Determination of active sites on the synthesis of novel Lewis acidic deep eutectic solvent catalysts and kinetic studies in microalgal biodiesel production

Abstract

Experimental and theoretical considerations for kinetic modeling of the transesterification reaction of microalgae lipids into biodiesel were investigated using Lewis acid deep eutectic solvents (DESs) as a catalyst. The acid sites involved in the reaction were characterized using acetonitrile as a probe to understand the mechanism. DES ChCl–SnCl₂ (choline chloride–tin II chloride) showed higher catalytic activity in transesterification due to its higher acidity compared to DES ChCl–ZnCl₂ (choline chloride–zinc chloride). This was illustrated by geometric optimization of the DES structures through density functional theory (DFT) which showed that the metal centers furthest from the choline moiety are the most acidic and the bond lengths of Sn–Cl were between 2.56 and 2.77 Å, and were greater than the Zn–Cl bond lengths from 2.30 to 2.48 Å, making the ChCl–SnCl₂ DES more acidic and more suitable for the biodiesel production. The fatty acid methyl ester (FAME) conversion from microalgae lipid was 36.75 mg g⁻¹ under ideal conditions (6 molar ratio methanol–lipid with 8 vol% DES dosage in methanol at 140 °C for 420 min). The activation energy is found to be 36.3 kJ mol⁻¹ based on the pseudo-first-order reaction, in addition, the DES catalyst (ChCl–SnCl₂) drove the reaction chemically and did not show mass transfer limitation. Information from this study can help to advance the development of an efficient and environmentally friendly industrial biodiesel production technology.