Thermal glycosylation and degradation reactions occurring at the reducing ends of cellulose during low-temperature pyrolysis

Abstract

Thermal glycosylation and degradation reactions of cellulose (Avicel PH-101) were studied in the presence or absence of alcohols (glycerol, mannitol, 1,2,6-hexanetriol, 3-phenoxy-1,2-propanediol, and 1-tetradecanol) under N 2 at 60–280 °C. In the presence of glycerol (heating time, 10 min), the reducing ends were converted into glycosides when the temperature of the glycerol was >140 °C without the addition of any catalysts. A temperature of 140 °C is close to that required for the initiation of thermal polymerization (glycosylation). Although the conversion was only around 20% in the range of 140–180 °C, the reactivity increased above 200–240 °C where the thermal expansion of cellulose crystals is reported to become significant. Finally, all reducing ends were converted into glycosides at 260 °C. Such heterogeneous reactivity likely arose from the lower reactivities of the reducing ends in the crystalline region due to their lower accessibility to glycerol, although the reactivity in the non-crystalline region was similar to that of glucose. Alcohols that have a lower OH/C ratio did not react with the reducing ends, suggesting that the hydrophilicity of the alcohol was critical for the glycosylation reaction to proceed. The glycosylated cellulose samples were found to be significantly stabilized against pyrolytic coloration. The results of neat cellulose pyrolysis indicated that two competitive reactions, thermal glycosylation and degradation, formed a dark-colored substance at the reducing ends while the internal glucose units in the cellulose were comparatively stable.