Production of levoglucosan and levoglucosenone from cellulose using Brønsted acid catalysts in polar aprotic solvents

Abstract

Levoglucosan and levoglucosenone are important platform molecules typically produced by pyrolysis of cellulosic feedstocks. Commercial manufacture of levoglucosenone uses phosphoric acid in sulfolane to convert cellulose at high temperatures and catalyst concentrations. We report the synthesis of levoglucosan and levoglucosenone from cellulose at moderate temperatures in polar aprotic solvents using low concentrations of Brønsted acid catalysts. We discovered that the basicity of the solvents has a linear relationship with the cellulose reaction rate. The initial turnover frequency of cellulose conversion increased as the basicity of the solvent decreased. Using a flow reactor with a reactive bed to produce levoglucosan and levoglucosenone we found that moderate temperatures promoted the production of levoglucosan and levoglucosenone, whereas higher temperatures promoted degradation reactions, decreasing the yields of levoglucosan and levoglucosenone. We also found that catalysts with high pK_a (lower strength) only promoted the production of levoglucosan and its isomerization, whereas using a catalyst with a low pK_a (higher strength) favored subsequent reactions and degradation products, like when we used high temperatures. At 430 K with a flow rate of 1.5 mL/min of 5 mM sulfuric acid in γ-valerolactone, we found yields of up to 34 and 24 wt% for levoglucosan and levoglucosenone, respectively.