Remarkable effect of alkalis on the chemoselective hydrogenation of functionalized nitroarenes over high-loading Pt/FeO:X catalysts

Article abstract of DOI:10.1039/c7sc00568g

The chemoselective hydrogenation of substituted nitroarenes to form the corresponding functionalized anilines is an important type of reaction in fine chemistry, and the chemoselectivity is critically dependent on the rational design of the catalysts. Thi

Full text of DOI:10.1039/c7sc00568g

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Remarkable Effect of Alkalis on Chemoselective Hydrogenation of
Functionalized Nitroarenes over High-Loading Pt/FeOx Catalysts
Haisheng Wei,+ a Yujing Ren,+ a,b Aiqin Wang,* a Xiaoyan Liu,a Xin Liu,a Leilei Zhang,a Shu Miao,a Lin
Li,a Jingyue Liu,c Junhu Wang,a Guofu Wang,d Dangsheng Su,a Tao Zhang* a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The chemoselective hydrogenation of substituted nitroarenes to the corresponding functionalized anilines is a type of
important reactions in fine chemistry, and the chemoselectivity is critically dependent on the rational design of catalysts.
In particular, this reaction has rarely been accomplished over high-loading Pt catalysts due to the formation of Pt crystals.
Here we for the first time reported that alkali metals (Li+, Na+, K+, etc.) could transform the non-selective high loading
Pt/FeOx catalyst to a highly chemoselective one. The best result was obtained over a 5%Na-2.16%Pt/FeOx catalyst, which
enhanced the chemoselectivity from 66.4% to 97.4% while the activity remained almost unchanged for the probe reaction
of 3-nitrostyrene hydrogenation to 3-aminostyrene. By using aberration-corrected HAADF-STEM, in situ XAS, 57Fe
Mössbauer and DRIFT spectroscopy, an active site of Pt-O-Na-O-Fe-like species was proposed, which ensure the Pt centers
isolated and positively charged for the preferential adsorption of –NO2 group.
www.rsc.org/
compromising their activity.
Introduction
Single-atom catalysts (SACs), which are featured with
exclusively atomic dispersion of the active metals,4 have
Chemoselective hydrogenation of substituted nitroarenes is a
class of reactions towards the production of functionalized
anilines, which are important intermediates for agrochemicals,
pharmaceuticals and dyes.1 A big challenge for this reaction is
the selective reduction of nitro group when other reducible
groups (e.g., C=C, C=O, -X, etc.) are present because most of
transition metal catalysts cannot discriminate different
functional groups for selective hydrogenation. Therefore, great
efforts have been made to seek for highly chemoselective
catalysts towards preferential hydrogenation of nitro group in
the presence of other reducible groups.2 Effective approaches
reported to date include introduction of a less active
component (e.g., Pt-Pb/CaCO3 catalyst),2a partially covering
active metal surface with reducible oxide support (e.g., Pt/TiO2
catalyst),2b as well as employing relatively inert metal (such as
Au and Ag) catalysts.3 All these strategies, however, enhance
the chemoselectivity at the cost of activity, which is
undesirable in practical applications. Therefore, it is keenly
desired to develop highly chemoselective catalysts without
demonstrated distinctive selectivities in
a
variety of
hydrogenation reactions without compromising intrinsic
activity.5 In particular, Pt1/FeOx SAC showed a turnover
frequency (TOF) of 1500 h-1 and a selectivity of close to 99% in
the chemoselective hdyrogenation of 3-nitrostyrene.5a The
outstanding performance of the Pt1/FeOx SAC was attributed
to the positively charged and isolated Pt center, which was
supposed to be favorable to the preferential adsorption of
nitro group. In spite of the superior activity per metal atom,
SACs are usually with pretty low metal loading (e.g., < 0.1wt%),
which is less attractive in practical applications due to the very
small number of total active sites per weight of catalyst. Taking
it into account that single atoms are susceptible to aggregation
upon increase of metal loading, it seems a formidable
challenge to prepare a stable high-atom-density SAC (e.g., 1-5
wt% loading) where all the metal atoms are isolated.
One distinctive feature of SACs from their nanoparticle (NP)
conterparts is that the active single atoms are usually
positively charged by bonding with more electronegative
elements (e.g., oxygen atoms) on the support surface.
Considering that alkalis are well known electronic modifiers in
transition metal catalysts,6 and even able to promote Pt
dispersed as single atoms,7 we envision that by modifying the
high-loading Pt/FeOx catayst with alkalis, one might be able to
prepare a high-atom-density SAC, and eventually transform
the less chemoselective high-loading Pt/FeOx into a highly
chemoselective one.
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Results and discussion
7.60%Naꢀ2.16%Pt/FeOx
30
300
94.2
97.4
Firstly, we prepared a series of Pt/FeOx catalysts with different
Pt loadings using co-precipitation of ferric nitrate (Fe(NO3)3)
and chloroplatinic acid (H2PtCl6) with ammonia carbonate
((NH4)2CO3) as the precipitating agent (details see ESI) and
then tested them for hydrogenation of 3-nitrostyrene, one of
the most demanding reaction in chemoselective
hydrogenation of nitroarens. Unless otherwise noted, all the
[a] Atomic ratio. Pretreatment condition: 5 ml toluene, 1 MPa H2, 40 oC, 1 h.
Reaction conditions: Pt/substrate = 0.22 mol%, 5 ml reaction mixture, 0.5 mmol
substrate, toluene as solvent, o-xylene as internal standard.
Table 2 Chemoselective hydrogenation of different substituted nitroarenes to the
corresponding anilines over the 5.03%Na-2.16%Pt/FeOx catalyst.
NO2
NH2
Catalyst
o
catalysts were reduced in 10% H2/He at 250 C for 0.5 h prior
R
R
to the reaction and characterizations. The reaction tests
showed that the selectivity to 3-aminostyrene decreased with
an increase of Pt loading: 0.12%Pt/FeOx yielded a selectivity of
97.6% whereas 2.16%Pt/FeOx gave only 66.4% (Table S1).
Apparently, the high selectivity was only achievable with low
levels of Pt loading, consistent with previous reports.2b,5a
Meanwhile, we found that the precipitating agent greatly
affected the selectivity of the resultant catalysts, e.g., a
relatively high selectivity could be obtained even at a high Pt
loading catalyst when Na2CO3 was used as the precipitating
agent (Table S2). we speculate that the residual Na ions in the
catalysts must play a key role in directing the chemoselectivity.
In order to study the effect of Na ions in a controllable manner,
we subsequently used the above Na-free 2.16wt%Pt/FeOx as
the starting catalyst and introduced a varying amount of
sodium with sodium nitrate as the precursor. The resulting
catalysts are designated as y%Na-2.16%Pt/FeOx, where y
refers to the weight content of sodium. The catalytic
performances are shown in Table 1. To our delight, the
selectivity to 3-aminostyrene increased monotonically with the
Na content until a plateau (~97%) was reached at 3.3%Na
while the activity did not change much with Na content.
Further increasing the Na content to 7.6% led to a dramatic
decline in activity. The best result was obtained at 5.03% Na
which corresponds to a Na/Pt atomic ratio of 19.8. This
catalyst afforded a TOF of 1083 h-1 and
Entry
1
Substrate
Product
Time (min)
Yield (%)
92.7
32
40
NH2
Cl
97.2
2
NO2
20
60
85
70
96.3
94.8
95.7
96.8
3
4
O
5*
6+
Pretreatment condition: 5 ml toluene, 1 MPa H2, 40 oC, 1 h. Reaction conditions:
T=40 oC, P=3 bar, Pt/substrate = 0.22 mol%; 5 ml reaction: mixture 0.5 mmol
substrate, toluene as solution. * Pt/substrate = 0.44 mol%, T=50 oC, P=6 bar.
+ T=50 oC, P=6 bar.
a selectivity to 3-aminostyrene of 97.5%. Such a high TOF level
is slightly lower than that over the 0.08%Pt/FeOx SAC we
previously reported,5a but the efficiency based on the total
weight of catalyst is increased by more than 20 times due to a
relatively high Pt loading. This result is very important to
practical applications since it suggests that the productivity can
be increased by 20 times. To our delight, in comparison with
Table 1 Effect of Na content on the chemoselective hydrogenation of 3-nitrostyrene.
Pt/TiO2,2b Pt/ZnO,2i Au/TiO2,3a Ag@CeO2 and RhIn/SiO2
3d
2j
catalysts which were earlier reported highly chemoselective
for the hydrogenation of 3-aminostyrene, our present
5.03%Na-2.16%Pt/FeOx catalyst is several-fold to two orders
of magnitude more active either based on per metal or per
weight of catalyst (Table S3). Moreover, the catalyst could be
reused several times without decrease in activity or selectivity
(Table S4), and could tolerate a wide spectrum of substrates
with different functional groups, such as triple bonds, ketones,
aldehydes, and halogen (Table 2). Akin to sodium ion, other
alkalis such as Li and K ions also showed remarkably positive
effects on the chemoselectivity (Table S5). These results clearly
demonstrate that alkali ions are able to transform the less
Na/Pt
ratio[a]
Time
(min)
Conv.
(%)
Sel.
(%)
Catalyst
2.16%Pt/FeOx
0
36
35
28
29
26
32
98.7
95.3
96.9
94.9
95.5
95.1
66.4
67.5
85.6
90.5
96.7
97.5
0.74%Naꢀ2.16%Pt/FeOx
1.75%Naꢀ2.16%Pt/FeOx
2.64%Naꢀ2.16%Pt/FeOx
3.30%Naꢀ2.16%Pt/FeOx
5.03%Naꢀ2.16%Pt/FeOx
2.9
6.9
chemoselective high-loading Pt/FeOx into
chemoselective one.
a
highly
10.4
13.0
19.8
To understand the remarkable effect of alkali ions on the
Pt/FeOx, we performed multifaceted characterizations
including X-ray diffraction (XRD), aberration-corrected high-
angle annular dark-field scanning transmission electron
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microscopy (HAADF-STEM), X-ray absorption spectroscopy crystals even at a relatively high loading of Pt. The XAS data
(XAS), Mössbauer spectroscopy and diffuse reflectance provided further support for this proposed model.
infrared Fourier transform spectroscopy (DRIFT). The XRD To uncover the nature of the interaction among Pt, Na and the
patterns of the samples, with or without Na ions, present only Fe3O4 support, we performed in situ XAS experiments during
Fe3O4 phase (Fig. S1). No any diffraction peaks of platinum catalysts reduction. In the XANES spectra, the whiteline
were detected,
intensity at Pt LIII edge decreases continuously with an increase
in the reduction temperature, with or without Na (Fig. S4).
However, comparing the Na-free and Na-containing samples,
one can clearly see that the latter has a higher whiteline
intensity than the former at the same reduction temperature
Fig.
2
Normalized XANES spectra at Pt LIII-edge of 2.16%Pt/FeOx catalysts with
different Na loadings after reduction.
Fig.
1
HAADF-STEM images and size distributions of different catalysts. (A)
2.16%Pt/FeOx; (B) 5.03%Na-2.16%Pt/FeOx; (C) Pt particle of 2.16%Pt/FeOx; (D) Pt
particle of 5.03%Na-2.16%Pt/FeOx. In (A) and (B), scale bar 5 nm; in (C) and (D), scale
bar 1 nm.
(Fig. S5). Moreover, with an increase in Na content, the
whiteline intensity increased monotonically (Fig. 2), indicating
that Pt became more positively charged with the addition of
Na. Consistently, the temperature programmed reduction of
H2 (H2-TPR) showed that with an increase of the Na content,
PtOx species became more difficult to reduce (Fig. S6). The
best-fitted EXAFS results are summarized in Table 3. Compared
with the Na-free sample, the Na-containing sample has a
larger Pt-O coordination number (2.5 vs. 1.2), smaller Pt-Fe
and Pt-Pt coordinations (totally 0.9 vs. 1.8), and a longer Pt-Pt
distance (2.75 vs. 2.68 Å), suggesting that the Pt centers in the
Na-containing sample are more oxidized, the Pt-Pt
coordination is more unsaturated, and the Pt-Pt metallic
bonding becomes weaker. This result is in good agreement
with the conclusion by
suggesting that Pt species is highly dispersed even at a high Pt
loading. Fig. 1 and Fig. S2 show the HAADF-STEM images of
two
typical
samples,
5.03%Na-2.16%Pt/FeOx
and
2.16%Pt/FeOx. At the first sight, the Na-containing sample
contains much bigger particles than the Na-free sample, and
the average particle size in the former is 0.9 nm while it is 0.4
nm in the latter. However, close examination of each particle
in the Na-containing sample led to a completely opposite
conclusion because the presence of Na ion greatly changed the
morphology of Pt particles. As shown in Fig. 1D, one can
observe apparent dark regions within the particles in the Na-
containing sample, which is in contrast with the well-
crystallized Pt particles in the Na-free sample (Fig. 1C). Since
both Fe and Na are lighter elements (compared to Pt), the
presence of either Fe or Na can give such contrasts in the
HAADF-STEM images. Examination of many different particles
revealed that almost each particle in the Na-containing sample
is assembled by loosely and randomly stacked single atoms or
clusters of only a few atoms. This can be visualized more
clearly in the catalysts reduced at a higher temperature (Fig.
S3). These observations suggest that the particles in the
HAADF images may contain Pt atoms as well as Na and/or Fe
atoms. The addition of Na ions prevented the formation of Pt
Table 3 EXAFS data fitting results of y%Na-2.16%Pt/FeOx samples.
Samples
Shell
N
R (Å)
σ2x102
(Å2)
∆E0
(eV)
rꢀfactor
(%)
Pt foil
PtO2
PtꢀPt
PtꢀO
PtꢀPt
PtꢀO
PtꢀFe
12.0
6.0
6.0
1.2
1.1
2.76
2.00
3.08
1.99
2.54
0.41
0.41
0.99
0.30
0.54
6.1
7.2
2.1
4.7
4.7
0.42
0.56
2.16%
Pt/FeOx
0.91
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PtꢀPt
PtꢀO
0.7
2.5
0.4
0.5
1.0
2.68
2.01
2.54
2.75
2.95
0.54
0.18
0.46
0.46
0.73
4.7
5.03%Naꢀ
2.16%
Pt/FeOx
9.6
9.6
9.6
9.6
0.92
PtꢀFe
PtꢀPt
PtꢀNa
N, the coordination number for the absorber-backscatterer pair. R, the average
absorber−backscaꢀerer distance. σ2, the Debye−Waller factor. ΔE0, the inner
potential correction. The accuracies of the above parameters were estimated as
N, ±20%; R, ±1%; σ2, ±20%; ΔE0, ±20%. The data range used for data fitting in k-
space (Δk) and R-space (ΔR) are 3.0-10.7 Å-1 and 1.2-3.5 Å, respectively.
HAADF-STEM imaging that the addition of Na greatly limits the
formation of Pt crystals. In addition, there is a new Pt-Na
contribution at a distance of 2.95 Å with a coordination
number of 1.0. Such a long bonding length between Pt and Na
can be ascribed to bridge linkage like Pt-O-Na. The formation
of Pt-O-Na interaction was also approved by a control
experiment, in which excess amount of Na was washed away
and the residue Na to Pt atomic ratio was close to 1 (0.83 and
0.88, see Table S6). The reaction test showed that the
performance of the catalyst remained almost unchanged after
such washing treatment (Table S7), indicating that the
effective Na/Pt atomic ratio was roughly 1/1.
Fig. 3
Mössbauer spectroscopy of (A) 2.16%Pt/FeOx-R250; (B) 5.03%Na-
2.16%Pt/FeOx-R250; (C) 2.16%Pt/FeOx-R400 and (D) 5.03%Na-2.16%Pt/FeOx-R400
catalysts.
support and then depositing Pt species (details see ESI). The
NaFeO2/FeOx structure was identified by Mössbauer
spectroscopy (Fig. S7 and Table S9). The HAADF-STEM images
of this sample clearly show the lattice fringes of Fe3O4
nanocystal, the amorphous layer of NaFeO2 on the surface of
Fe3O4, and the assembles of Pt single atoms preferentially
deposited on the NaFeO2 surface layer (Fig. S8). In consistence
with the series of y%Na-2.16%Pt/FeOx catalyst, the assembles
of Pt single atoms do not form Pt crystals due to the
interaction with NaFeO2. Interestingly, this Pt/NaFeO2/FeOx
catalyst exhibited 99.3% selectivity of 3-aminostyrene at full
conversion of the substrate (Table S10). Such an exceptionally
high selectivity at full conversion of substrate has never been
obtained on any Pt catalysts.
Considering that Pt interacts with Na ions via oxygen atoms on
the Fe3O4 support, we then performed 57Fe Mössbauer
spectroscopy to probe the local environment of iron oxides
surrounding the Pt species. As shown in Fig. 3 and Table S8,
compared with the Na-free sample which is characterized with
two sextets of magnetite, there is a new and well-defined
doublet with isomer shift value of 0.26 mm.s-1 and quadrupole
splitting value of 0.42 mm.s-1 in the Na-containing sample,
which can be assigned to high-spin Fe3+ of NaFeO2.8 Moreover,
the amount of NaFeO2 species became more pronounced
when the reduction temperature was raised to 400 oC, at
which all the magnetite was reduced to metallic Fe while Pt
was still positively charged (Fig.3 and Fig. S5). These results
lead us to deduce that the cationic Pt must arise from the
linkage to NaFeO2; otherwise, Pt would be reduced to metallic
state at such elevated temperatures. Therefore, we can
propose that Pt interacts with the NaFeO2 surface species via
bridging oxygen, forming Pt-O-Na-O-Fe- species, which creats
low-coordinated or even isolated, positively charged Pt centers
to ensure both high activity and chemoselectivity for
nitroarene hydrogenation. The particles with unusual contrast
observed in the HAADF-STEM images of the Na-containing
sample are most probably the assembly of Pt-O-Na-O-Fe-
species.
The formation of NaFeO2 surface layer not only promotes the
dispersion of Pt but also has a significant impact on the
preferential adsorption of nitro group. As shown in the DRIFT
spectra in Fig. 4, the adsorption of 3-nitrostyrene produced
two bands at 1531 cm-1 and 1350 cm-1, which are attributed to
asymmetric stretching (vas) and symmetric stretching (vss)
vibrations of nitro group,9a,
9b
respectively. Comparing the
DRIFT spectra on different catalysts, one can clearly see that
the 5.03%Na-2.16%Pt/FeOx catalyst adsorbs nitro group much
more strongly than the 2.16%Pt/FeOx catalyst does, suggesting
that the presence of Na cation intensify the adsorption of nitro
group. A control experiment with Na-modified FeOx support
reveals that the nitro group adsorbs on the support, rather
than on the Pt species (Fig. S9). Introduction of H2 on the
2.16%Pt/FeOx and 5.03%Na-2.16%Pt/FeOx samples which
were allowed to pre-adsorb 3-nitrostyrene led to the
appearance of new bands at 1589 cm-1 (-NO)9c, 9d, 1461 cm-1 (-
To further prove the above-proposed model that the surface
NaFeO2 species interacts with Pt and thus enhances its
dispersion, we prepared a sandwich catalyst composed of
Pt/NaFeO2/FeOx, by first forming a layer of NaFeO2 on the FeOx
NOH),9b 1620 cm-1, 1605 cm-1 and 1494 cm-1 (-NH2),9c,
9e
concurrently with the sharp decrease in the intensity of nitro
absorption bands, demonstrating the stepwise reduction of
nitro group to amino group. In contrast, the same procedure
on the FeOx support and Na-modified FeOx support did not
bring about any change in the nitro adsorbtion bands (Fig. S9).
Taken these results together,
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striking difference was observed under competitive reaction
conditions (both functional groups were present): the rates for
nitrobenzene hydrogenation were almost the same for both
catalysts while that for styrene hydrogenation over the Na-
containing catalyst was reduced by one order of magnitude as
compared to Na-free catalyst, thus greatly enhancing the
chemoselectivity for -NO2 hydrogenation.
Based on the above results, it can be reasonably concluded
that it was the surface layer of NaFeO2 that directed the
formation of pseudo-single atoms of Pt, by which the C=C
adsorption was almost completely suppressed while the –NO2
group was adsorbed preferentially, eventully leading to the
chemoselective hydrogenation of nitro group, as illustrated in
Fig. 5.
Conclusions
Fig.
4
DRIFT spectra of 3-nitrostyrene adsorption at 80 oC on (a, b) 5.03%Na-
2.16%Pt/FeOx-R250 and (c, d) 2.16%Pt/FeOx-R250 before (a, c) and after (b, d)
introduction of H2.
In conclusion, we have found that alkali metal cations are able
to transform less chemoselective high-loading Pt/FeOx catalyst
to a highly chemoselective one without compromising the
activity. Na cations interact with the Pt/FeOx catalyst to form
Pt-O-Na-O-Fe surface species, which not only prevents the
formation of Pt crystals, but also provides low-coordinated or
even isolated and positively charged Pt centers. Such Pt-O-Na-
O-Fe species is intrinsically more selective for –NO2
hydrogenation than for C=C hydrogenation. To the best of our
knowledge, this is the first report that both high activity and
chemoselectivity are achieved with a high loading transition
metal catalyst. Such a strategy for engineering the electronic
and geometric properties of Pt by doping alkali metals can be
extended to other supported metal catalysts for various
chemical reactions.
Acknowledgements
The authors are grateful to the supports from the National
Natural Science Foundation of China (21690080, 21690084,
21373206, 21522608 and 21673228), the Strategic Priority
Research Program of the Chinese Academy of Sciences
(XDB17020100), the National Key Projects for Fundamental
Research and Development of China (2016YFA0202801) and
the BL 14W beamline at the Shanghai Synchrotron Radiation
Facility (SSRF). J. Liu acknowledges the support by NSF under
CHE-1465057, Arizona State University, and the use of facilities
in the John M. Cowley Center for High Resolution Electron
Microscopy at Arizona State University.
Fig. 5 The proposed model for chemoselective hydrogenation of 3-nitrostyrene over
Na-modified Pt/FeOx catalysts.
we can conclude that the nitro group is adsorbed on the
NaFeO2 surface while H2 is activated on Pt species, and the
hydrogenation reaction occurrs most likely on the interface
between Pt and the NaFeO2 surface. The formation of NaFeO2
species make the nitro group strongly adsorbed on it, thereby
increasing the chemoselectivity to nitro group hydrogenation.
The control experiments with nitrobenzene and styrene as the
reaction substrates verify that the competive adsorption of –
NO2 over C=C group on the catalyst governs the
chemoselectivity. As shown in Table S11, under non-
competitive reaction conditions (only one functional group
was present), the presence of Na led to the reaction rate for
C=C hydrogenation reduced by more than 65%, while that for
nitrobenzene hydrogenation reduced by only 26%. It thus
demonstrates that the Pt-O-Na-O-Fe- species is intrinsically
selective for the –NO2 hydrogenation, which is quite different
from the Na-free sample that is almost non-selective. More
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Chemical Science
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DOI: 10.1039/C7SC00568G
Table of Contents
Alkali metals transform non-selective high-loading Pt/FeOx to a highly
chemoselective catalyst by helping to form isolated and electropositive Pt
centers.