Macropores, micropores

Carbon molecular sieves are produced by controlled pyrolysis and subsequent oxidation of coal, anthracite, or organic polymer materials. They differ from zeolites in that the micropores are not determined by the crystal structure and there is therefore always some distribution of micropore size. However, by careful control of the manufacturing process the micropore size distribution can be kept surprisingly narrow, so that efficient size-selective adsorption separations are possible with such adsorbents. Carbon molecular sieves also have a well-defined bi-modal (macropore-micropore) size distribution, so there are many similarities between the adsorption kinetic behavior of zeolitic and carbon molecular sieve systems. 

The definitions of the moments and their relationship to the system parameters for a biporous (macropore-micropore) adsorbent such as a commercial pelleted molecular sieve are given by the following equations(15,16) ... 

Hi. The monomer polymerization route. Compared with the resin-functionalization route, the homo- and copolymerization of organotin-containing monomers permits one to influence the polymer resin structure to a greater extent. In principle, it is possible to prepare gel-type, macroporous, microporous or nonporous polymers. The pore structure, tin loading, solubility and other factors which influence the reactivity of the polymer-supported organotin reagents can be controlled by appropriate... 

Membranes can be macroporous, microporous, or dense (nonporous). A microporous membrane contains interconnected pores that are small (on the order of 10 to 100,000 A), but large in comparison to the size of the molecules to be transferred. Only micoporous and dense membranes are permselective. However, macro-porous membranes are widely used to support thin microporous or dense membranes when significant pressure differences across the membrane are necessary to achieve a reasonable flux. 

The boundary between micro- and macropores is marked by an arrow on the porograms in Fig. 10.20. The surface area of the PAM is determined mainly by the surface of the micropores with a negligible contribution from the macropores. Hence, the boundary between the macro-and micropores is determined by the pore radius above which the PAM surface area starts to increase rapidly (Fig. 10.20b). As the agglomerates and particles that surround the micropores differ in size for PAMs obtained from different pastes, the macropore micropore boundary varies between 0.05 and 0.2 pm (Fig. 10.20b). In the literature, it is often assumed that the boundary micropore radius has an average value of 0.1 pm. 

Literature differentiates between dense membranes and porous membranes. The latter type is subdivided into macroporous, microporous, and nanopor-ous membranes according to their pore sizes. 

Figure 10.3 Exo-templating the method of Stein can be used to create hierarchical macroporous/microporous (or macroporous/mesoporous) silicas.

Macropore-Micropore diffusion This is the case often called the bimodal diffusion model in the literature. In this case the two diffusion processes both control the uptake. This is expected when the particle size is intermediate. 

External film resistance plus two intraparticlo diffusional resis-j tances (macropore-micropore). j ... 

Macropore—Micropore Diffusion with External Film Resistance... 

Sample % Metal content Surface area/m g Langmuir Meso+macropore Micropore volume/cm g ... 

The definition of the different types of pores is based on their width, which represents the distance between the walls of a slit-shaped pore or the radius of a cylindrical pore. This classification, which is not entirely arbitrary, is now widely accepted and used. It takes into account differences in the behaviour of molecules adsorbed in micropores and in mesopores. It appears that for pore widths exceeding 1.5-2.0 nm, the gaseous adsorbate condenses in a liquid-like state and a meniscus is formed. As a consequence, a hysteresis loop appears on desorption and its interpretation can lead to the distribution of the mesopores in the adsorbent [23]. The limit between mesopores and macropores at 50 nm is more artificial, and corresponds to the practical limit of the method for pore-size determination based on the analysis of the hysteresis loop. As a rule, the porous structure of the usual types of activated carbons is tridisperse, i.e. they contain micropores, mesopores and macropores. Micropores are of the greatest significance for adsorption owing to their very large specific surface area, and their large specific volume. At least 90-95% of the total surface area of an activated carbon can correspond to micropores. 

Carbon Carbon origin BET surface area (m g- ) Total pore volume (cm- g Total pore volume (cm g Macropores Micropores ) Mesopores Gold adsorption (mmolg )... 

Figure 4.5 Composite pellet containing macropores, micropores and crystals.

Macropore-micropore diffusion with external film resistance Kawazoe and Takeuchi (1974) Cen and Yang (1986) Rasmuson (1982)... 

Mass transfer through the external fluid film, and macropore, micropore and surface diffusion may all need to be accounted for within the particles in order to represent the mechanisms by which components arrive at and leave adsorption sites. In many cases identification of the rate controlling mechanism(s) allows for simplification of the model. To complicate matters, however, the external film coefficient and the intraparticle diffusivities may each depend on composition, temperature and pressure. In addition the external film coefficient is dependent on the local fluid velocity which may change with position and time in the adsorption bed. 

Fleury (2002) proposed a trimodal pore-size model for carbonates with three pore populations (micro, meso, macro). The electrical network comprises a series of mesopores and macropores micropores are in parallel. 

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