Selective dehydrogenation of aromatic alcohols photocatalyzed by Pd-deposited CdS–TiO₂ in aqueous solution using visible light

Abstract

A CdS/TiO₂ photocatalyst modified with a Pd co-catalyst was applied to a photocatalytic system for the selective dehydrogenation of benzyl alcohol to benzaldehyde with high selectivity (>99%) accompanied by the formation of H₂ in aqueous solution under visible-light irradiation.

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The selective oxidation of alcohols to carbonyl compounds is one of the most crucial reactions for scientific and industrial applications. Particularly, heterogeneously catalyzed alcohol oxidations using molecular oxygen (O₂) as the oxidant are attractive.¹

In recent years, the selective photocatalytic oxidation of alcohol by O₂ on titanium(IV) dioxide (TiO₂) has been extensively studied in organic solvents,² gas phases³ and aqueous solutions⁴ under UV-light irradiation. These photocatalytic systems can utilize only a small percentage of solar irradiation. Therefore, the development of a photocatalyst which works under visible light irradiation is strongly desired. We have previously reported that pure TiO₂ exhibits the selective photocatalytic oxidation of benzylic alcohols to the corresponding aldehydes by O₂ in acetonitrile solutions using visible light.⁵ However, this photocatalytic system does not work in aqueous solutions. Such green processes which work in water have been the focus of much attention since water is an abundant and eco-friendly solvent. It has been reported that benzyl alcohol in aqueous solutions suspended with Au/TiO₂ or Au/CeO₂ was converted to benzaldehyde by O₂ under visible light irradiation.⁶

On the other hand, various photocatalysts which work in water splitting to produce H₂ from aqueous methanol solutions have been extensively investigated,⁷ while oxidative products such as aldehyde, carboxylic acid and CO₂ are formed unselectively. The oxidant-free dehydrogenation of alcohols to carbonyl compounds and hydrogen by thermal catalysts at high temperature (>373 K) has also received considerable attention.⁸ However, more fundamental research focused on the selective dehydrogenation of alcohol to carbonyl compounds using visible light-responsive photocatalysts seems necessary.

In this paper, we have studied the selective dehydrogenation of aromatic alcohols to the corresponding carbonyl compounds and hydrogen on TiO₂-supported CdS (CdS/TiO₂) photocatalysts in aqueous solutions at 298 K under an inert gas atmosphere using visible light.

Fig. 1 shows the reaction time profiles for the dehydrogenation of benzyl alcohol in aqueous solution suspended with Pd (0.4 wt%)/CdS (15 wt%)–TiO₂ (hereinafter referred to as 0.4Pd/15CdS–TiO₂) under visible light irradiation. This reaction does not proceed without Pd/CdS–TiO₂ or irradiation. However, under irradiation, a decrease in the amount of benzyl alcohol was observed with an increase in the irradiation time, while an increase in benzaldehyde and hydrogen was observed. Moreover, neither oxidative products such as benzoic acid or CO₂, nor coupling products such as benzoin were formed in this system. The yield of benzaldehyde reached ca. 98%, while that of hydrogen was >80% after photo-irradiation for 6 h, as shown in Fig. 1. Theoretically, the dehydrogenation of benzyl alcohol leads to the formation of benzaldehyde and H₂ with an equivalent ratio. However, in fact, the ratio of H₂ to benzaldehyde was not observed to be 1 since H₂ and/or H-atoms may be partially adsorbed on the catalyst. Furthermore, this photo-assisted reaction was performed in a saturated solution of benzyl alcohol (0.30 mmol) suspended with 0.4Pd/15CdS–TiO₂ (4 mg) under visible light irradiation for 48 h. As a consequence, the yields of benzaldehyde reached 70% and the turnover number (TON), i.e., total number of photo-formed benzaldehyde per Cd atom in 0.4Pd/15CdS–TiO₂ exceeded 50, suggesting that this reaction system, in fact, takes place photocatalytically. It is known that the CdS photocatalyst photo-corrodes during the photochemical oxidation of water.⁹ However, the presence of benzyl alcohol as a hole scavenger was observed to retard the loss of CdS into the solution to less than 1%, as confirmed by ICP analysis. The anodic photo-corrosion of CdS was, thus, successfully suppressed.

Fig. 1 Reaction time profiles for the photocatalytic dehydrogenation of benzyl alcohol (50 μmol) on 0.4Pd/15CdS–TiO₂ (50 mg) under visible light irradiation. Amounts of (a) benzyl alcohol, (b) benzaldehyde, (c) H₂, (d) benzoic acid, (e) CO₂, and (f) benzoin. The initial amount of benzyl alcohol (ca. 40 μmol) shown in (a) is due to equilibrium adsorption on the catalyst for 1 h under dark conditions.

The photocatalytic activities of Pd-deposited CdS–TiO₂ were optimized by examining the effect of the amount of CdS and Pd loaded (Fig. S1 and S2, ESI†). From these results, the photocatalytic activity was found to depend on the loadings of the CdS and Pd species, which were optimized with 0.4Pd/15CdS–TiO₂. In order to understand the role of the components of the photocatalyst, various kinds of experiments were carried out and the results are listed in Table 1. Neither pure TiO₂ nor the Pd-deposited TiO₂ showed any activity for the reactions (runs 1 and 2). On the other hand, benzyl alcohol was converted to benzaldehyde with high selectivity (>99%) accompanied by H₂ on the pure CdS photocatalyst (run 3), and its activity was enhanced remarkably by modification with the Pd species (run 4). Furthermore, it was observed that the CdS (15 wt%) dispersed on TiO₂ showed higher activity than the pure CdS (run 5), and the activity of 15CdS–TiO₂ was also significantly enhanced by the deposition of the Pd species (run 6). From these results, it was found that the dispersibility of CdS on TiO₂ and the presence of the Pd species may work to favor improvement of the photocatalytic activities.

Table 1 Dehydrogenation of benzyl alcohol on various photocatalysts using visible light

Run	Catalyst^a	H2/μmol	Benzaldehyde /μmol	Sel.^b/%
a Reaction conditions: catalyst (7.5 mg of CdS); benzyl alcohol (50 μmol); reaction time (4 h). b The selectivity to benzaldehyde was calculated by 100 × (amounts of benzaldehyde)/(amounts of benzaldehyde, carboxylic acid, CO2 and benzoin).
1	TiO2	0	0	—
2	0.4Pd–TiO2	0	0	—
3	CdS	0.3	0.6	>99
4	0.4Pd/CdS	2.1	3.3	>99
5	15CdS–TiO2	2.0	4.5	>99
6	0.4Pd/15CdS–TiO2	36	48	>99

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Here, the dependence of the photo-irradiation wavelength on the activity of the dehydrogenation of benzyl alcohol was investigated in order to identify the photo-responsible center of the photocatalyst. Fig. 2 shows the UV-Vis absorption spectra of the CdS–TiO₂ composites in the presence and absence of the Pd species, and the apparent quantum yields (AQY) for the dehydrogenation of benzyl alcohol on Pd/CdS–TiO₂ under irradiation of monochromatic light. As can be seen in Fig. 2(a and b), 15CdS–TiO₂ exhibits visible light absorption due to CdS, while 0.4Pd/15CdS–TiO₂ exhibits absorption due to the CdS and Pd species. On the other hand, the AQY for 0.4Pd/15CdS–TiO₂ was determined as follows: 3.5% at 480 nm, 1.4% at 540 nm, and ca. 0% above 600 nm. The AQY plots, thus, form the action spectrum for the photocatalytic reaction on 0.4Pd/15CdS–TiO₂, as shown in Fig. 2(c). These results show the action spectrum to be in good agreement with the photo-absorption of 15CdS–TiO₂. It could, thus, be concluded that CdS works as the photo-responsible center for the visible-light induced photocatalytic dehydrogenation of benzyl alcohol, while the Pd species work as the co-catalyst to improve the photocatalytic activities.

Fig. 2 UV-Vis spectra of (a) 15CdS–TiO₂, (b) 0.4Pd/15CdS–TiO₂, and (c) AQY of the photocatalytic reactions on 0.4Pd/15CdS–TiO₂.

In order to characterize the photocatalysts, XRD and XPS analyses of Pd/CdS–TiO₂ were performed. XRD analysis of 0.4Pd/15CdS–TiO₂ confirmed that CdS and TiO₂ form cubic and anatase structures, respectively, and no other phases due to the Pd species were detected (Fig. S3, ESI†). From XPS analysis of 0.4Pd/15CdS–TiO₂, two kinds of Pd 3d5/2 peaks were observed at 336.3 and 335.3 eV, which were attributed to Pd(II) and Pd(0), respectively (Fig. S4, ESI†).¹⁰ Thus, the two kinds of Pd states in 0.4Pd/15CdS–TiO₂ can be assigned as follows: one is Pd(II) probably originating from PdS and the other is metallic Pd. A plausible mechanism is proposed as follows: a hole–electron separation is generated on CdS under visible-light irradiation. Subsequently, the photo-produced holes efficiently transfer to the PdS sites,¹¹ leading to the oxidation of benzyl alcohol to benzaldehyde, while the electrons participate in the reduction of protons to H₂ on the metallic Pd sites. Details on the location of the Pd species and the effect of these species on the activities are currently under investigation.

The photocatalytic activities for various kinds of aromatic alcohol were also investigated and the results are shown in Table 2. It was observed that not only benzyl alcohol but also its derivatives substituted by electron-donating groups (p-OH, m-OCH₃ and p-CH₃) and an electron-withdrawing group (p-Cl) were converted to the corresponding carbonyl compounds with high selectivity (>99%), accompanied by H₂ (Table 2, runs 2–5). Furthermore, 1-phenylethanol was also converted to acetophenone with high selectivity (>99%) (Table 2, run 6). These results indicate that this photocatalytic system is applicable to highly selective conversions of various benzylic alcohols on Pd-deposited CdS–TiO₂ under visible light irradiation.

Table 2 Selective dehydrogenation of various kinds of aromatic alcohols using visible light

Run	R1	R2	H2/μmol	Carbonyl compound/μmol	Conversion^a/%	Selectivity^b/%
a The conversion was calculated by 100 × (amounts of benzaldehyde)/(amounts of benzyl alcohol added). b The selectivity to benzaldehyde was calculated by 100 × (amounts of benzaldehyde)/(total amounts of benzaldehyde, carboxylic acid, CO2 and benzoin).
1	H	H	36	48	96	>99
2	H	OH	32	42	84	>99
3	H	OCH3	46	49	98	>99
4	H	CH3	43	48	96	>99
5	H	Cl	44	47	94	>99
6	CH3	H	46	40	80	>99

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This work is the first to achieve the selective photocatalytic dehydrogenation of aromatic alcohol in aqueous solution using visible light at 298 K under unaerated conditions. The CdS–TiO₂ photocatalyst was found to induce the highly selective formation of benzaldehyde from benzyl alcohol, and its activity was significantly enhanced by the presence of the Pd co-catalyst. These preliminary results are promising for various kinds of selective photocatalytic dehydrogenations in organic synthesis using visible light.

Experimental section

Materials

Commercially available TiO₂ powder (100% anatase; BET surface area: 320 m² g⁻¹, ST-01, Ishihara Co., Ltd.), benzyl alcohol and para-benzylic alcohols substituted by –OCH₃, –OH, –CH₃, –Cl and phenyl ethanol, and their corresponding carbonyl compounds, purchased from Wako Pure Chemical Industries, Ltd., were used as received.

Preparation of photocatalysts

The CdS-supported TiO₂ (CdS–TiO₂) was prepared by impregnation of TiO₂ with aq. Cd(NO₃)₂ at the desired concentrations, followed by stirring in aq. Na₂S (0.1 M) for 24 h at 298 K. Pure CdS was prepared as follows: 50 mL of aq. Cd(NO₃)₂ (0.1 M) was added to 50 mL of aq. Na₂S (0.1 M). The solids were washed in distilled water, filtrated and dried at 298 K for 12 h. Pd deposition on the catalysts (1.0 g for each) was performed in an aqueous solution (50 mL) of methanol (0.5 mL) and the desired amounts of PdCl₂ under UV-light (λ_max = 365 nm, 0.8 mW cm⁻²) from black light for 4 h under N₂ bubbling.

Photocatalytic reactions

Photocatalytic reactions were performed in a Pyrex cell (20 mL in volume) with an aqueous suspension (10 mL) of the photocatalyst (50 mg) involving 50 μmol of benzylic alcohol under an argon (1 atm) atmosphere by blue light emitted from a LED lamp (λ_max = 460 nm, ca. 10 mW cm⁻²). Photo-irradiation was performed after confirming the equilibrium adsorption of benzyl alcohol for 1 h under dark conditions. After the reactions, the catalysts were immediately separated from the suspension by filtration through a 0.20 μm membrane filter (Dismic-25, Advantec). The solution was then analyzed by high performance liquid chromatography: HPLC (Shimadzu LC10ATVP, UV-Vis detector, column: Chemcopak, mobile phase: a mixture of acetonitrile and 1.0% aqueous formic acid), while the gas phase (H₂) was analyzed by GC (TCD; Model-802, Ohkura; column: Molecular sieve 5A).

Apparent quantum yields

The apparent quantum yields (AQY) were measured in a quartz cell using a 100 W xenon lamp (Lax Cute II, Asahi Spectra Co., Ltd.) through appropriate band path filters with a FWHM of 10 ± 2 nm. The 0.4Pd/15CdS–TiO₂ photocatalyst was suspended in an aqueous solution (2 mL) of benzyl alcohol (10 μmol). The apparent quantum yield (Φ) at each centered wavelength of light was calculated using the following equation (eqn (1)): It was assumed that one photon forms one benzaldehyde due to the current doubling effect.¹² However, it should be noted that the AQY values calculated by eqn (1) are uncorrected for reflection and scattering losses. The AQY may, thus, show a lower value than the internal quantum yields. The flux of incident photons was measured using a power-meter (Ophir, ORION/PD), and photo-irradiation was conducted in the range of 7.24 × 10¹⁸–1.03 × 10¹⁹ photons h⁻¹.

Characterizations

Analyses of the optical properties of the samples were carried out in diffused reflectance mode using a UV-Vis spectrophotometer (UV-3100PC, Shimadzu). The UV-Vis reflectance spectra (R%) of the solids were obtained in diffused reflectance mode while these spectra were converted to absorbance (A) by the following equation: A = log (100/R). Powder X-ray diffraction (XRD) patterns were obtained with a RIGAKU RINT2000 diffractometer using Cu K_α radiation (λ = 1.5417 Å). The oxidation states of palladium were analyzed by X-ray photoelectron spectroscopy (XPS, ESCA 3200, Shimadzu). The binding energies were calibrated by using the containment carbon (C 1s = 284.6 eV). The loading amounts of Cd and Pd were analyzed by a multi-type ICP emission spectrometer (ICPE-9000, Shimadzu).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy20607b

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