Structural transformation of porous polyoxometalate frameworks and highly efficient biomimetic aerobic oxidation of aliphatic alcohols

Article abstract of DOI:10.1021/acscatal.7b01985

Due to their inherent inert nature, it is difficult to oxidize unactivated aliphatic alcohols with molecular oxygen under mild conditions. Inspired by enzymatic catalysis, numerous biomimetic systems have been therefore established. However, low catalytic efficiency and easy auto-oxidative deactivation nature remain the problematic issues. To meet these challenges, we report herein a 2D porous polyoxometalate (POM) framework Gd₄(H₂O)₂₆[WZn{Cu(H₂O)}₂(ZnW₉O₃₄)₂]·24H₂O (CZJ-11) and a 3D porous POM framework Gd₄(H₂O)₂₄[WZn{Cu(H₂O)}₂(ZnW₉O₃₄)₂]·11H₂O (CZJ-12) transferred from CZJ-11 by partial dehydration, consisting of scaffolded redox-active Cu(II) sites in the sandwich-type POM cluster [WZn{Cu-(H₂O)}₂(ZnW₉O₃₄)₂]^12- (abbreviated as {Zn₃Cu₂W₁₉}). To mimic the catalytic mechanism of enzymes, N-hydroxyphthalimide (NHPI) and tetramethylammonium bromide (TMAB) were introduced as cocatalysts, which performed as electron donor and electron-transfer mediator, respectively. The coupled catalyst systems demonstrate analogue properties with oxygenase enzymes in the aerobic oxidation of aliphatic alcohols under mild conditions. Compared with molecular POM counterpart and metalloporphyrins, the catalytic efficiency of these POM frameworks is predominant in aerobic oxidation of unactivated aliphatic alcohols by imitating the active sites and the catalytic mechanism of enzymes. Compared with metal-organic coordination complexes, such as metalloporphyrins, the pure inorganic frameworks offer significant superiority of robustness to auto-oxidation and simple recovery for recycling with retained high catalytic efficiency.

Full text of DOI:10.1021/acscatal.7b01985

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Article
Structural Transformation of Porous Polyoxometalate Frameworks and
Highly Efficient Biomimetic Aerobic Oxidation of Aliphatic Alcohols
Min Zhao, Xian-Wei Zhang, and Chuan-De Wu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01985 • Publication Date (Web): 21 Aug 2017
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Structural Transformation of Porous Polyoxometalate Frameworks
and Highly Efficient Biomimetic Aerobic Oxidation of Aliphatic Alco-
hols
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Min Zhao, Xian-Wei Zhang and Chuan-De Wu*
State Key Laboratory of Silicon Materials, Center for Chemistry of High-Performance & Novel Materials, Depart-
ment of Chemistry, Zhejiang University, Hangzhou, 310027, P. R. China
ABSTRACT: Due to their inherent inert nature, it is difficult to oxidize unactivated aliphatic alcohols with molecular oxy-
gen under mild conditions. Inspired by enzymatic catalysis, numerous biomimetic systems have been therefore estab-
lished. However, low catalytic efficiency and easy auto-oxidative deactivation nature remain the problematic issues. To
meet
these
challenges,
we
report
herein
a
2D
porous
a
polyoxometalate
3D porous
(POM)
POM
framework
framework
Gd4(H2O)26[WZn{Cu(H2O)}2(ZnW9O34)2]·24H2O
(CZJ-11),
and
Gd4(H2O)24[WZn{Cu(H2O)}2(ZnW9O34)2]·11H2O (CZJ-12) transferred from CZJ-11 by partial dehydration, consisting of scaf-
folded redox active Cu(II) sites in the sandwich-type POM cluster [WZn{Cu(H2O)}2(ZnW9O34)2]12- (abbreviated as
{Zn3Cu2W19}). To mimic the catalytic mechanism of enzymes, N-hydroxyphthalimide (NHPI) and tetramethylammonium
bromide (TMAB) were introduced as co-catalysts, which performed as electron donor and electron transfer mediator,
respectively. The coupled catalyst systems demonstrate analogue properties with oxygenase enzymes in the aerobic oxida-
tion of aliphatic alcohols under mild conditions. Compared with molecular POM counterpart and metalloporphyrins, the
catalytic efficiency of these POM frameworks is predominant in aerobic oxidation of unactivated aliphatic alcohols by
imitating the active sites and the catalytic mechanism of enzymes. Compared with metal-organic coordination complexes,
such as metalloporphyrins, the pure inorganic frameworks offer significant superiority of robustness to auto-oxidation
and simple recovery for recycling with retained high catalytic efficiency.
It has been well known that aerobic oxidation of inert
organic substrates is easily realized by enzymes under
INTRODUCTION
Oxidation of aliphatic alcohols into carbonyl and carbox-
ylic compounds is of considerable importance because
they are the feed-stocks in the fine chemical and pharma-
ceutical industry.1 Due to their low reactivity, these chem-
ical transformations are traditionally carried out using
stoichiometric amounts of oxidants, such as CrO3, MnO2
and hypervalent iodine, which are expensive and envi-
ronmental hazard.2 Even though many catalyst systems
have been developed for the oxidation of unactivated ali-
phatic alcohols, however, sacrificial oxidants (e.g. hydro-
gen peroxide, alkyl hydroperoxides or iodosylbenzene),
additives (e.g. bases, acids or aldehydes), or severe condi-
tions (e.g. high pressure and temperature) have limited
their practical applications.3 It is evident that the chal-
lenge is to design and cultivate effective, sustainable and
environmentally friendly alcohol oxidation catalyst plat-
forms with molecular oxygen as oxidant.4 Therefore, nu-
merous endeavors have been inspired associating with
this goal of molecular oxygen activation; however, only a
handful of noble metals, such as Pd, Ru, Pt, Rh and Au,
demonstrate high efficiency on the aerobic alcohol oxida-
tion reaction.5 A limitation of these systems is the high
catalyst loadings to result in low turnover numbers
(TONs), which is not suitable for the production of less
expensive and high volume fine chemicals.
mild conditions, and copper site has been recognized as
one of the important redox centers in many important
biological processes.6 Numerous synthetic Cu complexes
have been therefore established as biomimetic models of
enzymes, which demonstrate high catalytic efficiency in a
number of oxidation reactions.7 However, because cop-
per-catalyzed oxidation involves the formation of metal-
oxide intermediates, Cu-organic catalysts are suffered
from easy auto-oxidation and deactivation. In respect of
susceptible to oxidative degradation, adoption of pure
and stable inorganic-copper coordination complexes as
catalysts represents one of the ideal pathways to over-
come the easy auto-oxidation issue of copper-organic
catalysts. Polyoxometalates (POMs), as a class of metal-
oxide nanoclusters with unmatched molecular structural
diversity and versatile functions, exhibit high resistance
toward hydrolysis and oxidative degradation, which offer
great potential as stable inorganic scaffolds to immobilize
copper sites for biomimetic catalysis.8 For examples, the
sandwich-type POMs, [WM{M'(H2O)}2(MW9O34)2]n- (M,
M' = MnII, CoII, CuII and ZnII, etc.; abbreviated as
{M3M'2W19}), consist of two B-[α-MW9O34] truncated Keg-
gin-type fragments linked by four coplanar close-packed
transition metal atoms in the central rings, in which the
ring belt metal sites are systematically tunable to modify
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Page 2 of 9
the catalytic properties.9 It has been demonstrated that
the redox active transition metal ions, such as FeIII, RuIII,
MnII, RhIII, PdII and PtII, incorporated POMs exhibit ana-
logue properties with oxygenase enzymes.10
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Scaffolding redox Cu(II) sites in the ring belts of the
sandwich-type POMs might result in stable and efficient
catalysts for biomimetic aerobic oxidation of aliphatic
alcohols under mild conditions. Moreover, incorporation
of Cu-POMs into metal-oxide frameworks might facilitate
synergistic effect to improve the catalytic efficiency by
confining multiple redox active metal centers in the high-
ly ordered pore surfaces, and improve the stability for
long term recycling usage. The above considerations
prompted us to use the sandwich-type POMs, {M3M'2W19},
as redox active inorganic linkers to connect with metal
centers for the self-assembly of POM frameworks. Herein,
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we
report
a
2D
porous
POM
framework,
Gd4(H2O)26[WZn{Cu(H2O)}2(ZnW9O34)2]·24H2O (CZJ-11),
building from Gd3+ metal nodes linking up
[WZn{Cu(H2O)}2(ZnW9O34)2]12-
(abbreviated
as
{Zn3Cu2W19}) clusters consisting of redox active Cu(II)
sites. CZJ-11 can be simply transformed into a stable 3D
porous
POM
framework
Gd4(H2O)24[WZn{Cu(H2O)}2(ZnW9O34)2]·11H2O (CZJ-12)
by partial dehydration. Compared with molecular POM
counterpart and metalloporphyrins, these POM frame-
works demonstrate high catalytic efficiency on biomimet-
ic activation of molecular dioxygen in aerobic oxidation of
unactivated aliphatic alcohols. Compared with organic
metalloporphyrins, the pure inorganic frameworks offer
significant advantages of robustness to auto-oxidation
and simple recovery for recycling.
RESULTS AND DISCUSSION
Green needle crystals of the POM framework CZJ-11 were
synthesized by heating a mixture of {Zn3Cu2W19}, copper
nitrate and gadolinium chloride in acidified water solvent
at 50 °C for one week. It should be noted that additional
copper cations played vital roles for the formation of crys-
talline metal-oxide framework CZJ-11. Due to the highly
oxophilic nature of Gd3+ ions, {Zn3Cu2W19} and Gd3+ ions
would quickly combine together to form amorphous solid
in the absence of Cu2+ ions. Buffered with Lewis acid cop-
per ions, the reaction rate between {Zn3Cu2W19} anions
and Gd3+ cations is significantly reduced due to the com-
peted coordination between Cu2+ and Gd3+ cations to
{Zn3Cu2W19} anions. The formula of as-synthesized CZJ-11
was calculated on the basis of single crystal structure, and
characterized by thermogravimetric analysis (TGA) and
FT-IR (Figures S1 and S2 in supporting information). The
phase purity of the bulk material was confirmed by pow-
der X-ray diffraction (PXRD).
Figure 1. A) Ball-and-stick and polyhedral representations of
the polyoxoanion [WZn{Cu(H2O)}2(ZnW9O34)2]12- and its
coordination modes, and the coordination environments of
Gd3+ ions in CZJ-11. B) A view of the connections between
[WZn{Cu(H2O)}2(ZnW9O34)2]12- polyanions and Gd3+ ions in
the 2D lamellar network of CZJ-11, in which one polyanion is
highlighted in polyhedral model. C) Packing diagram of CZJ-
11 (the central lamellar network is highlighted as orange pol-
yhedra). Color scheme: Cu, cyan; W, green and yellow green;
Gd, purple; Zn, blue; O, red.
Keggin-type B-[α-ZnW9O34] fragments and a sandwiched
ring belt consisting of one W, one Zn and two Cu sites. It
is interesting to note that the chirality of {Zn3Cu2W19}
cluster was firstly observed in the crystal structure of CZJ-
11. Even though it has been suggested that {M3M'2W19}
clusters are chiral moieties in the literature, however,
because the M and W atoms in the ring belt are statisti-
Single crystal X-ray diffraction analysis revealed that
CZJ-11 crystallizes in the triclinic Pī space group, which
consists of one {Zn3Cu2W19} polyanion and four nine-
coordinated Gd3+ cations in the asymmetric unit (Figure 1).
The copper(II) cation disubstituted polyoxoanion
{Zn3Cu2W19} is of a dimer built from two tri-vacant
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ACS Catalysis
cally occupying two diagonal positions, the chirality of
{M3M'2W19} has never been clearly defined by crystallog-
1
2
raphy.11 As observed in the crystal structure of CZJ-11, the
specific positions of Zn and W atoms in the ring belt are
specifically defined, and thus to differentiate clearly the
enantiomers of {Zn3Cu2W19}. Without the interference of
distortion, the actual coordination environments of cop-
per(II) ions in the polyanions are also clearly defined for
comprehensively understanding the structure-function
relationship. In CZJ-11, each copper atom in the ring belt
is coordinating to one μ3-OZnWCu (Cu-O = 2.003-2.004 Å),
one μ3-OWWCu (Cu-O = 2.389-2.417 Å), one μ4-OWZnZnCu
(Cu-O = 2.001-2.014 Å), two μt-OW (Cu-O = 1.982-1.982 and
2.223-2.224 Å), and one water (Cu-O = 1.976-2.021 Å). It is
obvious that the copper atom comprises of highly disor-
dered octahedral coordination environment, which con-
sists of four strong Cu-O coordination bonds (1.982-2.021)
and two weak Cu-O bonds (2.223-2.417 Å). The Zn atom in
the central ring is also highly deviated the ideal octahe-
dral coordination environment with four equatorial Zn-O
distances of Zn-O = 1.919-2.015 Å and two axial Zn-O dis-
tances of 2.322-2.349 Å, which is rarely observed in metal-
organic coordination complexes. The unusual coordina-
tion spheres of these transition metal atoms in
{Zn3Cu2W19} suggest that the {CuO6} octahedra are flexi-
bly combined with two B-[α-ZnW9O34] fragments, which
are beneficial for molecular oxygen activation and subse-
quent substrate oxidation.
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Figure 2. PXRD patterns for calculated CZJ-11 based on single
crystal structure, as-synthesized CZJ-11, acetone-exchanged
o
CZJ-11, evaporated CZJ-11 by heating at 65 C under vacuum
for 12 h, transformed CZJ-12 from CZJ-11, and calculated CZJ-
12 based on single crystal structure, indicating the structural
transformation from 2D lamellar network of CZJ-11 to 3D
porous framework of CZJ-12.
As shown in Figure 2, the PXRD pattern of as-
synthesized CZJ-11 is basically identical to that of the cal-
culated one based on single crystal structure. Because
lattice water molecules in CZJ-11 are easily evaporated
during data collection, it is inevitable to result in partial
loss of its crystallinity with broadened diffraction peaks.
After CZJ-11 was guest-exchanged with dry acetone for 24
h (referred as acetone-exchanged CZJ-11) and further
In the crystal structure of CZJ-11, the {Zn3Cu2W19}
cluster acts as a nine-dentate ligand to coordinate with
nine GdIII cations through nine terminal oxygen atoms of
nine {WO6} octahedra (Figure 1a). According to the coor-
dination environments, there are three kinds of nine-
coordinated GdIII cations. The first one (Gd1) terminates
on the {Zn3Cu2W19} cluster (Gd-O = 2.476 Å) and further
coordinates with eight aqua ligands (Gd-O = 2.375-2.503
Å). The second one (Gd3) bridges two {Zn3Cu2W19} moie-
ties by coordinating to two oxygen atoms of two {WO6}
octahedra (Gd-O = 2.368-2.467 Å) and seven water mole-
cules (Gd-O = 2.407-2.603 Å). The remaining two GdIII
ions (Gd2 and Gd4) serve as three-connecting nodes to
combine with three {Zn3Cu2W19} moieties (Gd-O = 2.406-
2.577 Å) and terminated by six water molecules (Gd-O =
2.385-2.511 Å) for each Gd ion. The connections between
{Zn3Cu2W19} clusters and GdIII ions result in a 2D lamellar
network with micropore dimensions of about 7.8 × 10.4 Å2
(Figure 1b). Two neighboring redox active copper(II) sites
are located on the pore surfaces with average Cu…Cu dis-
tance of 7.85 Å. The layered networks are further con-
nected by extensive hydrogen bondings to form a 3D su-
pramolecular network (Figure 1c). TGA showed that a
weight loss of 13.9% occurred between 30 and 420 oC, cor-
responding to the loss of water ligands and guests (ex-
pected 14.5%; Figure S2). PLATON calculations indicate
that CZJ-11 consists of 27.5% (40.4%) void space to ac-
commodate water guests (water guests and ligands).12
o
heated at 65 C under vacuum for 12 h (referred as evap-
orated CZJ-11), PXRD patterns of the acetone-exchanged
and evaporated samples are different from that of the as-
synthesized CZJ-11. Moreover, the sharp diffraction peaks
cannot be restored to the original positions by immers-
ing the evaporated sample in water for 24 h (referred as
transformed CZJ-12). These results suggest that structur-
al transformations might occur upon the exchange and
removal of water guests and ligands. Single crystal X-ray
diffraction analysis revealed that the new crystalline
phase is a 3D porous metal-oxide framework CZJ-12,
formed from dehydration and rehydration of CZJ-11. CZJ-
12 crystallizes in the triclinic Pī space group, which con-
sists of one half {Zn3Cu2W19} polyanion and two Gd3+ cat-
ions in the asymmetric unit (Figure 3). Due to structural
distortion, the Zn and W atoms in the ring belt of
{Zn3Cu2W19} are statistically occupying two diagonal po-
sitions. Consequently, the chirality of {Zn3Cu2W19} can-
not be directly observed in the crystal structure of CZJ-12,
which is same to the literature structures.11 As a result,
the actual coordination environments of copper(II) and
zinc(II) ions in the ring belt of the polyanion cannot be
unambiguously defined in CZJ-12. The statistic Cu-O dis-
tances are ranging from 1.970-2.225 Å with statistic Zn-O
distances of 1.850-2.164 Å, which are significantly differ-
ent to those in CZJ-11. In the crystal structure of CZJ-12,
the {Zn3Cu2W19} cluster acts as a ten-dentate ligand to
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coordinate with ten GdIII cations (Figure 3a). According
atoms of three {WO6} octahedra (Gd-O = 2.504-2.511 Å)
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to the coordination environments, there are two kinds of
nine-coordinated GdIII cations. The first one (Gd1) serves
as a three-connecting node to coordinate with three
{Zn3Cu2W19} moieties (Gd-O = 2.475-2.535 Å) and termi-
nate by six water molecules (Gd-O = 2.363-2.518 Å).
Compared with the crystal structure of CZJ-11, Gd1 in
CZJ-12 is corresponding to Gd2 and Gd4 in CZJ-11 with
almost unchanged coordination environment (Figure 4).
The second one (Gd2) bridges two {Zn3Cu2W19} moieties
by coordinating to three oxygen
and six water molecules (Gd-O = 2.361-2.528 Å). Gd2 in
CZJ-12 is corresponding to Gd1 and Gd3 in CZJ-11. Compar-
ing the coordination environments of Gd ions in CZJ-11
and CZJ-12, one aqua ligand of Gd3 in CZJ-11 was replaced
by a μ3-OWWCu atom for Gd2 in CZJ-12. One aqua ligand of
the terminated Gd ion (Gd1) in CZJ-11 is substituted by an
oxygen atom from neighboring {Zn3Cu2W19} cluster for
Gd2 in CZJ-12. Substitution of water ligands with oxygen
atoms on {Zn3Cu2W19} for Gd ions resulted in an expanded
3D porous framework structure of CZJ-12, consisting of
almost unchanged micropore sizes of about 7.7 × 10.4 Å2
in dimensions (Figure 3). TGA showed that a weight loss
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of 13.3% occurred between 30 and 410 C, corresponding
to the loss of water ligands and guests (expected 13.6%;
Figure S3). PLATON calculations indicate that the void
space in CZJ-12 is slightly shrunk to 15.9% (31.8%) for wa-
ter guest molecules (water guests and ligands).12
Figure 4. Comparison of the connections between
[WZn{Cu(H2O)}2(ZnW9O34)2]12- and Gd3+ ions in CZJ-11 (A)
and CZJ-12 (B), in which the highlighted Gd ions (deep green
and deep purple balls) represent the ones with changed co-
ordination environments. Color scheme: Cu, cyan; W, green
and yellow green; Gd, purple; Zn, orange; O, red; ZnW, blue
polyhedra.
Figure 3. A) Ball-and-stick and polyhedral representations of
the polyoxoanion [WZn{Cu(H2O)}2(ZnW9O34)2]12- and its
coordination modes, and the coordination environments of
Gd3+ ions in CZJ-12. B) A view of the connections between
[WZn{Cu(H2O)}2(ZnW9O34)2]12- polyanions and Gd3+ ions in
the 3D framework of CZJ-12, in which one polyanion is high-
lighted in polyhedral model. C) The 3D porous structure of
CZJ-12. Color scheme: Cu, cyan balls; W, green balls and pol-
yhedra; ZnW, purple polyhedra and yellow green balls; Gd,
blue balls and polyhedra; Zn, orange; O, red.
The permanent porosity of CZJ-11 and CZJ-12 has
been examined by CO2 adsorption experiments at 273 K
(Figures S4 and S5). When the solid samples of CZJ-11 and
o
CZJ-12 were activated by heating at 65 C under vacuum
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phthalimido-N-oxyl radical (PINO), which would abstract
for 6 h, the activated solid samples take up 8.81 and 8.83
cm3 g-1 CO2 at 273 K and 1 bar, respectively. Compared
with those of traditional porous materials, these values
are relatively low, which should be ascribed to the very
large formula weights (6240-6470) and high densities
(4.181-4.653 g·cm−3) of these pure inorganic metal-oxide
frameworks. To check whether the inside pores are acces-
sible to small organic molecules, we studied the sorption
ability of activated CZJ-11 and CZJ-12. When the activated
samples of CZJ-11 and CZJ-12 were immersed in acetoni-
trile, ethanol and n-butanol for 12 h at room temperature,
GC-MS analysis suggests that their inside pores are acces-
sible to these small molecules. The uptakes of acetonitrile,
ethanol and n-butanol are of 8.9, 9.6 and 5.5 wt% for CZJ-
11, and 7.5, 5.2 and 1.9 wt% for CZJ-12, respectively. The
above results clearly indicate that the immobilized redox
active Cu(II) sites in the pore surfaces of CZJ-11 and CZJ-12
are readily accessible to small reactant molecules to
prompt their catalytic chemical transformations.
a hydrogen atom from organic substrate to initiate the
aerobic oxidation of alcohols in a radical propagation
mechanism.14 Therefore, the synergistic work between
these moieties in the coupled catalyst system is very simi-
lar to that of enzymes, which significantly improved the
catalytic efficiency in aerobic oxidation of aliphatic alco-
hols under mild conditions.
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Table 1. Aerobic oxidation of n-butanol for the formation
of n-butyric acida
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Yield
(%)b
Yield
(%)b
Entry Catalyst
Entry Catalyst
CZJ-11/NHPI
/TMAB
CZJ-11/NHPI
/TMAB
1
96
85
9
91c
84c
92
Direct oxidation of aliphatic alcohols into highly val-
uable carboxylic acids with molecular oxygen represents
one of the most elegant and environmentally friendly
pathways.13 To realize highly efficient catalysis on this
chemical transformation, we established a biomimetic
catalyst platform to mimic the catalytic mechanism of
enzymes, in which the immobilized copper(II) sites inside
the pore surfaces of inorganic framework CZJ-11 or CZJ-12
were used to mimic the roles of redox active sites in en-
zymes, and N-hydroxyphthalimide (NHPI) and tetrame-
thylammonium bromide (TMAB) as co-catalysts to mimic
the roles of co-enzymes.14 The catalytic properties of the
biomimetic catalyst platform were evaluated by aerobic
oxidation of aliphatic alcohols in acetonitrile at 50 °C un-
der 1 atm O2 atmosphere. As shown in Table 1 (entry 1),
the coupled catalyst system efficiently prompts aerobic
oxidation of n-butanol with 96% n-butyric acid yield and
high selectivity (>99%). When CZJ-12 was used as a cata-
lyst under the otherwise identical conditions, the yield of
n-butyric acid is of 85% (entry 2). Compared with that of
CZJ-11, the slightly decreased catalytic efficiency of CZJ-12
should be ascribed to the slight shrinkage of the pore
space which would reduce the diffusion rate of reactant
and product molecules. The catalytic efficiency of the
combined catalyst platform is significantly higher than
that of the single individuals such as CZJ-11, NHPI and
TMAB, and their simply combined mixtures (entries 3-8).
The superior aerobic oxidation efficiency of the combined
catalyst platform indicates that every constituent played
vital roles on the biomimetic catalysis. It has been known
that enzymatic oxidation of organic substrates involves in
a multiple active site system of molecular oxygen com-
plexation with redox active metal sites and oxygen reduc-
tion with co-enzyme centers.1,13 Compared with biocata-
lyst system, the copper ions in CZJ-11 and CZJ-12 perform
as redox active sites for O2 complexation, NHPI acts as an
electron donor to mimic the roles of reductases, and Br-
might act as an electron transfer mediator to improve the
electron transfer efficiency (Scheme 1).15 Additionally,
electron donation from NHPI results an intermediate of
CZJ-12/NHPI
/TMAB
CZJ-12/NHPI
/TMAB
2
3
4
5
6
7
8
10
{Zn3Cu2W19}/
NHPI/TMAB
CZJ-11
NHPI
TMAB
trace 11
trace 12
trace 13
Cu(ClO4)2/
NHPI/TMAB
61
Cu-TPP/
NHPI/TMAB
69
FeCl-TPP/
NHPI/TMAB
CZJ-11/NHPI 50
CZJ-11/TMAB 14
14
15
trace
25d
5e
{Zn3Cu2W19}/
NHPI/TMAB
Cu-TPP/
NHPI/TMAB
NHPI/TMAB trace 16
an-butanol (0.25 mmol), catalyst (0.001 mmol), NHPI (0.025
mmol) and TMAB (0.013 mmol) in acetonitrile (2 mL) were
o
stirred under an O2 atmosphere (balloon) at 50 C for 24 h.
The usage amounts of control catalysts are identical to those
b
of corresponding agents based on the effective centers. Y-
ield% was determined by GC-MS on a SE-54 column (Figures
S8-S17). cThe sixth cycle. dThe fourth cycle. eThe second cycle.
Scheme 1. Proposed catalytic cycles for the aerobic oxida-
tion of aliphatic alcohols by Cu-POM and NHPI
CZJ-11 and CZJ-12 are very stable, which can be easily
recovered by centrifugation, and reused in the successive
runs for six cycles with almost retained high catalytic effi-
ciency (Table 1, entries 9 and 10; Figures S6 and S7). A
PXRD pattern of the recovered solid after catalysis (re-
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reaction induces a serious loss of crystallinity in the sam-
ple of CZJ-11 because participation of the redox active Cu
sites in catalysis would significantly affect their local co-
ordination environments and partially lose the lattice
water guests in CZJ-11 (Figure 5). However, the structure
of CZJ-11 can be easily regenerated (referred as regenerat-
ed CZJ-11) when the recovered solid sample was immersed
in water for 24 h. Different to that of the 2D lamellar solid
CZJ-11, the structural integrity of CZJ-12 was maintained
after catalysis (referred as recovered CZJ-12) attributed to
the rigid 3D framework structure. After CZJ-11 or CZJ-12,
NHPI and TMAB in acetonitrile were stirred under an O2
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atmosphere at 50 C for 24 h, the mixture was filtered. n-
Butanol was added to the filtrate, which was reacted for
another 24 h under the otherwise identical conditions. GC
analysis showed that the conversion of n-butanol is iden-
tical to that of the background reaction catalyzed by
NHPI/TMAB (Table 1, entry 8). These results proved the
heterogeneous catalytic nature of CZJ-11 and CZJ-12.
Moreover, after the reaction proceeded for 12 h, the cata-
lytic reaction was interrupted, and the solid catalyst was
collected by centrifugation. GC-MS analysis showed that
the desorbed n-butanol and n-butyric acid from the re-
acted solids are of 8 and 23 mol for CZJ-11, and 5 and 8
mol for CZJ-12 per formula unit, respectively. These re-
sults proved that the catalytic aerobic oxidation did main-
ly occur inside the pore space of CZJ-11 and CZJ-12. To
make comparisons, we studied the catalytic properties of
molecular [WZn{Cu(H2O)}2(ZnW9O34)2]12-, copper(II) per-
chlorate, and traditional biomimetic molecular catalysts,
metalloporphyrins, CuII-TPP and FeIIICl-TPP (TPP =
tetraphenylporphyrin).16 As shown in Table 1 (entries 11-
14), the catalytic properties of these molecular catalysts
are inferior to those of CZJ-11 and CZJ-12. Compared to the
organic metalloporphyrin analogues, the superiority of
inorganic metal-oxide frameworks and molecular
[WZn{Cu(H2O)}2(ZnW9O34)2]12- is predominant in terms of
stability and sustainability (entries 9, 10, 15 and 16). We
also studied the kinetic behavior of solid catalyst CZJ-11 by
using large scale of substrate. As shown in Figure 6, when
enlarging the amount of the substrate, the ratio of mol of
substrate converted to mol catalyst is about 20750 after 58
h. Interestingly, the kinetic curve exhibits lag phase in a
sigmoidal behavior. The presence of a short induction
period should be ascribed to activation of the catalyst
CZJ-11 by NHPI and diffusion of the reactant molecules
into the pore space at the first step. After the induction
time, the catalytic reaction is therefore self-accelerated.
The subsequent decrease of the reaction rate is because
the substrate is almost exhausted. These results indicate
that the catalyst platform has the ability to self-accelerate
the catalytic reaction after a short induction period.
When CZJ-11 was replaced by Cu(ClO4)2 under the identi-
cal conditions, the ratio of mol of substrate converted to
mol catalyst is quickly reached to 3275 after 13 h. However,
the reaction rate is quickly slowdown after 13 h, which
indicates that its catalytic properties are much inferior to
those of CZJ-11.
Figure 5. PXRD patterns for as-synthesized CZJ-11, recovered
CZJ-11 after catalysis, regenerated CZJ-11 by immersing in
water for 24 h, as-synthesized CZJ-12 and recovered CZJ-12
after catalysis.
Figure 6. Aerobic oxidation of n-butanol catalyzed by the
combined use of CZJ-11/Cu(ClO4)2, NHPI and TMAB catalyst
system. Conditions: n-Butanol (125 mmol), CZJ-11/Cu(ClO4)2
(0.005 mmol), NHPI (12.5 mmol) and TMAB (1.33 mmol) in
acetonitrile (150 mL) were stirred at 50 C under an atmos-
phere of oxygen (balloon).
o
The biomimetic platform is also highly efficient and
selective for the aerobic oxidation of a range of alcohols
under the above-mentioned mild conditions (Table 2,
entries 1-14). The conversion of alcohol substrate decreas-
es slightly when the size of the substrate increases, which
might be attributed to their difficult access to the active
Cu(II) sites inside the pore space of CZJ-11 or CZJ-12. The
remarkable catalytic properties of the combined catalyst
system further encouraged us to study the universality of
CZJ-11 and CZJ-12 for the aerobic oxidation of different
kinds of organic substrates. As shown in Table 2 (entries
15-20), the coupled catalyst system demonstrates high
efficiency in aerobic oxidation of benzyl alcohol,
ethylbenzene and isochroman into the corresponding
oxygenated products with very high conversion and selec-
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bridging ligand to connect with highly oxophilic Gd3+ ions,
tivity. It is interesting to note that isochroman was almost
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2
3
4
5
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7
8
full oxidized within 1 h under the identical mild condi-
tions, which might be attributed to its special electronic
effect. There have been many reported homogeneous and
heterogeneous catalysts based on Pt, Ru, Rh, Pd, Ir, Ag
and Au noble metals for the aerobic oxidation of aliphatic
alcohols into carboxylic acids.17 Compared with these no-
ble metal catalysts, CZJ-11 and CZJ-12 present superior
features of high efficiency, sustainability and turnover
numbers, and environmental friend in the aerobic oxida-
tion of aliphatic alcohols.
we successfully synthesized a 2D inorganic porous POM
framework material CZJ-11, consisting of redox copper(II)
sites inside the pore space. CZJ-11 was simply transformed
into a stable 3D porous POM framework CZJ-12 by partial
dehydration. To realize highly efficient biomimetic activa-
tion of molecular oxygen for aerobic oxidation of aliphatic
alcohols, NHPI and TMAB were introduced as co-
catalysts to form a combined catalyst system. The coupled
catalyst platform exhibits enzyme-like features in aerobic
oxidation of aliphatic alcohol under mild conditions with
very high efficiency, selectivity and sustainability. This
work demonstrates that the use of pure porous inorganic
frameworks instead of metal-organic coordination com-
plexes would significantly improve the sustainability in
oxidation reaction. Moreover, the use of combined cata-
lyst system, consisting of multiple active sites to mimic
different functional sites of enzymes for synergistic catal-
ysis, represents one of the most feasible pathways to real-
ize highly efficient biomimetic catalysis under mild condi-
tions.
9
10
11
12
13
14
15
16
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Table 2. Aerobic oxidation of different organic substratesa
Time
(h)
Yield
(%)b
Entry Substrate
Catalyst
Product
1
CZJ-11
CZJ-12
CZJ-11
CZJ-12
CZJ-11
>99.9
93
21
2
3
4
5
96
24
85
ASSOCIATED CONTENT
98
Supporting Information. Experimental details, additional
figures, crystal data, and CIF files. This material is available
free of charge via the Internet at http://pubs.acs.org.
44
6
CZJ-12
91
7
CZJ-11
CZJ-12
CZJ-11
CZJ-12
CZJ-11
CZJ-12
CZJ-11
91
AUTHOR INFORMATION
44
44
44
8
79
87
87
84
80
82
Corresponding Author
*cdwu@zju.edu.cn
9
Author Contributions
10
11
12
13
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript.
Notes
The authors declare no competing financial interest.
39
14
15
16
17
18
19
20
CZJ-12
CZJ-11
CZJ-12
CZJ-11
CZJ-12
CZJ-11
CZJ-12
79
83
52
83
83
95
92
ACKNOWLEDGMENT
We are grateful for the financial support of the National Nat-
ural Science Foundation of China (grant nos. 21373180 and
21525312), and the Fundamental Research Funds for the Cen-
tral Universities (grant nos. 2017XZZX001-03A and
2017FZA3007).
24
24
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