Selective oxidation of glycerol by using a hydrotalcite-supported platinum catalyst under atmospheric oxygen pressure in water

Article abstract of DOI:10.1002/cssc.201000359

A hydrotalcite-supported platinum (Pt/HT) catalyst was found to be a highly active and selective heterogeneous catalyst for glycerol oxidation in pure water under atmospheric oxygen pressure in a high glycerol/metal molar ratio up to 3125. High selectivity toward glyceric acid (78%) was obtained even at room temperature under air atmosphere. The Pt/HT catalyst selectively oxidized the primary hydroxyl group of 1,2-propandiol to give the corresponding carboxylic acid (lactic acid) as well as glycerol. The activity of the catalyst was greatly influenced by the Mg/Al ratio of hydrotalcite. Glycerol conversion increased with increasing the Mg/Al ratio of hydrotalcite (from trace to 56%). X-ray absorption fine structure (XAFS) measurements indicated that the catalytic oxidation activity was proportional to the metallic platinum concentration, and more than 35% of metallic platinum was necessary for this reaction. TEM measurements and titration analysis by using benzoic acid suggested that the solid basicity of hydrotalcite plays important roles in the precise control of platinum size and metal concentration as well as the initial promotion of alcohol oxidation.

Full text of DOI:10.1002/cssc.201000359

DOI: 10.1002/cssc.201000359
Selective Oxidation of Glycerol by Using a Hydrotalcite-
Supported Platinum Catalyst under Atmospheric Oxygen
Pressure in Water
Akihiro Tsuji,[a] Kasanneni Tirumala Venkateswara Rao,[a] Shun Nishimura,[a]
Atsushi Takagaki,[a, b] and Kohki Ebitani*[a]
A hydrotalcite-supported platinum (Pt/HT) catalyst was found
to be a highly active and selective heterogeneous catalyst for
glycerol oxidation in pure water under atmospheric oxygen
pressure in a high glycerol/metal molar ratio up to 3125. High
selectivity toward glyceric acid (78%) was obtained even at
room temperature under air atmosphere. The Pt/HT catalyst se-
lectively oxidized the primary hydroxyl group of 1,2-propandiol
to give the corresponding carboxylic acid (lactic acid) as well
as glycerol. The activity of the catalyst was greatly influenced
by the Mg/Al ratio of hydrotalcite. Glycerol conversion in-
creased with increasing the Mg/Al ratio of hydrotalcite (from
trace to 56%). X-ray absorption fine structure (XAFS) measure-
ments indicated that the catalytic oxidation activity was pro-
portional to the metallic platinum concentration, and more
than 35% of metallic platinum was necessary for this reaction.
TEM measurements and titration analysis by using benzoic
acid suggested that the solid basicity of hydrotalcite plays im-
portant roles in the precise control of platinum size and metal
concentration as well as the initial promotion of alcohol oxida-
tion.
Introduction
Alternative energy resources are becoming important because
of decreasing petroleum reserves and increasing environmen-
tal concerns. In this regard, biomass is an intriguing candidate
as a renewable and carbon neutral resource.[1] As a biomass-
derivative, glycerol is currently produced in a large amount as
a byproduct in manufacturing biodiesel by transesterification
of vegetable oils. Biodiesel producers will soon look to new ap-
plications to convert the byproduct glycerol into value-added
products to make the biodiesel industry economically more at-
tractive.[2] Furthermore, glycerol is a highly functionalized mol-
ecule and a large number of valuable compounds can be ob-
tained by oxidation, hydrogenolysis, dehydration, esterification,
transesterification, polymerization, and so forth.[3]
From the environmental and green chemistry point of view,
homogeneous bases should not be used. Very recently, the oxi-
dation of glycerol in water without the addition of base was
examined.[9,10] Zheng et al. used Pt/multiwall carbon nanotubes
for base-free glycerol oxidation under O2 flow in water.[9] In
contrast, Prati et al. performed glycerol oxidation in water by
using an acidic zeolite as a support for AuÀPt nanoparticle cat-
alysts.[10] In both cases, high glycerol conversion (70%) and
high glyceric acid selectivity (70–80%) were achieved, but glyc-
erol/metal molar ratios were low (below 500) and the zeolite
reaction was carried out under 3 atm O2 pressure.
Hydrotalcites, a family of anionic clays, present positively
charged brucite-like layers (Mg(OH)2) in which some of Mg2+ is
replaced by Al3+ in the octahedral sites of the hydroxide
sheets. MgÀAl hydrotalcite itself was used as the catalyst for
base-catalyzed reactions, in particular, aldol condensations, the
Knoevenagel reaction, epoxidations, and transesterifica-
tions.[11,12] Metal-supported hydrotalcite catalysts have im-
mense importance in the field of catalysis, especially for the
oxidation of alcohols in the presence of oxygen under more
Oxidation reactions are important for the synthesis of fine
chemicals. In particular, the oxidation of alcohols and polyols
to chemical intermediates represents a challenging target. Oxi-
dation of glycerol leads to a large number of compounds, in-
cluding glyceraldehyde, glyceric acid, glycolic acid, tartronic
acid, and oxalic acid (Scheme 1) and a key problem is con-
cerned with the selectivity of desired compound. Therefore,
control of the reaction selectivity by careful design of the cata-
lyst is required. In this regard, supported platinum, palladium,
gold, and bimetallic (AuÀPd) catalysts have been extensively
studied for the oxidation of glycerol in the presence of
oxygen.[4–8] Most of these catalysts are highly efficient, but are
limited the use of high oxygen pressures and homogeneous
bases. Oxidations of glycerol over gold-based catalysts were
totally inactive in the absence of the base NaOH. Furthermore,
glyceric acid is the main product in the oxidation of glycerol
obtained in the form of sodium salt, which requires additional
neutralization and acidification to get the free glyceric acid.
[a] A. Tsuji, K. T. V. Rao, S. Nishimura, Dr. A. Takagaki, Prof. K. Ebitani
School of Materials Science
Japan Advanced Institute of Science and Technology (JAIST)
1-1 Asahidai, Nomi, 923-1292 (Japan)
Fax: (+81)761-51-1610
E-mail: ebitani@jaist.ac.jp
[b] Dr. A. Takagaki
Department of Chemical System Engineering
School of Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201000359.
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ChemSusChem 2011, 4, 542 – 548

Selective Oxidation of Glycerol
Comparison of glycerol oxidation activity for Pt/HT,
Au/HT, Ru/HT, and Pd/HT catalysts
The catalytic activity of a variety of metal supported
on HT, including Pt/HT, Au/HT, Pd/HT, and Ru/HT was
compared at 333 K. The results indicated that the Pt/
HT catalyst exhibited the highest catalytic activity
among those tested: Pt>Au>Pd>Ru (Figure 2).
Selective glycerol oxidation in water under ambient
oxygen pressure by using Pt/HT
Table 1 shows the catalytic performance of Pt/HT for
selective glycerol oxidation. A variety of supports, in-
cluding carbon, HT, Mg(OH)2, Al2O3, and SiO2, were
compared (Table 1, entries 1, 6–9) under the same re-
action conditions with a glycerol/metal molar ratio of
Scheme 1. A possible reaction pathway of glycerol oxidation.
environmentally friendly conditions.[13] Herein, a simple ap-
proach to replace homogeneous bases with heterogeneous
bases has been demonstrated. We reveal that basic hydrotal-
cite-supported platinum nanoparticles (Pt/HT) are highly active
and selective heterogeneous catalysts for glycerol oxidation at
high glycerol/metal molar ratios (800–3125) in water without
the addition of any homogeneous base under atmospheric
oxygen pressure.
Figure 1. Glycerol oxidation in water by using supported metal catalysts
with molecular oxygen. Reaction conditions: glycerol (0.5 mmol), H2O (5 mL),
glycerol/metal=20 (mol/mol), under O2 flow (100 mLminÀ1), 393 K, 6 h.
Results and Discussion
Glycerol oxidation by using gold and platinum catalysts
supported on carbon and HT
Pt/HT catalyst was prepared by adsorbing H2PtCl6·6H2O on HT
in water and subsequent metal reduction by using formalde-
hyde under reflux conditions. An X-ray diffraction measure-
ment confirmed that the crystal structure of Pt/HT was identi-
cal to that of unsupported HT. The amount of loaded Pt was
estimated to be 0.64 wt%, as determined by inductively cou-
pled plasma atomic emission spectroscopy.
Glycerol oxidation was performed by using gold and plati-
num catalysts supported on commercial carbon and hydrotal-
cite. The reaction was carried out in water at 363 K with a very
low glycerol/metal molar ratio of 20. The results are shown in
Figure 1. Although no activity was found for Au/C, the addition
of NaOH (NaOH/glycerol=2) led to glycerol conversion.[14] The
addition of base is essential for selective oxidation of glycerol
because of initial dehydrogenation through proton abstraction
from primary hydroxyl groups.[5] In contrast, an HT-supported
gold catalyst (Au/HT) could oxidize glycerol without NaOH ad-
dition and reached 100% glycerol conversion. A significant dif-
ference was clearly observed between Au/C and Au/HT, which
suggested that HT, an efficient solid base support, had enough
basicity for glycerol oxidation.
Figure 2. Glycerol oxidation in water by using HT-supported metal catalysts
with molecular oxygen. Reaction conditions: glycerol (0.5 mmol), H2O (5 mL),
glycerol/metal=20 (mol/mol), under O2 flow (100 mLminÀ1), 333 K, 6 h, HT
(Mg/Al=5).
800. Among them, Pt/HT exhibited highest conversion (55%)
with remarkable selectivity of glyceric acid (75%) (Table 1,
entry 1). The activity was much higher than that of Pt/Mg(OH)2,
Pt/Al2O3, Pt/SiO2, and Pt/C (Table 1, entries 6–9). For Pt/Al2O3,
Pt/C, and Pt/SiO2, glycerlaldehyde formation was observed to-
ChemSusChem 2011, 4, 542 – 548
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543

K. Ebitani et al.
reaction (1 h), a considerably high selectivity of glyceric acid
(96%) was obtained. With an increased reaction time, the glyc-
erol conversion increased and the selectivity of glyceric acid
gradually decreased. Unreduced Pt/HT obtained without the
use of formaldehyde, however, showed no activity (Table 1,
entry 5). Again, Pt metal was found to be best catalyst for glyc-
erol oxidation. When the glycerol/metal ratio was 800, Au/HT,
Pd/HT, and Ru/HT showed no activity (Table 1, entries 10–12).
These results clearly indicate that a platinum catalyst affords
much higher activity than gold. In addition, HT is a desirable
support for selective oxidation of glycerol towards glyceric
acid under such moderate reaction conditions, including base-
free and atmospheric O2 pressure. For the elucidation of the
reaction mechanism, the addition of a radical scavenger, 4-tert-
butylpyrocatechol, to the reaction medium hardly influenced
the oxidation of glycerol, which indicated that oxidation of
glycerol did not proceed by a radical mechanism. The Pt/HT
catalyst selectively oxidized the primary alcohol group of the
diol (1,2-propanediol) as well as the triol (glycerol). Lactic acid
was obtained from 1,2-propanediol with a high selectivity
(over 70%; see Figure S1 in the Supporting Information).
Table 1. Base-free glycerol oxidation with molecular oxygen in water by
using supported platinum catalysts.[a]
Entry
Catalysts
Conversion [%]
Selectivity [%]
GA
GLA
TA
1
Pt/HT[b]
55
54
40
16,[f] 47
0
18
9
5
8
0
0
0
0
75
74
74
96,[f] 78
0
85
28
51
21
0
0
0
0
0,[f]
0
5
3
12
0,[f]
3
0
2[c]
3[d]
4[e]
5
Pt/HT[b]
Pt/HT[b]
Pt/HT[b]
Pt/HT[b,g]
Pt/Mg(OH)2
0
6
7
8
9
10
11
12
13
0
0
[h]
Pt/Al2O3
58
49
76
0
0
0
trace
Pt/C[h]
Pt/SiO2
0
[h]
trace
Au/HT
Pd/HT
Ru/HT
blank
0
0
0
0
0
0
0
0
[a] Reaction conditions: glycerol (0.5 mmol), H2O (5 mL), glycerol/metal=
800(mol/mol), under O2 flow (10 mLminÀ1), 333 K, 6 h. GA=glyceric acid.
GLA=glycelaldehyde. TA=tartronic acid. [b] 0.64 wt% Pt. Mg/Al=5.
[c] Glycerol/metal=2500, 363 K, 24 h. [d] Glycerol/metal=3125, 363 K,
24 h. [e] Glycerol/metal=625, 303 K, 6 h, air atmosphere. [f] 1 h. [g] Unre-
duced. Without the use of formaldehyde during catalyst preparation.
[h] 1 wt% Pt. Purchased from Sigma–Aldrich.
gether with glyceric acid (Table 1, entries 7–9). Under our HPLC
measurement conditions, the peak of glycelaldehyde was de-
tected as a shoulder of the peak of glyceric acid at low glycerol
conversions. At the initial stages of the oxidation reaction,
glycerlaldehyde could be formed for all cases. These results
were obtained by slow oxidation at low conversions. Pt/HT
could catalyze this reaction at high glycerol/metal ratios of
2500 and 3125 (Table 1, entries 2 and 3). High glycerol conver-
sion with high glyceric acid selectivity was achieved even at
high glycerol/metal ratios at 363 K for 24 h with high turnover
numbers of at least 1350. It should be noted that Pt/HT selec-
tively gave glyceric acid even at 303 K under air atmosphere as
shown in Figure 3 (Table 1, entry 4). At the initial stages of the
Recyclability of the HT-supported platinum catalyst for
glycerol oxidation
The catalysts could be reused at least four times without the
loss of activity. The same glycerol conversion and glyceric acid
selectivity were obtained after the third reuse (55% glycerol
conversion and 75% glyceric acid selectivity). TEM images of
the reused catalyst showed no significant changes to the Pt
particle size or the morphology of support (see below). We
simply used uncalcined HT as the support. This type of HT
could be reused as the base catalyst for several reactions, in-
cluding transesterification,[12a] isomerization,[12b] and epoxida-
tion with aqueous hydrogen peroxide,[11a] without the loss of
activity. XRD measurements revealed that the crystal structure
of HT remained unchanged after the reaction (Figure 4).
The possibility of platinum leaching was confirmed by suc-
cessive reactions after removal of solid catalyst from the solu-
tion. The reactions were performed under the following condi-
tions: 0.1m aqueous solution of glycerol (5 mL) and Pt/HT cata-
lyst (0.64 wt%; glycerol/Pt molar ratio=800) at 333 K under an
Figure 3. Time course of glycerol oxidation over Pt/HT in water at 303 K
under an air atmosphere. Reaction conditions: glycerol (0.5 mmol), H2O
(5 mL), Pt/HT (0.64 wt%; Mg/Al=5), glycerol/metal=625 (mol/mol), air at-
~
&
*
mosphere, 303 K. : glycerol conversion, : glyceric acid yield, : tartronic
^
acid yield, : glycolic acid yield, and : glyceric acid selectivity.
Figure 4. XRD patterns for the Pt/HT catalyst before and after the reaction.
*
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ChemSusChem 2011, 4, 542 – 548

Selective Oxidation of Glycerol
oxygen flow (10 mLminÀ1). A glycerol conversion of 21.5% of
with >99% selectivity for glyceric acid was obtained after re-
action for 3 h in the presence of Pt/HT. After the quick removal
of the Pt/HT catalyst by filtration, the aqueous solution re-
mained at 333 K with stirring under an oxygen flow for a fur-
ther 3 h, resulting in same glycerol conversion (21.6%) and
glyceric acid selectivity (>99%). This result indicates that this
oxidation is performed over heterogeneous Pt catalyst.
Table 3. Glycerol oxidation in water by using HT-supported platinum cat-
alysts with molecular oxygen.[a]
Catalyst
Conversion [%]
Selectivity [%]
GLA
GA
TA
Pt/HT (Mg/Al=3)
Pt/HT (Mg/Al=4)
Pt/HT (Mg/Al=5)
Pt/HT (Mg/Al=6)
trace
8
55
N. D.
94
75
N. D.
N. D.
trace
5
0
0
0
56
70
9
[a] Reaction conditions: glycerol (0.5 mmol), H2O(5 mL), 0.64 wt% Pt/HT,
glycerol/metal=1250(mol/mol), under O2 flow (10 mLminÀ1), 333 K, 6 h.
N. D.=not detected. GA=glyceric acid. GLA=glycelaldehyde. TA=tar-
tronic acid.
Effect of the initial glycerol concentration on catalytic
activity
In addition, experiments at higher glycerol concentrations (0.2
and 0.3m) were carried out by using Pt/HT (0.64 wt%; Mg/Al=
5) at 343 K for 3 h under an oxygen flow (10 mLminÀ1) in
which the glycerol/Pt molar ratio was fixed at 2380 (Table 2).
Glycerol conversion increased
with increasing the HT Mg/Al
Table 2. Effect of initial glycerol concentration on glycerol oxidation by using HT-supported platinum catalysts
ratio (from trace to 56%). The
Pt/HTs with Mg/Al ratios of 5
and 6 showed high glycerol con-
versions with high selectivity to-
wards glyceric acid. We have fur-
ther examined glycerol oxidation
under different reaction condi-
with molecular oxygen.[a]
Initial glycerol
concn [m]
Conversion [%]
(glycerol consumed [mmol])
Yield [mmol]
Sum
[%][f]
GA (selectivity [%])[b]
TA[c]
HA[d]
OA[e]
0.1
0.2
0.3
34.0 (0.17)
52.3 (0.52)
46.1 (0.69)
0.13 (76)
0.24 (46)
0.25 (36)
trace
0.01
0.01
0.02
0.06
0.08
0.01
0.02
0
94
63
50
tions (at
a higher glycerol/Pt
[a] Reaction conditions: Pt/HT (0.64 wt%), glycerol/metal=2380 (mol/mol), H2O(5 mL), under O2 flow
(10 mLminÀ1), 343 K, 3 h. [b] GA=glyceric acid. [c] TA=tartronic acid. [d] HA=glycolic acid (hydroxyacetic
acid). [e] OA=oxalic acid. [f] (Sum of the products, including glyceric acid, tartronic acid, glycolic acid, and
oxalic acid)/amount of glycerol consumedꢁ100.
ratio of 2380 and 343 K) to com-
pare the catalytic activity of Mg/
Al=5 and Mg/Al=6. The time
course of product distribution is
shown in Figure 5. Although
The amount of glycerol consumed increased with increasing
the initial glycerol concentration. The amount of glyceric acid
produced increased from 0.13 to 0.24 mmol on going from 0.1
to 0.2m solutions, and then reached a plateau (0.24 to
0.25 mmol from 0.2 to 0.3m solutions). The selectivity towards
glyceric acid decreased with increasing the glycerol concentra-
tion (76, 46, and 36% for 0.1, 0.2, and 0.3m initial glycerol con-
centrations, respectively). In contrast, glycolic acid formation
gradually increased with increasing the initial glycerol concen-
tration. The sum of the products, including glyceric acid, tar-
tronic acid, glycolic acid, and oxalic acid, was nearly equal
(94%) to the glycerol conversion for the case of 0.1m glycerol
oxidation. However, at higher glycerol concentration, the sum
of the four products was lower than the glycerol conversion
(63 and 50% for 0.2 and 0.3m glycerol oxidation, respectively).
These results indicated that overoxidation (e.g., CO2 formation)
occurred at higher glycerol concentrations.
higher glycerol conversion was observed for the catalyst with
Mg/Al=6 at the initial stages of the reaction, the glyceric acid
yield and selectivity were almost the same between the two
catalysts over the reaction time monitored.
Oxidation state and particle size of Pt nanoparticles
supported on HT
Pt nanoparticles on HT were characterized by using X-ray ab-
sorption fine structure (XAFS) and transmission electron mi-
croscopy (TEM) measurements to examine the oxidation state
and particle size distribution. The intensity of the white line at
the Pt LIII-edge XANES spectra gives us valuable information
for the oxidation state of platinum.[15] Figure 6 shows Pt LIII-
edge XANES spectra for Pt/HTs and the results are listed in
Table 4; these results indicate that the concentration of Pt0 in-
creased with increasing the Pt/HT Mg/Al ratio. It was found
that glycerol conversion was proportional to the Pt0 concentra-
tion of the Pt/HT catalysts, and more than 35% of Pt0 was nec-
essary for this reaction. TEM measurements revealed that the
average size of supported Pt was approximately 2 nm for all
samples (Table 4 and Figure 7). The size distribution of Pt was,
however, very different for the Pt/HTs (Figure 8). Pt particle size
distribution gradually increased with increasing the Mg/Al ratio
of HT. Table 4 also includes the basicity of HT estimated by ti-
Effect of HT as a support on glycerol oxidation
We further examined the effect of HT as a support on glycerol
oxidation by varying the Mg/Al ratio (Mg/Al=3–6) of HTs that
were prepared by a conventional coprecipitation method.
Table 3 lists the glycerol oxidation activity obtained when
using Pt/HTs with various Mg/Al ratios. Interestingly, the activi-
ty of the catalyst was greatly influenced by the HT Mg/Al ratio.
ChemSusChem 2011, 4, 542 – 548
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545

K. Ebitani et al.
Table 4. Oxidation state and particle size of Pt nanoparticles supported
on Pt/HT catalysts and their solid basicity.
Catalysts
Pt0
[%][a]
Pt average
size[b] [nm]
Base amount[c]
[mmol]
Pt/HT (Mg/Al=3)
Pt/HT (Mg/Al=4)
Pt/HT (Mg/Al=5)
Pt/HT (Mg/Al=6)
35
39
52
54
2.1
2.1
2.1
2.5
0.11
0.11
0.16
0.18
[a] Estimated from the intensity of the white line at the Pt LIII-edge XANES
spectra. [b] Determined by TEM. [c] Determined by titration with benzoic
acid.
Figure 5. Time course of glycerol oxidation over Pt/HT with Mg/Al=5 (top)
and 6 (bottom) in water under air atmosphere. Reaction conditions: Pt/HT
(0.64 wt%; Mg/Al=5 and 6), glycerol/Pt=2380 (mol/mol), H2O (5 mL), under
an oxygen flow (10 mLminÀ1), 343 K. : glycerol conversion, : glyceric acid
&
*
~
^
yield, : tartronic acid yield, :glycolic acid yield, : and glyceric acid selec-
*
tivity.
Figure 7. TEM images of HT-supported platinum catalysts. a) Pt/HT (Mg/
Al=3), b) Pt/HT (Mg/Al=4), c) Pt/HT (Mg/Al=5), d) Pt/HT (Mg/Al=6), and
e) Pt/HT (Mg/Al=5) after the reaction.
tration with benzoic acid,[11] indicating that the basicity of Pt/
HTs increased with increasing the Mg/Al ratio.
By considering the Pt size distributions and the basicity of
the Pt/HTs, clear differences in Pt0 concentration among the
Pt/HTs could be explained as follows: First, H2PtCl6 was ad-
sorbed on basic HT to form Pt(OH)xCl6Àx as Pt hydroxide.[16] The
size of this Pt hydroxide should be influenced by the basicity
of the HTs. Small Pt hydroxide particles could be formed on
Figure 6. Pt LIII-edge XANES spectra for Pt/HT catalysts.
546
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Selective Oxidation of Glycerol
Experimental Section
Materials: H2PtCl6·6H2O (99.9%), HAuCl4·4H2O (99.9%), PdCl2, aque-
ous formaldehyde solution (36%), Mg(NO3)2·6H2O, Al(NO3)3·9H2O,
Na2CO3·10H2O, KCl, and activated carbon (Norit SX Plus) were pur-
chased from Wako Pure Chemicals. Pt/Al2O3 (platinum, 1 wt% on
alumina, dry), Pt/C (platinum, 1 wt% on activated carbon), Pt/SiO2
(platinum, 1 wt% silica granules, dry), and RuCl3·nH2O (99.98%)
were supplied from Sigma–Aldrich. NaOH and NaBH4 (90.0%) were
obtained from Kanto Chemicals.
Preparation of HT: Mg/Al HTs (Mg/Al=3–6) were prepared as fol-
lows: Mg(NO3)2·6H2O (30–60 mmol) and Al(NO3)·6H2O (10 mmol)
were dissolved in water (100 mL). The aqueous solution containing
Mg and Al ions was added to an alkaline solution composed of
Na2CO3·10H2O (30 mmol) and water (100 mL) by using peristaltic
pump at speed of 1 mLminÀ1 at room temperature. During the ad-
dition of Mg and Al ions, the pH value of the solution was main-
tained at 10 by adding a 1m aqueous solution of NaOH. After
aging for 1 h at 338 K, the white solid was filtered off and washed
with excess water (2 L) to remove sodium ions. The obtained solid
was dried at 373 K in an oven overnight.
Preparation of Pt/HT: HT (Mg/Al=5, 0.54 g) was added to an aque-
ous solution containing H2PtCl6·6H2O (2.5 mm, 11 mL). After stirring
overnight at room temperature, aqueous formaldehyde (36%,
1 mL) was added as a reducing agent and the solution was vigo-
rously stirred for 30 min. Then, the temperature increased to 373 K
and the solution was stirred for 30 min to reduce Pt species. The
resulting solid was recovered by filtration and washed with deion-
ized water. Finally, the gray-colored solid catalyst was obtained
after drying in oven at 373 K overnight.
Figure 8. Pt particle size distribution of the Pt/HT catalysts estimated from
TEM measurements.
Preparation of Au/HT and Ru/HT catalysts: Au/HT and Ru/HT were
prepared by same method as that used for the synthesis of Pt/HT.
HAuCl4·4H2O and RuCl3·nH2O were used as starting reagents for
Au/HT and Ru/HT, respectively.
HTs with low Mg/Al ratios, and large ones on HTs with high
Mg/Al ratios. Reflux in aqueous formaldehyde resulted in the
formation of Pt0 from Pt hydroxide species for all cases. How-
ever, the surface of Pt0 could be immediately oxidized when
exposed to air,[17] which greatly affects small Pt particles. There-
fore, Pt/HTs with low Mg/Al ratios (3 and 4) have small Pt parti-
cles with low Pt0 concentrations, resulting in poor activity. In
contrast, Pt/HTs with high Mg/Al ratios (5 and 6) have relatively
large Pt particles with high Pt0 concentrations and enough ba-
sicity to give good activity. These two characteristics of Pt0
concentration and HT basicity are advantageous to produce
good Pt/HT catalysts for the oxidation of glycerol.
Preparation of Au/C: Au/C was prepared according to the litera-
ture.[5] HAuCl4·4H2O (50 mm HAuCl4, 10 mL) was dropped into
water (90 mL) containing activated carbon (1.0 g, Norit SX Plus) at
a speed of 1 mLminÀ1 with vigorous stirring. After stirring 30 min,
aqueous formaldehyde (10 mL) was added and the solution was
further stirred for 30 min. Then, the temperature was increased to
373 K and kept for 30 min with stirring to reduce the Au species.
The resultant solid was filtered off, washed with water, and dried
at 373 K overnight.
Preparation of Pd/HT: Pd/HT was prepared according to the litera-
ture with a minor modification.[18] PdCl2 (0.25 mmol) and KCl
(2.5 mmol) were dissolved into water (500 mL), resulting in yellow-
ish brown aqueous solution. HT (1.0 g) was added to the aqueous
solution with stirring overnight. The resultant solid was filtered off,
washed with water (300 mL), and dried at 373 K overnight. After
the solid was redispersed into water, NaBH4 (50 mm, 10 mL) was
added to the solution at room temperature with vigorous stirring
for 30 min. Then, the solid was again filtered off, washed with
water, and dried overnight.
Conclusion
Base-free selective glycerol oxidation in water with molecular
oxygen was successfully demonstrated by using Pt/HT catalysts
under mild conditions. Pt/HT catalysts selectively afforded glyc-
eric acid with high activity, even at high glycerol/metal molar
ratios of up to 3125 under atmospheric oxygen pressure. This
remarkable catalytic activity was attributable for the precise
control of Pt0 concentration and solid basicity of the support.
Characterization: Samples were characterized by XRD (RINT-2000,
Rigaku), TEM (H-7100, Hitachi), and XAFS. Pt LIII-edge XAFS spectra
were recorded at room temperature by the fluorescence method
using a Si(111) monochromator at the BL01B1 station in the
SPring-8 synchrotron radiation facility, Japan. The oxidation state of
platinum was estimated from the height intensity of the white line
at the Pt LIII-edge XANES by using a linear relationship in which the
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Acknowledgements
The synchrotron radiation experiments were performed at the
BL01B1 in the SPring-8 with the approval of the Japan Synchro-
tron Radiation Research Institute (JASRI) (proposal nos.
2009B1497 and 2010A1598). This work was supported by a
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Keywords: biomass
catalysis · oxidation · sustainable chemistry
· green chemistry · heterogeneous
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Revised: December 6, 2010
Published online on January 26, 2011
548
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