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Author’s Accepted Manuscript
Encapsulation of Keggin-type manganesepolyoxomolybdates in MIL-100 (Fe) for efficient
reduction of p-nitrophenol
Waqas Ali Shah, Laila Noureen, Muhammad Arif
Nadeem, Paul Kögerler
www.elsevier.com/locate/yjssc
PII:
DOI:
Reference:
S0022-4596(18)30353-0
https://doi.org/10.1016/j.jssc.2018.08.024
YJSSC20342
To appear in: Journal of Solid State Chemistry
Received date: 24 May 2018
Revised date: 16 August 2018
Accepted date: 18 August 2018
Cite this article as: Waqas Ali Shah, Laila Noureen, Muhammad Arif Nadeem
and Paul Kögerler, Encapsulation of Keggin-type manganese-polyoxomolybdates
in MIL-100 (Fe) for efficient reduction of p-nitrophenol, Journal of Solid State
Chemistry, https://doi.org/10.1016/j.jssc.2018.08.024
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Encapsulation of Keggin-type manganese-polyoxomolybdates in MIL-100
(Fe) for efficient reduction of p-nitrophenol
Waqas Ali Shaha1, Laila Noureena1, Muhammad Arif Nadeema1*, and Paul Kögerler b,c*2
a
Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan
b
Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen,
Germany
c
Jülich-Aachen Research Alliance (JARA-FIT) and Peter Grünberg Institute (PGI-6),
Forschungszentrum Jülich, 52425 Jülich (Germany).
manadeem@qau.edu.pk
paul.koegerler@ac.rwth-aachen.de
Abstract
Confining polyoxometalates inside the cavities of metal-organic frameworks is a method to
utilize these versatile molecular metal oxide clusters as quasi-heterogeneous catalysts and to
thereby broaden their applications. In this study, manganese-polyoxomolybdate anions of the
Keggin structure type, [PVMoVI11MnII(H2O)O39]5–
(Mn-POM), are encapsulated in an
iron(III)-based metal-organic framework (MIL-100) to achieve 30 wt.-% loaded MnPOM/MIL-100 composite material (1). The composite compound 1 displays excellent
catalytic activity for the selective reduction of p-nitrophenol into p-aminophenol (96 %) at 20
1
2
+92-51-9064-2062
+49-241-80-93642
°C in 25 min. The rate constant and catalyst activity are calculated as 0.10 min –1 and 328 L g–
1
s–1, respectively. Interestingly, both individual components of the composite material are
themselves inactive toward this reaction.
Graphical Abstract
The synergism between a Keggin-type manganese-polyoxomolybdates and MIL-100 (Fe)
turn the two individual inactive components into an active catalyst (PMOF composite) toward
reduction of p-nitrophenol.
Keywords: Polyoxometalate; metal-organic framework; reduction; catalysis; p-nitrophenol
2
3
1. Introduction
p-Nitrophenol (PNP) is a common by-product of many industrial processes including
production of synthetic dyes, herbicides and pesticides, insecticides, leather and rubber
processing etc. and thus exists in industrial effluents in significant quantities [1, 2]. PNP is a
toxic pollutant that is known to induce headache, nausea and cyanosis, damage to the nervous
system, kidney and liver even at low concentrations [3, 4]. Synthesis of p-aminophenol (PAP)
on a commercial scale is demanding due to its importance and usage as chemical intermediate
in the manufacturing of several products including, paracetamol, acetanilide, and some
industrial dyes [5]. Although various methods have been reported for the synthesis of PAP,
reduction of PNP into PAP offers an effective solution to an environmental issue and the
isolation of a value-added product at the same time. Reduction of PNP does not proceed in
the absence of a catalyst, however, in the presence of a suitable catalyst, it could easily be
reduced by NaBH4 [6, 7]. Silver nanoparticles (NPs) were used for the first time as a catalyst
for this reaction and reported by Pal in 2002 [8]. After that, different types of catalysts have
been reported, including metal nanoparticles (Pd, Pt, Ni), oxides (CuO, Co3O4) and some
composites (Fe3O4@PtPd, Pt-Au-PDA/rGO, Pd/rGO/Fe3O4) [9, 10]. However, these catalysts
are associated with some limitations such as high cost owing to the use of noble metals, low
efficiency and complex preparation methods. There are some recent reports on the use of
metal-free catalysts for this purpose. For example, Q.-W. Chen et al. reported the use of Ndoped graphene for the catalytic reduction of PNP into PAP without any by-product
generation [11]. Likewise, Q. Chen reported ZIF-8-derived N-doped carbon for this purpose
[12]. However, challenges such as high-temperature annealing and optimization of nitrogen
doping in graphene are still associated with these catalysts. Therefore, the exploration of
alternative catalysts that are easy to synthesize, efficient and environment-friendly remains an
important goal.
4
Polyoxometalates (POMs) are metal oxygen clusters of the early transition metals (in
particular, V, Mo and W) in their higher oxidation states. Numerous POMs have been a
matter of extensive attention due to their fascinating structures, remarkable catalytic activities
and applications in electrochemistry and medicines [13, 14]. Among a wide variety of POMs,
the Keggin-type family of general composition [XM12O40]n– (where X = Si4+, P5+, As5+ etc.
and M = W5+/6+, Mo5+/6+, V4+/5+ etc.) has been explored extensively. Their transition metal
mono-substituted derivatives, i.e [XMʹ M11O39]n–, can be obtained by substitution of one M
site by a transition metal cation (Mʹ ). Only a few reports are available on the application of
POM materials toward organic reduction reactions. For example, Parida and Rana reported a
Pd-substituted Keggin-type phosphotungstate supported on mesoporous silica for the
hydrogenation of PNP to PAP at room temperature [5]. Similarly, a manganese-substituted
polyoxomolybdate (IMFC-101) adsorbed on Au-NPs has been reported previously for the
reduction of PNP. However, removal of Au-NPs from the POM surface leads to a reduction
in activity by up to 50 % [15]. We therefore aim at the exploration of POM units in the
context of such reduction reactions.
Metal-organic frameworks (MOFs) are an important class of porous solid materials formed
by using the multidentate organic ligands (linkers) and metal ions (nodes). MOFs have
attracted the attention of many scientists due to their multidimensional potential applications
in gas separation, adsorption, gas storage, optics, magnetism, and heterogeneous catalysis etc.
[16-19]. Due to the existence of highly-defined cavities and pores in many reported MOFs,
they have also been employed for the encapsulation of many active species such as metal
NPs, POMs etc. The cavities and pores of MOFs not only act as microreactors for the activity
of NPs but can also improve the catalytic characteristics of these NPs via synergic effects.
Encapsulation of POMs in the cavities of MOFs to form PMOF composites is a topic of
recent investigations. PMOFs offer many advantages such as converting the homogenous
POM catalysts into heterogeneous catalysts, allowing for their facile recovery. In some cases,
5
the particular activity of a POM was enhanced from almost zero to a very high value as a
result of synergic effects [20]. For instance, [CoW12O40]6– only becomes active as a catalyst
for the oxygen evolution reaction (OER) after encapsulation in ZIF-8 [21]. It is well
established that the catalytic activity of POMs in many applications is significantly boosted
when they synergize with other adsorbents, e.g. graphene and MOFs, through electrostatic
interactions [22, 23].
In
this
study,
we
have
encapsulated
Mn(II)-polyoxomolybdate(VI)
anions,
[PMo11Mn(H2O)O39]5– (Mn-POM), in an iron(III)-based MOF (MIL-100 (Fe)) to obtain the
Mn-POM@MIL-100 (Fe) composite 1, which we tested for the first time in the catalytic
conversion of PNP into PAP (scheme 1). MIL-100 (Fe) has previously been used for the
adsorption and removal of PNP [24]. Previously, the MIL-100 (Fe) has also been utilized as
a support material for the reduction of PNP by Pt and Pd-NPs [25, 26]. We found that the
[PMo11Mn(H2O)O39]5– anion in itself is inactive towards this reaction, while the composite 1,
i.e.
[Fe3O(OH)(H2O)2{C6H3(CO2)3}2][PMo11MnO39(H2O)]0.30·8H2O, manifests
catalytic
activity in the reduction of PNP to PAP comparable to those of noble metal-based catalysts.
Besides, it is stable, recoverable and recyclable.
6
Scheme 1. (a) Schematic representation of MIL-100 (Fe) showing two types of pores with
diameters of 29 Å and 25 Å. (b) Ball-and-stick representation of the molecular structure of
Mn-POM. (c) Scheme of the hydrothermal synthesis of composite 1 and the reduction of
PNP by sodium tetrahydroborane catalysed by 1.
2. Experimental
1.1.
Materials and Reagents
Sodium
molybdate
dihydrate,
iron(III)
chloride
hexahydrate,
triethyl-1,3,5-
benzenetricarboxylate, hydrochloric acid, Na2HPO4, K2S2O8, tetrabutylammonium bromide
(TBABr) and p-nitrophenol were purchased from Sigma Aldrich. All chemicals were used
without further purification.
1.2. Synthesis of TBA4H[PMo11Mn(H2O)O39]·2H2O (TBA4HMn-POM)
The tetra-n-butylammonium salt of Mn-POM was prepared by following a previously
reported method [27]. Briefly, an aqueous solution of Na2MoO4·2H2O (41 mmol, 10 g in 10
mL) was added to a 10 mL aqueous solution of Na2HPO4 (3 mmol, 0.39 g). The resulting
7
mixture was heated for 10 min at 90 °C, cooled and the pH was adjusted to 4.8 by dropwise
addition of a 6 M HCl solution. To this solution, 4 mL aqueous solution of MnSO4·H2O (3
mmol, 0.51 g) was added, followed by addition of 4 mL aqueous solution of K2S2O8 (2
mmol, 0.41 g). This solution was kept at 90 °C for 2 h, followed by the addition of an
aqueous solution of TBABr (20 mmol, 5 mL). A solid product immediately precipitated
which was filtered, washed with water, ethanol and ethyl ether, and was dried.
1.3. Synthesis of Mn-POM@MIL-100 (Fe) composite (1)
The composite material (1) was prepared by hydrothermal synthesis [28]. A total 300 mg of
TBA4HMn-POM were added in 25 mL of a water-acetonitrile (4:1 v:v) mixture. To this
solution, FeCl3·6H2O (1.89 g) and triethyl-1,3,5-benzenetricarboxylate (2.72 g ) were added.
The mixture was stirred for 10 min at room temperature and transferred into a 50 mL Teflonlined autoclave which was kept in an oven at 130 °C for 72 h. After cooling, the orangebrown product was filtered, washed with water, ethanol and ether and dried at ambient
condition. The percentage yield was found to be 45% based on Fe, as calculated by following
formula.
1.4. Synthesis of Fe3O(OH)(H2O)2{C6H3(CO2)3}2·14H2O [MIL-100 (Fe)]
The MIL-100 (Fe) compound was also synthesized for comparison experiments. The
synthetic procedure is identical to that for composite 1 except that TBA4HMn-POM was not
added into the reaction mixture. Briefly, FeCl3·6H2O (1.89 g) was dissolved in 20 mL of
distilled water; 2.72 g of triethyl-1,3,5-benzenetricarboxylate were added to this solution. The
mixture was then transferred into a 50 mL Teflon-lined autoclave and kept at 130 °C in for 72
h. After cooling, the orange product was filtered, washed with ethanol and ether and dried at
ambient conditions. Yield: 49%.
8
1.5. Leaching Test
To check the stability of composite 1 and the long-term retention of Mn-POM inside the
MOF cages, a leaching test was performed. Typically, 5 mg of synthesized composite 1 was
immersed in 25 mL of distilled water. The samples were taken after 1 h, 3 h, 5 h, 24 h and 72
h and their UV-Vis spectra were recorded in the range of 200 to 450 nm to probe for any
leached-out traces of manganese hexahydrate ions originating from Mn-POM or Fe3+ from
MIL-100. Likewise, the samples were also analysed before and after the catalytic
experiments by AAS spectroscopy. In the UV-Vis spectra, there was no appearance of
relevant peaks. The AAS analyses showed that there were no significant traces of Mn/Fe ions
in the filtered solutions. These results confirmed that the Mn-POM did not leach out of the
MOF cages under the investigated conditions. This is attributed to the narrow window size of
the MOF cavities in comparison to the effective size of the solvated Mn-POM ions.
1.6. Catalytic reduction of PNP
To study the catalytic conversion of PNP into PAP, a freshly prepared NaBH4 (0.1 M, 3 mL)
solution was added into an aqueous solution of PNP (100 mL, 20 ppm), which turned from
yellow to yellow green. This colour change is an indication of formation of the pnitrophenolate ion [29]. To this solution, 10 mg of catalyst 1 was added with continuous
stirring. The conversion of PNP to PAP was monitored by UV-Vis spectroscopy and spectra
were recorded in the range of 250 nm to 500 nm. To check the effect of temperature, the
procedure was repeated at different temperatures between 20 °C to 45 °C.
1.7. Characterization
Powder X-ray diffraction (PXRD) patterns were recorded at room temperature on an X-ray
diffractometer (Bruker AXS D8, Cu Kα, = 1.5406 Å, 40 kV and 20 mA) in the range of 3–
30°. The simulated PXRD patterns were calculated using single-crystal X-ray diffraction data
from Cambridge Crystallographic Data Center (CCDC). FT-IR spectra of KBr pellets were
9
obtained with a Bruker Tensor II spectrometer in the 1800–500 cm–1 range. The nitrogen
adsorption and desorption studies were carried out using a Quantachrome Autosorb-6. Before
adsorption and desorption studies, all samples were degassed at 150 °C for 12 h in vacuum.
Pore size distribution was obtained via the Barrett-Joyner-Halenda (BJH) method using the
adsorption curve of the isotherm. The total pore volume was estimated by the amount of
nitrogen adsorbed at the relative pressure, P/P0 = 0.99 (bar). X-ray photoelectron
spectroscopy (XPS) studies were carried out on a Kratos Axis Ultra DLD spectrometer
equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) operating at 150 W.
The morphological and structural examination and elemental mapping were accompanied by
transmission electron microscopy (TEM, JEOL, JEM-2100F) and energy-dispersive X-ray
(EDX) spectroscopy. Thermogravimetric analysis (TGA) were carried out in air atmosphere
(60 mL/min) using a Perkin Elmer electrobalance TGA-7 at a heating rate of 10 °C per min.
UV-vis spectra were collected with an Agilent 8453 UV-vis diode array spectrophotometer
and Agilent ChemStation software using a spectral bandwidth of 1.0 nm. Samples were
measured in a quartz cuvette with a 1 cm path length were used for the measurements of
samples.
3. Results and Discussion
The composite material 1 was prepared by hydrothermal synthesis. During our trial to
encapsulate Mn-POM in MIL-100, we experienced that the encapsulation proceeds more
successfully in water:acetonitrile mixtures than in water alone. Alhough Mn-POM has also
been reported with other counter ions (Na+ and Cs+), these POM salts are not sufficiently
soluble in organic solvents [30, 31]. For this reason, the corresponding TBA salt was used,
which provides adequate solubility in water-acetonitrile mixtures. Moreover, decomposition
of the TBA ions at ca. 130 °C makes this POM as perfect candidate for the encapsulation.
10
Figure 1 shows the PXRD patterns of composite 1 and that simulated from the native MIL100 structure. The PXRD pattern of composite matches the simulated pattern of MIL-100
(Fe) suggesting the integrity of structure. As expected, no peaks attributable to Mn-POM
appear in the PXRD of 1, as the Mn-POM units are not present in a regular translationsymmetric pattern inside the pores of MOFs and thus do not show any diffraction [32, 33].
The PXRD results also suggest that there are no significant levels of impurities present in the
composite material.
Figure 1. PXRD of MIL-100 (Fe) (black) and composite 1 (red)
FTIR spectroscopic studies of Mn-POM, MIL-100 (Fe) and their composite 1 were
performed to confirm the encapsulation and preservation of structural integrity of both
components after encapsulation. The IR spectrum of TBA4HMn-POM (Figure 2) shows
major peaks that are in agreement with previously reported spectra, i.e. 1059(m), 1039(m),
938(s), 874(m), 821(s), 755(s), 499(w) [27]. The IR spectrum of MIL-100 (Fe) shows major
peaks at 1460(s), 1380(s), 1140(w), 760(w), 710(m), also in line with a previous report [28].
It is obvious from Figure 2 that the IR spectrum of 1 exhibit major peaks of both of its
components, i.e. Mn-POM and MIL-100 (Fe). This indicates the encapsulation of Mn-POM
in MIL-100 with no significant structural changes to either component. Slight peak shifts of
Mn-POM that are observed in the spectrum of 1 originate from electrostatic interactions
11
between the components of 1. For example, the characteristic Mn-POM P-O vibrations at
1059 cm–1 and 1039 cm–1 merge into a single peak at 1046 cm–1 in 1. This shift is not
surprising and has already been reported for other related composites as a result of
electrostatic interactions between POMs and MOF frameworks [28, 34].
Figure 2. FTIR spectra of MIL-100 (Fe) (red), TBA4HMn-POM (black) and their composite
1 (blue).
Thermogravimetric analyses of the pure MOF and the composite 1 was carried out in order to
identify differences in the weight loss steps of both materials (Figure 3). In case of MIL-100,
the first weight loss (28 %) at ca. 120 °C is attributed to the loss of water molecules present
in its cavities. The second weight loss amounts to is ca. 48 % and is related to the collapse of
structure due to removal of organic ligands in MIL-100 (Fe) between 320-350 °C. It is
evident that 1 shows a smaller weight loss as compared to pure MIL-100 in both of these
steps, a direct consequence of the incorporation of POM units. The first weight loss is ca.
20% which has been assigned to loss of water molecules. This weight loss is comparatively
lower than that of MIL-100 (Fe) because a fraction of the water molecules inside the cavities
has been replaced by Mn-POM units. A comparison of the second weight loss step, ca. 33%,
takes place between 350-400 °C for 1, suggests the higher thermal stability of 1 than MIL100 (Fe). This weight loss is due to removal of organic ligand from composite 1.
12
Figure 3. TGA of MIL-100 (Fe) (black) and composite 1 (red)
In order to establish the exact percentage loading of Mn-POM in MIL-100, elemental
analyses of pure MIL-100 (Fe) and composite 1 were performed (Table S1). These results
point to a total 30 wt.-% loading of Mn-POM in MIL-100. Combined results of TGA and
elemental analyses suggest the formula Fe3C18H35O30 for MIL-100, which is in agreement
with previous report i.e. Fe3O(OH)(H2O)2{C6H3(CO2)3}2·14H2O. The formula for composite
1 was calculated by following previously reported method [28, 35]. The amount of Mo was
found
as
ca.
23%
by
elemental
analysis,
and
the
theoretical
formula
[Fe3O(OH)(H2O)2{C6H3(CO2)3}2][PMo11MnO39(H2O)]0.30·8H2O is in agreement with this
percentage of Mo along with other elements.
Figure 4 shows the results of an XPS analysis that confirms the presence of Fe, Mo, P, Mn, C
and O in 1. Binding energies at 712.1 eV and 726.0 eV (Figure 4a) are characteristic of the 2p
edges of Fe3+. The doublet in Figure 4d with binding energies of 232.2 eV and 235.6 eV
confirms the existence of Mo6+. A doublet at 531 eV and 532 eV has been assigned to oxygen
components [36]. In Figure 4b, two peaks at 284 eV and 289 eV are due to phenyl and
carboxyl groups, respectively. The peak appearing at 285 eV is attributed to C at the surface.
13
A single peak at 133.9 eV in Figure 4e is due to phosphorous [37]. Figure 4c shows the
binding energies at 641.3 eV and 653.8 eV which are attributed to Mn2+ [38]. The presence of
Mo, Mn and P confirms the presence of POMs units inside the cavities of MOFs.
Figure 4. XPS spectra of composite 1.
The EDX elemental mapping and TEM analyses were performed to study the spatial
distribution of Mn-POM in the MOF and the morphology of MIL-100 and composite 1.
Figure 5 shows elemental mapping images of Fe, Mn, Mo, P, O and C that confirm a uniform
distribution of Mn-POM in the cavities of MOF. Figure 6 shows TEM images of MIL-100
(a) and composite 1 (b). Crystalline particles of MIL-100 are shaped as distorted cubes of
approx. 80-120 nm size. The particles of composite 1 retain the size and shape of the parent
MIL-100 particles. The TEM of 1 also indicates the absence of more than one type of
particles suggesting that POM units are not just present in the form of physical mixture but by
14
necessity need to be present in the cavities of MIL-100. The POM units cannot be seen in
TEM images because they are deeply encapsulated in the MIL-100 cavities in a random
fashion.
Figure 5. EDX elemental mapping of composite 1. Fe (a), Mn (b), P (c), C (d), O (e), Mo (f)
Figure 6. TEM images of MIL-100 (Fe) (a), and composite 1 (b)
Figure S1 shows the nitrogen adsorption isotherms of MIL-100 (Fe) and 1. From this data,
the values of pore volume and Langmuir surface area of MIL-100 (Fe) were found to be
0.882 cm3 g–1 and 2800 m2 g–1, respectively, in agreement with previous reports [28]. Pore
15
volume and the surface area values of 1 have been reduced to 0.325 cm3 g–1 and 1050 m2 g–1,
respectively, when compared with pure MIL-100. This behaviour is attributed to the
existence of POM units inside the pores of MOF. Furthermore, these results reveal that there
is still a residual porosity in 1 despite the accommodation of POM units, which is imperative
in catalysis for the diffusion of reactants.
The reduction of PNP to PAP by a catalyst in the presence of NaBH4 is actually a reaction
model commonly engaged for the assessment of catalyst’s potential. PNP strongly absorbs
light at 317 nm. Addition of NaBH4 in solution of PNP shifts the absorbance to 400 nm with
higher intensity due to formation of p-nitrophenolate ion. The chemical reduction of PNP to
PAP was monitored and measured by UV-visible spectroscopy at regular time intervals. The
reduction starts with the addition of catalyst into an aqueous solution of PNP in the presence
of an excess of NaBH4. A series of UV-Vis spectra were recorded, which allows the
investigation of the phenomenological kinetics of PNP conversion to PAP. Reduction of PNP
is followed by a decrease in absorbance at 400 nm, and the appearance of a new peak at 300
nm is associated with the formation of p-aminophenol (PAP). This peak intensifies with the
time, corresponding to the increase in PAP concentration. Due to the reaction conditions,
both PAP and PNP exist in anionic form during the measurement. Figure 7 manifests a
typical example of absorption band for exhaustion of PNP and advent of PAP. Furthermore,
almost no conversion was observed without the addition of catalyst even after a period of 2 h.
Because the rate is dependent on the concentration of PNP only, the initial adsorption of PNP
at the surface of catalyst is in direct correlation with the rate constant for the whole reaction
[39]. It follows pseudo-first order kinetics as the concentration of NaBH4 was taken in excess,
and since a considerable part of hydride ions are lost at the surface of catalyst in the form of
gaseous molecular hydrogen [40]. Moreover, at 20 °C, the activity of catalyst was found to be
328 L g–1 s–1 with a rate constant value of kobs = 0.10 min–1, and 96 % of PNP was reduced in
just 25 min. The effect of temperature on the activity of catalyst and the reaction rate was
16
also investigated through a series of experiments to probe the thermodynamic parameters.
From Figures S2-S6 it is obvious that temperature has a significant effect on the reduction of
PNP, which proceeds faster at higher temperatures. The activity and rate constant values are
also improved at elevated temperature and reach to 683 L g–1s–1 and 0.23 min–1, respectively,
at 50 °C. At this temperature, reaction time was also reduced from 25 min to 12 min. Table 1
provides a summary of results of reduction experiments carried out at various conditions.
Table 1. Comparison of activity and rate constant of PNP reduction at different temperature
Series T (°C)
Time
k (min–1)
Activity
(L g–1 s–1)
(min)
1
20
25
0.10
328
2
35
20
0.13
410
3
40
16
0.16
512
4
45
13
0.20
630
5
50
12
0.23
683
The conversion of PNP to PAP was also confirmed by mass spectrometry, which revealed
that the reaction product was only PAP, thus the composite 1 can be attributed as selective for
PAP (Figure S7). Mass spectrometry analysis was conducted in negative mode, because our
product sample exists as p-aminophenolate ion. The peak appearing at m/z = 108 is attributed
to the p-aminophenolate ion, and the peak at m/z = 80 represents its first fragment, i.e.
C5H4NH2. Another peak at m/z = 53 is tentatively attributed to an rearranged fragment. The
fragmentation pattern is also in agreement with a previous report [41]. To compare the
results of our catalyst with other reports, we have listed a summary of activities of recently
reported catalysts in Table 2. From this data, it is evident that the activity of our catalyst is
17
6.99 times higher than CuOS NPs, 2.43 times higher than Ag NPs deposited on Si-based
POM.
Table 2. A comparison of the activity of our catalyst with the some recently reported catalysts
Sample
Kapp × 10–3 ( s–1)
catalyst quantity (mg)
activity factor (κapp × 10–3) mg–1s–1
ref.
CuOS
1.12
10
0.11
[42]
CuNiOS-0.6
70.0
10
7.0
[42]
Ag with poly(acrylic acid)
15.46
35
0.44
[43]
Au (with C19H42BrN)
0.0061
0.807
7.55
[44]
Co-C containing N species
0.003
10
0.3
[45]
Ag@ Si based POM
47.5
7
0.67
[46]
Cu NPs
1.6
1.1
1.5
[47]
Pd@MIL-100
6.3
0.525
12
[26]
Pt/MoS2 NSs
32
1.50
21.3
[29]
γ–Pt hybrid
3.20
1.09
2.94
[48]
Graphene carbocatalyst
5.70
2.50
2.28
[49]
Mn-POM@MIL-100 (Fe)
3.83
5
0.76
This work
Figure 7. UV-Vis spectroscopic results of catalytic reduction of PNP; in the presence of Mn-
POM only (a), MIL-100 (Fe) only (b), composite 1 (c) and kinetics of PNP reduction (d) by
composite 1 at 20 °C
18
The mechanism for the reduction of PNP reduction by NaBH4 is now well established,
whereby BH4– transfers an electron to PNP via a suitable pathway. Herein, we propose that
that PNP is initially adsorbed at the surface of MIL-100 (Fe). This adsorption occurs through
electrostatic interactions between the negatively charged PNP and the positively charged Fe3+
of the framework as well as due to π-π interactions between organic part of MIL-100 (Fe) and
the benzene ring of PNP [50]. The electron transfer takes place from BH4– to Mn-POM,
followed by an electron transfer to the Lewis acidic centres of the MOF, i.e. the Fe3+ sites,
which finally reduce PNP to PAP. It has already been reported that MIL-100 (Fe) alone is not
capable to reduce PNP [25], and we assume that this is because the Fe3+ cannot accept
electrons directly from BH4– to reduce PNP. Thus the presence of Mn-POM facilitates
electron transfer as it is well known that such kind of POM anions can act as electron
reservoirs.
The catalyst was recovered from the solution by simple filtration. It was sonicated in distilled
water to remove any adsorbed PAP or PNP and dried at room temperature. PXRD, FTIR and
XPS analysis were conducted to compare the identity of recovered catalyst material with
pristine 1. Figure S8 shows PXRD and FTIR of the recovered catalyst, which remained intact
after the catalytic reactions. A slight difference in relative peak intensities at 1480 and 1650
cm–1 may be due to adsorption of residual reduction product PNP over the surface of 1 as it
absorbs strongly 1640-1550 cm–1 [51].
Figure S9 shows the XPS of catalyst 1 before and after catalytic activity. It is obvious that
composition of catalyst remains intact and Fe3+ is not reduced in a solution of NaBH4. The
stability of MIL-100 (Fe) toward reduction reactions is also evident from previous reports
[25, 26].
19
4. Conclusions
Hydrothermal synthesis of an iron-based metal-organic framework (MIL-100) in the presence
of
Mn-polyoxomolybdate
anions
yields
the
PMOF
composite
[Fe3O(OH)(H2O)2{C6H3(CO2)3}2][PMo11MnO39(H2O)]0.30·8H2O (1). Compound 1 was
characterized and tested for catalytic conversion of PNP to PAP at various conditions. The
surface area of MIL-100, although reduced considerably after accommodating the POM units
inside the MOF cavities, still retains significant porosity (1050 m2 g–1, 0.325 cm3 g–1),
mandatory for catalytic applications. We confirmed that both individual components of 1, i.e.
MIL-100 (Fe) and Mn-POM, were inactive for catalytic reduction of PNP, however, after
encapsulation of Mn-POM into MIL-100 (Fe), the resulting composite 1 displays a
pronounced activity and efficiently reduces PNP into PAP in the presence of NaBH4. The
current work reveals the use of PMOF composites as a new class of cost-effective catalysts
toward the value-added reduction of PNP.
Supporting Information
Supporting information comprises the detailed experimental of synthesis and catalytic
studies, characterization techniques, results and discussion.
Acknowledgements
This work is supported by the Alexander von Humboldt (AvH) Foundation, Germany.
W.A.S. acknowledges the IRSIP scholarship awarded by Higher Education Commission
(HEC) of Pakistan for financial support for conducting PhD research at RWTH Aachen
University, Germany.
Conflicts of interest
The authors declare no conflicts of interest.
20
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