Activation of cellulose with CO2 and/or H2O: Evolution of functionalities of the biochar and environmental impacts

Abstract

CO2 and H2O are two important activators in production of activated carbon (AC) via physical activation method. They could oxidize the carbonaceous species in biochar or biomass via varied mechanisms and might also impact environments as well as the pore structures and evolution of functionalities of AC in different ways. In this study, the activations of the biochar from pyrolysis of cellulose with H2O, CO2 or mixed H2O/CO2 were performed. The results demonstrated that H2O as an activator led to higher weight loss than that of CO2 during the activation, originating from its higher efficiency for oxidation of the aliphatic structures in biochar to –OH and their further removal via deoxygenation or dehydration. Activation with CO2 made the AC more oxygen-rich, due to the retainment of abundant oxygen-containing functionalities, which suppressed further oxidation of carbon species and led to formation of main micropores. The higher capability of H2O for deoxygenation made the AC more carbon-rich with much more developed pore structures as well as higher proportion of mesopores and thus the superior capability for adsorption of phenol. In addition, during the activation in either CO2 or H2O, the unsaturated aldehyde/ketone and C-O-C structures was thermally more stable than lactones, saturated aliphatic acids and aliphatic aldehydes. The life cycle analysis (LCA) suggested that CO2 activation showed lower impact on environment by reducing emissions and consumption of fossil fuel.