Liquid-phase oxidation of propylene glycol using heavy-metal-free Pd/C under pressurized oxygen

Article abstract of DOI:10.1016/j.cattod.2012.02.064

The liquid-phase oxidation of propylene glycol using Pd/C undoped with any heavy metal such as tellurium in the presence of aqueous NaOH under pressurized oxygen resulted in a significant production of lactic acid and a minimal amount of hydroxyl acetone, although, under these conditions, sodium lactate as sodium analogue of lactic acid has been known to easily convert to sodium pyruvate. The advantageous effect of doping Pd/C with a heavy metal, which has been reported for various catalytic reactions in an aqueous phase, was not observed in the present oxidation. Previous papers have reported that oxidation using heavy-metal-free Pd/C and Pd/C doped with a heavy metal at atmospheric pressure and a pH 8 produced pyruvic acid via the formation of hydroxyl acetone and lactic acid, respectively, and that Pd/C doped with a heavy metal rather than Pd/C was suitable for the formation of pyruvic acid from propylene glycol. However the results of the present study show that the use of pressurized oxygen conditions as used in the present study can result in the exclusion of doping with a heavy metal in the preparation of the catalyst, which is amenable to green chemistry.

Full text of DOI:10.1016/j.cattod.2012.02.064

Catalysis Today 203 (2013) 116–121
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Catalysis Today
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Liquid-phase oxidation of propylene glycol using heavy-metal-free Pd/C under
pressurized oxygen
Shigeru Sugiyamaa,∗, Haruki Tanakab, Takuya Bandob, Keizo Nakagawaa, Ken-Ichiro Sotowaa,
Yuuki Katouc, Takanori Moric, Toshiya Yasukawac, Wataru Ninomiyac
a Department of Advanced Materials, Institute of Technology and Science, and Department of Resource Circulation Engineering, Center for Frontier Research of Engineering,
The University of Tokushima, Minamijosanjima, Tokushima 770-8506, Japan
b Department of Chemical Science and Technology, The University of Tokushima, Minamijosanjima, Tokushima 770-8506, Japan
c Mitshubishi Rayon Co. Ltd., 20-1, Miyuki-cho, Otake-shi, Hiroshima 739-0693, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 October 2011
Received in revised form 29 February 2012
Accepted 29 February 2012
Available online 12 April 2012
The liquid-phase oxidation of propylene glycol using Pd/C undoped with any heavy metal such as tel-
lurium in the presence of aqueous NaOH under pressurized oxygen resulted in a significant production of
lactic acid and a minimal amount of hydroxyl acetone, although, under these conditions, sodium lactate
as sodium analogue of lactic acid has been known to easily convert to sodium pyruvate. The advantageous
effect of doping Pd/C with a heavy metal, which has been reported for various catalytic reactions in an
aqueous phase, was not observed in the present oxidation. Previous papers have reported that oxidation
using heavy-metal-free Pd/C and Pd/C doped with a heavy metal at atmospheric pressure and a pH 8
produced pyruvic acid via the formation of hydroxyl acetone and lactic acid, respectively, and that Pd/C
doped with a heavy metal rather than Pd/C was suitable for the formation of pyruvic acid from propylene
glycol. However the results of the present study show that the use of pressurized oxygen conditions as
used in the present study can result in the exclusion of doping with a heavy metal in the preparation of
the catalyst, which is amenable to green chemistry.
Keywords:
Liquid-phase oxidation
Propylene glycol
Palladium catalyst
Pressurized oxygen
Unmodified catalyst
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
pressure and alkaline conditions [2]. The doping of Pd, Pt or Au
catalysts with heavy metals such as Pb, Te, and/or Bi [1–4] and
The reaction network concerning propylene glycol shown in
Scheme 1 has attracted attention, since this shows the possibil-
ity of an alternative route for the preparation of pyruvic acid [1–4].
Although pyruvic acid is used as an important feedstock for various
industries [5,6], it is still produced from the dehydrative decarboxy-
lation of tartaric acid using large quantities of KHSO4 per batch [7].
An alternative approach to pyruvic acid and the related compounds,
which bypasses the use of expensive tartaric acid and the waste of
KHSO4, has been accomplished by the catalytic oxidation of propy-
lene glycol and related compounds, as shown in Scheme 1. In the
liquid-phase oxidation of propylene glycol using Pd/C and related
catalysts in our laboratory (Scheme 1), Pd/C has shown great activ-
ity for the production of pyruvic acid through hydroxyl acetone,
while Pd/C doped with heavy metals has shown activity for the
formation of pyruvic acid through lactic acid under atmospheric
the use of alkaline conditions [1–4] made common sense for the
obtainment of great catalytic activities. Although the noble metal-
lic catalysts doped with those heavy metals are still used widely
in industry [8], green chemistry dictates that a heavy-metal-free
catalytic process should be developed. Indeed, recent reports have
indicated that heavy-metal-free Pd/C shows significant activity for
the conversion of sodium lactate to sodium pyruvate under pres-
surized oxygen without adjustment of the solution pH [9,10], which
indicates that the doping of Pd/C with heavy metals may not be
needed even in the reaction network (Scheme 1). To confirm the
advantageous effect of the use of pressurized oxygen using heavy-
metal-free Pd/C, the liquid-phase oxidation of propylene glycol was
examined using heavy-metal-free Pd/C and Pd/C doped with Te
under pressurized oxygen.
2. Experimental
∗
Pd/C (with Pd of 5 wt%) was purchased from Wako Pure Chem-
ical Industries, Ltd., Osaka, Japan and used as supplied. The BET
Corresponding author. Tel.: +81 88 656 7432; fax: +81 88 656 7432.
E-mail address: sugiyama@chem.tokushima-u.ac.jp (S. Sugiyama).
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.02.064

S. Sugiyama et al. / Catalysis Today 203 (2013) 116–121
117
1 2
1 0
8
6
4
Scheme 1. Reaction network from propylene glycol.
2
0
surface area of Pd/C was 946 m2/g. Pd/C catalyst doped with Te
(Te/Pd/C) was prepared by impregnating the commercially avail-
able Pd/C with a given amount of H6TeO6 (Wako) dissolved in
distilled water. Ethylene glycol (Wako) was added to the suspen-
sion and stirred for 5 h at 423 K to reduce Te cations [11]. After
the reduction, the suspension was cooled to room temperature
and isolated by filtration. The solid obtained was washed with dis-
tilled water and dried overnight at 343 K. Since previous reports
established that an atomic ratio of Te/Pd = 0.03 showed the greatest
activity for oxidative dehydrogenation of sodium lactate to pyru-
vate using Te/Pd/C [10], the catalyst containing the same ratio of
Te/Pd was used in the present study. The BET surface area of Te/Pd/C
was 883 m2/g. Unless otherwise noted, catalytic activity was tested
in a magnetically stirred stainless steel autoclave reactor (85 mL)
[10] using the following procedure. Into the stainless steel auto-
clave reactor, 25 mL of an aqueous reaction solution containing
propylene glycol (12.5 mmol: Kanto Chemical Co., Inc. Tokyo, Japan)
and a solution of NaOH (0–25 mmol: Kanto) was added. After the
autoclave was filled with 100% O2, the reaction temperature was
adjusted to 358 K in the presence of Pd/C (0.125 g), and the solu-
tion was stirred at 700 rpm. In the present study, the pressure in
the autoclave was kept constant (1 MPa) by the addition of 100%
O2 to the autoclave during the reaction. After the reaction was
allowed to proceed for 5 h, the solution was filtered. The reaction
solution thus obtained was analyzed using FID-GC (GC-8AP, Shi-
madzu Corp., Kyoto, Japan). Propylene glycol and hydroxyacetone
were directly analyzed with a 3 mm × 3 m stainless steel column
(Gasukuropack 55, Shimadzu). Lactic acid and pyruvic acid were
analyzed as ethyl esters after the following treatment. Aliquots
(5 mL) of the reaction solution were filtered to remove the cata-
lyst and then treated with aq. NaOH to completely convert these
acids to the corresponding sodium salts. After the solution was
completely evaporated to dryness, an ethanol solution of BF3-
etherate (Wako) was added to convert the sodium salts to the
corresponding ethyl esters. Ethyl pyruvate and ethyl lactate thus
obtained were analyzed with a 3 mm × 3 m stainless steel column
(PEG6000/Uniport B, GL Science Inc., Tokyo, Japan). The catalysts
were characterized by X-ray diffraction (XRD; Rigaku RINT 2500X
using monochromatized Cu K␣ radiation) and by extended X-ray
absorption fine structure (EXAFS). Analysis of EXAFS near the Pd K
edge was carried out at the High Energy Accelerator Research Orga-
nization with a storage ring current of 400 mA (6.5 GeV). The X-rays
were monochromatized with Si(3 1 1) at an NW-10A station. The
absorption spectra were observed using ionization chambers in the
transmission mode. Since it was impossible to compress Pd/C and
Te/Pd/C into a disk with diluents, the catalysts were carefully placed
into a hand-made sample holder with 2 polypropylene windows.
The photon energy was scanned in a range of 24,080–25,600 eV
for the Pd K edge. Details of the calculation procedure have been
previously reported [12]. Specific surface area was calculated from
adsorption isotherms obtained with a conventional BET nitro-
gen adsorption apparatus (BELSORP-18SP, Bell Japan Inc., Osaka,
Japan).
0
1
2
3
4
5
Reaction time [h]
Fig. 1. Time-course of the oxidative dehydrogenation of propylene glycol at 1.0 MPa
using Pd/C at 358 K. Symbols: (ꢀ) lactic acid (ꢀ) pyruvic acid (ꢁ) hydroxy acetone
(ꢁ) propylene glycol (ꢂ) by-product.
3. Results and discussion
3.1. Comparison of the catalytic activity for the oxidation of
propylene glycol under atmospheric pressure and that under
pressurized oxygen using Pd/C
As previously reported, the liquid phase oxidation of propylene
glycol (50 mmol in 100 mL) using Pd/C (2.0 g) at 338 K at a pH 8
Fig. 2. Effect of NaOH concentration on the oxidative dehydrogenation of propylene
glycol (12.5 mmol/25 mL) using Pd/C (A) and Te/Pd/C (B) (both 0.125 g) after 5 h at
358 K. Symbols: (
propylene glycol (ꢂ) by-product.
) lactic acid (
) pyruvic acid (
) hydroxy acetone (ꢁ)

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S. Sugiyama et al. / Catalysis Today 203 (2013) 116–121
Pd/C before
5 % Pd/C
the reaction
Te / Pd = 0.03
(EG reduction)
Te/Pd/C before
the reaction
After reaction
Pd/C after
Te/Pd/C after
After reaction
(NaOH = 0)
the reaction
(NaOH = 0)
the reaction
NaOH/Propylene glycol = 0
NaOH/Propylene glycol = 0
After reaction
Pd/C before
Te/Pd/C after
After reaction
(NaOH = 1.0)
the reaction
(NaOH = 1.0)
the reaction
NaOH/Propylene glycol = 1.0
NaOH/Propylene glycol = 1.0
24000
24500
25000
25500
24000
24500
25000
25500
Energy [eV]
Energy [eV]
Fig. 3. Pd K-edge EXAFS spectra of Pd/C and Te/Pd/C before and after the oxidation of propylene glycol under pressurized oxygen.
(adjusted with aq. NaOH) for 5 h at atmospheric pressure resulted in
the unselective formation of lactic acid, hydroxyl acetone and pyru-
vic acid (approximately 1:1:0.5 by molar ratio) with 85.5% of the
conversion of propylene glycol [2]. In contrast, lactic acid was selec-
tively produced from the oxidation of propylene glycol (12.5 mmol
in 25 mL) after 5 h at 1 MPa of oxygen gas using a small amount of
Pd/C (0.125 g) in the presence of NaOH (0.7 of NaOH/propylene gly-
col by molar ratio) with 61.7% of the conversion of propylene glycol
(Fig. 1). As previously reported, the oxidation of propylene glycol
at atmospheric pressure produced pyruvic acid through hydroxyl
acetone using Pd/C, and through lactic acid using Pd/C doped with
Pb or Te [2]. It should be noted that lactic acid was produced selec-
tively from the oxidation of propylene glycol under pressurized
oxygen even using Pd/C as shown in Fig. 1. When the oxidation of
sodium lactate was examined separately under pressurized oxy-
gen, sodium pyruvate acid was selectively produced, as shown in
our previous study [9,10]. Therefore, the reaction route from propy-
lene glycol to pyruvic acid would be strongly influenced by the
pressurized oxygen, but not by the dopant added to Pd/C. In our
previous paper, it has been already pointed out that the catalytic
activity of Pd/C for the oxidative dehydrogenation is improved due
to the enhancement of oxygen solubility in the reaction solution
under pressurized oxygen [10]. Furthermore the presence of NaOH
seems to be suitable for the present reaction using Pd/C since some
free organic acids react with metallic Pd to form Pd-salts of the free
organic acids, resulted in the decrease of the activity of the metallic
Pd catalyst. Similar roles of pressurized oxygen and the base has
been described in the oxidation of glycerol on Pt/C and Au/C in the
presence of NaOH by Carrettin et al. [13]. They proposed that role of
base was to aid the dehydrogenation through H-abstraction from
one of the primary OH groups of glycerol. Therefore the combina-
tion of these roles of pressurized oxygen and the base seem to result
in the enhancement of the catalytic activity for the present reaction
using Pd/C catalyst.
3.2. Effects of the concentration of NaOH on the oxidation of
propylene glycol under pressurized oxygen using Pd/C and Te/Pd/C
In the present system, the formation of free acids such as lactic
acid and pyruvic acid is unfavorable to the present catalytic reac-
tion since acidic conditions resulted in the deactivation of Pd/C and
Pd/C doped with heavy metal [10]. For example, the conversion
of propylene glycol remained unchanged from 3 h to 5 h (Fig. 1),
indicating that an amount of NaOH may be insufficient due to con-
sumption of NaOH by the reaction with the base and the free acids
formed during the reaction. Therefore, solution pH was controlled
to alkaline conditions by the addition of aq. NaOH into the reaction
solution, as shown in most of the studies of the present system, in
which free acid was converted to the corresponding sodium salt
[2–4]. Since it is rather difficult to add aq. NaOH solution into the
reaction solution in the present autoclave reactor, aq. NaOH solu-
tion was added into the reaction solution at the start of the reaction.
In Fig. 2(A and B), the effects of the molar ratio of NaOH through
propylene glycol in the reaction solution on the oxidative dehy-
drogenation of propylene glycol (12.5 mmol/25 mL) using 0.125 g
of Pd/C and Te/Pd/C catalysts after 5 h, respectively, are described.
The reaction temperature and the pressure were adjusted to 358 K
and 1.0 MPa using oxygen gas. As shown in Fig. 2(A), when the
concentration of NaOH was increased in the aqueous reaction solu-
tion before the reaction, the production of lactic acid also increased
while that of hydroxy acetone decreased. Finally, no hydroxy ace-
tone was detected at molar ratios of 1.0 and 2.0 of NaOH through
propylene glycol. Similar results were also observed using Te/Pd/C
(Fig. 2(B)) while the catalytic activity using Te/Pd/C was evidently
lower than that using Pd/C. It should be noted that the selectivi-
ties to by-product shown in Fig. 2(A and B), which were calculated
from the balance of propylene glycol converted and hydroxy ace-
tone and pyruvic acid formed, were from 0 to 20%. The selectivity to
by-product showed a tendency of the increase as the concentration

S. Sugiyama et al. / Catalysis Today 203 (2013) 116–121
119
Fig. 4. Fourier transformation of Pd/C and Te/Pd/C before and after the oxidation of propylene glycol under pressurized oxygen.
of NaOH was increased, regardless of the presence and absence of Te
in the catalyst. This indicates that the doping with Te of Pd/C is not
needed for the present reaction system, although previous reports
maintain that the selection of the pathway from propylene glycol to
pyruvic acid depended on the presence or absence of heavy metal
such as Te or Pb in Pd/C catalysts when doped with these heavy met-
als at atmospheric pressure [2]. However, the present results reveal
that the selection of the pathway depends on the amount of NaOH
added to the reaction solution under pressurized oxygen. It should
be noted that no pyruvic acid was detected in the present study,
although the oxidative dehydrogenation of propylene glycol gave
pyruvic acid using Pd/C and Pd/C doped with Pb [2] together with Pt
catalysts doped with some heavy metals under atmospheric pres-
sure [3]. Similar results such as the production of lactic acid, but not
pyruvic acid, from propylene glycol were also reported in the same
reaction in the presence of NaOH using a supported catalyst consist-
ing of Au and/or Pd at 1 MPa of oxygen gas [4]. In order to character-
ize the by-product in Figs. 1 and 2, GC–MS analysis was employed,
indicating that the by-product may be polyhydric alcohol.
glycol to pyruvic acid through intermediates such as hydroxyl ace-
tone or lactic acid. Therefore, the oxidations of hydroxyl acetone
and lactic acid (both 12.5 mmol/25 mL) were examined separately
using Pd/C (0.125 g) at 358 K and 1.0 MPa using oxygen gas in
the presence of NaOH (molar ratio of NaOH/intermediate = 0.7).
Although 99.6 and 15.7% of hydroxyl acetone and lactic acid,
respectively, were converted during 5 h, the formation of pyruvic
acid was not detected at all. Therefore, under pressurized oxygen,
deep oxidation of pyruvic acid further proceeded to unidentified
species, due to an excess solubility of oxygen gas into the reaction
solution. It should be noted that deep oxidation can be avoided
if sodium salt in the absence of NaOH instead of free acid in the
presence of NaOH is used as a substrate [9,10].
Table 1
Results of curve-fitting analyses for Pd/C and Te/Pd/C before and after the oxidation
of propylene glycol under pressurized oxygen.
Sample
Molar
ratioa
rPd–Pd (nm)b
Nc
ꢀd (nm)
Re
Pd/C
Pd/C
Pd/C
Te/Pd/C
Te/Pd/C
Te/Pd/C
Fresh
0
1
Fresh
0
1
0.274
0.274
0.275
0.273
0.275
0.275
8.18
7.44
8.01
9.29
7.60
9.74
0.066
0.074
0.075
0.066
0.071
0.075
0.482
1.540
0.889
0.342
0.622
1.419
3.3. The oxidation of hydroxyl acetone and lactic acid under
pressurized oxygen using Pd/C
It is odd that pyruvic acid was not detected in the present system
since previous studies found that the oxidative dehydrogenation
of sodium lactate using Pd/C under pressurized oxygen produced
sodium pyruvate under the same reaction conditions [9,10]. As
shown in Scheme 1, two possibilities for the formation of pyruvic
acid are present in the oxidative dehydrogenation of propylene
a
NaOH/Propylene glycol.
Nearest-neighbor distance around Pd.
Coordination number.
Debye–Waller (like) factor.
Reliability factor.
b
c
d
e

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S. Sugiyama et al. / Catalysis Today 203 (2013) 116–121
Fig. 5. Curve-fitting near Pd K-edge of Pd/C and Te/Pd/C before and after the oxidation of propylene glycol under pressurized oxygen.
3.4. Effects of the addition of NaOH into the reaction solution on
fine structure changes around Pd in Pd/C and Te/Pd/C
the metallic nature of Pd was maintained even after the oxidation
of propylene glycol in the presence and absence of NaOH using
Pd/C and Pd/Te/C under pressurized O2 (Fig. 5).
As shown in the present study, the catalytic activities for the
oxidation of propylene glycol under pressurized oxygen were
strongly influenced by the addition of NaOH into the reaction
solution. To examine the effects of the addition of NaOH into the
reaction solution on fine structure changes around Pd in Pd/C and
Te/Pd/C, EXAFS was analyzed near the Pd K edges using Pd/C and
Te/Pd/C previously used for obtaining the results shown in Fig. 2(A
and B), that is, before and after the reaction at NaOH/propylene
glycol = 0 or 1 (Fig. 3). The Pd K edge EXAFS spectra showed that
the absorption due to Pd in Pd/C and Pd/Te/C before and after the
reaction in the presence and absence of NaOH were essentially
identical, which was due to the metallic Pd [10]. Furthermore
XRD peaks due to metallic Pd (JCPDS 461043) were detected
from these samples (not shown). The corresponding Fourier
transformation near the Pd K-edge for these samples showed one
signal characteristic metallic Pd [10,14] (Fig. 4); the exception was
that obtained from Pd/C previously used in the absence of NaOH
(middle on the left column in Fig. 4), in which some signals were
also detected at a distance smaller than 0.2 nm. Regardless of the
presence and absence of Te in the catalysts, the nearest-neighbor
distance around the Pd increased from 0.273 to 0.274 nm before
the reaction to 0.274–0.275 nm after the reaction (Table 1). This
result may be explained by the incorporation of either hydrogen or
carbon into Pd bonds during oxidation [15]. Based on these results,
4. Conclusion
Previous reports on the oxidation system of propylene glycol
using Pd/C and related catalysts at atmospheric pressure found a
significant dependence on the doping of Pd/C with heavy metals.
However, the results of the present study obtained under pressur-
ized oxygen revealed that the catalytic activity is not dependent on
doping, but is instead dependent on the molar ratio of NaOH added
into the reaction solution against propylene glycol. Therefore, the
doping of Pd/C with heavy metals is unnecessary in the present
oxidation system under pressurized oxygen.
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