Pore surface fractal characterization in waste cotton-derived carbon materials and the effect of pyrolysis conditions on the level of sulfonation

Waste cotton-derived carbon materials were synthesized using various chemical activators, and their performance as solid acid catalyst supports was studied. The pore surface structures of the carbon materials were quantitatively characterized by surface fractal dimension, and the relationship between pore surface structure and the density of catalyst was also investigated. The pore sizes were grouped into four ranges, including fractal dimensions D1 (0.32 nm to 2.0 nm) and D2 (2.0 nm to 50 nm), as revealed by N2 adsorption and Frenkel-Halsey-Hill fractal theory. In addition, D3 (50 nm to1000 nm) and D4 (1000 nm to 200,000 nm) were revealed by mercury intrusion porosimetry and the Friesen-Mikula fractal theory. The surface fractal dimension D was found to be proportional to the density of SO3H groups from sulfonation at the same carbonization temperature. A larger D value corresponded to the exposure of more active sites, which was more favorable for improving the density of SO3H groups. With the rise of carbonization temperature, the surface fractal dimension also increased. However, the graphitization structure of carbon materials increased with the rise of carbonization temperature, which reduced the active sites of SO3H groups. Finally, the solid acid catalysts prepared via the sulfonation process were used in the esterification reaction of levulinic acid and n-butanol.