## 3 Results and Discussion
### Kinetics of the Acid Hydrolysis and Auto-Hydrolysis Step  
Application of a non-conventional process with auto-hydrolysis instead of the acid hydrolysis step, allowed to obtain apparently a better purer cellulose residue (Figure 2). For this reason, it could be interesting to further investigate and optimize the auto-hydrolysis step from the point of view of improving the cellulose yield. As explained in the materials and methods, for the kinetics study, the conditions of samples cooling were standardized as much as possible, and this might have led to slightly different results than in the previous trials. It was then decided to monitor the sugars (Table 5) release, since this first step is meant to hydrolyze the hemicelluloses with release of pentoses (mainly xylose), but also a fraction of cellulose can be hydrolyzed (giving glucose) depending on the severity of the working conditions. Furthermore, a too-severe process may lead to the thermal degradation of the released sugars with the formation of furans, and their concentration was monitored as well (Table 6). Indeed, as reported in a review work by [PERSON] and [PERSON] (2016), the pentoses and uronic acids, which come from the hydrolysis of the hemicelluloses, decompose, leading to the formation of furfural, while the hexoses degradation leads to HMF (5-hydroxymethyl-2-furaldehyde) formation. This was confirmed by xylose concentration profile (Table 5) which shows a maximum. This trend can be explained considering that, initially, the hemicellulose hydrolysis into xylose is the dominant reaction, while, when hemicellulose is almost completely hydrolyzed, the degradation of xylose becomes the dominant reaction. This results in higher concentrations of sugar degradation products in case of auto-hydrolysis (Table 6). On the other hand, xylose concentration of acid hydrolysis is higher and increases its value until a plateau. This means that hemicellulose was hydrolyzed, and that xylose was not completely converted into the related degradation products due to the lower temperature compared to auto-hydrolysis. Regarding glucose, the concentration is lower in case of auto-hydrolysis (Table 6) and this means that auto-hydrolysis is less aggressive and so it allows a higher cellulose recovery. It is interesting to highlight that the glucose concentration shows a maximum trend profile in case of acid hydrolysis. This could be due to the fact that glucose reaches a concentration such that glucose degradation reactions become relevant. \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & \multicolumn{2}{c}{**Auto-hydrolysis**} & \multicolumn{2}{c}{**Acid Hydrolysis**} \\ \cline{2-5}  & **mg/L** & **mg/gdw** & **mg/L** & **mg/gdw** \\ \hline Glucose & \(61.96\pm 1.70\) & \(0.81\pm 0.02\) & \(958.33\pm 16.95\) & \(3.71\pm 0.07\) \\ Xylose & \(169.56\pm 19.56\) & \(2.22\pm 0.26\) & \(15993.18\pm 538.20\) & \(61.97\pm 2.08\) \\ Acetic acid & \(132.27\pm 8.54\) & \(1.73\pm 0.11\) & \(2491.69\pm 4.86\) & \(9.66\pm 0.67\) \\
5-HMF & \(112.15\pm 0.23\) & \(1.47\pm 0.01\) & \(22.30\pm 0.46\) & \(0.17\pm 0.01\) \\ Furfural & \(1389.59\pm 1.13\) & \(18.23\pm 0.02\) & \(1139.61\pm 31.72\) & \(8.84\pm 0.25\) \\
5-MF & \(18.21\pm 0.16\) & \(0.24\pm 0.002\) & \(2.38\pm 0.07\) & \(0.02\pm 0.01\) \\ p-hydroxibenzoic acid & ND & ND & ND & ND \\ Gentisic acid & ND & ND & ND & ND \\ Vanillic acid & ND & ND & \(1.30\pm 0.09\) & \(0.01\pm 0.01\) \\ Caffeic acid & ND & ND & \(1.62\pm 0.16\) & \(0.01\pm 0.01\) \\ Sirringic acid & ND & ND & \(1.65\pm 0.07\) & \(0.01\pm 0.01\) \\ Vanillin & \(29.34\pm 0.09\) & \(0.385\pm 0.01\) & \(5.33\pm 0.15\) & \(0.04\pm 0.01\) \\ Syringaldehyde & \(3.53\pm 0.26\) & \(0.046\pm 0.01\) & \(3.06\pm 0.04\) & \(0.02\pm 0.01\) \\ Ferulic acid & ND & ND & ND & ND \\ Sinapic acid & ND & ND & ND & ND \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Phenolic profile and sugar and sugar degradation compounds content of auto-hydrolysis and acid hydrolysis liquors of wheat straw. HMF: hydroxyl-methyl-furfural; MF: methyl furural.