Zeolite Membranes in Separation and Reactive Processes

One of the most common liquid-liquid separation methods investigated is pervaporation. This technique is a low-energy alternative for the separation of mixtures that are difficult and expensive to separate by traditional means such as azeotropes and isomers [35]. In pervaporation a liquid feed is vaporized as ittravels 

Zeolite membranes have also been employed for organic-organic separations where selectivity is based on adsorption and diffusion differences of non-aqueous mixtures. NaX and NaY zeolite were used in the separation of methanol from MTBE and benzene (800 a 10000) exploiting the more polar nature of methanol which is attracted to the electrostatic poles of the high A1 content zeolites [38]. Other separations include (i) separation of n-hexane from 2,2-DMB using ZSM5, (ii) benzene from p-xylene using MOR/FER and (iii) xylene isomers [34]. 

Another challenging and industrially important separation that utihzes pervapo-ration through zeolite membranes is acid removal from H2O. In this case, the zeolite must have a high Si/Al ratio due to leaching of A1 by the acid. Both Ge-ZSM5 and silicalite have demonstrated significant stability and separation capability for the removal of acetic acid from H2O [35]. 

In weak-strong separations, one component does not strongly adsorb but has a high diffusivity the other component is slow but strongly adsorbs. In this case, the separation mechanism exhibited is either diffusion-controlled permeation or adsorption-controlled permeation, depending on the operating conditions [39]. 

Strong—strong mixture separations are most complex due to the interactions between the constituents and the zeolite framework as well as with each other [39]. 

---