Green Sulfonation of Carbon Catalysts via Gas–Liquid Interfacial Plasma for Cellulose Hydrolysis

Sulfonated carbon catalysts exhibit excellent catalytic performance for biomass transformation, yet most currently used sulfonation approaches still rely on concentrated sulfuric acid or chlorsulfuron acid and harsh reaction conditions. Herein, we investigate a “green” sulfonation system via a novel gas–liquid interfacial plasma (GLIP) process conducted in 1 M H2SO4 at 25 °C for carbon materials with diverse surface properties, including carbon nanotubes (CNTs), reduced graphene oxide (RGO), and activated carbon (AC). The sulfonic acid and total acid group densities of carbon acid catalysts are in the ranges of 0.36–0.59 and 3.47–3.63 mmol g–1, respectively, which are comparable to those of other carbon acid catalysts reported in the literature. In terms of the catalytic ability of cellulose conversion, GLIP-sulfonated carbon catalysts exhibit catalytic activity similar to that of those prepared by a hydrothermal method with concentrated H2SO4 under an identical cellulose hydrolysis process. Further, all GLIP-sulfonated carbons exhibit high recyclability of 96.6–98.2% after three consecutive runs. The performance of GLIP sulfonation is attractive thanks to the plasma-induced reactions at the gas–liquid interface, by which plasma discharge-induced active species (•OH, •H, SO3, electrons, etc.) graft the defects and edges of carbon materials directly without causing severe damage to the carbon structure. This study presents an innovative, “green”, and universal sulfonation process for synthesizing carbonaceous acid catalysts for biomass transformation.