Characteristics and Precipitation of Glucose Oligomers in the Fresh Liquid Products Obtained from the Hydrolysis of Cellulose in Hot-Compressed Water

Using a high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD), this study shows the presence of a wide range of glucose oligomers with the degrees of polymerization (DPs) up to 30 and their derivatives in the fresh liquid products produced from cellulose hydrolysis in hot-compressed water (HCW) using a semicontinuous reactor system at 280 degrees C and 20 MPa. None of those oligomers can be detected by a high-performance liquid chromatography with evaporative light scattering detector (HPLC-ELSD) that however can detect glucose oligomers with DPs up to 6 after the liquid solutions are concentrated by 25 times via vacuum evaporation at 40 degrees C, during which a large amount of precipitate is formed. While quantitative analysis of the glucose oligomers with DPs > 5 cannot be done due to the lack of standards, that of the glucose oligomers from glucose (DP = 1) to cellopentaose (DP = 5) using both HPAEC-PAD and HPLC-ELSD are in good agreement, suggesting that these low-DP glucose oligomers do not contribute to the precipitate formation. Results from a set of purposely designed precipitation experiments indicate that the precipitation starts as the fresh liquid sample is collected and is fast during the initial 8 h, levels off as the precipitation time increases further, and completes after 120 h (5 days). On the basis of a new approach developed for the quantification of glucose oligomer retention during the precipitation process, it is found that the contribution Of glucose oligomers to precipitate formation increases with DP. The higher the DP is, the lower the solubility of the glucose oligomer is. The glucose oligomers from glucose to cellopentaose and their derivatives (DPs = 1-5) contribute little to the precipitate formation, which explains why HPLC-ELSD can correctly analyze these glucose oligomers in the concentrated solutions prepared by vacuum evaporation. The glucose oligomers and their derivatives with DP > 5, which are soluble in HCW but become supersaturated in the solutions under ambient conditions, are responsible for precipitate formation. Most (but not all) of the glucose oligomers and their derivatives with DPs > 16 contribute to the precipitate formation as tiny peaks of these glucose oligomers are still shown in the chromatograms, suggesting that these glucose oligomers have very low (but nonzero) solubilities in ambient water. The retentions of glucose oligomers and their derivatives increase substantially with the DP decreasing from 16 to 6, indicating that less of these lower-DP oligomers contribute to the precipitate formation. To avoid the effect of precipitation on oligomer analysis, the fresh liquid products must be analyzed immediately after sample collection.