Mixed-matrix technology

For liquid-polymer MMMs, the physical state of the fillers incorporated into the continuous polymer matrix is liquid such as polyethylene-glycol (PEG) and amines [19]. Existing literature reveals that this new type of membrane is less developed [19]. Liquid-polymer MMMs are a less commonly used mixed-matrix technology, dne to the long-term stability encapsulated in the continuous polymer matrix. A new type of MMM has recently been developed in an attempt to deal with the disadvantages of liqnid-polymer MMMs. Solids, snch as activated carbon impregnated with liquid polymer (e.g., PEG), function as stabilizers of the liquid polymer in the continuous phase. Eurthermore, activated carbon increases the MMM performance. 

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. 

Figure 30.2 Material combinations in mixed-matrix technology.

The first known article concerning mixed-matrix membranes was published in 1960 by Barter and James. It examined several different zeolites dispersed in an inert polymer resin to create ion exchange membranes." " Voids were noted at the interface of the two media (i.e., where polymer and sieve meet). These voids resulted in a degradation of mixed-matrix performance. Today, researchers still struggle with material compatibility issues that exist at the solid-polymer interface, as well as other nonideal morphologies introduced by the solid-polymer mixed-matrix technology. ... 

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. 

Thin-film composite mixed-matrix membranes can be made by a coating method or by a dual-layer co-extrusion technology, as discussed in Section 11.5.2.2. 

Gas separation processes with membranes have undergone a major evolution since the introduchon of the first membrane-based industrial hydrogen separation process about two decades ago. The development of high selectivity mixed-matrix membranes will further advance the technology of membrane gas separation processes within the next decade. 

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... 

Husain, S. (2006). Mixed Matrix Dual Layer Hollow Fiber Membranes for Natural Gas Separation, in Chemical Biomolecular Engineering, PhD Dissertation, Georgia Institute of Technology. 

Of the three important technologies currently being pursued for CO2 capture, namely liquid absorption, solid adsorption [5], and membrane separations [6], membrane-based separation is promising since it is more energy efficient, mechanically simple and easy to scale-up with a smaller footprint. Polymeric, inorganic (silica, zeolite etc.), mixed matrix, and facilitated transport membranes are currently being investigated for this application [7]. 

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). 

P.S. Goh, A.F. Ismail, S.M. Sanip, B.C. Ng, M. Aziz, Recent advances of inorganic fillers in mixed matrix membrane for gas separation. Separation and Purification Technology... 

V. Naflsi, M.-B. Hagg, Gas separation properties of ZIF-8/6FDA-durene diamine mixed matrix membrane. Separation and Purification Technology 128 (2014) 31-38. 

H. Sun, C. Ma, B. Yuan, T. Wang, Y. Xu, Q. Xue, P. Li, Y. Kong, Cardo polyimides/ Ti02 mixed matrix membranes synthesis, characterization, and gas separation property improvement, Separation and Purification Technology 122 (2014) 367-375. 

Ismail, A. F., Goh, P. S., Sanip, S. M., Aziz, M. (2009). Transport and separation properties of carbon nanotube-mixed matrix membrane. Separation and Purification Technology, 70, 12-26. 

Various treatment methods based on conventional, modem and hybrid technologies have been applied for remediation of F , U and As in many parts of the world. These techniques have been critically reviewed in this chapter. Metal organic framework based mixed-matrix membranes have been reported to outperform state-of-art polymers. These composite membranes containing various adsorbents/fillers such as zeolites have high application potential and should be studied further for removal of heavy metals from wastewater. 

Jamshidi Gohari, R., Lau, W.J., Matsuura, T., Halakoo, E. Ismail, A.F. (2013) Adsorptive removal of Pb(II) from aqueous solution by novel PES/HMO ultrafiltration mixed matrix membrane. Separation and Purification Technology, 120, 59-68. 

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. 

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). 

Materials combirnation in mixed-matrix polymeric membrane technology. 

A next-future application of MOFs is predicted for MOF-oiganic polymer-based mixed matrix membranes in hollow fiber or spiral wound geometry. Established membrane production technologies such as hollow fiber spinning or foil casting can be used for the preparation of mixed matrix membranes. Further, MOFs match much better with organic polymers than zeolites, for example, even if the latter have been hydrophobized, such as by silylation. 

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. 

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