Location and bonding of cations in ETS-10 titanosilicate molecular sieve: A multinuclear n.m.r. investigation

Abstract

ETS-10 is a large pore titanosilicate molecular sieve with unique properties due to its peculiar pore structure and charge distributions. Extensive use of multinuclear n.m.r. in the present work has enabled elucidation of the effect of cations on the local structure and electronic environment in ETS-10. MAS 23Na n.m.r. shows that Na+ ions in NaETS-10 reside in both the 12- and 7-member ring channels to balance the negative charges from the octahedral TiO62 units. The cations bond weakly to the framework and can be exchanged reversibly without damaging the framework structure. After proton exchange, Na ions in both channels are replaced by more delocalized protons, as evidenced by efficient 1H-29Si cross-polarization, significant high field shift and line broadening of 29Si n.m.r. signals, and the great similarity between CP/MAS and MAS 29Si n.m.r. spectral patterns. Proton exchange causes a significant cation charge density dilution in the system, which could affect the properties of the molecular sieve, such as acidity and adsorption behavior. MAS 1H n.m.r. shows that silanol hydroxyl groups are the major H-containing species in HETS-10. Contrary to the prediction based on a currently established ETS-10 framework structure model, one Si(3Si, 1Ti) site was found to have quite different properties from the other three Si(3Si, 1Ti) sites.