Synthesis and Catalytic Properties of New Metalloporphyrin-Based Porous Organic Framework Materials with Single and Accessible Sites

Article abstract of DOI:10.1002/cctc.201600462

It is a big challenge to homogenize heterogeneous catalysts with molecular catalytic performance. With this target in mind, herein, we describe a facile strategy for direct incorporation of single catalytic sites in 3D open porous aromatic frameworks (PAFs). The synthesis of the PAF (denoted as PAF-76) as well as its derivatives (PAF-76-M, M=Fe, Mn, Zn) was achieved by the use of tetrakis(4-bromophenyl)methane as tetrahedral nodes and tetrakis(4-bromophenyl)porphyrin as planar nodes. The connection of monomers into an extended network of PAF-76 was monitored by ¹³C NMR and FTIR spectroscopies. The prepared PAF-76s showed 3D porous structures with surface areas of 450–700 m² g^?1, pore volumes of 0.3–0.4 cm³ g^?1, and pore sizes around 1.2 nm. The direct incorporation of metalloporphyrin components into the PAF-76-M frameworks has allowed the uniform distribution of metal ionic sites throughout the PAF-76-M particles. The combined merits of isolated metal sites and suitable pore size make PAF-76 a good candidate for heterogeneous catalysis. The catalytic performances of the porphyrin/metalloporphyrin-based active sites in the PAF-76s were evaluated by aerobic oxidation reactions of styrene, which are usually carried out with homogeneous systems. Metal-functionalized PAF-76s (PAF-76-M) exhibit enhanced turnover frequencies for styrene conversion (16.9–50.9 mol mol(M)^?1 h^?1) compared with molecular catalysts (0–35.0 mol mol(M)^?1 h^?1), and improved selectivity toward phenylacetaldehyde (85.7–99 %) in contrast to their corresponding monomers (0–75.5 %). The robustness of PAF-76 in terms of high thermal stability, good recyclability, and excellent solvent resistance showed that these PAF-76 materials hold great promise for developing heterogeneous catalysts.

Full text of DOI:10.1002/cctc.201600462

DOI: 10.1002/cctc.201600462
Full Papers
Synthesis and Catalytic Properties of New
Metalloporphyrin-Based Porous Organic Framework
Materials with Single and Accessible Sites
Shuang Meng,[a] Xiaoqin Zou,*[b] Chuanfang Liu,[b] Heping Ma,[c] Nian Zhao,[a] Hao Ren,[a]
Mingjun Jia,[a] Jia Liu,*[a] and Guangshan Zhu[a]
It is a big challenge to homogenize heterogeneous catalysts
with molecular catalytic performance. With this target in mind,
herein, we describe a facile strategy for direct incorporation of
single catalytic sites in 3D open porous aromatic frameworks
(PAFs). The synthesis of the PAF (denoted as PAF-76) as well as
its derivatives (PAF-76-M, M=Fe, Mn, Zn) was achieved by the
use of tetrakis(4-bromophenyl)methane as tetrahedral nodes
and tetrakis(4-bromophenyl)porphyrin as planar nodes. The
connection of monomers into an extended network of PAF-76
was monitored by 13C NMR and FTIR spectroscopies. The pre-
pared PAF-76s showed 3D porous structures with surface areas
of 450–700 m2 gÀ1, pore volumes of 0.3–0.4 cm3 gÀ1, and pore
sizes around 1.2 nm. The direct incorporation of metallopor-
phyrin components into the PAF-76-M frameworks has allowed
the uniform distribution of metal ionic sites throughout the
PAF-76-M particles. The combined merits of isolated metal
sites and suitable pore size make PAF-76 a good candidate for
heterogeneous catalysis. The catalytic performances of the por-
phyrin/metalloporphyrin-based active sites in the PAF-76s were
evaluated by aerobic oxidation reactions of styrene, which are
usually carried out with homogeneous systems. Metal-func-
tionalized PAF-76s (PAF-76-M) exhibit enhanced turnover fre-
quencies for styrene conversion (16.9–50.9 molmol(M)À1 hÀ1)
compared with molecular catalysts (0–35.0 molmol(M)À1 hÀ1),
and improved selectivity toward phenylacetaldehyde (85.7–
99%) in contrast to their corresponding monomers (0–75.5%).
The robustness of PAF-76 in terms of high thermal stability,
good recyclability, and excellent solvent resistance showed
that these PAF-76 materials hold great promise for developing
heterogeneous catalysts.
Introduction
Porous organic framework materials (POFs), including covalent
organic frameworks (COFs), polymers of intrinsic microporosity
(PIM), conjugated microporous polymers (CMPs), and porous
aromatic frameworks (PAFs), are emerging as a new family of
porous solids.[1–4] POFs are composed of light elements linked
by covalent bonds (CÀC, CÀN, CÀO, BÀO, etc.), resulting in
rigid/diversified structures, permanent porosity, high surface
areas, high thermal/chemical stability, tunable pore sizes, and
low framework density. Owing to these structural features,
POFs with adjustable functions have been widely applied in
different areas, particularly in materials science, such as for
clean energy storage, molecular separations, photoelectric ma-
terials, molecular motors, etc.[5–9] Very recently, intensive efforts
have been devoted toward developing catalytically active POF
materials with potential applications in chemical catalysis.[10,11]
As we know, homogeneous catalysts have the unique prop-
erties of high reactivity and selectivity thanks to their molecu-
lar nature.[12] Heterogeneous catalysts possess outstanding ad-
vantages for the facile separation of the catalyst from the
products, and for the possibility of continuous-flow processes
either in the gas or liquid phases. A major goal in catalysis is
to combine the merits of both homogeneous catalysis and
heterogeneous processes to ideally maintain (or even improve)
the activity and selectivity of the molecular catalysts as well as
to facilitate product recovery and catalysts recycling. To this
end, a series of materials, particularly those of porous materi-
als, have been successfully developed.[13–21] POFs as a subclass
of porous materials, have been proposed as an excellent candi-
date for homo/heterogeneous catalysts because of the tunabil-
ity of their functionalities (either inherited from the employed
monomers, or by post-synthesis modifications) and their chem-
ical stability in the reaction media.[22–42] With the challenge of
making homogeneous catalysts heterogeneous, we have
become interested in the designed synthesis of POF catalysts
by direct incorporation of molecular-like active sites in the
[a] S. Meng, N. Zhao, Dr. H. Ren, Prof. Dr. M. Jia, Dr. J. Liu, Prof. Dr. G. Zhu
College of Chemistry
Jilin University
Changchun 130012 (P.R. China)
E-mail: jia@jlu.edu.cn
[b] Prof. Dr. X. Zou, C. Liu
Faculty of Chemistry
Northeast Normal University
Changchun 130024 (P.R. China)
E-mail: xiaoqinzou123@gmail.com
[c] Dr. H. Ma
State Key Laboratory of Luminescence and Applications
Changchun Institute of Optics, Fine Mechanics and Physics
Chinese Academy of Sciences
Changchun 130033 (P.R. China)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cctc.201600462.
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open frameworks, and further evaluation of their catalytic
performances.
of styrene, which is a widely known industrial process for the
selective oxidation of organic alkenes to aldehydes.
Herein, porphyrin/metalloporphyrin compounds are chosen
as monomers in the construction of POFs based on the follow-
ing considerations:
Results and Discussion
1) Metalloporphyrins as active centers. Metalloporphyrins are
recognized as well-known analogs of the heme cofactor and
most metalloenzymes, which have been extensively used as
oxidation catalysts in the fine-chemicals syntheses of epoxides,
alcohols, ketones, and aldehydes.[43–45]
Structure determination
The formation of porphyrin-based PAF-76s involves the crea-
tion of new CÀC bonds between 5,10,15,20-tetrakis-(4-bromo-
phenyl)-porphyrin (TBPP) and tetrakis[4-(4’,4’,5’,5’-tetramethyl-
1’,3’,2’-dioxaborolane-phenyl)]methane (TTBPM) by cross-cou-
pling reactions, and the conjugated network results from ex-
tensive polymerization with the elimination of small molecules
(Scheme 1). For clarifying the linkage of monomers and the mi-
crostructure of PAF-76, solid-state 13C NMR spectroscopy is
a powerful technique, which was used to examine the carbon
2) Metalloporphyrins as building blocks. The utilization of
metalloporphyrins as rigid building units for constructing
stable frameworks can prevent the leaching of metalloporphy-
rin sites.[27,33,35,46,47]
In addition, the three-dimensional (3D) porous structure ren-
ders the active sites isolated from each other, and thus as
many active centers as possible are accessible to reactants
compared with homogeneous catalysts. Solid-state catalysts
guarantee a facile separation of catalysts from products, allow-
ing their long-term use. This concept is exemplified in this
study by the targeted synthesis of a new 3D microporous por-
phyrin-functionalized PAF and its derivatives, denoted as PAF-
76 and PAF-76-M (M=Fe, Mn, or Zn, see Scheme 1). Our previ-
ously reported strategy[4,5] has shown that 3D structures can
be made by judiciously choosing four-node (tetrahedral)
shaped monomers (e.g., PAF-1). Accordingly, a three-dimen-
sional network would be expected for PAF-76 because the
center carbon atoms of tetrakis(phenyl)methane adopt a sp3
tetrahedral configuration. An extended 3D network develops
between the tetrahedral and planar building units, which is
achieved by terminal CÀC bond formation between tetrakis(4-
bromophenyl)methane and tetrakis(4-bromophenyl)porphyrin
through coupling reactions. Most importantly, porosity would
be generated as a result of monomer rigidness, allowing full
exposure of the metalloporphyrin moieties. For the derivatives,
metalloporphyrins (Fe, Mn, Zn) are employed as monomers for
the constructions of identical PAF-76 structures (PAF-76-M).
Subsequently, all PAF-76s are studied in the aerobic oxidation
Figure 1. a) 13C solid-state MAS NMR spectrum of the as-synthesized PAF-76,
b) FTIR spectra of TBPP and TTBPM reactants and PAF-76 product, c) a repre-
sentative SEM image, and d) manganese distribution map of PAF-76-Mn.
connectivity of PAF-76. Figure 1a presents the NMR
spectrum of the PAF-76 product. As shown in Fig-
ure 1a, a carbon signal at 64 ppm is detected, which
is assigned to the center carbon with a tetrahedral
configuration. Additionally,
a shoulder peak at
147 ppm is observed, which is attributed to the
carbon atoms in the pyrrole ring. It can be found
that the carbon signals from boron esters disappear
(Figure S1a in the Supporting Information), suggest-
ing that the boron groups are cleaved off from the
monomers when the cross-coupling reaction takes
place at 383 K (1108C). Moreover, carbon signals from
the benzene ring merge together to form three
easily distinguished peaks around 119 ppm, 129 ppm,
140 ppm with the concurrent disappearances of CÀB
and CÀBr peaks at 127 ppm (Figure S1a) and
125 ppm (Figure S1b). The peaks merging and broad-
Scheme 1. A representation of the synthesis of PAF-76 employing TTBPM and TBPP reac-
tants.
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ening is due to the connection of TBPP and TTBPM monomers
through CÀC bonds with consequent formation of a more con-
jugated structure of PAF-76. Further, the PAF-76 structure was
characterized by infrared (IR) spectroscopy, and the result is
displayed in Figure 1b. The band corresponding to CÀBr vibra-
tions at 642 cmÀ1 (Figure 1b) in PAF-76 vanishes into the back-
ground (red line), indicating that the CÀBr bond is replaced by
a CÀC bond; which is in good agreement with the NMR re-
sults. The CÀB and BÀO associated bands at 1021 cmÀ1 and
1325 cmÀ1, respectively, were not observable in the PAF-76
sample, suggesting that the boron ester group was detached
from the carbon atom of the benzene ring during the cross-
coupling reaction, which is also consistent with the NMR
observations.
Clear bands at 1606 cmÀ1 and 1474 cmÀ1 are detected,
which suggest that the main backbone of the phenyl and pyr-
role rings is preserved in PAF-76. A similar NMR profile for PAF-
76-Zn suggests that the PAF-76 derivatives (Figure S2 in the
Supporting Information) possess the same structure as that of
PAF-76, which is further verified by the IR spectra (Figure S3 in
the Supporting Information). Elemental analysis data in
Table S1 (in the Supporting Information) shows the chemical
composition of C, H, and N for PAF-76 with trace Pd
(~0.5 wt%), affording a molecular formula of C69H42N4, which is
in agreement with the structure in Scheme 1. Based on the re-
sults from NMR, IR, and elemental analyses, it can be conclud-
ed that an extended network of PAF-76 is constructed from
TBPP and TTBPM by coupling reactions.
cal emission spectroscopy (ICP-OES). As shown from the metal/
porphyrin ring ratio of approximately 0.9 in Table S2 (in the
Supporting Information), almost a stoichiometric amount of
metal ions are incorporated into the porphyrin rings in PAF-
76s. Consequently, each metal ion is confined within one por-
phyrin ring, which favors the isolated and single metal centers.
A similar metal/porphyrin ratio (ꢀ0.8) of the polymer to the
corresponding monomers (ꢀ0.9) sheds light on the high sta-
bility of the metalloporphyrin components in the solvent (i.e.,
dioxane). In addition to the chemical stability, the thermal sta-
bility was also examined by thermogravimetric analysis (TGA).
A small weight loss of 2–6% is observed before 3008C (Fig-
ure S5 in the Supporting Information), attributed to solvent re-
lease (e.g., methanol). TGA results confirm that PAF-76s exhibit
relatively high thermal stability (up to 3008C), which permits
their subsequent catalysis applications.
The oxidation states of the metal species in PAF-76-M were
probed by X-ray photoelectron spectroscopy (XPS). Figure 2
shows the XPS spectra for PAF-76-Fe, PAF-76-Mn, and PAF-76-
Zn. As shown in Figure 2a, the Fe2p region has significantly
split spin–orbit components of Fe2p3/2 and Fe2p1/2. The Fe2p3/
spectrum of PAF-76-Fe displays a peak maximum near
2
710.8 eV and a satellite peak at 718.3 eV, characteristic of the
FeIII state. For PAF-76-Mn, the Mn2p spectrum contains
a Mn2P3/2 peak at 642.3 eV and Mn2p1/2 peak at 654.1 eV (Fig-
ure 2b). These features show that the Mn species in PAF-76-
Mn possess a high valence of MnIV.[48] The reason for the high
oxidation state of iron and manganese is the likelihood of ad-
ditional oxidation of FeII and MnII to the respective FeIII and
MnIV species with an exposure of PAF-76-Fe and PAF-76-Mn to
air.
The morphology and element distribution of PAF-76s were
studied by scanning electron microscopy (SEM). A representa-
tive SEM image of the PAF-76-Mn sample shows a hexagonal-
shaped bulk particle around tens of micrometers in size with
small-sized aggregates (Figure 1c). Figure 1d shows the distri-
bution map of manganese element in one of the PAF-76-Mn
particles, measured by energy-dispersive X-ray spectroscopy
(EDS). The EDS profile in Figure 1d reveals that manganese is
mainly located within the PAF-76-Mn particle and is homoge-
neously distributed throughout the polymer. The uniform dis-
tribution of metal ions is also observed in its analogs (Fe and
Zn derivatives, Figure S4 in the Supporting Information). The
quantitative determination of metal contents in PAF-76-M was
done by chemical analysis by inductively coupled plasma opti-
The permanent porosity and specific surface area of the
PAF-76s were determined by nitrogen physical sorption at
77 K. Figure 3a shows the N2 adsorption–desorption isotherms
for PAF-76s. A steep uptake at low relative pressures followed
by a steady increase in the adsorption branch was recorded in
the sorption isotherms of PAF-76 materials, typical for micropo-
rous materials. The possible reason for the observed hysteresis
could be the capillary condensation of N2 in some bottle-
shaped pores formed by interpenetrated polymerization or
inter-particle voids formed by aggregation of similar-in-size
small particles as seen by SEM (Figure 1c). The specific Brun-
Figure 2. XPS spectra of a) Fe2p, b) Mn2p, and c) Zn2p signals in PAF-76-Fe, PAF-76-Mn, and PAF-76-Zn, respectively.
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Catalytic properties
To assess the catalytic properties, PAF-76s with metalloporphy-
rin moieties in the framework were investigated for the aerobic
oxidation reaction of styrene as a substrate, which is an impor-
tant chemical feedstock for alkenes conversions.[47] The aerobic
oxidation reactions were performed by using 1 mmol of sty-
rene substrate and 3 mmol of isobutyraldehyde co-reductant
in 10 mL of 1,2-dichloroethane solvent (C2H4Cl2 used to com-
pletely dissolve metalloporphyrin monomers) with 10 mg of
PAF-76 catalysts for 2 h under ambient pressure (1 atm) and
temperature of 353 K. The catalytic results of conversion, turn-
over frequency, and selectivity for PAF-76s and their corre-
sponding homogenous metalloporphyrin catalysts are sum-
marized in Table 1. The PAF-76 catalysts exhibit excellent selec-
tivities for phenylacetaldehyde (85–99%, entries 1, 3, 5, 7). No
2-phenyloxirane side product was observed, and only a small
amount of benzaldehyde (<14.3%) was found after 2 h reac-
tion over PAF-76-M catalysts (entries 3, 5, 7). The formation of
trace benzaldehyde product is probably due to over oxidation.
The relatively small conversion of 27.4% with PAF-76 might
come from styrene activation on trace Pd metals or defect
sites. Interestingly, the respective turnover frequencies of 40.4,
41.5, 13.5 mmolgcatÀ1 hÀ1 are observed with PAF-76-Fe, PAF-76-
Mn, PAF-76-Zn after a reaction time of 2 h (entries 3, 5, 7 in
Table 1). The catalytic performance is quite reproducible as
seen from similar conversions for two independent runs (Fig-
ure S6 in the Supporting Information). Under the same condi-
tions, the metalloporphyrin monomers afforded a bit lower
substrate conversion rate with values of 32.0, 31.5, and
0 mmolgcatÀ1 hÀ1 (entries 4, 6, 8 in Table 1). The above experi-
mental results clearly demonstrate that PAF-76 catalysts per-
formed equally as well (or even outperformed) as their homo-
geneous metalloporphyrin analogs. The enhancements in
terms of turnover frequency (>26%) and selectivity (>18%)
suggest that the micro-environment plays a great role in sty-
Figure 3. a) N2 adsorption isotherms at 77 K and b) the DFT-derived pore
size distribution curves of PAF-76, PAF-76-Fe, PAF-76-Mn, and PAF-76-Zn.
auer–Emmet–Teller surface areas (SBET) and pore volumes are
summarized in Table S3 (in the Supporting Information). The
SBET and pore volumes of all PAF-76s materials are in the ranges
450–700 m2 gÀ1 and 0.3–0.4 cm3 gÀ1, respectively. These values
correspond to a highly porous material, which is in agreement
with our synthesis concept by using tetrahedral and planar
building blocks (Scheme 1). Pore size distribution curves are
presented in Figure 3b. Based on the non-local density func-
tional theory (NL-DFT), the pore size distribution (PSD) of PAF-
76 exhibits a dominant pore diameter of 1.2 nm, and the PSD
of PAF-76 metal derivatives exhibit similar dominant ones of
1.1–1.2 nm (Figure 3b and Table S3), suggesting that PAF-76
and PAF-76-M have the same porous structure. More impor-
tantly, the porous characteristics of PAF-76-Mn, PAF-76-Fe, and
PAF-76-Zn would have great benefits for high accessibility of
the metal sites as well as fast mass transport.
Table 1. Catalytic data for styrene oxidation over PAF-76 and its metallo-derivative PAFs as well as their corresponding porphyrin-based monomers.
Entry
Conversion [%][a]
Selectivity [%][b]
Phenylacetaldehyde
Turnover frequency[c]
[mmolgcatÀ1 hÀ1 [molmol(M)À1 hÀ1
]
Benzaldehyde
2-Phenyloxirane
]
PAF-76
TBPP
27.4
0
0
0
>99
0
0
0
0
0
0
0
0
0
13.5
0
–
–
PAF-76-Fe
Fe-TBPP
PAF-76-Mn
Mn-TBPP
PAF-76-Zn
Zn-TBPP
80.7
64.1
83.7
63.8
27.2
0
10.7
24.5
14.3
33.3
0
89.3
75.5
85.7
66.7
>99
0
40.4
32.0
41.5
31.5
13.5
0
48.3
35.0
50.9
34.1
16.9
0
0
[a] Reaction conditions: 1 mmol of substrate, 3 mmol of isobutyraldehyde (IBA), 10 mg of catalyst, 10 mL of 1,2-dichloroethane (C2H4Cl2), 1 atm, and 353 K
(808C) for 2 h. [b] Conversion and selectivity determined by GC by using benzene as an internal standard. [c] Turnover frequency is expressed as the con-
sumption (conversion) of styrene at 2 h per time in terms of catalyst weight and moles of metal.
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rene conversion as a result of the enrichment of reactants in
the micropores. Note that the turnover frequencies of PAF-76-
Fe and PAF-76-Mn (48.3 and 50.9 molmol(M)À1 hÀ1, respective-
ly) are almost three times the value of that over PAF-76-Zn
(16.9 molmol(M)À1 hÀ1). These enhancements with PAF-76-Fe
and PAF-76-Mn catalysts can be interpreted by the fact that
there are empty 3d orbits accepting the electrons from the re-
actants or co-reactant in PAF-76-Fe and PAF-76-Mn because
FeIII and MnIV adopt 3d5 and 3d3 electron configurations. How-
ever, in the case of PAF-76-Zn, no empty 3d orbits are available
in ZnII owing to its 3d10 configuration; that is, the poor electron
accepting ability results in low turnover frequency of styrene
conversion. This experimental result demonstrates that metal
centers with unsaturated orbits are of vital importance in sty-
rene conversion, as well as shedding light on the electron-
based mechanism for the oxidation of styrene to phenylacetal-
dehyde. This conclusion can be supplementary supported by
the results from homogeneous catalysts, as evident from the
turnover frequencies of, respectively, 35.0 and 34.1 mol
mol(M)À1 hÀ1 with Fe-TBPP and Mn-TBPP (entries 4 and 6 in
Table 1) in contrast to undetectable styrene conversion on Zn-
TBPP (entry 8, Table 1).
amount of free radical scavenger, 2,6-di-tert-butylphenol
(1 mmol), was added, and the catalytic activity with PAF-76-Mn
was examined. With the same reaction parameters specified in
Table 1, the turnover frequency of styrene conversion is signifi-
cantly decreased (4.5 molmol(M)À1 hÀ1) with a small conversion
of 7.4% after 2 h in comparison to that measured without ad-
dition of radical scavenger (entry 5, Table 1). This result, along
with the data in Table 1, confirms that free-radical-type species
occurs during the reaction. To further shed light on the in-
volvement of radicals in this oxidation, a reference experiment
in the absence of isobutyraldehyde was carried out under the
same reaction circumstances described in Table 1. A relatively
small conversion of 5.3% for styrene is found, suggesting that
styrene consumption is initiated by isobutyraldehyde radicals.
Based on the results in Table 1 and from the two control ex-
periments, a tentative reaction mechanism for the aerobic oxi-
dation of styrene is proposed, which is exemplified by the PAF-
76-Mn catalyst as shown in Scheme 2. Upon interaction of iso-
butyraldehyde with PAF-76-Mn, the isobutyraldehyde radical is
generated by hydrogen abstraction from a sacrificial co-reduc-
tant of isobutyraldehyde,[49] and intermediate Mn species 2 is
concurrently formed by electron and hydrogen transfers.[50]
The produced radical is rapidly captured by compound 2
and subsequently converted to a complex radical of 3.[51,52] The
To study the mechanistic behavior and also to identify the
active species formed in the oxidation of styrene, a small
Scheme 2. Possible mechanism for the aerobic oxidation of styrene to its corresponding aldehyde catalyzed by PAF-76-Mn in the presence of molecular
oxygen and isobutyraldehyde.
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activation of molecular oxygen occurs on 3 through the forma-
tion of active species 4.[53] The complex 4 is most likely decom-
posed to form metal-peroxo species of 5, with concerted re-
lease of isobutyric acid.[54] The cleavage of C=C bonds takes
place by an interaction of styrene with complex 5. Phenylacet-
aldehyde is yielded by the isomerization/rearrangement of the
styrene oxide intermediate inside complex 6, and the side
product of benzaldehyde may be produced by the deep oxida-
tion of the formed oxide species in 6.[55,56] Meanwhile, Mn spe-
cies in PAF-76-Mn is regenerated to the initial state of 1 after
the whole catalytic circle.
dress the recyclability of PAF-76 catalysts, the multiple repeat-
ed reactions were carried out with PAF-76-Mn. As expected, it
is found that the turnover frequency of styrene conversion is
well retained around 45 mmolgcatÀ1 hÀ1, and the selectivity for
phenylacetaldehyde (~85%) is preserved as well after recycling
for five times (Figure 5). These results confirm that PAF-76 cata-
More information on the catalytic properties of PAF-76-M
can be obtained from the oxidation of styrene derivatives,
which is exemplified by methylphenylene and chlorostyrene.
Figure 4 shows the oxidation results of 4-methylphenylene and
Figure 5. Turnover frequency of styrene conversion and the selectivity for
phenylacetaldehyde as a function of number of recycling tests.
lysts possess excellent recyclability. Almost all the catalyst is re-
cycled after each run by separating the solid from the liquid
with weight losses of <7%. The data from inductively coupled
plasma optical emission spectroscopy (ICP-OES) shows that the
concentration of Mn species in the supernatant after each run
is beyond our ICP detection limit (40 ppb). The small loss in
catalyst weight and negligible Mn leaching prove the high sta-
bility of PAF-76 catalysts. Additionally, catalytic tests of PAF-76-
Mn were also performed in different solvents (1,2-dichloro-
ethane, acetonitrile, and tetrahydrofuran), and the results are
summarized in Table S4 (in the Supporting Information). The
Figure 4. Turnover frequency of styrene, 4-methylphenylene, and 4-chloros-
tyrene conversion, and the respective selectivity for phenylacetaldehyde, 4-
methylphenylacetaldehyde, and 4-chlorophenylacetaldehyde products, over
PAF-76-Mn catalyst.
similar
turnover
frequency
of
styrene
conversion
(ꢀ41 mmolgcatÀ1 hÀ1) and selectivity for phenylacetaldehyde
(~80%) validate the high chemical robustness of PAF-76 cata-
lysts.
4-chlorostyrene over PAF-76-Mn catalyst. When 4-methylphe-
nylene is used as the substrate, its oxidation over PAF-76-Mn
proceeds at a frequency of 45.3 mmolgcatÀ1 hÀ1 and the major
product is 4-methylphenylacetaldehyde with a selectivity of
83.2%. In the case of 4-chlorostyrene oxidation, PAF-76-Mn dis-
plays a turnover frequency of 49.4 mmolgcatÀ1 hÀ1 for 4-chloro-
styrene conversion, with a selectivity of 72.8% for 4-chlorophe-
nylacetaldehyde. As shown in Figure 4, similar results for
4-methylphenylene and 4-chlorostyrene substrates to that of
styrene oxidation are observed in terms of turnover frequency
(41.5 mmolgcatÀ1 hÀ1 for styrene conversion) and product selec-
tivities for aldehydes (85.7% for phenylacetaldehyde). This ob-
servation indicates that the presence of the para-methyl and
-chloro functional groups in the substrates does not signifi-
cantly affect the oxidative conversions of styrenic compounds.
This finding also supports the radical-mediated mechanism of
styrene oxidation because an electron donor or acceptor on
the styrene molecules has little influence on C=C bond cleav-
age (Scheme 2).
Conclusions
In summary, we have developed a new synthesis strategy for
introducing functionalities into 3D microporous aromatic
frameworks (PAF-76s) by cross-coupling of porphyrin/metallo-
porphyrin-based monomers and tetrakis(4-bromophenyl)me-
thane. As-prepared PAF-76 possesses a conjugated network
linked by intensive and newly created CÀC bonds between
two building units, the structure of which has been verified by
NMR and IR measurements. N2 adsorption characterization has
revealed that PAF-76s exhibit permanent porosity, high surface
areas, as well as a molecular-level pore sizes. The direct incor-
poration approach favors uniform distribution of metal ions in
the PAF backbone, which has been demonstrated by energy-
dispersive X-ray spectroscopy and elemental analysis. Metallo-
porphyrin-functionalized PAF-76s have been further explored
as heterogeneous catalysts for the selective oxidation of sty-
rene to phenylacetaldehyde. Catalytic results have revealed
that the PAF-76 catalysts show superior catalytic performance
As metalloporphyrins are expensive organocatalysts, their
stability and recyclability is another important index. To ad-
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chloranil, FeCl2, MnCl2, ZnCl2, K2CO3, DMF, ethyl ether, hexane, ethyl
acetate, ethanol, CH2Cl2, 1,4-dioxane, tetrakis(triphenylphosphine)-
palladium(0) were used as received without any further
purification.
including high turnover frequency of styrene conversion and
high selectivity for phenylacetaldehyde product, which is due
to the single and highly accessible metal sites in the frame-
works and an enrichment of reactants in the micropores. The
radical catalytic mechanism for the aerobic oxidation of styrene
has been elucidated on PAF-76-Mn, which included a sequence
of radical generation and stabilization on the Mn center, the
formation of metal-peroxo intermediates assisted by molecular
oxygen, and C=C bond cleavage on the active Mn–O sites with
concurrent formation of phenylacetaldehyde. In addition, as-
synthesized PAF-76 catalysts have shown high thermal stability,
good recyclability, and excellent robustness toward solvents.
This study may provide us with some guidelines for the de-
signed synthesis of efficient porous heterogeneous catalysts,
and other new endeavors that are being explored in our
group.
Synthesis of PAF-76
5,10,15,20-Tetrakis-(4-bromophenyl)-porphyrin (204 mg, 0.2 mmol,
abbreviated as TBPP) and tetrakis[4-(4’,4’,5’,5’-tetramethyl-1’,3’,2’-di-
oxaborolane-phenyl)]methane (165 mg, 0.2 mmol, abbreviated as
TTBPM) were introduced into a dry 50 mL two-necked flask with
a condenser and a magnetic stirring bar inside the flask. After de-
gassing with argon, 1,4-dioxane (20 mL), anhydrous K2CO3
(220 mg) and tetrakis(triphenylphosphine)palladium(0) as a catalyst
(23 mg, 20 mmol, abbreviated as Pd(PPh3)4) were added into the
flask, and then the mixture was heated to 383 K for 24 h under an
inert atmosphere. After cooling to room temperature, the crude
product was collected by filtration, and then washed with ethanol
and methanol. Further purification of PAF-76 was carried out by
Soxhlet extraction with tetrahydrofuran (THF) and methanol for
24 h. The product was dried in vacuum at 393 K to give PAF-76 as
a purple powder. The syntheses of PAF-76-Fe, PAF-76-Mn and PAF-
76-Zn were performed by using the same procedure but by substi-
tuting TBPP with Fe-TBPP, Mn-TBPP, or Zn-TBPP, respectively.
Experimental Section
Synthesis of monomers
The synthesis of tetrakis[4-(4’,4’,5’,5’-tetramethyl-1’,3’,2’-dioxaboro-
lane-phenyl)]methane (TTBPM) was conducted according to an es-
tablished procedure.[57] Typically, tetrakis(4-bromophenyl)methane
(1.3 g, 2.0 mmol) was dissolved in dry ethyl ether (35 mL) and
cooled to 203 K. Under nitrogen, butyllithium/hexane solution
(6.0 mL of 2.5m) was added slowly in the above solution with stir-
ring. After warming to room temperature, another solution of tri-
methyl borate (3.2 g, 31 mmol) in dry ethyl ether (30 mL) was
added and stirred for 1 h. The ether was then removed under
vacuum at room temperature. The product was collected by filtra-
tion, washed with water, and dried in vacuum. The product (1 g,
2 mmol) and pinacol (1.1 g, 9.6 mmol) were added to ethyl acetate
(30 mL), and this reaction mixture was heated at reflux for 24 h.
After cooling down to room temperature, the crude product was
collected by filtration, washed with ethanol, and then dried in
Characterizations
The solid-state cross-polarization magic angle spinning 13C nuclear
magnetic resonance spectra (CP-MAS NMR) were collected with
a Bruker Avance III 400 MHz solid-state NMR spectrometer at
a MAS rate of 5 kHz. Fourier transform infrared spectra (FTIR) were
measured by using a Nicolet Impact 410 FTIR spectrometer at
room temperature in the range 400–4000 cmÀ1, with potassium
bromide pellets. Field emission scanning electron microscope (FE-
SEM) images were inspected with a Hitachi S-4800 with energy-dis-
persive X-ray spectroscopy (EDS). X-ray photoelectron spectroscopy
(XPS) measurements were performed with an ESCALAB 250 spec-
trometer. The nitrogen sorption measurements were carried out
with a Quantachrome Autosorb iQ2 analyzer. Prior to the measure-
ments, the samples were degassed at 393 K for 24 h. N2 adsorption
isotherms were recorded at 77 K by using ultra-high-purity grade
N2 (99.999%). The thermogravimetric analysis (TGA) was performed
by using a Netzch Sta 449c thermal analyzer system at a heating
rate of 10 KminÀ1 in an air atmosphere. The metal contents of Fe,
Mn, and Zn in the monomers and PAF-76 polymers were analyzed
by inductively coupled plasma optical emission spectrometer (ICP-
OES, PerkinElmer Optima 3300DV), and the C, H, N contents were
determined by using a PerkinElmer 2400 Series II CHNS/O analyzer.
1
vacuum at room temperature. H NMR (400 MHz, CDCl3) of TTBPM:
dH =7.6 (d, J=11.2 Hz, 8H), 7.3 (d, J=10.4 Hz, 8H), 1.3 ppm (s,
48H).
The preparation of 5,10,15,20-tetrakis-(4-bromophenyl)-porphyrin
(TBPP) was carried out as follows: 4-bromobenzaldehyde (1.5 g,
8.46 mmol), pyrrole (0.6 mL, 8.46 mmol), and CH2Cl2 (850 mL) sol-
vent were added to
a foil-covered 2.0 L three-necked flask
equipped with a reflux condenser, a magnetic stirrer, and a N2
inlet. The reaction mixture was first stirred for 15 min at room tem-
perature, and then BF3·OEt2 (105 mL, 0.85 mmol) was added. After
2 h, p-chloranil (1.56 g, 6.37 mmol) was added, and the reaction
mixture was heated to reflux for 2 h. The obtained liquid mixture
after reaction was evaporated to a volume of 50–100 mL, followed
by further purification by using silica gel.[58] 1H NMR (400 MHz,
CDCl3) of TBPP: dH =À2.9 (s, 2H), 7.8 (d, J=11.2 Hz, 8H), 8.0 (d,
J=11.2 Hz, 8H), 8.8 ppm (s, 8H).
The preparations of Fe-TBPP, Mn-TBPP and Zn-TBPP were done by
first mixing TBPP (900 mg, 0.97 mmol) and the metal source
(900 mg, 7.14 mmol FeCl2; or 900 mg, 7.14 mmol MnCl2; or
970 mg, 7.14 mmol ZnCl2) in DMF (100 mL), with subsequent reflux
for 5 h under an argon atmosphere. The reaction mixture was
cooled to room temperature, and then DMF was evaporated under
vacuum. The crude product was purified by silica gel column
chromatography.
Catalytic tests
Styrene oxidations were performed on the monomers (TBPP, Fe-
TBPP, Mn-TBPP, Zn-TBPP) and PAF-76 samples in a batch glass reac-
tor. The catalyst (10 mg) was loaded into a reactant solution con-
taining styrene (1 mmol, 0.12 mL), isobutyraldehyde (3 mmol,
0.27 mL), benzene (63.7 mg) as an internal standard, and 1,2-di-
chloroethane (10 mL) solvent in a 50 mL three-necked round-
bottom flask. An O2 stream (5 mLminÀ1, 1.0 atm, 99.999%) was in-
troduced into the reactor from one inlet, and the reactor was fur-
ther heated to 353 K (808C). The reaction process was monitored
by taking one sample every 30 min by using a syringe from anoth-
er outlet. Reactant and product concentrations were measured by
The chemicals of tetrakis(4-bromophenyl)methane, butyllithium, tri-
methyl borate, pinacol, 4-bromobenzaldehyde, pyrrole, BF3·OEt2, p-
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Received: April 20, 2016
Published online on && &&, 0000
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Homo-heterogeneous POF catalysts:
Porous organic framework (POF) materi-
als (PAF-76) with metalloporphyrin com-
ponents in the frameworks have been
prepared by a direct synthesis ap-
S. Meng, X. Zou,* C. Liu, H. Ma, N. Zhao,
H. Ren, M. Jia, J. Liu,* G. Zhu
&& – &&
Synthesis and Catalytic Properties of
New Metalloporphyrin-Based Porous
Organic Framework Materials with
Single and Accessible Sites
proach. The as-prepared PAF-76 materi-
als possess three-dimensional conjugat-
ed networks, permanent porosity, as
well as single and accessible metallo-
porphyrin active sites. The catalytic
properties of PAF-76 catalysts were eval-
uated for the aerobic oxidation of styr-
enes, and the catalytic results reveal
that PAF-76s exhibit superior per-
formance in terms of high turnover fre-
quency and good product selectivity.
The above merits along with the robust-
ness hold great promise for PAF-76 ma-
terials in heterogeneous catalysis.
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