Carbon adsorbents pore size

Valuable information about processes at the water/adsorbent interfaces can be furnished by measurements of the thickness of a layer of bound water and the value of free surface energy of an adsorbent in an aqueous medium. As it was shown earlier, these characteristics can be obtained by measuring dependences of the signal intensity for unfrozen (melted) water on temperature. However, while for porous carbons such dependences are limited by the adsorbent pore size, in the case of nonporons materials it is possible to investigate the whole layer of water subjected to the disturbing action of the surface (in approximation of a smooth surface). 

Lu Q, Serial GA. The role of adsorbent pore size distribution in multi-component adsorption on activated carbon. Carbon 2004 42(15) 3113-3142. 

Carbons prepared from sucrose have surface areas up to 1500 m g , and pore volumes up to 1 cm g (Table 1). These carbons also have micropore volumes of around 0.210 cm g, which is likely related to the intrinsic microporosity of the carbon precursor. For both samples a capillary condensation occurs in the p/po range between 0.1-0.4 and 0.3-0.6, which clearly indicates that the structural porosity of the silica framework is replicated in the carbon. The pore size distributions for these carbons are very narrow with peak maxima at 1.2 nm and 1 nm, for T30 SUC and T60 SUC, respectively. Additionally, a slight development of complementary mesoporosity (estimated from the amount of nitrogen adsorbed at p/po > 0.7) can be seen, accounting for 6 % and 7 % of the total pore volume. 

A factor militating against the use of other adsorptives for pore size determination at the present time is the lack of reliable r-curves. The number of published isotherms of vapours such as benzene, carbon tetrachloride or the lower alkanes, or even such simple inorganic substances as carbon dioxide, on a reasonable number of well-defined non-porous adsorbents, is very small. 

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). 

The models of Matranga, Myers and Glandt [22] and Tan and Gubbins [23] for supercritical methane adsorption on carbon using a slit shaped pore have shown the importance of pore width on adsorbate density. An estimate of the pore width distribution has been recognized as a valuable tool in evaluating adsorbents. Several methods have been reported for obtaining pore size distributions, (PSDs), some of which are discussed below. 

Mcntasty el al. [35] and others [13, 36] have measured methane uptakes on zeolites. These materials, such as the 4A, 5A and 13X zeolites, have methane uptakes which are lower than would be predicted using the above relationship. This suggests that either the zeolite cavity is more attractive to 77 K nitrogen than a carbon pore, or methane at 298 K, 3.4 MPa, is attracted more to a carbon pore than a zeolite. The latter proposition is supported by the modeling of Cracknel et al. [37, 38], who show that methane densities in silica cavities will be lower than for the equivalent size parallel slit shaped pore of their model carbon. Results reported by Ventura [39] for silica xerogels lead to a similar conclusion. Thus, porous silica adsorbents with equivalent nitrogen derived micropore volumes to carbons adsorb and deliver less methane. For delivery of 150 V./V a silica based adsorbent would requne a micropore volume in excess of 0.70 ml per ml of packed vessel volume. 

Activation of zeolites is a dehydration process aceomplished by the application of heat in a high vacuum. Some zeolite crystals show behavior opposite to that of activated carbon in that they selectively adsorb water in the presence of nonpolar solvents. Zeolites can be made to have specifie pore sizes that will increase their seleetive nature due to the size and orientation of the molecules to be adsorbed. Moleeules above a specific size could not enter the pores and therefore would not be adsorbed. 

Effectiveness of selective adsorption of phenanthrene in Triton X-100 solution depends on surface area, pore size distribution, and surface chemical properties of adsorbents. Since the micellar structure is not rigid, the monomer enters the pores and is adsorbed on the internal surfaces. The size of a monomer of Triton X-100 (27 A) is larger than phenanthrene (11.8 A) [4]. Therefore, only phenanthrene enters micropores with width between 11.8 A and 27 A. Table 1 shows that the area only for phenanthrene adsorption is the highest for 20 40 mesh. From XPS results, the carbon content on the surfaces was increased with decreasing particle size. Thus, 20 40 mesh activated carbon is more beneficial for selective adsorption of phenanthrene compared to Triton X-100. 

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