The structure of a molecule determines its reactivity and the amount of energy that is required to oxidize it to CO2. Unsaturated molecules such as formaldehyde, ethylene and propylene react readily at low temperatures. Saturated molecules such as methane, ethane and propane are very stable and require much higher temperatures to convert them to CO2 and H2O. Utilization of a catalyst lowers the energy required for reactions to occur, so they can take place at lower temperatures.