Copper-doped sulfonic acid-functionalized MIL-101(Cr) metal–organic framework for efficient aerobic oxidation reactions

Article abstract of DOI:10.1002/aoc.5445

A series of Cr-based metal–organic framework MIL-101-SO₃H bearing sulfonic acid functional groups were utilized for the immobilization of catalytically active copper species via a post-synthetic metalation method. The novel materials were fully characterized by scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), the Brunauer–Emmett–Teller method, and thermogravimetric analysis. XPS and the EDX element map both suggested that Cu²⁺ is coordinately bonded to the MIL-101-SO₃H, which forms the MIL-101-SO₃@Cu structure. The obtained copper-doped MIL-101-SO₃@Cu-1, MIL-101-SO₃@Cu-2, and MIL-101-SO₃@Cu-3 catalysts were utilized in the selective oxidation of alcohols and epoxidation of olefins using molecular oxygen as an oxidant. Catalytic aerobic oxidation optimization showed that MIL-101-SO₃@Cu-1 is the optimal catalyst and it can be reused ten times without compromising the yield and selectivity.

Full text of DOI:10.1002/aoc.5445

Received: 31 August 2019
DOI: 10.1002/aoc.5445
Revised: 9 November 2019
Accepted: 8 December 2019
F U L L P A P E R
Copper-doped sulfonic acid-functionalized MIL-101(Cr)
metal–organic framework for efficient aerobic oxidation
reactions
Xiujuan Li1 | Zihao Zhou1 | Yuzhen Zhao2 | Daniele Ramella3 | Yi Luan1,2
1School of Materials Science and
A series of Cr-based metal–organic framework MIL-101-SO3H bearing sulfonic
Engineering, University of Science and
Technology Beijing, 30 Xueyuan Road,
acid functional groups were utilized for the immobilization of catalytically
Haidian District, Beijing, 100083, China
2Key Laboratory of Organic Polymer
active copper species via a post-synthetic metalation method. The novel
materials were fully characterized by scanning electron microscopy, X-ray
Photoelectric Materials, School of Science,
Xijing University, Xi'an, Shaanxi Province,
710123, China
3Department of Chemistry, Temple
University-Beury Hall, 1901, N. 13th
Street Philadelphia, PA, 19122, USA
diffraction, energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron
spectroscopy (XPS), the Brunauer–Emmett–Teller method, and thermogravi-
metric analysis. XPS and the EDX element map both suggested that Cu2+ is
coordinately bonded to the MIL-101-SO3H, which forms the MIL-101-SO3@Cu
structure. The obtained copper-doped MIL-101-SO3@Cu-1, MIL-101-SO3@Cu-
2, and MIL-101-SO3@Cu-3 catalysts were utilized in the selective oxidation of
alcohols and epoxidation of olefins using molecular oxygen as an oxidant.
Catalytic aerobic oxidation optimization showed that MIL-101-SO3@Cu-1 is
the optimal catalyst and it can be reused ten times without compromising the
yield and selectivity.
Correspondence
Yi Luan, School of Materials Science and
Engineering, University of Science and
Technology Beijing, 30 Xueyuan Road,
Haidian District, Beijing 100083, China.
Email: yiluan@ustb.edu.cn
Funding information
Natural Science Basic Research Plan in
Shaanxi Province of China, Grant/Award
Numbers: No. 2017JM5134, No.
2018JM5047, No. 2018JQ5028,
K E Y W O R D S
metal–organic framework, heterogeneous catalyst, copper (II) catalyst, epoxidation reaction,
selective alcohol oxidation
2018JM5047, 2017JM5134, 2018JQ5028;
Natural Science Foundation of Beijing
Municipality, Grant/Award Numbers:
2172037, L182020; Beijing Natural Science
Foundation, Grant/Award Numbers:
L182020, 2172037
1 | INTRODUCTION
address these issues. General catalytic oxidation pro-
cesses require the use of large amounts of inorganic
Aerobic oxidation reactions are an important class of
reactions carried out in industry and widely used in the
production of fine chemicals,[1,2] pharmaceuticals,[3]
and food additives.[4] Many homogenous catalyst sys-
tems have been developed, but most of those methods
suffer from problems, including difficulty in recovering
the catalyst, and generation of harmful by-products.[5]
As a result, it is important to develop a fast and effi-
cient heterogeneous oxidation catalytic system to
oxidants, such as chromium compounds and permanga-
nates, which are toxic and expensive.[6] Serious environ-
mental problems have led to the development of
molecular oxygen as the oxidant source in order to
minimize chemical waste.[7,8] The use of heterogeneous
catalysis and molecular oxygen in oxidation reactions
provides a green alternative to traditional toxic chemi-
cal oxidants.[9] Extensive research has been carried out
to explore efficient catalytic systems for aerobic
Appl Organometal Chem. 2020;e5445.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5445

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LI ET AL.
oxidation reactions. Transition metals, including Pd,[10]
Ru,[11] Mn,[12] Co,[13] Zn,[14] Fe,[15] and Cu,[16] have
been developed to promote aerobic oxidation reac-
tions.[17] Copper is abundant, inexpensive, environmen-
tally friendly, and relatively high catalytic efficiency.
Several copper catalysts have been developed for aero-
bic oxidation reactions such as Cu(OTf)/TEMPO,[18]
Cu/TEMPO,[19] and Cu/DBED.[20] Copper p-toluene sul-
fonates are homogeneous complexes commonly used as
Lewis acid catalysts in organic synthesis; they show low
toxicity, moisture tolerance, and reusability.[21] A combi-
nation of copper ions and p-toluenesulfonyls displayed
good aerobic oxidation activity during our preliminary
study.[22] However, these homogeneous copper catal-
ysts are difficult to separate and recover from the reaction
system. Heterogeneous copper catalysts have been used
to overcome these issues and some heterogeneous
copper catalysts have been reported, such as MCM-
41-bpy-CuI,[23] CuTSPc@3D-(N)GFs,[24] PS/PANI@Cu
(OSO2CF3)2,[25] Cu/MOF,[26] PVA-stabilized copper oxide
nanoparticles,[27] copper-based coordination polymers,[28]
copper nanoparticles,[29] and Cu/graphene.[30] However,
some catalytic systems still suffer from instability, the
need for harsh reaction conditions, or low utilization of
catalytically active metal.
heterogeneous supports thanks to their large surface area
and highly porous structure.[44] Inspired by the unique
structure of sulfonic acid MOFs and the synthesis of cop-
per p-toluenesulfonate, we focused on introducing copper
ions to the sulfonic acid group of the MOF, as shown in
Scheme 1.
In this work, we report a copper-immobilized MOF
catalyst that enables the development of efficient het-
erogeneous catalysis. A Cr-based metal–organic frame-
work with sulfonic acid functional groups was
synthesized through a simple solvothermal method,
incorporating copper ions to form MIL-101-SO3@Cu.
The synthesized MIL-101-SO3@Cu catalyst served as
an efficient catalyst for aerobic oxidation using molecu-
lar oxygen (O2) as an oxidant. Great yields and selec-
tivities were obtained utilizing one of the as-
synthesized heterogenous copper catalysts, MIL-
101-SO3@Cu-1. Furthermore, the turnover number was
up to 99 under the optimized reaction conditions. The
sulfonic group of MIL-101-SO3H bonded to Cu2+
,
which stabilizes the catalytically active site. This sul-
fonic acid stabilizing strategy provided a novel copper
catalytic system with high oxidation efficiency and
high catalytic recyclability.
Metal–organic frameworks (MOFs) have attracted
considerable interest as highly efficient heterogeneous
catalysts due to their unique crystalline porosity, ability
to be tailored to need, and ultra-high specific surface
area.[31] As coordination polymers built up from various
organic linkers and inorganic connectors,[32] they are an
attractive prospect in heterogeneous catalysis.[33,34]
Several MOF catalysts have been utilized in aerobic
oxidation reactions,[35] including copper-based,[36] nickel-
based,[37] iron-based,[38] zinc-based,[39] chromium-
based,[40] cobalt-based,[41] and zirconium-based[42,43]
catalysts. Sulfonic acid-based MOFs can be utilized as
2 | EXPERIMENTAL
2.1 | Materials
2-Sulfoterephthalic acid and chromium trioxide were pur-
chased from Alfa Aesar. Copper sulfate pentahydrate
(CuSO4Á5H2O), copper nitrate trihydrate (Cu(NO3)2Á
3H2O), copper dichloride dihydrate (CuCl2Á2H2O), and
acetonitrile were purchased from the Beijing Chemical
Reagent Company. All reagents used in analytical grade
were used as received without further purification.
SCHEME 1 The synthesis of copper p-toluene sulfonate and MIL-101-SO3@Cu

COPPER-DOPED SULFONIC ACID-DERIVED MOFS FOR OXIDATION REACTIONS
3 of 13
2.2 | Preparation of MIL-101-SO3H
under nitrogen flow. The metal copper ion loadings of
the catalyst were analyzed using inductively coupled
plasma-atomic emission spectrometry (ICP-AES) on a
Varian 715-ES. The catalytic reaction product profile was
analyzed using a gas chromatography-mass spectrometer
(GC-MS, Agilent7890/597 5C-GC/MSD), and nitroben-
zene was employed as the internal standard.
MIL-101-SO3H was prepared by solvothermal reaction
according to the literature.[45] In a typical procedure,
CrO3 (2.50 g, 25.0 mmol), 2-sulfoterephthalic acid mono-
sodium salt (6.70 g, 25.0 mmol), and HF (16 mmol
40 wt%) were mixed with 25 ml of deionized water in a
50-ml round-bottomed flask and stirred for 15 min at
room temperature. The solution was then transferred
into a Teflon-lined stainless-steel autoclave. The mixture
was allowed to react in an oven at 180ꢀC for 24 hr. The
obtained green powder was treated in a solution of
diluted HCl (0.08 M in methanol and water) and washed
with a methanol/water mixture three times to remove
additional HCl. Finally, the sample of MIL-101-SO3H
was obtained after 6 hr in a vacuum desiccator at 150ꢀC.
2.5 | Catalytic test
2.5.1 | Aerobic oxidation of alcohols
MIL-101-SO3@Cu catalyst (1.0 mol% based on copper
content), NaHCO3 (1.0 mmol), TEMPO (0.5 mmol), and
alcohols (1.0 mmol) were added to solvent (5.0 ml). The
reaction was purged three times with oxygen gas, which
was supplied through a balloon, and stirred at 60ꢀC for
6 hr. The supernatant was analyzed by GC-MS using
nitrobenzene as the internal standard.
2.3 | Preparation of MIL-101-SO3@cu
catalyst
To
a 100-ml round-bottomed flask, MIL-101-SO3H
(0.16 g, 0.18 mmol), Cu (NO3)2Á3H2O (0.043 g, 0.18 mmol),
and 50 ml of ethanol were added. The mixture was stirred
at 60ꢀC for 12 hr. Subsequently, the solid was separated
by centrifugation and washed by a 1:1 mixture of water
and ethanol for three times. Then it was washed once by
acetonitrile and dried. Finally, the catalyst MIL-
101-SO3@Cu-1 was obtained as solid powder. Similar
standard procedures were used for the synthesis of MIL-
101-SO3@Cu-2 from CuCl2Á2H2O (0.03 g, 0.18 mmol) and
MIL-101-SO3@Cu-3 from CuSO4Á5H2O (0.045 g,
0.18 mmol).
2.5.2 | Epoxidation of olefins
Generally, the catalytic reaction was carried out under
the following conditions. MIL-101-SO3@Cu catalyst
(1.0 mol% based on copper content), the substrate
(1.0 mmol), and pivalaldehyde (1.2 mmol) were mixed in
acetonitrile (5.0 ml). The reaction mixture was purged
with oxygen gas that was supplied through a balloon and
stirred at 40ꢀC for 4 hr. After each catalytic cycle, the
solution was centrifuged and the filtered liquid solution
was analyzed via GC-MS using nitrobenzene as the inter-
nal standard.
2.4 | Material characterization
2.6 | Recyclability experiment
Scanning electron microscopy (SEM) images of samples
were obtained with a ZEISS SUPRA 55. Energy dispersive
X-ray spectroscopy Energy Dispersive X-Ray (EDX) ele-
mental maps were obtained using a VEGA TS 5130 LM
(Tescan). Transmission electron microscopy (TEM)
images were revealed by a JEOL JSM-6700. The Lang-
muir surface area (Brunauer–Emmett–Teller [BET]
method), pore volume, and pore diameter were measured
by an AUTOSORB-1C analyzer. The phase composition
of the products was characterized by X-ray powder dif-
fraction (XRD, Cu Kα radiation, λ = 0.1542 nm) via a
M21X diffractometer. Fourier transform infrared spectra
were collected by a Nicolet 6700 spectrometer. X-ray pho-
toelectron spectroscopy (XPS) data were obtained using a
Shimadzu ESCA-3200. Thermogravimetric analysis
(TGA) was performed using an STA 449F3 instrument
Under optimal reaction conditions, the selective epoxida-
tion of cyclooctene was examined using the recovered
MIL-101-SO3@Cu-1 catalyst. The catalyst was filtered off,
washed with ethanol for three times, and then dried.
Finally, it was reused for the next round of catalytic
oxidation.
2.7 | Leaching test
The standard catalytic epoxidation of cyclooctene was
carried out using the above method. After 1 hr, the MIL-
101-SO3@Cu-1 was separated from the reaction mixture
by centrifugation and the supernatant liquid was ana-
lyzed by GC-MS. The obtained supernatant was then

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LI ET AL.
directly stirred for an additional 3 hr and analyzed by
GC-MS. A comparison of the results of these two GC-MS
reactions provides evidence for the leaching of copper
from MIL-101-SO3@Cu-1 catalyst.
optimization. The distance between Cu and O was mea-
sured to be 1.70 Å and the angel of the O-Cu-O bonds
were measured to be 140.1o.
The fractions of the copper species in the MOFs for
MIL-101-SO3@Cu-1, -2, and -3 were determined to be
2.04, 1.59, and 2.58 wt% measured by ICP-AES. MIL-
101-SO3@Cu-1 was the optimal heterogeneous copper
catalyst according to the benzyl alcohol oxidation evalua-
tions in Table 1. As a result, MIL-101-SO3@Cu-1 was
chosen for further material characterizations.
3 | RESULTS AND DISCUSSION
3.1 | Characterization of the MIL-101-
SO3@cu catalyst
The crystalline structures of MIL-101-SO3H and MIL-
101-SO3@Cu-1 were evidenced by SEM and are shown in
Figure 2. MIL-101-SO3H crystals appear to form a typical
cage structure in Figure 2a and the structural morphol-
ogy was maintained after the immobilization of catalyti-
cally active Cu2+ , as shown in Figure 2b. The crystals of
MIL-101-SO3H are evenly distributed with a size of about
200 nm.
As shown in Scheme 2, the Cr-based MOF with sulfonic
acid functional groups (MIL-101-SO3H) was synthesized
using the solvothermal method according to the litera-
ture report,[46] followed by post-synthesis modification
with copper salts to obtain MIL-101-SO3@Cu. The varia-
tion of MIL-101-SO3@Cu catalyst was synthesized on
treatment with different copper salts. MIL-101-SO3@Cu-
1, -2, and -3 correspond to the Cu (NO3)2Á3H2O,
CuCl2Á2H2O, and CuSO4Á5H2O salts, respectively.
In order to analyze the elemental distribution of MIL-
101-SO3@Cu-1, we performed EDX imaging and spectral
measurements, as shown in Figure 3. The EDX elemental
The structural model of MIL-101-SO3@Cu was built
under the Materials Studio package. The state of the cop-
per ion is also demonstrated in Figure 1 after molecular
map results verified the successful incorporation of Cu2+
.
In addition, EDX showed that the copper was well dis-
SCHEME 2 Schematic illustration
of MIL-101-SO3@Cu synthesis
FIGURE 1 Illustration of copper
on MIL-101-SO3Cu. The geometry
optimization was performed using the
Materials Studio package

COPPER-DOPED SULFONIC ACID-DERIVED MOFS FOR OXIDATION REACTIONS
5 of 13
TABLE 1 Aerobic oxidation of benzyl alcohol under various
reaction conditionsa
found at 2θ = 2.82, 8.45, and 16.90, which is in agreement
with the literature report (Figure 4a).[47] The PXRD
pattern for MIL-101-SO3@Cu-1 (Figure 4c) indicates that -
MIL-101 (Figure 4a) and MIL-101-SO3H (Figure 4b)
have a high degree of similarity in terms of the main
framework, formed by
a
chromium cluster and
2-sulfoterephthalic acid ligands.
The Fourier transform infrared spectra of MIL-
101-SO3H and MIL-101-SO3@Cu-1 are compared in
Figure 5. The peaks at 621 cm−1 (a) and 624 cm−1 (b) can
be assigned to C-S stretching vibrations.[48] The peak at
1025 cm−1 can be attributed to the S-O stretching vibra-
tion. The peaks at 1180 cm−1 and 1224 cm−1 (a), and
1184 cm−1 and 1225 cm−1 (b) correspond to O=S=O sym-
metric and asymmetric stretching.[49] For MIL-
101-SO3@Cu-1, a new peak appears at 607 cm−1, which
can be attributed to the vibration mode of Cu-O.[50] There
is no peak around 607 cm−1 in the Fourier transform
infrared spectroscopy (FT-IR) pattern of MIL-101-SO3H,
indicating that the Cu2+ ion has bonded with O success-
fully and the strategy of introducing Cu2+ into the sul-
fonic acid group of the MOF is workable.
Con.
(%)b
Sel.
Entry Catalyst
Solvent
(%)b
1
–
Acetonitrile <5%
Acetonitrile <5%
Acetonitrile <10%
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
2
MIL-101
3
MIL-101-SO3H
4
MIL-101-SO3@Cu-1 Acetonitrile >99
MIL-101-SO3@Cu-2 Acetonitrile 83
MIL-101-SO3@Cu-3 Acetonitrile 90
5
6
7
Cu (NO3)2Á3H2O
Cu (NO3)2Á3H2Oc
CuSO4Á5H2O
Acetonitrile 87
Acetonitrile 99
Acetonitrile 43
Acetonitrile 68
8
9
10
11
12
13
14
CuCl2Á2H2O
MIL-101-SO3@Cu-1 Ethanol
MIL-101-SO3@Cu-1 Acetone
MIL-101-SO3@Cu-1 THF
MIL-101-SO3@Cu-1 Toluene
89
74
18
46
XPS measurements were performed for MIL-
101-SO3@Cu-1 to further investigate coordination envi-
ronments. Figure 6a shows the survey XPS data and
indicates that MIL-101-SO3@Cu contains five elements:
Cr, O, C, S, and Cu. The Cr 2p1/2 and Cr 2p3/2 signals
show two peaks at 587.4 and 577.7 eV (Figure 6b),
respectively. Both peaks corresponded to the typical
aReaction conditions: benzyl alcohol (1.0 mmol), catalyst (based on 1.0 mol%
copper, NaHCO3 (1.0 mmol), TEMPO (0.5 mmol), solvent (5.0 ml), O2
(1.0 atm), 60ꢀC, 6 hr.
bDetermined by GC-MS using nitrobenzene as the internal standard.
cReaction conditions: benzyl alcohol (1.0 mmol), catalyst (based on 9.0 mol%
Cu (NO3)2Á3H2O, NaHCO3 (1.0 mmol), TEMPO (0.5 mmol), solvent (5.0 ml),
O2 (1.0 atm), 60ꢀC, 6 hr.
binding energies of Cr3+ [51]
Figure 6c shows the high-
.
resolution Cu 2p spectrum with four peaks. The promi-
nent peaks at 934.1 and 954.7 eV can be ascribed to Cu
2p3/2 and 2p1/2, and the other two are satellite
peaks.[52] The four peaks confirm that the valence state
of the Cu ions is +2[53] and the Cu 2p spectrum corre-
sponds closely to the XPS spectrum of CuO,[54] which
means that the Cu and O might be covalently bonded.
The S 2p spectrum also splits into two peaks at 168.2
and 169.2 eV (Figure 6d), which could be attributed to
S-O and S=O bonds at an intensity ratio of 1:2.[55] The
O 1s spectrum exhibits two oxygen bonding features
persed in and on the surface of MIL-101-SO3H. This
observation fits our expectation that the copper(II) sites
are evenly distributed on the heterogeneous support.
The structural integrity of the MIL-101-SO3H and
MIL-101-SO3@Cu-1 catalysts was proved by powder
X-ray diffraction (PXRD) (Figure 4). The PXRD spectrum
revealed the great crystalline structure of MIL-101-SO3H
and the character diffraction peaks of MIL-101 were
FIGURE 2 SEM images of (a) MIL-
101-SO3H and (b) MIL-101-SO3@Cu-1

6 of 13
LI ET AL.
FIGURE 3 (a) TEM images of
MIL-101-SO3@Cu-1 and (b–f) EDX
elemental maps of Cu, Cr, C, O, and S
FIGURE 4 XRD patterns of (a) MIL-101, (b) MIL-101-SO3H,
FIGURE 5 FT-IR pattern of (a) MIL-101-SO3H and (b) MIL-
and (c) MIL-101-SO3@Cu-1
101-SO3@Cu-1

COPPER-DOPED SULFONIC ACID-DERIVED MOFS FOR OXIDATION REACTIONS
7 of 13
FIGURE 6 XPS spectra of MIL-
101-SO3@Cu-1: (a) survey spectrum,
(b) high-resolution Cr spectrum,
(c) high-resolution Cu spectrum,
(d) high-resolution S spectrum, (e) high-
resolution O spectrum, and (f) high-
resolution C spectrum
(Figure 6e). The peak at 531.7 eV is typical of the
[56]
metal–oxygen bond,
while the peak at 532.3 eV can
be attributed to OH groups from the adsorbed H2O
molecules or SO3H groups. Furthermore, the oxidation
state of the copper was confirmed. The C 1 s spectrum
in Figure 6f can be divided into two peaks at 284.8 and
289.2 eV, which can be attributed to C=C in aromatic
rings and Cr-O-C species.[57]
The thermal stability of the MIL-101-SO3H and MIL-
101-SO3@Cu MOF materials was further examined by
TGA in the temperature range of 100–800ꢀC (Figure 7).
The TGA curve of MIL-101-SO3H shows no significant
changes after incorporation of Cu2+. They all show a
two-step weight loss which can be attributed to loss of
the guest molecules before 300ꢀC and the other signifi-
cant weight loss after 500ꢀC. The TGA results suggest
FIGURE 7 TGA of (a) MIL-101-SO3H and (b) MIL-
101-SO3@Cu-1

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LI ET AL.
that our post-synthesis modification method does not
change the thermal stability of the catalyst and the opti-
mal epoxidation catalyst MIL-101-SO3@Cu can maintain
its stability at a temperature of up to 300ꢀC.
efficient catalytic activity of Cu2+ (Table 1, entries 4–6).
Among the three catalysts of MIL-101-SO3@Cu that were
found to be catalytically active towards benzyl alcohol,
MIL-101-SO3@Cu-1 (derived from Cu (NO3)2Á3H2O) was
found to be the optimal catalyst (Table 1, entry 4). Fur-
thermore, the homogeneous Cu (NO3)2Á3H2O catalyst
showed only modest conversion (Table 1, entry 7) under
the same reaction conditions. When 9.0 mol% of the cata-
lyst was used, 99% yield was achieved due to the large
excess of the required catalyst (Table 1, entry 8). The
other copper salts, CuSO4Á5H2O and CuCl2Á2H2O,
showed very low conversion (Table 1, entries 9 and 10).
In addition, solvent evaluation indicated that acetonitrile
is the most suitable reaction solvent for this reaction
(Table 1, entries 11–14). Oxygen-containing solvents,
such as ethanol (Table 1, entry 11), acetone (Table 1,
entry 12), and THF (Table 1, entry 13), showed com-
promised conversions, probably due to the undesired oxy-
gen coordination. A low-polarity solvent such as toluene,
only gave poor yields of the desired product (Table 1,
entry 14).
The porosity of MIL-101-SO3@Cu-1 was character-
ized by the N2 adsorption–desorption isotherm
(Figure 8). Typical type I isotherms of MIL-101-SO3@Cu-
1 were observed, revealing the microporous properties of
the material. The surface area of the sample was calcu-
lated using the BET method. MIL-101-SO3@Cu-1 had a
BET surface area of 822.438 m2/g, a total microporous
volume of 0.37 cm3/g, and a total pore volume of
0.50 cm3/g. In comparison with MIL-101-SO3H, which
has a BET surface area of 1066 m2/g (Figure S1, ESI†),
MIL-101-SO3@Cu-1 showed a slightly decreased surface
area because of copper loading.
3.2 | Catalytic activity of MIL-101-
SO3@cu for selective aerobic oxidation
The catalytic abilities of the MIL-101-SO3@Cu were opti-
mized by employing aerobic benzyl alcohol oxidation as
the model reaction. Various catalysts and solvents were
evaluated and the results are summarized in Table 1. The
aerobic oxidation of benzyl alcohol was carried out in the
presence of molecular oxygen as stoichiometric oxidant,
together with 0.50 equiv of TEMPO and 1.0 equiv
NaHCO3. A control experiment was performed to study
the background rate in the aerobic oxidation of benzyl
alcohol (Table 1, entry 1). There was almost no conver-
sion in the absence of catalyst. The MIL-101 and MIL-
101-SO3H catalysts only showed a low yield, which sug-
gests the crucial role of copper in the aerobic oxidation
reactions (Table 1, entries 2 and 3). When MIL-
101-SO3@Cu was utilized as the catalyst, the reaction sys-
tem showed a high conversion owing to the highly
After optimizing our catalytic oxidation reaction, we
studied the substrate scope of this methodology (Table 2).
Under optimal reaction conditions, the conversion of
benzyl alcohol was achieved to be 99%, together with 99%
selectivity of benzaldehyde (Table 2, entry 1). Various
copper catalysts from literature reports are summarized
in Table S1 (ESI†). The synthesized MIL-101-SO3@Cu-1
catalyst showed one of the best catalytic performance in
terms of turnover number and turnover frequency.
Electron-rich benzyl alcohols, such as 4-methylbenzyl
alcohol and 4-methoxybenzyl alcohol, were converted to
the corresponding aldehyde in a yield of 99% (Table 2,
entries 2 and 3). In addition, the 4-fluorobenzyl alcohol,
which has an electron-withdrawing group at the para
position, showed a slightly compromised conversion
(Table 2, entry 4) due to the lower electron density on the
aromatic ring. The aerobic oxidation of heteroaromatic
alcohols was also successful. 2-Pyridinemethanol gave an
excellent yield of 2-pyridinealdehyde (Table 2, entry 5).
However,
2-thiophenemethanol
provided
the
corresponding thiophene-2-carboxaldehyde with a 69%
conversion (Table 2, entry 6). Cinnamyl alcohol had a
good yield due to the presence of an electron-rich aro-
matic ring (Table 2 entry 7). The allylic alcohol 3-methyl-
2-buten-1-ol showed a decrnt conversion and selectivity
(Table 2, entry 8). The catalytic oxidation of secondary
alcohols such as 1-phenylethanol and cyclohexanol
(Table 2, entries 9 and 10) was also investigated. Only
poor to modest yields were achieved due to the steric hin-
drance and electronic effects of the secondary alcohols.
Furthermore, aerobic epoxidation of olefins was also
studied in the presence of MIL-101-SO3@Cu-1 catalyst
FIGURE 8 N2 adsorption/desorption of MIL-101-SO3@Cu-1

COPPER-DOPED SULFONIC ACID-DERIVED MOFS FOR OXIDATION REACTIONS
9 of 13
TABLE 2 Aerobic oxidation of alcohols catalyzed by MIL-101-SO3@Cu-1a
Entry
Substrate
Product
Con.(%)b
Sel.(%)b
1
>99
>99
2
3
4
>99
>99
91
>99
>99
>99
5
6
7
>99
69
>99
92
>99
>99
8
9
48
55
>99
>99
10
12
>99
aReaction conditions: alcohol (1.0 mmol), catalyst (based on 1.0 mol% copper), NaHCO3 (1.0 mmol), TEMPO (0.5 mmol), acetonitrile (5.0 ml), O2 (1.0 atm),
60ꢀC, 6 hr.
bDetermined by GC-MS using nitrobenzene as the internal standard.

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LI ET AL.
TABLE 3 Aerobic epoxidation of olefins catalyzed by MIL-101-SO3@Cu-1a
Entry
Substrate
Product
Time (h)
Con.(%)b
Sel.(%)b
1
4
>99
>99
2
3
4
4
>99
>99
>99
>99
4
5
6
5
73
69
82
>99
>99
85
5
20
7
8
20
20
>99
>99
>99
77
aReaction conditions: olefin (1.0 mmol), MIL-101-SO3@Cu-1 (based on 1.0 mol% copper), pivalaldehyde (1.2 mmol), acetonitrile (5.0 ml), 1.0 atm O2 balloon,
40ꢀC.
bDetermined by GC using nitrobenzene as the internal standard.
(Table 3). The cyclic olefins, such as cyclooctene,
cyclohexene, and 2-norbornene, showed great activity
with 99% conversion and 99% selectivity under standard
reaction conditions (Table 3, entries 1–3). Literature
reported catalysts for the epoxidation of cyclooctene are
summarized in Table S2 (ESI†). Our MIL-101-SO3@Cu-1

COPPER-DOPED SULFONIC ACID-DERIVED MOFS FOR OXIDATION REACTIONS
11 of 13
provided a good yield under mild reaction conditions.
Low activity can also be observed in linear olefins due to
their low electron density (Table 3, entries 4 and 5).
1,2-Dihydronaphthalene was also transformed into its
corresponding epoxide in decent conversion and selectiv-
ity (Table 3, entry 6). For 1,2-disubstituted aromatic ole-
fins, trans-stilbene provided a higher yield than its
corresponding cis-stilbene due to its steric effect (Table 3,
entries 7 and 8).
3.3 | Mechanism of oxidation
The mechanism for MIL-101-SO3@Cu-catalyzed aerobic
oxidation of alcohol under aerobic conditions is described
in Scheme 3. In the initial cycle, the alcohol substrate
combines with a Cu2+ site to form a metal–alkoxide
intermediate in order to generate intermediate b. An
alcohol proton exchange leads to a carbocation interme-
diate c. An oxidative elimination of the proton at alpha
position provides the desired aldehyde product, with the
generation of intermediate d. The regeneration of MOF
catalyst a is assisted by the oxygen atmosphere.
FIGURE 9 The recyclability of MIL-101-SO3@Cu-1 catalyst in
the epoxidation of cyclooctene
ten cycles. The results show that the MIL-101-SO3@Cu-
1 catalytic material exhibits excellent recyclability in
the epoxidation of cyclooctene. After ten cycles, the cat-
alyst was assessed by PXRD and the spectrum was simi-
lar to that of the fresh catalyst (Figure S2, ESI†),
suggesting that the crystalline structure and composi-
tion of the MIL-101-SO3@Cu-1 catalyst were well
maintained. Furthermore, the BET surface area of the
3.4 | Recyclability of the catalyst
In order to evaluate the recyclability of the MIL-
101-SO3@Cu-1 catalytic material, the catalytic material
recovered after the reaction under the foregoing condi-
tions was subjected to epoxidation of cyclooctene again
(Figure 9). After each reaction was completed, the cata-
lytic material was separated from the reaction mixture
by centrifugation. The MIL-101-SO3@Cu-1 catalytic
material remained catalytically active after each cycle
and maintained 99% conversion and 99% selection after
recycled sample was also measured.
A
slightly
decreased surface area of 805.326 m2/g (Figure S3,
ESI†) was found. XPS results indicated that the chemi-
cal state of copper and chromium remains unchanged
after ten cycles (Figure S4, ESI†).
FIGURE 10 The leaching test of MIL-101-SO3@Cu-1 catalyst
in the epoxidation of cyclooctene
SCHEME 3 Proposed reaction mechanism

12 of 13
LI ET AL.
3.5 | Leaching test
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In summary, a novel MIL-101-SO3@Cu catalyst has been
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How to cite this article: Li X, Zhou Z, Zhao Y,
Ramella D, Luan Y. Copper-doped sulfonic acid-
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framework for efficient aerobic oxidation reactions.
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