Nano-TiO2-P25-SO3H as a new and robust photo-catalyst: The acceleration effect of selective oxidation of aromatic alcohols to aldehydes under blue LED irradiation

Article abstract of DOI:10.1016/j.jphotochem.2018.06.035

Nano photo-catalyst ‘TiO₂-P25-SO₃H’ has been synthesized for the first time as a covalently grafted solid acid and used for the oxidation of different aromatic alcohols to the corresponding carbonyl compounds under visible light irradiation (blue LED, λ > 420 nm). This new and efficient nano photo-catalyst has selectively converted primary aromatic alcohols to the corresponding aldehydes in the presence of nitrobenzene as available industrial oxidant with excellent yields. This new powerful nano photo-catalyst provides a green and mild approach for photo-oxidation of alcohols.

Full text of DOI:10.1016/j.jphotochem.2018.06.035

Accepted Manuscript
Title: Nano-TiO2-P25-SO3H as a new and robust
photo-catalyst: The acceleration effect of selective oxidation
of aromatic alcohols to aldehydes under blue LED irradiation
Authors: Saber Hosseini, Ali Amoozadeh
PII:
S1010-6030(18)30284-3
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2018.06.035
JPC 11352
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
3-3-2018
30-5-2018
19-6-2018
Please cite this article as: Hosseini S, Amoozadeh A, Nano-TiO2-P25-SO3H as a new
and robust photo-catalyst: The acceleration effect of selective oxidation of aromatic
alcohols to aldehydes under blue LED irradiation, Journal of Photochemistry and
Photobiology, A: Chemistry (2018), https://doi.org/10.1016/j.jphotochem.2018.06.035
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Nano-TiO2-P25-SO3H as a new and robust photo-catalyst; the acceleration effect of selective
oxidation of aromatic alcohols to aldehydes under blue LED irradiation
Saber Hosseini and Ali Amoozadeh*
Department of Organic Chemistry, Faculty of Chemistry, Semnan University,
Semnan 35131-19111, Iran. E-mail: aamozadeh@semnan.ac.ir
Graphical abstract
Highlights
 Nano-TiO2-P25-SO3H was introduced for the first time as a covalently grafted
solid acid.
 Nano-TiO2-P25-SO3H was employed as a novel heterogeneous photo-
catalyst.
 Selective photo-oxidation of aromatic alcohols to aldehydes occurs with high
conversion using simple blue LED.
 Reusability of photo-catalyst was proved at least five times without suffering
any significant drop in activity.
1

Abstract
Nano photo-catalyst ‘TiO2-P25-SO3H’ has been synthesized for the first time as a covalently grafted solid
acid and used for the oxidation of different aromatic alcohols to the corresponding carbonyl compounds
under visible light irradiation (blue LED, λ > 420 nm). This new and efficient nano photo-catalyst has
selectively converted primary aromatic alcohols to the corresponding aldehydes in the presence of
nitrobenzene as available industrial oxidant with excellent yields. This new powerful nano photo-catalyst
provides a green and mild approach for photo-oxidation of alcohols.
Keywords: Nano-TiO2-SO3H, Photo-catalysis, Solid acid, Oxidation of alcohols, Blue LED
1. Introduction
Selective photo-oxidation of primary aromatic alcohols to the corresponding aldehydes is a prominent
chemical transformation in synthesis chemistry. This is due to carbonyl groups which are the result of the
first photo-oxidation stage of the wide range of organic compounds. Aldehydes derivatives are also useful
in the stuffing, confectionery, beverage industries and used as precursors for many drugs [1]. Usually, for
these types of transformations, titanium dioxide is one of the best practical and useful photo-catalyst. It has
different benefits such as high efficiency, availability, nontoxicity, high chemical stability and suitable
redox potential of the band gap; Degussa P25 (a mixture of rutile and anatase) as a commercial TiO2 has
attracted a great deal of attention [2]. As the light source, mercury lamps, UV lamps and sunlight have been
successfully used [3]. Recently, visible light emitting diodes (LEDs) have been used as an attractive means
to initiate photo-catalytic reactions. Using LED as visible light source has many advantages such as low
cost, readily available, high photon efficiency, needing low voltages of electricity and power stability during
long time operation [4-6]. In the direction of green chemistry, recently, researchers have interested in using
the combination of LED as visible light irradiation source and semiconductors (TiO2-P25) as photo-
catalysts [2]. In 2014, Zand et al. reported photo-reduction of nitro-aromatic compounds into corresponding
anilines using TiO2 Degussa P25 under blue LED irradiation. They showed that TiO2-P25 photo-catalyst
can be activated under high intensity of blue LED lamps [7]. Zhao and co-workers reported a procedure for
aerobic photo-oxidation of alcohols by using TiO2/TEMPO/alizarin red catalyst. Also, they used features
associated with dye-sensitized TiO2 for these types of oxidations [8]. On the other hand, Imamura and co-
2

workers studied the reduction of nitrobenzene and oxidation of 2-propanol in the presence of O2 by using
redox balance (RB) as an indicator. It could be deduced from results, after complete reduction of
nitrobenzene, even excessive photo-irradiation was not helpful for re-oxidation of aniline. Furthermore,
chemoselectivity was assessed and realized only nitro groups reduced in the existence of other reducible
groups such as chloro- or bromo- nitrobenzenes [9]. Also the photo-catalytic oxidation of benzyl alcohols
conducted by purging O2 gas under UV-light and blue LED lamp irradiation by Higashimoto et al. It was
the first report of performing such reactions under visible light irradiation which was resulted in high
conversion and high selectivity on TiO2 [10]. The coupling of two semiconductors with different band gap
could enhance the visible light activity because the charge separation and the energy range of photo-
excitation could be increased [11]. A mixture of CdS-TiO2 was also used by Higashimoto for one-pot
synthesis of imine from benzyl alcohol and nitrobenzene under visible-light irradiation. They showed that
loading CdS on TiO2 provides higher photo-catalytic activity [12].
The accelerating effect of adding external Bronsted acids on photochemical yields has been investigated by
Zhao in recent years [13]. Kozlov et al. investigated the photo-catalytic activity of TiO2 in the presence of
sulfuric acid and sodium hydroxide. Their results showed a direct relation between photo-catalytic activity
and sulfuric acid concentration while it has a reverse relation with NaOH’s concentration. Indeed, sulfuric
acid improved the adsorption constants of the reagents, intermediates and products on the surface of TiO2.
In fact by increasing acidic sites on the catalyst surface, photo-catalytic activity increased by 20-30% [14].
Furthermore, an extensive progress has been observed toward the surface modification of TiO2 with both
inorganic and organic acids significantly accelerated the conversion of the alcohol. As expected,
acceleration was observed in the photo-oxidation of benzyl alcohol on TiO2 when acid was directly added
to the reaction mixture [15]. However, these physically adsorbed acids which react as homogeneous
catalysts will cause the environmental difficulties such difficult separation. Furthermore, homogeneous acid
catalysts have another disadvantages such as using specialized reaction equipment, increasing operational
difficulties, high energy consumption and the formation of large amounts of waste products [16-21]. Due
to these problems, heterogenization of homogeneous catalysts by immobilization of sulfonic acid on the
surface of various solid supports has produced the heterogeneous solid acid catalysts. These solid acids are
appropriate for economically and environmentally friendly because they have some advantages such as
reusability of catalyst, mild reaction conditions, using less toxicity, readily available precursors [22-32].
Solid acids, which were introduced by zolfigol, have played a key role in the catalyzing. He developed a
reaction between silica gel and chlorosulfonic acid, in which sulfuric acid was immobilized on the surface
of silica gel by covalent bond. Applying silica sulfuric acid as a solid acid support could be great proton
source, which was considered for implementation reactions under heterogeneous condition [33].
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Amoozadeh research group, have done great efforts on the surface modification of nano-catalysts,
especially synthesis of heterogeneous solid acid catalysts for organic reactions. Nano-titania sulfonic acid
(n-TSA) was reported by the reaction of Nano-TiO2 with chloro-sulfonic acid, which represented the first
examples of new solid acid nano-catalysts [34]. In further researches, they introduced a number of new
heterogeneous solid acid Nano-catalysts such as Nano-Fe3O4@TDI@TiO2 [35], Nano-WO3-supported
sulfonic acid (n-WO3-SO3H) [36], Nano-zirconia-supported sulfonic acid (n-ZrO2-SO3H) [37] and TiO2-
coated magnetite nanoparticle-supported sulfonic acid (nano-Fe3O4–TiO2–SO3H) [38]. All these
heterogeneous solid acid catalysts were prepared for the first time by covalent grafting of sulfonic acid on
the surface of various solid supports and they were employed as a solid acid in synthesis of organic
reactions. Several advantages have been reported for these catalysts such as good stability, high reusability,
mild reaction conditions, environmentally benign conditions, easy separation and operational simplicity
[39-42].
Based on the above, we decided to prepare the TiO2-P25-SO3H as a new heterogeneous solid acid catalyst
and use it in the photo-catalyst reaction for the first time. This method provides both effects (acceleration
of adding acid and green chemistry) for the photo-oxidation of alcohols. In this context, TiO2-P25-SO3H
catalyst displayed a high potential toward the facile oxidation of aromatic alcohols using blue LED
irradiation. The environmentally friendly green chemical procedures showed by using this new photo-
catalyst and the LED light irradiation. The purpose of the present work is designing a selective method for
photo-oxidation of alcohols to the corresponding aldehydes or ketones by using the ‘TiO2-P25-SO3H’ as a
new powerful photo-catalyst.
2. Experimental
2.1. Materials and instruments
All chemicals were purchased from the Merck and Aldrich chemical companies with no need for further
purification. TiO2-P25 (ca. 70% anatase, 30% rutile; BET area ca. 50 ± 15 m2 g-1) powder was kindly
supplied by Degussa Co. Progress of reactions was assayed by thin layer chromatography (TLC) on
commercial plates coated with silica gel 60 F254. Fourier transform infrared spectroscopy (FT-IR) was
4

implemented on a Shimadzu FT-IR 8400 equipment using KBr pressed powder plates in the range of 400-
4000 cm-1. Thermogravimetric analyses (TGA) were performed by applying a Du Pont 2000 thermal
analysis apparatus under air atmosphere heated from 0 °C to 800 °C at a ramp rate of 5 °C min-1. X-ray
diffraction (XRD) measurements for the TiO2-P25-SO3H powder sample were conducted on a Siemens
D5000 diffractometer (Siemens AG, Munich, Germany) using Cu-Kα radiation of wavelength 1.54° A.
Energy dispersive X-ray (EDX) spectra of the photo-catalyst were reached by a Mira 3-XMU scanning
electron microscope. The X-ray fluorescence (XRF, Model ARL PERFOMIR’X) was used for the
elemental analysis. Diffuse reflectance spectroscopy (DRS, Model Sinco S4100, Korea) was used for
determination of the optical band gap (Eg) of TiO2-P25 and TiO2-P25-SO3H. Scanning electron microscope
(SEM) images were obtained with a BAL-TEC SCD 005 scanning electron microscope instrument
operating at 10 kV. TiO2-P25-SO3H was fixed on a double sided adhesive carbon disk and sputter-coated
with a thin layer of gold to avoid sample charging problems.
2.2. Preparation of nano-TiO2-P25-SO3H
TiO2-P25-SO3H was easily prepared by using of a suction flask that was equipped with a constant-pressure
dropping funnel and a gas inlet tube for conducting HCl gas over an adsorbing solution (i.e., water) and this
flask charged with the TiO2-P25 (crystalline with anatase/rutile) (4g) in dry CH2Cl2 (20 mL). Then the
mixture was stirred slowly in an ice bath and chlorosulfonic acid (1 ml, 15 mmol) was added drop-wisely
over a period of 30 minutes at room temperature while HCl gas evolved from the reaction vessel. Stirring
of mixture was continued until HCl evolution was seized. After the addition was completed, shaking of the
mixture was continued for 30 min. A white powder of nano-TiO2-P25 supported sulfonic acid was obtained.
Then, the solid powder was washed by ethanol (10 mL) and dried at 70◦C. The prepared TiO2-P25-SO3H
was dried in 120◦C for 6h.
2.3. Photo-catalytic conversion experiments
The photo-activity of TiO2-P25-SO3H was evaluated by photo-catalytic oxidation of benzyl alcohol under
visible light irradiation. The photo-catalytic oxidation of benzyl alcohol tests were conducted in a 15 mL
Pyrex glass equipped with a water circulating jacket to control the temperature of reactions. Photo-reactor
was fixed at room temperature during the experiments. Artificial irradiation into Pyrex reactor was provided
via 4 × 3W blue light emitting diode lamps as the visible light source. TiO2-P25-SO3H (0.05 g) was
suspended in 5 mL acetonitrile, then benzyl alcohol (150 µmol) and nitrobenzene (50 µmol) as an oxidant
5

to enhance the photo-catalytic oxidation activity, was added. Furthermore, in each experiment, the mixture
was magnetically stirred in the dark condition to prove the requirement of light irradiation for product
formation.
In order to ensure the photo-activity of TiO2-P25-SO3H in visible light wavelengths, the UV-Vis adsorption
spectra of photo-catalyst and emission spectrum of blue LED were investigated. Due to literatures, the
absorbance spectrum of TiO2-P25 overlaps with a part of emission spectrum corresponding to blue LED,
indicating the photo-activity of TiO2-P25 under blue LED irradiation [7]. The absorbance spectrum of TiO2-
P25 and TiO2-P25-SO3H was compared in Figure 1. The range of overlap region for TiO2-P25-SO3H has
been more extended than TiO2-P25. It means the photo-activity of TiO2-P25-SO3H in visible wavelengths
has been improved.
Scheme 1 shows the expected stoichiometry of photo-catalytic formation of benzaldehyde and aniline via
oxidation of benzyl alcohol and reduction of nitrobenzene. Photo-oxidation of benzyl alcohol to
benzaldehyde and photo-reduction of nitrobenzene to aniline need two holes and six electrons, respectively.
The atom efficiency (AE) [43] of photo-catalytic oxidation of benzyl alcohol under present condition is
71.14% as could be seen in Equation 1. The photo-oxidation of benzyl alcohol would be more noteworthy
if the reduction process led to formation of valuable compounds. The appropriate rate of AE indicates that
the negative electrons and positive holes are consumed for production of valuable compounds. According
to this viewpoint, the photo-catalytic oxidation of benzyl alcohol to benzaldehyde using nitrobenzene is
“green”.
(molecular weight of the desired product)
Atom efficiency (%) =
× 100
Eq. 1
(sum total of molecular weights of all substances produced)
3. Result and Discussion
We have designed, prepared and characterized nano-titania supported sulfonic acid as a novel class of nano
photo-catalyst (Scheme 2).
6

This novel photo-catalyst was characterized via FT-IR spectroscopy, thermal gravimetric analysis (TGA),
X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), X-ray fluorescence (XRF), diffuse
reflectance spectroscopy (DRS) and scanning electron microscopy (SEM).
3.1. Characterization of TiO2-P25-SO3H
Fourier transform infrared spectroscopy (FT-IR) is a significant technique for identification of the
functionalization of nanoparticles. So FT-IR spectra of the nano-TiO2-P25 and nano-TiO2-P25-SO3H were
analyzed (see Supplementary Fig. S1). Due to the reported IR spectra for –SO3H, the FT-IR spectrum of
nano-TiO2-P25-SO3H exhibiting the presence of sulfonic acid functional group [44].
In order to assessment the percentage of the chemisorbed sulfonic acid group on the surface of nano-TiO2-
P25, TGA was applied. The thermo gravimetric analysis comparison of nano-TiO2-P25-SO3H with nano-
TiO2-P25 in the temperature range of 0-800 °C was accomplished (Fig. S2). TGA curves demonstrate two
and three stages of weight loss, for TiO2-P25 and TiO2-P25-SO3H respectively, which has a reasonable
coordination with literatures [44-45]. On the basis of TGA results, it can be concluded that nano-TiO2-P25-
SO3H has a high thermal stability confirming that it can be safely used in organic reactions with high
temperature.
X-ray powder diffraction as is a noteworthy analytical technique, which was used for evaluating the phase
purity and crystal structure of photo-catalyst. The XRD patterns of nano-TiO2-P25 before and after
modification were compared (see details in Supplementary Fig. S3). In accordance to the Debye-Scherrer
equation, the average crystalline sizes of the nano-TiO2 were approximate to be around 32 nm from XRD
pattern. Similar peak intensities of nano-TiO2-P25-SO3H and nano-TiO2-P25 indicating the stability of the
crystalline phase of nano-TiO2-P25 during surface sulfonate-modification.
Energy dispersive X-ray spectroscopy (EDX) as a helpful analytical technique was operated for the
elemental analysis or chemical characterization of nanoparticles. The EDX analysis demonstrates the
existence of oxygen and sulfur in addition to titanium in the nano photo-catalyst, which approves the
presence of SO3H on nano-TiO2-P25 (Fig. S4).
X-ray fluorescence (XRF) analysis was applied for measuring SO3 weight percent in synthesized nano-
TiO2-P25-SO3H. In accordance to XRF results, the amount of SO3 on the surface of photo-catalyst was
weight percent of 5.85, confirmed that immobilization of SO3H on TiO2-P25 was carried out accurately.
The scanning electron microscopy (SEM) analysis was employed to study the surface morphology and
understand the particle shape and size distribution features of nanoparticles. It can be deduced from the
SEM images of nano-TiO2-P25-SO3H that the average particle size of spherical nano-TiO2-P25-SO3H
powder is about 25-45 nm which reveals that the photo-catalyst is in nano size (Fig. S5).
7

Diffuse reflectance spectroscopy (DRS)
The band gap energies of TiO2-P25 and modified sulfonated photo-catalyst are compared by diffuse
reflectance spectroscopy (DRS) (Figure 2). As indicated in figure 2, the band gap energies of these photo-
catalysts, (TiO2-P25 and TiO2-P25-SO3H) are 3.1 eV and 2.6 eV respectively. So, it is evident that a
considerable decrease on band gap energy and red shift take place by immobilizing the SO3H groups on the
surface of TiO2-P25. Therefore, photo-activity of TiO2-P25-SO3H would increase along visible wavelength.
The VB and CB potential edges of TiO2-P25 and TiO2-P25-SO3H are calculated according to the literature
[46-47] (equations 2 , 3).
ECB = X - Ee - 0.5 Eg
EVB = ECB + Eg
Eq. 2
Eq. 3
ECB and EVB are the conduction and valance band edge potentials of a semiconductor, respectively.
Electronegativity value of the semiconductor is shown by X, which is the geometric mean of the
electronegativities of constituent atoms. The energy of free electrons on the hydrogen scale is Ee (⁓4.5 eV).
The band gap energy of the semiconductor compound is expressed by Eg.
Based on above equations, ECB potentials of TiO2-P25 and TiO2-P25-SO3H are calculated -0.62 eV and
+0.42 eV respectively. Also, based on the same calculations, EVB potentials of TiO2-P25 and TiO2-P25-
SO3H are calculated +2.48 eV and +3.02 eV respectively. The results are shown in Figure 3. As indicated
in the literature and mentioned above, there is a reverse relation between the EVB amount and the oxidizing
ability of photo-catalyst. So TiO2-P25-SO3H is more powerful for photo-catalytic oxidation of benzyl
alcohol under visible light irradiation. In fact, the presence of covalently grafted -SO3H groups on the
surface of TiO2-P25 is at the origin of this enormous success.
8

3.2 Evaluation of photo-catalytic activity of TiO2-P25-SO3H for the photo-oxidation of
alcohols
As photo-oxidation has raised a wide attention in the realm of functional group interconversions in synthetic
organic chemistry, we have decided to investigate the applicability of nano-TiO2-P25-SO3H as a novel
heterogeneous photo-catalyst in the photo-oxidation of alcohols into corresponding aldehydes.
For this purpose, the oxidation of benzyl alcohol in the presence of nitrobenzene as an oxidant under blue
LED irradiation as a model reaction was explored to establish the possibility of the method and optimizing
the reaction conditions.
The obtained results are list in Table 1, which shows the condition reaction involving a solution of benzyl
alcohol (150µmol ) in acetonitrile (5mL) with 0.05g of nano-photo-catalyst in the presence of nitrobenzene
(5×10-2mmol) under N2 atmosphere (1atm) irradiated by four high blue LED 3W lamp at room temperature.
The reaction conversion was observed by gas chromatography (GC) and FT-IR. As can be seen,
neither irradiation alone nor the photo-catalyst without light irradiation acquired any conversion of benzyl
alcohol (Table 1, entries 1 and 6). Subsequently, the presence of nano-photo-catalyst under visible light
irradiation for this reaction was Obligatory. A 60% conversion of benzyl alcohol was obtained with 0.01g
of the photo-catalyst (Table 1, entry 2). It is evident that an increasing amount of photo-catalyst obtained a
considerable enhancement of the photo-oxidation (Table 1, entry 3). At the next step, the concentration of
nitrobenzene as an oxidant was investigated and the results showed a quantitative conversion (100%) for
5×10-2mmol nitrobenzene using 0.05g TiO2-P25-SO3H after blue LED irradiation for 7h.
Also, the results illustrate that a higher yield and shorter reaction time were achieved, when the reaction
was used the nano-TiO2-P25-SO3H, which confirms the high efficiency of this nano-photo-catalyst.
Notably, the greater photo-catalytic activity of TiO2-P25-SO3H was most likely related to the SO3H groups
of the photo-catalyst, which could provide efficient acidic sites.
Figure 4 represents the effect of irradiation time on the photo-catalytic oxidation of benzyl alcohol.
Additionally, the consumption of benzyl alcohol and nitrobenzene, and formation of benzaldehyde and
aniline in an acetonitrile suspension of TiO2-P25-SO3H for 420 min photo-irradiation under optimum
condition was monitored.
It is evident that after photo-irradiation, amounts of benzyl alcohol and nitrobenzene monotonously would
decrease, while amounts of benzaldehyde and aniline would increase as the photo-oxidation product of
benzyl alcohol and photo-reduction product of nitrobenzene, respectively.
It could be pointed out that the activity of TiO2-P25-SO3H under blue LED irradiation was remarkable,
while there is no photo-catalytic oxidation reaction when no photo-catalyst exists at the same condition.
9

The plausible mechanism for the preparation of benzaldehyde from photo-oxidation of benzyl alcohol using
TiO2-P25-SO3H as photo-catalyst is outlined in Figure 5. When TiO2-P25-SO3H absorbs visible light
irradiation from blue LED, it will produce a positive hole (h+) in valance band and electron pair (e-) in
conduction band which is the first stage of the reaction [48]. It is noteworthy to mention that, benzyl alcohol
oxidizes with the hole to benzaldehyde and the electron reduces nitrobenzene to aniline.
Recently specific emphasis has been gained on the use of visible light for photo-catalytic reaction [49-50].
In this report, experiments with TiO2-P25-SO3H under LED light were conducted for the oxidation of
alcohol compounds. The results indicated that nitrobenzene can be considered as expedient oxidant in
photo-catalytic oxidation of alcohols under visible light irradiation and in this condition TiO2-P25-SO3H
acts as a high efficiency nano-photo-catalyst.
Alcohol aromatic compounds possessing functional groups such as nitro, methoxy and halide were oxidized
selectively into their corresponding carbonyls and the results are summarized in Table 2. Interestingly, this
method is applicable for a broad variety of aromatic alcohols carrying both electron withdrawing and
electron releasing groups. As it is presented in Table 2, the alcohols with electron releasing groups (Table
2, entry 6 and 7) reacted faster compared to electron withdrawing substitutions (Table 2, entries 3 and 4).
All the aryl alcohols were oxidized in great yields affording a single product, which diminish the attempts
to separate unreacted starting compounds.
It is obvious from Table 2 that nano-TiO2-P25-SO3H can act as an effective photo-catalyst for photo-
oxidation of aromatic alcohols and it is highly efficient in minimizing the reaction time. It is notable that
the most efficient pathway for carbonyl production is ascribed to photocatalytic oxidation of alcohols by
TiO2-P25 supported sulfonic acid under blue LED irradiation. In this study, to assess the efficiency of the
new photo-catalyst for selective photo-oxidation of aromatic alcohols, it was compared by TiO2-P25 (Table
2).
At the next step, in order to promote greener organic synthesis and also for practical applications and
stability of this new nano-photo-catalyst, the level of reusability was also tested.
From this point of view, after completion of the reaction, photo-catalyst was separated by centrifuge and
washed with solvent. After washing in the second stage, color of TiO2-P25-SO3H was slightly yellowish.
The photo-catalyst can be reused at least five times without any remarkable drop in its photo-catalytic
10

activity or the yield of reactions (Fig. 6). This result shows that, no change in the nature of the photo-catalyst
is detectable, after each reaction and –SO3H groups were tightly connected with nano-TiO2-P25, most likely
because of a covalent linkage.
4. Conclusions
In summary, for the first time, we have introduced nano-TiO2-P25-SO3H as a novel and powerful covalently
grafted solid acid nano-catalyst for the selective photo-catalytic oxidation of aromatic alcohols to the
corresponding aldehydes under visible light irradiation. This efficient photo-catalyst can be generalized
well to the current procedures in the field of photo-catalysis reactions. TiO2-P25-SO3H was well
characterized by FT-IR, TGA, XRD, EDX and SEM analyses. The successful immobilization of sulfonic
acid groups was evaluated by FT-IR and TGA. The XRD presented crystallinity retention of nano-TiO2-
P25 after treatment by acidic reagent. SEM results confirmed nanometer-sized particles of the photo-
catalyst and the presence of Ti, S and O atoms was demonstrated in the EDX spectrum.
In this study, nitrobenzene as a new capable oxidant has been developed for the converting alcohols to
related carbonyl compounds in suitable yields. This method has some notable advantages such as
operational simplicity, excellent photo-catalytic performance, mild reaction condition, simple work-up
without column chromatographic purification, environmental friendly and reusability of the photo-catalyst
for five consecutive cycles with consistent activity.
Acknowledgements
We gratefully acknowledge the Faculty of Chemistry of Semnan University for supporting this work.
11

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15

Fig. 1. The emission spectrum of the blue LED lamp and UV-Vis absorption spectrum of TiO2-P25 and TiO2-P25-SO3H.
Fig. 2. Band gap of (a) TiO2-P25 and (b) TiO2-P25-SO3H.
16

Fig. 3. Conduction band and valance band potentials of TiO2-P25 and TiO2-P25-SO3H.
Fig. 4. Effect of irradiation time on the photo-catalytic oxidation of benzyl alcohol.
17

Fig. 5. proposed mechanism for the photo-oxidation of benzyl alcohol by nano-TiO2-P25-SO3H.
18

Fig. 6. Reusability of TiO2-P25-SO3H for the synthesis of benzaldehyde. Reaction condition: benzyl alcohol (150µmol), acetonitrile
(5ml), nitrobenzene (5×10-2) and TiO2-P25-SO3H (0.05g); Reaction under 4 × 3W blue LED irradiation.
NO2
O
NH2
TiO2-P25-SO3H
Acetonitrile
OH
H
3
+
3
+ 2 H2O
+
Scheme 1: Stoichiometric production of benzaldehyde and aniline.
O
S
ClSO3H
CH2Cl2 / r.t
Scheme 2: Preparation of TiO2-P25-SO3H nano photo-catalyst.
TiO2-P25
O
OH
+ HCl
TiO2-P25 OH
O
19

Table 1
Optimization of photo-oxidation of benzyl alcohol to benzaldehyde in the presence of nitrobenzene by TiO2-P25-SO3H under blue
LED irradiation
O
Photocatalyst / Acetonitrile (5mL)
OH
H
Nitrobenzene / LEDa / N2 (1atm)
298K , 7h
150
µmol
Entry TiO2-P25-SO3H (g)
Nitrobenzene (mmol)
5×10-2
Conversionb (%)
Time (h)
1c
2
0
0
60
100
100
20
0
10
9
0.01
0.05
0.08
0.08
0.08
5×10-2
3
5×10-2
7
4
5×10-2
6
5
5×10-3
8
6d
5×10-2
10
a) Four blue LEDs with 3 W electrical power, 80 Lumens.
b) Determined by integration of the signals in GC chromatograms.
c) Photo-oxidation of benzyl alcohol without TiO2-P25-SO3H.
d) In the dark
Table 2
Photo-oxidation of benzyl alcohol derivatives using TiO2-P25 and TiO2-P25-SO3H under blue light LED irradiation
Entry
Alcohols
Product
O
Timea (h)
Timeb (h)
OH
1
15
7
H
20

O
OH
16
18
7.5
11
H
2
3
Br
Br
O
OH
NO2
H
NO2
O
OH
17
10
H
4
O2N
O2N
O
OH
15
13
6.5
H
5
6
Cl
Cl
O
OH
4
H
H3CO
H3CO
O
OH
7
H
14
6
CH3
CH3
a) Conversion of benzyl alcohol derivatives using TiO2-P25.
b) Conversion of benzyl alcohol derivatives using TiO2-P25-SO3H.
21