Effect of hydrophilic/hydrophobic properties of carbon materials on plasma-sulfonation process and their catalytic activities in cellulose conversion

The plasma sulfonation process has shown promising results on modifying carbon materials to solid acid catalysts for cellulose conversion under dilute acid solution. However, surface hydrophilic/hydrophobic properties have a great impact on interfacial interactions between a material and aqueous reaction media. In order to explore the effects of hydrophilic/hydrophobic properties during the plasma sulfonation process, we conducted sulfonation under a 1 M sulfuric acid solution on two different carbon materials: hydrophilic carbon black (CB) and hydrophobic cup-stacked carbon nanotube (CSCNT). The total acidic and sulfonate group densities were 4.4 and 2.1 mmol g⁻¹, and 1.5 and 0.2 mmol g⁻¹, in plasma-sulfonated CB and CSCNT, respectively. The degree of sulfonation was strongly related to the surface chemical properties of the original carbon materials. A hydrophilic surface provided better interactions between the water and carbon surface, which increased the kinetics of the sulfonation reactions. As a result, the total densities of the acidic groups on the hydrophilic CB were much higher compared to that on the hydrophobic CSCNT under similar plasma sulfonation conditions. The cellulose conversion and glucose selectivity of plasma-sulfonated CB were 40% and 80%, respectively, which was significantly greater than that of plasma-sulfonated CSCNT (cellulose conversion of 6% and glucose selectivity of 64%). Nevertheless, both catalysts exhibited over 97% of their original catalytic activities after recycling. The results indicate that the hydrophilicity of original carbon materials is a key factor in the plasma sulfonation process.