Comparison of molecular sieve properties in microporous chars from low-rank bituminous coal activated by steam and carbon dioxide

A Polish high volatile bituminous coal was subjected to air oxidation, carbonization and gaseous activation. The activation with steam and carbon dioxide was performed to low levels of burn-off: 5-25%. Sorption measurements of CO2, as well as of organic vapours with increasing molecular sizes (CH2Cl2, C6H6, C6H12, CCl4) were applied to evaluate the porous structure of the activated chars. Steam and carbon dioxide develop the microporous system according to the same mechanism-opening (burn-off 5-10%) and then widening of the narrow micropores. For char from the oxidized coal mainly a widening of the narrow micropores takes place. Comparing both activating agents, it was stated that for steam greater micropore volumes were obtained. This was confirmed by other authors for chars from brown coal and coking coal, but was in disagreement with the results for olive stones and carbon fibres. This would indicate the importance of the carbon precursor in the formation of the porous structure of carbon materials by different activating agents. In the region of studied burn-offs, among the micropore sizes useful for separation of gases and vapours with small molecules, micropore volumes with widths close to 0.4-0.5 nm are dominating. At very low burn-offs (5-10%), steam activation renders greater micropore volumes within these sizes, than does activation with carbon dioxide. But with increasing burn-off (15-25%), this phenomenon becomes reversed. This effect is still more accentuated for the preoxidized coal.