The solvent effect on the sidewall functionalization of multi-walled carbon nanotubes with maleic anhydride

The Diels–Alder cycloaddition of maleic anhydride to multi-walled carbon nanotubes (MWCNT) was performed in dimethyl sulfoxide (DMSO) (190 °C, 24 h) and in 2-chorotoluene (150 °C, 48 h). The functionalized MWCNT were characterized by thermogravimetry, elemental analysis, X-ray photoelectron spectroscopy, and potentiometric titration allowing a clear perception of the solvent involvement on the functionalization reaction. The MWCNT functionalized in DMSO presented acidity arising mostly from reaction with the solvent. The MWCNT modified in 2-chlorotoluene did not evidence chemical transformations that could be assigned to the solvent alone. The cycloaddition of maleic anhydride to anthracene, used as model compound for the MWCNT surface, confirmed that the reaction was fast in DMSO. Hydrolysis of the anhydride was catalyzed by the MWCNT presence, but the cyclic anhydride structure was restored upon heating in this solvent at 185 °C. The work also confirmed the cycloaddition of maleic anhydride to CNT and DMSO was identified as the best solvent for the reaction, at 190 °C.