Synthesis of ethyl levulinate over sulfonated lignin-based carbon catalyst as a fuel additive to biodiesel-diesel blends towards engine emissions

Abstract

Biodiesel-diesel blend is a promising approach for petroleum diesel substitution but concurrently requires modifications to improve its properties. In this study, ethyl levulinate (EL) was synthesized via levulinic acid (LA) esterification over a sulfonated lignin-based carbon catalyst and employed as an additive to improve biodiesel-diesel blends. The catalyst was prepared from lignin as a carbon precursor, modified via hydrothermal sulfonation at 180 °C for 6 h, and finally characterized. The effects of reaction parameters for LA esterification, such as ethanol-to-LA molar ratio (3–15), catalyst loading (3–20 wt%), and reaction time (2–6 h) were studied. Ethyl levulinate with 80 mol% and 100 mol% concentrations were examined as a fuel additive for palm oil biodiesel-diesel blends of B10, B15, and B20. The effect of the additive on the density and viscosity of biodiesel-diesel blends, as well as carbon monoxide (CO) and nitrogen oxide (NOx) emissions from engine combustion, was investigated. The results show that the catalyst has good acidity (1.30 mmol/g) and suggested thermal stability up to 200 °C, as well as unique surface morphology with a large surface area of 193 m2/g contributed by mesopores and micropores. A high initial yield of EL of 84.3 mol% was obtained over LHS-400-1 at optimum conditions of 6 ethanol-to-LA molar ratio, 15 wt% of catalyst, and 5 h at 80 °C. The reusability study up to five cycles showed a gradual reduction in the LA conversion which indicated catalyst instability. As for the biodiesel-diesel blends, the presence of EL at 3 to 7 vol% reduced the viscosity and slightly increased the density. Furthermore, CO and NOx emissions trends decreased with the addition of EL. The presence of LA in 80 mol% EL for biodiesel-diesel blends resulted in slightly high CO and NOx emissions as compared to the blends with 100 mol% EL.