Artigo 2 - Oxidação de alcanos com Alumina e Argilas

7/28/2019 Artigo 2 - Oxidao de alcanos com Alumina e Argilas

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Hydrogen Peroxide Oxygenation of Saturated and UnsaturatedHydrocarbons Catalyzed by Montmorillonite or Aluminum Oxide

Dalmo Mandelli Anielle C. N. do Amaral Yuriy N. Kozlov Lidia S. Shulpina Anderson J. Bonon Wagner A. Carvalho Georgiy B. Shulpin

Received: 9 June 2009 / Accepted: 13 July 2009

Springer Science+Business Media, LLC 2009

Abstract Montmorillonites K-10, Na0.60K0.12Ca0.02(Al1.78

Fe0.12Mg0.10)oct(Si3.89Al0.11)tetO10(OH)2, and NT-25, Na0.10K0.04Ca0.08(Al1.28Fe0.45Mg0.28)

oct(Si3.94Al0.06)tetO10(OH)2,

and aluminum oxide, Al2O3, catalyze alkane hydroperox-

idation and olefin epoxidation with hydrogen peroxide.

Alkanes afford alkyl hydroperoxides as main primary

products which partially decompose to produce corre-

sponding ketones and alcohols. The oxidation of cis- and

trans-isomers of 1,2-dimethylcyclohexane on K-10 pro-

ceeds stereoselectively. Benzene in the presence of

montmorillonite K-10 is transformed into phenol. Styrene

is oxidized to afford benzaldehyde and styrene epoxide. It

is proposed on the basis of a kinetic study and measure-

ments of the selectivity parameters that in the case of

alumina the oxidation occurs with the participation of

hydroxyl radicals generated from an aluminum peroxo

derivative on the catalyst surface. In the cases of mont-morillonites iron centers are also responsible for the H2O2activation which gives hydroxyl radicals. Addition of tri-

fluoroacetic acid (TFA) leads to the noticeable accelera-

tion of all oxidation reactions and enhancement of the

product yield. It has been shown that in the presence of

TFA some parts of solid catalysts are transferred into the

homogeneous solution. Thus, very cheap natural solid

materials provide soluble species that are active in effi-

cient homogeneous oxidations of hydrocarbons with

hydrogen peroxide. In the absence of TFA the oxidation

reactions in the case of the three catalysts are truly het-

erogeneous in nature.

Keywords Alkanes Alkyl hydroperoxides Aluminum oxide Benzene Heterogeneous catalysis Hydrogen peroxide Montmorillonite Phenol

1 Introduction

Hydroxylation and hydroperoxidation of saturated hydro-

carbons is one of important goals of contemporary catalytic

chemistry. It is known that derivatives of various transition

metals efficiently catalyze hydrocarbon oxygenation with

molecular oxygen and peroxides including cheap and

ecologically friendly hydrogen peroxide [16]. It should be

emphasized that alkanes are very inert organic compounds

and yields in their oxidations are usually low. Yields of

around 30% based on the alkane can be considered as high

[4, 5]. In the commercial oxidation of cyclohexane by air at

160 C with the participation of Co(II) as a catalyst [6] the

desired compounds, cyclohexanol and cyclohexanone,

constitute 85% of the products at only 4% conversion.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10562-009-0103-z ) contains supplementarymaterial, which is available to authorized users.

D. Mandelli A. C. N. do Amaral A. J. BononFaculdade de Qumica, Pontifcia Universidade Catolica de

Campinas, Campus I, Rod. D. Pedro I, km 136, Pq. das

Universidades, Campinas, SP 13086-900, Brazil

Y. N. Kozlov G. B. Shulpin (&)

Semenov Institute of Chemical Physics, Russian Academyof Sciences, Ulitsa Kosygina, dom 4, 119991 Moscow, Russia

e-mail: [email protected]; [email protected]

L. S. Shulpina

Nesmeyanov Institute of Organoelement Compounds,

Russian Academy of Sciences, Ulitsa Vavilova, dom 28,

119991 Moscow, Russia

W. A. Carvalho

Centro de Ciencias Naturais e Humanas, Universidade Federal

do ABC, Rua Catequese, 242, Bairro Jardim, Santo Andre,

SP 09090-400, Brazil

123

Catal Lett

DOI 10.1007/s10562-009-0103-z
http://dx.doi.org/10.1007/s10562-009-0103-zhttp://dx.doi.org/10.1007/s10562-009-0103-z


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These valuable compounds are both also susceptible to

further oxidation, and at higher conversion the selectivity

will dramatically drop.

Solid aluminum-containing compounds, for example,

montmorillonites [713], aluminum oxide and some other

[1422], are widely used as supports for transition metal

complexes which in some cases are efficient catalysts in

transformations of organic compounds. Much less is knownabout reactions of alkanes and olefins in solutions cata-

lyzed by compounds of non-transition metals [2329].

For example, alumina has been demonstrated to catalyze

olefin epoxidation with hydrogen peroxide in ethyl acetate

[3038]. Aluminum nitrate has been recently used as a

catalyst for olefin epoxidation with H2O2 in ethyl acetate as

a solvent [39] and alkane hydroperoxidation in acetonitrile

[40]. Aluminum derivatives have been used to promote the

BaeyerVilliger oxidation [4143]. In the present work, we

have found for the first time that both montmorillonite and

aluminum oxide without any additives catalyze oxidation

of saturated as well as some unsaturated hydrocarbons.

2 Results and Discussion

In the present work, we studied for the first time alkane

oxidation with H2O2 catalyzed under relatively mild con-

ditions by solid aluminum derivatives (aluminum oxide

and montmorillonites).

2.1 Catalysis by Montmorillonites

We have found that montmorillonite K-10, Na0.60K0.12-Ca0.02(Al1.78Fe0.12Mg0.10)

oct(Si3.89Al0.11)tetO10(OH)2, i s a

relatively efficient catalyst for oxygenation of alkanes by

H2O2 in acetonitrile at 70 or 50 C. The oxidation of

cyclohexane and cyclooctane gave mainly the corre-

sponding alkyl hydroperoxides (Figs. 1, 2). Dependences

of the initial reaction rate on initial amounts of the catalyst

(Fig. 3) and cyclooctane (Fig. 4) have been determined.

Addition of water to 70% aqueous hydrogen peroxide leads

to some decrease in the initial oxidation rate (determined as

the yield after 15 min) and the total yield after 120 min

(Table S1). Some non-productive decomposition of

hydrogen peroxide to molecular oxygen and water has been

noticed (Fig. S3). Selectivity parameters measured for

oxidation of various saturated hydrocarbons on the mont-

morillonite K-10 catalyst are summarized in Table S2

(entry 1). It can be seen that these parameters are close to

the values obtained previously [4658] for the oxidations

by the systems that operate with the participation of

hydroxyl radicals (entries 39) and are different from that

obtained for the systems which are believed to operate via

other mechanisms (entries 1015). Cyclohexanol under

similar conditions was oxidized to cyclohexanone in yield

17% after 2 h (Fig. S4, curve 1). Cyclooctene was epoxi-

dized in acetonitrile solution at 70 C to afford the epoxide

25105 150 20

0

2

1

Amount/mmol

3

0.02

0.04

0.06

0.08

0.10

Time/ min

30

0.12

Fig. 1 Accumulation of the products (cyclohexyl hydroperoxide,

curve 1; cyclohexanol, curve 2; cyclohexanone, curve 3) in oxidation

of cyclohexane (2.7 mmol) with hydrogen peroxide (5.2 mmol) in the

presence of montmorillonite K-10 (50 mg) in MeCN at 70 C.

Amounts of cyclooctanone and cyclooctanol were determined twice,

before and after reduction of the aliquots with solid PPh 3 (for this

method, see Refs. [5, 44, 45]). Maximum total yield of oxygenates

(after 15 min) was 6% based on cyclohexane

1004020 600 80

0

2

1

Amount/mmol

3

0.04

0.08

0.12

0.16

Time/ min

120

Fig. 2 Accumulation of the products (cyclooctyl hydroperoxide,

curve 1; cyclooctanol, curve 2; cyclooctanone, curve 3) in oxidationof cyclooctane (2.5 mmol) with hydrogen peroxide (5.0 mmol) in the

presence of montmorillonite K-10 (50 mg) in MeCN at 50 C.



method, see Refs. [5, 44, 45]). Maximum total yield of oxygenates

(after 120 min) was 6% based on cyclooctane

D. Mandelli et al.

123


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in yield 21% after 5 h (Fig. S4, curve 2). It is noteworthy

that addition of trifluoroacetic acid (TFA) in a small

amount noticeably accelerates the alkane oxidation and

enhances the yield of the oxygenates (Fig. 5).

Benzene (0.2 mmol) after heating for 10 h at 70 C

with 35% aqueous H2O2 (0.2 mmol), trifluoroacetic acid

(0.1 mmol), acetonitrile (0.6 mL), and montmorillonite

K-10 (20 mg) was transformed into phenol (0.06 mmol,yield 3%). Styrene turned out to be more reactive: heating

for 3 h at 70 C a mixture of styrene (1.7 mmol), 35%

aqueous H2O2 (0.2 mmol), TFA (0.1 mmol), acetonitrile

(0.6 mL), and montmorillonite (20 mg) gave benzaldehyde

(0.14 mmol; yield 8.2%), styrene epoxide (0.04 mmol;

yield 2.4%), and acetophenone (0.008 mmol; yield 0.5%).

The reaction under the same conditions in the absence of

TFA gave only benzaldehyde in 2.1% yield. The oxidation

by peroxyacetic acid instead of H2O2 afforded benzalde-

hyde in 3% yield.

We have also found that another type of montmoril-

lonite, NT-25, Na0.10K0.04Ca0.08(Al1.28Fe0.45Mg0.28)oct

(Si3.94Al0.06)tetO10(OH)2, catalyzes alkane oxidation with

hydrogen peroxide. Initial rate of cyclooctane oxygenation

and the yield of the oxygenates after 120 min (see Table 1)

are ca. 2 times higher than in the reaction with montmo-

rillonite K-10 (see Fig. 2).

In order to check if there some leaching of solid catalyst

K-10 to the reaction solution we fulfilled the following

experiment. A mixture of K-10 (20 mg), H2O2 (70%

aqueous), and acetonitrile (4.7 mL) was stirred at 60 C for

30 min and then the solid catalyst was filtered off at 60 C.

Cyclooctane (2.5 mmol) was added to the homogeneous

solution and the reaction was continued at 60 C for 3 h.

The amount of oxygenates was only 0.0025 mmol (yield

0.1% based on cyclooctane). However, when a mixture of

K-10, H2O2, and acetonitrile was incubated for 30 min at

60 C in the presence of TFA (1 mL) yield of oxygenates

was 13% after 3 h (Fig. S5, curve 1). It is interesting that

when the incubation was in the absence of H2O2 (Fig. S5,

curve 2) the yield was even higher (32%). This can be due

to some decomposition of H2O2 during incubation period.

In another experiment, in the presence of TFA, solid cat-

alyst K-10 was filtered off from the reaction solution after

15 min and the reaction continued further (Fig. S6, Graph

A, curve 1). The reaction proceeded exactly as in the

experiment without filtering off the catalyst (compare with

curve 2 in Fig. S6, Graph A). If TFA is absent the oxidation

reaction stops immediately after filtering off the catalyst

(Fig. S6, Graph B). Thus, we can conclude that TFA

1004020 600 80

0

2

1Amount/mmol

0.02

0.04

0.06

0.08

Amount of montmorillonite/ mg

0

0.16

0.12

0.08

0.04

0.20

Amou

nt/mmol

Fig. 3 Oxidation of cyclooctane (2.5 mmol) with hydrogen peroxide

(5.0 mmol) catalyzed by montmorillonite K-10 in MeCN at 50 C.

Dependences of total yield of oxygenates after 3 min (mmol;

corresponds to the initial rate of oxygenate formation), curve 1, and

total amount of oxygenates (mmol) after 120 min (curve 2) onamount of montmorillonite are shown. Amounts of cyclooctanone and

cyclooctanol were determined after reduction of the aliquots with

solid PPh3. For the original full kinetic curves, see Fig. S1

521 30 4

0

2

1Amount/mmol

3

0.01

0.02

0.03

0.04

Amount of cyclooctane/ mmol

0

0.4

0.3

0.2

0.1

0.5

Amount/mmol

6 7

0.05

0.06

%

10

5

0

1/[cyclooctane]0 / mmol150

1/A3/M1

4321

0

20

40

60

80

A

B

Fig. 4 Oxidation of cyclooctane with hydrogen peroxide (5 mmol)

catalyzed by montmorillonite K-10 (20 mg) in MeCN at 50 C.

Graph A: Dependences of total yield of oxygenates after 3 min

(mmol; corresponds to the initial rate of oxygenate formation), curve

1, and total amount of oxygenates (mmol, curve 2, and % based on

cyclooctane, curve 3) after 120 min on initial amount of cyclooctane

are shown. Amounts of cyclooctanone and cyclooctanol were

determined after reduction of the aliquots with solid PPh3. For the

original full kinetic curves, see Fig. S2. Graph B: Linearization of

dependence 1 using coordinates 1/DA3 - 1/[cyclooctane]0 where

DA3 is the concentration (M) of products formed during first 3 min of

the reaction

Hydrogen Peroxide Oxygenation of Saturated and Unsaturated Hydrocarbons

123


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extracts soluble catalytically active (most probably iron-

containing) species from K-10 and NT-25 into the homo-

geneous solution. In the absence of TFA the catalytic

reaction is truly heterogeneous in nature. Some loss of

activity upon catalyst recycling has been found only for the

second cycle. Thus, in the cyclooctane oxidation (the

reaction at 70 C; for other conditions, see caption to

Fig. S6) with fresh sample of K-10 the yield of oxygenates

was 13% based on cyclooctane after 1 h. The catalyst was

filtered off, washed with acetonitrile and dried in air

(70 C, 4 h). The yield of oxygenates was 5.9%. It is

noteworthy, however, that the third, fourth and fifth cycles

gave the same yields: 5.4, 6.0, and 5.3%, respectively.

Thus, the catalyst can be easily isolated from the reaction

mixture and re-used many times without loss of activity.

2.2 Catalysis by Aluminum Oxide

Alumina is less efficient catalyst for the alkane oxidation

(compare Figs. 6 and 1). In this case, however, the reaction

is more selective: cyclooctyl hydroperoxide is formed as

the sole product. The initial rate is proportional to the

amount of the catalyst (Fig. 7) and does not depend on the

Table 1 Oxidation of cyclooctane with H2O2 catalyzed by montmo-

rillonite NT-25

Time

(min)

Cyclooctanone

(mmol)

Cyclooctanol

(mmol)

Yield (based on

cyclooctane; %)

1 0 0.025 0.9

2 0 0.047 1.7

3 0 0.061 2.3

15 0 0.16 5.9

60 0.03 0.22 9.3

120 0.04 0.24 10.4

Conditions Cyclooctane, 2.7 mmol; H2O2, 5.7 mmol; montmorillonite

NT-25, 20 mg; acetonitrile, up to 5 mL; 50 C. Concentrations of the

products were measured after reduction with PPh3

30012060 1800 240

0

2

1

Amount/mmol

0.01

0.02

0.03

0.04

Time/ min

360

Fig. 6 Accumulation of cyclooctyl hydroperoxide, in the oxidation

of cyclooctane (2.5 mmol) with hydrogen peroxide (5.0 mmol) in the

absence (curve 1) and in the presence of aluminum oxide (100 mg;

curve 2) in MeCN at 80 C. Amounts of cyclooctanone and

cyclooctanol were determined twice, before and after reduction of

the aliquots with solid PPh3 (for this method, see Refs. [5, 44, 45])

500200100 3000 400

0

2

1

Amount/mmol

0.005

0.010

0.015

0.020

Amount of aluminum oxide / mg

0

0.08

0.04

Amount/mmol

0.025

0.030


(5.0 mmol) catalyzed by aluminum oxide in MeCN at 80 C.



total amount of oxygenates (mmol) after 300 min (curve 2) on

amount of aluminum oxide are shown. Amounts of cyclooctanone and



1004020 600 80

0

2

1Amount/mmol

0.1

0.2

0.3

0.4

Time/ min

120


(5 mmol) catalyzed by montmorillonite K-10 (20 mg) in MeCN at

50 C in the absence (curve 1; yield was 6.4% after 120 min) and in

the presence of trifluoroacetic acid (0.044 g; curve 2; yield was 15.3%

after 120 min). Total amounts of oxygenates (cyclooctanol ? cyclo-

octanone) are shown. Amounts of cyclooctanone and cyclooctanol

were determined after reduction of the aliquots with solid PPh3

D. Mandelli et al.

123


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amount of cyclooctane at its amount[3 mmol (Fig. 8).

Similarly, the W0 value does not depend on the amount of

H2O2 in the interval 28 mmol (Fig. 9). The selectivity

parameters in this case are also low (Table S2, entry 2).

The reaction is accelerated if trifluoroacetic acid is added

in low concentration (Fig. 10).

In the leaching experiments, a mixture of alumina,

hydrogen peroxide and acetonitrile was incubated for 2 h.After that the solid catalyst was filtered off at 80 C,

cyclooctane and additional amount of H2O2 was added and

the homogeneous solution and the reaction continued for

additional 5 h the yield of the oxygenates was only 1%

(Fig. S11, curve 1). When the incubation was performed in

the presence of TFA the oxidation occurred even in the

absence of the solid alumina (Fig. S11, curve 2) giving the

oxygenates in 12% yield after 5 h.

2.3 Mechanistic Consideration

The experimental data discussed above and the selectivity

parameters summarized in Table S2 indicate that both

systems, H2O2/montmorillonite and H2O2/aluminum

oxide, generate hydroxyl radicals which induce the

hydrocarbon oxygenation. The results of kinetic study (the

dependences of the initial reaction rates on the initial

521 30 4

Amount of cyclooctane/ mmol

0

0.04

0.03

0.02

0.01

0.05

Amount/mmol

2

1

Amount/mmol

3

%

2

1

00

0.002

0.004

0.006

0.008

0.010

0.012

1/[cyclooctane]0 / mmol1100

1/A15/M1

8642

0

100

200

300

400

B

A

500

Fig. 8 Oxidation of cyclooctane with hydrogen peroxide (5 mmol)

catalyzed by aluminum oxide (100 mg) in MeCN at 80 C. Graph A:



total amount of oxygenates (mmol, curve 2, and % based on

cyclooctane, curve 3) after 300 min on initial amount of cyclooctane

are shown. Amounts of cyclooctanone and cyclooctanol were

determined after reduction of the aliquots with solid PPh3. For the

original full kinetic curves, see Fig. S9. Graph B: Linearization of

dependence 1 using coordinates 1/DA15 - 1/[cyclooctane]0 whereDA15 is the concentration (M) of oxygenates formed during first

15 min of the reaction

42 60 8

0

2

1

A

mount/mmol

0.002

0.004

0.006

0.008

Amount of H2O2 / mmol

0

0.08

0.04

Amount

/mmol


catalyzed by aluminum oxide (100 mg) in MeCN at 80 C. Depen-

dences of total yield of oxygenates after 15 min (mmol; corresponds

to the initial rate of oxygenate formation), curve 1, and total amount

of oxygenates (mmol, curve 2) after 300 min on initial amount of

hydrogen peroxide are shown. Amounts of cyclooctanone and



30012060 1800 240

2

1Amount/mmol

0

0.05

0.10

0.15

0.20

Time/ min

360

Fig. 10 Accumulation of cyclooctyl hydroperoxide in the oxidation

of cyclooctane (2.5 mmol) with hydrogen peroxide (5 mmol) cata-

lyzed by aluminum oxide (100 mg) in MeCN at 80 C in the absence

(curve 1; yield was 1.2% after 300 min) and in the presence of

trifluoroacetic acid (0.044 g; curve 2; yield was 7.2% after 300 min).



method, see Refs. [5, 44, 45])


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amount of the alkane) presented in Figs. 4 and 8 are in a

good agreement with this proposal. The shapes of these

kinetic dependences for both systems are typical for the

case of competition between the alkane and (most proba-

bly) acetonitrile for the same oxidizing species which is

hydroxyl radical. This competition can be described by the

following simple kinetic scheme of the process:

H2O2=catalyst ! HO Wi i

HO MeCN ! products k1 1

HO RH ! R H2O k2 2

ROO e H ! ROOH 3

where Wi is the rate of the oxidizing species generation by

the H2O2/catalyst system. Acetonitrile is oxidized in accord

with Eq. 1 to afford formaldehyde, formic acid and CO2[46]. The analysis of the scheme given above in the steady-

state approximation relative to hydroxyl radicals gave the

following equation for oxidation rate Wof cyclooctane, RH:

W d RH

dt

d ROOH

dt

Wi

1 k1MeCN

k2 RH

4

The experimental data shown in Figs. 4 and 8 can be

described by Eq. 4 because satisfactory linear depen-

dences of W-1 on [cyclooctane]0-1 have been obtained

(see Graphs B in Figs. 4, 8) in accord with Eq. 4. We

accepted in these calculations that W= (DA3 9 200)/

(3 9 60) M s-1 for the montmorillonite-catalyzed oxida-

tion and W= (DA15 9 200)/(15 9 60) M s-1 for the case

of aluminum oxide, where DA3 and DA15 are the amounts

of oxygenates produced during the first 3 or 15 min,respectively. It follows from these linear dependences that

under conditions described in captions to Figs. 4 and 8 the

rates of active species generation Wi are equal to

8.5 9 10-5 and 4.4 9 10-6 M s-1 for montmorillonite

K-10 and aluminum oxide, respectively. The k1[MeCN]/k2ratios were calculated to be 0.82 M for montmorillonite

K-10 at 50 C and 0.78 M for aluminum oxide at 80 C.

The values of the k1[MeCN]/k2 ratio for both heteroge-

neous catalysts as well as for soluble aluminum nitrate

[40] are close and this supports our kinetic model. At once

the hydroxyl radical generation rates Wi differ sufficiently

for these catalysts (Table 2).

The dependence of the initial reaction rate on initial

concentration of hydrogen peroxide for the catalysis

by aluminum oxide allowed us to roughly analyze the

model which involves the formation of an intermediate

peroxo derivative on the surface of alumina particles.

We conventionally denote this peroxo derivative as

(Al2O3H2O2)S:

H2O2 Al2O3 S) Al2O3 H2O2 0S K5 5

Al2O3 H2O2 0S! Al2O3 S2HO

6

In the frames of this proposal, the change in the alkane

oxidation rate is due to the change of the relative amount of

species (Al2O3H2O2)S that is the portion of aluminasurface H(H2O2) occupied by the peroxo derivative. We

can easily obtain in the quasi-equilibrium approximation

the following equation:

H H2O2 K5 H2O2

1 K5 H2O2 7

From Eq. 1 = K5[H2O2] we derive the condition when the

reaction rate is equal to the half of the maximum possible.

It follows from the data of Fig. 9 that the value

Wi = Wmax is attained at [H2O2] & 0.2 M, and it means

that K5 & 5 M-1.

As alumina samples used in the oxidations contained

only trace amounts of transition metals (Fe\0.01%) we

can propose that in the case of alumina hydroxyl radicals

are generated with the participation of an aluminum

peroxo derivative [59] although the mechanistic details

are not clear. The hydroxyl radicals attack the alkane to

generate the alkyl radical, R

, which rapidly reacts withmolecular oxygen: R ? O2 ? ROO

. The reduction of

the alkylperoxyl radical leads to the formation the main

primary product of the reaction, alkyl hydroperoxide:

ROO ? H? ? e- ? ROOH. Compounds containing

double bonds are epoxidized by the peroxo aluminum

fragment: Olefin ? AlOOH? Olefin epoxide ?

AlOH. It should be noted that many details of this

mechanism are still unclear.

Oxidations with the participation of montmorillonites

proceed noticeably faster than in the case of alumina. As

both used montmorillonites contain sufficient amounts of

iron it is reasonable to assume that iron ions are responsiblefor the generation of hydroxyl radicals in this case. High-

valent ions can be reduced by hydrogen peroxide:

Fe(III) ? H2O2 ? Fe(II) ? HOO

? H?. Low-valent

iron reacts with the second H2O2 molecule producing

hydroxyl radicals: Fe(II) ? H2O2 ? Fe(III) ? HO-?

HO. The reaction with montmorillonite K-10 exhibits

some stereoselectivity: predominant formation of the

alcohol with cis-orientation of the methyl groups (the

trans/cis ratio is 0.6) is noticed in the case of oxidation of

Table 2 Kinetic parameters for the cyclooctane oxidation with H2O2catalyzed by aluminum-containing compounds

Catalyst k1[MeCN]/k2(M)

106 9 Wi(M s

-1)

Montmorillonite K-10 at 50 C 0.82 17

Aluminum oxide at 80 C 0.78 4.4

Al(NO3)3 at 70 C 0.83 7.5

D. Mandelli et al.

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cis-1,2-DMCH. The oxidation of trans-1,2-DMCH gave

rise to the prevalence of the trans-alcohol (the trans/cis

ratio is 1.5). Oxidation reactions on mono- and polynuclear

iron centers in soluble [60, 61] and solid [7, 62] compounds

are well-known. In some cases reactions catalyzed by

compounds of iron and other transition-metals proceed

stereoselectively [6366]. The activation energy Ea value

measured (Fig. 11) for NT-25 (20 3) is lower than that

for K-10 (26 6 kcal mol-1).

3 Conclusions

In the present work, the first example of alkane oxidation

with H2O2 catalyzed by a solid non-transition metal

derivative (aluminum oxide) is described. The experi-

mental data indicate that in catalysis by montmorillonites,

there are iron ions which are responsible for the generation

of hydroxyl radicals from H2O2. The oxidation reactions

are truly heterogeneous in nature. It is known that stoi-

chiometric Fenton reagent (H2O2 ? FeII) and Fenton-type

catalytic systems containing FeIII ions [67, 68] in the

absence of N-ligands are not efficient in alkane oxidations

(low yields, tar formation, high catalase activity). In con-

trast, our systems which use natural iron-containing

montmorillonites allow us to oxidize alkanes very effi-

ciently in the presence of a small amount of TFA: the

product yields attain 32%, alkyl hydroperoxides are formed

as predominant products, hydrogen peroxide is not

decomposed non-productively to O2 and H2O. Such a

gentle behavior of the systems can be due to permanently

low concentration of iron ions in the solution. The solid

catalyst permanently feeds the reaction solution with cat-

alytically active iron ions. Evidently, the systems based on

very cheap montmorillonites can be used in industrial

oxidation of alkanes and other compounds.

4 Experimental

The chemical composition of montmorillonite K-10

(Fluka) was determined by X-Ray Fluorescence analysis

(spectrometer Philips, model PW2404): 65.34% SiO2,

12.89% Al2O3, 2.38% Fe2O3, 0.95% MgO, 0.52% TiO2,

0.24% CaO, 0.53% Na2O, 1.54% K2O, 7.85% P2O5. Loss

on ignition (LOI) was calculated from the difference in

weight before and after burning at 1,050 C of a sample

previously dried at 110 C (in order to remove remaining

moisture). LOI (loss on ignition at 1,050 K) was 8.06%.

For the BET surface area determination a Micrometrics

ASAP 2010 device was used to make adsorption-desorp-tion measurements at -196 C, under pressure ranging

from 10 to 925 mm Hg. Before each measurement the

samples were outgassed at 150 C and 1.3 9 10-3 Pa for

12 h. Specific area was 232 m2 g-1. The formula of one

half unit cell is Na0.60K0.12Ca0.02(Al1.78Fe0.12Mg0.10)oct

(Si3.89Al0.11)tetO10(OH)2. In this formula, Fe

2?, Mg2? and

Al3? are the isomorphically substituting cations, and Na?,

K? and Ca2? are the charge-compensating cations. The

symbols oct and tet are related to the octahedral and tet-

rahedral layers, respectively (see Fig. S12 [69]). Based on

the quantity of charge-compensating cations, the theoreti-

cal cation exchange capacity (CEC = 1.9 meq g-1) was

calculated. The experimental cation exchange capacity,

CECexp = 0.451 meq g-1 [70], of the original clay was

determined by the Cu-ethylenediamine method [71].

Scanning Electron Microscopy (the Leica-Zeiss micro-

scope LEO 440i coupled in an energy dispersive analyzer

of Si(Li) with Be window, Oxford 7060) showed (Fig. S13)

that the material contained some amount of large ([2 lm)

particles, which were removed by decantation.

The bentonite clay mineral containing montmorillonite

NT-25 as the main constituent (Bentonit Uniao Nordeste

S.A.) is originated from Campina Grande, Paraba State,

Brazil. The chemical composition obtained by X-Ray

Fluorescence analysis is 57.85% SiO2, 16.74% Al2O3,

8.78% Fe2O3, 2.73% MgO, 1.24% TiO2, 1.08% CaO,

0.73% Na2O, 0.43% K2O, 0.27% P2O5, and 10.10% LOI.

The formula of one half unit cell is Na0.10K0.04-Ca0.08(Al1.28Fe0.45Mg0.28)

oct(Si3.94Al0.06)tetO10(OH)2 with

a theoretical cation exchange capacity CEC = 0.8 -

meq g-1. The BET surface area is 139 m2 g-1. Diffracto-

grams of the two montmorillonites (a Shimadzu model

XD-3A diffractometer, with Cu Ka radiation; spectra were

3.23.02.9 3.1

3.5

logW0(

mm

olmin

1)

3.0

3.3

1

2.5

2.0

1.5

1.0

/K11000

T

2

Fig. 11 Arrhenius dependences of logW0 on 1/T for the cyclooctane

oxidations catalyzed by NT-25 (1) and K-10 (2) in the presence ofCF3COOH


123


8/9

registered between 2h = 2 and 40, with scanning speed

of 2 h min-1, using a cathode current of 25 mA and a

voltage of 35 kV) are shown in Fig. S14.

Size particle distributions of the samples of K-10 and

NT-25 are shown in Figs. S15 and S16, respectively. For

particle size distribution analysis, two different kinds of

techniques were employed: a laser diffraction and image

method. The laser diffraction analyses (in triplicate) werecarried out on the HORIBA LA-950 instrument; water was

used as dispersion medium, the circulation speed was 5 at

95.3% of transmittance without ultrasonic bath. The Por-

table FlowCAM VS.I (Fluid Imaging Technologies)

instrument was used for image particle analysis; water was

used as dispersion medium, with 109 optical amplification

lens and 300 lm flowcell. Photos of samples (shown in the

left sides of Figs. S15, S16) demonstrate that there is no

particle agglomerations.

Alumina (Fluka, type 507C, neutral, 100125 mesh,

pH = 7) was used. The surface area, obtained by BET

(Micrometrics ASAP 2010 instrument with nitrogen asprobe molecule), was 150 m2 g-1, and the average pore

size was 5.8 nm. X-ray diffraction (Shimadzu XD-3A

diffractometer) showed a Bohemite structure. Size particle

distribution of the samples is shown in Fig. S17. Photos of

samples (shown in the left side of Fig. S17) demonstrate

that there is no particle agglomerations. Energy dispersive

X-ray analysis (EDX, Philips CM30T electron microscope

with an LaB6 filament) showed that the Al2O3 only con-

tained traces of Si, Cl, Fe and S (\0.01%). Typically,

alumina was calcined prior the oxidation reaction by

heating at 500 C during 16 h.

The oxidations of hydrocarbons in acetonitrile were

carried out in air in thermostated Pyrex cylindrical vessels

with vigorous stirring. Hydrogen peroxide (usually 70%

aqueous; Peroxidos do Brasil) was used as received. The

total volume of the reaction solution was usually 5 mL.

Typically, the reaction started when the hydrogen peroxide

was added in one portion to the solution of the substrate

and the catalyst in MeCN. (CAUTION: the combination of

air and H2O2 with organic compounds at elevated tem-

peratures may be explosive!). In the case of alkanes,

samples of the reaction solutions were analyzed by GC.

The oxygenation of alkanes gives rise to the formation of

the corresponding alkyl hydroperoxides as the main prod-

uct. To demonstrate the formation of alkyl hydroperoxides

during this oxidation and to estimate its concentration in

the course of the reaction, we used a simple method

developed earlier by some of us [5, 44, 45]. In the kinetic

study of the cycloctane oxidation, we measured the con-

centrations of cyclooctanone and cyclooctanol only after

reduction with PPh3, because this reduction gives the most

precise values of total concentration of the products. Other

details are given in Figure captions. Concentrations of

products obtained in the oxidation of benzene and styrene

by 35% aqueous H2O2 were measured using1H NMR

method (solutions in acetone-d6; Bruker AMX-400

instrument, 400 MHz), signals were integrated using added

1,4-dinitrobenzene as a standard.

Acknowledgments This work was supported by the State of Sa o

Paulo Research Foundation (Fundacao de Amparo a Pesquisa doEstado de Sao Paulo, FAPESP; grants Nos. 2005/51579-2, 2006/

03996-6), the Brazilian National Council on Scientific and Techno-

logical Development (Conselho Nacional de Desenvolvimento

Cientifico e Tecnologico, CNPq, Brazil; grants Nos. 478165/2006-4,

305014/2007-2) and the Russian Foundation for Basic Research

(grant No. 06-03-32344-a). The authors thank the Radchrom Analtica

for help in particle size distribution analysis, Mr. Rafael Bogarin and

Miss Cristhiane Capelazzo for their help in the experiments. L. S. S.

and G. B. S. express their gratitude to the FAPESP (grant No. 2006/

03984-8), the CNPq (grant No. 300601/01-8), and the Faculdade de

Qumica, Pontifcia Universidade Catolica de Campinas for making it

possible for them to stay at this University as invited scientists and to

perform a part of the present work.

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Artigo 2 - Oxidação de alcanos com Alumina e Argilas