Structure, preparation, reactions
and uses of hydrogen peroxide;
Hydrogen peroxide, H2O2, was first discovered by Thenard among others in 1818 by reacting acids with barium peroxide, BaO2.
It resembles water in appearance being colourless in small quantities but blue when observed in thick layers.
It decomposes to oxygen and water and this decomposition is promoted by heat and alkalis.
Commercial grade H2O2 usually contains small amounts of stabilizers.
Hydrogen peroxide is a strong oxidising agent and is widely used as a bleaching agent. In dilute solutions it is an efficient antiseptic. The uses of hydrogen peroxide have been changing in recent years.
Wood pulp bleaching - Major user
Hydrogen peroxide is produced by reducing alkylanthraquinone with hydrogen in the presence of a catalyst to the hydroquinone. After the catalyst has been removed to prevent decomposition of the hydrogen peroxide, the hydroquinone is oxidised, usually with air, back to quinone with a resultant co-production of hydrogen peroxide.
The hydrogen peroxide is removed and purified and the quinone is regenerated and returned to the reaction.
The anthraquinone must be dissolved in a suitable solvent for the hydrogenation, oxidation and extraction steps - this is usually referred to as the working solution. The solvent is usually a mixture because quinones dissolve readily in non-polar aromatic solvents, such as alkylbenzene, whereas hydroquinones dissolve well in polar solvents, such as alcohols and esters. A variety of different mixtures are in use but the aim is to satisfy a number of criteria, namely good solubility of both quinone and hydroquinone, good stability in both hydrogenator and oxidiser, low solubility in water and aqueous hydrogen peroxide solutions, sufficiently higher or lower density than water to ensure separation of the two phases during extraction, low volatility, high distribution coefficient for hydrogen peroxide in the solvent-water system and low toxicity. 1
In the hydrogenator, the working solution is reacted with hydrogen in the presence of a catalyst. The process is exothermic and the heat of reaction is removed by cooling the working solution before it enters the hydrogenator, by cooling the reactor during hydrogenation and/or by cooling the hydrogenated working solution.
After the hydrogenation reaction, the working solution must pass through a filtration stage to remove all traces of catalyst. Even small traces of catalyst in the oxidation and extraction stages lead to significant losses of hydrogen peroxide and could present safety problems. During the oxidation stage, air is passed through the hydrogenated working solution to convert the dissolved hydroquinones to quinones and form the hydrogen peroxide. The air outlet is passed over activated carbon adsorbers to recover solvent.
Crude hydrogen peroxide is extracted from the oxidised working solution by treating with water. The working solution is then regenerated and fed back to the front of the process and the crude hydrogen peroxide (15-35 wt%) is fed to a treatment unit where the concentration is increased to 50-70 wt%.