(337d) Modeling Complex Reactions in Zeolites: Effects of Acid Site Location, Framework, and Reagent Structure on Methanol-to-Hydrocarbon Reactions
Acid site location directly impacts molecular adsorption energies, reaction energies, and activation barriers. Here, we examine the effects of acid site location using novel methods that rigorously examine molecular adsorption and reaction pathways at all acid sites within zeolite frameworks. Methanol adsorption free energies, calculated using dispersion-corrected energies within a periodic model of MFI, for example, vary from â126 kJ molâ1 to â16 kJ molâ1 at the 48 AlâO site pairs which define acid sites within MFI. Intrinsic free energy barriers for methanol dehydration to form surface methoxy species (CH3-Z) similarly vary from 80 kJ molâ1 to 150 kJ molâ1. Varying acid site location within MFI has the same impact on intrinsic rates as changing the entire zeolite framework, indicating the importance of NMR and synthesis techniques which attempt to locate and control the location of heteroatoms within zeolite frameworks. In this work, we will present methanol and ethanol dehydration pathways within all acid sites of TON, CHA, and MFI and use these data to demonstrate that rates, selectivities, and even mechanisms can be affected by the local zeolite environment by changing the framework or acid site location.
Methanol-to-hydrocarbon reactions are also impacted by the local zeolite environment, by certain environments permitting the formation of aromatic species which constitute a âhydrocarbon poolâ of species which co-catalyze further reaction cyclesâbut can also grow into coke precursors, leading to catalyst deactivation. This âhydrocarbon poolâ consists of methylated benzenes (from benzene to hexamethylbenzene, 13 species) and each of these species can 1) by methylated by methanol, 2) undergo intramolecular CâC bond rearrangements to form alkyl substituents, and 3) dealkylate to form light alkene products. The total number of elementary steps involved is enlarged by the large set of possible co-catalysts. Here, we use structural refinement methods to examine reaction energetics all co-catalysts for each elementary step. The free energy barrier for methylating an aromatic ring, for example, increases from 80 kJ molâ1 to form toluene to 140 kJ molâ1 to form hexamethylbenzene, indicating that methylation becomes more difficult as reagents become more sterically-crowded by methyl-substituents. These effects of co-catalyst structure, along with kinetic Monte Carlo methods, allow us to report not only the mechanism of light alkene formation during MTH, but also the hydrocarbon pool species predominantly responsible for light alkene production.
These two related topics focus on computational methods and strategies for examining reactions solvated by diverse zeolite environments (within one and across multiple frameworks) and for examining the effects of reagent structure through systematically varying âH and alkyl substituents in hydrocarbon pool species which facilitate methanol-to-hydrocarbon reactions. These approaches lay the groundwork for examining the complete MTH pathways in all known and predicted zeolite structures to ultimately use computation to direct zeolite synthesis efforts by identifying promising novel materials in which the hydrocarbon pool can be formed without concomitant coke buildup and catalyst deactivation.