is composed
volume
3
−1
(cm g
)
UiO-66
PTA@UiO-66
1054
952
0.83
0.80
1
3

Green Oxidation of Cyclohexanone to Adipic Acid over Phosphotungstic Acid Encapsulated in…
Fig. 4 a SEM image and b and
c TEM images of PTA@UiO-66
1
00
9
0
0
0
0
0
0
0
80
8
7
6
5
4
3
6
4
2
0
0
0
b
a
0
0
100
200
300
400
500
600
700
UiO-66
PTA@UiO-66
PTA
o
Temperature ( C)
Fig. 6 Catalytic performances of various catalysts for the oxidation
of cyclohexanone with H O . Reaction conditions: catalyst 0.9 g,
2
2
2
2
Fig. 5 TGA curves of (a) UiO-66 and (b) PTA@UiO-66
cyclohexanone 60 mmol, H O (30 wt%) 270 mmol, 90 °C, 8 h
3
.2 Catalytic Performance
result conꢁrms that the encapsulation of PTA by UiO-66 is a
feasible strategy to seek advantages and avoid disadvantages
of PTA. Interestingly, the pristine UiO-66 also showed a
little catalytic activity, giving a yield of AA to 12.9%. This
The catalytic performances of UiO-66, PTA@UiO-66, and
PTA are illustrated in Fig. 6. Notably, a high yield of AA
IV
(
80.3%) was achieved by PTA@UiO-66, which was compa-
is probably owing to the formation of Zr –peroxo groups
rable to the yield of AA (88.7%) using PTA as catalyst. This
on the surface of the MOF material by the interaction with
1
3

J. Feng et al.
IV
H O [33]. Oxygen from the Zr –peroxo groups can act as
2
2
80
active species to participate in the oxidation reactions [17,
3
3]. In the light of the high activity of PTA@UiO-66, PTA
is undoubtedly the main active centers for the oxidation of
cyclohexanone to AA.
70
The reaction temperature signiꢁcantly inꢂuenced the cat-
alytic performance of PTA@UiO-66 (Fig. 7). As the temper-
ature increased from 70 to 90 °C, the yield of AA increased
rapidly, after which the yield began to decrease. It is well
reported that H O will decompose into water and oxygen
6
5
4
3
0
0
0
0
2
2
at high temperature [3, 11]. Hence the decrease of AA yield
at a temperature above 90 °C is probably related to the rapid
decomposition of H O . In addition, excessively high tem-
2
2
3.0
3.5
4.0
4.5
5.0
perature may lead to the formation of byproducts such as
succinic acid and glutaric acid, which can also account for
the decrease of AA yield.
H O /cyclohexanone ratio
2
2
Fig. 8 Eꢀect of molar ratio of H O to cyclohexanone on the oxida-
2
2
2
The theoretical molar ratio of H O to cyclohexanone is
2
2
tion of cyclohexanone with H O over PTA@UiO-66. Reaction con-
2
3
for the target reaction, but the practical dosage of H O
2 2
ditions are the same as that in Fig. 6 but varied with molar ratio of
exceeded the theoretical value. This phenomenon can also
be explained by the undesirable decomposition of H O as
H O to cyclohexanone
2
2
2
2
we discussed above. It can be seen in Fig. 8 that the optimal
molar ratio of H O to cyclohexanone is 4.5. Too much of
The PTA loading also inꢂuences the catalytic perfor-
2
2
H O may cause side reactions, resulting in the decrease of
mance of PTA@UiO-66, and the results are shown in
Table S1. Initially, the yield of AA increased with increas-
ing PTA loadings (from 10.2 to 28.7 wt%) because of more
available active centers. However, keeping on increasing
the PTA loading resulted in the decrease of AA yield. A
reasonable explanation for this is that excessive amount
of PTA can intensify the decomposition of H O [1, 2],
2
2
AA yield.
The eꢀect of reaction time on the catalytic performance
of PTA@UiO-66 is presented in Fig. 9. Initially, the reaction
rate rapidly increased within 6 h, after which it proceeded
slowly. The highest yield of AA was achieved after 8 h of
reaction. The further extension of reaction time led to reduc-
ing of AA yield, which is probably due to the formation of
more byproducts [8]. In fact, the product obtained after reac-
tion for 16 h was impure based on the melting point, and it
had to be further puriꢁed by recrystallization.
2
2
leading to a part of H O consumed without participating
2
2
in the oxidation of cyclohexanone.
8
7
6
5
4
3
2
0
0
0
0
0
0
0
8
7
6
5
4
3
2
0
0
0
0
0
0
0
Hot filtration experiment
70
75
80
85
90
95
100
0
2
4
6
8
10
12
14
16
18
o
Reaction temperature
(
C)
Reaction time (h)
Fig. 7 Eꢀect of reaction temperature on the oxidation of cyclohex-
Fig. 9 Eꢀect of reaction time on the oxidation of cyclohexanone with
anone with H O over PTA@UiO-66. Reaction conditions are the
H O over PTA@UiO-66. Reaction conditions are the same as that in
2
2
2
2
same as that in Fig. 6 but varied with reaction temperature
Fig. 6 but varied with reaction time
1
3

Green Oxidation of Cyclohexanone to Adipic Acid over Phosphotungstic Acid Encapsulated in…
3
.3 Comparison of Catalytic Performance Between
8
7
6
5
4
3
0
0
0
0
0
0
PTA@UiO‑66 and Reported Solid Catalysts
The catalytic performance of PTA@UiO-66 is compared
with other reported solid catalysts, and the corresponding
results are summarized in Table 2. The turnover frequency
(
TOF) of the catalysts cannot be calculated because the spe-
ciꢁc active species are unknown in the literature. However,
all the AA yield values presented in Table 2 are the highest
one as reported in the corresponding references. Therefore,
the catalytic performance can be compared as far as the AA
yield is concerned. The optimal yield of AA in the present
work was 80.3%, which was much higher than most of the
reported values using zeolites, TIPO-1, Al O @Fe O , and
20
1
0
0
1
2
3
4
5
2
3
2
3
Reaction run
polyoxometalates as catalysts. Although 75.5% AA yield
was achieved over H Co PMo O , the reaction time was
1
1
12 40
Fig. 10 Catalytic reusability of PTA@UiO-66 for the oxidation of
much longer (20 h). Moreover, this polyoxometalate could
not be separated by a simple ꢁltration because of the forma-
tion of a homogeneous mixture after the reaction [6]. It is
worth mentioning that, besides TOF and the AA yield, the
average AA productivity can be also used to compare the
catalytic activity. In this case, we just need to take the weight
of PTA in the PTA@UiO-66 catalyst into account. The AA
cyclohexanone with H O . Reaction conditions are the same as that
2
2
in Fig. 6
signiꢁcant decrease of AA yield after ꢁfth successive reac-
tion cycles. Thus, PTA@UiO-66 exhibited good catalytic
stability for the oxidation of cyclohexanone to AA using
H O as oxidant. What’s more, the characterization results
−
1
−1
productivity in the present work is 23.2 mmol g
h
as
2
2
calculated, which is also the highest value for all the cata-
lysts listed in Table 2. Accordingly, PTA@UiO-66 shows a
better function as compared with other heterogeneous solid
catalytic systems under the investigated reaction conditions.
demonstrate that PTA@UiO-66 can maintain its skeleton
structure after the oxidation reaction. The XRD pattern of
the recycled PTA@UiO-66 catalyst in Fig. 1 shows that
all the crystalline phases are matched well with that of the
fresh catalyst. The FT-IR spectrum of the recycled catalyst
is almost identical to that of the fresh catalyst (Fig. 2). In
addition, besides the bands of PTA with Keggin structures,
the FT-IR spectra of UiO-66 and PTA@UiO-66 after the
reaction cycles seems to diꢀer slightly. This indicates that
H O doesn’t show obvious damage to the UiO-66 mate-
3.4 Catalyst Reusability
To evaluate the stability of the PTA@UiO-66 catalyst, the
reusability test was performed under the optimized reac-
tion conditions. After each run of the reaction, the catalyst
was recovered according to the procedure described in the
Sect. 2. It can be observed from Fig. 10 that there is no
2
2
rial surface sites along the reaction despite the formation of
IV
Zr –peroxo groups as discussed above.
Table 2 Comparison of
Catalysts
Conditions
Yield of AA (%) References
catalytic performance between
PTA@UiO-66 and reported
solid catalysts
Catalyst
amount
CHO
Tempera- Time (h)
ture (°C)
amount
(mmol)
(
g)
PTA@UiO-66
HTS-1
TIPO-1
0.9a
0.05
0.04
0.0625
0.0625
0.015
0.5
60
10
10
15
30
5
90
90
80
90
90
80
80
90
8
8
8
20
20
24
6
80.3
16.8
60.7
75.5
45.0
59.3
17.0
58.8
This work
[4]
[5]
[6]
[7]
[8]
[10]
[11]
H Co PMo O
1
1
12 40
(
NH4)0.5Ni1.25PMo O
12
40
Al O @Fe O
2
3
2
3
HTS
100
15
K P Mo W O
0.125
20
6
2
6
12 62
CHO cyclohexanone, HTS hollow titanium silicate zeolite, TIPO-1 oxyꢂuorinated titanium phosphate
a
PTA content=0.26 g
1
3

J. Feng et al.
Fig. 11 Reaction mechanism for
the oxidation of cyclohexanone
to AA over PTA@UiO-66
O
O
O
COOH PTA@UiO-66, H O
2 2
COOH
COOH
PTA@UiO-66, H O
Oxidation active sites
H O
Acid sites
2
2
2
Oxidation active sites
CH OH
2
A hot ꢁltration experiment was carried out to see if fur-
ther conversion occurs by the leached PTA from the catalyst,
and the results are shown in Fig. 9. The catalytic reaction
almost stopped after the removal of the catalyst, indicating
that the oxidation of cyclohexanone occurred on the surface
of the PTA@UiO-66 catalyst rather than in the solution.
The excellent immobilization eꢀect of the UiO-66 sup-
port for PTA is worthy of further comment. The pore sys-
tem of UiO-66 is constructed by two types of cages with
diꢀerent sizes (free apertures of~1.4 nm and~1.0 nm for
the octahedral and tetrahedral cages, respectively), which
are joined together through triangular windows with a size
of~0.6 nm [17]. PTA has a molecular size of~1.2 nm [34].
Accordingly, PTA can be encapsulated in the larger cages
of UiO-66 but cannot leak out from the windows. That is to
say, UiO-66 has a well matched window size to conꢁne PTA
molecule in its nanocages, which can be responsible for its
excellent immobilization eꢀect for PTA.
amount of ε-caprolactone were found. Combining the estab-
lished reaction pathway for the oxidation of cyclohexanone
to AA over other tungsten compounds [2, 3, 13], a plausi-
ble reaction mechanism is proposed and shown in Fig. 11.
The ꢁrst step is a typical Baeyer–Villiger oxidation process,
which occurs on the oxidation active sites of PTA@UiO-
66. These oxidation active sites may exist as peroxo species
2
−
of tungsten such as [WO(O ) (OH) ] , originating from
2
2
2
the oxidation of PTA in the presence of H O [2, 3, 13].
2
2
The ε-caprolactone obtained in the ꢁrst step is successively
hydrolyzed to 6-hydroxyhexanoic acid. This hydrolysis step
can readily take place on the acid sites of PTA@UiO-66
(including both Brønsted and Lewis acid sites). Finally, AA
is formed by the oxidation of hydroxyl group in 6-hydrox-
yhexanoic acid, which can also proceed on the oxidation
active sites.
To verify the window size of UiO-66 is important for
its good catalytic stability, a well-established MOF (MIL-
4
Conclusions
1
01) with larger window size was employed as support for
The encapsulation of PTA in a stable MOF UiO-66 was
achieved by a one-pot solvothermal method. The as-synthe-
sized PTA@UiO-66 composite material demonstrated to be
an eꢃcient catalyst for the green oxidation of cyclohexanone
comparison. PTA@MIL-101 with a same PTA loading
29.1 wt%) was prepared according to previous works [17,
7], and its catalytic reusability was evaluated under the
(
2
same conditions using for PTA@UiO-66 (Fig. S1). Obvi-
ously, the catalytic stability of PTA@MIL-101 for the oxi-
dation of cyclohexanone to AA was inferior, leading to low
AA yield (42.5%) only after three recycling runs. The ICP-
AES tests showed that the leaching of PTA into the reaction
solution was about 45%. This poor immobilization eꢀect of
to AA using H O as oxidant. High isolated yield of AA was
2
2
obtained under mild reaction conditions without the addition
of organic solvent or phase transfer agent. PTA@UiO-66
exhibited high stability and excellent reusability in the oxi-
dation process, which provides the possibility to its further
industrial applications.
MIL-101 for PTA is mainly due to its larger window size
2
(
1.6×1.6 nm ), which results in a serious leaching of PTA
Acknowledgements This work was supported by the National Natural
Science Foundation of China (21302237, 51708075) and the Scientiꢁc
and Technological Research Program of Chongqing Municipal Educa-
tion Commission (KJ1713335, KJQN201801527).
during the reaction process [17]. For PTA@UiO-66, the
leaching of PTA can be eꢀectively avoided because of its
well matched window size as discussed earlier.
In short, all the above results have conꢁrmed the good
structural stability of PTA@UiO-66, which is especially
important for its practical uses.
Compliance with Ethical Standards
Conflict of interest The authors declare no conꢂict of interest.
3.5 Plausible Reaction Mechanism
As discussed above, the oxidation of cyclohexanone to AA
over PTA@UiO-66 is really heterogeneous which occurs
on the catalyst surface. To investigate the possible reaction
mechanism, the mixture after the reaction was detected by
GC–MS. Besides AA, 6-hydroxyhexanoic acid and a small
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