Fast and efficient upgrading of levulinic acid into long-chain alkyl levulinate fuel additives with a tungsten salt catalyst at low temperature†
Levulinic acid (LA) is a promising renewable platform compound derived from biomass for the manufacture of various valuable chemicals, among which levulinate esters have been demonstrated to be good fuel additives. Many previous studies have focused on the production of short-chain levulinates for biofuels such as methyl and ethyl levulinates, but these esters suffer from some essential issues including a high oxygen content, low energy density (or heating value) and water solubility. Long-chain levulinate esters have similar structures to biodiesel. The long carbon chain may provide a higher carbon content and strong hydrophobicity, therefore improving the energy density and water insolubility. In this work, the synthesis of a long-chain levulinate ester, n-hexyl levulinate (HL), has been fast and efficiently achieved by the catalytic esterification of LA and 1-hexanol at low temperature. Tungsten hexachloride (WCl6) showed the most superior activity among the tested metal salt catalysts under mild conditions. A HL yield of ∼85% with almost complete LA conversion could be obtained at 50 °C after only 30 min. Other hexanols that have larger steric hindrances, such as 2-ethyl-1-butanol, 2-hexanol, 3-hexanol and cyclohexanol, could also promote efficient esterification with LA over the WCl6 catalyst, resulting in excellent product yields of ∼82–91%. The strategy applies to the efficient synthesis of longer chain levulinate esters such as n-octyl levulinate (OL) as well. The WCl6 catalyst was reusable several times in a consecutive batch reaction mode. The method reported herein offers a strategy for the efficient production of liquid fuels from LA, potentially bridging raw biomass feedstocks and terminal utilization of biobased products.