Aerobic Oxidation Of Alcohols Research Articles

Selective aerobic oxidation of benzyl alcohol under ambient conditions using polyionic nanoclay-supported gold/palladium nanoparticles

Oxidation of alcohols to their respective aldehydes, esters, and acids is a common value-added industrial process, though techniques often rely on harsh reaction conditions (e.g., high temperature, strong base, organic solvents, hydrothermal processes) to achieve rapid reaction turnovers. The incorporation of gold and palladium nanoparticles facilitates the dehydrogenation and O2 reduction processes, respectively, and these advantages can be dually incorporated and enhanced using supported nanoparticle alloys. Herein, we utilize polyionic nanoclays (PINCs) as two-dimensional inorganic-organic architectures for supporting monometallic and bimetallic gold and palladium nanocatalysts for the model oxidation of benzyl alcohol to the corresponding aldehyde, ester, and acid. In addition, we present two new PINC surface chemistries (N-methylpyrrolidinium- and pyridinium-functionalized PINCs) and compare their performances as catalyst supports to show that catalyst enhancement is altered by the PINC surface chemistry. Reaction conditions are mild (aqueous, room temperature, open to atmosphere, weak or no base) yet reaction turnovers are comparable to those of more demanding literature accounts.

Visible-light Aerobic Oxidation of Benzyl Alcohol over Porphyrin-sensitized Titanium Dioxide Photocatalysts

Visible-light Aerobic Oxidation of Benzyl Alcohol over Porphyrin-sensitized Titanium Dioxide Photocatalysts

Facile synthesis of CoSi alloy catalysts with rich vacancies for base- and solvent-free aerobic oxidation of aromatic alcohols

The rational design and green synthesis of low-cost, robust, and efficient catalysts for the selective oxidation of various alcohols are highly challenging. Herein, we report a fast and solvent-free arc-melting (AM) method to controllably synthesize a semimetallic CoSi alloy catalyst (denoted as AM-CoSi) that is efficient in the base- and solvent-free oxidation of six types of aromatic alcohols. X-ray absorption fine structure analysis, electron paramagnetic resonance spectroscopy, and aberration-corrected high angle annular dark field scanning transmission electron microscopy confirmed the successful synthesis of AM-CoSi with abundant Si vacancies (Siv). The as-prepared CoSi alloy catalysts exhibit an order of magnitude greater activity in the oxidation of a model reactant, benzyl alcohol (BAL) to benzyl benzoate (BBE), compared with their mono-counterparts, and provide 70% yield of BBE, the highest yield reported to date. Experimental results and density functional theory calculations suggest that the CoSi alloy structure improves the BAL conversion and that Si vacancy is the main contributor to the generation of BBE, based on which a potential reaction pathway is rationally proposed. Furthermore, the CoSi alloy maintains high stability and also exhibits high activity in the selective oxidation of various alcohols with different functional groups. This work demonstrates for the first time that semimetallic CoSi alloys can be robust catalysts for the green oxidation of various alcohols and proviedes a vast opportunity for the rational design and application of other semimetal alloy catalysts.

Aerobic, Efficient, Mild, and Heterogeneous Oxidation of Benzylic Alcohols Based on α-MnO2/GO Nanocatalyst

Aerobic, Efficient, Mild, and Heterogeneous Oxidation of Benzylic Alcohols Based on α-MnO2/GO Nanocatalyst

Bis(NHC)-Pd(II) and Pd(0) complexes supported on magnetic SBA-15 for the selective aerobic oxidation of benzyl alcohols to benzaldehydes and reduction of nitroarenes

The present study describes the synthesis, characterization, and use of two new supported magnetic Pd-NHC catalysts based on Pd(II) and Pd(0). A composite of magnetite-silica core shells and SBA-15 was successively treated with (3-aminopropyl) triethoxysilane (APTES), cyanuric chloride (CC), imidazole, 2-bromopyridine, and trans-[Pd(Cl)2(SMe2)2] complex. The obtained supported bis(NHC)-Pd(II) complex, Fe3O4@SiO2@SBA-AP-CC-bis(NHC)-Pd(II), was characterized by Fourier transform infrared spectroscopy (FT-IR), X‐ray diffraction spectroscopy (XRD), thermogravimetric analysis (TGA), vibrating sample magnetometry (VSM), energy‐dispersive X‐ray analysis (EDX), and Brunauer-Emmett-Teller surface area measurement (BET). The catalytic potential of the supported bis(NHC)-Pd(II) complex was explored for the selective preparation of benzaldehydes from the corresponding benzyl alcohols. The Pd(0) catalyst, Fe3O4@ SiO2@SBA-AP-CC-bis(NHC)-Pd(0), was obtained via reduction of the Pd(II)-catalyst with NaBH4. The obtained Pd(0)-catalyst efficiently reduced nitroarenes to anilines in the presence of hydrazine hydrate. The Pd contents of the Pd(II)-catalyst (2.83%wt.; 0.266 mmol/g) and Pd(0)-catalyst (2.26%wt., 0.212 mmol/g) were measured by atomic absorption spectroscopy. Both reactions are conducted under optimal reaction conditions of the solvent, the base, and the catalyst dosage. The catalysts enjoy easy magnetic separation, recyclability over eight runs without significant loss activity, and noticeable stability. They showed negligible palladium leaching (0.83% in the oxidation of benzyl alcohol and 0.1% in the reduction of nitroarenes).

Photocatalytic direct oxygen-isotopic labelings of carbonyls in ketones and aldehydes with oxygen-isotopic waters

Oxygen-isotopic labelings play important roles in identifying and understanding chemical and biological processes. Direct C=O to C=18O or C=17O conversion in a single step leading to labeled compounds can alleviate synthetic burdens without the need for resynthesis. Here we describe a photocatalytic oxygen-isotopic labeling protocol that can efficiently and selectively install 18O and 17O on carbonyls of ketones and aldehydes via oxygen isotope exchange with oxygen-isotopic waters (H218O or H217O) as the sources of oxygen isotopes, in which light and oxygen-enabled sodium alkanesulfinates catalyzed this process. This strategy was extended to the in-situ formed ketones from the photocatalytic aerobic oxidation of alkyl arenes and secondary alcohols. Furthermore, reduction of the oxygen-isotopically labeled aldehydes with NaBH4 provided the corresponding oxygen-isotopically labeled primary alcohols. We believe that the oxygen-isotopically labeling method will be widely used in chemistry, biology and medicine fields.

Structure-Based Redesign of a Methanol Oxidase into an "Aryl Alcohol Oxidase" for Enzymatic Synthesis of Aromatic Flavor Compounds.

Alcohol oxidases (AOxs) catalyze the aerobic oxidation of alcohols to the corresponding carbonyl products (aldehydes or ketones), producing only H2O2 as the byproduct. The majority of known AOxs, however, have a strong preference for small, primary alcohols, limiting their broad applicability, e.g., in the food industry. To broaden the product scope of AOxs, we performed structure-guided enzyme engineering of a methanol oxidase from Phanerochaete chrysosporium (PcAOx). The substrate preference was extended from methanol to a broad range of benzylic alcohols by modifying the substrate binding pocket. A mutant (PcAOx-EFMH) with four substitutions exhibited improved catalytic activity toward benzyl alcohols with increased conversion and kcat toward the benzyl alcohol from 11.3 to 88.9% and from 0.5 to 2.6 s-1, respectively. The molecular basis for the change of substrate selectivity was analyzed by molecular simulation.

Steering photocatalytic selectivity of Au/γ-Al2O3 for benzyl alcohol oxidation via direct photoexcitation

Nanometal photocatalysts are widely used for accelerating reactions and adjusting the selectivity of multipath reactions by energetic electrons activating specific adsorbates. However, the effect of excited charge carriers on the adsorption strength of reactant molecules has been overlooked. Herein, we demonstrate that light irradiation alters the selectivity of aerobic oxidation of benzyl alcohol over Au/γ-Al2O3 by changing the main product from benzaldehyde in dark to benzoic acid under light. It was found that the competitive adsorption of benzyl alcohol and benzaldehyde plays a crucial role in selectivity. Energetic electrons arising from ultraviolet excitation are directly injected into the metal-adsorbate antibonding orbital to weaken the adsorption strength of benzyl alcohol. The resulted d band holes facilitate more benzaldehyde adsorbed on the surface of Au nanoparticles (NPs), which are further oxidized to benzoic acid. This work proposes a new understanding of nanometal photocatalysis and a feasible approach to adjust the catalytic performance by optically excited charge carriers modulating the adsorption strength of adsorbates.

Photochemical Selective Oxidation of Benzyl Alcohols to Aldehydes or Ketones.

Using Eosin Y as a metal-free photocatalyst and O2 as an oxidant, the present study reports a new photochemical protocol that enables efficient aerobic oxidation of various benzyl alcohols to the corresponding aldehydes or ketones in excellent yields under mild reaction conditions. The catalyst system presents good functional-group tolerance and exquisite chemoselectivity, which also can easily be scaled-up to gram scale. Moreover, the methodological applications in practical synthesis of several organic molecules and the primary reaction mechanism were also discussed.

Partially Thiolated Au25 Cluster Anchored on Carbon Support via Noncovalent Ligand–Support Interactions: Active and Robust Catalyst for Aerobic Oxidation of Alcohols

Partially thiolated atomically precise Au cluster catalysts were developed by controlled calcination of presynthesized thiolate-protected Au clusters on a porous carbon support. X-ray absorption fine structure analysis, powder X-ray diffraction, and aberration-corrected high-angle annular dark field scanning transmission electron microscopy revealed that calcination of 1 wt % of Au25(SR)18 (RS = 2-phenylethanethiolate, 1-dodecanethiolate) on carbon at 450 °C for 8–12 h produced partially thiolated Au25 clusters Au25(SR)/C without sintering. The Au25(SR)/C catalyst exhibited comparable catalytic activity to that of the thiolate-free Au25 cluster Au25/C for benzyl alcohol oxidation, whereas Au25(SR)/C showed higher durability than Au25/C. These results suggested that the residual RS ligands were distributed at the interface between the clusters and the support and suppressed the diffusion of the clusters by multiple van der Waals interactions. Large kinetic isotope effects indicated that the hydride abstraction from the α-carbon of the alkoxide was the rate-determining step. The O2 pressure dependence on rate constants was comparable to that reported for negatively charged Au clusters stabilized by poly(N-vinyl-2-pyrrolidone). This comparison suggests that electron-rich Au atoms left behind by desorbed RS served as active sites, which was supported by density functional theory calculations on model clusters. The partially thiolated Au25 cluster catalysts oxidized various p-substituted benzyl alcohols and benzylic secondary alcohols, with turn-over numbers exceeding 1 × 104. This work demonstrates that partially ligated, atomically precise metal clusters anchored by noncovalent ligand–support interaction are promising model catalysts with both high activity and durability.

4‐Chloro‐2‐azaadamantane N‐Oxyl (4‐Cl‐AZADO): A Readily Preparable Organocatalyst for NOx Co‐catalyzed Aerobic Alcohol Oxidation

AbstractConcise synthesis and structure‐activity relationships study of 4‐substituted 2‐azaadamantane N‐oxyls reveals that 4‐chloro‐2‐azaadamantane N‐oxyl (4‐Cl‐AZADO) is an efficient organocatalyst toward the aerobic oxidation of alcohols in the presence of NOx as a co‐catalyst. The organocatalyst was easily synthesized from 2‐adamantanone in five steps, and it showed high activity compared with many previously reported nitroxyl radical catalysts.

Insights into the Effect of Metal Ratio on Cooperative Redox Enhancement Effects over Au- and Pd-Mediated Alcohol Oxidation.

The aerobic oxidation of alcohols and aldehydes over supported heterogeneous catalysts can be considered as comprising two complementary and linked processes: dehydrogenation and oxygen reduction. Significant rate enhancements can be observed when these processes are catalyzed by independent active sites, coupled by electron transport between the two catalysts. This effect, termed cooperative redox enhancement (CORE), could significantly influence how researchers approach catalyst design, but a greater understanding of the factors which influence it is required. Herein, we demonstrate that the Au/Pd ratio used in physical mixtures of monometallic catalysts and phase-separated Au and Pd bimetallic catalysts dramatically influences the degree to which CORE effects can promote alcohol oxidation. Perhaps more interestingly, the roles of Au and Pd in this coupled system are determined to be interchangeable. Preliminarily, we hypothesize that this is attributed to the relative rates of the coupled reactions and demonstrate how physical properties can influence this. This deeper understanding of the factors which influence CORE is an important development in bimetallic catalysis.

Promotion of Au nanoparticles on carbon frameworks for alkali-free aerobic oxidation of benzyl alcohol

We synthesized a series of modified Co-ZIF-67 materials with tunable morphology to support fine Au nanoparticles for the alkali-free aerobic oxidation of benzyl alcohol. Structure promotion was performed using Stöber silica as a hard template, which was subsequently removed by NaOH etching before gold immobilization. The texture structure of Au/(Si)C was greatly improved with increasing surface area and volume. CoOx was simultaneously introduced into the carbon shell from the Co-ZIF-67 precursor, which consequently facilitated the specific Au-support interaction via bimetallic synergy. XRD, XPS, and TEM images demonstrated the redispersion of both Au and CoOx as well as the electronic delivery between metals. Analysis of the chemical and surface composition suggested a surface rich in Auδ+ with abundant lattice oxygen contributed by CoOx in the final Au/(Si)C, which improved the transformation rate of benzyl alcohol even in an alkali-free condition. Au/(Si)C with finely dispersed Au particles showed excellent catalytic performance in the alkali-free environment, with 89.3% benzyl conversion and 74.5% benzaldehyde yield under very mild conditions.

Identification of the Optimal Framework for Nitroxyl Radical/Hydroxylamine in Copper-Cocatalyzed Aerobic Alcohol Oxidation

8-Azabicyclo[3.2.1]octan-8-ol (ABOOL) and 7-azabicyclo[2.2.1]heptan-7-ol (ABHOL) are the main homologues of hydroxylamine 2-azaadamantan-2-ol (AZADOL) and 9-azabicyclo[3.3.1]nonan-9-ol. Both homologues feature a small bicyclic backbone and are known to be stable; however, to date, they have not been used as catalysts for alcohol oxidation. Herein, we report that these hydroxylamines can efficiently catalyze the oxidation of various secondary alcohols to their corresponding ketones using molecular oxygen in ambient air as the terminal oxidant and copper cocatalysts at room temperature. Furthermore, we show that ABOOL and ABHOL can be easily synthesized from commercially available materials.

Mechanism-Guided Design of Robust Palladium Catalysts for Selective Aerobic Oxidation of Polyols

The palladium complex [(L1)Pd(μ-OAc)]2[OTf]2 (L1 = neocuproine) is a selective catalyst for the aerobic oxidation of vicinal polyols to α-hydroxyketones, but competitive oxidation of the ligand methyl groups limits the turnover number and necessitates high Pd loadings. Replacement of the neocuproine ligand with 2,2'-biquinoline ligands was investigated as a strategy to improve catalyst performance and explore the relationship between ligand structure and reactivity. Evaluation of [(L2)Pd(μ-OAc)]2[OTf]2 (L2 = 2,2'-biquinoline) as a catalyst for aerobic alcohol oxidation revealed a threefold enhancement in turnover number relative to the neocuproine congener, but a much slower rate. Mechanistic studies indicated that the slow rates observed with L2 were a consequence of precipitation of an insoluble trinuclear palladium species─(L2Pd)3(μ-O)22+─formed during catalysis and characterized by high-resolution electrospray ionization mass spectrometry. Density functional theory was used to predict that a sterically modified biquinoline ligand, L3 = 7,7'-di-tert-butyl-2,2'-biquinoline, would disfavor the formation of the trinuclear (LPd)3(μ-O)22+ species. This design strategy was validated as catalytic aerobic oxidation with [(L3)Pd(μ-OAc)]2[OTf]2 is both robust and rapid, marrying the kinetics of the parent L1-supported system with the high aerobic turnover numbers of the L2-supported system. Changes in ligand structure were also found to modulate regioselectivity in the oxidation of complex glycoside substrates, providing new insights into structure-selectivity relationships with this class of catalysts.

Enhanced Catalytic Activity of TEMPO-Mediated Aerobic Oxidation of Alcohols via Redox-Active Metal-Organic Framework Nodes.

Metal-organic frameworks (MOFs) are outstanding platforms for heterogeneous catalysis due to their tunable pore size, huge surface area, large porosity, and potential active sites. The design and synthesis of MOF/organocatalyst co-catalytic systems have attracted considerable interest owing to their high catalytic activity, low toxicity, and mild reaction conditions. Herein, we reported the synthesis of a bifunctional TEMPO-IsoNTA organocatalyst featuring a pyridyl group as an anchoring site and a TEMPO radical as a catalytic active site. By using the topologically isomorphic structures of MIL-101(Fe) and MIL-101(Cr) as co-catalysts, these MOF/TEMPO-IsoNTA systems enable the efficient aerobic oxidation of various alcohols to their corresponding aldehydes or ketones under mild conditions. Notably, the MIL-101(Fe)/TEMPO-IsoNTA system exhibits superior catalytic activity, thanks to their redox-active FeIII-oxo nodes, which facilitate the regeneration of TEMPO-IsoNTA. Our research not only solves the problem of potential heavy metal contamination in the TEMPO-based homogeneous catalytic system, but also enriches the understanding of synergism of MOFs/organocatalysts.

Photocatalytic aerobic oxidation of benzylic alcohols and concomitant hydrogen peroxide production.

The photochemical oxidation of benzylic alcohols using N-hydroxyphthalimide (NHPI) catalyst, with Rose Bengal as a singlet oxygen photosensitizer, and the production of hydrogen peroxide (H2O2) under metal-free conditions is presented. Computational and experimental investigations support 1O2 as the oxidant that converts NHPI to the active radical intermediate phthalimide-N-oxyl (PINO). This is a green alternative to current methods of H2O2 production.

Copper-catalyzed aerobic oxidation of primary alcohols to carboxylic acids.

Here, the first copper-catalyzed aerobic oxidation of primary alcohols to carboxylic acids with TEMPO and KHSO4 as the co-catalysts has been developed. The reaction exhibits excellent substrate scope and functional group compatibility under mild conditions. Even the very sensitive chiral alcohols, chiral amino alcohols, and alcohol-containing steroid skeletons may be oxidized to afford the corresponding carboxylic acids or lactones without racemization.

Improved catalytic performance by changing surface and textural properties of Ru supported bifunctional periodic mesoporous organosilicas in aerobic oxidation of alcohols

The catalytic activity of several supported catalysts composed of perruthenate anions incorporated into the mesochannels of a series of plugged and unplugged bifunctional periodic mesoporous organosilicas (Ru@BFPMOs) in the oxidation of alcohols were investigated.

Selective aerobic oxidation of biomass model compound veratryl alcohol catalyzed by air-stable copper(ii) complexes in water

Three new copper(ii) complexes were synthesized and successfully utilized as effective catalysts for the selective aerobic oxidation of biomass derived veratryl alcohol to veratraldehyde in water as a green and sustainable reaction medium.