Reaction Mechanism of Selective Photooxidation of Amines over

Article pubs.acs.org/JPCC

Reaction Mechanism of Selective Photooxidation of Amines over Niobium Oxide: Visible-Light-Induced Electron Transfer between Adsorbed Amine and Nb2O5 Shinya Furukawa,†,∥ Yasuhiro Ohno,† Tetsuya Shishido,*,†,‡ Kentaro Teramura,†,‡,§ and Tsunehiro Tanaka*,†,‡ †

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto Daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ‡ Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Kyoto 615-8520, Japan § Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: Mechanistic studies were performed using several spectroscopic techniques (ultraviolet−visible, Fourier transform infrared, and electron spin resonance spectroscpies) and quantum chemical calculations to elucidate the photoactivation process and catalytic cycle of photooxidation of amines over niobium oxide (Nb2O5) under visible light. The reaction mechanism was found to involve oxidative dehydrogenation of amines to imines via photoactivation of a Nb2O5−amide surface complex derived from dissociatively adsorbed amine on Nb2O5. Formation of the surface complex generates a donor level consisting of a N 2p orbital localized on the amide nitrogen atom within the forbidden band of Nb2O5. Direct excitation from this new donor level to the conduction band of Nb2O5 enables excitation at lower energy than the bandgap. Although Nb2O5 itself is transparent to visible light (λ > 390 nm), amine oxidation proceeds even under visible irradiation because of photoactivation of the Nb2O5−amide surface complex. When a primary amine is used as a substrate, the produced dehydrogenated imine is immediately hydrolyzed to aldehyde, followed by condensation with an amine to form an dimerized imine.

1. INTRODUCTION Heterogeneous semiconductor photocatalysts have been widely attempted to apply in various chemical conversions such as water splitting, aerobic oxidation, and decomposition of harmful organic compounds.1−3 In general, such photocatalytic reactions are driven by charge carriers derived from interband excitation, reduction by excited electrons in the conduction band, and oxidation by the resulting positive holes in the valence band. Therefore, the effective wavelength for reaction depends on the bandgap of the photocatalyst. This effective wavelength is often modified by the presence of an impurity because a donor or acceptor level is generated within the forbidden band of the photocatalyst. Doping of titanium oxide (TiO2) with nonmetallic elemental anions (N, S, C)4−10 is frequently used to enable excitation from the donor level to the conduction band of TiO2. Anpo and co-workers11−13 reported that implantation of a metal cation (V or Cr) also produces donor levels. Hashimoto et al. reported that Cu(II)14,15 and Fe(III)16 species grafted on TiO2 worked as acceptors. The resulting materials respond to visible light (>390 nm) irradiation, whereas bare TiO2 does not. It should be noted that such photocatalytic reactions are essentially triggered by excitation of the photocatalyst itself. © 2012 American Chemical Society

We recently reported a different type of photocatalysis; selective oxidation of amines to imines over niobium oxide (Nb2O5).17 The band structure of Nb2O5 is similar to that of TiO2,18 and Nb2O5 exhibits broad absorption caused by interband excitation only in the UV region (390 nm).17 This counterintuitive result cannot be explained by a classical electron transfer mechanism based on interband excitation. We also found that selective oxidation of alcohols proceeds over Nb2O5 even under visible irradiation.19 We proposed the following mechanism: (a) dissociative adsorption of the substrate alcohol (formation of a Nb2O5−alkoxide surface complex) generates donor levels consisting of O 2p orbitals within the forbidden band, and (b) direct excitation occurs from the new donor level to the conduction band of Nb2O5, which is composed of Nb 4d orbitals.19,20 This direct excitation from a new donor level to the conduction band of the photocatalyst can be categorized as a Received: October 23, 2012 Revised: December 12, 2012 Published: December 17, 2012 442

dx.doi.org/10.1021/jp310501h | J. Phys. Chem. C 2013, 117, 442−450

The Journal of Physical Chemistry C

Article

Irradiator) as a light source for 24 h. The QE was estimated as the number of moles of N-benzylidenebenzylamine produced per mole of incident photon. The number of incident photons was measured using a calibrated Si photodiode (Hamamatsu S2281, Japan). The rate of total incident photons at each wavelength range was follows: 3.7/300 ± 8, 12/330 ± 8, 13/ 360 ± 7, 12/380 ± 7, 20/400 ± 7, 25/420 ± 7, 30/460 ± 7, 24/500 ± 7, 19/550 ± 7 and 18/600 ± 7 (μmol h−1/nm). 2.4. UV−vis Spectra. Diffuse reflectance spectra (1 nm resolution) were measured with a UV−vis spectrometer (JASCO UV570). A slurry of a mixture of benzylamine (0.8 mL) and Nb2O5 (500 mg) was used as the sample of benzylamine adsorbed on Nb2O5. 2.5. FT-IR Measurement. FT-IR spectra of the samples before and during photoirradiation were recorded with a PerkinElmer SPECTRUM ONE FT-IR spectrometer. The resolution of the spectra was 4 cm−1. Nb2O5 catalyst was cast into a pellet with diameter of 12 mm. The molded sample was introduced into an in-situ IR cell equipped with CaF2 windows. Prior to obtaining measurements, the sample was pretreated with 6.7 kPa of O2 at 673 K for 1 h and evacuated at 673 K for 0.5 h. A certain amount of benzylamine was introduced to the pretreated Nb2O5 at room temperature, followed by evacuation. Before photoirradiation, the sample was exposed to 6.7 kPa of O2. A 200 W Hg−Xe lamp (SAN-EI ELECTRIC SUPERCURE-204S) was used as a light source. An L-42 cutoff filter was used for visible light irradiation (>390 nm). 2.6. Surface Models and DFT Calculation Method. Quantum chemical calculations using density functional theory (DFT) as implemented in Gaussian 0332 were carried out to estimate the electronic structure and excitation energy of Nb 2 O 5 and the Nb 2 O 5 −amide complex. The neutral Nb12O43H26 cluster (1), which was curved up from the H− Nb2O5(001) surface33 by saturating the peripheral oxygen atoms with hydrogen atoms (see Supporting Information Figure S1 and Table S1), was used in this study. Although the angles of Nb−O−H were determined by the structural parameters of H−Nb2O5, the distances of all O−H bonds were fixed at 0.96 Å. The Nb12O42H25(NHCH3) cluster (2) was used as a model of the Nb2O5−amide complex and was obtained by substitution of a surface hydroxyl (−OH) group of 1 with a methylamide (−NHCH3) group, followed by geometry optimization of the −NHCH3 moiety with the geometry of the Nb12O42H25 support fixed (see Figure S2 and Table S2). The calculation used Becke’s three-parameter hybrid (B3LYP) method involving the correlation function of Lee et al.’s34,35 and the LanL2DZ basis set for geometry optimization and single-point energy calculations with tight self-consisting field convergence criteria. One-electron excitation energies of the model clusters were also obtained by time-dependent (TD) calculations36−38 with singlet spin multiplicity. 2.7. ESR Measurement. ESR measurements were carried out using an X-band ESR spectrometer (JEOL JES-SRE2X) with an in-situ quartz cell. Prior to obtaining measurements, the sample was pretreated with 6.7 kPa of O2 at 673 K for 1 h and then evacuated at 673 K for 0.5 h. Degassed butylamine was introduced to the pretreated Nb2O5 at room temperature, and then the cell was held at 77 K. Unfortunately, using benzylamine as a substrate did not result in a recognizable signal, probably because of inadequate vapor pressure. ESR spectra of the sample were recorded before and after photoirradiation. A 500 W ultrahigh-pressure Hg lamp was used as a light source. An L-42 cutoff filter was used for visible

ligand-to-metal charge transfer (LMCT) transition in a broad sense, which has been reported in TiO2-based systems, for example, sulfoxidation of alkanes,21 activation of ammonia,22−24 oxidation of alcohols25−27 and amines,28 and other adsorbateinduced photocatalyses.29,30 According to these systems, it can be assumed that amine oxidation over Nb2O5 occurs through similar chemistry to that of alcohol oxidation. However, the detailed reaction mechanism of amine oxidation is still unclear. In addition, there are few studies on the mechanism of LMCTtype photocatalysis using multiple characterization techniques. In the present study, we investigated the mechanism of amine photooxidation over Nb2O5 in detail using several spectroscopic techniques (ultraviolet−visible (UV−vis), Fourier transform infrared (FT-IR), and electron spin resonance (ESR) spectroscopies) and quantum chemical calculations. The detailed catalytic cycle was also clarified and compared with those of other photocatalytic systems.

2. EXPERIMENTAL SECTION 2.1. Preparation. Niobium oxide hydrate (Nb2O5·nH2O, HY-340) was kindly supplied from CBMM (Brazil). Nb2O5 catalyst was prepared by calcination of niobium oxide hydrate in a dry airflow at 773 K for 5 h (BET surface area: 48 m2 g−1). BET surface areas of catalysts were determined from N2 adsorption isotherm at 77 K measured by a BELSORP 28SA (BEL Japan Corp., Japan). All reagents were of reagent grade and used without further purification. Reagents were obtained from Aldrich, Tokyo Kasei Kogyo Co., Ltd., and Wako Pure Chemical Industries, Ltd. N-Deuterated benzylamine (benzylamine-N-d2) was prepared by washing benzylamine (3 g) in CH2Cl2 (5 mL) with D2O (3 mL) three times, followed by separation, evaporation of CH2Cl2, and distillation.31 A 1H NMR spectrum indicated >90% deuterium content at the αpositions by comparison of the integral of the α-signals with that of the benzylic position. 2.2. Photocatalytic Reaction. The photocatalytic oxidation of benzylamine in the presence and absence of O2 was carried out in a batch system. Nb2O5 photocatalyst (100 mg) and a stirring bar were placed in a Pyrex glass reactor that cut off light below 300 nm and was equipped with a balloon. An amine as a substrate (5 mmol) in benzene (10 mL) was introduced into the reactor, and the reaction atmosphere was replaced with N2. The suspension was vigorously stirred at room temperature and irradiated from the flat bottom of the reactor through reflection by a cold mirror with a 500 W ultrahigh-pressure Hg lamp (USHIO Denki Co.). After 6 h of irradiation, the reactor was flushed with excess O2 to completely replace the reaction atmosphere with O2. Products were identified by gas chromatography−mass spectroscopy (GC-MS, Shimadzu QP-5050 equipped with a CBP-10 capillary column) and quantified by a flame-ionization detection gas chromatograph (FID-GC, Shimadzu GC14B equipped with a CBP-10 capillary column) using chlorobenzene as an external standard. 2.3. Measurement of Apparent Quantum Efficiency. The apparent quantum efficiency (QE) for benzylamine photooxidation was measured in a Pyrex batch reactor equipped with a balloon and quartz glass with a flat glass window in the ceiling to allow illumination from a top. Nb2O5 photocatalyst (200 mg) was dispersed in benzene (5 mL) with benzylamine (4 mmol), and the reaction atmosphere was replaced with O2. The mixture was then stirred and irradiated by a monochromatic irradiator (JASCO CRM-FA Spectro 443



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light irradiation (>390 nm). The g value of the radical species was determined using a Mn(II) marker. ESR spectral simulations were performed using the Win-EPR SimFonia software package from Bruker, software version 1.25, 1996.

the visible region (3100 cm−1, the bands

3. RESULTS 3.1. Wavelength Dependence on Apparent Quantum Efficiency. The QEs for benzylamine photooxidation over Nb2O5 were obtained under monochromatic irradiation from 300 to 600 nm. For each wavelength, the benzylamine conversion was lower than 2% and N-benzylidenebenzylamine was the sole product. Figure 1a shows UV−vis spectra of

Figure 1. (A) UV−vis spectra of (1) Nb2O5 and (2) Nb2O5 with adsorbed benzylamine. (B) (3) Difference UV−vis spectrum of (2) − (1) and (4) action spectrum of benzylamine photooxidation over Nb2O5.

Nb2O5 and QE as a function of the wavelength of incident light, the so-called action spectrum. The action spectrum extended to

Figure 2. FT-IR spectra obtained after Nb2O5 was exposed to 0.6 μmol of benzylamine for (a) 1, (b) 10, (c) 20, (d) 30, and (e) 40 min and (f) following evacuation. Difference spectrum between before and after the evacuation (f) − (e). The regions of (A) 3100−3900 cm−1 and (B) 1000− 1800 cm−1 are shown. 444



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at 3442 and 3245 cm−1 corresponding to νO−H and νN−H, respectively, increased in intensity with time (Figure 2A). The corresponding bands for νO−D and νN−D were observed at 2544 and 2400 cm−1, respectively, using benzylamine-N-d2 (Figure 3a−d). The experimental ratios of νO−H/νO−D and νN−H/νN−D

Scheme 1. Proposed Adsorption Fashion of Benzylamine on Nb2O5 and Water: (a) Physisorption of Benzylamine to an Isolated Surface Hydroxyl Group, (b) Chemisorption of Benzylamine To Form an Amide Species and a Bridging Hydroxyl Group, and (c) Dissociative Adsorption of Water

model clusters were performed. Figure 4a shows the model cluster of Nb2O5 (Nb12O43H26; 1) and that of an amide

Figure 3. FT-IR spectra obtained when (a) 0.45, (b) 0.98, (c) 1.6, and (d) 2.8 μmol of benzylamine-N-d2 was introduced or Nb2O5 was exposed to (e) 0.04, (f) 0.17, and (g) 0.28 kPa of D2O.

were both 1.35 and in good agreement with the theoretical ratio of 1.37 expected for H−D isotopic exchange. Here, the theoretical ratio of νO−H/νO−D was estimated by eq 1 derived from Hooke’s law: vO−H = vO−D

k μO−H k′ μO−D

(1) Figure 4. (a) Model cluster of Nb2O5 (Nb12O43H26; 1) and that of amide adsorbed on Nb2O5 (Nb12O42H25(NHCH3); 29. (b) Graphical illustrations of LUKS and HOKS of 1 and 2.

where k and k′ are spring constants being approximately equal to each other and μO−H(D) is reduced mass of the O−H(D) moiety. These findings strongly suggest that the appearance of O−H(D) bonds is related to dissociative adsorption of benzylamine at the N−H(D) moiety. The band at 3734 cm−1 assigned to νO−H of isolated surface hydroxyl groups decreased in intensity immediately after the introduction of benzylamine and recovered after evacuation (Figure 2A, spectra e and f), indicating interaction between isolated surface hydroxyl groups and physisorbed benzylamine. The corresponding band for νO−D was observed at 2700 cm−1 in addition to the band at 2545 cm−1 when Nb2O5 was exposed to D2O (Figure 3, spectra e−g). The experimental ratio of νO−H/νO−D (1.38; 3734 cm−1/ 2545 cm−1) was also consistent with the theoretical ratio of 1.37. On the basis of these results, the adsorption fashion of benzylamine on Nb2O5 can be summarized as shown in Scheme 1. The introduced benzylamine initially interacts weakly with an isolated surface hydroxyl group as a physisorbed species (Scheme 1a). Dissociative adsorption of benzylamine then occurs to form an amide species as a chemisorbed species and a bridging hydroxyl group (Scheme 1b). The chemisorbed species becomes dominant as the amount of adsorption increases. These two hydroxyl groups are also generated by heterolytic dissociative adsorption of water (Scheme 1c). 3.3. Quantum Chemical Calculations for Excitation Mechanism. To investigate the effect of amide formation on the electronic structure of Nb2O5, DFT calculations using

adsorbed on Nb2O5 [Nb12O42H25(NHCH3); 2]. In cluster 2, one of the hydroxyl groups is substituted by a methylamide. As shown in Figure 4b, the highest occupied and lowest unoccupied Kohn−Sham orbitals (HOKS and LUKS, respectively) of 1 consist of delocalized O 2p orbitals and Nb 4d orbitals, respectively. The so-called frontier orbitals of 1 from HOKS−4 to LUKS+4 have similar compositions (see Figures S3 and S4), which is consistent with the band structure of Nb2O5. However, the HOKS of 2 exhibits a quite different electronic structure compared to that of 1 (Figure 4b); the HOKS of 1 is composed of delocalized O 2p orbitals, whereas that of 2 is made up of a N 2p orbital localized on the amide nitrogen atom. Moreover, the HOKS of 2 is higher in energy than that of 1, whereas the LUKS of 1 and 2 are similar (Figure 5). These results clearly show that a donor level whose population is localized on the amide nitrogen is generated by formation of a Nb2O5−amide surface complex. Indeed, the electron excitation energies of 1 and 2 calculated by TD-DFT revealed that a lower energy transition takes place in 2 than in 1 (see Table S3). Thus, photooxidation induced by light of lower energy than the bandgap of Nb2O5 can be explained by excitation of the surface complex, i.e., the direct electronic transition from the N 2p orbital localized on the amide nitrogen 445



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other metal oxides such as TiO2, ZnO, and MgO.40−42 The formation of residual O− is probably caused by oxidative pretreatment at high temperature (O2−lattice → O− + e−). This signal did not change upon adsorption of butylamine, indicating that this paramagnetic species is inert to adsorbed butylamine or is not on the surface of Nb2O5, but in its bulk or near the surface. Considering the highly oxidizing character of surface O− even at low temperature,43,44 the latter situation seems more feasible. Visible irradiation of butylamine-adsorbed Nb2O5 resulted in the appearance of an ESR signal assigned to an organic radical at g = 2.005. A broad feature related to Nb(IV) was also observed at g ∼ 1.92.45,46 However, visible irradiation of neat butylamine at 77 K (without Nb2O5) did not give a signal (data not shown). Because Nb2O5 does not absorb visible light (>390 nm), the formation of this organic radical can be attributed to photoactivation of the surface complex mentioned above. The DFT calculation indicated that photoactivation of the Nb2O5−amide complex induced a direct electronic transition from the N 2p orbital localized on an amide nitrogen to the conduction band of Nb2O5 consisting of Nb 4d orbitals. Therefore, the observed signals from Nb(IV) and an organic radical can be attributed to reduction of Nb(V) by excited electrons and the resulting amide radical, respectively. A simulated ESR spectrum assuming an axially symmetric amide radical with one amino- and two α-protons showed good agreement with the experimental difference spectrum between before and after irradiation (Figure 6B; the parameters for the simulation are listed in Table 1). These

Figure 5. Energy diagram of the frontier Kohn−Sham orbitals of 1 and 2.

to the delocalized Nb 4d orbitals forming the conduction band of Nb2O5 (Scheme 2). Scheme 2. Schematic Illustration of Visible-Light-Induced Direct Electron Transition from a N 2p Orbital Localized on an Amide Nitrogen Atom to the Conduction Band of Nb2O5

Table 1. Parameters Used in the ESR Simulation Assuming an Amide Radical nuclei

3.4. ESR Study of Photogenerated Intermediates. Figure 6A shows (a) an ESR spectrum of Nb2O5, (b) that obtained when butylamine was adsorbed on Nb2O5, and (c) that following exposure to visible light (>390 nm) at 77 K. The Nb2O5 sample after oxidation pretreatment shows a small signal with axial symmetry (g∥ = 2.014, g⊥ = 1.998). This species can be assigned to mononuclear O−, which is widely observed in

14

N H 1 H 1

a

Ia

no. of nuclei

A∥b/mT

A⊥b/mT

1 1/2 1/2

1 1 2

1.33 3.30 1.10

0.20 1.40 0.55

I = nuclear spin quantum number. bA = hyperfine coupling constant.

findings strongly support the generation of an amide radical and the proposed photoactivation mechanism. On the basis of these results, we proposed a possible electron transfer mechanism that occurs during the photooxidation process, as shown in Scheme 3. Initially, photoirradiation of the Nb2O5−amide Scheme 3. Proposed Electron Transfer Mechanism during Photooxidation of Amine over Nb2O5: (a) Electron Excitation from the Amide Nitrogen Atom and Reduction of Nb(V), (b) α-C−H Activation by a Positive Hole on the Nitrogen Atom

surface complex excites an electron from the N 2p orbital localized on the amide nitrogen to the conduction band of Nb2O5. This is followed by reduction of Nb(V) to Nb(IV) by the excited electron and formation of an amide radical (Scheme 3, step a). This process can be triggered by light of lower energy (>390 nm) than the bandgap of Nb2O5. The resulting positive hole on the amide nitrogen then oxidizes an α-C−H bond to

Figure 6. (A) ESR spectra of (a) Nb2O5 after pretreatment and Nb2O5 with butylamine adsorbed (b) in the dark and (c) under illumination with visible light (hν > 390 nm) at 77 K. (B) Simulated spectrum of the proposed amide radical and experimental difference spectrum before and after irradiation; (c) − (b). 446



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Figure 7. FT-IR spectra of (a) benzaldehyde and (b) ammonia adsorbed on Nb2O5 after evacuation and changes in FT-IR spectra during visible (hν > 390 nm) irradiation of benzylamine chemisorbed on Nb2O5 for (c) 1, (d) 10, (e) 30, (f) 60, (g) 120, and (h) 180 min. The regions of (A) 2600− 3600 cm−1 and (B) 1000−1800 cm−1 are shown.

generate a dehydrogenated imine and Nb(IV) (Scheme 3, step b). 3.5. Change in FT-IR Spectra during Photoirradiation. The behavior of adsorbed amide species during photoirradiation was monitored by FT-IR spectroscopy. Figure 7 shows the changes in FT-IR spectra during irradiation of the chemisorbed benzylamine on Nb2O5 with visible light (hν > 390 nm). As irradiation time increased, the bands related to νC−N (1071 and 1129 cm−1), ωCH2 (1363 cm−1), δsCH2 (1456 cm−1), δsNH2 (1583 cm−1), and νN−H (3260 cm−1) that belonged to the amide species decreased in intensity (Figure 7, blue arrows). Correspondingly, several new bands were observed within the spectra. The positions of the new bands within 1200−1800 cm−1 are in good agreement with those of authentic benzaldehyde adsorbed on Nb2O5 (Figure 7, red arrows). The prominent bands at 1648, 1547, and 1410 cm−1 are assigned to ν CO , ν C−C (aromatic), and δ sCHO , respectively.39 However, the band around 1049 cm−1 and the broad feature in the region of 2800−3600 cm−1 were absent in the spectrum of authentic benzaldehyde. Instead, these features are consistent with those of ammonia adsorbed on Nb2O5 (Figure 7, green arrows). The broad feature around 2800−3600 cm−1 is attributed to an ensemble of various modes such as symmetric and asymmetric stretching vibrations (νsN−H and ν asN−H ) or overtone of asymmetric bending of NH 3 (δasNH3).47−49 Other bands characteristic of adsorbed ammonia (δsNH3, 1199 and 1431 cm−1; δasNH3, 1602 cm−1)47−49 appear to overlap with those of benzaldehyde in the spectra during irradiation. These results indicate that the adsorbed amide species was converted into benzaldehyde and ammonia under visible light irradiation. In the cases of secondary amines, corresponding dehydrogenated imines were yielded as main products. For primary amines, no dehydrogenated species were detected in FT-IR spectra at room temperature. This indicates that hydrolysis of the primary imine to an aldehyde occurs immediately. 3.6. Role of Oxygen. The photooxidation of benzylamine over Nb2O5 was performed under a N2 atmosphere to investigate the role of molecular oxygen. As shown in Figure 8, the reaction ceased within a few hours and a small amount of N-benzylidenebenzylamine (36 μmol/100 mg of cat., 6 h) was evolved as a sole product even in the absence of O2. This amount is close to that of chemisorbed ammonia on Nb2O5 (28 μmol/100 mg of cat.) determined in a previous study.50 In addition, the catalyst turned dark blue during the reaction under N 2 . This indicates an accumulation of excited

Figure 8. Change in amount of product during photooxidation of benzylamine over Nb2O5 under 1 atm of N2 (0−6 h) and 1 atm of O2 (6−9 h). Reaction conditions: Nb2O5 (100 mg), benzylamine (5 mmol), benzene (10 mL), hν > 300 nm. After 6 h of reaction under N2, the reaction atmosphere was replaced with excess O2.

electrons51,52 caused by the absence of O2 as an electron acceptor. These results suggest that complete reduction of the adsorption sites in the catalyst surface (Nb5+ + e− → Nb4+) results in deactivation of the catalyst and/or that accumulation of electrons in the conduction band inhibits further photoexcitation. After 6 h of reaction, the reactor was flushed with excess O2 so that the reaction atmosphere was completely replaced with O2. Upon exposure to O2, the catalyst returned to white and continuous evolution of N-benzylidenebenzylamine was observed. This indicates that reoxidation of the reduced Nb4+ sites by O2 is required for catalytic oxidation of benzylamine.

4. DISCUSSION 4.1. Photoactivation Process. On the basis of the obtained results, a plausible reaction mechanism for photooxidation of amines over Nb2O5 is proposed as shown in Scheme 4. First, the substrate amine is dissociatively adsorbed on Nb2O5 to form an amide species (step i). As discussed in Scheme 2, a new donor level is generated within the forbidden band of Nb2O5. Hence, the Nb2O5−amide surface complex absorbs light of lower energy than the bandgap of Nb2O5 (>390 nm), and an electron at the amide nitrogen atom is directly excited to the conduction band of Nb2O5. Thus, in this system, 447



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Scheme 4. Proposed Reaction Mechanism of Photooxidation of Amines over Nb2O5: (i) Dissociative Adsorption of Amine, (ii) Photoactivation of Nb2O5−Amide Surface Complex, (iii) α-C−H Activation by Hole Generated on Amide Nitrogen Atom To Form Dehydrogenated Imine, (iv) Hydrolysis of Imine, (v) Desorption of Imine and H2O, (vi) Desorption of Aldehyde and R′NH2, (vii) Condensation of Aldehyde and Primary Amine, and (viii) Reoxidation of Reduced Nb(IV) Sites by O2

reaction atmosphere of benzylamine photooxidation by thermal conductivity detector−gas chromatography.17 The proposed mechanism is partially based on FT-IR studies which were conducted in a gas phase environment. The presence of benzene solvent in the liquid phase photooxidation condition should also be taken into account. Although chemically inert benzene relative to amine does not seem to significantly contribute to the catalysis, some electronic and/or steric effects of benzene on the substrate and intermediate species could be involved. Moreover, the presence of benzene in the reaction environment may reduce adsorption rate of the substrate and affect overall reaction rate in contrast to the FTIR condition. 4.3. Comparison with Other Photocatalytic Systems. As mentioned above, amine oxidation proceeds by oxidative dehydrogenation of an amine to an imine via direct electron excitation from the adsorbed amine to Nb2O5. The photoactivation process is essentially identical to that of alcohol oxidation over Nb2O5. The major difference in the photooxidation of amines and alcohols is whether dimerization occurs or not. We previously proposed a similar pathway (direct electron excitation from an adsorbed molecule to the conduction band of Nb2O5) in photoactivation of ammonia molecule over TiO2.22−24 In this case, a TiO2−NH2 surface complex (dissociatively adsorbed ammonia on TiO2) can absorb visible light (>390 nm) to generate an amide radical and an excited electron in the conduction band of TiO2. However, both interband excitation and direct electron transfer are involved, in contrast to the present study. Moreover, the DFT study revealed that nondissociative adsorption (physisorption) of ammonia does not generate a donor level containing a N 2p orbital like dissociative adsorption.22 Similar optical properties dependent on the types of adsorption were also reported for a TiO2−catechol system.29 In this system, visible-light-induced

the dissociative adsorption of the substrate results in a catalyst with a visible response. The excited electron is trapped by Nb(V) to generate Nb(IV), and a positive hole remains at the amide nitrogen (step ii). This hole (amide radical) oxidizes an α-C−H bond to generate the corresponding dehydrogenated imine (step iii). 4.2. Catalytic Cycle. In the case of a primary amine (R′ = H), the dehydrogenated primary imine is immediately hydrolyzed into an aldehyde and ammonia by the neighboring water molecule generated by oxidative dehydrogenation or one adsorbed on Nb2O5 (step iv). The produced aldehyde and ammonia are then desorbed (step vi). Condensation of the aldehyde and substrate primary amine immediately takes place to generate secondary imine (vii). In the case of a secondary amine (R′ ≠ H), the dehydrogenated secondary imine and water are desorbed (v). For most secondary amines, benzaldehyde was observed as a byproduct, suggesting that part of the secondary imine was hydrolyzed into an aldehyde and a primary amine (R′NH2). Although this process can be reversible, overoxidation of the fragmented amine seems to result in the aldehyde remaining as a byproduct. Finally, the reduced Nb(IV) sites are reoxidized to Nb(V) by molecular oxygen. Two reaction pathways for dimer formation in amine oxidation are generally proposed: (1) condensation of substrate amine and hydrolyzed aldehyde53−55 and (2) direct coupling of primary imine and substrate amine.55,56 In the case of amine photooxidation over Nb2O5, the FT-IR spectra strongly support the former mechanism. We confirmed that the condensation of benzaldehyde and benzylamine to N-benzylidenebenzylamine process immediately and quantitatively when benzaldehyde is added to benzylamine in benzene in the absence of Nb2O5 in the dark. Moreover, gaseous ammonia was detected in the 448



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ACKNOWLEDGMENTS S.F. thanks the JSPS Research Fellowships for Young Scientists. A part of this work was performed under a management of “Elements Strategy Initiative for Catalysts & Batteries (ESICB)” supported by Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT).

electron transfer does not occur with physically adsorbed catechol but takes place with chemically adsorbed catecholate. Therefore, formation of a chemical bond between the photocatalyst and adsorbate is important in photoinduced direct electron transfer. Recently, Zhao et al. reported that photooxidation of amine proceeded over TiO2 even under visible light irradiation.28 They also proposed the involvement of a surface complex and a dimerization process. However, the proposed mechanism includes some differences from our system: (1) physisorbed amine is proposed as a visible light-harvesting species, (2) the photogenerated hole is located at an α-carbon atom, and (3) the photoactivated species is directly converted to an aldehyde upon attack of molecular oxygen in an Eley−Rideal-type fashion.28,55

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ASSOCIATED CONTENT

S Supporting Information *

Computational details, results of TD-DFT calculations, and complete list of ref 32. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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5. CONCLUSION The photoactivation process and reaction mechanism of amine photooxidation over Nb2O5 were investigated in detail. FT-IR spectra revealed that (1) the substrate amine absorbed predominantly as a dissociatively adsorbed amide species and that (2) primary amines were converted into aldehyde intermediate upon photoirradiation. ESR measurements showed that photoexcitation of the Nb2O5−amide surface complex resulted in the formation of an amide radical and Nb4+. The DFT calculations indicated the following: (1) dissociative adsorption of the substrate amine generates a donor level consisting of N 2p orbital localized on the amide nitrogen atom within the forbidden band of Nb2O5, and (2) excitation from this level to the conduction band of Nb2O5 is a transition of lower energy than the bandgap. In summary, the reaction was found to be oxidative dehydrogenation of an amine to an imine via photoexcitation of the Nb2O5−amide surface complex. For primary amines, the dehydrogenated imine rapidly hydrolyzes to an aldehyde, which reacts with substrate amine to secondary imine. In this system, dissociative adsorption of the substrate amine endows the Nb 2 O 5 photocatalyst with a visible response. We have called this effect “in situ doping”.23,24 It is likely that chemical bond formation between the photocatalyst and substrate is significant to enable this photoinduced direct electron transfer. The insights obtained in this study are applicable to other photocatalytic systems and may inspire further studies to develop visible light-driven green synthesis.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Tel +81-075-3832559, Fax +81-075-383-2561 (T.S.); e-mail tanakat@molemg. kyoto-u.ac.jp, Tel +81-075-383-2558, Fax +81-075-383-2561 (T.T.). Present Address

∥ Tokyo Institute of Technology, 2-12-1-E1-10 Ookayama, Meguro-ku, Tokyo 152-8550, Japan.

Notes

The authors declare no competing financial interest. 449



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Reaction Mechanism of Selective Photooxidation of Amines over

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