SILVER CATALYST SYSTEM HAVING A REDUCED PRESSURE DROP FOR THE OXIDATIVE DEHYDROGENATION OF ALCOHOLS

Abstract

The invention relates to a silver-comprising catalyst system for the preparation of aldehydes and/or ketones by oxidative dehydrogenation of alcohols, in particular the oxidative dehydrogenation of methanol to form formaldehyde, comprising a first catalyst layer and a second catalyst layer, wherein the first catalyst layer consists of a silver-comprising material in the form of balls of wire, gauzes or knitteds having a weight per unit area of from 0.3 to 10 kg/m.sup.2 and a wire diameter of from 30 to 200 m and the second catalyst layer consists of a silver-comprising material in the form of granular material having an average particle size of from 0.5 to 5 mm and the two catalyst layers are in direct contact with one another. The invention further relates to a corresponding process for the preparation of aldehydes and/or ketones, in particular of formaldehyde, by oxidative dehydrogenation of corresponding alcohols over a silver-comprising catalyst system.

Claims

1.-12. (canceled)

13. A silver-comprising catalyst system comprising a first catalyst layer A composed of a silver-comprising material which is present in a form consisting of wires and selected from the group consisting of balls of wire, gauzes and knitteds and a second catalyst layer B composed of a silver-comprising material which is present in the form of granular material having an average particle size in the range from 0.5 to 5 mm, with the catalyst layer B being in direct contact with the catalyst layer A, wherein the silver-comprising material of the catalyst layer A has a weight per unit area in the range from 0.3 to 10 kg/m.sup.2 for the entire catalyst layer A and a wire diameter in the range from 30 to 200 m.

14. The silver-comprising catalyst system according to claim 13, wherein the silver-comprising material of the catalyst layer A has a weight per unit area in the range from 0.3 to 3 kg/m.sup.2 for the entire catalyst layer A.

15. The silver-comprising catalyst system according to claim 13, wherein the silver-comprising material of the catalyst layer A has a wire diameter in the range from 30 m to 150 m.

16. The silver-comprising catalyst system according to claim 13, wherein the balls of wire, gauzes and knitteds of the catalyst layer A consist of wires which are silver-comprising fibers or threads having an essentially circular cross section.

17. The silver-comprising catalyst system according to claim 13, wherein the silver-comprising material of the catalyst layer B has an average particle size in the range from 0.75 mm to 4 mm.

18. The silver-comprising catalyst system according to claim 13, wherein the silver-comprising material of the catalyst layer A has a silver content of >98% by weight.

19. The silver-comprising catalyst system according to claim 13, wherein the silver-comprising material of the catalyst layer B has a silver content of >30% by weight.

20. An activated silver-comprising catalyst system obtainable by reacting the catalyst system according to claim 13 under operating conditions, wherein a feed stream comprising one or more C.sub.1-C.sub.10-alcohols and one or more oxidizing agents is passed at a temperature in the range from 350 C. to 750 C., a space velocity in the range from 36 000 h.sup.1 to 1 800 000 h.sup.1 and an inflow velocity in the range from 0.1 m s.sup.1 to 15 m s.sup.1 through the catalyst system or, under Activation conditions, a mixture comprising oxygen and an oxidizable material is passed at a temperature in the range from 350 C. to 750 C., a space velocity in the range from 36 000 h.sup.1 to 1 800 000 h.sup.1 and an inflow velocity in the range from 0.1 m s.sup.1 to 15 m s.sup.1 through the catalyst system, where the space velocity is the ratio of the volume stream which flows through the catalyst bed to the catalyst volume in the reactor.

21. The activated silver-comprising catalyst system according to claim 20 after at least two days of operation under operating conditions or activation conditions.

22. The activated silver-comprising catalyst system according to claim 20, wherein, during the reaction, at least 50% by weight of the silver-comprising material of the catalyst layer A ha.s been converted into a finely structured silver layer on the silver-comprising material of the catalyst layer B.

23. A process for the preparation of C.sub.1-C.sub.10-aldehydes and/or -ketones by oxidative dehydrogenation of the corresponding C.sub.1-C.sub.10-alcohols, wherein a feed stream comprising one or more C.sub.1-C.sub.10-alcohols and one or more oxidizing agents is passed at temperatures in the range from 350 to 750 C. through one or more catalyst systems according to claim 13 in such a way that the feed stream flows firstly through the catalyst layer A and immediately afterward through the catalyst layer B of the one or more catalyst systems.

24. The process according to claim 23, wherein formaldehyde is the C.sub.1-C.sub.10-aldehyde formed, methanol is the C.sub.1-C.sub.10-alcohol used and the feed stream is passed at temperatures in the range from 550 to 750 C. through the one or more catalyst systems according to the invention.

Description

FIGURES

[0071] FIG. 1: Schematic depiction of the arrangement of the catalyst system of the invention for the oxidative dehydrogenation of methanol to formaldehyde with a catalyst bed consisting of a first catalyst layer A (optionally in the form of a plurality of sublayers) and a second catalyst layer B optionally in the form of a plurality of sublayers) on a support C. Here, the feed stream (the fresh gas) X comprising methanol, oxygen, water and optionally nitrogen and formaldehyde flows through the catalyst bed and gives the reaction gas Y comprising methanol, water, hydrogen, carbon monoxide, carbon dioxide, formaldehyde and possibly nitrogen and oxygen.

[0072] FIG. 2: Schematic depiction of the experimental setup used for the examples. The starting materials air (1), nitrogen (2), deionized water (3) and methanol (4) are fed into a reactor column (5) having a preheating section (5a), catalyst packing (5b), electric heating (6) and quench cooler (7). The product mixture formed is fed into an absorption column (8) and from the bottom of the absorption column (9) is transferred into a condenser (10). Finally, the product is purified in a phase separator (11) with cryostat (12).

EXAMPLES

[0073] Detailed Description of the Experimental Apparatus and Procedure

[0074] The experiments were carried out in an adiabatic mode of operation in a fused silica reactor having an internal diameter of 20 mm and filled with catalyst. The adiabatic operation of the reactor was achieved by passive insulation and dispenses completely with compensatory heating. The reactions were carried out using a gaseous water/methanol mixture (molar ratio of water/methanol: 1.0), air (410 standard I/h) and nitrogen (150 standard I/h) in such amounts that the molar ratio of methanol to oxygen was 2.5. This mixture was heated to 140 C. in a preheater located upstream of the reactor and passed through the reactor.

[0075] When metering rates and preheated temperature are set as described above, the catalyst bed usually attains, when the adiabatic reaction has been ignited, temperatures in the range from 590 C. to 710 C. The space velocity is typically in the range from 85 000 h.sup.1 to 120 000 h.sup.1. The product mixture exiting from the catalyst bed is cooled directly to 120 C. in a heat exchanger. The composition of the product mixture is determined by gas-chromatographic analysis.

[0076] The methanol conversion is defined as the molar amount of methanol reacted divided by the molar amount of methanol used. The formaldehyde selectivity is defined as the molar amount of formaldehyde formed divided by the molar amount of methanol reacted. The input number, as measure of the selectivity of the reaction of methanol to form formaldehyde at a given conversion, is defined as the amount of methanol in kilogram which has to be fed into the reactor system for 1.00 kilogram of formaldehyde to be produced in the reactor. The initial activation of the catalyst and the pressure drop which increases over the later course of time lead to the input number going through an optimum (minimum) over the time on stream. In the following examples and comparative examples, the average input number, the average methanol conversion and the average formaldehyde selectivity are determined cumulatively over a period of 18.5 days after attainment of the respective input number optimum.

[0077] For starting up the catalyst, a methanol/water/air/nitrogen mixture was heated to 300 C. in order to ensure ignition of the adiabatic reaction over the silver catalyst, with the molar ratio of methanol/oxygen being 7:1 and the introduction of nitrogen being 300 standard I/h at these temperatures. The adiabatic ignition occurred in the temperature range of 300 C.-350 C. The above-mentioned composition of water/methanol/air/nitrogen was subsequently metered in stepwise.

[0078] The temperature measurement was carried out by means of temperature sensors which were installed distributed over the cross section in the catalyst bed.

[0079] The pressure difference is recorded by means of pressure sensors upstream and downstream of the reactor. The temperature of the catalyst is controlled via the amount of air fed in.

Comparative Example 1

Two-Layer Catalyst with Granular Material Composed of Electrolytic Silver

[0080] For this experiment, the reactor was filled with a two-layer catalyst bed. The lower layer consists of a granular material composed of electrolytic silver having a particle size in the range from 1 to 2 mm and has an average thickness of 25 mm. The purity of the silver is 99.99% and the weight per unit area of this layer is 34 kg/m.sup.2. The upper layer consists of a granular material composed of electrolytic silver having a particle size in the range from 0.5 to 1 mm and has an average thickness of 5 mm. The purity of this silver is 99.99% and the weight per unit area of this layer is 10.8 kg/m.sup.2. The space velocity over the catalyst is 100 000 h.sup.1. The average inflow velocity is 1.12 m/s. The initial pressure drop over the reactor is 65 mbar. The rate of increase of the pressure drop is 0.81 mbar per day. The average input number is 1.214, the average methanol conversion is 96.6% and the average formaldehyde selectivity is 90.9%, in each case determined cumulatively for a period of 18.5 days, commencing at the point in time of the input number optimum, 0.5 days after commencement of formaldehyde production.

Comparative Example 2

Single-Layer Catalyst with Silver Wire Knitted

[0081] For this experiment, the reactor was filled with layers of silver wire knitted. The wire knitted consists of silver wires having a wire diameter of 50 m. The purity of the silver is 99.99% and the weight per unit area of this layer is 18.4 kg/m.sup.2. The layers of silver wire knitted have a total thickness of 5 mm. The space velocity over the catalyst is 800 000 h.sup.1. The average inflow velocity is 1.12 m/s. The initial pressure drop over the reactor is 88 mbar. The rate of increase of the pressure drop is 12.23 mbar per day. The average input number is 1.213, the average methanol conversion is 96.6% and the average formaldehyde selectivity is 91.0%, in each case determined cumulatively for a period of 18.5 days, commencing at the point in time of the input number optimum, 7.4 days after commencement of formaldehyde production.

Example 1

Two-Layer Catalyst with Silver Wire Knitted and Granular Material Composed of Electrolytic Silver

[0082] For this experiment, the reactor was filled with a two-layer catalyst bed. The lower layer consists of granular material composed of electrolytic silver having a particle size in the range from 1 to 2 mm and has an average thickness of 20 mm. The purity of the silver is 99.99% and the weight per unit area of this layer is 22.7 kg/m.sup.2. The upper layer consists of individual layers of a silver wire knitted having a wire diameter of 50 m and a total thickness of 0.5 mm. The purity of the silver is 99.99% and the weight per unit area of this layer is 1.8 kg/m.sup.2. The space velocity over the catalyst is 220 000 h.sup.1. The average inflow velocity is 1.29 m/s. The initial pressure drop over the reactor is 60 mbar. The rate of increase of the pressure drop is 0.27 mbar per day. The average input number is 1.211, the average methanol conversion is 97.3% and the average formaldehyde selectivity is 90.5%, in each case determined cumulatively for a period of 18.5 days, commencing with the point in time of the input number optimum, 4.9 days after commencement of formaldehyde production.

Example 2

Two-Layer Catalyst with Silver Wire Gauze and Granular Material Composed of Electrolytic Silver

[0083] For this experiment, the reactor was filled with a two-layer catalyst bed. The lower layer consists of granular material composed of electrolytic silver having a particle size in the range from 1 to 2 mm and has an average thickness of 20 mm. The purity of the silver is 99.99% and the weight per unit area of this layer is 22.7 kg/m.sup.2. The upper layer consists of two superposed woven gauzes made of silver wire having a wire diameter of 100 m and has a total thickness of 2 mm. The mesh opening of the gauze is 25 mesh, the purity of the silver is 99.99% and the weight per unit area of this layer is 3.3 kg/m.sup.2. The space velocity over the catalyst is 190 000 h.sup.1. The average inflow velocity is 1.12 m/s. The initial pressure drop over the reactor is 48 mbar. The rate of increase of the pressure drop is 0.55 mbar per day. The average input number is 1.213, the average methanol conversion is 97.2% and the average formaldehyde selectivity is 90.5%, in each case determined cumulatively for a period of 18.5 days, commencing at the point in time of the input number optimum, 27.5 days after commencement of formaldehyde production.