## 4 Conclusions  
Three members of the acidic resins family featuring different morphologies, namely non-porous micronized Aquivion(r) PFSA (Aquivion mP98), macroreticular Amberlyst-15(r), and macroporous polystyrene-supported p-toluensulfonic acid (PS-pTsOH), were active catalysts in the esterification reaction of levulinic acid with 1-pentanol, both under conventional batch conditions and in flow. The activity generally tracked the progression Aquivion mP98 \(>\) PS-pTsOH \(>\) A-15, which was tentatively explained on the basis of the different porous structure of the catalytic systems. Particularly, non-porous Aquivion mP98 (10 mol %) allowed the highest levulinic acid conversion (92%) into pentyl levulinate under conventional batch conditions at 90 \({}^{\circ}\)C when employing a 1:10 LA/alcohol molar ratio (24 h). Moreover, the reaction was completely selective and the product was isolated (88% yield) by a straightforward procedure, encompassing the almost complete recovery (96%) of unreacted 1-pentanol by distillation, for later reuse. The protocol could be extended to aliphatic alcohols with short, moderate, and branched chains, leading to the preparation of levulinates in good-to-high yields (50-92%). Moreover, no byproducts coming from inter-/intramolecular dehydration of the different alcohols were observed under the tested reaction conditions. Recovery and reuse of all the three catalysts have been investigated. Notably, non-porous Aquivion mP98 substantially retained its activity even after five consecutive runs, while being fully recoverable (~5% weight loss). Calculation of the E-factors confirmed that our approach is very efficient for waste reduction, especially in the case of the flow conditions (E-factor as low as ca. 0.3). Importantly, the flow technique proved to be an efficient solution to maintain the physical integrity of the catalysts and also to allow for its easy recovery and recycling. Additionally, flow conditions enabled the production of levulinates on a g per hour scale when employing the low- to non-porous catalysts PS-pTsOH and Aquivion mP98, respectively. Further investigations are due in due course to demonstrate that resin-supported acid catalysts can improve substantially the overall efficiency of valuable fuel esters production. **Supplementary Materials:** The following are available online at www.mdpi.com/2073-4344/7/8/235/s1, E-factor calculation, Table S1: Esterification reaction of LA with 1-pentanol in batch conditions, Table S2: Recycling of Aquivion mP98, PS-pTsOH and Amberlyst-15 in esterification reaction of LA with 1-pentanol, Figure S1: Swelling in 1-pentanol of the three investigated resins, Figure S2: Effect of Aquivion mP98 catalyst loading on conversion of levulinic acid into pentyl levulinate (1:10 LA to pentanol molar ratio, 90 \({}^{\circ}\)C), Figure S3: Effect of the mole ratio on conversion of levulinic acid into pentyl levulinate (10 mol % catalyst, 24 h), Figure S4: FTIR spectra of Nafion NRS0 before use and after esterification reaction of LA with 1-pentanol, Figure S5: FTIR spectra of Aquivion mP98 before use and after five runs, Figure S6: FTIR spectra of PS-pTsOH before use and after five runs, Figure S7: FTIR spectra of Amberlyst 15 before use and after five runs, Figure S8: Hot filtration test for the heterogeneous acid catalyzed esterification reaction of LA and 1-pentanol (1:5 molar ratio, 10 mol % catalyst) at 90 \({}^{\circ}\)C, Figure S9: Comparison between the abilities of the investigated catalysts in producing pentyl levulinate **3** under flow conditions. **Acknowledgments:** This research has been developed and partially financed within the National project \"BIT3G\"--Italian Green Chemistry Cluster. We gratefully acknowledge [PERSON] (University of Rome \"Sapienza\") for FTIR analyses. **Author Contributions:** [PERSON], [PERSON], [PERSON], [PERSON], M.C. performed the experiments and analyzed the data; [PERSON] contributed reagents/materials/analysis tools; [PERSON] and [PERSON] conceived and designed the experiments and wrote the paper.