Spectroscopic Elucidation of Cu and Fe Active Sites in Zeolites

Stanford University, Stanford CA

Investigators

Abstract

Professor Edward I. Solomon of Stanford University is using funds from the Chemical Catalysis program of the Chemistry Division to study the selective hydroxylation of methane (and other simple hydrocarbons) at Cu/O and Fe/O active sites in Cu- and Fe-exchanged zeolites. The hydroxylation of methane (the chief component in natural gas) produces methyl alcohol that can be used as a liquid fuel. In fact, methane is the world's most abundant fossil fuel, but is a potent greenhouse gas. Its direct conversion to methanol would be a valuable source of green energy, decrease our dependence on oil reserves, and reduce one of the greenhouse gasses. This project is focused on understanding how Cu and Fe ions bound to zeolites (a family of porous minerals) facilitate the selective oxidation of methane and other simple hydrocarbons to produce liquid fuels and chemical precursors for industrially significant reactions. These studies will aid in rational catalyst design, enabling more efficient use of hydrocarbon feedstocks. Outreach activities coupled to this research program enhance professional development of local high school teachers and provide local high school students the opportunity to experience chemical research. The selective hydroxylation of methane (and other simple hydrocarbons) in Cu- and Fe-exchanged zeolites is being investigated using coupled spectroscopic and computational methods to elucidate the geometries, electronic structures, and contributions to hydrocarbon reactivity of the Cu/O and Fe/O active sites. Resonance Raman spectroscopy indicates a [Cu2O]2+ species to be the catalytic site in Cu-ZSM-5. Studies are now directed toward: 1) using a variety of site selective spectroscopic methods to further define this active site; 2) understanding the mechanisms of formation of [Cu2O]2+ from N2O and O2; 3) defining the complete reaction coordinate for CH3OH formation, the nature of the product binding site, and the mechanism of its over oxidation; 4) evaluating the Cu/O2 sites in other zeolites for structure/function correlations and enhanced reactivity; 5) extending these studies to Fe-ZSM-5 and other Fe zeolites to understand the mechanisms for selective oxidation and product binding; 6) understanding differences in the reactivity of Fe- vs. Cu-ZSM-5; and 7) extending these studies to other TMI/zeolite/O2 reactions and intermediates These studies are correlated to potentially analogous sites in Cu- and Fe-containing metalloenzymes to establish correspondence between biological and heterogeneous catalysis. Molecular-level insight derived from this research will inform rational catalyst design and enable more efficient use of hydrocarbon feedstocks such as methane. In particular, establishing methanol derived from methane as a viable source of green energy would protect the environment and decrease our dependence on oil reserves.