Sulfonated carbon nanotubes as catalysts for the conversion of levulinic acid into ethyl levulinate

• Activity of sulfonated carbon nanotubes in levulinic acid esterification correlates with acidity.
• Levulinc acid strongly adsorbs in the active sites of the sulfonated carbon nanotubes.
• Amberlyst-15 presents a good activity and reusability.

Multiwall carbon nanotubes were sulfonated at different temperatures (150, 180, 210, 230, 250 and 280 °C) and used as catalysts in the esterification of levulinic acid with ethanol. The materials sulfonated between 150 and 230 °C presented almost the same acidity (measured by ammonia chemisorption), which was higher than that obtained for samples sulfonated at 250 and 280 °C. Raman spectroscopy revealed that the treatment with sulfuric acid did not lead to the destruction of the carbon nanotubes structure, even for the higher temperature treatment. The activity results have shown that the nanotubes sulfonated below 250 °C presented a specific activity higher than those sulfonated at 250 and 280 °C. The association of these results with those obtained by temperature-programmed desorption of ammonia indicate that the activity in the esterification reaction is related to the number of acidic sites that desorb ammonia in temperatures around 220 °C. Temperature-programmed desorption experiments suggest that there is a strong adsorption of the levulinic acid on the active sites therefore not allowing the reuse of the materials.

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