Steven Butler and Faculty Mentor: Daniel Ess, Department of Chemistry and Biochemistry
There is now access to vast amounts of inexpensive natural gas, due in part to shale gas discoveries. Abundant natural gas provides significant motivation to develop methods for methane partial oxidation. One major target for methane oxidation is methanol. Natural gas is principally methane, but also includes significant amounts of ethane and propane. Methanol can be produced from natural gas via high-temperature generation of syngas (mixture of H2 and CO).
Oxidation of a compound is essentially the addition of oxygens and removal of hydrogens. Full oxidation of methane would yield carbon dioxide and therefore be a wasteful process. For this reason we are very interested in chemical reactions that can be used to achieve a partial oxidation to methanol, a chemical much more useful in industrial processes.
A method for partial oxidation via mercury catalysis has been developed and studied previously; it is at a much low temperature and pressure than the syngas method mentioned above. However, it is not an industrially viable process because of various problems. The exact mechanism of this reaction is debated. I sought to end this debate though the use of state-of-the-art molecular modeling computations that can be used in conjunction with the experimental data to draw a conclusion.
The molecular modeling software that I used was Gaussview and Gaussian 09. This software allows for the building of molecular models and computing the energies of each possible reaction pathway using a super computer. By computing all of the possible pathways I was able to determine the lowest energy and thus most probably reaction pathway and compare my results with experimental results.
There were many probable pathways to compute, but the major debate was between what are known as a CH activation pathway and a radical pathway. CH activation is much more controllable and a more favorable reaction while radical mechanisms are uncontrollable and often give a range of unwanted side products.
Each step in a proposed reaction pathway is modeled in Gaussview and then a calculation on the structure is run via Gaussian 09. After computing each of these steps for each pathway I was able to build energy pathway diagrams to determine which was the most favorable.
The CH Activation reaction pathway proved to be the most energetically favorable pathway at the experimental conditions. This along with the experimental data presented in papers by the Sen1 and Periana2 groups show that this is indeed a CH Activation pathway.
1 Periana, Roy A.; Taube, Douglas J.; Evitt, Eric R.; Löffler, Daniel G.; Wentrcek, Paul R.; Voss, George; Masuda, Toshihiko. A Mercury-Catalyzed, High-Yield System for the Oxidation of Methane to Methanol. Science, New Series, Vol. 259, No. 5093 (Jan. 15, 1993), pp. 340-343
2 Basickes, Naomi; Hogan, Terrence E.; Sen, Ayusman. Radical-Initiated Functionalization of Methane and Ethane in Fuming Sulfuric Acid. J. Am. Chem. Soc. 1996, 118, pp.13111-13112