Structures and vibrational spectra of Ti-MWW zeolite upon adsorption of H2O and NH3: A density functional theory study

The structures and vibrational spectroscopic features of framework Ti(IV) species in Ti-MWW zeolite upon adsorption of H2O and NH3 were investigated by density functional theory. The calculations were carried out on cluster models up to 36 tetrahedra at the B3LYP/6-31G(d,p) level of theory. The calculated results indicate that both Ti(OSi)4 and Ti(OSi)3OH species can interact with H2O or NH3 molecules to form five-coordinated complexes. The Ti(OSi)3OH species has higher Lewis acidity and adsorbs the ligands more easily than Ti(OSi)4. The Ti-specific band is attributed to the collective vibration of the antisymmetric stretching of Ti–O–Si bonds. The vibrational frequencies of coordinated Ti species can be divided into two regions: the Ti-specific vibration region and the hydroxyl group vibration region. After adsorption of H2O, the Ti-specific band of the Ti(OSi)4 species shifted from 960 to 970 cm−1, and the Ti-specific bands of the Ti(OSi)3OH species shifted from 990 cm−1 (T1 site) and 970 cm−1 (T3 site) to 980 cm−1. The frequencies of the corresponding NH3 adducts were about 5 cm−1 higher. The Ti(OSi)3OH species can also form hydrogen bonded complexes with H2O and NH3 through Ti–OH, resulting in the hydroxyl stretching band of Ti–OH red shifting by 500–1100 cm−1 and appearing in the 2700–3200 cm−1 region.

Graphical AbstractThe vibrational spectra of Ti-MWW upon adsorption of H2O and NH3 are investigated by density functional theory. The Ti(OSi)4 and Ti(OSi)3OH species interact with H2O to form five-coordinated complexes, with the Ti-specific bands at 971 and 982 cm−1, respectively.Figure optionsDownload as PowerPoint slide