I just started my internship in the Carngello chemical engineering lab today. The project I’ll be helping with (at least, to start) is designing catalysts for “Partial Methane Oxidation” or PMO. The PMO process is used to convert methane gas (CH4, commonly referred to as “natural gas”) into methanol (CH3OH, also known as wood alcohol).
Methane has found widespread use as a heating gas, but it has two properties that make it unfavourable. The first is that, as a gas with a very low boiling point, it takes a great deal of energy to transport it. To transport it as a gas requires very large vessels, and to hold any appreciable quantity of methane also requires the gas to be pressurized. The more you pressurize it, the more you can store in a container of the same volume (up until it becomes a liquid); however, this also increases the amount of energy you need to expend (pressurization can take up to xx% of the energy you get from burning the methane) and requires thicker containers to hold it (not to mention an increased risk of explosion). In the end, you get ships that look like this designed for transporting methane:
(Credit: Fortune Magazine)
This is fine on an industrial scale, but the high infrastructure costs mean that methane extraction is only practical on a large scale. Often, methane occurs in smaller deposits and is accidentally discovered when drilling for oil or minerals. Because of the high cost of liquefying methane, much of it is flared or released, contributing to climate change. Moreover, even when it is being liquefied, some escapes through small leaks in the system.
The other problem that caps methane’s potential is the difficulty of turning it into other chemicals. While we often think of methane as relative because it can be burned, it takes a great amount of energy to start the oxidation process and keep it going. This is part of the reason why methane only combusts at very specific ratios of methane: oxygen.
Converting methane into methanol (a substantially more reactive liquid), solves both of these issues. In order to convert methane into methanol – either for shipping or as a stepping stone towards other industrially useful products – we need to break one of the high-strength carbon-hydrogen bonds and replace it with oxygen.
This sounds simple enough, and it is (it’s the same first step as burning methane). The tricky part is stopping the reaction from going too far, and turning the methane into carbon dioxide.
This is already a challenging task, but the chemistry of methane makes it even more difficult. The first C-H bond is the hardest to break (converting CH4 to CH3), and each successive bond becomes easier to pull apart (So CH3 à CH2 à CH à C). This means that if you have a catalyst that can efficiently “abstract” the first hydrogen, that catalyst is likely to pull off all the other hydrogens as well, which is exactly what we’re trying to avoid.