Molecular sieving carbons

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

Despite the difference ia the nature of the surface, the adsorptive behavior of the molecular sieve carbons resembles that of the small pore zeoHtes. As their name implies, molecular sieve separations are possible on these adsorbents based on the differences ia adsorption rate, which, ia the extreme limit, may iavolve complete exclusion of the larger molecules from the micropores. 

Amorphous siHca gel, activated alumina, activated carbon, and molecular sieve carbons. 

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). 

Exit gases from the shift conversion are treated to remove carbon dioxide. This may be done by absorbing carbon dioxide in a physical or chemical absorption solvent or by adsorbing it using a special type of molecular sieves. Carbon dioxide, recovered from the treatment agent as a byproduct, is mainly used with ammonia to produce urea. The product is a pure hydrogen gas containing small amounts of carbon monoxide and carbon dioxide, which are further removed by methanation. 

Hatori, H., H. Takagi, and Y. Yamada, Gas separation properties of molecular sieving carbon membranes with nanopore channels, Carbon, 42, 1169-1173, 2004. 

Koresh, J.E. and A. Sofer, Study of molecular sieve carbon membranes, Part 1. Pore structure, gradual pore opening, and mechanism of molecular sieving, /. Chem. Soc., Faraday Trans. I, 76,2457,1980. 

Koresh, J.E. and A. Sofer, Molecular sieve carbon permselective membrane, Part I. Presentation of a new device for gas mixture separation, Sep. Sci. Technol., 18, 723, 1983. 

MOLPSA-nitrogen [Molecular sieve pressure swing adsorption] A version of the PSA process for separating nitrogen from air, developed by Kobe Steel. Most PSA processes for nitrogen use molecular sieve carbon as the adsorbent, but this one uses zeolite X. Water and carbon dioxide are first removed in a two-bed PSA system, and then the nitrogen is concentrated and purified in a three-bed system. 

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. 

In principle, molecular sieve carbons (MSC) can be achieved by the pyrolysis of thermosetting polymers such as polyvinylidene chloride, polyfurfuryl alcohol, cellulose, cellulose triacetate, polyacrylonitrile and phenol formaldehyde (Koresh 1980). An example is given by Trimm and Cooper (1970,1973) for the preparation of MSC (mixed with metallic compounds) for catalyst systems. A mixture of furfuryl alcohol, platinum oxide and formaldehyde was heated to 40°C and additional formaldehyde was added to ensure the... 

Table 2.8. Permeability, Selectivity and Separation Characteristics of Various Polymeric and Molecular Sieve Carbon (MSC) Membranes (Koresh and Soffer 1983) ...

Koresh, J. E. and A. Sofler. 1986. Mechanism of permeation through molecular-sieve carbon. J.C.S. Faraday I 82 2057-63. 

Three general types of solid sorbents are mainly used for trapping VOCs in air inorganic sorbents like silica gels or molecular sieves, carbon-based porous materials and porous organic polymers. 

J.E. Koresh and A. Soffer, Molecular Sieve Carbon Selective Membrane, Sep. Sci. Technol. 18, 123 (1983). 

Molecular sieves Carbon disulfide, hexane diethyl ether Used for the collection of aldehydes,... 

Synthetic diamond Molecular sieve Carbon molecular sieve XAD resins Porapak Q... 

In order to determine the PSD of the micropores, Horvath-Kawazoe (H-K) method has been generally used. In 1983, Horvath and Kawazoe" developed a model for calculating the effective PSD of slit-shaped pores in molecular-sieve carbon from the adsorption isotherms. It is assumed that the micropores are either full or empty according to whether the adsorption pressure of the gas is greater or less than the characteristic value for particular micropore size. In H-K model, it is also assumed that the adsorbed phase thermodynamically behaves as a two-dimensional ideal gas. 

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).

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

With respect to carbon membranes, the molecular sieving carbon membranes, produced as unsupported flat, capillary tubes, or hollow fibers membranes, and supported membranes on a macropo-rous material are good in terms of separation properties as well as reasonable flux and stabilities, but are not yet commercially available at a sufficiently large scale, because of brittleness and cost among other drawbacks [3,6],... 

Figure 1.6. Zero coverage energy of adsorption of n-alkanes versus carbon number Nc, on three microporous solids (and two others, for comparison). Molecular sieve carbon, after Carrott and Sing (1987) Silicalite after Canott and Sing (1986) NaX and macroporous silica, after Kiselev (1967) non-porous carbon, after Carrott and Sing (1987) and Avgul and Kiselev (1965).

Dubinin (1975) reported values of N between 2 and 6. Some molecular sieve carbons and zeolites gave N = 3. However, in view of the empirical nature of N, it is not surprising to find that usually the best values are not integers. The particular value of N may also depend on the range of the isotherm and the operational temperature. 

The most striking feature of Figure 8.2 is the effect of the additional degree of freedom provided by a parallel-sided slit. Indeed, this difference in the packing density in slits and cylinders will be seen to be of great importance when we consider the adsorptive properties of molecular sieve carbons and certain zeolites. 

By making use of the Kirkwood-Muller equation and substituting the available experimental data for the physical properties of nitrogen and carbon, Horvath and Kawazoe (1983) arrived at the following equation for the adsorption of nitrogen by molecular sieve carbons at 77 K ... 

The appearance of the initial section of a Type V isotherm is very similar to that of a Type III isotherm for a similar gas-solid system (e.g. water/carbon). In this case, however, the sharp increase in adsorption at higher p/pa is dependent on the pore size. For example, the ulbamicropores in a molecular sieve carbon are filled with water at a much lower p(p° than are the wider pores in a supermicroporous carbon. 

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