(689c) Influence of N,N,N-Trimethyl-1-Adamantyl Ammonium on Al and Defect Siting Energetics and Implications for Accessible Si/Al Ratios in CHA

Chabazite (CHA) synthesized with N,N,N-trimethyl-1-adamantyl ammonium (TMAda⁺) as sole organic structure directing agent (OSDA) can access a wide range of Si/Al ratios. Assuming a perfect CHA lattice with one OSDA per cage, charge-balance implies a Si/Al ratio of 11 if each CHA since each TMAda⁺ is charge compensated by a framework Al. Previous work indicates that the energetics of TMAda⁺ interaction with an Al-decorated CHA framework rationalizes observed Al siting preferences. Here we extend this approach to interrogate the influence of TMAda⁺ on Al and siloxy defects using ab initio molecular dynamics (AIMD). We explore over a range of relative densities and distributions of Al and defects over the CHA lattice. We find that siloxy defects can adopt different configurations in which three silanol protons point toward different framework oxygens. Relative energies show that TMADA+ does not bias the incorporation of defects over Al (or vice-versa), other than to bias towards distributions that minimize the distance between the positive charge on the OSDA and the negative charge on the framework. This is consistent with observed characterization of CHA materials at various compositions when synthesized with TMAda⁺.