Developing structure-function relationships for CO2-assisted ethane dehydrogenation through zeolite-supported chromium sites

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Domestic natural gas is an important contributor to the U.S. energy and chemical manufacturing sectors, with strong effects on the economy and national security. In the context of the chemicals industry, a unique opportunity exists to utilize light alkane components of natural gas, such as ethane, as feedstocks for production of value-added chemicals. Notably, ethane can be catalytically dehydrogenated to produce ethylene, which is a critically important building block feedstock for the manufacture of many types of polymers. Current ethane dehydrogenation process economics are limited by low yields, which result from the prevalence of side reactions and catalyst deactivation. The use of carbon dioxide (CO2) as a co-reactant enhances catalyst activity, facilitates high ethylene selectivity, and extends catalyst lifetimes. Additionally, the CO2 (an industrial waste product and greenhouse gas) is converted to carbon monoxide (CO), which is itself a value-added product used in the chemicals industry. Nevertheless, advances are needed in catalyst technology to assure commercial viability of carbon dioxide-assisted ethane dehydrogenation. The project develops fundamental insights into this conversion process through novel engineering of catalyst structures coupled with systematic investigation of the chemical reactions. Overall, the study will generate critical fundamental information on the mechanistic origins of high single-pass olefin yields, which will translate to improved process economics for combined alkane-CO2 conversion systems. The project is supported by broader efforts to enhance educational and research opportunities for women and underrepresented groups, especially the large Hispanic population served by the investigators’ university.The project focuses on zeolite-supported chromium (Cr) catalysts to develop insights into multiple aspects of the overall catalytic process: ethane activation, CO2 activation, competition and cooperation between co-occurring reactions, and the influence of both metal-support interactions and active site structure. Specifically, heteroatom-substituted zeotype supports (including framework Al, B, and Ga in MFI zeolites) are used to generate Cr active sites with consistent coordination geometry but with varied metal-support interaction (which will be determined by the targeted heteroatom elemental composition). Structural, chemical, and electronic properties of the dispersed Cr sites will be related to catalytic performance via a comprehensive suite of characterization tools, including probe molecule adsorption infrared spectroscopy, ex situ and operando synchrotron-based X-ray absorption spectroscopy, chemisorption and thermogravimetric analysis with mass spectrometry, and Raman spectroscopy. Using well-defined catalysts, the impact of CO2 on reaction kinetics will be isolated and interpreted in the context of the parallel ethane dehydrogenation and reverse water-gas shift (RWGS) reactions. The reaction network will be analyzed through reactor studies, employing varying feed compositions, probe reactions, and isotope labeling experiments. Catalyst structure-function relationships and optimized reaction conditions, obtained at low ethane conversion, will be further investigated in the high conversion regime, relevant to commercial single-pass reactor operation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.