DE10046672A1 - Production of acrolein or acrylic acid involves absorption of propane and propene from a gas mixture followed by desorption and oxidation, with no catalytic dehydrogenation of propane and no added oxygen - Google Patents

Abstract

Acrolein and acrylic acid are produced from propane and propene by absorption/desorption of propane and propene from a gas mixture followed by oxidation, in which no heterogeneously catalyzed dehydrogenation of propane occurs between the desorption and oxidation stages without addition of oxygen. Acrolein and acrylic acid are produced from propane and propene by: (a) separation of propane and propene from a gas mixture (A) by absorption in an absorption medium; (b) separation of propane and propene from the absorbent to give a mixture (B) containing propane and propene; and (c) using mixture (B) for oxidation to the required product(s). In this process, no catalytic dehydrogenation of propane without the addition of oxygen is carried out between stages (b) and (c).

Description

The present invention relates to a process for the preparation of acrylic acid by heterogeneously catalyzed gas-phase oxidation of propane with molecular oxygen at elevated temperature on a multimetal oxide composition of the general formula I.

Mo ₁ V _b M _c ¹ M _d ² O _n (I),

With
M ¹ = Te and / or Sb,
M ² = at least one of the elements from the group comprising Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Ga, Fe, Ru, Co, Rh, Ni, Pd, Pt, Bi, B and Ce,
b = 0.01 to 1,
c = 0.01 to 1,
d = 0.01 to 1 and
n = a number which is determined by the valency and frequency of the elements other than oxygen in (I),
whereby the X-ray diffractogram of the multimetal oxide mass (I) has diffraction reflections h, i and possibly k, whose apex points at the diffraction angles (2⊖) 22.2 ± 0.4 ° (h), 27.3 ± 0.4 ° (i ) and 28.2 ± 0.4 ° (k).

Multimetal oxide compositions which ent one of the general formula I. speak speaking stoichiometry are known (see e.g. EP-A 608 838, EP-A 529 853, JP-A 7-232 071, JP-A 10-57813, JP-A 2000-37623, JP-A 10-36 311, Proceedings ISO '99, Rimini (Italy), Sept. 10-11, 1999, G. Centi and 5th Perathoner Ed., SCI Pub. 1999, EP-A 767 164, Catalysis Today 49 (1999), pp. 141-153, EP-A 962 253, Applied Catalysis A: General 194 to 195 (2000), p. 479 to 485, JP-A 11/169 716, EP-A 895 809 and DE-A 198 35 247) and were already in the older application DE-A 100 29 338 proposed. Multimetal oxide materials that are chemical  Composition as the multimetal oxide materials (I) have also known from WO 00/29106.

In the prior art cited above is also already beaten, multimetal oxide masses of such a composition as catalysts for the heterogeneously catalyzed gas phases oxidation of propane to acrylic acid or for the heterogeneous cataly gated gas phase ammonium oxidation of propane to acrylonitrile turn.

Acrylic acid and acrylonitrile are important ethylenically unsaturated Connections, both as such and in the form of their Find alkyl esters for the preparation of polymers.

According to the teaching of WO 00-29106, multimetal oxide materials are suitable the relevant composition then as catalysts for the he terogenically catalyzed gas phase oxidation of propane to acrylic acid, if they have a substantially amorphous structure, which in the X-ray diffractogram in the form of very wide diffraction reflections with vertices in the diffraction angle (2⊖) range around 22 ° and around 27 ° is shown.

In contrast, EP-A 529 853 and the EP-A 608 838 both for use as catalysts for the heterogeneously catalyzed gas phase oxidation of propane to acrylic acid as well as for use as catalysts for the he Terogenically catalyzed gas phase oxidation of propane to acrylic nitrile required for the multimetal oxide masses of the rele have a specific crystalline structure indicate that in the form of very narrow X-ray diffractogram Diffraction reflexes is mapped and under which the diffraction reflex flexe at 2⊖ vertex positions of 22.1 ± 0.3 °, 28.2 ± 0.3 °, 36.2 ± 0.3 °, 45.2 ± 0.3 ° and 50.0 ± 0.3 ° the structural characteristics Main diffraction peaks.

In contrast to EP-A 529 853 and EP-A 608 838, the JP-A 11-169 716 for a corresponding usability of the multi metal oxide masses of the relevant composition a specific crystalline structure of the multimetal oxide masses, which in the X-ray diffractogram also in the form of very schma len diffraction reflexes is mapped, in addition to that in the EP-A 529 853 and the diffraction reflections required in EP-A 608 838 However, JP-A 11-169716 still considers the presence of Beu reflection reflections with 2⊖ vertices at 9.0 ± 0.3 °, 27.3 ± 0.3 °, 29.2 ± 0.3 ° and 35.4 ± 0.3 ° in the X-ray diffractogram as essential. JP-A 11/169716 considers one in particular simultaneous presence of the two diffraction reflexes with  2⊖ vertex positions at 28.2 ± 0.3 ° and 27.3 ± 0.3 ° with off weighed reflex intensities than for a satisfactory ammono Oxidation catalyst performance necessary, the diffraction flex with a 2⊖ vertex position at 27.3 ± 0.3 ° ins especially for adequate propane conversion and diffraction reflex with a 2⊖ vertex position at 28.2 ± 0.3 ° ins especially for a satisfactory selectivity of the acrylonitrile responsibility.

In "Ammoxidation of propane over Mo-V-Nb-Te mixed oxide catalysts "from" Spillover and Migration of Surface on Catalysts ", Ed. by Can Li and Qin Xin, Elsevier Science B.V. (1997), pp. 473 ff the inventors of JP-A 11-169716 explain that their spe culinary opinion after each of the above two diffraction rules flexe represents a crystal phase, which here corresponds to the Designation of the diffraction reflexes as "i-phase" and as "k-phase" should be designated. The ideal multimetal as a catalyst Oxide mass is now a balanced, intimate mixture of crystalline ner i-phase and crystalline k-phase, with the i-phase off finally the activation of the propane coincides and the k phase for a selective conversion of the activated on the i-phase Responsible for propane to acrylonitrile.

DE-A 198 35 247 confirms the above view and teaches that the proportion of the i phase, based on the sum of i and k phase (expressed in intensities of the corresponding x-ray diffraction reflections) for a reasonable catalyst performance the ammoxidation of propane to acrylonitrile in the range from 40 to 75% must move.

EP-A 895 809, which is a member of the family of industrial property rights DE-A 198 35 247, has the multimetal oxide materials DE-A 198 35 247 as well as for the heterogeneously catalyzed gas phases oxidation of propane to acrylic acid suitable catalysts.

In contrast, from JP-A 7-232071 it is known that in essence Chen only available in i-phase multimetal oxide masses of general my formula (I) also as catalysts for the heterogeneous kata lysed ammoxidation of propane to acrylonitrile are suitable.

In view of the aforementioned prior art, the object of the present invention was to provide a process for the preparation of acrylic acid by heterogeneously catalyzed gas-phase oxidation of propane with molecular oxygen at elevated temperature on a multimetal oxide composition of the general formula I,

Mo ₁ V _b M _c ¹ M _d ² O _n (I),

With
M ¹ = Te and / or Sb,
M ² = at least one of the elements from the group comprising Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Ga, Fe, Ru, Co, Rh, Ni, Pd, Pt, Bi, B and Ce,
b = 0.01 to 1,
c = 0.01 to 1,
d = 0.01 to 1 and
n = a number which is determined by the valency and frequency of the elements other than oxygen in (I),
whereby the X-ray diffractogram of the multimetal oxide mass (I) has diffraction reflections h, i and possibly k, whose apex points at the diffraction angles (2⊖) 22.2 ± 0.4 ° (h), 27.3 ± 0.4 ° (i ) and 28.2 ± 0.4 ° (k),
to provide, which shows that the multimetal oxide masses (I) need not be amorphous, nor must they contain any significant amounts of a k-phase in order to be suitable as catalysts for the heterogeneously catalyzed gas-phase oxidation of propane with molecular oxygen at elevated temperature to acrylic acid his.

Accordingly, a process for the preparation of acrylic acid by heterogeneously catalyzed gas-phase oxidation of propane with molecular oxygen at elevated temperature on a multimetal oxide composition of the general formula I,

Mo ₁ V _b M _c ¹ M _d ² O _n (I),

With
M ¹ = Te and / or Sb,
M ² = at least one of the elements from the group comprising Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Ga, Fe, Ru, Co, Rh, Ni, Pd, Pt, Bi, B and Ce,
b = 0.01 to 1,
c = 0.01 to 1,
d = 0.01 to 1 and
n = a number which is determined by the valency and frequency of the elements other than oxygen in (I),
whereby the X-ray diffractogram of the multimetal oxide mass (I) has diffraction reflections h, i and possibly k, whose apex points at the diffraction angles (2⊖) 22.2 ± 0.4 ° (h), 27.3 ± 0.4 ° (i ) and 28.2 ± 0.4 ° (k) are found, which is characterized in that

• - the diffraction reflex h within the X-ray diffractogram of the is the most intense as well as a full width at half maximum has at least 0.5 °,
• - The intensity P _{i of} the diffraction reflex i and the intensity P _{k of} the diffraction reflex k satisfy the relationship 0.8 ≦ R ≦ 1, in which R the formula
  R = P _i / (P _i + P _k )
  is defined intensity ratio, and
• - The full width at half maximum of the diffraction reflex i and one diffraction reflex k is ≦ 1 °.

According to the invention, preference is given to using multimetal oxide compositions (I) according to the invention where M ¹ = Te. Furthermore, those multimetal oxide materials (I) are favorable according to the invention in which M ² = Nb, Ta, W and / or titanium. Preferably M ² = Nb.

The stoichiometric coefficient b to be used according to the invention The multimetal oxide mass (I) is advantageously 0.1 to 0.6. The preferred range for the stoichiometric coefficient c to 0.05 to 0.4 and favorable Values for d range from 0.1 to 0.6. Particularly cheap fiction to be used according to multimetal oxide materials (I) are those at which the stoichiometric coefficients b, c and d simultaneously in the aforementioned preferred areas.

Further stoichiometries suitable according to the invention for the Multimetal oxide compositions (I) according to the invention are those which in the Writings of the prior art cited at the beginning, ins  in particular in the documents EP-A 608 838, WO 00-29106, JP-A 11/169716 and EP-A 962 253.

A targeted method for producing according to the invention using multimetal oxide materials (I), their intensity ratio n must be R ≧ 0.8 and ≦ 1 (i.e., it can also be ≧ 0.825, ≧ 0.85, ≧ 0.875, ≧ 0.9, ≧ 0.925, ≧ 0.95 and ≧ 0.975), of fenbart z. B. JP-A 11-43314, in which the relevant multi metal oxide masses as catalysts for the heterogeneously catalyzed Oxydehydrogenation of ethane to ethylene may be recommended.

Thereafter, in known per se, in the cited writings of State of the art disclosed, way a multimetal oxide mass of stoichiometry (I), which is a mixture of i- Phase and other phases (e.g. k-phase). In this mixture can now e.g. B. the proportion of i-phase can be increased in that the other phases, e.g. B. the k phase under the microscope reads out, or the multimetal oxide mass with suitable liquid washes. As such liquids come e.g. B. aqueous solution solutions of organic acids (e.g. oxalic acid, formic acid, vinegar acid, citric acid and tartaric acid), alcohols and aqueous water peroxide solution into consideration. Furthermore, the JP-A 7-232071 a process for the preparation of inventive Multimetal oxide materials (I).

Multi according to the invention are less systematic metal oxide compositions (I) can be obtained by using suitable Sources of their elementary constituents as intimate as possible, preferably finely divided, dry mixture produced and this Temperatures from 350 to 700 ° C or 400 to 650 ° C or 400 to 600 ° C thermally treated. The thermal treatment can be both under oxidizing, reducing as well as under inert atmosphere sphere. As an oxidizing atmosphere comes e.g. B. air with molecular oxygen-enriched air or oxygen enriched air. Preferably, the thermal loading action under an inert atmosphere, d. i.e., e.g. B. under molecular Nitrogen and / or noble gas. Usually done thermal treatment at normal pressure (1 atm). Selbstver thermal treatment can also be carried out under vacuum gene.

If the thermal treatment takes place in a gaseous atmosphere, it can both stand and flow.

Overall, the thermal treatment can last up to 24 hours or more Claim.  

The thermal treatment is preferably first carried out under oxi dating (oxygen-containing) atmosphere (e.g. under air) at a temperature of 150 to 400 ° C or 250 to 350 ° C. In the An finally the thermal treatment is expediently under Inert gas at temperatures from 350 to 700 ° C or 400 to 650 ° C or 400 to 600 ° C continued. Of course, the thermal Treatment also take place so that the catalyst precursor mass before their thermal treatment first (if necessary after Pulverization) tabletted (if necessary with the addition of 0.5 up to 2% by weight of finely divided graphite), then thermally treated and is subsequently split again.

The intimate mixing of the starting compounds as part of the Her position of multimetal oxide materials (I) according to the invention can be found in done dry or wet.

If it is in dry form, the starting compounds expediently used as a finely divided powder and after Mixing and optionally compacting the calcination (therm treatment).

However, the intimate mixing is preferably carried out in wet form. The starting compounds are usually in the form of a aqueous solution and / or suspension mixed together. to finally the aqueous mass is dried and after drying calcined. Appropriately, it is the aqueous mass around an aqueous solution. Preferably, the Drying process immediately after the production of the aqueous mixture and by spray drying (the outlet temperature doors are usually 100 to 150 ° C; spray drying can be carried out in cocurrent or in countercurrent), the one particularly intimate dry mix, especially when it the aqueous mass to be spray-dried is an aqueous one Solution.

As sources for the elementary constituents come in the frame the implementation of the manufacturing method described above multimetal oxide compositions (I) according to the invention all those in Consider the oxides and / or when heated (possibly in air) Capable of forming hydroxides. Of course, as such Starting compounds also oxides and / or hydroxides elementary constituents co-used or exclusively ver be turned.

Suitable sources for the element Mo according to the invention are e.g. B. Molybdenum oxides such as molybdenum trioxide, molybdates such as ammonium hepta molybdate tetrahydrate and molybdenum halides such as molybdenum chloride.  

Suitable starting compounds for element V to be used according to the invention are, for. B. vanadylacetylacetonate, vanadates such as ammonium metavanadate, vanadium oxides such as vanadium pentoxide (V ₂ O ₅ ), Va nadinhalogenide such as vanadium tetrachloride (VCl ₄ ) and Vanadinoxyhalo genide such as VOCl _3rd In this case, those which contain the vanadium in the oxidation state +4 can also be used as the starting vanadium compounds.

According to the invention, tellurium oxides such as tellurium dioxide, metallic tellurium, tellurium halides such as TeCl ₂ , but also telluric acids such as orthothelluric acid H ₆ TeO ₆ are suitable as sources for the element tellurium.

Advantageous antimony starting compounds are antimony halides, such as SbCl ₃ , antimony oxides such as antimony trioxide (Sb ₂ O ₃ ), antimonic acids such as HSb (OH) ₆ , but also antimony oxide salts such as antimony oxide sulfate (SbO ₂ ) SO ₄ .

Suitable niobium sources according to the invention are e.g. B. niobium oxides such as niobium pentoxide (Nb ₂ O ₅ ), niobium oxide halides such as NbOCl ₃ , niobium halides such as NbCl ₅ , but also complex compounds of niobium and organic carboxylic acids and / or dicarboxylic acids such as B. oxalates and alcoholates. Of course, the nb-containing solutions used in EP-A 895 809 also come into consideration as niobium sources.

With regard to all other possible elements M ² , suitable starting compounds according to the invention are, in particular, their halides, nitrates, formates, oxalates, acetates, carbonates and / or hydroxides. Suitable starting compounds are often their oxo compounds such. B. tungstates or the acids derived from these. Ammonium salts are also frequently used as starting compounds.

Furthermore come as starting connections for the creation of the Multimetal oxide materials (I) according to the invention also polyanions from Anderson type into consideration, e.g. B. in Polyhedron Vol. 6, No. 2, pp. 213-218, 1987. Another suitable Li The source of temperature for Anderson type polyanions is kinetics and Catalysis, Vol. 40, No. 3, 1999, pp. 401 to 404.

Other polyanions suitable as starting compounds are e.g. B. those of the Dawson or Keggin type. Preferably be according to the invention used such starting compounds that at elevated temperatures either in the presence or with exclusion  of oxygen releasing gaseous compounds into their Convert oxides.

The multimetal oxide compositions (I) obtainable according to the invention can as such [e.g. B. as a powder or after tableting the powder (often with the addition of 0.5 to 2% by weight of finely divided gra phit) and subsequent splitting into chips] or also shaped into shaped bodies for the process according to the invention be used. The catalyst bed can be both a Fixed bed, a moving bed or a fluidized bed.

Otherwise, the inventive method as in the EP-A 608838, WO 00/29106 and JP-A 10-36311 procedures.

That is, the reaction gas mixture with which the multi metal oxide active composition according to the invention at reaction temperatures of, for. B. 200 to 550 ° C or from 230 to 480 ° C or 300 to 440 ° C can be loaded, for. B. have the following composition:
1 to 15, preferably 1 to 7% by volume of propane,
44 to 99 vol .-% air and
0 to 55 vol% water vapor.

Reaction gas mixtures containing water vapor are preferred.

Other possible compositions of the reaction gas mixture are:
70 to 95 vol .-% propane,
5 to 30 vol .-% molecular oxygen and
0 to 25 vol% water vapor.

It is essential according to the invention that the X-ray diffractogram the multimetal oxide materials (I) Beu to be used according to the invention gung reflexes h, i and optionally k, the apex points at the diffraction angles (2⊖) 22.2 + 0.4 ° (h), 27.3 + 0.4 ° (i) and 28.2 ± 0.4 ° (k).

This, as well as all others in this document, on an X-ray diff Fractogram-related information refers to an under user Formation of Cu-Kα1 radiation as X-rays diffractogram (Siemens diffractometer Theta-Theta D-5000, Tube voltage: 40 kV, tube current: 40 mA, aperture diaphragm V20 (especially riabel), lens hood V20 (variable), secondary monochromator aperture (0.1 mm), detector aperture (0.6 mm), measuring interval (2⊖):  0.02, measurement time per step: 2.4 s, detector: scintillation counter pipe).

The intensity of a diffraction reflex in the X-ray diffraction program in this document refers to that in DE-A 198 35 247 never determination method.

That is, A ¹ denotes the vertex of a reflection 1 and B ¹ denotes B ¹ in the line of the X-ray diffractogram when viewed along the intensity axis perpendicular to the 2Θ axis, the nearest pronounced minimum (reflex shoulder-indicating minima are not taken into account) to the left of the vertex A ¹ and B ² in a corresponding manner, the closest pronounced minimum to the right of the apex A ¹ and denotes C ¹ the point at which a straight line drawn from the apex A ¹ perpendicular to the 2Θ axis connects a straight line connecting the points B ¹ and B ² intersects, then the intensity of the reflex 1 is the length of the straight section A ¹ C ¹ , which extends from the apex A ¹ to the point C ¹ . The expression minimum means a point at which the gradient gradient of a tangent applied to the curve in a base region of the refle xes 1 changes from a negative value to a positive value, or a point at which the gradient gradient serves towards zero, the coordinates of the 2⊖ axis and the intensity axis were used to determine the gradient.

The full width at half maximum is in a corresponding manner the length of the straight section which results between the two intersection points H ¹ and H ² if one draws a parallel to the 2⊖ axis in the middle of the straight section A ¹ C ¹ , where H ¹ , H ² mean the first intersection of these parallels with the line of the X-ray diffractogram to the left and right of A ¹ as defined above.

The accompanying FIG. 6 shows an exemplary implementation of the determination of the half-value width and intensity in the case of a diffraction reflex.

In addition to the diffraction reflections h, i and, if appropriate, k, the X-ray diffractogram of the multimetal oxide materials (I) to be used according to the invention generally also contains diffraction reflections whose apexes lie at the following diffraction angles (2⊖):
9.0 ± 0.4 ° (l),
29.2 ± 0.4 ° (m) and
35.4 ± 0.4 ° (n).

Often, the X-ray diffractogram of the multimetal oxide materials (I) to be used according to the invention additionally contains diffraction reflections, the apexes of which lie at the following diffraction angles (2⊖):
6.7 ± 0.4 ° (o),
7.9 ± 0.4 ° (p),
45.2 ± 0.4 ° (q).

If the X-ray diffractogram of the multi-metal oxide masses (I) according to the invention contains the diffraction reflex k, it generally contains further diffraction reflections, the apex of which lies at the following diffraction angles (2⊖):
36.2 ± 0.4 ° and
50.0 ± 0.4 °.

If the intensity 100 is assigned to the diffraction reflex h, the diffraction reflexes i, l, m, n, o, p, q often have the following intensities in the same intensity scale:
i: 5-95, often 5-80, sometimes 10-60
l: 1-30,
m: 1-40,
n: 1-40,
o: 1-30,
p: 1-30 and
q: 5-60.

Contains the X-ray diffractogram to be used according to the invention the multimetal oxide materials (I) from the aforementioned additional diffraction reflections, the full width at half maximum is the same Rule ≦ 1 °.

According to the invention, those multimetal oxide materials (I) to be used according to the invention are advantageous whose X-ray diffractogram not only fulfills the condition 0.8 ≦ R ≦ 1, but simultaneously the condition 0.8 ≦ R '≦ 1, where R' is the formula

R '= P' _i / (P ' _i + P' _k )

is defined intensity ratio and P ' _{i means} the product of P _i and the full width at half maximum of the diffraction reflex i (P' _k is there as defined as P ' _i ).

Of course, in the process according to the invention, a product gas mixture is obtained which does not consist exclusively of acrylic acid. Rather, the product gas mixture contains, in addition to unreacted propane, secondary components such as propene, acrolein, CO ₂ , CO, H ₂ O, acetic acid, propionic acid etc., from which the acrylic acid must be separated.

This can be done as it is catalyzed by the heterogeneous Gas phase oxidation of propene to acrylic acid is known.

In other words, the acrylic acid contained in the product gas mixture by absorption with water or by absorption with a high-boiling inert hydrophobic organic solvents (e.g. a mixture of diphenyl ether and diphyl if necessary may contain additives such as dimethyl phthalate) added become. The resulting mixture of absorbent and acrylic acid can then be rectified in a manner known per se, extractive and / or crystallized up to pure acrylic acid be prepared. Alternatively, the basic separation of acrylic acid from the product gas mixture also by fractional condensation done as it is e.g. B. is described in DE-A 199 24 532.

The resulting aqueous acrylic acid condensate can then z. B. by fractional crystallization (z. B. Suspensions crystallization and / or layer crystallization) further cleaned become.

The residual gas remaining in the basic separation of acrylic acid mixture contains in particular unreacted propane. This can from the residual gas mixture z. B. by fractional pressure rectifiers tion separated and then in the gas according to the invention phase oxidation are returned. However, it is cheaper that Residual gas in an extraction device with a hydrophobic or ganic solvent in contact (e.g. by same passage), which the propane prefers to absorb can.

Subsequent desorption and / or stripping with air can the absorbed propane is released again and into the invention appropriate procedures are returned. In this way we are host total propane sales achievable.  

According to the invention, it is surprising that when using Multi metal oxide masses of stoichiometry (I) as catalysts for the Gas phase catalytic oxidation of propane to acrylic acid, in the Un differed from the view held in the prior art that the i- and not the k phase and not a simultaneous existence be of crucial importance from the i and k phases seems. It is also unexpected that the i-phase Multimetal oxide materials (I) with regard to use as Catalysts for heterogeneously catalyzed gas phase oxidation from propane to acrylic acid have a higher activity than if they are in the k phase.

Furthermore, it is surprising that the multimetal oxide according to the invention mass (I) regarding their use as catalysts for hete Roe-catalyzed gas phase oxidation of propane to acrylic acid are of satisfactory stability.

Of course, the multi to be used according to the invention metal oxide masses (I) also with fine, z. B. colloidal, Materials such as silicon dioxide, titanium dioxide, aluminum oxide, Zirconium oxide, niobium oxide, diluted form in the ver driving can be used.

The dilution mass ratio can be up to 9 (thinner): 1 (Active mass). That is, possible diluent mass ver Ratios are z. B. 6 (thinner): 1 (active composition) and 3 (Thinner): 1 (active composition). The thinner can be incorporated before and / or after the calcination. Does the familiarization before the calcination, the thinner must be selected that he essentially received as such during the calcination remains. This is e.g. B. in the case of at a correspondingly high temperature Usually fired oxides are given.

The multimetal oxide catalysts to be used according to the invention are also suitable for the gas-phase catalytically oxidative production of methacrylic acid from its C ₄ precursors, such as, for. B. n-butane or isobutane.

Examples example 1

50.48 g of ammonium metavanadate (77.5% by weight of V ₂ O ₅ , from GfE in Nürnberg, DE) were placed at 80 ° C. in 1782 ml of water (3 l round-bottom flask, KPG stirrer, heating element) Stirring dissolved. A yellowish, clear solution resulted. This solution was cooled to 60 ° C. and then, while maintaining the 60 ° C. in the order mentioned, 67.3 g of telluric acid (99% by weight H ₆ TeO ₆ , from Fluka, DE) and 253.2 g of ammonium heptamolybdate (81.5% by weight of MoO ₃ , from Starck, in Goslar, DE). A deep red solution A resulted.

79.2 g of ammonium nioboxalate (20.2% by weight of Nb, from Starck in Goslar, DE) were dissolved in 300 ml of water at 60 ° C. in a 1-liter beaker to obtain a solution B. Solutions A and B were cooled to 30 ° C. and combined at this temperature with stirring, solution B being added to solution A. The addition took place over a period of 5 minutes. An orange suspension was formed. This suspension was then spray-dried (spray dryer Niro A / S Atomizer Portable Minor plant, centrifugal atomizer from Niro, DK;. The temperature of the storage container was 30 ° C; T ^a = 240 ° C, T ^out = 110 ° C) to give a orange spray powder. 100 g of the spray powder were calcined in a rotary kiln, as shown in the attached FIG. 1 (1 = oven housing, 2 = rotary lobe, 3 = heated room, 4 = nitrogen / air flow). For this purpose, the mixture was first heated linearly from 35 ° C. to 350 ° C. in the course of 35 minutes under an air flow of 50 Nl / h and this temperature was maintained for 1 hour while maintaining the air flow. Then the air flow was replaced by a nitrogen flow of 50 Nl / h and the temperature was increased linearly from 350 ° C to 600 ° C within 25 min. The nitrogen flow and the 600 ° C. were then maintained for 2 h and finally the entire rotary ball oven was allowed to cool to room temperature (while maintaining the nitrogen flow). The result was a black powder with the composition Mo _1.0 V _0.3 Te _0.2 Nb _0.12 O _x (determined by the method of X-ray fluorescence spectroscopy, Siemens SRS-3000) with a BET surface area of 1.42 m ² / G.

Fig. 2 shows the X-ray diffractogram of the Aktivmas senpulvers obtained including the assignment of the various diffraction reflexes.

The half-width of the diffraction reflex h is 0.20 °.  

The intensity ratio R (determined according to FIG. 3) is 0.87. The half-width of the diffraction reflex i is 0.19 ° and the half-width of the diffraction reflex k is 0.58 °.

If the intensity 100 is assigned to the diffraction reflex h, the diffraction reflexes i, l, m, n, o, p and q have the following intensities in the same intensity scale:
i: 58
l: 6.5
m: 13.1
n: 15
o: 4.2
p: 8.4
q: 21

The full width at half maximum of all the aforementioned diffraction reflections is ≦ 1 °.

The active material powder was with evenly distributed additive 1% by weight of finely divided graphite (graphite from Lonza, CH, dated Type TIM REX T44) to tablets (5 mm (outer diameter) × 3 mm (Height)) pressed (the applied pressure was 200 MPa). The resulting tablets were split in a mortar and the Sieve fraction (square sieve mesh) 0.6 to 1.2 mm for the subsequent performance test used in the tubular reactor.

A tubular reactor was used with 18.5 g of the aforementioned sieve fraction (Inner tube diameter: 8.5 mm; tube length: 140 cm; wall thickness of the Tube: 2.5 cm; Tube material: steel, contact chair height: 7 cm) as charged as follows: bed length of the catalyst of 14 cm with a 30 cm quartz chips (grain size: 0.6 to 1.2 mm) and a refill of 89 cm of the same quartz chips (the entire Fill on the contact chair). The catalyst was at a temperature of 330 ° C in air in the tubular reactor filled. Then the tubular reactor (reaction gas mixture inlet at the end opposite the contact chair) with a mixture the molar composition propane: air: water = 1:15:14 be sends (dwell time based on the pure active mass pouring = 0.7 s; Inlet pressure = 2 bar absolute) and the temperature of the Re action tube increased linearly to 380 ° C within 2 h (the reac The entire length of the tube was made with an electric heating tape heated).

Then the wall temperature was maintained at 380 ° C. To an operating time of the tubular reactor of 6 hours at 380 ° C. with a single pass, a conversion of the propane of 24 mol% and  a selectivity of acrylic acid formation of 54 mol% (at a Selectivity of fropene by-product formation of 14 mol%) achieved.

Example 2

93.9 g of ammonium metavanadate (77.5% by weight of V ₂ O ₅ , from GfE in Nurnberg, DE) were dissolved in 5400 ml of water (8 l round-bottom flask, KPG stirrer, patio heater) at 80 ° C. A yellowish, clear solution resulted. In this solution, while maintaining the 80 ° C. in the order mentioned, 91.8 g telluric acid (99% by weight H ₆ TeO ₆ , from Fluka, DE) and 383.7 g ammonium hepta molybdate (81.5% by weight) % MoO _{3 from} Starck in Goslar, DE). A deep red solution A was formed. 88.0 g of ammonium nioboxalate (21.0% by weight of Nb, from Starck in Goslar, DE) were obtained in a 2-liter beaker at 80 ° C. in 800 ml of water to obtain one Solution B solved.

Solutions A and B were combined at 80 ° C., solution B being added to solution A. The addition took place over a period of 5 minutes. An orange suspension was formed. This suspension was then spray-dried (spray dryer A / S as in Example 1 from the company Niro, DK, was the temperature of the receptacle 80 ° C;. T ^a = 330 ° C, T ^out = 110 ° C) to give an orange-colored spray-dried powder , 100 g of the spray powder were calcined as in Example 1. The result was a black powder of the composition Mo _1.0 V _0.37 Te _0.18 Nb _0.09 O _x (X-ray fluorescence spectroscopy) with a BET surface area of 8.43 m ² / g. Fig. 4 shows the X-ray diffraction of the active material powder obtained including the assignment of the various reflections.

The half-width of the diffraction reflex h is 0.19 °.

The intensity ratio R (determined according to FIG. 5) is 0.85.

The half-width of the diffraction reflex i is 0.26 ° and the Half-width of the diffraction reflex k is 0.50 °.

If the intensity 100 is assigned to the diffraction reflex h, the diffraction reflexes i, l, m, n, o, p and q have the following intensities in the same intensity scale:
i: 66
l: 12
m: 11
n: 18.7
o: 4.7
p: 13.6
q: 15

The full width at half maximum of all the aforementioned diffraction reflections is ≦ 1 °.

The active composition was compressed into tablets as in Example 1, the resulting tablets are subsequently split and the Sieve fraction 0.6 to 1.2 mm (as in Example 1) for the subsequent ß separated performance test in the tubular reactor.

A tubular reactor was used with 35.0 g of the aforementioned sieve fraction (Inner tube diameter: 8.5 mm; tube length: 140 cm; wall thickness of the Tube: 2.5 cm; Tube material: steel, contact chair height: 7 cm) as charged as follows: bed length of the catalyst of 50 cm with a 30 cm quartz chips (grain size: 0.6 to 1.2 mm) and a refill of 53 cm of the same quartz chips (the entire Fill on the contact chair). The catalyst was with a temperature of 320 ° C in air in the tubular reactor filled. Then the tubular reactor (reaction gas mixture inlet at the end opposite the contact chair) with a mixture the molar composition propane: air: water = 1:15:14 be sends (dwell time based on the pure active mass pouring = 2.6 s; Inlet pressure = 2 bar absolute). After an operating period of the tubular reactor from 22 h at 320 ° C (the reaction tube was opened the entire length was heated with an electric heating tape) one pass a conversion of propane of 31 mol% and one Selectivity of acrylic acid formation of 54 mol% (at a Selectivity of the propene by-product formation of 7 mol%) achieved.

Claims (8)

1. Process for the preparation of acrylic acid by heterogeneously catalyzed gas phase oxidation of propane with molecular oxygen at elevated temperature on a multimetal oxide composition of the general formula I.
Mo ₁ V _b M _c ¹ M _d ² O _n (1)
With
M ¹ = Te and / or Sb,
M ² = at least one of the elements from the group comprising Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Ga, Fe, Ru, Co, Rh, Ni, Pd, Pt, Bi, B and Ce,
b = 0.01 to 1,
c = 0.01 to 1,
d = 0.01 to 1 and
n = a number which is determined by the valency and frequency of the elements other than oxygen in (I),
the X-ray diffractogram of the multimetal oxide mass (I) having diffraction reflections h, i and possibly k, the apex of which at the diffraction angles (2⊖)) 22.2 ± 0.4 ° (h), 27.3 ± 0.4 ° (i) and are 28.2 ± 0.4 ° (k),
characterized in that

• the diffraction reflex h is the most intense within the X-ray diffractogram and has a half-value width of at most 0.5 °,
• - The intensity P _{i of} the diffraction reflex i and the intensity P _{k of} the diffraction reflex k satisfy the relationship 0.8 ≦ R ≦ 1, in which R the formula
  R = P _i / (P _i + P _k )
  is defined intensity ratio, and
• - The full width at half maximum of the diffraction reflex i and a diffraction reflex k contained in each is ≦ 1 °.

2. The method according to claim 1, characterized in that M ¹ = Te. 3. The method according to claim 1 or 2, characterized in that M ² = Nb. 4. The method according to any one of claims 1 to 3, characterized records that b = 0.1 to 0.6. 5. The method according to any one of claims 1 to 4, characterized indicates that c = 0.05 to 0.4. 6. The method according to any one of claims 1 to 5, characterized indicates that d = 0.01 to 0.6. 7. The method according to any one of claims 1 to 6, characterized shows that R ≧ 0.85 and ≦ 1. 8. The method according to any one of claims 1 to 6, characterized shows that R ≧ 0.9 and ≦ 1.