posted on 2005-03-15, 00:00authored byEllen Van Bavel, Vera Meynen, Pegie Cool, Kurt Lebeau, Etienne F. Vansant
Plugged hexagonal templated silica (PHTS) materials are synthesized using a high TEOS/EO20PO70EO20 ratio in the SBA-15 synthesis. This generates internal microporous nanocapsules or plugs in part
of the channels, which could be inferred from the two-step desorption branch. These materials exhibit a
tunable amount of open and plugged pores and a very high micropore volume (up to 0.24 mL/g) and are
more stable than the conventional micellar templated structures known so far. In this study the adsorption
properties of PHTS are investigated and compared to those of its plug-free analogue SBA-15. For this
purpose nitrogen, n-hexane, n-heptane, c-hexane, 3-methylpentane, 1-hexene, and water were adsorbed
on SBA-15 and PHTSs with a different ratio of open and plugged mesopores. The adsorption of n-hexane,
c-hexane, n-heptane, and 3-methylpentane on SBA-15 and PHTS-A demonstrated that the presence of the
plugs had an effect on the uptake of adsorbate in the low relative pressure region, the position of the
capillary condensation step, and the total adsorbed amount of adsorbate. The results showed that n-heptane
and 3-methylpentane cannot access part of the micropore system of SBA-15 and PHTS-A. Adsorption of
c-hexane and n-hexane on PHTS-A indicated that not only the kinetic diameter but also the shape of the
molecule is an important factor for being able to be adsorbed into the micropores or past the plugs. Moreover,
these two adsorbates were the most efficient in filling up the available pore volume. From the adsorption
of n-hexane on PHTSs with a different ratio of open and plugged pores, it was concluded that the size of
the plugs differed, which depends on the synthesis conditions. Water adsorption isotherms proved SBA-15
and PHTS-B to be more hydrophobic than PHTS-A. n-Hexane, 1-hexene, and toluene were adsorbed on
SBA-15 and the PHTSs to investigate the influence of the polarity of the adsorbate. The isotherms showed
higher uptakes for polar adsorbates on more hydrophobic materials and vice versa.