*2.3. Formosolv Fractionation Process*

Mixtures of Paulownia wood, water and formic acid (80–95%) at 8 g liquid/g dry Paulownia wood, were heated to boiling point in 250 mL Pyrex flasks. When the boiling started, hydrochloric acid was added, taking this as time zero. The selected concentrations of hydrochloric acid were in the range of 0.05 to 0.10%, according to the best results found in previous work [18,19]. The mixture was stirred

at atmospheric pressure along a desired time (30–60 min). Reactions were stopped by removing the flasks from the heating plates and by fast filtration through medium-pore Gooch crucibles to separate the pulp from black liquor. In order to prevent the reprecipitation of lignin on the solids, the pulps were subsequently washed with two formic acid solutions, the first one at the same concentration employed in the pretreatment, and the second one at half of its concentration. Finally, the solids were washed with distilled water until neutral pH (named Paulownia formosolv pulp, PFP). Pulp yield (PY; g oven-dried pulp/100 g oven-dried Paulownia wood) was determined gravimetrically after being oven-dried. The solids were analyzed as described in Section 2.2. and in addition, they were subjected to kappa number (KN) (Tappi T236) and, intrinsic viscosity (VIS) (Tappi T230) determinations.

The aliquots of the black liquors were subjected to posthydrolysis (121 ◦C, 40 min, 4% H2SO4) in order to quantify the solubilized hemicelluloses. In addition, another aliquot was employed in theh precipitation assays of the lignin by adding a specific amount of water to the black liquor. Precipitated solids were separated by centrifugation (4200 rpm, 10 min), repeatedly washed with water, and centrifuged until the supernatant was neutral. Afterwards, the precipitated lignin (Paulownia formosolv lignin, PFL) was lyophilized and used for structural characterization experiments. The quantitative acid hydrolysis of lignin was carried out following the same procedure as in the previous section (NREL/TP-510-42618, 2008).
