Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Patent
1998-04-14
2000-01-25
Straub, Gary P.
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
423324, 423325, 423326, 4233301, 423335, 423336, 423337, C01B 3300, C01B 3326, C01B 3312, C01B 3320
Patent
active
060175052
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a new process for preparing inorganic aerogels on the basis of oxides of the metals magnesium, aluminum, silicon, tin, lanthanum, titanium, zirconium, chromium and/or thorium by producing a hydrogel by a sol/gel process, replacing the water in the hydrogel by an organic solvent, and drying the solvent-moist gel.
DESCRIPTION OF THE BACKGROUND
Aerogels are highly porous materials which consist generally of metal oxides and normally have a porosity >0.85 (in other words, for SiO.sub.2, for example, a low density preferably in the range from 0.07 to 0.3 g/cm.sup.3) and a very high internal surface area (in the case of SiO.sub.2 generally from 700 to 1200 m.sup.2 /g). These properties make the aerogels outstandingly suitable as thermal and acoustic insulants and as catalyst supports.
The low density and high internal surface area are achieved by virtue of the hydrogel retaining its solids framework (metal oxide) on drying. The solid framework in the hydrogel comes about through aggregation processes which determine the corresponding properties (density, surface area). In general, the solid framework is described by fractal clusters. The aggregation clusters (solid framework, metal oxide) of the hydrogel are destroyed in the course of "simple" drying (heating of the untreated hydrogen under subcritical conditions) owing to the action of capillary forces and the condensation of the internal surface (2M.sub.surface -OH.fwdarw.M.sub.bulk -O-M.sub.bulk +H.sub.2 O). A xerogel is formed, whose internal surface area in the case, for example, of SiO.sub.2 is in the range of 25-700 m.sup.2 /g and whose density is in the range above 1.0 g/cm.sup.3.
Prior art preparation of inorganic aerogels proceeds by one of two fundamentally different routes.
On the one hand, supercritical drying processes are employed. This prevents the action of the capillary forces, since the phase transition liquid/gaseous is avoided by an appropriate temperature and pressure regime. The drying procedure starts from solvent-containing gels that are obtained by exchange of the original solvent, usually water. Solvents employed in this context are alcohols (methanol, ethanol, propanol) or liquid carbon dioxide (Phys. Chem. 36, (1932) 52-64; EP-A-171 722; DE-A-18 11 353; U.S. Pat. No. 3,672,833; DE-A-39 24 244; WO-A-95/06617). Because of the use of autoclaves, the processes are very cost-intensive.
On the other hand, there are also processes in existence in the prior art which permit the subcritical drying of aerogels.
The key feature of the process of WO-A15/25149 is an enlargement in the contact angle between solvent an solid framework. This reduces the capillary pressure, and the structure of the moist gel is retained almost completely on drying. The changing contact angle is achieved by modifying the internal surface of the solid framework in the moist gel by reaction with a compound of the formula R.sub.x MX.sub.y where M is the metal of the solid oxide, R is an organic group and X is a halogen. Subcritical drying of the gel, which is also subjected to mulitple solvent exchange, is carried out very slowly, in stages, and so renders the process described, together with the modification step lasting about 4 days, very time-consuming.
In the process of WO-A-94/26406 an SiO.sub.2 aerogel is produced, following surface modification with trimethylchlorosilane and multiple solvent exchange, with the aid of a dielectric drying procedure (microwave drying, high-frequency drying). Dielectric drying, however, is highly expensive in terms of energy and apparatus and must be matched precisely to the particular gel.
SUMMARY OF THE INVENTION
It is an object of the present invention to prepare inorganic aerogels, especially SiO.sub.2 aerogels, in a process which is shorter than those of the prior art, is less technically complex, and hence is cost-effective.
We have found that this object is achieved by a process for preparing inorganic aerogels based on oxides of th
REFERENCES:
patent: 3672833 (1972-06-01), Teichner et al.
Kistler, S.S., Coherent Expanded Aerogels, Phys. Chem., Bd. 36, 1932 S. 52-64.
Gerber Thomas
Ziegler Bernd
BASF - Aktiengesellschaft
Nguyen Cam N.
Straub Gary P.
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