Molecular-sieve carbon, pore size distribution

Molecular-sieve carbon, pore size distribution, 89... 

In addition, depending on the size of the adsorbate molecules, especially in the case of some organic molecules of a large size, molecular sieve effects may occur either because the pore width is narrower than the molecules of the adsorbate or because the shape of the pores does not allow the molecules of the adsorbate to penetrate into the micropores. Thus, slit-shaped micropores formed by the spaces between the carbon layer planes are not accessible to molecules of a spherical geomehy, which have a diameter larger than the pore width. This means that the specific surface area of a carbon is not necessarily proportional to the adsorption capacity of the activated carbon. Pore size distribution, therefore, is a factor that cannot be ignored. 

The extent of competition may also be a function of the adsorbate molecular size, correlated with the activated carbon pore size distribution. Activated carbon fibers tliat are exclusively microporous (more than 96 % of micropore volume) present a selectivity property for pesticides or phenol in the presence of higher molecidar weight compounds like humic substances, due to the direct connection of micropores to their external surface [41]. Using granular activated carbon, which does not have this molKUilar sieve property, a 20 to 70 % reduction in adsorption is obtained for atrazine in raw water compared with equilibria in distilled water [42]. 

Fig. 2. Pore size distribution of typical samples of activated carbon (small pore gas carbon and large pore decolorizing carbon) and carbon molecular sieve (CMS). A / Arrepresents the increment of specific micropore volume for an increment of pore radius.

Typical pore size distributions for these adsorbents have been given (see Adsorption). Only molecular sieve carbons and crystalline molecular sieves have large pore volumes in pores smaller than 1 nm. Only the crystalline molecular sieves have monodisperse pore diameters because of the regularity of their crystalline stmctures (41). 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

Horvath G and Kawazoe K. Method for the calculation of effective pore-size distribution in molecular-sieve carbon. J. Chem. Eng. Jpn., 1983 16(6) 470-475. 

Fig. 1. Pore-size distribution for activated carbon, silica gel, activated alumina, two molecular-sieve carbons, and zeolite 5A (Yang, 1997).

Porous carbonaceous materials are important in many application areas because of their remarkable properties, such as high surface areas, chemical inertness, and good mechanical stability. Carbon molecular sieves that are amorphous and microporous are commercially important for the separation of nitrogen from air, and activated carbons with a wide pore size distribution are also useful adsorbents for various applications. 

Two series of carbon molecular sieves have been prepared from coconut shells, with different pore size distribution. They have been characterised by carbon dioxide adsorption at 273 K and immersion calorimetry into liquids of different molecular sizes. The results have been related with the abihty of the CMS to separate the components of O2/N2, CO2/CH4 and n-C4H4/i-C4H4 gas mixtures. 

Figure 4 represents the evolution of the EoWo/EorcfWo ref ratio for the different carbon molecular sieves of the two series, as a function of the molecular size of the immersion liquid, and using CH2CI2 as a reference. A decrease of this ratio as the size of the immersion liquid increases indicates that the accessibility of the porosity is limited. It can be seen that the pore size distributions obtained by this method are comparable to those shown in Figure 3, corresponding to the surface area accessible to the different immersion liquids. In conclusion of this pore-size analysis, a variety of CMS with different pore size distribution, but always smaller than 0.7 nm, have been obtained. CMS with the narrowest pore diameter are prepared from the acid-washed precursors, i.e., without ashes able to catalyse the gasification reaction. 

Immersion calorimetry can be apply successfully to the characterisation of CMS to evaluate their pore size distribution and, in this way, their ability to separate gas mixtures as a function of their molecular size. On the other hand, carbon molecular sieves can be prepared from coconut shells by activation with C02. These materials can be used for the separation of gas mixtures such as O2/N2, CO2/CH4 and n-C4Hio/i-C4Hio. 

The polymer/zeolite host-guest precursor for preparation of porous carbon can also be obtained through direct contact of monomers in a carrier gas with zeolite molecular sieves followed by polymerization. For example, propylene can enter into zeolite Y under the carriage of N2 and polymerize to form polypropylene. After pyrolysis, the polypropylene undergoes carbonization, and the host zeolite framework of the carbonization product can be removed by dissolution in acids, leaving carbon material with characteristic pores.[93] However, the pore-size distribution of the porous carbons obtained through this approach is not uniform, and hence they are hardly used as molecular sieves for sieving small molecules. 

G. Horvath and K. Kawazoe, Method of calculation of effectiveness pore size distribution in molecular sieve carbons. /. Chem. Eng., pn., 16 (1983) 470. 

Since carbon molecular sieves are amorphous materials, the dimensions of their pore structures must be measured phenomenologically by the adsorption of small probe molecules with different critical dimensions. There is insufficient long range order to utilize standard x-Ray diffraction methods for characterization. The earliest reports of molecular sieving carbons dealt primarily with coals and charcoals. Sorption of helium, water, methanol, n-hexane, and benzene was measured and related to the porosity of the carbon. Pore-sizes were estimated to be two to six angstroms (3-6). In a classic paper P.H. Emmett described methods for tailoring the adsorptive properties and pore size distributions of carbon Whetlerites. 

Pore sizes and their distributions in adsorbents have to comply with requirements fi om different applications. Thus, relatively small pores are needed for gas adsorption and relatively large pores for liquid adsorption, and a very narrow PSD is required for molecular sieving applications. Macropores in carbon materials were found to be effective for sorption of viscous heavy oils. Porous carbons can respond to these widely ranged requirements from the applications, which is one of advantages of carbon materials even though pore size distributes in a certain range in the majority of porous carbons. Recent novel techniques to control pore structure in carbon materials (see Section 5) can be expected to contribute to overcome this limitation. 

Molecular sieving carbons (MSCs) have a smaller pore size with a sharper distribution in the range of micropores in comparison with other activated carbons for gas and liquid-phase adsorbates. They have been used for adsorbing and eliminating pollutant samples with a very low concentration (ethylene gas adsorption to keep fruits and vegetables fresh, filtering of hazardous gases in power plants, etc.) An important application of these MSCs was developed in gas separation systems [1-2]. 

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