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Formaldehyde can be produced commercially using mild oxidation or dehydrogenation methods with a molybdenum-iron oxide or silver catalyst. The reactions are exothermic and can generate heat for other purposes. The dehydrogenation process produces hydrogen, which is burned to produce steam for the boiler. Other catalysts include copper chromite and palladium acetate.
Most organic compounds, simple or complex, can be produced in several ways. Only the cheapest of these can be used for commercial production. The production of formaldehyde employs one of two catalytic methods involving methanol (CH3OH): mild oxidation or dehydrogenation. The catalyst used can be a mixture of molybdenum and iron oxide, or alternatively silver. A molybdenum catalyst requires a temperature of approximately 480-750°F (250-400°C) to sustain the reaction, while silver requires the much higher temperature of 1200°F (650°C).
It might appear that the production of formaldehyde using a strong oxidizer is a third option. However, such a route is not suitable, as the desired aldehyde itself would risk undergoing oxidation, to form a carboxylic acid, in this case formic acid (HCOOH). One feature that both mild oxidation and dehydrogenation methods share is the need for continuous heat to sustain the process. This may appear to increase the manufacturing cost of formaldehyde production prohibitively high. Both processes are exothermic, however, which means that each reaction gives off heat, making them both self-sufficient.
The use of a molybdenum iron oxide catalyst requires passing a mixture of methanol and vapors mixed with air over the catalyst. Stoichiometrically – or, speaking in terms of quantities of chemical reactants and products – the equation for this is 2 CH3OH + O2 → HCHO + 2 H2O + Δ. The Greek letter “delta” stands for heat. Although some of this heat is used to maintain the reaction process, some of it can be used for other purposes, such as powering plant turbines. Although the oxidative production of formaldehyde is occasionally used, it is less common than the dehydrogenation method.
A common catalyst used in the dehydrogenation process of formaldehyde production is silver, although silver can react, in part, via the oxidative route. As in the case of the oxidation catalyst, molybdenum oxide and iron, methanol vapors are combined with air and passed over the catalyst: the metal itself exists in granular and crystalline form. Both the reactions of molybdenum and silver take place over a steam boiler. The resulting vapors, which contain the formaldehyde product plus unreacted methanol vapors, are then condensed and purified. In the case of the dehydrogenation process, the remaining flue gas comprises hydrogen; the gas is burned to produce steam, which feeds the boiler.
The reaction equation for the dehydrogenation process is CH3OH → HCHO + H2. Further catalysts which can be used instead of silver as a dehydrogenating agent are copper chromite and palladium acetate. Special conditions are needed for its proper functioning. A form of heterogeneous catalyst, palladium acetate acts as a “phase transfer” agent. This means that it behaves much like a detergent, allowing the transfer of the reactant between two immiscible phases: one aqueous and one organic.
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