Alkaline-doped multiwall carbon nanotubes as efficient catalysts for the Knoevenagel condensation

Abstract

A series of alkaline doped multiwall carbon nanotubes, CNTOM (with M = Li, Na, K, Cs) has been prepared by wetness impregnation with alkaline acetates and subsequent pyrolysis of commercial multiwall carbon nanotubes previously oxidized with nitric acid, characterized by different physico-chemical techniques and tested as catalysts in the Knoevenagel condensation between benzaldehyde and different active methylene compounds. As determined by different techniques, caesium has been incorporated in a less extent than the rest of alkalines. The SBET values decrease in the CNTOM with respect to the raw material, mainly due to the blockage of micropores and small mesopores by the metallic phase formed. As deduced from PZC values, the character of the surface of nanotubes changes from acid for the oxidized sample to basic for the alkaline doped nanotubes, the basic character decreasing with the size of the alkaline cation. The order of activity observed in the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate seems to be correlated mainly with the amount of basic centres, CNTOCs being the less active catalyst due to its lower metal content. Conversion values of benzaldehyde comprised between 67 and 89% were achieved in only 120 min. [Publisher abstract]