Macropore rate-controlling step

Activated carbon fibers have attracted increasing attention in recent years as a better adsorbent than granular activated carbons, because they normally present much higher adsorption kinetics and adsorption capacity. Activated carbon fibers only have micropores, which are directly accessible from the external surface of the fiber. Thus, adsorptive molecules reach adsorption sites through micropores without the additional difiusion resistance of macropores, which is usually the rate-controlling step in granular adsorbents. 

Ammonia, which possesses a large dipole moment, has been used extensively as a probe molecule for the characterisation of both Lewis and Bronsted acidic sites. Figure 22 shows the significant difference in the FR data between ammonia in zeohte crystals and in pellets. The FR spectra of ammonia in zeolite crystals demonstrated that the rate of the ammonia adsorption on different acidic sites in the crystals controls the overall dynamics of the processes occurring in the systems, hi the case of pellets, the rate-controlhng step was found to be macropore diffusion with (Fig. 22a,2,b,2) or without (Fig. 22c,2) surface resistances [77]. 

Purification of Air Prior to Liquefaction. Separation of air by cryogenic fractionation processes requires removal of water vapor and carbon dioxide to avoid heat exchanger freeze-up. Many plants today are using a 13X (Na-X) molecular sieve adsorbent to remove both water vapor and carbon dioxide from air in one adsorption step. Since there is no necessity for size selective adsorption, 13X molecular sieves are generally preferred over type A molecular sieves. The 13X molecular sieves have not only higher adsorptive capacities but also faster rates of C02 adsorption than type A molecular sieves. The rate of C02 adsorption in a commercial 13X molecular sieve seems to be controlled by macropore diffusion 37). The optimum operating temperature for C02 removal by 13X molecular sieve is reported as 160-190°K 38). 

Alternatively one can in principle derive both micropore and macropore diffusivities from measurements of the transient uptake rate for a particle (or assemblage of crystals) subjected to a step change in ambient sorbate pressure or concentration. The main problem with this approach is that the overall uptake rate may be controlled by several different processes, including both heat and extraparticle mass transfer as well as intraparticle or intracrystalline diffusion. The intrusion of such rate processes is not always obvious from a cursory examination of the experimental data, and the literature of the subject is replete with incorrect diffusivities (usually erroneously low values) obtained as a result of intrusion of such extraneous effects. Nevertheless, provided that intraparticle diffusion is sufficiently slow, the method offers a useful practical alternative to the Wicke-Kallen bach method. 

The observed rate will depend on the molecular weight of the inert gas if it is influenced by the first step. External transport can also influence or control the rate of sorption/desorption if the sorbent consists of agglomerates of zeolite crystals such as pellets or layers. The rate of sorption or desorption will then depend on the size or shape of the agglomerates if it is influenced by the transport in the macropores between the crystals. 

As already underlined in the first part of the text, the mesoporosity can be controlled through the sol-gel process conditions, for example, when a two-step catalysis is applied to standard alkoxides such as TMOS [36, 82, 103]. If these simple tetia-alkoxides are mixed with a more exotic functionalized Si precursor, differences in the hydrolysis and condensation rates of the two precursors may drastically influence the final texture of the material. For example, when the functionalized precursor carries basic moieties such as an amine in 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS), 3-aminopropyltriethoxysilane (AES), or 3-aminopropyltrimethoxysilane (AMS), these functionalized precursors can act as nucleation centers for condensation and can lead to generation of large macropores [104, 105]. 

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