Cesium silicotungstate catalyzed solvent-free self-condensation of levulinic acid and its product identification.

The self-condensation of levulinic acid was studied using cesium silicotungstate (Cs_xH_4-xSiW₁₂O₄₀) catalyst without solvent. The Cs_xH_4-xSiW₁₂O₄₀ catalyst was characterized by FT-IR, Raman, XRD, NH₃-TPD, N₂ adsorption–desorption, SEM, and ICP. The results showed that the structure of Keggin was well retained in all samples of Cs_xH_4-xSiW₁₂O₄₀ catalyst. With a decrease of Cs content, the acid strength of Cs_xH_4-xSiW₁₂O₄₀ catalyst decreased, and thus the yield of cyclic C₁₀ products decreased accordingly, while that of linear C₁₀ products increased. The effect of reaction conditions on levulinic acid self-condensation was studied using Cs_2.5H_1.5SiW₁₂O₄₀ (H₂SO₄) catalyst. Under the conditions of catalyst amount of 15 wt.%, reaction temperature of 130 °C, and reaction time of 6 h, the conversion of levulinic acid was 53.8%, and the total selectivity of C₁₀ products were 83.1%. Through spectroscopic characterization and DFT calculation, the possible structures of C₁₀ products of levulinic acid aldol self-condensation were deduced. The results showed that levulinic acid condensed in two different paths to form two intermediates, 4-hydroxy-4-methyl-6-oxononanedioic acid and 3-acetyl-4-hydroxy-4-methylheptanedioic acid. The two intermediates were then subjected, respectively, to lactonization and intramolecular dehydration to three cyclic C₁₀ products: 5-(2-methyl-5-oxotetrahydrofuran-2-yl)-4-oxopentanoic acid, 3-(2-methyl-5-oxotetrahydrofuran-2-yl)-4-oxopentanoic acid, and 3-(3-acetyl-2-methyl-5-oxotetrahydrofuran-2-yl) propanoic acid, and to obtain the linear C₁₀ product, (E)-4-methyl-6-oxonon-4-enedioic acid. [ABSTRACT FROM AUTHOR]