Zeolite Membrane Separation Mechanisms

Molecular Sieving Controlled Excludes molecules too large to fit in zeolite pore 

Selectivity determined by relative size/shape of molecules and zeolite pore 

Permselectivity predicted by ideal selectivity at high temperature and low concentration 

H2/CO2 H2/light hydrocarbons, Hj/n-butane. Xylene isomers, Hexane isomers  

Selectivity determined by molecule-molecule interactions and membrane loading 

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The second part of the book covers zeolite adsorptive separation, adsorption mechanisms, zeolite membranes and mixed matrix membranes in Chapters 5-11. Chapter 5 summarizes the literature and reports adsorptive separation work on specific separation applications organized around the types of molecular species being separated. A series of tables provide groupings for (i) aromatics and derivatives, (ii) non-aromatic hydrocarbons, (iii) carbohydrates and organic acids, (iv) fine chemical and pharmaceuticals, (v) trace impurities removed from bulk materials. Zeolite adsorptive separation mechanisms are theorized in Chapter 6. 

The present review of zeolite membrane technology covers synthesis and characterization methods as well as the theoretical aspects of transport and separation mechanisms. Special attention is focused on the performance of zeolite membranes in a variety of applications including liquid-liquid, gas/vapor and reactive... 

Figure 10.5 Summary of the features of separation mechanisms exhibited by zeolite membranes along with common examples. Superscripts 1 high temperature and/or low partial pressure, 2 low temperature and/or high partial pressure.

This chapter provides a brief introduction to polymer and inorganic zeolite membranes and a comprehensive introduction to zeolite/polymer mixed-matrix membranes. It covers the materials, separation mechanism, methods, structures, properties and anticipated potential applications of the zeolite/polymer mixed-matrix membranes. 

Membranes made from zeolite materials provide separahon properties mainly based on molecular sieving and/or surface diffusion mechanism. Separation with large pore zeolite membranes is mainly based on surface diffusion when their pore sizes are much larger than the molecules to be separated. Separation with small pore zeolite membranes is mainly based on molecular sieving when the pore sizes are smaller or similar to one molecule but are larger than other molecules in a mixture to be separated. 

Some small-pore zeolite and molecular sieve membranes, such as zeolite T (0.41 nm pore diameter), DDR (0.36 x 0.44nm) and SAPO-34 (0.38nm), have been prepared recenhy [15-21]. These membranes possess pores that are similar in size to CH4 but larger than CO2 and have high CO2/CH4 selechvihes due to a molecular sieving mechanism. For example, a DDR-type zeolite membrane shows much higher CO2 permeability and CO2/CH4 selechvity compared to polymer membranes [15-17]. SAPO-34 molecular sieve membranes show improved selechvity for separation of certain gas mixtures, including mixtures of CO2 and CH4 [18-21]. 

Mixed-matrix membranes comprising small-pore zeolite or small-pore non-zeolitic molecular sieve materials will combine the solution-diffusion separation mechanism of the polymer material with the molecular sieving mechanism of the zeolites. The small-pore zeolite or non-zeolitic molecular sieve materials in the mixed-matrix membranes are capable of separating mixtures of molecular species... 

Current polymeric materials are inadequate to fully meet all requirements for the various different types of membranes (cf. Section 2.2) or to exploit the new opportunities for application of membranes. Mixed-matrix membranes, comprising inorganic materials (e.g., metal oxide, zeolite, metal or carbon particles) embedded in an organic polymer matrix, have been developed to improve the performance by synergistic combinations of the properties of both components. Such improvement is either with respect to separation performance (higher selectivity or permeability) or with respect to membrane stability (mechanical, thermal or chemical). 

Molecular sieving Fig. 4(e) where, due to steric hindrance, only small molecules will diffuse through the membrane, seems to be a useful principle for achieving good separations. To ensure this molecular sieving effect, ultramicroporous membranes have to be prepared. Moreover, such membranes should not only be defect free but must also present a very narrow pore size distribution to avoid any other (less selective) permeation mechanisms defect-free zeolite membranes appear to be good candidates for this type of separation. 

However, at least for separative applications, most hopes to find consistent application of inorganic-membrane reactors lie in the development of inorganic membranes having pores of molecular dimensions (<10 A, e.g., zeolitic membranes). Such membranes should moreover be thin enough to allow reasonable permeability, defect-free, resilient, and stable from the thermal, mechanical, and chemical standpoints. Such results should not be achieved only at a lab scale (a lot of promising literature has recently appeared in this context), but should also be reproducible at a large, industrial scale. Last, but not least, such membranes should not be unacceptably expensive, in both their initial and their replacement costs. 

Zeolite membranes have the potential to selectively separate gas molecules in a mixture operating under steady state, unsteady state, or under cyclic conditions whereas fixed bed adsorbers are typically operated under transient conditions. In addition, because of the inorganic nature of zeolite membranes, they have higher mechanical strength and greater thermal and chemical stability than their polymeric counterparts. Also, their ability to operate under very different conditions (total pressure. 

Although Knudsen diffusion, shape selectivity, and molecular sieving play an important role in the separation of mixtures, the mechanisms which control the majority of the multicomponent separations in zeolite membranes are surface diffusion, and sometimes, capillary condensation. In addition, molecular simulations and modeling of M-S diffusion in zeolites [69,70] show that the slower moving molecules are also sped up in some mixtures [71,72] in the presence of fast-diffusing molecules and other times, slower molecules inhibit diffusion of faster molecules because molecules have difficulty passing one another in zeolite pores [73]. 

Separation due to molecular sieving alone can only take place if the membrane is defect free and there is no adsorption of the permeating species on the zeolite crystals (i.e., at high temperatures) or the components in the mixture adsorb with similar heats of adsorption. For those mixtures, the shape and size of the components compared to the zeolite-pore sizes direct the separation toward molecular sieving. For example, the separation of H2/SF6 or N2/SF6 on MFl zeolite membranes at high temperatures where SFe does not adsorb significantly. In fact, the latter separation is commonly used to evaluate the membrane quality. Methane/f-octane represents another example of this mechanism since f-octane cannot fit into the MFl stmcture and the heat of adsorption of methane is very low in the zeolite [2]. 

So far, essentially three different approaches have been reported for the preparation of zeolitic membranes [119]. Tsikoyiannis and Haag [120] reported the coating of a Teflon slab during a "regular" synthesis of ZSM-5 by a continuous uniform zeolite film. Permeability tests and catals ic experiments were carried out with such membranes after the mechanical separation of the coating from the Teflon surface [121]. Geus et al. [122] used porous, sintered stainless steel discs covered with a thin top layer of metal wool to crystallize continuous polycrystalline layers of ZSM-5. Macroporous ceramic clay-type supports were also applied [123]. 

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