Heterogeneous photocatalysis for synthetic purposes: Oxygenation of cyclohexane with H3PW12O40 and (HBu4N)4W10O32 supported on silica

Article abstract of DOI:10.1039/a901051c

Heterogenisation of H₃PW₁₂O₄₀ and (nBu₄N)₄W₁₀O₃₂ with silica provides new photocatalytic systems able to oxygenate cyclohexane with O₂ without causing the oxidative mi

Full text of DOI:10.1039/a901051c

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Heterogeneous photocatalysis for synthetic purposes: oxygenation
of cyclohexane with H PW O and (nBu N) W O supported on
3
12 40
4
4
10 32
silica
Alessandra Molinari, Rossanno Amadelli, Leonardo Andreotti and Andrea Maldotti*
Dipartimento di Chimica, Centro di Studio su Fotoreattività e Catalisi del C.N.R.,
Università degli Studi di Ferrara, Via L. Borsari 46, 44100, Ferrara, Italy.
E-mail: mla@dns.unife.it
Received 8th February 1999, Accepted 26th February 1999
Heterogenisation of H PW O and (nBu N) W O with
W O /SiO photocatalyst does not cause any mineralisation
10 32 2
of the substrate, while maintaining a comparable efficiency for
the reaction under investigation.
3
12 40
4
4
10 32
silica provides new photocatalytic systems able to oxygen-
ate cyclohexane with O2 without causing the oxidative
mineralization processes which typically occur on the
surface of the photosensitive semiconductor TiO2.
H PW O was heterogenised on silica following the previ-
3
12 40
ously described† ‘impregnation’ procedure which results in the
fixation of the polyoxoanion to the support, possibly through
6
Catalytic oxidation of unactivated C–H bonds under mild con-
ditions (room temperature and atmospheric pressure), using
electrostatic interactions with the protonated silica surface.
We successfully applied the same method to support
environmentally friendly reagents such as O , is a field of ever
(nBu N) W O32.† Since this compound is present in the form
4 4 10
7
2
1
growing interest. In this context, a number of authors are
of cation–anion aggregates in organic solvent, it is likely that it
is adsorbed on silica as an ionic couple, with the tetraalkyl-
ammonium cations acting as a bridge between the surface and
the decatungstate anion. This interpretation is supported by
previous investigations on the modes of polyoxomolybdate
investigating photoexcited polyoxometalates, which exhibit a
noticeable activity in the oxidation of numerous organic com-
2
pounds, including saturated hydrocarbons. Some of our recent
contributions in this research area deal with the oxygenation of
alkanes photocatalysed by (nBu N) W O and by H PW O
12 40
8
adsorption on silica, as well as by infrared spectra, which
4
4
10 32
3
3
ϩ
4Ϫ
in homogeneous solution.
reveal the presence of (nBu
surface of the support.‡
4
N) cations and W10
O
32 on the
In view of the interest in the heterogenisation of polyoxo-
4
metalates, the photocatalytic activity for cyclohexane oxid-
Table 1 reports the photocatalytic properties of the so
obtained heterogeneous systems in the oxygenation of cyclo-
hexane at 20 ЊC and 760 Torr of O , in different dispersing
media and at excitation wavelengths chosen on the basis of
the absorption spectra of the two polyoxotungstates§
(entries 1,2,6,7). Table 1 also shows some data previously
obtained in homogeneous solution (entries 3 and 8), where
the cyclohexane photooxidation quantum yields are 0.35 at
ation of the two mentioned polyoxotungstates supported on
silica is here reported for the first time. We demonstrate that
both supported polyoxotungstates present the important
advantage of being more easily handled than in the homo-
geneous phase. An important aspect is that their photocatalytic
activity can be investigated in media where they are insoluble.
Specifically, we could investigate both of them in pure cyclo-
hexane and observe a quite good selectivity for the ketone
formation in the case of H PW O /SiO . Since we describe
2
325 nm and 0.03 at 254 nm for (nBu
N)
40, respectively. Finally, entries 4 and 5 report the
results obtained using TiO powder dispersions to induce
W O32 and
4
4 10
3
H PW12O
3
12 40
2
3
heterogeneous photosensible systems, the comparison with a
2
dispersed semiconductor such as TiO becomes natural. Con-
cyclohexane photooxidation, according to previously reported
2
5
9
trary to this well investigated semiconductor, the (nBu N) -
investigations.
4
4
Table 1 Photocatalytic oxidation of cyclohexane by O (760 Torr)
2
Excitation
a
Solvent or
dispersing
medium
wavelengths/
nm (irradiation
time/min)
Product ratio
Alcohol ϩ
ketone/
µmol
a
Photocatalytic
system
cyclohexanone:
cyclohexanol
CO2
µmol
/
Entry
b
1
2
(nBu N) W O /SiO
C H
12
λ > 280 (90)
λ > 280 (90)
1
1
<0.5
<0.5
22
36
4
4
10 32
2
2
6
b
(nBu N) W O /SiO
C H /CH Cl
4
4
10 32
6
12
2
2
1
/1
c
3
3
(nBu N) W O
CH Cl /C H /CH CN
λ > 280 (90)
1
<0.5
25
4
4
10 32
2
2
6
12
3
6
C H
C H /CH Cl
/3/1
d
9
4
5
TiO29
λ > 280 (90)
λ > 280 (90)
5.7
1.5
3
5
23
42
6
12
d
TiO2
6
12
2
2
1
C H
C H /CH Cl
1
CH Cl /C H /CH CN
6
/1
6
b
6
7
H PW O /SiO
λ = 254 (360)
λ = 254 (360)
2.3
1.1
<0.5
<0.5
4
4
3
12 40
2
12
b
H PW O /SiO
3
12 40
2
6
12
2
2
/1
c
3
8
H PW O
λ = 254 (360)
1
<0.5
6
3
12 40
2
2
6
12
3
/3/1
a
Analysis of cyclohexanol and cyclohexanone was carried out by gas chromatography and carbon dioxide was determined by a turbidimetric
b
Ϫ1
method using an absorbing solution of Ba(OH) in glycerol. Reported values are ±10ꢀ. 3 ml of dispersing medium containing 15 g l of supported
polyoxotungstate. 1.45 µmol of irradiated polyoxotungstate. 3 ml of solution 2 × 10 mol dm . 0.6 µmol of irradiated polyoxotungstate. CH CN
is necessary for the dissolution of the catalyst. 3 ml of dispersing medium containing 3 g l of TiO2.
2
c
Ϫ4
Ϫ3
3
d
Ϫ1
J. Chem. Soc., Dalton Trans., 1999, 1203–1204
1203

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In every case, photoexcitation of the two polyoxotungstates
leads to the oxidation of cyclohexane to cyclohexanol and
cyclohexanone as the main stable products (more than 90ꢀ of
the overall oxidised alkane). The ketone to alcohol concen-
tration ratio is always close to one except in entry 6 where
Notes and references
†
0.1 g of catalyst H PW O or (nBu N) W O was dissolved in a
3 12 40 4 4 10 32
suitable solvent (H O and CH CN, respectively) and then 1 g of
2
3
colloidal silica (0.012 µm, Strem Chemicals) was added. After stirring
and evaporation of the solvent, the obtained powder contained about
10ꢀ (w/w) of polyoxotungstate.
the H PW O /SiO presents a good selectivity for the ketone
3
12 40
2
formation. Iodometric analysis indicates that hydroperoxides,
which are proposed to be the primary products during the
‡
Infrared spectra of (nBu N) W O /SiO were recorded in KBr using
4 4 10 32 2
ϩ
a diffuse reflectance accessory. nBu N : stretching of –CH and –CH
4
3
2
4,10
Ϫ1
Ϫ1
oxygenation of alkanes by illuminated (nBu N) W O , are
(2963 and 2875 cm ), and bending of C–H (1481 and 1383 cm ).
4– Ϫ1
4
4
10 32
present only in negligible amounts (less than 5ꢀ of the overall
oxidized cyclohexane). As far as the stability of the photo-
catalysts is concerned, it is noteworthy that they can be used for
subsequent cycles of oxidation without any release of the poly-
oxotungstates during the experiments, and without any loss of
their photocatalytic activity.
W
10
O32 : 958, 890, 802 cm
.
§
Irradiation of (nBu N) W O /SiO and of TiO was performed with
4
4
10 32
2
2
a Hanau Q 400 Hg lamp, using a suitable cut-off filter (λ > 280 nm; 15
mW cm ), while H PW O /SiO system was irradiated with a Hg low
Ϫ2
3
12 40
2
Ϫ2
pressure lamp (λ = 254 nm; 3 mW cm ). All the experiments were
carried out at 20 ± 1 ЊC under oxygen at 760 Torr.
It is to be noted that entries 1–5 in Table 1 allow a semiquanti-
1
A. Bielanski and J. Haber, Oxygen in Catalysis, Marcel Dekker,
New York, Basel, Hong Kong, 1991; A. Kroty and J. P. Kingsley,
Chemtech, 1996, 39.
tative comparison of the photocatalytic properties of (nBu N) -
4
4
W O , (nBu N) W O /SiO and TiO because (i) excitation
10
32
4
4
10 32
2
2
wavelength range and irradiation time are the same; (ii) the
amounts of the photocatalysts have been chosen so as the maxi-
mum absorption of the incident light is the same for all the
systems (see notes b, c and d in Table 1). On this basis, we can
state that heterogenisation of (nBu N) W O inhibits only
partially its photocatalytic efficiency expressed as the ratio
between the value of the µmoles of oxidized cyclohexane,
reported in the last column of Table 1 (entries 2, 3), and the
µmoles of irradiated decatungstates: 25 in the heterogeneous
system and 41 µmol in the homogeneous one (see notes b, c and
d in Table 1).
2 M. T. Pope, Heteropoly and Isopoly Oxometalates, Springer Verlag,
Berlin, Heidelberg, New York, Tokyo, 1993; C. L. Hill and C. M.
Prosser-McCartha, Photosensitization and Photocatalysis using
Inorganic and Organometallic Compounds, Kluwer Academic
Publishers, Dordrecht, 1993.
4
4
10 32
3
A. Maldotti, A. Molinari, P. Bergamini, R. Amadelli, P. Battioni
and D. Mansuy, J. Mol. Catal., 1996, 113, 147; A. Maldotti,
A. Molinari, R. Argazzi, R. Amadelli, P. Battioni and D. Mansuy,
J. Mol. Catal., 1996, 114, 141; A. Maldotti, R. Amadelli, V. Carassiti
and A. Molinari, Inorg. Chim. Acta, 1997, 256, 309; A. Molinari,
A. Maldotti, R. Amadelli, A. Sgobino and V. Carassiti, Inorg. Chim.
Acta, 1998, 272, 197.
4
N. Mizuno and M. Misono, Chem. Rev., 1998, 98, 199; L. K.
Volkova, E. S. Rudakov and V. P. Tretyakov, Kinet. Katal., 1996, 37,
Despite a different product distribution, both in cyclohexane
entries 1, 4) and in mixed solvent (entries 2, 5), the µmoles of
(
5
40; 1995, 36, 373.
oxidised cyclohexane with (nBu N) W O /SiO are very close
4
4
10 32
2
5
6
M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann,
Chem Rev., 1995, 95, 69; P. Pichat, Catal. Today, 1994, 19, 313.
Y. Wu, X. Ye, X. Yang, X. Wang, W. Chu and Y. Hu, Ind. Eng.
Chem. Res., 1996, 35, 2546.
to those obtained with TiO . Although a quantitative com-
2
parison between the two heterogeneous systems is difficult, it
is reasonable to speculate that the photoactive centres on the
heterogeneous polyoxotungstate catalyst are sensibly lower
than for an equal amount of TiO . This makes the (nBu N) -
7 M. Fournier, R. Thouvenot and C. Rocchiccioli-Deltcheff, J. Chem.
Soc., Faraday Trans., 1991, 87, 349.
2
4
4
W O /SiO system even more interesting from an efficiency
8 C. Rocchiccioli-Deltcheff, M. Amirouche, M. Che, J. M. Tatibout
and M. Fournier, J. Catal., 1990, 125, 292; A. Chemseddine,
C. Sanchez, J. Livage, J. P. Launay and M. Fournier, Inorg. Chem.,
10
32
2
point of view. Another reason that makes the heterogenised
decatungstate very promising for applied synthetic purposes is
that, in contrast to TiO it does not induce any mineralisation
1
984, 23, 2609.
P. Boarini, V. Carassiti, A. Maldotti and R. Amadelli, Langmuir,
998, 14, 2080; R. Amadelli, M. Bregola, E. Polo, V. Carassiti and
2,
9
process of the substrate. In fact, it photocatalyses the oxygen-
ation of cyclohexane to cyclohexanone and cyclohexanol with-
out the formation of carbon dioxide.
1
A. Maldotti, J. Chem. Soc., Chem. Commun., 1992, 1355; W. Mu
and J. M. Pichat, Catal. Lett., 1989, 3, 73.
1
0 L. P. Ermolenko and C. Giannotti, J. Chem. Soc., Perkin Trans. 2,
996, 1205; L. P. Ermolenko, J. A. Delaire and C. Giannotti,
J. Chem. Soc., Perkin Trans. 2, 1997, 25.
1
Acknowledgements
This research was supported by M.U.R.S.T and C.N.R. (project
9
5/95-5ꢀ).
Communication 9/01051C
1
204
J. Chem. Soc., Dalton Trans., 1999, 1203–1204