Yujia Wang

Chemistry and Biochemistry

Faculty Advisor: William Schneider

Computational Investigation of the Identity and Reactivity of Exchange Cu Sites in Zeolites for Selective Activation of Methane

The increases in shale gas production in the world has driven the increase in global methane abundance. Methane is the main hydrocarbon in natural gas as well as a greenhouse gas. Converting methane to water and carbon dioxide through combustion can release energy. Converting methane to methanol, however, would form a liquid compound that can be transported more easily and subsequently converted into a variety of chemicals and fuels. Cu-exchanged zeolites, a type of heterogeneous catalysts with well-defined porous structures, have only recently been found to be active for partial methane oxidation to methanol (PMO), but the activities of the catalysts are still very low. Further advances require a fundamental molecular-level understanding of the structure and function of the active sites and the selective oxidation mechanisms in order to design catalysts that react with molecular oxygen to oxidize hydrocarbons. Cu-exchanged zeolites, on the other hand, are remarkable catalysts in terms of reactivity, selectivity and stability for NOx conversion to N2 (selective catalytic reduction of NOx with NH3, SCR). My previous research on dimeric Cu species in Cu-SSZ-13 zeolite for SCR shows Cu sites have a strong propensity to form oxygen-bridged dimers which would make O2 activation and CH4 activation possible for PMO reactions. Our experimentalists collaborators from Professor Gounder’s group at Purdue University are striving for the development of new tools and technologies for continuous methane oxidation to methanol on Cu-exchanged zeolites. Combining his group’s experiment effort with my molecular-level modeling and simulations, we would advance the fundamental mechanistic understanding of the reaction and the catalysts and eventually diversify the strategies available to use the world’s abundant hydrocarbon resources.

Systematic investigation of the stability and reactivity of exchange Cu sites at different locations of the zeolite (different framework Al proximity) and at various reaction conditions using density functional theory (DFT) will be conducted to identify the active Cu sites and to understand their oxidation-reduction behaviors. Monte Carlo simulations of global Al distributions will be conducted to bridge the knowledge of microscopic details of monomeric or dimeric Cu and reactivities of exchange Cu cations to the macroscopic composition of the zeolites (Si:Al and Cu:Al ratios) to predict the quantities of active Cu sites. This workflow can be applied broadly to other zeolites to predict speciation and distribution of exchange cations as consequences of Al distributions, and to find optimal zeolite compositions for catalysis applications.