## 2 Results and Discussion
The catalyst amount varied over a range of 4.3-10 mol %. Conversion of levulinic acid was found to generally increase proportionally with the catalyst amount, as the number of catalytic sites for the reaction increased. This is exemplified in Figure S2 (SM) for Aquivion mP98 (1:10 molar ratio, 90 \({}^{\circ}\)C). Also, the LA to 1-pentanol molar ratio was investigated at 1:5 and 1:10, and it was generally observed that the LA conversion into pentyl levulinate increased with an increasing molar ratio, as exemplified in Figure S3, SM. Within each set of reaction conditions, the catalysts activity generally tracks the progression Aquivion mP98 > PS-pTsOH > A-15. This might be, at least partially, explained by considering the porous structure of A-15 vs. the low to non-porous structure of PS-pTsOH and Aquivion mP98, respectively. Indeed, in the case of A-15, the water produced during the esterification may limit the accessibility of reactants to the fraction of the acid sites located inside the pores, due to the fill up of the latter with water itself. Conversely, PS-pTsOH and Aquivion mP98 (nearly) hamper the adsorption of water on the outer/inner surface, which ensures the maintenance of their performance for a longer amount of time [55]. In other words, the increase in surface area, due to the presence of the pores (Table 1), seems to inhibit the esterification process under the investigated conditions. This is not uncommon behavior [55; 56; 57; 58]. For comparison purposes, the reaction over commercially available Nafion NR50, a fluorinated congener of Aquivion mP98, has been also performed (Table S1, entry 2, SM) in optimal conditions (i.e., 1:10 LA/alcohol molar ratio, 10 mol % catalyst, 90 \({}^{\circ}\)C, 24 h). The lowest conversion value was achieved (60%), and a progressive marked increase of the catalyst pellets' size was observed, which, ultimately, was responsible for a badly efficient stirring of the reaction mixture. FTIR analysis (Figure S4) of Nafion NR50 after the esterification process suggested the LA and/or pentyl levulinate adsorption on the catalytic system, although there was repeated washing. Figure 1: Effect of the temperature on the efficiency of Aquivion mP98, PS-pTsOH and A-15 (10 mol %) in the reaction of LA with 1-pentanol (1:10 molar ratio, 24 h). Figure 2: Effect of time and temperature on the reaction of LA with 1-pentanol (1:10 molar ratio), catalyzed by 10 mol % of Aquivion mP98, PS-pTsOH and A-15. Furthermore, the esterification reaction over a homogeneous reference catalyst, i.e., p-toluensulfonic acid (pToOH), gave a LA conversion value lower than that achieved by Aquivion mP98 under the optimal experimental conditions (Table S1, entry 3 and Table 2, entry 36, respectively). Similar results were obtained by comparing the PS-pTSOH catalyzed esterification process (Table 4, entry 36) carried out under identical conditions. On the other hand, Amberlyst 15 (Table 3, entry 36) gave lower LA conversions, thereby indicating the presence of diffusional limitations and/or catalyst instability at the temperature conditions of 90 \({}^{\circ}\)C [54]. To broaden the substrate scope, a range of aliphatic alcohols **4-10** with short, moderate, and branched chains were reacted with LA using the optimized batch conditions for each catalyst to give the corresponding alkyl levulinates **11-17** (Scheme 2, Table 5). Aquivion mP98 revealed good versatility, giving a good to high conversion (50-84%) with all of the alcohols under investigation. Notably, it was effective with branched-chain alcohols (entries 16 and 19, Table 2). This is particularly attractive since skeletally branched chain esters are considered more effective [59] as additives and diluents to improve, e.g., the cold-flow properties of biodiesel. Additionally, no byproducts coming from the inter-/intramolecular dehydration of the different alcohols were observed under the tested reaction conditions.