Wood biorefinery concept based on γ-valerolactone/water fractionation

This thesis introduces a novel biorefinery concept based on the uncatalyzed pulping of hardwood biomass in an aqueous solution of gamma-valerolactone (GVL). The optimum conditions were determined as 50 wt% GVL, liquor-to-wood ratio (L:W) of 3 – 4 L/kg, 180°C and 90 – 150 minutes. Eucalyptus globulus (E. globulus) wood was effectively fractionated in a single step into its principal components. The pulp fraction was of viscose grade with similar properties to those of a dissolving pulp produced from acid sulfite pulping of hardwood. GVL unbleached and bleached pulps were readily converted to textile fibers with mechanical properties comparable to the current commercial products. Lignin-containing nanofibrillated cellulose (NFC) was also produced from unbleached GVL pulps and subjected to morphological and rheological characterizations, revealing potential applications related to the water retention and structural recovery of the NFC suspensions. Most of the hemicelluloses and lignin in wood were extracted and dissolved into the spent liquor. The dissolved hemicellulose-based fraction can be upgraded to furanic platform chemicals in subsequent catalytic conversion processes in GVL-rich medium. Lignin was precipitated by water addition or by GVL removal. The sulfur-free lignin was characterized by low impurity content, rich in functionalities, relatively low polydispersity and low molecular mass. Fundamental study on the solubility of lignin in GVL/water resulted in a ternary phase diagram with information applicable to the recovery process. Hildebrand solubility theory estimated an optimum dissolution of lignin in a solution containing about 92 – 96 wt% GVL (70 – 80 mol% GVL). The solvent recovery was conducted with vacuum distillation and liquid CO2 extraction as the core techniques. A lignin and GVL recovery rate of ca. 90% was achieved, which can still be improved by the equipment modification, operational parameters optimization and the adoption of advanced lignin treatments. The GVL fractionation process is sulfur-free and the high pulp bleachability offers the possibility for a total-chlorine-free bleaching sequence, thus making the process environmentally friendly. Furthermore, initial assessment demonstrated that the proposed GVL biorefinery could be energetically self-sustained by the steam and electricity generated from the combustion of isolated lignin. Altogether, our concept fulfills the requirements of a modern biorefinery.