The depletion of fossil fuels necessitates the exploration of renewable energy sources, with biodiesel emerging as a promising alternative. Biodiesel, composed of fatty acid alkyl esters, is typically produced through transesterification of triglycerides (TG) or esterification of free fatty acids (FFAs) using alcohol, often methanol. While base-catalyzed transesterification offers faster kinetics, it's highly sensitive to impurities like water and FFAs, limiting the use of low-cost, high-FFA feedstocks. Acid catalysts, conversely, can efficiently convert both TG and FFAs without soap formation, but inorganic acids suffer from slow transesterification kinetics and equipment corrosion. Organic acid catalysts, such as DBSA, mitigate these drawbacks, but reaction times remain relatively long with conventional heating methods. This study addresses this limitation by employing microwave heating to accelerate the DBSA-catalyzed transesterification process, aiming for high biodiesel yield in significantly shorter reaction times under mild operating conditions. The use of RSM ensures optimization of process parameters for maximum efficiency and cost-effectiveness.

Previous research has explored biodiesel production using various methods. Base-catalyzed transesterification, while efficient, is sensitive to FFAs and water. Two-step processes involving acid-catalyzed esterification followed by base-catalyzed transesterification have been developed, but they are complex and costly. Acid-catalyzed one-step methods using inorganic acids such as sulfuric acid, phosphoric acid or hydrochloric acid offer a simpler approach, but their slow transesterification kinetics and corrosive nature hinder their widespread application. Several studies have demonstrated that organic acid catalysts like DBSA offer improved solubility of both alcohol and oil phases, leading to enhanced mass transfer and faster reaction rates. However, even with DBSA, conventional heating requires extended reaction times to achieve high conversion. Microwave-assisted transesterification has attracted attention due to its ability to rapidly heat polar molecules, leading to accelerated reaction rates. Existing literature shows successful biodiesel production using microwave heating with various acid catalysts including solid and liquid acids, but information on microwave-assisted transesterification using DBSA remains scarce. Therefore, this research aims to fill the knowledge gap by investigating the effectiveness of microwave heating to enhance the rate of transesterification reaction with DBSA as a catalyst.

This study employed microwave-assisted transesterification of refined sunflower oil (pure triglycerides) with methanol in the presence of DBSA as the catalyst. The reaction was conducted in a sealed Teflon vessel within a microwave reactor (Milestone Flexi Wave) equipped with a magnetic stirrer and infrared temperature sensor. The RSM, specifically a central composite design (CCD), was utilized to optimize three independent variables: reaction temperature (°C), catalyst-to-oil molar ratio, and methanol-to-oil molar ratio. Design Expert 11 software was used for experimental design and analysis. Twenty experiments were conducted according to the CCD matrix (Table 2), varying the levels of the three parameters (Table 1). The reaction progress was monitored at different times (10, 30, 60, and 120 min) by analyzing the unreacted triglycerides and formed FAME using 1H NMR spectroscopy. The conversion of triglycerides to biodiesel (X) was calculated using equation 1, which relates the areas under the curves of glyceridic protons (TG) and methyl ester protons (ME). The biodiesel product was purified by phase separation and water washing, followed by drying. The quality of the biodiesel was assessed by determining various physicochemical properties (Table 9) according to ASTM standards. Data analysis included ANOVA to determine the significance of the factors and their interactions, generation of main effect plots, interaction plots, contour plots, and response surface plots, to understand the impact of the independent variables on the biodiesel conversion. Model discrimination was carried out to select the best model to represent the process. The model was then validated experimentally using the determined optimal conditions.

The RSM analysis revealed that the main effects of reaction temperature, catalyst-to-oil molar ratio, and methanol-to-oil molar ratio significantly influenced biodiesel conversion. The catalyst-to-oil molar ratio exhibited the most dominant effect throughout the reaction (Figure 2). The main effects were positive, indicating that increasing each parameter led to higher biodiesel conversion (Figure 3). Initially (10 min), a quadratic effect of temperature was observed while at 60 min, a quadratic effect of methanol-to-oil ratio was significant. Two-factor interactions were mostly independent or slightly synergistic, with the interaction between temperature and methanol-to-oil molar ratio showing a more prominent synergistic effect (Figure 8). The initial full quadratic model was refined through model discrimination (Table 7) to a modified model (Equation 4) that excluded insignificant terms while maintaining a good fit (R² = 0.9321, Adj. R² = 0.9140, Pred. R² = 0.8613; Table 10). The optimal conditions predicted by the model were validated experimentally. Under microwave heating at 76 °C, a catalyst-to-oil molar ratio of 0.09, and a methanol-to-oil molar ratio of 9.0, nearly 100% conversion was achieved in 30 min (Table 2, Run order 4). This compares favorably with the conventional heating method, which yielded near 100% conversion in 6 h under similar conditions. The microwave method also facilitated easy separation of the biodiesel layer. The quality of the produced biodiesel conformed to ASTM D6751 standards (Table 9).

The findings demonstrate that microwave-assisted transesterification using DBSA significantly accelerates biodiesel production, reducing reaction time from 6 hours to 30 minutes while maintaining high conversion rates. This acceleration is attributed to the efficient absorption of microwave radiation by the polar molecules, leading to localized heating and potentially non-thermal effects that enhance reaction kinetics. The superior performance of microwave heating over conventional heating is evident in the drastic reduction of reaction time. The developed RSM model provides a valuable tool for optimizing the biodiesel production process, allowing for the selection of optimal operating conditions based on desired conversion levels and economic considerations. The model's robustness is validated by the good agreement between predicted and experimental results. The findings highlight the potential of microwave-assisted acid-organocatalyzed transesterification as a commercially viable and cost-effective biodiesel production technology. The use of DBSA, with its lower corrosiveness compared to inorganic acids, also reduces capital and operational costs associated with equipment.

This study successfully demonstrated the feasibility of significantly enhancing biodiesel production using microwave-assisted acid-organocatalyzed transesterification. The microwave method proved far superior to conventional heating, shortening reaction times drastically while achieving near-complete conversion. The RSM-based optimization and the validated model provide valuable tools for process control and scale-up. Future research can explore other intensification technologies such as ultrasound or microchannel reactors, and also investigate the reaction kinetics in detail.

The current study focused on refined sunflower oil, which contains minimal FFAs. Future studies should investigate the applicability of the microwave-assisted DBSA-catalyzed transesterification with low-cost feedstocks containing high FFA levels. The study used a single type of oil and further research would be needed to understand the impact of different feedstock compositions on the optimal reaction parameters. While the model provides a good prediction of biodiesel conversion, it's based on a limited set of experimental conditions. More extensive testing across a wider range of parameters might provide a more comprehensive model.