The commercial production of formaldehyde was first started in Germany in the 1880s but the development of a methanol synthesis route in the 1920s gave the spur to the development of large-scale manufacture. Today there are two main routes: oxidation-dehydrogenation using a silver catalyst involving either the complete or incomplete conversion of methanol; and the direct oxidation of methanol to formaldehyde using metal oxide catalysts (Formox process).
In the silver catalyst route, vapourised methanol with air and steam is passed over a thin bed of silver-crystal catalyst at about 650oC. Formaldehyde is formed by the dehydrogenation of methanol. The heat required for the endothermic reaction is obtained by burning hydrogen contained in the off-gas produced from the dehydrogenation reaction.
The other route involves the oxidation of methanol over a catalyst of molybdenum and iron oxide. A mixture of air and methanol is vapourised and passed into catalyst-packed reactor tubes. The reaction which takes place at 350oC is highly exothermic and generates heat to provide steam for turbines and process heating.
Perstorp offers a high pressure version of the Formox process which can be retrofitted to existing plants to boost capacity. The high conversion rate of the Perstorp process eliminates the need for methanol recovery via distillation and it can produce formaldehyde at concentrations up to 57%.
Yields from both processes are around 90% to 92% but the oxidation route has a lower reaction temperature and the metal catalyst is cheaper than silver. However, the partial oxidation-dehydrogenation route is still the most prevalent.
A wide range of alternative feedstocks have been considered but not found to be economic. For example, a tiny amount is produced from the non-catalytic oxidation of propane-butane mixtures. Formaldehyde can be produced from methane but a mixture of products needs to be separated. It is also a byproduct of the oxidation of naphtha to acetic acid.