Aerobic Oxidation Of Alcohols Research Articles

Deposition-Precipitation Synthesized CuCo/Activated Carbon Catalysts for Benzyl Alcohol Oxidation Reaction

The effect of Cox addition in the range of 0.1 to 1 mole into Cu supported on activated carbon (AC) catalysts fabricated by deposition-precipitation method was assessed. CuCox/AC prepared via the same method under different calcination temperature was done to evaluate the effect on the properties and activity of the catalysts. The bimetallic Cu-Co catalysts are structurally characterized by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). It was found that Co was clearly incorporated in Cu/AC lattice and improved the crystalline structure with optimum Co loading. CuCox/AC nanoparticles formed with uniform shapes and size under the present synthesis conditions. The catalytic performances of the as-prepared catalysts were evaluated towards the aerobic oxidation of benzyl alcohol using hydrogen peroxide as oxidant. Cu/AC, Co/AC and CuCox/AC showed ability to oxidize benzyl alcohol to benzaldehyde. CuCo0.2/AC calcined at 450°C showed highest catalytic activity exhibiting 86% conversion of benzyl alcohol due to the high crystallinity as compared to the monometallic counterpart. It was found that the crystallinity of the catalysts could be tuned by varying the amount of Co. The synergistic effects of Cu-Co in optimum mole ratio and calcination temperature influenced the availability of active sites participating in the catalytic activity.

Three‐Dimensional Radical Covalent Organic Frameworks as Highly Efficient and Stable Catalysts for Selective Oxidation of Alcohols

AbstractWith excellent designability, large accessible inner surface, and high chemical stability, covalent organic frameworks (COFs) are promising candidates as metal‐free heterogeneous catalysts. Here, we report two 3D radical‐based COFs (JUC‐565 and JUC‐566) in which radical moieties (TEMPO) are uniformly decorated on the channel walls via a bottom‐up approach. Based on grafted functional groups and suitable regular channels, these materials open up the application of COFs as highly efficient and selective metal‐free redox catalysts in aerobic oxidation of alcohols to relevant aldehydes or ketones with outstanding turn over frequency (TOF) up to 132 h−1, which has exceeded other TEMPO‐modified catalytic materials tested under similar conditions. These stable COF‐based catalysts could be easily recovered and reused for multiple runs. This study promotes potential applications of 3D functional COFs anchored with stable radicals in organic synthesis and material science.

Unraveling the Mechanism of Aerobic Alcohol Oxidation by a Cu/pytl-β-Cyclodextrin/TEMPO Catalytic System under Air in Neat Water.

The mechanism for the oxidation of p-tolylmethanol to p-tolualdehyde catalyzed by a Cu/pytl-β-cyclodextrin/TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidinyl-1-oxy) catalytic system under air in neat water is fully investigated by density functional theory (DFT). Four possible pathways (paths A → D) are presented. The calculated TOF = 0.67 h-1 for path A is consistent with the experimental TOF = 1.9 h-1 but much lower than that for path D (TOF = 1.1 × 105 h-1). The results demonstrate that path A is the dominant pathway under the optimal experimental conditions, even though path D is more kinetically favorable. This is because the concentration of precatalyst 11 [(pytl-β-CD)CuII(OH)] in path D is too low to start path D, so p-tolylmethanol oxidation can only proceed via path A. This finding implies that the relative concentration of precatalysts in a one-pot synthesis experiment plays a vital role in the aerobic alcohol oxidation reaction. Based on this finding, we speculate that the direct use of the presynthesized precatalyst 11 or addition of an appropriate amount of NaOH to the reaction solution, but with the total amount of the base added unchanged, is a good way to improve its catalytic activity. Meanwhile, the solvent water was not found to directly participate in the catalytic active sites for the oxidation of alcohols but rather inhibited it by forming the hydrogen-bonded network.

Heteroarylation of Ethers, Amides, and Alcohols with Light and O2.

An efficient protocol for the Cα-H heteroarylation of ethers, amides, and alcohols using air and light under mild conditions is described. The reaction is applicable to a wide spectrum of functional groups. The generation of C-radicals via photoinduced aerobic oxidation of ethers, amides, and alcohols is the key feature of the process. Control experiments suggest a radical pathway for the reaction.

Recyclable Pd‐based polysaccharide catalyst for aerobic oxidation of benzyl alcohol

AbstractPalladium acetate that was heterogenized together with triphenylphosphine trisulfonate ligand on renewable polysaccharides was employed in the aerobic oxidation of benzyl alcohol in three representative solvents—hexane, ethyl acetate, and ethanol. Solvent polarity was found to affect the reaction rate, as both the solubility of benzyl alcohol and oxygen depend on the nature of the solvent. The conversion rate was decreased in the order of hexane > ethanol > ethyl acetate (conversion rates in the presence of the heterogenized iota‐based catalyst after 24 h of reaction were 38, 20, and 15%, respectively). In addition, the nature of the polysaccharide, that is, the type and numbers of functional groups on its backbone, also affected the reaction rate, where xanthan gum yielded the highest conversion rate of 58% in hexane. Finally, though hexane was found to be the preferred solvent for the heterogeneous system, catalytic performance in hexane decreased during recycling while in ethanol it increased due to the formation of palladium nanoparticles. To reveal the structural changes undergone by the catalyst during the reaction, several techniques were used, including energy dispersive X‐ray spectrometry‐scanning electronic microscopy, X‐ray photoelectron spectroscopy, Transition electronic microscopy, and thermal gravimetric analysis. This simple and straightforward heterogenization procedure can be also used for immobilization of different other metal complexes in variety of organic reactions.

Microgels as Soluble Scaffolds for the Preparation of Noble Metal Nanoparticles Supported on Nanostructured Metal Oxides

An approach for the preparation of noble metal nanoparticles supported on nanostructured metal oxides is described herein. The approach is based on the sequential generation of the noble metal nanoparticles and of a metal oxide phase inside a cross-linked polymer colloid (microgel). By tuning the properties of the employed microgel, the nature and amount of both the noble metal nanoparticles and the metal oxide phase can be independently varied. The resulting composite colloids are colloidally stable and, upon isolation by precipitation and subsequent calcination, produce noble metal nanoparticles dispersed on a crystalline, nanostructured oxide phase. Preliminary catalytic tests provide information on the accessibility of the noble metal nanoparticles and, particularly in the case of gold, result in promising catalytic performances in the aerobic oxidation of alcohols.

3D printed catalytic reactors for aerobic selective oxidation of benzyl alcohol into benzaldehyde in continuous multiphase flow

In this work, novel, patterned monolithic reactors were devised to explore more efficient routes for reactant conversion in order to investigate their potential to replace the packed bed and batch reactors conventionally employed in chemical industries. Well-defined bimetallic formulations were developed to substitute platinum group metals and critical raw materials such as palladium and cobalt, at least in part, by less active, but more sustainable and cost-effective metals such as earth-abundant iron. FePd and FeCo based monoliths were 3D printed and stacked in a continuous flow tubular reactor for testing the selective oxidation of benzyl alcohol (BA) into benzaldehyde (BZ) under mild conditions (80–100 °C and atmospheric pressure). The novel monolithic reactors were evaluated against current state-of-the-art reactor technologies, conventional packed bed and batch reactors. The FeCo- and FePd-Al2O3-supported monolithic catalyst beds showed higher conversion and TOF than their packed bed counterparts under the same operating conditions, revealing the impact of the novel design on both regular geometry and composition. What is of particular interest in the catalytic measurements shown is that the combined stacking of two monoliths in a flow reactor, Al2O3-supported Fe and GO-supported FePd catalysts, can significantly improve the performance with an increase in TOF of up to 90% in comparison to their FePd analogues. Mathematical modelling was used to obtain additional insights into the physical and chemical processes governing the rate of BA conversion. It was found that due to the flow regime inside the microchannels, an axial dispersion model was appropriate, which allowed for mapping the concentration profiles of the reactants and products within the respective monolith geometries.

Steering accessible oxygen vacancies for alcohol oxidation over defective Nb2O5 under visible light illumination

We provide a distinct understanding on the role of oxygen vacancies (OVs) for visible light-triggered aerobic oxidation of alcohols via comparative studies on three kinds of Nb2O5, i.e., undoped, Ti-doped and Ce-doped Nb2O5. With benzyl alcohol and 5-hydroxymethylfurfural as representative alcohol substrates, we show that fast alcohol oxidation with high selectivity can be achieved on these doped Nb2O5 containing abundant OVs. However, the functionality of OVs is dependent on the properties of dopants as Ce-doped Nb2O5 exhibits a better performance than Ti-doped one. Investigations indicate that the Ce dopants incur little distortion to the structure of Nb2O5 and induce more accessible OVs than Ti dopants for the dissociative chemisorption of alcohol molecules which supports favorable interfacial electron migration process. The present work not only reveals the difference of oxygen vacancies caused by varied metal doping but provides a unique insight in the relation between OVs structure and photocatalytic alcohol oxidation.

Benzyl alcohol to benzaldehyde oxidation on MnOx clusters: Unraveling atomistic features

• Catalytic oxidation of benzyl alcohol to benzaldehyde over MnO x catalysts is analyzed in the DFT framework. • Oxidation reactivity and selectivity over cluster-sized Mn 4 O 8 and Mn 4 O 9 catalytic models is compared. • Catalyst poisoning is addressed by investigating the formation of benzoic acid following that of benzaldehyde. • Reaction steps and the related oxidation pathways are collected and analyzed by an original kinetic approach. • Experimental strategies aimed at decreasing catalyst deactivation hence at increasing selectivity are proposed. The catalytic oxidation of benzyl alcohol with O 2 is a promising option for the production of benzaldehyde, from both environmental and economical viewpoints. In particular, highly dispersed MnO x systems feature good activity and selectivity in a wide range of temperatures, although deactivation phenomena by over-oxidation and/or poisoning of active sites are generally recorded. On this account, a density functional theory study was performed on cluster-sized catalyst models, namely Mn 4 O 8 and over-oxygenated Mn 4 O 9 fragments, to predict the reactivity pattern of MnO x catalysts in the selective aerobic oxidation of benzyl alcohol. Several pathways concur to determine the whole reaction process and all of them were compared to unveil the atomistic details of the alcohol oxidation mechanism. Moreover, assuming that the consecutive formation of benzoic acid affects the activity-stability pattern of the MnO x based catalyst, also the benzaldehyde oxidation mechanism was computationally addressed. A systematic comparison of the benzyl alcohol and benzaldehyde oxidation mechanisms on the Mn 4 O 8 and Mn 4 O 9 fragments reveals some experimental strategies to test the reaction mechanisms and design alternative catalytic routes to decrease undesired parasitic reactions leading to catalyst deactivation. The matching structural, energetic and kinetic data are published in the Data in Brief journal [1].

Cobalt-Based Quasi-Metal–Organic Framework as a Tandem Catalyst for Room-Temperature Open-Air One-Pot Synthesis of Imines

The catalytic activity of MOFs derives in a large extent from defects. To generate these defects, a cobalt-based metal–organic framework TMU-10, [Co6(oba)5(OH)2(H2O)2(DMF)4]n·2DMF has been subjected in the present study to controlled thermal treatment under air at different temperatures ranging from 100 to 700 °C. This treatment produces the removal of ligands, generation of structural defects, and additional porosity in an extent that depends on the conditions of the thermal treatment. The resulting defective materials, denoted according to the literature as quasi-MOFs, were subsequently employed as heterogeneous tandem catalysts in the one-pot synthesis of N-benzylideneaniline from aniline and benzyl alcohol in open air as a terminal oxidant at 60 °C under solvent-, base-, and dehydrating agent-free conditions. The quasi-TMU-10 framework obtained at 300 °C (QT-300) can efficiently promote imine synthesis within 2 h, forming water as the only byproduct. Unsaturated cobalt sites and the presence of micro- and mesopores in QT-300 are responsible for this excellent catalytic performance as tandem catalysts. The influence on QT-300 catalytic activity of catalyst amount, reactant ratio, and reaction temperature was investigated, as well as the stability and recyclability of the catalyst. The one-pot imine formation promoted by QT-300 follows alcohol aerobic oxidation and subsequent anaerobic condensation of the aldehyde intermediate with aniline.

Cerium-photocatalyzed aerobic oxidation of benzylic alcohols to aldehydes and ketones.

We have developed a cerium-photocatalyzed aerobic oxidation of primary and secondary benzylic alcohols to aldehydes and ketones using inexpensive CeCl3·7H2O as photocatalyst and air oxygen as the terminal oxidant.

Accelerating Selective Oxidation of Biomass-Based Hydroxyl Compounds with Hydrogen Bond Acceptors.

Hydrogen-bonding-initiated self-association makes the valorization of biomass-based hydroxyl compounds a formidable challenge at high concentration. Apart from enhancing the dehydration reaction of hydroxyl compounds with the noncovalent medium effects, insights into how these effects can be exploited to optimize the oxidative reactivity of concentrated hydroxyl compounds remain unclear. Herein, we elucidate that deaggregation of hydroxyl groups with a catalytic number of hydrogen bond acceptors is essential in improving the reactivity of the aerobic oxidation of biomass-based neat aromatic alcohols over the vanadium-based catalyst. The neat 5-hydroxymethylfurfural (HMF) deaggregated with 25 mol % N,N-dimethylformamide (DMF) shows a >7-fold increase in reactivity to produce corresponding aldehydes with excellent selectivity, in stark contrast to the contrary deactivation of reaction in excessive DMF.

Homogeneous CuCl2/TMEDA/TEMPO-Catalyzed chemoselective base- and halogen- free aerobic oxidation of primary alcohols in mild conditions

This article describes the developing of a base- and halogen- free homogeneous system aiming to chemoselectively oxidize allyl, furyl, aryl and heteroaryl primary alcohols. The current easy-to-handle aerobic system uses few amounts of CuCl2/TMEDA/TEMPO system under mild reaction conditions to produce aldehydes in high yields. Moreover, the CuCl2/TMEDA cyclic voltammetry was measured for the first time, disclosing that TMEDA as ligand substantially affects the redox potential (E1/2) of the couple E1/2Cu2+/Cu+ to E1/2Cu2+/Cu+-TMEDA by 454 mV in the redox system.

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).

Iron(III) Nitrate/TEMPO-Catalyzed Aerobic Alcohol Oxidation: Distinguishing between Serial versus Integrated Redox Cooperativity.

Aerobic alcohol oxidations catalyzed by transition metal salts and aminoxyls are prominent examples of cooperative catalysis. Cu/aminoxyl catalysts have been studied previously and feature "integrated cooperativity", in which CuII and the aminoxyl participate together to mediate alcohol oxidation. Here we investigate a complementary Fe/aminoxyl catalyst system and provide evidence for "serial cooperativity", involving a redox cascade wherein the alcohol is oxidized by an in situ-generated oxoammonium species, which is directly detected in the catalytic reaction mixture by cyclic step chronoamperometry. The mechanistic difference between the Cu- and Fe-based catalysts arises from the use iron(III) nitrate, which initiates a NOx-based redox cycle for oxidation of aminoxyl/hydroxylamine to oxoammonium. The different mechanisms for the Cu- and Fe-based catalyst systems are manifested in different alcohol oxidation chemoselectivity and functional group compatibility.

Chemically functionalized MIL-101-NH2 with cobalt(II) tetrasulfophthalocyanine: an efficient catalyst for the aerobic oxidation of alcohols and one-pot tandem conversion of alcohols to propargylamines

In this investigation, MIL-101-NH2 chemically functionalized with cobalt(II) tetrasulfophthalocyanine (CoTSPc) through a superior electrostatic interaction reaction. The configuration of the synthesized composite was characterized using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermal gravimetric analysis (TGA), energy-dispersive X-ray spectroscopy (EDS), scanning electron microscope (SEM), and inductively coupled plasma optical emission spectrometer (ICP-OES). This spongy composite showed excellent catalytic activity in the aerobic oxidation reaction of primary and secondary benzylic alcohols. The generated catalyst's behavior was examined to synthesize propargylamines via tandem oxidative A3 coupling process to illustrate the maximum potential of the synthesized catalyst. Utilizing air as an oxidant, operational simplicity, easy preparation, apt yielding, non‐hazardous nature of the catalyst, and reusability are the essential advantages of this catalyst.

Solvent-free aerobic selective oxidation of benzyl alcohol catalyzed by palladium nanoparticles supported on nitrogen-containing ordered mesoporous carbon

A series of nitrogen-containing ordered mesoporous carbon (NOMC) materials have been prepared by a one-pot soft-templating method under an aqueous solution by adopting various nitrogen-containing precursors. The synthesized NOMC materials were utilized as catalyst supports to load palladium (Pd). The ordered mesostructures of NOMC have been reserved after the loading of Pd, and Pd nanoparticles were well dispersed on the surface of NOMC materials. The analytic results of XPS confirmed that the supported Pd species existed mainly in the form of metallic Pd (Pd0) and there was a probable interaction between Pd and nitrogen species of NOMC. In the solvent-free aerobic oxidation of benzyl alcohol, the Pd/NOMC materials demonstrated high catalytic activities, affording a maximum yield of benzaldehyde up to 95% at 95 °C of reaction temperature.

Selective aerobic oxidation of benzylic and allylic alcohols catalyzed by Cu(OAc)2/TEMPO/Et2NH

Selective oxidation of benzylic and allylic alcohols to their corresponding aldehyde/ketone derivative without affecting saturated alcohols is still a challenging endeavor in organic synthesis. Various metal complexes, especially the copper complexes in the presence of TEMPO are being used very often for such transformations under aerobic conditions, but they are not selective to allylic and benzylic alcohols. The use of copper salt for oxidation of alcohols in the absence of a ligand are very scarcely studied except for the one catalyzed by CuCl/TEMPO where chloride inhibition and lack of selective oxidation have been noted upon use of CuCl 2 . Herein we report a Cu(OAc) 2 catalyzed and TEMPO mediated selective aerobic oxidation of benzylic and allylic alcohols to aldehyde/ketone in the presence of Et 2 NH. The method avoids pre-synthesis of the catalyst as in the case of Cu(II)/(I) complexes/TEMPO catalyzed oxidation reactions, requires low catalyst loading, employs cheaper copper salt, and gives excellent selectivity for oxidation of benzylic and allylic alcohols.

A meso-scale ultrasonic milli-reactor enables gas–liquid-solid photocatalytic reactions in flow

The handling of solid reagents, catalysts and by-products is a daunting challenge in continuous-flow micro- and milli-reactors. Suspensions tend to settle over time leading to irrevocable clogging of the reaction channels. Herein, we describe our efforts to develop an ultrasonic milli-reactor which can handle such challenging solid-containing transformations. The reactor consists of a Langevin-type transducer, a sonotrode and an irradiating cylinder, on which a coiled glass capillary (12.88 mL) was attached. The ultrasonic milli-reactor was combined with an LED illuminating box and its efficacy was showcased in the photocatalytic aerobic oxidation of benzyl alcohol enabled by TiO2 particles exposed to UV-A irradiation. Ultrasound irradiation generates cavitation bubbles and causes a vigorous oscillation of both the cavitation and the Taylor bubbles. This improves the liquid mixing, the gas–liquid mass transfer and ensures resuspension of the settled particles. Moreover, these effects enhance the photon absorption by the semiconductor catalyst, which has an overall positive effect on the photocatalytic transformation.

Highly dispersive Pd nanoparticles decorated strontium niobate nanosheets: Efficient and recyclable catalyst for base-free aerobic oxidation of benzyl alcohols in water

2D perovskite Sr2Nb3O10 (SNO) nanosheets were synthesized by cation exchange-assisted liquid exfoliation and applied as supports for the immobilization of palladium (Pd) nanoparticles through the electrostatic assembly with Pd(NH3)42+ precursors and successive in-situ photoreduction. Pd nanoparticles with an average diameter of 9.22 nm are uniformly dispersed on the surface of strontium niobate nanosheets. The obtained nanohybrid Pd-SNO could serve as an efficient heterogeneous catalyst for the atmospheric pressure oxidation of benzyl alcohol to benzaldehyde by using air as an oxidizing agent and water as the sole solvent at low temperature (80 °C). Furthermore, the catalyst could be easily separated from the reaction mixture by simple centrifugation and reused five times without significant degradation in catalytic activity under the investigated conditions. We believe that the synthetic strategy described in this work is a feasible means of developing effective noble metal/layered transition metal oxide hybrid catalysts for organic reactions.