Biodiesel production from microalgae biomass using Caprolactam-based ionic liquids

Abstract

Biodiesel (fatty acids methyl esters, FAMEs) is a clean form of energy that could replace petroleum- diesel. Whereas microalgae is a promising renewable and sustainable lipid source for biodiesel production, the main limitation of this process is lack of inexpensive and efficient lipid extraction methods. Lately, the eco-friendly protic ionic liquids (PILs) such as caprolactam ionic liquids (CPILs) and its sulphonic-functionalized form (SO 3 -bCPILs), have gained prominence in lipids extraction and biodiesel production. This is because of their lower cost relative to common ionic liquids, but they are rarely synthesized or applied. The aim of this study was to investigate the capacity of CPILs and SO 3 - bCPILs for lipid extraction and biodiesel production from Spirulina platensis microalgae. Specifically, the study aimed to synthesize and characterize the CPIL/SO 3 -CPIL. To extract lipids from both dry and wet algae using a conventional method and synthesized ionic liquids. To study direct and indirect conversion of the triglycerides (lipids) to the corresponding free fatty acids methyl esters (FAMEs- biodiesel). To characterize biodiesel products. To investigate the reusability of these compounds. Nine CPILs/SO 3 -CPIL, namely, caprolactam chloride (CPHA), caprolactam methylsulphonate (CPMS), caprolactam trifluoromethane sulfonate (CPTFS), sulfonic-butyl caprolactam chloride (SO 3 -bCPHA), sulfonic-butyl caprolactam methyl sulphonate (SO 3 - bCPMS), caprolactam acetate (CPAA), caprolactam hydrogen sulphate (CPSA), caprolactam trifluoromethane acetate (CPTFA), and sulphonic-butyl caprolactam hydrogen sulphate (SO 3 -bCPSA), were prepared through neutralization of caprolactam cations with different Brønsted acids. Lipid extraction from microalgal was performed under reflux at 95 o C for 2 h using pure CPILs, mixtures of CPILs/methanol and SO 3 -CPILs/methanol in a ratio of 1:1 (w/w), and the control (Hexane: Methanol 54: 46, v/v, 10 h), with a dry/wet microalgae biomass: solvent ratio of 1:19 (w/w). The synthesized SO 3 - bCPILs/CPILs and lipid/biodiesel extracts were characterized by techniques including Fourier transform infrared (FTIR), Raman, and UV/VIS spectroscopy, thermogravimetric analysis (TGA), gas chromatography-flame ionization detector (GC- FID), the biomass surface by scanning electron microscope (SEM) analysis and predictive methods to estimate the physicochemical properties of biodiesels. Characteristic absorption bands of caprolactam such as N-H (3296, 3209.33 and 3074.32 cm -1 ), C=O (1658 cm -1 ), in FTIR spectra and C-N (1485 cm -1 ) and N-H (697 cm -1 ), in Raman spectra, showed a large change in position/intensity, indicating successful formation of the ionic liquids. The first five of the synthesized compounds are novel. The TGA of SO 3 - bCPIL/CPILs showed their stability between 108 - 221 °C. The extracted lipids yield was 9.5%/4.1% (dry/wet biomass, control), and 14.2%/13%/11% (CPAA/CPHA/CPTFA, dry biomass, methanol mixtures, P ˂ 0.05). SO 3 - bCPMS/methanol and SO 3 -bCPSA/methanol mixtures showed no significant difference compared from control, for wet/dry biomass. SEM analysis showed the absence of intact microalgae cells after extraction by CPAA/CPHA. FTIR spectra of the CPILs/Methanol extracts showed characteristic peaks of FAMEs (C=O, 1739 and 1741 cm -1 ), indicating direct conversion of lipids to biodiesel, and confirming that CPILs act as both solvents and catalysts. GC-FID analysis of the CPIL/methanol and SO 3 - bCPIL/methanol extracts showed the presence of major FAMEs components found in biodiesel, including palmitic, palmitoleic and oleic acids, except for CPAA, which had the lowest acidity strength - and hence least catalytic activity. The biodiesel yield from indirect conversion using conventional (H 2 SO 4 ) catalyst was 31%, whereas that of the synthesized compounds with high catalytic activity was 60 – 80%. The physicochemical properties results showed that the quality characteristics of produced biodiesels were in a good agreement with the specifications of international biodiesel standards (ASTM D6751 and EN 14214). The reusability of selected SO 3 - bCPIL/CPILs (based on yield and extract clarity) was investigated, and there were insignificant differences over six runs. Therefore, the SO 3 -bCPIL/CPILs method is simple, cost-effective, and faster and has higher or comparable lipid/biodiesel yields to the conventional method and it is recommended that the SO 3 -bCPIL/CPILs be used as green solvents/catalysts for biodiesel production from other microalgae species