Room temperature selective conversion of aniline to azoxybenzene over an amorphous niobium oxyhydroxide supported on δ-FeOOH

Synthesis of Azoxy Compounds: from Copper Compounds to Mesoporous Silica-Encaged Ultrasmall Copper Catalysts

Azoxy compounds have aroused extensive attention due to their unique biological activities, but the chemical synthesis of these compounds often suffers from limitations due to their requirement for stoichiometric oxidants, high costs, and restricted substrate range. Herein, a series of azoxy compounds were constructed via facile coupling reactions by using cost-effective N-methoxyformamide and nitroso compounds over Cu-based catalysts, affording high product yields with excellent tolerance of functional groups. Significantly, the mesoporous silica nanosphere-encapsulated ultrasmall Cu (Cu@MSN) catalyst was developed via a one-pot synthetic method and first used for the synthesis of azoxy compounds. As compared with copper salt catalysts, the Cu@MSN catalyst exhibited remarkably enhanced catalytic activity and superior recycling stability. Such a Cu@MSN catalyst overcame the inherent drawbacks of low activity, fast deactivation, and difficult recycling of traditional metal salt catalysts in organic reactions. This work provides a green and efficient method for the construction of azoxy compounds and also creates new prospects for the application of nanoporous materials confined metal catalysts in organic synthesis.

Niobium: The Focus on Catalytic Application in the Conversion of Biomass and Biomass Derivatives

The world scenario regarding consumption and demand for products based on fossil fuels has demonstrated the imperative need to develop new technologies capable of using renewable resources. In this context, the use of biomass to obtain chemical intermediates and fuels has emerged as an important area of research in recent years, since it is a renewable source of carbon in great abundance. It has the benefit of not contributing to the additional emission of greenhouse gases since the CO2 released during the energy conversion process is consumed by it through photosynthesis. In the presented review, the authors provide an update of the literature in the field of biomass transformation with the use of niobium-containing catalysts, emphasizing the versatility of niobium compounds for the conversion of different types of biomass.

Low-Valent Tungsten-Catalyzed Controllable Oxidative Dehydrogenative Coupling of Anilines

Herein, we have developed an efficient tungsten-catalyzed homogeneous system for oxidative dehydrogenative coupling of anilines to selectively produce various azoaromatics and azoxyaromatics as well as 2-substituted indolone N-oxides by simply regulating the reaction solvent with peroxide as a terminal oxidant under additive-free conditions. These findings provide an experimental framework for exploring tungsten catalysis in organic synthesis and offer an efficient and convenient tactic for the selective oxidation of anilines.

Catalytic selective oxidation of aromatic amines to azoxy derivatives with an ultralow loading of peroxoniobate salts

Tartaric acid-coordinated peroxoniobate salts demonstrate an exceptionally high TOF value (up to 4435 h−1) even at an ultralow catalyst loading for the oxidation of aromatic amines to azoxy compounds under green and very mild conditions.

Zr(OH)4 Catalyzed Controllable Selective Oxidation of Anilines to Azoxybenzenes, Azobenzenes and Nitrosobenzenes

The selective oxidation of aniline to metastable and valuable azoxybenzene, azobenzene or nitrosobenzene has important practical significance in organic synthesis. However, uncontrollable selectivity and laborious synthesis of the expensive required catalysts severely hinder the uptake of these reactions in industrial settings. Herein, we have pioneered the discovery of Zr(OH) 4 as an efficient heterogeneous catalyst capable of the selective oxidation of aniline, using either peroxide or O 2 as oxidant, to selectively obtain various azoxybenzenes, symmetric/unsymmetric azobenzenes, as well as nitrosobenzenes, by simply regulating the reaction solvent, without the need for additives. Mechanistic experiments and DFT calculations demonstrate that the activation of H 2 O 2 and O 2 is primarily achieved by the bridging hydroxyl and terminal hydroxyl groups of Zr(OH) 4 respectively. The present work provides an economical and environmentally friendly strategy for the selective oxidation of aniline in industrial applications.

Bulk and surface theoretical investigation of Nb-doped δ-FeOOH as a promising bifunctional catalyst

The development of bifunctional catalysts is of great interest in fine chemistry, since they are capable of promoting multicatalytic reactions involved in several important industrial processes. Iron oxyhydroxides have been identified as low-cost bifunctional catalysts. However, their applications are limited due to their weak acid character. Thus, elaborated modifications of these systems can significantly contribute to increasing their activities and selectivity. This work consists in the study, through DFT calculations, of the properties of the bulk and the surface of feroxyhyte (δ-FeOOH) doped with niobium, as a potential bifunctional catalyst. We identified the formation of stronger van der Waals interactions among the doped δ-FeOOH layers, which can increase the thermal stability of the catalyst. In addition, evidence has been found that the insertion of Nb increases Brönsted acidity and gives rise to new Lewis acid sites on the catalyst surface.

New Approach to Dehydration of Xylose to 2-Furfuraldehyde Using a Mesoporous Niobium-Based Catalyst

Furfural chemistry is one of the most promising platforms directly derived from lignocellulose biomass. In this study, a niobium-based catalyst (mNb-bc) was synthesized by a new fast and simple method. This new method uses microemulsion to obtain a catalyst with a high specific surface area (340 m² g^-1), defined mesoporosity, and high acidity (65 μmol g^-1). Scanning electron microscopy revealed that mNb-bc has a rough surface. The mNb-bc was used to catalyze the conversion reaction of xylose into 2-furfuraldehyde in a monophasic system using water as a green solvent. This reaction was investigated using a 2³ experimental design by varying the temperature, time, and catalyst-to-xylose ratio (CXR). The responses evaluated were xylose conversion (X _c), reaction yield (Y), and selectivity to 2-furfuraldehyde (S). The optimized reaction conditions were used to evaluate the reaction kinetics. At milder reaction conditions of 140 °C, 2 h, and a CXR of 10%, mNb-bc led to an X _c value of 41.2%, an S value of 77.1%, and a Y value of 31.8%.

Selective oxidation of aniline into azoxybenzene catalyzed by Nb-peroxo@iron oxides at room temperature

Nb-peroxo@iron oxides show high selectivity and activity in aniline conversion to azoxybenzene.

Low-temperature catalytic oxidation of aniline to azoxybenzene over an Ag/Fe2O3 nanoparticle catalyst using H2O2 as an oxidant

Highly dispersed Ag-supported α-Fe₂O₃ is found to be a recyclable and efficient catalyst for one-pot conversion of aniline to azoxybenzene.