Mixed matrix materials, membrane

Zeolite/polymer mixed-matrix membranes can be fabricated into dense film, asymmetric flat sheet, or asymmetric hollow fiber. Similar to commercial polymer membranes, mixed-matrix membranes need to have an asymmetric membrane geometry with a thin selective skin layer on a porous support layer to be commercially viable. The skin layer should be made from a zeohte/polymer mixed-matrix material to provide the membrane high selectivity, but the non-selective porous support layer can be made from the zeohte/polymer mixed-matrix material, a pure polymer membrane material, or an inorganic membrane material. 

Widjojo, N., Chung, T.S., and Kulprathipanja, S. (2008) The fabrication of hollow fiber membranes with doublelayer mixed-matrix materials for gas separation. J. Memhr. Sci., 325 (1), 326-335. 

One should note that the methods to prepare such mixed-matrix membranes and the resulting properties are strongly dependent on the interactions between the different materials, and a homogeneous, regular distribution and interface compatibility are the key issues. Techniques to prepare mixed-matrix materials have been reviewed recently [21]. Mixed-matrix membranes are typically prepared by ... 

A wide variety of polymeric membranes with different barrier properties is already available, many of them in various formats and with various dedicated specifications. The ongoing development in the field is very dynamic and focused on further increasing barrier selectivities (if possible at maximum transmembrane fluxes) and/ or improving membrane stability in order to broaden the applicability. This tailoring of membrane performance is done via various routes controlled macro-molecular synthesis (with a focus on functional polymeric architectures), development of advanced polymer blends or mixed-matrix materials, preparation of novel composite membranes and selective surface modification are the most important trends. Advanced functional polymer membranes such as stimuli-responsive [54] or molecularly imprinted polymer (MIP) membranes [55] are examples of the development of another dimension in that field. On that basis, it is expected that polymeric membranes will play a major role in process intensification in many different fields. 

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... 

These concerns can be addressed partially through the use of mixed-matrix membranes [77-79]. Dispersing the microporous material in the form of small particles within a polymeric matrix simplifies membrane formation dramatically. Mixed matrix materials possess transport properties intermediate between those of the polymer matrix and the microporous particle and operating temperatures are limited by the thermal stability of the polymer matrix. However, proper selection of the matrix, control of particle volume fraction, and development of a membrane formation process can yield materials with properties that approach those of the particles [77-78]. Special attention must be given to the particle-polymer interface. If the interface morphology is uncontrolled, the matrix may 1) not wet the particle leaving a non-selective void around the particle, 2) enter the particle and block pores, or 3) rigidify around the particle and block access to it [79]. 

However, membranes can be symmetric or asymmetric. Many porous or dense membranes are asymmetric and have one or several more porous supporting layers and a thin skin layer which, in fact, gives selectivity. If these two layers are made of different materials, the membrane is a composite one. On some occasions, dense membranes have inclusions of other materials these are, of course, also composite membranes. In the case of gas separation membranes it has became usual to include inorganic charges in a polymeric membrane to get what is called a mixed matrix composite membrane. 

Finally, other possible way to produce useful membranes is to mix a certain material (usually a polymer with good mechanical and chemical properties) with another material (an activated carbon for example) with better selectivity-permeability performances but which it is not advisable to use alone in the actual gas environment for a given application or which is difficult to obtain in a resistant film layer. This constitutes what is called a mixed matrix composite membrane (MMCM). 

The polymer blend carbonization method will become an important tool for producing carbon membranes, with mixed-matrix materials, to overcome the challenges and limitations of membrane technology used in the gas separation industry. 

Materials combirnation in mixed-matrix polymeric membrane technology. 

Incorporation of selective flakes into mixed-matrix membranes promises lowered permeability and enhanced selectivity compared to the pure polymer. This concept was discussed by Cussler, based on existing theory concerning impermeable flakes. Data exists for aluminophosphate flakes dispersed in a polyimide matrix, and the separation trends from Cussler s theory hold for all gases examined. This technology is applicable to highly permeable polymers that require selectivity enhancement to meet industrial needs. Flakes are a promising mixed-matrix material, but loss of membrane productivity, in the end, may limit the application of this technology unless adequate intrinsic flake permeability can be achieved. 

FIGURE 14.2 Illustration of in situ synthesis process of mixed-matrix nanocomposite membranes (a) metal ions are preloaded within polymer matrix to serve as nanoparticle precursor (b) monomers of polymer matrix and the nanofillers as the starting materials and (c) blending of the nanoparticle precursors and monomers of polymers in solvent. 

Zi02 has been used as a bulk material in mixed matrix polysulfone membranes, although in the size range of micrometers (Genne et al. 1996). Zeolites are known to enhance the fluxes and selectivities for membranes used in gas separation and pervaporation (Rezakazemi et al. 2012). 

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. 

Both zeolitic and non-zeolihc inorganic materials have been used as the dispersed phase for making mixed-matrix membranes. 

Up to now, a variety of non-zeolite/polymer mixed-matrix membranes have been developed comprising either nonporous or porous non-zeolitic materials as the dispersed phase in the continuous polymer phase. For example, non-porous and porous silica nanoparticles, alumina, activated carbon, poly(ethylene glycol) impregnated activated carbon, carbon molecular sieves, Ti02 nanoparticles, layered materials, metal-organic frameworks and mesoporous molecular sieves have been studied as the dispersed non-zeolitic materials in the mixed-matrix membranes in the literature [23-35]. This chapter does not focus on these non-zeoUte/polymer mixed-matrix membranes. Instead we describe recent progress in molecular sieve/ polymer mixed-matrix membranes, as much of the research conducted to date on mixed-matrix membranes has focused on the combination of a dispersed zeolite phase with an easily processed continuous polymer matrix. The molecular sieve/ polymer mixed-matrix membranes covered in this chapter include zeolite/polymer and non-zeolitic molecular sieve/polymer mixed-matrix membranes, such as alu-minophosphate molecular sieve (AlPO)/polymer and silicoaluminophosphate molecular sieve (SAPO)/polymer mixed-matrix membranes. 

Figure 11.2 Selection of proper zeolite material for a mixed-matrix membrane (MMM) using the Maxwell model.

The Maxwell model can also guide the selection of a proper polymer material for a selected zeolite at a given volume fraction for a target separation. For most cases, however, the Maxwell model cannot be applied to guide the selection of polymer or zeolite materials for making new mixed-matrix membranes due to the lack of permeabihty and selectivity information for most of the pure zeolite materials. In addition, although this Maxwell model is well-understood and accepted as a simple and effective tool for estimating mixed-matrix membrane properties, sometimes it needs to be modified to estimate the properties of some non-ideal mixed-matrix membranes. 

Material Selection for Zeolite/Polymer Mixed-Matrix Membranes... 

The development of a successful zeolite/polymer mixed-matrix membrane with properties superior to the corresponding polymer membrane depends upon good performance match and good compatibility between zeolite and polymer materials, as well as small enough zeolite particle size for membrane manufacturing on a large scale. 

It has been demonstrated by many studies that mixed-matrix membranes with a good match between the permeabihty of proper zeolite materials and these glassy polymers exhibit separahon properties superior to the corresponding pure glassy... 

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... 

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. 

Research has shown that good compatibihty and good adhesion between the polymer matrix and the zeoHte particles in mixed-matrix membranes are of particular importance in forming successful mixed-matrix membranes with enhanced selectivity [41, 60, 61]. Despite aU research efforts, issues of material compatibihty and adhesion at the zeoHte/polymer interface of the mixed-matrix membranes have not been completely addressed. 

Most recently, significant research efforts have been focused on materials compatibility and adhesion at the zeoHte/polymer interface of the mixed-matrix membranes in order to achieve enhanced separation property relative to their corresponding polymer membranes. Modification of the surface of the zeolite particles or modification of the polymer chains to improve the interfacial adhesion provide new opportunity for making successful zeolite/polymer mixed-matrix membranes with significantly improved separation performance. 

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