WO1994008706A1

WO1994008706A1 - Microporous silica membrane, utilisation and fabrication thereof

Info

Publication number: WO1994008706A1
Application number: PCT/ES1993/000087
Authority: WO
Inventors: Mª José MUÑOZ-AGUADO; Miguel Gregorkiewitz
Original assignee: Consejo Superior Investigaciones Cientificas
Priority date: 1992-10-22
Filing date: 1993-10-21
Publication date: 1994-04-28
Also published as: ES2056016A1; ES2056016B1; AU5178493A

Abstract

The invention relates to a new microporous membrane having a purely inorganic composition based essentially on isometric silica particles with an acurately defined size distribution. The material has a high chemical and thermal stability and has, deposited as internal partition walls or ultrafine films on a surface of ceramic supports, molecular sieve properties. The membrane may be fabricated according to one of the following two processes: a) from sols obtained by acidification of a soluble silicate with a mineral acid, producing an asymmetric multilayer membrane when depositing said sols on a ceramic support, and b) by floculation of silica in the inside of a ceramic support, when contacting the support impregnated with a soluble silicate with a solution of a bivalent or trivalent metal salt, thereby producing a microporous partition wall by filling the porous volume of the support with microporous silica.

Description

Title

Microporous silica membranes, their use and manufacture

Technical field

Inorganic silica membranes.

Separation procedures using semipermeable membranes (B01D612).

Separation of gases or vapors by diffusion (B01D53)

Introduction

In response to environmental problems and the limitation of natural resources, it is observed, beginning with the so-called energy crisis in 1972, a growing importance of alternative membrane-based processes that, due to their greater specificity and lower energy consumption, are they introduced both to fulfill new functions and to replace older technologies where they have been obsolete. Since its consolidation in the early years of the 1980s, the membrane market is still growing with an annual increase of = 20% and an increasing diversification in specific applications [A Crull. Prospects for the inorganic membrane business. Key Engineering Materials (1991) 61 & 62: 279-288.].

The membranes are used today as a "filter" for the separation of mixtures such as microfiltration, ultrafiltration, electrodialysis, dialysis, gas separation and reverse osmosis, and as partitions for compartmentalization in combustion cells, ionoselective electrodes , chemical sensors, slowed dosage of drugs, etc.

An ideal membrane would be based on a material that, with a precisely defined chemical composition and pore size, was permoselective specifically for a species or group of particles and impermeable for the remaining components of the mixture, and that at the same time it was possible to prepare film form with minimum thicknesses to optimize the flow. State of the art

Inorganic membranes have chemical and thermal properties that make them indispensable in many existing processes, and highly desirable for the development of new technologies [A Crull, lc]. However, so far only few inorganic materials in membrane form and with well defined porosity are achieved. These include alumina membranes that exist with pore diameters from a few μm to a minimum of 30-50 A [BE Yoldas. A Transparent porous alumina. Amer Ceram Soc. Bull. (1975) 54: 286-288; BE Yoldas. Alumina gels that form porous transparent Al ₂ O ₃ . J. Mater. Sci. (1975) 10: 1856-1860]. This type of membrane has a high porosity («50% [BE Yoldas, lc; AFM Lenaars, K Keizer, AJ Burggraaf. The preparation and characterization of alumina membranes with ultrafine pores. Pari 1. Microstructural investigations on non-supported membranes J. Mater. Sci. (1984) 19: 1077-1088]) and acceptable flows that depend on the pore size and thickness and give values, eg for water in a membrane of 2 μm thickness and with pore diameter of 60 A, of = 3 μl-cm -s [L Cot, C Guizard, A Larbot. Novel ceramic material for liquid separation process: present and prospective applications in microfiltration and ultrafiltration. Industrial Ceramics (1988) 8: 143-148]. Its selectivity, however, is limited to the field of ultrafiltration and micro filtration (50 A - 2 μm), since it was not possible [AFM Lenaars et al, lc] to manufacture Al ₂ O ₃ membranes with pores of smaller diameter of 30 A. Similar results have also been obtained for ZrO ₂ and TiO ₂ membranes [A Larbot, JA Alary, C Guizard, JP Fabre, N Idrissi, L Cot. Strati sottili di ceramici ottenuti with the sol-gel technique (membrane inorganiche) per la filtrazione di liquidi. Ceramurgia (1988) 18: 216-218] without exceeding the lower limit of 30 A for pore diameters.

On the other hand it is known that the pore size increases inversely with the specific surface. One of the materials with the largest specific surface area is silica, SiO ₂ , with up to 700 m ² / g [RK Iler. The Chemistry of Silica. Ed. John Wiley & Sons 1979. 866 pag.]. A silica membrane promises, therefore, essentially smaller pores (micropores of 5 <0 <20 A) which would allow the introduction of inorganic materials in the field of separation at the molecular level (gas separation, reverse osmosis, etc. .). Until now, attempts to make silica membranes have been based on the polycondensation of an organic precursor such as Si (OC ₂ H ₅ ) ₄ [A Kaiser, H Schmidt. Generation of SiO ₂ membranes from alkoxysilanes on porous supports. J Non-Cryst. Solids (1984) 63: 261-271.] Or of neutralized silica solutions mixed with organic additives [A Larbot, A Julbe, C Guizard, L Cot. Silica membranes by the sol-gel process. J. Membrane Science (1989) 44: 289-293], but without obtaining microporous membranes, free of fractures and of fine thickness.

Patents of this type include the following:

- EP 0 094 060 (16-11-83), priority DE 3217047 820506 "Membranes constituted by heteropolycondensates of silicic acid, process for their manufacture and use".

- JP 03127615 (30-5-91) "Manufacture of porous composite for e.g. filtration membrane comprises filling communicating pores of inorganic material with gel by sol-gel reaction".

EP 0 332 789 (17-3-88), "Membrane de filtration et proceed de fabrication"

EP 0 248 748 (9-12-87), priority FR 8607920 "You come from the manufacture of mineral et poudre d'oxydes mixtes de titane et de silicium"

Brief Description of the Invention

Unlike the membranes known so far, the membrane object of the present invention is a purely inorganic composition material based essentially on silica (SiO ₂ ) with a microporous structure, that is, with pore diameters of less than 2 nm, It consists of silica particles of the same radius, between 1 and 10 nm, which are linked to each other by siloxane bridges forming continuous barriers without cracks as partitions inside or as films on the surface of mesoporous supports. This membrane is achieved by polycondensation of silica from exclusively inorganic precursors, avoiding structural indefinitions as well as the possible destructive effects that the organic remains could introduce, either during the drying step in the preparation of the membrane, or in its subsequent use at high temperatures.

The structure with well-defined pore size distribution, and with a morphology without fractures as thin as possible, is achieved through the rigorous control of:

a) Polycondensation by varying pH, concentration and gelation time of the suns, b) gel formation on or inside a ceramic support, c) drying and, d) adjustment to the structure of the support eventually repeating the deposit varying the particle size of the gel.

The membranes obtained are purely inorganic and have microporos of defined size, which gives them high selectivity in separations at the molecular level (use as molecular sieves).

Detailed description of the invention

The detailed description of the invention will be done by dividing this into the three parts for which protection is requested: the membrane object of the invention, its use as a molecular sieve and the way to obtain it.

Membrane description

The microporous silica membrane is composed of silicon dioxide SiO ₂ with varying amounts of water H ₂ O, between 5 and 15% by weight, and mono-, di-, and trivalent cations as minor components. Its structure is amorphous in X-ray diffraction and is based on isometric SiO ₂ particles inside which SiO ₄ tetrahedra are found that share all their vertices with neighboring tetrahedra so that each oxygen is shared by two tetrahedra giving rise to a framework three-dimensional In the particle surface, finishing said fabric, are Si - O ^- Si - OH and Si - OM where M is a cation mono-, di-, or trivalent. The proportion of silicon tetrahedra on the surface and inside the framework, determined by high-resolution nuclear magnetic resonance spectroscopy for the core of ²⁹ Si, is inversely proportional to the radius of the particle that can vary between 1 and 20 nm

Said SiO ₂ particles are packed in the membrane, leaving micropores whose size is essentially defined by the particle size and can go down to diameters below 20 Á with a porosity of up to 50% and a specific surface area of up to 800 m ² -g ^" \ determined by the BET interpretation of nitrogen adsorption-desorption isotherms. Among the particles there are strong bonds based on chemical bonds, especially Si-O-Si siloxane type, which confer high chemical and thermal stability. to the membrane; the microporous structure supports the attack with acids of concentrations of up to 4 molar, the treatment with organic solvents and temperatures up to 550-600 ° C, without losing its properties.

Morphologically, said microporous silica forms permoselective barriers that, to increase their mechanical stability, are attached to a porous support (with pore diameter about 10 times or greater than those of silica) or by filling the porous volume as microporous partitions. of the support, or deposited as such a microporous membrane on one of the surfaces of the support.

When the silica is deposited on the support, its assembly has an asymmetric multilayer morphology that allows minimum thicknesses for the microporous active layer without prejudice to its mechanical stability. To increase this effect, in turn the silica active membrane itself can also be manufactured in multilayer morphology, making the successive layers applied on the support present increasingly smaller pore diameters. The barriers thus formed (both in fill and in layer) are continuous, without cracks or interruptions in the same silica or between it and the support, showing a virtually total absence of capillary flow of the Hagen-Poiseuille type. Membrane use

These membranes show a thermally activated transport with selectivity in the separation of mixtures such as oxygen and nitrogen gas, typical of a molecular sieve.

The mechanism, according to which the diffusion of gases takes place, is intermediate between diffusion of Knudsen and diffusion of Einstein-Smoluchowski, which gives rise to a high chemical selectivity, which in turn gives it great interest for certain applications, such such as gas separation, reverse osmosis and hyperfiltration, electrodialysis or as a chemical sensor.

When the membrane contains Si-OH, Si-O ^- or Si-OM groups in its walls, the pores are hydrophilic and said membrane has a fixed negative charge, which may be of interest for separations and processes where water is one of the constituents of the mixture as occurs, for example, in reverse osmosis. In addition, if these groups are replaced by more bulky, organic or inorganic ones, using conventional techniques, lower pore sizes can be achieved or the fixed electrostatic charge of the membrane can be defined, resulting in improved performance or new applications, especially in reverse osmosis or electrodialysis.

Obtaining procedure

Essentially, the microporous membrane is obtained from a commercially concentrated solution of soluble silicate, preferably sodium (of the order of 6 M in SiO ₂ and with a SiO ₂ / Na ₂ O ratio of about 3.3), from which the silica, either by gelation by the addition of a mineral acid or by flocculation by the saline effect that is produced by adding a salt of a di or trivalent cation. The first is preferable in the case where the membrane is going to cover the surface of the porous support, the second in the case in which the silica formed is going to plug the macropores of the support with a microporous filler.

Surface film formation by gelation. To a container of appropriate volume, provided with a device to produce vigorous agitation, with a detergent With a large volume of water, a mineral acid, preferably hydrochloric acid, is added, establishing a final pH between 0 and 9, although usually 1 to 8 is sufficient, and a solution of soluble silicate in predetermined amounts to achieve a final SiO ₂ content on the order of 0.1 to 10% by weight.

The final pH is related to the pore size that is desired to be obtained in the layer that is much higher the higher the pH as can be seen in Table I.

The final silica content in the formed sun has an impact on gelation time and depends on the support and the morphology of the membrane to be obtained. In the case of ZrO ₂ supports, which have a pore diameter of 100 nm, and using deposition techniques of the dip-coating type (immersion coating), SiO ₂ concentrations between 0.5 and 4% lead to layers with thicknesses of the order of 0.1 to 3 μm that gel at times, inversely proportional to the concentration, between 24 h and 5 min.

(The particle diameter has been measured by neutron diffraction and the pore diameter by BET) When the pore diameter of the support is of the order of 100 times greater than that which is to be obtained in the last layer, it proceeds according to the following sequence of operations:

a.) The sun is deposited on the ceramic support. In the event that the support is tubular [1] it is supplemented with another tube [2] of the same inner diameter blinded in its lower part [3] as shown in Figure 1 and is charged by the top with the sun obtained.

b The sun is immediately discharged by removing the lower part [3] and, when all the sun has drained, the extra tube [2] is removed, thus preventing them from forming

"tears" at the lower end of the tube which, resulting in a greater thickness of the deposit, would result in the formation of cracks in drying.

C _j ) The tube is rotated on its horizontal axis at low revolutions (preferably 8 rpm) inside an airtight enclosure to maintain moisture until the sun has gelled, an operation that usually lasts less than 24 h.

dj Then it is washed with water to remove the anion of the acid used and then treated with a solvent of low surface tension, such as acetone, to prevent the breakage of the film by creating capillary tensions that are both greater the higher the tension surface of the solvent used.

e Next, drying is carried out, which is carried out in two steps, first at a temperature below 60 ° C for a time between 4 and 24 h, and then, in an oven, at more than 105 ° C, for more than 2 h.

This sequence of operations is repeated using gels more diluted in SiO ₂ or lower final pH to obtain the different layers to be obtained, in order to achieve, on the one hand, that the transition between the particle size of the support and the membrane is gradual, thus reducing tensions in the subsequent stages of drying and, on the other hand, to prevent the following deposits from diffusing into the ceramic, by formation of a silica barrier obtained from the deposit of the first suns with a minimum gelation time, greater concentration and larger particle size.

When the pore size of the support is of the order of 10 times the finest pore size that it is desired to obtain, the deposits of the different soles can be succeeded without going through the drying stage, to avoid fractures that would form in a deposit too thick. Only after the last deposit is the washing and drying as described.

Filling the pores of the support by the flocculation method. When the deposition of the microporous silica is carried out by flocculation inside a ceramic support, proceed as follows to fully or partially load it, that is, to fill all the pores or only the part closest to the surface, which It depends on the residence time, as indicated below in a ₂ , of the sun in contact with the support:

a ₂ ) Start from the commercial solution of soluble silicate, preferably sodium (of the order of 6 M in SiO ₂ and with a SiO ₂ / Na ₂ O ratio of about 3.3), as such or diluted to 1:10 with water , with which the support is filled, for example a tube, as in the previous variant but the solution remains in it between 5 seconds and 10 minutes.

b ₂ ) The solution is discharged by removing the lower part [3] (figure 1) and, when all has drained, the extra tube [2] is removed, thus preventing "tears" from forming at the lower end of the tube they would lead to inhomogeneities in the microporous deposit.

c ₂ ) It is now loaded with a solution between 0.1 to 2 M of a di or trivalent cation salt, the solution remaining inside the support for 1 second to 30 minutes. d ₂ ) The saline solution is discharged by removing, again, the lower part [3] and, when all has drained, the supplementary tube [2] is removed avoiding inhomogeneities in the microporous reservoir.

e ₂ ) Then wash, first with a mineral acid to remove hydrolysis products and then with water to remove the acid used and then treated with a solvent of low surface tension, such as acetone, to prevent breakage of the deposit due to the creation of tensions that are much greater, the greater the surface tension of the solvent used.

f ₂ ) Drying is then carried out, which is carried out in two steps, first at a temperature below 60 ° C for a time between 4 and 24 h, and then, in an oven, at more than 105 ° C, for more 2 h.

At the end you get a microporous silica partition that fills the windows or channels of the support.

Examples

Example 1 The support used is a ZrO ₂ tube supplied by the company Societé Ceramiques Techniques, 30 mm long and 10 mm 0 _exl and 7 mm 0 _jn, with an average pore size of 100 nm.

It is based on a commercial soluble silicate solution with a weight ratio of SiO ₂ / Na ₂ O equal to 3.3 and a molar concentration of SiO ₂ equal to 6; it is diluted with water to reach a concentration of SiO ₂ equal to or less than 3% and acidified, under vigorous stirring, with C1H 2N, until reaching the necessary pH value according to the desired pore size (see Table I) .

Once the sun is obtained, immediately after its formation, it is deposited on the ceramic support in one or several layers according to the desired morphology. For the deposit of the suns, the tube used as a support [1] is placed on top of another [2] of the same diameter but with the lacquered inner surface and it rests on a surface [3] so that it is closed at that end as shown in figure 1. Both tubes are filled with the sun so that when the two tubes are lifted together from the surface, the sun drains through the inner wall of the lacquered tube, and the tubes can now be separated. The deposit is made in both directions, inverting the tube [1], and the sun keeps in contact with the support only the time necessary to fill the tube and then empty it.

Then the support is introduced into a rotating system (figure 2) where it is rotated, in a horizontal position at 8 r.p.m. for 24 h so that the sun is distributed homogeneously over the entire surface of the support, until gelation occurs.

In the rotor the supports are covered to maintain a humid atmosphere in order to avoid a rapid evaporation of the solvent that would cause the appearance of fractures.

The next steps are, the washing of Cl ^" , until with AgNO ₃ they are not detected; the treatment with acetone for the replacement of water and the drying that is carried out in two stages first at room temperature for 5 h and then in the stove to 120 ° C, for 24 hours, an operation that is repeated for each of the tanks.

Six soles are deposited on the ZrO ₂ support, the first two at pH 7 with decreasing silica concentrations, and the last four at pH 3 also with decreasing concentrations, (because at this pH the silica is obtained with the size of smallest pore: r _p = 10 Á), as indicated in Table II.

Example

The same type of ceramic support (ZrO ₂ from SCT with 0 _p = 10 nm) is embedded with the commercial solution of sodium silicate (SiO ₂ / Na ₂ O 3.3, [SiO ₂ ] = 6M) which, in this case , it is previously diluted to 50% with water. The ceramic tube is filled with silicate and held for four minutes, then the solution is removed and when there are no left drops on the inner walls of the tube, it is filled with a solution of a salt of a metal, eg Cl ₃ Fe 0.5M, being in this case in contact with the embedded support for 10 minutes, after which the Cl solution is removed ₃ Faith

When the tube is filled, with sodium silicate or with the salt solution, the support must be kept completely full throughout the process, since during it the solutions diffuse into the support.

Once the solution is removed from the salt, it is washed with 4M C1H to remove the hydroxide from the metal, in this case Fe (OH) ₃ . It is then washed with Cl ^- (until the AgNO ₃ test is negative), treated with acetone and dried as in Example 1.

Description of the figures

Figure 1 Diagram of the mounting of the support tube for inner film formation [1] Support tube [2] Complementary tube of the same internal diameter as [1] (non-porous or with 'lacquered inner surface) [3] Support and closure. Figure 2 Experimental gelation device [1] Bell for maintaining humidity [2] Motor with variable speed between 0 and 30 rpm [3] Spindle [4] Tube support head

[5] Tube [6] Base

Claims

1. Microporous silica membranes consisting of a material of purely inorganic composition based essentially on silica (SiO ₂ ) characterized by its microporous structure with pores of diameters smaller than 2 nm, consisting of silica particles of the same radius, between 1 and 10 nm, which are linked to each other by siloxane bridges forming continuous barriers without cracks as partitions inside mesoporous supports or as films on their surface.

2. Microporous silica membranes according to claim 1, characterized in that the membrane is composed of silicon dioxide SiO ₂ with varying amounts of water H ₂ O, between 5 and 15%, and mono-, di-, and cations trivalent as minor components.

3. Microporous silica membranes, according to claims 1 and 2, characterized in that their structure is amorphous and is based on isometric SiO particles, inside which SiO ₄ tetrahedra are found that share all their vertices with neighboring tetrahedra so that each Oxygen is shared by two tetrahedra giving rise to a three-dimensional framework; on the surface of the particles, finishing said fabric, are Si - O ^- Si - OH and Si - OM where M is a cation mono-, di-, or trivalent.

4. Microporous silica membranes according to claims 1 to 3, characterized in that the pore dimension is less than 20 A with a porosity of up to 50% and a specific surface area of up to 800 πr-g ^-1 , and with a uniform pore size distribution.

5. Microporous silica membranes according to claims 1 to 4, characterized in that they support the attack with acids of concentrations up to 4 molar, the treatment with organic solvents and temperatures up to 550-600 ° C, without losing their properties.

6. Microporous silica membranes according to claims 1 to 5, characterized in that their assembly has an asymmetric multilayer morphology that allows minimum thicknesses for the microporous active layer without prejudice to its mechanical stability.

7. Use of microporous silica membranes, according to claims 1 to 6, characterized by their application in the separation of gases, in reverse osmosis and hyperfiltration (by the hydrophilic character of their pores and having said membrane fixed negative charge) , in electrodialysis or as a chemical sensor, due to its typical behavior of a molecular sieve that results in high chemical selectivity.

8. Method for obtaining microporous silica membranes, according to claims 1 to 7, characterized by polycondensation of silica from exclusively inorganic precursors, essentially a soluble silicate such as sodium silicate, by the addition of an inorganic acid or a salt of a di or trivalent cation. In the case of obtaining in the form of a surface film, it is preferably started from a sol obtained by diluting the commercial soluble silicate solution to a SiO ₂ content of less than 10% by weight, and acidifying with a mineral acid, preferably hydrochloric acid, to achieve the final pH between 0 and 9; in the case that the silica is to be obtained inside the ceramic support, it is impregnated with the silicate and then coagulated by impregnating the support with a solution of a salt of a di or trivalent cation.

9. Method for obtaining microporous silica membranes, according to claim 8, characterized in that, for the formation of multilayers, in each of them, a sol obtained with a lower pH or SiO ₂ content than the one used in the previous layer

10. Procedure for obtaining microporous silica membranes, according to claims 8 and 9, characterized in that, when the pore diameter of the support is of the order of 100 times greater than that which is to be obtained in the last layer, it is proceed, to form each layer, according to the following sequence of operations: a) deposit of the sun on the support; b) drained immediately avoiding the formation of "tears" at the end; c) gelation in a humid environment by rotating the tube horizontally; d) washing with water until removal of the anion from the acid used and then with a solvent of low surface tension such as acetone and e) drying, which is carried out in two steps, first at a temperature below 60 ° C for a time between 4 and 24 h, then, in an oven, at more than 105 ° C, for more than 2 h.

11. Method for obtaining microporous silica membranes according to claims 8 to 10, characterized in that, when the pore size of the support is of the order of 10 times the finest pore size to be obtained, the deposits of the different soles can be succeeded according to claim 10 but without going through the drying stage until after the last deposit in which if washing and drying proceeds.

12. Method for obtaining microporous silica membranes, according to claim 8, characterized in that, when the deposition of the microporous silica is carried out by flocculation inside a ceramic support, proceed according to the following steps: a) Filling with commercial solution soluble silicate as such or diluted to 1:10; b) discharge of the silicate once it has been embedded in the desired thickness; c) loading with a solution, between 0.1 to 2 M, of a di or trivalent cation salt, the solution remaining inside the support between 1 second and 30 minutes; d) discharge of the saline solution; e) removal of the hydrolysis products by acid washing and acid by washing with water and final washing with acetone or other solvent of low surface tension and f) drying according to the specifications of section e) of claim 10.