DE2812875C2 - - Google Patents

Description

The invention relates to a crystalline α- alumina monohydrate of the formula Al₂O₃ · x H₂O, wherein x is greater than 1 and less than 2, a process for the preparation of this α- alumina monohydrate and its use for the production of automotive exhaust catalysts.

From DE-OS 15 92 108 is a process for the preparation of toner gels known in which an alkali aluminate solution with an aluminum sulfate solution as precipitant is mixed. The previously known method is carried out at room temperature and gives a product, which is not a boehmite pseudo-intermediate form but is probably pseudoboehmite.

Catalysts usually consist of a carrier and a catalytic on-carrier Material, wherein the carrier is usually made of particles of a porous material, eg. B. alumina exists. The particles typically have a size corresponding to spherical Particles with a diameter of 1 mm to to 15 mm, so that the support material optionally in corresponding particle shape, z. B. spheres, spheroids, Pills and cylinders must be formed. The activity, the efficiency, the stability and durability of a Catalysts are dependent on the structural properties the carrier material and the particles formed therefrom as well as the nature and distribution of the catalytic substances on the carrier particles. Accordingly, it is desirable that positively affect the catalytic activity Properties of the carrier material by the Shaping to particles not impaired but possible completely preserved.  

The initial porous structure of the catalyst particles and whose starting material is decisive for the size and the absorption capacity for the contact of the catalyst material and the reaction components available surface. By larger pore diameter to achieve better diffusion rate for the reaction components and the products to the catalyst particles and away from them, which is often too leads to improved catalyst activity. However, the extent to which the pore size can be increased advantageously limited. As the pore size increases, so does the surface on which reactions can take place. With a good catalyst, there should be balance between high specific surface, total pore volume and macroporosity. High macroporosity means a pore size distribution, in the a relatively high percentage of pores with a diameter of more than 1000Å is present. Alumina and formed alumina with less Density and correspondingly low thermal insulation leads to a Catalyst that gets to reaction temperature faster.  

A particularly useful catalyst support material is alumina because it is naturally high in porosity has and a comparatively high surface in the temperature range where most normally catalytic reactions take place. If however, during long periods of high temperature Alumina may be due to overheating sinter and change its crystal structure. This will the catalytic activity, for example because of catalytic reactions available surface lost, sink.

Because the physical and chemical properties of Alumina largely from the process measures are dependent on the production, are already numerous Manufacturing process has been developed with which tried which was for use as a catalyst support material to optimize the required properties. alumina is mostly by combination of a water-soluble acidic aluminum compound, for example an aluminum salt, such as aluminum sulfate, aluminum nitrate or aluminum chloride, with an alkali aluminate, such as Sodium or potassium aluminate precipitated (see z. B. US-PS 29 88 520, US-PS 38 64 461, US-PS 35 20 654, US-PS 30 32 514 and US-PS 31 24 418). However, they were So far, the properties of the resulting products after washing and drying usually in one or other direction, for example, in terms of high Surface, macroporosity, phase stability or lower Dense, unsatisfactory.  

In a process for the formation of larger particles Alumina should not only be the shaped alumina the surface properties, porosity and density properties of the starting alumina material, but also low shrinkage and high abrasion resistance and have good crushing strength. Usual carrier materials low density usually have insufficient structural strength. If they are not stabilized, subject Alumina particles of considerable shrinkage and Change in geometric volume when during the Are exposed to high temperatures. By too strong Shrinkage produces unfilled channels in the catalyst bed, through which the reaction components can flow, without getting in contact with the catalyst.

High abrasion resistance ensures structural stability and retention the activity under high mechanical stress. During transport, filling in the reaction zone and prolonged use, catalyst particles are numerous Subjected to collisions, and thereby arises in the outer layers material loss. Attrition of catalytic active layer, which is in the outer volume range of the particles  is present, affects the catalytic performance and also causes the volume of material in the Reaction zone is lower. Volume loss due to shrinkage and / or attrition in high-density close-fitting Particles in a fixed bed catalyst can cause the particles are loosened and an increased movement and more collisions are possible by vibration. When a tightly packed bed loosens, the abrasion decreases still too. During storage, the catalyst is often in packed large tower-like containers. The catalyst must, so that it is the force caused by the weight of the particles to resist, to have a high crushing strength.

Catalysts are used to convert polluting Car exhaust used to less disturbing substances. When Mainly catalytically active components can be precious metals use or in small quantities for acceleration the activity of the base metal systems admit. In the US PS 31 89 563 and 39 32 309 are noble metal catalysts for Control of car exhaust emission described. The US PS 34 55 843 may serve as an example of a base metal catalyst, which is formed with noble metal as a promoter serve. Not For example, accelerated base metal catalysts are disclosed in US-PS 33 22 491.  

A common disadvantage of catalytic converters is their decreasing activity at high temperatures, mechanical Vibration and poisoning by lead present in the exhaust gas, Phosphorus and sulfur compounds after prolonged service life in the order of about 80,000 kilometers. An effective one Catalyst should be active against poisoning, Change of the crystalline phase and structure degradation by resistance retained against noble metal crystallite growth.

An optimal high-temperature alumina Catalyst support should be as low density and high Have porosity and at the same time its surface substantially maintain and good properties and abrasion resistance respectively. He should also be considering his crystalline phases and its geometric volume resistant his. There are difficulties, a suitable combination to achieve these partly conflicting properties and so impregnating an aluminum support material, that creates a catalyst that has the ability has to detoxify car exhaust emissions as much as it currently has prescribe existing official regulations.  

The invention therefore an object of the invention find material suitable as a catalyst support, that the disadvantages of the catalyst carrier of the state the technique avoids or the above stated aspired properties closer than these.

To achieve this object, a crystalline α alumina monohydrate of the type mentioned is suggested, which is characterized in that it has a microcrystalline boehmite pseudoboehmite intermediate form with a [020] - d -Netzebenenabstand of 6.2 to 6.5 Å and a and that after a one-hour thermal treatment at 1010 ° C, the X-ray diffraction spectrum of R- alumina and a pore volume of the pores having a diameter of 10 to 600 Å from 0.60 to 0.75 cc / g.

The invention further provides the process according to claim 2 for the production of the α- alumina monohydrate according to the invention and its use for the production of automotive exhaust catalysts according to claim 3.

The process according to the invention is reproducible and results in crystalline α- alumina monohydrate, from which process impurities can be easily removed by washing with water and filtering. As a result of the control of temperature, reaction time, rate of addition, concentrations and pH values, an α- alumina monohydrate is obtained which has a substantially pure pseudoboehmite-boehmite microcrystalline intermediate structure and in which about 70 to 85% by weight, based on the total amount of existing Al₂O₃, in crystalline form.

The process for the preparation of the inventive Alumina monohydrate comprises five phases. In Phase I is the formation of boehmite seed crystals at acidic pHs in very dilute aqueous Media; this phase is called the nucleation phase.

In phase II, which is the main phase, precipitation occurs and crystallization of alumina at a pH of 7 to 8. During this phase crystallites grow boehmite or pseudoboehmite precipitated from the aqueous Alumina on the seed crystals. The phase II will referred to as precipitation and crystallite growth phase.

In phase III, the pH of the system is increased by addition changed an alkaline solution and thus the electrical Reduced surface charge on the alumina precipitation. During this phase, the positive charge of the Gradually remove alumina particles until they are at a pH from 9.4 to 9.6 becomes substantially zero. In this condition the alumina precipitation is considered to be isoelectric. This is the point in the pH range where the surface is practical shows no electrical charge. For this reason, called the Phase III as a reduction phase for the electrical charging of the surface.  

In phase IV, the aging of the system takes place advantageously during a predetermined period of time, and in the phase V the resulting slurry is filtered and washed and unwanted electrolytes and impurities removed.

Advantageously, the washed in a last stage of the process Filter cake dried to a powdery material. This can be used with or without incorporation of special additives that are used to Reduction of the adsorption of impurities provided can happen.

The reaction components which are used to carry out the inventive Used are water-soluble aluminum salts, such as aluminum sulfate, aluminum nitrate and aluminum chloride, and an alkali aluminate such as sodium aluminate and Potassium. In specific embodiments if one sets as reaction components for cost reasons, because they are easily available and because they are the ones you want Properties in carrying out the method according to the invention have aluminum sulfate and sodium aluminate. you uses the reaction components in the form of aqueous Solutions. The aluminum sulfate is preferably used in high concentrations of about 6 to about 8 wt .-% of Al₂O₃ equivalents.  

As a sodium aluminate solution should advantageously a relative freshly prepared solution can be used in the no Precipitation or undissolved alumina is present. you can sodium aluminate by its purity and its concentration to characterize alumina equivalents. These equivalent Al₂O₃ concentration is 18 to 22 wt .-%.

In addition, the should Alkaline content, for example as Na₂O equivalents, so sufficient be high, that the alumina completely safe is brought into solution. The sodium aluminate should be a Na₂O to Al₂O₃ molar ratio of more than about 1.2, preferably more as about 1.35. Of course, should be economic and practical reasons, the upper limit of the molar ratio not too high; for an economically working practical Process control will therefore not the molar ratio higher than about 1.5. Impurities, one not sufficient cause high alkali content and too high dilutions, that the sodium aluminate becomes unstable.

Before the start of the process, the reaction solutions on a temperature between 54 and 72 ° C, preferably heated to about 60 ° C. The reaction is started with the nucleation phase, in the first filling of deionized Water is added to a suitable reaction vessel. The water is stirred and heated to a temperature of 60 to  Heated to 77 ° C. In general, the temperature is the Fill at a value of about 1.5 to 3 ° C below the Final temperature up to which the reaction is to be conducted.

A first batch of aluminum sulfate will be sufficient in one small amount of water added to the pH of the mixture to a value between 2 and 5, preferably between about 3 and about 4 is set. To This time should be the concentration of alumina equivalents in the mixture about 0.1 wt .-%, preferably about 0.05 wt .-% does not exceed. If you are that way very much low concentration, low pH and high temperature combined, the result is partial hydrolysis of aluminum sulfate and the simultaneous formation of extremely small boehmite crystallites. This nucleation process is going very quickly, and the beginning of Phase II is set soon after the first addition of aluminum sulfate. However, it is advantageous, up to 10 minutes, preferably about 5 minutes, wait to be sure that the nucleation phase has properly expired and the system with a sufficient amount of seed crystals is supplied.

The second phase of the process is carried out so that the Mixture of the water filling and the seeds contained therein simultaneously the sodium aluminate and the aluminum sulphate  Adds reaction components. The solutions will be simultaneous in the form of separate streams with fixed and essentially constant additional speeds in the Reactor initiated; This produces the alumina precipitation and forms an alumina slurry. Because the reaction at a pH between 7 and 8 are performed must, is the rate of addition of one of the two expediently Reaction components during the process so controlled, that the desired pH range of the slurry is reached quickly becomes.

During this phase, the pH rapidly increases from 2 to 5 to 7 to 8. To the extent that the reaction components added to the initial filling increases the alumina concentration in the resulting slurry gradually. Under the specified conditions, the Aluminiumoxidausfällung to, in crystalline alumina crystallize a boehmite pseudo-boehmite intermediate form. If the precipitation rate the crystallization rate exceeds the specified conditions, remains in the precipitate excess hydrous Alumina as alumina gel. The inventively prepared Alumina is characterized by a balance between the amount of crystalline boehmite pseudoboehmite Intermediate form and the amount of gel. This requires that  the precipitation rate, that of the rate of addition depends on the reaction components, the rate of crystallization of hydrous alumina to boehmite Pseudoboehmite intermediate form by a certain set amount exceeds. The pH and temperature of the slurry and the Additional rates of the reaction components are during the precipitate is held so that a crystalline boehmite Pseudoböhmit- Forms intermediate form. As the crystallization rate primarily determined by the temperature of the system is, the additional rates of the reaction components vary depending on the specific temperature at which the reaction is carried out. At low operating temperatures the rate of crystallization is relatively slow and As a result, the addition of the reaction components with a slow speed.

For example, the additional speed is in phase II at temperatures in the range of 60 to about 65 ° C for 60 minutes and is preferably more than 70 minutes.

On the other hand, if in the upper region of the reaction temperature is working, for example at about 75 up to 85 ° C, the rate of addition significantly increased be so that the phase II in shorter times, for example in about 15 to about 30 minutes can be performed. at  a preferred precipitation reaction is the temperature between about 65 and 77 ° C and the precipitation rate should be adjusted so that the reaction within a period of about 30 to 70 minutes, preferably within 40 to 60 minutes.

In a specific embodiment of the invention Process is carried out at 68.5 to 74 ° C, and thereby the reaction components over a period of time from about 48 to 52 minutes with substantially constant Feed rate throughout the precipitation phase added and the relative inflow will, if necessary, adjusted so that programmed pH values are reached as follows become:

Time in minutes from the beginning of Phase II pH range of the slurry 5 ± 2 7.0-7.4 10 ± 2 7.2-7.5 20 ± 2 7.3-7.5 50 ± 2 7.35-7.45

In general, since the reaction is exothermic, sufficient heat will be released to maintain the temperature at the desired level provided that the water fill and reaction components are preheated as described. If there is difficulty maintaining the desired temperature, external cooling or heating can provide the necessary temperature control. Conducting the precipitation reaction under the conditions described above ensures the formation of such an alumina precipitate having the desired balance between crystalline α- alumina monohydrate (boehmite pseudo-boehmite intermediate form) and gel.

At the end of phase II, the concentration is as Al₂O₃ equivalents the slurry in the range of 5 to 9 wt .-%, preferably at about 6 to about 8 wt .-%. The material balance between the reaction components added to this point should be set so that the ratio of sodium, expressed as moles of Na₂O, to sulfate ions as mole H₂SO₄ equivalent, more than 0.80, preferably more 0.88, but less than 0.97.

After completion of Phase II, after the desired amount of Alumina is precipitated, the inflow of aluminum sulfate off. Then the phase III is carried out and the electric charge on the surface of the precipitated Alumina of strongly positive charge to zero or possibly reduced to negative charge. this happens in such a way that the pH of the at the end of phase II existing value from 7 to 8 close to a value  or slightly above the isoelectric point for alumina, which is somewhere between 9.4 and 9.6, is brought. In general, the pH of the slurry is adjusted 9.5 to 10.5, preferably about 9.6 to about 10.0.

The pH change can be made by adding a strong alkali solution such as sodium hydroxide. For practical reasons, however, it is expedient to use the initially used sodium aluminate reaction component solution as an alkaline solution. An additional amount of alumina is then added to the slurry as the pH changes, thereby improving yield and reducing costs. Accordingly, the process according to the invention in phase III is preferably carried out by further adding sodium aluminate, but at a reduced rate of addition. This ensures that the final pH is not exceeded and that there is no localized over-concentration of sodium aluminate due to the undesirable precipitation of alumina under high pH and hence the formation of undesirable phases such as bayerite ( β -alumina trihydrate). could take place. During phase III, as during phases I and II, the temperature of the slurry is maintained and stirred constantly. The sodium aluminate is first fed continuously at a slow addition rate until the pH has risen to 9.5 to about 10.0; then sodium aluminate is added discontinuously as necessary to reach the final pH. During this phase, the sulfation fixed to the surface of the alumina particles becomes free as the positive charge of the alumina surface is reduced to zero or slightly negatively adjusted. When a pH of 9.5 to 10.5, preferably a pH of about 9.6 to about 10.0 has been reached, the inflow of sodium aluminate is interrupted. Phase III can be as short as 4 minutes or as long as 20 minutes over a long period of time. Advantageously, they are carried out within a period of about 6 to about 12 minutes.

In the phase IV one can, after all reaction components were added, the slurry for several hours aging. Generally aging one for about 30 minutes. If during Phase III large batch of alumina slurry has been prepared, which one can not filter in a single approach, but continuously during a period of time is subject to a part of the material necessarily undergoes aging, while it is kept ready for the filtration treatment.  

The alumina slurry is then filtered, and then the filter cake is washed; in this way become unwanted Impurities eliminated. As a washing liquid it is advantageous to use deionized water; leave it with that remove the water-soluble impurities. It is impractical, impurities such as calcium, Containing magnesium, chloride, carbonate or bicarbonate To use water. However, depending on the end use purpose for the alumina, some of these impurities, such as volatile impurities, are tolerated. Metallic impurities, such as calcium, Magnesium, iron, silicon and nickel can not, at most in extremely low concentrations, be tolerated.

The term "essentially pure," as used in the present Description used refers to Alumina containing impurities on a dry basis, does not exceed the following upper limits: Sodium, expressed as Na₂O, 0.15 wt .-%, calcium, given as CaO, 0.15% by weight; Magnesium, indicated as MgO, 0.15% by weight; Silicon, reported as SiO, 0.80 wt .-%; Iron, given as Fe₂O₃, 0.07 wt .-%; Nickel, indicated as NiO, 0.07 wt .-%.  

You work with deionized water to make it pure Alumina product. The deionized water becomes usually heated, causing the removal of electrolytes is facilitated; It can reach a temperature of about 50 to 80 ° C have. Electrolytes can be due to the crystalline Nature of precipitation relatively easy to remove by washing. Crystalline material has less tendency to contaminants include, and the washout is due to the improved Filtration properties easier. The amount of deionized water, which is needed to make a product good Achieving quality depends on the specific used Filtration apparatus. In general, however, 0.453 kg each to Al₂O₃ (dry basis) in the filter cake water in the Range from 9 to 45 kg used.  

In a preferred embodiment of the invention Process, the filter cake to an alumina powder dried, which, without it decomposes, comfortably over longer Time can be stored before it is processed further. The drying of the filter cake can be done in various ways, for example, on hordes or on the conveyor belt. In a particularly advantageous However, you give the filter cake water to work to bring it into a pumpable slurry and then uses a method with which the water quickly can be removed again, for example, spray drying or evaporation drying. In these cases, the pumpable alumina slurry about 15% to about 20% by weight on solids. The slurry is finely divided Droplets are sprayed through a spray nozzle into a drying chamber. The droplets come into contact with hot drying gases. For example, the spray dryer inlet temperature of the Drying gases may be adjusted to about 430 to about 370 ° C. The feed rate of the slurry is adjusted that an outlet temperature of more than about 120 ° C, however, does not exceed about 200 ° C and expedient between about 150 and about 180 ° C is reached. If you are among such Working conditions, it is ensured that the partial removal of water without destroying the crystalline Structure of the alumina takes place.  

If oil combustion products are used as drying gases, These gases can produce significant levels of carbon dioxide contain. When carbon dioxide with the alkaline Slurry comes into contact, it is absorbed and can possibly chemically react on the surface. In such Cases then contains the final product small amounts of carbon dioxide as an impurity. However, this carbon dioxide has in In most of the following stages no greater importance. If it However, it is necessary, the carbon dioxide uptake can be by reducing the pumpable slurry with Trace amounts of acid to a pH of 7 or less acidified. For this one uses preferably thermally decomposable organic acids such as acetic acid and formic acid. Mineral acids may possibly be considered impurities be held, and thereby the process can be used later Process steps to be impaired. In the case of the use of nitric acid become unwanted Pollution causing components received.

During spray drying, depending on the specific working conditions, a subset of the gel fraction in the precipitate crystallize to boehmite pseudo-boehmite intermediate form. The spray-dried Product is not completely dry but contains still a certain amount of water. Generally located less than about 18% by weight in the spray dried product.  Water; however, the amount of water may increase up to about 33% by weight. Advantageously, the water content should be between about 20 and about 28 wt .-% are.

The alumina used in the invention is an alumina material, the Al₂O₃, water of hydration and Anhängendes Contains water. The degree of dryness of the Aluminum oxide may be present in the form of the weight percentages Proportion of Al₂O₃ be specified. It is assumed, the 100% Al₂O₃ are produced when three hours long at 650 ° C has been dried.

The partially dried hydrous aluminum oxide formed by the controlled reaction of sodium aluminate and aluminum sulfate is an intermediate between boehmite and pseudoboehmite. This form of alumina is α- alumina monohydrate with additional water molecules trapped in the crystal structure; it corresponds to the formula Al₂O₃ · x H₂O, wherein x is greater than 1 and less than 2. The boehmite pseudo-boehmite structure of the product, including the crystal structure, the degree of crystallization and the average size of the individual crystallites can be determined by X-ray diffraction interruption.

Pseudoboehmite is described by Papee, Tertian and Biais in whose work "Research on the Constitution of the Gels et des  Hydrates Cristallises d'Alumine ", published in Bulletin de la Societe Chimique de France 1958, pages 1301-1310.

Boehmite is well-defined and well-known for many years Mineral, its crystalline structure and X-ray diffraction pattern are known (see ASTM Card No. 5-0190).

Other characteristic of the product according to the invention Properties are: Its behavior in case of aging under alkaline Conditions; its ability to produce anions, such as sulfates, chemically absorb at different pH's; his crystalline nature and stability after treatments under strong Heat; and the high temperature stability of its surface, Pore volume and pore size distribution.

All the above properties are based on the particular Balance between the crystalline component and the amorphous gel component in the product, as well as its excellent overall chemical purity.  

The degree of crystallinity is checked by X-ray diffraction. The images are compared with the data given in the ASTM chart no. 5-0190 for boehmite. If all the diffraction maxima coincide, this is an indication that the product is boehmite. However, if the [020] -d grid level spacing is shifted from 6.6 to 6.7 Å, while the remaining maxima have remained substantially unchanged, this is an indication that pseudo-boehmite is present. Intermediate values of 6.2 to 6.5 Å indicate that the material is an intermediate form. By comparing the area below the [020] maximum with the corresponding area in the X-ray diffraction spectrum of a material with a known high content of boehmite, which is defined as "100% boehmite", the degree of crystallinity of the examined material can be determined beyond that.

For a particular diffraction peak, the average crystallite size is inversely proportional to the width of the maximum at half its maximum intensity. Relative measurements can easily be made by measuring the width at half maximum intensity of the [020] diffraction maximum. Large values of width correspond to small crystallite sizes, while small values of width correspond to large crystallite sizes. For example, 100% crystalline α- alumina monohydrate obtained by dehydration of well-defined large crystals of α- alumina trihydrate gives very small and narrow diffraction peaks. In this material, the [020] reflection appears at 6.1 Å, an indication that the product is boehmite rather than pseudo-boehmite, and the half maximum intensity width is only about 0.2 Å Sign of the presence of large crystallites.

In contrast, when it comes to microcrystalline Pseudoboehmite, the [020] maximum at values in the range from 6.6 to 6.7 Å. This material shows wider maxima Values for the half maximum intensity of 2 Å or more.  

The product according to the invention results in a boehmite pseudoboehmite intermediate molded structure which is characterized by a [020] - d -Netzebenenabstand in the range of 6.2 to 6.5 Å, preferably about 6.3 to about 6.4 Å. The half maximal intensity width of the [020] maximum is between 1.65 to 1.85 Å, preferably about 1.75 to about 1.80 Å. This indicates a small crystallite size, that is, a crystallite size closer to the small size of pseudoboehmite than the large size of boehmite. The crystallite size is a factor that determines the microporosity of the support material formed from the new crystalline alumina and thus the behavior of the catalyst.

Expressed as relative crystallinity are in the inventive Product about 70 to about 85 wt .-% of the total amount Al₂O₃ in crystalline form. The boehmite Pseudoboehmite product is characterized by high crystalline Purity, small crystallite sizes, that is microcrystallinity, and a high relative crystallinity content. Regarding These properties make the product according to the invention unique and this is due to the manufacturing conditions, with which is a high ratio of crystalline material to  amorphous gel made. As a result, the invention differ Products from the previously known materials, in the proportion of amorphous gel in the product is either very high or not at all, such as at Boehmite.

The nature of the balance between crystalline and amorphous Product contents in the product according to the invention leaves by converting the gel components into undesirable crystalline phases, such as bayerite, and the Chemisorption of anions at the surface at characterize different pH values even further.

Amorphous anhydrous alumina products tend to crystallize. The crystalline phase obtained in a particular case depends on the nature of the environment around the alumina during crystallization. A product consisting of boehmite and pseudoboehmite containing a high proportion of gel components tends to crystallize as β- trihydrate (bayerite) when exposed to elevated temperatures in an aqueous alkaline environment for a prolonged period. In contrast, from a material containing little or no gel components, no crystalline bayerite phase is formed under similar alkaline aging conditions. For example, when aging a low oxide alumina primarily of pseudoboehmite with high levels of gel dispersed therein for at least about 18 hours at about 50 ° C in a high pH aqueous sodium hydroxide solution such as 10, bayerite forms while the product otherwise remains essentially unchanged in its crystalline structure. This is an indication that the formation of bayrite does not occur at the expense or by remodeling of pseudo-boehmite, but that the bayerite is formed from the amorphous alumina gel phase. In contrast, when the product of the present invention is treated under the same conditions, there is no presence of bayerite. This is an indication that the amount of gel in the product according to the invention is either very low or much more stable.

For the anion chemisorption test, a slurry of the to be examined alumina powder in deionized water prepared, and this slurry is diluted with sulfuric acid known normality over a pH range in which the Alumina is insoluble, titrated by potentiometric. you Titrate slowly, ensuring that there is enough time for the penetration of the acid into the structure of the alumina product is available. About the issues in question  pH ranges from about 9 to about 4, alumina is insoluble, so that the titration with sulfuric acid as a measure of the Amount of sulfate, which at the surface of the alumina at be held at a given pH or chemisorbed can, serves. For different aluminas is the one to reach a certain pH of one usual initial value from required amount of acid an indirect Measure of the amount of alumina intermediate surface, which is exposed to the aqueous medium. Materials with a lot high degree of crystallinity and very large crystal dimensions have small interface areas and therefore require low levels of acid to change a certain pH achieve. In contrast, materials have very high Gel content a high intermediate surface area and require correspondingly large amounts of acid to the same pH change to achieve. The products with crystal in between Gel structure require medium amounts of acid for the equal pH change.

For example, when 100% of crystalline α- alumina monohydrate consisting of well-defined large crystallites is present, only about 53 milliequivalents of sulfuric acid per mole of alumina is needed to bring the pH from an initial value of about 8.3 to a final value of about 4 to change. In contrast, alumina produced at low temperatures and having a pseudoboehmite structure,% of crystallinity, and crystallite size indicates a low degree of crystallinity and a high content of gel requires about 219 milliequivalents of sulfuric acid per mole of alumina for the same pH change.

Compositions according to the invention are characterized that mean amounts of sulfuric acid for the pH change required are. With about 130 to 180 milliequivalents, preferably about 140 to 160 milliequivalents of sulfuric acid per mole of alumina, the pH of a slurry can be a composition of the invention from about 8.3 to about 4.0 change.

For the use of the invention Product for the production of suitable carrier material for automotive gas There must be good stability of the catalysts have structural properties at elevated temperatures. For example, pore volume and surface should be when they are after strong heat treatment, with a catalyst stress is simulated during use, it is determined keep their high value and be stable. These properties at high temperatures depend largely on the purity the material used and its structural nature as well as the crystalline nature of the product after heat treatment.  

The product of the present invention, when heated, gradually loses its water of hydration and otherwise adhered or attached thereto. This dehydration causes a transformation of the crystalline structure to q -alumina. Upon further heating to higher temperatures, the γ- alumina is converted to δ and optionally to R -alumina. All of these alumina structures are transitional aluminas with high surface area and pore volume. If heated even higher, α- alumina or corundum, which is not a transitional alumina, is formed and has a very small surface area and pore volume. The final conversion to α- alumina is so intrusive that there is a very large decrease in pore volume and surface area associated with this transformation. A good alumina powder capable of conversion to automotive exhaust catalysts should be thermally stable and not convert to a- alumina at fairly high temperatures, such as 980 to 1050 ° C. In general, high gel content aluminas at relatively moderately high temperatures, such as 980 to 1050 ° C, tend to sinter to alpha- alumina. For substances which have been produced at low temperatures and which have a high content of gel, the formation of undesired α- alumina shows when heated to 1010 ° C for one hour.

In contrast, products according to the invention which contain only a very small amount of amorphous gel remain stable under the same heat treatment and show no formation of α- alumina. Inventive compositions have after one hour of heating at 1010 ° C X-ray diffraction patterns of R- alumina, γ- alumina and δ -alumina. Moreover, products made according to this invention retain substantially their surface area and pore volume at these temperatures, and remain stable even when subjected to high heat treatment for extended periods of time.

After one hour of treatment at about 1010 ° C have inventive Products BET nitrogen surfaces from 100 to 150 m² / g, most often from about 110 to about 140 m² / g. They point furthermore, a nitrogen pore volume of 0.60 to 0.75 cc / g, usually from about 0.64 to about 0.72 cc / g.

Furthermore, shows the pore structure of the thus heat treated product no undesirable high proportion of microporosity determined by nitrogen pore size distribution. The invention Product typically has no pore volumes below  about 80 Å, usually not less than 100 Å.

If in the present description of "Nitrogen pore volume" is mentioned, this refers Indication of pore volumes, which according to the in the work of S. Brunauer, P. Emmett and E. Teller, J. Am. Chem. Soc., Vol 60, page 309 (1938). This method is based on the condensation of nitrogen in the pores, and it manages to pores with pore diameters ranging from 10 to 600 Å.

If from surface is meant, the nitrogen BET surface is meant which after the at the aforementioned reference at Brunauer, Emmett and Teller described method has been.  

The α- alumina monohydrate according to the invention is suitable for the production of spherical aluminum oxide particles. This may be done by various known methods, but preferably by a novel method in which the alumina monohydrate of the present invention and an acidic aqueous medium are mixed to form a slurry from which droplets are formed, the droplets to form spherical particles through the air downward passed through a column having an upper portion with a water-immiscible liquid and ammonia and a lower portion of aqueous ammonia, the resulting particles are aged in aqueous ammonia, and the aged particles are dried and calcined (for details, see the equivalent US-PS 41 79 408). The spherical particles produced in this way are particularly suitable for the production of automotive exhaust catalysts.

The α- alumina monohydrate according to the invention which is suitable as a carrier material is distinguished by low density, high degree of macroporosity, high breaking strength, good shrinkage resistance at high temperature, good abrasion resistance and adjustable size and shape.

A catalyst in which the carrier material according to the invention with a catalytically effective amount of at least one catalytic active metal or a metal component impregnated is, shows high catalytic activity in many catalytic  Systems, especially those that operate at high temperatures. Although the alumina itself can be active as a catalyst, However, it is usually with the purpose of strengthening its activity a suitable catalytic component impregnated and activated. The catalytic component, its amount, the impregnating and Activation type will vary depending on the nature of the reaction, for the the catalyst is to be used, selected. A preferred catalytically active component is a platinum group metal, such as platinum, palladium, ruthenium, iridium, rhodium, Osmium or mixtures thereof.  

The use of platinum group metals for automotive exhaust catalysis is due to the limited natural occurrence of these Restricted metals. Since a large proportion of the world's out South Africa comes, and the bulk of the metals platinum, palladium and rhodium is that in nature in proportions of approximately 68 parts platinum, 27 parts palladium and 5 parts Rhodium, most of the work with platinum group Metals are carried out in these proportions. The amount of noble metals in automotive catalytic converters is typically based on the weight of the catalyst, at about 0.005 to 1.00 wt .-%, preferably at about 0.03 to 0.30 wt .-%. The in individual cases optimal metal combination depends on the specific desired performance of the catalyst; for example achieved one with a higher content of palladium a faster one Oxidation of carbon monoxide and better thermal stability, more platinum gives better resistance to catalyst poisons and a prolonged hydrocarbon oxidation, while increased rhodium concentration is the conversion of nitrogen oxides improved to nitrogen. One can If desired, catalysts with only one to form single platinum group metal; usually becomes but you can use several metals of this group. The relationship the metal can vary over a wide range; preferably  However, a catalyst contains platinum and palladium in one Weight ratio of 5 parts to 2 parts or platinum, Palladium and rhodium in an amount of about 68 parts or 27 parts or 5 parts.

There are many different precious metal compounds known and described in the literature. Depending on the type of surface of the carrier material and the interaction with it can also such compounds are used. For example, you can anionic or cationic forms of platinum in the carrier incorporate it by adding H₂PtCl₆ or Pt (NH₃) ₄ (NO₃) ₂ starts.  

It has been found that especially such complex compounds are as described in US-PS 39 32 309. In this procedure, the catalyst is impregnated the support material with sulfite treated platinum and Palladium salt solutions impregnated. When these sulfite complexes on the carrier, they decompose, that a high degree of dispersion is present. The impregnation depth of the metal in the particle can be distinguished by different cationic form of the complex, for example NH₄⁺- or H⁺ shape vary.

It was further found that for high initial activity and, more importantly, for lasting high activity Precious metals should be applied so that about 50% of entire effective precious metal surface at a depth of more than 50 μm. It has also been found that the Precious metals should be at such a depth that 50% of the active metals are located deeper than about 75 microns. The determination The precious metal surface is made by the method by D.E. Mears and R.C. Hansford, J. of Catalysis, Volume 9, 125-134 (1967).  

Catalysts threefold on both carbon monoxide, hydrocarbons and nitrogen oxides are effective because of require the different reducing or oxidizing nature the exhaust gas atmosphere a good resistance. It is still In particular, such a support material necessary that a low Dense and high macroporosity has, because in itself difficult is a good carbon monoxide removal and for a good one General performance required rapid oxydation to be achieved. Rhodium is typically used as a catalytic component for Improving effectiveness used in all three directions. Since rhodium is a rather rare material, you have to Use it with good efficiency and arrange it carefully. Similar to oxidation catalysts have in three directions acting catalysts have a high initial activity, and their performance also stays good when the active ones Metals are arranged in a suitable manner. Regarding the activity of the triple-acting Catalyst using of the aluminum oxide monohydrate according to the invention has been described in the US-PS 41 79 408, the US PS 42 79 779 and the US-PS 43 90 456 described laboratory investigations.  

Example 1

Alumina trihydrate was completely in sodium hydroxide dissolved and so a sodium aluminate solution with 20% Al₂O₃ and a Na₂O / Al₂O₃ molar ratio of 1.4 prepared. 495 g of water were poured into a reaction vessel, and then 631 ml of 50% sodium hydroxide solution added. This volume of sodium hydroxide solution was at the specific gravity of the solution of 1.53 g / cc 966 g. The mixture was stirred gently and heated to 93.3 ° C heated. Over a period of 30 minutes were after and after a total of 672 g of alumina trihydrate added. During the addition of the alumina trihydrate, the Continue to warm mixture until lightly boiled slowly stirred. Then it was another 60 minutes or until the trihydrate was completely dissolved kept at a gentle simmer and stirred. Then the Heat source turned off, and the mixture was stirred cooled to 60 ° C.

By adding 290 g of water at a temperature of 60 ° C and stirring the mixture was the specific gravity and the temperature of the sodium aluminate solution at 1.428 g / cc or 54.4 ° C. 2016 g of the solution were for the Production of the alumina used.  

By dissolving 1373 g of aluminum sulfate crystals in 1963 g of water 2286 g of an aluminum sulfate solution with 7% Al₂O₃, a specific gravity of 1.27 g / cm³ at 25 ° C and a SO₄ ⁼ / Al₂O₃ molar ratio of 3.01 were prepared.

The sodium aluminate solution and the aluminum sulfate solution were heated to 62.8 ° C. In a precipitation tank were 3160 g of water filled, the stirrer was started and the amount of water was heated to 68.4 ° C.

By adding 6 ml of aluminum sulfate at an addition rate of 36 ml / min. became the amount of water acidified to pH 3.5 and aged for 5 minutes. After this aging was with the inflow of sodium aluminate at a rate of 28 ml / min. began. Within 5 seconds was with the inflow of Aluminum sulfate at a rate of 36 ml / min. started, and this addition was made during the 50 minutes long Ausfällphase kept constant. The inflow of Sodium aluminate was adjusted so that the pH of the reaction mixture was kept at 7.4. The precipitation temperature was by heating the Ausfällbehälters kept at 72.8 ° C.  

Within 50 minutes, the total amount of the aluminum sulfate solution was was added, and there remained 317 g of sodium aluminate left.

At the end of the precipitation, the pH of the reaction mixture was raised to 10.0 by addition of another 29 g of sodium aluminate solution. The final molar ratio of Na₂O to SO₄ ⁼ 1.00. The solution was aged for 30 minutes at a constant temperature of 72.8 ° C and thus stabilized.

After aging, the reaction mixture was filtered and washed. For each gram of alumina in the mixture were 50 g wash water used. A standard filtration wash test was done as follows. Reaction slurry (600 ml) was using a filter cloth, at 254 mm vacuum in a diameter of 20.32 cm Pitcher filtered. It was washed with 2.5 l of water. The filtration time was 2.1 minutes, and the Filter cake was 7 mm thick.

The filter cake was reslurried to a slurry with 15% solids content and with an outlet temperature spray dried from 121.1 ° C to a powder with a Total volatile content of 27.5%, determined by loss on ignition at 1010 ° C. The dried powder was calcined for 1 hour at 1010 ° C.  

The properties of the dried and the calcined product are given in Table 1.

Dry powder Wt .-% Na₂O 0.02 Wt .-% SO₄ ⁼ 0.20 % By weight of volatiles 27.5 Particle size of the agglomerate 21.5 μ bulk density 0.386 g / cm³ X-ray phase (Boehmite-pseudo-boehmite intermediate form) no α- or β- trihydrate phases present Maximum for [020] crystallographic network plane falls on a lattice plane distance d of 6.37 Å. Calcined powder at 1010 ° C for 1 hour @ N₂ surface 136 m² / g N₂ pore volume @ <600 Å 0.72 cc / g a total of 0.95 cc / g X-ray phase Thetaaluminum oxide, no α- alumina present Pore size distribution A pore size distribution measurement with nitrogen showed that all pores had a diameter of greater than 100 Å and 50% of the pores had a diameter in the range of 100 to 200 Å.

example 2

In Table 2 below, the results of 13 Experimental approaches according to the process conditions described in Example 1 compiled.

properties average Weight Al₂O₃ / batch (kg) 0,454 Precipitation ratio Na₂O / SO₄ ⁼ 0.93 Standard filtration test (min.) 2.4 Spray dried powder @ Wt .-% Na₂O 0.03 Wt .-% SO₄ ⁼ 0.19 % By weight of volatiles 27.9 Bulk density (g / cm³) 0.384 N₂ surface after 30 minutes at 400 ° C (m² / g) 420 N₂ pore volume after 30 minutes at 400 ° C (cm³ / g) <600 Å 0.82 X-ray structure Boehmite pseudo intermediate form Calcined powder at 1010 ° C for 1 hour @ Surface area (m² / g) 131 Pore volume (cm³ / g) @ a total of 1.01 <600 Å 0.73 X-ray structure Thetaaluminum oxide, no α- alumina present.

example 3

Table 3 below shows the results of in 6 large-scale experimental approaches derived mixed products listed. The process conditions were the same as described in Example 1, with the difference that per process batch 88.5 kg of alumina (Dry basis) were manufactured. The size of the apparatus and the amounts of materials were correspondingly larger.

The results were the same as at the laboratory approaches. This shows that the inventive Process without difficulty in the large-scale Manufacturing can be transferred.

Spray dried powder Wt .-% Na₂O 0.059 Wt .-% SO₄ ⁼ 0.31 Wt .-% CO₂ 1.37 % By weight of volatiles 29.6 Bulk density (g / cm³) 0.481 N₂ surface at 400 ° C (m² / g) 413.0 N₂ pore volume at 400 ° C (cm³ / g) <600 Å 0.77 X-ray phase Boehmite / pseudoboehmite intermediate form At 1010 ° C for 1 hour calcined powder @ N₂ surface (m² / g) 131 N₂ pore volume (cm³ / g) @ a total of 0.97 <600 Å 0.70 X-ray phase Thetaaluminum oxide, no α- alumina present.

example 4

In this example, the washed, filter cake prepared as described in Example 3 treated with acetic acid before spray-drying. The Acetic acid causes a decrease in carbon dioxide absorption during spray-drying. In each approach was the Filter cake glacial acetic acid added until a pH of 6.0 is reached was. The mixture was stirred. The spray-dried Product contained 3.8% acetic acid and 64.5% Alumina. This corresponds to 0.1 moles of acetate each Mole of alumina.

The properties of Example 3 and according to the alumina products prepared in this example are illustrated in Table 4. The carbon dioxide content was 0.76%, while the alumina according to Example 3 1.37%.  

Table 4

example 5

There were the relative proportions of crystalline and amorphous material in the alumina according to the invention examined, and to this were samples of as in Example 1 described alumina and for comparison of Aluminum oxides A and B, which have a lower or a showed a higher degree of crystallinity in deionized water to Al₂O₃ concentrations of 100 g Al₂O₃ (dry basis)  slurried in 1 liter of water. Each approach was made by 1.1 N sulfuric acid at an additional rate of 1 ml / min. over the pH range of 8.3 to 4.0, in the Alumina is insoluble, titrated slowly potentiometrically.

Table 5

The results show that the aluminum oxide Compositions for the same pH change mean Require amounts of acid and consequently had a gel content, which was between those of samples A and B.  

The degree of crystallinity of the alumina compositions of this invention was further demonstrated by X-ray structural measurements of the evolution of β- alumina trihydrate in alkaline aging and heating. 100 g (dry basis) of the alumina A sample shown in Table 5 and the alumina sample prepared as in Example 3 and shown in Table 3 were slurried in 250 ml of deionized water and brought to pH 10 by addition of 1N alkali. The aging times and temperatures and the magnitude of the strong and weak X-ray intensity maxima of the alumina A for β- trihydrate are given in Table 6. Under the same aging and heating conditions as for alumina A, no β- trihydrate was seen in the alumina composition of Example 3.

Table 6

The facile formation of β- trihydrate in Sample A under alkaline conditions indicates that it has a higher gel content therein than in the alumina of Example 3.

Claims (3)

1. Crystalline α alumina monohydrate of the formula Al₂O₃ · x H₂O, where x is greater than 1 and less than 2, characterized in that it has a microcrystalline boehmite pseudoboehmite intermediate form with a [020] - d -Netzebenenabstand from 6.2 to Is 6.5 Å and a half maximum intensity width of the [020] X-ray diffraction peak between 1.65 to 1.85 Å and that after a one hour thermal treatment at 1010 ° C, the X-ray diffraction spectrum of R- alumina and a pore volume of the pores with a Diameter of 10 to 600 Å from 0.60 to 0.75 cm³ / g. 2. A process for the preparation of crystalline α- alumina monohydrate according to claim 1 by combining an alkali aluminate solution and an aluminum salt solution of a mineral acid, followed by washing and drying of the resulting product, characterized in that

• (A) an aqueous solution of an aluminum salt of a mineral acid with an equivalent Al₂O₃ concentration of 5 to 9 wt .-% and a temperature of 54 to 72 ° C to 60 to 77 ° C warm water until the pH of the mixture set to 2 to 5,
• (b) adding to the mixture a further amount of a solution of an aluminum salt of a mineral acid and simultaneously, but separately, an aqueous alkali aluminate solution having an equivalent Al₂O₃ concentration of from 18 to 22% by weight and a temperature of from 54 to 72 ° C; raises the pH of the slurry of precipitated alumina to a value of 7 to 8,
• (c) while maintaining the pH of the slurry from 7 to 8 and the temperature of the slurry from 60 to 85 ° C, adding the solution of the aluminum salt of the mineral acid and the alkali aluminate solution,
• (d) then adjusting the pH of the slurry to 9.5 to 10.5,
• (E) the slurry is filtered, the filter cake is washed out, optionally re-slurried and then dried.

3. Use of the crystalline α- alumina monohydrate according to claim 1 for the preparation of particular platinum, palladium, ruthenium, iridium, rhodium and / or osmium-containing automotive exhaust catalysts.