Magnetically recyclable acidic polymeric ionic liquids decorated with hydrophobic regulators as highly efficient and stable catalysts for biodiesel production

Biodiesel, typically produced from non-food crops with heterogeneous catalysts, deems to be a promising and sustainable alternative biofuel to petroleum-derived fuels. In this study, to improve biodiesel production process efficiency by acids, biodiesel production were successfully developed using a series of magnetically acidic poly(ionic liquid) catalysts with varying hydrophobicity and controllable acidity. The detailed characterization results demonstrated that the FnmS-PIL (1a, C8) core-shell structure catalyst had large Bruner Emmett and Teller (BET) surface area (128.1 m²/g), uniform mesoporous structure (4.2 nm), strong magnetism (12.4 emu/g), a high number of acid sites (2.14 mmol/g), strong acid strength (strong electron-withdrawing anion CF₃SO₃ ⁻ effect, −8.2 < H₀ < −5.6) and strong hydrophobicity (water contact angle, 115.4°). For the biodiesel production, high biodiesel yields could be achieved by the esterification of oleic acid (by response surface methodology, RSM) and (trans)esterification of crude Euphorbia lathyris L. oils with an acid value of 24.59 mg KOH/g (by single factor optimization). Furthermore, a relative kinetic study was conducted wherein the activation energy was calculated as 39.2 kJ/mol and a pseudo-first-order model was determined for esterification. More importantly, the FnmS-PIL (1a, C8) catalyst was separated simply using a magnet and exhibited constant activity with biodiesel yield of 87.5% after five cycles in (trans)esterification. In addition, the biodiesel yield was maintained above 90% even with a water content of 6 wt% with respect to crude Euphorbia lathyris L. oils, and the fuel properties of Euphorbia lathyris L. biodiesel were found to satisfy the EN 14212 and ASTM D6751 standards, highlighting its potential in industrial biodiesel production as derived from crude non-food oil resources. © 2018 Elsevier Ltd