Study of the Reaction Mechanism of Selective Photooxidation of

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J. Phys. Chem. C 2009, 113, 17018–17024

Study of the Reaction Mechanism of Selective Photooxidation of Cyclohexane over V2O5/Al2O3 Kentaro Teramura,*,† Tai Ohuchi,‡ Tetsuya Shishido,‡ and Tsunehiro Tanaka*,‡ Kyoto UniVersity Pioneering Research Unit for Next Generation, Kyoto UniVersity, Kyoto 615-8510, Japan, and Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Kyoto 615-8510, Japan ReceiVed: April 1, 2009; ReVised Manuscript ReceiVed: August 5, 2009

The selective oxidation of a hydrocarbon to a ketone over an orthovanadate-like (V)O)O3 species in the presence of O2 consists of five elementary steps: (1) photoactivation of orthovanadate-like (V)O)O3 species, (2) adsorption of hydrocarbon, (3) formation of an alkoxide intermediate, (4) evolution of ketone and water, and (5) desorption of the ketone. In this study, the alkoxide intermediate was isolated and characterized by in situ FT-IR spectroscopy. By using C6D12, we found that the rate-determining step involved the elimination of a proton. A kinetic study was carried out on the basis of our proposed photocatalytic cycle to identify the rate-determining step. Further, the rate of evolution of cyclohexanone produced by the photooxidation of cyclohexane was measured under various conditions to determine the reaction order. The rate-determining step was found to be the elimination of a proton from the adsorbed hydrocarbon under photoirradiation. Introduction It is well-known that orthovanadate-like (V)O)O3 species show good photocatalytic activity for CO oxidation1 and selective oxidation of alkanes,2-6 olefins,7-15 and alcohols10 in the presence of molecular oxygen. The active species is in the triplet excited state, where an electron-hole pair is localized on the V)O bond. Recently, we found that it is formed by intersystem crossing between two singlet excited states (designated as S1 and S2 in our previuos paper 16) and the lowest triplet excited state, T1. S1 and S2 are assigned to the excitation between the three oxygen anions (in the legs) in the basal plane of the orthovanadate structure and the vanadium cation in the center of the tetrahedral structure and localization of an electron-hole pair on the V)O bond, respectively. When selective photooxidation is carried out in the presence of O2 over the orthovanadate-like (V)O)O3 species dispersed on a support, the active oxygen that is incorporated in the oxygenated product is the lattice oxygen of the V)O bond in the orthovanadate-like (V)O)O3 species. In general, silica, silicabased zeolite, and mesoporous silica, which have high specific surface areas, can isolate vanadium species on their surfaces; this facilitates the formation of photoactive orthovanadate-like (V)O)O3 species.17-21 However, it is known that in the presence of water, orthovanadate-like (V)O)O3 species supported on silica and silica-based supports aggregate into poly-VOx species and easily leach out into the liquid phase.21 Therefore, in the liquid phase, these species, do not function as photocatalysts. We confirmed that orthovanadate-like (V)O)O3 species supported on alumina exhibit tolerance to water. In previous studies,22-24 we reported that an orthovanadate-like (V)O)O3 * Corresponding authors. E-mail: [email protected]. kyoto-u.ac.jp (K.T.), [email protected] (T.T.). Telephone: +8175-585-6095 (K.T.), +81-75-383-2558 (T.T.). Fax: +81-75-585-6096 (K.T.), +81-75-383-2561 (T.T.). † Kyoto University Pioneering Research Unit for Next Generation, Kyoto University. ‡ Department of Molecular Engineering, Graduate School of Engineering, Kyoto University.

species supported on alumina exhibits photocatalytic activity in selective oxidation of hydrocarbons in the liquid phase. Onestep insertion of oxygen atoms from hydrocarbons (alkanes and olefins) into alcohols, aldehydes, ketones, and epoxides using molecular oxygen is one of the most attractive reactions because partially oxidized derivatives are raw materials of manufactured products such as polymers, perfumes, agricultural chemicals, drug medicines, and synthetic compounds. We have reported that the conversion of cyclohexane and selectivity to the selective oxidation products were achieved 0.49% and 85% after 24 h of photoirradiation using 0.8 g of 3.5 wt % aluminasupported vanadium oxide (V2O5/Al2O3), respectively, where the ratio of ketone to alcohol (K/A) was 6.2 in the photocatalytic oxidation of cyclohexane.24 In addition, V2O5/Al2O3 exhibited photocatalytic activity for selective oxidation of various hydrocarbons, particularly for oxidation of benzene to phenol.23 As mentioned above, our research groups have investigated the photocatalysis of orthovanadate-like (V)O)O3 species dispersed on various supports and proposed realistic reaction mechanisms of selective oxidation of various hydrocarbons in the gas and liquid phases. The photoexcitation process of these species was clarified in detail. However, the photoexcitation process of hydrocarbon substrates and the formation process of the reaction intermediate are still unclear. In this study, we carried out photocatalytic oxidation of various hydrocarbons and recorded infrared absorption spectra of the species adsorbed on V2O5/Al2O3 under photoirradiation in order to identify the photoactive species and intermediates during the selective photooxidation of cyclohexane, which contributed to the development of the photocatalytic cycle. In addition, we determined the kinetic order of reactions and deduced the ratedetermining step from the analysis of reaction kinetics on the basis of our proposed photocatalytic cycle. Experimental Section V2O5/Al2O3 was prepared by the impregnation of γ-alumina (JRC-ALO-8) supplied by the Catalysis Society of Japan with an aqueous solution of ammonium metavanadate (NH4VO3) at

10.1021/jp902955b CCC: $40.75  2009 American Chemical Society Published on Web 09/03/2009

TRH: Photooxidation of Cyclohexane over V2O5/Al2O3

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TABLE 1: Evolutions of Selective Oxidation Products and Carbon Dioxide (CO2) in the Photooxidation of Various Hydrocarbons in the Liquid Phase

353 K for 2 h, followed by evaporation, drying, and calcination at 773 K for 5 h in the same manner as that reported previously.22-24 The specific surface areas of γ-alumina as a support and V2O5/Al2O3, which were evaluated by the BrunauerEmmett-Teller (BET) method using the N2 adsorption isotherm at 77 K, were 140 and 132 m2g-1, respectively. Accordingly, the surface density of vanadium atoms in 3.5 wt % V2O5/Al2O3 is 1.76 atom nm-2. Further, orthovanadate-like (V)O)O3 species was confirmed to be the main species in 3.5 wt % V2O5/Al2O3 by UV-vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, and XAFS analysis. The photocatalytic reaction and analysis of reaction kinetics were carried out in a quasi-flowing batch system. V2O5/Al2O3 was used as the photocatalyst and various hydrocarbon reactantes were introduced to a Schrenck flask-like reactor and irradiated from the flat bottom of the reactor through a reflection by a cold mirror with a 500 W ultrahigh pressure Hg lamp (supplied by USHIO, Inc.) in flowing O2 gas with a velocity of 2 cm3 min-1 at 310 K. Organic products were analyzed by a FID gas chromatograph (Shimadzu Corporation, GC-14B) and gas chromatograph mass spectrometer (Shimadzu Corporation, GCMS-2010). Fourier transform infrared absorption (FT-IR) spectra were recorded with a Fourier transform infrared spectrometer (Spectrum One, PerkinElmer Co., Ltd.) in the transmission mode. The catalyst sample was pressed into a wafer (diameter ) 10 mm) and introduced in an in situ cell equipped with BaF2 windows. Before in situ measurement, the catalyst wafer was pretreated at 673 K for 1 h in the presence of O2. Gaseous deuterated cyclohexane was used as the reactant. For each spectrum, the data from 10 scans were accumulated at a resolution of 4 cm-1. Results and Discussion Table 1 shows the evolutions of selective oxidation products and carbon dioxide (CO2) in the photooxidation of various hydrocarbons in the liquid phase over 3.5 wt % V2O5/Al2O3 and JRC-TIO-11 supplied by the Catalysis Society of Japan as a standard TiO2 sample, a typical photocatalyst under photoir-

radiation for 24 h. JRC-TIO-11 consists of anatase (91%) and rutile (9%) phases, and its specific surface area is evaluated to be 78 m2 g-1.25 All photocatalytic selective oxidations were carried out characteristically without solvent, although lowconcentration substrates had been used in many other researches in order to achieve high conversion of reactants. We have already reported that with an increase in the loading amount of vanadium species, the evolution of selective oxidation products increases until a loading amount of 3.5 wt % is reached, and then the photocatalytic activity decreases.23 V2O5/Al2O3 and TiO2 exhibited photocatalytic activity for selective oxidation of benzene to phenol; cyclohexane to cyclohexanol and cyclohexanone; toluene to benzyl alcohol, benzaldehyde, and benzoic acid; and ethyl benzene to 1-phenylethonol and acetophenone. Mul et al. recently reported photooxidation of cyclohexane over the anatase phase of TiO2, analyzed by in situ ATR-FTIR spectroscopy.26,27 Large amounts of CO2 were, however, generally formed over irradiated TiO2, which shows that TiO2 is advantageous for complete oxidation.28-32 It has been actually reported that different types of active oxygen species are generated on TiO2, and many oxidation processes proceed indiscriminately. Accordingly, TiO2 is not effective in selectively oxidizing a hydrocarbon. However, V2O5/Al2O3 promotes the formation of much smaller amounts of CO2 than TiO2. For cyclic hydrocarbons containing a side chain such as toluene and ethyl benzene, we achieved high selectivity (approximately 100%) in selective oxidation. In a previous study,16 we reported that a low loading amount of V2O5/Al2O3 contains orthovanadate-like (V)O)O3 species (so-called isolated tetrahedral vanadium oxide species) and that the triplet excited state of orthovanadate-like (V)O)O3 species is a photoactive site, where an electron-hole pair is localized on the V)O bond. It is concluded that an oxygenated intermediate is generated between a hydrocarbon substrate and orthovanadate-like (V)O)O3 species containing the exclusive active oxygen species. We have already carried out photooxidation of hydrocarbons over orthovanadate-like (V)O)O3 species in the presence of 18O2 in the gas phase and obtained only 16O oxygenated compounds in the initial stage.8,9,33 This implies that the lattice oxygen atom of orthovanadate-like

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TABLE 2: Evolutions of 1-Adamantanol, 2-Adamantanone, and 2-Adamantanol in the Photooxidation of Adamantane in the Presence of 18O2 over 3.5 wt % V2O5/Al2O3

(V)O)O3 species is activated under photoirradiation and inserted into the hydrocarbon substrate. Therefore, it is important to generate the exclusive active oxygen species in order to achieve selective photooxidation of a hydrocarbon. Table 2 shows the evolutions of 1-adamantanol, 2-adamantanone, and 2-adamantanol in the photooxidation of adamantane with acetonitrile as a solvent in the presence of 18O2 over 3.5 wt % V2O5/Al2O3 under photoirradiation for 12 and 24 h. 2-Adamantanone mainly contained the 16O atom derived from the lattice oxygen of the orthovanadate-like (V)O)O3 species. 1-Adamantanol and 2-adamantanol were almost 18O oxygenated products. In addition, the ratio of the amount of an 18O oxygenated product to that of an 16O oxygenated product (18O/ 16 O ratio) was larger after 24 h photoirradiation than after 12 h photoirradiation, which suggests that the lattice defect generated in the orthovanadate-like (V)O)O3 species is reoxidized by molecular oxygen. However, it is speculated that an oxygen radical species derived from molecular oxygen is involved in the formation mechanism of alcohol because the 18O/16O ratios of 1-adamantanol and 2-adamantanol were very high. Further, the photofragmentation of ketone (Norrish type I reaction) is known to generate a radical species.24 We proposed a photocatalytic cycle for selective photooxidation of a hydrocarbon to a ketone over an orthovanadate-like (V)O)O3 species as shown in Scheme 1. In step 1, the orthovanadate-like (V)O)O3 species is activated under photoirradiation, and an electron-hole pair is localized on the V)O bond. In our previous paper,16 we described the photoactivation mechanism of orthovanadate-like (V)O)O3 species in detail. In step 2, a substrate such as alkane, olefin, and alcohol is adsorbed on a support such as Al2O3. Then, the substrate would spill over to the orthovanadate-like (V)O)O3 species with elimination of a proton in step 3. At present, it is speculated that an alkoxide intermediate is generated between the orthovanadate-like (V)O)O3 species and hydrocarbon substrate. Gaseous molecular oxygen attacks this intermediate to generate ketone and water in step 4. The generated water would be desorbed from the orthovanadate-like (V)O)O3 species. Finally,

SCHEME 1: Photocatalytic Cycle for Selective Photooxidation of Hydrocarbon to Ketone over Orthovanadate-Like (V)O)O3 Species

in step 5, the selective oxidation product such as ketone is also desorbed, and the orthovanadate-like (V)O)O3 species reverts back to the ground state. As mentioned above, we confirmed steps 1 and 5 empirically;16 however, steps 2, 3, and 4 have not been demonstrated as it now stands. FT-IR spectroscopy was employed to clarify the adsorption and desorption processes that occur in steps 2, 3, and 4. Figures 1 and 2 show the FT-IR spectra of deuterated cyclohexane (C6D12) on Al2O3 and V2O5/Al2O3, respectively. Three sharp bands (H1-H3) at 3772, 3727, and 3674 cm-1 and one broadband (H4) between 3600 and 3200 cm-1 were assigned as follows: O-H stretching vibration of a terminal hydroxyl group coordinated to a single tetrahedral Al3+ cation; Al-OH (H1 mode), a bridging OH group that links to a tetrahedral and an octahedral Al3+ cation; Al2-OH (H2 mode), an OH group coordinated to three cations in octahedral interstices; Al3-OH (H3 mode); and hydrogen-bonded hydroxyl group (H4 mode) as shown in Figures 1a and 2a.34,35 The bands of the hydroxyl

TRH: Photooxidation of Cyclohexane over V2O5/Al2O3

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17021 TABLE 3: Yields of Cyclohexanol (Alcohol) and Cyclohexanone (Ketone) in 24 h Photooxidation of C6H12 and C6D12 together with Ratios of the Formation Rate of a Product to Oxygenated Products, kH/kDa ketone alcohol

C6H12 (/µmol)

C6D12 (/µmol)

kH/kD

C6H10O (37.1) C6H11OH (14.2)

C6D10O (25.5) C6D11OD (15.4)

1.45 0.92

a Reaction time, 24 h; light source, 500 W ultrahigh pressure Hg lamp; amount of C6H12 or C6D12, 10 mmol; and amount of benzene, 10 mL.

Figure 1. FT-IR spectra of deuterated cyclohexane (C6D12) on Al2O3 (a) after pretreatment, (b) introduction of C6D12 and photoirradiation, and (c) evacuation.

Figure 2. FT-IR spectra of deuterated cyclohexane (C6D12) on V2O5/ Al2O3 (a) after pretreatment, (b) introduction of C6D12 and photoirradiation, and (c) evacuation.

group became broad after introduction of C6D12 and photoirradiation as shown in Figure 1b, which shows that C6D12 is adsorbed on Al2O3 with hydrogen-bond coordination to surface hydroxyl groups. In addition, C-D stretching vibration bands appeared between 2400 and 2000 cm-1, although these bands vanished after evacuation (Figure 1c). C6D12, therefore, is physisorbed on Al2O3 and easily desorbed by evacuation. However, in the case of V2O5/Al2O3, the V)O stretching vibration band36 that was present at 1030 cm-1 disappeared (Figure 2a) after introduction of C6D12 and photoirradiation (Figure 2b). As in the case of Al2O3, the bands due to hydroxyl groups became broad after introduction of C6D12. Figure 2b shows that bands appeared between 2800 and 2600 cm-1 as well as between 2400 and 2000 cm-1 (C-D stretching vibration). The band between 2800 and 2600 cm-1 is assignable to the O-D stretching vibration corresponding to the H4 mode.37 The appearance of the O-D stretching band suggested that the D atom is abstracted from cyclohexane. The C-D and O-D stretching vibrations were still observed after evacuation as shown in Figure 2c. The intensity of the C-D stretching band reduced on evacuation, but the spectral feature essentially remained unchanged. This indicates that consequent to evacuation the cyclohexane adsorbed on the Al2O3 surface was removed, while that adsorbed on surface vanadium species remained adsorbed and maintained its molecular structure. It is very likely that cyclohexane is retained as cyclohexanoxide bonding to vanadium ion species, and that abstracted hydrogen leads to the formation of the hydrogen-bonded OD group. In addition, the V)O stretching vibration and O-H stretching

vibration of the H1 mode were not restored. Nonrestoration of OH groups of the H1 mode showed that the oxygen atom of each of these OH groups interacted with another proton, resulting in a change from the H1 mode to the H4 mode. From these results, we propose that under photoirradiation, the substrate is adsorbed onto the orthovanadate-like (V)O)O3 species with the elimination of a proton, which interacts with the O-H stretching vibration of the H1 mode and forms an alkoxide intermediate as shown in Scheme 1(iii). Table 3 shows the yields of cyclohexanol (alcohol) and cyclohexanone (ketone) in 24 h photooxidation of C6H12 and C6D12 together with the ratios of the formation rate of a product to oxygenated products, kH/kD. The kH/kD values for the evolutions of cyclohexanol and cyclohexanone were 0.92 and 1.45, respectively. In general, if the free radical reaction proceeds, the kH/kD value is expected to be 1. The kH/kD value, 0.92, for the formation of cyclohexanol is close to 0.94, the square root of the molecular weight ratio of C6H12 to C6D12. Except for the contribution from the diffusion rates of C6H12 and C6D12, the kH/kD value is equal to 1. The oxygen radical species derived from molecular oxygen is involved in the formation mechanism of cyclohexanol; the radical species produced by the photofragmentation of cyclohexanone attacks molecular oxygen to generate the oxygen radical species. The oxygen radical species would contribute to the generation of cyclohexanol. However, as for the formation of cyclohexanone, hydrogen abstraction from cyclohexane is involved in the ratedetermining step, judging from the kH/kD value, 1.45. In fact, the FT-IR spectra showed that the cyclohexane adsorbed on Al2O3 interacts with the orthovanadate-like (V)O)O3 species with the elimination of a proton to generate the alkoxide intermediate. In addition, the photooxidation of adamantane shows that the lattice oxygen of the orthovanadate-like (V)O)O3 species would contribute to the generation of ketone. From this, it is concluded that ketone such as cyclohexanone is generated via the alkoxide intermediate on the orthovanadate-like (V)O)O3 species. In the photocatalytic cycle shown in Scheme 1, it is considerable that the elimination of a proton has a decisive influence on the time-determining step. There are two proton elimination processes in our proposed photocatalytic cycle. One process occurs in step 3, where the substrate adsorbed on Al2O3 spills over to the orthovanadate-like (V)O)O3 species with the elimination of a proton. The other process occurs in step 4, where the molecular oxygen in the gas phase attacks the alkoxide intermediate to generate ketone and water. We carried out a kinetic study to verify which proton elimination process is a time-determining step. The reaction rate is shown as the following formula

r ) kCC6H12RPO2βIγ

(1)

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We obtained the evolution rate of cyclohexanone for photooxidation of cyclohexane under various conditions so as to determine each reaction order. Figure 3 shows the logarithms of evolution rates of cyclohexanone in the photooxidation of cyclohexane under the condition of various concentrations of cyclohexane in benzene as a solvent, O2 diluted by N2 in the gas phase, and different light intensities. These logarithmic plots are reasonably fitted by straight lines as follows

r ) kCC6H120.77PO20.07I0.25

(5)

This rate equation is compared to each rate equation derived from our proposed reaction mechanism. Rate constants and equilibrium constants in each elementary step are defined as follows k1

ln CC6H10O ) -21.0 + 0.77 ln CC6H12

(2)

Sv + hV f Sv* k2

C6H12 + SA 98 C6H12 - SA ln CC6H10O ) -19.1 + 0.07 ln PO2

(6)

(Step 1)

(Step 2)

(3) k3

Sv* + C6H12 - SA 98 C6H12 - Sv* + SA ln CC6H10O ) -19.1 + 0.25 ln I

(7)

(Step 3)

(4)

The reaction rate is approximately first order against the concentration of cyclohexane. It depends on the adsorption of cyclohexane on Al2O3 or orthovanadate-like (V)O)O3 species. However, it does not depend on the concentration of O2. It is expected that the reactions involving O2 in the gas phase as shown in Scheme 1 do not constitute a time-determining step. The light intensity is physically controlled by various metal meshes and is estimated by UV-vis spectroscopy. The reaction rate is 0.25th order against the light intensity, which implies that one photon is involved in this photooxidation system. The results are summarized as the following rate equation

(8) k4

C6H12 - Sv* + O2 98 C6H10O - SV + H2O

(Step 4)

(9) k5

C6H10O - SV 98 C6H10O + SV

(Step 5)

(10)

We derived the following rate equations by assuming that each step is a rate-determining step.

Figure 3. Logarithms of evolution rates of cyclohexanone in the photooxidation of cyclohexane under the condition of (a) various concentrations of cyclohexane in benzene as a solvent, (b) O2 diluted by N2 in the gas phase, and (c) different light intensities.

Figure 4. Reciprocals of the reaction rates of photooxidation of cyclohexane against the reciprocal of (a) cyclohexane concentration and (b) light intensity.

TRH: Photooxidation of Cyclohexane over V2O5/Al2O3

r1 )

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k1K2K3K4K5CC6H12PO2I[Sv]0 CH2OCC6H10O + K2K3CC6H12CH2OCC6H10O + K2K3K4CC6H12PO2CC6H10O + K2K3K4K5CC6H12PO2 (11) r2 )

k2K1K3K4K5CC6H12PO2I[SA]0 CH2OCC6H10O + K1K3K4K5PO2I

(12)

the liquid phase. Adsorption of alkanes by orthovanadate-like (V)O)O3 species in the triplet excited state (T1) leads to selective photooxidation. The photocatalytic reaction proceeds as per the typical Langmuir-Hinshelwood mechanism. An alkoxide intermediate is generated on the orthovanadate-like (V)O)O3 species by the elimination of a proton from the hydrocarbon substrate. Molecular oxygen in the gas phase attacks the alkoxide intermediate to generate a ketone and water. The time-determining step is the elimination of the proton from the adsorbed hydrocarbon. Nomenclature

r3 )

k3K1K2K4K5CC6H12PO2I[SV]0[SA]0 (1 + K2CC6H12)(CH2OCC6H10O + K4PO2CC6H10O + K4K5PO2 + K1K4K5PO2I) (13)

r4 )

k4K1K2K3K5CC6H12PO2I[SV]0 CC6H10O + K5 + K1K5I + K1K2K3K5CC3H12I

(14) r5 ) k5K1K2K3K4CC6H12PO2I[SV]0 CH2O + K1CH2OI + K1K2K3CC6H12CH2OI + K1K2K3K4CC6H12PO2I

(15) The eqs 11-15 were compared with eq 5 obtained in the present study. The reaction orders of eqs 11, 12, and 14 are not compatible with those of eq 5. Consequently, steps 1, 2, and 4 are not time-determining steps of the selective photocatalytic oxidation of cyclohexane to cyclohexanone. On the other hand, eqs 13 and 15 are converted into eqs 16 and 17 as follows, assuming K4 . 1 and CC6H10O ≈ 0

r3 )

k3CC6H12I[SV]0[SA]0 (1/K2 + CC6H12)(1/K1 + I) r5 ) k5[SV]0

(1/K2 + CC6H12)(1/K1 + I) k3CC6H12I[SV]0[SA]0

Acknowledgment. This work was partially supported by Grant-in-Aid for Scientific Research(B) 19360365 from the Japan Society for the Promotion of Science (JSPS) and Program for Improvement of Research Environment for Young Researchers from Special Coordination Funds for Promoting Science and Technology (SCF) commissioned by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. References and Notes

(16)

(17)

The reaction order of eq 17 is not consistent with those of eq 5, whereas eq 16 can explain the kinetic order of eq 5. Equation 18 is the reciprocal formula of eq 16.

1/r )

r: cyclohexanone production rate (mol s-1) k: rate constant CC6H12: concentration of cyclohexane PO2: partial pressure of oxygen CC6H10O: concentration of cyclohexanone CH2O: concentration of water I: light intensity R: reaction order of cyclohexane β: reaction order of oxygen γ: reaction order of light intensity ki: rate constant at Step i Ki: equilibrium constant at Step i SV: site of the orthovanadate-like (V)O)O3 species SV*: site of the photoactivated orthovanadate-like (V)O)O3 species SA: alumina site

(18)

In Figure 4, we have plotted reciprocals of the reaction rates of photooxidation of cyclohexane against the reciprocal of the cyclohexane concentration and light intensity. Evidently, eq 16 describes the reaction rate very well. In conclusion, Step 3, which is elimination of the proton from the hydrocarbon adsorbed on Al2O3 by activated orthovanadate-like (V)O)O3 species, is the time-determining step. Conclusion Orthovanadate-like (V)O)O3 species exhibited high activity for selective photooxidation of various neat hydrocarbons in

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