A Perspective on Heterogeneous Catalysts for the Selective Oxidation of Alcohols

Selective oxidation of higher alcohols using heterogeneous catalysts is an important reaction in the synthesis of fine chemicals with added value. Though the process for primary alcohol oxidation is industrially established, there is still a lack of fundamental understanding considering the complexity of the catalysts and their dynamics under reaction conditions, especially when higher alcohols and liquid‐phase reaction media are involved. Additionally, new materials should be developed offering higher activity, selectivity, and stability. This can be achieved by unraveling the structure–performance correlations of these catalysts under reaction conditions. In this regard, researchers are encouraged to develop more advanced characterization techniques to address the complex interplay between the solid surface, the dissolved reactants, and the solvent. In this mini‐review, we report some of the most important approaches taken in the field and give a perspective on how to tackle the complex challenges for different approaches in alcohol oxidation while providing insight into the remaining challenges.

Hydrogen Peroxide as a Green Oxidant for the Selective Catalytic Oxidation of Benzylic and Heterocyclic Alcohols in Different Media: An Overview

Among a plethora of known and established oxidant in organic chemistry, hydrogen peroxide stands in a special position. It is commercially and inexpensively available, highly effective, selective, and more importantly it is compatible with current environmental concerns, dictated by principles of green chemistry. Several chemicals or their intermediates that are important in our daily life such as pharmaceuticals, flavors, fragrances, etc. are products of oxidation of alcohols. In this review, we introduce hydrogen peroxide as an effective, selective, green and privileged oxidant for the catalyzed oxidation of primary and secondary benzylic and heterocyclic alcohols to corresponding carbonyl compounds in different media such as aqueous media, under solvent-free conditions, various organic solvent, and dual-phase system.

Selective Catalytic Oxidation of Benzyl Alcohol by MoO2 Nanoparticles

Selective oxidation of benzyl alcohol to benzaldehyde was carried out with MoO2 nanoparticles (MoO2 NPs). MoO2 NPs were synthesized by two different approaches and characterized by several techniques. The synthesis was done by a hydrothermal procedure using ethylenediamine and either Fe2O3 or hydroquinone. In the latter case, an additional calcination step under N2 was performed to eliminate passivating agents at the surface of the nanoparticles. The synthesized nanocatalysts showed similar catalytic properties, being efficient catalysts in the oxidation of benzyl alcohol. High substrate conversion and product selectivity were achieved under all tested conditions. Studies were conducted using two different oxidants: tert-butyl hydroperoxide and hydrogen peroxide, in our continuous effort to obtain more efficient catalysts for more sustainable catalytic processes. When H2O2 was used as the oxidant, 94% yield was achieved with 100% selectivity for benzaldehyde, which was a very promising result to undergo other studies with this system. Moreover, to elucidate some aspects of the reaction mechanism, a study was conducted, and it was possible to conclude that the reaction undergoes, to some extent, through a radical mechanism with both oxidants.

Effect of Ultrasound on the Green Selective Oxidation of Benzyl Alcohol to Benzaldehyde

Oxidation of alcohols plays an important role in industrial chemistry. Novel green techniques, such as sonochemistry, could be economically interesting by improving industrial synthesis yield. In this paper, we studied the selective oxidation of benzyl alcohol as a model of aromatic alcohol compound under various experimental parameters such as substrate concentration, oxidant nature and concentration, catalyst nature and concentration, temperature, pH, reaction duration, and ultrasound frequency. The influence of each parameter was studied with and without ultrasound to identify the individual sonochemical effect on the transformation. Our main finding was an increase in the yield and selectivity for benzaldehyde under ultrasonic conditions. Hydrogen peroxide and iron sulfate were used as green oxidant and catalyst. Coupled with ultrasound, these conditions increased the benzaldehyde yield by +45% compared to silent conditions. Investigation concerning the transformation mechanism revealed the involvement of radical species.

Counter Anion-Directed Growth of Iron Oxide Nanorods in a Polyol Medium with Efficient Peroxidase-Mimicking Activity for Degradation of Dyes in Contaminated Water

Development of nanozymes, which are nanomaterials with intrinsic enzymatic properties, has emerged as an appealing alternative to the natural enzymes with tremendous application potential from the chemical industry to biomedicine. The self-assembled growth of micrometer-sized oxide materials with controlled nonspherical shapes can be an important tool for enhancing activity as artificial enzymes, as the formation of these superstructures often results in high surface area with favorable impact on catalytic activity. Herein, the growth of rod-shaped Fe3O4 microstructures via a one-pot microwave-based method and using a water-poly(ethylene glycol) mixture as a solvent is reported, without the involvement of external shape-directing agents. The precursor metal salt played a key role in the size, shape, and phase selective evolution of iron oxide micro/nanomaterials. Whereas self-assembled microrod superstructures were obtained using Fe(NO3)3 as the metal salt precursor, use of FeCl3 or Fe-acetate as precursors afforded hollow Fe2O3 microparticles and Fe3O4 nanoparticles, respectively. A graphitic layer was deposited on the Fe3O4 surface, imparting a negative surface charge as a result of a high-temperature treatment of poly(ethylene glycol). The rod-shaped Fe3O4 microcrystals show efficient peroxidase-mimicking activity toward 3,3,5,5'-tetramethylbenzidine and pyrogallol as peroxidase substrates with a Michaelis-Menten rate constant (K m) value of 0.05 and 0.52 mM, respectively. The proficient enzyme mimicking behavior of these magnetic superstructures was further explored for the degradation of organic dyes that includes rhodamine B, methylene blue, and methyl orange with a rate constant (k) of 0.038, 0.011, and 0.007 min-1 respectively, using H2O2. This fast and simple method could help to develop a new pathway for differently shaped oxide nanoparticles in a sustainable and economical manner that can be harnessed as nanozymes for industrial as well as biological applications.

Versatile Sulfathiazole-Functionalized Magnetic Nanoparticles as Catalyst in Oxidation and Alkylation Reactions

Catalyst design and surface modifications of magnetic nanoparticles have become attractive strategies in order to optimize catalyzed organic reactions for industrial applications. In this work, silica-coated magnetic nanoparticles with a core-shell type structure were prepared. The obtained material was successfully functionalized with sulfathiazole groups, which can enhance its catalytic features. The material was fully characterized, using a multi-technique approach. The catalytic performance of the as-synthesized material was evaluated in 1) the oxidation of benzyl alcohol to benzaldehyde and 2) the microwave-assisted alkylation of toluene with benzyl chloride. Remarkable conversion and selectivity were obtained for both reactions and a clear improvement of the catalytic properties was observed in comparison with unmodified γ-Fe2O3/SiO2 and γ-Fe2O3. Noticeably, the catalyst displayed outstanding magnetic characteristics which facilitated its recovery and reusability.

Multifunctional nanozymes: enzyme-like catalytic activity combined with magnetism and surface plasmon resonance

Over decades, as alternatives to natural enzymes, highly-stable and low-cost artificial enzymes have been widely explored for various applications. In the field of artificial enzymes, functional nanomaterials with enzyme-like characteristics, termed as nanozymes, are currently attracting immense attention. Significant progress has been made in nanozyme research due to the exquisite control and impressive development of nanomaterials. Since nanozymes are endowed with unique properties from nanomaterials, an interesting investigation is multifunctionality, which opens up new potential applications for biomedical sensing and sustainable chemistry due to the combination of two or more distinct functions of high-performance nanozymes. To highlight the progress, in this review, we discuss two representative types of multifunctional nanozymes, including iron oxide nanomaterials with magnetic properties and metal nanomaterials with surface plasmon resonance. The applications are also covered to show the great promise of such multifunctional nanozymes. Future challenges and prospects are discussed at the end of this review.

Facile construction of leaf-like WO3 nanoflakes decorated on g-C3N4 towards efficient oxidation of alcohols under mild conditions

Selective oxidation of aryl and alkyl alcohols with H₂O₂ in aqueous media catalyzed by a well-defined WO₃/g-C₃N₄ composite.

Ionothermal synthesis of Fe3O4 magnetic nanoparticles as efficient heterogeneous Fenton-like catalysts for degradation of organic pollutants with H2O2

Fe₃O₄ magnetic nanoparticles (MNPs) are attractive heterogeneous Fenton-like catalysts for oxidative degradation of organic pollutants with H₂O₂. Herein highly efficient and stable Fe₃O₄ MNPs (Fe₃O₄-op-DES, ca. 10nm) were successfully prepared via a novel oxidative precipitation-combined ionothermal synthesis, which comprised oxidative precipitation of FeSO₄·7H₂O in choline chloride:2urea deep eutectic solvent. Among five different Fe₃O₄ particles tested, Fe₃O₄-op-DES MNPs exhibited the highest catalytic activity with the activation energy of 47.6kJmol^-1 for degradation of Rhodamine B (RhB) with H₂O₂ under the same conditions (Fe₃O₄ dosage of 0.50gL^-1, H₂O₂ concentration of 40mmolL^-1, pH 6.4, 55°C, 2h). Fe₃O₄-op-DES MNPs were magnetically recoverable, and had good catalytic stability and recyclability without the need of regeneration (>98% degradation efficiency of RhB in 2h and pseudo-first-order rate constant of 0.0376min^-1 after having been continuously running for 12h). The superior catalytic performance of Fe₃O₄-op-DES MNPs was attributed to the combination of multiple technologically important features, including the nanometer size, high Fe²⁺ content, large surface area, high density of surface active sites and stable crystal structure (no phase transformation, negligible iron leaching and particle aggregation after reaction). The wide applicability of Fe₃O₄-op-DES MNPs was also demonstrated by the degradation of four other organic pollutants.

Effect of TiO2 morphology on the benzyl alcohol oxidation activity of Fe2O3–TiO2 nanomaterials

Three series of Fe loaded (1, 3, 5 and 7 mol% Fe) TiO₂ anatase nanomaterials with different morphologies; nanoparticles, nanotubes and nanofibers were synthesized and used as catalysts for oxidation of benzyl alcohol.

One-step solvothermal synthesis of Al-promoted Fe3O4 magnetic catalysts for the selective oxidation of benzyl alcohol to benzaldehyde with H2O2 in water

The in situ modification of Fe₃O₄ by aluminium introduced new active sites besides the primary iron sites. Their synergistic effect contributes to much better catalytic activity of Al-promoted Fe₃O₄ in the title reaction.