| In this text I explore the importance of including the Madelung potential when modeling reactions in zeolites. The Madelung potential was included in quantum mechanical calculations with a novel embedded cluster method, so Chapters 1 and 2 are devoted to the verification of the model, as well as the examination of the Madelung potential's contribution to neutral-pair and ion-pair adsorption species in zeolites. Results for NH3 and NH4 + adsorption in H-chabazite (Chapter 1) and H-zeolite Y (Chapter 2) were compared with previous embedded cluster, cluster, and periodic results, and show that it is important to include both structural relaxation and the Madelung potential to properly model these interactions. Chapter 3 focuses on the effect of the Madelung potential on transition states for hydrocarbon reactions in zeolites, with the finding that the Madelung potential stabilizes a carbonium-like transition state for the HID exchange reaction of CD4 with H-zeolite Y, and reduces the reaction barrier by 17--23 kJ/mol.; The static and dynamic properties of the acidic protons in H-zeolite Y are examined in Chapters 4 and 5. In Chapter 4 it is shown that including the Madelung potential destabilizes the acidic protons, relative to acid site O3, by 36.8 to 72.2 kJ/mol, yielding the following ranking of proton stabilities: O3 > O1 > O2 > O4, in agreement with previous experimental and computational studies. Without the Madelung potential the relative proton stabilities lie within 10.5 kJ/mol of each other, and are ranked as: O1 > O3 > O4 > O2. Barrier heights for the transfer of the acidic proton from one acid site to another are presented for all paths in Chapter 5. These barriers include contributions due to: the Madelung potential, vibrational energy, and tunneling. Tunneling was found to be significant up to 500 K, effectively reducing barrier heights by as much as 14.3 kJ/mol at that temperature. Transition state theory was used to predict rate constants for all reaction paths, leading to the conclusion that O1 to O2, O3 to O2, and O2 to O3 are the dominant paths for proton migration. |
