A new methodology to probe shape selectivity in porous adsorbents

Low coverage adsorption properties of C5-C9 n- and iso-alkanes, obtained from pulse chromatographic experiments, were used to characterize microporous and mesoporous adsorbents. Zeolites Beta and ZSM-5, the clay material Saponite, the mesoporous solid MCM-48, and also Silica gel and Alumina as "amorphous" structures were studied. On all evaluated materials, iso-alkanes have lower Henry constants and adsorption enthalpies compared to the linear alkanes. The relationship between the Henry constant and some typical properties of the molecules (boiling point, vapour pressure, molecular surface and Van der Waals volume) was evaluated. It was found that MCM-48, which contains only mesopores and lacks micropores, shows a unique relation when the logarithm of the Henry constants of linear and branched alkanes is plotted as a function of the logarithm of the vapour pressure. When shape selective micropores are present, a deviation of this tendency is observed for branched alkanes, as a result of steric constraints imposed on the adsorption of the more bulky species. (C) 2008 Elsevier Inc. All rights reserved