Recent Advances in Polyoxometalate-Catalyzed Reactions

Photocatalysis has been invariably considered as an unselective process (especially in water) for a fairly long period of time, and the investigation on selective photocatalysis has been largely neglected. In recent years, the field of selective photocatalysis is developing rapidly and now extended to several newer applications. This review focuses on the overall strategies which can improve the selectivity of photocatalysis encompassing a wide variety of photocatalysts, and modifications thereof, as well as the related vital processes of industrial significance such as reduction and oxidation of organics, inorganics, and CO2 transformation. Comprehensive and successful strategies for enhancing the selectivity in photocatalysis are abridged to reinvigorate and stimulate future investigations. In addition, nonsemiconductor type photocatalysts, such as Ti–Si molecular sieves and carbon quantum dots (CQDs), are also briefly appraised in view of their special role in special selective photocatalysis, namely e...

Selectivity Enhancement in Heterogeneous Photocatalytic Transformations.

BiFeO3 magnetic nanoparticles (BFO MNPs) were prepared with a sol−gel method and characterized as a catalyst. It was found that BFO MNPs effectively catalyzed the decomposition of H2O2 into •OH radicals, being confirmed with electron spin resonance spin-trapping technique and other radical probing techniques. The strong H2O2-activating ability of BFO MNPs showed promising applications in the oxidative degradation of organic pollutants. When BFO MNPs were used as a heterogeneous Fenton-like catalyst to degrade Rhodamine B, the apparent rate constant for the RhB degradation at 25 °C at pH 5.0 in the BFO MNPs-H2O2 system was evaluated to be 2.89 × 10−2 min−1, being about 20 folds of that obtained with Fe3O4 MNPs as the catalyst under similar conditions. Moreover, BFO MNPs were demonstrated to have excellent stability and reusability. The catalytic mechanism of BFO MNPs was also investigated with Monte Carlo simulations and density functional theory calculations.

Efficient removal of organic pollutants with magnetic nanoscaled BiFeO3 as a reusable heterogeneous fenton-like catalyst.

Advances in selective conversions by heterogeneous photocatalysis.

This review provides the reader with a general overview on heterogeneous photocatalytic oxidation mechanisms in the presence of TiO 2 , with a special address to conversion of aliphatic and aromatic organic species. The aim was to clarify the steps of the photo-oxidation of the various classes of compounds and to relate them with the properties of the catalysts and the experimental conditions used. Reactions carried out to perform complete degradation and photocatalytic partial oxidations have been deeply discussed. Recent isotopic studies highlighted new reaction pathways concerning partial oxidation of alcohols to aldehyde and oxidation of benzene while EPR investigations confirmed that not only the photogenerated hole but also the OH radicals are involved in the oxidation of the substrates.

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Overview on oxidation mechanisms of organic compounds by TiO2 in heterogeneous photocatalysis

Bismuth ferrite (BiFeO3) nanopowder was prepared by the novel combustion technique using sucrose as a fuel. The catalyst was characterized by using XRD, FT-IR, scanning electron microscope (SEM), transmission electron microscope (TEM), and BET surface area measurement and used as the heterogeneous catalyst for the acetylation reaction under solvent-free conditions. Efficient and selective acetylation of various amines, alcohols and phenols was carried out over BiFeO3 nanopowder by using acetic anhydride and/or acetyl chloride as the acetylating agents at room temperature under solvent-free conditions. The method is highly chemoselective – alcoholic hydroxyl group can be protected while phenolic hydroxyl group remains intact and the amine group can be acetylated in the presence of hydroxyl. This method is fast, high yielding, clean, safe, cost effective, compatible with the substrates having other functional groups and very suitable for practical organic synthesis. The catalyst can also be reused for acetylation without loss of catalytic activity.

Bismuth ferrite (BiFeO3) nanopowder prepared by sucrose-assisted combustion method: A novel and reusable heterogeneous catalyst for acetylation of amines, alcohols and phenols under solvent-free conditions

A polyoxometalate–zirconia (POM/ZrO2) nanocomposite was prepared through entrapment of H3PW12O40 polyoxometalate into zirconia matrix by sol–gel technique involving the hydrolysis of zirconium (IV) n-butoxide, Zr(n-OBu)4, as the ZrO2 source. The catalyst was characterized by using XRD, FT-IR, SEM, TEM and surface area measurement and used as an efficient heterogeneous photocatalyst for aerobic oxidation of alcohols. Various primary and secondary benzylic alcohols were oxidized into the corresponding aldehydes and ketones in high yields under O2 atmosphere in the presence of photoexcited POM/ZrO2 nanocomposite. Also, nonactivated aliphatic alcohols were oxidized in good yields albeit after longer reaction times in comparison with benzylic alcohols, so that the selective oxidation of a benzylic OH group in the presence of a non-benzylic OH group could be achieved by the appropriate choice of the reaction time. The photocatalyst has been reused several times, without observable loss of activity and selectivity. Indeed, the photocatalytic activity of the POM/ZrO2 was higher than that of pure POM.

Polyoxometalate–zirconia (POM/ZrO2) nanocomposite prepared by sol–gel process: A green and recyclable photocatalyst for efficient and selective aerobic oxidation of alcohols into aldehydes and ketones

A new Keggin-type heteropoly acid, namely decamolybdodivanadogermanic acid (H6GeMo10V2O40•16H2O), with nanosized particles (5-8 nm), has been synthesized by a hydrothermal method and characterized by elemental analysis, thermogravimetric analysis (TGA), powder X-ray diffractometry (XRD), Fourier-transform infrared spectroscopy (FTIR), UV-Visible spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and potentiometric titration. H6GeMo10V2O40•16H2O revealed high catalytic activity for acetylation of various alcohols and phenols with acetic anhydride at room temperature (298 ± 2 K) and under solvent-free conditions. The catalyst can be easily recovered and used repeatedly for five cycles with a slight loss of activity. The catalytic activity of H6GeMo10V2O40•16H2O was higher than that of other Keggin-type heteropoly acids, such as H3PW12O40, H3PMo12O40 and H4SiW12O40.

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H6GeMo10V2O4016H2O nanoparticles prepared by hydrothermal method: a new and reusable heteropoly acid catalyst for highly efficient acetylation of alcohols and phenols under solvent-free conditions

Masoumeh Zaidi

Papers

Bismuth ferrite (BiFeO3) nanopowder prepared by sucrose-assisted combustion method: A novel and reusable heterogeneous catalyst for acetylation of amines, alcohols and phenols under solvent-free conditions

Polyoxometalate–zirconia (POM/ZrO2) nanocomposite prepared by sol–gel process: A green and recyclable photocatalyst for efficient and selective aerobic oxidation of alcohols into aldehydes and ketones

H6GeMo10V2O4016H2O nanoparticles prepared by hydrothermal method: a new and reusable heteropoly acid catalyst for highly efficient acetylation of alcohols and phenols under solvent-free conditions