~ APPLIED CATALYSIS A: GENERAL

ELSEVIER Applied Catalysis A: General 142 (1996) 243-254

Polymer support with Schiff base functional group with cobaltous palmitate as oxidation catalyst for

cyclohexane

Sanjay Kulkarni, Mahesh Alurkar, Anil Kumar * Department of Chemical Engineering, LI.T. Kanpur, 208016 Kanpur, India

Received 8 January 1996; accepted 24 January 1996

Abstract

Macroporous cross-linked beads of styrene-divinyl benzene copolymer have been prepared with Schiff base (with one of its components as L-tyrosine) active sites on them. Cobalt palmitate has been chelated with these and the metal does not leach out under the reaction conditions of liquid phase oxidation of cyclohexane. The oxidation of cyclohexane has been carried out using this catalyst with different promoters. Cyclohexanone and cyclohexanol were obtained as products with the former in larger concentration. Experiments reveal that there is no acid formation, demonstrating the specificity of the catalyst. It appears from these studies that the polymer support is playing a significant role in giving these products in high yield.

Keywords: Cyclohexane; Cobaltous palmitate; Oxidation catalyst; Schiff base functional group; Styrene-di- vinylbenzene; L-tyrosine

1. Introduction

In recent years large number of homogeneous catalysts consisting of transi- tion metal complexes have been discovered which exhibit high selectivity and activity. Some of the disadvantages of these are corrosion problems, product contamination and difficult catalyst recovery. To overcome these difficulties one 'heterogenises' the metal either by dispersing it in the pores of a suitable solid support (called solid supported liquid phase catalysts or SLPC) or by binding it to a polymer support (called polymer bound catalysts [1-5]). The latter leads to

* Corresponding author. Manuscript previously received 20 June 1995.

0926-860X/96/$15.00 © 1996 Published by Elsevier Science B.V. All rights reserved. PII S0926- 860X(96)00048- 8

244 S. Kulkarni et al. / Applied Catalysis A: General 142 (1996) 243-254

simplification of reaction work-up and the supported species are easily separated from the reaction mass by filtration.

Polymer supports are cross linked resins having large surface area (macropor- ous or macroreticular resins [6-12]), and the catalyst particles swell in the reacting medium thereby giving the advantage of high capacity. These beads and their inside pores have a great influence on the final reaction [13-18] and have been used as reagents [19-31], chelating agents for separation of metals [24-31] and sorbents [32,33]. Literature is full of modifications of these [34-44] and several novel beads [45-57] have been prepared for usage as phase transfer catalysts [58-66] and catalysts [67-81]. Some of the problems associated with the long term usage of polymer supported catalysts in continuous, fixed bed reactors is the loss of metal complex. This occurs because the attached metal is involved in dissociation equilibria under catalytic conditions. The leaching of the metal is a serious problem and metal loss been reduced either by increasing the ligand concentration in polymer beads, avoiding the use of coordinated solvents, or by using chelating ligands [3].

In this paper, we have developed an oxidation catalyst for cyclohexane using cobalt metal on a polymer support from which the metal does not leach. The oxidation of cyclohexane has been studied in literature by several workers and find that its oxidation using molecular oxygen yields a mixture of adipic acid, cyclohexanone, cyclohexanol along with many other acids. In these studies, various metal salts have been employed and the yield and selectivity of adipic acid has been stressed [82-97]. In this communication we have studied oxida- tion of cyclohexane using the polymer catalyst developed herein and found this to give a high yield of cyclohexanone and cyclohexanol with no acid formation.

2. Experimental

2.1. Chemicals used

All chemicals used were of Laboratory reagent grade.

2.2. Macroreticular beads for catalyst support

Styrene-divinyl benzene copolymer beads were produced by suspension polymerization technique [29]. 50 ml of toluene, 70 g styrene, 20 g divinyl benzene and 1 g azobis isobutyronitrile (AIBN) are mixed and added to a solution consisting of 225 g water, 24 g sodium sulphate, 0.07 g gelatin and 12 g calcium carbonate. The suspension polymerization was carried out under con- trolled stirring at 60°C for four h and subsequently at 90°C for one h. The beads of 10-20 mesh size were experimentally found to be most effective as supports

S. Kulkarni et al. / Applied Catalysis A: General 142 (1996) 243-254 245

for catalyst and were separated by sieving. The steps involved in the modifica- tion of the beads having Schiff base groups are as follows.

Chloromethylation of beads was performed using the technique given in Ref. [13]. A stock solution consisting of 90 g (3 tool) paraformaldehyde, 250 g ( ~ 3 mol) methanol and 225 ml HC1 were prepared. 150 ml of this was mixed with 10 g of beads and was refluxed for 48 h. The beads thus generated were of light yellow in colour and were refluxed with diethanol amine (10 g of beads with 3 g of diethanol amine in solution with excess chloroform) for 24 h and then further refluxed in presence of pyridine for three h. This leads to formation of (A) as follows [9,12].

.-CH2CH2OH ~ (~ CH N ICHzCH20H (~-CH2C[ +NH~.cHzCH20H - 2- "CH2CH2OH + HCt

(A)

(1) The beads change to cream colour, were separated and washed with tetrahydro- furan followed by cold water and then dried.

Two grams of L-tyrosine was dissolved in methanol by adding 3 -4 ml thionyl chloride dropwise in ice cold condition. 10 g beads were added to this and the mixture was stirred in ice cold condition for 4 h. It was further stored in cold water for about 24 h and then washed with methanol. As a result of this, the beads underwent following modifications [9,12]:

INH2 0 SOCt2 I Ol I NH2 OH(~CH2-CH-C-OH • C t - C - C H - C H z ~ O H + S02 + HC[

(t}

(2)

I + A • (~-CH2-N "CH2 CH20-C-CH-NH~CL "CH 2 CH 20-C-CH-NH2 HCI

I I I O CH2 (~ ' -OH

(B}

(3) The resultant beads are of light pink in color and are mixed with 0.1 M NaOH and the mixture is refluxed for about 4 h to remove the HC1. The beads were then washed with water to give

0 CH2 ("O"~-OH C II I

.... -* H2 CH20-C-CH-NH~

L~2 - ~rI2'~'~CH2 CH20-C-CH-NH2 II I 0 CH2 (~--OH

IC)

(4)

246 S. Kulkarni et a l . /Appl ied Catalysis A." General 142 (1996) 243-254

2 ml salicyaldehyde and these beads, C, were then taken in ethanol and kept in cold condition for 8 h. This gave rise to the formation of Schiff base functional groups upon it by the following reaction [45,49]:

OH C + ~}.CHO

0 CH2 ~--OH / CH' CH20-~-CH-N=C ~ >

~" ~)-CH2 N HO \CH~CH20- I~ I-C'H-N=C-~

O CH20~0H

(5)

2.3. Preparation of cobaltous palmitate

On mixing 25 g (0.1 mol) of palmitic acid dissolved in 150 ml ethanol (95%) and 4 g (0.1 tool) of NaOH in 75 ml of ethanol, white precipitate of sodium palmitate was formed. 56 g (0.2 mol) of dried sodium palmitate was dissolved in water at 60°C and to the clear solution, 13 g (0.1 mol) of cobaltous chloride C o C I 2 • 2H20 dissolved in water is added. The cobaltous palmitate appeared as purple precipitate which was separated, washed with methanol and air dried for 6h .

2.4. Loading of cobalt metal

4 g of cobaltous palmitate was dissolved in 50 ml of n-octanol by slowly heating at 60°C. 10 g beads were then mixed with this and refluxed for 20 h. After this, the beads are separated and washed with 100 ml of 95% ethanol followed by 100 ml of acetone. These are then dried in the oven for 4 h. The beads become greenish in colour indicating the chelation of the metal with the Schiff base sites and are expected to have the formula shown in Fig. 1.

CH2CH20-C-CH

O--Co---O.~

CH2CH20-.C.-C. H II I 0 CH2(~}'-OH

Fig. 1. Formula of cobalt-loaded beads.

S. Kulkarni et al. / Applied Catalysis A: General 142 (1996) 243-254

3. Experimental set up

247

Experiments on oxidation of n-heptane were carried out in a batch pressure reactor set up shown in Fig. 2. The apparatus consisted of a 250 ml stainless steel autoclave equipped with a stirrer with four blades. Its speed was varied by a variable speed motor and the reactor was heated by an external electrical heating coil. The temperature was controlled by a temperature controller (pro- portional type) with a chrome alloy thermocouple for temperature sensing. The reactor has provisions for oxygen inlet, liquid sampling out, pressure indicator and cooling coil. Experiments were conducted batchwise with respect to cyclo- hexane, constant temperature, constant pressure mode. In a typical run the reaction feed consisted of cyclohexane polymer bound catalyst and a suitable promoter (here AIBN was used) in the concentration range of 1000 ppm. After applying the oxygen pressure, the reactor was heated to the desired temperature, and the oxygen valve was opened for supply of oxidant at fixed pressure with the stirrer speed adjusted to 200 rpm.

I S ,R ER t xO 1' ~ ] ~ ? - - , ~LIQUID Y ~ SAMPLING OUT C PI E CWO •

N CWI ' '('T~ I

Y I L 1 N D

REACTOIt IIEATI.~G

COIL

PI - PRESSURE INDICATOR TE- TIIERMOCOUPLE

PR- PRESSURE RECORDER TIC-TEMPERATURE INDICATOR ASD CONTROLLER

CWO - COOL[SC WATER OUT TY- TIIERMO[IEATER

CWI- COOLISG WATER IN

Fig. 2. Apparatus used for ox idat ion o f n-hexane.

248 S. Kulkarni et al. /Applied Catalysis A: General 142 (1996) 243-254

4. Results and discussion

The suspension polymerization technique described in Ref. [7] gives rise to porous beads having a high internal surface area. Its surface area was measured using Pulse Chemisorb 2700 from Micromentics, U.S.A. and found it to be 10 mZ/g. Among these, beads having particle size larger than 10 mesh were found to break under stirring and are therefore not regarded as proper for the study of the catalytic reaction. In order to get the Schiff base group on the beads, we modified them using techniques given in literature through four chemical reaction steps, viz., chloromethylation, amination using diethanol amine [9,12], esterification with L-tyrosine and Schiff base formation using salicylaldehyde [45,49,50]. We carried out the FTIR of the solids and found that it gave signals corresponding to the bond formation. The study of oxidation reaction using particles less than 30 mesh, for some unknown reasons, did not give repro- ducible results. As a result of this, we separated 10-20 mesh beads for our studies. We also found that these particles were mechanically very strong and did not break under stirring.

The extent of chloromethylation was measured by the usual technique of boiling the beads in the NaOH solution. In order to get high extent of chloromethylation (in our case, it was measured to be 6%), we added more methanol to the refluxing reaction mass. At the end of the reaction, the beads were of light brown colour, indicating high extent of chloromethylation. The 6% chloromethylation indicates, that the -CH2C1 may have been generated in sufficient yield not only on the surface, but also within their internal pores. It was found that after chloromethylation, the surface area reduced to 5.2 m2/g. The remaining steps of amination using diethanol amine (surface area 6.3

Table 1

Oxidation of cyclohexane with respect to time a

Time Cyclohexane Yield c (mol%) Selectivity d(%)

(h) conversion b (mol%) one e O1 r one ol

1 5.01 2.00 3.01 39.92

2 6.90 2.82 4.08 40.87

3 7.70 3.22 4.48 41.82

4 8.69 3.77 4.92 43.38

5 9.01 3.91 5.10 43.40

6 10.07 4.34 5.73 43.10

10 20.48 11.82 8.66 57.71

60.08

59.13

58.18

56.62

56.60

56.90

42.29

a Reaction conditions: cyclohexane = 39 g; P = 5.44 atm; T = 150°C; A I B N promoter = 0.1

g.

b Conversion = (moles of cyclohexane reacted)/(total moles of cyclohexane) x 100%.

c Yield = (moles of the product formed)/(total moles of cyclohexane)X 100%.

d Selectivity = (moles of the product formed)/(moles of cyclohexane reacted)x 100%.

e one = cyclohexanone. f ol = cyclohexanol.

g; c a t a l y s t = 1.0

S. Kulkarni et al./Applied Catalysis A: General 142 (1996) 243-254 249

m2/g), esterification using tyrosine (surface area 6.3 m2/g) and Schiff base formation (surface area 6.5 m2/g) were found to be smooth reactions and the final support after loading metal (surface area 6.3 m2/g) gave reproducible results in catalyzing oxidation of cyclohexane. After loading cobalt palmitate, the amount of Co 2+ loaded was determined to be 0.40 mmol cobalt per gram of particle using EDS. To confirm whether polymer support plays any role in the oxidation, cobalt palmitate was added to cyclohexane and the reaction carried out as described in the experimental section. The reaction product was mostly adipic acid which separated and deposited at the bottom of the reactor as expected according to Ref. [90].

In order to demonstrate the efficiency of the catalyst developed in this work, experiments were carried out as described, for oxidation of cyclohexane using molecular oxygen. Experiments revealed that there was a large induction period and to reduce this an AIBN promoter was used. Samples were taken out every hour to determine the yield and the product distribution with time and some of the results are given in Table 1. The product of oxidation does not have any acid and gives cyclohexanone and cyclohexanol. Also the conversion of cyclohexane and selectivity of cyclohexanone increased with time, while the selectivity to cyclohexanol decreased with time. These observations are consistent with the proposed mechanism which we will discuss later. The products were analyzed using a Carbowax 20-M column under flame ionization detector (FID) mode on a Gas-Liquid chromatograph. The standard retention times were checked using authentic compounds and found to be 0.747 min for cyclohexane, 3.733 min for cyclohexanone and 4.345 min for cyclohexanol. Also, the infrared analysis of the product was done, IR (v cm -1) 1710-1720 (C=O stretch), 2960 (C-H stretch), 1100-1200 (C-O stretch), 1450 (C-H bend), 3450 (O-H bend).

The effect of temperature on the oxidation of cyclohexane was examined and the results are given in Table 2. The conversion of cyclohexane and the selectivity to cyclohexanone is found to increase with temperature. The effect of

Table 2

Effect of temperature on oxidation of cyclohexane a

Temp. Cyclohexane Yield c (mol%) Selectivity d(%)

(°C) conversion b (mol%) o n e e o l f one ol

120 7.34 3.40 3.94 46.32 53.68

130 13.12 6.49 6.63 49.47 50.53 140 22.42 12.38 6.84 55.22 30.51

150 20.48 11.82 8.66 57.71 42.29

a Reaction conditions: cyclohexane = 39 g; P = 5.44 atm; duration = 10 h, catalyst = 1.0 g; AIBN promoter - 0 . 1 g .

b Conversion = (moles of cyclohexane reacted) / ( to ta l moles of cyc lohexane)× 100%. c Yield = (moles of the product formed) / ( to ta l moles of cyc lohexane )x 100%.

J Selectivity = (moles of the product fo rmed) / (moles of cyclohexane reac ted)× 100%. e one = cyclohexanone. f ol = cyclohexanol.

250 S. Kulkarni et al. / Applied Catalysis A: General 142 (1996) 243-254

Table 3

Effect of pressure on oxidation of cyclohexane a

Pressure Cyclohexane Yield c (mol%) Selectivity d (%)

(atm) conversion b mol% o n e e O1 f one e ol f

3.40 7.95 5.29 2.66 66.54 33.46 5.44 14.15 9.46 4.69 66.86 33.14

8.16 13.13 7.51 5.62 57.20 42.80

a Reaction conditions: cyclohexane = 39 g; duration = 6.6 h; cyclohexane promoter = 0.1 g; catalyst = 1.0 g; T = 150°C.

b Conversion = (moles of cyclohexane reac ted) / ( to ta l moles of cyc lohexane )x 100%.

Yield = (moles of the product formed) / ( to ta l moles of cyclohexane) X 100%.

d Selectivity = (moles of the product fo rmed) / (moles of cyclohexane reacted) × 100%. e one = cyclohexanone.

f ol = cyclohexanol.

oxygen pressure on oxidation of cyclohexane was studied by carrying out experiments at different oxygen pressures at 150°C in the presence of catalyst. On increasing the pressure the solubility of oxygen increases giving higher cyclohexane conversion and reaches a maximum at about 80 p.s.i. (1 p.s.i. = 6894.76 Pa). The results are given in Table 3. The effect of promoter on oxidation of cyclohexane was examined and results are given in Table 4. The induction period is considerably reduced by the use or promoters and the reaction is accelerated. Cyclohexane is found to be the most effective promoter, giving maximum conversion and selectivity for cyclohexanone. The promoter plays an important role in the product distribution by initiating the reaction. Also, the cobalt loaded catalyst plays a dual role in the initiation and termination of the reaction as shown by the proposed mechanism. It is generally accepted

Table 4 Effect of promoters on cyclohexane oxidation a

Promoter Induction Cyclohexane Yield c (mol%) Selectivity d (%)

period (h) conversion b (mol%) one e ol f one ol

Cyclohexane 1.6 15.37 9.96 5.41 64.80 35.20

Cyclohexanol 2.2 12.11 5.72 6.39 47.23 52.77

AIBN 0 10.61 4.79 5.82 45.15 54.85 Acetaldehyde 1.6 3.66 1.23 2.43 33.61 66.39

a Reaction conditions: cyclohexane = 39 g; P = 80 p.s.i.; T = 150°C; promoter = 0.1 g; Catalyst = 1.0 g. Duration = 5 h after induction period.

b Conversion = (moles of cyclohexane reac ted) / ( to ta l moles of cyc lohexane)× 100%. c Yield = (moles of the product formed) / ( to ta l moles of cyc lohexane)× 100%.

d Selectivity = (moles of the product fo rmed) / (moles of cyclohexane reacted)X 100%. e o n e = cyclohexanone. f o l = cyclohexanol.

S. Kulkarni et al. / Applied Catalysis A: General 142 (1996) 243-254 251

Table 5 Accepted mechanism for thermal oxidation of alkanes

Initiation 2 R H + O 2 ~ R +H202 RH + 0 2 --* R + H O O

Propagation R + O 2 ~ ROO' R O O ' + R H ~ R ' + R O O H

Termination R + O H O ~ ROOH

2 RH + 2 R O O ---, 2 ROH + 0 2 + 2 R

R' + ROO' ~ ROOR

R', R O O , and H O O are free radicals

that the thermal oxidation proceeds through homolytic pathway involving free radical intermediates [84], and the basic steps in autoxidation of alkanes are given in Table 5. In the presence of the metal loaded catalyst a similar mechanism is expected to be valid and is modified as follows (where RH represents a cyclohexane molecule):

Initiation

RH + Co 3+ ---) R' + H + + Co 2+

} 0 2 + RH + Co 3+

Propagation

R + 0 2 ~ R O O

(6)

AIBN ---) O = R + H + + C o 2+ (7)

(8) R = O + RH ~ ROH + R (9)

R O O + RH ~ ROOH + R (10)

O = R O O + RH ~ O = R O O H + R (11)

R O O + R = O ~ ROOH + O = R (12)

Termination

O = R O O H --* ROH + O 2 (13)

2ROOH ~ 2 R O H + O 2 (14)

2 R O O + 2Co 2+ ~ 2 R = O + 2Co 3+ + O 2 (15)

O = R O O + Co 2+ ---) R = O + Co 3+ + 02 (16)

Step 7 is initiated by the promoter AIBN and this accelerates the reaction. The radicals formed during initiation, give the formation of R O O . , O = R O O . radicals (steps (8) and (9) and alcohol (cyclohexanol step [5]). The ROO- and O - R O O . radicals react with the metal loaded catalyst more easily than with cyclohexane or cyclohexanone. The observation that cyclohexane conversion and cyclohexanone formation in Table 1 increase, while cyclohexanol formation decreases with time is explained by the proposed mechanism. Consistent with

252 S. Kulkarni et al. /Appl ied Catalysis A: General 142 (1996) 243-254

the observation that no acids is formed, we suggest that steps [15] and [16] are overwhelmingly facile and give formation of cyclohexanone. The intermediates yield cyclohexanol in steps [13] and [14]. Clearly the metal loaded catalyst and the ROO. , O=ROO. radicals determine the product distribution.

5. Conclusions

In this paper we have chemically modified the crosslinked macroporous polystyrene beads to have Schiff Base functional groups (with one of its component as L-tyrosine). We have chelated Cobalt Palmitate upon these and have shown that metal does not leach out for at least 100 h of experimentation. We have carried out oxidation of cyclohexane with molecular oxygen using this as catalyst and the analysis shows, 1. Higher conversion of cyclohexane to give cyclohexanone as a major product

and cyclohexanol as minor product. There is no acid formation. 2. Increase in temperature and oxygen pressure increased the conversion of

cyclohexane. Cyclohexanone as a promoter gave maximum conversion of cyclohexane and product selectivity to cyclohexanone. It appears that the polymer support and the promoter play an important role in

catalyzing the oxidation of cyclohexane to cyclohexanone and cyclohexanol without any acid.

Acknowledgements

Authors wish to acknowledge the financial support from the Department of Science and Technology, New Delhi, India.

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