N2/Ar plasma-induced surface sulfonation on graphene nanoplatelets for catalytic hydrolysis of cellulose to glucose

Highlights: • N₂/Ar plasma induces active species of SO₃, H, OH to form acid groups on GNPs. • The addition of Ar gas exerts a positive effect on improving the sulfonation performance. • The balance of physical/chemical effect is the crux for GLIP sulfonation chemistry. • GLIP-GNPs exhibited supreme recyclability (95.9%) in three cycle tests. The surface functionalization of graphene nanoplatelets catalysts (GNPs) with OH, COOH, and SO₃H groups for cellulose hydrolysis was investigated via gas–liquid interfacial plasma (GLIP) sulfonation system under 1 M H₂SO₄. The sulfonation chemistry of the GLIP process was examined under different gas ratios of N₂/Ar (100%N₂, 70%N₂–30%Ar, 30%N₂–70%Ar). The balance between plasma-induced physical activation and chemical reaction plays an essential part in the sulfonation performance. Experiments on acidic group determination and catalysis ability test demonstrated that the addition of 30% Ar to N₂ exerts a positive effect on enhancing the sulfonation activity and catalytic stability of GNPs. The 70%N₂–30%Ar-GNPs shown higher total acid and sulfonic acid density of 5.56 and 0.45 mmol g⁻¹, respectively. The optimized catalyst achieved a conversion rate of 41.5% with high glucose selectivity of 84.3%. Sulfonation of GNPs over traditional hydrothermal method under 18 M H₂SO₄ at 200 °C for 24 h further reflected the "less hazardous" and "high efficiency" properties of GLIP sulfonation in 1 M H₂SO₄ for 1 h. Significantly, the GLIP-GNPs also exhibited superior recyclability of 95.9% in 3 times cycle tests for cellulose hydrolysis, which is much higher than that of H-GNPs (49.5%).