CAREER: Elucidating Mechanisms and the Effects of Zeolite Framework, Acid Site Location and Strength in Methanol-to-Hydrocarbon Reactions

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Zeolites are crystalline materials that have been used commercially for many years as agents for separating gas mixtures and as catalysts for refining crude oil to fuels and chemicals. Alternatively, zeolites can be used to process methanol, which can be formed from natural gas, biomass, or coal refining, into a wide range of fuels and chemicals. Such processes are collectively known as methanol-to-hydrocarbons, or MTH, reactions. This research project will explore reaction mechanisms for MTH and how zeolite structure and acid strength influence the efficiency, selectivity, and stability of MTH catalysts. The outcomes of this research project will improve how we utilize our nation's natural gas, shale gas, and biomass resources for the production of liquid transportation fuels and chemicals. The computational methods developed during this project will be released to the public, thereby providing researchers around the globe access to the protocols. The project researchers will develop a three-week on-campus internship for secondary school science teachers to help them implement computational science-based lesson plans for their classes. The project researchers will provide opportunities for high school and undergraduate students to participate in computational catalysis research.Methanol reacts over zeolites with a pool of hydrocarbon co-catalysts in two cycles: the alkene and aromatic cycles. In both cases, methanol reacts with alkenes and arenes to form carbon-carbon bonds prior to isomerization and carbon-carbon bond scission reactions which form a mixture of alkene and aromatic products. Hydride transfer reactions can form alkanes and dienes, the latter of which can form aromatic species and polyaromatic coke precursors that deactivate catalysts. An atomistic understanding of these pathways will be built by dispersion corrected density functional theory (DFT) calculations that estimate the free energies of adsorption, reaction, and activation. Zeolites can contain multiple unique acid site locations, and reacting species and transition states can take many distinct orientations around each site. Experimental techniques currently cannot distinguish between those orientations, but the development of novel calculation techniques allows discrimination between acid sites and their roles in the overall reaction scheme. The role of acid site location will be investigated using models of the H-ZSM-5 zeolite (MFI framework) by contrasting the behavior of various T-site locations. The role of zeolite framework will be determined by contrasting H-ZSM-5 with H-SSZ-13 (CHA framework), which has distinct pores and connectivity, which should show differences in kinetic and transport behavior. Acid strength effects will be examined by comparing H-SSZ-13 with the zeotype H-SAPO-34 (both CHA) which have the same structure but differ in their composition and acid strength. Together, these results will describe the unconfounded effects of acid site location (e.g., channels vs. intersections in MFI), zeolite framework (MFI vs. CHA), and acid site strength (H-SSZ-13 vs. H-SAPO-34). DFT-calculated free energies will be used to parameterize kinetic Monte Carlo (KMC) simulations which will identify reactive and unreactive pathways and account for transport effects to understand crystallite size effects when combined with DFT-calculated diffusion barriers for transport-limited species.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.