Bilayer MOF@MOF and MoOx species functionalization to access prominent stability and selectivity in cascade-selective biphase catalysis

Article abstract of DOI:10.1016/j.mcat.2021.111818

A novel bilayer metal-organic framework (viz., MOF@MOF, based on Zr-MOFs via epitaxial growth of UiO-bpy on preformed UiO-66) assembled with two-dimensional MoO_x species was designed and synthesized. The MoO_x@UiO-66@MoO_x@UiO-bpy composite would improve substrates/products diffusion, avoid active metals leaching, and crucially provide a new platform via trapping intermediates and regulating the desired reaction direction. As confirmed via characterization results, MoO_x@UiO-66@MoO_x@UiO-bpy with micro-/mesopores and strong acid sites (Lewis and Br?nsted) exhibited good thermal and chemical stability. The bilayer MoO_x@UiO-66@MoO_x@UiO-bpy was further tested in cascade-selective biphase cyclopentene oxidation and exhibited 10.3% cyclopentene conversion and 16.9% glutaric acid selectivity higher than those of MoO_x@UiO-66, due to its large specific surface area, high content of MoO_x species, micro-/mesoporous structure, and bilayer catalytic effect. Its good reusability (> 10 runs) in the solvent-free-biphase cyclopentene oxidation suggested multidimensional encapsulation of active species in bilayer MOFs providing new idea for designing high-selectivity/stability heterogeneous catalysts. Besides, a set of reaction kinetic models with detailed kinetic parameters and a detailed reaction mechanism were provided, reconfirming the key to develop the technology of cyclopentene to glutaric acid was to conquer cyclopentene oxide hydrolysis activation energies (E₂ and E₄). Compared to our previous study, E₂ and E₄ values were decreased 93.8 and 31.7 kJ?mol^?1, respectively.

Full text of DOI:10.1016/j.mcat.2021.111818

Molecular Catalysis 513 (2021) 111818
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Bilayer MOF@MOF and MoOx species functionalization to access
prominent stability and selectivity in cascade-selective biphase catalysis
,
,1,
Qingtao Niua 1, Manman Jina *, Guodong Liua, Zhiguo Lvb, Chongdian Sia, Hong Hana
a Department of Chemistry and Chemical Engineering, Jining University, No.1 Xingtan Road, Qufu 273155, China
b State Key Laboratory Base for Eco-chemical Engineering, College of Chemical Engineering, Qingdao University of Science and Technology, No.53 Zhengzhou Road,
Qingdao 266042, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A novel bilayer metal-organic framework (viz., MOF@MOF, based on Zr-MOFs via epitaxial growth of UiO-bpy
on preformed UiO-66) assembled with two-dimensional MoOx species was designed and synthesized. The
MoOx@UiO-66@MoOx@UiO-bpy composite would improve substrates/products diffusion, avoid active metals
leaching, and crucially provide a new platform via trapping intermediates and regulating the desired reaction
direction. As confirmed via characterization results, MoOx@UiO-66@MoOx@UiO-bpy with micro-/mesopores
and strong acid sites (Lewis and Brønsted) exhibited good thermal and chemical stability. The bilayer
MoOx@UiO-66@MoOx@UiO-bpy was further tested in cascade-selective biphase cyclopentene oxidation and
exhibited 10.3% cyclopentene conversion and 16.9% glutaric acid selectivity higher than those of MoOx@UiO-
66, due to its large specific surface area, high content of MoOx species, micro-/mesoporous structure, and bilayer
catalytic effect. Its good reusability (> 10 runs) in the solvent-free-biphase cyclopentene oxidation suggested
multidimensional encapsulation of active species in bilayer MOFs providing new idea for designing high-
selectivity/stability heterogeneous catalysts. Besides, a set of reaction kinetic models with detailed kinetic pa-
rameters and a detailed reaction mechanism were provided, reconfirming the key to develop the technology of
cyclopentene to glutaric acid was to conquer cyclopentene oxide hydrolysis activation energies (E2 and E4).
Compared to our previous study, E2 and E4 values were decreased 93.8 and 31.7 kJ⋅molꢀ 1, respectively.
UiO-66@UiO-bpy
MoOx species
Encapsulation
Cascade-selective catalysis
1. Introduction
stability obtained via selecting chelating ligands with pyridyl moieties
(e.g., bipyridines, terpyridines, or phenanthrolines) [22,23].
Metal-organic frameworks (MOFs), an emerging family of organic-
inorganic hybrid materials, have aroused tremendous attention in
academia and industry due to their potential capacity in catalysis [1,2],
separation [3,4], gas storage [5,6], and carbon capture [7,8], etc..
Compared with other porous materials [9–11], MOFs provide greater
structure versatility [12], facility to design [13], adjustable porosity
[14], much higher surface area [15], and relatively effortless confine-
ment of guest species [16]. These features permit MOFs acting as a novel
type of platform to assemble bi-/multifunctional characteristics into one
system for plentiful catalysis, especially for cascade catalysis [17–20].
Particularly, Zr-MOFs, assembled via linear dicarboxylate and
Zr6O4(OH)4(O2CR)12 units [21], have become attractive candidates for
constructing novel catalytic materials due to their extraordinary ther-
mal/chemical stability, low toxicity of Zr, as well as robust redox
Many efforts to the development of active species encapsulated
MOFs (denoted as species@MOFs), for a large scale of catalysis system,
were done [24]. The fascination depends on the variety of nearly un-
limited combinations, referring to both employed MOFs and immobi-
lized species. The frameworks of MOFs feature individual properties
because of their variable metal clusters and the functional organic en-
tities, whereas the confined species represent functional and catalytic
sites [25]. Combination of both generates synergetic effects, beneficial
to a large amount of application [26]. Nevertheless, the microporosity of
the conventional MOFs may generally result in diffusion limitation of
the anticipate catalytic transformation [27], and the leaching of metal
particles during the reaction process is inevitable [28]. Accordingly,
confining species in hollow MOFs composites has become possible
which can accelerate an effective transport of reactants to the internal
* Corresponding author.
E-mail address: 13864220985@163.com (M. Jin).
1
These authors contributed equally.
https://doi.org/10.1016/j.mcat.2021.111818
Received 15 June 2021; Received in revised form 14 July 2021; Accepted 2 August 2021
Available online 13 August 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

Q. Niu et al.
Molecular Catalysis 513 (2021) 111818
Fig. 1. XRD patterns (a) and BET isotherms (b, pore size distribution (inset)) of different materials.
surface with simultaneously improved desorption of products [29], but a
continuous activity decrease because of the loss of active particles from
open-ended pores is still a critical problem. The conventional MOFs are
sometimes difficult to meet special needs leading to the emergence of
hybrid or multifunctional MOFs composites [30,31]. Therefore, the
so-called MOF@MOF with bilayer hetero-structure were developed via
epitaxial growing another MOF on preformed MOF to bring changes in
their physical/chemical properties compared with a single MOF mate-
rial or intrigue unexpected synergies. Zhuang et.al [32] designed a
fracture-free Pd-UiO-66@ZIF-8 MOF, which displayed excellent
size-selective hydrogenation catalysis and high stability for prolonged
reactions (17 h). Compared with UiO-67-BPY-Ag, UiO-66@UiO-67-B-
PY-Ag showed a 22% improvement of 3-phenylpropiolic acid yield, and
just < 0.0013% Ag was detected in the filtrate after a series cycles,
suggesting the relatively better stability of UiO-66@UiO-67-BPY-Ag
[33].
2. Experimental section
2.1. Catalysts synthesis
ZrCl4 (99.5 wt%), 1,4-benzene dicarboxylic acid (99 wt%), glacial
acetic acid (99.5 wt%), DMF (99.5 wt%), MeOH (99.5 wt%), 2,2′-
bipyridine-5,5′-dicarboxylic acid (98 wt%), and (NH4)6Mo7O24⋅4H2O
(99 wt%) were purchased from Macklin Company. Cyclopentene (98 wt
%) and H2O2 (35 wt%) were obtained from Kangderui Chemical Tech-
nology Co., Ltd.
2.1.1. UiO-66 and UiO-bpy
UiO-66 and UiO-bpy were synthesized on the basis of the reported
literatures [28,46,47]. The detailed synthetic procedure was exhibited
in Supporting Information.
Obviously, selective oxidation of cyclopentene to glutaric acid with
H2O2, an important green and efficient cascade-selective biphase
oxidation, has been explored for many years [34–38], but there is no
commercial installation so far. The fundamental reason is that it is
difficult to coordinate and balance the catalytic activity, target product
oriented selectivity, and catalyst stability, especially the poor directional
selectivity is particularly unacceptable in cascade liquid catalysis. Ho-
mogeneous catalysts (tungstic acid or molybdophosphate heteropoly
acid, etc.) with excellent catalytic activity always generate poor reus-
ability and contamination via residual catalyst. Transition metal oxides
of Mo, W, Fe, and Co have been confirmed serving as heterogeneous
catalysts active sites for many reactions [38–42]. Among numerous
heterogeneous catalysts, Mo/W-based catalysts have showed promising
activity because of their oxidation potentials appropriate for actuating
the oxidation reaction and multiple Lewis acidity sites [43–45]. Based
on the above-mentioned description, it still remains an enormous chal-
lenge to design suitable Mo/W-based heterogeneous catalysts with su-
perior directional selectivity to the target product while facilitating
reactants/products diffusion and inhibiting leaching of active species for
cyclopentene selective oxidation.
2.1.2. MoOx@UiO-66
UiO-66 (0.5 g) was added to (NH4)6Mo7O24⋅4H2O aqueous solution
(20 mg⋅mLꢀ 1, 12 mL), and the mixture was stirred at 40 ◦C for 48 h to
obtain
a suspension solution. The resulting MoOx@UiO-66 was
collected, washed, purified, and dried with similar procedure as the
preparation of UiO-66 or UiO-bpy, and calcined at 300 ◦C for 5 h.
2.1.3. MoOx@UiO-66@UiO-bpy
ZrCl4 (0.047 g, 0.2 mmol), 2,2′-bipyridine-5,5′-dicarboxylic acid
(0.049 g, 0.2 mmol), glacial acetic acid (0.4 mL), and DMF (30 mL) were
placed in a 50 mL flask. The mixture was sonicated for 2 h before
MoOx@UiO-66 (0.1 g) introducing. Afterwards, the suspension solution
was sonicated for another 40 min, and then transferred to Teflon-lined
stainless steel autoclave (50 mL) and heated at 120 ◦C for 24 h. After
cooling, sample was isolated and washed with DMF. After soaking with
MeOH, the solid was dried at 100 ◦C overnight to obtain MoOx@UiO-
66@UiO-bpy.
2.1.4. MoOx@UiO-66@MoOx@UiO-bpy
The same synthetic procedure was employed as MoOx@UiO-66
except that UiO-66 was replaced with MoOx@UiO-66@UiO-bpy.
Herein, inspired via the double layers structure of kidney which has a
porous cystic cavity structure facilitating the transport of H2O and
electrolyte and previous research work, we designed a hybrid metal-
organic framework MoOx@UiO-66@MoOx@UiO-bpy with bilayer
structure all assembled with MoOx species (active sites). In the solvent-
free selective oxidation of cyclopentene with H2O2, MoOx@UiO-
66@MoOx@UiO-bpy exhibits three superior functions: 1) improving the
transport of reactants and products due to the increased specific surface
area and mesopores; 2) largely avoiding the leaching of active MoOx
species owing to the confinement of bilayer structure; 3) crucially
trapping intermediates and regulating the desired reaction direction
with 92.5% selectivity to glutaric acid.
2.2. Characterization
Textural properties of the materials were measured via N2 sorption at
77 K on a Micromeritics ASAP 2020 sorptometer. Powder X-ray
diffraction (PXRD) was conducted on a Rint 2000 vertical goniometer
with Cu Kα radiation. Fourier-transform infrared spectroscopy (FT-IR)
was performed on a Nicoet 460 from 4000-400 cmꢀ 1. Thermogravi-
metric measurements (TG) were carried out on a STA 449C. UV-Vis
diffuse reflectance spectra (UV-Vis DRS) were recorded on CARY 500
UV-VIS NIR spectrophotometer. Surface acidity was monitored from FT-
IR spectra collected after pyridine adsorption.
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Molecular Catalysis 513 (2021) 111818
Fig. 2. FT-IR (a) and UV-Vis DRS spectra (b) of related materials.
Fig. 3. TG curves (a) and Py-FT-IR spectra (b) of related materials.
nt(cyclopentene)/V
Fig. 4. Cyclopentene oxidation catalyzed over different composites under optimized reaction conditions (a,Ct/C0
=
) and hot filtration test (b).
n0(cyclopentene)/V
Table 1
Table 2
Kinetic data k1~5 of cyclopentene oxidation with the two catalysts.
Kinetic data of cyclopentene oxidation with MoOx@UiO-66@MoOx@UiO-bpy.
Samples
k (L⋅molꢀ 1⋅sꢀ 1
)
A (L⋅molꢀ 1⋅sꢀ 1
)
E (kJ⋅molꢀ 1
)
k1
k2
k3
k4
k5
A1
A2
A3
A4
A5
2.48 × 108
9.66 × 1014
1.34 × 107
2.30 × 1016
2.01 × 107
E1
E2
E3
E4
E5
49.07
97.99
39.42
105.55
40.06
MoOx@UiO-66
MoOx@UiO-
6.8193
0.5811
3.4952
4.6094
1.7350
7.8111
10.9500
25.2519
18.0305
19.8143
66@MoOx@UiO-bpy
2.3. Selective oxidation of cyclopentene
66@MoOx@UiO-bpy (0.1 g). The mixture was stirred and the temper-
ature was raised to 45~50 ◦C, then, H2O2 (35 wt%, 4.08 g, 42 mmol)
was slowly added to the system. Samples were periodically extracted
and analyzed via GC. After 3 h reaction, MoOx@UiO-66@MoOx@UiO-
bpy was filtered hot and the reaction was permitted to continue without
catalyst.
Cyclopentene selective oxidation was performed in a three-necked,
round-bottomed flask provided with a condenser, a constant pressure
dropping funnel, and a thermometer. Typically, cyclopentene (0.6813 g,
10 mmol) was added to the flask containing MoOx@UiO-
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Molecular Catalysis 513 (2021) 111818
Fig. 5. Reusability test (a), XRD (b), and FT-IR (c) patterns of recovered MoOx@UiO-66@MoOx@UiO-bpy.
Scheme 1. Bilayer structure of MoOx@UiO-66@MoOx@UiO-bpy in nature and by design. Left: multilayer structure of glomerulus, which own large surface area to
enhance the exchange of H2O and electrolyte. Right: bilayer structure of UiO-66@UiO-bpy assembly with MoOx species.
Scheme 2. Synthetic strategy of UiO-66@UiO-bpy assembly with MoOx species.
3. Results and discussion
Scheme
closely matched the reported patterns [48]. After the loading of MoOx
species, the peak positions of MoOx@UiO-66 were in line with UiO-66,
suggesting UiO-66 frame structure was well-preserved. The same was
true for MoOx@UiO-66@UiO-bpy and MoOx@UiO-66@MoOx@-
UiO-bpy. Nevertheless, the peak intensities of UiO-66 were reduced,
perhaps due to the slight change in framework structural regularity
when the MoOx species were deposited [49]. In comparison with
MoOx@UiO-66@UiO-bpy, the peak intensities of MoOx@-
UiO-66@MoOx@UiO-bpy showed lower intensities of crystallinity,
suggesting that MoOx@UiO-66@UiO-bpy peak intensities decreased
after the introduction of MoOx species. In addition, no characteristic
peaks of MoOx were detected in the diffraction patterns of
1
illustrates the bilayer structure of MoOx@UiO-
66@MoOx@UiO-bpy in nature and by design. Meanwhile, Scheme 2
depicts the strategy for synthesizing bilayer UiO-66@UiO-bpy composed
of MoOx species.
3.1. Catalyst characterization
Powder X-ray diffraction (XRD) studies (Fig. 1a) revealed the crystal
structure of MoOx@UiO-66@MoOx@UiO-bpy and related precursor
materials. Patterns of both UiO-66 (layer B) and UiO-bpy (layer A)
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Molecular Catalysis 513 (2021) 111818
Scheme 3. Cascade-selective cyclopentene oxidation over MoOx@UiO-66@MoOx@UiO-bpy.
MoOx@UiO-66@UiO-bpy
and
MoOx@UiO-66@MoOx@UiO-bpy,
growth of UiO-bpy layer. The growth of UiO-bpy could improve
MoOx@UiO-66 porosity resulting in increased surface area. Comparison
of the pore distribution curves (Fig. 1b, inset) revealed that MoOx@UiO-
66@MoOx@UiO-bpy possessed an extended porosity in mesopores re-
gion, distinctly indicating that the epitaxial growth of UiO-bpy layer
enhanced the mesoporosity. These results evidenced that UiO-66@UiO-
bpy owned multimodal porosities and larger surface area, which were
significant for mass transfer of reactants and products.
implying that MoOx species were highly dispersed or/and the loading of
MoOx species content was too low to generate a signal. Generally, each
XRD peak of MoOx@UiO-66@MoOx@UiO-bpy could be attributed to
either UiO-66 or UiO-bpy, while some were a result of peak overlap. The
well-defined peaks suggested the decent crystallinity of MoOx@-
UiO-66@MoOx@UiO-bpy, UiO-66, as well as UiO-bpy.
N2 sorption data were recorded at 77 K. The BET isotherms exhibited
the porosity of these composites, as a type I sorption isotherms were
observed (Fig. 1b). The surface area of UiO-66, MoOx@UiO-66,
MoOx@UiO-66@UiO-bpy, and MoOx@UiO-66@MoOx@UiO-bpy were
892, 619, 1121, 935 m2⋅gꢀ 1, respectively. The smaller surface area of
MoOx@UiO-66 and MoOx@UiO-66@MoOx@UiO-bpy over related pre-
cursor materials (UiO-66 and MoOx@UiO-66@UiO-bpy, respectively)
was contributed to the additional introduction of MoOx species.
Nevertheless, the surface area improved greatly after the epitaxial
Fig. 2a depicted the FT-IR spectra of different composites. The
spectra of MoOx@UiO-66 not only had typical infrared bands assigning
to UiO-66, but also contained typical infrared bands attributing to MoOx
species, revealing that MoOx structure maintained intact even when
confined in UiO-66 pores. Furthermore, there were characteristic bands
of MoOx, UiO-66, and UiO-bpy in the vibrational spectra of MoOx@UiO-
66@UiO-bpy and MoOx@UiO-66@MoOx@UiO-bpy. Nevertheless, the
characteristic bands of MoOx intensity became stronger, owing to the
5

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Molecular Catalysis 513 (2021) 111818
Scheme 4. Cyclopentene oxidation over MoOx@UiO-66@MoOx@UiO-bpy with H2O2.
secondary encapsulation of MoOx contents [50]. Besides, the change on
color, from pale yellow to yellow, offered an evidence of the encapsu-
lation (Fig. S1a, Supporting Information). After the multidimensional
confinement of MoOx species, the color of the corresponding catalyst
was obviously deepened. In order to further confirm the existing of
MoOx species in UiO-66@UiO-bpy, the EDS mapping of C, O, Zr, and Mo
elements (Fig. S1b, Supporting Information) that displayed uniform
distribution of MoOx in MoOx@UiO-66 and MoOx@UiO-66@
MoOx@UiO-bpy, and the Energy Dispersive Spectrometer (EDS) analysis
of MoOx@UiO-66 and MoOx@UiO-66@MoOx@UiO-bpy were also
measured (Fig. S1c, Supporting Information). Besides, the Inductively
Coupled Plasma-Optical Emission Spectrometry (ICP-OES) manifested
the Mo contents were 2.49 wt% versus 4.93 wt%, respectively. The solid
state UV-Vis DRS spectra of related composites were depicted in Fig. 2b.
MoOx@UiO-66 exhibited an absorption peak at ca. 295 nm due to the
ligand to metal charge transfer O2ꢀ → Mo6+ [38]. Impressively,
6

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Molecular Catalysis 513 (2021) 111818
cyclopentene conversion) and 92.5% glutaric acid selectivity was
assigned to the more active MoOx content (4.93 wt% versus 2.49 wt%),
the enhanced diffusion of substrates/products obtained by multimodal
porosities containing micro-/mesopores and the larger internal surface
(935 versus 619 m2⋅gꢀ 1), and the platform provided via bilayer structure
to capture the intermediates and convert them to the target product.
Compared with high glutaric acid selectivity (92.5%), the relative low
cyclopentene conversion (24.7%) was assigned to the mass transfer
resistance existed between the oil and H2O phases.
To quantitatively compare the catalytic activity of MoOx@UiO-66
and MoOx@UiO-66@MoOx@UiO-bpy, the reaction rate constants (k1~5
,
Scheme 5. Circulation of cyclopentene distillate.
Table 1, Scheme 3) were calculated via adopting the established reaction
models [2].
MoOx@UiO-66@UiO-bpy showed a large red shift of 310 nm compared
to MoOx@UiO-66, and a peak located at ca. 375 nm attributed to
UiO-bpy was observed [51] indicating the successful epitaxial growth of
UiO-bpy on UiO-66. An intense absorption peak (i.e., 310 nm) for
MoOx@UiO-66@MoOx@UiO-bpy was assigned to the secondary MoOx
species confinement, corresponding to the color change from pale yel-
low for MoOx@UiO-66 or MoOx@UiO-66@UiO-bpy to yellow for
MoOx@UiO-66@MoOx@UiO-bpy.
Apparent rate constants k1~5 were determined to be 6.8193 versus
18.0305, 0.5811 versus 3.4952, 4.6094 versus 19.8143, 1.7350 versus
7.8111, 10.9500 versus 25.2519 L⋅molꢀ 1⋅sꢀ 1 for MoOx@UiO-66 versus
MoOx@UiO-66@MoOx@UiO-bpy, indicating the reaction was easier to
occur when MoOx@UiO-66@MoOx@UiO-bpy was employed as the
catalyst. The reaction rate constants k1~5 for cyclopentene oxidation
over MoOx@UiO-66@MoOx@UiO-bpy were also investigated at
different temperatures (70, 85, and 90 ◦C, Table S3, Supporting Infor-
mation) to calculate the activation energy.
Thermal stability of different composites were investigated via ther-
mogravimetric analysis (TG, Fig. 3a), and the results both exhibited high
thermal stability (up to 450 ◦C) of MoOx@UiO-66@MoOx@UiO-bpy and
suggested that the successful epitaxial growth of UiO-bpy and encapsulated
MoOx species. The mass loss of MoOx@UiO-66@MoOx@UiO-bpy at
450~550 ◦C was higher than that of MoOx@UiO-66@UiO-bpy and
MoOx@UiO-66 owing to the secondary encapsulation of MoOx species and
the epitaxial growth of UiO-bpy, respectively. Pyridine adsorption measured
via IR spectroscopy was carried out to estimate the types and strength of acid
sites of MoOx@UiO-66@MoOx@UiO-bpy. Fig. 3b exhibited the FT-IR
spectra recorded after pyridine adsorption and subsequent evacuation at
40, 120, and 350 ◦C. The Py-FT-IR spectrum recorded at 40 ◦C on
According to Arrhenius equations ((1)~(2), Where A, E, R, and T are
pre-exponential factor, activation energy, gas constant, and absolute
temperature, respectively), lnk versus 1000⋅Tꢀ 1 plots (Fig. S2, Support-
ing Information) provided the corresponding activation energy values
(Table 2).
k = Aeꢀ (E/RT)
(1)
E
RT
lnk = lnA ꢀ
(2)
The conclusion suggested that the key to develop the technology of A
to F was to overcome the activation energies (E2 and E4) of D hydro-
lyzation, and this was consistent with our previous research. Never-
theless, the lower E2 (97.99 (this work) versus 191.82 kJ⋅molꢀ 1 [2]) and
E4 (105.55 (this work) versus 137.29 kJ⋅molꢀ 1 [2]) for MoOx@-
UiO-66@MoOx@UiO-bpy acted as catalyst might be due to its larger
surface area and multimodal porosities (micro- and mesopores), which
were beneficial to the transfer and diffusion of the molecules.
MoOx@UiO-66@MoOx@UiO-bpy revealed bands at 1444 and 1604 cmꢀ 1
,
assigned to pyridine coordinately bonded to weak surface Lewis (viz., L) acid
sites, which maintained almost entirely after outgassing at 120 ◦C. Besides,
the spectra recorded displayed bands at 1492, 1549, and 1641 cmꢀ 1
because of protonated pyridine bonded to surface Brønsted (i.e., B) acid
sites. The intensity of these bands reduced after outgassing with increasing
temperature, but they were still existed even after outgassing at 350 ◦C,
indicating that L and B acid sites were rather strong, and the relative amount
of L, B, and total acidity was listed (Table S1, Supporting Information).
Based on Py-FT-IR, it can be concluded that both L and B acid sites were
evidenced over MoOx@UiO-66@MoOx@UiO-bpy and that the strong L and
B acid sites were essential to cyclopentene selective oxidation [52].
These characterization results depicted that UiO-66@UiO-bpy was
formed in the existence of MoOx and MoOx had been in situ confined in
the micro- and mesopores of UiO-66@UiO-bpy. That is, the MoOx@UiO-
66@MoOx@UiO-bpy composite where MoOx species uniformly distrib-
uted within the framework of UiO-66@UiO-bpy has been synthesized
and fully characterized.
Since MoOx species were confined in double layers, their leaching
was distinctly suppressed, which was confirmed via hot filtration test in
cyclopentene oxidation reaction. MoOx@UiO-66@MoOx@UiO-bpy was
separated from the reaction system via centrifugation after 3 h and the
resulting filtration was continued reacted at 85 ◦C for another 5 h
(Fig. 4b). No distinct reduction of Ct/C0 was detected after the separa-
tion of catalyst (a slight decrease assigned to the fraction that was
thermodynamically converted even without catalyst). According to the
above results and previous studies [2,53,54], we reasoned that such an
efficient performance was owing to the unique feature of MoOx@-
UiO-66@MoOx@UiO-bpy for mediating the cascade-selective reaction,
and for the first time, the detailed reaction mechanism was proposed
(Scheme 4).
3.2. Catalytic tests
As exhibited in Scheme 4, in layer A, H2O2 molecules might first
attack MoOx species and coordinate with Mo center to generate Mo-
OOH (b), subsequently, Mo-OOH radical (c) reacted with C=C of
cyclopentene to generate the important intermediate product D. On the
one hand, under the action of b and H2O, the ring of D opened to form 2-
hydroperoxycyclohexanol (1), which was rearranged to E. Glutaralde-
hyde radical (2) reacted with H2O2 and E producing glutaraldehyde
peroxyacid (3) which could be further reacted with E to (4). Aldehydic
acid (5) was formed via the rearranged of 4, and 5 continued to be
oxidized into F. On the other hand, D hydrolyzed to give G, which was
transformed to cyclopentanone (6) with the existence of H+. 6 under-
went Baeyer-Villiger oxidation reaction producing valerolactone (7),
which further converted to 5-hydroxypentanoic acid (8) via
MoOx@UiO-66@MoOx@UiO-bpy catalytic performance was evalu-
ated in cyclopentene oxidation employing H2O2 as an oxidant without
any solvent. In order to investigate the optimal reaction conditions, the
related experiments were based on an orthogonal design (L25 matrix,
Table S2, Supporting Information).
Fig. 4a compared catalytic activity under optimized reaction condi-
tions obtained via range analysis over MoOx@UiO-66, MoOx@UiO-
66@MoOx@UiO-bpy, UiO-66@UiO-bpy, and commercial MoO3 as a
benchmark, and the corresponding cyclopentene conversion data was
also listed. As exhibited in Fig. 4a, there was significant difference in the
catalytic performance. The higher catalytic activity of the bilayer
MoOx@UiO-66@MoOx@UiO-bpy with Ct/C0 = 75.3% (viz., 24.7%
7

Q. Niu et al.
Molecular Catalysis 513 (2021) 111818
hydrolyzation. Subsequently, 8 was oxidized to 5, and finally 5 was
further oxidized to F.
Declaration of Competing Interest
Based on the catalytic performance, we presumed that the above-
mentioned reaction process reoccurred in layer B, causing A and the
intermediate products (D, E, and G) further converted into F, thereby
high F selectivity exhibited. Nevertheless, due to the existence of mass
transfer resistance between the oil and H2O phases, unsatisfactory A
conversion showed.
The authors declare no conflict of interest.
Acknowledgment
This work was supported by grants from Natural Science Foundation
of Shandong Province [ZR2020QB025, ZR2020ME104]; National Nat-
ural Science Foundation of China [NSFC21376128]; Natural Science
Foundation of Shandong Province, China. Young Innovative Talents
Introduction & Cultivation Program for Colleges and Universities of
Shandong Province: Innovative Research Team on Optoelectronic
Functional Materials.
3.3. Catalyst reusability
Fig. 5a depicted that MoOx@UiO-66@MoOx@UiO-bpy maintained
~75.71% Ct/C0 (i.e., ~24.3% cyclopentene conversion) and ~91.2%
glutaric acid selectivity in the 10 repeated uses, and when the reaction
completely finished, the metal leaching in the reaction solution were
0.009, 0.005, 0.004, 0.004, 0.003, 0.003, 0.001, 0.0013, 0.001, 0.001
wt% for each of 10 repeated cycles, respectively. Additionally, the metal
leaching (0.475 wt%) of MoOx@UiO-66 was also detected after the first
run. Compared to MoOx@UiO-66, the metal leaching was decreased via
more than one order of magnitude which was a significant improvement
of the catalyst reusability. What’s more important, trace leaching of Mo
was beneficial to glutaric acid purification, and the colorless glutaric
acid crystals was obtained through the process depicted in Scheme S1
(Supporting Information).
Supplementary materials
Supplementary material associated with this article can be found, in
the online version, at doi:10.1016/j.mcat.2021.111818.
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9