WO2008151666A1

WO2008151666A1 - Sol-gel process for wet-gel modification

Info

Publication number: WO2008151666A1
Application number: PCT/EP2007/055832
Authority: WO
Inventors: Lorenzo Costa; Gabriele Lugli; Lucia Gini
Original assignee: Degussa Novara Technology S.P.A.
Priority date: 2007-06-13
Filing date: 2007-06-13
Publication date: 2008-12-18
Also published as: TW200914373A

Abstract

The invention concerns a sol-gel process to treat and modify aquagels with the purpose of achieving specific properties in the aerogels and/or glasses there from to be obtained. In a different embodiment of this invention the object of the innovative sol-gel treatment is a 'wet-gel' system obtained form aerogels rewetted. The purpose remain the same.

Description

Sol-gel process for wet-gel modification

The invention concerns a sol-gel process to treat and modify aquagels with the purpose of achieving specific properties in the aerogels and/or glasses there from to be obtained.

In general, all sol-gel processes which are used to make a gel, an aerogel or a glass monolith, start with the preparation of the sol.

It is well known, that the sol might be prepared in several different ways.

One example of sol preparation is by hydrolizing metal- alkoxides (US 5,264,197).

Alternatively sols can be prepared from mixtures of alkoxides and oxides in fine powder, specially silicon alkoxides and fumed silica (US 4,801,318 and US 5,207,814).

Furtheron from EP 0 974 557 Al it is known that a sol can be prepared from oxides and particulary from silicon oxide in the form of fumed silica only.

This document EP 0 974 557 Al describes a process for fabricating a silicon body, comprising the steps of:

Providing a silica dispersion having at least 500 ppm of dissolved silica; inducing at a pH of about 10.5 or greater, gelation of the dispersion into a gel body and drying the gel body to remove water.

The dispersion of silica is made by dispersing fumed silica in water and the addition of TMAH

(Tetramethylammoniumhydroxide) to raise the pH value from 2 to about 10 and to stabilize the dispersion. The gelation can be induced by adding a gelling agent, which lowers the pH of the dispersion to the gel point. The gelling agent can be a water-soluble liquid that under goes hydrolysis to consume base, e.g. an ester, amide or an alkyl halide and thereby lowers the pH.

Furtheron the document EP 0 974 557 Al describes that the addition of ammonium salts of weak acids, such as ammonium formate or ammonium citrate tend to cause inhomogeneous gelation .

Therefore it seems to be without any advantage to use ammonium salts of weak acids to lower the pH.

The document EP 0 974 557 Al does not describe the use of TEOS (Tetraethoxysilane) or other alkoxides.

Sols can be differentiated by the precursors used in their preparation; but that is not all, concentration also can play an important role in this respect, since it can characterize the properties of several kind of sols (Journal of Non-Crystalline Solids 71 (1985), 361-371).

Moreover, several other features can be indicated as important for identify sol typology. Among these the one that eventually plays the central role in determining sol- characteristics, particularly its activity, is the value of the hydrogen ions concentration in the sol, or pH of the sol (Journal of Sol-Gel Science and Technology 26, 63-66, 2003, Lorenzo Costa "High purity glass forms by a colloidal sol-gel process; Sol-Gel Science by C. Jeffrey Brinker and George W. Scherer, pages 99-107, Academic Press, 1990).

It is common practice to prepare acid sols, specially if very fine microstructure of the gel is sought (Journal of Sol-gel Science and Technology 26, 63-66, 2003; Sol-gel science, C. Jeffrey Brinker, George W. Scherer, p. 99-107, Academic Press, 1990) . Acid sols, particularly silicic acid sols, always contain alkoxides precursors, since silica does not hydrolize under acid conditions (US 7,216,509).

Such acid sols, either containing only silicon-alkoxides, or alternatively fumed silica and silicon alkoxides (in a kind of composite sol), are prepared at pH around 2, since for silicon alkoxides at this pH hydrolysis prevails over silanols condensation.

It is commo n practice, under these conditions, after hydrolysis is completed and before the sol is poured into moulds, to condition the same for gelation in a reasonable time, typically from one to a few hours, by rising its pH value from 2 to vicinity of 4. This is accomplished with the use of a free base, which is forcebly dilute and slowly added to a well stirred sol in order to avoid local conditions of pH non-homogeneity, that would result into local spots of irreversible microgelation .

To the best of our knowledge, patent applications NO 2003 A 000012 (EP 1 667 938) and patent application NO 2005 A 000012 (WO 2007/017454) are the only examples of pH modification in aquagel-systems in order to bind to the gels metal-cations. Motivation of the treatment in one case is to obtain optical glasses with specific properties, in another case is to remove undesirable metal contaminants from solution. In both cases the control of pH was exercised by the conventional method of adding a free base (ammonia) to the liquid phase of the wet-gel.

From the WO 2007/017454 is it known to add concentrated ammonia to an aquagel, in order to increase the pH-value in the aquagel liquid phase. This pH change cause, at the equilibrium, substantial modification of the Al³⁺ concentration in the liquid phase. The aluminium amount lacking in the liquid phase was immobilized in the aquagel. (see WO 2007/0174555 example 4) .

Unfortunately, the aluminium immobilized in the aquagel is not 100% bonded to silica in atomic units, as glasses obtained with this procedure exhibit light-scattering, particularly in the UV Spectral Range, a clear indication of traces of aggregation among the aluminium atoms. This precipitation of the aluminium in the aquagel has the disadvantage that the glass received by sintering the aquagel becomes opaque and contains internal tensions. Which provoke undesired cracks.

On the other hand, also the so called "alkaline sols" (or high pH sols) have a substantial role in sol-gel technology. Typical "alkaline sols" are made with only silicon-dioxide powders of adequate purity and morphology. Examples of silicon dioxide powders suitable for the purpose are the so called "fumed silica", "pyrolytic silica" like AEROSIL^® etc.

To hydrolyze silica a pH value superior to about 11,5 is required, typically around 12,5 achievable with amines and/or ammonia.

In the case of "alkaline sols" the alkoxide is not required for the process of gelation, since at the pH of "alkaline sols" oxides usually hydrolize, particulary silica.

It is necessary instead, after enough hydrolysis is achieved, to lower the pH to obtain gelation of the sol.

The operation is the opposite of that described for the acid sol analogy. To achieve the gelation it is now necessary to lower the pH value of the sol, from about 12,5 to about 10. This is achieved by the addition of water-soluble hydrolysable esters, typically methyl or ethyl formate to a dispersion of fumed silica (see Debra A. Fleming et al . in US 4.775.401 or US 5.240.488).

Other gelling agents like esters or amides are known from EP 0 974 557 Al.

The US 5,379,364 describes a "pH - increasing ingredient" and a "pH - decreasing ingredient" used to reach a pH of about 9.5 in the sol prior to gelation, where "pH- increasing ingredient is at least one quaternary ammonium hydroxide " (claim 10), or (alternatively) where " pH - increasing ingredient is at least one compound selected from the group consisting of tetra-methyl-ammonium hydroxide and tetra-ethyl-ammonium-hydroxide, and where pH- reducing ingredient consist essentially of water soluble aliphatic ester of an acid selected from the group consisting of formic acid, lactic acid, and glycolic acid.

The change of the pH-value represents always a delicate step for mantaining control of process conditions, as it is well known, that substantial pH modifications by adding a dilute free-base or free free-acid will clearly increase the volume of the liquid-phase, and this increase of the volume must be predicted and compensated in an accurate programmation for mantaining the liquid-phase characteristic. Moreover good stirring of the liquid while adding the pH-modifier is one necessary requirement, but further on, avoiding as much as possible to dissolve gases into the liquid is another necessary, even if conflicting, requirement .

Further difficulties may be the non compatibility of alkaline bases with the sol-gel process, leaving for the purpose only amines, most frequently ammonia, the volatility of which often is another problem. In addition to the described difficulties, typical for any pH modification in pre-formulated liquid-bodies, there are other more severe difficulties that are specific of aquagel liquid-phases or rewetted aerogels containing in solution relevant amount of salts. In this case any non-uniformity of pH across the liquid solution, even slight, will trigger precipitation of metals, practically ruining the process.

Aquagel is defined in this content as the result of gelation of an aqueous sol.

It is always intended immerged in aqueous liquid, its original "mother liquor" or other aqueous liquid. Typically is covered with extra water and sealed in its contained to protect it from evaporation and exposure to air.

It is considered relatively stable dimensionally and structurally, after the initial syneresys (shrinkage) that occours in a few hours after gelation.

For the same context is here defined aerogel as a dry , porous oxide obtained from an aquagel (or alcogel, or equivalent wet-gel in different liquid) by extracting the relevant liquid phase at conditions equal or superior than critcal conditions of the liquid (or liquids) extracted from the gel.

For the sake of clarity, only for the context of the present document, we define as "wet-gel" the gel-systems formed either by aquagels or by any wet-gel system including rewetted-aerogels .

In a preferred feature of the invention "wet-gel" means aquagel produced by the sol-gel process.

The authors of the present invention have found a sol-gel innovative technique, that properly applied to the very delicate operation of modifying pH value of wet-gel liquid- phases, makes it possible to avoid all the difficulties previously described, including the general ones connected with undesirable dilution of pre-formulates liquid solutions and the specific ones connected with wet-gel liquid-phases containing relevant salts amount. The newly found way of pH modification is based on the use of high concentrated solutions, or even the dry powder, of suitable salts, that will provoke the rapid change of pH, without producing dilution of the liquid-phase, of even trigger metal precipitations.

Moreover, "suitable salts" often have the function of pH buffer in aqueous solutions. In those cases, when the buffered pH value coincides with the desired value of pH of the liquid-phase, the most desirable condition for the process is verified.

The subject of the invention is a sol-gel process, whereby in a "wet-gel" system the gel is modified either in its chemical composition, and/or in its macro and/or micro structure by a treatment involving an active role of the liquid-phase; such an active role being stimulated by a change in the pH value of the liquid phase characterized in that in order to change the pH value from a starting pH- value to a final pH-value at least one suitable salt is added to the liquid-phase of the "wet-gel" system.

The suitable salt can be added to increase or to rise the pH-value. In another form of the invention the suitable salt can be added to lower the pH-value.

The starting pH-value can be lower than 0, preferred -0.1 to 2.9. Then the final pH-value can be 5 to 6.

If an alkaline dispersion of the silica is used the final pH-value can be 7 to 9.5, whereas the starting pH-value can be 10.5 or greater.

The suitable salt can be a salt of a weak acid and of a weak base. In another form of the invention the suitable salt can be a salt of a strong acid and a weak base. In a further form of the invention the suitable salt can be a salt of a weak acid and a strong base.

The suitable salt can be added to the wet-gel liquid-phase after having been solved in a solvent. The suitable salt can be added to lower the pH-value. In a preferred feature of the invention the suitable salt can be added to the liquid in order to increase the pH-value.

The quantity of the suitable salt can be calculated by the dissociation constant, confirmed by titration in an experimental way. The salt can be added in any concentration, if used in solution or even as a dry powder. The solvent can be water. The aqueous solution can contain ethanol or acetone.

It is the objective of the present invention to use the "suitable salts" method in a context of wet-gels systems, for modifying the pH of the liquid phase of the same.

The technical literature on the field reports examples of aquagels being treated by systematic increasing of pH in their relevant liquid phases, in order to modify the chemical composition of the gel, by binding to it metal cations, purposely extracted from the liquid-phases of the system.

In a preferred feature of the invention the sol-gel process that produced the aquagel object of treatment is a method using TEOS as one educt, which is hydrolysed to result in a sol, whereby the pH-value of the sol an acidic one. In a preferred feature of the invention the pH-value of the sol is lower than 3, especially between -0.5 and 2.9.

Furtheron the aquagel can contain silica, which can be produced pyrogenically . These sol-gel processes are known methods, except the use of the gelling agent.

The method according to the invention can be used in the sol-gel process of the type described in US 5,207,814 or WO 2004/063105 Al (PCT/EP 2003/014759) or US 2006/0059709 Al. These methods for preparing monoliths of an aerogel of at least one oxide comprises the following operating steps:

(a) mixing an alkoxide of at least one metal with water in the presence of an acidic catalyst to form a mixture;

(b) hydrolyzing the mixture obtained in step (a) to form a hydrolysate;

(c) adding an oxide of at least one metal as a fine powder having a relatively high specific surface area to the hydrolysate of step (b) to form a colloidal solution;

(d) causing the colloidal solution of step (c) to become a gel;

(e) washing the gel;

(f) drying the gel at values of temperature and pressure which are higher than the critical pressure and temperature values of the solvent used for the washing to form said monolith.

In another configuration of this invention, that present substatial advantages under particular conditions, the preferred "wet-silica" to be modified is the rewetted- aerogel .

According to the invention the suitable salt can be added in the step (d) of the process. Furtheron the sol can be made by the addition of an tetraalkylorthosilan to a dispersion of fumed silica as it is known from WO 2006/094874.

According to WO 2006/094874 the process for the production of monoliths, in particular of glass, by means of the invert sol-gel process comprises the following steps:

(a) dispersion of a pyrogenically prepared oxide of a metal and/or metalloid to form an aqueous or water- containing dispersion;

(b) addition of metal alkoxide and/or metalloid alkoxide to the dispersion, which is optionally hydrolysed by means of water before the addition;

(c) mixing of the components to form a homogeneous colloidal sol;

(d) optional removal of coarse contents from the colloidal sol;

(e) gelling of the colloidal sol in a mould;

(f) replacement of the water contained in the aerogel by an organic solvent;

(g) drying of the aerogel;

(h) heat treatment of the dried gel.

The suitable salt can be added after the step c) eventually step d) .

A "suitable salt" can be defined according to simple criteria referred to the sol: first it should be compatible with the gel and its planned objectives, typically avoiding to contaminate it with undesirable cationic species; secondly, at equilibrium in aqueous solution, it can exibit a pH-value suitable to modify the pH of the gel in the desired direction. For example: if the need is to rise the pH-value of the gel before gelation, the "suitable salt" solution should have a pH value higher than the original pH of the gel. Viceversa, in the opposite case, where the need instead would be to lower the gel's pH, the "suitable salt" solution should have a pH value lower than the pH of the original gel. The suitable salt can be added in the form of a powder or in the form of a concentrated solution in water. This has the advantage that the volume of the reaction mixture remains nearly unchanged.

Identified suitable salts for this invention are listed in table 1, table 2 and table 3. The suitable salt has to fulfil two general criteria:

First, it must be compatible with the gel and its planned objectives,

Secondly, it must exhibit a pH-value suitable to modify the pH of the sol in the desired direction.

The suitable salt according to the invention can be:

A: All the salts of weak acids and weak bases. Since metal cations are already excluded by the first of the two general criteria, to this class will belong only the ammonium and/or tetraalkyl ammonium salts of weak acid.

B: All the salts of strong acids and weak bases. Since metal cations are excluded by the first general criteria, will belong to this class only the salts of ammonium and/or tetraalkyl ammonium and mineral acids.

C: All the salts of weak acids and strong bases. Strong bases are here intended alkaline metals. All the alkaline metals are excluded in general by the first criterium, however there may be special cases, when the presence of an alkaline metal is not uncompatible with the sol objectives. In these cases, even if unfrequent, the salt of alkaline metals and weak acids should be considered "suitable salts".

Table 1:

A: WEAK ACID AND WEAK BASE

NAME FORMULA pKal pH solution (approximate)

Ammonium Formate CH₅NO₂ 3,75 6,5

Ammonium Acetate C₂H₇NO₂ 4, 76 7,0

Ammonium Propionate C₄HnNO₂ 4, 87 7,1

Ammonium Butirate C₅H₁₃NO₂ 4, 82 7,0

Ammonium Hexanate C₆Hi₅NO₂ 4, 85 7,1

Ammonium 2 - Ethylhexanate CsHi₉NO₂ 4, 75 7,0

Ammonium Caprilate C₈Hi₉NO₂ 4, 89 7,1

Ammonium Pivalate C₅Hi₀NO₂ 5, 03 7,1

Ammonium Palmitate Ci₈H₃₉NO₂

Ammonium Stearate Ci₆H₃₅NO₂

Ammonium C₇Hi₅NO₂ 4, 90 7,1 Cyclohexancarboxylate

Ammonium benzoate C₇H₉NO₂ 4, 19 6,7

Ammonium Saliciate C₇H₉NO₃ 2, 98 6,1

Ammonium Oleate Ci₈H₃₇NO₂

Ammonium Lactate C₃H₉NO₂ 3, 6,2

Ammonium Oxalate (mono! C₂H₅NO₄ 1,23 5,2 Ammonium Malonate (mono) C3H7NO₄ 2,83 6,0

Ammonium Succinate (mono) C₄H₉NO₄ 4,16 6,7

Ammonium Maleate (mono) C₄H₇NO₄ 1,83 5,5

Ammonium Malate (mono) C₄H₉NO₅ 3,40 6,3

Ammonium Fumarate (mono) C₄H₇NO₄ 3,03 6,1

Ammonium Tartrate (mono) C₄H₉Nθ6 3,22 6,2

Ammonium CsHi₅NO₄

Cyclohexanbicarboxylate (mono)

Ammonium Phthalate (mono) C₈H₉NO₄ 2,89 6,1

Ammonium Terephthalate (mono) CsH₉NO₄ 3,51 6,4

Ammonium Citrate (mono) C_GH_HNO₇ 3,14 6,2

Ammonium Citrate (dibasic) CeHi₄N₂O₇ 4,2

Ammonium Citrate (tribasic)

Ammonium CioHi₉N3θs 1,70 5,5 Ethylendiamminotetraacetate (mono)

The series of the weak acids, that can form "suitable salts" is more extended, of course, than examples in table 1 illustrate. To better identify the weak acids series it has been somewhat organized in the following:

1) Salts of organic acids monocarboxylic, aliphatic, or aromatic. Typical examples of these acids are: Formic Acid, Acetic Acid, Propionic Acid, Hexanoic Acid, 2Ethylhexanoic Acid, Caprilic Acid, Versatic Acid, Palmitic Acid, Cyclohexancarboxylic Acid, Cyclohexaneacetic Acid, Benzoic Acid, Salicylic Acid, etc . 2) Salts of organic acids monocarboxylic, aliphatic unsatuturate as, for example oleicacid.

3) Salts of organic acid monocarboxylic containing in the aliphatic chain one OH group as, for example, in the lactic acid.

4) Salts or organic acids dicarboxylic, aliphatic, or aromatic, as for example, Oxalic Acid, Malonic Acid, Succinic Acid, Malic Acid, Fumaric Acid, Tartaric Acid, Cyclohexandicarboxylicacid, Phthalic Acid.

5) Salts of organic acid, aliphatic, or aromatic, tri and tetracarboxylic as, for example Citric Acid and the Ethylendiamminotetraacetic Acid.

Table 2:

B: STRONG ACIDS AND WEAK BASES

NAME FORMULA pkb Moles/1 pH solution

(approximate)

Ammonium Chloride H₄Cl N 4 , 74 0 , 10 5, 10

4 , 7 6 1 , 00 4, 63

Ammonium Sulfate 0 , 10 5,50

Ammonium Phosphate H6NO4P 0 , 20 4,20 monobasic

Ammonium Phosphate H9N2O4P 8,00 Dibasic

Ammonium Nitrate 0,10 5,43

Ammonium Iodide 0, 10 4, 60

Ammonium Fluoride Ammonium Chloride

Table 3:

C: WEAK ACIDS AND STRONG BASES

NAME pkal Moles/1 pH solution

(approximate

Na or K Acetate 4,74 1,00 9,4

4,74 0, 10 8,9

Na or K Formate 3,75 1,00 8,9

3,75 0, 10 8,4

Na or K Henanate 4,85 1,00 9,4

4, 85 0, 10 8,9

Na or K Citrate 3,14 1,00 8, 6

3, 14 0, 10 8,1

Na or K NDTA

Na (HO₂CCH₂) 2 N(CH₂; CH₂CO₂H₂) >₂ 1,70 1, 00 7,9 i₂CO₂H₂)₂ 1,70 0,10 7,4

The sol-gel process according to the invention shows the following advantages:

The process of the invention is a way of pH modification based on the use of high concentrated solutions or even dry powders of suitable salts, which will provoke the rapid change of pH, without the producing of dilution of the liquid-phase or even trigger the metal precipitations.

Example 1

Doping treatment of aquagels using a free-base as pH modifier .

A series of standard aquagels was prepared according to the following method: 30Og of H3PO4 at a concentration of 3M were weighed in a "duran" glass laboratory cup. A mechanical stirrer of the laboratory type RW20 IKA-WERK was set on the cup with the stirring anchor into the liquid contained in the cup. At the beginning of the experiment the mixer was set at a rate "1" equal to 250 rpm. The temperature of the liquid was registered at 26 ⁰C. Refrigeration was provided by an external ice-water bath.

At temperature 12 ⁰C lOOg of TEOS were started to be added through a dripping funnel, mixer rate at "4". TEOS addition was completed in 5 minutes. The temperature was at 12 ⁰C after 10 minutes the cup was set under degassing by ultrasounds, and the cup was cooled in an ice bath. Sol was completed and pored into cilindric moulds. Gelification occurred in about 15 hours.

3 aquagel disks corresponding to a total silica weight of 28,84 g were set inside a reactor for aquagel-doping, as described in Italian Patent Application NO 2005 A 000012 (= WO 2007/017454) .

1000 g of aqueous H₃PO₄ (pH = O) were used to dissolve

1,2713 g of FePO₄2H₂O, Equivalent to 0,540 g of Fe. The solution was added to the reactor and cycled through the silica aquagels for diffusing the Fe³⁺ ions into the aquagels and binding the metal cations to the silanol group of which the aquagel is particularly rich.

Ammonia, a free-base, was used for increasing the pH of the recycling solution to a controlled level to stimulate the hydrolysis of the metal cations. The addition operations were forcebly slow, because of the formation of turbity in the solution, with a tendency to precipitate.

The results of the Fe-binding are monitored by sampling the recycling solution and analyzing for Fe content. The summary is given in Figure 1 were the ordinate axis on the left represents the Fe concentration in ppm, and refers to the dotted diagram, the ordinate axis on the right represents pH and refers to the smooth diagram. On the abscissa axis is represented time.

Example 2

Doping treatment of aquagels using is "suitable salt" as pH modifier .

The identical aquagels were used as in example 1; the same reactor was prepared for an aquagel-doping treatment.

1000 g of aqueous H₃PO₄ (pH =0) were used to dissolve 3,646 g of FePO₄2H₂O, equivalent to 1,09 g of Fe. The solution was added to the reactor and cycled through the silica aquagels for diffusing the Fe³⁺ ions into the aquagels and bind the metal cations to the silanol group of which the aquagel is particularly rich. A "suitable salt" was used for increasing the pH the recycling solution to a controlled level to stimulate the hydrolization of the metal cations. The salt used was ammonium acetate (CH₃COO NH₄) in concentrate solution.

A first "very important result is apparent by the absence of turbidity in the solution, in coincidence with the additions of the pH modifier.

The results of the Fe-binding are monitored by sampling the recycling solution and analyzing for Fe content. The summary is given in figure 2 were the ordinate axis on the left represent Fe concentration in ppm, and refers to the diagram, (see dotted line) the ordinate axis on the right represents pH and refers to the diagram, (see smooth line) .

On the abscissa axis is represented time.

The comparison of Examples 1 and 2 clearly indicates the advantages of the operation of pH-modification, made with the "suitable salt". The serious problem evidentiated by example 1 is the reversibility of the Fe-binding represented by the several peaks of Fe concentration in the dotted diagram of figure 1. Absent in figure 2. In addition, the undesirable turbidity appearing in the solution in coincidence with pH-modification, is absent with the suitable salts.

Claims

1. Sol-Gel process, whereby in a "wet-gel" system the gel is modified either in its chemical composition, and/or in its macro and/or micro structure by a treatment involving an active role of the liquid-phase; such an active role being stimulated by a change in the pH value of the liquid phase characterized in that in order to change the pH value from a starting pH-value to a final pH-value at least one suitable salt is added to the liquid-phase of the "wet-gel" system.

2. Sol-gel process according to claim 1, characterized in that the suitable salt is a salt of a weak acid and a weak base.

3. Sol-gel process according to claim 1, characterized in that the suitable salt is a salt of a strong acid and a weak base.

4. Sol-gel process according to claim 1, characterized in that the suitable salt is a salt of a weak acid and a strong base.

5. Sol-gel process according to claim 1, characterized in that the suitable salt is added after having been solved in a solvent.

6. Sol-gel process according to claim 1, characterized in that the suitable salt is added to lower the pH-value.

7. Sol-gel process according to claim 1, characterized in that the suitable salt is added to increase the pH- value .