4-Benzoylbenzoate intercalated in layered double hydroxides: A new catalyst for photo-oxidation of sulfides in solution and in the gas phase

Article abstract of DOI:10.1016/j.tetlet.2004.03.149

A new mixed organic-inorganic photosensitizer, based on 4-benzoylbenzoate intercalated into a layered double hydroxide has been prepared, characterized and successfully tested for the photo-oxidation of dialkylsulfides, both in acetonitrile and in the gas phase.

Full text of DOI:10.1016/j.tetlet.2004.03.149

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 4047–4050
4
-Benzoylbenzoate intercalated in layered double hydroxides:
a new catalyst for photo-oxidation of sulfides in solution
and in the gas phase
Thierry Pigot, Thomas Arbitre, Herv ꢀe Martinez and Sylvie Lacombe*
UMR CNRS 5624, Universit ꢀe de Pau et Pays de l’Adour, CURS, BP 1155, 64013 Pau Cedex 13, France
Received 3 March 2004; revised 23 March 2004; accepted 26 March 2004
Abstract—A new mixed organic–inorganic photosensitizer, based on 4-benzoylbenzoate intercalated into a layered double
hydroxide has been prepared, characterized and successfully tested for the photo-oxidation of dialkylsulfides, both in acetonitrile
and in the gas phase.
Ó 2004 Elsevier Ltd. All rights reserved.
Photo-oxidation of sulfides is a well documented and
1
trans-cinnamate and 4-benzoyl benzoate ions in sus-
pension resulted in efficient formation of a syn head-to-
head dimer. The synthesis of the material involved anion
exchange or reconstruction from commercial LDH,
without any detailed characterization of the intercal-
extensively studied reaction. At short wavelength, the
2
reaction is achieved through photolysis or, at longer
wavelengths, sensitized with singlet oxygen or electron
3;4
5;6
transfer photosensitizers. In the latter case, benzo-
phenone derivatives (either soluble or grafted on silica)
already proved to be efficient photocatalysts, both in
11;12
ation or adsorption.
5;7
water and in acetonitrile solutions. Grafted silicas
have several advantages like easier separation from
reaction products and reusability. However, chemical
grafting presents some disadvantages: multistep syn-
thesis has to be performed and the amount of grafted
Generally speaking, numerous examples of insertion of
organic hosts in LDH are described in the recent liter-
13
ature, and the field of applications of such hybrid
organic–inorganic materials is still growing: catalysis of
14
organic reaction, adsorption of organic and inorganic
15
ꢀ
1
benzophenone remains low (0.5 mmol g ). Due to the
weak molar absorption coefficient of benzophenone at
pollutants,
controlled release of pharmaceutically
16
active compounds. In the field of catalytic oxidation,
sulfides were efficiently converted to sulfones by an
osmate molecular oxygen system immobilized on MgAl-
ꢀ
1
ꢀ1
3
50 nm (e ¼ 240 L mol cm ), large amounts of silica
grafted photosensitizers are needed to achieve high
photooxidation yields.
17
LDH, recently developed by Choudary et al.
Silica is not the only inorganic matrix used in the field of
supported photosensitizers. For example, polyoxomet-
We report here the preparation and characterization of
a benzophenone derivative, 4-benzoylbenzoate (4-BB)
intercalated in a MgAl-LDH by direct synthesis. This
material is used for the first time for the photo-oxidation
of di n-butylsulfide (DBS) in acetonitrile solution
(Scheme 1). Its photo-oxidizing ability towards gaseous
dimethyl sulfide (DMS) has also been studied at the gas-
solid interface.
8
9
allate or pthalocyanines supported photosensitizers
have been prepared by their intercalation in lamellar
materials like Mg–Al layered double hydroxides (MgAl-
LDH). However, the use of intercalated organic
10
photosensitizers has been rather seldom described. For
example, irradiation of a clay co-adsorbed mixture of
Sens, hν
RSR
RS(O)R + RS(O) R + R-S-S-R + RSO H + H SO
2 3 2 4
Keywords: 4-Benzoylbenzoate; Layered double hydroxide; Photooxi-
dation; Sulfide.
O2
1
2
3
4
5
*
Corresponding author. Tel.: +33-559-407-579; fax: +33-559-407-451;
e-mail: sylvie.lacombe@univ-pau.fr
Scheme 1. Photo-oxidation of di n-butylsulfide in acetonitrile.
0
040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.03.149

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T. Pigot et al. / Tetrahedron Letters 45 (2004) 4047–4050
1
1
20
00
c
b
8
0
0
0
0
6
4
2
a
1
651
589
539
1284
317
1
1
1
1404
0
4
000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1
)
Figure 1. FTIR spectra of: (a) nonintecalated LDH (HT0), (b) 4-BB/HT1, (c) pure 4-BB.
The materials 4-BB/HT1 and 4-BB/HT2, which differ by
the Mg/Al ratio, are prepared by direct synthesis at
controlled pH . Microanalyses are consistent with
extensive incorporation of the organic anion. The
amount of intercalated 4-BB does not differ significantly
ꢀ
1 18
between the two materials (1.91 and 1.81 mmol g )
despite the different Mg/Al ratio. In the following, all the
results are presented on 4-BB/HT1, as the other material
gives comparable spectra.
Nitrate anions are still present in the material as con-
ꢀ
1
firmed by the band at 1400 cm in the FTIR spectra in
KBr pellets (Fig. 1). Neither significant shift nor new
signals are observed relative to the spectra of pure 4-BB,
indicating that 4-BB is not grafted into the clay but only
intercalated. Direct evidence for intercalation is
obtained from XRD analysis. The parent LDH sample
exhibits an XRD pattern (Fig. 2a), characteristic of a
well-crystallized layered structure of hydrotalcite-like
à
type (JCPDS file No. 37-0630) . The d spacing calcu-
lated is 7.70 A ((0 0 3) line). The height of the interlayer
ꢁ
ꢁ
is estimated to be ca. 2.9 A as previously referred by
19
Miyata and the thickness of the brucite-like octahedral
ꢁ
hydroxide layer made with Mg and Al is 4.78 A from the
interlayer distance. The c (basal spacing) and a lattice
parameters (determined by least squares refinement) are,
ꢁ
respectively, 23.12 and 3.02 A. The XRD pattern of 4-
Figure 2. X-Ray diffraction pattern of (a) nonintercalated LDH
HT0), (b) 4-BB/HT1.
BB/HT1 (Fig. 2b) highlights the structural changes fol-
lowing the insertion. It clearly indicates the presence of a
new phase (associated with 4-BB intercalated in the in-
terlayer region) together with the hydroxyl phase.
(
Relative to the parent LDH pattern, the presence of
supplementary reflections (indexed by an asterisk on
Fig. 2) is noted. The (0 0 3) and (0 0 6) reflections are
shifted to the smaller angles, as previously reported for
2
0;21
1
00 mL of
Mg(NO , 6H
ratio of 2 (4-BB/HT1) or 2.5 (4-BB/HT2) and 100 mL of a 4-BB
a water (treated by inverse osmosis) solution of
some other hybrid structures.
enlargement of the basal spacing corresponding to an
This reveals an
ꢀ1
3
)
2
O and Al(NO ) , 9H O (0.1 mol L ) in a Mg/Al
2 3 3 2
ꢁ
expansion of the interlayer space to about 15.1 A. Note
ꢀ
1
ꢀ
1
(0.11 mol L ) and NaOH (2 mol L ) solution are added dropwise
that simultaneously, an increase of the structural dis-
order was observed. The symmetric (1 1 0) and (1 1 3)
reflection merged, possibly indicating a disturbance of
the brucite-like layer structure. The coexistence of the
pure Mg–Al hydrotalcite phase reflections with the
hybrid phase reflections on the same pattern indicates an
while carefully controlling the pH between 8 and 10. The slurry is
stirred for 12 h at 60 °C, then filtered and washed thoroughly with hot
water. The filter cake is then dried at 353 K for 15 h.
JCPDS files numbers are from the Mineral Powder Diffraction Data
Files Library, International Center for Diffraction Data, Swarth-
more, PA, USA.
à

T. Pigot et al. / Tetrahedron Letters 45 (2004) 4047–4050
4049
inhomogeneous insertion of 4-BB in the host structure,
and, probably, remaining nonintercalated zones.
structure, some 4-BB is found in acetonitrile (5%)
contrary to the blank experiments. This exchange
is closely related to the initial concentration of sul-
fide, since with more concentrated DBS solutions
(10 mol L ), greater amounts of 4-BB are released
in solution (42%) upon irradiation. This can be
accounted for by surface exchange between 4-BB
and formed sulfate and sulfonate as suggested by
the XPS analysis of the irradiated 4-BB/HT1.
Photo-oxidation experiments are performed both in
solution and in the gas phase. In the first case, 10 mL of
ꢀ
1
ꢀ1
ꢀ
2
ꢀ1
1
.15 ꢁ 10 mol L
di n-dibutylsulfide (DBS) aceto-
nitrile solutions containing 30 mg of the solid photo-
catalyst and cyclododecane as internal standard in a
Pyrex reactor are used. The stirred slurry is continuously
bubbled with pure oxygen and irradiated with four
(3) Third, 4-BB/HT1 appears much more selective
towards sulfoxide formation and only minor
amounts of disulfide, sulfone and acidic compounds
are obtained. In both homogeneous and hetero-
geneous case, it may be noticed that sulfone is
mainly observed at long irradiation time. It is
known from previous studies that disulfides and
acids arise, respectively, from the dimerization or
from the oxidation by ground state oxygen of thiyl
â
fluorescent 350 nm lamps in a RAYONET device.
After filtration, the composition of the irradiated solu-
tion is determined by GC-FID for volatile compounds,
by ion exchange chromatography for acidic species and
by UV spectrometry in order to check the possible
presence of 4-BB in solution.
In the second case, preliminary attempts were performed
with a fluidized bed of 4-BB/HT1 in a 20 mL Pyrex
vertical reactor (500 mg of 4-BB/HT1). The gaseous
effluent (100 ppmv of dimethylsulfide (DMS)) is con-
tinuously introduced at a constant flow (250 mL/mn).
The gas flow at the outlet of the reactor is analyzed each
seven minutes with a GC-FID chromatograph with an
automatic injection loop.
2
radicals. Moreover, it has been shown that under
benzophenone sensitization, both electron transfer
Åþ
Åꢀ
(with formation of an ion-pair RSR , O ) and sin-
7
glet oxygen addition to sulfides are competitive.
Our results thus strongly suggest that the electron
transfer mechanism, which is only able to lead to
acidic products, is less favored with the lamellar
material. This may be tentatively accounted for by
Åþ
In acetonitrile solution, blank experiments without sul-
fide indicate that the lamellar structure is preserved
under irradiation in the presence of oxygen and that less
than 1% of 4-BB is released in the solution. We first
verified that with this amount of 4-BB in solution, no
oxidation of sulfide occurred under homogeneous con-
ditions. Second, we checked that DBS did not react
when HT0 (without 4-BB) was used. Comparative
photooxidation experiments were then performed with
the unfavorable formation of charged RSR
between the positive brucite sheets.
Finally we examined the turnover of 4-BB/HT1 for
sulfide oxidation. In these experiments, a slurry of the
photocatalyst was irradiated for 150 min. The clay was
then removed by filtration, and the filtrate analyzed.
After washing with acetonitrile, the clay was then re-
used in successive trials. Table 1 shows that 4-BB/HT1 is
active during at least three consecutive runs and that
sulfoxide is always the major product. The formation of
sulfone decreases from the first to the third trial. The
absence of other by-products and the stability of the
amount of 4-BB found in solution after irradiation (5%)
are also noteworthy. After the last run, the catalyst is
removed by filtration. After evaporation of acetonitrile,
the crude product is diluted in a chloroform solution,
washed with a carbonate aqueous solution to take off
the released 4-BB, and purified on a silicagel column.
The yield in isolated sulfoxide is 80% (eluent chloro-
form).
3
0 mg of the intercalated materials (corresponding to a
known amount of 4-BB deduced from the microanalysis
data) or with equivalent amounts of 4-benzoyl benzoic
acid (4-BBac) in homogeneous solution (Table 1). Three
main conclusions may be drawn from this table.
(
1) First, the intercalated material reacts more slug-
gishly than soluble 4-BBAc, since after 1 h irradia-
tion, 30% of the sulfide is not converted. This
probably results from the hindered diffusion of the
reactants inside the lamellar structure in the hetero-
geneous case, as already reported with phthalocya-
9
nines intercalated in LDH.
2) Second, although the XRD pattern of the irradiated
(
Some experiments have been attempted towards the gas
phase photo-oxidation of dimethylsulfide (DMS) with
4
-BB/HT1 is still characteristic of a preserved LDH
Table 1. Composition of the solutions after irradiation
b
b
b
b
b
c
Time (h)
1
2
3
4
5
4-BB released
a
4
4
4
4
4
-BBAc
-BB/HT1
1
58
76
74
81
88
2
1
12
1
5
1
0.5
0.1
Nd
Nd
Nd
––
4
a
2
d
d
d
d
d
d
-BB/HT1 (cycle 1)
-BB/HT1 (cycle 2)
-BB/HT1 (cycle 3)
2.5
2.5
2.5
10
6
0
Nd
Nd
Nd
5
0
5
5
0
5
a
b
c
Time for total consumption of sulfide.
%
%
Relative to the initial sulfide concentration.
Released in solution relative to the initial amount of inserted 4-BB.
d
Not determined.

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T. Pigot et al. / Tetrahedron Letters 45 (2004) 4047–4050
1
20
00
References and notes
1. Ando, W. Sulf. Rep. 1981, 1, 147–213.
Starting of irradiation at 350 nm
1
2
3
4
. Robert-Banchereau, E.; Lacombe, S.; Ollivier, J. Tetrahe-
dron 1997, 53, 2087–2102.
. Janssen, F.; Greer, E.; Clennan, E. J. Am. Chem. Soc.
1998, 120, 4439–4449.
. Clennan, E. L. Acc. Chem. Res. 2001, 34, 875–884.
DMS concentration at the inlet of the photocatalytic reactor
8
0
0
0
0
6
4
2
DMS concentration at the outlet of the photocatalytic reactor
5. Hug, G.; Marciniak, B.; Bobrowski, K. J. Photochem.
Photobiol., A: Chem. 1996, 95, 81–88.
6. Baciocchi, E.; Del Giacco, T.; Elisei, F.; Gerini, M.;
Guerra, M.; Lapi, A.; Liberali, P. J. Am. Chem. Soc. 2003,
0
0
1
25, 16444–16454.
. Lacombe, S.; Cardy, H.; Simon, M.; Koukh, A.; Soum-
.00
5.00
10.00
15.00
20.00
25.00
7
Time (hours)
illon, J. P.; Ayadim, M. Photochem. Photobiol. Sci. 2002,
1, 347–354.
. Guo, Y.; Li, D.; Hu, C.; Wang, Y.; Wang, E. Int J. Inorg.
Mater. 2001, 3, 347–355.
. Iliev, V.; Ileva, A.; Bilyarska, L. J. Mol. Cat. A: Chem.
Figure 3. Evolution of dimethylsulfide concentration in the gas phase
outside the reactor and through the reactor under irradiation.
8
9
1
997, 126, 99–108.
0. Madhavan, D.; Pitchumani, K. Tetrahedron 2001, 57,
391–8394.
the same material. In the absence of irradiation, the
DMS concentration in the gas phase is the same when
the effluent flows outside or through the reactor even in
the first chromatographic runs: this means that no sig-
nificant adsorption of DMS occurs on the modified
LDH. Irradiation of the reactor at 350 nm with 12
fluorescent lamps lowers the concentration of DMS by
about 30%, with simultaneous observation of dimethyl-
sulfoxide. Under these experimental conditions, the
photoactivity of 4-BB/HT1 holds on for more than one
day without any decrease (Fig. 3).
1
1
8
1. Shichi, T.; Takagi, K.; Sawaki, Y. Chem. Lett. 1996, 781–
782.
12. Takagi, K.; Saito, N.; Shichi, T.; Sawaki, Y. Chem. Lett.
1999, 275–276.
3. Newman, S.; Jones, W. New J. Chem. 1998, 105–115.
4. Prevot, V.; Casal, B.; Ruiz-Hitzky, E. J. Mater. Chem.
1
1
2
001, 11, 554–560.
5. You, Y.; Zhao, H.; Vance, G. Environ. Technol. 2001, 22,
447–1457.
1
1
1
1
1
6. Khan, I.; Lei, L.; Norquist, A.; O’Hare, D. Chem.
Commun. 2001, 2342–2343.
7. Choudary, B.; Reddy, C.; Prakash, B.; Kantam, M.;
Sreedhar, B. Chem. Commun. 2003, 754–755.
This result implies that 4-BB/HT1 can achieve the par-
tial photo-oxidation of DMS at the gas–solid interface.
8. 4-BB/HT1:Mg/Al ¼ 2, %C, expt (theor): 31.88 (32.01);
In summary, intercalation of 4-BB inside an LDH
material has been achieved, with a noticeable increase of
the interlamellar distance. This material proved to be an
efficient and reusable oxidation photocatalyst in solu-
tion. The enhanced selectivity of the material towards
sulfoxide can be attributed to unfavourable electrostatic
effects. In the gas phase, optimization of the adsorption
of the sulfide on the material and of the contact time
have to be sought in order to improve the DMS photo-
oxidation yield.
%H,
expt
(theor):
4.49
(4.54);
Formula:
4-BB/
ꢀ ꢀ
[Mg0:67Al0:33(OH) ]4-BB0:27 (NO )0:06 ,H O
2 3 2
HT2:Mg/Al ¼ 2.5, %C, expt (theor): 29.72 (29.10); %H,
expt (theor): 4.49 (4.34); Formula: [Mg0:71Al0:29(OH) ]-
2
ꢀ
ꢀ
4
-BB0:23 (NO
3
)
0:06 , 0.7H
2
O.
9. Miyata, S. Clays Clay Miner. 1975, 23, 369–375.
0. Kannan, S.; Awate, S.; Agashe, M. Stud. Surf. Sci. Catal.
998, 927–935.
1
2
1
2
1. Aramendia, M.; Boreau, V.; Jimenez, C.; Luque, J.;
Marinas, J.; Ruiz, J.; Urbano, F. Appl. Catal., A: General
2001, 216, 257–267.