Catalytic conversion of lactic acid into propylene glycol over various metals supported on silica

Article abstract of DOI:10.1007/s11164-011-0392-x

Catalytic hydrogenation of lactic acid to propylene glycol was performed over various metals (Ag, Co, Cu, Ni, Pt, and Ru) supported on silica prepared by an incipient wetness impregnation method. The loading amount of each metal was 5 wt%. Crystallinity of the synthesized catalysts was investigated by X-ray diffraction (XRD), and the BET method was utilized to examine the surface area. Pore volume and pore size of catalysts were determined using BJH analysis of the N₂ adsorption isotherm. Particle sizes of various metals were determined from transmission electron microscopy (TEM) images. The catalytic activity was found to be strongly dependent on the supported metal. Among catalysts tested, Ru/SiO₂ showed the highest propylene glycol yield. The yield of propylene glycol increased with pressure, and the highest yield was achieved at 130 °C.

Full text of DOI:10.1007/s11164-011-0392-x

Res Chem Intermed (2011) 37:1275–1282
DOI 10.1007/s11164-011-0392-x
Catalytic conversion of lactic acid into propylene glycol
over various metals supported on silica
•
Tae Young Jang Ku Bong Chung Hye Ri Eom
•
•
•
•
•
Dong Kyun Noh In Kyu Song Jongheop Yi
Sung-Hyeon Baeck
Received: 31 May 2011 / Accepted: 27 July 2011 / Published online: 17 September 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Catalytic hydrogenation of lactic acid to propylene glycol was
performed over various metals (Ag, Co, Cu, Ni, Pt, and Ru) supported on silica
prepared by an incipient wetness impregnation method. The loading amount of each
metal was 5 wt%. Crystallinity of the synthesized catalysts was investigated by
X-ray diffraction (XRD), and the BET method was utilized to examine the surface
area. Pore volume and pore size of catalysts were determined using BJH analysis of
the N2 adsorption isotherm. Particle sizes of various metals were determined from
transmission electron microscopy (TEM) images. The catalytic activity was found
to be strongly dependent on the supported metal. Among catalysts tested, Ru/SiO2
showed the highest propylene glycol yield. The yield of propylene glycol increased
with pressure, and the highest yield was achieved at 130 °C.
Keywords Lactic acid Á Propylene glycol Á Hydrogenation Á Ruthenium
Introduction
Lactic acid has been widely used in food, the medical industry and cosmetic
applications. Because the consumption of lactic acid has been steadily increasing,
the production of lactic acid by fermentation of biomass is of importance to meet the
increase of consumption. In 1987, lactic acid was produced equally by both
chemical synthesis and fermentation processes [1], and currently, most lactic acid is
being produced by the biotechnological methods or by fermentation of renewable
sources such as refined carbohydrates derived from agricultural crops [2, 3] and
T. Y. Jang Á K. B. Chung Á H. R. Eom Á D. K. Noh Á S.-H. Baeck (&)
Department of Chemical Engineering, Inha University, Incheon, Korea
e-mail: shbaeck@inha.ac.kr
I. K. Song Á J. Yi
Department of Chemical and Biological Engineering, Seoul National University, Seoul, Korea
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T. Y. Jang et al.
O
+2H2
-H2O
HO
OH
HO
OH
Propylene glycol
Lactic acid
Fig. 1 Reaction scheme of propylene glycol production from lactic acid
waste biomass [3]. Lactic acid has been known as one of the most reactive species
because it has both hydroxyl and carboxy functional groups. It can be converted into
a variety of chemicals through polymerization [4], condensation [5–7], dehydration
[8], esterification [9, 10], and oxidation [11].
Propylene glycol is a very useful material for foods, consumer products, and
chemical applications. Although propylene glycol has been produced industrially by
hydration of propylene oxide [12], propylene glycol production from lactic acid has
been reported from the 1930s as a very promising method (Fig. 1). In general,
however, the catalytic conversion of lactic acid into propylene glycol requires high
temperature and pressure. Dumesic et al. [13] reported conversion of lactic acid to
propylene glycol over silica-supported copper catalyst. They proposed that the
dissociative adsorption of lactic acid to form an hydroxyl acryl species were the key
step, and the successive hydrogenations of the adsorbed species were proposed as
the main mechanism for the production of propylene glycol. Although copper/
chromite catalyst is also reported as an effective catalyst, catalytic activity for lactic
acid hydrogenation was achieved only at elevated pressure up to 20 MPa [14].
Miller et al. [15] investigated propylene glycol production from lactic acid over
various catalysts, and reported high yield of propylene glycol over ruthenium
supported on activated carbon prepared by the impregnation method. According to
their work, carbon is the promising support due to its inertness and stability in
aqueous solution.
The goal of the present study is to investigate the catalytic activities of various
metals for hydrogenation of lactic acid. Six different metals (Ag, Co, Cu, Ni, Pt, and
Ru) were supported on silica by an incipient wetness impregnation method, and the
supported catalysts were characterized by XRD, BET, and HR-TEM. Liquid-phase
hydrogenation of lactic acid to propylene glycol was carried out in a high pressure
batch reactor (150 mL), and the influence of reaction temperature and pressure on
lactic acid conversion was also studied.
Experimental details
Preparation of metal supported on silica catalyst and characterizations
The silica (AEROSIL 200, BET surface area 200 m2/g) used as a support in this
study was obtained from commercial sources. The silica-supported catalyst was
prepared according to similar methods reported in the literature [16–21]. The six
different metals (Ag, Co, Cu, Ni, Pt, and Ru) were supported on silica by an wetness
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Catalytic conversion of lactic acid into propylene glycol
1277
Table 1 Precursor, calcinations and H2 reduction temperature to the synthesis of catalysts
Catalyst
Precursor
Calcination
H2 reduction
Ag/SiO2
Co/SiO2
Cu/SiO2
Ni/SiO2
Pt/SiO2
Ru/SiO2
AgNO3
500 °C, 4 h
450 °C, 2 h
500 °C, 2 h
700 °C, 2 h
500 °C, 4 h
500 °C, 3 h
120 °C, 3 h
400 °C, 10 h
400 °C, 14 h
500 °C, 2 h
500 °C, 2 h
450 °C, 2 h
RuCl3
Co(NO3)2Á6H2O
Ni(NO3)2Á6H2O
H2PtCl6
RuCl3
impregnation method, and the loading amount of each metal was fixed at 5 wt%.
Precursors for each metal, calcinations temperature, and H2 reduction temperature
are given in Table 1. After drying at 100 °C overnight, the dried powder was
calcined at elevated temperature under air flow. Prior to the reaction, all synthesized
catalysts were reduced under a flow of hydrogen. After cooling, the catalysts were
passivated in nitrogen for 30 min before exposing to air [22].
All of the synthesized catalysts underwent detailed analyses. The crystallinity of
the synthesized samples was investigated by X-ray diffraction analysis (XRD,
Rigaku, D/max-2200). Transmission electron microscope (TEM, JEOL/JEM2100F)
was utilized to examine the surface morphology and metal particle size. The surface
area of the synthesized catalyst was determined by the Brunauer–Emmett–Teller
(BET, MICROMERITICS/ASAP 2020) method, and both pore volume and average
pore size were calculated using the BJH model.
Hydrogenation of lactic acid
Liquid-phase catalytic hydrogenation of lactic acid to propylene glycol was
performed in a 150-mL batch reactor at various reaction pressures (6–9 MPa) and
temperatures (110–170 °C).
Amounts of 50 mL of 1 M lactic acid solution and 0.5 g of prepared catalyst
were charged into a reactor with agitation at 700 rpm. After purging the reactor with
nitrogen for 5 min, the hydrogen was then flowed into the reactor while heating.
When the reactor temperature reached the desired value, the hydrogen pressure was
set to the reaction pressure (6–9 MPa). The reaction was carried out for 7 h, and
products were analyzed by a gas chromatograph (HP 6890) equipped with flame-
ionization detector (FID). A free fatty acid phase (FFAP) capillary column was used
for the separation of reactants and products.
Results and discussion
Characterization of supported catalysts
X-ray diffraction analyses were performed for all samples, and XRD patterns of
Ni/SiO2 and Ru/SiO2 catalysts after reduction under hydrogen flow are shown in
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T. Y. Jang et al.
RuO
Ru
NiO
Ni
(b)
(a)
(calcined)
(calcined)
(reduced)
(reduced)
20
30
40
50
60
70
80
20
30
40
50
60
70
80
2 Theta (deg.)
2 Theta (deg.)
Fig. 2 X-ray diffraction patterns of a Ni/SiO2, b Ru/SiO2 before and after reduction under hydrogen flow
Fig. 2. For all synthesized catalysts, one broad peak at around 2h = 22° was
observed, which can be assigned as the diffraction of amorphous silica. Ni and Ru
were confirmed to exist as metal oxide on the surface of silica after calcination.
After reduction under hydrogen flow, none of the reduced catalysts exhibited the
characteristic XRD peak of metal oxide, which means complete reduction into metal
during the reduction process.
Figure 3 shows the TEM image of Ag/SiO2 and Ni/SiO2 catalysts after reduction
under hydrogen flow. Average metal particle sizes measured from TEM images are
summarized in Table 2. Cu, Co, and Ru were observed to be not well distributed on
SiO2 support, resulting in a large particle size. However, in the cases of Ag, Pt, and
Ni, well-dispersed metal particles were clearly observed on the surface of the silica.
Pt particle size was the smallest among the synthesized catalysts (4.5 nm), and Cu
exhibited the largest particles (34.9 nm)
BET surface area, pore volume, and average pore size of each catalyst are also
summarized in Table 2. Interestingly, Cu/SiO2 showed the highest surface area
(194.2 m2/g), and the other catalysts exhibited BET surface areas in the range of
110–140 m2/g. Considering the BET surface area of silica (200 m2/g) used as
supports, metal supported on silica catalysts showed a lower surface area than
silica support, probably due to the pore blocking by metal particles during the
impregnation step.
Hydrogenation of lactic acid to propylene glycol
The catalytic hydrogenation of lactic acid were conducted in a high pressure batch
reactor, and Fig. 4 compares lactic acid conversion after 7-h reaction over various
metals (Ag, Co, Cu, Ni, Pt, and Ru) supported on silica. Reaction temperature was
150 °C, and hydrogen pressure was 8 MPa. Three metal (Ag, Co, Cu) catalysts
showed no catalytic activity for hydrogenation of lactic acid, while Ni, Pt, and Ru
supported on silica showed pretty high activities. Parmon et al. [23] reported liquid-
phase hydrogenation of lactic acid to propylene glycol over silica-supported copper
catalyst. The lactic acid hydrogenation over the 14 wt% Cu/SiO2 catalyst leads to
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Catalytic conversion of lactic acid into propylene glycol
1279
Fig. 3 TEM images of a Ag/SiO2, b Ni/SiO2 after reduction under hydrogen flow
Table 2 BET surface area, pore volume, average pore size, and average metal particle size of synthe-
sized catalysts
Average metal
Ag/SiO2
Co/SiO2
Cu/SiO2
Ni/SiO2
Pt/SiO2
Ru/SiO2
Particle size (nm)
Surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
9.2
25.5
135.2
0.8
34.9
194.2
0.8
13.3
117.57
0.7
4.5
18.6
138.1
0.7
116.1
0.5
139.8
0.6
18.4
24.8
15.8
24.6
17.0
19.4
the 45% conversion of lactic acid with 50% selectivity of propylene glycol at the
hydrogen pressure of 0.1 MPa. In this work, Cu/SiO2 did not exhibit any catalytic
activity for lactic acid hydrogenation. The huge difference can be explained by the
difference in Cu loading amount. The doping density of Cu in our study was only
5 wt%. Among catalysts tested, 5 wt% Ru/SiO2 catalyst showed the highest
conversion of lactic acid (35%).
Catalytic activity (lactic acid conversion, propylene glycol selectivity, and
propylene glycol yield) over 5 wt% Ru/SiO2 catalyst was investigated with reaction
time, which is plotted in Fig. 5a. Reaction temperature was 150 °C and pressure was
8 MPa. Lactic acid conversion was increased with reaction time, because the
reaction was performed in a batch reactor. Selectivity of propylene glycol was
observed to decrease slightly with time on stream. The decrease of selectivity is
mainly due to the reduction in hydrogen pressure with reaction time. The other main
products were propionic acid and 2-hydroxy propionaldehyde. C1 or C2 compounds
and propanol were not detected during the reaction. These results are somewhat
different from those previously reported [15]. Miller et al. [15] investigated lactic
acid conversion over various catalysts, and they reported that, at temperatures below
170 °C,the only liquid product detected was propylene glycol, and the major
byproducts of the reaction were the light hydrocarbons, methane, ethane, and
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T. Y. Jang et al.
50
40
30
20
10
0
Fig. 4 Lactic acid conversion
over various silica-supported
metal catalysts. Reactant: 50 mL
of 1 M lactic acid solution.
Reaction: time = 7 h,
T = 150 °C, P = 8 MPa,
catalyst 0.5 g
propane. However, Dumesic et al. [13] reported that the main byproducts over a
silica-supported copper catalyst were propionic acid and 2-hydroxy propionalde-
hyde, and no light carbon product (C1 or C2 compounds) was detected. The
difference in product distribution can be explained by the difference in the catalysts
and reaction conditions used. The lactic acid conversion is strongly dependent on
catalyst and reaction conditions.
Figure 5b shows the catalytic activity of 5 wt% Ru/SiO2 with respect to reaction
pressure at 150 °C. Both lactic acid conversion and propylene glycol selectivity
increased with reaction pressure, which is probably due to the increase of hydrogen
solubility in aqueous solution with reaction pressure. With the the increase of
hydrogen concentration in liquid, the reaction rate was increased resulting in the
increase of propylene glycol yield. Reaction pressure must be determined by
considering equipment cost.
The influence of reaction temperature on the hydrogenation of lactic acid over
5 wt% Ru/SiO2 catalyst was investigated, and is plotted in Fig. 5c. Generally,
conversion of lactic acid increased with reaction temperature in the range of
110–170 °C. However, propylene glycol selectivity reached a maximum at 130 °C,
and then decreased with temperature increase. The decrease of selectivity at high
temperature could be attributed to cracking and secondary reaction of propylene
glycol. Because hydrogenolysis of products occurs competitively with hydrogena-
tion of lactic acid, the selection of catalyst and reaction conditions is very important.
In this work, the maximum yield of propylene glycol was achieved at 130 °C.
Conclusion
In the present study, all metals (Ag, Co, Cu, Ni, Pt, and Ru) supported on silica
catalysts were prepared by an incipient wetness impregnation method. After H2
reduction process, XRD analysis revealed that metal oxide species in all catalyst
were completely reduced into metal. The catalytic hydrogenations of lactic acid
were performed in a high pressure reactor, over various metals supported on silica.
Among catalysts tested, 5 wt% ruthenium on silica catalysts showed the highest
activity for hydrogenation of lactic acid.
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Catalytic conversion of lactic acid into propylene glycol
1281
100
100
80
60
40
20
0
yield
selectivity
Conversion
yield
selectivity
conversion
(a)
(b)
80
60
40
20
0
5
6
7
8
6
7
8
9
Time (hr)
Pressure (bar)
100
80
60
40
20
0
yield
selectivity
conversion
(c)
110
130
150
170
Temperature (°C)
Fig. 5 Catalytic activity of 5 wt% Ru/SiO2 with a time on stream (reaction T = 150 °C, P = 8 MPa),
b reaction pressure (reaction T = 130 °C, time = 7 h), c reaction temperature (reaction P = 8 MPa,
time = 7 h)
Conversion of lactic acid and selectivity of propylene glycol increases with
pressure over Ru/SiO2, and the highest yield was achieved at 130 °C. High
temperature ([150 °C) intensifies secondary reactions to by-products. Thus, there
exists an optimal reaction temperature for maximizing PG yield. The most
promising catalyst was found to be ruthenium supported on silica catalyst.
Acknowledgment This subject is supported by Korea Ministry of Environment as ‘‘Converging
technology project (202-091-001)’’.
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