Molecular silica

The choice of the inorganic supports was based on their different structural features. The MCM-41 material consists of uniform hexagonal arrays of linear channels that are constructed with a silica matrix like a honeycomb (Fig. 6), whereas HMS (hexagonal molecular silica) has a wormlike or sponge structure (Fig. 7) [94-100],... 

Periodic nanoporous silicates have been prepared in a wide variety of conditions. Different sources of molecular, and non molecular silica have been used. This includes TEOS, TMOS, fumed, colloidal and precipitated silicas. Depending on the synthesis conditions, particularly on the nature of the silica source, crystallization may take place in seconds at subambient temperatures [82], or at room temperature [60,61,69,72,83]. However, in most cases the crystallization temperature was set in the 80 - 120 °C range. Liu et al. [84,85] found that the use of small amounts of colloidal particles (silica or titania) promotes the formation of ordered structures by providing nucleation seeds. The pH conditions varied from extremely acidic [60,61], to neutral [69,72] to very basic [48,49]. Ryoo and Kim [86]... 

Molecular silicas (closed cage) very stable, trisilanols good stabihty... 

Molecular silicas a closed cage with an inert organic group at each vertex (e.g., alkyl, fluoroalkyl, phenyl, PEG). Usually all the eight groups are the same but alternatives exist with seven identical groups plus one different one. 

Ihe literature " describes a number of dendrimers and the closely related star-like 120-123 polysiloxanes. Ihe hyperbranched polysiloxanes are the primary example of more random structures. Although the emphasis has been on synthesis and characterization,i i modeling on hyperbranched polymers has also been carried out. Some of the most interesting species involve polysiloxane chains. Star polymers, some with nanosized silica cores, have also been synthesized.i °-i i Hyperbranched polysiloxanes have been prepared with controllable molecular weights and polydispersities,i -1 - with epoxy terminal groups some are UV-curable ° and some serve as a source of molecular silica. Hyperbranched polysiloxanes have also been used in the sol-gel preparation of polypropylene/silica nanocomposites. ... 

Most silica deposits are probably built up in this way. They may be hard and very adherent, yet are microporous, since usually there is not enough molecular or soluble silica present to fill the pores between the colloidal particles. In nature, almost any imaginable set of conditions can occur at one time or another. Further deposition of molecular silica can occur until the siliceous deposit becomes completely impervious. This effect is seen in some opal and in the walls of geodes. 

The results of Bolt (184) (Figure 4.10) and Heston, Her, and Sears (185) on nonporous silica particles are compared with those of Yates and Healy on precipitated silica (BDH) which is porous until heated to high temperature. It is evident that the surface charge density on the sol particles is of the same order as that on the precipitated powder that has been heated. The sol particles haVe never been heated over lOO C yet are dense because they were made by the slow deposition of molecular silica. Unless particles are grown in this way, porosity will be present as shown by the higher charge density. However, it appears that even these sol particles... 

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