1 polymerization silica

Column chemistry Silica Polymeric Poor separation and recovery Good separation and recovery... 

The effects of restructuring at short length scales are evident in several studies of sol-gel silica polymerization. Based on... 

Polymerization. The basic features of silica polymerization and its technical relevance have been summarized (9). A general polymerization scheme is shown in Figure 2. When a solution of Si(OH)4 is formed, eg, by acidification of a solution of a soluble silicate, at a concentration greater than... 

Surely the Phillips Cr/silica polymerization catalyst, discovered by J. P. Hogan and R. L. Banks in the early 1950s (/), can claim one distinction—to have been one of the most studied and yet most controversial systems ever. Today we seem to be debating the same questions posed over thirty years ago, being no nearer to a common view. 

The Phillips Cr/silica polymerization catalyst is prepared by impregnating a chromium compound onto a wide pore silica and then calcining in oxygen to activate the catalyst. This leaves the chromium in the hexavalent state, monodispersed on the silica surface. Chromium trioxide (Cr03) has been impregnated mast commonly, but even a trivalent chromium salt can be used since oxidation to Cr(VI) occurs during calcining. 

Type of base material (silica polymeric zirconia etc)... 

The model materials were obtained under the assumption that silica polymerization is negligible before the ordering of the liquid crystal structure. Such systems have been observed experimentally for silica at very basic conditions or for transition metal oxides at acid conditions. 

Model materials similar to the M41 family can be modeled using lattice MC simulation under the assumption of non silica polymerization conditions. The structures observed are in agreement with experimental evidence with respect to the surfactant/silica ratio, temperature and surfactant architecture. Even when the resulting structures have a strong influence of the lattice constraints, adsorption properties on such materials are in good agreement with experimental observations. Adsorption properties of modeled materials are similar to experimental observations on MCM-41 type materials and show heterogeneity at low pressures. Such behavior is not observed in smooth cylinders. 

Galameau et al. [81] used a similar approach to design porous clay heterostructures. They intercalated layered fluorohectorite by CjgTMA cations followed by treatment in a solution of neutral amine and TEOS, then drying and calcination. In brief, the authors proposed that the interlayer galleries of the intercalated clay are further swollen by the amine followed by insertion of TEOS, formation of rod-like micelles and silica polymerization. 

The synthesis of molecular precursors responsible for tailored nucle-ation and growth is a key feature in the chemical control of biomineralization. Here we highlight recent studies using simplified model systems that aim to identify aspects of solution chemistry important in (1) silica polymerization and aggregation and (2) the morphological design of calcite crystals. 

A serious problem in the control of silica polymerization is that, in general, initial particle formation is a result of random bond formation between a range of polysilicate species in solution. Moreover, the aggregation process is uncontrolled under conventional preparation conditions. It is important to understand the basic mechanisms of particle formation and aggregation in aqueous solution, as the physical properties of such silicas are determined primarily by the coordination number and nature and strength of interactions between essentially spherical primary particles. 

Composition of standard silica polymeric sol. The figures in parenthesis give the ratios X(reac-tant)/TEOS as defined by rw, raicohoi and ra... 

The activity and termination rate of the Phillips Cr/silica polymerization catalyst have been examined in relation to the surface hydroxyl population. 

This paper examines some factors which affect not only the overall activity, but also the rate of termination of polyethylene chains growing on the Phillips Cr/silica polymerization catalyst. Although the theme of this symposium is not the termination but the initiation of polymer chains, the two aims are not inconsistent because on the Phillips catalyst the initiation and termination reactions probably occur together. They are both part of a continuous mechanism of polymerization. One possibility, proposed by Hogan, is shown below. The shift of a beta hydride simultaneously terminates one live chain while initiating another ... 

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