Unexpectedly ambivalent O2 role in the autocatalytic photooxidation of 2-methoxybenzyl alcohol in water

Article abstract of DOI:10.1016/j.molcata.2015.03.021

Abstract An unusual autocatalytic photooxidation of 2-methoxybenzyl alcohol has been observed under UV irradiation in aqueous medium. The homogeneous oxidation is catalyzed by the corresponding aldehyde that is also the main oxidation product. The trend of alcohol disappearance rate matches the typical shape of an autocatalytic process, where a crucial and ambivalent role is played by the presence of molecular oxygen. Low oxygen concentrations give rise to a zero-order reaction since the beginning of irradiation, while higher amounts of oxygen reduce the alcohol oxidation rate until the aldehyde reaches a concentration high enough to speed up the alcohol's conversion. Experiments performed by varying alcohol, aldehyde and oxygen concentrations in both aqueous and organic media suggest that the alcohol oxidation is initially favored by the presence of oxygen. Once the formed aldehyde competes for photon absorption, the oxidation is driven by aldehydes alpha cleavage leading to reactive oxygenated species in aqueous medium. An excess of oxygen quenches the latter processes thus inhibiting/slowing down the reaction.

Full text of DOI:10.1016/j.molcata.2015.03.021

Journal of Molecular Catalysis A: Chemical 403 (2015) 37–42
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Unexpectedly ambivalent O2 role in the autocatalytic photooxidation
of 2-methoxybenzyl alcohol in water
Giovanni Palmisanoa,1, Gabriele Scandurab,1, Vincenzo Augugliarob, Vittorio Loddob,
Andrea Pacec, Bilge Sina Tekd, Sedat Yurdakald, Leonardo Palmisanob,∗
a Department of Chemical and Environmental Engineering (CEE), Institute Center for Water and Environment (iWater) – Masdar Institute of Science and
Technology, PO Box 54224, Abu Dhabi, United Arab Emirates
b “Schiavello-Grillone” Photocatalysis Group, Deim, Università degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy
c Dipartimento di Scienze e Tecnologie Biologiche Chimiche e Farmaceutiche, Stebicef, Università degli Studi di Palermo, Viale delle Scienze, Edificio 17,
90128 Palermo, Italy
d Kimya Bölümü, Fen-Edebiyat Fakültesi, Afyon Kocatepe Üniversitesi, Ahmet Necdet Sezer Kampüsü, 03200 Afyon, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
An unusual autocatalytic photooxidation of 2-methoxybenzyl alcohol has been observed under UV irradi-
ation in aqueous medium. The homogeneous oxidation is catalyzed by the corresponding aldehyde that is
also the main oxidation product. The trend of alcohol disappearance rate matches the typical shape of an
autocatalytic process, where a crucial and ambivalent role is played by the presence of molecular oxygen.
Low oxygen concentrations give rise to a zero-order reaction since the beginning of irradiation, while
higher amounts of oxygen reduce the alcohol oxidation rate until the aldehyde reaches a concentration
high enough to speed up the alcohol’s conversion. Experiments performed by varying alcohol, aldehyde
and oxygen concentrations in both aqueous and organic media suggest that the alcohol oxidation is ini-
tially favored by the presence of oxygen. Once the formed aldehyde competes for photon absorption,
the oxidation is driven by aldehydes alpha cleavage leading to reactive oxygenated species in aqueous
medium. An excess of oxygen quenches the latter processes thus inhibiting/slowing down the reaction.
© 2015 Elsevier B.V. All rights reserved.
Received 26 November 2014
Received in revised form 25 March 2015
Accepted 25 March 2015
Available online 27 March 2015
Keywords:
Autocatalytic photooxidation
2-Methoxybenzyl alcohol
2-Methoxybenzaldehyde
O2 quenching
Homogeneous photocatalysis
1. Introduction
nane, have been proposed [2b]. Photochemistry in homogeneous
media has been deeply studied during the last decades [3] and it
Oxidation of alcohols has been usually carried out with sto-
ichiometric amounts of metallic oxidants, i.e., permanganates,
chromium(VI) reagents, and ruthenium(VIII) oxide, which are envi-
ronmentally unsafe species containing heavy metals [1a–e]. Both
stoichiometric procedures utilizing a variety of oxidants as well
as catalytic methods based on the use of coordination compounds
of transition metals have been reported [1f]. Then the develop-
ment of efficient systems to oxidize organic substrates is of great
importance both by an economical and an environmental point of
view.
has been applied both in preparation of chemicals [4] and degra-
dation of contaminants [5]. Air purification can be achieved by
photochemical reactions, while irradiation of water can lead to
bactericidal results [6].
Nevertheless only very few studies have been published on
the oxidation of alcohols to aldehydes in homogeneous media,
although this type of photoreaction has been deeply investigated
in heterogeneous systems [7]. In these photooxidations, the con-
centration of oxygen generally enhances the disappearance rate
of the reagent through first order kinetics [8]. In homogeneous
systems, for example, ruthenium complexes [9] or ferric ions [10]
have been used as the photocatalysts. Chen et al. [9] report that
photocatalytic oxidations of aliphatic and aromatic alcohols under
sunlight in water at ambient conditions produce aldehydes and
ketones with high selectivity and turnover numbers higher than
100. A chromophore-catalyst dyad of ruthenium polypridyl com-
plexes was used. Spasiano et al. [10a] also report a homogeneous
photo synthesis of benzaldehyde from benzyl alcohol in water by
The selective oxidation of primary alcohols to the corresponding
aldehydes is very important and widely used in organic synthe-
ses [2a]. Many reagents as for instance TEMPO/NaClO, activated
DMSO, pyridium chlorochromate (PCC), and Dess-Martin periodi-
∗
Corresponding author. Tel.: +39 091 23863746.
E-mail address: leonardo.palmisano@unipa.it (L. Palmisano).
These authors contributed equally.
1
http://dx.doi.org/10.1016/j.molcata.2015.03.021
1381-1169/© 2015 Elsevier B.V. All rights reserved.

38
G. Palmisano et al. / Journal of Molecular Catalysis A: Chemical 403 (2015) 37–42
using Fe(III) aquo-complex at very high pH (0.5) and high yield and
selectivity values of aldehyde were obtained.
MBAD (Sigma–Aldrich, >98% assay) was added to MBA solutions
before irradiation to have different initial concentration ratios of
CMBAD/CMBA. During the photoreactivity runs the samples were
withdrawn from the reactor at fixed times.
In this context, to the best of our knowledge, autocatalytic
photooxidations have never been reported and the role of molec-
ular oxygen has not been investigated. The present study reports
an unexpectedly autocatalytic photooxidation of 2-methoxybenzyl
alcohol (MBA) to 2-methoxybenzaldehyde (MBAD). The reaction is
catalyzed by the same MBAD, which is able to significantly increase
the disappearance rate of the alcohol in water under UV irradia-
tion. Depending on its concentration, molecular oxygen played the
double role of MBA oxidant and MBAD excited state quencher [11].
The identification and quantitative determination of MBA, the
other investigated alcohols, and their oxidation products were
performed by means of a Beckman Coulter HPLC (System Gold
126 Solvent Module and 168 Diode Array Detector), equipped
with a Phenomenex Kinetex 5 ␮m C18 100 column (150 mm
long × 4.6 mm id) at 298 K. The eluent consisted of a mixture of ace-
tonitrile and 13 mM trifluoroacetic acid aqueous solution (20:80
v:v); its flow rate was 0.8 mL min−1. Species were identified by
comparing their retention times and UV–vis spectra with those of
standards. Mineralization extent was evaluated by total organic
carbon (TOC) determination carried out by a 5000 A Shimadzu
analyser. H2O2 presence was confirmed by the absorption of the
titanium(IV)–hydrogen peroxide complex (ca. 405–410 nm) in a
strong acidic medium [12]. The samples were added to TiCl4 in 6 M
HCl solution.
2. Experimental section
Photoreactivity runs were performed by using a cylindrical
batch photoreactor (internal diameter: 100 mm; height: 126 mm)
of Pyrex. The photoreactor (Fig. 1) was provided with ports in its
upper section for the inlet and outlet of gases and for sampling.
The presence of a trap at the outlet of the reactor gas line ensured
the insulation from external air so that the gas concentration in
the reacting solution was constant. The aqueous solution (temper-
ature of ca. 298 K; volume of 800 mL) was magnetically stirred and
was bubbled with a gas containing different ratios of oxygen and
nitrogen for 0.5 h before turning on the lamp and during the run
(the ratios were set by means of a Bronkhorst high-tech mass flow
controller). The oxygen concentration was measured before each
run by using a HD22559.2 apparatus (Delta Ohm). When 100% O2
was bubbled, a 1.2 mM concentration was reached in water, and
the latter figure decreased linearly with the percentage of the bub-
bled O2. A 125 W medium pressure Hg lamp (Helios Italquartz,
Italy), axially positioned within the photoreactor, was cooled by
water circulating through a Pyrex thimble. The radiation energy
3. Results and discussion
Fig. 2a shows the disappearance of the alcohol under UV irradi-
ation and in the presence of different concentrations of dissolved
oxygen, which were kept constant during each run by continu-
ously purging the solution with N2/O2 mixtures. Freshly prepared
aqueous solution of MBA contained a 0.7% amount of MBAD; how-
ever, under these conditions, no photooxidation was observed in
the absence of oxygen. When N2/O2 mixtures containing less than
2% of O2 were purged in the solution, MBA concentration decreased
since the beginning of irradiation with zero-order kinetics (Fig. 2).
In this range, the reaction rate increased with O2 percentage since
a higher amount of oxidant was present in solution. Unexpectedly,
by increasing the oxygen content above 2% in the purging gaseous
mixture, the MBA oxidation process showed an induction period
during which both MBA decreased and MBAD increased very slowly
(Fig. 2a and b). A similar but unexplained lag time was observed
in the photo-chemical oxidation of phenols in air saturated aque-
ous solution of various benzaldehydes [13]. Here, such induction
time showed an almost linear dependence with the oxygen con-
centration in the purging gas mixture (Fig. 2c), and under oxygen
saturated conditions (solution purged with 100% O2), the observed
induction time increased to more than 16 h (Table 1). This finding
suggests that the involvement of MBAD as potential singlet oxy-
gen sensitizer is marginal and can be ruled out, since it should
have led to a reaction rate enhancement with increasing oxygen
concentration. After the induction period, once the MBAD prod-
uct reached a concentration high enough to compete with MBA for
photon absorption and to overcome the quenching of its excited
impinging on the solution had an average value of 4.4 mW cm−2
;
it was measured between 315 and 400 nm by using a radiome-
ter (Delta Ohm DO 9721). The initial 2-MBA (Sigma–Aldrich, >99%
assay) concentrations were 0.15, 0.35 and 0.5 mM. For a few runs
Table 1
Induction times of MBA photooxidation at different initial CMBAD,0/CMBA,0 ratios
(CMBA,0 = 0.5 mM) and in the presence of various molar percentages of oxygen in
the bubbling N2–O2 mixture. Initial pH 5. T = 298 K. Gas flow rate: 150 ml/min.
C
MBAD,0
Induction time[min]
× 100
O2[%]
C
MBA,0
0
10
60
120
310
490
1000
60
0.7
0.7
0.7
0.7
0.7
0.7
0.7
5
1
2
8
20
40
80
100
20
20
20
20
Fig. 1. Scheme of the reaction system: (A) cooling water inlet; (B) cooling water
outlet; (C) nitrogen inlet; (D) oxygen inlet; (E) gas outlet from mass flow controller;
(F) inlet of gas by diffuser; (G) lamp; (H) magnetic stirrer; (I) lamp connectors to
power supply; (L) outlet of gases from reactor; (M) withdrawal point for liquid sam-
ples; (N) mass flow controller; (O) Erlenmeyer flask containing distilled water; (P)
Pyrex-made reactor containing the reacting solution.
36
18
8
10
14
30

G. Palmisano et al. / Journal of Molecular Catalysis A: Chemical 403 (2015) 37–42
39
1
0.8
0.6
0.4
0.2
0
1
1% O2
C
MBAD,0/CMBA,0
2% O2
0.8
0.7%
8% O2
5%
10%
14%
30%
20% O2
0.6
40% O2
80% O2
0.4
0.2
0
0
200
400
t [min]
600
800
0
50
100
t [min]
150
200
(a)
Fig. 3. MBA decrease (CMBA/CMBA,0) vs. irradiation time in runs at different initial
CMBAD,0/CMBA,0 ratios (CMBA,0 = 0.5 mM) under 20% O2 bubbling. Initial pH 5. T = 298 K.
Gas flow rate: 150 ml/min.
0.4
0.3
0.2
0.1
0.0
initial amount of MBAD affected significantly the induction time
(Table 1). The MBAD amount required to reach a significant
reaction rate was indeed achieved sooner. Interestingly, when
starting with a sufficient amount of MBAD (30% with respect to
MBA, see Fig. 3), the reaction started immediately with approx-
imate zero-order kinetics and only a short induction time was
observed. On the other hand, the influence of a change in MBA ini-
tial concentration was less important and strictly related to the
consequently lower production rate of MBAD.
Other aromatic alcohols, in addition to MBA, were also
tested for a preliminary investigation on the correlation between
structural features and autocatalytic behavior. Thus, benzyl alco-
hol (BA), 3-methoxybenzyl alcohol (3-MBA), 4-methoxy-benzyl
alcohol (4-MBA), 2,4-dimethoxybenzyl alcohol (2,4-DMBA), and
2,3,4-trimethoxybenzyl alcohol (2,3,4-TMBA) were irradiated by
following the same protocol, and only 2,4-DMBA showed a similar
behavior to MBA.
The main emission peaks of the used lamp in the UV region are
at 315 and 365 nm (Fig. S1) and it is well known that Pyrex glass
cuts off all the radiation below 300 nm. Indeed, both MBAD and
2,4-dimethoxybenzaldehyde (2,4-DMBAD) show good absorption
above 300 nm (Fig. 4). The presence of some extent of absorp-
tion for these two aldehydes can be considered necessary but
not determinant as also other methoxy-substituted aldehydes,
which did not play any catalytic role, showed absorption above
300 nm (Fig. 4). The lack of catalytic activity of 3-methoxy- and 4-
methoxybenzaldehyde suggests that ortho-substitution is crucial.
In this context, the absence of photocatalytic activity of 2,3,4-
1% O2
2% O2
8% O2
20% O2
40% O2
80% O2
0
100
200
300
400
t [min]
500
600
700
800
(b)
500
400
300
200
100
0
0
20
40
O2 [%]
60
80
(c)
4.0
Fig. 2. (a) MBA decrease and (b) MBAD formation vs. irradiation time in runs at dif-
ferent O2 amounts. (c) Induction time vs. percentage of bubbled O2 (CMBA,0 = 0.5 mM).
Initial pH 5. T = 298 K. Gas flow rate: 150 ml/min.
2 MBAD
3 MBAD
4 MBAD
2,4 DMBAD
3.0
2,3,4 TMBAD
state by molecular oxygen, a knee-shaped decrease of MBA was
recorded with a change of slope paralleling the production of MBAD
and leading to the rapid decrease of the alcohol (Fig. 2b). While a
total conversion of the MBA was achievable, MBAD yield was lim-
ited by its further photoreactivity, since prolonged irradiation time
leads to aldehyde degradation products.
2.0
1.0
Nevertheless total organic carbon (TOC) determinations per-
formed for some selected experiments, indicated that a low extent
of mineralization occurred after the partial conversion of MBA
(>50%) under the experimental conditions adopted.
0
320
330
340
350
380
310
370
300
360
Wavelength [nm]
Given the unusual photoreactivity results, a study of MBA oxi-
dation in the presence of different starting amounts of MBAD was
carried out by fixing the O2 content at 20% (Fig. 3). An increased
Fig. 4. UV spectra of 2-methoxybenzaldehyde (2-MBAD); 3-methoxybenzaldehyde
(3-MBAD); 4-methoxybenzaldehyde (4-MBAD); 2,4-dimethoxybenzaldehyde (2,4-
DMBAD); 2,3,4-trimethoxybenzaldehyde (2,3,4-TMBAD) (C = 0.5 mM).

40
G. Palmisano et al. / Journal of Molecular Catalysis A: Chemical 403 (2015) 37–42
Table 2
0.5
Results of photooxidation of MBA at different molar percentages of oxygen in the
bubbling N2-O2 mixture. Initial MBA concentration: 0.5 mM.
C MBA [mM]
0.4
0.3
0.2
0.1
0.0
C MBAD [mM]
O2 [%]
tX = 50% [min]
tX = 70% [min]
SX=50% [%]
SX = 70% [%]
1
2
8
Air
139
76
81
205
107
93
20
40
55
98
13
31
52
85
135
141
Note: Irradiation time, t; MBA conversion, X; selectivity to MBAD, S.
trimethoxybenzaldehyde could be tentatively explained in terms
of steric hindrance affecting the conjugation between the phenyl
ring and the three methoxy groups which could be maximized by
the co-planarity of the substituents.
0
200
400
600
800
1000
t [min]
The photooxidation of 2-chlorobenzyl alcohol (2-ChlBA),
2-hydroxybenzyl alcohol (2-HydBA), 2-nitrobenzyl alcohol (2-
NitroBA) and 2-methylbenzyl alcohol (2-MethylBA) was also
investigated in order to understand if the primary effect of ortho-
substitution depends on the nature of the substituent group. The
only significant activity was observed for 2-NitroBA that was pho-
tooxidized, with bad selectivity, to its aldehyde. Notably, it was
previously reported that the presence of a nitro group produces
a detrimental effect on the selectivity toward aldehyde forma-
tion [7c]. However, in this case, the aldehyde was not acting as
catalyst since no beneficial effects were observed when a certain
amount of 2-nitrobenzaldehyde was added in the reacting solution.
Moreover, even the 2-hydroxybenzaldehyde (2-HydBAD), which
showed a good absorption above 300 nm (Fig. S9), did not accelerate
the photooxidation of 2-HydBA. Finally, the absence of photocat-
alytic activity of the chloro- (EWG, electron withdrawing group)
and methyl- (EDG, electron donor group) derivatives rules out the
involvement of electronic effect in the observed behavior.
The attention was focused on the effect of oxygen concentra-
tion toward the selectivity of aldehyde formation (Table 2) for MBA
conversions (X) of 50% and 70%. The highest selectivity values (98
and 85% for 50% and 70% conversions, respectively) were obtained
when air was bubbled. In fact, the presence of a low amount of
oxygen induced parallel reactions by producing secondary prod-
ucts whereas a very high oxygen concentration was detrimental
for the conversion of MBA to MBAD favoring further degradation of
MBAD.
(a)
0.6
0.5
0.4
0.3
0.2
0.1
0.0
C MBA [mM]
C MBAD [mM]
0
50
100
t [min]
150
200
(b)
Fig. 5. Variation of MBA and MBAD concentration upon irradiation in the absence
of oxygen (bubbling N2) in water (a) and hexane (b).
oxygen in water, MBA alcohol conversion virtually stopped as soon
as the MBAD concentration dropped to zero (Fig. 5a). The observed
behavior could be rationalized by considering Scheme 1, where
the photooxidation of MBA is favored by intramolecular hydrogen
bonding. In this process the hydride abstraction can occur involv-
ing molecular oxygen. Small amounts of H2O2 were qualitatively
detected by a colorimetric method (see Section 2).
Table 3 reports conversion and selectivity obtained in runs
where air was bubbled, starting from different initial concentration
ratios of MBAD and MBA. Addition of a certain amount of MBAD to
the reacting solution before irradiation produced a beneficial effect
on the conversion as expected, whereas selectivity decreased when
the MBAD to MBA ratios exceeded 0.05 value. The latter finding
suggests an active role of MBAD not limited to simple sensitiza-
tion. This was proven by further testing the photoreactivity of a 1:1
mixture of MBA and MBAD in water and in hexane in the absence
of oxygen (bubbling N2) (Fig. 5).
However, the mechanism reported in Scheme 1 will be quickly
overcome by that reported in Scheme 2, once MBAD concentra-
tion in water reaches a sufficient value to compete for photon
absorption. MBAD can be photochemically reduced producing the
ketyl radical species MBAD-H•. Ketyl radicals can be formed under
acidic conditions by reaction of protonated triplet excited states of
non-phenolic aromatic aldehydes with electron donors [13]. Under
the experimental conditions used in this paper, instead, the triplet
state of the excited aldehyde can be reduced by hydrogen donor
species [13]. When the reductant is the substrate MBA, the transfer
of a benzylic hydrogen to the aldehyde triplet state produces two
ketyl radicals (Scheme 2). The latter can react with dissolved O2
thus producing a peroxide radical species which decomposes into
the hydroperoxide radical and MBAD [13]. This O2 addition would
be nearly diffusion controlled. Alternatively, we cannot exclude
that the ketyl radicals can be oxidized back to MBAD by reaction
with any hydrogen atom acceptor species (e.g., the hydroperox-
ide radical), even if the concentration of any H-abstractor should
be effectively lower than the oxygen concentration (even at low
partial pressures).
These experiments showed that a polar protic solvent was
crucial to observe alcohol conversion whereas the aldehyde was
photoreactive in both solvents. More importantly, in the absence of
Table 3
Results of photooxidation of MBA in air at different initial ratios CMBAD/CMBA. Initial
MBA concentration: 0.5 mM.
C
MBAD,0
× 100
tX = 50%[min]
135
66
47
28
17
tX = 70%[min]
141
70
52
36
22
SX = 50%[%]
SX = 70%[%]
C
MBA,0
0.7
5
10
14
30
98
100
86
74
74
85
90
86
62
76
The processes reported in Scheme 1 are less favorite than those
showed in Scheme 2, due to the scarce photon absorption of MBA
Note: Irradiation time, t; MBA conversion, X; selectivity to MBAD, S.

G. Palmisano et al. / Journal of Molecular Catalysis A: Chemical 403 (2015) 37–42
41
Scheme 1. Initial photochemical oxidation of MBA.
photocatalytic behavior which is inhibited when an excess of
molecular oxygen is dissolved in the reaction medium. The quench-
ing effect can be compensated by increasing the initial amount
of MBAD. A reaction mechanism is suggested by considering the
involvement of the ortho-methoxy substituent in MBA direct oxi-
dation and the ambivalent role of molecular oxygen which can act
both as an oxidant and as a quencher. Encouraging preliminary
results about the use of MBAD as a co-catalyst for the photooxi-
dation of other alcohols will be subject to further study.
Acknowledgement
GS, VA, VL and LP wish to thank MIUR (Rome) for economical
Support (Project: PON02 00153 2849085).
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.
2015.03.021.
Scheme 2. MBAD photocatalyzed oxidation of MBA. (isc: Intersystem crossing).
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