Modification of Cu2+ into Zr-based metal–organic framework (MOF) with carboxylic units as an efficient heterogeneous catalyst for aerobic epoxidation of olefins

Article abstract of DOI:10.1016/j.mcat.2018.06.023

A series of metal-organic framework (MOF) based catalysts for aerobic epoxidation were reported. A post-synthetic modification strategy using a solvothermal method enabled the incorporation of three different copper salts on a Zr-based MOF material UiO-66-(COOH)₂; catalysts Cu@UiO-1, Cu@UiO-2 and Cu@UiO-3 were obtained. When analyzed by SEM and EDX, no impact of the post-synthetic modification on the core structure of the UiO-66-(COOH)₂ framework was noticed. Studies showed Cu@UiO-1 to be the optimal catalyst for aerobic epoxidation thanks to its broad substrate scope, thermal stability, excellent recyclability. Observed catalytic properties are also better than those of previously reported copper catalysts.

Full text of DOI:10.1016/j.mcat.2018.06.023

Molecular Catalysis 456 (2018) 57–64
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Modification of Cu2+ into Zr-based metal–organic framework (MOF) with
carboxylic units as an efficient heterogeneous catalyst for aerobic
epoxidation of olefins
a
b
c
d
a,⁎
Jian Zhao , Wenyu Wang , Houliang Tang , Daniele Ramella , Yi Luan
a
School of Materials Science and Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, PR China
The Broad Institute, 415 Main Street, Cambridge, MA, 02142, USA
b
c
Department of Chemistry, Southern Methodist University, 3215 Daniel Avenue, Dallas, 75275, TX, USA
Temple University-Beury Hall, 1901, N. 13th Street, Philadelphia, PA 19122, USA
d
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of metal-organic framework (MOF) based catalysts for aerobic epoxidation were reported. A post-syn-
thetic modification strategy using a solvothermal method enabled the incorporation of three different copper
Metal-organic framework
Post-synthetic modification
Aerobic epoxidation
Catalytic properties
Copper catalyst
salts on a Zr-based MOF material UiO-66-(COOH)
tained. When analyzed by SEM and EDX, no impact of the post-synthetic modification on the core structure of
the UiO-66-(COOH) framework was noticed. Studies showed Cu@UiO-1 to be the optimal catalyst for aerobic
2
; catalysts Cu@UiO-1, Cu@UiO-2 and Cu@UiO-3 were ob-
2
epoxidation thanks to its broad substrate scope, thermal stability, excellent recyclability. Observed catalytic
properties are also better than those of previously reported copper catalysts.
1. Introduction
utilization of expensive metals, such as molybdenum(VI) [9], titanium
(
IV) [10], and manganese (II) [11]. Copper, a classic non-noble tran-
Among all oxidative transformations [1], olefin epoxidation has
sition metal, is a more appealing catalyst source; it is abundant, in-
always been recognized as an important and useful reaction in both
industry and academia due to the broad range of synthetic applications
of epoxides in medicinal chemistry [2], natural product synthesis [3]
and material synthesis [4]. Therefore, many catalytic epoxidation
methods have been designed to achieve this transformation [5]. How-
ever, development of a catalytic epoxidation under mild conditions and
with low cost is still in need from an industrial perspective.
Although many oxidants have been utilized in epoxidation metho-
dology development, using molecular oxygen as an oxidant for the
epoxidation reaction is beneficial because of its low cost, high abun-
dance in nature, and lack of toxicity [6]. Various reported aerobic
epoxidation catalysts are based on homogeneous solutions of redox-
active transition metal [7]. Metal-organic frameworks (MOFs) [8] are
highly tailorable microporous materials; they could be applied to het-
erogeneous catalysis due to their ability to incorporate a broad range of
chemical functionalities.
expensive and non-toxic, it also displays high catalytic efficiency [12].
UiO-66–(COOH) exhibited high selective capture performance of Cu
2
from aqueous solution. [13] Herein, we reported our concise prepara-
tion of post-synthesis-modified metal-organic frameworks (MOFs) and
its application in olefin epoxidation reactions using environmentally
2
+
friendly molecular oxygen (O
2. Experimental section
2.1. Materials
2
) as oxidant.
1,2,4,5-benzenetetracarboxylic acid and zirconium tetrachloride
were purchased from Alfa Aesar. Copper sulfate pentahydrate
(CuSO
4
·5H
2
O), copper nitrate trihydrate (Cu(NO
3 2 2
) ·3H O), copper
dichloride dihydrate (CuCl
2
·2H O) and acetonitrile were purchased
2
from Beijing Chemical Reagent Company. All reagents used in analy-
tical grade were used as received.
Taking advantage of the structural characterizability of MOFs,
several elegant transition metal-based epoxidation catalysts have been
developed using MOFs syntheses in combination with post-synthesis
modification. However, several of the literature reports require
⁎
Corresponding author.
E-mail address: yiluan@ustb.edu.cn (Y. Luan).
https://doi.org/10.1016/j.mcat.2018.06.023
Received 10 April 2018; Received in revised form 10 June 2018; Accepted 27 June 2018
2468-8231/ © 2018 Elsevier B.V. All rights reserved.

J. Zhao et al.
Molecular Catalysis 456 (2018) 57–64
2.2. Synthesis of UiO-66-(COOH)
2
2.5. Catalytic testing
The UiO-66-(COOH)
ported in literature [14].
2
crystals were prepared using a procedure re-
The reactivity of catalysts was examined via aerobic epoxidation of
olefins. 1 mmol of the substrate, 2 mmol of pivalaldehyde, 5 ml of
2
acetonitrile, and the Cu@UiO-66-(COOH) catalyst (The amount of
catalyst being applied to the reaction was calculated based on the wt%
of copper in the MOF catalysts and the equivalence of copper was
2.3. The synthesis of Cu@UiO-1
0.31 mol % for each reaction.) were added into a 25-mL flask.
In a 100 ml round bottom flask equipped with magnetic stirrer, UiO-
6-(COOH) (0.4 g, 0.18 mmol), Cu(NO ·3H O (0.043 g, 0.18 mmol)
Subsequently, the reaction mixture was purged with oxygen gas for 3
times and stirred at 40 °C for 4 h with an oxygen balloon installed. After
the reaction was complete, the heterogeneous catalyst was separated
via filtration. Conversion and selectivity were measured by GC–MS
using nitrobenzene (0.2 mmol) as the internal standard, and by H NMR
spectroscopy [15].
6
2
3
)
2
2
and 50 ml of ethanol were added. The mixture was maintained at 60 °C
for 12 h (Inert gas purge is not needed for this experiment).
Subsequently, the solid was separated by centrifugation and washed by
a 1:1 mixture of water and ethanol for three times and then washed
once by acetonitrile, and finally dried. Catalyst Cu@UiO-1 was ob-
tained as a blue solid. A similar standard procedure was used for
1
2.6. Recyclability experiment
synthesis of Cu@UiO-2 from CuCl
2 2
·2H O (0.03 g, 0.18 mmol) and Cu@
UiO-3 from CuSO ·5H O (0.045 g, 0.18 mmol). The percentage of the
4
2
Under optimal reaction conditions, aerobic epoxidation of cy-
copper species being introduced into the MOFs scaffolds for Cu@UiO-1,
Cu@UiO-2 and Cu@UiO-3 were determined to be 1.87 wt%, 0.59 wt%
and 1.95 wt% via plasma atomic emission spectroscopy (ICP-AES).
clooctene was examined using the recovered Cu@UiO-66-(COOH)
2
catalyst. The catalyst was filtered off, washed with ethanol for three
times, and then dried. Finally, it was reused for the next round of cat-
alytic epoxidation.
2.4. Characterization
2.7. Leaching test
The phase composition of the derived catalysts was characterized
A standard catalytic epoxidation reaction of cyclooctene was con-
using a scanning electron microscope (SEM, ZEISS SUPRA55) (FE-SEM)
at 200 kV. EDX elemental maps were obtained using a VEGA TS 5130
LM (Tescan) SEM equipped with an EDAX Sapphire Si(Li) X-ray mi-
croanalysis detector. Nitrogen adsorption − desorption isotherms were
measured at 77 K with a Micromeritics ASAP 2420 adsorption analyzer.
The specific surface areas of the catalysts were calculated by the BET
ducted using the above-mentioned procedure. After 2 h, the Cu@UiO-
6-(COOH) catalyst was separated from the reaction mixture via cen-
6
2
trifuge and the supernatant was analyzed by GC–MS. Subsequently, the
obtained supernatant was directly stirred for another 3 h and analyzed
by GC–MS again. The comparison of these two GC–MS reaction profiles
provided indication of the copper leaching information from the Cu@
(
Brunauer-Emmett-Teller) method, and the pore-size distributions of
UiO-66-(COOH)
. Results and discussion
.1. Structure and composition of Cu@UiO-66-(COOH)
We first prepared the Zr-based MOF material, UiO-66-(COOH)
2
catalysts.
the catalysts were derived from the adsorption branches of isotherms by
using t-plot method. The structure and phase of the samples were in-
vestigated by X-ray powder diffraction (XRD, M21X) with Cu Kα ra-
diation (40 kV, 150 mA, λ = 1.5406 Å). Copper contents of the samples
were obtained using inductively coupled plasma-atomic emission
spectrometry (ICP-AES). X-ray photoelectron spectroscopy (XPS) data
were obtained using a Shimadzu ESCA-3200. The Fourier transform
infrared spectra (FTIR) were obtained on the potassium bromide (KBr)
pellets using a Nicolet 6700 Fourier spectrometer. UV–vis diffuse re-
flectance spectra (DRS) were recorded on a Shimadzu UV-2550. Barium
3
3
2
2
,
using a solvothermal method which was subjected to a post synthesis
modification using various copper salts to afford the Cu@UiO-66-
(
COOH)
2
catalysts as shown in Scheme 1.
We then conducted scanning electron microscope (SEM) measure-
4
sulphate (BaSO ) was used as a reflectance standard. The catalytic re-
ments to characterize the surface morphology of Cu@UiO-1 catalyst;
octahedral crystals with a size of about 150 nm were observed (Fig. 1a).
After Cu2+ incorporation, no change in the morphology and particle
size of Cu@UiO-1 was observed. In order to analyze the element
action product profile was analyzed using a Gas Chromatography-Mass
Spectrometer (GC–MS, Agilent7890/597 5C-GC/M SD), and ni-
trobenzene was employed as the internal standard.
Scheme 1. Post Synthesis Modification of Zr-based MOF Using Cu2+
.
58

J. Zhao et al.
Molecular Catalysis 456 (2018) 57–64
Fig. 1. (a) SEM Image of Cu@UiO-1, (b–e) EDX Imaging of All the Corresponding Elements in Cu@UiO-1, (f) EDX Spectrum of Cu@UiO-1.
distribution of the Cu@UiO-66-(COOH)
2
catalysts, energy-dispersive X-
in comparison to UiO-66-(COOH)
crystalline structure with UiO-66-(COOH)
conclude that post synthesis modifications using copper ions have no
2
indicating a high resemblance in the
ray spectroscopy (EDX) imaging and spectroscopic measurement were
carried out. The C and O signals outside the sample are due to the
conductive paste used as the substrate. EDX imaging shows that copper
2
(Fig. 2). Therefore, we can
2
influence on the structural backbone of the UiO-66-(COOH) MOF
(
II) is uniformly distributed on the surface of the Cu@UiO-1 catalyst
material.
without any aggregation; it can be concluded that this loading method
successfully provides copper dispersion on the MOF catalyst (Fig. 1e).
Similar SEM and EDX analyses of the Cu@UiO-2 and Cu@UiO-3 cata-
lysts were also conducted and showed very similar results [16].
Consistent with literature-reported diffraction peaks of UiO-66-
To confirm the oxidation states of the copper ion in the Cu@UiO-66-
(COOH) catalysts, X-ray photoelectron spectroscopy (XPS) measure-
ments were conducted. Specifically, the Cu 2p region for all the Cu@
UiO-66-(COOH) catalysts showed two typical peaks at 932.6 eV and
952.4 eV, which could be attributed to Cu 2p3/2 and Cu 2p1/2 electronic
configurations, respectively. Presence of Cu, Cu O or CuO complexes on
2
2
(
COOH)
confirmed by powder XRD [17] (Fig. 2a). The powder XRD patterns of
the Cu@UiO-66-(COOH) catalysts also showed no significant change
2
, the structural integrity of the UiO-66-(COOH)
2
was also
2
the catalyst surface is likely (Fig. 3). We also realized that with more
elemental Cu incorporated in the MOF scaffold (Cu@UiO-3 > Cu@UiO-
2
59

J. Zhao et al.
Molecular Catalysis 456 (2018) 57–64
2
Fig. 2. X-ray diffraction patterns of copper MOFs: (a) UiO-66-(COOH) , (b)
Cu@UiO-1, (c) Cu@UiO-2, and (d) Cu@UiO-3.
Fig. 4. XPS Analysis of the Cu(LMM) Auger Region for (a) Cu@UiO-1, (b) Cu@
UiO-2, (c) Cu@UiO-3.
1
> Cu@UiO-2), high-resolution Cu 2p spectrum peaks shifted barely
toward lower binding energies, which also indicated the modified local
electronic structure of Cu [18]. Accordingly, this study confirmed that
Cu was incorporated during the hydrothermal process and formed Cu
complexes on the MOF supports.
We next measured Cu(LMM) Auger peaks and utilized their shapes
to distinguish between Cu (0) and oxidized Cu (Fig. 4); good agreement
2
with the reported Cu(LMM) data for CuO and Cu O complexes was
observed []. Therefore, the aforementioned data clearly supports that
the copper element we have successfully incorporated on the MOF
support is copper ions.
3.2. Aerobic epoxidation of cyclooctene
By choosing cyclooctene as a substrate [19], the epoxidation cata-
lytic reactivity of all Cu@UiO-66-(COOH)
Fig. 5). Although all three catalysts were found to be able to catalyze
the epoxidation reaction to complete conversion after 6 h, Cu@UiO-1
derived from Cu(NO ·3H O) was found to be the most active catalyst
2
catalysts were studied
(
(
3
)
2
2
Fig. 5. Reaction Progress Kinetic Analysis of the Aerobic Epoxidation Using
to provide complete conversion after only 4 h in acetonitrile at 40 °C.
Subsequently, we optimized our catalytic epoxidation reaction; re-
presentative conditions are summarized in Table 1. The epoxidation
reaction was conducted using cyclooctene as substrate and molecular
Cu@UiO-66-(COOH)2 Catalysts.
Fig. 3. XPS Analysis of Cu 2p Regions for (a) Cu@UiO-1, (b) Cu@UiO-2, (c) Cu@UiO-3.
60

J. Zhao et al.
Molecular Catalysis 456 (2018) 57–64
Table 1
a
2
Aerobic Epoxidation of Cyclooctene by Cu@UiO-66-(COOH) . .
Entry
Catalyst
Solvent
Con.(%)b
Sel.(%)b
1
2
3
4
5
6
7
8
9
–
CH
CH
CH
CH
CH
CH
CH
CH
Ethanol
Acetone
Toluene
THF
3
3
3
3
3
3
3
3
CN
CN
CN
CN
CN
CN
CN
CN
11
> 99
78
96
13
71
32
65
< 5
58
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
Cu@UiO-1
Cu@UiO-2
Cu@UiO-3
UiO-66-(COOH)
CuSO
CuCl
Cu(NO
Cu@UiO-1
Cu@UiO-1
Cu@UiO-1
Cu@UiO-1
2
4
·5H
2
O
O
2
·2H
2
3
)
2
·3H
2
O
1
1
1
0
1
2
10
< 5
Fig. 7. TGA Diagrams of UiO-66, UiO-66-(COOH)2, and Cu@UiO-1.
a
Reaction condition: cyclooctene (1.0 mmol), catalyst (Cu 0.31 mol %), pi-
(1 atm.), 40 °C, 4 h.
Determined by GC–MS using nitrobenzene as the internal standard.
−1
and a total pore volume of 0.44 cm³ g−1. The surface
0
.065 cm³ g
area and pore volume of Cu@UiO-1 is close to the theoretical surface
area of UiO-66-(COOH) on the core–shell structure indicating that the
valaldehyde (2.0 mmol), acetonitrile (5.0 mL), O
2
b
2
method of post synthesis modification using copper made no change to
the microporous property of the MOF material. FTIR (Fig. S4) and UV-
vis analyses (Fig. S5) indicate that there is no significant change in
terms of morphology and internal structure [16,25].
The thermal and structural stability of the Cu@UiO-1 catalyst was
examined using thermal gravimetric analysis (TGA); UiO-66 and UiO-
oxygen [20] as stoichiometric oxidant, by stirring the reaction at 40 °C
for 4 h. Additionally, 2 equivalents of pivalaldehyde were added which
was previously identified as an excellent epoxidation initiating additive
[
21]. A control experiment was conducted using standard reaction
conditions without catalyst; only 11% conversion was observed (Entry
) []. On the other hand, all Cu@UiO-66-(COOH) catalysts were highly
1
2
2
66-(COOH) were used as measurement control (Fig. 7). For UiO-66,
efficient in facilitating the reaction. This observation is in good agree-
ment with our prediction that Cu@UiO-1 would be the optimal catalyst
the TGA diagram showed a two-step weight loss which was attributed
to loss of the guest molecules (before 500 °C) and the other significant
weight loss after 500 °C [26]. The UiO-66-(COOH) and Cu@UiO-1
2
catalysts showed slightly better thermal stability than UiO-66 with a
decomposition temperature of approximately 147 °C and 234 °C in air.
(
Entries 2–4). A series of control experiments were then conducted by
using UiO-66-(COOH) and same amounts of copper salts as epoxida-
2
tion catalysts (Entries 5–8). Although other solvents were studied,
acetonitrile was found to be the optimal solvent (Entries 9–12). Several
other variables were investigated; the full condition screening is re-
ported in the SI [16,22].
1
0% weight loss of the UiO-66-(COOH)
at around 202 °C and 250 °C. The TG curves demonstrate that the
weight loss values of UiO-66, UiO-66-(COOH) , Cu@UiO-1 catalyst are
8.4 wt%, 50.9 wt%, 52.8 wt% at 800 °C, respectively. This study
showed a weight loss difference of 1.9% between UiO-66-(COOH) and
2
and Cu@UiO-1 were observed
To further characterize our best catalyst, the porosity of Cu@UiO-1
2
5
was assessed by N
material UiO-66-(COOH)
type I isotherms for both UiO-66-(COOH)
2
adsorption–desorption isotherms using the MOF
as a measurement control (Fig. 6). Typical
and Cu@UiO-1 were ob-
2
2
Cu@UiO-1 which equaled the amount of copper loading in Cu@UiO-1.
Accordingly, our post-synthesis modification method did not change
the thermal stability of the catalyst and the optimal epoxidation catalyst
Cu@UiO-1 could retain its stability at a temperature of up to 250 °C.
2
served which revealed the microporous property of the material [23].
The surface area of the samples was calculated using the Brunauer–-
Emmett–Teller (BET) method. The microporous volume was calculated
by using the t-plot method, and the total pore volume is obtained from
the volume of nitrogen adsorption at P/P° = 0.99832. The BET surface
3.3. Aerobic olefin epoxidations substrate scope study
2
−1
2
area of UiO-66-(COOH) is 422 m g , the total microporous volume is
−1
−1
0
.083 cm³ g , and the total pore volume is 0.42 cm³ g , which is
After the optimal catalyst (Cu@UiO-1) had been identified, we
studied the substrate scope of this methodology (Table 2). Cyclic olefins
such as cyclooctene, cyclohexene, 2-norbornene and α-pinene showed
consistent with previous literature report [24]. On the other hand, Cu@
2
−1
UiO-1 has a surface area of 326 m g , a total microporous volume of
2 2
Fig. 6. Nitrogen Adsorption-Desorption Isotherm of UiO-66-(COOH) and Cu@UiO-1. (a) UiO-66-(COOH) , (b) Cu@UiO-1.
61

J. Zhao et al.
Molecular Catalysis 456 (2018) 57–64
Table 2
a
Aerobic epoxidation of olefins catalyzed by Cu@UiO-1 .
Entry
1
Substrate
Product
Time(h)
4
Con.b
%)
Sel.b
(%)
(
> 99
> 99
2
4
> 99
> 99
3
4
4
4
> 99
> 99
> 99
> 99
5
5
> 99
> 99
6
7
5
12
65
76
> 99
96
8
24
68
95
9
1
6
5
> 99
> 99
50
80
0c
1
1
2
6
> 99
> 99
> 99
> 99
1
24
1
1
3
4
24
24
> 99
79
> 99
> 99
a
Reaction condition: olefin (1.0 mmol), Cu@UiO-1 (Cu 0.31 mol %), pivalaldehyde (2.0 mmol), acetonitrile (5.0 mL), 1 atm. O
Determined by GC using nitrobenzene as the internal standard.
Reaction was performed at 50 °C.
2
balloon, 40 °C.
b
c
full conversion under standard condition after 4 h (Entries 1–4). Linear
oxide is therefore exposed to the reaction conditions for a shorter
period of time and less decomposition would occur. Accordingly, we
conducted reaction progress kinetic analysis of the styrene epoxidation
reaction and found that the reaction rate increased significantly as the
temperature increased from 40 °C to 60 °C (Fig. 8). When the reaction
temperature was raised to 50 °C, complete conversion of styrene was
obtained after 5 h with 80% selectivity. However, at 60 °C, styrene was
completely consumed after 4 h with 78% of styrene oxide selectivity.
Therefore, we were able to improve the styrene oxide formation se-
lectivity to reach a conversion of 80%, by increasing the reaction
temperature to 50 °C compared to the previous 50% at 40 °C (cf.
Table 2).
To compare our novel MOF catalyst Cu@UiO-1 with previously
reported Cu-based MOF catalysts or homogeneous Cu catalysts, epox-
idations of cyclooctene as comparison experiment were investigated
(Fig. 9). Although the homogeneous copper nitrate catalyst showed a
slightly higher initial reaction rate, the rate dramatically decreases
when 60% of the reagent has been converted. A previously reported
Cu@UiO-67-bpydc catalyst was also synthesized [32]; this catalyst did
not provide complete conversion at 6 h while the Cu@UiO-1 catalyst
showed complete conversion under the standard condition.
olefins gave mixed results; 1-hexene showed high conversion and se-
lectivity under standard condition [16,27] while diminished conversion
was observed for 1-decene (65%) after 6 h (Entries 5–6). Unfortunately,
aliphatic olefins were inert for the epoxidation reaction over time and
the outcomes were almost low volume, only 11% conversion was ob-
tained (entry 7). In the case of 1-dodecene as the substrate resulted in a
6
8% conversion after 24 h (entry 8). The results indicated that Cu@
UiO-1 exhibited limited effects on the epoxidation of aliphatic olefins
28]. For aryl-substituted olefins, although trans-β-methylstyrene was
[
transformed into the corresponding epoxide with high selectivity and
conversion (Entry 11), epoxidation of styrene under identical condi-
tions provided 50% selectivity after 6 h (Entry 9) and 80% selectivity
after 5 h (Entry 10); by-products, including benzaldehyde and phenyl-
propanal, were also observed. This is due to the kinetic instability of
styrene oxides [29]. For 1,2-dihydronaphthalene and stilbenes, since
more steric interaction was introduced, the reaction was slower and
24 h were required to achieve high conversion [30]. High selectivity
was observed for all three substrates (Entries 12–14).
As shown in Table 2, styrene was found to be a more challenging
substrate for aerobic epoxidation with the production of significant
amount of by-products [31]. To minimize by-product formation, we
raised the temperature to increase the reaction rate; the derived styrene
62

J. Zhao et al.
Molecular Catalysis 456 (2018) 57–64
Fig. 10. The Recyclability of the Cu@UiO-1 Catalyst in Epoxidation of
Cyclooctene.
using a magnet. The Cu@UiO-1 catalyst remained active during each
recycle and maintained 99% conversion and 99% selectivity after 8
recycles. The results suggested Cu@UiO-1 catalyst exhibited excellent
recyclability in epoxidation of cyclooctene. XRD and XPS analysis (Fig.
S6) indicate that there is no significant change in terms of morphology
and internal structure after 8 recycles.
3.5. Leaching test for the selective aerobic oxidation of cyclooctene
In order to verify the absence of leaching of Cu during the reaction
time, a standard aerobic epoxidation of cyclooctene was set up and the
catalyst was separated from the reaction mixture after 1 h via cen-
trifugation; 52% conversion was observed (Fig. 11). Subsequently, the
supernatant was stirred at 40 °C for an additional 4 h; no further con-
version was observed after removal of the catalyst (Fig. 11). In addition,
the inductively coupled plasma-atomic emission spectrometry (ICP-
AES) analysis of the supernatant mixture indicated a copper element
level of 0.04 ppm, indicating that no significant copper leaching hap-
pened during the reaction.
Fig. 8. Condition Optimization of Epoxidation of Styrene: (a) Reaction
Conversion, (b) Reaction Selectivity.
Fig. 9. The Conversion of the Aerobic Epoxidation of Cyclooctene.
3.4. Recyclability of Cu@UiO-1
To evaluate the recyclability of the Cu@UiO-1 catalyst, the epox-
idation of cyclooctene was performed using batches of recovered cat-
alyst under previously mentioned conditions (Fig. 10). After completion
of each reaction, the catalysts were separated from the reaction mixture
Fig. 11. Leaching Test for Epoxidation of Cyclooctene Over the Cu@UiO-1
Catalyst.
63

J. Zhao et al.
Molecular Catalysis 456 (2018) 57–64
4. Conclusions
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In summary, we have successfully prepared three novel Cu@UiO-
6-(COOH) catalysts using a concise solvothermal method and various
2
(
6
1
analytical methods were introduced to fully characterize the derived
catalysts. The epoxidation was optimized for several parameters and
catalyst Cu@UiO-1 demonstrated excellent catalytic reactivity; a broad
substrate scope was also obtained. Furthermore, outstanding thermal
stability and high recyclability proved Cu@UiO-1 to be an active and
reliable catalyst for olefin aerobic epoxidation reactions.
[
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This work is supported by Beijing Natural Science Foundation
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