Green photocatalytic organic transformations by polyoxometalates vs. mesoporous TiO2 nanoparticles: Selective aerobic oxidation of alcohols

Article abstract of DOI:10.1039/c4pp00268g

In this study, the catalytic activity of decatungstate (W₁₀O₃₂^4-) supported on mesoporous TiO₂ nanoparticle assemblies (DT-MTA) was compared with that of homogeneous [Bu₄N]₄W₁₀O₃₂ catalysts under mild conditions. Our experiments showed that both catalytic systems achieve exceptionally high activity and selectivity under UV-visible light oxidation of various para-substituted aryl alcohols, using molecular oxygen as a "green" oxidant. The chemoselective transformation of aryl alcohols into the corresponding ketones was investigated with gas chromatography (GC) and NMR spectroscopy. Product analysis and kinetic results also indicated that these photooxidation reactions proceed via both electron transfer (ET) and hydrogen atom transfer (HAT) mechanisms over the DT-MTA catalyst, with the former one as the predominant, whereas a HAT route was adopted to account for the decatungstate homogeneous catalyzed reactions. This journal is

Full text of DOI:10.1039/c4pp00268g

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Green photocatalytic organic transformations by
polyoxometalates vs. mesoporous TiO2 nano-
particles: selective aerobic oxidation of alcohols†
Cite this: DOI: 10.1039/c4pp00268g
Theodoros S. Symeonidis,a Ioannis Tamiolakis,b Gerasimos S. Armatasb and
Ioannis N. Lykakis*a
In this study, the catalytic activity of decatungstate (W10O324−) supported on mesoporous TiO2 nanoparticle
assemblies (DT-MTA) was compared with that of homogeneous [Bu4N]4W10O32 catalysts under mild con-
ditions. Our experiments showed that both catalytic systems achieve exceptionally high activity and selecti-
vity under UV-visible light oxidation of various para-substituted aryl alcohols, using molecular oxygen as a
“green” oxidant. The chemoselective transformation of aryl alcohols into the corresponding ketones was
investigated with gas chromatography (GC) and NMR spectroscopy. Product analysis and kinetic results also
indicated that these photooxidation reactions proceed via both electron transfer (ET) and hydrogen atom
transfer (HAT) mechanisms over the DT-MTA catalyst, with the former one as the predominant, whereas a
HAT route was adopted to account for the decatungstate homogeneous catalyzed reactions.
Received 13th July 2014,
Accepted 13th October 2014
DOI: 10.1039/c4pp00268g
www.rsc.org/pps
chemical stability and interesting photocatalytic activity.8
Nevertheless, reports concerning the synthesis of hetero-
Introduction
For catalytic oxo-functionalization processes,1 an attractive genized decatungstate catalysts are scarce and are mostly
approach is the use of a solid, recyclable catalyst in the pres- based on the stabilization of W10O324− anions on silica and zir-
ence of environmentally friendly oxidants such as molecular conia mesoporous supports.8,9 These hybrid mesostructures
oxygen.2 The use of supported catalysts on well-ordered metal have been successfully applied for the photooxidation of
oxide surfaces offers several advantages over homogeneous aliphatic alkanes, alkenes and alcohols.9–12
systems, including catalyst recovery and controlled regio- and
In a previous work, we have shown that decatungstate clus-
chemo-selective reactions.3 In addition, supported catalysts ters deposited on silica and γ-alumina surfaces, by wet impreg-
may exhibit larger catalytically active surface area and better nation at different pH values, catalyze the photooxidation of
accessibility of reactants to the active sites. So far, various selected aromatic alcohols to the corresponding carbonyl pro-
photocatalytic reactions with metal oxide semiconductors such ducts with high yields.13 Following our general interest on
as TiO2, ZnO CeO2, Fe2O3, CdS and WO3 have been extensively the homogeneous and heterogeneous catalytic oxidation
studied.4 These include degradation of organic pollutants, oxi- of aromatic compounds, we report here the chemoselective
dation of small organic molecules and photocatalytic splitting oxidation of various para-alkyl substituted aryl alcohols by a
of water into hydrogen and oxygen.5
decatungstate catalyst supported on high-surface-area meso-
During the past few years, important advances have been porous TiO2 nanoparticle assemblies (DT-MTA).14 For compari-
made in the use of various polyoxometalate (POM) clusters son, the relevant oxidation reactions over the homogeneous
supported on metal oxide surfaces as heterogeneous catalysts [Bu4N]4W10O32 (TBADT) catalyst is also discussed. Finally, a
for the oxidation of organic substrates.6,7 Among them, deca- proposed mechanistic pathway for the synthesis of oxidation
tungstate anion (W10O324−) is one of the most prominent products is also suggested.
examples, which has been widely studied due to its excellent
aDepartment of Chemistry, Aristotle University of Thessaloniki, University Campus,
Experimental
Synthesis of the DT-MTA catalyst
GR-54124 Thessaloniki, Greece. E-mail: lykakis@chem.auth.gr
bDepartment of Materials Science and Technology, University of Crete, Vasilika
Vouton, GR-71003 Heraklion, Crete, Greece
Mesoporous sensitized TiO2 nanoparticle assemblies (MTA)
were synthesized using the previously reported two-step soft-
templating and ion-exchange chemical process.14 The meso-
†Electronic supplementary information (ESI) available: Catalysts characteriz-
ation, reaction time profiles, NMR data and copies of spectra. See DOI: 10.1039/
c4pp00268g
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porous structure of MTA was investigated by small-angle X-ray
scattering (SAXS) and transmission electron microscopy
(TEM). Analysis of the TEM images gives an average particle
size of ∼7.8 nm for TiO2 nanocrystals. In addition, N2 adsorp-
tion–desorption measurements showed that the MTA catalyst
contains narrow-sized mesopores between the nanoparticles.
The specific Brunauer–Emmett–Teller (BET) surface area and
the average pore diameter of MTA were calculated to be 157 m2
g−1 and 7.7 nm, respectively.14
Tetrabutylammonium decatungstate was synthesized and pur-
ified according to literature procedures.15 The [W10O32]4− anions
supported on mesoporous titania (DT-MTA) were prepared as
follows: 1 g of Brij-58(HO(CH2CH2O)20C16H33) surfactant was
dissolved in 10 mL of anhydrous ethanol with stirring at room
temperature. Then, 11 µmol of tetrabutylammonium decatung-
state ([Bu4N]4W10O32), 1.56 mmol of titanium tetrachloride
(TiCl4) and 5.55 mmol of titanium(IV) propoxide were slowly
added to the surfactant solution with continuous stirring. The
mixture was kept at room temperature for 2 h and then was
transferred to a Petri dish and stored in an oven at 40 °C for
7 days. A mesoporous solid was prepared by heating the
gel product at 100 °C under vacuum for 12 h, and then gently
calcining in air to 260 °C for 4 h and, subsequently, to 350 °C
for 6 h with a heating rate of 0.5 °C min−1
.
Physical characterization
Small-angle X-ray scattering (SAXS) measurements were per-
formed on a Rigaku S-MAX 300 high-brilliance system using
Cu Kα radiation (80 kV and 40 mA). The sample-to-detector
distance and center of the beam were precisely measured
using a Ag-behenate standard (d001 = 58.38 Å). The two-
dimensional diffraction images were integrated into a one-
dimensional diffraction pattern, as a function of q, with the
Fit2D program. The scattering data were corrected for dark
current and empty tube scattering. Transmission electron
microscopy (TEM) images were taken using a JEOL JEM-2100
electron microscope (LaB6 filament) operating at an acceler-
ated voltage of 200 kV (see Fig. S1, ESI†). Elemental com-
positions were obtained using a JEOL JSM-6390LV scanning
electron microscope (SEM) operated at 20 kV. Data acquisition
was performed using 100 s accumulation time. Nitrogen
adsorption and desorption isotherms were measured at
−196 °C with a Quantachrome Nova 3200e sorption analyzer.
Before analysis, the samples were degassed at 150 °C under
vacuum (<10−4 mbar) for 12 h. The surface areas were calcu-
lated using the Brunauer–Emmett–Teller (BET) model on the
adsorption data in the 0.05–0.25 relative pressure (P/Po) range,
the total pore volumes were calculated from the adsorbed
amount at the P/Po = 0.95, and the distributions of pore size
was derived from the adsorption branch of isotherms, using
the non-local density functional theory (NLDFT) method for
slit pore geometry (see Fig. S2, ESI†).
Fig. 1 Time profiles of para-substituted aryl alcohols (1–5) photooxida-
tion over (A) mesoporous DT-MTA and (B) TBADT compound.
acetonitrile (HPLC-grade) in the presence of 45 mg of the cata-
lyst in an open cap vial. Before switching on the lamp, the
catalytic grains were kept in suspension by vigorous magnetic
stirring for 10 min. Then the reaction mixture was irradiated
for an appropriate time with a Variac Cermax 300 W xenon
lamp (λ > 320 nm) as the light source under continuous bub-
bling with O2 (ca. 20 mL min−1). During irradiation, the reac-
tion mixtures were cooled with ice water bath (ca. 5 °C). The
reaction conversion and the assignment of products were
determined by a combination of gas chromatography-mass
spectrometry (Shimadzu GC-MS QP2010 Ultra) and 1H and
13C NMR spectroscopy (Bruker AMX 300 MHz) by withdrawing
small aliquots from the reaction mixture. To establish the
identity of the products unequivocally, the retention times and
spectral data from the crude aliquots were compared to those
of commercially available compounds. Aryl alcohols 1–5 were
commercially available, while alcohols 6–8 were synthesized
following the literature procedure.16 The entire carbonyl
Photocatalytic reactions
The photosensitized oxidations of aryl alcohols 1–8 (0.1 mmol)
(Fig. 1 and Table 1) were carried out in a solution of 3 mL of
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oxidation reactions, control experiments were conducted
under similar conditions under a nitrogen atmosphere. The
results showed no photocatalytic activity (2% conversion of 1
in 1 h was observed). Moreover, in the absence of the catalyst
or visible light again no oxidation products were obtained,
confirming that the oxidation reaction is really a photocatalytic
process.
Table 1 DT-MTA and TBADT catalyzed photooxidation of the para-
alkyl substitute 1-phenylethanols 3 and 6–8
Besides the product evolution study, a formal kinetic ana-
lysis for the para-X-substituted 1-phenylethanols was also per-
formed. Assuming that the oxygen concentration remains
constant during the reaction process, the reaction can be con-
sidered as pseudo-first order with respect to the benzyl alcohol
substrate (eqn (1)).
Relative product yieldb (%)
Conversiona
(%)
Time
(min)
Sub.
Catalyst
a
b
c
d
lnð1 ꢀ xÞ ¼ ꢀkt
ð1Þ
3
3
6
6
7
7
8
8
TBADT
DT-MTA
TBADT
DT-MTA
TBADT
DT-MTA
TBADT
DT-MTA
90
90
20
90
20
90
60
90
82
35
78
41
55
51
80
35
>99
>99
71
90
85
91
80
92
—
—
5
7
1
2
8
—
—
—
10
—
12
4
—
—
14
3
2
3
where k is the rate constant and x is the conversion of para-
substituted benzyl alcohol at irradiation time t.
Thus, a plot of the ln(1 − x) versus time yields a straight line
with the slope equal to the first-order reaction rate constant
(k). As seen in Fig. S3 (ESI†), a remarkable affect is observed
due to the nature of the para-substituent group, in which the
photooxidation reaction proceeds faster as the electron-donat-
ing ability of the substituent group increases. For example, the
photooxidation of 2 (MeO-substituted) proceeds faster than
the corresponding oxidation of 1 (H-substituted), as inferred
by the relative rate constant ratio of kMeO/kH = 1.4. In
contrast the photooxidation of 5 (NO2-substituted) proceeds
with a lower rate constant than the oxidation of 1 with a ratio
6
—
6
2c
a Alcohol (0.1 mmol), decatungstate (5 × 10−4 M), 2 mL CH3CN, at 5 °C.
b Determined by 1H NMR spectroscopy and GC-MS after reduction of
hydroperoxides to the corresponding alcohols with PPh3, error 1%. c 6%
of the corresponding 4-(1-hydroxyethyl) benzophenone was observed by
NMR.
compounds were identical with the authentic samples based
on NMR and GC-MS. Conversion of the alcohols and selectivity
for the oxidation products were calculated as follows: conver-
sion = 100 × (Co − Ct)/Co, selectivity = 100 × (Cp/Co − Ct) where,
Co, Ct are the initial and final (after time t) molar concen-
trations of alcohol, respectively and Cp is the concentration of
the corresponding product at a certain time t of the photo-
catalytic reaction.
of kNO /kH ∼ 0.1 (Fig. S3, ESI†). This result suggests that an
electron transfer (ET) mechanism is predominant during the
2
oxidation process.
In comparison, the [Bu4N]4W10O32 (TBADT) homogeneous
photocatalyzed oxidations of the 1–5 alcohols were carried out
on the same reaction scale as described above, using 1 mol%
catalyst based on the amount of alcohol. In all cases, the
corresponding para-substituted ketones (1a–5a) were produced
in good-to-high isolated yields (78–85%) within 1 h of
irradiation (Fig. 1B). Different from the above kinetic results
using DT-MTA, homogeneous oxidation of aryl alcohols con-
taining NO2 and Cl (5 and 4) electron-withdrawing substitutes
proceeds faster than the oxidation of para-methoxy 1-phenyl-
ethanol (2), i.e. bearing an electron-donor group (see Fig. S4,
ESI†). This result suggests that a hydrogen atom transfer (HAT)
mechanism is preferred for this system.8
Results and discussion
Product analysis in the DT-MTA and TBADT catalyzed
photooxidation of para-X substituted 1-phenylethanols 1–5
In order to test the catalytic activity of the DT-MTA we per-
formed photooxidation of various para-X-substituted 1-phenyl-
ethanols 1–5 in the presence of molecular oxygen (Fig. 1A). It
was found that 1-phenylethanols oxidized to the corres-
ponding ketones (1a–5a) in appreciable yields (13–75%) within
1 h of irradiation, as identified by GC-MS and NMR spectro-
scopy, see Fig. 1. All the photocatalytic experiments were per-
Stability and reusability of the DT-MTA catalyst
formed under similar conditions, using 0.1 mmol alcohol, Because of the polymeric matrix of the DT-MTA, this catalyst
45 mg catalyst, 3 mL CH3CN, 20 mL min−1 O2, at 5 °C, under can be easily recovered from the reaction mixture by simple fil-
UV-visible irradiation (λ > 320 nm). In consistent with the elec- tration and it can be reused for the next catalytic run. The
trophilic character of the catalyst, the photooxidation of the stability of the DT-MTA was examined by repeating the catalytic
para-methoxy and para-methyl 1-phenylethanols 2 and 3 gives experiments, in which 3 was oxidized under the same reaction
ketones 2a and 3a with 70% and 75% conversion, respectively. conditions described above. As shown in Fig. S5 (ESI†), the
On the other hand, electron-deficient alcohol 5 was oxidized to DT-MTA catalyst can be used at least three times without sig-
the corresponding ketone 5a but in significantly lower yield nificant loss of its catalytic activity (∼85–78% conversion)
(13%). To confirm that molecular oxygen is essential for and selectivity (100%). Elemental X-ray analysis (EDS) and N2
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physisorption results showed that the composition and the the presence of a bulky isopropyl or phenyl group in alcohols 7
porous structure of the three times reused DT-MTA catalyst is and 8 leads to a higher chemoselectivity, resulting in the for-
well preserved after the oxidation reactions (results not mation of the corresponding ketones 7a and 8a in 85% and
shown), thereby, indicating high durability and recyclability.
80% yield, respectively. In the case of DT-MTA catalyzed photo-
oxidations, although lower reaction rates were observed, a
higher chemoselectivity to the corresponding ketones (3a and
6a–8a) was obtained (>90% relative yield, Table 1), thus allows
the separation of the corresponding ketones in >99% isolated
yields. These results reflect that there is a stronger and more
consistent preference for C–H bond cleavage from the benzylic
carbon compared with the para-alkyl substituent of 3 and 6–8
substrates. To rationalize these results, we propose that the
polar surface of the solid support probably favors accumu-
lation of the alcohol moiety, resulting in the selective oxidation
to the corresponding ketones. It is worth noting that these
results clearly demonstrate the important role of the present
Product analysis in the DT-MTA and TBADT catalyzed
photooxidation of para-alkyl substituted 1-phenylethanols
So far, we have reported the catalytic behavior of the meso-
porous DT-MTA and [Bu4N]4W10O32 (TBADT) compound with
respect to the photooxidation of para-X-substituted 1-phenyl-
ethanols 1–5 that contain a benzylic hydrogen atom. To
further study the applicability of these catalytic systems, a
series of p-alkyl-substituted 1-phenylethanols, 6–8, were also
oxidized under the same conditions. These substrates bear two
distinguishable benzylic hydrogen atoms, one on the alcohol
carbon and the other on the para-alkyl substituent, both of
them can undergo C–H cleavage under the present conditions.
According to this reaction scheme, a hydrogen atom abstrac-
catalytic
systems
in
synthetically
useful
oxidative
transformations.
tion or proton release in the case of a radical cation inter- Proposed mechanistic pathways
mediate from the Ca(OH)–H group is expected to form the
Based on the product analysis and kinetic results, we suggest
that the decatungstate-catalyzed homogeneous oxidation of
benzyl alcohols proceeded via a HAT mechanism. Accordingly,
a HAT can be realized from the substrate to the excited state of
the catalyst (wO), leading to the formation of an alpha-hydroxy
carbon-centered radical intermediate (RI) and the reduced
form of the catalyst HW10O324−. The latter can be re-oxidized
by molecular oxygen, producing hydroperoxy radicals (HOO•),
whereas the RI intermediate can be trapped by O2 or the in situ
produced HOO• to form the corresponding aryl ketone
(Scheme 2). This proposed oxidation pathway is consistent
corresponding ketone, while similarly from the benzylic
C(R)–H group of the para-substituent the deoxygenated
product could be obtained, as shown in Scheme 1. Therefore,
the selectivity of the catalyst by means of preferential hydrogen
abstraction can be evaluated.
As shown in Table 1, both the catalytic systems showed a
strong preference for oxidation of the carbon atom that bears
the hydroxyl group, resulting in the formation of the corres-
ponding aryl ketones 3a, 6a–8a. It is worth noting that the
TBADT catalyzes the oxidation of the corresponding alcohols
faster than the heterogeneous DT-MTA catalyst, although lower
chemoselectivity was consistently observed, see Table 1.
Indeed in these oxidation reactions, the selectivity depends on
the nature of para-substituted group and the extent of the
reaction time (see Table S1, ESI†). For instance, in the oxi-
dation of 4-ethyl-1-phenylethanol (6) a highly selective for-
mation of the corresponding ketone 6a was observed (71%)
within a short irradiation time (20 min), while the corres-
ponding 1,4-diketone (6d) was produced in 72% yield at a pro-
longed time (120 min) (see Table S1, ESI†). On the other hand,
with previous mechanistic studies on the oxidation of aromatic
4− 17
alcohols with W10O32
.
Regarding the DT-MTA catalyzed aerobic oxidation of
aryl alcohols, a reasonable mechanistic pathway can be
assessed by the kinetics and selectivity of the reactions. On the
basis of these results, we conclude that aromatic alcohols
are oxidized on DT-MTA by the reaction routes shown in
Scheme 3. First, the TiO2 nanocrystals are excited under the
UV-vis irradiation to generate the TiO2(e−) and TiO2(h+) pairs.
Subsequent oxidation of TiO2(e−) by electron transfer to mole-
cular oxygen can produce the superoxide anions (O2•−).
Although an electron transfer can also be realized from the
Scheme 1 Intramolecular selectivity in the C–H bond oxidation of
para-alkyl substituted 1-phenylethanols.
Scheme 2 Proposed mechanisms for the photooxidation of para-sub-
stituted aryl alcohols catalyzed by the decatungstate anion.
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Acknowledgements
Financial supports by the European Union and the Greek
Ministry of Education (ERC-09, MESOPOROUS-NPs) and
(ARISTEIA-2691) are kindly acknowledged. INL is acknowled-
ging COST action CM1201.
Notes and references
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Scheme 3 Proposed mechanistic pathway for the photocatalyzed oxi-
dation of aryl alcohols over mesoporous DT-MTA.
conduction band of TiO2 (−0.5 V vs. NHE) to the lowest un-
occupied molecular orbital (LUMO) of decatungstate clusters (or
to the long-lived intermediate wO),18 leading to the formation
of W10O325− reduced species. Once these species are produced,
they can be readily re-oxidized by molecular oxygen, forming
hydroperoxy and/or superoxide anion radicals. On the other
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Finally, these radical intermediates can be further reacted with
in situ produced superoxide anions to form the corresponding
carbonyl products. However, under our irradiation conditions
(λ > 320 nm), W10O324− are also be excited to the locally excited
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long lived intermediate wO.8,18 Subsequent hydrogen atom
abstraction from the benzylic position of the alcohol produces
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the one-electron-reduced species H+W10O32 and a radical
intermediate. Thus, a HAT route between the excited states of
wO and aryl alcohols cannot be excluded, giving a significant
amount of the observed deoxygenated products.
Conclusions
In conclusion, the aerobic photooxidation of various aryl
4−
alcohols by the [Bu4N]4W10O32 (TBADT) complex and W10O32
supported on mesoporous TiO2 nanoparticle assemblies
(DT-MTA) has been reported. Our experiments showed that
both catalytic systems are highly active and selective for the
oxidation of aryl alcohols, under mild conditions. The DT-MTA
used in oxidation reactions was found to be stable and active
under the present conditions, providing a unique system that
combines long-term stability and high activity. From the
mechanistic point of view, a HAT mechanism was proposed
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