Facile and sustainable functionalization of graphene layers with pyrrole compounds

A facile and sustainable functionalization of graphene layers was performed with pyrrole compounds (PyC) prepared through the Paal–Knorr reaction of a primary amine with 2,5-hexanedione. A good number of primary amines were used: hexanamine, dodecanamine, octadecanamine, 2-aminoacetic acid, 2-amino-1,3-propanediol, 3-(triethoxysilyl)propan-1-amine. The reactions were characterized by good yield, up to 96 %, and indeed satisfactory atom efficiency, up to 80 %. The functionalization of graphene layers was obtained by mixing PyC with a high surface area graphite and heating at a temperature range from 130 °C to 150 °C for 3 h. The yield of functionalization reaction was larger than 60 % and also up to about 90 % for the pyrrole compounds from dodecanamine and 2-amino-1,3-propanediol, respectively. The cycloaddition reaction between the graphene layers and the pyrrole compound, oxidized in two position, is proposed as working hypothesis to account for such efficient functionalization. Raman spectroscopy revealed that the structure of the graphitic substrate remained substantially unaltered, after the reaction. Stable dispersions of HSAG adducts with different PyC were prepared in solvents with different solubility parameters and HRTEM analysis showed the presence of aggregates of only few layers of graphene. Qualitative results of dispersion tests were used to calculate the Hansen sphere for the HSAG adduct with the pyrrole compound based on dodecanamine so to provide a first estimate of its Hansen solubility parameters. This work paves the way for the facile and sustainable modification of the solubility parameters of graphene layers and for the predictive assessment of their compatibility with different environments.