MOF-Mediated Synthesis of Supported Fe-Doped Pd Nanoparticles under Mild Conditions for Magnetically Recoverable Catalysis**

Article abstract of DOI:10.1002/chem.202001895

Metal–organic framework (MOF)-driven synthesis is considered as a promising alternative for the development of new catalytic materials with well-designed active sites. This synthetic approach is used here to gradually transform a new bimetallic MOF, with Pd and Fe as the metal components, by the in situ generation of aniline under mild conditions. This methodology results in a compositionally homogeneous nanocomposite formed by Fe-doped Pd nanoparticles that, in turn, are supported on iron oxide-doped carbon. The nanocomposite has been fully characterized by several techniques such as IR and Raman spectroscopy, TEM, XPS, and XAS. The performance of this nanocomposite as an heterogeneous catalyst for hydrogenation of nitroarenes and nitrobenzene coupling with benzaldehyde has been evaluated, proving it to be an efficient and reusable catalyst.

Full text of DOI:10.1002/chem.202001895

Accepted Article
Accepted Article
Title: MOF-Mediated Synthesis of Supported Fe-doped Pd
Nanoparticles under Mild Conditions for Magnetically
Recoverable Catalysis
Authors: Mohanad D. Darawsheh, Jaime Mazarío, Christian W. Lopes,
Mónica Giménez-Marqués, Marcelo E. Domine, Debora M.
Meira, Jordan Martínez, Guillermo Mínguez Espallargas, and
Pascual Oña-Burgos
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202001895
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01/2020

10.1002/chem.202001895
Chemistry - A European Journal
MOF-Mediated Synthesis of Supported Fe-doped Pd
Nanoparticles under Mild Conditions for Magnetically
Recoverable Catalysis
Mohanad D. Darawsheh,a† Jaime Mazarío,b† Christian W. Lopes,c Mónica
Giménez-Marqués,a Marcelo E. Domine,b Debora M. Meira,d,e Jordan
Martínez,b Guillermo Mínguez Espallargas*a and Pascual Oña-Burgos*b,f
a) Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, c/Catedrático
José Beltrán, 2, 46980 Paterna, Spain.
b) Instituto de Tecnología Química, Universitat Politècnica de València-Consejo
Superior de Investigaciones Científicas (UPV-CSIC), Avda. de los Naranjos s/n,
46022 Valencia, Spain.
c) Laboratory of Reactivity and Catalysis – Institute of Chemistry, Universidade
Federal do Rio Grande do Sul, 91501-970 Porto Alegre, Brazil.
d)CLS@APS sector 20, Advanced Photon Source, Argonne National Laboratory,
9700 S. Cass Avenue, Lemont, IL 60439, USA.
e) Canadian Light Source Inc., 44 Innovation Boulevard, Saskatoon, SK S7N 2V3,
Canada.
f) Department of Chemistry and Physics, Research Centre CIAIMBITAL, University of
Almería, Ctra. Sacramento, s/n, Almería, E-04120, Spain.
Abstract
MOF-driven synthesis is considered as a promising alternative for the development of
new catalytic materials with well-designed active sites. This synthetic approach is
used here to gradually transform a new bimetallic MOF, composed of Pd and Fe as
metal components, via the in situ generation of aniline under mild conditions. This
methodology results in a compositionally homogeneous nanocomposite formed by Fe-
doped Pd nanoparticles and these, in turn, supported on an iron oxide-doped carbon.
The nanocomposite has been fully characterized by several techniques such as IR,
Raman, TEM, XPS, XAS, among others. The performance of this nanocomposite as
an heterogeneous catalyst for hydrogenation of nitroarenes and nitrobenzene coupling
with benzaldehyde has been evaluated, proving it to be an efficient and reusable
catalyst.
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Introduction
Nanostructured materials and their composites have attracted a lot of attention
over the last decades in catalysis,[1] sensing,[2] environmental[3] and biomedical[4]
applications. Specifically, the use of nanomaterials as catalysts in organic
compound synthesis is an important pathway in order to develop viable synthetic
procedures in pharmaceutical and chemical industry.[1] In this regard, supported
metal nanoparticles (NPs) are highly valued due to their high activity, selectivity
and the possibility to modulate their activity by controlling their size, distribution,
composition and the nature of the support.[5]
However, despite the broad applicability in different areas, high-temperature
reduction of metal salts or coordination compounds is still the most common way
to prepare supported metal NPs.[6] A different route to successfully control the NP
size under mild conditions consists in using metal complexes of low oxidation state
in combination with long-chain aliphatic amines,[7] although it would require the
use of an inert atmosphere. An alternative approach is based on the use of
templates,[8] with the precursor serving as the source of both, metal and support.
In this sense, metal-organic frameworks (MOFs)[9] offer a unique scenario.
These porous crystalline materials formed by metal clusters and organic linkers
have been studied in the last years for numerous applications, such as gas sorption
and separation,[10] catalysis,[11] sensing,[12] electronic[13] and magnetic devices,[14]
among others. Their ordered structures act as scaffolds that result in ideal
candidates for self-template precursors upon thermal conversion to well-defined
nanocomposites[15] that can be used in several fields such as catalysis,[16]
electrocatalysis,[17] and energy storage and conversion.[18]
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An additional benefit that accompanies the use of MOFs as precursors is the
possibility of preparing dispersed and homogenous multimetallic NPs, which can
be easily obtained by encapsulation of metal complexes or metal NPs in the pores
of the MOF prior to the thermal conversion,[19] or by the use of core-shell MOFs.[20]
Indeed, a better control of the homogeneity of the NPs can be achieved by the use
of heterometallic MOFs. Being an example of this, the preparation of Ni-Co nano-
alloy from a pre-synthesized coordination polymer using temperatures over 500
°C.[21]
Herein, we present the formation of a composite material based on
Nanoparticles of PdFe (PdFe-NPs) under mild conditions (25 °C) using a
heterometallic PdFe-MOF that has also been prepared under soft conditions (120
°C) in a solvent-free manner. These PdFe-NPs are directly supported on
nanometric FeOx homogeneously inserted into a carbon mantle, resulting in
excellent catalysts for hydrogenation reactions, forming in situ and in the absence
of previous treatment.
Discussion and Results
Synthesis and characterization of PdFe-MOF..
The grinding of the solid reactants Fe(NO3)3∙9H2O and the palladium metalloligand
PdCl2(PDC)2 (H4L where PDC: pyridine-3,5-dicarboxylic acid), followed by heating at
120 °C under vacuum results in the formation, after 72 h, of the MOF
[Fe3O(L)1.5(H2O)3(NO3)], hereafter denoted as PdFe-MOF. Figure 1 represents the MOF
in which the [Fe3(µ3-O)] building blocks are connected with six different tetra-carboxylic
acid ligands to form a 3D structure with soc topology, isoreticular to an In analogue
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recently reported by Steriotis, Trikalitis and co-workers.[22] This solvent-free
methodology, uncommon for the preparation of MOFs, has been adapted from previously
reported methods[23] and allows the formation of the Fe analogue, which resulted as
unfruitful using the conventional synthetic route for the PdIn-MOF analogue[22] yielding
instead an Fe-2D network with no Pd in the structure.[24] The porous nature of PdFe-MOF,
as established by N2 sorption, reveals a total N2 uptake of 250 cm3ꞏg–1 at 77 K and 0.8 bar
with a calculated BET surface area of 830 m2ꞏg–1 for the activated material, which is
consistent with those found in the isoreticular PdIn-MOF. Further characterization of the
MOF includes powder X-ray diffraction, SEM, infrared spectroscopy, thermal
gravimetric analysis, XPS spectroscopy, EDS and ICP elemental analysis, which confirm
the homogeneity of the sample (see ESI and Figures S1-9 for a more detailed discussion).
Figure 1. Schematic diagram of the metalloligand (H4L), the oxo-centred iron carboxylate SBU,
[Fe3O(COO)6(H2O)3]+ and the cuboidal cage of the PdFe-MOF. Colour scheme: orange = Fe, grey
= C, blue = N, red= O, dark teal = Pd, green = Cl, and yellow sphere represents the cavity. Hydrogen
atoms are omitted for clarity.
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Synthesis and characterization of PdFe-NPs
Scheme 1. Flow chart of the synthesis process to yield the nanocomposite (PdFe‐NP) from the
PdFe‐MOF.
Preparation of the PdFe-NPs from PdFe-MOF has been achieved by adapting a
previously reported procedure by Chaudret and co-workers,[7] based on the use of
an amine (aniline) in the presence of H2 (5 bar) at room temperature. This modified
approach consists in the in situ generation of aniline, instead of using it as a solvent,
in order to provide the reaction mixture with a slow supply of the amine. As a
result, the heterometallic nanocomposite (PdFe-NP) is slowly formed, therefore
providing a well-controlled and reproducible material. Specifically, PdFe-MOF
was mixed with toluene and nitrobenzene together with a H2 atmosphere (5 bar) at
room temperature, and after 1.5 h the final nanocomposite was obtained. In this
way the PdFe-MOF results in a nanocomposite (PdFe-NP) comprising both,
metallic nanoparticles and a carbonaceous support, whose true nature will be
thoroughly discussed over the next pages. For comparison purposes, the
nanocomposite using the standard approach described by Chaudret[7] was also
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prepared together with the traditional high temperature methodology based on
thermal treatment of the MOF (500 °C) with a H2 flow in a fixed bed reactor (see
Supporting Section, Figure S11).[25] The main materials prepared in this work are
summarized in Table 1.
Figure 2. (a) TEM (b) STEM micrographs and (c) particle size distributions of PdFe-NP.
Electronic microscopy techniques reveal that the novel chemical methodology
for the in situ preparation of NPs from a MOF results in ultra-small PdFe-NPs with
a very narrow distribution, and an average size of 1.0±0.2 nm (Figure 2), embedded
in a carbonaceous support containing FeOx. By HR-TEM, the obtained NPs are
found to be smaller, and remarkably with a narrower distribution, than the NPs
formed by the straightforward use of aniline (2.0±1.8 nm, Figure S11c) or by the
thermal treatment of the MOF (3.1±3.1 nm, Figure S11b), and significantly smaller
than other previously reported synthetic approaches based either on thermal and
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chemical decomposition of metallic precursors (≈ 5 nm)[26] or high temperature
thermal treatment methods (≈ 10 nm).[25] Based on that, it is clear that this novel
methodology gives rise to NPs smaller than previously described. Moreover, in an
attempt to clean the surface of the nanocomposite prior to its use in catalysis,
different thermal treatments under vacuum were applied over PdFe-NP. It was
concluded that these treatments did not to have any considerable effect as far as the
nanoparticle size is concerned (see Figure S10).
Table 1. Summary of the main materials prepared.
Material code
PdFe-MOF
PdFe-NP
Description
Original MOF
NP sizea
–
Chemically as-synthesized NP
PdFe-NP heated for 6h at 300 °C under vacuum
1.0±0.2
1.2±0.3
PdFe-NP-300
a: Measured by HR-TEM by considering a minimum number of 200 particles.
The remarkably reduced size of the PdFe-NPs is consistent with a heterometallic
nature of the NPs, in accordance with previous reports that show this effect on Pd-NPs
upon doping with other metals.[27] In fact, the possible presence of Pd and Fe in the NPs
has been studied by electron diffraction (Figure S13), EDAX (Figures S14-16), X-ray
powder diffraction (Figure 3a and S18), XPS (Figures S19-21) and XAS (Figures S24-
25). The interplanar distances of the electron diffraction patterns indicate the absence of
undesired homometallic Pd crystalline phases (Figure 3b), which are however found with
the common synthetic routes (Figure S13c). The presence of Fe in the NPs is confirmed
by an EDAX punctual analysis of the isolated nanoparticles (Figure S14a). Further
confirmation is provided by XRD (Figure 3a and S18b), which shows the centered cubic
(fcc) pattern characteristic of Pd with a decrease in the cell parameter with respect to
elemental palladium (a = 3.8935(4) vs 3.8972), in accordance with Vegard’s law.[28]
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Besides that, no impurities corresponding to metallic iron, iron oxide or palladium oxide
are detected. Moreover, EDAX mapping of the composite reveals that iron is present in
large amounts in the carbon support (Figure S15), confirmed by Raman spectroscopy to
be FeOx (see Figure S22). However, the absence of any diffraction peak corresponding to
iron oxide indicates a very small size of iron oxide in the carbonaceous support. On the
contrary, the classic thermal methodology ends in the formation of bimetallic NPs
together with other NPs based on metallic iron (Figure S18a). These data also indicate
that mild reaction conditions result in NPs with higher homogeneity in composition.
Figure 3. (a) XRD patterns for PdFe-NP and PdFe-MOF, PdFe-NP spectrum corresponds to the positions
of the (111), (200), (220), (311) and (222) peaks; (b) and (c) SAED patterns of the sample PdFe-NP.
With the aim to provide further insights into the bimetallic nature of the nanoparticle
composites, XPS measurements were carried out. XPS spectra interpretation and peak
fitting of Fe2p, N1s, O1s and C1s can be found in the Supporting Information (Figures
S19-21). As can be seen in Figure 4, the Pd3d5/2 signal for the chemically prepared PdFe-
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NP is shifted towards lower binding energies than what is usually observed for Pd0. On
the contrary, the NPs obtained by traditional thermal treatment barely present this shift
(see Figure S21 and Table S4). This indicates that Pd is electronically richer in the
chemically synthesized nanoparticles, which could be due to a charge transfer from either
the Fe in the NP[29] or the FeOx in the support,[30] both situations having been previously
described. However, as the nanocomposite obtained by the thermal treatment of the
MOF, where no shift is seen, does not present FeOx in the support, this finding points
to FeOx as the main responsible for this effect. These observations have been made taking
as a reference the XPS spectra for commercial Pd@C (mainly Pd0, Figure 4) and they
could be of great significance in catalytic applications, among others.
Figure 4. Pd3d XPS signal of a) PdFe-MOF, b) PdFe-NP-300, c) Pd@C (commercial).
Further details on the nature of iron species were obtained by XAS. According to both
XANES and EXAFS features (Figure 5), the iron species present a structure very similar
to magnetite, in good agreement with the Raman spectrum of PdFe-NP (Figure S22). The
nanosized character of the FeOx is confirmed by the flattening of EXAFS signal features
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in comparison with those of Fe3O4 pattern, which is also reflected on the diminished Fe-
O coordination number in comparison with the bulk counterpart (Figure 5, Figure S24
and Table S5). In addition, this fitting is further improved when including a Pd
contribution (Figure S25 and Table S6). The position and shape of the absorption edge in
the spectrum of PdFe-NP sample indicate that Pd atoms are in the reduced state in an fcc
local structure.[31] Therefore, the intensity of these oscillations points to the presence of a
fraction of low coordinated atoms, causing the low amplitude of the EXAFS oscillations
and, consequently, smaller nanoparticles in comparison with the bulk counterpart.
However, the Pd EXAFS does not reveal this Pd-Fe interaction. This can be explained as
a consequence of XAS being a bulk-sensitive technique. In this sense, the presence of
some NP agglomerations (Figure S17), better detected than isolated NPs by this
technique, may explain why, on average, this contribution is very small and cannot be
distinguished.
0.2 Å-3
FeO
Fe2O3
0.2
Fe3O4
PdFe-MOF
PdFe-NP
FeO
Fe2O3
Fe3O4
PdFe-MOF
PdFe-NP
(b)
(a)
7100
7120
7140
7160
7180
0
1
2
3
4
5
6
Energy (eV)
R (Å)
Figure 5. Normalized XANES spectra at Fe K-edge (a) and |FT| of the k2-weighted (k) functions (b) for
PdFe-MOF, PdFe-NP and Pd-based standards.
In order to understand the mechanism of the formation of the PdFe-NP from PdFe-
MOF, a series of ATR-IR measurements were done upon exposing the MOF to different
conditions for 30 minutes. Immersion of the MOF in toluene in the presence of 5 bar of
H2 at 25 °C, or the addition of 1 mmol of nitrobenzene, neither of them causes any major
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changes in the IR spectrum (see Figure S23), thus indicating that the PdFe-MOF remains
unmodified. On the contrary, the presence of aniline significantly modifies the structure
of the material, as evidenced by a shift to lower frequencies of the bands corresponding
to the Fe-O bond (600-650 cm–1 and 450-500 cm–1), the disappearance of the band
corresponding to the coordinated pyridine (1445 cm–1),[32] as well as by the presence of
the characteristic bands of the N-H bending associated to aniline (1616 and 1556 cm–
1).[33] Thus, aniline plays a major role in the NP formation, likely via coordination to the
Pd centres, displacing the pyridine and therefore initiating the rupture of the MOF.
Accordingly, the evolution of XPS for N1s also supports an effect of replacement of the
pyridinic ligand from the coordination sphere of Pd by aniline (Figure S20a).
Catalytic tests for nitroderivative hydrogenation
Considering the small size and the enhanced electronic properties of PdFe-NPs, we
analysed their catalytic activity in nitroarene hydrogenations (Scheme 2). Transformation
of nitroarenes into anilines is of great importance, as they are one of the main building
blocks for dye and pharmaceutical industries.[34]
Scheme 2. Nitroarene hydrogenation.
More specifically, we investigated the capability of the prepared NPs in
nitrobenzene hydrogenation. The specific activity (SA) for different Pd-based
materials was calculated as the initial reaction rate (Figure 6d). Interestingly, when
the Fe-doped Pd nanoparticles were chemically prepared and their surface properly
cleaned after the common thermal treatment used for NP activation, i.e. 6 h at 300
⁰C under vacuum (PdFe-NP-300), their activity plainly surpassed that of the
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commercial Pd@C (SA = 153 vs 55, see Figure 6d). This observed improvement
of the catalytic activity of our nanocomposites with respect to the commercial
catalyst Pd@C could be due to the higher electronic density, observed by XPS
measurements, together with the reduction in particle size (from 3 nm to 1 nm, see
Figures S12 and S10, respectively).[29-30] Moreover, SA values of this bimetallic
composite are ranked within the best catalysts under similar/identical reaction
condition, thereby clearly improving many of the materials previously reported
(see Table S7) for this particular reaction.
Figure 6. a) Nitrobenzene reduction with PdFe-MOF. b) Leaching test for PdFe-MOF in the nitrobenzene
hydrogenation. Reaction conditions: 0.123 g nitrobenzene, 5 mg PdFe-MOF, at 5 bar H2 pressure and 25
⁰C. 1 h of induction. c) Reusability of PdFe-NP formed “in-situ” from PdFe-MOF. Reaction conditions:
0.123 g nitrobenzene, 5 mg PdFe-MOF, at 5 bar H2 pressure and 25 ⁰C during 2 h (after 1 h of induction).
d) Initial reaction rates (as SA) for different PdFe-based materials and comparison with a commercial
catalyst. SA has been calculated after the first 30 minutes of reaction plus the corresponding induction time
observed.
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However, the most remarkable results arise from the fact that the PdFe-NPs can be
formed in situ. Thus, since the product of nitrobenzene hydrogenation is aniline, we
hypothesised that the direct use of PdFe-MOF should also act as catalyst for this reaction,
as NP formation would take place in situ. In this sense, we barely see difference when
comparing the activity of PdFe-MOF against the Pd@C (SA values for PdFe-MOF and
Pd@C are 47 and 55, respectively). Thus, the PdFe-MOF is acting as a Pd reservoir that,
after an induction time, can generate active Pd species in the same reaction media, without
the need of any thermal pre-treatment with H2.
Looking at the overall reaction kinetics, nitrobenzene hydrogenation takes place after
1 h of induction, corresponding to the already explained MOF to NP transformation,
reaching yields very closed to 100 % (conversion ≈ 99 % and selectivity ≈ 99 %) in 2 h
of reaction (Figure 6a). These NPs formed in situ can be reused several cycles, with a
minor catalytic deactivation (Figure 6c). This small deactivation observed could be
related to the deposition of organic matter on the active centres, as it is suggested by the
attenuation of the Pd3d XPS signal of the material PdFe-NP (in situ formed, no thermal
treatment, Figure S21) and/or to an increase in the average particle size after the catalytic
process, from 1.0±0.2 to 3.8±1.4 nm, as it has been observed by HR-TEM (Figure S11c).
Nonetheless, the loss of metals (Pd and/or Fe) from the solid to the solution during
reaction could be practically discarded. In this sense, the catalyst was filtered at 30
minutes of reaction (C≤50%). The reaction was continued in the absence of catalyst until
it reached 16 h, and no significant changes were observed in the conversion levels of
nitrobenzene. This finding meaning that the process is purely heterogeneous. (Figure 6b).
With this operation, the possibility of suffering from possible desorption and re-
absorption of the active species of the catalyst during the reaction was also discarded, a
phenomenon that has been widely discussed in the literature when similar materials were
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used as catalysts.[35] Moreover, the fact that the PdFe-NP were synthesized in the same
reaction media used in the catalytic tests is a proof of the structural stability of the
nanocomposite during the reaction and points to carbon deposition and particle size
growth as the only probable reasons of deactivation. Specially the deactivation by coke
deposition is in compliance with what has been stated in the literature when using Pd as
the active phase for nitrobenzene hydrogenation. [36]
An additional benefit of the chemically prepared solid catalysts is the possibility of
magnetically recovering them after having been used in reaction (Figures S27 and S28),
an advantage which recently has started to be eagerly sought when using the synthetic
approach herein described. [37] For this purpose, the thermal treatment of the material
helps to maximize the magnetic response, allowing for a successful separation after
reaction. As a result, we have achieved a catalytic composite material with a higher
capability to hydrogenate nitrobenzene at room temperature than that exhibited by a
commercial catalyst, owing to the Fe-doped Pd-NPs, and with the additional advantage
of being easily recovered with the help of a magnet, due to the FeOx species present in
the support.
Table 2. Catalytic activity of “in-situ” formed PdFe-NPs when using different nitroderivatives.
Reactant
Observed Product
Yield (mol.%)a
Induction time (h)
Nitrobenzene
Aniline
99
1.0
4-methylnitrobenzene
4-chloronitrobenzene
Nitrostyrene
4-methylaniline
4-chloroaniline
4-ethylaniline
99
73
3.0
4.5
7.0
92
a
Reaction conditions: 0.123 g nitroderivative, 5 mg PdFe-MOF, at 5 bar H2 pressure and 25 ⁰C. At 2 h of reaction discarding the
induction time in each case. Note: In the case of nitrostyrene the observed product (4-ethylaniline) is the complete hydrogenated
molecule, both nitro and vinyl groups being reduced to amine and ethyl moieties, respectively.
Moreover, the PdFe-NPs can also hydrogenate other nitroarenes besides
nitrobenzene
such
as nitrostyrene, 4-methylnitrobenzene and 4-
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chloronitrobenzene, as summarized in Table 2. Both, the induction time and the
formation of the corresponding aniline, vary with the nitro-derivative used. The
higher induction time observed for chloronitrobenzene with respect to
methylnitrobenzene could be ascribed to the lesser reactivity of the nitro group in
the presence of an electron attractor such as chloride, which can be rationalized
with the Hammett parameter (σ = -0.01 for CH3; σ = 0.47 for Cl).[38] Nitrostyrene
does not follow this trend as two hydrogenations take place, with both the nitro and
the C=C being hydrogenated. The explanation for the higher induction time
observed in this case is therefore more complex.
Tandem reaction
Finally, the possibility of using this new catalyst in tandem reductive amination
reactions was tested (Figure 7). With this aim, the reduction reaction of
nitrobenzene to aniline was coupled with the nucleophilic addition of benzaldehyde
and the subsequent reduction of the formed adduct. Instead of working with an
excess of benzaldehyde in the reaction mixture, slow addition of this reagent was
used to decrease the rate of undesired secondary reactions resulting from
benzaldehyde hydrogenation. While the one-pot reaction affords only 40 % yield
to the desired product (with a 60 % selectivity to aniline, Fig. S26), slow addition
of the carbonyl compound causes an increase of the selectivity to 75 % to the
benzylaniline (with a subsequent decrease of the selectivity to aniline to 20 %, see
figure 7). Thus, this reduction of the unreacted aniline is clear proof that the
benzaldehyde is not being hydrogenated so fast when lower concentrations of it are
present in the reaction mixture.
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Figure 7. Coupling reaction between nitrobenzene and benzaldehyde, with slow addition of the latter.
Reaction conditions: 0.123 g nitrobenzene, 0.106 g benzaldehyde (slow addition, v=102 μL/h), 5 mg
PdFe@MOF, at 5 bar H2 pressure and 25 ⁰C during 6.5 h (addition starts after 30 min of reaction, taken as
t0). The main nitrogenated by-products detected apart from the imine was aniline (≈22 mol.%)
Conclusions
A new family of nanocomposites based on Fe-doped Pd nanoparticles supported
on an iron oxide-doped carbon has been obtained from a new bimetallic PdFe-
MOF. The heterometallic nanocomposite precursor PdFe-MOF is isostructural
with a previously reported PdIn-MOF and has been obtained for the first time,
using a solvent-free method.
A controlled decomposition of the PdFe-MOF upon in situ generation of aniline
leads to the formation of ultra-small PdFe-NP, smaller and more homogeneous
than the NPs obtained by conventional thermal procedures, which also provide
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other NPs based on metallic iron. In addition, PdFe-NPs obtained with our
approach present excellent catalytic properties in the hydrogenation of nitroarenes,
surpassing those of commercial Pd@C and offering the possibility of being
magnetically recovered. Moreover, PdFe-MOF can be directly used to generate the
catalyst in the reaction media. In addition, compelling evidence is provided with
respect to the beneficial synergy stablished when both, Fe and Pd are so close
enough that they can interact with each other. The size control of the Pd
nanoparticles by the presence of iron, as well as the modification of their electronic
charge by the FeOx in the support, seem to be the main facts that would define the
nature of this synergy.
In summary, the results obtained in this work should further strengthen the
confidence in the MOF-driven synthesis as a powerful tool to prepare novel
nanocomposites and catalytic systems with well-defined active sites. In particular,
the methodology developed in this work could be a good starting point for the
controlled transformation of MOFs having similar building units into
multifunctional nanomaterials.
Experimental
Synthesis of PdFe-MOF
The metallo-ligand H4L was prepared according to the published procedure.[39]
Concerning the MOF, a solid mixture of Fe(NO3)3ꞏ9H2O (47.2 mg, 0.117mmol), H4L (30
mg, 0.0582 mmol) and benzoic acid (3 mg, 0.0246 mmol) was briefly grounded. The
mixture was placed in a thin glass tube which sealed after a cycle of vacuum. The mixture
was placed in a thin glass tube, sealed under vacuum and heated during 72 h at 120 °C.
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The resulting mixture was washed with DMF and isopropanol for 1.5 days. The as-
synthesized MOF was activated using MeOH for 3 days and heated at 130°C for 4 hours.
Nanoparticle synthesis
As a general procedure for the in situ chemically synthesized NP, 10 mg of PdFe-MOF
were added to a solution containing 2 mmol of nitrobenzene derivative and 2 mL of
toluene. The system was sealed and pressurized until 5 H2 bar at 25 ⁰C under vigorous
stirring for 1.5 h. Then, the solid was filtered and repeatedly washed with MeOH during
24 h (as-synthesized PdFe-NP). In the case of thermal treatment of the as-synthesized
PdFe-NP, it was placed at 300 ⁰C during 6h under vacuum (PdFe-NP-300). For other
times of activation (12 h or 48 h) see Supporting Information.
For conventional thermal-treating synthesis of nanoparticles, the PdFe-MOF was
activated in a tubular fix-bed reactor under a H2 flow at 500 °C during 3 h (see
Supporting Information).
Material characterization
PdFe-MOF was characterized by XRD, IR spectroscopy, TG analysis, SEM and
sorption measurements, while PdFe NPs and nanocomposites here prepared were
characterized by means of chemical analysis (ICP), XRD, XPS, XAS, Raman and ATR
spectroscopies, and HR-TEM (for more details see Supporting Information).
Catalytic tests
Hydrogenation reactions for nitrobenzene derivatives were carried out in a 6 ml
batch glass micro-reactor equipped with a probe for sampling and a pressure gauge
for pressure measurement. For the first catalytic experiments, 0.123 g of
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10.1002/chem.202001895
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nitrobenzene (1 mmol), 1 mL of toluene and 5 mg of solid catalyst (PdFe-MOF or
PdFe-NP) were added in the same vessel. The reactor was sealed and pressurized
with 5 bars of H2 at 25 ⁰C and maintained at 800 rpm throughout the process. In all
cases, the pressure of H2 in the system was kept constant at the selected value.
Reusability tests were carried out for the in situ formed PdFe-NP. The catalyst
underwent four cycles, being washed with toluene for 30 minutes in-between. The
tandem reaction was performed in a similar manner albeit adding 0.106 g of
benzaldehyde (1 mmol) to the mixture previously described.
The progress of the reaction was followed by gas chromatography. Liquid
samples (≈50 μL) were collected at different time intervals, and then diluted in a
solution of 1wt% chlorobenzene (internal standard) in MeOH. The analysis of the
reaction mixtures was carried out by a 3900-Varian GC equipped with FID detector
and a capillary column (HP-5, 30 m length). Product identification was done by
GC–MS (Agilent 6890N GC System coupled with an Agilent 5973N mass
detector).
In all cases, conversion (
X
) and selectivities (Si) to the different products “
i
”, have
been estimated using the formulas below at different reaction times “
taking nitrobenzene as a reference.
t”, always
ꢐ
ꢊ
ꢑꢇ
ꢀꢁ ꢂꢃꢄ. %ꢅ ꢆ ꢇ
ꢈꢉꢊꢋꢌꢍꢎꢈꢏꢎꢈꢎ ⋅ 100
(eq. 1)
ꢈꢉꢊꢋꢌꢍꢎꢈꢏꢎꢈꢎ
ꢐ
ꢇ
ꢈꢉꢊꢋꢌꢍꢎꢈꢏꢎꢈꢎ
ꢊ
ꢇ
ꢒꢓꢁ ꢂꢃꢄ. %ꢅ ꢆ ꢇ
⋅ 100 (eq. 2)
ꢉ
ꢊ
ꢊꢌꢊꢔꢕ ꢖꢗꢘꢋꢌꢙꢚꢛꢊꢜ
SA (Specific Activity) was also calculated for nitrobenzene hydrogenation and
defined as the mol of aniline produced per mol of Pd present in the catalyst per
time. Finally, carbon balances were estimated for each reaction with respect to
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10.1002/chem.202001895
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nitrobenzene, considering the total amount of products detected by GC analysis
along with the remnant nitrobenzene.
ꢡ
ꢝ
ꢞꢝꢟꢄꢟꢝꢠ
SA ꢆ ꢝ
(eq. 3)
0
∙ ꢤ.ꢡꢟꢂꢠ
ꢢꢣ
ꢩ
ꢩ
ꢐ
∑
ꢨꢧꢇ
ꢑꢇ
ꢅ ∙ ꢪꢫ ꢬꢁꢭꢮꢯꢰꢱ ꢧꢇ
∙ ꢲ ꢫ ꢬꢁꢭꢮꢯꢅ
ꢈꢉꢊꢋꢌꢍꢎꢈꢏꢎꢈꢎ
ꢈꢉꢊꢋꢌꢍꢎꢈꢏꢎꢈꢎ
ꢘꢋꢌꢙꢚꢛꢊ
ꢧ
ꢅ
ꢥꢦ ꢂꢃꢄ. % ꢆ
⋅ 100 (eq. 4)
ꢐ
ꢇ
∙ ꢪꢫ ꢬꢁꢭꢮꢯ
ꢈꢉꢊꢋꢌꢍꢎꢈꢏꢎꢈꢎ
Associated Content
Supporting Information.
The following files are available free of charge.
Electronic Supplementary Information (ESI) available: [General methods and materials,
synthesis of PdFe-MOF, PdFe-NPs and PdFe nanocomposite, and their data
characterization by XRPD, IR, TG, SEM, HRTEM, XPS, XAS and Raman].
Author Information
Corresponding Author
* G. M. E. guillermo.minguez@uv.es; P. O-B. pasoabur@itq.upv.es.
Notes
† These authors contributed equally to this work.
Conflicts of interest
There are no conflicts to declare.
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10.1002/chem.202001895
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Acknowledgements
This work has been supported by the European Union (ERC-2016-CoG 724681-S-
CAGE), by the Spanish MICINN (Structures of Excellence Severo Ochoa SEV-
2016-0683 and María de Maeztu MDM-2015-0538; projects CTQ2017-89528-P,
CTQ2015-67592, PGC2018-097277-B-100, and RTI2018-096399-A-I00 co-
financed by FEDER). We also thank the Generalitat Valenciana
(PROMETEO/2018/006 and PROMETEU/2019/066). G.M.E., and P.O.-B. thank
MICINN for their “Ramón y Cajal” fellowships. M.G.-M thanks support of a
fellowship from “la Caixa” Foundation (LCF/BQ/PI19/11690022). J.M. thanks
MICINN for his PhD fellowship (CTQ2015-67592). Authors also thank the
Electron Microscopy Service of Universitat Politècnica de València for their
support, M.P. Romero for her assistance with TEM measurements and Prof. E.
Rodríguez-Castellón for discussions on the XPS spectra interpretation. This
research used resources of the Advanced Photon Source, an Office of Science User
Facility operated for the U.S. Department of Energy (DOE) Office of Science by
Argonne National Laboratory and was supported by the U.S. DOE under Contract
No. DE-AC02-06CH11357, and the Canadian Light Source and its funding
partners.
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