Role of antioxidants in enhancing oxidation stability of biodiesels

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Role of Antioxidants in Enhancing Oxidation Stability of Biodiesels Shilpi Agarwal,*,† Shailey Singhal,† Manjeet Singh,† Shefali Arora,† and Manisha Tanwer‡ †

Department of Chemistry, University of Petroleum & Energy Studies, Bidholi, Dehradun 248007, India D. Y. Patil Institute of Engineering, Management and Research, Arkudi, Pune 411044, India

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‡

ABSTRACT: Chemical composition of biodiesel plays an important role in deciding its oxidation stability. Bisallylic positions in fatty acid ester chain are especially prone to attack by oxygen, leading to the formation of gums, acids, and polymer deposits that cause failing or plugging problems in the engine and filter as consequences. Although oxidation of biodiesel cannot be checked completely, it can be slowed using antioxidants, which prevent the process from continuing by scavenging the free radicals, formed by the attack of oxygen on reactive sites of fatty acid ester chain. Antioxidants having phenolic and amine moiety have been found to be especially effective to enhance the stability. The mode of action and activity of various antioxidants have also been discussed. KEYWORDS: Biodiesel, Oxidation stability, Antioxidants, Phenols, Amines

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INTRODUCTION Biodiesel is an environmentally benign fuel with high biodegradability and lubricity when compared to the conventional diesel, but suffers with the disadvantage of having deterioration property due to its tendency to oxidize in the presence of oxygen at ambient temperature. The process results in the formation of innumerous undesirable oxidation products, 1−3 which may either cause high molecular compounds as polymers, gums, and sediments4 or decompose to lower aldehydes, ketones, alcohols, acids, epoxy compounds, olefins, etc.5 Acids formed during the process can corrode metals used in vehicle and distribution fuel handling systems severely. Polymers, deposits, and other insoluble materials formed during oxidation can cause fuel filter blocking. Some of the products are soluble in the fuel, pass through the fuel filter, but stick to the surfaces of the fuel pump and fuel injectors. Hydroperoxides damage the elastomeric gasket materials in the engine combustion mechanical system.4 Short-chain carboxylic acids such as formic acid and acetic acid are miscible with water and dissociate to form reasonably strong acids (pKa 3.77 and 4.76, respectively), which lead to the increase in acid value of biodiesel. At low temperature, polymers that resulted by the oxidation of methyl ester contain dimers mainly with C−O− O−C and C−O linkage between the fatty acid units,6 which gets deposited at the bottom of the tank. Along with causing specific problems in engine, oxidation products also change the physicochemical properties of biodiesel (Table 1) and thereby affect the quality of fuel adversely. Therefore, the extent of oxidation can be predicted by studying various parameters as acid value (AV), peroxide value (PV), density, viscosity, flash point (FP), iodine value (IV), etc. The significance and consequences of change in these fuel properties are summarized in Table 2. It leads to an increase in density, viscosity, acid value, and peroxide value, but a decrease in iodine value.18 Therefore, in © XXXX American Chemical Society

Table 1. Standard Values of Some Parameters (Related to Oxidation) of Biodiesel as per European and ASTM Specifications property induction period peroxide value density viscosity acid value iodine value fatty acid methyl ester (FAME) content

units

biodiesel ASTM D6751

biodiesel EN14214

h

min 3

min 6

(g/cm3) at 15 °C (mm2/s) at 40 °C mg of KOH/g mg of iodine/ 100 g % m/m

0.860−0.900 (ASTM D 4052) 1.90−6.00 (ASTM D 445) 40 2.5 1.6 1.0 0.2

Hence, the relative oxidation rates of common C18 unsaturated fatty acid esters found in biodiesels can be ordered as linolenate > linoleate > oleate.47,48 The bisallylic methylene group has high reactivity to form radicals due to less dissociation energy of C−H bond at this position. The radials formed at the bis-allylic sites immediately isomerize to form a more stable conjugated structure. All of the possible types of radicals react with O2 to form peroxide radicals. The existence of these molecules is an early indication of oxidation taking place, and it is measured in terms of peroxide value. Later, aldehydes, ketones, furans, alcohol, and

Figure 5. Common fatty acid methyl esters in biodiesel.

acids are formed. Finally, during the polymerization process, resins are produced making the fuel unusable. Presence of Metal Contaminants. Other than the structural features, traces of metal contaminants as Cu, Zn, Ni, Fe, Co, Mn, etc., present in biodiesel act as catalysts for oxidation and decrease the stability.49−52 In palm oil biodiesel, jatropha biodiesel, and rapeseed biodiesel, Cu had the strongest detrimental and catalytic effect on oxidation D

DOI: 10.1021/acssuschemeng.8b02523 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering stability,52,53 while Fe had the least effect on degradation of jatropha biodiesel.53 In rapeseed oil, 70 ppm of Cu concentration increases the level of hexanal and 2-hexenal 70-fold and 200-fold, respectively, relative to rapeseed oil without copper.54 Therefore, utmost care should be taken while selecting the storage tank material for biodiesel as the presence of copper, zinc, bronze, etc., may hasten the oxidation process and result in the formation of insoluble sediments. Along with affecting the biodiesel adversely, metal contaminants also get deposited in engine parts as undesirable residue, which may release into the atmosphere along with combustion products of fuel and cause air pollution.40,55 They induce ready decomposition of hydroperoxides to alkoxyl radical intermediates (RO•) and (ROO•) (Figure 6), which in turn attack other fatty acid species to propagate the chain reaction.34

where IPx is the induction period when antioxidant is present and IPo is the induction period when antioxidant is not present. Antioxidants give better results at lower concentration in undistilled oil esters due to some naturally present antioxidants like α-tocopherol and flavonoids in them, but because they get volatilized off during distillation, a higher concentration of antioxidants is required in distilled oil esters.62,63 The most common antioxidants currently used are substituted monohydroxy or polyhydroxy phenolic compounds, which, due to their low activation energy, can donate the hydrogen easily and interrupt the propagation step of the free radical chain. In this manner, they prevent the continuous formation of reactive free radicals and thereby slow the process of oxidation. The resulting antioxidant free radical stabilizes itself by delocalization of the odd electron, and also it can react with another free radical to make stable complex compounds. Antioxidants might show significantly different antioxidant competences depending upon the type and positions of the substituents present in their structures.

Figure 6. Formation of hydroperoxides for reaction with transition metals.

Temperature. Temperature is another significant factor as it makes the oxidation process of biodiesel intense 56 irrespective of the type of oil.57 High temperature favors the formation of conjugated dienes, which may react with another double bond through Diels−Alder reaction to form high molecular weight polymers. Other important parameters affecting the oxidation process of biodiesel include the presence of air, light, humidity, and the surface area between biodiesel and air.58,59

ROO• + AH → ROOH + A•

(8)

R• + AH → ROH + A•

(9)

ROO• + A• → ROOA

(10)

RO• + A• → ROA

(11)

These reactions are exothermic in nature. As the bond strength of “AH” decreases, the efficiency of antioxidant increases. Phenolic compounds are considered as powerful chain breaking antioxidants. They have a very important scavenging property due to the presence of the hydroxyl groups. The resulting phenoxy radical moiety stabilizes itself by resonance (Figure 7). Phenol itself does not show antioxidant behavior, but when its ortho or para positions are occupied by alkyl groups, it shows an effective antioxidant activity due to the increase in electron density on the OH group, which makes the release of H radical easy. Bulky tert-butyl groups on ortho positions cause steric repulsion with the OH group and force it to bend from its natural angle. The process leads to the decrease of O−H bond strength.64 The resulting phenoxyl radical is sterically protected by bulky alkyl groups, and therefore the stability of the phenoxy radical increases many folds as in BHT (Figure 8). The number of hydroxyl groups present in the molecule is also the deciding factor in establishing the oxidation preventing tendency of an antioxidant. tert-Butylhydroxyquinone (TBHQ) is a diphenolic antioxidant with two OH groups on para positions. It can react with peroxyl radicals to form semiquinone radical stabilized by resonance. The semiquinone radical can further combine with another lipid free radical or another radical of the same type to form more stable compounds (Figure 9). Pyrogallol possesses three hydroxyl groups (OH) in its aromatic rings, which scavenge reactive free radical to interrupt the chain propagation and thereby inhibit the rate of oxidation in methyl esters. The resulting antioxidant radical becomes stable by intramolecular hydrogen bonding, and can react with other fatty acid free radicals to further inhibit the oxidation process (Figure 10). The two antioxidant free radicals also can react together to give stable products. Nordihydroguaiaretic acid contains four OH groups in its molecule. Through stabilization of phenoxy and diphenoxy

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PREVENTION OF OXIDATION/METHODS TO INCREASE OXIDATION STABILITY: ANTIOXIDANTS Although oxidation of biodiesel becomes inevitable due to its composition and storage conditions, it can be minimized by following certain measures like decreasing its contact with air, light, moisture, storing it at low temperature, etc. Even after taking the utmost precautions during its storage for a long period, oxidation occurs, which is mainly the subject of its composition. As the oxidation initiates, the external factors such as air, light, temperature, etc., accelerate the reaction. Therefore, use of antioxidants becomes substantially vital and is the most widely accepted practice for improving the oxidation stability of biodiesel (Table 4). Two types of antioxidants are commonly used in biodiesel: primary and secondary. Primary antioxidants are the free radical quenchers, which play an important role in removing the free radicals formed during the steps of initiation and propagation by donating hydrogen atoms and interrupting the chain reaction.60 The resulting antioxidant radical stabilizes itself through resonance without causing any further chain reaction. The most widely used primary antioxidants are sterically hindered phenols and aromatic amines. The efficiency of an antioxidant depends on many factors such as the structure of antioxidant, prevailing storage conditions, composition of the biodiesel, etc.,23,27 and is represented by the stabilization factor F:46,61 IP F= x IPo E


Research Article

ACS Sustainable Chemistry & Engineering Table 4. List of Commercial Primary Antioxidants

The reaction of ADPA starts with the abstraction of hydrogen atom by alkyl peroxy free radical. The resulting aminyl radical reacts with another peroxy free radical to form nitroxyl radical and alkoxy radical (Figure 14). On further reactions with free radical, it forms compounds like nitroxide and benzoquinone. On an equal mole basis, ADPA can quench 4 free radicals per molecule. While comparing their efficiency to the hindered phenolic antioxidants, in general, secondary aromatic amines can scavenge 50−500 peroxy radicals per molecule,70 while hindered phenolic antioxidants can trap only two peroxy radicals per molecule.71 However, they have the drawback in that they form many colored products as the oxidation proceeds. Secondary antioxidants are the hydroperoxide decomposers, which break hydroperoxides (ROOH) into nonreactive stable products before they get converted into radicals.72,73 The most commonly used secondary antioxidants are organic phosphite esters and sulfides, which convert into phosphates and sulfoxides respectively during the reaction. In general, sulfide antioxidants are active at high temperature (>100 °C), while phosphites decompose hydroperoxides at substantial lower

Figure 7. Stabilization of phenoxy radical.

radicals and further reactions, it prevents oxidation of fatty acid methyl esters effectively (Figure 11). Another important class of antioxidants is aromatic amines. Primary aromatic amines scavenge the free radicals and form imines (Figure 12). Aminophenol with OH and NH2 groups in the 1,2 position has the ability to form an intramolecular hydrogen bond, and the H atom that is not involved in this bond will then be abstracted by free radicals to result in a stable phenoxy radical (Figure 13).68,69 Secondary aromatic amines such as alkylated diphenyl amines (ADPA) are also very effective free radical scavengers. F


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ACS Sustainable Chemistry & Engineering

Figure 8. Antioxidant behavior of BHT.65

Figure 9. Antioxidant behavior of TBHQ.

temperature.74 Sulfoxides again decompose another molecule of peroxide and itself gets oxidized to sulfones (Figure 15). They are mostly used in combination with primary antioxidants to achieve a synergistic inhibition.

beef tallow,77 whereas BHT is effective for refined soybean oilbased biodiesel.82 In karanja biodiesel, BHT showed better efficiency than BHA at different concentrations (250, 500, and 750 ppm) of antioxidant.79 Depending upon the initial induction period of biodiesel, the optimum concentration of different antioxidants varies and has been summarized in Table 6. In an another study in which biodiesel B-1 was prepared from a mixture of oils containing 36% used frying oils, 20% palm oil, and 44% soybean oil, it was found to have IP > 6 h at 1000 ppm concentration of propyl gallate (PG), pyrogallol (PA), and tert-butyl hydroquinone (TBHQ), whereas BHA and BHT could not be able to produce such good results. The reason behind it was explained on the basis of their molecular structure. Because of the presence of 2OH groups and thereby their resulting electronegativities, PG, PA, and TBHQ offered more sites for the formation of stable complex between ester free radical and antioxidant free radical.60 The efficiency was found to be less in BHA and BHT due to the presence of only one OH group. Low volatility of BHA and BHT was probably another reason for their poor performance because much of

(RO)3 P + ROOH → (RO)3 PO + ROH

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EFFECT OF ANTIOXIDANTS ON DIFFERENT BIODIESELS Antioxidants generally have a positive impact on oxidation stability of all different types of biodiesel, although their efficiency may be very different for biodiesels from different feedstocks, and it depends on the composition of fatty acid esters present in the biodiesel. The biodiesel having higher unsaturation is more prone to oxidation. Table 5 summarizes the inhibition effectiveness of antioxidant at a constant concentration of 1000 ppm for biodiesel derived from different feedstocks with unsaturation of fatty acid esters. Among the different antioxidants investigated, PG and PY are very effective for biodiesels obtained from karanja oil,79 rapeseed oil,77 safflower oil,80 honge oil,81 used frying oil,77 and G


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Figure 10. Antioxidant reactions of pyrogallol.66

Figure 11. Antioxidant action of nordihydroguaiaretic acid.67

Figure 13. Antioxidant mechanism of 2-aminophenol.68

the additive might have been lost while performing the standard oxidation test method.60 Few studies showed that the effectiveness of BHA and BHT on the oxidative stability of rapeseed oil biodiesel, and tallow-

Figure 12. Antioxidant mechanism of p-phenylenediamine.68 H


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Figure 14. Antioxidant mechanism of alkylated diphenyl amines.65

Table 5. Antioxidant Efficiency at the Concentration of 1000 pppm in Different Biodiesels unsaturation (wt %)

antioxidant order at 1000 ppm (induction time in h)

soybean

82−85

jatropha

77−78

rapeseed

85−91

yellow grease used frying oil

78−81

PY (11.5) > TBHQ (11) > PG (10) > BHA (7) > > BHT (6) PY (53.73) > PG (24.45) > BHA (11.15) > BHT (8.59) > TBHQ (5.56) TBHQ (38.53) > PG (27.36) > PY (26.31) > BHA (24.30) > BHT (9.85) PY (19) > PG (14) > TBHQ (6.5) > BHA (5.5) > BHT (4) PY (31.95) > PG (29.90) > TBHQ (29.44) > BHA (13.80) > BHT (10.84) PY (15) > PG (12) > TBHQ (6.5) > BHA (5.5) > BHT (3.5) PG (7) > BHA (3) > TBHQ (3) PY (26) > PG (24) > TBHQ (18) > BHA (14) > BHT (8)

biodiesel

Figure 15. Oxidation of phosphites and sulfides by hydroperoxide.

based biodiesel was maximum at the optimum concentration of 400 ppm.94 PG increased the induction period of acai biodiesel from 1.5 to 21.3 h at 500 ppm concentration.78 In all, the optimum concentration and effectiveness of antioxidants vary with the change in the feedstock, production procedures, composition, and properties of biodiesel.95−97 Various antioxidants, 2,6-ditertiarybutyl hydroxytoluene (AO-1), bis-2,6-ditertiary butyl phenol derivative (AO-2), and octylated butylated diphenylamine (AO-3), were found to enhance oxidation stability of jatropha biodiesel up to an optimum concentration. However, AO-1 was reported to be most effective among all three antioxidants.15 Catechol and 4-allyl-2,6-dimethoxyphenol are also used as antioxidants in rapeseed and soybean biodiesel.98 The use of catechol in very small amounts as low as 0.05% mass fraction in soybean biodiesel and 0.10% mass fraction in rapeseed biodiesel enables compliance with the restrictive limit imposed by the EN14214 standard. On the other hand, 4-allyl-2,6dimethoxyphenol works satisfactorily only in rapeseed biodiesel blended with additive contents above 0.3% mass

75−78

sunflower seed linseed distilled used frying oil neem

85−88

65−67

beef tallow

54−56

87−89 78−80

PY (44.50) > PG (30.20) > BHA (4.54) > BHT (2.41) > TBHQ (2.15) PY (37) > TBHQ (15.5) > PG (14.5) > BHA (9) > BHT (6)

ref 75 76 76 75 77 77 78 77 76 77

fraction. However, the unblended biodiesel is far from meeting the threshold value. Natural antioxidants such as sage and thyme extracts prevented the oxidation of rapeseed oil and made the induction period double.99 Also, it improved other fuel properties like lubricity, kinematic viscosity, etc., of the oil. I


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idants also reduce NOx emissions from the diesel engine significantly.81,103 As antioxidants differ in their antioxidative characteristics, various correlations can be established among the properties of different antioxidants under different conditions. However, as per the studies made by Zhou et al. in 2016, few important antioxidants can be ranked in the following order with respect to their important parameters: antioxidant efficiency (TBHQ > PY > PG > BHA > BHT), consumption rate (TBHQ < PG < PY < BHA < BHT), and predicted shelf life (TBHQ > PG > PY > BHT > BHA). The lower consumption rate of antioxidant seemed to lead to a longer induction period and predicted shelf life of biodiesel.107

Table 6. Optimum Concentration of Various Antioxidants in Biodiesels from Different Feedstock To Meet EN 14214 Specifications biodiesel

initial IP (h)

karanja biodiesel

2.75

soybean biodiesel

4.34 0.7

1.36

jatropha biodiesel

distilled palm oil biodiesel cottonseed biodiesel karanja biodiesel

terminalia belerica biodiesel poultry fat-based biodiesel yellow greasebased biodiesel

3.27

3.23 3.52 4.9 2.24 3.17 2.54 3.76

0.67 2.25

min conc for bringing IP > 6 h (EN 14214)

ref

500 ppm 500 ppm 1000

79 79 83

1500 3000 >6000 NPPD > EHN,103 whereas aldimines, N,N′-bis(5-chloro-2-hydroxybenzylaldehyde)-1,2-phenylenediamine (A), 5-(3-chloro-2-hydroxypropoxy)-2-{(E)-[(2- hydroxyphenyl)imine] methyl} phenol (B), and 5-(3-chloro-2-hydroxypropoxy)- 2-{(E)-[(2hydroxy-1,1-dimethyl-ethyl)imine]methyl} phenol (C) enhanced the stability of soyabean biodiesel B7 blend with ULSD (ultralow sulfur diesel) in the order A > B > C.104 Even with biodiesels with pronounced oxidizability due to metal contamination such as from copper, zinc, chromium, iron, nickel, cobalt, titanium, and manganese, etc., antioxidants work efficiently to bring stability to meet ASTM and EN specifications.105,106 Along with enhancing stability, antioxJ


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bulky groups, etc., as well as nature of biodiesel, its storage conditions, etc., are the factors responsible to decide the efficiency of an antioxidant. Although a lot of studies have been made so far to establish the efficiency of various antioxidants, still more focus is required to ascertain the relation of antioxidant structure with its activity. Only a few selected antioxidants have been studied in detail. Some more advanced antioxidants are required, which can be synthesized using a green route along with having a high activity to inhibit the oxidation process. Because the efficiency of antioxidants is a subject of a number of parameters like structure of antioxidant and composition of biodiesel, an intense study is required to establish a relationship between antioxidant structure and individual component of biodiesel also using model compounds. It will help the researchers to design an antioxidant with an activity that is multiple times higher than those of existing ones and also to predict the antioxidant efficiency for a particular biodiesel just by knowing its exact composition.

Figure 16. Isomers of BHA.

Isomer I is considered to be a better antioxidant than isomer II, still less effective than BHT due to the presence of bulky tert-butyl group on only one ortho position of the OH group. High volatility of BHA also limits its efficiency and application.118 If the ortho or para position of OH group is occupied by unsaturated hydrocarbon substituent, the unpaired electron of resulting phenoxyl radical is highly delocalized in the substituent, making the radical highly stable. The bond dissociation energy of O−H group decreases and the efficiency of antioxidants increases.115 If the para position of the OH group is occupied by −SMe or −OMe group, the bond dissociation energies of the O−H group are 78.2 and 81.6 kcal/mol, respectively. Therefore, phenol with p-SMe group shows better antioxidant activity.119 It may therefore be a better choice to use sulfur in place of oxygen while designing a phenolic antioxidant structure. The number of OH groups in the structure also plays an important role in deciding the antioxidant tendency. In case of the compound having two OH groups at positions ortho to each other, the activity of one phenolic group increases much, as after scavenging the radical the resulting phenoxy radical gets stabilized through hydrogen bonding with another OH group. Therefore, PY and PG with polyhydroxy groups are superior in activity than BHA and BHT having only one hydroxyl group.120 As the number of OH groups increases further, it makes the compound more hydrophilic than lipophilic. Therefore, the concentration of antioxidant decreases in the oil phase and so the activity. On the other hand, electron-withdrawing groups such as COOH and COOR at ortho and para positions destabilize the phenoxy radical and thereby decrease the antioxidant efficiency.121 Aromatic amines are also effective antioxidants in the protection of biodiesel against auto-oxidation. Their role in the stabilization of unsaturated ester chain is more complex.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shilpi Agarwal: 0000-0003-0935-5134 Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS AND FUTURE PROSPECTS Oxidation of biodiesel is a spontaneous process, which starts immediately after its production and makes its long-term storage a challenge. Because of the presence of some susceptible sites of fatty acid esters to oxygen, oxidation starts following free radical chain mechanism and affects the life and performance of the engine adversely. It also degrades the quality of biodiesel by altering its physicochemical properties such as density, viscosity, acid value, etc., which in turn causes additional problems such as of fuel pump catastrophe, filter plugging, and corrosion. The severity of the problem can be decreased by the use of antioxidants. In comparison to the neat biodiesel, its blend with diesel fuel is more stable, and then less concentration of antioxidant is required to enhance the stability of the mixture to the recommended value. Substituted phenolic antioxidants and aromatic amine antioxidants interrupt the oxidation reaction of biodiesel by providing the hydrogen atom. Antioxidants vary in their competencies to prevent the oxidation. Structural features such as the nature of other substituents present, number of OH groups, presence of K


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Role of antioxidants in enhancing oxidation stability of biodiesels

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