Molecular surface pore dimension

On this basis the porosity and surface composition of a number of silicas and zeolites were varied systematically to maximize retention of the isothizolinone structures. For the sake of clarity, data is represented here for only four silicas (Table 1) and three zeolites (Table 2). Silicas 1 and 3 differ in their pore dimensions, these being ca. 20 A and 180 A respectively. Silicas 2 and 4, their counterparts, have been calcined to optimise the number and distribution of isolated silanol sites. Zeolites 1 and 2 are the Na- and H- forms of zeolite-Y respectively. Zeolite 3 is the H-Y zeolite after subjecting to steam calcination, thereby substantially increasing the proportion of Si Al in the structure. The minimum pore dimensions of these materials were around 15 A, selected on the basis that energy-minimized structures obtained by molecular modelling predict the widest dimension of the bulkiest biocide (OIT) to be ca. 13 A, thereby allowing entry to the pore network. 

Besides the molecular probe method using gas adsorption,107 162 recently, the TEM image analysis method163"167 has been applied to evaluate the surface fractal dimension of porous materials. The most attractive fact in this method is that the pores in different size ranges can be extracted from the TEM images which include contributions from many different pore sizes by the inverse fast Fourier transform (FFT) operation by selecting the specific frequency range.165 167... 

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. 

When a solid contains very fine micropores that have pore dimensions only a few molecular diameters, the potential field of force from the neighboring walls of the pores will overlap causing an increase in the interaction energy between the solid surface and the gas molecules. This will result in an increase in adsorption, especially at low relative pressures. There is a possibility and considerable evidence that the... 

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. 

CMS are amorphous materials. Their pore structure below 5 A can not be studied by X-ray diffraction, in contrast to most mineral molecular sieves. Transmission electron microscopy has also not been found suitable for determining such small pore dimensions. The most effective method for characterization is the analysis of adsorption isotherms of small probe molecules with different critical dimensions, viz. O2, N2, CO2, CH4. These adsorption isotherms are useful in determining the pore size distribution, surface area, pore volumes and separation capacity of CMS. In addition, these isotherms give information on the potential industrial applications of these materials, e.g. for the separation of nitrogen from air or of carbon dioxide and methane from flue gases. 

The mechanism of this apparent slip effect is thought to be molecular surface exclusion, resulting in a layer depleted in polymer at the pore wall (Auvray, 1981 Chauveteau and Zaitoun, 1981 Chauveteau, 1982 Chauveteau et al, 1984). This phenomenon is caused by the entropic exclusion of polymer molecules from the wall of the porous medium and is of particular significance when the dimensions of the macromolecule approach those of typical pore sizes. This effect has been observed in both non-adsorbing (Chauveteau and Zaitoun, 1981 Chauveteau, 1982 Lecourtier and Chauveteau, 1984) and adsorbing (Chauveteau et al, 1984) porous media. 

Atomic force microscopy is finding more use in examination of membranes, but artifacts must be addressed, as was done by Bowen and Doneva [197], who noted changes in pore size and structure and used Fast Fourier Transform (FFT) filtering to show the true pore shape. Samples for AFM were prepared by attaching them to steel disks with double sided tape. These same authors used AFM to characterize ultrafiltration membranes [198, 199] and characterized the pore dimensions and quantified the interaction or adhesion of cellulose with two polymeric UF membranes. Atomic force microscopy was also used to characterize molecularly imprinted composite polyethersulfone membranes for quantification of the pore size and surface roughness [200]. 

CO2, and in hydrocarbon adsorption and separation. Adsorption-based separation is an important process for hydrocarbon mixtures in the petrochemical industry. While zeolite-based molecular sieves have been successfully applied for such commercial applications since the middle of the past century, " MMOFs offer distinct advantages, thanks to their adjustable pore dimensions, unique pore geometries, and functionalized pore surfaces. In this chapter, we give a concise summary of the various aspects of hydrocarbon and alcohol adsorption and separation using MMOFs as the adsorbent. 

Film surface areas as a probe for of surface fractal dimension agree with previous SAXS and molecular tiling results for materials which are not self-affine. Pore size distributions calculated via film surface areas seem to provide reasonable results in the microporous region although the method should always underestimate pore size and pore volume but a correction could be developed for this systematic deviation. However, because of the sensitivity of vapor volume uptake to P/Po/ this method should not be used for mesopores. A more rigorous test of the film surface area PSD method awaits results for solids with better described pore structure such as zeolites. REFERENCES... 

Sofer et al. ° is one of the earliest works in this topic. These membranes are important due to the improved trade-off upper limit between permeabiUty and selectivity compared with their polymer precursor membranes. Singh-Ghosal and Koros reported the Robeson s plot (Oj/Nj selectivity versus O2 permeability) for some carbon membranes and corresponding polymer membranes (Fig. 10.2). It is obvious that the performance of carbon membranes is much better than that of corresponding polymer membranes. Moreover, the permeance of the carbon membranes depends on the surface characteristics and the interactions between pores and gas molecules rather than on the bulk properties as for the polymer membranes. When carbon membranes separate molecules based on the molecular-sieving mechanism, the molecules have to overcome an energetic barrier created by the differences between pore dimension and gas molecules. 

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