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Most organic compounds, whether simple or complex, can be produced in a number of ways. Only the most cost-effective of these can be used for commercial production. Formaldehyde production employs one of two catalytic methods involving methanol (CH3OH): mild oxidation, or dehydrogenation. The catalyst used may be a mixture of molybdenum and iron oxide, or alternatively, silver. A molybdenum catalyst requires a temperature of about 480-750° F (250-400° C) to sustain the reaction, whereas silver requires the much higher temperature of 1200° F (650° C).
It might seem formaldehyde production using a strong oxidizer would constitute a third option. That route is not suitable, however, as the desired aldehyde itself would be in danger of undergoing oxidation, to form a carboxylic acid — in this instance, formic acid (HCOOH). One feature both the mild oxidative and the dehydrogenation methods share is the need for continuing heat to sustain the process. This might seem to prohibitively increase the cost of manufacture of formaldehyde production. Both processes are exothermic, however — meaning each reaction gives off heat — making them both self-sustaining.
Using a molybdenum and 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. While oxidative formaldehyde production is occasionally used, it is less common than the dehydrogenation method.
One common catalyst used in the dehydrogenation process of formaldehyde production is silver, although silver may react, in part, via the oxidative pathway. As is the case with the oxidative, molybdenum-iron oxide catalyst, the methanol vapors are combined with air and passed over the catalyst — the metal itself exists in granular, crystalline form. Both molybdenum and silver reactions 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 effluent gas includes hydrogen; the gas is burned to produce steam, which feeds the boiler.
The reaction equation for the dehydrogenation process is CH3OH → HCHO + H2. Additional catalysts that may be employed in place of silver as the dehydrogenating agent are copper chromite and palladium acetate. Special conditions are required for its successful operation. A form of heterogeneous catalyst, palladium acetate acts as a “phase-transfer” agent. This means it behaves much like a detergent, allowing the transfer of reactant between two immiscible phases — one aqueous, one organic.
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