## 2 Results and Discussion
The recovery and reuse of the three catalysts have been investigated, and the results reported for the reaction of LA with pentyl levulinate carried out at 70 \({}^{\circ}\)C (1:5 molar ratio, 10 mol % catalyst, 24 h). After the reaction was completed, the catalyst used was recovered by filtration. The recovered \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Entry** & **Alcohol** & **Catalyst Type** & **T (\({}^{\circ}\)C)** & **C (\%) 2** \\ \hline
1 & & Aquivion mP98 & 90 & 58 \\
2 & OH & PS-p-ToOH & 90 & 58 \\
3 & & A-15 & 70 & 60 \\ \hline
4 & & Aquivion mP98 & 90 & 55 \\
5 & OH & PS-p-ToOH & 90 & 50 \\
6 & & A-15 & 70 & 65 \\ \hline
7 & & Aquivion mP98 & 90 & 75 \\
8 & OH & PS-p-ToOH & 90 & 26 \\
9 & & A-15 & 70 & 70 \\ \hline
10 & & Aquivion mP98 & 90 & 76 \\
11 & & PS-p-ToOH & 90 & 58 \\
12 & & A-15 & 70 & 58 \\ \hline
13 & & Aquivion mP98 & 90 & 84 \\
14 & & OH & PS-p-ToOH & 90 & 62 \\
15 & & A-15 & 70 & 80 \\ \hline
16 & & Aquivion mP98 & 90 & 50 \\
17 & & PS-p-ToOH & 90 & 32 \\
18 & OH & A-15 & 70 & 12 \\ \hline
19 & & Aquivion mP98 & 90 & 56 \\
20 & & PS-p-ToOH & 90 & 25 \\
21 & OH & A-15 & 70 & 10 \\ \hline \hline \end{tabular} \({}^{1}\) Reaction conditions: 1:10 LA/alcohol molar ratio, 10 mol % catalyst, 90 \({}^{\circ}\)C, 24 h. 2 LA conversion to alkyl levulinates **11-17**, determined by 1H-NMR analysis, using 4-bromoanisole as internal standard; the remaining material was the unreacted mixture of 1 and 4-10.  
\end{table} 
Table 5: SERTification reaction of LA with various alcohols 1.  
catalyst was then washed with a minimum amount of ethyl acetate, dried, and subsequently reused. All the three catalysts substantially retained their activity, even after five consecutive runs (Table S2 and Figure 3). FTIR analyses indicated that there is LA or pentyl levulinate adsorption to some extent on all resins washed, except Aquivion mP98 (Figures S4-S7, SM). The loss of catalysts has been also evaluated. Aquivion mP98 mass was fully recovered even after five consecutive runs (\(\sim\)5% weight loss), whereas PS-pTSOH was \(\sim\)80% recovered, and Amberlyst-15 loss was as high as \(\sim\)40% (Table S2 and Figure 3). The losses were due to the mechanical disintegration of the catalyst particles, which made their recovery difficult. Indeed, the low to no-porosity of PS-pTSOH and Aquivion mP98, respectively, guaranteed their superior mechanical strength. To confirm that there was no leaching of the catalyst -SO3H groups at the reaction temperature, a hot filtration test was carried out. Esterification of levulinic acid was therefore performed at 90 \({}^{\circ}\)C, using the three investigated catalysts. Then, after 30 min, the catalyst was filtered off and the reaction mixture was allowed to react further. We found that no significant further reaction occurred after this hot filtration procedure, which suggests there is no significant -SO3H leaching from the catalysts during the reaction progress (Figure S8). To evaluate the efficiency of our synthetic strategy, we calculated the Environmental factor (E-factor, kg waste per kg product) [60]. Values in the range of 0.4-0.9 were obtained, which includes the recovery (and reuse) of catalysts, as well as that of unreacted 1-pentanol--the optimized esterification process employing Aquivion mP98 (Table 2, entry 36) giving the minimal production of waste. Unfortunately, it was not possible to compare the efficiency of our synthetic strategy to those of already published protocols, since essential experimental details to properly calculate the metrics are missing in these latter. As a reference, we therefore calculated the E-factor for the homogeneous p-TsOH catalyzed protocol, which showed a value of 76.36, with a consequent reduction of waste for our protocol greater than 99% (see Section 3 and SM). To overcome the issue of the catalysts recoverability, we decided to apply the flow technology.