TiO 2 Nanoparticles-Photocatalytic Oxidation of Selected Cycloalkanols

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.


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][3][4][5][6].
Photocatalytic oxidation and degradation of alcohols catalyzed by TiO 2 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 TiO 2 .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 H 2 PtCl 6 and CuCl 2 .
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 TiO 2 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 International Journal of Photoenergy 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 Co 2+ -modified P25-TiO 2 powders for the H 2 production from aqueous ethanol solution.In this paper, the long-term stability of photocatalytic activity of Co 2+ doped P25-TiO 2 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.

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 spectrophotometer-Shimadzu using thin film and NaCl disks.The electronic absorption spectra were recorded using UV-2101 PC, UV-VIS scanning spectrophotometer-Shimadzu. 1 H-NMR spectra were carried out using 90 MHz Varian 390 in CDCl 3 and TMS as an internal standard.GC analy-ses 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.

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.

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 TiO 2 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.
Results obtained in this work will be presented for each individual alcohol; and collective table and figures are also available in the following discussion.
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 Table 1, 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].
(1) This supports the electron/hole mechanism in photocatalytic oxidation of the investigated alcohols.

PCO of 1 in acetonitrile-water (4 : 1)
Irradiation products of  1).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 • OH 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).

PCO of 1 in acetonitrile-30% H 2 O 2 (4 : 1)
Irradiation of 1 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  GC chromatogram and the suggested mechanismof these products showed that most of products formed in acetonitrile-30% H 2 O 2 (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 TiO 2 to hydrogen peroxide because it is a stronger oxidant than molecular oxygen [30].When hydrogen peroxide accepts an electron, • OH radical may be produced according to (1) [35], so the • OH 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% H 2 O 2 , respectively, and new products were formed such as glutaric acid (21) and adipic acid (22).
The addition of H 2 O 2 is a well-known technique for increasing the rate of photocatalytic degradation with higher quantum yield of formation of • OH [30].Consequently, the increased oxidation rate is expected by the addition of H 2 O 2 as shown in Table 1 (Exps.[1][2][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 (H 2 O 2 , TiO 2 and light) in the photocatalytic oxidation process.
The suggested mechanism of these products is in agreement with the suggested mechanism of photocatalytic oxidation of cyclopentanol.

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 (CH 3 CN) 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, TiO 2 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-578.0nm (4.18 − 2.15 eV), the obtained products are therefore originated through initial band gap excitation of TiO 2 (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.
Despite the contributions from a number of research groups, detailed mechanisms of the photocatalytic oxidation processes at the TiO 2 surface remain elusive for many important organic materials particularly regarding the initial steps involved in the redox radical reactions, which may Omima S. Mohamed et al.  involve one or more of the following radicals O 2 •− , • OH, 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 TiO 2 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 of αhydrogens 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 superoxide O 2 •− 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 O 2 •− (superoxide).
O 2 (solution) O 2 (ads), 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 producing α-hydroxy radical of reasonable stability [17] as follows (5).•− ) 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: 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.

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.
From these results, we can conclude that the photocatalytic oxidation of secondary alcohols 1, 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.
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 TiO 2 suspension can be described by the first-order kinetic model; LnC = −kt + LnC o , where C o 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].

Scheme 1 :
Scheme 1: Suggested mechanism of PCO of 1 in acetonitrile.

5 )
CH − OH ads + h + VB CH − OH •+ ads −H + • C − OH ads (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 of O 2 •− and/or O 2 •− : An alternative mechanism is the trapping of the α-hydroxy radical by oxygen forming the peroxy radical or by superoxide (O 2

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 (Table