Quaternary Ammonium Salts and their Low-Melting Eutectics for the Conversion of Cellulose and Cellulose-Derived Sugars.

Abstract

The use of Ionic Liquids (ILs) as solvents presents a promising route for the conversion of cellulosic biomass feedstock to value-added products, like 5-hydroxymethylfurfural (HMF), levulinic acid – chemicals from which polyethers and polyesters can be made, and 2,5-dimethylfuran, a biofuel superior to ethanol. These products form the basis for the production of renewably-sourced fuels and chemicals. 1,3-dialkylimidazolium ILs in particular, have become a popular and thoroughly-studied reaction media for this purpose. Reactions like the dissolution and hydrolysis of cellulose, and the dehydration of glucose have been demonstrated. However, their widespread adoption has been limited due to their toxicity and prohibitive costs.

There is a clear need for less-expensive, and environmentally benign alternatives. In this work, quaternary ammonium salts and their eutectics are explored as possible solvents. These materials can either be renewably sourced, or are already being produced in large quantities for alternate uses. Solvent properties such as hydrogen-bonding ability and acidity have been studied for different classes of ILs to examine their effects on reactivity. Using probe reactions like the isomerization and dehydration of sugars, and the hydrolysis of disaccharides and cellulose, relationships between chemical properties and reactivity are discussed. Reaction networks and mechanisms are analyzed, and differences between the different classes of ILs are highlighted. It was found that in the eutectics, it was possible to tune selectivities of the products from HMF to levulinic acid, based on the acidity of the partner molecules. The role of hydrogen bonding in these solvents was also studied, and it was found that the lack of hydrogen bonding in the eutectics inhibited isomerization reactions. In studying the depolymerization of cellulose, it was found that even though the eutectics lacked the ability to solvate cellulose, it was still possible to perform hydrolysis of the polysaccharide, and dehydrate the resulting monosaccharides to HMF or levulinic acid, based on the choice of the solvent.

This work has culminated in new and valuable insights into rationally selecting ILs to perform targeted chemistries. It represents a significant step towards the development of economical and environmentally-friendly conversion methods for biomass to fuels and chemicals.