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Mechanism of Photooxidation of Alcohol over Nb2O5 Tetsuya Shishido,*,† Toshiaki Miyatake,† Kentaro Teramura,‡ Yutaka Hitomi,§ Hiromi Yamashita,| and Tsunehiro Tanaka*,† Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Kyoto 615-8510, Japan, Kyoto UniVersity Pioneering Research Unit for Next Generation, Kyoto UniVersity, Katsura, Kyoto 615-8510, Japan, Department of Molecular Chemistry and Biochemistry, Doshishya UniVersity, Kyotanabe, Kyoto 610-0321, Japan, and DiVision of Materials and Manufacturing Science, Graduate School of Engineering, Osaka UniVersity, Osaka 565-0871, Japan ReceiVed: February 21, 2009; ReVised Manuscript ReceiVed: May 12, 2009
Photooxidations of alcohols to carbonyl compounds proceed selectively at a low temperature over niobium oxide (Nb2O5) without organic solvents. Although Nb2O5 is not able to absorb light at >390 nm wavelengths, the photooixdation of 1-pentanol proceeded under irradiation up to ca. 480 nm. This observation indicates that the photoactivation mechanism of alcohol over Nb2O5 is different from the classical electron transfer mechanism found in semiconductor photocatalysis (the formation of an excited electron in the conduction band and the positive hole in the valence band). On the basis of FT-IR and ESR measurements, the following mechanism is proposed: Alcohol is adsorbed onto Nb2O5 as an alcoholate species in the dark. Alcoholate adsorbed on Nb2O5 is activated by transferring an electron to the conduction band reducing Nb5+ to Nb4+, and leaving a hole on the alcoholate. The formed alkenyl radical is converted to a carbonyl compound. The product is desorbed and then the reduced Nb4+ sites are reoxidized by the reaction with molecular oxygen. The rate-determining step of the photooxidation of alcohol over Nb2O5 is the process of desorption of the formed carbonyl compound. 1. Introduction Catalytic alcohol oxidation to carbonyl compounds is one of the most important chemical transformations used in industrial chemistry and organic syntheses.1-3 Noncatalytic methods with stoichiometric, toxic, corrosive, and expensive oxidants such as dichromate and permanganate, which produce a large amount of heavy metal waste, under stringent conditions of high pressure and/or temperature have been widely used for alcohol oxidations.1-4 In addition, these reactions are often carried out with environmentally unfriendly organic solvents. Therefore, in the last few years, much attention has been paid to the development of heterogeneous catalytic systems that use clean and atom efficient oxidants like molecular oxygen or H2O2 without organic solvents.4-16 Recently, the aerobic oxidation of alcohols was successfully carried out by using heterogeneous catalysts such as tetrapropylammonium perruthenate (TPAP)/MCM-41,8 Ru/ CeO2,9 Ru-hydrotalcite,10 Ru/hydroxyapatite(Ru-HAP),11 [RuCl2(pcymene)]2/activated carbon,12 Ru/Al2O3,13 Pd-hydrotalcite which requires the addition of pyridine,14 and Pd or Pt on activated carbon.4,15 These systems require the use of organic solvents like toluene and trifluorotoluene. Although Wu et al. reported on solvent-free aerobic oxidation of alcohols by Pd/Al2O3,16 the * Tetsuya Shishido and Tsunehiro Tanaka, Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan, E-mail:
[email protected] and tanakat@ moleng.kyoto-u.ac.jp, Tel: +81-75-383-2558, Fax: +81-75-383-2561. † Department of Molecular Engineering, Graduate School of Engineering, Kyoto University. ‡ Kyoto University Pioneering Research Unit for Next Generation, Kyoto University. § Department of Molecular Chemistry and Biochemistry, Doshishya University. | Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University.
use of the noble metal Pd is an essential requirement. In this respect, photoreactions are promising processes and the development of photocatalysts is a subject that is now receiving noticeable attention. TiO2 has been identified as one example of a practical and useful photocatalysts.17-20 However, in these reports TiO2 is used in vapor phase oxidations at high temperature,17 oxidation of only lower alcohols,18,19 oxidation with solvents such as benzene,20 and a low selectivity to partial oxidation products due to excess photoactivation of target products which leads to deep oxidation. This study demonstrates an effective organic solvents-free photooxidation of alcohols by using molecular oxygen without additives over niobium oxide (Nb2O5) [eq 1]. The study presents a detailed reaction mechanism of the photooxidation of alcohols.
2. Experimental Section Preparation. Niobic acid, niobium oxide hydrate (Nb2O5 · nH2O, AD/2872, HY-340) was kindly supplied from CBMM. Niobium oxide catalyst was prepared by calcinations of niobic acid in a dry air flow at 773 K for 5 h. After calcination, the catalyst was ground into powder under 100 mesh (0.15 mm). The TT-Nb2O5 phase with a pseudohexagonal structure was obtained by using the XRD pattern and Raman spectra data.21-23 Photocatalytic Activity and the Measurement of the Action Spectrum. The photocatalytic oxidation of alcohol was carried out in a quasi-flowing batch system under atmospheric oxygen. Nb2O5 (100 mg), an alcohol as a substrate (10 mL), and a stirring bar were introduced to the Pyrex glass reactor. No solvent was
10.1021/jp901603p CCC: $40.75 2009 American Chemical Society Published on Web 10/05/2009
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TABLE 1: Photooxidation of Various Alcohols over Nb2O5 with Molecular Oxygena,b
Reaction conditions were as follows: alcohol (10 mL), Nb2O5 (100 mg), 323 K, under 1 atm of O2, O2 flow rate (2 cm3 min-1): conversion and selectivity were determined by gas chromatography with an internal standard. b Numbers in parentheses are the results of photochemical reaction without catalysts. a
used. The catalyst was not evacuated nor pretreated in the presence of O2. Moreover, substrate was used without further purification. The suspension was vigorously stirred at 323 K and irradiated from the flat bottom of the reactor through a reflection by a cold mirror with a 500 W ultra-high-pressure Hg lamp (USHIO Denki Co.). Oxygen was flowed into the reactor at 2 cm3 min-1. Organic products were analyzed by FIDGC and GC mass spectrometry. Furthermore, at the down stream of the flow reactor, a barium hydroxide solution (Ba(OH)2) was equipped to determine the quantity of carbon dioxide (CO2) as barium carbonate (BaCO3). In the measurement of an action spectrum, a SHIMADZU MONOCHROMATER GRATING 1200 GROOVES/MM was used and located between the 300 W Xe lamp (Perkin-Elmer CERAMAX PE300BUV) and the quartz reactor. UV-Vis Spectrum. The diffuse reflectance spectrum was measured with a UV-vis spectrometer (Perkin-Elmer UV/vis/ NIR Spectrometer Lambda 19). The resolution of the spectra was 1 nm. The Nb2O5 sample was introduced into an in situ quartz cell. Prior to the measurements, the sample was heated in air and evacuated for 1 h at ambient temperature followed by treatment with 10.7 kPa of O2 for 2 h and evacuation for 1 h. FT-IR Spectra. FT-IR spectra of the sample before and during the reaction were recorded with a Perkin-Elmer SPECTRUM ONE Fourier transform infrared spectrometer. The resolution of the spectra was 4 cm-1. The Nb2O5 sample was cast into a pellet (diameter )12 mm). The molded sample was introduced into an in situ IR cell equipped with BaF2 windows. Prior to the measurements, the sample was heated in air and evacuated for 1 h at 773 K followed by treatment with 10.7 kPa of O2 for 2 h and evacuation for 1 h at 773 K. As a light
source, a 300 W Xe lamp (Perkin-Elmer PE300BF) was used. An L-42 cutoff filter was used for the visible light irradiation (>390 nm). O2 was introduced onto the Nb2O5 with adsorbed alcohol under the measurement conditions. ESR Spectra. ESR measurements were carried out by using an X-band ESR spectrometer (JEOL JES-SRE2X) with an in situ quartz cell. Prior to EPR measurements, the Nb2O5 sample was heated in air and evacuated for 0.5 h at ambient temperature followed by treatment with 10.7 kPa of O2 for 1 h and subsequently evacuated for 0.5 h. The g value of radical species was determined by using an Mn marker. ESR spectra of the alkenyl radical were obtained by illumination of Nb2O5 with adsorbed alcohol. As a light source, a 500 W ultra-high-pressure mercury lamp was used. To confirm the reactivity of the formed alkenyl radical, O2 was introduced onto the Nb2O5 with adsorbed alcohol under measurement conditions. 3. Results and Discussion The niobium oxide catalyst showed high catalytic activities for the oxidation of alcohols with 1 atm of molecular oxygen under irradiation as shown in Table 1. The photogenerated products were the corresponding aldehydes, carboxylic acid, ketones, and carbon dioxide (CO2). No product was detected in the dark with the Nb2O5 catalyst. The evolution of photogenerated products responded to illumination. Autooxidation proceeded when 1-phenylethanol, cyclohexanol, and benzyl alcohol were irradiated without catalyst. This was due to the formation of radical species by the photodecomposition of carbonyl compounds (Norrish type I reaction [eq 2]) which were impurities present in the alcohols (entries 1-3 in Table 1). The conversions and/or selectivities to carbonyl compounds were greatly improved by the presence of the Nb2O5 catalyst.
Mechanism of Photooxidation of Alcohol over Nb2O5
In the cases of aliphatic secondary alcohols, the conversions remarkably increased without decreasing the selectivities to the corresponding ketones (entries 4 and 5 in Table 1). The less reactive primary alcohol, 1-pentanol, was also photooxidized over the Nb2O5 catalyst; however, the reaction did not proceed under photoirradiation without catalyst (entry 6, Table 1), showing that photooxidation of 1-pentanol over the Nb2O5 catalyst was due entirely to a photocatalytic reaction. The evolution of products (1-pentanal + pentanoic aicd) linearly increased with irradiation time (see the Supporting Information). The niobium oxide catalyst was reusable and showed the same conversion and selectivity without any pretreatment as the catalyst as prepared. Although TiO217-20 is known to be active for photooxidation of alcohols, the Nb2O5 catalyst showed higher selectivity than TiO2 at the same conversion level.24 This suggests that the photoactivation mechanism of alcohol over TiO2 is different from that over Nb2O5. Figure 1 shows the FT-IR spectra of adsorbed cyclohexanol on Nb2O5. The bands at 1467 and 1452 cm-1 were assigned to δs(CH2). The peaks at 1363 and 1347 cm-1 were assigned to ω(CH2). The formation of the alcoholate species was confirmed by FT-IR spectra of adsorbed cyclohexanol on Nb2O5. The formation of the alcoholate species by the adsorption of alcohol is usually accompanied by a shift of the stretching mode of a C-O bond to a higher wavenumber.25-27 Pure cyclohexanol has a band at 1068 cm-1, which is assignable to the stretching mode of a C-O bond. This band is similar to physisorbed cyclohexanol (Figure 1c). After the adsorption on Nb2O5, the bands at 1091 and 1126 cm-1 were assigned to the stretching mode of a C-O bond in the alcoholate species on the Nb2O5. Figure 2 represents the effect of UV irradiation (390 nm) as shown in Figure 3. Figure 4 shows the apparent quantum efficiency of photooxidation of 1-pentanol as a function of the wavelength of the incident light (action spectrum) and a UV-vis spectrum of Nb2O5 whose band gap energy is estimated to be 3.2 eV (the photoexcitation wavelength is 390 nm). Although Nb2O5 catalyst is not able to absorb light at wavelengths longer than 390 nm, the photooxidation of 1-pentanol proceeds under irradiation up to ca. 480 nm. This result indicates that the photoactivation mechanism of alcohol over Nb2O5 is different from the classical electron transfer mechanism in semiconductor photocatalysis (the formation of an excited electron in the conduction band and the positive hole in the valence band). Recently, we reported that the photo-SCR (selective catalytic reduction) of NO with NH3 over TiO2 (the band gap energy is 3.28 eV (385 nm)) proceeded even under visible light irradiation (400-450 nm) through the direct electron transfer from the N 2p orbital of adsorbed NH3 to the conduction band consisting of Ti 3d orbitals.28 On the basis of DFT calculations, it was
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Figure 1. FT-IR spectra of adsorbed cyclohexanol on Nb2O5: (a) cyclohexanol was exposed to Nb2O5 for 1 h (physisorption + chemisorption), (b) evacuated for 2 h (chemisorption), (c) difference spectrum (a-b: physisorption). Nb2O5 was evacuated at 773 K for 1 h and oxidized at 773 K with 10.7 kPa of O2 and then evacuated at 773 K for 1 h before FT-IR measurements.
Figure 2. FT-IR spectra of adsorbed species on Nb2O5 in the photoreaction of adsorbed cyclohexanol with O2: (a) cyclohexanol was exposed to Nb2O5 for 1 h and evacuated for 2 h, (b) under UV irradiation for 1, (c) 5, (d) 7, (e) 10, (f) 15, and (g) 30 min. Nb2O5 was evacuated at 773 K for 1 h and oxidized at 773 K with 10.7 kPa of O2 and then evacuated at 773 K for 1 h before FT-IR measurements.
supported that this N 2p electron donor level is located in the forbidden band and enables the photo-SCR to proceed under visible light. In the case of the photooxidation of alcohols, it appears that the electron transfer from an O 2p orbital of the adsorbed alcoholate species to a conduction band consisting of Nb 4d orbitals takes place by absorption of visible light (>390 nm) and hence this transfer enables the transformation from alcoholate to carbonyl compounds. A broad ESR signal around g ) 1.9 was observed at 123 K (Figure 5c), when an excess of 1-pentanol was adsorbed on Nb2O5 under UV irradiation. The identical signal was also obtained by the reduction with H2 at 673 K. On the basis of these results and previous reports,29,30 the broad signal at g ) 1.9 was assigned to Nb4+. The signal at g ) 1.9 disappeared with the introduction of O2. This indicates that Nb4+ was oxidized to Nb5+ even at 123 K (Figure 5d). ESR signals (g ) 2.006, AH1 ) 2.0 mT, AH2 ) 4.4 mT) assigned to alkenyl radical species were obtained (Figure 6) when 1-pentanol was adsorbed on Nb2O5 under UV irradiation at 77 K. The signal was stable in the dark at 77 K, but disappeared at room temperature (Figure 6e). The signal was restored by photoirradiation at 77 K. Moreover, the signal hardly changed when molecular oxygen was introduced. This was also observed even under UV irradiation (Figure 6f). This indicates that the alkenyl radical species does not react with molecular oxygen. Thus it is
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Figure 3. FT-IR spectra of adsorbed species on Nb2O5 in the photoreaction of adsorbed cyclohexanol with O2: (a) Cyclohexanol was exposed to Nb2O5 for 1 h and evacuated for 2 h, (b) under visible light irradiation (>390 nm) for 1, (c) 5, (d) 7, (e) 10, (f) 15, and (g) 30 min. Nb2O5 was evacuated at 773 K for 1 h and oxidized at 773 K with 10.7 kPa of O2 and then evacuated at 773 K for 1 h before FT-IR measurements. Figure 6. ESR spectra of Nb2O5 recorded at 77 K: (a) after pretreatment, (b) under UV irradiation, (c) after the exposure to 1-pentanol in the dark, (d) under UV irradiation in the presence of 1-pentanol, (e) in the dark, after the sample was heated up to room temperature and then cooled to 77 K, and (f) under reirradiation. Nb2O5 was evacuated at 773 K for 1 h and oxidized at 773 K with 10.7 kPa of O2 and then evacuated at 773 K for 1 h before ESR measurements. (g) The alkenyl radical.
SCHEME 1: Reaction Mechanism of Alcohol Photooxidation over Nb2O5 with Molecular Oxygen
Figure 4. Action spectrum of photooxidation of 1-pentanol (dot) and UV-vis spectrum of Nb2O5 (linear). Reaction conditions of the action spectrum were as follows: 1-pentanol (10 mL), Nb2O5 (100 mg), 323 K, under 1 atm of O2, O2 flow rate (2 cm3 min-1).
Figure 5. ESR spectra of Nb2O5 recorded at 123 K: (a) after pretreatment, (b) in the dark in the presence of 1-pentanol, under irradiation, (c) under UV irradiation in the presence of 1-pentanol, and (d) after the introduction of O2 at 123 K. Nb2O5 was evacuated at 773 K for 1 h and oxidized at 773 K with 10.7 kPa of O2 and then evacuated at 773 K for 1 h before ESR measurements.
concluded that the carbonyl compound was formed by dehydrogenation (the reduction of Nb5+ to Nb4+ proceeded simultaneously) and that molecular oxygen contributes to the reoxidation of Nb4+ to Nb5+. On the basis of these results, the reaction mechanism shown in Scheme 1 is proposed: Alcohol is adsorbed on Nb2O5 as alcoholate species I in the dark (step i). Alcoholate adsorbed
on Nb2O5 is activated by transferring an electron to the conduction band reducing Nb5+ to Nb4+ and leaving a hole on alcoholate (step ii). The formed alkenyl radical II is converted to carbonyl compound III (step iii). The product is desorbed (step iv) and then the reduced Nb4+ sites are reoxidized by the reaction with molecular oxygen (step v). Oxygen anion radical species (O2- and O3-), which are formed by irradiation over TiO231-35 and often responsible for deep oxidation, do not participate in the photooxidation over Nb2O5 catalyst as shown in Scheme 1. For instance, when Nb2O5 was irradiated in the presence of molecular oxygen, no ESR signal due to oxygen anion radical species was observed. This presumably explains why the photooxidation of alcohol to carbonyl compound proceeds selectively over the Nb2O5 catalyst. According to the dependencies of the concentrations of substrate and O2 and light intensity, the reaction rate (r) of photooxidation of alcohol is expressed as
Mechanism of Photooxidation of Alcohol over Nb2O5
r ) k[S]0.9PO0.92 I0.65
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(3)
Here, the rate constant, the substrate concentration, and the light intensity and the pressure of the oxygen are abbreviated to k, [S], I, and PO2, respectively. To verify the proposed mechanism shown in Scheme 1 and to determine the rate-determining step of the photooxidation of alcohol over Nb2O5, the obtained rate equation (eq 3) is compared with the rate equation derived from the proposed reaction mechanism by steady-state approximation. Rate constants and equilibrium constants in each elementary step are defined as shown in Scheme 2. The following rate equations were derived by assuming that each step is a rate-determining step:
r)
k1K2K3K4K5[A]PO0.52 I[S]0 K2K3K4K5PO0.52 I + K2K3K4I + K2K3[C]I + [C][H2O] (step i) (4)
r)
a S: vacant active site. AS: active site adsorbed 1-pentanol. B: alkyl radical. S*: reduced site. C: pentanal.
surface was slow. Thus we decided that the rate-determining step of the photooxidation of alcohol over Nb2O5 is the desorption process of the formed carbonyl compound (step iv). 4. Conclusions
k2K1K3K4K5[A]PO0.52 I[S]0 K1K3K4K5[A]PO0.52
+
K3K4K5[A]PO0.52
+ K3K4 + K3[C] + 1 (step ii) (5)
r) k1K2K3K4K5[A]PO0.52 I[S]0 K1K2K4K5[A]PO0.52 I + K1K4K5[A]PO0.52 + K4K5PO0.52 + K4 + [C] (step iii)
r)
SCHEME 2: Elementary Steps in Photooxidation of 1-Pentanola
(6)
k1K2K3K4K5[A]PO0.52 I[S]0 {(K1K2K3K5 + K1K2K5)[A]PO0.52 I + K1K5[A]PO0.52 + K5PO0.52 + 1}[H2O] (step iv) (7)
r) k1K2K3K4K5[A]PO0.52 I[S]0 (K1K2K3K4 + K1K2K3[C] + K1K2[C][H2O])[A]I + K1[C][H2O][A] + [C][H2O] (step v)
(8)
The reaction orders of the substrate concentration, the light intensity, and the pressure of the oxygen in steps i, ii, and v are not compatible with the experimental data. Consequently, step iii (alkenyl radical is converted to carbonyl compound) or step iv (desorption of product) is the rate-determining step of the photooxidation of alcohol over Nb2O5. The alkenyl radical species were not obtained at 123 K, whereas Nb4+ was observed. This suggests that step iii (the alkenyl radical species to carbonyl compounds) proceeded even at a low temperature due to the high reactivity of the alkenyl radical species. FT-IR spectra showed that the carbonyl compounds remained on the surface of Nb2O5 even at room temperature (Figures 3 and 4), indicating that the desorption of the carbonyl compound from the Nb2O5
In conclusion, photooxidation of alcohol to a carbonyl compound proceeds selectively at low temperature over the Nb2O5 catalyst without the need of organic solvents. The first step of the photooxidation is the dissociative adsorption of alcohol on Nb2O5, resulting in the formation of the alcoholate species and a hydroxy group. A surface complex consisting of Nb2O5 and the alcoholate species is photoactivated to generate an alkenyl radical, and then dehydrogenated to the carbonyl compounds. Molecular oxygen does not react with this alkenyl radical but reoxidizes reduced Nb2O5. Steady-state kinetics indicate that the rate-determining step in photo-oxidation of alcohol is the desorption of carbonyl compounds. Acknowledgment. This work was partially supported by Grants-in-Aid for Scientific Research on Priority Areas (No. 17034036, Molecular Nano Dynamics, and No. 20037038, Chemistry of Concerto Catalysis) and Scientific Research (No. 19360365, B) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. K.T. is supported by the 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. Supporting Information Available: Figure showing the time course of the photooxidation of 1-pentanol over the Nb2O5 catalyst. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Sheldon, R. A.; Kochi J. K. Metal-Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. (2) Hudlicky, M. Oxidations in Organic Chemistry; ACS Monograph Series; American Chemical Society: Washington, DC, 1990. (3) Hill C. L. AdVances in Oxygenated Process; Baumstark, A. L., Ed.; JAI: London, UK, 1998; Vol. 1; p 1. (4) Larock, R. C. ComprehensiVe Organic Transformations; VCH: New York, 1989. (5) Sheldon, R. A.; Arends, I. W. C. E.; Dijksman, A. Catal. Today 2000, 57, 157. (6) Mallat, T.; Baiker, A. Chem. ReV. 2004, 104, 3037. (7) Srinivas, N.; Rani, V. R.; Kishan, M. R.; Kulkarni, S. J.; Raghavan, K. V. J. Mol. Catal. A: Chem. 2001, 172, 187. (8) Bleloch, A.; Johnson, B. F. G.; Ley, S. V.; Price, A. J.; Shephard, D. S.; Thomas, A. W. Chem. Commun. 1999, 1907.
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