C-037 (P)
The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
21-23 November 2006, Bangkok, Thailand
Thermal Stability of Biodiesel Fuel as Prepared by Supercritical Methanol Process
Hiroaki Imahara, Eiji Minami, Shusaku Hari and Shiro Saka*
Graduate School of Energy Science, Kyoto University, Kyoto, Japan
Abstract: Non-catalytic biodiesel production technology from oils/fats in vegetables and animals has been developed in our
laboratory employing supercritical methanol. However, due to conditions in high temperature and high pressure of the supercritical
fluid, biodiesel prepared may possibly be thermally degraded. In this study, therefore, thermal stability of fatty acid methyl esters and
actual biodiesel fuels from various feedstocks was studied in supercritical methanol, and discussed the effect of thermal degradation
on fuel properties, mainly cold flow properties. As a result, it was found that poly-unsaturated methyl esters were partly decomposed
and isomerized from cis-type to trans-type at the temperature higher than 350oC. These behaviors were also observed for actual
biodiesel fuels prepared from linseed and safflower oils, which are high in poly-unsaturated fatty acids. On the other hand, their
temperatures of cloud point and pour point remained almost unchanged after supercritical methanol exposure. From these results, it
was clarified that biodiesel is not deteriorated in terms of cold flow properties after exposure to supercritical methanol.
Keywords: Biodiesel, Fatty Acid Methyl Esters, Supercritical Methanol, Cis-trans isomerization, Cold Flow Properties
1. INTRODUCTION
For pursuit of sustainable energy, biodiesel fuel (fatty acid methyl esters) has been given much attention as a substitute for fossil
diesel fuel. Currently, biodiesel is commonly produced by alkali-catalyzed method. In this method, however, waste oils/fats rich in
free fatty acids and water are difficult to be utilized efficiently since the former results in producing undesirable saponified products,
while the latter hinders complete conversion of oils/fats.
The non-catalytic supercritical methanol technologies are attractive processes to overcome such problems. Our research group has
developed the one-step supercritical methanol method (Saka process) in which oils/fats can be converted into biodiesel fuel through
transesterification. However, this process requires rather high temperature and high pressure conditions (350oC/20~50MPa) [1-4].
Although the two-step method (Saka-Dadan process), which consists of oils/fats hydrolysis in subcritical water and following
methyl esterification in supercritical methanol, can offer relatively moderate reaction conditions (<300oC/7~20MPa) [5], these are
still higher than those of the conventional alkali-catalyzed method. In such severe conditions of supercritical fluid, thermal stability
of biodiesel is a major concern. As is well known, especially, poly-unsaturated fatty acids are rather reactive and thus vulnerable to
denaturations such as oxidation and cis-trans isomerization.
In this study, therefore, thermal stability of various kinds of fatty acid methyl esters was studied in supercritical methanol to
discuss its effect on fuel properties of biodiesel, focusing on fuel cold properties such as cloud point and pour point.
2. MATERIALS AND METHODS
As main components of biodiesel, four kinds of fatty acid methyl esters (methyl stearate (18:0), oleate (18:1), linoleate (18:2) and
linolenate (18:3) purchased from Aldrich-Sigma) were, respectively, exposed to supercritical methanol. These unsaturated fatty acid
methyl esters used in this study have only cis-type double bonds since unsaturated fatty acids are cis-type only in nature. The
exposure was made by placing fatty acid methyl esters in methanol using a 5mL batch-type reaction vessel made of Inconel-625 at
temperatures between 270 and 380oC for a designated reaction time. After the exposure, the obtained reactant was directly analyzed
by high performance liquid chromatography (HPLC) with a Shimadzu LC-10A system under the following conditions: column,
Cadenza CD-C18 (ID4.6mm x L250mm); flow rate, 0.8mL/min; eluent, methanol; detector, refractive index detector; temperature,
40oC. The remaining sample was then evaporated at 70oC for 20min to remove methanol on a rotary evaporator and analyzed by
Fourier transform infrared spectrometry (Shimadzu, FT-IR 8300).
As actual biodiesel samples, on the other hand, linseed oil, safflower oil, rapeseed oil and palm oil were chosen as these oils have
different chemical compositions of fatty acids in their triglycerides as in Table 1. These oils were converted into fatty acid methyl
esters by alkali-catalyzed method reported by Freedman et al. [6]. The obtained esters were then subjected to a similar exposure to
supercritical methanol and subsequent analysis mentioned above. In addition, cloud point and pour point were measured by a mini
pour/cloud point tester (TANAKA Scientific, MPC-102).
3. RESULTS AND DISCUSSION
3.1 Thermal stability of fatty acid methyl esters
To study the thermal stability of fatty acid methyl esters, fatty acid methyl ester itself was evaluated for its stability against an
exposure to supercritical methanol at 350oC/43MPa. As a result, in Fig.1, methyl stearate (18:0) was found out to be stable for the
first 20 min and slightly decomposed in further reaction time. Methyl oleate (18:1) was also stable but a little more decreased in its
amount. In the case of methyl linoleate (18:2), however, a monotonous decrease was obvious to be approximately 75wt% in 40 min,
and even more in the case of methyl linolenate (18:3). These results clearly demonstrate that poly-unsaturated fatty acid is much
more vulnerable to thermal treatment than mono-unsaturated one.
Therefore, methyl linolenate was studied more in detail over various exposure conditions as in Fig.2. At 270 and 300oC, thermal
Corresponding author: saka@energy.kyoto-u.ac.jp
This paper is the revised manuscript presented at the 14th European Biomass Conference held in Paris, France in October 2005,
being planned to submit as an original paper to the Journal of the Japan Institute of Energy.
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C-037 (P)
The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
21-23 November 2006, Bangkok, Thailand
degradation of methyl linolenate could not be significant even after 60 min of exposure. Therefore, it seems likely that methyl
linolenate is rather stable in supercritical methanol at the temperatures below 300oC.
Under the temperature above 320oC, on the other hand, the degradation became noticeable when an exposure was prolonged. This
decomposition was getting more obvious with a rise in temperature; at 350oC for 20 min, 20 wt% of methyl linolenate remained,
while at 380oC for 15 min, only 10 wt%. From these results, it seems apparent that methyl linolenate is stable in supercritical
methanol below 300 oC, but unstable above 320oC.
100
18:0
18:1
80
Recovery (wt%)
18:2
60
40
18:3
20
0
0
10
20
30
40
Exposure time (min)
50
60
Fig. 1 Recovery of various fatty acid methyl esters (cis-type plus trans-type)
as exposed to supercritical methanol at 350oC/43MPa
100
270oC/17MPa
Recovery (wt%)
80
300oC/19MPa
60
320oC/29MPa
40
350oC/43MPa
20
0
380oC/56MPa
0
10
20
30
40
Exposure time (min)
50
60
Fig. 2 Recovery of methyl linolenate (cis-type plus trans-type) as exposed
to supercritical methanol at various reaction conditions
3.2 FT-IR analyses for fatty acid methyl esters
To evaluate the fatty acid methyl esters and actual biodiesel after exposure to supercritical methanol, FT-IR analyses were carried
out as shown in Fig.3. It is apparent that for each fatty acid methyl ester, IR absorbance peaks for ester and alkyl groups, described by
C=O stretching, C-H deformation, C-O stretching and (CH2)4 skeleton, remained unchanged after exposure to supercritical methanol.
These results indicate that these groups are stable after supercritical methanol exposure at 350 oC.
In the case of unsaturated fatty acid methyl esters, on the other hand, the absorbance peak for C=C (cis) stretching was observed at
the wavenumber around 690cm-1[7]. With regard to methyl oleate, the peak remained unchanged even after 40min of exposure
(Fig.3(b)). For the other unsaturated esters, the absorbance peak for C=C (trans) stretching was newly formed as observed at 970cm-1
[7] after supercritical methanol exposure (Fig.3(c, d)). For methyl linoleate, the peak was getting higher with increasing exposure
time, followed by decrease in the peak for C=C (cis). For methyl linolenate, in contrast, both peaks corresponding to C=C decreased
in 40min of exposure in supercritical methanol.
Fig.4 shows the possible pathway of the cis-trans isomerization for methyl linoleate (18:2). At first, a hydrogen atom is
incidentally withdrawn from poly-unsaturated methyl ester (LH) producing a radical species (L·). If oxygen exists in the reaction
system, then, it reacts with the radical species to form hydroperoxide (LOO·) accompanied by geometrical isomerization. Without
oxygen, however, only the isomerization occurs as in Fig.4. As is well known in lipids chemistry, a hydrogen atom at the position of
methylene group between two double bonds can be withdrawn more easily than that in other methylene groups. Therefore, methyl
oleate, which has only one double bond, is more stable as in saturated methyl esters than poly-unsaturated ones such as methyl
linoleate and linolenate.
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The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
21-23 November 2006, Bangkok, Thailand
C-037 (P)
C=O
stretching
C-O
stretching
C=O
stretching
C-H
deformation
C-O
stretching
(CH2)4
skeleton
C-H
deformation
(CH2)4
skeleton
40min
40min
20min
20min
0min
2000
C=C
(cis)
b) Methyl oleate (18:1)
a) Methyl stearate (18:0)
0min
1800
1600
1400
1200
1000
800
600
2000
1800
1600
C=C
(cis)
c) Methyl linoleate (18:2)
C-O
stretching
C=O
stretching
1400
1200
1000
800
600
Wavenumber (cm-1)
Wavenumber (cm-1)
C=C
(trans)
C-H
deformation
C=C
(cis)
d) Methyl linolenate (18:3)
C=O
stretching
(CH2)4
skeleton
C-O
stretching
C=C
(trans)
C-H
deformation
(CH2)4
skeleton
40min
40min
20min
20min
0min
2000
0min
1800
1600
1400
1200
1000
800
600
2000
1800
Wavenumber (cm-1)
1600
1400
1200
1000
800
600
Wavenumber (cm-1)
Fig. 3 FT-IR spectra of various methyl esters as exposed to supercritical methanol at 350oC/43MPa
12
9
B
A
●
●
●
trans-10, cis-12
cis-9, cis-12
cis-9, trans-11
●
●
●
●
trans-9, cis-12
trans-9, trans-11
cis-9, trans-12
trans-10, trans-12
●
trans-9, trans-12
Fig. 4 Possible cis-trans isomerization of methyl linoleate in supercritical methanol
(A, -(CH2)7COOCH3; B, -(CH2)4CH3)
3.3 FT-IR analyses for biodiesel (BDF)
To investigate the effect of thermal degradation on actual biodiesel, BDF from rapeseed oil, safflower oil, linseed oil and palm oil
were studied in supercritical methanol. As shown in Table 1, rapeseed oil is rich in unsaturated fatty acid content, especially in oleic
acid; safflower oil and linseed oil are also rich in unsaturated fatty acid content, but especially in linoleic and linolenic acid,
respectively. Palm oil, on the other hand, is high in saturated fatty acid content, namely palmitic acid. These oils were chosen as a
representative of oleic acid, linoleic acid, linolenic acid and palmitic acid.
Fig.5 shows FT-IR spectra of biodiesel fuels as exposed to supercritical methanol at 350oC. With regard to palm oil BDF,
formation of the peak corresponding to C=C (trans) was not observed as in Fig.5(a). In the case of rapeseed oil BDF, safflower oil
BDF and linseed oil BDF, in contrast, formation of C=C (trans) was confirmed after 20 min in Fig.5(b, c and d) as in a similar
manner to the poly-unsaturated methyl esters mentioned above.
These behaviors are due to the difference in fatty acid composition of oils. Since most of the vegetable oils contain higher amount
of poly-unsaturated fatty acids, it tends to be vulnerable to thermal degradation. However, some of them are rich in saturated fatty
acid content as in the case of palm oil and thereby become more stable than other oils.
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The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
21-23 November 2006, Bangkok, Thailand
C-037 (P)
Table 1 Various oils used in this study and their chemical composition of fatty acids in triglycerides
Oils and fats
Fatty acid composition (wt%)
16:0
18:0
18:1
18:3
Others
Palm oil
39.5
4.1
43.2
10.6
00.2
2.4
Rapeseed oil
04.3
1.9
61.5
20.6
08.3
3.4
Safflower oil
06.4
2.2
13.9
76.0
00.2
1.3
Linseed oil
06.7
3.7
21.7
15.8
52.1
0.0
C=C
(cis)
a) Palm oil BDF
C-O
stretching
C=O
stretching
C=O
stretching
C-O
stretching
C-H
deformation
C=C
(trans)
40min
40min
20min
20min
C=C
(trans)
(CH2)4
skeleton
0min
0min
1800
1600
1400
1200
1000
800
600
2000
1800
1600
Wavenumber (cm-1)
C=O
stretching
1400
1200
1000
800
600
Wavenumber (cm-1)
C=C
(cis)
c) Safflower oil BDF
C-O
stretching
C=C
(trans)
C-H
deformation
C=C
(cis)
d) Linseed oil BDF
C=O
stretching
(CH2)4
skeleton
C-O
stretching
C-H
deformation
40min
40min
20min
20min
0min
2000
C=C
(cis)
b) Rapeseed oil BDF
(CH2)4
skeleton
C-H
deformation
2000
18:2
C=C
(trans)
(CH2)4
skeleton
0min
1800
1600
1400
1200
1000
800
600
2000
1800
Wavenumber (cm-1)
1600
1400
1200
1000
800
600
Wavenumber (cm-1)
Fig. 5 FT-IR spectra of biodiesel fuels from (a) palm oil, (b) rapeseed oil, (c) safflower oil and (d) linseed oil as exposed to
supercritical methanol at 350oC/43MPa.
3.4 Cold flow properties of biodiesel
The cis-trans isomerization mentioned above may have adverse effect on cold properties of biodiesel since trans-isomer generally
has higher melting point than cis-isomer. For instance, a melting point of α-linolenic acid methyl ester, which exists in nature with
having three cis-type double bonds, is lower than -50oC, whereas that of linolenelaidic acid methyl ester, which has three trans-type
ones, is about 16oC [8,9]. Therefore, the effect of supercritical methanol exposure on cold flow properties should be investigated.
To evaluate the effect of supercritical methanol exposure on cold flow properties, cloud point and pour point were analyzed and
the obtained results are shown in Table 2. It can be seen from these results that both cloud point and pour point of biodiesel from
linseed oil increase slightly by 2oC from –5oC to –3oC and –6oC to –4oC, respectively after 40 min of exposure. Similar behavior can
be seen in the case of biodiesel from safflower and rapeseed oil. With regard to biodiesel from palm oil, in contrast, both cloud point
and pour point remain unchanged even after 40min of exposure. Taking it into account that these measurements are conducted with
an interval of ±1oC, these results clearly indicate that thermal degradation and cis-trans isomerizaiton have little effect on cold flow
properties.
Table 2 Cold flow properties of biodiesel fuels prepared from various
oil/fat feedstocks as exposed to supercritical methanol (350oC, 43MPa)
Oils and fats
Palm oil
Rapeseed oil
Exposure time
Cloud point
Pour point
(min)
(oC)
(oC)
0
11
11
20
11
11
40
11
11
0
-7
-17
20
-7
-17
4
C-037 (P)
The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
21-23 November 2006, Bangkok, Thailand
Safflower oil
Linseed oil
40
-6
-15
0
-6
-8
20
-6
-8
40
-4
-6
0
-5
-6
20
-4
-5
40
-3
-4
4. SUMMARY
For high-quality biodiesel production by supercritical methanol process, its degradation behaviors were studied in supercritical
methanol by exposing fatty acid methyl ester and biodiesel itself to supercritical methanol. As a result, it was found that polyunsaturated methyl esters such as methyl linoleate (18:2) and methyl linolenate (18:3) were partly degraded and isomerized into
trans-type one with a rise in treatment temperature, especially above 300oC. Moreover, these changes were more likely to occur for p
oly-unsaturated fatty acids with higher degree of unsaturation. The effect of thermal degradation and cis-trans isomerization on cold
flow properties is, however, not noteworthy as exposed to supercritical methanol. From these lines of evidence, it was clarified that
biodiesel is not deteriorated in terms of cold flow properties by the supercritical methanol treatment.
5. ACKNOWLEDGMENTS
This work has been done in the Kyoto University 21 COE program “Establishment of COE on Sustainable-Energy System”,
Grant-in-Aid for Scientific Research (B) (2) (No.13556058, 2001.4~2003.3) from the Ministry of Education, Science, Sports and
Culture, Japan, and in part in NEDO “High Efficiency Bioenergy Conversion Projects”, for all of which the authors are highly
acknowledged.
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