Photocatalysis with Na4W10O32 in water system: Formation and reactivity of OH radicals

Journal of Molecular Catalysis A: Chemical 372 (2013) 23–28

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Photocatalysis with Na4 W10 O32 in water system: Formation and reactivity of OH• radicals Alessandra Molinari a,∗ , Roberto Argazzi b , Andrea Maldotti a a b

Dipartimento di Chimica, Università di Ferrara, Via Luigi Borsari 46, 44123 Ferrara, Italy Centro ISOF CNR, Via Luigi Borsari 46, 44123 Ferrara, Italy

a r t i c l e

i n f o

Article history: Received 17 September 2012 Received in revised form 21 December 2012 Accepted 25 January 2013 Available online xxx Keywords: Sodium decatungstate Heterogeneous photocatalysis OH• radicals EPR-spin trapping Alcohol oxidation

a b s t r a c t Photoexcitation of Na4 W10 O32 dissolved in water leads to the formation of hydroxyl radicals. These species originate both from H2 O oxidation and H2 O2 reduction. EPR-spin trapping investigation and laser flash photolysis experiments contribute to clarify their formation mechanism and their involvement in the oxidation of propan-2-ol. Continuous irradiation of water solutions of Na4 W10 O32 leads to the overoxidation of propan-2-ol to CO2 with high yield. This result may be of interest for the development of photocatalytic systems aimed to pollutants degradation. Entrapment of Na4 W10 O32 into a microporous silica matrix gives a rather robust photocatalyst where the decatungstate structure is preserved. This material is able to catalyze the photooxidation of propan-2-ol to acetone with appreciable chemoselectivity. In particular, it is seen that the solid photocatalyst yields acetone as main product inhibiting its over-oxidation to carbon dioxide. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Tungsten oxygen anion clusters, known collectively as polyoxotungstates (POT), form a large and structurally diverse class of inorganic compounds with significant applications in photocatalysis [1–7]. In particular, their ability to promote photooxidation reactions of various substrates has been investigated by several authors [2,5–7]. This research topic moves toward a “sustainable chemistry” opening to new oxidative routes either for syntheses or for wastewater treatments that require O2 as available and cheap oxidant, mild temperature and pressure conditions and use of light as renewable source of energy. The decatungstate anion W10 O32 4− is of particular interest from this point of view, since its absorption in the near UV region (max = 323 nm) partially overlaps the UV solar emission, opening the possibility to carry out solar-assisted applications [1,2,7]. The proposed mechanism for W10 O32 4− -based photocatalysis in organic solvent involves absorption of light by the decatungstate ground state leading to an oxygen to metal [O2− –W6+ ] charge transfer excited state (W10 O32 4−* , Scheme 1 step a) [8,9] that decays in few picoseconds in both aerated and deaerated solutions to a very reactive non-emissive transient wO (step b). This species has

∗ Corresponding author. Tel.: +39 0532455147; fax: +39 0532240709. E-mail address: [email protected] (A. Molinari). 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.01.037

an oxyradical-like character with a longer lifetime of about 50 ns. wO is able to oxidize many organic substrates (RH) by hydrogenatom abstraction or electron transfer mechanisms, depending on the chemical nature of RH. In both cases, the reaction pathways lead to the one electron reduced form of the decatungstate (W10 O32 5− ) and to the substrate derived radical (R• , step c) [3,5,7,8,10]. Oxidation of W10 O32 5− by O2 restores starting W10 O32 4− , closing the photocatalytic cycle (step d). O2 consumption is accompanied by its reductive activation to peroxy species [8,11]. In aqueous solution, the formation of the highly reactive hydroxyl radicals (OH• ) through the direct reaction of water with wO (Eq. (1)) is likely, since the excited state potential of the decatungstate is more positive than the one-electron oxidation of water [1]. However, this possibility is still a matter of debate [17–20]. On the one hand, several kinetics reports suggest that direct photoinduced electron/hydrogen transfer between organic substrates and photoexcited decatungstate may occur also in water without the involvement of OH• radicals [19,20]. On the other hand, some experimental observations are in agreement with the formation of these intermediates. In particular, it has been observed that illumination of aqueous solutions of K4 W10 O32 induces the hydroxylation of aromatic hydrocarbons, a result consistent with OH• -based mechanism but not exclusive to it [17]. It has been also reported that photoexcitation of the polyoxotungstate [NH3 Pri ]5 [W6 O20 (OH)] in aqueous solution in the presence of the spin trap DMPO led to the evolution of an electron

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A. Molinari et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 23–28

hν

W10O324-

of (nBu4 N)4 W10 O32 in silica [14]. The obtained material contained 30% (w/w) of decatungstate. UV–vis spectra of washing water aliquots showed that Na4 W10 O32 was not released into the solution. A sample not including decatungstate (SiO2 ) was also prepared, following the same procedure mentioned above.

W10O324- *

(a) (b)

2.2. EPR spin-trapping experiments

wO RH

EPR spin-trapping experiments were carried out with a Bruker ER200 MRD spectrometer equipped with a TE 201 resonator (microwave frequency of 9.4 GHz). The samples were H2 O/CH3 CN (85/15) solutions containing 5,5-dimethylpyrroline Noxide (DMPO, 3 × 10−2 M) as spin trap, Na4 W10 O32 (2 × 10−3 M) and, when requested, propan-2-ol (0.1 M). In heterogeneous experiments, Na4 W10 O32 /SiO2 was suspended in the solution containing DMPO and propan-2-ol as described above. Then, samples were put into a flat quartz cell and directly irradiated into the EPR cavity at  > 300 nm with a Hg medium pressure mercury lamp. No EPR signals were obtained in the dark and during irradiation of the solution in the absence of photocatalyst.

(c)

.

R + H+

(d)

W10O325O2 + H+

ROOH

Scheme 1. Photocatalytic behavior of W10 O32 4− upon aerobic conditions.

paramagnetic resonance spectrum similar to that obtained from the reaction between this spin trap and OH• [21]. wO + H2 O → W10 O32

5−

+ OH•

+H

+

(1)

The partial oxidation of alcohols by O2 is a reaction with a high synthetic value [12] and it is one of the most widely investigated W10 O32 4− -photocatalyzed reactions. Considering that this reaction has been extensively studied in organic solvent, namely in acetonitrile solutions, we believe that an effort should be done in order to investigate this process in water, which is a benign and economically advantageous solvent able to minimize the environmental impact of organic synthesis. Herein, we report evidences of the formation of hydroxyl radicals during photoexcitation of Na4 W10 O32 in water solutions. In particular, we focus our attention on the formation mechanism of these intermediates and on their involvement in the oxidation of propan-2-ol through EPR spin trapping and laser flash photolysis experiments. The ability to control the chemoselectivity of the propan-2-ol oxidation process is another objective of this work. With this in mind, we also investigated the possible effect of a microporous siliceous matrix to exert a specific substrate recognition for improving the selectivity of the process. In fact, recent developments in the field of photocatalysis indicate that heterogenisation of Na4 W10 O32 with solid supports is a suitable means in order to provide selective and recyclable photocatalytic systems [2,13–16]. In particular, it has been shown that decatungstate entrapped inside polymeric membranes induces the partial oxidation of several water soluble alcohols with accumulation in solution of carbonylic products up to a substrate conversion of 30% without appreciable evidence of CO2 [22]. Moreover, we reported recently that photoexcitation of Na4 W10 O32 heterogenized with microporous silica induces glycerol oxidation mainly to glyceraldehyde and dihydroxyacetone with only negligible amounts of CO2 in spite of the observed formation of OH• radicals [23]. 2. Experimental 2.1. Photocatalyst preparation and characterization Reagents and solvents were purchased from Sigma in the highest purities available and used without further purification. Sodium decatungstate (Na4 W10 O32 ) was synthesized following a literature procedure [24]. The heterogeneous photocatalyst (Na4 W10 O32 /SiO2 ) was prepared by hydrolysis of tetraethylorthosilicate (TEOS) in the presence of an acid aqueous solution of Na4 W10 O32 , as recently published in detail for the entrapment

2.3. Laser flash photolysis experiments Nanosecond transient measurements were performed with a custom laser spectrometer comprised of a Continuum Surelite II Nd:YAG laser (FWHM 6–8 ns) with frequency doubled, (532 nm, 330 mJ) or tripled, (355 nm, 160 mJ) option, an Applied Photophysics xenon light source including a mod. 720 150 W lamp housing, a mod. 620 power controlled lamp supply and a mod. 03–102 arc lamp pulser. Laser excitation was provided at 90◦ with respect to the white light probe beam. Light transmitted by the sample was focused onto the entrance slit of a 300 mm focal length Acton SpectraPro 2300i triple grating, flat field, double exit monochromator equipped with a photomultiplier detector (Hamamatsu R3896) and a Princeton Instruments PIMAX II gated intensified CCD camera, using a RB Gen II intensifier, a ST133 controller and a PTG pulser. Signals from the photomultiplier (kinetic traces) were processed by means of a LeCroy 9360 (600 MHz, 5Gs/s) digital oscilloscope, while transient difference spectra were recorded with the CCD camera in a single beam configuration, acquiring first the light transmitted by the sample ground state and then after laser excitation at various delays and gate widths. A pulse energy below 10 mJ/pulse was used in all measurements. Samples were purged 20 with argon when requested. 2.4. Photocatalytic experiments Photocatalytic experiments were carried out inside a closed Pyrex tube at 298 ± 1 K. Na4 W10 O32 (2 × 10−4 M) or Na4 W10 O32 /SiO2 (8 g/L) was placed in an aqueous solution (3 mL) containing propan-2-ol (10−1 M). The photoreactor was joined to a balloon filled with O2 and irradiated by an external Helios Q400 Italquartz medium-pressure Hg lamp, selecting wavelengths higher than 300 nm with a cut off filter (photon flux was 0.075 W cm−2 ). Then, samples were analyzed by a HP 6890 gas chromatograph equipped with a FID and with a HPWAX capillary column. Quantitative analysis for acetone was performed with calibration curve obtained with authentic sample. Each experiment was repeated three times in order to evaluate the error, which remained in the ±5% interval around mean values. Control experiments were carried out by irradiating SiO2 suspended in the solution containing propan-2-ol (10−1 M) or keeping the photocatalyst dispersed in the solution in the dark. The amount of CO2 eventually formed was measured following a procedure described in detail in Ref. 23 that uses a pH meter BasiC 20 CRISON equipped with a gas sensing probe (Crison 9666). The yield of CO2

A. Molinari et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 23–28

25

(b)

(a) 6

emission intensity

Intensity (a.u.)

6,0x10

(b)

(c)

6

3,0x10

(a) 0,0 400

3425

3450

3475

3500

500

600

wavelength (nm)

magnetic field (gauss) Fig. 1. EPR spectra obtained after 30 s irradiation ( > 300 nm) of aqueous solutions of DMPO (3 × 10−2 M) containing: (a) Na4 W10 O32 (2 × 10−3 M), (b) powder dispersions of Na4 W10 O32 /SiO2 .

was referred to the number of carbon atoms present in the alcohol molecule. Other photochemical experiments have been carried out irradiating ( > 300 nm, 30 ) aerated aqueous solutions (3 mL) containing Na4 W10 O32 (2 × 10−4 M) and coumarin (Sigma, 1 × 10−4 M). After irradiation the fluorescence spectrum (exc = 332 nm) of 7-hydroxycoumarin, eventually formed, was recorded (emiss = 455 nm) at room temperature with a Jobin Yvon Spex Fluoromax II spectrofluorimeter equipped with a Hamamatsu R3896 photomultiplier. Both emission and excitation slits were set at 5.0 nm during the measurements. 3. Results and discussion 3.1. OH• radicals formation EPR spin-trapping investigation is a powerful technique for detecting the formation of short-lived radicals and it has been fruitfully employed in photochemical studies on polyoxometalates in order to better understand primary processes occurring after the excitation [25–28]. This technique is based on the ability of some molecules such as nitrones to trap short-lived radicals to give paramagnetic nitroxides stable enough to be successfully detected and studied [29]. The nature of the trapped radical can be often identified by the parameters obtainable from the EPR spectrum. Aqueous solutions of Na4 W10 O32 (2 × 10−3 M) have been irradiated inside the EPR cavity ( > 300 nm) in the presence of DMPO (3 × 10−2 M) as spin trap. Few seconds photoirradiation causes the prompt formation of a quartet (1:2:2:1, aN = aH = 14.6 G), which is shown in Fig. 1 (curve a). Signal pattern and coupling constant values are in agreement with the trapping of OH• radicals to form the paramagnetic adduct [DMPO–OH]• according to Eq. (2) [25]. Control experiments show that no signal is observed neither in the dark nor during irradiation but in the absence of decatungstate. H

+ OH. N O

H

N O

OH

(2)

This result suggests that reaction (1) is occurring in our system. However, some other sources of the quartet relative to the paramagnetic [DMPO–OH]• species should be ruled out. First, it has been previously reported that the [DMPO–OH]• species may originate from degradation of the adduct [DMPO–OOH]• between DMPO

Fig. 2. Changes in emission intensity measured at 455 nm of: (a) aqueous solution of Na4 W10 O32 (2 × 10−4 M) and coumarin (1 × 10−4 M) before irradiation, (b) sample (a) after 30 irradiation ( > 300 nm), (c) sample (a) irradiated (30 ,  > 300 nm) but in the presence of catalase.

and O2 − , which presents a typical 12-lines EPR spectrum [30,31]. Literature data indicate that a maximum level of approximately 3% [DMPO–OH]• should originate from [DMPO–OOH]• . Considering that the spectrum of Fig. 1 (curve a) is the only observed during irradiation, we conclude that, although in our photocatalytic system reductive activation of O2 by W10 O32 5− can lead to superoxide species, [DMPO–OOH]• is a negligible source of [DMPO–OH]• . Second, it has been demonstrated that in an oxidant environment (such as photoexcited aqueous suspensions of TiO2 ) DMPO can be directly oxidized to the corresponding radical cation. Its subsequent reaction with water leads to the formation of [DMPO–OH]• [32]. In order to confirm that in our case Eq. (1) is the source of OH• radicals and that the detection of [DMPO–OH]• is not the result of other redox processes involving the employed spin trap, we decided to use coumarin as fluorescent probe. In fact, it is reported that coumarin produces 7-hydroxycoumarin, a strongly luminescent compound, by reaction with hydroxyl radicals according to Eq. (3). This method has been already successfully applied for the detection of hydroxyl radicals generated by photoexcited TiO2 [33]. Therefore, some experiments have been carried out irradiating aqueous solutions containing Na4 W10 O32 (2 × 10−4 M) and coumarin (1 × 10−4 M). The comparison between emission spectra before and after irradiation (Fig. 2 curves (a) and (b)) points out the formation of the fluorescent 7-hydroxycoumarin.

O

O

HO

O

O

+ OH. (3) 3.2. OH• radicals origin Laser flash photolysis experiments have been carried out on deaerated solutions of Na4 W10 O32 in the presence of DMPO. The absorbance growth at 780 nm after a 7 ns laser pulse is in agreement with the formation of the transient wO (Fig. 3 curve (a)) [9]. Moreover, transient absorbance changes in the wavelength range 450–900 nm in the insert of Fig. 3 show the formation of the one-electron-reduced form of the decatungstate W10 O32 5− , which absorbs at 780 nm too [3]. It is also seen that this species survives on a much longer time scale. The above results are a clear indication that photochemical excitation induces W10 O32 4− reduction to W10 O32 5− , so confirming that Eq. (1) does occur. Curve (b) of Fig. 3 shows that W10 O32 5− is not accumulated in detectable amounts in the absence of DMPO, likely because the photogenerated hydroxyl

A. Molinari et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 23–28

Intensity of [DMPO-OH]. (a. u.)

26 0,08

ΔA

0,04

0,04

ΔA

0,00 450

600

750

900

wavelength (nm)

0,02

(a) (b)

0

0,00

25

50

75

100

125

Time (s) 200

400

600

Time (ns) Fig. 3. Kinetics profiles observed at 780 nm obtained after laser excitation at 355 nm of a deaerated aqueous solution of Na4 W10 O32 (2 × 10−4 M). Curve (a): in the presence of DMPO (3 × 10−3 M). Curve (b): in the absence of DMPO. Insert: corresponding transient absorbance spectrum.

radicals are able to quickly reoxidize W10 O32 5− if they are not scavenged by the spin trap. The decatungstate in its phoreduced form is able to initiate reductive activation processes of O2 leading up to hydrogen peroxide. Therefore, the one-electron reduction of H2 O2 according to Eq. (4) should be considered as an additional reaction pathway for the formation of hydroxyl radicals. Detection of OH• radicals during photolysis of acetonitrile solutions of decatungstate in the presence of an organic substrate has been previously explained on this basis [25]. H2 O2 + W10 O32 5− → W10 O32 4− + OH• + OH−

(4)

Intensity of adduct [DMPO-OH]. (a. u.)

Some experiments carried out in the presence of catalase enable us to establish the origin of hydroxyl radicals during irradiation of water solution of Na4 W10 O32 . This enzyme is able to catalyze H2 O2 disproportionation with very high efficiency, so avoiding its accumulation. On this basis, we can assume that the contribution of Eq. (4) to the production of hydroxyl radicals is negligible when photoirradiation is carried out in the presence of catalase. Fig. 4 shows the EPR signal intensity changes of the [DMPO–OH]• adduct in the absence (curve a) and in the presence of catalase (curve b). A comparison between these curves allows us to conclude that, the hydroxyl radicals formed during photoexcitation of Na4 W10 O32 originate from both H2 O oxidation and H2 O2 reduction. In fact, it is

(a)

(b)

(c)

0

25

50

75

100

125

Time (s) Fig. 4. Fixed-field EPR signal intensity of the [DMPO–OH]• adduct vs. time upon irradiation of: (a) aerated aqueous solution of Na4 W10 O32 (2 × 10−3 M) and DMPO (3 × 10−2 M); (b) as in sample (a) but in the presence of catalase; (c) deaerated aqueous solution of Na4 W10 O32 (2 × 10−3 M) and DMPO (3 × 10−2 M) in the presence of catalase.

Fig. 5. Curve (a): fixed-field EPR signal intensity of the [DMPO–OH]• adduct in time upon irradiation ( > 300 nm) of an aqueous solution containing Na4 W10 O32 (3 × 10−3 M) and DMPO (3 × 10−2 M); curve (b): the same experiment but in the presence of propan-2-ol (10−1 M).

seen that the signal intensity of [DMPO–OH]• is significantly lower, but not negligible, when irradiation is carried out in the presence of catalase. Curve (c) shows the EPR signal intensity when the previous experiment with catalase is carried out also in the absence of O2 . It is seen that the direct H2 O oxidation leads to the formation of detectable amounts of [DMPO–OH]• just after few seconds irradiation. Later, when most of the decatungstate has been reduced and can not be reoxidized due to the absence of O2 , the [DMPO–OH]• signal decreases quickly. In line with the statement that both Eqs. (1) and (3) contribute to the formation of OH• radicals, Fig. 2 (curve c) shows that a lesser luminescence by 7-hydroxycoumarin is observed when aqueous solutions of Na4 W10 O32 and coumarin are irradiated as described in Section 3.1 but in the presence of catalase. This can be due to the fact that a lower amount of H2 O2 can be accumulated and, consequently, lesser amounts of hydroxyl radicals are formed. 3.3. Photoexcitation in the presence of propan-2-ol Additional spin trapping investigations have been carried out on aqueous solutions of Na4 W10 O32 containing propan-2-ol (10−1 M). The only detected EPR signal is the same quartet of the [DMPO–OH]• adduct (Fig. 1), indicating that Eq. (1) occurs also in the presence of alcohol. However, Fig. 5 shows that the intensity of this adduct is significantly lower than that observed in pure water. This behavior can be ascribed to two possible reactions involving the alcohol. In fact, propan-2-ol can compete with water for the reaction with wO according to Eq. (5) and, with DMPO for the reaction with OH• radicals (Eq. (6)). It is expected that the reaction between OH• radicals and propan-2-ol leads to the formation of water and hydroxy alkyl radicals. wO + (CH3 )2 CHOH → (CH3 )2 C• OH + W10 O32 5− + H+

(5)

OH•

(6)

+ (CH3 )2 CHOH → (CH3 )2

C• OH

+ H2 O

Laser flash photolysis experiments have been performed on deaerated aqueous solutions of Na4 W10 O32 in the presence of propan-2-ol. The kinetics profile is in line with the formation of the mono-reduced form of decatungstate (Fig. 6). This is further confirmed by the transient absorbance changes in the wavelength range 450–900 nm reported in the insert of Fig. 6. It is also seen an absorption at about 630 nm which can be ascribed to the formation of W10 O32 6− [3]. This result is in agreement with previous observations indicating that a further electron transfer from hydroxy alkyl radicals to W10 O32 5− can generate the two electron reduced polyoxoanion and the carbonyl derivative according to Eq. (7) [34,35]. This very fast reaction gives reason of the lack of any EPR signal due

A. Molinari et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 23–28

27

4.0E-03

CO2 acetone CO2 acetone

0,02

Concentration (M)

0,03

ΔA

0,12

ΔA

0,08

0,01

0,04

3.0E-03

2.0E-03

1.0E-03 0,00 450

600

750

900

wavelength (nm)

0,00 150

300

450

0.0E+00

600

0

Time (ns)

40

60

80

Irradiation time (min)

Fig. 6. Kinetics profile observed at 780 nm obtained after excitation at 355 nm of a deaerated aqueous solution of Na4 W10 O32 (2 × 10−4 M) and propan-2-ol (10−1 M). Insert: corresponding transient absorbance spectrum.

to the adduct between DMPO and hydroxy alkyl radicals during the EPR spin trapping experiments. (CH3 )2 C• OH + W10 O32 5− → (CH3 )2 CO + W10 O32 6− + H+

20

(7)

Gas chromatographic analyses indicate that acetone concentration increases quite linearly during the first thirty minutes irradiation of aerated aqueous solutions of Na4 W10 O32 (2 × 10−4 M) and propan-2-ol (10−1 M) (see Supplementary data). It is also observed that, when the experiment is performed in the presence of catalase, which reduces the amount of OH• radicals, the acetone yield decreases of about 60% This result allows us to conclude that OH• radicals are involved in the photocatalytic oxidation of propan-2-ol. CO2 deriving from over-oxidation of propan-2-ol must be counted among the possible reaction products, since hydroxyl radicals are known to be very powerful reacting species that are able to overoxidize various organic substrates. Indeed, the results reported in the first row of Table 1 shows that the photocatalytic oxidation of propan-2-ol by Na4 W10 O32 yields considerable amounts of CO2 . This result may be of interest for the development of photocatalytic systems for pollutants abatement but, on the other hand, it is a serious problem for possible application in synthesis. Efficiency of a photocatalyst is usually evaluated also in terms of its stability. The system here investigated presents a serious inconvenient from this point of view. In fact, spectroscopic evidences indicate that at neutral pH Na4 W10 O32 undergoes quite fast degradation to smaller fragments that do not absorb light at  > 300 nm. 3.4. Photocatalytic properties of heterogenized Na4 W10 O32 Heterogeneous systems represent a suitable means to tune efficiency and selectivity of photocatalytic processes through the control of the microscopic environment surrounding the photoactive center [2,13–16,22]. In particular, it has been already evidenced that silica surface is able to discriminate among reactants, favoring alcohol adsorption in close proximity to the photoactive species [16,23,36,37]. As a consequence, partially oxidized products can accumulate in the solution bulk without over-oxidation to carbon dioxide. We reported recently about the entrapment of Na4 W10 O32 in a silica matrix by a sol–gel procedure [23]. This material (Na4 W10 O32 /SiO2 ), that contains 30% (w/w) of decatungstate and ˚ and mesopores that is characterized by micropores (7 A˚ and 13 A) ˚ (30 A), strongly affects the selectivity of glycerol oxidation process. In particular, contrary to what obtained with

Fig. 7. Acetone and CO2 concentrations vs. irradiation time obtained irradiating ( > 290 nm) Na4 W10 O32 (2 × 10−4 M, circles) and Na4 W10 O32 /SiO2 (8 g/L, triangles) in aqueous solutions containing propan-2-ol 1 M.

homogeneous Na4 W10 O32 , photoexcited Na4 W10 O32 /SiO2 accumulates primary oxidation products with negligible amounts of over-oxidation products. On this basis, we were prompted here to evaluate the possibility to increase the chemoselectivity of Na4 W10 O32 in the photocatalytic oxidation of propan-2-ol by using Na4 W10 O32 /SiO2 . EPR spin-trapping investigation indicate that heterogenization does not affect the ability of photoexcited Na4 W10 O32 to produce hydroxyl radicals. In fact, photoexcitation of aqueous suspensions of Na4 W10 O32 /SiO2 in the presence of DMPO leads to the formation of the [DMPO–OH]• adduct (Fig. 1, curve b), in agreement with the results obtained in homogeneous solution. The results concerning the photocatalytic activity of Na4 W10 O32 /SiO2 (8 g/L) in aqueous solutions containing propan2-ol (10−1 M) are shown in the second row of Table 1. Propan-2-ol is oxidized to acetone and CO2 . A comparison between the acetone/CO2 ratios of rows 1 and 2 allows us to conclude that the solid support inhibits in some extent the over-oxidation to CO2 . A main advantage of heterogenization is that Na4 W10 O32 /SiO2 is a robust photocatalyst. In fact, the same photocatalyst has been used for repeated experiments after simple washing and drying at 70 ◦ C for 10 min. The decrease in activity, in terms of detected end products, is less than 10% after three repeated cycles. Moreover, control experiments show that any kind of photoactivation of the silica matrix can be excluded since irradiation of a dispersion of SiO2 does not lead to any oxidation product. Heterogenized decatungstate does not oxidize propan-2-ol in the absence of light. Finally, UV–vis spectra of solutions recovered after the irradiation of Na4 W10 O32 /SiO2 show that decatungstate is not released from the support. A significant increase of the matrix effect on the selectivity is observed when the photocatalytic experiments are carried out in the presence of propan-2-ol ten times more concentrated (1 M). Fig. 7 shows the yields of acetone and CO2 as a function of irradiation time for both homogeneous Na4 W10 O32 (circles) and Na4 W10 O32 /SiO2 (triangles). It is seen that CO2 is the main reaction product in the homogeneous experiment, while, its formation is quite negligible when Na4 W10 O32 /SiO2 is used. In particular, acetone/CO2 ratios, obtained at similar propan-2-ol conversion, are 7.5 with Na4 W10 O32 /SiO2 and only 0.7 with homogeneous decatungstate. This very important result can be ascribed to the known ability of silica of promoting alcohol adsorption, allowing the enrichment of substrate on the surface and favoring the detachment of acetone, which prevents its over-oxidation [23,36,37].

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A. Molinari et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 23–28

Table 1 Photocatalytic properties of Na4 W10 O32 and of Na4 W10 O32 /SiO2 in the oxidation of propan-2-ol.a Run

Photocatalytic system

Acetone (mol/L)

CO2 (mol/L)

Molar ratio acetone/CO2

1 2

Na4 W10 O32 propan-2-ol 0.1 M Na4 W10 O32 /SiO2 propan-2-ol 0.1 M

1.42 × 10−3 6.47 × 10−4

1.74 × 10−3 4.60 × 10−4

0.82 1.41

a In a typical experiment, Na4 W10 O32 (2 × 10−4 M) or Na4 W10 O32 /SiO2 (8 g/L) was suspended in an aqueous solution containing propan-2-ol and irradiated (30 min,  > 290 nm) at 298 ± 1 K and 760 Torr of O2 . Reported values are the mean of three repeated experiments with ±5% of precision. The chosen amount of photocatalysts warrants the absorption of 90% of the incident radiation at 313 nm, which is the emission line of the employed light source closest to the absorption maximum of decatungstate at 323 nm.

4. Conclusions This study demonstrates that hydroxyl radicals can be formed irradiating aqueous solutions of Na4 W10 O32 . We show that these species originate from both H2 O oxidation and H2 O2 reduction and that they cause propan-2-ol oxidation. Photocatalytic investigation indicates that Na4 W10 O32 is able to oxidize propan-2-ol up to CO2 with high yield. This result may be of interest for the development of photocatalytic systems aimed to pollutants degradation. On the other hand, we have evidence that the decatungstate dissolved in water is not stable and undergoes complete degradation in few hours. The strong oxidative powerful of Na4 W10 O32 is a drawback in photocatalysis when the main purpose is the selective accumulation of valuable reaction intermediates. Entrapment of Na4 W10 O32 into a microporous silica matrix gives a robust photocatalyst where the decatungstate structure is preserved. This material is able to catalyze the photooxidation of propan-2-ol to acetone with appreciable chemoselectivity. In particular, optimal conditions have been found allowing the solid photocatalyst to yield acetone as main product inhibiting over-oxidation processes that lead to carbon dioxide. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.molcata.2013.01.037. References [1] [2] [3] [4] [5]

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