Significant Differences in the Hydrolysis Behavior of Amorphous and Crystalline Portions within Microcrystalline Cellulose in Hot-Compressed Water
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Due to the presence of amorphous structure in microcrystalline cellulose, the reactivity of microcrystalline cellulose exhibits a considerable reduction in the initial stage during hydrolysis in hot-compressed water (HCW). Further analysis of the liquid products obtained at various temperatures suggests that the amorphous portion within microcrystalline cellulose contains some short glucose chain segments hinged with crystalline cellulose via weak bonds (e.g., hydrogen bonds). These short chain segments are reactive components responsible for the formation of C4−C13 oligomers in the primary liquid products during hydrolysis in HCW at temperatures as low as 100 °C. The minimal temperature for breaking the glycosidic bonds in those short chain segments to form glucose monomer from amorphous portion within microcrystalline cellulose is ~150 °C. However, the minimal temperature at which glucose monomer starts to be produced from the crystalline portion within microcrystalline cellulose is around 180 °C, apparently due to the limited accessibility of the glycosidic bonds in the crystalline portion to HCW as a result of the strong intra- and intermolecular hydrogen bonding networks. The differences of chain length and hydrogen bonding pattern between amorphous and crystalline cellulose also greatly affect the distribution of glucose oligomers in their liquid products during hydrolysis in HCW. Generally, amorphous cellulose produces more glucose monomers and oligomers at the same hydrolysis temperature, but the selectivity ratios of glucose oligomers in the primary liquid products from amorphous and crystalline portions do not show a monotonic trend with the degree of polymerization, at least partly resulting from the presence of shorter glucose chain segments in the amorphous portion within the microcrystalline cellulose.
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