NL8402656A - Thin inorganic semipermeable membranes prodn. - by immersing microporous alumina carrier in boehmite sol and drying and heating lyogel layer obtd - Google Patents

Abstract

Dry, tear-free, mechanically and chemically stable, thin semipermeable inorganic membranes with a thickness of less than 20 microns are made by (1) coating a dry microporous inorganic carrier with a thickness of 50 microns to 5 mm and a pore dia. of up to 5 microns and made of sintered inorganic oxides, by immersion in a stable sol. of colloidal metal oxide or hydroxide particles with a particle size of 3 nm to 2 microns in which the concn. of the disperse phase in a non-solvent for the metal (hydr)oxide particles is 0.01-25 wt.% (esp. 0.5-5 wt.%), and in which the sol. in the border layer is first converted into a lyogel; (2) removing the carrier from the liq. and air drying to form a xerogel 1-20 microns thick and (3) heating to at least 390 deg.C.

Description

Method for the production of tear-free semi-permeable inorganic membranes.

Applicant mentions as inventors: A.F.M. Leenaars, A.J. Burggraaf, K. Keizer, Department of Chemical Technology, TH-Twente.

The present invention relates to a process for the production of dry, tear-free, mechanically and chemically stable, thin semi-permeable, inorganic membranes, by coating a micro-porous support, preferably consisting of sintered inorganic oxides, with a stable suspension of an inorganic membrane-forming coating material in a non-solvent for the coating material and then heating.

EP-OA-0 040 282 discloses such a method, wherein a microporous support consisting of carbon or sintered inorganic oxides, with a pore volume of 5 to 60% and pores of 5 µm to 40 µm, is coated with a 0.5 to 20% by weight suspension of an inorganic membrane-forming coating material in a non-solvent for the membrane-forming coating material, in the presence of a volatile liquid, which is miscible with the continuous phase of the suspension, which liquid forms the membrane causes coating material to penetrate into the pores of the support without gel formation, to impregnate the coating of the volatile liquid and subsequently heat at more than 25 ° C and sinter it to 1500 ° C.

Considered drawbacks of this known method are: 1. Nothing is stated about the pore size distribution of the membranes. However, the coating slurry used is relatively coarse (specific surface area less than 300 ml per cm 2). solid). The pores to be reached can hereby be relatively large, depending on the pore size of the support. The pore size distribution of the support, which according to the application is not very narrow, will also affect the pore size and size distribution in the membrane itself. Good control of these properties will therefore be difficult.

2. The thickness of the actual membrane layer, ie the layer deposited in the pores of the support, depends on the pore size distribution in the support, and in particular on any gradients over the thickness of the support and of local variants herein.

3. The imperative required the use of a volatile liquid that is miscible with the continuous phase of the coating slurry, which is non-solvent for the coating to be formed, excluding the use of water alone.

Mention may also be made of US-3,926,799 and US-3,977,967. In both cases this is a layer that is deposited on a porous support, but the material must remain wet to prevent the formation of drying shrinkage cracks. Little is said about the pore size.

However, the object of the invention is to achieve dry, tear-free, mechanically and chemically stable, semi-permeable inorganic membranes with a thickness of at most 20 µm with a pore distribution within narrow limits, which is at least on the side of the large pores ftherp 15 limited.

According to the present invention this is achieved by coating a dry microporous inorganic support with a thickness of 50 µm to 5 mm, with pores with a diameter of up to 5 µm, preferably not more than 0.5 µm, by dipping in a stable sol of colloidal metal oxide or metal hydroxide particles with a particle size from 3 nm to 2 µm; whose concentration of the disperse phase in a non-solvent (preferably water) for those metal oxide or metal hydroxide particles is 0.01 to 25% by weight! and preferably 0.5-5% by weight, and wherein the sol in the boundary layer is first converted, in a 25 lyo gel, to remove the carrier from the liquid, to air dry to a xerogel of thickness from 1 to 20ym and then to dry and heat to a temperature of at least 390 ° C.

If desired, the formed membrane is then sintered to a maximum temperature of about 1500 ° C.

The advantages which the method according to the invention offers over the known prior art are the following: - a good control of the pore size (by good control of the particle size and by the favorable particle shape, namely platelets); - a very narrow pore size distribution; 35 - the possibility of obtaining very small pores.

This is the true value of the invention. By the last point 8402656 • * *. The application is not limited to liquid phase separations, eg dewatering processes, but also extends to gas separation to relatively high temperatures.

The preparation of stable sols of colloidal metal oxide or metal hydroxide particles is not part of the invention and can preferably be carried out according to the instructions in US Pat. Nos. 3,941,719 and 3,944,658. A stable Boehmite sol is preferably used.

Fig. 1 schematically shows the method according to the invention; this is based on stable Boehmite sol (γ-Α100Η). According to method A only air-dried, according to method B partially air-dried and then supercritically dried and, according to method C, only supercritical dried.

In Fig. 2, the distribution is summarized. the pore diameter when using 0.07 mol HNO3 per mol boehmite of samples having a temperature of 500 ° C. 6Q0 "C resp. Have had 780 ° C through curves 2, 3 and 4 indicated; curve 1 indicates the distribution of the pore size of the unheated boehmite (γ-AlQQH). These curves are based on cylindrical pores. The heating of the mentioned samples was effected with a heating temperature of 10 ° C per hour and the samples were kept at the final temperature value for 24 hours.

Fig. 3 shows the porosity and pore diameter of samples calcined at 50QeC as a function of the acid concentration in the sol, using HClQ4 as acid. The influence of the acid concentration on the pore structure is absent when HNO3 is used.

The suitability of the support is determined in particular by the pore size distribution of the support. If the pores are too large, no layer will be formed because then the sol particles will be drawn into the support, so that no lyo-gel is formed on the support. If the pores are too small, a layer will be formed, but the flow resistance of the final product is too high. Examples of suitable dry microporous carriers are; α-Αΐ2θ3 carriers with a porosity of 60 vol.Z and a modal pore size of Q, 34ym, whereby 90% of the pore volume is found between 0.1 and 0.53ym and α-AI2O3 carriers with a porosity of 46 vol. % and a modal pore size of 0.12ym, 8402656 '- 4 - with 90% of the pore volume being between 0.08 and 0.1ym.

Boehmite sols contain plate-shaped particles. This is important both for obtaining a very small pore diameter and for a small spread in the pore size distribution. Precisely this plate-5 shape also makes it possible to obtain very thin, tear-free membranes.

When using Boehmite, a phase transition takes place when heated to 390 ° C, which can be represented by the following equation: 10 2A100H + AI2Ö3 + H20

During this conversion, the pore volume increases. The diameter of the pores depends on the temperature treatment and increases from 3.5 nm on dried samples (200eC), to 4.4 nm when heated at 500eC rusp. 8.2 nm at sintering at 780 ° C. The total porosity is virtually constant in this temperature range. At sintering temperatures above 900eC, the pore size increases rapidly as shown in Table 1. The pore size distribution of a sample is small as shown in Figure 2. In particular, pores larger than the modal pore size are almost completely absent. Pores with a diameter greater than 20 nm were not found in samples treated at 800 ° C or lower temperature when tested with a mercury porosimeter.

It has been found that when using HCIO4 as an acid, the diameter of the pores decreases and the porosity of the final product decreases as the acid concentration of the sol increases. Thus, when using 0.05 mole HCIO4 per mole Y-A100H, a porosity of 48% and a pore diameter of 4.4 nm is obtained, while when using 0.11 mole HCIO4 per mole Y-A100H, a porosity of 38% and a pore diameter of 3.5 nm. is obtained. These results apply to samples treated at 500 ° C and are shown in Figure 3 based on a cylindrical pore shape.

Only the shape and size of the primary crystallites is important for the pore size of the membrane, provided that the crystallites are only loosely bound in agglomerates. The size of the agglomerates is not critical, but it does determine which pore size of the carrier 35 is maximum permissible.

When the sol is brought into contact with the microporous support 8402656-5, unlike in the above-mentioned known processes, the dispersant (preferably water) present in the sol is removed by means of capillary suction of the pores of the support from the the boundary layer of carrier and sol is removed, as a result of which the sol in this boundary layer becomes unstable and is converted into a lyo-gel of which the density of solids is low.

After removal of the carrier from the liquid, the gel layer is found to be present on the carrier and air-dried to a xerogel. The drying speed used therein is a function of the layer thickness, that is, as the layer thickness is larger (smaller), the drying speed is smaller (larger). Thus, a 2 µm thickness crack-free membrane will be obtained if the intermediate is calcined immediately after lyogel preparation at 10 ° C per hour to the final temperature while one-eleventh treatment in the preparation of a final product with a membrane thickness of 7 µm results in a ruptured membrane. This cracking can then be prevented by air drying the intermediate product for 24 hours before calcining.

On drying, large capillary pressures occur and the gel layer shrinks more strongly than in the above known processes, eg from an initial relative density of 10 vol.Z in the iyogel phase to a density of 60 vol.% In the xerogel- phase.

In the calcination, the strongly bound water is removed from the xerogel and the (Boehmite) hydrated oxide or hydroxide is converted into crystalline oxide. At this stage, the material acquires mechanical stability and chemical resistance and its final microstructure, with respect to final porosity and pore diameter.

It has been found that as the sintering temperature of the ceramic membrane is higher, the pore diameter becomes larger. This is illustrated with the table below (see also fig. 2):

30 Table I

TS-5Ö0 ° C TS = * 600 ° C TS-780 ° C TS = 1000 ° C

Porosity in% 48 48 47 41

Pore diameter in nm (I) 4.4 5.1 8.2 78

Pore diameter (slit width) in nm (II) 3.0 3.5 5.2 39 (I) * based on cylindrical pores (Isometric particles) (II) based on splined pores (plate particles).

8402656

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The layer thickness of the membranes can be varied within certain limits through combinations of pore diameter and porosity of the support, sol concentration and immersion time. In Fig. 4 the effect of the sol concentration and the immersion time on the thickness of the membrane after calcination is indicated. The sol used therein contains 0.07 mol HNO3 per mol boehmite. The carriers used herein have a modal pore size of 0.34 * 0.2 and a porosity of 60%. It can be seen from Fig. 4 that the lyogel-forming process is very rapid and suitable for mass production. In that figure, the straight (1) refers to a sol concentration of 1.3 mol / liter (7.4 wt% Boehmite) and the straight (2) refers to a sol concentration of 0.8 mol / liter (4 .7 wt.% Boehmite).

In a particular embodiment, which makes it possible to obtain tear-free membranes with larger layer thicknesses and porosities than the above-mentioned, the drying is carried out in whole or in part under supercritical conditions. In this procedure, the liquid phase is removed from the lyogel without a meniscus being present and without large capillary forces acting. For this purpose, the water is removed from the lyogel by washing out with a liquid which is inert with respect to the oxide or the hydroxide and preferably has a low critical pressure and temperature. The liquid should also not peptize the lyogel. After the gel layer is completely saturated with regard to this liquid, for which, for example, butanol can be used, the whole is heated in a pressure vessel to above the critical point of the liquid, after which the supercritical vapor is removed and the gel membrane is cooled. The usual calcination and optionally sintering procedure can then be applied.

The membranes obtained according to the method of the invention can be used with temperatures of up to 900 ° C, while retaining a small pore diameter, for example for gas separation.

The following examples illustrate the manufacture and use of the inorganic membranes.

Example I

In a Boehmite sol with 0.07 mole of HNO3 per mole in which the Boehmite concentration is 3% by weight, a microporous round plate of aau-Al2 O3 is added for 2 seconds. a diameter of 4 cm, a thickness of 2.3 mm 8402656 Λ ¨ · -? s - 7 - and a modal pore size of 0.22μχη dipped. The resulting lyo-gel on the support is air dried for 24 hours and then heated to 500 ° C at a rate of 10 ° C per hour and held for 18 hours. The layer thickness of the membrane is 3.5 µm and the pore size distribution of the membrane material is as shown in Fig. 2. The clean water flux of the membrane on support is 1.0 cm 2 per cm 2. membrane area per bar pressure difference per hour. To determine the separating properties of the membrane, a solution of 300 µg serum albumin (a protein with a molecular weight of 69,000 Daltons) per liter of demineralized water was used. In a so-called stirred "dead-end" permeation cell, a retention of 96% and a flux of 0.3 cm? Cm, ^, bar- *,, hour- · "are measured.

Example II

In a Boehmite sol, 0.39 mole HCIO4 per mole Boehmite in which 15 the Boehmite concentration is 14% by weight, becomes a microporous round plate of α-AI2O3 with a diameter of 2 mm and a thickness of 2 mm for 2 seconds. dipped a modal pore size of 0.34um. The resulting lyogel on the support is air dried for 24 hours after which the residual water is washed with seeun-20 dair butanol. This product is supercritically dried in a pressure vessel at 280 ° C and 100 bar and then heated at a rate of 10 Ce per hour to 500 ° C and held for 24 hours. The layer thickness of the membrane is 7 µm and the modal ear size is 7 on the basis of a cylindrical shape of the pores. The clean water flux of the membrane on support 25 is 0.8 cm · cm, cm- ^ bar- ^ ·, hour- ^. With ultrafiltration of a solution of 250 mgr. albumin per liter of demineralized water, the retention is 43% and the flux 0.4 cm cm-2, bar -1, hour-1.

In general, it can be assumed that a microporous support is used of α-Al 2 O 3, with a modal pore size of up to 5 µr, but preferably of Q, 10 µm to 0.50 µm, that a Boehmite sol is used with dispense particles of 3 nm to 21-1¾ which sol contains HNO3, or 0.01 to 0.20 mole HCIO4 per mole Boehmite, while the sol concentration is 0.01-25% by weight at a contact time of support 35 and sol from 1 to 5 seconds, preferably about 2 seconds, depending on the applied sol concentration and the desired thickness of the 8402656 s. - 8 - a membrane layer of at most 20 µm, after which the intermediate product is dried and heated to a temperature of at least 390 ° C and at most 1500 ° C.

Finally, the invention also relates to a membrane on a microporous support, which membrane thickness is less than 20 µm thick and manufactured according to the above specification.

8402656

Claims (12)

1. A method for the production of dry, crack-free, mechanically and chemically stable, thin semipermeable, inorganic membranes, by coating a microporous support from sintered inorganic oxides, with a suspension of an inorganic membrane-forming coating material and subsequently heating, characterized in that inorganic membranes with a thickness of less than 20 µm are prepared by coating a dry microporous inorganic support with a thickness of 50 µm to 5 mm, with pores with a diameter of up to 5 µm, by dipping in a stable colloid sol metal oxide or metal hydroxide particles with a particle size of 3 nm to 2 µm, the concentration of the disperse phase in a non-solvent for which metal oxide or metal hydroxide particles 0.01 to 25% by weight (and preferably 0, 5 to 5 wt. Z) and wherein the sol in the boundary layer is first converted into a lyogel, remove the carrier from the liquid and air dry until a xerogel with a thickness of 1 to 20 and subsequently heating to a temperature of at least 390 ° C. 2. Process according to claim 1, characterized in that CK-AI2O3 is used as the microporous ceramic support. 3. Process according to claim 1 or 2, characterized in that a microporous α-AI2O3 carrier with a modal pore size of QlQUm t0t 0.50W is used. 4. Process according to claims 1-3, characterized in that a microporous ceramic support with a modal pore size of 0.12 to 0.34 µm is used. 5. Process according to claims 1-4, characterized in that a sol of Boehmite in water is used with disperse particles from 3 nm to 2Wa. 6. Process according to claims 1-5, characterized in that a sol containing 0.01 to 0.20 mol HCl 4 per mol Boehmite is used and the intermediate product is heated at 500 ° C for 24 hours. 7. Process according to claims 1-6, characterized in that a sol containing 0.03 to 0.11 mole HCIO4 per mole Boehmite is used and the intermediate is heated at 500 ° C for 24 hours. 8. Process according to claims 1-7, characterized in that a stable Boehmite sol is prepared using HNO3. 8402656 r .... · · - / ··· - · ~ - - "V" "'. -' - 10 - Process according to claims 1-8, characterized in that a sol concentration of 3% by weight is used and a contact time of 1 to 5 seconds is set between the microporous support and the Boehmite sol. 10. Process according to claims 1-9, characterized in that the intermediate product is sintered at a temperature between 390 ° C and 1500 ° C. 11. Inorganic membrane with a thickness of less than 20 µ on a microporous support of 01-AI2O3, with a modal pore size of up to 5 µm, but preferably 0.10 µm to 0.50 µm, characterized in that the carrier is immersed for 1 to 5 seconds, preferably 2 seconds, in a Boehmite sol with disperse particles from 3 nm to 2ym, which sol contains HNO3, or 0.01 to 0.20 mole HCIO4 Per mo ^ Boehmite, and a sol concentration of 0.01 - 25% by weight, the loaded carrier then being dried. and heated to a temperature of at least 390 ° C, at most 1500 ° C. Inorganic membrane manufactured according to the method of claims 1-10 and as described and illustrated by examples. 8402656