Specific pore volume, determination

Characterization. When siHca gel is used as an adsorbent, the pore stmcture determines the gel adsorption capacity. Pores are characterized by specific surface area, specific pore volume (total volume of pores per gram of solid), average pore diameter, pore size distribution, and the degree to which entrance to larger pores is restricted by smaller pores. These parameters are derived from measuring vapor adsorption isotherms, mercury intmsion, low angle x-ray scattering, electron microscopy, gas permeabiHty, ion or molecule exclusion, or the volume of imbibed Hquid (1). 

Wheeler [16] proposed that the mean radius, r, and length, L, of pores in a catalyst pellet (of, for that matter, a porous solid reactant) are determined in such a way that the sum of the surface areas of all the pores constituting the honeycomb of pores is equal to the BET (Brunauer, Emmett and Teller [17]) surface area and that the sum of the pore volume is equed to the experimental pore volume. If represents the external surface area of the porous particle (e.g. as determined for cracking catalysts be sedimentation [18]) and there are n pores per unit external area, the pore volume contained by nSx cylindrically shaped pores is nSx nr L. The total extent of the experimentally measured pore volume will be equal to the product of the pellet volume, Vp, the pellet density, Pp, and the specific pore volume, v. Equating the experimental pore volume to the pore volume of the model... 

The total pore volume, Vp, sometimes called specific pore volume when referred to unit mass, is the total internal volume per unit mass of catalysts. Some of this pore volume may be completely enclosed and thus inaccessible to molecules participating in a catalytic reaction. The total accessible pore volume is often derived from the amount of vapour adsorbed at a relative pressure close to unity, by assuming that the pores are then filled with liquid adsorptive. The accessible pore volume may be different for molecules of different sizes. It may be useful to determine the dead space by means of a nonsorbable gas (normally helium) in conjuction with the determination of the bulk volume of the catalyst by means of a non-wetting liquid (mercury). 

The estimation of the amount of adsorbent in the column is based on the comparison of the total pore volume (Vptot [niL]) of the adsorbent in the column with the specific pore volume (Vp [mL/g]) of the same adsorbent determined from the full nitrogen adsorption isotherm. The ratio of these two values will give the adsorbent mass. Total pore volume in the column is determined as the difference of the column void volume (Vo) and the interparticle volume (Vip). 

The total pore volume Vpi has been estimated from the amount of gas. Fads, adsorbed at a relative pressure PI Po=Q.99. Assuming a straight cylindrical pore model, V(d) has been determined according to the BJH (BARRET, JOYNER and HALENDA) method. The porosity is defined by the silica network density 2.2 g/cm and the specific pore volume 0.227 cm /g. It follows c 30%. 

Micromeretics GeoPyc 1360 measures envelope density this includes the volume of pores and surface crevices. When absolute density is inputted, specific pore volume and percent porosity are determined. The instrument uses a flowing dry medium, DryFlo , to surround the powder and generate the displacement volume. 

Nitrogen isotherms were measured by using an ASAP (Micromeritics) at 77K. Prior to each analysis, the samples were outgassed at S73K for 10 - 12 h to obtain a residual pressure of less than 10 torr. The amount on nitrogen adsorbed was used to calculate specific surface area, and the micro pore volumes determined from the BET equation [14] and t-plot method [15], respectively. Also, the Horvath-Kawazoe model [16] was applied to the experimental nitrogen isotherms for pore size distribution. 

The specific pore volume can be determined from nitrogen adsorption measurements if the adsorbent is meso- or microporous. For macroporous adsorbents with pore diameters above 1000 A, the pore volume can be determined by mercury penetration measurements by integrating the pressure volume curve. The total pore volume of meso- and microporous adsorbents can be calculated by assuming that, in the range 0.95 < pjpo < 1, all pores in the adsorbent are filled with condensed gas. The total pore volume is then simply calculated as ... 

Hg specific pore volume measured by mercury porosimetry Kc m<75 m diameter between 2 and 7.5 nm determined by Broekhoff-de-Boer theory V volume obtained by addition of tag, Tc m<7 5mii and Sbet specific surface si02 silica particle diameter measured by TEM. 

The specific pore volume, Vp, and the pore size distribution for the support as well as the catalyst samples were determine by mercury porosimetry (Carlo Erba, Porosimeter Model 2000). 

The morphologic characterization of the immobilized enzyme is important to correlate the biocatalyst performance with porous structure parameters. BET analysis, which is usually based on N2 isothermal adsorption at 77 K, allows determining the solid-specific surface area, total pore volume, pore size distribution, and mean pore diameter. It is not recommended for solids with a low specific surface area (<5 m g ). Table 2 shows the specific smface area, mean pore diameter, and total pore volume determined by BET for the pure sol-gel silica matrix having TEOS as the precursor and the same matrix with the encapsulated CGTase. 

When the behavior of inorganic packings is studied in detail, it is found that the primary parameter that determines a particle s strength is the specific pore volume. Particles with a high pore volume break more easily than do particles with a small pore volume. However, since the pressure per unit length in most chromatographic applications rarely exceeds 2 MPa/cm, the commonly used particles with a pore volume of 1 mL/g are of sufficient strength. 

For the actual catal5dic reaction, the distribution of meso- and micropores is of greater importance. The specific pore volume, pore size, and pore size distribution of microporous materials are determined by gas adsorption measurements at relatively low pressures (low values of p/po = pressure/saturation pressure). The method is based on the pressure dependence of capillary condensation on the diameter of the pores in which this condensation takes place. To calculate the pore size distribution, the desorption isotherm is also determined. Thus a distinction can be made between true adsorption and capillary condensation. The latter is described by the Kelvin equation (Eq. 5-95). 

Specific Surface Area and Pore Volume Determination... 

The total surface area of a porous material is given by the sum of the internal and external surface areas. Pores are classified as micropores (pore width less than 2 nm), mesopores (pore width between 2 and 50 nm), and macropores (pore width greater than 50 nm) according to the definitions proposed by lUPAC (6). The specific pore volume, pore widths, and pore size distributions for micro-and mesopores are determined by gas adsorption. In the case of mesopores, the method is based on the relationship between the pressure of capillary condensation and the radius of a cylindrical pore in which condensation takes place, given by the Kelvin equation ... 

Specific pore volume (vp, ml g O the total volume of pores per gram of solid. This is usually determined by measuring the volume of a liquid required to fill the pores but not the spaces between aggregates. 

N. E. Fisher and A. Y. Mottlau [193] have described a simple method for approximate determination of the specific pore volume. It is based on the observation that whereas dried, fine-grained silica gel is a mobile powder, the grains adhere to each other and cake when the pores have been filled with liquid (including water). 

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