GB1586364A

GB1586364A - Porous inorganic materials

Info

Publication number: GB1586364A
Application number: GB2520976A
Authority: GB
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1976-06-17
Filing date: 1976-06-17
Publication date: 1981-03-18
Also published as: JPS6323158B2; SE7706990L; JPS52154814A; SE8207520L; SE451715B; SE8207520D0; DE2727143A1; SE435717B; NL7706730A; DE2727143C2

Description

(54) IMPROVEMENTS IN OR RELATING TO POROUS INORGANIC MATERIALS (71) We, UNITED KINGDOM ATOMIC ENERGY AUTHORITY, London, a British Authority, do hereby declare the ~ invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to porous materials and more particularly to the production of porous materials suitable for the selective retention of macromolecules from a fluid substance containing said macromolecules.

According to one aspect of the present invention there is provided a process for the production of a porous material suitable for the selective retention of predetermined macromolecules from a fluid substance containing said macromolecules which includes the steps of treating a slurry comprising particles of a finely divided, substantially insoluble, sorptive, inorganic material, an additive and a solvent for the additive, said additive being capable of conversion to an insoluble form, to convert the additive to the insoluble form thereby to produce an intermediate material comprising particles of the inorganic material bound together by the insoluble form of the additive and treating the intermediate material to remove at least some of the insoluble form of the additive thereby to produce a porous material having an interconnected internal porosity, the inorganic material being selected such that, and the additive and amount of additive being selected such that, the porous material has a pore structure capable of allowing the macro molecules to permeate the porous material.

"Sorptive" as used in this specification with reference to an inorganic material from which the porous material may be formed means that the inorganic material either is of its nature sorptive and capable of sorbing molecules or may be treated to make it sorptive and capable of sorbing molecules. Furthermore it is to be understood that sorptive processes with which the present invention is concerned may embrace absorption and adsorption (which includes physisorption and chemisorption) and combinations thereof.

The term "substantially insoluble" as used herein means that the inorganic material is substantially insoluble in the substances with which it comes into contact in the manufacture of the porous material according to the invention. It will be appreciated that the inorganic material should also be substantially insoluble in substances with which it comes into contact in the use of the porous material for its intended purpose.

The additive and the insoluble form of the additive may, optionally, have the same or a different chemical constitution.

The slurry may be treated to precipitate the insoluble form of the additive to produce intermediate material comprising particles of the inorganic material bound together by precipitated insoluble form of the additive.

We have found that it is in general convenient, to facilitate handling and/or storage, at least partially to dry the material after its production by conversion of the additive to the insoluble form. Drying may be, for example, in air or methanol.

In accordance with the present invention the slurry is preferably formed into droplets and contacted with a reagent capable of precipitating the insoluble form of the additive thereby to give substantially spherical discrete particles of intermediate material such that substantially spherical discrete porous particles are produced after removal of the precipitated insoluble form of the additive.

It will be understood that the intermediate material is conveniently separated from the precipitating reagent prior to being treated to remove the precipitated form of the additive.

In carrying out the process of the present invention the insoluble form of the additive is preferably removed by heating the intermediate material to a temperature at which the insoluble form of the additive is "burnt out" to leave a porous material. (i.e. the in soluble form of the additive acts as a fugitive additive).

In addition to the additive an additional porosity forming fugitive material can be incorporated in the intermediate material and subsequently used to generate porosity in the porous material. The additional porosity forming fugitive material may thereby serve to increase or modify the porosity produced.

by removal of the insoluble form of the ad ditive. An additional porosity forming fugitive material may, for example, be incorporated in the intermediate material by addition to the slurry from which the intermediate material is formed.

Our British Patent Application No.

58374/71 (Now British Patent No.

1,421,531) discloses inter alia a method for producing discrete porous particles having an interconnected internal porosity for the selective retention of predetermined macromolecules from a fluid substance containing said macromolecules. The present invention provides another process which can be used to produce discrete porous particles for such a purpose.

In order that the porous material can be used for the selective retention of macromolecules from a fluid substance containing said macromolecules it will be understood that the porous material should have a pore structure such as will allow the molecules to permeate the material and be sorbed by the sorptive inorganic material.

Reference should be made to the Complete Specification of British Patent No. 1,421,531 for a discussion of pore size and its relationship to the size of molecules which can be sorbed or excluded from sorption.

The pore size of the porous material and thus the size of molecules excluded or allowed to permeate may be varied by varying the amount of additive or fugitive material or by incorporating appropriate additive or fugitive material at the manufacturing stage.

Our British Patent No. 1,421,531 hereinbefore mentioned refers to enhancement of molecular separation by use of "molecular sieving" and the production of material having sorptive and molecular sieving properties.

By appropriate choice of additive or fugitive material such molecular sieving properties may be introduced into porous materials fabricated in accordance with the present invention by controlling the pore size.

In some cases inorganic ions may become incorporated in the intermediate material (e.g.

by interacting with the additive to form a complex) and give rise to impurities in the porous material products after heating.

If these impurities are soluble then they may be removed by treating the porous material with a suitable solvent to dissolve them out.

Examples of additives capable of conversion to an insoluble form which can be used in accordance with the present invention are high molecular weight materials such as alginic acid, alginates, pectin, polygalacturonic acid and proteins.

The foregoing additives are preferred on economic grounds, but other, more expensive, additives capable of conversion to an insoluble form may be used (e.g. carboxymethyl derivatives of cellulose, gums (which give insoluble products with acids or cationic solutions), derivatives of polyacrylic acid, polymers of styrene-maleic anhydride and nucleic acids).

It will be appreciated that by "insoluble form" in respect of the additive, we mean a form substantially insoluble in substances used during formation of the intermediate material in accordance with the present invention (e.g. reagents used to cause precipi- tation) and substances used in subsequent processing of the intermediate material (e.g.

drying reagents).

Examples of additional porostiy forming fugitive materials which may be incorporated in the intermediate material in addition to the additives which can be converted to an insoluble form are haemoglobin and soluble starch. Also, powdered insoluble polymers can be used as additional porosity forming fugitive materials.

Certain other substances can also be used as additional porosity forming fugitive materials (e.g. casein, gelatine and vegetable proteins) as can "fugitive additives" disclosed in our British Patent No. 1,421,531, for example: ammonium carbonate, polyvinyl alcohol, dextran, bovine serum albumin and ovalbumin.

In selecting an additional porosity forming fugitive material generally it is, of course, preferable to choose a substance which will not readily diffuse or leach from the slurry during conversion of the additive to the insoluble form.

Also, it is to be understood that where particles are to be formed, it is very preferable to avoid additional porosity forming fugitive materials which can give rise to early precipitation (i.e. before droplet formation).

For example, it is very preferable, when an alginate is the additive, to avoid certain casein preparations which contain calcium ions as an impurity since these ions may cause premature precipitation of calcium alginate.

The additive capable of conversion to an insoluble form and the additional porosity forming fugitive material (when used) may contribute to the formation of the inter connected internal porosity (porous network) in the porous material.

Examples of inorganic materials which can be formed into porous materials in accordance with the present invention are titania, calcium phosphate, natural earths such as bentonite and Celite, and other inorganic materials disclosed in our British Patent No. 1,421,531 (i.e. aluminium oxide, barium sulphate, zinc oxide and calcium sulphate). (Celite (Registered Trade Mark) is a natural diatomaceous earth produced by Johns-Manville Corporation).

In one preferred embodiment of the invention discrete porous particles of a material suitable for the selective retention of macromolecules are prepared by a process comprising forming a slurry by mixing a finely divided, substantially insoluble, sorptive inorganic material and an aqueous solution of a soluble alginate (e.g. sodium alginate), forming the slurry into droplets, contacting the droplets with a reagent (e.g.

aqueous calcium chloride solution) capable of causing the soluble alginate to precipitate as an insoluble alginate thereby to produce intermediate particles containing particles of the inorganic material bound together by the precipitated alginate, and heating the intermediate particles to remove at least some of the alginate thereby to -produce discrete porous particles.

In another preferred embodiment of the invention discrete porous particles of a material suitable for the selective retention of macromolecules are prepared by a process comprising forming a slurry by mixing a finely divided, substantially insoluble, sorptive inorganic material and an aqueous solution of a soluble alginate (e.g. ammonium alginate), forming the slurry into droplets, contacting the droplets with an acidic reagent to precipitate alginic acid thereby to produce intermediate particles containing particles of the inorganic material bound together by the precipitated alginic acid and heating the intermediate particles to remove at least some of the alginic acid thereby to produce discrete porous particles.

In both of the foregoing preferred embodiments the slurry can, optionally, contain a further porosity forming fugitive material to give increased or modified porosity in the discrete porous particles as hereinbefore disclosed.

Discrete porous particles capable of selectively retaining macromolecules from a fluid substance containing said macromolecules have been prepared from titania, Celite, bentonite and calcium phosphate in the size range 200-500 diameter having pores of a size suitable for allowing macromolecules to permeate the particles can be sorbed. It will be appreciated that particles can be made in sizes suited to particular uses and can be made with sizes outside the foregoing range if desired. Thus, depending upon the use to which the particles are to be put, particles may be prepared in, for example, the size range 50a5 mm diameter.

An example of a macromolecular species which has been selectively retained by- discrete porous particles prepared in accordance with the present invention is haemoglobin, which was selectively retained from an aqueous solution containing haemoglobin and bovine serium albumin. Other macromolecules such as those disclosed in our British Patent No.

1,421,531 may also be selectively retained by the porous particles of the present invention.

According to another aspect the present invention provides a porous material, suitable for the selective retention of macromolecules prepared by the process of the invention.

Preferably the porous material is in the form of discrete porous particles.

We have found that a number of factors influence the size of discrete porous particles prepared in accordance with the present invention. Thus size depends upon the inorganic material content of the slurry, the droplet size of the slurry before the conversion (e.g.

precipitation) step and the degree of porosity of the particles. Droplet size is dependent upon the viscosity of the slurry and, where a nozzle is used to produce the droplets, the nozzle orifice diameter, the type of nozzle (e.g. needle, vortex or atomiser nozzle) and the velocity of ejection of slurry from the orifice.

The foregoing factors can be varied to produce particles of a particular desired size.

Our British Patent No. 1,421,531 mentions applications of the invention disclosed therein and it is to be understood that these applications also apply to the porous material of the present invention.

Thus, for example, discrete porous particles in'accordance with the present invention may be used in chromatographic separations.

Also, for example, porous materials in accordance with the present invention may be used as a support material upon which biologically active substances, such as enzymes, may be immobilized (e.g. in accordance with the invention disclosed in our co-pending British Patent Application No. 28212/74, now British Patent Specification No. 1,514,707, (corresponding to German Offenlegungsschrift No. 2 527 884 published 15th January, 1976).

Thus, amyloglucosidase has been immobilized on discrete porous particles of TiO2 and Celite (prepared in accordance with the present invention) using the invention disclosed in British Application No. 28212/74 (now British Patent Specification No. 1,514,707).

The present invention will now be further described, by way of example only, as follows: Example 1.

200 g TiO2 were slurried in 1 litre of a 1% aqueous solution of sodium alginate (Welgum Alginate Industries Ltd.) by ball-milling for 5 hours to give a slurry of a suitable viscosity for droplet formation. The slurry was added dropwise through a pipette nozzle (1 mm diameter) to a solution of 0.1M calcium chloride to form discrete particles of intermediate material containing TiO2 particles bound together by precipitated calcium alginate.

These particles were sufficiently robust to be handled and were transferred to methanol in which they were allowed to dehydrate to 25 % of their original volume. Subsequently the particles were heated in an oven to 1000C for 1-2 hours and were finally sintered at 9000C to yield discrete porous particles of titania (500,u diameter).

Example 2.

200 g of Celite were slurried in 1 litre of a 2% aqueous solution of sodium alginate (Welgum Alginate Industries Ltd.) by ballmilling for 5 hours. In a manner similar to that in Example 1 the slurry was added dropwise to an 0.1M calcium chloride solution to produce discrete particles of intermediate material containing particles of Celite bound together by precipitated calcium alginate.

These particles were dried in air and then in an oven at 1000C, and finally sintered at 115001= to produce discrete porous particles of Celite (500,u diameter).

Example 3.

200 g of TiO2 were slurried in 1 litre of a 20 aqueous solution of ammonium alginate (Collatex (Registered Trade Mark) A/RK EXTRA, Alginate Industries Ltd.) by use of a homogeniser for 20 mins. The slurry was added dropwise through a pipette nozzle (1 mm diameter) into 0.1M HCI to form discrete particles of intermediate material containing TiO2 particles bound together by alginic acid.

These were dried in air and heated to give discrete porous particles of titania.

Example 4.

To a sample of the slurry prepared in accordance with Example 1 10% (w/v) haemoglobin was added as an additional porosity forming fugitive material.

The slurry was formed into droplets and treated as in Example 1 to give discrete porous particles.

Example 5.

The procedure of Example 4 was repeated, except that 10% soluble starch was used as the additional porosity forming material, to give discrete porous particles.

Example 6.

600 g of Celite (ex Koch-Light Laboratories Ltd.) and 10 g soluble starch (an additional porosity forming material) (ex BDH Ltd.) were slurried in 61 of a 1 % aqueous solution of sodium alginate (Welgum Alginate.

Industries Ltd.)-by homogenising in an homogeniser for 20 minutes.

The resulting slurry was added dropwise into a 0.1M aqueous CaCl solution to form discrete particles of intermediate material containing Celite particles and starch bound together with calcium alginate.

The discrete particles were dried in meth anol and then in air at 800C and subsequently heated to 11500C to give discrete porous particles of Celite (500X,u diameter).

Example 7.

20 g of TiO2 and 5 g of the commercially available polymer resin Amberlite (Registered Trade Mark) CG50 100--200 mesh (an additional porosity forming material) were slurried in 100 ml of 1% aqueous solution of sodium alginate (Welgum Alginate Industries Ltd). The resulting slurry was added dropwise to a 0.1M Cacti2 solution to form discrete particles of intermediate material con taining TiO2 particles and polymer resin particles bound together by calcium alginate.

The discrete particles were dried in methanol and subsequently heated to poroduce discrete porous particles.

Example 8.

20 g of calcium orthophosphate were stirred into 100 mls of 1 % aqueous solution of sodium alginate (of the kind used in Example 1) and, without delay, the resulting slurry was added dropwise to 0.1M Cacti2 solution to produce discrete particles of material comprising calcium phosphate bound together by precipitated calcium alginate.

It was found that the particles of material could be dried in air or methanol to give particles sufficiently robust to be handled.

The particles were heated to produce discrete porous particles of calcium phosphate.

Example 9.

This example demonstrates the selective retention of a macromolecular species by discrete porous particles prepared in accordance with Example 1.

Thus, porous titania particles prepared as in Example 1 were loaded into a column to form a bed 5 cm in length and 0.5 cm diameter.

The bed was equilibrated with 20 mM Tris buffer (pH6.8) and subsequently 2 mls of an aqueous solution containing 2.5 mg bovine serum albumin and 2.5 mg haemoglobin were passed through the bed.

The bovine serum albumin was not sorbed by the particles and appeared in the eluate from the bed. It was determined by trichloroacetic acid precipitation.

Haemoglobin was sorbed and thereby retained by the particles from which it was subsequently eluted using 0.1M potassium pyrophosphate (pH 9.6) and determined spectrophotometrically.

Example 10.

This example demonstrates the selective retention of a macromolecular species by discrete porous particles prepared in accordance with Example 2.

The procedure of Example 9 was followed using porous Celite particles prepared in accordance with Example 2 for the bed in place of the titania particles.

It was found that once again bovine serum albumin was not sorbed by the particles, but that haemoglobin was sorbed and could be subsequently removed as in Example 9.

Example 11.

In this Example a biologically active substance (in this case the enzyme papain) was immobilised on discrete porous particles prepared in accordance with the present invention.

Thus, 5 ml of discrete porous particles of Celite (prepared as in Example 6) were mixed thoroughly with 1.5 ml of Pasol on ice ("Pasol" is a papain preparation in solution available from Powell and Schofield).

A 4% solution of Tannia (Registered Trade Mark) (synthetic polyphenol ex Harshaw Chemicals) in 2:1 water : acetone was prepared and the pH adjusted to 7 with NaOH.

To 2.25 ml of this solution was added 0.6 ml of formaldehyde.

The resulting solution was added to the Celite particles and Pasol, and the whole left on ice for 1w hours.

The resulting Celite particles upon which papain enzyme had been immobilised were washed in demineralised water and assayed for enzyme activity using benzoyl arginine ethyl ester as substrate and measuring the esterase activity.

The insoluble activity was 10% of the soluble activity.

WHAT WE CLAIM IS: 1. A process for the production of a porous material suitable for the selective retention of predetermined macromolecules from a fluid substance containing said macromolecules which includes the steps of treating a slurry comprising particles of a finely divided, substantially insoluble, sorptive, inorganic material, an additive and a solvent for the additive, said additive being capable of conversion to an insoluble form, to convert the additive to the insoluble form thereby to produce an intermediate material comprising particles of the inorganic material bound together by the insoluble form of the additive and treating the intermediate material to remove at least some of the insoluble form of the additive thereby to produce a porous material having an interconnected internal porosity, the inorganic material being selected such that, and the additive and amount of additive being selected such that, the porous material has a pore structure capable of allowing the macromolecules to permeate the porous material.

2. A process as claimed in Claim 1 wherein the slurry is treated to precipitate the insoluble form of the additive to produce the intermediate material comprising particles of the inorganic material bound together by the precipitated insoluble form of the additive and the intermediate material is treated to remove at least some of the precipitated insoluble form of the additive.

3. A process as claimed in Claim 2 wherein the slurry is formed into droplets and contacted with a reagent capable of precipitating the insoluble form of the additive thereby to give substantially spherical discrete particles of intermediate material such that substantially spherical discrete porous particles are produced after removal of the precipitated insoluble form of the additive.

4. A process as claimed in any one of Claims 1 to 3 wherein the insoluble form of the additive is removed by heating the intermediate material to a temperature at which the insoluble form of the additive is "burnt out" to leave a porous material.

5. A process as claimed in any preceding claim wherein the inorganic material is titania, calcium phosphate, aluminium oxide, barium sulphate, zinc oxide, calcium sulphate or a natural earth.

6. A process as claimed in Claim 5 wherein the natural earth is a natural diatomaeous earth or bentonite.

7. A process as claimed in any preceding claim wherein the additive capable of conversion to an insoluble form is alginic acid, an alginate, pectin, polygalacturonic acid, a protein, a carboxymethyl derivative of cellulose, a gum, a derivative of polyacrylic acid, a polymer of styrene-maleic anhydride, or a nucleic acid.

8. A process as claimed in any preceding claim wherein an additional porosity forming fugitive material is incorporated in the intermediate material.

9. A process as claimed in Claim 8 wherein the additional porosity forming fugitive material is haemoglobin, a soluble starch, a powdered polymer, ammonium carbonate, dextran, polyvinyl alcohol, bovine serum albumin, ovalbumin, casein gelatine or a vegetable protein.

10. A process as claimed in any one of Claims 1 to 9 wherein discrete porous particles of a material suitable for the selective retention of macromolecules are prepared by a process comprising forming a slurry by mixing a finely divided, substantially insoluble, sorptive inorganic material and an aqueous solution of a soluble alginate, forming the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. pyrophosphate (pH 9.6) and determined spectrophotometrically. Example 10. This example demonstrates the selective retention of a macromolecular species by discrete porous particles prepared in accordance with Example 2. The procedure of Example 9 was followed using porous Celite particles prepared in accordance with Example 2 for the bed in place of the titania particles. It was found that once again bovine serum albumin was not sorbed by the particles, but that haemoglobin was sorbed and could be subsequently removed as in Example 9. Example 11. In this Example a biologically active substance (in this case the enzyme papain) was immobilised on discrete porous particles prepared in accordance with the present invention. Thus, 5 ml of discrete porous particles of Celite (prepared as in Example 6) were mixed thoroughly with 1.5 ml of Pasol on ice ("Pasol" is a papain preparation in solution available from Powell and Schofield). A 4% solution of Tannia (Registered Trade Mark) (synthetic polyphenol ex Harshaw Chemicals) in 2:1 water : acetone was prepared and the pH adjusted to 7 with NaOH. To 2.25 ml of this solution was added 0.6 ml of formaldehyde. The resulting solution was added to the Celite particles and Pasol, and the whole left on ice for 1w hours. The resulting Celite particles upon which papain enzyme had been immobilised were washed in demineralised water and assayed for enzyme activity using benzoyl arginine ethyl ester as substrate and measuring the esterase activity. The insoluble activity was 10% of the soluble activity. WHAT WE CLAIM IS:

1. A process for the production of a porous material suitable for the selective retention of predetermined macromolecules from a fluid substance containing said macromolecules which includes the steps of treating a slurry comprising particles of a finely divided, substantially insoluble, sorptive, inorganic material, an additive and a solvent for the additive, said additive being capable of conversion to an insoluble form, to convert the additive to the insoluble form thereby to produce an intermediate material comprising particles of the inorganic material bound together by the insoluble form of the additive and treating the intermediate material to remove at least some of the insoluble form of the additive thereby to produce a porous material having an interconnected internal porosity, the inorganic material being selected such that, and the additive and amount of additive being selected such that, the porous material has a pore structure capable of allowing the macromolecules to permeate the porous material.

slurry into droplets, contacting the droplets with a reagent capable of causing the soluble alginate to precipitate as an insoluble alginatc thereby to produce intermediate particles containing particles of the inorganic material bound together by the precipitated alginate, and heating the intermediate particles to remove at least some of the alginate thereby to produce discrete porous particles.

11. A process as claimed in Claim 10 wherein the soluble alginate is sodium alginate.

12. A process as claimed in Claim 10 wherein the reagent is aqueous calcium chloride solution.

13. A process as claimed in any one of Claims 1 to 9 wherein discrete porous particles of a material suitable for the selective retention of macromolecules are prepared by a process comprising forming a slurry by mixing a finely divided, substantially insoluble, sorptive inorganic material and an aqueous solution of a soluble alginate, forming the slurry into droplets, contacting the droplets with an acidic reagent to precipitate alginic acid thereby to produce intermediate particles containing particles of the inorganic material bound together by the precipitated alginic acid and heating the intermediate particles to remove at least some of the alginic acid thereby to produce discrete porous particles.

14. A process as claimed in Claim 13 wherein the soluble alginate is ammonium alginate.

15. A process as claimed in any preceding claim wherein the material is dried after conversion of the additive to the insoluble form.

16. A porous material, suitable for the selective retention of predetermined macromolecules from a fluid substance containing said macromolecules, whenever prepared by a process as claimed in any one of Claims 1 to 15, said porous material having a pore structure such as will allow the macromolecules to permeate the material.

17. A porous material as claimed in Claim 16 in the form of discrete porous particles.

18. A porous material as claimed in Claim 17 wherein the discrete porous particles have a diameter in the size range 50 e to 5mm.

19. A porous material as claimed in Claim 18 wherein the discrete porous particles have a diameter in the size range 200500/1.

20. A process for the selective retention of predetermined macromolecules from a fluid substance containing said macromolecules which includes the step of contacting the fluid substance with a porous material as claimed in any one of claims 16 to 19.

21. A porous material as claimed in any one of Claims 16 to 19 having immobilized thereon a biologically active substance.

22. A process for the production of a porous material substantially as hereinbefore described with reference to any one of Examples 1 to 8.

23. A process for the selective retention of macromolecules from a fluid substance containing said macromolecules substantially as hereinbefore described with reference to Example 9 or Example 10.

24. A porous material substantially as hereinbefore described with reference to any one of Examples 1 to 8.

25. A porous material having immobilized thereon a biologically active substance substantially as hereinbefore described with reference to Example 11.