SK172198A3 - Catalyst for dehydrogenation of cyclohexanol, process for the preparation of said catalyst and use thereof - Google Patents

Abstract

The invention relates to a catalyst comprising alpha -aluminium oxide as the support material and copper as the active constituent, the BET surface (as defined in DIN 66131) of the aluminium oxide being not lower than 30 m<2>/g. It also relates to a process for preparing said catalyst according to the invention, and the use thereof in dehydrogenation of cyclohexanol to form cyclohexanone, and also to the use of alpha -aluminium oxide, the BET surface (as defined in DIN 66131) of said aluminium oxide being not lower than 30 m<2>/g, to prepare a catalyst.

Description

Technical field

The present invention relates to an improved copper catalyst comprising α-alumina, wherein the BET surface area (measured according to DIN 66131) of alumina is not less than 30 m ² / g.

The invention further relates to a process for the preparation of a catalyst according to the invention and its use in the dehydrogenation of cyclohexanol to cyclohexanone and to the use of an aluminum oxide having a BET surface area (determined according to DIN 66131) of not less than 30 m ² / g.

BACKGROUND OF THE INVENTION

Cyclohexanone is an important intermediate for nylon-6 and nylon-6,6 and is produced in large quantities in the catalytic dehydrogenation of cyclohexanol industry. There are two methods of catalytic dehydrogenation of cyclohexanol. The high temperature method is carried out at a temperature of 320 to 420 ° C, the low temperature method is carried out at a temperature of 220 to 260 ° C.

Dehydrogenation at high temperature is disadvantageous because of the low selectivity of cyclohexanol, since various side reactions such as dehydration of cyclohexanol to cyclohexene or dimerizations such as the formation of cyclohexanylcyclohexanone occur at high temperatures. By-products require complex processing and reduce process economy.

The dehydrogenation of cyclohexanol at low temperature is catalyzed primarily by copper catalysts. These catalysts make it possible to reduce the reaction temperature to about 240 to 280 ° C and thus provide greater selectivity of cyclohexanone. However, since the equilibrium is shifted at a comparatively lower temperature, the conversion is not very high.

One class of these low temperature catalysts includes copper and ceramic carrier compositions, which may be SiO ₂ or Al ₂ O _3, or a mixture thereof. The copper content of these catalysts may be up to 50% by weight. These catalysts may further contain small amounts of alkali metals as promoters.

For example GB-A 1 081 491 discloses Cu / Al ₂ O ₃ , SU-A 465217 discloses Cu / Al / SiO ₂ and SU-A 522853 discloses Cu / K / Al ₂ O ₃ for the non-oxidative dehydrogenation of cyclohexanol. Said copper catalyst is usually prepared by applying the active copper component either to a prefabricated carrier, precipitating the copper salt or impregnating it with a suitable copper salt solution, or co-precipitating the catalyst constituents. Another way to prepare copper catalysts is to dry the mixture of ingredients and calcine the mixture.

Non-electrical copper or reductive precipitation of copper on a support using a reducing agent in the presence of a complexing agent in the preparation of catalysts for the non-oxidative dehydrogenation of cyclohexanol is known from Chang et al., Appl. Catal. A 103 (1994) 233-42. The carrier used is α-Al ₂ O ₃ having a surface area of 22.6 m ² / g. According to Chang, selectivity is affected by the acidity of the carrier, and acidic carriers such as Al ₂ O ₃ or SiO ₂ will catalyze side reactions such as dehydration to cyclohexene or dimerization. By adding alkali metal or alkaline earth metal ions, acidity can be reduced (see, for example, Appl. Cat. A: General Vol. 83, No. 2 (1992), 201-11), but this measure also reduces activity. Thus, the advantage of the higher activity of copper catalysts in high temperature catalysts is again lost to some extent by the addition of an alkali metal or an alkaline earth metal.

The catalysts described in Appl. Catal. A 103 (1994), 233-42, but are not suitable for industrial use because they are powdered. Compression into shaped articles such as tablets is difficult because they are poorly tabletable. In addition, due to the low hardness of the resulting molded articles, they have low abrasion resistance in the reactor, resulting in an increased pressure drop in the reaction process. Thus, their full use in industry is generally not possible.

Another disadvantage of using α-Al ₂ O ₃ as a carrier material is the low interaction with the active copper component. Because of this low interaction, rapid agglomeration of the active ingredient occurs resulting in a loss of the catalytically effective surface area and thus a decrease in activity. In order to achieve a high conversion without loss, the temperature of the reactor should be increased during dehydrogenation, but the agglomeration will be further accelerated and the catalyst inactivated simultaneously.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a catalyst that does not have the above drawbacks. In addition, the catalyst should, in particular, exhibit a long service life and provide dehydrogenation of cyclohexanol to cyclohexanone in high yield and selectivity at comparatively low reaction temperatures. Constant readjusting of the reaction temperature is unlikely to be necessary. In addition, the catalyst will be able to be processed directly into shaped articles, especially tablets, strips, rings, rollers, without cost, and can be used in such a form, based on good hardness and abrasion resistance.

We have found that this object can be achieved by an improved copper catalyst on α-alumina, whereby the BET surface area (measured according to DIN 66131) of alumina is not less than 30 m ² / g.

The invention further provides a process for preparing a catalyst according to the invention, its use in the dehydrogenation of cyclohexanol to cyclohexanone and the use of α-alumina having a BET surface area (measured according to DIN 66131) of not less than 30 m ² / g for catalyst preparation. .

The catalyst of the invention is prepared by depositing the active ingredient copper in a conventional manner, such as impregnation, precipitation, dry mixing or non-electrical metering, on an α-Al ₂ O ₃ support having a BET surface area of not less than 30 m ² / g.

The α-Al ₂ O _{3 used} has a BET surface area in the range of 50 to 300 m 2 / g, particularly preferably in the range of 100 to 250 m 2 / g. Such high-BET α-Al ₂ O ₃ are commercially available (e.g., Alcoa).

Impregnation basically involves saturating the carrier with an aqueous solution of copper salts, preferably nitrates, sulphates, acetates or copper chlorides, drying and then calcining the impregnated carrier.

Precipitation generally involves admixing an aqueous copper salt solution (see above) in the presence of a carrier with a precipitating agent, resulting in the formation of a sparingly soluble copper compound. Sodium carbonate is particularly preferred for copper precipitation. This is followed by drying and calcination in a conventional manner.

Dry mixing essentially involves mixing the carrier with the desired copper salt and subsequent calcination.

In another embodiment, the catalyst is prepared by non-electrical deposition in the presence of high BET α-alumina according to the invention (see also Appl. Catal. A 103 (1994), 233-242). a metal such as platinum, rhodium, iridium, gold or palladium, preferably palladium, to form crystallization-initiating centers. Then the copper is released from the complex by means of a reducing agent to be applied to the support.

To avoid premature copper precipitation at the usually high desired pH, it is preferred to add a strong complexing agent such as ethylenediaminetetraacetate, its alkali metal salts such as tetrasodium salt, ethylenediamine or phenanthroline. The reduction (or self-sweeping) is then generally carried out with a reducing agent capable of depositing copper (0) from copper salt solutions such as formaldehyde or sodium formate.

From the previous observations, it is concluded that non-electric deposition produces mainly small copper particles. Their size is generally 50 nm, preferably 20 nm, as measured by X-ray diffraction.

Regardless of the method of depositing copper on the support, the resulting powder or correspondingly shaped article is calcined at a temperature in the range of 250 to 450 ° C in air for a period of from 1 to 24 hours. The shaped articles may be shaped before or after calcination.

The catalyst powder obtained in the non-electric deposition or other processes mentioned above is preferably compressed into shaped articles such as tablets, strips, rings, discs, stars, monoliths, beads, granules or extrudates, preferably tablets, essentially by admixing. tableting auxiliary pros5 classes. Conventional tableting aids may be used. Examples are carbon, magnesium stearate, methylcellulose (such as WaloceP), Cu powder or mixtures thereof.

The copper content of the catalyst is usually chosen in the range from 0.01 to 50% by weight, preferably in the range from 2 to 30% by weight, particularly preferably in the range from 5 to 20% by weight, based on the total weight of the catalyst material.

The BET surface area (measured according to DIN 66131) of the catalyst is generally not less than 30 m ² / g, preferably in the range of 50 to 300 m ² / g, particularly preferably in the range of 100 to 250 m ² / g.

The dehydrogenation of cyclohexanol to cyclohexanone is generally carried out in the gas phase at a temperature of from 180 to 400 ° C, preferably at a temperature of from 200 to 350 ° C, particularly preferably at a temperature of from 220 to 260 ° C. The pressure is generally selected in the range from 50 kPa to 5 MPa, particularly preferably at atmospheric pressure.

The starting material for the reaction is generally a mixture of cyclohexanol and cyclohexanone. Naturally, it is also possible to use pure cyclohexanol. The mixture used usually contains from 50 to 100, preferably from 60 to 99, in particular 96% by weight of cyclohexanol and from 50 to 0, preferably from 40 to 1, in particular 4% by weight of cyclohexanone. Cyclohexanone and cyclohexanol can conveniently be obtained by oxidation of cyclohexane followed by concentration of cyclohexanol by removal of cyclohexanone and other low-boiling components by distillation.

Generally, the catalyst is reduced with hydrogen prior to the actual reaction. This is generally done by allowing a stream of hydrogen diluted with an inert gas, preferably nitrogen, to pass through the catalyst at a certain temperature, preferably at a temperature in the range of from 120 to 300 ° C. The hydrogen content of the reducing gas is then usually continuously increased until further significant temperature changes are observed.

In a preferred embodiment, the starting material is passed in the form of a gas through the catalyst, wherein the LHSV (liquid hourly space velocity) is preferably from 0.1 to 20 h ^-1 . The starting material may be mixed with an inert gas such as nitrogen or steam. The dehydrogenation product can be worked up in a conventional manner (see for example DE-A 1,296,625 and DE-A 1,443,462) and further processed.

In another preferred embodiment, hydrogen is separated from the reaction mixture leaving the reaction zone and added to the gaseous mixture entering the reaction zone. Further, it is preferred to recycle the reaction mixture until the desired conversion is achieved.

The catalyst of the present invention is very active and can therefore be used at significantly lower temperatures than existing catalysts used in industry, activates rapidly and provides high selectivity and near-equilibrium conversions. In addition, appreciable deactivation occurs only after considerably longer periods than with existing catalysts.

The catalyst according to the invention is characterized by good tabletability, adequate hardness, high conversion at low operating temperatures, high cyclohexanone selectivities and long life.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

Preparation of the catalyst

To a mixture of 27.27 g of a 11% by weight solution of Pd (NO ₃ ) ₂ (calculated as Pd), 495 ml of redistilled water and 495 ml of ethanol was added 5 g of polyvinylpyrrolidone ("PVP", from Merck, order). No. 7443 (average molecular weight 25,000 g / ml) and the resulting mixture was heated to reflux for 4 hours. The obtained sol ("Pd sol") had a palladium content of 0.34% by weight (based on the total weight of the Pd sol).

To 20.6 ml of the Pd-sol prepared above was admixed with 23 ml of water and 125 g of aluminum oxide (from Alcoa, BET surface area 156 m ² / g, water sorption 0.35 ml / g Al ₂ O ₃ ). The saturated support was then air dried. The pretreated and dried carrier was then suspended in 3.884 ml of a freshly prepared solution containing 0.1 M Cu (OAc) ₂ , 0.2 M Na ₄ EDTA, 0.2 M formaldehyde and 0.125 M pyridine. The pH of the solution was adjusted to 12.5 using 35% (w / w) NaOH with vigorous stirring. A gray-black to red-brown color was observed. The suspension was heated to 70 ° C and the pH was maintained in the range of from 12 to 12 ° C

12.5 by further addition of NaOH (NaOH consumption: about 400 mL). The suspension was stirred at 70 ° C for 30 minutes and then cooled to room temperature. The suspension was filtered and the filter residue was washed with water until the water showed a neutral pH (the filtrate was pale blue). The washed solid was then dried at 110 ° C under a nitrogen atmosphere for 16 hours and then calcined at 300 ° C for 2 hours.

The resulting catalyst contained 16.5 wt% copper and 0.046 wt% sodium based on the total weight of the gray-black catalyst.

The X-ray diffraction assay showed the following data for the crystalline components: Al ₂ O ₃ (19 nm), CuO (13.0 nm), Cu ₂ O (traces).

Preparation of tablets (length 5 mm, diameter 3 mm)

100 g of the catalyst powder prepared above were pre-pressed with 1 wt. % Cu-powder FFL (Norddeutsche Affinerie # 10914) and 1 wt. Magnesium stearate powder (Riedel de Haen # 4162757), based on the weight of the catalyst, for tablets having a length of 20 mm and a diameter of 1 mm. These tablets were then passed through a 1.6 mm sieve and, after addition of 2 wt. The graphite was compressed into tablets having a length of 5 mm and a diameter of 3 mm.

The lateral compressive strength of the tablets was 53 N with a standard deviation of 16 N (measured using a Frank apparatus, Model No. 81557).

Example 2

Catalyst testing

Catalyst tests were performed in a tubular reactor with a diameter of 5 mm and a length of 60 cm. For each test, 20 ml of the catalyst, which had been activated with hydrogen prior to the reaction, were loaded. Before the starting material was introduced, the catalyst was activated with 150 IN ₂ / h and 1.5 IH ₂ / h at 120 ° C. The hydrogen flow stopped when the temperature rose to more than 10 ° C. The temperature was then increased in increments of 20 ° C to 240 ° C while keeping the amount of hydrogen constant. At 240 ° C, the catalyst was then activated with 150 IN ₂ / h and 7.5 IH ₂ / h. After activation, the catalyst was contacted with a mixture of 96% cyclohexanol / 4% cyclohexanone, with an LHSV of about 0.7 h ^-1 . The components leaving the reactor were analyzed by gas chromatography. The results are summarized in the table below.

Comparative example

The dehydrogenation assay of Example 2 was repeated using a commercial copper catalyst (Catalyst CU 940 from Procatalyse).

catalyst Temperature [° C] Time [h] Conversion [%] Selectivity [%] Example 2 229 42 57 97.7 228 150 56 99.2 228 246 59 99.4 226 438 55 99.9 229 558 55 99.7 237 630 61 99.6 242 774 60 99.5 comparison 277 0 68 98.3 275 12 63 98.4 275 24 62 98.5 275 36 61 98.6 276 48 61 98.5 280 168 60 98.1

The catalyst of the invention provides near-equilibrium conversion at a very high selectivity of > 99% at a temperature < 230 ° C. There was no observable deactivation of the catalyst at this temperature. With a slight increase in the reaction temperature to about 240 ° C, the conversion increased to about 60% without loss of selectivity. Also at this temperature only very slow deactivation was observed.

As can be seen, in the comparative test, a temperature adjustment was required as early as 168 hours to maintain the conversion, while such a correction is required for the catalyst prepared according to the present invention only after 600 hours. In addition, the temperatures of the comparative example are approximately 40 ° C higher than the temperatures of the present invention. Finally, it should be pointed out that the selectivities in the example of the invention are optimal. This was unexpected for alumina with a BET surface area of less than 30 m ² / g.

Claims (6)

PATENT CLAIMS Catalyst consisting of α-alumina as carrier, characterized in that the BET surface area (measured according to DIN 66131) of alumina is not less than 30 m ² / g. 2. Catalyst consisting of α-alumina as carrier and copper as active ingredient, characterized in that the BET surface area (measured according to DIN 66131) of alumina is not less than 30 m ² / g obtainable by copper deposition by impregnation, precipitation, dry mixing or non-electric copper plating in a manner known per se, onto the support of α-alumina. A process for the preparation of a catalyst according to claim 1 or 2, wherein the copper is impregnated, precipitated, dry blended or non-electrically converted by a known method to an α-alumina support, characterized in that α-alumina with a BET surface area is used. surface area (determined according to DIN 66131) not less than 30 m ² / g. Process according to claim 3, characterized in that the catalyst is processed into shaped products, in particular tablets, strips, rings, rollers. Use a catalyst according to claim 1 or 2 or prepared according to claim 3 or 4 for the preparation of cyclohexanone by dehydrogenation of cyclohexanol. 6. Use α-alumina, with a BET surface area (determined according to DIN 66131) of not less than 30 m ² / g, to prepare the catalyst.