## 3 Results and Discussion
### Hydrothermal Bio-Oil Liquid Fraction
#### 3.2.1 Composition of Hydrothermal Bio-Oil Liquid Fraction
Indeed, dark-brown insoluble products were formed in experiments carried out at 240 \({}^{\circ}\)C and 300 \({}^{\circ}\)C. As expected, under the reaction conditions adopted very low concentrations of levulinic acid (LA) were registered in all investigated reaction temperatures. A noticeable amount of LA can be seen at 300 \({}^{\circ}\)C due to the higher concentration of protons driving from the dissociation promoted by high reaction temperatures. Having found 180 \({}^{\circ}\)C as the best reaction temperature that maximizes FU derivatives, the conversion of OPW was also investigated at different reaction times (15, 30, 60, 180 and 300 min) (Figure 5). The highest 5-HMF and FU yields were already gained after only 30 min. A slight decrease in the production of furans was registered after 360 min and may be related both to the thermal degradation of furan derivatives as well as to the fact that a prolonged hydrolysis time increases formation of humins [47]. Figure 4: Effect of HTC reaction temperature on the production of furfural (FU), 5-hydroxymethylfurfurral (5-HMF) and levulinic acid (LA) from OPW under neutral conditions (Reaction conditions: 20 g of OPW; 20 mL of H\({}_{2}\)O; time: 60 min; autogenous pressure; stirring: 600 rpm). On the other hand, by changing the conditions using a sulfuric acid solution as reaction media, a decrease in the production of furans together with a higher production of levulinic acid is observed as a consequence of the acid hydrolysis (Figure 6). Upon increasing the H\({}_{2}\)SO\({}_{4}\) content (from 0.2 to 1.0 M), a slight decrease in the overall LA can be noticed. Analogous results are registered in the presence of simple alcohols as co-solvents, such as methanol and ethanol, that permit the direct formation of alkyl levulinates (Figure 7), used as flavoring/fragrance agents or as fuel bio-additives [48; 49]. Moreover, methyl- and ethyl-levulinate now represent a valid starting bio-based feedstock for the preparation of \(\gamma\)-valerolactone [50; 51] and find many applications ranging from flavoring agent to green solvent or as an intermediate in the synthesis of bio-based chemicals and polymers [52; 53]. Figure 5: Effect of HTC reaction time on the production of furfurral (FU), 5-hydroxymethylfurral (5-HMF) and levulinic from (LA) OPW under neutral conditions (Reaction conditions: 20 g of OPW; 20 mL of H\({}_{2}\)O; temperature: 180 \({}^{\circ}\)C; autogenous pressure; stirring: 600 rpm). Figure 6: Effect of acid conditions on the production of furural (FU), 5-hydroxymethylfurral (5-HMF) and levulinic acid (LA) (Reaction conditions: 20 g of OPW; 20 mL solution of H\({}_{2}\)SO\({}_{4}\) (from 0.1 to 1.0 M) in H\({}_{2}\)O; temperature: 180 \({}^{\circ}\)C; time: 60 min; autogenous pressure; stirring: 600 rpm). Indeed, the best results in terms of alkyl-levulinate production were obtained when using a 0.2 M sulphuric acid solution with higher yields from the methanol esterification that, being characterized by a shorter carbon chain, is definitely a better entering group than ethanol (Table 5).