Suspending ionic single-atom catalysts in porphyrinic frameworks for highly efficient aerobic oxidation at room temperature

Article abstract of DOI:10.1016/j.jcat.2017.11.022

Room temperature and atmosphere pressure are highly desired catalytic conditions for aerobic oxidation of inert sp³ C–H bonds. To meet this challenge, we developed a simple strategy by suspending ionic single atoms (ISAs) inside the anionic pores of metal–organic frameworks (MOFs). CZJ-22-Cu, consisting of suspended ISA copper(II) inside the anionic pores, exhibits exceptionally high catalytic efficiency in aerobic oxidation of ethers to esters at room temperature and atmosphere pressure, in which turnover number (TON) 77,100 and turnover frequency (TOF) 7710 h^?1 have been realized for aerobic oxidation of isobenzofuran. The unmatched catalytic properties of CZJ-22-Cu are attributed to the unique features of suspended ISAs inside the anionic pores, which result in high surface free energy of redox-active centers, and the substrate-selective accumulation nature of the inside pores, which would significantly improve the reactivity and reaction rate.

Full text of DOI:10.1016/j.jcat.2017.11.022

Journal of Catalysis 358 (2018) 43–49
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Suspending ionic single-atom catalysts in porphyrinic frameworks
for highly efficient aerobic oxidation at room temperature
⇑
Wei-Long He, Xiu-Li Yang, Min Zhao, Chuan-De Wu
State Key Laboratory of Silicon Materials, Center for Chemistry of High-Performance and Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027,
People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Room temperature and atmosphere pressure are highly desired catalytic conditions for aerobic oxidation
of inert sp CAH bonds. To meet this challenge, we developed a simple strategy by suspending ionic single
Received 3 October 2017
Revised 16 November 2017
Accepted 18 November 2017
3
atoms (ISAs) inside the anionic pores of metal–organic frameworks (MOFs). CZJ-22-Cu, consisting of sus-
pended ISA copper(II) inside the anionic pores, exhibits exceptionally high catalytic efficiency in aerobic
oxidation of ethers to esters at room temperature and atmosphere pressure, in which turnover number
ꢀ1
(
TON) 77,100 and turnover frequency (TOF) 7710 h have been realized for aerobic oxidation of isoben-
Keywords:
zofuran. The unmatched catalytic properties of CZJ-22-Cu are attributed to the unique features of sus-
pended ISAs inside the anionic pores, which result in high surface free energy of redox-active centers,
and the substrate-selective accumulation nature of the inside pores, which would significantly improve
the reactivity and reaction rate.
Metal-organic frameworks
Single-atom catalysts
Biomimetic
Ionic single atoms
Aerobic oxidation
Ó 2017 Elsevier Inc. All rights reserved.
1
. Introduction
Catalysts, which can alter chemical reaction paths, are corner-
of metal coordination sites by organic ligands and/or counterions
would reduce the activity of valence electrons, and thus signifi-
cantly affect their catalytic efficiency. The challenge is how to
decrease the coordination saturability and increase the surface free
energy of SACs inside porous MOFs.
There are many anionic MOFs, consisting of endogenous cations
inside the pore space, that are systematically exchangeable to tune
their properties by ion exchange, and thus to realize applications in
different fields [22–27]. Anionic MOFs provide an opportunity to
develop ionic single-atom catalysts (ISCAs) inside the anionic pores
by eliminating the ligation of counterions. Sequestered by the
anionic pores, most of the coordination sites of ISACs would be
vacant, resulting in very high surface free energy. Moreover, con-
fining ISACs in negatively charged pores would solve the leaching
and aggregation issues. Here, we report an anionic porphyrinic
stones in the chemical industry. Single-atom catalysts (SACs) are
an emerging class of heterogeneous catalysts in which active met-
als, such as Pt, Ir, Rh, Pd, Au, and Ag atoms, are atomically dispersed
onto solid surfaces [1,2]. Due to high metal atom usage and high
surface free energy of active centers, SACs demonstrate superior
catalytic properties in many chemical reactions [3–12]. To prevent
single-atom mobility and sinter under realistic reaction conditions,
SACs are fixed onto solid surfaces by strong interactions. This tech-
nique inevitably results in part of coordination sites being inacces-
sible to reactant molecules, which would compromise their
catalytic efficiency. Additionally, low surface areas of nonporous
solid supports would result in low loadings of active atoms and
low spatial utility.
framework, [(CH
22; Cu-H OCPP = Cu -5,10,15,20-tetrakis(3,5-biscarboxylphenyl)p
orphyrin), consisting of endogenous [(CH
3
)
2
NH
2
]
2
[Y (Cu-OCPP)(H O)
2 2 4
]ꢁ8DMFꢁ22H
2
O (CZJ-
II
Metal-organic frameworks (MOFs) are an emerging class of por-
ous materials with tunable framework structures and properties
8
+
3 2 2
) NH ] cations that are
[
13–15]. MOFs with high porosity, high surface areas, and tunable
systematically exchangeable with redox-active metal ions to tune
the catalytic properties. The copper (II)-encapsulated material Cu
hydrophobic and hydrophilic pore nature have been targeted as a
tunable class of crystalline porous supports for single-site catalysts
by in situ synthesis or postmodification [16–21]. However, ligation
[Y (Cu-OCPP)(H O)
2 2 4
]ꢁ10DMFꢁ18H
2
O (CZJ-22-Cu), consisting of sus-
II
pended ionic single-atom (ISA) Cu inside the anionic pores,
demonstrates exceptionally high catalytic activity in aerobic oxida-
tion of ethers to esters at room temperature and atmosphere
pressure.
⇑
Corresponding author.
E-mail address: cdwu@zju.edu.cn (C.-D. Wu).
https://doi.org/10.1016/j.jcat.2017.11.022
021-9517/Ó 2017 Elsevier Inc. All rights reserved.
0

4
4
W.-L. He et al. / Journal of Catalysis 358 (2018) 43–49
2
. Experimental
1231(w), 1208(w), 1149(w), 1112(w), 1074(m), 1002(m), 939(m),
9
20(w), 828(w), 778(m), 711(m), 686(s), 543(w), 416(w).
2.1. Materials and methods
2.5. Synthesis of Zn-CZJ-22-Cu
All of the chemicals were obtained from commercial sources
and were used without further purification, except that 5,10,15,2
-tetrakis(3,5-biscarboxylphenyl)porphyrin was prepared accord-
ing to the literature [28,29]. FT-IR spectra were collected from
KBr pellets on an FTS-40 spectrophotometer. Thermogravimetric
The preparation procedure for Zn-CZJ-22-Cu is the same as that
0
for CZJ-22-Cu, except that Zn-CZJ-22 was used instead of CZJ-22.
Anal. calcd. for C85 ZnCu (%): C, 41.10; H, 5.88; N,
.46; Cu, 2.56; Zn, 2.63; Y, 7.16. Found: C, 39.57; H, 5.65; N,
145 15 50 2
H N O Y
8
analyses (TGA) were carried out under N
2
on a NETZSCH STA
ꢀ1
ꢀ1
8.63; Cu, 2.43; Zn, 2.15; Y, 7.19. FT-IR (KBr pellet, m/cm ):
4
09PC/PG instrument at a heating rate of 10 °C min . Elemental
1
1
7
696(m), 1546(s), 1437(s), 1371(s), 1233(w), 1210(w), 1149(w),
121(w), 1076(w), 1004(m), 939(m), 828(w), 796(w), 779(m),
13(m), 664(w), 623(w), 541(w), 421(w).
analyses were performed on a ThermoFinnigan Flash EA 1112 ele-
ment analyzer. GCꢀMS spectra were recorded on a SHIMADZU
GCMS-QP2010. Powder X-ray diffraction (PXRD) data were
recorded on a RIGAKU D/MAX 2550/PC for Cu K
a
radiation (k = 1.
5
406 Å). A Micromeritics ASAP 2020 surface area analyzer was
2.6. X-ray crystallography
2
used to measure N gas adsorption/desorption isotherms. Induc-
tively coupled plasma mass spectrometry (ICP-MS) was performed
on an X-Series II instrument. X-ray photoelectron spectra (XPS)
were recorded on a VG ESCALAB MARK II machine. Transmission
electron microscopy (TEM) with an energy-dispersive X-ray
The determination of the unit cells and data collection for the
crystals of CZJ-22, CZJ-22-Cu, and Zn-CZJ-22 were performed on
an Oxford Xcalibur Gemini Ultra diffractometer with an Atlas
detector. The data were collected using graphite-monochromatic
enhanced ultra Cu radiation (k = 1.54178 Å) at 293 K. The data sets
were corrected by empirical absorption correction using spherical
harmonics, implemented in the SCALE3 ABSPACK scaling algorithm
(
EDX) detector was carried out on JEM 2100F equipment, and the
samples were deposited onto ultrathin carbon films on carbon
grids. Electron paramagnetic resonance (EPR) spectra were
recorded on a Bruker ESRA-300 at room temperature.
[
30]. The structures of all compounds were solved by direct meth-
ods and refined by full-matrix least-squares methods with the
SHELX-97 program package [31]. Because the solvent molecules
in these compounds are highly disordered, the SQUEEZE subrou-
tine of the PLATON software suit was used to remove the scattering
from the highly disordered guest molecules [32,33]. The resulting
new files were used to further refine the structures. The H atoms
on C atoms were generated geometrically. The X-ray crystallo-
graphic coordinates for structures reported in this paper have been
deposited at the Cambridge Crystallographic Data Centre (CCDC)
under deposition number CCDC 1569598–1569600. These data
can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
2
.2. Synthesis of CZJ-22
Cu-H
.26 mmol) were dissolved in a mixture of DMF (2 mL) and dilute
(4 M, 0.5 mL). The mixture was sealed in a screw cap vial and
heated at 80 °C for 1 week. Brown crystals of CZJ-22 were collected
by filtration, washed with DMF, EtOH, and Et O, and dried at room
temperature. Yield: 83% (based on Cu-H OCPP). Anal. calcd. for
(%): C, 41.00; H, 6.19; N, 8.37; Cu, 2.71; Y,
.59. Found: C, 41.03; H, 5.97; N, 8.28; Cu, 2.39; Y, 7.71. FT-IR
8
OCPP (1 mg, 0.001 mmol) and Y(NO
3
)
3 2
ꢁ6H O (100 mg,
0
HNO
3
2
8
C
7
80 50 2
H144CuN14O Y
ꢀ1
(
KBr pellet, m/cm ): 1692(w), 1610(s), 1547(s), 1437(s), 1371(s),
1
251(w), 1148(w), 1114(w), 1080(w), 1008(m), 938(m), 781(m),
7
13(s), 614(w), 416(w).
2.7. A typical procedure for the aerobic oxidation of isobenzofuran
2.3. Synthesis of CZJ-22-Cu
Isobenzofuran (0.3 mmol), CZJ-22-Cu (0.6 mol), and N-hydrox-
l
yphthalimide (NHPI) (0.015 mmol) in acetonitrile (2 mL) were stir-
red at room temperature (25 °C) under an atmosphere of oxygen
(balloon) for 1 h. The solid was recovered by centrifugation and
4 2
Brown crystals of CZJ-22 (50 mg) were immersed in Cu(ClO )
DMF solution (0.1 M, 2 mL) in a 5 mL screw cap vial and heated
at 50 °C for 8 h. After the supernatant solution was decanted, the
3 2
thoroughly washed several times with THF, CH CN, and Et O. The
vial was refilled with fresh Cu(ClO
was repeated for 5 days. After cation exchange was completed,
mL MeOH was added to the vial to wash out excess Cu(ClO
in the solid sample for 3 days (3 times per day), and afforded
4
)
2
DMF solution. This procedure
recovered solid was reused in the successive run. The identity of
the product was determined by GC–MS and compared with that
of the authentic samples analyzed under the same conditions,
while the conversion of substrate, the yield, and the selectivity of
product were obtained by GC analysis using a flame ionization
detector (FID) with a capillary SE-54 column in the presence of
an internal standard of naphthalene.
2
4 2
)
CZJ-22-Cu as brown crystals. Anal. calcd. for C82
2
H136Cu N
14
O
48
Y
2
(
%): C, 41.19; H, 5.73; N, 8.20; Cu, 5.32; Y, 7.44. Found: C, 40.37;
ꢀ1
H, 5.53; N, 8.36; Cu, 5.52; Y, 7.51. FT-IR (KBr pellet,
m/cm ):
1
1
7
695(w), 1611(s), 1546(s), 1439(s), 1372(s), 1341(w), 1211(w),
121(w), 1109(w), 1082(w), 1008(m), 939(m), 835(w), 780(m),
12(m), 623(w), 427(w).
2
.8. Study of the turnover numbers (TONs) for the aerobic oxidation of
isobenzofuran catalyzed by CZJ-22-Cu
2.4. Synthesis of Zn-CZJ-22
Isobenzofuran (120 mmol), NHPI (1.2 mmol), and CZJ-22-Cu
(
1.5 lmol) in acetonitrile (150 mL) were stirred at 25 °C under an
The preparation procedure for Zn-CZJ-22 was the same as that
atmosphere of oxygen (balloon) for 10 h. The solid catalyst was
recovered by centrifugation and washed three times with acetoni-
trile. Aliquots were regularly taken out for GC analysis to deter-
mine the conversion of isobenzofuran, the yield of phthalide, and
the turnover number (TON), which refers to mol of product per
mol of catalyst.
for CZJ-22, except that Zn-H
Cu-H OCPP. Yield: 75% (based on Zn-H
Zn (%): C, 40.32; H, 6.40; N, 8.50; Zn, 2.64; Y,
.19. Found: C, 39.46; H, 6.19; N, 8.51; Zn, 2.38; Y, 7.09. FT-IR
8
OCPP was used instead of
8
8
OCPP). Anal. calcd. for
C
7
(
83 157 15 54 2
H N O Y
ꢀ1
KBr pellet, m/cm ): 1697(m), 1605(s), 1547(s), 1433(s), 1374(s),

W.-L. He et al. / Journal of Catalysis 358 (2018) 43–49
45
2
.9. A typical procedure for substrate adsorption experiments
the bulk solution using GC in the presence of triphenylmethane
as an internal standard and compared with the authentic sample
analyzed under the same conditions.
A sample of CZJ-22-Cu (5 mg) was treated under vacuum at
5
0 °C for 12 h. The activated sample was subsequently immersed
in isobenzofuran (0.25 mL) at room temperature for 12 h and cen-
trifuged. The recovered solid was thoroughly washed with ethyl
ether to remove the surface-adsorbed molecules, which was subse-
2
.10. A typical procedure for studying the chemical species adsorbed
inside the pores of CZJ-22-Cu during aerobic oxidation of
isobenzofuran
3
quently submerged in CH CN under stirring at room temperature
for 18 h. The mixture was determined by analyzing an aliquot of
Isobenzofuran (0.3 mmol), CZJ-22-Cu (0.6
lmol) and NHPI
(
0.015 mmol) in acetonitrile (2 mL) were stirred at 25 °C under
an atmosphere of oxygen (balloon) for 0.5 h. The solid was subse-
quently collected by centrifugation and thoroughly washed with
Et O. The solid was immersed in acetonitrile for 12 h and cen-
2
trifuged. The supernatant liquid was subjected to GC analysis and
compared with the authentic samples analyzed under the same
conditions.
3
. Results and discussion
II
CZJ-22 was synthesized by heating Cu -H
8
OCPP and yttrium
(
III) nitrate in a mixture of DMF and nitric acid at 80 °C for 1 week.
CZJ-22 crystallizes in the orthorhombic Cmca space group, consist-
III
ing of one Cu-OCPP and two Y ions per formula unit. To balance
+
3 2 2
the anionic charge of the framework of CZJ-22, [(CH ) NH ]
cations were encapsulated inside the anionic pores. CZJ-22 is a
D noninterpenetrated anionic framework, building from four-
connected [Y(COO) (H O) ] nodes and octatopic Cu-OCPP metallo-
3
4
2
2
porphyrin linkers (Fig. 1a). The directional connections between
these subunits resulted in nanocages with pore sizes of about 1.4
+
and 2.6 nm to accommodate [(CH
molecules (Fig. 1b).
3
)
2
NH
2
]
cations and solvent
Since the pore window size (1.4 nm) in CZJ-22 is very large, the
+
endogenous [(CH
3
)
2
NH
2
]
cations are readily exchanged with
exogenous redox-active cations to develop ISACs. Treatment of
CZJ-22 with a DMF solution of copper (II) perchlorate at 50 °C for
five days resulted in a new MOF material, CZJ-22-Cu, in which
+
the extra-framework [(CH
3
)
2
NH
2
]
cations were almost fully
exchanged for copper (II) ions, as confirmed by ICP-MS, EDX spec-
troscopy, and elemental analysis. In the crystal structure of CZJ-22-
Cu, ISA copper (II) is suspended in the anionic pores, which are
weakly coordinated to two carboxyl oxygen atoms of Cu-OCPP
with CuAO distances of 2.28 and 2.44 Å (Fig. 1c). The long CuAO
bonds indicate that the coordination field of ISA copper is very
Fig. 1. (a) Ball-and-stick and polyhedron representations of the anionic framework
of CZJ-22 viewed down the b axis. (b) A fragment structure of CZJ-22, highlighting
the substrate-accessible nanocage. Color scheme: Y, green polyhedra; Cu, cyan; O,
red; N, blue; C, deep gray; H, light gray. (c) Top view of the nanocage in CZJ-22-Cu
II
(
the top porphyrin was omitted for clarity), highlighting the porphyrin copper (cyan
Fig. 2. Cu 2p XPS for Cu(ClO
4
2
) , CZJ-22, Zn-CZJ-22-Cu, and CZJ-22-Cu in the solid
balls) and suspended ISA copper (violet balls) on the pore surfaces.
state at room temperature.

4
6
W.-L. He et al. / Journal of Catalysis 358 (2018) 43–49
weak. Accordingly, the surface free energy of copper species would
be very high for activation of reactant molecules in catalysis. The
permanent porosity of CZJ-22 and CZJ-22-Cu has been examined
significantly negatively shifted. These results indicate that the
coordination field of ISA copper is significantly affected by the spe-
cial coordination environment. Indeed, the differences of the bind-
ing energies for porphyrin and ISA copper in CZJ-22-Cu are
negligible, with the Cu2p3/2 signal centered at 934.75 eV. The
solid-state EPR spectrum of CZJ-22 showed a typical monomer
2
by N adsorption experiments at 77 K. After the as-synthesized
samples were heated at 50 °C under vacuum for 12 h, the activated
3
solid samples of CZJ-22 and CZJ-22-Cu took up 452 and 545 cm
ꢀ1
II
g
N
2
at 77 K and 1 bar, resulting in BET surface areas of 242.4
m /g and 236.8 m /g, respectively (Figs. S16–S19 in the Supporting
Information).
Cu -porphyrin signal (g = 2.0664), which is identical to that of
2
2
Cu-Me
the EPR signals for ISA Cu in Zn-CZJ-22-Cu are significantly differ-
ent from those of Cu(ClO (g = 2.16676) in acetonitrile, indicating
8
OCPP (g = 2.0666) in acetonitrile (Fig. S24) [35]. However,
II
XPS of copper (II) perchlorate showed that the binding energies
4 2
)
II
II
for Cu 2p3/2 and 2p1/2 are 935.98 and 955.68 eV, respectively [34].
that the coordination field of ISA Cu is significantly tuned by the
anionic pore microenvironment. As expected, CZJ-22-Cu, consist-
II
In contrast, the signals for porphyrin Cu 2p3/2 and 2p1/2 in CZJ-22
II
are negatively shifted to 934.8 and 948.08 eV, respectively
ing of both Cu -porphryin and ISA copper(II) species, exhibits
(Fig. 2). It is interesting that the binding energies for Cu2p3/2 and
merged signals in the EPR spectrum.
2
p
1/2 in Zn-CZJ-22-Cu, consisting of suspended ISA copper(II),
Oxidation of ethers to esters offers straightforward access to
high-value chemicals from less expensive raw materials, which is
important and fundamental for the synthesis of natural products
and bioactive molecules [36]. The use of molecular oxygen as an
oxidant represents the most environmentally friendly pathway
which are centered at 935.13 and 951.08 eV, respectively, are also
[
37]. The practical barriers are the limited efficiency, low selectiv-
ity, limited scope, and extreme catalytic conditions. It is remark-
able that CZJ-22-Cu is highly active in aerobic -oxidation of
a
ethers to esters. As shown in Fig. 3, the heterogeneous catalyst
CZJ-22-Cu afforded almost complete conversion of isobenzofuran
(
91%) for 0.5 h in the presence of N-hydroxyphthalimide (NHPI)
at room temperature (25 °C) under 1 atm O atmosphere. The
2
II
heterogeneous catalyst CZJ-22, consisting of Cu -porphyrins,
exhibited much lower activity, with 9% isobenzofuran conversion,
whereas the conversion is less than 1% catalyzed by homogeneous
copper perchlorate under identical conditions. In other word, CZJ-
2
2-Cu is 10 and 100 times more active than CZJ-22 and copper per-
chlorate, respectively. CZJ-22-Cu is also much superior to a simple
mixture of its constituents of Cu-Me OCPP and Cu(ClO (3.8%
conversion) under identical conditions.
8
4 2
)
We further studied the catalytic properties of Zn-CZJ-22-Cu,
II
consisting of ISA Cu only. As shown in Fig. 3, the isobenzofuran
Fig. 3. Aerobic oxidation of isobenzofuran catalyzed by CZJ-22, CZJ-22-Cu, Zn-CZJ-
conversion is 36% after 0.5 h under identical conditions. These
2
2-Cu, Cu(ClO
4 2
)
, and a mixture of Cu-Me
8
OCPP and Cu(ClO
)
4 2
at room temperature
II
results indicate that the suspended ISA Cu should be the main
and atmosphere pressure.
active site in the aerobic oxidation. Because the catalytic results
Fig. 4. Illustration of the sorption ability of CZJ-22-Cu as probed by GC.

W.-L. He et al. / Journal of Catalysis 358 (2018) 43–49
47
of CZJ-22-Cu are not the simple additive effect of porphyrin copper
and ISA copper, the superior catalytic efficiency should be the
result of synergistic work between two kinds of redox-active
copper sites. However, even though metalloporphyrins have been
recognized as a class of highly efficient biomimetic catalysts, their
catalytic properties are much inferior to those of ISACs in the
aerobic oxidation reaction [21]. The super catalytic efficiency of
CZJ-22-Cu should mainly be attributed to suspended ISAs with
very weak coordination field, highly active valence electrons, and
almost full availability of coordination sites.
To probe the accessibility of ISAs, we studied the solvent
sorption properties of CZJ-22-Cu by immersing the activated solid
samples in various solvents at room temperature for 12 h and cen-
trifuging. GC analysis of the supernates indicates that CZJ-22-Cu
takes up large numbers of different solvent molecules, such as
ethanol, acetone, acetonitrile, toluene, isobenzofuran, phthalide,
and isochroman (Fig. 4). We further studied the selective sorption
capability of CZJ-22-Cu for reactant and product molecules. After
an activated sample of CZJ-22-Cu was immersed in a mixture of
isobenzofuran and phthalide (1:1 M ratio) at room temperature
for 12 h, GC analysis of the supernate showed that CZJ-22-Cu
preferably takes up isobenzofuran (35 molecules per formula unit)
over phthalide (2 molecules per formula unit), suggesting that
CZJ-22-Cu exhibits preferred sorption capability for reactant over
product molecules. This property is very important for heteroge-
neous catalysts, which would enrich low-polar substrates and
release high-polar products to improve the catalytic efficiency
and reaction rate. To verify this hypothesis, we probed the included
reactant and product molecules inside the pores of CZJ-22-Cu dur-
ing the aerobic oxidation of isobenzofuran. After the aerobic oxida-
tion of isobenzofuran for 0.5 h, the catalysis was deliberately
interrupted by centrifugation. The collected solid was immersed
in acetonitrile to release the included guest molecules. GC analysis
showed that there are about 3.5 isobenzofuran reactant molecules
inside the pores of CZJ-22-Cu per formula unit, whereas the encap-
sulated phthalide product is negligible, even though the conversion
of isobenzofuran has been 91%.
Compared with homogeneous catalysts and SACs immobilized
on solid surfaces, the selective accumulation property of
CZJ-22-Cu is unique, which is one of the important factors illustrat-
ing the superior catalytic properties of CZJ-22-Cu in the aerobic
oxidation. When additional isobenzofuran reactant (equal to the
original) was added to the reaction mixture without separation
of phthalide product after catalysis, the additional isobenzofuran
can be fully oxidized for 1 h under the identical conditions.
Isobenzofuran can also be fully oxidized for 1 h even in the pres-
ence of 10-fold phthalide. These results confirmed the distinctively
selective accumulation of CZJ-22-Cu toward reactant over product
molecules. The remarkable catalytic properties of CZJ-22-Cu fur-
ther prompted us to study the kinetic process by using a large
amount of substrate. As shown in Fig. 5, the turnover number
(
TON) reached 77,100 with an average turnover frequency (TOF)
ꢀ1
of 7710 h for 10 h with persistent high catalytic efficiency.
To check the stability of CZJ-22-Cu, we studied the leaching and
recycling experiments. When the filtrate of the reaction mixture
after catalysis was used instead of CZJ-22-Cu under the identical
conditions, the oxidation of additional isobenzofuran is negligible.
ICP-MS analysis indicates that copper species was not leached in
the supernate. These results demonstrate the heterogeneous
Fig. 5. Kinetic process for the aerobic oxidation of isobenzofuran catalyzed by CZJ-
2
2-Cu at room temperature and atmosphere pressure.
Fig. 6. Catalytic aerobic oxidation of isobenzofuran by CZJ-22-Cu at different runs. The byproducts are phthalaldehyde and hemiacetal (Fig. S30). Color scheme: blue column,
conversion of isobenzofuran; red column, selectivity of phthalide.

4
8
W.-L. He et al. / Journal of Catalysis 358 (2018) 43–49
nature of the catalyst. CZJ-22-Cu can be recovered simply by
centrifugation and reused in the successive run for eight cycles
with almost retained high catalytic efficiency (Fig. 6). The PXRD
pattern of the recovered solid is almost identical to that of
the as-synthesized one, which proved the structural integrity of
CZJ-22-Cu after catalysis (Fig. S10).
of CuII species. It is interesting to note that the CuII species in CZJ-
22-Cu is much more easily activated (room temperature) than cop-
per perchlorate (50 °C) and porphyrin copper, as revealed by EPR
spectroscopy (Figs. S27–S29), suggesting that the Cu species in
CZJ-22-Cu is highly active. According to the free radical pathway,
II
II
I
the Cu species is first reduced by NHPI to form Cu , which would
I
To evaluate the applicability of the ISAC system in aerobic oxi-
bind with dioxygen to form the Cu –O
complex is then reduced by H derived from NHPI to result in a
hydroperoxo intermediate, Cu AOAOH, which would be further
reduced by another H to lead to heterolytic cleavage of the OAO
bond, forming an oxo–metal intermediate, Cu @O. Cu @O is
highly reactive, which would insert the oxygen atom into the sp3
CAH bond to realize aerobic oxidation [42]. It is very clear that
the present catalyst system is similar to the synergistic work
between multiple active sites of enzymes [43]. Compared with
2
species (Scheme 1). The
Å
dation, we investigated the catalytic
a-oxidation of isochroman
II
and its derivatives. It is interesting that isochroman was almost
fully oxidized, with 92% isochromanone selectivity for 2.5 h under
Å
III
III
1
atm O
2
atmosphere at room temperature (Fig. 7). CZJ-22-Cu is
also highly efficient and selective for the aerobic oxidation of a
range of aromatic ethers to afford valuable esters with high con-
versions and significant functional group tolerance under the iden-
tical conditions. These results demonstrate that CZJ-22-Cu is a
super catalyst for the production of synthetically and biologically
valued isochromanones from readily available cheap raw materials
with a broad substrate scope and functional group tolerance [37].
It has been suggested that such catalytic oxidation reactions
often involved reactive radical and oxo–metal intermediates [38].
We first examined the radical intermediates in aerobic oxidation.
When free radical scavengers such as hydroquinone and iodine
were added to the reaction mixture, the conversion of isobenzofu-
ran was drastically decreased to 3.1 and 0.9%, respectively. We fur-
ther employed EPR spectroscopy to monitor the radical
intermediates. When CZJ-22-Cu and NHPI were mixed in acetoni-
Å
enzymatic catalysis, H acts as a combined proton and electron
donor that is similar to reductases in the biological system,
trile under N
nate displayed
2
at room temperature, the EPR spectrum of the super-
triplet hyperfine splitting signal for the
a
N
phthalimido-N-oxyl (PINO) radical with g = 2.0087 and A = 4.72
G (Fig. S25) [39,40]. When a diamagnetic spin trap agent, 5,5-
dimethyl-1-pyrroline-N-oxide (DMPO), was added to the reaction
mixture during catalysis. The EPR spectrum of the supernate exhi-
N
bits a four-line hyperfine pattern with the coupling constants of a
H
=
12.9 G and a
b
= 13.7 G with g = 2.0085, which is the characteris-
Å–
tic signal for the spin adduct resulting from addition of O
DMPO (Fig. S26) [41].
2
to
It is known that hydrogen is easily abstracted from NHPI to
form PINO and hydrogen radicals during catalytic aerobic oxida-
tion [39,40]. The hydrogen radical is highly active for the reduction
Scheme 1. Proposed catalytic cycles for the aerobic oxidation of organic substrates
by CZJ-22-Cu in the presence of the co-catalyst NHPI.
Fig. 7. Aerobic oxidation of aromatic ethers catalyzed by CZJ-22-Cu at room temperature and atmosphere pressure. The numbers in the parenthesis represent the reaction
time (h). Color scheme: green column, conversion of ether; red column, selectivity of ester.

W.-L. He et al. / Journal of Catalysis 358 (2018) 43–49
49
whereas ISA CuII in CZJ-22-Cu performs as a redox-active center,
corresponding to the prosthetic groups in enzymes. Additionally,
PINO would abstract hydrogen atoms from organic substrates to
initiate aerobic oxidation in a radical propagation mechanism
[4] G. Kyriakou, M.B. Boucher, A.D. Jewell, E.A. Lewis, T.J. Lawton, A.E. Baber, H.L.
Tierney, M. Flytzani-Stephanopoulos, E.C.H. Sykes, Science 335 (2012) 1209.
[
5] G. Vile, D. Albani, M. Nachtegaal, Z. Chen, D. Dontsova, M. Antonietti, N. Lopez,
J. Perez-Ramirez, Angew. Chem. Int. Ed. 54 (2015) 11265.
[6] P. Liu, Y. Zhao, R. Qin, S. Mo, G. Chen, L. Gu, D.M. Chevrier, P. Zhang, Q. Guo, D.
Zang, B. Wu, G. Fu, N. Zheng, Science 352 (2016) 797.
[
39,40]. In other words, there are two parallel pathways in the cat-
[
7] J. Jones, H. Xiong, A.T. DeLaRiva, E.J. Peterson, H. Pham, S.R. Challa, G. Qi, S. Oh,
M.H. Wiebenga, X. Hernández, Y. Wang, A.K. Datye, Science 353 (2016) 150.
alytic aerobic oxidation, in which NHPI would initiate aerobic oxi-
dation by ISAs to form PINO, whereas PINO can abstract hydrogen
from organic substrates to initiate radical aerobic oxidation. The
synergistic work between these parallel pathways would signifi-
cantly improve the catalytic efficiency and the reaction rate of
the aerobic oxidation reaction. Compared with enzymes, the cou-
pled catalyst system is more economical in terms of oxygen usage,
[8] S. Yang, J. Kim, Y.J. Tak, A. Soon, H. Lee, Angew. Chem. Int. Ed. 55 (2016) 2058.
[
9] M. Yang, S. Li, Y. Wang, J.A. Herron, Y. Xu, L.F. Allard, S. Lee, J. Huang, M.
Mavrikakis, M. Flytzani-Stephanopoulos, Science 346 (2014) 1498.
[
[
[
10] K. Ding, A. Gulec, A.M. Johnson, N.M. Schweitzer, G.D. Stucky, L.D. Marks, P.C.
Stair, Science 350 (2015) 189.
11] P. Hu, Z. Huang, Z. Amghouz, M. Makkee, F. Xu, F. Kapteijn, A. Dikhtiarenko, Y.
Chen, X. Gu, X. Tang, Angew. Chem. Int. Ed. 53 (2014) 3418.
12] H. Yan, H. Cheng, H. Yi, Y. Lin, T. Yao, C. Wang, J. Li, S. Wei, J. Lu, J. Am. Chem.
Soc. 137 (2015) 10484.
[13] H.C. Zhou, J.R. Long, O.M. Yaghi, Metal-organic frameworks special issue,
Chem. Rev. 112 (2012) 673.
14] H.C. Zhou, S. Kitagawa, Themed issues on metal-organic frameworks, Chem.
Soc. Rev. 43 (2014) 5415.
15] G. Maurin, C. Serre, A. Cooper, G. Férey, Metal-organic frameworks and porous
polymers – current and future challenges, Chem. Soc. Rev. 46 (2017) 3104.
16] M. Yoon, R. Srirambalaji, K. Kim, Chem. Rev. 112 (2012) 1196.
17] T. Zhang, W. Lin, Chem. Soc. Rev. 43 (2014) 5982.
18] J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang, C.Y. Su, Chem. Soc. Rev. 43 (2014) 6011.
[19] W.Y. Gao, M. Chrzanowski, S. Ma, Chem. Soc. Rev. 43 (2014) 5841.
20] M. Ranocchiari, C. Lothschutz, D. Grolimund, J.A. van Bokhoven, Proc. R. Soc. A
468 (2012) 1985.
21] M. Zhao, S. Ou, C.D. Wu, Acc. Chem. Res. 47 (2014) 1199.
22] B. Chen, S. Xiang, G. Qian, Acc. Chem. Res. 43 (2010) 1115.
23] W.P. Lustig, S. Mukherjee, N.D. Rudd, A.V. Desai, J. Li, S.K. Ghosh, Chem. Soc.
Rev. 46 (2017) 3242.
[24] A. Karmakar, A.V. Desai, S.K. Ghosh, Coord. Chem. Rev. 307 (2016) 313.
25] Q.L. Zhu, Q. Xu, Chem. Soc. Rev. 43 (2014) 5468.
26] D.T. Genna, A.G. Wong-Foy, A.J. Matzger, M.S. Sanford, J. Am. Chem. Soc. 135
because both oxygen atoms of O
2
are participated in the oxidation
0
0
reaction (RCH
2
R + O
2
? RCOR + H O), whereas one oxygen of
2
0
+
O
2
is sacrificed in enzymatic catalysis (RCH
2
R + 2O
2
+ 4H + 4e ?
[
0
RCOR + 3H
2
O).
[
4
. Conclusions
In summary, to make use of eight-branched metalloporphyrin
[
[
[
II
III
Cu -OCPP connecting with four-connected Y ions, we successfully
[
constructed an anionic porphyrinic framework, CZJ-22, in which
+
[
[
[
the extra-framework [(CH
3
)
2
NH
2
] cations inside the anionic pores
are easily exchanged to synthesize porous ISACs. The single-crystal
structure of CZJ-22-Cu shows that the encapsulated ISA copper is
suspended on the pore surfaces and almost fully available at the
coordination sites, which results in the largest number of dangling
bonds and empty d atomic orbitals to maximize the surface free
energy for activation of molecular oxygen. CZJ-22-Cu demonstrates
exceptionally high catalytic efficiency in aerobic oxidation of
ethers to esters at room temperature and atmosphere pressure.
The operational simplicity and environmental friendliness of this
methodology afford a new pathway for the development of porous
ISACs as promising alternatives to traditional catalysts, and thus a
way to tackle the challenge issue of activation of inert reactants.
[
[
(
2013) 10586.
[27] R.J. Comito, K.J. Fritzsching, B.J. Sundell, K. Schmidt-Rohr, M. Dinc a˘ , J. Am.
Chem. Soc. 138 (2016) 10232.
28] W. Fudickar, J. Zimmermann, L. Ruhlmann, J. Schneider, B. Röder, U. Siggel, J.H.
Fuhrhop, J. Am. Chem. Soc. 121 (1999) 9539.
[29] M. Skupin, G. Li, W. Fudickar, J. Zimmermann, B. Röder, J.H. Fuhrhop, J. Am.
Chem. Soc. 123 (2001) 3454.
30] CrysAlisPro, version 1.171.33.56, Oxford Diffraction Ltd., Abingdon, UK, 2010.
31] G.M. Sheldrick, Program for Structure Refinement. University of Göttingen,
1997.
32] A.L. Spek, Single-crystal structure validation with the program PLATON, J. Appl.
Crystallogr. 36 (2003) 7.
33] A.L. Spek, PLATON, A Multipurpose Crystallographic Tool, Utrecht University,
Utrecht, The Netherlands, 2008.
34] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg (Eds.),
Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation
Physical Electronics Division, Minnesota, 1979.
[
[
[
[
[
[
Acknowledgment
This work was financially supported by the National Natural
Science Foundation of China (Grants 21373180 and 21525312).
[
[
35] D. Kivelson, R. Neiman, N. Robert, J. Chem. Phys. 35 (1961) 149.
36] T. Janecki, Natural Lactones and Lactams: Synthesis, Occurrence and Biological
Activity, Wiley-VCH, Weinheim, 2013.
Appendix A. Supplementary material
[
37] J.E. Backvall, Modern Oxidation Methods, 2nd ed., Wiley-VCH, Weinheim,
2010.
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jcat.2017.11.022.
[38] V.L. Pecoraro, M.J. Baldwin, A. Gelasco, Chem. Rev. 94 (1994) 807.
[39] Y. Ishii, S. Sakaguchi, T. Iwahama, Adv. Synth. Catal. 343 (2001) 393.
[40] F.C. Recupero, F. Punta, Chem. Rev. 107 (2007) 3800.
References
[41] N.A. Belikova, Y.Y. Tyurina, G. Borisenko, V. Tyurin, A.K.S. Arias, N. Yanamala, P.
G. Furtmüller, J. Klein-Seetharaman, C. Obinger, V.E. Kagan, J. Am. Chem. Soc.
1
31 (2009) 11288.
[1] X.F. Yang, A. Wang, B. Qiao, J. Li, J. Liu, T. Zhang, Acc. Chem. Res. 46 (2013) 1740.
[2] W. Zhang, W. Zheng, Adv. Funct. Mater. 26 (2016) 2988.
[3] B. Qiao, A. Wang, X. Yang, L.F. Allard, Z. Jiang, Y. Cui, J. Liu, J. Li, T. Zhang, Nat.
Chem. 3 (2011) 634.
[
[
42] W. Keown, J.B. Gary, T.D.P. Stack, J. Biol. Inorg. Chem. 22 (2017) 289.
43] D. Hamdane, H. Zhang, P. Hollenberg, Photosynth. Res. 98 (2008) 657.