0023-1584/01/4202- $25.00 © 2001

MAIK “Nauka

/Interperiodica”0217

Kinetics and Catalysis, Vol. 42, No. 2, 2001, pp. 217–222. Translated from Kinetika i Kataliz, Vol. 42, No. 2, 2001, pp. 242–248.Original Russian Text Copyright © 2001 by Kholdeeva, Maksimovskaya, Maksimov, Kovaleva.

1

INTRODUCTION

Heteropolyanions (HPAs) substituted by transitionmetals (M-HPAs) are of interest as homogeneous oxi-dation catalysts due to the inorganic nature of the coor-dination sphere, which makes them resistant to oxida-tive media [1, 2]. On the other hand, the nature of HPAsallows them to be considered as discrete fragments ofmetal-oxide lattices, and monosubstituted HPAs of the

Keggin type, PW

11

M

(

L

)

(Fig. 1a), because theirstructures can serve as models of catalytic centershomogeneously distributed in the inorganic matrix.Ti

−

HPAs are of special interest as homogeneous mod-els for mechanistic studies of the catalytic effect ofthe practically important heterogeneous catalysts:Ti-containing micro- and mesoporous materials.Although many studies have been carried out, themechanism of catalysis and the nature of active speciesin these systems remain disputable [3–5]. The inor-ganic nature, thermodynamic stability, and nonlabilityof the tungsten-oxygen framework of HPAs stipulateseveral advantages for HPA-based model systems oversystems based on transition metal complexes withorganic ligands, which are prone to oxidative degrada-tion, ligand exchange, and polymerization [6]. In addi-tion, the presence of the central phosphorus atomallows the

31

P NMR monitoring of the catalyst state atall stages of the catalytic process. In this work, wepresent data on various forms of Ti–HPAs, their reac-tivity toward hydrogen peroxide, and catalytic activity

1

Proceedings of the II All-Russian Workshop on Highly OrganizedCatalytic Systems.

O39n–

in the oxidation of thioethers. We succeeded in detect-ing the active form of the catalyst (titanium hydroper-oxo complex) for the first time using spectroscopy anddemonstrate its reactivity toward thioether under sto-

Titanium-Substituted Heteropolytungstates as Model Catalysts for Studying the Mechanisms of Selective Oxidation

by Hydrogen Peroxide

1

O. A. Kholdeeva, R. I. Maksimovskaya, G. M. Maksimov, and L. A. Kovaleva

Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia

Received July 20, 2000

Abstract

—The

31

P NMR method shows that four forms of titanium(IV)-monosubstituted Keggin-type het-eropolytungstate (Ti–HPA) exist in MeCN: the dimer

(

Bu

4

N

)

7

[{

PTiW

11

O

39

}

2

OH

]

(in the abbreviated form,

(

PW

11

Ti

)

2

OH or

H

1

), its conjugate base (PW

11

Ti

)

2

O

(

1

), and two monomers, PW

11

TiO

(

2

)

and PW

11

TiOH

(H

2

)

. The ratio between the forms depends on the concentrations of H

+

and H

2

O. Dimer

H

1

is produced from

2

in MeCN when H

+

(1.5 mol) is added, and monomer

H

2

is the key intermediate in this process. The catalyticactivity of Ti–HPA in the oxidation of thioethers by H

2

O

2

correlates with their activity in peroxo complex for-mation and decreases in the order H

2

>

H

1

>

2

. The reaction of

2

with H

2

O

2

in MeCN occurs slowly to formthe inactive peroxo complex PW

11

TiO

2

(

A

)

. The addition of H

2

O

2

to H

1

and H

2

most likely results in the for-mation of the active hydroperoxo complex PW

11

TiOOH

(

B

).

Complexes

A

and

B

transform into each otherwhen H

+

or OH

–

(1 mol) is added per 1 mol of

A

or

B

, respectively. The activity of

B

toward thioethers in thestoichiometric reaction is proven by

31

P

NMR and optical spectroscopy.

(a)

(b)

Fig. 1.

Polyhedral model of the heteropolyanion of the Keggin

type (a) PW

11

Ti and (b) dimer

[{

PW

11

O

39

Ti

}

2

OH

]

7–

.

Octahedron TiO

6

and central tetrahedron PO

4

are full.

O405–

218

KINETICS AND CATALYSIS

Vol. 42

No. 2

2001

KHOLDEEVA

et al

.

ichiometric conditions. Based on the data obtained inthis work, we attempt to explain several phenomenaobserved for the heterogeneous Ti-containing catalysts.

EXPERIMENTAL

Acetonitrile (Fluka) was dried and stored over acti-vated molecular sieves 4A. Methyl phenyl sulfide(MPS), methyl

p

-tolyl sulfide, methyl

p

-methoxyphe-nyl sulfide, and benzyl phenyl sulfide (BPS) (Fluka)were used as received. Methyl

p

-bromophenyl sulfideand methyl

p

-nitrophenyl sulfide (Fluka) were recrys-tallized from methanol. The concentrations of tetrabu-tylammonium hydroxide (TBAOH) (1.0 mol/l solutionin MeOH, Aldrich) and trifluoromethanesulfonic acid(TFMSA, Merk) were determined by titration before use.

Hydrogen peroxide (35%) was concentrated under areduced pressure to 86%, and its concentration was deter-mined by iodometry directly before use. Other reagentswere reagent or analytical grade and used as received. Het-eropolytungstates (Bu

4

N

)

7

[{

PW

11

O

39

Ti

}

2

OH

] (

1

)and (Bu

4

N

)

5

PW

11

TiO

40

(

2

), as well as the

(

Bu

4

N

)

5

PW

11

TiO

41

peroxo complex (

A

), were preparedaccording to the procedures described in [7]. The activetitanium hydroperoxo complex (PW

11

TiOOH

,

B

) wasgenerated

in

situ

by the addition of an equivalentamount of TFMSA to a solution of

A

directly before anexperiment.

Thio ethers were oxidized in a glass well stirredconstant-temperature reactor. The products were iden-tified by

1

H NMR spectroscopy and mass spectrometry.The yields of sulfoxides and sulfones and the conver-sion of starting sulfide were determined by GLC usingbiphenyl as an internal standard. The conversionof H

2

O

2 was determined from its concentration(by iodometry). The stoichiometric oxidation of thioet-hers was studied by 31P NMR and optical spectroscopyby adding the thioether to the preliminarily generatedin situ hydroperoxo complex B. The kinetics of the sto-ichiometric reaction was studied under the conditionsof a pseudo-first order by monitoring the disappearanceof B (λmax = 410 nm and ε = 2000 l mol–1 cm–1 inMeCN).

A Tsvet-500 chromatograph with a flame-ionizationdetector and a glass capillary column (15 m × 0.3 mm)with an SE-30 phase were used for GLC analysis.Analysis by coupled chromatography/mass spectrome-try was carried out on an LKB-2091 instrument.31P NMR spectra were recorded on an MSL-400 Brukerinstrument at 161.98 MHz. Chemical shifts δ (in ppm)were taken with reference to 85% H3PO4. Electronicabsorption spectra were recorded on a ShimadzuUV-300 spectrophotometer.

RESULTS AND DISCUSSION

We found several years ago that alkylammoniumsalts of Ti-substituted heteropolytungstate catalyze theoxidation of alkyl aryl sulfides by H2O2 [8]. The reac-tion occurs at room temperature to form sulfoxide andsulfone in almost 100% yield. The data on the oxidation

Table 1. Oxidation of methyl phenyl sulfide by H2O2 in the presence of various Ti-containing catalysts*

Catalyst Time, h Conversion of MPS, % Sulfoxide yield, % Sulfone yield, %

Ti–HPA 4.0 97 80 20

TS-2 [9] 2.5 98 78 22

Ti–MMM [10] 0.2 97 77 23

* Reaction conditions: MPS, 0.30 mmol; 35% H2O2, 0.33 mmol; Ti(IV), 0.006 mmol; MeCN, 3 ml; 25°C.

Table 2. Oxidation of thioethers by H2O2 in the presence ofH1 and 2*

Catalyst Substrate** Initial rate***,×106 mol l–1 s–1

H1 MTS 2.9 ± 0.2

H1 MTS 5.0 ± 0.3****

2 MTS 0.45 ± 0.02

H1 – 0.0063

2 – 0.0016

H1 MPS 3.0 ± 0.3

H1 MPS 4.5 ± 0.3****

* Reaction conditions: 2.5 × 10–4 M H1 or 5 × 10–4 M 2, 0.5 Msulfide, 0.1 M H2O2 (added as a 86% aqueous solution),MeCN, 25°C.

** MTS is methyl p-tolyl sulfide, MPS is methyl phenyl sulfide. *** Consumption of H2O2 (–d[H2O2]/dt).**** A 35% aqueous solution of H2O2 was used.

KINETICS AND CATALYSIS Vol. 42 No. 2 2001

TITANIUM-SUBSTITUTED HETEROPOLYTUNGSTATES 219

of methyl phenyl sulfide by hydrogen peroxide in thepresence of Ti–HPAs and heterogeneous titanium-con-taining catalysts, such as the microporous titanium-sil-icalite TS-2 [9] and mesoporous mesophase titanium-silicate (Ti–MMM) [10], are presented in Table 1. Thesulfoxide/sulfone ratio is close for all catalysts, whichcan indicate a similar mechanism of their catalyticaction.

At the initial stage of the study, it was surprising thatthe samples of tetrabutylammonium salts Ti–HPAobtained at different pH had very different catalytic

activities [8, 11], whereas only one titanium-monosub-

stituted Keggin-type HPA, PW11Ti (2), wasdescribed [12, 13]. This prompted us to study system-atically the interrelation between the method of HPApreparation, its physicochemical parameters, and cata-lytic properties. When tetrabutylammonium bromide(TBABr) was added to an aqueous solution of het-eropolyacid H5PW11TiO40 (in the molar ratio 3.5 : 1), apreviously unknown dimer (Bu4N)7[{PTiW11O39}2OH](H1) (Fig. 1b) was formed whose structure was provenby various physicochemical methods [7].

As other spectroscopic parameters, the 31P NMRspectra for dimer H1 and monomer 2 differ substan-tially: –12.76 and –13.34 ppm in MeCN, respectively.Dimerization was observed in acetonitrile with theacidification of 2, which has the terminal Ti=O bondaccording to published data (Fig. 2). The third, previ-

O405–

–10 –11 –12 –13 –14 –15 –16δ, ppm

(a)

(b)

(d)

(c)

–11 –12 –13 –14 –15 δ, ppm

(a)

(b)

(c)

(d)

(e) (Bu4N)7[{PTiW11O39}2OH]

+1.5 mol H+

+1.25 mol H+

+0.75 mol H+

(Bu4N)5PTiW11O40

Fig. 2. 31P NMR spectra of 2 (0.005 mol/l) (a) before theaddition of acid and after the addition of (b) 0.75, (c) 1.25,and (d) 1.5 mol of H+ per 1 mol of HPA; (e) 31P NMR spec-trum of H1 (0.005 mol/l). All spectra were recorded inMeCN at 20°C.

Fig. 3. 31P NMR spectra of H1 and 2 (0.005 mol/l): (a) 2 36 hafter the addition of 77% H2O2 (20 µl), (b) after the additionof H+ (1.0 mol per 1 mol of 2) to the (a) sample, (c) H1 4.5 hafter the addition of 77% H2O2 (20 µl), and (d) after theaddition of OH– (2.0 mol per 1 mol of H1) to the (c) sample.

220

KINETICS AND CATALYSIS Vol. 42 No. 2 2001

KHOLDEEVA et al.

ously unknown form of Ti–HPA, namely protonatedmonomer H2 with the terminal Ti–OH bond (–13.44 ppm),was spectroscopically found. The monomers are notexchanged in the NMR time scale; hence, two individ-ual signals are observed when 0.75 mol of H+ per 1 molof 2 is added (Fig. 2b). In anhydrous acetonitrile, H2 isdimerized, which is characteristic of titanium com-pounds [14]. The dimer also exists in two forms, proto-nated and nonprotonated, which, unlike the monomericforms, are in fast exchange resulting in one averagedsignal with a downfield shift when the acidity increases(Figs. 2c and 2d). After the addition of 1.5 mol of H+

per 1 mol of 2, the position of the signal coincides withthat observed for the protonated dimer synthesized byan independent method (Figs. 2d and 2e). In turn, 2 can

easily be prepared from H1 by the addition of TBAOH(3 mol per 1 mol of H1). The ratio between all formsdepends on the concentrations of H+ and H2O. Only H1and 2 can be isolated in the individual state.

The data on the catalytic activity of H1 and 2 in theoxidation of thioethers by hydrogen peroxides are pre-sented in Table 2. The dimer is much more active, andthe oxidation rate is higher for dilute H2O2 than for theconcentrated peroxide. To explain this phenomenon,we studied the interaction of H2O2 with various formsof Ti–HPA. The addition of H2O2 to a solution of 2 inMeCN results in a very slow formation of the orange

–11 –12 –13 –14 δ, ppm

(a)

(b)

(c)

(d)

(e)

(Bu4N)5PW11TiO41

–11 –12 –13 –14

(a)

(b)

(c)

(d)

TiOOH

Ti–O–Ti TiOH

δ, ppm

Fig. 4. 31P NMR spectrum of H1 in MeCN–H2O (a) beforeand (b) 6, (c) 20.5, and (d) 73 min after the addition ofH2O2. All spectra were recorded in MeCN (4 ml) at 20°C,[H1]0 = 0.0025 mol/l, [H2O2] = 0.1 mol/l, 10 vol % H2O.

Fig. 5. (a) 31P NMR spectra of peroxo complex A; (b) afterthe addition of H+ (1 mol per 1 mol of A); and the (b) sample(c) 2, (d) 12, and (e) 25 min after the addition of methylp-tolyl sulfide (MTS). All spectra were recorded in MeCN(4 ml) at 20°C, [A] = 0.02 mol/l, [MTS] = 0.1 mol/l.

KINETICS AND CATALYSIS Vol. 42 No. 2 2001

TITANIUM-SUBSTITUTED HETEROPOLYTUNGSTATES 221

peroxo complex PW11TiO2 (A) described in [13] withthe 31P NMR signal at –13.0 ppm, whereas under thesame conditions, H1 easily transforms into another, previ-ously unknown peroxo complex with –12.35 ppm (B)(Fig. 3). Compound A transforms into B and vice versaby the addition of H+ and OH– (1 mol per 1 mol of A orB, respectively). Thus, these peroxo complexes differin the number of protons, and B is likely the so-calledtitanium hydroperoxo complex PW11TiOOH. Note thatthe catalytic oxidation of thioethers is efficient onlywhen B is formed in the reaction mixture.

Unlike nonprotonated monomer 2, protonatedmonomer H2, as well as dimer H1, reacts with H2O2 togive the active hydroperoxo complex B. The rate of for-mation of B is higher for H2 than for H1. When bothforms are present in a solution, the addition of H2O2first results in the disappearance of the H2 form (thesignal of B appears), and then the dimer graduallytransforms into B (Fig. 4). The ratio between H1 + 1and H2 at different concentrations of water and the ini-tial rates of the formation of B estimated from the 31PNMR data are presented in Table 3. A slight upfieldshift of the signal of H2 is likely stipulated by a changein the magnetic susceptibility of the solution with anincrease in the content of water in acetonitrile, whereasthe strong upfield shift of the signal of the dimer is dueto the deprotonation of the Ti–O(H)–Ti bridge in thepresence of H2O to form Ti–O–Ti bonds [7]. Thus, anincrease in the concentration of water favors dimer dis-sociation to form H2, which increases the rate of B for-mation. As mentioned above, an increase in the concen-tration of water also leads to an increase in the catalyticreaction rate. Thus, the catalytic activity of the Ti–HPAforms in sulfide oxidation by H2O2 correlates with therates of their interaction with H2O2 to form peroxocomplexes and decreases in the order H2 > H1 @ 2.

The nonprotonated complex A, as all known tita-nium peroxo complexes [15], is inactive toward thioet-hers in the stoichiometric reaction. By contrast, using31P NMR and UV-VIS spectroscopy, we succeeded in

confirming the activity of hydroperoxo complex Bunder stoichiometric conditions. Complex B was gen-erated in situ from A by the addition of 1 mol of H+ per1 mol of A, then organic sulfide was added, and the dis-appearance of the signal from B was monitored(Fig. 5). The rate of B decomposition in the absence ofa substrate was an order of magnitude lower. As far aswe know, this is the first example that directly demon-strates the capability of the titanium hydroperoxo com-plex to oxidize an organic substrate.

The study of the stoichiometric reaction of B withorganic sulfides revealed that it obeys the first orderwith respect to both sulfide and peroxo complex, whichindicates the absence of a strong complex formationbetween the sulfide and B. The oxidation of p-substi-tuted phenyl methyl sulfides exhibits the absence of theHammett correlation (Table 4). This implies that themechanism via the electrophilic transfer of oxygen,which is commonly accepted for Mo(VI) and W(VI)peroxo complexes [16, 17], is not the key mechanismfor the Ti(IV) hydroperoxo complex. Both stoichiomet-ric and catalytic oxidations of the test substrate, benzylphenyl sulfide (BPS), along with sulfoxide and sulfone,give benzaldehyde and disulfide, whose appearance isexplained by the fragmentation of the intermediate

Table 3. Influence of the concentration of H2O in MeCN on the dimer/monomer ratio and initial rate of the formation of hy-droperoxo complex B

H2O, vol % ([1] + [H1])* × 103, mol/l (–δ**) [H2] × 103, mol/l (–δ**) × 106, mol l–1 s–1

0 2.5 (12.76) – 0.071

1 2.4 (12.80) 0.2 (13.39) 0.47

3 2.2 (12.98) 0.6 (13.50) 1.8

10 1.4 (13.45) 2.2 (13.59) 6.4

* Total concentration of the protonated (Ti–O(H)–Ti) and nonprotonated (Ti–O–Ti) dimeric forms. ** Chemical shift of the signal in the 31P NMR spectra (ppm).*** Initial rate of formation of B at 20°C.

w0***

Table 4. Stoichiometric oxidation of p-X-phenyl methylsulfides by hydroperoxo complex B

Substituent X Hammett parameter σ k × 102,l mol–1 s–1

CH3O –0.27 5.9

CH3 –0.17 2.8

– 0 1.4

Br 0.23 5.3

NO2 0.78 2.0

222

KINETICS AND CATALYSIS Vol. 42 No. 2 2001

KHOLDEEVA et al.

thioether radical cation [18]. We obtained the sameproducts in the oxidation of BPS by hydrogen peroxidein the presence of the Ti–MMM mesoporous catalyst.This also indicates that the oxidation mechanisms forTi–HPAs and heterogeneous titanium-silicates are sim-ilar. Thus, the reaction of the titanium hydroperoxocomplex with thioethers occurs via the radical mecha-nism.

CONCLUSION

The data obtained in this work allow us to revealsome analogy with heterogeneous titanium-containingcatalysts and explain several phenomena observed inheterogeneous systems.

Titanium hydroperoxo complexes were assumed tobe active oxidizing species in the heterogeneous tita-nium silicates, but this has to be proven by spectros-copy [3–5].

It is known that the presence of the so-called extra-framework titanium in the titanium-silicates decreasesthe catalyst activity [3]. Our data suggest that this isbecause the Ti=O bond (a bond present in the extra-framework titanium) reacts much more slowly withH2O2 compared to the Ti–O–Si and Ti–OH bonds,which prevail in high-quality catalysts.

A decrease in the activity of the titanium-silicatesamples with an increase in the Ti concentration (thephenomenon observed for the mesoporous titanium-sil-icates [10]) is probably related to the fact that the Ti–O–Tibond reacts more slowly with H2O2 compared to theTi−OH or Ti–O–Si bonds, which dominate in the sam-ples with a minor quantity of Ti. The same observationscan explain the fact that the calcined samples of the cat-alysts containing supported titanium are less activethan noncalcined samples in which Ti–OH bonds dom-inate.

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research, project no. 96-03-34215. Theauthors thank V.N. Romannikov for providing us with

the Ti–MMM sample and N.N. Trukhan for carryingout experiments with it.

REFERENCES1. Hill, C.L. and Prosser-McCartha, C.M., Coord. Chem.

Rev., 1995, vol. 143, p. 407.2. Kholdeeva, O.A., Grigoriev, V.A., Maksimov, G.M., and

Zamaraev, K.I., Top. Catal., 1996, vol. 3, p. 313.3. Duprey, E., Beaunier, P., Springuel-Huet, M.-A., et al.,

J. Catal., 1997, vol. 165, p. 22.4. Zecchina, A., Bordiga, S., Lamberti, C., et al., Catal.

Today, 1996, vol. 32, p. 97.5. Karlsen, E. and Schoffel, K., Catal. Today, 1996, vol. 32,

p. 107.6. Talsi, E.P. and Babushkin, D.E., J. Mol. Catal. A: Chem.,

1996, vol. 106, p. 179.7. Kholdeeva, O.A., Maksimov, G.M., Maksimovskaya, R.I.,

et al., Inorg. Chem., 2000, vol. 39, no. 17, p. 3828.8. Kholdeeva, O.A., Maksimovskaya, R.I., Maksimov, G.M.,

and Zamaraev, K.I., React. Kinet. Catal. Lett., 1998,vol. 63, no. 1, p. 95.

9. Reddy, R.S., Reddy, J.S., Kumar, R., and Kumar, P.,J. Chem. Soc., Chem. Commun., 1992, p. 84.

10. Trukhan, N.N., Derevyankin, A.Yu., Shmakov, A.N.,et al., Micropor. Mesopor. Mater. (in press).

11. Kholdeeva, O.A., Maksimov, G.M., Maksimovskaya, R.I.,et al., React. Kinet. Catal. Lett., 1999, vol. 66, no. 2,p. 311.

12. Tourné, C. and Tourné, G., Bull. Soc. Chim. Fr., 1969,p. 1124.

13. Maksimov, G.M., Kuznetsova, L.I., Matveev, K.I., andMaksimovskaya, R.I., Koord. Khim., 1985, vol. 11,no. 10, p. 1353.

14. Doeuff, S., Dromzee, Y., Taulelle, F., and Sanchez, C.,Inorg. Chem., 1989, vol. 28, p. 4439.

15. Mimoun, H., Postel, M., Casabianca, F., Fisher, J., andMitschler, A., Inorg. Chem., 1982, vol. 21, p. 1303.

16. Di Furia, F. and Modena, G., Pure Appl. Chem., 1982,vol. 54, no. 10, p. 1853.

17. Ballistreri, F.P., Bazzo, A., Tomaselli, G., andToscano, R.M., J. Org. Chem., 1992, vol. 57, p. 7074.

18. Baciocchi, E., Lanzalunga, O., and Malandrucco, S.,J. Am. Chem. Soc., 1996, vol. 118, p. 8973.