(594d) Development of Small-Pore Rho Zeolites for Gas Separation Applications

Gas separations, such as the kinetic separation of oxygenfrom either nitrogen or argon by adsorption, requires adsorbent structures with very specific pore dimensions. The Lennard-Jones 6-12 kinetic diameter of Ar (3.4Å) is smaller than that of O₂ (3.46Å). Unlike Ar, O₂ is not spherical and has a minimum molecular dimension of 2.8Å, and this 0.6Å difference in diameters between O₂ and Ar can be exploited for kinetic separation of these gases. For example, a small-pore zeolite with an effective diameter of 2.8Å would allow O₂ diffusion and restrict Ar. With such an adsorbent, an Ar pressure swing adsorption (PSA) process could be developed for the production of crude argon in an efficient and safe manner. In this paper, we will discuss the development of novel RHO zeolites with a Si/Al ratio of 3.9, which are useful as kinetically selective adsorbents for oxygen and/or nitrogen.

The RHO zeolites useful for Ar PSA, which will be the focus of this work, can contain a variety of extra-framework cations. The composition of these zeolites is tunable through ion-exchange, and we will discuss how the choice of extra-framework cation(s) causes structural change in the 8-ring windows that act as the selective pores. This structural change is the key component to controlling the selectivity of the zeolite toward gas molecules of interest. These RHO zeolites have superior O₂ uptake rate, capacity, and selectivity relative to carbon molecular sieves at sub-ambient to ambient temperatures.