Activated carbons with narrow micropore size distributions

In previous sections, it was explained how different precursors can be used to produce activated carbons and the type of porosity developed depending on the type of activation method applied. Thus, thermal activation imrmally yields adsorbents with a medium to liigh adsorption capacity, a medium micropore size distribution and no mesopore formation (except in the case of high bum-off ratios, where micropores may be of a large size). Phosphoric acid activation yields a carbon with a higher adsorption capacity than thermal activation and a wider micropore size distribution (even in the low mesopore range), whereas KOH yields extremely narrow microporous carbons. 

Manso et studied the formation of CMS by carbon vapor deposition (CVD) over activated carbons from four different rank coals. The deposition of carbon was carried out by pyrolyzing benzene vapors at 725°C. This produced gradual closing of the micropores, due to the formation of constrictions at their entrances. As a result the MSC with a narrow micropore-size distribution around 0.35 to 0.5 nm were obtained. Samples with diameters smaller than 0.33 mn obtained by a high degree of deposition were able to separate O2/N2 and CO2/CH4 mixtures. 

High porosity carbons ranging from typically microporous solids of narrow pore size distribution to materials with over 30% of mesopore contribution were produced by the treatment of various polymeric-type (coal) and carbonaceous (mesophase, semi-cokes, commercial active carbon) precursors with an excess of KOH. The effects related to parent material nature, KOH/precursor ratio and reaction temperature and time on the porosity characteristics and surface chemistry is described. The results are discussed in terms of suitability of produced carbons as an electrode material in electric double-layer capacitors. 

Activation with KOH was recognized originally as an efficient way of producing microporous carbons with relatively narrow pore size distribution and extremely high surface area. The results of present study demonstrate a considerable flexibility of the process in terms of porosity development and, to some extent, surface properties. 

Thermogravimetry/mass spectrometry was used to determine adsorptive capacity of several commercially available activated carbons produced from coal, coconut, and petroleum pitch precursors. The range of their N2 BET surface areas was between 400 to 2000 m /g. Although, carbons with high adsorption capacity contained similar C, N, and O contents, proximate analyses, surface areas and micropore volumes, no significant correlations were found between chemical and physical properties and the NO, adsorptive capacity. One possibly important characteristic of the carbons correlated with NO, adsorption capacity was specific and narrow pore size distribution with an effective pore diameter of 0.56 nm. 

By these means it is possible to prepare carbon sieves with effective micropore diameters ranging from about 4 to 9 A. The micropore size distribution of such sieves is much narrower than in a typical activated carbon and the porosity and therefore the adsorptive capacity are generally very much smaller, as may be seen from Figure 1.2. The ability to modify the effective pore size by adjusting the conditions of the manufacturing process makes it relatively easy to tailor a carbon sieve to achieve a particular separation. However, it is difficult to achieve absolute reproducibility between different batches, and the existence of a distribution of pore size, even if narrow, means that the molecular sieving selectivity of a carbon sieve seldom approaches the almost perfect separation achievable under fav orable circumstances with a zeolite sieve. Nevertheless, the kinetic selectivities which may be attained with a well-prepared carbon sieve are remarkably high. 

The DR equation (Equation 2.121) is applicable more accurately to microporous activated carbons containing a narrow distribution of micropores. Strongly activated carbons that have a wider distribution of micropore volume as a function of the micropore size can be described more accurately by the superposition of two distributions " of micropores with effective radius r less than 0.6 to 0.7 nm and of supermicropores with radii of 0.7 to 1.6 nm as... 

Carbon molecular sieves (CMS) have played a critical role in the commercialization of the pressure swing adsorption process for the separation of nitrogen from air. They differ from activated carbon mainly in the pore size distribution and surface area. While activated carbons have a broad range of pores, with a typical average pore diameter of 20 A, carbon molecular sieves have a more narrow pore size distribution, with pore sizes in the range of 3 - 5 A. A molecular probe method is one of the best approaches to determine the effective micropore size distribution of carbon molecular sieves [3,4]. Typical surface areas for a carbon molecular sieve are in the range of 250-400 m /g, while the micropore volume is about 0.15-0.25 cm Vg [2,5]. 

Activated carbons are adsorbents with a wide pore size distribution and consequently the precise determination of their porous structure is a rather difficult task. Since activated carbons are es.sentially microporous, most work devoted to their characterization is centred around the determination of the microporosity. Several methods have been extensively used to analyze the adsorption isotherms of nitrogen and other ad.sorptives. The micropore volume filling theory of Dubinin has been successfully used but there are well-known problems when the micropore size distribution is heterogeneous (refs. 1,2). The n-nonane preadsorption technique has also been used in the last few years but it provides information only on narrow microporosity and the results are conditioned by interconnectivity network of the porosity (refs. 3,4). The t- and a-plot methods have also been widely applied using different non-porous reference materials, the selection of which may be critical (refs. 5,6). 

CNTs have a different porous structure than activated carbon. The specific surface area of CNTs can range from 50 m2/g (multi-walled CNTs with 50 graphene walls) to 1315 m2/g (single-walled CNTs). Theoretically, the porous structure of CNTs is identical to the tubular structure of CNTs and the pore sizes of CNTs correspond to the inner diameters of opened CNTs and should have a narrow distribution. Activated carbons usually have a broad pore distribution covering micropore, meso-pore and macropore ... 

We have an excellent activated carbon of fiber morphology, so called activated carbon fiber ACF[3]. This ACF has considerably uniform slit-shaped micropores without mesopores, showing characteristic adsorption properties. The pore size distribution of ACF is very narrow compared with that of traditional granular activated carbon. Then, ACF has an aspect similar to the regular mesoporous silica in particular in carbon science. Consequently, we can understand more an unresolved problem such as adsorption of supercritical gas using ACF as an microporous adsorbent. 

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. 

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