Self-esterification of partially maleated castor oil using conventional and microwave heating
Highlights
► Reaction proceeded more quickly when microwaves were used. ► Reaction was first-order respect to hydroxyl-groups concentration. ► Reaction was first-order respect to acid-groups concentration. ► Presence of non-thermal effects of microwave heating. ► 10% decrease in the activation energy and 182% increase in the pre-exponential factor.
Introduction
Maleinization reactions have been used to chemically modify vegetable oils through Diels–Alder reactions (conjugated dienes), esterifications (alcohols), and “ene-reactions” (compounds with allylic hydrogens) [1], [2]. These reactions have been employed to increase the hydrophilicity of olefinic compounds and unsaturated fatty acids. These products and their derivatives are used as drying oils, water-soluble paints, surfactants, and biomodifiers for biodegradable polymers [3], [4]. For this last application, maleated castor oil (MACO) has been widely investigated [5], [6], [7]. MACO is produced by the esterification of castor oil with maleic anhydride and this reaction proceeds in two stages, as shown in Fig. 1. The first stage (without condensation) is fast, non-catalytic, and first-order with respect to each reactant [8], while the second stage (self-esterification, with condensation) is a slow equilibrium reaction that can be catalyzed with acids, such as sulfuric acid, and is enhanced by removing water [9]. Therefore, environmental concerns related to the disposal of salts formed during the final neutralization of acids and technical problems associated with their use, such as corrosion and separation operations, constitute a strong driving force to search for alternatives.
The use of microwaves as heating source is an interesting alternative technology to promote esterification reactions; for instance, reductions in reaction times have been observed using microwave heating in the esterifications of lauric acid with 1,2-propanediol using commercial lipases [10], propionic acid with ethanol using Amberlyst-15 [11], free fatty acids from palm oil with methanol using H2SO4 and cationic resins [12], [13].
Microwave heating is an efficient and a relatively new tool in chemistry, which widens the scope of conventional thermal heating and gives chemists novel and exciting possibilities. Microwave irradiation produces efficient internal heating by direct coupling of microwave energy with the molecules of solvents, reagents or catalysts. The radiation passes through the walls of the vessel directly into the bulk reaction mixture volume and an inverted temperature gradient, as compared to conventional thermal heating, results. Three main effects of microwave irradiation on reactions are distinguished: (i) thermal effects, (ii) specific thermal effects, and (iii) non-thermal effects. Thermal effects are those resulting from dielectric heating which may cause a different temperature regime. In the majority of cases the reason for the observed rate enhancement of chemical processes is purely a thermal effect, i.e., the high reaction temperature achieved rapidly under microwave irradiation of polar materials increases the chemical reaction rate according to the Arrhenius law.
Specific thermal effects are caused by the unique nature of microwave heating. These effects should be defined as accelerations of chemical transformations in a microwave field that cannot be achieved or duplicated by conventional heating, but essentially are still thermal effects. Hot spots and liquid overheating are the most common specific thermal effects. Most non-thermal effects are electrostatic polar effects, which lead to dipole–dipole-type interactions between the dipolar molecules and charges in the electric field. Therefore, more polar states appear to be more stable in the electric field. These effects lead to (i) increased pre-exponential factor A of the Arrhenius equation (collision efficiency can be improved by mutual orientation of polar molecules involved in the reaction); (ii) decreased activation energy (ΔG≠), which is the main non-thermal effect. When considering the contribution of enthalpy and entropy to ΔG≠ (ΔG≠ = ΔH≠ − TΔS≠), it may be predicted that the magnitude of the −TΔS≠ term would decrease in a microwave-induced reaction, this being more organized when compared with classical heating as a consequence of dipolar polarization. To justify the reduction in the energy of activation one must assume stabilization of the transition state by the microwaves. This is presumably true when the transition state is more polar than the ground state [14], [15], [16].
This manuscript presents results on the kinetics of the self-esterification of partially maleated castor oil, using conventional and microwave heating. Kinetic parameters were determined, i.e., rate constants, activation energies, pre-exponential factors, as well as thermodynamic properties of activation. Comparison of these kinetic and thermodynamic properties was done to investigate the effects of microwave irradiation. The product of this reaction (SEMACO) is an interesting polyol for the synthesis of polymers such as polyurethanes and polyesters, because the hydroxyl value is decreased and the molecular weight is increased, compared to MACO. Besides, it can be prepared keeping some acid groups that introduce more functionality to the molecule [7].
Section snippets
Materials
Castor oil (CO) USB-grade (hydroxyl value 159.5 mg of KOH/g, 930 g/mol) and commercial-grade (Merck, Whitehouse Station, NJ, USA) maleic anhydride (MA), were used. A thermocouple J was used to measure and control the temperature. The microwave oven was an ETHOS (MLS GmbH, Leutkirch, Germany) with a nominal power of 1000 W.
Synthesis of partially maleated castor oil (PMACO)
Maleic anhydride (32.7 g) and castor oil (311 g) were added to a 250 mL four-necked and round-bottom flask equipped with a mechanical stirrer (turbine, 500 rpm), a thermometer, and
Results and discussion
Reaction progress was assessed by measuring the acid number and results are shown in Fig. 2, Fig. 3 for conventional and microwave heating, respectively. It is evident that the reaction proceeds more quickly when microwaves are used.
Because none of the reactants is in excess, reaction rate should depend on the concentration of both reactants (acid and hydroxyl groups). Therefore, assuming that the reaction is first-order with respect to acid groups (CA) and first-order with respect to hydroxyl
Conclusions
The self-esterification of partially maleated castor was non-catalytic, first-order with respect to hydroxyl-groups concentration and first-order with respect to acid-groups concentration. This reaction proceeded more quickly when microwaves were used and showed the presence of non-thermal effects of microwave heating, which were evidenced by the 10% decrease in the activation energy and the 182% increase in the pre-exponential factor.
Acknowledgments
Authors thank to “Universidad de Antioquia”, “Departamento administrativo de ciencia, tecnología e innovación-COLCIENCIAS (Colombia)” and “Agencia Nacional de Promoción Científica y Tecnológica, ANPCyT” (Argentina) for the financial support.
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