Mixed matrix membranes molecular separation

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... 

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. 

Mixed-matrix membranes containing dispersed zeolites in a continuous polymer matrix may retain polymer processabibty and improved selectivity for separation appHcations due to the superior molecular sieving property of the zeolite materials. 

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]. 

Mixed matrix membranes with molecular sieves incorporated combine the high separation capacity of molecular sieving materials (see Section 4.3.2) with the desirable mechanical properties and economical processing attributes of polymeric materials. 

An example is provided by mixed matrix membranes (MMM). Basically they are constituted of inorganic molecular nanoparticles (such as zeolites, carbon molecular sieves, etc.) imbedded in polymers. MMMs open up new perspectives in gas separation. A main application for sustainable development is the purification of... 

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... 

S. J. Miller, W. J. Koros, D. Q. Vu, Mixed matrix membrane technology enhancing gas separations with polymer/molecular sieve composites. Studies Surf. Sci. Cated., 170, 1590-1596 (2007). 

R. Nasir, H. Mukhtar, Z. Man, M.S. Shaharun, M.Z. Abu Bakar, Effect of fixed carbon molecular sieve (CMS) loading and various di-ethanolamine (DEA) concentrations on the performance of a mixed matrix membrane for CO2/CH4 separation, RSC Advances 5 (2015)60814-60822. 

This can be a very efficient and economical way of separating components that are suspended or dissolved in a liquid. The membrane is a physical barrier that allows certain compoimds to pass through, depending on their physical and/or chemical properties. Polymeric membrane materials are intrinsically limited by a tradeoff between their permeability and their selectivity. One approach to increase the selectivity is to include dispersions of inorganic nanoparticles, such as zeolites, carbon molecular sieves, or carbon nanotubes, into the polymeric membranes - these membranes are classified as mixed-matrix membranes. 

Gas separation membranes combining the desirable gas transport properties of molecular sieving media and the attractive mechanical and low cost properties of polymers are considered. A fundamental analysis of predicted mixed matrix membrane performance based on intrinsic molecular sieve and polymer matrix gas transport properties is discussed. This assists in proper materials selection for the given gas separation. In addition, to explore the practical applications of this concept, this paper describes the experimental incorporation of 4A zeolites and carbon molecular sieves in a Matrimid matrix with subsequent characterization of the gas transport properties. There is a discrepancy between the predicted and the observed permeabilities of O2/N2 in the mixed matrix membranes. This discrepancy is analyzed. Some conclusions are drawn and directions for further investigations are given. 

In the past 25 years, relatively few attempts to increase gas separation membrane performance with dense film mixed matrices of zeolite and rubbery or glassy polymer have been reported. Table I summarizes practically all of the reported O2/N2 mixed matrix membranes. Permeabilities and permselectivities are specified as a range to encompass the various zeolite volume fractions studied. In general, an increase in permeability is observed with zeolite addition coupled with a slight increase in permselectivity. Despite the wide variety of combinations of zeolites with rubbery and glassy polymers, reported mixed matrix membranes fail to exhibit the desired O2/N2 performance increases. These failures have generally been attributed to defects between the matrix and molecular sieve domains. While this is certainly a possible practical source of failure, our work earlier 8) has addressed a more fundamental source caused by inattention to matching the transport properties of the molecular sieve and polymer matrix domains. This topic is discussed briefly prior to consideration of the practical defect issue noted above. 

Polymer matrix selection determines minimum membrane performance while molecular sieve addition can only improve membrane selectivity in the absence of defects. Intrinsically, the matrix polymer selected must provide industrially acceptable performance. For example, a mixed matrix membrane using silicone rubber could exhibit properties similar to intrinsic silicone rubber properties, O2 permeability of 933 Baiters and O2/N2 permselectivity of 2.1 (8). The resulting mixed matrix membrane properties would lie substantially below the upper boimd trade-off curve for gas permeability and selectivity. In contrast, a polymer exhibiting economically acceptable permeability and selectivity is a likely candidate for a successful polymer matrix. A glassy polymer such as Matrimid polyimide (PI) is an example of such a material because it exhibits acceptable properties and current technology exists for formation of asymmetric hollow fibers for gas separation (10). 

Besides zeolites and traditional silica, carbon molecular sieves have been investigated as dispersed phases. Mixed-matrix membranes have been created using carbon molecular sieves dispersed in polyetherimide (Ultem) and Matrimid, separately. These mixed-matrix membranes displayed an increase in both permeability and selectivity over their neat polymer counterparts." The effect of trace amounts of toluene impurity in the feed stream of these carbon-polymer membranes was tested, and the membranes showed promising stability over time against the impurity. Zeolite-carbon mixed-matrix membranes have recently been developed where the carbonized polymer matrix is derived from a pure Matrimid membrane." These mixed-matrix membranes double the CO2/CH4 selectivity of the pure carbonized Matrimid membranes tested but lose over half of their productivity in the process. While these properties are well above Robeson s upper bound, other researchers have achieved better separation properties using only pure carbonized Matrimid membranes. ... 

Successful spinning of mixed-matrix hollow-fiber membranes for gas separation has so far been only demonstrated in apatentby Ekineret al. ° A major hurdle to the commercial implementation of mixed-matrix membranes has been the lack of reproducibility in forming successful mixed-matrix membranes. Challenges with poor polymer- sieve interaction, variability in molecular sieve transport, surface characteristics, and effects of contaminants on molecular sieve performance have been identified in dense mixed-matrix... 

While porous defects between the solid and polymer are undesirable in microporous solid-polymer gas separation membranes, the opposite is tme for ultrafiltrafion and ion exchange mixed-matrix membranes.The presence of such voids enhances membrane performance as the solid acts only in the adsorbent capacity and not as a molecular sieve as required in the case of the microporous solid. 

Most of the more recent research has focused on developing membrane materials with a better balance of selectivity and productivity (permeability) as that seems the most likely route for expanding the use of this technology. There appear to be natural upper bounds [9,10] on this tradeoff that limit the extent of improvement that can be realized by manipulating the molecular structure of the polymer used for the selective layer of high-flux membranes, at least in many cases. This has led to interest in nonpolymeric and so-called mixed-matrix materials for membrane formation [8] however, at this time, polymers remain the materials of choice for gas-separation... 

Overcoming the current limitation faced by gas separation membranes may be accommodated by introducing two classes of materials that lie between conventional polymers and the high-performance molecular sieving materials. These two classes, illustrated in Fig. 11 and Fig. 12, respectively, are (i) crosslinked polymers and (ii) blends of molecular sieving domains in polymers, usually referred to as mixed matrix materials. Such materials... 

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