Mechanical depolymerisation of acidulated cellulose: understanding the solubility of high molecular weight oligomers

Water soluble oligomers of cellulose were produced by milling acidulated microcrystalline cellulose. Acids with low pK(a) were found to be more effective for the treatment. The yield of water soluble fraction was proportional to the increasing severity of milling or the acid amount. Soluble oligomers were found to have an average degree of polymerisation of similar to 7 monomer units. High resolution NMR spectroscopy was used to determine the chemical structures of soluble oligomers. It was found that branched oligomers with a (1 -> 6) linkages were formed, which increased their solubility in water and reduced the generation of monomers and dimers which may degrade during milling or subsequent hydrolysis hydrogenation. The soluble oligomers showed excellent reactivity towards hydrolysis hydrogenation in the presence of bi-metallic Ni-Pt/alumina catalyst. High yields (similar to 90%) of sorbitol and mannitol were obtained with only 1 h of reaction time.