WO2016156790A1

WO2016156790A1 - Method for preparing particle-stabilized foams comprising amino acids

Info

Publication number: WO2016156790A1
Application number: PCT/GB2016/050697
Authority: WO
Inventors: Andrea Celani; Elizabeth Margaret Holt
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2015-03-31
Filing date: 2016-03-15
Publication date: 2016-10-06
Also published as: GB201505558D0

Abstract

A method is described for preparing a particle-stabilised foam comprising the steps of (i) forming a suspension of a particulate material in an aqueous medium containing an amphiphile and (ii) introducing a gas into the dispersion to generate a foam, wherein the amphiphile is an amino acid.

Description

METHOD FOR PREPARING PARTICLE-STABILIZED FOAMS COMPRISING

AMINO ACIDS

The present invention relates to a method for preparing particle-stabilised foams, in particular particle-stabilised foams suitable for use as catalysts or sorbents or supports therefor.

WO2007/068127 A1 discloses a method to prepare wet foams exhibiting long-term stability wherein partially lyophobized colloidal particles are used to stabilize the gas-liquid interface of a foam. In one aspect, the particles are partially lyophobized in-situ by treating initially hydrophilic particles with amphiphilic molecules of specific solubility in the liquid phase of the suspension. Amphiphiles specifically disclosed were C2-C6 carboxylic acids, including butyric acid and propionic acid, and propyl gallate. The carboxylic acids are unpleasant to handle and attack pH-sensitive materials. The low pH may also require adjustment in order to generate foams. We have found that surprisingly effective foams may be produced using amino acid amphiphiles.

Accordingly the invention provides a method for preparing a particle-stabilised foam comprising the steps of (i) forming a suspension of a particulate material in an aqueous medium containing an amphiphile and (ii) introducing a gas into the dispersion to generate a foam, wherein the amphiphile is an amino acid.

The invention further provides a method for preparing a solid foam from the particle-stabilised foam, and a solid foam obtained by the method.

The method for preparing the particle-stabilised foam is a direct foaming method, in which a suspension of a particulate material in a suitable liquid is foamed by addition of a gas to create a wet particle-stabilised foam, which may be subsequently heated to remove liquid and form a solid foam material.

The particulate material may be a ceramic material, a catalyst material or a sorbent material. (By "sorbent" we include adsorbent and absorbent). Thus the particle-stabilised foam may be prepared using a suspension of a ceramic powder, a catalyst powder or a sorbent powder. The foam may also be prepared using a metal powder.

Thus, the particulate material may be a ceramic powder, such as a metal oxide or other ceramic material, such as SiC. Metal oxides are preferred and suitable foams may be prepared using alumina, metal-aluminates, magnesia, silica, lanthana, ceria, titania, zirconia and mixtures of these. Zeolite powders may also be considered herein to be a ceramic material. The particulate ceramic materials are preferably those suitable for use as supports for catalysts and sorbents. The resulting ceramic foams may be treated with catalyst or sorbent materials in subsequent processing steps. The particulate material may be a catalyst powder. A catalyst powder may be any composition containing a catalytically active metal or a precursor thereto. Catalyst powders include one or more of the oxides, hydroxides, carbonates and/or hydroxycarbonates of metals. The metals may be supported or unsupported. Suitable catalytic metals may comprise one or more metals selected from Na, K, Mg, Ca, Ba, Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Sn, Sb, La, Hf, W, Ag, Re, Ir, Pt, Au, Pb, or Ce. Hence the catalyst powder may comprise one or more transition metals such as nickel, cobalt, iron or copper, and/or one or more precious metals such as silver, gold, platinum, palladium, rhodium iridium or ruthenium that are present in the form of the metal, an oxide, hydroxide, carbonate or hydroxycarbonate. Such catalyst materials may be supported on a refractory oxide such as alumina, calcium aluminate, magnesium aluminate, titania or zirconia. Examples of particulate transition metal catalyst powders include copper/alumina compositions, copper/zinc oxide/alumina

compositions, cobalt/alumina compositions, cobalt titania compositions, nickel/alumina compositions, nickel/metal aluminate compositions, iron/copper/alumina compositions, iron/molybdenum compositions and zinc oxide/alumina compositions. Examples of particulate precious metal compositions include platinum/alumina compositions, palladium alumina compositions and ruthenium alumina compositions. The transition metal and precious metal content in such catalysts may be up to 85% by weight, but is preferably in the range 0.1 -60% by weight. The sorbent powder may be a zeolite material or a reactive composition that is capable of adsorbing contaminants such as sulphur compounds, mercury or arsenic compounds and halogen or halogen compounds from gaseous or liquid process fluids. Sorbent compositions suitable for adsorbing sulphur compounds may comprise one or more metals selected from Mn, Fe, Cu and Zn, which may be present as an oxide, hydroxide, carbonate and/or

hydroxycarbonate of the metals. Sorbent compositions suitable for adsorbing mercury and/or arsenic compounds may comprise one or more metals selected from Mn, Fe, Cu and Zn, which may be present as a sulphide of the metals. Pd supported on an metal oxide support may also be used. Sorbent compositions suitable for adsorbing mercury and/or arsenic compounds may comprise one or more metals selected from Mn, Fe, Cu and Zn, which may be present as a sulphide of the metals. Sorbent compositions suitable for adsorbing halogen, particularly chlorine compounds may comprise one or more metals selected from Na, K, Ca and Mg, which may be present as an oxide, carbonate or hydrogen carbonate. The mean particle size of the particles in the suspension may be in the range 1 nm to 20 μηι, but is preferably in the range 2 nm - 10 μηι, more preferably 2 nm - 2 μηι. Where powders with larger mean particle sizes are used, preferably the suspension is subjected to milling to attain the desired mean particle size in these ranges. Good foams can be obtained with narrow as well as with broad particle size distributions. Particulate materials that have been subjected to pre-treatment with a surface modifier may be used, including powders that have been pre- treated with an amphiphile.

Thus a suitable method for preparing a foam may include steps where:

1 . A particulate material is suspended in an aqueous phase;

2. An amino acid amphiphile is included in the suspension. This adsorbs at the particles surfaces forming modified particles that are less hydrophilic.

3. Air or another gas is then included into the suspension forming bubbles and causing attachment of the modified particles at the gas/aqueous phase interface in the wet foam, thereby stabilising the wet foam, and,

4. The particle-stabilised foam is further processed, e.g. by shaping, drying and/or

calcination.

The stabilizing colloidal particles are initially hydrophilic and are partially hydrophobized, preferably in-situ, by the adsorption of amino acid amphiphilic molecules on the particle surface.

The amphiphilic amino acid molecules consist of a tail part coupled to a head group. The tail part may generally be described as non-polar and can be aliphatic (linear alkyl or cycloalkyi) or aromatic (phenyl or naphthyl) and can carry one or more substituents. Such substituents may be an alkyl group, e.g. -C_nH₂n₊i with n<8, and/or an aryl group (such as phenyl or substituted phenyl). Preferred tail parts are optionally substituted linear carbon chains comprising 2 to 8 carbon atoms. The head group that is coupled to the tail part may be generally described as polar and comprises a carboxylate group (i.e. COOH). In the present invention amphiphile comprises an amino group (i.e. an -NH₂ substituent) such that the amphiphile is an amino-acid.

Particularly preferred amphiphiles are alpha-amino acids, where the amine group is attached to a carbon atom adjacent the carboxylate head group. The amphiphile may then be considered to be of formula; R -CR²(NH₂)COOH, where R is the tail group and R² is H, alkyl or aryl. Preferred alpha-amino acids include one or more of valine, isoleucine, leucine, phenyl alanine and tryptophan. Methionine may be used where the sulphur content of the resulting foam can be tolerated. Less preferred alpha-amino acids include alanine and tyrosine. Particularly preferred alpha-amino acid amphiphiles include one or more of leucine, isoleucine and phenylalanine. D, L or DL forms of each may be used. An especially suitable amino acid is phenyl alanine. DL-phenyl alanine may be used.

Using an alpha-amino acid amphiphile has the advantage of producing stable foams with small bubbles without the handling difficulties of the carboxylic acids or problems caused by the acidic pH of carboxylic acids.

The pH of the suspension may be adjusted before the amphiphile is included. For example, the pH may be adjusted to a pH at which the surface charge of the particulate material is high enough for electrostatic stabilization. However one advantage of the amino acid amphiphile is that pH adjustment is not usually necessary. Preferably the foaming stage is performed without pH adjustment of the suspension.

For the in-situ lyophobization of particles, the amino acid amphiphile may be applied in amounts of less than 1 % by weight of the particles, preferably in amounts of <0.8 % by weight. The minimum amount of amphiphile that should be present may be about 0.001 %, preferably about 0.1 %. Amounts in the range 0.2 - 1 .0 mmol amphiphile/g particles may be used. Since the amphiphile, besides of other ingredients of the suspension, also influences the viscosity of the foam, the actual amount of modifier used is chosen dependent on the desired final viscosity.

It has been found that particles with different shapes can be used as foam stabilizers, i.e. the particles may be spherical, polygonal plates, needles, fibres, rods, single crystals etc., provided that their particle size is within suitable dimensions. The particles themselves may be dense, i.e. non-porous, or porous, or mixtures of dense and porous particles may be used.

The particles are preferably present in amounts of at least about 5 % v/v in the suspension. The upper limit is provided by the viscosity that must not be too high. In general said viscosity should not exceed 10 Pa.s at a shear rate of 100 s The minimum amount needed to foam the whole suspension depends on the particle size and can easily be determined by the skilled person. In general the smaller the particles are, the lower the amount of particles to produce the stabilised wet foam will be. In the present invention, the solids concentration of the suspension is preferably in the range 10-45% by weight, preferably 25-35% by weight.

The wet foams are suitably formed at temperatures up to 90°C. The lower temperature is limited by freezing of the aqueous phase. The method appears to perform best where it is operated below the Krafft temperature for the amphiphile in question. The Krafft temperature (also known as Krafft point, or critical micelle temperature) is the minimum temperature at which surfactants form micelles. Below the Krafft temperature, there is no value for the critical micelle concentration (CMC), i.e., micelles cannot form.

The wet foam can be prepared using different methods, for example by incorporating bubbles of gas into the suspension. The incorporated bubbles may be small bubbles, or they may be big bubbles that upon shearing of the suspension are divided into the desired amount of small bubbles.

The air or another gas such as nitrogen, oxygen, argon and carbon dioxide, may be introduced to the suspension to generate the wet foam by any suitable means. For example foams may be produced by subjecting the suspension to a high intensity and/or high speed agitation while exposed to the atmosphere. The agitation maybe carried out using a mixer, e.g. a mechanical mixer rotated at high speed. The agitation is carried out for a sufficient period to introduce bubbles of air into the suspension until expansion has been achieved according to the desired physical and other properties of the end product. The expansion ratio, i.e. the volume of foam formed compared to the volume of the starting suspension, can be between about 1 .5 and about 15. Hence dip tubes or sparger apparatus may be used. In one embodiment, a gas- inducing impeller may be used. A gas-inducing impeller introduces gas bubbles from the blades of the impeller as it rotates within the suspension. Alternatively, the gas may be introduced by bubbling it through a filter of a defined pore size into the suspension while being stirred. In this case the final pore size of the foam may be dependent on the pore size of the filter. In a variation, high pressure gas is forced through a fine filter, then intimately mixed with the suspension in a suitable chamber and the aerated mixture is then ejected from a nozzle. Alternatively an aerosol method may be used, in which the suspension is placed in a pressurised vessel and gas such as air or carbon dioxide is injected under pressure into the suspension to produce a foam when the pressure is released, e.g. via a nozzle. Bubbles may also be formed in the suspension by including a gas-forming reagent in the suspension and activating it by heat or chemical reaction to evolve a gas. Such gas forming reagents include sodium azide and hydrogen peroxide.

The formation of bubbles in the suspension may be accomplished in a batch-wise manner or continuously.

The aqueous phase comprises water, e.g. mains water or demineralised water, which may further comprise a hydrophilic solvent such as alcohols, glycols, etc. and mixtures thereof. Further additives, such as acids or bases can be added e.g. to adjust the pH and/or the ionic strength. The ionic strength can be adjusted to favour the close-packing of the attached particles at the interface and the attraction of particles within the foam lamella. However, the ionic strength should be kept low enough to ensure a sufficiently low viscosity of the suspension exists to allow sufficient introduction of air or good foaming.

The viscosity of the suspension preferably is such that the viscosity is less than the level at which the introduction of gas cannot take place and above the level at which entrapped gas bubbles will tend to escape. In the absence of amphiphile addition, the viscosity may be 5 mPas to l OOOmPas at a shear rate of 100 s The viscosity of the suspension when the amphiphile is present may be in the range of about 5 mPas to about 10,000 mPa.s at a shear rate of 100 s preferably 25 mPa.s to about 5000 mPa.s. The preferred range is dependent on the method of gas entrapment.

The bubble size of the wet foam is dependent on all the above parameters, in particular the viscosity, the amount of additives, the amount of particles and the apparatus or the apparatus dependent method parameters used to get air into the suspension. The bubble size, e.g. diameter, may range from 1 μηι to 1 mm, preferably from 1 μηι to 500 μηι. The bubbles ultimately provide the cells in the solid foam. The wet foams may be cast into moulds for shaping or may be shaped by extrusion using conventional extrusion equipment suitably adjusted for the wet foam viscosity. The cast or extruded foams may then be subjected to a drying step to create a solid foam with sufficient green strength for further processing. The drying can be carried out using a conventional oven at up to about 120°C. The drying time may be varied from 0.1 to 48 hours as desired to preserve the foam structure. The drying may be done at atmospheric or reduced pressure. At reduced pressure the foam may expand before the green strength is developed. The degree of expansion and hence the cell size of the foam will depend on the pressure selected. Drying at elevated temperature may also cause a slight expansion of the foam. It is preferred to control the humidity during the drying step, to prevent uneven shrinkage and drying cracks. Temperature-assisted or vacuum-assisted unidirectional drying leads to an even shrinkage of the sample without inducing stresses which would result in cracks. Freeze drying may also be used. The suspension may include other ingredients, which play a role at the drying stage. Examples of such ingredients include binders such as resins, e.g. polyvinylchloride, gums, cellulose, starch, polyvinyl alcohol, oligo- and poly-saccharides to increase green strength.

Polymerisable materials may also be included although this is less preferred. Although the addition of binders in general is not needed to produce suitable wet foams, such additives may have advantages if high green strength after drying is desired. The body formed in the presence of binders or polymerizable materials after drying is relatively robust, and the addition of binders or polymerizable materials can be preferred when the article to be formed is of a complex shape.

The cast or extruded foams may be subjected to additional shaping steps, including for example milling or grinding the foam to a desired particle size, sieving, granulating pelleting and other shaping procedures known in the art. The foam may be subjected to a heating step or calcination in which the particles are fused together. Such heating may be done at temperatures in the range 200-1600°C or higher depending on the particulate material, although to retain a suitable pore structure in the cell walls, it may be desirable to calcine the foams below about 1200°C. The calcination may be performed for 1 -24 hours, preferably 1 -8 hours, depending on the temperature chosen. The heating step may be performed under air or an inert atmosphere such as nitrogen or argon. The latter may be preferred where decomposition of organic residues is expected.

A solid foam results from the drying and/or calcination of the particulate-stabilised wet foam. The solid foam is a solid porous material comprising many gas-filled cells or voids formed therein. In the present invention the ceramic foam comprises a closed cell structure. By the term, "closed cell structure", we mean that the majority of the cells or voids within the foam are generally not inter-connected, although a proportion, e.g. up to 50% of the cell volume, but preferably less than 20% of the cell volume, may be interconnected. In a closed cell structure, unlike an open-cell structure, there is generally no open flow-path through the foam from one side to another such that a gas passing though the foam has to diffuse through a cell wall from one cell to the next. The shape of the cells in the closed cell structure will vary depending upon the manufacturing method and conditions, but typically are spheres or distorted spheres having a circular cross section. The cells may have a diameter in the range 0.1 -500 μηι, although the majority (i.e. >50% in number) of the cells desirably have a diameter in the range 0.1 -250 μηι. The average cell diameter may be in the range 0.2-150 μηι, preferably 1 -100 μηι. The cell diameters may be determined using microscopy.

The void fraction of the foams may be in the range 50-95% by volume, preferably 75-90% by volume. Hence the solids content of the foams may be 5-50%, preferably 10-25% by volume.

Because the foam comprises a closed cell structure, there are walls between at least some of the cells. The cell walls are porous, i.e. the cell walls may comprise pores through which a gas may pass. The pore size or width may be >50 nm in diameter although preferably the pores are < 50 nm in diameter. The thickness of the cell walls may be controlled to provide an optimum diffusion path for reacting gases through the catalyst or sorbent. The thickness of the cell walls may also be controlled to impart a desired strength to the final product. Cell wall thicknesses may be up to 500 μηι, preferably up to 200 μηι, with an average in the range 10- 100 μηι, preferably 25-85 μηι. The control of wall thickness may, for example, by performed by controlling the void fraction in the foam during its manufacture.

The solid foam may be subjected to treatment with one or more metal compounds to form a catalyst or sorbent. This may be accomplished by applying a wash-coat slurry containing an insoluble metal compound such as a metal oxide, but this is less preferred because the solid foam comprises a closed cell structure and so the wash-coat will not be able to penetrate the foam beyond the surface cells. Therefore preferably the solid foam is treated using one or more soluble metal compounds in solution. One or more of the metals recited above may be used to treat the solid foam. The metal compound may be an organic metal compounds such as metal acetylacetonate, a metal complex such as metal ammine complex, or may be a metal salt such as metal acetate and/or metal nitrate. One or more soluble metal compounds may be used. Thus in a preferred method, a soluble metal compound is dissolved in a suitable solvent, such as water, to form a metal solution and the metal solution applied to the solid foam, e.g. by spraying or dipping, to impregnate the cells and cell wall pores. One or more treatments using the same or different metal compounds may be applied to the foam. The treated foam may then be subjected to drying to remove the solvent and leave the metal deposited within the foam. The metal may be deposited as a salt or complex or may be metal oxide depending upon the method used to impregnate the foam. The metals are preferably uniformly deposited within the ceramic foam. The metal compounds in the solid foam may if desired be subjected to a heating step or calcination to cause their decomposition to form the corresponding metal oxide. This may accompany the drying step, or the calcination step may be performed separately. The conversion of metal compounds to metal oxide may conveniently be accomplished by heating the impregnated or dried metal -containing foam to temperatures up to about 800°C, preferably 200-800°C, more preferably 200-600°C under air or an inert gas such as nitrogen or argon. The calcination of the metal compounds may be performed over 0.5-24 hours depending on the temperature.

The foams may be subjected to various treatments such as reduction with a hydrogen- and/or carbon monoxide-containing gas stream or sulphidation, e.g. with hydrogen sulphide, to render them active in use. The post treatment may be carried out ex-situ or in-situ, i.e. before or after installation in the vessel where it is to be used. Foams containing one or more catalytic metals prepared according to the present invention may be applied to any heterogeneous catalytic process, but are preferably applied to processes using gaseous reactants. The catalytic process may comprise contacting a reactant mixture, preferably a gaseous reactant mixture, with the catalyst under conditions to effect the catalysed reaction. The catalytic process may be selected from hydroprocessing including hydrodesulphurisation, hydrogenation, steam reforming including pre-reforming, catalytic steam reforming, autothermal reforming and secondary reforming and reforming processes used for the direct reduction of iron, catalytic partial oxidation, catalytic cracking, water-gas shift including isothermal-shift, sour shift, low-temperature shift, intermediate temperature shift, medium temperature shift and high temperature shift reactions, methanation, hydrocarbon synthesis by the Fischer-Tropsch reaction, methanol synthesis, methanol oxidation to formaldehyde, ammonia synthesis, ammonia oxidation and nitrous oxide decomposition reactions.

Foams containing one or more sorbent metals may be used to trap sulphur compounds, halogen compounds or heavy metals such as mercury and arsenic from contaminated gaseous or liquid fluid streams.

The invention will now be further described by reference to the following examples.

Example 1 Titania foam preparation

a) A ceramic foam was prepared using a titania suspension containing 30% wt solids.

128 g titania powder (P25 available from Evonik) were added slowly to 300 ml demineralised water in a stirred vessel. Then, without pH adjustment, 0.36 mmol DL- phenylalanine were added per gram of titania (7.6 g DL-phenylalanine). The pH of the suspension was 4.59. Air was then introduced into the suspension to form bubbles using a gas inducing impeller for 30 minutes. The foam was cast into a tray and dried at room temperature and at atmospheric pressure. The cast foam was then calcined under a nitrogen purge by heating at 2 °C/ min to 600 °C. After 45 minutes at 600 °C the nitrogen flow was replaced by air. The total dwell at 600 °C was 4 hours. The resulting ceramic foam support was crushed and sieved to 0.5 mm.

A porosity measurement was made by immersing the ceramic foam in demineralised water at room temperature for 4 days and measuring the water up-take by the increase in weight. The porosity measured in this way includes the volume of the cells and accessible pores and indicates the volume of metal solution that may be incorporated into the foam. The porosity of the foamed titania was 0.8 cm³/g after 4 days. b) The method was repeated for different titania contents in the suspension as follows; Titania DL-phenylalanine PH Porosity weight % mmol/g titania cm^"3g^"1

17.5 0.82 4.65 2.2

20.0 0.36 4.50 1 .6

20.0 0.54 4.52 1 .5

20.0 0.72 4.56 1 .4

25.0 0.39 4.56 0.9

25.0 0.54 4.58 1 .0 c) The method was repeated, replacing the phenyl alanine with other alpha-amino acids as follows;

Titania DL-amino acid PH Porosity weight % mmol/g titania cm^"3g^"1

20 0.22 DL-tryptophan 3.25 0.50

20 0.74 DL-leucine 3.96 2.84

30 0.54 DL-valine 4.02 0.47 d) The method was repeated but with nitric acid adjustment of the pH of the slurry prior to air entrainment as follows:

Titania DL-amino acid PH Porosity weight % mmol/g titania cm^"3g^"1

25 0.72 DL-leucine 1 .29 1 .1 1

25 0.72 DL-tryptophan 1 .20 0.38

25 0.72 DL-phenylalanine 1 .36 0.78

25 0.72 DL-isoleucine 1 .28 0.63

25 0.72 DL-methionine 1 .21 0.44

25 0.72 DL-valine 1 .34 0.41

The results suggest that the pH adjustment does not enhance porosity. The order of effectiveness of the amphiphiles at an amphiphile concentration of 0.20-0.25 mol/litre at a pH 1 .0-1 .4 is DL-leucine >DL-phenylalanine >DL-isoleucine >DL-methionine >DL-valine > DL- tryptophan.

Example 2: Catalyst preparation

a) Cobalt nitrate impregnation of titania foam of Example 1 (a).

7 g of cobalt nitrate hexahydrate (Co(N0₃)2.6H₂0) and 2.2 ml demineralised water were heated until the melting and dissolution of the salt were complete. This was added in aliquots to 15 g of the ceramic foam product of Example 1 (a) in a plastic bag. After each addition the material was kneaded into the support. The impregnated foam was dried for 2 hours at 105°C and calcined for 2 hours at 300°C. The process was then repeated. 5.5 g Co(N0₃)2.6H₂0 and 2.74 ml demineralised water were heated until the melting and dissolution of the salt were complete. This was added in aliquots to 16.5 g of the impregnated ceramic foam product from the first impregnation in a plastic bag. After each addition the material was kneaded into the support. The resulting material was dried for 2 hours at 105°C and calcined for 2 hours at 300°C. The cobalt content of the catalyst by ICPAES was 10.8 %wt. b) Cobalt ammine-carbonate impregnation of titania foam of Example 1 (a).

A cobalt ammine carbonate solution was prepared as follows; 198 ml of a 28% ammonia solution was added to 20.4 g ammonium carbonate in a round bottomed flask and diluted with 193.4 ml demineralised water. The resulting solution was stirred for 20 minutes then 23.7 g of cobalt basic carbonate was added over 15 minutes and the solution stirred at 150 rpm for a further 2.5 hr to give a purple solution. 30% hydrogen peroxide solution was added drop wise while the solution was stirred at 234 rpm until the Oxido-reduction potential (Metier Toledo transmitter M 700) was near to -100 mV. Stirring was continued for a further 10 minutes and then the solution was filtered.

385 ml of the Co ammine carbonate solution (2.6% w/w Co) was added to a 2 L four-necked round bottom flask. A stirrer, temperature probe, lute and condenser were fitted to the flask. 52 g of the ceramic foam product of Example 1 (a) was added. This mixture was then diluted with 385 mL water and 165 ml_ ammonia before being heated and agitated for 80 minutes to cause evolution of the ammonia and deposition of cobalt oxide in the cells and pores of the ceramic foam. The mixture was filtered and washed with 80 ml demineralised water. The catalyst was dried at 105°C for 8 hours. It was not calcined. The cobalt content of the catalyst by ICPAES was 1 1 .2 %wt.

Example 3. Direct foaming of catalyst powder

a) CuO/ZnO/alumina catalyst. 99.7 g of a co-precipitated copper/zinc oxide/alumina catalyst powder comprising 45% wt CuO, 45% wt ZnO and 10% wt silica-doped alumina were dispersed in 300 ml of water using a Silverson L 5M mixer at 6000 rpm. The initial pH was 7.18. The suspension was subsequently stirred with an impeller and, without pH adjustment, 8.2 g of DL-phenylalanine were dissolved into the suspension. The pH of the resulting suspension was 6.48. Air was introduced into the suspension using a gas inducing impeller at 2000 rpm for 20 minutes. Some stable foam was produced. The foam was poured into a tray and dried at room temperature. The sample was subsequently heated to 590°C under a nitrogen purge and held at 590°C for 2 hours to produce a catalyst foam. Example 4: Comparative Example

The method of Example 1 was repeated, replacing the phenyl alanine with n-butyric acid. a) Without pH adjustment.

A 20% suspension was prepared by adding 75 g titania powder (P25 available from Evonik) to 300 ml demineralised water in a beaker stirred with an impeller. The suspension pH was 4.49. 2.5 ml of butyric acid were added to provide 0.36 mmol butyric acid/g titania. The resulting pH was 3.86. Air was entrained with a gas inducing impeller for 20 minutes but no foaming was observed. The pH upon addition of the butyric acid was too low for a stable foam to be produced.

b) With pH adjustment.

Example 4(a) was repeated. 0.54 ml of KOH were added to increase the pH of the suspension to 4.65. Airwas then entrained with a gas inducing impeller for 20 minutes. The foam was dried at room temperature. It was calcined with a nitrogen purge by heating at 2 °C/ min to 600 °C. After 45 minutes at 600 °C the nitrogen flow was replaced by air. The total dwell at 600 °C was 4 hours. The porosity of the foamed titania was 2.2 cm³/g.

The method was repeated for different titania contents in the suspension as follows. pH adjustment was required before air entrainment using potassium hydroxide.

*pH was not increased further since an increase in viscosity was observed

Claims

Claims.

1 . A method for preparing a particle-stabilised foam comprising the steps of (i) forming a suspension of a particulate material in an aqueous medium containing an amphiphile and (ii) introducing a gas into the dispersion to generate a foam, wherein the amphiphile is an amino acid.

2. A method according to claim 1 wherein the particulate material is a ceramic powder, a catalyst powder or a sorbent powder.

3. A method according to claim 2 wherein the ceramic powder comprises a metal oxide selected from alumina, metal-aluminates, magnesia, silica, lanthana, ceria, titania, zirconia and mixtures of these; or a zeolite.

4. A method according to claim 2 wherein the catalyst powder comprises an oxide,

hydroxide, carbonate or hydroxycarbonate of one or more metals selected from the group consisting of Na, K, Mg, Ca, Ba, Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Sn, Sb, La, Hf, W, Ag, Re, Ir, Pt, Au, Pb, and Ce.

5. A method according to claim 2 wherein the sorbent powder comprises an oxide,

hydroxide, carbonate, hydroxycarbonate or sulphide of one or more metals selected from Na, K, Ca Mg, Pd, Mn, Fe, Cu and Zn.

6. A method according to any one of claims 1 to 5 wherein the mean particle size of the particles in the suspension is in the range 1 nm to 20 μηι, preferably in the range 2 nm - 10 μηι, more preferably 2 nm - 2 μηι.

7. A method according to any one of claims 1 to 6 wherein the amino acid consists of a non- polar tail part, a carboxylate head group and an amino group.

8. A method according to any one of claims 1 to 7 wherein the amino acid is an alpha-amino acid, where the amine group is attached to a carbon atom adjacent a carboxylate head group.

9. A method according to any one of claims 1 to 8 wherein the amino acid is of formula;

R -CR²(NH₂)COOH, where R is a tail group and R² is H, alkyl or aryl.

10. A method according to any one of claims 1 to 9 wherein the amino acid is one or more of valine, isoleucine, leucine, phenylalanine or tryptophan, preferably one or more of leucine, isoleucine and phenyl alanine, more preferably phenylalanine.

1 1 . A method according to any one of claims 1 to 10 wherein the pH of the suspension is not adjusted by addition of an acid or a base.

12. A method according to any one of claims 1 to 1 1 wherein the foam contains bubbles

having a diameter from 1 μηι to 1 mm, preferably from 1 μηι to 500 μηι.

13. A method for making a solid foam comprising the steps of (i) preparing a particle-stabilised foam according to any one of claims 1 to 12, (ii) shaping the particle stabilised foam and (iii) drying and/or calcining the shaped foam.

14. A method according to claim 13 wherein the solid foam is prepared from a ceramic powder and is subjected to treatment with one or more metal compounds to form a catalyst or sorbent.

15. A solid foam obtainable by the method of claim 13 or claim 14.