Photocatalytic Selective Oxidation Of Alcohols Research Articles

Tailoring the catalytic sites by regulating photogenerated electron/hole pairs separation spatially for simultaneous selective oxidation of benzyl alcohol and hydrogen evolution

Photocatalytic selective oxidation of alcohols into aldehydes and H2 is a green strategy for obtaining both value-added chemicals and clean energy. Herein, a dual-purpose ZnIn2S4@CdS photocatalyst was designed and constructed for efficient catalyzing benzyl alcohol (BA) into benzaldehyde (BAD) with coupled H2 evolution. To address the deep-rooted problems of pure CdS, such as high recombination of photogenerated carriers and severe photo-corrosion, while also preserving its superiority in H2 production, ZnIn2S4 with a suitable band structure and adequate oxidizing capability was chosen to match CdS by constructing a coupled reaction. As designed, the photoexcited holes (electrons) in the CdS (ZnIn2S4) were spatially separated and transferred to the ZnIn2S4 (CdS) by electrostatic pull from the built-in electric field, leading to expected BAD production (12.1 mmol g−1 h−1) at the ZnIn2S4 site and H2 generation (12.2 mmol g−1 h−1) at the CdS site. This composite photocatalyst also exhibited high photostability due to the reasonable hole transfer from CdS to ZnIn2S4. The experimental results suggest that the photocatalytic transform of BA into BAD on ZnIn2S4@CdS is via a carbon-centered radical mechanism. This work may extend the design of advanced photocatalysts for more chemicals by replacing H2 evolution with N2 fixation or CO2 reduction in the coupled reactions.

Amino-functionalized zirconium and cerium MOFs: Catalysts for visible light induced aerobic oxidation of benzylic alcohols and microwaves assisted N-Alkylation of amines

Herein we report that mixed zirconium-cerium-MOFs with amino-functionalized linkers (2-amino-1,4-benzenedicarboxylate, BDC-NH2, and 4-aminonaphthalene-2,6-dicarboxylate, NDC-NH2) act as effective non-noble-metal-based heterogeneous catalysts, for both the aerobic photooxidation of benzylic alcohols and for the direct microwaves activation N-alkylation of amines with alcohols. The catalytic performance has been studied considering two different approaches: the effect of NH2 functionalized linkers and the effect of hosting CeIII species on Zr-MOFs. Zr- and ZrCe-MOFs resulted efficient visible-light-driven photocatalysts with high stability and considerable recyclability in the photocatalytic selective oxidation of alcohols to aldehydes using molecular oxygen as oxidant. These materials are also effective catalysts for the microwaves-assisted N-alkylation of amines with alcohols yielding selectively the imine (ZrCe-MOF) or the corresponding secondary amine (Ce-MOFs).

Modulating oxygen vacancies on bismuth-molybdate hierarchical hollow microspheres for photocatalytic selective alcohol oxidation with hydrogen peroxide production

Photocatalytic selective oxidation of alcohols into high value-added carbonyl compounds accompanied by producing hydrogen peroxide (H2O2) is undoubtedly a more efficient solar energy conversion strategy with high atom economy. Herein, we have developed an efficient photocatalyst of bismuth-molybdate (Bi2MoO6) hierarchical hollow microspheres with tunable surface oxygen vacancies (OVs) for promoting the photocatalytic selective alcohol oxidation with H2O2 production. The effect of surface OVs on the photocatalytic efficiency is studied systematically by comparing the performance of different photocatalysts. The benzaldehyde and H2O2 production rates over the OV-rich Bi2MoO6 photocatalyst reach up to 1310 and 67.2 μmol g−1 h−1, respectively, which are 2.3 and 4.0 times those generated from the OV-poor Bi2MoO6 hollow microspheres. The roles of various active radicals in the photocatalytic reaction are probed by a series of controlled experiments and in situ ESR measurements, revealing that both superoxide radical (•O2-) and carbon-centered radical are the key active intermediates. The introduction of surface OVs on Bi2MoO6 hollow microspheres accelerates the separation and transfer of photo-generated charge carriers as well as enhances the adsorption and activation of reactant molecules, thereby greatly promoting the photocatalytic selective oxidation of alcohols along with H2O2 production. This work not only demonstrates a facile strategy for the preparation of high-efficiency photocatalysts by simultaneous modulations of morphology and surface defects, but also offers insight into developing the dual-functional photocatalytic reactions for the full utilizations of photoinduced electrons and holes.

TiO2/Ti3C2 as an efficient photocatalyst for selective oxidation of benzyl alcohol to benzaldehyde

Photocatalytic selective oxidation of alcohols into corresponding aldehydes has received enormous attention. However, it remains a great challenge to explore photocatalysts with high conversion efficiency and selectivity. Herein, we developed a composite consisted of TiO2 and Ti3C2 through a simple calcination of Ti3C2 nanosheets. It was found that Ti3C2 could stabilize the oxygen vacancies and Ti3+ species in the TiO2/Ti3C2 composite, which could subsequent promote the production of active intermediates during the photocatalytic oxidation of benzyl alcohol (BA) to benzaldehyde (BAD). Additionally, the valence band of TiO2 in the composites was found to be up-shifted, which would avoid the further oxidation of BAD and enhance selectivity. As a result, the optimal composite exhibits the BA conversion efficiency of 97 % and BAD selectivity of 98 %, which is 2.8 and 1.2 folds higher than that of pristine TiO2. Our work may provide insights for designing efficient photocatalysts for selective oxidation of alcohols.

Hydrogen reduction treatment of boron carbon nitrides for photocatalytic selective oxidation of alcohols

Light-driven selective oxidation of aromatic alcohols allows a sustainable and eco-friendly manner to convert solar energy into highly valuable aromatic aldehydes. However, the low separation and transfer efficiency of photogenerated charge carriers and the sluggish reaction kinetics seriously restrict the efficiency of the organic photosynthesis of desired compounds. Herein, a facile strategy is adopted to regulate the boron carbon nitrides (BCN) semiconductors by hydrogen reduction to precisely tune the structural and surface properties. The reduced BCN materials can effectively enhance charge separation and migration as well as promote O2 activation and mass transfers. As a result, this BCN catalyst therefore displays a remarkable enhancement in photosynthesis of aromatic aldehydes from the alcohols with high conversion and selectivity compared to pristine BCN.

Nature-mimic fabricated polydopamine/MIL-53(Fe): efficient visible-light responsive photocatalysts for the selective oxidation of alcohols

PDA/MIL-53(Fe) nanocomposites prepared via a nature-mimicking method were efficient catalysts towards the photocatalytic selective oxidation of alcohols to aldehydes or ketones by a direct holeoxidation process under visible-light illuminated conditions.

Bi2WO6-x nanosheets with tunable Bi quantum dots and oxygen vacancies for photocatalytic selective oxidation of alcohols

Herein, Bi2WO6-x nanosheets with tunable Bi quantum dots and oxygen vacancies have been synthesized by a facile hydrothermal treatment combining in-situ redox reaction route, which exhibited the highly enhanced activity for selective oxidation of aromatic alcohols to carbonyl compounds in aqueous medium under visible-light irradiation for the first time. Results revealed that the highly enhanced activity of this photocatalyst was mainly attributed to the synergetic effect of Bi quantum dots and oxygen vacancies, which resulted in the narrowed energy band gap by inducing the formation of defect state level below the conduction band, the enhanced light-harvesting ability and the improved separation efficiency of photocharges. Meanwhile, this photocatalyst also displayed excellent durability owing to the strong interaction between Bi quantum dots and robust Bi2WO6-x nanosheets against significant deactivation caused by either the leaching of Bi quantum dots or the structural collapse of Bi2WO6-x nanosheets, showing its good potential in practical applications.

Toward a Comprehensive Understanding of Enhanced Photocatalytic Activity of the Bimetallic PdAu/TiO2 Catalyst for Selective Oxidation of Methanol to Methyl Formate.

Photocatalytic selective oxidation of alcohols over titania supported with bimetallic nanoparticles represents an energy efficient and sustainable route for the synthesis of esters. Specifically, the bimetallic PdAu/TiO2 system was found to be highly active and selective toward photocatalytic production of methyl formate (MF) from gas-phase methanol. In the current paper, we applied the electronic structure density functional theory method to understand the mechanistic aspects and corroborate our recent experimental measurements for the photocatalytic selective oxidation of methanol to MF over the PdAu/TiO2 catalyst. Our theoretical results revealed the preferential segregation of Pd atoms from initially mixed PdAu nanoclusters to the interface of PdAu/TiO2 and subsequent formation of a unique structure, resembling a core@shell architecture in close proximity to the interface. The analysis of the calculated band gap diagram provides an explanation of the superior electron-hole separation capability of PdAu nanoparticles deposited onto the anatase surface and hence the remarkably enhanced photocatalytic activity, in comparison to their monometallic counterparts. We demonstrated that facile dissociation of molecular oxygen at the triple-point boundary site gives rise to in situ oxidation of Pd. The in situ formed PdO/TiO2 is responsible for total oxidation of methanol to CO2 (no MF formation) in the gas phase. Our investigation provides theoretical guidance for designing highly selective and active bimetallic nanoparticles-TiO2 catalysts for the photocatalytic selective oxidation of methanol to MF.

Visible-light-induced aerobic oxidation of alcohols in a green catalytic system of carbonate-like species doped TiO2

Although all kinds of the promising catalytic approaches have been developed in recent years, pursuing a complete green catalytic system with high efficiency for the production of various aldehydes is still an interesting challenge. Moreover, the low chemo-selectivity or chemical deactivation for particular primary aldehydes with complicated substituent groups, however, is an inevitable bottleneck for achieving industrial application. Here, we construct a new green pathway in which carbonate-like species doped TiO2 (C/T) can function as a photocatalyst to exhibit excellent chemoselectivity and stability toward aerobic oxidation of structurally diverse and dysfunctional alcohols (20 examples) with dimethyl carbonate as a green reaction solvent. The control experiments with various species scavengers reveal the role of the active species in the photocatalytic system, and the ESR results further confirm O2− as the dominate oxidative species for the reaction. Through that the plausible reaction mechanism for the photocatalytic selective oxidation of alcohols over the C/T catalysts is proposed.

Synthesis of Titanate Nanotube–CdS Nanocomposites with Enhanced Visible Light Photocatalytic Activity

CdS-1D titanate nanotubes (CdS/TNTs) nanocomposites have been synthesized via a facile one-step in situ hydrothermal method. The structure and properties of CdS/TNTs nanocomposites have been characterized by X-ray diffraction, UV-vis diffuse reflectance spectra, transmission electron microscopy, photoluminescence spectra, nitrogen adsorption-desorption, and electron spin resonance spectra. The results show that (i) as compared to blank-CdS, it is found that the morphology of CdS in the CdS/TNTs nanocomposites can be finely tuned by TNTs formed during the one-step in situ hydrothermal process; and (ii) the CdS/TNTs nanocomposites exhibit remarkably much higher visible light photocatalytic activity than both blank-CdS and blank-TNT toward aerobic selective oxidation of alcohols under mild conditions. Three integrative factors lead to such a drastic photoactivity enhancement for CdS/TNTs nanocomposites. The first one is the different morphology of CdS in the CdS/TNTs nanocomposites from blank-CdS. The second one is the prolonged lifetime of photogenerated electron-hole pairs from CdS in CdS/TNTs nanocomposites under visible light irradiation. The third one is the higher surface area and adsorption capacity of CdS/TNTs nanocomposites than blank-CdS. In addition, the possible reaction mechanism for photocatalytic selective oxidation of alcohols over CdS/TNTs nanocomposites has also been investigated using the radical scavengers technique. It is hoped that this work could promote further interest in fabrication of various 1D TNT-based composite materials and their application to visible-light-driven photocatalytic selective organic transformations.

Ordered mesoporous TiO2with exposed (001) facets and enhanced activity in photocatalytic selective oxidation of alcohols

Single-crystal TiO2 with 2D and 3D ordered mesoporous structures and exposed (001) facets were synthesized by SBA-15 or KIT-6 templated alcoholysis of TiOSO4 under solvothermal conditions. In the liquid phase photocatalytic selective oxidation of alcohols to aldehydes under UV-light irradiation, the single-crystal TiO2 exhibited much higher activity than the polycrystalline TiO2, since the high crystallization degree favored the transfer of photoelectrons, which might reduce their rate of recombination with holes. Meanwhile, the TiO2 with ordered mesopores showed higher activity than that with disordered mesopores, and the 3D ordered mesoporous channels were more favorable for photocatalytic oxidation than the 2D ordered mesoporous channels, which could be attributed to both the enhanced light harvesting resulting from the multiple light reflections in the pore channels, and the facilitated diffusion and adsorption of the reactant molecules. Also, the exposed (001) facets favored the photocatalytic oxidation due to its high surface energy and increased oxygen vacancies, which reduced the photocharge recombination rate by capturing photoelectrons.

Multifunctional NH2-mediated zirconium metal–organic framework as an efficient visible-light-driven photocatalyst for selective oxidation of alcohols and reduction of aqueous Cr(vi)

Metal-organic frameworks (MOFs) have been arousing a great interest owing to their unique physicochemical properties. In this work, Zr-benzenedicarboxylate (UiO-66) and its derivative, Zr-2-NH2-benzenedicarboxylate (UiO-66(NH2)), are successfully prepared via a facile solvothermal method and applied to photocatalytic reactions. Powder X-ray diffraction (XRD) confirms the isoreticular nature of UiO-66 and UiO-66(NH2), while Fourier transformed infrared spectra (FTIR) prove the effective presence of amino group. UV-vis diffuse reflectance spectra (DRS) show the photoabsorption edge of UiO-66 could be shifted to the visible light region by simply introducing the amino group (-NH2) on the organic ligand. Importantly, UiO-66(NH2) is proved to perform as an efficient multifunctional visible-light-driven photocatalyst with high stability and considerable recyclability in both the photocatalytic selective oxidation of alcohols to aldehydes using molecular oxygen as oxidant and catalytic reduction of aqueous Cr(VI) to Cr(III) under ambient conditions. Furthermore, the possible reaction mechanism has also been investigated in detail. This work makes a systematic attempt to understand the reaction of photocatalytic selective oxidation of alcohols over MOFs and represents the first example to report the identification of MOFs as promising visible-light photocatalysts toward reduction of aqueous Cr(VI). More significantly, our finding also provides a new way to design MOFs-based photocatalysts, that is, by tuning the predesigned ligands with specific functional groups, the optical absorption properties of MOFs can be flexibly modulated, and then the effective solar energy conversion can be expected.

One-pot, high-yield synthesis of one-dimensional ZnO nanorods with well-defined morphology as a highly selective photocatalyst

One-dimensional (1D) semiconductor ZnO nanorods have been successfully prepared via a facile surfactant-free hydrothermal method. Their structure and properties have been characterized by a series of techniques, including X-ray diffraction (XRD), UV-vis diffuse reflectance spectra (DRS), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), nitrogen adsorption–desorption, photoluminescence spectra (PL) and electron spin resonance spectra (ESR). It is found that ZnO nanorods with diameters around 20–30 nm and lengths up to several micrometers are well-distributed. ZnO nanorods have been used to photocatalyze the aerobic oxidation of alcohols to aldehydes under ambient conditions. The results show that ZnO nanorods exhibit a markedly enhanced photoactivity and selectivity compared to their precursor, commercial ZnO, which can be ascribed to the special 1D morphology of ZnO nanorods leading to its favorable, selective adsorptivity toward the reactant of alcohols instead of the product of aldehydes, and the efficient separation of photogenerated charge carriers. A possible reaction mechanism for the photocatalytic selective oxidation of alcohols over ZnO nanorods has also been proposed. Our present work not only provides a facile, high-yield and surfactant-free approach for the fabrication of ZnO nanorods, but also enriches the chemistry of morphology-dependent functional properties of ZnO materials in the application of heterogeneous photocatalytic selective oxidation, which therefore could open a new doorway to utilize 1D semiconductor materials such as nanorods, nanotubes and nanowires as photocatalysts in selective transformation under ambient conditions.