Efficient Synthesis of Liquid Fuel Intermediates from Furfural and Levulinic Acid via Aldol Condensation over Hierarchical MFI Zeolite Catalyst
A water-tolerant, basic, and hierarchical MFI zeolite catalyst was synthesized and applied in the aldol condensation reaction between biomass-derived furfural and levulinic acid. The results showed that the addition of poly(diallyl dimethylammonium chloride) significantly affected the textural and acid–base properties of hierarchical zeolite, which subsequently influenced the catalytic performance of hierarchical zeolite. In the aqueous phase, potassium-modified, hierarchical MFI zeolite (K/H-MFI-n) was more active for aldol condensation between furfural and levulinic acid than the potassium-modified, conventional MFI zeolite (K/MFI). This was ascribed to higher basic sites density and improved diffusion limitation of K/H-MFI-n. A 70.6% yield of aldol condensation product was achieved with a complete conversion of furfural at 100 °C for 9 h by K/H-MFI-0.6. However, only 27.4% yield of aldol condensation product with 55.1% furfural conversion was obtained by K/MFI at the same condition. Two major isomeric aldol products, β-furfurylidenelevulinic acid and δ-furfurylidenelevulinic acid (β-FDLA and δ-FDLA), were obtained after acidification. K/H-MFI-n displayed an enhanced selectivity (54.9%) to δ-FDLA, owing to the stronger basicity of K/H-MFI-n. However, K/MFI showed a preferred selectivity to β-FDLA (42.7%), owing to the dominant Lewis acidity. Recyclability research showed that the catalytic performance of potassium-modified, hierarchical MFI zeolite was acceptable after five runs.
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