Catalyst – solid sorbent – or support therefor: product or process – Solid sorbent – Silicon containing
Patent
1997-07-22
1999-08-31
Lewis, Michael
Catalyst, solid sorbent, or support therefor: product or process
Solid sorbent
Silicon containing
502401, B01J 2010
Patent
active
059453737
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to processes for modifying the surface chemistry of silica and to silica products so modified. A particular application is to silica packing media for chromatographic applications, specifically the preparation of a "reverse phase" silica medium.
BACKGROUND
Chromatography is a widely used technique for separating mixtures. Generally it involves two relatively moving phases, e.g. a stationary phase (solid) and a mobile phase (fluid). "Normal phase" chromatography uses a polar stationary phase and a non-polar mobile phase. In "reverse phase" chromatography the stationary phase is non-polar. A common stationary phase is silica; particulate amorphous silica. Reverse phase chromatography conventionally uses silica which has been modified by covalent attachment of long hydrocarbon chains to give a non-polar surface. This "reverse phase" silica is protected from hydrolysis in aqueous mobile phases because of the absence of polar interactions at its surface; the consequent use of aqueous mobile phases permits a variety of new separations. This is important in the medical field where many compounds are at least to some extent soluble in water, and includes analytical chromatography techniques such as thin layer chromatography (TLC).
However, reverse phase chromatography is often avoided for reasons of expense. Reverse phase silica media are up to ten times as expensive as their normal phase counterparts, because of the multi-step reactions with various silanes involved in manufacture.
THE INVENTION
We have found a new and simple process for treating the surface of silica to render it essentially non-polar, and usable to convert a `normal` particulate silica chromatography medium into a `reverse phase` medium.
We use a metal complex which has a hydrocarbon chain coordinated with the metal ion through a ligand moiety, and other ligands which enable reaction with the silanol groups at the silica surface to bind the metal ion and its attached hydrocarbon chain to that surface, and also preferably enable reaction between adjacent complex molecules to bind them together over the surface.
The hydrocarbon chain preferably has at least 7, more preferably at least 9 and most preferably at least 12 carbon atoms. Alkyl chains are preferred. It confers relatively non-polar properties on the resulting silica surface. Its ligand moiety is generally some electron-donor group such as an oxygen-containing group e.g. a carboxylic or carboxylate group: it may conveniently be a fatty acid. It may coordinate with more than one metal ion: carboxylate can coordinate with two, for example.
Other entities coordinated to metal ion are preferably electronegative entities susceptible to forming anions in solution on hydrolysis (or neutralisation by base) of the complex. This enables the complex to acquire a positive charge in solution, believed to be significant in achieving reaction with the silica surface. It can also assist the formation of bridges e.g. hydroxyl links between complex and silica silanol, and between adjacent complexes: these links can eliminate solvent on curing to form permanent bonds via oxygen.
Halide ligands are preferred, especially Cl, Br, I.
The metal ion is preferably a transition metal ion: chromium (III) has been found suitable but others may be used provided that they can coordinate the appropriate ligands in such a way that the required binding reactions can occur.
PREFERRED AND OPTIONAL FEATURES
A preferred complex is representable (omitting associated solvent molecules such as water and/or alcohol) by the formula: ##STR1## in which M is a trivalent transition metal ion such as Cr (III), R is a hydrocarbon chain, preferably of at least seven and more preferably at least twelve carbon atoms, and X is halogen other than fluorine.
In aqueous or alcoholic solution solvent molecules can coordinate e.g. thus: ##STR2## where R' is water or alcohol. Further hydrolysis
eutralisation frees X.sup.- anions and coordinates further solvent; solvent groups can then lose p
REFERENCES:
patent: 2273040 (1942-09-01), Iler et al.
patent: 3886070 (1975-05-01), Martineau et al.
patent: 4520122 (1985-05-01), Arena
Patents Abstracts of Japan, vol. 12, No. 56 (C-477) & JP A 62 201640 (Daicel Chem. Ind.) Sep. 5, 1987.
Patent Abstracts of Japan, vol. 13, No. 171 (C-588) & JP A 64 003130 (Daicel Chem. Ind.) Jan. 6, 1989.
Downstream Media Limited
Ghyka Alexander G.
Lewis Michael
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