~ APPLIED CATALYSIS A: GENERAL

ELSEVIER Applied Catalysis A: General 155 (1997) 253-263

Oxidation of sulfoxides to sulfones by hydrogen peroxide over Ti-containing zeolites

Patrice Moreau*, Vasi le H u l e a l, S y l v i e G o m e z , D a n i e l Brunel ,

Francesco Di R e n z o

Laboratoire de Matdriaux Catalytiques et Catalyse en Chimie Organique, CNRS UMR 5618, Ecole Nationale Supirieure de Chimie de Montpellier 8, rue de l 'Ecole Normale 34296, Montpellier Cedex 5, France

Received 26 August 1996; received in revised form 13 November 1996; accepted 13 November 1996

Abstract

The oxidation reaction of dibutylsulfoxide to the corresponding sulfone by dilute hydrogen peroxide is investigated over Ti-containing zeolites as catalysts; Ti-beta is more active than TS-1, as for the oxidation of sulfides under similar conditions. The kinetic parameters are determined over Ti-beta in t-butanol as solvent, and the kinetic law of the sulfoxide oxidation reaction is established. The influence of the reaction temperature and effect of the solvent are also reported.

Keywords: Dibutylsulfoxide; Dibutylsulfone; Oxidation; Hydrogen peroxide; Titanosilicates

1. Introduction

Among the various methods available for the preparation of sulfones, the most widely used approach involves direct oxidation of sulfides and sulfoxides

RI - S - R2 2[_~0] R1 - SO2 - R2 [0] R1 - SO - R 2 ( 1 )

The oxidizing agents for these transformations are hydrogen peroxide, peracids, hydroperoxides, chlorine, nitrogen oxides, oxygen or ozone [1-3]. The most common oxidant is hydrogen peroxide, generally in acetic acid [4,5], but the reaction can be carried out in non-acidic media, in the presence of catalytic

* Corresponding author. Tel.: +33 4 67 144323; fax: +33 4 67 144349. ~Pennanent address: Faculty of Industrial Chemistry, 71 Blv. D. Mangeron, PO Box 2007, 6600 IASI,

Romania.

0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. P I I S 0 9 2 6 - 8 6 0 X ( 9 6 ) 0 0 4 0 0 - 0

254 P Moreau et al./Applied Catalysis A: General 155 (1997) 253-263

amounts of tungsten, molybdenum or vanadium [6,7]. Oxidation of sulfides with H202 in a stoichiometric ratio leads generally to the corresponding sulfoxides. The same transition metal (W, Mo, V) compounds are used as catalysts for the preparation of sulfones from sulfoxides, in homogeneous medium [8-11].

Recent papers showed that Ti-containing zeolites were able to catalyze the sulfoxidation reaction of thioethers into sulfoxides [12-14]. It is expected that these catalysts are also active in the oxidation reaction of sulfoxides to sulfones.

The purpose of this paper is to examine the effect of various parameters on the kinetics of n-butylsulfoxide oxidation by hydrogen peroxide over TS-1 and Ti-beta zeolites.

2. Experimental

2.1. Materials

n-Butylsulfoxide (96%) from Aldrich was used as supplied. Hydrogen peroxide (aqueous solution 31 wt%) was obtained from Prolabo.

t-Butanol, HPLC grade (Sigma-Aldrich), methanol, analytical grade (SDS), acetonitrile, analytical grade (SDS) and dioxane-l,4, analytical grade (SDS) were used as solvents.

2.2. Catalysts

Two Ti-containing zeolites, Ti-beta and TS-1 have been tested in oxidation reaction of sulfoxides with H202. The composition of the catalysts was Ti/ (Ti+Si)=0.008 and 0.011 for Ti-beta and TS-1, respectively, and their main physical characteristics have been reported [14]. The presence of absorption band at 48 000 cm 1 in the DR-UV-vis spectra indicates that for both Ti-beta and TS-1 samples, Ti(IV) is incorporated in the framework of molecular sieves [ 14]. No band corresponding to segregated TiO2 particles was found to be present in our UV-vis spectra. Crystal size as determined by scanning electron microscopy (Cambridge Stereoscan 260) was 0.1 ~tm for TS-1 isometric crystal and 0.4 ~tm for zeolite beta spheroidal grains.

2.3. Catalytic experiments

The oxidation of sulfoxides was carried out in a thermostatic glass flask equipped with a magnetic stirrer and a condenser. In a typical run, a Ti-beta or TS-1 sample was stirred with sulfoxide (10 mmol), a solvent (3 ml) and hydrogen peroxide (10 mmol) at a constant temperature. Aliquots of the reaction mixture

P Moreau et al./Applied Catalysis A: General 155 (1997) 253-263 255

were withdrawn at various times and analyzed for sulfoxide content by GLC on a capillary column (Methyl silicone Gum, 25 m x0.2 mm x 0.33 gm film thickness) using a Hewlett Packard chromatograph.

The hydrogen peroxide was measured by standard iodometric titration.

3. Results and discussion

The oxidation reaction of n-butylsulfoxide (Bu2SO), chosen as a model sub- strate, by H202 over Ti-containing zeolites led to n-butylsulfone (Bu2SO2) as the sole oxidation product. In our experimental conditions, the analysis of the residual hydrogen peroxide showed no direct decomposition. The influence of the nature and the amount of the catalyst on the sulfoxide conversion has been first examined. The kinetics of the reaction over the Ti-beta catalyst has then been studied in more details, and the effect of the reaction temperature and nature of the solvent on the reaction rate investigated.

3.1. Influence of the nature of the catalyst

The catalytic activity of two Ti-containing zeolites, TS-1 and Ti-beta with MFI and BEA structures, respectively, has been studied in the oxidation of Bu2SO using dilute H202 as oxidant, at 323 K. The concentration of both sulfoxide and hydrogen peroxide was 1.8 M in tert-butanol (t-BuOH) as solvent. The concen- tration of the catalyst was 52.3 g 1-1.

A blank reaction using a titanium-free dealuminated beta zeolite (Si/AI=40) has been carried out under the same conditions.

Fig. 1 shows the sulfoxide conversion versus time for both catalytic and non- catalytic reactions; the non-catalytic reaction was carried out under the same conditions, but without catalyst.

Under these conditions, the conversion of dibutylsulfoxide is nearly the same and is very low in the non-catalytic reaction and in the reaction over the dealuminated beta sample without titanium. Over the two Ti-containing zeolites, the conversion is more important, especially in the case of Ti-beta, which can be the first evidence of the role of the titanium sites in the oxidation reaction.

As clearly shown on Fig. 1, the catalytic activity of Ti-beta is higher than that of TS-1, although the Ti concentration in the reaction medium is higher for the TS-1 sample (catalyst: 300mg, Ti/(Ti+Si)=0.011 for TS-1 and 0.008 for Ti-beta). Diffusion limitation of the reactants and shape-selectivity towards the transition state might be taken into account to explain the lower activity of the medium pore (0.55 nm) TS-1 compared with the large pore (0.7 nm) Ti-beta. The critical diameters of dibutylsulfoxide and dibutylsulfone in their more stable conformation have been calculated, on the basis of the

256 P. Moreau et al./Applied Catalysis A: General 155 (1997) 253-263

100

8O

O g o 60 ~D

O

o 4 0

¢I1

20

0 50 100 150 200 250 300 350

Reaction time (min)

Fig. 1. Influence of the nature of the catalyst on the oxidation reaction of Bu2SO by H202 ([Bu2SO]o=[H202]o=l.8 M, [cat.]=52.3 g l ~, solvent - t-BuOH, T-323 K): (V) Ti-beta, ( ~ ) TS-1, (©) beta, (A) without catalyst.

intermolecular distances computed by the molecular mechanics method [15], usin~ the software "Hyperchem 4.5" [16]. The values obtained, 5.5 and 5.3 A, respectively, confirm the diffusion limitation of both reactant and product in the medium pore TS-1. The same difference has been reported in the case of oxidation of cyclic or branched alkanes and alkenes over the two catalysts [17,18].

The comparison of the results obtained for the oxidation of Bu2SO (Fig. 1) and Bu2 S (in [13, Fig. 3]) leads to the following remarks:

The same order of activity is observed for both reactions: Ti-beta>>TS- l>without catalyst. In all cases, the reactivity of the sulfide is higher than that of the sulfoxide; indeed, the sulfide conversion is always higher than the sulfoxide conversion (Table 1), even when the reaction conditions are more severe in the case of BuzSO (temperature, substrate concentration, catalyst concen- tration).

These results are in agreement with a mechanism involving the same heterolytic process as that described for the sulfide oxidation [14] and

P. Moreau et al./Applied Catalysis A: General 155 (1997) 253-263 257

Table 1 Comparison of the conversion of BuzS and BuzSO in the oxidation reaction by H202 in t-BuOH ([substrate]/ [H202]=1)

Catalyst Conversion %a

Bu2 Sb Bu2SO c

None 20.0 7.0 TS-1 20.5 15.5 Ti-beta 78.0 51.7

aConversion of the substrate after 120 min reaction. bT--303 K, Co 0.19 M, Ccat.=4.75 g 1 1 [14]. CT=323 K, Co=1.8 M, Ccac=52.3 g 1-1 (this work).

R I

SiO O H \ / :

SiO ~ Ti

SiO / ~ 0 H O I

R-SO-R

Fig. 2. Proposed complex intermediate for the oxidation of BueSO by HzOe over Ti-containing zeolites.

consisting in a nucleophilic attack of the sulfur atom on the peroxidic oxygen of the complex depicted in Fig. 2. The difference in the reactivity of the sulfoxide can be explained by the decrease of the nucleophilicity of the sulfur atom due to the presence of the oxygen atom of the sulfoxide (>S-O).

3.2. Kinetics study

The kinetics of the oxidation reaction of dibutylsulfoxide by hydrogen peroxide has been studied over Ti-beta, the more active catalyst for which no steric constraint towards the reactants is observed [14].

It must be mentioned first that a global order of zero is observed for the non- catalytic reaction, as shown by the linear dependence of the Bu2SO conversion versus time (Fig. 1). The high value of the ratio of initial rates of the oxidation reaction over Ti-beta and without catalyst (r0 (Ti-beta)/r0 (uncatal.)= 2 x 10-4/0.17 × 10 - 4 = 11.7) allows to consider the contribution of the non-catalytic reaction as insignificant in the oxidation over Ti-beta.

Moreover, no decomposition of hydrogen peroxide is observed under our conditions, indicating a one-to-one stoichiometry of the sulfoxide versus hydrogen peroxide.

258 t~ Moreau et al./Applied Catalysis A: General 155 (1997) 253-263

-4

-5

y = -5,4126 + 1,0374x R-- 0 ,99654

O

_=

-6

-7 i I H I i -2 -1 0

ln[Bu2SO]

Fig. 3. Observed first-order from the linear form of Eq. (5) in the oxidation of Bu2SO over Ti-beta ([Bu2SO]0=[H202]o=l.8 M, [cat.]=52.3 g 1 1, solvent - t-BuOH, T--323 K).

In these conditions, the general rate equation of the oxidation of Bu2SO by H202 over Ti-beta according to

[cat.] BuzSO + H202 ~ Bu2SO2 + H20 (2)

can be expressed as follows:

R = kl [Bu2SO]"' [H202] n2 [cat.] "3 , (3)

where nl, n2, n3 are the partial orders in Bu2SO, H202 and catalyst, respectively. For a constant amount of catalyst, Eq. (3) can be simplified to the form:

(4) R = k~l [Bu2SO] nl [H202] n2 ,

where U 1 = kl [cat.] n3. For an equimolecular ratio of Bu2SO/H202, Eq. (4) becomes

R = k' 1 [ B u 2 S O ] hI+n2 . (5)

The plot of the linear form of Eq. (5) gives the global order of the reaction. As shown in Fig. 3, this order is equal to 1.

The kinetic study has then been carried out under pseudo-first-order conditions, by using first a large excess of hydrogen peroxide over the substrate. The reaction rate was determined by following the disappearance of dibutylsulfoxide. A very good correlation (r=0.998) is obtained between log[Bu2SO] and time (Fig. 4), indicating a first-order towards the substrate.

-0,2

I

-0,4

I ' I ' I I

y = -0,34328 + -0,0062482x R= 0,99818

-0,6

I I J I i I I I I

0 i0 20 30 40 50 Reaction time (min)

Fig. 4. Observed partial order in Bu2SO in the oxidation reaction over Ti-beta using excess of hydrogen peroxide ([Bu2SO]o=0.71 M, [H202]o=7.1 M, [cat.]=52.3 g I 1, solvent - t-BuOH, T-323 K).

80

g

40

20

0

P. Moreau et al./Applied Catalysis A: General 155 (1997) 253-263 259

0 50 100 150 200 250 300 350

Reaction time (min)

Fig. 5. Influence of the concentration of the Ti-beta catalyst on Bu2SO conversion ([Bu2SO]o=[H202]o 1.8 M, solvent - t-BuOH, T--323 K): (m) 52.3 g 1-1, ( 0 ) 39.2 g 1-1, ( 0 ) 31.2 g 1 x, (A) 13.0 g 1-1.

260 P. Moreau et al./Applied Catalysis A: General 155 (1997) 253-263

6

E "---" 5

"" 4

2 i

10 60

| ~ I ~ I ~ I

~ y -- 1,2671 + 0,098624x R-, 0,99695

I I I I I I I I

20 30 40 50

Conc. of catalyst (g 1 1)

Fig. 6. Observed partial first-order in catalyst, in the oxidation reaction of BuzSO by H202 over Ti-beta (conditions as in Fig. 5).

For a large excess of Bu2SO over hydrogen peroxide, a linear dependence of the H202 concentration versus time is found, which is characteristic of a zero-order towards H202. Fig. 5 shows the sulfoxide conversion versus time for various amounts of catalyst (Ti-beta); a global first-order is obtained in each case.

A linear relation between the observed kinetic rate constant and catalyst concentration is obtained, which confirms a first-order kinetics towards the catalyst (Fig. 6).

As a result of this study, it can be assessed that the oxidation reaction is first- order with respect to the amount of catalyst and sulfoxide concentration and zero- order to hydrogen peroxide concentration:

R = k[BuzSO][cat.].

These results are in agreement with the proposed mechanism where the active species of the oxidation reaction is the peroxo or hydroperoxo complex between H202 and titanium sites (Fig. 2). A first-order in sulfoxide indicates that the determining step is the nucleophilic attack of the sulfur atom on the peroxydic oxygen.

A similar kinetic relation R=k[substrate] [Mo(VI)] has been found by Bortolini et al. [3] for the oxidation reaction of various sulfoxides (dibutyl, diphenyl, phenyl methyl) by H202 over Mo(VI) catalysts.

3.3. Influence of the reaction temperature

The effect of the reaction temperature on the Bu2SO oxidation by hydrogen peroxide over Ti-beta catalyst is shown in Fig. 7, where the conversion of sulfoxide

P. Moreau et al./Applied Catalysis A." General 155 (1997) 253-263

lOOv

261

80

o

60

0 ~ 40 0

20

0 50 100 150 200 250 300

Reaction time (rain)

Fig. 7. Effect of reaction temperature on the Bu2SO conversion in the oxidation reaction by H202 over Ti-beta ([Bu2SO]o=[H202]o=l.8 M, [cat.]=31.2 g l 1, so lvent - t-BuOH): (11) 333 K, ( 0 ) 323 K, ( 0 ) 313 K, (A) 303 K.

versus time is plotted for reactions carried out at 303, 313,323, and 333 K, using a 1/1 molar ratio of substrate to hydrogen peroxide.

From these results, it can be seen that temperature influences strongly the catalytic activity of the Ti-beta sample. A linear Arrhenius plot of the first-order reaction rate constants is obtained. The corresponding apparent activation energy, Ea=68 kJ mo1-1, indicates that the reaction is kinetically controlled. Such a value is higher than that obtained for the oxidation of butylsulfide over Ti-beta in t-butanol as solvent (51 kJ mo1-1 [19]), which confirms that sulfoxides are less easily oxidized than the corresponding sulfides [2].

3.4. Influence of the nature of the solvent

The effect of the nature of the solvent in the oxidation of Bu2SO by H202 over Ti-beta catalyst has been studied by using, respectively, t-BuOH, acetonitrile, 1,4- dioxane and methanol as solvents, at 323 K (C0=1.8 M, Ccat=31.2 g l-t) . The results obtained under these conditions are shown in Fig. 8.

Even if the Bu2SO conversion is slightly higher in the presence of acetonitrile, the effect of the nature of the solvent is much less important in the oxidation of

262 P. Moreau et al./Applied Catalysis A: General 155 (1997) 253-263

lO0"r

80

o

= 60 Q

o

0 40

20

0 ~ 0 50 100 150 200 250 300 350 400

Reaction time (rain)

Fig. 8. Effect of the nature of the solvent on BuaSO conversion in the oxidation reaction by H202 over Ti-beta ([Bu2SO]o=[H202]o=l.8 M, [cat.] 31.2 g l a, T=323 K): ( 0 ) acetonitrile, ( I ) 1,4-dioxan, (&) methanol, (©) t-butanol.

sulfoxides than in the oxidation of sulfides under similar conditions [20]. In the latter case, it was shown that protic solvents (MeOH, EtOH, t-BuOH) were more efficient than aprotic solvents (MeCN, THF, acetone) in the presence of Ti-beta as catalyst. A possible explanation could be related to the limited step of the oxidation process. In the oxidation of sulfides, the formation of the peroxo complex, in which the solvent is involved [14], can be considered as the determining step of the kinetic process. This is not the case for the sulfoxide oxidation for which we have shown that the kinetics is totally controlled by the attack of the sulfoxide on the peroxo complex.

4. C o n c l u s i o n

The oxidation of sulfoxides to sulfones by hydrogen peroxide can be carried out efficiently in an organic solvent over Ti-containing zeolites. The catalytic activity of Ti-beta is higher than that of TS- 1; such a difference in catalyst efficiency can be explained, as in the case of sulfide oxidation, by the higher diffusivity constants of the large pore Ti-beta in comparison with the medium pore TS-1. The kinetic study

P Moreau et al./Applied Catalysis A: General 155 (1997) 253-263 263

over Ti-beta indicates that the oxidation reaction is first-order with respect to the amount of catalyst and sulfoxide concentration and zero-order to hydrogen peroxide concentration: R=[Bu2SO][catalyst]. Moreover, it is shown that the solvent effect is less important for the sulfoxide oxidation to sulfone than for the corresponding sulfide oxidation to sulfoxide. In conclusion, the kinetics of the system is totally controlled by the attack of the substrate on the peroxo complex (Fig. 2), and not by the rate of formation of such a complex in which the solvent is involved.

Acknowledgements

Financial support from Elf-Aquitaine is gratefully acknowledged. Thanks are due to the Groupement de Recherches de Lacq for providing samples of TS-1 zeolite, and to Dr. J. Joffre for the calculations of the critical diameters of dibutylsulfoxide and dibutylsulfone.

References

[1] W.E. Truce, T.C. Klinger, W.W. Brand, in: S. Oae (Ed.), Organic Chemistry of Sulfur, Plenum Press, New York, 1977, p. 527, and references therein.

[2] S. Oae, Organic Sulfur Chemistry: Structure and Mechanism, CRP Press, Boca Raton, 1991, p. 253. [3] O. Bortolini, S. Campestrini, E Di Furia and G. Modena, J. Org. Chem., 52 (1987) 5093. [4] M. Portelli and B. Soranza, Ann. Chim., 52 (1962) 1280. [5] N. Furukawa, Y. Konno, H. Tsuruoka, H. Fujihara and S. Ogawa, Chem. Lett., (1989) 1501. [6] H.S. Schultz, H.B. Freyermuth and S.R. Buc, J. Org. Chem., 26 (1963) 1140. [7] L. Kuhnen, Angew, Chem., Eng. Int. Ed., 78 (1966) 937. [8] O. Bortolini, E Di Furia and G. Modena, J. Mol. Catal., 14 (1982) 53. [9] O. Bortolini, E Di Furia and G. Modena, J. Mol. Catal., 16 (1982) 69.

[10] K.A. Vassel and J.M. Espenson, Inorg. Chem., 33 (1994) 5491. [11] M. Madesclaire, Tetrahedron, 42 (1986) 5459 and references therein. [12] R.S. Reddy, J.S. Reddy, R. Kumar and P. Kumar, J. Chem. Soc., Chem. Commun., (1992) 84. [13] A.V. Ramaswamy and S. Sivasanker, Catal. Lett., 22 (1993) 239. [14] V. Hulea, R Moreau and E Di Renzo, J. Mol. Catal., 111 (1996) 325. [15] W. Burkart, N.L. Allinger, ACS Monograph, 177, 1982. [16] R Moreau, A. Finiels, R Geneste, J. Joffre, E Moreau and J. Solofo, Catal. Today, 31 (1996) 11. [17] A. Corma, M.A. Camblor, R Esteve, A. Martinet and J. Perez-Pariente, J. Catal., 145 (1994) 151. [18] M.A. Camblor, A. Corma and J. Perez-Pariente, Zeolites, 13 (1993) 82. [19] V. Hulea, R Moreau, E Di Renzo, Stud. Surf. Sci. Catal., in press. [20] V. Hulea, P. Moreau, J. Mol. Catal., 113 (1996 499.