Microporous carbon materials molecular sieves

In addition to studies focusing exclusively on the catalyst surface, the catalyst support (when employed) can play a major role in enhacing the activity/selectivity via morphologic, electronic, and physico-chemical effects. These factors have been extensively explored in the case of thermochemical heterogeneous reactions where a variety of compounds and structures have been successfully used on an industrial scale as catalyst supports (e.g., oxides, sulfides, meso- and microporous materials (molecular sieves), polymers, carbons [251-256]). In electrocatalysis, on the other hand, the practical choice of support in gas diffusion electrodes has been largely limited thus far to carbon black particles. The high electronic conductivity requirement, combined wifli electrochemical stability and cost effectivness, imposes serious restrictions on the type of materials that could be employed as supports in electrocatalysis. 

Size of micropore of the activated carbon is determined during pyroiyzing and activation steps. Thus small and defined micropores that have molecular sieving effects can be prepared by using proper starting materials and selecting conditions such as carbonizing temperature. 

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. 

Carbon molecular sieves are produced by controlled pyrolysis and subsequent oxidation of coal, anthracite, or organic polymer materials. They differ from zeolites in that the micropores are not determined by the crystal structure and there is therefore always some distribution of micropore size. However, by careful control of the manufacturing process the micropore size distribution can be kept surprisingly narrow, so that efficient size-selective adsorption separations are possible with such adsorbents. Carbon molecular sieves also have a well-defined bi-modal (macropore-micropore) size distribution, so there are many similarities between the adsorption kinetic behavior of zeolitic and carbon molecular sieve systems. 

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. 

This work focuses on the application of water adsorption to the characterisation of narrow microporosity (uitramicroporosity) on carbon molecular sieves (CMS). In addition, the mechanism of water adsorption in carbonaceous solids is analysed. Despite of the presence of surface groups in these materials, they present low adsorption at low relative pressures (P/Po <0.2), indicating the significant role of micropore size in the starting of water adsorption. Interestingly, it has been seen that water adsorbs even on samples, which do not present CO2 adsorption. 

Similarly, there is a great potential in the use of water vapour for the analysis of the porous texture, because it has considerable potential due to both the easy experimental conditions (at room temperature the whole range of relative pressures can be covered) and the characteristics of the molecule itself (polar molecule and small kinetic diameter-0.28 nm). This vapour is widely used in the characterisation of inorganic porous solids, such as zeolites, silicas, and clays. However, its interaction with carbon materials (microporous carbons coals, activated carbon fibres, carbon molecular sieves and porous carbons activated carbons), is more complex than the interaction of non-polar molecules [8]. 

By using potassium as a carbon gasification catalyst, it is possible to obtain activated carbons of large adsorption capacity (large micropore volume), but with micropores of small dimensions. Nevertheless, these materials could not be converted into carbon molecular sieves by carbon deposition from benzene pyrolysis. Success was achieved with chars which were activated only to a limited extent [16]. 

Successful separation of alkanes and alkenes has been documented when microporous membranes have been used [79,138]. The physiochemical properties, size, and shape of the molecules will play an important role for the separation, hence critical temperatures and gas molecule configurations should be carefully evaluated for the gases in mixture. On the basis of gas properties and process conditions, the separation may be performed according to selective surface flow or molecular sieving (refer to Section 4.2 on transport). The transport may also be enhanced by having a Ag compound in the membrane. The Ag ion will form a reversible complex with the alkene, and facilitated transport results. Selectivities in the range of 200-300 have been reported for separation of ethene-ethane and propene-propane [138]. Successful separation of alkanes and alkenes will be important for the petrochemical industry. Today the surplus hydrocarbons in the purge gas are usually flared. Membranes which should be suitable for this application are the carbon molecular sieves (see Section 4.3.2) and nanostructured materials (Section 4.3.3). 

In materials containing micropores (and mesopores), such as molecular sieves and activated carbons, the boundary conditions (multilayer adsorption) are not fulfilled and the BET analysis is not applicable here. When (traditionally) reported in the literature, the BET surface area should be understood as a niunber proportional to the pore volume rather than the specific surface area. Note also that conceptually the term specific surface area is inapplicable once the solid material surroimds sorbed molecules. 

The history of mesoporous material synthesis is unintentionally or intentionally duplicating the development of zeolites and microporous molecular sieve. It starts from silicate and aluminosilicate, through heteroatom substitution, to other oxide compounds and sulfides. It is worth mentioning that many unavailable compositions for zeolite (e.g., certain transition metal oxides, even pure metals and carbon) can be made in mesoporous material form. 

The main difference between the synthesis of MCM-41 mesoporous material and traditional synthesis of zeolite or silica molecular sieve is the use of different templates. An individual organic molecule or metal cation is used for the traditional synthesis of silica microporous molecular sieve. For example, the typical template for ZSM-5 synthesis is tetrapropylammonium ion the crystal is formed through the condensation of silicate species around the template molecule, while for the formation of MCM-41, the typical template is the assembly of large molecules containing one hydrophobic chain with more than 10 carbons. 

More promising for reactive separations involving gas phase reactions appears to be the development and use in such applications of microporous zeolite and carbon molecular sieve (Itoh and Haraya [2.25] Strano and Foley [2.26]) membranes. Zeolites are crystalline microporous aluminosilicate materials, with a regular three-dimensional pore structure, which are relatively stable to high temperatures, and are currently used as catalysts or catalyst supports for a number of high temperature reactions. One of the earliest mentions of the preparation of zeolite membranes is by Mobil workers (Haag and Tsikoyiannis... 

The analysis of the previous results indicates that tire molecular sieving effect would appear in aqueous medium for carbon materials with micropores smaller than 0.58 nm which, depending on the electrolyte, will participate or not in the charging of the double layer. This effect will be more pronounced when using organic electrolytes in which the solvating molecules are larger than water [38]. 

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