Carbonization of Cellulose in Supercritical CO2 for Value-Added Carbon

In this paper, carbonization of biomass in the presence of supercritical CO2 is investigated to obtain carbon solids with enhanced properties and potential to provide a sustainable pathway for high-value solid products which are currently resourced from expensive and carbon driven fossil-fuel routes. Carbonization of cellulose was carried out in supercritical CO2 at temperatures of 523 K and 623 K at similar to 100 bar pressure in a stirred reactor for 1-8 h of residence times. The obtained solid residue was characterized for morphology using scanning electron microscopy (SEM), surface graphitization using Raman spectroscopy, thermal stability using thermogravimetric analysis (TGA), and crystallinity using powder X-ray diffraction (XRD). The solid chars were found to be dominated by clusters of microspheres (<5 mu m), especially at temperatures of 623 K. Raman spectroscopy revealed the formation of graphitic crystallite units connected by sp(3) carbons (i.e., aliphatic) suggesting significant graphitization. G-band peak ratio was found to be highest for a residence time of 5 h for both the temperatures. TGA data revealed that higher carbonization temperature led to higher thermal decomposition peaks of the chars. The peak value of thermal decomposition ranged between 700 and 800 K for char obtained at 523 K and between 750 and 900 K for char at 623 K. The values were significantly higher than the decomposition peak cellulose at similar to 610 K. Proximate analysis results revealed significant increase of fixed carbon content compared with cellulose. Fixed carbon to volatile content ratios revealed increase from 0.052 in cellulose to values ranging from 1.4 to 4.3 making these chars similar in character to coal (with ranking of bituminous coal and petroleum coke). The net yield of solid chars from carbonization was around 50-66% depending upon the extent of carbonization. These results suggest this pathway to produce high yields of high-quality carbon solids with low volatile content, high thermal stability, and significant graphitization. The graphitized carbon offers potential applications in catalysis, electrode materials, pollutant absorption, and energy storage and solid fuels while avoiding drying to remove moisture unlike pyrolysis.