Aqueous selective photocatalytic oxidation of salicyl alcohol by TiO2 catalysts: Influence of some physico-chemical features

Article abstract of DOI:10.1016/j.cattod.2021.06.030

Partial photocatalytic oxidation of salicyl alcohol (2-hydroxybenzyl alcohol) to salicylaldehyde in water was investigated under environmental friendly conditions in the presence of home-prepared and commercial TiO₂ (Merck and Aeroxide P25) samples under UVA irradiation. The photocatalysts were characterized by using BET, XRD, SEM and/or TEM techniques. The effects of crystallinity degree, pH (3–11) and presence of a hole trap (ethanol) on the photocatalytic activity and product selectivity were investigated. 4-Hydroxybenzyl alcohol was also used to study the influence of the position of the substituent group in the aromatic ring. High alcohols conversion and product selectivity values were obtained at pH = 11 by using well crystallized TiO₂ samples. The conversion values significantly decreased by increasing the hole trap concentration, whereas the selectivity values increased slightly. The selectivity towards the corresponding aldehyde after 30% of alcohol conversion was significantly higher for 4-HBA (48%) than for 2-HBA (32%), due to the role of the para position of the substituent group. In order to clarify the different selectivity of the products, various experiments have been also performed starting from the products; these results indicate that the selectivity is also strongly dependent on the stability of the formed products under the experimental conditions used. By concluding, this article reports that the conversion and selectivity values for the studied reaction depend both on the TiO₂ type and on the substrate.

Full text of DOI:10.1016/j.cattod.2021.06.030

Catalysis Today xxx (xxxx) xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Aqueous selective photocatalytic oxidation of salicyl alcohol by TiO2
catalysts: Influence of some physico-chemical features
,
Sedat Yurdakala, Marianna Bellarditab *, Ivana Pibiric, Leonardo Palmisanob, Vittorio Loddob
a
¨
Kimya Bolümü, Fen-Edebiyat Fakültesi, Afyon Kocatepe Üniversitesi, Ahmet Necdet Sezer Kampüsü, 03200 Afyonkarahisar, Turkey
b Dipartimento di Ingegneria, University of Palermo, Viale delle Scienze Ed. 6, Palermo 90128, Italy
c Dipartimento di Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche (STEBICEF), University of Palermo, Viale delle Scienze Ed. 16-17, Palermo 90128, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords:
Partial photocatalytic oxidation of salicyl alcohol (2-hydroxybenzyl alcohol) to salicylaldehyde in water was
investigated under environmental friendly conditions in the presence of home-prepared and commercial TiO2
(Merck and Aeroxide P25) samples under UVA irradiation. The photocatalysts were characterized by using BET,
XRD, SEM and/or TEM techniques. The effects of crystallinity degree, pH (3–11) and presence of a hole trap
(ethanol) on the photocatalytic activity and product selectivity were investigated. 4-Hydroxybenzyl alcohol was
also used to study the influence of the position of the substituent group in the aromatic ring. High alcohols
conversion and product selectivity values were obtained at pH = 11 by using well crystallized TiO2 samples. The
conversion values significantly decreased by increasing the hole trap concentration, whereas the selectivity
values increased slightly. The selectivity towards the corresponding aldehyde after 30% of alcohol conversion
was significantly higher for 4-HBA (48%) than for 2-HBA (32%), due to the role of the para position of the
substituent group. In order to clarify the different selectivity of the products, various experiments have been also
performed starting from the products; these results indicate that the selectivity is also strongly dependent on the
stability of the formed products under the experimental conditions used. By concluding, this article reports that
the conversion and selectivity values for the studied reaction depend both on the TiO2 type and on the substrate.
Photocatalysis
Salicyl alcohol
Salicylaldehyde
Salicylic acid
Selective oxidation
TiO2
Environmental friendly conditions
1. Introduction
attack of strong oxidizing agents as hydroxyl radicals, which unselec-
tively degrade and mineralize almost all organic compounds especially
Heterogeneous photocatalysis is a “green” and economic technology,
which can be carried out under mild experimental conditions of pressure
and temperature in the presence of inexpensive and nontoxic semi-
conductors as photocatalysts, and generally by using water as the sol-
vent. Moreover, O2 present in air can be used as the oxidizing agent, and
solar light or artificial light with low-energy consumption as the irra-
diation sources [1–6]. Due to the mild operative conditions and inex-
pensiveness, the photocatalytic method can compete with the classical
technologies that require usually more drastic conditions. However,
photocatalysis has some drawbacks such as the need for UV light irra-
diation to activate most semiconductors, the low selectivity towards
partial oxidation products (when water is used as the solvent) and the
difficulty of working with highly concentrated solutions.
in water. However, the method can be also used for selective oxidation
and reduction reactions to produce chemicals with high added value
starting from various organic substrates such as alcohols [6–10].
On the irradiated surface of a solid photocatalyst, in fact, oxidizing
and reducing species form simultaneously making the photocatalytic
method suitable for organic synthesis through oxidative or reductive
pathways or through the combination of both. It is possible to find in the
pertinent literature examples of organic synthesis via oxidative [10–17]
or via reductive pathways [18–22].
The efficiency of photocatalytic organic synthesis can be improved
by optimizing some characteristics of the solid photocatalysts such as
type of polymorph, degree of crystallinity, surface acid-base properties,
exposure of particular crystalline facets, coupling of different semi-
conductors, position of the valence and conduction bands, and addition
of dopants. Moreover, also parameters related to the reaction system
such as reactor geometry, setup configuration, type of solvent, presence
Heterogeneous photocatalysis has been generally used for waste-
water treatment to degrade harmful compounds as pesticides, dyes,
drugs, and their intermediates [4]. Photocatalytic reactions involve fast
* Corresponding author.
E-mail address: marianna.bellardita@unipa.it (M. Bellardita).
https://doi.org/10.1016/j.cattod.2021.06.030
Received 5 October 2020; Received in revised form 11 May 2021; Accepted 25 June 2021
Available online 10 July 2021
0920-5861/© 2021 Elsevier B.V. All rights reserved.
Please cite this article as: Sedat Yurdakal, Catalysis Today, https://doi.org/10.1016/j.cattod.2021.06.030

S. Yurdakal et al.
Catalysis Today xxx (xxxx) xxx
alcohol in the product distribution was examined. The main oxidation
products were 4-hydroxybenzaldehyde, 3,4-dihydroxybenzyl alcohol,
and hydroquinone. By increasing the substrate concentration, 3,4-dihy-
droxybenzyl alcohol yield decreased significantly. Moreover, in the
presence of isopropyl alcohol, no 3,4-dihydroxybenzyl alcohol as a
product was detected, and hydroquinone yields increased by increasing
the substrate concentration. Also, the reaction rate decreased signifi-
cantly but 4-hydroxybenzaldehyde selectivity increased. 4-Hydroxyben-
zyl alcohol was oxidized to 4-hydroxybenzaldehyde in the presence of a
commercial (Sigma Aldrich, rutile) and home-prepared rutile TiO2
photocatalysts in water at neutral pH and under UV–Vis irradiation [33].
Only 13% and 8.5% 4-hydroxybenzaldehyde selectivity values were
obtained using home-prepared and Sigma-Aldrich catalysts, respec-
tively. Marotta and co-workers [34] studied the photocatalytic oxidation
of benzyl alcohols (i.e., 2-HBA and 4-HBA) in water under UV irradia-
tion using a commercial TiO2 catalyst (Sigma Aldrich, anatase). This
work [34], different from the present investigation, was performed in
deaerated and very acidic conditions (pH = 2) in the presence of Cu2+
ion (0.5 mM) as electron trap instead of oxygen (usually used by other
researchers). After 2 h of irradiation, conversions of 42% and 39% were
obtained for 2-HBA and 4-HBA, respectively. The selectivity values after
2 h towards the corresponding aldehyde and acid were ca. 43% and 12%
for 2-HBA and 56% and 20% for 4-HBA.
Homogeneous photocatalytic oxidation of 2-HBA was also investi-
gated using 2-methoxybenzaldehyde as a catalyst, and almost no 2-
hydroxybenzaldehyde selectivity (0.6% for 50% conversion) was ob-
tained [35]. The same reaction was also performed by using Degussa
P25 and a home-prepared anatase TiO2 catalyst in water under UV–Vis
irradiation and at neutral conditions. Only 7.5% and 8% selectivity
values (for 50% conversion) towards aldehyde were obtained using
Degussa P25 and the home-prepared sample, respectively.
Fig. 1. XRD patterns of Aeroxide P25, Merck and home prepared (HPA) pho-
tocatalysts. A = Anatase. R = Rutile.
In this work, the photocatalytic oxidation of salicyl alcohol (2-
hydroxybenzyl alcohol) to salicylaldehyde and salicylic acid is reported.
The reaction was performed under environmental friendly conditions in
the presence of home-prepared (HP) and commercial TiO2 samples. The
photocatalysts were characterized and the effects of crystallinity degree,
pH and the presence of a hole trap (ethanol) on the photocatalytic ac-
tivity and product selectivity were studied. 4-Hydroxybenzyl alcohol
was also used to study the influence of the substituent group on the
aromatic ring, and photocatalytic experiments using the partial oxida-
tion product of 2-hydroxybenzyl alcohol and 4-hydroxybenzyl alcohol
as the starting substrates were performed to explain the obtained results.
Table 1
Crystal phase, BET specific surface area, and crystallinity of the catalysts.
Catalyst
Phase
Primary particle size
(nm)
Crystallinity
(%)
A.S.S. (m2
gꢀ 1
)
Merck
A
60
79
10
50
P25
A + R
25.0 (A) 33.0 (R)
37 (A) 62 (R)
HPA
A
A
35
24
35
165
43
HPA-600-2
139
h
HPA-600-8
h
A
150
46
34
2. Experimental
A = Anatase, R = rutile.
2.1. Materials
of gas, temperature, type and quantity of photocatalyst, and initial pH
appear to be essential [8,9,23].
Titanium oxysulfate hydrate (TiOSO4⋅xH2O 29% Sigma), 2-hydroxy-
benzyl alcohol (> 98% Aldrich), 2-hydroxybenzyl aldehyde (99.8%
Aldrich), 2-hydroxybenzoic acid (98% Sigma-Aldrich), 4-hydroxybenzyl
alcohol (> 98% Aldrich), 4-hydroxybenzyl aldehyde (99.8% Aldrich), 4-
hydroxybenzoic acid (98% Sigma-Aldrich) and ethanol were used
without further purification. Commercial TiO2 samples (Merck and
Aeroxide P25) were used as reference photocatalysts.
TiO2 is the most used photocatalyst, since it is highly photoactive,
cheap, and resistant to photocorrosion. The most common polymorphs
of TiO2 are anatase, rutile and brookite which have different intrinsic
electronic and surface physicochemical properties [24–29] resulting in a
different photocatalytic activity and selectivity. From a photocatalytic
point of view, anatase is generally considered the most active phase and
rutile the least active, while, until a few years ago, brookite activity was
less studied due to the difficulty in obtaining it as a pure phase.
Salicylaldehyde (2-hydroxybenzaldehyde) is a key precursor for a
variety of chelating agents, by condensation with amines, some of which
are commercially important [30]. Salicylic acid, derived from the
metabolism of salicin (an alcoholic β-glucoside), is widely used in
organic synthesis and functions as a plant hormone. Salicylic acid is also
a starting molecule for synthesis of acetylsalicylic acid (aspirin) [31].
Photocatalytic oxidation of 4-hydroxybenzyl alcohol using a com-
mercial ZnO in water under UVA irradiation was investigated [32]. The
effect of substrate concentration and absence/presence of isopropyl
2.2. Samples preparation
25 g of TiOSO4xH2O were added to 100 mL of distilled water in a
Teflon vessel. The solution was heated at 180 ◦C for 2 h inside a
stainless-steel autoclave. After the thermal treatment, the precipitate
was washed three times with distilled water, and then dried overnight at
60 ◦C. The powder was labeled as HPA, where A refers to anatase TiO2
polymorphic phase. In order to promote the powder crystallization, two
different aliquots were calcined at 600 ◦C for 2 h and 8 h respectively.
These samples were named HPA-600-2 h and HPA-600-8 h.
2

S. Yurdakal et al.
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Fig. 2. SEM (HPA, Aeroxide P25) and TEM (Merck, HPA, HPA-600-2 h) images of TiO2 samples, respectively.
determined by using the Scherrer equation [36] and the Bellardita et al.
method [37], respectively. The specific surface areas (SSA) were
measured in a FlowSorb 2300 apparatus (Micromeritics) by using the
single-point BET method. Before the measurement the samples were
degassed by flowing a N2/He mixture 30/70 (v/v) for 0.5 h at 523 K.
Nitrogen adsorption–desorption measurements were acquired by
using a Micromeritics ASAP 2020 apparatus. The samples were degassed
below 1.3 Pa at 200 ◦C prior to the measurement. The pore size distri-
butions (PSD’s) were obtained by using the Barrett Joyner Halenda
(BJH) method applied both on the adsorption and desorption branches
of the isotherms.
Table 2
Photocatalytic activity results of 2-HBA (0.5 mM) under UV irradiation for
different pH’s in the presence of the commercial TiO2 sample Merck.
X = conversion; S = selectivity. X,4 h: conversion after 4 h irradiation. Sald 4 h
,
and Sald 30%: salicylaldehyde selectivity after 4 h of irradiation and at 30%
,
conversion, respectively. Sacid,4 h
: salicylic acid selectivity after 4 h of
irradiation.
pH
X4 h
SAld,4 h
SAld,30%
Sacid,4 h
3.0
37
52
53
11
7.4
22
12
9.0
32
0.0
0.3
1.3
5.3
11
SEM (Scanning electron microscopy) images were obtained using a
FEI Quanta 200 ESEM microscope, operating at 30 kV on specimens
upon which a thin layer of gold had been evaporated. TEM images were
acquired by using a Tecnai G2 transmission electron microscope oper-
ating at 200 kV.
2.3. Samples characterization
The X-ray diffraction (XRD) analysis of the samples was performed
by a Philips diffractometer using the Cu Kα (k = 1.5406 Å) radiation at
40 kV and a current of 30 mA with a 2θ scan rate of 1.28◦ minꢀ 1. The
primary particle size and the crystallinity of the samples were
Fig. 3. Photocatalytic oxidation of 2-HBA (full symbols) to salicylaldehyde (empty symbols) at pH 3 (▴), 5.3 ( ) and 11 ( ) by using Merck. Aldehyde concentrations
are reported in the right ordinate.
3

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carried out twice, and the differences between two of them were about
2%.
Table 3
Photocatalytic activity results of 2-HBA (0.5 mM) under UV irradiation in the
presence of ethanol as hole trap at pH 11 by using commercial TiO2 sample
Merck. X = conversion; S = selectivity. X4 h: conversion after 4 h irradiation.
Sald,4 h and Sald,30%: salicylaldehyde selectivity after 4 h of irradiation and at
30% conversion, respectively. Sacid,4 h: salicylic acid selectivity after 4 h of
irradiation.
2.5. Analytical techniques
A Beckman Coulter HPLC (System Gold 126 Solvent Module and 168
Diode Array Detector) equipped with a Phenomenex Kinetex 5 mm C18
100A column (4.6 mm × 150 mm) was used for the analysis of the re-
action solution after irradiation. The eluent consisted of a mixture of
acetonitrile and 13 mM trifluoroacetic acid aqueous solution (30:70
volumetric ratio), and the flow rate was 0.8 mL minꢀ 1. Total Organic
Carbon (TOC) analyses were performed by using a Total Organic Carbon
analyser (Shimadzu, TOC-LCPN model) to evaluate amount of substrates
that mineralized to CO2.
X4 h (%)
Sald,4 h (%)
Sald,30% (%)
Sacid,4 h (%)
Ethanol (mM)
53
48
38
10
22
18
29
32
32
24
33
–
1.3
1.4
1.6
3.5
–
0.10
0.50
5.0
3. Results and discussion
Fig. 1 shows XRD patterns of Aeroxide P25, Merck and home pre-
pared (HPA) photocatalysts. In Table 1 are reported crystal phase, BET
specific surface area, and crystallinity of the used samples. The XRD
peaks at 2Ɵ = 25.58◦, 38.08◦, 48.08◦, 54.58◦ can be attributed to the
anatase TiO2 phase, while those at 2Ɵ = 27.5◦, 36.5◦, 41.0◦, 54.1◦, 56.5◦
to the rutile one. All of the catalysts are in the anatase phase, however
P25 also includes a fraction of rutile (ca. 80% A, 20% R). P25 and Merck
samples presented the highest crystallinity percentage (90% and 80%,
respectively), while the HP ones exhibited low crystallinity values that
increased by calcination and/or duration of the thermal treatment. HPA
showed the highest surface area (169 m2/g) and a significant decrease
was observed after calcination. On the contrary, the crystallites size
increased with the thermal treatment, as expected.
Fig. 4. Photocatalytic oxidation of 2-HBA. X = conversion; S = selectivity. X4 h
:
In Fig. S2 are reported the nitrogen adsorption–desorption isotherms
and the corresponding pore size distribution curves (inset) of the HPA
and HPA-600-2 h samples. They display Type IV isotherms with hys-
teresis loops at relative pressure between 0.5 and 1.0 and between 0.7
and 1, suggesting the contemporaneous presence of mesopores
(2–50 nm) for HPA and macropores (> 50 nm) for HPA-600-2 h,
respectively [38]. The hysteresis curve is type H3 and can been associ-
ated to slit-like pores formed by plate-like particles. After the thermal
treatment an increase in the pore size distribution is observed.
XPS spectra of similar samples (the TiO2 samples were prepared
starting from TiOSO4 by thermal treatment in a Pyrex bottle at 100 ◦C
and successively calcined at different temperatures) were acquired in a
previous study [39]. The spectra revealed the Ti 2p peak at
459.5 ± 0.2 eV which is typical of Ti4+ and the presence of residual
sulfur quantities. In fact, the amount of residual sulfur was low in all of
the samples (S/Ti atomic ratio equal to ca. 0.1) and scarcely influenced
by temperature and duration of calcination. The spectra of the samples
calcined at 600 ◦C for 3 h and 10 h were similar to those of the
non-calcined sample.
the conversion after 4 h irradiation. Sald,4 h and Sald,30%: selectivity towards
salicylaldehyde after 4 h of irradiation and at 30% conversion, respectively.
Sacid,4 h: selectivity towards salicylic acid after 4 h of irradiation.
Table 4
Photocatalytic activity results of 4-HBA (0.5 mM) under UV irradiation at pH 11.
X = conversion; S = selectivity. X4 h: the conversion after 4 h irradiation. S4 h
and SAld,30%: 4-hydroxybenzyl aldehyde selectivity after 4 h and at 30% con-
version, respectively. Sacid,4 h
: 4-hydroxybenzoic acid selectivity after 4 h
irradiation.
Catalyst
X4 h (%)
SAld,4 h (%)
SAld,30% (%)
Sacid,4 h (%)
Merck
48
48
19
38
42
38
36
40
33
35
48
40
9.1
8.5
0
P25
HPA
HPA-600-2 h
HPA-600-8 h
35
37
3.8
5.5
2.4. Photocatalytic experiments
Fig. 2 shows selected SEM and TEM images of the used samples. HPA
samples exhibit clusters of small roundish particles, whilst P25 irregular
shaped agglomerates. For the commercial TiO2 samples spherical par-
ticles whose size ranges from 100 to 200 nm can be observed by TEM
analysis, whilst the home prepared HPA samples consist of agglomerates
with lower dimensions. A slight increase in particle size can be observed
for the HPA-600-2 h sample due to sintering of the aggregates after heat
treatment.
Aqueous partial oxidation of 2-hydroxybenzyl alcohol (2-HBA) and
4-hydroxybenzyl alcohol (4-HBA) along with additional selected tests
starting from the products were carried out in liquid-solid regime in a
Pyrex reactor externally irradiated by six Actinic BL TL MINI 15 W/10
Philips lamps. The main emission peak was in the near-UV region at 365
nm (Fig. S1). The volume of solution was 150 mL, the alcohols initial
concentration was 0.5 mM and the used photocatalyst amount was 0.2
g/L. The pH of the suspension was adjusted by using 0.1 M HCl or NaOH
solutions. The experiments were carried out at ca. 30 ◦C and atmo-
spheric air was used as the oxygen source. The lamps were switched on
30 min after the TiO2 introduction inside the reactor to ensure the
achievement of the adsorption–desorption equilibrium of the substrate
on the catalyst surface. Samples were withdrawn at fixed time intervals
and filtered through 0.2 µm membranes for the quantitative determi-
nation of the substrates and their intermediates. The experiments were
Many commercial and home-prepared TiO2 samples were tested for
the partial oxidation of 2-HBA at pH 11 with the aim to compare their
photoactivity as far as both the conversion of the substrate and the
selectivity towards the aldehyde are concerned. The results are reported
in Table S1. The best results were obtained in the presence of two
commercial samples (Merck and P25) and only one of the home pre-
pared (HPA).
In Table 2 (Fig. S3) the photocatalytic activity results of 2-HBA
oxidation by using TiO2 Merck are reported at different pH’s. At pH’s
4

S. Yurdakal et al.
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Fig. 5. Photocatalytic oxidation of 4-HBA ( ) to 4-hydroxybenzaldehyde ( ) and 4-hydroxybenzoic acid ( ) at pH 11 in the presence of Merck.
Scheme 1. The scheme of photocatalytic oxidation of 2-HBA and 4-HBA.
factors. However, although an interaction between the deprotonated
Table 5
fraction of 2-HBA and the low fraction of positively charged catalyst
surface (with respect to the lower pH value) cannot be excluded at pH
= 11, that interaction cannot be considered important. Indeed, at pH
= 11 the concentration of the deprotonated form of 2-HBA (negatively
charged) is higher than that of its protonated form, but the surface of
TiO2 is predominantly negatively charged. A more significant role in
increasing the degradation rate of the substrate, in the basic system, is
probably played by the hydroxide ions adsorbed on the surface of the
catalyst, since their significant quantity can induce a greater formation
of OH radicals mainly by reaction with the photo-produced holes. On the
other hand, 2-HBA is present almost as protonated form (and then
neutral) at pH 3 being its pKa = 9.92, whilst the TiO2 surface is posi-
tively charged being the pH value of 3 below the zero charge point [16].
Therefore, the interaction between 2-HBA and TiO2 surface was possibly
very weak, the OHꢀ species concentration low and consequently the
activity less significant than at pH 11. Moreover, at pH 3 an intra-
molecular hydrogen bonding can be formed between the two adjacent
Photocatalytic activity results (substrates initial concentration 0.5 mM) in water
under UV irradiation for different pH’s by using Merck TiO2. X = conversion;
S = selectivity. X3 h: conversion after 3 h irradiation. SAld,30%: aldehyde selec-
tivity after 30% conversion. SAcid,30%: salicylic acid selectivity after 30% con-
version. The CO2 selectivity’s was considered after 3 h of reaction (X3 h).
Substrate
pH
X3 h
(%)
SAld,30%
(%)
SAcid,30%
(%)
SCO2/6, X3 h
(%)
2-HBA
6
11
6
55
41
85
33
45
45
46
26
9
0
0
8
5
9.7
low
41
2-HBA
28
salicylaldehyde
salicylaldehyde
salicylic acid
salicylic acid
salicylic acid
4-hydroxybenzoic
acid
11
3
52
8.7
3.9
1.2
2.5
6
11
11
5.3 and 11, significant alcohol conversion was observed (ca. 52%),
whilst at pH 3 just 40% conversion was obtained after 4 h of irradiation.
Different factors contribute to the higher activity observed at basic pH
conditions. Dark adsorption percentages of the alcohol were quite
similar at pH’s 5.3 and 11 (5% and 8%, respectively); on the contrary
almost no adsorption occurred at pH 3. Therefore, it can be concluded
that a slight higher interaction between the alcohol and the TiO2 surface
occurred by increasing the pH.
–
OH groups that stabilizes the molecule.
The selectivity was also surprisingly the highest at pH 11. This trend
is opposite to what observed in a previous work concerning the partial
oxidation of 4-methoxybenzyl alcohol to the corresponding aldehyde
[7], where it was reported that higher activity and product selectivity
were obtained at low pH’s. Therefore, the pH effect on the activity and
product selectivity depends also on the structure of the substrate.
The most important factor addressing the product selectivity is the
stability of the produced intermediate under the working conditions. In
this case, the high selectivity values obtained at basic pH indicate that
salicylaldehyde was very stable in a basic water environment in the
presence of TiO2 and irradiation, and therefore it underwent a low
The variation of pH of the reaction medium can give rise to various
effects depending both on the preparation of the photocatalyst and the
molecular structure of the substrate. In the latter case, the higher ac-
tivity under basic conditions cannot be attributed to electrostatic
5

S. Yurdakal et al.
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Fig. 6. Photocatalytic oxidation of salicylaldehyde (full symbols) to salicylic acid (empty symbols) at pH 6 ( ) and 11 ( ). Salicylic acid concentrations are reported
in the right ordinate.
degradation (see Table 5). It can be noticed that almost no selectivity to
salicylic acid was found. Probably, the produced acid was easily
decomposed to open ring products and CO2. 2-HBA and salicylaldehyde
concentrations during irradiation time at different pH’s are plotted in
Fig. 3. As the irradiation time increases, a progressive decrease in 2-HBA
concentration and a simultaneous increase in salicylaldehyde can be
noted. At pH = 3 the degradation of alcohol is slower, and increases at
higher pH values. The maximum aldehyde concentration (ca.
0.057 mM) was obtained at pH 11.
As far as the selectivity is concerned, higher values than those of 2-HBA
were obtained for all of the photocatalysts, in particular towards the acid
(1.3% starting from 2-HBA and 9.1% starting from 4-HBA). Probably,
both 2-HBA and its oxidation products decomposed more easily than 4-
HBA and its derivatives, due probably to a higher affinity with the TiO2
surface (see Fig. 5 and Table 4). In particular, a good interaction on the
titania surface can occur for salicylaldehyde and 2-hydroxybenzoic acid
in which the functional groups are in the ortho position. Furthermore,
–
OH
these molecules show intramolecular hydrogen bonds, while
In Table 3 the photocatalytic activity results of 2-HBA in the presence
of ethanol as hole trap are reported. By increasing the ethanol concen-
tration, the activity values considerably decreased due to the competi-
tion for holes between the substrate and ethanol. However, the
selectivity values increased slightly. Therefore, the presence of ethanol
as a hole trap was detrimental for the oxidation of 2-HBA and beneficial
for selectivity in accordance to what reported in our previous work for
the oxidation of benzyl alcohol [33]. When high amount of ethanol was
added (i.e., 5 mM), a drastic reduction of the 2-HBA oxidation occurred
as it competed with the main substrate for the holes.
functional groups in the para position give rise to intermolecular
hydrogen bonds (which can lead to dimerization) [42].
The adsorption of the two starting molecules and consequently the
attack by radicals could be different (for instance the greater symmetry
of 4-HBA could favor its planar adsorption on the surface of the pho-
tocatalyst). Nevertheless, it is very difficult to understand clearly this
phenomenon by a spectroscopic investigation (for instance by IR spec-
troscopy) due to the presence of OH groups both in the TiO2 surface and
in the substrates which give rise to strong absorption covering a large
part of the spectrum.
Fig. 4 shows the photocatalytic activity results of 2-HBA performed
at pH 11 by using HP and commercial samples. In particular the 2-HBA
conversion and the selectivity to the corresponding aldehyde and acid
after 4 h of irradiation are compared. Furthermore, to allow a more
direct comparison between the different samples, the selectivity towards
aldehyde was also calculated for the same alcohol conversion (30%).
Merck, P25 and HP-600-8 h samples showed the highest conversion
values, but the selectivity was maximum with the Merck one (Table S2).
For the HP samples, the photocatalytic activity also increased by
increasing the crystallinity. Indeed, the most crystalline HPA sample
(HPA-600-8 h) showed a higher conversion with respect to HPA (53 vs
41%), but the selectivity towards 2-hydroxybenzaldehyde (20% at 30%
conversion) was virtually the same as for the other HP samples. In this
case, the most crystalline sample showed the maximum conversion, in
accordance with the general observation that the oxidant power of a
photocatalyst increases with the crystallinity due to a lower defects
amount [40].
An important influence on the photoactivity is that of the crystal-
linity degree of the photocatalyst. In fact, the photocatalytic activity (in
terms of 4-HBA conversion) of HP samples significantly increased by
increasing the crystallinity from 19% to 42%, whilst the selectivity
slightly decreased at the high conversion values. The conversion values
obtained with 4-HBA are in agreement with those previously found for
4-methoxybenzyl alcohol [40], and indicate that the least crystalline
samples were less oxidant. On the contrary, during the partial oxidation
of 4-methoxybenzyl alcohol, the selectivity towards the corresponding
aldehyde strongly depended on TiO2 crystallinity [40]. In particular, the
highest selectivity values were obtained with the less crystalline cata-
lysts. The highest product selectivity value for 4-HBA, after 4 h of irra-
diation, was obtained by using the HPA sample (40%), which afforded
the lowest conversion (19%). By considering the same degree of con-
version (30%), however, the highest selectivity values were measured
with the commercial samples, even if the data were not dramatically
different from those found in the presence of the calcined HPA samples
(see Table 4). Therefore, even for 4-HBA, in the presence of commercial
samples, the most crystalline photocatalysts were more oxidant and
more selective.
In the presence of Merck, one of the most crystalline samples, both
the conversion and the selectivity presented the highest values, and this
result was different from that observed with other monohydroxylated
aromatic alcohols [7], but in agreement with what reported for the
photocatalytic partial oxidation of glycerol [41].
Scheme 1 shows a sketch of the photocatalytic oxidation of 2-HBA
and 4-HBA. The initial steps could be the interaction of h+ or hydroxyl
radicals with the substrate (2-HBA or 4-HBA) producing hydrox-
ybenzaldehyde that is successively oxidized to hydroxybenzoic acid. The
over-oxidation products of these valuable molecules are mainly
aliphatic species and CO2.
Table 4 shows the conversion and selectivity values measured during
the partial oxidation of 4-HBA. The conversion values were slightly
lower than those obtained for 2-HBA when commercial samples were
used, whilst a more evident decrease was observed with the HP samples.
6

S. Yurdakal et al.
Catalysis Today xxx (xxxx) xxx
Fig. 7. A comparison between 2-HBA and 4-HBA, corresponding 2- and 4-aldehydes partial oxidation products and related 2-and 4-acids resonance structures
respectively.
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S. Yurdakal et al.
Catalysis Today xxx (xxxx) xxx
Fig. 5 shows a representative run of 4-HBA oxidation to 4-hydroxy-
4. Conclusions
benzaldehyde and 4-hydroxybenzoic acid performed at pH 11 by
using Merck TiO2. Throughout the run, the substrate concentration
decreased, while that of both products increased, although the variation
of concentration for both aldehyde and acid between ca. 200 and
250 min was virtually negligible. It is very likely, as observed usually in
photocatalysis, that long irradiation times favor the formation of open
ring unidentified products which subsequently can be mineralized.
Further experiments were performed (Table 5) to highlight the sta-
bility/instability of 2-HBA, salicylaldehyde, salicylic acid and 4-hydrox-
ybenzoic acid under neutral and basic conditions. The results show that
both the oxidation of 2-HBA and mineralization to CO2 were more sig-
nificant at pH 6, evidencing higher tendency to the oxidation at this pH
and justifying the lower selectivity values to aldehyde. Salicylaldehyde
oxidation was much higher at pH 6 than at pH 11 (conversion after 3 h of
irradiation equal to 85 vs 33%, see Fig. 6). These results point out that
the 2-HBA partial oxidation product is less stable at low pH and explains
the enhanced selectivity to salicylaldehyde at higher pH values. Runs
carried out by starting from salicylic acid showed almost the same ac-
tivity results at pH 3, 6 and 11.
In this work selective photocatalytic oxidation of salicyl alcohol was
investigated under environmental friendly conditions in the presence of
some home prepared and commercial TiO2 samples. The photocatalysts
were characterized and the effects of crystallinity degree, pH and hole
trap presence (ethanol) on the photocatalytic activity and product
selectivity were studied. High alcohols conversion and product selec-
tivity values were obtained at basic conditions, and by using well crys-
talline TiO2 samples. By increasing the added concentration of ethanol
as a hole trap, the activity values considerably decreased while the
selectivity values increased slightly. 4-Hydroxybenzyl alcohol was also
used as the starting substrate to study the influence of the position of the
substituent group. Selectivity to the corresponding aldehyde and acid
was much higher for 4-HBA than 2-HBA due to the less tendency of the
first substrate (more stable to the oxidant attacks) to give rise to a
complete mineralization and probably to open ring intermediates
products. This work demonstrates that the selectivity depends on both
the photocatalyst and substrate, and the different photocatalyst pa-
rameters (as crystallinity degree) can play a different role in the pres-
ence of different substrates. It would be useful as future work to attempt
a spectroscopic investigation (difficult due to the presence of OH groups
both in the substrates and on the surface of the photocatalysts) to
correlate conversion and selectivity of 2-hydroxybenzyl alcohol and 4-
hydroxybenzyl alcohol to the position of the OH substituent.
The comparison of salicylic acid and 4-hydroxybenzoic acid oxida-
tion at pH 11 showed that the conversion of salicylic acid is almost twice
higher than that of 4-hydroxybenzoic acid. For this reason, ca. 9%
selectivity towards 4-hydroxybenzoic acid was obtained at pH 11
starting from 4-hydroxybenzyl alcohol, whilst almost no salicylic acid
was formed during the partial oxidation of 2-HBA. These results indicate
that the selectivity is strongly dependent on the stability of the products
under the experimental conditions used. This behavior can be due to a
different interaction of the two substrates with the photocatalyst surface
probably ascribable to a different adsorption way and/or a different
strength of adsorption.
CRediT authorship contribution statement
Sedat Yurdakal: Conceptualization, Investigation, Writing – orig-
inal draft, Writing – review & editing. Marianna Bellardita: Concep-
tualization, Investigation, Writing – original draft, Writing – review &
editing. Ivana Pibiri: Investigation. Leonardo Palmisano: Conceptu-
alization, Writing – review & editing. Vittorio Loddo: Conceptualiza-
tion, Writing – review & editing.
By comparing the resonance structures of each couple of 2- and 4-
substituted phenols isomers (2-HBA and 4-HBA) and those of the cor-
responding partial oxidation products, i.e. 2- and 4-aldehydes and the
related 2-and 4-acids (see Fig. 7), the number and the position of the
charges are the same. Consequently, their stability appears virtually the
same. Nevertheless, it must be stressed that in heterogeneous photo-
catalysis, considerations on the reacting molecules without considering
their interaction with the catalyst surface are not informative and cannot
be related straightforwardly to the observed photoreactivity. For these
reasons, we conclude that the major conversion and minor selectivity
observed for 2-HBA isomer, when compared to the 4-HBA isomer, is
probably due to its major interaction with the catalyst surface, due to the
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgment
–
proximity of phenolic OH to the alcohol/aldehyde/acid moieties. A
The authors thank Dr. Melike Kalkan (University of Ankara, Turkey)
for her help in carrying out some photocatalytic tests and Professor
Giovanni Palmisano (Department of Chemical Engineering, Khalifa
University of Science and Technology, Abu Dhabi, United Arab Emir-
ates) for TEM analysis.
thorough investigation on this specific point could be useful to try to
understand more, but this is not the aim of this work.
In order to compare the mineralization pathways, the CO2 selectivity
values, divided by 6 for stoichiometric normalization, after 3 h irradi-
ation were also determined. Notably, the highest CO2 selectivity values
were obtained for salicylaldehyde oxidation (ca. 41–52%), whilst low
figures were found for salicylic acid and 4-hydroxybenzoic acid oxida-
tion. Moreover, the selectivity values towards salicylic acid from sali-
cylaldehyde oxidation (up to 8%, for 30% conversion) were also low.
The above finding suggests that the salicylaldehyde reaction pathway
from 2-HBA preferentially proceeds towards mineralization rather than
further partial oxidation to salicylic acid.
Appendix A. Supplementary material
Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.cattod.2021.06.030.
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