CODEN (USA): IJCRGG, ISSN: 0974-4290, ISSN(Online):2455-9555 Vol.10 No.10, pp 206-215, 2017
Abstract : This work present the methodology for preparing epoxidized wild safflower oil (EWSO) using cation exchange resin as heterogeneous catalyst and its subsequent ring-opening hydrolysis in presence of phosphoric acid under varying experimental parameters. The influence of various reaction conditions such as type and relative ratio of solvent, phosphoric acid dosing and water content on the ring-opening response was investigated. The hydrolyzed products, WSO-phosphate polyols were analytically characterized for hydroxyl content, oxirane content, acid value and viscosity. The chemical confirmation of the synthesized polyols was done using FTIR spectral analysis. The rate of degradation of oxirane rings in acid hydrolysis of EWSO was moderately rapid in aqueous acid media, when t-butyl acetate used as solvent, with higher hydroxyl value of the derived polyols. Keywords : Wild safflower oil, epoxidation, oxirane content, ring-opening, polyols.
Vegetable oils and fats are the important renewable resources and are nowadays increasingly replacing petroleumderived materials in various fields as they are biodegradable, low in cost and readily available. Vegetable oils and their derivatives can serve as a feedstock for many applications such as fuels (bio-diesel)1,
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polymers, lubricants, surfactants, plasticizersand specialty chemicals.Thus, there is an immediate need of attention towards effective utilization of these bio-based products taking into the consideration of current environmental harm and steadily rising petroleum prices. Moreover, this trend will promote the sustainable agricultural initiative.
Epoxidation is one of the most important reactions thatintroduce the high strain energy threememberoxirane ring into the unsaturated part of the natural oil. The newly attached epoxide groups in triglyceride structure being highly reactive make them to be readily transformed into desired functionality. The chemistry of epoxidation of unsaturated fatty compounds is well known and usually involves peracid formation by reaction with molecular oxygen or hydrogen peroxide either in a separate step or in situ10. Industrially epoxides have been prepared from castor, linseed, rapeseed, soybean and sunflower oils containing high to
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moderate unsaturation.The utilization of other seed oils of semidrying nature like canola, karanja, jatropha,
141516
rubber seed, cottonseed, mahuaetc for epoxide synthesis have also been reported.Safflower is an important oilseed crop of the tropical countries, of which most species are wild and weed. Wild safflower oil (WSO) consists of a mixture of triglycerides derived mostly from C16 to C18 saturated and unsaturated fatty acids17.However, taking into account of higher proportion of unsaturation in fatty acid composition of the WSO, it is characterized as semidrying18 and therefore, can be used as renewable relatively inexpensive resource in preparation of epoxides.
Epoxidized vegetable oils are currently used on industrial scale as plasticizers for polyvinyl chloride, reactivediluents and toughening materials for epoxy resins. Besides, epoxidized oils can be modified through
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the ring-opening of oxirane groups using nucleophilic reactions with alcohols,acids, water or via hydrogenation25 to obtain polyols having multiple hydroxyl functional groups. These biobasedpolyols are reported to have application in electroinsulating casting compounds26 and, serve as the most predominant reactant replacing the conventional petrochemical-based polyol, reacting with isocyanates to produce polyurethane foams, elastomers, and coatings20.The other possible synthesis routes to obtain polyols from vegetable oils are hydroformylation, ozonolysis, transesterification, metatesis and alkoxycarbonylation, and thiol-ene coupling27. However, some deliberate attempts have been made to study the ring-opening hydrolysis
23,28
of epoxides with polar solventin presence of phosphoric acid to prepare soy-based phosphate polyolsand reported to improve the corrosion resistance property in coating applications.
In present work, we report epoxidation of wild safflower oil (EWSO) using cation exchange resin as a catalyst and investigate the effects of catalyst, solvent and the amount of water on thering-opening hydrolysis of synthesized EWSO. The hydrolyzed products, WSO-phosphatepolyols were analytically tested for different propertiesby standard laboratory methods and analyzedusing FTIR spectroscopy.
About 10Kg seeds of wild safflower(Carthamus oxyacantha)were collected from the adjoining regions of Punjab district and extracted according to AOCS Official Method Ba 3-38 with Rankosolv SG solvent (distillation range 63.0-70.0°C; water content, as determined by KF method, 0.1%) obtained from Rankem RFCL, New Delhi, India. The seed yielded 28.6% (wt basis)of crude yellow oil, which waspurified, refined and bleached by usual methods.Refined WSO,after due analytical study, was usedfor further experimentation.The laboratory grade chemicals glacial acetic acid (98%) ando-phosphoric acid (PA, 85%) were obtained from Qualigens India Ltd., Mumbai, India.AR grade hydrogen peroxide (30 wt%), Iodine monochloride and Wij’s solution were procured from S.D. Fine Chemicals Ltd., Mumbai, India. HBr in acetic acid (33wt%) was obtained from Fischer Scientific, Mumbai, India and then diluted with glacial acetic acid to make 0.1N HBr.The other chemicals, solvents and reagents used in the study were of LR grade and procured from CDH Ltd., Mumbai, India. Strongly acidic cation exchange resin, Amberlite® IR-122(sulfonic acid functionality, 16-50 wet mesh size, macroreticular form) witha total cation exchange capacity 2.1meq/mLwas procured from Sigma– Aldrich (USA).
The batch mode epoxidation of WSO was carried out in a glass reactor consisting of a fourneckround bottom flask of 2 Lcapacity fitted with mechanical stirrer, temperature measuring device, dropping funnel and condenser.The reactor assembly was placed in a thermostatic oil bath equipped with controller to maintain the system temperature within ±2 °C. A calculated amount of WSO was introduced in the reactor and heated at constant stirring (800 rpm) upto 40°C for 15 min. The requisite amount of acetic acid and catalyst (20wt %) was added and the mixture was continuously stirred fornext 30 min. This was followed by dropwise addition of hydrogen peroxide (1.5 moleper mole of ethylenic unsaturation)over a period of 30 min under continuous stirring with gradual increase in temperature upto 60°C and the reaction was continued for next sixhours (Scheme 1). At the end of reaction,heating was stopped and stirring was continued till temperature of the product cool down to room temperature. The derived epoxideproduct was extracted with diethyl ether followed by successive washing with warm water to make it acid free.The upper ethereal layer was separated and the solvent was evaporated in rotary vacuum evaporator to obtain pure EWSO.The dried epoxide sample was analyzed foroxirane content and iodine value.
Scheme 1: Epoxide synthesis reaction from wild safflower oil (WSO)
The above prepared EWSO was used to studythe oxirane ring-opening using acid hydrolysis route. The hydrolysis experiments wereperformed as per the procedurereported in literature23. Briefly, arequisiteamount of EWSO was dissolved separately in a portion of reaction solvent and the remaining amount of solvent along with PA and water, in a specified proportion, was charged into the fully baffled glass reactor.The reaction mixture was brought to the reflux condition by heating at the slow rate under constant stirring. The temperature of the reaction mixture was in the range of 60-120 oC depending upon the choice of solvent. The prepared slurry of EWSO in solvent was then added dropwise into the reactor over a period of 30 min. After the complete addition of EWSO slurry, the reaction was continued fornext five hours under continuous stirring, to ensure fine dispersion of the epoxide. The course of the hydrolysis reaction was followed by withdrawing samples at regular intervals. The collected samples were purified to make them acid free by washing its ethereal extract (1:1v/v) with deionizedwater to enhance the separation of the oil product from water phase, and dried in a rotary vacuum evaporator.The extent of hydrolysis was monitored by determination of the oxirane contentof the washed aliquots of the reaction mixture.
Scheme 2: Synthesis route for WSO-phosphate polyol by ring-opening hydrolysis
Acid value (A.V.), saponification value (S.V.), iodine value (I.V.) andspecific gravity (Sp. Gr.) of the refined WSO were determined according to AOCS Method Cd 3d-63, AOCS Method Cd 3-25, AOCS Method Cd 1b-87 and MS 817:1989 respectively. The extracted WSO was subjected to gas chromatography to find out the fatty acid composition.WSO ester sample was analyzed with Shimazdu GC-14B gas chromatograph using Chromatopack Silar 10C column (12 m x 0.53 mm ID x 0.31 m). The samples were injected at port temperature 230oC with N2 carrier gas flow of 10mL/min using FID detector maintained at 270oC. The oven was programmed from an initial temperature of 180oC to 200oC at heating ramp of 2oC/min with a 5 min hold, raised to 230oC at 7 oC/min and kept for 5 min, then upgraded to 250oCat 7 oC /min with a 2 min hold for a total run time of 20 min.
The percent oxirane oxygen content ofthe prepared WSO-phosphate polyolswas determined using AOCS Official Method Cd 9-57 (1997)in which the oxygen was titrated directly with HBrsolution in acetic acid.Hydroxyl value (H.V.)and acid value (A.V.)of the polyolsamples were determined by standard methods of ASTM D-1639-90 (1996) e1and AOCS Tx 1a-66 (1997) respectively.The viscosity was measured usingSV-10 sine wave vibroviscomter (A&D make, Tokyo, Japan) at 30oC. The vibrational changes in the structural functional groups of WSO-phosphate polyol samples were characterizedwith FTIR spectra, measured on Shimazdu FTIR-8400spectrometer in the range of 4000 to 400 cm−1.
The properties of WSO with fatty acid composition were obtainedas follows: Sp. Gr.= 0.918(at 25oC);
A.V. (mg KOH/g) = 0.2; I.V. (g I2/100 g) = 146; S.V. (mg KOH/g) = 192; oleic acid, C18:1 = 12.6%; linoleic acid, C18:2 = 70.5%. In situ epoxidation reaction was carried out with 1:0.5:1.5 molar ratio of ethylenic unsaturation: acetic acid: hydrogen peroxide at 60°C. In epoxidation reaction, acetic acid was converted to peracetic acid and was constantly being regenerated, an excess of 0.5 mole relative to ethylenic unsaturation was considered reasonable. The theoretical maximum oxirane content in 100 g ofoil was calculated to be 8.42%. Under the experimental epoxidation conditions, 7.87% oxirane content in prepared ESWO was obtained. The reduction in I.V.was achieved to13.5(g I2/100 g) with A.V. of 0.12 (mg KOH/g).
In present work, the phosphoric acid being a polybasic acid not only catalyze the ring-opening hydrolysis of EWSO with water but also expected that PA chemically bind with EWSO to produce mono-, di-, and tri-phosphate esters (Scheme 2). The presence of phosphateester functionality in hydrolyzed product, WSOpolyol is desired to improve corrosion resistance and fire retardant properties, when used in coating applications. However, the extent of formation and hydrolysis of phosphate esters depends on several factors including relative nucleophilicity of reactants, stability ofintermediates, solvent, temperature, and relative concentrationof reactants 23.Therefore, we have investigated the effect of different solvents and varyingthe relative ratio of solvent, water and phosphoric acid on ring-opening reaction and studied the characteristics of the resultant WSO-phosphate polyols.
The effect of solvents duringhydrolysis reaction on the reductionof oxirane content was determinedusing five differentsolvents: acetone, tetrahydrofuran (THF), Isopropanol(IPA), ethyl acetate (EA)and t-butyl alcohol(TBA). In each case,EWSO was treated with PA-2%, solvent-50% and water-10% on weight basis. The characteristics of WSO-phosphate polyolsobtained at the end of hydrolysis are presentedin Table1.
Table 1: Properties of WSO-phosphate polyols using different solvents
Parameter | Solvents | ||||
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Acetone | THF | IPA | EA | TBA | |
EWSO/solvent (w/w) | 1.0/0.5 | 1.0/0.5 | 1.0/0.5 | 1.0/0.5 | 1.0/0.5 |
Phosphoric acid (%) | 2 | 2 | 2 | 2 | 2 |
Water (%) | 10 | 10 | 10 | 10 | 10 |
Oxirane content (%) | 2.9 | 2.2 | 1.4 | 1.0 | 0.28 |
Acid value (mg KOH/g) | 1.2 | 1.9 | 2.4 | 55 | 3.2 |
Hydroxyl value (mg KOH/g) | 95 | 123 | 169 | 234 | 270 |
Viscosity (cPs, 30 oC) | 3200 | 3540 | 4560 | 3890 | 3490 |
The extent of ring-opening during hydrolysis was monitored by determination of the percent oxirane content over the course of reaction time at regular time intervals. This variation of the percent oxirane content with reaction time for different solvents is depicted in Fig.1.The use of acetone and THF as solventduring hydrolysis showed depletionof oxirane content in the hydrolyzed products to 2.90 and 2.22 at the end of reaction, which corresponds to approximately 63.1% and 71.8% completionof the reaction, respectively. The higher rate of oxirane ring degradation (81.7 % conversion) was achieved in the polyol product based on IPA than that for acetone and THF,signified by the percent of residual epoxy groups in their products. EA as solvent resulted into 87.2% ring-opening, while for the product based on TBA a much faster rate of depletion of oxirane groups was observed,which indicatedthe higher rate of hydrolytic rupture.A very high A.V. (55 mg KOH/g) of the hydrolyzed product was found usingEA as solvent.This can be attributed to the possible acid catalysis of this ester-type solvent in the presence of water,resulting in the formation of ethanol and acetic acid. From Table1,it is evident thatTBAwas the best solvent for EWSO hydrolysis, since the degradation rate of oxirane ring wasmoderatelyswiftin aqueous acid media (96.4 % conversion)along with a higher hydroxyl value (270 mg KOH/g)of the derived product. The relatively low nucleophilicity of TBA solvent system tends to rearrange its thermodynamically stable ion by multiple migrations and in turn responsible for a higher degree of hydrolysis. Therefore, further experiments were performed with TBAto study the effect of other parameters on oxirane ring-opening.
Fig. 1:Degree of hydrolysis of EWSO in different solvents a function of reaction time.
The solvent dosing is an important parameter because this factor substantially affects the oxirane ring-opening. The influence of relative solvent ratio on the rate of reaction was investigated by performing hydrolysis at four different TBAconcentrationsof 0.2, 0.5, 1.0 and 1.5wt%in the reaction mixture. The effect of varying TBAconcentrations on the percentoxirane content is presented in Fig.2, which shows a gradual decline in oxirane content in the hydrolyzed product with increased EWSO dilution with TBA.An addition of 1.0% TBA in a reaction mixture enabled to reduce the oxirane content to 1.30%.However, there was no significant difference obtained for oxirane ring-opening capability beyond 0.5% TBA under the given conditions. Moreover, using 0.5 % TBA formed the product with substantialH.V.(270 mg KOH/g) at the end reaction. Therefore, 0.5% TBA concentration was considered to be optimum for subsequent hydrolysis experiments.
Fig. 2:Effect of relative solvent dosing on degree of hydrolysis of EWSO. Table 2: Characteristics of WSO-phosphate polyols prepared using various reaction blends
Parameter | Solvent, t-butyl alcohol | |||||||||
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EWSO/solvent (w/w) | 1.0/0.2 | 1.0/0.5 | 1.0/1.0 | 1.0/1.5 | 1.0/0.5 | 1.0/0.5 | 1.0/0.5 | 1.0/0.5 | 1.0/0.5 | 1.0/0.5 |
Phosphoric acid (%) | 2 | 2 | 2 | 2 | 3 | 4 | 5 | 10 | 2 | 2 |
Water (%) | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 20 | 30 |
Oxirane content (%) | 0.08 | 0.28 | 1.3 | 2.4 | 0.18 | 0.11 | 0.06 | 0.02 | 0.08 | 0.05 |
Acid value (mg KOH/g) | 9.2 | 3.2 | 2.85 | 2.15 | 10.8 | 31.2 | 68.8 | 118.2 | 2.95 | 2.6 |
Hydroxyl value (mg KOH/g) | 256 | 270 | 245 | 218 | 241 | 248 | 259 | 275 | 275 | 279 |
Viscosity (cPs, 30 oC) | 3650 | 3490 | 2460 | 1550 | 4160 | 4550 | 4780 | 3160 | 3360 | 3270 |
PA plays a significant role in hydrolysis of the oxirane ring due to acidcatalyzed behavior of the reaction.The influence of PA concentration varied from 2 to 10% on the relative rate of ring-opening is shown in Table2.The increase in PA content lead to absolute ring-openingindicated by the increasing trend in hydroxyl values of the product. At the same time,the products exhibited higher final acid values that can be attributed to the progressive hydrolysis of higher phosphate esters to lower esters23. The viscosity of the hydrolyzed products swiftly shoots as a function of PA content up to 5%. This might be due to presence of increasing amounts of higher phosphate esters. The product obtained with10%PAdosing showedthe higher hydroxylvalue but comparatively reducedviscosity.This can be related to the availability of less higher phosphate esters resulted fromthe reduced rate of possible oligomeraization during the course of reaction23 . At a higher acid concentration, there is the rapid utilization of oxirane groups in the formation of phosphate estersresulting in a significant drop in oligomerization.
The influence of water content on the oxirane ring-opening viaacid hydrolysis was investigated. From Table 2, it can be seen that increase in the concentration of water from 10% to 30% does not contribute any noteworthy effect besides a small increase in the rate of hydrolysis was evident.
FTIR spectra of the WSO anf epoxide adduct (EWSO) are shown in Fig. 3. For WSO the characteristic
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peaks at 3008 cm, 1650 cmand 721cmare attributed to the stretching vibration of the double bonds: =C-H, C=C, cis-CH=CH, respectively.
Upon epoxidation the almost complete disappearance of double bonds band at 3008 cm−1was observed and formation of new peaks at 841 cm−1, ascribed to the epoxy groups, confirmed that almost all the C=C-H had taken part in the epoxidation reaction.The subsequent acid catalyzed epoxy ring-opening was monitored for the disappearance of characteristic peaks of EWSOand formation of other new peaksappearing at 3397-3410 cm −1,assigned to the hydroxyl functional groups and at1075-1084 cm−1, phosphate ester groups (Fig.4).
Fig. 4: FTIR spectrum of EWSO-PA2% polyol (A), EWSO-PA3% polyol (B), EWSO-PA5% polyol (C) and EWSO-PA10% polyol (D) (Conditions: Temperature, 60 oC; Reaction time, 5 hours; EWSO:TBA, 1:0.5; Water content, 10 %).
The epoxide adduct, EWSO was successfully synthesized by epoxidation using peroxy acid generated in situ. Theacid catalyzed ring-opening hydrolysis of EWSO formedthe WSO-phosphate polyols that showed variation in hydroxyl values. The completion of the reaction was judged and confirmed with the formation of single layerviscous product and FTIR analysis.The viscosity of the resultant product increased with the increase in PA concentration, suggesting theformation of phosphate estersfrom the appearance of IR absorptionband at1075-1084 cm−1.The synthesized EWSO and WSO-phosphate polyolscan serveas the potential intermediates for the producing various industrially important products.
Author is thankful to TEQIP-II, funded by MHRD, Government of India, Delhi (India) for financial assistance to undertake this work.
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