DE19922113A1 - Multimetal oxide composition, useful as oxidation catalyst, especially for gas phase production of (meth)acrylic acid, comprises crystallites of constant composition - Google Patents

Abstract

Multimetal oxide composition is a multimetal oxide (I) of molybdenum with vanadium, niobium, tantalum, tungsten and/or rhenium and boron, silicon, phosphorus, germanium, arsenic and/or antimony and optionally other metal(s) in the form of crystallites of constant composition. Multimetal oxide composition is of formula (I); (A)p(B)q (I) Mo12X<1>aX<2>bX<3>cX<4>dX<5>eOx (II) MofX<6>gX<7>hOy (III) A = a multimetal oxide of formula (II); B = a multimetal oxide of formula (III); X<1> = hydrogen, up to 97 mole-% of which may be replaced by ammonium, potassium, rubidium and/or cesium; X<2> = vanadium, niobium (Nb), tantalum (Ta), tungsten and/or rhenium; X<3> = boron, silicon, phosphorus, germanium, arsenic and/or antimony (Sb); X<4> = chromium, manganese, iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), magnesium (Mg), calcium (Ca), strontium (Sr) and/or barium (Ba); X<5> = sulfur; X<6> = Cu, Fe, Co, Ni, Zn, cadmium, Mn, Mg, Ca, Sr and/or Ba; X<7> = Nb, Ta and/or Sb; a = 1-3; b = 0.1-2; c = 0-5; d, e = 0-1; f = 0-2; g = 0.5-1.5; h = 2-4; x, y = values depending on the valency and frequency of elements other than oxygen; p = 1; and q = 0; or p, q not = 0 and p/q = (160-1):1 The standard deviation of the stoichiometric coefficient a of the variable X<1> of individual crystallites of component A is less than 0.40, preferably less than 0.20, especially less than 0.11. An Independent claim is also included for the preparation of multimetal oxide (I).

Description

The invention relates to multimetal oxide compositions, processes for their preparation, as well as the use of the multimetal oxide masses as oxidation catalysts, in particular for the gas-phase catalytic oxidative production of (meth) acrylic acid.

The term multimetal oxide mass used here denotes salt-like built-up heteropoly compounds derived from phosphoromolybdic acid. In addition to molybdenum and phosphorus, they usually contain metallic and transition metal elements, in particular copper, vanadium, arsenic, antimony, Cesium and potassium (see DE-A-43 29 907, DE-A-26 10 249, JP-A- 7/185354). Multimetal oxide masses have been in the past 20 years Industrial importance as catalysts both in purely acid-catalyzed Reactions as well as in oxidation reactions, especially in the oxidative Production of methacrylic acid obtained. A serious disadvantage compared to However, catalyst types of other classes of compounds consisted of the reduced ones. Service life.

DE-A 44 05 060 proposes multimetal oxide compositions with a two-phase structure, the gross element composition of which essentially corresponds to that of the multimetal oxide compositions according to the invention, with improved catalytic activity and selectivity and with an improved service life. The multime talloxidmassen according to DE-A 44 05 060 correspond to formula I.

[A] _p [B] _q (I),

in which the variables have the following meaning:
A Mo ₁₂ X _a ¹ X _b ² X _c ³ X ⁴ S _e X _f ⁵ O _X
BX ⁶ _g X ⁷ _h O _y
X ¹ phosphorus, arsenic, boron, germanium and / or silicon,
X ² vanadium, niobium and / or tungsten,
X ^{3 is} hydrogen, of which up to 97 mol% can be replaced by potassium, rubidium, cesium and / or ammonium (NH ₄ ),
X ⁴ antimony and / or bismuth,
X ⁵ rhenium and / or rhodium,
X ⁶ copper, iron, cobalt, nickel, zinc, cadmium, manganese, magnesium, calcium, strontium and / or barium,
X ⁷ niobium, tantalum and / or antimony,
a 1 to 6, preferably 1 to 3 and particularly preferably 1.5 to 2.5
b 0 to 6, preferably 0.2 to 4 and particularly preferably 0.5 to 2,
c 3 to 5,
d 0 to 6, preferably 0 to 3 and particularly preferably 0.5 to 1.5,
e 0 to 3, preferably 0.01 to 1 and particularly preferably 0.01 to 0.2,
f 0 to 3, preferably 0.01 to 1 and particularly preferably 0.1 to 0.5,
g 0.5 to 1.5, preferably 0.9 to 1.1,
h 2 to 4, preferably 2 to 3 and particularly preferably 2 to 2.5,
x, y numbers which are determined by the valency and frequency of the elements in I other than oxygen and
p, q numbers other than zero, whose ratio p / q is 12: 0.1 to 12: 48, preferably 12: 0.25 to 12: 12 and particularly preferably 12: 0.5 to 12: 4,
that the portion [A] _p in the form of three-dimensionally extended areas A of the chemical composition, which are delimited from their local environment due to their different chemical composition

A Mo ₁₂ X _a ¹ X _b ² X _c ³ X _d ⁴ S _e X _f ⁵ O _x

and the proportion [B] _q in the form of three-dimensionally extended regions B of the chemical composition which are differentiated from their local surroundings on account of their chemical composition which differs from their local surroundings

BX ⁶ _g X ⁷ _h O _y

included, the areas A, B relative to each other as in a finely divided Mixture of A and B are distributed.

In contrast, it is an object of the invention, the catalyst life of To further improve multimetal oxide materials. A procedure is to continue Manufacture of the multimetal oxide materials with improved tool life available be put.

According to the invention, the object is achieved by multimetal oxide compositions of the formula

[A] _p [B] _q (I),

solved, in which the variables have the following meaning:
A Mo ₁₂ X _a ¹ X _b ² X _c ³ X _d ⁴ X _e ⁵ O _X
B MO _f X ⁶ _g X ⁷ _h O _y
X ¹ H of which up to 97 mol% can be replaced by ammonium, K, Rb, and / or Cs,
X ² V, Nb, Ta, W and / or Re,
X ³ B, Si, P, Ge, As and / or Sb,
X ⁴ Cr, Mn, Fe, Co, Ni, Cu, Zn, Mg, Ca, Sr and / or Ba,
X ⁵ p
X ⁶ Cu, Fe, Co, Ni, Zn, Cd, Mn, Mg, Ca, Sr and / or Ba,
X ¹ Nb, Ta and / or Sb,
a 1 to 3,
b 0.1 to 2,
c 0 to 5,
d 0 to 1,
e 0 to 1,
f 0 to 2,
g 0.5 to 1.5,
h 2 to 4,
x, y numbers which are determined by the valency and frequency of the elements other than oxygen in (I),
p = 1 and q = 0 or
p, q integers other than zero, whose ratio p / q is from 160: 1 to 1: 1,
where the standard deviation of the stoichiometric coefficient a of the variable X ^{1 of} individual crystallites within component A of the multimetal oxide mass is less than 0.40, preferably less than 0.20, in particular less than 0.11.

It has been found that the service life of multimetal oxide catalysts depends crucially on the spread with respect to the composition of the individual crystallites of component A within the multimetal oxide mass [A] _p [B] _q .

Technically simple measures, namely the choice of defined parameters in the calcination stage, surprisingly made it possible, according to the invention, to achieve a small scatter in the concentration of the variable X ¹ of component A, which is initially undesirably large after the precipitation and drying. It was particularly unexpected that this effect, which is decisive for the catalytic properties, is only achieved with calcination in a defined temperature range and decreases with a further increase in temperature.

A long calcination time for the process result according to the invention is that is at least 6 hours, essential. The upper limit of the calcination time is open, limitations only arise in practice for reasons of Economics.

A calcination time in the range of 6 to 12 hours and one is preferred Calcination temperature selected in the range of 300 to 385 ° C.

A calcination time of 6 to 12 hours and one is particularly preferred Calcination temperature of 350 to 380 ° C, more preferably a calcination time from 6 to 9 hours and a calcination temperature from 360 to 370 ° C.

Multimetal oxide materials have a largely crystalline structure: Primary structure is the structure of the anion of the salt-like heteropolyver bonds referred to. The anion is a molecular ion, built up over a complex Linking oxometalate polyhedra or a heteroatom with Oxometalate polyhedra.

Some of these primary structures were named after their discoverers, for example the Keggin structure, PMo ₁₂ O ₄₀ , or the Dawson structure, P ₂ Mo ₁₈ O ₆₄ . The primary structure is the same in the solid as in the solution. The anions of the primary structure and the cations on the lattice sites are defined in the secondary structure. With the same primary structure, the secondary structure is determined, for example, by the degree of hydration, by the ion radius of the counterions (cations) and by the charge of the anions of the primary structure. Component A of the multimetal oxide mass is composed of individual crystallites that have this secondary structure. Crystalline particles with grain sizes in the range from 0.1 to 10 μm are defined as crystallite, the grain size denoting the diameter of the particles in a known manner.

The constancy of the is essential for uniform catalyst properties Composition of these individual crystallites.

The constancy of the cesium content of individual crystallites is of particular importance, since otherwise the crystallites with a relatively low cesium content predominantly on Catalytic events are involved, leading to an in-depth reduction of molybdenum to molybdenum oxides, and thereby the primary structure for example the Keggin unit disintegrates. The alkali metal released in the process cations migrate into neighboring crystallites, in which the heteropoly structure still exists is present, replace those still present there for the catalytic activity crucial protons and thus lead to an exponential drop in the Catalyst activity.

The standard deviation s is defined in a known manner by the formula (II).

in which
s = standard deviation
n = number of measurements
j = measurement number
x = measured value
x = arithmetic mean of all measured values. (see E. Kreyszig: Advanced Engineering Mathematics, J. Wiley, New York, 5 ^th Edition, 1983, p 895th).

A manufacturing method for multimetal oxide compositions according to the invention comprises the following steps:

• a) Preparation of one or more, preferably 2 or 3, in particular aqueous solutions and / or dispersions of element sources of the Multi metal oxide masses,  
• b) optionally entering pH-lowering agents or preparation of solutions that are predominantly or exclusively pH-lowering agents contain,
• c) mixing the solutions and / or dispersions,
• d) optionally heating, especially in the presence of inert or Reactive gases,
• e) drying, in particular evaporation or spray drying,
• f) shaping, in particular into tablets or hollow cylinders and
• g) calcining.

In process stage a), preference is given to aqueous solutions and / or dispersions of the element sources of the multimetal oxide mass. With the element sources it is either already oxides, or such compounds that by Heating, optionally in the presence of oxygen, can be converted into oxides. In addition to the oxides, the main starting compounds are halides, Hydrates, formates, oxalates, acetates, carbonates, hydroxides, Ammonium compounds and / or inorganic acids into consideration. For the Particularly suitable sources of individual elements are in DE-A 44 05 060 specified. Individual elements become a solution and / or Dispersion summarized, which are as homogeneously miscible as possible and none or cause almost no precipitation of heteropoly compounds. In at least one of the solutions and / or dispersions can contain a pH-lowering agent, preferably nitric acid, are entered (process stage b).

In process stages a) and b), warm, d. H. a Temperature of about 40 to 60 ° C, preferably from 45 to 55 ° C Solutions and / or dispersions produced.

Working up by mixing the solutions and / or dispersions Precipitation product is produced in a known manner. With individual recipes it may be advantageous to first heat the dispersion produced, in particular in the presence of inert or active gases, whereas other recipes have one  immediate separation of the precipitate from the liquid phase with subsequent Drying, especially by evaporation or spray drying presuppose. The masses obtained are then shaped, for example as tablets or hollow cylinders and finally calcined.

According to a further process, a, preferably aqueous, solution is first made or dispersion of the element sources and optionally a pH depressant, especially nitric acid, registered.

It has been found that the constancy of the composition of the multimetal oxide mass can be improved via the parameters of the calcination step g): through a relatively long calcination time of at least 6 hours as well Calcination temperatures in the range of 300 to 385 ° C, finds a wide range Compensation of the decisive concentration for the catalytic activity Cesium cations by solid diffusion between different single crystallites instead. The calcination temperature is down due to the mobility of cesium cations and capped by the stability of the crystal structure. The calcination is usually carried out in air. However, it can also be advantageous, the calcination in nitrogen or in air / nitrogen mixtures perform.

The multimetal oxide compositions according to the invention are called oxidation catalysts with improved service life, especially for gas phase catalysis oxidative production of (meth) acrylic acid used.

The multimetal oxide materials are preferably in the form of full catalysts sensors used, with the geometry of a hollow cylinder with an outside diameter and a length of 2 to 10 mm and a wall thickness of 1 to 3 mm.  

Examples

The examples relate to the production of multimetal according to the invention oxide masses and multimetal oxide masses for comparison (the content of That puts hydrogen, ammonium and oxygen of the resulting masses respective manufacturing processes fixed; the values were not determined regularly and are therefore not regularly included in the stoichiometries indicated).

The solvent or dispersant is in all examples and comparative examples of water.

The terms conversion, selectivity, deactivation rate and standard deviation are defined here as follows, unless stated otherwise:

The deactivation rate is defined as the temperature increase of the heating medium in Degrees Kelvin between the time after 120 hours of operation and 720 Hours of operation is necessary to achieve the given sales of (meth) acrolein hold.

The standard deviation s is defined in a known manner by the formula (II) already listed. In the present case, the standard deviation with regard to the concentration of the variable X ^{1 was carried out} by evaluating 20 measurements in each case. The concentration measurements were carried out with the aid of analytical transmission electron microscopy: for this purpose, the multimetal oxide materials were embedded in polymethylacrylate and cut into slices approximately 100 nm thick using an ultramicrotome from Reichert. These disks were installed as samples in a transmission electron microscope model 400 KVFX from JEOL. The element concentration was determined with the aid of an energy-dispersive X-ray detector and the associated evaluation unit (model Noran TN5500) with a resolution of 0.05 µm (compare Goldstein et al .: Principles of Analytical Electronmicroscopy, Plenum Press 1989, page 155).

First, the starting materials AM1 for component A and AM2 for component B were produced in the following way:
For the starting mass AM1, 4260 g (NH ₄ ) ₆ Mo ₇ O ₂₄ .4H ₂ O and 140.6 g NH ₄ VO _{3 were dissolved} in 5 l of H ₂ O heated to 40 to 45 ° C. with constant stirring. At the same time, 393.1 g of CsNO _{3 was dissolved} in 850 ml of warm H ₂ O and then added. 388.3 g of a 76% strength by weight H ₃ PO ₄ solution were then added dropwise to the starting solution, and 10.9 g of diammonium sulfate and 293.1 g of Sb ₂ O _{3 were} added in succession. The resulting mixture was heated to 85 to 95 ° C. in the course of 30 minutes, 413.9 g of a copper nitrate solution (15.5% by weight of Cu) were added dropwise and spray-dried at an outlet temperature of 110 to 120 ° C. The resulting initial mass M, measured over the entire mass, is based on the following stoichiometry (after calcination):

Mo ₁₂ P _1.5 V _0.6 Cs _1.0 Cu _0.5 Sb _1.0 S _0.04 O _X.

To produce the starting mass AM2 for component B, 880.0 g of Sb ₂ O ₃ with an Sb content of 83.0% by weight (= 6.0 mol Sb) were suspended in 4 l of water with stirring. A solution of 724.8 g of Cu (NO ₃ ) ₂ × 3 H ₂ O (= 3.0 mol of Cu) in 4 l of water was then added to the suspension. The suspension was stirred at 80 ° C. for 2 hours and spray-dried (inlet temperature = 350 ° C., outlet temperature = 110 ° C.).

The spray powder obtained was gradually heated to 150 ° C. in the course of 1 hour, to 200 ° C. in the course of 4 hours, to 300 ° C. in the course of 2 hours and to 400 ° C. in the course of 2 hours while passing sufficient pure oxygen through it. Subsequently, the pre-calcined powder was also heated to 900 ° C. under pure oxygen within 2 days. The powder obtained had a BET specific surface area of 0.3 m ² / g. The powder X-ray diagram of the powder obtained essentially showed the diffraction reflections of CuSb ₂ O ₆ (comparison spectrum 17-0284 from the JCPDS-ICDD file). The resulting initial mass AM2 is based on the following stoichiometry:

CuSb ₂ O ₆ .

The starting materials AM1 and AM2 were then treated in the manner described below, Examples 1 to 7 and Comparative Examples 1 and 2. The process parameters and the respectively obtained values for conversion, selectivity, deactivation rate and standard deviation of the stoichiometric coefficient a of the variable X ¹ are listed in the table below.

example 1

617.2 g solid of the starting mass AM1 was mixed with 54 g of the solid Initial mass AM2 mixed. Then about 2 wt .-% graphite added and the mass formed into hollow cylinders (7 mm × 7 mm × 3 mm). The hollow cylinders were calcined in a forced air oven under constant air flow (500 NI / h air per kg solid), where they were heated to 270 ° C at 4 ° C / h and wherein at 180 ° C, 220 ° C and 270 ° C, the temperature for 30 min was held. In conclusion, the temperature was at a rate of 2 ° C / h increased to 385 ° C and maintained for 4 h.

Example 2

As Example 1 with the difference that the final temperature in the calcination at Was 380 ° C and was held for 6 h.

Examples 4 to 7

Analogous to Example 1, with the difference that the final temperature during the calcination  as well as the calcination time to those listed in the table below Values were set.

Comparative Example 1

Analogous to Example 1, with the difference that the hollow cylinder is not calcined were.

Comparative Example 2

Analogous to Example 1, with the difference that the final temperature at Calcination was held at 395 ° C and for 4 h.

Table 1

From the examples it can be seen that the lowest deactivation rate and the smallest standard deviation of the stoichiometric coefficient a of the variable X ^{1 were achieved} for the examples, which were run at final temperatures for the calcination of 360 to 370 ° C and a calcination time between 6 and 12 hours were.

Claims (8)

1. Multimetal oxide compositions of the formula
[A] _p [B] _q (I),
in which the variables have the following meaning:
A Mo ₁₂ X _a ¹ X _b ² X _c ³ X _d ⁴ X _e ⁵ O _X
B Mo _f X ⁶ _g X ⁷ _h O _y
X ¹ H, of which up to 97 mol% can be replaced by ammonium, K, Rb and / or Cs,
X ² V, Nb, Ta, W and / or Re,
X ³ B, Si, P, Ge, As and / or Sb,
X ⁴ Cr, Mn, Fe, Co, Ni, Cu, Zn, Mg, Ca, Sr and / or Ba,
X ⁵ S,
X ⁶ Cu, Fe, Co, Ni, Zn, Cd, Mn, Mg, Ca, Sr and / or Ba,
X ⁷ Nb, Ta and / or Sb,
a 1 to 3,
b 0.1 to 2,
c 0 to 5,
d 0 to 1,
e 0 to 1,
f 0 to 2,
g 0.5 to 1.5,
h 2 to 4,
x, y numbers which are determined by the valency and frequency of the elements other than oxygen in (I),
p = 1 and q = 0 or
p, q integers other than zero, the ratio p / q of which is from 160: 1 to 1: 1, the standard deviation of the stoichiometric coefficient a of the variable X ^{1 of} individual crystallites within component A of the multimetal oxide mass being less than 0.40, is preferably less than 0.20, in particular less than 0.11. 2. A process for the preparation of multimetal oxide compositions according to claim 1, which comprises the following process steps:

• a) preparation of one or more, preferably 2 or 3, in particular aqueous solutions and / or dispersions of element sources of the metal oxide masses,
• b) if appropriate, adding pH-lowering agents or preparing solutions which predominantly or exclusively contain pH-lowering agents,
• c) mixing the solutions and / or dispersions,
• d) optionally tempering, in particular in the presence of inert or reactive gases,
• e) drying, in particular evaporation or spray drying,
• f) shaping, in particular into tablets or hollow cylinders and calcining, characterized by the following process parameters in stage g):
  Calculation time at least 6 h and
  Calcination temperature in the range of 300 to 385 ° C.

3. The method according to claim 2, characterized by a calcination time of 6 to 12 hours and a calcination temperature of 300 to 385 ° C.   4. The method according to claim 2, characterized by a calcination time of 6 to 12 hours and a calcination temperature of 350 to 380 ° C. 5. The method according to claim 2, characterized by a calcination time of 6 to 9 hours and a calcination temperature of 360 to 370 ° C. 6. Multimetal oxide materials, obtainable by a process according to one of the Claims 2 to 5. 7. Use of the multimetal oxide materials according to claim 6 as Oxidation catalyst, especially for the gas phase catalytic oxidative Production of (meth) acrylic acid. 8. Use according to claim 6, characterized in that it is in the used catalyst is a full catalyst, the geometry that of a hollow cylinder with an outer diameter and a length of 2 to 10 mm and a wall thickness of 1 to 3 mm.