###### Abstract
In particular, the development of efficient, highly functional, group-tolerant, controlled, and living polymerization methods has allowed for diverse ranges of structures and the tuning of the density of catalyst sites along polymers, as well as the easy incorporation of the catalytic center into polymer structures [43; 44; 45]. Yet, there have been only sporadic attempts at the production of alkyl levulinates by esterification of LA with resin-supported acid catalysts [40; 41; 42; 46]. The only report to date focusing exclusively on such a topic has been very recently published by [PERSON] et al. [47], who demonstrated the potential of a range of acidic ion-exchange resins in promoting the esterification of LA with 1-butanol, selectively obtaining LA conversion into butyl levulinate in the range 64-94%. The present paper is focused on the development of a sustainable approach to alkyl levulinates from LA by using heterogeneous catalysis, safe media and flow conditions aimed at demonstrating how crucial this approach is to access chemically and environmentally efficient protocols [48; 49; 50; 51]. Notably, to the best of our knowledge, this is the first report on the preparation of levulinates under these conditions. To this end, a set of acidic resins with different morphologies has been investigated, i.e., Amberlyst 15(r) (A-15) and polystyrene-supported p-toluensulfonic acid (PS-pTsOH), both available in the market, as well as perfluorosulfonic polymer Aquivion(r) PFSA in the form of micronized pellets (Aquivion mP98) from Solvay Specialty Polymers S.p.A., which has not yet become commercially available. It is well-known in fact that the morphology of polymer-supported catalysts is a critical parameter from the point of view of their use under flow conditions [52]. Thus, A-15 and PS-pTsOH are nearly spherical beads of sulfonated co-polymers of styrene (S) and divinylbenzene (DVB). More specifically, they are classified as macroporous (macroreticular) resins, i.e., a degree of permanent porosity is induced into the resin beads so that a high amount of the acidic sites, although distributed through the bulk of the resin, may be accessible to non-swellingsolvents and reactants. Macroreticular resins are macroporous, typically showing a high cross-linking ratio. In this case, swelling does not usually produce a noticeable change of the overall resin volume. The permanent porous structure on the resins facilitates, in principle, the flow through the beads of the solvents/reactants, and, therefore, the accessibility of the catalytic sites. However, a high cross-linking degree may limit the access to the sites embedded in the inner polymer matrix regions, since it precludes proper swelling of those regions. On the other hand, Aquivion mP98 is a co-polymer of tetrafluoroethylene and sulfonylfluoride vinyl ether which, after hydrolysis of the sulfonyl fluoride, yields the strongly acidic terminal -CF\({}_{2}\)CF\({}_{2}\)SO\({}_{3}\)H groups. It has no porosity in the dry state and, as such, depends on being swollen by the reaction medium, giving rise to a porous structure and therefore enabling the diffusion of reactants through the polymer matrix. From the point of view of flow application, it is important to bear in mind that swelling leads to a variation in the overall non-porous resin volume that can be significant. This needs to be taken into account when configuring the flow reactors to avoid, e.g., overpressures. It should also be noted here that the micronized form of Aquivion PFSA (Aquivion mP98) exhibits an average particle size which is smaller than the commercially available Aquivion pellets P98 counterpart (about 0.7 vs. 2.5 mm), and this likely makes it more useful as catalytic material. In fact, due to the smaller dimension of the pellets, the stirring of the reactants and the isolation of the product become easier under conventional batch conditions. Also, this form of Aquivion PFSA is expected to be more useful in view of our final set of a flow procedure [53].