(23b) Characterization of the Microporosity in Mesoporous Adsorbents by Hydrocarbon Adsorption

Mesoporous molecular sieves have attracted much attention because of their large pore size and related accessibility to larger molecules as compared to zeolites. Unfortunately, common mesoporous materials such as MCM-41 lack the catalytic shape selectivity, adsorption hyperselectivity and hydrothermal and mechanical stability of zeolites. On the other hand, zeolites with their microporous structure often show intracrystalline diffusion limitations, as a result of difficult transport of reactants to the active sites in the channels or back-diffusion of products. Thus, generation of microporosity within the pore walls of mesopores might create an ideal bimodal pore system for catalysts and adsorbents enhancing the diffusional properties, because molecules are first transported through mesopore channels and then strongly adsorbed in the micropores. A particular class of such mesoporous materials is formed by SBA-15 and PHTS, which have a much better mechanical hydrothermal en mechanical stability as compared to MCM-41. SBA-15 has large mesopores containing micropores that interconnect the mesopores, while PHTS; a plugged variant of SBA-15, contains extra silica nanoparticles (plugs) in the mesoporous channels.

In this work, we have studied the adsorption and separation of linear, monobranched and dibranched C5-C9 alkanes on SBA-15 and PHTS using pulse chromatography and by performing vapour phase breakthrough experiments at low, intermediate and high degree of pore filling. Remarkably, these materials with their large mesopores have even higher adsorption enthalpies than Faujasite zeolites with pores of 12.3 Å. This is explained by the existence of micropores in SBA-15 and PHTS silicates that act as preferential adsorption sites at low coverage, thus contributing to this higher interaction energy. Also higher entropy losses of alkane molecules were observed on these materials compared to the faujasites, again pointing at a strong confinement at very low partial pressure. Both SBA-15 and PHTS show shape-selectivity, where linear alkanes are preferentially adsorbed over the branched molecules.

Pure component adsorption isotherms indicate that hexane is slightly more adsorbed than 2,2-dimethylbutane at low partial pressure, whereas at higher partial pressures, the capacity for hexane is significantly higher than that of 2,2-dimethylbutane on both materials. However, when plotted as a function of reduced pressure (p/psat), isotherms of the linear and branched molecules coincide exactly, showing that a pore size induced condensation effect rather than a pure physisorption effect causes differences in adsorption between both components. This effect occurs well below the point at which capillary condensation is expected. At low loading, PHTS shows a higher capacity for hexane and 2,2-dimethylbutane compared to SBA, which is attributed to strong adsorption of both components in the microporous plugs in PHTS.

Separation of binary (hexane, 2,2-dimethylbutane) and quinary (pentane, 2-methylbutane, hexane, 3-methylpentane and 2,2-dimethylbutane) alkane mixtures is possible on both biporous materials. In all cases, the branched molecules elute before their linear homologue. The separation factor is not affected by the degree of pore filling. Besides, both SBA-15 and PHTS materials show similar separation factors, indicating that the plugs in PHTS only affect adsorption capacity but not the selectivity between n- and iso-alkanes.