Hydrogen peroxide (H 2 O 2 ) is a very pale blue liquid, slightly more viscous than water, that appears colorless in dilute solution. It is a weak acid, has strong oxidizing properties, and is a powerful bleaching agent. It is used as a disinfectant, antiseptic, oxidizer, and in rocketry as a propellant. The oxidizing capacity of hydrogen peroxide is so strong that it is considered a highly reactive oxygen species.
Hydrogen peroxide is naturally produced in organisms as a byproduct of oxygen metabolism. Nearly all living things possess enzymes known as peroxidases, which harmlessly and catalytically decompose low concentrations of hydrogen peroxide to water and oxygen.
Structure and properties
As with all molecules, the physical properties of hydrogen peroxide are the result of its molecular mass, structure and distribution of atoms within the molecule.
The preferred molecular structure of any molecule is the configuration which has the lowest internal stress. For hydrogen peroxide, there are two basic structural forms (conformers) available for the molecule. While flat shape of the anti conformer would minimize steric repulsions, the 90° torsion angle of the syn conformer would optimize mixing between the filled p-type orbital of the oxygen (one of the lone pairs) and the LUMO of the vicinal O-H bond.
The resulting anticlinal "skewed" shape is a compromise between the two conformers.
Despite the fact that the O-O bond is a single bond, the molecule has a remarkably high barrier to complete rotation of 29.45 kJ/mol (compared with 12.5 kJ/mol for the rotational barrier of ethane). The increased barrier is attributed to repulsion between one lone pair and other lone pairs. The bond angles are affected by hydrogen bonding, which is relevant to the structural difference between gaseous and crystalline forms; indeed a wide range of values is seen in crystals containing molecular H 2 O 2 .
Analogues of hydrogen peroxide include the chemically identical deuterium peroxide and malodorous hydrogen disulfide. Hydrogen disulfide has a boiling point of only 70.7°C despite having a higher molecular weight, indicating that hydrogen bonding increases the boiling point of hydrogen peroxide.
Aqueous hydrogen peroxide solutions have specific properties that are different from those of the pure chemical due to hydrogen bonding between water and hydrogen peroxide molecules. Specifically, hydrogen peroxide and water form a eutectic mixture, exhibiting freezing-point depression. While pure water melts and freezes at approximately 273K, and pure hydrogen peroxide just 0.4K below that, a 50% (by volume) solution melts and freezes at 221 K.
History
Hydrogen peroxide was first isolated in 1818 by Louis Jacques Thénard by reacting barium peroxide with nitric acid. An improved version of this process used hydrochloric acid, followed by sulfuric acid to precipitate the barium sulfate byproduct. Thénard's process was used from the end of the 19th century until the middle of the 20th century. Modern production methods are discussed below.
For a long time it was believed that pure hydrogen peroxide was unstable, because attempts to separate the hydrogen peroxide from the water, which is present during synthesis, failed. This was because traces of solids and heavy metal ions led to a catalytic decomposition or explosions of the hydrogen peroxide. 100% pure hydrogen peroxide was first obtained through vacuum distillation by Richard Wolffenstein in 1894. At the end of 19th century, Petre Melikishvili and his pupil L. Pizarjevski showed that of the many proposed formulas of hydrogen peroxide, the correct one was H-O-O-H.
The use of H 2 O 2 sterilization in biological safety cabinets and barrier isolators is a popular alternative to ethylene oxide (EtO) as a safer, more efficient decontamination method. H 2 O 2 has long been widely used in the pharmaceutical industry. In aerospace research, H 2 O 2 is used to sterilize satellites.
The FDA has recently granted 510(k) clearance to use H 2 O 2 in individual medical device manufacturing applications. EtO criteria outlined in ANSI/AAMI/ISO 14937 may be used as a validation guideline. Sanyo was the first manufacturer to use the H 2 O 2 process in situ in a cell culture incubator, which is a faster and more efficient cell culture sterilization process.
Manufacture
Formerly inorganic processes were used, employing the electrolysis of an aqueous solution of sulfuric acid or acidic ammonium bisulfate (NH 4 HSO 4 ), followed by hydrolysis of the peroxodisulfate ((SO 4 ) 2 ) 2− which is formed.
However, today hydrogen peroxide is manufactured almost exclusively by the autoxidation of a 2-alkyl anthrahydroquinone (or 2-alkyl-9,10-dihydroxyanthracene) to the corresponding 2-alkyl anthraquinone. Major producers commonly use either the 2-ethyl or the 2-amyl derivative. The cyclic reaction depicted below, shows the 2-ethyl derivative, where 2-ethyl-9,10-dihydroxyanthracene (C 16 H 14 O 2 ), is oxidized to the corresponding 2-ethylanthraquinone (C 16 H 12 O 2 ) and hydrogen peroxide. Most commercial processes achieve this by bubbling compressed air through a solution of the anthracene, whereby the oxygen present in the air reacts with the labile hydrogen atoms (of the hydroxy group) giving hydrogen peroxide and regenerating the anthraquinone. Hydrogen peroxide is then extracted out and the anthraquinone derivative reduced back to the dihydroxy (anthracene) compound using hydrogen gas in the presence of a metal catalyst. The cycle then repeats itself.
This process is known as the Riedl-Pfleiderer process, having been first discovered by them in 1936. The overall equation for the process is deceptively simple:
It is important to point out that the economics of the process depend heavily on effective recycling of the quinone (which is very expensive) and extraction solvents, and of the hydrogenation catalyst.
In 1994, world production of H 2 O 2 was around 1.9 million tonnes and grew to 2.2 million in 2006, most of which was at a concentration of 70% or less. In that year bulk 30% H 2 O 2 sold for around US $0.54 per kg, equivalent to US $1.50 per kg (US $0.68 per lb) on a "100% basis".
New developments
A new, so called "high productivity/high yield" process, based on an optimized distribution of isomers of 2-amyl anthraquinone has been developed by Solvay. In July 2008, this process allowed the construction of a "mega" scale single-train plant in Zandvliet (Belgium). The plant has an annual production capacity more than two times that of the world's next largest single train plant. An even larger plant is scheduled to come onstream at Map Ta Phut (Thailand) in 2011. It can be imagined that this leads to reduction in the cost of production due to economies of scale.
A process to produce hydrogen peroxide directly from the elements has been of interest to producers for many years. The problem with the direct synthesis process is that thermodynamically, the reaction of hydrogen with oxygen favors production of water. It had been recognized for some time that a finely dispersed catalyst was beneficial in promoting selectivity to hydrogen peroxide but while selectivity was improved it was still not sufficiently high to permit commercial development of the process. However, an apparent breakthrough was made in the early 2000s by researchers at Headwaters Technology. The breakthrough revolves around development of a minute (nanometer sized) phase-controlled noble metal crystal particles on carbon support. This apparently led, in a joint venture with Evonik Industries, to the construction of a pilot plant in Germany in late 2005. The pilot plant trials to test the commercial feasibility of the process are presumably ongoing since little has been revealed about the results or progress of the operation. It is claimed that there are reductions in investment cost because the process is simpler and involves less equipment; however, the process is also more corrosive and unproven. It should be noted that this process results in low concentrations of hydrogen peroxide (about 5–10 wt% versus about 40 wt% through the anthraquione process), and is therefore only suitable for "over the fence" applications.
In 2009, another catalyst development was announced by workers at Cardiff University. This development also relates to the direct synthesis, but in this case, specifically using gold–palladium nanoparticles. Normally the direct synthesis must be carried out in an acid medium to prevent immediate decomposition of the hydrogen peroxide once it is formed. While hydrogen peroxide has a tendency to decompose on its own (which is why, even after production, it is often necessary to add stabilisers to the commercial product when it is to be transported or stored for long periods), the nature of the catalyst can cause this decomposition to accelerate rapidly. It is claimed that the use of this gold-palladium catalyst re
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