## 3 Results and Discussion
### Effect of Using Alkaline Catalysts  
Different catalysts were used to evaluate their effect on the resulting DRs. According to reports by other studies, alkaline catalysts at concentrations around 0.1 M [19, 21, 28] can achieve a significant improvement in the depolymerization reaction kinetics. In this way, higher concentrations have been discarded in order to use the minimum amounts of additives that have demonstrated good efficiencies in the process. Alkaline catalysts considerably accelerate hydrothermal solvolysis processes due to the following mechanisms: increasing the concentration of hydroxyl ions (OH\(-\)) that attack the electrophilic carbon atoms of the target compound; enhancing the electrostatic interaction between these negative hydroxyl ions and the positively charged hydrogen atoms so that the bonds between the atoms of the target compound are weakened, making them sensitive to breakage; and promoting the ionization of water molecules that generate hydronium ions (H\({}_{3}\)O+) that promote the hydrolysis reaction [29]. For this purpose, a number of experiments were performed, as follows: catalyst-free (P9), 0.1 M KOH (ac) (P4), 0.1 M NaOH (ac) (P7), and 0.1 M K\({}_{2}\)CO\({}_{3}\) (ac) (P8). All the tests were performed at 300 \({}^{\circ}\)C, 130 bar, 60 rpm, 80 min reaction time, 300 ml\({}_{\text{solv}}\)/g\({}_{\text{comp}}\) solvent/sample ratio, and 5 wt.% IPA solution as co-solvent. Figure 2 shows the DR values (%) obtained from the different experiments, where no relevant differences could be observed between the results that could be attributed to the catalyst used. The tests that used 0.1 M KOH and K\({}_{2}\)CO\({}_{3}\) presented the highest DR values, at 95% and 95%, respectively. The test where NaOH was used showed a slightly poorer removal performance, with a 90% DR. Finally, the experiment that was performed without any catalysts showed a significantly lower DR (27%) than those in which a catalyst had been used. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**Test** & **P1** & **P2** & **P3** & **P4** & **P5** & **P6** & **P7** & **P8** & **P9** & **P10** & **P11** & **P12** \\ \hline Stirring (rpm) & 0 & 60 & 60 & 60 & 60 & 60 & 60 & 60 & 60 & 60 & - & - \\ T (C) & 300 & 300 & 300 & 300 & 300 & 300 & 300 & 300 & 300 & 300 & 400 & 400 \\ P (bar) & 125 & 132 & 104 & 132 & 134 & 132 & 132 & 132 & 129 & 136 & 242 & 244 \\ t (min) & 35 & 35 & 120 & 80 & 80 & 80 & 80 & 80 & 80 & 80 & 35 & 35 \\ Solv. (wt.\%\%) & 5\% IPA & 5\% IPA & 5\% IPA & 5\% IPA & 5\% IPA & 5\% IPA & 5\% IPA & 5\% IPA & Water & 5\% \\ Cat. (0.1 M) & KOH & KOH & KOH & KOH & KOH & KOH & NaOH & K\({}_{2}\)CO\({}_{3}\) & - & KOH & - & KOH \\ Ratio (ml\({}_{\text{solv}}\)/g\({}_{\text{comp}}\)) & 303 & 300 & 299 & 301 & 448 & 223 & 301 & 301 & 302 & 222 & 311 & 302 \\ S.S. (mm\({}^{2}\)/g) & 924 & 927 & 925 & 973 & 950 & 850 & 972 & 932 & 973 & 716 & 849 & 826 \\ DR (\%) & 49 & 51 & 97 & 95 & 94 & 90 & 90 & 95 & 27 & 84 & 94 & 98 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Test operating conditions and results. Figure 2: DRs obtained using different catalysts or no catalyst at 300 \({}^{\circ}\)C, 130 bar, 60 rpm, 80 min, 300 ml\({}_{\text{solv}}\)/g\({}_{\text{comp}}\), IPA 5 wt.%. The comparison of these results allows us to confirm that the decomposition of the resin was substantially improved when a catalyst was added to the hydrothermal treatment under subcritical conditions, as in the tests where a catalyst was used, the decomposition rates were three times higher than when no catalyst was used. Similar results have been reported in the literature. For example, [PERSON] et al. [30] used methanol as a solvent and KOH as a catalyst under subcritical conditions to recycle CFRPs.