Tuning porosity of coal-derived activated carbons for CO2 adsorption

A simple method was developed to tune the porosity of coal-derived activated carbons (ACs), which provided a model adsorbent system to investigate the volumetric CO2 adsorption performance. Specifically, the method involved the variation of the activation temperature in a K2CO3 induced chemical activation process which could yield ACs with defined microporous (<2 nm, including ultra-microporous <1 nm) and meso-micro-porous structures. CO2 adsorption isotherms revealed that the microporous AC has the highest measured CO2 adsorption capacity (6.0 mmol·g–1 at 0 °C and 4.1 mmol·g–1 at 25 °C), whilst ultra-microporous AC with a high packing density exhibited the highest normalized capacity with respect to packing volume (1.8 mmol·cm−3 at 0 °C and 1.3 mmol·cm–3 at 25 °C), which is significant. Both experimental correlation analysis and molecular dynamics simulation demonstrated that (i) volumetric CO2 adsorption capacity is directly proportional to the ultra-micropore volume, and (ii) an increase in micropore sizes is beneficial to improve the volumetric capacity, but may lead a low CO2 adsorption density and thus low pore space utilization efficiency. The adsorption experiments on the ACs established the criterion for designing CO2 adsorbents with high volumetric adsorption capacity.

Keywords coal-derived activated carbons, porosity, CO2 adsorption, molecular dynamics