Oxygen
From Captain of Industry Wiki
Research:Biofuel unlocks the Air Separator which pulls Oxygen and Nitrogen directly from the air. 78% of air is Nitrogen. Oxygen comprises around 21% of the atmosphere. The Fermentation Tank requires Oxygen and either Corn Mash or Sugar
to make Ethanol. Research:Electrolysis unlocks the Electrolyzer which can make small amounts of Hydrogen and Oxygen from liters of Water, but one Air Separator will produce enough Oxygen for three Oxygen Furnace or two Oxygen Furnace II
to forge Molten Steel out of Molten Iron using the Bessemer process. Diesel can be made with Oxygen and Fuel Gas in a Cracking Unit. This process produces Carbon Dioxide and requires an Air Separator direct connect.