Efficient solvent-free hydrogenation of ketones over flame-prepared bimetallic Pt-Pd/ZrO2 catalysts

Article abstract of DOI:10.1002/cssc.201200126

Named and flamed: Bimetallic Pt-Pd/ZrO₂ catalysts with different Pt/Pd atomic ratios and high dispersion of the metal nanoparticles are prepared by a single-step flame-spray pyrolysis. The catalysts show excellent activity and tunable product s

Full text of DOI:10.1002/cssc.201200126

DOI: 10.1002/cssc.201200126
Efficient Solvent-Free Hydrogenation of Ketones over Flame-Prepared
Bimetallic Pt–Pd/ZrO2 Catalysts
Yijiao Jiang,[a, d] Robert Bꢀchel,[a, b] Jun Huang,[c] Frank Krumeich,[a] Sotiris E. Pratsinis,[b] and Alfons Baiker*[a, e]
The hydrogenation of ketones is an important reaction in the
syntheses of fine chemicals and pharmaceuticals,[1] and more
recently it has gained significance due to its application in the
production of biofuels and high-value renewable chemicals
from bio-oils.[2–6] A range of supported palladium, platinum,
nickel, and ruthenium catalysts have been employed in these
reactions.[1,7] However, the use of various organic, aqueous, or
acidic solvents in ketone hydrogenation causes serious prob-
lems in product separation, and environmental issues.[1–3,5,7]
Therefore, it is desirable to develop solvent-free hydrogenation
processes for efficient and clean fine-chemical and biorefinery
processes, aimed at promoting the sustainable development
of the chemical industry.
tion.[10–13] This promising technique not only offers a very effi-
cient route for the production of solid catalysts, but also a rela-
tively easy means of controlling the distribution of various
atoms on the surface.[14] With this in mind we explored the po-
tential of FSP for producing high-performance bimetallic Pt–
Pd/ZrO2 catalysts for the solvent-free hydrogenation of ke-
tones. Cyclopentanone and acetophenone were chosen as ali-
cyclic and aromatic ketone model compounds. The latter is the
simplest aromatic ketone for testing the chemoselective hydro-
genation of phenyl and carbonyl groups.[15–19]
For FSP synthesis, appropriate amounts of platinum acetyl-
acetonate and palladium acetylacetonate were added to zirco-
nium(IV) isopropoxide and diluted 1:2 (v/v) with toluene.
5 mLminÀ1 of this clear precursor solution were sprayed and
dispersed with 5 LminÀ1 oxygen and ignited with a flame of
premixed CH4 and O2 of 1 and 2 LminÀ1, respectively. The as-
prepared 3 wt% Pt–Pd/ZrO2 catalysts with different Pt and Pd
atomic ratios (Table 1) are denoted as PtxPdy/ZrO2, where x and
Herein, we target the solvent-free hydrogenation of ketones.
One major challenge is the development of suitable highly
active and selective catalysts. The activity and selectivity of bi-
metallic Pt–Pd catalysts have been reported to be enhanced
by creating electron-deficient Pt sites, which exist when Pd is
in close vicinity to Pt.[8] However, during the synthesis and cal-
cination of bimetallic Pt–Pd catalysts, the Pd atoms tend to
segregate towards the surface and are therefore dominant as
surface-active sites.[9] Support properties are well-known to
play a crucial role in optimizing the catalytic performance of
metal nanoparticles. Possessing intrinsic structural defects for
enhanced interaction with metal particles, zirconia has been
used as a support in many catalytic reactions.[7] Recently, we
developed a simple flame spray pyrolysis (FSP) method to pre-
pare catalysts in one step, combining synthesis and calcina-
Table 1. Structural properties of 3 wt% Pt–Pd/ZrO2 catalysts.
Catalyst
SSA
[m2 gÀ1
d
[nm]
XRD ZrO2
Dispersion
[%]
]
Pt/ZrO2
70
69
67
82
86
22
22
23
18
18
66
56
42
44
32
Pt75Pd25/ZrO2
Pt50Pd50/ZrO2
Pt25Pd75/ZrO2
Pd/ZrO2
[a] Dr. Y. Jiang, Dr. R. Bꢀchel, Dr. F. Krumeich, Prof. Dr. A. Baiker
Department of Chemistry and Applied Sciences
ETH Zꢀrich
Hçnggerberg, HCI, 8092 Zꢀrich (Switzerland)
Fax: (+41)44-632-1163
E-mail: baiker@chem.ethz.ch
y correspond to the atomic ratios of Pt and Pd, respectively.
The specific surface areas of these FSP made catalysts were in
the range of 67 and 86 m2 gÀ1, as determined by N2 adsorption
using the Brunauer–Emmett–Teller (BET) method at 77 K
(Table 1).
[b] Dr. R. Bꢀchel, Prof. Dr. S. E. Pratsinis
Department of Mechanical and Process Engineering
ETH Zꢀrich
8092 Zꢀrich (Switzerland)
Powder X-ray diffraction (XRD) patterns showed only reflec-
tions of tetragonal ZrO2 and no reflections due to Pt or Pd
(Supporting Information, Figure S1) consistent with the low
loading (3 wt%) and high dispersion of the noble metals. How-
ever, the XRD patterns revealed that the different loadings of
the metal constituents had a slight effect on the crystallite size
of ZrO2, which varied between 18 and 23 nm (Table 1). The
ZrO2 particle sizes correlate with the surface areas of the
catalysts.
[c] Dr. J. Huang
School of Chemical and Biomolecular Engineering
The University of Sydney
Sydney, NSW 2006 (Australia)
[d] Dr. Y. Jiang
School of Chemical Engineering
The University of New South Wales
Sydney, NSW 2052 (Australia)
[e] Prof. Dr. A. Baiker
Chemistry Department
Faculty of Science
Typical scanning transmission electron microscopy (STEM)
and TEM images of the catalysts are shown in Figure 1. The
crystalline ZrO2 support of all catalysts showed an egg-like
shape. The majority of Pt particles in the Pt/ZrO2 sample
King Abdulaziz University
Jeddah 21589 (Saudi Arabia)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200126.
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Figure 2. DRIFT spectra of CO adsorption on reduced a) Pt/ZrO2, b) Pt75Pd25/
Figure 1. STEM images of a) Pt/ZrO2, c) Pt50Pd50/ZrO2, and e) Pd/ZrO2. TEM
ZrO2 c) Pt50Pd50/ZrO2, d) Pt25Pd75/ZrO2, and e) Pd/ZrO2.
images of b) Pt/ZrO2 and d) Pt50Pd50/ZrO2. f) EDX analysis of Pt50Pd50/ZrO2.
(bright spots in Figure 1a) were smaller than 1 nm, indicating
a narrow size distribution. The TEM image of this catalyst
showed spherical Pt particles (Figure 1b). When adding Pd pre-
cursor to the FSP feed, the resulting bimetallic catalysts
showed slightly increased particle sizes, as shown in the STEM
image of Pt50Pd50/ZrO2 (Figure 1c). The average particle size of
the Pt–Pd nanoparticles was ca. 1 nm, which is considerably
smaller than those normally obtained by classical incipient
wetness impregnation (2 nm).[20] As the TEM picture (Fig-
ure 1d) indicates, the bimetallic particles were well-dispersed
on the ZrO2 surface, which showed a similar morphology to
monometallic Pt/ZrO2. Energy-dispersive X-ray (EDX) investiga-
tions (one representative example is shown in Figure 1 f) con-
firmed that the ratio of the noble metal constituents (Pt and
Pd) was close to the nominal ratio of the precursors used in
the synthesis. The dispersion of bimetallic Pt–Pd particles
(Table 1), as determined by CO chemisorption, decreased with
increasing Pd loading; it was highest for the monometallic Pt/
ZrO2 and lowest for Pd/ZrO2.
a new band at ca. n˜ =2055 cmÀ1, indicating that a new type of
site was created. Unlike the spectrum for Pt/ZrO2 (Figure 2a),
the DRIFT spectrum of Pd/ZrO2 (Figure 2e) showed that a band
at n˜ =2085 cmÀ1 is dominant in the high-wavenumber range
attributed to linearly coordinated CO on (111) and (100) terra-
ces of highly coordinated Pd atoms.[23] In the low-wavenumber
range, the broad band at n˜ =1925 cmÀ1 and the narrow one at
n˜ =1980 cmÀ1 are ascribed to bridged CO on Pd(111) and
Pd(100) facets, respectively.[24] Obviously, the spectra in Fig-
ure 2b–d cannot be represented by a simple superposition of
the spectra of CO adsorbed on monometallic Pt/ZrO2 (Fig-
ure 2a) and Pd/ZrO2 (Figure 2e), confirming that bimetallic Pt–
Pd particles were formed on the ZrO2 support, typical for
flame-made noble metal particles supported on refractory
oxides.[26–27] In addition, the broad band of bridged CO shifted
to high wavenumber from n˜ =1775 to 1925 cmÀ1 correlating
well with the increasing Pd content from 0 to 100 at%.
The solvent-free hydrogenations of cyclopentanone and ace-
tophenone were carried out in an autoclave at a hydrogen
pressure of 60 bar and a temperature of 1608C. Figure 3 shows
the conversions of cyclopentanone and corresponding average
turnover frequencies (TOFs) as a function of Pd content of the
Pt–Pd/ZrO2 catalysts. The conversion of cyclopentanone was
82% and 75% on monometallic Pt/ZrO2 and Pd/ZrO2, respec-
tively, while it reached 99%, 92%, and 89% with Pt75Pd25/ZrO2,
Pt50Pd50/ZrO2, and Pt25Pd75/ZrO2, respectively. The bimetallic
catalysts clearly showed enhanced activity: TOF (at 1608C) in-
creased from 0.53 sÀ1 to 0.64 sÀ1 for bimetallic catalysts with
25 at% Pd and reached a maximum of 0.69 sÀ1 for Pt50Pd50/
ZrO2. Further increase of the Pd content resulted in a TOF de-
crease to 0.54 and 0.52 sÀ1, for Pt25Pd75/ZrO2 and Pd/ZrO2, re-
spectively. During the hydrogenation of cyclopentanone, cyclo-
CO adsorption combined with diffuse reflectance infrared
Fourier transform (DRIFT) spectroscopy was applied to gain
some information about the structural and electronic proper-
ties of the supported noble metal nanoparticles. Mainly, two
band regions can be distinguished for adsorbed CO: a band at
n˜ =2000–2100 cmÀ1 due to linearly coordinated CO, and
a band at n˜ =1770–2000 cmÀ1 assigned to CO adsorbed in
bridged coordination mode. The spectrum of Pt/ZrO2 in Fig-
ure 2a shows a narrow signal at n˜ =2085 cmÀ1 and a broad
band at ca. n˜ =2010 cmÀ1 assigned to linearly coordinated CO
on (111) and (100) terraces and on low-coordinated Pt sites at
step-edges, corners, and defects.[21,22] Spectra in Figure 2b–d
show that the formation of bimetallic Pt–Pd particles results in
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Figure 5. Influence of Pd content of Pt-Pd/ZrO2 catalysts on the selectivity to
products at an acetophenone conversion of ca. 60%: Selectivity to C=O
Figure 3. Influence of Pd content of Pt-Pd/ZrO2 catalysts on the conversion
*
*
(
) and TOF ( ) of cyclopentanone hydrogenation. Conditions: 1608C,
&
bond hydrogenation affording EB ( ); Selectivity to phenyl ring hydrogena-
60 bar hydrogen pressure, 3 h reaction time.
&
tion products affording CE+EC+CMK ( ). Beside these products, 1-phenyl-
ethanol (PhE) was formed, reaching up to 64% over Pt75Pd25/ZrO2. Selectivi-
ties to each product are listed in Table S1 of the Supporting Information.
Conditions: 1608C, 60 bar hydrogen pressure, 3 h reaction time.
pentanol was the only product, and no cyclopentane was
detected.
Unlike in the hydrogenation of cyclopentanone, Pd/ZrO2
showed higher activity than Pt/ZrO2 in the hydrogenation of
the aromatic ketone acetophenone. As shown in Figure 4, the
consumes limited hydrogen during hydrogenation and has
a high research octane number (RON) of 124. Due to the com-
petitive hydrogenation of the carbonyl group and the phenyl
group, different possible products can be formed during hy-
drogenation of acetophenone, as shown in Scheme 1. Beside
Figure 4. Influence of the Pd content of Pt-Pd/ZrO2 catalysts on the conver-
*
*
sion ( ) and TOF ( ) of acetophenone. Conditions: 1408C, 60 bar hydrogen
pressure, 3 h reaction time.
Scheme 1. Hydrogenation of acetophenone to different products.
conversion of acetophenone and TOF (at 1408C) were 80%
and 0.42 sÀ1 for Pd/ZrO2, and 32% and 0.15 sÀ1 for Pt/ZrO2, re-
spectively. Increasing the Pd content from 25 to 75 at% in the
bimetallic catalysts was accompanied by an increase of aceto-
phenone conversion from 53% to 96%. However, the TOF still
reached a maximum of 0.5 sÀ1 for Pt50Pd50/ZrO2 at a conversion
of 87%. Again the activity was enhanced by applying the bi-
metallic catalysts. Although a direct comparison of the activity
of the bimetallic catalysts to others reported in the litera-
ture[17–19,25–29] is difficult due to different conditions applied
(e.g., temperature, solvent), we can safely conclude that the
flame-derived bimetallic catalysts are among those possessing
the highest activity.
its higher activity, Pd/ZrO2 also showed higher selectivity to EB
(SEB =64%) than Pt/ZrO2 (SEB =14%; Figure 5). For bimetallic
Pt–Pd/ZrO2, the selectivity to EB increased from 8% to 50%
and 83% when the Pd content was increased from 25% to
50% and 75%. Reversely, when the Pt content was increased
from 0% to 100%, the selectivity of Pt–Pd/ZrO2 to phenyl ring
hydrogenation products (CE+EC+CMK, Scheme 1) increased
from 0% to 62%. Pt sites seem to favor hydrogenation of the
phenyl group and Pd sites that of the carbonyl group. For bi-
metallic catalysts, the surface Pd and Pt sites still kept their
specific catalytic properties though their electronic properties
were changed and activity was enhanced.
Pt–Pd/ZrO2 catalysts were able to totally remove the oxygen
from acetophenone (hydrodeoxidation, HDO) and converted
this activated ketone to ethylbenzene (EB) as shown in
Figure 5. EB is a desired compound in biofuels, since it only
Pd segregation to the surface has been demonstrated exper-
imentally in bimetallic Pt–Pd particles prepared by wetness im-
pregnation.[9] Pt–Pd/ZrO2 prepared by wetness impregnation
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Before starting the reaction, the autoclave was purged 3 times
with nitrogen and subsequently 3 times with hydrogen. The reac-
tions were carried out at 60 bar in the autoclave with auto-heating
controller and a magnetic stirrer. The autoclave was equipped with
a valve for sample collection. 100 mg of the catalysts and 6 mL re-
actants were stirred magnetically (1000 rpm) at the specified reac-
tion temperatures. The conversion and product composition were
analyzed by using a Termo Quest Trace 2000 gas chromatograph
equipped with a HP-FFAP (Agilent HP-FFAP 30 mꢂ0.32 mmꢂ
0.25 mm) capillary column. Bicyclohexyl was used as internal stan-
dard (0.1 mmol). Average turnover frequencies (TOFs) over 3 h re-
action time were calculated based on the total number of metal
atoms (Pt and Pd) determined by CO chemisorption assuming
a stoichiometric factor of 1 for both metals. The selectivity to a spe-
cific product(s) i (Si) was calculated by using the expression Si(%)=
100ꢂ(i)/[(APh)0À(APh)], where (i) is the molar concentration of the
product(s) and (APh)0 and (APh) correspond to the molar concen-
tration of acetophenone before and after reaction, respectively.
did show enhanced activity in tetraline hydrogenation, and
showed poor activity in decahydroquinoline hydrodenitrogena-
tion as compared to a Pt/ZrO2 monometallic system.[20] Probing
the surface by combined DRIFT spectroscopy and CO adsorp-
tion, and the catalytic results obtained with the flame-made bi-
metallic Pt–Pd/ZrO2, did not reveal substantial surface segrega-
tion of Pd and indicated highly dispersed nanoparticles. These
may be the properties responsible for the high activity of
these catalysts in the solvent-free hydrogenation of cyclopen-
tane and acetophenone.
Finally, the most active Pt50Pd50/ZrO2 catalyst was subjected
to repeated catalytic tests and showed the same activity and
selectivity for the hydrogenation of cyclopentanone and aceto-
phenone after washing (in n-hexane), filtering, and drying (in
vacuum, overnight at 308C).
In summary, flame-derived bimetallic Pt–Pd/ZrO2 catalysts
show excellent catalytic performance in the solvent-free hydro-
genation of alicyclic and aromatic ketones, such as cyclopenta-
none and acetophenone. The synthesis of bimetallic Pt–Pd cat-
alysts by flame-spray pyrolysis results in a high dispersion of Pt
and Pd and allows easy control of the metal loading on the
ZrO2 surface. The as-prepared bimetallic Pt–Pd/ZrO2 catalysts
afford excellent activity in the hydrogenation of both cyclo-
pentanone and acetophenone. Among the Pt–Pd/ZrO2 cata-
lysts with different atomic Pt/Pd ratios, Pt50Pd50/ZrO2 shows the
highest activity. Although the electronic properties of bimetal-
lic catalysts are changed compared to their monometallic
counterparts, the Pd and Pt active sites still kept their specific
properties concerning product selectivity. The catalysts could
be reused without significant loss in activity and selectivity
after appropriate treatment. The substrate/metal molar ratio
under solvent-free conditions was in the range between 1814
and 4406, which is much higher than that in the hydrogena-
tion with solvent. This efficient solvent-free hydrogenation of
alicyclic and aromatic ketones may present a significant step
towards the development of green chemistry.
Acknowledgements
Financial support by ETH Zurich (TH-0906-2) and the European
Research Council/European Community (under FP7) is kindly ac-
knowledged. The electron microscopy study was performed at
EMEZ (Electron Microscopy ETH Zurich).
Keywords: bimetallic catalysts · heterogeneous catalysis ·
hydrogenation · ketones · pyrolysis
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Received: February 20, 2012
Revised: April 5, 2012
Published online on && &&, 0000
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COMMUNICATIONS
Y. Jiang, R. Bꢀchel, J. Huang, F. Krumeich,
S. E. Pratsinis, A. Baiker*
Named and flamed: Bimetallic Pt–Pd/
ZrO2 catalysts with different Pt/Pd
atomic ratios and high dispersion of the
metal nanoparticles are prepared by
a single-step flame-spray pyrolysis. The
catalysts show excellent activity and
tunable product selectivity for the sol-
vent-free hydrogenation of the ketone
model compounds cyclopentanone and
acetophenone.
&& – &&
Efficient Solvent-Free Hydrogenation
of Ketones over Flame-Prepared
Bimetallic Pt–Pd/ZrO2 Catalysts
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 5
ÝÝ
These are not the final page numbers!