Studies on the kinetics of epoxidation of soybean oil

Dai Honghai, Yang Liting, Lin Bo, Yi Aihua, Shi Guang
(School of Chemistry and Environment, South China Normal University GuangZhou   510631, China)

AbstractThe kinetics of epoxidation of soybean oil (SBO) by peroxyacetic acid (PAA) in the presence of sulphuric acid catalyst are investigated at 45 C, 65 C and 75 C. Epoxidation with higher conversion of unsaturated carbon and lower oxirane cleavage can be attained by the in situ technique. The rate constant for epoxidation of SBO was found to be of the order of 10-6mol-1-1 and activation energy of epoxidation of 43.11kJ·mol-1. The enthalpy, entropy and free energy activation were 40.63 kJ·mol-1, -208.80kJ·mol-1 and 102.88 kJ·mol-1, respectively.

Epoxides of all kinds of plant oils are well known commercially since the they can undergo many important reactions. Epoxidation of long chain olefins, and unsaturated fatty acid derivatives such as soybean oil and other plant oils have been carried out on an industrial scale [1]. Nowadays, one of the most important epoxidized vegetable oils is the epoxidized soybean oil (ESO) and its worldwide production is about 200 000 t/year [2]. Fatty epoxides are used directly as plasticizers that are compatible with polyvinyl chloride (PVC) and as stabilizers for PVC resins to improve flexibility, elasticity, and toughness and to impart stability of polymer towards heat and UV radiation. Due to high reactivity of the oxirane ring, epoxides also act as a reactant raw material for a variety of chemicals, such as alcohols, glycols, alkanolamines, carbonyl compounds, olefinic compounds and polymers like polyesters, polyurethanes (PU) and epoxy resins.
The modified soy-based vegetable oil polyols could be incorporated as a replacement for conventional polyols, reacting with Toluene dicocyanate (TDI) and 4,4′-Diphenylmethane dicyanate (MDI), to produce flexible slabstock polyurethane foams, elastomers, coating. Soybean oil (SBO) is highly hydrophobic, so an excellent weather stability of the derived PU can be expected, the thermal and oxidative stability of the soybean oil-based PU are comparable with those of the polypropyleneoxide-based PU [3-6]
As the demands of energy increase and fossil fuel reserves are limited, there has been a growing interest in the utilization of renewable resources as an alternative to petroleum based polymers. Consequently, much attention has been focused on the development of polymeric materials from vegetable oils, a sustainable resource [7C9]. SBO which is readily available and is a comparatively inexpensive material can be used to synthesize various types of polymers.
There are four known technologies to produce epoxides from olefinic type of molecules: [a] epoxidation with percarboxylic acids [10], the most widely used in industry, can be catalyzed by acids or by enzymes [1,11]; [b] epoxidation with organic and inorganic peroxides which includes alkaline and nitrile hydrogen peroxide epoxidation as well as transition metal catalyzed epoxidation [12]; [c] epoxidation with halohydrins, using hypohalous acids (HOX) and their salts as the reagents for the epoxidation of olefins with electron deficient double bonds [10]; and [d] epoxidation with molecular oxygen [10]
But during the ways in the epoxidation of vegetable oils, the available technologies that need to be explored are only [a] and [b] described above, namely, epoxidation with percarboxylic acids and epoxdation with organic and inorganic peroxides [13] they are clean and efficient. These technologies can be rendered cleaner by using heterogeneous catalysts replacing traditional homogeneous ones [14]. In the present work the kinetics of epoxidatin of SBO by PAA generated in situ and the relation between the extent of epoxidation and temperature was investigated.

2.1 Materials
The refined soybean oil with an iodine value 127.23 (g I/100g oil) was kindly provided by Nanhai Oil Co. Ltd. Glacial acetic acid (99.5%), sodium carbonate, sodium hydroxide, hydrogen peroxide (30%) and sulphuric acid (98%) are AR grade and purchased from Guangzhou Chemical Reagent Co.. They were used as received.
2.2 Epoxidation of soybean oil

Epoxidation of SBO was carried out in a 1000-ml four-necked round-bottom flask equipped with a thermometer sensor, a mechanical stirrer, a condenser, and an isobaric funnel. The whole apparatus was kept in a water bath to maintain the reaction temperatures at 45, 65, 75 ± 1. SBO (150 g, 0.172 mol) was placed in the round-bottom flask. PAA,was prepared in situ by reacting various mixtures of 35g (0.58 mol) of 99.5% glacial acetic acid and 165 g (1.46 mol) of 30% H in the presence of small quantities (0.1 mL) of concentrated sulphuric acid for about 12 hours and added slowly through an isobaric funnel over two hours (±1 minute). This precaution was taken to prevent overheating of the system due to the exothermic nature of epoxidation reactions. The stirring rate was controlled at 800 r/min so that the oil in the mixture was well dispersed. The reaction was monitored by withdrawing aliquots of the reaction mixture at various time intervals. At the end, the reaction expired, the samples were quenched by cooling to 5 C to stop the epoxidation reaction and the reaction mass delaminated soon.
The samples were poured into a separating funnel the aqueous layer was drawn off and the oil layer was washed successively with warm dilute sodium carbonate solution until the pH was neutral, and then washed with saturated sodium chloride solution and distilled water. The solvent was removed by using a rotary evaporator, with the help of a water vacuum pump. The oil phase was further dried on anhydrous magnesium sulfate and then filtered. The hydroxyl values of the polyols were determined according to the ASTM E 1899-97 standard test method for hydroxyl groups using reaction with p-toluenesulfonyl isocyanate (TSI) and potentiometric titration with tetrabutylammonium hydroxide. The Epoxy Oxygen Content (EOC) in polyols was determined by direct titration of epoxy groups with HBr according to the standard method for oils and fats. [15]
2.3 Method and test apparatus
The IR spectra were recorded on a PerkinElmer Specrum-1000 Fourier transform infrared (FTIR) spectrometer. Samples were prepared as thin films on KBr plates. The oxirane ring formation reactions were monitored by the appearance of the oxirane C―O double peaks at 823 cm-1and 833 cm-1

3.1 FTIR Spectra of SBO and ESBO
The FTIR spectra of SBO and ESBO are shown in the Figure 1, and 2. As it can be seen from the FTIR spectrum of the SBO (Figure 1 and Figure 2), the characteristic peak at 3009 cm-1 was attributed to the CCH stretching of SBO C=CCH. The peak at 3009 cm-1 decreased after the epoxidation reaction and disappeared almost completely after 6 hours reaction at 65 and 75 C, indicating that almost all the C=C bands have taken part in the epoxidation reaction. The peaks due to C=O stretching vibrations at 1725 cm-1 remained unaltered after the epoxidation reaction. The presence of new peak in the FTIR spectra of ESBO at 823 cm-1 and 833 cm-1, attributed to epoxy group, corroborated the conclusion that the success of the epoxidation reaction of SBO. The other new peak at 3463 cm-1 was attributed to the hydroxyl O�CH stretching, indicating that the epoxy group might be opened. The extent of hydroxyl group of ESBO increases with an increase on reaction temperature at the same reacting time.
Figure 1 FTIR spectra: spectra of SBO and ESBO prepared at various temperature reacting for 6 hours
Figure 2 FTIR spectra: part spectra of SBO and ESBO prepared at various temperature reacting for 6 hours

3.2 Effect of temperature on epoxidation
The extents of epoxidation of SBO by PAA generated in situ at various temperatures are showed in Figure 3. The results indicated that increasing temperature showed a favorable effect on the extent of epoxidation of SBO by PAA generated in situ. The initial increase in the extent of epoxidation of SBO with reaction time reached maximum values (for epoxidation at 75 C) , and then decreased with further increase in reaction time, indicating that the epoxy group was opened more and more seriously with further reaction time at 75 C. The decrease in the extent of epoxidation value at 75 C was at long reaction time very obvious, while the extent of epoxidation value at 65 C hardly decreased during initial 8h. Reaction at lower temperature showed lower rate but gave more stable oxirane ring (for epoxidation at 45 C).
Figure 3 Extent of epoxidation of SBO by peroxyacetic acid at various temperatures

   The epoxy oxygen content, iodine value and OH value of the epoxidation products at 75 C, 65 C and 45 C for 6 hours were shown in Table 1. As can be seen from Table 1, EOC increases with increasing reaction temperature while iodine value decrease. The OH value, due to oxirane cleavage, also increases with increasing of reaction temperature. These results suggested that the maximum optimum level of epoxidation could be obtained in a shorter time at moderate reaction temperature range of 65 and 75C, at which the rate of epoxide degradation was not high relatively. Consequently, only the initial stage of the reaction used for the kinetic study.

Table 1 Chemical properties of epoxidation products at different temperatures

   45 65 75
EOC, mol/100 g 0.240 0.350 0.378
Iodine Value, g I/100 g oil 60.50 26.65 12.93
OH Value, mg KOH/g 6.44 19.16 33.88

3.3 Kinetics of epoxidation
The in situ epoxidation reaction generally takes place in two steps (i) formation of peroxyacidand (ii) reaction of peroxyacid with the unsaturation [16]
(i) Formation of peroxy acid
If the first step is considered to be the rate determining and the concentration of peroxy acid is assumed to be constant throughout the reaction, then the rate of epoxidation will be given by the following expression [17]
Where, subscript 0 denotes initial concentrations and EP denotes epoxides.
R{[H – [EP]} = -k·[RCOOH]·t + R[H (2)

According to Eq.(2), plot of {[H – [EP]} vs. time should yield straight lines for those reactions with negligible degradation of oxirane, and so the rate constants were obtained with the initial slopes. Figure 4 shows the plots for in situ epoxidation of SBO at different temperatures. The rate constants obtained for the reaction were of the order of 10-6mol-1-1 (Table 2). As the epoxidation reagent (PAA) had been formed, it is mixed with SBO to prepare ESBO; then the rate of the epoxidation of SBO by PAA was greater than the published values [18]. The rate constants in situ epoxidation of SBO by PAA increased with temperature (Figure 5), and the activation energy was determined to be 43.11kJ·mol-1. The value of activation energy was smaller than the published values of 63.26 and 65.69 kJ·mol-1[17, 18] for the reasons discussed above.


Figure 4Kinetics of epoxidation of SBO by PAA
Figure 5 Activation energy, E for the epoxidation of SBO by PAA

Table 2 Rate constants of SBO epoxidation at various temperatures.

Temperature(C) Rate constant of epoxidation K·10(1 mol-1-1
45 7.4
65 18.0
75 29.6

3.4 Thermodynamics of Epoxidation of SBO
The enthalpy of activation,wascalculated using the equation(3) [19]
H=E-RT (3)
Enthalpy of activation was found to be 45.59 kJ·mol-1
The average energy of activation,S, and free energy of activation,F,
were obtained using the relationship (4) in [20]:
image  (4)
Where, k is the rate constant; R, gas constant; T, absolute temperature; N, Avogadro constant; and h, Plank’constant.
The average values of the thermodynamic parameter were found to be
S=-208.80 J·mol-1andF =107.84 kJ·mol-1

The results from this study show that the epoxidation of SBO by PAA generated in situ can be carried out at moderate temperatures with minimum epoxide degradation. The kinetic and thermodynamic parameters of epoxidation obtained indicate that an increase in the process temperature would promote the rate of epoxide formation and useful for the scale-up production of ESBO in situ technique.

ACKNOWLEDGMENTS The authors are grateful to the Natural Science Foundation of Guangdong Province (No. 06025028) for financial support.