Reaction Mechanism of Aromatic Ring Hydroxylation by Water over

Article pubs.acs.org/JPCC

Reaction Mechanism of Aromatic Ring Hydroxylation by Water over Platinum-Loaded Titanium Oxide Photocatalyst Hayato Yuzawa,†,§ Masanori Aoki,† Kazuko Otake,† Tadashi Hattori,† Hideaki Itoh,‡ and Hisao Yoshida*,† †

Department of Applied Chemistry, Graduate School of Engineering and ‡Division of Environmental Research, EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan S Supporting Information *

ABSTRACT: Direct aromatic ring hydroxylation of benzene derivatives by using water as an oxidant over platinum-loaded titanium oxide photocatalyst proceeded with high selectivity under the light of selected wavelength. Two kinds of electrophilic active species, which depended on the reaction condition, were proposed: a surface oxygen radical in neutral or acidic conditions and a hydroxyl radical in basic condition, both of which were produced by a photoformed hole on the surface of titanium oxide. In both cases, these active species can attack the aromatic ring to form an intermediate, followed by the formation of a hydroxylated product. Thus, the reaction proceeds through the addition−elimination mechanism. On the other hand, a photoformed electron reduces a proton to form a hydrogen radical, which reacts with the intermediate to produce a hydroxylated product and a molecular hydrogen or with another hydrogen radical to form a molecular hydrogen.

1. INTRODUCTION Phenol is one of the most important chemical intermediates for producing resin, medicine, and so on. It is industrially synthesized by the famous cumene process,1 which consists of three steps, i.e., aromatic ring alkylation with propylene to produce cumene, oxidation of cumene by oxygen to produce cumene hydroperoxide, and hydrolysis of it to produce both phenol and acetone. Although this process is a well-established synthetic route, it is a multistep process with the production of equimolar acetone as a byproduct. Thus, the development of novel and simple synthetic routes has been desired from the viewpoint of green chemistry. Direct aromatic ring hydroxylation with an oxidant has been one of the most challenging reactions because undesired reactions such as side reactions and successive oxidation can easily occur to reduce the selectivity for the objective phenols. Thus, various kinds of oxidants and catalysts have been investigated to realize the selective direct aromatic ring hydroxylation. For example, both hydrogen peroxide with Fe2(SO4)3,2 TS-1,3 and Ti-MCM-414 catalysts and nitrous oxide with Fe-ZSM-55 and H-[Al]ZSM-56 catalysts attained the selective phenol synthesis (phenol selectivity: 87−100%). However, these oxidants are not appropriate for the practical use because of the cost for the synthesis of them. Although the use of oxygen as an ideal oxidant with Re10-cluster/ HZSM-57 recently allowed the selective phenol synthesis, this reaction required a consumption of ammonia to recover the Re10cluster. Further, these catalytic reactions have not realized the selective phenols synthesis of substituted benzenes yet. Photocatalytic direct hydroxylation of aromatic ring has also been intensively studied8−19 because it has unique potential to © 2012 American Chemical Society

enable various organic syntheses in mild conditions by using photoformed hole (h+) and electron (e−).20−24 Nevertheless, most of studies could not attain the selective phenols synthesis over titanium oxide,8−14 TiO2-pillared clay,15 ionexchanged zeolites,16 and so on, in the presence of water and oxygen. Although a few studies realized the selective phenol synthesis from benzene over polyoxometalate-modified titanium oxide,17 mesoporous titanium oxide,18 and tungsten oxide,19 these photocatalytic reactions have not been applied to the selective phenols synthesis from substituted benzenes yet, except for our system.25 In our previous rapid communication, it is reported that the direct aromatic ring hydroxylation of benzene and substituted benzenes by using water as an oxidant can occur with high selectivity over a platinum-loaded titanium oxide.25 In this photocatalytic reaction system, selection of incident light wavelength, exclusion of oxygen, and optimization of platinum loading amount were revealed as the important factors to realize the selective phenol synthesis. However, the reaction mechanism has not been clarified. The aspects for the reaction mechanism will provide further possibilities for the selective oxidation with water molecule in a mild condition. Thus, in the present study, we investigated the reaction mechanism of the photocatalytic aromatic ring hydroxylation with water in detail. Received: August 25, 2012 Revised: October 16, 2012 Published: October 16, 2012 25376

dx.doi.org/10.1021/jp308453d | J. Phys. Chem. C 2012, 116, 25376−25387

The Journal of Physical Chemistry C

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2. EXPERIMENTAL SECTION 2.1. Reagents. Titanium oxide samples, JRC-TIO-4 (anatase/rutile, 50 m2 g−1), JRC-TIO-6 (rutile, 100 m2 g−1), and JRC-TIO-8 (anatase, 338 m2 g−1) were supplied by Catalysis Society of Japan, and they are referred to as TiO2(A/R), TiO2(R), and TiO2(A), respectively. Precursor of platinum was chloroplatinic acid hexahydrate (H2PtCl6·6H2O, Kishida, 99.9%). Other reagents were nitrobenzene (Kishida, 99.5%), benzonitrile (Kishida, 99%), chlorobenzene (Kishida, 99.5%), fluorobenzene (Kishida, 99.5%), benzene (Kishida, 99.5%), benzene-d6 (Cambridge Isotope Laboratories, 99.5%), toluene (Kishida, 99.5%), ethylbenzene (Kishida, 99%), para-xylene (Kishida, 99%), tertbutylbenzene (Acros Organics, 99%), anisole (Kishida, 99%), aniline (Kishida, 99.5%), phenol (Wako, 99%), methanol (Wako, 99.5%), D2O (Acros Organics, 99.95%), H218O (Euriso-top, 95.4%), NaOH−H2O solution (Wako, conc. 30% w/v), NaOD− D2O solution (Acros Organics, conc. 30% w/v), H2SO4−H2O solution (Kishida, conc. 96%), D2SO4−D2O solution (Merck, conc. 96%), Fe(II)SO4·7H2O (Wako, 99.5%), and hydrogen peroxide (Wako, H2O2 content, 30%). These reagents were used without further purification. 2.2. Preparation and Characterization of Photocatalysts. Platinum-loaded titanium oxide samples were prepared by a photodeposition method. The titanium oxide powder (4 g) was dispersed into an aqueous methanol solution (400 mL, methanol concentration 25%) containing the platinum precursor (chloroplatinic acid hexahydrate) in a beaker (500 mL) with vigorous stirring, followed by irradiation from the upper side with a 300 W Xe lamp, which emitted both UV and visible light, for 3 h in air atmosphere. Then, the suspension was filtered off with suction, washed with water, and dried at 323 K overnight. In this article, titanium oxide sample loaded with x wt % platinum is described as Pt(x)/TiO2. Measurements of ICP-AES with an Optima 3300 DV (Perkin-Elmer Japan) confirmed that the objective amount of platinum was correctly loaded on the titanium oxide. The UV−vis absorption spectra of substrates were recorded at room temperature on a JASCO V-570 spectrophotometer. The diffuse reflectance UV−vis spectra of the bare TiO2(A) sample and the TiO2(A) samples adsorbing alkylbenzenes were recorded on the same instrument equipped with an integrating sphere covered with BaSO4. The TiO2(A) samples adsorbing alkylbenzenes were prepared as reported in the literature.26 2.3. Photocatalytic Reaction Tests. Photocatalytic aromatic ring hydroxylation of benzene and substituted benzenes was carried out in a closed reactor for all of the substrates or a fixed-bed flow reactor for benzene. In the closed reactor, the photocatalyst sample in a quartz cell was irradiated by the 300 W xenon lamp from beneath (18 cm2) for 1 h in air atmosphere as a pretreatment, and the gas phase was purged by argon. Then, benzene (0.05 or 1 mL, corresponding to 0.57 or 11.3 mmol, respectively) or substituted benzenes (for phenol 0.94 g, corresponding to 10 mmol; for others 1 mL, corresponding from 6.4 to 11.0 mmol) and distilled water (1.0 or 4.0 mL, corresponding to 55.6 or 222.4 mmol, respectively) were introduced and irradiated from the 300 W xenon lamp for 3 h at 308 K in argon atmosphere with magnetically stirred. When the irradiated light wavelength was limited, a cutoff filter or one of the three band-pass filters (described later) was used. After the irradiation, the gas phase products were collected by a gastight syringe and analyzed by a GC-TCD (Shimadzu, GC-8A). Then, methanol (5 mL) was added to the liquid phase, and it was filtered. The filtrate was analyzed by an HPLC (Shimadzu, LC-20AT), a GC-FID

(Shimadzu, GC-14B), and a GC-MS (Shimadzu, QP-5050A). In the case of the recycle use test, the photocatalyst sample was separated by decantation and washed with 4 mL of benzene, which was carried out three times, and dried at 333 K overnight before use for the next photocatalytic reaction as the same manner. Isotopic experiments for the reaction were carried out by using benzene-h6 or benzene-d6 and H2O, D2O, or H218O. After the photocatalyst sample was irradiated from the 300 W xenon lamp for 1 h in air atmosphere in the quartz cell and evacuated at 473 K for 2 h, Ar (750 Torr), benzene-h6, or benzene-d6 (1.0 mL) and H2O, D2O, or H218O (1.0 mL) were introduced to the reactor. Then, the same procedure as described above was carried out. Irradiation light wavelength for the photocatalytic reaction was limited to 365 ± 20 nm through the bandpass filter. When the H−D distribution of the product was investigated, argon (750 Torr), benzene (100 μmol), and D2O (3 mL) were introduced to the reactor and irradiated for 3 h from the 300 W xenon lamp through the band-pass filter, which permits the light of 405 ± 20 nm in wavelength. After the irradiation, methanol (7 mL) was added to the liquid phase, and the mixed solution was analyzed with the GC-MS (Shimadzu, QP-5050A). In this analytical procedure, it was confirmed that the D atom in the hydroxyl group of the produced phenol was almost exchanged by H atom in the introduced methanol. In the flow reactor, the photocatalyst sample (0.2 g) mixed with quartz sand (0.9 g) in another quartz cell (60 × 20 × 1 mm3) connected to a Y-shaped glass tube was irradiated from the 300 W xenon lamp for 1 h in air atmosphere, and then, the gas phase in the reactor was purged by Ar for 1 h. Benzene (44 mL/h) and distilled water (44 mL/h) were mixed through the Y-shaped glass tube and successively flowed for 10 min. Then, the photocatalyst was irradiated from the 300 W xenon lamp in the flow of the mixture. The effluent was periodically collected and mixed with acetone. The obtained solution was analyzed with the GC-MS (Shimadzu, QP-5050A) and the GC-FID (Shimadzu, GC-14B). 2.4. Fenton Reaction. Aromatic ring hydroxylation through the Fenton reaction (Fe2+ + H2O2 → Fe3+ + ·OH + OH−) was carried out in the closed reactor. After benzene derivatives (0.5 or 2 mL) and 50 mM FeSO4/H2SO4 solution (pH = 2.4, 2 mL) were introduced to the quartz cell and cooled down to 280 K, the gas phase in the reactor was purged by argon for 120 min. Hydrogen peroxide (50 mM, 2 mL) was then introduced into the reactor by using a syringe. After the mixture was magnetically stirred at 280 K for 30 min, the gas and liquid phases were analyzed by the same procedure, as described above.

3. RESULTS AND DISCUSSION 3.1. Outline of the Photocatalytic Reaction. Table 1 shows the results of the photocatalytic reaction tests. When the Pt(0.1)/TiO2(A) sample was used without the wavelength limitation of the irradiated light, phenol (13.5 μmol), small amount of biphenyl (0.4 μmol), cyclohexanol, cyclohexanone (0.2 μmol), and carbon dioxide (4.8 μmol) were yielded (Table 1, entry 1). Although the calculated phenol selectivity based on the aromatic products was high (88%), an excess amount of hydrogen (95 μmol) was produced. The remarkable color change of the Pt(0.1)/TiO2(A) sample from light gray to dark brown suggests that the hydrogen would be produced together with the formation of undetected products, such as surface phenoxy species and polyphenyls, which might be strongly adsorbed on the titanium oxide surface. Except for the excess amount of 25377



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Table 1. Photocatalytic Aromatic Ring Hydroxylation of Benzene over the Various Pt(0.1)/TiO2 Samplesa products (μmol) entry

photocatalyst

irradiation light wavelength (nm)

1 2 3h 4 5

Pt(0.1)/TiO2(A) Pt(0.1)/TiO2(A) Pt(0.1)/TiO2(A) Pt(0.1)/TiO2(A/R) Pt(0.1)/TiO2(R)

alle >385f >385f >385f >385f

phenol biphenyl 13.5 12.0 5.2 4.4 0.1

0.4 n.d.g n.d.g n.d.g n.d.g

cyclohexanol + cyclohexanone

H2

CO2

phenol yieldb (%)

phenol selectivityc (%)

H2 obs./ H2 exp.d

0.2 1.2 n.d.g 0.4 n.d.g

95 14 n.d.g 4.1 n.d.g

4.8 n.d.g 26 n.d.g n.d.g

2.4 2.1 0.9 0.8 0.02

88 91 54 92

3.8 1.3 1.3

a

Reactor, a closed reactor; catalyst, 0.2 g; benzene, 0.05 mL (0.57 mmol); H2O, 4 mL; reaction time, 90 min. Detection limits of hydrogen and carbon dioxide were 0.1 and 1.0 μmol, respectively. bPhenol yield (%) = 100 × Yphenol/(introduced benzene), where Y stands for the yielded amount of product. cPhenol selectivity (%) = 100 × Yphenol/(Yphenol + 2 × Ybiphenyl + Ycyclohexanol+cyclohexanone + YCO2/6). dH2 observed/H2 expected = YH2/ (Yphenol + Ybiphenyl + 5/2 × YCO2 − 2 × Ycyclohexanol − Ycyclohexanone). eIrradiated light wavelength was not limited. Irradiated light intensity was 8.5, 51, and 165 mW/cm2 when measured at 254 ± 10, 365 ± 15, and 405 ± 30 nm, respectively. fOptical cutoff filter permitting the light longer than 385 nm was used. Irradiated light intensity was 1.7, 0.27, and 127 mW/cm2 when measured at 254 ± 10, 365 ± 15, and 405 ± 30 nm, respectively. g n.d. = not detected. hThe reaction was carried out in air atmosphere.

area of the TiO2(A) sample than that of the TiO2(A/R) sample. This suggests that the number of surface active sites for the TiO2(A) surface was larger than the TiO2(A/R) surface, as reported in literature.29,30 However, with the Pt(0.1)/TiO2(R) sample, phenol was hardly obtained (0.1 μmol, Table 1, entry 5), although the specific surface area of the TiO2(R) sample was larger than that of the TiO2(A/R) sample. As reported in the literature, the potential of the photoformed hole in the rutile (3.04 V vs NHE)31 is lower than that in the anatase (3.12 V vs NHE).32 Considering the band edge potential for each polymorph, the rutile would less promote the oxidation of water (H2O + h+ → ·OH + H+, 2.8 V vs NHE)33 than the anatase. Thus, anatase titanium oxide with large surface area would be effective for the production of phenol. Figure 1a shows the relationship between the platinum loading amount and the products yield for the photocatalytic aromatic ring hydroxylation of benzene. The Pt(0.1)/TiO2(A) sample exhibited the highest phenol yield. With the Pt(0.05)/ TiO2(A) sample, the largest amount of hydrogen was yielded although the other product yield was smaller than those with the Pt(0.1)/TiO2(A) sample. This result indicates that the undetectable products were more easily formed on the photocatalyst with the lower amount of platinum loading. As shown in Figure 1b, the hydrogen yield is almost consistent with the produced organic compounds on the Pt(0.1)/TiO2(A) and Pt(0.5)/TiO2(A) samples. However, with the Pt(1.0)/TiO2(A) sample, no hydrogen was yielded although the organic compounds were produced, where some undetectable hydrogenation products might be formed and adsorbed on the titanium oxide surface. Thus, the platinum loading amount was also important for the selective aromatic ring hydroxylation, and the optimum amount for the phenol production was 0.1 wt %, at least on the present TiO2(A) sample. Also for the selective aromatic ring hydroxylation of alkylbenzenes, the wavelength limitation of irradiated light and the platinum loading amount were crucial. Table 2 shows the result of toluene hydroxylation over the Pt(0.1)/TiO2(A) sample under various wavelengths of irradiation light, where the light intensity was controlled to be 3 mW/cm2 except for entry 1. When the reaction was carried out without the limitation of irradiation light wavelength, cresols (11 μmol) were obtained with some byproducts, such as the coupling products (mainly dibenzyl, 7.8 μmol), benzaldehyde (0.3 μmol) and carbon dioxide (11 μmol), and an excess amount of hydrogen (236 μmol, Table 2, entry 1). In this condition, benzyl radical would be easily

hydrogen, it is deduced that the obtained products would be formed according to the following equations, i.e., hydroxylation of benzene with water (eq 1),25 coupling of benzene (eq 2),25 reduction of the produced phenol and successive oxidation of the produced cyclohexanol (eq 3),27 and decomposition of benzene with water (eq 4).

When the wavelength of the irradiation light was limited to longer than 385 nm, higher selectivity for the phenol (91%) was obtained without the production of biphenyl, carbon dioxide, and excess amount of hydrogen (Table 1, entry 2). As a result, the amount of the detected hydrogen became close to the value expected from the amount of the detected products. Thus, the limitation of the irradiation light wavelength would be one of the critical factors for the selective aromatic ring hydroxylation of benzene. In this system, no reactions occurred in the dark with the Pt(0.1)/TiO2(A) sample or under irradiation without the photocatalyst. These facts confirmed that the aromatic ring hydroxylation of benzene proceeded photocatalytically as shown in eq 1. In contrast, when the reaction was carried out in the presence of air with the limitation of wavelength (Table 1, entry 3), less amount of phenol and large amount of carbon dioxide (26 μmol) were obtained although the conversion of benzene, which was calculated from the yield of the products, was as low as the result of entry 2 (2.3% for entry 2 and 1.7% for entry 3). This result clearly indicates that the oxygen dominantly promotes the decomposition of benzene (eq 5), as reported in literature.28

Thus, the elimination of oxygen from the reactor was required for the high selectivity in the aromatic ring hydroxylation in the present photocatalytic reaction system. The phenol yield over the Pt(0.1)/TiO2(A) sample was higher than that over the Pt(0.1)/TiO2(A/R) sample (Table 1, entry 4), which was consistent with the higher specific surface 25378



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moted the formation of benzyl radical. Figure 2c shows DR UV−vis spectrum of toluene adsorbed on the TiO2(A) sample. In addition to the large absorption bands assigned to the molecular toluene (99%) regardless of the irradiation light wavelength (Supplementary Table S1, entries 7−9). Although tert-butylbenzene adsorbed on the TiO2(A) also exhibited the similar band at around 360 nm (Supplementary Figure S1d), the corresponding dehydrogenated radical can not be stabilized by resonance, as generally known. The tert-butyl group has thus lower reactivity than the other alkyl groups, which is consistent with the present result. The effect of platinum loading amount on the hydroxylation of alkylbenzenes over the various Pt(x)/TiO2(A) samples was also investigated (Supplementary Figure S2). In the case of toluene, the yield of coupling products decreased with increasing platinum loading amount (Supplementary

Figure 1. Relationship between platinum loading amount and the yield of the products (a), and phenol selectivity and the ratio of observed hydrogen to expected hydrogen (b) over the various Pt(x)/TiO2(A) samples. Red circles, black triangles, green squares, and blue diamonds in panel a correspond to the yield of phenol, biphenyl, cyclohexanol + cyclohexanone, and hydrogen, respectively. Orange inverse triangles and purple crosses in panel b correspond to phenol selectivity and ratio of H2 observed to H2 expected, respectively. As for the reaction condition, see footnotes a and f of Table 1.

formed from toluene to yield the coupling product (dibenzyl). When the light was limited to 254 ± 20 nm in wavelength (Table 2, entry 2), although the selectivity of cresols was improved (88%), the strong absorption by the toluene (Figure 2a) prevented the incident light from reaching the photocatalyst and reduced the total yield including cresols. In contrast, when limited to 365 ± 20 nm (Table 2, entry 3), dibenzyl was produced, and the selectivity of cresols was not improved (50%), meaning that the light of 365 ± 20 nm in wavelength pro-

Table 2. Results of Photocatalytic Aromatic Ring Hydroxylation of Toluene over the Pt(0.1)/TiO2(A) Samplea products (μmol) entry

irradiation light wavelength

1f 2 3 4

allg 254 ± 20h 365 ± 20h 405 ± 20h

cresols

coupling productsb

benzaldehyde

H2

CO2

cresols yieldc (10−2%)

cresols selectivityd (%)

H2 obs./ H2 exp.e

11 1.5 3.5 5.7

7.8 0.1 1.7 0.4

0.3 n.d.i 0.1 0.1

236 3.0 31 31

11 n.d.i n.d.i n.d.i

12 2 4 6

38 88 50 86

5.0 1.9 5.7 4.9

a

Reactor, the closed reactor; catalyst, 0.2 g; toluene, 1.0 mL (9.4 mmol); H2O, 1.0 mL; reaction time, 180 min. Detection limits of hydrogen and carbon dioxide were 0.1 and 1.0 μmol, respectively. bDibenzyl, etc. cCresols yield (%) = 100 × Ycresols/(introduced toluene), where Y stands for the yielded amount of product. dCresols selectivity (%) = 100 × Ycresols/(Ycresols + 2 × Ycoupling products + Ybenzaldehyde + YCO2/7). eH2 observed/H2 expected = YH2/(Ycresols + Ycoupling proudcts + 18/7 × YCO2). fAlso detected were benzyl alcohol (0.1 μmol) and methylcyclohexanol (0.5 μmol). gIrradiation light wavelength was not limited. Irradiation light intensity was: 10, 50, and 176 mW/cm2 when measured at 254 ± 10, 365 ± 15, and 405 ± 30 nm, respectively. hBand-pass filter was used. Intensity of the irradiation light, which could be absorbed by titanium oxide, was estimated to be 3 mW/cm2. i n.d. = not detected. 25379



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Figure 3. Phenol yield in the closed reactor (red circles), that with the recycle use of the photocatalyst (red triangles), and that in the flow reactor (orange squares) (a), and time course of phenol yield in the flow reactor by periodic irradiation (b). In the closed system, benzene, 1 mL (11.3 mmol); H2O, 1 mL. As for the other reaction conditions, see footnotes a and f of Table 1. The recycle use of the Pt(0.1)/ TiO2(A) sample was carried out for every reaction for 1.5 h. In the flow system, catalyst, 0.2 g; benzene, 44 mL/h; H2O, 44 mL/h; irradiated light wavelength, >385 nm; irradiated light intensity, 3.2, 0.32, and 187 mW/cm2 when measured at 254 ± 10, 365 ± 15, and 405 ± 30 nm in wavelength, respectively. Figure 2. UV−vis absorption spectrum of toluene diluted by methanol (a); diffuse reflectance UV−vis spectra of the TiO2(A) sample (b) and toluene that was adsorbed on the TiO2(A) sample and diluted by MgO powder (c). The spectrum in panel c was subtracted by the spectrum of the TiO2(A) sample similarly diluted by MgO. Dashed lines in each figure show the transmission spectra of the optical bandpass filters, which permits 254 ± 20, 365 ± 20, and 405 ± 20 nm in wavelength, respectively.

period, the rate became almost constant as 2.2 μmol/h after 1.5 h (Figure 3a, circles). In the case of recycle use of the Pt(0.1)/ TiO2(A) sample for 1.5 h each (Figure 3a, triangles), the integral yield of phenol was larger than that for the continuous reaction, and the reaction rate achieved to be 5.7 μmol/h. These results suggest that the adsorption of the products on the photocatalyst surface would reduce the reaction rate. When a flow reactor was used, the production rate of phenol was almost constant to be 10.4 μmol/h after 1.5 h, and the total yield of phenol was much higher than that in the closed reactor (Figure 3a, squares). The periodic irradiation in the flow reactor provided a high phenol yield proportionally increasing with the irradiation time at the rate of 22.7 μmol/h (Figure 3b). This result indicates that the products covering the titanium oxide surface would be washed out with the flowing benzene and water in the dark. These results clearly elucidated that the elimination of the adsorbed products from the titanium oxide surface was effective for the continuous progress of the photocatalytic aromatic ring hydroxylation. 3.2. Isotopic Experiments. Table 3 shows the results of isotopic experiments in various conditions, where the reaction

Figure S2a). In addition, the ratio of the produced hydrogen yield to the expected hydrogen yield was close to unity on the Pt(1.0)/TiO2(A) sample. As a result, cresols was selectively obtained with the Pt(1.0)/TiO2(A) sample (97%). The other alkylbenzenes exhibited almost the same tendency (Supplementary Figure S2b−d), and selective phenols synthesis was attained on the Pt(1.0)/TiO2(A) sample (>87%). Since it is known that phenols are easily adsorbed on the titanium oxide as a phenoxy species,38,39 the elimination of the phenols from the surface would be required for the continuous progress of the reaction. Figure 3a shows the time courses of the phenol yield from benzene. In the closed reactor, although the phenol production rate decreased in the initial 25380



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Table 3. Isotopic Effect of Photocatalytic Aromatic Ring Hydroxylation of Benzene over the Pt(0.1)/TiO2(A) Samplea substrates

products (μmol)

entry

benzene

water

phenol

biphenyl

cyclohexanol + cyclohexanone

H2b

1 2 3 4 5 6 7 8 9 10

benzene-h6 benzene-h6 benzene-d6 benzene-h6 benzene-h6 benzene-d6 benzene-h6 benzene-h6 benzene-d6 benzene-h6

H218O H2O H2O D2O H2O−H2SO4c H2O−H2SO4c D2O−D2SO4c H2O−NaOHd H2O−NaOHd D2O−NaODd

Ph−18OH (90%) Ph−16OH (10%) 10.6 11.2 10.0 8.2 7.9 7.6 10.4 11.2 2.7

0.2 0.1 0.2 0.3 0.2 0.3 0.2 0.1 0.2

0.7 0.7 0.6 0.5 0.5 0.4 0.7 0.6 0.2

18 17 7.1 7.2 7.0 2.4 29 31 13

a

Reactor, the closed reactor equipped with vacuum line; catalyst, 0.2 g; benzene, 1.0 mL (11.3 mmol); H2O, 1.0 mL; reaction time, 45 min; irradiated light wavelength, 365 ± 20 nm; irradiated light intensity, 30 mW/cm2 when measured at 365 ± 15 nm. Detection limits of hydrogen and carbon dioxide were 0.1 and 1.0 μmol, respectively. In this reaction condition, carbon dioxide was not detected. Before the photocatalytic reaction, the Pt(0.1)/TiO2 sample was evacuated at 473 K for 2 h to remove physisorbed water. bThe yield of hydrogen is shown as the total amount of H2, HD, and D2. Distribution of hydrogen molecule was not quantified. cConcentration of H2SO4 or D2SO4 was 0.18 M. Solution pH of H2O−H2SO4 was 0.8. dConcentration of NaOH or NaOD was 0.3 M. Solution pH of H2O−NaOH was 13.4.

time was shortened to 45 min to reflect the initial rate on the yields. When the benzene-h6 was used with H218O, 90% of the produced phenol contained the 18O and 10% of that contained 16 O (Table 3, entry 1). This result clearly shows that the introduced water dominantly reacts with benzene, while other components such as surface hydroxyl group and surface lattice oxygen on the titanium oxide were also consumed to form the product. When benzene-d6 was used with H2O, the isotopic effect was not observed for the yield of phenol and hydrogen (Table 3, entries 2 and 3). This result confirmed that the C−H bond cleavage in benzene was not the rate-determining step for the aromatic ring hydroxylation. When benzene-h6 was used with D2O, the isotopic effect was observed for the yield of hydrogen (kH/kD = 2.5), as reported in hydrogen production by water electrolysis on various metal electrodes such as platinum (Table 3, entries 2 and 4).40 This result indicates that the hydrogen production is the rate-determining step for the phenol production. In this reaction condition, the yield of hydrogen was smaller than that of the phenol. This would be because the formation rate of phenol was independent of the hydrogen production in the initial stage of the reaction in the present condition. Detail will be discussed later. When the isotopic experiments were carried out in an acidic condition (pH = 0.8) with sulfuric acid, the same feature as the case in the neutral water was observed (Table 3, entries 5−7). In contrast, in the case of a basic condition with NaOD−D2O solution (pH = 13.4), the isotopic effect was observed for both phenol and hydrogen yield, i.e., kH/kD = 3.9 for phenol and kH/kD = 2.2 for hydrogen, respectively (Table 3, entries 8 and 10). This result indicates that the cleavage of O−H bond in the activation of the water molecule is the rate-determining step for the production of both phenol and hydrogen in the basic condition. Thus, reaction mechanism in basic condition would be different from that in the neutral and acidic conditions. In the literature,41 it was proposed that the photoformed hole would be trapped by deprotonated terminal oxygen in basic conditions (pH ≥ 13), as shown in eq 6.

Considering this mechanism, the obtained isotopic effect for the present study in the basic condition (pH = 13.4) would originate from the reaction between the surface oxygen radical and water molecule through the O−H bond cleavage to produce a hydroxy radical, as shown in eq 7.

In addition to eq 7, one may consider that the oxidation of the hydroxide ion (OH−) in water by the trapped hole (OH− + Ti−O· → ·OH + Ti−O−) is also a possible reaction route. However, this route can not explain the isotopic effect because the difference of the property between H and D hardly influences the rate of electron transfer from O atom in the OH− to the trapped hole. Further, the OH− would not be easily accessible to the surface due to the electrostatic repulsion between them. Thus, not OH− but H2O would be oxidized by the trapped hole in this reaction condition. In neutral or acidic conditions, another mechanism was proposed, i.e., the bridging Ti−O−Ti site trapping the photoformed hole would receive the nucleophilic attack of water to produce surface oxygen radical (eq 8).41

Considering the reaction mechanism for the production of the active surface oxygen species, the bond formation between the O atom of water and the Ti cation of the titanium oxide would induce the O−H bond cleavage as deprotonation in water, which should occur more easily (faster) than the water activation in basic conditions mentioned above. Thus, these mechanisms could explain the two different rate determining steps for the aromatic ring hydroxylation, i.e., the cleavage of the O−H bond of the water molecule was the rate-determining step in basic conditions, while it was not the rate-determining step in neutral or acidic conditions. 3.3. Reaction of the Active Oxygen Species with Benzene. Considering the reaction between benzene and the active oxygen species in the neutral condition, two reaction 25381



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Table 4. Results of Photocatalytic Aromatic Ring Hydroxylation of Benzene and Substituted Benzenes over the Pt(0.1)/TiO2(A) Samplea

Reactor, the closed reactor; catalyst, 0.2 g; substrate, 1.0 mL (9.2−11.3 mmol); H2O, 1.0 mL; irradiated light wavelength, 405 ± 20 nm; irradiated light intensity, 85 mW/cm2 when measured at 405 ± 30 nm; reaction time, 180 min. Detection limits of hydrogen and carbon dioxide were 0.1 and 1.0 μmol, respectively. In this reaction condition, carbon dioxide was not detected. bPhenols yield (%) = 100 × Yphenols/(introduced substrate), where Y stands for the yielded amount of product. cPhenols selectivity (%) = 100 × Yphenols/(Yphenols + Yphenol + 2 × Ycoupling products + Yreduced products). dEWG = electronwithdrawing group. en.d. = not detected. fAniline. gBiphenyl; trace = a trace amount. hCyclohexanol and cyclohexanone. iAlso detected was benzaldehyde (0.2 μmol). jDibenzyl. k1,2-Diphenoxyethane. lAzobenzene. mEDG = electron-donating group. nHydroxydiphenylether. a

paths are possible; one is the coupling reaction between the active oxygen and phenyl radical produced through abstraction of H atom from benzene by the photoformed hole, and the other is the addition−elimination mechanism between the active oxygen species and benzene. In order to discuss these possibilities, the photocatalytic hydroxylation of various monosubstituted benzenes over the Pt(0.1)/TiO2(A) sample was carried out (Table 4 and Figure 4). The investigated substrates except for the strongly electron-donating group, such as phenol, aniline, and anisole, were selectively hydroxylated to produce the corresponding phenols (70 to >99%; Table 4, entries 1−6). For the phenols yield among the substrates except for phenol, aniline, and anisole, the substrate having the stronger electron-donating substituent exhibited a higher yield of phenols (Figure 4a). As for the isomer distribution of the corresponding phenols, the substrates having an electron-withdrawing group exhibited metaorientation, while those having an electron-donating group exhibited ortho−para-orientation (Figure 4b). Both the order of the phenols yield and the isomer distribution were similar to those in the usual aromatic electrophilic substitution reaction, which is known to proceed in the addition−elimination mechanism as shown in eq 9, confirming that the active oxygen species has electrophilic property.

If the reaction path through the formation of the phenyl radical is the dominant reaction mechanism, the position where the C−H bond dissociation energy is the lowest will be preferentially activated to become a radical center and hydroxylated through the coupling with the surface oxygen radical.

Figure 4. Yield and selectivity for the hydroxylation products (a) and isomer distribution of the produced phenols (b) for the photocatalytic aromatic ring hydroxylation of benzene derivatives over the Pt(0.1)/TiO2 sample. As for the reaction condition, see footnote a of Table 4. 25382



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In the case of chlorobenzene, the C−H bond strength in the metaposition is estimated as the lowest,42 and thus the C−H bond at the meta-position could be preferentially hydroxylated. Since this expectation is inconsistent with the isomer distribution of the chlorophenols in Figure 4b, the aromatic ring hydroxylation would dominantly proceed without the formation of phenyl radical. In the case of phenol and aniline, both of which have each strongly electron-donating substituent, the corresponding phenols were not obtained though the coupling products were obtained (Table 4, entries 8 and 9). In our previous study for benzene derivatives adsorbed on titanium oxide surface by using solid state NMR,26 it was proposed that the phenol was adsorbed through the oxygen atom in the hydroxyl group and that the aromatic ring was relatively separate from the surface. Thus, the surface active oxygen species on the Pt(0.1)/TiO2 sample could attack only the substituent, which was closer to the surface compared to the aromatic ring. This is the reason why no hydroxylated products were obtained from phenol, which would hold true for aniline also because the amino group in aniline has an electrophilic property as strong as the hydroxyl group in phenol. In the case of anisole, the phenols selectivity was low (49%) because of the side reaction at the substituent producing phenol and 1,2-diphenoxyethane (Table 4, entry 7). The previous NMR study26 proposed that anisole was strongly interacted with the titanium oxide surface through the oxygen atom in the methoxy group and that the orientation of the molecule would be almost parallel to the surface. Further, the substituent of anisole has high electron density. Thus, the lower phenols selectivity would be because the electrophilic active oxygen species would attack the substituent as well as the aromatic ring. As known, in the Fenton reaction in homogeneous system, hydroxyl radicals (·OH) are produced as the active radical species. Figure 5 shows the results of the aromatic ring hydro-

similar to the hydroxyl radical in the homogeneous system. However, in these substrates, the photocatalytic reaction system exhibited slightly higher selectivity to the meta-isomer compared with the Fenton system, indicating that the nature of the active oxygen species in the present heterogeneous reaction would be different from the free radical. Especially in the case of nitrobenzene, although the Fenton system provided three isomers almost evenly, the photocatalytic reaction selectively produced the meta-isomer only. This might be because the electron in the benzene derivatives adsorbed on the titanium oxide surface was furthermore withdrawn by the titanium oxide, as reported in our previous study,26 and the reactivity in the ortho−para-position of nitrobenzene decreased. Figure 6 shows the isomer distribution of phenols in the photocatalytic reaction in neutral and basic conditions by using

Figure 6. Isomer distribution of phenols in the photocatalytic aromatic ring hydroxylation over the Pt(0.1)/TiO2(A) sample by using water (a) and aqueous sodium hydroxide solution (b). Irradiated light wavelength, 365 ± 20 nm; irradiated light intensity, 30 mW/cm2 when measured at 365 ± 15 nm. In the case of basic condition, concentration of sodium hydroxide was 0.3 M. Solution pH of aqueous sodium hydroxide was 13.4. As for the other reaction conditions, see the footnote a of Table 4. Arrows and values in the structural formula of the substrates show the direction and magnitude of the dipole moment calculated by PM6 method43 in MOPAC 2009.44

water and an aqueous solution of sodium hydroxide, respectively. The dipole moments of the substrates were calculated by PM6 method43 in MOPAC 2009.44 In the case of toluene, whose substituent positively polarizes (0.682 D), the ratio of the orthoisomer was higher in the basic condition compared with the neutral condition. Since titanium oxide surface is negatively charged in basic conditions, the attractive electrostatic interaction between the substituent and the titanium oxide surface would make the ortho-position be close to the titanium oxide surface, where the active oxygen species could attack. In the case of fluorobenzene, whose substituent negatively polarizes (1.632 D), the ratio of para-isomer increased in the basic condition. This would be because the repulsive electrostatic interaction between the substituent and the negatively charged surface would preferentially lead the para-position to the surface. Although the obtained isomer distribution of phenols in the basic condition was influenced by the interaction between the substrates and the negatively charged surface, the feature of the isomer distribution of the phenols was still similar to the aromatic ring electrophilic substitution.

Figure 5. Isomer distribution of phenols in the photocatalytic aromatic ring hydroxylation over the Pt(0.1)/TiO2(A) sample (a) and in aromatic ring hydroxylation by using Fenton reagent (b). As for the condition of the photocatalytic reaction, see the footnote a of Table 4. As for the aromatic ring hydroxylation by using Fenton reagent, substrate, 2.0 mL for nitrobenzene or 0.5 mL for the others; 50 mM FeSO4/H2SO4 solution (pH = 2.4), 2 mL; hydrogen peroxide (50 mM), 2 mL; reaction time, 30 min.

xylation by the heterogeneous photocatalysis and the Fenton reaction. In the Fenton system, ortho−para-orientation was obviously observed for all the substrates except for nitrobenzene, and it became clear with increasing the electron-donating property of the substituent. This result indicates that the active oxygen species in the present photocatalytic reaction would be 25383



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This fact indicates that the reaction in the basic condition also proceeds through the addition−elimination mechanism, and the surface active oxygen species in this reaction would be electrophilic. In the case of nitrobenzene, whose substituent negatively polarizes more strongly (5.284 D) than that of fluorobenzene, the corresponding phenols were not observed in the basic condition. Although the repulsive electrostatic interaction between the substituent and the surface might spatially increase the chance for the hydroxylation at the para-position of nitrobenzene, no detection of para-nitropehnol would be because the electrophilic active species has difficulty in the hydroxylation of the carbon having low electron density, as shown in Figure 4. However, no detection of meta-nitropehnol would be because the active oxygen species hardly attacks the meta-position of nitrobenzene in the basic condition farther from the surface active sites compared to that in neutral condition. As for the low reactivity of the ortho-position, both of these two factors would be included. This confirms the mechanism mentioned above. As reported for the aromatic ring hydroxylation by using the Fenton reaction,45 the reaction of the intermediate can not proceed without the hydrogen abstraction agent, i.e., an oxidant. In the present reaction condition, two oxidants would be proposed; one is the surface active oxygen species as shown in eq 10, and the other is the hydrogen radical produced on the platinum site as shown in eq 11.

Figure 7. Distribution of the H−D exchanged phenol (a) and benzene (b) in the photocatalytic aromatic ring hydroxylation of benzene-h6 by using D2O over the various Pt(x)/TiO2(A) samples (x = 0.01, 0.05, 0.1, 0.5, and 1.0), where deuterium exchanged at the hydroxyl group of phenol was not counted as exchanged number. Catalyst, 0.2 g; benzene-h6, 100 μmol; D2O, 3 mL; irradiation light wavelength, 405 ± 20 nm. Intensity of the irradiation light, which could be absorbed by titanium oxide, was 3 mW/cm2. Before the photocatalytic reaction, the Pt(0.1)/TiO2 sample was evacuated at 473 K for 2 h to remove physisorbed water.

As descried above, when the reaction was carried out by using D2O instead of H2O, the yield of phenol was not changed although the yield of hydrogen was less than that of phenol (Table 3). In eq 11, the yield of hydrogen should always be no less than that of phenol, which is inconsistent with the result. Thus, the surface active oxygen species should abstract the hydrogen from the intermediate species (eq 10). However, the possibility of the hydrogen abstraction by the hydrogen radical could not be ruled out. Figure 7 shows the distribution of aromatic ring H−D exchanged phenol and benzene in the photocatalytic hydroxylation by using benzene-h6 and D2O over the various Pt(x)/TiO2 samples. In the present analysis method, since the D atom of the hydroxyl group in the produced phenol can be exchanged easily with H of methanol that was introduced for analysis after the reaction, the detected D atoms should be of the aromatic ring. As shown, the aromatic ring D labeled phenols were produced with any Pt(x)/TiO2(A) samples (Figure 7a). This result confirmed that the isotopic exchange occurred in the reaction system. In addition, the ratio of H−D exchanged phenol increased with increasing the platinum loading amount. In the case of the introduced benzene, the same tendency as phenol was observed although the ratio of D-labeled benzene was lower than that of D-labeled phenol (Figure 7b). This result clearly indicates that hydrogen radical actually attacks the aromatic ring. Thus, it is considered that the abstraction of hydrogen from the intermediate would be occurred by both the surface active oxygen species (eq 10) and the hydrogen radical (eq 11).

3.4. Proposed Reaction Mechanism of Photocatalytic Aromatic Ring Hydroxylation. Through the above results and discussions, the reaction mechanism of the photocatalytic aromatic ring hydroxylation is summarized as shown in Figure 8. The mechanism depends on the pH of the solution. In acidic or neutral conditions (Figure 8a), first, a water is absorbed (Figure 8a, i). The titanium oxide is photoexcited to produce a hole and an electron, and the hole is trapped by the surface bridge oxygen site of the titanium oxide, and the electron migrates to a platinum site (Figure 8a, ii). The trapped hole induces the nucleophilic attack of a water to produce a surface oxygen radical and a proton41 (Figure 8a, iii). A benzene is adsorbed on the titanium oxide and would mainly interact with hydrogen of adsorbed water or surface hydroxyl group.46 The surface oxygen radical attacks the aromatic ring to produce the surface intermediate (Figure 8a, iv). This electrophilic attack is supported by the facts that the tendency of both the yields of the hydroxylation products and the isomer distributions were similar to those in the aromatic electrophilic substitution reaction (Figure 4). Then, the titanium oxide is further photoexcited to produce a hole and an electron. The additionally produced hole abstracts the hydrogen from the surface intermediate species to produce phenoxy species on the titanium oxide surface (Figure 8a, v). Finally, the phenol is desorbed from the titanium 25384



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Figure 8. Proposed reaction mechanism of the photocatalytic aromatic ring hydroxylation of benzene in neutral or acidic conditions (a) and basic condition (b).

However, in basic condition (Figure 8b), the reaction step in the production of the active oxygen species is different from the case of acidic or neutral conditions, i.e., photoformed hole is trapped on the terminal deprotonated hydroxyl group (Ti−O−, Figure 8b, ii), and it induces the cleavage of O−H bond in the water molecule to produce the surface hydroxyl radical as the active oxygen species (Figure 8b, iii). The active species, the hydroxyl radical, attacks the aromatic ring to form the intermediate (Figure 8b, iv and vii). In the steps vi and vii, one may consider the possibility of water reduction by the photoformed electron on the platinum sites because of the low concentration of proton in the basic condition. However, the reduction potential of water to produce a hydrogen molecule (2H2O + 2e− → H2 + 2OH−) is −0.828 V vs NHE,47 which is much higher than the potential of a photoformed electron in the conduction band of titanium

oxide (Figure 8a, vi), while the two photoformed electrons reduce protons to form a hydrogen molecule (Figure 8a, vii). Instead of the steps iv−vii, another reaction path is also possible as mentioned above. The photoformed electron reduces the proton to produce a hydrogen radical (Figure 8a, viii), which abstracts a hydrogen in the intermediate species to produce a phenol and a molecular hydrogen (Figure 8a, ix). These processes are supported by the production of the H−D exchanged benzene and phenol (Figure 7). In this mechanism, the lattice oxygen atom of the titanium oxide surface is introduced to the product, the oxygen atom in the water molecule is introduced to the surface in the first cycle, and the surface oxygen atoms originated from the water molecule are then introduced into the product in the successive cycles. This is supported by the isotopic experiment with H218O (Table 3). 25385



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oxide (anatase, −0.08 V vs NHE).32 Thus, the photoformed electron on the platinum sites would dominantly reduce a proton to produce a hydrogen molecule (2H+ + 2e− → H2, 0 V vs NHE47) even though the concentration of proton is low. In each condition (Figure 8a,b), both one-photon reaction and two-photon reaction would be included. A more detailed kinetic study will reveal the contribution of these routes.

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4. CONCLUSIONS Reaction mechanism of the direct aromatic ring hydroxylation of benzene and substituted benzenes by water over platinum loaded titanium oxide photocatalyst was clarified as follows: the titanium oxide is photoexcited to form a hole and a photoexcited electron. In neutral or acidic conditions, the surface oxygen radical activated by the photoformed hole attacks the aromatic ring to form the surface intermediate, from which hydrogen is abstracted to form phenoxy species and then to produce phenols. However, in basic condition, the first radical species is the hydroxyl radical, which attacks the aromatic ring, followed by hydrogen elimination to produce phenols. The electrophilic oxygen radical species preferentially reacts with the aromatic ring compared to the substituent of the substituted benzenes. However, there are some limitations for the selective reaction, i.e., the orientation of the substrates on the surface of the titanium oxide was important for the progress and the selectivity of the reaction because the surface oxygen radical could not attack the aromatic ring apart from the surface. In addition, the limitation of the irradiated light wavelength was effective for the selective aromatic ring hydroxylation because it inhibited the direct photoactivation of the substrates to produce byproducts (coupling products). When these limitations were satisfied, a wider range of substrates would be applied to the selective aromatic ring hydroxylation with the photocatalyst.

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ASSOCIATED CONTENT

* Supporting Information S

Aromatic ring hydroxylation of alkylbenzenes over the Pt(0.1)/ TiO2(A) sample (Table S1), UV−vis absorption spectra of alkylbenzenes and DR UV−vis spectra of alkylbenzenes adsorbed on the TiO2(A) sample (Figure S1), and the effect of platinum loading amount on the aromatic ring hydroxylation of alkylbenzenes over the Pt/TiO2(A) sample (Figure S2). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-52-789-4609. Fax: +81-52-789-3178. E-mail: [email protected]. Notes

The authors declare no competing financial interest. § Research Fellow of the Japan Society of Division for the Promotion of Science.

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ACKNOWLEDGMENTS This research was financially supported by Grant-in-Aid for JSPS fellows (22-7799) and the fund for doctoral students in Nagoya University.

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REFERENCES

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Reaction Mechanism of Aromatic Ring Hydroxylation by Water over

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