Selective liquid phase oxidation of glycerol to glyceric acid over novel supported Pt catalysts

Article abstract of DOI:10.2298/JSC121203037S

Several supported platinum catalysts were prepared by the extractive- pyrolytic method for the selective production of glyceric acid from glycerol. Al₂O₃, Y₂O₃, Lu₂O ₃, ZrO₂-Y₂O₃ TiO₂, SG, Fe₂O₃, -AlO(OH) and C were used as the catalyst supports. Glycerol oxidation was performed in alkaline solutions and oxygen was used as the oxidant. The optimal catalyst preparation parameters and glycerol oxidation conditions to obtain glyceric acid were determined. The best result (57 % selectivity to glyceric acid with 92 % glycerol conversion) was achieved on a 4.8 % Pt/Al2O3 catalyst. Copyright 2013 SCS.

Full text of DOI:10.2298/JSC121203037S

J. Serb. Chem. Soc. 78 (9) 1359–1372 (2013)
JSCS–4503
UDC 547.426.1+66.094.3+544.478+
546.92:547.474.3:66.061
Original scientific paper
Selective liquid phase oxidation of glycerol to glyceric acid over
novel supported Pt catalysts
1
1
1
*
ELINA SPROGE , SVETLANA CHORNAJA , KONSTANTINS DUBENCOVS ,
SVETLANA ZHIZHKUN1, VALDIS KAMPARS1, VERA SERGA2, LIDIJA KULIKOVA2
and ERIKS PALCEVSKIS2
1 Faculty of Material Science and Applied Chemistry, Riga Technical University, Azenes Str.
14/24, Riga, LV-1048, Latvia and 2 Institute of Inorganic Chemistry, Riga Technical
University, Miera str. 34, Salaspils, LV-2169, Latvia
(Received 3 December 2012, revised 26 February 2013)
Abstract: Several supported platinum catalysts were prepared by the extrac-
tive–pyrolytic method for the selective production of glyceric acid from gly-
cerol. Al2O3, Y2O3, Lu2O3, ZrO2–Y2O3 TiO2, SG, Fe2O3, γ-AlO(OH) and C
were used as the catalyst supports. Glycerol oxidation was performed in alka-
line solutions and oxygen was used as the oxidant. The optimal catalyst prepa-
ration parameters and glycerol oxidation conditions to obtain glyceric acid
were determined. The best result (57 % selectivity to glyceric acid with 92 %
glycerol conversion) was achieved on a 4.8 % Pt/Al2O3 catalyst.
Keywords: glycerol oxidation; supported platinum catalysts; glyceric acid;
extractive–pyrolytic method.
INTRODUCTION
Glycerol, which is potentially a valuable building block, is obtained as a by-
product in the production of biodiesel. Glycerol is produced in large amounts
during the transesterification of fatty acids into biodiesel – its yield can reach
1,2
approximately 10 %. In recent years, because of increasing biodiesel produc-
tion, glycerol utilization has become a significant problem. The possibilities of
1–28
5,29–31
glycerol transformation into useful compounds,
sources
materials,
or energy
5,32–34
are being investigated. With green chemistry and environmentally
friendly manufacturing in mind, the selective oxidation of glycerol using sup-
ported noble metal catalysts has attracted much attention. Liquid phase oxidation
of glycerol with oxygen or air over heterogeneous Pt, Pd or Au catalysts affords
many important compounds – glyceric acid (GLYA), lactic acid (LACT), tart-
ronic acid (TART), glycolic acid (GLYC), dihydroxyacetone (DIHA), glyceral-
*Corresponding author. E-mail: Elina.Sproge@rtu.lv
doi: 10.2298/JSC121203037S
1359
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SPROGE et al.
1,6–12,14–26
dehyde (GLYAD), etc.
Glyceric acid and its derivatives are important
compounds as raw materials for chemical products, such as bioplastics, phar-
maceuticals for acceleration of alcohol metabolism or liver disease treatment and
3,9,11
cosmetics.
Most of the studies concerning the heterogeneous oxidation of glycerol were
dedicated to the selective oxidation of glycerol over supported gold catalysts
12–14
using oxygen as the oxidant.
Gold catalysts are very selective to glyceric
acid. Using an Au/C catalyst, Carretin et al. reported a selectivity to glyceric acid
14
of 100 % at a glycerol conversion of 56 %. For gold catalysts, many supports
have been investigated, i.e., carbon materials, several metal oxides and poly-
8,15,16
mers.
The main drawback of gold catalysts is that they are active only in
the presence of base, whereas platinum catalysts work in basic, acidic and neutral
17–19
conditions.
6
Liang et al. reported that multi-wall carbon nanotubes (MWNT)-supported
Pt catalysts were more active and selective than Pt/activated carbon (AC) catal-
ysts, but this type of support is expensive and the preparation of catalyst is
complicated, so in further studies, the authors returned to activated carbon. Liang
et al. compared glycerol oxidation over 5 % Pt/MWNT catalyst under alkaline
8
and base-free conditions. The 5 % Pt/MWNT catalyst in a base-free aqueous
solution was more selective to glyceric acid (68 %) at the same glycerol
conversion (90 %). The authors also concluded that the activity and selectivity to
glyceric acid increased with decreasing particle size of the carbon support (from
253 to 9 μm). The most active were catalysts with a Pt particle size of less than 6
1,6,8
nm.
A general drawback of platinum-containing catalysts is poisoning with
molecular oxygen and deactivation. Therefore, Pt catalysts are used in oxidation
processes at low partial pressures of oxygen or the platinum is alloyed with gold,
20–23
thereby allowing higher oxygen partial pressures. Prati et al.
reported that in
the alkaline solutions at 3 atm bimetallic Pt–Au/C catalysts were more active and
selective to glyceric acid compared with Pt/C catalysts. For example, in the pre-
sence of 1 %Pt–Au/C, the glycerol conversion was 46 % higher than in the pre-
22
sence of 1 %Pt/C and the selectivity to glyceric acid was increased by 13 %.
It is obvious that during the past years, many authors dedicated their time to
the investigation of glycerol oxidation only in base-free solutions because these,
1,8,23–25
together with recyclable catalysts, are the environmentally friendly route.
17
Since 1993, when Kimura et al. reported glycerol oxidation over Pt/C,
mainly carbon material supports have been studied for monometallic Pt catalysts,
i.e., activated carbon, graphite and MWNT. Recently, several authors reported
about selective glycerol oxidation over bimetallic Au–Pt catalysts supported on
35,36
37
38
MgO, Mg(OH) ,
H-mordenite and Al O . The preparations of Pt catal-
2
2 3
ysts are based on 3 methods – impregnation, immobilization and ion
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SELECTIVE OXIDATION OF GLYCEROL TO GLYCERIC ACID
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1,6,8,11,18
39
exchange.
Recently, a new extractive–pyrolytic method was described
for the preparation of a fine-disperse platinum coating on nanopowder supports,
such as Al2O3, γ-AlO(OH), Y2O3, CeO2 and SiO2. It was determined that the Pt
particles were spherical and depending on the nature of the supports nature and
parameters for the preparation of the catalyst, the sizes of the Pt crystallites range
from 5 to 35 nm.
Herein, the results of an investigation of glycerol oxidation by molecular
oxygen over novel Pt catalysts to obtain glyceric acid are reported. The catalysts
were prepared by the new extractive–pyrolytic method and Al2O3, Y2O3, Lu2O3,
ZrO2–Y2O3 TiO2, silica gel (SG), Fe2O3, γ-AlO(OH) and AC were used as the
supports.
EXPERIMENTAL
Materials
The following reagents were used for the preparation of the catalysts precursors: plati-
num powder (99.99 %; Sigma–Aldrich), HCl (35 %; Lachema); HNO3 (65 %; Lachema), tri-
octylamine ((C8H17)3N) (95 %; Fluka) and toluene (analytical grade; Stanchem). For the syn-
thesis of the catalysts, several powders were used as supports – Al2O3 and Y2O3 (nanopow-
ders of aluminium and yttrium oxide obtained in plasma by procedure described in the lite-
rature39), ZrO2–Y2O3 (nanopowder of zirconium oxide (86 %), which was stabilized with
yttrium oxide (14 %) and prepared by a literature procedure40), Fe2O3 (nanopowder of iron
(III) oxide prepared by the extractive–pyrolytic method described in the literature41), TiO2
(nanopowder of titanium dioxide prepared by the sol–gel method described previously42),
Lu2O3 nanopowder (Sigma–Aldrich), γ-AlO(OH) nanopowder (boehmite, PURAL® SB,
Sasol, Germany); γ-Al2O3 (nanopowder of aluminium oxide obtained in burning commercial
γ-AlO(OH), C (Norit®, Sigma–Aldrich), SG – silica gel obtained through sol–gel technology
from S. I. Vavilov State Optical Institute (St. Petersburg, Russia)43. Glycerol (≥98 %; Fluka),
NaOH (reagent grade, Sigma–Aldrich) and oxygen (98 %; AGA SIA) were used in the gly-
cerol oxidation experiments. H2SO4 (95–98 %; Sigma–Aldrich) was used in the samples of
the reaction mixture preparation and analysis. For the identification of the glycerol oxidation,
several possible products were used: DL-glyceraldehyde dimer (≥97 %; Aldrich), 1,3-dihyd-
roxyacetone dimer (≥97 %; Aldrich), glyceric acid calcium salt hydrate (≥99 %; Fluka), sodium
β–hydroxypyruvate hydrate (≥97 %; Fluka), lithium lactate (≥97 %; Fluka), tartronic acid
(≥98 %; Alfa Aesar), sodium mesoxalate monohydrate (≥98 %; Aldrich), glycolic acid (≥99
%; Acros Organics), glyoxylic acid monohydrate (≥98 %; Aldrich), oxalic acid (98 %; Ald-
rich), acetate standard for IC (1.000 g L-1; Fluka) and formate standard for IC (1.000 g L-1; Fluka).
Catalyst preparation and characterization
Supported platinum catalysts were prepared by the extractive–pyrolytic method.39,44. In
order to obtain platinum-containing organic extracts by the liquid extraction method, chlo-
roplatinic acid (H2PtCl6) solution in hydrochloric acid (2 M HCl) was added to the trioctyl-
amine ((C8H17)3N) solution in toluene. After shaking the mixture for 3 min, the organic phase
was separated from the aqueous phase and filtered. The analysis of the aqueous solution after
extraction using a Hitachi 180-50 atomic absorption spectrometer evidenced that the platinum
had been completely extracted into the organic phase. The obtained organic phase, which was
a solution of [(C8H17)3NH]2PtCl6 in toluene, is a precursor. The precursor was added to the
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SPROGE et al.
support. Alkylammonium salts are typical cationic surfactants and wet properly surfaces of
different nature.45 To uniformly distribute the precursor over the carrier surface, the mixture
was thoroughly stirred, then dried to remove the solvent and calcinated. In the preparation of
the catalyst, several parameters, i.e., the concentration of the precursor (0.016–0.4 M), impre-
gnation time of the supports with the precursor (5–180 min), the temperature and time of dry-
ing (18–110 °C and 3–120 min, respectively) and the temperature and time of calcination
(300–500 °C and 5–120 min, respectively), were varied.
Example of catalyst precursor preparation. To 48.25 mL of 1 M trioctylamine solution
in toluene, 38.7 mL of 0.5 M chloroplatinic acid solution in 2 M HCl was added. After shak-
ing the mixture for 3 min, organic phase was separated and filtrated. During extraction, the
precursor, which was a 0.4 M solution of [(C8H17)NH]2PtCl6 in toluene was formed.
Example of 2.4 % Pt/Al2O3 catalyst preparation. To 1.0 g Al2O3 powder, 0.32 mL of a
0.4 M precursor solution was added. The obtained mixture was stirred until the support had
been completely impregnated with precursor and then dried for 5 min at a temperature between
80–100 °С. After drying, the mixture was heated at a rate of 10 °С min-1 and calcinated at 300
°С for 5 min under atmospheric pressure.
Catalyst characterization
The prepared catalysts were characterized by X-ray diffraction (XRD) analysis using a
Bruker AXS D-8 Advance diffractometer with CuKα radiation (λ = 1.5418 Å) over a wide
range of Bragg angles (10° < 2θ < 75°) at a scanning rate of 0.02° s-1 at room temperature.
Specific surface area (SSA) of the powders was measured using HROM-3 chromatograph
(Laboratorni Pristroje, Praha) at the liquid nitrogen temperature and calculated by the Brun-
auer–Emmett–Teller (BET) method.
Glycerol oxidation
The oxidation was performed at elevated temperatures using a thermostatted slurry bub-
ble reactor (50 mL capacity), equipped with gas supply system. After the catalyst had been fed
into the reactor, the desired amount of distilled water and an aqueous solution of glycerol were
added. If necessary, an aqueous solution of NaOH was added to the reaction mixture. When
the required temperature had been reached, the oxygen supply to the reactor was turned on
(300 mL min-1). Samples were removed periodically and analysed by high-performance liquid
chromatograph (HPLC). Several glycerol oxidation process parameters like NaOH initial
concentration (0–1.5 M), glycerol and platinum molar ratio (300–500 mol/mol) as well as
oxidation temperature (50–65 °C) were varied.
Example of glycerol oxidation over 4.8 % Pt/Al2O3 catalyst in basic solution. Oxidation
conditions: initial concentration of glycerol c0(C3H8O3) = 0.3 mol dm-3; initial concentration
of sodium hydroxide c0(NaOH) = 0.7 mol dm-3; glycerol and platinum molar ratio
n(C3H8O3)/n(Pt) = 300; oxygen partial pressure pO = 1 atm; oxidation temperature 60 °C.
2
0.0585 g of dry 4.8 % Pt/Al2O3 catalyst, 11.77 mL of distilled water and 1.13 mL of 3.99 M
aqueous glycerol solution were added into the reactor on the thermostatted stage (10 min).
After that an oxygen supply to the reactor was turned on (300 mL min-1) and 2.10 mL of an
aqueous 5.00 M NaOH solution was added.
Product analysis
Analysis of the reaction mixture was performed on a Waters 2487 HPLC instrument
equipped with an ultraviolet (UV 210 nm) and a refractive index (RI) detector.
The reactants and the products were separated on an anion exclusion column (IC–PAK
Ion–Exclusion 50 A, 7 µm (300 mm×7.8 mm)) maintained at 60 °С. The eluent was 2.5 mM
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SELECTIVE OXIDATION OF GLYCEROL TO GLYCERIC ACID
1363
H2SO4 solution. Filtered samples of the reaction mixtures (20 µL) were diluted 50 times with
eluent. The injection volume was 10 µL, and run time and eluent flow rate were set at 20 min
and 0.6 mL min-1, respectively. The possible products were identified by comparison with ori-
ginal samples.
RESULTS AND DISCUSSION
Influence of catalyst support and Pt loading on glycerol oxidation
Several products were obtained in the oxidation process, i.e., glyceric acid,
tartronic acid, lactic acid, glyceraldehyde, dihydroxyacetone, glycolic acid, oxa-
lic acid and formic acid. The reaction pathways are presented in Scheme 1.
Scheme 1. Reaction pathway for glycerol oxidation in the presence of Pt-supported catalysts.
The results for the oxidation of glycerol over the different novel Pt catalysts
are given in Table I. The activity of the catalysts was estimated by glycerol con-
version. The most active catalysts with glycerol conversion above 83 % were 1.2 %
Pt/Y O , Pt/ZrO –Y O , Pt/γ-Al O , 1.2 % Pt/SG, 4.8 % Pt/γ-AlO(OH) and 2.4
2
3
2
2
3
2 3
% Pt/Lu O . For almost all these catalysts, the Pt crystallite size d was less than
2
3
Pt
39
10 nm. As was reported previously, X-ray diffraction analyses showed that with
increasing Pt loading, the characteristic peak Pt(111) became more apparent. In
the case of Pt loading less than 2.4 %, this Pt peak was almost fully overlapped
by peaks arising from some of the supports, which makes the determination of
the average crystallite size of the metal in the catalyst impossible. As was men-
1,6,8
tioned above, Liang et al.
who investigated the influence of Pt particle size
on the activity and selectivity of Pt/C and Pt/MWNTs catalysts, concluded that
catalysts with Pt particle sizes less than 6 nm were the most active. Simulta-
neously, the selectivity to glyceric acid gradually increased with increaseing size
of the Pt particles. They considered that the lower activity of the larger-sized Pt
particles was due to their lower metal surface area, which restricted the adsorp-
tion of substrate. In the study, the specific surface area (SSA) of the supports and
catalysts, determined by the BET method, were different depending on the nature
of the support. The data presented in Table I show that the carbon support (337
2
–1
2
–1
m g ) and the Pt catalysts on the carbon support (161–213 m g ) have the
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SELECTIVE OXIDATION OF GLYCEROL TO GLYCERIC ACID
1365
largest surface areas. However, catalysts of this type were less active than
catalysts with Y O , ZrO –Y O , γ-Al O , SG, γ-AlO(OH) and Lu O as supports.
2
3
2
2
3
2
3
2 3
It was shown in earlier investigations of glycerol oxidation in the presence of
46,47
48
gold catalysts supported on different carbons,
CeO , Nb O , V O , Ta O
2 2 5 2 5 2 5
46
and Al O
that there was no direct dependency between specific surface area
2
3
of the support and the catalytic properties of gold. The results presented in Table
I confirm this statement. For example, the glycerol conversions with the 4.8 %
Pt/ZrO –Y O and 4.8 % Pt/γ-Al O catalysts, having d =10 nm, are the same
2
2
3
2
3
Pt
(85–89 %) but the specific surface areas of the supports and catalysts differ two-
fold. Moreover, the activities of the 4.8 % Pt/Al O and 4.8 % Pt/Y O catalysts
2
3
2 3
were similar, but the d of the 4.8 % Pt/Al O catalyst was 3 times greater and
Pt
2 3
the SSA of the support was 2 times greater than the corresponding values for the
4.8 % Pt/Y O catalyst.
2
3
The data in Table I show that depending on the nature of the catalyst support
and the loading of Pt on the support, glyceric acid, lactic acid, tartronic acid or
glycolic acid were produced as the main product. It is visible from Table I that in
the presence of catalysts with Al O , Lu O , TiO , SG, γ-AlO(OH) and C as
2
3
2
3
2
supports, glyceric acid was the main product regardless of the Pt loading. The
Pt/Fe O catalyst was selective to glycolic acid. When Y O , ZrO –Y O and
2
3
2
3
2
2 3
γ-Al O were used as supports, the selectivity of the catalyst was dependent on
2
3
the Pt loading. If the Pt loading was 2.4–4.8 %, the main product of glycerol
oxidation was glyceric acid. Decreasing the Pt content in the catalyst from 2.4 to
0.6 % led to changes in the main product of the reaction. Over the 0.6–1.2 %
Pt/Y O catalyst, the main product was lactic acid with a selectivity of 42 %, but
2
3
the 1.2 % Pt/γ-Al O catalyst was the most selective to tartronic acid (40 %). 1.2
2
3
% Pt/(ZrO –Y O ) catalyst was not selective; the amounts of formed glyceric,
2
2 3
tartronic and lactic acids were similar.
It could be concluded from Table I that the most effective Pt catalysts to
produce glyceric acid are Pt/TiO , Pt/Al O , Pt/Y O , Pt/Lu O , Pt/ZrO –Y O
2
2
3
2
3
2
3
2
2 3
and Pt/γ-Al O . The catalysts with a Pt loading of 2.4–4.8 % were both active
2
3
(glycerol conversion for these catalysts was 65–94 %) and selective to glyceric
acid (39–62 %). The Pt/Al O and Pt/Y O catalysts were used in the further
2
3
2 3
investigation to determine how the catalyst preparation and glycerol oxidation
parameters affect the activity and selectivity of the catalysts.
Effect of the catalyst preparation parameters on the activity and selectivity of the
catalyst
The experiment results showed that some catalyst preparation parameters,
such as concentration of precursor, impregnation time of the supports with the
precursor, temperature and drying time did not significantly influence the activity
and selectivity of the catalyst. In the further investigation these preparation para-
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SPROGE et al.
meters were kept constant: concentration of precursor 0.4 M, support impreg-
nation time with precursor 10 min, temperature and drying time 80–100 °C and
10 min, respectively. The calcination temperature and time considerably influ-
enced the activity and selectivity of the catalyst, as given in Table II.
TABLE II. Effect of catalysts calcination temperature on glycerol conversion and oxidation
product selectivity; reaction conditions: c0(C3H8O3) = 0.3 mol dm-3, c0(NaOH) = 1.5 mol dm-3,
n(C3H8O3)/n(Pt) = 300, T = 60 °C, pO2 = 1 atm and reaction time = 7 h; Pt catalyst calcination
time = 5 min
Selectivity, %
GLYA TART LACT GLYC OXAL FORM
Calcination tem-
perature, °C conversion, %
Glycerol
Catalyst
1.2 % Pt/Al2O3
300
400
500
300
400
500
300
400
500
300
400
500
300
400
500
69
54
61
69
56
54
75
16
22
88
87
85
66
59
87
51
7
18
50
6
20
53
15
15
38
8
10
54
12
25
17
26
23
9
26
14
13
2
16
57
49
24
57
58
19
65
67
42
62
62
27
49
48
12
0
0
13
0
0
10
15
12
6
6
6
1
6
4
1
6
2
3
0
0
2
5
5
3
6
3
3
4
6
3
5
6
2
3
3
1
1
1
1
4
1
2.4 % Pt/Al2O3
4.8 % Pt/Al2O3
1.2 % Pt/Y2O3
2.4 % Pt/Y2O3
3
11
18
16
9
29
15
6
0
8
From Table II it is visible that the 4.8 % Pt/Al O catalyst prepared with a
2
3
calcination temperature of 300 °C demonstrated the highest activity and selec-
tivity to glyceric acid. On increasing the catalyst calcination temperature from
300 to 500 °C, the glycerol conversion for the 4.8 % Pt/Al O catalyst decreased
2
3
from 75 to 22 % and the selectivity to glyceric acid decreased from 53 to 15 %.
Meanwhile, the selectivity to lactic acid rose from 19 to 67 %. The changes in the
glycerol conversion in the presence of the 1.2–2.4 % Pt/Al O catalysts were not
2
3
so drastic (69–54 %) in comparison with those observed in the presence of the
4.8 % Pt/Al O catalyst. The data presented in Table III showed the Pt/Al O
2
3
2 3
catalysts became more active when the catalyst calcination time was extended
from 5 to 120 min. The glycerol conversion increased by 9–19 %. The selectivity
of the 4.8 %Pt/Al O catalyst to glyceric acid also increased from 53 to 59 %
2
3
with increasing calcination time, but the 1.2–2.4 % Pt/Al O catalysts became
2
3
non-selective.
It can be seen from the data presented in Tables II and III that in the glycerol
oxidation processes over the Pt/Y O catalysts, the effect of calcination tempe-
2
3
rature and time on catalyst activity and selectivity was different and depended on
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SELECTIVE OXIDATION OF GLYCEROL TO GLYCERIC ACID
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the Pt loading. Using the 1.2 % Pt/Y O catalyst, the glycerol conversion was in
2
3
the range from 83 to 88% and was independent of the temperature and time of
calcination. The main product of the reaction was lactic acid and the selectivity
increased with increasing temperature and time of calcination. The 1.2 %
Pt/Y O catalyst calcinated at 400–500 °C was the most selective catalyst to
2
3
lactic acid (62 %, with a glycerol conversion of 85–87 %). In the presence of the
2.4 % Pt/Y O catalyst, the main product obtained was glyceric acid and the
2
3
glycerol conversion was higher when the catalyst was prepared with a calcination
temperature of 300 °C and a calcination time of 120 min.
TABLE III. Effect of catalyst calcination time on glycerol conversion and oxidation product
selectivity; reaction conditions: c0(C3H8O3) = 0.3 mol dm-3, c0(NaOH) = 1.5 mol dm-3,
n(C3H8O3)/n(Pt) = 300, T = 60 °C; pO2 = 1 atm and reaction time = 7 h; Pt catalyst calcination
temperature: 300 °C
Selectivity, %
GLYA TART LACT GLYC OXAL FORM
Calcination Glycerol
time, °C conversion, %
Catalyst
1.2 %Pt/Al2O3
5
30
120
5
30
120
5
120
5
30
120
5
69
86
88
69
89
88
73
82
88
83
83
66
87
84
51
26
27
50
36
35
53
59
38
18
8
17
27
31
9
20
21
14
10
11
25
23
9
16
29
24
24
28
29
19
20
42
43
55
27
24
27
12
11
11
13
12
11
10
7
6
8
7
6
1
7
6
1
3
3
2
1
2
5
7
3
2
2
3
0
1
3
1
1
2
3
1
1
0
1
1
1
2.4 %Pt/Al2O3
4.8 %Pt/Al2O3
1.2 %Pt/Y2O3
2.4 %Pt/Y2O3
54
41
46
30
120
21
14
11
10
By comparing the data presented in Tables II and III, it could be concluded
that the most effective catalyst for glycerol oxidation to glyceric acid was the 4.8
% Pt/Al O catalyst that had been calcinated at 300 °C for 120 min.
2
3
Effect of glycerol oxidation parameters on the glycerol conversion and oxidation
selectivity in the presence of the 4.8 % Pt/Al2O3 catalyst
23,49–51
It was reported
that catalysts based on platinum group metals suf-
fered oxygen poisoning proportional to the oxygen partial pressure. When using
Pt catalysts, a low oxygen partial pressure must be applied to limit the rate of
oxygen supply to the surface of such catalysts. Under these conditions, the oxi-
dation process must be performed in the oxygen transport limited region when
the temperature effect on the oxidation ratio is insignificant. To investigate
influence of oxygen partial pressure on glycerol oxidation, experiments were
–3
performed under the following conditions: c (C H O ) = 0.3 mol dm ;
0
3 8 3
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SELECTIVE OXIDATION OF GLYCEROL TO GLYCERIC ACID
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–3
c (NaOH) = 0.7 mol dm ; n(C H O )/n(Pt) = 500. From the data presented in
0
3 8 3
Table IV, it could be seen that the best result was attained when p = 1 atm was
O2
applied. The glycerol conversion increased by 7 % when the oxidation was
realized at p = 3 atm in comparison with that at p = 1 atm. Moreover, it was
O2
O2
found that with further oxidation, the glycerol conversion did not change but the
selectivity to glyceric acid decreased. It was impossible to reach a conversion
above 45 % when glycerol was oxidised at p = 0.2 atm. Atmospheric oxygen
O2
pressure was used in the further studies on the effect of other oxidation para-
meters on glycerol conversion and oxidation selectivity.
Data from Table IV show that at p = 1 atm, a higher glycerol conversion
O2
of 92 % was reached when the initial concentration of NaOH was 0.7 M, which
was not dependent on the n(C H O )/n(Pt) ratio. In a 1.5 M NaOH solution, the
3
8 3
glycerol conversion decreased by 25 % if the n(C H O )/n(Pt) was increased
3
8 3
from 300 to 500. Selectivity to the main product, glyceric acid, was similar and
ranged from 52 to 59 % in both aqueous 0.7 and 1.5 M NaOH solutions when the
glycerol conversion was 82–92 %, respectively. In addition, in the base-free solu-
tion, the glycerol conversion decreased noticeably to 49 % and the selectivity to
glyceric acid decreased from 59–57 to 47 % with new products, such as glyceral-
dehyde and dihydroxyacetone, being formed. As was mentioned above, Liang et
8
al. reported that a 5 % Pt/MWNTs catalyst in a base-free aqueous solution was
more selective to glyceric acid (68 %) at the same glycerol conversion (90 %).
This apparent contradiction with the present results could be explained by an
effect of the nature of the support on glycerol oxidation.
The influence of temperature on the oxidation rate of glycerol data is shown
in Fig. 1, from which it could be seen that temperature had almost no effect.
Fig. 1. Effect of temperature on
glycerol oxidation over the 4.8 %
Pt/Al2O3 catalyst. Reaction con-
ditions: c0(C3H8O3) = 0.3 mol dm-
3, c0(NaOH) = 1.5 mol dm-3,
n(C3H8O3)/n(Pt) = 300 and pO2 = 1
atm.
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1370
SPROGE et al.
The changes in selectivity to glyceric acid with oxidation time at different
temperatures are shown in Fig. 2. Analyzing the data shown in Figs. 1 and 2, it
could be concluded that at a glycerol conversion of 90 %, the selectivity to
glyceric acid was similar (50–56 %) at 50–60 °C. At the higher temperature (65
°C), the selectivity to glyceric acid was only 40 % at the same glycerol
conversion. It should be noted that only at 60 °C was the selectivity to glyceric
acid higher and independent of glycerol conversion.
Fig. 2. Effect of temperature on
the selectivity to glyceric acid
during glycerol oxidation over
the 4.8 % Pt/Al2O3 catalyst.
Reaction conditions: c0(C3H8O3)
= 0.3 mol dm-3, c0(NaOH) = 1.5
mol dm-3, n(C3H8O3)/n(Pt) = 300
and pO2 = 1 atm.
CONCLUSIONS
It was demonstrated that the extractive–pyrolytic method could be employed
for the preparation of supported platinum catalysts for the selective oxidation of
glycerol. By optimization of catalyst preparation and glycerol oxidation process
parameters, the optimal conditions for the selective production of glyceric acid
were found for the most effective novel 4.8 % Pt/Al O catalyst to be: catalyst
2
3
calcination temperature, 300 °C; calcination time, 120 min; c (C H O ), 0.3 mol
0
3 8 3
–3
–3
dm ; c (NaOH), 0.7 mol dm ; n(C H O )/n(Pt), 300; temperature, 60 °C and
0
3 8 3
P , 1 atm, when a glycerol conversion of 92 % and a selectivity to glyceric acid
O2
of 57 % were achieved..
Acknowledgment. This work was supported by the European Regional Development
Fund – Project No. 2010/0304/2DP/2.1.1.1.0/10/APIA/VIAA/087.
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___________________________________________________________________________________________________________________
Copyright (C)2013 SCS

SELECTIVE OXIDATION OF GLYCEROL TO GLYCERIC ACID
1371
И З В О Д
СЕЛЕКТИВНА ОКСИДАЦИЈА ГЛИЦЕРОЛА ДО ГЛИЦЕРИНСКЕ КИСЕЛИНЕ
У ТЕЧНОЈ ФАЗИ СА Pt КАТАЛИЗАТОРИМА
ELINA SPROGE1, SVETLANA CHORNAJA1, KONSTANTINS DUBENCOVS1, SVETLANA ZHIZHKUN1, VALDIS
KAMPARS1, VERA SERGA2, LIDIJA KULIKOVA2 и ERIKS PALCEVSKIS2
1
Faculty of Material Science and Applied Chemistry, Riga Technical University, Azenes str 14/24,
2
Riga, LV-1048, Latvia и Institute of Inorganic Chemistry, Riga Technical University,
Miera str 34, Salaspils, LV-2169, Latvia
Неколико чврстих Pt катализатора за селективно добијање глицеринске киселине
из глицерола су припремљени поступком који је комбинација екстракције и пиролизе.
Al2O3, Y2O3, Lu2O3, ZrO2–Y2O3, TiO2, SG, Fe2O3, γ-AlO(OH) и C су коришћени као носачи
катализатора. Оксидација глицерола је вршена у алкалним растворима и кисеоник је
коришћен као оксиданс. Одређени су оптимални параметри за припрему катализатора и
услови оксидације глицерола при којим се добија глицеринска киселина. Најбољи
резултат (селективност за глицеринску киселину 57 % са конверзијом глицерола 92 %)
постигнут је у присуству катализатора 4,8 % Pt/Al2O3.
(Примљено 3. децембра 2012, ревидирано 26 фебрауара 2013)
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