A Brønsted–Lewis acidic ionic liquid as a dual-acidity catalyst for direct cellulose liquefaction to ethyl levulinate

Abstract

The use of biomass carbohydrates as feedstock in conversion processes aims to mitigate climate change by eliminating the emission of greenhouse gases. In this study, the conversion of cellulose to biofuel additive ethyl levulinate (EL) using a newly synthesized Brønsted–Lewis acidic ionic liquid (BLAIL), [SMIM][ZnCl₃], was explored as a green approach. Three different ionic liquids (ILs), namely, Lewis acidic ionic liquid [BMIM][ZnCl₃], Brønsted acidic ionic liquid [SMIM][Cl], and BLAIL [SMIM][ZnCl₃], were synthesized and characterized, and their catalytic performances in the direct conversion of cellulose to EL was compared. The effect of the reaction parameters, namely temperature (120–200 °C), time (2–10 h), cellulose loading (0.2–1.0 g), IL loading (1–4 g), water content (0–1.5 mL), IL Brønsted : Lewis ratio (1 : 2, 1 : 1, and 2 : 1), and the IL flexibility in converting biomass samples, polysaccharides and monosaccharides, were investigated in this work. BLAIL was found to possess the highest acid value (0.44 g NaOH per g IL) and Hammett acidity (H₀) of 3.33 among other ILs. The [SMIM][ZnCl₃] BLAIL also exhibited the best catalytic activity in EL production via direct cellulose conversion compared with other ILs. Additionally, the [SMIM][ZnCl₃] BLAIL showed good potential in the conversion of hexose sugars. Direct cellulose conversion resulted in 23.53 wt% EL yield under the conditions of 180 °C, 6 h, 0.6 g cellulose, and 2 g 2 : 1 [SMIM][ZnCl₃], which is the highest EL yield reported to date for cellulose conversion using ILs. Moreover, glucose conversion yielded 33.98 wt% EL under the conditions of 180 °C, 4 h, 0.6 g cellulose, and 2 g 2 : 1 [SMIM][ZnCl₃]. This study provides insights into the potential of [SMIM][ZnCl₃] as a catalyst in directly converting hexose sugars to EL.