Microporous silica adsorption properties

We have an excellent activated carbon of fiber morphology, so called activated carbon fiber ACF[3]. This ACF has considerably uniform slit-shaped micropores without mesopores, showing characteristic adsorption properties. The pore size distribution of ACF is very narrow compared with that of traditional granular activated carbon. Then, ACF has an aspect similar to the regular mesoporous silica in particular in carbon science. Consequently, we can understand more an unresolved problem such as adsorption of supercritical gas using ACF as an microporous adsorbent. 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

Separation of gas streams by adsorption is becoming increasingly popular as improved technology comes on the market. Some examples of commercially practiced adsorption processes are shown in Table 1. These processes take advantage of the selective adsorption properties of a number of microporous adsorbents, including activated carbon, silica, alumina, and various synthetic and natural zeolites. 

In a series of papers, Ma and his co-workers [1 ] systematically examined the interrelationship between adsorption, permeation and diffusion in microporous silica membranes. Both equilibrium and nonequilibrium properties of the microporous inorganic gas separation membranes were studied. Both high pressure and low pressure gravimetric units were used in their adsorption measurements. 

The implication of the theoretical considerations given above is that the permeation can be increased in cases of low adsorption amd sticking coefficients by application of a mesoporous top layer with better sorption properties on top of the microporons membranes. Selective sorption should then also lead to an enhanced separation factor (see Eq. (9.71)). Indications for this effect are reported for dense membranes by Deng et al. [106] and for microporous silica membranes by Nair [107]. 

Before the distinctive adsorptive properties of porous silica can be described, the different ranges of pore size that are of special importance to the mechanisms of physisorption must be identified. Micropores are the pores of the smallest width (d < 2 nm) mesopores are of intermediate size (id 2-50 nm) macropores are the widest pores (d > 50 nm) (5). Amorphous silica gels tend to be mesoporous or microporous, whereas the crystalline zeolitic silicas possess intracrystalline microporosity. The precipitated silicas are macroporous and also, to a small extent, microporous. These and other aspects of the microstructures will be discussed in the following sections. 

Clearly, highly active mesoporous or microporous silicas cannot be produced by the compaction of nonporous powders, but Avery and Ramsay s (8) and other compaction studies (10-12) confirmed the importance of particle coordination in determining porosity and hence, adsorptive properties. 

Il in, Turutina, and co-workers (Institute of Physical Chemistry, the Ukrainian S.S.R. Academy of Sciences, Kiev) (113-115) investigated the cation processes for obtaining crystalline porous silicas. The nature of the cation and the composition of the systems M20-Si02-H20 (where M is Li+, Na+, or K+) affect the rate of crystallization, the structure, and the adsorption properties of silica sorbents of a new class of microporous hydrated polysilicates (Siolit). These polysilicates are intermediate metastable products of the transformation of amorphous silica into a dense crystalline modification. The ion-exchange adsorption of alkali and alkaline earth metals by these polysilicates under acidic conditions increases with an increase in the crystallographic radius and the basicity of the cations under alkaline conditions, the selectivity has a reverse order. The polysilicates exhibit preferential sorption of alkali cations in the presence of which the hydrothermal synthesis of silica was carried out. This phenomenon is known as the memory effect. 

In the 1950s, A. Kiselev, Zhdanov, and co-workers (12, 84, 155-159) showed that when the adsorption isotherms of water are expressed as absolute isotherms (referred to as the unit surface of the SiC>2 sample), widely different forms of amorphous silica having a completely hydroxyl-ated state adsorb the same amount of water at the same relative pressure (p/po <0.3). Thus the plots of absolute adsorption isotherms for different samples showed that the surfaces of these samples are of a similar nature. The adsorption properties of nonporous silica and silica having large pores (i.e., an absence of micropores) depend above all on the presence of OH groups and on the degree of hydroxylation of the surface. 

The second major objection to the Langmuir and BET formulations derives from consideration of the adsorbent surface. Both models assume a finite number of uniform sites available for adsorption, but even cursory microscopic evaluation of surfaces of interest in chromatography demonstrates that with the possible exception of smooth glass beads, this is rarely the case. Surfaces such eis diatomaceous earth, silica, etc. are highly heterogeneous and, in addition, possess microporous structure, the adsorptive properties of which can be much different from those of the surface (14). 

Four main types of porous silica adsorbents have been identified compacts of pyrogenic powders, precipitated silicas, silica gels, and zeolitic silicas. The importance of porosity relative to the adsorptive properties of each group is reviewed, with particular reference to the adsorption of nitrogen, argon, and water vapor. The differences in size and specificity of these adsorptive molecules may be exploited to explore the surface properties of each grade of silica. A notable feature cf Silicalite I, which is the best known of the zeolitic silicas, is its remarkable hydrophobic character. Furthermore, the uniform tubular pore structure of this microporous silica is responsible for other highly distinctive properties. 

Before the distinctive adsorptive properties of porous silica can be described, the different ranges of pore size that are of special importance to the mechanisms of physi-sorption must be identified. Micropores are the pores of the smallest width (d < 2 nm) mesopores are of intermediate... 

OXYGEN ADSORPTION PROPERTIES OF MICROPOROUS SILICA DERIVED FROM LAYERED SILOXENE BY OXIDATION... 

The Si/Al ratio in a zeolite is never less than 1.0 but there is no upper limit and pure silica analogs of some of the zeolite structures have been prepared. The adsorptive properties show a systematic transition from the aluminum-rich sieves, which have very high affinities for water and other polar molecules, to the microporous silicas such as silicalite which are essentially hydrophobic and adsorb n-paraffins in preference to water. The transition from hydrophilic to hydrophobic normally occurs at a Si/Al ratio of between 8 and 10. By appropriate choice of framework structure, Si/Al ratio and cationic... 

Adsorption of hard sphere fluid mixtures in disordered hard sphere matrices has not been studied profoundly and the accuracy of the ROZ-type theory in the description of the structure and thermodynamics of simple mixtures is difficult to discuss. Adsorption of mixtures consisting of argon with ethane and methane in a matrix mimicking silica xerogel has been simulated by Kaminsky and Monson [42,43] in the framework of the Lennard-Jones model. A comparison with experimentally measured properties has also been performed. However, we are not aware of similar studies for simpler hard sphere mixtures, but the work from our laboratory has focused on a two-dimensional partly quenched model of hard discs [44]. That makes it impossible to judge the accuracy of theoretical approaches even for simple binary mixtures in disordered microporous media. 

---