(109c) 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.