When Dense or Microporous Materials Control the Overall Process Performance

For membrane operations in which dense or microporous materials control the overall process performance there is no doubt that process intensification will follow directly from improvement of material properties. With the development of new materials having properties controlled at the nanoscale level, operations ofthis kind seem promised a really bright future.

An example is provided by "mixed matrix membranes" (MMM). Basically they are constituted of inorganic molecular nanoparticles (such as zeolites, carbon molecular sieves, etc.) imbedded in polymers. MMMs open up new perspectives in gas separation. A main application for sustainable development is the purification of

Figure 4.4 Mixed matrix membranes (MMM) with covalent linking of rigid entities like buckyballs into polymers.

hydrogen cheaply and efficiently. Figure 4.4 pictures a membrane developed at Twente University [3] by the chemical modification of polymers with covalent linking of rigid entities like buckyballs or nanotubes.

Free volume changes are observed and it appears that covalent bonding enhances the permeability without compromising the selectivity: a strong contrast to dispersing the entities in the polymer matrix as most often classically done.

Another interesting example has been provided by Eric Hoek from the University of California [4]. The membrane presented by the author promises to reduce the cost of sea-water desalination and wastewater reclamation, with a driving pressure lower than in conventional systems, and a considerably reduced fouling; thus, a reduction of about 25% of the overall cost of desalination, including energy consumption and environmental issues, follows. This new membrane is also of MMM type, with a uniquely crosslinked matrix of polymers and engineered nanoparticles structured at the nanoscale. Indeed, molecular tunnels are formed and water flows through them much more easily than nearly all contaminants. The nanoparticles are designed to attract water and are highly porous, soaking up water like a sponge, while repelling dissolved salts and other impurities such as organics and bacteria, which tend to clog up conventional membranes.

A recent publication of Silvestri et al. reports a synthesis on molecular imprinted membranes designed for an improved recognition of biomolecules [5]. The unique feature of molecularly imprinted membranes is the interplay of selective binding and transport properties, making them potentially superior to state-of-the-art synthetic separation membranes already applied in various biological and biomedical fields. By using such materials based on the concept of "key-lock" (ideally, only molecules used for the imprinting will be recognized and thus able to pass the selective layer) it is possible to envisage the highly selective separation of highly toxic substances from drinking water (hormones, pesticides, antibiotics, etc.).

Another interesting example was provided by Teplyakov et al. [6] at the last EuroMembrane Congress in Taormina, in September 2006. The authors deal with processes using porous ceramics with catalytic coating in microchannel walls. This design is valuable in creating high speeds (residence time less than 10~3 s) and improve reactor compactness. Catalytic microporous inorganic membranes combining selective gas transport and catalytic activity can be considered as an "ensemble" of nanoreactors, and as then opening up new fields of applications of heterogeneous catalysis [7]. In such a configuration the classical counter-diffusion in catalyst particles is replaced by an unidirectional transport with the potential of intensified catalysis and increased selectivity.

This example can be applied to a broad class of catalytic reactions but it is much more obvious for partial oxidation reactions where secondary reactions (total combustion) result in a dramatic decrease of selectivity. This is the case with methanol decomposition and methane conversion, where the intensification of gas-phase catalytic operations in micro- or nanochannels clearly appears.

Other promising routes to preparing new kinds of efficient materials include molecular self-assembly, adaptative supramolecular and dynamic chemistry, and hydrothermal synthesis for zeolites [8-10].

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