Transforming nonselective into chemoselective metal catalysts for the hydrogenation of substituted nitroaromatics

Article abstract of DOI:10.1021/ja800959g

It is generally accepted that good hydrogenation noble and nonnoble metal catalysts such as Pt, Ru, or Ni are not chemoselective for hydrogenation of nitro groups in substituted aromatic molecules. We have found that it is possible to transform nonchemoselective into highly chemoselective metal catalysts by controlling the coordination of metal surface atoms while introducing a cooperative effect between the metal and a properly selected support. Thus, highly chemoselective and general hydrogenation Pt, Ru, and Ni catalysts can be prepared by generating nanosized crystals of the metals on the surface of a TiO₂ support and decorating the exposed (111) and (100) crystal faces by means of a simple catalyst activation procedure. By doing this, it has been possible to change the relative rate for hydrogenating competitive groups present in the molecule by almost 2 orders of magnitude, increasing the chemoselectivity from less than 1% to more than 95%.

Full text of DOI:10.1021/ja800959g

Published on Web 06/14/2008
Transforming Nonselective into Chemoselective Metal
Catalysts for the Hydrogenation of Substituted Nitroaromatics
Avelino Corma,*,† Pedro Serna,† Patricia Concepcio´n,† and Jose´ Juan Calvino‡
Instituto de Tecnolog´ıa Qu´ımica (UPV-CSIC), AV. de los Naranjos s/n, 46022 Valencia, Spain,
and Departamento Ciencia de Materiales e Ingenier´ıa Metalu´rgica y Qu´ımica Inorga´nica,
UniVersidad de Ca´diz, Campus R´ıo San Pedro, Puerto Real, 11510 Ca´diz, Spain
Received February 7, 2008; E-mail: acorma@itq.upv.es
Abstract: It is generally accepted that good hydrogenation noble and nonnoble metal catalysts such as
Pt, Ru, or Ni are not chemoselective for hydrogenation of nitro groups in substituted aromatic molecules.
We have found that it is possible to transform nonchemoselective into highly chemoselective metal catalysts
by controlling the coordination of metal surface atoms while introducing a cooperative effect between the
metal and a properly selected support. Thus, highly chemoselective and general hydrogenation Pt, Ru,
and Ni catalysts can be prepared by generating nanosized crystals of the metals on the surface of a TiO2
support and decorating the exposed (111) and (100) crystal faces by means of a simple catalyst activation
procedure. By doing this, it has been possible to change the relative rate for hydrogenating competitive
groups present in the molecule by almost 2 orders of magnitude, increasing the chemoselectivity from less
than 1% to more than 95%.
iron,5 tin,6 or zinc in ammonium hydroxide7 are still being used,
with the concomitant formation of at least 1 mol of residue per
Introduction
Catalysis is a key discipline for sustainability since it allows
chemical transformations to be performed at lower temperatures
while avoiding or minimizing the formation of byproduct. The
impact is not only on energy saving but also for preserving a
cleaner environment, provided that more active, and especially
more selective, catalysts could be prepared. Selectivity becomes
crucial when molecules present two or more reacting groups
and only one of them has to be transformed. Thus, a selective
catalyst has to “recognize” and preferentially interact with the
desired chemical group, while avoiding the transformation of
the others. This is, for instance, the case for chemoselective
hydrogenations, for which noble metals are frequently used as
catalysts.1–3 However, the problem often arising is that the
highly active noble metals are not chemoselective and they must
be modified by alloying or poisoning with other metal oxides
or molecules that improve selectivity, but in most cases at the
expenses of activity. The chemoselective hydrogenation of sub-
stituted nitroaromatics to the corresponding substituted anilines,
which are important products for pharmaceuticals, pesticides
and dyes, among others, is a difficult scientific and technological
problem. It is generally accepted that commercially available
Ni or Pt catalysts cannot be used to hydrogenate substituted
nitrobenzenes with H2, since they are not chemoselective.
Because of this lack of selectivity, it is common to see that
stoichiometric reducing agents such as sodium hydrosulfite,4
mole of desired product. Researchers have attempted to
overcome this limitation by use of H2 as reducing agent and
addition of PbO or H3PO2 to supported Pt catalysts. Then,
though the total catalytic activity decreased, the selectivity to
substituted aromatic amines was increased.8 However, the
chemoselective hydrogenation of substituted nitroaromatics is
an especially complex case, since one type of byproduct formed
with Pt-PbO or Pt-H3PO2 catalysts was phenylhydroxylamine
derivatives, which can be explosive even at low levels.
Intelligently, Siegrist et al.8 used Pt-H3PO2 and Pt-PbO and
added vanadium or iron salts, respectively, to the reaction media
in amounts of up to 20 wt % to convert the hydroxylamines
and improve the final process. By using these catalytic systems,
the authors have presented a large number of examples
comprising differently functionalized nitroaromatics, showing
that a broad spectra of substitutes anilines can be synthesized
through a chemoselective hydrogenation.9 Despite the important
advance achieved, the resultant catalyst, which involves large
amounts of transition metal salts in solution, should be improved
by designing a fully heterogeneous chemoselective catalyst.
Recently, the reaction was studied from another direction by
starting with a less active hydrogenation catalyst, such as gold,10
that gave very high selectivity to substituted anilines.11 Nev-
(5) Suchy, M.; Winternitz, P.; Zeller, M. (Ciba-Geigy). WO Patent 91/
02278, 1991.
(6) Butera, J.; Bagli, J. (American Home Products). WO Patent 91/09023,
1991.
† Instituto de Tecnolog´ıa Qu´ımica.
‡ Universidad de Ca´diz.
(7) Burawoy, A.; Critchley, J. P. Tetrahedron 1959, 5, 340.
(8) Siegrist, U.; Baumeister, P.; Blaser, H.-U. Chem. Ind. (Dekker) 1998,
75, 207.
(1) Bond, C. G. Chem. Soc. ReV. 1991, 20 (4), 441.
(2) Gallezot, P.; Richard, D. Catal. ReV. Sci. Eng. 1998, 40, 81.
(3) Claus, P. Appl. Catal., A 2005, 291, 222.
(4) Kovar, R. F.; Armond, F. E. (U.S. Air Force). U.S. Patent 3,975,444,
1976..
(9) Blaser, H. U.; Siegrist, U.; Steiner, H.; Studer, M. In Fine Chemicals
Through Heterogeneous Catalysis: Aromatic Nitro Compounds; Shel-
don, R. A., van Bekkum, H., Eds.; Wiley: New York, 2001; p 389.
9
8748 J. AM. CHEM. SOC. 2008, 130, 8748–8753
10.1021/ja800959g CCC: $40.75
2008 American Chemical Society

Transformation into Chemoselective Metal Catalysts
A R T I C L E S
Samples for electron microscopic studies were prepared by
depositing small amounts of the powders directly onto holey carbon-
coated Cu grids. Excess powder was removed from the grids by
gentle blowing with a nozzle.
ertheless, a catalyst with the same selectivity as gold but with
a higher activity would be desired for industrial application,12
and there is a scientific and technological challenge to transform
active but nonselective noble metals (Pt, Ru) and a nonnoble
metal (Ni) into highly active and chemoselective catalysts.
C. Hydrogenation of Nitroaromatics. Catalytic testing was
performed in reinforced glass reactors equipped with a temperature
and pressure control. For each reaction, a 1 mL mixture of reactants
and solvent was placed into the reactor (2 mL capacity) together
with an appropriate amount of catalyst. All the reactants used in
this paper were commercially available from Sigma-Aldrich Co.
with purities above 96%. o-Xylene was always used as internal
standard for the determination of conversion level and yields. Details
on the composition of feed for the different substrates tested can
be found in Table S1 in Supporting Information. After the reactor
was sealed, air was purged by flushing two times with 10 bar of
hydrogen. Then the autoclave was heated up to the required
temperature, and finally it was pressurized with H2 at the selected
set point. During the experiment, the pressure was maintained
constant and the stirring rate was fixed at 1000 rpm (magnetic
stirring). Aliquots were taken from the reactor at different reaction
times until the end of the experiment. The product composition
was determined by means of gas chromatography, once the catalyst
particles were removed from the solution by centrifugation at 12 000
rpm. The products were identified by gas chromatography/mass
spectrometry (GC-MS) and also by comparison with commercially
pure products (Sigma-Aldrich Co.). Only experiments with mass
balances >95% were considered.
In this paper, we will show that control of the crystal di-
mensions of the metal and the corresponding exposed domains,
together with proper selection of the support, are key issues to
directly transform nonselective Ni, Pt, or Ru into highly active
and chemoselective catalysts for the hydrogenation of substituted
nitroaromatics to the corresponding anilines, without forming
hydroxylamine derivatives while avoiding the addition of
transition metal salts.
Experimental Section
Catalyst Preparation. Pt/Al2O3, Pt/C, Pt/TiO2, Ni/TiO2, and Ru/
TiO2 samples were prepared by incipient wetness technique at the
desired metal contents. H2PtCl6 (hexahydrate, Aldrich, >37.5% as
Pt), Ni(NO3)2 (hexahydrate, Fluka, >98.5%), and RuCl3 (Aldrich,
Ru content 45-55%) precursors were used to impregnate the
γ-Al2O3 (Merck), C (activated carbon, Aldrich), and TiO2 (Degussa
P-25) supports. As an example, 20 mL of an aqueous solution
containing 53.1 mg of H2PtCl6 ·6H2O was impregnated on 10 g of
TiO2 to prepare the 0.2 wt % Pt/TiO2 catalyst. After a perfect mixing
of the corresponding slurries, samples were always dried at 373 K
for 5 h and then reduced under pure H2 flow at 473 or 723 K for
3 h. Only in the case of Ni catalysts, a calcination process in air
atmosphere was performed at 823 K for 3 h before the reduction
of the sample. The Au/TiO2 catalyst has been supplied by the World
Gold Council and was prepared by a deposition-precipitation
procedure.
Catalyst Characterization: A. FTIR Experiments. Infrared
transmission spectra were recorded with a Bio-Rad FTS-40A
spectrometer equipped with a mercury-cadmium-telluride (MCT)
detector. The infrared cell was designed to treat the samples in
situ under vacuum or under flow conditions. CO adsorption
experiments were performed at room temperature at increasing CO
pressure (2-100 mbar). Prior to adsorption, the samples were
evacuated at 298 K and 10-5 mbar for 1 h.
B. Electron Microscopy. Studies have been performed in a
JEOL 2010F microscope operating at 200 kV in both transmission
(TEM) and scanning-transmission modes (STEM). This micro-
scope has a structural resolution of 0.19 nm and allows formation,
in STEM mode, of electron probes with diameters down to 0.5 nm
suitable for high-spatial resolution analytical investigation. High-
resolution transmission electron microscopic (HRTEM) images were
acquired digitally on a 1024 × 1024 CCD camera and analyzed
by the routines of the Diffpack module of Gatan Digital Micrograph
software. Crystallographic phase analysis has been performed on
the basis of FFTs applied on small selected areas of the HRTEM
micrographs. Particle size distributions were obtained by use of a
program developed at UCA. STEM images were obtained by use
of a high-angle annular dark field detector (HAADF), which allows
Z-contrast imaging. Electron energy loss spectra (EELS) were
recorded on a GIF2000 spectrometer. To monitor the presence of
TiOx moieties on top of the Pt metal particles in the Pt/TiO2
catalysts, the spectrum-line mode was employed. In this operation
mode, a fine electron probe (about 0.5 nm diameter) is rastered
along a predefined path on the sample. EELS spectra, spanning a
defined energy-loss window, are collected at successive points on
this path; in our case with a separation slightly larger than that of
the probe diameter, 0.7 nm. Changes in the EELS spectra allow
performing element distribution mapping. Collecting EELS spectra
along electron-beam paths going from vacuum through surface
positions of the metal particles and ending in the bulk of the metal
particles has allowed us to detect the presence of patches of support
(fingerprint of Ti L2,3 edge) covering the metal nanoparticles.
Results and Discussion
Structure of Pt Nanoparticles and Catalytic Behavior. It is
well recognized that different crystallographic planes of metals
exhibit different catalytic performance. On the other hand, it is
also known that the nature of the support strongly affects the
shape of the metal particles, and this can influence which
crystallographic planes participate in the reaction. For instance,
it has been found that the hydrogenolysis activity of Ni particles
on silica is a function of the relative percentage of (100) and
(111) facets on the particle surfaces.13 Also, during the NO +
CO reaction over Pd catalysts, it was demonstrated that the
stabilization of inactive atomic nitrogen species occurred on
highly reactive undercoordinated sites, which are more abundant
on smaller particles.14,15 In the case of Ni metal catalysts, the
formation of thin hexagonal morphologies generated on graphite
nanofibers strongly increases the selectivity of partial hydro-
genation of 1,3-butadiene to 1-butene.16 An important effect of
the metal particle morphology was also reported for hydrogena-
tion of crotonaldehyde,17–19 indicating that the reaction tends
to occur on Pt(111), Pt(100), and Pt(110) terraces. While there
is an important amount of work on the influence of different
crystallographic planes of metals on the activation of alkenes,
alkynes, carbonyl groups, or R,ꢀ-unsaturated carbonyls, only a
few examples20 dealing with nitro-group activation on the
(10) Corma, A.; Boronat, M.; Gonza´lez, S.; Illas, F. Chem. Commun. 2007,
32, 3371.
(11) Corma, A.; Serna, P. Science 2006, 313, 332.
(12) Blaser, H.-U. Science 2006, 313, 312.
(13) Rainer, D. R.; Goodman, D. W. J. Mol. Catal. A: Chem. 1998, 131,
259.
(14) Rainer, D. R.; Xu, C.; Koranne, M.; Oh, W. S.; Goodman, D. W. J.
Catal. 1997, 167, 234.
(15) Vesecky, S. M.; Rainer, D. R. J. Vac. Sci. Technol. A 1996, 14, 1457.
(16) Park, C; Baker, R. T. K. J. Phys. Chem. B 1998, 102, 5168.
(17) Englisch, M.; Jentys, A.; Lercher, J. A. J. Catal. 1997, 166, 25.
(18) Delbeq, F.; Sautet, S. J. Catal. 1995, 152, 217.
(19) Beccat, P.; Bertolini, J. C.; Gauthier, Y.; Massardier, J.; Ruiz, P. J.
Catal. 1990, 126, 451.
(20) Zhao, F.; Ikushima, Y.; Aray, M. J. Catal. 2004, 224, 479.
9
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A R T I C L E S
Corma et al.
surface of these particles have been presented, showing that this
process goes from a non-structure-sensitive relationship to a
moderate dependency under different reaction conditions.
Moreover, our previous research on gold catalysts21 showed that
the adsorption of nitro groups is very slightly modified when
working with nanoparticles of different morphologies. When
all the above factors are taken into account, it could be expected,
in a first approximation, that if one is able to prepare Pt catalysts
with a smaller fraction of metal atoms in faces with respect to
Pt atoms in corners of the crystallites, it could be possible to
decrease the reaction rate of a structure-sensitive reaction, such
as the hydrogenation of a double bond or a carbonyl, while
maintaining the activity for the reduction of the nitro group.
This effect would certainly produce an increase of the chemose-
lectivity toward the reduction of the nitro group in substituted
nitroaromatics containing olefinic or carbonyl groups. To check
this hypothesis, a series of Pt/Al2O3 catalysts have been prepared
containing different amounts of Pt, in which the average metal
crystallite size, as determined by STEM, decreases when
decreasing the Pt content is decreased (see Figure S1 in
Supporting Information). The electron microscopic results have
been complemented with a structural characterization of the Pt
metal surface by adsorption of CO followed by IR spectroscopy.
This was done in order to characterize the relative amount of
Pt atoms located on corners and terraces in the different Pt/
Al2O3 samples. It has been reported22 that CO adsorbs in a
bridge form mainly on large Pt particles, while linear adsorbed
CO predominates on small particles. More specifically, in the
case of 0.2 wt % Pt/Al2O3 sample, with the smallest average
metal crystallite size according to STEM measurements (Figure
S1), an intense CO adsorption band is observed at 2056 cm-1
(Figure S2 in Supporting Information), which is related to CO
linearly adsorbed on low-coordination Pt surface sites.23–25 No
bands at higher frequencies (2110-2070 cm-1) related to
extended Pt surfaces, such as Pt(111) and Pt(100), have been
observed on this sample, and a much less intense IR band
appears at 1823 cm-1, which can be associated with CO
adsorbed in a bridge form on larger particles. We can then
conclude from STEM and IR characterization that, in the 0.2
wt % Pt/Al2O3 sample, most of the Pt atoms are within very
small particles (∼2 nm), and consequently the ratio of Pt on
low-coordinated sites (corners and steps) to metal atom on
terraces is high.
Figure 1. Effect of Pt loading on the intrinsic reaction rate of Pt/Al2O3
catalysts in the hydrogenation of (O) styrene and (0) nitrobenzene. Reaction
conditions: 313 K, 3 bar of hydrogen, 0.0044 mol of Pt/mol of substrate.
Feed composition: 94.75 mol % toluene, 4.25 mol % styrene or nitrobenzene,
1 mol % o-xylene.
Figure 2. Effect of the support in hydrogenation of 3-nitrostyrene with
0.2 wt % Pt/C [0.00124 mol of Pt/mol of 3-nitrostyrene; (9) yield of
3-aminostyrene, (0) yield of hydroxylamine derivatives] and with 0.2 wt
% Pt/TiO2 [0.00310 mol of Pt/mol of 3-nitrostyrene; (b) yield of
3-aminostyrene, (O) yield of hydroxylamine derivatives] reduced at 450
°C. Reaction conditions: 313 K, 3 bar of hydrogen. Feed composition: 90.5
mol % toluene, 8.5 mol % 3-nitrostyrene, 1 mol % o-xylene.
When the Pt content is increased, the particle sizes increases
(Figure S1) together with the intensity of IR bands of CO
associated with Pt in Pt(111) and Pt(100) faces, while the
intensity of the IR bands associated with low-coordinated surface
sites decreases (Figure S2). If this is so, we should expect that
the specific activity, that is, the activity per metal atom, for a
well-proven structure-sensitive reaction such as styrene hydro-
genation to ethylbenzene, would decrease with decreasing metal
dispersion. Indeed, this can be clearly observed in Figure 1.
On the contrary, in the case of the reduction of nitrobenzene to
aniline we observe no influence of crystal size on the specific
catalytic activity (Figure 1), in agreement with our previous
hypothesis. From the results presented above, we could expect
that the chemoselective reduction of 3-nitrostyrene to 3-ami-
nostyrene will increase when the crystallite size of Pt is
decreased. Results from Table S2 in Supporting Information
confirm this hypothesis, though the maximum 3-nitrostyrene
selectivity with the best Pt/Al2O3 catalyst (0.2 wt % Pt/Al2O3)
is still not high enough (∼60%) when working at high levels
of conversion. Nevertheless, these results already show that the
direction we should follow to increase the chemoselectivity of
Pt catalysts is to further decrease the metal crystallite size. This
was done by preparing a catalyst with a low Pt content (0.2 wt
%) on a high-surface-area support such as activated carbon
(∼850 m2 ·g-1). Under the preparation conditions used here, a
very large metal dispersion should be obtained. STEM analysis
of this sample reveals a rather large number of very small Pt
particles (particle diameters in the range 0.5-1.7 nm) as well
as a much smaller fraction of larger ones (3-4 nm) (Figure S3
in Supporting Information). The large fraction of crystallites
with very small size is in agreement with the IR results of
adsorbed CO (Figure S4 in Supporting Information), which
shows only one IR band at 2040 cm-1 that is associated with
(21) Boronat, M.; Concepcio´n, P.; Corma, A.; Gonza´lez, S.; Illas, F.; Serna,
P. J. Am. Chem. Soc. 2007, 129 (51), 16230.
(22) Sarkany, J.; Gonzalez, R. D. Appl. Catal. 1983, 5, 85.
(23) Jin, T.; Zhou, Y.; Mains, G. J.; White, J. M. J. Phys. Chem. 1987, 91,
5931.
(24) Rasko´, J. J. Catal. 2003, 217, 478.
(25) Boccuzzi, F.; Chiorino, A.; Guglielminotti, E. Surf. Sci. 1996, 368,
264.
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Transformation into Chemoselective Metal Catalysts
A R T I C L E S
Figure 3. (a) HRTEM images of 0.2 wt% Pt/TiO2 catalyst after reduction at 473 K. The surface of the crystallites appears free from any support material.
(b) IR spectra of CO adsorbed on this sample reveal the presence of Pt terraces Pt(111) and Pt(100), as well as oxidized metal sites.
cm-1 region correspond to CO molecules adsorbed on Pt(111)
terraces (sites with coordination number 9). The IR bands in
Table 1. Product Distribution for Hydrogenation of 3-Nitrostyrene
in Toluene by Different Catalytic Systemsa
catalytic system
reduction temp (K)
% X
% Sa
% Sb
% Sc
% Sd
the 2077-2071 cm-1 region have been related to CO molecules
adsorbed on Pt(100) (Pt sites with coordination number 8). The
IR bands at lower frequencies (2066, 2050, and 2040-2020
cm-1) indicate the presence of Pt sites of low surface coordina-
tion, like steps and corners.23–25 In other words, the metal
crystallites on the Pt/TiO2 catalyst are larger than in the case of
Pt/C and contain a larger proportion of Pt atoms on terraces. It
is not surprising, then, that the selectivity at 90% conversion is
lower with this 0.2 wt Pt % TiO2 catalyst than with 0.2 wt %
Pt/C (see Table 1). Remarkably, and as was also observed with
the Au/TiO2 catalysts,26 no phenylhydroxylamine was detected
any more with Pt/TiO2, and the only undesired observed
products were 3-nitroethylbenzene and 3-ethylaniline.
At this point, and to further increase the selectivity of the
Pt/TiO2 sample, one should reduce the fraction of the Pt atoms
in terraces and increase the Pt-Ti interface sites. It has been
reported27,28 that high reduction temperatures of Pt/TiO2
catalysts produce a decoration of the Pt crystal terraces with
TiO2, decreasing the number of Pt atoms exposed on terraces
as well as increasing the number of Pt-Ti sites. Therefore, the
0.2 wt % Pt/TiO2 sample was reduced at 723 K instead of 473
K, and the resultant sample was characterized by means of
HRTEM (Figure 4a) and IR spectroscopy of adsorbed CO
(Figure 4b). The presence of TiOx species on top of the Pt
particles reduced at 723 K has been observed by nanoanalytical
techniques (STEM EELS; Figure S6 in Supporting Information).
Evidence of Ti features in EELS spectra acquired at certain
positions on top of the particles shows a partial coverage effect
after reduction at high temperature. This was confirmed by CO
adsorption results: only one band at 2048 cm-1 due to CO
adsorption on Pt-Ti interface sites is observed with the high
temperature (723 K) activated 0.2 wt % Pt/TiO2 catalyst, while
CO adsorption on Pt terraces was mainly observed on the sample
activated at 473 K. This decorated catalyst gives 93% selectivity
at 95% conversion in 6.5 h under very mild reaction conditions
(313 K, 3 bar of H2 pressure) and without formation of
phenylhydroxylamine (Table 1, Figure 2). Interestingly, in the
1.5% Au/TiO2
5% Pt/C
0.2% Pt/C
0.2% Pt/TiO2
0.2% Pt/TiO2
5% Ni/TiO2
5% Ni/TiO2
1% Ru/TiO2
1% Ru/TiO2
98.5 95.9
95.6 29.0
95.1 90.5
89.3 41.5
95.1 93.1
1.0 55.3
93.0 90.2
85.0 71.4
95.1 96.3
0.0
7.8
3.0
6.4
0.1
4.0
0.2
4.3
0.2
3.9 0.2
723
723
473
723
473
723
473
723
63.1 0.1
3.9 2.6
52.1 nd
6.8 nd
35.6 5.1
7.3 2.3
20.7 3.6
2.8 0.7
a a,3-aminostyrene;b,3-nitroethylbenzene;c,3-ethylaniline;d,hydroxyl-
amine derivatives.
CO adsorbed on highly uncoordinated Pt sites. This result
explains the fact that 90% selectivity to 3-aminostyrene at 95%
conversion is obtained with the 0.2 wt % Pt/C catalyst.
Interestingly, 0.2 wt % Pt/C is much more active than the Pt-
PbO/CaCO3 or Au/TiO2 catalysts previously reported,8,11 since
the former works at 313 K and 3 bar, while the others require
393 K and H2 pressures between 9 and 20 bar. In other words,
by controlling the crystallite size of Pt, we can make selective
the nonselective Pt without the necessity of poisoning the metal
surfaces, while avoiding the corresponding decrease in activity.
Unfortunately, with the 0.2 wt % Pt/C catalyst, an important
amount of the highly undesired phenylhydroxylamine intermedi-
ates (up to 25% at low conversion levels; Figure 2) is produced,
which makes the catalyst unsuccessful, at least from an industrial
point of view.
Importance of the Support. From our previous work on gold
catalysis,11,21 we learned that a proper selection of the support
could improve selectivity through a cooperative effect between
the support (TiO2) and the metal, avoiding the formation of
hydroxylamine derivatives. If this could be extrapolated to Pt,
a highly active and selective catalyst that will not produce
phenylhydroxylamine intermediates could be achieved with
small crystallites of Pt on a TiO2 support. Following this
hypothesis, 0.2 wt % Pt/TiO2 catalyst was prepared and reduced
at 473 K. HRTEM images show that Pt particles preferentially
grow along TiO2 interphases, resulting in larger particles than
expected (Figure 3a, and Figure S5 in Supporting Information).
The IR spectrum of adsorbed CO shows bands at 2120, 2111,
2089, 2074, and 2042 cm-1 (Figure 3b). The high-frequency
band at 2120 cm-1 has been assigned to CO adsorbed on
oxidized Pt surface sites, while IR bands in the 2111-2086
(26) Corma, A.; Concepcio´n, P.; Serna, P. Angew. Chem., Int. Ed 2007,
46 (38), 7266.
(27) Datye, A. K.; Kalakkad, D. S.; Yao, M. H.; Smith, D. J. J. Catal.
1995, 155, 148.
(28) Salama, T. M.; Hattori, H.; Kita, H.; Ebitani, K.; Tanaka, T. J. Chem.
Soc., Faraday Trans. 1993, 89 (12), 2067.
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A R T I C L E S
Corma et al.
Figure 4. (a) HRTEM images of 0.2 wt % Pt/TiO2 catalyst after reduction at 723 K. The Pt particle surface appears partially covered by TiOx species as
a decoration effect. (b) IR spectra of CO adsorbed on this sample show only one band corresponding to low surface coordination sites, indicating the absence
of Pt terraces Pt(111) and Pt(100).
Table 2. Catalytic Results for 0.2 wt % Pt/TiO2 Catalyst Reduced
nitroaromatics, that Pt with larger crystallites was more
selective.24 Nevertheless, reduction of the Pt/TiO2 catalysts
at 723 K for the Hydrogenation of Different Substituted
Nitroaromatics
at higher temperatures also improved the chemoselectivity
for hydrogenation of crotonaldehyde.17,29,30 However, it has
Pt/nitro
(mol, ×
P
time conversion selectivity
(%)
102) T (K) (bar) (h)
(%)
to be pointed out that chemoselectivity to crotyl alcohol on Pt/
TiO2 was always below 40% at high levels of conversion, while
the optimized Pt/TiO2 catalyst achieves a selectivity above 93%
at 95% conversion in the case of nitrostyrene. We have also
studied here the relative rate of hydrogenation of the nitro and
carbonyl groups by reacting 4-nitrobenzaldehyde on 0.2 wt %
Pt/TiO2 catalyst reduced at 723 K. The results presented in Table
2 show a selectivity to 4-aminobenzaldehyde of 98% at 99%
conversion level. These results indicate that, on this catalyst,
the nitro reacts faster than the carbonyl group, probably due to
a preferential adsorption through the former. We have seen that
the optimized Pt/TiO2 catalyst is of general application for the
chemoselective reduction of substituted nitroaromatics, since it
gives very high yields of the amino derivatives in the presence
of double and triple bonds, carbonyl, nitriles, and halogens
(Table 2).
Extension of the Concept to Other Metals. The first objective
of this work was already achieved by converting the nonselective
Pt into a selective Pt catalyst while avoiding the formation of
phenylhydroxylamine without using any other catalyst additive.
A second objective was then to find if the concept could be
generalized to other metals. Results from Table 1 show that Ru
and Ni, which are not chemoselective for the reduction of
3-nitrostyrene, can now be converted into selective catalysts
by supporting them on TiO2 and reducing at 723 K. Notice that,
with the preparation and activation procedures described here,
Ru gives 96% selectivity at 95% conversion, and even Ni gives
90% selectivity to 3-aminostyrene at 93% conversion. These
catalysts have also been tested in the hydrogenation of different
substituted nitroaromatics, always showing high chemoselec-
tivity toward the nitro group reduction. Table 3 summarizes the
results of catalysts optimized in this paper for 3-nitrostyrene,
4-nitrobenzaldehyde, 4-nitrobenzonitrile, and 4-iodonitroben-
zene. (Detailed distribution of products can be observed in
Tables S3-S5 in Supporting Information.)
substrate
solvent
3-nitrostyrene
toluene 0.31 313
3
3
6
6
3
4
6.50 95.1
3.00 99.2
2.20 99.5
0.35 99.5
5.60 98.9
1.00 99.0
93.1
98.2
98.6
99.5
90.1
99.6
4-nitrobenzaldehydea THF
0.92 313
0.40 323
1.02 333
4-nitrobenzonitrile
4-iodonitrobenzene
4-nitrophenylacetylene toluene 0.24 313
3-chloronitrobenzene THF 0.75 318
THF
THF
a Mass balance ∼85% because of an unavoidable polymerization of
4-aminobenzaldehyde
Table 3. Catalytic Results of Chemoselective Hydrogenation of
Substituted Nitroaromatics into the Corresponding Aminoaromatics
on Various Catalystsa
catalystsb Pt/nitro (mol, × 102) T (K) P (bar) time (h) conversion (%) selectivity (%)
3-Nitrostyrene in Toluene
Au/TiO2
Pt/TiO2
Ni/TiO2
Ru/TiO2
0.23
0.31
2.06
1.19
393
313
393 15
393 15
9
3
6.00
6.50
3.00
1.50
98.5
95.1
93.0
95.1
95.9
93.1
90.2
96.3
4-Nitrobenzaldehydec in THF
Au/TiO2
Pt/TiO2
Ni/TiO2
Ru/TiO2
1.14
0.92
20.40
8.89
373 10
313
363 11
363 11
1.25
3.00
1.75
2.17
99.0
99.2
99.6
98.4
96.8
98.2
98.7
96.5
3
4-Nitrobenzonitrile in THF
Au/TiO2
Pt/TiO2
Ni/TiO2
Ru/TiO2
0.60
0.40
6.71
2.92
413 25
323
413 15
413 15
1.25
2..20
0.75
3.75
99.0
99.5
99.1
90.0
97.2
6
98.6
88.3d
85.4d
4-Iodonitrobenzene in THF
413 25 10.0
Au/TiO2
Pt/TiO2
Ni/TiO2
Ru/TiO2
1.20
1.02
16.90
5.89
0.5
99.5
93.0
98.0
100.0
99.5
93.8
98.8
333
6
0.35
1.50
1.75
413 15
413 15
a Reactions were performed under isobaric conditions. Conversion
was determined by gas chromatography (GC) with o-xylene as internal
standard. Products were identified by means of GC-MS. b Catalysts were
1.5 wt % Au/TiO2 and decorated 0.2 wt % Pt/TiO2, 5 wt % Ni/TiO2,
and 1 wt % Ru/TiO2. c Mass balances were between 80-90% because
of an unavoidable polymerization of 4-aminobenzaldehyde. d Loss of selec-
tivity through hydrolysis of nitrile function to the amide group.
In order to explain the strong effect of the catalyst activation
conditions on Pt/TiO2, as well as on the other metal catalysts,
(29) Vannice, M. A.; Sen, B. J. Catal. 1989, 115, 65.
(30) Claus, P.; Schimpf, S.; Scho¨del, R.; Kraak, P.; Mo¨rke, W.; Ho¨nicke,
D. Appl. Catal., A 1997, 165, 429.
case of chemoselective hydrogenation of R,ꢀ-unsaturated alde-
hydes (crotonaldehyde) it was found, contrary to the substituted
9
8752 J. AM. CHEM. SOC. VOL. 130, NO. 27, 2008

Transformation into Chemoselective Metal Catalysts
A R T I C L E S
we can assume that a strong metal-support interaction (SMSI)
occurs between the metals and the TiO2 support.31 SMSI can
be manifested by modifying the electron density of small clusters
by charge transfer or polarization from partially reducible
supports,32 through the unique properties of metal-support
borderline sites,33 and/or by decoration of the metal by mobile
support.34 In our case, we have seen that chemoselective
reduction of nitro groups on supports such as γ-Al2O3 and
carbon, in where SMSI are not to be expected. This should allow
us to disregard electronic effects as being the main cause of
the high selectivity observed with Pt/TiO2 catalysts. In a similar
way, it would be difficult to explain the high selectivity of the
properly activated Ni/TiO2 catalysts with large Ni crystallite
sizes, on the bases of electronic effects. On the other hand, the
fact that high chemoselectivity is observed when the formation
of high-density metal plains is avoided and when the number
of metal sites in corners and axes is increased leads us to assume
that the main role of SMSI is, in our case, due to the decoration
of metal surfaces by TiO2. This decreases the assemblies of
accessible metal atoms, with a strong effect on the adsorption
of the substituted nitroaromatic, in such a way that a preferential
adsorption through the nitro groups occurs. Molecular modeling
and kinetic studies are in process to elucidate the selective
surface interactions occurring between substituted nitroaromatics
and decorated and nondecorated Pt/TiO2 catalysts.
selectivity of some of the catalysts presented here breaks the
previous paradigm that high chemoselectivity in the hydrogena-
tion of substituted nitroaromatics was in parallel with a reduction
of the intrinsic hydrogenation capacity of the catalyst, together
with the requirement of a second catalytic function (iron or
vanadium salts) to avoid the accumulation of undesired reaction
products. These conclusions not only are relevant from a
fundamental point of view but also have allowed preparation
of very active, chemoselective, and environmentally friendly
catalysts for the reduction of nitro groups in substituted
nitroaromatics, which accomplishes the requirements for in-
dustrial application.
Acknowledgment. We thank the Spanish government (Project
MAT 2006-14274-C02-01 and Grant FPU AP2003-4635) and the
generalitat Valenciana (Project GV06/262) for financial support.
Supporting Information Available: Characterization of Pt/
Al2O3, Pt/C, and Pt/TiO2 samples by FTIR of CO, STEM,
HRTEM, STEM-HAADF, and STEM-EELS spectroscopy and
detailed distribution of products for the hydrogenation of
3-nitrostyrene, 4-nitrobenzaldehyde, 4-nitrobenzaldehyde, and
4-iodonitrobenzene. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA800959G
Conclusions
(31) Tauster, S. J.; Fung, S. C.; Garten, R. J. Am. Chem. Soc. 1978, 100,
170.
It is possible to transform nonchemoselective hydrogenation
metals such as Pt, Ru, and Ni into chemoselective catalysts by
supporting nanosized crystals of the metals on TiO2 and
decorating the exposed (111) and (100) crystal faces by means
of a simple catalyst activation. The very high activity and
(32) Boudart, M.; Djega-Mariadasson, G. In Kinetics of Heterogeneous
Catalytic Reactions; Princeton University Press: Princeton, NJ, 1984;
p 157.
(33) Schwab, G. M. Discuss. Faraday Soc. 1950, 8, 166.
(34) Haller, G. L.; Resasco, D. E. AdV. Catal. 1989, 36, 173.
9
J. AM. CHEM. SOC. VOL. 130, NO. 27, 2008 8753