DE102009054229B4 - A process for preparing a molybdenum mixed oxide precursor and molybdenum mixed oxide precursor obtainable therewith - Google Patents

Abstract

A process for the preparation of a molybdenum mixed oxide precursor, comprising the steps a) of preparing a first solution which contains a molybdenum starting compound and at least one further metal-containing starting compound selected from a vanadium-containing and / or tellurium-containing and / or antimony-containing and / or tungsten-containing starting compound, b) des Preparation of a second solution, which contains a niobium-containing and / or iron-containing and / or bismuth-containing and / or antimony-containing starting compound, c) introducing the first and the second solution into a micromixing reactor, which has a mixing / reaction chamber and a temperature control device, which are located in the Mixing / reaction chamber is located, wherein the first and the second solution in substantially opposite partial streams separately in the mixing / reaction chamber of the micromixing reactor and mixed there, whereby a molybdenum mixed oxide precursor is formed, and d) the off bringing the molybdenum mixed oxide precursor obtained in step c) from the mixing / reaction chamber.

Description

The present invention relates to a process for producing a molybdenum mixed oxide precursor and to the molybdenum mixed oxide precursor obtainable by the process. Further, the present invention relates to methods of making a molybdenum mixed oxide material using the molybdenum mixed oxide precursor.

Molybdenum mixed oxide materials are obtained in the prior art by precipitation methods, sol-gel methods or solid-state reactions. It should be noted that the term "molybdenum mixed oxide" in the context of the present application to the term "molybdenum-containing polyoxometalate" is synonymous.

Precursors for such molybdenum mixed oxide materials, which are generally suspensions of educt compounds or solutions of iso- or heteropoly compounds with various additional atoms (so-called addenda atoms), have hitherto been prepared in the prior art by conventional mixing of educt solutions. For example, a mixed solution of ammonium heptamolybdate tetrahydrate, ammonium metavanadate and telluric acid is mixed with a solution of ammonium niobium oxalate by combining and stirring, e.g. B. mixed with a stirrer. However, these solutions are incompatible with each other, so that when mixing the two solutions partially an inhomogeneous precipitate fails, but also metal ions remain in solution. Furthermore, there may be structural and morphological changes in the inhomogeneous precipitation caused by aging.

Due to this inhomogeneous formation of precipitates and due to aging phenomena arise in the further processing of the precipitate-containing suspension such. As by spray drying, often nonuniform particles and complex phase mixtures, which must be purified by post-treatments (leaching with acid, hydrothermal or supercritical treatments).

The WO 2009/121624 A1 describes a process for producing a nanocrystalline molybdenum mixed oxide having a uniform particle size. This is achieved by first introducing a solution, suspension or slurry containing a molybdenum starting compound and another starting metalloid compound into a reaction chamber by means of a carrier fluid and then subjecting it to a pulsating flow thermal treatment.

From the DE 10 2004 001 852 A1 A method for mixing at least two fluids is known in which the mixing reaction takes place in both exothermic and endothermic processes at a constant, adjustable process temperature. For this purpose, the fluids are introduced as part streams into a mixing / reaction chamber in such a way that they impinge on and flow around a centrally located tempering cylinder, whereby the mixing takes place. By isothermal temperature control and additional tempering the mixing reaction can be controlled.

The object of the present invention was therefore to provide a process by means of which a molybdenum mixed oxide precursor which does not have the abovementioned problems can be obtained in the simplest possible manner.

• a) des Herstellens einer ersten Lösung, die eine Molybdänausgangsverbindung und wenigstens eine weitere metallhaltige Ausgangsverbindung, ausgewählt aus einer vanadiumhaltigen und/oder tellurhaltigen und/oder antimonhaltigen und/oder wolframhaltigen Ausgangsverbindung, enthält,
• b) des Herstellens einer zweiten Lösung, die eine niobhaltige und/oder eisenhaltige und/oder bismuthaltige und/oder antimonhaltige Ausgangsverbindung enthält,
• c) des Einbringens der ersten und der zweiten Lösung in einen Mikromischreaktor, der eine Misch/Reaktionskammer und eine Temperiereinrichtung, die sich in der Misch/Reaktionskammer befindet, aufweist, wobei die erste und die zweite Lösung in im Wesentlichen gegenüber liegenden Lösungsteilströmen getrennt in die Misch/Reaktionskammer des Mikromischreaktors zugeführt und dort gemischt werden, wobei eine Molybdänmischoxidvorstufe gebildet wird, und
• d) des Ausbringens der in Schritt c) erhaltenen Molybdänmischoxidvorstufe aus der Misch/Reaktionskammer.

The object is achieved by a method for producing a Molybdänmischoxidvorstufe, comprising the steps

• a) the preparation of a first solution containing a molybdenum starting compound and at least one further metal-containing starting compound selected from a vanadium-containing and / or tellurium-containing and / or antimony-containing and / or tungsten-containing starting compound,
• b) producing a second solution containing a niobium-containing and / or iron-containing and / or bismuth-containing and / or antimony-containing starting compound,
• c) introducing the first and second solutions into a micromixer reactor having a mixing / reaction chamber and a tempering device located in the mixing / reaction chamber, wherein the first and second solutions are separated into substantially opposite solution substreams Mixed / mixed reaction chamber of the micromixer reactor and mixed there, wherein a Molybdänmischoxidvorstufe is formed, and
• d) applying the molybdenum mixed oxide precursor obtained in step c) from the mixing / reaction chamber.

The residence time of the solutions introduced into the mixing / reaction chamber in step c) of the process according to the invention in the mixing / reaction chamber is preferably less than 1 second, more preferably less than 0.5 seconds and in particular less than 0.25 seconds.

The micromixing reactor used in the process according to the invention can, for. B. be a micromix reactor, as in the DE 10 2004 001 852 A1 is described, wherein the DE 10 2004 001 852 A1 with respect to the disclosure of the micromixing reactor is fully included in the present description.

The micromixer reactor according to the DE 10 2004 001 852 A1 is a micromixing reactor for mixing at least two fluids A and B in substantially opposite partial fluid streams in a mixing / reaction chamber in which a cylindrical temperature control device is preferably located centrally in the mixing / reaction chamber, wherein the cylindrical temperature control preferably a round or having an elliptical cross-section and having a diameter of less than 1 mm in at least one horizontal direction, and wherein the mixing / reaction chamber preferably has a round or elliptical cross section and has a diameter of less than 2 mm in at least one horizontal direction, and wherein outside the mixing / reaction chamber at least one further tempering device is arranged, in which the heating or cooling media are returned to the heating or cooling media again. This micromixing reactor is operated such that the at least two fluids A and B are introduced as partial streams into the mixing / reaction chamber, whereby they impinge on the temperature control cylinder located centrally in the mixing / reaction chamber and flow around it at least partially, the mixing process through this temperature control cylinder and / or controlled by the outside of the mixing / reaction chamber adjacent tempering and the resulting mixture is discharged through an opening in the bottom or in the lid of the mixing / reaction chamber.

The molybdenum starting compound used is preferably a molybdate, preferably an ammonium (mono-, di-, hepta-, para-molybdate, a Mo-containing heteropolyacid, such as, for example, Mo ₁₀ V ₂ (P, Sb, As) O ₄₀ , more preferably a solution of MoO ₃ in H ₂ O ₂ (resulting in unspecified peroxomolybdenum complexes), more preferably ammonium heptamolybdate tetrahydrate is used However, it will be apparent to those skilled in the art that other molybdates and molybdenum compounds known in the art are also used can.

In order to obtain a molybdenum mixed oxide precursor, at least one further metal-containing starting compound is used, which is selected from a vanadium-containing and / or tellurium-containing and / or antimony-containing and / or tungsten-containing starting compound. Furthermore, according to the invention, in addition to these metal-containing starting compounds, a niobium-containing and / or iron-containing and / or bismuth-containing and / or antimony-containing starting compound is used. Particularly preferred are ammonium metavanadate, telluric acid and ammonium niobium oxalate as metal-containing starting compounds.

In a preferred development of the method according to the invention, a first solution containing ammonium heptamolybdate tetrahydrate, ammonium metavanadate and telluric acid dissolved, and a second solution containing ammonium niobium oxalate dissolved are used.

The molybdenum concentration of the first solution is preferably 0.5M to 1.5M, preferably 0.7M to 1.5M, and more preferably 0.9M to 1.3M.

In a further preferred embodiment of the method according to the invention, the temperature set by the tempering device in the mixing / reaction chamber is 4 ° C to 95 ° C, more preferably 30 ° C to 90 ° C and especially 40 ° C to 70 ° C.

The total volume flow of the first and the second solution, which are supplied to the mixing / reaction chamber, is preferably 5 ml / min to 60 ml / min, more preferably 15 ml / min to 60 ml / min and especially 30 ml / min to 60 ml / min per micromixer module (micromixing chamber). The term "module" is understood in this context to mean a stack of films or plates into which microchannels are etched, as described in greater detail in the operating instructions of the turbulence microparticle reactor below, for example.

Larger total volume flows and thus productivities can be realized simply by connecting several micromixer modules in parallel. The volumetric flow ratios of the volume of the first solution to the second solution are preferably in the range of 3: 1 to 1: 3

In a preferred embodiment of the method according to the invention, the concentration of the first and the second solution and / or the temperature within the mixing / reaction chamber and / or the total volume flow of the first and the second solution is / are adjusted during introduction into the mixing / reaction chamber, the molybdenum mixed oxide precursor is formed in step c) in the form of a suspension or a colloidal solution.

For the purposes of the present application, a suspension is defined as a heterogeneous substance mixture comprising a liquid with finely divided solids, the solids having an average particle size of more than 10 μm.

In the context of the present application, a colloidal solution is defined as a liquid in which particles having an average particle size of 1 nm to 10 μm are finely distributed.

The skilled artisan can determine by a few experiments how to set the conditions mentioned to obtain a suspension or a colloidal solution.

In particular, for the preparation of a suspension, the molybdenum concentration of the first solution and / or the temperature within the mixing / reaction chamber and / or the total volume flow of the first and the second solution when introducing into the mixing / reaction chamber are adjusted so that they are in the lower range of the above Ranges of values lie.

Preferred values for preparing a suspension are a molybdenum concentration of the first solution of 0.25 M to 1.5 M and / or a temperature within the mixing / reaction chamber from room temperature (RT) to 95 ° C and / or a total volume flow of the first and the second solution when introduced into the mixing / reaction chamber from 10 ml / min to 70 ml / min.

The suspension formed according to this preferred embodiment has a precipitate which, compared to the precipitate obtained with the prior art processes, has the advantage that it is due to the special mixing conditions present in a micromix reactor and the continuous flow Operation is uniform and has no complicated phase mixtures.

In the context of the present application, "uniform" means that the precipitation is invariable in terms of its chemical composition, morphology, particle size distribution, pH, conductivity, etc., temporally and spatially.

This is particularly due to the fact that the mixing of the individual components of the first and the second solution in the micromixer is much better due to the immediate mixing in the microchannels of the micromixer, than is the case with conventional mixing methods. For example, in a large mixing vessel (with a volume of, for example, 100 liters or more), despite the use of an intensive stirrer, it only takes several minutes for a suspension to form.

Further, for the preparation of a colloidal solution, the molybdenum concentration of the first solution and / or the temperature within the mixing / reaction chamber and / or the total volume flow of the first and second solutions are adjusted upon introduction into the mixing / reaction chamber to be in the range of the above range of values. A colloidal solution (or a sol) is the precursor to a gel; Preferred ranges of values for the preparation of a colloidal solution are therefore generally higher concentrations, in particular a higher molybdenum concentration and higher temperatures. A sol can also be stabilized in less preferred embodiments of the present invention by the addition of stabilizers / ligands (such as ammonium acetate, acetic acid, surfactants, betaines, etc.), thereby preventing subsequent precipitation / gelation.

Preferred values for producing a colloidal solution are a molybdenum concentration of the first solution of 0.7 M to 1.5 M and / or a temperature within the mixing / reaction chamber from room temperature (RT) to 40 ° C and / or a total volume flow of the first and the second solution when introduced into the mixing / reaction chamber from 10 to 60 ml / min.

The colloidal solution formed in accordance with this particularly preferred embodiment of the invention, after removal of the solvent, results in a precipitate which is even more uniform as compared to the precipitate which is obtained in the form of a suspension. Furthermore, the colloidal solution has the advantage over the suspension that it is easier to pump in the technical embodiment of the method.

The uniform precipitate obtained by the process according to the invention results in the further processing by spray drying with subsequent calcination or during processing in a pulsation reactor, as it is for example in the DE 101 09 892 A1 to a correspondingly uniform Molybdänmischoxidmaterial.

The present invention therefore also provides a molybdenum mixed oxide precursor obtainable by the process described above and having the said advantages.

Further, the present invention provides a process for producing a molybdenum mixed oxide material in which the molybdenum mixed oxide precursor of the present invention is subjected to the steps of spray drying and calcining, and which optionally comprises activating the above molybdenum mixed oxide precursor in suspension form under inert gas at 600 ° C.

The calcination is preferably carried out at a temperature in the range of from 250 ° C to 450 ° C, more preferably from 270 ° C to 350 ° C, and preferably for a period of from 1 hour to 8 hours, more preferably from 2 hours to 4 hours, preferably in air, more preferably carried out statically. Less preferably, the calcining takes place in a stream of air, that is non-static.

In a preferred embodiment of this method, prior to spray-drying, a step of aging the molybdenum mixed oxide precursor and / or a step of chemically altering the molybdenum mixed oxide precursor (eg, by changing the pH or by adding further metal ions, such as e.g. B. of Ni, Cu, Fe, Co, Cr and Bi).

When the molybdenum mixed oxide precursor obtainable in accordance with the invention is processed in suspension or in the form of a colloidal solution by first spray-drying and then calcining and optionally before spray-drying by the further steps indicated above, a molybdenum mixed oxide material is obtained which has very uniform particles.

In a preferred embodiment of the method according to the invention, the concentration of the first and the second solution and / or the temperature within the mixing / reaction chamber and / or the total volume flow of the first and the second solution is / are adjusted during introduction into the mixing / reaction chamber, the molybdenum mixed oxide precursor is formed in step c) in the form of a gel.

A gel is defined in the context of the present application as a finely divided system of at least one solid and one liquid phase. This solid phase forms a sponge-like, three-dimensional network whose pores are filled by a liquid, with both phases completely penetrating each other.

The skilled artisan can determine by a few experiments how to set the conditions mentioned to obtain a gel.

In particular, for the preparation of a gel, the molybdenum concentration of the first solution and / or the temperature within the mixing / reaction chamber and / or the total volume flow of the first and the second solution when introducing into the mixing / reaction chamber are set to be in the upper range of the above Ranges of values lie.

Preferred values for producing a gel are a molybdenum concentration of the first solution of 0.7 to 1.5 and / or a temperature within the mixing / reaction chamber from room temperature (RT) to 40 ° C and / or a total volume flow of the first and the second solution when introduced into the mixing / reaction chamber from 10 to 60 ml / min.

By having the molybdenum mixed oxide precursor in the form of a gel, segregation phenomena such as are obtained in a synthesis of the precursor without the use of a microreactor are avoided because the metals are immobilized in the gel. This allows the Molybdänmischoxidvorstufe in a simple manner z. Into a corresponding mixed molybdenum oxide material by allowing the molybdenum mixed oxide precursor to be allowed to dry in gel form without special precautions or drying, followed by calcination of the dried precursor.

Therefore, the present invention further provides a process for producing a molybdenum mixed oxide material in which the molybdenum mixed oxide precursor of the present invention in the form of a gel Is subjected to drying and calcination steps, and which optionally comprises activating the aforesaid molybdenum mixed oxide precursor in gel form under inert gas at 600 ° C.

• a) des Einbringens der vorstehend beschriebenen Molybdänmischoxidvorstufe in Suspensionsform oder in Form einer kolloidalen Lösung in eine Reaktionskammer mittels eines Trägerfluids,
• b) des thermischen Behandelns der Molybdänmischoxidvorstufe in einer Behandlungszone mittels einer pulsierenden Strömung bei einer Temperatur von 200 bis 1000°C,
• c) des Bildens eines nanokristallinen Molybdänmischoxidmaterials und
• d) des Ausbringens des in Schritt c) erhaltenen nanokristallinen Molybdänmischoxidmaterials aus der Reaktionskammer umfasst.

Further, the present invention provides a process for producing a nanocrystalline molybdenum mixed oxide material comprising the steps

• a) introducing the above-described molybdenum mixed oxide precursor in suspension form or in the form of a colloidal solution into a reaction chamber by means of a carrier fluid,
• b) the thermal treatment of the Molybdänmischoxidvorstufe in a treatment zone by means of a pulsating flow at a temperature of 200 to 1000 ° C,
• c) forming a nanocrystalline molybdenum mixed oxide material and
• d) comprising discharging the molybdenum mixed oxide nanocrystalline material obtained in step c) from the reaction chamber.

In a preferred embodiment of the method, the carrier fluid is a gas.

The method can be used with a pulsation reactor, as described for example in the DE 101 09 892 A1 (whose disclosure of the pulsation reactor and the corresponding method is fully incorporated into the present description), preferably at relatively low temperatures of 200 ° C to 800 ° C, more preferably from 250 ° C to 750 ° C, most preferably from 300 ° C to 650 ° C are performed. By means of the method according to the invention, the crystallization process of the molybdenum mixed oxide can be specifically controlled, in particular the size of the crystallites and the pore size distribution of the corresponding molybdenum mixed oxides. This can be further influenced advantageously by the residence time in the flame or by the reactor temperature. The pulsed thermal treatment prevents the resulting nanocrystalline molybdenum mixed oxide particles from agglomerating. Typically, the nanocrystalline particles are immediately transferred by the stream of hot gas to a colder zone where the molybdenum mixed oxide crystallites are obtained, in part, with diameters of 60 to 120 nm, sometimes less than 20 nm.

This leads to the Molybdänmischoxidkristalliten thus obtained to significantly increased BET surface areas of> 1 m ² / g, more preferably 2 m ² / g to 200 m ² / g and particularly preferably 5 m ² / g to 25 m ² / g. The BET surface area is determined according to DIN 66132 (according to the method of Brunauer, Emmett and Teller).

With this method, the suspensions or colloidal solutions of a molybdenum mixed oxide precursor provided according to the invention can be calcined within a very short period of time, typically within a few milliseconds, without additional filtration and / or drying steps or addition of additional solvents. The resulting molybdenum mixed oxide nanocrystallites have significantly increased BET surface areas and thus provide, compared to the corresponding prior art catalysts, a molybdenum mixed oxide catalyst with increased reactivity, improved conversion and selectivity, especially with respect to conversion of propane to acrylic acid, propene to acrolein, acrolein to acrylic acid, propene to acrylonitrile, methanol to formaldehyde, i-butene to methacrolein, methacrolein to methacrylic acid, o-xylene to phthalic anhydride, and benzene to maleic anhydride.

Furthermore, due to the approximately equal residence time of each molybdenum mixed oxide particle in the homogeneous temperature field produced by the process, an extremely homogeneous end product with a narrow monomodal particle distribution is produced. The device for carrying out the process according to the invention in the production of such monomodal nanocrystalline metal oxide powder is, as already mentioned, for example from US Pat DE 101 09 892 A1 known. However, unlike the apparatus and method disclosed therein, the present method does not require an upstream evaporation step in which the starting material, ie, the molybdenum starting compound, is heated to an evaporation temperature.

The suspension or colloidal solution of the molybdenum mixed oxide precursor provided according to the invention is introduced directly into a so-called reaction chamber, ie into the combustion chamber of the pulsation reactor according to the invention, directly via a carrier fluid, in particular a carrier gas, preferably an inert carrier gas, for example nitrogen, etc., more preferably a reducing gas mixture DE 101 09 892 A1 , introduced. On the exhaust gas side of the reaction chamber, a resonance pipe is connected with a flow cross-section which is significantly reduced in relation to the reaction chamber. The combustion chamber bottom is equipped with several valves for the combustion air to enter the combustion chamber. The aerodynamic valves are fluidically and acoustically tuned with the combustion chamber and the resonance tube geometry such that the pressure waves of the homogeneous "flameless" temperature field generated in the combustion chamber are predominantly pulsating in the resonance tube. It forms a so-called Helmholtz resonator with pulsating flow with a pulsation frequency between 3 and 150 Hz, preferably 5 to 110 Hz, from.

The feeding of the suspension or the colloidal solution in the reaction chamber is typically carried out with a nozzle. In addition to single-fluid nozzles, multicomponent nozzles can also be used.

Preferably, the suspension or colloidal solution of the molybdenum mixed oxide precursor provided according to the invention is introduced in a troublesome form into the reaction chamber, so that a fine distribution in the region of the treatment zones is ensured.

After the thermal treatment, the resulting nanocrystalline molybdenum mixed oxides, if possible by means of the carrier fluid, immediately transferred to a colder zone of the reaction chamber, so that they can be deposited and discharged in the colder zone. The yield of this process is almost 100% because the resulting product can be completely discharged from the reactor. Typically, the process is carried out at a pressure ranging from atmospheric pressure to 40 bar.

An inventive Molybdänmischoxidmaterial can be described, for example, by the general formula MoV _v Te _y Nb _z O _x, wherein v, y, z and x preferably in the areas (for v, y, z +/- 20% for x +/- 50%) of the following preferred compositions. However, the Molybdänmischoxidmaterial the above formula may contain other elements such. B. Sb, W, Fe and Bi. Preferred compositions within this general formula are, for example, MoV _0.3 Te _0.23 Nb _0.125 O _x , MoV _0.2 Te _0.2 Nb _0.1 O _x , MoV _0.4 Te _0.2 Nb _0.2 O _x , MoV _0.21 Te _0.22 Nb _0.11 O ₄₀ , MoV _0.35 Te _0.23 Nb _0.15 O _x , MoV _0.35 Te _0.27 Nb _0.13 O ₄₀ , MoV _0.35 Te _0.27 Nb _0.16 O _x , MoV _0.35 Te _0.24 Nb _0.13 O _x , Mo ₁₀₀ V ₃₀ Te ₂₃ Nb _16.7 , Mo ₁₀₀ V ₃₄ Te ₁₉ Nb ₁₅ , Mo ₁₀₀ V ₃₀ Te ₁₈ Nb ₁₅ just to name a few.

The molybdenum mixed oxide material according to the invention is outstandingly suitable for use as a catalyst, for example for the conversion of propane to acrylic acid, propene to acrolein, acrolein to acrylic acid, propene to acrylonitrile, methanol to formaldehyde, i-butene to methacrolein, methacrolein to methacrylic acid, o-xylene to phthalic anhydride and benzene to maleic anhydride.

This is in particular due to the fact that the proportion of so-called M1 phase which is selective for the selectivity of the catalyst-relevant M1 (Desanto et al, Z. Kristallogr., 219 (2004) pp. 152-165) is more than 60%, preferably more than 70% % and the proportion of "unselective" M2 phase is significantly lower than with mixed oxides of the prior art.

The invention thus also provides the molybdenum mixed oxide material obtainable by the processes according to the invention, in which the proportion of molybdenum mixed oxide material in orthorhombic modification is more than 60%.

In a particularly preferred embodiment, the molybdenum mixed oxide material used as a catalyst has the composition MoVNbTe, further comprising Sb as a promoter. In this context, particularly preferred Molybdänmischoxidmaterialien compositions have 41.7 wt .-% Mo, 7.07 wt .-% V, 7.43 wt .-% Nb, 9.82 wt .-% Te, 0.54 wt. % Sb and 32.6% by weight O, and also 39.5% by weight Mo, 6.90% by weight V, 8.79% by weight Nb, 11.2% by weight Te, 0.64 wt .-% Sb and 32.2 wt .-% 0 on.

The invention will now be explained in more detail with reference to the following and not to be understood as limiting embodiments. In the following embodiments, a turbulence microparticle reactor TMP T30 A / B 11065D (available from SYNTICS GmbH, Bochum, Germany) was used as a micromix reactor. The pulsation reactor used was a pulsation reactor from IBU-tec advanced materials AG, Germany.

Examples

Embodiment 1

Preparation of molybdenum mixed oxide precursors with a micromix reactor

For the embodiments, starting solutions according to Tables 1 and 2 were prepared as first and second solution. Table 1 (Mo concentration 0.7 M) connection n metal (mol) m compound (g) VH ₂ O (ml) of ammonium 0,350 61.793 500 Solution 1 ammonium 0.105 12.283 telluric 0.081 18.483 Ammoniumnioboxalatmonohydrat 0,044 20,133 150 Solution 2 Table 2 (Mo concentration 1.3 M) connection n metal (mol) m compound (g) VH ₂ O (ml) of ammonium 0.65 114.758 500 Solution 1 ammonium 0.195 22.811 telluric .1495 34.325 Ammoniumnioboxalatmonohydrat 0.0815 37.39 150 Solution 2

The solution 1 was introduced by means of a pump and after reaching constant ratios, the solution 2 was added. Solutions 1 and 2 were each preheated in an electric microheater and then fed into the turbulence microparticle reactor TMP so that the predetermined temperature in the turbulence microparticle reactor was rapidly established by the tempering device in the turbulence microparticle reactor. The parameters in the implementation of the embodiments were set according to Table 3, wherein the total volume flow V _ges of solution 1 and solution 2 according to Table 4 composed. Another experiment (experiment no. 8) was carried out at a somewhat lower volume flow rate of 45 ml / min (instead of 50 ml / min as in the gel formation in experiment 7, ie at a flow rate 10% lower than in gel formation) Sol obtained: Table 3: Experimental parameters attempt Concentration (mol / liter) V _tot (ml / min) T (° C) Remarks 1 0.7 40 10 Gel after 10 min 2 0.7 10 40 Gel after 15 min 3 0.7 10 60 Turbidity (precipitate) after 2 min, gel after 5 min 4 0.7 10 40 Turbidity (precipitation) after 3 min 5 1.3 10 40 Turbidity (precipitation) after 7 min 6 1.3 40 40 Turbidity (precipitate) after 2 min, gel after 3 min 7 1.3 50 40 Gel after 2 min 8th 1.3 45 40 Sol after 2 min Table 4: Total volume flows V _tot V _tot (ml / min) 10.0 40.0 50.0 Solution 1 7.7 30.8 38.5 Solution 2 2.3 9.2 11.5

According to Table 3 above, the parameters were varied such that they started with low solution concentrations and initially the temperature and the total volume flow were varied, whereupon high solution concentrations, high temperatures and volume flows were transferred.

Results

In all experiments carried out, an orange-red product was discharged from the turbulence microparticle reactor. Depending on the experimental conditions, the orange-red product was present either as a precipitate in a suspension or in a colloidal solution, or as a gel. In particular, it was found that at higher solution concentrations, gelation occurred earlier than at lower solution concentrations. With increasing total volume flows, a better mixing of the educts resulted in the turbulence microparticle reactor, which also began a faster gelation. The same tendency was observed with an increase in temperature. The formation of a molybdenum mixed oxide precursor in gel form can thus be achieved by increased solution concentrations, higher total volume flows and / or elevated temperatures (see the comments on Tests 6 and 7 in Table 3).

Embodiment 2:

Preparation of a molybdenum mixed oxide from a molybdenum mixed oxide precursor in precipitate form (suspension form)

There was prepared a molybdenum mixed oxide precursor in precipitation form (suspension form) according to Experiment 4 of Embodiment 1. The suspension was then introduced in a conventional manner in a spray-drying apparatus and spray-dried, wherein the inlet temperature of the spray dryer was adjusted (between 200 ° C and 300 ° C) that the temperature at the outlet was more than 100 ° C. The material obtained after spray-drying was then calcined at 320 ° C for 3 hours in air. The calcined powder was characterized by means of an X-ray diffraction analysis (XRD) (the assignment of the signals took place in all examples according to the abovementioned reference by Desanto et al.). X-ray diffraction analysis revealed ( 1a ) after activation of the powder at 600 ° C for 2 h under nitrogen, a proportion of M1 phase of 75%, M2 phase of 13% and Mo ₅ O ₁₄ of 12% wherein a mixed oxide formation was detected.

Embodiment 3

Preparation of a molybdenum mixed oxide from a molybdenum mixed oxide precursor in precipitate form (suspension form)

There was prepared a molybdenum mixed oxide precursor in precipitation form (suspension form) according to Experiment 4 of Embodiment 1. The suspension was then heated at 450 ° C in a pulsation reactor according to DE 101 09 892 A1 atomized and calcined. Thereby, a black powder was obtained, which is a nanocrystalline molybdenum mixed oxide material. X-ray diffraction analysis after activation at 600 ° C. for 2 h under nitrogen showed a proportion of M1 phase of 73% of M2 phase of 15% and Mo ₅ O ₁₂ of 12% ( 1c ).

Embodiment 4

Preparation of a molybdenum mixed oxide from a molybdenum mixed oxide precursor in gel form

A molybdenum mixed oxide precursor in gel form was prepared according to Experiment 7 of Embodiment 1. The gel was dried overnight at 120 ° C. The resulting dry powder was then calcined at 320 ° C for 3 hours in air. The calcined powder was activated at 600 ° C. for 2 h under nitrogen to improve the crystallinity and then characterized by means of an X-ray diffraction analysis ( 1b ), whereby a mixed oxide formation was detected.

The proportion of M1 phase (orthorhombic) was 66% and M2 phase (hexagonal) was 34%.

Comparative example

As a comparative example, a molybdenum mixed oxide precursor was conventionally carried out by precipitation in a large stirred tank. To this was added 3.34 kg of ammonium heptamolybdate tetrahydrate, 0.663 kg of ammonium metavanadate and 1.0 kg of telluric acid in 26.4 liters of dist. H ₂ O and then an aqueous solution of 1.426 kg of ammonium niobium oxalate in 5.7 liters distilled with stirring. H ₂ O added. The suspension obtained was processed in a pulsation reactor as in the preceding examples.

However, it was found that the conductivity and the pH of the suspension were not constant during the spray calcination (about 5 hours duration). Thus, from the time of combining the solutions until the end of the spray calcination, the conductivity changed from 57.5 mS to 53.6 mS and the pH in the same period from 5.2 to 3.29.

In particular, at the moment of addition of the niobium-containing solution (pH 0.9) to the initially introduced MOVTe solution in the stirred tank, a very pronounced decrease in pH from pH 5.2 to 3.4 occurs within a few minutes.

The material obtained after spray-drying was then calcined at 320 ° C for 3 hours in air. The calcined powder was characterized by means of an X-ray diffraction analysis, although a mixed oxide formation was detected, but the mixed oxide obtained was not uniform in the sense of the present application. The spray-calcined MoVNbTe powder after activation at 600 ° C / 2 h / Ar had a phase mixture of ~ 55% M1 phase (orthorhombic), ~ 30% M2 phase (hexagonal) and 15% MoVNb ₅ O ₁₄ ( 2 ).

It is striking that the powders according to the invention (continuously precipitated) no longer have a disruptive MoVNb ₅ O ₁₄ phase, but are composed only of M1 and M2 phases (+ if necessary Mo ₅ O ₁₄ ), the proportion of M1 typically being more than 60%. A high M1 content is desirable since M1 leads to high selectivities in the propane oxidation to acrylic acid and is responsible for the long-term stability of the catalyst (Desanto et al, Z. Kristallogr., 219 (2004) pp. 152-165).

catalyst tests

The material obtained in Example 4 and Comparative Example was tested as a catalyst for propane oxidation to acrylic acid. The materials were previously applied to spherical steatite supports using a Ludox AS 40 binder (loading: 24.4 wt.% Active mass). A 16-fold parallel tube reactor was used, and 500 mg of activated catalyst were introduced per tube. The reactor used is described, for example, by C. Hoffmann et al. in Angew. Chem. Int. Ed. 38, 2800 (1999) and C. Hoffmann et al. in J. Catalysis 198, 348-354 (2001). However, any reactor known to those skilled in the art and suitable for carrying out the catalytic reaction may, of course, be used to carry out the invention.

Subsequently, a flow rate of 20 ml / min / tube for a gas addition of propane: Kr: O ₂ : He: H ₂ O = 3: 1: 6: 20: 70 was set and then reacted at normal pressure and 400 ° C for 6 h to let. The results were as follows: Table 5 Example 4 Comparative example Propane conversion (%) 62 12 Acrylic acid yield (%) 44 4 Acrylic acid selectivity (%) 72 33

As can be seen from Table 5, the catalyst according to the invention is superior in terms of yield and selectivity to that of the prior art from the comparative example.

Claims (18)

A process for producing a molybdenum mixed oxide precursor comprising the steps a) preparing a first solution which contains a molybdenum starting compound and at least one further metal-containing starting compound selected from a vanadium-containing and / or tellurium-containing and / or antimony-containing and / or tungsten-containing starting compound, b) producing a second solution which contains a niobium-containing and / or iron-containing and / or bismuth-containing and / or antimony-containing starting compound, c) introducing the first and second solutions into a micromixer reactor having a mixing / reaction chamber and a tempering device located in the mixing / reaction chamber, wherein the first and second solutions are separated into substantially opposite solution substreams Mixed / mixed reaction chamber of the micromixer reactor and mixed there, wherein a Molybdänmischoxidvorstufe is formed, and d) applying the molybdenum mixed oxide precursor obtained in step c) from the mixing / reaction chamber. Process according to claim 1, characterized in that the molybdenum starting compound is ammonium heptamolybdate tetrahydrate. A method according to claim 1 or 2, characterized in that the at least one further metal-containing starting compound is telluric acid and / or ammonium metavanadate. Method according to one of claims 1 to 3, characterized in that the niobium starting compound is ammonium niobium oxalate or niobium oxalate. Method according to one of claims 1 to 4, characterized in that the molybdenum concentration of the first solution is 0.5 to 1.5 M. Method according to one of claims 1 to 5, characterized in that the adjusted by the tempering temperature in the mixing / reaction chamber is 4 ° C to 95 ° C. Method according to one of claims 1 to 6, characterized in that the concentration of the first and the second solution and / or the temperature within the mixing / reaction chamber and / or the total volume flow of the first and the second solution when introduced into the mixing / reaction chamber so is set or be that the molybdenum mixed oxide precursor is formed in step c) in the form of a suspension. Method according to one of claims 1 to 6, characterized in that the concentration of the first and the second solution and / or the temperature within the mixing / reaction chamber and / or the total volume flow of the first and the second solution when introduced into the mixing / reaction chamber so is set that the Molybdänmischoxidvorstufe in step c) is formed in the form of a colloidal solution. Molybdenum mixed oxide precursor obtainable by the process according to claim 7 or 8. A process for producing a molybdenum mixed oxide material, characterized in that a molybdenum mixed oxide precursor according to claim 9 is subjected to the steps of spray drying and calcination. A process for producing a nanocrystalline molybdenum mixed oxide material, characterized in that it comprises the steps of a) introducing the molybdenum mixed oxide precursor of claim 9 into a reaction chamber by means of a carrier fluid, b) thermally treating the molybdenum mixed oxide precursor in a treatment zone by means of a pulsating flow at a temperature of 200 to 1000 ° C., c) forming a nanocrystalline molybdenum mixed oxide material, and d) applying the nanocrystalline molybdenum mixed oxide material obtained in step c) from the reaction chamber. A method according to claim 11, characterized in that the carrier fluid is a gas. Method according to one of claims 1 to 6, characterized in that the concentration of the first and the second solution and / or the temperature within the mixing / reaction chamber and / or the total volume flow of the first and the second solution when introduced into the mixing / reaction chamber so is set that the Molybdänmischoxidvorstufe in step c) is formed in the form of a gel. Molybdenum mixed oxide precursor in the form of a gel, obtainable by the process according to claim 13. A process for producing a molybdenum mixed oxide material, characterized in that a molybdenum mixed oxide precursor according to claim 14 is subjected to the steps of drying and calcining. A mixed molybdenum oxide material obtainable by the process of any one of claims 10 to 12 and 15, wherein the proportion of molybdenum mixed oxide material in orthorhombic modification is greater than 60%. A molybdenum mixed oxide material according to claim 16, which has the composition MoVNbTe, further comprising Sb, Bi, Fe, Co, Ni, Cu, Mn as a promoter. Use of the molybdenum mixed oxide material according to claim 16 or 17 as a catalyst.

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