Hypochlorination of the olefinic acid followed by dehydrohalogention, is one of the earliest preparations of the epoxy acids via chemical means (Scheme 2) (Nicolet and Poulter, 1930). This method, however, is not suitable for large-scale experiments as the dehydrohalogenation step rarely exceeds 50% yield, in contrast with small scale experiments affording an excellent yield of epoxides (Swern, 1955).

In industrial processes, the Prilezhaev reaction, i.e. the epoxidation reaction using peracids as the oxygen transfer reagent to the olefin, is the traditional, most commonly used method for the epoxidation of fatty acids and triacylglycerols (Köckritz and Martin, 2008). Two common oxidants employed in the industry are performic acid and peracetic acid, generated in-situ using hydrogen peroxide as the oxygen donor and a carboxylic acid such as formic acid (Ni and Salimon, 2012) or acetic acid. While the use of formic acid can be carried out with no catalyst requirement (Ni and Salimon, 2012), acetic acid requires an acid catalyst, either a strong inorganic acid (Dinda et al., 2008), such as HCl, HNO₃, H₃PO₄ and the most widely used is H₂SO₄, or acidic ion exchange resins (AIER) (Cooney et al., 2011; Mungroo et al., 2008). The mechanism for this reaction can be seen in Scheme 3. Despite having a higher rate of epoxide formation when formic acid is employed (Dinda et al., 2008), acetic acid is found to be a better choice as 10% higher conversion of ethylenic unsaturation to epoxides is observed compared to formic acid (Mungroo et al., 2008).

When epoxidation is carried out using hydrogen peroxide and the unsaturated fatty acid without the presence of organic peracid, no reaction is observed, as the peracid is essential for transfering the active oxygen from the aqueous to the oil phase (Nowak et al., 2003).The use of the strong mineral acids, however, has a few drawbacks including non-selectivity and low epoxide content in the final product as the highly strained epoxide ring is susceptible to ring-open under acidic conditions, giving rise to a number of side products such as diols, hydroxyl esters, estolides and other dimer formation (Figure 2) (Saurabh et al., 2011). The handling of the peracid on a large scale is very hazardous due to its instability and explosive property in high concentrations (Cooney et al., 2011). The use of mineral acids also resulted in several problems such as equipment corrosion and environmental issues, and the acid must be neutralized and removed from the end product (Cai and Wang, 2011).

Conversion as high as 98% was reported for the methyl ester of palm fatty acid distillate (PFAD-ME) producing a respective epoxide compound using a molar ratio of 1:1:4 (PFAD-ME:formic acid:H₂O₂) at 50 ^oC (Lee et al., 2009).

Thus, over the last ten years, many studies have been reported on homogeneous and heterogeneous catalytic epoxidation, from the enzyme-to metal-catalyzed reaction of vegetable oils with different oxidants considering the advantages offered by the modified fatty acids derived from vegetable oils.

Acidic ionic exchange resin was then introduced as a heterogeneous catalyst replacing the inorganic acid catalyst with improved yield and/or selectivity. Mungroo et al. (2008) and Dinda et al. (2008) studied the catalytic activity of AIER on the epoxidation of canola and cottonseed oil, respectively, using peroxyacetic acid as the oxidant with good oxirane conversion, up to 65%, and selectivity as high as 90%. Borugadda and Goud (2014) reported the epoxidation of castor oil fatty acid methyl esters using Amberlite IR-120(H) for the synthesis of bio-lubricant base stock. This catalyst, however, has some disadvantages which limit its application for large scale production, such as high mass transfer resistance, long reaction time, high cost and non-uniform acid sites (Cai and Wang, 2011).

In 2011, Cai and co-worker (Cai et al, 2011) brought into light the use of SO₃H-functional Brønsted acidic ionic liquid as catalyst for the epoxidation of fatty acid methyl esters giving as high as 84.4% oxirane conversion (Cai and Wang, 2011). Cai suggested that this “task-specific ionic liquid” (TSIL) catalyst shows great potential as a replacement of conventional homogeneous/heterogeneous acid catalysts with properties like flexibility, non-volatility, non-corrosive and immiscibility in many organic solvents and has been reported to be applied in other organic reactions, such as esterification, alcoholysis, Claisen-Schmidt condensation, nitration and hydrolyzation.

Molecular oxygen (O₂) can act as oxygen donor via the radical epoxidation pathway. The presence of a metal catalyst is said to affect the initiation, rate of reaction and selectivity to epoxides in competition with allylic oxidation. The aldehyde will be co-oxidized into carboxylic acid along with epoxide formation.

The metal complex catalyst can be recycled up to ten times as reported in the Ru-catalyzed epoxidation of methyl oleate in a perfluorinated solvent (Klement et al., 1997; Ragagnin and Knochel, 2004).

Tert-butyl hydroperoxide (TBHP), ethylbenzene hydroperoxide and cumyl hydroperoxide (CHP) are examples of organic hydroperoxides that have been used as oxidants in the presence of metal catalysts, such as Mo, Ti, and Ru.

The industrial production of propylene oxide mediated by TBHP and ethylbenzene hydroperoxide was first developed by The Halcon Corp. and Atlantic Richfield Oil Corp. (later ARCO) in 1970s using isobutane and ethylbenzene, respectively. Interestingly, this method produced t-butanol (starting material of methyl tert-butyl ether, MTBE) and α-methylbenzyl alcohol (starting material of styrene) as co-product (Scheme 4).

Titanium-derived catalysts are used both as homogeneous and heterogeneous catalysts. The latter are found to be more efficient for the epoxidation reaction. The use of a titanium tartrate complex as a homogeneous catalyst in the Sharpless-Katsuki epoxidation is one of the well-known examples of asymmetric epoxidation (Li and Wang, 1997). Omar et al. (2003) reported the application of this reaction on (13S )-hydroxy-9Z, 11E-octadecadienoate (13S-HODE) giving 76% of diastereomeric excess.

Guidotti et al. (2006) reported high epoxide and diepoxide yields from the epoxidation of methyl ester derivatives of fatty acids from different vegetable oils, obtained using TBHP as oxidant and Ti-MCM-41 (ordered mesoporous titanium-grafted silica). Catalytic activity is highly dependent on textural properties as the catalytic reaction was reported to occur at the external surface area of these materials. Superior catalytic activity of the well-structured titanosilicates compared to non-ordered Ti-SiO₂ has been proven in several reports. This method offers several advantages, including acid-free conditions, easy removal of catalyst by filtration and low oxidant excess.

However, simple amorphous Ti-SiO₂ is shown to be a good alternative to titanosilicates (especially for much more expensive Ti-MCM-41 material) as a comparable yield of up to 60% epoxidised methyl oleate can be obtained with good selectivities (>90%). Rios et al. (2005) reported that catalyst structure does not play any important role in catalytic reactivity.

The use of organic hydroperoxides is not limited to metal catalysts. In 2001, Piazza et al. reported the use of TBHP in enzymatic epoxidation.

The biocatalyzed reaction is more environmental friendly and it is more selective. This method is considerably favored as it suppresses undesirable ring opening of the epoxide (Biermann et al., 2000), and can lead to high selectivity. On top of that, the stability and separation of enzymes are improved by the immobilization of the enzymes (Köckritz and Martin, 2008). Immobilized Candida antarctica lipase B is one of the enzymes used in most of the chemo-enzymatic epoxidation reactions of several vegetable oils i.e. soybean, rapeseed, linseed and sunflower oil. The lipase enzyme is reusable, selective and exhibits excellent stability and catalytic reactivity with a conversion rate of over 90%.

The earliest report of enzymatic epoxidation was made in the early 1930s using cytochrome P450 monooxygenase (CYP) on arachidonic acid, oleic acid, linoleic acid, α-linoleic acid, 5, 8, 11-eicosatrienoic acid, 5, 8, 11, 15, 17-eicosapentaenoic acid and docosahexaenoic acid producing the corresponding mono-epoxides (Oliw, 1994; Zhang et al., 2014).

The first chemo-enzymatic epoxidation was carried out in 1990 (Björkling et al., 1990). Studies made by Vlček and Petrović (2006) revealed that in the epoxidation of soybean oil, the rate of reaction was dependent on the concentration of the lipase enzyme. Enzyme sensitivity towards temperature was reported by Sun et al. (2011) in the epoxidation of Sapindus mukorossi seed oil (SMSO) using H₂O₂ as the oxidant; the reaction temperature and enzyme load had significant effects on epoxidation while reaction time and substrate ratio were less significant. Sun also suggested the epoxidation of SMSO is enhanced with the use of stearic acid as active oxygen carrier (Sun et al., 2011).

Orellana-Coca et al. (2007) investigated the effect of the reaction variables on the lipase-mediated epoxidation of linoleic acid. This study revealed that the excess amount of H₂O₂ with respect to the number of double bonds present in the fatty acid is crucial to cut short the reaction time to compensate for H₂O₂ decomposition for reactions above 50 ^oC and to increase the reaction rate (H₂O₂ concentration between 10–50% wt) at the expense of enzyme inactivation.

The use of oat seed (Avena sativa) peroxygenase immobilized onto a hydrophobic support was also reported in the enzymatic epoxidation of sodium oleate. 80% conversion to epoxide was obtained using t-butyl hydroperoxide (TBHP) as the oxidant at 65 ^oC after 24 hours. This study also revealed pH and temperature dependence of this stereospecific reaction (Piazza et al., 2001). Later, they reported the improvement of the epoxidation of vegetable oil amides from linseed oil in hexane with the addition of a small amount of isopropyl ether and buffered water containing Tween 20 using TBHP and cumene hydroperoxide as oxidants (Piazza and Foglia, 2005).

The effect of different epoxidation methods has been investigated by Saithai et al. (2013) employing two enzymes: Novozyme^® 435 (CALB) and a homemade lipase/acyltransferase (CpLIP2), and in-situ chemical epoxidation using H₂O₂ and formic acid.

A recent publication of enzymatic-mediated epoxidation was reported by Grausem et al. (2014). They reported on the epoxidation of linoleic acid (C18:2) by CYP77A19, one of the two cDNA clones of new cytochrome P450s from potato tubers (Solanum tuberosum).

In general, low stability of the enzyme under certain reaction conditions limits the potential of this method compared to chemical treatment due to the sensitivity of the enzyme to the concentration of hydrogen peroxide. The enzyme has been reported to be stable at a temperature of up to 50 ^oC. Nevertheless, this reaction condition is not favorable due to the decomposition of H₂O₂ and possible reduced catalytic activity of the enzyme.

Metalloporphyrins are a combination of a porphyrin and a metal such as heme. These compounds have been shown to selectively catalyze various reactions, i.e. oxygenation, oxidation, oxidative chlorination and dismutation.

A recent work concerning the catalytic epoxidation of fatty acid methyl esters (FAMEs) was reported by Zhang et al. (2014) using tetraphenylporphyrins modified with variable metals (such as Mn, Fe, Co, Ni, Zn) and electron-donating substituents (Figure 3).

Dioxiranes have been shown to be one of the most versatile reagents for epoxidation as they transfer an oxygen atom efficiently to a variety of organic substrates (Curci et al., 1984). The preparation can be either in an isolated solution or in-situ (Scheme 5) (Frohn et al., 1998). Dioxiranes are powerful and selective oxidants for electron-rich alkenes, whereas electron-poor alkenes such as α,β-unsaturated acids, esters and ketones (Adam et al., 1990; Armstrong and Ley, 1990; Messeguer et al., 1991), and β-oxo enol ethers (Adam and Hadjiarapoglou, 1990) will require elevated temperatures (up to 30 ^oC), extended reaction times (up to 2 days), and an excess of dioxirane (up to 3 equiv) for complete conversion. Numerous synthetically useful transformations of a wide variety of organic substrates have been carried out employing dioxiranes.

Dimethyldioxirane (DMDO) is the simplest form of dioxirane. This reagent is not commercially available due to instability during transport and storage but it can be prepared by reacting acetone and the triple salt Oxone^® (2KHSO₅·KHSO₄·K₂SO₄) in the laboratory, both on small and large scales (Adam et al., 1987).

Adam et al. (1989) suggested that pH control between 7.0 and 7.5 by means of phosphate or bicarbonate buffer is essential for in-situ generation of dioxirane as low pH will suppress the critical deprotonation step (from 2 to 3, Scheme 5). In addition, high pH leading to the destruction of dioxirane by oxygen transfer competition from the dioxirane to the substrate or SO₄^2-, and autodecomposition of Oxone^® is a concern. Nevertheless, Frohn et al. (1998) reported that some of the chiral ketones studied were highly pH dependent, and the reactions at higher pH are more efficient. He suggested that the formation of oxy-anion 3 (Scheme 5) is preferred at a high pH, which led to efficient formation of DMDO.

There have been previous studies on the epoxidation of vegetable oil mediated by ethylmethyldioxirane (EMDO), an analogue of the dioxiranes. In 2007, Akintayo reported complete conversion of the double bond in the epoxidation of Plukenetia conophora oil (Akintayo, 2007). Epoxidation of methyl oleate and methyl ricinoleate were reported by Sonnet affording the corresponding epoxides in excellent yield (Sonnet and Foglia, 1996). Both of these reactions were accomplished under biphasic conditions employing 2-butanone as the solvent as reported by Curci et al. (1980).

One of the primary markets for epoxidized vegetable oils (mainly soybean) is as plasticizers and hydrogen chloride acceptors for use in plastic materials, especially polyvinyl chloride (PVC). As PVC exposed to heat and/or light, the HCl evolved which may cause the development of undesirable color and rapid deterioration of the polymer (Greenspan and Gall, 1953; Swern, 1955). Plasticizers are additives integrated into rigid polymeric material to improve their flexibility, workability, and distensibility, thereby increasing the plastic utility in numerous applications.

A recent study by Kandula et al. (2014) showed that fatty acid methyl ester ketals derived from soybean oil can be synthesized via the epoxide as intermediates. The synthesized soy ketal compounded with PVC exhibited better thickening and aging behaviors as compared to petroleum-based plasticizers.

Corrosion is a common problem faced in our daily lives. The idea of using renewable resources in coating technology is crucial as an effort to reduce our reliance on non-renewable petroleum-based products (Alam et al., 2014). Furthermore, the development of coating materials from renewable sources such as vegetable oil can provide low cost in the large scale production due to availability. Naturally occurring epoxidized oil (such as Euphorbia lagascae and Vernonia galamensis) is uncommon. However, in the case of palm oil, the active functional groups introduced can play an important role in the downstream chemical industry as well as its applications. The epoxides of methyl ester of oleic acid have been reported to have good corrosion inhibitor properties and polymeric binders (Feldmann and Schäfer, 2001) which are important properties as coating materials attributed to the long hydrophobic chains of the fatty acids which also contribute to the high flexibility of the substance.

Dihydroxystearic acid which is derived from the hydroxylation of epoxidized palm fatty acid has shown to exhibit rust inhibition properties evaluated using stainless steel rod in bis (2-ethylhexyl) sebacate and tris (2-ethylhexyl) phosphate (Rahman and Sadi, 1998).

Lubricant is a substance employed to reduce wear or tear of one or both surfaces in mutual contact and moving in relation to each other. In general, vegetable oils possess most of the necessary lubricity properties such as good contact lubrication, high viscosity index (lowest changes in viscosity with temperature), high flash point and low volatility. Above all, the presence of a high degree of unsaturation in vegetable oils led to low thermal and oxidative stability, which can be improved by epoxidation (Akintayo, 2007).

The evaluation of palm kernel oil’s (PKO) properties as lubricant has been reported previously. The properties of PKO were compared with heavy duty oil (SAE 40) and light duty oil (SAE 30) and it was shown that PKO is a promising base oil for the production of value-added lubricating products with comparable properties that meet SAE specifications (Musa, 2009). Nevertheless, there are still limitations on some properties that requires modification such as poor low-temperature properties, susceptibility to oxidative degradation and tendency to undergo hydrolysis under acidic conditions (Campanella et al., 2010). Epoxidized castor oil was showed to possess enhanced properties of lubricity like higher density, kinematic viscosity and pour point. This is probably due to the stronger interaction between the molecules of the epoxides (Borugadda and Goud, 2014).

The study of the production of polyols with branched ether and ester compounds, with epoxidized vegetable oils as the intermediates has been carried out for the improvement of their properties. The epoxides will then be subjected to ring-opening reaction under acidic conditions to produce alcohols which will then be subjected to esterification reactions (Adhvaryu et al., 2002).

Moser et al. (2007) reported improved oxidative stability and fluidity at low temperatures via esterification of alkyl 9,10-epoxyoctadecanoates using propionic and octanoic acids under catalyst- and solvent-free environments.

The reaction of the epoxygenated fatty acids with low-molecular-weight mono- or polyfunctional alcohols or acids via the ring-opening of the oxirane afforded polyols (Hazimah et al., 2011) that can be used in a polyurethane system, adhesives and casting resins (Norhayati et al., 2013). Polyols can also serve as raw material in the production of palm-based flexible polyurethane foams (Shaari et al., 2006) to manufacture some daily products such as carpet underlay, sandwich panels and headrests (Hazimah, 2012). Polyols can also be used in a wide range of coating processes from thin decorative or protective coating to industrial flooring applications due to their relatively low viscosity and compatibility with methylene di (phenylisocyanate) (MDI). A study on the alcoholysis of epoxidized palm olein catalyzed by K10 Montmorillonite produced palm-based polyol which can be used as a component in coating applications (Norhayati et al., 2013).

Preparation of low viscosity and light-colored fatty acid-based polyols can be carried out in a three-step reaction, depending on several parameters at each step such as temperature, pressure, amount of catalyst used, molar ratio of reactant, and reaction time. Catalyst loading was reported as crucial factor to be controlled to produce light-colored polyol in the first step (Hoong et al., 2008).

A patent was filed for the conversion of used frying oils into polyol via epoxidation as one of the intermediates followed by reaction with diisocyanate to afford the corresponding polyurethane product. The epoxidation was carried out using peroxyacetic acid or performic acid at 65–70 ^oC (Shaari et al., 2006).

Self-levelling polyurethane or epoxy/polyurethane multilayer systems are widely used in the flooring industry because of good chemical and mechanical properties such as minimal shrinkage, high mechanical strength and durability, and the reasonable cost of installation offered by these materials. Apart from that, they can also be used to bind porous filler materials and rubber particles to produce composites for sport tracks and playing fields (Höfer, 1999).

The epoxygenated palm olein acrylate derived from the respective epoxide was studied for use in the synthesis of radiation curable resins (Mohd Nor et al., 1992). The presence of the “high strain energy” rings on the fatty acid chains promote cross-linking when the epoxy resin is cured. The higher the number of epoxy rings that are opened by the process, the more cross linking can occur, and the higher the quality of the resulting plastic (Goud et al., 2007).

Most of commercial ionic and non-ionic surfactants are derived from C12 and C14 fatty acids which are abundant in PKO. Fatty acids with longer carbon chains are barely used due to their hydrophobic property and less suitable for micelle formation. As for palm oil that contains longer chain fatty acids, the modification of the fatty acids has been carried out to increase the polarity of the fatty acids. Epoxide can serve as an intermediate to attach the polar groups to internal positions of the fatty acids (Dierker and Schäfer, 2010). Internal diols can be obtained by acid-catalyzed ring opening of the epoxidized fatty acids or by dihydroxylation in the presence of osmium tetroxide as catalyst. Conversion of methyl oleate into methyl vic-oligoethyleneglycol hydroxy oleate was carried out in the presence of tin tetrachloride (Scheme 6).

The surfactants with tensidic properties synthesized by the modification of long chain fatty acids were shown to be comparable or with better properties than those of commercial surfactants of C12 and C14 fatty acids (Dierker and Schäfer, 2010).

Ring-opening of epoxidized methyl oleate occurred when reacted with bifunctional levulinic acid to form cyclic ketals, by controlling the temperature and acid concentration, led to potential new surfactants derived from oleochemicals (Doll and Erhan, 2008).

In a pharmacological study, the epoxidation of arachidonic acid catalyzed by cytochrome P450 (CYP) epoxygenase generated the corresponding epoxygenated fatty acid, the epoxyeicosatrienoic acid – an important mediator involved in regulating various biological processes including inflammation, pain and angiogenesis (Zhang et al., 2014).