Mixed-matrix membranes types

Small-pore zeolite Nu-6(2) has a NSI-type structure and two different types of eight-membered-ring channels with limiting dimensions of 2.4 and 3.2 A [54]. Gorgojo and coworkers developed mixed-matrix membranes using Nu-6(2) as the dispersed zeolite phase and polysulfone Udel as the continuous organic polymer phase [55]. These mixed-matrix membranes showed remarkably enhanced H2/ CH4 selectivity compared to the bare polysulfone membrane. The H2/CH4 selectivity increased from 13 for the bare polysulfone membrane to 398 for the Nu-6(2)/ polysulfone mixed-matrix membranes. This superior performance of the Nu-6(2)/ polysulfone mixed-matrix membranes is attributed to the molecular sieving role played by the selected Nu-6(2) zeoHte phase in the membranes. 

The chemical composihons of the zeolites such as Si/Al ratio and the type of cation can significantly affect the performance of the zeolite/polymer mixed-matrix membranes. MiUer and coworkers discovered that low silica-to-alumina molar ratio non-zeolitic smaU-pore molecular sieves could be properly dispersed within a continuous polymer phase to form a mixed-matrix membrane without defects. The resulting mixed-matrix membranes exhibited more than 10% increase in selectivity relative to the corresponding pure polymer membranes for CO2/CH4, O2/N2 and CO2/N2 separations [48]. Recently, Li and coworkers proposed a new ion exchange treatment approach to change the physical and chemical adsorption properties of the penetrants in the zeolites that are used as the dispersed phase in the mixed-matrix membranes [56]. It was demonstrated that mixed-matrix membranes prepared from the AgA or CuA zeolite and polyethersulfone showed increased CO2/CH4 selectivity compared to the neat polyethersulfone membrane. They proposed that the selectivity enhancement is due to the reversible reaction between CO2 and the noble metal ions in zeolite A and the formation of a 7i-bonded complex. 

Rubbery polymer, polydimethylsiloxane (PDMS), was used as the polymer matrix to prepare zeolite/PDMS mixed-matrix membranes [82, 89]. This type of mixed-matrix membrane, however, did not exhibit improved selectivity for n-pentane/i-pentane separation relative to the neat PDMS membrane. 

Another type of mixed-matrix membranes for alcohol/water pervaporation applications was developed utilizing hydrophiUc poly(vinyl alcohol) (PVA) and ZSM-5. The ZSM-5/PVA mixed-matrix membranes demonstrated increased selectivity and flux, compared to pure PVA, for the water/isopropyl alcohol separation [97]. This type of mixed-matrix membranes, however, may have membrane swelling issue due to the hydrophilic nature of the PVA polymer. Mixed-matrix membranes comprising modifled poly(vinyl chloride) and NaA zeolite have shown both enhanced flux and selectivity for the ethanol/water separation at high NaA loadings [98]. 

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. 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

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). 

Hasse, D.J. et al. (September 2003) Mixed matrix membranes incorporating chabazite type molecular sieves. US Patent 6,626,980. 

To overcome these limitations, mixed matrix membranes (MMM) started to emerge as an alternative approach in membrane technology. In this approach, the superior gas separation properties of the molecular sieve materials combine with the desirable mechanical properties and economical processability of polymers (Moore et al. 2004). A mixed matrix is a blend of inorganic particles (such as nanoparticles) in a polymer matrix, which are well dispersed. The effect of the inorganic dispersed phase on the MMM properties is related to its chemical structure, surface chemistry, and the type of particles. The inorganic materials used... 

Based on the need of a more efficient membrane than polymer and inorganie membranes, a new type of membranes, mixed-matrix membranes, has been developed recently. Mixed-matrix membranes are hybrid membranes containing solid, liquid, or both solid and liquid fillers embedded in a polymer matrix." The various material combinations possible with mixed-matrix technology are represented in Figure 30.2. All of these combinations, with the exception of supported liquids, will be covered in this chapter. 

The strategy for the development of mixed-matrix membranes is to combine the advanced features of polymer membrane and inorganic membrane into one composite membrane. As discussed in the previous section, this is done by incorporating dispersed fillers into continuous polymer matrices. As noted in the introduction, there are three main types of mixed-matrix membranes reported in the literature solid-polymer, liquid-polymer, and solid-liquid-polymer mixed-matrix membranes. The polymer matrices providing low cost and easy processability are selected from either glassy polymers (e.g., polyimide, polysulfone, polyethersulfone, or cellulose acetate) or rubbery polymers (e.g., silicone rubber). The dispersed fillers include solid, liquid, or both solid and liquid. 

For the liquid-polymer mixed-matrix membranes, the physical state of the fillers incorporated into the continuous polymer matrix is liquid such as poly(ethylene glycol) (PEG). Because of the long-term stability concern of liquid polymer encapsulated in the continuous polymer matrix, a new type of mixed-matrix membranes, solid-hquid-polymer mixed-matrix membrane, has been developed recently. The solid such as activated carbon impregnated with liquid polymer such as PEG is functioned as stabilizer of the liquid polymer in the continuous polymer phase. Besides, activated carbon enhances the mixed-matrix membrane performance. 

Research has shown that the interfacial region, which is a transition phase between the continuous polymer and dispersed sieve phases, is of particular importance in successful mixed-matrix membrane formation.The type of morphology that forms at the interfacial region has a direct impact on a membrane s separation properties, and its abUity to reach the predicted Maxwell model properties. As shown in Figure 30.4, the ideal mixed-matrix membrane will exhibit both an increase in selectivity and permeability as the solid-phase volume fraction is increased, and the Maxwell model ean be used to estimate these separation properties (as discussed earlier). 

For liquid-polymer mixed-matrix membranes, the physical state of the IUIcts incorparated into the continuous polymer matrix is hquid such as PEG and amines. The following section provides an introduction to this less common mixed-matrix technology. Since the existing hterature on this new type of membrane is less developed, more detail is provided here. 

Similar efforts have been made in other application areas. In gas separation, the addition of Ti02 nanoparticles to polyvinyl acetate improved the thermal stability of the resulting membranes, which was demonstrated by an inaease in the glass transition temperature (Ahmad and Hagg 2013). In this case, it was found that the addition of Ti02 up to 10 wt% improved both the permeability and selectivity of the membranes for gas separation, including H2, CO2,02, and N2. Similar observations were made on the effect of silica nanoparticles on the permeability of CO2 and CH4 for two types of nanocomposite membranes based on polyester urethane and polyether urethane (Hassanajili et al. 2013). Khan et al. (2013) studied mixed matrix membranes composed... 

The use of polymeric catalytic membranes in distributor/contactor-type reactors for hydrogenation or oxidation reactions has been widely described in several former and recent reviews. Mixed-matrix membranes of PDMS hlled with Pd particles, composite membranes of ionic liquid-polymer gels filled with Pd/C, ° ionic liquids containing rhodium complexes and supported in polystyrene sheets in a corrugated configuration, have been used for the selective gas-phase hydrogenation of hydrocarbons in contactor-type membrane reactors. 

These are usually prepared by casting from polymer solution followed by solvent evaporation. It results in formation of a dense membrane. Membrane may be (1) homogeneous, (2) blend, (3) mixed matrix, (4) polyelectrolyte, or (5) polymer-ceramic composite depending on the type of polymer and other additives used for making the membrane. 

Separation of different organic components from each other is still a matter of laboratory investigation. In the past 15 years considerable efforts have been devoted to develop polymeric membranes to separate, for example, aromatic hydrocarbons from aliphatic ones which resulted in several patents [25, 26], or olefins from paraffins or to separate isomers, e.g. para- and ortho-xylenes, from each other. In the last years additional membranes [27] have become available and the first industrial applications have been reported, e.g. the separation of sulfur-containing aromatics from gasoline [28] and of benzene from a stream of saturated hydrocarbons [29], Further development of membranes, especially of the mixed-matrix type, may lead to improved selectivity and a broadening of these applications. 

V. Vatanpour, S.S. Madaeni, A.R. Khataee, E. Salehi, S. Zinadini, H.A. Monfared, Ti02 embedded mixed matrix PES nanocomposite membranes influence of different sizes and types of nanoparticles on antifouling and performance. Desalination 292 (2012) 19-29. 

Contactor-type polymeric membrane reactors have been also applied to liquid-phase reactions other than hydrogenation or oxidation. The hydration of a-pinene has been carried out successfully over polymeric membranes consisting of mixed matrixes of PDMS embedded USY or beta zeolites or sulfonated activated carbon. The membranes were assembled in a flat contactor-type reactor configuration, separating the aqueous and organic phases. Sulfonated PVA membranes were also reported to be effective in the acid catalysed methanolysis of soybean oil carried out in a flat contactor-type membrane reactor configuration. ... 

The mechanical stability and ion exchange capacity of these condensation resins were modest. A better approach is to prepare a suitable crosslinked base membrane, which can then be converted to a charged form in a subsequent reaction. Ionics is believed to use this type of membrane in many of their systems. In a typical preparation procedure, a 60 40 mixture of styrene and divinyl benzene is cast onto a fabric web, sandwiched between two plates and heated in an oven to form the membrane matrix. The membrane is then sulfonated with 98 % sulfuric acid or a concentrated sulfur trioxide solution. The degree of swelling in the final membrane is controlled by varying the divinyl benzene concentration in the initial mix to control crosslinking density. The degree of sulfonation can also be varied. The chemistry of the process is ... 

---