WO1998010864A1

WO1998010864A1 - Catalyst for preparing cyclohexanone by dehydrogenation of cyclohexanol and process for the preparation thereof

Info

Publication number: WO1998010864A1
Application number: PCT/EP1996/003982
Authority: WO
Inventors: Jong-Sik Jeong; Kyung-Su Jeon
Original assignee: Basf Aktiengesellschaft
Priority date: 1996-09-11
Filing date: 1996-09-11
Publication date: 1998-03-19
Also published as: AU7127896A

Abstract

A catalyst having a formula Cu/SiO2 used for preparing cyclohexanone by dehydrogenation of cyclohexanol and a process for making the same. The catalyst prepared according to the present invention has a higher activity and a higher selectivity than a conventional catalyst.

Description

CATALYST FOR PREPARING CYCLOHEXANONE BY DEHYDROGENATION OF CYCLOHEXANOL AND PROCESS FOR THE PREPARATION THEREOF

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a copper-based catalyst for pre- paring cyclohexanone by dehydrogenation of cyclohexanol and a process for preparing the same, particularly to a highly active and selective copper catalyst in which a relatively small amount of copper is dispersed minutely at the surface of high -surface - area silica as a carrier by precipitating onto the carrier a solu- tion of a copper salt with the aid of an alkali solution and by reducing the precipitated copper salt, and a process for the preparation thereof.

Description of the Prior Art

Cyclohexanone is a raw material for preparing caprolactam, which is used as a material for the synthesis of nylon. Among many processes for producing cyclohexanone, the dehydrogenation of cyclohexanol is known to be the most economical.

Of the catalysts used in the production of cyclohexanone, a ■ copper-metal catalyst is the best. For example, a composite oxide catalyst of copper and magnesium, CuMgO, is used as a catalys in Soviet Patent No. 697,179 (1979), and a catalyst consisting of copper and zinc as the major components, CuZnO, is disclosed ir. European Patent No. 204,046 (1986).

Other copper-based catalysts have been suggested. For example, Japanese Patent Publication No . 80/136,241 (1980) and Soviet Patent No. 936,989 disclose a CuCoO-based catalyst, and Japanese Patent Publication No. 83/157,741 (1988) suggests that a composite oxide catalyst of CuCrO or CuCrMnV is useful as a catalyst.

The reaction for the production of cyclohexanone is a reversible thermodynamically endothermic reaction, and, therefore, it is advantageous to carry it out at a high temperature in order to increase the conversion rate. However, high temperatures, considerably decrease the activity of the catalyst (deterioration) as well as its selectivity (formation of by-products) . A typical by-product, cyclohexene, is produced by dehydration of cyclohexanol by acidic sites at the surface of the catalyst.

In order to decrease the production of this by-product and to stabilize the copper metal, a basic or weakly basic magnesium- or zinc-based metal oxide is conventionally used as carrier.

However, since the carriers of such semiconductor metal oxides such as magnesium- and zinc-based metal oxides are themselves structurally unstable, said carriers are disadvantageously reduced with hydrogen. Thus, the coordination at the surface of said carriers is changed by removing coordination partners, and compounds such as phenol are easily produced due to a severe dehydrogenation reaction.

Furthermore, since the above carriers themselves are basic, they have the disadvantage that the amount of polymerized products such as cyclohexene -cyclohexanone and cyclohexylidene -cyclohexanone is increased, favouring a sintering of the metallic copper particles. Thus, the size of said particles is increased and accordingly the catalytic activity is decreased.

Generally speaking, the disadvantage of heat-resistant oxides of dielectrics such as silica, alumina, zirconia, etc., which are commonly used as carriers, is that they form by-products more readily because of their acidic properties (C. Sivaraj et al., J. of Molecular Catalysis, 60, L23 (1991)).

In order to solve the above-described problems, Brazil Patent No. 8,903,027 (1989), teaches a process for producing a catalyst by coprecipitation using an alkali silicate, such as a CaSiOj solution, which has a relatively low acidity, and using a relatively high (e.g. 50 - 90 wt%) content of the copper solution, based on the weight of silica. In this reference, the product is conven^¬ tionally precipitated at pH 7, and then washed, aged for eight hours, and then formed into particles, to which sodium hydroxide is added in order to remove the acidic sites. The product is then dried, oxidized, and reduced. A catalyst produced as described above exhibits a high selectivity of more than 99%, when more than 0.5 weight % of sodium is added. However, since water-soluble CaSi0 is used, a CuSi0₃ catalyst, which is inactive for dehydrogenation, is produced by the reaction of copper ions and CaSi0 , thus creating the problem that the content of copper in the catalyst should be more than 50 weight % in order to increase reactive centers in the reduced copper metal. Further, in order to remove acidic sites formed at the surface of the silicate, the catalyst has to be again treated with an alkali solution. SUMMARY OF THE INVENTION

The present invention addresses the problem of the conventional method of producing a catalyst for preparing cyclohexanone.

It is therefore an object of this invention to provide a highly active and selective catalyst for preparing cyclohexanone by dehydrogenation of cyclohexanol by using a small amount of copper, and a process for the preparation thereof .

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:

Figure 1 is a graph showing selectivity and yield versus tne content of copper at 280°C (liquid weight hourly space velocity) (LWHSV = 7 hr"¹) ;

Figure 2 is a graph showing selectivity versus the precipitation pH at various temperatures (LWHSV = 7 hr"¹) ; and

Figure 3 is a graph showing yield versus the precipitation pH at various temperatures (LWHSV = 7 hr^-1) .

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

In the present invention, weakly acidic but high-surf ce-area silica is used as a carrier for dispersion of the copper metal, onto which a salt of copper is directly precipitated in the form of a hydroxide by using a copper salt solution, while keeping the precipitation pH at a weak alkalinity of at least pH 9. In this manner the acidic sites at the surface of the carrier are neutralized and the copper is simultaneously dispersed as minute particles .

Silica powder is added to a 0.05 - 4.0 mole concentration of aqueous solution of a copper salt, and the reaction mixture is heated to 20 - 95°C. To this reaction mixture, an aqueous alkaline solution is added dropwise to precipitate the copper salt on the surface of the silica in the form of copper hydroxide. The aqueous alkaline solution is added dropwise for two to ten hours until the mixture reaches the final pH of 7 - 11. The precipitates are filtered, dried without being washed, shaped into proper-sized particles, i.e., 5 - 8 mm, and heated gradually to from 25 to 300°C for oxidation for 4 - 12 hours in a gaseous atmosphere containing oxygen such as air. The products thus prepared are transferred to a reaction vessel for reduction at 200 - 300°C under a reducing gas such as hydrogen. Then, the final product, a catalyst 5 having a formula Cu/Si0₂ according to the present invention, is obtained.

The catalyst according to the present invention contains 5 - 40, preferably 7 - 25, weight % of copper, on the silica carrier. The 10 copper dispersed on the surface of the silica carrier is minute particles having a diameter of less than 200A, preferably 50 - lOOA.

If the content of copper on the above carrier is less than 5 15 weight %, the desired selectivity of cyclohexanone cannot be achieved, since acidic sites of the catalyst are exposed to the environment and a side reaction, dehydration, occurs; when tne content of copper is more than 40 weight %, tne desired yield cannot be obtained. 20

Examples of copper salts for preparing the catalyse according to the present invention are any compounds which contain copper ions and which are soluble in an alkaline solvent such as an aqueous solution or ethanol. For example, cupric acetate, cupric formate, 25 cupric nitrate, etc. , can be used alone or in the form of a mixture .

The aqueous alkaline solution contains a cation of an alkali metal or alkaline earth metal of Groups IA and IIA of the Periodic

30 Table, such as sodium, potassium, calcium, etc., and an anion in the form of a strongly basic salt, for example hydroxide and carbonate. Examples of such aqueous alkaline solution are sodium carbonate, sodium hydroxide, etc., either alone or in the form of a mixture.

35

A high-surface-area porous silica gel, a fumed non-porous silica manufactured by a dry method, etc. can be used as carrier. A high-surface-area silica gel is one having a surface area of 15 - 400 m²/g. An example of fumed silica is manufactured by Dong Yang

40 Chemical Company under the trade name Zeosil.

Using the catalyst manufactured according to the present invention for the dehydrogenation of cyclohexanol gives a conversion rate of more than 90% at 200 - 300°C as well as a selectivity of more than 45 99%. The present invention is described in more detail in the following preferred embodiments of the invention, in which the conversion rate (%) is defined as:

Amount of cyclohexanol Amount of cyclohexanol at reactor inlet X at reactor outlet X 100

Amount of cyclohexanol at reactor inlet

and, the selectivity (%) is defined as:

Amount of cyclohexanone produced X 100

Amount of all products produced excluding reactants

The liquid weight hourly space velocity (LWHSV) (hr^-1) is defined as :

input of cyclohexanol feedstock (g)

hour weight of catalyst (g)

Example 1

250 g of Zeosil manufactured by Dong Yang Chemical Company was dispersed in 2,500 ml of distilled water, and then the temperature was raised to 95°C. 47 g of Cu(N0₃)₂ dissolved in distilled water was added thereto, until the entire aqueous solution had a volume of 3 liters. 0.4N of Na₂C0 solution was added slowly and dropwise for about 3 hours until the solution reached pH 9. Thereafter, it was kept for about 2 hours to age, immediately filtered, dried for 8 hours at 100°C, and oxidized in air for 5 hours at 300°C.

The catalyst thus manufactured had a copper content of 6% by weight based on weight of silica, and the average size of the copper particles was 7.5 nm. 1 g of the catalyst powder and 20 g of 5 x 7 mm particles formed from the said catalyst powder were reduced for 2 hours at 240°C in a hydrogen atmosphere. Two such catalysts were reacted in a fixed-bed reactor of 25 x 800 mm; the result is shown in Table 1. The flow rate of cyclonexanol was set at LWHSV = 7 hr"¹.

Table 1

Reaction temperature Conversion Rate (%) Selectivity (%) (°C) Powder Particle Powder Particle

250 46.47 39.83 99.2 99.80

260 53.71 45.15 99.2 99.83

270 62.74 52.66 99.1 99.85

280 72.17 59.24 99.2 99.87

290 77.45 64.58 99.0 99.86

300 81.64 69.83 99.0 99.84

Reaction Product Composition (Mole %) Temperature (°C)

Phenol Cyclohexene CHA* CHLA** pwd ptc pwd ptc pwd ptc pwd ptc

250 0.02 0.02 0.77 0.15 no 0.01 no 0.02 detectable detectable

260 0.01 0.02 0.78 0.12 0.02 " 0.01

270 0.02 0.01 0.β7 0.11 0.01 „ 0.02

280 0.06 0.02 0.78 0.09 " 0.01 " 0.01

290 0.20 0.03 0.80 0.09 0.01 „ 0.01

300 0.19 0.05 0.81 0.09 0.01 " 0.01

* Cyclohexene -cyclohexanone pwd = powder

** Cyclohexylidene- cyclohexanone ptc = particle

As shown in Table 1, both catalyst^'s exhibited selectivi ties of more than 99% within the reaction temperature range of 250°C - 300°C; whereas catalysts formed from particles exhibited even higher selectivity. The amount of cyclohexanone produced due to acidic sites was increased, whereas the amount of the other two reactants showed a dramatic decrease, as expected.

Example 2

Catalyst particles were produced according to the same method as Example 1 except that the amount of Cu(N0₃)₂ was changed to con- trol the copper content in the catalyst and then the selectivity and the yield of the catalyst were measured at 280°C. The result is given in Fig. 1, which implies that a catalyst produced accord* ing to the present invention has a superior performance within the range of 10 - 40% of copper content, which is relatively low.

Example 3

A particulate catalyst was produced according to the same method as Example 1 except that the copper content of the final catalyst was 15% by weight and the pH of preparation solution in the reactor changed when NaC0₃ solution was added dropwise. The results of the reaction to the catalysts at different temperatures are shown in Figs. 2 and 3, and the results of selectivity (Fig.2) are shown in Table 2.

Table 2

Reaction Selectivity (%) Temperature

(°C) 250 260 270 280 290 300

Precipitation pH

7 18.5 18.0 17.2 16.3 15.1 12.1

9 99.4 99.4 99.3 99.1 98.9 99.0

10 99.0 99.0 98.9 98.7 98.5 98.2

11 98.9 98.9 98.7 98.4 98.2 98.0

As shown in Figs. 2 and 3, the selectivity for cyclohexanone was very inferior at pH 7, whereas the yield of cyclohexanone was best at pH 9.

Example 4

250 g of Zeosil manufactured by Dong Yang Chemical Company was dispersed in 2,500 ml of distilled water, and then the temperature was raised to 90°C. An aqueous solution of 47 g of Cu(N0₃)₂ dissolved in distilled water was added thereto until the entire aqueous solution had a volume of 3 liters. 0.3N NaOH was added slowly and dropwise to the above solution for about 3 hours to a final pH of 9 and then it was kept for about 2 hours to age. It was filtered, dried for 8 hours at 120°C and then in the presence of 6.5 g of KN0₃ dissolved in 100 ml of distilled water was oxidized by means of air for 5 hours to further neutralize acidic sites on the surface of the catalyst.

A catalyst thus manufactured was shaped into 5 x 7 mm particles and then 20 g of the catalyst was used to carry out the reaction test, the result of which was the same as a catalyst to which no potassium was added, showing a conversion rate of 75.2% and a selectivity of 99.4% at 300°C.

Example 5 5

A catalyst powder was produced under the same conditions as Example 1 except that a silica gel (manufactured by Aldrich Company, grade 15 m²/g) was used instead of Zeosil, and the amount of Cu(N0₃)₂ was increased to a copper content in the catalyst of 10 15% by weight. The reaction test result of using this powdered catalyst was the same as that using Zeosil, as shown in Table 3.

Table 3 (1 g catalyst, LWHSV = 7 hr'¹)

15

Temperature (°C) 250 260 270 280 290 300

Conversion Rate (%) 55.2 58.3 65.0 71.9 78.5 80.4

Selectivity (%) 99.4 99.7 99.5 99.2 99.0 98.7

20 Example 6

104 g of Cu(N0₃) was dissolved in distilled water to form 1.5 liters of solution, and was transferred to a reactor equipped with a reflux condenser. 200 g of Zeosil manufactured by Dong Yang

25 Chemical Company was then added and the whole was heated to 80°C with stirring. To this solution, a 0.3 N KOH solution was added dropwise for 2 hours. During this time the pH of the solution in the reactor was raised to 9.5, and thereafter, when dropwise addition was complete, it was kept in for 4 hours, immediately filte-

30 red, and dried for 8 hours at 120°C. The dried catalyst was heated gradually to 300°C over 2 hours at atmospheric pressure, and then oxidized at 300°C for 5 hours. It was compression molded on a compression molding machine to form 5 x 8 mm particles, heated to 250°C for 3 hours under a hydrogen atmosphere, and reduced at

35 250°C for 2 hours. The average size of reduced copper particles on the surface of the carrier was 150* 10^"10 m (150A).

40 g of this catalyst was placed in a 250 x 800 mm fixed-bed reactor and the amount of cyclohexanol at 245°C at atmospheric

40 pressure was brought to LWHSV = 1.0 hr-1. The result is shown in Table 4.

45 Table 4

[Reaction Time (days) 3 7 10 "

Cyclohexanol 27.2 26.3 24.6 25.7

Cyclohexanone 62.3 63.3 65.2 63.9

Phenol 0.02 0.02 0.01 0.01

Cyclohexene 0.25 0.17 0.10 0.18

CHA 0.01 0.01 0.01 0.01

CHLA 0.01 0.01 0.021 0.01

Others 0.21 0.17 0.18 0.23

Conversion Rate (%) 62.3 63.3 65.1 63.9

Selectivity (%) 99.2 99.4 99.5 99.4

Claims

We claim :

1. A catalyst for preparing cyclohexanone by dehydrogenation of cyclohexanol, wherein copper is dispersed on a surface of a silica carrier in a particulate form of less than 200A and in an amount of 5 to 40 weight %, based on the weight of said silica carrier.

2. A catalyst as defined in claim 1, wherein said silica carrier is a porous silica gel having a surface area of 15 - 400 m²/g or a fumed silica.

3. A process for producing a catalyst for preparing cyclo- hexanone, comprising the steps of:

dispersing a silica carrier in a solution of a copper salt;

neating a mixture of said silica carrier and said copper salt;

adjusting a pH of said mixture to 7 - 11 with an aqueous alkaline solution;

filtering said mixture;

drying a filtered product;

oxidizing a dried product; and

reducing an oxidized product.

4. A process as defined in claim 3, wherein said copper salt is cupric acetate, cupric formate, or cupric nitrate, or a mix- ture thereof.

5. A process as defined in claim 3, wherein said solution of a copper salt has a mole concentration of 0.05 to 4.0.

6. A process as defined in claim 3, wherein said mixture is heated to 20 to 95°C.