ROMP polymer supported manganese porphyrins: Influence of C[dbnd]C bonds along polymer chains on catalytic behavior in oxidation of low concentration Fe2+

Article abstract of DOI:10.1016/j.reactfunctpolym.2020.104530

One unsaturated polymer support was prepared through ring opening metathesis polymerization (ROMP) of norbornene-2,3-dip-toluene sulfonate initiated by Grubbs 2nd initiator and manganese porphyrins were immobilized on polymer through transesterification reaction. To investigate the effect of C[dbnd]C bonds along polymer chains on the catalytic behavior, the obtained polymer supported catalyst (P-PPIXMnCl) was applied in oxidation of low concentration Fe²⁺ to mimic catalytic behavior of Ceruloplasmin. In the presence of P-PPIXMnCl, the conversion of Fe²⁺ reaches to 91.92% and 96.46% at 10 °C and 37.5 °C (body temperature), respectively. Compared to manganese porphyrins, P-PPIXMnCl can dramatically increase oxidation rate of Fe²⁺ and the catalytic kinetic shows that the oxidation reaction changes from second-order to third-order. Upon hydrogenation of ROMP polymer, the oxidation reaction still conforms to the second-order kinetics. Density functional theory (DFT) calculation shows that the C[dbnd]C bonds along polymer chains play an important role in the coordination with Fe²⁺ in the catalytic microenvironment. The real time morphology of supported catalysts in aqueous environment characterized by Cryo-TEM indicates that hydrogenation can shrink the morphology of polymer-water skeleton. The catalyst could be recycled six times without any significant loss in activity. The liner heterogeneous catalyst is expected to be used as drugs for treating excessive iron accumulation in the human body.

Full text of DOI:10.1016/j.reactfunctpolym.2020.104530

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ROMP polymer supported manganese porphyrins: Influence of
CC bonds along polymer chains on catalytic behavior in oxidation
of low concentration Fe2+
Fanfan Li, Xuan Wang, Huanhuan Zhao, Yanwu Zhang
PII:
S1381-5148(19)31269-6
DOI:
https://doi.org/10.1016/j.reactfunctpolym.2020.104530
REACT 104530
Reference:
To appear in:
Reactive and Functional Polymers
Received date:
Revised date:
Accepted date:
27 November 2019
8 February 2020
10 February 2020
Please cite this article as: F. Li, X. Wang, H. Zhao, et al., ROMP polymer supported
manganese porphyrins: Influence of CC bonds along polymer chains on catalytic behavior
in oxidation of low concentration Fe2+, Reactive and Functional Polymers (2019),
https://doi.org/10.1016/j.reactfunctpolym.2020.104530
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ROMP Polymer Supported Manganese Porphyrins:
Influence of C=C Bonds along Polymer Chains on Cata-
lytic Behavior in Oxidation of Low Concentration Fe2+
Fanfan Li, Xuan Wang, Huanhuan Zhao and Yanwu Zhang*
School of Chemical Engineering and Energy, Zhengzhou University, 100# Science Road, Zheng-
zhou, 450001, P.R. China
*Corresponding author.
*E-mail address: zhangyanwu@zzu.edu.cn
ABSTRACT
One unsaturated polymer support was prepared through ring opening metathesis polymerization
(ROMP) of norbornene-2,3-dip-toluene sulfonate initiated by Grubbs 2nd initiator and manganese por-
phyrins were immobilized on polymer through transesterification reaction. To investigate the effect of
C=C bonds along polymer chains on the catalytic behavior, the obtained polymer supported catalyst (P-
PPIXMnCl) was applied in oxidation of low concentration Fe2+ to mimic catalytic behavior of Cerulo-
plasmin. In the presence of P-PPIXMnCl, the conversion of Fe2+ reaches to 91.92% and 96.46% at 10
°C and 37.5 °C (body temperature), respectively. Compared to manganese porphyrins, P-PPIXMnCl can
dramatically increase oxidation rate of Fe2+ and the catalytic kinetic shows that the oxidation reaction
changes from second-order to third-order. Upon hydrogenation of ROMP polymer, the oxidation reac-
tion still conforms to the second-order kinetics. Density functional theory (DFT) calculation shows that
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the C=C bonds along polymer chains play an important role in the coordination with Fe2+ in the catalyt-
ic microenvironment. The real time morphology of supported catalysts in aqueous environment charac-
terized by Cryo-TEM indicates that hydrogenation can shrink the morphology of polymer-water skele-
ton. The catalyst could be recycled six times without any significant loss in activity. The liner heteroge-
neous catalyst is expected to be used as drugs for treating excessive iron accumulation in the human
body.
Fe2+
Fe3+
C
-OTs
Manganese porphyrin
Keywords: Ring-opening metathesis polymerization; Polymer support; Manganese porphyrins; Bionic
catalyst; Catalytic oxidation; Cryo-TEM
1. Introduction
In the organism, natural oxidase surrounded by polypeptides can fully play the catalytic effect in
the local hydrophobic microenvironment that formed by polypeptide chains. Similarly, macromolecular
chains provide the bionic hydrophobic microenvironment in catalytic process when catalysts are sup-
ported on macromolecules. This kind of biomimetic microenvironment can not only protect the catalyst
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from the destruction and deactivation caused by autoxidation and aggregation, but also improve activity
and selectivity of catalysts [1-4].
According to the principle of enzyme catalysis in life, the bionic chemical catalysts are designed to
mimic some active groups or parts in the biocatalysts [5-7]. Iron is a necessary metal element for human
body, and it plays an important role in oxygen transport, cell biological oxidation, DNA replication and
electron transfer. The blood circulation process is dependent on oxidization of Fe2+ catalyzed by serum
Ceruloplasmin (Cp) and transferrin Helper Protein (Hephaestin) [8, 9]. In the oxidation process of Fe2+,
Fe2+ ions firstly replace Cu2+ ions in Cp and then Fe2+ ions are oxidized to Fe3+ ions [10-13]. When Cp
is deficient or incapacitated, it can cause iron deposition in local tissues [14]. Therefore, a bionic cata-
lyst simulating the function of Cp in the oxidation of Fe2+ is required to treat disease arising from defi-
ciency or a function of Cp. In order to explore the influence of the microenvironment formed by poly-
mers on the biomimetic catalytic mechanism, oxidation of Fe2+ in the presence of polymer-supported
catalyst mimicking Cp was studied.
Over the last decades, metalloporphyrins have been successfully used as models for the cyto-
chrome P-450 enzymes. Porphyrins and metalloporphyrins with a large conjugated ring structure are
widely applied in catalysis, special materials and targeted drugs [15, 16]. However, the catalytic effi-
ciency of porphyrins will be reduced due to agglomeration in normal conditions [17-19]. Many studies
have shown that anchoring metalloporphyrins on suitable carrier materials can improve catalyst activity,
stability and product selectivity owing to the support environment. The common inorganic carriers in-
clude activated carbon, montmorillonite, silica, zeolite and molecular sieve [20-25]. Further, organic car-
riers such as polystyrene and resins are also used to immobilize metalloporphyrins [26, 27]. Among all
kinds of solid scaffolds, polymer scaffolds have attracted much attention due to their inherent character-
istics. Compared to rigid inorganic carriers, polymer with abundant conformations can build a flexible
catalytic microenvironment formed by C=C bonds along polymer chains and therefore catalytic activity
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and stability of metalloporphyrins can be improved obviously [28-31]. Ring-opening metathesis
polymerization (ROMP) has high functional group tolerance initiated by Grubbs catalysts and is widely
applied in synthesis of well-defined polymers [32-36]. The living features of ROMP are helpful to de-
sign and synthesize advanced structures such as metallocarbene-containing polymers, functional nano-
particles and topologic polymers [37-42].
For mimicking the natural enzyme, designing the skeleton of the model to select the access of the
substrate to metal center is a crucial object [43]. In our previous work, one bifunctional chelating su-
pramolecular polymer was prepared to down-regulate cellular iron uptake through chelating and reduc-
tion of Fe3+ [44]. Moreover, the catalytic oxidation of Fe2+ to Fe3+ by manganese complexes was previ-
ously reported [45-47]. Herein, we immobilized manganese porphyrins onto linear polymer and cata-
lyzed oxidation of low concentration Fe2+. To simulate polypeptide of Cp, polymer support was synthe-
sized by ROMP of 5-norbornene-2,3-dip-toluene sulfonate (NDT), π electrons of double bonds along
the polymer chains provided coordination sites of Fe2+ ions. Manganese porphyrins as active sites were
immobilized on poly (5-norbornene-2,3-dip-toluene sulfonate) (PolyNDT) through transesterification
reaction (Complete experimental process is shown in Scheme 1). The catalytic behavior of supported
manganese porphyrins (P-PPIXMnCl) at low temperature in oxidation of low concentration Fe2+ was
studied. The microscopic images obtained under vitrified, frozen hydrated condition confirm the state of
the catalyst in the water environment analyzed by Cryo-TEM imaging technique [48-50]. DFT method
was used to calculate the interaction energy between Fe2+ and polymer.
2. Experimental
2.1. Materials
The Grubbs 2nd catalyst (Sigma-Aldrich) and ethyl vinyl ether (98%, TCI), cis-5-norbornene-
endo-2,3-diacid anhydride (99%, J&K), dicyclopentadiene (99%, J&K), maleic anhydride (95%, Sigma-
Aldrich), methyl tert-butyl ether (99%, J&K), lithium aluminium hydride (97%, Sigma-Aldrich), 4-
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dimethylaminopyridine (DMAP, 99%, J&K), pyridine (99%, Sigma-Aldrich) was used after distillation.
Tetrahydrofuran (THF), anhydrous ether and dichloromethane (CH2Cl2) were used after refluxed onto
sodium or calcium hydride, 4-toluene sulfonyl chloride (99%, J&K), hemin (95%, J&K), benzene
(99.5%, J&K), tributyl phosphate (TBP, 99%, Sigma-Aldrich). MnCl2·4H2O (98%, TCI), N,N-
Dimethylformamide (DMF, 99.9%, Sigma-Aldrich), FeSO4·7H2O (99.9%, Sigma-Aldrich), H2O2 (31%,
Sigma-Aldrich).
2.2. Characterization
1H NMR spectra was recorded on Bruker AV 400 spectrometer with Si (CH3)4 as an internal stand-
ard. Gel permeation chromatography (GPC) measurement was carried out by an Agilent 1100 series
equipped with an RI-G1362A RI detector and a PL gel Mixed-C column using DMF as the mobile
phase at a flow rate of 1.0 mL·min−1 at 25 °C. UV-Visible (UV-Vis) absorption measurement was car-
ried out on Shimadzu UV-2450 spectrometer. Fourier Transform-Infrared Spectroscopy (FT-IR) was
recorded on an FT-IR Nicole spectrometer over the range of 4000-500 cm−1. Inductively Coupled plas-
ma mass spectrometry (ICP-MS) measurement was carried out with Optimass 9500 NWR-213. Ele-
mental Analyzer (EA) was carried out on vario EL III. The transmission electron microscopy (TEM)
was observed by FEI TalosF200S. Cryo-Transmission Electron microscopy (Cryo-TEM) was observed
by Talos F200C 200KV.
2.3. Experiment
2.3.1. Preparation of PolyNDT
In a vacuum glove box, Grubbs 2nd catalyst (0.009 g, 0.01 mmol) was dissolved in dichloro-
methane (1 mL), and then the solution was added in a single flask and stirred for 15 minutes at 25 °C.
After that, the monomer NDT (0.460 g, 3 mmol) dissolved in dichloromethane (15 mL) was added in
flask, the mixture was stirred for 6 h at 25 °C. Subsequently, the terminator vinyl ether (600 equiv) was
added and the stirring was continued for 0.5 h. After the reaction, the mixture was added into the cold
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ether (100 mL) and the light yellow solid was precipitated. Then the precipitate was dissolved in THF,
and the polymer solid was got again with ether as precipitator. Then the solid product was dried in vac-
uum at 25 °C to obtain the PolyNDT (0.228 g). Yield: 91%.
Scheme 1. Manganese porphyrins immobilized on PolyNDT and HPolyNDT.
2.3.2. Synthesis of P-PPIXMnCl
PolyNDT (0.560 g, 1.2 mmol) was dissolved in DMF (100 mL). Then the mixture was added into
three-necked flask. Manganese porphyrins (0.225 g, 0.4 mmol) and DMAP (0.015 g, 0.12 mmol) were
added, the compound was stirred for 3 days at 28 °C. After the reaction, the crude product was washed
by CH2Cl2 (100 mL × 3) and water (100 mL × 3) respectively. After centrifugation and vacuum drying
at 50 °C, then the dark product was obtained (0.644 g). Yield: 82%.
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2.3.3. Hydrogenation of PolyNDT
PolyNDT was dried for 24 h at 80 °C to remove trace moisture before the experiment. Then,
PolyNDT dissolved in DMF was placed in a high pressure hydrogenation reactor. N2 was used to purge
the reactor for 0.5 h firstly. Afterwards, the reaction solution was gradually heated to 150 °C under the
hydrogen pressure 1 MPa. H2 was continuously filled until the pressure in the reactor reached 4 MPa.
After 42 h of reaction, the liquid was cooled to room temperature before the reaction device was de-
compressed. And then the mixture was taken out and removed solvent by rotary evaporation, the Hy-
drogenated PolyNDT (HPolyNDT) was got.
The method that loaded manganese porphyrins on the HPolyNDT was consistent with the above
mentioned process 2.2, and the product HP-PPIXMnCl was obtained.
2.3.4. Catalytic oxidation of Fe2+ to Fe3+
FeSO4·7H2O was dissolved in 10 mL distilled water. 5 mL of the solution was fixed volume in 1 L
volumetric flask. And then 15 clean reaction bottles (20 mL) were marked as 10-0, 10-1, 10-2, 20-1, 20-
1, 20-2, 30-0, 30-1, 30-2, 40-0, 40-1, 40-2, 50-0, 50-1 and 50-2. 10 mg (n (Mn3+) = 15.38 μmol) manga-
nese porphyrins were added in the reaction bottles which labeled 10-1, 20-1, 30-1, 40-1 and 50-1. And
10 mg (n (Mn3+): 10.98 μmol) P-PPIXMnCl was added in the reaction bottles which labeled 10-2, 20-2,
30-2, 40-2 and 50-2. 15 mL of Fe2+ solution (3.53 μmol/L) was added into all reaction bottles, and each
bottle was injected with 50 μL 30% H2O2 (32.65 mmol/L). Then the mixture was homogenized by mag-
netic stirrer at 10 °C. Reaction time of the solution in bottles which labeled 10-0, 10-1 and 10-2 was 10
minutes; in 20-0, 20-1 and 20-2 was 20 minutes; 30-0, 30-1 and 30-2 was 30 minutes; 40-0, 40-1and 40-
2 was 40 minutes, and 50-0, 50-1 and 50-2 was 50 minutes. At the end of the reaction, the compounds in
flasks which labeled 10-1, 10-2, 20-1, 20-2, 30-1, 30-2, 40-1, 40-2, 50-1and 50-2 were filtered. Reaction
processes are shown in Scheme 2.
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Scheme 2. Catalytic oxidation of Fe2+.
2.4 Computational methods
As a complement to experimental results, DFT calculation is a powerful method to understand the
geometry structure and interaction between catalyst and reactant. The DFT studies on catalytic oxidation
of manganese porphyrin have been reported in previous literatures [51].
In this work, interaction energy between Fe2+ and P-PPIXMnCl (a)/HP-PPIXMnCl (b) were calcu-
lated by Accelrys Material Studio Dmol3 module. Catalyst molecules P-PPIXMnCl (a)/HP-PPIXMnCl
(b) and reactant molecule Fe2+ were optimized by the GGA and PBE funcational form, DNP (double
numeral plus polarization) basis set was used to describe the valence orbitals of the atoms [52]. The
convergence criterion of self-consistent-field calculation, tolerances of energy, maximum force, and
maximum displacement for the geometry optimization were 1×10-6 Hartree, 1×10-5 au, 0.002 au and
0.005 au, respectively. The interaction energy between Fe2+ and polymer was described as:
Einter = EFe2+ - polymer - Epolymer - EFe2+
3. Results and discussion
3.1. Materials Characterization
GPC was used to characterize ROMP polymer. Corresponding curves of monomer NDT and poly-
mer PolyNDT are shown in Fig. 1. The shift to left of the characteristic peak in Fig. 1 indicates that the
polymerization of NDT is achieved and the calculated PDI value of ROMP polymer is 1.56. The left
small shoulder peak is ascribed to the coupling termination of living chains [53]. The polymerization is
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also proved by 1H NMR spectra in Fig. 2. After ROMP, the position of C=C peaks shifts from 5.93 (a)
to 5.32 (a´). The monomer conversion is 54.54% according to the ratio between two characteristic peak
areas. The values of Mn, Mw, Mp and PDI of PolyNDT are shown in Table 1.
Fig. 1. GPC Curves of NDT (a), PolyNDT (b) and P-PPIXMnCl (c).
Table 1
The values of Mn, Mw, Mp and PDI for PolyNDT and P-PPIXMnCl.
Mn
Mw
Mp
PDI
PolyNDT
2.6748e5
4.1879e5
4.2163e5
1.56
P-PPIXMnCl
4.2009e6
4.3670e6
4.0194e6
1.04
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Fig. 2. 1H NMR spectra of comparison between vinyl peaks of NDT and PolyNDT in CDCl3.
Fig. 3. FT-IR spectra of PolyNDT (a), manganese porphyrin (b) and P-PPIXMnCl (c).
(a)
(b)
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Fig. 4. 1H NMR spectrum (a) and UV-Vis spectra (b) of P-PPIXMnCl.
a)
C
b)
1μm
1μm
c)
Mn
d)
O
1μm
1μm
Fig. 5. TEM and EDS mappings of C element (b), Mn element (c) and O element (d) for P-
PPIXMnCl.
When manganese porphyrins are immobilized onto PolyNDT, the GPC peak continues to shift to
left because of large size of manganese porphyrins in Fig. 1. The values of Mn, Mw, Mp and PDI of P-
-1
-1
PPIXMnCl are shown in Table 1. In FT-IR spectra (Fig. 3), 3059 cm and 1494 cm are attribute to the
=C-H and C=C absorption vibration peaks in benzene ring, S-O and S=O stretching vibration peaks ap-
-1
-1
-1
pear at 1210 cm and 1171 cm , respectively. 1121 cm belong to C-O stretching vibration peaks, and
-1
with characteristic peaks at 1654, 1453 and 1398 cm belong to the telescopic vibration absorption peak
-1
of C=C and C=N on porphyrin ring. The C-O-C stretching vibration peak at 1019 cm is observed and
which means manganese porphyrins are successfully immobilized through transesterification reaction.
The synthesized P-PPIXMnCl was characterized by 1H NMR. As shown in Fig. 4(a), the 1H NMR spec-
trum of P-PPIXMnCl displays an absorption peak at 5.07-5.14 ppm, which is the -CH=CH2 (o). Peaks at
1.916-2.177 ppm can be attributed to the chemical displacement of porphyrin ring (t), and the peaks at
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6.938-7.372 ppm belong to the absorption peak of at position 2, 7, 12 and 18 in porphyrin ring (b). The
chemical displacement of -CH2- absorption peak (m) that contacts with the ester bond moves to 4.44-
4.86 ppm at higher field. The absorption peak at 3.47-3.85 ppm corresponds to the chemical displace-
ment of -CH2- (h), the characteristic peaks at 2.21-2.31 ppm are ascribed to the -CH2-CH2- (g, i) at the
position of transesterification, the chemical shifts of -CH2- (k) and -CH2- (n) in porphyrin ring appear at
2.82-3.01 ppm and 2.62-2.68 ppm. In addition, the absorption peak at 1.62-2.17 ppm is the -CH2- signal
peak (f) on the main chain, and peak at 5.28-5.39 ppm is attributed to the -CH=CH- (a) on the main
chain. So the above analysis indicates successful synthesis of the P-PPIXMnCl. UV-Vis detection was
used to characterize PolyNDT and P-PPIXMnCl, and corresponding patterns are shown in Fig. 4(b). It
can be seen that P-PPIXMnCl shows a typical characteristic absorption peak (461 nm) at ultraviolet re-
gion. This peak exhibits a blue shift compared with the Soret band (481nm) of manganese porphyrin
(Fig. S9 line c) and it also has two absorption peaks (551 nm, 777 nm) of Q band for manganese por-
phyrin at 500-800 nm in the visible region. The absorption band at 283 nm can be ascribed to PolyNDT.
Results above provide a further proof that manganese porphyrins had been successfully immobilized on
the PolyNDT. Based on the calculation of elemental analysis, the transesterification ratio is 46.90%. In
order to indicate the distribution of manganese porphyrins in polymer support, EDS mappings of P-
PPIXMnCl are shown in Fig. 5. The consistent distributions of C, Mn and O elements show that man-
ganese porphyrins are successfully loaded on polymer chains.
1
The synthesis of HPolyNDT and PolyNDT were characterized by H NMR. As shown in Fig. 6,
the characteristic peaks at 5.2-6.0 ppm are ascribed to C=C in PolyNDT. After hydrogenation, the
chemical shift of the C=C in PolyNDT disappears, and the characteristic peaks at 1.69-2.01 ppm ap-
peared which belong to C-C in HPolyNDT backbone. This phenomenon indicates that PolyNDT hydro-
1
genation was completed. The H NMR of HP-PPIXMnCl is shown in Fig. 7(a), the absorption peaks at
3.36-3.59 ppm and 4.17-4.53 ppm correspond to the chemical displacement of -CH2- (h) and -CH2 (m).
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The appearance of new characteristic peaks (-CH2- (m)) indicates that the esterification reaction has
been carried out successfully. The chemical shift of -CH2- on the main chain appears at 1.79-1.91 ppm.
In addition, the absorption peaks at 1.93-2.06 ppm are the chemical displacement of porphyrin ring (t).
Results of 1H NMR analysis indicate the successful synthesis of the HP-PPIXMnCl. Fig. 7(b) shows the
UV-Vis spectra of HPolyNDT and HP-PPIXMnCl. The Soret band (459 nm) of HP-PPIXMnCl has a
blue shift compared with the Soret band (481nm) of manganese porphyrin (Figure S9 line c). In addi-
tion, there are two absorption peaks attributed to Q band of manganese porphyrin at 549 nm and 777 nm
in the visible region. The absorption band at 279 nm could be assigned to the HPolyNDT. These provide
a further proof that manganese porphyrin had been successfully immobilized on the HPolyNDT.
Fig. 6. 1H NMR spectra of HPolyNDT and PolyNDT.
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(a)
(b)
Fig. 7. 1H NMR spectrum (a) and UV-Vis spectra (b) of HP-PPIXMnCl.
Since the reaction medium of the catalyst is water, the morphological structures of catalyst before
and after hydrogenation (P-PPIXMnCl and HP-PPIXMnCl) in water system were analyzed by using
Cryo-TEM imaging technique. The microscopic images obtained under vitrified, frozen hydrated condi-
tion confirm the state of the catalyst in the water environment, as depicted in Fig. 8. Cryo-TEM analyses
allow direct, visual investigation of state of the catalyst in the water environment. In this study, the cata-
lyst structure is retained by freezing and subsequent preservation involved during sample preparation,
displaying a dispersion of catalyst (Black structure) in continuous aqueous medium (Gray background).
(a)
(b)
200nm
200nm
Fig. 8. Cryo-TEM micrographs of P-PPIXMnCl (a) and HP-PPIXMnCl (b).
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Fig. 8 shows the Cryo-TEM images of P-PPIXMnCl and HP-PPIXMnCl. It can be seen from Fig. 8
that the skeleton of the hydrogenated catalyst HP-PPIXMnCl is looser than P-PPIXMnCl. This may be-
cause that manganese ion interacts with C=C of the skeleton in the catalyst so that making it contract.
After hydrogenation, there is a weak interaction between C=C and manganese ion in the catalyst, result-
ing in a loose skeleton of the catalyst, which indicates that the catalyst provides a flexible microenvi-
ronment for the active center. In addition, from Cryo-TEM images, it can be seen that there is a large
amount of hydrophilic C=C in the catalyst before hydrogenation, which leads to a thicker skeleton in the
water medium. After hydrogenation, there is a lot of C-C with low hydrophilicity in the hydrogenated
catalyst, which finally makes the skeleton of HP-PPIXMnCl slightly thinner than that of P-PPIXMnCl.
Scheme 3. The likely mode of inter-
action of
manganese porphyrin and H2O2.
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Fig. 9. The curves of C (Fe3+)-reaction time t (min) at 10 °C using different catalysts (a, Catalyst - free;
b, 10 mg manganese porphyrins (n (Mn3+) = 15.38 μmol); c, 10 mg P-PPIXMnCl (n (Mn3+) = 10.98
μmol).); d, Blank experiment: the degree of oxidation of Fe2+ by air.
3.2. Catalytic behavior
As shown in Scheme 3, when H2O2 was used as oxidant and metalloporphyrin as catalyst, the in-
termediate of metalloporphyrin-hydrogen peroxide adduct (Compound I) is produced firstly. And then
the hemolysis of O-O bond for Compound I in aqueous solution or aprotic solvent resulted in the for-
mation of Compound II which is a porphyrin oxide intermediate with a hydroxyl radical and single elec-
tron. Finally, Compound III is obtained by transferring an electron from the hydroxyl radical [54-58].
To study the catalytic behavior of the supported manganese porphyrins, P-PPIXMnCl is used to catalyze
oxidation of low concentration Fe2+ and H2O2 as oxidizer. The conversion-reaction time curves are plot-
ted in Fig. 9. Both manganese porphyrins and P-PPIXMnCl have good catalytic activity. The conversion
of Fe2+ in the presence of P-PPIXMnCl rapidly increases to 77.27% in 10 min and is much higher than
that of manganese porphyrins. The maximum value of conversion is over 91.92% at 50 min. The re-
markable catalytic performance of P-PPIXMnCl is ascribed to the flexible micro-environment for man-
ganese porphyrins built by ROMP polymer. When manganese porphyrins are used, relatively few active
sites take part in the reaction because manganese porphyrins are easy to agglomerate. The polymer sup-
porter disperses manganese porphyrins and therefore more sites can take effect. Besides, we did a blank
experiment to study the degree of oxidation of Fe2+ by air rather than the H2O2. Blank experiment indi-
cates that air can oxidize Fe2+ slightly in the presence of P-PPIXMnCl, as it shown in Fig. 9 point d.
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Fig. 10. The curves of C (Fe3+)-reaction time t (min) at different amount of catalyst (a, 0 mg P-
PPIXMnCl; b, 5 mg P-PPIXMnCl (n (Mn3+) = 5.49 μmol); c, 10 mg P-PPIXMnCl (n (Mn3+) = 10.98
μmol); d, 15 mg P-PPIXMnCl (n (Mn3+) = 16.47 μmol)) at 10 °C.
Fig. 11. The curves of C (Fe3+)-reaction time t (min) at different temperature (a, 10 °C and b, 37.5 °C)
under the catalytic oxidation of 10 mg P-PPIXMnCl (n (Mn3+) = 10.98 μmol).
The catalyst is a key factor in the reaction and directly affects the efficiency of the reaction. As
shown in Fig. 10, the conversion of Fe2+ increases with the rise of catalyst dosage from 0 mg to 10 mg.
However, the conversion of Fe2+ did not change much when the catalyst dosage increases from 10 mg to
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15 mg. This shows that with the increase of catalytic dosage, the reaction is controlled by diffusion
when the amount is small, and the mass transfer efficiency cannot be synchronized when the amount
reaches a certain level, the reaction is controlled by activation energy, so the reaction conversion rate no
longer changes dramatically with the increase of the catalytic dose. In order to simulate the human body
environment, the catalytic oxidation reaction at the human body temperature (37.5 °C) is conducted and
the reaction curve is plotted in Fig. 11. The conversion rate of Fe2+ at 37.5 °C is faster than that at 10 °C
and 96.46% of conversion can be obtained. Therefore, P-PPIXMnCl can work more efficiently at the
human body temperature.
To explore the catalytic mechanism of P-PIXMnCl, the apparent kinetics of three reactions pro-
cesses were studied. The regression lines derived from kinetic data are plotted in Fig. 12 and the corre-
sponding kinetic equations are listed in Table 2. As shown in Fig. 12 and Table 2, the apparent rate con-
stants of different catalysts at the same temperature follow the order: k3(P-PIXMnCl) > k2(Manganese porphyrins)
>
k1(free catalyst). When the oxidation of Fe2+ is catalyzed by manganese porphyrins, the order of reaction is
still 2. But the order of reaction increases from 2 to 3 when P-PPIXMnCl is used as catalyst. This indi-
cates that polymer support changes the order of reaction. The parameters of the kinetic curves are shown
in Table (A), Table (B) and Table (C) in Fig. 12, respectively. To explain the effect of ROMP polymer
on the reaction mechanism, the apparent energy of activation (Ea) and diffusion factor (A) were calcu-
lated. The corresponding values are listed in Table 3. Because the catalytic active sites reduce the appar-
ent activation energy, the value of Ea of reaction 2 and 3 is lower than that of reaction 1. In the compari-
son of reaction 2 and reaction 3, Ea of reaction 2 is less than that of reaction 3 since the number of the
active sites in P-PPIXMnCl was lower than manganese porphyrins with the same catalytic dosage; the
diffusion factor A is enhanced three orders of magnitude in the presence of P-PPIXMnCl. This is be-
cause the dispersion of manganese porphyrins in the flexible polymer conformation leads to a sharp de-
crease in the diffusion resistance.
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19

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Fig. 12. (A) The apparent kinetic curves of reaction (2) and (3) at 10 °C; (B) The apparent kinetic
curves of reaction (2) and (3) at 37.5 °C; (C) The apparent kinetic curves of reaction (3) at different
amount of catalyst P-PPIXMnCl at 10 °C.
Table 2
The apparent kinetic models of three reactions.
d(Fe3+
)
=k[Fe2+]m[H2O2]n
dt
Apparent
Rate
Temperature
Reac-
tion
Catalyst
Reaction
Kinetic Equation
(°C)
m, n
Constant
k
Order
[Fe2+]0([H2O2]0-x)
[H2O2]0 ([Fe2+]0-x)
1
ln
ln
=0.281t+4.99
10
0.28
1.38
Second
Order
[H2O2]0-[Fe2+
]
0
m=1
n=1
H2O2
→
Fe2+
Fe3+ (1)
Free catalyst
[Fe2+]0([H2O2]0-x)
[H2O2]0 ([Fe2+]0-x)
1
=1.38t+21.4
37.5
Reaction
[H2O2]0-[Fe2+
]
0
10 mg
10
37.5
10
1.14
2.40
1.66
manganese
porphrins
(n(Mn3+):
[Fe2+]0([H2O2]0-x)
[H2O2]0 ([Fe2+]0-x)
Second
Order
1
ln
=1.14t+4.52
H2O2
m=1
n=1
[H2O2]0-[Fe2+
]
0
Fe2+ Manganese Fe3+ (2)
→
porphyrin
[Fe2+]0([H2O2]0-x)
[H2O2]0 ([Fe2+]0-x)
Reaction
1
ln
=2.40t+23.8
[H2O2]0-[Fe2+
]
0
15.38 μmol)
[Fe2+]0-x [H2O2]0-[Fe2+
]
1
[H2O2]0 [H2O2]0
2(ln
+
0+ln
-
+1)=1.66t+11.3
+1)=7.29t+25.8
10 mg
[Fe2+]0-x
[Fe2+
]
[Fe2+
]
[H2O2]0-x
([Fe2+]0-[H2O2]0)
0
0
Third
Order
P-PPIXMnCl
(n(Mn3+):
m=2
n=1
H2O2
Fe2+
Fe3+(3)
→
P-PPIXMnCl
[Fe2+]0-x [H2O2]0-[Fe2+
]
Reaction
1
[H2O2]0 [H2O2]0
2(ln
+
0+ln
-
10.98 μmol)
37.5
7.29
[Fe2+]0-x
[Fe2+
]
[Fe2+
]
[H2O2]0-x
([Fe2+]0-[H2O2]0)
0
0
5 mg
P-PPIXMnCl
(n(Mn3+):
[Fe2+]0-x [H2O2]0-[Fe2+
]
1
[H2O2]0 [H2O2]0
2 (ln
+
0 +ln
-
+1)=0.363t+0.425
10
0.36
[Fe2+]0-x
[Fe2+
]
[Fe2+
]
[H2O2]0-x
([Fe2+]0-[H2O2]0)
0
0
Third
Order
5.49 μmol)
m=2
n=1
H2O2
Fe2+
Fe3+(3)
→
P-PPIXMnCl
Reaction
15 mg
P-PPIXMnCl
(n(Mn3+):
[Fe2+]0-x [H2O2]0-[Fe2+
]
1
[H2O2]0 [H2O2]0
2(ln
+
0+ln
-
+1)=1.33t+19.4
10
1.33
[Fe2+]0-x
[Fe2+
]
[Fe2+
]
[H2O2]0-x
([Fe2+]0-[H2O2]0)
0
0
16.47 μmol)
Table 3
The Ea (apparent activation energy) and A (diffusion factor) values of the three reactions.
H2 O2
H2O2
H2O2
3+ ( )
3+ ( )
Fe2+
Fe
2
3+ ( )
Fe 3
Fe2+
Fe
1
Fe2+
→
→
→
Manganese porphyrin
P-PPIXMnCl
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(n (Mn3+): 15.38 μmol)
(n (Mn3+): 10.98 μmol)
4.10×104
Ea
A
4.43×104
3.51×107
2.59×104
6.57×104
2.37×107
To study the possible effect of double bonds on Fe2+, the polymer was added to the Fe2+ solution
and stirred for 0.5 h in the atmosphere of N2, the concentration of Fe2+ was measured before and after
stirring. It was found that the concentration of Fe2+ decreased from 0.198 mg/L to 0.189 mg/L after stir-
ring. The result shows that the C=C bonds of polymer chains have the enrichment effect on Fe2+ and as
a result the local concentration of Fe2+ around the catalytic active sites is relatively higher than that in
the bulk. Therefore, the oxidation of Fe2+ catalyzed by P-PPIXMnCl is a diffusion control process and
the increase of reaction order may be ascribed to the coordination effect of C=C bonds in ROMP poly-
mer.
In order to explore the influence of C=C on the oxidation of Fe2+ in the reaction, we hydrogenated
the carrier PolyNDT, and then loaded manganese porphyrins to get HP-PPIXMnCl to catalyze oxidation
of low concentration Fe2+. Fig. 13 shows the apparent kinetic curve of catalytic oxidation of Fe2+ by the
HP-PPIXMnCl. The catalytic oxidation of Fe2+ by HP-PPIXMnCl conforms to the second-order kinetic,
which is consistent with the reaction order of manganese porphyrins as catalyst and without catalyst.
The result well verifies our conjecture and proves that the apparent reaction kinetic in the reaction pro-
cess for catalytic oxidation of low concentration Fe2+ is influenced by C=C in P-PPIXMnCl. Table (D)
in Fig. 15 shows the parameters of the kinetic curve.
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Fig. 13. Apparent kinetic curve of reaction catalyzed by HP-PPIXMnCl at 10 °C.
a
b
Fig. 14. Optimized structures of Fe2+ with C=C of P-PPIXMnCl (a) and with C-C of HP-PPIXMnCl (b).
Table 4
The calculated interaction energy Einter ofFe2+ and ploymers.
Ploymers
Einter (kJ·mol-1)
-840.4
P-PPIXMnCl
HP-PPIXMnCl
-302.8
The Optimized structures of Fe2+ with C=C of P-PPIXMnCl (a) and with C-C of HP-PPIXMnCl (b)
are shown in Fig. 14. And the corresponding energies are listed in Table 4. The absolute value of inter-
-1
action energy between Fe2+ and C=C of P-PPIXMnCl (a) (840.4 kJ·mol ) is larger than that of Fe2+ and
22

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-1
C-C of HP-PPIXMnCl (b) (302.8 kJ·mol ). What’s more, the distance between Fe2+ and C-C is larger
than that of Fe2+ and C=C. In summary, larger absolute value of interaction energy as well as shorter
interatomic distance means the stronger interaction. These outcomes indicate that C=C has a stronger
interaction with Fe2+ so that the substrate Fe2+ gathers near the carrier and influences the reaction pro-
cess of catalytic oxidation of the substrate. This phenomenon is consistent with the kinetic experiment
results.
3.3. Catalyst recycling
The recyclability of P-PPIXMnCl was examined using the oxidation of Fe2+. The particles were
separated at the end of the reaction by centrifugation, washed and reused six times. Fig. 15 shows that
the P-PPIXMnCl is effective on the first three consecutive runs, the conversion rates are all above 90%.
By the sixth cycle, conversion rate of Fe2+ was 88.29%. They showed outstanding reusability without
significant catalytic activity losses, and the slightly reduced conversion rate (α (%)) of Fe2+ is attributed
to the very slow decomposition of the catalytic system in the oxidizer. The filtrate was also examined by
UV-Vis spectrophotometer for the presence of the manganese porphyrins after each recycling experi-
ment (Fig. S10). No detectable trace of manganese porphyrins is detected in the range 300-800 nm
(compare with line c in Fig. S9), and that confirms the absence of the manganese porphyrins in the fil-
trate. The results have strongly explained that it is induced by the better stabilization of the manganese
porphyrins after immobilizing. In general, the control experiments indicate that the observed catalytic
results are obtained from the heterogeneous catalysts. Moreover, there is no active component leaching
in the catalytic process. These data are consistent with the results of catalyst activity in cyclic experi-
ments.
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Fig. 15. Catalytic activity of the recovered catalyst. Recycling reaction conditions: P-PPIXMnCl 10 mg,
Fe2+ solution (3.53 μmol/L), H2O2 (32.65 mmol/L) at 10 °C for 50 min.
4. Conclusions
ROMP of norbornene-2,3-dip-toluene sulfonate is initiated by the Grubbs 2nd catalyst and manga-
nese porphyrins are successfully immobilized onto ROMP unsaturated polymer. The supported catalyst
(P-PPIXMnCl) can effectively catalyze oxidation of low concentration Fe2+ at low temperature. Because
polymer support overcomes agglomeration of manganese porphyrins, the diffusion resistance is in the
same order of magnitude with homogeneous reaction. In the presence of P-PPIXMnCl, the oxidation
reaction changes from second-order to third-order due to coordination effect of C=C bonds along poly-
mer chains. Upon hydrogenation of ROMP polymer, the oxidation reaction still conforms to the second-
order kinetic. And the interaction force between Fe2+ and C=C of P-PPIXMnCl is larger than that be-
tween Fe2+ and C-C of HP-PPIXMnCl according to DFT calculation. Therefore, the C=C bonds along
polymer chains play an important role through the coordination with Fe2+ in the catalytic microenvi-
ronment. Morever, The ture morphology of P-PPIXMnCl and HP-PPIXMnCl in aqueous environment
characterized by Cryo-TEM indicate that hydrogenation can shrink the morphology of polymer-water
skeleton. It is well explained that the C=C bonds along polymer chains provides a flexible microenvi-
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ronment for the active center of the catalyst so as to give full play to the catalytic performance of the
catalyst. The catalyst P-PPIXMnCl in the aqueous solution could be recycled to oxidize Fe2+, and con-
version rate of Fe2+ reduced slightly after six recycles, from 91.92% to 88.29%. From the studies of UV-
Vis spectra, we speculate that this is induced by the better stabilization of the manganese porphyrins af-
ter immobilizing. Therefore, P-PPIXMnCl is one potential candidate of Cp to maintain the iron homeo-
stasis of the body.
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the
data also forms part of an ongoing study.
Declaration of Competing Interest
The authors declare no competing financial interest.
Author information
Corresponding Author
*E-mail: zhangyanwu@zzu.edu.cn
ORCID
Yanwu Zhang: 0000-0002-5441-9961
Acknowledgments
We gratefully acknowledge the financial support from the National Natural Science Foundation of
China (21574119). We thank the Supercomputing Center in Zhengzhou University (Zhengzhou, China).
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