.........1ft \ Bulelin Kimia

\.~ j 2005, Jld. 21 , Bil. 1&2 49-54-

Titanium Doped Octahedral Manganese Oxide HybridCatalyst in the Oxidation of Cyclohexene

Fitri Hayati, Hadi Nur and Halimaton Hamdan

Ibnu Sina Institute far Fundamental Science Studies, Universiti TeknalagiMalaysia, 81310 UTM Skudai, Johor Bahru.

Abstract- Octahedral manganese oxide molecular sieves (OMS-2) was doped with

titanium to form hybrid catalyst (Ti-OMS-2) by precipitation method. Atomic absorption

and X-ray diffraction analysis indicate the incorporation of titanium into the framework of

OMS-2. Infrared and pyridine adsorption measurements demonstrated that Lewis acid

sites were created by the presence of titanium in the OMS-2. Ti-OMS-2 is the most active

catalyst for the oxidation of cyclohexene with tert-butylhydroperoxide (TBHP) compared to

litan ium oxide (TiO,) and OMS-2; suggesting a synergetic effect belween Ti and OMS-2 in

the hybrid catalyst.

Abstrak -Penapis mo/ekul mangan oksida oktahedra/ (OMS-2) didopkan dengan tilanium

untuk memben tuk mangkin hibrid (Ti -OMS-2) den gan menggunakan kaeda h

pemendakan. Analisis menggunakan Spektroskopi Serapan Atom dan Pembe/auan

Sinar-X menunju kkan penggab ungan titanium ke dalam bingkai OMS-2. Pengukuran

penjerapan piridina di/engkapi dengan Spektroskopi Inframerah mengesahkan bahawa

tapak asid Lewis terbentuk dengan keha diran ti tanium da/am OMS-2. Ti-OMS-2

merupakan mangkin yang paling aktif untuk pengoksidaan sik/oheksena dengan ten­

bulil hidroperoksida (TBHP) berbanding mangkin litanium oksida (Tia) dan OMS-2;

mencadangkan adanya kesan sinergi antara Ti dan OMS-2 da/am mangkin hibrid .

Keywords : Octahedral manganese oxide, titanium oxide, oxidation, hybrid catalyst

0127-8711 © Jabatan Kimia, Universiti Tekno/ogi Malaysia 49

Titanium Doped Octahedral Managanese Oxide Hybrid Catalysl in the Oxidation of Cyclohexene

1. Introduction

Heterogeneously catalyzed oxidation ofolefins has beenwidely applied in numerouschemical. biological and pharmaceuticalindustries. Besides oxidation of oletins inliquid phases with molecular oxygen ,aqueous or organ ic hydrogen peroxide inparticular, has been of interest in recentyears. Aqueous hydrogen peroxide is anefficientand environmentallyfriendlyoxidant;since water or alkyl alcohol is the only by­product obtained as the result of itsapplication in such reaction . Products ofselective oxidation of otefins are importantstart ing materia ls toward the production ofmany other fine chemicals and polymers [1].

The un ique catalytic activity andselectivity demonstrated by titaniumsilical ite- 1 (T S- 1) in photocatalyt icdecomposition of halogen compound andoxidation of alkenes, reported recently [2],have generated much interest to probe thepotential of titanium incorporated catalysts .

Octahedral manganese oxide molecularsieves (OMS-2) is curren tly considered asone of the most potential cata lyst in theoxidation of alcohols and olefins [1,3,4J.Similar to zeol ite molecular sieves, OMS-2materials are manganese oxide (MnO,; x =1.85-2.00) of molecular dimensions. Theframework structure of OMS-2 consists of 2x 2 type tunnels : built up of MnO. octahedrawith pore diameter of 46 A[5]. Unlike othermanganese oxide materials, OMS-2molecular sieves can be easily synthesized.In addition, OMS-2 uniquely catalyzes liquidphase oxidation of alcohols with molecularoxygen via the Mars van Krevelen mecha­nism to selectively generate the desiredproduct [3]. It is therefore desirable to studythe catalytic performance of OMS-2 materialwith the addition or incorporation of suitablemetal such as titanium.

This paper reports on the study oftitanium doped OMS-2 (Ti-OMS-2) as hybridcatalyst in the oxidation of cycfohexene withtert-butyl hydroperox ide (TBHP) as the

50

oxidant. The physical properties of OMS-2and Ti-OMS-2 were characterized and theircatalytic activity tested . The effect ofincorporation of Ti on the catalytic activityof OMS-2 mater ial was also studied.

2. Experimental

Catalyst Preparation

OMS-2 molecular sieves was prepared byprecipitation method . 225 mL of 0.4Msolution of KMnO, was added to 67.5 mLof1.75M solution of MnSO, .H,o and 6.8 mLconce ntrated HNO, (Merck) fo llowed byvigorous stirring. The mixture was thenrefluxedat 373 K for 24 h. The resultingblackprecipitate was filtered , washed and driedat393 K. Ti-OMS-2 was prepared followingthe same procedure and conditions as in(i)where, 225 mL of OAM solution of KMnO,was added to 150 mL of 15 w/v% solution ofTi2SO, in H2SO, insteadof MnSO, andHNO,solution .

Characterization ofsamples

X-raydiffractionanalysisof the sampleswereperformed using powdered X-ray diffrac­tometer (Bruker 08 Advance) with Cu K,radiation (I = 0.1542 nm at 40kV and 40mAl, step size of 0.02' per minute and ascan rate of 10 s/step. The XRO patternswere recorded at28 of 5'-70'.

Infrared spectra were measured usingaPerkin Elmer Fourier Transformed Infrared(FTIR) spectromete r wit h a spectra lresolution of 4 em" , 10 s scan time, 20'Ctemperature using the KBr pellet technique.The IR spectra were recorded in thewavelength region of 400 cm"-1400 ern".

Acidity study was performed using thepyridine adsorption technique. 12 mg of thesample was pressed under 5 tonnespressure for 10 s to form a 13 mm self­supporting wafers. The sample waferswerethen placed in the IR cellwith calcium fluorite

window.The sample was heated at 400'C invacuum for 4 h. The IR spectra werereco rded at room temperature usi ngShimadzu 2000 FTIR spectrometer with 2em" spectral resolution . The types of acidsites were determined using pyridine as theprobe molecule. Pyridine was adsorbed atroom temperature and desorbed at 150'Cfor one hour.

Elemental analysis was carried outusing atomic absorption spectrometer(MS). The samples were analyzed for Mn,Ti and K. The solid samples were preparedby hydrofluor ic acid me thod . 50 mg ofsample was placed in a Teflon vessel with0.5 mL aqua regia (vlv HNO,:HCI = 1:3). 3mL of HF (48%) was added and the tightlysealed vessel was placedin an oven at 11O'Cfor 1h. After cool ing, the products werequantitatively transferred to a 50 mL plasticbeaker containing 28 g of HBO,. 10 mL ofwater was added followed by stirring until aclear solution was obtained. The solutionwas fu rther dilu ted 10 100 mL pr ior toelemental analysis.

Catalytic reaction

Oxidation of cyclohexene was carr ied outusing tert-butylhydroperoxide (TBHP) as theoxidant. Typically, the reaction mixtu reconsists of catalyst (50 mg), cyclohexenesubstrate (10 mmol), TBHP (5 mmol), solvent(10 mL acetonitrile) and cyclooctane (0.5mmol) as an internal standard. The mixturewas placed in a round-bottomed flask witha reflux condenser. The assembly wasperformed under a semi-bath conditions for2 h. Upon completion, the liquid productswere separated from the solid catalyst byfiltration and analyzed by gas chromato­graphy (GC) using OB-1MS column fittedwith FlO.

3. Results and Discussion

Figure 1 shows the XRO patterns of OMS-2and Ti-QMS-2. The reflectionswere matched

Bul. Kim. (2005) 21(1&2)

,•

10 20 30 40 50 60 70

Figure 1. X-ray diffractograms of {a} OMS-2,(b) Ti-OMS-2 and(c) cryptomelane(JCPDS 29. 102).

to those of cryptomelane Q [6]; the naturalcounterpart of OMS-2 material. The resultsconfirmed that both OMS-2 and Ti-OMS-2materia ls co ns ist of the cryptomelanestructure : 2 x 2 tunnels with a pore size of4.6 A, composed of double chains of edge­sharing and corner-sharing MnO. octahedraas depicted in Figure 2. The more intensereflections forTi-OMS-2 samplesuggestthatthe hybrid material is of higher crystallinitythan the pure OMS-2 material.

Figure 2. Structure of OMS-2 with crypto­melane structure.

51

Titanium Doped Octahedral Managanese Oxide Hybrid Catalyst in the Oxidation of Cyclohexene

Table 2. Composition of rnanqanese andTilMn ratios in OMS-2 and Ti-OMS-2. (~

-

723

(al

I589534

I ' .",1112

1043

I

1111

1200 1100 1000 9(X) 800 700 600 500 400

V\tavert.JlTt:ler I an 1

Figure 3, FTIR spectra of (a) Ti-OMS-2 and(b) OMS-2.

0.05

0.02

K/(Mn+Ti)TIIMn

o2.43

22.40

13.38

Mn (mg)"

Lattice parameter'V"Sampl es

a c

OMS-2 9.741 2.859 271 .29

Ti-OMS-2 9.827 2871 277.20

* In 50 mg of catalyst.

OMS-2

Ti-OMS-2

Samples

I h' +k' I'I I d1 ;:;~ + c1

bl V == a 1 c

Table 1. Lattice parameters (a and c) andcell volumes (V) OMS-2 and Ti­OMS-2 hybrid catalysts ca lcu-latedfrom X-ray diffraction data.

The lattice parameters (a and c) and ce llvolumes of OMS-2 and Ti-OMS-2 samplescalculated from XRD data given in Table 1are typical for materials having tetragonalstructure. However, the increase in cellvolume of Ti-OMS-2 material indicates thatTi has been incorporated into the frameworkof OMS-2. This is further supported by theelemental analysis by AAS given in Table2. The lower amount of Mn in T i-OMS-2hybrid material than OMS-2 suggests that

i 'I " i i i Ii if ii' i 'I iii i

FTIR spectra of (e) Ti-OMS-2 and(b) OMS-2 after evacuation undervacuum at 400 ' C for 4 h followedby pyridine adsorption at roomtemperature and evacu ation at 150°C for an hour in the pyridineregions.

Ti has been incorporated in the frameworkof the latter. A low K/(Ti + Mn) rat io in Ti­OMS-2 also indicates that potassium ionsin OMS-2 malerial has been substituted bythe titanium ions. IR spectra of OMS-2 andTi-OMS-2 are shown in Figure 3. The spectraacquired in the wavenumber range of 400crn'-- 800 em" of both sam ples show thepresence of peaks typically attributed to Mn­o vibrations found in OMS-2 materials [7,8].

Figure 4.

Ulll '5'll 15!l 14/D

V\9.e'UTt:l:r Iarl '

1440 14lJ

52

However, for Ti-OMS-2 sam ple, the peaksare generally shifted to longer wavenumbersdue to the bigger size of Tl. In addition thespectrum of Ti-OMS-2 in Figure 2(a) showsa small peak at 967 ern' and 1000 cm ' ­1050 crn' assigned to tetrahedral Ti and Ti­°stretching in Ti -OMS-2 respectively [9] .

IR spectrum of acidity study by pyridineadsorption after evacuation under vacuumat 400 'c and 150 ·C in Figure 4(a) showsthat Lewis acid sites are form ed in Ti-OMS­2 hybrid material as indi cated by theappearance of peaks at 1447 ern", 1489ern and 1604 ern". In contrast, no peaks

Bul Kim. (2005) 21(1&2)

are observed for OMS-2 sample in Figure4(b). The absence of peaks at 1540 ernconfirms that Brensted ac id sites are notfound in both samples.

Oxidation of cyc lohexene using TBHPas the oxidant cata lyzed by TiO, (anatasephase) , OMS-2 an d T i- O MS-2 wereanalysed by GC. The yield of 2-cyclo­hexenone , 2-cyclohexenol and cyclohexeneoxide as the reaction products are shown inFigure 4. As indicated in Figure 4, thereaction catalyzed by Ti-OMS-2 produ cedthe highest yie ld of 2-cyclohe xe no ne .Furthe rmo re, the distribution data of the

Table 3. Distribution of products of oxidation of cyclohexene with TiO" OMS-2 and Ti-OMS-2 ascatalysis .

c onversion- TONSelectivity ' (%)

Catalyst cyclohexene z-cyclohexen-t -ol 2-cyclohexen-1-(%) for Ti oxide one

TiO, 11 0.15 0 40 60OMS-2 22 25 2 38 60Ti-OMS·2 38 32 2 26 72

• conversion (%) based on substrate = [1 - (concentration of substrate left after reaction/initial concentrationof substrate)) X 100

e Selectivity (%) of product A = (concentration of product Motal concentration of all product) x 100

Figure S. The yield of z-cvcrohexen-t-one, z-cvc lohexen-t-ol and cyclohexene oxide in theoxidation of cyclohexene using TiO" OMS·2 and Ti-OMS-2 as catalysts.

53

Titanium Doped Octahedral Managanese Oxide Hybrid Catalyst in the Oxidation of Cyclahexene

oxidation products of cyclohexene in Table3 evidently demon-strate that Ti-OMS-2 givesthe best conversion (38%) , highest turnovernumber (TON = 32%) and selectivity for 2­cyclohexenone (72%). The superiorperformance of Ti-OMS-2 hybrid materialstrongly suggests a synergetic effect of Tiand OMS-2 in Ti-OMS-2 catalyst. Theexplanation of such characteristic and itsmechanism is currently being studied.

4. Conclusion

Titanium doped octahedral manganese oxide(Ti-OMS-2) was successfully synthesizedby precipitation method using titaniumsulphate as the source of Ti. The structureof Ti-OMS-2 is similar to cryptomelane. Ti­OMS-2 is highly active and selective towardthe oxidation of cyclohexene to give 2­cyclohexenone as the main product. Theexistence of Ti in the tunnel and frameworkstructure of OMS-2 materials induces asynergetic effect that enhances the catalyticactivity of the resultant hybrid catalyst.

54

References

1. R. Ghosh, Y-C.Son, V. D. Maknawaand S. L. Suib. J. Cata/. 224 (2003) 228­296.

2. M. Taramasso, G. Perego, B. Notari,US Patents, No. 4,410,501 , 1983.

3. V. D. Makwana., Y -C . Son, A. R.Howell and S. L. Suib, J. Cata/. 210(2002) 46-52.

4. V. D. Makwana., L. J. Garces, J. uu , J.Cai, Y -C. Son, and S. L. Suib. Cata/.Today, 85: (2003) 225-233.

5. S. L. Suib, Curro Opin . Solid StateMater. Sci., 3 (1998) 63-70.

6. Database ofJoint Committee for PowderDiffraction Studies (JCPDS) No. 29,102.

7. J. Cai, J. Liu, WS. Willis and S.L. Suib,Chern. Mater., 13 (2001) 2413-2422 .

8. L. J .Garces, B. Hincapie, V. D.Maknawa, K. Laubernds, A. Sacco, S.L. Suib, Micropor. Mesopor. Mafer.,63(2003) 11-20.

9. D. K. Pattanayak, V. Mathur, B. T. Raoand T. R. R. Mohan, Trend. Biomafer.Artit Organs. 17 (2003) 8-12.