Molecular-sieve carbon, pore size

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

This solvent is employed widely in analytical and coordination chemistry. At its boiling point it undergoes partial decomposition to yield dimethylamine and carbon monoxide. The decomposition is catalyzed by various substances, particularly those with acidic or basic properties this must be taken into consideration in the selection of the material used for drying. Under no circumstances may this solvent be refluxed with, for example, potassium hydroxide, sodium hydroxide or calcium hydride. Dimethylformamide can be dehydrated most advantageously with a molecular sieve of pore size 0.4 nm however, calcium sulphate, magnesium sulphate or silica gel may also be employed. After dehydration, the solvent may be purified by vacuum distillation. 

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). 

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. 

Molecular sieves are porous aluminosilicates (zeolites) or carbon solids that contain pores of molecular dimensions which can exhibit seleaivity according to the size of the gas molecule. The most extensive study on carbon molecular sieve membranes is the one by Koresh and Soffer (1980,1987). Bird and Trimm (1983) also described the performance of carbon molecular sieve membranes, but they were unable to prepare a continuous membrane. Koresh and Soffer (1980) prepared hollow-fiber carbon molecular sieves, with pores dimensions between 0.3 and 2.0 run radius (see Chapter 2). 

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).

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. 

We are currently in the process of publishing reference data for various adsorptive molecules of different size and polarity which we hope other research groups will be able to evaluate. In this paper, we will present a summary of some of the results obtained with benzene, dichloromethane and methanol, and show how application of the reference data can be used to obtain useful information about the pore structure of molecular sieve carbons and superactivated carbons. 

G. Horvath and K. Kawazoe, Method of calculation of effectiveness pore size distribution in molecular sieve carbons. /. Chem. Eng., pn., 16 (1983) 470. 

The nanoporous carbon membrane consists of a thin layer (<10pm) of a nanoporous (3-7 A) carbon film supported on a meso-macroporous solid such as alumina or a carbonized polymeric structure. They are produced by judicious pyrolysis of polymeric films. Two types of membranes can be produced. A molecular sieve carbon (MSC) membrane contains pores (3-5 A diameters), which permits the smaller molecules of a gas mixture to enter the pores at the high-pressure side. These molecules adsorb on the pore walls and then they diffuse to the low-pressure side of the membrane where they desorb to the gas phase. Thus, separation is primarily based on differences in the size of the feed gas molecules. Table 7 gives a few examples of separation performance of MSC membranes. ° Component 1 is the smaller component of the feed gas mixture. 

The pore structure in molecular sieving carbon is typically ascribed to arise from at least three different mechanisms. In the first the structure of the carbon is derived from the precursor, but in shrunken form. In this way the structure of wood charcoal is similar to the cellular structure of the wood (2). Another type of porosity arises in the fissures and cracks left behind in the carbon matrix. These faults relieve the thermally-induced, mechanical stresses brought on by pyrolysis. Ultramicroporosity can also originate from the volatilization of small molecules. These molecules are formed during pyrolysis, and leave molecularly-sized channels in the solidifying carbon matrix (3-6). Pyrolysis of polyfurfuryl alcohol, polyvinylidenechloride, and polyacrylonitrile lead to the formation of formaldehyde and water, hydrogen chloride, and hydrogen cyanide, respectively. 

Since carbon molecular sieves are amorphous materials, the dimensions of their pore structures must be measured phenomenologically by the adsorption of small probe molecules with different critical dimensions. There is insufficient long range order to utilize standard x-Ray diffraction methods for characterization. The earliest reports of molecular sieving carbons dealt primarily with coals and charcoals. Sorption of helium, water, methanol, n-hexane, and benzene was measured and related to the porosity of the carbon. Pore-sizes were estimated to be two to six angstroms (3-6). In a classic paper P.H. Emmett described methods for tailoring the adsorptive properties and pore size distributions of carbon Whetlerites. 

Shortly thereafter, it was realized that molecular sieving carbons could be prepared by controlled pyrolysis of polymeric precursors (2.). Early estimates of pore sizes for this carbon were seven to eight angstroms, but progressive activation increased surface area, pore volume, and pore dimensions. 

The use of carbon molecular sieves (CMSs) as catalysts for the oxidative dehydrogenation of alkyl aromatics was described in a patent by Lee [54]. Higher conversions and selectivities were reported with molecular sieve carbons with pore sizes in the range 0.5 to 0.7 nm (Carbosieve G from Supelco, and MSC-V from Calgon) than with activated carbon. This work may have triggered subsequent interest for CMS in ODE. 

In addition, depending on the size of the adsorbate molecules, especially in the case of some organic molecules of a large size, molecular sieve effects may occur either because the pore width is narrower than the molecules of the adsorbate or because the shape of the pores does not allow the molecules of the adsorbate to penetrate into the micropores. Thus, slit-shaped micropores formed by the spaces between the carbon layer planes are not accessible to molecules of a spherical geomehy, which have a diameter larger than the pore width. This means that the specific surface area of a carbon is not necessarily proportional to the adsorption capacity of the activated carbon. Pore size distribution, therefore, is a factor that cannot be ignored. 

The extent of competition may also be a function of the adsorbate molecular size, correlated with the activated carbon pore size distribution. Activated carbon fibers tliat are exclusively microporous (more than 96 % of micropore volume) present a selectivity property for pesticides or phenol in the presence of higher molecidar weight compounds like humic substances, due to the direct connection of micropores to their external surface [41]. Using granular activated carbon, which does not have this molKUilar sieve property, a 20 to 70 % reduction in adsorption is obtained for atrazine in raw water compared with equilibria in distilled water [42]. 

Molecular sieving carbons (MSCs) have a smaller pore size with a sharper distribution in the range of micropores in comparison with other activated carbons for gas and liquid-phase adsorbates. They have been used for adsorbing and eliminating pollutant samples with a very low concentration (ethylene gas adsorption to keep fruits and vegetables fresh, filtering of hazardous gases in power plants, etc.) An important application of these MSCs was developed in gas separation systems [1-2]. 

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