## 4 Discussion  
The effects of the process parameters temperature, treatment time, and biomass/water ratio on the structure of hydrocarbons obtained by HTC of olive tree trimmings was qualitatively investigated by FTIR spectroscopy, then studied in more detail and more quantitatively by \({}^{13}\)C SSNMR spectroscopy. The differences observed in the FTIR spectra recorded on hydrocarbon samples with respect to that of the feedstock (Figure 1) can be explained in terms of the reactions known to occur during the HTC process [2; 3; 4]. In particular, dehydration and decarboxylation reactions can be associated with the loss of intensity of the O-H stretching band between 3700 and 3000 cm\({}^{-1}\) and bands of carbonyl and alcohol groups in the \"fingerprint region\", with both reactions being favoured at higher treatment temperature, longer heating times, and higher B/W ratio. These reactions mainly occur on carbohydrate components, as indicated by the evolution of the bands at 1732 cm\({}^{-1}\) (from hemicellulose), 895 cm\({}^{-1}\) (from cellulose) and 1165 cm\({}^{-1}\), whereas lignin seems to remain almost unreacted after HTC in the adopted conditions. On the other hand, aliphatic alkyl structures, aromatic and furanic units, and carbonyl functional groups increase in content during the HTC process, as indicated by the bands at 3000-2800 cm\({}^{-1}\) (alkyl chains), 3065, 1610, 1512, 1527, 885-782 cm\({}^{-1}\) (aromatic structures), 1265 and 1210 cm\({}^{-1}\) (C-O and phenolic O-H), and 1700 and 1652 cm\({}^{-1}\) (C=O groups). A more detailed picture of the transformations occurring during HTC was achieved from the \({}^{13}\)C CP-MAS NMR spectra (Figures 2 and 3). The first effect of HTC at lower temperatures and short carbonization time is the hydrolysis of hemicellulose and amorphous cellulose [54], as shown by the narrowing of carbohydrate signals in the 60-110 ppm region. Indeed, in previous studies, extractives and hemicellulose were found to be easily hydrolys and completely solubilized in hot compressed water, with the consequent presence of monomers and oligomers in the liquid aqueous phase [1; 2; 3; 4; 55; 56]. Moreover, part of the amorphous cellulose and soluble lignin is fragmented into smaller molecules, mainly into oligosaccharides and 5-hydroxymethylfurfurral the former, into phenolic derivatives the latter [7]. On the contrary, crystalline cellulose was found to be resistant to hydrolysis, as expected for a lignocellulosic biomass, where cellulose may be protected from water by lignin and by hemicellulose. By increasing the temperature and prolonging the HTC treatment, cellulose is also transformed, as indicated by the decrease in intensity of the O-alkyl and di-O-alkyl signals in the 60-110 ppm region. In fact, at temperatures >230 \({}^{\circ}\)C, subcritical water can disrupt the hydrogen bonds and crystalline structure of cellulose, hydrolysing the \(\beta\)-(1-4)-glycosidic bonds to form glucose monomers [1]. Nonetheless, carbohydrate components are still detected in the \({}^{13}\)C CP-MAS NMR spectra of hydrocarbons at the longest HTC reaction time of 180 min, even for those obtained with the lower B/W ratio. The solubilization and hydrolysis products are generally highly reactive, undergoing condensation, oligomerization and polymerization reactions. These reactions occur in parallel [1; 3; 4], ultimately leading to the formation of polymeric furanic and aromatic structures connected by aliphatic chains. The solid residue formed through these reactions is often called secondary char. These reactions may be accompanied by dehydration, decarboxylation, and decarbonylation reactions. When precipitates form on the surface of cellulose, water access is inhibited. Consequently, cellulose is subject to pyrolysis and not dissolution and hydrolysis [57], giving carbonaceous materials with aromatic structures instead of furanic ones [58]. Additional contributions to the aromatic structures arise from lignin. Indeed, soluble lignin fragments are quickly polymerized.