Theoretical investigation of Ti substitution, defect localization, and acidic sites in MFI-type zeolites

T4, T11, T10, and T8 are the most favorable Ti substitution sites in S-1. • In MFI-type zeolites, hydroxyl nest defects preferentially form at the T10-T1, whereas tripodal(1I) defects are most favorable at the T11-T10. • Compared with tripodal(1I) defects, the formation of hydroxyl nest defects is more sensitive to Ti incorporation and easily formed. • In TS-1, hydroxyl nest defects show stronger Lewis and Brønsted acidity compared with tripodal(1I) defects. Although MFI-type zeolites have been extensively studied, the formation of framework defects and their preferential siting at different framework positions remain difficult to accurately predict, which largely hampers the realization of defect engineering and precise defect control. In this study, density functional theory was used to study the preferred Ti substitution sites in S-1 zeolite. The results showed that T4, T8, T10, and T11 were the most favorable sites for Ti incorporation, and cluster models centered on these four representative sites were constructed as the main research objects to further analyze the formation sites and spatial distributions of hydroxyl nests and tripodal(1I) defects. These findings revealed that hydroxyl nest defects are most stable at Ti-T10-T1 sites in TS-1, whereas tripodal (1I) defects preferentially form at Si-T11-T10 sites in S-1, indicating that framework positions strongly affected the preference of defect site. Moreover, Ti incorporation markedly lowers defect formation energies, indicating that TS-1 is intrinsically more prone to defect formation than S-1, although the most stable defect configurations remain unchanged. Hydroxyl nest defects were more sensitive to Ti substitution, while tripodal(1I) defects were relatively unaffected. Furthermore, the optimal formation sites and acidity of two representative framework defects (hydroxyl nests and tripodal(1I) defects) in both S-1 and TS-1 were analyzed, and demonstrated that hydroxyl nest configurations in TS-1 exhibited stronger Lewis and Brønsted acidity. This study deepens the understanding of preferential framework defect sites and Ti substitution effects in MFI-type zeolites, providing theoretical guidance for defect-engineered design. Schematic illustration of the preferential formation sites of hydroxyl nest and tripodal(1I) defects in S-1/TS-1 and their Lewis and Brønsted acidity in TS-1. Download: Download high-res image (232KB) Download: Download full-size image