Process for the production of organofunctionalized aerogels

Details Process for the production of organofunctionalized aerogels Process for the production of organofunctionalized aerogels

1998-06-15

2001-02-20

Warden, Jill (Department: 1721)

Colloid systems and wetting agents; subcombinations thereof; pro

Continuous or semicontinuous solid phase

The solid phase contains organic material

C423S338000, C428S404000, C427S220000

Reexamination Certificate

active

06191173

ABSTRACT:

The invention relates to a process for preparing organofunctionalized aerogels, to organofunctionalized aerogels, and to their use.
Aerogels are highly porous materials of low density, which are prepared by forming a gel and then removing the liquid while retaining the structure of the gel.
According to a more precise definition (see, for example, Gesser and Goswani, Chem. Rev. 1989, 89, 767), aerogels are materials in which the liquid is removed from the gel under supercritical conditions, while the term xerogels is used if the gel is dried under subcritical conditions and cryogels in cases where the liquid is removed by sublimation from the frozen state.
For the purposes of the present invention, aerogels include all of these materials and may also contain, besides air, any other desired gases.
Because of their high porosity, aerogels have interesting physical properties which give them suitability for use as, inter alia, thermal insulation materials, acoustic materials, luminescent solar collectors, gas filters, catalysts and support materials.
For many of these applications, it is desirable to be able to modify the chemical properties of the aerogels, for example by incorporating functional groups.
DE-A-40 02 287 describes functionalized inorganic xerogels. However, the preparation conditions described therein do not give any products having gel structures, and therefore these are not aerogels for the purposes of the present invention.
U.S. Pat. No. 5,270,027 describes aerogels which have been etherified with amino alcohols. However, ether bridges of this type are not particularly stable in the long term, so that there is gradual cleavage of the organic groups.
EP-A 0 629 442 discloses aerogels which contain chelated transition metals and are catalysts, and Cao and Hunt (Mat. Res. Soc. Symp. Proc. 1994, Vol. 346, 631) describe amino functionalized aerogels.
Schubert et al. (Mat. Res. Soc. Symp. Proc. 1994, Vol. 346, 151, German Patent Application P 195 33 851.0) have synthesized aerogels having various functionalized radicals.
A common feature of all of these materials is that the aerogel on which they are based is prepared from tetraalkoxysilanes. However, these starting compounds represent, inter alia, an exceptionally high cost, which creates difficulties for preparation of the aerogels on an industrial scale.
It was therefore desirable to find a cost-effective process which is suitable for preparing organofunctional aerogels on an industrial scale.
Surprisingly, it has now been found that organofunctionally modified aerogels may be prepared using water glass as starting material.
It is known (see, for example, EP-A 0 396 076) that aerogels can be prepared from water glass and, if desired, (see, for example, EP-A 0 658 513) can be modified with unreactive organic radicals.
However, a possible modification with functional organic radicals cannot be found in these texts.
The invention therefore provides a process for preparing organofunctionally modified aerogels by gelling a water glass by means of polycondensation and then converting it, by drying while retaining the structure of the gel, to give an aerogel, which process comprises reacting the gel, before drying, with an at least bifunctional organic compound, at least one functional group serving as a bond to the aerogel, while the remainder are retained.
For the purposes of the invention, the term organofunctionally modified means that the aerogel, preferably on the inner surface, contains identical or different organic radicals which have at least one, preferably one, two or three, functional groups. The term functional groups here means structures which are in the organic radicals and have polar atomic bonds, preferably resulting from the presence of heteroatoms.
Examples of functional groups are halogens, pseudohalogens, hydroxyl, thio, amino, amido, ether, ester, acid, formyl and keto.
According to the invention, the aerogels are prepared from water glass. The term water glass is taken to mean water-soluble alkali metal silicates, preferably sodium and/or potassium silicates. For the purposes of the invention, the term also includes water-soluble complex alkali-metal metallates of other metals, such as Al, Ti, Zr and Sn, but Si water glass is preferred. Commercially available water glasses comprise 2 [lacuna].
The functional organic compound contains at least one group, for example a halo, preferably chloro, alcohol, ether, acid, preferably carboxylic acid, or ester function, which enables it to react with the original hydroxyl or ether groups of the aerogel surface. The compounds may, for example, be amino alcohols of the formula R
1
—NH—R
2
—OH, where R
1
=H, C
1
-C
4
-alkyl or —(CH
2
)
2-3
—OH and R
2
=C
1
-C
4
-alkylene.
However, preference is given to functional organic compounds of the formula
R
n
—MX
m
where M is Si, Al, Zr, Ti or Sn, preferably Si, and n and m are integers greater than zero, the sum of which corresponds to the valence of M. R is a C
1
-C
22
-hydrocarbon radical which contains at least one functional group, preferably selected from the class consisting of OH, OR
3
, COOR
3
, OCOR
3
, SO
3
R
3
, SO
2
Cl, F, Cl, Br, NO
2
, CN, SCN, —NCO, —OCN, NR
3
2
, —CONR
2
, —O—CO—O—, oxiran-2,3-diyl and SR
3
, where R
3
may be identical or different and is H, C
1
-C
20
-alkyl or C
4
-C
10
-aryl; the radicals R
3
, independently of one another, are identical or different or may be bridged. X is a halogen, preferably Cl, or a radical —OR
4
, —SR
4
or —NR
4
2
where R
4
is H, a straight-chain or branched C
1
-C
18
aliphatic radical, benzyl or C
4
-C
10
-aryl.
If m is at least two, the radicals X, independently of one another, may be identical or different or bridged. The index n is preferably 1 or 2, particularly preferably 1.
It is also possible to use mixtures of two or more functional organic compounds.
Particular preference is given to functional organic compounds in which R is —Y—Z, where
Y is C
1
-C
8
-alkylene, preferably straight-chain, C
1
-C
8
-alkenylene or —[(CH
2
)
2
O]
n
—CH
2
, where n=1, 2 or 3;
Z is F, Cl, Br, I, CN, SCN, —NCO, —OCN, NR′R″, OR′, SR′, PR′R″ or oxiran-2-yl;
R′ and R″ are identical or different and are H, C
1
-C
12
-alkyl, C
4
-C
10
-aryl or benzyl.
Other preferred functional organic compounds are those of the type R
n
R
5
s
MX
m
, where n, s and m are natural numbers and together correspond to the valence of M and where R, M and X are as defined above. R
5
is hydrogen or an unreactive organic linear, branched, cyclic, aromatic or heteroaromatic radical, such as C
1
-C
18
-alkyl, preferably C
1
-C
6
-alkyl, particularly preferably methyl or ethyl, cyclohexyl or phenyl; the radicals R
5
, independently of one another, are identical or different or may be bridged.
The functional organic compound is generally used in a proportion of up to 30% by weight, preferably 20% by weight, particularly preferably 10% by weight, based on the undried aerogel.
By reacting with functional organic compounds, the original surface groups are replaced by groups of the type MR
n
and/or MR
n
R
5
s
.
The compounds of the formula R
n
MX
n
are commercially obtainable or can be prepared by methods known per se and familiar to the person skilled in the art.
Methods of this type are described, for example, in the article “Hybrid Inorganic-Organic Materials by Sol-Gel Processing of Organofunctional Metal Alkoxides” (U. Schubert et al., Chem. Mat., 1995, 7, 2010).
Preparation methods for the preferred organoalkoxysilanes, such as the hydrosilylation of unsaturated compounds with a subsequent alcoholysis:
are described, inter alia, in W. Noll, Chemie und Technologie der Silicone [Chemistry and Technology of the Silicones], Verlag Chemie, Weinheim, 1968 and U. Deschler, P. Kleinschmit, P. Panster, Angew. Chem. 1986, 98, 237 (Int. Ed. Engl. 1986, 25, 236).
There are numerous individual examples in Silane Coupling Agents, E. P. Plueddermann, 2nd Edition, Plenum Press, New York, 1991, pp. 31-54.
Examples o

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