Involvement of adsorption effects in the TiO2-sensitized photooxidation rate of benzylic derivatives in CH3CN

Article abstract of DOI:10.1002/poc.583

Competitive kinetic experiments in the TiO₂-sensitized photooxidation of some series of differently ring-substituted benzylic derivatives (ArCHROR′) in aerated and/or deaerated CH₃CN and in the presence of Ag₂SO₄ were carried out. From logk_rel vs E_p plots it was hypothesized that a changeover of the electron abstraction site occurs on going from electron-donating to electron-withdrawing groups on the ring, from the aromatic moiety to the OCH₃ group of benzyl methyl ethers, probably owing to the preferential adsorption of this group on TiO₂ with respect to the aromatic ring. This phenomenon is not observed with benzyltrimethylsilanes because the orbital resulting from the overlap between the C-Si bond and the aromatic π system is the only adsorption site. The steric hindrance of an α-methyl group (in benzylic alcohols) or a tert-butyl group (in benzyl tert-butyl ethers) reduces the degree of adsorption but does not influence the shape of the plot because OR′ should still be the preferential adsorption group. This also provides useful information about the surface structure of TiO₂ suspended in CH₃CN. Copyright

Full text of DOI:10.1002/poc.583

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2003; 16: 127–132
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.583
Involvement of adsorption effects in the TiO2-sensitized
photooxidation rate of benzylic derivatives in CH3CN
Tiziana Del Giacco,1 Cesare Rol1* and Giovanni V. Sebastiani2*
1Dipartimento di Chimica, UniversitaÁ di Perugia, Via Elce di Sotto 8, 06123Perugia, Italy
2Dipartimento di Ingegneria Civile ed Ambientale, UniversitaÁ di Perugia, Via G. Duranti 93, 06125 Perugia, Italy
Received 28 February 2002; revised 21 August 2002; accepted 10 September 2002
ABSTRACT: Competitive kinetic experiments in the TiO2-sensitized photooxidation of some series of differently
ring-substituted benzylic derivatives (ArCHROR') in aerated and/or deaerated CH3CN and in the presence of Ag2SO4
were carried out. From logkrel vs Ep plots it was hypothesized that a changeover of the electron abstraction site occurs
on going from electron-donating to electron-withdrawing groups on the ring, from the aromatic moiety to the OCH3
group of benzyl methyl ethers, probably owing to the preferential adsorption of this group on TiO2 with respect to the
aromatic ring. This phenomenon is not observed with benzyltrimethylsilanes because the orbital resulting from the
overlap between the C— Si bond and the aromatic p system is the only adsorption site. The steric hindrance of an a-
methyl group (in benzylic alcohols) or a tert-butyl group (in benzyl tert-butyl ethers) reduces the degree of adsorption
but does not influence the shape of the plot because OR' should still be the preferential adsorption group. This also
provides useful information about the surface structure of TiO2 suspended in CH3CN. Copyright  2003 John Wiley
& Sons, Ltd.
KEYWORDS: titanium dioxide; adsorption; photooxidation; benzylic derivatives
INTRODUCTION
information about the involvement of the adsorption
phenomena on substrate reactivity in the TiO2-sensitized
photooxidation of some series of benzylic derivatives 1–4
in aerated or deaerated CH3CN. Through this study,
further information was obtained about the surface of this
heterogeneous catalyst in the above-mentioned non-
aqueous solvent.
The importance of substrate adsorption in the TiO2-
photosensitized oxidation of polluting organic com-
pounds in waste water is known.1 In previous papers
we reported some of the implications of the substrate
adsorption in a mechanistic study of the above-mentioned
heterogeneous photooxidation of alkylaromatic com-
pounds, in both CH3CN2–4 and water.5
Based on a competitive kinetic study, we recently
reported6 that the reactivity of ring-substituted benzyl
alcohols in CH3CN can be drastically influenced by the
known7 preferential adsorption of an OH group, with
respect to the aromatic ring, on TiO2. In particular, this
phenomenon could be responsible for the changeover of
the electron abstraction (by the photogenerated hole) site
from the aromatic ring, in the presence of electron-
donating groups, to the hydroxyl moiety, in the presence
of electron-withdrawing groups.
RESULTS AND DISCUSSION
Kinetic studies were conducted to acquire further
The only product obtained from the TiO2-photosensitized
oxidation of benzyl methyl ethers 1a–f, in deaerated
CH3CN, was the corresponding benzaldehyde (Table 1).
After 2 h, the observed yield decreased on going from
electron-donating ring-substituted compounds (lower Ep
values) to benzyl methyl ether, whereas in the presence of
electron-withdrawing groups (higher Ep values) the yield
was practically unchanged. To evaluate this behavior
*Correspondence to: C. Rol, Dipartimento di Chimica, Laboratorio di
Chimica Organica, Universita` di Perugia, Via Elce di Sotto 8, 06123
Perugia, Italy. E-mail: rol@unipg.it or G. V. Sebastiani, Dipartimento
di Ingegneria Civile ed Ambientale (Sesione di Techologie Chimiche e
dei Materiali per l’Ingegneria), Facotta´ di Ingeneria, Universita` di
Perugia, Via G. Duranti 93, 06125 Perugia, Italy. E-mail: gseb@
tech.ing.unipg.it
Contract/grant sponsor: Ministero dell’Universita` e Ricerea Scienti-
fica e Tecnologica (MURST).
Contract/grant sponsor: Consiglio Nazionale delle Ricerche (CNR).
Copyright  2003 John Wiley & Sons, Ltd.
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128
T. DEL GIACCO, C. ROL AND G. V. SEBASTIANI
Table 1. Products yields in the photooxidation of benzyl
methyl ethers sensitized by TiO2 in deaerated CH3CN and in
the presence of Ag2SO4 (reaction time 2 h)
potentials (1d–f) had similar reactivity that was higher
than expected based on the correlation. This behavior
could be due to the involvement of an alternative
electron-transfer process from the OCH3 group (via b
in Scheme 1), in which the substituent would have much
less kinetic effect. The changeover of the electron
abstraction site cannot be justified on a thermodynamic
basis. In fact, the reduction potential of an aliphatic ether
such as diethyl ether, Ep >3 V vs SCE (only the solvent/
electrolyte signal is observed up to 3 V in the voltammo-
gram), is higher than that of the least oxidizable benzyl
methyl ether 1f, Ep = 2.80 V. Therefore, this behavior
should be in line with the increased oxidizability of the
OCH3 group due to the adsorption of the molecule by this
moiety, as reported for the OH group of benzyl
alcohols.3,6,7 A recent study, based on oxidation peak
current measurements, supports this hypothesis, indicat-
ing that adsorbed species are consistently characterized
by less positive potentials (are therefore more oxidizable)
than those attributed to solution free species.9 The
increased oxidizability of the OCH3 group would favor
competition for electron abstraction with respect to the
aromatic site when deactivated by substituents having a
sufficiently high electron-withdrawing effect (such as 3-
Cl, 3-CF3 and 4-CF3).
X
Benzaldehyde (%)
Ep (V vs SCE)
4-OCH3
4-CH3
H
62
19
8
1.55
1.98
2.22
2.30
2.70
2.80
3-Cl
9
3-CF3
4-CF3
9
10
Similar results obtained with benzylic alcohols6
support the hypothesis that preferential adsorption (with
respect to the aromatic ring) on the semiconductor
surface occurs at the OR' (R' = H, CH3) group. Whereas
in our previous study6 it was not possible to establish
whether the preferential OH adsorption occurred through
the H or O atom, it is now plausible to suggest that only
the oxygen atom of the OR' group is involved. This
interaction probably involves the oxygen lone pair which
facilitates the abstraction of the electron by the
corresponding orbital.9
Figure 1. Plot of log(kX/kH) vs Ep for TiO2-photosensitized
oxidation of ring-substituted benzyl methyl ethers
(XPhCH2OCH3) to the corresponding benzaldehydes in
deaerated CH3CN and in the presence of Ag2SO4
X
more precisely, the relative reaction rates were deter-
mined by the competitive method. In the plot of logkrel
(krel = kX/kH) vs Ep (Fig. 1), ethers 1a–c are linearly
correlated and show a rate decrease as the redox potential
increases, which indicates a rate-determining electron-
transfer step from the aromatic ring to the photogenerated
hole, (TiO2)h‡, of the semiconductor (via a in Scheme 1).
The slope (À2.2 VÀ1) is characteristic of a substrate-like
transition state in a slightly exoergonic electron-transfer
step.8 In contrast, the ethers with higher reduction
The final product, aldehyde, is probably derived from
the benzylic cation (path c in Scheme 2) obtained from
the oxidation, probably by (TiO2)h‡,
10 of the correspond-
ing benzylic radical (path a).
The logkrel vs Ep profile, relative to the heterogeneous
photooxidation of ethers 1a–f in aerated medium (in this
case the aldehyde is accompanied by the corresponding
methyl benzoate, Table 2), maintained the same shape as
that in the deaerated medium. This behavior is expected
Scheme 1
Copyright  2003 John Wiley & Sons, Ltd.
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ADSORPTION EFFECTS ON TiO2-SENSITIZED PHOTOOXIDATION RATE
129
Scheme 2
since oxygen should not be involved in the kinetically
significant steps (Scheme 1) but rather could participate
in the subsequent steps.11–13 In fact, the benzylic radical
can be captured by oxygen (path b in Scheme 2) or
oxidized by the hole (path a); the rate ratio of the two
competitive paths depends on the reduction potential of
this intermediate radical (the relative amount of ester
increases on going from the electron-donating to the
electron-withdrawing ring-substituted substrates, Table
2).11 To confirm that the anomalous relationship between
the log k and Ep values is due to the presence of two
electron abstraction sites on the same molecule (benzyl
alcohols6 or ethers), the reactivity behavior of a series of
ring-substituted benzyltrimethylsilanes was evaluated.
These compounds have only one adsorption site, the
orbital resulting from the overlap between the C—Si
bond and the p system, from which the electron is
abstracted through a process kinetically influenced by the
substituent effect. In the photooxidation of silanes 3a–d
sensitized by TiO2 in deaerated medium to the corre-
sponding 1,2-diarylethanes2 the diagram of logkrel (from
a previous study2) vs Ep values [determined in this work:
Ep = 1.49 (3a), 1.69 (3b), 1.89 (3c) and 2.22 V (vs SCE)
(3d)] shows that the rate decreases linearly as the
reduction potential increases (Fig. 2).
adsorption phenomena is represented by the reactivity
behavior of the ethers in homogeneous oxidation in the
presence of 2,4,6-triphenylpyrylium tetrafluoroborate,11
a photosensitizer with a reduction potential in the excited
state similar to that of the valence band edge of TiO2 (ca
2.4 V in CH3CN14). In contrast to what happens in the
heterogeneous oxidation with TiO2, substrates with high
Ep, such as 3- and 4-CF3 ring-substituted ethers, are less
reactive than the unsubstituted ether. Moreover, the
reactivities of the two substrates are linearly correlated in
the endoergonic zone of the plot of logkq(fluorescence
quenching) vs DGet(electron transfer) derived from
Marcus–Rehm–Weller treatment (see Fig. 4 in Ref. 11).
This behavior is expected in the homogeneous phase
(absence of adsorption phenomena) because the ring is
the electron abstraction site for all the substrates. [It is
also interesting to observe that, unlike the heterogeneous
phase, 4-MeO and 4-Me substituted ethers appear in the
plateau of the logkq vs DGet diagram, where the rate-
determining step is the diffusion process. This step is not
kinetically influential in the heterogeneous phase since
the pre-adsorption of the substrate is required for its
A confirmation that the observed anomalous reactivity
pattern in the heterogeneous photooxidation of methyl
ethers 1a–f should be due to the involvement of
Table 2. Product yields in the photooxidation of benzyl
methyl ethers sensitized by TiO2 in aerated CH3CN and in the
presence of Ag2SO4
X
Reaction time (h) Benzaldehyde (%) Ester (%)
CH3O
CH3
H
1
30
12
2
41
23
28
20
Figure 2. Plot of log(kX/kH) vs Ep for TiO2-photosensitized
2.5
2.5
3
oxidation of
X
ring-substituted benzyltrimethylsilanes
[XPhCH2Si(CH3)3] to the corresponding 1,2-diarylethanes in
deaerated CH3CN and in the presence of Ag2SO4
CF3
—
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130
T. DEL GIACCO, C. ROL AND G. V. SEBASTIANI
Table 3. Product yields in the photooxidation of a-
methylbenzyl alcohols sensitized by TiO2 in deaerated
CH3CN and in the presence of Ag2SO4
Table 4. Product yields in the photooxidation of benzyl tert-
butyl ethers sensitized by TiO2 in aerated CH3CN and in the
presence of Ag2SO4
X
Reaction time (min)Acetophenone (%)Ep (V vs SCE)
Reaction time Benzaldehyde Ester
Ep
X
(h)
(%)
(%) (V vs SCE)
4-OCH3
4-CH3
H
30
10
30
30
97
40
20
50
1.60
2.00
2.28
2.73
CH3O
CH3
H
0.5
2
30
2
2
2
38
55
31
1.53
1.98
2.25
2.65
4-CF3
3
3
CF3
—
photooxidation1,7a,b and, therefore, the rate for the two
ethers is correlated with that for the unsubstituted ether in
the plot of logkrel vs Ep.]
adsorption of oxygen with respect to the aromatic ring.
However, this structural change would generate steric
hindrance to the adsorption of alcohols 2a–d on TiO2
with respect to the corresponding a-unsubstituted alco-
hols 1. In fact, we have observed that the adsorption
constant of 2a is nearly 1.5 times higher than that of 4-
methoxybenzyl alcohol.16
Steric hindrance to the OR' adsorption could be
increased if benzyl tert-butyl ethers were the starting
material. The photooxidation reaction products of these
substrates in aerated CH3CN are again the corresponding
aldehydes and esters (see Table 4) in relative amounts
depending on the reduction potential of the intermediate
benzylic radicals, as observed for methyl benzyl ethers.
The plot in Fig. 4, similar to those obtained for the above
a-OR' benzyl derivatives, also shows that the introduc-
tion of a very bulky tert-butyl group directly linked to the
oxygen atom furnishes greater steric hindrance than the
methyl group bound to oxygen (as shown above
comparing the adsorption constants of 1a with that of
4-methoxybenzyl alcohol); the steric hindrance, how-
It must be noted that the introduction of a methyl group
in place of a hydrogen atom linked to the adsorption site
(oxygen) generates steric hindrance. In fact, it has been
shown that the adsorption constant of 4-methoxybenzyl
alcohol is higher than that of the corresponding methyl
ether 1a.15 Therefore, the analogous reactivity behavior
observed for alcohols and ethers leads to the hypothesis
that the steric effect of the methyl group is not high
enough to avoid the preferential adsorption of the oxygen
of the OCH3 group with respect to the aromatic ring.
An analogous study on the TiO2-sensitized photooxi-
dation of a-methylbenzyl alcohols in deaerated CH3CN
evaluated the steric hindrance induced by introducing an
a-methyl group. In all cases, the reaction product was the
corresponding acetophenone (Table 3), which was
probably formed by the oxidation of the benzylic radical
(paths a and d in Scheme 2). The plot of logkrel vs Ep (Fig.
3) is similar to the plots for benzyl alcohols6 and ethers
(Fig. 1), showing that an a-methyl group on the benzylic
carbon is not bulky enough to avoid the preferential
Figure 3. Plot of log(kX/kH) vs Ep for TiO2-photosensitized
oxidation of X ring-substituted a-methylbenzyl alcohols
(XPhCH(CH3)OH) to the corresponding acetophenones in
deaerated CH3CN and in the presence of Ag2SO4
Figure 4. Plot of log(kX/kH) vs Ep for TiO2-photosensitized
oxidation of X ring-substituted benzyl tert-butyl ethers to the
corresponding benzaldehydes and tert-butyl benzoates in
aerated CH3CN and in the presence of Ag2SO4
Copyright  2003 John Wiley & Sons, Ltd.
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ADSORPTION EFFECTS ON TiO2-SENSITIZED PHOTOOXIDATION RATE
131
sodium alkoxide obtained from the reaction of the
corresponding benzyl alcohol with NaH in the same
solvent. The crude material obtained from the usual
work-up was purified by column chromatography on
silica gel, eluting with n-hexane. 1a–c and f were
1
characterized by H NMR18,19 and GC–MS. 1d, b.p.
(15 mmHg) = 74°C; ꢀH 7.4–7.2 (4H, m, ArH), 4.42 (2H,
‡
s, CH2), 3.38 (3H, s, OCH3); m/z 156 (M ), 155, 127,
125, 121 (100%), 91, 89, 77. 1e, b.p. (15 mmHg) = 52°C;
ꢀH 7.6–7.4 (4H, m, ArH), 4.51 (2H, s, CH2), 3.42 (3H, s,
‡
OCH3); m/z 136 (M ), 135, 121, 105 (100%), 104, 91, 77,
65, 51. Benzyl tert-butyl ethers 4a, b and d were prepared
by mixing the corresponding alcohols with tert-butyl
alcohol and concentrated sulfuric acid as described,20
whereas 4c was prepared by silane reduction of
benzaldehyde in acidic tert-butyl alcohol;21 all the ethers
Figure 5. Possible interaction (via hydrogen bonding)
between benzyl tert-butyl ethers 4a±d and TiO2 hydroxy-
lated surface
1
ever, is not high enough to limit the interaction between
the oxygen lone pairs of the OR' group and TiO2 with
respect to the interaction through the p-system of the
aromatic ring.
were characterized by H NMR20,22–24 and GC–MS.
Photochemical oxidation. A solution of benzylic alcohol
or ether (0.22–0.24 mmol) in N2- or O2-purged CH3CN
(20 ml, HPLC grade), in the presence of 130 mg of TiO2
and 0.30 mmol of Ag2SO4, was externally irradiated
using a Helios Italquartz 500 W high-pressure mercury
lamp (through a Pyrex filter), with stirring at room
temperature. The TiO2 powder was then filtered through
double paper and repeatedly washed with CH3CN and
diethyl ether; the reaction mixture was poured into water
and extracted with diethyl ether. Quantitative analysis of
the crude product was performed by 1H NMR and/or GC
with a suitable internal standard.
The latter result confirms the hypothesis4,5 that, in
CH3CN, the TiO2 surface structure has hydroxyl groups
that are derived from water physiadsorption or dissocia-
tive chemiadsorption, as in aqueous medium.17 If a
hydroxylated surface structure derived from dissociative
chemiadsorption is considered, the tert-butyl group does
not significantly hinder the interaction between the
oxygen lone pair and the hydroxyl group on TiO2 via
hydrogen bonding, allowing a greater distance between
the substrate and the TiO2 surface (Fig. 5). In contrast, if
the interaction involves the oxygen lone pairs and the
superficial Ti atoms (Lewis acid sites), the tert-butyl
steric hindrance would become so important as to move
the adsorption site from the oxygen atom to the ring and,
in this case, the logkrel vs Ep profile would be linear, as
observed for silanes.
Reaction products. The products were identified directly
1
from the crude material by comparison of the H NMR
and GC–MS data with those for commercial samples
(aldehydes and ketones) or literature spectroscopic data
(esters).
Competitive experiments. Kinetic experiments were
performed at 25°C by irradiating (Applied Photophysics
multilamp photochemical reactor, l = 355 Æ 20 nm)
mixtures containing the same amounts of TiO2, Ag2SO4,
CH3CN and substrate (corresponding to the sum of
equimolar quantities of the two compounds) as reported
in the above photooxidation procedure. The amounts of
reaction products were determined by GC with respect to
an internal standard at different times and the values were
inserted in a suitable kinetic equation.25
EXPERIMENTAL
1
Instrumentation. H NMR spectra were measured on a
Bruker AC 200 (200 MHz) spectrometer from solutions
in CDCl3 with TMS as internal standard. GC–MS
analyses were performed on a Hewlett-Packard Model
6890A gas chromatograph (HP-Innovax capillary col-
umn, 15 m) coupled with a MSD-HP 5973 mass-selective
detector (70 eV). GC analyses were carried out on a HP
Model 5890 gas chromatograph (HP-Innovax capillary
column, 15 m).
Cyclic voltammetry. Ep values were obtained using an
AMEL 552 potentiostat controlled by a programmable
AMEL 568 function generator (at 100 mV sÀ1, 1 mm
Materials. TiO2 (anatase) (Aldrich, 99.9%, dried at
110°C), CH3CN (HPLC grade, water content 0.02%
from Karl Fischer coulometry), Ag2SO4 and a-methyl
benzyl alcohols 2a–d were commercial samples. Benzyl-
trimethylsilanes 3a–d were available from previous
studies.2 Benzyl methyl ethers 1a–f were prepared as
described18 by methylation (with CH3I in DMSO) of the
diameter platinum disk anode) in CH3CN–LiClO4 (0.1 M).
Acknowledgements
This work was carried out with the financial support of
Copyright  2003 John Wiley & Sons, Ltd.
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132
T. DEL GIACCO, C. ROL AND G. V. SEBASTIANI
11. Del Giacco T, Lipparoni L, Ranchella M, Rol C, Sebastiani V. J.
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Tecnologica (MURST) and the Consiglio Nazionale delle
Ricerche (CNR).
14. Kabir-ud-Din, Owen RD, Fox MA. J. Phys. Chem. 1981; 85:
1679–1682.
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