# TiO2 Nanoparticles-Photocatalytic Oxidation of  Selected Cycloalkanols

Authors: Omima S. Mohamed; Saleh A. Ahmed; Mostafa F. Mostafa; Aboel-Magd A. Abdel-Wahab
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

Photocatalytic oxidation of cyclohexanol (1), cyclopentanol (2), and cycloheptanol (3) was investigated by using titanium dioxide (Degussa P25) as a semiconductor photocatalyst .The effect of different operational parameters such as the catalyst, the solvent, time, and oxidant was also studied. Results showed a high percentage of conversion for 1–3 (71.6%, 94.2%, and 100%, resp.) and that the primary photocatalytic oxidation products are the corresponding cycloalkanones (4–6). They were formed with high selectivity (>85%). Several other products were also identified using GC, GC/MS techniques and authentic samples. The photocatalytic activity is explained by a photoinduced electron transfer mechanism through the formation of electron-hole pair at the surface of the semiconductor particles. A first-order kinetic model was observed for the photocatalytic oxidation of the investigated cycloalkanols.


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## Body

## 1. Introduction

Photocatalysis represents one of the promising technologies for
photoenergy conversion started with the pioneer findings of Fujishima and Honda [1]. Its applications in water
recycling, air-pollution treatment, as well as in organic synthesis  have found a great interest [2–6].There are many studies focused on the photocatalytic oxidation and degradation of several
classes of organic compounds catalyzed by TiO2 semiconductor
particulates [7–13].Photocatalytic oxidation and degradation of alcohols catalyzed by TiO2 and other
semiconductor particulates have found some interest. For example, Harvey et al. [14] studied photocatalytic oxidation of liquid propan-2-ol to propanone
using suspensions of titanium dioxide irradiated with filtered UV radiation.
The dependence of reaction rate on the square root of the intensity of the
incident radiation, together with low quantum yields, reflects the dominance of
photoelectron and photohole recombination within the TiO2. Cameron
and Bocarsly [15] studied the photocatalytic oxidation of ethanol, benzyl
alcohol, cinnamyl alcohol, n-hexyl alcohol, isopropyl alcohol, cyclopentanol,
and cyclohexanol which converted to the corresponding aldehyde or ketone by
using oxygen gas and visible light 488 nm (85 mW) illumination from an Ar laser. The process was catalyzed by the
presence of H2PtCl6 and CuCl2.Nishimoto et al. [16] studied the photocatalytic dehydrogenation of aliphatic
alcohols by aqueous suspensions of platinized titanium dioxide, photoirradiation
(λex>300 nm) of Ar-purged aqueous propan-2-ol solution gave
hydrogen and acetone. The regioselectivity in the semiconductor-mediated
photooxidation of 1,4-pentanediol was studied in 1989 by Fox et al.
[17].We also studied the photocatalytic oxidation of selected aryl alcohols [18, 19].
The main oxidation products were the corresponding aldehydes or ketones and
acids. Kinetic studies revealed a second-order reaction rate for benzyl alcohol
as a model alcohol. Also photocatalytic oxidation of fluoren-9-ol and
4,5-diazafluoren-9-ol in nonaqueous oxygen saturated TiO2 suspension
to the corresponding ketones was formed in a high yield, in addition to minor
amount of the corresponding hydrocarbon and other decomposition products.
Interestingly, photocatalytic oxidation of the parent fluorene afforded fluorenone
under the same conditions. Similarly, Phenyl-4-pyridylmethanolas an
acyclic simulant gave the corresponding ketone and hydrocarbon. Furthermore,
the effect of solvent polarity has been tested [18, 19].Pillai and Salhe-Demessie [20] studied the
selective oxidation of primary and secondaryaliphatic alcohols to their
corresponding carbonyl compounds in gas phase. In
2006, Wu et al. [21] studied the long-term photocatalytic stability of Co2+-modified
P25-TiO2 powders for the H2 production from aqueous ethanol
solution. In this paper, the long-term stability of photocatalytic
activity of Co2+ doped P25-TiO2 nanoparticles for
hydrogen production from aqueous ethanol
solution was discussed in detail using a photoelectrochemical method.It is clear from the
previous survey that the effect of ring size and conditions on the
photocatalytic oxidations of secondary cycloalkanols has not been tackled. So,
this study will be of great interest.


## 2. Experimental

### 2.1. Materials

Titanium dioxide P25 (Degussa-Hűs AG, powder)
was dried at 120°C for 24 hrs before use, these particles are
composed mostly of anatase with surface area of 37.7 m2g−1,
average particle size 21 nm, and pore diameter 31.0. Oxygen was
dried by passage through a drying calcium chloride tube. Acetonitrile and
acetone (Sigma Aldrich, St. Louis, USA, HPLC grade) were distilled before use.
Hydrogen peroxide was 30% (v/v). Cyclohexanol (1), cyclopentanol (2) and cycloheptanol (3) were purchased from Aldrich chemical company and were distilled before
use. The authentic samples cyclohexanone (4), cyclopentanone (5),
cycloheptanone (6), and 1,4-cyclohexadione (7) were obtained
from Aldrich, whereas cyclohexyl formate (8) [22], 2-cyclohexenone (9) [23], 2-hydroxycyclohexanone (10) [24], n-hexanoic acid (11) [25],
cyclohexyl hexanoate (12) [22], n-pentanoic acid (13) [22, 26], cyclopentyl pentanoate (14) [22], and n-heptanoic acid (15)
[27] were synthesized as described in literatures. All the prepared authentic
samples were analyzed by IR, H1-NMR, GC, and GC/MS analyses as synthetic
mixture.


### 2.2. Apparatus

A 450 W medium pressure mercury lamp (ACE
glass, immersion type) with a pyrex well was used as the light source, which has
a maximum emission range 296.7–578 nm (4.18−2.15 eV). The system was covered
with aluminum foil which served as light reflector to decrease light loss and
the apparatus was set up in a metallic cabinet. The distance between sample and
irradiation source was 5 cm.IR spectra were recorded using
IR-470, IR spectro-photometer-Shimadzu using thin film and NaCl disks. The
electronic absorption spectra were recorded using UV-2101 PC, UV-VIS scanning
spectrophotometer-Shimadzu.1H-NMR spectra were carried out using 90 MHz Varian 390 in CDCl3 and TMS as an internal standard.  GC  analyses were performed
employing Perkin Elmer, Autosystem XLGC using capillary column (5% diphenyl–95% dimethyl polysiloxane), length 30 m, internal diameter 0.32 mm, film
thickness 0.25 μm. The initial temperature of column is 40°C and the
final temperature is 280°C, the rate of heating is 20°C/min.
The injector temperature is 300°C and the detector used is a flame
ionization detector at 250°C. GC/MS analysis were carried out using
GC model: GC 2000 thermo, capillary column DB-5 (5% phenyl–95% methyl polysiloxane, L. 30 m, I.D. 0.25
mm, F.T. 0.25 μm, temperature of column from 50°C to 300°C,
10°C/min. and injector temperature 250°C),  attached with mass spectrometer: model SSQ
7000 produced by Finnigan. All melting points were determined on a Gallen-Kamp
melting point apparatus.


### 2.3. General Procedure

A 75 ml (0.1 M) solution of the investigated cycloalkanols (1–3) was
prepared in the used solvent, 75 mg of titanium dioxide (P25 Degussa-Hűs AG) was added
to the solution, the mixture was transferred to the reaction vessel and
sonicated in an ultrasonic bath for 15 minutes. The stirred suspension was
bubbled with a stream of oxygen gas at a flow rate (30 ml/min) and then
illuminated for 20 hours using 450 W medium pressure mercury lamp.The
reaction progress was monitored by GC. After irradiation, the titanium dioxide
powder was removed by filtration using a medium porosity frit and the solvent
was evaporated by using rotatory evaporator. The reaction mixture was analyzed by
using GC and GC/MS techniques, and then the products were identified and
estimated with convincing match qualities compared with the standard mass
spectra in the mass spectral libraries and with their retention time matched
with authentic samples.


## 2.1. Materials

Titanium dioxide P25 (Degussa-Hűs AG, powder)
was dried at 120°C for 24 hrs before use, these particles are
composed mostly of anatase with surface area of 37.7 m2g−1,
average particle size 21 nm, and pore diameter 31.0. Oxygen was
dried by passage through a drying calcium chloride tube. Acetonitrile and
acetone (Sigma Aldrich, St. Louis, USA, HPLC grade) were distilled before use.
Hydrogen peroxide was 30% (v/v). Cyclohexanol (1), cyclopentanol (2) and cycloheptanol (3) were purchased from Aldrich chemical company and were distilled before
use. The authentic samples cyclohexanone (4), cyclopentanone (5),
cycloheptanone (6), and 1,4-cyclohexadione (7) were obtained
from Aldrich, whereas cyclohexyl formate (8) [22], 2-cyclohexenone (9) [23], 2-hydroxycyclohexanone (10) [24], n-hexanoic acid (11) [25],
cyclohexyl hexanoate (12) [22], n-pentanoic acid (13) [22, 26], cyclopentyl pentanoate (14) [22], and n-heptanoic acid (15)
[27] were synthesized as described in literatures. All the prepared authentic
samples were analyzed by IR, H1-NMR, GC, and GC/MS analyses as synthetic
mixture.


## 2.2. Apparatus

A 450 W medium pressure mercury lamp (ACE
glass, immersion type) with a pyrex well was used as the light source, which has
a maximum emission range 296.7–578 nm (4.18−2.15 eV). The system was covered
with aluminum foil which served as light reflector to decrease light loss and
the apparatus was set up in a metallic cabinet. The distance between sample and
irradiation source was 5 cm.IR spectra were recorded using
IR-470, IR spectro-photometer-Shimadzu using thin film and NaCl disks. The
electronic absorption spectra were recorded using UV-2101 PC, UV-VIS scanning
spectrophotometer-Shimadzu.1H-NMR spectra were carried out using 90 MHz Varian 390 in CDCl3 and TMS as an internal standard.  GC  analyses were performed
employing Perkin Elmer, Autosystem XLGC using capillary column (5% diphenyl–95% dimethyl polysiloxane), length 30 m, internal diameter 0.32 mm, film
thickness 0.25 μm. The initial temperature of column is 40°C and the
final temperature is 280°C, the rate of heating is 20°C/min.
The injector temperature is 300°C and the detector used is a flame
ionization detector at 250°C. GC/MS analysis were carried out using
GC model: GC 2000 thermo, capillary column DB-5 (5% phenyl–95% methyl polysiloxane, L. 30 m, I.D. 0.25
mm, F.T. 0.25 μm, temperature of column from 50°C to 300°C,
10°C/min. and injector temperature 250°C),  attached with mass spectrometer: model SSQ
7000 produced by Finnigan. All melting points were determined on a Gallen-Kamp
melting point apparatus.


## 2.3. General Procedure

A 75 ml (0.1 M) solution of the investigated cycloalkanols (1–3) was
prepared in the used solvent, 75 mg of titanium dioxide (P25 Degussa-Hűs AG) was added
to the solution, the mixture was transferred to the reaction vessel and
sonicated in an ultrasonic bath for 15 minutes. The stirred suspension was
bubbled with a stream of oxygen gas at a flow rate (30 ml/min) and then
illuminated for 20 hours using 450 W medium pressure mercury lamp.The
reaction progress was monitored by GC. After irradiation, the titanium dioxide
powder was removed by filtration using a medium porosity frit and the solvent
was evaporated by using rotatory evaporator. The reaction mixture was analyzed by
using GC and GC/MS techniques, and then the products were identified and
estimated with convincing match qualities compared with the standard mass
spectra in the mass spectral libraries and with their retention time matched
with authentic samples.


## 3. Results and Discussions

Application of photocatalysis in our daily life and chemical industries has
found a great interest. As reported in the introduction, the superior TiO2 photocatalyst was elegantly used for several photocatalytic oxidation
processes. The present work is directed for synthesis, mechanistic, and
selectivity studies as well as for decontamination purposes.The target starting materials
include three selected alicyclic alcohols; cyclohexanol (1),
cyclopentanol (2), and cycloheptanol (3); of different ring sizes.Results obtained in this work will be presented for each individual
alcohol; and collective table and figures are also available in the following
discussion.


### 3.1. PCO of Cyclohexanol (1) and Cyclohexanone (4)

#### 3.1.1. PCO of 1 in Acetonitrile

After irradiation of1 in dry acetonitrile, the photolysis product, using GC/MS technique,
were cyclohexanone (4, 45.6%), 1,4-cyclohexanedione  (7,
3.2%), cyclohexyl formate (8,
2.5%), 2-cyclohexenone  (9, 1.8%), 2-hydroxycyclohexanone (10,
1.1%), n-hexanoic acid  (11, 4.0%), 5-hexenal(16, 2.9%),
4-hydroxycyclohexanone  (17,
4.4%), and cyclohexyl hexanoate (12, 3.0%), and other product (19,
2.9%) (Table 1, Exp. 1).Table 1
Photocatalytic oxidation of the investigated cyclalkanols1–3.
Exp.SubstrateSolventsTime (hrs)Conv. (%)Products (No, yield%)1Cyclohexanol (1)CH3CN2071.6Cyclohexanone (4, 45.6), 1,4-Cyclohexanedione
(7, 3.2), Cyclohexyl formate (8, 2.5), 2-Cyclohexenone (9, 1.8), 2-Hydroxycyclohexanone
(10, 1.1) n-Hexanoic
acid (11, 4.0), Cyclohexyl hexanoate (12, 3.0), 5-Hexenal (16,
2.9), 4-Hydroxycyclohexanone (17, 4.4), Other product (19, 2.9).2Cyclohexanol  (1)CH3CN+H2O (4 : 1)2074.3Cyclohexanone (4, 39.7), 1,4-Cyclohexadione
(7, 3.7), Cyclohexyl formate (8, 1.4), 2-Cyclohexenone (9, 2.4), 2-Hydroxycyclohexanone
(10, 7.8), 5-Hexenal (16,
1.8), 4-Hydroxycyclohexanone (17, 11.0), 1,4-Cyclohexanediol   (20,
6.5).3Cyclohexanol  (1)CH3CN+H2O2 (4 : 1)2098.0Cyclohexanone (4, 13.6), 1,4-Cyclohexadione
(7, 8.7), 2-Cyclohexenone (9, 5.1), 2-Hydroxycyclohexanone (10,
10.2), 4-Hydroxycyclohexanone
(17, 10.5), 1,4-Cyclohexanediol   (20,
8.2), Glutaric acid (21, 12.0), Adipic acid (22, 29.8).4Cyclohexanol  (1)H3COH32045Cyclohexanone (4, 25.2), 1,4-Cyclohexadione
(7, 3.4), Cyclohexyl formate (8, 2.3), 2-Hydroxycyclohexanone (10,
0.5) n-Hexanoic acid (11,
5.4), 5-Hexenal (16, 1.9), 4-Hydroxycyclohexanone (17, 3.5),
1,1′-Bicyclohexyl-1,1′-diol (18, 2.5).5Cyclopentanol  (2)CH3CN2094.2Cyclopentanone (5, 72.7), n-Pentanoic
acid (13, 4.3), Cyclopentyl pentanoate (14, 2.8), 4-Pentenal (23,
3.7), Other products (24, 3.1), (25, 5.8), (26, 1.8).6Cycloheptanol  (3)CH3CN20100Cycloheptanone (6, 79.9), n-heptanoic acid
(15, 3.2), 6-Heptenal (27, 1.8), 4-Hydroxycycloheptanone (28, 4.1),
1,4-Cycloheptadione (29, 4.7), 2-Hydroxycycloheptanone (30, 1.5).Cyclohexanol (1) is a sensitive probe, for it has been established that two electron
oxidation will give cyclohexanone (4) [17] as a primary product, whereas
single electron oxidation selectively produces the open-chain unsaturated
aldehyde 5-hexenal (16) [17], also 5-hexenal (16) formed by intramolecular
hydrogen transfer reaction of cyclohexanone. Cyclohexanone was also oxidized to
n-hexanoic acid (11) [28, 29].Cyclohexanone (4) can be
hydroxylated by O•H radical to 4-hydroxycyclohex-anone (17) and 2-hydroxycyclohexanone (10). 1,4-cyclohexanedione (7) is most
probably formed by oxidation of 4-hydroxycyclohexanone (17) [30]
and  2-cyclohexenone (9) was
probably formed by dehydration of 2-hydroxycyclohexanone (10) [14, 17],
respectively. 3-hydroxycyclohexanone may be formed also, but not detected, and
dehydrated to 2-cyclohexenone (9). This is because the heat of formation
[31] of the three hydroxylated cyclohexanones are close to each other (−101.61, −99.88, −103.64 Kcal/mol, resp.). This means that they are close in
stability and probability of formation. 5-hexenal (16) oxidation by
double bond cleavage to formaldehyde [32] which by further oxidation gives
formic acid. Cyclohexanol (1) can react with formic acid to give cyclohexyl
formate (8), and with n-hexanoic acid (11) to give cyclohexyl hexanoate (12),  (Scheme 1).Scheme 1
Suggested mechanism of PCO of1 in acetonitrile.


#### 3.1.2. PCO of 1 in Acetone:

After irradiation of1 in acetone, the photolysis products analyzed by GC/MS technique were
cyclohexanone  (4, 25.2%),
1,4-cyclohexanedione  (7, 3.4%) cyclohexyl
formate  (8, 2.3%),
2-hydroxycyclohexanone  (10, 0.5%), n-hexanoic acid  (11, 5.4%),
5-hexenal (16, 1.9%), and 4-hydroxycyclohexanone (17, 3.5%),1,1′-bicyclohexyl-1,1′-diol  (18, 2.5%) (Table 1, Exp. 4).GC chromatogram and the suggested mechanism of these products showed that nearly
all products formed in acetone are the same as the products formed in
acetonitrile, but in lower conversion. From Table1, the conversion of
cyclohexanol (1) in dry acetonitrile was 71.6%, while in dry acetone
equals to 45% (Figure 1). This is because the dielectric constant (DEC)
of acetonitrile is much higher than that of acetone, 37.5 and 21.4, respectively
[33]. A plausible explanation is that increase of solvent polarity increases
the stabilization of the intermediate radical cations which in turn raise the
percentage of the oxidation products [33]. This supports the electron/hole
mechanism in photocatalytic oxidation of the investigated alcohols.Figure 1
Effect of solvents on photocatalytic oxidation of cyclohexanol (1).


#### 3.1.3. PCO of 1 in Acetonitrile-Water (4 : 1)

Irradiation products of1 in acetonitrile-water mixture (4 : 1), as analyzed by GC/MS, were cyclohexanone (4, 39.7%),
1,4-cyclohexanedione (7, 3.7%), cyclohexyl formate (8,
1.43%), 2-cyclohexenone  (9, 2.4%), 2-hydroxycyclohexanone (10,
7.8%), 5-hexenal (16, 1.8%), 4-hydroxycyclohexanone  (17, 11.0%) and
1,4-cyclohexanediol  (20, 6.5%), (Table 1, Exp. 2).The products formed in aqueous acetonitrile are almost similar to those formed
in acetonitrile, but the experimental conversion and yield distribution
are different (Table1). Conversion of 1 in aqueous acetonitrile
(4 : 1) is 74.3% more than in dry acetonitrile (71.6%) this is because the dielectric
constant (DEC) of water is 78.3 [34] (Figure 1). Also primary oxidant in
this reaction is expected to be the strong oxidant O•H radical
generated by trapping a photohole at an adsorbed water molecule [35]. The percentage of hydroxylated compounds increased from 5.5% to 25.3%, and a new
product is formed 1,4-cyclohexanediol (20).


#### 3.1.4. PCO of 1 in Acetonitrile-30%H2O2 (4 : 1)

Irradiation of1 in acetonitrile-30% hydrogen peroxide  (4 : 1) without oxygen passage, using GC/MS analyses of the photolysat gave cyclohexanone  (4, 13.6%), 1,4-cyclohexanedione  (7, 8.7%), 2-cyclohexenone (9, 5.1%), 2-hydroxycyclohexanone (10, 10.2%),
4-hydroxycyclohexanone  (17, 10.5%)
and 1,4-cyclohexadiol  (20,
8.2%), glutaric acid  (21, 12.0%) and
adipic acid (22, 29.8%) (Table 1, Exp. 3).GC chromatogram and the suggested mechanismof these products showed that most
of products formed in acetonitrile-30%H2O2 (4 : 1)
mixture are the same as formed in dry acetonitrile with
almost complete conversion of 1 (98%), that is, more than in dry
acetonitrile or in aqueous acetonitrile (Figure 1). Electrons are
donated from TiO2 to hydrogen peroxide because it is a stronger
oxidant than molecular oxygen [30]. When hydrogen peroxide accepts an
electron, O•H radical may be produced according to (1) [35], so
the O•H radicals are generated in large amounts, so the
percentage of hydroxylated compounds increased, for example, the % of compound 10 increased from 1.1% to 7.8% to 10.2% in cases of acetonitrile, aqueous
acetonitrile or acetonitrile-30%H2O2,
respectively, and new products were formed such as glutaric acid (21) and adipic acid (22).(1)TiO2(e−)+H2O2⟶TiO2+OH¯+O⋅H.The addition ofH2O2 is a well-known technique for increasing the rate of photocatalytic degradation
with higher quantum yield of formation of O•H [30].
Consequently, the increased oxidation rate is expected by the addition of H2O2 as shown in Table 1 (Exps. 1–3).It is worthy to mention that no oxidation products could be detected
with cyclohexanol in the absence of catalyst and/or light [30]. This is a good
proof for the role of each parameter (H2O2, TiO2 and light) in the photocatalytic oxidation process.


#### 3.1.5. PCO of Cyclohexanone (4) in Acetonitrile

To shed more light on the formation and fate of the primary
photooxidation products, cyclohexanone was irradiated at the same previous
conditions. The conversion was only 19.8% after 20 hours irradiation. The GC/MS analyses of the photolysis products were
1,4-cyclohexanedione (7,
4.1%), 2-cyclohexenone (9, 1.4%), 2-hydroxycyclohexanone (10, 0.3%),
n-hexanoic acid (11, 6.2%), 5-hexenal (16, 3.5%), and
4-hydroxycyclohexanone (17,
3.8%) (Table 2, Exp.7).Table 2
Photocatalytic Oxidation of the Investigated Ketones4–6:
Exp.SubstrateSolventsTime (hrs)Conv. (%)Products (No,%)7Cyclohexanone (4)CH3CN2019.81,4-Cyclohexadione (7, 4.1), 2-Cyclohexenone
(9, 1.4), 2-Hydroxycyclohexanone (10, 0.3) n-Hexanoic acid (11, 6.2), 5-Hexenal
(16, 3.5), 4-Hydroxycyclohexanone (17, 3.8).8Cyclopentanone  (5)CH3CN2026.3n-Pentanoic acid (13, 15.9), 4-Pentenal
(23, 10.4).9Cycloheptanone  (6)CH3CN2029.9n-heptanoic acid (15, 6.8), 6-Heptenal
(27, 4.1), 4-Hydroxycycloheptanone (28, 7.8), 1,4-Cycloheptanedione
(29, 7.8), 2-Hydroxycycloheptanone (30, 3.4).GC chromatogram and the suggested
mechanism of these products matches those obtained from photocatalytic
oxidation of cyclohexanol (1).


### 3.2. PCO of Cyclopentanol (2) and Cyclopentanone (5)

#### 3.2.1. PCO of 2 in Acetonitrile Using TiO2

After irradiation of2 in
acetonitrile, the residue was
analyzed by GC/MS. The photolysis products were cyclopentanone (5, 72.7%), n-pentanoic
acid (13, 4.3%), cyclopentyl pentanoate (14, 2.8%),
4-pentenal (23, 3.7%), and some other unidentified low
concentration products (24, 3.1%, 25, 5.8%, and 26, 1.8%) (Table 1, Exp.  5).GC chromatogram and the suggested mechanism of these products showed that
cyclopentanol (2) via two electron oxidation gave cyclopentanone (5) [17] as a primary product, whereas single electron oxidation selectively
produces open chain unsaturated aldehyde 4-pentenal (23) [17]. Also
4-pentenal (23) may be formed by intramolecular hydrogen atom transfer
reaction of cyclopentanone [28]. Cyclopentanone was also oxidized to
n-pentanoic acid (13), which reacted with cyclopentanol (2) to
give cyclopentyl pentanoate (14) (Scheme 2).Scheme 2
Photocatalytic oxidation of2 and 3.


#### 3.2.2. PCO of Cyclopentanone (5) in Acetonitrile

Great attention was focused on the
oxidation of the primary oxidation product, cyclopentanone. Therefore, the
photocatalytic oxidation of5 under the same previous condition was
investigated. Low conversion (26.3%) was observed and the photolysis products
were n-pentanoic acid (13, 15.9%), and 4-pentenal (23,
10.4%) (Table 2, Exp. 8).The suggested mechanism of these products is
in agreement with the suggested mechanism of photocatalytic oxidation of
cyclopentanol.


### 3.3. PCO of Cycloheptanol (3) and Cycloheptanone (6)

#### 3.3.1. PCO of 3 in Acetonitrile Using TiO2

After irradiation of3 in acetonitrile,  the
photolysis products, analyzed by GC/MS, were cycloheptanone  (6, 79.9%), n-heptanoic acid  (15, 3.2%), 6-heptenal (27, 1.8%),
4-hydroxycycloheptanone  (28, 4.1%),
1,4-cycloheptadione  (29, 4.7%)
and 2-hydroxycycloheptanone (30,
1.5%) (Table 1, Exp.  6).The suggested mechanism of these
products showed that cycloheptanol (3) is a sensitive probe, therefore
it has been established that two electron oxidation will give cycloheptanone (6) [17], whereas a single electron oxidation selectively produces the open-chain unsaturated
aldehyde 6-heptenal (27) [17]. Also 6-heptenal (27) was formed by
intramolecular hydrogen transfer reaction for cycloheptanone [28]. Cycloheptanone
oxidiation affored n-heptanoic acid (15). Also cycloheptanone (6) was hydroxylated by O⋅H radical to give
4-hydroxycycloheptanone (28) and 2-hydroxycycloheptanone (30). 1,4-cycloheptandione (29) was most probably generated by oxidation of
4-hydroxycycloheptanone (28) [30] (Scheme 2).


#### 3.3.2. PCO of Cycloheptanone (6) in Acetonitrile Using TiO2

Photocatalytic irradiation of6 in dry acetonitrile, resulted in 29.9% conversion. The photolysis
products were 6-heptenal (27, 4.1%), n-heptanoic acid (15,
6.8%), 4-hydroxycycloheptanone (28, 7.8%), 1,4-cycloheptadione  (29, 7.8%), and
2-hydroxycycloheptanone (30, 3.4%) (Table 2, Exp. 9).GC chromatogram and the suggested
mechanism of these products are similar to the photocatalytic oxidation of cycloheptanol (3).The obtained results indicated that the percentage conversion of3>2>1, the same as that of
corresponding ketones where 7>6>5. This is in agreement with
their heat of formation
(ΔHf=−60.83, −66.98, and −80.22)Kcal/mol, respectively. Of
course cyclohexanol (1), the most stable one [31] (ΔHf=
−80.22 Kcal/mol), has the lowest percentage of conversion compared with 2 and 3.


## 3.1. PCO of Cyclohexanol (1) and Cyclohexanone (4)

### 3.1.1. PCO of 1 in Acetonitrile

After irradiation of1 in dry acetonitrile, the photolysis product, using GC/MS technique,
were cyclohexanone (4, 45.6%), 1,4-cyclohexanedione  (7,
3.2%), cyclohexyl formate (8,
2.5%), 2-cyclohexenone  (9, 1.8%), 2-hydroxycyclohexanone (10,
1.1%), n-hexanoic acid  (11, 4.0%), 5-hexenal(16, 2.9%),
4-hydroxycyclohexanone  (17,
4.4%), and cyclohexyl hexanoate (12, 3.0%), and other product (19,
2.9%) (Table 1, Exp. 1).Table 1
Photocatalytic oxidation of the investigated cyclalkanols1–3.
Exp.SubstrateSolventsTime (hrs)Conv. (%)Products (No, yield%)1Cyclohexanol (1)CH3CN2071.6Cyclohexanone (4, 45.6), 1,4-Cyclohexanedione
(7, 3.2), Cyclohexyl formate (8, 2.5), 2-Cyclohexenone (9, 1.8), 2-Hydroxycyclohexanone
(10, 1.1) n-Hexanoic
acid (11, 4.0), Cyclohexyl hexanoate (12, 3.0), 5-Hexenal (16,
2.9), 4-Hydroxycyclohexanone (17, 4.4), Other product (19, 2.9).2Cyclohexanol  (1)CH3CN+H2O (4 : 1)2074.3Cyclohexanone (4, 39.7), 1,4-Cyclohexadione
(7, 3.7), Cyclohexyl formate (8, 1.4), 2-Cyclohexenone (9, 2.4), 2-Hydroxycyclohexanone
(10, 7.8), 5-Hexenal (16,
1.8), 4-Hydroxycyclohexanone (17, 11.0), 1,4-Cyclohexanediol   (20,
6.5).3Cyclohexanol  (1)CH3CN+H2O2 (4 : 1)2098.0Cyclohexanone (4, 13.6), 1,4-Cyclohexadione
(7, 8.7), 2-Cyclohexenone (9, 5.1), 2-Hydroxycyclohexanone (10,
10.2), 4-Hydroxycyclohexanone
(17, 10.5), 1,4-Cyclohexanediol   (20,
8.2), Glutaric acid (21, 12.0), Adipic acid (22, 29.8).4Cyclohexanol  (1)H3COH32045Cyclohexanone (4, 25.2), 1,4-Cyclohexadione
(7, 3.4), Cyclohexyl formate (8, 2.3), 2-Hydroxycyclohexanone (10,
0.5) n-Hexanoic acid (11,
5.4), 5-Hexenal (16, 1.9), 4-Hydroxycyclohexanone (17, 3.5),
1,1′-Bicyclohexyl-1,1′-diol (18, 2.5).5Cyclopentanol  (2)CH3CN2094.2Cyclopentanone (5, 72.7), n-Pentanoic
acid (13, 4.3), Cyclopentyl pentanoate (14, 2.8), 4-Pentenal (23,
3.7), Other products (24, 3.1), (25, 5.8), (26, 1.8).6Cycloheptanol  (3)CH3CN20100Cycloheptanone (6, 79.9), n-heptanoic acid
(15, 3.2), 6-Heptenal (27, 1.8), 4-Hydroxycycloheptanone (28, 4.1),
1,4-Cycloheptadione (29, 4.7), 2-Hydroxycycloheptanone (30, 1.5).Cyclohexanol (1) is a sensitive probe, for it has been established that two electron
oxidation will give cyclohexanone (4) [17] as a primary product, whereas
single electron oxidation selectively produces the open-chain unsaturated
aldehyde 5-hexenal (16) [17], also 5-hexenal (16) formed by intramolecular
hydrogen transfer reaction of cyclohexanone. Cyclohexanone was also oxidized to
n-hexanoic acid (11) [28, 29].Cyclohexanone (4) can be
hydroxylated by O•H radical to 4-hydroxycyclohex-anone (17) and 2-hydroxycyclohexanone (10). 1,4-cyclohexanedione (7) is most
probably formed by oxidation of 4-hydroxycyclohexanone (17) [30]
and  2-cyclohexenone (9) was
probably formed by dehydration of 2-hydroxycyclohexanone (10) [14, 17],
respectively. 3-hydroxycyclohexanone may be formed also, but not detected, and
dehydrated to 2-cyclohexenone (9). This is because the heat of formation
[31] of the three hydroxylated cyclohexanones are close to each other (−101.61, −99.88, −103.64 Kcal/mol, resp.). This means that they are close in
stability and probability of formation. 5-hexenal (16) oxidation by
double bond cleavage to formaldehyde [32] which by further oxidation gives
formic acid. Cyclohexanol (1) can react with formic acid to give cyclohexyl
formate (8), and with n-hexanoic acid (11) to give cyclohexyl hexanoate (12),  (Scheme 1).Scheme 1
Suggested mechanism of PCO of1 in acetonitrile.


### 3.1.2. PCO of 1 in Acetone:

After irradiation of1 in acetone, the photolysis products analyzed by GC/MS technique were
cyclohexanone  (4, 25.2%),
1,4-cyclohexanedione  (7, 3.4%) cyclohexyl
formate  (8, 2.3%),
2-hydroxycyclohexanone  (10, 0.5%), n-hexanoic acid  (11, 5.4%),
5-hexenal (16, 1.9%), and 4-hydroxycyclohexanone (17, 3.5%),1,1′-bicyclohexyl-1,1′-diol  (18, 2.5%) (Table 1, Exp. 4).GC chromatogram and the suggested mechanism of these products showed that nearly
all products formed in acetone are the same as the products formed in
acetonitrile, but in lower conversion. From Table1, the conversion of
cyclohexanol (1) in dry acetonitrile was 71.6%, while in dry acetone
equals to 45% (Figure 1). This is because the dielectric constant (DEC)
of acetonitrile is much higher than that of acetone, 37.5 and 21.4, respectively
[33]. A plausible explanation is that increase of solvent polarity increases
the stabilization of the intermediate radical cations which in turn raise the
percentage of the oxidation products [33]. This supports the electron/hole
mechanism in photocatalytic oxidation of the investigated alcohols.Figure 1
Effect of solvents on photocatalytic oxidation of cyclohexanol (1).


### 3.1.3. PCO of 1 in Acetonitrile-Water (4 : 1)

Irradiation products of1 in acetonitrile-water mixture (4 : 1), as analyzed by GC/MS, were cyclohexanone (4, 39.7%),
1,4-cyclohexanedione (7, 3.7%), cyclohexyl formate (8,
1.43%), 2-cyclohexenone  (9, 2.4%), 2-hydroxycyclohexanone (10,
7.8%), 5-hexenal (16, 1.8%), 4-hydroxycyclohexanone  (17, 11.0%) and
1,4-cyclohexanediol  (20, 6.5%), (Table 1, Exp. 2).The products formed in aqueous acetonitrile are almost similar to those formed
in acetonitrile, but the experimental conversion and yield distribution
are different (Table1). Conversion of 1 in aqueous acetonitrile
(4 : 1) is 74.3% more than in dry acetonitrile (71.6%) this is because the dielectric
constant (DEC) of water is 78.3 [34] (Figure 1). Also primary oxidant in
this reaction is expected to be the strong oxidant O•H radical
generated by trapping a photohole at an adsorbed water molecule [35]. The percentage of hydroxylated compounds increased from 5.5% to 25.3%, and a new
product is formed 1,4-cyclohexanediol (20).


### 3.1.4. PCO of 1 in Acetonitrile-30%H2O2 (4 : 1)

Irradiation of1 in acetonitrile-30% hydrogen peroxide  (4 : 1) without oxygen passage, using GC/MS analyses of the photolysat gave cyclohexanone  (4, 13.6%), 1,4-cyclohexanedione  (7, 8.7%), 2-cyclohexenone (9, 5.1%), 2-hydroxycyclohexanone (10, 10.2%),
4-hydroxycyclohexanone  (17, 10.5%)
and 1,4-cyclohexadiol  (20,
8.2%), glutaric acid  (21, 12.0%) and
adipic acid (22, 29.8%) (Table 1, Exp. 3).GC chromatogram and the suggested mechanismof these products showed that most
of products formed in acetonitrile-30%H2O2 (4 : 1)
mixture are the same as formed in dry acetonitrile with
almost complete conversion of 1 (98%), that is, more than in dry
acetonitrile or in aqueous acetonitrile (Figure 1). Electrons are
donated from TiO2 to hydrogen peroxide because it is a stronger
oxidant than molecular oxygen [30]. When hydrogen peroxide accepts an
electron, O•H radical may be produced according to (1) [35], so
the O•H radicals are generated in large amounts, so the
percentage of hydroxylated compounds increased, for example, the % of compound 10 increased from 1.1% to 7.8% to 10.2% in cases of acetonitrile, aqueous
acetonitrile or acetonitrile-30%H2O2,
respectively, and new products were formed such as glutaric acid (21) and adipic acid (22).(1)TiO2(e−)+H2O2⟶TiO2+OH¯+O⋅H.The addition ofH2O2 is a well-known technique for increasing the rate of photocatalytic degradation
with higher quantum yield of formation of O•H [30].
Consequently, the increased oxidation rate is expected by the addition of H2O2 as shown in Table 1 (Exps. 1–3).It is worthy to mention that no oxidation products could be detected
with cyclohexanol in the absence of catalyst and/or light [30]. This is a good
proof for the role of each parameter (H2O2, TiO2 and light) in the photocatalytic oxidation process.


### 3.1.5. PCO of Cyclohexanone (4) in Acetonitrile

To shed more light on the formation and fate of the primary
photooxidation products, cyclohexanone was irradiated at the same previous
conditions. The conversion was only 19.8% after 20 hours irradiation. The GC/MS analyses of the photolysis products were
1,4-cyclohexanedione (7,
4.1%), 2-cyclohexenone (9, 1.4%), 2-hydroxycyclohexanone (10, 0.3%),
n-hexanoic acid (11, 6.2%), 5-hexenal (16, 3.5%), and
4-hydroxycyclohexanone (17,
3.8%) (Table 2, Exp.7).Table 2
Photocatalytic Oxidation of the Investigated Ketones4–6:
Exp.SubstrateSolventsTime (hrs)Conv. (%)Products (No,%)7Cyclohexanone (4)CH3CN2019.81,4-Cyclohexadione (7, 4.1), 2-Cyclohexenone
(9, 1.4), 2-Hydroxycyclohexanone (10, 0.3) n-Hexanoic acid (11, 6.2), 5-Hexenal
(16, 3.5), 4-Hydroxycyclohexanone (17, 3.8).8Cyclopentanone  (5)CH3CN2026.3n-Pentanoic acid (13, 15.9), 4-Pentenal
(23, 10.4).9Cycloheptanone  (6)CH3CN2029.9n-heptanoic acid (15, 6.8), 6-Heptenal
(27, 4.1), 4-Hydroxycycloheptanone (28, 7.8), 1,4-Cycloheptanedione
(29, 7.8), 2-Hydroxycycloheptanone (30, 3.4).GC chromatogram and the suggested
mechanism of these products matches those obtained from photocatalytic
oxidation of cyclohexanol (1).


## 3.1.1. PCO of 1 in Acetonitrile

After irradiation of1 in dry acetonitrile, the photolysis product, using GC/MS technique,
were cyclohexanone (4, 45.6%), 1,4-cyclohexanedione  (7,
3.2%), cyclohexyl formate (8,
2.5%), 2-cyclohexenone  (9, 1.8%), 2-hydroxycyclohexanone (10,
1.1%), n-hexanoic acid  (11, 4.0%), 5-hexenal(16, 2.9%),
4-hydroxycyclohexanone  (17,
4.4%), and cyclohexyl hexanoate (12, 3.0%), and other product (19,
2.9%) (Table 1, Exp. 1).Table 1
Photocatalytic oxidation of the investigated cyclalkanols1–3.
Exp.SubstrateSolventsTime (hrs)Conv. (%)Products (No, yield%)1Cyclohexanol (1)CH3CN2071.6Cyclohexanone (4, 45.6), 1,4-Cyclohexanedione
(7, 3.2), Cyclohexyl formate (8, 2.5), 2-Cyclohexenone (9, 1.8), 2-Hydroxycyclohexanone
(10, 1.1) n-Hexanoic
acid (11, 4.0), Cyclohexyl hexanoate (12, 3.0), 5-Hexenal (16,
2.9), 4-Hydroxycyclohexanone (17, 4.4), Other product (19, 2.9).2Cyclohexanol  (1)CH3CN+H2O (4 : 1)2074.3Cyclohexanone (4, 39.7), 1,4-Cyclohexadione
(7, 3.7), Cyclohexyl formate (8, 1.4), 2-Cyclohexenone (9, 2.4), 2-Hydroxycyclohexanone
(10, 7.8), 5-Hexenal (16,
1.8), 4-Hydroxycyclohexanone (17, 11.0), 1,4-Cyclohexanediol   (20,
6.5).3Cyclohexanol  (1)CH3CN+H2O2 (4 : 1)2098.0Cyclohexanone (4, 13.6), 1,4-Cyclohexadione
(7, 8.7), 2-Cyclohexenone (9, 5.1), 2-Hydroxycyclohexanone (10,
10.2), 4-Hydroxycyclohexanone
(17, 10.5), 1,4-Cyclohexanediol   (20,
8.2), Glutaric acid (21, 12.0), Adipic acid (22, 29.8).4Cyclohexanol  (1)H3COH32045Cyclohexanone (4, 25.2), 1,4-Cyclohexadione
(7, 3.4), Cyclohexyl formate (8, 2.3), 2-Hydroxycyclohexanone (10,
0.5) n-Hexanoic acid (11,
5.4), 5-Hexenal (16, 1.9), 4-Hydroxycyclohexanone (17, 3.5),
1,1′-Bicyclohexyl-1,1′-diol (18, 2.5).5Cyclopentanol  (2)CH3CN2094.2Cyclopentanone (5, 72.7), n-Pentanoic
acid (13, 4.3), Cyclopentyl pentanoate (14, 2.8), 4-Pentenal (23,
3.7), Other products (24, 3.1), (25, 5.8), (26, 1.8).6Cycloheptanol  (3)CH3CN20100Cycloheptanone (6, 79.9), n-heptanoic acid
(15, 3.2), 6-Heptenal (27, 1.8), 4-Hydroxycycloheptanone (28, 4.1),
1,4-Cycloheptadione (29, 4.7), 2-Hydroxycycloheptanone (30, 1.5).Cyclohexanol (1) is a sensitive probe, for it has been established that two electron
oxidation will give cyclohexanone (4) [17] as a primary product, whereas
single electron oxidation selectively produces the open-chain unsaturated
aldehyde 5-hexenal (16) [17], also 5-hexenal (16) formed by intramolecular
hydrogen transfer reaction of cyclohexanone. Cyclohexanone was also oxidized to
n-hexanoic acid (11) [28, 29].Cyclohexanone (4) can be
hydroxylated by O•H radical to 4-hydroxycyclohex-anone (17) and 2-hydroxycyclohexanone (10). 1,4-cyclohexanedione (7) is most
probably formed by oxidation of 4-hydroxycyclohexanone (17) [30]
and  2-cyclohexenone (9) was
probably formed by dehydration of 2-hydroxycyclohexanone (10) [14, 17],
respectively. 3-hydroxycyclohexanone may be formed also, but not detected, and
dehydrated to 2-cyclohexenone (9). This is because the heat of formation
[31] of the three hydroxylated cyclohexanones are close to each other (−101.61, −99.88, −103.64 Kcal/mol, resp.). This means that they are close in
stability and probability of formation. 5-hexenal (16) oxidation by
double bond cleavage to formaldehyde [32] which by further oxidation gives
formic acid. Cyclohexanol (1) can react with formic acid to give cyclohexyl
formate (8), and with n-hexanoic acid (11) to give cyclohexyl hexanoate (12),  (Scheme 1).Scheme 1
Suggested mechanism of PCO of1 in acetonitrile.


## 3.1.2. PCO of 1 in Acetone:

After irradiation of1 in acetone, the photolysis products analyzed by GC/MS technique were
cyclohexanone  (4, 25.2%),
1,4-cyclohexanedione  (7, 3.4%) cyclohexyl
formate  (8, 2.3%),
2-hydroxycyclohexanone  (10, 0.5%), n-hexanoic acid  (11, 5.4%),
5-hexenal (16, 1.9%), and 4-hydroxycyclohexanone (17, 3.5%),1,1′-bicyclohexyl-1,1′-diol  (18, 2.5%) (Table 1, Exp. 4).GC chromatogram and the suggested mechanism of these products showed that nearly
all products formed in acetone are the same as the products formed in
acetonitrile, but in lower conversion. From Table1, the conversion of
cyclohexanol (1) in dry acetonitrile was 71.6%, while in dry acetone
equals to 45% (Figure 1). This is because the dielectric constant (DEC)
of acetonitrile is much higher than that of acetone, 37.5 and 21.4, respectively
[33]. A plausible explanation is that increase of solvent polarity increases
the stabilization of the intermediate radical cations which in turn raise the
percentage of the oxidation products [33]. This supports the electron/hole
mechanism in photocatalytic oxidation of the investigated alcohols.Figure 1
Effect of solvents on photocatalytic oxidation of cyclohexanol (1).


## 3.1.3. PCO of 1 in Acetonitrile-Water (4 : 1)

Irradiation products of1 in acetonitrile-water mixture (4 : 1), as analyzed by GC/MS, were cyclohexanone (4, 39.7%),
1,4-cyclohexanedione (7, 3.7%), cyclohexyl formate (8,
1.43%), 2-cyclohexenone  (9, 2.4%), 2-hydroxycyclohexanone (10,
7.8%), 5-hexenal (16, 1.8%), 4-hydroxycyclohexanone  (17, 11.0%) and
1,4-cyclohexanediol  (20, 6.5%), (Table 1, Exp. 2).The products formed in aqueous acetonitrile are almost similar to those formed
in acetonitrile, but the experimental conversion and yield distribution
are different (Table1). Conversion of 1 in aqueous acetonitrile
(4 : 1) is 74.3% more than in dry acetonitrile (71.6%) this is because the dielectric
constant (DEC) of water is 78.3 [34] (Figure 1). Also primary oxidant in
this reaction is expected to be the strong oxidant O•H radical
generated by trapping a photohole at an adsorbed water molecule [35]. The percentage of hydroxylated compounds increased from 5.5% to 25.3%, and a new
product is formed 1,4-cyclohexanediol (20).


## 3.1.4. PCO of 1 in Acetonitrile-30%H2O2 (4 : 1)

Irradiation of1 in acetonitrile-30% hydrogen peroxide  (4 : 1) without oxygen passage, using GC/MS analyses of the photolysat gave cyclohexanone  (4, 13.6%), 1,4-cyclohexanedione  (7, 8.7%), 2-cyclohexenone (9, 5.1%), 2-hydroxycyclohexanone (10, 10.2%),
4-hydroxycyclohexanone  (17, 10.5%)
and 1,4-cyclohexadiol  (20,
8.2%), glutaric acid  (21, 12.0%) and
adipic acid (22, 29.8%) (Table 1, Exp. 3).GC chromatogram and the suggested mechanismof these products showed that most
of products formed in acetonitrile-30%H2O2 (4 : 1)
mixture are the same as formed in dry acetonitrile with
almost complete conversion of 1 (98%), that is, more than in dry
acetonitrile or in aqueous acetonitrile (Figure 1). Electrons are
donated from TiO2 to hydrogen peroxide because it is a stronger
oxidant than molecular oxygen [30]. When hydrogen peroxide accepts an
electron, O•H radical may be produced according to (1) [35], so
the O•H radicals are generated in large amounts, so the
percentage of hydroxylated compounds increased, for example, the % of compound 10 increased from 1.1% to 7.8% to 10.2% in cases of acetonitrile, aqueous
acetonitrile or acetonitrile-30%H2O2,
respectively, and new products were formed such as glutaric acid (21) and adipic acid (22).(1)TiO2(e−)+H2O2⟶TiO2+OH¯+O⋅H.The addition ofH2O2 is a well-known technique for increasing the rate of photocatalytic degradation
with higher quantum yield of formation of O•H [30].
Consequently, the increased oxidation rate is expected by the addition of H2O2 as shown in Table 1 (Exps. 1–3).It is worthy to mention that no oxidation products could be detected
with cyclohexanol in the absence of catalyst and/or light [30]. This is a good
proof for the role of each parameter (H2O2, TiO2 and light) in the photocatalytic oxidation process.


## 3.1.5. PCO of Cyclohexanone (4) in Acetonitrile

To shed more light on the formation and fate of the primary
photooxidation products, cyclohexanone was irradiated at the same previous
conditions. The conversion was only 19.8% after 20 hours irradiation. The GC/MS analyses of the photolysis products were
1,4-cyclohexanedione (7,
4.1%), 2-cyclohexenone (9, 1.4%), 2-hydroxycyclohexanone (10, 0.3%),
n-hexanoic acid (11, 6.2%), 5-hexenal (16, 3.5%), and
4-hydroxycyclohexanone (17,
3.8%) (Table 2, Exp.7).Table 2
Photocatalytic Oxidation of the Investigated Ketones4–6:
Exp.SubstrateSolventsTime (hrs)Conv. (%)Products (No,%)7Cyclohexanone (4)CH3CN2019.81,4-Cyclohexadione (7, 4.1), 2-Cyclohexenone
(9, 1.4), 2-Hydroxycyclohexanone (10, 0.3) n-Hexanoic acid (11, 6.2), 5-Hexenal
(16, 3.5), 4-Hydroxycyclohexanone (17, 3.8).8Cyclopentanone  (5)CH3CN2026.3n-Pentanoic acid (13, 15.9), 4-Pentenal
(23, 10.4).9Cycloheptanone  (6)CH3CN2029.9n-heptanoic acid (15, 6.8), 6-Heptenal
(27, 4.1), 4-Hydroxycycloheptanone (28, 7.8), 1,4-Cycloheptanedione
(29, 7.8), 2-Hydroxycycloheptanone (30, 3.4).GC chromatogram and the suggested
mechanism of these products matches those obtained from photocatalytic
oxidation of cyclohexanol (1).


## 3.2. PCO of Cyclopentanol (2) and Cyclopentanone (5)

### 3.2.1. PCO of 2 in Acetonitrile Using TiO2

After irradiation of2 in
acetonitrile, the residue was
analyzed by GC/MS. The photolysis products were cyclopentanone (5, 72.7%), n-pentanoic
acid (13, 4.3%), cyclopentyl pentanoate (14, 2.8%),
4-pentenal (23, 3.7%), and some other unidentified low
concentration products (24, 3.1%, 25, 5.8%, and 26, 1.8%) (Table 1, Exp.  5).GC chromatogram and the suggested mechanism of these products showed that
cyclopentanol (2) via two electron oxidation gave cyclopentanone (5) [17] as a primary product, whereas single electron oxidation selectively
produces open chain unsaturated aldehyde 4-pentenal (23) [17]. Also
4-pentenal (23) may be formed by intramolecular hydrogen atom transfer
reaction of cyclopentanone [28]. Cyclopentanone was also oxidized to
n-pentanoic acid (13), which reacted with cyclopentanol (2) to
give cyclopentyl pentanoate (14) (Scheme 2).Scheme 2
Photocatalytic oxidation of2 and 3.


### 3.2.2. PCO of Cyclopentanone (5) in Acetonitrile

Great attention was focused on the
oxidation of the primary oxidation product, cyclopentanone. Therefore, the
photocatalytic oxidation of5 under the same previous condition was
investigated. Low conversion (26.3%) was observed and the photolysis products
were n-pentanoic acid (13, 15.9%), and 4-pentenal (23,
10.4%) (Table 2, Exp. 8).The suggested mechanism of these products is
in agreement with the suggested mechanism of photocatalytic oxidation of
cyclopentanol.


## 3.2.1. PCO of 2 in Acetonitrile Using TiO2

After irradiation of2 in
acetonitrile, the residue was
analyzed by GC/MS. The photolysis products were cyclopentanone (5, 72.7%), n-pentanoic
acid (13, 4.3%), cyclopentyl pentanoate (14, 2.8%),
4-pentenal (23, 3.7%), and some other unidentified low
concentration products (24, 3.1%, 25, 5.8%, and 26, 1.8%) (Table 1, Exp.  5).GC chromatogram and the suggested mechanism of these products showed that
cyclopentanol (2) via two electron oxidation gave cyclopentanone (5) [17] as a primary product, whereas single electron oxidation selectively
produces open chain unsaturated aldehyde 4-pentenal (23) [17]. Also
4-pentenal (23) may be formed by intramolecular hydrogen atom transfer
reaction of cyclopentanone [28]. Cyclopentanone was also oxidized to
n-pentanoic acid (13), which reacted with cyclopentanol (2) to
give cyclopentyl pentanoate (14) (Scheme 2).Scheme 2
Photocatalytic oxidation of2 and 3.


## 3.2.2. PCO of Cyclopentanone (5) in Acetonitrile

Great attention was focused on the
oxidation of the primary oxidation product, cyclopentanone. Therefore, the
photocatalytic oxidation of5 under the same previous condition was
investigated. Low conversion (26.3%) was observed and the photolysis products
were n-pentanoic acid (13, 15.9%), and 4-pentenal (23,
10.4%) (Table 2, Exp. 8).The suggested mechanism of these products is
in agreement with the suggested mechanism of photocatalytic oxidation of
cyclopentanol.


## 3.3. PCO of Cycloheptanol (3) and Cycloheptanone (6)

### 3.3.1. PCO of 3 in Acetonitrile Using TiO2

After irradiation of3 in acetonitrile,  the
photolysis products, analyzed by GC/MS, were cycloheptanone  (6, 79.9%), n-heptanoic acid  (15, 3.2%), 6-heptenal (27, 1.8%),
4-hydroxycycloheptanone  (28, 4.1%),
1,4-cycloheptadione  (29, 4.7%)
and 2-hydroxycycloheptanone (30,
1.5%) (Table 1, Exp.  6).The suggested mechanism of these
products showed that cycloheptanol (3) is a sensitive probe, therefore
it has been established that two electron oxidation will give cycloheptanone (6) [17], whereas a single electron oxidation selectively produces the open-chain unsaturated
aldehyde 6-heptenal (27) [17]. Also 6-heptenal (27) was formed by
intramolecular hydrogen transfer reaction for cycloheptanone [28]. Cycloheptanone
oxidiation affored n-heptanoic acid (15). Also cycloheptanone (6) was hydroxylated by O⋅H radical to give
4-hydroxycycloheptanone (28) and 2-hydroxycycloheptanone (30). 1,4-cycloheptandione (29) was most probably generated by oxidation of
4-hydroxycycloheptanone (28) [30] (Scheme 2).


### 3.3.2. PCO of Cycloheptanone (6) in Acetonitrile Using TiO2

Photocatalytic irradiation of6 in dry acetonitrile, resulted in 29.9% conversion. The photolysis
products were 6-heptenal (27, 4.1%), n-heptanoic acid (15,
6.8%), 4-hydroxycycloheptanone (28, 7.8%), 1,4-cycloheptadione  (29, 7.8%), and
2-hydroxycycloheptanone (30, 3.4%) (Table 2, Exp. 9).GC chromatogram and the suggested
mechanism of these products are similar to the photocatalytic oxidation of cycloheptanol (3).The obtained results indicated that the percentage conversion of3>2>1, the same as that of
corresponding ketones where 7>6>5. This is in agreement with
their heat of formation
(ΔHf=−60.83, −66.98, and −80.22)Kcal/mol, respectively. Of
course cyclohexanol (1), the most stable one [31] (ΔHf=
−80.22 Kcal/mol), has the lowest percentage of conversion compared with 2 and 3.


## 3.3.1. PCO of 3 in Acetonitrile Using TiO2

After irradiation of3 in acetonitrile,  the
photolysis products, analyzed by GC/MS, were cycloheptanone  (6, 79.9%), n-heptanoic acid  (15, 3.2%), 6-heptenal (27, 1.8%),
4-hydroxycycloheptanone  (28, 4.1%),
1,4-cycloheptadione  (29, 4.7%)
and 2-hydroxycycloheptanone (30,
1.5%) (Table 1, Exp.  6).The suggested mechanism of these
products showed that cycloheptanol (3) is a sensitive probe, therefore
it has been established that two electron oxidation will give cycloheptanone (6) [17], whereas a single electron oxidation selectively produces the open-chain unsaturated
aldehyde 6-heptenal (27) [17]. Also 6-heptenal (27) was formed by
intramolecular hydrogen transfer reaction for cycloheptanone [28]. Cycloheptanone
oxidiation affored n-heptanoic acid (15). Also cycloheptanone (6) was hydroxylated by O⋅H radical to give
4-hydroxycycloheptanone (28) and 2-hydroxycycloheptanone (30). 1,4-cycloheptandione (29) was most probably generated by oxidation of
4-hydroxycycloheptanone (28) [30] (Scheme 2).


## 3.3.2. PCO of Cycloheptanone (6) in Acetonitrile Using TiO2

Photocatalytic irradiation of6 in dry acetonitrile, resulted in 29.9% conversion. The photolysis
products were 6-heptenal (27, 4.1%), n-heptanoic acid (15,
6.8%), 4-hydroxycycloheptanone (28, 7.8%), 1,4-cycloheptadione  (29, 7.8%), and
2-hydroxycycloheptanone (30, 3.4%) (Table 2, Exp. 9).GC chromatogram and the suggested
mechanism of these products are similar to the photocatalytic oxidation of cycloheptanol (3).The obtained results indicated that the percentage conversion of3>2>1, the same as that of
corresponding ketones where 7>6>5. This is in agreement with
their heat of formation
(ΔHf=−60.83, −66.98, and −80.22)Kcal/mol, respectively. Of
course cyclohexanol (1), the most stable one [31] (ΔHf=
−80.22 Kcal/mol), has the lowest percentage of conversion compared with 2 and 3.


## 4. Discussion

The main objectives of the present
work include photocatalytic oxidation (PCO) of the selected alicyclic alcohols (1–3) of different ring sizes in the presence of aerated titanium dioxide (Degussa
P25) suspensions in anhydrous acetonitrile and other solvent mixtures. A polar,
nonhydroxylic solvent (CH3CN) represents the best opportunity for
controlling oxidative reactivity at the interface between a liquid reaction
mixture and a solid irradiated photocatalyst [18, 19, 36]. To evaluate the role of
each parameter in the system using cyclohexanol (1) as a model alcohol, we studied the effect of catalyst, light,
oxygen, solvent polarity, and the effect of hydrogen peroxide as oxidizing
agent instead of oxygen gas.No oxidation products were detected
upon irradiation of cyclohexanol (1) in the absence of catalyst and/or
of light. Furthermore, experiments carried out in the absence of oxygen showed
also no significant evidence of products formed. All these results confirm the
essential role of oxygen, TiO2 and light in the photocatalytic
oxidation process [18, 19, 33].Furthermore, the UV absorption
spectra of the target compounds have been inspected to shed some light on their
behavior upon irradiation. The maximum absorption of the investigated alcohols (1–3) ranges between 277.1 nm for 2 and 287.1 nm for 3, a range that
will be cutoff by
pyrex glass [18, 19]. As the maximum emission of the used 450 W medium
pressure mercury lamp ranges between 296.7 nm–578.0 nm (4.18−2.15 eV), the
obtained products are therefore originated through initial band gap excitation
of TiO2 (3.23 eV) [7, 37], but not through direct
excitation of the alcohol.In a photocatalytic process, the primary step following the radiation
absorption by the photocatalyst is the generation of electron-hole pairs (2), which must be trapped in order to avoid recombination, that is, charge
separation is essential for any photocatalytic electroprocess.(2)TiO2⟶hv[TiO2*]⟶eCB−+hVB+.Despite the contributions from a
number of research groups, detailed mechanisms of the photocatalytic oxidation
processes at the TiO2 surface remain elusive for many important
organic materials particularly regarding the initial steps involved in the
redox radical reactions, which may involve one or more of the following
radicals O2•−, O•H, HOO•, Cl•, and so on.
The use of nonaqueous reaction medium rules out the participation of hydroxy
radicals generated by water trapping of the hole, in the oxidation process [38].From the product distribution and
reaction products profiles, the reaction mechanism of TiO2 sensitized photooxidation of the target alcohols (1–3) in aerated
acetonitrile has been proposed.The efficiency of the oxidation was
governed by the oxidation potential of the alcohol, the avability ofα-hydrogens and the tightness of the association with the photocatalyst.Cyclic voltammetric measurement
using glassy carbon electrode revealed anodic oxidation potential of the model
alcohol,  cyclohexanol equals to (+0.85 V) which is less positive than the hole potential (+2.35 V), meaning that
photocatalytic oxidation of cyclohexanol is thermodynamically possible. Also as
the reduction potential of oxygen (−0.39 V) is less negative than the
conduction band electron (−0.88 V), formation of the superoxideO2•− is
thermodynamically allowed in the reaction medium [18, 19, 33].It is useful to stress on the point
that in a photoreaction occurring on a semiconductor catalyst powder both the
oxidation and reduction processes must occur on the same particle, although reaction
sites for these processes may be different. Adsorbed oxygen species are the
traps for electrons according to the following (3), (4), forming
the radical anion O2•− (superoxide).(3)(4)On the other hand, the adsorbed
alcohol species in dry acetonitrile are the likely traps for holes forming the
radical cation, by single electron transfer, which rapidly deprotonated
producingα-hydroxy radical of
reasonable stability [17] as follows (5).(5)Two fates are available to such
radical. (1)A second electron oxidation: the latter intermediate is
extremely easily oxidized [17] by loss of a second electron generating a
protonated carbonyl intermediate. This intermediate gives the proton to the
environmental forming the corresponding carbonyl compound (two-electrons
mechanism).   (2) Trapping ofO2•− and/or O2•−: An
alternative mechanism is the trapping of the α-hydroxy radical by oxygen
forming the peroxy radical or by superoxide (O2•−) forming
peroxy anion, decomposition of which would be expected to occur rapidly to
produce the carbonyl products [17] (6), (7).Formation of the open-chain carbonyl compounds (8) can be
produced by ring rupture of the α-hydroxy radical cation [17]
(single-electron mechanism).These
suggested mechanisms can be represented in general as follows:(6)(7)(8)Accordingly, the proposed
photocatalytic oxidation (PCO) of cyclohexanol (1), for example, could
be present as in Scheme 1. Also the PCO of 2 and 3 are
summarized in Scheme 2.


## 5. Kinetic Studies

The photocatalytic oxidation reactions of cyclohexanol (1) and cyclopentanol (2) in
acetonitrile were followed up by GC, as shown in Figures 2 and 4, respectively.Figure 2
Photocatalytic Oxidation of cyclohexanol
(1) in acetonitrile.Selectivity of photocatalytic
oxidation of cyclohexanol (1), cyclopentanol (2) are shown in Figures
3 and 5, respectively.Figure 3
Selectivity of products of PCO of1
in acetonitrile.Figure 4
Photocatalytic oxidation of cyclopentanol2 in acetonitrile.Figure 5
Selectivity of products of PCO of2 in acetonitrile.From
these results, we can conclude that the photocatalytic oxidation of secondary
alcohols1, 2 have high selectivity to form ketones by more than 85%.The kinetic disappearance [39] of the investigated alcohols (1, 2) from an initial concentration in acetonitrile was shown in Figures 6 and 8, respectively.Figure 6
Photocatalytic disappearance of1 in acetonitrile.The semilogarithmic plots [39] of concentration
data gave a straight line for cyclohexanol (1) and
cyclopentanol (2) in acetonitrile as shown in Figures 7 and 9, respectively. These findings indicate that the
photocatalytic oxidation of the investigated alcohols in TiO2 suspension can be described by the
first-order kinetic model; LnC=−kt+LnCo, where Co is the initial concentration and C is the concentration of alcohols at time t.
This is in agreement with the former work of Cinar on m-cresol [39].Figure 7
First-order PCO of1 in acetonitrile
(r=0.9886, k=−0.083 hr−1).Figure 8
Photocatalytic disappearance of2 in acetonitrile.Figure 9
First-order PCO of2 in acetonitrile (r=0.9918, k=−0.094 hr−1).



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*Source: 205358-2008-06-15.xml*