## 3 Results and Discussion
### Hydrothermal Bio-Oil Liquid Fraction  
#### 3.2.1 Composition of Hydrothermal Bio-Oil Liquid Fraction  
The general chemical composition of the light bio-oil obtained at different reaction temperatures and at a fixed residence time of 60 min was analyzed by GC-MS (Table 4). The reaction mechanism involved in the formation of bio-oil during the HTC of various lignocellulosic biomasses has been described by several authors [38; 44] and consists of a series of consecutive and parallel reactions starting with acid hydrolysis of polysaccharides to form monosaccharides. Glucose and fructose can be dehydrated by acids into 5-hydroxymethylfurral and furfural. Furans and other reaction intermediates can \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{5}{c}{**Sample**} \\ \cline{3-6}  & **Compounds Name** & **L-HC\({}_{\bf 150-60}\)** & **L-HC\({}_{\bf 180-60}\)** & **L-HC\({}_{\bf 240-60}\)** & **L-HC\({}_{\bf 300-60}\)** \\ \cline{3-6}  & & \multicolumn{5}{c}{**Peak Area\%**} \\ \hline Furans & Furfural & 18.94 & 16.69 & 1.63 & - \\  & 2-Furancarboxaldehyde, 5-methyl & 14.86 & 7.88 & 5.05 & - \\  & 5-Hydroymethylfurral & 41.97 & 61.82 & 28.61 & - \\ \hline Phenols & Phenol & 0.57 & 0.10 & 4.72 & 9.62 \\  & Catechol & 0.60 & 0.29 & 4.52 & 8.64 \\  & 1,2-Benzenedi, 3-methyl & - & 0.11 & 0.78 & 1.04 \\  & Hydroquinone & - & - & 1.91 & 7.26 \\  & p-Cresol & - & - & 0.79 & 2.17 \\  & Phenol, 2-methyl & - & - & 0.27 & 0.69 \\ \hline Acids & Benzoic acid & 2.27 & 0.61 & 2.67 & 6.21 \\  & 2-Pentenoic acid & - & - & 0.32 & - \\ \hline Ketones & 2-Pentanone, 4-hydroxy-4-methyl & 0.40 & 0.68 & 3.87 & 1.20 \\  & Ethanone, 1-(2-furanyl) & 0.69 & 0.46 & 0.40 & 1.86 \\  & 1,2-Cyclopentanedione, 3-methyl & 0.63 & 0.81 & - & - \\  & 2-Cyclopenten-1-one, 2-hydroxy-3-methyl & - & - & 4.37 & 0.99 \\  & 2-Cyclopenten-1-one, 2-methyl & - & - & 0.61 & 5.83 \\  & 2-Cyclopenten-1-one, 3-methyl & - & - & 0.61 & 7.41 \\ \hline Aldehydes & 1H-Pyrrole-2-carboxaldehyde & 0.86 & 0.41 & 0.66 & - \\  & Benzaldehyde, 3-hydroxy & - & 0.17 & 0.83 & - \\  & Vanillin, acetate & - & - & 1.07 & - \\ \hline Alcohols & \(\alpha\)-Terpineol & 0.91 & 0.24 & - & 1.01 \\  & Benzyl alcohol & - & - & 0.29 & - \\  & 3-Pyridinol & - & - & 10.14 & 4.63 \\ \hline Alkenes & 1-Nonadecene & 0.24 & 0.33 & 1.65 & - \\  & 1-Pentadecene & - & 0.59 & 3.51 & - \\  & 1-Heptadecene & - & - & 3.64 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Major chemical components of the light bio-oil determined by GC-MS analysis. undergo further transformation processes (e.g., isomerization, condensation, rehydration, hydrations) or they can be degraded into humins [45]. As illustrated in Table 4, the light bio-oil is mainly divided into seven categories: furans, phenols, acids, ketones, aldehydes, alcohols and alkenes (Figure S5 in ESI). At lower HT temperatures, the main compounds at 200 \({}^{\circ}\)C are furan derivatives. When increasing the temperature, the composition of the light bio-oil becomes more complex, showing a higher concentration of acid phenolic compounds as a consequence of humins formation. 2.2 Production of Furan Derivatives and Levulinates from Hydrothermal Upgrading of Orange Peel Waste  
To investigate the best reaction conditions that maximize the production of furan and levulinate derivatives, a systematic study on the effect of (i) time; (ii) temperature; (iii) initial acid concentration; and (iv) co-solvent was carried out. Reaction temperature is surely a crucial parameter for investigating the overall productivity of furan derivatives starting from OPW. The yield profiles of FU, 5-HMF and levulinic acid are reported in Figure 4. Under neutral conditions, FU and 5-HMF yields gradually increase, reaching the highest value at 180 \({}^{\circ}\)C. The observed decrease in the production of furans at higher reaction temperatures is in line with other reports [46] and can be related to the formation of humin type by-products.