Depolymerization of Cellulose with Superheated Steam: Remarkable Obstruction Effects of Sodium and High Reactivity of Crystalline Cellulose

A superheated steam (SHS) treatment was found to be effective for conversion of cellulose to levoglucosan (LG) and other organic compounds. Yields of water-soluble organic compounds (TOC) and LG reached 88% and 41%, respectively, with the SHS treatment at 300 °C for 30 min. The reaction temperature was far lower than temperatures employed in conventional pyrolysis treatments (≥500 °C), and the TOC and LG yields were comparable to values reported for pyrolysis. Product distributions were significantly altered by sodium content in the parent cellulose. Sodium at 0.05 wt % or more in cellulose lowered the LG yield to almost zero, giving approximately constant TOC values other than LG, and increased solid residue, which indicated that sodium greatly reduced the reactivity of cellulose to form LG. In the absence of sodium, the reactivity of the crystalline phase of cellulose was much higher than the amorphous phase, indicating that the widely reported phenomena in which amorphous cellulose has higher reactivity than crystalline cellulose might result from a sodium impurity. In addition, the effect of ball-milling treatment was investigated to reveal the reactivity of crystalline and amorphous phases of cellulose with or without sodium contamination. The crystalline phase was converted to an amorphous phase distinct from the original amorphous phase, which could be distinguished by XRD patterns. The reactivity of the new amorphous phase was higher than that of the original crystalline phase. The obstruction of the LG formation by sodium was suggested to be due to its coordination onto a terminal glucose unit of a cellulose nanocrystal, based on the cellulose structure and amounts of sodium.