Method for producing porous inorganic materials

Details Method for producing porous inorganic materials Method for producing porous inorganic materials

1999-06-22

2001-03-27

Dixon, Merrick (Department: 1774)

Plastic and nonmetallic article shaping or treating: processes

Direct application of electrical or wave energy to work

Producing or treating porous product

C264S413000, C264S415000, C264S419000, C264S425000, C264S426000

Reexamination Certificate

active

06207098

ABSTRACT:

This invention relates to an improved method for producing porous inorganic materials. The method of this invention is favorably utilized in manufacture of sorbents for chromatography, fixed phases for thin-layer chromatography, porous materials for blood separation, moisture-absorbing porous materials, porous materials for adsorbing low-molecular materials for deodorization, or porous materials for enzyme carriers.
Porous inorganic materials are favorably applied to producing columns for chromatography, porous filters for separating blood, porous catalysts, or enzyme supports. Such inorganic porous columns can be favorably applied to liquid- and gas-chromatography. These columns can be used unmodified or modified e.g. by covering their surface with molecules like hydrophobic hydrocarbon ligands (e.g. octadecyl ligands) or like hydrophilic ligands like 2,3-dihydroxypropyl derivatives. Other derivatives useful for separations are known in the art. The ligands of such modified columns can be further modified using known procedures. Porous catalysts or enzyme supports can be prepared by binding enzymes, e.g. glucose isomerase, or catalytic metal elements, e.g. platinum and palladium. Such inorganic porous columns can also be attached to an injector or a catheter for blood injection. Due to their mechanical rigidness and heat stability, which allows sterilization by heat, inorganic base materials are preferred.
The sol-gel method is one liquid phase reaction path to produce inorganic porous materials, especially silica gels. The sol-gel method denotes widespread processes in which polymerizable low molecular weight species are first generated, and through polymerization reactions, aggregated or polymerized materials are finally obtained. For example, the sol-gel method can be applied by hydrolyzing metal alkoxides, metal chlorides, metal salts or coordinated compounds which typically contain carboxyl or beta-diketone ligands. A process of this kind is disclosed in Japanese Patent Publication No. 8-29952 and corresponding EP 0 363 697 and U.S. Pat. No. 5,009,688. In this process an organic polymer is used, which is compatible with the solution of the metal alkoxide or its polymer, and which undergoes phase separation during the hydrolysis-polymerization step. This method comprises preparing a gel which has a solvent-rich phase capable of giving macropores of not smaller than about 100 nanometers in size, through sol-gel conversion in the presence of a pore forming agent, and finally drying and calcining the material. The porous inorganic materials produced by this process display connected open macropores. Examples of pore forming agents disclosed in these documents are: sodium polystyrene sulfonate, polyacrylic acid, polyallylamine, polyethylene-imine, polyethylene oxide, and polyvinyl pyrrolidone. Adding lower alkyl alcohols like methanol or ethanol to the gelling mixture can also be used to modify the size of the macropores.
Especially for use in chromatography the porous inorganic material should display another set of pores to enhance the specific surface of the material. These pores have to be accessible for the analyte; therefore mesopores of appropriate size (2 to 100 nm) should be present in the walls of the macropores. A method for manufacturing such porous materials which possess interconnected continuous macropores with a median diameter larger than 0.1 &mgr;m and additional mesopores in the walls of said macropores, said mesopores having a median diameter between 2 and 100 nm has been disclosed in Japanese Patent Application Laid-open No. 7-41374 and corresponding EP 0 710 219 and U.S. Pat. No. 5,624,875. This method comprises preparing a gel which has a solvent-rich phase capable of giving macropores of not smaller than about 100 nanometers in size, through sol-gel conversion in the presence of a pore forming agent, followed by dipping the wet monolithic gel in an aqueous solutions containing a matrix dissolving agent thereby modifying the distribution of mesopores in the gel, and finally drying and calcining the porous inorganic material. Examples of matrix dissolving agents disclosed in these documents are basic substances like sodium hydroxide or aqueous ammonia, or hydrofluoric acid.
Additional disclosure of technical background is found in the documents mentioned above.
Where porous materials are used as carriers for various purposes, it is necessary that they have an optimal median pore diameter and have pore size distribution as narrow as possible. The diameter of their macropores often controls the flow-rate and the pressure drop in the system. The accessibility of the surface of the pores is usually controlled by the diameter of the mesopores. Therefore, for porous materials to be obtained through sol-gel conversion, various attempts have heretofore been made at controlling the reaction conditions for the formation of gel to thereby control the pore size of the porous material to be derived from the gel.
However, as using a water-soluble polymer, this method still causes problems in that the preparation of the reaction system takes much time, and that the characteristics of the product depends on the molecular weight distribution thereof. In addition, since this method comprises separate steps for the formation of the gel and for the solvent substitution thereof, it is a complicated one.
It had been found that further improvements of the porous inorganic materials would be highly needed. Such improvement refer to controling the formation of both the macro- and the mesopores. Such improvements are especially needed where the porous inorganic materials are used as base materials for monolithic columns for chromatography; especially for use in preparative chromatography where larger columns (e.g. diameter typically larger than 1 cm) are needed.
The inventors found improvements related to the manufacture of porous inorganic materials. These improvements comprise using a nonionic surfactant as pore forming agent and novel methods of applying matrix dissolving agents by using precursors of matrix dissolving agents. When the precursor of a matrix dissolving agent that has been dissolved in the gel is reacted to yield the matrix dissolving agent (e.g. by thermolysis, sometimes also called pyrolysis), the gel can easily be converted into a porous material with mesopores of at most about 100 nanometers in inner size and having a narrow pore size distribution. Because the matrix dissolving agent typically is freed from its precursor by thermolysis, a precursor of a matrix dissolving agent can also be called a thermolyzable (sometimes also called pyrolyzable) compound.
The object of the present invention is a process for producing inorganic porous materials, mainly composed of glass or glass-ceramic components, having interconnected continuous macropores with a median diameter larger than 0.1 &mgr;m, and having mesopores in the walls of said macropores, said mesopores having a median diameter between 2 and 100 nm via sol-gel route which includes the steps of:
(a) Dissolving a water-soluble polymer or some other pore forming agent and a precursor for a matrix dissolving agent in a medium that promotes the hydrolysis of the metalorganic compound (see step b);
(b) mixing a metalorganic compound which contains hydrolyzable ligands to promote hydrolysis reaction;
(c) solidifying the mixture through the sol-gel transition, whereby a gel is prepared which has threedimensional interconnected phase domains one rich in solvent the other rich in inorganic component in which surface pores are contained;
(d) setting the matrix dissolving agent free from its precursor, whereby the matrix dissolving agent modifies the structure of said inorganic component;
(e) removing the solution by evaporation drying and/or heat-treatment;
(f) calcining the gel to form the porous material.
In preferred embodiments of the invention silica is the base material of the porous inorganic materials and urea or compounds having an amido group or an alkylamido group are used as precursor of the matrix disso

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