High Resolution Sorption Studies of Argon and Nitrogen on Large Crystals of Aluminophosphate AlPO4-5 AND ZEOLITE ZSM-5

Abstract

High resolution adsorption (HRADS) with argon and nitrogen at 77 K in the pressure range of 10−6 < p/p° < 0.5 were performed on large crystals of zeolite ZSM-5 (180 μm) and aluminophosphate AlPO4-5 (150 μm) using a novel volumetric device. Multi-step isotherms of both adsorptives on ZSM-5 could be observed for the first time. The adsorption followed by low temperature microcalorimetry resulted in distinct exothermic signals at the steps in the adsorption isotherms. Based on the results of atom-atom potential energy calculations (AAP) as well as independent model building it was shown that 24 ‘kinetic’ adsorbate molecules can be filled into a ZSM-5 unit cell. Experimental results are reasonably interpreted assuming a primary filling of narrow channels and a secondary adsorption in the wider channel intersections. Localized adsorption is understood as a possible filling mechanism. In ZSM-5 and nitrogen as adsorbate there is evidence for a transition of fluid-like to solid-like adsorbate phase. AlPO4-5 behaves as a homogeneous sorbent with a micropore capacity of four molecules per unit cell for argon and nitrogen. For both adsorptives in the molecular sieves under investigation the initial isosteric heat of adsorption for nitrogen was found to give values comparable to the enthalpies of adsorption derived from the temperature dependence of experimentally determined HENRY constants. HENRY constants and the initial isosteric heat of adsorption are indicative of a stronger adsorption of nitrogen compared to argon which is thought to be due to additional interactions between the nitrogen quadrupole moment and the crystalline molecular sieve framework.