Method for a Controlled Change of the Pore Size in Solids

This invention relates to a method for pore size modification in such a way that specific molecular sieving effects can be obtained by controlled pore size reduction, and that encapsulation of compounds, elements or ions can be performed by pore closure. It is known that porous materials such as zeolites can adsorb gases and other materials, and that they can take in ions by ion exchange, provided that the pores of the substrate are large enough. A purely physical sorption or ion exchange, however, is a equilibrium process depending on concentration, pressure, temperature etc. It is therefore essentially reversible and not suitable for immobilising ions or molecules in a stable manner in a solid matrix.

In U.S. Pat. No. 3,316,691, it is proposed to encapsulate gases or fluids in a suitable zeolite, whose pores are not large enough to take in the molecules concerned. In that process the molecules are forced into the structure at high temperatures (250.degree.-350.degree. C.) and high pressures (2000 bar). After cooling to room temperature, the pressure is released, and the molecules remain trapped in the zeolite.

The European Pat. No. 80103117.0 (Penzhorn) describes a similar method for encapsulating gas molecules in zeolites under high pressures and high temperatures. The encapsulation, however, is based on a thermal vitrification of the zeolite in the presence of a pressurised gas. Under this pressure, and at higher temperature, the zeolite transforms into amorphous stable material, containing the enclosed gas molecules.

The European Pat. No. 81201137.7 describes the possibility to encapsulate gas molecules and other molecules by closing or narrowing the zeolite pores after sorption under normal conditions of temperature and pressure. The pore size reduction is obtained by a structural modification process, based on chemisorption of a modifier such as SiH.sub.4, B.sub.2 H.sub.6, etc. on structural hydroxyl groups, followed by further reaction with O.sub.2, H.sub.2 O, CH.sub.3 OH, etc.

In the present application we claim very stable effective pore size reductions by using reactions of nitrogen (or phosphorous) compounds such as NH.sub.3, NH.sub.4 + or their alkyl derivatives or compounds such as pyridine etc. with diborane in aluminosilicates or zeolites (hereafter called the "material"). Some possible examples of such reactions are given below: ##STR1##

Similar reactions can occur when these compounds react with each other in any zeolite or clay mineral, whatever its structure, chemical composition, or ionic form is. Analogous reactions take place if one of the reaction components is previously chemisorbed on the substrate. Thus, if B.sub.2 H.sub.6 is previously chemisorbed in a and H zeolite, the reaction of NH.sub.3 or other N or P compounds with the boranated samples is carried out, a further pore size reduction is obtained. Another type of procedure consists in reaction of diborane in zeolites which have been subject to chemisorption of NH.sub.3 or other N or P compounds. In addition, successive alternating treatments with NH.sub.3 and with B.sub.2 H.sub.6 are possible. Boron-nitrogen compounds, connected to the structural framework of the substrate, are formed, yielding stable obstructions with a strong pore size reducing effect. Of course, these reactions can always be combined in many ways with hydrolysis reactions such as reactions with structural OH groups and/or hydration water.

A mordenite sample was degassed (dehydrated) in a vacuum at 450.degree. C. and 1.95 meq NH.sub.4.sup.+ /g dry material was then adsorbed. The following treatments were applied: boranation at 20.degree. C., heating at 150.degree. C., boranation at 150.degree. C., heating at 400.degree. C., oxidation with H.sub.2 O and dehydration at 400.degree. C. After each step sorption capacities were measured. The results, tabulated below, indicate that a small boranation results in a dramatic exclusion of N.sub.2 at -196.degree. C. At 150.degree. C. boranation reactions proceed intensively, and a further strong pore size reduction is observed. After oxidation, adsorption appears to be limited to the external surface and the zeolite pores are closed completely. An argument for NH to be really involved in the reactions is the fact that deammoniation did not occur when the zeolite was heated at 400.degree. C. after boranation.

A CH.sub.3 NH.sub.3.sup.+ -mordenite was reacted with B.sub.2 H.sub.6 and the exclusion for Kr and N.sub.2 was followed:

A mordenite sample was prepared in its H-form, dehydrated in a vacuum at 400.degree. C., and reacted with 1.60 mmol B.sub.2 H.sub.6 per g mordenite at 150.degree. C. Then the boranated sample was reacted with NH.sub.3 at room temperature. During this reaction H.sub.2 was evolved, indicating a real reaction between the NH.sub.3 and the BH.sub.n groups in the zeolite. When heated at 400.degree. C., again H.sub.2 was evolved and only a small amount of NH.sub.3 was liberated by the zeolite. Deammoniation did not occur and therefore formation of stable B--N bonds in the zeolite is concluded. When treated with water, again a small amount of H.sub.2 was evolved. The sorption properties are measured after each step, and tabulated below.