Specific pore volume

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). 

Commercially available pre-coated plates with a variety of adsorbents are generally very good for quantitative work because they are of a standard quality. Plates of a standardised silica gel 60 (as medium porosity silica gel with a mean porosity of 6mm) released by Merck have a specific surface of 500 m /g and a specific pore volume of 0.75 mL/g. They are so efficient that they have been called high performance thin layer chromatography (HPTLC) plates (Ropphahn and Halpap J Chromatogr 112 81 1975). In another variant of thin layer chromatography the... 

Because of the preceding reasons, sorbents for partition chromatography should have rather small specific surface areas in combination with large specific pore volumes. 

Figure 6. Plot of the specific electrode pore volume vs. the specific pore volume measuredfor the corresponding carbon powders.

Fig. 4.11 (A) Adsorption isotherms of lysozyme meters of the CMK materials (a) specific surface on CMK-type mesoporous carbon materials at pH area (b) specific pore volume (c) pore diameter. 11 (a) CMK-1 (b) CMK-3 (c) CMK-3-130 (d) Reprinted with permission from [152], A. Vinu CMK-3-150. (B) Dependence of the adsorption etal.,J. Phys. Chem. B 2005, 109, 6436. 2005, capacities of lysozyme on the structural para- American Chemical Society.

Fig. 4.16 Structural parameters of carbon nanocage materials as a function of the sucrose to silica ratio in their synthesis (A) pore diameter (B) specific surface area (C) specific pore volume.

If we apply another restriction, p=const., in addition to porosity, all other volume-related characteristics, such as (5, specific pore volume, Fpore, etc. become independent of scaling. Indeed, if the space is isotropic the unit volume is directly proportional to the unit mass. Scaling results in change of sizes and surface areas, but the volume related to the mass remains the same. 

The pore volume j)er unit mass, Vg, (a measure of the catalyst pellet porosity) is also a parameter which is important and is implicitly contained in eqn. (14). Since the product of the particle density, Pp, and specific pore volume, V, represents the porosity, then Pp is inversely proportional to Fg. Therefore, when the rate is controlled by bulk diffusion, it is proportional not simply to the square root of the specific surface area, but to the product of Sg and Vg. If Knudsen diffusion controls the reaction rate, then the overall rate is directly proportional to Vg, since the effective Knudsen diffusivity is proportional to the ratio of the porosity and the particle density. 

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 specific pore volume is important in size-exclusion chromatography (SEC) [10] because the separation takes place there. In retentive chromatography, it is necessary to provide the surface area... 

Mean particle size Surface area Specific pore volume Dispersion (pm) (m /g) (ml/g) (%)... 

Run nr. Catalyst mass (g) Temperature Pressure (C) (bar) Surface area (mTg) Specific pore volume (ml/g)... 

A novel synthesis route was developed to produce spherical silica particles. The synthesis is based on a modified Stoeber method and the room-temperature synthesis of MCM 41S-materials applying tetraethoxysilane, alcohol, water, ammonia and homopolymers as template. The specific surface area, the specific pore volume and the average pore diameter were varied in the following ranges 5 - 1,000 m2/g 0.1 — 1.0 cm3/g and 2-50 nm. With respect to catalytic applications hetero-atoms e.g. Al were incorporated into the silica framework. 

Table 2 shows that SC-155(G40.60) and C-l55(G20.20), which have comparable values of specific surface area, have very different CO conversion values. Both materials were produced under different gelling and drying conditions, and as a consequence the C-155 precursor (SC-I55/G20.20) was different in pore size distribution than the SC-I55(G40.60), but that difference has not sense when silica was eliminated for to produce C-155, having the last one other new characteristics then the observed differences in CO conversion are mainly attributed or to the higher value of specific pore volume of C-155, or because some of the metallic iron were on silica surface of the SC-155 diminishing its catalytic activity, but not attributed to the different gelling and drying conditions. CON material, in spite of its low specific surface area and its low specific pore volume is a fully carbon material like C-155 is, then its lower performance in CO conversion is attributed to the specific surface and pore characteristics. 

The specific pore volume is the total internal void volume per unit mass of adsorbent. Some of the pore volume may be completely enclosed, and thus inaccessible to molecules participating in a catalytic reaction. 

Apparent density is related to the specific pore volume and can be measured relatively easily. Silica gels with low apparent density do not... 

---