Research Article pubs.acs.org/journal/ascecg
An Efficient, Visible Light Driven, Selective Oxidation of Aromatic Alcohols and Amines with O2 Using BiVO4/g‑C3N4 Nanocomposite: A Systematic and Comprehensive Study toward the Development of a Photocatalytic Process Subhajyoti Samanta,† Santimoy Khilari,‡ Debabrata Pradhan,‡ and Rajendra Srivastava*,† †
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab-140001, India Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal-721302, India
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‡
S Supporting Information *
ABSTRACT: In this study, BiVO4 was prepared by a hydrothermal synthesis route in the presence of sodium dodecyl sulfate using aqueous NH3 as precipitant. g-C3N4 was prepared by a combustion method using melamine. In order to develop highly efficient photocatalyst, a heterojunction catalyst based on g-C3N4 and BiVO4 was prepared. Different amounts of BiVO4 and g-C3N4 were mixed and annealed to obtain heterojunction photocatalysts. FeVO4 and LaVO4 were also prepared for the comparative catalytic investigation. Catalysts were characterized by a series of complementary combinations of powder X-ray diffraction, thermogravimetric analysis, elemental analysis, N2 adsorption−desorption, scanning electron microscopy, transmission electron microscopy, temperature-programmed desorption of NH3 and CO2, diffuse reflectance ultraviolet visible spectroscopy, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, and photoelectrochemical studies. Catalysts were investigated in the visible light driven oxidation of benzyl alcohol, benzyl amine, and aniline with O2. In order to propose the electrons, holes, and radicals mediated reaction pathways, reactions were performed in the presence of an electron/hole/radical scavenger. Further, in order to confirm various products formed during the photocatalytic oxidation of benzyl alcohol, benzyl amine, and aniline, several model reactions were carried out. Based on the results obtained, the reaction mechanism and structure−activity relationship were established. KEYWORDS: Metal vanadate, Carbon nitride, Heterojunction photocatalyst, Visible light assisted photocatalysis, Acidity, Basicity, Selective oxidation
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INTRODUCTION The development of a sustainable and eco-friendly catalytic process is challenging and interesting to academicians and industrialists. Heterogeneous, easily recoverable, and recyclable catalysts have been developed for this purpose. Additionally, efforts are being made to develop a catalytic process based on renewable synthetic intermediates.1,2 In recent times, significant efforts are being made to carry out reactions using renewable energy resources such as solar energy.3,4 This decade is witnessing the development and utilization of solar energy to produce sustainable energy resources and chemicals.5,6 Artificial photosynthesis is one of the most extensively studied and emerging research areas which demonstrate that solar energy can be used in chemical synthesis.7−9 The development of a novel, eco-friendly, and sustainable heterogeneous photocatalyst is an important and challenging task for catalysis researchers.10,11 Electron−hole (e−−h+) pairs are produced from photoexcitation of a photocatalyst. Photogenerated electrons can be used in a reduction process, whereas holes can be utilized in an oxidation process. Electron scavengers (such as oxygen) can capture the photogenerated electrons and form various reactive © 2017 American Chemical Society
species (such as superoxide radicals, hydoxyl radicals, and oxyhydoxyperoxide radicals) that perform various reactions, such as oxidation, C−H activation, and C−C or N−N coupling reactions.12−15 On the other hand, holes can be trapped by different organic species, leading to their oxidized products. But the inherent tendency of e−−h+ pairs recombination hampers their efficient utilization.16 Recombination of electrons and holes occurs unless an electron acceptor (for example, O2) is available to scavenge the electrons to form various (oxo and hydroperoxo) species.16 In order to use abundant solar energy for photocatalysis, a catalyst should have (i) an appropriate band gap for efficient absorption of visible light (e.g. LaVO4 > FeVO4 and BiVO4/g-C3N4 (5/5) > BiVO4/g-C3N4 (1/9) > BiVO4/g-C3N4 (9/1). However, the catalytic activity with O2 is more when compared to H2O2 (Compare Table 1 and Table S1). From the catalytic data obtained using H2O2 and O2, it can be concluded that the economical, sustainable, and eco-friendly oxidant O2 is capable of producing benzaldehyde in more yield under visible light conditions when compared to H2O2. H2O2 decomposes into reactive oxygen species (2HO•) with strong oxidation ability in the presence of photocatalyst, which abstract (H•) from the O−H and C−H bonds of benzyl alcohol in a sequential manner to form benzaldehyde (Scheme 1(c)). Further, photogenerated (HO•) can also abstract the (H•) from benzaldehyde to form benzoic acid.72 Based on
these observations, one can conclude that the oxidation is catalyzed by O2 and in situ generated H2O2. Benzyl benzoate is obtained in large amount using metal vanadate, which shows that the catalyst exhibits strong oxidizing ability along with good acidity. The acidity of the catalyst facilitates the esterification reaction between the oxidized product benzoic acid and the reactant benzyl alcohol to form benzyl benzoate. However, the selectivity of benzaldehyde is more for the heterojunction catalyst BiVO4/g-C3N4 (5/5), which shows that BiVO4/g-C3N4 (5/5) is more suitable for mild oxidation and also exhibits acidity (benzyl benzoate is formed in this case also). Such high selectivity for benzaldehyde and acidity of BiVO4/g-C3N4 (5/5) would be interesting to obtain the selective production of dimethoxy-methylbenzene (an important and reactive synthetic intermediate). In order to achieve this product, reaction was carried out in methanol under the optimized reaction conditions illustrated in Table 3. Reaction did not take place in dark conditions or in the absence of catalyst (Table 3, entry 1 and 2). Under the illumination of visible light, all photocatalysts are found to be active. In this case, four products were observed during the GC analysis. GCMS analysis showed that the products were benzaldehyde, dimethoxy-methylbenzene, benzyl benzoate, and methyl benzoate. Oxidation of benzyl alcohol leads to formation of benzaldehyde and benzoic acid as products. Acid catalyzed methylation of benzaldehyde takes place to form dimethoxymethylbenzene, whereas acid catalyzed esterification of benzoic acid with benzyl alcohol/methanol leads to formation of benzyl benzoate/methyl benzoate. Since the reactivity of benzyl alcohol is more when compared to methanol; therefore, a larger amount of benzyl benzoate is obtained. As expected, high benzyl alcohol conversion and selectivity for dimethoxymethylbenzene is obtained using BiVO4/g-C3N4 (5/5) (Table 3). Having shown that the catalysts are good for the photocatalytic oxidation for benzyl alcohol, it would be interesting to investigate the photocatalytic oxidation of amines such as benzyl amine and aniline. First, oxidative coupling of benzyl amine was investigated using molecular oxygen as oxidant (Table 4). The reaction did not proceed under dark conditions and exhibits very low product yield (Table 4, entries 1−2). All 2570
DOI: 10.1021/acssuschemeng.6b02902 ACS Sustainable Chem. Eng. 2017, 5, 2562−2577
Research Article
ACS Sustainable Chemistry & Engineering
(within 15 min) took place and produced a quantitative yield of N-benzylidene benzylamine in the case of BiVO4/g-C3N4 (5/ 5), whereas it took a longer time for the reaction to complete using g-C3N4. These results indicate that a rapid condensation takes place using BiVO4/g-C3N4 (5/5). Based on these observations, the following reaction mechanism is proposed (Scheme 2a).54 The adsorption of benzyl amine on the catalyst surface takes place due to the electrophilicity of V sites that would form a surface complex (I), which plays the role of the antenna to absorb visible light (step 1). Upon visible light absorption, electrons and holes are generated: the holes localized on the oxidized substrate, and the electrons localized within the BiVO4/g-C3N4 lattice. The deprotonation of the oxidized substrate generates a radical species (intermediate II) using holes (step 2). Dioxygen and the trapped electron in the conduction band of BiVO4/g-C3N4 produced superoxide radical anion (O2•−). Intermediate II reacts with O2•− to form benzaldehyde via intermediate (III) and to form intermediate (IV) (step 3). The regeneration of the surface VV sites of BiVO4/g-C3N4 completes the photocatalytic oxidation cycle through H+ (step 4). The aldehyde condenses with benzyl amine to produce N-benzylidene benzylamine. Formation of benzonitrile in the case of g-C3N4 will be discussed in the next paragraph. Primary benzyl amine derivatives bearing various functional groups (OMe, Me, and Cl) were converted to the corresponding coupled imines with excellent yields (Table 4, entries 10−12). Electron-rich benzyl amines (OMe and Me derivatives) exhibit better yield than electron-deficient ones (Cl). This can correlate well with the first step of the proposed mechanism, which shows that the more the electron donates amines, the more the reaction will be facilitated. In order to verify the oxygenated pathway, reaction was performed with dibenzyl amine. It yields the formation of a small amount of benzaldehyde (2.2%) and a large amount of Nbenzylidene benzylamine (96%). This confirms that the reaction proceeds through an oxygenated pathway that leads to formation of benzaldehyde, which, upon condensation with benzyl amine, gives N-benzylidene benzylamine. In order to verify the involvement of electrons and holes as reactive species in the photocatalytic oxidation of benzyl amine, experiments were performed with 0.1 mM AgNO3 and 0.1 mM ammonium oxalate as electron scavenger and hole scavenger,27 respectively. Results show that benzyl amine conversion increased when 1 mL of 0.1 mM AgNO3 was used (Figure S6(b)). AgNO3 facilitates the electron separation because the separated electrons are used up in making metallic silver. After the reaction, Ag nanoparticles were observed in the reaction vessel. When 1 mL of 0.1 mM ammonium oxalate was used, benzyl amine conversion was decreased. This provides evidence that holes are also involved in the reaction. Having received satisfactory results with benzyl amine, it is interesting to investigate the reaction with aniline where no C− H activation is possible. g-C3N4, BiVO4, and BiVO4/g-C3N4 composite photocatalysts were investigated under the optimized reaction conditions for aniline. In this case, two products, azobenzene and N-phenyl benzene-1,4-diamine, were obtained (Table 5). It is interesting to observe that, using g-C3N4, azobenzene was formed in high selectivity (Table 5, entry 6), whereas, with BiVO4, a significant amount of N-phenylbenzene1,4-diamine was formed. This provides evidence that the acidity of the catalyst plays a role in the formation of side product. This side product is formed only when a high concentration of BiVO4 is present in the composite catalyst. Upon light
Table 4. Photocatalytic Oxidation of Benzyl Amine in the Presence of O2 over Various Photocatalysts Investigated in This Studya
Entry No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Catalyst None BiVO4b BiVO4 FeVO4 LaVO4 g-C3N4c BiVO4/g-C3N4 BiVO4/g-C3N4 BiVO4/g-C3N4 BiVO4/g-C3N4 BiVO4/g-C3N4 BiVO4/g-C3N4 BiVO4/g-C3N4 BiVO4/g-C3N4
(1/9) (5/5) (9/1) (5/5)d (5/5)e (5/5)f (5/5)g (5/5)h
Benzyl amine conv. (%)
Product selectivity (%)
TONi
