Preparation of Carbon Molecular Sieves CMS or MSC

The pyrolysis of benzene over the active carbon surface results in the deposition of the carbon on the surface of the substrate carbon as weU as in the microporous system and at some preferred sites. The adsorption isotherms of organic molecules of varying sizes and shapes indicated that the carbon gets deposited preferentially in the pore entrances reducing entrance diameter resulting in the formation of carbon molecular sieves. Pore-size distribution curves indicated that the treatment with benzene between 3 and 6 hrs reduces the mean pore dimensions to 0.6 nm, and a larger time of treatment reduces pore entrances to less than 0.6 nm. 

The blocking of pores by PVC impregnation was carried out by treatment of the active carbons with suspensions of PVC in alcohol under reflux followed by carbonization at 600°C. This resulted in the deposition of appreciable amounts of carbonaceous material into the microporous structure causing a reduction in pore dimensions and producing carbon molecular sieves. These MSC were found to permit the adsorption of smaller molecules such as benzene or cyclohexane but prevented the adsorption of larger molecules such as isooctane and a-pinene. 

Lilan et al. prepared MSC for air separation by the carbonization of two Chinese anthracite coals. The coals were ground, mixed with a binder, carbonized, and subsequently activated with CO2. The activated material thus obtained was modified by the pyrolysis of toluene and tar fractions of oil at 1073 K to obtain molecular sieve materials. These MSC materials were found to have predominantly a micropore size of 0.33 to 0.40 mn and were used to separate air to a nitrogen purification of 98.6%. 

De Salazar et al. prepared CMS by carbon deposition by the pyrolysis of benzene on an activated carbon. The carbon was prepared by the carbonization of peach stone followed by its activation with CO2. The effect of preparation variables such as temperature, time, and benzene partial pressure on the characteristics of MSC was studied. The MSC obtained in this study were completely accessible to both dichloromethane and benzene as shown by similar values of immersion enthalpy for both liquids. However, the accessibility of a particular liquid into the MSC was found to depend upon the deposition time. A linear relationship was found between the decrease of accessibility of these two liquids to the microporosity of the MSC and the deposition time. The separation ability of different MSC for N2/O2 and CO2/CH4 mixtures was analyzed by measuring the adsorption kinetics of pure gases. It was observed that the carbon sample without any deposition was not selective for the separation of these mixtures, but the separation could be obtained after pyrolytic 

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. 

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