2.1. Eligibility Criteria

This review paper is based on previously published research articles, related to epoxy formation, its different strategies, and epoxy curing techniques. All topic-related data have been regarded as reliable to write result and discussion.

2.2. Focus Questions

Major focus questions were about problem, investigation, comparison, and outcome (PICO). Research questions were as follows: What is epoxidation? What is bio-based epoxidation? What are the methods of epoxy curing? What are epoxidation strategies? Which techniques are applicable at industrial scale? Which technique gives limited side reactions? What are the benefits of epoxidation method? What are the future trends for epoxidation?

2.3. Search Strategy

2.4. Study Selection

The authors screened the titles and abstracts of all selected research articles independently, as mentioned in Table 1. They finalized research papers and wrote down different components of review paper.

2.5. Data Extraction Process

Data have been collected from topic-related research articles. Authors made self-grouping for data collection and extraction. First group collected introduction-based data, along with data for different curing techniques, including thermal and photochemical. Another group collected and screened data about epoxidation strategies while the third group collected data about epoxidation advantages as well as conclusion.

3.1. Eligible Study

During searching, a huge number of 400,742 articles were identified, but after title and abstract screening, only 206 articles were shortlisted for full text review (Figure 2). Out of these, 159 articles were excluded due to certain reasons, like 104 articles did not explain epoxidation clearly and 55 articles had short follow-up time period. 47 articles were finally selected for qualitative and quantitative analysis.

4.1. Thermal Curing

Thermal curing mechanism utilizes a two-component system, i.e., epoxy and hardener (Table 3) [62]. Typical examples of commonly used hardeners are amines, anhydrides, acids, thiols, phenols, or alcohols [63]. The choice of hardener for polymerization of epoxies is critical as its structure determines thermomechanical behavior of composites. Bifunctional reagent (hardener) gives linear polymeric structure, i.e., low cross-linking ratio, whereas trifunctional hardener produces three-dimensional cross-linked network of polymers. Among various curing agents, amines are most popular and common ones for epoxies because they have the capability of cross-linking both as bifunctional as well as trifunctional hardener [64]. For thermal curing, microwave radiation technique (MWR) has been reported with steady effectiveness and eco-friendliness [65, 66]. It effectively works for polar compounds and speeds up the reaction. MWR heats sample equally, hence reported for food preservation from antimicrobial activates [65, 66]. Thermally cured epoxy composites are most commonly used materials with significant industrial importance. DGEBA is a popular epoxy monomer which is cured by various curatives, but in two-component systems, it is mostly amines, i.e., DETA (Scheme 1). Anhydrides like PA, second most important curing agents for epoxies, form more stable bond with epoxies than amines. Hardener upon reaction with epoxy releases energy (exothermic reaction) that affects quality of product, and almost 100% cured material may be obtained.

Table 3. Curing agents in two-component epoxy systems.

Hardener	Structure	Composites	Ref.
Amines (50%)		Epoxy composites	[67]
Polyamides (14%)		Multifunctional superhydrophobic composites	[68]
Anhydrides (15%)		Bio-composites	[69]
Polymercaptan and polysulphides (14%)		Nanocomposites	[70]
Urea formaldehyde (4.5%)		Nanocomposite foams	[71]
Percarboxylic polyesters (22%)		Fire retardant recyclable and bio-based composites	[72]

4.2. Photocuring

Photo-induced epoxy polymerization is carried out in the presence of PI that generates active centers to initiate polymerization. These reactions follow either cationic or anionic curing protocols. Since this curing system consumes only epoxy to generate matrix, it is termed as one-component curing [73]. PI absorbs UV light to generate active species. In cationic curing, two types of mechanisms (activated chain end mechanism and activated monomer mechanism) are involved, where protonated epoxide attacks on the other epoxide ring for polymerization (Scheme 2) [74].

Due to noticeable increase in environmental issue sensitivity during last decades, petrochemical-based epoxies are becoming crucial. Polymer materials with bio-based origin have gained much attention because of their feasibility to replace petroleum-based materials. Epoxy synthesis from bio-based oils has taken significant attention in various fields (Figure 3) because of nontoxic, economical, and renewable competences. Epoxy adhesives have wide range of applications in industry, i.e., construction [75], electronics [76], automotive [77], and aerospace [78, 79]. Bio-resourced epoxy foams are extensively used for thermo-insulating and dye-decontaminating applications [33, 80]. Epoxy resins are used for coating applications on commercial level, divided in two categories: structural coatings and protective coatings. High biomass content epoxy resins have also been prepared for green coating and composite applications [27]. Recently, radiation curable coatings have been growing fast because of energy efficient and environment friendly coatings [81]. In biomedical field, epoxy resins have been utilized in food and human clinical applications [82–84]. Bio-based epoxy (BBE) paints and coatings have been reported for anti-corrosive properties [85, 86]. Cross-linked epoxy polymers are high-cost materials in comparison to other resins. Combination of properties or high performance/engineering properties is incorporated into final products to realize added value of epoxy resins [87]. Several epoxy products on commercial level are prepared with a mixture of epoxies in order to attain desired processing and properties. In order to attain higher T_g, modulus, or stiffness, epoxides with higher functionality are required. Epoxides are available in market for various applications from low-cost simple adhesives to highly engineered conductive adhesives, to high-strength structural composites. A wide range of interesting properties of epoxides could be obtained at a reasonable cost. Researchers have reported various synthetic methods (Table 1) for BBEs with specific properties.

5.1. Hypochlorination

HC of olefins is the most primitive process for epoxy monomer synthesis, on a small scale [107]. DGEBA has been synthesized from bisphenol-A and epichlorohydrin: chloro-addition of propylene produced allyl chloride which upon HC gave epichlorohydrin [108]. However, this method has not been found suitable for large-scale production as dehydrohalogenation and side product formation affect % yield [109]. Other researchers have prepared DGEBA from glycerol and epichlorohydrin [110]. Compared to the former preparation method, higher selectivity (93%) has been reported with minimum side products. However, in DGEBA synthesis, 67% of BPA is derived from fossil fuel.

5.2. Epoxidation Through Peroxy Acids

HC method failure regarding high yield on industrial level has brought in situ generated peroxy acid method in industry [106]. Reaction mechanism is presented in Scheme 3.

It was observed that sometimes peroxy acids require catalyst. Major peroxy acid oxidants are peracetic acid and performic acid with H₂O₂ (as oxygen donor), where the former requires catalyst and the latter does not [111]. Unsaturated fatty acids do not show any reaction with H₂O₂ in absence of organic peracids because peracids are responsible for oxygen transfer to oils [112].

5.2.1. Homogeneous Catalyst

Inorganic acids, e.g., HCl, HNO₃, H₂SO₄, or H₃PO₄, work as catalyst in peracetic acid epoxidation [113]. However, these acids have the drawbacks of low epoxy content and nonselectivity of final material. Highly strained epoxy group undergoes ring opening and side reactions that is hydroxyl esters, dimer, and diol formation in the presence of acid (Scheme 4) [114]. Ninety-one percent epoxy yield has also been reported from cyclohexene and peroxy acids prepared by catalytic (Fe-N/C) oxidation of iso-butyraldehyde in air or O₂ [115].

On large scale, peracid handling is very difficult and hazardous because of their instability and explosive nature in high concentrations. The acid used in reactions must be removed from products or neutralized because it corrodes the equipment and causes many health and environmental issues [116]. Hong et al. [117], reported the optimized conditions for epoxidation process of Jatropha curcas oil for in situ generation of peroxy formic acid (Scheme 5). Seventy percent yield with 80.4% oxirane conversion was found at 45°C for 2 h, having molar ratios of formic acid:oil and H₂O₂:oil as 2:1 and 12:1, respectively.

Epoxidation of SBO was performed by in situ peroxy-formic acid generation with hydrogen peroxide and formic acid in the presence of inorganic sulfuric acid as catalyst. The optimized conditions for the reaction were checked by varying mole ratio of hydrogen peroxide (1, 1.5, 2), temperature (50°C, 60°C), and time (2 h, 4 h). Thirty-eight percent yield and 2.86% OOC were reported within 50°C–60°C for 4 h. Experimental analysis revealed the low epoxide yield with some side products. It was concluded that there is longer time required for epoxidation reaction [118].

Nahor oil is nonedible, easily available feedstock with high iodine value and bio-lubricant for chemical industries. Dinda et al. [119] worked on Nahor oil epoxidation by in situ peracetic acid generation in batch reactor. The reaction was performed at 60°C for 5 h in the presence of H₂SO₄ and 72% yield was obtained. Drawbacks of this process were negligible mass transfer resistance and increased glycol formation. In 2017, epoxidation of Nahor oil was again carried out with formic acid, acetic acid, propionic acid, and Amberlite IR120. It was observed that formic acid use is better than acetic and propionic acid. Glycol was also formed as by-product in case of formic acid with H₂SO₄ [120]. Ion exchange Amberlite was found best from all other carboxylic acids with 70% yield. Glycol was also formed as by-product in case of formic acid with H₂SO₄ [120].

Researchers reported the epoxidation of economically inexpensive and efficient used/waste cooking oil having highly unsaturated content. The mineral acids HCl, HNO₃, and H₂SO₄ were utilized in this process as homogeneous catalysts. Conventional process was performed for in situ peracetic acid generation in double bond oxidation. For sulfuric acid (2%) catalyst, 20% oxirane conversion, 3.1% oxygen content, and 74.9% conversion of iodine value were obtained for 5 h at 60°C higher than other used acids. Although less epoxidation was concluded in this method, higher temperature could reduce selectivity or initiate the ring opening reaction [121].

Crude oleic acid epoxidation was carried out by in situ preparation of peroxy formic acid with acid catalyst (H₂SO₄). In the presence of acid, highly reactive oxirane ring opens and undergoes side reactions, i.e., synthesis of α-glycol or diol. It was investigated that low concentration of formic acid should be used as its high amount in in situ generation decreases the yield. It was also studied that maximum yield with formic acid could be attained as compared to acetic acid [122].

Comparative study of peracetic acid and performic acid has been reported [123]. Stoichiometric amounts of SBO, formic/acetic acid, and hydrogen peroxide were taken as 1:0.5:2, 1:0.5:1, and 1:0.5:0.5 for the synthesis with constant temperature (50°C). The reaction time was kept at 2, 4, and 6 h as per requirement. Formic acid was proved to be effective than acetic acid for high yield and oxirane contents. Moreover, more reaction time and high concentration of hydrogen peroxide are required for acetic acid. The calculated/estimated optimized conditions for the reaction were 2 mol of hydrogen peroxide with the time of 6 h giving 98% yield and 7.45% oxirane contents.

Jalil [124] has also worked in 2019 on epoxidation of palm oleic acid by performic acid (in situ generation). Most stable and high yield (85.6%) of epoxide ring was obtained by optimized reaction conditions, i.e., 55°C, 200 rms speed, and 1:1:1.5 ratios of oil, formic acid, and H₂O₂.

In 2019, epoxidation of CSO was carried out on reflux condenser using RBS [28]. CSO was heated on condenser at 55°C. After reaching the required temperature (55°C), sulfuric acid (0.8 mL) and acetic acid (9.8 mL) were added. 58 mL hydrogen peroxide was poured drop by drop and temperature was increased (70°C) for 8 h. Synthesized product was separated by solvent extraction process. Ninety-five percent yield was obtained. These epoxidized oils were used to make polymers of higher stability and 1 MPa tensile strength.

5.3. Chemoenzymatic Epoxidation

Enzyme catalysts were preferred for more environment friendly and avoiding side product processes. The immobilized Candida antarctica lipase is extensively utilized as enzyme catalyst and added step wisely to improve epoxidation reaction. It is better alternative to chemical process for epoxidation, but low stability under reaction conditions is the main limitation of the enzymatic method [135]. It was investigated that enzyme activity may be lost in bio-catalyzed reactions due to elevated temperature or higher H₂O₂ concentrations. However, Candida antarctica B lipase showed retained stability above 50°C in recent studies, although decomposition of H₂O₂ is unfavorable for it. At higher temperatures, catalytic activity of enzymes is decreased due to nonaccessibility in the inner regions [136]. Enzymatic epoxidation provides selective properties to the obtained products and flexibility of chemical reactions [137]. Medium temperature is required in this method, and therefore it also has the advantage of heat safety. Björking et al. were the first who reported chemoenzymatic-based epoxidation results. Later, more acceptable procedures were introduced to make enzymes better alternatives for oil-based transformations [138]. Ursula et al. investigated that lipase catalysts show excellent activity and stability in epoxide formation reactions. They have also the ability to reuse many times [105].

In 2016, scientists worked on activity of different catalysts for epoxidation of SBO [139]. They used H₂SO₄, AIER (Zeolite 325, Amberlite 15, and Zeolite Y), and enzymatic catalyst (Novozyme 435). They studied that enzymatic catalyst showed excellent activity (increased selectivity) with highest epoxide value (0.345 mol/100 g), then H2SO4 (0.323 mol/100 g), and ion exchange resins (0.277 mol, 0.260 mol, 0.020 mol/100 g) in the end. The highest unsaturation conversion was obtained for Novozyme 435 (99.7%). 24 h reaction at 25°C was performed for enzymatic epoxidation.

Bajwa et al. [140] carried out three different lipase-mediated chemoenzymatic epoxidation reactions of Karanja oil using three different solvents (toluene, hexane, and benzene). Seventy-two percent and 82% epoxide conversion were obtained for benzene and toluene, respectively, by Novozyme 435 catalyst without side products. Chemoenzymatic epoxidation is a green and clean technology with four run reusability. It is expecting 100% selectivity of this method for future. One reason for high selectivity was the absence of sulfuric acid that causes ring opening of epoxy. Moreover, relatively fewer amounts for hydrogen peroxide are required compared to chemical epoxidation. To further improve the chemoenzymatic method, Bhalerao et al. [141] have reported lipase-mediated (Novozyme 435) epoxidation of SBO. This process was performed in ultrasonic irradiation to reduce the reaction time and 91.22% oxirane conversion was calculated within 5 h. The enzyme lipase exhibited remarkable stability, later recovery, and six times reusability for epoxidation. Optimized conditions (4 wt. % enzyme, 40°C, and 200 rms stirring speed) for the reaction were investigated. When conventional enzymatic method was compared with ultrasonication method, three significant results were obtained. For the conventional method, 27% yield in 5 h and 85% in 20 h was obtained with stirring. By ultrasonication process, 91% yield with stirring, 45% yield without stirring and significant rate increment was investigated. Ultrasound method is more energy efficient because only 11.44 kJ/g per unit wt. was needed as compared to conventional process (21.02 kJ/g).

The influence of various parameters on VO epoxidation by enzymatic method was reviewed by Sustaita-Rodriguez et al. [142]. They investigated enzyme inactivation with increasing temperature and increasing amount of hydrogen peroxide. Therefore, their amounts should be carefully added to avoid deactivation of enzyme catalyst. Sapindus mukorossi fatty oil was epoxidized by Candida lipase enzyme [143]. They observed the effect of reaction temperature, time, H₂O₂ addition, and different solvents on the epoxidized product. Eighty-three percent yield was obtained with optimum conditions of 35°C for 2 h, 30°C for 6 h, and 15 wt % enzyme loading. Shorter time was needed as compared to Novozyme 435 catalyst at low temperature that gave similar conversion. Among various organic solvents, n-hexane was the only one that increased the oxirane value. In 2019, group of researchers reported enzymatic epoxidation of SBO using three different lipase catalysts and checked their stability. They observed that Aspergillus oryzae lipase exhibited excellent conversion with maximum stability (Table 5) [144].

5.4. Metal-Catalyzed Epoxidation

The use of metal catalysts is more advanced process which improves epoxy contents and epoxidation performance. To improve latest existing technology, scientists and researchers are always in continuous struggle and search to find new methods and techniques. Many workers and their coworkers have contributed to improve efficiency and oxirane contents of epoxidation processes. An ideal catalyst should not only have best performance in reactions but also: (1) easy and cheap to form, (2) robust, stable and usable, (3) to be used for many types of substrates, versatile, etc. [146]. Metal-catalyzed materials are typically used as catalysts for epoxidation. Low oxidation potential and high Lewis acidity of metals in their highest oxidation states make them superior catalysts which shows following order in activity: Mo > W > Ti > V [147]. The use of peracid is unfavorable for health and environment due to safety requirements in handling and wastage of considerable amounts of acid. Thus, it has dealt in recent papers to overcome these drawbacks using more efficient and sustainable technologies and compounds.

For radical pathway of epoxy synthesis, molecular oxygen (O₂) is used as oxygen donor. Aldehyde is utilized as co-oxidation in epoxide synthesis process. High oleic or methyl oleate materials were epoxidized obtained from different VOs [148]. Cumene-O₂ catalyst with Cu-mediated materials was used for epoxidation process. Single-pot reaction was performed at 100°C. The selectivity and conversion results against two different catalysts are shown in Table 6.

VOs and their derivatives were epoxidized using silica-based complex catalyst [149]. This catalyst exhibited highest pore volume, surface area, peroxide value, and excellent activity with 5 times recyclability. 92.76% yield was gained for methyl linoleate at rt. High yield of linseed oil and olive oil was also successfully obtained at rt. 0.3 mL of oil with 12.5 mL DCM was poured in RBS and stirred at rt. The reaction was performed for 1 h after adding catalyst. The obtained solid was filtered and washed. DCM was removed using vacuum evaporator and epoxide product was obtained. Palm oil–derived oleic acid was used for epoxy synthesis with performic acid [150]. Eighty-six percent oxirane conversion of oleic acid has been reported with sulfuric acid as catalyst at 90°C for 10 h. This work involved synthesis of dihyroxysteric acid by hydrolysis of palm oil–derived oleic acid, for cosmetic applications.

Methyl oleate–based epoxide was synthesized using molecular oxygen with aldehyde reductant (Mukaiyama reaction). The reaction was performed in highly efficient and safer continuous flow process. Improved productivity, better mass transfer, and high selectivity (> 99%) were achieved through this method [151]. Only in 4 min, 99% selectivity was obtained at rt with ∼25/74% cis/trans ratio. A special reactor was used in which reactant solution of 50 mL was loaded. Oxygen and organic phase were fed in reactor using T-mixer. 5 bar pressure was applied.

FAMEs were epoxidized using niobium mesoporous silica catalysts (Nb-SiO₂) [152]. They exhibited excellent selectivity toward monoepoxides over Nb grafted silica catalysts. Sixty-five percent conversion and 95% selectivity were obtained in the presence of hydrogen peroxide.

Campanella et al. reported the epoxide synthesis method of SO and soybean methyl esters using heterogeneous Ti/SiO₂ catalyst in ter-butyl alcohol. No degradation of epoxy ring was observed under the conditions applied in this experiment. 6 wt. % diluted solution of H₂O₂ was used with the catalyst. Highly effective oxidation has been shown recently with Ti on amorphous silica in the presence of H₂O₂ [153]. Gunam Resul et al. [154] have worked on tungsten-based polyoxometalates for epoxidation of limonene using hydrogen peroxide as an oxidant. Highly selective (100%) product was achieved with excellent yield (95%). Saturated sodium sulfate was used to prevent hydrolysis. The mole ratios for limonene/H₂O₂/catalyst were 4:1:0.005 with 373 K temperature. No solvent is required for epoxidation. Activation energy for the reaction was ∼36 kJ/mol. Less than 0.04 M acid concentration was used for maximum selectivity. H₂O₂ was added with H₃PO₄ and Na₂SO₃ to achieve complex catalyst. Then, H₂SO₄ was poured with them for the maintenance of pH. Limonene was added and reaction was further proceeded to complete the epoxidation.

Charbonneau et al. [155] reported the catalyst-free epoxidation of limonene and some other terpenes. Both the conventional and semibatch heating techniques were utilized for the reaction. Cheapest oxidizing agent (Oxone) was used with acetone and sodium bicarbonate for in situ generation of DMDO. DMDO is a green oxidant, comparable to m-CPBA, and reduces the time for reaction under ultrasound process. However, DMDO cannot epoxidize double bonds of low electron density. The results obtained through semibatch heating methods are shown in Table 7 which showed high yield.

The use of sulfated zirconia for epoxidation of castor oil has been reported by Yuan et al. [156]. Autoclave reactor was utilized for the reaction in which 20 g castor oil, 2 g formic acid, and 3 g catalyst were placed at 25°C. 38 g H₂O₂ was added during first 2 h, then reaction was performed for 2–10 h at 65°C. The product was extracted by centrifugation and vacuum distillation. However, catalytic activity of this zirconia-based catalyst was not so valuable because of less surface area and nonordered pores in catalyst. SiO₂-supported TiO₂ was used as catalyst for epoxidation of FAMEs [157]. Methyl oleate, naphthalene, and acetonitrile were mixed with 1:100:0.6 ratio, at 50°C, and 70 mg hydrogen peroxide was added slowly. The reaction was carried out for 5 h. Catalyst was removed by filtration and product was obtained after workup. The metal-based epoxidation of various oils is discussed in Table 8.

Recently, various epoxidation methods have been compared with cheap and nonedible cardoon oil, as starting material [158].Various metal-based heterogeneous catalysts have been used, where high-quality products have been achieved with Al₂O₃. Epoxidation was performed in batch reactor that was kept in oil bath for heating. Catalyst, solvent, and hydrogen peroxide were added in it together with the cardoon oil. The reaction was performed for 5 h at 70°C–80°C.