## 3 Results
### \({}^{13}\)C SSNMR Spectroscopy  
For a more detailed characterization of the chemical structure of our materials, feedstock and hydrochar samples were investigated by \({}^{13}\)C CP-MAS NMR. In agreement with FTIR, the NMR spectrum of FS (Figure 2a) presented the characteristic signals of lignocellulosic materials [18; 39; 40; 41; 42]. In particular, the 60-110 ppm spectral region was dominated by the signals of cellulose [43], with less intense signals deriving from hemicellulose (C-1 to C-6 carbons) [39] and from aliphatic side chains of lignin [44; 45; 46; 47; 48]. Indeed, cellulose signals are expected at 104.5 ppm for carbon C-1 of \(\beta\)-D-glucose units, at 72.1 and 74.6 ppm for C-2, C-3, and C-5, at 83.3 and 88.1 ppm for C-4, and at 62.2 and 64.5 ppm for C-6. The signals at 88.1 and 64.5 ppm, due to C-4 and C-6 of crystalline or internal cellulose, and those at 83.3 and 62.2 ppm, due to amorphous cellulose and cellulose on the surface of fibers, were not clearly distinguished due to the higher intensity of the broader amorphous signals. The shoulder at about 102 ppm was ascribed to amorphous cellulose or hemicellulose C-1. The signal of carboxyl carbons from acetate groups of hemicellulose was observed at 172.5 ppm [41; 43; 49]. Conversely, the signal of acetate methyl carbons was not clearly distinguished at 21.0 ppm, due to the superposition with signals of other alkyl groups in the 0-45 ppm region, arising from carbohydrates and lignin, but also from organic extractives, cutin, and suberin [5]. In particular, the strong signal at about 30 ppm is typical of long aliphatic chains. Aromatic lignin carbons, resonating between 115 and 160 ppm [44; 45; 46; 47; 48] (in particular, protonated aromatic carbons between 115 and 128 ppm and oxygen-bonded aromatic carbons between 147 and 160 ppm) could be observed, even though their signals had low relative intensity. The methoxyl group signal typical of lignin was detected at 55.7 ppm [39]. The \({}^{13}\)C CP-MAS NMR spectra of the hydrocarbons obtained from the different treatments are shown in Figure 2b-h. Three main changes were observed in the spectra of the carbonized samples as temperature and reaction time increased: (1) The carbohydrate signals (in the 60-110 ppm spectral region) became narrower and decreased in intensity; (2) The relative intensities of alkyl (0-45 ppm) and aromatic C (110-165 ppm) signals increased; (3) New signals appeared due to carboxyl (165-190 ppm) and carbonyl (190-220 ppm) functional groups. In fact, after HTC at 180 \({}^{\circ}\)C (Figure 2b), the narrowing of the signals in the 60-110 ppm region, mainly due to cellulose and hemicellulose, indicated a significant reduction in the contribution of amorphous cellulose and hemicellulose compared to crystalline cellulose. The broad signals of the amorphous components strongly decreased for the hydrocarbons obtained at 250 \({}^{\circ}\)C, as highlighted by the narrow linewidth of the carbohydrate signals, even after 30 min of treatment (Figure 2c,d). The latter signals decreased in intensity as treatment time increased (Figure 2c-h). However, cellulose was not completely transformed even at the longest reaction time, and a slower decreasing trend was observed when HTC was performed with a lower B/W ratio (B/W = 7% \(w/w\)_vs._ B/W = 25% \(w/w\)) (Figure 2g,h). Also the signal of residual lignin at \(\sim\)56 ppm was observable in all the hydrocarbon spectra. New signals appeared in the aliphatic (0-45 ppm) and aromatic (110-165 ppm) regions, as well as in the carboxyl (165-190 ppm) and carbonyl (190-220 ppm) regions, due to the formation of new chemical structures. In particular, a sharp signal at 168.0 ppm was observed in the spectra of all hydrocarbons produced at 250 \({}^{\circ}\)C (Figure 2c-h), ascribable to inorganic carbonates [50]. The formation of carbonyl groups, mainly ketones, was clearly observed for treatment times \(\geq\)60 min (Figure 2e,h). This finding agrees with results reported in the literature on HTC of carbohydrate-based materials [18; 19; 21; 51; 52; 53].