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Catalytic oxidation
Catalytic oxidation
Catalytic oxidation
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Catalytic oxidation are processes that rely on catalysts to introduce oxygen into organic and inorganic compounds. Many applications, including the focus of this article, involve oxidation by oxygen. Such processes are conducted on a large scale for the remediation of pollutants, production of valuable chemicals, and the production of energy.[1]

Oxidations of organic compounds

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Carboxylic acids, ketones, epoxides, and alcohols are often obtained by partial oxidation of alkanes and alkenes with dioxygen. These intermediates are essential to the production of consumer goods. Partial oxidation is challenging because the most favored reaction between oxygen and hydrocarbons is combustion.

Oxidations of inorganic compounds

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Sulfuric acid is produced from sulfur trioxide which is obtained by oxidation of sulfur dioxide. Food-grade phosphates are generated via oxidation of white phosphorus. Carbon monoxide in automobile exhaust is converted to carbon dioxide in catalytic converters.

Examples

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Industrially important examples include both inorganic and organic substrates.

Substrate Process Catalyst Product Application
sulfur dioxide contact process vanadium pentoxide
(heterogeneous)		 sulfuric acid fertilizer production
ammonia Ostwald process platinum
(heterogeneous)		 nitric acid basic chemicals, TNT
hydrogen sulfide Claus process vanadium pentoxide
(heterogeneous)		 sulfur remediation of byproduct of
oil refinery
methane,
ammonia		 Andrussow process platinum
(heterogeneous)		 hydrogen cyanide basic chemicals, gold mining extractant
ethylene epoxidation mixed Ag oxides
(heterogeneous)		 ethylene oxide basic chemicals, surfactants
cyclohexane K-A process Co and Mn salts
(homogeneous)		 cyclohexanol
cyclohexanone		 nylon precursor
ethylene Wacker process Pd and Cu salts
(homogeneous)		 acetaldehyde basic chemicals
para-xylene terephthalic acid synthesis Mn and Co salts
(homogeneous)		 terephthalic acid plastic precursor
propylene allylic oxidation Mo-oxides
(heterogeneous)		 acrylic acid plastic precursor
propylene,
ammonia		 SOHIO process Bi-Mo-oxides
(heterogeneous)		 acrylonitrile plastic precursor
methanol Formox process Fe-Mo-oxides
(heterogeneous)		 formaldehyde basic chemicals, alkyd resins
butane Maleic anhydride process vanadium phosphates
(heterogeneous)		 maleic anhydride plastics, alkyd resins

Catalysts

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Oxidation catalysis is conducted by both heterogeneous catalysis and homogeneous catalysis. In the heterogeneous processes, gaseous substrate and oxygen (or air) are passed over solid catalysts. Typical catalysts are platinum, and redox-active oxides of iron, vanadium, and molybdenum. In many cases, catalysts are modified with a host of additives or promoters that enhance rates or selectivities.

Important homogeneous catalysts for the oxidation of organic compounds are carboxylates of cobalt, iron, and manganese. To confer good solubility in the organic solvent, these catalysts are often derived from naphthenic acids and ethylhexanoic acid, which are highly lipophilic. These catalysts initiate radical chain reactions, autoxidation that produce organic radicals that combine with oxygen to give hydroperoxide intermediates. Generally the selectivity of oxidation is determined by bond energies. For example, benzylic C-H bonds are replaced by oxygen faster than aromatic C-H bonds.[2]

Fine chemicals

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Main article: Asymmetric catalytic oxidation

Many selective oxidation catalysts have been developed for producing fine chemicals of pharmaceutical or academic interest. Nobel Prize–winning examples are the Sharpless epoxidation and the Sharpless dihydroxylation.

Biological catalysis

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Catalytic oxidations are common in biology, especially since aerobic life subsists on energy obtained by oxidation of organic compounds by air. In contrast to the industrial processes, which are optimized for producing chemical compounds, energy-producing biological oxidations are optimized to produce energy. Many metalloenzymes mediate these reactions.

Fuel cells, etc

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Fuel cells rely on oxidation of organic compounds (or hydrogen) using catalysts. Catalytic heaters generate flameless heat from a supply of combustible fuel and oxygen from air as oxidant.

Challenges

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The foremost challenge in catalytic oxidation is the conversion of methane to methanol. Most methane is stranded, i.e. not located near metropolitan areas. Consequently, it is flared (converted to carbon dioxide). One challenge is that methanol is more easily oxidized than is methane.[3]

Catalytic oxidation with oxygen or air is a major application of green chemistry. There are however many oxidations that cannot be achieved so straightforwardly. The conversion of propylene to propylene oxide is typically effected using hydrogen peroxide, not oxygen or air.

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Catalytic oxidation

View on Grokipedia
from Grokipedia
Catalytic oxidation is a chemical process in which a catalyst accelerates the oxidation of organic or inorganic substrates, typically using molecular oxygen or air as the oxidant, to produce oxidized products such as carbonyl compounds, acids, or and water, thereby lowering the and improving reaction efficiency and selectivity compared to non-catalytic routes. This process encompasses both homogeneous systems, where the catalyst is dissolved in the reaction medium, and heterogeneous systems, where it is in a separate phase, often a solid surface, facilitating and adsorption of reactants. Key mechanisms in catalytic oxidation include the Langmuir-Hinshelwood model, where both the substrate and oxidant adsorb onto the catalyst surface before reacting, the Eley-Rideal model, involving reaction between an adsorbed species and a gas-phase molecule, and the Mars-van Krevelen mechanism, which utilizes lattice oxygen from the catalyst in a cycle. These mechanisms are supported by kinetic studies and (DFT) simulations, which reveal how catalyst composition influences reaction pathways and rates. Common catalysts feature transition metals such as , , , and , frequently supported on metal oxides like CeO₂ or Al₂O₃, or in nanostructured forms like single-atom catalysts to maximize surface area and stability. In industrial applications, catalytic oxidation is essential for producing bulk chemicals, including the oxidation of to in the and p-xylene to for . Environmentally, it plays a critical role in control, such as the catalytic conversion of volatile organic compounds (VOCs) and (CO) in exhaust gases to harmless products, and in through processes like catalytic wet oxidation, which degrades refractory pollutants under high-temperature aqueous conditions. Recent advances emphasize sustainable catalysts, including metal-organic frameworks and non-precious metal oxides, to enhance principles by reducing energy use and byproduct formation.

Fundamentals

Definition and Principles

Catalytic oxidation is the process by which a accelerates the rate of an oxidation reaction—the addition of oxygen to a substrate or the removal of electrons or from it—without itself being consumed or undergoing a net change. This catalytic intervention lowers the barrier, enabling reactions to proceed under milder conditions than in uncatalyzed systems. The process is central to numerous chemical transformations, relying on the catalyst's ability to provide an alternative reaction pathway that enhances and often improves selectivity. At its core, catalytic oxidation operates through principles that emphasize the catalyst's role in mediating and activating molecular oxygen. Catalysts, typically transition metals or metal oxides, facilitate the stepwise reduction of triplet O₂ via electron donation, generating reactive intermediates such as (O₂⁻) or (O₂²⁻) that interact more readily with substrates. This oxygen activation mechanism not only drives the oxidation but also imparts selectivity in complex, multi-step reactions by preferentially stabilizing certain transition states or intermediates, thereby directing the outcome toward desired products over side reactions. Thermodynamically, catalytic oxidations are predominantly exothermic, as the formation of stable oxygenated products releases energy due to the strength of new bonds formed. Reaction feasibility is governed by the Gibbs free energy change (ΔG), with spontaneous processes requiring ΔG < 0 under given conditions; this criterion distinguishes viable partial oxidations (e.g., alcohol to aldehyde) from complete oxidations (e.g., to CO₂ and H₂O), where the latter often have more negative ΔG values but require control to prevent over-oxidation. Kinetically, catalysts enhance reaction rates by stabilizing the transition state, thereby reducing the activation energy (E_a) in the Arrhenius equation, which describes the temperature dependence of the rate constant k as: k=Ae−Ea/RTk = A e^{-E_a / RT} Here, A is the pre-exponential factor, R is the gas constant, and T is the absolute temperature; the lowered E_a results in exponentially faster rates compared to uncatalyzed processes. The early recognition of these principles dates to the 19th century, exemplified by the 1868 patenting of the Deacon process for HCl oxidation, marking a foundational industrial milestone in heterogeneous catalysis.

Reaction Mechanisms

Catalytic oxidation reactions proceed through distinct mechanistic pathways that can be broadly classified as homogeneous or heterogeneous, depending on whether the catalyst and reactants are in the same phase or involve solid surfaces. In homogeneous catalysis, typically involving soluble transition metal complexes, the mechanism often begins with the coordination of molecular oxygen to a metal center, forming activated oxygen species that interact with the substrate via inner-sphere electron transfer. This contrasts with heterogeneous catalysis, where reactions occur on solid catalyst surfaces, such as metal oxides. Common mechanisms include the Langmuir-Hinshelwood (LH) model, in which both the substrate and oxidant adsorb onto the catalyst surface before reacting; the Eley-Rideal (ER) model, involving reaction between an adsorbed species and a gas-phase molecule; and the Mars-van Krevelen (MvK) pathway, in which lattice oxygen from the oxide directly oxidizes the substrate, creating oxygen vacancies that are subsequently refilled by gaseous O₂. The MvK mechanism is particularly prevalent in selective oxidations over reducible metal oxides, enabling redox cycling of the catalyst without relying on surface-adsorbed molecular oxygen for the initial oxidation step. The LH and ER models are widely used to describe adsorption-driven kinetics in heterogeneous oxidations, with LH often dominating at high coverage and ER at low pressures. Key steps in these mechanisms universally involve oxygen activation, substrate binding, electron or proton transfer, and catalyst regeneration. Oxygen activation can occur via dissociative adsorption, where the O-O bond breaks to form atomic oxygen species bound to the catalyst (e.g., on metal surfaces like Pt), or associative adsorption, preserving the O-O bond to generate peroxo (η²-O₂²⁻) or superoxo (η¹-O₂⁻) intermediates. In homogeneous systems, this often leads to high-valent metal-oxo species (M=O), while in heterogeneous systems, associative modes may form bridged peroxides on oxide lattices. Substrate binding follows, typically through coordination to the activated oxygen or metal site, facilitating electron transfer that oxidizes the substrate and reduces the catalyst; proton transfer may accompany this in cases involving O-H or N-H bonds. Catalyst regeneration completes the cycle, often by reoxidation with O₂, restoring the initial state. A simplified representation of the catalytic cycle is: \ceCat(ox)+SubH−>Cat(red)+Sub(ox)\ce{Cat(ox) + SubH -> Cat(red) + Sub(ox)} \ceCat(red)+1/2O2−>Cat(ox)\ce{Cat(red) + 1/2 O2 -> Cat(ox)} This cycle highlights the redox nature of the process, with variations depending on the catalysis type. Common reactive intermediates include metal-oxo species, which act as potent oxygen atom donors in both homogeneous and heterogeneous settings; peroxo complexes, featuring intact O-O bonds that enable two-electron transfers; and radical species, such as hydroxyl (•OH) or alkyl radicals, often generated in radical-chain mechanisms or via homolytic O-O cleavage. In homogeneous catalysis, metal-oxo intermediates like those in iron or manganese porphyrins mimic enzymatic oxidations, while heterogeneous systems may involve surface-bound peroxides on titania or vanadia. Radical intermediates are implicated in non-selective pathways but can be controlled for desired outcomes. The choice of mechanism profoundly influences selectivity, as it dictates the nature and availability of activated oxygen, thereby controlling product distribution and minimizing over-oxidation. For instance, the MvK mechanism enhances selectivity in partial oxidations by delivering lattice oxygen in a controlled manner, reducing the risk of unselective gas-phase , whereas associative in homogeneous systems allows fine-tuned oxygen transfer to prevent deep oxidation. The LH and ER mechanisms affect selectivity through adsorption competition and surface coverage. Factors like oxygen coverage and binding strength modulate this, with low O₂ partial pressures favoring selective pathways over total oxidation. Spectroscopic techniques provide critical evidence for these mechanisms by identifying intermediates and tracking changes . (EPR) detects paramagnetic species such as metal-centered radicals or superoxo complexes, revealing their role in oxygen activation and transfer. (XAS), including XANES and EXAFS, probes metal oxidation states and local coordination environments, confirming the formation of high-valent oxo species during ; for example, shifts in the Ru K-edge have evidenced Ru(V)=O intermediates in water oxidation analogs. These methods, often coupled operando, validate mechanistic proposals by correlating spectral features with reaction kinetics.

Organic Applications

Oxidations of Organic Compounds

Catalytic oxidation plays a pivotal role in transforming organic substrates by selectively introducing oxygen functionality, enabling the synthesis of valuable intermediates from readily available feedstocks. Common transformations include the oxidation of primary alcohols to aldehydes or carboxylic acids, secondary alcohols to ketones, alkenes to epoxides or diols, and alkanes to alcohols or ketones. For instance, the Wacker process converts ethylene to acetaldehyde using palladium(II) chloride and copper(II) chloride in aqueous solution with oxygen, achieving high selectivity under mild conditions. These reactions are essential in both laboratory synthesis and industrial production, where control over regioselectivity and functional group tolerance is critical. In , transition metals such as and , often coordinated with ligands like bipyridines or phosphines, facilitate efficient oxygen transfer under mild temperatures and pressures. A representative example is the aerobic oxidation of alcohols, depicted by the equation: R-CH2OH+12O2→[Pd] or [Ru]R-CHO+H2O\text{R-CH}_2\text{OH} + \frac{1}{2}\text{O}_2 \xrightarrow{\text{[Pd] or [Ru]}} \text{R-CHO} + \text{H}_2\text{O}
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