Single step conversion of cellulose to levulinic acid using temperature-responsive dodeca-aluminotungstic acid catalysts

Abstract

The conversion of cellulose into platform chemicals is essential for the sustainable development of the chemical industry. With this aim, the single step conversion of cellulose to industrially important levulinic acid (LA) using heteropolyacids (HPAs) as catalysts has been investigated. A new series of heteropolyacids (HPAs) [(CH₃)₃NCH₂CH₂OH]_nH_5−nAlW₁₂O₄₀ (abbreviated as Ch_nH_5−nAlW₁₂O₄₀, n = 0–5) have been synthesized by a precipitation/ion exchange method using choline chloride and H₅AlW₁₂O₄₀ as precursors. The resulting HPA nanohybrids exhibited novel switchable properties based on temperature variation due to the incorporation of the choline cation, which dissolved in the reaction mixture at higher temperatures to form a homogeneous catalytic system and then precipitated spontaneously from the mixture at room temperature. The three synergistic effects of temperature-stimulus combined with its dual Lewis and Brønsted acidity endowed ChH₄AlW₁₂O₄₀ with more efficiency for catalyzing the conversion of cellulose into glucose in water with 75.9% yield and 94.8% conversion at 140 °C for 3 h. Meanwhile, MIBK (methyl isobutyl ketone), a green co-solvent, produced the highest reported yield of LA directly from cellulose in a highly effective single phase conversion strategy, with 74.8% yield and 98.9% conversion in one pot, which is the best result compared to previous reports. Moreover, the recycling of such HPA catalysts was easily achieved without structural changes or loss of weight by lowering the reaction temperature. In addition, the conversion of cellulose to glucose could be promoted by microwave assistance.