Title: Building Structure-Function Relationships for the Catalytic Conversion of Light Alkenes and Carbon Dioxide

Abstract: The valorization of CO2 and shale gas-derived light alkenes will be key components of technologies for decarbonizing the fuel and chemical industries. Light alkenes can be oligomerized to transportation fuel-range alkenes over solid Brønsted acid catalysts, while CO2 can be converted to oxygenates (e.g., methanol) through hydrogenation on transition metal-derived catalysts. Regulating rates and directing selectivity during the thermocatalytic conversion of both feedstocks poses fundamental challenges for the industrialization of such processes. Herein, the influences of thermodynamic, kinetic, and transport barriers on the rates and selectivity of propene oligomerization and CO2 hydrogenation to methanol are evaluated.

Propene oligomerization proceeds through dimerization and subsequent propene additions to yield oligomers with carbon numbers that are integer multiples of propene; concurrent β-scission cracking and co-oligomerization reactions form products of other carbon numbers. Medium-pore MFI zeolites synthesized with independently varied Brønsted acid site density (H⁺/u.c.) and crystallite size enabled evaluating the effects of these material properties on propene oligomerization rates and selectivity. Systematic decreases in propene dimerization rates on MFI samples of fixed H⁺/u.c. with crystallite size, transients in rates upon step-changes in reaction temperature and pressure, and reaction-transport formalisms together evidence that propene oligomerization rates and selectivity are strongly influenced by transport barriers imposed by products that occlude within the zeolitic micropores during catalysis. The composition of these products, and consequently the transport barriers they impose, evolve with reaction conditions and H⁺/u.c., providing new avenues to tune rate and selectivity in alkene oligomerization.

Achieving high methanol yields during CO2 hydrogenation requires kinetically directing selectivity towards methanol and activating CO2 at low temperatures (< 423 K) where methanol conversion is not significantly limited by thermodynamic equilibrium. Unsupported Mo2C catalyzes continuous CO2 hydrogenation at low temperatures (348–408 K) with high selectivity to methanol (up to ~80%). Product formation rates measured over widely varying reactant and product concentrations in conjunction with reversibility formalisms afforded the dependence of forward kinetic rates on reactant and product concentrations, from which mechanisms of methanol synthesis, reverse water-gas shift, and methanation could be deduced. A kinetic model revealed that Mo2C surfaces are highly covered with partially hydrogenated CO- and CO2-derived intermediates during steady-state catalysis, the coverages of which dictate relative rates of methanation and methanol synthesis, respectively. Importantly, these findings suggest that COx- and H-derived intermediates do not compete for surface occupancy on Mo2C, but adsorb cooperatively, thereby enabling low temperature CO2 activation.

Bio: Elizabeth Bickel Rogers received her Bachelor of Science degree with Distinction in Chemical Engineering from Tennessee Tech University in 2017. She earned her Ph.D. in Chemical Engineering from Purdue University in 2022 under the supervision of Professor Rajamani Gounder. At Purdue her research focused on the synthesis of zeolite materials with well-defined properties and their application as catalysts for upgrading light alkenes. She is currently a postdoctoral scholar in Professor Aditya Bhan’s research group in the Department of Chemical Engineering and Materials Science at the University of Minnesota where her research has centered on low temperature CO2 hydrogenation over transition metal carbide catalysts. Elizabeth is the lead author of seven journal articles and has been recognized for her research and mentorship with several honors, including the Kokes Award from the North American Catalysis Society, the Philips 66 Fellowship, and the Outstanding Graduate Mentor Award from Purdue.