Photocatalytic Oxidation Of Benzyl Alcohol Research Articles

Supramolecular Assemblies of a Water-Soluble Diprotonated Porphyrin and Heteropolyoxometalates

Photocatalytic oxidation of organic substrates has been of great interest in terms of utilization of photon energy to produce useful materials from abundant feedstock. As a photocatalyst that can mediate electron transfer (ET) from an oxidation catalyst to an oxidant upon photoexcitation, porphyrins can be useful in light of the strong visible-light absorption and clear redox behavior. Although porphyrins have been used as photocatalysts for reduction of organic substrates, photocatalytic oxidation of organic substrates has been seldom reported by using porphyrins as photocatalysts. So far, we have established the utility of diprotonated porphyrins as electron acceptors in photoinduced ET.1 On the basis of the concept, we prepared a water-soluble and saddle-distorted porphyrin, which can act as an electron acceptor in photoinduced ET upon diprotonation. The porphyrin (H2P4+) possesses four N-methylpyridinium-4-yl substituents at the para positions of the four meso-phenyl groups of dodecaphenylporphyrin. We observed proton-coupled electron transfer processes of the diprotonated form of H2P4+, H4P6+, in water to establish the Pourbaix diagram of H4P6+. We also determined the pKa value of the first protonation/deprotonation equilibrium to form the monoprotonated H3P5+ (pKa1 = 4.04) and that of the second one to afford H2P4+ (pKa2 = 7.83). The highly positively charged H4P6+ was revealed to form supramolecular assemblies with negatively charged heteropolyoxometalates (POMs) in the ratio of H4P6+ : POM = 1 : 2 in water at pH 3 as indicated by Job's plots. Based on the redox properties of H4P4+ and the formation of supramolecular assemblies, we examined intra-supramolecular photoinduced ET from heteropolyoxometalates as electron donors to H4P6+ as an electron acceptor in water at pH 3 using femto-second laser flash photolysis upon photoexcitation at 393 nm. First-order rate constants of ET were analyzed on the basis of the Marcus theory of ET to determine the reorganization energy of ET to be 0.74 eV and the electronic coupling matrix element (V) to be 43 cm-1. We also examined its application to photocatalytic oxidation of organic substrates in water, using H4P6+ as a photocatalyst, Na2S2O6 as an oxidant, and a Ru-substituted POM (RuPOM) as an oxidation catalyst, under the visible light irradiation. Preliminary results on the photocatalytic oxidation of benzyl alcohol were 560 turnovers for 4h; blank tests clearly suggest that H4P6+, RuPOM, the oxidant, and photoirradiation are all required to perform the photocatalytic reaction.

Optimization of plasmon-induced photocatalysis in electrospun Au/CeO2 hybrid nanofibers for selective oxidation of benzyl alcohol

Exploiting photocatalysts with improved properties for solar-driven chemical reactions is of great significance in developing green chemistry. Here, a series of Au/CeO2 hybrid nanofibers with different Au loadings have been fabricated by a simple method of electrospinning followed by calcination in air. The particle size and plasmonic absorption of Au nanoparticles (NPs) loaded in the nanofibers were analyzed and found to vary with the dosage of chloroauric acid in the precursor solution. The Au/CeO2 hybrid nanofibers were used as photocatalysts for selective oxidation of benzyl alcohol to benzaldehyde with O2 under simulated sunlight and visible light (>420nm), respectively. The results showed that introducing Au NPs into CeO2 nanofibers induced a great improvement in photocatalysis. The degree of improvement increased first and then decreased with the increase in Au loading, reaching an optimal level over 0.5wt.% Au-loaded CeO2 nanofibers. The photocatalytic reaction presents a very high selectivity of 100% for benzaldehyde, which is important for organic synthesis. The transient photocurrent responses of the Au/CeO2 catalysts were also tested for corroborative evidence. After detailed discussion of various factors including plasmonic absorption, charge transfer, and surface activity of the photocatalyst, a possible mechanism for the photocatalytic oxidation of benzyl alcohol occurring at the Au–CeO2 interface was proposed.

Insight into the synthesis procedure of Fe3+/TiO2-based photocatalyst applied in the selective photo-oxidation of benzyl alcohol under sun-imitating lamp

Fe3+/TiO2/zeolite Y photocatalyst synthesized by using sonophotodeposition method was compared with photocatalysts prepared by simple photodeposition and sonodeposition methods in order to clarify the role of light irradiation and ultrasounds while they are used simultaneously. To gain an insight into the mechanism of this method a detailed characterization of the photocatalysts was carried out by means of the following techniques: UV–vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, Mössbauer measurements and photocatalytic test reaction. Basing on the results from these techniques the chemical role of light and mainly mechanical role of ultrasound were observed. The selective photocatalytic oxidation of benzyl alcohol into benzaldehyde in liquid phase was a test reaction verifying the utility of the prepared materials. The best photocatalytic efficiency in this reaction was performed by photocatalyst synthesized using compilation of ultrasound energy with photoexcitation.

The Study of Photocatalytic Oxidation of Benzyl Alcohol with g-C<sub>3</sub>N<sub>4</sub> under Visible Light: Effect of pH and Salt

Carbon nitride is a representative metal-free photocatalyst for green chemical technology. In this study, synthesized g-C3N4 carbon nitride was employed to study effect of pH and potassium salt loading on photocatalytic oxidation of benzyl alcohol. The result indicated that the maximum conversion could be achieved to 20.75% at a pH of 8.66 with a concentration of K2HPO4 of 0.6 mmol Suitable pH and salt concentration were important factor for promoting of proton on surface and electronic conductivity to separate electron in photocatalysis.

Bimetallic (Au–Cu core)@(ceria shell) nanotubes for photocatalytic oxidation of benzyl alcohol: improved reactivity by Cu

Supported bimetallic AuCu@CeO2 nanocomposites with high Cu-content demonstrate high catalytic activity, selectivity and stability for photocatalytic oxidation of benzyl alcohol.

Selective photocatalytic oxidation of benzyl alcohol into benzaldehyde with high selectivity and conversion ratio over Bi4O5Br2 nanoflakes under blue LED irradiation

Novel Bi4O5Br2 nanoflakes with a thickness of approximately 5nm and a band gap energy of 2.54eV were synthesized by a rapid and energy-saving microwave route. Under blue light emitting diode (LED) irradiation and using the Bi4O5Br2 nanoflakes as a photocatalyst, selective catalytic oxidation of benzyl alcohol (BA) into benzaldehyde (BAD) was successfully achieved with a high selectivity and conversion ratio. Compared to Bi12O17Cl2, which has a similar blue LED light absorption capability and a band gap energy of 2.37eV, these Bi4O5Br2 nanoflakes exhibit superb conversion efficiency (>99%) and selectivity (>99%) toward the photocatalytic oxidation of BA into BAD. Based on the structural characterization of the as-synthesized photocatalyst, comparison of photocatalytic performances, investigation of active radicals, and quantum chemical calculations, a possible photoreaction pathway is explored and proposed. It is revealed that the high selectivity of the system comes from direct hole oxidation of alkoxide anions (BA−) and the appropriate valence band potential (+2.41V vs. NHE) of Bi4O5Br2. And the high conversion ratio is attributed to the positively charged surface, large specific surface area with micro-nano structures, and effective separation of photogenerated carriers of the as-synthesized photocatalyst. In addition, the as-synthesized Bi4O5Br2 catalyst remains stable during the photocatalytic conversion process and can be utilized repeatedly, suggesting its potential for practical applications.

To boost photocatalytic activity in selective oxidation of alcohols on ultrathin Bi2MoO6 nanoplates with Pt nanoparticles as cocatalyst

Photocatalytic oxidation reactions provide a promising strategy for green synthesis of fine chemicals. However, the low photocatalytic efficiency and selectivity are the main obstacles. Herein, we report the ultrathin Bi2MoO6 nanoplates with thickness of ∼17nm which exhibit a superior performance in alcohol oxidation to corresponding aldehyde using O2 under visible light irradiation, achieving ca. 29 times enhancement of activity compared with Bi2MoO6 synthesized from traditional hydrothermal method (thickness of ∼250nm). When Pt nanoparticles as cocatalyst are loaded on the surface of the ultrathin Bi2MoO6 nanoplates, the photocatalytic performance is further enhanced. The remarkable enhancement of activity is attributed to the ultrathin 2D geometry of Bi2MoO6 nanoplates, promoting the charge-separation efficiency and affording abundant catalytically active sites. The excellent selectivity of photocatalytic oxidation of benzyl alcohol on the ultrathin Bi2MoO6 is attributed to the different adsorption behaviors of benzyl alcohol and benzaldehyde. This study could also provide an efficient strategy for improving the photocatalytic performance by rational fabrication of two-dimensional semiconductor materials for organic synthesis.

Selective photocatalytic oxidation of benzyl alcohol to benzaldehyde by using metal-loaded g-C3N4 photocatalysts

Replacement of traditional thermal synthesis of fine chemicals by selective photocatalysis can lead to more environmental friendly processes, carried out at milder conditions, using ambient temperature, atmospheric pressure, with more ecological and economic solvents such as water. Because benzaldehyde (BAL), as fine chemical building block, has an enormous range of applications in pharmaceutical, fragrance and agricultural industries, we analyzed its route of synthesis by the photocatalytic oxidation of benzyl alcohol (BA).In this work, we used graphitized-carbon nitride (g-C3N4) as photocatalyst for the selective synthesis of benzaldehyde from benzyl alcohol. The solid matrix of the photocatalyst was modified by loading different metals such as Au, Ru, Pd, Pt, Ir, Ag and Rh. Conversion, yield and selectivity were compared against benchmark commercial photocatalyst P25 TiO2 (by Evonik). The influence of the physical-chemical nature of the medium, by means of the pH, on the efficiency of the photocatalytic process was also evaluated. The 1wt.% Ru/g-C3N4 photocatalyst had the best compromise between conversion (73%) and selectivity (72%) for a 4h of irradiation time using 390nm activation by a light emitting diode source and it proved to be stable up to 3 consecutive runs.

AgBr@TiO2/GO ternary composites with enhanced photocatalytic activity for oxidation of benzyl alcohol to benzaldehyde

AgBr@TiO2/GO (graphene oxide) ternary composite photocatalyst was synthesized by fabricating core–shell-structured AgBr@TiO2 and anchoring it onto the surface of GO. The obtained samples were characterized by transmission electron microscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy, ultraviolet–visible (UV–Vis) diffuse reflectance spectrum, and photoluminescence (PL) spectroscopy. It was found that the AgBr nanoparticles were prone to aggregation while the core–shell-structured AgBr@TiO2 possessed excellent dispersity. PL analysis revealed that the ternary-structured AgBr@TiO2/GO could effectively promote the separation rate of electron–hole pairs. Photocatalytic oxidation of benzyl alcohol to benzaldehyde under visible-light irradiation was selected as probe reaction to evaluate the photocatalytic activity of the different samples. It was found that the AgBr@TiO2/GO ternary composite exhibited evidently improved photocatalytic activity compared with AgBr, AgBr@TiO2, and AgBr/GO. On the basis of the experiment results, the photocatalytic oxidation mechanism of benzyl alcohol over AgBr@TiO2/GO is tentatively discussed.

Constructing Bi24O31Cl10/BiOCl heterojunction via a simple thermal annealing route for achieving enhanced photocatalytic activity and selectivity.

This work reports on the construction of a Bi24O31Cl10/BiOCl heterojunction via a simple thermal annealing method. The X-ray diffraction (XRD) results indicated that the phase transformation from BiOCl to Bi24O31Cl10 could be realized during the thermal annealing process. The high-resolution transmission electron microscopy (HRTEM) images, X-ray photoelectron spectroscopy (XPS) binding energy shifts, Raman spectra and Fouier transform infrared spectroscopy (FT-IR) spectra confirmed the formation of the Bi24O31Cl10/BiOCl heterojunction. The obtained Bi24O31Cl10/BiOCl photocatalyst showed excellent conversion efficiency and selectivity toward photocatalytic conversion of benzyl alcohol to benzaldehyde under visible light irradiation. The radical scavengers and electron spin resonance (ESR) results suggested that the photogenerated holes were the dominant reactive species responsible for the photocatalytic oxidation of benzyl alcohol and superoxide radicals were not involved in the photocatalytic process. The in-situ generation of Bi24O31Cl10/BiOCl heterojunction may own superior interfacial contact than the two-step synthesized heterojunctions, which promotes the transfer of photogenerated charge carriers and is favorable for excellent photocatalytic activities.

Polydopamine-Coated TiO2 Nanotubes for Selective Photocatalytic Oxidation of Benzyl Alcohol to Benzaldehyde Under Visible Light.

TiO2 nanotube arrays grown by anodization were coated with thin layers of polydopamine as visible light sensitizer. The PDA-coated TiO2 scaffolds were used as photocatalyst for selective oxidation of benzyl alcohol under monochromatic irradiation at 473 nm. Benzaldehyde was selectively formed and no by-products could be detected. A maximized reaction yield was obtained in O2-saturated acetonitrile. A mechanism is proposed that implies firstly the charge carrier generation in polydopamine as a consequence of visible light absorption. Secondly, photo-promoted electrons are injected in TiO2 conduction band, and subsequently transferred to dissolved O2 to form O*2- radicals. These radicals react with benzyl alcohol and lead to its selective dehydrogenation oxidation towards benzaldehyde.

Roles of the active species involved in the photocatalytic oxidation of benzyl alcohol into benzaldehyde on TiO2 under UV light: Experimental and DFT studies

Active species such as holes, electrons, hydroxyl radicals (OH), and superoxide radical anions (O2−) involved in the photocatalytic oxidation of benzyl alcohol (BA) into benzaldehyde (BAD) on TiO2 under ultraviolet (UV) light were investigated by adding corresponding scavengers. The experimental results show that the oxidation of BA into BAD is driven mainly by the holes and OH radical. The role of dissolved O2 was studied by bubbling N2 through the reaction. The results show that the dissolved O2 is, to some extent, the source of OH. Additional density functional theory (DFT) calculations identified two H-abstraction pathways (RabsOH and Rabsα). The structure of the RabsOH product is less stable and tends to transfer resulting in the Rabs α product. The holes are the main driving force for the abstracting of H from BA. The OH radical combines with the H-abstract product to form a precursor of product BAD, detected by LC–MS technique (m/z 123), which then undergoes an intramolecular dehydration reaction to form BAD. The results will help to select the optimal active species for the photocatalytic oxidation of BA into BAD on TiO2 under UV light.

Iron‐Containing Titania Photocatalyst Prepared by the Sonophotodeposition Method for the Oxidation of Benzyl Alcohol

AbstractThe sonophotodeposition method was applied in the synthesis of an iron‐containing titania photocatalyst. This procedure enables the use of sustainable energy sources, such as sonication and light, and eliminates the use of hazardous reducing agents. For the first time an ultrasonic horn and sun‐imitating Xenon lamp were used in this method. The following techniques: diffuse reflectance UV/Vis spectroscopy, high‐resolution transmission electron microscopy with X‐ray microanalysis (HRTEM‐EDS), X‐ray photoelectron spectroscopy (XPS) and energy‐dispersive X‐ray fluorescence (EDXRF) were applied for the characterization of the prepared material. The photocatalytic oxidation of benzyl alcohol into benzaldehyde under sun‐imitating Xenon lamp irradiation was applied as a test reaction. The photocatalyst prepared by the sonophotodeposition method showed better results, in terms of alcohol conversion and yield of benzaldehyde, in comparison with the photocatalyst prepared by an ordinary wet‐impregnation method.

Photocatalytic Oxidation of Benzyl Alcohols by Three-Dimensional Reduced Graphene Oxide/Nano Zinc Oxide Catalyst under Visible Light

A three-dimensional reduced graphene oxide/nano zinc oxide (3D-RGO/ZnO) composite photocatalyst has been prepared through an in situ reduction of GO and growth of nano ZnO particles process. The 3D-RGO/ZnO catalyst displayed enhanced photocatalytic activity towards commercial ZnO nanoparticles and can be applied to the oxidation of a broad series of benzyl alcohols including primary and secondary alcohols to corresponding carbonyl compounds under the irradiation of visible light. A plausible photocatalytic mechanism referring to photo electron generation and transfer has been proposed.

Photocatalytic Oxidation of Benzyl Alcohol by Homogeneous CuCl2/Solvent: A Model System to Explore the Role of Molecular Oxygen

The oxidation of alcohols to the corresponding carbonyl compounds is a pivotal reaction in organic synthesis. Under visible light irradiation, the homogeneous CuCl2 and cheap solvent oxidized benzyl alcohol into benzaldehyde with a selectivity higher than 95% using molecular oxygen as an oxidant. The formation of a visible light responsive complex between Cu(II) and solvent is responsible for the occurrence of the oxidation of benzyl alcohol. During the photocatalytic process, molecular oxygen was not incorporated into the final benzaldehyde and only involved in the oxidation of Cu(I) into Cu(II) in which it served as a terminal hydrogen acceptor to form H2O. A similar role of molecular oxygen has also been observed in the heterogeneous TiO2 photocatalytic system. The understanding of the role of molecular oxygen helps us to further design new classes of synthetic organic reactions by photocatalytic processes.

Effectively photocatalytic aerobic oxidation of benzyl alcohol catalyzed by spinel Co–Ni ferrite under visible light irradiation

Magnetic ferrite Co x Ni1−x Fe2O4 (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) nanocrystalline complex oxides were prepared by sol–gel auto-combustion method and well characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, diffuse reflectance UV–visible spectroscopy and N2 adsorption–desorption analysis. The catalytic performances of the as-prepared samples were evaluated in photocatalytic oxidation of benzyl alcohol using molecular oxygen as oxidant under visible light irradiation. The results have shown that the catalysts exhibit remarkably high activities in the photocatalytic oxidation of benzyl alcohol. In particular, 18.9 % of benzyl alcohol conversion and 90.1 % of selectivity for benzaldehyde were obtained when the sample Co0.3Ni0.7Fe2O4 was used as catalyst under optimum conditions. The catalyst could be magnetically separated easily for reuse, and no obvious loss of activity was observed when reused in six consecutive runs. Spinel Co–Ni ferrites were prepared by a facile, mild and environment-friendly route. The catalysts exhibit remarkably high activities in the photocatalytic oxidation of benzyl alcohol under visible light irradiation. The catalyst can be magnetically separated easily and has shown prominent recyclability.

Multifunctional radical-doped polyoxometalate-based host-guest material: photochromism and photocatalytic activity.

An effective strategy to synthesize multifunctional materials is the incorporation of functional organic moieties and metal oxide clusters via self-assembly. A rare multifunctional radical-doped zinc-based host-guest crystalline material was synthesized with a fast-responsive reversible ultraviolet visible light photochromism, photocontrolled tunable luminescence, and highly selective photocatalytic oxidation of benzylic alcohols as a result of blending of distinctively different functional components, naphthalenediimide tectons, and polyoxometalates (POMs). It is highly unique to link π-electron-deficient organic tectons and POMs by unusual POMs anion-π interactions, which are not only conducive to keeping the independence of each component but also effectively promoting the charge transfer or exchange among the components to realize the fast-responsive photochromism, photocontrolled tunable luminescence, and photocatalytic activity.

Layered C3N3S3 Polymer/Graphene Hybrids as Metal-Free Catalysts for Selective Photocatalytic Oxidation of Benzylic Alcohols under Visible Light

Metal-free carbon-based hybrids composed of reduced graphene oxide (rGO) and C3N3S3 polymers with a layered “sandwich” structure were fabricated by an in situ two-step polymerization strategy and used as visible light photocatalysts for selective aerobic oxygenation of benzylic alcohols to the corresponding aldehydes, at good efficiency and high selectivity. This work shows the great potential and prospective application of the metal-free C3N3S3 polymer in the photocatalyzed synthesis of fine chemicals.

Synthesis of Au@ZIF-8 single- or multi-core-shell structures for photocatalysis.

We explored the use of PVP-Au NPs as nucleation seeds for zeolitic imidazolate framework-8 (ZIF-8) to selectively synthesize Au@ZIF-8 single- or multi-core-shell structures by epitaxial growth or coalescence of nuclei. Photocatalytic oxidation of benzyl alcohol was studied based on LSPR-induced light absorption.

Monolayer HNb3O8 for Selective Photocatalytic Oxidation of Benzylic Alcohols with Visible Light Response

AbstractMonolayer HNb3O8 2D nanosheets have been used as highly chemoselective and active photocatalysts for the selective oxidation of alcohols. The nanosheets exhibit improved photocatalytic activity over their layered counterparts. Results of in situ FTIR, DRS, ESR, and DFT calculations show the formation of surface complexes between the Lewis acid sites on HNb3O8 2D nanosheets and alcohols. These complexes play a key role in the photocatalytic activity of the material. Furthermore, the unique structural features of the nanosheets contributed to their high photocatalytic activity. An electron transition from the coordinated alcohol species to surface Nb atoms takes place and initiates the aerobic oxidation of alcohols with high product selectivity under visible light irradiation. This reaction process is distinct from that of classic semiconductor photocatalysis.