Microporous silica beads

It has been shown that for naphthalene contained in coal tar globules, the area-dependent mass transfer coefficient for globules was 10 greater than when the substrate was coated on microporous silica beads, and that this was an important factor in determining the rate of mineralization (Ghoshal and Luthy 1996). 

Porous silica beads have been used in GSC for separating mixtures of gases and volatile organic hydrocarbons (VOCs). Retention characteristics arise from adsorption by the micropores, 50-500 nm in diameter, and polar properties of the surface silanol groups. Retention of VOCs decreases as the pore size decreases and surface area increases. Hydrothermal treatment with steam at 850°C for up to 24 h is used to increase pore size to over lOOOnm and chemical modification by silanisation of the silanol groups reduces polar character. Esterification of the silanol groups with methanol and ethanol has also been used. 

Microporous particles are available in two sizes 20 to 40 jam diameter with longer pores and 5 to 10 fim with short pores (see Figure 3.14B). These are now more widely used than the porous layer beads because they offer greater resolution and faster separations with lower pressures. The micro-porous beads are prepared from alumina, silica, ion-exchanger resins, and chemically bonded phases (see next section). 

Selected signature libraries may be immobilized on a solid matrix such as activated silica resin, cellulose microporous modified membranes [66], Sepharose , magnetic beads based on MagaPhase technology. The affinity support obtained is used for IgM antibodies parting. 

In microporous particles, pore depths are decreased by decreasing dp. Particles of 20-40 g diameter with longer pores and of 3-10 g diameter with short pores are available. Microporous beads are prepared from porous materials like silica or alumina. 

Both microporous and pellicular bead silica and alumina can be used for HPLC in the adsorption mode (liquid-solid chromatography). Pellicular beads, however, are rarely used now. While the pellicular beads (Table 10.2) mostly are spherical in shape, microporous particle column materials (Table 10.3) can be either irregular in shape or spherical. Theoretically irregular particles should give higher efficiencies but spherical materials pack together better. 

The silica penetrates the catalyst bead via a shell progressive mechanism and deposits in the micropores of the catalyst, deposition is non-selective and silica masks the noble metal active sites... 

The silica loading and thickness in beads measured by electron microprobe method are shown in Figure 8 and Table 6. Table 7 summarizes the pore structure of fresh and aged catalysts (top 1 inch). The surface area and micropore volume of the aged catalyst was lower than that of the fresh catalyst, suggesting that silica has been preferentially deposited within the micropores of the catalyst, probably as a gaseous silica compound rather than as a particulate. 

These gaseous precursor compounds diffuse into the bead catalyst and deposit as silica particles in the micropores. This deposition results in silica penetration further into the particle than the outer layer containing the active metals. 

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). 

Enzymes immobilized microporous matrices, such as organic polymer beads, silica particles, and ceramic carriers, have been widely used for the production... 

The most common supports are insoluble particles of polystyrenes (PS), cross-linked with divinylbenzene (DVB), and silica gel (SG). Soluble polymers have been used as supports and separated by precipitation or by ultrafiltration. Insoluble PS beads are prepared by suspension polymerization. They can be either microporous or macroporous (synonymous with macroretic-ular). " Typical average particle diameters are 50 gm for peptide synthesis and 500 jum for ion exchange. PS beads are used in the form of solvent-swollen gels. The micropores are created by solvent, and removal of solvent collapses the pores. A macroporous polymer retains pores in the dry state and may have as much as 700 m g" of internal surface. The macropores are created during polymerization by a solvent from which the polymer precipitates as it is formed. A macroporous polymer is usually, but not necessarily, highly cross-linked. In a good solvent, the polymer phase of a macroporous PS also becomes a microporous, solvent-swollen gel. 

---