Surface pore diameter distribution

A number of analytical instruments helped us to probe and understand our catalysts in greater detail. Although these may be standard equipment for a large manufacturer or large petroleum research laboratory, we had to acquire them and train our personnel in their use before we could proceed. Some of the more vital instruments included equipment for measuring surface area, pore volume and pore diameter distribution, and X-ray... 

Fortunately, the effects of most mobile-phase characteristics such as the nature and concentration of organic solvent or ionic additives the temperature, the pH, or the bioactivity and the relative retentiveness of a particular polypeptide or protein can be ascertained very readily from very small-scale batch test tube pilot experiments. Similarly, the influence of some sorbent variables, such as the effect of ligand composition, particle sizes, or pore diameter distribution can be ascertained from small-scale batch experiments. However, it is clear that the isothermal binding behavior of many polypeptides or proteins in static batch systems can vary significantly from what is observed in dynamic systems as usually practiced in a packed or expanded bed in column chromatographic systems. This behavior is not only related to issues of different accessibility of the polypeptides or proteins to the stationary phase surface area and hence different loading capacities, but also involves the complex relationships between diffusion kinetics and adsorption kinetics in the overall mass transport phenomenon. Thus, the more subtle effects associated with the influence of feedstock loading concentration on the... 

Adsorption isotherms were recorded with a Micromeritics ASAP 2010 instrument. Experimental porous data (surface area, volume, pore diameter distribution) have been... 

The pore structure parameters were included in Table 2. It was found that the BET s surface area and the pore volume were 803 mVg and 0.83 cm /g, respectively. Also, the BJH pore size distribution shows MCM-41 material with a quite narrow pore diameter distribution centered around 25.8A. Thus, the framework walls are 22.5A thick. 

In the selection of the stationary and mobile phase, a variety of chemical and physical factors of the chromatographic system that may contribute to the variation in the resolution and recovery of natural products need to be considered. The stationary phase contributions relate to the ligand composition, ligand density, surface heterogeneity, surface area, particle size, particle size distribution, particle compressibility, pore diameter, and pore diameter distribution. The mobile phase contributions relate to the type of organic solvents, eluent composition, ionic strength, pH, temperature, loading concentration, and volume. 

Surface pore diameters were measured by visual inspection of the line profiles of 50 pores of each membrane. All membranes have a wide pore size distribution. The deviation between 20 % and 40% from the average value is noticeable in most cases and is higher for membranes with larger pores (or higher MWCOs). The pore density was obtained by observing several AFM images from different sample areas of the same membrane and counting the number of pores in a unit area. Smface porosity is defined as the ratio of the pore area to the total area of the membrane. The porosity is low and varies between 0.6% and 7%. No relationship between MWCO and porosity was found. 

Is it possible to estimate the influence of the pore-filling degree on the remaining surface area for a supported IL system, if the initial surface area, pore volume, and pore diameter distribution of the porous solid are known ... 

Hydrogel texture is evaluated by thermoporometry, which is a calarimetiic method (17) giving pore diameter distribution (4 nm to 150 nm), pore surface and volume of wet or dry materials. 

Many properties have been investigated as a function of porosity (pore volume) but this parameter alone is a simplification of the overall morphology. Certain properties will scale better with internal surface area, others with skeleton size distribution, yet others with pore diameter distribution. Gas diffusion and liquid diffusion through porous silicon, for example, are primarily dependent on pore diameter and interconnectivity. Visible photoluminescence depends primarily on skeleton size distribution in combination with surface chemistry. 

Each of the adsorption applications mentioned above is best done with a different activated adsorbent. One may choose from a product with a different smallest pore diameter, distribution of pore sizes and volumes, total amount of surface area per volumetric charge and especially, the method of activation . 

The porous structure of active carbons is the defining factor of their adsorption performance The pore diameter distribution determines the adsorption energy and therefore, the slope of adsorption isotherm, whereas in mainly microporous carbons the micropore volume limits the adsorption capacity at the higher end of t.he adsorptive concentration. Furthermore, the chemical compositor of the carbon surface influences the selectivity of adsorption, e.g. the competition of the water adsorption when working in aqueous solution. However, here only the structural properties of carbons are taken into account. 

Product name Mean pore diameter (nm) Pore diameter distribution (%) Specific pore volume (cm /g) Specific surface area (mVgr... 

Fig. 26. Screen filters contain pores of a uniform size and retain all particulates greater than the pore diameter at the surface of the membrane. Depth filters contain a distribution of pore sizes. Particulates entering the membrane are trapped at constrictions within the membrane. Both types of filters are rated 10...

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). 

Important physical properties of catalysts include the particle size and shape, surface area, pore volume, pore size distribution, and strength to resist cmshing and abrasion. Measurements of catalyst physical properties (43) are routine and often automated. Pores with diameters <2.0 nm are called micropores those with diameters between 2.0 and 5.0 nm are called mesopores and those with diameters >5.0 nm are called macropores. Pore volumes and pore size distributions are measured by mercury penetration and by N2 adsorption. Mercury is forced into the pores under pressure entry into a pore is opposed by surface tension. For example, a pressure of about 71 MPa (700 atm) is required to fill a pore with a diameter of 10 nm. The amount of uptake as a function of pressure determines the pore size distribution of the larger pores (44). In complementary experiments, the sizes of the smallest pores (those 1 to 20 nm in diameter) are deterrnined by measurements characterizing desorption of N2 from the catalyst. The basis for the measurement is the capillary condensation that occurs in small pores at pressures less than the vapor pressure of the adsorbed nitrogen. The smaller the diameter of the pore, the greater the lowering of the vapor pressure of the Hquid in it. 

Pore diameter and distribution are important factors. Small pores limit the accessibihty of internal surface because of increased resistance to diffusion of reactants inwards. Diffusion of products outwards also is slowed, and degradation of those produces may result. 

When the catalyst is expensive, the inaccessible internal surface is a liabihty, and in every case it makes for a larger reactor size. A more or less uniform pore diameter is desirable, but this is practically reahz-able only with molecular sieves. Those pellets that are extrudates of compacted masses of smaller particles have bimodal pore size distributions, between the particles and inside them. Micropores have diameters of 10 to 100 A, macropores of 1,000 to 10,000 A. The macropores provide rapid mass transfer into the interstices that lead to the micropores where the reaction takes place. 

---