Catalytic oxidation
Catalytic oxidation are processes that oxidize compounds using catalysts. Common applications involve oxidation of organic compounds by the oxygen in air. Such processes are conducted on a large scale for the remediation of pollutants, production of valuable chemicals, and the production of energy. In petrochemistry, high-value intermediates such as carboxylic acids, aldehydes, ketones, epoxides, and alcohols are obtained by partial oxidation of alkanes and alkenes with dioxygen. These intermediates are essential to the production of consumer goods. Partial oxidation presents two challenges. The first is that the most favored reaction between oxygen and hydrocarbons is combustion. The second challenge is the considerable difficulty to activate dioxygen, viz. the splitting of the molecule into its constituent atoms, which has an energy barrier of 498 kJ/mol. The usual strategy to activate oxygen in a controlled manner is to use molecular hydrogen or carbon monoxide as sacrificial reductants in presence of a heterogeneous catalyst, such that the activation barrier is lowered to < 10 kJ/mol and hence milder reaction conditions are required.
One of the most challenging selective oxidations, which has been achieved with supported gold catalysts, is the epoxidation of propylene.
An illustrative catalytic oxidation is the conversion of methanol to the more valuable compound formaldehyde using oxygen in air:
This conversion is very slow in the absence of catalysts. Typical oxidation catalysts are metal oxides and metal carboxylates.
Examples
Industrially important examples include both inorganic and organic substrates.| Substrate | Process | Catalyst | sulfuric acid | fertilizer production |
| ammonia | Ostwald process | platinum | nitric acid | basic chemicals, TNT |
| hydrogen sulfide | Claus process | vanadium pentoxide | sulfur | remediation of byproduct of oil refinery |
| methane, ammonia | Andrussow process | platinum | hydrogen cyanide | basic chemicals, gold mining extractant |
| ethylene | epoxidation | mixed Ag oxides | ethylene oxide | basic chemicals, surfactants |
| cyclohexane | K-A process | Co and Mn salts | cyclohexanol cyclohexanone | nylon precursor |
| ethylene | Wacker process | Pd and Cu salts | acetaldehyde | basic chemicals |
| para-xylene | terephthalic acid synthesis | Mn and Co salts | terephthalic acid | plastic precursor |
| propylene | allylic oxidation | Mo-oxides | acrylic acid | plastic precursor |
| propylene, ammonia | SOHIO process | Bi-Mo-oxides | acrylonitrile | plastic precursor |
| methanol | Formox process | Fe-Mo-oxides | formaldehyde | basic chemicals, alkyd resins |
| butane | Maleic anhydride process | vanadium phosphates | maleic anhydride | plastics, alkyd resins |
Catalysts
Applied catalysis
Oxidation catalysis is conducted by both heterogeneous catalysis and homogeneous catalysis. In the heterogeneous processes, gaseous substrate and oxygen 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.