Production of Magnetic Metal Nanoparticles Embedded in a Silica-Alumina Matrix

Abstract

Nanostructured metalceramic composites with powdery consistency are disclosed, comprising nanoparticles of ferromagnetic metals (Fe, Ni, Co) dispersed in a ceramic matrix mainly based on amorphous silica and alumina as well as relevant processes for producing these materials are disclosed.

Claims

1. A nanostructured metalceramic composite with powdery consistency, comprising nanoparticles of ferromagnetic metals (Fe, Ni, Co) having dimensions in the order of nanometers or tens nanometers, dispersed in a ceramic matrix mainly based on amorphous silica and alumina protecting said nanoparticles from oxidation.

2. The nanostructured metalceramic composite according to claim 1, containing a variable quantity between 0 and 22% by weight of metallic Fe, Ni and Co.

3. The nanostructured metalceramic composite according to claim 1, wherein the raw material for the ceramic matrix comprises zeolites of the type A, X, LSX, chabazite and phillipsite.

4. The nanostructured metalceramic composite according to claim 1, wherein the raw material for the ceramic matrix comprises any other zeolitic material, such as microporous or mesoporous material consisting of atoms of Si, Al or other species, tetrahedrally coordinated, sharing the O atoms at the tetrahedron corners and having ion exchange properties.

5. The nanostructured metalceramic composite according to claim 1, wherein the raw material for the ceramic matrix comprises nanocrystals of zeolites, having dimensions of tens or hundreds nanometers, obtained in laboratory by proper synthesis in processes of commercially available zeolites.

6. A process for producing nanostructured metalceramic composites with powdery consistency according to claim 1, wherein the dispersion of nanoparticles of ferromagnetic metals in the ceramic matrix is carried out by thermal treatments in a reducing environment of zeolites previously exchanged with Fe, Ni or Co.

7. The process for producing metalceramic composites according to claim 6, wherein the thermal treatments are carried out at a temperature between 600 and 1000 C. with a short stay time at the maximum temperature and rapid heating and cooling velocities.

8. A process for producing monoliths starting from nanostructured metal-ceramic composites produced according to claim 6, wherein such nanostructured metal-ceramic composite is dispersed in any polymeric binder that is initially fluid and then becomes stiff in the previously imparted shape.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] A concise description is now given hereinafter of the features illustrated by way of non-limiting example in the various figures of the accompanying drawings.

[0047] FIGS. 1 and 2 show the X-ray diffractograms of two specimens of nanostructured metal-ceramic composites obtained by the process described in the present application. The diffractogram of FIG. 1 relates to a specimen referenced as M, containing 15% by weight of metallic Ni. Only the diffraction peaks of metallic Ni are observed in it. These diffraction peaks are not of a high intensity and their basis appears rather wide, thus letting perceive that the size of the metal particles should be rather restrained. The absence of other diffraction peaks suggests that the ceramic matrix consists of amorphous silica and alumina. The diffractograms of all the other specimens of nanostructured metal-ceramic composites, obtained in the frame of this experimentation, arising from thermal treatments under reducing atmosphere of Ni exchanged zeolites A and X, were very similar to this illustrated in FIG. 1 and therefore were not included.

[0048] The diffractogram of FIG. 2 relates to a specimen referenced as Q, containing 17.5% by weight of metallic Fe. Only the diffraction peaks of metallic Fe are observed in it. These diffraction peaks are not of a high intensity and their basis appears rather wide, thus letting perceive that the size of the metal particles should be rather restrained. The absence of other diffraction peaks suggests that the ceramic matrix consists of amorphous silica and alumina. The diffractograms of all the other specimens of nanostructured metal-ceramic composites, obtained in the frame of this experimentation, arising from thermal treatments under reducing atmosphere of Fe exchanged zeolites A and X, were very similar to this illustrated in FIG. 2 and therefore were not included.

[0049] FIGS. 3a, 3b and 3c are TEM (transmission electronic microscopy) micrographs of the above described specimen M, taken at different magnifications. In these micrographs the Ni metal particles appear dark, while the ceramic matrix based on amorphous silica and alumina appears light.

[0050] FIG. 3a demonstrates the very high amount of existing metal particles, whose dimensions cannot be correctly assessed because of the low magnification and TEM detects not only the surface particles but also some located thereunder.

[0051] FIG. 3b taken at an intermediate magnification, demonstrates again the very high amount of Ni metal particles and allows to assess that they have dimensions between about 5 and 25 nm.

[0052] FIG. 3c taken at the maximum magnification, shows the detail of some Ni metal particles whose dimensions are in the range between 5 and 15 nm. These results appear to have an absolute value, since it was reported that the size of Ni particles, below which they behave as a single magnetic domain is 55 nm [3]. Referring again to FIG. 3c, inside Ni particles some straight striae are detected. These striae are the traces of some reticular planes of metal Ni and it is even possible to assess the interplane distance. This operation is carried out in FIG. 4, wherein the TEM image is shown of a nanostructured metal-ceramic composite references as H, containing 14.4% by weight of metal Ni. The graphic analysis of the interplane distance gives a value of about 0.23 nm, which is a value very close to those reported in literature for some Ni reticular planes.

[0053] During the experimentation forming the basis of this disclosure, other specimens of powders consisting of metal Ni nanoparticles dispersed in a matrix based on amorphous silica and alumina (starting from both zeolite A and zeolite X), the relevant TEM micrographs appear to be similar to those shown in FIGS. 3a, 3b and 3c, therefore they were not illustrated with the exception of specimen H.

[0054] FIGS. 5a, 5b and 5c are TEM (transmission electronic microscopy) micrographs of the above described specimen Q, taken at various magnifications. Also in these micrographs the metal Fe particles appear dark, while the ceramic matrix based on amorphous silica and alumina appears light.

[0055] FIG. 5a demonstrates the very high amount of existing metal particles, whose dimensions cannot be well assessed because of the low magnification and TEM detects not only the surface particles, but also some of those located under said surface.

[0056] FIG. 5b taken at an intermediate magnification level, again demonstrates the very high amount of metal Fe particles and allows to assess that they have a size between about 5 and 30 nm.

[0057] FIG. 5c taken at the maximum magnification, shows the detail of a metal Fe nanoparticle of about 25 nm. Still in FIG. 5c, some rectilinear striae are detected inside the Fe nanoparticle. These are traces of some reticular planes of metal Fe.

[0058] During the experimentation forming the basis of this disclosure, other specimens of powders consisting of metal Fe nanoparticles dispersed in a matrix based on amorphous silica and alumina (starting from both zeolite A and zeolite X), the relevant TEM micrographs appear to be similar to those shown in FIGS. 5a, 5b and 5c, therefore they were not illustrated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0059] As already pointed out in the paragraph summary of the invention, the first stage of the process for producing nanostructured metal-ceramic composites of the present disclosure consists of the ionic exchange of zeolites. Therefore this first stage will now be discussed in detail hereinafter.

[0060] The operations of cationic exchange are carried out by contacting zeolite with an aqueous solution of the cation that should enter the zeolitic lattice and stirring the system. There are various parameters controlling the cationic exchange operations, and they will be discussed one at a time as follows.

[0061] 1) Concentration of the cation in the solutionWhen a zeolite is being contacted with an aqueous solution of a cation intended to be exchanged with that/those contained in zeolite, the system tends to a condition of chemical balance, wherein the cation originally present in zeolite (generally Na.sup.+) and the cation originally present in the solution spread in the solution phase and in the zeolite phase according to ratios dictated by the affinity of zeolites for the selected cations. These ratios are also affected by the lack of ideal status of the system, given by the coefficients of activity of the various elements in the various phases. In order to obtain an increase of the amount of cation to be introduced into the zeolite, it is necessary to repeat the operation of cationic exchange. In this way the solution balanced with zeolite is being replaced by a fresh solution which is not balanced with it. This procedure generally comprising 7-8 iterations, allows to approximate reasonably the maximum achievable exchange level. Higher starting concentrations of the cation may make this procedure faster and perhaps reduce the number of iterations required to achieve the desired goal. However it is to be noted that concentrations above 0.2-0.3 M appear to be a useless waste of raw materials. Indeed above said concentrations, the benefits resulting from higher starting concentrations of the cation are negligible. Thus it can be said that the operations of ionic exchange may be conducted at any starting concentration of the incoming cation, but its sensible values are in the range between 0.05 and 0.30 M.

[0062] 2) Solid/Liquid (S/L) ratioThe ration between the amount of solid (zeolite) and exchange solution (liquid) should be neither too high nor too low. When there is too much solid relative to liquid (high S/L ratios), at each iteration a little amount of cation will enter the zeolite, and consequently the number of iterations will increase with equal quantity of cation entering zeolite. When there is too much liquid relative to solid (low S/L ratios), the number of final iterations required to achieve a given level of cationic exchange will be probably lower, but the amount of zeolite used for the production of metal-ceramic composites will be definitely low. On the basis of these remarks, the cationic exchange may be conducted at any S/L ratio, but the recommended range is between 1/20 and 1/200.

[0063] 3) TemperatureLike most chemical reactions, also those of cationic exchange are accelerated by higher temperatures. Therefore reactions of cationic exchange at 60-70 C. allow to approximate the above cited balance condition in a shorter time, which is certainly desirable. Higher temperatures are not advisable because trend to evaporation of the exchange solution would increase too much. In some cases, such as in Fe.sup.2+ exchange, it is advisable to conduct the cationic exchange at low temperatures (6-7 C.) to prevent oxidation to Fe.sup.3+. For this purpose it is also useful to scrub Ar in the exchange solution so as to strip out oxygen that would cause oxidation to Fe.sup.3+.

[0064] 4) Number of iterationsThe value of this parameter is bound by the quantity of Fe.sup.2+, Ni.sup.2+, Co.sup.2+ that should be inserted into zeolite. At the above recommended values of incoming cation and S/L ratio, a number of iterations of 7-8 (in any case not above 10) allows to reach the maximum achievable exchange level, corresponding to a content up to 20-22% by weight of metal particles in the final metal-ceramic composite. Obviously a lower number of iteration will correspond to a lower final content of metal particles. Therefore the recommendation on the choice of this operative parameter is that it should be taken according to the desired content of metal particles in the final metal-ceramic composite material.

[0065] 5) Type of zeoliteIn principle any zeolitic material, thus having properties of cationic exchange, may be subject of the proposed processes of the present invention for the production of nanostructured metal-ceramic composite materials. In practice the most sensible choice is substantially directed to some synthetic zeolites such as zeolite A, X and LSX. Natural zeolites are indeed to be discarded as they contain various impurities that would pollute the final product. Within the synthetic zeolites it is advisable to turn to those having the highest capacity of cationic exchange, allowing to introduce higher amounts of metal particles into the zeolite and consequently in the final product of metal-ceramic composite, and showing fast exchange kinetics. Thus practically zeolites A, X and LSX.

[0066] Another reason for turning the choice of zeolites to be transformed into metal-ceramic composites, to zeolites A, X and to a lesser extent LSX, is that the synthesis methods of these zeolites (more particularly zeolites A and X) are well known and used for some time. This achieves low costs (in the order of tens of Euro cents per kilogram) of the main raw material that should be transformed into metal-ceramic composite.

[0067] However it might be interesting to use samples of synthetic cabasite or phillipsite for the production of nanostructured metal-ceramic composites of the present invention. Such zeolites, although they have an exchange capacity lower than zeolites A, X and LSX, have a more symmetric distribution of cationic sites, that could be useful to obtain particularly small nanoparticles.

[0068] It has to be underlined that in this disclosure commercial samples of zeolites A and X were used for sake of simplicity. However one can easily understand that use of samples of zeolites lab synthesized expressly for their subsequent transformation into metal-ceramic composite materials, may further improve the already obtained good results. Indeed in ref [21] the granulometric distribution of commercial samples of zeolites A and X used also for the present experimentation is reported. From this reference it can be seen that more than 90% of the zeolite grains have a size between 5 and 32 microns. In literature examples of synthesis of zeolite nanocrystals are reported, having a size lower than 100-200 nm [23-26]. It is clear that metal nanoparticles of Fe, Ni or Co which would be obtained starting from these nanocrystals of lab synthesized zeolites, would be much smaller than those obtained starting from commercial zeolites, having much bigger grains. Indeed let us suppose that all the metal (at most 20-22% by weight) contained in a 100 nm grain of zeolite, after thermal treatment under reducing atmosphere, gathers to form a single metal nanoparticle, which is the worst condition that may practically occur; considering that density of Fe, Ni or Co is about three times bigger than the density of the ceramic matrix based on amorphous silica and alumina, it results that the single metal nanoparticle takes no more than 7-8% of the 100 nm volume of the original grain; thus also the linear dimensions of such particle would be no more than some nanometers. Obviously these results would be still better if the formed metal nanoparticles are more than only one.

[0069] The sequence of operations to be carried out for obtaining the nanostructured metal-ceramic composites of the present invention as well as their basic rationale were already outlined in the paragraph Summary of the invention and will now be explained in detail as follows. Accordingly, the zeolite specimen must be heated with the fastest possible heating rate (in any case higher than 10 C./min) to a temperature which is slightly higher than the temperature at which all the cations Fe.sup.2+, Ni.sup.2+ and Co.sup.2+ result reduced to metal Fe, Ni and Co. Unfortunately these data are available only in some cases and for the others they should be determined experimentally by the TPR (temperature programmed reduction) method, relying upon the experience. This suggests that said temperatures are in the range of 600-1000 C. and the definition of the final temperature of the most suitable thermal treatment is a question of optimization of each production process of a determined nanostructured metal-ceramic composite. Once selected the maximum temperature to be reached during the thermal treatment under a reducing atmosphere, the time at which the maximum temperature is to be kept will be certainly of few minutes. This time may even be 0 minutes if cooling is being started at once after reaching the maximum temperature of thermal treatment under reducing atmosphere. Also the cooling step from maximum temperature to room temperature should be effected at the highest possible cooling rate. Usually this may be done by interrupting the system heating and continuing to scrub the reducing gaseous mixture of Ar and H.sub.2 (2% vol. H.sub.2) on the materials that were thermally treated under a reducing atmosphere.

[0070] Finally, to conclude the description of the process leading to the nanostructured metal-ceramic composites of the present invention, it has to be pointed out that the outcome is the production of materials having a powdery consistence. If the production of articles in monolith form is required, the powder sintering procedure should be excluded, because the necessary thermal treatment would involve an unavoidable increase of volume of the metal particles. The production of monoliths mainly comprising particles of ferromagnetic metals (Fe, Ni, Co) having dimensions in the order of nanometers or tens of nanometers (hereinafter indicated as nanoparticles), dispersed in a ceramic matrix mainly based on amorphous silica and alumina, protecting said nanoparticles from oxidation, may in any case be easily obtained by dispersing the so obtained nanostructured metal-ceramic composites in any initially fluid polymeric binder that subsequently becomes stiff in the form previously imparted to it. The paragraph Object of the invention mentions the obtained nanostructured metal-ceramic materials.

EXAMPLES

[0071] The following examples illustrate the samples of nanostructured composite materials obtained through the methods reported in the present disclosure, together with the detailed description of the procedures required for their achievement.

[0072] Sample G

[0073] Preparation: A sample of commercial zeolite A was contacted with a 0.2 M aqueous solution of NiCl.sub.2.6H.sub.2O in a solid/liquid ratio 1/20 at a temperature of about 60-70 C. The contact lasted about six hours and was iterated ten times. This sample of Ni exchanged zeolite A, resulted to have a content of Ni revealed by its equivalent fraction x.sub.Ni=0.75, was heated under reducing atmosphere (generated by a flow of a gaseous mixture ArH.sub.2 at 2% volume of the latter) at a rate of 15 C./min up to 735 C., it was kept at this temperature for 10 minutes and subsequently let cool up to room temperature in the closed and off oven.

[0074] The diffractogram of the so obtained sample, which resulted to have a content of metal Ni of 15% by weight, is very similar to that shown in FIG. 1, this indicating that the sample consists of particles of metal Ni dispersed in a matrix based on amorphous silica and alumina. The TEM micrographs of this sample are similar to those shown in FIGS. 3a, 3b and 3c, indicating that the nanoparticles of metal Ni have a size between 5 and 25 nm.

[0075] Sample H

[0076] Preparation: A sample of commercial zeolite X was contacted with a 0.2 M aqueous solution of NiCl.sub.2.6H.sub.2O in a solid/liquid ratio 1/20 at a temperature of about 60-70 C. The contact lasted about six hours and was iterated six times. This sample of Ni exchanged zeolite X, resulted to have a content of Ni revealed by its equivalent fraction x.sub.Ni=0.79, was heated under reducing atmosphere (generated by a flow of gaseous mixture ArH.sub.2 at 2% volume of the latter) at a rate of 15 C./min up to 735 C., it was kept at this temperature for 10 minutes and subsequently let cool up to room temperature in the closed and off oven.

[0077] The diffractogram of the so obtained sample, which resulted to have a content of metal Ni of 14.4% by weight, is very similar to that shown in FIG. 1, this indicating that the sample consists of particles of metal Ni dispersed in a matrix based on amorphous silica and alumina. The TEM micrographs of this sample are similar to those shown in FIGS. 3a, 3b and 3c, indicating that the nanoparticles of metal Ni have a size between 5 and 25 nm.

[0078] Sample I

[0079] Preparation: A sample of commercial zeolite A was contacted with a 0.2 M aqueous solution of NiCl.sub.2.6H.sub.2O in a solid/liquid ratio 1/20 at a temperature of about 60-70 C. The contact lasted about six hours and was iterated ten times. This sample of Ni exchanged zeolite A, resulted to have a content of Ni revealed by its equivalent fraction x.sub.Ni=0.75, was heated under reducing atmosphere (generated by a flow of gaseous mixture ArH.sub.2 at 2% volume of the latter) at a rate of 15 C./min up to 750 C., it was kept at this temperature for 15 minutes and subsequently let cool up to room temperature in the closed and off oven.

[0080] The diffractogram of the so obtained sample, which resulted to have a content of metal Ni of 15.0% by weight, is very similar to that shown in FIG. 1, this indicating that the sample consists of particles of metal Ni dispersed in a matrix based on amorphous silica and alumina. The TEM micrographs of this sample are similar to those shown in FIGS. 3a, 3b and 3c, indicating that the nanoparticles of metal Ni have a size between 5 and 25 nm.

[0081] Sample L

[0082] Preparation: A sample of commercial zeolite X was contacted with a 0.2 M aqueous solution of NiCl.sub.2.6H.sub.2O in a solid/liquid ratio 1/20 at a temperature of about 60-70 C. The contact lasted about six hours and was iterated six times. This sample of Ni exchanged zeolite X, resulted to have a content of Ni revealed by its equivalent fraction x.sub.Ni=0.79, was heated under reducing atmosphere (generated by a flow of gaseous mixture ArH.sub.2 at 2% volume of the latter) at a rate of 15 C./min up to 750 C., it was kept at this temperature for 15 minutes and subsequently let cool up to room temperature in the closed and off oven.

[0083] The diffractogram of the so obtained sample, which resulted to have a content of metal Ni of 14.4% by weight, is very similar to that shown in FIG. 1, this indicating that the sample consists of particles pf metal Ni dispersed in a matrix based on amorphous silica and alumina. The TEM micrographs of this sample are similar to those shown in FIGS. 3a, 3b and 3c, indicating that the nanoparticles of metal Ni have a size between 5 and 25 nm.

[0084] Sample M

[0085] Preparation: A sample of commercial zeolite A was contacted with a 0.2 M aqueous solution of NiCl.sub.2.6H.sub.2O in a solid/liquid ratio 1/20 at a temperature of about 60-70 C. The contact lasted about six hours and was iterated ten times. This sample of Ni exchanged zeolite A, resulted to have a content of Ni revealed by its equivalent fraction x.sub.Ni=0.75, was heated under reducing atmosphere (generated by a flow of gaseous mixture ArH.sub.2 at 2% volume of the latter) at a rate of 15 C./min up to 750 C. and then was let cool up to room temperature in the closed and off oven (time of thermal treatment at 750 C. equal to 0 minutes).

[0086] The diffractogram and the TEM micrographs of the so obtained sample, which resulted to have a content of metal Ni of 15% by weight, are reported in FIG. 1 and FIG. 3, respectively. This indicates that the sample consists of nanoparticles of metal Ni dispersed in a matrix based on amorphous silica and alumina and that these nanoparticles of metal Ni have a size between 5 and 25 nm.

[0087] Sample N

[0088] Preparation: A sample of commercial zeolite X was contacted with a 0.2 M aqueous solution of NiCl.sub.2.6H.sub.2O in a solid/liquid ratio 1/20 at a temperature of about 60-70 C.: The contact lasted about six hours and was iterated six times. This sample of Ni exchanged zeolite X, resulted to have a content of Ni revealed by its equivalent fraction x.sub.Ni=0.79, was heated under reducing atmosphere (generated by a flow of gaseous mixture ArH.sub.2 at 2% volume of the latter) at a rate of 15 C./min up to 750 C. and then was let cool up to room temperature in the closed and off oven (time of thermal treatment at 750 C. equal to 0 minutes).

[0089] The diffractogram of the so obtained sample, which resulted to have a content of metal Ni of 14.4% by weight, is very similar to that reported in FIG. 2, this indicating that the sample consists of particles of metal Ni dispersed in a matrix based on amorphous silica and alumina. The TEM micrographs of this sample are similar to those reported in FIGS. 3a, 3b and 3c, thus indicating that the nanoparticles of metal Ni have a size between 5 and 25 nm.

[0090] Sample O

[0091] Preparation: A sample of commercial zeolite A was contacted with a 0.1 M aqueous solution of FeSO.sub.4.7H.sub.2O in a solid/liquid ratio of 1/50. To avoid oxidation of Fe.sup.2+ to Fe.sup.3+, the exchange was conducted at 7 C. and in the aqueous solution of Fe.sup.2+, Ar was continuously scrubbed. The contact lasted about six hours and was iterated ten times. This sample of Fe exchanged zeolite A, resulted to have a content of Fe revealed by its equivalent fraction x.sub.Fe=0.92, was heated under reducing atmosphere (generated by a flow of gaseous mixture ArH.sub.2 at 2% volume of the latter) at a rate of 15 C./min up to 800 C., was kept at this temperature for 30 minutes and then was let cool up to room temperature in the closed and off oven.

[0092] The diffractogram of the so obtained sample, which resulted to have a content of metal Fe of 17.5% by weight, is very similar to that reported in FIG. 2, this indicating that the sample consists of particles of metal Fe dispersed in a matrix based on amorphous silica and alumina. The TEM micrographs of this sample are similar to those reported in FIGS. 5a, 5b and 5c, thus indicating that the nanoparticles of metal Fe have a size between 5 and 30 nm.

[0093] Sample P

[0094] Preparation: A sample of commercial zeolite X was contacted with a 0.1 M aqueous solution of FeSO.sub.4.7H.sub.2O in a solid/liquid ratio of 1/50. To avoid oxidation of Fe.sup.2+ to Fe.sup.3+, the exchange was conducted at 7 C. and in the aqueous solution of Fe.sup.2+, Ar was continuously scrubbed. The contact lasted about six hours and was iterated eight times. This sample of Fe exchanged zeolite X, resulted to have a content of Fe revealed by its equivalent fraction x.sub.Fe=0.82, was heated under reducing atmosphere (generated by a flow of gaseous mixture ArH.sub.2 at 2% volume of the latter) at a rate of 15 C./min up to 800 C., was kept at this temperature for 30 minutes and then was let cool up to room temperature in the closed and off oven.

[0095] The diffractogram of the so obtained sample, which resulted to have a content of metal Fe of 14.3% by weight, is very similar to that reported in FIG. 2, this indicating that the sample consists of particles of metal Fe dispersed in a matrix based on amorphous silica and alumina. The TEM micrographs of this sample are very similar to those reported in FIGS. 5a, 5b and 5c, thus indicating that the nanoparticles of metal Fe have a size between 5 and 30 nm.

[0096] Sample Q

[0097] Preparation: A sample of commercial zeolite A was contacted with a 0.1M aqueous solution of FeSO.sub.4.7H.sub.2O in a solid/liquid ratio of 1/50. To avoid oxidation of Fe.sup.2+ to Fe.sup.3+, the exchange was conducted at 7 C. and in the aqueous solution of Fe.sup.2+, Ar was continuously scrubbed. The contact lasted about six hours and was iterated ten times. This sample of Fe exchanged zeolite A, resulted to have a content of Fe revealed by its equivalent fraction x.sub.Fe=0.92, was heated under reducing atmosphere (generated by a flow of gaseous mixture ArH.sub.2 at 2% volume of the latter) at a rate of 15 C./min up to 800 C. and subsequently was let cool up to room temperature in the closed and off oven (time of thermal treatment at 800 C. equal to 0 minutes).

[0098] The diffractogram and the TEM micrographs of the so obtained sample, which resulted to have a content of metal Fe of 17.5% by weight, are reported in FIG. 2 and FIG. 5, respectively. This indicates that the sample consists of nanoparticles of metal Fe dispersed in a matrix based on amorphous silica and alumina, and these nanoparticles of metal Fe have a size between 5 and 30 nm.

[0099] Sample R

[0100] Preparation: A sample of commercial zeolite X was contacted with a 0.1 M aqueous solution of FeSO.sub.4.7H.sub.2O in a solid/liquid ratio of 1/50. To avoid oxidation of Fe.sup.2+ to Fe.sup.3+, the exchange was conducted at 7 C. and in the aqueous solution of Fe.sup.2+, Ar was continuously scrubbed. The contact lasted about six hours and was iterated eight times. This sample of Fe exchanged zeolite X, resulted to have a content of Fe revealed by its equivalent fraction x.sub.Fe=0.82, was heated under reducing atmosphere (generated by a flow of gaseous mixture ArH.sub.2 at 2% volume of the latter) at a rate of 15 C./min up to 800 C. and subsequently was let cool up to room temperature in the closed and off oven (time of thermal treatment at 800 C. equal to 0 minutes).

[0101] The diffractogram of the so obtained sample, which resulted to have a content of metal Fe of 14.3% by weight, is very similar to that shown in FIG. 2, and this indicates that the sample consists of particles of metal Fe dispersed in a matrix based on amorphous silica and alumina. The TEM micrographs of this sample are very similar to those reported in FIGS. 5a, 5b and 5c, thus indicating that the nanoparticles of metal Fe have a size between 5 and 30 nm.

INDUSTRIAL APPLICABILITY

[0102] As already stated in the preceding paragraphs, the international scientific and technologic community show a great interest for the materials consisting of magnetic nanoparticles covered by a ceramic matrix protecting them from oxidation, thus making the particles stable. This interest is justified by the various applications that said materials may have in the following sector: magnetic fluids, catalysis, biotechnologies/biomedicine/bioengineering, diagnostics by magnetic resonance, data storage and environmental improvement and reclamation, production of stealth aircrafts, whose flight cannot be detected by radar systems. In the frame of these applications, that appearing particularly appealing and probably having the widest and immediate prospect of success, is the field of biotechnologies, biomedicine and bioengineering. Indeed in these sectors, research based on use of magnetic nanoparticles stabilized in various ways, is particularly active and comprises the following topics: electrochemical biosensors, detection and separation with purification of biomolecules (nucleic acids and proteins) and cells, targeted delivery of genes and drugs to highly selected organic regions, regeneration of biological tissues, detoxication of biological fluids and magnetic hyperthermia. Use of conditional in predicting such applications is justified by the following considerations. Although on the one hand application of magnetic nanoparticles in biotechnologies is already a reality, such as their use in the human genome project for DNA purification, on the other hand just the difficulty of obtaining reliable and stable magnetic nanoparticles is a restraint to their massive application. In view of this, the implementation of a simple, reliable and economic technique like that disclosed by the present invention, might give a great boost to applications in the above mentioned fields. On the basis of these considerations, products obtained by the processes disclosed in the present invention might reasonably and probably find application in the above mentioned fields.

[0103] Although the present invention was described as an illustrative but non limiting example through its preferred embodiments, it has to be understood that variations and/or modifications may be resorted hereto, without departing however from its scope of protection, as defined in the appended claims.

LITERATURE REFERENCES

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