Zeolites membranes

Fig. 7.n SEM images of the 3D macroporous functionalized macroporous zeolitic membrane zeolitic membrane used as a support for enzyme bioreactor was prepared via the LbL electrostatic immobilization by the LbL procedure. Images assembly of PEs and enzyme (catalase) on the 3D (A-D) are cross-sections of the membrane at macroporous membrane. (Reprinted from [59] different magnifications. A biomacromolecule- with permission of Wiley-VCH). 

Various zeolites have been studied as the dispersed phase in the mixed-matrix membranes. Zeolite performance in the zeolite/polymer mixed-matrix membrane is determined by several key characteristics including pore size, pore dimension, framework structure, chemical composition (e.g., Si/Al ratio and cations), crystal morphology and crystal (or particle) size. These characteristics of zeolites are summarized in Chapter 6. 

D. Photocycloaddition and Photocyclization Within Nation Membranes, Zeolites, and Vesicles... 

A zeolite membrane is a membrane in which the transfer is controlled by the porous structure of the zeolite. Compared to sol-gel membranes, zeolite membranes can present some advantages for CMR applications. The most useful feature is that the pores of zeolites arc in the ultramicroporous range and have a very narrow size distribution (in this case pores are linked to the structure and not to the texture as in sol gel membranes). These characteristics of zeolite... 

In order to achieve high selectivities with thermostable zeolite-based membranes, zeolites can be choosen with pore apertures matching the kinetic diameters of the molecules to be separated. Moreover, the hydrophobicity of all-silica zeolites provides continuous separation, independently of traces of water in the gas streams applied. In the total spectrum of tectosilica(te)s there is only one all-silica 8-ring system Deca-dodecasil 3R (DD3R) but several all-silica 6-ring systems Table 6). 

M. Matsukata, N. Nishiyama, and K. Ueyama, Preparation of a thin zeolitic membrane, Zeolites and related microporous materials State of the art 1994 Part B. Proc. 10th Ini. Zeol. Conf., Garmisch-Partenkirchen (J. Weitkamp, H.G. Karge, H. Pfeifer, and W. Holderich, eds.), Elsevier, Amsterdam, 1994, p. 1183. 

The inorganic silica membranes, also commercial, have solved the problem of thermal and chemical stability however, these membranes are only used for dehydration purposes, leaving the problem of separation of organic mixtures unsolved. As we have seen previously, due to the versatility and special feamres of zeolites, new applications in pervaporation that are not possible with other membranes could be developed with zeolite membranes. GaUego-Lizon et al. [110] compared different types of commercial available membranes zeolite NaA from SMART Chemical Company Ltd., sUica (PERVAP SMS) and polymeric (PERVAP 2202 and PERVAP 2510) both from Sulzer Chemtech GmbH, for the pervaporation of water/f-butanol mixtures. The highest water flux was obtained with the silica membrane (3.5 kg/m h) while the zeolite membrane exhibited the highest selectivity (16,000). 

Tetrahydrofuran, THF, is an important industrial solvent and forms an azeotropic mixture at 5.3 wt% with water (see Table 10.3). To separate water/THF, Li et al. [148] tested the pervaporation performance of different hydrophihc zeolite membranes, zeolite A, zeohte Y, MOR, and ZSM-5. The preliminary test showed that the separation factor increased as the Si/Al ratio of the zeohte decreased, except for the case of zeolite A. This fact is probably due to the lower quality of this membrane with respect to the others since in the permeation of triisopropylbenzene (TIPB), showed the highest flux, 3.1 g/m h, indicating the presence of nonselective defects. Therefore, the best results were obtained with zeolite Y, rendering a separation factor of 300 with a water flux of 2.24 kg/m h at 60°C. The water flux increased with water concentration in the feed, up to a value of 15 wt%, indicating that the zeolite was saturated, as was the same for the case of water/ethanol mixtures in zeolite A, previously described. At the same time, the separation factor decreases as water concentration decreased. The stabihty of the membrane was also studied, showing a stable performance after 35 h of operation. 

More promising for reactive separations involving gas phase reactions appears to be the development and use in such applications of microporous zeolite and carbon molecular sieve (Itoh and Haraya [2.25] Strano and Foley [2.26]) membranes. Zeolites are crystalline microporous aluminosilicate materials, with a regular three-dimensional pore structure, which are relatively stable to high temperatures, and are currently used as catalysts or catalyst supports for a number of high temperature reactions. One of the earliest mentions of the preparation of zeolite membranes is by Mobil workers (Haag and Tsikoyiannis... 

So many membrane reactors were reported in the literature for the WGS reaction. Along with Pd membranes, studies on silica membranes, zeolite membranes, hollow membrane reactors, electrochenucal WGS reactor are also available in the literature. However, Pd-based membranes were investigated extensively for the WGS reaction. In the following sections we have given detailed description about various membrane reactors investigated for the WGS reaction. A detailed description about the theoretical and simulation studies of the membrane reactors for the WGS reaction was also discussed. 

Vankelecom et al. (1997a) studied three types of hydrophobic porous fillers (carbon blacks, in situ methylated silicas, and silylated silicas) incorporated in PDMS membranes in order to find out under which conditions these membranes were advantageous for PV of aqueous solutions. The properties of these fillers were changed systematically in order to maximize the fluxes and selectivities of the PV of aqueous EtOH, tert-hutyl alcohol (TEA), or aroma solution. The effect of incorporation of carbon black in PDMS for the PV of a 6 wt% alcohol solution in water is based on the level of the PDMS. When using PDMS membranes filled with hydrophobic silicas, the best results were obtained with silylated silicas. Vankelecom et al. (1997b) investigated the PV of aroma compounds using zeolite-filled PDMS composite membranes. Zeolite-filled PDMS membranes were supported on the PAN asymmetric membrane coated on a nonwoven polyester. Table 9.6 shows the influence of the filler on the fluxes and overall enrichment factors. 

The manufacture of DPMs based on infiltrated molten carbonates in the porosity of perovskite membranes constitutes the most recent and innovative application of perovskites for CO2 capture. Since the first studies reported by Wade and coworkers [33] and Lin and coworkers [34], many efforts have been made to develop stable high-flux membranes for CO2 separation. Table 39.5 collects the most remarkable results, whereas Figvue 39.12 plots the permeance versus permeability plots for dual-phase perovskite-carbonate materials compared with low-temperature CO2 separating membranes (zeolites and MOFs/ZIFs). 

In CMRs, no separate catalyst is used and reactions take place on the membrane. Zeolite membranes can be intrinsically catalytic due to the presence of catalytic sites (Bronsted acid sites, Lewis acid sites, metal ions in cationic positions, transition metal ions in zeolite lattice positions, extra-lattice transition metal compounds in channels and cavities of a zeolite, metal particles in zeolite cavities) and their high internal surface area. 

Zeolite Membranes Zeolites are porous aluminosilicate crystals composed of Si04 and AIO4 tetrahedra arranged in various geometric patterns. The tetrahedra arc... 

Polyvinyl acetate (PVAc) is another rubbery polymer used in mixed-matrix research. Its flexible nature helps to prevent void formation at the solid-polymer interface. Although it may not have practical industrial applications, PVAc aids in developing proof-of-concept associated with mixed-matrix membranes. Zeolite 4A-PVAc membranes have been proven to enhance membrane selectivity in mixed-matrix membranes with only 15 vol% zeolite " however, the permeability is lower than predicted presumably due to matrix rigidification. The rubbery nature of PVAc allows for more polymer relaxation at the solid-polymer interface as compared to the case with traditional, glassy polymers." ... 

Some of the available membrane separation processes can already be applied on an industrial scale. Hence, inorganic ceramic membranes (zeolites and their derivatives, e.g. silico aluminophosphates), organic polymer membranes and facilitated transport membranes, which rely on a carrier molecule with high CO2 affinity to achieve selective CO2 transport (such as metallic ions or liquid amines), have been used in separating CO2 from flue gas in post-combustion. As single-stage separation with these membranes is still difficult, new membrane materials are being developed [1]. Typically, the initial separation of carbon dioxide accounfs for 60-80% of the total cost of CO2 sequestration [24,25]. 

Key words zeolite, zeolite membranes, zeolite membrane reactors. 

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