As one of the substitutes for traditional fossil energy, the biomass resource has attracted extensive attention. Levulinate esters (LEs) are important green and high value-added chemical molecules. Catalytic routes based on acid catalysts are developed for the production of levulinate esters from various biomass-derived materials. In this review, the catalytic route for preparation of levulinate esters from C5/C6 carbohydrates is summarized. The effects of homogeneous and heterogeneous acid systems on the levulinate ester yields and the catalyst recyclability were expounded. The effect of reaction solvents and heating methods on LE selectivity was presented. Given the apparent differences in the synthesis of LEs with different heating methods in current publications, current challenges and prospects for the synthesis of LEs from biomass-derived compounds are provided. The lab-scale catalytic synthesis of levulinate esters from a biomass process has been developed. This review will facilitate and inspire future research on LE synthesis from biomass.
Bifunctional Lewis (L) acid (Ni-or Hf-) site−Brønsted (B) acid catalysts designed to promote transfer hydrogenation reactions were prepared via hydrothermal and solvothermal methods using safe mixed solvents and sustainable precursors. By using N,Ndimethylformamide as a basis for the desired basicity, mixed solvents could be identified that allowed catalysts to be prepared with tunable ratios of Lewis to Brønsted acid sites (L/B). The as-prepared catalysts promoted transfer hydrogenation of furfural and ethanolysis to form ethyl levulinate (EL) using ethanol as a solvent and hydrogen donor source. Among the catalysts, sulfonated Hf-catalysts prepared with a cyclopentanone/formic acid mixed solvent (Hf-CPN/FA) with an L/B ratio of 6.4 gave 95% furfural conversion with 51.9% yield of EL, while the sulfonated Hf catalyst prepared with a cyclopentanone/γ-valerolactone mixed solvent (Hf-CPN/GVL) with a total Lewis and Brønsted acid site amount of 85.1 μmol/g gave 100% furfuryl alcohol (FAL) conversion with 72.5% yield of EL. Brønsted acid sites promoted reversible acetalization of furfural with ethanol into 2-furaldehyde diethyl acetal, while Lewis acid sites promoted furfural transfer hydrogenation into FAL and EL and further conversion into γ-valerolactone. The methods developed in this work eliminate dipolar aprotic solvents and harsh acids used in catalyst synthesis and allow sustainable production of EL from biomass-related chemicals.
5−hydroxymethylfurfural (HMF), as one of the top ten important platform chemicals, can be used to produce 2,5−furandicarboxylic acid (FDCA), 2,5−dimethyl furan (DMF), levulinic acid, and other chemicals. An environmentally friendly system for the synthesis of sulfonated carbon materials from discarded masks has been proposed. A series of mask−based solid acid catalysts (bMC−SO3H) were prepared by a simple two−step process. Mechanochemical pretreatment (ball milling) of waste mask and sulfonated group precursor, followed by thermal carbonization under nitrogen gas, were used to synthesize sulfonated porous carbon. The total acid amount of the prepared bMC−SO3H was measured by the Boehm method, which exhibited 1.2–5.3 mmol/g. The addition of the sulfonated group precursor in the mechanochemical treatment (ball milling) process caused intense structure fragmentation of the discarded masks. These sulfonated porous carbons (bMC(600)−SO3H) as solid acid catalysts achieved fructose conversion of 100% and HMF yield of 82.1% after 120 min at 95 °C in 1−butyl−3−methylimidazolium chloride. The bMC−SO3H could be reused five times, during which both the HMF yield and fructose conversion were stable. This work provides a strategy for the synthesis of sulfonated carbon from discarded masks and efficient catalyzed fructose upgrading to HMF.
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