Subcritical and Supercritical Water as a Cellulose Solvent

Cellulose's recalcitrance to dissolve in common solvents is a major obstacle for the utilization of lignocellulose biomass into fuels and chemicals. As a promising answer to this challenge, subcritical and supercritical water have been proposed as a green medium for cellulose hydrolysis. Prior reports also indicate that the solubility of cellulose substantially increases near the critical point of water. This thesis investigated whether subcritical or supercritical water can dissolve cellulose as a polymer and whether the dissolved polymers can be recovered without extensive degradation. Two reactor systems were used to treat microcrystalline cellulose at 245-380 °C with treatment times between 0.2 and 6.0 s. Solid cellulose residue samples were investigated for evidence of swelling via scanning electron microscopy, wide angle X-ray, and solid-state NMR techniques. The depolymerization pattern of cellulose polymers was determined by viscosity analysis and size exclusion chromatography, and the results were compared to random chain cleavage and end-attack models. Dehydration and fragmentation reactions in dissolved polymers were analyzed by GC-MS, HPAEC-PAD-MS, and ESI-MS/MS techniques. In addition to the experimental work, the Gibbs energy of dissolution in supercritical water at 400 °C was analyzed via atomistic molecular dynamics simulations. The results showed that subcritical water below 300-320 °C is a poor cellulose solvent. This was demonstrated by the lack of polymeric dissolution products; also no evidence of cellulose swelling was found. The solubility of cellulose was enhanced when the temperature exceeded 300-320 °C. Then the structure of cellulose crystallites was transformed into cellulose II allomorph, likely caused by the enhanced solvent power and concomitant cellulose depolymerization. Increased solubility resulted in cellulose dissolution as a polymer of sufficiently high molar mass to precipitate at ambient temperature. At the same time the dissolved polymers were subjected to fast depolymerization which rendered the molar mass of precipitated cellulose low. The yield of cellulose precipitate depended on the characteristics of microcrystalline cellulose feedstock and spanned from 0 to 68 wt%. Significant amounts of levoglucosan and erythrose end-groups were detected in the dissolved cellulose chains. The molecular dynamics simulations indicated that the thermodynamics favor cellulose dissolution in supercritical water and an increasing water density or pressure decreases the Gibbs energy of dissolution while it predicted insolubility in ambient water.