Some Examples of Cation and Framework Influences On Selective N2 Adsorption

Some Examples of Cation and Framework Influences on Selective N₂ Adsorption

Charles G. Coe^1*, Thomas. R. Gaffney, John. F. Kirner, Herbert. C. Klotz, James. E, MacDougall, Scott. J. Weigel ,and Brian. H. Toby

¹ Corporate Science and Technology Center, Air Products and Chemicals, Inc., 7201 Hamilton Blvd, Allentown, PA 18195 (*presently at Chemical Engineering Department, Villanova University, PA)

Selective N₂ adsorption in cation-rich zeolites has been known for over 40 years and provides the basis for the equilibrium-based production of O₂ from air.  At the heart of this important process is a highly tailored adsorbent that combines the proper composition and pore structure to allow the economical production of 100 ton/day quantities of oxygen.

After giving an introduction to adsorptive separations, examples from our past research at Air Products will be reviewed and used to illustrate the remarkable importance of cation siting and structure in controlling selective N₂ adsorption.  A multidisciplinary approach involving experimental synthesis, modeling, and extensive analytical characterization elucidated some new insights leading to improved air separation processes as well as other new applications.  We found that both the N₂ capacity and selectivity are strongly influenced by not only the framework structure, but the type, size, location, and number of accessible and effective cations present in the zeolite .   Comparisons of adsorptive properties on a series of Na, Ca, and Li exchanged X and Y type zeolites were made. In combination with known cation sitings for some of the compositions along with some additional structural findings, these studies showed that the site specific effectiveness of Li and Ca for selective N₂ adsorption was very different.  Whereas Ca is effective in site II, Li is only effective in site III.  Structural studies on CaLSX (Si/Al = 1.0) and ab initio calculations on six-ring clusters explain the observations and show there is a substantial difference in partial charges of Li and Ca due to local coordination geometries.

The importance of cation siting is not limited to X-type adsorbents.  We also found an unexpected maximum in the N₂ capacity at ambient conditions for chabazite at a specific Si/Al ratio of 2.0 and developed a model to describe the strong variation in N₂ capacity with composition and aluminum distribution.  The model also allowed us to define chromatographic chabazite-based adsorbents that can analyze trace oxygen in argon at ambient conditions, greatly simplifying the analytics for this important analysis.   This review will hopefully reinforce the critical importance of using fundamental approaches to optimize adsorbent properties for a given application.