Density Functional Theory Insights into Ti Substitution Location and Acidity in MFI Zeolite with Varying Ti/Si Ratios

This computational study provides fundamental insights into the structure-acidity relationships of titanosilicates (TS-1) by systematically examining Ti substitution thermodynamics and NH3 adsorption behavior across single- to multi-Ti configurations (1Ti to 5Ti). The substitution stability exhibits pronounced site dependence, with energy variations up to 51.62 kJ/mol, where T4 emerges as the universally preferred site regardless of Ti loading, followed by T3, T8, T11, and T12. Intriguingly, Ti–Ti interactions display unconventional distance-independent characteristics, distinct from classical Al–Al interactions in aluminosilicates. NH3 adsorption energetics reveal substantial acidity variations (∼51.83 kJ/mol) among Ti sites, with T2 showing the strongest binding (−108.80 kJ/mol). Multi-Ti systems exhibit optimized acidity at Ti/Si = 1:31 (3Ti, −108.48 kJ/mol), beyond which higher Ti content diminishes NH3 affinity. No correlation is observed between the adsorption strength, Ti–N distance, and charge transfer. These findings provide atomic-level insights into Ti distribution preferences and acid site characteristics in TS-1, offering valuable guidance for the rational design of Ti-zeolite catalysts with tailored acidity and adsorption properties.