Silica adsorption, kinetics properties

More elaborate and ambitious studies on the dissolution reactions of silica were conducted by Xiao and Lasaga (1994, 1996). Their objective was to provide full descriptions of the reaction pathway of quartz dissolution in acidic and basic solutions, from the adsorption of H2O or OH on a site, the formation of possible reaction intermediates and transition states, to the hydrolysis of the Si-O-Si bonds. Also, their aim was to extract kinetic properties such as changes in activation energy, kinetic isotope effects, catalytic and temperature effects, and the overall rate law form. The reaction mechanisms investigated were... 

Physical adsorption and covalent attachment of both native and reconibinant OPH onto various supports such as nylon membranes, porous glass and nanometer size silica beads have been enqiloyed (3,21,22).. Unfortunately, physical adsorption offers poor and nonspecific binding, vdiile coval modifications to OPH often results in reduction of enzyme activity and kinetic properties (22,23). In addition to reducing catalytic activity, there is no controlled orientation of the immobilized enzymes, leading to inaccessibility of the substrate to the enzyme active site. In the case of covalent bondii, the immobilization support is not reusable since the formed covalent bond is irreversible. In addition, the tedious and costly protocol for purification of OPH limits its use in large-scale enzymatic degradation. 

It must be concluded that the quantitative determination of micropore size is still an ambiguous problem new theories, models, mechanisms and simulations are still under study [56-58]. Therefore isotherm interpretations must be used carefully and can be considered as useful mainly for qualitative studies. No reliable method has been developed for the determination of the micropore size distribution. At present the most promising approach appears to be that of pre-adsorption linked with the use of various probe molecules of known size and shape [59-61]. For example, this approach has been applied successfully for silica compacts characterisation in [61] using spherical symmetrical inert molecules, such as neopentane and trimethylsiloxysilane [(CH3)3SiO]4Si with diameters of 6.5 and 11.5 A respectively. In general the limited availability of volatile probe molecules with diameters extending above 10 A puts a restriction on the applicability of this method. Furthermore effective pore sizes determined by this technique depend on the kinetic and thermodynamic properties of the... 

A key factor in the development of adsorption technology for the fluid separation has been the availability of appropriate adsorbents. The most frequently used categories include crystalline materials like zeolites, and amorphous materials like activated carbons, silica and alumina gels, polymeric sorbents, and ion-exchange resins. These materials exhibit a large spectrum of pore structures (networks of micro- and mesopores of different shapes and sizes) and surface chemistry (degrees of polarity), which provide a large choice of core adsorptive properties (equilibria, kinetics, and heat) to be utilized in... 

Commercial availability of a spectrum of adsorbents like activated carbons, zeolites, aluminas, silica gels, and polymeric materials having a variety of pore structures and surface properties and exhibiting a variety of core adsorptive properties like equilibria, kinetics, and heats for the gas separation or purification application. 

Modification of the oxide surfaces can lead to changes in their structural characteristics, which were studied with respect to different silicas [11,36,41,74]. Notice that systematic investigations of different structural, adsorptive, spectroscopic, electrophysical, electropheretic, kinetic and other properties using SVD-regularization applied to appropriate integral equations allow us to obtain a... 

The states of intact and dissociatively adsorbed water, the impact of nonstandard synthesis conditions on the structural-adsorptive properties of fumed silicas and related oxides, developments of overall conceptions and ideas related to the kinetics, dynamics, and mechanisms of surface reactions, the classifications of these reactions, developments in synthesis of stable Si-C bonds, different CVD-oxides on the fumed silica surfaces, and other results obtained at the Institute of Surface Chemistry (Kiev, Ukraine) are noteworthy and show new possibilities in preparation of materials based on fumed silicas and related oxides for different applications in medicine, biotechnology, environmental, and human protection, as well as in industry. 

In addition to changes to the cationic structure, the Si/Al ratio can be varied during manufacture from unity to well over 1000. Thus zeolites with widely different adsorptive properties may be tailored by the appropriate choice of framework structure, cationic form and silica to alumina ratio in order to achieve the selectivity required for a given separation. Many zeolites are extremely polar and therefore separations may be effected using both molecular sieving and internal surface property effects. The kinetic selectivity is determined from the free diameters of the windows in the intra-crystalline channel structure. Examples of such diameters, together with the principal properties and main uses of zeolites, are given in Table 2.4. 

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