Effect of the surface oxidation of carbon nanotubes on the selective cyclization of citronellal

• Oxidation of commercial carbon nanotubes in liquid media and air was studied.
• The samples were tested on the cyclization of citronellal.
• The previous ball-milling treatment increases the carboxylic acid concentration.
• The activity is ascribed to the carboxylic groups and the selectivity to isopulegol seems to be related to the pore size.

Commercial carbon nanotubes have been oxidized with HNO3, H2O2 and air and they have been tested on the cyclization of citronellal. The materials have been characterized by N2 adsorption/desorption isotherms, transmission electron microscopy, X-ray diffraction, temperature programmed desorption, potentiometry titration and X-ray photoelectron spectroscopy. The HNO3-treated samples displayed the largest concentration of carboxylic acids which increased by increasing the oxidation time and by a previous ball-milling treatment. The carboxylic acid groups were found to be the active groups on the reaction and the activity increased by increasing its content. The selectivity to isopulegols was higher (98%) over the most active samples (activity 0.20–0.68 mmol/g min), which has been explained by their smaller pore size caused by the oxidation treatment. The stability of the most active CNT sample was also proved by recycling the catalyst in 3 cycles.

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