Microwave-assisted green synthesis of levulinate esters as biofuel precursors using calix[4]arene as an organocatalyst under solvent-free conditions

Abstract

Levulinic acid, one of the top 12 value-added chemicals, can be obtained by the transformation of biomass by acid catalysis. Alkyl levulinate has been widely explored as a precursor for obtaining renewable fuel additives. In this work, the organocatalyst CX4SO₃H was employed for the first time as an organocatalyst for levulinic acid esterification reactions. Various parameters, such as the temperature, reaction time, and catalyst load, were investigated. The optimized reaction conditions were a reaction temperature of 80 °C (MWI), a reaction time of 2.5 min, a CX4SO₃H catalyst load of 1 mol% and solvent-free conditions. Ten different alcohols were evaluated for the synthesis of alkyl levulinate with high yields (ca. 99%), with the exception of tert-butyl alcohol (13% yield). The levulinic acid esterification reaction, a type of green chemistry reaction, has many advantages such as (i) creation of a new C–O bonds, (ii) water being the sole waste, (iii) 100% carbon economy, (iv) metal- and solvent-free processes, (v) short time and (vi) nontoxic and reusable organocatalysts. These advantages, along with the simple workup procedure, make this efficient protocol a greener alternative to the traditional methods used for the synthesis of levulinate esters to generate biofuels.