Gas adsorption over
zeolites is at the basis of important applications
of this class of microporous crystalline solids, notably as separation
media and catalysts, but it may also be complex and not straightforward
to understand. Here we report that for temperature below 323 K propane
adsorption on the small-pore pure-silica zeolite ITW exhibits a clear
step (pseudosaturation). This is absent in the case of propene and
the other small linear alkanes. An intermediate plateau, clearly observed
in the 293 K isotherm, always occurs when one molecule of propane
is loaded in every other cage, i.e., at half-saturation. The simulation
results show a swelling of the ITW structure upon propane adsorption.
The strong dependence of available pore volume on the adsorbate loading
level implies that adsorption cannot occur on the void structure while
saturation can only be reached on highly loaded structures. To account
for this unprecedented adsorption phenomenon, we propose the term
“guest-modulated effect”
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