Evaluation of antioxidants on the thermo-oxidative stability
of soybean biodiesel
Maria A. S. Rios•Francisco F. P. Santos•
Francisco J. N. Maia•Selma E. Mazzetto
Received: 4 January 2012/Accepted: 17 August 2012
? Akade ´miai Kiado ´, Budapest, Hungary 2012
oxidants (2,6-di-t-butyl-4-methylphenol (BHT)—synthetic
antioxidant, hydrogenated cardanol (HC), and alkyl
hydrogenated cardanol (AHC)—both derived from cashew
nut shell liquid) on the thermo-oxidative stability of the
soybean biodiesel. The antioxidants were added at con-
centrations of 200, 300, and 400 ppm, and the oxidative
stability of the biofuel with and without antioxidants were
investigated by thermogravimetric analysis (TG-DTG and
IPDT) and Metrohm 743 Rancimat per the EN 14112
method. The results showed that all antioxidants contrib-
uted for the thermo-oxidative stability of the soybean
biodiesel as follows: soybean biodiesel\soybean biodie-
sel ? BHT\soybean biodiesel ? HC\soybean biodie-
sel ? AHC. In the Rancimat method, the results showed
that the antioxidants influenced the biodiesel stability with
an increase of at least 71 %.
This work shows the evaluation of three anti-
CNSL ? Rancimat ? IPDT
Biodiesel is an alternative fuel derived from vegetable oils,
animal fats, or used frying oils [1–3]. This biofuel is made
from transesterification of the lipid with a simple alcohol,
such as methanol or ethanol, and can be used as a neat fuel,
or it can be blended with petroleum or Fischer–tropsch
diesel. Because many vegetable oils contain a significant
amount of fatty acids with double bonds, oxidative stability
is of concern, especially when storing biodiesel over an
extended period of time [4, 5].
The oxidation stability of biodiesel is a function of the
fatty acid composition, and decreases with higher con-
tents of linoleic and linolenic acids [6–8]. This structural
fact is a key to understanding both oxidative and thermal
instability of these esters. In linoleic acids, polyunsatu-
rated fatty acid chains contain two (C18:2) double bonds,
while linolenic acids contain three (C18:3) double bonds.
As a result, undesirable products, like gums, organic
acids, and aldehydes, formed as a result of degradation,
may cause injector and filter blockages resulting in
engine problems [8, 9].
Despite its many advantages, the biodiesel has a poor
thermo-oxidative stability. Its oxidation follows a free
radical mechanism that starts with the abstraction of a
hydrogen atom. The radical form (R?) rapidly reacts with
oxygen to form a peroxy radical via a free radical chain
reaction and the peroxy radical (ROO?) can gain a hydro-
gen atom to form a hydroperoxide (ROOH) [2, 10]. Once
that, these oxidation reactions can cause a negative effect
on the biodiesel performance, the biofuel industries have
been using antioxidants to improve the quality of their
esters. These additives, used in most formulations, control
the oxygen-initiated degradation of the esters, protecting
them from oxidation [11, 12].
M. A. S. Rios (&)
Departamento de Quı ´mica–Grupo de Inovac ¸o ˜es Tecnolo ´gicas e
Especialidades Quı ´micas–GRINTEQUI, Universidade Federal
do Piauı ´, Campus Ministro Petro ˆnio Portella, Teresina, PI CEP
F. F. P. Santos
Departamento de Engenharia de Produc ¸a ˜o–Grupo de Inovac ¸o ˜es
Tecnolo ´gicas e Especialidades Quı ´micas–GRINTEQUI,
Universidade Federal do Piauı ´, Campus Ministro Petro ˆnio
Portella, Teresina, PI CEP 64.049-550, Brazil
F. J. N. Maia ? S. E. Mazzetto
Departamento de Quı ´mica Orga ˆnica e Inorga ˆnica–Laborato ´rio de
Produtos e Tecnologia em Processos–LPT, Universidade Federal
do Ceara ´, Campus do Pici, Bloco 935, Fortaleza, CE CEP
J Therm Anal Calorim
Antioxidants are organic compounds that are added to
oxidizable organic materials to retard oxidation and, in
general, to prolong the useful life of the substrates .
These additives are classified as either radical trapping
(chain breaking) or peroxide decomposing, terms that
describe the mechanisms by which they function. The
radical-trapping antioxidants, such as hindered phenols and
secondary aromatic amines react with oxygen radicals
(peroxy and alkoxy), while phosphites and phosphates
function as peroxide decomposing by abstracting peroxidic
oxygen from hydroperoxides and peroxides and reducing
them [14, 15].
Among the radical-trapping antioxidants, the hindered
phenol is one of the most important types of compound of
this class. Thus, the present work evaluated three antioxi-
dants of this class [2,6-di-t-butyl-4-methylphenol (BHT),
hydrogenated cardanol (HC), and alkyl hydrogenated
cardanol (AHC)] on the thermo-oxidative stability of soy-
bean biodiesel . Each antioxidant was added at con-
centrations of 200, 300, and 400 ppm. The oxidative
stability of biodiesel with and without antioxidant was
investigated by thermogravimetric analysis (TG-DTG) and
Metrohm 743 Rancimat per the EN 14112 method. The
integral procedure decomposition temperature (IPDT) was
determined in this study too . The antioxidants HC and
AHC were derived from cashew nut shell liquid (CNSL),
an important regional biomass.
Materials and methods
Hydrogenated cardanol (HC) and AHC were supplied by
Laborato ´rio de Inovac ¸o ˜es Tecnolo ´gicas e Especialidades
Quı ´micas–GRINTEQUI–Brazil. The HC and AHC (Fig. 1)
were purified by column chromatography on silica gel
using hexane as eluant . Refined soybean oil was
obtained in the local trade and the soybean biodiesel was
synthesized by catalytic transesterification [2, 17]. The
reagents and solvents were supplied by Aldrich (analytical
Synthesis of soybean biodiesel
Biodiesel from soybean oil was synthesized by the catalytic
transesterification of soybean oil using methanol as ali-
phatic alcohol (molar ratio 1:8—oil:alcohol) and KOH as
base . The mixture was heated under reflux for 1 h and
was monitored by thin-layer chromatography. After this
time, the mixture was poured into a separating funnel and
for difference of absolute density; the biodiesel was sepa-
rated of the glycerin (major by-product). The light phase
(rich in biodiesel) was separated, washed with hydrochloric
acid solution (5 %) and water, dried with anhydrous
sodium sulfate, and concentrated using rotary vacuum
evaporator at 70 ?C (±1 ?C). The biodiesel was charac-
terized by GC–MS, TG-DTG, and Rancimat per the EN
Formulation antioxidant/soybean biodiesel
The soybean biodiesel, obtained by the methyl route, was
additivated with BHT, HC, and AHC antioxidants at con-
centrations of 200, 300, and 400 ppm by simple mixtures.
The soybean biodiesel analysis was carried out using a
GC–MS system. The configuration used here was the
(30 m 9 0.25 mm id 9 0.25 lm film) used an oven tem-
perature program that initiated data collection at a tem-
perature of 100 ?C and ramped at 10 ?C min-1to 300 ?C,
holding this temperature for the remaining duration of the
data collection. Electron impact (EI, 70 eV) mode was
used and sample of 1 lL was injected into the column.
Table 1 provides an overview of all the instrument
Thermoanalytical measurements were obtained in a
Mettler Toledo TGA/SDTA85 using alumina pans and a
heating rate of 10 ?C min-1in the temperature range
25–800 ?C, and mass of approximately 10 mg. The sam-
ples were carried out in synthetic air atmosphere
(50 mL min-1).
The oxidative stabilities of the samples were determined
by a Metrohm 743 Rancimat per the EN14112 method. In
this test, a 10 L h-1stream of dry air is bubbled into 3 g
samples maintained at 110 ?C, volatile oxidation products
are carried through the detector chamber containing
deionized water. The change in conductivity is measured
and recorded every 36 s. The increase in conductivity is
measured as a function of time until maximal change which
reflects the oxidative stability.
Fig. 1 Chemical structures of hydrogenated cardanol (HC) and alkyl
hydrogenated cardanol (AHC)
M. A. S. Rios et al.
Results and discussion
GC–MS, TG-DTG, IPDT, and Rancimat—soybean
Figure 2 presents an identified chromatogram with fatty
acid methyl esters present in the soybean biodiesel.
According to the results, the chromatogram denotes the
preponderance of methyl palmitate, methyl linoleate,
methyl cis-9-octadecanoate, and methyl octadecanoate in
the mixture of methyl esters (Table 2). The GC–MS peak
report showed that the overall amount of methyl esters was
100 %, what confirms the efficiency of the purification
process carried out after the biodiesel synthesis, once that,
according to the European Standard EN 14103, the overall
ester content must be higher than 96.5 % [1, 9].
The TG-DTG curves of soybean biodiesel show that the
sample is thermally stable up to 175 ?C (Fig. 3), between
175 and 350 ?C, the DTG curve shows mass loss occurring
in one degradation event. The Figs. 4, 5, and 6 show the
DTG curves of the formulations of antioxidant/soybean
biodiesel. According to the results, it is noteworthy that
soybean biodiesel exhibits oxidative stability smaller than
the formulations (Tmax). Table 3 summarizes the thermal
parameters of soybean biodiesel with and without antiox-
idants. This thermo-oxidative study reported the relative
stabilities of the samples, which are as follows: soybean
biodiesel\soybean biodiesel ? BHT 200 ppm\soybean
biodiesel ? HC 200 ppm\soybean
200 ppm; for the other proportion of 400 ppm, the samples
proceeded with the same behavior, when evaluated the
second event. This result is compatible with the literature of
the hindered phenolic antioxidants [18–20], which says that
their bulky substituents influence the specificity of the
phenols by blocking phenoxyl radicals from abstracting
hydrogen atoms from organic substrates , and AHC
possesses two substituents in its ring, in the ortho and meta
positions. The first and second events are due to volatiliza-
tion of methyl esters.
The values of integral procedural decomposition tem-
perature (IPDT) calculated by Doyle’s  method are in
the range of 226–260 ?C. These values represent an overall
thermal stability of the materials. The IPDT is calculated as
biodiesel ? AHC
Table 1 Instrument parameters of GC–MS analysis
Inlet temperature250.0 ?C
Column inlet pressure72.3 kPa
1.0 mL min-1
50.0 mL min-1
37.2 cm s-1
Equilibrium time1.0 min
Fig. 2 Chromatogram of soybean biodiesel
Table 2 GC–MS peak report
dm/dT/ mg °C–1
Fig. 3 TG-DTG curves of soybean biodiesel in air atmosphere
Thermo-oxidative stability of soybean biodiesel
ðÞ ¼ A?? K?Tf? Ti
ð Þ þ Ti
where A* is the area ratio of total experimental curve
dividedby total TG
(D1? D2? D3)], K* is the coefficient [(D1? D2)/(D1)],
Ti the initial experimental temperature and Tf the final
experimental temperature. Figure 7 shows a schematic
representation of D1, D2, and D3for calculation of the
parameters A* and K*. According to the results (Table 3),
the formulations that present the better stabilities were:
soybean biodiesel ? AHC 200 ppm and soybean biodie-
sel ? AHC 400 ppm. This result confirms once again the
potentiality of the AHC. On the other hand, the BHT
presented low stability potential, probably due to its com-
plete vaporization [5, 22].
Soybean biodiesel + HC 200 ppm
Soybean biodiesel + HC 400 ppm
Fig. 4 DTG curves of soybean biodiesel ? HC 200 and 400 ppm in
Soybean biodiesel + AHC 200 ppm
Soybean biodiesel + AHC 400 ppm
Fig. 5 DTG curves of soybean biodiesel ? AHC 200 and 400 ppm
in air atmosphere
Soybean biodiesel + BHT 200 ppm
Soybean biodiesel + BHT 400 ppm
Fig. 6 DTG curves of soybean biodiesel ? BHT 200 and 400 ppm
in air atmosphere
Table 3 Thermal parameters of soybean biodiesel with and without
Soybean biodiesel ? HC 200 ppmI140241
Soybean biodiesel ? HC 400 ppmI140260
Soybean biodiesel ? AHC 200 ppmI144258
Soybean biodiesel ? AHC 400 ppmI144260
Soybean biodiesel ? BHT 200 ppmI251241
Soybean biodiesel ? BHT 400 ppmI252226
Residual mass/ %
Fig. 7 Schematic representation of D1, D2, and D3to determining the
M. A. S. Rios et al.
0.00.51.0 1.5 2.02.5 3.03.5
4.0 4.55.0 5.5 6.06.5
0.0 0.51.0 1.5 2.02.53.03.54.0 4.55.0 5.5 6.0
0.00.5 1.01.52.0 2.53.03.54.0
Fig. 8 Rancimat data of
soybean biodiesel (a), soybean
biodiesel ? HC 300 ppm (b),
soybean biodiesel ? AHC
300 ppm (c) and soybean
biodiesel ? BHT 300 ppm (d)
Thermo-oxidative stability of soybean biodiesel
For the induction period (IP) determination, the samples
were analyzed by Rancimat (EN14112) method. This
accelerated oxidation test is used frequently for predicting
the oxidative stability (shelf life) of fats, oils, and biofuels,
and the efficacy of antioxidants for increasing their sta-
bilities [2, 13] (Fig. 8). Table 4 summarizes the parameters
of oxidative stability of the samples. In this procedure, the
soybean biodiesel was additivated with BHT, HC, and
AHC antioxidants at concentration of 300 ppm, an average
of the extreme values of 200 and 400 ppm.
The addition of the antioxidants, HC, AHC, and BHC in
the soybean biodiesel at concentration of 300 ppm, influ-
enced the biodiesel stability with an increase of at least
71 % (HC antioxidant), as determined by the Rancimat
method (Table 4). The IP of the soybeanbiodieselincreased
101.7 % when 300 ppm of BHT was added, and the offered
protection by AHC when 300 ppm was added was of
111.9 %. According to the results, although the induction
periods of the samples have presented values below 6 h, the
addition of the antioxidants in low concentrations in the
biodiesel was very important for its oxidative stability.
Once that this work presented promising results, the authors
are working in formulations with the use of larger con-
centrations of the antioxidants HC, AHC, and BHT. The
results will be presented in another paper.
The presence of methyl esters from the unsaturated fatty
acids in the soybean biodiesel denotes its low induction
period. In spite of this result, the addition of the antioxidants
HC, AHC, and BHC in the soybean biodiesel at concen-
trations of 200, 300, and 400 ppm, influenced the biodiesel
stability. The TG-DTG curves of soybean biodiesel shows
that the sample is thermally stable up to 175 ?C and the
thermo-oxidative study reported that the relative stability of
the samples are as follows: soybean biodiesel\soybean
biodiesel ? BHT\soybean
biodiesel ? AHC. The results obtained by TG-DTG, IPDT,
and Rancimat method were consistent in terms of the
potentiality of AHC. The TG-DTG and IPDT were used as
alternative techniques to determination of the oxidative
stability of the soybean biodiesel.
biodiesel ? HC\soybean
476568/2010-2) and FAPEPI for the financial support, and the
Organic and Inorganic Chemistry Department of Federal University
of Ceara ´ by chemical and thermal analyses.
The authors acknowledge CNPq (Process No.
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Soybean biodiesel ? AHC 300 ppm4.98
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Thermo-oxidative stability of soybean biodiesel