Often, the heavier fractional species are sent through a reaction chamber under heat, pressure, and catalysis to “crack” the large molecules into smaller, more volatile molecules. These post-reaction by-products are then sent back to be refined again, thus get more valuable gasoline from the heavy oils.
Is it possible to “un-crack” natural gas molecules? Build them up into a liquid stock?
Absolutely. That’s exactly how synthetic motor oil is made. In a nutshell, they take ethylene gas and polymerize it into molecules called polyalphaolefins, which have most of the characteristics of an ideal oil and then blend that with additives and then you have a much more stable oil for your car.
Alpha-olefins can also be used to make detergents and other useful products. If I had remembered certain details of the Shell Higher Olefins Process, a method of making linear alpha-olefins from ethylene, at a certain time interval two years ago, I have reason to believe the subsequent time would have been much different for me…
Another way of producing heavier molecules from natural gas is by using the methane to make synthesis gas, which is mostly carbon monoxide and hydrogen. This can be done done simply by heating the methane to high temperature (~850°) at moderate pressure (~10-20 atm) in the presence of a nickel catalyst. Synthesis gas can then be used to make methanol or ethanol, and it can be used as a feedstock in making other compounds. I don’t imagine that it would be practical to make hydrocarbons from synthesis gas, but it’s not really necessary to do so. Ethanol can be used as a fuel or as a fuel additive, and methanol can be used as a fuel, an additive, or in the manufacture of biodiesel.