The most varied membrane materials and adsorbents are known for separating mixtures of materials and are continually being improved with respect to their technical suitability and economy. In the treatment of effluent waters, it is desirable for instance by developing asymmetrical membranes to achieve the throughput loadings necessary for technical application. On the other hand, other problems arise in other areas of use. For example, known membrane materials, such as cellulose acetate membranes, are not very temperature or pressure resistant and swell rapidly in organic solvents. The low temperature, pressure and solvent resistance has the result that the pore sizes continually alter during technical application and this can lead to non-reproducible results as well as short periods of use for the membranes.
Also, adsorbents for purifying fluid or gaseous media are known in enormous numbers, for example, active charcoal and oxides with high specific surfaces, such as aluminium oxide. With these known adsorbents, adsorption normally takes place non-specifically and not very selectively. Moreover, the adsorption process is often only poorly reversible at high adsorption grades, that is, a high consumption of energy is necessary for desorption.
Known adsorbents and membranes with surfaces which have been chemically modified also have the disadvantage that, as a rule, they have only a modified mono-layer at the surface, so that they are extraordinarily sensitive to mechanical and chemical effects, such as hydrolytic action.
The inventors have already developed certain silicic acid heteropolycondensates, which are particularly suitable for the preparation of temperature, pressure and solvent-resistant membranes for the most varied separation problems. In the granular or similar state, these polycondensates are usable as adsorbents, even for highly specific separations, which either cannot be carried out or only with unsatisfactory results with known adsorption agents.
These porous silicic acid heteropolycondensates usable as membranes and adsorbents are made by:
(i) forming a reaction mixture comprising: PA0 (ii) subjecting the reaction mixture to hydrolysis and condensation in the presence of at least the amount of water stoichiometrically required for hydrolysis, optionally a solvent and in the presence of a condensation catalyst, wherein the amounts of the components (a) to (d) are selected so that the resultant silicic acid heteropolycondensate, calculated as oxide units, contains, by weight, 35% to 90% of component (a), 10% to 50% of component (b), 0% to 15% of component (c) and 0% to 40% of component (d).
(a) at least one hydrolysable silicic acid derivative of the general formula I: EQU SiR.sub.4 (I) PA1 wherein each R is selected from the group consisting of hydrogen, halogen, alkoxy and --NR'.sub.2 radicals, wherein each R' in the last-mentioned radical independently represents a hydrogen atom or an alkyl group, with the proviso that not all the R radicals are hydrogen atoms, and PA1 (b) at least one substituted silane of the general formula II: EQU SiR.sub.n R".sub.(4-n) (II) PA1 wherein R has the meaning defined above, each R" is selected from the group consisting of alkyl, alkenyl, aryl and aralkyl radicals and n is an integer from 1 to 3, and PA1 (c) optionally at least one functional silane of the general formula III: EQU SiR.sub.n (R'"Y).sub.(4-n) (III) PA1 wherein R and n have the meanings defined above, each R'" is selected from the group consisting of alkylene, phenylene, alkylphenylene and alkylenephenylene radicals and Y is selected from the group consisting of halogen, amino, anilino, aldehyde, keto, carboxy, hydroxy, mercapto, cyano, hydroxyphenyl, diazo, carboxylic alkyl ester, sulphonic acid (--SO.sub.3 H) and phosphoric acid (--PO.sub.3 H.sub.2) radicals, and/or PA1 (d) optionally at least one oxide component selected from the group consisting of non-volatile oxides soluble in the reaction medium and compounds capable of forming non-volatile oxides soluble in the reaction medium, the oxide component being an oxide of an element selected from Groups Ia to Va, Group IVb and Group Vb of the Periodic System, and
In this method, the condensation catalyst is used in an amount of not more than 3% by weight, based on the weight of the reaction mixture.