New insights on monosaccharides’ isomerization, dehydration and fragmentation in hot-compressed water

Typical monosaccharides, which constitute hemicelluloses and cellulose of lignocellulosic materials, were treated in batch hot compressed water to investigate monosaccharides’ isomerization and decomposition. It was found that glucose and mannose were isomerized to each other while galactose was isomerized to tagatose and talose. On the other hand, arabinose was isomerized to ribulose and ribose while xylose was isomerized to xylulose and lyxose. Additionally, these isomerization products’ formation was not only affected by initial monosaccharides but also the reaction conditions. Similarly the dehydration and the fragmentation reactions also depended on reaction conditions, in which possibly dehydration occurred firstly and fragmentation reaction followed. Furthermore, furfural was also detected when hexoses were treated in hot compressed water and it was proposed that furfural was formed via hexose fragmentation reactions through a pentose and formaldehyde pathway. On the other hand, the detection of 5-HMF when pentoses were treated in hot compressed water provided a hint that not only retro-aldol condensation reaction but also aldol condensation occurred when monosaccharides were treated in hot-compressed water. Based on these experiments, some new isomerization, fragmentation and dehydration reaction pathways were proposed to supplement previous knowledge on monosaccharides’ decomposition scheme in hot compressed water.