Liquid-phase oxidation of cyclohexane to adipic acid catalysed by cobalt containing β-zeolites

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

577

Liquid-phase oxidation of cyclohexane to adipic acid catalysed by cobalt containing 13-zeolites I. Belkhir 1., A. Germain 1, F. Fajula 1 and E. Fache 2 1 Laboratoire de Mat~riaux Catalytiques et Catalyse en Chimie Organique, UA/IR-CNRS 5618, ENSCM, 8, Rue de l'Ecole Normale,34296 Montpellier Cedex 5, France. Tel: 33 (0) 4 67.14.43.90 ; Fax: 33(0) 4 67.14.43.49. 2 Rh6ne-Poulenc Industrialisation, CRIT-Carribres, 85, Avenue des Frbres Perret, BP 62, 69192 Saint-Fons Cedex, France.

Abstract Cobalt exchanged 13-zeolites obtained by impregnation and solid state ion exchange and cobalt substituted 13-zeolites obtained by incorporation of cobalt in the synthesis gel were studied towards the oxidation of cyclohexane into adipic acid. The Co-substituted 13-zeolites were found to be effective catalysts for the oxidation of cyclohexane in acetic acid. In contrast, the use of Co-exchanged 13-zeolites always led to inhibition of the oxidation. It was demonstrated that the catalytic activity came as a result of the dissolved cobalt in the reaction medium, while inhibition was ascribed to the accessible uncompensated aluminic sites of the zeolites. 1. I N T R O D U C T I O N Adipic acid is an important intermediate extensively used for the manufacture of nylon 66. It is currently produced from cyclohexane oxidation by a two steps process [1 ]. During the first step, oxidation of cyclohexane by air in the liquid phase forms cyclohexanol and cyclohexanone. Further oxidation of this mixture by nitric acid gives adipic acid. In addition to its cost, the use of nitric acid generates corrosion risks and requires recovery of the nitrogen oxides effluents. Direct aerial oxidation of cyclohexane in a single step implies a partial and selective oxidation of the substrate. Oxidation without catalyst but in the presence of initiator gives adipic acid as a minor product [2]. Homogeneous catalysis by cobalt acetate in acetic acid provides good selectivity for adipic acid (88% at 21% conversion) [3, 4]. Recently, solid CoAPO were found to be effective heterogeneous catalysts [5]. However, the adipic acid selectivities were low [5-7] and the heterogeneous nature of the catalysis was not clear [8]. Moreover, redox properties of the framework cobalt ions are now subject to debate [9-10] and the reactive redox process could be attributed to non-framework cobalt species. Thus we have decided to explore the catalytic activity of cobalt containing zeolites and compare cobalt exchanged (or impregnated) zeolites to cobalt substituted zeolites obtained by incorporation

578 of cobalt into the zeolite synthesis gel as it was already achieved to obtain cobalt silicalite [11-13]. In order to favour adsorption of the organic apolar substrate and subsequent desorption and diffusion out of the catalyst of the polar products, the zeolite must possess both surface hydrophobicity and an open large pored structure. Taking into account these reasons, high silica 13-zeolites were chosen. Since 13-zeolites cannot be obtained without trivalent metal, cobalt substituted 13-zeolites were synthetised in the presence of aluminium or boron. The aim of the present work is to investigate and compare the cyclohexane oxidation activities of cobalt exchanged zeolites prepared by conventional impregnation or solid-state ion-exchange methods and cobalt substituted zeolites, in order to gain insight into the type of catalysis involved. Herein, the results obtained for oxidation of cyclohexane to adipic acid catalysed by cobalt exchanged zeolites (Co/BEA) and cobalt substituted 13-zeolites (Co-BEA) are presented. 2. E X P E R I M E N T A L BEA stands for 13-zeolites and the numbers after the structure type code of zeolites denote Si/AI or Si/B ratio (determined by analysis).

2.1. Materials Cobalt (II) acetate tetrahydrate, Cobalt (II) nitrate hexahydrate, cobalt chloride, sodium aluminate (Na20.A1203.3H20), boric acid (H3BO3) and tetraethylammonium hydroxide (Aldrich), acetic acid purex and cyclohexane for analysis (SDS) were used as received. Ludox HS-40 colloidal silica solution was obtained from Dupont. Zeolites BEA 15 and BEA 27 were synthesized in the presence of tetraethylammonium hydroxide (TEAOH) according to the procedure described by Wadlinger and al. [ 14]. Dealuminated BEA 1100 was obtained by treating BEA 15 with concentrated nitric acid [ 15].

2.2. Catalysts Zeolite impregnation: The zeolites were impregnated with 1 to 2% Co using a cobalt (II) acetate-water solution [ 16]. After evaporation until dryness at 343 K, the solids were calcined at 823 K for 6 hours (Co/BEA 15, Co/BEA 27 and Co/BEA 1100). Solid state exchange: Mechanical mixtures of powders of the zeolite and COC12 were grounded and calcined in air at 823 K for 6 hours [ 17] (Co/BEA 15S). Cobalt substituted zeolites synthesis: A: 0.97 g of Co(NO3)2.6H20 was dissolved in 13 cm 3 distilled water. Next, 0.55 g of sodium aluminate was added to this solution. Then, 48 g of Ludox HS-40 were dissolved in the mixture. A second solution was prepared by dissolving 0.56 g of sodium hydroxide in 36.8 g of a 40% aqueous solution of tetraethylammonium hydroxide. The final gel composition was 10Na20.CoO.AI203.110SIO2.1170H20. After 4 hours of stirring, the gel solutions were transfered into autoclaves and crystallised at 403 K for various periods from 1 to 2 weeks (Co-AI-BEA). B: 0.57 g of H3BO3 was added to 32.5 g of a 35% aqueous solution of tetraethylammonium hydroxide. Then, 0.27 g of Co(NO3)2.6H20 were dissolved in this solution. Next, 25 g of Ludox HS-40 were added to the mixture. The final gel composition was 3Na20.CoO.5B203.190SIO2.2170H20. After 4

579 hours of stirring, the gel solutions were transfered into autoclaves and crystallised at 423 K for various periods from 2 to 3 weeks (Co-B-BEA). Characterization of catalysts: The zeolite structure was checked by X-ray diffraction patterns recorded on a CGR Theta 60 instrument using Cu Kal filtered radiation. The chemical composition of the catalysts was determined by atomic absorption analysis after dissolution of the sample (SCA-CNRS, Solaize, France). Micropore volumes were measured by N2 adsorption at 77 K using a Micromeritics ASAP 2000 apparatus and by adsorption of cyclohexane (at P/Po=0.15) using a microbalance apparatus SETARAM SF 85. Incorporation of tetrahedral cobalt (II) in the framework of Co-A1-BEA and Co-B-BEA was confirmed by electronic spectroscopy [ 18] using a Perkin Elmer Lambda 14 UV-visible diffuse reflectance spectrophotometer. Acidity measurements were performed by Fourier transform infrared spectroscopy (FT-IR, Nicolet FTIR 320) after pyridine adsorption. Self-supported wafer of pure zeolite (20 mg/cm 2) was outgassed at 673 K for 6 hours at a pressure of 10 -1 Pa. After cooling at 423 K, the zeolite was saturated with pyridine vapour (30 kPa) for 5 min, evacuated at this temperature for 30 min and the IR spectrum was recorded.

2.3. Procedure Cyclohexane oxidation was carried out in a 300 cm 3 titanium, semi-batch, mechanically stirred Parr-type reactor. A typical procedure used for the oxidation was described in detail for an experiment at 383 K and 21 bars of total pressure. The reaction feed consisted of cyclohexane (45 cm3; 690 mmol), glacial acetic acid (68 cm3), catalyst (0.5 to 3 g) and acetaldehyde (0.24 g; 5 mmol) used as promoter. The autoclave was brought to the operating temperature and pressure, then held there for 3 hours under a constant flow of 20 dm3.h 1 of oxygen and nitrogen (10/90). Oxygen consumption was followed by the measure of the oxygen concentration and the flow rate in the output. The reactor was cooled and depressurized, and the product mixture was removed. The reaction mixture (2 g) was esterified at reflux with methanol (15 cm 3) in the presence of 2 drops of concentrated H2SO 4 to obtain the diacids in the diesters form. The products were analysed using a Hewlett Packard gas chromatograph equipped with a Carbowax 52 CB polar capillary column and a flame ionization detector assembled with a Shimadzu programmed and computerized Chromatopac CR6A. The reaction products consisted of adipic, glutaric, succinic and 6-hydroxycaproic acids, cyclohexanone, cyclohexanol and butyrolactone. Filtrates of Co-containing zeolites were obtained by treatment of 1 to 2g of zeolites in 75 cm 3 acetic acid at reflux overnight. After centrifugation of the solid, the desired amount of filtrate was fed into the reaction system. 3. R E S U L T S

AND DISCUSSION

3.1. Cyclohexane oxidation catalysed by cobalt exchanged zeolites We observed that the aerial oxidation of cyclohexane without catalyst, but in the presence of initiator (acetaldehyde) and in acetic acid as a solvent, occurred at 110~ The first step of the mechanism was the formation of the cyclohexylhydropero• which was converted to cyclohexanol and cyclohexanone. As cyclohexanone catalysed the decomposition of the hydroperoxide, the oxidation was autocatalytic.

580 Table 1 summarizes the activity of aluminic and boric zeolites in the proton form in the oxidation of cyclohexane. The results show that the reaction rate was reduced in the presence of aluminic zeolites. The addition of aluminic H-zeolites thus inhibited the oxidation of cyclohexane in acetic acid and the inhibition effect was stronger the larger the amount of aluminic sites was. Thus, the inhibition of the uncatalysed oxidation is attributable to the presence of the strong Br6nsted acid sites of the zeolites. It might come from the proton assisted heterolytic decomposition of the cyclohexyl hydroperoxide which is an intermediate in the autoxidation of cyclohexane [19]. Such a decomposition in acetic acid leads to the formation of 6-acetyloxy-hexanal [20] due to an ionic mechanism. Later, this aldehyde is oxidised into the acid analogue, leading to 6-hydroxycaproic acid after methanolysis. This reaction contributes to the termination of the free radical chain mechanism. In contrast to aluminic zeolites, the use of a boric H-zeolite led to an activity level equal to that of the uncatalysed reaction. The oxidation of cyclohexane was thus not inhibited by boric Hzeolites. It has been showed that boron atoms incorporated into the framework of 13-zeolites during synthesis, were removed from the solid after activation [21]. This could explain the very weak acidity presented by the solid. Table 1 Catalytic activity of zeolites in the proton form in the oxidation of cyclohexane. Zeolite

Amount (g)

None

Acid sites * Reaction rate*** (mmol)

(mmol/min)

Cyclohexane

Adipic acid

conversion **

yield **

(%)

(%)

0

0

0.36

6.6

1.1

H-A1-BEA 15

1.96

1.74

-~0

0

0

H-A1-BEA 1100

1.01

0.021

0.30

6.1

0.85

H-B-BEA 15

1.0

0.96

0.38

7.7

1.5

Cyclohexane: 690 mmol; acetaldehyde: 5 mmol; acetic acid: 68 cm3; N2/O2 90/10; 21 bars; flow: 20 dm3.hl; 110~ * Overall aluminium (or boron) content in the reaction medium. ** The reaction lasted for 3 hours. *** Rate of oxygen consumption measured after 2 hours of reaction. The use of Co-exchanged zeolites always led to an activity level below that of the uncatalysed reaction (Table 2). This was true for the impregnated Co/BEA and for the Co/BEA prepared by solid state ion exchange. Taking into account the amount of cobalt cations and considering that each cation compensates two negative charges of the framework, the number of residual (noncompensated) acid sites was calculated and the activity of samples was plotted as a function of the latter in Figure 1. It was observed that all the Co/BEA zeolites exhibited the same behaviour as the aluminic zeolites in the proton form whatever the exchange method used. The oxidation activity was thus a decreasing function of the number of aluminic acid sites. In order to understand the mechanism occuring during the oxidation, the Co-exchanged zeolites were treated in acetic acid at reflux overnight. After centrifugation of the solid, the filtrate

581 was fed into reaction. The results of the activities of Co-exchanged zeolites filtrates are reported in Table 3 and in Figure 1. Table 2 Catalytic activity of Co-exchanged zeolites in the oxidation of cyclohexane. Catalyst

Co content* Non exchanged Reaction rate (mmol)

acid sites

(mmol/min)

Cyclohexane

Adipic acid

conversion

yield

(%)

(%) 0.7

(mmol) Co/BEA 15

0.84

0.52

0.10

5.6

Co/BEA 27

0.26

0.11

0.20

6.0

0.6

Co/BEA 1100

0.60

0

0.34

7.0

0.87

Co/BEA 15S** 0.42 1.03 0.04 0.7 0 Same conditions as Table 1. * Total cobalt content in the reaction medium. ** Obtained by solid state ion exchange.

0.5

~L

LL

r-

E

0.4

o E E

0.3 tO

o

0.2

0 L_

L_ 0

> 0

0.1

0

0.3

0.6

0.9

1.2

1.5

1.8

Non compensated aluminic sites (mmol) FIG. 1 Dependency on the rate of oxygen consumption as a function of the free aluminic sites of zeolites present in the reaction medium (N none, II H/BEA, 5 Co/BEA, A Co/BEA filtrates linked to the corresponding Co/BEA, O Co-BEA)

582 The filtrates exhibited effective catalytic activity higher than the solids. This confirmed that the inhibition was due to the zeolitic aluminic sites. Thus, the catalytic activity of the filtrates must be attributed to the dissolved cobalt. We also observed that the addition of an aluminic 13-zeolite (Si/AI=I 5) in the proton form to an active filtrate inhibited the oxidation reaction. All these results demonstrate that the catalysis is homogeneous and that the zeolitic aluminic sites are responsible for the inhibition. If the Co-exchanged zeolites are not catalysts in the oxidation of cyclohexane, it is due to the presence of uncompensated aluminic sites in the solids. Table 3 Catalytic activity of the Co-exchanged zeolite filtrates in the oxidation of cyclohexane. Filtrate

Zeolite cobalt

Reaction rate

Cyclohexane

Adipic acid

content

(mmol/min)

conversion

yield

(%)

(%)

9.8

2.5

(mmol) Co/BEA 15

0.66

0.62

Co/BEA 15S

0.42

0.60

9.2

1.8

Co/BEA 1100

0.60

0.62

10.1

1.9

Co/BEA 15+H-AI-BEA 15

0.66

0

0

0

Same conditions as Table 1.

3.2. Catalytic activity of cobalt substituted zeolites The Co-exchanged zeolites were not effective catalysts for the oxidation of cyclohexane. The cobalt exchanged ions were not stabilized enough by the zeolite interactions and part of these cations were released in the oxidation medium. Thus, we decided to explore the activity of 13-zeolites in which cobalt ions were incorporated into the framework. We hoped that the incorporation would increase the stability of the cation within the solid. We studied the catalytic activities of cobalt substituted 13-zeolites containing aluminium (Co-A1-BEA) and boron (Co-B-BEA) towards the oxidation of cyclohexane into adipic acid. Table 4 Catalytic activity of the Co-substituted 13-zeolites in the oxidation of cyclohexane. Catalyst

Zeolite cobalt

Reaction rate

Cyclohexane

Adipic acid

content

(mmol/min)

conversion

yield

(%)

(%)

(mmol) Co-AI-BEA

0.27

0.62

9.1

18

calcined Co-AI-BEA

0.27

0.64

9.2

19

Co-A1-BEA filtrate

0.29

0.60

8.3

14

Co-B-BEA

0.24

0.66

9.4

25

calcined Co-B-BEA

0.24

0.64

9.3

2.1

Co-B-BEA filtrate

0.24

0.68

11.5

3.4

Same conditions as Table 1.

583 Three types of catalytic experiments were achieved on each Co-substituted p-zeolites. First, the catalytic activity of the as-made zeolite was evaluated. Then, the activity of the calcined one was investigated. Finally, the as-synthetised Co-substituted zeolite was treated in acetic acid at reflux and the filtrate was fed into reaction. The results are reported in Table 4. The boric (as-made or calcined) Co-substituted p-zeolites presented a catalytic activity higher than the activities of the uncatalysed reaction and the reaction with boric zeolite in the proton form. As the boric Co-substituted zeolites were not acid, they did not decrease the reaction rate of the oxidation. The filtrate of the boric Co-substituted zeolite was as active as the solids. This demonstrated that the catalysis resulted from the cobalt in solution. The reaction rates of the aluminic (as-made or calcined) zeolites were higher than the rate of reaction whithout catalyst and they did not follow the inhibition curve of the aluminic Coexchanged zeolites. As the as-made zeolite still contained the templates inside the pores, the solid certainly prevented access of cyclohexane to the aluminic sites responsible for the inhibition, but allowed the dissolution of cobalt. The activity of the as-made Co-substituted zeolite filtrate was similar to the activity of the zeolite. So, as for the Co-exchanged zeolites, the catalysis is homogeneous and came as a consequence of the dissolved cobalt. In contrast to calcined cobalt exchanged p-zeolites, the accessible acid sites of the calcined Cosubstituted aluminic p-zeolite did not inhibit the oxidation of cyclohexane. This result shows that the aluminic sites of the Co-substituted zeolite did not inhibit the oxidation of cyclohexane. The nature of the acidity was investigated in order to explain the catalytic activity of the calcined Co-substituted p-zeolite and the role played by the aluminic sites of this solid. A pyridine adsorption followed by IR spectroscopy measurements was performed on the calcined catalyst. It has been shown that adsorption of pyridine emphasized two distinct bands at 1548 c m -1 and 1451 cm 1 corresponding respectively to the adsorption on Br6nsted and Lewis sites [22]. In the case of the calcined Co-substituted zeolite, only a weak band at 1548 -1 cm appeared in the IR spectrum. Thus, we deduced that very few Br6nsted sites were present in the catalyst. This could explain that the oxidation of cyclohexane into adipic acid in the presence of calcined Co-substituted aluminic p-zeolite was not inhibited. 4. C O N C L U S I O N In the oxidation of cyclohexane into adipic acid, we have shown that the aluminic sites of proton form zeolites inhibited the reaction. When the aluminic sites of a Co-exchanged [3zeolites were not totally compensated, the solids inhibited also the oxidation. The activity was not influenced by the cobalt exchange method. As the acetic acid filtrates of Co-exchanged zeolites presented catalytic activities, we demonstrated that the catalysis is homogeneous and is due to the dissolved cobalt. In this precise case, the cobalt exchanged ions were not sufficiently stabilized by the zeolite and were dissolved in acetic acid. As we expected a better stabilization of the cobalt ions introduced in the zeolite framework, we studied the activity of Co-substituted p-zeolites. They showed an effective catalytic activity towards the oxidation of cyclohexane also linked to the cobalt in solution. We demonstrated that the Cosubstituted p-zeolites were more active than the Co-exchanged zeolites. In both cases, however, when a catalysis occurs, it came as a result of the dissolved cobalt.

584

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