Carbon molecular sieve membranes pores size

Molecular sieves are porous aluminosilicates (zeolites) or carbon solids that contain pores of molecular dimensions which can exhibit seleaivity according to the size of the gas molecule. The most extensive study on carbon molecular sieve membranes is the one by Koresh and Soffer (1980,1987). Bird and Trimm (1983) also described the performance of carbon molecular sieve membranes, but they were unable to prepare a continuous membrane. Koresh and Soffer (1980) prepared hollow-fiber carbon molecular sieves, with pores dimensions between 0.3 and 2.0 run radius (see Chapter 2). 

K. Between 973°K and 1023°K, the membranes were highly effective in the separation of absorbable and non-absorbable spedes which were considered as ASCM instead of carbon molecular-sieving membranes. Over 1073 K the carbon structure becomes more ordered, which implies decrease of pore size causing molecular-sieving behavior. 

With appropriate membrane pore size and a narrow distribution, membrane selectivity for smaller gas molecules can be high but the overall permeability is generally low due to a high flow resistance in fine pores. Several studies are being conducted to develop molecular sieve-type membranes using different inorganic materials, for example, those based on carbon (Liu, 2007), silica (Pex and van Delft, 2005), and zeolites (Lin, 2007). 

Recently, carbon molecular sieves have been fabricated in the form of planar membranes and hollow tubes by the pyrolysis of polyacrylonitrile in suitable forms (12-16). Very high separation selectivities have been reported with these materials. Their pore sizes are in the range from 3 to 5.2A. Selectivities of greater than 100 1 are observed between molecules which differ by as little as 0.2A in their critical dimensions. Kinetics of adsorption on these materials have been determined (2.,ii,l ) -... 

Molecular sieves such as zeolites or carbon molecular sieves show a much higher selectivity for many gas mixtures than polymeric membranes due to their very defined pore sizes. For example it can be calculated from reported sorption and diffusivity data that zeolite 4A has an oxygen permeability of 0.77 Barter and an O2/N2 selectivity of approximately 37 at 35 °C [308]. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Activated charcoal was originally regarded as a relatively inexpensive adsorbent with an assortment of pores of ill-defined size and shape. However, in recent years considerable progress has been made in the development of tailor-made porous carbons such as molecular sieves, activated carbon fibres and carbon composites (Marsh et al., 1997). Superactive carbons are now made on a commercial scale with BET areas of around 3000 m2g-1. Activated carbons can be manufactured as fine particles or granules or in the form of a cloth, felt or consolidated membrane. The properties of some of these special types of activated carbon are discussed in Chapter 12. 

The best answer to the permeability/permselectivity optimization would be to synthesize very thin layers of materials having a comparatively high porosity and pore sizes in the range 5-8 A so as to achieve molecular sieving effects. Instead of the modification of already available membranes, the synthesis of new membranes seems more appropriate to reach the above goal. The two most promising candidates in this context are carbon and zeolite membranes. 

Figure 4.11 illustrates a carbon membrane with pores in the range suitable for molecular sieving [78]. As expected, there is a clear and indisputable correlation between flux and molecular size. In Figure 4.12, the carbon membrane is more open (pore size in the range 6-10 A). The gas pair reported is CO2 and CH4, and as can be seen, the selectivity is clearly in favor of CO2 indicating selective surface flow. The critical temperatures, 7)., and Lennard-Jones diameters, for the two gases are... 

The nanoporous carbon membrane consists of a thin layer (<10pm) of a nanoporous (3-7 A) carbon film supported on a meso-macroporous solid such as alumina or a carbonized polymeric structure. They are produced by judicious pyrolysis of polymeric films. Two types of membranes can be produced. A molecular sieve carbon (MSC) membrane contains pores (3-5 A diameters), which permits the smaller molecules of a gas mixture to enter the pores at the high-pressure side. These molecules adsorb on the pore walls and then they diffuse to the low-pressure side of the membrane where they desorb to the gas phase. Thus, separation is primarily based on differences in the size of the feed gas molecules. Table 7 gives a few examples of separation performance of MSC membranes. ° Component 1 is the smaller component of the feed gas mixture. 

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