Nanocomposite Membranes

There are a large number of nanofiltration and gas separation applications with membranes made from a polymeric or an inorganic material. The application of, for example, a polymeric material for a separation membrane depends of course upon both the throughput and the purity of the product transported through the membrane. This means that the permeability coefficient and the selectivity should be as large as possible. However, it has been found that simple structural modifications, which lead to an increase in product flux, usually cause a loss in permselectivity and vice versa [37-41]. This so-called "trade-off" relationship is well described in the literature [42, 43]. In recent years, the efforts and successes in synthesizing a variety of nano-structured hybrid materials have provided a new degree of freedom for the development of advanced materials with enhanced separation properties [44]. See Figure 13.

Nanocomposites represent a current trend in developing novel nanostructured materials. They can be defined as

Figure 13. Gas molecules (red) move through the spaces between the polymer chains (in green). The larger the spaces, the faster and more selective the movement. The finding, which appeared in Science, describes a new type of nanoparticle-enhanced filter for separating compounds at the molecular level. Reprinted with permission from [46], A. Hill et al., Science 296, 519 (2002). © 2002, American Association for the Advancement of Science.

Figure 13. Gas molecules (red) move through the spaces between the polymer chains (in green). The larger the spaces, the faster and more selective the movement. The finding, which appeared in Science, describes a new type of nanoparticle-enhanced filter for separating compounds at the molecular level. Reprinted with permission from [46], A. Hill et al., Science 296, 519 (2002). © 2002, American Association for the Advancement of Science.

a combination of two or more phases containing different compositions or structures, where at least one of the phases is in the nanoscale regime. Kusakabe and his co-workers [45] already reported in 1996 that the permeability of CO2 in a polyimide/SiO2 hybrid nanocomposite membrane is 10 times larger than in the corresponding polyimide.

Another application is the use of zeolite nanocrystals (10-100 nm) to combine the advantages of polymers and zeolites while overcoming the shortcomings of both. Polymer-zeolite nanocomposite membranes can be developed for air separation as a promising alternative to the conventional energy-intensive cryogenic distillation. Utilization of polymer-zeolite nanocomposite membranes shows a route to achieve a modified polymer matrix offering high molecular sieving selectivities (O2/N2 > 20) while maintaining polymer processing conditions.

For any molecule to move across a membrane, it must go through a solution-diffusion process. The molecule first has to get into the membrane—the solution part of the process—then diffuse through it. Recently novel gas separation nanomembranes have been obtained with inorganic silica nanoparticles, with contradictory, but very useful, properties. It was shown that larger molecules dissolved much faster into the membrane, and once in, moved right through to the other side before the smaller molecules had completed the first step.

Silica nanoparticles embedded in a carbon base may help in the future to produce gases free of impurities. Because of the nanocomposite's ability to trap molecular-sized impurities it could be further used in processes such as biomolecule purification, environmental remediation, sea water desalination, and petroleum chemicals and fuel production [46].

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