Selective oxidation of glycerol by highly active bimetallic catalysts at ambient temperature under base-free conditions

Article abstract of DOI:10.1002/anie.201101772

Au-Pt alloy nanoparticles deposited on Mg(OH)₂ (see STEM-HAADF image) show high activity in the selective oxidation of polyols using molecular oxygen as oxidant at mild and base-free conditions.

Full text of DOI:10.1002/anie.201101772

Communications
DOI: 10.1002/anie.201101772
Selective Oxidation
Selective Oxidation of Glycerol by Highly Active
Bimetallic Catalysts at Ambient Temperature under
Base-Free Conditions**
Gemma L. Brett, Qian He, Ceri Hammond, Peter J. Miedziak,
Nikolaos Dimitratos, Meenakshisundaram Sankar, Andrew A. Herzing,
Marco Conte, Jose Antonio Lopez-Sanchez, Christopher J. Kiely,
David W. Knight, Stuart H. Taylor, and Graham J. Hutchings*
Dedicated to the Fritz Haber Institute, Berlin,
on the occasion of its 100th anniversary
Angewandte
Chemie
1
0136
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Angew. Chem. Int. Ed. 2011, 50, 10136 –10139

Glycerol is a by-product of the
manufacture of biodiesel and its
supply as a sustainable raw mate-
rial is anticipated to grow steadily
due to a shift towards “greener”
fuels. Since glycerol provides a
precursor to various industrially
valuable products, such as glyceric
acid, tartronic acid and hydroxya-
cetone, this increasing glycerol
supply represents an industrially
important feedstock for synthesiz-
ing such chemicals by oxidative
reaction. Heterogeneous catalysts
play a key role in the promotion
and control of such reactions, and
can also lead to “greener” synthe-
Table 1: The oxidation of glycerol under base free conditions using selected gold bimetallic catalysts at
low temperatures.
Selectivity [mol% C]
Catalyst
T [8C]
t [h] Conv.
Oxalic Tartronic Glyceric Glycolic Formic
[mol%] acid
acid
acid
acid
acid/CO2
[a]
AuPd(1:1)/MgO
AuPt(1:1)/MgO
60
60
60
60
40
4
4
4
4
4
5.9
0.4
0.4
0.1
0.4
0.4
0.2
0.7
13.3
12.3
12.5
14.9
9.4
74.2
78.4
74.4
72.2
80.4
85.1
66.7
4.0
3.9
6.4
3.3
3.9
8.1
5.0
6.6
9.2
5.9
[
a]
29.2
14.5
42.9
29.4
42.5
29.7
[
a]
AuPd(1:3)/MgO
[
a]
AuPt(1:3)/MgO
[
a]
AuPt(1:3)/MgO
[
b]
AuPt(1:3)/MgO
ambient (23) 24
ambient (23) 24
3.8
8.5
4.8
11.5
6.1
12.6
[b]
AuPd(1:3)/MgO
[
(
a] Reaction conditions: catalyst metal ratios by mole fraction with 1% metal loading by mass, water
ꢀ
1
10 mL), 0.3 molL glycerol, mole fraction of glycerol/metal=1000, CO was found to be negligible
2
(
see Supporting Information). [b] Mole fraction of glycerol/metal=500, p(O )=300 kPa, products
2
expressed as mol% C.
sis routes when compared with alternatives, such as the
addition of stoichiometric reagents during synthesis.
alloying gold with palladium can lead to a 25-fold enhance-
[
12]
ment in the activity of alcohol oxidation.
Prati and co-
[13]
Nanoparticulate gold dispersed on a variety of oxide
supports has been studied for a wide range of catalytic redox
workers
have recently demonstrated the oxidation of
glycerol under base-free conditions using a gold–platinum
catalyst supported on carbon and mordenite. These previous
studies have focused on the catalytic oxidation of glycerol at
elevated temperatures. Here we show that high glycerol
[
1–3]
reactions,
and the observation that supported gold cata-
lysts are effective for glycerol oxidation is currently attracting
considerable attention. In their initial seminal work Rossi and
Prati were the first to demonstrate that supported gold
nanoparticles are effective for alcohol oxidation in the
conversion and selectivity to specific C products (glyceric
3
and tartronic acids) can be achieved at ambient temperatures
without employing a base. We further report microstructural
characterization of these bimetallic catalysts using aberration-
corrected scanning transmission electron microscopy
(STEM), high-angle annular dark-field (HAADF) imaging,
and energy-dispersive X-ray spectroscopy (XEDS).
[
5,6]
presence of a base. We subsequently extended this concept
to show that glycerol could be converted into glyceric acid
[7,8]
with a high yield using basic reaction conditions.
In
contrast to palladium and platinum catalysts, gold has been
shown to promote the reaction without causing over-oxida-
[
8]
tion. More recent studies of glycerol oxidation have
Bimetallic catalysts were synthesized by immobilizing
colloidal metal particles on an MgO support. X-ray diffrac-
tion (XRD) analysis (Supporting Information, Figure S2)
revealed that the support material is converted to Mg(OH)2
due to hydrolysis during the immobilization process, which is
carried out in aqueous solution.
[9]
concentrated on the role of the gold particle size, the
[10]
catalyst pre-treatment,
method.
and the catalyst preparation
[11]
While these previous studies have focused on monome-
tallic nanoparticulate gold, we have since demonstrated that
ꢀ
1
Initial catalyst testing was performed at 608C (0.3 molL
glycerol solution, glycerol/metal mole fraction 1000,
pO2 ¼300 kPa, 4 h) with supported Au–Pt (1:3 mole fraction,
1% by mass total metal loading). A substantially higher
activity was observed (Table 1) than has previously been
[
*] G. L. Brett, C. Hammond, Dr. P. J. Miedziak, Dr. N. Dimitratos,
Dr. M. Sankar, Dr. M. Conte, Dr. J. A. Lopez-Sanchez,
Prof. D. W. Knight, Dr. S. H. Taylor, Prof. G. J. Hutchings
Cardiff Catalysis Institute, School of Chemistry, Cardiff University
Main Building, Park Place, Cardiff, CF10 3AT (UK)
E-mail: hutch@cardiff.ac.uk
[13]
reported for Au–Pt catalysts, suggesting that the selection
of the metal molar ratios and support material for this
bimetallic catalyst has a strong influence on activity. The Au–
Pt catalyst with 1:3 mol fraction retained significant activity
when the temperature was decreased to 408C. By extending
the reaction time to 24 h and further decreasing the reaction
temperature to ambient (238C), high conversion was retained
Q. He, Prof. C. J. Kiely
Center for Advanced Materials and Nanotechnology
Lehigh University
5
East Packer Avenue, Bethlehem, PA 18015-3195 (USA)
Dr. A. A. Herzing
National Institute of Standards and Technology
Surface and Microanalysis Science Division
with a simultaneous increase in the C product selectivity
3
100 Bureau Drive, Gaithersburg, MD 20899-8371 (USA)
(> 88% by mol). In contrast, Au–Pd catalyst prepared with a
[
**] This work formed part of the Glycerol Challenge. The Sasol company
and the Technology Strategy Board are thanked for their financial
support. This project is co-funded by the Technology Strategy
Board’s Collaborative Research and Development programme,
following an open competition. The Technology Strategy Board is an
executive body established by the UK Government to drive
innovation.
1
:3 mol fraction and similar metal loading demonstrated
significantly lower activity under these conditions, and the
selectivity to glyceric acid was lower than that displayed for
the Au–Pt catalyst, despite the lower conversion observed.
These results suggest that at similar conversion levels, the Au–
Pd bimetallic catalysts are significantly less selective to the
desired C products and data at iso-conversion shows this is
the case (Table S1).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101772.
3
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Communications
Time-on-line data for the reaction at ambient temperature
trum imaging was performed. The results for the 1:1 mol
fraction AuPt/MgO catalyst (Figure 2) show that there is a
good spatial correspondence of the Au and Pt elemental maps
formed with the intensities of the La lines. These results
is shown in Figure 1. It is apparent from the reaction profiles
that the Pt-containing catalysts are considerably more active
than the Pd-containing catalysts at all reaction times. Fur-
Figure 2. HAADF image of the AuPt(1:1)/MgO catalyst (dried at
1
208C), along with the corresponding Au-L , Pt-L , Mg-K , and (Au-
a a a
M +Pt-M ) elemental maps from the same area.
a
a
Figure 1. Time-on-line data for selective oxidation of glycerol by Au–M/
MgO catalysts, with M=Pt (^) or Pd (&). Reaction conditions: 1:3
molar fraction Au:M, total metal loading 1% by mass, water (10 mL),
ꢀ1
suggest that the individual metal particles do contain both
elements. However, this analysis is complicated by the
inherently low XEDS signal collected from such small
0
.3 molL glycerol, mole fraction of glycerol/metal=500, T=ambient,
p 2
O
¼300 kPa. Data points represent the mean value obtained from 10
reaction runs, and the error bars are defined by the range of these
measurements.
particles, pathological overlaps between the Au-M and Pt-
a
M peaks, and the degradation of the underlying Mg(OH )
a
2
support under the electron beam. Whether the metal particles
are homogeneous or phase-separated within each particle
(e.g. a core–shell morphology similar to that observed for Au–
Pd) could not be resolved due to the extremely small size of
the particles and the difficulty in obtaining spatially resolved
XEDS data from such small particles at near atomic
resolution.
A representative selection of STEM-HAADF images of
the Au–Pd and Au–Pt (both 1:3 mole fraction) particles
supported on MgO are presented in Figure 3. The sol-
thermore the rate exhibits no discernable reduction as the
reaction proceeds; suggesting that with extended reaction
times greater conversion levels could be achieved. The Au–Pt
catalyst also displays significantly higher selectivity to glyceric
acid, when compared with Au–Pd catalysts (Table S1).
Specifically, for catalysts exhibiting 30% conversion, Au–Pt
gives 85% selectivity to glyceric acid, while the selectivity of
Au–Pd is 67%. In the reaction with MgO alone less than
0
.8% conversion was observed after 4 h.
It is possible that MgO may be acting as a sacrificial base
during the reaction and hence we investigated possible
2
+
leaching of Mg during reaction but found it to be negligible
2
+
2+
(
77 ppm Mg ). Indeed, the [Mg ] is 2–3 orders of magnitude
lower in amount than the products observed. Furthermore
addition of this minimal level of base did not significantly
affect the conversion and there was no reaction in the
2
+
presence of Mg .
Transmission electron microscopy (TEM) measurements
demonstrate that the Au–Pd catalysts have a broader particle-
size distribution than the Au–Pt catalysts (Figure S3). The
microstructure of the bimetallic catalysts was also investi-
gated using aberration-corrected STEM. The binary phase
diagrams of the Au–Pt and Au–Pd systems indicate that,
whereas Au–Pd exhibits a complete solid solution at all
temperatures below the melting point, Au–Pt exhibits a large
miscibility gap with limited solubility between the two face-
centered cubic (fcc) end member phases at ambient temper-
[15]
atures. However, it is well known that bulk phase relations
[
15]
can be substantially altered for nanoscale particles. In order
to determine if the ca. 2 nm particles are homogenous or
phase-separated, aberration-corrected STEM-XEDS spec-
Figure 3. High-magnification STEM-HAADF image and corresponding
particle-size distribution of a,b) AuPd(1:3)/MgO and c,d) AuPt(1:3)/
MgO.
1
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Angew. Chem. Int. Ed. 2011, 50, 10136 –10139

immobilized Au–Pd particles (Figure 3a,b) were found to
have a mixture of characteristic cuboctahedral, decahedral
and icosahedral particle morphologies. The latter two particle
morphologies, which are multiply twinned crystals that
preferentially expose {111}-type surface facets, were domi-
nant. By virtue of the large difference in atomic number (Z)
between Pd (Z = 46) and Au (Z = 79), the Au–Pd particles
were determined to be homogeneous. If these particles
experienced substantial core–shell segregation, it would
have been clearly visible because the HAADF cross-sections
enhanced glycerol conversion and selectivity towards C3
products relative to Pd. These Au–Pt catalysts may permit
bench-top reaction procedures for alcohol oxidation to be
more commonly practiced in synthetic methodology.
Received: March 11, 2011
Revised: July 18, 2011
Published online: August 24, 2011
Keywords: alloys · gold · heterogeneous catalysis ·
.
[
16–18]
nanoparticles · platinum
of the two elements differ by more than a factor of 2.
In
contrast, STEM-HAADF is insensitive to such segregation in
the Au–Pt particles, since Pt and Au have consecutive atomic
numbers. The vast majority of Au–Pt particles exhibited a
simple cuboctahedral structure, as illustrated in Figure 3c,d,
exposing a mixture of {111} and {200} facets. XEDS point
spectrum was acquired by scanning the beam over one
particle and therefore the averaged composition information
of individual particle can be obtained. As shown in Figure S4
and S5, particles from both sides of the size distribution are all
alloyed. However in the Au–Pd system there is a size-
dependant composition variation, which we have reported
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