The process intensification for transesterification has been progressing rapidly to make biodiesel synthesis economically viable, and one of the methods available is microwave irradiation. Microwave-assisted transesterification of oils offers the ben­efits of shorter times and energy efficient operation. Electromagnetic radiation at microwave wavelength is transmitted and influence molecular motions. However, it does not alter the molecular structure of substances. Molecules move and vibrate with the alternating electric field of the microwaves, which leads to intense local­ized heating and accelerates the chemical reaction as the result of molecular friction and collisions (Kumar et al. 2011). The intensification allows the chemical reaction to produce higher yield at much shorter time.

The transesterification under microwave heating was conducted with nonedible feedstocks, Jatropha oil, and waste frying palm oil (WFPO) as feedstocks (Yaakob et al. 2009). The reaction was conducted using NaOH as the catalyst. The conver­sion of both feedstocks produced highest biodiesel yield in 7 min and temperature of 65°C, whereas the purity reached as high as 99%. Catalyst concentration lower than 1 wt% reduced biodiesel purity, while higher value did not improve its purity significantly. Assessment of the fuel properties showed that biodiesel obtained from Jatropha and WFPO were within the specified limits of the international standards of EN 14214.

Kumar et al. (2011) evaluated the capability of NaOH and KOH for the FAME production from Pongamia pinnata seed oil with the inclusion of microwave irra­diation. 96% and 97% of biodiesel yield were obtained when the catalyst concentra­tion of 0.5% w/w NaOH and 1.0% w/w KOH were used, respectively. A complete gel formation was observed when the catalysts concentration of NaOH increased to 1.5% w/w. Both catalysts managed to enhance biodiesel yield when the reaction time was increased from 3 to 10 min. Several properties of the resultant biodiesel meet the limit of international standard ASTM D6751. The author mentioned that these properties were influenced by the reaction time of the transesterification.

Microwave-assisted process is also appropriate for transesterification reaction utilizing heterogeneous catalysts. Zhang et al. (2010) used heteropolyacid catalyst for converting yellow horn under microwave heating. The biodiesel productivity obtained using Cs25H05PW12O40 catalyst was the highest among other catalysts used and even higher than the conversion using homogeneous acid catalyst (i. e., H2SO4) at the same operating conditions. The comparison between microwave-assisted transesterification (MAT) and conventional method of transesterification (CMT) showed that the former reached high conversion at lower methanol to oil molar ratio and shorter time in comparison to the latter method.

Heterogeneous catalyst ZnO/La2O2CO3 was applied for catalyzing biodiesel pro­duction from canola oil assisted by microwave source (Jin et al. 2011). The process reached equilibrium in 5 min, and extended time shows no improvement in FAME yield. The solid catalyst was shown to be more active than homogeneous catalyst of KOH in terms of reaction rate. Furthermore, the reaction assisted by microwave irradiation reached 95% FAME yield in less time compared to conventional heating. The determination of Zn-La oxide content in the biodiesel by XRF analysis proved that there was no leaching of Zn and La into the biodiesel even when the process was exposed to microwave irradiation. The results showed that high biodiesel pro­ductivity can be achieved in very short operating time with the microwave assistance.

Although microwave-assisted reaction shows positive impacts for the transesteri­fication process, especially less reaction time, there are some disadvantages related to the technology (Vyas et al. 2010). The first problem is related to the applicability of the method at industrial scale, as it is difficult to scale up the microwave process and is limited to laboratory scale synthesis. Another drawback in implementing this technology at larger scale is the penetration depth of microwave radiation into the absorbing materials, which is limited to only a few centimeters.