抄録
Heterogeneous catalysis is a promising technology in biomass valorization owing to the high reactivity, facile post-separation process, and exceptional catalyst recyclability, which outperform homogeneous catalysts. Various functionalized catalytic materials have emerged as promising substitutes for homogenous liquid acid catalysts. These materials include carbon materials, metal–organic frameworks, and metal nanoparticles. Among these, carbon materials such as carbon nanotube, graphene oxide, and carbon black have been investigated as catalysts for cellulose hydrolysis to produce glucose.
Carbon materials exhibit notable catalytic activity upon functionalization with active acidic groups like hydroxyl groups (-OH), carboxyl groups (-COOH), and sulfonic groups (-SO3H). For carbon catalysts, hydroxyl and carboxyl groups play pivotal roles as connection points, facilitating access to cellulose, while sulfonic groups serve as active sites, mediating the cleavage of the linkages between the glucose units in cellulose. Consequently, the surface modification of carbon materials with these functional groups, i.e., the carbon sulfonation process, is essential for producing efficient carbon acid catalysts. Conventional methodologies for carbon sulfonation include hydrothermal and reflux methods that require the use of hazardous chemicals such as concentrated sulfuric acid (95–98%), chlorosulfonic acid, fuming sulfuric acid, or 4-benzenediazoniumsulfonate under elevated temperatures for dozens of hours. Consequently, as an eco-friendly and efficient carbon sulfonation process, the application of gas–liquid interfacial plasma (GLIP) has been investigated.
Within the framework of the GLIP process, plasma is generated between the tips of multi-needles and the surface of dilute sulfuric acid, which contains carbon particles in a N2/Ar gas mixture. The optimal catalyst synthesized from graphene nanoplatelets by the GLIP process achieved 41.5% conversion, with a high glucose selectivity of 84.3%, which is superior to the performance of catalysts synthesized by the hydrothermal method. The GLIP process required a sulfuric acid concentration of 1 mol/L, reaction temperature of 40°C, and reaction duration of 0.75 h, whereas the hydrothermal method required a sulfuric acid concentration, temperature, and reaction duration of 18 mol/L, 200°C, and 24 h, respectively. Thus, the GLIP process is not only a safer, but also a more proficient alternative. Remarkably, the catalyst synthesized by the GLIP process exhibited superior recyclability of 95.9% over three cycles for cellulose hydrolysis.
Of late, we investigated the mechanism of carbon sulfonation in the GLIP processes. Active species produced near the gas–liquid interface, such as •OH, SO3, and HOSO2•, probably play important roles in carbon sulfonation. Efforts are being taken to identify the gas-phase active species using Fourier transform infrared spectroscopy and quadrupole mass spectrometry and the liquid-phase counterparts using electron spin resonance spectroscopy.
