Carbon sulfonation by plasma in contact with liquid to synthesize carbon catalysts for biofuel production

Heterogeneous catalysis is a promising technology for the conversion of biomass to biofuels due to its high reactivity, ease of post-reaction separation, and excellent recyclability compared to homogeneous catalysis. Various functionalized catalytic materials have emerged as viable alternatives to homogeneous liquid acid catalysts. Carbon materials—such as carbon nanotubes, graphene nanoplatelets, and carbon black—have been investigated as catalysts in biofuel synthesis processes, particularly in the hydrolysis of cellulose to produce glucose.

Carbon materials exhibit significant catalytic activity when functionalized with acidic groups such as hydroxyl (-OH), carboxyl (-COOH), and sulfonic (-SO₃H) groups. The hydroxyl and carboxyl groups on carbon catalysts serve as anchoring sites for cellulose, while sulfonic groups act as active sites that cleave the glycosidic bonds between glucose units in cellulose. Therefore, surface modification of carbon materials with these functional groups—referred to as the carbon sulfonation process—is a critical step in the production of efficient carbon-based acid catalysts.

Hydrothermal method, a conventional carbon sulfonation method, requires concentrated sulfuric acid and is conducted at elevated temperatures over extended durations.

As a greener and more efficient alternative, we have employed plasma generated in contact with dilute sulfuric acid for carbon sulfonation. Recently, we have investigated the underlying mechanism of carbon sulfonation in the plasma processes. Reactive species generated near the gas–liquid interface, such as OH radicals, SO₃, and HOSO₂ radicals, are believed to play crucial roles in the sulfonation process. Additionally, SO₄⁻ radicals, produced via the reaction between OH radicals and HSO₄⁻ in the solution, are also key contributors to effective carbon sulfonation. We are currently working to scale up the treatment capacity of this process by employing dielectric barrier discharge systems.