(495f) Acid-Catalyzed Depolymerization of Cellulose in Aprotic Solvent for Production of Precursor Chemicals

Various aprotic polar solvents have the ability to depolymerize cellulose at elevated temperatures and pressures to form to anhydromonosaccharides and other soluble carbohydrates. It was found that polarity of the solvent strongly affects the yield and selectivity of the anhydromonosugar during non-catalytic conversion. The conversion was further enhanced by addition of small amounts of homogeneous acid catalyst. Aprotic solvents tested in this work include 1,4-dioxane, ethyl acetate, tetrahydrofuran (THF), methyl isobutyl ketone (MIBK), acetone, acetonitrile, and gamma-valerolactone (GVL). Cellulose conversion was studied at reaction temperatures of 200^o-350^oC with 0.2-20 mM sulfuric acid for each solvent system. Results show that the effectiveness of the acid catalyst is highly dependent on the type of aprotic solvent. While the presence of acid only increased the yield of levoglucosan by 12-21% for GVL, sugar yield was enhanced by over 100% for THF and 1,4-dioxane solvents when 0.5 mM acid was added to the solvent system operated at 350^oC. Levoglucosenone was only found as a major product when acid catalyst was included in the solvent system. 5-HMF and furfural were relatively minor products. A reaction network for primary depolymerization of cellulose and secondary degradation reactions was proposed. Reaction rate constants and apparent activation energies for each step involved in the reaction network were determined and compared for different solvent systems. The effectiveness of the various solvent systems was related to acidity (pK_a) of the solvent phase. The degree of depolymerization of cellulose and the resulting yields of products might be controlled by the relative stability and reactivity of the acidic protons in different solvents. Understanding how the choice of aprotic solvent and catalytic effect of acid on production and stability of products enable us to develop an effective model for maximizing targeted precursor chemicals.