Methane to methanol
Methane is produced naturally by methanobacteria breaking down organic material in mammal intestines. Cultures of these bacteria in methane digesters can turn animal and vegetable waste into useful biogas. Although very useful for cooking food, heating water and other household related tasks, there are barriers to using biogas to propel internal combustion powered vehicals. In addition to methane, biogas also contains other variable components such as hydrogen sulfide and hydrogen, which are corrosive. These need to be removed before upgrading. Several thousand Pounds per square inch, hundreds of times atmospheric pressure, are needed to compress the gas to useful densities. Liquefying it requires greater pressures and perhaps very low temperatures.
Methane is chemically very similar to the liquid fuel methanol (CH4 and CH3OH, where one hydrogen is replaced by a hydroxyl). Industrially, Methane is converted to methanol by partial oxidation to hydrogen gas and carbon monoxide (synthesis gas or syn gas) at high temperatures (several hundred degrees Celsius) in a process called cracking. Syngas is then catalytically converted to methanol over a copper or platinum surface, also at a couple hundred degrees Celsius. This process is only around five or ten percent efficient due to accidental total oxidation to carbon dioxide and water.
If one could simply replace one hydrogen with a hydroxyl, there would be no need to produce syngas and run the risk of complete oxidation. The recent field of photocatalysis offers another pathway to liquid fuel from methane. Here, ultra violet light breaks water into a hydrogen and hydroxyl free radical, which are highly reactive. When a hydroxyl radical reacts with a methane molecule, a hydrogen is displaced and methanol is produced. With the use of tungsten oxide or a similar semiconductor, photons of lower energy than ultraviolet (down to blue) can be used.
This process has been demonstrated by several groups using ultraviolet flash bulbs and also with lasers.
The paper "Photo-catalytic conversion of methane into methanol using visible laser" by M.A. Gondal et al. details the creation of a tungsten oxide semiconducting powder and experimental details of an apparatus for carrying out this conversion in batches.
Ultimately, direct focused sunlight could be used. Though only a portion of the solar spectrum is useable, and the process would likely be highly energy inefficient, the use of sunlight directly avoids the use of highly inefficient lasers (typically less than 2% efficent), any electricity production, storage or transportation, as well as the machinery required to produce lasers. The light that is not absorbed by the semiconductors could be used for further purposes, and the heat generated could be used to distill the methanol (thus allowing for a continuous rather than batch production). Also, the process proceeds more easily at higher temperatures.