Nano N-TiO₂ mediated selective photocatalytic synthesis of quinaldines from nitrobenzenes

Abstract

N-Doped TiO₂ using a new nitrogen precursor hydrazine hydrate was synthesized by a simple wet method. This photocatalyst was characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area, high resolution transmission electron microscopy (HR-TEM), UV-Vis diffused reflectance spectra (DRS), photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). N-Doping does not change the phase of TiO₂. It is found that the size of N-TiO₂ is 15.6 nm with 134.72 m² g⁻¹ surface area. XPS analysis reveals the presence of anionic nitrogen in TiO₂ as O–Ti–N. Substitution of N in place of oxygen in the TiO₂ lattice causes a decrease in oxygen vacancies which inhibits the recombination of electron–hole pairs. This catalyst was used for the selective one-pot synthesis of quinaldines from nitrobenzenes in ethanol under UV and visible light. N-TiO₂ on irradiation induces a combined redox reaction with nitrobenzene and alcohol and this is followed by condensation-cyclization of aniline with oxidation products to give quinaldines. N-Doped TiO₂ is found to be more efficient than metal doped TiO₂ in quinaldine synthesis under visible light. Higher activity of the N-TiO₂ could be attributed to its stronger absorbance of visible light.

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1. Introduction

Nano-structured TiO₂ materials have been studied extensively due to their high surface area and efficient electron–hole charge separation. Such materials hold great promise as efficient photocatalysts and for solar cells.¹ Titanium dioxide (TiO₂) has attracted elaborate interest because of its good photocatalytic activity, low cost and long-term stability. However, it can be activated only with UV light due to its large band gap of 3.2 eV. Present research is focussed on the development of doped photocatalysts for solar energy utilisation.^2,3 Among the doped TiO₂ catalysts, nitrogen-doped TiO₂ plays a vital role in the field of solar photocatalysis.

Many researchers have shown that doping nitrogen into TiO₂ to form TiO₂–_xN_x can efficiently shift the optical response to the visible spectral range.^4–6 The synthesis of N-doped TiO₂ nanophotocatalysts can be achieved by a sol–gel method,⁷ hydrothermal method,⁸ solvothermal method,⁹ chemical vapour deposition,¹⁰ emulsion precipitation,¹¹ electrical oxidation,¹² sputtering, ion implantation,¹³ spray pyrolysis⁵ and oxidation of TiN.⁶

Most of the above methods need a higher temperature or complicated and expensive equipment. Therefore, it is promising to develop a simple and lower temperature method for the preparation of the nitrogen-doped TiO₂.

Li et al. reported the wet method for the synthesis of excellent visible-light responsive (from 400 to 550 nm) TiO₂–_xN_x photocatalysts at low temperature.¹⁴ This catalyst was prepared by treating self-made amorphous TiO₂ powder with hydrazine hydrate at 110 °C. On the other hand, the solution combustion technique (CS) has been extensively used for the synthesis of nanotitania [CS–TiO₂].^15–17 TiO₂ obtained by the CS method is nanosized (8–12 nm) with high surface area. Therefore, nitrogen-doping on CS–TiO₂ using hydrazine hydrate is expected to produce an efficient photocatalyst in UV and visible light. This nitrogen-doping method is very simple and does not need calcination at high temperature. However, N-TiO₂ was mostly used for photodegradation of organic pollutants.^18,19

Synthetic methods reported by Skraup, Doebner–Von Miller, Friedlander, and Combes have been developed for the preparation of quinolines.^20–22 But many of these methods are not fully satisfactory with regard to operational simplicity, cost of the reagent and isolated yield. Photocatalytic synthesis of quinoline derivatives from nitrobenzene using TiO₂, metal doped TiO₂ and others had been reported earlier.^23–25 In the present study, an attempt has been made for the preparation of N-doped TiO₂ by a simple wet method using hydrazine hydrate with CS–TiO₂. The efficiency of the N-TiO₂ photocatalyst has been evaluated by the photocatalytic synthesis of quinaldines from nitrobenzenes.

2. Experimental

2.1. Materials and methods

All chemicals were of the highest purity available and were used as received. TiO₂-P25 is a mixture of anatase and rutile (80 : 20) supplied by Degussa, Germany. Nanosized titania (CS–TiO₂) was prepared by the solution combustion synthesis method. This method involves the combustion of stoichiometric amounts of aqueous titanyl nitrate (obtained by the hydrolysis and subsequent nitration of the precursor compound titanium tetraisopropoxide) and fuel, glycine at 350 °C in a muffle furnace. A detailed description of the synthesis protocol is available elsewhere.¹⁵

CS–TiO₂ was dipped in hydrazine hydrate (80%) for 12 h, then filtered and dried at 110 °C for 3 h in air. Finally, the yellow nitrogen-doped TiO₂ powder was formed. In the drying process, during evaporation, a glow was detected on the surface of the powder for a moment. Then the catalyst turned yellow.

2.2. Apparatus

X-Ray diffraction (XRD) patterns of catalysts were obtained using a Model D/Max 2550 V with Cu anticathode radiation. The diffractograms were recorded in a 2θ range between 10 and 80° in steps of 0.02° with a count time of 20 s at each point. The crystalline phase can be determined from integration intensities of anatase (101), rutile (110) and brookite (120) peaks and the average crystallite sizes were determined according to the Debye–Scherrer equation using the full width half maximum (FWHM) data of each phase.

D = Kλ/β cos θ

Where D is the crystal size of the catalyst, λ is the X-ray wavelength (0.154 nm), β is the full width half maximum of the catalyst, K = 0.89 and θ is the diffraction angle.

The phase formation, particle size, surface morphology and crystallinity of pure and loaded catalysts were examined using transmission electron microscope (TEM) (Model JEOL TEM-3010) operated at 300 keV. The samples for TEM analysis were prepared by dispersing the catalysts in ethanol under sonication and depositing on a copper grid. High resolution TEM (HR-TEM) measurements were carried out using a JEOL-JEM-2010 UHR instrument operated at an acceleration voltage of 200 kV with a lattice image resolution of 0.14 nm. X-Ray photoelectron spectra (XPS) of the catalysts were recorded in an ESCA-3 Mark II spectrometer (VG Scientific Ltd., England) using Al Kα (1486.6 eV) radiation as the source. The spectra were referenced to the binding energy of C1s (285 eV).

The DRS of all the catalysts were recorded in a Shimadzu UV 2450 model UV-visible spectrophotometer in the range 800–190 nm equipped with an integrating sphere and using powdered BaSO₄ as a reference.

The specific surface areas of the catalysts were determined using a Micromeritics ASAP 2020 sorption analyzer. The samples were degassed at 423 K for 12 h and analysis was performed at 77 K with N₂ gas as the adsorbate. The Brunauer–Emmett–Teller (BET) multipoint method least-square fit provided the specific surface area.

Photoluminescence (PL) spectra at room temperature were recorded using a Perkin Elmer LS 55 fluorescence spectrometer. The nanoparticles were dispersed in carbon tetrachloride and excited using light of wavelength 300 nm.

2.3. Photocatalytic synthesis of quinaldines

In a typical experimental run, 50 mg N-TiO₂ was suspended in 25 mL of an absolute ethanolic solution containing 25 mM of the nitrobenzene and irradiated by a 365 nm medium-pressure mercury lamp (Sankyo Denki, Japan; intensity I = 1.381 × 10⁻⁶ einstein L⁻¹ s⁻¹) after purging with N₂ for 30 min. N₂ bubbling (flow rate = 6.1 mL s⁻¹) and magnetic stirring of the suspension were continued throughout the reaction while the temperature was maintained at 30 ± 1 °C. Progress of the reaction was monitored by TLC. Product analysis was performed by a Perkin-Elmer GC-9000 with a capillary column of DB-5 and flame ionization detector. GC-MS analysis was carried out using a Varian 2000 Thermo with the following features: capillary column VF5MS (5% phenyl-95% methylpolysiloxane), 30 m length, 0.25 mm internal diameter, 0.25 μm film thickness, temperature of column range from 50–280 °C (10 °C min⁻¹) and injector temperature 250 °C, attached with mass spectrometer model SSQ 7000. The isolation was performed by column chromatography on a silica gel column by eluting with a co-solvent of hexane and ethyl acetate (volume ratio 8 : 2).

3. Results and discussion

3.1. XRD analysis

X-Ray diffraction (XRD) patterns of prepared TiO₂ (bare TiO₂), TiO₂-P25 and N-TiO₂ are shown in Fig. 1(a–c). The XRD patterns of bare TiO₂ and N-TiO₂ (Fig. 1a and 1c) are identical with the standard pattern of anatase (JCPDS: 01-078-2486C) and the rutile lines (01-089-0553C) are absent. The peaks at 25.43°, 37.92°, 48.03°, 53.97°, 55.05°, 62.70°, 68.80°, 70.39° and 75.05° are the diffractions of the (101), (004), (200), (105), (211), (204), (116), (200) and (215) crystal planes of anatase TiO₂, respectively. This clearly reveals that the bare TiO₂ and N-TiO₂ are of the anatase phase. Fig. 1b displays the XRD pattern of TiO₂-P25. Since it is a mixture of 80% anatase and 20% rutile, the XRD pattern shows both anatase and rutile lines. There is no diffraction peak of Ti–N compound in N-TiO₂, which can be attributed to the small amount of the doped nitrogen.

Fig. 1 XRD patterns from (a) bare TiO₂, (b) TiO₂-P25 and (c) N-TiO₂.

3.2. UV-Visible diffuse reflectance spectra

The UV-Vis DRS absorption spectra of N-TiO₂, bare TiO₂ and TiO₂-P25 are shown in Fig. 2. N-TiO₂ has a higher absorption of visible light in the region 400–800 nm than bare TiO₂. The band gap energy (E_g) of N-TiO₂, determined using the equation E_g = 1239.8/λ, where λ is the wavelength of the absorption edge in the spectrum, is 2.96, which is less than E_g of bare TiO₂ (3.18 eV). The band gap narrowing may be caused by introduction of nitrogen from hydrazine hydrate into the lattice of titanium. Thus, the sample of N-TiO₂ showed excellent visible light absorption, indicating the increase of photocatalytic activity in the visible region.

Fig. 2 Diffuse reflectance spectra of (a) bare TiO₂, (b) N-TiO₂ and (c) TiO₂-P25.

3.3. Photoluminescence spectra of N-TiO₂

Photoluminescence (PL) spectra of bare TiO₂, N-TiO₂ and TiO₂-P25 are displayed in Fig. 3. TiO₂ exhibited two photoluminescence emission peaks at 420 and 485 nm. Since PL emission is the result of the recombination of excited electrons and holes,^26,27 the lower PL intensity of N-TiO₂ indicates a lower recombination rate of excited electrons and holes (Fig. 3).²⁸

Fig. 3 Photoluminescence spectra of (a) bare TiO₂, (b) TiO₂-P25 and (c) N-TiO₂.

3.4. HR-TEM analysis

High resolution transmission electron microscope images at two different regions are shown in Fig. 4a and 4b. It consists of intergrown fundamental particles with a rough and uneven surface. The intergrowth of small primary particles results in aggregates with significant extra framework void space, which is consistent with the textural mesoporosity observed in N₂ adsorption isotherms, and it is due to the wormhole mesoporous structure. The selected area electron diffraction (SAED) pattern taken from these nanocrystalline particles (Fig. 4c) clearly reveals the presence of (101) plane concentric diffraction rings of anatase TiO₂ phase. The high intense (101) feature highlighting the TiO₂ crystallites is preferentially oriented along the (101) plane. The high-resolution lattice image shown in Fig. 4d confirms that the sample is composed of aggregates of crystalline titania nanoparticles with a d-spacing (interplanar spacing) of 0.35 nm, which corresponds to the (101) plane of anatase TiO₂ phase (strongest intensity in SAED).^29,30 Such high anatase crystallinity in the mesoporous TiO₂ is highly desirable for photocatalysis. Fig. 4e shows the particle size distribution for N-TiO₂. It can be seen that the N-TiO₂ particle sizes are in the range from 5 to 35 nm. The distribution could be fitted well by a Gaussian function. The nanocrystallite size distribution of N-TiO₂ samples is found to be a narrow one with an average particle size of 15.6 nm.

Fig. 4 HR-TEM analysis: (a, b) images at two different regions of N-TiO₂, (c) SAED pattern of N-TiO₂, (d) lattice fringes of N-TiO₂ and (e) particle size distribution of N-TiO₂.

3.5. XPS analysis

The chemical state of nitrogen in the catalyst has been analyzed through X-ray photoelectron spectra. The XPS survey spectrum of N-TiO₂ sample in Fig. 5a shows the presence of the elements Ti, N and O for Ti2p, N1s and O1s core levels.

Fig. 5 X-Ray photoelectron spectra of N-TiO₂: (a) survey spectrum, (b) Ti2p peak, (c) O1s peak, (d) N1s peak and (e) C peak.

N-TiO₂ shows a single peak at 398.1 eV (Fig. 5d) for nitrogen N1s core level. Nitrogen from simple chemisorbed nitrogen or TiN should appear at ≤397.5 eV and nitrogen in NO or NO₂ type species appear above 400 eV.^31–33 Hence this N1s peak at 398.1 eV can be attributed to the anionic N in O–Ti–N linkages. Further the direct interaction between N and O in the lattice is ruled out as this interaction will increase the BE of the N1s level. It is also supported by the low BE of Ti2p of 458.1 eV compared to that of pure TiO₂ (459.3 eV). From the above observations it can be concluded that the peak observed in the present study at 398.1eV is due to the N⁻ anion incorporated in the TiO₂ as a O–Ti–N structural feature.

Fig. 5b shows that the high intense peaks of Ti2p_3/2 and Ti2p_1/2 core levels of N-TiO₂ appear at 458.1 and 463.1 eV, respectively. But with pure TiO₂, binding energy peaks of Ti2p_3/2 and Ti2p_1/2 were reported at 459.3 and 465.0 eV, respectively.³⁴ This reveals that nitrogen doping decreases binding energies of Ti2p_3/2 and Ti2p_1/2 peaks. Lower binding of Ti2p in N-TiO₂ indicates the electronic interaction between Ti and N⁻ anions, which can cause partial electron transformation from N to Ti. Electron density on Ti increases due to the lower electronegativity of nitrogen compared to oxygen. Earlier Miao et al. also assigned lower binding energy peaks to TiO₂–_xN_x.³⁵ This further confirms that nitrogen is incorporated into the lattice and substitutes for oxygen.

It can be seen from Fig. 5c that the O1s spectra of the N-doped TiO₂ consisted of two peaks: one was centered at 529.7 eV and the other was centered at 531.6 eV, which is ascribed to nitrogen dopant. This indicates the presence of another type of oxygen due to the more covalent nature of N-TiO₂. The carbon peak is attributed to the residual carbon from the sample and adventitious hydrocarbon from XPS instrument itself (Fig. 5e).

3.6. BET surface area analysis

In general, the surface area of the catalyst is the most important factor influencing the catalytic activity. N₂ adsorption and desorption isotherms of bare N-TiO₂ are shown in Fig. 6a. It can be seen from Fig. 6a, that the hysteresis loop in this material is of type IV and can be attributed to the mesoporous nature of the N-TiO₂. The pore size distribution has been studied using B.J.H. method and is shown in Fig. 6b. The specific surface area and pore volume of bare N-TiO₂ sample are 134.7 m² g⁻¹ and 0.99–0.13 cm³ g⁻¹, respectively. It can be stated that the maximum pore volume is contributed by the pores with an average size of 21 Å. The bare TiO₂ has a specific surface area of 74 m² g⁻¹.

Fig. 6 (a) N₂ adsorption–desorption isotherms of N-TiO₂ and (b) its pore size distribution.

3.7. Photocatalytic synthesis of quinaldine from nitrobenzene with N-TiO₂

Initial experiments were carried out with ethanolic solution of nitrobenzene containing TiO₂ nanoparticles under different conditions. Irradiation of nitrobenzene and TiO₂ in ethanol with 365 nm UV light produced cyclized product of quinaldine. Either the irradiation of ethanolic solution of nitrobenzene alone or the solution of nitrobenzene and the catalyst without light did not give any product. This indicates that both light and TiO₂ are essential for the formation of quinaldine. The catalytic activities of N-TiO₂ and TiO₂-P25 for the synthesis of quinaldine were investigated for the reaction of nitrobenzene with ethanol.

The changes in the concentrations of nitrobenzene, aniline and quinaldine during the photocatalytic reaction with TiO₂, TiO₂-P25 and N-TiO₂ were determined at different times (Fig. 7). With TiO₂-P25 and bare TiO₂ (Fig. 7a and 7b), 6 h photoirradiation of nitrobenzene gave only 55 and 60% yield of the product quinaldine, respectively. In contrast, 70% yield of quinaldine was obtained in 5 h with N-TiO₂ (Fig. 7c). This indicates that N-TiO₂ promotes rapid and selective quinaldine production. Based on the results, it is obvious that N-TiO₂ has higher photocatalytic activity compared with pure TiO₂, which is ascribed to the N dopant. This is consistent with the analytical results. According to the XRD analysis, the N dopant of TiO₂ decreased the crystalline size of TiO₂ which resulted in the increase of surface area of the TiO₂ from 74 to 134.0 m² g⁻¹. N₂ adsorption–desorption isotherms indicate the mesoporous nature of N-TiO₂. Higher surface area and mesoporous structure increase the photocatalytic activity of N-TiO₂. Furthermore the entry of N into the TiO₂ lattice suppressed the particle growth and consequently caused a decrease of oxygen vacancies, which minimized the electron–hole recombination during the photocatalytic synthesis of quinaldines.

Fig. 7 Time-dependent change in the concentrations of nitrobenzene and products during photoirradiation of nitrobenzene with (a) TiO₂, (b) TiO₂-P25 and (c) N-TiO₂. [Nitrobenzene] = 25 mM, N₂ flow rate = 6.1 mL s⁻¹, I = 1.381 × 10⁻⁶ einstein L⁻¹ s⁻¹, irradiation time = 5/6 h.

GC-MS chromatograms recorded at different reaction times of the photocatalytic conversion of nitrobenzene in ethanol are presented in Fig. 8. GC-MS chromatograms reveal the formation of nitrosobenzene, aniline, N-hydroxyaniline, N-(propan-2-yl)aniline, 2-methyl-1,2,3,4-tetrahydroquinolin-4-yl(phenyl)amine during the quinaldine formation. The formation of byproducts, aniline and 2-methyl-1,2,3,4-tetrahydroquinoline after 6 h is also indicated by the GC-MS chromatogram. Very recently, we reported a similar reaction with a plausible reaction mechanism of the formation of quinaldine from nitrobenzene catalyzed by Au–TiO₂.²⁵ Since the intermediates formed are same, this reaction also follows the same mechanism. To find out the optimum conditions for maximum efficiency of this reaction, various experiments under different conditions were carried out.

Fig. 8 GC-MS chromatograms at different reaction times for the photocatalytic conversion of nitrobenzene with N-TiO₂.

3.8. Influence of the amount of catalyst

Keeping all other factors constant, we investigated the influence of the amount of N-TiO₂ on the photocatalytic cyclization of quinaldine. For the same concentration of nitrobenzene, the amount of N-TiO₂ was varied from 25 to 150 mg in 25 mL. Percentage of product formation increased with increasing amount of N-TiO₂ up to 50 mg/25 mL (Fig. 9). Exceeding 50 mg, all incident photons might be scattered by catalyst particles. Therefore, in all studies, the optimum concentration of N-TiO₂ for the photocatalytic oxidation has been fixed as 50 mg in 25 mL.

Fig. 9 Percentage of quinaldine formed under different catalyst dosage. [Nitrobenzene] = 25 mM, N₂ flow rate = 6.1 mL s⁻¹, I = 1.381 × 10⁻⁶ einstein L⁻¹ s⁻¹, irradiation time = 5 h.

3.9. Influence of the substrate concentration

In another set of experimental runs, the influence of the initial concentration of nitrobenzene (15–45 mmol L⁻¹) on the percentage conversion of quinaldine formation was investigated and the results are given in Table 1. It becomes obvious that the reaction rate decreases with increasing initial NB concentration. When 15 mmol L⁻¹ NB was used 78% conversion was achieved within 5 h of UV irradiation, while only 37% conversion occurred in the case of 45 mmol L⁻¹.

Table 1 Effect of [nitrobenzene] on the photocatalytic cyclization^a

Run	Concentration of nitrobenzene (mM)	% of quinaldine
a Catalyst suspended = 50 mg/25 mL, N2 flow rate = 6.1 mL s^−1, I = 1.381 × 10^−6 einstein L^−1 s^−1, irradiation time = 5h.
1	15	78
2	25	70
3	35	56
4	45	37

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3.10. Effect of substituents

To demonstrate the generality and scope of this method, we examined this reaction with different nitroarenes in ethanol using N-TiO₂ catalyst under UV light and the results are summarized in Table 2. This process is tolerant for the synthesis of various substituted quinaldines. Photoirradiation of alcohol solutions of different nitroarenes with N-TiO₂ catalyst successfully afforded the corresponding quinaldines. 3-Nitrotoluene and 4-nitrotoluene gave 2,7-dimethylquinoline (80%) and 2,6-dimethylquinoline (75%), respectively. The quinaldine yield for 3-nitrotoluene is higher than for 4-nitrotoluene. In the synthesis of quinaldine, reduction of nitro group is followed by condensation with aldehyde and cyclization.

Table 2 Photocatalytic synthesis of various substituted quinaldines using N-TiO₂^a

Reactant	Products yield (%)	Byproduct (%)	Conversion
a All reactions were performed with a 25 mM alcoholic solution of a reactant 50 mg of N-TiO2 suspension, I = 1.381 × 10^−6 einstein L^−1 s^−1, irradiation time = 5 h.
Nitrobenzene	Quinaldine (70)	29	99
3-Nitrotoluene	2,7-Dimethylquinoline (80)	18	98
4-Nitrotoluene	2,6-Dimethylquinoline (75)	23	98
4-Methoxy-nitrobenzene	6-Methoxy-2-methylquinoline (70)	26	96
3-Methoxy-nitrobenzene	7-Methoxy-2-methylquinoline (72)	22	94
3,5-Dimethyl-nitrobenzene	2,5,7-Trimethylquinoline (66)	19	85
4-Chloro-nitrobenzene	6-Chloro-2-methylquinoline (36)	64	99
4-Fluoro- nitrobenzene	6-Fluoro-2-methylquinoline (20)	79	99

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It seems that the electron releasing group at the para position inhibits the condensation of the amino group with aldehyde. This is also revealed by 70% quinaldine formation by 4-methoxynitrobenzene which has a strong electron releasing group at the p-position. In the case of 3,5-dimethylnitrobenzene, the cyclization reaction is hindered due to the steric effect and this decreases the product yield (66%) when compared to 3-nitrotoluene (80%). In the case of 4-chloro- and 4-fluoronitrobenzenes, the yield of quinaldines was very low. This is attributed to photoinduced dehalogenation. Dehalogenated anilines have been identified in the GC-MS analysis.

Earlier it was reported that the irradiation of nitrobenzene (2 mmol) and TiO₂ (0.5 g) in nitrogen saturated ethanol solution for 8–12 h gave ethoxy-tetrahydroquinolines as the major product.²³ We obtained a different product with continues purging of nitrogen during the reaction. Our results reveal that of N-TiO₂ has good oxidizing power under these experimental conditions.

3.11. Effect of additives

It was reported that additives such as KI, NaI could improve the reduction of nitrophenol. Hence, we tested the effect of the additives in the synthesis of quinaldines from nitrobenzene and the results are given in Table 3. It is found that the addition of KI and NaI increases the product yield. As the initial reaction in the quinaldine synthesis is the reduction of nitrobenzene, a similar trend reported for the reduction of nitrophenol is observed.¹⁶

Table 3 Effect of various additives on synthesis of quinaldine with N-TiO₂^a

Run	Additives	% of quinaldine
a [Nitrobenzene] = 25 mM, catalyst suspended = 50 mg/25 mL, N2 flow rate = 6.1 mL s^−1, I = 1.381 × 10^−6 einstein L^−1 s^−1, irradiation time = 5 h.
1	Without additive	70
2	KI	76
3	KBr	71
4	NaI	84
5	NaBr	59
6	Na2SO4	36
7	C6H7SO3Na	66

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Increase in product yield is due to the hole scavenging effect of the iodide ion. The iodide ion was oxidized to iodine by holes generated by the irradiation of N-TiO₂. Consequently the decrease in oxidation by holes will increase the reduction of nitrobenzene (Scheme 1).

Scheme 1 Mechanism for iodide participating in photocatalytic conversion of nitrobenzene to quinaldine.

3.12. Photoredox cyclization in visible light

Since the synthesized N-TiO₂ photocatalyst has a higher absorption in the visible region, we have investigated the photocatalytic activity of this catalyst under visible light (300 W tungsten lamp illumination) on the synthesis of quinaldine from nitrobenzene and the results are shown in Table 4. We have compared the photocatalytic performance of other metal doped photocatalysts such as Ag–TiO₂, Pt–TiO₂ and Au–TiO₂ in this reaction using UV and visible light under the same experimental conditions.

Table 4 Photocatalytic synthesis of quinaldine from nitrobenzene using doped photocatalysts in UV and visible light^a

Run	Catalyst	UV light (%)	Visible light (%)
a All reactions were performed with a 25 mM alcoholic solution of reactant, 50 mg of catalyst, N2 flow rate = 6.1 mL s^−1, 300 W Tungsten lamp illumination, irradiation time = 5 h.
1	Prepared TiO2	60	16
2	Ag–TiO2	65	45
3	Au–TiO2	75	51
4	Pt–TiO2	70	56
5	N-TiO2	70	62

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No product was obtained without the catalyst under visible light (>400 nm) after 5 h, while the yield of quinaldine from nitrobenzene was 16% with bare TiO₂ in 5 h. The results reveal that N-TiO₂ shows the best catalytic activity among the five catalysts under visible light. Order of activity of the catalysts for the synthesis of quinaldine from nitrobenzene using visible light is N-TiO₂ > Pt–TiO₂ > Au–TiO₂ > Ag–TiO₂ > bare TiO₂.

All doped catalysts are more efficient than bare TiO₂. The highest efficiency of N-TiO₂ is due to its increased visible light absorption when compared to other doped catalysts.

4. Conclusions

N-TiO₂ enables efficient quinaldine production from nitroarenes through a combined photoredox reaction. This process has following advantages when compared to other methods: (i) a cheap and stable reactant (alcohol), (ii) it does not require the use of acids, oxidants or reductants and (iii) the reaction proceeds under mild ambient conditions. N-TiO₂ is more efficient than other metal doped catalysts in quinaldine synthesis under visible light. Therefore, this process has the potential to enable a more sustainable quinaldine synthesis from nitrobenzenes in UV and visible light. The combination of a photocatalytic redox-cyclization reaction presented here may help to develop a new strategy towards photocatalysis based organic synthesis.

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Footnote

† Electronic Supplementary Information (ESI) available: See DOI: 10.1039/c2ra01178f/

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