Synthesis of Three-Dimensional-Ordered Mesoporous Cobalt Oxides for Selective Oxidation of Benzyl Alcohol

Radical-driven selective oxidation of benzyl alcohol on MnCoOx catalysts with no oxidant other than air in reactor

Mn reinforced Co3O4 catalysts (MnCoOx) were prepared by a facile solid phase mixed foaming method with an in-situ heating enhancement for the formation of spinel phase mixed oxide species, and studied in the selective oxidation of benzyl alcohol just the air in reactor as oxygen donor. It was found that the MnCoOx catalysts are composed of relatively minimal spinel MnCo2O4 mixed oxide and massive Co3O4 to form MnCo2O4-Co3O4 oxide pair. The micro-domains of MnCo2O4-Co3O4 oxide pair present two redox couples of Mn3+/Mn2+ and Co3+/Co2+ instead of the single one of Co3+/Co2+ in Co3O4, and then dramatically enhance the formation of superoxide radicals (•O2-) species from the O2 in air, which can efficiently initiate the conversion of benzyl alcohol to benzaldehyde in a Fenton-like processes. With no oxidant other than air in reactor, the interaction between MnCo2O4 and Co3O4 in MnCoOx catalysts leads to a benzyl alcohol conversion up to 98 % with a 100 % benzaldehyde selectivity at atmospheric pressure while single component Co3O4 can only present a benzyl alcohol conversion at 37 %. This embodiment of highly efficient heterogeneous selective oxidation just with air as oxidant provides a probability for developing a low-cost and super-facile radical-induced selective oxidation process for alcohols.

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.

Synthesis and Characterization of CoxOy–MnCO3 and CoxOy–Mn2O3 Catalysts: A Comparative Catalytic Assessment Towards the Aerial Oxidation of Various Kinds of Alcohols

CoxOy–manganese carbonate (X%)(CoxOy–MnCO3 catalysts (X = 1–7)) were synthesized via a straightforward co-precipitation strategy followed by calcination at 300 °C. Upon calcination at 500 °C, these were transformed to CoxOy–dimanganese trioxide i.e., (X%)CoxOy–Mn2O3. A relative catalytic evaluation was conducted to compare the catalytic efficiency of the two prepared catalysts for aerial oxidation of benzyl alcohol (BzOH) to benzaldehyde (BzH) using O2 molecule as a clean oxidant without utilizing any additives or alkalis. Amongst the different percentages of doping with CoxOy (0–7% wt./wt.) on MnCO3 support, the (1%)CoxOy–MnCO3 catalyst exhibited the highest catalytic activity. The influence of catalyst loading, calcination temperature, reaction time, and temperature and catalyst dosage was thoroughly assessed to find the optimum conditions of oxidation of benzyl alcohol (BzOH) for getting the highest catalytic efficiency. The (1%)CoxOy–MnCO3 catalyst which calcined at 300 °C displayed the best effectiveness and possessed the largest specific surface area i.e., 108.4 m2/g, which suggested that the calcination process and specific surface area play a vital role in this transformation. A 100% conversion of BzOH along with BzH selectivity >99% was achieved after just 20 min. Notably, the attained specific activity was found to be considerably larger than the previously-reported cobalt-containing catalysts for this transformation. The scope of this oxidation reaction was expanded to various alcohols containing aromatic, aliphatic, allylic, and heterocyclic alcohols without any further oxidation i.e., carboxylic acid formation. The scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and Brunauer–Emmett–Teller (BET) specific surface area analytical techniques were used to characterize the prepared catalysts. The obtained catalyst could be easily regenerated and reused for six consecutive runs without substantial decline in its efficiency.

A Highly Efficient Bifunctional Catalyst CoOx/tri-g-C3N4 for One-Pot Aerobic Oxidation–Knoevenagel Condensation Reaction

A highly efficient bifunctional catalyst of an s-triazine-based carbon-nitride-supported cobalt oxide is developed for the aerobic oxidation–Knoevenagel condensation tandem reaction of benzyl alcohol and malononitrile, whereby 96.4% benzyl alcohol conversion with nearly 100% selectivity towards benzylmalononitrile can be obtained in 6 h at 80 °C. The excellent catalytic performance derives from the high basicity of carbon nitride and strong redox ability of Co species induced by carbon nitride. The catalyst is also quite stable and can be reused without any regeneration treatment, whose product yield is only an 11.5% reduction after four runs.

Designing and Fabricating Ordered Mesoporous Metal Oxides for CO₂ Catalytic Conversion: A Review and Prospect

In the past two decades, great progress has been made in the aspects of fabrication and application of ordered mesoporous metal oxides. Ordered mesoporous metal oxides have attracted more and more attention due to their large surface areas and pore volumes, unblocked pore structure, and good thermal stabilities. Compared with non-porous metal oxides, the most prominent feature is their ability to interact with molecules not only on their outer surface but also on the large internal surfaces of the material, providing more accessible active sites for the reactants. This review carefully describes the characteristics, classification and synthesis of ordered mesoporous metal oxides in detail. Besides, it also summarizes the catalytic application of ordered mesoporous metal oxides in the field of carbon dioxide conversion and resource utilization, which provides prospective viewpoints to reduce the emission of greenhouse gas and the inhibition of global warming. Although the scope of current review is mainly limited to the ordered mesoporous metal oxides and their application in the field of CO₂ catalytic conversion via heterogeneous catalysis processes, we believe that it will provide new insights and viewpoints to the further development of heterogeneous catalytic materials.

Pd Nanoparticles Incorporated Within a Zr-Based Metal–Organic Framework/Reduced Graphene Oxide Multifunctional Composite for Efficient Visible-Light-Promoted Benzyl Alcohol Oxidation

Metal–organic frameworks (MOFs) in photocatalysis oxidation reactions have been arousing great interest because of their unique properties. Zr-based MOFs (mainly 1,4-dicarboxybenzene MOF (UiO-66)) appear to be very attractive candidates. In this study, a Pd@UiO-66/reduced graphene oxide (rGO) nanocomposite was successfully prepared via a facile solvothermal method and was characterised by several techniques, including field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), nitrogen adsorption–desorption isotherms, and photoluminescence (PL) spectroscopy. Subsequently, the as-obtained Pd@UiO-66/rGO composite was used as a photocatalyst for the selective oxidation of benzyl alcohol to benzaldehyde with O2 under visible light irradiation (>420nm); it exhibited superior photocatalytic activity due to the synergistic effect of coupling Pd nanoparticles (NPs) with UiO-66 and rGO. Importantly, the Pd@UiO-66/rGO composite showed high stability and considerable recyclability to preserve most of its initial photocatalytic activity after five cycles of the oxidation reaction.

Facilely fabricating mesoporous nanocrystalline Ce–Zr solid solution supported CuO-based catalysts with advanced low-temperature activity toward CO oxidation

The synergistic effect between CuO and mesoporous Ce–Zr solid solution greatly enhanced the advanced low-temperature catalytic activity toward CO oxidation.