EP2365953A1

EP2365953A1 - Catalysed cyclohexane oxidation

Info

Publication number: EP2365953A1
Application number: EP09799082A
Authority: EP
Inventors: Corinne Daguenet; Rudy Francois Maria Jozef Parton; Johan Thomas Tinge
Original assignee: DSM IP Assets BV
Current assignee: DSM IP Assets BV
Priority date: 2008-12-17
Filing date: 2009-12-11
Publication date: 2011-09-21
Also published as: WO2010069870A1; EA201100959A1; BRPI0923002A2; JP2012512225A; KR20110098818A; TW201033164A; CN102256921A

Abstract

The invention is directed to a method for preparing a mixture comprising cyclohexanol and cyclohexanone. In accordance with the invention a mixture comprising cyclohexanol and cyclohexanone is prepared in a molar cyclohexanone/cyclohexanol ratio of at least 1.2 at a cyclohexane conversion of below 5 mol%. The method comprises oxidising cyclohexane in a liquid phase, said oxidising being catalysed by a cobalt compound and a chromium compound, wherein the atomic cobalt to chromium ratio is in the range of 0.05-0.8, wherein the sum of the concentrations of cobalt and chromium is 0.05-0.9 ppm by weight based on the total weight of the reaction mixture, and wherein the cobalt compound and chromium compound are dissolved in the liquid phase.

Description

CATALYSED CYCLOHEXANE OXIDATION

The invention is directed to a method for preparing a mixture comprising cyclohexanol and cyclohexanone. Cyclohexanol and cyclohexanone can be commercially produced from cyclohexane. The first step in such a process is oxidation of the cyclohexane by an oxygen-containing gas to produce cyclohexanol, cyclohexanone and cyclohexylhydroperoxide. The mixture of cyclohexanol (A) and cyclohexanone (K) is commonly referred to as "KA". Conventionally, in this first step cylcohexane is oxidised in the liquid phase with air. This oxidation is normally conducted on industrial scale, either uncatalysed or with a soluble cobalt catalyst, in one or more reactors at temperatures in the range of 130-200 ⁰C. The vaporised cyclohexane and other products in the gaseous effluent are condensed and recovered, and the off-gases leave the system. The KA product is recovered from the liquid effluent from the reactor or reactors, and the unreacted cyclohexane is recycled (Kirk-Othmer, Encyclopedia of Chemical Technology, John-Wiley & Sons, New York, 1979, 3^rd Edition, Vol. 7, pp. 410-416 and Ullmanns, Encyklopadie der technischen Chemie, Verlag Chemie, Weinheim, 1975, 4^th Edition, Vol. 9, pp. 689-698).

A process of oxidising cyclohexane to produce a product essentially consisting of cyclohexanone and cyclohexanol in the presence of a binary catalyst system of cobalt and chromium is known from US-A-3 987 100. The only cobalt to chromium ratios disclosed for the step wherein cyclohexane is oxidised are in the preferred embodiment (Examples, Table 2), mentioning a cobalt concentration of 0.1 ppm and a chromium concentration of 0.04 ppm, which corresponds to an atomic ratio of cobalt to chromium of 2.2. Hence, there is always more cobalt present than chromium. In addition, it is not specified whether the ratio of cobalt to chromium has an effect on the oxidation process.

US-A-3 598 869 describes a process wherein cyclohexane is oxidised to obtain a mixture of (a) cyclohexanol and cyclohexanone (KA) and (b) a mixture of carboxylic acid derivatives of cyclohexane (COP acids) in a particularly suitable ratio (Ae. of about 1 :1 ) of COP acids to KA for further conversion to nylon salts in the presence of at least one catalyst of the group of cobalt and chromium. Catalyst concentrations can be between 1 ppm and 200 ppm, calculated as the metal, or higher. In a preferred embodiment chromium is used and no cobalt. Moreover, it is noted that this document discloses high cyclohexane conversions such as in between about 14 mol% and 30 mol%.

EP-A-O 063 931 relates to a process for the catalytic oxidation of a liquid cycloparaffin to partial oxidation products thereof, which comprises introducing amolecular oxygen-containing gas into a cycloparaffin of from 5 to 12 carbon atoms in the presence of an oxidation catalyst comprising a combination of a cobalt compound and a chromium compound, said oxidation catalyst being soluble in the cycloparaffin, said cobalt compound having ligands selected from the group consisting of dialkyl phosphate, dicycloalkylphosphate and alkylcycloalkylphosphate, and mixtures thereof, the ratio of cobalt compound to chromium compound being greater than 1 :1 on an atomic basis. This means that there is more cobalt than chromium.

SU-A-929 213 describes a catalyst for the oxidation of pure cyclohexane. Although two experiments are shown with different cobalt to chromium ratios (respectively of 38 vs. 62 %; and of 23 vs. 77 % of the naphthenates), both experiments show substantially the same selectivity. In addition, the concentration of the sum of cobalt and chromium in the reaction medium is 1 ppm by weight in each of these experiments, and the cyclohexane conversion is higher than 5 mol%. Furthermore, the highest K/A ratio is 1.05.

It is an object of the invention to provide a novel process for the preparation of a mixture comprising cyclohexanol and cyclohexanone, with improved selectivity towards desired products, namely cyclohexylhydroperoxide, cyclohexanol and cyclohexanone, while at the same time obtaining a high molar cyclohexanone/cyclohexanol ratio (K/A ratio) in comparison with processes described in the prior art. A precise definition of the term "selectivity" as used herein is given hereinafter.

One or more objects which may be accomplished in accordance with the invention will become apparent from the description and/or claims below.

It has now been found possible to meet at least one of said objects by oxidising cyclohexane in the presence of a specific form of catalytic cobalt and a specific form of catalytic chromium, and using said catalysts in a specific ratio and under specific conditions.

Accordingly, in a first aspect the invention is directed to a method for preparing, at high selectivity towards desired products, a mixture comprising cyclohexanol and cyclohexanone in a molar cyclohexanone/cyclohexanol ratio of at least 1.2 and with a cyclohexane conversion below 5 mol%, comprising oxidising cyclohexane in a liquid phase, said oxidising being catalysed by a cobalt compound and a chromium compound, the atomic cobalt to chromium ratio being in the range of 0.05 to 0.8, wherein the sum of the concentrations of cobalt and chromium is 0.05-0.9 ppm by weight based on the total weight of the reaction mixture, and wherein the cobalt compound and chromium compound are dissolved in the liquid phase. The inventors found that the process of the invention, wherein a specific cobalt chromium catalyst is utilised at a cyclohexane conversion below 5 mol%, advantageously allows achieving, at a specific conversion level (e.g. of 3 or 4 mol%), both a high value of K/A and a higher selectivity than is achieved with prior art processes. Selectivity in this regard is defined as the sum of desired products (cyclohexanol, cyclohexanone, and cyclohexylhydroperoxide) divided by the total amount of cyclohexane that has been converted on a molar basis. Moreover, the process of the invention at the same time yields an advantageously high K/A ratio (such as a molar K/A ratio of 1.2 or more, or even a molar K/A ratio of 1.4 or more). A high K/A ratio is desirable, because it minimises subsequent conversion of remaining cyclohexanol to the final product cyclohexanone. The K/A ratio can for instance be as high as 1.6.

The catalyst used in the process of the invention is a combination of a cobalt compound and a chromium compound. The term "compound" as used in this context includes complexes, salts, and the like. In the cobalt compound, the cobalt can be present either as Co" or as Co'". In terms of solubility, compounds wherein the cobalt is present as Co" are preferred. In the chromium compound, the chromium can be present as Cr'" or even higher oxidation states. In practice, the chromium will usually be present as Cr'" in the chromium compound.

Suitably, the cobalt compound and/or chromium compound can comprise one or more ligands selected from porphyrins, phthalocyanines,

1 ,3-bis(arylimino) isoindolines, Schiff bases, hydroxides, carboxylates (such as acetates, propanoates, butyrates, octanoates, benzoates, naphthenates, and stearates), picolinates, acetylacetonates and other beta-diketones, 1 ,3-bis(arylimino) isoindolinates, salen, saleph, porphyrinates, porphycenates, porphenates, phthalocyanates, bipyridines, terpyridines, phenanthrolines, dithiocarbamates, xanthates, salicylaldimines, cyclam, dioxocyclam, pyrazoylborates, and tetraazamacrocyclic compounds. In particular, the cobalt compound and/or chromium compound can comprise one or more ligands selected from the group consisting of acetylacetonates, octanoates, naphthenates, bipyridines, porphyrinates, and phthalocyanates. In case the cobalt compound and/or chromium compound comprise one or more ligands, the one or more ligands can be present in excess. In the process of the invention, the atomic ratio of cobalt to chromium in the oxidation catalyst of cyclohexane is in the range of 0.05-0.8. In particular, the ratio of cobalt to chromium can be in the range of 0.1-0.5, more in particular in the range of 0.15-0.35. The atomic ratio of cobalt to chromium in the catalyst is of particular importance in obtaining a product having a high K/A ratio, along with a high selectivity. It was found that either the K/A ratio or the selectivity considerably decreases if the atomic cobalt to chromium ratio is higher than 0.8 and/or if the total metal concentration is higher than 0.9 ppm. On the other hand, if the amount of cobalt becomes too low, such the atomic ratio of cobalt to chromium in the catalyst becomes lower than 0.05, the K/A ratio remains high but the selectivity drops to too low values (e.g. to less than 86% at 3 mol% conversion; respectively less than 84% at 4 mol% conversion).

Furthermore, the cobalt compound and the chromium compound are dissolved in the liquid phase. This means that the cobalt/chromium catalyst is a homogeneous catalyst. In an advantageous embodiment, the cyclohexane oxidation process of the invention is a homogeneous catalysis process.

The selectivity and/or the K/A ratio were found to increase, generally at decreasing values for the sum of the concentrations of cobalt and chromium, at an atomic ratio of cobalt to chromium in the range of from 0.05 to 0.8, and at conversions in the range of from 3 mol% to just below 5 mol%. The optimum of the total concentration of cobalt and chromium is in the range of from 0.05 to 0.9 ppm.

Thus, according to the invention, the sum of the concentrations cobalt and chromium is 0.05-0.9 by weight based on the total weight of the reaction mixture, in particular 0.1-0.7 ppm by weight, more in particular 0.2-0.6 ppm by weight, especially at about 0.5 ppm. The cobalt concentration during the oxidation reaction can suitably be

0.01-0.4 ppm by weight of the total reaction mixture, in particular 0.05-0.2 ppm by weight. The chromium concentration during the oxidation reaction can suitably be 0.04-0.8 ppm by weight of the total reaction mixture, in particular 0.1-0.5 ppm by weight. Such low amounts of cobalt and chromium are advantageous in view of environmental aspects, as well as in reducing fouling problems. Moreover, it was found that these relatively low amounts of catalyst still yield satisfactorily high conversion (such as 2-just below 5 mol%) with acceptable residence times.

The process of the invention has a cyclohexane conversion of below 5 mol%, in particular of 2.5-4.5 mol%, more in particular 3-4 mol%. The conversion can largely be influenced by the residence time of the reaction products in the oxidising reactor(s). A cyclohexane conversion of below 5 mol% is advantageous, because the selectivity decreases quickly at or above a conversion of 5 mol%. At a conversion below 2.5 mol%, and in particular at a conversion below 2 mol%, the amount of cyclohexane to be evaporated becomes rather high.

The cyclohexane oxidation of the invention can suitably be conducted in a reactor system, comprising one or more oxidation reactors (such as 3-8), which system preferably has plug flow characteristics, such as multiple reactors in series, for instance multiple CSTRs (Continuous Stirred Tank Reactors) in series or multiple bubble columns in series. Normally, the reaction temperature will normally be at least 130 ⁰C, in particular at least 140 ⁰C. It is not preferred that the reaction temperature exceeds 180 ⁰C, in particular 170 ⁰C, because at higher temperatures there is so much decomposition of cyclohexylhydroperoxide that the selectivity goes down. Depending on the temperature and the pressure during the reaction, the residence time is usually in the range of 1-120 minutes, such as 5-90 minutes, in particular 10-60 minutes.

After appropriate purification (which typically involves a number of distillation steps) the cyclohexanone/cyclohexanol mixture can be used for the preparation of adipic acid. Furthermore, after appropriate isolation of cyclohexanone from the obtained mixture, the isolated cyclohexanone can be used for the preparation of cyclohexanone oxime, caprolactam and nylon.

The invention is further demonstrated by the following examples and comparative examples. In the experimental part the following abbreviations are used: acac: acetylacetonate EH: 2-ethylhexanoate which is often also referred to as octanoate CH_x: cyclohexane NL/hr: normal litre per hour

Examples Comparative Example A

A mixture of 96.26 g of a cyclohexane feed as used in a cyclohexane oxidation plant (hereinafter referred to as "CH_x feed"), 1.0326 g of biphenyl (internal standard), and 0.1673 g of a solution of Cr(acac)₃ in cyclohexane (249.9 ppm Cr) was placed in a Hastelloy fed-batch reactor. In this comparative example only chromium was used with a concentration of 0.43 ppm in the CH_x feed. The system was pressurised with N₂ up to 10-1 1 bar and heated up to 156 ⁰C under a 30 NL/h nitrogen flow while stirring at a rate of 1300 rpm. At reaction temperature, the nitrogen flow was changed for a 30 NL/h flow of 14 % O₂ in N₂. Samples were taken at regular intervals and analysed by gas chromatography. During reaction, CO and CO₂ were continuously monitored in the off-gas.

Comparative Example B A mixture of 97.80 g of CH_x feed, 1.0090 g of biphenyl (internal standard), and 0.3364 g of a solution of Cr(acac)₃ in cyclohexane (249.9 ppm Cr) was placed in the same reactor as comparative example A. In the present comparative example only chromium was used with a concentration of 0.86 ppm in the CH_x feed. The reaction was carried out using the same conditions and in the same way as in Comparative Example A.

Comparative Example C

A mixture of 98.20 g of CH_x feed, 1.0254 g biphenyl (internal standard), 0.34901 g of a solution of Cr(acac)₃ in cyclohexane (249.9 ppm Cr) and 0.67525 g of a solution of Co(EH)₂ in cyclohexane (149 ppm Co) was placed in the same reactor as comparative example A. In the present comparative example the chromium and cobalt concentrations used were 0.88 ppm and 1.02 ppm in the CH_x feed, respectively, giving an atomic Co/Cr ratio of 1.02. The reaction was carried out using the same conditions and in the same way as in Comparative Example A.

Comparative Example D

A mixture of 96.97 g of CH_x feed, 1.0880 g of biphenyl (internal standard), 0.9147 g of a solution of Cr(acac)₃ in cyclohexane (60.52 ppm Cr) and 0.6716 g of a solution of Co(EH)₂ in cyclohexane (149 ppm Co) was placed in the same reactor as comparative example A. In the present comparative example the chromium and cobalt concentrations used were 0.57 ppm and 0.35 ppm in the CH_x feed, respectively, giving a molar ratio of 1.60. The reaction was carried out using the same conditions and in the same way as in Comparative Example A. Comparative Example E

A mixture of 99.21 g of CH_x feed, 1.0002 g of biphenyl (internal standard), 0.0389 g of a solution of Co(EH)₂ in cyclohexane (637 ppm Co) was placed in the same reactor as comparative example A. In the present comparative example only cobalt was used with a concentration of 0.25 ppm in the CH_x feed. The reaction was carried out using the same conditions and in the same way as in Comparative Example A.

Comparative Example F. A mixture of 98.85 g of CH_x feed, 1.0074 g of biphenyl (internal standard), 0.33 g of a solution of Co(EH)₂ in cyclohexane (149 ppm Co) was placed in the same reactor as comparative example A. In the present comparative example only cobalt was used with a concentration of 0.50 ppm in the CH_x feed. The reaction was carried out using the same conditions and in the same way as in Comparative Example A.

Comparative Example G

A mixture of 95.66 g of CH_x feed, 1.0186 g of biphenyl (internal standard), 1.1798 g of a solution of Cr(acac)₃ in cyclohexane (60.52 ppm Cr) and 0.1716 g of a solution of Co(EH)₂ in cyclohexane (149 ppm Co) was placed in the same reactor as comparative example A. In the present comparative example the chromium and cobalt concentrations used were 0.75 ppm and 0.27 ppm in the CH_x feed, respectively, giving a molar ratio of 0.32. The reaction was carried out using the same conditions and in the same way as in Comparative Example A.

Comparative Example H

A mixture of 101.90 g of CH_x feed, 1.0345 g of biphenyl (internal standard), 1.086 g of a solution of Cr(acac)₃ in cyclohexane (60.52 ppm Cr) and 0.2130 g of a solution Of Co(EH)₂ in cyclohexane (149 ppm Co) was placed in the same reactor as comparative example A. In the present comparative example the chromium and cobalt concentrations used were 0.65 ppm and 0.31 ppm in the CH_x feed, respectively, giving a molar ratio of 0.42. The reaction was carried out using the same conditions and in the same way as in Comparative Example A. Comparative Example I

A mixture of 99.85 g of CH_x feed, 1.2357 g of biphenyl (internal standard), 7.849 g of a solution of Cr(acac)₃ in cyclohexane (60.52 ppm Cr) and 0.725 g of a solution of Co(EH)₂ in cyclohexane (149 ppm Co) was placed in the same reactor as comparative example A. In the present comparative example the chromium and cobalt concentrations used were 4.4 ppm and 1.0 ppm in the CH_x feed, respectively, giving a molar ratio of 0.2. The reaction was carried out using the same conditions and in the same way as in Comparative Example A.

Example 1

A mixture of 95.52 g of CH_x feed, 1.0189 g of biphenyl (internal standard), 0.74260 g of a solution of Cr(acac)₃ in cyclohexane (60.52 ppm Cr) and 0.10880 g of a solution of Co(EH)₂ in cyclohexane (149 ppm Co) was placed in the same reactor as comparative example A. In the present example the chromium and cobalt concentrations used were 0.47 ppm and 0.07 ppm in the CH_x feed, respectively, giving a molar ratio of 0.13. The reaction was carried out using the same conditions and in the same way as in Comparative Example A.

Example 2 A mixture of 96.02 g of CH_x feed, 1.0041 g of biphenyl (internal standard), 0.6913 g of a solution of Cr(acac)₃ in cyclohexane (60.52 ppm Cr) and 0.16721 g of a solution of Co(EH)₂ in cyclohexane (149 ppm Co) was placed in the same reactor as comparative example A. In the present example the chromium and cobalt concentrations used were 0.44 ppm and 0.26 ppm in the CH_x feed, respectively, giving a molar ratio of 0.52. The reaction was carried out using the same conditions and in the same way as in Comparative Example A.

Example 3

A mixture of 101.02 g of CH_x feed, 1.4460 g of biphenyl (internal standard), 1.10698 g of a solution of Cr(acac)₃ in cyclohexane (60.52 ppm Cr) and

0.0629 g of a solution of Co(EH)₂ in cyclohexane (149 ppm Co) was placed in the same reactor as comparative example A. In the present example the chromium and cobalt concentrations used were 0.66 ppm and 0.09 ppm in the CH_x feed, respectively, giving a molar ratio of 0.12. The reaction was carried out using the same conditions and in the same way as in Comparative Example A. Example 4

A mixture of 97.82 g of CH_x feed, 0.9932 g of biphenyl (internal standard), 1.10045 g of a solution of Cr(acac)₃ in cyclohexane (60.52 ppm Cr) and 0.09878 g of a solution of Co(EH)₂ in cyclohexane (149 ppm Co) was placed in the same reactor as comparative example A. In the present example the chromium and cobalt concentrations used were 0.68 ppm and 0.15 ppm in the CH_x feed, respectively, giving a molar ratio of 0.19. The reaction was carried out using the same conditions and in the same way as in Comparative Example A.

Example 5

A mixture of 96.00 g of CH_x feed, 1.0330 g of biphenyl (internal standard), 1.0675 g of a solution of Cr(acac)₃ in cyclohexane (60.52 ppm Cr) and 0.06899 g of a solution of Co(EH)₂ in cyclohexane (149 ppm Co) was placed in the same reactor as comparative example A. In the present example the chromium and cobalt concentrations used were 0.67 ppm and 0.10 ppm in the CH_x feed, respectively, giving a molar ratio of 0.13. The reaction was carried out using the same conditions and in the same way as in Comparative Example A.

Example 6 A mixture of 93.70 g of CH_x feed, 1.0192 g of biphenyl (internal standard), 0.7500 g of a solution of Cr(acac)₃ in cyclohexane (60.52 ppm Cr) and 0.01931 g of a solution Of Co(EH)₂ in cyclohexane (149 ppm Co) was placed in the same reactor as comparative example A. In the present example the chromium and cobalt concentrations used were 0.48 ppm and 0.03 ppm in the CH_x feed, respectively, giving a molar ratio of 0.06. The reaction was carried out using the same conditions and in the same way as in Comparative Example A.

Example 7

A mixture of 95.73 g of CH_x feed, 0.9900 g of biphenyl (internal standard), 1.13181 g of a solution of Cr(acac)₃ in cyclohexane (60.52 ppm Cr) and

0.0236 g of a solution of Co(EH)₂ in cyclohexane (149 ppm Co) was placed in the same reactor as comparative example A. In the present example the chromium and cobalt concentrations used were 0.72 ppm and 0.04 ppm in the CH_x feed, respectively, giving a molar ratio of 0.05. The reaction was carried out using the same conditions and in the same way as in Comparative Example A. In all Comparative Examples and Examples mentioned above, the main products analysed and included for the determination of conversion and selectivity were:

Cyclohexylhydroperoxide (CHHP) Cyclohexanone

Cyclohexanol

Adipic acid

Hydroxycaproic acid

Caproic acid Valeric acid

Butyric acid

Pentanol

Butanol

Caprolactone Formic acid

CO

CO₂

The results obtained with the Examples and Comparative Examples above are provided in Tables 1-3 below for three different conversion levels (3, 4, and 5 mol%, as obtained by interpolation from the available series of experimental data). All results at the 5 mol% conversion level are comparative results.

Table 1. Results for conversion of 3 mol%

Example [Co] [Cr] Atomic ([Co] + Selectivity to Molar

(ppm in (ppm in Co/Cr [Cr]) CHHP K/A

CHx CHx ratio (ppm in + cyclohexanol + ratio feed) feed) CHx cyclohexanone feed) (mol%)

Comp.Ex. A / 0.43 0 0.43 85.12 1.72

Comp.Ex. B / 0.86 0 0.86 85.07 1.53

Comp.Ex. C 1.02 0.88 1.02 1.9 84.73 1.38

Comp.Ex. D 1.04 0.57 1.60 1.61 87.63 1.02

Comp.Ex. E 0.25 / / 0.25 88.11 0.54

Comp.Ex. F 0.50 / / 0.50 87.05 0.59

Comp.Ex. G 0.27 0.75 0.32 1.02 85.25 1.34

Comp.Ex. H 0.31 0.65 0.42 0.96 84.89 1.26

Comp.Ex. I 1.00 4.40 0.20 5.40 83.60 1.00

Ex. 1 0.07 0.47 0.13 0.54 87.72 1.36

Ex. 2 0.26 0.44 0.52 0.70 86.80 1.38

Ex. 3 0.09 0.66 0.12 0.75 87.17 1.43

Ex. 4 0.15 0.68 0.19 0.83 86.37 1.32

Ex. 5 0.10 0.67 0.13 0.77 86.64 1.38

Ex. 6 0.04 0.48 0.07 0.51 86.21 1.36

Ex. 7 0.04 0.72 0.05 0.76 86.12 1.30

Table 2. Results for conversion of 4 mol%

Example [Co] [Cr] Atomic ([Co] + Selectivity to Molar

(ppm in (ppm in Co/Cr [Cr]) CHHP K/A

CHx CHx ratio (ppm in + cyclohexanol + ratio feed) feed) CHx cyclohexanone reed) (mol%)

Comp.Ex. A / 0.43 0 0.43 83.53 1.60

Comp.Ex. B / 0.86 0 0.86 83.71 1.49

Comp.Ex. C 1.02 0.88 1.02 1.9 84.73 1.36

Comp.Ex. D 1.04 0.57 1.60 1.61 85.75 0.90

Comp.Ex. E 0.25 / / 0.25 86.6 0.56

Comp.Ex. F 0.50 / / 0.50 85.68 0.61

Comp.Ex. G 0.27 0.75 0.32 1.02 84.53 1.34

Comp.Ex. H 0.31 0.65 0.42 0.96 84.13 1.26

Comp.Ex. I 1.00 4.40 0.20 5.40 82.60 1.09

Ex. 1 0.07 0.47 0.13 0.54 85.87 1.24

Ex. 2 0.26 0.44 0.52 0.70 85.63 1.32

Ex. 3 0.09 0.66 0.12 0.75 85.86 1.42

Ex. 4 0.15 0.68 0.19 0.83 85.44 1.31

Ex. 5 0.10 0.67 0.13 0.77 85.74 1.35

Ex. 6 0.04 0.48 0.07 0.51 85.21 1.26

Ex. 7 0.04 0.72 0.05 0.76 85.45 1.31

Table 3. Results (all comparative) for conversion of 5 mol%

Example [Co] [Cr] Atomic ([Co] + Selectivity to Molar

(ppm in (ppm in Co/Cr [Cr]) CHHP K/A

Comp.Ex. A / 0.43 0 0.43 81.83 1.46

Comp.Ex. B / 0.86 0 0.86 82.33 1.43

Comp.Ex. C 1.02 0.88 1.02 1.9 82.39 1.33

Comp.Ex. D 1.04 0.57 1.60 1.61 83.50 0.81

Comp.Ex. E 0.25 / / 0.25 84.98 0.57

Comp.Ex. F 0.50 / / 0.50 84.21 0.62

Comp.Ex. G 0.27 0.75 0.32 1.02 83.82 1.31

Comp.Ex. H 0.31 0.65 0.42 0.96 83.08 1.24

Comp.Ex. I 1.00 4.40 0.20 5.40 81.83 1.12

Ex. 1 0.07 0.47 0.13 0.54 83.96 1.12

Ex. 2 0.26 0.44 0.52 0.70 84.23 1.22

Ex. 3 0.09 0.66 0.12 0.75 84.76 1.38

Ex. 4 0.15 0.68 0.19 0.83 84.30 1.27

Ex. 5 0.10 0.67 0.13 0.77 84.50 1.27

Ex. 6 0.04 0.48 0.07 0.51 83.72 1.17

Ex. 7 0.04 0.72 0.05 0.76 84.56 1.29

Claims

1. Method for preparing a mixture comprising cyclohexanol and cyclohexanone in a molar cyclohexanone/cyclohexanol ratio of at least 1.2 and with a cyclohexane conversion of below 5 mol%, comprising oxidising cyclohexane in a liquid phase, said oxidising being catalysed by a cobalt compound and a chromium compound, wherein the atomic cobalt to chromium ratio is in the range of 0.05-0.8, and wherein the sum of the concentrations of cobalt and chromium is 0.05-0.9 ppm by weight based on the total weight of the reaction mixture, and wherein the cobalt compound and chromium compound are dissolved in the liquid phase.

2. Method according to claim 1 , wherein the cobalt compound comprises at least one ligand selected from the group of acetylacetonates, octanoates, naphthenates, bipyridines, porphyrinates, and phthalocyanates.

3. Method according to claim 1 or 2, wherein the chromium compound comprises at least one ligand selected from the group of acetylacetonates, octanoates, naphthenates, bipyridines, porphyrinates, and phthalocyanates.

4. Method according to any one of the preceding claims, wherein the atomic cobalt to chromium ratio is in the range of 0.1-0.5, in particular in the range of 0.15-0.35.

5. Method according to any one of the preceding claims, wherein the sum of the concentrations of cobalt and chromium is 0.1-0.7 ppm by weight based on the total weight of the reaction mixture, in particular 0.2-0.6 ppm by weight.

6. Method according to any one of the preceding claims, wherein the cobalt concentration is 0.01-0.4 ppm by weight based on the total weight of the reaction mixture, in particular 0.05-0.2 ppm by weight.

7. Method according to any one of the preceding claims, wherein the chromium concentration is 0.04-0.8 ppm by weight based on the total weight of the reaction mixture, in particular 0.1-0.5 ppm by weight.

8. Method according to any one of the preceding claims, wherein the cyclohexane conversion is 2.5-4.5 mol%, in particular 3-4 mol%.