Chemoselective Hydrogenation of Nitroaromatics at the Nanoscale Iron(III)–OH–Platinum Interface

Article abstract of DOI:10.1002/anie.202003651

Catalytic hydrogenation of nitroaromatics is an environment-benign strategy to produce industrially important aniline intermediates. Herein, we report that Fe(OH)_x deposition on Pt nanocrystals to give Fe(OH)_x/Pt, enables the selective hydrogenation of nitro groups into amino groups without hydrogenating other functional groups on the aromatic ring. The unique catalytic behavior is identified to be associated with the Fe^III-OH-Pt interfaces. While H₂ activation occurs on exposed Pt atoms to ensure the high activity, the high selectivity towards the production of substituted aniline originates from the Fe^III-OH-Pt interfaces. In situ IR, X-ray photoelectron spectroscopy (XPS), and isotope effect studies reveal that the Fe³⁺/Fe²⁺ redox couple facilitates the hydrodeoxygenation of the -NO₂ group during hydrogenation catalysis. Benefitting from Fe^III-OH-Pt interfaces, the Fe(OH)_x/Pt catalysts exhibit high catalytic performance towards a broad range of substituted nitroarenes.

Full text of DOI:10.1002/anie.202003651

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Accepted Article
Title: Chemoselective hydrogenation of nitroaromatics at the nanoscale
Fe(III)-OH-Pt interface
Authors: Yu Wang, Ruixuan Qin, Yongke Wang, Juan Ren, Wenting
Zhou, Laiyang Li, Jiang Ming, Wuyong Zhang, Gang Fu, and
Nanfeng Zheng
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Chemoselective hydrogenation of nitroaromatics at the nanoscale
Fe(III)-OH-Pt interface
Yu Wang, Ruixuan Qin, Yongke Wang, Juan Ren, Wenting Zhou, Laiyang Li, Jiang Ming, Wuyong
Zhang, Gang Fu,* and Nanfeng Zheng*
Abstract: Catalytic hydrogenation of nitroaromatics is deemed as an
environment-benign strategy to produce industrially important aniline
intermediates. Herein, we report that the Pt nanocrystals with Fe(OH)x
deposition, Fe(OH)x/Pt, enable the selective hydrogenation of nitro
group into amino group without hydrogenating other functional groups
on aromatic ring. The unique catalytic behavior is identified to be
associated with the Fe(III)-OH-Pt interfaces. While H2 activation
occurs on exposed Pt atoms to ensure the high activity, the high
selectivity towards the production of substituted aniline is originated
from the Fe(III)-OH-Pt interfaces. In situ IR, XPS and isotope effect
studies reveal that the Fe3+/Fe2+ redox couple facilitates the
oxide, also have been demonstrated to reduce diverse
nitroarenes into anilines with high selectivity.[10] In such cases, the
oxophilic low-valence metal would effectively capture the nitro
group, thus switching off the undesired hydrogenation processes.
However, the low activity for FeOx towards H2 activation made it
less economically attractive as the reaction needed to be
operated under harsh conditions (150 C and 50 bar H2 for >15
h).[5a]
We demonstrate herein that a combined system with
patching Fe(OH)x on Pt nanocrystals (NCs) readily exhibits a
significant synergistic effect for the selective hydrogenation of
o
hydrodeoxygenation of the -NO2 group during hydrogenation catalysis. nitroaromatics to aminoaromatics. A wet-chemical method was
Benefited from rich Fe(III)-OH-Pt interfaces, the Fe(OH)x/Pt catalysts
employed to fabricate Fe(OH)x/Pt interfaces by depositing
Fe(OH)x species on the surface of Pt nanocrystals (NCs).[11]
Therein, Fe(OH)x species on the surface of the Pt nanocrystals
are in the form of sub-monolayer islands, thus building up tunable
Fe-OH-Pt interfaces with various Fe/Pt ratio. The hydrogention of
3-nitrostyrene (3-NS) containing both C=C and -NO2 groups was
chosen as a model reaction to evaluate the chemoselectivity
induced by the Fe-OH-Pt interfaces. Under ambient condition, the
Fe(OH)x/Pt catalysts exhibited excellent catalytic performance in
the hydrogenation of 3-NS with the turnover frequency (TOF) of
11065 h-1 and the selectivity to 3-aminostyrene (3-AS) of 99.3%.
Comprehensive characterizations and Density functional theory
(DFT) calculations revealed that the Fe(III)-OH-Pt interface
favored the adsorption and reduction of nitro group over vinyl
group as Fe3+/Fe2+ redox would initiate the hydrodeoxygenation
pathway.
As illustrated in Figure 1a, Pt NCs with an average size of 5
nm were prepared by using wet-chemical method (Figure S1a). A
small amount of Fe(acac)3 was then added to deposit sub-
monolayer Fe(OH)x onto the surface of Pt NCs (Figure S1b). The
mole ratio of Fe to Pt was determined to be 0.3 by inductively
coupled plasma-mass spectrometry (ICP-MS) so that as-
synthesized catalyst was denoted as 0.3Fe(OH)x/Pt. While
STEM-EDS images (Figure 1b) demonstrated the successful
deposition of Fe species on Pt NCs, high-sensitivity low-energy
ion scattering spectroscopy (HS-LEIS) confirmed the surface
exposure of Pt even after the Fe(OH)x deposition.[12] In addition,
HS-LEIS (He+) spectra (Figure 1c) showed that there were three
main O, Fe, and Pt peaks. After ion etching to partially remove
surface Fe(OH)x, Pt underneath the surface became detectable
by HS-LEIS. Interestingly, the Fe/Pt ratio on the outermost atomic
layer decreased from 3.9 to 3.1 after bombarding the
polycrystalline 0.3Fe(OH)x/Pt surfaces by Ne+, suggesting that the
polycrystalline had a Fe-rich surface. Furthermore, XPS spectra
(Figure 1d) revealed that most of the Fe species were in +3
valence with Fe 2p peaks at 725.4 eV and 711.8 eV. Although the
presence of Fe(III) species on Pt should result in partially oxidized
surface Pt sites, Pt XPS spectra (Figure S2) suggested that Pt
was mainly retained near zero valence (with Pt 4s peak at 726.5
eV, and Pt 4f peaks at 71.2 eV and 74.5 eV) probably due to the
exhibit high catalytic performance towards
substituted nitroarenes.
a broad range of
Substituted anilines are important intermediates in fine
chemical industry for manufacturing highly value-added
chemicals, such as dyes, pigments, pharmaceuticals, and
agrochemicals.[1] Stoichiometric reduction by using base metals
(usually iron, tin, zinc and aluminum) in acidic media as reducing
agents to produce aniline from nitrobenzene has been broadly
applied for more than a century because of its low cost and easy
manipulation.[2] However, stoichiometric reduction process
inevitably lead to forming a large amount of solid wastes. Instead,
the environment-benign catalytic hydrogenation protocol with the
aid of metal catalysts has been gradually adopted and took an
important part in industry. Pt-based catalysts are usually
employed in heterogeneous hydrogenation due to their high
activity for H2 dissociation and the following H atom addition as
well. However, the unmodified Pt catalysts have poor
chemoselectivity,[3] when the substituted aromatics consists of
other functional groups, such as C=C, C=O, or -Cl. This is
because nitroarmatic is prone to adsorb in a flat manner with both
the benzene ring and conjugated groups directly interacting with
the Pt surface, resulting in an indiscriminate attack by H atoms.
To circumvent this problem, numerous efforts have been made,
[4]
e.g. downsizing Pt nanoparticles to clusters,
even to single
atoms,[5] confining Pt nanoparticles in microporous solids,[6] or
adding organic ligands[7], another metals[8] or metal oxides.[4b, 9]
These modifications shared the same strategy to modulate the
adsorption behaviour of nitrobenzene to prevent the falt
adsorption through electronic and/or stetic effects. Alternatively,
some reducible metal oxides/hydroxides themselves, such as iron
[a]
Y. Wang, R. X. Qin, Y. K. Wang, R. Juan, W. T. Zhou, L.Y. Li, J.
Ming, W. Y. Zhang, Prof. G. Fu, Prof. N. F. Zheng
State Key Laboratory for Physical Chemistry of Solid Surfaces,
Collaborative Innovation Center of Chemistry for Energy Materials,
National & Local Joint Engineering Research Center of Preparation
Technology of Nanomaterials, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005 (China)
E-mail: nfzheng@xmu.edu.cn, gfu@xmu.edu.cn
Supporting information for this article is available on the WWW
under http://
1
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large deep detection.[13] As shown in Figure 1e, the O1s XPS
spectrum displayed two peaks with one major peak at 531.9 eV
and a shoulder peak at 530.3 eV, corresponding to the existence
of OH- and O2-, respectively. CO adsorbed FT-IR spectra showed
a same adsorption peak at around 2050 cm-1 on both Pt and
0.3Fe(OH)x/Pt (Figure S3), which could be assigned to the linear
adsorption of CO on metallic Pt.[14] All these findings came to the
conclusion that Fe(III)-OH-Pt interfaces have been created.
of Fe to Pt from 0.0 to 0.3 led to a decrease in Pt dispersion from
19% to 3.2%, indicating that the Pt atoms on the surface were
partially covered by the Fe(OH)x species. Interestingly, the
selectivity to 3-AS were however significantly enhanced by the
presence of Fe(OH)x species. This implied that the addition of Fe
would alter the chemoselectivity in which -NO2 hydrogenation
became preferred. To illustrate the remarkable selectivity
enhancement, the hydrogenation of styrene and nitrobenzene
mixture (1:1) over these catalysts was also performed (Figure S6).
As shown in Figure 2e, unmodified Pt catalyst preferred to
hydrogenate C=C bond rather than -NO2 group. Such a
preference could be rationalized by considering the flat adsorption
mode.[8b, 15] On the contrary, the addition of Fe modulated the
hydrogenation pattern. With the Fe/Pt mole ratio increasing, the
TOF of styrene hydrogenation decreased sharply, while the TOF
of nitrobenzene hydrogenation were nearly a constant when
Fe(OH)x was introduced. While the reduced C=C hydrogenation
activity might be due to the decreased exposure extent of Pt
domains, the enhanced activity towards -NO2 can be ascribed to
the synergetic effect of Fe(III)-OH-Pt interfaces.
Figure 1. a) Schematic illustration of synthetic route of Fe(OH)x/Pt. b) STEM-
EDS elemental mapping of 0.3Fe(OH)x/Pt. c) HS-LEIS spectra of as-prepared
and sputtered 0.3Fe(OH)x/Pt. d,e) Fe2p and O1s XPS spectra of 0.3Fe(OH)x/Pt.
The catalytic properties of 0.3Fe(OH)x/Pt were investigated
in the hydrogenation of 3-NS (Figure 2a) in comparison with Pt
NCs. As shown in Figure 2b, 3-NS was completely converted on
Pt NCs at 40 min with a low selectivity towards 3-AS (15%).
Figure 2. a) Possible hydrogenation products of 3-NS. b,c) 3-NS conversion
and product distribution over Pt nanocrystals (b) and 0.3Fe(OH)x/Pt (c). d) 3-AS
selectivity as a function of Fe/Pt mole ratio and Pt dispersion over different
catalysts. e) Comparison of TOF of catalysts with different Fe/Pt mole ratios in
the hydrogenation of nitrobenzene and styrene mixture, which possesses single
NO2 group and single C=C group, respectively. Reaction conditions: 60 oC, 1bar
H2, 0.001 mol of Pt/ mol of each substrate, 10mL ethanol as solvent.
Besides
3-AS,
3-nitroethylbenzene
(3-NE)
and
3-
aminoethylbenzene (3-AE) were also obtained. However, over
the 0.3Fe(OH)x/Pt catalyst (Figure 2c), the selectivity of 3-AS was
improved to 92.8% at 2h. In addition to the catalytic activity and
selectivity, the durability of a catalyst is another important criterion
to evaluate the potential application of catalysts. As shown in
Figure S4, the 0.3Fe(OH)x/Pt catalyst could be reused for at least
five times without significant decay, indicating that the as-formed
interfaces were be relative robust.
To understand the promotional effects of the Fe(OH)x/Pt
combined system, a series of catalysts with different Fe/Pt mole
ratios were synthesized by varying the amount of Fe(acac)3
introduced during the synthesis process. Together with ICP-MS
and CO titration measurements, the catalytic performances of
these catalysts were also investigated (Table S1 and Figure S5).
As shown in Figure 2d, it was clear that increasing the mole ratio
Previously, we have demonstrated the Fe(III)-OH-Pt
interfaces (I) could effectively boost the CO oxidation.[11] We
propose here that the same interfaces can also play a pivot role
to boost the catalytic deoxygenation of 3-NS. As illustrated in
Figure 3a, H2 is readily dissociated on exposed Pt atoms into H
atoms (II),[16] which can then migrate to react with the interfacial
OH to produce H2O and Fe2+ ions (III). In the next step, the release
of H2O leads to the formation of a vacancy close to the five-
coordinated Fe2+ site (IV), which can capture and reduce the nitro
group (V) rather than the C=C bond due to the oxophilicity of Fe2+.
2
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Subsequently, proton coupling electron transfer occurs with the
proton migration from the adsorbed H2O to O end of nitro group
and the simultaneous electron transfer from Fe2+ to the N end (VI).
In such a process, the N-O bond is broken so that the Fe(III)-OH-
Pt interface is recovered (VII). In the hydrogenation of
nitroaromatics over the interface, the activated H atoms and nitro
groups are playing roles very similar to those of CO and O2 in CO
oxidation, nicely explaining why the same interface works
effectively for both oxidation and hydrogenation reactions.
Moreover, our DFT calculation results (Figure 3b and Figure S7)
showed that when Fe(OH)x deposited on the Pt(111) surface, the
preferential adsorption orientation for 3-NS would be changed in
which the reactant preferred to adsorb on the interface through
the -NO2 group rather than the vinyl group. Similar to previous
work,[17] we suggested that the following intermediates and the
product aninline preferred the N end adsorption mode, thus
inhibiting the hydrogenation of C=C bond.
0.3Fe(OH)x/Pt catalyst after being treated in D2 atmosphere
(Figure S9). The formation of D2O was observed by the mass
spectrometry when the 0.3Fe(OH)x/Pt catalyst was treated by D2
flow at 60 oC (Figure S10). More importantly, when 0.3Fe(OH)x/Pt
was treated with H2, the change of oxidation state from Fe3+ to
Fe2+ with the binding energy of 2p1/2 shifted from 725.4 to 723.1
eV was also identified by in situ Fe2p XPS spectra (Figure S11).
As shown in Figure S12, the hydrogenation rate of nitrobenzene
over 0.3Fe(OH)x/Pt in D2 was found approximately 2.3 times
slower than that in H2. A similar isotope effect uisng D2O to
replace H2O in the reaction solvent was also observed (Figure
S13). These isotope effects suggested the direct involvement of
O-H cleavage in the proposed hydrogenation mechanism.
To further verify the involvement of proton transfer in the
hydrogenation of nitro group but not in the hydrogenation of C=C
group on Fe(III)-OH-Pt interfaces,[18] the influence of H2O on the
hydrogenation of nitrobenzene and styrene mixture was
examined. As illustrated in Figure S14, with the increased volume
percentage of H2O in the EtOH-H2O solvent from 0 to 10%, the
conversion of nitrobenzene was only slightly decreased. In
contrast, the conversion of styrene was dramatically suppressed.
Similarly, in the anhydrous reaction condition (Figure 3c), 3-NS
was completely converted at 2h with the selectivity of 92.8%
towards 3-AS. When the reaction time was extended to 26 h, 3-
AS was further hydrogenated to 3-AE. However, when the
reaction solvent was EtOH/H2O (volume ratio of 9:1) (Figure 3d)
,
the selectivity of 3-AS maintained at a high level of 99.0% even
after 26-h hydrogenation. All these results clearly manifested
the critical role of interfacial OH species in the selective
hydrogenation of nitro groups. Introducing a small amount of
H2O in the reaction helps to stabilize the Fe(III)-OH-Pt
interfaces during catalysis and thus achieve high stability of
the catalytic activity.
The understanding of the important role of Fe(III)-OH-Pt
interfaces motivated us to explore the application of the
Fe(OH)x/Pt catalyst in the selective hydrogenation of a wide range
of functional nitroaromatics with various substituents. As
summarized in Table S2, the Fe(OH)x/Pt catalyst did exhibit both
excellent activity and selectivity in the hydrogenation of
substituted nitroaromatics having aldehyde, ketone groups and
halogen atoms, respectively. The TOF values based on the
exposed Pt sites on the 0.3Fe(OH)x/Pt catalyst readily reached a
level of over 30,000 h-1, suggesting the high activity of the Fe(III)-
OH-Pt interfaces in the chemoselective hydrogenation of
substituted nitroaromatics, which was superior to many already
reported Fe-based and Pt-based catalysts (Table S3). Although
TOF values were related to reaction conditions, such as
temperature, gas pressure, reactant concentrations and so on,[19]
the excellent selectivity achieved by the Fe(III) -OH-Pt interfaces
makes by the Pt/Fe(OH)x combination promising for catalysts for
green production of substituted anilines from nitroaromatics.
Figure 3. a) Proposed mechanism of catalytic deoxygenation of nitrobenzene
on Fe(OH)x/Pt surface. b) Calculated adsorption energy (in eV) for different
molecules on Pt(111) and Fe(OH)x/Pt surfaces. Effect of H2O for 0.3Fe(OH)x/Pt
catalyst on hydrogenation of 3-NS c) 10mL EtOH as solvent, d) 9mL EtOH and
1 mL H2O as solvent. Reaction conditions: 60 oC, 1bar H2, 0.001 mol of Pt/ mol
of substrate, 10mL solvent.
In summary, we reported a combined catalyst with superior
chemoselectivity towards the hydrogenation of nitrostyrene by
patching Fe(OH)x species on the surface of Pt NCs. We
demonstrated that with the abundance of Fe(III)-OH-Pt interfaces
increased, the hydrogenation of nitro group was favored over that
of vinyl group. During the hydrogenation of 3-NS, the fabricated
Fe(III)-OH-Pt interfaces promoted the chemoselectivity
hydrogenation with a high TOF value (11065 h-1) in 1bar H2 at 60
oC. In situ IR spectra and isotopic studies illustrated that the
hydrogenation at the Fe(III)-OH-Pt interfaces occurred in a
The proposed mechanism was further confirmed by isotopic
experiments. When D2 was used in styrene hydrogenation over
0.3Fe(OH)x/Pt catalyst, the reaction rate was slowed down by a
factor of 1.05 (Figure S8). This kinetic isotope effect (KIE) was
consistent with most hydrogenation reactions involved with
homolytically activated H atoms. FT-IR spectroscopic studies
revealed the existence of O-D species around 2500 cm-1 on the
3
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