From Carbon Molecular Sieves to VOCs filters: Carbon gels with tailored porosity for hexane isomers adsorption and separation

Resorcinol-formaldehyde aerogel and xerogel were tested as selective adsorbents of hexane isomers under static and dynamic conditions. The aim of this work is to further study the carbon molecular sieve behaviour of some of these samples for linear and branched hydrocarbons separation in gas phase, and the possibility of tuning the adsorptive properties of the xerogel by means of physical activation with carbon dioxide. While the aerogel presented a larger micropore volume, the micropore mean width was narrower than in the case of the xerogel, therefore its behaviour as selective adsorbent of n-hexane was better; in fact, equilibrium adsorption demonstrated that the carbon aerogel acts as a carbon molecular sieve with respect to the four isomers tested, being able to separate all of them. The ratio for adsorbed amounts at saturation obtained from Langmuir resulted 16.1 for n-hexane, 4.4 for 3-methylpentane and 2.2 for 2,3-dimethylbutane compared with the one for 2,2'-dimethylbutane. These values decreased significantly when xerogel was used as adsorbent (1.7 for n-hexane, 1.5 for 3-methylpentane and 1.3 for 2,3-dimethylbutane) and for the carbon dioxide activated samples. In such a way, when adsorption equilibrium isotherms for the four isomers onto the adsorbent with a highly developed microporosity (activated up to 40%) were assessed, there were almost no differences between them. The excellent performance of the aerogel in the separation of n-hexane and 2,2'-dimethylbutane was finally proven by competitive dynamic adsorption experiments, while the carbon dioxide activated samples showed great potential for VOCs removal due to their enhanced capacity.