## 3 Materials and Methods  
### General Remarks  
All reagents were purchased from commercial sources and used without further purification, unless otherwise noted. Commercial Amberlyst-15(r) (4.7 mmol SO3H/g, particle size < 0.3 mm), polystyrene-bound p-toluenesulfic acid (PS-pTsOH; 2.9 mmol SO3H/g, particle size: 0.25-0.59 mm), and Nafion(r) NR50 (0.8 mmol SO3H/g, particle size \(\sim\)3.5 mm) were purchased at Sigma-Aldrich, Si Milan, Italy. Aquivion mP98 (1.2 mmol SO3H/g, particle size \(\sim\)0.7 mm) was provided by Solvay Specialty Polymers S.p.A. Elemental microanalyses were performed using a Fison's EA1106 CHN analyzer using atropine, 2,5-bis-2-(5-tertbutylbenzoxazol-yl)-thiophene (BBOT), and phenanthrene as a reference standard, with an accuracy of ca. 2 umol g\({}^{-1}\). NMR spectra were recorded on a Bruker DRX-ADVANCE 400 MHz (\({}^{1}\)H at 400 MHz and \({}^{13}\)C at 100.6 MHz) in CDCl3, using TMS as an internal standard. Fourier Transformed Infrared (FTIR) spectra were recorded on a VERTEX 70 Bruker Optics Instrument (Billerica, MA, USA), with a spectral range of 4000-400 cm\({}^{-1}\) and a resolution of 4 cm\({}^{-1}\), equipped with a single reflection diamond ATR cell.  
Representative Experimental Batch Procedure for the Preparation of Pentyllevulinate **3** over Heterogeneous Acid Catalysts  
In a round-bottomed flask capped with a PTFE stopper and equipped with a magnetic stirrer, levulinic acid (1 g, 8.6 mmol), 1-pentanol (9.35 mL, 86 mmol) and Aquivion mP98 catalyst (10 mol % 0.7 g) were consecutively added. The mixture was left under stirring at 90 \({}^{\circ}\)C for 24 h. Then the reaction mixture was cooled down to room temperature and the catalyst filtered off and washed with ethyl acetate (80 mL). The filtrate was concentrated by removing solvent and unreacted 1-pentanol was recovered (96%) by distillation at 75 \({}^{\circ}\)C under vacuum (80 mmHg). Subsequent filtration using a basic Al2O3 pad (0.15 g) to remove the residual LA gave pentyl levulinate **3** as a pale yellow oil (1.43 g,  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Entry** & **Catalyst Type** & **Cycle** & **C (\%)2** & **gkh\({}^{-1}\)** & **TON 3** & **TOF (h\({}^{-1}\))4** \\ \hline
1 & & 1 & 93 & 0.93 & 11.5 & 0.88 \\
2 & A-15 & 2 & 93 & 0.93 & 11.5 & 0.88 \\ \hline
3 & PS-p-TsOH & 1 & 98 & 4.73 & 47.8 & 4.30 \\
4 & & 2 & 98 & 4.73 & & \\ \hline
5 & Aquivion mP98 & 1 & \textgreater{}99 & 4.8 & 37.4 & 4.68 \\
6 & & 2 & \textgreater{}99 & 4.8 & & \\ \hline \hline \end{tabular}  
* 1:10 LA/1-pentanol molar ratio, 90 \({}^{\circ}\)C. 2 LA conversion to pentyl levulinate **3**, determined by \({}^{1}\)H-NMR analysis, using 4-bromoaisole as internal standard; the remaining material was the unreacted mixture of **1** and **2**. 3 Turnover Number = total number of moles of pentyl levulinate produced per mole of catalyst. 4 Turnover frequency = number of substrate molecules reacted at each available catalytic site per unit time.  
\end{table}
Table 6: Reusability of catalysts in flow reactor for representative reaction to 1-pentanol 1.  
7.57 mmol, 88% yield). 1H-NMR 8 (ppm) 3.9 (t, 2H); 2.6 (m, 2H); 2.4 (m, 2H); 2.1 (s, 3H); 1.5 (m, 2H); 1.2 (m, 4H); 0.81 (m, 3H). E-factor: [1 g (LA) + 7.6 g (1-pentanol) + 0.2 g (Al2O3) \(-\) 1.63 g (pentyl levulinate 3) \(-\) 6.5 g (96% recovered 1-pentanol)]/1.63 g (pentyl levulinate 3) = 0.4.  
The Aquivion mP98 catalyst was not considered in the calculation due to its complete recovery.