EP2349968A2 - Catalyst for oxidizing methanol into formaldehyde - Google Patents

Abstract

The invention relates to a supported catalyst comprising a molded support and a layer that is made of a metal oxide of a main group metal, an early transition metal, or a lanthanoid, is arranged on the molded support, and is composed of individual particles. An oxide composition, the surface density of which corresponds at least to a monolayer and which contains molybdenum and/or vanadium and/or iron, is provided on the layer or on the individual particles. The invention further relates to the use of said catalyst for reacting methanol into formaldehyde.

Description

 Catalyst for the oxidation of methanol to formaldehyde

The present invention relates to a supported catalyst, in particular for the oxidation of methanol to formaldehyde, comprising a carrier shaped body on which a layer of an insoluble metal oxide of a main group metal, an early transition metal or a lanthanide is arranged and wherein on the layer or on the layer forming individual particles at least one of a monolayer corresponding surface density of a molybdenum and / or vanadium and iron-containing oxide composition is present.

Heterogeneously catalyzed oxidation reactions are of particular importance to the chemical industry as they serve to large scale, selective production of many end products from inexpensive and abundant raw materials (e.g., alcohols).

A representative of this class of reactions is the partial oxidation of methanol to formaldehyde. In addition to the coupled dehydrogenation and oxidation of silver contacts, partial oxidation with excess air on porous solid catalysts of iron molybdate is customary for large-scale production. Side reactions include the "total oxidation" of the methanol to CO, as well as the acid-catalyzed formation of dimethyl ether.

Ironmolybdate full catalysts are commonly used in industry today, but these are commonly used in the industry Have relative to their active mass only a small active surface.

Next molybdenum-vanadium-iron or vanadium-iron catalysts for the partial oxidation of methanol to formaldehyde are known.

The atomic ratios between molybdenum or vanadium and iron vary considerably in the catalysts known from the prior art. Some of the active components may be further partially replaced by so-called promoters such as titanium, antimony, tin, nickel, chromium, cerium, aluminum, calcium, magnesium, niobium, silver and / or manganese in metallic form or in the form of compounds.

Typically, two types of (solid) catalysts are distinguished in so-called heterogeneous catalysis (J. Weitkamp and R. Gläser in: Winnacker / Kuchler "Chemische Technik: Processes and Products", Volume 1, Chapter 5, Wiley-VCH, 2004 ):

On the one hand, there are so-called "supported catalysts", also known as coat or coating catalysts, which are understood to mean solid catalysts prepared by coating a (typically non-porous) carrier body with a porous layer containing the actually catalytically active species.

In contrast, in the so-called "supported catalysts" by impregnation, the catalytically active species (eg, noble metals such as Pd, Pt, Au, Ag, etc., or transition metals, such as Ni, Co, Cu, Fe, etc.) as a solution a compound of these species dispersed on a porous support, such as SiO ₂ , Al ₂ O ₃ , TiO ₂ , ZrO ₂ , etc.).) In the supported catalysts prepared by the impregnation process mostly chemical-physical interactions between carriers and active species, which have a decisive influence on the catalytic process.

In the case of the supported catalysts, the carrier body merely serves for structural support. In contrast to supported catalysts in which the active elements are distributed disperse in the porous support - optionally also in an outer shell (= shell catalyst) arranged on the support, the carrier species which is typically non-porous in the supported catalyst is one of the active species wrapped layer containing.

No. 3,975,302 describes an Fe / Mo catalyst prepared by the impregnation process for the oxidation of methanol to formaldehyde (generally referred to as the Formox process). Thus, iron and molybdenum are dissolved as MoO ₄ ^{2 "} and Fe ³⁺ salts in a solvent such as water and then applied to a porous support having a BET surface area of 1 to 20 m ² / g Catalyst for the oxidation of methanol to formaldehyde, which is prepared by impregnation in the fluidized bed process.

No. 5,217,936 describes a catalyst for the preparation of aldehydes from the corresponding alcohols, in particular formaldehyde from methanol, the catalytically active composition being applied to a monolithic support. In addition to molybdenum oxide, the active species may also contain oxides of chromium, vanadium, aluminum, iron, tungsten, manganese and mixtures thereof. To enhance binding of the active mass to the monolithic carrier, the active species may contain a binder. As a suitable binder, silicon dioxide and titanium dioxide are specified. DE 10 2004 014 918 describes a catalyst with a silver-vanadium oxide phase and a promoter phase based on titanium dioxide and vanadium pentoxide, which is suitable for the preparation of aldehydes, carboxylic acids and carboxylic anhydrides from aromatic or heteroaromatic hydrocarbons by gas-phase oxidation. The catalyst is preferably designed as a shell catalyst, wherein the two phases are arranged as concentric shells on an inert carrier.

In WO 2005/037427 a catalyst bed of a physical mixture of catalytically active and catalytically inactive moldings is described, wherein the catalytically inactive moldings have rounded edges on the outer friction surfaces. If the catalyst bed is used for the oxidation of methanol to formaldehyde, for example Eisenmolybdate can be used as catalytically active species.

Formaldehyde technical catalysts for the Formox process (also known as the Perstorp-Reichhold Formox process) suffer from the fact that they are rapidly deactivated. So technical lifetimes of only about a year are common and lead to significant costs by the necessary catalyst change. This deactivation, which is mainly caused by the thermal discharge of the Mo component, leads to mechanical instability of the catalyst and to a strong pressure increase in the reactor and thus ultimately to a catalyst change.

There is therefore a need for further, improved catalysts for the oxidation of methanol to formaldehyde, the have good activity and at the same time selectivity for formaldehyde.

The object of the present invention was therefore to provide improved catalysts, in particular for the oxidation of methanol to formaldehyde, which have a particularly high abrasion resistance and good activity and at the same time selectivity for formaldehyde and avoid total oxidation of methanol.

This object is achieved by a supported catalyst, in particular for the oxidation of methanol to formaldehyde, comprising an inert support body on which a built up of individual particles carrier layer of a particular non-or hardly reducible metal oxide of a main group metal, an early transition metal or a lanthanoid arranged Noids and wherein there is on the layer or on the individual particles forming the layer as active component a molybdenum and / or vanadium and iron-containing oxide composition whose surface density corresponds to at least one monolayer.

It has been found that the most preferred catalysts of the invention have a surface density of the active component of at least one monolayer on the individual particles of the carrier layer or on the carrier layer itself in preferred developments of the invention of even at least 2 monolayers.

The term "monolayer" as used herein refers to the surface density of the active component. The surface density in the context of the invention results from the experimentally determined BET surface area (DIN 66131) and the theoretical area requirement of the active components.

The surface density of the invention depends in particular on the nature of the selected active component.

For example, for pure molybdenum oxide (MoO ₃ ), the monolayer surface density is 5 Mo atoms / nm ² (see Adv. Catal 37,1 (1990)). For vanadium oxide (V ₂ O ₅ ), this value is approximately

4.5 V atoms / nm ² (IE Wachs, Cat. Today, 27 (1996) 437). For mixed Fe / Mo or Fe / V and Fe / V / Mo oxides, analogous values are given. As a general rule for all active components in the context of the present invention, the monolayer surface density is typically in the range of 4 to 5 metal atoms / nm ² .

The surface density of the active component, that is, the actual catalyst, greatly affects the effectiveness of the catalyst. In extreme cases, catalysts of the iron molybdate, molybdenum oxide or vanadium oxide type are relatively isolated oxide species, for example monomolybdate or monovanadate species which have relatively less active sites on the support surface. Therefore, these catalysts tend to retain their oxygen and therefore have relatively low reaction rates in the oxidation of methanol to formaldehyde.

If the catalyst contains a monolayer of both molybdenum and vanadium oxides, one species may be in the form of, for example, oligomerized iron molybdate or iron vanadate on the carrier and on the particles, and possibly a second such species as a second layer on the first monolayer. The term "methanol" in the context of the present invention refers both to pure methanol streams, paper mill exhaust gas streams containing methanol, other exhaust gas streams containing methanol, methylmercaptan and / or mixtures thereof.

The term "selectivity" as used herein is determined by dividing the number of moles of formaldehyde by the number of moles of methanol consumed (based on the feed flow in the reactor) and multiplying by 100. Thus, the selectivity is expressed as a percentage The selectivity therefore indicates the percentage of formaldehyde formed as compared to the percentage of non-formaldehyde oxidation products of methanol, such as carbon monoxide, carbon dioxide, etc.

The term "conversion" or "conversion" as used herein is determined by the difference between the number of moles of methanol that are introduced into the reactor minus the number of moles of methanol leaving the reactor, by the total number of moles of in divided methanol is divided and multiplied by 100.

Consequently, the conversion is also given in percent. The conversion therefore gives the percentage of moles of methanol that have been oxidized to formaldehyde and all other non-formaldehyde oxidation products of methanol. That is, if two moles of methanol were introduced into the reactor and the reactor each leave one mole of formaldehyde and one mole of methanol, there is a selectivity of 100%, but a conversion of 50%.

Further advantages of the catalyst according to the invention in the oxidation of formaldehyde to methanol are in particular its higher activity compared to conventional unsupported catalysts, whereby high selectivities can be achieved even at very low temperatures (vide infra).

It was further found that in the preparation of the catalyst according to the invention no calcination at high temperatures of more than 500 ⁰ C after application of the coating is necessary, as required in the prior art. Rather surprisingly enough annealing at temperatures <500 ⁰ C in order to obtain the catalyst of the invention. The term "annealing" thus finds use in contrast to the term "calcination" for a temperature treatment of 500 ⁰ C.

Therefore, no thermally induced loss of active Mo species or Fe-Mo and Fe-V species occurs during operation of the catalyst according to the invention. In particular, this leads to a longer service life of the catalyst according to the invention in the reactor than comparable catalysts of the prior art have. The elimination of the high-temperature calcination further increases the mechanical stability, especially during transport and when filling a reactor with the catalyst according to the invention. It therefore also occurs no dust during filling, so that it does not come to a possible health risk to the person entrusted with the filling and further no active mass losses occur.

The support layer of a metal oxide of a main group metal, an early transition metal or a lanthanide which is preferably difficult or impossible to reduce can be selected, for example, from SiO ₂ -SoI or its precursors, Al ₂ O ₃ -SoI or its precursors, ZrO ₂ -SoI or its precursors, TiO ₂ - Sol, CeO ₂ - Sol or its precursors, water glass, MgO, etc produced become. The carrier layer has in particular a BET surface area of> 15 m ² / g, very particularly preferably> 20 m ² / g and

> 50 m ² / g.

It has surprisingly been found that a supported catalyst according to the invention in which the coating composition of the carrier molding, as defined above, exhibits a particularly high abrasion resistance. It was unexpectedly found that even the non-tempered supported catalysts are particularly resistant to abrasion, which is even enhanced after annealing the catalysts of the invention.

According to the invention, the supported catalyst first comprises a (catalytically) inert carrier molding. In principle, all inert carrier shaped bodies familiar to the person skilled in the art can be used here. Preferably, however, the carrier tablets should be substantially non-porous. Substantially nonporous means that the carrier moldings used in the context of the present invention have a BET surface area (determined to DIN 66131) of less than about 1 m ² / g. The pore volume of the inert, substantially non-porous carrier shaped body is preferably less than 0.1 ml / g, determined in accordance with DIN 66133. The material density of the carrier molded body is preferably in the range from 2.0 to 4.5 g / cm ³ , in particular preferably in the range of 2.3 to 3.5 g / cm ³ .

Non-limiting examples of materials suitable for carrier moldings according to the invention are magnesium silicate (steatite), quartz (SiO ₂ ), porcelain, magnesium oxide, tin dioxide,

Silicon carbide, rutile, alumina (Al ₂ O ₃ ), zirconium silicate, aluminum silicate, cersilicate or mixtures thereof and metals or alloys such as stainless steel. Carrier bodies made of steatite are particularly preferred. In a preferred embodiment according to the invention, hollow cylinders or rings which are known per se to those skilled in the art are used as inert carrier shaped bodies which permit a flow rate adapted to the reaction in the reactor. Such moldings are described, for example, in WO2007 / 059974, EP 1 127 618 A1 or WO 2005/037427 A1, to the disclosure content of which reference is made here in their entirety.

The inventively preferred moldings form when loading the reactor less easily locally ordered dense packs, but are rather irregularly arranged, with more turbulence in the gas flow through the catalyst bed, which a slightly onset of overheating (formation of so-called "hot-spots"), especially in the Oxidation of methanol too

Counteract formaldehyde both in the molding as well as in the reactor as it is e.g. in monoliths, such as typically occurs according to US 5,217,936. The reduction of overheating also leads to an extension of the lifetime of the catalyst.

At least one carrier layer consisting of particles of a metal oxide of a main group metal, an early transition metal or a lanthanide is applied to the inert carrier body. These can, as already mentioned above, be applied either in the form of their powders (as suspension) and / or also as sols.

Depending on the sol, the solids content is between 10 and 50% by weight, for example SiO ₂ sols having 20 to 40% by weight, ZrO ₂ sols having 10 to 20% by weight, CeO ₂ sols having 15 to 20% by weight. 25 wt .-% solids content and TiO ₂ -SoIe with 10 - 20 wt -.% Solids content is preferred. TiO ₂ - brine or TiO ₂ powder are very particularly preferred in the context of the invention. Particularly preferred is a Position using a powder suspension of the carrier oxide with a solids content of 1 to 60 wt.% Plus a proportion of 0.05 wt .-% to 5 wt -.% Of a sol of the same carrier oxide.

In a preferred embodiment of the invention, the integral pore volume (determined by Hg porosimetry, DIN 66133) of the carrier layer is between about 100-800 mm ³ / g, preferably between about 200 and 700 mm ³ / g, more preferably between about 250 and 600 mm ³ / g. The mean pore radius determined by this method is preferably between about 50 and 1000 nm, preferably between about 100 and 700 nm, more preferably between about 150 and 500 nm.

According to a very particularly preferred embodiment of the invention, the sol has a particle size of 1 to 100 nm, preferably from 2 to 50 nm, particularly preferably <40 nm. The determination of these particle sizes was carried out according to ASTM B822-97. Alternatively, ISO 13320-1 can also be used.

Preferred oxides used as carriers in the context of the present invention are characterized by the following particle size distribution:

Di ₀ : 0.1 to 2.5, D ₅₀ : 0.1 to 19, D ₉₀ : 0.5 to 40, very particularly preferred are the particle size distributions of Di ₀ : 0.1 to 0.5, D ₅₀ : 0.1 to 0.9, D ₉₀ : 0.5 to 2.5.

Oxides of molybdenum and / or vanadium and iron and / or their mixed oxides (such as Fe ₂ (MoO ₄ ) ₃ ) or compounds which can be converted into the corresponding oxides or mixed oxides, such as, for example, acetates, oxalates, acetylacetonates, are preferred as the active component , Citrates, nitrates, chlorides, Phosphates, sulfates or ammonium compounds of iron or molybdenum and vanadium used.

In a particularly preferred embodiment according to the invention, the active component is a non-stoichiometric iron-molybdenum mixed oxide which does not have the composition (Fe ₂ (MoO ₄ ) _3. Preference is given to Mo / Fe ratios of from 0.5 to 20 Mo / Fe ratios of 2 to 15 are preferred.

Further optional compounds which may be present in the active component are metallic or metal oxide components or compounds which can be converted into the corresponding oxides or mixed oxides, and those skilled in the art with regard to the use of the catalyst for the oxidation of methanol to formaldehyde Non-limiting examples are compounds of titanium, antimony, tin, nickel, cerium, aluminum, calcium, magnesium, chromium, niobium, silver and / or manganese, which can partially replace Fe, V and Mo. ,

Another aspect of the present invention relates to a process for the preparation of a coating catalyst according to the invention, in particular for the oxidation of methanol to formaldehyde.

In the method according to the invention, it is preferred that the provided aqueous suspension of the carrier layer is applied to the inert carrier body by means of a fluidized bed process. The use of a fluidized bed coater is preferred, as described, for example, in DE-A-12 80 756, DE-A-197 09 589, DE 40 06 935 A1, DE 103 44 845 A1 or WO 2005/030388 is. It has been found that the aqueous suspension with the coating composition by means of a fluidized bed process, such as outlined above, particularly uniform and good adhesion and can be applied to the nonporous carrier molding with unexpectedly low spray losses.

Furthermore, it has been found to be advantageous that the pH of the suspension is adjusted. It has been shown that for this purpose generally a pH of the suspension to be applied to the carrier body between about 1 and 5 is advantageous. It has been found, in particular, that a pH of about 3-5 in the case of using a ZrO ₂ sol, in particular an acetate-stabilized ZrO ₂ sol, a pH between 1 and 5, preferably between 1 and 3 in the case the use of a TiO ₂ -SoIs, in particular a nitric acid-stabilized TiO ₂ -SoIs, is advantageous. In the case of a acetic acid-stabilized CeO ₂ sol, a pH of 2-4 is particularly advantageous.

In a further preferred embodiment of the invention, the aqueous suspension is applied to the inert carrier body in the fluidized bed process at a temperature of less than 100 ⁰ C, in particular less than 80 ° C.

Preferably, the particles in the aqueous suspension have a particle size D ₉₀ value of less than 5 μm, preferably less than 3 μm. Compliance with this particle size contributes to a particularly uniform and abrasion-resistant coating on the inert support bodies and leads to low spray losses during the coating process.

The above particle size can be adjusted by conventional milling before, during or after the preparation of the aqueous suspension. According to a further preferred embodiment, after application of the coating composition to the inert carrier body by temperature, the coated carrier body is thermally treated. In general, any temperature and time period familiar to the person skilled in the art can be used for tempering. According to the invention, a temperature between 200 and 500 ° C is preferred. On the other hand, "calcining" in the context of the present invention means that temperatures of more than 500 ^° C. are used.

The duration of the annealing is preferably between 0.5 and 20 h, in particular between 1 and 15 h.

In a preferred embodiment of the method according to the invention, the coated carrier body is placed in a suitable heating cabinet, e.g. a tray furnace with sheets, which contains the coated carrier body as a particle bed with a preferred bed height of 1 to 5 cm, particularly preferably from 1 to 3 cm. The oven is then preferably heated at a constant rate over a period of 1 h to 20 h, preferably 5 h to 15 h, from room temperature to a temperature Tl of preferably between 130 ° to 350 °, particularly preferably 200 to 300 °.

It is particularly preferred to carry out the annealing directly in situ in a Formox reactor during the first heating of the reactor after replacement of the catalyst, especially when methanol is already passed over the catalyst.

The impregnation of the particles of the carrier layer or the finished carrier layer with the active component, for example, the iron-molybdate or iron-vanadate active component can either by known per se methods such as the incipient wetness method or by spreading (solid wetting) etc., as described in the examples below, take place. In this case, either the titanium dioxide particles can be impregnated before their slurry or, in still further preferred embodiments of the present invention, the carrier layer can first be applied to the carrier body, for example starting from brines or suspensions, and then the optionally. annealed carrier layer are impregnated with the active component, so that a monolayer of active component is formed on the surface of the carrier layer.

In yet another preferred embodiment, the powder of the carrier layer and the powder of the active components are intimately physically mixed and the catalyst at 200 to 500 ⁰ C in air or water or methanol vapor-containing air for 2 to 72 h tempered.

A further aspect of the present invention relates to the use of the catalyst according to the invention for the oxidation of methanol to formaldehyde, in particular in a fixed-bed process. A suitable method is known for example from DE 103 61 517, EP 0 001 570 and US 3,852,361. However, in principle, other applications of the catalyst, in particular in partial (gas-phase) oxidation of hydrocarbons are possible.

The gas-phase oxidation is per se carried out in the reactors known for this reaction under customary conditions. Preference is given to tubular reactors, wherein the tubes are cooled for heat removal, for example with a salt bath or a temperature-resistant oil. The tubes preferably have one

Diameter in the range of 15 to 30 mm, more preferably from 20 to 25 mm, and a length in the range of preferably 80 to 200 cm. The catalyst of the invention is introduced into the tubes. According to the invention, the following physical determination methods were used to characterize the catalyst:

1. Test for determining the adhesion of the active material (abrasion test):

The adhesion of the active composition was investigated in a device of ERWEKA, type TAR 10. For this purpose, a total of 50 g of catalyst at a revolution speed of 75 revolutions / min was rotated a total of 10 times (= 10 revolutions of the drum). It was then determined the abrasion as a ratio of the loose, no longer bound to the support body coating to the total applied coating.

2. BET surface area:

The determination is made according to the BET method according to DIN 66131; a publication of the BET method can also be found in J. Am. Chem. Soc. 60, 309 (1938).

3. Pore radius distribution:

The pore radius distribution was determined by means of mercury porosimetry according to DIN 66133; maximum pressure:

2,000 bar, Porosimeter 4000 (Porotec, DE), according to the manufacturer.

4. Determination of particle sizes (particle sizes):

The particle sizes were determined by the laser diffraction method using a Fritsch Particle Sizer Analysette 22 Economy (Fritsch, DE) according to the manufacturer's instructions, also with regard to the sample pretreatment deionized water without the addition of auxiliaries homogenized and sonicated for 5 minutes. The specified D values are based on the sample volume.

The invention will now be explained in more detail with reference to the following non-limiting examples and with reference to the figures.

Show it:

FIG. 1 shows first-order activity constants of inventive catalysts against surface coverage, FIG.

2 shows the application of the formaldehyde selectivity of inventive catalysts with different coverages and a Mo / Fe ratio of 1.5 at 95% methanol conversion,

3 shows the formaldehyde yields of the monolayer catalysts according to the invention against the Mo / Fe atomic ratio,

4 shows the formaldehyde yield during a long-term stress test at 100% methanol conversion.

embodiments

example 1

Preparation of the active component

General:

Ammonium heptamolybdate and ferric nitrate were purchased from Merck KGAa, Darmstadt, Eisenmolybdat from Süd-Chemie Catalysts Italia. Commercially available titanium dioxides were used. The BET surface areas of the titanium dioxides ranged from 21 m ² / g to 101 m ² / g, and the pH values obtained when slurrying the suspensions ranged from 3.89 to 7.62.

The pH values of the titanium dioxide suspensions were determined with a glass electrode by slurrying 9 g of the respective oxide in 400 ml of distilled water at room temperature and stirring for 12 hours.

Impregnation of titanium dioxides

The impregnation was carried out by means of different methods:

1. Incipient wetness method ("dry impregnation ^)

Typically, ferric nitrate and ammonium heptamolybdate were used as precursor compounds. The solvent used was distilled water. To prepare the iron (III) nitrate solution, the water was adjusted to pH with nitric acid 0 - 1 in order to keep the volume constant during each impregnation. The solution volume has therefore been adjusted specifically for the respective titanium dioxides in order to comply with the conditions of the incipient wetness method.

The iron-impregnated titanium dioxide was dried at a heating rate of 1 ⁰ C / min at 150 ⁰ C for 4 hours. Subsequently, ammonium heptamolybdate was applied. The water used for the ammonium heptamolybdate solution was adjusted to pH 8 with ammonia.

2. Modified impregnation

The amount of iron (III) nitrate and then ammonium heptamolybdate to cover the carrier surface with a

Monolayer Iron molybdate was dissolved in 1.3 times the amount of water compared to the incipient wetness method and applied to the titanium dioxide. The drying was carried out at 50 ^° C. and 42 mbar overnight in a rotary evaporator. The dried powder was again added with a 1.3-fold excess of water compared to the incipient wetness method and dried. This process was repeated twice. Before the impregnation with ammonium nitrate, the iron-impregnated titania at 120 ⁰ C and 300 mbar for 12 hours, was treated. The thus separately impregnated titanium dioxide was dried at a heating rate of 1 ⁰ C / min at 150 ⁰ C for 2 hours and immediately annealed at a heating rate of 3 ⁰ C / min at 500 ⁰ C for 6 hours.

3. Spread Method (Solid Wetting)

Another possibility for applying a monolayer of iron molybdate to, for example, a titanium dioxide carrier by spreading method is the heating of an iron molybdate Titanium oxide mixture at the Tammann- (1/2 melting point) or preferably at the Hüttig temperature (1/3 melting point) is.

In this method it is assumed that the released enthalpy of the uniform distribution of the

Iron molybdate on the surface of titanium dioxide is greater than the enthalpy of clustering (reduction of surface free energy). The higher the temperature, the greater the risk of sintering, which can be far below the melting temperature of the active component. Typically, the surface diffusion of metal atoms can (üttig = 0.3 T _Sc hmeiz T _H) and that of Metallagglomeraten in Tammann temperature (T _Tamma nn = 0.5 Ts _c hm _e iz) are already used in the Hüttig temperature.

Annealing at Tammann temperature was repeated for 14, 36, and 48 hours. Immediately after the last treatment lasting 24 hours heat treatment at 297 ⁰ C (T _H üttig) was carried out in order to resolve the remaining defects in the monolayer. Between the tempering, the mixture was mortared in a mortar to ensure a uniform distribution as possible. Before each anneal, a sample was taken for analysis to control the monolayer formation.

The amount of iron molybdate calculated for a monolayer was mixed with the respective titanium dioxide powder. The TiO ₂ used particularly preferably has a BET surface area of> 20 m ² / g and <50 m ² / g. Therefore, it was necessary to impregnate 250 g of TiO ₂ and 40.68 g of iron molybdate. The mixture was heated with a heating rate of 4 ⁰ C at 495 ⁰ C (T emperature _Schme i _zt = 990 ⁰ C). The annealing was repeated for 14, 36 and 48 h. After the last annealing immediately a 24 h lasting temper was carried out at 297 ⁰ C (T _Hütt ig). The coating composition thus obtained was applied in the form of thin layers at a temperature between 60 to 80 ⁰ C on the steatite body in a fluidized bed. The layer thickness of the catalytically active material was up to 300 microns. Of course, the same applies to a subsequent impregnation of a previously applied to the carrier molding TiO ₂ layer.

After impregnation with ammonium heptamolybdate and calcination, the titanium dioxides had a yellow color indicative of iron molybdate Fe ₂ (MoO ₄ ) ₃ .

Example 2

1. Preparation of the Monolayer Catalyst

TiO ₂ powder was impregnated with a theoretical monolayer of iron molybdate according to the "incipient wetness" method (1.4% based on the total steatite support).

The TiO _{2 support} was impregnated with iron nitrate as in Example 1, dried and then impregnated with ammonium heptamolybdate and dried again. The ratio of Fe: Mo was set to 1: 1. The impregnated TiO ₂ powder was annealed at 500 ⁰ C for 6 hours, then annealed at 297 ⁰ C (Hüttig temperature) for 24 hours.

The coating was carried out under the following conditions:

1% commercial TiO ₂ -SoI (81 g) as an inorganic binder was at a pH of 2.5 and (84 g) of organic binder, for example, a vinyl acetate-ethylene copolymer binder, such as in tile adhesives, was added to the suspension of the FeMo / TiO ₂ powder (184.2 g in 1000 ml H ₂ O). Subsequently, the suspension was with stirring at 10,000 min ^"1 for 3 minutes at 70 ⁰ C is homogenized and then placed 2.5 mm steatite rings of 5 x 5 x.

2. Preparation of Monolayer Catalyst (Example 3)

To prepare the monolayer catalyst (Example 3) TiO ₂ powder (175 g) physically mixed with Eisenmolybdatpulver (FAMAX ^®, Süd-Chemie Catalysts Italia) (9.2 g).

Example 3 Production of the Full Catalyst

The unsupported catalyst FAMAX ^® was used as Süd-Chemie Catalysts Italia related.

Example 4

The catalysis tests listed below were performed in a tubular reactor with 21 mm inside diameter and a length of 100 mm at 330, 250, 290 ⁰ C average catalyst temperature and carried out under the GHSV of 5, 10, 15 and 20th

Formaldehyde, methanol, dimethyl ether and water were determined by gas chromatography, CO and CO ₂ by IR measurements.

The supported Fe-Mo-O catalysts according to the present invention having a Mo / Fe ratio of 0.25 to 4 and a surface loading of 0.5 to 2 monolayers were added in terms of evaluation of their activity and selectivity in the oxidation of methanol Formaldehyde tested and compared with the commercially available unsupported catalyst. The following Table 1 summarizes the experimental results of the catalytic tests.

Table 1: Results catalytic test

Comparative Example 1

Comparative Example 2

Comparative Example 3 Inventive Example 1

Comparative Example 4

Comparative Example 5

Comparative Example 6 Inventive Example 2 Inventive Example 3

Comparative Example 7

Inventive example 4 Different Mo / Fe ratios of catalysts according to the invention:

1 monolayer of active component

First, Fe-Mo-O catalysts having a monolayer of the active component were tested with different Mo / Fe ratios. From Table 1 it can be seen that the Fe excess catalyst (Mo / Fe ratio = 0.25) has a significantly lower activity than the remaining catalysts tested. The catalyst, whose surface consists of pure MoO ₃ (Mo / Fe ratio = ∞) is indeed more active than at high Fe content, but much less active than the catalysts with the same Fe and Mo content (Mo / Fe = 1) or with Mo excess (Mo: Fe = 4).

In order to achieve a conversion of 85%, the catalyst with Mo / Fe ratio of 0.25, a temperature of 330 ⁰ C was required. A comparable conversion was achieved at a Mo / Fe ratio of 1 already at 290 ⁰ C. With a triple Mo excess, only 250 ^° C. was needed to reach about 70% conversion. A higher conversion is therefore achieved only with a catalyst whose Mo content is at least equal to that of Fe. Sales of greater than 95% were achieved at the same Fe and Mo content, and at pure MoO ₃ at a temperature of 330 ⁰ C. For Mo excess, however, only a temperature of 290 ⁰ C was required.

The pure MoO ₃ catalyst had no higher activity than the catalyst with a Mo: Fe = 4 ratio, suggesting that the activity is not due solely to the Mo content. When comparing the selectivities for formaldehyde in Table 1 it can be seen that the catalysts with Mo / Fe = 1 and Mo: Fe = 4 are not significantly different from each other. At full conversion, the catalyst with the same Fe and Mo content (Mo / Fe = 1) has a higher formaldehyde selectivity (<85%) than that with Mo excess (Mo: Fe = 4) (about 80%).

The Fe excess catalyst (Mo / Fe = 0.25) had a very low selectivity to formaldehyde. Even with comparatively small conversions, the formaldehyde selectivity drops sharply and also results in a higher CO _x selectivity.

The catalyst without iron also has a low formaldehyde selectivity. However, from the slightly better activity than the catalyst with Fe excess, it follows that this low selectivity is expressed only at higher conversions than in the catalyst with Fe excess. With technically non-relevant sales below 50%, however, the selectivity of MoO _{3 is} at a similar level to that of a Mo / Fe ratio of 1 or 4.

These results therefore allow the conclusion that a high formaldehyde selectivity is not only due to a high Mo0 ₃ content, since the formaldehyde selectivities do not increase with the Mo content, but decrease again from a certain Mo excess.

However, the results show that pure MoO _{3 is} not even more selective than the Fe / Mo mixed oxides.

The dimethyl ether selectivities were generally very low for the catalysts tested and higher conversions (Table. 1) . With almost quantitative sales, they mostly fell to zero. At the same Fe and Mo content, the dimethyl ether selectivity is about 80% conversion through a maximum, which is very low at about 0.3%.

In summary, for catalysts with a monolayer of active components, the statement can be made that an excess of iron has a negative effect on the activity and on the formaldehyde selectivity. However, the absence of iron also has a negative effect on activity and selectivity. Catalysts, which however do not have an excess of iron or even contain more Mo than Fe, are similar in a first approximation.

2 monolayers of active component

Similar results to a monolayer active component were obtained when catalysts with two monolayer active components and different Mo / Fe ratios were investigated. In comparison to each other, they behave similarly to the catalysts with only one monolayer of active component. However, the catalyst with pure MoO ₃ was slightly more active with 2 monolayer of active component compared to a monolayer of active component and almost reached the activity of the catalyst with a Mo / Fe ratio of 4 (Table 1).

Also when comparing the formaldehyde selectivities it can be seen (Table 1) that 2 monolayers of pure MoO _{3 give} significantly better results than is the case with a monolayer.

A somewhat different picture compared to a monolayer active component results when comparing the dimethyl ether selectivities in Table 1. Different As with a monolayer, the selectivities at two monolayers decrease with increasing turnover, without undergoing a clear maximum.

In summary, in addition to the results with monolayer catalysts, a lack of Fe content in pure MoO _{3 has} less negative effects on activity and selectivity.

Different Mo / Fe ratios of the active component and correlation with the layer thickness of the active component:

Mo / Fe = 0.25

Comparing catalysts with the same Mo / Fe ratio of 0.25 and different loading of the active component, it is noticeable that the 2 monolayer catalyst is the more active of the two (Table 1). However, at the same time the formaldehyde selectivity decreases with increasing conversion below 50%.

Mo: Fe = 6/1 and 3/1

In contrast, with a Mo / Fe ratio of 6/1 and 3/1, the catalyst with a monolayer of active component is somewhat more active (Table 1) than the catalyst with 2 monolayers. This manifests itself among other things by larger sales at the same reaction temperature. As expected, formaldehyde selectivities are significantly better for both catalysts (1 and 2 monolayers) with this Mo / Fe ratio than for the catalysts with a threefold iron excess. It was found that the formaldehyde selectivity of the catalyst with a monolayer active component is slightly higher under the same reaction conditions than in the case of the two-monolayer catalyst. At full conversion, however, the catalyst is with two monolayer active component slightly better than that with only one monolayer.

The dimethyl ether selectivities in both catalysts decrease with increasing conversion until they reach approximately zero with conversions of <95%.

Mo / Fe = 1/1

With the same Mo and Fe content, the catalyst with two monolayers of active component is again more active than that with only one. However, this two monolayer catalyst is characterized by a slightly lower formaldehyde selectivity with low methanol conversions (Table 1).

Mo / Fe = 1/0

If the catalysts without iron content in the active component are compared, it can be seen (Table 1) that the catalyst with two monolayers of active component performs considerably better. In addition to about twice as high activity, it also has a much higher formaldehyde selectivity at industrially relevant high conversions.

In summary, the catalysts that have two monolayers of active component are more active. Only with a large Mo excess (Mo: Fe = 6) is the catalyst slightly more active with only one monolayer.

The formaldehyde selectivities show a different picture: the larger the Mo content in the active component, the higher the formaldehyde selectivities for the two monolayer catalysts compared to the monolayer catalysts. Summary of results:

Considering only the activities, for one and two monolayers, the catalysts with a high Mo / Fe ratio of 3-6 proved the most active (Figure 3). Most inactive are the high iron catalysts (Mo / Fe = 0.25).

If one considers the absolute activities on the basis of the activity constant (Table 1, FIG. 1), it can be seen that the 2-monolayer catalysts have up to 1.5-fold activity in comparison to the 1-monolayer catalysts or to the reference own catalysts.

In the formaldehyde selectivities, the catalyst with a monolayer and a Mo / Fe ratio of 1.5 is the most selective (Figure 2). Second is the more active catalyst with Mo excess (Mo: Fe = 3 and 6).

For catalysts with two monolayers of active component, the catalyst with the same Fe and Mo contents is the most selective, followed by that with Mo: Fe = 3 and 6.

Mo / Fe ratios between 1 and 6 thus have the greatest advantages in terms of activity and selectivity on methanol oxidation to formaldehyde.

Figure 4 shows the yield of formaldehyde over 2400 h reaction time carried out according to Example 4 with a catalyst according to the invention (2 monolayers, ratio Mo: Fe = 6). The formaldehyde yield remained constant over the entire reaction time. Comparison of inventive catalysts with solid catalysts

Table 1 lists the first order activity constants for various catalysts.

The activity of the catalysts constants increase with 330 ⁰ C salt bath temperature in the following order:

1) full body catalyst, 2) impregnated monolayer catalyst and 3) spiked monolayer catalyst. The difference in the activity at 330 ⁰ C between the impregnated and the spread-monolayer catalysts was insignificant and shows that both methods lead to manufacture active FeO monolayer on the carrier.

Claims

claims

1. A supported catalyst comprising a support layer arranged on an inert carrier body constructed from individual particles and containing a metal oxide of a main group metal, an early transition metal or a lanthanide, wherein on the carrier layer or on the individual particles of the carrier layer comprises an active component comprising a molybdenum and / or vanadium and / or iron-containing oxide composition is arranged, whose surface density corresponds to at least one monolayer.

2. Catalyst according to claim 1, wherein the surface density of the active component corresponds to at least two monolayers.

3. Catalyst according to claim 1 or 2, wherein the active component is selected from the group consisting of molybdenum, iron and vanadium.

4. Catalyst according to claim 3, wherein the active component contains molybdenum and iron.

5. A catalyst according to claim 4 wherein the active component contains further vanadium.

6. Catalyst according to claim 3, wherein the active component contains iron and vanadium.

7. Catalyst according to one of the preceding claims further comprising a promoter selected from the group consisting of Li, Na, K, Cs, Rb, Ca, Sr, Ba, P, Sb, Bi, Cu, Ag, Au, Sn, Ce and Mixtures thereof.

8. A catalyst according to claim 4 or 5, wherein the ratio of molybdenum to iron (Mo / Fe) is in the range of 0.5 to 20.

9. A catalyst according to any one of the preceding claims, wherein the BET surface area of the support layer is greater than 15 m ² / g.

10. Catalyst according to claim 9, wherein the metal oxide of the support layer is selected from the oxides of aluminum, silicon, titanium, zirconium, tin, magnesium and mixtures or mixed oxides thereof.

The catalyst of claim 9, wherein the metal oxide is titanium dioxide.

12. A catalyst according to any one of the present claims, wherein the material of the carrier body is selected from magnesium silicate (steatite), quartz (SiO ₂ ), porcelain, magnesium oxide, tin dioxide, silicon carbide, rutile, alumina (Al ₂ O ₃ ) zirconium silicate, aluminum silicate, cersilicate or mixtures thereof and metals or alloys.

13. A catalyst according to claim 11, wherein the inert carrier body has a porosity of less than 0.1 ml / g and a BET surface area of less than 1 m ² / g.

14. A process for preparing a catalyst according to any one of the preceding claims comprising the coating of the carrier layer or particles of the carrier layer with a molybdenum and / or vanadium and / or iron-containing oxide composition of an active component.

15. The method of claim 14, wherein the active component is then treated at a temperature corresponding to its Hüttig- temperature in the range of 280-310 ⁰ C is treated.

16. The method of claim 14, wherein the active component is then treated at a temperature corresponding to its Tammann- temperature in the range of 300-550 ⁰ C is treated.

17. The method of claim 15 or 16, wherein the carrier layer is prepared via a starting compound containing a sol.

18. The method of claim 14 or 17, wherein in the sol further a solid metal oxide is introduced.

19. The method of claim 18, wherein the pH of the sol is adjusted to a range of 2 to 8 after introduction of the metal oxide.

20. The method of claim 19, wherein an organic binder is further introduced into the sol.

21. The method of claim 20, wherein in the sol further soluble inorganic salts or organometallic complexes are introduced.

22. Use of a catalyst according to one of the preceding claims 1 to 13 for the oxidation of methanol to formaldehyde.