Investigation of the sulfonation mechanism by gas–liquid interfacial plasma under atmospheric pressure conditions

Although sulfonated carbon catalysts are considered promising solid acid catalysts for cellulose conversion, most carbon sulfonation processes require concentrated sulfuric acid (18 M) at elevated temperatures. This work investigates a novel sulfonation mechanism for carbon acid catalysts via a gas–liquid interfacial plasma sulfonation system under atmospheric pressure conditions with a dilute sulfuric acid solution (1 M) at 38 °C, and the by-products of the plasma sulfonation process were investigated by in-situ and ex-situ diagnosis. The results show that a high gas temperature (>1050 K) around the plasma allows H 2 SO 4 droplet decomposition, and active species (•OH, SO 3 , and HOSO 2 •) generated at the gas–liquid interface were grafted on the defects of the carbon materials and subsequently formed sulfonic acid groups (0.36 mmol g −1 ) and total acid groups (4.16 mmol g −1 ) on the carbon network. This study aimed to provide significant insight into the understanding of the sulfonation mechanisms of an emerging plasma-based process for carbon acid catalysts, which is important for the further development of an environmentally friendly sulfonation process for acid catalysts for biomass conversion.