An acceleration of microwave-assisted transesterification of palm oil-based methyl ester into trimethylolpropane ester

The global lubricant market is large and growing, but conventional petroleum-based lubricants raise environmental concerns. Biolubricants derived from vegetable oils offer advantages including biodegradability, low toxicity, and renewability. However, native vegetable oils suffer from poor oxidative and thermal stability due to unsaturation, necessitating chemical modification to polyol esters to improve viscosity index, low-temperature performance, and stability. Trimethylolpropane (TMP) is a practical polyol for synthesizing lubricant-range esters from palm oil methyl ester (PME), a widely available Malaysian palm-based product. Conventional transesterification with thermal heating and catalysts can be slow and energy intensive, with side reactions (hydrolysis/saponification) reducing yield. This study investigates microwave-assisted, vacuum-operated transesterification of PME and TMP to produce trimethylolpropane esters (TMPE) as a biolubricant base oil, aiming to intensify heating, shorten reaction time and reduce energy consumption while suppressing soap formation, and compares performance to conventional heating.

Transesterification to polyol esters typically uses polyhydric alcohols such as neopentylglycol, trimethylolpropane (TMP), and pentaerythritol; TMP is cost-effective and easier to handle. Catalytic systems include enzymatic, acid, and alkaline catalysts. Enzymatic routes can require hours (e.g., ~7.4 h at 70 °C). Acid catalysis is corrosive and toxic, while alkaline catalysis under vacuum shortens time but may induce saponification, reducing yield. Prior reports with calcium or sodium methoxide achieved high TMPTE yields (~98%) but with substantial soap formation and longer times (e.g., 8 h yielding 9.2 mg soap/g with CaOCH3; 1 h yielding 46 mg soap/g with NaOCH3). Conventional batch systems with sodium methoxide also produced high soap (85.6 mg/g) with 80.1% TMPTE at 1 h. Microwave-assisted esterification/transesterification with short-chain alcohols (methanol/ethanol) has delivered 30 s–15 min reaction times and ~93% yields with minimal alkaline catalyst, reducing saponification. However, studies of branched polyol (TMP) with PME under vacuum using microwave pulsed width modulation (PWM) are scarce, and the role of hydrolysis/saponification side reactions under microwave conditions is underreported.

Design: One-variable-at-a-time (OVAT) approach varying temperature, catalyst amount, reaction time, TMP:PME molar ratio, and vacuum pressure; each run triplicated and analyzed with averages and standard deviations. Microwave system: Custom microwave-PWM Dixson 2301 (2.45 GHz) enabling temperature control and pulsed modulation to avoid overheating. Reaction setup: 250 mL three-neck flask with thermocouple and sampling port, magnetic stirring, connected to a vacuum pump. PME (80 g) was oven-dried at 105 °C overnight. PME and TMP were preheated by microwave for 3 min to set temperature before catalyst addition; sodium methoxide solution (commercial 70% in methanol) was added in specified wt.% relative to total reactants. Vacuum pressure was ramped to the setpoint to continuously remove methanol by-product and air, driving the reversible reaction forward. Parameters: Temperature 90, 110, 130, 150 °C; catalyst 0.2–1.0 wt.% (as NaOCH3 solution); time 3, 5, 7, 10, 15, 25 min; TMP:PME molar ratios 1:3 to 1:4.5; pressures 10, 20, 30, 50 mbar. Dielectric properties (ε′, ε″) of TMP, PME, water, air, sodium methoxide were measured at room temperature using an open-ended coaxial probe (HP 85070B) with a network analyzer (HP 8753C) over 1–4 GHz. Post-reaction processing: Fatty soap was separated at room temperature using Whatman No. 5 filter paper and quantified as mass% of total reactants. Excess PME was removed by vacuum distillation (oil bath to yield ~180 °C internal temperature; pressure ~1.4×10⁻² mbar; 4 h) to obtain a base oil enriched in TMPTE. Analytical methods: GC-FID (Perkin Elmer) with DB-5HT column (15 m × 0.32 mm, 0.1 μm), oven: 80 °C 3 min, ramp 6 °C/min to 360 °C, hold 5 min; H2 carrier (26.67 mL/min), N2 makeup; 0.1 μL injection; 27 min analysis; sample derivatized with BSTFA in ethyl acetate (40–50 °C, 10 min). FTIR (Perkin Elmer 1650) 4000–500 cm⁻¹. TGA/DSC (Mettler Toledo TGA/DSC 823 IHT) in air, 25–600 °C, 10 °C/min, 50 mL/min. 1H-NMR (JEOL ECX500, CDCl3, 500 MHz). Material and energy balances: Transesterification modeled as three reversible steps (mono-, di-, tri-ester formation) with hydrolysis of PME to FFA and methanol, and saponification of FFA with NaOCH3 to soap and methanol. Assumptions: 100% conversion of H2O to FFA; no hydrolysis of TMPDE/TMPTE during transesterification; catalyst moisture negligible; PME represented as single-component C18 (dominant methyl oleate ~75% w/w). Enthalpy calculations included Cp integration, heats of formation, vaporization, and fusion as applicable. Energy consumption was computed as Total = heating energy + vacuum pump energy − reaction enthalpy absorbed; compared for microwave vs conventional heating at identical nominal conditions (130 °C, 10 min, 10 mbar, 0.6 wt.% NaOCH3, 1:4 TMP:PME).

Optimal microwave-assisted conditions: 130 °C, 10 mbar, 0.6 wt.% sodium methoxide (solution), TMP:PME molar ratio 1:4, 10 min. Under these, product composition by GC showed TMPTE 46.6 wt.%, TMPDE 14.6 wt.%, TMPME 1.1 wt.%, and fatty soap 17.4 wt.%. Yield calculation (relative to theoretical) gave TMPTE yield 66.9–67.0%. Temperature: Increasing temperature increased TMPTE up to 130–150 °C (45.4–45.8 wt.% at fixed conditions), with more reverse reaction to TMPDE at 150 °C and higher soap (up to 17.7%). At 110 °C, soap peaked (~21.8%) with lower TMPTE; at 90 °C reaction incomplete with low soap. Catalyst: 0.2 wt.% was insufficient (methanol removal hindered), 0.4–0.8 wt.% increased conversion but also soap; 0.6 wt.% selected as best trade-off (complete conversion, 17.4% soap). Reaction time: Rapid preheating (to 130 °C in ~3 min). Optimum 10 min; beyond 10 min TMPTE decreased (enhanced reverse reactions) and soap increased (e.g., 10.3% at 3 min vs 13–13.4% at 15–25 min). Molar ratio: Best at 1:4 (TMPTE 46.6 wt.%); 1:3 left residual TMP; higher PME ratios (1:4.5) lowered soap (15.1%) but increased dilution and reverse reactions, reducing TMPTE. Pressure: Strongest vacuum (10 mbar) maximized TMPTE despite higher soap (17%); at 50 mbar soap minimized (~10%) but TMPTE lower, indicating competition between transesterification and saponification and insufficient methanol removal at higher pressures. Energy and rate benefits: Microwave-assisted reaction reached 130 °C in ~3 min vs 40–60 min conventional; at 10 min reaction time and identical setpoints, microwave achieved 46.6 wt.% TMPTE vs 25.5 wt.% conventional (TMPDE 14.6 vs 16.3 wt.%, TMPME 1.1 vs 2.1 wt.%). Total energy consumption: microwave 1194.6 kJ vs conventional 3786.4 kJ (≈68.4% saving); reaction enthalpy absorbed 11.97 kJ (microwave) vs 17.57 kJ (conventional). After fractionation to remove excess PME, base oil contained 63 wt.% TMPTE. Base oil properties: kinematic viscosity 45.3 cSt (40 °C) and 9.0 cSt (100 °C); pour point −18 °C. Thermal analysis: PME began mass loss ~170 °C (50% at 284 °C; mostly decomposed by 304 °C). TMP esters onset ~312 °C; 50% mass loss at 449.9 °C; DTG peak at 452.3 °C; DSC endotherm associated with volatilization (−805.94 J/g). Spectroscopy: FTIR confirmed ester carbonyl and C–O stretching features with disappearance of TMP O–H; 1H-NMR showed characteristic methyl ester peaks enabling quantification.

Microwave heating accelerates the PME–TMP transesterification via dipolar rotation and ionic conduction mechanisms, particularly leveraging the high polarity of methanol (both catalyst solvent and reaction by-product) to create localized hotspots and rapid, volumetric heating. Under vacuum, continuous removal of methanol drives the reversible sequence toward TMPTE (Le Chatelier’s principle). Optimal TMPTE formation occurred at 130 °C, 10 mbar, 0.6 wt.% catalyst, and 10 min, balancing forward reaction enhancement against reverse reactions and side reactions. Stronger vacuum (10 mbar) enhanced both TMPTE formation and saponification, indicating competition between desired transesterification and soap-forming pathways initiated by hydrolysis of PME in the presence of moisture. Catalyst loading influenced both conversion and soap proportion; 0.6 wt.% was sufficient to complete reaction without excessive soap escalation. Microwave intensification reduced preheating time by an order of magnitude and cut total energy consumption by ~68% compared to conventional heating, while delivering a 3.1-fold faster progression toward TMPTE within 10 min. The base oil produced after removing excess PME had viscosities comparable to literature TMPTE oils, though a higher TMPDE fraction likely raised the pour point relative to previous reports. Thermal analysis confirmed that TMP esters possess substantially higher thermal stability than PME, supporting their suitability as biolubricant base stocks.

Microwave-PWM assisted transesterification of PME with TMP substantially accelerates reaction rate and reduces energy consumption compared to conventional heating. At 130 °C, 10 mbar, 0.6 wt.% sodium methoxide, TMP:PME = 1:4, and 10 min, the process achieved a TMPTE yield of ~67% (46.6 wt.% in crude product) with fatty soap ~17.4%. The method reduced total energy consumption by ~68% and cut preheating time from ~30–60 min to ~3 min. After fractionation, the biolubricant base oil contained ~63 wt.% TMPTE and exhibited viscosities comparable to established TMPTE products, though with a higher pour point attributed to TMPDE content. Microwave-assisted transesterification thus provides an efficient, potentially lower-cost route to biodegradable biolubricant base stocks. Future work should further elucidate the coupled microwave heating–reaction kinetics, optimize to suppress hydrolysis/saponification and TMPDE content, and explore scale-up and continuous operation.

Microwave heating also accelerates competing hydrolysis and saponification, increasing fatty soap formation under strong vacuum; drying of feeds is critical. The optimized product still contained substantial TMPDE, elevating the pour point relative to literature. The optimization used OVAT rather than multivariate design, and PME was modeled as a single-component (C18) in energy/material balances, which simplifies its complex composition. Results were obtained at lab scale and under specific vacuum and catalyst conditions; scale-up effects and continuous operation were not assessed within this study.