Synthesis of alkyl levulinates via the esterification of levulinic acid and transesterification of methyl levulinate with alkyl alcohols over montmorillonite K10†

Alkyl levulinates are bio-based chemicals with great potential for application in the fields of energy and fine chemical synthesis. They are synthesized via the esterification of levulinic acid with the corresponding alkyl alcohols over Brønsted acid catalysts. Here, three types of commercially available, low-cost, and environmentally friendly layered clay minerals (montmorillonite K10, halloysite, and kaolinite) were applied to the esterification of levulinic acid with ethanol as heterogeneous Brønsted acid catalysts. This is because of their surface hydroxyl groups, which can function as Brønsted acid sites. The catalytic activity followed the order of montmorillonite K10 ≫ halloysite ≈ kaolinite ≈ blank (no catalyst). This was most likely attributable to the difference in the thickness of a layer with one interlayer space. The most effective clay mineral, montmorillonite K10, was used to synthesize the target product (ethyl levulinate) at an excellent yield of 96.5% under optimized reaction conditions (N₂ pressure, 0.6 MPa; temperature, 443 K; time, 3.75 h). The clay mineral was observed to be reusable at least thrice for the esterification reaction without any significant decrease in its catalytic activity. Furthermore, it could be used to synthesize various alkyl levulinates in excellent yields by varying the selection of alkyl alcohols used. In addition, it was applied to the transesterification of methyl levulinate with various alkyl alcohols, producing the corresponding alkyl levulinates in extremely good yields. This study provides an environmentally friendly, economical, and effective route to biomass utilization.