Bottom-up approach to engineer two covalent porphyrinic frameworks as effective catalysts for selective oxidation

Article abstract of DOI:10.1039/c4cy00969j

Two kinds of novel functional covalent organic frameworks were assembled with the porphyrin building block and terephthalaldehyde or squaric acid via bottom-up approach. Here, our reported covalent porphyrinic frameworks with coordinated manganese(iii) io

Full text of DOI:10.1039/c4cy00969j

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Bottom-up approach to engineer two covalent
porphyrinic frameworks as effective catalysts for
Cite this: DOI: 10.1039/c4cy00969j
selective oxidation†
Received 25th July 2014,
Accepted 2nd September 2014
a
Weijie Zhang, Pingping Jiang, a Ying Wang,a Jian Zhangb and Pingbo Zhanga
*
DOI: 10.1039/c4cy00969j
www.rsc.org/catalysis
Two kinds of novel functional covalent organic frameworks were
assembled with the porphyrin building block and terephthalaldehyde
or squaric acid via bottom-up approach. Here, our reported covalent
porphyrinic frameworks with coordinated manganeseIJIII) ions
(Mn–CPF-1 and Mn–CPF-2) present promising catalytic properties
for the selective oxidation of olefins.
To be more precise, the covalent porphyrinic frameworks
(CPFs) with accessibility of the open channels can be consid-
ered as self-supported catalysts with enhanced performance
due to their high-density active sites in the frameworks. The
tetrapyrrolic macrocycles of porphyrins play an important
role in the design of extended supramolecular lattices
because of their robust structure, remarkable thermal and
oxidative stability, and unique catalytic properties. In addi-
tion, the heterogeneous nature of CPFs can be very useful to
separate the catalyst from the products of interest, recover it
after simple filtration procedures and finally regenerate it for
successive catalytic runs. Therefore, it may be a valuable trial
to target efficient heterogeneous catalysts with open coordi-
nation frameworks via bottom-up strategy, which is essential
to catalysis application. Here, the bottom-up method enables
direct visualization of the structure and makes it possible to
get a detailed insight into the relationship between the struc-
ture and the catalytic activity by structural interrogation.
With this background in mind, we reported the develop-
ment of new CPFs based on the hydrazone and squaraine
linkage (Fig. 1). It has been previously studied that con-
densation of hydrazide with aldehyde6 or squaraine7 was a
thermodynamically controlled reaction. During this reaction,
the formation of hydrazone or squaraine derivatives and
water by-products proceeded. Accordingly, we employed
meso-tetraIJ4-hydrazidocarbonylphenyl)porphyrin (Fig. 1, THCPP)
with four hydrazide groups at the periphery as the aldehyde
or squaraine component to condense with hydrazine, with
the aim of demonstrating the feasibility of the strategy and
features of a new class of CPFs. Furthermore, the crystal
data of THCPP (Fig. S1†) obtained by a solvent diffusion
method substantiated the point that the macrocycles of
porphyrin units could be classified as 2D-C4 blocks3c on a
simplified symmetry notation with a rigid nature and dis-
crete bonding direction of arenes in order to make aro-
matic π systems.
Activation of unsaturated bonds is a field of increasing
interest in relation to the design of highly active catalysts for
selective oxidation reactions. Among them, synthetic
metalloporphyrin has long been known as an effective
catalyst for oxidation reactions due to its excellent catalytic
performance and high product selectivity.1 However, the
application of metalloporphyrin as a catalyst in solution is
particularly difficult to achieve because of the formation of
catalytically inactive dimers and fast degradation in
homogeneous catalysis.2 Therefore, it is of great significance
to circumvent these challenges.
Covalent organic frameworks (COF) are a novel class of
porous crystalline organic materials assembled from molecular
building blocks by linking light elements (e.g. B, C, N, and O)
via covalent bond formation (boronic acid trimerization,
boronate ester formation, the Schiff base reaction, hydrazone
and squaraine linkage) in a periodic manner.3 In terms of cata-
lytic application, COFs have been successfully projected to
bridge the gap between heterogeneous materials and homoge-
neous catalysis.4 Currently, porphyrins and metalloporphyrins
afford ideal synthons for building blocks in the construction of
covalent porphyrinic networks analogous to robust inorganic
zeolites for versatile application.5
a The Key Laboratory of Food Colloids and Biotechnology, School of Chemical and
Material Engineering, Jiangnan University, Wuxi 214122, PR China.
E-mail: ppjiang@jiangnan.edu.cn
b School of Chemistry and Environmental Science, Lanzhou City University,
Lanzhou 730000, PR China
† Electronic supplementary information (ESI) available: HNMR, FT-IR, PXRD, TG
analysis, crystal data and catalytic details. CCDC 955396. See DOI: 10.1039/
c4cy00969j
All CPFs were synthesized by solvothermal reactions
(see the ESI†). In order to introduce catalytically active sites
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Compounds CPF-1 and CPF-2 both exhibited a typical type III
isotherm with surface areas of 158 m2 g−1 for CPF-1 and
92 m2 g−1 for CPF-2 (Fig. S8†). The prepared COFs showed N2
adsorption isotherms with low surface areas. It can be
speculated that these COFs adopt a thin-layer morphology.
Due to the thin-layered structures, long-range pore formation
is hindered, rendering N2 adsorption possible only in the
shallowest, most accessible pores.8 Alternatively, it may be
helpful to prepare the crystalline CPF-1 or CPF-2 first, subse-
quently introducing Mn to get a Mn catalyst with a crystalline
structure. These studies will be reported separately in the
near future.
FE-SEM revealed that CPF-1 and CPF-2 were composed of
a uniform micrometre-scale bet morphology with a dimension
of ca. 200 nm and a larger particle of ca. 0.5–1 μm, respec-
tively (Fig. S9†). Both samples were stable in various solvents.
TGA of the activated CPF-1 and Mn–CPF-1 had no obvious
weight loss until 350 °C (Fig. S10†). One may notice that the
frameworks of CPF-2 and Mn–CPF-2 started to decompose
around 230 °C, which can be attributed to a weak thermal
stability of squaraine linkage (Fig. S11†).
The inductively coupled plasma atomic mass spectrometry
analysis (ICP-MS) results showed that the density of active
Mn sites reached as high as 1.7 mmol g−1 for Mn–CPF-1 and
1.6 mmol g−1 for Mn–CPF-2. The Mn content of Mn–CPF was
found to be more than 10 times higher than the record
reported amount of Mn complex grafted on insoluble mate-
rials, like a mesoporous sieve.9 In this regard, the bottom-up
strategy could facilitate a metalloporphyrinic catalyst with a
high-density active site, owing to their accessibility, facile
derivatization and ability to bind a wide variety of metal ions,
like Cu and Fe ions.
Recent research also showed that framework catalysts with
an AB-stacked 3D net, constructed by Mn–porphyrin as a
bridging linker and Zn as a node, provided accessibility for
the MnIJIII) active site to a molecular substrate and accord-
ingly exhibited excellent catalytic properties.10 Thus, we were
encouraged to demonstrate the catalytic efficiency of metallic
CPF materials. First of all, styrene was employed to investi-
gate their different catalytic performance. Although the
conversion of styrene was very high, a selectivity of 68% for
Mn–CPF-1 and a selectivity of 60% for Mn–CPF-2 were
revealed after 24 h (Table 1, entries 1 and 2). Mn–CPF-1 led
to a faster reaction rate compared to Mn–CPF-2 as shown in
Fig. S12.† The comparably low selectivity of styrene epoxides
was related to the low electron density of styrene which
usually reduced their nucleophilicity toward the electrophilic
oxygen of porphyrin–MnIJV)O.
Fig. 1 Schematic representation of molecular building blocks of CPF-1
and CPF-2.
into monomers, metal–porphyrin complexation was employed.
As expected, the resultant Mn–HTCPP showed characteristic
peaks as m/z = 899 in the MALDI-MS spectra (Fig. S3†). CPF-1
and CPF-2 were obtained as purple solids in 60% and 55% iso-
lated yields, respectively. Mn–CPF-1 and Mn–CPF-2 were
obtained as green powder in 52% and 46% yields, respectively.
As shown in Fig. S4 and S5,† the FT-IR spectra of CPF-1 and
Mn–CPF-1 showed stretching modes at 1660 cm−1 that are
characteristic of CN moieties. For CPF-2 and Mn–CPF-2, the
FT-IR exhibited a vibration band at 1521 cm−1, which is charac-
teristic of a CO band from squaraine (SQ).
The crystalline structure of the synthesized CPF materials
was resolved by XRD measurements in conjunction with
structural simulation. Atomic positions and cell sizes of
modelled COF layers were optimized using Material studio
software. XRD studies on CPF-1 and CPF-2 in Fig. S6 and S7†
indicated certain accordance between experimental patterns
and the simulated patterns based on the modelled structure.
Although the intensities of the XRD patterns for CPF-1 and
CPF-2 were poor, the experimental XRD patterns of CPF-1
and CPF-2 also partly matched well with the simulated
pattern of the AB stacking model in Fig. S6 and S7† relative
to the AA stacking model. Obviously, it was still not yet clear
for other diffraction peaks (40° > 2θ > 15°) in CPFs. The
cross-linking of CPFs may give rise to uncertain conforma-
tion. Nevertheless, Mn–CPF-1 and Mn–CPF-2 did not reveal
any obvious diffraction peaks under the same solvothermal
growth conditions, implying that they were composed of
an amorphous network. This was probably due to the dif-
ferent solution between porphyrins and metalloporphyrins.
Excellent heterogeneous catalysts should not only have
high catalytic activity and selectivity but also be structurally
stable and thus be easily recovered for continuous usage.
Compound Mn–CPF-1 can be simply recycled by filtration,
which was subsequently reused in successive runs (Fig. S13†).
Although the reaction rate decreased, the recycled Mn–CPF-1
still exhibited a very high conversion of 96% and a selectivity
of 62% when styrene was being used as a substrate, thus
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Table 1 Scope of Mn–CPF-catalysed epoxidation of alkenesa
Entry
1
Substrate
Product
Catalyst
Conversionb (%)
99/96d
Selectivityc (%)
68/62e
Mn–CPF-1
2
3
4
Mn–CPF-2
Mn–CPF-1
Mn–CPF-1
93
99
95
60
>99
>99
5
6
7
Mn–CPF-1
Mn–CPF-1
Mn–CPF-1
81
66
20
>99
>99
>99
a
Olefin (1.0 mmol), TBHP (1.5 mmol), the catalyst (0.01 mmol), acetonitrile (3.0 mL) and bromobenzene (50 mg) as internal standards sealed
b
c
in Teflon-lined screw cap vials were stirred at 80 °C for 24 h. Conversion (%). Selectivity (%) was determined by GC using an SE-54 column
(50 °C for 1 min, then 10 °C min−1 up to 140 °C and 140 °C for 15 min). After the third cycle. The by-products were benzaldehyde and
d
e
benzenacetaldehyde.
indicating that Mn–CPF-1 was indeed a heterogeneous
catalyst for styrene epoxidation (Table 1, entry 1).
35% (Table 2, entry 3). No increase of trace product was
detected after the filtrate from a mixture of Mn–CPF-1 in
acetonitrile (Table 2, entry 4). Low epoxide yield was also
obtained when no catalytic active sites were added
(Table 2, entries 5 and 6). The different catalytic stabilities of
Mn–CPF-1 and Mn–HTCPP in the catalytic epoxidation of ole-
fins can be attributed to the homogeneous metalloporphyrin
catalyst having very high suicidal inactivation by formation of
the catalytically inactive u-oxometalloporphyrin dimers,11
whereas integration of metalloporphyrins within the pore
surfaces of Mn–CPF-1 can significantly enhance and sustain
the catalytic activities by blocking formation of the inactive
species.
Although Mn–CPF-1 showed superior catalytic efficiency
compared to Mn–CPF-2, the nature of Mn sites in Mn–CPFs
has not been revealed. To identify this point, XPS analysis
can help to reveal the oxidant state of Mn sites of molecules
in Mn–CPFs. As shown in Fig. 2, one may notice that the peak
value of Mn2p3/2 in Mn–CPF-1 shifted to a higher value of
642.6 eV, with respect to the value of 642.1 eV in Mn–CPF-1
and 642.2 eV in the previous research.12 In this case, the
redox properties of the Mn complexes in Mn–CPF-1 were
greatly influenced after being assembled into extended π
systems; that is, the charge was positively transferred from
the MnIJIII) centre to the linkage. There was a desirable
The catalytic activity of the epoxidation of cyclohexene
and cyclooctene with different steric sizes was further investi-
gated under the same conditions (Table 1, entries 3 and 4).
Obviously, Mn–CPF-1 exhibited high catalytic activity and
selectivity for epoxidation of cyclohexene. However, cyclo-
octene with a larger steric size led to a reduced conversion,
which suggested that the active catalytic sites should be the
Mn–porphyrin moieties within the pores of Mn–CPF-1. In
order to further understand the steric effect on catalytic
efficiency, a range of natural alkenes were selected to be
oxidized in this catalytic system. Increasing the length of lin-
ear alkenes triggered lower epoxide yield due to the different
steric sizes of the substrates (Table 1, entries 5–7).
To confirm the aforementioned claim, we compared the
catalytic activities of Mn–CPF-1 with their molecular compo-
nents of MnCl2 and Mn–HTCPP under identical reaction
conditions. Mn–HTCPP showed quite moderate catalytic
activity that could transform 79% of the cyclohexene into
cyclohexene oxide (Table 2, entry 2), but it was still not as
efficient as heterogeneous Mn–CPF-1 (99% conversion).
MnCl2 showed lower catalytic activity with a conversion of
Table 2 Scope of catalysed epoxidation of cyclohexenea
Entry
Catalyst
Conversionb (%)
Selectivityc (%)
1
2
3
4
5
6
Mn–CPF-1
Mn–HTCPP
MnCl2
99
79
35
20
19
32
>99
53
78
4
5
7
Filtrated
Blank
CPF
a
Olefin (1.0 mmol), TBHP (1.5 mmol), the catalyst (0.01 mmol),
acetonitrile (3.0 mL) and bromobenzene (50 mg) as internal
standards sealed in Teflon-lined screw cap vials were stirred at 80 °C
Fig. 2 (a) XPS spectra for the clean Mn–CPF-1 surface, the fresh
catalyst (black line) and Mn–CPF-2 (red line) and (b) XPS spectra for
Mn–CPF-2, Mn 2p3/2 and the high-valent oxo region.
b
c
for 24 h. Conversion (%). Selectivity (%) was determined by GC
using an SE-54 column. d After catalytic assay for Mn–CPF-1.
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catalysis-promoted electronic environment around the MnIJIII)
center in Mn–CPF-1. Therefore, Mn–CPF-1 possesses a more
efficient performance than Mn–CPF-2, which derives from
their catalytic data. In Mn–CPF-2, for example, there was a
satellite peak at 646.4 and 654.2 eV, respectively, beside the
main peak of Mn2p3/2. The resulting satellite peak may be
attributed to the manganese–oxo species from the surface of
the catalyst.
In summary, we successfully constructed two kinds of
covalent porphyrinic frameworks based on the crystal struc-
ture of HTCPP via bottom-up approach, namely, CPF-1 with a
hydrazone linkage and CPF-2 linked by the squaraine link-
age. After incorporating the catalytic MnIJIII) centre into
monomers, the catalytic activity was carefully examined. The
results showed that Mn–CPF-1 was the most effective among
those tested involving homogeneous and heterogeneous sys-
tems. Of further importance, a detailed XPS study evidenced
that Mn–CPF-1 had a more desirable electronic environment
than Mn–CPF-2. This could help to further understand the
relationship between the material structure and catalytic
activity.
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Acknowledgements
The authors give special thanks to Dr. Liu from East China
University of Science and Technology for his help in the
analysis of XPS data. This work was supported financially by
the National “Twelfth Five-Year” Plan for Science & Technology
(2012BAD32B03), the National Natural Science Foundation of
China (20903048) and the Innovation Foundation in Jiangsu
Province of China (BY2013015-10).
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