Article pubs.acs.org/EF
Cite This: Energy Fuels XXXX, XXX, XXX−XXX
Long-Term Storage Stability of Epoxides Derived from Vegetable Oils and Their Methyl Esters Venu Babu Borugadda and Vaibhav V. Goud* Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India S Supporting Information *
ABSTRACT: Epoxidized plant seed oils have received much attention in recent years to replace conventional lubricant basestocks in the current lubricant market. Although there is an increase in the productivity of epoxides, showing a solution for future energy insecurity, there still remains some concern for commercialization due to its susceptibility during long-term storage. Therefore, in order to commercialize the epoxides, they should maintain their integrity (physical and chemical) in all aspects. The objective of this study is to investigate the effect of various storage conditions on quality-indicative parameters for epoxides, such as acid value, oxirane oxygen content, and alpha glycol content for epoxidized waste cooking oil, castor oil, and their epoxidized methyl esters. Aforementioned quality indicative parameters for epoxides were investigated after every 3 months over a period of 12 months. During the storage period, epoxides were stored in three different groups at different temperatures (room temperature, 4 °C) and different environmental conditions (closed to air in the dark, closed to air and exposed to light). The analysis was carried out at regular intervals to monitor the quality-indicative parameters for four epoxides (two oil derived epoxides and two methyl esters derived epoxides). The results of the study revealed that epoxides stored at ambient temperature (closed to air and exposed to light) were highly more unstable than those at the other storage conditions. Likewise, epoxidized methyl esters stored at the same condition were found to degrade at a faster rate than epoxidized oils.
1. INTRODUCTION In the present era, the need to develop technical and economically feasible biofuel and biolubricant technology is an unyielding challenge throughout the world due to sustainably grown plant seed oil derivatives. Plant seed oils and their derivatives are seen as plausible feedstocks owing to their potential to improve the existing global warming burden and the necessity to diminish pollution from fossil fuels.1,2 Thus, commercialization of plant seed oil derivatives is confronting two major challenges: finding a technical and economically viable route, so that it will be competitive with inexpensive fossil fuels and the high cost of feedstocks. Therefore, one potential solution anticipated to overcome this technical problem is to utilize renewable, cost-effective, nonedible feedstocks and edible waste oils. Waste cooking oil (WCO), therefore, can be considered as a potential waste which can be used for the production of a number of valueadded products.3,4 Also, in order to avoid food vs fuel crisis, nonedible vegetable oils which are considered as low-grade oils could be pleasing for industrial applications. Nonedible plants can be grown in rural, unproductive, degraded lands as they are well adaptive to a semiarid and low-moisture/fertility environment.5 Generally, plant seed oils deteriorate when processed inadequately, with the primary decomposition reaction being oxidation in the presence of atmospheric air (oxygen). The stability of plant seed oil derivatives is a significant indicator of performance and shelf life, and it depends on the fatty acid composition and storage conditions.6 Inadequate stability, poor low-temperature properties, filter-clogging tendency, unpleasant odor, and formation of oxidative compounds make plant seed oil derivatives unsuitable for long-term storage and usage.7 Oxidation of plant seed oil takes place through a free radical © XXXX American Chemical Society
mechanism upon prolonged heating and air exposure. This is characterized by an initial unpleasant odor that becomes progressively worse until it attains a rancid fat smell, yielding peroxides and unstable hydroperoxides as primary products.8 Oxidation of vegetable oils cannot be avoided as it is affected by a large number of factors, such as fatty acid composition, storage conditions, presence of the air, temperature, light, and contaminants.9 During long-term exposure, these primary products degrade easily to secondary oxidation products such as aldehydes, ketones, acids, and alcohols.10 However, plant seed oil derivatives such as epoxides and ring-opening products will not form any primary and secondary oxidation products unless there is any unsaturation in their fatty acid composition. Among several plant seed oil derivatives, structurally modified derivatives at their unsaturation (epoxides) are gaining a lot of attention due to their ability to develop various value-added products such as lubricant basestocks, greases, polymers, paints, fuels, surfactants, etc. Among all these applications, liquid lubricant basestocks serve as a great application in industries, automobiles, and aviation machinery. They are performing vital functions such as cutting down friction, removal of wear, enhancing efficiency, minimizing energy losses, and uniformly distributing the heat. The majority of conventional lubricants are mineral oil based and are originated from petroleum derivatives having very low biodegradability, having a high price, being harmful to the ecosystem, and affecting the health of the people who handle them throughout production and usage, until disposal. Received: September 17, 2017 Revised: January 20, 2018
A
DOI: 10.1021/acs.energyfuels.7b02351 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
months, epoxides were analyzed for AV, OOC, and AGC, which are highly significant quality-indicative parameters for epoxides. The effects of real storage conditions on the aforementioned chemical properties of different basestocks are investigated at various storage conditions. To the best of our knowledge, this is the first comprehensive report which investigates the long-term storage stability of epoxides of WCO, castor oil (CO), and their methyl esters. The outcomes of this study will provide information on the factors influencing the degradation of epoxides quality during storage, which will be immensely helpful in maintaining the quality of epoxides over long-term storage.
Currently, the consumption of conventional lubricants basestocks comprises 53% automotive transmission fluids, 32% industrial lubricants, 5% marine oils, and 10% process oils.11 Irrespective of the problems associated with conventional lubricant basestocks, global demand for aforementioned lubricants is anticipated to increase 1.6% per year, and in India, the increase is 7% annually.11,12 But, during the last 25 years, high concerns for the ecosystem and utilization of biodegradable products have grown worldwide. By concerning the above global issues and negative effects of mineral oil derived lubricants on the ecosystem, plant seed oil based resources are attractive as a potential alternative to replace conventional lubricant basestocks. However, direct utilization of plant seed oils is not acceptable due to their lower thermooxidative stability, unfavorable cold flow properties, and low hydrolytic stability. Thus, plant seed oils can only be utilized potentially by modifying their chemical structure via different chemical/structural modification techniques, so that the end products can be utilized as a potential alternative to conventional lubricants for a variety of applications.13 However, in order to ensure end users acceptance, standardization and quality assurance are fundamental factors to introduce plant seed oil derived lubricant basestocks for various applications in the current lubricant market. One of the principal standards for the quality of lubricant basestocks is their storage stability. The trouble arises due to the deterioration of physicochemical properties of basestocks during long-term storage, and usage is anticipated to be more serious than conventional basestocks. Resistance to aerobic degradation during storage is a significant factor for the productive growth and viability of alternate basestocks.8 Due to oxidation/aging, physicochemical changes occur for epoxides such as acid value (AV), oxirane oxygen content (OOC), alpha glycol content (AGC), thermo-oxidative stability, viscosity, and density in long-term storage. When there is a change in aforementioned physicochemical properties, quality of the basestocks reduces; therefore, it cannot be used for any further application. Each and every property plays a major role and has its own significance in preserving the physical properties and chemical structures of the plant seed oil derived basestocks.14 From the reported literature on long-term storage stability, it was clearly noticed that the stability of plant seed oil derivatives can worsen due to storage conditions such as light exposure, air exposure, and storage in dark (aerobic and anaerobic), which catalyze harmful reactions to degrade the quality of the plant seed oil derived basestocks.15 Similarly, Bouaid et al. reported that resistance to aerobic degradation during storage is a highly significant issue for the successful development and viability of alternative basestocks.16 Oxidation accelerates the unwanted side reactions, thereby degrading the chemical structure of epoxides, which may harm the quality of basestocks and ultimately their performance. One of the DOW Chemicals technical bulletins on epoxides revealed that, for liquid epoxides, the epoxy content (OOC), epoxide equivalent weight, and viscosity are primary qualitative indicators against product deterioration.17 Besides Dow Chemical’s technical bulletin, there was no literature available on how epoxide properties change at different storage conditions. Therefore, in the present communication, the storage stability of biolubricant basestocks prepared from four different feedstocks (waste cooking oil, castor oil, and their methyl esters) was investigated over a storage time of 12 months under different storage conditions. After every three
2. MATERIALS AND METHODS 2.1. Materials. Fish-fried soybean waste cooking oil (WCO) was collected from the hostel mess, IIT Guwahati, India, and castor seeds were collected from Cherukupalli, Andhra Pradesh, India. After collection, seeds were cleaned and oil was extracted by Soxhlet extraction setup using hexane as a solvent. Afterward, WCO and CO methyl esters were prepared by base-catalyzed transesterification at optimized conditions reported in previous studies.18,19 The waste cooking oil methyl esters (WCOME) used in this study are found to be within the standards of ASTM D6751 as per the properties reported in Table S1 (Supporting Information). Likewise, some of the castor oil methyl esters (COME) properties were found to be inconsistent with the ASTM D6751 specifications, which signifies that COME is not suitable for fuel application (Table S2; Supporting Information). Further, detailed physicochemical characterizations of WCOME and COME are reported in the Supporting Information (Tables S1 and S2). Epoxidized WCO, CO, and their respective epoxidized methyl esters were prepared at laboratory conditions via insitu epoxidation. All other chemicals and reagents like ethanol, phenolphthalein indicator, hydrogen bromide, acetic acid, methyl violet indicator, sodium hydroxide pellets, and potassium hydrogen phthalate were of analytical grade and purchased from M/s Merck India Pvt. Ltd. 2.2. Storage Conditions and Analytical Techniques. Initially, AV, OOC, and AGC analysis of freshly prepared basestocks (epoxides) were evaluated and recorded as zero-month values. The basestocks were stored for 12 months as three individual groups at different storage conditions. The groups were made on the basis of different storage temperatures, i.e., 4 °C and room temperature (30 °C). At 4 °C epoxide samples were sealed and kept in the refrigerator. In the same way, at room temperature (30 °C) all the epoxides were stored in two different environmental conditions (subgroups); in the first subgroup epoxides were sealed and kept in the dark environment (closed to air in the dark). In the second subgroup, epoxides were sealed but exposed to light (closed to air and exposed to light). The storage conditions selected are highly significant considering exposure of the sample to light or dark conditions during transportation and storage. Hence, conditions such as dark, exposed to light, and closed to air were selected for this study. Epoxides from each group were analyzed periodically at an interval of three months for aforementioned quality parameters; each analysis was replicated, and the average values are reported with standard deviations. The acid value (AV) and free fatty acid (FFA) content of all the epoxides were determined according to the AOCS official method (Te 1a-64, 1997). Oxirane oxygen content of the epoxides was estimated by HBr Method (AOCSCd-9, 120) using a hydrobromic acid solution in glacial acetic acid. Theoretical oxirane oxygen (OOthe) and relative percentage conversion to oxirane were calculated from the following expressions
⎡ ⎤ (IVo/2A i ) OOthe = ⎢ ⎥Ao × 100 ⎣ 100 + (IVo/2A i )Ao ⎦ B
DOI: 10.1021/acs.energyfuels.7b02351 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels where Ai (126.9) and Ao (16) are the atomic weights of iodine and oxygen, respectively, and IVo is the initial iodine value of the respective feedstocks.
epoxidation reaction, prior to analysis, epoxides were washed repeatedly with warm Millipore water (40 °C) until all the catalyst and other reactants were removed to make it neutral. Finally, rotary evaporator was used to distill off solvent and trace water. The detailed fatty acid composition of WCO, CO, WCOME, and COME and physicochemical properties of epoxidized WCO, CO, and their methyl esters are reported in Tables 2 and 3. Also, the structures of each fatty acid molecule with epoxide groups are shown in Figure S1 (Supporting Information). Physicochemical properties of the aforementioned epoxides were compared with the conventional lubricants. Comparative analysis of epoxide viscosity revealed that epoxidized WCO and CO show that the values are within the acceptable limits of ISO VG 32, 46, 68, and 100; paraffin VG 95, R150, SAE20W40, AG 100, 75W-90, 75W-140 standards (Table 4). Likewise, pour point of epoxidized WCO and CO is found to be within the specifications of ISO VG 32, 46, 68, and 100 lubricant standards (Table 4).25 Further, the thermo-oxidative stability of the epoxidized COME was compared to servohydraulic lube oil and found to have higher thermo-oxidative stability.26
⎡ OOexp ⎤ relative percentage conversion to oxirane = ⎢ ⎥ × 100 ⎣ OOthe ⎦ where OOexp is the experimentally determined content of oxirane oxygen, and OOthe is the theoretical maximum oxirane oxygen content. AGC was determined according to the method reported by May (1973) and Weiss (1970) based on the oxidation of glycol with benzyl trimethylammonium periodate in a nonaqueous medium. The following expression was used for the estimation of α-glycol of the alkoxides20
α‐glycol content (mol/100 g) = ((VB − VS) × N )/(20 × w) where VB is the volume of Na2S2O3 solution consumed for the blank test (mL), VS is the volume of Na2S2O3 solution consumed by the alkoxides (mL), N is the normality of Na2S2O3 solution; and w is the weight of alkoxide (g). 2.3. Synthesis Procedure of Epoxides. The in-situ synthesis of all the epoxides was carried out in a 250 mL 3-necked glass reactor with a flat bottom; 5-blade glass stirrer was introduced into the reactor, and the reaction mixture was stirred using an overhead motor. The whole setup was immersed in a heating water bath. During the preparation, WCO, WCOME, CO and COME, hydrogen peroxide, and acetic acid quantities were measured in a molar ratio; ionexchange resin (IR-20) as a heterogeneous acidic catalyst was used in terms of weight percent. Optimized synthesis conditions for each epoxide are reported in Table 1. Initially, the unit amount of respective
3. RESULTS AND DISCUSSION The current study investigates the effect of real storage conditions on the chemical properties of epoxides derived from WCO, CO of different fatty acid compositions. The main attention was to evaluate the effect of temperature, light, and the dark environment on the chemical properties of epoxides. For that reason samples were analyzed periodically at every three-month interval; initial epoxide content of given samples was measured and considered as zero (0)-month values. The following paragraphs give detailed information about variation in epoxide quality while maintaining its physical, chemical, and structural integrity. 3.1. Effect of Various Storage Conditions on Acid Value. Storage conditions play a significant role in degradation or maintaining the integrity of epoxides. Mazumdar et al. have reported that the process of oxidation accelerates when the plant seed oil derivatives are exposed beyond ambient temperature,27 which results in increased density and viscosity of the stored epoxides. Acid value (AV) is one of the qualityindicative parameters to monitor the degradation of epoxides; typically 0.8 mg KOH/g is the maximum allowable acid value for biodiesel to use them for fuel application. In the present study, epoxides were considered to use them for lubricant application, and they still have double bonds (iodine value 0.6, 0.25 g I2/100 g of epoxides for WCO, WCOME; 51.86, 1.27 g I2/100 g of epoxides for CO, COME) (Table 3). Presence of the unsaturation leads them to auto-oxidation; thereby, epoxides AV and viscosity increase, which makes them unfit for the end application. Therefore, epoxides AV also should be as minimum as possible to avoid the oxidation process from occuring. In this regard, it is anticipated that the AV of 0.8 mg KOH/g could be an acceptable AV for epoxides. Figure 1
Table 1. Optimum In-Situ Epoxidation Reaction Conditions for Epoxidized WCO, CO, and Their Epoxidized Methyl Esters name of epoxide
H2O2 (mol)
catalyst loading (wt %)
waste cooking oil (ref 21 ) waste cooking oil methyl esters (ref 22) castor oil (ref 23 ) castor oil methyl esters (ref 24)
1.68
16.75
54
11.45
1.72
28.17
53.71
7.51
1.65 2.34
15.14 19.43
52.81 50.02
2.81 5.26
a
a
temp (°C)
reaction time (h)
H2O2: hydrogen peroxide.
feedstock (in moles) was heated to a reaction temperature, and then glacial acetic acid followed by the catalyst was added. Thereafter, hydrogen peroxide was added dropwise to the reaction mixture in the first 30 min when the temperature was 5 °C below the reaction temperature to avoid explosion. Stirring speed of 1100−1500 rpm was maintained to ensure uniform mixing. After the complete addition of hydrogen peroxide, the reaction was continued for the desired time duration as mentioned in Table 1. Upon completion of the
Table 2. Chemical Composition (wt %) of WCO, CO, and Their Methyl Esters (Ref 19)a name of fatty acid
WCO/WCOME
CO/COME
chemical name of fatty acid
structure (xx:y)b
formula
palmitic stearic oleic linoleic linolenic recinolic
14.20 ± 1.4 4.07 ± 0.41 23.96 ± 1.6 39.16 ± 1.7 5.25 ± 0.6 -
1.4 ± 0.23 1.8 ± 0.35 4 ± 1.1 6 ± 0.9 0.6 ± 0.08 84 ± 2.3
hexadecanoic octadecanoic 9-octadecenoic 9,12-octadecadienoic 9,12,15-octadecadienoic 12-hydroxy-9-octadecenoic
16:0 18:0 18:1 18:2 18:3 18:1-OH
C16H32O2 C18H36O2 C18H34O2 C18H32O2 C18H30O2 C18H34O3
a
WCO: waste cooking oil; WCOME: waste cooking oil methyl esters; CO: castor oil; COME: castor oil methyl esters. bxx indicates number of carbons and y the number of double bonds in the fatty acid chain. C
DOI: 10.1021/acs.energyfuels.7b02351 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 3. Physicochemical Properties of the Epoxidized WCO, CO, and Their Epoxidized Methyl Estersa
a
physicochemical properties of epoxides
WCO
WCOME
CO
COME
acid value (mg KOH/g) density (kg/m3) free fatty acid (mg KOH/g) iodine value (g I2/100 g) kinematic viscosity (cSt) at 40 °C moisture content (wt %) pour point (°C) refractive index (at 27.6 °C) onset temperature (°C) oxidative onset temperature (°C) oxirane content (experimental, mass %) oxirane content (theoretical, mass %) relative percentage conversion of oxirane (%) glycol content (theoretical) glycol content (experimental) relative percentage conversion α-glycol (mol/100 g)
0.4 ± 0.08 798.14 ± 3.6 0.2 ± 0.04 0.6 ± 0.1 282.38 ± 4.7 0.2 ± 0.03 −6.2 ± 1.5 1.47 ± 0.3 330 ± 5 320 ± 5.2 6.2 ± 0.9 7.72 ± 0.8 80.31 ± 2.5 0.44 ± 0.02 0.18 ± 0.01 40.9 ± 3.1
0.96 ± 0.1 773.82 ± 4.2 0.48 ± 0.03 0.25 ± 0.07 10.42 ± 1.2 0.16 ± 0.02 10 ± 1.2 1.45 ± 0.6 187 ± 4.3 200 ± 3.4 5.8 ± 0.5 7.72 ± 1.5 76.42 ± 3.2 0.44 ± 0.01 0.14 ± 0.01 31.81 ± 2.8
1.46 ± 0.23 837.24 ± 3.8 0.73 ± 0.07 51.86 ± 1.34 249.84 ± 3.8 0.21 ± 0.04 −15 ± 1 1.47 ± 0.5 308 ± 3.6 320 ± 4.7 3.85 ± 0.7 5.35 ± 0.9 71.96 ± 2.6 0.31 ± 0.02 0.18 ± 0.02 58.06 ± 1.9
1.62 ± 0.28 956 ± 5 0.54 ± 0.06 1.27 ± 0.42 35.81 ± 2.5 0.18 ± 0.05 8 ± 1.8 1.47 ± 0.6 340 ± 4.8 305 ± 2.8 4.86 ± 0.3 5.06 ± 1.3 96.04 ± 2.7 0.31 ± 0.01 0.15 ± 0.01 48.38 ± 2.4
WCO: waste cooking oil; WCOME: waste cooking oil methyl esters; CO: castor oil; COME: castor oil methyl esters.
Table 4. Comparison of Prepared Lubricant Basestocks Properties with Conventional Lubricant Standardsa
a
lubricant requirement
viscosity 40 °C (cSt)
viscosity 100 °C (cSt)
ISO VG32 (ref 25) ISO VG46 (ref 25) ISO VG68 (ref 25) ISO VG100 (ref 25) paraffin VG95 (ref 25) R150 (ref 25) SAE20W40 (ref 25) AG 100 (ref 25) 75W-90 (ref 25) 75W-140 (ref 25) epoxidized WCO (ref 21) epoxidized WCOME (ref 22) epoxidized CO (ref 23) epoxidized COME (ref 24)
>28.8 >41.4 >61.4 >90 95 150.04 105 216 120 175 282.38 ± 3.5 10.42 ± 1.2 249.84 ± 3.4 35.81 ± 1.3
>4.1 >4.1 >4.1 >4.1 10 − 13.9 19.6 15.9 24.7 33.33 ± 2.6 3.29 ± 0.8 34 ± 1 6.49 ± 0.6
viscosity index >90 >90 >198 >216 102 − 132 103 140 174 162.15 212.51 188.92 135.71
± ± ± ±
4.1 3.9 4.3 2.3
pour point (°C) −6 −6 −6 −6 − − −21 −18 −48 −54 −6.2 ± 0.8 10 ± 1.2 −15 ± 1.5 8 ± 0.9
ISO: international organization for standardization; VG: viscosity grade; SAE: society for automotive engineers; AG: agricultural grade.
shows fluctuations in the AV of samples stored under different conditions (a, exposed to light; b, dark; c, 4 °C). The epoxides degradation show a direct correlation with AV, 0 month AV of epoxidized WCO, WCOME, and CO, COME was found to be 0.4, 0.96, and 1, 1.62 mg KOH/g, respectively. After 3 months of storage, highest AV was observed as 22 mg KOH/g (Figure 1a) for epoxidized COME stored at room temperature and exposed to light. However, epoxidized WCO stored at 4 °C showed the lowest AV (0.98 mg KOH/g) (Figure 1c). Even further extending the storage period to 6, 9, and 12 months demonstrated a similar AV trend. Throughout the study it was noticed that the epoxides stored at 4 °C (Figure 1c) showed increase in the AV from 0.98 (3 months) to 1.6 mg KOH/g (12 months) for epoxidized WCO, 2.7 to 3.4 mg KOH/g for epoxidized WCOME, 1.62 to 3.34 mg KOH/g for epoxidized CO, and 4.6 to 11.39 mg KOH/g for epoxidized COME. Similarly, epoxides stored at room temperature under the dark condition (Figure 1b) showed an increase in AV from 1.1 to 1.9 mg KOH/g for epoxidized WCO, 3.3 to 5.17 mg KOH/g for epoxidized WCOME, 3.87 to 14.02 mg KOH/g for epoxidized CO, and 6.2 to 14.62 mg KOH/g for epoxidized COME. On the other hand, for epoxides stored at room temperature under light condition (Figure 1a), AV was found to increase from 1.2
to 2.2 mg KOH/g for epoxidized WCO, 3.9 to 8.21 mg KOH/g for epoxidized WCOME, 7.7 to 17.23 mg KOH/g for epoxidized CO, and for epoxidized COME it was found to be 22 to 48 mg KOH/g. This clearly indicates that long-term storage of epoxides at room temperature and exposed to the light condition can accelerate the oxidation process and deteriorate epoxide structural integrity (quality). As reported by Bouaid et al., oxidation undergoes a complex reaction which leads to the formation of reactive aldehydes; in addition to that, the presence of moisture can hydrolyze the esters present in the epoxides to alcohol and acids.16 Therefore, the outcomes of the effect of AV on different storage conditions revealed that at all storage conditions AV increases with storage time due to the accelerated oxidation process in the lipid molecules.16 Epoxides exposed to light at room temperature led to increasing AV among all storage conditions used in this study. The probable causes are the formation of hydroperoxides and free radicals by the auto-oxidation mechanism28, which could be responsible for the increase in AV. Besides the lower temperature storage condition (4 °C), at all other storage conditions, AV was found to be increased at a faster rate. This is mainly attributed to photo-oxidation along with auto-oxidation reaction under the light condition which enhances the D
DOI: 10.1021/acs.energyfuels.7b02351 Energy Fuels XXXX, XXX, XXX−XXX
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of oil derived epoxides and methyl esters derived epoxides. Due to lower viscosity of methyl esters, atmospheric oxygen can more easily penetrate into the epoxidized methyl esters than epoxidized oils. Therefore, among all the epoxides, degradation of the epoxidized methyl esters was more than that of the epoxidized oils. In addition to that, the free fatty acids (FFA) content of CO was higher than that of the WCO. Therefore, during the storage, high FFA content in the epoxides, atmospheric air, and light accelerated the oxidation process which may result in higher AV. The experimental investigation of this study also revealed that AV has gone out of the threshold value after three months of storage; alike observations were reported by Gurau et al.8 From the outcomes of this study, it could be concluded that epoxides stored at room temperature and exposed to light increase their AV at a faster rate than epoxides stored at 4 °C in the dark environment. 3.2. Effect of Various Storage Conditions on Oxirane Value. Oxirane value is a highly considerable and significant parameter to maintain epoxide structural integrity. The structure of an epoxide group of fatty acid molecules is represented by the carbon−carbon−oxygen ring as shown in Figure S1 (Supporting Information). The structure of the epoxides which are derived from natural triglycerides is located along a hydrocarbon chain so that each carbon of the cyclic ether bears an alkyl substituent.32,33 In order to make use of epoxides for a specific application, they should maintain their physical, chemical, and structural integrity at all storage conditions. The effects of different storage conditions in maintaining the physical, chemical, and structural integrity of epoxides are investigated. Figure 2 represents the change in oxirane value at different time intervals under diverse storage conditions. The initial oxirane content was found to be 6.2, 5.8, 3.85, and 3.3 mass % for epoxidized WCO, WCOME, CO, and COME, respectively. During the storage time, over a period of three months, oxirane values of the epoxides were found to be decreased. Comparative study for all the epoxides at threemonths storage time interval revealed that epoxides exposed to light (Figure 2a) have lost the structural integrity, and percentage oxirane oxygen decrease was found to be 8, 10.2, 53.2 and 71.2% for epoxidized WCO, WCOME, CO, and COME respectively. The degradation of oxirane oxygen content was more in epoxides exposed to the light condition, owing to the absorption of ultraviolet (UV) radiation, which would result in the photo-oxidation.8 Jose et al. also concluded the faster degradation during their study on Karanja biodiesel when exposed to the light condition.34 Similarly, for the epoxides stored at 4 °C (Figure 2c), percentage oxirane oxygen decrease was found to be 4.8, 8.2, 30.9 and 40.7% for epoxidized WCO, WCOME, CO, and COME, respectively, and the epoxides stored in dark (Figure 2b) showed intermediate oxirane oxygen degradation. In the case of epoxidized CO and COME stored at 4 °C, major OOC loss was found during 9− 12 months. Similar observations were noticed at different time intervals as well. During this study, it was also observed that maximum epoxide degradation occurred during the first threemonths storage period, and degradation of epoxides was attributed to the storage condition, acid value, moisture content, light exposure, and diffused air into the storage container. As discussed in the previous section and from the reported literature on the degradation of oxirane oxygen,35,36 it was clearly noticed that temperature plays a major role in
Figure 1. Evaluation of acid value of epoxides over 12-months storage period: (a) exposed to light, (b) dark, (c) 4 °C.
degradation of plant seed oil derivatives.29 Similar results were obtained by other studies as well on storage stability testing of various biodiesels.30,31 Compared to all other epoxides used in this study, epoxidized COME showed the highest AV at the end of the storage period, which might be due to higher initial AV (1.62 mg KOH/g) among all the epoxides. On the other hand, this can be explained with the help of viscosity difference E
DOI: 10.1021/acs.energyfuels.7b02351 Energy Fuels XXXX, XXX, XXX−XXX
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presence of hydroxyl group in the ricinoleic acid triggers the degradation process, which is susceptible to thermal and hydrolytic degradation. Similarly, epoxidized methyl esters degraded at a faster rate than epoxidized oils. This may be due to low viscosity of methyl esters as atmospheric oxygen can easily diffuse into the epoxidized methyl esters and can accelerate the degradation process.27 3.3. Effect of Various Storage Conditions on α-Glycol Content. In the current study, an effort has been made to evaluate the capability of epoxides to transform into ringopened products during long-term storage conditions. Figure 3 represents the change in AGC over 12-months time period at various storage conditions. Initial (zero-months) glycol content values were found to be 0.18, 0.14, 0.18, and 0.15 for epoxidized WCO, WCOME, CO, and COME, respectively. After three-months storage period, glycol content was found to be 0.181, 0.142, 0.183, and 0.154 for epoxidized WCO, WCOME, CO, and COME, respectively, for epoxides exposed to light at room temperature (Figure 3a). For epoxidized WCO significant change in the AGC was detected after three-months storage period in light and dark condition; this may be due to absorption of moisture from surroundings which may lead to oxirane cleavage.22 Further, insignificant change in the AGC values was noticed for three-months storage period (Figure 3), and a similar trend was observed for all epoxides stored up to 6, 9, and 12 months. AGC of the epoxides stored at room temperature and exposed to light (Figure 3a) degraded at a faster rate compared to other storage conditions (Figure 3b and 3c). As already mentioned in the earlier sections, photooxidation along with auto-oxidation are predominant in the oxidation of epoxides when exposed to light. Nevertheless, according to Markovnikov’s principal, when alkenes react with water in the presence of a catalytic amount of acid, this leads to the formation of alcohols. Further, these alcohols undergo dehydration process to form alkenes on the treatment of heat in the presence of a catalyst. Thus, during the entire study we suspect that these reactions might take place; however, a detailed advanced study is needed with UV and FTIR spectral techniques to detect structural changes during degradation along with chromatographic studies for the improvement of epoxides handling, trading, and use. Nevertheless, due to longterm storage in diverse conditions covered in this study, it was anticipated that moisture absorbed from atmosphere might be responsible for slight enhancement in the glycol values compared to initial AGC values (Figure 3). Above all, it is anticipated that the chemical structure of the epoxides could be maintained by addition of suitable antioxidants which are natural (L-ascorbic acid 6-palmitate, caffeic acid, tannic acid, tocopherols, ascorbic acid) and synthetic (tert-butylhydroquinone (TBHQ), butylated hydroxy anisole (BHA), butylated hydroxy toluene (BHT), and propylgallate (PG)). However, antioxidant study for epoxidized oils and their methyl esters is beyond the scope of the current study, and we considered this theme for our future study.
Figure 2. Evaluation of oxirane oxygen content of epoxides over 12months storage period: (a) exposed to light, (b) dark, (c) 4 °C.
4. CONCLUSIONS In conclusion, for effective commercialization of epoxides, maintaining their quality during storage period is of great significance. Present study experimentally investigated the effect of different storage environments on epoxidized WCO, CO, and their methyl esters quality-indicative factors, such as AV, OOC, and AGC. Experimental outcomes disclosed that all the three quality-indicative parameters chemical structures deteri-
maintaining the structural and chemical integrity of epoxides. Hence from this study, it is clear that long-term exposure to light, increased the degradation rate of epoxides compared to the dark and 4 °C storage conditions. In the case of epoxidized CO and COME, oxirane degradation was more rapid than that of other epoxides, which might be due to their higher initial AV and presence of the unsaturation in the epoxides. Also, the F
DOI: 10.1021/acs.energyfuels.7b02351 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
epoxidized WCO, WCOME at all storage conditions, and an abrupt increase in the AV was noticed for the epoxides stored in air and light-exposed condition. Therefore, suitable additives need to be added to the epoxides before storage. Oxirane oxygen content of epoxidized WCO and WCOME values was found to be more stable at all storage conditions. In the case of epoxidized CO and COME, OOC was found to be decreased abruptly after every three-months storage time due to higher AV of the feedstocks and epoxides. More than the OOC, epoxides can show excellent physicochemical properties; due to rapid decrement in the OOC of epoxidized CO and COME they are unfit for their end use after three months of storage period at two different storage conditions (dark, exposed to light and air). But, the same epoxides stored at 4 °C can be used potentially up to nine months. However, epoxidized WCO and WCOME can be used efficiently up to 12 months of storage period in spite of the storage environment considered in this study. Similar findings were noticed for all epoxides in the analysis of AGC, as epoxidized CO and COME degraded rapidly. Epoxides stored in light condition have shown little increase in the AGC values 0.001, 0.002, 0.003, and 0.004 mol/ 100 g for epoxidized WCO, WCOME, CO, and COME, respectively at three months of storage period. The slight significant difference was noticed in degradation of AGC between 3- and 12-months value. Compared to all the storage conditions, degradation was considerably higher in air and lightexposed condition than dark and 4 °C storage conditions. From the comparative analysis of all the data, it can be concluded that among all storage conditions from the current study at room temperature, light and air exposure are the two strong factors in fastening the epoxides degradation. It was also understood that higher AV feedstocks (epoxidized CO, COME) and their derivatives cannot maintain epoxides structural integrity, and they have a faster rate of degradation compared to lower AV feedstocks (epoxidized WCO, WCOME) at room temperature in the presence of light. In the scope of this study, epoxidized WCO, WCOME were found to be the most stable by maintaining their chemical structure. Therefore, enough care should be taken for these quality-indicative parameters during storage to maintain their chemical and structural integrity; otherwise, epoxides may be ineffectual very soon, and they may become unfit for their end use.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02351. Detailed physicochemical characterization of the WCO, CO, and their methyl esters along with the epoxidized structures of the different fatty acids present in the WCOME and COME: (1) physicochemical characterization of waste cooking oil methyl esters; (2) comparison of physicochemical properties of castor oil methyl esters; (3) epoxidized structures of the different fatty acids present in the WCOME and COME; (4) references (PDF)
Figure 3. Evaluation of alpha glycol content of epoxides over 12months storage period: (a) exposed to light, (b) dark, (c) 4 °C.
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orate over the long-term storage period. Increase in the value of qualitative parameters proves that the process of degradation increases with an increase in storage period. Epoxides deterioration was evident when they are exposed to light condition rather than at dark and 4 °C. Among all the epoxides, epoxidized CO, COME deteriorated at a faster rate than
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Corresponding Author
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DOI: 10.1021/acs.energyfuels.7b02351 Energy Fuels XXXX, XXX, XXX−XXX
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(35) Campanella, A.; Balanas, M. A. Latin Am. Appl. Res. 2005, 35, 211−216. (36) Lee, C. S.; Ooi, T. L.; Chuah, C. H. Am. J. Appl. Sci. 2009, 6, 72−78.
Vaibhav V. Goud: 0000-0001-7755-6451 Notes
The authors declare no competing financial interest.
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