Oxidant-free Dehydrogenation of Glycerol to Lactic Acid by Heterogeneous Platinum Catalysts

Article abstract of DOI:10.1002/cctc.201700099

Among various supported metal nanoparticle catalysts, Pt-loaded carbon (Pt/C) catalysts, including a commercial one, showed the highest yield for selective dehydrogenation of glycerol to lactic acid under oxidant-free, solvent-free, and alkaline conditions. For the reaction with Pt/C, the effects of conditions (type of basic additives, the amount of water, atmosphere, reactor) on the catalytic activity and selectivity were studied. Combined with a reaction pathway in the literature and our result of control reactions, we proposed a strategy for decreasing the selectivity of byproducts. The optimized conditions gave a high turnover number of 69 000. To clarify the structural factors affecting the catalytic activity, we show the effect of electronic states of various transition metals on the activity as well as the effect of Pt particle size on the turnover frequency per surface Pt atom.

Full text of DOI:10.1002/cctc.201700099

Accepted Article
Title: Oxidant-free Dehydrogenation of Glycerol to Lactic Acid by
Heterogeneous Platinum Catalysts
Authors: Ken-ichi Shimizu, S. M. A. Siddiki, Abeda Touchy, Kenichi
Kon, and Takashi Toyao
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To be cited as: ChemCatChem 10.1002/cctc.201700099
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10.1002/cctc.201700099
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Oxidant-free Dehydrogenation of Glycerol to Lactic Acid by
Heterogeneous Platinum Catalysts
S. M. A. H. Siddiki, [a] Abeda S. Touchy, [a] Kenichi Kon, [a] Takashi Toyao,[a,b] Ken-ichi Shimizu*[a,b]
free[6,7] or O2-free[8] dehydrogenation of glycerol to LA in alkaline
Abstract: Among various supported metal nanoparticle catalysts,
conditions. Most of these studies used homogeneous catalysts,[6]
Pt-loaded carbon (Pt/C) catalysts, including a commercial one,
such as Ir,[6a-c] Ru,[6d] Fe[6e] complexes, and the methods generally
showed the highest yield for selective dehydrogenation of glycerol
showed high selectivity to LA and high TON. Considering
to lactic acid (LA) under oxidant-free, solvent-free, and alkaline
industrial application of the catalytic system, it is important to
conditions. For the reaction with Pt/C, the effects of conditions
develop the oxidant-free method with heterogeneous catalysts,
(type of basic additives, the amount of water, atmosphere,
reactor) on the catalytic activity and selectivity were studied.
Combined with a reaction pathway in the literature and our result
of control reactions, we proposed a strategy for decreasing the
selectivity of byproducts. The optimized conditions gave a high
turnover number (TON) of 69000. To clarify the structural factors
affecting the catalytic activity, we show the effect of electronic
states of various transition metals on the activity as well as the
effect of Pt particle size on the turnover frequency (TOF) per
surface Pt atom.
especially a commercially available heterogeneous catalyst.
However, a few heterogeneous catalysts[7] reported so far have
serious problem of low selectivity to LA due to the formation of
1,2-propanediol and C1-C3 alcohols as side products as well as
lower TON than the homogeneous catalysts.[6] For example,
Chaudhari et al. [7a] reported that oxidant-free dehydrogenation of
glycerol in alkaline aqueous solution by a commercial Pt/C
catalyst under 1.4 MPa N2 at 160 °C for 6 h resulted in 36.5%
selectivity to LA, 25.6% selectivity to 1,2-propanediols, and 21.6%
selectivity to C1-C3 alcohols at 78.7% conversion. The yield of LA
(28.7%) corresponds to TON of 184, which is lower than the
homogeneous catalysts for the same reaction.[6]
We have previously found that heterogeneous Pt catalysts are
effective for the oxidant-free dehydrogenation of alcohols.[9] In this
report, we show a detailed optimization study of catalyst and
reaction conditions for the dehydrogenation of glycerol to LA.
Under the optimized conditions, a commercial Pt/C catalyst
shows 93% yield of LA and two orders of magnitude higher TON
than previously reported heterogeneous catalysts. On the basis
of control experiments and catalyst structure-activity relationship
study, we show a strategy for increasing the activity and selectivity
of the present catalytic system.
Introduction
Biodiesel fuel is manufactured by transesterification of
renewable resources, triglycerides. Rapid increase in the
production of biodiesel resulted in increasing production of
glycerol as the byproduct. For upgrading of this waste material, a
number of catalytic methods for glycerol conversion to chemicals
(propanediols, lactic acid, etc.) have been developed.[1] Most of
the known methods use O2 or H2 as reagents, which promote
over-reduction or over-oxidation of the target product, resulting in
low selectivity. The low selectivity is one of the most critical issue
in the glycerol conversion.
Among the glycerol-derived products, lactic acid (LA) is
important as it is a monomer of a commercial bioplastic, poly LA,
with growing demand.[2] Industrially, LA is produced by
fermentation of glucose, which has problems of low space-time
yield, complicated purification and co-production of large amount
of wastes. Recent studies [3-5] reported the synthesis of LA via
oxidation of glycerol by O2 with heterogeneous catalysts. Various
transition-metal-based solid catalysts can promote the reaction.
However, the methods[3-5] have a general drawback of low chemo-
selectivity caused by oxidation of intermediates to side products
(glyceric, tartronic, and glycolic acids) which are more
thermodynamically stable than LA. [5a] To prevent the unselective
oxidation reactions, recent efforts have been focused on oxidant-
Results and Discussion
Catalyst characterization
A Pt-loaded carbon purchased from Sigma-Aldrich (Pt/CSA, Pt = 5
wt%) was used as a catalyst precursor. Table 1 lists the mean
size (D) of Pt metal particles on as-purchased Pt/CSA and Pt/CSA
reduced by H2 at different temperatures (TH2). The mean Pt size
(D) was estimated by a conventional CO adsorption method. The
as purchased Pt/CSA sample showed the smallest Pt particle size
of 2.4 nm. The Pt particle size in Pt/CSA increased with increases
in the reduction temperature. Thus, the Pt/CSA catalysts with the
same Pt content (5 wt%) and different Pt particle size (2.4-7.0 nm)
were prepared. To confirm the accuracy of the Pt particle size,
TEM analysis of the samples was carried out. Figure 1 shows
typical TEM pictures and Pt size distributions of the Pt/CSA
samples. The volume-area mean diameter[10] of Pt particles
estimated by TEM analysis (Figure 1, right) is listed in Table 1.
The volume-area mean diameter is consistent with the mean size
estimated by the CO adsorption experiment within the
experimental error of TEM analysis. This confirms the accuracy of
the mean Pt size estimated by the CO adsorption method.
[a]
[b]
Dr. S. M. A. H. Siddiki, Dr. A. S. Touchy, Dr. K. Kon, Dr. T. Toyao,
Prof. Dr. K. Shimizu
Institute for Catalysis, Hokkaido University, N-21, W-10, Sapporo
(Japan)
E-mail: kshimizu@cat.hokudai.ac.jp
Dr. T. Toyao, Prof. Dr. K. Shimizu
Elements Strategy Initiative for Catalysis and Batteries, Kyoto
University, Katsura, Kyoto 615-8520 (Japan)
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Optimization of Catalysts and Conditions
Table 1. Preparation conditions, Pt particle size and catalytic
activity of 5 wt% Pt/C catalysts.
[a]
TH2
[°C]
-
D [b]
[nm]
2.4
3.9
7.0
D [c]
[nm]
2.8 ± 0.5
3.6 ± 0.5
5.3 ± 1.0
Rate [d]
[mol molPt h-1]
554
TOF [e]
[h-1]
1119
2086
3967
A catalyst screening study was carried out for dehydrogenation of
neat glycerol (6.84 mmol) in the presence of 1.1 equiv. of KOH
and various supported transition metal catalysts pre-reduced at
300 °C containing 0.0021 mmol of the metal (0.03 mol % with
respect to glycerol). The reaction was initially carried out under 1
atm N2 in a closed reactor. Soon after heating the mixture at
160 °C, a needle-shaped pipe was put on the septum rubber on
the reactor in order to release H2 gas produced during the reaction.
Table 2 compares the conversion of glycerol and the yield of LA.
The reaction under the catalyst-free conditions (in the presence
of KOH, entry 1) gave no conversion of glycerol to LA. This
indicates that KOH itself does not promote the reaction at 160 °C.
The Pt/CSA catalyst reduced at 300 °C (entry 2) showed higher
yield (93%) than as-purchased Pt/CSA (73%, entry 4). Exposure
of the reduced Pt/CSA catalyst to air for 0.5 h resulted in low yield
of 61% (entry 3). This could be due to oxidation of the surface Pt0
species by air to Pt oxides. On the basis of these results, hereafter
the catalysts reduced at 300 °C were tested for the reaction
without exposure of the reduced catalyst to air. Various metal
oxide-supported Pt catalysts (entries 5-12) were tested, but these
catalysts showed lower yields (17-64%) than Pt/CSA (93%, entry
2). The metal oxides have basic or acidic nature, whereas carbon
is basically neutral. Hence, the result suggests that the acidity or
basicity of the support material has negative impact on the Pt-
catalyzed dehydrogenation of glycerol in the presence of KOH.
Then, various transition metal-loaded carbon catalysts were
prepared using an active carbon support and tested for the
reaction (entries 13-21). The platinum-group metal (Pt, Rh, Ir, Ru,
Pd) catalysts and Ni/C showed higher yields than the other
catalysts. Among the metals-loaded carbon catalysts, the home-
made Pt/C (entry 13) showed the highest yield. Summarizing the
screening results, and considering wide availability of the
commercial Pt/CSA catalyst, we have selected the Pt/CSA catalyst
reduced at 300 °C (entry 2) as the standard catalyst.
300
500
651
684
[a] Temperatures of reduction under H2.
[b] Mean particle diameter of the supported Pt particles estimated
by CO adsorption experiments.
[c] Volume-area mean diameter of Pt particles estimated by TEM.
[d] Initial rate defined as moles of LA h-1 per mole of Pt.
[e] TOF defined as moles of LA h-1 per mole of surface Pt.
Table 2. Catalyst screening for LA synthesis form glycerol. [a]
OH
OH
0.03 mol% catalyst
OH
+ H2 + H2O
HO
OH
6.84 mmol
Entry
1.1 equiv. KOH
160 °C, 18 h
O
Catalysts
Conv. [%]
0
Yield [%]
0
1
2
blank
Pt/CSA
Pt/CSA-air
Pt/CSA
Pt/Al2O3
Pt/CeO2
Pt/MgO
Pt/CaO
Pt/Nb2O5
Pt/TiO2
Pt/SiO2
Pt/Hβ
Pt/C
Rh/C
Ir/C
Ru/C
Pd/C
Ag/C
Cu/C
Ni/C
95
67
80
37
66
58
55
38
25
21
21
98
68
38
49
64
3
93
61
73
35
64
56
53
36
23
17
20
96
63
33
42
61
1
3[b]
4[c]
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
3
45
4
0
39
0
Co/C
[a] Conversion and yield of LA were determined by 1H-NMR.
[b] Pt/CSA reduced at 300°C was exposed to air at 25°C for 0.5 h.
[c] As-purchased Pt/CSA without reduction.
Figure 1. Typical TEM images (left) and Pt particle size
distributions (right) of Pt/CSA: (a) as-purchased Pt/CSA, Pt/CSA
reduced in H2 at 300°C (b) and 500°C (c). The volume-area mean
diameter of Pt particles is shown in the right figures.
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Table 3. Dehydrogenation of glycerol to LA by Pt/CSA with
Figure 2 shows the time course of the yields of LA and
unconverted glycerol for the reaction by Pt/C (entry 13 in Table 2).
After 18 h, the conversion of glycerol to LA leached the highest
value. So, the reaction time of 18 h is adopted in the standard
conditions. Then, we carried out optimization of the reaction
conditions for the reaction of glycerol by Pt/CSA (Table 3). The
Pt/CSA-catalyzed reaction in the absence of base (entry 1) gave
no yield of LA. Addition of basic additives (entries 2-6) increased
the yield. Among the various bases (1.1 equiv. with respect to
glycerol), KOH (entry 6) gave the highest yield (93%). The yield
decreased with the decreasing amount of KOH (entries 6-8). In
the presence of 1.1 equiv. of KOH, the yield decreased with
decrease of the reaction temperature (entries 6,9,10). Under the
best conditions (entry 6), the yield of LA, estimated by H1 NMR,
was 93%, and the yield of pure LA isolated from the mixture was
81%.
various base.
Entry
Base (x)
-
Na2CO3 (1.1)
K2CO3 (1.1)
NaOH (1.1)
KOtBu (1.1)
T [°C] Conv. [%]
Yield [%]
0
1
2
3
4
5
160
160
160
160
160
<1
19
22
71
82
12
16
63
78
93
6
KOH (1.1)
160
95
(81) [a]
81
7
8
9
KOH (1.0)
KOH (0.5)
KOH (1.1)
KOH (1.1)
160
160
140
120
86
50
88
57
42
82
51
Table 4 shows the effect of atmosphere and reactor on the yields
of LA and byproducts: PD (1,2-propanediol), EG (ethylene glycol),
C1-C3 alcohols (methanol, ethanol, propanols), FA (formic acid)
under the optimized conditions (1.1 equiv. KOH, 160 °C). The
yield of LA under a flow of 1 atm O2 (entry 1, 75 %) was lower
than that under N2 flow (entry 2, 93%). This indicates that O2 is
not an effective hydrogen-acceptor for the dehydrogenation of
glycerol under the present conditions. High yield of LA (93%) was
obtained by the standard reactor (entry 3) used in the experiments
in Tables 1-3, where a needle-shaped pipe was put on the upper
side of the reactor to release H2 gas from the system. The reaction
under 1 atm N2 in a closed reactor, stainless autoclave, (entry 4)
gave the lower LA yield (59%) and higher PD yield (11%) than
those under N2 flow conditions (93% LA and 1% PD in entry 2).
The reaction under 1 atm O2 in the closed autoclave (entry 5) also
showed a relatively low LA yield (56%) and a high PD yield (10%).
Taking into account that PD can be produced by hydrogenation
of glycerol, these results suggest that PD is produced by
hydrogenation of glycerol by the H2 produced by the
dehydrogenation of glycerol and removal of H2 by N2 flow
suppress the hydrogenation of glycerol to PD. To verify this
hypothesis, we carried out the reaction under 0.8 MPa H2 in the
closed autoclave (entry 6). As expected, the reaction under H2
gave lower yield of LA (36%) and higher yields of PD (31%) and
C1-C3 alcohols (11%) than the standard conditions (entry 3) and
the N2 flow conditions (entry 2). In the literature, water has been
generally used as solvent for the oxidation (or dehydrogenation)
of glycerol to LA by heterogeneous catalysts.[4,7] We have also
carried out the reaction in the presence of excess water (275
mmol) in the closed autoclave (entry 7), which results in lower
conversion (73%), lower LA yield (40%) and higher PD yield
(16%) than those in the absence of excess water with the same
reactor (entry 4): 90% conversion, 59% LA yield, and 11% PD
yield. The result demonstrates that water has negative impact on
the reaction; water inhibits the dehydrogenation of glycerol to LA,
while it promotes formation of PD via hydrogenation of glycerol by
the H2 produced by the dehydrogenation of glycerol.
10
[a] Isolated yield.
Table 4. Dehydrogenation of glycerol to LA by Pt/CSA with KOH.[a]
Conv.
[%]
Yield [%]
LA PD EG ROH FA
Entry Atmosphere
1
2
O2 flow[b]
N2 flow[b]
82
95
95
90
88
82
73
75
93
93
4
1
1
2
<1
<1
2
0
0
1
<1
<1
4
[c]
3
1 atm N2
1 atm N2
1 atm O2
0
[d]
[d]
4
59 11
56 10
36 31
40 16
8
5
2
8
7
[d]
6
0.8 MPa H2
4
11
0
7[e]
1 atm N2
4
7
2
[d]
[a] LA (lactic acid), PD (1,2-propanediol), EG (ethylene glycol), ROH
(methanol, ethanol, propanols), FA (formic acid).
[b] 20 cm3 min-1.
[c] a needle-shaped pipe was put on the upper side of the reactor to
release H2 gas from the system.
[d] In a closed system (stainless autoclave).
[e] Water (275 mmol) was added to the mixture.
100
80
LA
60
40
20
glycerol
5
10
t [h]
15
20
25
0
Figure 2. Time course of the yields of glycerol () and LA () for
the reaction of entry 13 in Table 3.
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OH
O
H2
Pt
number of Pt atoms in the catalyst, which is larger than those of
the previously reported heterogeneous catalysts[7] for the oxidant-
free dehydrogenation of glycerol to LA.
H2
O
H2
Pt
CH3OH
OH
HO
C
HO
Pt
H
OH
EG
FA
GLA
KOH
OH
H2
Pt
OH
PD
OH
OH
H2
Pt
H2
Pt
HO
O
HO
HO
OH
OH
GA
H2
w
o
l
s
y
r
e
OH
v
OH
LA
O
Structural factors affecting the catalytic activity
Scheme 1. Possible pathways for the conversion of glycerol to LA and
byproducts.
The effect of Pt particle size of Pt/CSA was tested using the
catalysts in a Pt size range of 2.4-7.0 nm. The initial rates of LA
formation, shown in Table 1, were measured under the conditions
where conversions were below 25%. Using the number of the
surface Pt atoms estimated by the CO adsorption experiment,
TOF with respect to the surface Pt atoms were calculated as listed
in Table 1. The TOF increased with the size of Pt particles (Figure
3). Taking into account the well-established fact that the larger the
size of a metal particle the lower the fraction of under-coordinated
sites and the larger the fraction of the plane sites, the result
suggests that the Pt atoms at flat surfaces have higher activity
than the Pt atoms at under-coordinated sites.
Considering the proposed reaction mechanism in the literature,[3-
presumable pathways from glycerol to LA and byproducts are
8]
proposed in Scheme 1. Glycerol can be dehydrogenated by Pt
sites to from H2 and glyceraldehyde (GA), which can be converted
to LA via dehydration and subsequent benzylic acid
rearrangement of GA. If H2 is present in the system, GA can be
hydrogenated by Pt sites to yield PD, which can be further
hydrogenated to propanol. Also, GA can undergo a retro-aldol
reaction to give formic acid (FA) and glycolaldehyde (GLA), and
these can be hydrogenated to methanol and ethylene glycol (EG),
respectively. To verify the reaction of GA to PD, we tested a
control reaction of GA with Pt/CSA and KOH under 0.8 MPa H2 as
shown in eqn. (1). The result shows 43% yield of PD, which is
consistent with the proposed pathway of PD formation. As shown
in eqn. (2), hydrogenation of LA under the same conditions gave
no conversion of LA and no yield of PD. This indicates that LA is
stable under H2 and is not hydrogenated to PD.
5000
500 oC
4000
3000
300 oC
2000
1000
as-purchased
2
4
6
8
0
Mean diameter of Pt [nm]
Figure 3. TOF for conversion of glycerol to LA vs Pt particle size
by Pt/CSA H2-reduced at various temperature.
By adopting the pathways in Scheme 1, combined with the results
in Table 4, we discuss a strategy for suppression of the
undesirable side reactions. First, removal of H2, produced by the
dehydrogenation of glycerol, from the gas phase is effective to
inhibit the hydrogenation of GA to PD. For the removal of H2,
oxidation of H2 by O2 (a H2-acceptor) is not effective, but
spontaneous release of H2 from the needle pipe on the reactor or
purging by flowing N2 is effective. So, a closed reactor such as a
stainless autoclave should not be used. Second, water should not
be used as solvent, because it promotes hydrogenation of the
intermediate to PD and C1-C3 alcohols.
1.5
Pt
1
Pd
Rh
Ru
Ni
Ir
0.5
0
Ag
Cu
Co
The present catalytic method was applicable for a gram-scale
reaction with small amount of the catalyst. As shown in eqn. (3),
the reaction of 250 mmol of glycerol with 275 mmol KOH and
0.0027 mmol of Pt/CSA (0.0011 mol%) gave 75% yield of LA. The
yield corresponds to TON of 69000 with respect to the total
-4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0
d-band center (d - EF ) [eV]
Figure 4. Effect of the d-band center [11] of the metals relative to
the Fermi energy on the reaction rates by metal-loaded carbon.
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To discuss the effect of the electronic properties of metals on the
catalytic activity of the carbon-supported transition metals (entries
13-21 in Table 2), the initial rates of LA formation by metal-loaded
carbon catalysts is plotted as a function of the d-band center (εd)
relative to the Fermi energy (EF), εd - EF, calculated by Hammer
and Nørskov (Figure 4).[11] The plot shows a volcano-type
dependence of the catalytic activity on the d-band center, except
for the Ir/C catalyst. Generally, there is a tendency that the
interaction between metal surfaces and adsorbed species is
weaker for a metal with deeper the d-band center. [11] Hence, our
result suggests that platinum, having an intermediate d-band
center level, shows higher catalytic activity than the other metals,
because the interaction between surface intermediates and the
metal surface is moderately strong, which can be suitable to the
metal-catalyzed dehydrogenation step.
metal solution was evaporated at 50 °C, followed by drying at
90 °C for 12 h. Before the reaction and characterization
experiments, the precursor was reduced under H2 flow (20 cm3
min−1, 300 °C, 0.5 h) in a pyrex tube. The Pt catalysts loaded on
various metal oxides were prepared by the same method as Pt/C.
Catalyst Characterization
The number of surface Pt0 atoms on Pt/CSA, pre-reduced by H2 at
a temperature in Table 1, was estimated by the CO uptake of the
samples at room temperature using the pulse-adsorption of CO in
a flow of He by BELCAT (MicrotracBEL). The mean size of Pt
particles is estimated from the CO uptake based on the
assumption that CO is adsorbed on the surface of spherical Pt
particles with a stoichiometry of CO/(surface Pt atom) = 1/1.
Transmission electron microscopy (TEM) experiment was carried
out by a JEOL JEM-2100F TEM operated at 200 kV.
Catalytic tests
Conclusions
Organic compounds were purchased from Tokyo Chemical
Industry and were used without further purification. Typically, 5
wt% Pt/CSA reduced at 300 °C was used as a standard catalyst.
Catalytic tests were carried out using a batch-type reactor without
exposing the reduced catalyst to air as follows. Glycerol (6.84
mmol) was injected to the pre-reduced catalyst inside the reactor
(cylindrical glass tube, 17 mL) through a septum inlet, followed by
adding KOH (7.52 mmol) and filling with N2. The tube was heated
at 160 °C for 18 h and magnetically stirred (500 rpm). For the
standard conditions for Tables 1-3 and entry 3 of Table 4, a
needle-shaped pipe was put into the septum on the upper side of
the reactor to release H2 gas from the system. For the reactions
in a closed system (entries 4-7 in Table 4), glycerol was injected
through the septum inlet to a glass tube with the reduced Pt/CSA
catalyst, and then, the septum was removed under air, and a
magnetic stirrer was put in the tube, followed by inserting the tube
inside stainless autoclave (28 cm3), followed by charging with
various gas (N2, O2 or H2). After 18 h of the reaction, D2O (2 mL)
and an aqueous solution of sodium acetate (internal standard)
were added to the products, and 1H NMR analysis (JEOL-ECX
600 operating at 600.17 MHz) of the crude mixture was carried
out to determine to conversion of glycerol and yield of LA and
byproducts. To estimate the yield of isolated LA (entry 6, Table 3),
pure LA was obtained as follows. After the reaction, 1 M HCl
solution was added to the reaction mixture, followed by extraction
of the products with diethylether (10 mL, 3 times), evaporation of
the ether solution reduced pressure, and by removal of the
byproducts by a column (silica gel 60, spherical, 63-210 µm,
Kanto Chemical Co. Ltd.) using hexane/ethylacetate (80/20) as
the eluting solvent. The purity of the obtained LA was confirmed
by 1H NMR.
Carbon-supported Pt metal nanoparticles, including a commercial
Pt/CSA catalyst, was found to be effective for oxidant-free
dehydrogenation of glycerol to lactic acid under oxidant-free,
solvent-free, and alkaline conditions. The effects of conditions on
the catalytic activity and selectivity were studied. Considering the
mechanism in the literature, we proposed the following strategy
for suppression of the undesirable side reactions. Removal of H2,
produced by the dehydrogenation of glycerol, from the gas phase
is effective to lower the yields of hydrogenation products (1,2-
propanediol and C1-C3 alcohols). For the removal of H2, oxidation
of H2 by O2 is not effective, but spontaneous release of H2 from
the needle pipe on the reactor or purging by flowing N2 is effective.
So, a closed reactor (a stainless autoclave) should not be used.
Water should not be used as solvent, because it promotes the
hydrogenation of glycerol to 1,2-propanediol and C1-C3 alcohols.
The structure-activity relationship study showed the following
trends. The TOF increased with the size of Pt particles, which
suggests that the Pt atoms at flat surfaces have higher activity
than the Pt atoms at under-coordinated sites. Among various
metals, platinum, having an intermediate d-band center level,
showed higher catalytic activity than the other metals.
Experimental Section
Catalyst Preparation
The standard catalyst, Pt-loaded carbon catalyst (Pt/CSA, Pt = 5
wt%), was purchased from Sigma-Aldrich. The Pt/CSA powder
was reduced in a flow of H2 for 0.5 h at different temperatures
(300, 500, 700 °C). Thus, Pt/CSA catalysts with different Pt particle
size were prepared as listed in Table 1.
Acknowledgements
Active carbon (296 m2 g-1) was purchased from Kishida Chemical.
γ-Al2O3 (124 m2 g-1) was prepared by calcination of γ-AlOOH
(Catapal B Alumina from Sasol) at 900 °C for 3 h. SiO2 (Q-10, 300
m2 g-1) was supplied from Fuji Silysia Chemical Ltd. TiO2 (JRC-
TIO-4, 50 m2 g-1), MgO (JRC-MGO-3, 19 m2 g-1), and HBEA
zeolite (SiO2/Al2O3 = 25±5, JRC-Z-HB25) were supplied from
Catalysis Society of Japan. Nb2O5 (54 m2 g-1) was prepared by
calcination of Nb2O5nH2O (CBMM) at 500 °C for 3 h.
This work was supported by a Grant-in-Aid for Scientific Research
on Innovative Areas "Nano Informatics" (25106010) from JSPS
and a MEXT program "Elements Strategy Initiative to Form Core
Research Center". The authors thank the technical division of
Institute for Catalysis, Hokkaido University, for their help in
building the experimental equipment.
Metal (5 wt%)-loaded active carbon (C) catalysts, M/C (M = Pt,
Pd, Rh, Ir, Ru, Ni, Cu, Co, Ag), were prepared by impregnation
method using aqueous HNO3 solution of Pt(NH3)2(NO3)2 (Furuya
Metal Co., Ltd.), Pd(NH3)2(NO3)2 (Kojima Chemicals Co., Ltd.),
HNO3 solution of Rh(NO3)3 (Furuya Metal Co., Ltd.) or aqueous
solution of metal nitrates (for Ni, Cu, Co, Ag) or RuCl3 or
IrCl3·nH2O. A mixture of the carbon (Kishida Chemical) and the
Keywords: dehydrogenation • glycerol • lactic acid • biomass •
platinum
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S. M. A. H. Siddiki, A. S. Touchy, K.
Kon,a T. Toyao, K. Shimizu
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Oxidant-free Dehydrogenation of
Glycerol to Lactic Acid by
Heterogeneous Platinum Catalysts
Pt-loaded carbon catalysts showed high yield and high turnover number for
selective dehydrogenation of glycerol to lactic acid under oxidant-free, solvent-free,
and alkaline conditions. Catalytic and structural studies are shown to discuss a
strategy to improve activity and selectivity of the reaction.
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