Membrane Large pore zeolite

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. 

LARGE PORE ZEOLITE-BASED MEMBRANES (12 membered oxygen ring)... 

Eguizdbal A, Lemus J, Pina MP (2013) On the incorporation of protic ionic liquids imbibed in large pore zeolites to polybenzimidazole membranes for high temperature proton exchange membrane fuel cells. J Power Sources 222 483-492. doi 10.1016/j.jpowsour.2012.07.094... 

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. 

This is the area of zeolitic materials, which can be characterized as crystalline, porous materials built predominantly of oxygen, silica, and alumina (or phosphor) [1,2]. These crystals contain straight or sinusoidal channels that can be interconnected, resulting in a one-, two-, or three-dimensional pore network. At the intersections, small or large cavities may exist. Due to the crystallinity, the pores are uniform and can be prepared with great reproducibility, which is the biggest advantage over attempts to produce amorphous membranes with pores of molecular dimensions. Therefore, in the last decade much research effort has been put into the development of zeolitic membranes, and with success [3-29]. 

The pores of zeolites are of molecular dimensions (Fig. 1). In principle molecules can be excluded from the pores if their diameter is larger than the pore apertures. The zeolite framework is, however, not a rigid structure. Especially at higher temperatures, the pores become more flexible. As a result, molecules with larger diameters than the dimensions of the pores will be able to penetrate the channels. The maximum diameter of molecules that are able to adsorb within the zeolite is called the adsorption cutoff diameter. This diameter is about 0.95 nm for zeolite X and Y [30], 0.65 nm for ZSM-5 [30], and 0.4 nm for zeolite-4 A [1] at 3(X) K. This means that, in principle, all molecules listed in Fig. 1 can permeate through a zeolitic membrane made from zeolite X, Y or ZSM-5, except for the large amines. Complete exclusion of molecules from the pores will therefore occur only when using a zeolite-A membrane. 

A schematic picture of different t5q)es of pores is given in Fig. 9.1 and of main types of pore shapes in Fig. 9.2. In single crystal zeolites the pore characteristics are an intrinsic property of the crystalline lattice [3] but in zeolite membranes other pore types also occur. As can be seen from Fig. 9.1, isolated pores and dead ends do not contribute to the permeation under steady conditions. With adsorbing gases, dead end pores can contribute however in transient measurements [1,2,3]. Dead ends do also contribute to the porosity as measured by adsorption techniques but do not contribute to the effective porosity in permeation. Pore shapes are channel-like or slit-shaped. Pore constrictions are important for flow resistance, especially when capillary condensation and surface diffusion phenomena occur in systems with a relatively large internal surface area. 

Torres, Gutierrez, Mugica, Romero, and Lopez (2011) synthesized P-zeolite membranes with different acidities. They demonstrated that catalytic membrane shows high activity toward isobutene dimers and attributed this to the surface acidity of the membrane and the control of short residence time within the zeolite membrane active pores contributing to the large butane oligomer Cie elimination. 

Synthesis of membranes with high permeability and selectivity, that is, oriented and thin zeolite membranes. Optimal MR operation requires the membrane flux to be in balance with the reaction rate. A large number of factors - such as the support, organic additives, temperature, and profile - have a significant influence on the microstructure and overall quality of the membrane. However, the precise correlation between the synthesis procedure and conditions and the properties of the resultant zeolite membranes is not clear. In contrast, the majority of membranes synthesized so far are MFI-type zeolite membranes that have pore diameters 5 A, which are still too big to separate selectively small gaseous molecules. Zeolite membranes with pores in the 3 A range should be developed for membrane reactors, to separate small gas molecules on the basis of size exclusion. In addition, a method to produce zeolite membranes without non-zeolite pores or defects has to be found. 

Other MFl-type zeolite-sorbate systems are known to exhibit similar behavior. In a recent study, Yu et al. [34] reported that at saturated loadings of -hexane a single MFl-type zeolite unit cell has an overall volume expansion of 2.3%, which can correlate to shrinkage in non-zeoUtic pores up to 7 nm for a 1 tm crystal when isotropic expansion is assumed. It was demonstrated that, even in membranes with large number of defects, the crystallite swelling caused the membrane to achieve significant separation between n-hexane and trimethylbenzene, iso-octane and 2,2-dimethylbutane using pervaporation [34]. 

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