Ag-doped TiO2 enhanced photocatalytic oxidation of 1,2-cyclohexanediol

Article abstract of DOI:10.1002/poc.2974

The superiority of silver-doped titania for photocatalytic oxidation (PCO) of organic compounds inspired us to investigate PCO of 1,2-cyclohexanediol. Ag/TiO₂ was prepared, characterized (nanosize 19-24 nm) and used for oxidation of 1,2-cyclohexanediol (1) in acetonitrile. The photolysate was analyzed using GC and GC/MS techniques. The PCO products are 2-hydroxylcyclohexanone (2), 1,2-cyclohexandione (3), 2-cyclohexenone(4), cyclohexanone (5), and adipic acid (6).The formation of electron-hole pair at the surface of the catalyst followed by oxidation reactions was the suggested mechanism. Kinetic studies revealed first-order mechanism for PCO of 1 and rate constant (k) = -0.145 h^-1. Copyright

Full text of DOI:10.1002/poc.2974

Short Communication
Received: 4 March 2012,
Revised: 29 April 2012,
Accepted: 6 May 2012,
Published online in Wiley Online Library:
(wileyonlinelibrary.com) DOI: 10.1002/poc.2974
Ag-doped TiO2 enhanced photocatalytic
oxidation of 1,2-cyclohexanediol†
Aboel-Magd A. Abdel-Wahaba*, Omima S. Mohameda, Saleh A. Ahmeda,b
and Mostafa F. Mostafaa
The superiority of silver-doped titania for photocatalytic oxidation (PCO) of organic compounds inspired us to
investigate PCO of 1,2-cyclohexanediol. Ag/TiO2 was prepared, characterized (nanosize 19–24 nm) and used for
oxidation of 1,2-cyclohexanediol (1) in acetonitrile. The photolysate was analyzed using GC and GC/MS techniques.
The PCO products are 2-hydroxylcyclohexanone (2), 1,2-cyclohexandione (3), 2-cyclohexenone(4), cyclohexanone
(5), and adipic acid (6).The formation of electron–hole pair at the surface of the catalyst followed by oxidation
reactions was the suggested mechanism. Kinetic studies revealed first-order mechanism for PCO of 1 and rate
constant (k) = ꢀ0.145 h–1. Copyright © 2012 John Wiley & Sons, Ltd.
Keywords: Ag /TiO2 nanoparticle; 1,2-Cyclohexanediol; photocatalysis; photocatalytic oxidation
from Alfa Aeser GmbH & Co and 1,2-cyclohexanedione (3) from
Merck Schuchardt.
INTRODUCTION
Heterogeneous photocatalysis has been started with the pioneer
findings of Fujishima and Honda.[1] Application of photocatalytic
Preparation of 2-hydroxycyclohexanone (2)
oxidation (PCO), as one of the promising technologies, in water
recycling, air pollution treatment,[2–14] as well as in organic
A solution of dimethyl adipate (4.0 g, 0.23 mole) in 40-ml dry toluene was
synthesis,[15–22] has been a great interest. The need for sustainable
added for 1 h to a mixture of freshly wired sodium metal (2.3 g, 0.1 gA) in
120-ml dry toluene placed in a three-necked flask. The reaction was
green chemistry technology to obtain valuable chemicals appears
today of primary importance. PCO may decisively contribute to this
target, but a main drawback is that TiO2 is a UV absorber and has to
be modified to make use of the renewable solar energy. In this
processed as reported.[27] Colorless crystals of 2 were obtained (1.2 g, 47%).
The circumstances were as follows: mp 95–98 ꢁC, lit. mp = 93–98 ꢁC,[27] mass
spectra, m/z (%) = 114 (26, P+.), 70 (100, B+.).
domain, extensive work has been carried out to enhance the
anatase PCO activity via deposition of noble metals (Ag, Au, Pt),
Preparation of 2-cyclohexenone (4)
non-noble metals (Cu, Zn, Fe, Pd, W, Co, Rb, Y, La, Mo), or other
A solution of 15 g chromic anhydride in a mixture of 15-ml water and
50-ml glacial acetic acid was dropwise added to a mixture of 13.66 g
(0.16 M) cyclohexane in 33.0 g glacial acetic acid placed in a three-
necked flask at 25–35 ꢁC. The reaction was processed as reported.[28]
The product 2-cyclohexane (3.3g, 37%) was collected at 55–56 ꢁC/17 mm
Hg, lit. bp 67 ꢁC/25 mm Hg,[28] and mass spectra, m/z (%) = 96 (46, P+.),
68 (100, B+.).
elements(N, C, S, F, P).[5–12]
Several factors have a role on the rate of PCO process, such as
type and size of TiO2 particles, silver precursor, method of
preparation of the doped catalyst, nature of the substrate, the
pH, and the rate of oxygen flow.[7] Ag deposits generate a great
number of holes by acting as electron sinks and assisting their
transfer to species in solution (e.g., dissolved O2). This reduces
the possibility of electron/hole recombination. The organic com-
pound must be adsorbed on the semiconductor catalyst surface
for photo-generated holes are capable of direct oxidation at pH
lower than the point of zero charge of the catalyst.
Preparation of Ag/TiO2 catalyst. The catalyst Ag/TiO2 (1%, w:w) was
prepared from TiO2 (Degussa P25), and the aqueous solution of AgNO3
was prepared as reported by the impregnation method.[8,29] A mixture
of TiO2 (1.0 g, Degussa P25 ) and AgNO3 (0.0157 g) in 5 ml bidistilled
Continuing our interest on PCO of cyclic and acyclic
alcohols,[17,20,21] as well as other interests on PCO of biomole-
cules,[23–26] the present article embraces PCO of 1,2-cyclohexane-
diol (1). Cyclohexane moiety represents the base structure of many
biomolecules such as sterols and estrogens hormones (estriol). So,
selective PCO of 1 to the corresponding carbonyl derivatives has a
great significance and is of interest.
* Correspondence to: A. A. Abdel-Wahab, Chemistry Department, Faculty of
Science, Assiut University, 71516 Assiut, Egypt.
E-mail: wahab_47@hotmail.com
†
Presented in part at the 3rd International Semiconductor Photochemistry
Conference (SP-3), 12–16 April 2010, University of Strathclyde, Glasgow, UK.
a A.-M. A. Abdel-Wahab, O. S. Mohamed, S. A. Ahmed, M. F. Mostafa
Chemistry Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt
EXPERIMENTAL
Materials
b S. A. Ahmed
Acetonitrile (HPLC grade), cyclohexanone, and adipic acid were
purchased from Aldrich; 1,2-cyclohexanediol (1, 98%) was obtained
Chemistry Department, Faculty of Applied Science, Umm Al-Qura University,
Makkah, Saudi Arabia
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Copyright © 2012 John Wiley & Sons, Ltd.

A.-M. A. ABDEL-WAHAB ET AL.
water was magnetically stirred for 48 h at room temperature; after that,
the water was removed at 100 ꢁC. Finally, the solid residue was calcined
at 500 ꢁC for 5 h, according to the literature procedure.[8,29]
RESULTS AND DISCUSSION
The selectivity of Ag/TiO2 catalyst was demonstrated for en-
hancement of PCO of organic compounds, which are predomi-
nantly oxidized by holes.[4,7] This advantage was applied for
the PCO of the target compound 1,2-cyclohexanediol. The
silver-doped titanium dioxide was prepared as reported[8] and
characterized using transmission electron microscopy (TEM).
TEM indicated the formation of Ag/TiO2 particles in nanoscale
size of 19–24 nm (Fig. 2). The silver loading was 1% (w: w), and
the optimum concentration was reported by Liu et al.[30]
Instrumentation
The electronic absorption spectra were recorded on UV-2101 PC, UV–vis
Spectrophotometer (Shimadzu). GC analyses were performed employing
Perkin Elmer, Auto System XLGC using capillary column (30 m ꢂ 2.23 mm)
packed with 5% diphenyl–95% dimethyl polysiloxane and 0.25 mm film
thicknesses. GC/MS analysis was carried out using GC model: GC 2000
thermo, capillary column DB-5 (5% phenyl–95% methyl polysiloxane),
30 m length, 0.25 mm internal diameter, and 0.25 mm film thickness.
TEM results
Photoreactor
The TEM micrograph of Ag/TiO2 showed uneven distribution of
Ag on TiO2. The loaded silver covers only part of the TiO2 surface,
and the silver cluster differed as silver loading varied (Fig. 2).
All the photocatalytic reactions were carried out in a Vycor glass cell
(29.5 ꢂ 2.5 cm), fitted with a reflux condenser capped with CaCl2 drying
tube, and externally irradiated with a 450 W medium pressure mercury
lamp (ACE glass, immersion type) with a Pyrex well, which has maximum
emission at 296.7–578 nm (4.18–2.15 eV; Fig. 1).The distance between the
sample cell and the radiation source is 5 cm. The system was covered with
aluminum foil served as light reflector to decrease light loss, and
the apparatus was set up in a metallic cabinet.[21]
Results of photocatalysis
Silver deposits generate a number of holes by acting as electron
sinks and assisting their transfer to the dissolved oxygen species
in solution forming the superoxide O.–. This reduces the e–/h+
recombination possibilities and therefore enhances the oxidation–
reduction process.
Analysis of the photolysate mixtures using GC and GC/MS
techniques revealed that the percentage conversions of 1,2-
cyclohexanediol are 41%, 61%, 83%, and 95% after irradiation
for 5, 10, 15, and 20 h, respectively (Fig. 3). Also, the products
after irradiation for 20 h included 2-hydroxylcyclohexanone
(2, 24%), 1,2-cyclohexanedione (3, 26%), 2-cyclohexenone (4, 8%),
and cyclohexanone (5, 5%), in addition to the acyclic acidic
oxidation product adipic acid (6, 10%).
General procedure of photocatalytic experiment
A 75-ml solution (0.1 M) of 1,2-cyclohexanediol (1) was prepared in dry
acetonitrile, 75 mg of Ag/TiO2 was added to the solution, and the mixture
was transferred to the reaction vessel and sonicated in an ultrasonic bath
for 15 min. The stirred suspension was bubbled with a stream of oxygen
gas (at a flow rate of 30 ml/min) and then illuminated for 20 h. The
reaction progress was monitored by GC at different intervals. After
irradiation, the titanium dioxide powder was removed by filtration using
a medium-porosity frit, and the solvent was evaporated by using rotatory
evaporator.[20] The reaction mixture was analyzed by GC and GC/MS
techniques using authentic samples
Figure 1. The photocatalysis system
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AG/TIO2 ENHANCED PCO OF 1,2-CYCLOHEXANEDIOL
the Ag/TiO2–acetonitrile interface, it traps the holes, forming
the radical cation 1a via single electron transfer, which is rapidly
deprotonated to form the a-hydroxyl radical 1b.
Formation of 2, 3, 4, and 5 may be suggested starting from
1b via three different routes (A, B, and C). Route A takes place
via a second electron oxidation of 1b with generation of the
protonated carbonyl intermediate 1C, which in turn transfers
to 2 via a proton loss. The second route (B) embraces trapping
of O2– with the formation of the peroxy anion radical 1d. Also,
trapping of O2 gives the peroxy radical 1e. Decomposition of
1d and 1e would be expected to occur rapidly to form the
cyclic oxidation products 2, 3, 4, and 5.[31] Formation of 4 could
be expected via dehydration of 2.[21,32] A multistep process
(scheme 20) may be proposed for the formation of 5 from 2.[33]
Furthermore, open-chain carbonyl compounds such as adipic
acid (6) can be produced by single electron mechanism via ring
rupture of 1a to 1g with a proton loss (route D).
The use of water-free acetonitrile solvent rules out the
participation of hydroxyl radicals in the photocatalyzed oxida-
tion process and the elimination of the metal (Ag) by photocor-
rosion. Acetonitrile, as a polar nonhydroxylic solvent, represents
the best solvent for controlling the oxidation reactivity at the
interface, between a liquid reaction and a solid irradiated photo-
catalyst.[19–22] Also, direct photolysis is excluded as the lmax of
1,2-cyclohexanediol was observed at 224 nm (Є = 2.28 mV),
whereas the maximum emission of the used 450-Wt medium
pressure Hg lamp is in the range 296.7–578.0 nm. Accordingly,
the obtained products obviously originated through initial band
gap excitation of the Ag/TiO2 catalyst rather than substrate 1.
Also, no oxidation products could be detected upon irradiation
of 1,2-cyclohexanediol (1) in the absence of catalyst and/or of light.
Furthermore, experiments carried out in the absence of oxygen
showed also no evidence of products formed.
All these results confirm the essential role of oxygen, TiO2, and
light in the PCO process. This indicates the predominance of
the e–/h+ mechanism via the formation of the e–/h+ pair at the
surface of the Ag/TiO2 nanoparticles followed by oxidation of
the adsorbed 1,2-cyclohexandiol, as indicated in Scheme 1.
Figure 2. TEM images of Ag/TiO2
1.2
1.0
0.8
0.6
0.4
0.2
0.0
KINETIC STUDIES
The semilogarithmic plot of the concentration of 1,2-cyclohexa-
nediol (1) at different intervals (Fig. 4) indicates that PCO of 1
can be described by the first-order kinetic model:
ln C ¼ ꢀkt þ ln C0
where C0 is the initial concentration, and C is the concentration
at time t.
This result is in agreement with the results of Cinar[19] and Liu[30]
on PCO m-cresol and phenol, respectively, as well as with our work
on cyclohexyl alcohols.[20,21] The rate constant (k) of 1 under the
working conditions was found to be equal to ꢀ0.145 h–1.
0
5
10
15
20
25
t(hrs)
Figure 3. Photocatalytic disappearance of 1 in acetonitrile
CONCLUSION
Ag/TiO2 catalyst was prepared in nanoscale size (19–24 nm) and
used for PCO of 1,2-cyclohexandiol in the presence of molecular
oxygen. The PCO products included 2-hydroxylcyclohexanone
(2), 1,2-cyclohexanedione (3), 2-cyclohexenone (4), cyclohexa-
none (5), and adipic acid (6). The formation of electron–hole pair
at the surface of the semiconductor followed by oxidation
Reaction mechanisms
The reaction mechanism (Scheme 1) has been proposed
depending on the reaction products and conditions. The target
1,2-cyclohexanediol (1) is a sensitive probe; when adsorbed at
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A.-M. A. ABDEL-WAHAB ET AL.
Scheme 1. Photocatalytic oxidation of 1 in acetonitrile
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
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0
5
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25
t(hrs)
Figure 4. First-order PCO of 1 in acetonitrile (r = 0.9886, k = ꢀ0.145 h–1
)
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reactions was the suggested mechanism. PCO of 1 under the
applied conditions was found to follow a first-order mechanism
with rate constant (k) equal to –0.145 h–1.
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