Solid-polymer mixed-matrix membranes

Liu C, Kulprathipanja S, Schott ME, Wilson ST. Ionic liquid-solid-polymer mixed matrix membranes for gas separations. U.S. Patent 7943543 Bl 2011. 

For solid-polymer mixed-matrix membranes, the solid fillers dispersed in the polymer... 

While most solid-polymer mixed-matrix membranes rely on sieve micropores to generate high selectivities within the membrane by discriminating between gas penetrants, mesoporous molecular sieves have been studied as well. These mesoporous sieves arc used to increase permeabUity in mixed-matrix membranes, instead of enhancing selectivity. Mesoporous MCM-41 incorporated into a polysulfone matrix improved membrane permeability for aU gases tested. The selectivity of pure polysulfone was maintained, as expected. Mesoporous sieves are an excellent material for highly selective polymer membranes that require increased productivity however, nanoscopic-scale crystals will probably be required to enable accommodation within the submicron selective layer typical for high-performance asymmetric membranes in use today. 

How to Formulate and Process Solid-Polymer Mixed-Matrix Membranes... 

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. 

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. ... 

Several polymers other than PDMS and PVA have been examined for solid-polymer mixed-matrix pervaporation. An activated earbon-block polyether-amide mixed-matrix membrane was investigated for the separation of volatile organie eompounds (VOCs). An enhancement in both trichloroethane and water flux was noted, as well as increase in trichloroethane-water selectivity compared to the neat polymer. Zeolite-polyimide mixed-matrix membranes were explored for xylraie isomo- separation. Solid-polymer adhesion was poor, resulting in a sieve-in-a-eage morphology therefore, no separation properties that exceeded neat polymer membranes were identified. Methanol-toluene separation was successfully performed with NaX—Viton mixed-matrix membranes, though other solid-polymer combinations were attempted and failed to form an enhanced membrane. 

Preparation of Solid-Liquid-Polymer Mixed-Matrix Membranes A... 

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. 

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). 

A number of polymers have been embedded with zeolites to form mixed-matrix membranes however, like Barrer, many researchers note poor adhesion at the solid-polymer interface. The use of silane coupling agents to chemically link polymer to the zeolite particles, membrane formation under high-temperature conditions, and membrane annealing are common tools used to heal a poorly adhered, sieve-in-a-cage, interface. ... 

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." ... 

Poly (vinyl alcohol) (PVA) has also been applied to alcohol-water pervaporation. ZSM-5 was again utilized, and mixed-matrix enhancements resulted. The ZSM-5/PVA mixed-matrix membranes demonstrated increased selectivity and flux, compared to pure PVA, for the water-isopropyl alcohol separation. Membrane swelling and fluxes increase as the water concentration in the feed increases. " Sorption studies of ZSM-5/PVA systems indicate that this hybrid material is a good candidate for the extraction of water from alcohol. Modified poly (vinyl chloride) embedded with NaA zeolite has demonstrated both flux and selectivity enhancements for the ethanol-water separation at high zeolite loadings. Voids at the solid-polymer interface prevented seleetivity enhaneement at low zeolite loadings. ... 

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. 

Figure 11.22 Mixed-gas ethylene/ethane selectivity of a solid polymer electrolyte membrane as a function of AgBF4 concentration in the polyamide-polyether matrix [62]...

Membrane-like heteropoly acid-blended polymer film catalysts were prepared using a common solvent (or mixed solvents) and they were tested as fixed-bed catalysts for the ethanol conversion reaction in a continuous flow reactor. It was found that heteropoly acid catalyst was finely and uniformly distributed through the polymer matrix. All the film catalysts showed the higher selectivity to acetaldehyde than the bulk solid catalyst. Conversion and selectivity over the film catalysts were also affected by the nature of solvent and polymer. Microporosity of the film catalyst was controlled by the phase separation method. The microporous film catalyst could be regarded as a highly dispersed heteropoly acid catalyst supported on polymer matrix. The film catalysts were characterized by IR, TPD, SEM, EDX, DSC, and ESCA. 

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