Molecular Structure and Reactivity of Model Mn/Na2WO4/SiO2 Oxidative Coupling of Methane Catalyst under Operating Conditions

Lehigh University, Bethlehem PA

Investigators

Abstract

This research project addresses the direct catalytic conversion of methane, the primary component of natural gas, to value-added chemical feedstocks. In particular, the research provides fundamental insight into the mechanisms of how two methane molecules couple together to form various molecules with two carbon atoms, the OCM reaction. The latter types of molecules are chemicals and fuels of interest to chemical industries. The OCM reaction is of current industrial interest as a more economical alternative to large-scale, capital-intensive routes for methane conversion, especially for remotely located methane reservoirs whose methane cannot be economically transported to centralized chemical plants. The project combines research, teaching and an outreach plan focused on understanding limitations of the best performing OCM catalyst, and exploring options for improving its activity and selectivity beyond current levels. This research is being showcased in annual summer workshops at the Lehigh Valley Da Vinci Science Center where visitors of all ages will be exposed to the new technologies being developed that convert methane to chemicals. Workshops, curricula development, and research integration into a senior Chemical Engineering Project Design capstone course will impact approximately180 middle-, high-school and undergraduate students of diverse socioeconomic backgrounds per year. The research is being conducted in collaboration with international researchers, which will allow access to cutting edge research facilities not available at Lehigh, enhance visibility of the research, and expose the participating students to the research culture in leading international laboratories. The project is built on the hypothesis that the active site for OCM by supported Mn/Na2WO4/SiO2 catalysts is an isolated surface WOx species anchored to the silica support. The investigators' preliminary studies have identified, for the first time, the presence of unpromoted and promoted isolated surface WOx sites during OCM that catalyze this reaction. The objectives of the project are to (1) establish the fundamental catalyst structure-activity relationships by application of modern in situ and operando spectroscopy during OCM combined with kinetic studies and density functional theory (DFT), and (2) apply the new fundamental insights to guide rational design of advanced active and selective OCM catalysts functioning at lower temperatures. Various permutations of unpromoted and Na-, Mn- and Na/Mn-promoted SiO2-supported WOx sites are being synthesized. The silica support is also being surface-modified with sodium metal and nanolinkers (TiOx, ZrO2, Al2O3) that will increase the number of isolated surface WOx sites and tune their reducibility and acid-base characteristics. The promoters-to-W ratios is being systematically varied to explore their effects on both the number and structure of the WOx sites and OCM activity/selectivity. The catalysts are being characterized as prepared and during OCM reaction conditions with in situ and operando spectroscopy (Raman, UV-vis and NAP-XPS) to determine the molecular and electronic structures of the WOx sites on silica. Experimental findings are being complemented with molecular level DFT calculations to provide additional insights into structure-activity relationships. Corresponding kinetic studies with isotopic CH4/CD4 and 16O2/18O2 are addressing the rate-determining-step and relative participation of different WOx sites, respectively, - the latter via time-resolved Raman-Mass Spectroscopy. Information about gas phase radicals and their relationship to specific catalyst structures is being obtained with Molecular Beam Mass Spectroscopy through collaboration with European partners. The new insights will lead to conceptually new and realistic catalyst models that will have the potential to enable development of one-step OCM catalytic processes. Successful execution of the proposed research has the potential to lead to economically viable production of C2 hydrocarbons from cheap, abundant, yet difficult and costly to transport methane, and thus move the US closer to energy independence while also providing a longer timeframe to transition into sustainable chemicals. The project will involve collaborations with two companies, Siluria and SABIC, that will facilitate transfer of new catalysts to the chemical industry.