Mixed-matrix membranes concept

Concept of Zeolite/Polymer Mixed-Matrix Membranes... 

Concept of Zeolite/Polymer Mixed-Matrix Membranes 335 Pz+2Pp-2[Pg.335]

Geong and coworkers reported a new concept for the formation of zeolite/ polymer mixed-matrix reverse osmosis (RO) membranes by interfacial polymerization of mixed-matrix thin films in situ on porous polysulfone (PSF) supports [83]. The mixed-matrix films comprise NaA zeoHte nanoparticles dispersed within 50-200 nm polyamide films. It was found that the surface of the mixed-matrix films was smoother, more hydrophilic and more negatively charged than the surface of the neat polyamide RO membranes. These NaA/polyamide mixed-matrix membranes were tested for a water desalination application. It was demonstrated that the pure water permeability of the mixed-matrix membranes at the highest nanoparticle loadings was nearly doubled over that of the polyamide membranes with equivalent solute rejections. The authors also proved that the micropores of the NaA zeolites played an active role in water permeation and solute rejection. 

The new concept of using mixed-matrix membranes with commercially attractive thin-film composite geometry for desalination of water has been demonstrated by Jeong and coworkers [83]. 

Mixed-matrix membranes have been a subject of research interest for more than 15 years [28-33], The concept is illustrated in Figure 8.10. At relatively low loadings of zeolite particles, permeation occurs by a combination of diffusion through the polymer phase and diffusion through the permeable zeolite particles. The relative permeation rates through the two phases are determined by their permeabilities. At low loadings of zeolite, the effect of the permeable zeolite particles on permeation can be expressed mathematically by the expression shown below, first developed by Maxwell in the 1870s [34],... 

For example, Ronco [48] suggested the possibility of mixing the biomaterial of a specific membrane with a sorbent material based on a significant enhancement of internal and backfiltration in hollow-fiber hemodialyzers. Thus, the single membrane will have both characteristics, i.e., diffusion and adsorption for removal of uremic acid, and is called mixed matrix membrane (MMM) [49,50]. The MMM concept had been proposed earlier as an alternative to traditional chromatographic column [51]. 

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. 

To explore the difficulties in practical implementation of the above concepts, mixed matrix membranes, with 20% molecular sieves (by volume), were prepared by solution deposition on top of a porous ceramic support. The ceramic supports used were Anodise membrane filters which had 200 A pores that open into 2000 A pores and offer negligible resistance to gas flow. Initially the molecular sieve media, zeolites (4A crystals) or carbon molecular sieves, was dispersed in the solvent, dichloromethane, to remove entrapped air. After two hours, Matrimid was added to the mixture, and the solution was stirred for four hours. The solutions used varied in polymer content from 1-5 wt %. The solution was then deposited on top of the ceramic support, and the solvent was evaporated in a controlled manner. The membranes were then dried overnight at 90°C under vacuum. This was followed by a reactive intercalation post treatment technique 15) to eliminate defects. This technique involves imbibing a reactive monomer (e.g. diamine) from an inert solvent (e.g. heptane) into any micro defects. Next, a second reactive monomer (e.g. acid chloride) was introduced to reactively close defects by forming a low permeability polymer. The membranes were dried again to remove the inert solvent. Individual membrane thickness was determined by weight gain and varied from 5 to 25 Jim. 

Mixed matrix membranes (MMM) consist of a nanopaiticle filler like zeolite, metal-organic framework ionic liquid or carbon, in a continuous polymer phase thereby combining the molecular sieving or another property of the filler with the established processability of the polymers in one membrane. The concept of zeolite-based mixed matrix membranes is followed for more than 30 years. However, in most cases these zeolite-based MMMs did not show an improvement of the selectivity because of an imperfect embedding of the zeolite crystals into the polymer matrix as shown schematically in Fig. 19. 

The concept of mixed-matrix membranes has been demonstrated at UOP ° in the mid-1980s using silicalite-cellulose acetate mixed-matrix membranes for CO2/H2 separation. In the demonstration, a feed mixture of 50/50 CO2/H2 with a differential pressure of 50 psi was used. The calculated separation factor for CO2/H2 was found to be 5.15 + 2.2. In contrast, a CO2/H2 separation factor of 0.77 + 0.06 was found for cellulose acetate membrane. This indicates that silicalite in the membrane phase reversed the selectivity from H2 to CO2. Experimental results and modeling predictions indicate that mixed-matrix membranes with the incorporation of fillers within polymeric substrates provide potential possibilities to achieve enhanced membrane performance, which will open up new opportunities for the separation and purification processes. Highlighted applications for mixed-matrix membranes include separation and purification of gas mixtures such as separation of N2 from CO2 removal from natural and separation... 

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." ... 

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. 

Mixed-matrix interfacial polymerization has been developed to embed nanoparticles throughout the polyamide thin film layer. This concept aims to improve the membrane performances. Super-hydrophilic zeolite nanoparticles are used to enhance the water permeation while maintaining high salts rejection [113]. A similar approach has been developed to fabricate aquaporin-based biomimetic membranes that have superior separation performance [114]. 

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