Renewable and Sustainable Energy Reviews xxx (xxxx) xxx–xxx
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Renewable and Sustainable Energy Reviews
journal homepage: www.elsevier.com/locate/rser
Screening of antioxidant additives for biodiesel fuels
⁎
K. Varatharajana, , D.S. Pushparanib
a
b
Department of Mechanical Engineering, Velammal Engineering College, Surapet, Chennai 600066, India
Department of Biochemistry, SRM University, Ramapuram, Chennai 600089, India
A R T I C L E I N F O
A BS T RAC T
Keywords:
Biodiesel stability
Antioxidants
Phenolic
Aromatic amine
Chelating agents
Fuel additives
One of the major issues associated with the use of biodiesel is to maintain the fuel at specified standards for a
longer period. Biodiesel is more prone to oxidization than a mineral diesel, and it starts turning rancid within a
week or less, and complete degradation occurs after a period of 4 weeks. The final products of oxidation alter the
physical and chemical properties of fuel which results in the formation of insoluble gums that can plug fuel
filters. This instability of biodiesel is a long-standing issue, and it has not as yet been satisfactorily resolved. As
the use of biodiesel has increased massively, this oxidation issue could become a significant barrier to market
expansion. One very promising and cost-effective approach to improving the stability of biodiesel is that the
addition of appropriate antioxidants to the biodiesel fuels. Antioxidants perform better or worse in different
biodiesel fuels, and there is no unique inhibitor that suit for every kind of biodiesel fuels. To screen the
antioxidants for a specific biodiesel, it is necessary to understand the chemistry of antioxidants and the key
factors that influence their effectiveness against biodiesel oxidation. Most of the published studies had been
carried out solely with some common antioxidants, and still, hundreds of antioxidants are out there for testing.
This article provides an insight into the factors to be considered for the selection of antioxidants to improve the
storage stability of biodiesel fuels.
1. Introduction
1.1. Factors affecting the oxidation stability
Biodiesel is a renewable fuel that reduces engine emissions and
provides greater lubrication when compared to mineral diesel. Despite
its many advantages, it has a poor oxidation stability and increased
NOx emitting tendency over the conventional diesel fuel. Oxidation of
biodiesel leads to the formation of hydroperoxides which can produce
insoluble gums and sediments that can plug fuel filters or make
deposits on the fuel injector. The final products of oxidation also
increase the viscosity of the fuel that leads to poor fuel atomization.
This consequently leads to the biodiesel getting into the crankcase and
making sludge with the lubricating oil which can lead to catastrophic
engine failure [1]. In general, biodiesel is considered as a safe fuel
because of its higher flash point. However, in oxidized state, it reacts
with water and makes micro-explosion that can cause fire hazards [2].
Karavalakis et al. [3] observed a sharp increase in emissions of human
carcinogens such as formaldehyde, acetaldehyde, acrolein, and PAHs
when the engine is fueled with oxidized biodiesel blends. Moreover, the
unsaturated biodiesel fuels have more reactivity towards oxygen that
leads to the formation of more prompt NO during combustion which
results in increased NOx emissions [4].
Hydrolytic, ketonic, microbiological and oxidative deterioration are
the common degradation processes of fatty acids. It is generally
accepted that, apart from microbial deterioration, oxidation is the
primary process by which fatty acid or its ester degrades [5].
Autoxidation, photooxidation, thermal and enzymatic oxidation are
the major oxidation processes that cause quality deterioration in
biodiesel fuels. Among all, autoxidation is the most common process
and it is defined as the spontaneous free radical reaction of fatty acids
with atmospheric oxygen [6]. Factors that influence the oxidation rates
of biodiesel fuels include the FAME composition and its structure,
presence of natural antioxidants, exposure to heat, light, air and
moisture, the presence of metal ion catalysts, enzymes and other
impurities [7].
⁎
1.1.1. Chemical composition
There are two types of fatty acids, viz. saturated fat and unsaturated
fat. The stearic, palmitic and hydroxystearic acids are saturated fatty
acid group while the oleic, linoleic, ricinoleic, palmitoleic, linolenic and
eicosenoic acids are unsaturated fatty acid group [8]. In general, the
rate of oxidation of saturated fatty acids is extremely slow when
Corresponding author.
E-mail address: varathas11@gmail.com (K. Varatharajan).
http://dx.doi.org/10.1016/j.rser.2017.07.020
Received 19 August 2016; Received in revised form 2 March 2017; Accepted 6 July 2017
1364-0321/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Varatharajan, K., Renewable and Sustainable Energy Reviews (2017), http://dx.doi.org/10.1016/j.rser.2017.07.020
Renewable and Sustainable Energy Reviews xxx (xxxx) xxx–xxx
K. Varatharajan, D.S. Pushparani
1.1.4. Presence of transition metals
All biodiesel fuels contain small amounts of transition metals such
as copper, iron, and nickel that accelerate the rate of autoxidation even
at quite low concentrations. Iron and copper are the most common
transition metals found in biodiesel fuels and they decompose the
peroxides into free radicals. When the transition metal ions in their
lower valence state (Mn + ), it reacts rapidly with hydroperoxides
(ROOH) and produces free radicals RO* and ROO* [18].
compared to unsaturated ones and their contribution to the oxidation
of biodiesel has usually been considered insignificant. Therefore, fuel
stability research is primarily focused on the study of the reactions of
unsaturated fatty acids. Though Iodine Value (IV) is an indicator of
unsaturation, it cannot be a predictor of oxidative stability [9]. Knothe
and Razon [9] reported that the oxidation rates of biodiesel increases
with the total number of the bis-allylic sites (methylene CH directly
adjacent to the two double bonds) in its structure and not with the total
number of double bonds or Iodine value. The allylic position equivalent
(APE) and bis-allylic position equivalent (BAPE) value are important
parameters that decide the oxidation stability which can be calculated
by using the following relations [10].
APE = 2 (A C18:1 + A C18:2 + A C18:3)
(1)
BAPE = (A C18:2 + A C18:3)
(2)
ROOH + Mn+ → RO* + −OH + M(n+1)+
(3)
ROOH + M(n+1)+ → ROO* + H+ + Mn+
(4)
The Eq. (4) shows that metal ions are regenerated and generates
more RO* and ROO* radicals which result in a reduction of the
induction period (IP). This catalytic effect can be suppressed with the
use of Metal chelators or deactivators. They form catalytically inactive
complexes with the metal cations making them unavailable to promote
oxidation.
Where A is the amount of each fatty acid compound.
C18:1 – Oleic acid
C18:2 – Linoleic acid
C18:3 – Alpha-linolenic acid
1.1.5. Other factors
It has long been known that exposure to light accelerates the
oxidation rate of organic materials. In liquids like biodiesel, light
penetrates in depth which results in larger portions of FAME become
deteriorated. The carotenoids naturally present in vegetable oils helps
to protect oils against light-induced oxidation [19]. The biodiesel
sensitivity to light depends on the type and amounts of sensitizers
present, FAME composition and other constituents. The pigments such
as chlorophylls and pheophytins act as a photo-sensitizer when
exposed to light and promote photooxidation and in the dark, they
show antioxidant activity [20]. Biodiesel with a high content of monoand diglycerides or glycerol absorb more water and is potentially
subject to hydrolytic degradation. During this process, biodiesel is
reconverted into alcohols and free fatty acids [21]. High initial acid
value, free fatty acid content and humid and warm climate aggravate
this hydrolytic reaction. The water content of biodiesel reduce its
calorific value, increase corrosion rate and can serve as a breeding
ground for microbes [21].
The BAPE value is the more significant than the APE value since a
higher rate of oxidation occurs in bis-allylic positions only [9]. The
linoleic and linolenic acids contain bis-allylic sites in their structure
(linoleic – 1 and linolenic – 2) and are more susceptible to oxidation
than saturated acids. The relative oxidation rates of the fatty acid series
stearic, oleic, linoleic and linolenic acid are to be in the ratio of
1:100:1200:2500 [11]. Kumar et al. [12] reviewed the possibilities of
improving the stability of biodiesel by increasing the saturated fatty
acid content of the feedstock. However, higher saturation leads to poor
cold flow that can cause choking of fuel lines and filters [13].
1.1.2. Presence of natural antioxidants
The results of many researches show that even highly saturated
ester loses its stability rapidly if it contains a low level of natural
antioxidants. Though the biodiesel fuels produced from land animal
fats such as lard and beef tallow contain large amounts of saturated fats
they have poor stability because they contain a lesser amount of natural
antioxidants [14]. Moreover, Lima et al. [15] reported that the highly
unsaturated Amazonian buriti oil (80% unsaturation) has more oxidation stability irrespective of its high oleic acid content because of the
presence of large amounts of tocopherols and carotenoids. Most of
these natural antioxidants might be reduced or destroyed during the
transesterification or in the refining process.
1.2. Methods to improve the stability of biodiesel fuels
A wide variety of oxidation inhibiting techniques is being pursued,
including vacuum technology, inert gas packaging, low-temperature
storage, enzyme deactivation, reducing the partial pressure of oxygen
in contact and the use of antioxidants. Structural modifications such as
changing the location of unsaturation closer to the ester head group,
lessening the number of double bonds, removal of hydroxyl groups and
the conversion of cis unsaturation to trans are some of the advanced
techniques to improve the stability of the biodiesel fuels [22]. Sundus
et al. [23] have discussed in detail about the methods to enhance the
oxidation stability of biodiesel. In order to avoid the water contact they
suggested using membrane technology for the purification of biodiesel.
Preventing or delaying the oxidation process with the use of small
quantity antioxidants is the most cost-effective way than any other
methods. Antioxidants donate an electron or hydrogen to the free
radicals to neutralize the oxidation reaction. There is no universal
antioxidant that prolongs the shelf life of all kinds of biodiesel fuels.
With the abundance of available antioxidants, it is necessary to find the
most appropriate antioxidants or its combination to optimize the
storage stability of each biodiesel fuel. The objective of this paper is
to identify the main factors that influence the effectiveness of antioxidants which can be considered in the screening of antioxidants for
biodiesel fuels.
1.1.3. Storage temperature
It has been well established that the rates of most oxidation
reactions increase when the temperature is increased. Typically,
oxidative stability measured by Rancimat method is based on the
principle that degradation rate linearly increases with temperature. Xin
et al. [16] reported that the degradation rate of biodiesel fuel is low
when it is stored at a lower temperature. On the other hand, in realworld conditions, low-temperature storage of fuels has more practical
difficulties and usually stored at ambient temperature. During the
storage periods, the fuels are not subjected to higher temperature and
the effect of temperature on oxidation is nominal. However, the engine
spray nozzles and needles, the combustion chamber walls and piston
are subjected to a higher temperature which leads to rapid oxidation of
fuels that cause deposits on them. The sludge deposition rate is high
within a range of temperature and is dependent on the type of fuel.
Beyond this range, sludge formation rate decreases due to the increase
of vapor pressure of the fuel and decrease in partial pressure of oxygen
[17].
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K. Varatharajan, D.S. Pushparani
ing the weak part of the FAME structure to form a hydroperoxide
(ROOH*) radical and an R*. The Bis-allylic hydrogen which has low
bond enthalpy is more prone to attack by peroxy radicals and collapses
the FAME structure.
2. Antioxidants
Antioxidants are compounds that delays, controls or inhibits
autoxidation processes of substrates and decrease the yields of
unwanted secondary products. There are two kinds of antioxidants:
chain breaking or primary antioxidants, and hydroperoxide decomposers or secondary antioxidants. Primary antioxidants (AH) delay or
interrupt the propagation reaction by donating a hydrogen atom to a
free radical (especially peroxyl radical ROO*). The hydrogen is released
from the active OH or NH groups of primary antioxidants and then
donated to the free radicals [24]. Substituted phenolic compounds,
secondary aromatic amines, and thiophenols are the common primary
antioxidants in use. The natural antioxidants tocopherols and flavonoids are also of this class.
The secondary antioxidants decompose the hydroperoxides or
replenish hydrogen to chain breaking antioxidants or scavenge singlet
oxygen, metal ions and pro-oxidative enzymes [25]. In recent years, the
hybrid or bifunctional antioxidants such as aminophenols, hydroxy
hydro quinolines, and hydroxyl benzimidazoles have been increasingly
popular because of their effectiveness in scavenging free radicals. They
contain both OH and NH groups in their molecular structure and can
donate more hydrogen to the peroxyl radicals [26]. Primary antioxidants cannot inhibit the light-induced photooxidation process. The
Hindered Amine Light Stabilizer (HALS) can be used to trap the free
radicals initiated by light or UV rays. However, HALS should be used
with caution since it interacts with phenolic antioxidant additives that
result in loss of antioxidants [27].
ROO* + RH → ROOH* + R*
The new R* from Eq. (3) can then react with diatomic oxygen to
make the peroxy radicals (Eq. (2)), and the chain reaction propagates.
The primary or chain breaking antioxidants (AH) reacts with this
peroxy (ROO*) and hydroperoxide (ROOH*) radicals and interrupts
the propagation reaction.
(8)
RO* + AH → ROH + A*
(9)
ROOH + A* → ROOA
(10)
ROH + A* → ROA
(11)
4. Screening of antioxidants for biodiesel fuels
Antioxidant additives should be added immediately after the
production of biodiesel; their addition will not be useful once the
biodiesel has started to get rancid. Ideal antioxidants for biodiesel
should possess the following requirements: (1) no toxicity; (2) low
volatility; (3) effective in low concentrations; (4) high thermal and light
stability; (5) availability; (6) high solubility in biodiesel; (7) long shelf
life; and (8) inexpensive [29]. Technically, an excellent antioxidant
should be an efficient free radical scavenger and at the same time, they
should not make any side reactions.
Autoxidation, photooxidation, thermal and enzymatic oxidation are
the major oxidation processes that cause quality deterioration in
biodiesel fuels. Among all, autoxidation is the most common process,
and it is the process of introduction of atmospheric oxygen in a C–R
bond to form a hydroperoxide. To understand the means by which an
antioxidant act it is necessary to know how the oxidation reaction
propagates. The stages involved in the autoxidation are an initiation,
propagation, and termination. During the transesterification process,
the fatty acid chain is not altered and therefore the degradation
mechanism of biodiesel should be the same as that of the source fatty
oils [28].
4.1. Type of antioxidants
Some antioxidants are highly effective in one kind of biodiesel but
quite ineffective in another. To screen the suitable antioxidants for a
specific biodiesel, it is essential to understand the types of antioxidants
and their properties. There are three basic types of antioxidants:
phenolic, amines and thiophenols. Most natural antioxidants contain
polyphenols, and they act similar to phenolic antioxidants. Due to
possible sulfur emissions, thiophenols are not used to improve the
stability of biodiesel.
3.1. Effects of antioxidants on the initiation reaction
In the initiation period, initiators such as heat, light, high energy
radiation, metal ion catalysts, humidity remove a hydrogen atom from
the unsaturated fatty acid structure and generate free radicals.
4.1.1. Phenolic antioxidants
In general, phenolic type of antioxidants is most commonly used in
biodiesel industry because of their cost, availability, and performance
[30]. The major phenolic antioxidants used to increase the stability of
biodiesel are butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), tertiary-butylhydroquinone (TBHQ), propyl gallate (PG),
octyl gallate (OG), dodecyl gallate (DG), pyrogallol (PY) and ethoxyquin
(EQ) [31].
Butylated hydroxytoluene (BHT) is one of the most volatile antioxidants of the phenolic groups and extremely soluble in fat and
insoluble in water. It is an efficient antioxidant for liquid hydrocarbons
(especially in gasoline fuel) and low-level biodiesel blends but is
ineffective in protecting neat biodiesel or high-level biodiesel blends.
Moreover, it is more effective in preserving animal fat than vegetable
oil. In combination with BHA and TBHQ or propyl gallate, it offers a
synergic effect. Rapid depletion resulting from high volatility and poor
thermal stability are the major drawbacks of BHT in biodiesel. The two
t-butyl groups of the BHT molecules offer greater steric hindrance that
affects its hydrogen donating ability [32]. However, the cost of BHT is
relatively cheaper than other phenolic antioxidants [33], and it can be
(5)
RH represents an organic substrate being oxidized (FAME), R* is a
carbon-centered alkyl radical derived from RH, and HO2* is hydroperoxyl radical. Primary antioxidants do not inhibit the initiation
reaction of the biodiesel degradation. However, the chelating agents
sometimes called antioxidant synergists can slow down the initiation
reaction by deactivating the metal ion catalysts.
3.2. Effects of antioxidants on the propagation reaction
The carbon-centered radicals (R*) formed in the initiation reaction
are highly reactive, and combines with available oxygen, giving peroxy
radicals (ROO*).
R* + O2 → ROO*
ROO* + AH → ROOH + A*
The resulting products are inactive and do not initiate further
oxidation reaction [24].
3. Antioxidant mechanism
RH + O2 → R* + HO2*
(7)
(6)
The peroxy (ROO*) radical is not as reactive as R*. However, it is
sufficiently reactive and quickly removes another hydrogen by attack3
Renewable and Sustainable Energy Reviews xxx (xxxx) xxx–xxx
K. Varatharajan, D.S. Pushparani
tert-Butylphenol [45] can also be considered as antioxidant additives
for biodiesel fuels.
used as a synergist with other antioxidants.
Butylated hydroxyanisole (BHA) is a white and waxy solid that is
highly soluble in oil and insoluble in water. It is a mixture of 3-tertiarybutyl-4-hydroxyanisole (90%), and 2-tertiary-butyl-4-hydroxyanisole
(10%) and its effectiveness are slightly better than BHT. The steric
hindrance effect is responsible for its poor effectiveness in vegetable
oils and at the same time, this effect helps to protect the OH group and
to enhance the thermal stability especially in animal fats [34]. BHA
inhibits the growth of some gram positive and gram negative bacteria
and yeasts which could help to prevent microbial contamination in oils
[35]. Furthermore, BHA is effective in improving the stability of short
chains fatty acids such as palm and coconut oils [36]. It can be used
either alone or in combination with BHT, TBHQ or gallates. In general,
BHA is less effective in animal fats but in combination with citric acid,
it performs better and its induction period is almost doubled. A
combination of 0.02% BHA and 0.01% citric acid is more effective in
improving the stability of yellow grease and tallow [36]. One of the
problems with the use of BHA is it reacts with alkaline metals such as
sodium or potassium present in the lipids and generates a pink color
[38].
Tertiary butyl hydroquinone (TBHQ) is a white, crystalline solid
moderately soluble in fats and oils and slightly soluble in water. It is
one of the most effective antioxidants in many types of biodiesel fuels.
TBHQ exhibits good synergism with BHT or BHA and not with propyl
gallate (PG) [32]. Chelating agents such as citric acid and monoglyceride citrate can be used along with TBHQ to enhance the effectiveness of
the vegetable oil. However, the use of TBHQ in animal fats such as lard
is less effective. It is highly effective in polyunsaturated oils and does
not cause discoloration [32]. TBHQ is even effective in biodiesel fuels
produced from poultry and animal fats. The lack of steric hindrance
improves the electron releasing ability of TBHQ and enhances its
activity. The thermal stability of TBHQ is slightly better than BHA and
BHT. To improve the thermal stability and also the oil solubility, Zhang
et al. [39] developed lauryl TBHQ and lauryl TBQ (tertbutyl quinone)
antioxidants that showed better antioxidant activity than TBHQ even at
temperatures higher than 140 °C. Nanditha and Prabhakar [40]
reported that TBHQ has antimicrobial properties similar to BHA.
Moreover, TBHQ forms a protective film layer that protects copper
and carbon steel from corrosion reactions [41].
The derivatives of a gallic acid such as propyl, octyl, and dodecyl
gallates are effective in stabilizing polyunsaturated fats. The trihydroxy
structure of the propyl gallate is crucial for its antioxidant potency.
Effective synergism can be obtained with BHT and BHA, but BHA
performs better. The solubility of propyl gallate is very poor in oils and
fats and good in water. The volatility of propyl gallate is relatively less
than that of TBHQ, BHA, and BHT. The gallates react with metal ions
and darken the substrate; hence, it should always be used with a
chelating agent [32]. It is deactivated quickly in alkaline systems and at
elevated temperatures. The longer chain length derivatives octyl and
dodecyl gallates have better solubility and heat resistance than propyl
gallate [42]. However, their antioxidant effectiveness is reduced
significantly because of their larger molecules. Gallates are effective
in preventing the enzyme (lipoxygenases) induced oxidation in fatty
acids [43].
The phenolic compound pyrogallol (1,2,3-benzenetriol) is highly
successful in preventing oxidation of most biodiesel fuels. The pyrogallol molecule contains three phenolic OH groups and is highly
soluble in water and partially soluble in biodiesel. Pyrogallol is the
strongest reducing agent, and it has long been used for the removal of
oxygen from gasses. However, only limited quantities of pyrogallol
were available because isolating its starting material gallic acid from
plants is expensive [44]. Apart from these common antioxidants other
phenolic derivatives such as 2,4,6-Tri Tertiary Butylphenol, 2,6-DiTertiary-Butylphenol, 2-tert-Butyl-5-methylphenol, 2-(tert-Butyl)-4,6dimethylphenol, 4-tert-Butyl-2-methylphenol, 2-tert-Butyl-4-methylphenol, 2-tert-Butyl-6-methyl-phenol, 2-tert-Butylphenol and 2,4 di-
4.1.2. Natural antioxidants
There has been a long-standing interest in the use of natural
additives in fuels for health reasons. Some plant materials with high
phenolic contents can act as antioxidants in oils and fats. In general,
vegetable oils possess natural antioxidants such as tocopherols, tocotrienols, polyphenols, chlorophylls, ascorbates, lignin, and carotenoids
that exert an essential protective role in the fatty acid oxidation. Most
of these antioxidants might be reduced or destroyed during the
transesterification or in the refining process [46]. Biodiesels produced
from unrefined vegetable oils contains more natural antioxidants and
better stability, but they do not meet other fuel requirements [47].
Plant-based phenolic compounds such as tocopherols, carotenoids,
lycopene, zeaxanthin, canthaxanthin, astaxanthin, gallic acid, caffeic
acid, vanillin, sinapic acid, ferulic acid, protocatechuic acid, p-coumaric
acid, eugenol, sesamol, vanillic acid, cinnamic acid, and resveratrol
have antioxidant properties and are produced commercially on a large
scale. Herbal extracts of sage, rosemary, clove, allspice, thyme,
cinnamon, oregano, marjoram, eucalyptus, artichoke, and turmeric
have been identified as effective antioxidants in food products [48].
However, except for tocopherols, only very few studies have made on
biodiesel fuels using these natural antioxidants. Tocopherols are
effective only if their concentration is approximately equal to their
concentration in vegetable oils and at a higher level, they could act as a
prooxidant [49]. Most studies have reported that tocopherols have a
limited antioxidant activity on biodiesel fuels when compared to
synthetic antioxidants. Tocopherols are more effective in hydrocarbon
oils than in vegetable oils or its ester [50].
There are four forms of tocopherols are available in nature: α, β, γ,
and δ. The α- and γ-tocopherols are found in vegetable oils and αtocopherol is common in animal fat. According to the results obtained
from the BIOSTAB project [51] supported by the European
Commission, γ-tocopherol was proved to be the most effective of the
three isomers (α, β, and γ) tested and α-tocopherol is the least effective.
Moreover, γ- tocopherol is significantly effective in tallow and waste
cooking oil methyl esters and ineffective in sunflower and rapeseed
methyl esters. Warner and Moser [52] also found that gammatocopherol is more effective antioxidant than alpha or delta tocopherols. Moreover, tocopherols are readily oxidized by air, and they are
stable only in the absence of air. Also, the presence of enzyme
lipoxygenase in oil degrades the tocopherols, and this enzyme should
be deactivated to slow down the degradation [53].
The carotenoid β-Carotene is primarily found in palm oil is a potent
antioxidant and its effectiveness mainly depends on the partial
pressure of oxygen in contact. At higher oxygen concentration it acts
as a prooxidant and favor oxidation [54]. The minor components such
as polyphenols, citric acid also help to enhance the stability by
sweeping the metal ions which accelerate the oxidation. The ascorbic
acid present in the oil acts as a secondary antioxidant and reduce the
formation of hydroperoxides [55].
The BIOSTAB research group tested the effect of carotenoids
astaxanthin and retinoic acid on sunflower and rapeseed methyl ester
and found no significant effect on stability [50]. Damasceno et al. [56]
tested the antioxidant activity of caffeic acid (CA) at a level of
1000 ppm in soy methyl ester and reported that CA could meet EN
14214 specifications even after three months. In addition to the
hydrogen donation by the caffeic acid, the free radicals generated from
the CA form a dimer that has excellent antioxidant properties and
further protective. The reaction product 2, a 5-cysteinylcaffeic acid
obtained from the reaction of CA with cysteine has higher antioxidant
effectiveness than caffeic acid alone [57]. Moser [58] investigated the
effectiveness of myricetin, a flavonol obtained from the extracts of
Moringa oleifera seed oil in soybean oil methyl ester and reported that
myricetin shows better antioxidant activity than α-tocopherol. Also,
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K. Varatharajan, D.S. Pushparani
good improvement in stability of highly unsaturated moringa oleifera
oil based biodiesel when it was treated with secondary aromatic amines
N, N′-diphenyl-1,4-phenylenediamine, and N-phenyl-1,4-phenylenediamine. However, Joshi et al. [78] didn’t find any improvement in the
stability of the jatropha methyl ester with the use of diphenylamine.
Some of the oil soluble secondary aromatic amines are butyl octyl
diphenylamine, N-p-methyl phenyl-alpha naphthylamine, N-phenylalpha-naphthylamine, di-sec-butyl diphenylamine and tertiary-octyl
diphenylamine.
Fernandes et al. [59] showed that the ethanolic extract of Moringa
oleifera leaves is a better antioxidant than the TBHQ in soy biodiesel.
Serqueira et al. [60] tested the effectiveness of tetrahydro curcuminoid, a natural antioxidant derived from curcumin in biodiesel fuels
produced from cottonseed and residual cooking oils and reported that
the performance of tetrahydro curcuminoid is superior to the BHT. In
addition, De Sousa et al. [61] tested the antioxidant activity of
curcumin and β-carotene in soybean biodiesel and reported that
curcumin more effective than the β-carotene. Moreover, their study
showed that β-carotene acts as a pro-oxidant and reduced the induction period of the biodiesel significantly. Medeiros et al. [62] observed a
better synergism between the natural antioxidant rosemary extract (in
ethanol) and a synthetic antioxidant TBHQ in cottonseed biodiesel.
Moreover, Spacino et al. [63] found a significant synergistic activity
with the addition of alcoholic extracts of rosemary and oregano at a 1:1
ratio in soy methyl ester. Natural antioxidants have not achieved
notable commercial success until recently primarily because of their
higher cost. Furthermore, Deyab et al. [64] showed that ethanol
extracts of rosemary leaves reduce the corrosion rate of aluminum in
biodiesel. More recently, Garcia et al. [65] claimed that extracted
fractions of lignocellulosic bio-oil have antioxidant properties and their
addition of 4% by weight to biodiesel improved the oxidation stability
by 475%.
4.1.4. Secondary antioxidants
Secondary or preventative antioxidants work either by chelating of
metal ions or by decomposing of hydroperoxides. In general, secondary
antioxidants are added to fatty acids to enhance the life of expensive
primary antioxidants. Metal chelators make complexes with transition
metal ions and prevent metals from entering in the catalytic reaction.
Ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), sodium tripolyphosphate, N,N′-disalicylidene-1,2diaminopropane and citric acid are widely used as chelating agents.
And the salts of phytic acid, phosphoric, tartaric, malic and ascorbic
acids also have possessed significant metal binding activities [79].
Chelating agents which form σ -bonds with a metal are most suitable to
bind the metal ions whereas chelating agents which form π complexes
increase the redox potential thereby not preferable as secondary
antioxidants [80].
The performance of a chelator is also dependent on the pH of the
matrix, and its activity decreases with the reduction of pH since the
solubility of metal ions increases at lower pH. The
presence of other chelatable ions such as calcium also reduces the
activity of chelating agents [81].
EDTA is preferred to chelate ferric irons and at the same time, its
concentration should be higher than the metal ions. If the EDTA: iron
ratio is less than one, EDTA will become prooxidative [82]. Citric acid
can also be used to chelate Fe, Cu, and Ni ions but its effectiveness is
lesser than EDTA since it has higher pKa values and lowers iron
stability constant. Phosphoric acid can bind Cu and Ni ions and not
ferric ions. Effective synergism can be obtained with the combinations
of
citric
acid,
phosphoric
acid,
and
lauryl
gallate.
Dipalmitoylphosphatidylethanolamine, a phospholipid shows no antioxidant activity if it is used alone and in combination with BHT, BHA,
PG and tocopherols it enhances the activity [83].
The surface passivators or metal deactivators form a protective
layer between metal ions and fuel, thereby preventing metal–fuel
contact. Benzotriazole,2-Mercaptobenzothiazole, triazole derivatives
and tolu triazole derivatives are some of the surface passivators used
in lubricants. Organosulfur or organophosphorus compounds with
ascorbic acid can act as hydroperoxides decomposer and convert the
hydroperoxides into stable alcohols and other inactive products. At
0.01% concentration, the oil soluble ascorbyl palmitate is more
effective than the BHT and BHA in improving the stability of vegetable
oils [84]. Some secondary antioxidants such as ascorbic acid can
regenerate the primary antioxidants which could reduce the consumption of antioxidants [6].
4.1.3. Secondary aromatic amine antioxidants
After phenols, secondary aromatic amines are widely used as
antioxidants to improve the stability of organic compounds. The
formation of nitroxides and benzoquinone imine compounds are
responsible for the antioxidant properties of secondary aromatic
amines. Primary aromatic amines have no antioxidant properties since
they do not generate nitroxides. The tertiary amines are oxidation
promoters rather than inhibitors [66]. The use of aromatic amines as
stabilizers in biodiesel is less popular because most of the aromatic
amines tend to discolor the substrates as oxidation progresses.
Furthermore, the insoluble oxidation products of aromatic amines
can cause piston deposits and suitable detergents, and dispersants need
to be used that increases the cost of additives [67]. However, secondary
aromatic amines have many advantages over phenolic antioxidants.
The hydrogen-donating ability of aromatic amine is superior to phenols
since the N―H bond in aromatic amine is not as strong as the O―H
bond of phenols [68]. The hindered phenolic antioxidants could trap
only two peroxy radicals per molecule [69], whereas secondary
aromatic amines can scavenge 50 (alkylated diphenylamines) to 500
(hindered cyclic secondary amine derivatives) peroxy radicals per
molecule even at 130 °C [70].
The aromatic amine antioxidants follow a cyclic process in which a
nitroxyl radical is regenerated and consumes more radicals. Because of
this regeneration, one molecule of the aromatic amine can trap a large
number of peroxyl radicals before the nitroxyl radical is destroyed [71].
Therefore, the quantity of antioxidants required is much lower for
aromatic amine antioxidants. Another major advantage of using
aromatic amines in fuel is that they offer better resistance to heat than
phenolic antioxidants. Corrosion inhibition is another benefit that can
be obtained with the use of aromatic amines which can save on the cost
of corrosion inhibitors. Though the aromatic amine antioxidants have a
built in “N” atom in its molecules, it reduces the NOx emissions
effectively since they are highly efficient free radical quenchers that
trap NOx- forming radicals [72,73]. Rashed et al. [74] and Hess et al.
[75] also recommend aromatic amines for biodiesel stability because of
their high antioxidant capacity at low concentrations.
Alberici et al. [76] tested the effectiveness of an aromatic amine
N,N′-Di-sec-butyl-p-phenylenediamine in soybean, canola, and sunflower based biodiesel fuels and found that the additive is most
effective even at a concentration of 0.2 ppm. They also reported that
at a concentration of 10 ppm, the induction period of canola biodiesel
fuel is drastically increased from 5.2 to 50 h. Rashed et al. [77] found a
4.2. Bond dissociation enthalpy (BDE)
The hydrogen donating ability is one of the major factors to be
considered for the screening of antioxidants. Hydrogen atom transfer
reaction is primarily a function of the bond dissociation enthalpy
(BDE). It is the driving force for the hydrogen transfer from the
antioxidant to free radical [85]. The activation energy of an antioxidant
is linearly increased with BDE and hence the effectiveness of antioxidant increases with the decrease of BDE. To efficiently quench the free
radicals, the bond energy of OH and NH group of an antioxidant should
be considerably lower than the BDE values of peroxides and hydroperoxide free radicals (should be less than 368–376 kJ mol−1). A
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Renewable and Sustainable Energy Reviews xxx (xxxx) xxx–xxx
K. Varatharajan, D.S. Pushparani
Table 1
Bond dissociation energies of phenols and diphenylamines [87].
Phenols
OH bond BDE kJ mol−1
Diphenylamines
NH bond BDE kJ mol−1
C6H5OH (phenol)
2,6-(t-Bu)2,C6H3OH
2,4,6-(t-Bu)3,C6H3OH
2,6-(t-Bu)2-4-MeC6 H2OH
2,6-(t-Bu)2-4-Et C6 H2OH
2,6-(t-Bu)2-4-MeOC6 H2OH
2,6-(t-Bu)2-4-MeOCOC6 H2OH
2,6-(t-Bu)2-4-NO2C6 H2OH
379
346
344
335
335
333
353
361
(C6H5)2NH (Diphenylamine)
p-MeOC6H4NHPh
p-Me C6H4NHPh
m-Me C6H4NHPh
(p-MeOC6H4)2NH
p-PhNHC6H4NHPh
(p-Br C6H4)2NH
p-NO2C6H4NHPh
366
358
364
367
352
331
369
378
preferable BDE for a good antioxidant is about 40 kJ mol−1 lower than
the BDE of hydroperoxide free radicals [86].
Phenolic antioxidants donate hydrogen from the hydroxyl group
[OH] and secondary aromatic amines from nitroxyl group (NH). The
introduction of electron donating groups to the phenols or amines
decreases the bond energy whereas the addition of electron withdrawing groups increases the BDE. Zhu et al. [87] analyzed the effect of
introducing electron donating groups to phenols and secondary amines
on BDEs in detail. The BDEs of OH bonds of phenolic antioxidants and
NH bonds of secondary amines are presented in Table 1. Examination
of Table 1 provides some insight into the BDE of phenolic and amine
components with different substituent groups.
Simple phenol has BDE of 379 kJ mol−1 could efficiently quench the
alkoxyl and hydroxyl radicals, but they cannot scavenge peroxyl or
hydroperoxyl radicals. To make the BDE of the phenolic components
considerably lower than those of peroxides and hydroperoxide some
strong electron donating groups such as p-Me, p-Et, p-OH, and p-MeO
are introduced at either the ortho- or para-position [87]. The introduction of two t-butyl groups at the ortho positions reduced the BDE of
phenol from 379 to 344 kJ mol−1. The addition of one more t-butyl
group didn't reduce the BDE significantly. The introduction of additional methyl or ethyl group to the two t-butyl groups lowered the BDE
considerably. The lowest BDE is achieved with the introduction of MeO
group (333 kJ mol−1). The introduction of electron withdrawing group
such as p-CO2Me and p-NO2increases the BDEOH values significantly.
The basic secondary aromatic amine diphenylamine also has
significant antioxidant property because of its low bonding energy.
The introduction of the electron withdrawing groups such as Cl, NO2,
Br increased the bond energy, and the introduction of electron
donating groups such as MeO, PhNH at p position lowered the BDE
values about 14–35 kJ mol−1 lesser than the BDE of diphenylamine.
However, unlike phenols, there has been a very limited success in
reducing BDE of diphenylamine with the introduction of electron
donating substituents. Valgimigli and Pratt [88] suggested that heterocyclic diarylamines have more antioxidant properties than the diphenylamines. They also found that NH group of diarylamines are more
reactive than the OH of phenols for the same bond dissociation energy.
Denisova and Denisov experimentally calculated the BDEs of various
antioxidants and tabulated the data [89]. The BDEs of isomers of
tocopherols α, β,γ,andδ are 330, 335.3, 334.9 and 341.5 respectively
[90]. Although the alpha-tocopherol has the lowest BDE value among
the tocopherols, its antioxidant effectiveness is lesser than others.
Therefore it should be re-emphasized that the antioxidant activity does
not solely depend on BDE and other factors also have some impact.
antioxidants. And the presence of electron acceptor groups like NO2,
COOH and halogens and the para-alkyl groups branched in the α
position reduce the effectiveness [91]. The ortho and para alkyl groups
in the structure help to stabilize the phenoxyl free radical generated by
antioxidants (A*) and hence prevent the side reactions. Also, the ortho
alkyls provide steric hindrance to stop the undesirable pro-oxidation
reaction [92]. However, in most cases, steric hindrance suppresses
electron releasing rate and hence affects the antioxidant activity. The
tertiary butyl groups of BHA and BHT molecules offer greater steric
hindrance than the TBHQ which results in a reduction of activity in
vegetable oils. Even though the O-H bond dissociation enthalpy of BHT
is lower than the mesitol (2,4,6-Trimethylphenol), the effectiveness of
BHT is 6-times lesser than the mesitol because of its steric effect [85].
The number of hydroxyl groups in the structure also has effects on
antioxidant activity. Propyl gallate and other longer chain gallates have
polyhydroxy groups, and their performance is superior to the monohydric BHT and BHA antioxidants. However, the antioxidant activity
starts to decrease for the structure having more than three hydroxy
groups. Moreover, polyhydroxy substituent makes the substance as
partial water soluble that depletes the concentration of antioxidants in
oil phase [93].
In general, BHA is more effective than the BHT because of the
presence of para oxygen in the BHA structure. However, the para
oxygen could reduce the stoichiometric factor (number of radicals
neutralized per hindered phenol moiety) significantly. The effectiveness
of antioxidants containing no para oxygen linearly increases with the
concentration Meanwhile, the para oxygen containing antioxidants
loses their activity at concentration [94]. Denisov and Denisova [89]
identified the major physical factors that have an impact on the
effectiveness of antioxidants include enthalpy of reaction (∆H e), force
constants of ruptured and generated bonds, bond dissociation energies
of OH or NH bonds, difference between electronegativities of antioxidants and the substrate (∆AE ), radii of the reactants, presence of polar
group in the molecule, multidipole interaction effect, presence of π
bond or aromatic ring in the α position relative to the CH bond,
existence of bulky groups next to the reaction center and the solvent
polarity. Higher differences in electronegativities, smaller radii of
reactants are the favorable factors that increase the effectiveness of
antioxidants. The presence of polar compounds such as methanol
residue in biodiesel affects the effectiveness since antioxidants form a
hydrogen bond with alcohol that retards the reaction of antioxidants
with free radicals [89].
4.3. Structure of antioxidants
Antioxidants intended to provide stability must be capable of
migrating freely throughout the of biodiesel mass to reach a large
number of initiation sites that are generated during the storage period.
A low-molecular weight antioxidant can mix thoroughly with the
biodiesel and reaches the initiation sites easily. However, low-molecular-weight antioxidants are volatile in nature, and evaporation loss of
antioxidants is inevitable. For this reason, low-molecular weight
antioxidants are not preferred for long-term storage [95]. The high
4.4. Molecular weight
Antioxidants perform differently from one another in depending
upon the type or the positions of substituents attached to the aromatic
ring of the compound. The molecular structure of some of the common
antioxidants is presented in Fig. 1. It is well known that electron
donating substituents such as alkyl, alkoxyl (CH3O) at the ortho or para
positions of the aromatic ring of phenols are the desirable structure for
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Renewable and Sustainable Energy Reviews xxx (xxxx) xxx–xxx
K. Varatharajan, D.S. Pushparani
Fig. 1. Structure of common antioxidants.
molecular weight antioxidants contain more hydrogen per molecule for
donation. Moreover, they have poor volatility which is desirable for
long-term protection. On the other hand, the increase of molecular
weight decreases the mobility of antioxidants to the initiation sites
which leads to a non-uniform distribution of antioxidants. Because of
this reason, the antioxidants concentration should be 10 times higher
than the actual concentration of initiating radicals [96].
The antioxidant activity of natural antioxidants increases as the
alkyl chain lengthens (the increase of molecular weight) until a
threshold is reached, after which further chain length extension leads
to a drastic loss of activity [97]. Ingendoh [98] tested the antioxidant
effectiveness of BHT and higher molecular weight Bis-BHT in soybeanbased biodiesel and found that Bis-BHT is more effective than the
lesser molecular weight BHT. Therefore, higher molecular weight
antioxidants are preferred for long-term protection of biodiesel fuels.
Fig. 2. Effect of concentration on antioxidant activity.
Prankl [51] reported that the concentration requirement of antioxidants in biodiesel is strongly dependent on the feedstock and production technology. He also reported that natural antioxidants are more
sensitive to concentrations and at higher concentrations, they offer prooxidant effects. The optimum range of tocopherol concentration is
0.043–0.13% in weight and at higher concentration (greater than
0.2%) it shows pro-oxidant effect [5]. Furthermore, Moser [58]
reported that the optimum concentration for the α-tocopherol in
soybean methyl ester is 600–700 ppm and after that, no antioxidant
activity was observed. In general, a higher concentration of antioxidant
additives should be avoided in fuels since it could have a significant
effect on the increase of the delay period in combustion and also the
cost increase.
4.5. Concentrations requirement
Antioxidants cannot function until it reaches a critical concentration beyond which their effectiveness linearly increases with concentration. At the same time, there is a final finite saturation value beyond
that concentration no further improvement in antioxidant activity. In
most cases, a higher concentration of phenolic antioxidants acts as prooxidant and promotes the degradation reaction [99]. The concentration
in between critical concentration and the saturation value is called
optimum range. Different antioxidants exhibit different optimum range
with various substrates. Furthermore, Zhong and Shahidi [29] reported
that the relationship between antioxidant activity and concentration is
not linear and follows a parabolic manner. Fig. 2 [11] shows the effect
of polar and nonpolar antioxidants concentration on their antioxidant
activity in bulk oil. Both critical and saturation point are not same for
different antioxidants and their value is higher for polar antioxidants.
Chen and Luo [100] reported that the critical concentration of
antioxidants for highly unsaturated (more than 90%) biodiesel is
approximately 100 ppm to have a noticeable increase in the induction
period (IP).
The critical concentration always increases with temperature and
higher quantity of antioxidant is necessary to obtain a good stability at
high temperatures. Lapuerta et al. [101] tested the effect of temperature on the induction period of biodiesel fuels derived from animal fat,
soybean oil and used cooking oil with BHT antioxidant and found that
at higher temperature (130 °C) the required concentration of antioxidant is too high ( > 25,000 ppm) to achieve the threshold of 8 h.
4.6. Synergism
Antioxidant synergism is the combined use of two or more
antioxidants to produce an effect greater than the sum of any individual
antioxidant. Synergism can be obtained either by adding two primary
antioxidants (homo synergism) or a combination of primary and
secondary antioxidants (hetero synergism). Synergism not only improves stability but also reduces the amount of antioxidants needed
and the cost of additives. For an effective combination, the synergistic
action S always should be greater than zero [102].
S = IP1,2 − (IP1 + IP2)) > 0
Where IP1 and IP2 are the induction periods of individual antioxidants
and IP1,2 is for the combination. In homo synergism, a primary
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Renewable and Sustainable Energy Reviews xxx (xxxx) xxx–xxx
K. Varatharajan, D.S. Pushparani
catalyst poisoning effect in the after treatment device of the engine.
Recently, Roveda et al. [118] reported that addition of low-cost
anthraquinone compound DHQ (1,4-dihydroxyanthraquinone) to the
BHT and PG provides a better synergetic effect in biodiesel and reduces
the final cost of the fuel.
antioxidant with lower oxidation potential acts a peroxy radical
scavenger, whereas the other antioxidant with higher oxidation potential donates hydrogen and helps in the regeneration of the former
antioxidant. In biodiesel fuels, if used alone BHT is less effective than
other phenolic antioxidants but its effectiveness increases with added
synergists. Omura [103] studied the mechanism of synergism between
BHT and BHA and reported that BHT regenerates BHA by donating
hydrogen atoms to BHA which results in higher antioxidant activity
than used singly in oils or fuels. And this synergism can be significantly
altered by adding base or acid. However, Borsato et al. [104] conducted
an experimental investigation on the oxidative stability of soybean
biodiesel with the addition of BHT, BHA and TBHQ and their various
combinations. The results of this study showed that the BHT/BHA in
1:1 (w/w) is a better performer than the BHA/BHT/TBHQ (1:1:1)
combinations at 30 °C. In another experiment, they reported that BHT
and BHA combination is least effective in biodiesel produced from
soybean oil (90%) and lard (10%) mixture based, but TBHQ and PG
combination is most efficient [105].
Buck [106] reported that propyl gallate demonstrates better synergism with BHT and BHA antioxidants and negative synergism with
TBHQ. Although the antioxidant effectiveness of BHT in biodiesel is
less than that of other commercial antioxidants, it should be considered
because of its low cost and its synergistic effects with other expensive
antioxidants. Tang et al. [107] tested the synergistic effects of 500 ppm
of TBHQ/BHA (2:1), TBHQ/PY (1:1), and TBHQ/PG (1:1) in soy
methyl ester and found that except TBHQ/BHA other combinations are
ineffective. Moreover, Orives et al. [108] conducted experiments to find
the synergistic effect of phenolic antioxidants based on their cost and
efficiency and reported that 75% TBHQ and 25% BHA combination is
optimum for soy biodiesel. Guzman et al. [109] experimented
TBHQ:BHA (2:1), TBHQ:PY (2:1) and TBHQ: PG (1:1) in soybean
and poultry fat based methyl esters and reported that best synergy was
produced by the TBHQ:BHA and TBHQ:PY combinations.
Furthermore, Rawat et al. [110] evaluated the effectiveness of binary
antioxidants PY:PG, PY:TBHQ and PY:BHA at different weight ratios in
Jatropha and Pongamia biodiesels and observed the best synergism
with PG:PY and TBHQ:PY and antagonism with PY:BHA.
An effective synergistic effect is observed between secondary
aromatic amines and phenols where the phenolic antioxidant could
regenerate the amine or nitroxyl radical [111]. Initially, amines
deactivate the alkyl peroxy radicals by donating hydrogen atoms. The
amines are regenerated by receiving hydrogen atoms from the phenolic
antioxidants and the remaining phenols undergo reaction with alkyl
peroxy radicals until it is consumed fully. After that there generated
aromatic amines again begins to react with peroxy radicals [112].
However, negative synergism also possibly occurs with hindered amine
photo antioxidants and hindered phenols combinations [113].
Synergism is also displayed between primary antioxidants and preventive secondary antioxidants. Marinova and Yanishlieva [114] demonstrated that oil soluble ascorbyl palmitate act as a synergist with
tocopherols and regenerates and recycle the tocopherol radical to the
parent phenol. Merril et al. [115] also observed similar results in high
oleic sunflower oil. The synergistic effect of the secondary antioxidant
citric acid have been investigated by Serrano et al. [116] and they
reported that biodiesel washed with citric acid showed better stability
with all kinds of antioxidants (TBHQ, propyl gallate, and mixed
tocopherols). This synergistic effect of citric acid is primarily due to
the chelation of metal ions.
Significant synergism effect is obtained when primary chain breaking antioxidants are used together with peroxide decomposer. Larson
and Marley [117] found an excellent synergism between the peroxide
decomposer triphenylphosphine (TPP) and propyl gallate (PG) in soy
methyl ester. Triphenylphosphine improved the half-life of PG significantly at higher concentration. However, due to phosphate content,
the use of TPP as an additive in biodiesel is questionable. The presence
of sulfur and phosphates in the fuel antioxidants possibly lead to
4.7. Source of biodiesel
The antioxidant behavior in biodiesel fuels primarily depends on
the source of feedstock and the degree of unsaturation. Several
researchers carried out stability studies on biodiesel fuels using
different antioxidant additives and measuring different physical and
chemical parameters. Experiments were conducted on the effect of
antioxidants on the long-term storage stability of biodiesel fuels in the
research centers of University of Graz [51], Austria and Wayne State
University, USA [119]. The results have shown that the antioxidants
TBHQ, PY, PG are most effective in rapeseed, soybean, and cottonseed
biodiesel fuels. According to the ‘polar paradox’ theory proposed by
Porter [120] polar or hydrophilic antioxidants such as TBHQ, PY, PG
and ascorbic acid are highly effective in less polar media such as oils,
biodiesels whereas lipophilic or non-polar antioxidants (BHT, BHA,
ascorbyl palmitate, tocopherols etc.) are superior in their action at
water-oil emulsion. Zhong and Shahidi [121] stated that the polar
paradox theory is valid only above a critical concentration. Biodiesel
fuels contain more oxygen in their molecule (about 11%), and hence
they are more polar than the mineral diesel. Because of its polar nature,
it absorbs water from the air which could alter the solubility of
antioxidants. The antioxidants TBHQ, PY, and PG have larger hydrophilic-lipophilic balance and are enriched in the surface of the biodiesel
that effectively prevents oxidation reaction at the air-oil interface [120].
Table 2 shows the composition of various biodiesel fuels and the
effectiveness of antioxidants in them. Most researchers conducted
stability tests using common phenolic antioxidants such as PY,
TBHQ, PG, BHA, BHT, and α-T, etc. The distilled biodiesel fuels have
shown poor induction periods compared to undistilled one because of
their poor natural antioxidant contents. Pyrogallol (PY) is identified as
potential antioxidants for most biodiesel fuels. It is more effective in
preventing degradation of inexpensive high FFA feedstocks based
biodiesel fuels. The three hydroxyl groups on the aromatic ring of
pyrogallol help to absorb more ground state oxygen readily than many
organic compounds. However, PY has some drawbacks such as higher
cost, poor solubility in biodiesel and toxicity [51]. The partial solubility
of this compound in biodiesel releases the toxic pyrogallol emissions
through the exhaust gas of engines that can cause irritation of nose,
throat, and eye. Pyrogallol has a high affinity for oxygen and the
inhalation of vapor reduces the oxygen in the blood which results in
bluish discoloration of the skin (cyanosis) and possible coma [126].
The solubility of the PY can be increased with the use of surfactants but
it increases the cost of additives [127].
It has been observed (Table 2) that TBHQ is more effective in
soybean, cottonseed and rapeseed methyl esters and ineffective in high
FFA feedstocks such as yellow grease, poultry fat, jatropha, neem, and
Pongamia. Chen and Luo [100] tested the stability of high FFA
feedstocks based biodiesel fuels and reported that TBHQ is least
effective among the tested antioxidants. Linseed methyl ester has the
poorest stability among all kinds of biodiesel fuels because of its higher
linolenic acid content (more than 52%) and the quantity of antioxidant
needed to get the desired stability is also high. Pantoja [125] reported
that TBHQ is totally ineffective in linseed methyl ester up to 1500 ppm
concentration and above this limit; the IP was linearly increased with
the concentration of TBHQ. The antioxidant activity of TBHQ is greater
in most vegetable oil based biodiesel fuels. The two para hydroxyl
groups are responsible for its effectiveness [37]. PG showed moderate
to high antioxidant activity in most biodiesel fuels except poultry fat
based biodiesel. BHA and BHT are more soluble in fats and both are
considered to be more effective in animal fats than vegetable oils. In
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K. Varatharajan, D.S. Pushparani
Table 2
Effectiveness of antioxidants in biodiesel fuels.
Biodiesel
Fatty acids
∑UFA
Antioxidants effectiveness at 1000 ppm (induction time in h)
PY (11.5) > TBHQ (11) > PG (10) > BHA (7) > DTBHQ (6.7) > BHT (6)
> α-T (3.5) [119]
TBHQ (30) > PY (26) > PG (21) > DTBHQ (20) > BHA (9.2) > BHT (9)
> α-T (5.7) [119]
PY (19) > PG (14) > TBHQ (6.5) > BHA (5.5) > DTBHQ (5) > BHT (4)
> α-T (2.5) [119]
PY (13) > BHA (12) > BHT (6.5) > PG (5) > TBHQ (4) > DTBHQ (2) >
α-T (1) [119]
TBHQ (38.53) > PG (27.36) > PY (26.31) > BHA (24.30) > 2t-BHT
(10.54 > BHT (9.85) [122]
PY (18.5) > PG (17.5) > BHA (14.50) > TBHQ (5.70) > BHT (5) [122]
PY (31.95) > PG (29.90) > TBHQ (29.44) > BHA (13.80) > BHT (10.84)
[122]
PY (26) > PG (24) > TBHQ (18) > BHA (14) > BHT (8) [122]
PY (15) > PG (12) > TBHQ (6.5) > BHA (5.5) > BHT (3.5) [122]
TBHQ (6.5) > PG (5.5) > BHA (5) > PY (4.8) > BHT (3.5) [122]
C14:0
C16:0
C16:1
C18:0
C18:1
C18:2
C18:3
Soybean
0
14.1
0.7
5.15
25.29
48.7
6.08
80.8
Cottonseed
0.76
24.74
0.37
2.68
18.45
52.99
0
71.8
Yellow grease
0.14
16.12
0.02
3.96
31.43
46.05
2.28
79.78
Poultry fat
1.04
21.82
3.71
7.61
36.59
27.02
1.78
69.1
Rapeseed
0.09
5.95
0
2.07
60.34
20.87
8.15
89.36
Distilled rapeseed
Used frying oil
0
0.41
2.05
14.38
0
0.39
2.61
4.26
62.20
57.17
21.05
17.08
7.90
2.08
91.15
76.72
Distilled used frying oil
Sunflower seed
Distilled Sunflower
seed
Tallow
Distilled Tallow
Pongamia
Neem
0.27
0
0
11.55
5.98
7.22
0.34
0
4.43
4.66
4.20
58.04
23.95
27.40
19.18
63.74
61.18
2.25
0
0
79.81
87.69
88.58
2.20
2.84
0
0
21.88
21.61
10
16
1.57
2.11
0
0
17.03
27.95
6
17
45.12
37.61
72
48
8.05
4.71
12
15
1.09
0.58
0
3
55.83
45.01
84
66
Jatropha
0
14
0
7
43
34
0
77
Croton Megalo-carpus
oil
Açaí
Passion fruit
Linseed
0.1
6.5
0.1
3.8
11.6
72.7
3.9
88.3
0.2
0.1
0.1
24.6
10.9
6.4
4.1
0.1
0.1
2.4
2.8
4.5
52.9
17.3
21.7
8.3
68.1
13.5
0.7
0.3
52.7
66
85.8
88
PY (37) > TBHQ (15.5) > PG (14.5) > BHA (9) > BHT (6) [122]
PG (60) > PY (48) > TBHQ (30) > BHT (9) > BHA (7) [122]
PY (34.35) > TBHQ (6.19) > BHA (5.02) > BHT (4.88) > GA (0.88) [123]
PY (44.50) > PG (30.20) > BHA (4.54) > BHT (2.41) > TBHQ (2.15)
[123]
PY (53.73) > PG (24.45) > BHA (11.15) > BHT (8.59) > TBHQ (5.56)
[123]
PY (25.7) > PG (20.5) > BHA (7.67) [124]
PG (45) > BHA (23) > TBHQ (9) at [125]
PG (18) > BHA (7) > TBHQ (4) [125]
PG (7) > BHA (3) > TBHQ (3). TBHQ outperformed PG above 2000 ppm
[125]
resulting from dissolved oxygen because of their uneven distribution in
biodiesel [129]. The dissolved oxygen can be minimized by directing
the biodiesel jet on the walls rather than at the middle of the biodiesel
surface while filling in the container. And the container should always
be filled from the bottom to avoid pouring biodiesel through the air.
poultry based biodiesel fuel both BHA and BHT improved stability
significantly but not in tallow biodiesel.
The pH of biodiesel also has a significant impact on the effectiveness of antioxidants. At higher pH environment, substituent groups of
antioxidants will undergo deprotonation that further enhance their
electron donating ability. Moreover, higher pH reduces the solubility of
transition metal ion catalysts which could reduce the oxidation rate
[128]. Also, the viscosity of biodiesel has some significant effects on the
performance of antioxidant additives. Viscosity affects the mobility of
antioxidants in the oil matrix. The higher viscosity of biodiesel fuels
leads to a non-uniform distribution of antioxidants which results in
some portion of the biodiesel fuels left unprotected [29]. The presence
of free fatty acids in biodiesel also reduces its stability because FFA
oxidizes faster than the methyl esters. And the FFA facilitates
incorporation of transition metal ions from storage tanks and thereby
increases the oxidation rate [49].
4.9. Temperature effects
Though the dissolved oxygen decreases as the temperature increases, most primary antioxidants (except tocopherols) lose their
activity with the increasing temperature. The hydroperoxides decompose at high temperature (100–150 °C), and a significant quantity of
phenolic antioxidants is consumed by their deactivation. Réblová [130]
studied the effects of temperature on natural phenolic antioxidants
such as gallic, gentisic, protocatechuic, syringic, vanillic, ferulic, caffeic
and sinapic acids. He observed that less oxidizable antioxidants for
example syringic, ferulic and sinapic, and vanillic acids rapidly losing
their activity with increasing temperature when compared to easily
oxidizable phenols (gallic, gentisic, protocatechuic, and caffeic acids).
The antioxidants BHT, BHA, TBHQ and their combination, shows no
antioxidant activity when these additives are kept above 80 °C temperature [104]. The increase in temperature alters the mechanism of
action of some antioxidants which results in a change in the effectiveness of antioxidants.
Frankel [93] reported that at 45 °C ascorbic acid and ascorbic
palmitate are less effective in stabilizing soybean oil than the primary
antioxidants, PG and TBHQ. At the same time at 98 °C, the relative
effectiveness of ascorbic acid and ascorbic palmitate are reversed and
they are more effective than PG and TBHQ. According to Illinois
Sustainable Technology Center report [117], the antioxidant activity of
propyl gallate is rapidly decreased above 60 °C and hence the accelerated stability tests such as a raincoat and active oxygen method have
limited validity and these tests should not be conducted at standard
4.8. Solubility in biodiesel
Solubility is an important parameter that ensures the uniform
distribution of antioxidants in biodiesel which is crucial for their
effectiveness. The partially fat-soluble polar antioxidants such as
pyrogallol, TBHQ, and propyl gallate are more effective in protecting
biodiesel than the fat-soluble BHT and BHA antioxidants. In general,
the oxidation of biodiesel or any oil is initiated at the surface and
propagates to the inner parts. The partially fat-soluble polar antioxidants are enriched at the oil-air interface and effectively reduce the
free radicals on the surface [121]. Frankel [129] suggested that the
polar antioxidants in bulk oil are mostly concentrated at the water-oil
interface and not at the oil-air interface as previously believed.
Biodiesel is hygroscopic in nature and absorbs some quantity of water
from the air that could be the reason for the water-oil interface. On the
other hand, polar antioxidants are ineffective in preventing oxidation
9
Renewable and Sustainable Energy Reviews xxx (xxxx) xxx–xxx
K. Varatharajan, D.S. Pushparani
maintenance problem especially the countries where the use of
biodiesel blends is mandatory. Except for BHT most synthetic phenolic
or natural antioxidants are expensive to use and therefore inexpensive
antioxidant additives need to be developed. At the same time, it should
be safe and environmentally benign and should not increase the flash
and fire points of the biodiesel. The following conclusions were drawn
from this review:
110 °C. In general, biodiesels are stored at ambient temperature and
not subjected to extreme heat, however, while passing through the hot
fuel injector they get oxidized easily. Phenolic antioxidants are effective
in room temperature, but they cannot prevent thermoxidation for
example at engine nozzle effectively. Antioxidants with good carrythrough properties (the ability of antioxidants to escape destruction on
heating) with low volatility are most suitable for this application.
Antioxidants with more number of benzene rings and longer aliphatic
chains are most suitable for high-temperature use. And the secondary
aromatic amines are more tolerable to heat than phenolics [131].
1. Antioxidant selection involves compromises between conflicting
desirable properties. Some of the highly desirable properties are
good solubility, effective in low concentrations, long shelf life, and no
toxicity.
2. Antioxidants with low bond dissociation enthalpy (BDE) are preferred because they can release the hydrogen very easily. The BDE of
a good antioxidant should be about 40 kJ mol−1 lower than the BDE
of hydroperoxide free radicals (376 kJ mol−1). Secondary aromatic
amines have lower BDE than the phenolic antioxidant, but most of
them are toxic and darkening the fuels when added.
3. Higher molecular weight antioxidants are favored for long-term
storage of biodiesel fuels since they contain more hydrogen for
donation.
4. Antioxidants that contain polyhydroxy groups in their structure
performs better than the monohydric BHT and BHA antioxidants.
However, there is no improvement in the activity beyond three
hydroxy groups.
5. Effective synergism can be obtained between primary and secondary
antioxidants and also between primary antioxidants. The optimum
combination of primary, secondary antioxidants and chelating
agents can reduce the quantity of antioxidants needed and also
reduce the cost of fuel.
6. Antioxidants performance is also affected by the source of biodiesel
fuels. Pyrogallol is identified as the most effective antioxidant for
biodiesels produced from low-cost high FFA feedstock. Biodiesel
fuels that contain low level of natural antioxidants such as tocopherols and carotenoids have poor oxidation stability.
7. Partially fat-soluble polar antioxidants are effective in protecting
biodiesel than the fat-soluble antioxidants.
8. Temperature, pH, and viscosity of biodiesel significantly affect the
effectiveness of antioxidants. Antioxidants with more number of
benzene rings and longer aliphatic chains have good heat resistance.
And also the secondary antioxidants perform better than the
primary antioxidants at elevated temperatures. The increase in pH
reduces the concentration of metal ions and promotes stability.
Moreover, the viscosity of biodiesel significantly affects the uniform
distribution of antioxidants in biodiesel.
9. In biodiesel blends, cetane additives act as a free radical initiator and
reduce the concentration of antioxidant additives. Thus, in the
biodiesel blend where the cetane improver levels are high will prove
detrimental to fuel stability.
4.10. Effectiveness on hydrocarbon blends
Use of biodiesel blends in the engine is more common than the
B100 since it requires no engine modification. Tang et al. [119] have
reported that the effects of antioxidants on B20 and B100 fuels are
similar and suggesting that mineral diesel has a negligible effect on
stability. But, in certain cases, mineral diesel also initiates the autoxidation and it depends on their composition, additives, and age at the
time of blending. The effectiveness of antioxidant additives also will
depend considerably on the composition of the mineral diesel fuel.
Higher aromatic content diesel fuel is most compatible with polar
biodiesel and it can hold oxidation products in solution. Moreover, the
presence of sulfur compounds in diesel also could increase the stability
since sulfur can function as an antioxidant [132]. However, higher
aromatic and sulfur content in diesel fuel is not favorable in the
emissions point of view.
Straight-run diesel fractions from refinery usually have a higher
cetane number (52-54). But owing to increased refining efficiency the
concentration of cracked, low-cetane gas oils (obtained from pyrolysis,
coking, and thermal and catalytic cracking and also by broadening the
distillation) in the diesel fuels increased [133]. To improve the ignition
quality, cetane improvers such as alkyl nitrates and peroxides are
added to the diesel. Ignition improvers act as free radical initiators
because of their low bonding energy (about 150 kJ mol−1) and they
promote gum formation in fuel. The addition of cetane improvers
especially amyl nitrate and Ethylhexyl nitrate in diesel leads to the
formation of corrosive nitric acid and also tends to reduce the
concentration of antioxidant additives. The cetane additive Di-tertbutyl peroxide is relatively better than nitrates but it is quite expensive
[133].
4.11. Toxicity
Toxicity of antioxidants is also one of the prime considerations
while selecting antioxidants for biodiesel fuels. Generally, hindered
phenol antioxidants such as BHT, BHA and TBHQ are colorless,
odorless, cheap and less-toxic [39]. On the other hand most aromatic
amine antioxidants are toxic in nature and some of them are carcinogens [134]. Inhalation toxicity should be considered as the main
criteria while selecting the antioxidants for biodiesel rather than the
oral toxicity. As per material safety data sheets [135–137], both
phenolic and amine antioxidants are respiratory tract irritants and
therefore they must be selected carefully. Many studies were conducted
on the toxicity and mutagenicity of biodiesel combustion products;
however, until recently, no toxicological studies were found specifically
for antioxidants treated biodiesel.
Acknowledgements
The authors would like to thank Dr. M.V. Muthuramalingam, the
Chairman, Mr. M.V.M. Velmurugan, CEO, Dr. N. Duraipandian,
Principal and Dr. G. Prabhakaran, HoD (Mechanical Engineering) of
Velammal Engineering College, Chennai, India for their support.
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