PROCESSES

The present invention relates to the chemical industry, specifically, to a method for producing a granular catalyst for processes of oxidation of alkanes, in particular for the oxidation of propane so as to produce acrylic acid, a catalyst produced by said method, use of a combination of talc and graphite to mold a dry catalyst precursor followed by preparation of a granular catalyst, and to processes for the oxidation of propane so as to produce acrylic acid, which provide for use of the catalyst according to the invention.

Background of the invention

Acrylic acid is a valuable raw material for the chemical industry. Particularly, acrylic acid is utilized as a monomer in the production of polymers, including polyester resins.

One of the industrially applicable methods is a method for producing acrylic acid by heterogeneous catalytic partial oxidation of propane in the presence of a catalyst. The following catalysts are known among catalysts for the oxidation of propane to acrylic acid: vanadium-phosphorus binary oxides, catalytic compositions on the basis of heteropolyacids, and compositions on the basis of mixed transition element oxides with several oxidation states [M.M. Lin. Selective oxidation of propane to acrylic acid with molecular oxygen // Appl. Catal. A: Gen. - 2001. - C. 207. - P. 1-16]. The highest catalytic activity in oxidation processes, particularly, in propane oxidation processes, is manifested by multicomponent systems on the basis of transition element oxides with several oxidation states, which obligatorily comprise molybdenum (Mo) and vanadium (V), niobium (Nb), and tellurium (Te).

EP0608838 discloses a catalyst used in the process for producing acrylic acid by the oxidation of propane, which is a mixed oxide catalyst having the formula of °aVbTecXdOx, wherein X is at least one element selected from the group consisting of Nb, Ta, W, Ti, Al, Zr, Cr, Mg, Fe, Ru, Co, Rh, Ni, Pd, Pt, Sb, Bi, B, In, and Ce, 0.25<a<0.98, 0.003<b<0.5, 0.003<c<0.5, 0.003<d<0.5. Use of a catalyst having the formula Mo^ _{Q j} Te _Q ^Nb _{Q 12} O _{x jn} accordance with the invention represented in EP0608838 allows achieving a propane conversion as high as 65.7%, the selectivity to acrylic acid of 58%, the acrylic acid yield of 38.1%. The yield of acrylic acid is raised to 48% by a further treatment of this catalyst, the treatment comprising the following steps: providing an aqueous slurry, subjecting the slurry to heat treatment, compacting the powder, scattering and final calcination of a catalyst fraction in a nitrogen flow at 600°C for 2 hours.

Also, known in the art are niobium (Nb)- and/or tantalum (Ta)-promoted catalysts on the basis of Mo, V, Te and/or antimony (Sb) of the general formula Mo _j V _a (Te and/or Sb) _b (Nb and/or Ta) _c Si _d O _x where a = 0.006-1, b = 0.006-1, c = 0.001- 0.5, d - 0-3.5. Subject to W02006100128, when a catalyst having the formula M°l Vo,33Sb() _i was used at a temperature of 340°C, the propane conversion rate was 41.7%, the selectivity to acrylic acid was 27%. The introduction of niobium contributed to an increase in selectivity to acrylic acid to 52%.

Documents JP3924824, US5198580 disclose multicomponent metal oxide catalysts employed in oxidation processes, in particular, in the oxidation of propane for producing acrylic acid. For example, US5198580 discloses a catalyst having the general formula Bi _b Mo _c B _b A _a D _d E _e O _x , in which A is one or more of K, Na, Li, Cs and Tl, D is one or more of Fe, Ni, Co, Zn, Ce and La, wherein a, d, e, and b are from 0 to 10, c is from 0.1 to 20. Vanadium compounds are used as a promoter additive. The metal oxide catalyst of patent JP3924824 comprises Mo, V, Sb, and component A, provided that A is an element selected from the group consisting of Nb, Ta, Sn, W, Ti, Ni, Fe, Cr, Co.

Such multicomponent catalysts based on mixed transition metal oxides may be produced by evaporating mixed solutions of starting compounds of transition elements under normal conditions (a so-called“slurry method”) or by the hydrothermal synthesis. At that, the most widespread method for producing Mo, V, Nb, Sb and/or Te-containing catalysts is the evaporation of mixed solutions of starting compounds. Such a method of producing a dry catalyst precursor, which is known, for example, over patent RU2342991, comprises the following steps:

- providing solutions of starting compounds (as a rule, ammonium paramolybdate and ammonium metavanadate, telluric acid, or antimony compounds), mixing with a separately prepared solution of niobium oxalate and promoter additives;

- drying the so-obtained slurry;

- stepwise thermal treatment at 275-600°C. It is worth observing that the distinction of MoVTe(Sb)Nb-systems, i.e. the presence of several elements with several oxidation states, determines strong dependency of catalyst structure formation on methods and parameters of synthesis, redox conditions of heat treatment, and the subsequent molding step to obtain a granular catalyst. This is confirmed by the fact that, regardless of a close chemical formulation, prior art catalysts differ in terms of their catalytic characteristics (Table 1):

Table 1 - Catalytic properties of prior art catalysts based on mixed transition metal oxides

The reason for the difference in catalytic activity of catalysts, their identical chemical formulation notwithstanding, may be the essential role of certain crystal phases and a ratio thereof. The complexity of a catalyst and dependency of chemical reactions within a system on preparation conditions results in formation of a complex phase composition. It is known that two crystal phases are active in MoVTe(Sb)Nb oxide catalysts - orthorhombic (Ml) and hexagonal (M2). At that, pure Ml phase is active and selective in the propane oxidation, while M2 phase is active and selective in the propylene oxidation. The need for providing an optimal combination of Ml and M2 phases in a multicomponent catalyst based on mixed transition metal oxides stems from these conceptions. Irreproducibility of a phase composition of a catalyst and, correspondingly, of its catalytic properties sets up the problem of developing preparation techniques for catalysts which catalytic properties would permit utilizing them in propane oxidation processes for producing acrylic acid.

It is generally known that use of catalysts in industrial processes for producing acrylic acid requires a step of molding dry catalyst precursors in order to obtain granules having an optimal size and shape. Noteworthy, the catalyst synthesis methods set out in the above-referenced patent documents differ solely by the step of making a dry catalyst precursor, while the subsequent heat treatment steps are not substantially different, and molding steps are often not even mentioned. At the same time, the step of molding a catalyst is of great importance, because this step may have an effect upon a catalyst structure and, as a consequence, upon it catalytic properties, and may also determine mechanical strength of catalyst granules to a large extent.

The authors of the present invention have discovered that the molding step, and namely, conditions for carrying out the same, may affect adversely catalytic properties of produced catalysts, in particular, selectivity and yield of a target product. However, the prior art does not contain sufficient information concerning techniques that should be employed for molding the multicomponent catalysts represented above so as to satisfactorily preserve their structure and catalytic properties.

Methods of molding catalysts known in the art include pressing, extrusion, pelletization, and agglomeration with formation of spheroidal particles by spray drying. These methods may be carried out with or without addition of an inert processing additive (binder).

The most widely used method of molding a catalyst is pressing a dry catalyst precursor, when, under the influence of externally exerted pressure, the initial volume of a powder material reduces, cohesion among its particles occurs and, as a consequence, a solid body forms - a granule or a pellet having a certain shape and size. At that, various special inert processing additives (binders) or plasticizers, which play the role of substances contributing to pore formation, catalyst hardening, and facilitation of molding catalyst granules, are used during the compaction. Clays, hydrogels, organic and inorganic acids, polyhydric alcohols may be utilized as such additives. In particular, graphite, starch, sodium stearate, talc, etc. are used as said additives.

For example, patent RU2377068 suggests a method for producing a continuous

1 2 3 4 annular oxide catalyst material of the general formula Mo ₁₂ Bi _a Fe _b X _C X _d X _e X which finds its use in oxidation processes, especially in oxidation processes for producing (meth)acrolein. Molding of catalysts includes several steps. First, a finely dispersed plastic mixture is obtained from the active mass of a catalyst precursor, intermediate compaction is performed, then annular articles are molded from a pre- compacted plastic mixture with addition of auxiliary agents for molding and/or auxiliary reinforcing materials. When carrying out the method according to the invention, for example, carbon black, stearic acid, starch, polyacrylic acid, mineral or vegetable oils, water, boron trifluoride or graphite are used as such auxiliary agents for molding (lubricating materials). Glycerol and cellulose ester may also be utilized as lubricating materials.

RU2559337 discloses a process for producing an unsaturated acid or unsaturated nitrile that involves a complex oxide catalyst, which latter is represented by the formula M ^o _i V _a Nb _b Sb _c W _d Z _e O _n , wherein Z is an element selected from the group of La, Ce, Pr,

Yb, Y, Sc, Sr, and Ba. According to this technical solution, mechanical strength of the catalyst is achieved by adding at least 20% silica to the carrier, although document RU2559337 is silent on hydrodynamic characteristics of the catalyst - a size, a shape, density of catalyst grains, stability - operational life.

The prior art molding techniques aiming to prepare a granular catalyst turned out not to be applicable for molding the catalyst produced in accordance with the present invention. In this connection, it is crucial to develop a reproducible method for the production of a catalyst characterized by acceptable catalytic properties in oxidation processes, particularly, in propane oxidation processes, during molding of which catalyst a structure of active centers would not be destroyed and, consequently, its catalytic properties would not decrease.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a granular catalyst based on mixed transition metal oxides, which is characterized by relatively high activity, high selectivity in oxidation processes, particularly, in the process of producing acrylic acid by the partial oxidation of propane.

A technical result is carrying out a method for making a granular catalyst, under which catalytic properties do not deteriorate during the process of molding a dry catalyst precursor: the propane conversion is maintained over 60%, the selectivity to the target product is over 60-67%.

An additional technical result is improvement of mechanical strength of the granular catalyst and its stability in oxidation processes.

In order to accomplish the above object and achieve the desired technical result, the present invention provides the claimed method for producing a granular catalyst for oxidation processes. The method proposed in accordance with this invention comprises the following steps:

a) providing a dry catalyst precursor comprising at least one compound of each element selected from the group consisting of molybdenum (Mo), vanadium (V), tellurium (Te) and/or antimony (Sb), as well as niobium (Nb) and/or tantalum (Ta), and optionally further comprising one or more metal compounds selected from the group including bismuth (Bi), germanium (Ge), cerium (Ce), gallium (Ga), cobalt (Co), iron (Fe), and nickel (Ni) (hereinafter referred to as the dry precursor);

b) molding the dry precursor obtained in step a) to form a granular catalyst.

The method according to the invention is characterized in that the step of molding to produce a granular catalyst comprises the following stages:

bi) mixing the dry precursor obtained in step a) with talc taken in an amount from 1 to 4% by weight; b ₂ ) forming a catalyst fraction having a particle size from 0.25 to 0.5 mm from the mixture obtained in stage (bi);

b ₃ ) mixing the catalyst fraction obtained in stage (b ₂ ) with graphite taken in an amount from 0.5 to 3% by weight;

b ₄ ) pressing the composition formed in stage (b ₃ ) to produce a granular catalyst; b ₅ ) subjecting the granular catalyst obtained in stage (b ₄ ) to heat treatment.

The authors of the present invention have surprisingly discovered that use of a combination of talc and graphite in different stages of the molding step allows obtaining catalyst granules characterized by high mechanical strength and stability under propane oxidation reaction conditions. At that, when such a combination of additives is used during the catalyst molding, considerable deterioration of catalytic properties does not occur, whereas use of other additives or combinations thereof that are known in the art leads to serious deterioration of strength of catalyst granules and catalytic properties thereof.

In another aspect, the claimed invention relates to a method for molding a dry catalyst precursor to produce a granular catalyst, characterized in that it comprises the following subsequent stages: (i) mixing a dry catalyst precursor with talc taken in an amount from 1 to 4% by weight; (ii) molding a catalyst fraction from the mixture obtained in stage (i); mixing the fraction obtained in stage (ii) with graphite taken in an amount from 0.5 to 3% by weight; (iv) pressing the composition obtained in stage (iii) to produce a granular catalyst.

In yet another aspect, the claimed invention relates to a granular catalyst of the general formula MO _j V _a B _b C _c X _d O _n , produced by the method according to the invention, wherein X denotes at least one element selected from Bi, Ge, Ce, Ga, Co, Fe, and Ni, B represents Te and/or Sb, C is Nb and/or Ta, a, b, c, d are atomic ratios of metals, where 0.006<a<l, 0.006<b<l, 0.00l<c<0.5, 0<d<0.l, n is dependent on the oxidation state of the elements included in the catalyst.

In one more aspect, the present invention relates to use of a combination of talc taken in an amount from 1 to 4% by weight and graphite taken in an amount of 0.5 to 3% by weight for molding a dry catalyst precursor to produce a granular catalyst. In one more aspect, the present invention relates to a method of oxidizing propane to produce acrylic acid performed in the presence of the catalyst according to this invention.

In yet another aspect, the present invention relates to a method of oxidizing propane to produce acrylic acid in the presence of the catalyst according to this invention, comprising the steps of:

i) supplying a feed gas stream comprising propane, water vapor, and an oxidizing agent to a propane oxidation reaction zone in a reactor, which zone contains the granular oxidation catalyst according to this invention;

ii) supplying the gas mixture comprising acrylic acid, unreacted propane and propylene, carbon oxides, obtained at an outlet of the reaction zone to a separation step of separating liquid and gas products followed by recovering acrylic acid.

Brief description of drawings

Fig. 1 describes schematically a sequence of steps of producing a granular catalyst.

Detailed description of the invention

Various aspects and embodiments of the present invention are described in detail below.

The method for making a granular catalyst based on transition metal oxides proposed in this invention comprises the following steps (Fig. 1):

a) providing a dry catalyst precursor comprising at least one compound of each element selected from the group consisting of molybdenum (Mo), vanadium (V), tellurium (Te) and/or antimony (Sb), as well as niobium (Nb) and/or tantalum (Ta), and optionally further comprising one or more metal compounds selected from the group including bismuth (Bi), germanium (Ge), cerium (Ce), gallium (Ga), cobalt (Co), iron (Fe), and nickel (Ni) (hereinafter referred to as the dry precursor);

b) molding the dry precursor obtained in step a) to form a granular catalyst.

The method is characterized in that the step of molding to produce a granular catalyst comprises the following stages:

bi) mixing the dry precursor obtained in step a) with talc taken in an amount from 1 to 4% by weight;

b ₂ ) forming a catalyst fraction having a particle size from 0.25 to 0.5 mm; b ₃ ) mixing the catalyst fraction obtained in stage (b ₂ ) with graphite taken in an amount from 0.5 to 3% by weight;

b ₄ ) pressing the composition formed in stage (b ₃ ) to produce a granular catalyst; b ₅ ) subjecting the granular catalyst obtained in stage (b ₄ ) to heat treatment.

In accordance with the present invention, the content of talc and graphite is based on 100% of the catalyst composition comprising a dry catalyst precursor and processing additives: talc and graphite.

Step a) of the present invention comprises providing a dry particulate catalyst comprising at least one compound of each element selected from the group consisting of molybdenum (Mo), vanadium (V), tellurium (Te) and/or antimony (Sb), as well as niobium (Nb) and/or tantalum (Ta), and optionally further comprising one or more metal compounds selected from the group including bismuth (Bi), germanium (Ge), cerium (Ce), gallium (Ga), cobalt (Co), iron (Fe), and nickel (Ni) (hereinafter referred to as the dry precursor). Any suitable method known in the prior art may be employed for producing said dry precursor. For example, said dry precursor can be produced by precipitating one or more solutions, preferably aqueous solutions comprising molybdenum, vanadium, niobium and tellurium, and also by hydrothermal synthesis (W02006100128).

According to the present invention, step a) of producing a dry precursor of a multicomponent catalyst based on transition metal oxides comprises the following stages:

ai) mixing with water at least one compound of each element selected from the group consisting of Mo, V, Te and/or Sb, as well as Nb and/or Ta, and optionally one or more metals selected from the group including Bi, Ge, Ce, Ga, Co, Fe, and Ni to produce an aqueous slurry of transition metals;

a ₂ ) drying the aqueous slurry of transition metals to remove the solvent and obtain a dry catalyst precursor;

a ₃ ) optionally impregnating the dry catalyst precursor with a modifying additive solution and subsequently drying to produce a modified dry catalyst precursor.

The production of a dry catalyst precursor (step (a) which includes stages (ai- af) is described in detail below. Stage ai) comprises mixing with water at least one compound of each element selected from the group consisting of Mo, V, Te and/or Sb, as well as Nb and/or Ta, and optionally one or more metals selected from the group including Bi, Ge, Ce, Ga, Co, Fe, and Ni to produce an aqueous slurry of transition metals.

In order to obtain an aqueous slurry, it is preferable to provide two aqueous solutions first. One of said solutions (solution 1) comprises Mo, V, Te and/or Sb, while the other solution (solution 2) comprises Nb and/or Ta and optionally one or more metals selected from the group including Bi, Ge, Ce, Ga, Co, Fe, and Ni. Solution 1 is prepared by adding at least one compound of elements Mo, V, Te and/or Sb to distilled water under constant stirring at a temperature from 80 to l00°C, preferably from 50 to 90°C, followed by further cooling down the resulting solution to room temperature. Solution 2 is prepared by adding at least one compound of Nb and/or Ta and, if necessary, other metal compounds selected from, for example, Bi, Ge, Ce, Ga, Vo, Fe, and Ni to distilled water under constant stirring, wherein said solution 2 is prepared at a temperature from 15 to 45°C, preferably from 20 to 35°C. Thereafter, solution 1 and solution 2, which were cooled down to room temperature, are mixed using any prior art method to produce a slurry comprising Mo, V, Te and/or Sb, Nb and/or Ta, and optionally one or more metals selected from the group including Bi, Ge, Ce, Ga, Co, Fe, and Ni. Upon mixing said solutions, the resulting slurry is maintained at room temperature for a period of time from 0.5 to 24 hours, preferably from 1 to 10 hours.

During the preparation of the aqueous slurry, it is preferable to maintain the pH in the range between 2.5 to 3.5 under which a heteropoly compound having the Anderson-type structure (HP A) is formed in the aqueous mixture. As the subsequent steps of producing a dry precursor are carried out, said heteropoly compounds decompose to form a precursor in the form of dry nanoparticles characterized by M2 and Ml phases.

The amount of distilled water for dissolving transition metal compounds therein may vary depending on solubility of the used starting compounds. Therefore, the amount of distilled water should be at least sufficient for formation of a slurry of reactants, which are a mixture of stirrable solid and liquid substances.

Stirring all the starting compounds for preparing a homogeneous aqueous slurry may be carried out by any suitable method known in the art using stirring devices (Edward L. Paul; Victor Atiemo-Obeng; Suzanne M.Kresta, eds. (2003). Handbook of Industrial Mixing: Science and Practice). The resulting slurry can be cooled down to room temperature by any suitable method known in the art, for example, using a thermostat.

According to the present invention, compounds that form a heteropoly acid or a salt thereof as a result of drying (stage a ₂ ) are utilized as starting transition metal compounds. In particular, water-soluble salts, such as molybdenum bromide, molybdenum (VI) dioxydibromide, molybdenum (VI) dioxydichloride, potassium molybdate, calcium molybdate, magnesium molybdate, sodium molybdate and vanadium bromide, vanadium (IV) oxydifluoride, vanadium (IV) oxydibromide, vanadium (II) sulfate, vanadium (II) chloride, potassium metavanadate, sodium metavanadate, magnesium metavanadate, preferably ammonium paramolybdate and ammonium metavanadate, are used as starting compounds of molybdenum and vanadium. A starting tellurium compound may be telluric acid, ammonium tellurate or tellurium dioxide. A starting antimony compound may be antimonic acid, ammonium stibate, etc.

A niobium and/or tantalum compound is used as a solid, a mixture or a dispersion in a suitable medium. Niobic acid and niobium oxalate may be a starting niobium compound. Tantalic acid may be used as a starting tantalum compound. In the event that acids are employed, it is preferable to remove acidic impurities, which may contaminate these acids, by washing with an aqueous ammonia solution and/or water. The concentration of a compound of niobium and/or tantalum (on the basis of a metal) in a preliminary niobium-containing and/or tantalum-containing aqueous solution is maintained in the range between 0.2 and 0.8 mol/kg of the solution or mixture. Corresponding nitrates may be starting bismuth compounds. Chlorides, sulfates, nitrates, oxides or acetates of transition metal elements may also be utilized as starting compounds of transition metals other than Mo, V, Te, Sb, Nb, and Ta.

In one of the preferred embodiments of the invention, the catalyst is modified with silica. It has been found that adding silica sol in an amount from 5 to 50% by weight, preferably from 10 to 25% by weight, in the step of preparing an aqueous slurry leads to improvement of catalytic properties of the produced catalyst, which, particularly, may be associated with extension of the specific surface of the catalyst in regard to a non-modified catalyst. In order to produce a silica-modified catalyst, silica sol is added at any moment of the preparation of an aqueous slurry; silica sol is advantageously added to a slurry obtained by mixing solutions 1 and 2.

Stage a2) further comprises drying the aqueous slurry of transition metals for removing a solvent to obtain a dry residue. Among prior art methods (JP2003053190, EP0962253, JP2006159190) - evaporation, freezing, rotary drying, vacuum drying or spray drying - the most preferable for accomplishing the object of the present invention is spray drying. Spray drying rests on drying an aqueous suspension onto a hot plate or on injecting an aqueous suspension into a carrier gas stream, usually air heated to a temperature from 100 to 300°C, followed by separating solid particles. At that, water is rapidly removed from the suspension, with a heteropoly acid being preserved, which, under subsequent heat treatment, allows producing a dry precursor which is more homogeneous in terms of phase composition than the one obtained by slow drying.

Stage a3). Stage (a ₃ ) comprises optionally impregnating the dry catalyst precursor with a modifying additive solution and subsequently drying to produce a modified dry catalyst precursor.

The transformation of propane to acrylic acid is known to run through intermediate steps of forming propylene and acrolein. Each of said steps requires active centers determined by acidic properties of the generating product and the reactant to be oxidized. Catalyst efficiency may be raised by suppressing deep oxidation of intermediates and target products by adjusting acid-base properties of active centers. One of the methods for changing acid-base properties is the modification of a catalyst with additives which electronegativity differs from that of cations of the major composition.

For these purposes, in accordance with the present invention, the dry precursor is post-synthetically modified with a promoter additive based on at least one compound of transition metals selected, for example, from the group of Bi, Ge, Ce, Ga, Co, Fe, Ni; preferably, at least one compound of Ce and/or Ge is used as the promoter additive.

It is advantageous to subject the dry precursor to calcination at a temperature from 300 to 600°C prior to impregnating the dry catalyst precursor with a modifying additive. The impregnated catalyst precursor is dried at a temperature from 100 to 200°C, preferably l20°C, for a period of time from 10 to 20 hr to produce a post- modified dry catalyst precursor.

Therefore, a dry catalyst precursor, which may optionally be modified with promoter additives, is prepared by means of step (a) comprising stages (ai-a ₃ ). The so- obtained dry precursor may be used as a catalyst for the gas-phase oxidation of propane. However, as stated above, for a dry catalyst precursor to be utilized in industrial propane oxidation processes, it should be molded to prepare a granular catalyst based on mixed oxides of transition metals having the general formula MO _j V _a B _b C _c X _d O _n , wherein X denotes at least one element selected from Bi, Ge, Ce, Ga, Co, Fe, and Ni, B represents Te and/or Sb, C represents Nb and/or Ta, a, b, c, d are atomic ratios of metals, wherein 0.006<a<l, 0.006<b<l, 0.00l<c<0.5, 0<d<0.l, preferably 0.25<a<0.35, 0.l7<b<0.23, 0.l0<c<0.l4, 0<d<0.l, n is dependent on the oxidation state of elements included in the catalyst. At that, the catalyst of the formulation set out above may optionally be modified by silica (Si0 ₂ ) sol in an amount from 5 to 50% by weight, preferably from 10 to 25% by weight.

According to the present invention, the molding process is granulation, i.e. formation of solid catalyst granules having a certain size and shape. The process is based on pressing a dry catalyst precursor using inert processing additives (binders) on special machinery. The molding process according to the invention comprises recycling fine catalyst particles to the pressing step thereby performing return granulation. The authors have discovered that use of a combination of talc and graphite in different molding steps allows making a granular catalyst without reduction of its catalytic properties, said catalyst being characterized by high mechanical strength and stability under reaction conditions.

The following indices are understood under the term“catalytic properties of a catalyst” with respect to the oxidation reaction in accordance with the present invention:

- conversion (conversion degree) is the amount of an reacted reagent relative to its starting amount;

- selectivity is characterized by a ratio of the rate of formation of a target product to the overall rate of conversion of a stating substance under a certain composition of the reaction mixture and a temperature. According to this aspect of the present invention, after the dry precursor has been prepared in step (a), it is subjected to molding (step b) comprising the following stages:

bi) mixing the dry precursor obtained in step a) with talc taken in an amount from 1 to 4% by weight;

b ₂ ) forming a catalyst fraction having a particle size from 0.25 to 0.5 mm;

b ₃ ) mixing the catalyst fraction obtained in stage (b ₂ ) with graphite taken in an amount from 0.5 to 3% by weight;

b ₄ ) pressing the composition formed in stage (b ₃ ) to produce a granular catalyst; b ₅ ) subjecting the granular catalyst obtained in stage (b ₄ ) to heat treatment.

In accordance with the present invention, the content of talc and graphite is based on 100% of the catalyst composition comprising a dry catalyst precursor and processing additives: talc and graphite.

The molding of a dry catalyst precursor (step (b) which includes stages (bi-bs )) is described in detail below.

Stage bi). The dry catalyst precursor obtained in step (a) is mixed with talc, if necessary, using any prior art mixing equipment (Chemical Technology, F.A.Henglein, p. 49-58). According to the invention, talc is used in an amount from 1 to 4% by weight, preferably 2% by weight.

As talc, the present invention employs any known mineral of the layer-silicate subclass having the general formula Mg ₃ [Si ₄ O, ₀ ](OH) ₂ , wherein magnesium is substituted with Fe, Ni, Al, Cr. It is preferable to use talc ground to powder with a predominant particle size of not greater than 12 pm, preferably not greater than 8 pm. In addition, the talc used in the present invention is characterized by water (moisture) content of less than 0.5% by weight.

Stage b ₂ ). According to the invention, after the step of mixing the dry precursor with talc, a catalyst fraction having a particle size from 0.25 to 0.5 mm is formed in stage (b ₂ ). For this purpose, the mixture of the dry catalyst precursor and talc obtained in stage bi) is pressed to produce granules or pellets. The pressing is performed on a hydraulic press at a pressure from 20 to 50 t/cm ² , preferably 30 t/cm ² . A fraction having a particle size from 0.25 to 0.5 mm is produced by comminuting the obtained pellets. Said comminution is performed by any suitable method known in the prior art, for example, using a teeth granulator. The remaining fraction with a particle size of less than 0.25 mm is added to the starting dry precursor mixed with talc and is subjected to compaction again.

According to the present invention, stages (b ₃ ) and (b ₄ ) relate to formation of catalyst granules having a specific size and shape through pressing the fraction having a particle size from 0.25 to 0.5 mm obtained in stage (b ₂ ).

Stage b3). The catalyst fraction having a particle size from 0.25 to 0.5 mm obtained in stage (b ₂ ), is mixed with an additive, which latter is graphite. In accordance with the present invention, graphite is used in an amount from 0.5 to 3% by weight, preferably 1% by weight. The particle size of graphite is preferably from 2 to 25 pm. When selecting graphite for the application according to this invention, it is also advantageous to consider such indices as ash content, i.e. a percentage of a noncombustible residue generated after complete combustion, and the presence of moisture, i.e. weight content of water. It is preferable to use graphite with an ash content of not greater than from 13 to 25% by weight, preferably not greater than 18% by weight, and with a water content of less than 1% by weight.

Stage b4) comprises pressing the composition formed in stage (b ₃ ) to produce a granular catalyst of the desired shape and size, having sufficient mechanical strength. The optimal size and shape of the catalyst are such that permit achieving the predetermined performance and minimizing hydraulic resistance with minimum reactor expenses. In order to run a reaction of the selective oxidation of propane, a catalyst may be used in the form of granules having a shape selected from spherical, cylindrical, annular, and combinations thereof. Use of cylindrical granules is preferred in the process of propane oxidation.

Step bs). The catalyst granules obtained in stage (b ₄ ) are subjected to heat treatment to produce a calcined granular catalyst. The heat treatment of the catalyst granules is preferably carried out by calcining at a temperature from 300 to 600°C. In accordance with the present invention, in cases when tellurium is additionally contained in a multicomponent catalyst, it is advantageous to calcine the catalyst granules in an air stream at a temperature from 275 to 350°C, more preferably 3 l0°C, for a period of time from 10 to 20 min so as to minimize loss of tellurium. Afterwards, the catalyst should be subjected to calcination in an inert gas (nitrogen, argon, helium) stream or in a stream of air with an inert gas, or in vacuum, or in the absence of air at a temperature of 600°C for a period of time from 1.5 to 3 hours, preferably 2 hours.

During said high-temperature heat treatment, a final phase and chemical formulation of a catalyst according to yet another aspect of the present invention is formed: Mo,V _a B _b C _c X _d O _n , wherein X is at least one of elements selected from Bi, Ge, Ce, Ga, Co, Fe, and Ni, B is Te and/or Sb, C represents Nb and/or Ta, a, b, c, d are atomic ratios of metals, wherein 0.006<a<l, 0.006<b<l, 0.00l<c<0.5, 0<d<0.l, preferably 0.l<a<0.5, 0.l<b<0.5, 0.00l<c<0.5, 0<d<0.05, most preferably 0.25<a<0.35, 0.l7<b<0.23, 0.l0<c<0.l4, 0<d<0.l, n is dependent on the oxidation state of the elements included in the catalyst formulation, said catalyst may optionally be modified by silica sol in an amount from 5 to 50% by weight, preferably from 10 to 25% by weight.

The catalyst obtained in accordance with the present invention is preferably characterized by the presence of 2 crystal phases which have characteristic peaks at 20 of 6.6, 7.9, 9.0, 22.1, 27.2° (Ml phase), and at 20 of 22.1, 28.2, 36.1, 45.1, 50.0° (M2 phase), where 20 is a diffraction angle of incidence of an X-ray beam. It is also preferable to provide the M1/M2 ratio in the catalyst of about 9/1, which allows enhancing process selectivity, because the propane activation occurs in the Ml phase, while the M2 phase ensures the propane oxidation to acrylic acid. Also preferable are the instances when the catalyst is substantially enriched by the active Ml phase, while the M2 phase ranges from 2.2 to 20%. The active phase of VMoOs is further present in catalysts of the formulas MoVTeNbGa, MoVTeNbNi, MoVTeNbSe.

The specific surface of the granular catalyst is preferably from 2 to 12 m ² /g, preferably from 5 to 7 m ² /g.

The granular catalyst produced by any of the methods described above is used in the gas-phase oxidation of alkanes, in particular, in the oxidation of propane for producing acrylic acid, which is an aspect of the present invention, too.

A method for producing acrylic acid by the oxidation of propane in the presence of the granular catalyst obtained according to this invention, comprises the following steps: i) supplying a feed gas stream comprising propane, water vapor, and an oxidizing agent to a propane oxidation reaction zone in a reactor, which zone contains the granular catalyst for the oxidation of propane to acrylic acid;

ii) supplying the gas mixture comprising acrylic acid, unreacted propane and propylene, carbon oxides, and optionally trace amounts of acrolein and acetic acid, obtained at an outlet of the reaction zone to a separation step of separating liquid and gas products followed by recovering acrylic acid.

The method for producing may comprise the recycling step of recycling unreacred propane and/or propylene to the reaction zone. In this instance, a target product is separated from the gas mixture escaping the reaction zone of the propane oxidation, while the unreacted propane left in said gas mixture, as a rule, together with the unreacted propylene contained in the mixture, is recycled to the oxidation step.

As an oxidizing agent according to the invention, molecular oxygen may used, which latter is added to the reaction gas mixture, for example, as such or in a mixture with gases, mainly inert ones.

Once the process for producing acrylic acid is accompanied by massive heat release, the hydrocarbon feedstock supplied to the reaction zone is preferably diluted with a gas, which, owing to its thermal capacity, is capable of absorbing the heat generated during the reaction. At least one gaseous inert diluent selected from N ₂ , H ₂ 0, C0 ₂ , He, Ar, saturated Ci-C ₅ hydrocarbons is employed as such a gas (for example, subject to DE-A1924431 and EP-A 293224, etc.).

The propane oxidation for producing acrylic acid is preferably carried out at a temperature from 320 to 420°C, especially preferably from 350 to 390°C. The working pressure of the process is preferably from 0.5 to 5 bar, especially preferably from 1 and 3 bar.

Propane can be oxidized in a reactor in the presence of a fixed-bed granular catalyst. In order to increase the yield of acrylic acid, the oxidation of propane is preferably performed not in a single reactor, but in two or more consecutively or concurrently connected reactors.

According to the invention, the vapor-phase catalytic oxidation can be carried out in a tubular reactor. Performing oxidation processes in tubular reactors allows adjusting the reaction temperature by means of efficient withdrawal of a large amount of heat released during the vapor-phase catalytic oxidation reaction, in which the substance to be oxidized contacts molecular oxygen in the presence of a solid catalyst. This method protects the catalyst from destruction caused by local overheating of a catalyst bed (hot spot formation).

This invention will now be further described by reference to the examples given below. Such examples are purely illustrative and are not to be taken as exhaustive of the invention.

Examples of carrying out the invention

Example 1. Preparation of a dry precursor Mo _j V _{0 3} Te ₀₂₃ Nb _{0 12} + 10 % Si0 ₂ l7.7g of ammonium paramolybdate, 5.28 g of telluric acid, and 3.51 g of ammonium metavanadate are dissolved in 100 ml of distilled water at a temperature of 80°C and under intense stirring (solution I). 19.8 ml of a niobium oxalate solution, in which the Nb concentration is 56.3 mg Nb/ml, is added to cooled solution I. Si0 ₂ sol in an amount of 10% by weight is also added to the resulting solution. Using a laboratory spray dryer, water is removed from the so-obtained wet precursor - a bright orange gel. The produced dry precursor having the cationic formulation Mo,V _{0 3} Te ₀₂₃ Nb ₀ , ₂ , modified by Si0 ₂ in an amount of 10% by weight, is further used to produce a granular catalyst.

Example 2. Preparation of a dry precursor Mo _j V _{0 25} Te _{0 17} Nb _{0 1} + 10 % Si0 ₂ l7.7g of ammonium paramolybdate, 3.90 g of telluric acid, and 2.925 g of ammonium metavanadate are dissolved in 100 ml of distilled water at a temperature of 80°C and under intense stirring (solution I). 19.8 ml of a niobium oxalate solution, in which the Nb concentration is 56.3 mg Nb/ml, is added to cooled solution I. Si0 ₂ sol in an amount of 10% by weight is also added to the resulting solution. Using a laboratory spray dryer, water is removed from the so-obtained wet precursor - a bright orange gel. The produced dry precursor having the cationic formulation Mo, ^ _0.25 ^ ^e _0.17 ^ _{0. P} modified by Si0 ₂ in an amount of 10% by weight, is further used to produce a granular catalyst.

Example 3. Preparation of a dry precursor MO _j V ₀₃₅ Te _{0 23} Nb _{0 14} + 10 % Si0 ₂ l7.7g of ammonium paramolybdate, 3.90 g of telluric acid, and 4.095 g of ammonium metavanadate are dissolved in 100 ml of distilled water at a temperature of 80°C and under intense stirring (solution I). 23.1 ml of a niobium oxalate solution, in which the Nb concentration is 56.3 mg Nb/ml, is added to cooled solution I. Si0 ₂ sol in an amount of 10% by weight is also added to the resulting solution. Using a laboratory spray dryer, water is removed from the so-obtained wet precursor - a bright orange gel. The produced dry precursor having the cationic formulation Mo ₁ V _{0 35} Te ₀₂₃ Nb _{0 14} , modified by Si0 ₂ in an amount of 10% by weight, is further used to produce a granular catalyst.

Example 4. Preparation of a dry precursor Mo _i n ₀₃ Te _{0 23} N5 _{0 12} Bi _{0 02} + 10 %

Si0 ₂

This example is similar to Example 1 with the exception that it contains a bismuth modifying additive. The bismuth content is 0.02 mo.% with respect to molybdenum.

Example 5. Preparation of a dry precursor MO _j V ₀₃ Sb _{0 23} Ta _{0 12}

l7.7g of ammonium paramolybdate, 3.93 g of antimonic acid, and 3.51 g of ammonium metavanadate are dissolved in 100 ml of distilled water at a temperature of 80°C and under intense stirring (solution I). 19.8 ml of a solution of tantalic acid freshly made from tantalum chloride, in which the Ta concentration is 109.6 mg Ta/ml, is added to cooled solution I. Using a laboratory spray dryer, water is removed from the so-obtained wet precursor. The produced dry precursor having the cationic formulation Mo _j V _{Q j} Sb _Q ^Ta _{Q 12} is further used to produce a granular catalyst.

Example 6. Molding a catalyst using processing additives in the stage of production of a granulated material (additive-1)

For the purpose of studying influence of processing additives during molding on catalytic properties of catalysts, additives of a different nature have been tested: graphite, sodium stearate, starch, talc.

The dry precursor powder obtained according to Examples 1-5 is mixed with a processing additive (additive- 1). The resulting mixture is then subjected to pressing on a hydraulic press at a pressure of 30 t/cm ² to produce granules in the form of pellets having a diameter of 50 mm and a height of 8 to 10 mm. Using a teeth granulator, a fraction with a particle size from 0.25 to 0.5 mm (hereinafter referred to as the granulated material) is produced from the obtained pellets. The remaining fraction with a particle size of less than 0.25 mm is added to the starting dry precursor mixed with additive- 1 and is subjected to pressing on a hydraulic press again. For the purpose of producing a granular catalyst, the fraction obtained above (granulated material) is further pressed. However, use of additives exclusively in the step of producing the granulated material does not allow forming catalyst granules of the desired shape due to their full destruction during the compaction.

Therefore, in order to determine influence of additives on catalytic properties, a calcined granulated material representing a catalyst fraction with a particle size from 0.25 to 0.5 mm mixed with an additive was tested.

Catalytic properties of the obtained samples are set out in Tables 2-6.

Example 7. Molding a catalyst using processing additives in the stage of production of a granulated material and in the stage of pressing (pelletizing) (additive 1 + additive 2)

The granular catalyst was produced similarly to Example 6, with the exception that the granulated material was further mixed with a processing additive (additive-2), followed by pressing (pelletizing) said mixture into cylindrical granules of an annular shape, having an outer diameter of 5 mm, a wall thickness of 1.5 mm, and a height of 5 mm, on the Courtoy R-53 UE tablet press.

The so-obtained catalyst granules were then calcined in two stages: a low- temperature stage (280 to 320°C) under air and a high-temperature stage (550 to 570°C) under an inert gas stream.

Graphite, talc, and their combinations with different weight ratios were tested as additive- 1 and additive-2.

Catalytic properties of the obtained samples are set out in Table 7.

Example 8. Oxidation of propane to acrylic acid

Catalytic properties of the catalysts were determined in the propane oxidation reaction in a laboratory flow-through plant at a temperature of 380°C. A reaction mixture having the following composition:

propane (C ₃ H ₈ ) - 5 vol.%;

oxidizing agent (air) - 65 vol.%;

water vapor - 30 vol.%,

was passed through the catalysts produced according to Example 6 and 7.

As a reference sample, the dry precursor produced according to Example 1 was taken, which precursor, prior to the oxidation, had been subjected to calcining in two stages: a low-temperature stage (280 to 320°C) under air and a high-temperature stage (550 to 570°C) under an inert gas stream.

Results of the tests are set out in Tables 2-7.

Table 2 - Catalytic properties of the catalysts (based on the dry precursor according to Example 1), produced according to Example 6 o

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Table 3 - Catalytic properties of the catalysts (based on the dry precursor according to Example 2), produced according to Example 6

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Table 4 - Catalytic properties of the catalysts (based on the dry precursor according to Example 3), produced according to Example 6

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Table 5 - Catalytic properties of the catalysts (based on the dry precursor according to Example 4), produced according to Example 6

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Table 6 - Catalytic properties of the catalysts (based on the dry precursor according to Example 5), produced according to Example 6

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Table 7 - Catalytic properties of the granular catalysts produced according to Example 7

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Tables 2-6 show the influence of the processing additives used during molding of the dry precursor on catalytic properties of the produced granulated materials. It has been demonstrated that talc is preferable as processing additive 1 , because considerable reduction of the catalytic properties of the obtained granulated material in regard to the additive-free dry catalyst precursor does not occur in this instance. The inventors have failed to carry out further compaction of the produced granulated materials so as to obtain a granular catalyst having the desired shape and size.

It is apparent from Table 7 that the best technical result is achieved by means of using the following combination of processing additives: processing additive 1, namely talc, in the step of producing the granulated material, and processing additive 2, namely graphite, in the step of pressing. At that, it is advantageous to use the additives in the amounts of 2 wt.% and 1 wt.% for talc and graphite, correspondingly.

Example 9. Determination of mechanical strength of the granular catalysts produced according to Example 7 (graphite, talc - combinations thereof)

Crush strength was determined on a MA.TEC.CRUSH-BK apparatus according to ASTM D4179-11. A sample of 24 granules for axial crush and a sample of 24 granules for radial crush were formed by random selection. The statistical treatment was performed in conformity with ASTM D4179-11. 2 minimal and 2 maximal values were rejected from 24 obtained results. The average crush strength was calculated on the basis of the remaining 20 values using formulas (l)-(3).

The axial crush strength for cylindrical granules, s _r kg/mm ² , was determined by the formula:

where P is the force under which a granule crushes, kg;

S is the sectional area of a granule, mm ² ;

d is the diameter of a granule, mm.

The radial crush strength for cylindrical granules, s _r kg/mm ² , was determined by the formula:

— P ^p

sR ^~ s ^~ id ( ²

where P is the force under which a granule crushes, kg;

S is the sectional area of a granule, mm ² ;

d is the diameter of a granule, mm; 1 is the length of a granule, mm.

The average crush strength, kg/mm ² , is calculated on the basis of the remaining 20 values according to the formula:

where s _r. - is the crush strength of granules tested in a certain test, kg/mm ² .

The obtained results are set out in Table 8.

Table 8 - Mechanical strength of the granular catalysts

**s _r - average crush strength

As apparent from the results set out in Table 8, granular catalysts molded by means of a combination of additives, talc and graphite, differ in terms of their mechanical strength. The catalyst obtained with the addition of graphite in the step of producing the granulated material and in the step of compacting (pressing) (graphite + graphite) has both less axial crush strength (by 8.6 relative %) and less radial crush strength (by 29.4 relative %) in comparison with catalyst samples which are molded with the addition of talc in the step of producing the granulated material. In this case, the relative percentage is given for the catalyst produced from the precursor according to Example 1.

An endurance test has been performed in Example 10 in order to confirm stability of granular catalysts molded in the presence of the additives: talc (2 wt.%) and graphite (1 wt. %).

Example 10. Endurance test of granular catalysts molded in the presence of additives: talc (2 wt. %.) and graphite (1 wt. %.)

The method according to Example 8, in which stability of granular catalysts molded by means of additives: talc in an amount of 2 wt. % in the step of granulating and graphite in an amount of 1 wt.% in the step of compacting (pressing), was tested.

In accordance with the Example, a fraction of a granular catalyst was loaded into a reactor having a diameter of 9.6 mm. Said catalyst was reacted with a reaction mixture having the following composition: 55 vol.% of propane (C ₃ Hg), 20 vol.% of water vapor, 25 vol.% of an oxidizing agent (oxygen). The contact time was 2 s, the reaction temperature was 360°C. Results of the experiment are given in Table 9.

Table 9 - results of the endurance test according to Example 10

It can be seen from the results of Example 10 that the granular catalyst molded using the following additives: talc in an amount of 2 wt.% in the step of producing granulate and graphite in an amount of 1 wt.% in the step of compaction, maintains relatively high activity and selectivity to the target product within the entire propane oxidation process (over 230 h).

Therefore, it has been demonstrated that the method for producing a dry catalyst precursor is reproducible and is capable of providing a catalyst with catalytic properties that allow using such a catalyst in oxidation processes, particularly, in processes of the propane oxidation for producing acrylic acid.

Moreover, it has been shown that use of processing additives in the steps of molding a dry precursor makes it possible to produce catalyst granules having the desired shape and size without considerable deterioration of catalytic properties with respect to the starting dry precursor (Table 7). Noteworthy, it is important to use the following combination as such additives: talc in the step of producing the granulated material (step (bi) in accordance with the present description) and graphite in the stage of compacting said granulated material (step (b ₃ )). It is preferable to use talc in an amount between 1 and 4% by weight, most preferable 2% by weight. It is preferable to use graphite in an amount between 0.5 and 3% by weight, most preferable 1% by weight.

The so-obtained catalyst is characterized by high mechanical strength (Example 9, Table 8), as well as performance stability in oxidation processes (Example 10, Table 9)·