###### Abstract  
Biomass-derived levulinic acid (LA) is an excellent substrate to obtain high-value esters that can be used as second-generation biofuels and biofuel additives. The present study focuses on the identification and definition of the key parameters crucial for the development of chemically and environmentally efficient protocols operating in continuous-flow for the preparation of structurally diverse alkyl levulinates via the esterification of LA. We have focused on the use of solid acid catalysts consisting of sulfonated cation exchange resins and considered different aliphatic alcohols to prepare levulinates **3** and **11-17** regioselectively, and in good to high yields (50-92%). Direct correlations between several reaction parameters and catalyst activity have been investigated and discussed to set proper flow reactors that allow minimal waste production during the workup procedure, enabling Environmental factor (E-factor) values as low as ca. 0.3, full recoverability and reusability of the catalysts, and the production of levulinates up to ca. 5 gxh\({}^{-1}\) scale. biomass; levulinic acid; biofuel additives; catalyst recyclability; heterogeneous catalysis; green chemistry; flow technology +
Footnote †: journal: Physics Letters B  
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olefins, aromatics, and sulfur. Longer hydrocarbon chain esters exhibit volatility, polarity, and ignition properties that are suitable for diesel [9; 10; 11]. Alkyl esters of LA have significant potential as blend components in diesel formulations [12; 13; 14; 15; 16]. Such esters are structurally related to the biodiesel fatty acid esters that are used in some low-sulfur diesel formulations, but they do not have their principal drawbacks, i.e., poor oxidative stability, poor low temperature properties, and a tendency towards gum formation. The addition of alkyl levulinates to biodiesel is therefore expected to address these issues. The most studied is a formulation for low-smoke diesel developed by Biofine and Texaco [17] that uses ethyl levulinate as an oxygenated additive. Additionally, ethyl levulinate has been tested as an additive for transportation fuels to improve emissions of nitrogen oxides in high compression diesel engines [18; 19; 20]. Also, technological initiatives are being taken to use ethyl levulinate as a 100% biodegradable neat fuel in the near future [21]. Notably, levulinates are currently in the focus of multiple R&D initiatives in the field, due to their straightforward accessibility from levulinic acid. Besides, levulinic acid esters may find an application as alternative green solvents, polymer plasticizers, and fragrances [22]. Consequently, the development of new routes for the production of levulinates from bio-based platform molecules have attracted ever more attention [22; 23]. The direct esterification of LA with alcohols, which is typically acid catalyzed by homogeneous catalysts, (e.g., sulfuric acid, phosphoric acid) still remains the most frequently utilized approach [22]. A variety of robust and industrially more benign heterogeneous acid catalysts have been used since recent times for the esterification of levulinic acid (mostly with ethanol), including supported heteropolyacids [24; 25; 26], zeolites [27; 28], hybrid catalysts [29; 30; 31; 32], carbon-based materials functionalized with sulfonic groups [33; 34; 35; 36], sulfated metal oxides [37; 38; 39], and silicas [40; 41; 42]. In this context, it should be noted that organic polymer-supported acid catalysts and, particularly, polystyrene sulfonic acid resins are among the most important heterogeneous acids in industry and have been widely used in reactions such as esterification, etherification, alkylation of phenols, and hydration of olefins [43; 44; 45].