72 __________________________________Acta Chim. Slov. 2006, 53, 72-76__________________________________
Scientific Paper
Sol-Gel Silica-Supported 12-Tungstosilicic Acid
(H4SiW12O40/SiO2) as a Heterogeneous Photocatalyst for
Efficient and Selective Oxidation of Benzylic Alcohols with
O2 under Photoirradiation
Saeid Farhadi,* Zaynab Babazadeh and Mansoureh Maleki
Department of Chemistry, University of Lorestan, Khoramabad, Iran E-mail:farhadi.s@lu.ac.ir
Received 13-07-2005
Abstract
Sol-gel silica-supported heteropolyoxotungstate (H4SiW12O40/SiO2), prepared by incorporating H4SiW12O40 cluster
I into a silica matrix via sol-gel technique, was used as an efficient heterogeneous photocatalyst for oxidation of alcohols by O2. Irradiation of a series of primary and secondary benzylic alcohols in acetonitrile solvent over H4SiW12O40/SiO2 under O2 atmosphere resulted in the corresponding aldehydes and ketones selectively and in high yield. Non-benzylic alcohols were less reactive under the conditions estabilished here. The photocatalyst has been reused several times, without observable loss of activity and selectivity. Eventually, heterogeneous photocatalyst considerably is more active than the unsupported H4SiW12O40.
Key words: Photooxidation, Benzylic alcohols, Heterogeneous, Polyoxometalates, Photocatalyst, Sol-gel silica-supported.
Introduction
The importance of environmentally friendly processes has been recognized over all fields of industry and, needless to say, in the field of synthetic organic chemistry as well. In this context, photochemical processes that take place under mild conditions, i.e. room temperature and atmospheric pressure or, even more, photocatalytic ones that avoid stoichiometric reagents and consume cheap photons are particularly attractive for synthetic purposes from an economic and environmental point of view.1 In recent years, the semiconductor metal oxides (mainly TiO2) widely used as green photocatalyst in the field of synthetic organic chemistry.2-4 However, the TiO2 photocatalysis method generally leads to the total mineralization of organic compounds into CO2 and H2O. Moreover, the ultrafine TiO2 particles used are easy to lose and it is difficult to separate them from the reaction systems due to the formation of a milky dispersion of the TiO2 particles. These findings suggest that the development of other photocatalytic materials is important.
Another class of photochemically active materials that has received less attention, in comparison with semiconductors, are the polyoxometalates (POMs).5 Many of them, such as W10O324-, Mo7O246-, PW12O403-, PMo12O403- and SiW12O404-, share the same general photochemical characteristics of the semiconductor
photocatalysts, that is, they can be photoexcited (POM*), abstract one or two electron(s) from organic substrate without decomposition (POMn-), be reoxidized and return to their original oxidation state (POM) by electron acceptors, such as the dissolved dioxygen in solution. Thus, they complete a net photocatalytic cycle. The proposed photocatalytic cycle of POMs, which is analogous to the well-known photocatalytic cycle of semiconductor photocatalysts, is shown in Scheme 1. 6-9
hv
POM*
SubH2
POM        POM-     ^ Sub + 2H+
H20           02
Scheme 1. The photocatalytic cycle of a polyoxometalate (POM). POM*= Photoexcited POM, POMn- = Reduced POM, SubH2= Organic substrate, Sub= Oxidized substrate.
The photocatalytic oxidation of organic substrates by POMs has been studied extensively in homogeneous systems and it indicates that the photo-oxidation efficiency of POMs is comparable to that of the semiconductor TiO2.10-17 The major drawback to the practical applications of POM photocatalytic systems
Farhadi et al. Sol-Gel Silica-Supported 12-Tungstosilicic Acid (H4SiW12O40/SiO2)
Acta Chim. Slov. 2006, 53, 72–76
73
is the high solubility of them in water and in highly polar organic media which impedes ready recovery and reuse of the photocatalyst.18,19 Moreover, the specific surface area of the solid POMs is very low (<10 m2/g), leading to very few active sites on their surfaces.20 Thus, immobilization of POMs onto solid support to create heterogeneous photocatalysts is necessary for their recovery and possibly their recycling. Moreover, their catalytic activities may be improved. In this framework, the POMs Na4W10O32, H3PW12O40, H4SiW12O40 and H3PMo12O40 supported into porous inorganic solids e.g. silica or zeolite networks have been used as the effective and recyclable photocatalysts for photo-degradation and mineralization of aqueous organic pollutants.21-30 However, there are less report regarding the application of supported POMs as the photocatalysts for synthetic organic chemistry, so far.31-32
In the present work, the sol-gel silica-supported H4SiW12O40 was used, for the first time, as an efficient and recyclable heterogeneous photocatalyst for aerobic oxidation of various primary and secondary benzylic alcohols into the corresponding carbonyl compounds. In addition, the photocatalytic behaviour of the H4SiW12O40/SiO2 composite was also compared with pure H4SiW12O40 in homogeneous system.
Results and discussion
The silica-supported polyoxometalate, H4SiW12O40/ SiO2, containing ca. 20 wt. % of H4SiW12O40 was synthesized through encapsulating into a silica matrix on the basis of the sol-gel method reported recently, by adding H4SiW12O40 during the hydrolysis of tetraethyl orthosilicate (Si(OEt)4, TEOS) as the SiO2 source.18,21 During TEOS hydrolysis-condensation sol-gel process in the presence of this POM, the H4SiW12O40 molecules were entrapped in silica matrix. Photocatalytic activity of the supported H4SiW12O40 was tested via oxidation of a series of various benzylic alcohols under O2 atmosphere.
The oxidation of benzyl alcohol was first investigated as a standard substrate using H4SiW12O40/ SiO2 under O2 atmosphere in acetonitrile. It was found that the benzyl alcohol was oxidized to benzaldehyde in 82 % yield within 4 h irradiation and ca. 16 % of the benzyl alcohol was recovered (Scheme 2).
/r~\   OH
('    V^     +1/2 O
H
H
H4SiWn04l/Si02 , hv
CH CN ,  r. t.
/ Vj
O
H
H2O
Scheme 2. Aerobic oxidation of benzyl alcohol.
On the other hand, when the pure SiO2 support was used as a catalyst for aerobic oxidation of benzyl
alcohol, no observable oxidation product was detected from GC analysis after 4 h irradiation, suggesting that the photocatalytic activity of the H4SiW12O40/SiO2 system is due to H4SiW12O40. Control experiments without H4SiW12O40/SiO2 or light under same reaction conditions showed no reaction. Using pure H4SiW12O40, as a homogeneous photocatalyst, we found from GC analysis that 35 % of the benzyl alcohol was oxidized to benzaldehyde and about 64 % of benzyl alcohol was recovered within 4 h irradiation under the same reaction conditions. These results clearly indicate that silica-supported H4SiW12O40 is much more reactive than the bulk H4SiW12O40. The significantly higher activity of H4SiW12O40/SiO2 composite, as compared with that of unsupported H4SiW12O40, is attributed to the much higher density of active sites in H4SiW12O40/SiO2.
Only a small amount of benzaldehyde from photooxidation of benzyl alcohol was observed when the reaction with H4SiW12O40/SiO2 was carried out under N2 and almost concomitantly, SiW12O405- was obtained, as evidenced from the appearance of a dark blue colour in the reaction suspension. The blue colour was also observed when benzhydrol was used as the substrate. Since there is no catalyst regeneration, according with Scheme 1 in the absence of O2, the photoexcited H4SiW12O40/SiO2 stoichiometrically oxidizes the alcohol to aldehyde with the negligible yield. Therefore, it is confirmed that H4SiW12O40/SiO2, light and O2 are essential for this reaction. A decrease in catalytic activity and selectivity for benzaldehyde was not observed, providing 79 % yield of benzaldehyde even after being reused five times.
The fact that the reaction solution contained no dissolved H4SiW12O40, which was determined by UV-VIS, IR and ICP-AES, confirms that this POM did not leach from the silica support during the photocatalytic reaction. No leaching of entrapped catalyst is important since many heterogeneous oxidations can actually be promoted by catalyst leached in solution.33 Further evidence that confirms that the reaction mediated by the catalytic amount of H4SiW12O40/SiO2 is heterogeneous in nature drawn from the fact that when the catalyst separated from the reaction mixture shortly (30 min) after the begining of the irradiation and the reaction filtrate was irradiated under O2, no extra formation of benzaldehyde from benzyl alcohol was observed via GC and/or TLC even after 4 h irradiation.
Under the explored standard conditions, a range of primary and secondary alcohols such as aliphatic, benzylic, allylic and even sulfur/nitrogen containing alcohols were evaluated as substrates for the aerobic oxidation. All the tested primary benzylic alcohols were converted into their corresponding aldehydes and no overoxidation to acids was observed in the case of aldehyde products. (Table 1).
+
Farhadi et al. Sol-Gel Silica-Supported 12-Tungstosilicic Acid (H4SiW12O40/SiO2)
74
Acta Chim. Slov. 2006, 53, 72–76
Table 1.  Photooxidation of various benzylic alcohols with O2
catalysed by H4SiW12O40/SiO2
Table 2. Photooxidation of   secondary benzylic alcohols and vicinaldiols with O2 catalysed by H4SiW12O40/SiO2
Ar—CH2OH -
H4SiWnOJSi02 , hv
O2 , CH3CN ,  r. t.
Ar— CHO
Entry   Alcohol
Producta
Time (h)   Yield (%)b
1
2 c
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Bu-ff     V-O
FC
3 CVJ^
/O              OH
^S             OH
n    t—'
Br—


NV
,OH                                           ^O
CO
-^-^/—O
CO
4 4
2
2
2
2.5 1.5 1.25
2 1.5
2
6
6
4
4
4
3
4
4
4
3
6
6
12
82 35 95
80 95 92 86 90 88 35 30 74 78 76 80 66 74 70 80 30 32 56
a All products were characterized on the basis of mass, IR and 1H-NMR spectral data. b Yields are for isolated and chromato-graphically pure products. cUnsupported H4SiW12O40 was used as the photocatalyst.
Benzylic alcohols having electron-donating groups such as Me-, MeO-, tert-Bu-, iso-Pr in the aromatic ring and also halobenzyl alcohols were oxidized to corresponding aldehydes in excellent yields (Table 1, entries 3-11). However, 4-nitro- and 4-trifluoromethyl-benzyl alcohols which possess an electron-withdrawing group, resisted this oxidation conditions, and the corresponding benzaldehydes were formed in 35 and 30 % yields respectively after 6 h irradiation (Table 1, entries 12 and 13). Furthermore, furfuranol, thiophene-2-methanol and pyridin-4-methanol, were oxidized to the corresponding aldehydes in high yields and heteroatoms such as S or N did not result in any
Entry    Alcohol
Product a
Time(h)    Yield (%)b
1
2 c
3
4
5
6
7
8
9 10 11 12 13 14
Cro     crc
u
xOHxO
CAD
JCrO
O
O
V    //        OH
HO
Me-d  y \
^   ff        OH
HO
Cl
Me
\_J/       OH
O,
ff          OH
O,
O O
OH
1.5 1.5
1 1.25 1.5 1.5
1
1.5 2.5 1.5 1.5 1.25
2 2.5
92 30 98 95 90 86 94 85 78 86 85 90 60 75
a All products were characterized on the basis of mass, IR, and1H-NMR spectral data. b Yields are for isolated and chromatographically pure products. cUnsupported H4SiW12O40 was used as the photocatalyst.
notorious effects on the catalytic system (Table 1, entries 14-16). Other Arylmethyl alcohols gave the corresponding benzaldehydes in good yields (Table 1, entries 17-20). Allylic alcohol such as cinnamyl alcohol was also good substrate, and in this case no isomerisation of the conjugated double bond was observed (entry 21)
Whereas various primary benzylic and allylic alcohols were oxidized in a highly efficient way, non-benzylic arylalcohols such as 2-phenylethanol and 3-phenyl-1-propanol were less reactive and the corresponding aldehydes were obtained in 30-32 % yields (Table 1, entries 22 and 23) and therefore longer irradiation time was required to obtain the aliphatic carbonyl compound in moderate yield (entry 24).
With this method, various secondary benzylic alcohols were converted with high selectivity to their corresponding ketones and were more efficient than the primary benzylic alcohols (Table 2, entries 1-9). This reaction is highly selective for vicinaldiols in oxidizing only the secondary hydroxy group ? to the benzene ring (Table 2, entries 10-14). Therefore, it is a method of choice for the oxidation of benzylic OH in the presence of non-benzylic OH.
To study the possible chemoselectivity, a mixture of 1-phenylethanol and 2-phenylethanol was next
O
OH
OH
O
OH
O
OH
OH
O
m

OH
O
OH
OH
OH
t-Bu
Me
OH
MeO
MeO
OHO
OMe
OMe
O
MeO
HO
Cl
Cl
OH
OH
O
HO
Cl
Cl
Cl
O
OH
Cl
Cl
O
OH
OH
ON
2      \>   k
OH
OH
HO   OH
O
OH
OH
O
OH
OH
O
Farhadi et al. Sol-Gel Silica-Supported 12-Tungstosilicic Acid (H4SiW12O40/SiO2)
Acta Chim. Slov. 2006, 53, 72–76
75
subjected to 1 h photoirradiation in the presence of H4SiW12O40/SiO2 under O2, GC analysis indicated that the former was oxidized to acetophenone in 84 % yield and the latter gave 2-phenylacetaldehyde in < 5 % yield. This study clearly reveals that this method can be applied for the efficient oxidation of benzylic alcohols in the presence of non-benzylic alcohols. However, upon prolonged irradiation (6 h) of this mixture, 2-phenylethanol formed aldehyde in 25 % yield.
Conclusions
In conclusion, a novel, efficient and environmentally benign photocatalytic method is presented for the oxidation of alcohols to aldehydes and ketones. This metodology is also compatible in the presence of other functionalities such as methoxy, nitro, halogen and alkene double bonds. Benzylic alcohols can be chemioselectively converted into corresponding carbonyl compounds in the presence of non-benzylic hydroxy groups. Heterocyclic alcohols are also selectively oxidized without oxidation of heteroatoms like N, S. It is noteworthy that the aldehydes do not undergo further oxdation to carboxylic acids. In addition, it is easy to separate and recover the catalyst for another photocatalytic recycling. The present work provided a new type of heterogeneous photocatalytic materials for potential synthetic application.
Experimental
Preparation of H4SiW12O40was adapted from the literature methods.34 Alcohols were either purchased commercially or synthesized as reported in the literature.35, 36 The light source was a 400W high-pressure mercury lamp ( HPML, ?>320 nm).
Preparation of the H4SiW12O40 /SiO2:
The H4SiW12O40/SiO2 photocatalyst was prepared by the improved literature methods.18,21 To a mixture of water (2 mol), 1-BuOH (0.2 mol) and H4SiW12O40 (1.04×10-3 mol, 3g), was added tetraethyl orthosilicate (TEOS, 0.2 mol), and the mixture was stirred at 80 oC for 3 h. The hydrogel thus obtained was dehydrated slowly at 80 oC for 3 h and dried in a vacuum at 60 oC for 12 h. The dried gel was calcined in a vacuum at 150 oC for 2 h to fasten the silica network and gave ca. 15 g of silica-included heteropolyacid, as may have been expected. The spectra of IR, UV-VIS and ICP-AES analysis are in agreement with data reported for this composite.18,21 The loading of H4SiW12O40 in the H4SiW12O40/SiO2 composite was ca. 20 %, estimated by ICP-AES analysis. The silica-included H4SiW12O40 was dried in a vacuum at 150 oC for 3 h prior to use for photocatalytic reactions.
General procedure for photocatalytic oxidation of benzylic alcohols by using H4SiW12O40/SiO2 under O2:
A solution of benzylic alcohol (1 mmol) in acetonitrile (25 mL) was added to a Pyrex cell containing a teflon-coated stirring bar. To this solution was added H4SiW12O40/SiO2 (100 mg). Oxygen was passed through, and the reaction mixture was kept under an oxygen atmosphere (O2 balloon). The suspension was vigorously stirred and irradiated. The temperature of the suspension was maintained at 25±2 oC by water circulation through an external cooling coil. The reaction was followed by TLC and/or GC and after an appropriate irradiation time, the photocatalyst was filtered off. After removal the solvent, the products were purified by silica-gel plate or column chromatography. Yields are shown in Table 1. All of the products were characterized by mass, IR and 1H-NMR spectra and by comparison with the known compounds.37-41
Acknowledgement
Financial support from the Lorestan University Research Council is gratefully acknowledged.
References
1.  W. M. Horspool, P. S. Song, CRC Handbook of Organic Photochemistry and Photobiology, CRC Press, Boca Raton, 1995.
2.  M. A. Fox, M. T. Dulay, Chem. Rev. 1993, 93, 341-357.
3.  M. R. Hoffmann, S. T. Martin, W. Choi, D. W. Bahnemann, Chem. Rev. 1995, 95, 69-96.
4.  A. Fujishima T. N. Rao, D. A. Tryk, Photochem. Photobiol. C: Photochem. Rev. 2000, 1, 1-21.
5.  M. T. Pope, A. Muller, Angew. Chem. Int. Ed. 1991, 30, 34-48.
6.  E. Papaconstantinou, J. Chem. Soc., Chem. Commun. 1982, 12-13.
7.  C. L. Hill, D. A. Bouchard, J. Am. Chem. Soc. 1985, 107, 5148-5157.
8.  K. Nomiya, Y. Sugie, T. Miyazaki, M. Miwa, Polyhedron 1986, 5, 1267-1271.
9.  T. Yamase, T. Usami, J. Chem. Soc., Dalton Trans. 1988, 183-190.
10.  E. Papaconstantinou, Chem. Soc. Rev. 1989, 18, 1-31.
11.  A. Hiskia, E. Papaconstantinou, Inorg. Chem. 1992, 31, 163-167.
12.  A. Maldotti, R. Amadelli, G. Varani, S. Tollari, F. Porta, Inorg. Chem. 1994, 33, 2968-2973.
13.  D. C. Duncan, T. L. Netzel, C. L. Hill, Inorg. Chem. 1995, 34, 4640-4646.
14.  A. Mylonas, E. Papaconstantinou, J. Photochem. Photobiol. A: Chem. 1996, 94, 77-82.
15.  C. Tanielian, Coord. Chem. Rev. 1998, 178-180, 1165-1181.
Farhadi et al. Sol-Gel Silica-Supported 12-Tungstosilicic Acid (H4SiW12O40/SiO2)
76
Acta Chim. Slov. 2006, 53, 72–76
16.  T. Yamase, Chem. Rev. 1998, 98, 307-325.
17.  A. Hiskia, A. Mylonas, E. Papaconstantinou, Chem. Soc. Rev. 2001, 30, 62-69.
18.  Y. Izumi, K. Hisano, T. Hida, Appl. Catal. A: Gen. 1998, 181, 277-282.
19.  A. Molinari, R. Amadelli, V. Carassiti, A. Maldotti, Eur. J. Inorg. Chem. 2000, 6, 91-96.
20.  N. Mizuno, M. Misono, Chem. Rev. 1998, 98, 199-217.
21.  Y. Guo, Y. Wang, C. Hu, Y. Wang, E. Wang, Y. Zhou, S. Feng, Chem. Mater. 2000, 12, 3501-3508.
22.  Y. Guo, D. Li, C. Hu, Y. Wang, E. Wang, Y. Zhou, S. Feng, Appl. Catal. B: Environ. 2001, 30, 337-349.
23.  G. Peng, Y. Wang, C. Hu, E. Wang, S. Feng, Y. Zhou, H. Ding, Y. Liu, Appl. Catal. A: Gen. 2001, 218, 91-99.
24.  R. R. Ozer, J. L. Ferry, J. Phys. Chem. B. 2002, 106, 4336-4342.
25.  D. Li, Y. Guo, C. Hu, L. Mao, E. Wang, Appl. Catal. A: Gen. 2002, 235, 11-20.
26.  Y. Guo, C. Hu, S. Jiang, C. Guo, Y. Yang, E. Wang, Appl. Catal. B: Environ. 2002, 36, 9-17.
27.  Y. Guo, C. Hu, C. Jiang, Y. Yang, S. Jiang, X. Li, E. Wang, J. Catal. 2003, 217, 141-151.
28.  S. Anandan, S. Y. Ryu, W. Cho, M. Yoon, J. Mol. Catal. A: Chem. 2003, 195, 201-208.
29.  C. Jiang, Y. Guo, C. Hu, C. Wang, D. Li, Mater. Res. Bull. 2004, 39, 251-261.
30.  Y. Yang, Q. Wu, Y. Guo, C. Hu, E. Wang, J. Mol. Catal. A: Chem. 2005, 225, 203-212.
31.  A. Molinari, R. Amadelli, L. Andreotti, A. Maldotti, J. Chem. Soc. Dalton Trans. 1999,1203-1204.
32.  A. Maldotti, R. Molinari, A. Amadelli, Chem. Rev. 2002, 102, 3811-3836.
33.  R. A. Sheldon, M. Wallau, I. W. C. E. Arends, U. Schuchardt, Acc. Chem. Res. 1998, 31, 485-493.
34.  M. Filowitz, R. K. C. Ho, W. G. Klemperer, W. Shun, Inorg. Chem. 1979, 18, 2968-2973.
35.  F. Fringerio, R. Germani, F. Pizzo, G. Sarelli, Synth. Commun. 1989, 19, 1939-1944.
36.  M. A. Reddy, L. R. Reddy, N. Bhanumathi, K. R. Rao, Org. Prep. Proced. Int. 2002, 34, 527-529.
37.  M. Mulbaier, A. Giannis, Angew. Chem. Int. Ed., 2001, 40, 4393-4394.
38.  J. D. Moore, S. N. Finney, Org. Lett. 2002, 4, 3001-3003.
39.  G. Sorg, A. Mengel, G. Jung, J. Rademann, Angew. Chem. Int. Ed. 2001, 40, 4395-4397.
40.  N. N. Reed, M. Delgado, K. Hereford, B. Clapham, K. D. Janda, Bioorg. Med. Chem. Lett. 2002, 12, 2047-2049.
41.   R. M. Moriarty, H. Hu, S. C. Gupta, Tetrahedron Lett. 1981, 22, 1283-1286.
Povzetek
S sol-gel tehniko smo pripravili 12-volframsilicijevo kislino na podlagi silicijevega dioksida (H4SiW12O40/SiO2) in produkt uporabili kot učinkovit heterogen fotokatalizator za oksidacijo alkoholov z O2. Pri obsevanju vrste primarnih in sekundarnih benzilnih alkoholov v acetonitrilu kot topilu v prisotnosti H4SiW12O40/SiO2 v kisikovi atmosferi smo dobili ustrezne aldehide in ketone z visokim izkoristkom. Alkoholi brez benzilne skupine so manj reaktivni pod opisanimi pogoji. Fotokatalizator smo uporabili večkrat brez znatnega zmanjšanja aktivnosti in selektivnosti. Heterogeni katalizator ima znatno višjo aktivnost kot H4SiW12O40 brez sol-gel podlage.
Farhadi et al. Sol-Gel Silica-Supported 12-Tungstosilicic Acid (H4SiW12O40/SiO2)