Green process for fatty acid production from soybean oil through microwave-mediated autocatalytic synthesis

Highlights

• Autocatalytic oil hydrolysis using microwave irradiation was successfully developed.

• Microwave irradiation was more efficient than traditional heating in oil hydrolysis.

• Oil hydrolysis using microwave irradiation achieved a 96.6 % yield within 4 h.

• A kinetic model was developed to describe the performance of oil hydrolysis.

• Autocatalytic oil hydrolysis is a promising green process for fatty acid production.

Abstract

This study developed a new method that involved autocatalytic hydrolysis of soybean oil using microwave irradiation for fatty acid production. To enhance the conversion yield, a surfactant and fatty acid were added to the reaction mixture. The oil hydrolysis using microwave irradiation had 96.6 % conversion yield within a reaction time of 4 h at 195 °C. Furthermore, this study established a model for describing the kinetics of oil hydrolysis performed using traditional heating and microwave irradiation under different reaction conditions. The model exhibited a good fit to the data (R² of 0.934 to 0.995), demonstrating the reliability of the developed model. The rate constants and pre-exponential factor observed for the oil hydrolysis process using microwave irradiation were higher than those observed for the oil hydrolysis process using traditional heating, indicating that performing oil hydrolysis by using microwave irradiation achieved a reaction more quickly than using traditional heating. These results suggest that autocatalytic oil hydrolysis using microwave irradiation is an ecofriendly and promising method for fatty acid production.

Introduction

Fatty acids are valuable oleochemicals used in various industrial applications including the production of food, surfactants, soap, and biomedical products [1,2]. In addition, scholars have recently highlighted the promising use of fatty acids as a renewable source for biodiesel preparation; this is because biodiesel is produced worldwide to address problems pertaining to fossil fuel depletion and environmental pollution [[3], [4], [5]]. Such attention has thus stimulated the development of effective processes for fatty acid production.

Fatty acids are primarily produced by hydrolyzing molecules of triglycerides (oils/fats) at the ester bonds between glycerol and fatty acids [6,7]. Several methods can be used to hydrolyze such oils [8,9]. The oils are commonly hydrolyzed using acids or alkalis as catalysts [8,10,11]. Although such chemical-catalyzed methods produce a high yield of fatty acids, they cause corrosive damage to equipment and require a complicated product purification process [7,12]. Moreover, the use of chemical catalysts raises concerns about environmental pollution because of problems pertaining to the disposal of such catalysts [1,12]. To address these problems, enzyme-catalyzed oil hydrolysis is considered an alternative to chemical-catalyzed hydrolysis [13,14]. Enzymes have high selectivity toward substrates and can be processed under mild reaction conditions, thus facilitating the achievement of an ecofriendly process for fatty acid production [[12], [13], [14]]. However, the high cost and unstable activity of biocatalysts limit their application [15].

Noncatalytic reactions are inherently executed without the presence of catalysts and have increasingly received interest as green processes for chemical production [[16], [17], [18]]. Noncatalytic processes do not have the drawbacks associated with both chemical and enzymatic methods [6,18]. Supercritical subcritical water have been applied in noncatalytic oil hydrolysis because water in supercritical and subcritical states acts as both a solvent and cleavage reagent for effectively hydrolyzing oil molecules into fatty acids [19,20]. Nevertheless, this approach requires high temperature (250–375 °C) and pressure (10–22 MPa) levels, and consequently causes the degradation of both reactants and products because of pyrolysis, isomerization, or decomposition processes [19,21]. To avoid this shortcoming, Kansedo and Lee [6] performed a noncatalytic process for hydrolyzing oil below subcritical conditions. Although this process efficiently hydrolyzed the oil into fatty acids, it required a long reaction time (up to 15 h), leading to high energy consumption. Therefore, developing an efficient oil hydrolysis method is imperative.

Several chemical synthesis methods have been efficiently executed using microwave irradiation [[22], [23], [24]]. Microwaves are electromagnetic waves that induce certain molecules to vibrate at an extremely high frequency (billions of times per second) [25]. In microwave irradiation technology, energy is transferred through microwave radiation to the reactants through either relaxation or resonance, which increases movement, collision, oscillation, and friction of chemical molecules and thus increases the reaction efficiency [26,27]. Consequently, microwave irradiation technology shortens the reaction time, reduces energy consumption, and increases product yield [27]. Furthermore, the energy cost of microwave irradiation is about 67 % less than that of traditional heating processes (the process in which heat is delivered from the surface of a reactor to reactants through conduction, convection, and radiation) [28,29]; accordingly, microwave irradiation is effective for pilot- and large-scale production [25,30]. Taken together, microwave irradiation can be a promising technique for promoting oil hydrolysis. Autocatalytic oil hydrolysis performed using microwave irradiation can facilitate the achievement of green methods of producing energy-efficient and cost-effective fatty acids. Although microwave-assisted processes have been widely applied in catalytic reactions [26,27,31], their use in the autocatalytic hydrolysis of oils has been limited.

The current study developed a method involving autocatalytic hydrolysis of soybean oil using microwave irradiation to produce fatty acids. The study compared the effectiveness of the proposed oil hydrolysis method using microwave irradiation with that of oil hydrolysis using traditional heating. The effects of initial concentrations of the applied surfactant (potassium oleate) and fatty acid on the conversion yield of oil hydrolysis were examined. Furthermore, the influences of reaction factors (temperature, microwave power, and reaction time) on the conversion yield were systematically explored. Also, this work established a model for describing the kinetics of autocatalytic oil hydrolysis in a condition of excessive water.