Metal Cation Exchanged in Zeolite

One of the most signiflcant variables affecting zeolite adsorption properties is the framework structure. Each framework type (e.g., FAU, LTA, MOR) has its own unique topology, cage type (alpha, beta), channel system (one-, two-, three-dimensional), free apertures, preferred cation locations, preferred water adsorption sites and kinetic pore diameter. Some zeolite characteristics are shown in Table 6.4. More detailed information on zeolite framework structures can be found in Breck s book entitled Zeolite Molecular Sieves [21] and in Chapter 2. 

The variety of the different framework structures result in different adsorbent characteristics acid strength, size of molecule adsorbed, adsorption/desorption rate of different molecules, capacity and stability. As a result, these differences characterize the adsorbent s selectivity to a specific molecule and adsorbent-adsorbate interactions. Take for example, the difference in selectivity of BaY and Ba-Mordenite [24] to p-xylene (PX), m-xylene (MX) and o-xylene (OX)  

Ba-Modenite s selectivity to MX is higher than OX, but the opposite is true for BaY. This reversal in selectivity is a result of differences in adsorbent framework characteristics mordenite has higher acid strength compared to Y zeolite. Adsorption and desorption rates of xylenes are expected to be faster in BaY compared to Ba-Mordenite because Mordenite is a one-dimensional channel system while Y zeoUte is a three-dimensional channel. With the reason stated, a three-dimensional channel ZeoUte is the preferred mass separating agent of choice compared to one-or two-dimensional channels for the liquid adsorption separation. 

One of the parameters in the broad class of equilibrium-selective adsorption mechanisms is the interaction between the acidic and basic sites of the adsorbent and the adsorbate. ZeoUtes can be ion-exchanged with a variety of metal cations 

Zeolite type Channel system Pore openings (A hydrated form) Typical Sl02 Al203 mole ratio Theoretical ion exchange capacity (meq/g Na form, anhydrous) 

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