Characterization of the microporosity in mesoporous adsorbents by hydrocarbon adsorption

The adsorption and separation of linear, monobranched, and dibranched C₅-C₉ alkanes on SBA-15 and PHTS were studied using pulse chromatography and by performing vapor phase breakthrough experiments at low, intermediate, and high degree of pore filling. These materials with their large mesopores had even higher adsorption enthalpies than Faujasite zeolites with pores of 12.3 Å. Higher entropy losses of alkane molecules were observed on these materials compared to the faujasites. Both SBA-15 and PHTS showed shape-selectivity, where linear alkanes were preferentially adsorbed over the branched molecules. Pure component adsorption isotherms indicated that hexane is slightly more adsorbed than 2,2-dimethylbutane at low partial pressure, whereas at higher partial pressures, the capacity for hexane was significantly higher than that of 2,2-dimethylbutane on both materials. Separation of binary (hexane, 2,2-dimethylbutane) and quarternary (pentane, 2-methylbutane, hexane, 3-methylpentane and 2,2-dimethylbutane) alkane mixtures was possible on both biporous materials. In all cases, the branched molecules eluted before their linear homologue. The separation factor was not affected by the degree of pore filling. Besides, both SBA-15 and PHTS materials showed similar separation factors, indicating that the plugs in PHTS only affected adsorption capacity but not the selectivity between n- and iso-alkanes. This is an abstract of a paper presented at the 2006 AIChE National Meeting (San Francisco, CA 11/12-17/2006).