A new porous Co(ii)-metal-organic framework for high sorption selectivity and affinity to CO2and efficient catalytic oxidation of benzyl alcohols to benzaldehydes

Article abstract of DOI:10.1039/d1ce00250c

Herein, we report a new 3D porous Co(ii)-based metal-organic framework catalyst (Me2NH2)2[Co3(L)2(H2O)2]·2DMF (MOF I), which has been successfully prepared by using Co(ii) ions and rigid V-shaped 3,5-di(2,4-dicarboxylphenyl)pyridine (H4L) via the solvothermal reaction. Structural analysis reveals that I displays a porous structure with the pore size of 16.2 × 7.2 ?2 based on the trinuclear [Co3(COO)4(H2O)2N2] secondary building units (SBUs). Gas sorption experiments on the guest free sample I′ reveals a high capacity and selectivity to CO2 over CH4. And further, the catalytic explorations of the I′-catalyzed system (I′: 3 mol%; proline: 40 mol%; CH3CN: 2 mL) reveal that benzyl alcohols with different structures can be efficiently transformed into benzyl alcohols without by-products under mild conditions.

Full text of DOI:10.1039/d1ce00250c

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A new porous Co(II)-metal–organic framework for
high sorption selectivity and affinity to CO and
2
Cite this: DOI: 10.1039/d1ce00250c
efficient catalytic oxidation of benzyl alcohols to
benzaldehydes†
Yun-Long Wu, *ab Rong-Rong Yang, Guo-Ping Yang,
a
b
a
Yang-Tian Yan,
a
a
a
b
b
Xiao-Lei Su, Xin-Hai He, Yan-Yan Song, Zheng-Sheng Ma and Yao-Yu Wang
Herein, we report a new 3D porous Co(II)-based metal–organic framework catalyst (Me
O) ]·2DMF (MOF I), which has been successfully prepared by using Co(II) ions and rigid V-shaped
,5-di(2,4-dicarboxylphenyl)pyridine (H
2 2 2 3 2 2
NH ) [Co (L) (H -
2
3
4
L) via the solvothermal reaction. Structural analysis reveals that I
2
Received 20th February 2021,
Accepted 4th April 2021
displays a porous structure with the pore size of 16.2 × 7.2 Å based on the trinuclear [Co (COO) (H O) N ]
3
4
2
2 2
secondary building units (SBUs). Gas sorption experiments on the guest free sample I′ reveals a high
capacity and selectivity to CO over CH . And further, the catalytic explorations of the I′-catalyzed system
I′: 3 mol%; proline: 40 mol%; CH CN: 2 mL) reveal that benzyl alcohols with different structures can be
efficiently transformed into benzyl alcohols without by-products under mild conditions.
2
4
DOI: 10.1039/d1ce00250c
(
3
rsc.li/crystengcomm
friendly and reasonable transformation procedures in the
Introduction
conversion of benzyl alcohol to benzaldehyde without other
by-products for the pharmaceutical and chemical industries
Benzaldehydes are not only essential reagents in modern
chemical syntheses, but also very important functionally
active compounds in spices, dyes, medicines and so on.
1
3–15
and other areas.
Thus, catalysts played a decisive role for the final products in
1
,2
16,17
Now, benzyl alcohol is always selected as the original material
the whole synthetic procedure.
Catalyst systems consisting
for preparing aldehydes and ketones via oxidation
of molecular oxygen, air, or hydrogen peroxide by combination
with appropriate transition metal catalysts (Cu, Pd, Au, Fe, etc.)
have been overwhelmingly explored in the conversion of benzyl
alcohol to benzaldehyde, and present superior alternatives
according to the principles of green, atom economical and
3–5
reactions.
They are the main reactions for converting
alcohols into the corresponding aldehydes or ketones in
chemical syntheses both at the laboratory and industrial
levels. This conversion has been achieved by oxidation of the
substrate alcohols, and many oxidation methods have been
18–20
sustainable chemistry and so on.
However, the selectivities
6–11
developed.
But in fact, the oxidation procedure using a
of these catalysts are usually low and may produce other by-
21–23
stoichiometric amount of oxidants (metal oxides, methyl
sulfoxides, hypervalent iodine, etc.) may lead to a low atom
economy and possibly produce toxic wastes and other by-
products.
With the growing concern related to economic
and environmental acceptability, it should be more attractive
from an environmental standpoint that a conversion process
using an atomically economic catalyst system does away with
these unfavorable elements mentioned above during the
1
2
products, such as benzoic acid and so on, which may bring
lots of inconvenience for obtaining the target compounds.
Thus, it is of great necessity to explore facile, environmentally
24,25
conversion of alcohols to aldehyde compounds.
Catalysts
with confined permanently porous or channel frameworks have
been the prioritized alternatives in homogeneous and
26,27
a
heterogeneous catalytic organic reactions.
Considered to be
School of Materials Science & Engineering, Xi'an Polytechnic University, Xi'an
new promising analogues of carbon nanotubes, single-walled
tubular metal–organic frameworks (MOFs), constructed with
organic linkers and metal ions/clusters, have attracted lots of
attention because of their attractive structures and properties.
The typical MOFs, such as ZIF-8, ZIF-9, HKUST-1, etc., have been
always used as efficient catalysts in chemical reactions because
7
10048, P. R. China. E-mail: 20180717@xpu.edu.cn
b
Key Laboratory of Synthetic and Natural Functional Molecule of Ministry of
Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of
Chemistry & Materials Science, and, School of Chemical Engineering, Northwest
University, Xi'an 710127, P. R. China
†
Electronic supplementary information (ESI) available. CCDC 2011190. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1ce00250c
28,29
of their confined nanotubes.
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Based on our previous studies on the porous MOF-based
General procedure for the oxidation of benzyl alcohols into
benzaldehydes
3
0–32
catalysts and the above discussion,
herein, V-shaped
L) (Fig. S1†) and
Co(NO ) ·6H O were used as the building blocks to construct
3
,5-di(2,4-dicarboxylphenyl)pyridine (H
4
A mixture of benzyl alcohol 1a (0.2 mmol) and L-proline (40
mol%), I′ (3 mol%), and CH CN (2 mL) were sequentially added
3
3
2
2
the target porous MOFs. Fortunately,
heterogeneous Co(II)-MOF (Me NH [Co (L)
a
(H
new porous
O) ]·2DMF (I)
to a 15 ml sealed tube using a syringe in an air atmosphere. The
mixture was stirred under ambient conditions and monitored
by thin-layer chromatography (TLC) until the completion of the
reaction. Then, the reaction was partitioned between water and
ethyl acetate. The layers were separated and the organic layer
was washed with water (3 × 5 mL) and washed with aqueous
saturated brine solution (3 × 5 mL), dried over Na SO , filtered
2 4
and concentrated under reduced pressure. Purification by flash
chromatography [ethyl acetate (EA); petroleum ether (PET)] (EA :
2
2
)
2
3
2
2
2
2
(
16.2 × 7.2 Å ) was successfully synthesized. The gas sorption
explorations for I′ show the high adsorption selectivity to CO2
over CH . And due to its catalytic properties, the I′-catalyzed
4
system can efficiently catalyze the oxidation of benzyl
alcohols into benzaldehydes without by-products under the
mild conditions.
PET = 1 : 50) afforded 2a. The structures of the target products
Experimental
1
(2a–2j) were identified by H NMR.
Materials and measurements
Crystallographic data collection and refinement
The starting materials were commercially obtained and
directly used. Co(NO ) ·6H O and organic solvents were
X-ray diffraction data for I were obtained on a Bruker SMART
APEX II CCD detector with graphite monochromated Mo-Kα
radiation (λ = 0.71073 Å) by using a ϕ/ω scan technique at 298(2)
K. The structure of I was solved by direct methods and refined
3
2
2
purchased from Xi'an Chemical Reagent Factory. The organic
ligand was purchased from Jinan Camolai Trading Company.
Elemental analyses of C, H, and N were carried out with a
Perkin-Elmer 2400C elemental analyzer. The infrared spectra
2
anisotropically on F by the full-matrix least-squares technique
−
1
33–35
were obtained with KBr pellets in 4000–400 cm on a Bruker
EQUINOX-55 FT-IR spectrometer. Thermogravimetric
using the SHELXTL program.
Crystal data and structure
refinement information are displayed in Table 1. The selected
bond lengths and angles are displayed in Table S1 .† The crystal
structure has been deposited at the Cambridge Crystallographic
Data Center (CCDC No.: 2011190).
analyses (TGA) were performed with a NETZSCH STA 449C
microanalyzer under a nitrogen stream at a heating rate of 5
−
1
°
C min . Powder X-ray diffraction (PXRD) data were recorded
on a Bruker D8 ADVANCE X-ray powder diffractometer
determined with Cu-Kα radiation (1.5418 Å) at room
temperature. The sorption isotherms were obtained using a
Results and discussion
1
Structure description of (Me NH ) [Co (L) (H O) ]·2DMF (I)
2
2 2
3
2
2
2
Micromeritics ASAP 2020 M. H NMR data were obtained
with a Bruker Ascend 400 spectrometer (400 MHz).
Single crystal data analyses reveal that I is a 3D porous
network based on the trinuclear [Co (COO) (H O) N ] SBUs,
3
4
2
2 2
Synthesis of (Me
A mixture of H L ligand (0.1 mmol, 40.8 mg), Co(NO
0.2 mmol, 58.2 mg), DMF (5 mL), CH CN (2 mL) and H
2 2 2 3 2 2 2
NH ) [Co (L) (H O) ]·2DMF (I)
Table 1 Crystal data and structure refinements for I
4
3
)
2
·6H
2
O
(
3
2
O (2
Complex
I
mL) was placed in a 25 mL glass beaker and stirred for 30
min at ambient temperature. After that, the clear liquid was
transferred to a Teflon-lined (20 ml) stainless steel vessel.
And then, the vessel was heated at 145 °C for 72 h and cooled
to room temperature. Red clubbed crystals were obtained
Empirical formula
Formula mass
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
52 3 6 20
C H52Co N O
1257.78
Monoclinic
P2 /n
1
13.979(2)
12.436(2)
16.164(3)
90
90.613(3)
90
2809.8(8)
2
1.487
(yield 49% based on H
4
L). Elemental analysis calcd (%) for
C H N Co O : C, 49.65; H, 4.17; N, 6.68. Found: C, 49.58;
5
2
52
6
3 20
−
1
H, 4.25; N, 6.72. FT-IR (KBr, cm ): 3402 (m), 3048 (m), 2781
m), 2482 (w), 1664 (s) 1612 (s), 1491 (m), 1377 (s), 1174 (w),
034 (m), 795 (s), 682 (m), 481 (w) (Fig. S7†).
γ [°]
V [Å ]
3
(
1
Z
D
−3
calcd.[g cm
]
−
1
μ [mm
]
0.956
F [000]
θ [°]
Reflections collected
Goodness-of-fit on F
Final R indices [I > 2σ(I)]
1294
Synthesis of I′
1.916–26.369
15 219/5710
1.01
2
The synthesized bulk product I was immersed in CH
2
Cl
2
for
a
R
1
= 0.0478
7
2 2
2 h with exchange of the CH Cl solvent every 24 h and
wR = 0.1333
2
heated at 100 °C under vacuum for 6 h to obtain the guest-
free phase I′.
P
P
P
P
a
2
o
2 2
2 2 1/2
R
1
=
‖F
o
| − |F
c
‖/ |F
o
|, wR
2
= [ w(F
− F
c
) / w(F
o
) ]
.
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crystallizing in the P2 /n monoclinic space group. The
1
asymmetric unit of I contains two center Co(II) ions with
similar coordination configurations (Fig. 1). The center
equatorial plane of the center Co1 was connected with four
4
−
oxygen atoms of different L ligands and the axial direction
was coordinated with two aqueous molecules. Co2 shows a
similar coordination geometry, the only difference is that one
of the axial aqueous molecules was instead one nitrogen
atom [Co–X = 2.024(2)–2.155(2) Å; X–Co–X = 83.33(10)–180° (X
=
O, N)]. The oxygen atoms of the aqueous molecules bridge
three center Co(II) ions, giving the trinuclear SBU in I
Fig. 2a).
As for the fully deprotonated L
carboxylate groups adopt two coordination models (Fig. S2†):
(
4
−
ligands, the four
2
2
1
1
η μ
4
2
χ ; η μ
1
χ with five Co(II) ions. The main skeleton of the
−
L
ligands extends and supports the trinuclear SBUs in a
2
porous network with the pore size of 16.2 × 7.2 Å (Fig. 2b),
showing the 3D porous structure of I. There exist the solvent
molecules and Me NH in the channel, which produce the
+
2
2
Fig. 2 (a) The trinuclear SBU [Co
structure of I.
3
(COO)
4
(H
2
O)
2
N
2
] in I; (b) the porous
H-bonds in the channel (Fig. S3 and Table S2†) to support
the network and balance the charge. Topological analysis
shows that the trinuclear SBU [Co (COO) (H O) N ] motifs
3
4
2
2 2
4
−
and L ligands are viewed as 8- and 4-connected nodes (Fig.
S4†). So, the skeleton of I can be viewed as a bimodal (4,8)-
DMF molecule. And the second step is losing the coordinated
aqueous molecule. The last step is the collapse of the main
framework for I beyond the stable temperatures. As for I′, the
uncoordinated DMF molecules have been moved from the
main structure of I. Thus, in the curve of I′, the first step of
weight loss is losing the coordinated water molecules. And
after that, the main frameworks begin to collapse similar to
that of I.
1
2
12
4
6
connected flu topological net with the (4 ·6 ·8 )(4 ) symbol
2
(Fig. 3a and b and S5†).
PXRD and TGA discussion
The PXRD experiments were carried out to check the purity
and the stability of I and I′ (after catalytic and gas sorption
experiments). The peaks of the bulk product I were closely
consistent with those of the simulations from the single-
crystal X-ray diffraction data, implying the high purity of the
obtained products (Fig. S6†). Also, the thermal stabilities of I
and I′ were also detected via TGA analysis (Fig. S8†). As for I,
from structure analysis, there exist coordinated water
molecules and uncoordinated DMF molecules in I, so the
TGA curve shows three step weight losses following the
temperature rise. The first step is losing the uncoordinated
Gas sorption properties
After the hypothetical removal of the non-coordinated solvent
molecules, the potential solvent area volume of I′ is determined
3
to be ∼42.7% (1201 Å ) out of the total volume of the unit cell
3
(2809.8 Å ) by the PLATON program. Before the gas sorption
experiments, the non-coordinated solvent molecules were
removed from the as-synthesized bulk samples to obtain the
guest-free phase I′ to obtain the guest-free bulk samples of I′.
From the N
2
sorption isotherm of I′ with a maximum of 9.9
−3
−1
cm
g
(Fig. S9†), the Brunauer–Emmett–Teller (BET) and
Fig. 1 The coordination environment of Co(II) in I. Symmetry codes:
#
1: 1.5 − x, −0.5 + y, 0.5 − z; #2: 1.5 − x, −0.5 + y, 1.5 − z; #3: 0.5 + x,
1
1
.5 − y, 0.5 + z; #4: 0.5 + x, 1.5 − y, −0.5 + z; #5: 2 − x, 1 − y, 1 − z; #6:
− x, 1 − y, 1 − z.
Fig. 3 (a) The 3D porous framework of I; (b) topological net of I.
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Langmuir surface areas of I′ are evaluated to be 13.28 and 26.23
2
−1
m g , respectively. Further, the porosity characteristics of I′ are
assessed via CO and CH gas adsorption. As shown in Fig. 4a,
2
4
the isotherms exhibit the typical type-I isotherms for CO and
2
CH4 at 195 K. It can be seen that the CO adsorption of I′
2
3
−1
3 −1
4
reaches 60 cm g and the lower CH adsorption is 5.8 cm g
under the same adsorption conditions. The big difference in
the gas sorption made us explore its selectivity adsorption
properties under ambient conditions, and the gas sorption
capacity for CO2 and CH4 was measured at 273/298 K. The
3
−1
sorption capacity for CO and CH reaches 32.5/2.4 cm g (273
K) and 19.8/1.0 cm g (298 K), respectively (Fig. 4b).
2
4
3
−1
From the discussion above, the great differences in the gas
sorption capacity encouraged us to explore the selectivities to
2 4
the mixtures of CO and CH . The selective gas adsorption
performance for CO over CH of I′ was simulated by the ideal
2
4
36
adsorbed solution theory (IAST) (Fig. S10†). As shown in
Fig. 5, the selective gas adsorption properties of I′ were obtained
by using the gas adsorption data for CO and CH at 273 K and
2
4
2 4
298 K (the component: CO /CH = 50/50). The simulation curves
of the selective gas adsorption properties for the mixture of
CO /CH gases are obtained. As illustrated in Fig. 5, the
2
4
2 4
predicted selectivities to CO /CH are dependent on the
temperature and the pressure. The values of the selectivities to
CO /CH are ranging from 112.3 to 20 (273 K) and 12.8 to 2 (298
2
4
K) following the change of the gas pressure, and I′ outperforms
many reported MOFs under the same conditions because of the
high selectivities. The selective gas adsorption performance of I′
Fig. 5 The curve of the selective gas adsorption of I′ for CO
mixtures simulated by IAST: at (a) 273 K and (b) 298 K.
2
/CH
4
for CO was also verified by the adsorption heat (Q ). The Q
2
st
st
2
was calculated by using the CO gas sorption isotherms at 273/
298 K via the virial equation in Fig. S11.† The high selectivity of
I′ is better than many reported MOFs with similar porous
structures (Table S3 and S4†). The high selectivity and affinity to
+
2
CO
2
may be caused by the synergistic effect of Me
2
NH
within
the 1D channel of the porous I′, which may produce a highly
polar wall of the channel to strengthen the interactions between
2
the framework and CO . And also, the natural differences for
the size of the molecular structure (CO : 3.30 Å; CH : 3.80 Å) or
2
4
the narrow hole screening effect may promote I′ to have a high
3
7–41
adsorption capacity for CO
2
.
As shown in Fig. 6, the Qst
−1
curve for CO is up to a maximum of 31.5 kJ mol . The high
2
Qst value may be related to the polarity of the porous structure,
which may cause the strong interactions between I′ and
4
2,43
CO2.
prediction for the high uptake and selective adsorption of the
mixture gases (CO and CH ). The results reveal that I′ can be
used as a potential gas adsorption and separation material for
post combustion CO capture applications in industrial areas or
daily life.
Thus, the research may provide a theoretical
2
4
2
Catalytic oxidation of benzyl alcohols into benzaldehydes
The oxidation of benzyl alcohols into benzaldehydes is one of
the basic transformations in organic syntheses, which
Fig. 4 (a) Gas adsorption isotherms for CO
gas adsorption isotherms for CO and CH at 273 and 298 K.
2 4
2
and CH
4
at 195 K of I′; (b)
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2
Fig. 6 CO adsorption heat Qst for I′ calculated by the virial equation.
attracts interest in basic research and industrial production.
And because of the high potential solvent accessible volume,
the evenly distributed metal centers and attractive chemical
properties of I′, I′ was used as the platform for heterogeneous
catalysts for transforming benzyl alcohols to benzaldehydes.
Initially, benzyl alcohol 1a was chosen as the model substrate
to explore the catalytic properties of I′. Considering that the
oxidation of alcohols to benzaldehydes always produces
benzoic acids as the by-products, proline was introduced into
the I′-catalyzed system as the co-catalyst to stabilize the
benzaldehydes. Proline was also replaced by acids such as
benzoic acid, anthranilic acid, amino acid, alanine and
glycine during the oxidation process. The results show that
there exists a certain amount of by-products (∼30% for
benzoic acid and ∼50% for anthranilic acid), and amino
acid, alanine and glycine give a low yield for the target
products. Thus, proline was selected as the ideal co-catalyst
in the oxidation reaction. Fortunately, 1a oxidation reactions
proceeded smoothly under the conditions [I′ (3 mol%) and
proline (40 mol%) with toluene as the solvent] at ambient
temperature for 3 h, which produce benzaldehyde 2a in the
yield of 21% (Table S5,† entry 1) without benzoic acid as the
by-product. The oxidation reaction is discontinued
(monitored by TLC) when I′ was removed from the reaction
system. Also, the optimized reaction conditions of the
oxidation reactions are displayed in Table S5.†
And furthermore, we also optimized the reaction
conditions by using different solvent systems (Table S5,†
3
entries 1–9), and the results show that CH CN was the
optimal solvent to afford the desired products in ∼95% yield.
In addition, the effects of the concentration of proline and I′
on the products have also been systematically explored. The
whole catalytic reactions were hard to conduct or produce the
benzoic acid as the by-products (Table S5,† entries 10–18).
Accordingly, the reaction conditions were optimized as
Scheme 1 Oxidation of various alcohols under optimal conditions.
follows: I′ (3 mol%); proline (40 mol%.); CH CN (2 ml); room
3
temperature. After that, different substituted benzyl alcohols
were selected as the original materials to investigate the
catalytic effect of the I′-catalyzed system. As shown in
Scheme 1, the different substrates containing either electron-
withdrawing groups (F, Cl, Br) or electron-donating groups
2b–2i). And in addition, other aromatic substrates, such as
1-naphthyl alcohol, also reacted successfully, providing 2j in
91% yield (Scheme 1). The H NMR spectra are shown in Fig.
1
(Me, OMe) on the aromatic ring can react smoothly, providing
S12.† The results reveal high yields for the catalytic oxidation
the corresponding benzaldehydes in high yields (89–95%,
of benzyl alcohols into benzaldehydes based on the
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Conclusions
In summary, a new porous Co(II)-based MOF (I) with the pore
2
size of 16.2 × 7.2 Å based on the trinuclear secondary
building unit (SBU): [Co
3
(COO)
4
(H
2
O)
2
N
2
]
has been
successfully prepared. Gas sorption experiments show that
the guest-free bulk product I′ may be viewed as a new porous
material with high capacity and selectivity to CO over CH .
2 4
Due to its catalytic properties, I′ may provide a new catalytic
system based on 3D porous Co-based MOFs for catalyzing the
oxidation of benzyl alcohols into benzaldehydes without by-
products under mild conditions.
Scheme 2 Tentative mechanism for the reaction.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the NSFC (22071194 and
2
2
1971207), the NSF of Shaanxi Province (2021JQ-666 and
020JQ-821), the Scientific Research Program Funded by
Shaanxi Provincial Education Department (20JK0655), the
China Postdoctoral Science Foundation (2020M673621XB),
the Research Start-up Foundation of Xi'an Polytechnic
University (BS201864), the Young Talent Fund of University
Association for Science and Technology in Shaanxi, China
Fig. 7 Five-cycle experiments of the oxidation of benzyl alcohol into
benzaldehyde under ideal conditions.
(
20190608) and the Open Foundation of Key Laboratory of
I′-catalyzed system, which are comparable with the reported
publications on the oxidation of benzyl alcohols into
benzaldehydes based on MOFs or other complex catalyzed
Synthetic and Natural Functional Molecule Chemistry of the
Ministry of Education, China (KLSNFM2020006).
4
4–51
systems.
The tentative mechanism for the reaction is shown in
Scheme 2. Firstly, H O was removed from the trinuclear
Co (COO) (H O) N ] by a protonation procedure, which
Notes and references
2
1 D. Wang, P. Wang, S. Wang, Y.-H. Chen, H. Zhang and A.
Lei, Nat. Commun., 2019, 10, 2796.
[
3
4
2
2 2
produces the activated LCo–H intermediate S1. S1 was
activated in an atmosphere to form oxygen–cobalt
and further generating intermediate S3
2 G. Tojo and M. Fernandez, Basic Reactions in Organic
Synthesis: Oxidation of Alcohols to Aldehydes and Ketones,
Springer, New York, 2010.
3 S. E. Davis, B. N. Zope and R. J. Davis, Green Chem.,
2012, 14, 143–147.
4 C. E. Chan-Thaw, A. Savara and A. Villa, Catalysts, 2018, 8, 431.
5 M. M. Dell'Anna, M. Mali, P. Mastrorilli and A. Monopoli,
J. Mol. Catal. A: Chem., 2014, 386, 114–119.
6 R. A. Sheldon, I. W. C. E. Arends, G.-J. T. Brink and A.
Dijksman, Acc. Chem. Res., 2002, 35, 774–781.
O
2
5
2–55
complexes S2,
−
and OH . Complex A was obtained via the oxidation reaction
of benzyl alcohol. The consequent deprotonation of the
radical cation A generates the radical B. And next, the single-
electron oxidation and deprotonation of C produces the
target benzaldehyde (2a). Lastly, S3 obtains the proton to
complete the catalytic cycle. The study on related oxidation
transformation mechanisms is being conducted by our
group.
7 A. M. F. Phillips, A. J. L. Pombeiro and M. N. Kopylovich,
ChemCatChem, 2017, 9, 217–246.
8 S. M. A. H. Siddiki, T. Toyao and K.-I. Shimizu, Green Chem.,
2018, 20, 2933–2952.
In addition, the circulation experiments were also
conducted to explore the stability of the catalyst system
(Fig. 7). Also, the catalyst leaching experiments were
conducted by filtering the reaction system, and the results
show that I′ can maintain a high stability and catalytic
activity after 5 cycles in the circulation experiment. After the
circulation experiment, the crystalline structure of I′ was
verified by FTIR and PXRD, and its original crystalline
structure was retained after the reaction (Fig. S5†), providing
us with an economical, environmentally friendly and
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