CO₂ selectivity in CO₂:CH₄ and CO₂:N₂ mixtures on carbon microfibers (CMFs) and carbon microspheres (CMSs)

The CO₂ selectivity in CO₂:CH₄ and CO₂:N₂ mixtures has been studied in carbon materials; carbon microspheres (CMSs) and carbon microfibers (CMFs). These materials have different morphologies and textural properties but with similar chemical composition, which was demonstrated by SEM, nitrogen adsorption, and XPS techniques. The isosteric enthalpy of adsorption shows that the interaction between the CO₂ molecule and CMFs releases a higher energy than CMSs. CMSs have a higher surface area and thus superior CO₂ adsorption only at pressures above 6 bar, for example, i) at 323 K and 1 bar, 1.43 mmol CO₂ (CMFs) and 0.82 mmol CO₂ (CMSs), and ii) at 323 K and 10 bar, 3.24 mmol CO₂ (CMFs) and 3.70 mmol CO₂ (CMSs). At 1 bar, CMFs is better than CMSs, improving it by 40 %. And at 10 bar, although CMSs adsorb more than CMFs, it is only 14 % more. In addition, CMFs have a higher microporosity, which allows them to exhibit better CO₂ selectivity in CO₂:CH₄ mixtures, even at relatively high pressures (5.4 mmol CO₂ / mmol CH₄ at 298 K and 1 bar, 4.6 mmol CO₂ / mmol CH₄ at 323 K and 10 bar). These values are like those reported in the literature for carbon materials synthesized by more complex and expensive methods.