Parameters Characterizing Porous Adsorbents

The parameters characterizing porous adsorbents are the following. Specific surface area is represented by S and measured in [m2/g] (the surface area is the outer surface, to be precise, the area outside 

Dr where Vp is the pore volume accumulated up to the pore width Dp measured in [cc-STP/g A] [1,2], The unit, cc-STP, denotes the quantity adsorbed, measured in cubic centimeters at standard temperature and pressure (STP), that is, 273.15 K and 760 Torr, that is, 1.01325 x 10Pa. 

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The performance of a conventional method of protein purification may generally be characterized by its capacity and resolution, in the case of a primary recovery operation the additional criteria of clarification efficiency and reduction of process volume become important parameters describing a successful procedure. In packed bed chromatography of proteins on porous adsorbents there are four main system parameters influencing the overall performance ... 

In Chapters 4 and 6, several methods of characterization of solids that are normally used for catalyst testing were described. In particular, the parameters which characterize the surface morphology of a porous catalyst are the same that characterize a porous adsorbent, that is, the specific surface area, S [m2/g], the micropore volume, W1 [cm3/g], the sum of the micropore andmesopore volumes, that is, the pore volume, W [cm3/g], and the pore size distribution (PSD), AVp/ADp (see Chapter 6). 

The volume of nitrogen adsorbed on the sieve CMS - R is remarkable. In the case of the other two sieves (CMS-Kl, CMS-K2), the adsorption was so low that, the calculation of parameters characterizing the texture, such as pore size distribution, pore volume, etc., was impossible. In this way it was found that the adsorption of nitrogen at 77 K on the kinetic sieves did not allow to characterize their porous structure. 

Much of our information about the nature of the adsorbed gas layer comes from studies of the amount of gas adsorbed on a surface a (the surface coverage) as a function of gas pressure P at a given temperature. The o-P curves derived from these experiments are called adsorption isotherms. Adsorption isotherms are used primarily to determine thermodynamic parameters that characterize the adsorbed layer (heats of adsorption, and the entropy and heat capacity changes associated with the adsorption process) and to determine the surface area of the adsorbing solid. The latter measurement is of great technical importance because of the widespread use of porous solids of high surface area in various industrial processes. The effectiveness of participation by a porous solid in a surface reaction is often proportional to the surface area of the solid. The simplest adsorption isotherm at a constant temperature is obtained from Eq. 3.85, which we can rewrite as... 

Since almost all practically important adsorbents are porous solids a key parameter which is required to characterize an adsorbent is the specific surface area. The specific areas of microporous solids are very large, and values of several hundred square meters/gram are not uncommon. Accurate measurement of the surface area of a microporous solid presented a significant problem in early studies of adsorption and catalysis. 

Precipitation of the growing polymer from the initial solution of styrene and DVB in an inert diluent during crosshnking copolymerization results in the formation of a two-phase heterogeneous network, in which one phase is presented hy the highly crosshnked and rigid polymer, while the rejected diluent forms another phase. After removing the diluent, permanent voids remain in the copolymer beads. The total pore volume, and the inner surface area, S, are the major characteristics of the porous structure these are intimately related to pore size and pore size distribution. These parameters determine the practical apphcation frelds of the polymeric adsorbent resins therefore, a precise quantitative characterization of resin porosity becomes an important task. 

A whole series of methods has been developed for the investigation of porous solids such as activated carbons, porous glasses, silica gels, and zeolites. Together with some new suggestions, they aU have been applied to the characterization of the porosity of polymeric adsorbents. We will briefly review these methods related to the porosity parameters and emphasize the problems that may arise on their application to polymers, when the specific properties of the polymeric materials are not taken into account. 

Also, the ratio of these two variables, m = Ou, serves as a very informative parameter, which characterizes the intensity of the given stressed state (absolute value) and its stiffness (slope). While being universal, the Davidenkov-Friedman scheme still has limitations, one of which is the absence of negative values in Ou = Oj - v (03 + O3) < 0, that is, it assumes the absence of damage under hydrostatic compression. This is not the case for a large number of porous bodies, such as grounds, sponges, various catalysts, and adsorbents. 

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