WO1995025590A1

WO1995025590A1 - Shell-type catalyst, its use in the preparation of aziridines, and process for preparing aziridines

Info

Publication number: WO1995025590A1
Application number: PCT/EP1995/000839
Authority: WO
Inventors: Uwe Dingerdissen; Günter Lauth; Klaus Harth; Ulrich Steuerle
Original assignee: Basf Aktiengesellschaft
Priority date: 1994-03-18
Filing date: 1995-03-07
Publication date: 1995-09-28
Also published as: EP0750525A1; DE4409310A1; JPH09510462A

Abstract

The invention concerns a shell-type catalyst comprising a metallic carrier material and a catalytically active layer which adheres firmly thereto, which contains elements of the alkali, alkaline earth and/or lanthanide group of the periodic system of elements and which can be obtained by physical or chemical vapour deposition on the carrier material. The invention further concerns the use of shell-type catalysts and a process for preparing aziridines by the catalytic intramolecular splitting off of water in the gaseous phase from compounds of formula (I), in which R?1, R2, R3, R4 and R5¿, independently of one another, signify H or C¿1? - C4 alkyl and R?5¿ additionally stands for cyclohexyl, phenyl, benzyl, C¿1? - C8 hydroxyalkyl or C1 - C8 aminoalkyl.

Description

Shell catalyst, its use in the production of aziridines and process for the production of aziridines

description

The invention relates to coated catalysts made of a metallic support material and a catalytically active layer firmly adhering to it, which by physical or chemical deposition via the vapor phase under reduced pressure

(physical or chemical vapor deposition) is available on the support material, the use of coated catalysts in the production of aziridines and processes for the production of azididines by catalytic intramolecular dehydration in the gas phase of alkanolamines of the formula

R ¹ * R ² HO CC NH R- ^* (I), I

R ³ R ^* *

in which R ¹ , R ² , R ³ , R ^* * and R ⁵ independently of one another are H or C -.- to Cö-alkyl or aryl and R ⁵ additionally for cyclohexyl, phenyl, benzyl, C -, - to Cs -Hydroxyalkyl or Ci- to Cβ-aminoalkyl.

Palladium catalysts are known from EP-A-0 412 415, which can be used, for example, by tempering a mesh made of stainless steel at temperatures up to 1000 ° C. and then continuously applying palladium as an active component in a vacuum evaporation system with the aid of an electron beam getting produced. The monolith catalysts thus obtainable are used in the selective hydrogenation of triple bonds to double bonds.

From EP-B-0 166 831 and EP-B-0 172 280 shell catalysts are known which are obtained by sputtering inert support materials such as silicon dioxide or aluminum oxide with metals such as silver, platinum, palladium, rhodium and iridium. The catalysts are used in the production of isocyanates from N-monosubstituted formamides or in the synthesis of formaldehyde from methanol.

EP-A-0 576 944 relates to coated catalysts which, by physical and / or chemical vapor deposition (physical or chemical vapor deposition), of an alloy on molded articles for example, glass, quartz, ceramic, graphite or metal are available. The essential elements of the alloys include, for example, aluminum, gallium, silicon, germanium, tin, lead, bismuth, titanium, zirconium, vanadium, chromium, molybdenum, tungsten, iron, zinc, copper or silver. The shell catalysts that can be produced in this way are used for hydrogenation, oxidation and isomerization reactions.

Various catalysts are used to prepare aziridines, in particular ethyleneimine, by dehydrating alkanolamines in the gas phase. For example, SU-A-230 166 discloses ethyleneimine by reacting ethylene oxide and ammonia to form ethanolamine and dehydrating the ethanolamine at temperatures of about 450 ° C. with calcium phosphate impregnated with, for example, copper chloride or cobalt (II) chloride to produce as a catalyst.

JP-B-50/10593 discloses the production of ethyleneimine by dehydrating ethanolamine in the gas phase over tungsten oxide and oxides of catalysts containing Li, Mg, Ni, Mo, Bi, Sn, Si or Al. According to the teaching of WO-A-89/05797, molecular sieves made of aluminum silicates, aluminum phosphates or silicon-aluminum phosphates are used in the dehydration of ethanolamine in the gas phase.

Catalysts are disclosed in US-A-4,289,656, US-A-4,301,036, US-A-4,337,175, US-A-4,358,405, US-A-376,732 and US-A-4,477,591 Based on niobium / tantalum oxide with the addition of alkaline earth metal oxides and / or iron / chromium oxides for the catalytic gas phase dehydration of monoethanolamine to ethyleneimine.

Further catalysts for the intramolecular elimination of water from alkanolamines in the gas phase are oxide materials which contain silicon or phosphorus as essential components. Catalysts of this type are described, for example, in EP-B-0 227 461, EP-B-0 228 898 and EP-B-0 230 776.

If the catalysts described in the abovementioned references are used in the production of aziridines, oligomers and polymers of aziridine are formed to an undesirable extent. In addition, many catalysts have a too short service life.

The object of the present invention is therefore to provide a new catalyst. With regard to aziridine formation from alkanolamines in the Gas phase have a high selectivity and deliver smaller amounts of oligomeric or polymeric products than the known catalysts and have a long service life.

The object is achieved according to the invention with a shell catalyst made of a metallic carrier material and a catalytically active layer firmly adhering to it, which can be obtained on the carrier material by physical or chemical deposition via the vapor phase under reduced pressure (physical or chemical vapor deposition), if the catalytically active layer contains elements of the alkali, alkaline earth and / or lanthanide group of the periodic system.

The invention also relates to the use of the shell catalysts in the production of aziridines by catalytic intramolecular elimination of water in the gas phase from compounds of the formula

in the R ^] , R ^? , R ³ , R ⁴ and R ⁵ independently of one another are H or C -.- to Cs-alkyl or aryl and R ⁵ additionally represents cyclohexyl, phenyl, benzyl, Ci to Cβ-hydroxyalkyl or Ci to Cs-aminoalkyl . The substituents R ¹ , R ² , R ³ , R ⁴ and R ⁵ can also represent a C ₃ - to Cs-cycloalkyl group, for example cyclopropyl, cyclohexyl, cyclopentyl or cyclooctyl.

Another object of the invention is a process for the preparation of aziridines of the formula

in which R ¹ , R ² , R ³ , R ⁴ and R ⁵ independently of one another are H or C- _. - to Cs-alkyl or aryl and R ⁵ additionally represents cyclohexyl, phenyl, benzyl, C- to Cs-hydroxyalkyl or Ci- to Cs-aminoalkyl, by intramolecular dehydration of compounds of the formula R ⁱ R ²

HO CC NH R ⁵ (i),

^{R3 R} '

in which the substituents R ¹ , R ² , R ³ , R ⁴ and R ^{5 have} the meaning given for this in formula II, on solid catalysts in the gas phase, the dehydration being carried out on a coated catalyst according to the invention or a coated catalyst ¬ sator used, which can be obtained by impregnating metallic support materials with a catalytically active composition which contains elements of the alkali, alkaline earth and / or lanthanide group of the periodic system. All substituents in formula II can represent a C ₃ - to Cs-cycloalkyl group, for example cyclopentyl or cyclohexyl.

The coated catalysts according to the invention consist of a metallic support material. These can be molded articles made of a single metal or alloys. The metallic support material is dimensionally stable under the conditions of intramolecular catalytic elimination of water from alkanolamines. It forms an inert, thermally and mechanically stable base for the catalytically active layer. The carrier material should have the largest possible geometric surface. For the metallic carrier materials, geometrical forms include, for example, powder, grit, spheres, wires, stretch plates, nets, fabrics, nonwovens, foils, perforated foils and chips. The carrier materials are preferably used in the form of woven fabrics or foils, which can optionally be perforated. Shaped bodies of this type can be shaped very well into monolithic catalysts. As is known, monolithic metallic catalysts enable a very high catalyst load with a low pressure drop, thorough mixing of the reactants and rapid heat supply and heat dissipation.

Suitable metallic carrier materials are metals of the subgroup such as iron, cobalt, nickel, titanium, tungsten, copper, silver, gold, platinum, rhodium, palladium, chromium, molybdenum, vanadium, zirconium and all types of steel. In addition to steels, for example, all alloys of the above metals can be used as the carrier material. The alloys can also contain elements of main groups of the periodic table, for example carbon, silicon, boron, aluminum, phosphorus, antimony, bismuth, nitrogen, lithium, sodium, potassium, calcium, magnesium and / or barium. Iron and iron alloys are preferably used as the metallic carrier material because these Carriers are easily accessible and can be easily brought into forms that are suitable for catalysts.

The metallic support material can optionally be subjected to a pretreatment with a catalytically active layer before coating. Such pretreatment can e.g. consist in a calcination, a chemical treatment with an acid or an alkali or in a plasma treatment (noble gas plasma or reaction gas plasma). This increases the roughness of the carrier surface, which increases the adhesive strength of the catalytically active layer which is applied thereon.

A firmly adhering, catalytically active layer is applied to the metallic supports by physical or chemical deposition via the vapor phase under reduced pressure. Suitable materials for the catalytically active layers are all compounds which are described in the literature for the intramolecular elimination of water from the compounds of the formula I, cf. references SU-A-230 166, JP-B-50/10593 given above for the prior art,

WO-A-89/05797, EP-B-0 227 461, EP-B-0 228 898 and EP-BC 230 776. Preference is given to using alkali, alkaline earth and / or lanhanide compounds which are used in the Conditions of azidine synthesis are stable and in particular are not volatile and do not form volatile compounds. Examples include phosphates, sulfates, niobates, tungstates, vanadates, manganates, molybdates, rhenates, titanates and salts of heteropolyacids. In addition to the elements of the alkali, alkaline earth and / or lanthanide group of the periodic system, the catalytically active compositions preferably contain phosphorus and / or transition metals. Examples of such catalytically active compositions are lithium-doped tungsten oxide, sodium-doped calcium tungstate, cesium or barium-doped iron sulfate or nickel sulfate.

Catalytically active materials are used, for example, in the

EP-B-0 227 461 characterized by the following formula

Si _a XχY _y O _b (III),

in which Si is silicon, X is at least one element from the group of the alkali and alkaline earth metals, Y is an element from the group B, Al, Ti, Zr, Sn, Zn and Ce and 0 is oxygen, and the indices a , x, y and b indicate the respective atomic ratios of the elements Si, X, Y and 0, where if a = 1, x = 0.005-1, y = 0-1 and b has a value which is given by a, x and y is determined. According to EP-B-0 228 898, the catalytically active compositions have the composition

X _a P _b X _c O _d (IV),

in which X denotes at least one element selected from alkali and alkaline earth metals. P is phosphorus, Y is at least one element selected from B, Al, Si, S, Sc, Ti, Cu, Y, Zr, Nb, Mo, Sn, Sb, La, Ce, Ta, W, Ti, Pb, Bi and Th, 0 is oxygen and the indices a, b, c and d indicate the atomic ratios of the elements X, P, Y and 0 and, if a = 1, b = 0.01-3 and c = 0-100 and d is a value, by a, b and c and the binding state of the individual elements and according to EP-B-0 230 776 the formula

X _a b cO _d (V),

in which X is at least one element selected from elements from Group IIIA, elements from Group IVA, elements from Group VA, transition metal elements from Groups I to VIII, lanthanide and actinide elements from the periodic table, P is phosphorus, Y is at least one element , selected from alkali and alkaline earth metals, O denotes oxygen and the indices a, b, c and d indicate the atomic ratios of the elements X, P, Y and 0 and, if a = 1, b = 0.01-6 and c = 0-3 and d is a value determined by a, b and c and the binding state of the respective elements.

Further suitable catalytically active compositions are those of US-A-4,337,175, US-A-4,289,656, US-A-4,301,036, US-A-4,337,175, US-A-4,358,405, US A-4 376 732 and ÜS-A-4 477 591 known oxides of tantalum or niobium with an alkaline earth metal oxide as a promoter. Furthermore, catalysts of the composition M-oFeo, 5-2, .Cro, 3-ι, 7 or Mι ₀ Fe ₁ , ₀ -2, 1Cr ₀ , 6- ₁ , 7 with M = niobium, tantalum are suitable.

Instead of alkaline earth metal oxides, alkali metal oxides and or oxides of elements of the lanthamide group of the periodic system can also be used.

The catalytically active materials are reduced by physical or chemical vapor deposition

Pressure on the backing material firmly adhered. These methods are referred to in the literature as physical vapor deposition (PVD) or as chemical vapor deposition (CVD), cf. RF Bhunshah et al., Deposition Technologies for Films and Coatings, Noyes Publications, 1982 and P. Wirz, Vakuum-Technik, 38, 208-214 (1989). Known PVD processes are vapor deposition, cathode sputtering, which is used in technical parlance Is called sputtering, and the arc coating. Known CVD processes include thermal and plasma-assisted coating. These processes are important, for example, for releasing phosphorus from organophosphorus and incorporating it into an active catalyst layer.

The catalytically active compositions can be applied to the metallic support material in a layer thickness of 0.1 nm to 10 μm, preferably 1 nm to 1 μm. Shell catalysts in which the metallic support material is a mesh, a fabric or a film and in which the catalytically active layer has been applied by sputtering are particularly advantageous. The carrier material preferably consists of a single metal or its alloys. For example, it can consist of transition metals or their alloys. The coated catalysts according to the invention are preferably produced by sputtering elements or their compounds, in particular their oxides. This makes it easy to apply a layer consisting of several components to the metallic carrier or to deposit layers of different composition on it.

Furthermore, the microstructure of the coating can be influenced by sputtering by varying the process gas pressure and / or by applying a negative bias (bias). For example, a process gas pressure in the range of 4 ^• 10 ^{~ 3} to 8 ^• 10 ^{~ 3} mbar leads to a very dense, fine crystalline layer with high corrosion stability. If a negative bias voltage is applied during the coating, this results in more intensive ion bombardment of the metallic carrier to be coated, which generally results in a denser layer and an improved adhesion of the active catalytically active layer on the metallic carrier.

This is generally the case with sputtering

Coating material, ie the catalytically active composition, is applied in solid form as a so-called target to the cathode of a plasma system, then under reduced pressure, for example from 1 • 10 ^{~ 4} to 1 mbar, preferably 5 • 10 ^{~ 4} to 5 • 10 ^{~ 2} mbar, atomized in a process gas atmosphere by applying a plasma and deposited on the substrate to be coated, anode, ie the metallic support. In general, at least one noble gas such as helium, neon or argon, preferably argon, is selected as the process gas. The plasma usually consists of charged (ions and electrons) and neutral (partly radical) constituents of the process gas, which interact with one another via shock and radiation processes.

Various methodological variants of cathode sputtering such as magnetron sputtering, DC and RF sputtering or bias sputtering and their combinations can be used to produce the coatings. In magnetron sputtering, the target to be atomized is usually in an external magnetic field, which concentrates the plasma in the area of the target and

10 causes an increase in the atomization rate. In DC or RF sputtering, the atomization plasma is generally excited by a direct voltage (DC) or by an alternating voltage (RF), for example with a frequency in the range from 10 kHz to 100 MHz, preferably 13.6 MHz. Bias sputtering

Usually, the substrate to be coated is coated with a generally negative bias (bias), which generally leads to intensive bombardment of the substrate with ions during the coating.

To produce the coated catalysts according to the invention, a multicomponent target is generally atomized, or two or more targets of different compositions are sprayed simultaneously (simultaneous atomization) or in succession. The desired layer thickness as well as the chemical composition and the micro

The structure of the layer can essentially be influenced by the process gas pressure, the atomization performance, the sputtering mode, the substrate temperature and the coating time. The atomization power is the power that is used to excite the plasma and is usually in the range of

30 50 W to 10 kW.

The substrate temperature is generally selected in the range from room temperature to 350, preferably from 150 to 250 ° C.

35 The coating time essentially depends on the desired layer thickness. Typical coating rates for sputtering are usually in the range of 0.01 to 100 nm / s.

Another preferred embodiment is the production of 40 coatings by vapor deposition (see L. Holland, Vacuum Deposition of Thin Films, Chapman and Hay Ltd., 1970). The coating material is expediently introduced in a manner known per se into a suitable vapor deposition source, such as electrically heated evaporator boats or electron beam evaporators. The coating material is then evaporated under reduced pressure, usually in the range from 10 ^"7 to 10 ^-3 mbar, whereby forms the desired coating on the metallic supports inserted into the vacuum system.

When producing multi-component layers, the evaporation material can be evaporated either in the appropriate composition from a common source or simultaneously from different sources.

Typical coating rates during vapor deposition are generally in the range from 1 nm / s to 10 μm / s.

In one embodiment, the substrate to be coated can be bombarded with ions before or during the vapor deposition process by means of an RF plasma or by means of a conventional ion gun in order to improve the microstructure and the adhesion of the layers. Furthermore, the microstructure and the adhesion of the layers can also be influenced by heating the substrate.

The coated catalysts can, for example, also contain transition metals in the catalytically active composition. These transition metals are preferably different from the transition metals contained in the carrier material. Shell catalysts in which the catalytically active composition contains phosphorus and those in which the catalytically active composition contains compounds of main group elements of the 3rd to 7th main group are also of technical importance.

The coated catalysts described above can, if appropriate, be further treated. The aftertreatment can consist, for example, of a calcination or a chemical treatment. In the case of chemical aftertreatment, additional components can optionally be introduced into the catalytically active layer. During the aftertreatment, for example, the mechanical properties and / or the porosity of the shell catalyst can be changed.

The shell catalysts described above are used in the production of aziridines by catalytic intramolecular elimination of water from compounds of the formula I in the gas phase. Aziridines of the formula II given above are obtained. The preparation of the unsubstituted aziridine (ethyleneimine) is particularly preferred. The temperatures during the intramolecular dehydration of alkanolamines are, for example, 200 to 600, preferably 300 to 500 ° C. The reaction is preferably carried out at normal pressure or in vacuo, but it is also possible to work at elevated pressure. When working under normal pressure, the dilution of the compounds of formula I with inert gases such as N ₂ , noble gases, H ₂ 0 etc. advantageous. The coated catalysts according to the invention can, for example, be arranged stationary in a tubular reactor in the form of nets, fabrics or perforated foils. However, they can also be used in the form of a finely divided powder or granulate in a fluidized bed reactor, in a shell reactor or in a fluidized bed reactor.

The active catalyst compositions described above can also be deposited by precipitation on a metallic support material or by impregnation of a metallic support material, preferably by spray impregnation. The coatings obtained in this way are not as firmly adhering to the supports as the tray catalysts produced using the PVD or CVD processes. The catalysts obtained by impregnation or precipitation on metal supports are also to be mentioned as shell catalysts and can be used as catalysts in the preparation of aziridines by intramolecular catalytic elimination of water from compounds of the formula I. They are particularly suitable as catalysts in the production of ethyleneimine.

For example, polyethyleneimines with different molecular weights are produced from ethyleneimine. The polyethyleneimines are process aids in the manufacture of paper.

Examples

Manufacture of coated catalysts

Steel mesh from Kufferath was used in all examples. The material numbers are given in the examples.

example 1

A steel mesh material number 1.4767 annealed at 900 ° C. (mesh size = 180 μm, wire thickness = 112 μm) was introduced into a cathode sputtering system. At a distance of 56 m, a pressed target made of Ca phosphate of the composition 30.7% Ca, 23.7% P, 0.11% Na, rest 0 was introduced. The system was evacuated to a pressure of 10 ^{~ 6} mbar using a two-stage pump system. Thereafter, argon was admitted to a pressure of 5 x 10 ^{~ 3} mbar. By applying an RF voltage to the carrier with an output of 250 W, the carrier was subjected to a sputter etching treatment for 1 min. After the end of the etching treatment, the Ar pressure was set to 5 × 10 ^-3 mbar. By applying an RF voltage of 500 W at the target was deposited a 50 nm thick layer on the front of the metal mesh. The back of the fabric was then coated in the same way.

Example 2

A steel mesh material number 1.4767, annealed at 900 ° C. (mesh size = 180 μm, wire thickness = 112 μm), was introduced into a cathode sputtering system. A pressed target of Ca phosphate of the composition 30.7% Ca, 23.7% P, 0.11% Na, rest 0 was introduced at a distance of 56 mm. The system was evacuated to a pressure of 10 " ⁶ mbar using a two-stage pump system. Then argon was admitted to a pressure of 5 x 10" ³ mbar. By applying an RF voltage to the carrier with an output of 250 W, the carrier was subjected to a sputter etching treatment for 1 min. After the end of the etching treatment, oxygen was let in with a partial pressure of 5 * 10 ^{~ 4} mbar, the Ar pressure set to 5 x 10 ^{~ 3} mbar. A 50 nm thick layer was deposited on the front of the metal mesh by applying an RF voltage of 500 W to the target. The back of the fabric was then coated in the same way.

Example 3

Example 1 was repeated with the exception that the layer thickness of the catalytically active composition on the support was 200 nm.

Example 4

Example 2 was repeated with the exception that the layer thickness of the catalytically active composition on the support was 200 nm.

Example 5

Example 1 was repeated with the exception that a metal mesh made of material number 3.0280 was used.

Example 6

Example 1 was repeated with the exception that a metal mesh made of material number 1.4541 was used. Example 7

Example 1 was repeated with the exception that a metal mesh made of material number 1.4310 was used.

Example 8

A steel net (material number 1.4767) was placed in a cathode sputtering system. A pressed target made of Ca phosphate of the composition 30.7 Ca, 23.7% P, 0.11% Na, rest 0 was introduced at a distance of 56 mm. The system was evacuated to a pressure of 10 ^{~ 6} mbar using a two-stage pump system. Thereafter, argon was admitted to a pressure of 5 x 10 ^{~ 3} mbar. By applying an RF voltage to the carrier with an output of 250 W, the carrier was subjected to a sputter etching treatment for 1 min. After completion of the etching treatment, oxygen was let in with a partial pressure of 5 * 10 ^{~ 4} mbar, the Ar pressure set to 5 x 10 ^{~ 3} mbar. A 50 nm thick layer was deposited on the front side of the metal mesh by applying an RF voltage of 500 W to the target. The back of the fabric was then coated in the same way. The catalyst was then heated at 900 ° C.

Example 9

A steel mesh annealed at 900 ° C (material number 1.4767) was placed in a vapor deposition system. The vapor deposition system contained an evaporator device with two controllable, resistance-heated evaporator boats. In one of the two boats fine-grained (Ca-PO) powder with the composition 30.7% Ca, 23.7% P, rest 0 was poured. (Ca-O) powder was poured into the second boat. The system was evacuated to a pressure of 10 ^{~ 5} mbar using a two-stage pump system. O was let into the vacuum chamber with a partial pressure of 5 x 10 ^{~ 4} mbar. The evaporator boats were then heated, an evaporation rate of 10 nm / min from boat 1 and an evaporation rate of 20 nm / min from boat 2 being set. A 50 nm thick layer was thereby deposited on the carrier.

Claims

claims

1. Shell catalyst made of a metallic support material and a catalytically active layer firmly adhering to it, which is obtainable by physical or chemical deposition via the vapor phase under reduced pressure (physical or chemical vapor deposition) on the support material, characterized in that the catalytically active Layer contains elements of the alkali, alkaline earth and / or lanthanide group of the periodic table.

2. coated catalyst according to claim 1, characterized in that the metallic carrier material is a mesh, a fabric or a film and that the catalytically active layer has been applied thereto by sputtering.

3. coated catalyst according to claim 1 or 2, characterized gekenn¬ characterized in that the support material consists of a single metal or its alloys.

4. coated catalyst according to one of claims 1 to 3, characterized in that the catalytically active composition contains transition metals.

5. coated catalyst according to one of claims 1 to 4, characterized in that the catalytically active composition contains compounds of main group elements of the 3rd to 7th main group.

6. Use of the coated catalysts according to one of claims 1 to 5 in the production of aziridines by catalytic intramolecular elimination of water in the gas phase from compounds of the formula (I)

R ^l R ²

HO CC NH R ⁵ (I),

R ³ R ⁴

in which R ¹ , R ² , R ³ , R ⁴ and R ⁵ independently of one another are H or C 1 -C 1 -alkyl or aryl and R ⁵ additionally represents phenyl, benzyl, C 1 -C 4 -hydroxyalkyl or C -.- to Cs-Amino¬ alkyl.

7. Process for the preparation of aziridines of the formula

in which R ¹ , R ² , R ³ , R ⁴ and R ⁵ independently of one another are H or Ci * to Cs-alkyl or aryl and R ⁵ additionally for cyclohexyl, phenyl, benzyl, C -.- to Cs-hydroxyalkyl or C 1- to Cs-aminoalkyl is by intramolecular dehydration of compounds of the formula

R ⁱ R ²

HO- NH ^* R-- (I ⁾ .

R ³ R ⁴

in which the substituents R ¹ , R ² , R ³ , R ⁴ and R ^{5 have} the meaning given for this in ^' Formula II, on solid catalysts in the gas phase, characterized in that the dehydration is carried out on a coated catalyst according to one of Claims 1 to 5 or uses a coated catalyst which can be obtained by impregnating metallic support materials with a catalytically active composition which contains elements of the alkali, alkaline earth and / or lanthanide group of the periodic system.