ES2366848A1 - Method for obtaining doped carbon gels, gels thus obtained and use thereof as catalysts - Google Patents

Abstract

The invention relates to a method for the synthesis of carbon gels doped superficially with metal nanoparticles, based on the formation of macromolecules comprising a phenolic compound, an aldehyde and at least one surfactant, performing steps in a particular order and under controlled experimental conditions that allow the porosity, the nanostructure and the surface chemistry of the support to be adjusted, thereby obtaining high surface metal dispersion with high sintering resistance. The invention also relates to the carbon gels (aerogels, xerogels and cryogels) obtained using this method and to the catalytic uses thereof.

Description

Method of obtaining doped carbon gels, gels obtained by said method and its application as catalysts

Technical sector

The present invention is framed within the field of the manufacture of coal gels, more specifically in the manufacture of carbon gels doped with metals, materials commonly used in electrochemistry, as catalysts in synthesis reactions, energy or environmental interest, or as adsorbents of gases or other polluting molecules.

State of the art

The metal catalysts supported on carbon materials have been used successfully in a multitude of industrial applications Carbon gels are materials modern, successfully used in very different applications. Though inorganic gels (eg silica gel) are classic materials, coal gels were only developed and patented by Pekala in the late 80s [US Patent 4,873,218]. These materials they are prepared by carbonization of organic gels, obtained in turn by polymerization of resorcinol (or other monomers) and formaldehyde in aqueous solution and catalyzed by Na2CO3.

Since the late 1990s, the inventors of this patent have been working on the synthesis of carbon aerogels doped with metals with the idea of directly obtaining materials with catalytic activity [ Carbon , 37 (1999), p. 1199-1205] and in its catalytic applications [ Applied Catalysis A: General , 183 (1999), p. 345-356], The initial doping method consisted of dissolving the precursor salt of the corresponding metal active phase in the aqueous solution that also contains the organic monomers. The gelation produces an organic polymer doped with the metal, which when carbonized, simultaneously produces the carbon gel and the decomposition of the metal salt, with formation of catalytic active metal nanoparticles dispersed in the gel matrix. Being formed next to the polymer matrix, its mobility and sintering possibility is lower, and consequently, the stability of the catalysts can be increased with respect to those prepared by impregnation or other classical methods. In the state of the art there are works that describe the preparation of metal catalysts based on carbon gels by other techniques, such as ion exchange, direct impregnation or equilibrium adsorption. Methods in general have several disadvantages. In this sense, for example, methods based on ion exchange, adsorption, etc. they do not allow adjusting the surface load of the metal and require prior chemical treatments for the generation of anchor points in the support, for example by oxidation of the surface. The impregnation methods with the precursor salt generate little interaction of the support with the metal, so that the sintering and deactivation of the catalyst is favored, especially at high metal loads.

Regarding the catalytic activity of gels of doped coal, these have been used successfully in multiple Applications: reactions of energy interest (isomerization of alkenes, transformation of alcohols), reactions of interest environmental (combustion of organic air pollutants or reduction of nitrogen oxides). They have also been successfully applied as catalysts for advanced oxidation processes type Fenton, for the removal of coloring contaminants in water or as adsorbents of contaminants in the gas phase. Other uses are the hydrogen storage or in electrochemical applications, such as in fuel cells. Finally, the application of catalysts based on carbon gels in various reactions of synthesis and in fine chemistry, for the production of molecules with Pharmaceutical, perfumery, or agrochemical applications.

The known methods of preparing catalysts based on doped carbon gels for applications Catalytic present various problems.

On the one hand, the dissolution of the metal salt in the solution implies that a fraction (uncontrolled) of metal particles is trapped within the organic structure of the gel (Figure 1), and consequently is isolated from the reagents, so that they will remain inactive during the reaction [ Catalysis B: Environmental , 54 (2004), pp. 217-224], The effective catalyst charge is therefore uncertain, not directly controllable, and part of the active phase that is usually wasted be expensive metals (Pt, Pd, Au, Mo, Co).

On the other hand, catalysts with higher metal charges on their surface, specifically for electrochemical applications, are progressively required. However, another additional disadvantage of the known methods is the impossibility of doping at high metal concentrations, since precipitation of the metal from the solution and the formation of heterogeneous phases is caused. In this sense, the doping of the original solution with high metal loads causes the precipitation of the solution metal, generating heterogeneous phases [ Carbon 42 (2004) 3217-3227], so this technique is limited to low loads. For high metal loads doping is not resorted to, but to the impregnation of the support. However, impregnation results in low dispersion values and rapid sintering of the metal, specifically at high metal loads and severe heat treatments, so the catalysts are not very active. In these cases, to improve the dispersion of the metal on the support, the reduction of the corresponding salts is carried out by chemical methods that are usually expensive, slow and generate unwanted residues [ Carbon , 44 (2006) p. 2516-2522].

The processes of salt breakdown precursor also lead to sintering processes. Alternatively to heat treatment, chemical agents are used reducers, which make the synthesis process more difficult and difficult.

In view of the foregoing the present invention faces the problem of providing a method of preparing carbon gels doped with metal that exceed at least part of the mentioned disadvantages.

The method of the present invention is based on a part in the use of a surfactant that introduces large number of anchor centers in the formed hydrogel and in the realization after the doping stage on the hydrogel previously obtained. In this way the doped gels resulting from The present invention has a stable phase dispersion metallic (nanoparticles) on the surface of its nanostructure. This high dispersion is advantageous during the subsequent carbonization of the doped gel, as it gives the metal phase high sintering resistance. Also the method of This invention allows the obtaining of aerogels, xerogels or cryogels, depending on how the drying stage is carried out of the doped gel, doped with various metals that are active in Multiple catalytic processes

The method also contemplates the possibility of vary and control synthesis conditions, such as type and concentration of phenolic compound and aldehyde, ionic nature and concentration of the surfactant, and optionally of co-surfactant, the pH, the temperature of the different stages, such as the gelation temperature, that of curing, carbonization, the times of each of them, the presence or absence of agitation. This versatility of parameters and conditions allows to obtain gels with different characteristics controlled morphology (microspheres, nanospheres, nanofibers, or amorphous materials), porosity (micro, mesoporosity and macroporosity), surface distribution of nanoparticles metallic (and therefore its accessibility to reagents in later catalytic reaction) and chemical nature of the nanoparticles metallic with high dispersion and high resistance to sintering, which present advantages at the time of its application in Catalytic processes in which they will be used.

Description of the figures

Figure 1. Image obtained by microscopy high resolution transmission electronics (HRTEM) of a metal particle surrounded by the carbon gel matrix, generated by a previous conventional method.

Figure 2. Electron microscopy image of Sweep (SEM) of a comparative carbon xerogel doped with Mo, synthesized in the absence of surfactants and co-surfactants

Figure 3. SEM image of a coal xerogel doped with Mo synthesized by the method of the invention using hexadecyltrimethylammonium bromide (CTAB) as a surfactant and 1,3,5-trimethylbenzene (TMB) and t-butanol (t-BuOH) as co-surfactants that leads to the formation of Highly coated nanofibers of metal nanoparticles.

Figure 4. SEM image of a coal xerogel doped with Mo synthesized by the method of the invention in the presence of CTAB and t-BuOH (in the absence of TMB) leading to the formation of nanospheres (note that these are much smaller than those shown in Figure 2, which are of the order of microspheres).

Figure 5. HRTEM image of a xerogel of coal doped with amorphous Mo.

Figure 6. HRTEM image of a xerogel of carbon doped with Mo formed by carbon nanospheres.

Figure 7. HRTEM image of a xerogel of coal doped with Mo formed by carbon nanofibers.

Figure 8. Spectroscopy spectrum X-ray photoelectronics (XPS) of a doped carbon xerogel with Mo (S13), synthesized in the absence of surfactants that only shows the presence of molybdenum oxide.

Figure 9. XPS spectrum of a xerogel of carbon doped with Mo, synthesized in the presence of surfactant cationic showing a mixture of carbide and oxide molybdenum.

Figure 10. Activity comparison catalytic reaction of propanol to propene dehydration of two doped carbon gels S7 and S10 obtained according to the method of The present invention.

Description of the invention

In one aspect the invention relates to a method for preparing a doped carbon gel with at least a metal, which comprises the use of water as a solvent, thus avoiding the use of organic solvents and minimizing the waste formation Said method hereinafter method of the The invention comprises the following steps:

(i)

obtaining a hydrogel from a compound phenolic (R), a surfactant (S) and an aldehyde, (F)

(ii)

doping of the hydrogel resulting from step (i), with at least one metal,

(iii)

curing of the doped gel resulting from step (ii) Y

(iv)

carbonize the cured gel resulting from the stage (iii).

       \ vskip1.000000 \ baselineskip
    

Obtaining a hydrogel first It also includes the stages of:

to)

preparing an aqueous solution comprising a phenolic compound and at least one surfactant;

b)

heating said aqueous solution;

C)

add an aldehyde drop by drop over the solution watery; Y

d)

constant temperature gelation until Obtaining the hydrogel.

       \ vskip1.000000 \ baselineskip
    

The surfactant useful to implement the method of the invention can be any selected surfactant of the group formed by anionic, cationic and non-ionic In a particular embodiment it is used a surfactant selected from the group consisting of bromide of hexadecyltrimethylammonium, sodium dodecylbenzenesulfonate, and sorbitan sodium monooleate In a preferred embodiment it is used hexadecyltrimethylammonium bromide as cationic surfactant (CTAB). In another preferred embodiment, it is used sodium dodecylbenzenesulfonate (DCBS) as an anionic surfactant. In Another preferred embodiment is sorbitan monooleate (SPAN80), as a nonionic surfactant. The surfactant concentration can vary between wide margins. In a preferred embodiment the R / S molar ratio is 2/1, regardless of the nature of the surfactant

Optionally obtaining the hydrogel (stage (i)) may include the use of at least one co-surfactant which is typically added to the aqueous solution prepared in a). Saying co-surfactant can be added in concentrations variables In a preferred embodiment it is used as t-butanol co-surfactant or 1,3,5 trimethyl benzene (TMB). The R / cosurfactant molar ratio is in a particular embodiment 1/1 or 2/1.

In principle the method of the invention can put into practice with any phenolic compound. By compound phenolic in the context of the invention a compound should be understood organic comprising a benzene ring with at least one -OH substituent. In a particular embodiment the compound Phenolic is for example, resorcinol, phenol or catechol. In a Preferred embodiment is resorcinol. The method of the invention. It can be implemented with any aldehyde. By aldehyde has to understood in the present invention any organic compound that It has an aldehyde function. Preferably said aldehyde no contains functionalities that interfere with the reactions that they take place in the method of the invention. In one embodiment Particularly the aldehyde is formaldehyde.

The aqueous solution obtained in a) that it comprises a phenolic compound, a surfactant and optionally a co-surfactant, is then heated by above room temperature. Typically heated to a temperature between 40 ° C and 60 ° C, preferably at 50 ° C. The temperature reached is then maintained during the subsequent steps of adding aldehyde c), gelation d) and doping (ii).

At this temperature typically between 40-60 ° C the gelation reaction occurs instantly with the addition of each drop of aldehyde so that the appearance of new hydrogel in the suspension is observed after Addition of each new drop. The gelation stage d) is completed when all the added aldehyde has reacted. The time of gelation depends on temperature, concentration and molar ratio of reagents (phenolic compound, aldehyde, surfactant, and optionally co-surfactant). In a particular embodiment the gelation time is two hours at temperature between 40-60ºC which ensures that all the F added has reacted. The gelation is carried out under agitation.

The phenolic compound aldehyde (RF) aggregates of the hydrogel are formed in situ , avoiding the redispersion process in organic solvents used by Tonanon (Carbon 41 (2003) p. 2981-2990) and others. The structure of the organic hydrogel is defined in step d) of gelation and is a function of the particular experimental conditions in each case. Said structure is homogeneous and can be a microstructure, nanostructure or be amorphous. Sometimes fractions of amorphous material may appear next to the structured material.

Molar ratios of reagents (compound phenolic, aldehyde, surfactant) remain constant during the Preparation method. In a particular embodiment the relationship R / F molar is ½, the R / S molar ratio is 2/1 and the concentration of Reagents is 10% by weight.

The doping stage (ii) is carried out using a metal precursor with which you want to dop the gel. In a particular embodiment step (ii) of doping is carried to after adding the hydrogel resulting from step (i) drop by drop a saturated metal precursor solution. During this stage typically the gel is kept under stirring and at the temperature above indicated between 40-60ºC for a while variable. In general, the doping time is one hour after the addition.

       \ newpage
    

The method of the invention is based on the fact that once that the hydrogel is morphologically defined, and before completing its healing, that is, before it acquires too much rigidity and it lose anchor sites, the precursor solution of is added metal. In this way the hydrogel has a surface that is composed of charged species from the RFS macromolecule generated, and therefore the metallic species are incorporated easily to said surface from dissolution. Unlike other methods in the state of the art metallic species not they get trapped inside the polymer matrix generating embedded particles since gelation has already been completed previously, although not the cure. The metallic phase that is generated it is exclusively on the surface forming at the end of the method of the invention a high dispersion of metal nanoparticles.

The metal precursor can be any salt of any conventional metal such as for example nitrates, sulfates, acetates, nitrites, sulphites, chlorides, fluorides, sulphides, iodides, etc., with metals in different oxidation states, as well as mixtures thereof. In particular a salt of any catalytically active metal, such as Pt, Pd, Rh, Ru, Ni, Mo, etc, which is selected based on the reaction that goes to later want to catalyze. In a particular embodiment the metal is Mo.

The use of a surfactant in obtaining the hydrogel forms molecular aggregates compound phenolic-aldehyde-surfactant (RFS) with a load that will depend on the nature of the surfactant. Also the metal in the form of salt, which is a species charged (cationic or anionic), will then be attracted or repelled by the hydrogel depending on the experimental conditions and the nature of the RFS of the gel. In this sense the method of invention favors the establishment of interactions electrostatic between the functional groups of the gel cores that are polymerizing and the species of the metal in solution.

The interactions that metals present with the surface of the hydrogel are different depending on the original composition of the solutions (types and concentration of phenolic compound, aldehyde, surfactant / s, co-surfactant / s, pH, etc.) so that the metal particles are transformed into different chemical species during the remaining steps of the method of the invention.

The step (iii) of curing the doped gel is carried carried out at a temperature higher than gelation between 40-60 ° C In a particular embodiment it is performed at a temperature between 80 and 90 ° C. Curing takes out generally under agitation. The cure time is variable, although 24 hours are sufficient in general to complete the cure. Through curing takes place the crosslinking of the polymer.

The method of the invention is carried out. controlling the pH, reagent concentration and the speed of agitation. The pH of the can vary between wide ranges. In a particular realization is around 5.5. Higher pH values 7.5-9.5 are obtained by adding NaOH to the aqueous solution.

Then the cured gel obtained is filtered, It is dried and charred.

Drying can be done according to treatments different giving rise to a xerogel, an airgel or a cryogel: In a particular embodiment an airgel is obtained by supercritical drying with CO 2. For this, the gel resulting from step (iii) is filtered and then quickly suspended in acetone to produce a water-acetone ion exchange, since Water is not soluble in liquid CO2. The exchange takes out for a variable time, usually 48 hours with renewal of the solvent every 12 h, and finally the filtered gel is dried in Supercritical CO 2.

Xerogels can be obtained directly by oven drying, preferably by first low drying temperature (for example 12 h at 50 ° C) followed by another drying during for example 24 h at 110 ° C.

The cryogels are obtained by any conventional cryogenic technique.

Once dry the xerogel, the airgel, or the obtained cryogel is carbonized to yield the doped carbon gel of the invention. The carbonization stage (iv) is done in an atmosphere inert and under variable temperature conditions final, temperature gradient, time, etc. In a particular embodiment is carried out in flow of N2. In inert atmosphere, at higher temperatures and longer carbonization progressively obtain more metallic phases reduced (oxides in a lower oxidation state) and finally they form metals in a state of zero oxidation or even reach occur reactions with carbon forming metal carbides.

In a particular embodiment the temperature of Carbonization is between 400ºC and 900ºC. In other particular embodiment the carbonization is performed in current of N2 (preferably at 100 cc / min) using a ramp of slow heating between 1 and 2ºC / min, up to a temperature given, preferably 900 ° C. Reaching a high temperature in This step allows the doped carbon gel of the invention to be stable up to this temperature in any catalytic application later. In another particular embodiment the temperature of Carbonization is maintained for 5 h. The doped carbon gel The resulting result is slowly cooled in the same flow of N2.

The method of the invention has numerous advantages. In this sense it allows:

-

Adjust particle morphology from the gel to advanced forms such as nanofibers, nanospheres, microspheres, amorphous form.

-

Adjust the porosity of the gels, it is say, micro-, meso- and macroporosity.

-

Adjust the surface chemistry of gels varying reagents and surfactant (introduction of monomers with heteroatoms).

-

Avoid the formation of metallic particles trapped inside the organic matrix and / or precipitation of the metal phase before the gelation

-

Get high metal loads homogeneously distributed on the surface of the support forming nanoparticles of defined chemical nature and with high resistance to sintering.

-

Get active catalysts, selective and stable, based on the properties previous.

-

Be improves dispersion and resistance to sintering phase metallic dispersed on the organic gel, so that the performance of doped carbon gels in very applications varied within heterogeneous catalysis: energy, medium environment, pharmaceutical industry, synthesis of compounds such as herbicides, phytosanitary, etc., both from the point of view of its activity and selectivity, as of the stability of the catalyst.

       \ vskip1.000000 \ baselineskip
    

In another aspect the invention relates to the gel of carbon doped with at least one metal obtained by the method of the present invention, hereinafter doped carbon gel of the invention.

The doped carbon gels of the invention are They can characterize by various methods.

The inventors of the present invention have Chemically and texturally characterized carbon gels doped according to the present invention by application of conventional techniques. The porous texture has been analyzed by physical adsorption of gases (CO 2 at 273 K and N 2 at 77 K) and immersion calorimetry in Benzene, these techniques allow you to analyze different ranges of porosity (micro-mesopores) as well as determining the values of the external and internal surfaces of the materials.

The chemical structure of the organic polymer and of Carbon gels have also been determined by various techniques, thermogravimetry (TG), x-ray diffraction (DRX), Spectroscopy infrared with Furrier transforms (FTIR) or spectroscopy X-ray photoelectronics (XPS). These techniques allow to analyze the variations suffered by the doped gels during stage (iv) of carbonization, as well as studying the chemical state of metals and of the support of the doped carbon gels of the invention.

The morphology of the gels is studied by scanning electron microscopy (SEM), while the dispersion, particle size and crystal structure is analyzed by high resolution transmission electron microscopy (HRTEM) and DRX.

In a further aspect the invention relates to the use of the doped carbon gel of the invention as a catalyst heterogeneous. The method of the invention improves dispersion and sintering resistance of the metal phase dispersed on the organic gel of the doped carbon gel of the invention, so that its performance is improved in any of its possible and varied applications within heterogeneous catalysis: energy, medium environment, pharmaceutical industry, synthesis of compounds such as herbicides, phytosanitary, etc., both from the point of view of its activity and selectivity, as of its stability.

By way of illustration the inventors have studied the isopropanol to propene decomposition reaction, using doped carbon gels according to the present invention. This reaction It is in itself a test reaction to characterize the acidity of the catalysts, but in this case a product like it is the propene of high demand for example for the manufacture of plastics

According to the method of the invention the Gel morphology and surface chemistry is defined before the metal precursor addition, so they can be adjusted in function of the type and concentration of monomers (phenolic compound (R) and aldehyde (F)) and surfactants (S). Since the polymerization of R and F occurs in the solution that contains the micelles of surfactant, redispersion used by other methods of synthesis as mentioned above. When defining the morphology, meso and macroporosity associated with the space between Particles in this type of gels can be controlled in this way. Porosity will also depend on the drying process and carbonization, where microporosity is generated by the output of Pyrolysis gases. During the synthesis, the surface of the hydrogel that is formed, will consist of several chemical functions in the RFS hydrogel macromolecule that attract to the solution metal and fix it to the surface of the particle Hydrogel In this way a high dispersion of metallic nanoparticles on the hydrogel nanostructures previously defined.

The inventors have verified that in the absence of surfactants and co-surfactants (sample S13), the gel of coal with doped Mo obtained has an area of micropores of 173 m2 / g and no ability to detect adsorb N2. This indicates an exclusively microporous character. of the carbon gel, where the micropores are so narrow that they present diffusional restrictions. Morphologically, this gel It is composed of microspheres (Figure 2). Although also they observe a high concentration of Mo both by HRTEM and XPS superficial, it is observed that next to the deposited nanoparticles on the surface of the microspheres, there are certain particles of acicular form of metallic character. In the absence of surfactant, thus produces a low gel-metal interaction, possibly as a result of a lower concentration of related species at the surface and at the low surface (porosity) of These microspheres. This leads to particle formation large metal ones, which appear independent (segregated) of support and that lead to loss of activity due to crystal growth The DRX and the XPS analysis confirm that the Mo in this case remains 100% in the oxide state (Figure 8).

In the presence of a surfactant the porosity and surface of a carbon xerogel according to the invention increases, because the decomposition of the structure surfactant Organic during carbonization favors the formation of pores. The combination of synthesis parameters (surfactants, co-surfactants, pH, etc.) allows to obtain different morphologies, from microspheres (such as those shown in the Figure 2), nanofibers (Figure 3), nanospheres (Figure 4), or amorphous materials. In any case (Figures 5-7), a homogeneous metal coating is observed with a high concentration of metal nanoparticles and without large segregated crystals. They are all doped with Mo and they were carbonized at 900 ° C, that is, at a very high temperature. Still, I know observes a high dispersion of nanoparticles (the size of particle is very small and does not sinter even after a heat treatment at 900 ° C). There can be no metal particles sausages since these are formed after the formation of the Nanostructure of the organic hydrogel. In any case, they are observed large segregated metal particles, as before, of In fact, no DRX peaks were obtained in these samples, indicating that the metal particles are less than 4 nm, as consequence of the metal being strongly anchored to the hydrogel structure, which hinders its mobility during heating.

The surface concentration of metal and Chemical nature of metal nanoparticles has been studied by elementary analysis, XPS and DRX. Metal particles that they are formed by decomposition of the precursor salt during carbonization, are reduced, first to oxides, progressively from lower oxidation state, then to oxidation metals zero, even reaching the formation of metal carbides.

In previous work [Applied Catalysis A: General 183 (1999) 345-356], is revealed than by doping with Cr, Mo, W at a total charge of 2.7% metal in the carbon gel, only the W generated 5% of the metal in the form of carbide (CW) when carbonized at 1000 ° C. In the case of the doped airgel with Mo and carbonized at 1000 ° C, it presented a surface charge of Mo (determined by XPS) of 3.2%, being composed of a mixture of oxides of Mo (VI), (IV) and (III). The formation of Mo_ {C} was forced given its importance in certain industrial processes of hydrodesulfurization by high temperature treatment in mixture H 2 / Ar and after such treatment is formed around 30% of Mo_ {C} C without modifying the amount of surface Mo [Carbon 41 (2003) 1291-1299].

The analysis by XPS and DRX of the series of samples obtained by this new method of the invention allows conclude that: a) very high surface concentrations are obtained (as already observed by TEM, Figures 5-7) and which favors the formation of metal carbides directly during carbonization. This avoids long and dangerous treatments in H2 and high temperature in the case of having to synthesize these chemical forms.

For example, a coal xerogel doped with Mo (S7), with metal loading similar to the one described above (around 3% total), synthesized in the presence of a surfactant cationic to obtain nanofibers doped with Mo and carbonized to 900 ° C, presented a surface composition of Mo of 22.4%, of the which, 63% is in the form of carbide, and the remaining 37% as oxide, as evidenced by XPS analysis (Figure 9).

It is observed, then, that being the metal exclusively on the gel surface, the concentration Superficial increases around 10 times compared to methods known, where although high dispersions are achieved and a homogeneous distribution of the metal in the total structure of the hydrogel, most of it does not appear on the surface, where it is active in reaction. Similarly, the different interactions metal-hydrogel RFS, also induce a high degree of carburization, even greater than that obtained previously after H2 treatment.

In the absence of surfactants, the metal is It presented 100% as oxide.

The inventors have also studied the influence of the nature of the surfactant. Examples were made comparatives in which the molar concentration of surfactant was the same, the pH is around 5.5 and preferably, TMB and t-BOH as co-surfactants. Load metal obtained on the surface of the doped xerogels with Mo, and the chemical species they form, varied greatly with the type of surfactant, as shown in the following table.

one

It is observed that the cationic surfactant favors to a greater extent the fixation of the Mo species in solution as well as, its transformation into carbide. This is because the salt precursor used in this case was ammonium heptamolybdate, of so that the anions Mo_ {O} {24} <-6} are more attracted to the presence of cationic species on the surface. The Mo concentration when synthesis is carried out in the presence of anionic surfactant is the lowest in the series, probably as a consequence of repulsions, in addition, the formation was not observed of carbide. The behavior of the xerogel synthesized in the presence of nonionic surfactants is intermediate between the cationic and anionic, both in its ability to fix the metal and in the chemical species that form. The type of interactions, and consequently, both the charge of the metal and its chemical state In the end, they depend on the nature of the surfactant and its ability to form RFS aggregates.

The inventors have studied the influence of Hydrogel pH at the charge and chemical form of the final Mo that Summarized in the following table:

2

In this case, the attractive interactions are optimal at pH around the neutral while simultaneously favoring carburization of Mo.

       \ vskip1.000000 \ baselineskip
    

Examples

Synthesis examples are shown below. and characteristics of two xerogels doped with Mo according to the invention under identical experimental conditions, in which it only varies slightly the initial dissolution composition. These are chosen two samples, among the many synthesized ones, to show the importance of strict control of synthesis conditions. Small variations in composition, pH, temperature, stirring, etc., can lead to significant changes in the properties of the samples. Thus, the selected samples were prepared carrying out a strict control of them, in the conditions and order described above, using the relationships molars:

3

       \ vskip1.000000 \ baselineskip
    

The porosity and surface of the xerogels of Carbon obtained were:

4

       \ vskip1.000000 \ baselineskip
    

Both xerogeles are eminently microporous. The microporosity is similar as well as its surface values. The difference between Smic and SBET indicates diffusional restrictions on inside of said microporosity. Note however, that the Presence of the surfactant in the synthesis mixture leads to a significant increase in the surface of the hydrogel with respect to the sample synthesized as blank (Smic = 173 m2 / g). In both cases, the surface values are similar because it is also the type and concentration of surfactant.

       \ vskip1.000000 \ baselineskip
    

Morphology

The morphology of hydrogel particles undergoes an important transformation due to the small change in initial composition of the system. The type of particle is independent of metal, since it is formed before its presence, and is due, in this case, to the presence or not of the TMB, which is the only synthesis variable between both samples. In the absence of TMB nanospheres are obtained (Figures 4 and 6), in the presence of TMB, nanofibers (Figures 3 and 7).

       \ vskip1.000000 \ baselineskip
    

Metallic dispersion: nanoparticle formation superficial

5

In both cases a high degree of surface dispersion, note that the concentration of Mo on the Total reagents is 1% weight (in volume at 3% in the xerogel of final carbon), while the XPS shows concentrations of Mo around 25% by weight. In both cases a high is also obtained degree of carbonization when carbonized at 900 ° C. These grades of carburization may be adjusted by decreasing / increasing the temperature or treatment time, during carbonization.

       \ vskip1.000000 \ baselineskip
    

Applications

The activity of the catalysts obtained by This method will depend on the deposited metal, its chemical form and dispersion. To test the catalytic activity of these gels has been proven its efficiency in the transformation of propanol in propene The catalysts work in this reaction based on their acidity. In this case, said parameter is linked to the concentration surface oxide, thus, the activity is higher in the case of S10 than in S7 (Figure 10), according to the results shown previously. With both catalysts, it is obtained selectively propene, increasing the conversion as the temperature increases. For him on the contrary, increasing the content in C_ {2} Mo is expected to increase Efficiency in hydrodesulfurization reactions (not tested here). The deposition of other metals opens the way for applications electrocatalytic, environmental, energy reactions, etc.

Claims (16)

1. Method of synthesis of a doped carbon gel with at least one metal, which comprises the following stages:

(i)

obtaining a hydrogel from a compound phenolic, a surfactant and an aldehyde,

(ii)

doping of the hydrogel resulting from step (i), with at least one metal,

(iii)

curing of the doped gel resulting from step (ii) Y

(iv)

carbonize the cured gel resulting from the stage (iii).

         \ vskip1.000000 \ baselineskip
      

2. Method of synthesis of a carbon gel doped with at least one metal according to claim 1, characterized in that the obtaining step (i) comprises:

to)

preparing an aqueous solution comprising a phenolic compound and at least one surfactant;

b)

heating said aqueous solution;

C)

add an aldehyde drop by drop over the solution watery;

d)

constant temperature gelation until Obtaining a hydrogel.

         \ vskip1.000000 \ baselineskip
      

3. Method of synthesis of a doped carbon gel with at least one metal according to claim 1 or 2 wherein the doping stage (ii) is carried out by adding to the hydrogel resulting from step d) drop by drop a saturated solution of metal precursor. Method according to any one of claims 1 to 3, characterized in that the aqueous solution comprising a phenolic compound and at least one surfactant is heated at a temperature between 40 and 60 ° C and this is maintained during the steps of adding the aldehyde c ), gelation d) and doping (ii). 5. Method of synthesis of a doped carbon gel with at least one metal according to any one of claims 1 to 4 in which step (iii) of curing is carried out at a temperature higher than gelation. 6. Method according to claim 5, wherein the curing step (iii) of the doped gel is performed at a temperature between 80 and 90 ° C. 7. Method according to any one of claims 1 to 6, characterized in that:

to)

the molar ratio phenolic compound / aldehyde is 1/2, the phenolic compound / surfactant molar ratio is 2/1 and these remain constant and because the concentration of compound Phenolic, surfactant, aldehyde is 10% by weight.

         \ vskip1.000000 \ baselineskip
      

Method according to any one of claims 1 to 7, characterized in that the doped gel obtained from step (iii) has a structure selected from nanofibers, microspheres, nanospheres and amorphous. 9. Method according to any one of claims 1 to 8, characterized in that the doped gel obtained from step (iii) is dried and then carbonization (iv) is carried out in an inert atmosphere at a temperature between 500-900 ° C. 10. Method according to any one of the claims 1 to 9, wherein the phenolic compound used is resorcinol 11. Method according to any one of the claims 1 to 10, wherein the aldehyde used is formaldehyde. 12. Method according to any one of the claims 1 to 11, wherein the surfactant is selected from group formed by cationic, anionic and non-ionic 13. Method according to claim 12 wherein the surfactant is selected from the group consisting of bromide of hexadecyltrimethylammonium, sodium dodecylbenzenesulfonate, and sorbitan sodium monooleate 14. Method according to any one of the claims 1 to 10, wherein the aqueous solution comprises also a co-surfactant, preferably selected from 1,3,5 trimethyl benzene and t-butanol. 15. Carbon gel doped with at least one metal obtained according to any one of the claims previous. 16. Use of doped carbon gel with at least a metal according to claim 15, as catalyst heterogeneous.