Intensification and optimization of biodiesel production using microwave-assisted acid-organo catalyzed transesterification process

The study addresses the challenge of producing cost-effective biodiesel from feeds that may contain high free fatty acids (FFAs), which complicate base-catalyzed transesterification due to soap formation and yield loss. Acid catalysts can process both triglycerides and FFAs without soap, but commonly used mineral acids exhibit slow transesterification kinetics and cause equipment corrosion. The authors propose the use of an organo-sulfonic acid catalyst, 4-dodecylbenzene sulfonic acid (DBSA), which improves mutual solubility of oil and alcohol phases and enhances mass transfer. The research question is whether microwave heating can further intensify and shorten the DBSA-catalyzed transesterification of triglycerides under mild conditions. The purpose is to optimize key process variables (temperature, catalyst-to-oil ratio, methanol-to-oil ratio) using response surface methodology (RSM) to maximize conversion while minimizing severity. The significance lies in accelerating acid-catalyzed transesterification, reducing corrosion issues, lowering energy/time demands, and meeting biodiesel quality standards.

Prior work shows mineral acid catalysts (e.g., H2SO4, H3PO4, HCl) tolerate FFAs and can conduct esterification and transesterification but demand high temperatures/pressures and are far slower for triglyceride transesterification (reported ~4000× slower than base catalysis), with mass transfer limitations and corrosion drawbacks. DBSA, an alkylbenzene sulfonic acid, is a strong yet less corrosive organic acid with amphiphilic character that enhances oil–alcohol solubility, improving mass transfer and reaction rates. Conventional heating with DBSA can achieve high conversions but typically requires around 6 h for triglyceride transesterification or higher temperatures/catalyst/alcohol loading. Microwave (MW) heating has been widely reported to cut reaction times dramatically for biodiesel synthesis with various catalysts, enhancing rates via rapid volumetric heating and potential non-thermal effects. Studies cited include microwave-assisted reactions achieving high yields within minutes to hours with sulfuric acid, solid acids, and ionic liquids, and evidence of improved phase behavior and separation under MW. However, literature on MW-assisted transesterification specifically using DBSA for triglycerides is scarce, motivating this study.

Feedstocks and reagents: Refined sunflower oil (pure triglycerides), methanol (>99.8%, Sigma-Aldrich), and 4-dodecylbenzene sulfonic acid (DBSA, ≥95%, Sigma-Aldrich) were used as received. Equipment: A Milestone FlexiWave microwave reactor (300 W) with magnetic stirring (~300 rpm) and infrared temperature control was used with a sealed 100 mL Teflon vessel. Experimental procedure: For each run, specified catalyst-to-oil and methanol-to-oil molar ratios were prepared. Oil and DBSA were charged to the vessel, brought to the preset temperature; preheated methanol was added, and the mixture irradiated at 300 W with stirring. Reaction temperatures and compositions were varied per a central composite design (CCD). Samples were taken at 10, 30, 60, and 120 min. Purification: After reaction, the vessel was cooled in water to stop reaction. Phase separation (enhanced under MW) produced a lower glycerol layer and an upper biodiesel layer. Biodiesel was water-washed 4–5 times to neutral litmus and dried at 100 °C. Acid number (ASTM D974) confirmed compliance with ASTM D6751. 1H NMR confirmed catalyst removal post-wash. Analysis: Quantitative 1H NMR (Bruker Avance 400, CDCl3) measured the methyl ester (ME) signal at 3.67 ppm and glyceridic methylene signal at 4.07–4.35 ppm. Conversion X (%) was computed as X = [4 I_ME / (4 I_ME + 9 I_TG)] × 100, where factors 4 and 9 reflect methylenic hydrogens in TG and hydrogens in ester products. Design of experiments and statistical modeling: A three-factor CCD (20 runs: 8 factorial, 6 axial, 6 center points) explored ranges: Temperature A = 64–76 °C (axial 60–80 °C), Catalyst-to-oil molar ratio B = 0.03–0.09 (axial 0.0095–0.11), Methanol-to-oil molar ratio C = 3–9 (axial 0.95–11). Responses were conversions at 10, 30, 60, and 120 min. Multiple linear regression and ANOVA (Design-Expert 11) evaluated full quadratic and reduced models. Diagnostics included residual normality, predicted vs. actual plots, and outlier assessment (|studentized residual| > 3.67). Model reduction considered main effects, two-factor interactions, and quadratic terms; final model selection used R², adjusted and predicted R², standard deviation, and outlier counts. Optimization and verification: Numerical optimization sought near-100% conversion at 60 min subject to limits (e.g., minimizing methanol, limiting temperature ≤76 °C). Optimal points were validated experimentally. Product characterization: Biodiesel properties (flash point, viscosity, density, carbon residue, cloud point, cetane number, acid value, distillation, copper strip corrosion) were measured per ASTM methods and compared with ASTM D6751.

- Microwave-assisted DBSA-catalyzed transesterification achieved near-complete conversion under mild conditions. At 76 °C, catalyst-to-oil molar ratio B = 0.09, and methanol-to-oil ratio C = 9, conversion was >99.5% within 30 min (Table 2, run order 4). - Compared to conventional heating with DBSA (≈6 h required for similar conversion), microwaves reduced reaction time to 30 min under comparable temperature and dosage conditions. - RSM outcomes (60 min response): Initial full quadratic model: X(60 min) = 67.25 + 13.79A + 14.73B + 9.69C + 0.26AB + 2.06AC + 0.56BC + 1.11A² + 0.02B² − 4.27C², with R² = 0.9393, adj R² = 0.8846, but lower predicted R² (0.5361). - Model refinement identified that the main effects dominated, with C² contributing curvature; two-factor interactions were mostly insignificant or weakly synergistic. The final modified model (60 min) was: X (%) = 68.10 + 13.79A + 14.73B + 9.69C − 4.37C². ANOVA: F = 51.49, p < 0.0001; R² = 0.9321, adj R² = 0.9140, pred R² = 0.8613 (Table 10). - Factor importance: Catalyst-to-oil ratio (B) and temperature (A) had stronger positive effects than methanol-to-oil ratio (C); across times up to 60 min, relative contributions generally followed B > C > A (early) and B > A > C (later). - ANOVA at 60 min (full quadratic): Significant main effects A, B, C (p < 0.001); only C² significant among quadratic/interaction terms (p ≈ 0.039). - Optimization examples (60 min): • Minimize methanol with T ≤ 76 °C: Optimal A = 76 °C, B = 0.110, C = 4.09 predicted 98.48%; experimental 97.38%. • Lower temperature with C ≤ 9: A = 69.5 °C, B = 0.110, C = 8.77 predicted 96.69%; experimental 97.43%. • Lower catalyst with A ≤ 76 °C, C ≤ 9: A = 76 °C, B = 0.078, C = 9 predicted 96.02%; experimental 94.68%. - Microwave heating enhanced phase separation; biodiesel layer separation time was reduced relative to conventional heating. - Product quality met ASTM D6751: e.g., flash point 161 °C (>130), kinematic viscosity 4.55 mm²/s (1.9–6.0), density 0.879 g/cc (0.87–0.90), cetane number 51 (≥47), acid value 0.05 mg KOH/g (≤0.5).

Using DBSA, an amphiphilic organo-sulfonic acid, under microwave heating intensified triglyceride transesterification by improving phase interactions and rapidly delivering energy to polar methanol and catalytic sites. This reduced reaction time from hours (conventional heating) to minutes while maintaining moderate temperatures (≤76 °C) and moderate methanol and catalyst dosages. RSM/CCD showed that the primary determinants of conversion were catalyst loading and temperature, with methanol ratio contributing positively but exhibiting diminishing returns (captured by a significant negative C² term). The final reduced polynomial provided accurate predictions across the design space (good agreement between predicted and experimental conversions), enabling practical optimization to balance conversion with reagent usage and temperature constraints. The findings directly answer the research question: microwave-assisted DBSA catalysis is an effective, milder, and faster route for biodiesel synthesis from triglycerides, with improved separation and product quality compliant with ASTM standards. The significance extends to potential processing of low-cost, high-FFA feeds by leveraging DBSA’s dual esterification/transesterification capability, with microwaves mitigating prior kinetic limitations that hindered acid-organocatalyst commercialization.

Microwave irradiation markedly intensifies DBSA-catalyzed transesterification of triglycerides, achieving near-100% conversion in 30 min at 76 °C with B = 0.09 and C = 9, compared to ≈6 h under conventional heating. A modified RSM-derived polynomial model with strong statistical metrics (R² = 0.9321; F = 51.49; p < 0.0001) accurately describes conversion at 60 min and highlights the dominant influence of catalyst loading and temperature. Optimization demonstrated the ability to reach high conversions while constraining methanol usage, temperature, or catalyst loading, and experimental verifications matched predictions closely. Produced biodiesel met ASTM D6751 specifications. Future work suggested by the authors includes exploring other process intensification technologies (ultrasonics, microchannel reactors, static mixers) and conducting kinetic studies of DBSA-catalyzed transesterification under microwave irradiation to deepen mechanistic understanding and support scale-up.

- Feedstock was refined sunflower oil (pure triglycerides); performance on actual low-cost, high-FFA oils was not experimentally demonstrated in this study, limiting generalizability. - The temperature and reagent ranges were constrained (A ≤ 76 °C; C ≤ 11; B ≤ 0.11) due to the sealed vessel and methanol boiling considerations; extrapolation beyond these ranges is uncertain. - The full quadratic model at 60 min showed significant lack of fit before reduction; although the final reduced model performed well, model adequacy outside the experimental region remains unverified. - Detailed reaction kinetics under microwave irradiation with DBSA were not studied, as noted by the authors, which limits mechanistic interpretation and scale-up modeling.