Solubilities of Oligomer Mixtures Produced by the Hydrolysis of Xylans and Corn Stover in Water at 180 °C

This paper investigates the solubility limitations of oligomers in solutions produced by the hydrolysis of commercial xylans and corn stover in just water at 180 °C. These experiments show that higher degree of polymerization (higher-DP) oligomers precipitate when hydrolyzates of birchwood and oat spelt xylan are cooled from 80 to 26 °C. Furthermore, even dilution by a factor of 10 at 80 °C is not sufficient to prevent precipitation upon cooling to 26 °C. Fewer high-DP oligomers precipitate from corn stover hydrolyzate solutions than from commercial xylans at the same solids concentrations, likely because of its lower fraction of hemicellulose. Overall, solubility limits do not seem to be significant during reaction up to the highest solids concentration investigated (18% at 180 °C), although these limitations are very important as the liquid hydrolyzate is cooled from 80 °C to 26 °C. These results indicate that oligomer yields from batch xylan hydrolysis could be considerably greater if the oligomers were separated from the solids while hot, rather than allowing the hydrolyzates to cool before analysis, as typically reported.