Hydrothermal treatment of furfural and sugar monomers and oligomers: a model-compound approach to probe the cross-polymerization reactions in heating bio-oil

The high tendency of bio-oil towards polymerization renders bio-oil with the application as the precursor for carbon materials. However, the pathway of the cross-polymerization of the major components of bio-oil was still unclear. Therefore, the cross-polymerization of furfural and the typical carbohydrates (monosaccharides and oligosaccharides) in bio-oil was studied. The results suggested that the cross-polymerization between furfural and sugars existed and proceeded to a significant extent, producing a much higher yield solid polymer than that of single feedstock and the soluble polymers with the varied pi-conjugated structure. In addition, the characterization also indicated that the polymerization of furfural was accompanied with the substantial deoxygenation reactions such as dehydration, forming the polymer with a higher carbon content than that from the sugars. Although sugars contain multiple hydroxyl group, the dehydration reactions for elimination of the hydroxyl group did not take place to a remarkable extent during the polymerization, retaining significant hydrogen and oxygen content of polymer. In addition, the cross-polymerization of furfural and the sugars significantly changed the elemental composition, functionalities, thermal stability, and morphologies of the polymer, owing to the varied pathway from the polymerization of the single feedstock.