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INorganic
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frontiers
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: Q. Sun, M. Liu, K.
Li, Y. Han, F. Chai, Y. Zuo, C. Song, G. zhang and X. Guo, Inorg. Chem. Front., 2016, DOI:
10.1039/C6QI00441E.
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Page 1 of 36
Inorganic Chemistry Frontiers
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DOI: 10.1039/C6QI00441E
frameworks and their catalytic activity for phenol
degradation at mild conditions
Qiao Suna, Min Liua, Keyan Lia, Yitong Hana, Yi Zuoa, Fanfan Chaia, Chunshan
Song*a,b, Guoliang Zhangc, and Xinwen Guo*a
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a
State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy
Research, School of Chemical Engineering, Dalian University of Technology, Dalian
116024, People’s Republic of China
b
EMS Energy Institute, PSU-DUT Joint Center for Energy Research and Department
of Energy & Mineral Engineering, Pennsylvania State University, University Park,
Pennsylvania 16802, United States
c
College of Biological and Environmental Engineering, Zhejiang University of
Technology, Hangzhou 310014, People’s Republic of China
*X. Guo. E-mail: [email protected]; Fax: +86-0411-84986134; Tel:
+86-0411-84986133
*C. Song. E-mail: [email protected]; Fax: +1-814-863-4466; Tel: +1-814-865-3573
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Synthesis of Fe/M (M=Mn, Co, Ni) bimetallic metal organic
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Abstract
Partial isomorphic substitution of iron in Fe(BDC)(DMF,F) metal organic framework
by manganese, cobalt, and nickel was described for the first time. Specifically,
different amounts of Mn, Co and Ni have been incorporated into the Fe-based
framework during solvothermal crystallization procedure. Several characterization
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techniques, including XRD, FT-IR, SEM, EDS, TG, XPS and ICP-AES, strongly
support the effective incorporation of Mn, Ni and Co into the material frameworks.
The catalytic performance of these materials was examined in liquid-phase
degradation of phenol at 35 C and near neutral pH of 6.2. Results present that the
degradation efficiency can be evidently improved by the incorporation of Mn, while it
can be inhibited by the incorporation of Ni. The incorporation of Co shows no
remarkable influence on the degradation process. Moreover, the ratios of n(Fe)/n(Mn)
in the bimetallic MOFs have strong impact on the degradation process. The stability
and reusability of these catalysts under mild conditions were also demonstrated in this
study. This work illustrates the potential of bimetallic MOF structures in developing
active heterogeneous catalysts for toxic compounds degradation process.
1. Introduction
Increasing contamination and deterioration of water quality has been a serious
concern for human health and the sustainable development. Wastes from chemical
plants, paper mills and farming usually contain aromatics, heavy metals, organic dyes
and sulfur- or nitrogen-containing compounds, which are considered as the most
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phenolic compounds are toxic to aquatic organisms even at quite low concentration
and hence treatment of phenolic pollutants is essential before disposal2. Advanced
oxidation processes, including Fenton, photo-Fenton, sonolysis, ozone oxidation and
their combination, are used for the removal of phenol. Fenton process is of great
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practical significance due to its high efficiency, simplicity, easy reproducibility and
easy handling. Despite its environmentally friendly nature, the application of Fenton
process is limited by the final production of ferric iron sludge, which can increase
excess costs of disposal. In order to solve this problem, most researchers concentrate
on heterogeneous Fenton-like catalysts, such as pillared clays3, zeolites and
mesoporous silica materials4-7. However, most studies were carried out at 50 C or
even higher temperature at acidic pH value8. Therefore, effective catalysts should be
designed and fabricated to make the process take place at mild conditions.
Metal-organic frameworks (MOFs) are porous coordination polymers, which are
constructed from metal-containing nodes and organic ligands9-12. Owing to their
particular high surface area, large pore volume, low density and easily tunable
framework, MOFs have many potential applications such as separation, gas storage,
catalysis, drug delivery, and battery13-19. Recently, MOF-based materials have been
studied
in
heterogeneous
selective
oxidation
of
aromatic
substrates
and
cyclooctene20-22, olefin hydrogenation, 4-nitrophenol reduction and CO oxidation23,
Knoevenagel condensation and Suzuki coupling reaction24,25. Besides, some studies
have dealt with photocatalytic properties of MOFs26, such as hybrid CdS/UiO-66,
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important reason for water pollution1. Among these hazardous chemicals, phenol
and
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graphitic
carbon
nitride/MIL-125(Ti),
DOI: 10.1039/C6QI00441E
Fe(II)@MIL-100(Fe),
Ag2O/Cu(tpa)(dmf) and MIL-53(Fe)27-33.
The catalytic activity observed for MOFs is concerned with its metallic
components, either as isolated metal centers or metal clusters10. Pristine MOFs doped
or incorporated with another one or more metal centers have attracted much attention
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in recent years, because this combination may enhance their particular activity34,35.
For example, MOF-5 is moisture sensitive even under atmospheric conditions. In
order to overcome this drawback, Ni(II)-doped MOF-5 was obtained through a
solvothermal method36. HKUST-1 is a MOF material widely used as an adsorbent for
the separation of CO2, which can be chemically reduced by doping alkali metals (Li,
Na and K)37. CO2 adsorption tests showed that the storage capacity of HKUST-1
doped with moderate quantities of Li+, Na+ or K+ individually was greater than that of
the unmodified HKUST-1. Zn and Cu are adjacent in the periodic table of elements,
and they have similar features, thus a bimetallic Zn/Cu-BTC material was synthesized
via a microwave-assisted method38. Due to the synergetic effect between Cu and Zn,
this Zn/Cu-BTC exhibited higher desulfurization capacity than Cu-BTC and those
reported zeolites.
Inspired by the possibility to enhance catalytic performance through introducing a
new metal component into MOFs, a series of new bimetallic Fe/M-MOFs (Mn, Co, Ni)
were successfully synthesized via a direct solvothermal method in this work. The
major objective of this work is to evaluate the performance of the new bimetallic
MOFs as heterogeneous Fenton-like catalysts. The textural and surface chemistry
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UiO-67,
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phenol degradation. As expected, the catalytic performance of the Fe/Mn-MOFs
samples was higher than that of the original Fe-MOFs, which could be attributed to
the cooperative effect of Fe and Mn. Further, the stability of the catalysts was also
assessed in consecutive runs. A comparison of catalytic property was made among the
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bimetallic Fe/Mn-MOF, Fe/Co-MOF and Fe/Ni-MOF materials. However, it turned
out that the Fe/Co and Fe/Ni samples didn’t perform as well as the Fe/Mn-MOFs.
2. Experimental
2.1 Materials
Terephthalic acid (1,4-BDC) was purchased from Aladdin. Ferrous chloride
tetrahydrate (FeCl2·4H2O), manganese(II) chloride tetrahydrate (MnCl2·4H2O), cobalt
chloride hexahydrate (CoCl2·6H2O), nickel chloride hexahydrate (NiCl2·6H2O), N,
N-dimethylformamide (DMF), methanol, hydrofluoric acid (HF), phenol and
hydrogen peroxide (H2O2) were obtained from Tianjin Kemiou Chemical Reagent Co.,
Ltd. (China). All reagents were used as received without further purification.
2.2 Sample Preparation
Fe(BDC)(DMF,F) was prepared through a solvothermal method similar to MIL-53(Fe)
in the literature39,40. In a typical synthesis, terephthalic acid (4 mmol, 0.67 g) and
FeCl2·4H2O (4 mmol, 0.80 g) were dissolved in 20 mL DMF, respectively.
Ultrasonic-assisted or stirring method was adopted to accelerate the dissolution. Then
the two solutions were transferred into a 100 mL Teflon-lined autoclave, and HF (0.8
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features of these materials were analyzed and linked to their catalytic behaviors
in
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minutes by ultrasonic instrument. Finally, the autoclave was kept in an oven at 150 C
for 3 days. After being cooled to room temperature in air, the sample was obtained by
centrifugation and washed by DMF, methanol and water. The product was dried at 60
C in a vacuum oven. Moreover, a series of Fe/M-MOFs (Fe/Mn, Fe/Co and Fe/Ni)
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was synthesized, individually, based on the above procedures. The specific description
was present in Table 1.
In order to compare the structural change, Mn-MOFs, Co-MOFs and Ni-MOFs
were synthesized, respectively, adopting a method similar to the above
Fe(BDC)(DMF,F). 4 mmol MCl2·xH2O (M=Mn, Co or Ni) and terephthalic acid (4
mmol, 0.67 g) were separately dissolved in 20 mL DMF. HF (0.8 mL, 5 M) was added
into the mixed solution. Then the mixture was poured into a 100 mL autoclave, stirred
and put in an oven at 150 C for 3 days. The subsequent treatment was analogous to
that of the Fe(BDC)(DMF,F) samples.
2.3 Characterization
Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku SmartLab(9)
diffractometer, using Cu Kα X-rays at a scanning rate of 8 /min between 5 and 50 .
Fourier transform infrared spectra (FT-IR) were recorded on an EQUINOX55
spectrometer (Bruker, Germany) by means of the KBr pellet technique. The
morphology and energy dispersive spectrometry (EDS) of the synthesized samples
were observed using a field-emission scanning electron microscopy (NOVA
NanoSEM 450) at an accelerating voltage of 10.0 kV. Thermal behavior was
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mL, 5 M) was added to the solution. The resulting mixture was agitatedDOI:
for10.1039/C6QI00441E
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photoelectron spectra (XPS) of all samples were recorded on a VG ESCALAB250
Electron Spectrometer with a monochromatic Al Kα (1486.6 eV) at 15 kV and 10mA,
and all binding energies were referenced to the C 1s peak (284.6 eV). The amount of
iron and manganese was analyzed by inductively coupled plasma-atomic emission
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spectrometer (Optima 2000DV, USA). The pH value was measured by a pH meter
(PHSJ-3F) at room temperature.
2.4 Degradation Experiments
The degradation test of phenol was executed in a batch reactor. A total of 10 mg
previously treated catalyst, 25 mL aqueous solution of 1000 mg L1 phenol and 6.22
mL 0.60 mol L1 hydrogen peroxide (H2O2) were added into the batch glass reactor.
The molar ratio of H2O2/phenol was 14, which was consistent with their
stoichiometric ratio in Fenton process. The pH of the reaction mixture was not
controlled, and the initial pH was 6.2. The reaction was conducted at 35 C for 3
hours, and the residual H2O2 was measured by iodometric titration. The liquid product
was analyzed on an Agilent high performance liquid chromatograph 1200 series
(HPLC 1200) with an Eclipse XDB-C18 (150 mm×4.6 mm×5 μm) column. The
conversion of COD was determined by potassium dichromate oxidation method.
3. Results and discussion
3.1 Structure analysis
Fig. 1 showed that the sample synthesized with FeCl2 had the characteristic peaks of
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evaluated by a SDT Q600 thermogravimetric analyzer (TA Instruments, USA).
X-ray
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peaks differed from that of the simulated pattern, which may be due to the difference
in the synthetic methods. The four Mn-incorporated MOFs displayed the same XRD
patterns as Fe(BDC)(DMF,F) with little decrease in intensity, suggesting that there
was no change in the crystal structure after the incorporation of manganese. No peak
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belonging to manganese species, like manganese oxides or manganese salts, was
detected, possibly due to the partial isomorphic substitution of iron atoms by
manganese atoms. Similar with the four Mn-incorporated samples, the XRD patterns
of the Fe/Co and Fe/Ni samples were characteristic Fe(BDC)(DMF,F) without extra
peaks (shown in Fig. S1 and S2). The sample synthesized with CoCl2 turned out to be
cobalt oxide, and the one prepared with NiCl2 was a Ni-BDC material42. The XRD
patterns of the synthesized Mn-MOFs (Fig. S3) were in good agreement with that of
the typical Mn3(1,4-BDC)3(µ-DMF)243.
The FT-IR spectra illustrated that these Fe/Mn-MOFs, Fe/Co-MOFs and
Fe/Ni-MOFs all displayed similar peaks in general, as shown in Fig. S4. The typical
absorption peaks were observed at 1657, 1560, 1373, 1017, 750 cm-1, which could be
attributed to the vibration of the carboxylate groups33. The broad peak at 3440 cm-1 is
referred to the stretching vibration of the O-H from the surface adsorbed water
molecules. The peak at 750 cm-1 corresponds to C-H bending vibration of the benzene.
In order to clearly demonstrate the difference among these curves, partial
magnification of the FT-IR spectra is shown in Fig. 2a. For Fe(BDC)(DMF,F), the
absorption band at 540 cm-1 is the characteristic stretching vibration of Fe-Olinker
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Fe(BDC)(DMF,F), as previously reported in the literature41. The intensity of
these
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Mn-Olinker mode was seen at 509 cm-1
46
. All of the four Mn-incorporated samples
displayed a weak band at 532 cm-1, which was 8 cm-1 lower than that of the
Fe(BDC)(DMF,F). This decrease signified modification in these Fe/Mn-MOFs, which
was caused by the incorporation of Mn into these frameworks. As for the
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Fe/Co-MOFs and Fe/Ni-MOFs (Fig. 2b and 2c), there present absorption bands at 537
cm-1 and 544 cm-1, respectively, which reflected slight structure modification in these
frameworks. Therefore, the results of XRD analysis and FT-IR spectra doubly
confirmed the formation of bimetallic Fe/Mn-MOFs, Fe/Co-MOFs and Fe/Ni-MOFs.
3.2 Morphology and elemental analysis
In order to investigate the morphologies of these Fe/Mn-MOF samples, SEM images
were collected and shown in Fig. 3. The images confirmed that these four
Mn-incorporated samples present uniform triangular prism structure similar to that of
the Fe(BDC)(DMF,F). This illustrates that the incorporation of Mn does not lead to
significant changes in morphology and particle size. In addition, EDS elemental
mapping was performed for the bimetallic Fe/Mn-MOFs as shown in Fig. 4. For the
four samples, the SEM images almost completely correspond to the images of the
EDS mapping of Fe and Mn. The Fe mapping follows the structures of the
Fe/Mn-MOF crystals, and the Mn mapping is simultaneously detected and consistent
with the Fe mapping, thereby indicating that Mn atoms are well dispersed in the
Fe/Mn-MOF crystals. The SEM images of the Fe/Co-MOFs and Fe/Ni-MOFs are
present in Fig. S5 and S6, respectively. Similarly, these two series of Fe/M-MOFs all
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mode44,45. While for the Mn-MOFs, Mn3(1,4-BDC)3(µ-DMF)2, the absorption
of
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reveal the existence of the well-dispersed Fe, Co and Ni ions (Fig. S7 and S8). These
results also demonstrate that the formation of Fe/Mn-MOFs, Fe/Co-MOFs and
Fe/Ni-MOFs.
As present in Table 2, the contents of Fe and Mn, and the real ratio of n(Fe)/n(Mn)
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in these four Fe/Mn-MOFs were determined by ICP-AES. The ratios of n(Fe)/n(Mn)
in these samples are close to the ratios of n(FeCl2)/n(MnCl2) in the feed solutions. The
data of the Fe/Co-MOFs and Fe/Ni-MOFs are shown in Table S1 and S2. The real
ratios of n(Fe)/n(Co) are quite far away from the experimental ratios, while the ratios
of n(Fe)/n(Ni) approach the experimental ratios. The contents of cobalt in the
Fe/Co-MOFs are much smaller than the manganese in the Fe/Mn-MOFs and the
nickel in the Fe/Ni-MOFs. This illustrates that the incorporation of cobalt into the
framework is more difficult than that of the manganese and nickel.
3.3 Thermogravimetric analysis
As shown in Fig. 5, thermogravimetric analysis profiles of Fe(BDC)(DMF,F) and the
four Fe/Mn-MOF materials all displayed two main steps of weight loss. The first
weight loss in the temperature range of 20-300 C can be attributed to the loss of
guest molecules, such as adsorbed water and DMF. The second step is due to the
decomposition of organic linkers. While Mn-MOFs exhibited two main steps of
weight loss in the temperature range from 320 to 550 C, indicating that the
framework is stable up to 320 C. A little weight loss of 4.96% was observed between
20 and 320 C, which was assigned to desorption of moisture and solvent molecules.
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10.1039/C6QI00441E
had the same shape in morphology like the Fe/Mn-MOFs. The EDS of these DOI:
samples
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they all present similar descending trend as that of the Fe(BDC)(DMF,F). This
demonstrates that the incorporation of Mn, Co and Ni has little impact on the thermal
stability of the frameworks.
3.4 Chemical states analysis
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XPS tests were carried out to identify the chemical state and binding energy of Fe and
Mn on the surface of the Fe/Mn-MOF materials. The XPS spectra of Fe 2p for
Fe(BDC)(DMF,F) are shown in Fig. 6a. The binding energies centered at 710.0 and
724.2 eV were ascribed to Fe(III) cations47. The shake-up satellite peak at 715.5 eV
was the fingerprint of Fe(III) species, which indicated that the surface iron is present
mainly in the Fe(III) oxidation state. The spectra for all the four Fe/Mn-MOF samples
were similar to that of the Fe(BDC)(DMF,F), implying that the chemical state of the
surface iron was the same after the incorporation of Mn in these frameworks.
However, the binding energy of Fe 2p3/2 varied after the incorporation of Mn in the
framework. As can be seen in Fig. 6, the binding energy of the four bimetallic samples
was 0.4 eV higher than that of the pristine Fe-MOFs, which illustrated that the cloud
density of the surface Fe of these samples were lower after the incorporation of Mn.
Fig. 7 displayed the Mn 2p spectra of the Mn-MOFs and the four Fe/Mn-MOFs.
Two main peaks corresponding to Mn 2p3/2 and Mn 2p1/2 were observed in the range
of 635-660 eV. The asymmetric Mn 2p3/2 peak confirmed the presence of
mixed-valence manganese. Therefore, the manganese oxidation states were further
analyzed by deconvolution. For the Mn-MOFs (a), the Mn 2p3/2 peak can be
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The TG curves of the Fe/Co-MOFs and Fe/Ni-MOFs are shown in Fig. S9 and
S10,
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Mn(III) (641.4 eV), and Mn(IV) (643.8 eV), respectively. This is consistent with the
literature48. However, the spectra of the four Mn-incorporated samples were different
from that of the Mn3(1,4-BDC)3(µ-DMF)2. The Mn 2p3/2 peak consisted of two
overlapping peaks, Mn(II) at 639.4 eV, and Mn(III) at 641.2 eV, which indicated that
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the Mn species mainly existed in the form of Mn(II) and Mn(III) with no Mn(IV) on
the surface of these four samples. In contrast to Mn3(1,4-BDC)3(µ-DMF)2, the
binding energy of Mn(II) and Mn(III) was a little lower in these four Fe/Mn-MOFs,
which means that the cloud density of the surface Mn of these four samples was
higher. This result is consistent with the above Fe 2p result, which further verifies the
incorporation of Mn into these frameworks.
3.5 Catalytic performance
The catalytic performance of the bimetallic MOFs was evaluated in the liquid-phase
degradation of phenol at mild conditions. The results of several blank runs are present
in Table S3. Based on the summary of the blank runs, phenol conversion and thermal
decomposition of H2O2 were negligible in either case. To rule out the possibility that
the catalysis occurs homogeneously in the phenol solution, a series of hot filtration
experiment was performed under the same reaction conditions (Fig. S11). After 30
minutes of reaction, the solid catalyst was removed from the reaction mixture by
filtration at the reaction temperature, and the resulting solution was continually
analyzed at the same reaction temperature for an additional 150 minutes. However,
the conversions of phenol on the Fe/Mn-MOFs, Fe/Co-MOFs and Fe/Ni-MOFs
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deconvoluted into three characteristic peaks that are assigned to Mn(II) (640.1
eV),
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heterogeneous and stops completely after the removal of solid catalysts. This hot
filtration test verifies the heterogeneity of these samples in this liquid-phase catalysis.
All the bimetallic MOFs were tested in the degradation of phenol, carried out
under mild conditions described above. The catalytic performance of the Mn-MOFs
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(Mn3(1,4-BDC)3(µ-DMF)2) was also studied in the degradation process, but the
conversion of phenol was below 1 %, which denoted that the catalytic performance of
this material can be ignored under this particular condition. The evolution of
conversion on the Fe/Mn-MOFs is presented in Fig. 8. It is clear that all catalysts are
able to decompose more than 90 % of the initial phenol content after 3 h of reaction.
The best performance was obtained with the Fe/Mn-MOF-71 sample, followed by the
Fe/Mn-MOF-91, Fe/Mn-MOF-51, and Fe/Mn-MOF-31 ones. The Fe(BDC)(DMF,F)
sample showed the lowest catalytic activity among all the Fe/Mn bimetallic catalysts.
The phenol removal rate seemed to be well related with the real n(Fe)/n(Mn) ratios of
these materials. Based on the results of ICP-AES, the n(Fe)/n(Mn) ratios were 2.88,
4.55, 7.01 and 8.88 for the Fe/Mn-MOF-31, Fe/Mn-MOF-51, Fe/Mn-MOF-71 and
Fe/Mn-MOF-91 samples, respectively. This analysis leads to the conclusion that the
n(Fe)/n(Mn) ratio has a strong impact on the reaction rate, and 7.01 is the optimal
value for the phenol degradation under conditions employed. Further, the degradation
results of the Fe/Co-MOFs and Fe/Ni-MOFs are shown in Fig. 9 and Fig. 10.
According to the conversion curves, the Fe/Co-MOF samples perform approximately
similar catalytic activity as Fe(BDC)(DMF,F), while the Fe/Ni-MOFs perform more
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remained unchanged, implying that the degradation of phenol is predominantly
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incorporation of Mn can efficiently improve the catalytic performance of Fe-MOFs,
the incorporation of Ni can impede this catalytic activity, and the incorporation of Co
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practically does no harm on the activity.
Fe(II) + H2O2 → Fe(III) + HO∙ + OH-
(1)
Fe(III) + H2O2 → Fe(II) + HO2∙ + H+
(2)
Mn(II) + H2O2 → Mn(III) + HO∙ + OH-
(3)
To explain the unique behavior of these bimetallic MOF samples, the reaction
·
mechanism should be considered. Generally, HO is taken as the active oxidizing
intermediate in the Fenton and Fenton-like process. According to this mechanism, the
reaction can be initiated by the reaction of H2O2 with Fe(II) or Fe(III) to produce
HO· or HO∙2 (Eqs. (1) and (2)), respectively49. As a transition metal ion with variable
valence, manganese ion can participate in the chain reaction of radicals in the catalytic
system. On one hand, Mn could exhibit the redox pairs Mn(II)/Mn(III), which can
produce HO· through the decomposition of H2O2 (Eqs. (3)). On the other hand,
electron transfer among the Mn species and the Fe species is a beneficial factor during
the degradation process50. According to the standard reduction potentials for Fe and
Mn (Eqs. (4) and (5)), the reduction of Mn(III) by Fe(II) is thermodynamically
·
favorable as shown in Eqs. (6). Briefly, the Mn species take a part in producing HO ,
and Mn(II) can be efficiently regenerated through electron transfer, thus better results
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worse than the former two series of bimetallic MOFs. We may presume DOI:
that10.1039/C6QI00441E
the
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mechanism among the Fe species and the Mn species is present in Fig. 11. Actually,
Co has similar effects as Mn, that is, HO· production and easy regeneration via
electron transfer process in the Fe-containing materials50,51. In our case, the contents
of Co in the Fe/Co-MOFs were much lower than that of Mn in the Fe/Mn-MOFs.
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Therefore, the Fe/Co-MOFs exhibit no remarkable activity as the Mn-incorporated
samples. According to Zhong51, only Ni(II) species are stable during the reaction. This
means Ni cannot work with H2O2 to produce hydroxyl radicals and further initiate the
degradation process. Besides, this inhibitory impact becomes more obvious as the Ni
contents increase. This exactly explains the unfavorable performance of the
Fe/Ni-MOFs.
Fe(III) + 1e- → Fe(II),
E° = 0.77 V
(4)
Mn(III) + 1e- → Mn(II),
E° = 1.51 V
(5)
Mn(III) + Fe(II) → Mn(II) + Fe(III),
E° = 0.73 V
(6)
3.6 Stability and reusability
The stability of these four Fe/Mn-MOFs under catalytic conditions was demonstrated
by comparing XRD patterns before and after catalysis, shown in Fig. S12. Apparently,
all the XRD patterns of the used materials were almost the same as the fresh ones with
a little decline in crystallinity. This signified that these catalysts were quite stable
under the conditions employed. In the case of these catalysts, the main metal portions
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were achieved on the Fe/Mn-MOFs rather than the Fe(BDC)(DMF,F). The synergetic
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ions, the amounts of Fe and Mn species leached into the solution were determined
after the degradation reactions. As illustrated in Table S4, the concentrations of Fe and
Mn species were quite low in the solution, suggesting that this impact can be
insignificant. Besides, the Fe/Mn-MOF-71 was recovered by filtration after the
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degradation tests in order to evaluate its reusability. The material was washed at room
temperature, and then dried at 70 C under vacuum overnight. Afterwards, the catalyst
was reused with fresh phenol solution. Fig. 12 showed that the phenol conversion
remained almost constant in three runs. While the H2O2 conversion decreased from
68.6 % to 65.3 %, the COD conversion dropped from 75.9 % to 72 %. These results
demonstrated that this catalyst was essentially stable and could be reused in this
degradation reaction.
4. Conclusion
We have successfully prepared three series of bimetallic MOFs via a direct
solvothermal
method,
Fe/Mn-MOFs,
Fe/Co-MOFs
and
Fe/Ni-MOFs.
The
characterization results with XRD, FT-IR and EDS confirmed the incorporation of Mn,
Co and Ni into the Fe-based MOF structures. The catalytic performance of these
materials was studied in the oxidative degradation of phenol at near neutral pH. It
turned out that the incorporation of Mn could promote the catalytic process, Co
exhibited no obviously favorable behavior, and Ni presented an apparently inhibitory
impact. The best degradation result was achieved on the Fe/Mn-MOF-71 in a 3 h
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of the MOFs are Fe and Mn. In order to assess the possible influence of the DOI:
leached
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the reaction efficiency. Moreover, the catalyst showed almost negligible leaching of
iron and manganese after the reaction. The catalyst can be easily reused after washing
and drying under vacuum.
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Supporting information
XRD patterns of the Fe/Co-MOF, Fe/Ni-MOF, Mn-MOF, Co-MOF and Ni-MOF
samples. FT-IR spectra of the Fe/Mn-MOFs, Fe/Co-MOFs and Fe/Ni-MOFs. SEM
images of the Fe/Co-MOFs and Fe/Ni-MOFs. ICP results of the Fe/Co-MOFs and
Fe/Ni-MOFs. TG curves of the Fe/Co-MOFs and Fe/Ni-MOFs. Hot filtration tests of
the Fe/Mn-MOFs, Fe/Co-MOFs and Fe/Ni-MOFs. Results of several blank runs.
XRD patterns of Fe(BDC)(DMF,F) before and after reactions.
Acknowledgements
This work was supported by the State Key Program of National Natural Science
Foundation of China (Grant No. 21236008, 21401017).
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Fig. 1 XRD patterns of the samples synthesized with FeCl2 and MnCl2
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Fig. 2 Partial magnification of the FT-IR spectra of the Fe/Mn-MOFs (a),
Fe/Co-MOFs (b) and Fe/Ni-MOFs (c)
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Fig. 3 SEM images of the Fe/Mn-MOFs
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Fig. 4 EDS mapping of metal elements in the four Fe/Mn-MOFs
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Fig. 5. TG curves of the Fe/Mn-MOFs
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Fig. 6. XPS spectra of Fe 2p for the Fe/Mn-MOFs and Fe(BDC)(DMF,F)
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Fig. 7. XPS spectra of Mn 2p for the Fe/Mn-MOFs and Mn-MOFs
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Fig. 8. Catalytic performance of the Fe/Mn-MOFs (Conditions: initial phenol
concentration, 1000 mg L1; n(H2O2):n(phenol)=14; initial pH 6.2; cat 0.064 g L1; 35
C, 1 atm, 3 h).
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Fig. 9. Catalytic performance of the Fe/Co-MOFs (Conditions: initial phenol
concentration, 1000 mg L1; n(H2O2):n(phenol)=14; initial pH 6.2; cat 0.064 g L1; 35
C, 1 atm, 3 h).
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Fig. 10. Catalytic performance of the Fe/Ni-MOFs (Conditions: initial phenol
concentration, 1000 mg L1; n(H2O2):n(phenol)=14; initial pH 6.2; cat 0.064 g L1; 35
C, 1 atm, 3 h).
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Fig. 11 The synergetic mechanism between the Fe and Mn species
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Fig. 12. The reusable test of the Fe/Mn-MOF-71 (Conditions: initial phenol
concentration, 1000 mg L1; n(H2O2):n(phenol)=14; initial pH 6.2; cat 0.32 g L1; 35
C, 1 atm, 3 h).
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Sample
n(FeCl2)/mmol
n(MCl2)/mmol
n(FeCl2)/n(MCl2)
Fe(BDC)(DMF,F)
4.0
0

Fe/M-MOF-91
3.60
0.40
9:1
Fe/M-MOF-71
3.50
0.50
7:1
Fe/M-MOF-51
3.33
0.67
5:1
Fe/M-MOF-31
3.0
1.0
3:1
M-MOF
0
4.0
0
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Table 1 Synthetic parameters of the bimetallic MOF samples
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Table 2 The contents of Fe and Mn and the real ratio of n(Fe)/n(Mn) in these
DOI: 10.1039/C6QI00441E
Fe/Mn-MOF-91
Fe/Mn-MOF-71
Fe/Mn-MOF-51
Fe/Mn-MOF-31
Fe/wt%
12.48
11.35
10.0
9.35
Mn/wt%
1.38
1.59
2.16
3.19
n(Fe)/n(Mn)
8.88
7.01
4.55
2.88
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Sample
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Fe/Mn-MOFs
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Results present that the incorporation of Mn can significantly promote the catalytic
process, Co exhibits no obviously favorable behavior, and Ni presents an apparently
inhibitory impact.
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Graphical abstract
DOI: 10.1039/C6QI00441E
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