MXPA00011547A - Zinc aluminate with high specific surface area, preparation method and use for treating motor vehicle exhaust gases - Google Patents

Abstract

The invention concerns a zinc aluminate characterised in that it has, after calcination at 800°C for 8 hours, a specific surface area of at least 85m2/g. The invention also concerns a precursor composition of said aluminate. The method for preparing the aluminate and the composition comprises the following steps:contacting in a solvent medium a salt, a zinc sol or alkoxide and an aluminium alkoxide;hydrolyzing the resulting mixture by adding an amount of water in excess relative to the aluminium alkoxide;recuperating the formed precipitate and optionally drying it, thereby obtaining the precursor composition;if required, calcining said precipitate, thereby obtaining the aluminate. Finally, the invention concerns the use of aluminate for treating motor vehicle exhaust gases.

Description

YOUR PROCESS OF PREPARATION AND ITS USE IN THE TREATMENT OF GASES OF ESCAPE OF AUTOMOTIVE VEHICLES The present invention comprises a zinc aluminate with a high specific surface area, a precursor composition thereof, an aluminate preparation process and its composition, as well as the use of aluminate in the treatment of gases, specifically exhaust gases of motor vehicles. It is known that the reduction of emissions of nitrogen oxides (NOx) derived from the exhaust gases of the engines of vehicles or industrial plants represents a serious problem of environmental protection. In the case of automotive vehicles, the "three-way" catalysts are used in particular, which are used stoichiometrically in the reduction of gases present in the mixture. However, any excess gas is reflected in a sudden deterioration of the catalyst. However, some engines, such as the diesel engine or oil engines that operate in a low combustion mode, save fuel, but emit exhaust gases with a large constant amount of oxygen, for example 5% minimum. Therefore, a standard three-way catalyst does not represent a big help in terms of NOx emissions from this type of engine. In addition, the adjustment of the standards in relation to the subsequent combustion of motor vehicles, which currently cover other types of engines, makes it absolutely necessary to limit NOx emissions. For these motors, spiraled catalysts based on aluminum and zinc were proposed. However, these catalysts must be refined since they do not have high specific surface areas at a high temperature. At this time, one aspect of catalyst performance that is likely to be improved is surface stability, i.e. having the ability to maintain a high surface area at an elevated temperature. Accordingly, there is a need for a spiked catalyst with a high specific surface area. At this point, the zinc aluminate of the invention is characterized by having a specific surface area of at least 85 m2 / g. after calcining at 800 ° C for a period of eight hours. Likewise, the invention deals with a zinc aluminate precursor composition, whose composition is characterized by having zinc and aluminum components, as well as by the ability to form, after being calcined, a zinc aluminate with a surface area of specific zinc. at least 85 m2 / g, after calcining 8 hours at 800 ° C. Another point of this invention is a process for the preparation of an aluminate or a composition as described above, which is characterized by being integrated by the following steps: a zinc salt and an aluminum alkoxide are united in a solvent medium; - the mixture thus formed is hydrolyzed by adding a greater amount of water to the aluminum alkoxide; - the formed precipitate is recovered and optionally dried, thus obtaining the precursor composition; - if necessary, said precipitate is calcined, thus obtaining aluminate. Other characteristics, details and benefits of the invention will be more evident after reading the following description and the different specific examples, but not exhaustive. The aluminate of the invention is a zinc aluminate having a spire-like structure, ZnAl204. It is possible that it is in the form of one or more zinc depleted or rich phases in zinc relative to Zn? L204. These phases comply with the formula Zn? -xAl2? 4-5 and Zn? + XAl204 + 5, where 0 < x 0.95. The values of x can more specifically satisfy the following ratios: 0 < x 0.85, 0 < x 0.8 and even more specifically 0 < x o.5. Finally, x can meet the ratio 0.4 x 0.85. In addition, aluminate may contain one or more additives that may be one of the elements of Groups IA, IIA, VIIA to IB of the Periodic Table, as well as tin, gallium and rare earths. The Periodic Table of the Elements referred to is the one published in Supplement to the Bulletin of the Société Chimique de France (Supplement to the Bulletin of the Chemical Society of France) No. 1 (January 1966). Likewise, the term "rare earths" shall be understood as the group of elements formed by yttrium and the elements of the Periodic Table with an atomic number between 57 and 71, inclusive. Within the VIIA Group, more direct reference is made to manganese; from Group VIII, to iron; of the IB Group, copper and silver. It is possible to find these additives especially in aluminate as a partial substitution of zinc or aluminum. A characteristic of the aluminate of this invention is its specific surface area. The term "specific surface area" shall be understood, for purposes of this document, as the BET specific surface area determined by the absorption of nitrogen in accordance with ASTM D 3663-78 stipulated in the Brunauer-Emmett-Teller method described. in the publication "The Journal of the American Chemical Society, 60, 309 (1938)". Even after being calcined at a high temperature, the aluminate of this invention still has a high surface area value. Thus, after calcining the aluminate at 800 ° C for eight hours, a specific surface area of at least 85 m2 / g is obtained. Also, it can be obtained from at least 90 m2 / gr. and more specifically of at least 100 m2 / g., even after having been calcined at 800 ° C for 8 hours. It is possible to obtain values of at least 120 m2 / gr. The high values of this surface area are maintained even at higher temperatures because the aluminate of this invention can have, after having been calcined two hours at 900 ° C, a specific surface area of at least 70 m2 / gr. and more specifically of at least 80 m2 / gr. In addition, after calcination at a temperature of 1000 ° C for six hours, specific surface areas of at least 50 m2 / g can be obtained. , more in particular of at least 70 m2 / gr. Which means that the stability of the aluminate surface area is maintained within a wide range of temperature. The values that have just been provided reach the calcination in the air. Also, it can be seen that the aluminate of this invention has a high resistance to thermal relaxation. The above means that, under particular conditions of calcination, its specific surface area varies a little. Then, after the six-hour calcination process at 1000 ° C in a H20 / N medium with 10% H20 by volume, the surface area remains essentially the same as that obtained after calcination in air at the same temperature and during the same time, that is to say at least 50 m2 / gr. The same results of the calcination are obtained in a medium of 02 / H20 / N2 with 10% of H20 and 10% of 02 by volume. Also, the aluminate of this invention may have a pore volume of at least 0.6 ml / g; This porosity is determined by the intrusion porosimetry of mercury. The measurements were taken using a Auto Pore 9220 Micromeritic machine on powders degassed overnight in an oven at a temperature of 200 ° C. The operating parameters are the following: penetrometer constant: 21.63, capillary volume: 1.1, contact angle: 140 °. It is possible that the porosity is more specifically at least 2 ml / gr. and it can be, for example, between 2.5 and 3.5 ml / gr. Another point of the invention is an aluminate precursor composition, such as that described above. This composition consists of zinc and aluminum compounds and, if necessary, of the above-mentioned additive compounds. The main characteristic of the precursor composition is its ability to produce a zinc aluminate, after calcination. The above calcination temperature that the aluminate causes is about 500 ° C. Also, the aluminate obtained in this way possesses the aforementioned characteristics, that is, if the aluminate is calcined at a temperature of 800 ° C for eight hours, it maintains a high specific surface area of at least 85 m2 / g., more specifically of at least 90 m2 / gr. and even more specifically of at least 100 m2 / gr. Of course, all above-mentioned surface area values in relation to aluminate at temperatures of 800 ° C and 900 ° C are also applied here. Next, the process for the preparation of the aluminate and its precursor composition will be described. The first step in this process is to incorporate salt, sol or zinc alkoxide together with an aluminum alkoxide in a solvent medium, at the discretion of adding salt, sol or alkoxide of at least one of the aforementioned additives. The zinc or alkoxide salt, as well as the salt or alkoxide of the additive, must be soluble in the solvent medium. For example, the zinc or additive salt is an inorganic salt, such as nitrate or chloride, or any other organic salt, such as citrate, oxalate or acetate. The aluminum alkoxide may be an ethoxide, butoxide, isopropoxide, among others. The solvent medium is chosen from any medium in which the zinc or alkoxide salt and the aluminum alkoxide are soluble. In general, an alcohol solvent is used. Among the alcohol solvents, reference can be made to saturated monoalcoholics and more particularly to those with a short chain (eg, at most Ca), such as methanol, ethanol, propanol and butanol. It is also possible to use unsaturated alcohols and polyalcohols, such as ethylene glycol, propylene glycol, hexylene glycol, propanediol and butanediol. Also, ketona can be used. The reactants can be incorporated in the solvent medium in any way. However, according to a specific inclusion of the invention, the zinc salt and the aluminum alkoxide are combined when the zinc salt in the solvent medium is added to the aluminum alkoxide, which constitutes a stock, that is to say beforehand dissolved in this medium. In this part it will be noted that, according to another version of the invention, it is possible to heat the mixture obtained in this way. What facilitates the dissolution of the salts and continue with the next step that can be better controlled: hydrolysis and precipitation.

The second step of the process of the invention is the hydrolysis of the mixture obtained in the previous step. Said hydrolysis is carried out when water is added to the mixture. According to a characteristic of the process of the invention, the hydrolysis is carried out using a quantity of water greater than that of the aluminum alkoxide. Said amount is determined by the molar ratio between H20 and the aluminum alkoxide. In general, this index can be at least 6, more specifically at least 10 and still more specific at least 20. However, in the case of the preparation of a zinc depleted aluminate, this index can be less. Specifically, in the case of the preparation of an aluminate with a Zn / Al index of less than 0.4, the molar ratio of H20 (aluminum alkoxide can be at least 3, more specifically at least 4. It is not necessary that the water is in its natural state, it can be found in a mixture of water and alcohol, as long as the latter is one of those already mentioned in relation to the solvent medium, it is possible to refer more specifically to ethanol. A precipitate of elements is formed The precipitate obtained is separated from the reaction mixture by any known means, in particular by centrifugation.

If necessary, the precipitate can be washed. It is optional to dry it or not. In this step, the precursor composition of the invention is obtained. The aluminate is prepared by calcining the precipitate (precursor composition) at a temperature of at least 500 ° C. It is possible to resort to other processes for the preparation of an aluminate or a precursor according to the invention as long as this aluminate or precursor contains an additive of the type already mentioned. In this process the additive is not added during the synthesis of the aluminate but by impregnation, either of the precursor composition, for example the dry precipitate, or the aluminate itself, ie the calcined precipitate. Said impregnation is carried out using a salt solution of one of the aforementioned additives in the example. Dry impregmentation is much more used, which consists in adding to the product to be impregnated a volume of aqueous solution of the element equal to the volume of pores of the solid to be impregnated. In addition, the invention includes a process for the treatment of exhaust gases of automotive vehicles, in which a catalyst system uses an aluminate as described above.

The invention also comprises a process in which a catalyst system uses the same aluminate as is used in the treatment of gases possibly containing nitrogen oxides, in order to reduce the emissions of such nitrogen oxides. In this case, the gases that can be treated are those that give off the gas turbines, the boilers of thermal power stations, the internal combustion engines, in particular the diesel or low combustion engines, among others. Therefore, the aluminate of the invention is applied to the treatment of gases with a high content of oxygen and nitrogen oxides. The phrase "gases with a high oxygen content" should be understood as gases with a higher amount of oxygen compared to the amount necessary for stoichiometric combustion and, in particular, gases that permanently have a greater amount of oxygen in relation to the stoichiometric value? = 1. The value? it is correlated with the air and fuel index in a manner known per se, especially as regards internal combustion engines. That is to say, the aluminate of the invention is used in the treatment of gases derived from systems such as those described in the previous paragraph and that operate in a constant manner under conditions such that always? is strictly greater than 1. In the case of gases with a high oxygen content, the aluminate of the invention is then used both in the treatment of gases discharged by a motor that operates in low combustion and with a high content of oxygen ( expressed in volume) of between 2.5 and 5%, in general, as in the treatment of gases with a much higher oxygen content, for example the gases fired by diesel engines, that is to say with a content greater than at least 5%, in particular 10%, the margin being of content between 5 and 20%. The gases contain a reducing active that can be one or more hydrocarbons and one of the reactions that is sought to catalyze; in this case it is the HC (hydrocarbons) NOx reaction. The hydrocarbons that can act as reducing agents in the elimination of NOx are in particular the gases or liquids of the families of saturated carbides, ethylenic carbides, acetylenic carbides, aromatic carbides and oil-cut hydrocarbons such as methane, ethane, propane, butane, pentane, hexane, ethylene, propylene, acetylene, butadiene, benzene, toluene, xylene, kerosene and gas oil. Also, gases, as reducing agents, can contain organic compounds with oxygen. Basically, such compounds can be alcohols, for example saturated alcohols (methanol, ethanol or propanol), ethers (such as methyl ether, ethyl ether) and esters (methyl acetate and ketones). Similarly, gases may contain ammonia as a reducing agent. As regards the treatment of gases, aluminate can be used in catalytic compositions of different forms, such as granules, beads, cylinders or hives with varying dimensions, it being possible for these compositions to contain the aluminate of the invention on any basis that is generally applied in the field of catalysis, such as Zr02, A1203, Ti02, Ce02, Si02 or the mixture of some of these. The invention more specifically introduces a catalytic system for the aforementioned gas treatment. The main characteristic of this system is that it has an aluminate on a substrate. In general, such a system contains a thin coating incorporating the aluminate and a support as mentioned above. The first is deposited on a substrate, either metal or ceramic monolith. The systems are installed in the known way in the exhaust pipes of the vehicles when it comes to the treatment of exhaust gases. Finally, the invention also encompasses the use of an aluminate or a precursor composition, such as those mentioned above, for the manufacture of said catalyst system. Here are some examples.

EXAMPLE 1 The preparation of ZnA204 is exemplified below. The following raw materials will be used: Zn (N03) 2 6H20 crystallized and 99% pure, molecular weight: 297.47 - 97% pure tri-sec-butoxide aluminum (C2H5CH2CH20) 3AI 99% pure hexylene glycol (2-methyl 2, 4- pentanediol) absolute ethanol, molecular weight: 46.07; d: 0.79 g / cm3 1.25 mol of zinc salt dissolves in one liter of hexylene glycol. This solution is added to 2.5 mol of aluminum tri-sec-butoxide previously introduced into the reactor, in a simple rotation and by vigorous stirring (500 rpm). The mixture is heated to 70 ° C and maintained at this temperature for two hours. Then, a mixture of water and ethanol ((50/50% by volume) is added at a rate of 5 ml per minute.The index of aluminum tri-sec-butoxide / water is equal to 28. The The mixture obtained is stirred overnight, the precipitate obtained is separated by centrifugation, and then dried in the oven as a thin layer at 70 ° C for 48 hours, finally, the product is calcined, the temperature is increased by 5 °. C per minute, then it is maintained at a constant level with the value and the aforementioned times After the calcination process for six hours at 600 ° C, the specific surface area of the product is 136 m2 / g. of the calcination process for six hours at 700 ° C, the specific surface area of the product is 125 m2 / g After the calcination process for eight hours at 700 ° C, the specific surface area of the product is 115 m2 / gr. After the calcination process for two hours At 900 ° C, the specific surface area of the product is 101 m2 / gr. After the calcination process for two hours at 1000 ° C, the specific surface area of the product is 76 m2 / gr. and 53 m2 / gr. after six hours at the same temperature. After the calcination process for six hours at 1000 ° C in a mixture of H20 / N2 with 10% H20 by volume, the specific surface area remains stable at 53 m2 / g.

The same result is obtained when calcining for six hours at 1000 ° C in a medium of 02 / H2? / N2 with 10% H20 and 10% of 02 by volume. EXAMPLE 2 The precipitate obtained in Example 1 is dried 48 hours a 70 ° C. Then, it is impregnated with a solution of SnCl2 2H20 dissolved in ethanol. The technique used is dry impregnation. The amount of Sn deposited is equivalent to 1.6% by weight in relation to zinc aluminate oxide. The product obtained in this way is baked in the oven for two hours at 110 ° C and then burned for eight hours at 800 ° C (at an increase rate of 5 ° C per minute). The specific surface area of the product obtained in this way is equivalent to 115 m2 / gr. EXAMPLE 3 The preparation of ZnAl? 8Ga0.204 is exemplified below.

For the purposes of this example, the same raw materials of Example 1 are used, in addition to a solution of Ga (N03) 3 of 1. 807 mol / 1, d = 1,365 gr./cm3. 1.25 mol of zinc salt is dissolved in one liter of hexylene glycol and then 0.25 mol of gallium nitrate is added. This solution is added to 2.25 moles of aluminum tri-sec-butoxide previously incorporated in the reactor, by vigorous stirring (500 rpm). The mixture is heated to 70 ° C and maintained at this temperature for two hours. Then, a mixture of water and ethanol (50/50% by volume) is added at a rate of 5 ml. per minute. The water and aluminum tri-sec-butoxide index is equivalent to 25. The precipitate obtained is allowed to cool overnight by stirring; it is separated by centrifugation; it is dried in the form of a thin layer at 70 ° C for 48 hours and then calcined for eight hours at 800 ° C (at an increase rate of 5 ° C per minute). The specific surface area of the product obtained from this way is equivalent to 113 m2 / gr. EXAMPLE 4 The preparation of Z o.95Cao.o5 l2 is exemplified below. . For purposes of this example, the same raw materials of Example 1 are used, in addition to a Ca (N03) 2 4H20 99% pure solution. 1.19 mol of zinc salt is dissolved in one liter of hexylene glycol and then 0.06 mol of calcium nitrate is added. This solution is added to 2.5 moles of aluminum tri-sec-butoxide previously incorporated in the reactor, by vigorous stirring (500 rpm). The mixture is heated to 70 ° C and maintained at this temperature for two hours. Then, a mixture of water and ethanol (50/50% by volume) is added at a rate of 5 ml. per minute. The water and aluminum tri-sec-butoxide index is equivalent to 28. The precipitate obtained is left to cool overnight with stirring; it is separated by centrifugation; it is dried in a thin layer at 70 ° C for 48 hours and then calcined for eight hours at 800 ° C (at an increase rate of 5 ° C per minute) The specific surface area of the product obtained in this way is equivalent to 119 m2 / gr. EXAMPLE 5 For the purposes of this example, the same raw materials as in Example 1 are used in addition to a pure Li (N03) 99% solution. 1.19 mol of zinc salt is dissolved in one liter of hexylene glycol and then 0.06 mol of lithium nitrate is added. This solution is added to 2.5 moles of aluminum tri-sec-butoxide previously incorporated in the reactor, by vigorous stirring (500 rpm). The mixture is heated to 70 ° C and maintained at this temperature for two hours. Then, a mixture of water and ethanol (50/50% by volume) is added at a rate of 5 ml. per minute. The water and aluminum tri-sec-butoxide index is equivalent to 28. The mixture is allowed to cool overnight with stirring. The precipitate obtained is separated by centrifugation; it is dried in the form of a thin layer at 70 ° C for 48 hours and then calcined for eight hours at 800 ° C (at an increase rate of 5 ° C per minute). The specific surface area of the product obtained from this way is equivalent to 108 m2 / gr. EXAMPLE 6 The following example refers to an aluminate of formula ZnAl204 containing silver as an additive. The precipitate obtained in Example 1 is dried 48 hours at 70 °. The technique used is dry impregmentation.

The amount of silver deposited is equivalent to 1.6% by weight in relation to zinc aluminate oxide. What is obtained is baked in the oven for two hours at 110 ° C and then calcined for eight hours at 800 ° C (at an increment rate of 5 ° C per minute) The specific surface area of the product obtained in this way is equivalent to 90 m2 / gr. EXAMPLE 7 The process as in Example 1 is carried out in order to prepare zinc aluminates with different Zn / Al indexes.

They use the same raw materials, in necessary quantities. In the case of the products from 7-1 to 7-3, 28 is the molar ratio of H20 / aluminum alkoxide. In the case of products 7-4 to 7-6, it is 4. Below are the characteristics of the products prepared.

EXAMPLE 8 In this example, the products obtained in the previous examples are examined in order to evaluate their catalytic performance. A quartz reactor is charged with 0.2 gr. of the catalyst powder. The powder used was previously compacted and then cemented and sifted in order to separate the particles with a size range between 0.125 and 0.250 mm. The reaction mixture that is incorporated into the reactor is formed in the following manner (by volume): NO = 300 ppmv C3H6 = 150 ppmv or 450 ppmv C3H8 = 150 ppmv or 450 ppmv CO = 350 ppmv - 02 = 10% C02 = 10% H20 = 10% N2 = qsp 100% The general fluid rate is 30 Nl / h . The HSV is approximately 200,000 h "1. The HC signals (C3H6 + C3H8), NO and N0X (N0x = NO + N02) are continuously recorded, as well as the reactor temperature.The HC signal is obtained by a Beckman detector. of total HC based on the principle of ionization detection per conductor.The NO and NOx signals are obtained by an N0X analyzer of Ecophysics based on the principle of chemiluminicence: it gives the values of NO, N0X and N02, being calculated the latter of the difference between the NOx and NO signals.The catalytic activity is measured from the HC, NO and NOx signals as a function of temperature during a controlled temperature increase of 150 to 700 ° at a rate of 15 ° C per minute and from the following equations: The degree of conversion of NO (TNO) in%, which is obtained by: TNO = 100 (NO0 -NO) NO0 where NO0 is the NO signal at time t = 0, which corresponds to the signal obtained with the reaction mixture that passes at raves of the catalytic reactor and is NOT the NO signal at time t. The degree of conversion of HC (THC) in%, which is obtained by: THC = 100 (NO0 - HC) / NO ° where NO0 is the HC signal at time t = 0, which corresponds to the HC signal obtained with the reaction mixture passing through the catalytic reactor and HC is the HC signal at time t. The degree of conversion of NOx (TNOx) in%, which is obtained by: TNOx = 100 (NOx ° - N0x) / NOx ° where NOx ° is the NOx signal at time t = 0, which corresponds to the Nox signal obtained with the reaction mixture passing through the catalytic reactor and Nox is the Nox signal at time t.

Table 1 shows the results obtained with the product of Example 1 calcined eight hours at 800 ° and for a reaction mixture in which NO = 300 ppmv, C3H6 = C3H8 = 150 ppmv, ie an HCi / NO ratio of 3 (HCi which is expressed as the carbon number, in this case 6 x 150/300).

Table 1 Table 2 shows the results obtained with the product of Example 1 calcined eight hours at 800 ° and for a reaction mixture in which NO = 300 ppmv, C3HS = C3H8 = 450 ppmv, that is, an HCi / NO index of 9.

Table 2 Table 3 shows the results obtained with the product of Example 1 calcined for two hours at 900 ° and for the same reaction mixture of Example 1, ie an index Table 3 Table 4 shows the results obtained with the product of Example 1 calcined two hours at 1000 ° and for the same reaction mixture of example 1, that is, an index of Table 4 Table 5 shows the results obtained with the product of Example 2 calcined for two hours at 800 ° and for the same reaction mixture as the first case of the product of example 1, that is, an HCi / NO index of 3.

Table 5 Table 6 shows the results obtained with the product of Example 2 calcined for two hours at 800 ° and for a reaction mixture in which NO = 300 ppmv, C3HS = C3H8 = 450 ppmv, ie, an HCi / NO index of 9.

Table 6 Table 7 shows the results obtained with the product of Example 7 calcined for two hours at 800 ° and for a reaction mixture with an HCi / NO index of 3. Furthermore, the product is used in an amount of 100 mg. of powder with 100 mgr. of SiC.

Table 7 Table 8 shows the maximum values in% of NOx obtained, and their corresponding temperature, with the products of Example 8 that are calcined two hours at 800 ° C and for a reaction mixture with a HC? / NO index of 3. In addition, the products are used in the amount of 100 mgr. of powder with 100 mgr. of SiC.

Table 8

Claims (19)

1. Zinc aluminate is characterized by having a specific surface area of at least 85 m2 / g, after having been calcined at 800 ° C for eight hours.

2. The zinc aluminate according to claim 1 has a specific surface area of at least 100 m2 / g. , after calcination for eight hours at 800 ° C.

3. The characteristic of zinc aluminate according to any of the two aforementioned claims, is to count, after calcination for two hours at 900 ° C, with a specific surface area of at least 70 m / g. , more specifically at least 80 m2 / gr.

4. The zinc aluminate according to any of the preceding claims, is characterized by having, after calcination of six hours at 1000 ° C, a specific surface area of at least 50 m2 / g, more in concrete 70 m2 / gr.

5. The characteristic of zinc aluminate according to any of the above-mentioned claims is to have, after calcination at 1000 ° C for six hours, in a medium of H20 / N2 with 10% H0 by volume, an area of Specific surface area of at least 50 m2 / gr.

6. The zinc aluminate according to one of the claims described above, is characterized by incorporating at least one additive of the elements of Groups IA, IIA, VIIA to IB of the Periodic Table, as well as between tin, the gallium and the rare earths.

7. The precursor composition for a zinc aluminate, is characterized by containing zinc and aluminate compounds, as well as by its ability to form, after being calcined, a zinc aluminate, which has a specific surface area of at least 85 m2 / gr. , after calcination for eight hours at 800 ° C.

8. The composition according to claim 7, characterized by its ability to form an aluminate with a specific surface area of at least 90 m2 / g. , more specifically of at least 100 m2 / gr., after the calcination of eight hours at 800 ° C.

9. The composition according to claim 7 or 8 also incorporates at least one additive which is chosen from the elements of Groups IA, IIA, VIIA to IB of the Periodic Table, as well as from tin, gallium. and the rare earths.

10. The process for preparing an aluminate according to any one of claims 1 to 6 or of a precursor composition according to one of claims 7 to 9, is characterized by having the following steps: - a salt, sol or zinc alkoxide plus an aluminum alkoxide result in a solvent medium, optionally with salt, sol or alkoxide of at least one of the aforementioned additives; - the mixture obtained in this way is hydrolyzed by adding water in a greater quantity in relation to the aluminum alkoxide; - the formed precipitate is recovered and optionally dried, thus obtaining the precursor composition; if necessary, this precipitate is calcined, thus obtaining aluminate.

11. The process of preparing an aluminate with an additive according to claim 6 or a precursor composition with a compound of an element according to claim 9, characterized by containing the following steps: a salt, sol or zinc alkoxide plus an aluminum alkoxide result in a solvent medium; - the mixture obtained in this way is hydrolyzed by adding water in a greater quantity in relation to the aluminum alkoxide; - the precipitate formed is recovered and, if desired, dried, thereby obtaining the precursor composition; if necessary, this precipitate is calcined, thus obtaining aluminate; - the precursor composition or the aluminate is impregnated with a solution of a salt of the additive or of the aforementioned element.

12. The process according to claim 10 or 11, characterized by an alcoholic solvent is used as a solvent medium.

13. The main feature of the process according to claim 10, 11 or 12 is that the water that is added is in a mixture of water and alcohol.

14. The main feature of the process according to one of claims 10 to 13 is that the zinc salt and the aluminum alkoxide are obtained jointly by adding the zinc salt in the solvent medium to the aluminum alkoxide .

15. The main feature of the process according to one of claims 10 to 14 is that the precipitate is calcined at a temperature of at least 500 ° C.

16. The main characteristic of the process for the treatment of gases in order to reduce the emissions of nitrogen oxides is that the catalyst system uses an aluminate according to claims 1 to 6.

17. The main characteristic of the process for the treatment of exhaust gases of automotive vehicles is that the catalytic system uses an aluminate according to one of the claims from 1 to 6.

18. a main characteristic of the process for the treatment of exhaust gases. automotive vehicles is that the catalytic system uses an aluminate according to one of claims 1 to 6, gases with a high content of gases.

19. The catalytic system for the application of a process according to claims 16, 17 or 18 is characterized by containing an aluminate according to one of claims 1 to 6 about a substrate. The use of an aluminate according to one of claims 1 to 6 or of a precursor composition according to one of claims 7 to 9 for the production of a catalyst system according to claim 1. 19