(Invited) Efficient Carbon Sulfonation Using Plasma in Contact with Dilute Sulfuric Acid

Abstract

Heterogeneous catalysis is a promising technology in biomass valorization because of its high reactivity, easy post-separation, and great recyclability compared to homogeneous catalysis. Various functionalized catalytic materials have emerged as promising alternatives to the homogeneous liquid acid catalyst, such as carbon materials, metal–organic frameworks, and metal nanoparticles. Among them, carbon materials such as carbon nanotube, graphene oxide, carbon black, and so on have been investigated as catalysts for cellulose hydrolysis to produce glucose.Carbon materials show considerable catalytic activity by functionalization with active acidic groups including hydroxyl groups (-OH), carboxyl groups (-COOH), and sulfonic groups (-SO3H). Hydroxyl and carboxyl groups on the carbon catalysts act as connection center to access cellulose, where sulfonic group serves as the active site to cut off the linkage between glucose units in cellulose. So, the surface modification of carbon materials with these functional groups, namely, carbon sulfonation process, becomes the determining step to producing efficient carbon acid catalysts. Conventional methods for the carbon sulfonation are the hydrothermal and reflux method, in which hazardous chemicals are necessary, such as concentrated sulfuric acid (95–98%), chlorosulfonic acid, fuming sulfuric acid, or 4-benzenediazoniumsulfonate under elevated temperature for dozens of hours. As a green and effective carbon sulfonation process, we have utilized gas–liquid interfacial plasma (GLIP).In the GLIP process, plasma is generated between the tips of multi-needles and the surface of dilute sulfuric acid which contains carbon particles in N2/Ar gas mixture. The optimized catalyst synthesized from graphene nanoplatelets by the GLIP process achieved a conversion rate of 41.5% with high glucose selectivity of 84.3%, which is superior to performance of catalysts synthesized by the hydrothermal method. The concentration of sulfuric acid, reaction temperature, and reaction time for the GLIP process were 1 M, 40℃, and 0.75 h, respectively while those of the hydrothermal method were 18 M, 200℃, and 24 h, respectively. Thus, the GLIP process is less hazardous and more efficient. Significantly, the catalyst synthesized by the GLIP process also exhibited superior recyclability of 95.9% in 3 times cycle tests for cellulose hydrolysis.Recently, we have investigated the mechanism of carbon sulfonation in GLIP processes. Active species produced near the gas–liquid interface such as ·OH, SO3, and HOSO2· probably play important roles in carbon sulfonation. We are trying to observe gas-phase active species by Fourier transform infrared spectroscopy and quadrupole mass spectrometer, and liquid-phase active species by electron spin resonance spectroscopy.