ZA200200015B

ZA200200015B - Zinc oxide-based dehydrogenating catalysts.

Info

Publication number: ZA200200015B
Application number: ZA200200015A
Authority: ZA
Inventors: Bernd Pennemann; Andreas Schulze-Tilling; Jorg Dietrich Jentsch; Heinz Peter Meier; Erika Meier-Reiter; Petra Meier; Ute Nadalini; Jutta Meier
Original assignee: Bayer Ag
Priority date: 1999-07-19
Filing date: 2002-01-02
Publication date: 2003-01-02
Also published as: HK1048613A1; JP2003504194A; AU6432100A; EP1202799A1; DE19933079A1; CN1361718A; WO2001005499A1; KR20020013968A

Description

DEHYDROGENATION CATALYSTS BASED ON ZINC OXIDE

The present invention relates to dehydrogenation catalysts based on zinc oxide as active component and the use thereof in the dehydrogenation of secondary alcohols to the corresponding ketones.

DE-A 19626587 discloses a process for producing cyclohexanone by dehydrogenation on a catalyst containing Cu as active component and ALO, as support. Such catalysts generally permit lower reaction temperatures than do those without copper.

EP-A 0 204 046 describes a process for producing cyclohexanone, wherein a catalyst consisting of copper, zinc oxide and of an alkali metal compound, preferably sodium carbonate, is used for the dehydrogenation. The selectivities of such catalysts are frequently low, but this can be remedied by a suitable method of production and by favourable combinations of active substance and support. Besides this, it is frequently necessary to admix additives such as water to the reaction mixture, in order to obtain an acceptable selectivity. It is not possible to remedy the comparatively low conversion, which results from the more unfavourable position of equilibrium at a low reaction temperature. In the case of these catalysts, an elevation ot the reaction teiiperatiite is coiiiifioily associdted with 4 distifictiy accelerated ageing of the catalysts owing to sintering of the copper, and consequently the catalysts thus used have uneconomically short useful lives.

These disadvantages are avoided in processes which use copper-free catalysts.

DE-A 1443462 describes catalysts for the dehydrogenation of cyclohexanol, which are produced by precipitation of a solution of zinc salts by means of basic precipitating agents. No information is given about the Na content of the catalysts.

DE-A 19609954 discloses a process for the dehydrogenation of secondary alcohols

RE WO 01/05499 PCT/EP00/06376 at elevated temperature in the vapour phase. A mixture of zinc oxide and calcium carbonate is used as catalyst. The catalyst is obtained by precipitating zinc nitrate and calcium nitrate by means of sodium carbonate, filtering off, washing until free from nitrate and drying the filter cake. The product is subsequently calcined and pressed to form mouldings. However, a selectivity which enables this process to be used economically is achieved only if hydrogen is added to the gas flowing into the reactor. This is a serious disadvantage of the process, as the addition of hydrogen diminishes the conversion of the reaction, owing both to the more unfavourable position of equilibrium of the reaction and to the decreased residence time of the gas mixture in the reactor. The space-time yield of this process, according to the

Examples, was 0.46 g cyclohexanone per g catalyst and per hour.

In all the known processes of prior art, dianone is an undesirable secondary product.

Accordingly, the object of the present invention was to find a catalyst for a process whereby secondary ketones, preferably cyclohexanone, can be obtained with high selectivities accompanied by high conversions. In addition, the catalyst is to have an adequate useful life. In the process according to the invention, secondary alcohols are dehydrogenated at elevated temperature in the vapour phase, in the presence of a catalyst containing zinc oxide with 0.1 to 0.6 wt.% sodium. Alcohols which can be used according to the invention are cycloaliphatic alcohols as well as secondary aliphatic alcohols, preferably cyclohexanol.

The Application accordingly provides a catalyst containing zinc oxide as active compound. Catalysts which contain 0.1 to 3%, preferably 0.1 to 0.6%, sodium are preferred. Catalysts containing 0.15 to 0.4% sodium are particularly preferred. The specific BET surface is preferably between 5 and 30 m*/g; from 8 to 20 m%/g is particularly preferred.

a ' | ' WO 01/05499 PCT/EP00/06376

A catalyst according to the invention can be obtained by precipitation of a sparingly soluble zinc compound from water-soluble zinc compounds by means of a base and subsequent processing of the precipitated product in a manner familiar to the person skilled in the art. A production according to the invention consists, for example, in using aqueous sodium carbonate solution and adding zinc sulfate thereto. The precipitated product is filtered off, washed, dried and subsequently calcined at a temperature not higher than 650°C. The product obtained is optionally ground and pressed to form mouldings, for example, by mixing it with a tabletting aid and tabletting it in a tabletting machine.

One generally proceeds from aqueous solutions of zinc salts. Examples of zinc salts which can be used are zinc sulfate, zinc nitrate, zinc chloride or zinc acetate. Zinc sulfate and zinc chloride are preferred. Water-soluble sodium salts such as sodium hydroxide, sodium hydrogen carbonate or sodium carbonates, for example, can be used for the precipitation; sodium carbonate and sodium hydroxide are preferred.

Here, a solution of one of the two salts, preferably the base, will be placed in a vessel and the aqueous solution of the other salt will be run in until the required pH value has been attained. Precipitation is preferably carried out up to a pH of 6 to 9.

Such a preparation is described, for example, in DE-A 3900243. The temperature chosen is generally within the range of 20°C to 90°C, preferably 50°C to 80°C. In another variant of the process, both sait solutions are introduced into the vessel simultaneously, the addition being regulated in such a way that a constant pH value is established in the vessel. A pH value of 6 to 9 is preferred during the precipitation.

The precipitate is filtered off, washed and dried. The washing is carried out preferably by passing liquid through in such a way that the calcined catalyst precursor still contains 0.1 to 0.6% sodium that can be washed out, particularly preferably 0.15 to 0.4% sodium that can be washed out. Here the content of sodium that can be washed out is ascertained by determination of the Na content before and after the catalyst precursor has been washed with 100 1 distilled water per kg catalyst precursor.

a : | : WO 01/05499 PCT/EP00/06376

During the washing of the catalyst precursor, care is taken to ensure that the anion of the zinc salt used is completely washed out, as these anions can adversely affect the selectivity of the catalyst according to the invention. In another preferred procedure, the quantity of sodium is added deliberately to the processed powder, for instance, by making the precipitated product obtained into a paste with the basic solution, for example, a sodium carbonate solution, and subsequently drying it. The concentration of the solution is chosen in such a way that the catalyst has the sodium content according to the invention. Thus the washing step is particularly important, as here the sodium content - which is easily accessible to the person skilled in the ar, for example, by elemental analysis - of the catalyst can be influenced.

The powder obtained may optionally be precalcined, preferably in a spray dryer. The powder is then subjected to a shaping step and the product is calcined, the calcining conditions and in particular the temperature being chosen in such a way that the resulting catalyst has a specific internal BET surface of at least 5 and at most 30 m®/g, particularly preferably 8 to 20 m?/g. In this connection, it holds in preliminary orientating experiments that the specific surface is the smaller, the higher the calcining temperatures and/or the longer the calcining times chosen. The sequence of shaping and calcining may optionally also be reversed.

In a preferred embodiment, calcining is carried out first, the product obtained is mixed with 0.1 to 5 wt.%, preferably 1 to 5 wt.%, particularly preferably 2 to Swt.% auxiliary substances and the product obtained is pressed to form mouldings, such as tablets, stars, rings, chippings, cartwheels, spheres - preferably tablets. The preferred tabletting aid is graphite, synthetic graphite being particularly preferred owingto its higher purity, as contamination of the catalyst should be avoided, in any case during the processing steps. Quite generally, care should be taken to ensure that those metal oxides which are known by the person skilled in the art to have acidic properties do not enter the catalyst. These include, for example, activated aluminium oxide. The tablets are calcined so that the specific internal BET surface of at least 5 and at most h : | ' WO 01/05499 PCT/EP00/06376 30 m’/g, particularly preferably 8 to 20 m%g, is attained. Particularly preferred in this connection are catalysts having a lateral crush strength of 20 to 500 N, particularly preferably 40 to 100 N. These crush strengths can be established by methods conventional in the field of tabletting, for example, adjustment of the ram force and measurement on an Instron Mini 44, diameter of moulding plug 8 mm.

When the catalyst is used in the process according to the invention, the vapour of the secondary alcohol to be dehydrogenated is brought into contact with the catalyst. To this end, the alcohol can, for example, be vaporised in an evaporator and passed through a heated flow tube containing a catalyst bed. In the case of cyclohexanol, the chosen quantity of alcohol charged per unit of time is preferably 0.5 to 3 kg, particularly preferably 1 to 2 kg, cyclohexanol per litre catalyst bed and per hour; in the case of other alcohols, the quantity corresponds to their molecular weight in proportion to cyclohexanol. A particularly preferred procedure is to use the product obtained from the cyclohexanone oxidation and subsequent purification, a mixture of cyclohexanol and cyclohexanone. The reaction temperature here is preferably between 200°C and 500°C, particularly preferably 300°C to 450°C, without any resulting unacceptable changes in the useful life of the catalyst. In a preferred procedure, the temperature is chosen in such a way that a conversion of the secondary alcohol of preferably 60% to 90%, particularly preferably 65% to 80%, is established. The particularly preferred range of the reaction temperature is then from 300°C to 450°C. The product obtained is conventionally condensed, freed from hydrogen and worked up by distillation.

The catalyst according to the invention has a high selectivity accompanied by a long useful life and sufficiently high activity.

Cyclohexanone produced according to the invention is an important precursor for polyamide -6 and polyamide -6.6.

h ‘ | ’ WO 01/05499 PCT/EPOOB6376

Examples

Example 1 (Production of catalyst) 18 1 of distilled water and 18 1 of an 18% solution of sodium carbonate were placed in a 60 1 vessel and heated to 60°C. Approximately 20 1 of a solution of zinc sulfate containing 100 g zinc/l, obtained by dissolving zinc sulfate hydrate (ZnSO, * 7H,0) in distilled water and acidifying with sulfuric acid to a pH of 4.1, was heated to 60°C and run into the vessel over a period of 60 minutes, with stirring. The addition of the zinc sulfate solution was concluded when a pH value of 7.0 had been attained in the vessel. The reaction mixture was stirred for a further 30 minutes and then left to stand for 30 minutes, the supernatant solution was filtered under suction, the suspension was distributed on three nutsch filters, filtered off and washed with a total of 72 1 of distilled water. The filter cake was spread out on several metal sheets and dried overnight in a forced-air oven at 125°C. 3570 grams of product was obtained. This product was then heated to 400°C in a stationary oven and calcined for one hour. The product obtained was ground, then mixed with 4 wt.%, based on the total solid substance, of a synthetic graphite (average particle diameter 44 um) and pressed in a tabletting machine into tablets of 5 mm in diameter and 3 mm in height. The tablets were tempered in a laboratory oven for 4 hours at 500°C. The catalyst had a lateral breaking hardness of 104 N, a specific internal BET surfice of 11 m*/g, a sodium content of 0.20% and a sulfate content of 0.02%.

Example 2 (Production of catalyst)

As in DE 3900243, a solution of zinc salts consisting of zinc sulfate and zinc chloride in a molar ratio of 1:2 was used. A solution of sodium carbonate and sodium hydroxide in a molar ratio of 40:60 was used as base. The two solutions were metered into the vessel simultaneously, the pumping speed being regulated in such a way that a pH of 8 was established. The product obtained was filtered. It was then washed on nutsch filters. Otherwise, the procedure was as in Example 1. The final prepared catalyst had a sodium content of 0.39%, a lateral breaking hardness of 45 N, a specific internal BET surface of 9 m%/g.

Example 3 (Comparison Example)

The procedure was as in Example 2, but the tablets of catalyst were not calcined at the end of the production process. This resulted in a specific internal BET surface of 64 m’/g. The sodium content of the catalyst was 0.39% and its lateral breaking hardness was 48 N.

Example 4 (Comparison Example)

The procedure was as in Example 2, but 5.0 kg of the precipitated product was additionally washed with 530 1 of distilled water on a vacuum nutsch filter. The mouldings were produced as in Example 2. The resulting catalyst had a sodium content of 0.022%, a lateral breaking hardness of 35 N and a specific internal BET surface of 10 m’/g.

Example 5 (Comparison Example) 4 kg of ZnO powder of the type "Zinkweill Pharma 8" (product of the firm Grillo) was made into a paste with water, the product was dried overnight in a laboratory drying oven at 125°C, the resulting cakes were broken and the fraction with a particle size of 1.5 to 3 mm was screened out by means of a screening machine. The resulting grain was very soft and had a specific internal BET surface of 7.5 m*/g and a sodium content of 12 ppm.

Example 6 (Test of the catalysts)

The catalysts obtained were tested in a flow tube having an internal diameter of 55 mm and a length of 1 m. The reaction tube was heated electrically; the volume of the catalyst bed was 2 | in each case. A mixture of 94% cyclohexanol and 6% cyclohexanone was fed at a rate of 3 kg per hour to the reactor via an evaporator.

The catalysts were operated over a period of about 7 to 10 days and in the course of this, the reaction temperature was increased in steps to the extent that the resulting cyclohexanone content of the product was 73-75%. Then the dianone content of the product, as determined by gas chromatography, and the required reaction temperature for the assessment of the catalyst were recorded. In the case of the catalyst in Example 3, the conversion was not increased to more than 63%; comparable measured values for Example 1 and Example 2 are given.

If Example 1 of DE 14 43 462 is repeated, a catalyst containing 3.67 wt.% sodium is obtained.

Table 1: Results of the dehydrogenation tests

Example 1 0.20 11 97 309 64 0.19 243 327 73 0.24

Example 2 | 0.39 93 333 64 0.09 189 347 75 0.17

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Claims

Claims:

1. Dehydrogenation catalyst containing zinc oxide, characterised in that it contains between 0.1 and 3 wt.% sodium, based on the total weight of the catalyst with a specific internal BET surface of 5 to 30 m?/g.

2. Dehydrogenation catalyst according to claim 1, characterised in that it contains between 0.1 and 1 wt.% sodium, based on the total weight of the catalyst.

3. Dehydrogenation catalyst according to claim 2, characterised in that it contains between 0.1 and 0.6 wt.% sodium, based on the total weight of the catalyst.

4 Dehydrogenation catalyst according to claim 3, characterised in that it contains between 0.15 and 0.4 wt.% sodium, based on the total weight of the catalyst.

5. Dehydrogenation catalyst according to any one of claims 1 to 4 characterised in that the internal BET surface is between 8 and 20 m%g.

6. Dehydrogenation catalyst according to any one of claims 1 to 5, characterised in that it contains additional auxiliary substances, to form mouldings in Yuaiitities uf fiviin 0.1 lu 3 Wi.%.

7. Dehydrogenation catalyst according to claim 6, characterised in that it contains additional auxiliary substances in quantities of from 1 to 5 wt. %.

8. Dehydrogenation catalyst according to claim 7, characterised in that it contains additional auxiliary substances in quantities of from 2 to 5 wt. %.

9. Dehydrogenation catalyst according to any one of claims 6 to 8, characterised in that the additional auxiliary substances is graphite.

10. Dehydrogenation catalyst according to any one of claims 6 to 9, characterised AMENDED SHEET in that the mouldings are tablets.

11. Moulding containing one of the dehydrogenation catalysts according to any one of claims 1 to 10, characterised in that it has a lateral indentation hardness of between 20N and SOON.

12. Moulding according to claim 11, characterised in that it has a lateral indentation hardness of between 40N to 100N.

13. Process of producing secondary ketones by dehydrogenation of secondary alcohols in the presence of a catalyst according to any one of claims 1 to 10 in the vapour phase at elevated temperature between 200°C and 500°C.

14, Process for producing secondary ketones by dehydrogenation of secondary alcohols in the presence of a moulding according to claim 11 or 12 in the vapour phase at elevated temperature between 200°C and 500°C.

15. Process for producing secondary ketones by dehydrogenation of secondary alcohols in the presence of a catalyst according to any one of claims 1 to 10 and of a moulding according to claim 11 or 12 in the vapour phase at elevated temperature between 200°C and 500°C. 18, Process according to any one of claime 13 to 15 in which the elevated temperature is between 300°C and 450°C. AMENDED SHEET