Indian Journal of Chemistry VoL38A, August 1999, pp. 792-796

Oxidation of p-cresol catalyzed by neat and zeolite encapsulated cobalt salen complexes

Trissa Joseph, C S Sajanikumari, S S Deshpande & Sarada Gopinathan*

Inorganic & Catalysis Division , National Chemical Laboratory, Pune 411 008, India

Received 22 March 1999; revised 24 May 1999

Salen and substituted salen schiff base comp lexes of cobalt(ll) have been synthesized . These complexes have been encapsulated in the supercages of Na-Y zeolite employing flexible ligand method . The neat as well as the zeolite encapsulated complexes have been characteri~ed by XRD, FTIR and UV-vis spectroscopy. Aerial oxidation of p-cresol using these catalysts in presence of a base affords p-hydroxybenzaldehyde (PHBA) in good yield . Among the catalysts studied, cobalt-chlorosalen-Y g ives a lmost 100% conversion of p-cresol with 97% selectivity towards PHB A with a turnover frequency of I 049/h. The spent encapsulated catalysts retai n their identity afte r the oxidati on reacti ons as seen from their IR spectra .

p-Hydroxybenzaldehyde (PHBA) is obtained as a by-product from the Reimer-Tiemann reaction of sa licylaldehyde with phenol. PHBA is an important intermediate in agriculture and pharmaceutical industry. It is also used as an additive for metal plating brightners and in perfumes. Cobalt salts are frequently used as catalysts in oxidation reactions' ) . The liquid phase aerial oxidation of p-cresol was reported by Sharma et ctl' . using coba ltous ch lor ide as cata lyst in presence of excess sod iulll hydroxide to give an overa ll p-creso l conversion of 90% with 60% se lecti vity to PHBA. Other by-products incl uded j1-

hydroxybenzyl alcohol, p-hydroxybenzoic ac id, p­hydroxybenzyl methyl ether and minor quantities of tarry material s. An increased yield of PHBA has been ach ieved using heterogeneous catalysts such as CoAPO-S and CoA PO-1 1'1 . Recently, compounds like hydrotalcite also have been used as catalysts to get a hi gh p-uesol conversion' .

In this paper we report aerial oxidati on of p-cresol under a lka line conditions using catalysts consisting of neat sa len complexes of coba lt(lI) as well as these complexes heterogenized by encapsul ation in the voids of Y -type zeo l ites for the first time with a view to obtaining an increased se lectivity towards PHBA.

Materials and Methods p-Creso l, coba lt chloride hexahydrate and coba lt

acetate tetrahydrate and the . Ii gands such as sa li cylaldehyde, S-ch lorosal icylaldehyde, S-

bromosalicylaldehyde, S-nitrosa licylaldehyde and ethylenediamine were of Aldrich make.

Synthesis a/salen sch(ffbase Salicylaldehyde (12.2 g, 0.1 mol) was diluted with

ethyl alcohol (100 ml) and mixed with a so lution of ethylened iamine (3 g, O.OS mol) dissolved in ethyl alcohol (SO ml) dropwise to get sa len ligand as ye llow flakes. The slurry was then refluxecl for I h to complete the reaction . The solid product was filtered , washed with cold ethyl alcohol and dried in vacuo;. yield 13.2 g. The substituted sa lens were likewise prepared .

Synthesis ofcobalt(Il)-salen The cobalt(IJ)-salen was freshly prepared by

following the procedure described in the literature6.

The schiff base salen (7.34 g, 0.03 mol) was dissol ved in ethyl alcohol (200 ml) by warming in nitrogen atmosphere. An ethanolic solution of C (OAC)2.4 H20 (6.46 g, 0.03 mol in 100 ml ethanol ) was bo iled until all the crystalline solid had gone into so lu tion. Both the solutions were mixed and refluxed for 4 h unJer nitrogen, cooled to obtain a wine red precipitate which was fi ltered and dried in vacuo;. yie ld II g. Coba It-ch lorosalen, cobalt -bromosalen and coba I t­nitrosa len were prepared in a similar man ner.

Synthesis ofCo-Y by ion-exchange CO(OAC)2.4 H20 (3.S g) was dissolved in warm

distilled water (700 1111) to which Na-Y (7.S g) was

"

JOSEPH e/ al. : CATALYTIC EFFECT OF NEAT & ZEOLITE ENCAPSULATED COBALT COM PLEXES 793

added and the contents were refluxed for 8 h. A pink so lid obtained was collected by filtration , and washed several times with water. This solid was again treated wi th a soluti(ln of CO(OAC)2 (3.5 g) for a second loading. S imil arly, a third loadin g was a lso carri ed out. The cobalt exchanged zeo lite thus obtained was fi ltered, washed w ith wate r and dried in a ir overnight

at II ODe. The product was characterised by XRD, UV and I R spec.tra.

Synthesis of cobalt-salen-Y by flexible ligand method T he salen li gand (3 g) was me lted in a round­

bottolll fl ask kept in an o il -bath at 160°C. To the molten salen was added , Co-Y (I g) and the contents were kept at the sa me temperature for 24 h. The mo lten mass was then coo led to get a brown solid which was powdered and soxh let-extracted with methy lene chloride . The product was then refluxed wi th 0.1 N NaC I so lution for 4 h to remove the uncompl exed Co2

; ions adhering to the zeolite . The so lid was then washed with hot water till free from chloride ions. Sim il arly , coba lt-chl orosa len, cobalt­bromosa len and coba lt-n itrosa len were encapsulated us ing the above procedure. The coba lt content in the encapsulated complexes, estimated by atom ic absorption spectroscopy after destroying the zeo lite structure by HF treatment, was found to be in the range of2 .8-3 % by wt.

Cataly tic oxidation reaction p-Creso l (6 g, 0 .06 mo l) was added to a warm

methano lic so luti on (25 ml ) of sod ium hydrox ide (6 .2 g, 0 . 16 mol ) in a Parr autoc lave of 300 ml capacity and to that was added neat cobalt(lI) sa len catalyst (0.7 mgJg of p-cresol). T he reactor was then pressurised w ith air up to 600 psi and heated to 100°C with stirring. A drop in pressure suggested com mencement of reac tion which was continued over a period of 3 h. After the reaction, the autoc lave was coo led, gases were released and the product was obtained as a brown solid s lurry . It was homogeni sed by dissolving in water and ana lysed by HPLC us ing external standard method.

Similarly, kinetic runs us ing neat and encapsulated complexes were carri ed out and samples were analysed after every 30 min . The results show that p­creso l convers ion was only 40-50% after I h and was a lmost complete at the end of 3h .

Characterization T he electronic spectra of the neat and encapsulated

complexes were taken on a Shimadzu UV-VIS scanning spectrophotometer (Model 2101 PC). IR spectra were scanned on a Shimadzu FTIR instrument (Model 820 I PC). X-ray diffraction analysis was ca rried out using a Ri gaku DMAX-IIIB set-up w ith

CuKu radiation and a graphite monochromator, sGan speed 16°/min and scanning from 5 to 500

. Tlie reaction prod uct was analysed by HPLC us ing a

Shimadzu liquid chromatograph (Model LC-9A) equipped with Chrompak C 18 15 cm column.

Results and Discussion The XRD pattern of the coba lt-sa len encapsulated

zeolite catalysts was comparable with that of neat Na­y suggesting retention of zeo lite crysta llinity.

The UV-vi s e lectroni c spectra of the sa len liga nds

show the typica l absorptions at -253 and -326 nm attributable to ligand n-1[* and 1[-1[* charge transfer bands. In the coba lt compl exes, these bands are red­shifted by 10 to 20 nm suggesting complex formation . Further, an addit iona l peak in the higher wavelength between 470 and 540 nm present in a ll the comp lexes

(as shown in Fig. I) is suggested to be due to O~Co

li gand-to-meta l charge transfer band . lR spectra display characteristic difference between

the spectra of coba lt-sa lens and the corresponding free schiff base ligands. T he ligand s show absorption due to v (C=N) at 1635 cm-1

• In the spectra of all the complexes in nujo l thi s band is shifted to lower frequencies and is observed in the region 1600-16 14 cm-1

• This shift is characteri sti c of coordination of azomethine nitrogen to cobalt as a result of complex

formation . T he phenolic v(C-O) of the free ligand shifts to higher wave numbers in the complexes due to

2 .000r--l-r-·---------------,

'" § ~ 1.000 '" .c «

2~~ :

200.0 550

Wavelength (nm)

O' Complex

b · Ligand

900

Fig. J- lJV-vis spec tra orO. 1 mM so lution of sal en and coba lt salen in acetonitrile .

794 INDIAN J CHEM SEC A, AUGUST 1999

Table I-Activities of neat and encapsulated Co(lI)----salen[p-cresol=6 g,; aOH=6.2 g; catalyst (neat)=0.7 mg of cobaltlg of p-cresol ; temp. 80°C, catalyst (encapsulated)=0.175 mg of cobaltlg of p-creso l]

Catalyst Product selectivity (%) p -cresol PHBA Ether PHBAlc. PHB Acid Others TOFIh

onv. (%)

CoCI26H2O 80 80 10 4.5 3.5 2 209 C...Q(OAc)2 85 75 10 4 10 I 222 Co-salen 90 85 8 2 4 I 235 Co-salen-Y 96 90 5 2.5 1.5 I 982 Co-chlorosalen 98 96 1.5 1.5 0.5 0.5 254 Co-chlorosalen-Y 100 97 I I 0.5 0.5 1049 Co-bromosalen 94 92 5 2.5 1.5 I 245 Co-bromosalen-Y 98 95 3 2 1.5 0 .5 1011 Co-nitrosalen 92 89 3.5 2.5 3.6 1.4 240 Co-nitrosalen-Y 97 93 4.5 2.5 2 1049

Table 2-Effect of temperature IIsing neat cobalt-chlorosalen [p-cresol=6 g; NaOH=6.2 g [catalyst=cobalt-chlorosalen=0.6 mg of cobaltlg of p-cresol]

Temp.(°C) Conv.of Product selectivity % Cresol (%) PI-IBA Ether Alcohol Acid Others

40 30 65 60 70 80 70 88 89 80 98 96

increased CoO bond order compared to the hydrogen­bonded structure in the schiff base.

IR spectra of zeolite encapsulated complexes in DRS mode exhibit characteristic absorption bands of coordinated ligand along with a broad peak between 1100 and 1000 cm- I due to Si-O-Si of the zeolite framework. IR spectra 01' the spent catalysts also exhibit a broad band betwelen 1100 and 1000cm-1

similar to the fresh catalyst, which indicates that the zeolite framework is intact and has not undergone any distortion. Absorption due to v(C=N) at I637cm-1 is also observed showing that the ligand has not been leached out from the zeolite during the reaction.

Aerial oxidation of p-cresol was carried out using neat as well as zeolite encapsulated cobalt-salens as catalysts. Table 1 gives p-cresol converswn values and product selectivities. The results of control reactions using CoCh.6H20 and Co(OAc)2.4H20 as catalysts have also been included in the table. Among the neat catalysts studied, the chlorosalen derivative gives maximum substrate conversion and PHBA selectivity. The bromo- and nitro-salen complexes also show marginally better performance compared to the unsubstituted cobalt-salen. This increased activity shown by the catalysts contammg electron withdrawing substituents on salen ligand could be attributed to the stabilization of cobalt peroxo radical

25 9.5 5

1.5

5 4 I 4.5 4.5 1.5 2.5 2.5 I 1.5 0.5 0.5

1PO

90

80

70

60 C " v 50 ... ~ p-crt.sol 01 0..

-a--. PHBA 40 -tr- Bh-,fr

30 """""*- Akohol

-f-20

10

0 40 60 70 80

Fig. 2-EtTect of reaction temperature using neat catalyst on the rate of ji-cresol conversion and product selectivity. Reaction conditions :p-cresol= 6g, NaOH =6.2g, catalyst(cobalt-chlorosalen)~O.6mg of cobalt/g of p-cresol, reaction time =3h.

[CO(III)-O-O'] formed during the chain radical oxidation reaction of the substrate.

The zeolite encapsulated cobalt salens as catalysts are found to b~ far superior to the corresponding neat

."

~

JOSEPH et al. : CAT AL YTIC EFFECT OF NEAT & ZEOLITE ENCAPSULATED COBALT COMPLEXES 795

complexes as seen from the increased p-cresol conversion and PHBA selectivity even at lower concentrations of cobalt per gram of substrate. The turnover frequency (TOF) for these catalysts is also increased by a factor of -4. This increased selectivity for PHBA could be attributed to an optimum residence time for the substrate near site isolated active catalyst intermediate in the supercage of the zeolite. A considerable reduction in tar formation also has been noticed with these catalysts.

roo gO

BO

70

C 60

., -.- p- cresol v 50 ... 01 ---- PHM a.

40 -l:r- BhH

~ Alcohol 30 -f- Acid

20 --*- OtherS

10

0 40 60 70 80

Temp. inoC

Fig. 3-Effect of reaction temperature using encapsulated complex on the rate of p -cresol conversion and product selectivity. Reaction conditions: p -cresol= 6g, NaOH =6.2g, catalyst(encapsulated cobalt-chlorosalen)=O.6mg cobalt/g of p-cresol ; reaction time=3h.

Effect of temperature The reactions have been conducted using neat and

encapsulated chlorosalen catalysts in the temperature range 40-80°C to study the effect of temperature on the rate of reaction and aldehyde selectivity. The results are furnished in Tables - 2 (Fig 2) for neat complex and in Table - 3 (Fig 3) for encapsulated complex. The reaction is found to be incomplete at lower temperatures (say, 40°C) even after increasing the reaction titTle up to 5 h. The selectivity towards PHBA IS also decreased considerably due to unwanted benzyl methyl ether formation . An

1 o0r='-----~~======::_-I

c:

90

80

70

60

~ 50 ... . 01 a. 40

30

20

10

-.- p-cresol

---- PHBA

-l:r- Bher

~ Alcohol

-f- Acid

--*- Others

Cobolt content(mg)/6g of cresol

Fig 4--Effect of cobalt content on the rate of p-cresol conversion and product se lectivity. Reaction conditions: p­cresol= 6g; NaOH =6.2g; catalyst(neat cobalt-chlorosalen); ~eaction time=3h; reaction temperature=80°C

Tab le 3-Effect of temperature on encapsulated cobalt-chlorosalen-Y [p-cresol=6 g; Na9!-l=6.2 g; catalyst= cobalt-chlorosalen-Y=0. 175 mg of cobaltlg Of creso l; time=3 h]

Temp.(°C) Conv.of Product selecti vity % cresol (%) PH BA Ether PH B alc. PHB acid Others

40 63 70 20 4 4 2 60 85 82 8.5 3.5 4 2 70 93 93 2 1. 5 2. 5 I 80 100 97 I 0.5 0.5

Table 4-Effect of cobalt content on neat cobalt-chlorosalen [p-cresol=6 g; NaOH =6.2 g, catalyst=neat cobalt-chlorosalen; lemp.=80°C time=3h]

Amt. of Co(mg) Conv.of Selecti vity % /6g of cresol creso l (%) PHBA Ether Alcohol Acid Others 1.02 83 79 10 5 4 2 2.1 90 90 4 3 2 I 4.2 98 96 I.5 I.5 0.5 0.5 28.8 95 90 4 2 3 I 57.6 83 79 8 4 5 4

796 rNDlAN J C HEM SEC A, AUGUST 1999

enhancement of reaction rate has been observed with an incremental increase in reaction temperature between 60 and 70°C. A further increase in temperature gives products having less PHBA yield due to tar formation.

Effect of cobalt content Reactions have been carried out at different p­

cresol to catalyst ratios at 80°C. The results (Fig 4, Table 4) show that on increasing the amount of catalyst, the percentage conversion of p-cresol increases to an optimum value and then decreases with further increase in the amount of catalyst which may be attributed to a catalytically inactive dimer formation in case of neat complexes. Thus, in order to get maximum conversion and product selectivity, the cobalt content for the reaction is found to be in the narrow range of 4-5 mg of cobalt /6g of p-cresol in case of neat complexes. Similar conversion and selectivity can be achieved us ing one-fourth amount

of cobalt in the case of encapsulated complexes which may be suggested to be due to site iso lation of the complex in the zeolite super cage.

Catalytic behaviour of neat and encapsulated cobalt

complexes in the reaction of p-cresol oxidation was studied. Results show that encapsulated complexes have higher TOF than neat complexes. The cobalt content and NaOH to p-cresol ratio have an obvious effect on the catalytic activity. Choice of a suitable reaction temperature and reaction time improves the rate of reaction. Cobalt-chlorosalen gives the best selectivity towards PHBA. Encapsulated cobalt schiff base complexes are expected to be novel and efficient catalysts for selective oxidation of p -creso l to p­hydroxybenzaldehyde.

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