Pore volume sorption

Figure 3.23. Pore volume distributions (Nt physi.sorption) of a. wide-pore silica, b. y-alumina, c. a-alumina, d. activated carbon, e. Raney Nickel and f. ZSM-5.

The uptake values obtained for MEC adsorption are in good agreement with the results obtained for nitrogen sorption. As expected, the lower adsorption capacity belongs to SBA-15. After Al incorporation by direct-synthesis, the surface areas and pore volumes... 

Thus, either type I or type IV isotherms are obtained in sorption experiments on microporous or mesoporous materials. Of course, a material may contain both types of pores. In this case, a convolution of a type I and type IV isotherm is observed. From the amount of gas that is adsorbed in the micropores of a material, the micropore volume is directly accessible (e.g., from t plot of as plot [1]). The low-pressure part of the isotherm also contains information on the pore size distribution of a given material. Several methods have been proposed for this purpose (e.g., Horvath-Kawazoe method) but most of them give only rough estimates of the real pore sizes. Recently, nonlocal density functional theory (NLDFT) was employed to calculate model isotherms for specific materials with defined pore geometries. From such model isotherms, the calculation of more realistic pore size distributions seems to be feasible provided that appropriate model isotherms are available. The mesopore volume of a mesoporous material is also rather easy accessible. Barrett, Joyner, and Halenda (BJH) developed a method based on the Kelvin equation which allows the calculation of the mesopore size distribution and respective pore volume. Unfortunately, the BJH algorithm underestimates pore diameters, especially at... 

In column sorption/desorption tests, a dimensionless time is often used and termed pore volume [268]. One pore volume (e.g., Vp) is the volume of pores (e. g., voids) present in the column that may be filled with water. The number of pore volumes passed through the column is thus ... 

Modern N2 sorption porosimeters are very sophisticated and generally reliable. Typically they come supplied with customized user-friendly software which enables the experimental data to be readily computed using the above models and mathematical expressions. Usually the raw isotherm data is displayed graphically along with various forms of the derived pore size distribution curve and tabulated data for surface area, pore volume and average pore diameter. 

The identification of the solid phases and the evaluation of their crystallinity were performed by X-ray powder diffraction while the pore volume of the ZSM-48 materials was evaluated by isothermal (90 °C) sorption of n-hexane, followed in the thermobalance (Stanton Redcroft ST-780 combined TG-DTA-DTG thermoanalyser). 

Roberts, B. F. (1963), A Procedure for Estimating Pore Volume and Area Distributions from Sorption Isotherms, National Meeting of the American Chemical Society. 

In Silicalite. A variety of papers are concerned with sorption of methane in the all-silica pentasil, silicalite. June et al. (87) used a Metropolis Monte Carlo method and MC integration of configuration integrals to determine low-occupancy sorption information for methane. The predicted heat of adsorption (18 kJ/mol) is within the range of experimental values (18-21 kJ/ mol) (145-150), as is the Henry s law coefficient as a function of temperature (141, 142). Furthermore, the center of mass distribution for methane in silicalite at 400 K shows that the molecule is delocalized over most of the total pore volume (Fig. 9). Even in the case of such a small sorbate, the channel intersections are unfavorable locations. 

A low temperature nitrogen sorption was carried out on an automated physisorption instrument (ASAP 2000, Micromeritics Instrument Corporation). Before the measurement, the sample was degassed at 350 C for 4-5 h until the vacuum of system was better than 0.67 Pa. The data for micropore were obtained from t-plot, and those for mesopore and distribution of mesopore were calculated by BJH method (using desorption curve). The single point total pore volume at high relative pressure was taken as the total volume. 

Our results show that there is a clear difference between the sorption of /ert-butyl alcohol by the microporous phenyl-modified silica compared with the sorption of w-butyl alcohol. Comparison of total pore volumes and monolayer capacities with those for other sorptives leads us to conclude that sorption of the straight-chained isomer is significantly more sterically hindered than its branched analogue. This may be explained by the difference in the shapes of the two molecules. The spherical ten-butyl alcohol molecule is able to easily penetrate into the micropores of the unswollen sample without blocking the pore-entrance, while the straight-chain w-butyl alcohol molecule may block the pores upon adsorption. 

The LPS silica is prepared by extracting the water from a low solids hydrogel with an organic solvent to avoid the compression of aqueous surface tension. This leaves a fragile catalyst of very high pore volume. Such preparations often provide dramatic examples of the effect of pore volume because the same gel dried directly in an oven will frequently be less active or even completely dead. Invariably, nitrogen sorption shows the inactive silica to contain a low pore volume mainly inside small pores, e.g., less than 60 A diameter. In contrast, the active sample dried by extraction usually has equal or greater volume inside small pores, and, in addition, considerable volume... 

Roberts BF. A procedure for estimating pore volume and area distributions from sorption isotherms. 

In addition, the large pore volume, pore size flexibility, and structural variety of MCM-41 can be extensively used for the selective adsorption of a diversity of gases and liquids [39,40], An extremely high sorption capacity for benzene has been demonstrated [40], Widespread work has been carried out on the sorption properties of some adsorbates, such as nitrogen, argon, oxygen, water, benzene, cyclopentane, toluene, and carbon tetrachloride, as well as certain lower hydrocarbons and alcohols on MCM-41 [122],... 

Porous texture of the different materials was all characterized using nitrogen (N2) physisorption at 77 K and up to a pressure of 0.1 MPa. From the nitrogen physisorption data, obtained with the High Speed Gas Sorption Analyser NOVA 1200, the BET-surface area, total pore volume, microporous volume and t-volume were derived. The BET surface area (SBet) is the surface area of the sorbent according to the model formulated by Brunauer et al. [8] for planar surfaces. 

The micropore volume is defined as the pore volume of the pores < 2 nm. Microporous volumes calculated from the application of the Dubinin-Radushkevich equation to the N2 adsorption isotherms at 77 K. The mean pore size of each sample obtained from N2 adsorption was determined by applying Dubinin-Radushkevich equation. The hydrogen sorption isotherms were measured with the High Speed Gas Sorption Analyser NOVA 1200 at 77 K in the pressure range 0-0.1 MPa. 

Characterization of the crosslinked polymer in the dry state [apparent density (16), surface measurement by N2-ad sorption (17,18), Hg-intrusion for measurement of the pore volume (iS)] is not conclusive for the properties as polymeric reagent, However, extensive knowledge about the porous structure and the accessibility of different regions in the polymer network can be obtained by gel-permeation chromatography (GPC) (20). GPC is used in an inverse mode. Well-characterized samples are keys for the pore structure. 

The intracrystalline pore volume of the catalysts was evaluated by n-hexane sorption as shown in Fig. 6. Sorption capacities for samples SI to S3 are comparable to that of the zeolite before Ga impregnation and correspond to the value expected for an unaltered ZSM-5 type material (S10). Sorption capacity decreases for samples S3, S4, S5, and S6, because of intracrystalline volume blockage by coke deposits and possibly also (silica)-alumina debris [6] in the aged catalyt S6. In addition, the sorption rate for S6 is about twice the rate observed for the other samples, suggesting that adsorption occurs mostly at the external surface of the S6 catalyst crystallites. Thus, it appears that coke deposited on S6, probably as polyaromatic species, has almost blocked the channel pore mouths and/or practically occupied the whole intracrystalline pore volume. It explains the poor catalytic performance of S6. 

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