The global lubricants market is experiencing significant growth, with an increasing demand for environmentally friendly alternatives to petroleum-based lubricants. Biolubricants, derived from renewable resources, offer a sustainable solution, addressing concerns about environmental impact and fossil fuel depletion. Vegetable oils, historically used as lubricants, possess inherent advantages such as biodegradability and renewability. However, their susceptibility to oxidation and lower thermal stability compared to synthetic esters necessitates chemical modification. This study focuses on the synthesis of trimethylolpropane ester (TMPE) from palm oil methyl ester (PME) and trimethylolpropane (TMP), leveraging the advantages of microwave-assisted synthesis. Microwave irradiation is known to accelerate chemical reactions, offering potential benefits in terms of reduced reaction time and energy consumption compared to conventional thermal methods. The research aims to optimize the microwave-assisted transesterification process by investigating the influence of key reaction parameters and to compare the performance with conventional methods. This will contribute to the development of a more cost-effective and environmentally sustainable approach for biolubricant production, addressing the growing need for eco-friendly alternatives in the lubricant market.

Existing literature highlights the use of various polyhydric alcohols, including neopentylglycol (NPG), trimethylolpropane (TMP), and pentaerythritol (PE), in the production of polyol esters. TMP is preferred due to its cost-effectiveness and ease of handling. Traditional transesterification methods employing thermal heating often involve extended reaction times and are prone to side reactions such as saponification, leading to reduced product yield. Enzymatic and acid-catalyzed approaches have limitations, including long reaction times and the corrosive or toxic nature of some catalysts. Studies on microwave-assisted biodiesel production have shown significantly reduced reaction times compared to conventional methods. However, research on microwave-assisted transesterification of branched alcohols such as TMP with PME under vacuum conditions is limited. This study addresses this gap by investigating the potential of a vacuum-operated microwave reactor to enhance the synthesis of TMPE while simultaneously minimizing undesired by-products like fatty soap.

The study employed a one-variable-at-a-time (OVAT) experimental design to optimize the microwave-assisted transesterification of PME and TMP. A microwave-pulsed width modulation (PWM) reactor operating at 2.45 GHz was used. The influence of five process parameters was investigated: reaction temperature (90, 110, 130, and 150 °C), catalyst amount (0.2, 0.4, 0.6, 0.8, and 1.0 wt.% sodium methoxide), reaction time (3, 5, 7, 10, 15, and 25 min), molar ratio of TMP to PME (1:3, 1:3.5, 1:3.7, 1:4, and 1:4.5), and vacuum pressure (10, 20, 30, and 50 mbar). The reaction was conducted in a three-necked flask equipped with a thermocouple, a sample port, and a magnetic stirrer. The mixture was preheated for 3 minutes before catalyst addition. Vacuum pressure was controlled to remove the methanol by-product. The product composition was analyzed using gas chromatography-flame ionization detection (GC-FID). The properties of the resulting TMPE (viscosity at 40 and 100 °C, pour point) were determined using standard ASTM methods. The energy consumption of both microwave-assisted and conventional methods were compared. Additional characterization techniques employed included Fourier Transform Infrared (FTIR) spectroscopy, Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC), and Nuclear Magnetic Resonance (NMR) spectroscopy to confirm the chemical structure and thermal stability of the synthesized TMPE. Fatty soap content was determined by filtration and weighing, and excess PME was removed by vacuum distillation at 260°C.

The microwave-assisted transesterification significantly accelerated the reaction compared to the conventional method. Optimal conditions for TMPE synthesis were determined as follows: temperature of 130 °C, 0.6 wt.% sodium methoxide catalyst, 10 min reaction time, a 1:4 molar ratio of TMP to PME, and a vacuum pressure of 10 mbar. Under these conditions, a 66.9 wt.% yield of TMP triester and 17.4% fatty soap were obtained. The microwave-assisted approach reduced the reaction time by approximately 3.1 folds compared to the conventional method (1 hr vs 10 min). The total energy consumption was markedly reduced with the microwave system (1194.6 kJ) compared to conventional heating (3786.4 kJ), resulting in a 68.4% energy saving. The viscosity of the TMPE base oil at 40 and 100 °C was comparable to those reported in previous studies. The pour point of the microwave-synthesized TMPE (-18 °C) was slightly higher than values reported in previous studies, potentially attributed to the TMPDE content. FTIR analysis confirmed the presence of ester functional groups in the TMPE product. TGA and DSC analysis showed enhanced thermal stability of the TMPE compared to PME. ¹H-NMR analysis confirmed the successful synthesis of TMPE.

The study's findings demonstrate the efficacy of microwave-assisted transesterification for producing TMPE, a potential biolubricant. The significant reduction in reaction time and energy consumption offer considerable economic and environmental advantages over conventional methods. Microwave heating's effectiveness stems from its ability to rapidly generate heat within the reaction media due to the interaction of the electromagnetic field with polar molecules, thus increasing reaction kinetics. The optimized parameters identified in the study contribute to high TMPE yield while minimizing the formation of undesirable by-products like fatty soap. However, it is noteworthy that microwave heating not only accelerates the desired transesterification but also competing reactions such as hydrolysis and saponification. The slightly higher pour point observed compared to previous studies might require further optimization of reaction conditions or product purification techniques. These results highlight the potential of microwave-assisted transesterification as a sustainable and cost-effective method for producing high-quality biolubricants from renewable feedstocks.

This study successfully demonstrated the feasibility of microwave-assisted transesterification for producing TMPE from PME and TMP. The method significantly reduced reaction time and energy consumption while achieving comparable product properties to those obtained using conventional methods. The optimized conditions offer a promising route for the sustainable and economic production of biolubricants. Future research should focus on further optimizing the process, investigating the kinetics and thermodynamics in greater detail, and exploring different catalysts to minimize fatty soap formation and improve the overall process efficiency.

The study used a one-variable-at-a-time optimization approach. Employing more sophisticated experimental designs such as response surface methodology (RSM) or factorial design might reveal more nuanced interactions between the process parameters and could potentially lead to further process optimization. The assumption of PME as a single component (C18) to simplify the reaction model is a limitation, as PME is a complex mixture of fatty acid methyl esters. The effect of this simplification on the overall process kinetics and yield requires further investigation. Further studies are needed to fully assess the long-term performance of the produced biolubricant in real-world applications.