Methane to methanol
Methane is produced naturally by methanobacteria breaking down organic material (e.g. in the intestines of animals). Cultures of these bacteria in methane digesters can turn animal and vegetable waste into useful biogas. Although very convenient for cooking food, heating water and other household related tasks, there are barriers to using biogas to propel internal combustion-powered vehicles. In addition to methane, biogas also contains other variable components such as hydrogen sulfide and hydrogen, which are corrosive. Carbon dioxide is often up to one third of the biogas volume. These need to be removed for many uses ("upgrading") such as feeding into existing natgas infrastructure ("biomethane"). 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.
The solution is to convert methane to the chemically very similar but liquid fuel methanol (CH4 and CH3OH, where one hydrogen is replaced by a hydroxyl group). Industrially, methane is converted to methanol by partial oxidation to hydrogen gas and carbon monoxide (synthesis gas or syngas) 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, ultraviolet 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.
Tungsten Oxide Catalyst
The paper "Photo-catalytic conversion of methane into methanol using visible laser" by M.A. Gondal et al. (here and here) details the creation of a tungsten oxide semiconducting powder and experimental details of an apparatus for carrying out this conversion in batches.
Photo-Catalytic Conversion Of Methane Into Methanol Using Visible Laser - Gondal, MA; Hameed, A; Suwaiyan, A ELSEVIER SCIENCE BV, APPLIED CATALYSIS A-GENERAL; pp: 165-174; Vol: 243 King Fahd University of Petroleum & Minerals http://www.kfupm.edu.sa
Summary The photo-catalytic conversion of methane into methanol was investigated under different experimental parameters such as catalyst concentration, laser power, laser exposure time, effects of free radical generator (H2O2) and electron capture agent (Fe3+), using visible laser light. The study was carried out at room temperature with a simple set up using a laser light, water and a semiconductor photo-catalyst WO3. The reaction products (methanol, O-2 and CO2) were characterized using gas chromatography. The use Of WO3 as photo-catalyst replaces the UV laser light with visible laser light. This greatly simplifies reactor design and permits flexibility in the selection of a laser source in the visible region. The oxygen to tungsten ratio in WO3 at different temperatures was studied by XPS. (C) 2002 Elsevier Science B.V. All rights reserved
Direct focused sunlight could be used. Though only a portion of the solar spectrum is usable, 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% efficient), 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.
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