Silanol group replacement silica

Reversed-phase PLC precoated plates are based on silica gel matrices with chemical modifications in such a manner that the accessible polar, hydrophilic silanol groups at the silica gel surface are replaced by nonpolar, hydrophobic alkyl chains via silicon-carbon bonds. For preparative purposes, up to now only PLC precoated RP plates with C-18 modification are available. This abbreviation is often also designated as RP-18, meaning that an octadecyl alkyl chain is chemically bonded to the silica gel surface. 

Fused silica capillary tubes were variously coated for capillary electrophoresis. The chemical process involving a Grignard reaction is shown in equation 30. The silanol groups on the silica surface are treated with alkali, dried, convert to chlorosUanes with thionyl chloride and vinylmagnesium bromide replaces the chlorine atoms with vinyl... 

FIGURE 4 Silanol group concentrations on Cr/silica catalysts as a function of activation temperature. The binding of Cr(VI) on silica replaces OH groups. 

To prepare a titanated catalyst, surface silanol groups on the silica react with a titanium ester or halide. Unreacted organic or halide groups left on the Ti are then replaced by oxide during subsequent calcination. Titanation can be accomplished in various solvents, such as hydrocarbons, alcohols, or even sometimes water, or by vapor-phase deposition. The simplicity of the approach allows any commercial silica to be so modified with up to 5-8 wt% Ti, at which point (depending on the surface area) saturation is reached. Catalyst manufacturers practice titanation but some procedures have also been developed by many polyethylene producers as well. These recipes can be practiced in a commercial polyethylene plant, because the titanium compound is applied as a vapor during the catalyst activation step. 

Her (324) prepared a series of sols modified with various amounts of aluminate. One series was made from an unmodified sol of 22 nm particles (Ludox TM) and another of 14 nm particles (Ludox HS). The coverage by aluminate ions ranged from 1.8 to 25% of the total silanol groups on the surface (assuming 8 SiOH nm ). Stabilization as an aluminate ion in the particle surface requires that each aluminum atom be surrounded by three oxygens linked to silicon, which means that no more than 25% of the surface silicon sites can be replaced by aluminum atoms. This assumes, of course, that the underlying silica surface is nonporous and not accessible to reaction with Al(OH)4" ions. 

More than 95% of the silanol groups can be replaced by the stable Si—C bonds. Column materials with Si—C bonded functions on the silica surface are commercially available, but have not achieved the same position as the more conventional Si—O bonded materials. 

---