Shape selectivity in adsorption of microporous/mesoporous hybrid n- and iso-alkanes on a Zeotile-2 and mesoporous MCM-48

The adsorption of linear and branched C5-C9 alkanes in the temperature range 50-250 degrees C on mesoporous MCM-48 material and its microporous/mesoporous variant Zeotile-2 at low surface coverage is investigated using the pulse chromatographic technique. On MCM-48, the differences in adsorption between linear and branched alkanes are merely a result of differences in volatility, indicating that the MCM-48 material does not present shape-selective adsorption sites. On Zeotile-2, there is a preferential adsorption of linear over branched alkanes. The difference arises from a difference in adsorption entropy rather than enthalpy. Upon their adsorption on Zeotile-2 branched alkanes lose relatively more entropy than their linear isomers do. Zeolitic molecular pockets embedded in the walls of the mesoporous Zeotile-2 impose steric constraints on the bulky isoalkanes. Zeotile-2 combines adsorption properties from microporous and mesoporous materials. Compared to the nitrogen molecule, linear and branched C5-C9 alkanes are superior probes for investigating micropores and micropockets in hierarchical materials.