Surface of the Silica Support

Electron microscopy shows that polymerization starts at active centres on the surface of the particle. During this initial stage, a thin polymer cover is formed on and just below the outer surface of the silica support. This thin cover consists of highly crystalline polypropylene, which acts as a diffusion barrier for the monomer. Diffusion of propylene through this layer thus becomes rate-limiting for polymer formation consequently the high initial polymerization activity decreases sharply after a few minutes and a period of relatively low activity is reached. 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

The problems of stationary phase erosion can be largely overcome by solvent-generated LLC, where the stationary liquid phase is generated dynamically by the mobile phase, in this approach, one of the phases of an equilibrated liquid-liquid system is applied as a mobile phase to a solid support which is better wetted by the other phase of the liquid-liquid system. The support is usually silica when the stationary phase is aqueous or a polar solvent and a reversed-phase chemically bonded support when the stationary phase is a nonpolar solvent. Under these Conditions a multimolecular layer is formed on the surface of the solid support which has the properties of the liquid phase in... 

In most cases, the SAPCs are less active than their homogeneous counterparts, but more active than their liquid biphasic catalyst analogue, a phenomenon very simply interpreted as resulting from the increase of the interphase surface area which, in the case of the SAPCs, is similar to that of the silica support. 

Various novel imprinting techniques have also been presented recently. For instance, latex particles surfaces were imprinted with a cholesterol derivative in a core-shell emulsion polymerization. This was performed in a two-step procedure starting with polymerizing DVB over a polystyrene core followed by a second polymerization with a vinyl surfactant and a surfactant/cholesterol-hybrid molecule as monomer and template, respectively. The submicrometer particles did bind cholesterol in a mixture of 2-propanol (60%) and water [134]. Also new is a technique for the orientated immobilization of templates on silica surfaces [ 135]. Molecular imprinting was performed in this case by generating a polymer covering the silica as well as templates. This step was followed by the dissolution of the silica support with hydrofluoric acid. Theophylline selective MIP were obtained. 

Bonded-Phase Supports The bonded-phase supports usually overcome plethora of the nagging problems which is mostly encountered with adsorbed-liquid phases. Here the molecules, comprising the stationary phase, i.e., the surfaces of the silica particles, are covalently bonded to a silica-based support particle. 

The alumina or silica-alumina supports used in bifunctional catalysts have been shown to be acidic in nature. The acidic properties are readily demonstrated by the affinity of these solids for adsorption of basic compounds such as ammonia, trimethylamine, re-butylamine, pyridine, and quinoline (01, R5). Furthermore, adsorption of certain acid-base indicators such as butter yellow gives a coloration similar to that observed in acid media (B3, B4). With regard to the origin of the acidity, Tamele (Tl) has suggested in the case of silica-alumina that aluminum atoms replace silicon atoms in the surface of the silica structure, giving rise to surface sites of the form... 

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