Oxidation of Alcohols with Molecular Oxygen on Solid Catalysts

An aerobic oxidation of alcohols into carbonyl synthons using bipyridyl-cinchona based palladium catalyst

We have reported an aerobic oxidation of primary and secondary alcohols to respective aldehydes and ketones using a bipyridyl-cinchona alkaloid based palladium catalytic system (PdAc-5) using oxygen at moderate pressure. The PdAc-5 catalyst was analysed using SEM, EDAX, and XPS analysis. The above catalytic system is used in experiments for different oxidation systems which include different solvents, additives, and bases which are cheap, robust, non-toxic, and commercially available on the industrial bench. The obtained products are quite appreciable in both yield and selectivity (70-85%). In addition, numerous important studies, such as comparisons with various commercial catalysts, solvent systems, mixture of solvents, and catalyst mole%, were conducted using PdAc-5. The synthetic strategy of oxidation of alcohol into carbonyl compounds was well established and all the products were analysed using 1H NMR, 13CNMR and GC-mass analyses.

Oxidative Catalytic Fractionation of Lignocellulosic Biomass under Non-alkaline Conditions

Biomass pretreatment methods are commonly used to isolate carbohydrates from biomass, but they often lead to modification, degradation, and/or low yields of lignin. Catalytic fractionation approaches provide a possible solution to these challenges by separating the polymeric sugar and lignin fractions in the presence of a catalyst that promotes cleavage of the lignin into aromatic monomers. Here, we demonstrate an oxidative fractionation method conducted in the presence of a heterogeneous non-precious-metal Co-N-C catalyst and O2 in acetone as the solvent. The process affords a 15 wt% yield of phenolic products bearing aldehydes (vanillin, syringaldehyde) and carboxylic acids (p-hydroxybenzoic acid, vanillic acid, syringic acid), complementing the alkylated phenols obtained from existing reductive catalytic fractionation methods. The oxygenated aromatics derived from this process have appealing features for use in polymer synthesis and/or biological funneling to value-added products, and the non-alkaline conditions associated with this process support preservation of the cellulose, which remains insoluble at reaction conditions and is recovered as a solid.

Identification of active catalysts for the acceptorless dehydrogenation of alcohols to carbonyls

Acceptorless dehydrogenation into carbonyls and molecular hydrogen is an attractive strategy to valorize (biobased) alcohols. Using 2-octanol dehydrogenation as benchmark reaction in a continuous reactor, a library of metal-supported catalysts is tested to validate the predictive level of catalytic activity for combined DFT and micro-kinetic modeling. Based on a series of transition metals, scaling relations are determined as a function of two descriptors, i.e. the surface binding energies of atomic carbon and oxygen. Then, a volcano-shape relation based on both descriptors is derived, paving the way to further optimization of active catalysts. Evaluation of 294 diluted alloys but also a series of carbides and nitrides with the volcano map identified 12 promising candidates with potentially improved activity for alcohol dehydrogenation, which provides useful guidance for experimental catalyst design. Further screening identifies β-Mo2N and γ-Mo2N exposing mostly (001) and (100) facets as potential candidates for alcohol dehydrogenation.

Oxidative N-Formylation of Secondary Amines Catalyzed by Reusable Bimetallic AuPd-Fe3O4 Nanoparticles

Bimetallic catalysts are gaining attention due to their characteristics of promoting reactivity and selectivity in catalyzed reactions. Herein, a new catalytic N-formylation of secondary amines using AuPd–Fe₃O₄ at room temperature is reported. Methanol was utilized as the formyl source and 1.0 atm of O₂ gas served as an external oxidant. The bimetallic catalyst, consisting of Au and Pd, makes the reaction more efficient than that using each metal separately. In addition, the catalyst can be effectively recycled owing to the Fe₃O₄ support.

Gold Nanoparticles as Efficient Catalysts in Organic Transformations

This review summarizes the utilization of gold nanoparticles as efficient catalysts for a variety of chemical transformations like oxidation, hydrogenation, and coupling reactions as compared to conventional catalytic materials. This review explores the gold nanoparticles-based catalysts for the liquid phase chemo-selective organic transformations which are proving to be evergreen reactions and have importance for industrial applications. Apart from organic transformation reactions, gold nanoparticles have been found to be applicable in removing the atmospheric contaminants and improving the efficiency of the fuel cells by removing the impurities of carbon monoxide.

A Multifunctional Au/CeO2-Mg(OH)2 Catalyst for One-Pot Aerobic Oxidative Esterification of Aldehydes with Alcohols to Alkyl Esters

Au nanoparticles bound to crystalline CeO2 nanograins that were dispersed on the nanoplate-like Mg(OH)2, denoted as Au/CeO2-Mg(OH)2, were developed as the highly active and selective multifunctional heterogeneous catalyst for direct oxidative esterification of aldehydes with alcohols to produce alkyl esters under base-free aerobic conditions using oxygen or air as the green oxidants. Au/CeO2-Mg(OH)2 converted 93.3% of methacrylaldehyde (MACR) to methyl methacrylate (MMA, monomer of poly(methyl methacrylate)) with 98.2% selectivity within 1 h, and was repeatedly used over eight recycle runs without regeneration. The catalyst was extensively applied to other aldehydes and alcohols to produce desirable alkyl esters. Comprehensive characterization analyses revealed that the strong metal-support interaction (SMSI) among the three catalytic components (Au, CeO2, and Mg(OH)2), and the proximity and strong contact between Au/CeO2 and the Mg(OH)2 surface were prominent factors that accelerated the reaction toward a desirable oxidative esterification pathway. During the reaction, MACR was adsorbed on the surface of CeO2-Mg(OH)2, upon which methanol was simultaneously activated for esterifying the adsorbed MACR. Hemiacetal-form intermediate species were subsequently produced and oxidized to MMA on the surface of the electron-rich Au nanoparticles bound to partially reduced CeO2-x with electron-donating properties. The present study provides new insights into the design of SMSI-induced supported-metal-nanoparticles for the development of novel, multifunctional, and heterogeneous catalysts.

Trends in Sustainable Synthesis of Organics by Gold Nanoparticles Embedded in Polymer Matrices

Gold nanoparticles (AuNPs) have emerged in recent decades as attractive and selective catalysts for sustainable organic synthesis. Nanostructured gold is indeed environmentally friendly and benign for human health; at the same time, it is active, under different morphologies, in a large variety of oxidation and reduction reactions of interest for the chemical industry. To stabilize the AuNPs and optimize the chemical environment of the catalytic sites, a wide library of natural and synthetic polymers has been proposed. This review describes the main routes for the preparation of AuNPs supported/embedded in synthetic organic polymers and compares the performances of these catalysts with those of the most popular AuNPs supported onto inorganic materials applied in hydrogenation and oxidation reactions. Some examples of cascade coupling reactions are also discussed where the polymer-supported AuNPs allow for the attainment of remarkable activity and selectivity.

Oxidation of Benzyl Alcohol Using Cobalt Oxide Supported Inside and Outside Hollow Carbon Spheres

Cobalt oxide nanoparticles (6 nm) supported both inside and outside of hollow carbon spheres (HCSs) were synthesized by using two different polymer templates. The oxidation of benzyl alcohol was used as a model reaction to evaluate the catalysts. PXRD studies indicated that the Co oxidation state varied for the different catalysts due to reduction of the Co by the carbon, and a metal oxidation step prior to the benzyl alcohol oxidation enhanced the catalytic activity. The metal loading influenced the catalytic efficiency, and the activity decreased with increasing metal loading, possibly due to pore filling effects. The catalysts showed similar activity and selectivity (to benzaldehyde) whether placed inside or outside the HCS (63 % selectivity at 50 % conversion). No poisoning was observed due to product build up in the HCS.

Hierarchically Porous, Biphasic, and C-Doped TiO2 for Solar Photocatalytic Degradation of Dyes and Selective Oxidation of Benzyl Alcohol

Macroporous TiO2 monoliths were synthesized by self-sustained combustion reactions of molded pellets made up of a mixture of TiCl4 as a precursor, urea as a fuel, ammonium nitrate as an oxidizer, and starch as a binder. The porous TiO2 monoliths were found to be a heterostructure of anatase and rutile phases, in addition to being doped with carbon. Variation in the amount of starch yielded porous monoliths of different anatase-rutile ratios (increasing rutile component from 0 to 40%) but comparable Brunauer-Emmett-Teller (BET) surface area (∼30 m2 g-1). The porous monoliths obtained, where the TiCl4/starch mass ratio was 2.17, exhibit exceptional photocatalytic activity in the degradation of dyes (methylene blue and methyl orange) and selective oxidation of benzyl alcohol to benzaldehyde under natural sunlight. The synergistic combination of high surface area, porous network, lowered band gap due to heterostructured anatase-rutile polymorphs, and the presence of doped carbon renders the macroporous TiO2 an efficient photocatalyst.

Recent Development in the Synthesis and Catalytic Application of Iridacycles

Cyclometallated complexes are well-known and have found many applications. This article provides a short review on the progress made in the synthesis and application to catalysis of cyclometallated half-sandwich Cp*Ir(III) complexes (Cp*: pentamethylcyclopentadienyl) since 2017. Covered in the review are iridacycles featuring conventional C,N chelates and less common metallocene and carbene-derived C,N and C,C ligands. This is followed by an overview of the studies of their applications in catalysis ranging from asymmetric hydrogenation, transfer hydrogenation, hydrosilylation to dehydrogenation.

Bioinspired Cooperative Photobiocatalytic Regeneration of Oxidized Nicotinamide Cofactors for Catalytic Oxidations

Inspired by water-forming NAD(P)H oxidases, a cooperative photobiocatalytic system has been designed to aerobically regenerate the oxidized nicotinamide cofactors. Photocatalysts enable NAD(P)H oxidation with O₂ under visible-light irradiation, producing H₂ O₂ as a byproduct, which is subsequently used as an oxidant by the horseradish peroxidase mediator system (PMS) to oxidize NAD(P)H. The photobiocatalytic system shows a turnover frequency of 8800 min^-1 in the oxidation of NAD(P)H. Photobiocatalytic NAD(P)H oxidation proceeds smoothly at pH 6-9. In addition to natural NAD(P)H, synthetic biomimetics are also good substrates for this regeneration system. Total turnover numbers of up to 180000 are obtained for the cofactor when the photobiocatalytic regeneration system is coupled with dehydrogenase-catalyzed oxidations. It may be a promising protocol to recycle the oxidized cofactors for catalytic oxidations.

Room-Temperature Catalyst Enables Selective Acetone Sensing

Catalytic packed bed filters ahead of gas sensors can drastically improve their selectivity, a key challenge in medical, food and environmental applications. Yet, such filters require high operation temperatures (usually some hundreds °C) impeding their integration into low-power (e.g., battery-driven) devices. Here, we reveal room-temperature catalytic filters that facilitate highly selective acetone sensing, a breath marker for body fat burn monitoring. Varying the Pt content between 0-10 mol% during flame spray pyrolysis resulted in Al2O3 nanoparticles decorated with Pt/PtOx clusters with predominantly 5-6 nm size, as revealed by X-ray diffraction and electron microscopy. Most importantly, Pt contents above 3 mol% removed up to 100 ppm methanol, isoprene and ethanol completely already at 40 °C and high relative humidity, while acetone was mostly preserved, as confirmed by mass spectrometry. When combined with an inexpensive, chemo-resistive sensor of flame-made Si/WO3, acetone was detected with high selectivity (≥225) over these interferants next to H2, CO, form-/acetaldehyde and 2-propanol. Such catalytic filters do not require additional heating anymore, and thus are attractive for integration into mobile health care devices to monitor, for instance, lifestyle changes in gyms, hospitals or at home.

Highly selective gas sensing enabled by filters

Portable and inexpensive gas sensors are essential for the next generation of non-invasive medical diagnostics, smart air quality monitoring & control, human search & rescue and food quality assessment to name a few of their immediate applications. Therein, analyte selectivity in complex gas mixtures like breath or indoor air remains the major challenge. Filters are an effective and versatile, though often unrecognized, route to overcome selectivity issues by exploiting additional properties of target analytes (e.g., molecular size and surface affinity) besides reactivity with the sensing material. This review provides a tutorial for the material engineering of sorption, size-selective and catalytic filters. Of specific interest are high surface area sorbents (e.g., activated carbon, silica gels and porous polymers) with tunable properties, microporous materials (e.g., zeolites and metal-organic frameworks) and heterogeneous catalysts, respectively. Emphasis is placed on material design for targeted gas separation, portable device integration and performance. Finally, research frontiers and opportunities for low-cost gas sensing systems in emerging applications are highlighted.

Light-Driven Alcohol Splitting by Heterogeneous Photocatalysis: Recent Advances, Mechanism and Prospects

Splitting of alcohols into hydrogen and corresponding carbonyl compounds, also called acceptorless alcohol dehydrogenation, is of great significance for both synthetic chemistry and hydrogen production. Light-Driven Alcohol Splitting (LDAS) by heterogeneous photocatalysis is a promising route to achieve such transformations, and it possesses advantages including high selectivity of the carbonyl compounds, extremely mild reaction conditions (room temperature and irradiation of visible light) and easy separation of the photocatalysts from the reaction mixtures. Because a variety of alcohols can be derived from biomass, LDAS can also be regarded as one of the most sustainable approaches for hydrogen production. In this Review, recent advances in the LDAS catalyzed by the heterogeneous photocatalysts are summarized, focusing on the mechanistic insights for the LDAS and aspects that influence the performance of the photocatalysts from viewpoints of metallic co-catalysts, semiconductors, and metal/semiconductor interfaces. In addition, challenges and prospects have been discussed in order to present a complete picture of this field.

Preparation, Functionalization, Modification, and Applications of Nanostructured Gold: A Critical Review

Gold nanoparticles (Au NPs) play a significant role in science and technology because of their unique size, shape, properties and broad range of potential applications. This review focuses on the various approaches employed for the synthesis, modification and functionalization of nanostructured Au. The potential catalytic applications and their enhancement upon modification of Au nanostructures have also been discussed in detail. The present analysis also offers brief summaries of the major Au nanomaterials synthetic procedures, such as hydrothermal, solvothermal, sol-gel, direct oxidation, chemical vapor deposition, sonochemical deposition, electrochemical deposition, microwave and laser pyrolysis. Among the various strategies used for improving the catalytic performance of nanostructured Au, the modification and functionalization of nanostructured Au produced better results. Therefore, various synthesis, modification and functionalization methods employed for better catalytic outcomes of nanostructured Au have been summarized in this review.

Carboxyboronate as a Versatile In Situ CO Surrogate in Palladium-Catalyzed Carbonylative Transformations

The application of carboxy-MIDA-boronate (MIDA=N-methyliminodiacetic acid) as an in situ CO surrogate for various palladium-catalyzed transformations is described. Carboxy-MIDA-boronate was previously shown to be a bench-stable boron-containing building block for the synthesis of borylated heterocycles. The present study demonstrates that, in addition to its utility as a precursor to heterocycle synthesis, carboxy-MIDA-boronate is an excellent in situ CO surrogate that is tolerant of reactive functionalities such as amines, alcohols, and carbon-based nucleophiles. Its wide functional-group compatibility is highlighted in the palladium-catalyzed aminocarbonylation, alkoxycarbonylation, carbonylative Sonogashira coupling, and carbonylative Suzuki-Miyaura coupling of aryl halides. A variety of amides, esters, (hetero)aromatic ynones, and bis(hetero)aryl ketones were synthesized in good-to-excellent yields in a one-pot fashion.