Sulfonic-functionalized MIL-101 as bifunctional catalyst for cyclohexene oxidation

Article abstract of DOI:10.1016/j.mcat.2019.110746

Metal-organic frameworks (MOFs) are newly emerging and versatile platforms for designing catalysts, and catalytic oxidation of cyclohexene has attracted much academic and industrial attention for the versatile reactivity of the substrate and the great importance of the various oxygenated products. Here we report the bifunctional catalytic properties of a sulfonic-containing MOF, MIL-101-SO₃H, for cyclohexene oxidation. The sulfonic group and the Cr(III) site acts in a complementary or collaborative way. The Cr(III) framework promotes the oxidation to 3-hydroperoxycyclohex-1-ene (perox) and 2-cyclohexen-1-one (1-one) (route A), whereas the sulfonic group in collaboration with the Cr(III) framework promotes the oxidation to diol (route B) and also enhances further conversions in route A: from perox to 1-one, to 2-cyclohexen-1,4-dione (dione) and even to benzoquinone. With the bifunctional MOF, molecular oxygen alone cannot oxidize cyclohexene but participates as oxidant cooperating with tert-butyl hydroperoxide (TBHP) to accelerate the reactions and to alter the product distribution in favor of dione.

Full text of DOI:10.1016/j.mcat.2019.110746

MolecularCatalysis482(2020)110746
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Sulfonic-functionalized MIL-101 as bifunctional catalyst for cyclohexene
oxidation
Weng-Jie Sun, En-Qing Gao*
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200062,
China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Metal-organic frameworks (MOFs) are newly emerging and versatile platforms for designing catalysts, and
catalytic oxidation of cyclohexene has attracted much academic and industrial attention for the versatile re-
activity of the substrate and the great importance of the various oxygenated products. Here we report the
bifunctional catalytic properties of a sulfonic-containing MOF, MIL-101-SO3H, for cyclohexene oxidation. The
sulfonic group and the Cr(III) site acts in a complementary or collaborative way. The Cr(III) framework promotes
the oxidation to 3-hydroperoxycyclohex-1-ene (perox) and 2-cyclohexen-1-one (1-one) (route A), whereas the
sulfonic group in collaboration with the Cr(III) framework promotes the oxidation to diol (route B) and also
enhances further conversions in route A: from perox to 1-one, to 2-cyclohexen-1,4-dione (dione) and even to
benzoquinone. With the bifunctional MOF, molecular oxygen alone cannot oxidize cyclohexene but participates
as oxidant cooperating with tert-butyl hydroperoxide (TBHP) to accelerate the reactions and to alter the product
distribution in favor of dione.
Metal-organic frameworks
Difunctional catalyst
Cyclohexene
Oxidation
Mechanism study
1. Introduction
dependent upon the nature of the catalysts.
Metal-organic frameworks (MOFs), constructed by linking metal
Cyclohexene is a cheap, abundant, and easily obtained industrial
raw material and can be oxidized to different oxygenated chemicals
that are important intermediates for the production of medicines, pes-
ticides, spices, surfactants and polymers [1–4]. so the oxidation of cy-
clohexene has received great attention from both industrial and aca-
demical viewpoints [5,6]. Cyclohexene has two potential sites for
oxidation transformation, usually giving rise to a mixture of products
with different oxygenated functional groups. Oxidation at the allylic
CeH bond can lead to unsaturated compounds such as 2-cyclohexen-1-
one (1-one), 2-cyclohexen-1-ol (1-ol), 3-hydroperoxycyclohex-1-ene
(perox) or 3-(tert-butylperoxy)cyclohex-1-ene (Scheme 1, route A),
whereas oxidation at the C]C bond usually produces cyclohexene ep-
oxide (epox) and 1,2-cyclohexanediol (diol) (Scheme 1, route B) [6].
Many efforts have been made in recent years to explore the selectivity
and mechanism of cyclohexene oxidation over various catalysts, in-
cluding homogeneous or immobilized metal-based species [7–13] and
non-metal ones (such as carbon nanotubes [4] and C3N4 [14,15]). The
most popular oxidants have been molecular oxygen, hydroperoxide and
tert-butyl hydroperoxide (t-BuOOH, TBHP), which are clean or rela-
tively clean [10,16,17]. The choice of the oxidants is strongly
ions or metal clusters with organic ligands and featured by well-defined
crystalline structure, tunable porosity and functionalities, have recently
emerged as versatile and attractive platforms for the design of hetero-
geneous porous catalysts and many other functional materials [18–21].
Some MOFs with different metal centers or topologies, such as MFU-1
[22], MIL-47 [23], MIL-101 [24], MOF-74 [25], and UiO-66-NH2 [17],
have been employed for catalytic oxidation of cyclohexene in the last
decade, with diverse activity and selectivity [10,26,27]. MOF-based di-
or multifunctional catalysts for cyclohexene oxidation has seldom been
explored. Polyoxometalates have been encapsulated inside MIL-101 to
enhance the activity [28,29]. Mo(VI) [27,30], Nb(V) [31], Ti (IV) [32]
oxide sites have been installed onto the metal cluster nodes by post-
synthetic solvothermal deposition in NU-1000 or onto the organic lin-
kers by postsynthetic coordination modification of HKUST-1 and MIL-
47, the Mo(VI) oxide functionalized and showing high selectivity for
epoxidation by hydroperoxide. In this article, we report the catalytic
study of a facile difunctional MOF, MIL-101-SO3H [33], for cyclohexene
oxidation. It is demonstrated that the sulfonic group and the Cr(III)
center acts in a complementary way to promote the oxidation reactions.
In particular, the Cr(III) center promote the transformation to perox and
⁎ Corresponding author at: Shanghai Key Laboratory of Green Chemistry and Chemical Processes, College of Chemistry and Molecular Engineering, East China
Normal University, Shanghai, 200062, China.
E-mail address: eqgao@chem.ecnu.edu.cn (E.-Q. Gao).
https://doi.org/10.1016/j.mcat.2019.110746
Received 7 September 2019; Received in revised form 9 December 2019; Accepted 16 December 2019
2468-8231/©2019PublishedbyElsevierB.V.

W.-J. Sun and E.-Q. Gao
MolecularCatalysis482(2020)110746
0.020 mmol SO3H, 4 mol% with respect to cyclohexene) was dried in
vacuum at 110 °C for 4 h and added under stirring to a 10 mL vessel
containing cyclohexene (0.5 mmol) and TBHP (5 mmol) in n-decane
(5.5 mol/L). The mixture was stirred at 40 °C for 14 h. The catalyst was
filtered and washed with chloroform (10 mL) for three times. The
conversion and yield was determined by gas chromatography (GC) with
the combined liquid phase. The washed catalyst was dried in vacuum at
110 °C for 4 h and used for recycle tests.
2.4. Control test with 1-one
Scheme 1. Possible reaction routes of cyclohexene oxidation.
The test was performed following the above procedure, using 1-one
in the place of cyclohexene.
1-one (route A), and the incorporation of the sulfonic group into the
MOF enhances the conversion to diol (route B) and the further con-
versions in route A: from perox to 1-one and further to 2-cyclohexen-
1,4-dione (dione). The dione product is rarely identified in cyclohexene
oxidation [24], can become the major product in our catalytic system.
We also demonstrate that molecular oxygen alone cannot oxidize cy-
clohexene but serves as co-oxidant in cooperation with TBHP. Tentative
mechanisms for the formation of different products are proposed.
2.5. Control test with epox
TBHP (5 mmol) or H2O (45 μL, 2.5 mmol) was added under stirring
to a 10 mL vessel containing epox (0.1 mmol) and MIL-101-SO3H (8 mg,
4 mol%). The mixture was stirred at 40 °C for 14 h. The diol yield was
determined by GC.
3. Results and discussion
2. Experimental
3.1. Synthesis and characterization of MIL-101-SO3H catalyst
2.1. Physical measurements
MIL-101-SO3H was synthesized according to the procedures re-
ported the literature before [34]. The powder X-ray diffraction (PXRD,
Fig. 1a) is in good agreement with those of MIL-101 (simulated and
experimental), indicating the formation of the MIL-101-type structure.
Compare with MIL-101, MIL-101-SO3H shows three new FTIR bands
assignable to the sulfonic group (Fig. 1b). The asymmetric and sym-
X-ray powder diffraction patterns of the samples were measured
using a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation (λ
=1.5418 Å) at a scanning rate of 10° /min, with accelerating voltage
and current of 35 kV and 25 mA, respectively. Nitrogen adsorption and
desorption isotherm measurements were performed on a Micromeritics
ASAP2020 analyzer at 77 K. Scan electron microscopy (SEM) images
were carried out with a S-4800 HITACHI scanning electron microscope.
FT-IR spectra were recorded in the range 500 − 4000 cm−1 using KBr
pellets on a Nicolet NEXUS 670 spectrophotometer. The temperature-
programmed desorption of ammonia (NH3-TPD) were performed on a
Micromeritics autochem 2920 instrument. For the purpose, 0.050 g
sample was put into a U-shaped quartz tube and treated in argon
(20 mL/min) at 150 °C for 30 min. The TPD curves were obtained at the
10 °C / min rate. The catalytic products were analyzed by gas chro-
matography (Shimadzu: GC-2014) and gas chromatograph-mass spec-
trometer (Agilent Technologies, GC: 7890B, mass: 5977A). Elemental
analysis was performed on an Elementar Vario ELIII analyzer. The
metal amount was detected by inductively coupled plasma atomic
emission spectroscopy (ICP-AES) using an IRIS Intrepid II XSP spec-
trometer.
metric stretching peaks of O]S]O appears at 1226 and 1179 cm−1
,
respectively, and the peak at 1025 cm−1 is ascribable to the SeO
stretching vibration [35–37]. The peaks at 1080 and 770 cm−1 for MIL-
101-SO3H can be assigned to the in-plane and out-of-plane skeletal CeH
vibrations of the benzene ring, which are blue shifted compared to the
corresponding peaks (1018 and 748 cm-1) for MIL-101 [35,38]. The
changes should be due to sulfonation of the ligand. According to ni-
trogen adsorption isotherm at 77 K (Fig. 1c), the Bru-
nauer − Emmett − Teller (BET) specific surface area is 1603 m2/g.
SEM (Fig. 1d) displayed an octahedral morphology with particle sizes of
100−250 nm.
3.2. Catalytic studies for cyclohexene oxidation
To evaluate the catalytic performance of MIL-101-SO3H for cyclo-
hexene oxidation, different reaction conditions were tested over MIL-
101-SO3H and TBHP. Two commercially available TBHP reagents,
70 wt% (water) and 5.5 mol/L (n-decane), were used as the oxidant to
react with cyclohexene over MIL-101-SO3H at 40 °C for 14 h under air
atmosphere. According to the results, the reactions in both solvent
systems produce 1-one, dione and diol, with similar product distribu-
tions (Table 1, entries 1 and 2). TBHP/n-decane leads to somewhat
higher conversion than the aqueous solution. This can be explained by
the blocking of active sites by water molecules. TBHP/n-decane was
used for further investigation. Various additional solvents were tested
to investigate the solvent effects. From Table 1 we can see that except
CH3CN, the additional solvents would greatly decrease the conversion
of cyclohexene. The catalytic reactions without additional solvents gave
the highest conversion. The negative effects of additional solvents could
partially be attributed to the decreased concentrations of TBHP and
cyclohexene. Another reason could be that the formation and stability
of radical intermediates during the catalytic process is affected by the
solvents (especially DMF and acetic acid).
2.2. Catalyst preparation and characterization
MIL-101-SO3H was prepared under the hydrothermal conditions
according to a procedure described [34] with some modification. Sul-
foterephthalic acid (3.35 g, 12.5 mmol), CrO3 (1.25 g, 12.5 mmol) and
concentrated aqueous hydrochloric acid (0.8 mL, 25 mmol) was dis-
solved in water (50 mL) and then transferred to a Teflon-lined stainless
steel autoclave. The solution was heated at 180 °C for six days. The
reaction product was harvested by centrifugation, washed with deio-
nized water (50 mL × 3) and methanol (50 mL × 3), and dried in air.
Elemental
analysis:
Anal.
calcd.
for
[Cr3O(OH)
(H2O)2(C8H4O7S)3]·12H2O: Cr, 13.3; C, 24.5; S: 8.2 %. Found: Cr, 13.2;
C, 24.8; S, 8.2 %. According to the analytic results, the amounts of Cr
and SO3H are both 2.55 mmol/g.
2.3. Typical procedure for cyclohexene oxidation
The effects of the oxidant dosage were investigated. When TBHP
was slightly in excess of cyclohexene, 1-one was the main product
MIL-101-SO3H (8.0 mg, corresponding to 0.020 mmol Cr and
2

W.-J. Sun and E.-Q. Gao
MolecularCatalysis482(2020)110746
Fig. 1. (a) PXRD profiles of MIL-101-SO3H
(fresh) and MIL-101 (observed and simulated).
The profile for used MIL-101-SO3H (after one
catalytic run) is also included. (b) FTIR spectra
of MIL-101 and MIL-101-SO3H. The spectrum
for used MIL-101-SO3H (after one catalytic
run) is also included. (c) Nitrogen adsorption
and desorption isotherms of MIL-101-SO3H,
(d) SEM image of MIL-101-SO3H.
worth noting. This product was rarely noticed in the literature of cy-
clohexene oxidation, except for only a few reports that mentioned it as
a minor product [24,39–41]. According to the stoichiometry of the
reactions [10,42], at least two equivalent TBHP is required to oxidize
cyclohexene to 1-one or diol. Similarly, four equivalents of the oxidant
are required to oxidize cyclohexene to dione. It is surprising that 1.5
equivalent TBHP leads to a 95 % conversion of cyclohexene, with the
yields of 57, 22 and 16 % for 1-one, dione and diol, respectively. Ac-
cording to the stoichiometric proportions, the conversion would need
nearly 2.3 equivalents of TBHP. This implies a utilization efficiency of
TBHP is up to 156 %, which is unrealistic. To explain the phenomenon,
we propose that dioxygen in the air should participate as additional
oxidant. The dioxygen should be responsible for the conversion that at
least corresponds to the actually nonexistent 56 % TBHP.
Table 1
Conversion of cyclohexene and yields of various oxidation products in different
solvent systems.a
Entry
Solvent
Conv (%)
1-one (%)
dione (%)
diol (%)
1b
2
3
–
–
91
95
89
8
64
67
16
56
57
69
8
46
58
5
24
22
18
–
3
7
11
16
2
–
3
acetonitrile
DMF
toluene
ethyl acetate
acetic acid
4
5c
6
2
11
7
–
a
Reaction conditions: cyclohexene (0.5 mmol), TBHP (0.75 mmol, 1.5
equivalent) in n-decane (5.5 mol/L), MIL-101-SO3H (4 mol%), additional sol-
vent (0.5 mL, if added), air atmosphere, 40 °C, 14 h. b 0.75 mmol TBHP in water
c
To further confirm the involvement of dioxygen, control reactions
were performed in different atmospheres (dinitrogen, air and dioxygen)
(Table 2). Under the dinitrogen atmosphere instead of air, 1.5 equiva-
lent TBHP leads to sharp declines in conversion of cyclohexene (58 %)
and yield of route-A products (42 % for 1-one + dione) (Table 2, entry
4). The reaction shows a reasonable efficiency of TBHP (79 %). Under
dioxygen, the conversion is almost complete, the selectivity for diol
being somewhat increased compared with that under air (Table 2, en-
tries 1 and 6). Again, an unrealistic efficiency of TBHP (153 %) was
obtained under dioxygen, indicating the participation of dioxygen.
Excessive TBHP (10 eqv.) leads to decreased TBHP efficiency (∼ 30 %)
but complete conversions under the different atmospheres (entries 3, 5
and 7). The dione yield increases as the atmosphere changes from di-
nitrogen to air and to dioxygen, in line with the decrease in the 1-one
yield. The yield of diol is relatively high under dioxygen. These results
not only confirm that dioxygen is involved in the oxidation reactions
but also suggest that it facilitates the production of dione and diol. We
also performed the reaction under dioxygen in the absence of TBHP
(Table 2, entry 8). No conversion was observed, which indicates that
dioxygen alone cannot oxidize cyclohexene but serves as co-oxidant in
cooperation with TBHP. A small amount of TBHP (0.5 eqv.) in the
presence of pure dioxygen leads to only 1-one (Table 2, entry 9). The
(70 wt %). 12 % epox was also found.
Table 2
Conversion of cyclohexene and yields of various oxidation products with dif-
ferent TBHP dosages and under different atmospheres.a
Entrya TBHP (eqv.) atmosphere Conv (%) 1-one (%) dione (%) diol (%)
1
2
3
4b
5
6
7
8
9
1.5
5
air
air
air
nitrogen
nitrogen
oxygen
oxygen
oxygen
oxygen
95
96
99
58
98
96
99
0
57
41
36
37
42
54
13
–
22
36
43
5
35
19
61
–
16
19
20
12
21
23
25
–
10
1.5
10
1.5
10
0
0.5
31
31
–
–
a
Reaction conditions: cyclohexene (0.5 mmol), TBHP with MIL-101-SO3H
b
(4 mol%) at 40 °C for 14 h. 4 % epox was also found.
(Table 2, entry 1). Increasing the TBHP dosage leads to less 1-one, more
dione and slightly more diol (entries 2 and 3). The results could suggest
that dione be the secondary product arising from further oxidation of 1-
one. The formation of a large amount of dione in the catalytic system is
3

W.-J. Sun and E.-Q. Gao
MolecularCatalysis482(2020)110746
Table 3
Control reactions were performed to further confirm the two routes
(Scheme 2). Under the typical catalytic conditions, 1-one can be oxi-
dized to dione by TBHP under dinitrogen atmosphere (Scheme 2a). The
reaction under dioxygen gave higher conversion but would not proceed
if TBHP were not used. The results proved that, as mentioned above for
oxidation of cyclohexene, dioxygen serves to oxidize 1-one only in
cooperation with TBHP. To verify the epox-to-diol conversion in route
B, epox was allowed to react with TBHP under the typical catalytic
conditions (Scheme 2b). The yield of diol was 76 %. For comparison,
using water instead of TBHP to react with epox, a significantly lower
yield of diol (41 %) was observed. Obviously, the epoxide ring-open
mechanism by TBHP should be different from that by water. The ring-
open reaction is a catalytic process for the reaction does not proceed
without MIL-101-SO3H. Furthermore, no diol (or dione) was detected in
the catalytic reaction starting from 1-one (or epox), confirming that
routes A and B are mutually independent without cross conversion.
When a radical inhibitor, BHT (butylated hydroxytoluene) was
added to the reaction mixture at the beginning, no conversion of cy-
clohexene was observed within 14 h (Scheme 3a), suggesting the in-
volvement of radical species in the oxidation processes. Furthermore,
when BHT was added after the catalytic reaction proceeded for 1 h (19
% conversion to perox, 1-one and epox), no further conversion of cy-
clohexene was detected, and only the radical-independent elimination
reaction from perox to 1-one proceeded to completion (Scheme 3b). It is
worth noting that the addition of BHT suppress the ring-open reaction
of epox, which indicates the epox-to-diol reaction also involves radicals.
This was further confirmed by the results of the control reactions
starting with epox (Schemes 2b and 3 c): the conversion of epox re-
acting with TBHP was dramatically reduced from 76 to 28 % when BHT
was added.
Next, the effective catalytic sites in MIL-101-SO3H were explored.
Under catalyst-free conditions, 68 % cyclohexene converts to perox (56
%) and a small portion of 1-one (12 %) in 14 h (Table 5, entry 1). In the
presence of MIL-101 (entry 2), the conversion is enhanced to 82 %,
where perox remains to be the main product but the yield of 1-one
increases obviously, with a little amount of dione (7 %). No products of
route B appear in above reactions. For comparison, MIL-101-SO3H leads
to 99 % cyclohexene conversion (entry 3). Dione becomes the main
product in the expense of 1-one, and a considerable amount of diol
appears. According to the results, the Cr(III) center should promote
route A but cannot trigger route B, whereas the introduction of the
sulfonic acid group could trigger route B and meanwhile promotes
perox decomposition and the 1-one-to-dione conversion. The effect of
sulfonic acid was checked by using p-toluene sulfonic acid (PTSA) in-
stead of the MOFs (entry 4). It seems that PTSA is an impediment to
route A because it leads to lower conversion of cyclohexene and lower
yields of perox and 1-one compared with the catalyst-free reaction.
However, it indeed leads to diol formation, confirming the positive
effect of sulfonic acid on route B. Using PTSA together with MIL-101
leads to much improved cyclohexene conversion through route A (entry
5). With either PTSA or MIL-101 alone, the major product is perox.
With PTSA + MIL-101, however, perox disappears and 1-one becomes
the major product, suggesting that the framework and the sulfonic acid
act in synergy for the conversion from perox to 1-one. Compared with
PTSA + MIL-101, MIL-101-SO3H leads to higher conversion, more
dione and more diol. All in all, combining the Cr(III) and sulfonic sites
in the MOF can significantly enhance the activity for cyclohexene oxi-
dation. In particular, the bifunctional MOF promotes the oxidation
through route B and also the further reactions in route A (the elim-
ination reaction of perox to 1-one and the deeper oxidation of 1-one to
dione). It could be because the framework provides an appropriate
environment for synergistic action of the different catalytic sites. In-
terestingly, the promotion of route B by the sulfonic group is similar to
the results reported recently for cyclohexene oxidation with H2O2 as
oxidant and with Ti or Zr MOFs as catalyst, where the addition of an
acid leads to significantly increased selectivity for route-B products
Conversion of cyclohexene and yields of various oxidation products with dif-
ferent catalyst amounts and at different temperatures.a.
Entrya MOF
(mol%)
Temperature(°C) Conv (%) 1-one
(%)
dione (%) diol (%)
1
2
3
4
5
2
40
40
40
30
50
98
99
99
99
99
52
13
11
37
–
40
61
60
51
52
6
4
6
4
4
25
28
6
b
c
37
a
Reaction conditions: cyclohexene (0.5 mmol), TBHP (5 mmol) with MIL-
b
c
101-SO3H in oxygen atmosphere for 14 h. 5 % epox was also found. 10 %
benzoquinone was also found.
conversion (31 %) is higher than the value (25 %) expected for 0.5 eqv.
TBHP, once again confirming participation of dioxygen in oxidation.
The conversion is much lower than those for higher TBHP dosages,
indicating that dioxygen cannot oxidize cyclohexene any more once the
small dosage of TBHP is completely consumed up (entries 6 and 7). In
addition, the absence of dione in the products implies that dioxygen
alone cannot oxidize 1-one.
The effects of catalyst dosage and reaction temperature were tested
(Table 3). When the catalyst dosage was increased from 2 to 4 mol%,
the yields of dione and diol were significantly increased at the cost of
less 1-one (entries 1 and 2). Therefore, more catalyst is more beneficial
to route B and secondary oxidation of 1-one. The effects become weaker
upon further increasing the catalyst dosage to 6 mol% (entry 3): a slight
increase in diol yield and a slight decrease in 1-one yield were observed.
When the temperature was reduced from 40 to 30 °C with 4 mol%
catalyst, the yield of diol was significantly reduced, and the conversion
from 1-one to dione was slowed down (entry 4). In addition, a small
amount of epox was observed (5 %), which could be the precursor to
diol. On the other hand, increasing the temperature to 50 °C leads to
significant increase in diol yield, complete disappearance of 1-one
(entry 5), which is presumed to be further oxidized to diol. A certain
amount of benzoquinone was also detected (10 %) at 50 °C, which
should arise from further oxidation of dione. On the whole, the product
distribution is strongly dependent upon temperature. Increasing tem-
perature facilitates route B and further conversion of the products of
both routes.
To gain further insight into the reaction process and intermediates,
the product distribution at different time was investigated (Fig. 2 and
Table 4). The conversion of cyclohexene under dinitrogen reaches 42 %
after 2 h and is almost complete after 10 h. An intermediate product,
identified as perox by GC–MS, was found at the early stage of the re-
action (Fig. 2a). The intermediate disappeared after 6 h through rapid
conversion to 1-one [4,43,44], which increases rapidly in the first 4 h
and then decreases. The decrease of 1-one is accompanied by the in-
crease of dione, which was not detected within the first 2 h. The pre-
cursor to diol, epox, was also detected within 10 h and disappeared
after prolonged reaction time, accompanied by an increase in diol yield.
This suggests the conversion of epox to diol [4,45], The data confirm
that the oxidation of cyclohexene proceeds through two routes: route A,
through perox to 1-one and then to dione (and finally to DDQ at higher
temperature); route B, through epox to diol. The total yield for each
route increases at the early stage and remains unchanged after complete
conversion of cyclohexene, indicating that the two route are in-
dependent without cross transformation (Fig. 2c and d). The kinetic
data for the reaction under dioxygen atmosphere reveal the same in-
termediates and products (Fig. 2b). However, the reaction is sig-
nificantly accelerated by dioxygen, the conversion of cyclohexene
reaching 45 % after 1 h and almost completed after 6 h. Close inspec-
tion suggests that the effects of dioxygen are more significant on route A
than on route B. Particularly, the production of perox from cyclohexene
and of dione from 1-one is significantly enhanced.
4

W.-J. Sun and E.-Q. Gao
MolecularCatalysis482(2020)110746
Fig. 2. Time-dependent conversion and product distribution for cyclohexene oxidation in N2 (a, c) and O2 (b, d) atmosphere (other conditions: 0.5 mmol cyclo-
hexene, 5 mmol TBHP, 8 mg MIL-101-SO3H (4 mol%), 40 °C, 14 h). (a) and (b) show the yields for individual products, and (c) and (d) show the total yields for the
two different routes.
Table 4
Conversion of cyclohexene and yields of various oxidation products in N2 and
O2 atmosphere.a.
Atmosphere Time (h) Conv (%) 1-one (%) dione (%) epox (%) diol (%)
dinitrogen
dioxygen
2
4
6
10
14
42
81
89
97
98
24 (7b)
57 (3b)
56
46
42
–
4
13
30
35
10
14
14
9
–
3
6
12
21
–
b
1
2
4
6
45
58
88
95
98
99
61
15(24
)
–
–
6
–
–
6
13
15
25
–
36 (11b)
54 (4b)
45
11
13
9
9
–
11
28
41
61
–
10
33
14
13
22(32
1 + 5c
)
6
b
a
Reaction condition: cyclohexene (0.5 mmol), TBHP (5 mmol) with MIL-
b
101-SO3H (8 mg, 4 mol%) at 40 °C for 14 h. 3-hydroperoxycyclohex-1-ene
(perox) yield. The catalyst was removed after 1 h, and the filtrate was ana-
c
lyzed after standing under the same conditions for 5 h.
Scheme 2. Control tests with the intermediates. (a) 1-one. (b) epox. Conditions:
0.5 mmol 1-one (or 0.1 mmol epox), 5 mmol TBHP (or 2.5 mmol H2O, if used),
8 mg MIL-101-SO3H (4 mol%, unless otherwise specified), 40 °C, 14 h.
[46,47].
The heterogeneous nature of the catalysis was examined by “hot”
filtration tests. The solid catalyst was removed by filtration after the
catalytic reaction proceeded for 1 h (conversion 45 %), and the filtrate
was stirred under the same conditions as those before filtration. As can
be seen Fig. 3, the conversion in the filtrate stirred for 5 h is only 60 %,
which is much lower than the value (95 %, 6 h) without filtration. The
small increase of conversion after filtration is consistent with the fact
that the reaction can proceed slowly under catalyst-free conditions
(Table 5, entry 1). The reaction after catalyst filtration only led to in-
creased yields for perox and 1-one. No further conversion of cyclo-
hexene to epox occurred, and no secondary products (dione and diol)
were observed (Table 4, the last entry). These are also consistent with
the data obtained for the catalyst-free reaction and indicate that the
catalyst is required for route B and the 1-one-to-dione conversion in
route A. The results of the filtration tests support that the catalytic
process is heterogenous, with the active sites in the solid.
On the basis of our experimental results and previous reports, the
oxidation processes of cyclohexene over MIL-101-SO3H can be tenta-
tively proposed in Scheme 4. First, different radical species are gener-
ated from TBHP [4,48], which is promoted by Cr(III) centers (eqn 1–4).
Dioxygen participates in the oxidation by reacting with tert-butyl hy-
droperoxide to generate tert-butyl peroxyl and hydroperoxyl radicals
(eqn 4). The fact that dioxygen alone cannot oxidize cyclohexene sug-
gests that it cannot generate oxidative radicals by itself. In route A,
5

W.-J. Sun and E.-Q. Gao
MolecularCatalysis482(2020)110746
radicals. However, as discussed above, the epox to diol conversion in
the MIL-101-SO3H/TBHP system is even more efficient than that in
MIL-101-SO3H/water, and the process should involve radicals. A ten-
tative radical mechanism can be speculated as follows (Scheme 4).
Attack of epox by the hydroxyl radicals leads to ring-opening, and then
the intermediate radical (IV) undergoes hydrogen exchange with TBHP
to give diol. Nevertheless, the contribution from the hydrolyzation
mechanism cannot be precluded because water is an inevitable copro-
duct in the reaction system. It is possible that the two mechanisms
coexist in the catalytic reaction.
On the basis of the mechanisms, the functions of the sulfonic group
in MIL-101-SO3H can be satisfactorily explained as follows. MIL-101
structures show interconnecting mesopores up to 29 and 34 Å (free
internal diameters) with entrances up to 16 Å, while cyclohexene and
its oxidation products are only 7–8 Å in their maximum dimension. It is
reasonable to assume that the large and open pore system should set no
limitation on internal diffusion of the small molecules [46]. The sul-
fonic group makes the pore surface more polar and hydrophilic than the
“bare” surface of MIL-101 and thus facilitates adsorption and retention
of the oxidant and various oxygenated intermediates within the pores
through electrostatic interactions and hydrogen bonding. The fact that
MIL-101-SO3H, compared with MIL-101, gives enhanced selectivity for
route B could be due to stabilization of the peroxide radical inter-
mediate (III) by the interactions afforded by the sulfonic-functionalized
surface. The relatively reduced total selectivity for route A could be
because the first radical intermediate (I) of the route is non-oxygenic
and not favored by the sulfonic group. The promotion effects of the
sulfonic group on further transformations along route A can be justified
as follows. Perox can be activated by hydrogen bonding with the sul-
fonic group in favor of the elimination transformation to 1-one; the
sulfonic group can also help to keep 1-one inside the pore to facilitate
further oxidation to dione. The hydrogen-bonding catalytic activity of
the sulfonic group has also been demonstrated elsewhere for oxidation
of olefins to diols, where, however, the oxidant and the catalyst are
H2O2 and silica-supported sulfonic acid, respectively [54]. To further
reveal the effects, the MOF with neutralized sulfonate groups (MIL-101-
SO3Na [55]) was also tested for the reaction (entry 6, Table 5). Com-
pared with MIL-101-SO3H, the deprotonated catalyst led to lower
conversion (12 %) through route B and also lower secondary conver-
sions (1-one to dione, and epox to diol) for both routes. The results
corroborate the assumption that -SO3H promotes route B and the sec-
ondary conversions through the OH group, which can form hydrogen
bonds with the oxygenated intermediates involved in route B and the
secondary reactions. The hydrogen bonds are absent for MIL-101-
SO3Na to the disadvantage of route B and the secondary reactions.
Nevertheless, compared with the “bare” MIL-101 (entry 2), MIL-101-
SO3Na is more active, leading to higher conversions for both routes and
for the perox-to-1-one and 1-one-to-dione reactions. This indicates that
the electrostatic interactions enhanced by the sulfonate group also play
important roles in promoting the reactions. The sulfonate group may
also form hydrogen bonds with the OH-containing species (such as
perox and TBHP) in favor of the reactions.
Scheme 3. The influence of radical inhibitor BHT. (a) and (b) Oxidation of
cyclohexene. (c) Reaction of epox. Conditions: 0.5 mmol cyclohexene or
0.1 mmol epox, 5 mmol TBHP, 8 mg MIL-101-SO3H (4 mol%), 5 mmol BHT,
40 °C, 14 h.
Table 5
Comparison of different catalytic systems for cyclohexene oxidation.a.
Entry Catalyst
Conv. (%) perox (%) 1-one
(%)
dione (%) diol (%)
1
2
3
4
5
6
no catalyst
MIL-101
MIL-101-SO3H
PTSA
MIL-101+PTSA 84
MIL-101-SO3Na 99
68
82
99
49
56
43
–
36
–
12
32
13
7
64
50
–
7
61
–
14
37
–
–
25
6
6
–
6(6b)
a
Reaction conditions: cyclohexene (0.5 mmol), TBHP (5 mmol) with MIL-
b
101-SO3H (4 mol%, 8 mg) at 40 °C in oxygen atmosphere for 14 h. yield of
epox.
Fig. 3. Filtration test for cyclohexene oxidation (solid symbols for the reaction
without filtration and open symbols for the reaction after filtering off the cat-
alyst). Conditions: 0.5 mmol cyclohexene, 5 mmol TBHP, 8 mg MIL-101-SO3H
(4 mol%), 40 °C, O2 atmosphere, 14 h.
hydrogen transfer from an α-C-H of cyclohexene to ROO⋅ or RO⋅ radi-
cals generates the cyclohexenyl radical (I), which combines with the
hydroperoxyl radical to give perox. According to some previous reports
[49–51], the cyclohexenyl radical could react with tert-butyl peroxyl to
produce 3-(tert-butylperoxy)cyclohexene, but the product was not de-
tected in our system. Perox undergoes the elimination reaction to
produce 1-one, which is catalyzed by synergetic action of sulfonic acid
and Cr(III). The 1-one to dione transformation occurs through a radical
mechanism similar to that for cyclohexene to 1-one. For route B, the
tert-butyl peroxyl radical is added to the C]C bond to generate the
intermediate 2-peroxide cyclohexyl radical (III), which undergoes fast
ring-closing to give epox [52,53]. For the formation of diol, the
common route is the ring-opening hydrolyzation of epox, through a
well-established acid-catalyzed mechanism without involvement of
Finally, the recyclability of the catalyst was tested. As can be seen
from Table 6, the conversion of cyclohexene remains almost unchanged
in the recycle runs, but the product selectivity changes. The yield of 1-
one increases and those of dione and diol decrease in the recycle run
compared to those in
the preceding run, along with increasing total selectivity for route A
relative to route B. The XRD, IR and N2 adsorption for the used catalyst
are compared with those for the fresh catalyst (Fig. 1). There are no
essential changes in IR absorption, including the bands characteristic of
the sulfonic group, so the chemical integrity of the material is retained.
The XRD profiles are similar on the whole, but the used catalyst shows
slightly decreased reflection intensity. The N2 adsorption capacity de-
creases after use, and the surface area changes from 1603 to 1291 m2/g.
According to ICP analysis with the supernatant obtained by filtering off
6

W.-J. Sun and E.-Q. Gao
MolecularCatalysis482(2020)110746
Scheme 4. Proposed reaction pathways of MIL-101-SO3H catalyzed cyclohexene oxidation.
relationships which may be considered as potential competing interests:
CRediT authorship contribution statement
Table 6
Recycle tests of MIL-101-SO3H catalyzed cyclohexene oxidation.a.
Number of cycles
Conv. (%)
1-one (%)
dione (%)
epox (%)
diol (%)
1
2
3
99
99
97
13
29
52
61
49
34
–
10
8
25
11
3
Weng-Jie Sun: Formal analysis, Investigation, Validation, Writing -
original draft, Writing
-
review
&
editing. En-Qing Gao:
Conceptualization, Methodology, Formal analysis, Funding acquisition.
a
Reaction condition: cyclohexene (0.5 mmol), TBHP (5 mmol) with MIL-
101-SO3H (8 mg, 4 mol%) at 40 °C for 14 h.
Acknowledgements
This work is supported by the National Natural Science Foundation
of China (NSFC No. 21773070).
the catalyst, the amount of leached Cr(III) after one cycle accounts for
1.9 % of the total Cr(III) content in the catalyst. It is unlikely that the
insignificant leaching can cause the obvious changes in catalytic
properties. The XRD and N2 adsorption data indicate that the porous
MOF undergoes some degree of structural degradation, which could be
responsible for the changes in catalytic properties. The details are open
to further investigation.
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In this study, MIL-101-SO3H was demonstrated as a bifunctional
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