Alteration and curing of framework defects by heating different as-made silica zeolites of the MFI framework type

Zeolite materials of the MFI-structure type have been synthesized with tetrapropylammonium (TPA(+)) as organic structure directing agent, (i) applying the all-silica route ((TPA)[Si-MFI-type]), (ii) with fluoride as mineralizer ((TPA,F)[Si-MFI-type]) and (iii) including a boron source ((TPA)[B,Si-MFI-type]). These three materials served as models for different states of defect and perfect framework zeolites in their as-made and calcined states. The materials themselves and their silanol/siloxy type defects were characterized using PXRD, TG/DTA, SS-NMR and IR spectroscopy, i.e. techniques probing long range order and local structure. The combination of these analytical techniques proved to be very helpful for a most complete understanding of their local structure as well as the changes that occur upon heating. If defects were detected, the curability was analysed by performing calcination. Whereas the as-made (TPA,F)[Si-MFI-type] already exhibits a perfect, defect-free silica framework, (TPA)[B,Si-MFI-type] and (TPA)[Si-MFI-type] both show a substantial concentration of defects after synthesis. (TPA)[Si-MFI-type] contains predominantly a complex local defect structure, which is completely curable during calcination. The defects for (TPA)[B,Si-MFI-type]-calc but remained, to a small degree, after calcination and deboronation in (TPA)[B,Si-MFI-type]-deb, where silanol nests had formed intermediately. An IR band at 3730 cm(-1) characteristic for isolated Si-OH indicates that some silanol defects remained due to imperfect condensation of the silanol nests.